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Cite this:DOI: 10.1039/d0cp00262c

Enantiospecificity in achiral zeolites forasymmetric catalysis†

Tianxiang Chen, ab Ching Kit Tommy Wun,ab Sarah J. Day,c Chiu C. Tangc andTsz Woon Benedict Lo *ab

This article highlights the recent fundamental study in using achiral and chiral porous materials for the

potential applications in asymmetric catalysis. Thanks to the new-generation synchrotron X-ray powder

diffraction (SXRD) facilities, we reveal the presence of the unique ‘chiral region’ in achiral zeolites with the

MFI topology. Both the inherent site-isolation effect of the active sites and internal confinement restraints

in zeolites are critical for creating ‘chiral regions’ that can aid the design of more enantioselective catalytic

reactions. We also offer an outlook on the challenges and opportunities of this research area.

1. Introduction

The use of chiral catalysts has been a promising solution tomany enantioselective chemical and pharmaceutical synthesisprocesses. The homogeneous Sharpless and Jacobsen catalystsare excellent examples for asymmetric alkene epoxidation. It isachieved through directing functional groups, such as thehydroxyl group in the Sharpless catalyst, or through rationaltailoring of the ligands of metal complexes, such as by usingchiral salen-Mn(III) centre in the Jacobsen catalyst (salen: trans-(R,R)-1,2-bis(salicylideneamino)cyclohexane). This has accordinglyinspired the heterogeneous community to design chiral catalystsfor enantioselective reactions by the formation of ion-pair oradsorption,1 by the incorporation of chiral metal centres within amicroporous support material,2,3 and so forth. By heterogenisinghomogeneous catalysts, product-catalyst separation can be morereadily achieved by simple filtration or centrifugation. Dueto site-isolation, the catalytic properties of the heterogenisedcatalysts (e.g., thermal stability and product selectivity) are oftenfound comparable to the homogeneous analogues.4,5 Variousstrategies to heterogenise homogeneous catalysts have beendiscussed and reviewed by McMorn and Hutchings.6

In general, to achieve diastereoselective or even enantio-selective catalysis over porous materials, there are two commonlyaccepted strategies. The first strategy is to engineer ‘homochiral

framework’,7 such as the first chiral mesoporous ITQ-37 germano-silicate zeolite with one single gyroidal channel reported bySun et al.8 Dryzun et al. revealed the existence of 20 chiralsilicate zeolites which have not been recognised as chiral intheir original publications, as revealed by comparative adsorp-tion calorimetric experiments with D-/L-histidine.9 The use ofchiral organic structure-directing agents can transfer the chiralfunctionality into the zeolite-type frameworks as discussedextensively by Gomez-Hortiguela and Bernardo-Maestro,10 andby Davis.11,12 The chirality induction effect can be oftenobserved by templating with an enantiopure organic linker.13

Calero et al. have investigated the potential for enantiomericseparation of chiral zeolites based on molecular simulations.14

Adsorption with different geometries and enthalpies have beenshown, as seen in the illustration in Fig. 1.

Metal–organic frameworks (MOFs) are another class ofmicroporous materials that can possess frameworks with chirality.

Fig. 1 Snapshots of pure L (left), and pure R (right) CHBrClF isomersadsorbed in STW at 298 K and a total fugacity of 100 kPa. The centralcarbon atom of the molecule is coloured red for the L-isomer and lightblue for the R-isomer. The different atoms of the molecule are alsocoloured: Br = grey; Cl = yellow; F = green; H = white. Reprinted (adapted)with permission from ref. 14. Copyright (2010) American Chemical Society.

a The Hong Kong Polytechnic University Shenzhen Research Institute,

Shenzhen Hi-tech Industrial Park, Shenzhen 518000, Chinab State Key Laboratory of Chemical Biology and Drug Discovery,

Department of Applied Biology and Chemical Technology, The Hong Kong

Polytechnic University, Hong Kong, China. E-mail: benedict.tw.lo@polyu.edu.hkc Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot,

Oxfordshire OX11 0DE, UK

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cp00262c

Received 16th January 2020,Accepted 25th February 2020

DOI: 10.1039/d0cp00262c

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Duan et al. reported an enantiomorph pair of Ce-based MOFsresulted using opposite enantiomeric organic linkers.15 Similarto the syntheses of achiral MOFs, the inherent structures of theorganic linkers play a pivotal role in governing the dimensionof the channels/pores.16,17 Wu, Lin and co-workers reported aCd-based homochiral MOF (1) which is constructed by linkinginfinite one-dimensional [Cd(m-Cl)2]n zigzag chains with axiallychiral bipyridine bridging ligands ((R)-6,60-dichloro-2,20-dihydroxy-1,10-binaphthyl-4,40-bipyridine) containing orthogonal secondarydihydroxyl groups18,19 (see Fig. 2). The secondary dihydroxyl groupswere utilised as the asymmetric catalytic active sites for highlyselective conversion of aromatic aldehydes to secondary alcohols.Lin and co-workers also employed a similar strategy to prepare aCu-based (using 4,40,6,60-tetrabromo-2,20-diethoxy-1,10-binaphthylas the organic linker)17 and a Ru-based (using (R,R)-1,2-cyclo-hexanediamino-N,N0-bis(3-tert-butyl-salicylidene) as the organiclinker) homochiral MOFs20 for asymmetric catalysis. Cui et al.reported a series of Cd-based homochiral MOFs by employingCo- and Ni-based salen complexes as the precursors.21 Inaddition to the typical characterisation techniques for solid-state materials (such as X-ray diffraction), solid-state circulardichroism (CD) spectroscopy is a handy tool to reveal thechirality of the MOF framework. Readers can find more infor-mation in the detailed review covering various spectroscopictechniques by Castiglioni et al.22 However, the synthesis ofchiral zeolite-type materials remains a significant challenge,limiting their potential in broader commercial applications.

The second strategy is to incorporate external chirality intothe pores/channels; achiral zeolites and metal–organic frame-works (MOFs) have gained increasing attention as they can hostchiral metal complexes to achieve asymmetric catalysis. Forinstance, Li et al. reported the encapsulation of Cu-salen complex

into zeolite Y (see Fig. 3) for the selective oxidation of benzylalcohol.23 Hutchings and co-workers have employed a similarapproach to design asymmetric heterogeneous catalysts for alcoholdehydration,24 aziridination and epoxidation of alkene,25,26 andcarbonyl-ene and imino-ene reactions.27 These catalysts possesshigh activity associated with homogeneous catalysts and facilerecyclability adherent to heterogeneous catalysts. In these studies,the structure–activity relationship of asymmetric catalysts has beencorrelated to the intrinsic confinement effect exerted by the rigidframework of zeolites. The highly ordered hierarchical structures ofzeolites can isolate the metal complexes through electrostaticinteraction. These two effects (i.e. active site isolation and zeoliticconfinement) can work in synergy to offer steric specificity toenhance the catalytic properties in asymmetric catalysis. Forinstance, the supercage of Y zeolite can only accommodate aCu-salen species with quasi-planar tetragonal coordinate.23 Thesupercage, meanwhile, can prevent the Cu-salen from leachingduring catalysis. Even after six catalytic cycles, high catalyticperformance (activity and selectivity) can be maintained.Another example is the aziridination of styrene over Cu–Yzeolite; the confinement effect exerted by the micropores of Yzeolite introduces notable diffusion limitations for the reactantsand products. The active site in the Cu–Y catalyst is proposed to bemore constrained than the homogeneous counterparts. It is alsofound that water has a slightly deleterious effect over enantio-meric excess (e.e.) during the aziridination of styrene.25,28

In some cases, the zeolitic framework can exert additionalconfinement effect onto the substrate, rendering various reac-tions with higher enantioselectivity than the homogeneouscounterparts.29 It should be noted that for most immobilisedcatalysts in zeolites, the enantioselectivity (in terms of e.e.)typically remains or declines with the extent of reaction. Thereare extensive reports over the use of encapsulated chiral metal-salen complexes for asymmetric catalysis.2,30–36 Besides zeolitic

Fig. 2 Crystal structure of the Cd-based homochiral MOF reported byWu et al.18 (A) The 2D square grid in 1. (B) The 1D zigzag polymeric chain in 1.(C) Schematic representation of the 3D framework of 1. (D) Schematicrepresentation of the twofold interpenetration of 1. (E) Space-filling model of 1as viewed down the c axis showing the chiral 1D channels of 13.5 � 13.5 Å2 indimensions. Colour scheme for (A), (B), and (E): cyan: Cd, green: Cl, red: O,blue: N, grey: C, and light grey: H. Reprinted (adapted) with permission fromref. 18. Copyright (2007) John Wiley and Sons.

Fig. 3 Schematic of zeolite encapsulated metal-salen complexes. Reprinted(adapted) with permission from ref. 23. Copyright (2018) Elsevier.

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materials, siloxane is another class of support material. Ogunwumiand Bein have immobilised Mn–salen complex into a siloxaneelastomer network.3 However, the reported enantioselectivities formany cases without the use of a rigid chiral ligand, like salen, arequite short-lived.

Asymmetric catalysis reactions have been documented overachiral zeolites,37–44 particularly for b zeolites as summarised inthe early review by Davis.45 Renz et al. reported the catalyticepoxidation of chiral allylic alcohols by the Ti-b and TS-1zeolites.46 Akin to enzymatic catalysis, zeolites can stabiliseone transition state of a stereoisomer over another due to therestriction and confinement of the pores. The location of theactive sites with respect to the porous frameworks is pivotal.The intrinsic reactivity of the metal active site is anotherimportant factor for catalysis.16,47,48 For example, the reactivity(in terms of conversion and yields) of Rh-BINAP-MOF is higherthan the Ru analogue.48 The reaction activity of heterogeneousRh-BINAP-MOF catalyst was compared with a homogeneouscontrol; the reactivity of Rh-BINAP-MOF was found three timesbetter than the homogeneous analogue, which has beenascribed to the site isolation effect within solid-state materials.It prevents the catalysts from intermolecular deactivation.However, it is still challenging to achieve highly selective chiralreactions (high e.e.) because of the high framework rigidity ofthese types of solid-state materials.

Based on the density functional theory (DFT) calculations,specific ‘chiral cells’ can be tailored by doping Ca2+ into Al-richzeolites with the MFI topology to create an enantiospecificenvironment for 4-ethyl-4-methyloctane.49 Das and Marek haveemployed R- and S-4-ethyl-4-methyloctane (the simplest chiralsaturated hydrocarbon with a quaternary stereogenic centre) asa chiral probe molecule to demonstrate the enantioselectiveadsorption preference on MFI zeolites.50 For non-bulky andflexible polar organic molecules, there is a high degree ofrotational freedom with respect to the zeolitic channels/pores,leading to the absence of enantioselective adsorption. But forbulky and rigid adsorbates, like ibuprofen, the comparable sizebetween the adsorbate molecule and the pore/channel (in achiralMIL-47 and MIL-5349,51–53) creates notable confinement effect,promoting the formation of intermolecular hydrogen bonds.Even with the aid of computation modelling, it is challengingto predict a very detailed molecular knowledge of the system.Neither a chiral MOF can necessarily separate small isomersnor is an achiral MOF unselective towards some mixtures ofenantiomers, as summarised by the computation predictions byvan Erp and co-workers.49,51

The key factors to incorporate chiral functionality intoporous materials are (a) the site-isolation effect and (b) theconfinement restraints exerted by the rigid frameworks. Giventhe low-cost and high abundance of achiral zeolites and MOFs,it is valuable to study how to design specific ‘chiral regions’ thatcan offer extensive possibilities and potential for asymmetriccatalysis. We have recently discussed the differential adsorptionproperties of lysine (Lys) by achiral MFI zeolites.54 It has beenascribed to the difference in the local adsorption geometries ofL- and D-Lys on the zeolitic Brønsted acid sites (BASs), as revealed

by combined thermogravimetric analyses (TGA) and the Rietveldrefinements of the high-quality synchrotron X-ray powder dif-fraction (SXRD) data.

2. Enantiospecificity in achiral zeolites

This work will further investigate the enantiospecificity inachiral zeolites with MFI and MOR topologies. Circular dichroism(CD) spectroscopy, TGA and the Rietveld refinements of thehigh-resolution SXRD data will be employed to understand theadsorption chemistry. The inherent site-isolation effect andthe confinement restraints will shed light on the future designof asymmetric catalysis reactions over achiral materials.

As presented in Fig. 4, the difference in the Bragg’s peakintensities of the SXRD patterns of L- and D-Lys in MFI (‘MFI-L-Lys’ and ‘MFI-D-Lys’) can be noticeably observed. However,as seen in the normalised intensity of L- and D-Lys in MOR(‘MOR-L-Lys’ and ‘MOR-D-Lys’), no significant difference isobserved, inferring that the adsorbate structures are similar. Thestructural parameters as determined by the Rietveld refinements ofthe SXRD patterns are summarised in Table S1 (ESI†). The unit cellvolume expands by 0.47% and 0.42% upon the adsorption of L- andD-Lys on MFI. It expands to a similar extent upon L-Lys (0.16%) andD-Lys (0.06%) adsorption on MOR. The marginal change in the unitcell volumes can be ascribed to the high rigidity of the zeoliticframeworks that can host Lys without affecting the original T–O–T(where T = Al or Si) covalent arrangements.

The derived crystal structures from the Rietveld refinementsof the high-resolution SXRD data of MFI-L-Lys and MFI-D-Lys arepresented in Fig. 5. The MFI structures have two independentsites, I and II (both Wyckoff letters of 4c, Fig. 5(a) and (b)),whereas MOR-L-Lys and MOR-D-Lys only reveal a binding site,I (Wyckoff letter of 16h, Fig. 5(c) and (d)). The OLys� � �Oframework

and NLys� � �Oframework interatomic distances in both zeolites arein the range between 2.4–3.2 Å, which falls in the typical range ofmolecular adsorption in zeolites. As reported, both L- and D-Lysexhibit an end-on interaction to the protonic BASs (d+) via anoxygen of the a-carboxylate group (d�).54 In MOR, Site I residesalong the major [0 0 1] 12-membered ring (12-MR) straightchannel (channel diameter of 6.5 � 7.0 Å), which agrees withthe adsorption findings by Arletti et al.55,56 Note that MOR isoften represented by isolated cylindrical columns, which alsocontains [0 0 1] 8-MR straight channels but its dimension of2.6 � 5.7 Å is too small to host Lys molecules. Due to theisolated-cylindrical-column system of MOR, the L- and D-Lysadsorbate structures in MOR are highly comparable. On theother hand, the adsorbate structures of L- and D-Lys on MFI arenoticeably dissimilar, where the lysyl side chains are directed todifferent channel exits. It can be ascribed to the unique frame-work structure, with the interconnecting straight-sinusoidalchannel along the [1 0 0] axis. The much more stable conforma-tion of the L-Lys adsorption within MFI has been verified by theDFT calculations.54 In the cases of MOR-L-Lys and MOR-D-Lys,the adsorbate geometries of L- and D-Lys can hardly be affectedby the MOR framework. The combination of site-isolation effect

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(inherent BASs) and confinement restraints (by the zeoliticframework) gives us a much more extensive perspective inunderstanding why MOR does not offer differential adsorptionon Lys.

To reveal the enantiospecific desorption behaviours of Lysfrom MFI, in situ SXRD measurements as a function of tem-perature were conducted. The change in the atomic parametersof L- and D-Lys in MFI at temperature-resolved conditions can

provide a more thorough understanding of the adsorptionchemistry. The in situ synchrotron data were collected onBeamline BL02B2 at SPring-8 (Japan) with the X-ray energyoptimised at 18 keV. The wavelength of the synchrotron X-raywas calibrated as l = 0.689556(2) Å and 2y-zero-point =�0.000015(1)1 using SRM674b CeO2 standard (space group:Fm%3m, a = 5.41153(30) Å). The temperature-programmeddesorption conditions were applied on the capillary samplesusing an N2 gas flow device. From 25 to 500 1C (Fig. 6), in situSXRD profiles with different shapes can be noted. Taking thefirst peak at around 3.51 (merged Bragg (101) and (011) reflec-tions) as an example, the profile dips at around 225–250 1C. Anupward trend is typical due to the removal of adsorbate speciesfrom the microporous frameworks. Notably, the mergingof (200) and (020) at elevated temperatures were observed inthe in situ SXRD profile of MFI-D-Lys. The variations in thecrystallographic parameters are presented in Fig. S2(a)–(d)(ESI†). The unit cell volume decreases gradually at elevatedtemperatures, signifying the removal of L-/D-Lys adsorbate formthe host. For MFI-L-Lys, the unit cell shrinks by 0.23% (from5403.29 Å3 to 5391.27 Å3) from 25 to 500 1C; for MFI-D-Lys, theunit cell shrinks by 0.21%.

The Rietveld refinements of the in situ synchrotron datawere applied to study the change in the site occupancy factors(SOFs) of Sites I and II with respect to the sample temperature.Rigid body Z-matrices were employed to describe L- and D-Lysspecies within the zeolite hosts. The variations in the atomicparameters from the Rietveld refinements are presented inFig. S2(e) and (f) (ESI†). The changes in SOFs show general

Fig. 5 The crystal structures from the Rietveld refinements of the synchrotrondata of (a) MFI-L-Lys, (b) MFI-D-Lys, (c) MOR-L-Lys and (d) MOR-D-Lys. Twoadsorption sites are observed in MFI at the interconnecting straight-sinusoidalchannel region, whereas only one adsorption site is observed in MOR along thecylindrical channel. The closer interatomic distances between the frameworkand L- and D-Lys are determined as 2.4–3.2 Å. For clarity, hydrogen atoms andthe mirror symmetry of the Lys adsorbates are disregarded. Rietveld refinementdetails are given in the Experimental section of ESI.† The atomic parameters aregiven in Table S2 (ESI†). A comparison of the straight-channel-only structure ofMOR (orthorhombic) and the interconnecting straight-sinusoidal-channelstructure of MFI (orthorhombic). Models generated by ZEOMICS by First et al.59

Fig. 4 SXRD patterns and the structural profiles of (a and b) MFI (H-ZSM-5, SiO2:Al2O3 = 46) and (c and d) MOR (H-MOR, SiO2:Al2O3 = 25) pre-adsorbedwith L- or D-Lys measured at 25 1C. The experimental data (open red circles), fitted results (black lines) and the differences (grey traces) are shown. Thecorresponding normalised SXRD patterns are shown, with noticeable difference in the peak intensity in MFI but not in MOR. The SXRD patterns andstructural profiles of pristine MFI and MOR are presented in Fig. S1 (ESI†).

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decreasing trends with two apparent regions of desorptionbehaviours. In both SOF curves, the slopes change consistentlyat ca. 225–250 1C. This agrees with our TGA findings that thesharpest differential peak of each sample lies at ca. 235 1C(discussed later). Interestingly, the SOF of Site I in MFI-D-Lysdecreases at a much slower rate beyond 250 1C than thatobserved in MFI-L-Lys; we can also observe a similar patternin the variation in the unit cell volume.

We also employed CD spectroscopy and TGA to verify ourcrystallographic findings. CD, the most common technique tostudy the chirality of different species, can directly measure theextent of polarised light passing through a sample. As seen inFig. S3 (ESI†), upon sequential additions of MFI zeolites, thechanges in the normalised CD intensities of the L- and D-Lysdiffer by 18% (cf. k = �0.0648 (for L-Lys) vs. k = �0.0794 (forD-Lys)). In contrast, the corresponding changes upon the additionof MOR zeolites are alike (cf. k = �0.0997 vs. k = �0.0951). OurTGA data also reveal a similar extent of differential adsorption ofLys by MFI and MOR (Fig. S4, ESI†). From the TGA of Lys fromMFI, the first desorption behaviour peaks at around 160 1C,followed by a sharp desorption peak at around 235 1C. The peakdesorption temperature of L-Lys is 11.3 1C higher than that ofD-Lys from MFI. For the adsorption of Lys by MOR, the TGA resultsare consistent with the CD findings, with comparable peakdesorption temperature (cf. 206.8 1C vs. 207.7 1C). Based on theTGA results as presented in Fig. S4 (ESI†), the D-/L-Lys ratio ofadsorption by MFI was determined to be 1 : 1.12 (cf. 73 mgL-Lys pergMFI and 82 mgD-Lys per gMFI). The D-/L-Lys ratio of adsorptionby MOR was determined to be 1 : 1 (cf. 69 mgL-Lys per gMOR and69 mgD-Lys per gMOR). The difference in the quantity adsorbedsuggests that an obvious differential adsorption behaviour ofD-/L-Lys only over MFI. It should be noted that these calculations

were based on the weight difference from 100 1C as measured inTGA, where the weight loss below 100 1C was disregarded that canbe primarily caused by physisorption.

The presence of ‘chiral regions’ can be ascribed to the site-isolation effect of the protonic BASs and the confinementrestraints by the MFI framework. We further testify this postu-lation by incorporating an extra-framework adsorption site byion-exchange with Cu(NO3)2. As presented in Fig. S5 (ESI†), thepowder X-ray diffraction patterns of Cu2+-exchanged MFI-L-Lysand MFI-D-Lys show noticeable differences in the Bragg’s peakintensities, which is consistent with the synchrotron data onthe undoped MFI collected in SPring-8. The structural para-meters are summarised in Table S5 (ESI†). From the Rietveldrefinement of the diffraction data, the content of Cu2+ per unitcell of H-ZSM-5 was determined as 1.89(3). It suggests thatabout a low concentration of BASs (about 0.2 per unit cell) wereremaining after ion-exchange, by taking the structural formulaof the pristine zeolite sample (H4Al4Si92O192) into account. Theunit cell volume decreases marginally by 0.015% and 0.058%respectively. Interestingly, the first peak desorption tempera-ture from the differential TGA of the two samples doped withCu2+ become 204.0 1C and 204.2 1C (see Fig. S6, ESI†), suggest-ing almost identical sorption properties. It contrasts with thesamples without Cu2+ doping, where the first peak desorptiontemperatures differ by 11.3 1C. Without high-temperaturecalcination treatment, the extra-framework hydrated metal ionstypically locate about 3 Å away from the framework O atoms.5,57,58

This provides a much less confined environment for Lys adsorptionon the Lewis acidic Cu2+ ions, where the lysyl side chains of bothL- and D-Lys possess more spatial freedom. The ‘strain-free’ adsorp-tion environments on the Lewis acidic Cu2+ sites have almosteliminated the strain as observed over the BASs in the pristineMFI samples. Based on previous computational findings, theasymmetric positioning of the extra-framework cationic sites (suchas Ca2+) in unique framework structures can create chiral environ-ments due to the difference in channel exits.49 Despite that we havereported a ‘strain-free’ environment for the adsorption of lysinemolecules by the doping of Cu2+ in H-ZSM-5, this experimentalresult has inferred the realisation the rational tuning of theenantiospecific properties of microporous structures.

3. Conclusion and outlook

In summary, we have revealed the enantiospecific adsorbatestructures of L- and D-Lys in zeolites with the MOR and MFItopologies. The difference is originated from the host zeolitestructures. In particular, the adsorption sites at the intercon-necting straight-sinusoidal channel framework of MFI renderspecific adsorption geometries with respect to the chirality ofL- and D-Lys. Whereas, in the straight-channel-only MOR structure,the corresponding adsorption geometries are similar. Both theinherent site-isolation effect of the BASs and internal confinementrestraints are critical for creating ‘chiral regions’ that leads toenantioselective adsorption behaviours (Scheme 1). While achievinghighly enantioselective reactions over heterogeneous catalysts

Fig. 6 The in situ SXRD-temperature-programmed desorption measurementsand the corresponding contour plots of (a) MFI-L-Lys and (b) MFI-D-Lys.Apparent differences in the shape of the desorption profiles can be seen.The crystallographic and atomic parameters are summarised in Tables S3and S4 (ESI†).

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is challenging, the advantages of using solid-state materials forgas/liquid phase reactions should be recognised. This funda-mental understanding and insights will help the rational designof enantiospecific sites in both achiral and chiral materials formore selective asymmetric catalysis reactions. For instance, it ispromising to introduce the site-isolation effect in the channel ofporous materials by utilising the metal-doping strategy with theintrinsic confinement effect by the channels.

In the near future, the application of porous material can beextended to asymmetric catalysis based on the rational designof ‘chiral region’ in achiral solid-state materials. We believethat the product selectivity (in terms of e.e.) in asymmetriccatalysis can be enhanced by establishing active sites with well-fitted structural specificity for the adsorption and stabilisationof certain reaction substrates and intermediates (or transitionstates) over another stereoisomer. Elucidating the adsorbatestructures of enantiomeric pairs on the cationic sites will thusbe pivotal for the utilisation of the ‘chiral regions’. It will offer arational guide towards where the cationic sites should beplaced with respect to the microporous frameworks for theoptimisation of adsorption geometries in asymmetric catalysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Hong Kong Research Grants Council (PolyU253009/18P) and the National Natural Science Foundation ofChina (21902139) for financial support. We thank SPring-8(2018B1081), Diamond (NT23230-1) for the provision of beam-time, and UMF, UCEA and ULS of HKPU for the support inmaterial characterisation.

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Scheme 1 A schematic illustration of the chiral region for enantiomerdifferential adsorption in zeolites with the MFI topology.

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