TECHNICAL ARTICLE
Lattice response of the porous coordination framework Zn(hba)
to guest adsorption
Josie E. Auckett,1 A. David Dharma,2 Marina P. Cagnes,3 Tamim A. Darwish,3 Brendan F. Abrahams,2
Ravichandar Babarao,4 Timothy A. Hudson,2 Richard Robson,2 Keith F. White,2 and
Vanessa K. Peterson1,a)
1Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, NSW 2234, Australia
2School of Chemistry, University of Melbourne, VIC 3010, Australia
3National Deuteration Facility, Australian Nuclear Science and Technology Organisation, NSW 2234, Australia
4School of Science, RMIT University, VIC 3000, Australia
(Received 28 April 2017; accepted 20 June 2017)
Analysis of in situ neutron powder diffraction data collected for the porous framework material
Zn(hba) during gas adsorption reveals a two-stage response of the host lattice to increasing CO2
guest concentration, suggesting progressive occupation of multiple CO2 adsorption sites with different
binding strengths. The response of the lattice to moderate CH4 guest concentrations is virtually indis-
tinguishable from the response to CO2, demonstrating that the influence of host–guest interactions on
the Zn(hba) framework is defined more strongly by the concentration than by the identity of the
guests. © 2017 International Centre for Diffraction Data. [doi:10.1017/S0885715617000720]
Key words: gas sorption, metal–organic framework, neutron powder diffraction
I. INTRODUCTION dianion of coumaric acid; internal ∼8 Å squares) and Zn(hbpc)
′
Sorbents capable of separating gas mixtures ef ciently (hbpc = the dianion of 4 -hydroxy-4-biphenylcarboxylic acid;fi
and cost-effectively are in high demand for a large number ∼10 Å squares) (White et al., 2015). An attractive feature of
of industrial applications, including pre- and post-combustion this series of frameworks is the ease with which the bridging
carbon capture, natural gas sweetening, air puri cation, and ligands can be functionalised, raising the prospect of tailoringfi
isotopic separations (Li et al., 2009; Sumida et al., 2012; framework setswith a range of pore sizes for specific applications.
Wang and Zhao, 2017). One broad category of materials For example, a recent study of the adsorption of inhalation
under investigation for such applications are porous solid anaesthetics by members of this series (Abrahams et al., in
frameworks, including metal–organic frameworks (MOFs) press) exemplified the utility of pore size tunability by identifying
constructed of metallic ions or clusters linked by organic a size-related trade-off between maximum uptake and
ligands in a porous crystalline array. Despite the discoveries low-pressure adsorption performance in these materials.
to date of many MOFs exhibiting promising gas separation The fully ordered form of Zn(hba) is depicted in Figure 1.
behaviour (Millward and Yaghi, 2005; Li and Yang, 2007; The framework is constructed by Zn nodes stacked along the
Hamon et al., 2009; Yang et al., 2012), a need still exists c-axis at the corners of the square channels, with the hba link-
for the development of novel MOF separators, which combine ers on the a–c and b–c unit-cell faces forming the pore
strong performance under realistic operating conditions with “walls”. Because of the asymmetry of the hba linker, the Zn
the ful lment of additional pragmatic criteria, such as high nodes are displaced from the corners of the square in afi
cyclability and low cost. The targeted design of MOFs for sep- 41-type screw arrangement, and the planar linker molecules
aration applications is reliant upon a fundamental understand- tilt away from the unit-cell faces at an angle of ∼14°.
ing of the mechanisms of selective adsorption characteristics. Zn(hba) has been shown experimentally to exhibit selec-
Considerable efforts are therefore being directed toward tive uptake of CO2 and CH4 relative to H2 and N2 at moderate
atomic-scale characterisation of sorbent–guest interactions pressures near room temperature (White et al., 2015).
and the structural responses of the host framework to guest Interaction energies of almost 30 kJ mol
−1 for CO2 and−1
binding, in order to better understand and exploit the origins 21.5 kJ mol for CH4 were obtained by fitting virial equa-
of guest selectivity in MOFs. tions to adsorption isotherms and supported by the results of
Zn(hba) (hba = the dianion of 4-hydroxybenzoic acid) is a dispersion-corrected density functional theory (DFT-D2) cal-
tetragonal square-channel MOF (P4122 symmetry) with an inter-
culations. However, little information has yet been reported
nal square dimension of ∼6 Å between opposite van der Waals regarding the structural aspects of guest adsorption in this
surfaces (White et al., 2015). It belongs to a set of isotopological MOF family, including the effects of guest loading on the
MOFsconstructedusingZnnodeswithdifferent phenolic carbox- host framework, and even the guest binding locations in
ylate anion linkers, the larger members being Zn(cma) (cma = the Zn(hba) have so far been determined only theoretically for
CO2 by classical simulated annealing (White et al., 2015).
While an indication of guest–host interactions within the
a)Author to whom correspondence should be addressed. Electronic mail: material was gained from the DFT-D2 calculations, the
vanessa.peterson@ansto.gov.au structure of the empty framework that formed the basis of
S49 Powder Diffraction 32 (S2), December 2017 0885-7156/2017/32(S2)/49/5/$18.00 © 2017 JCPDS-ICDD S49
Downloaded from https://www.cambridge.org/core, subject to the Cambridge Core terms of use.
Figure 1. (Colour online) The structure of Zn(hba) depicted along the [001] and approximate [100] projections. Black lines denote the unit cell [Figure 4 drawn
using VESTA (Momma and Izuma, 2008)].
these calculations was unconfirmed experimentally under B. Data collection and analysis
CO2-loaded conditions. The as-prepared sample was sealed and evacuated at
This paper presents results of in situ neutron powder dif- 490 K for 18 h. The desolvated sample (1.198 g) was trans-
fraction (NPD) measurements performed on a deuterated sam- ferred to a 9 mm diameter V can inside a He-filled glove
ple of Zn(hba), which yield the first insights into the box and attached to an air-isolated cryofurnace centrestick.
dependence of the framework lattice on the concentration of This centrestick supports gas delivery to the sample as well
CO2 and CD4 guests, and reveals that the framework lattice as temperature sensing and control, and is described elsewhere
response is dependent only on the number of guests for (Lee et al., 2016). Using this apparatus, the sample was posi-
these two gas species. tioned inside a He cryofurnace mounted on the high-intensity
neutron diffractometer WOMBAT (Studer et al., 2006) at the
II. EXPERIMENTAL OPAL reactor facility in Australia. WOMBAT features an area
detector covering 120° in 2θ, and NPD data were collected
A. Sample preparation continuously during isothermal adsorption and desorption
4-Hydroxybenzoic acid-h (7.0 g, 51 mmol) was placed procedures using 2 min acquisition times with an incident
into a 450 ml Parr vessel, followed by deuterium oxide wavelength of λ = 2.9589(7) Å (as determined using the
(D2O, 200 ml), 10 wt% platinum on carbon (1.5 g), and NIST La
11B6 standard reference material 660b). The simulta-
40 wt% sodium deuteroxide (10.4 ml, two equivalents). The neously measured adsorption isotherms were controlled using
vessel was affixed to the Parr reactor, sealed, and the stirrer a Hiden Isochema IMI computerised manometric dosing
turned on. The vessel was purged with N2, then H2, before system, and were performed at 273 K.
heating to 453 K for 18 h. The vessel was then cooled, and For each isotherm experiment, an “equilibration” data set
the reaction mixture filtered through celite. The celite was was compiled by extracting the last NPD pattern acquired
washed with water (2 × 100 ml). The filtrate was acidified to before application of each dose in the isotherm sequence.
pH 2 by slow addition of 1 N HCl. Saturated sodium chloride These limited data sets were subjected to Pawley fitting,
solution (150 ml) was then added, followed by extraction into which was performed using the sequential refinement feature
ethyl acetate (3 × 200 ml). The organic phase was dried over available in the GSAS2 software package (Toby and Von
Na2SO4, filtered, and concentrated to provide the crude prod- Dreele, 2013). The tetragonal unit-cell parameters a and c
uct mixture as an orange solid (6.1 g). This was adsorbed onto were refined. The Pawley intensities of 56 reflections were
silica and purified by column chromatography (3:7–1:0 ethyl selected for refinement after initial test fits, with the intensities
acetate/petroleum ether) to yield 4-hydroxybenzoic acid-d4 as of most unobserved reflections manually fixed to zero to avoid
a white-cream solid [1.6 g; overall 78% D by mass spectrom- biasing the background model. Peak profiles were modelled
etry (MS) and 13C nuclear magnetic resonance (NMR) analy- using the Gaussian parameters U, V, and W, and the back-
sis; 58% D at the 2-position and 98% D at the 3-position by ground was modelled using a four-term Chebyschev polynomial.
quantitative 13C NMR analysis (Darwish et al., 2016); details The zero shift and peak asymmetry parameters, along with the
of the NMR and MS spectra and deuteration analysis are pro- centre, area, and width of a spurious peak (at 2θ≈ 93°) arising
vided in the on·line Supplementary Material]. from the sample environment, were allowed to refine onlyZn(OAc)2 2H2O (240 mg) and D2hba-d4 (150 mg) were for the first pattern in the CO2 isotherm series, after which
dissolved in hot methanol (20 ml), and pentanol (20 ml) was they were fixed to the same values for all subsequent model
added to the heated solution. The slow evaporation·ofmethanol refinements against data collected for both guests.at 323 K yielded colourless crystals of Zn(hba-d4) x pentanol, Following the initial sequential fitting procedure, it
which were filtered and air dried. Repetition of this procedure became apparent that the background was poorly fitted at
yielded a batch of seven individual samples (190–232 mg), high loadings of both CO2 and CD4 because of the emergence
which were combined prior to the NPD experiment. of a very broad peak centred near 2θ = 40–50°. After refining
S50 Powder Diffr., Vol. 32, No. S2, December 2017 Auckett et al. S50
Downloaded from https://www.cambridge.org/core, subject to the Cambridge Core terms of use.
the position, intensity, and width of this peak against the final evidence for a phase transition requiring modification of the
NPD pattern in each sequence (in which the broad feature was unit cell at elevated guest loadings was observed in any of
most intense), the position and width values were fixed and the the NPD data series.
sequential Pawley fit was repeated with only the peak intensity The Zn(hba) lattice responds anisotropically to CO2 load-
allowed to refine across the series. ing and exhibits a two-stage behaviour (Figure 3). At low
guest loadings, the a-axis contracts with increased guest con-
centration, while the c-axis remains unchanged within error,
III. RESULTS AND DISCUSSION resulting in an overall decrease of the unit-cell volume.
A. Adsorption isotherms Guest-induced contraction arising from the attractive force
between host and guests is commonly observed in flexible
Adsorption isotherms measured for the Zn(hba) sample
framework systems (Coudert et al., 2014). Although the loca-
during the collection of in situ NPD data (Figure 2) are in
tion of the CO binding sites in Zn(hba) has not been deter-
good agreement with those reported previously by White 2
mined experimentally, simulated annealing methods
et al. (2015). Because of a technical issue, no data were
previously identified a binding site interacting with the hba
obtained for Zn(hba):CD4 at equilibrium pressures higher phenolate and carboxylate oxygen atoms near the channel cor-
than 5.43 bar. Nevertheless, the form of the isotherm curve
ner, with a large calculated binding energy of −33 kJ mol−1
and the previously published data both indicate that the max-
∼ (White et al., 2015). Strong binding at this location may resultimum CD4 uptake achieved during this experiment was 80% in contraction of the a-axis as the ends of perpendicular hba
of the expected saturated uptake. As observed by White et al.
linkers are drawn closer together, reducing the square diameter
(2015), Zn(hba) demonstrates a modest selectivity of around
without greatly affecting the c axis.
2:1 for CO2 over CD4 at 273 K. At high CO2 loadings, both the a- and c-lattice parameters
increase rapidly with guest concentration, resulting in positive
B. Sequential Pawley fits volume expansion. The changeover point between the two
regimes lies in the range 0.5–0.7 CO2:Zn, but is difficult toThe in situ NPD data collected for Zn(hba) were well determine with precision because of the errors on the refined
indexed using a tetragonal unit cell with P4122 symmetry parameters. Pseudo-isotropic expansion is characteristic of
[a = 9.1178(3), c = 12.6306(19) Å for the desolvated frame- weaker binding interactions between the framework and
work]. This model represents the structure reported by incoming guests, and is usually associated with the more com-
White et al. (2015) having fully ordered hba ligand orienta- plete filling of pores and consequential reduction in empty
tions (Figure 1). However, single-crystal X-ray diffraction space leading to sterically driven expansion. The observed
studies previously revealed evidence for considerable orienta- change in lattice expansion behaviour therefore implies that
tional disorder among the hba linkers and Zn nodes (White a second, less energetically favourable binding site is being
et al., 2015). For this reason, an alternative model was con-
filled in this regime. The location of this possible binding
structed incorporating fourfold disorder of the framework by site has not yet been identified.
reflection along [100] and [010], resulting in P42/mmc sym- The response of the Zn(hba) framework to CD loading
metry and a c-axis half that of the starting model. This cell 4follows the same trends as for CO in the low-concentration
failed to index several very weak reflections corresponding 2regime. The maximum 0.62 CD :Zn loading achieved during
to l-odd reflections of the data, despite Pawley refinement 4the experiment was insufficient to determine whether a com-
yielding good agreement values, so the larger unit cell of parable change in expansion behaviour occurs at high loadings
the ordered model was adopted for all sequential Pawley of CD4. However, given the appearance of the data inrefinements. The low intensity of the l-odd reflections is Figure 3, it is doubtful that any such change would be observ-
noted as likely evidence of poor orientational order. No able before the expected loading of <0.8 CD4:Zn was reached
at the high-pressure limit of our experiment.
A broad diffuse feature is also clearly evident in the NPD
data at higher guest concentration (Figure 4). This feature
was modelled using a peak centred around d≈ 3.3 Å for CO2
and d≈ 4.0 Å for CD4, and in both cases its intensity increased
with guest concentration. The onset of this feature is indicative
of a form of poor structural order emerging among the guest
molecules or framework components. For the CO2 data series,
the intensity of the diffuse feature increases more rapidly above
∼0.5 CO2:Zn loading (the approximate commencement point
of the change in lattice expansion behaviour), suggesting that
the feature probably corresponds to the progressive occupation
of a secondary guest binding site that is poorly ordered. The fact
that the refined peak centre is guest-dependent also supports the
view that the feature is likely to arise directly from partial guest
disorder rather than partial framework disorder.
It is particularly interesting to note that in the low-
concentration regime, the Zn(hba) unit-cell behaviour is
Figure 2. (Colour online) Adsorption isotherms recorded for Zn(hba) during essentially indistinguishable for both CO2 and CD4 at equal
simultaneous collection of NPD data. loadings. Although the binding enthalpy determined for
S51 Powder Diffr., Vol. 32, No. S2, December 2017 Lattice response of the porous coordination framework Zn(hba) to guest adsorption S51
Downloaded from https://www.cambridge.org/core, subject to the Cambridge Core terms of use.
Figure 3. (Colour online) Lattice parameters of Zn(hba) as a function of guest loading, determined by Pawley refinement against in situ NPD data.
Figure 4. (Colour online) (a) Pawley refinement against NPD data collected at a guest concentration of 1.36 CO2:Zn, exhibiting a diffuse feature centred around
d∼ 3.3 Å. Arrowheads indicate reflections for which the Pawley intensity was refined; intensities of the unmarked reflections were fixed to zero. (b) Refined
intensity of the diffuse feature as a function of concentration of CO2 and CD4 guests.
CO2 in Zn(hba) was 8 kJ mol
−1 higher than that for CH4 by expansion at high concentrations, with no evidence for
(White et al., 2015), as is typical in MOFs because of the any accompanying change in symmetry. The experimental
quadrupole moment of CO2 (D’Alessandro et al., 2010; results point to the probable occupation of two binding sites,
Chevreau et al., 2015), the Zn(hba) lattice expansion appears the first strongly interacting and the second weakly interacting
to be independent of guest interaction strength and dependent and poorly ordered. The lattice behaviour up to 0.62 Guest:Zn
only on the total concentration of guest molecules. It can be loading is indistinguishable for CO2 and CD4 guests, indicat-
speculated that the lower observed uptake of CH4 by Zn(hba) ing that the mechanism of lattice contraction is not strongly
results from a steric inability of the bulkier CH4 molecule dependent on the interaction strength of the guest, and that
to access the proposed secondary binding site occupied the higher observed uptake of CO2 may be related to steric
by CO2 at higher loadings. Further investigation of the struc- considerations.
tural changes associated with guest loading, along with the
determination of preferred binding site locations for both SUPPLEMENTARY MATERIAL
guests, is required in order to fully understand the origins of
CO2/CD4 selectivity in this material. The supplementary material for this article can be found at
https://doi.org/10.1017/S0885715617000720.
IV. CONCLUSION
In situ NPD data collected during CO2 adsorption in
ACKNOWLEDGEMENTS
Zn(hba) reveal a two-stage lattice response characterised by This research was supported by the Science and Industry
framework contraction at low guest concentrations followed Endowment Fund (RP02-035). The authors thank the sample
S52 Powder Diffr., Vol. 32, No. S2, December 2017 Auckett et al. S52
Downloaded from https://www.cambridge.org/core, subject to the Cambridge Core terms of use.
environment team at the Australian Centre for Neutron Lee, S., Chevreau, H., Booth, N., Duyker, S. G., Ogilvie, S. H., Imperia, P.,
Scattering for assistance and support with respect to the gas- and Peterson, V. K. (2016). “Powder sample-positioning system for neu-
delivery equipment used for the NPD experiment. The tron scattering allowing gas delivery in top-loading cryofurnaces,” J.
Appl. Crystallogr. 49, 705–711.
National Deuteration Facility is partly funded by the Li, J.-R., Kuppler, R. J., and Zhou, H.-C. (2009). “Selective gas adsorption
National Collaborative Research Infrastructure Strategy and separation in metal-organic frameworks,” Chem. Soc. Rev. 38,
(NCRIS), an initiative of the Australian government. RB 1477–1504.
acknowledges the Australian Research Council for a Li, Y. and Yang, R. T. (2007). “Gas adsorption and storage in metal-organic
DECRA fellowship (DE160100987), and the National framework MOF-177,” Langmuir 23, 12937–12944.
Computational Infrastructure (NCI) and Pawsey Millward, A. R. and Yaghi, O. M. (2005). “Metal-organic frameworks with
Supercomputing facility (Magnus) for providing computing exceptionally high capacity for storage of carbon dioxide at room temper-
ature,” J. Am. Chem. Soc. 127, 17998–17999.
support. Momma, K. and Izuma, F. (2008). “VESTA: a three-dimensional visualization
system for electronic and structural analysis,” J. Appl. Crystallogr. 41,
653–658.
Abrahams, B. F., Dharma, A. D., Donnelly, P. S., Hudson, T. A., Kepert, C. J., Studer, A. J., Hagen, M. E., and Noakes, T. J. (2006). “Wombat: the high-
Robson, R., Southon, P. D., and White, K. F. “Tunable porous coordina- intensity powder diffractometer at the OPAL reactor,” Physica B 385–86,
tion polymers for the capture, recovery and storage of inhalation anesthet- 1013–1015.
ics,” Chem. Eur. J. 23, 7871–7875. Sumida, K., Rogow, D. L., Mason, J. A., McDonald, T. M., Bloch, E. D.,
Chevreau, H., Liang, W., Kearley, G. J., Duyker, S. G., D’Alessandro, D. M., Herm, Z. R., Bae, T.-H., and Long, J. R. (2012). “Carbon dioxide capture
and Peterson, V. K. (2015). “Concentration-dependent binding of CO2 in metal–organic frameworks,” Chem. Rev. 112, 724–781.
and CD4 in UiO-66(Zr),” J. Phys. Chem. C 119, 6980–6987. Toby, B. H. and Von Dreele, R. B. (2013). “GSAS-II: the genesis of a modern
Coudert, F.-X., Fuchs, A. H., and Neimark, A. V. (2014). “Comment on open-source all purpose crystallography software package,” J. Appl.
Volume shrinkage of a metal-organic framework host induced by the dis- Crystallogr. 46, 544–549.
persive attraction of guest gas molecules”, PCCP 16, 4394–4395. Wang, Y. and Zhao, D. (2017). “Beyond equilibrium: metal–organic frame-
D’Alessandro, D. M., Smit, B., and Long, J. R. (2010). “Carbon dioxide cap- works for molecular sieving and kinetic gas separation,” Cryst. Growth
ture: prospects for new materials,”Angew. Chem. Int. Ed. 49, 6058–6082. Des. 17, 2291–2308.
Darwish, T. A., Yepuri, N. R., Holden, P. J., and James, M. (2016). White, K. F., Abrahams, B. F., Babarao, R., Dharma, A. D., Hudson, T. A.,
“Quantitative analysis of deuterium using the isotopic effect on quaternary Maynard-Casely, H. E., and Robson, R. (2015). “A new structural family
(13)C NMR chemical shifts,” Anal. Chim. Acta 927, 89–98. of gas-sorbing coordination polymers derived from phenolic carboxylic
Hamon, L., Llewellyn, P. L., Devic, T., Ghoufi, A., Clet, G., Guillerm, V., acids,” Chem. Eur. J. 21, 18057–18061.
Pirngruber, G. D., Maurin, G., Serre, C., Driver, G., Van Beek, W., Yang, S., Sun, J., Ramirez-Cuesta, A. J., Callear, S. K., David, W. I. F.,
Jolimaître, E., Vimont, A., Daturi, M., and Férey, G. (2009). Anderson, DP., Newby, R., Blake, A. J., Parker, J. E., Tang, C. C., and
“Co-adsorption and separation of CO2-CH4 mixtures in the highly flexi- Schröder,M. (2012). “Selectivity and direct visualization of carbon dioxide
ble MIL-53(Cr) MOF,” J. Am. Chem. Soc. 131, 17490–17499. and sulfur dioxide in a decorated porous host,” Nat. Chem. 4, 887–894.
S53 Powder Diffr., Vol. 32, No. S2, December 2017 Lattice response of the porous coordination framework Zn(hba) to guest adsorption S53
Downloaded from https://www.cambridge.org/core, subject to the Cambridge Core terms of use.