Luis
Gómez-Hortigüela
*a,
Álvaro
Mayoral
bcd,
Haining
Liu
a,
Laura
Sierra
a,
Laura
Vaquerizo
a,
Cristina
Mompeán
a and
Joaquín
Pérez-Pariente
a
aInstituto de Catálisis y Petroleoquímica, ICP-CSIC, C/Marie Curie 2, 28049, Madrid, Spain. E-mail: lhortiguela@icp.csic.es
bInstitute of Materials Science of Aragon (ICMA), CSIC-University of Zaragoza, 12, Calle de Pedro Cerbuna, 50009 Zaragoza, Spain
cLaboratorio de Microscopias Avanzadas (LMA), University of Zaragoza, Spain
dCenter for High-resolution Electron Microscopy (CħEM), School of Physical Science and Technology, ShanghaiTech University, 393 Middle Huaxia Road, Pudong, Shanghai 201210, China
First published on 23rd June 2020
In this work, we perform an in-depth experimental and computational study about the structure-directing effect of two new chiral organic quaternary ammonium dications bearing two N-methyl-prolinol units linked by a xylene spacer in para or meta relative orientation, displaying four enantiopure stereogenic centers in (S) configuration. Synthesis results show that the para-xylene derivative is an efficient structure-directing agent, promoting the crystallization of ZSM-12 (in pure-silica composition), beta zeolite (as pure-silica, or in the presence of Al or Ge), and a mixture of polymorphs C, A and B of zeolite beta (in the presence of Ge). In contrast, the meta-xylene derivative showed a much poorer structure-directing activity, yielding only amorphous materials unless Ge is present in the gel, where beta and polymorph C (together with A and B) zeolites crystallized. Molecular simulations showed that the para-xylene dication displays a cylindrical shape suitable for confining in zeolite pores, while the meta-xylene derivative has an angular shape that shifts from the typical dimensions required for 12MR zeolite channels. Despite enantio-purity of the para-xylene dication with (S,S,S,S) configuration, no enrichment in polymorph A of the zeolite beta samples obtained was observed by Transmission Electron Microscopy. With the aid of molecular simulations, the failure in transferring chirality to the zeolite is explained by the loose fit of this SDA in the large-pores of zeolite beta, and a lack of close geometrical fit with the chiral element of polymorph A, as evidenced by the very similar interaction of the cation with the two enantiomorphic space groups of polymorph A. Nevertheless, the molecular-level knowledge gained in this work can provide insights for the future design of more efficient SDAs towards the synthesis of chiral zeolites.
Confinement effects associated to zeolite microporous frameworks enable the discrimination between guest species (sorbates, reactants, transition states or products) with small steric differences.5 This has been widely exploited in catalytic uses, but it is also essential during the synthesis of these materials where guest extra-framework species are confined within the zeolite pores and cavities during the crystallization process.6 In particular, the addition of organic cations with particular geometric properties (size and shape) to the zeolite synthesis gels has enabled to gain control on the zeolite porous architecture that crystallizes through host–guest chemistry and, in turn, confinement effects. These organic species, which are usually referred as structure-directing agents (SDA), direct the crystallization process towards a particular framework type through a geometric relationship between the size and shape of the organic species and that of the porosity of the zeolite framework.7–10
One of the greatest challenges in zeolite science is the development of enantiomerically pure chiral zeolites or at least enriched in one of the two enantiomorphic crystals.11–13 These chiral zeolites should be able to perform enantioselective operations, both in adsorption and catalysis processes, because of an asymmetric confinement of guest species in the chiral pores and/or cavities.14–18 Indeed, several chiral zeolite frameworks do actually exist.12,19–23 However, they usually crystallize as mixtures of crystals with the two handednesses, either as racemic mixtures of enantiopure crystals (like STW, that can crystallize in P6122 or P6522 enantiomorphic space groups)24 or as intergrown polymorphs of chiral frameworks, like zeolite beta where polymorph A is chiral (and can crystallize in P4122 or P4322 space groups).19 Once again, confinement in a restricted space is crucial for developing enantio-discriminating properties, not only for potential applications but also during crystallization in the presence of organic SDAs. In this context, the usual host–guest geometrical relationship between organic SDAs and zeolite frameworks provides a straightforward tool to promote the crystallization of chiral zeolite frameworks through the use of chiral organic species as SDAs.11,25 For this imprint of chirality to occur, a true template effect, in the sense of establishing a close geometrical relationship between the guest molecular shape and the host framework walls, should be established. Although this strategy has been used for very long, only very recently a single successful example of enantio-enrichment of a chiral zeolite (STW) through the use of a rationally-designed chiral organic SDA has been reported.26 In this context, an in-depth knowledge at molecular level of the structure-directing role played by chiral SDAs during crystallization of zeolite frameworks is vital for successfully promoting a transfer of chirality from organic guests to the zeolite host framework.
In recent years, we have successfully studied several organic SDAs prepared from chiral precursors derived from the chiral pool, on the one hand from chiral alkaloids (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine,27–32 and on the other from L-prolinol (derived from L-proline amino acid).33–38L-Prolinol is a useful chiral precursor since it provides a rigid ring with an N atom that can be quaternized with two different alkyl substituents, providing an additional stereogenic centre. Interestingly, a careful selection of the synthesis protocol during the alkylation reactions enables a preferential attack by one particular side of the molecule, leading to enantiopure stereogenic N atoms in addition to the enantiopure C atoms of the original L-prolinol units.33 In previous works, we studied the structure-directing effect of N-methyl-N-benzyl-prolinol, and observed the crystallization of several zeolite materials, including MTW,33 MWW38 and FER frameworks,36 as a function of the synthesis conditions. Another report with similar but smaller prolinol derivatives with methyl and ethyl substituents showed the production of layered precursors of CDO zeolite.39
In order to maximize the transfer of chirality to zeolite frameworks, it is essential to adapt the chiral dimension of the organic SDAs, which is expressed at a molecular level, to that of the zeolite frameworks, which is usually expressed at a long-range level in the form of helicoidal channels. Based upon these grounds, in this work we build new SDAs based on two L-prolinol units linked by a xylene ring in different positions, resulting in large SDAs with four enantiopure stereogenic centres that should enhance the asymmetry of the organic species (Scheme 1). In order to understand from a molecular level the structure-directing activity of these SDAs, we perform a combined experimental and computational study of the zeolite materials obtained.
Synthesis of pDMDPx and mDMDPx was carried out by alkylation of (S)-2-pyrrolidinemethanol (L-prolinol) with α,α′-dibromo-para-xylene or α,α′-dibromo-meta-xylene, respectively. In a typical synthesis of pDMDPx, 10.00 g of (S)-2-pyrrolidinemethanol were carefully added to a cooled solution of 13.05 g of α,α′-dibromo-para-xylene in 350 mL of acetonitrile with 20.50 g of potassium carbonate (careful, exothermic reaction). The reaction mixture was kept refluxing overnight, after which the inorganic solids were removed by filtration, and the solvent was rotoevaporated, yielding a yellowish solid (14.30 g, yield 95%). 13C NMR (CDCl3): 23.4; 27.8; 54.5; 58.3; 61.9; 64.3; 128.7; 138.2.
13.70 g of this solid were dissolved in 300 mL of cooled acetonitrile, and 12.80 g of methyl iodide were added dropwise (careful, exothermic reaction). The mixture was kept at room temperature for 5 days, after which the solvent was rotoevaporated, and the obtained yellow oil was washed with diethyl ether. The resulting product was 1,1′-(1,4-phenylenebis(methylene))bis(2-(hydroxymethyl)-1-methyl-pyrrolidin-1-ium iodide (pDMDPx+I−) (20.70 g, yield 91%). 13C NMR (D2O): 18.9; 23.5; 42.5; 58.8; 64.3; 67.2; 74.5; 130.4; 133.5.
In order to confirm the production of the (S,S,S,S)-isomer, the organic synthesis was also carried out by reverting the order of alkylation, first adding a methyl group to L-prolinol through the Leucart reaction, and then adding the dibromo-para-xylene derivative. In this case, we obtained a product where 13C NMR signals corresponding to the C atoms directly attached to N were doubled (Fig. S1†), evidencing the presence of the two isomers, (S,R,S,R) and (S,S,S,S), and confirming the isomeric purity of our original product.
The corresponding derivative in meta-position, (1,1′-(1,3-phenylenebis(methylene))bis(2-(hydroxymethyl)-1-methyl-pyrrolidin-1-ium iodide (mDMDPx+I−), was obtained in the same way using the α,α′-dibromo-meta-xylene derivative. 13C NMR (D2O): 18.9; 23.5; 42.4; 58.8; 64.3; 67.3; 74.6; 129.1; 130.1; 134.9; 136.8.
The hydroxide forms of the quaternary ammonium iodide salts were obtained by ion exchange with an anionic resin (Amberlite IRN-78; exchange capacity, 4 meq g−1; Supelco), and the hydroxide solutions were concentrated to ∼30 wt%.
Electron microscopy analyses were carried out in a cold FEG JEOL GrandARM 300 operated at 300 kV. The microscope was equipped with a double spherical aberration (Cs) corrector from JEOL Company. Images were recorded under low-dose conditions to minimize the electron beam damage using an annular dark field detector (ADF). Prior to observation, the samples were deeply crushed using mortar and pestle dispersed in ethanol and few drops of the suspension were placed onto holey carbon copper grids.
Calculation of the NMR chemical shielding of the different isomers was carried out with the gauge-including projector augmented-wave method (GIPAW) developed by Pickard and Mauri,43 as implemented in the CASTEP code, using a σref value of 176 ppm, the same as in our previous works.29,32,44
The conformational behaviour of the SDA cations in water was studied by NVT Molecular Dynamics simulations, in the same way as reported in our previous work.35 8 SDA cations, 16 Cl− anions (for charge-compensation) and 160 water molecules were included in the simulation cell, and 10 ns of MD simulations in NVT ensemble were run at 423 K.
SDA | R | H2O | Temperature (°C) | Time (days) | Phase |
---|---|---|---|---|---|
pDMDPx | 0.25 | 5.0 | 130 | 10 | MTW |
150 | 5 | MTW | |||
150 | 10 | MTW | |||
3.0 | 130 | 10 | BEA + MTW | ||
175 | 10 | MTW + Am | |||
1.0 | 130 | 7 | BEA | ||
0.125 | 1.0 | 130 | 7 | MTW + Am | |
mDMDPx | 0.25 | 5.0 | 130 | 10 | Am |
150 | 5 | Am | |||
150 | 10 | Am | |||
2.0 | 130 | 6 | Am | ||
150 | 6 | Am | |||
180 | 6 | Am |
SDA | Si/Al | R | H2O | Temperature (°C) | Time (days) | Product |
---|---|---|---|---|---|---|
pDMDPx | 15 | 0.25 | 5.0 | 130 | 10 | BEA |
150 | 5 | BEA | ||||
150 | 14 | BEA | ||||
15 | 0.125 | 5.0 | 130 | 14 | Am | |
150 | 14 | Am | ||||
15 | 0.25* | 5.0 | 130 | 13 | Am | |
150 | 6 | Am | ||||
150 | 13 | Am | ||||
5 | 0.25 | 5.0 | 130 | 13 | Am | |
150 | 5 | Am | ||||
150 | 13 | Am | ||||
22 | 0.25 | 5.0 | 130 | 13 | BEA | |
30 | 0.25 | 5.0 | 130 | 13 | BEA | |
30 | 0.25 | 3.0 | 130 | 9 | BEA | |
40 | 0.25 | 5.0 | 130 | 13 | Am | |
mDMDPx | 15 | 0.25 | 5.0 | 130 | 10 | Am |
150 | 5 | Am | ||||
150 | 10 | Am | ||||
15 | 0.25 | 2.2 | 150 | 6 | Am |
In order to limit the crystallization of zeolite beta in an attempt to promote the crystallization of polymorph A, the SDA concentration was reduced (0.125); however, this prevented the formation of zeolite beta (Table 2). Similarly, an increase of the HF concentration also impeded the crystallization of zeolite beta. An increase of the amount of Al (Si/Al = 5) resulted in no crystalline products. In contrast, under the typical SDA and H2O contents (0.25 and 5 molar compositions, respectively), zeolite beta was able to crystallize up to a Si/Al ratio of 30, but a further reduction of the Al content (Si/Al = 40) prevented its formation.
In line with the previous result for pure-silicate composition, the conditions that promoted the crystallization of highly crystalline zeolite beta with pDMDPx did not result in any crystalline material when the SDA was the meta-derivative (mDMDPx), further evidencing the poor structure-directing ability of this cation.
SDA | Si/Ge | H2O | Temperature (°C) | Time (days) | Product |
---|---|---|---|---|---|
pDMDPx | 1 | 5.0 | 150 | 7 | BEC |
150 | 14 | BEC | |||
175 | 7 | BEC + AST | |||
5 | 5.0 | 150 | 7 | BEC | |
150 | 14 | BEC | |||
175 | 7 | BEC | |||
5 | 3.0 | 150 | 7 | BEC | |
175 | 7 | BEC + AST | |||
15 | 5.0 | 150 | 7 | BEA* | |
175 | 7 | BEA* | |||
30 | 5.0 | 130 | 10 | BEA | |
150 | 7 | BEA | |||
150 | 14 | BEA | |||
mDMDPx | 5 | 5.0 | 130 | 11 | BEC |
30 | 5.0 | 130 | 11 | BEA | |
40 | 5.0 | 130 | 10 | BEA | |
30 | 3.0 | 130 | 11 | BEA | |
15 | 2.7 | 130 | 6 | BEA* | |
150 | 6 | BEA* | |||
30 | 2.0 | 130 | 6 | Am | |
150 | 6 | BEA | |||
5 | 1.0 | 150 | 6 | BEC + BEA |
We then analysed the structure-directing ability of mDMDPx. In contrast to previous cases where no crystalline products were observed with this SDA, polymorph C of zeolite beta (together with some minor amounts of polymorphs A and B) was obtained with a Si/Ge ratio of 5 with high water content (H2O = 5), while an increase of the concentration (to 1H2O) resulted in the crystallization of a mixture of the three beta polymorphs (Fig. 1E). As with pDMDPx, an increase of the Si/Ge ratio beyond 15 results in the crystallization of zeolite beta (BEA), being more crystalline the samples obtained with a lower water content (Fig. 1F). Hence, only in the presence of Ge did the less efficient mDMDPx SDA enabled the crystallization of zeolite materials related to beta.
The chemical integrity of the SDAs occluded within the different zeolite materials was studied by 13C CP MAS NMR of the solid samples (Fig. 2). All the zeolite materials obtained with pDMDPx (left) showed several bands corresponding to the different C atoms of the pristine cation (as represented by the iodide salt in D2O solution), confirming the integral incorporation of the cation in the solids. Nevertheless, some additional bands with lower intensity between 115–120 ppm and at ∼30 ppm reveal the presence of minor amounts of other organic fragments in certain samples, probably as a result of a partial decomposition; this is particularly true in zeolite beta obtained in the presence of Al and in polymorph C (blue and green lines). Similarly, 13C NMR spectra of the solids obtained with mDMDPx also showed the presence of all the bands characteristic of the cation (Fig. 2-right), again evidencing the integral incorporation of this cation within the zeolite beta-related materials. The same bands corresponding to degradation products are also observed in these samples, especially in BEA. Overall, these results demonstrate the incorporation of pDMDPx and mDMDPx in the zeolite frameworks.
The BET surface area and porosity of the sample of zeolite beta crystallized from gels with Si/Al = 15 by using pDMDPx were determined in order to compare with those of zeolite beta samples synthesized from conventional templates. The micropore volume was 0.17 cm3 g−1 and the total surface 621 m2 g−1, of which 408 m2 g−1 corresponds to micropores and 213 m2 g−1 to external surface area. These results are comparable to those of Al-containing beta zeolites prepared from a conventional template, namely tetraethylammonium hydroxide, taking into account the well-known observed trend for the micropore volume of zeolite beta and other zeolites to decrease as the external surface area increases, being the relative proportion of micropore-to-external surface a function of crystal size and specific synthesis parameters.54,55
Selected samples obtained with pDMDPx were studied by electron microscopy with the intention of obtaining some insights of the materials formed. In the case of Al-containing beta zeolite (obtained after 14 days at 150 °C with a Si/Al ratio of 15), Fig. 3a presents the high-resolution Cs-corrected STEM-ADF image of a typical crystallite, which already proves the excellent crystallinity of the sample and where the existence of the structural defects is evidenced as a consequence of the intergrowth of both polymorphs of zeolite beta, polymorphs A and B. The Electron diffraction (ED) pattern (inset) obtained from the same crystal exhibits well-defined spots together with diffuse lines along c* axis produced as a consequence of the mixture of both polymorphs. Such types of images allowed the calculation of the relationship of both polymorphs on different zeolite crystallites, obtaining a ratio for this Al-beta sample of 40% of polymorph A and 60% of B. The stacking sequence is marked showing the zig-zag pillaring of PA in yellow, while the regions of polymorph B are marked by straight red lines.
In the case of Ge-containing materials obtained in the presence of pDMDPx, samples prepared with different Ge amounts (with 5 H2O molar ratio, Table 3) were analysed. For materials with a low Ge content (Si/Ge = 30), an excellent crystallinity was observed, displaying the typical morphology for zeolite beta, as shown in Fig. 3b. This image allows the observation of several zeolite crystallites with truncated octahedral shaped particles of few hundreds of nm (the atomic resolution data of the framework, again along ‘b’ axis, is shown in the inset). As for the previous case, the stacking sequence can be also followed if the data is sufficiently good to distinguish the different polymorphs (Fig. 3c). In this sample, a mixture of 50% of polymorph B, 49% of A and 1% of C was observed (Fig. 3c uses the same colour code to denote the polymorphs, including green lines for polymorph C).
Very similar results were also obtained when the amount of Ge was increased to a Si/Ge of 15 (Fig. 3d and e), obtaining a product with very good crystallinity where the predominant polymorphs were A and B (Fig. 3d). In this case, polymorph A was found to be 42% of the sample, while B was 54% with a slight increment of polymorph C (3%) (Fig. 3e). In conclusion, no evidence of an enrichment in polymorph A is observed in the Ge-containing beta materials, while minor amounts of C-stacking sequences are observed, evidencing that the slightly increased peaks at 7 and 9.7° observed previously in the XRD patterns (Fig. 1D) are due to minor amounts of polymorph C.
Increasing the amount of Ge to Si/Ge = 5 made a significant influence on the structure and morphology of the materials obtained. Fig. 3f shows the low magnification image of few particles representative of this material. The first difference in comparison with the previous zeolites is the particle size, which was much smaller, obtaining crystallites of sizes under 100 nm. The atomic resolution observation is depicted in Fig. 3g, corresponding to a zeolite nanoparticle of ∼50 nm with the different polymorphs marked. Interestingly, the increase of polymorph C is evident in this micrograph, reaching up to 18%, being 44% and 38% for A and B, respectively. A closer look of the occurrence of the three polymorphs is displayed in Fig. 3h, showing how the three polymorphs stack onto each other. From these observations, we conclude that Ge has a big influence on the final material obtained, where high Ge contents increases the amount of polymorph C while reducing the crystal size.
In terms of the relative crystallinity of the materials, no significant differences were observed for beta zeolites obtained with Si/Al = 15, Si/Ge = 30 or Si/Ge = 15, presenting in every case excellent crystallinities without the observation of amorphous phases. Despite no amorphous material was visualized for Si/Ge = 5 either, the crystallinity was not as good as for the others, although this aspect could be related to the small size of the particles obtained and maybe the entire structure was not completely formed along the particle. Nevertheless, most of the particles observed displayed a reasonable well-defined framework.
Isomer | C7 | C8 | C11 | C10 | C6 | C5 | C9 | C1 | C2 | |
---|---|---|---|---|---|---|---|---|---|---|
Theor. | SSSS | 21.4 | 25.4 | 38.7 | 68.3 | 70.3 | 76.2 | 84.9 | 138.0 | 140.0 |
SRSR | 22.2 | 25.8 | 53.4 | 67.8 | 66.0 | 58.3 | 86.4 | 136.8 | 140.4 | |
Exper. | 1 | 18.9 | 23.5 | 42.5 | 58.8 | 64.3 | 67.2 | 74.5 | 130.4 | 133.5 |
2 | 19.3 | 23.8 | 49.6 | 58.0 | 58.4 | 61.9 | 78.1 | 130.0 | 133.4 |
Energy results for pDMDPx showed that the most stable conformation in vacuum was for a C11–N–C5–C1 torsional angle of ∼180°, where methyl C11 and phenyl ring are in anti configuration (labelled as pDMDPx-opp-180180), followed closely by conformers with torsional angles of ∼60°, where they are in syn configuration, having a relative energy of 0.8 kcal mol−1 (labelled as pDMDPx-opp-6060) (both cases with prolinol units in opposite sides) (Fig. 4-top and middle and Table S1†). Similar results were found for mDMDPx, with the same two most stable conformers with relative energies of 0.0 and 0.2 kcal mol−1, respectively (Fig. S3†).
Due to the similar stability of these two conformations (opp-180180 and opp-6060), we studied their relative occurrence in aqueous solution in order to analyse which one will be responsible for the structure-direction of zeolite materials. SDA cations were initially arranged in the most stable conformation in vacuum (opp-180180), and were then allowed to relax in the presence of water. Fig. S4† shows the torsional angle distribution at the beginning (0–0.5 ns time interval, left) and at the end of the simulation (7.5–10 ns, right) (top: pDMDPx; bottom: mDMDPx). We can clearly observe that both cations tend to shift from opp180180 towards opp-6060 conformations in aqueous solution, despite the former being slightly more stable in vacuum. These MD results suggest that the opp-6060 conformations will dominate in aqueous solution and, consequently, the molecular shape of these conformers will determine the porosity of the resulting zeolite materials.
We then analysed the geometrical properties of the two cations in their most stable conformations (opp-6060) by running 1 ns of MD NVT simulations (at 25 °C) in vacuum, and calculated the molecular shadow lengths during these MD simulations (Fig. S5†). Results showed that pDMDPx displays a roughly cylindrical shape, with two similar dimensions centred at 7.3 and 8.5 Å and a large molecular axis of 16.5 Å, while mDMDPx displays a more elliptical shape with dimensions of 7.3 and 9.6 Å, and a slightly shorter molecular main axis of 15.2 Å.
Finally, packing of pDMDPx cations along the MTW channels can take place with adjacent dications in the same orientation, or rotated consecutively by 180° (Fig. S6†). Energy results showed a higher stability for the latter case by 3.1 kcal mol−1 SDA because of a better packing of the pyrrolidinium rings (Fig. 5-bottom), giving a packing value of 1.33 SDAs per u.c.
We started by loading one SDA in 4 × 1 × 1 BEA supercells, i.e. neglecting packing interactions. After an extensive search by simulated annealing calculations, we found one particularly stable location for pDMDPx along the BEA channels of both enantiomorphic polymorphs (Fig. 6); pDMDPx dications site aligned with the [100] (or the equivalent [010]) channels, with the bulkier prolinol units spanning consecutive channel intersections, displaying the opp-6060 conformation. Despite enantiopurity of pDMDPx, we found a very similar stability of the SDA in the two chiral enantiomorphic polymorphs, with interaction energies of −163.85 and −164.00 kcal mol−1 in P4122 and P4322 frameworks, respectively; such energy difference was very similar when energies were averaged from MD simulations (<0.3 kcal mol−1). This lack of chiral recognition might be associated to the chirality of BEA being imposed by the stacking sequence of bea sheets along the ‘c’ axis, while pDMDPx SDAs tend to site aligned with the [100] and [010] channels, thus not expressing its chirality in the appropriate orientation for the BEA framework to feel it. Therefore, these SDAs should not impose any particular stacking sequence with +90 or −90° rotation of sheets along ‘c’, and therefore a random sequence would be expected, resulting in the crystallization of zeolite beta with the typical mixture of polymorphs A (in the two space groups) and B. This is in line with our experimental observations where no particular enrichment of this zeolite beta was observed by XRD or STEM.
However, worth is noting that the location of hydroxyl groups in the dication follows always (in the two enantiomorphic frameworks of BEA) an asymmetric orientation where they rotate ∼45° in clockwise direction, regardless of the polymorph (Fig. 6-right); this particular orientation is imposed by the handedness of (S,S,S,S)-pDMDPx. If one assumes that the location of Al (which generates a negative framework charge) could be at some extent directed by these polar hydroxyl groups through the formation of H-bonds, an asymmetric orientation of these active sites could be expected, what could potentially lead to asymmetric catalysts. At present, this is just a mere hypothesis, but work is in progress in order to study this issue.
We next studied the packing of pDMDPx along the [100] (or [010]) channels in an attempt to analyse if a potential asymmetric packing arrangement could lead to an enantio-differentiation of polymorphs. Imposing the condition of maximum load of organic cations while preventing too short contacts, we estimated that 3 SDAs could be loaded in 4 u.c. along each channel. Such packing arrangement gives a total SDA loading of 3 SDAs per u.c. (there are 4 channels per u.c.), corresponding to an organic content of 21%, which is in line with the values obtained by TGA in the different zeolite beta materials. In this fully-loaded systems, we found interaction energies of −190.91 and −190.98 kcal mol−1 of SDA for pDMDPx in P4122 and P4322 frameworks, respectively, being extremely similar, as occurred in the absence of packing interactions; Fig. 7 (top) shows the final arrangement of pDMDPx cations in P4322 framework. These energy results evidence that the packing of pDMDPx cations in the BEA channels does not impose any asymmetric arrangement. Indeed, RDFs (during MD simulations) between H atoms of the SDAs and framework O atoms (Fig. 7-bottom) show a very similar profile for the two diastereomeric pairs, evidencing that the host–guest fitting in the fully-loaded systems is very similar for (S,S,S,S)-pDMDPx in the two BEA chiral frameworks. Again, this would explain the non-prevalence of a particular stacking sequence of bea sheets in the presence of this cation. The lack of such host–guest chiral recognition might be associated to the looseness of the host–guest fit, given the high porosity and large size of BEA channels and the limited geometric fit with the molecular shape of pDMDPx, as indicated by the large distance between framework O and pDMDPx H atoms (higher than 3 Å, Fig. 7-bottom).
Despite having 4 enantiomerically-pure stereogenic centres, (S,S,S,S)-pDMDPx does not display enantioselective discrimination properties between the two enantiomorphic BEA chiral frameworks. This might be associated to the small size of the substituents attached to the stereogenic centres (C and N, with methyl and hydroxymethyl groups, respectively), which promotes small asymmetric shapes that are not sufficient to be imprinted on the stacking of bea sheets due to the loose host–guest fit. For this reason, we studied the effect of having larger substituents attached to N; in particular, we analysed the interaction energy difference in the two frameworks of a related cation but where the methyl substituents attached to N were replaced by ethyl groups (pDEDPx). In this case, the larger size of ethyl groups involved that these pointed to the perpendicular adjacent channels along the ‘c’ axis (which determines the stacking sequence) (Fig. S7†). As a consequence, the interaction energy difference of (S,S,S,S)-pDEDPx dications in P4122 and P4322 frameworks notably increased from 0.15 (for pDMDPx) to 1.12 kcal mol−1. Although this evidences the validity of this argument, the energy difference between the two chiral polymorphs is still not very high. In any case, the arrangement of these SDAs with ethyl groups partially filling the perpendicular adjacent channels (Fig. S7†) could potentially disrupt an efficient loading of those channels, what might prevent an effective crystallization of zeolite beta with this SDA. Anyhow, unfortunately attempts to experimentally synthesize this cation failed because of the higher steric hindrance of the ethyl substituent.
We finally studied the incorporation of mDMDPx in the two BEA enantiomorphic frameworks. In this case, two different stable orientations of the SDAs were found. In the most stable orientation, mDMDPx cations locate in the intersections spanning perpendicular [100] and [010] channels, roughly aligned with the ‘c’ axis, which is a consequence of the angular shape of this cation (compared with the linear shape of pDMDPx) (Fig. 8-top). In principle, this orientation would be interesting since the stacking sequence of bea sheets along ‘c’ determines the chirality of the resulting polymorph. However, again we observed a negligible difference in the interaction energy of mDMDPx with the two chiral frameworks P4122 and P4322, being −213.42 and −213.27 kcal mol−1, respectively (giving an enantio-differentiation energy of 0.15 kcal mol−1). Once again, this might be a consequence of the small size of the asymmetric methyl and hydroxymethyl substituents. Nevertheless, in this case we also observed another stable orientation where mDMDPx located aligned with the [100] (or [010]) channels (Fig. 8-bottom), giving interaction energies of −207.31 and −208.20 kcal mol−1 for P4122 and P4322, respectively; note that the energy difference in this case is slightly higher (0.89 kcal mol−1), though still low. Nevertheless, we note that the conformational energy penalty for this orientation of mDMDPx aligned with BEA channels is higher than in the previous orientation. In any case, once again we did not find any evidence for an enrichment of polymorph A in this zeolite. Replacement of methyl by ethyl substituents did result in an increase of the enantio-discrimination energy (to 1.51 kcal mol−1) (Fig. S8†), although in this case a very low interaction energy was found (−80.72 and −79.21 kcal mol−1 for P4122 and P4322, respectively), evidencing a poor fit of this cation in BEA.
![]() | ||
Fig. 8 Location of mDMDPx in chiral BEA polymorph A with P4122 (left) and P4322 (right) space groups in the two possible orientations, along ‘c’ (top) or aligned with ‘a’ (or ‘b’) channels (bottom). |
We note that the crystallization of zeolite materials in the presence of mDMDPx invariably required the presence of Ge. Therefore, it seems that the main structure-directing agent in this system was provided by Ge rather than by mDMDPx, which might act more as a space-filling agent. The same as for the MTW framework, the low structure-directing efficiency of mDMDPx might be associated to the angular shape of this cation, which shifts away from the cylindrical shape typical of pore-based zeolites.
For the sake of completeness, we also studied the most stable location of these cations in polymorph C (BEC) (Fig. S9†). Similar orientations of the SDAs were observed, with pDMDPx aligned with the [100] (or [010]) channels, and mDMDPx spanning the two types of channels along ‘c’ axis. On the other hand, the interaction energies of each SDA were also very similar as those for BEA.
Because of the presence of the chiral polymorph A in zeolite beta, we analysed the stability of our enantiomerically-pure para-xylene SDA in the two enantiomorphic space groups, yielding a similar interaction energy in both cases. This might be associated to the large size of the BEA zeolite channels and the loose fit of the SDA, which prevents an effective transfer of the asymmetric nature of the chiral SDA to the stacking of beta sheets along the ‘c’ direction that determines the BEA chirality. As a consequence, zeolite beta obtained in this work seems to have the usual proportion of A and B polymorphs. Nevertheless, our results suggest that larger substituents associated to the stereogenic centres should result in larger energy differences between the chiral polymorphs, providing clues about the future design of new chiral SDAs.
Footnote |
† Electronic supplementary information (ESI) available: Fig. S1, Additional liquid 13C NMR, Fig. S2, Thermogravimetric analyses, Table S1, Relative energies, Fig. S3, Additional pictures of molecular structure, Fig. S4, Torsion angle distributions, Fig. S5, Geometric properties of SDAs, Fig. S6, Packing of SDAs in MTW, Fig. S7 and S8, Location of alternative SDAs, Fig. S9, Location of SDAs in BEC. See DOI: 10.1039/d0dt01834a |
This journal is © The Royal Society of Chemistry 2020 |