Facile synthesis of layered spinel ferrite from fly ash waste as a stable and active ketonisation catalyst Sasha Yang a, Jinxing Gu a, Binbin Qian a,b, Jim Mensah c, Adam F. Lee d,*, Karen Wilson d, Barbara Etschmann e, Xiya Fang f, Jisheng Ma g, Qinfen Gu h, Lian Zhang a,* a Department of Chemical and Biological Engineering, Monash University, Victoria 3800, Australia b School of Chemistry and Environmental Engineering, Yancheng Teachers University, Yancheng 224002, China c School of Science, RMIT University, Melbourne, Victoria 3000, Australia d Centre for Catalysis and Clean Energy, School of Environment and Science, Griffith University, Queensland 4222, Australia e School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia f Monash Centre for Electron Microscopy (MCEM), Monash University, Clayton, Victoria 3800, Australia g Monash X-ray Platform, Monash University, Clayton, Victoria 3800, Australia h Australian Synchrotron, 800 Blackburn Rd, Clayton, Victoria 3168, Australia A R T I C L E I N F O Keywords: Waste-derived catalyst Fly ash Layered spinel ferrite Multimetallic Ketonisation A B S T R A C T Spinel catalysts exhibit superior activity and structural stability across a wide range of catalytic reactions. Nevertheless, few studies have delved into the synthesis of spinels containing more than four metal cations, for which conventional syntheses from pure chemical precursors are costly and generate significant waste. Here we demonstrate a facile, rapid and scalable synthesis of layered spinel ferrite catalysts from fly ash waste that is otherwise detrimental to landfill ecosystems. The optimum waste-derived catalyst primarily comprised MgAl0.2Fe1.8O4, with a distorted structure due to the substitution of various cations (Ca2+, Mn2+, Mn3+, and Ti4+) at tetrahedral and/or octahedral iron sites, and demonstrates high activity (1.26 mmol⋅g− 1⋅min− 1) and stability (>100 h) for acetic acid ketonisation at a modest temperature (300 ◦C). Acidity measurements yield a corresponding turnover frequency of 2.21 min− 1. Strong synergies are observed between the different metallic cations and octahedral Fe2+ species; XANES and in-situ DRIFTS indicate the latter is the primary active sites for ketonisation in fly ash-derived spinel ferrites, promoting both acetic acid adsorption as bidentate acetate and subsequent C–C coupling to acetone. 1. Introduction Global decarbonisation of the energy sector is recognised as a critical goal to combat climate change and associated extreme weather events. However, transitioning CO2 emissions from ‘hard-to-abate’ sectors such as aviation and maritime transportation present unique challenges due to their reliance on energy-dense liquid fuels for which there are currently few viable alternatives to traditional fossil derivatives. Renewable bio-oil derived from biomass pyrolysis is one such promising solution for decarbonising these sectors as part of a broader low-carbon energy landscape. However, the abundance of carboxylic acids (notably acetic acid) in pyrolysis bio-oils renders these fuels corrosive and un- stable, limiting their practical application [1–3]. Many catalytic tech- nologies exist to upgrade oxygenated primary bio-oils [4], including hydrogenation, decarboxylation and esterification, but require addi- tional chemical inputs (e.g. H2 or short-chain alcohols) and/or pres- surised reactors, hindering process intensification. In contrast, acetic acid ketonisation [5] (2CH3COOH→CH3COCH3 + CO2 + H2O) requires no additional chemical inputs, and can be operated as a continuous vapour phase process, and hence is more economically scalable [6–9]. Ketone products are also valuable building blocks for carbon chain growth through subsequent aldol condensation and hydrogenation, of- fering a route to sustainable diesel and jet fuels [10–13]. Another advantage of ketonisation is that the requisite solid acid catalysts comprise Earth-abundant elements, typically as metal oxides, rather than scarce and costly noble metals commonplace in catalytic hydro- genation/hydrogenolysis. A diverse range of heterogeneous solid acids, encompassing * Corresponding authors. E-mail addresses: adam.lee@griffith.edu.au (A.F. Lee), lian.zhang@monash.edu (L. Zhang). Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej https://doi.org/10.1016/j.cej.2024.157797 Received 1 March 2024; Received in revised form 16 October 2024; Accepted 17 November 2024 Chemical Engineering Journal 502 (2024) 157797 Available online 19 November 2024 1385-8947/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). mailto:adam.lee@griffith.edu.au mailto:lian.zhang@monash.edu www.sciencedirect.com/science/journal/13858947 https://www.elsevier.com/locate/cej https://doi.org/10.1016/j.cej.2024.157797 https://doi.org/10.1016/j.cej.2024.157797 http://crossmark.crossref.org/dialog/?doi=10.1016/j.cej.2024.157797&domain=pdf http://creativecommons.org/licenses/by/4.0/ amphoteric, acidic and basic oxides have been evaluated for ketonisa- tion (Table S1). Amphoteric oxides have demonstrated the highest ac- tivity of single metal oxides due to the existence of surface acid-base pairs that promote the formation of the critical β-ketoacid intermediate [14,15]. However, their practical application is hindered by severe deactivation by coke formation [16,17]. The addition of a second metal can enhance catalytic activity by creating oxygen vacancies and coor- dinatively unsaturated cations [18], and/or tuning acid/base character [19] and redox properties [20–23]. Spinel ferrite and mixed metal ox- ides have also been studied for ketonisation, water gas shift reaction, chemical looping and CO production (Table S2). Spinel ferrites have the chemical formula MFe2O4 (M: divalent ions) and are classified based on their crystal structure as: normal spinel; inverse spinel; and complex spinel [24,25], depending on the type of divalent cations. For all three structures, judicious cation selection can increase the concentration of defects [26] and oxygen vacancies [27], improve oxygen storage ca- pacity [28,29], and influence the density and type of active sites [30]. The inclusion of divalent Zn2+, Co2+, Cu2+ and Mg2+ and Sr2+ into an Fe3+ spinel (Table S2) can localise lattice oxygen vacancies, and pro- mote Fe active sites [31]. Site substitution can also modify the cation distribution, promoting reactant adsorption and electron transport [32,33]. Metal dopants can drive the restructuring of normal spinels to inverse or mixed spinel structures, with concomitant tuning of the sur- face electronic properties that generate new catalytically active sites [24,32,34]. Spinel ferrites and their derivative composites have shown considerable promise in a wide array of applications, such as hydroge- nation, steam reforming, catalytic cracking, electrocatalysis, photo- catalysis, and adsorption [24,35]. However, despite their potential, spinel ferrites as catalysts for ketonisation have been rarely explored; to our knowledge only FeCr2O4, Ce-doped alumina ferrite, and red mud (containing mixed metal oxides) have been reported [30,36–38] (Table S2). The mechanism of ketonisation over spinel ferrite catalysts has also yet to be established. Coal fly ash forms during high-temperature coal combustion which induces significant sintering, and although it may have an iron content close to that of iron ores, is unsuitable as an iron ore substitute in blast furnaces due to its large particle size and presence of alkali elements that can corrode blast furnace walls.[39] These issues also hinder the use of such fly ash in metallurgical applications (e.g. recovery of nickel ferrite and vanadium oxide which affords low yield [40]), while the high Fe, Mg, and Ca content are a barrier to traditional low value applications in cement [41], road base construction [42], and soil amendment [43]. Fly ash is therefore predominantly landfilled where metal leaching poses a threat to ecosystems. We recently demonstrated the use of fly ash waste as a precursor to the synthesis of an Fe@Fe3O4 core–shell catalyst for acetic acid ketonisation at > 300 ◦C [44]. This catalyst contained 89 wt % Fe, predominantly as Fe2O3, with Al (6.9 wt%) being the second abundant element. Here we explore the impact of additional metal im- purities on fly ash waste, including bivalent (Ca2+, Mg2+, Fe2+ and Mn2+), trivalent (Fe3+, Al3+ and Mn3+) and tetravalent (Ti4+) cations, as a route to multimetallic spinel ferrites (Fig. 1). As noted above, spinel ferrites are potential heterogeneous catalysts, composed of Earth- abundant elements, for megaton scale, sustainable chemical manufacturing of chemicals and fuels [45–47]. Spinel ferrites are also magnetic materials with potential market values of US$3.9 billion in 2020 and a forecast compound annual growth rate of 5 % [48]. We hypothesised that high-entropy ferrites might exhibit superior physico- chemical and catalytic properties (for acetic acid ketonisation) to con- ventional metal oxides and low-entropy ferrites due to synergies between dopants. Selective leaching of Si and Al from fly ash, subse- quent reduction of the resulting precipitate under H2, and Mn doping of the resulting spinel ferrite, resulted in an active and stable catalyst for vapour phase acetic acid ketonisation under mild conditions. This work outlines new opportunities for fly ash waste valorisation, in alignment with 2030 UN Sustainable Development Goal 12, and the application of resulting ferrites in the manufacture of sustainable biofuels. Moreover, it contributes to the broader understanding of using iron-rich waste ma- terials, such as red mud [49], steel slag [50], and iron ore tailings [51], in catalytic applications. The results are also expected to extend the synthesis formulation of ferrite catalysts which are mostly limited to maximum four metals [38]. 2. Experimental 2.1. Synthesis of spinel ferrites Analytical grade oxides and chlorides purchased from Sigma-Aldrich include hematite (Fe2O3), magnetite (Fe3O4), FeCl3⋅6H2O, MgCl2⋅6H2O, AlCl3⋅6H2O, CaCl2⋅2H2O, MnCl2⋅4H2O, and TiCl4. The coal fly ash sample was sourced from a lignite-fired power generation station in Victoria, Australia. It was first water-washed to remove unburnt carbon and water-soluble species (such as alkali sulfates), and subsequently with Na2CO3 to remove water-insoluble sulfates. In a practical setting, the wastewater generated in these two steps are reused by being circu- lated back to the electro-static precipitator of the coal-fired power sta- tion to collect the dry fly ash as slurry which is then pumped into ash dam. The resulting material was dried, and manually crushed and sieved to produce a particle size fraction of 100–500 μm, and mainly comprised elements as Fe (54.72 wt%) and Mg (22.67 wt%), with smaller amounts of Ca, Al and Si, and trace Mn, Ti and K (Table S3). Synthesis of the spinel ferrite was achieved by treating the water-washed, dried and sized material with cycles of metal leaching (using 32 wt% HCl) and metal hydroxide precipitation (using 2 M NaOH) a pH-swing process whereby the pH of the resultant leachate was increased gradually to ~ 10 by adding 2 mol⋅L− 1 NaOH dropwise [44,52]. The precipitate was rinsed with deionised water to completely remove water-soluble cations (notably Na+) and anions (Cl-), dried at 110 ◦C for 24 h and then annealed at 400 ◦C (ramp rate 10 ◦C⋅min− 1) in flowing N2 for 1 h to yield Fig. 1. Mixed spinel ferrite synthesis from fly ash waste. S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 2 the fly ash-derived derived ferrite (FAF). The aqueous chloride waste from the leaching procedure can be reclaimed and converted back into HCl [53,54], whilst the resulting acid-insoluble residue was rich in Al and Si (Table S3) and hence suitable for use as a cementitious additive [41]. The FAF sample was further subjected to reduction at 400 ◦C under flowing 10 % H2/He (200 mL⋅min− 1) for 1–6 h. The resulting samples are designated rFAF-xh, where r indicates reduction and xh the reduc- tion time in hours. To investigate the influence of individual metal cations, a bottom-up approach was also employed to synthesise ferrites containing similar concentrations of each metal as found in the FAF material. Water-soluble chloride precursors of each metal were dis- solved in deionised water and then subjected to an identical precipitation-annealing-reduction (5 h only) protocol as mentioned above. The detailed procedure is described in the Supporting Infor- mation. The resultant catalysts are named: rFe-5 h for reduced Fe2O3 in 5 h. Fe2O3 was synthesised from pure FeCl3 only, and r stands for reduced; rMgFe-5 h for co-existence of Fe and Mg at the same ratio with that in fly ash, synthesised from FeCl3 and MgCl2 solution; rMgFeAl-5 h for reduced Fe2O3 with Mg and Al dopants; rMgCaFeAl-5 h for reduced Fe2O3 with Mg, Al and Ca dopants; rMgCaMnFeAl-5 h for reduced Fe2O3 with Mg, Al, Ca and Mn dopants; and finally, rMgCaMnFeAlTi-5 h for reduced Fe2O3 with Mg, Al, Ca, Mn and Ti dopants. To better understand the role of a trace metal cation such as Mn on the catalyst activity, varying amounts of MnCl2 were also added into the fly ash leachate to obtain 2.5 wt%, 5 wt%, and 9 wt% MnO doping within the fly ash derived ferrite (see Supporting Information). The resultant ferrites were denoted as rFAF2.5Mn-5 h, rFAF5Mn-5 h and rFAF9Mn-5 h, where the numbers refer to the content of MnO dopant, and 5 h refers to the reduction duration of 5 h in H2 at 400 ◦C. Note that all the catalysts were stored in a vacuum desiccator after reduction to avoid its re-oxidation by the ambient air. 2.2. Catalyst characterisation Fresh, reduced and spent ferrite catalysts were characterised by: inductively coupled plasma mass spectroscopy (ICP-MS), laboratory- based and synchrotron X-ray diffraction (XRD), BET (Brunauer- Emmett-Teller) surface area analysis, SEM, scanning transmission elec- tron microscopy (STEM) with high-angle annular dark-field (HAADF) detector, X-ray photoelectron spectra (XPS), Fe and Mn K-edge absorp- tion near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) in transmission mode. In addition, H2 temperature programmed reduction (H2-TPR) and in-situ synchrotron XRD were conducted to investigate the reducibility of catalysts. The acid properties were measured by NH3 temperature-programmed desorption (NH3- TPD) and pyridine chemisorption, and basicity was measured via CO2- TPD. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was also carried out to track the transient species and their adsorption/desorption on the catalyst surface. Detailed measurement parameters are described in the Supporting Information. 2.3. Acetic acid ketonisation The ketonisation experiments of acetic acid were carried out in a fluidised-bed quartz micro-reactor (1.27 cm in ID and 56 cm in length, Fig. S1). For each experiment, 20–200 mg of a sample premixed with five times the quantity of SiC (80 grit) was placed in the middle of the reactor tube between two quartz wool plugs. After being annealed via heating to 400 ◦C (ramp rate 10 ◦C⋅min− 1) in 200 mL⋅min− 1 of N2, 10 % H2 in N2 with an identical flow rate to that of N2 was fed into the reactor for in-situ reduction, with a varying duration of 1––6 h. Afterwards, pure N2 was switched back to purge any residual gases such as H2 for 20 min, and finally acetic acid vapor was introduced into the reactor via bubbling high-purity N2 through a temperature-controlled bubbler containing pure acetic acid. The total gas flow remained at 200 mL⋅min− 1 and the acetic acid concentration in N2 was 1.7 vol%. The corresponding catalyst mass to acetic acid mass flow ratio (W/F, h) was set at 0.04–0.29 h which is in the low-end of the 0.04–0.73 h range tested in the literature [37,55,56]. The outlet gas from the furnace was protected by a heating tape at 120 ◦C which is above the boiling point of the unreacted acetic acid (118 ◦C), water (100 ◦C) and acetone (56 ◦C). Subsequently, it passed through two impingers filled with 100 mL of deionised water in a dry ice bath to condense the liquid products. Af- terwards, the permanent gas products were sent to an ETG 9500 FTIR gas analyser for real-time monitoring. Each experiment was conducted in triplicate, and in some cases up to five times, to obtain reliable errors (based on the standard deviation). As equimolar amounts of CO2 and acetone are produced from the ketonisation reaction, and a carbon mass balance of > 95 % without any noticeable by-product generation was confirmed (Table S4), the acetone yield in this study was calculated based on the concentration of CO2 in the outlet gas, per Eq. (1) below, to present a continuous reaction trend. The acetic acid conversion during this process was also determined according to the experimental and theoretical CO2 yields. The average specific activity and turnover fre- quencies (TOFs) were calculated based on Eq. (2) and (3) during the first 2 h of the reaction period. The number of mols of the active site in Eq. (3) was obtained from the acid sites determined via NH3 temperature-programmed desorption (NH3-TPD). Acetone yield, mol% = Experimental acetone (mol) Theoretical acetone (mol) × 100 = Outlet CO2 (mol⋅min− 1) 0.5 × inlet acetic acid (mol⋅min− 1) × 100 (1) Average specific activity = Total acetic acid converted (mol) Amount of catalyst (g) × time (min) × 100 (2) TOF = Acetic acid converted (mol) Acid sites (mol) × time (min) (3) 3. Results and Discussion 3.1. Physicochemical properties of fly ash-derived ferrite The fly ash-derived derived ferrite (FAF) hearafter consists of 64.32 wt% Fe, 27.92 wt% Mg, 5.09 wt% Al, and minor Ca (1.53 wt%), Mn (0.90 wt%) and Ti (0.24 wt%) (Table S5). The as-synthesised FAF exhibited type IV nitrogen adsorption–desorption isotherms (Fig. 2a), with H3 hysteresis, indicative of aggregates of plate-like particles giving rise to groove-shaped pores [57,58]. The corresponding BET surface area (90 m2⋅g− 1), pore volume (0.26 cm3⋅g− 1) and mean pore diameter (8.3 nm) determined by the Barret-Joyner-Halenda method, are higher than reported for other spinel ferrites (e.g. MgAlxFeyOz) with a compa- rable Fe2O3 content and composition. Singh et. al. [26] and Buelens et. al. [59] reported surface areas of 42 m2⋅g− 1 and 30 m2⋅g− 1 for a Mg-Fe- Al-O spinel synthesised via co-precipitation of 50 wt% Fe2O3 with MgAl2O4, respectively. Textural properties from N2 physisorption were consistent with SEM images (Fig. 2b) which revealed a layered material whose layers were on average 10–20 nm thick and extended for ~ 400 nm, likely formed by topotactic transformation of the layered hydroxide precipitate (Fig. S2). Co-precipitation of multiple cations with different valence favours the formation of brucite-like layered hydroxides [60] which can retain their layered structure even after thermally induced transformation to the corresponding mixed oxides (the so-called ‘memory effect’) [61,62]. As per Pauling’s radius-ratio rule, metallic radii of 55–98 pm occupy octahedral coordination sites in such layered hydroxides [63]; this is the case for all metal cations in FAF except Ca2+ (ionic radius ~ 115 pm) which is only present in trace amounts. The powder XRD pattern of FAF (Fig. 2c) evidences a single spinel oxide phase similar to a MgAl0.2Fe3+ 1.8O4 reference material (Fig. S3), with Fe3+ S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 3 occupying primarily octahedral sites. Rietveld refinement of the XRD pattern for FAF suggests that the minor elements Ca, Mn and Ti were also present in this spinel phase, inducing expansion of the lattice parameter a and cell volume V relative to the MgAl0.2Fe3+ 1.8O4 reference (Table S7). Additionally, concerning the heterogeneity of fly ash in terms of compositions, ten different fly ash samples were further tested, five of which were used to synthesize ferrite catalysts. As shown in Table S6 and Fig. S4, although the original ten fly ash samples have a reasonable variability on both composition and crystalline structure, the resultant FAFs samples exhibited comparable composition and struc- tural features based on XRD patterns, suggesting reproducible catalytic performance. Additional insight into the local coordination of Fe cations was ob- tained from XANES analysis (Fig. 2d), which revealed pre-edge features for Fe3+ associated with 1 s → 3d transition [64–66] indicative of a distorted environment intermediate between hematite (corundum structure) and magnetite (inverse spinel). Least squares fitting to he- matite, magnetite and metallic Fe references indicate the presence of ~ 78 mol% Fe3+ and 22 mol% Fe2+ in the FAF (Fig. S5), the latter potentially in an amorphous phase undetectable by XRD. Fitting of the corresponding EXAFS spectra (Fig. S6) resulted in a k3-weighted Fourier transform (Fig. 2e) exhibiting an Fe-O nearest neighbour distance of 1.97 Å (Table S8), close to the typical value of ~ 2.06 Å for Fe occupying octahedral sites in magnetite [67], but shorter than values of 2.12 Å and 2.02 Å for Fe2+-O and Fe3+-O respectively in the same sites in spinels [68,69]. The Fe-O nearest neighbour coordination number (N) of 4.9 was slightly lower than that for Fe2O3 (6), but still consistent with an octahedral site [70]. The next-nearest neighbour distance of 2.98 Å is similar to that for Feoct-Fe scatterers in Fe2O3 (~3.0 Å), albeit the co- ordination number (CN) of 2.3 was lower than that in Fe2O3 (CN = 4) [70]. Together, these fits suggest that Fe primarily occupies distorted octahedral sites in the spinel FAF, with distortion potentially reflecting partial occupancy of octahedral sites by other metal cations [68]. The absence of a distinctive Fe-O interatomic scattering distance at ~ 1.8 Å, and the weak Fe-Fe peak at 3.7 Å (CN = 0.5), indicates negligible Fe occupancy of tetrahedral sites in FAF [68]. 3.2. Physicochemical properties of synthetic ferrites A range of synthetic FAF analogues was prepared by a bottom-up approach to explore the influence of metals on iron oxide; chloride precursors were used in all cases, with the same precipitation and annealing protocols adopted as for FAF. Annealing of iron hydroxide alone resulted in a pure phase hematite (sFe, Table S5) with a BET surface area of 33 m2⋅g− 1. The addition of Mg (the second most abun- dant metal in FAF) had minimal impact on the surface area, but more than doubled the mean pore volume and pore diameter relative to Fe2O3 and induced the partial formation of MgFe2O4 ferrite phase (sMgFe, Figs. S7 and S8) with a normal spinel structure, alongside Fe2O3. In contrast, the subsequent addition of ~ 7 wt% Al significantly increased the surface area to 75 m2⋅g− 1 (sMgFeAl, Table S5), with XRD indicating incorporation of Al3+ cations into MgFe2O4 resulting in a new MgAl0.2Fe1.8O4 ferrite spinel (analogous to the FAF). The smaller ionic radius of Al3+ versus Fe3+ is expected to increase the density of MgAl0.2Fe1.8O4 versus MgFe2O4, however, both crystalline phases exhibit similar sizes and hence are expected to confer similar surface areas with an analogous composition of ~ 2 wt% Fe2O3 and 98 wt% spinel structure in each. The further introduction of trace Ca, Mn and Ti slightly increased the BET surface area and decreased the size of MgAl0.2Fe1.8O4 crystallites (sMgCaFeAl, sMgCaMnFeAl and sMgCaMn- FeAlTi), but did not result in new crystalline phases which remained dominated (~99 wt%) by MgAl0.2Fe1.8O4. This reflects a successful incorporation of these metal cations into the MgAl0.2Fe1.8O4 spinel. Fig. 2. Physicochemical properties of annealed fly ash-derived ferrite (FAF): (a) N2 adsorption–desorption isotherms; (b) SE-SEM images; (c) synchrotron XRD patterns (λ = 0.7752545 Å); (d) normalised Fe K-edge XANES spectra; and (e) k3-weighted Fourier transformed radial distribution functions (nearest neighbour coordination numbers and interatomic scattering pairs labelled). S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 4 3.3. Reducibility of FAF The effect of chemical reduction by molecular hydrogen, often used to activate mixed metal oxide catalysts, was subsequently examined for the FAF and synthetic metal oxides by H2-TPR (Fig. 3a). FAF exhibited a broad, asymmetric reduction peak centred ~ 475 ◦C, with shoulders at 380 ◦C and 531 ◦C. In comparison, the sFe (hematite) exhibits two well- resolved peaks associated with the reduction of Fe2O3 to Fe3O4 (~446 ◦C) and subsequent Fe3O4 reduction to Fe0 (~620 ◦C) [71]. The addition of Mg to hematite retards the former process, with a reduction to magnetite occurring at 490 ◦C in SMgFe, but promotes magnetite reduction to metallic iron which now occurs at 558 ◦C. Substitution of metal cations for Fe3+ on the octahedral site is beneficial in increasing the lattice oxygen content and inhibiting the over-reduction of ferrite. These reduction temperatures are in good agreement with those observed for FAF, and suggest that the substitution of divalent cations at octahedral sites facilitates electron transfer and thermal reduction of Fe3+ to Fe2+ [24]. Further metal doping had little impact on Fe3+/Fe2+ reduction temperature, but slightly decreased the proportion of magnetite formed from hematite (490 ◦C versus 558 ◦C peak areas). The discrepancy between reduction profiles of sMgCaMnFeAlTi and FAF may reflect slight differences in their compositions (Table S5), or the sequence selected for the progressive introduction of each metal cation which differs from the order in which metal cations will precipitate from fly ash leachate [72]. The impact of chemical reduction under flowing H2 on crystalline phases present in the FAF material was also assessed by in-situ syn- chrotron XRD (Fig. 3b and Fig. S9). New ferrite species including Fe3+ 2 MgO4 (JCPDS: 00–001-1120/00–017-0465, inverse structure) [73], Fe2+/3+ 3 O4 (JCPDS: 00–003-0863, inverse spinel) [74] and AlFe2+/3+ 2 O4 (JCPDS: 01–089-7408, inverse structure) formed at reduction temper- atures < 450 ◦C, approaching that of the principal H2 TPR peak in Fig. 3a (475 ◦C) [75]. An Al1.97Fe3+ 0.23Mg0.7O4 (JCPDS: 01–089-8728, normal structure) [76] phase was also observed at ~ 500 ◦C, whereas low valence iron phases, including wustite (Fe0.924O, JCPDS: 04–003-1444, normal structure) [77] and metallic iron (Fe0, JCPDS: 04–002-7176) required reduction ≥ 650 ◦C. Prolonged reduction of FAF at 400 ◦C was also examined (Fig. 3b and Fig. S10), being the lowest temperature at which phase changes were observed and hence lattice oxygen becomes mobile, with resulting samples designated rFAF-xh. Extended reduction times facilitated the substitution of Al3+ for Fe3+ cations in octahedral sites, evidenced by the formation of an Al1.97Fe3+ 0.23Mg0.7O4 (JCPDS: 01–089-8728) phase, and the aforementioned spinels observed between 450–500 ◦C in Fig. S9, for rFAF-5 h. The complete reduction of a few percent of Fex+ species to metallic iron was also apparent in the rFAF-5 h material from quantitative Rietveld analysis (Table S9). The majority of lattice parameters for phases observed in rFAF-5 h are larger than those of corresponding reference materials, attributed to the presence of low concentrations of metal dopants. Fe K-edge XANES spectra evidence a significant decrease in the white line position for rFAF-5 h versus FAF (Fig. 3c), accompanied by a decrease in the energy of the 1 s → 3d pre-edge feature from ~ 7114.3 eV to ~ 7113.8 (Fig. 3c inset). The shape and energy of this pre-edge feature are indicative of the oxidation state of iron; pure Fe3+ in he- matite exhibits a well-resolved peak with a centroid ~ 7114.6 eV, whereas that for pure magnetite (equimolar in Fe3+ and Fe2+) appears as a shoulder on the rising 1 s → 4 s white line with a centroid ~ 7113.5 eV [78]. Changes in the pre-edge features are thus consistent with quanti- tively least squares fitting of the XANES spectra (Fig. S11) which confirm that the Fe3+ content decreases from 78 % for FAF to 61 % for rFAF-5 h, accompanied by an increase in Fe2+ and Fe0 species. Fitting of the corresponding Fe K-edge EXAFS spectrum (Fig. S11 and Fig. 3d) reveals that the Fourier transform (radial distribution function) of rFAF- 5 h is dominated by Feoct-O (bond length 2.03 Å, CN = 2.9), Feoct-Fe (2.51 Å, CN = 2.5) and Fetet-M (3.37 Å, CN = 2.6) scattering pairs, where M refers to Mg or Al cations substituted for Fe3+ in the ferrite (Table S10) [68]. This local structure is distinct from synthetic hematite subjected to the same reduction treatment (rFe-5 h, Fig. S12), which is dominated by Feoct-Fe scattering (CN = 4.5) with weak Feoct-O scat- tering (1.91 Å, CN = 0.7). Evidence for Mn and Ti cation substitution in FAF and rFAF-5 h is also apparent from their corresponding XANES data (Fig. 3e-f and Fig. S13). As-prepared FAF contains ~ 60 mol% Mn3+ with the balance Mn2+; reduction increases the Mn2+ concentration to 65 mol%, which were attributed to Mn2+ substitution for Fe3+ at both tetrahedral and octahedral sites in the resulting mixed spinel [68]. The Ti K edge XANES spectra of FAF and rFAF-5 h materials closely resemble that of FeTiO3, which contains Ti4+ and Fe2+ in octahedral sites [79], and hence again indicates the presence of a mixed spinel ferrite. Fig. 3. (a) H2-TPR profiles of as-prepared FAF and synthetic samples; (b) synchrotron XRD patterns of as-prepared and reduced FAF (rFAF-5 h), and reference compounds (λ = 0.7752545 Å); (c) normalised Fe K-edge XANES spectra and (d) corresponding k3-weighted Fourier transformed radial distribution functions (interatomic scattering pairs labelled); (e) normalised Mn K-edge XANES spectra and (f) normalised Ti K-edge XANES spectra of as-prepared and reduced FAF. S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 5 SEM imaging of rFAF-5 h (Fig. 4a) reveals aggregates of densely packed platelets with widths spanning ~ 100–300 nm and thicknesses spanning ~ 20–50 nm, akin to the parent FAF (bright-field TEM, Fig. S14). All elements are uniformly distributed through rFAF-5 h, except for Al which is concentrated in a few regions (a discrepancy consistent with XRD wherein Al3+ cations are not present in all crys- talline phases) (Fig. 4b). However, complete phase segregation of Al3+ as Al2O3 crystallites was not observed by XRD. Selected area electron diffraction (SAED) reveals various diffraction spots for rFAF-5 h with multiple rings at different orientations (Fig. 4c). These diffraction fea- tures are consistent with those expected for AlxFe3+ y MgzO4, AlFe2+/3+ 2 O4 and Fe3+ 2 MgO4 spinels, and magnetite [80], consistent with powder XRD (and associated interplanar spacings) of reference materials (Fig. S3). Uniform element concentrations and speciation (oxidation state) across rFAF-5 h were also observed by XPS mapping (Figs. S15 and S16). 3.4. Catalytic performance Temperature-programmed acetic acid ketonisation was subsequently conducted on FAF, rFAF-5 h, a reduced synthetic analogue and pure iron oxides. The resulting acetic acid conversions for FAF, hematite (Fe2O3) and magnetite (Fe3O4) were negligible < 300 ◦C (Fig. 5a), with FAF attaining only ~ 30 % conversion at 425 ◦C. In contrast, both rFAF-5 h and sFeMg-5 h exhibited significant activity < 300 ◦C, and attained acetic acid conversion > 80 % by 450 ◦C. Parametric testing (Fig. 5b-d) determined that ketonisation was optimal for rFAF-xh reduced at 400 ◦C for 5 h under H2, and tested with a W⋅F− 1 of 0.29 h at 400 ◦C reaction temperature. The reduced synthetic MgFe2O4 (rMgFe) exhibited a su- perior acetone yield (~47 mol% in 2 h, Fig. 5e) to either pure iron oxide phase, with the sequential addition of Al, Ca, Mn and Ti monotonically increasing the yield; reactivity of most complex mixed synthetic phase approached that of rFAF-5 h evidencing the importance of dopants in the spinal ferrite (Fig. 5e and Fig. S17). The stability of rFAF-5 h catalyst was assessed at 350 ◦C (to avoid mass-transport limitations at high conversion), with no evidence of deactivation observed for 100 h time- on-stream (Fig. 5f). Additionally, the five ferrite samples derived from different fly ash sources exhibit nearly reproducible performance, with slight deviations attributed to compositional differences (Fig. S17). Comparison of the chemical state of Fe in rFAF-5 h and the same catalyst post-ketonisation by XANES and EXAFS (Fig. 5g-h, Fig. S18 and Table S10) revealed an increase in the white line intensity and sharp- ening of the pre-edge feature, both indicative of an increase in Fe2+, and the Feoct-O CN, possibly due to oxidation of the Fe0 component (10 mol % of all iron) by acetic acid [44]. The shoulder peak can be attributed to the impurity metals, such as Mg2+ in the sFeMg-5 h catalyst (synthesised from pure FeCl3 and MgCl2 reagents, annealed and reduced by H2 at 400 ◦C in 5 h) which shows the same shoulder peak (Fig. S19). The inclusion of Mn2+/3+ into Fe-based materials has been observed to enhance the stability of vulnerable Fe2+ sites [81,82]. We therefore intentionally added 2.5–9 wt% Mn2+ to the as-prepared FAF material in the hope of stabilising Fe-O bonds during the 400 ◦C reduction used to activate the catalyst. This was achieved by adding MnCl2 to the fly ash leachate prior to the standard precipitation, annealing and reductive treatments used to prepare FAF. The addition of 9 wt% Mn to create rFAF9Mn-5 h resulted in a homogeneous dispersion of Mn (and other elements) across the catalyst matrix from EDX elemental maps (Fig. S20). Powder XRD showed that manganese addition stabilised Fe2+ and Fe3+ against reduction to Fe0 (Fig. 6a), evidenced by loss of the metallic iron reflection at 22.5◦ and weakening of the 31.5◦ reflection compared to rFAF-5 h (Fig. 3b). Corresponding XANES of rFAFxMn-5 h (Fig. 6b and Fig. S21) showed a progressive increase in the energy of the Fe pre-edge feature with Mn content (x). The fitting results are also consistent with a higher Fe2+ (up to 37.6 mol %) and stable Fe3+ con- centration than present in rFAF-5 h. These changes coincide with an increase in the width of Fe-O scattering pairs (Fig. 6c), associated with the emergence of Fetet-O bonds (~1.87 Å, Table S10) [68], and a rise in overall Fe-O nearest neighbour coordination (ΣCN[Fetet-O + Fetet-O] ~ 5.0 for rFAF9Mn-5 h versus 2.9 for rFAF-5 h), evidencing stabilisation of Fe3+ species and increase in Fe2+ against reduction. Mn doping increased the acid site loading determined by NH3-TPD (Fig. 6d, Table S11 and Fig. S22), from ~ 0.40 mmol⋅g− 1 for rFAF-5 h to ~ 0.56 mmol⋅g− 1 for rFAF9Mn-5 h, with an inverse trend observed for base site loading determined by CO2-TPD (Fig. S23). The nature of acid sites was further probed by chemisorbed pyridine (Fig. 6e): Mn doping increased the number of Lewis acid sites, which are reported to promote the dissociative adsorption of carboxylic acids (as carboxylates) [83]. The benefits of increased Lewis acidity on acetic acid ketonisation following Mn doping are striking (Fig. 6f), with all rFAFxMn-5 h cata- lysts more active than rFAF-5 h at 350 ◦C, and activity proportional to Mn content. From the temperature-programmed experiment, rFAF9Mn- 5 h achieved almost completed acetic acid conversion ~ 310 ◦C (Fig. S24), at which temperature rFAF-5 h only delivers 25 % conver- sion. Isothermal ketonisation of the as-prepared (Fig. 6g) rFAF9Mn-5 h achieved higher acetone yields at 275 ◦C and 300 ◦C compared to rFAF- 5 h and exhibited excellent stability for 100 h on-stream (Fig. 6h); the high acetone yield was retained for at least 6 h following washing and Fig. 4. (a) SEM image of rFAF-5 h; (b) STEM image and corresponding EDX elemental maps of rFAF-5 h; and (c) SAED of rFAF-5 h. Note similar colour intensities from EDX maps are not indicative of similar concentrations, as these elements exhibit different electron scattering cross-sections and fluorescence yield. S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 6 Fig. 5. (a) Temperature-programmed ketonisation of acetic acid over Fe-containing catalysts (10 ◦C⋅min− 1); (b-f) acetone yields from acetic acid ketonisation as a function of reaction temperature, catalyst composition and reduction time, and W⋅F− 1 (mass of catalyst to acetic acid vapor flow rate) ratio, after 2 h time-on-stream; (g) normalised Fe K-edge XANES spectra; and (h) corresponding k3-weighted Fourier transforms of reduced and post-reaction FAF catalyst (interatomic scattering pairs labelled). Fig. 6. (a) Synchrotron XRD patterns (λ = 0.7752545 Å), with the same labels for minerals as in Fig. 3b, (b) Fe K edge XANES spectra and (c) corresponding k3- weighted Fourier transformed EXAFS spectra, (d) acid and base site loadings from integration of respective NH3-TPD and CO2-TPD profiles, (e) DRIFT spectra of chemisorbed pyridine (L: Lewis acid, B: Brønsted acid), and (f-h) acetone yields from acetic acid ketonisation over rFAF-5 h and reduced Mn-doped fly ash samples as a function of reaction temperature (each measurement after 2 h under isothermal conditions) and time-on-stream. S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 7 filtering of the spent catalyst (Fig. S25). Note there was no correlation between ketonisation activity and surface area, and a physical mixture of rFAF-5 h and 9 wt% MnO exhibited a significant induction period and gave a lower steady-state acetone yield than rFAF9Mn-5 h (Fig. S26). Collectively, the preceding data demonstrate a strong synergy between Mn cations doped into the octahedral site of spinel ferrite and the sta- bility of Lewis acidic Fe2+/3+ species. The improved catalytic perfor- mance is attributed to the elevated concentration of Fe2+ by Mn doping, which is directly proportional to acetone yield (Fig. S27). The specific activities for acetic acid ketonisation over rFAF9Mn-5 h at 275 ◦C, 300 ◦C and 350 ◦C (calculated from the acetone yield averaged over 2 h reaction in Fig. S25) were 0.62 mmol⋅g− 1⋅min− 1, 1.26 mmol⋅g− 1⋅min− 1 and 1.31 mmol⋅g− 1⋅min− 1 (Table 1), respectively. These values far exceed that for rFAF-5 h at the corresponding temperatures, and are an order of magnitude greater than those achieved with our recently re- ported fly ash-derived Fe@Fe3O4 catalyst (Fig. S25) [44], as well as other reported catalysts. The turnover frequency (TOF) for rFAF9Mn-5 h of 2.30 min− 1 at 350 ◦C (Table 1), representing the intrinsic activity per acid site, was likewise an order of magnitude greater than those rFAF-5 h and Fe@Fe3O4, and comparable to that for the more readily activated propionic acid over CeO2 at a similar temperature [22]. Acetic acid ketonisation proceeded with an apparent activation energy of 71 kJ⋅mol− 1 over rFAF9Mn-5 h, at the lower end of literature values (58–185 kJ⋅mol− 1). 3.5. Synergistic roles of multiple metal cations and mechanistic study To understand the benefits of reducing the parent FAF, and doping the resulting rFAF-5 h with Mn, in-situ DRIFTS measurements were conducted following the thermal evolution of saturated acetic acid layers. The rFAF-5 h catalyst was pre-loaded with 20 µL pure acetic acid, and dried at 110 ◦C for 30 min in a Harrick DRIFTS cell housed in an FTIR spectrometer to remove the physisorbed molecule. Chemisorbed acetic acid was then subject to temperature-programmed desorption at a ramp rate of 5 ◦C⋅min− 1 under 20 mL.min− 1 of flowing N2. The resulting spectra exhibited two strong absorption bands at 2938 cm− 1 and 3100 cm− 1 at 100 ◦C due to νasC-H stretches of the molecular adsorbate (Fig. 7a) and a weaker band at 3585 cm− 1 due to the νO-H stretch [88]. The band at 1727 cm− 1 is attributed to residual physisorbed acetic acid [84,89], whereas the two broad absorptions at 1583 and 1428 cm− 1 are respectively assigned as the asymmetric and symmetric stretches of the CO2 – function. The difference of 142 cm− 1 between the latter two bands is indicative of bridging bidentate acetate configuration (Table S12) [84,89,90]. Acetate adsorption is expected to predominantly occur at Fe2+ octahedral sites [44,91], due to the tendency of spinel oxide sur- faces to expose octahedral cations [92]. Two weak bands at 1347 cm− 1 and 1296 cm− 1 are assigned as C–H and O–H deformations, respectively. Physisorption features at 1727 cm− 1 and 1296 cm− 1 were completely lost > 175 ◦C, concomitant with the emergence of a new νC = O adsorption band at 1660 cm− 1 evidencing the formation of acetone at Lewis acid sites (Table S12) [93]. Acetone formation coincided with the appearance of very weak bands at 2360 cm− 1 and 2306 cm− 1, ascribed to the O = C = O stretch of linearly adsorbed CO2 [94], the by-product of ketonisation. CO2 band intensities reached a maximum between 265–300 ◦C before falling to zero, whereas acetate bands persisted to ~ 400 ◦C (at which point the rate of acetic acid ketonisation is maximal, Fig. 5b). A subsequent isothermal experiment was conducted at 300 ◦C, a temperature at which the preceding in-situ study demonstrated acetic acid undergoes ketonisation. DRIFTS spectra were continuously collected while acetic acid vapour was flowed across the rFAF-5 h catalyst (Fig. 7b), resulting in the immediate appearance of adsorption bands for ketonisation products, notably the νC = O band of acetone at 1645 cm− 1, νas,sCO2 - bands for CO2 at 2360 cm− 1 and 2306 cm− 1, and the νOH band for water at 3585 cm− 1 [95]. However, after 10 min time-on- stream, new bands at 1565 cm− 1 and 1455 cm− 1 emerged associated with bridging bidentate acetate, indicating that ketonisation is rate- limited by acetic acid dehydroxylation at 300 ◦C [17]. This experi- ment was repeated for the rFAF9Mn-5 h catalyst (Fig. 7c) and corre- sponding time-dependent intensities of vibrational modes for this catalyst and the undoped rFAF-5 h are shown in Fig. 7d. Manganese addition increased the amount (but not the kinetics) of reactively- formed acetone (1640 cm− 1 band) at the catalyst surface, but weak- ened H2O (3585 cm− 1) and CO2 (2360 and 2306 cm− 1) adsorption. A corresponding study of pure MnO showed no evidence for acetone for- mation (Fig. S28a). These observations suggest that acetic acid decar- boxylation occurs over Fe2+ octahedral sites, and hence that Mn doping of rFAF-5 h only increases the number of catalytically active Fe2+ sites present rather than creating a new type of active site. In contrast, pure CaO and anatase TiO2 were able to form surface acetone under the same conditions (Fig. S28b-c), as were the synthetic rFeMgAlCa-5 h and rFeMgAlCaMnTi-5 h catalysts (Fig. S28d-e). However, Ca2+ and Ti4+ Table 1 Comparison of carboxylic acid ketonisation over reported catalysts. Catalyst Reactant W/F/ h Temperature/ ◦C Acetone yield/ mol% Average specific activity/ mmol⋅g¡1⋅min¡1 TOF/ min¡1 Ea/ kJ⋅mol¡1 Reference rFAF-5 h Acetic acid 0.29 350 33 0.49 1.08a 78 This work 0.29 300 25 0.37 0.82a 0.29 275 22 0.33 0.72a rFAF9Mn-5 h Acetic acid 0.29 350 100 1.31 2.30a 71 0.29 300 96 1.26 2.21a 0.29 275 47 0.62 1.08a 0.10 275 27 0.35 0.62 ​ Fe@Fe3O4 Acetic acid 0.29 350 9 0.09 0.17a 127 0.29 300 4 0.04 0.08a 0.29 275 3 0.03 0.06a CeO2 Acetic acid 2.00 300 100 0.14 − 58 [84] ZrO2 2.00 300 73 0.10 − 106 TiO2 2.00 300 48 0.07 − 115 Al2O3 2.00 300 3.1 0.05 − 110 CeO2 Propionic acid 0.17 350 − − 2.17b − [22] Ru/TiO2 Acetic acid 0.38 275 37 0.27 − 185 [85] 0.10 275 18 0.23 − − HZSM-5 Acetic acid 0.34 300 16 − − − [55] ZrO2/C Acetic acid (2 M in H2O) − 340 − 0.63 − − [86] La2O3/ZrO2 Acetic acid (10 wt% in H2O) 0.26 295 − 0.78 − − [87] a TOF = (mols acetic acid converted) / (mols acid sites × time); bTOF = (mols acetic acid converted) / (mols basic sites × time). S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 8 are only present in low amounts at the surface of rFAF-5 h (Fig. S15), while Mg2+, which predominantly resides at tetrahedral sites in rFAF-5 h, exhibited poor activity for acetic acid adsorption and acetone for- mation as MgO (Fig. S29a) and when present in rFeMg-5 h (Fig. S29c). It is therefore unlikely that Mn, Ca, Ti or Mg cations are themselves the active sites for ketonisation in rFAF-5 h or rFAF9Mn-5 h. The contri- bution of Al3+, which is present at ~ 10 atom% in the surface of rFAF-5 h, to acetic acid ketonisation is more complex. Pure γ-Al2O3 is a well- known Lewis acid, but literature reports suggest it is almost inactive towards acetic acid at 300 ◦C [84]. Under isothermal conditions, a strong υ(C = O) band at 1680 cm-1 was observed for acetic acid over alumina, accompanied by δ(C–H) bands at 1385 cm− 1 and 1325 cm− 1 (Fig. S29b), characteristic of an acetyl intermediate (–COCH3) [96], resulting from cleavage of the hydroxyl group in acetic acid (Table S12) [15,97]. Acetyl formation is generally accepted as a critical step in the ketonisation mechanism, however, the predominance of reactively- formed acetyl versus acetone on alumina suggests that the former may be too strongly bound to react with another acetic acid molecule. Sub- stitution of Al3+ for Fe3+ cations at the octahedral site in rFeMgAl-5 h switches the surface chemistry from that of alumina, with bidentate acetate now the favoured surface species at 300 ◦C (Fig. S29d). We conclude that Al3+ cations are also unlikely to directly participate in ketonisation but increase the surface area (Table S5) and hence the surface concentration of active Fe2+ species. Fig. 7. In-situ DRIFTS spectra of acetic acid (a) as a function of temperature, and (b-c) time at 300 ◦C over rFAF-5 h and rFAF9Mn-5 h; (d) corresponding intensities of acetone (1640–1645 cm− 1), CO2 (2360 cm− 1) and H2O (3585 cm− 1) bands. S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 9 4. Conclusions In summary, through the facile processing of coal fly ash, an other- wise valueless solid waste, we have synthesised a green spinel catalyst with a principal formula of MgAl0.2Fe1.8O4 in a layered structure. More uniquely, this ferrite spinel is different from the traditionally synthesised ones in having its structure highly distorted by the substitution of a variety of metallic cations (i.e., Ca2+, Mn2+, Mn3+ and Ti4+) in trace quantity on both tetrahedral and octahedral Fe sites. Stepwise bottom- up synthesis of samples revealed that the introduction of Mg2+ into Fe2O3 formed MgFe2O4 with a modest effect on the surface area, but a further addition of ~ 9 wt% Al2O3 led to a significant increase in the surface area, along with the formation of MgAl0.2Fe1.8O4. Substituting Mg2+ and Al3+ for Fe3+ respectively on the tetrahedral and octahedral site can inhibit the over-reduction of ferrite while substituting Ca, Mn and Ti cations on the octahedral site can regenerate Fe2+. The fly ash- derived spinel ferrite can even take up an extra 9 wt% Mn2+ into its lattice matrix, and the Mn-doped spinel ferrite (rFAF9Mn-5 h) demon- strates superior activity and long-term stability over all the reported heterogeneous catalysts for the ketonisation of acetic acid. Mn, Ca, Ti and/or Mg cations enhance the surface concentration of active Fe2+ octahedral sites and stabilise cationic iron against reduction to the metal, promoting acetic acid adsorption as bidentate acetate. Octahedral Al3+ cations increase the total surface area and hence surface concen- tration of active Fe2+ sites. C–C coupling to acetone likely occurs over octahedral Fe2+ sites. CRediT authorship contribution statement Sasha Yang: Writing – original draft, Validation, Investigation, Formal analysis, Data curation, Conceptualization. Jinxing Gu: Writing – review & editing, Methodology. Binbin Qian: Writing – review & editing, Methodology. Jim Mensah: Writing – review & editing, Methodology. Adam F. Lee: Writing – review & editing, Supervision, Resources, Methodology. Karen Wilson: Writing – review & editing, Supervision, Resources. Barbara Etschmann: Writing – review & editing, Methodology. Xiya Fang: Methodology, Formal analysis. Jisheng Ma: Methodology, Formal analysis. Qinfen Gu: Methodology, Formal analysis. Lian Zhang: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank the Australian Research Council (DP160102818, DP200103287, DP200100204, DP200100313, LP190100849 and LP220100365) for financial support. Synchrotron ex-situ and in-situ XRD and XAS analyses were undertaken on the Powder Diffraction beamline (10BM1) and X-ray Absorption Spectroscopy beamline under the Australian Synchrotron (beamtime awards M17119, M19329 and M19819). Dr. Tim Williams at the Monash Centre of Electron Micro- scopy (MCEM), Monash University, is acknowledged for the TEM anal- ysis. Monash X-ray Platform (MXP) is also acknowledged for the XPS analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.cej.2024.157797. Data availability Data will be made available on request. References [1] D. Lee, H. Nam, M.W. Seo, S.H. Lee, D. Tokmurzin, S. Wang, Y.-K. Park, Recent progress in the catalytic thermochemical conversion process of biomass for biofuels, Chem. Eng. J. 447 (2022) 137501, https://doi.org/137510.131016/j. cej.132022.137501. [2] Y. Huang, Y. Gao, H. Zhou, H. Sun, J. Zhou, S. Zhang, Pyrolysis of palm kernel shell with internal recycling of heavy oil, Bioresour. Technol. 272 (2019) 77–82, https://doi.org/10.1016/j.biortech.2018.1010.1006. [3] Y.-K. Park, J.-M. Ha, S. Oh, J. Lee, Bio-oil upgrading through hydrogen transfer reactions in supercritical solvents, Chem. Eng. J. 404 (2021) 126527, https://doi. org/126510.121016/j.cej.122022.137501. [4] P. Lahijani, M. Mohammadi, A.R. Mohamed, F. Ismail, K.T. Lee, G. Amini, Upgrading biomass-derived pyrolysis bio-oil to bio-jet fuel through catalytic cracking and hydrodeoxygenation: A review of recent progress, Energy Conv. Manag. 268 (2022) 115956, https://doi.org/115910.111016/j. enconman.112022.115956. [5] Y. Wang, M. Peng, J. Zhang, Z. Zhang, J. An, S. Du, H. An, F. Fan, X. Liu, P. Zhai, Selective production of phase-separable product from a mixture of biomass-derived aqueous oxygenates, Nat. Commun. 9 (2018) 5183, https://doi.org/5110.1038/ s41467-41018-07593-41460. [6] C.A. Mullen, A.A. Boateng, Chemical composition of bio-oils produced by fast pyrolysis of two energy crops, Energy & Fuels 22 (2008) 2104–2109, https://doi. org/2110.1021/ef700776w. [7] R. Aguado, M. Olazar, M.J. San José, G. Aguirre, J. Bilbao, Pyrolysis of sawdust in a conical spouted bed reactor. Yields and product composition, Ind. Eng. Chem. Res. 39 (2000) 1925–1933, https://doi.org/1910.1021/ie990309v. [8] L. Wu, T. Moteki, A.A. Gokhale, D.W. Flaherty, F.D. Toste, Production of fuels and chemicals from biomass: condensation reactions and beyond, Chem 1 (2016) 32–58, https://doi.org/10.1016/j.chempr.2016.1005.1002. [9] O. Norouzi, M. Heidari, F. Di Maria, A. Dutta, Design of a ternary 3D composite from hydrochar, zeolite and magnetite powder for direct conversion of biomass to gasoline, Chem. Eng. J. 410 (2021) 128323 https://doi.org/128310.121016/j. cej.122020.128323. [10] H. Ling, Z. Wang, L. Wang, C. Stampfl, D. Wang, J. Chen, J. Huang, Composition- structure-function correlation of Ca/Zn/AlOx catalysts for the ketonization of acetic acid, Catal. Today. 351 (2020) 58–67, https://doi.org/10.1016/j. cattod.2019.1001.1057. [11] T.N. Pham, D. Shi, D.E. Resasco, Evaluating strategies for catalytic upgrading of pyrolysis oil in liquid phase, Appl. Catal. B: Environ. 145 (2014) 10–23, https:// doi.org/10.1016/j.apcatb.2013.1001.1002. [12] E.L. Kunkes, D.A. Simonetti, R.M. West, J.C. Serrano-Ruiz, C.A. Gartner, J. A. Dumesic, Catalytic conversion of biomass to monofunctional hydrocarbons and targeted liquid-fuel classes, Science 322 (2008) 417–421, https://doi.org/ 410.1126/science.1159210. [13] S. Rawat, B. Singh, R. Kumar, C. Pendem, S. Bhandari, K. Natte, A. Narani, Value addition of lignin to zingerone using recyclable AlPO4 and Ni/LRC catalysts, Chem. Eng. J. 431 (2022) 134130 https://doi.org/134110.131016/j.cej.132021.134130. [14] S. Wang, E. Iglesia, Experimental and theoretical assessment of the mechanism and site requirements for ketonization of carboxylic acids on oxides, J. Catal. 345 (2017) 183–206, https://doi.org/110.1016/j.jcat.2016.1011.1006. [15] A. Pulido, B. Oliver-Tomas, M. Renz, M. Boronat, A. Corma, Ketonic decarboxylation reaction mechanism: a combined experimental and DFT study, ChemSusChem 6 (2013) 141–151, https://doi.org/110.1002/cssc.201200419. [16] M. Gliński, G. Zalewski, E. Burno, A. Jerzak, Catalytic ketonization over metal oxide catalysts. XIII. Comparative measurements of activity of oxides of 32 chemical elements in ketonization of propanoic acid, Appl. Catal. A: Gen. 470 (2014) 278–284, https://doi.org/210.1016/j.apcata.2013.1010.1047. [17] T.N. Pham, T. Sooknoi, S.P. Crossley, D.E. Resasco, Ketonization of carboxylic acids: mechanisms, catalysts, and implications for biomass conversion, ACS Catal. 3 (2013) 2456–2473, https://doi.org/2410.1021/cs400501h. [18] H. Teterycz, R. Klimkiewicz, M. Łaniecki, The role of Lewis acidic centers in stabilized zirconium dioxide, Appl. Catal. a: Gen. 249 (2003) 313–326, https://doi. org/310.1016/S0926-1860X(1003)00231-X. [19] M. Gliński, J. Kijeński, A. Jakubowski, Ketones from monocarboxylic acids: Catalytic ketonization over oxide systems, Appl. Catal. a: Gen. 128 (1995) 209–217, https://doi.org/210.1016/0926-1860X(1095)00082-00088. [20] L. Ilieva, G. Pantaleo, I. Ivanov, A. Venezia, D. Andreeva, Gold catalysts supported on CeO2 and CeO2-Al2O3 for NOx reduction by CO, Appl. Catal. b: Environ. 65 (2006) 101–109, https://doi.org/110.1016/j.apcatb.2005.1012.1014. [21] R.W. Snell, B.H. Shanks, CeMOx-promoted ketonization of biomass-derived carboxylic acids in the condensed phase, ACS Catal. 4 (2014) 512–518, https://doi. org/510.1021/cs400851j. [22] F. Lu, B. Jiang, J. Wang, Z. Huang, Z. Liao, Y. Yang, Insights into the improvement effect of Fe doping into the CeO2 catalyst for vapor phase ketonization of carboxylic acids, Mol. Catal. 444 (2018) 22–33, https://doi.org/10.1016/j. mcat.2017.1005.1022. [23] S.D. Randery, J.S. Warren, K.M. Dooley, Cerium oxide-based catalysts for production of ketones by acid condensation, Appl. Catal. a: Gen. 226 (2002) 265–280, https://doi.org/210.1016/S0926-1860X(1001)00912-00917. S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 10 https://doi.org/10.1016/j.cej.2024.157797 https://doi.org/10.1016/j.cej.2024.157797 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0005 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0005 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0005 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0005 https://doi.org/10.1016/j.biortech.2018.1010.1006 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0015 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0015 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0015 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0020 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0020 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0020 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0020 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0020 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0025 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0025 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0025 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0025 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0030 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0030 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0030 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0035 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0035 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0035 https://doi.org/10.1016/j.chempr.2016.1005.1002 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0045 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0045 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0045 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0045 https://doi.org/10.1016/j.cattod.2019.1001.1057 https://doi.org/10.1016/j.cattod.2019.1001.1057 https://doi.org/10.1016/j.apcatb.2013.1001.1002 https://doi.org/10.1016/j.apcatb.2013.1001.1002 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0060 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0060 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0060 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0060 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0065 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0065 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0065 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0070 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0070 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0070 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0075 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0075 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0075 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0080 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0080 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0080 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0080 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0085 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0085 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0085 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0090 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0090 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0090 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0095 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0095 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0095 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0100 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0100 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0100 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0105 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0105 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0105 https://doi.org/10.1016/j.mcat.2017.1005.1022 https://doi.org/10.1016/j.mcat.2017.1005.1022 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0115 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0115 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0115 [24] H. Qin, Y. He, P. Xu, D. Huang, Z. Wang, H. Wang, Z. Wang, Y. Zhao, Q. Tian, C. Wang, Spinel ferrites (MFe2O4): Synthesis, improvement and catalytic application in environment and energy field, Adv. Colloid Interface Sci. 294 (2021) 102486 https://doi.org/102410.101016/j.cis.102021.102486. [25] D.S. Mathew, R.-S. Juang, An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions, Chem. Eng. J. 129 (2007) 51–65, https://doi.org/10.1016/j.cej.2006.1011.1001. [26] V. Singh, L.C. Buelens, H. Poelman, M. Saeys, G.B. Marin, V.V. Galvita, Carbon monoxide production using a steel mill gas in a combined chemical looping process, J. Energy Chem. 68 (2022) 811–825, https://doi.org/810.1016/j. jechem.2021.1012.1042. [27] L. Liu, J. Sun, J. Ding, Y. Zhang, T. Sun, J. Jia, Highly active Mn3–xFexO4 spinel with defects for toluene mineralization: insights into regulation of the oxygen vacancy and active metals, Inorg. Chem. 58 (2019) 13241–13249, https://doi.org/ 13210.11021/acs.inorgchem.13249b02105. [28] B. Yan, Z. Liu, J. Wang, Y. Ge, J. Tao, Z. Cheng, G. Chen, Mn-doped Ca2Fe2O5 oxygen carrier for chemical looping gasification of biogas residue: Effect of oxygen uncoupling, Chem. Eng. J. 446 (2022) 137086 https://doi.org/137010.131016/j. cej.132022.137086. [29] N.A. Dharanipragada, L.C. Buelens, H. Poelman, E. De Grave, V.V. Galvita, G. B. Marin, Mg-Fe-Al-O for advanced CO2 to CO conversion: carbon monoxide yield vs. oxygen storage capacity, J. Mater. Chem. A . 3 (2015) 16251–16262, https:// doi.org/16210.11039/C16255TA02289D. [30] C. Chen, Y. Li, W. Ma, S. Guo, Q. Wang, Q.X. Li, Mn-Fe-Mg-Ce loaded Al2O3 catalyzed ozonation for mineralization of refractory organic chemicals in petroleum refinery wastewater, Sep. Purif. Technol. 183 (2017) 1–10, https://doi. org/10.1016/j.seppur.2017.1003.1054. [31] J.I. Orege, G.A. Kifle, Y. Yu, J. Wei, Q. Ge, J. Sun, Emerging spinel ferrite catalysts for driving CO2 hydrogenation to high-value chemicals, Matter 6 (2023) 1404–1434, https://doi.org/1410.1016/j.matt.2023.1403.1024. [32] A.A. Zahrani, W. Yang, T. Wu, Inhibition of bromate formation in plasmon- enhanced catalytic ozonation over silver-doped spinel ferrite, Water Research 242 (2023) 120173 https://doi.org/120110.121016/j.watres.122023.120173. [33] M. Ahmed, H. Hassan, M. Eltabey, K. Latka, T. Tatarchuk, Mössbauer spectroscopy of MgxCu0. 5-xZn0. 5Fe2O4 (x= 0.0, 0.2 and 0.5) ferrites system irradiated by γ-rays, Physica B: Condensed Matter, 530 (2018) 195-200. https://doi.org/110.1016/j. physb.2017.1010.1125. [34] J. Choi, D. Kim, S.J. Hong, X. Zhang, H. Hong, H. Chun, B. Han, L.Y.S. Lee, Y. Piao, Tuning the electronic structure and inverse degree of inverse spinel ferrites by integrating samarium orthoferrite for efficient water oxidation, Applied Catalysis b: Environmental 315 (2022) 121504 https://doi.org/121510.121016/j. apcatb.122022.121504. [35] T. Tatarchuk, L. Soltys, W. Macyk, Magnetic adsorbents for removal of pharmaceuticals: A review of adsorption properties, Journal of Molecular Liquids 384 (2023) 122174 https://doi.org/122110.121016/j.molliq.122023.122174. [36] M.A. Jackson, Ketonization of model pyrolysis bio-oil solutions in a plug-flow reactor over a mixed oxide of Fe, Ce, and Al, Energy & Fuels 27 (2013) 3936–3943, https://doi.org/3910.1021/ef400789z. [37] J. Weber, A. Thompson, J. Wilmoth, V.S. Batra, N. Janulaitis, J.R. Kastner, Effect of metal oxide redox state in red mud catalysts on ketonization of fast pyrolysis oil derived oxygenates, Appl. Catal. b: Environ. 241 (2019) 430–441, https://doi.org/ 410.1016/j.apcatb.2018.1008.1061. [38] S. Yang, J. Gu, B. Dai, L. Zhang, A critical review of the synthesis and applications of spinel-derived catalysts to bio-oil upgrading, ChemSusChem, e202401115. https://doi.org/202401110.202401002/cssc.202401115. [39] T.K. Choo, Y. Song, L. Zhang, C. Selomulya, L. Zhang, Mechanisms underpinning the mobilization of iron and magnesium cations from Victorian brown coal fly ash, Energy & Fuels 28 (2014) 4051–4061, https://doi.org/4010.1021/ef500618r. [40] A. Hamidi, S. Shakibania, A. Mahmoudi, F. Rashchi, E. Vahidi, Valorization of fly ash by nickel ferrite and vanadium oxide recovery through pyro- hydrometallurgical processes: Technical and environmental assessment, J. Environ. Manag. 344 (2023) 118442 https://doi.org/118410.111016/j. jenvman.112023.118442. [41] A. Palomo, M. Grutzeck, M. Blanco, Alkali-activated fly ashes: A cement for the future, Cem. Concr. Res. 29 (1999) 1323–1329, https://doi.org/1310.1016/S0008- 8846(1398)00243-00249. [42] E. Mulder, A mixture of fly ashes as road base construction material, Waste Manag. 16 (1996) 15–20, https://doi.org/10.1016/S0956-1053X(1096)00026-00028. [43] V.C. Pandey, N. Singh, Impact of fly ash incorporation in soil systems, Agric. Ecosyst. Environ. 136 (2010) 16–27, https://doi.org/10.1016/j. agee.2009.1011.1013. [44] S. Yang, B. Qian, Y. Wang, K. Taira, Q. Zhou, K. Wilson, A.F. Lee, L. Zhang, Fly ash waste-derived Fe@Fe3O4 core-shell nanoparticles for acetic acid ketonization, Appl. Catal. B: Environ. 322 (2023) 122106 https://doi.org/122110.121016/j. apcatb.122022.122106. [45] S. Mitchell, A.J. Martín, J. Pérez-Ramírez, Transcending scales in catalysis for sustainable development, Nat. Chem. 1 (2024) 13–15, https://doi.org/10.1038/ s44286-44023-00005-44281. [46] A. Abbas, M. Cross, X. Duan, S. Jeschke, M. Konarova, G.W. Huber, A.F. Lee, E. C. Lovell, J.Y.C. Lim, A. Polyzos, R. Richards, K. Wilson, Catalysis at the intersection of sustainable chemistry and a circular economy, One Earth 7 (2024) 738–741, https://doi.org/710.1016/j.oneear.2024.1004.1018. [47] J.A. Tickner, K. Geiser, S. Baima, Transitioning the chemical industry: elements of a roadmap toward sustainable chemicals and materials, Environment: Science and Policy for Sustainable Development 64 (2022) 22–36. [48] F.M. Insights, Ferrite Market Overview (2022 to 2032), 2022. [49] K. Evans, E. Nordheim, K. Tsesmelis, Bauxite residue management, Light Metals 2016 (2012) 63–66, https://doi.org/10.1007/1978-1003-1319-48179-48171_ 48111. [50] O. Gencel, O. Karadag, O.H. Oren, T. Bilir, Steel slag and its applications in cement and concrete technology: A review, Constr. Build. Mater. 283 (2021) 122783 https://doi.org/122710.121016/j.conbuildmat.122021.122783. [51] L.T. Da Silva Ramos, R.C. De Azevedo, A.C. Da Silva Bezerra, L.M. Do Amaral, R. D. Oliveira, Iron ore tailings as a new product: A review-based analysis of its potential incorporation capacity by the construction sector, Clean. Waste Syst. (2024) 100137 https://doi.org/100110.101016/j.conbuildmat.102021.122783. [52] B. Qian, C. Liu, J. Lu, M. Jian, X. Hu, S. Zhou, T. Hosseini, B. Etschmann, X. Zhang, H. Wang, Synthesis of in-situ Al3+-defected iron oxide nanoflakes from coal ash: A detailed study on the structure, evolution mechanism and application to water remediation, J. Hazard. Mater. 395 (2020) 122696 https://doi.org/ 122610.121016/j.jhazmat.122020.122696. [53] B. Qian, T. Hosseini, X. Zhang, Y. Liu, H. Wang, L. Zhang, Coal waste to two- dimensional materials: Fabrication of α-Fe2O3 nanosheets and MgO nanosheets from brown coal fly ash, ACS Sustain. Chem. Eng. 6 (2018) 15982–15987, https:// doi.org/15910.11021/acssuschemeng.15988b03952. [54] C. Liu, J. Gu, S. Zhou, B. Qian, B. Etschmann, J.Z. Liu, D. Yu, L. Zhang, Silica- assisted pyro-hydrolysis of CaCl2 waste for the recovery of hydrochloric acid (HCl): Reaction pathways with the evolution of Ca(OH)Cl intermediate by experimental investigation and DFT modelling, J. Hazard. Mater. 439 (2022) 129620 https:// doi.org/129610.121016/j.jhazmat.122022.129620. [55] A. Gumidyala, T. Sooknoi, S. Crossley, Selective ketonization of acetic acid over HZSM-5: The importance of acyl species and the influence of water, J. Catal. 340 (2016) 76–84, https://doi.org/10.1016/j.jcat.2016.1004.1017. [56] Z. Yang, Q. Yu, Y. Guo, X. Wu, H. Wang, J. Han, Q. Ge, X. Zhu, Effect of postsynthesis preparation methods on catalytic performance of Ti-Beta zeolite in ketonization of propionic acid, Microporous Mesoporous Mater. 330 (2022) 111625 https://doi.org/111610.111016/j.micromeso.112021.111625. [57] S.A.A. Abdel Aziz, Y. GadelHak, M.B.E.D. Mohamed, R. Mahmoud, Antimicrobial properties of promising Zn-Fe based layered double hydroxides for the disinfection of real dairy wastewater effluents, Sci. Rep. 13 (2023) 7601, https://doi.org/ 7610.1038/s41598-41023-34488-y. [58] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 87 (2015) 1051–1069, https://doi.org/1010.1515/pac-2014- 1117. [59] L.C. Buelens, A. Dharanipragada, H. Poelman, Z. Zhou, G.B. Marin, V.V. Galvita, Exploring the stability of Fe2O3-MgAl2O4 oxygen storage materials for CO production from CO2, J. CO2 Util. 29 (2019) 36–45, https://doi.org/10.1016/j. jcou.2018.1011.1008. [60] M. Xu, M. Wei, Layered double hydroxide-based catalysts: recent advances in preparation, structure, and applications, Adv. Funct. Mater. 28 (2018) 1802943, https://doi.org/1802910.1801002/adfm.201802943. [61] O. Kikhtyanin, Z. Tǐsler, R. Velvarská, D. Kubička, Reconstructed Mg-Al hydrotalcites prepared by using different rehydration and drying time: Physico- chemical properties and catalytic performance in aldol condensation, Appl. Catal. A: Gen. 536 (2017) 85–96, https://doi.org/10.1016/j.apcata.2017.1002.1020. [62] J. Han, Y. Dou, M. Wei, D.G. Evans, X. Duan, Erasable nanoporous antireflection coatings based on the reconstruction effect of layered double hydroxides, Angew. Chem. 122 (2010) 2217–2220, https://doi.org/2210.1002/ange.200907005. [63] M. Kim, I. Oh, H. Choi, W. Jang, J. Song, C.S. Kim, J.-W. Yoo, S. Cho, A solution- based route to compositionally complex metal oxide structures using high-entropy layered double hydroxides, Cell Rep. Phys. Sci. 3 (2022) 100702 https://doi.org/ 100710.101016/j.xcrp.102021.100702. [64] H. Hayashi, T. Azumi, A. Sato, Y. Udagawa, A cartography of Kβ resonant inelastic X-ray scattering for lifetime-broadening-suppressed spin-selected XANES of α-Fe2O3, J. Electron Spectros. Relat. Phenomena 168 (2008) 34–39, https://doi. org/10.1016/j.elspec.2008.1008.1006. [65] C. Yang, J. Chiou, H. Tsai, C. Pao, J. Jan, S.C. Ray, C. Yeh, K. Huang, H. Hsueh, W. Pong, Electronic structure and magnetic properties of Al-doped Fe3O4 films studied by x-ray absorption and magnetic circular dichroism, Appl. Phys. Lett. 86 (2005) 062504 https://doi.org/062510.061063/062501.1863450. [66] S. Shen, J. Zhou, C.-L. Dong, Y. Hu, E.N. Tseng, P. Guo, L. Guo, S.S. Mao, Surface engineered doping of hematite nanorod arrays for improved photoelectrochemical water splitting, Sci. Rep. 4 (2014) 6627, https://doi.org/6610.1038/srep06627. [67] M.E. Fleet, The structure of magnetite, Acta. Crystallogr. B. Struct. Sci. Cryst. 37 (1981) 917–920, https://doi.org/910.1107/S0567740881004597. [68] C. Henderson, J. Charnock, D. Plant, Cation occupancies in Mg Co, Ni, Zn, Al ferrite spinels: a multi-element EXAFS study, J. Condens. Matter Phys. 19 (2007) 076214 https://doi.org/076210.071088/070953-078984/076219/076217/076214. [69] C. Henderson, G. Cressey, S. Redfern, Geological applications of synchrotron radiation, Radiat. Phys. Chem. 45 (1995) 459–481, https://doi.org/410.1016/ 0969-1806X(1095)92799-92795. [70] C.J. Jia, L.D. Sun, Z.G. Yan, L.P. You, F. Luo, X.D. Han, Y.C. Pang, Z. Zhang, C. H. Yan, Single-crystalline iron oxide nanotubes, Angew. Chem. Int. Ed. 44 (2005) 4328–4333, https://doi.org/4310.1002/anie.200463038. [71] G. Magnacca, G. Cerrato, C. Morterra, M. Signoretto, F. Somma, F. Pinna, Structural and surface characterization of pure and sulfated iron oxides, Chem. Mater. 15 (2003) 675–687, https://doi.org/610.1021/cm021268n. [72] A. Monhemius, Precipitation diagrams for metal hydroxides, sulphides, arsenates and phosphates, Pascal and Francis Bibliographic Databases (1977). S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 11 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0120 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0120 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0120 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0120 https://doi.org/10.1016/j.cej.2006.1011.1001 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0130 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0130 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0130 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0130 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0135 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0135 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0135 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0135 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0140 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0140 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0140 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0140 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0145 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0145 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0145 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0145 https://doi.org/10.1016/j.seppur.2017.1003.1054 https://doi.org/10.1016/j.seppur.2017.1003.1054 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0155 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0155 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0155 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0160 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0160 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0160 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0170 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0170 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0170 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0170 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0170 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0175 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0175 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0175 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0180 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0180 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0180 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0185 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0185 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0185 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0185 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0195 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0195 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0195 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0200 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0200 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0200 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0200 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0200 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0205 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0205 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0205 https://doi.org/10.1016/S0956-1053X(1096)00026-00028 https://doi.org/10.1016/j.agee.2009.1011.1013 https://doi.org/10.1016/j.agee.2009.1011.1013 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0220 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0220 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0220 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0220 https://doi.org/10.1038/s44286-44023-00005-44281 https://doi.org/10.1038/s44286-44023-00005-44281 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0230 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0230 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0230 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0230 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0235 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0235 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0235 https://doi.org/10.1007/1978-1003-1319-48179-48171_48111 https://doi.org/10.1007/1978-1003-1319-48179-48171_48111 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0250 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0250 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0250 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0255 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0255 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0255 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0255 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0260 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0260 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0260 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0260 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0260 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0265 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0265 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0265 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0265 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0270 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0270 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0270 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0270 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0270 https://doi.org/10.1016/j.jcat.2016.1004.1017 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0280 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0280 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0280 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0280 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0285 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0285 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0285 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0285 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0290 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0290 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0290 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0290 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0290 https://doi.org/10.1016/j.jcou.2018.1011.1008 https://doi.org/10.1016/j.jcou.2018.1011.1008 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0300 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0300 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0300 https://doi.org/10.1016/j.apcata.2017.1002.1020 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0310 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0310 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0310 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0315 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0315 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0315 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0315 https://doi.org/10.1016/j.elspec.2008.1008.1006 https://doi.org/10.1016/j.elspec.2008.1008.1006 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0325 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0325 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0325 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0325 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0330 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0330 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0330 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0335 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0335 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0340 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0340 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0340 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0345 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0345 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0345 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0350 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0350 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0350 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0355 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0355 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0355 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0360 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0360 [73] V. Stevanovic, M. d’Avezac, A. Zunger, Universal electrostatic origin of cation ordering in A2BO4 spinel oxides, J. Am. Chem. Soc. 133 (2011) 11649–11654, https://doi.org/11610.11021/ja2034602. [74] S. Nasrazadani, A. Raman, The application of infrared spectroscopy to the study of rust systems-II. Study of cation deficiency in magnetite (Fe3O4) produced during its transformation to maghemite (γ-Fe2O3) and hematite (α-Fe2O3), Corros. Sci. 34 (1993) 1355–1365, https://doi.org/1310.1016/0010-1938X(1393)90092-U. [75] Y. Dong, H. Lu, J. Cui, D. Yan, F. Yin, D. Li, Mechanical characteristics of FeAl2O4 and AlFe2O4 spinel phases in coatings-A study combining experimental evaluation and first-principles calculations, Ceram. Int. 43 (2017) 16094–16100, https://doi. org/16010.11016/j.ceramint.12017.16008.16142. [76] A. Pavese, G. Artioli, U. Russo, A. Hoser, Cation partitioning versus temperature in (Mg0.70Fe0.23)Al1.97O4 synthetic spinel by in situ neutron powder diffraction, Phys. Chem. Miner. 26 (1999) 242–250, https://doi.org/210.1007/s002690050183. [77] A. Gorton, G. Bitsianes, T. Joseph, Thermal expansion coefficients for iron and its oxides from X-ray diffraction measurements at elevated temperatures, Trans. Metall. Soc. AIME 233 (1965) 1519. [78] A.J. Berry, H.S.C. O’Neill, K.D. Jayasuriya, S.J. Campbell, G.J. Foran, XANES calibrations for the oxidation state of iron in a silicate glass, Am. Mineral. 88 (2003) 967–977, https://doi.org/910.2138/am-2003-0704. [79] S. Fukushima, T. Kimura, K. Nishida, V.A. Mihai, H. Yoshikawa, M. Kimura, T. Fujii, H. Oohashi, Y. Ito, M. Yamashita, The valence state analysis of Ti in FeTiO3 by soft X-ray spectroscopy, Mikrochim. Acta 155 (2006) 141–145, https://doi.org/ 110.1007/s00604-00006-00532-y. [80] B. Qian, S. Yang, J. Zhang, S. Zhou, B. Etschmann, C. Liu, B. Dai, J. Cashion, Y. Wang, H. Wang, Waste to worth: A high-temperature water-gas shift magnetite catalyst with encapsulated core-shell structure from coal fly ash, Fuel Process. Technol. 232 (2022) 107265 https://doi.org/107210.101016/j. fuproc.102022.107265. [81] M. Feyzi, M. Irandoust, A.A. Mirzaei, Effects of promoters and calcination conditions on the catalytic performance of iron-manganese catalysts for Fischer- Tropsch synthesis, Fuel Process. Technol. 92 (2011) 1136–1143, https://doi.org/ 1110.1016/j.fuproc.2011.1101.1010. [82] Y.-D. Dong, Y. Shi, Y.-L. He, S.-R. Yang, S.-Y. Yu, Z. Xiong, H. Zhang, G. Yao, C.- S. He, B. Lai, Synthesis of Fe-Mn-based materials and their applications in advanced oxidation processes for wastewater decontamination: a review, Ind. & Eng. Chem. Res. 62 (2023) 10828–10848, https://doi.org/10810.11021/acs. iecr.10823c01624. [83] J.A. Bennett, C.M. Parlett, M.A. Isaacs, L.J. Durndell, L. Olivi, A.F. Lee, K. Wilson, Acetic acid ketonization over Fe3O4/SiO2 for pyrolysis bio-oil upgrading, ChemCatChem 9 (2017) 1648, https://doi.org/1610.1002/cctc.201601269. [84] S. Almutairi, E. Kozhevnikova, I. Kozhevnikov, Ketonisation of acetic acid on metal oxides: Catalyst activity, stability and mechanistic insights, Appl. Catal. A: Gen. 565 (2018) 135–145, https://doi.org/110.1016/j.apcata.2018.1008.1008. [85] T.N. Pham, D. Shi, D.E. Resasco, Kinetics and mechanism of ketonization of acetic acid on Ru/TiO2 catalyst, Top. Catal. 57 (2014) 706–714, https://doi.org/ 710.1007/s11244-11013-10227-11247. [86] K. Wu, M. Yang, W. Pu, Y. Wu, Y. Shi, H.-S. Hu, Carbon promoted ZrO2 catalysts for aqueous-phase ketonization of acetic acid, ACS Sustain. Chem. Eng. 5 (2017) 3509–3516, https://doi.org/3510.1021/acssuschemeng.3507b00226. [87] J.A. Lopez-Ruiz, A.R. Cooper, G. Li, K.O. Albrecht, Enhanced hydrothermal stability and catalytic activity of LaxZryOz mixed oxides for the ketonization of acetic acid in the aqueous condensed phase, ACS Catal. 7 (2017) 6400–6412, https://doi.org/6410.1021/acscatal.6407b01071. [88] Z.-F. Pei, V. Ponec, On the intermediates of the acetic acid reactions on oxides: an IR study, Appl. Surf. Sci. 103 (1996) 171–182, https://doi.org/110.1016/0169- 4332(1096)00453-00459. [89] M. Hasan, M. Zaki, L. Pasupulety, Oxide-catalyzed conversion of acetic acid into acetone: an FTIR spectroscopic investigation, Appl. Catal. a: Gen. 243 (2003) 81–92, https://doi.org/10.1016/S0926-1860X(1002)00539-00532. [90] K. Nakamoto, Infrared and Raman spectra of inorganic and coordination compounds, part B: applications in coordination, organometallic, and bioinorganic chemistry. 1 applications in coordination chemistry. 1.9 Complex of Alkoxides, Alcohols, Ethers, Ketones, Aldehydes, Esters, and Carboxylic Acids., 2009. pp. 64- 65. [91] A.A. Taimoor, A. Favre-Réguillon, L. Vanoye, I. Pitault, Upgrading of biomass transformation residue: influence of gas flow composition on acetic acid ketonic condensation, Catal, Sci. Technol. 2 (2012) 359–363, https://doi.org/310.1039/ C1031CY00346A. [92] Y. Zhou, S. Sun, C. Wei, Y. Sun, P. Xi, Z. Feng, Z.J. Xu, Significance of engineering the octahedral units to promote the oxygen evolution reaction of spinel oxides, Adv. Mater. 31 (2019) 1902509, 1902510.1901002/adma.201902509. [93] E.V. Fufachev, B.M. Weckhuysen, P.C. Bruijnincx, Crystal phase effects on the gas- phase ketonization of small carboxylic acids over TiO2 catalysts, ChemSusChem 14 (2021) 2710–2720, 2710.1002/cssc.202100721. [94] S. Yin, X. Zhao, E. Jiang, Y. Yan, P. Zhou, P. Huo, Boosting water decomposition by sulfur vacancies for efficient CO2 photoreduction, Energy Environ. Sci. 15 (2022) 1556–1562, 1510.1039/D1551EE03764A. [95] H.A. Al-Abadleh, V. Grassian, FT-IR study of water adsorption on aluminum oxide surfaces, Langmuir 19 (2003) 341–347, 310.1021/la026208a. [96] Y. Zhang, P. Gao, F. Jiao, Y. Chen, Y. Ding, G. Hou, X. Pan, X. Bao, Chemistry of ketene transformation to gasoline catalyzed by H-SAPO-11, J. Am. Chem. Soc. 144 (2022) 18251–18258, 18210.11021/jacs.18252c03478. [97] B. Boekaerts, B.F. Sels, Catalytic advancements in carboxylic acid ketonization and its perspectives on biomass valorisation, Appl. Catal. b: Environ. 283 (2021) 119607, 119610.111016/j.apcatb.112020.119607. S. Yang et al. Chemical Engineering Journal 502 (2024) 157797 12 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0365 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0365 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0365 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0370 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0370 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0370 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0370 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0375 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0375 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0375 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0375 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0380 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0380 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0380 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0385 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0385 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0385 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0390 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0390 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0390 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0395 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0395 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0395 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0395 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0400 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0400 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0400 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0400 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0400 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0405 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0405 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0405 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0405 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0410 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0410 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0410 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0410 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0410 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0415 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0415 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0415 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0420 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0420 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0420 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0425 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0425 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0425 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0430 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0430 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0430 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0435 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0435 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0435 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0435 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0440 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0440 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0440 https://doi.org/10.1016/S0926-1860X(1002)00539-00532 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0455 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0455 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0455 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0455 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0460 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0460 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0460 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0465 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0465 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0465 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0470 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0470 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0470 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0475 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0475 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0480 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0480 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0480 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0485 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0485 http://refhub.elsevier.com/S1385-8947(24)09288-X/h0485 Facile synthesis of layered spinel ferrite from fly ash waste as a stable and active ketonisation catalyst 1 Introduction 2 Experimental 2.1 Synthesis of spinel ferrites 2.2 Catalyst characterisation 2.3 Acetic acid ketonisation 3 Results and Discussion 3.1 Physicochemical properties of fly ash-derived ferrite 3.2 Physicochemical properties of synthetic ferrites 3.3 Reducibility of FAF 3.4 Catalytic performance 3.5 Synergistic roles of multiple metal cations and mechanistic study 4 Conclusions CRediT authorship contribution statement Declaration of competing interest Acknowledgments Appendix A Supplementary data datalink5 References