PST‐24: A Zeolite with Varying Intracrystalline Channel Dimensionality

Abstract Herein we report the synthesis, structure solution, and catalytic properties of PST‐24, a novel channel‐based medium‐pore zeolite. This zeolite was synthesized via the excess fluoride approach. Electron diffraction shows that its structure is built by composite cas‐zigzag (cas‐zz) building chains, which are connected by double 5‐ring (d5r) columns. While the cas‐zz building chains are ordered in the PST‐24 framework, the d5r columns adopt one of two possible arrangements; the two adjacent d5r columns are either at the same height or at different heights, denoted arrangements S and D, which can be regarded as open and closed valves that connect the channels, respectively. A framework with arrangement D only has a 2D 10‐ring channel system, whereas that with arrangement S only contains 3D channels. In actual PST‐24 crystals, the open and closed valves are almost randomly dispersed to yield a zeolite framework where the channel dimensionality varies locally from 2D to 3D.


Introduction
Zeolites and related microporous materials are among the most important classes of industrial catalysts and adsorbents. [1] To discover new zeolite structures with unprecedented shape and/or surface selectivity properties,m any rational synthetic strategies,l ike the long-lasting organic structuredirecting agent (OSDA) design approach, have been devel-oped. [2] Despite the fact that ah uge number of chemically feasible hypothetical zeolite structures have already been proposed, only 252 framework types are included in the Database of Zeolite Structures by the Structure Commission of the International Zeolite Association. [3] This implies that there is still as trong need to develop more effective approaches for synthesizing unprecedented structures,e specially materials with the aluminosilicate composition that show high Brçnsted acidity and structural stability,which are of industrial relevance for catalysis and adsorption.
Since the seminal work of Flanigen and Patton in 1978, [4] the use of fluoride anions first as am ineralizing agent and later on as an inorganic structure-directing agent has been of major interest in zeolite synthesis. [5] This is particularly true when the structure-directing effect of Ge toward the formation of double 4-ring (d4r or [4 6 ]) units is properly combined with that of an OSDAi nc oncentrated fluoride media (H 2 O/ SiO 2 < 10). Over the past two decades,infact, alarge number of new zeolite structures containing d4r units have been discovered. [5] Them olar concentrations of F À and OSDAi n the synthesis mixture are generally identical to each other, but F À encapsulation within various types of small cages like d4r units is competitive with Al substitution in the zeolite framework. This led us to hypothesize that increasing the F À concentration in aluminosilicate synthesis mixtures containing ap articular OSDAc an alter the phase selectivity of the crystallization. Indeed, we have recently been able to discover three novel aluminosilicate zeolites,i.e., PST-21 (PWO), PST-22 (PWW), and PST-30 (PTY), using this so-called excess fluoride approach. [6] Most zeolites with disordered structures are characterized by one-dimensional (1D) stacking disorder of the same building layers,l eading to ap artial peak broadening in their powder X-ray diffraction (PXRD) patterns. [7] Different stacking sequences of the same building layer can also result in different ordered zeolite polytypes.Acommon feature of zeolites generated by different layer stackings is that their channel dimensionality is preserved. [7] On the other hand, the diffusion path length in zeolites,w hich has been mainly adjusted by the control of crystal size and mesoporosity so far, is ac rucial factor governing their catalytic and adsorption behavior. One excellent example is that alarge (3 mm) ZSM-5 (MFI) crystal has significantly higher p-xylene selectivity in toluene alkylation and disproportionation than as mall (0.5 mm) one. [8] Nevertheless,t he control of intracrystalline diffusivity in zeolites by changing the channel dimensionality has never been reported. Here we present the synthesis, structure solution, and catalytic properties of anew medium-pore zeolite with varying intracrystalline channel dimensionality,d enoted PST-24.

Results and Discussion
PST-24 was found when the pentamethylimidazolium (PMI + )c ation is employed as an OSDAi nh ighly excess F À conditions.T able 1lists the products from the PMI + -mediated synthesis of zeolites using reaction mixtures with different fluoride compositions.I tc an be seen that when the HF/ PMIOH ratio in the reaction mixture was fixed to 3.0, the Si/ Al ratio leading to pure PST-24 formation was 50 or higher ( Figure 1a). When the HF/PMIOH ratio was 2.0 or lower, however, SSZ-50 (RTH) and HPM-1 (STW) were the zeolite phases that crystallized at Si/Al = 5-20 and 1,respectively,as already reported. [9] PST-24 formation is observed at fairly lower Al and higher F À concentrations compared with prior crystallization conditions (Si/Al = 10 and HF/OSDA n+ = 2n, where OSDA n+ is 1,2,3-trimethylimidazolium, 1,2,3,4-tetramethylimidazolium, and 1,1'-(1,4-butanediyl)bis(2,5-dimethyl-1H-pyrazol-2-ium) ions for PST-21, PST-22, and PST-30, respectively). [6] It thus appears that the compositional range in which the excess fluoride approach is viable for finding novel zeolite structures is rather wide.I np articular, the fact that the use of ap ure-silica reaction mixture with HF/ PMIOH = 3.0 gave PST-24 instead of HPM-1 implies that the structure-directing ability of OSDAs can differ significantly according to the concentration of F À ions,even in the absence of heteroatoms like Al.
TheS EM image reveals that as-made,p ure-silica PST-24 (Si-PST-24) are typically rectangular plate-shaped crystals with ca. 0.06 0.25 1.5 mm 3 in size (Figure 1a). The 13 CNMR spectrum of as-made Si-PST-24 shows that the occluded PMI + ions remain intact (Supporting Information, Figure S1). Thef ramework of PST-24 maintains structural stability after calcination at 600 8 8C, which was verified by PXRD,t hermal analysis,N 2 adsorption, and 29 Si MAS NMR ( Figure 1a;Supporting Information, Figures S2-S4). Notable differences between the 29 Si NMR spectra of the as-made and calcined forms of PST-24 suggest that its local framework structure has been substantially altered upon removal of occluded OSDAm olecules during calcination at 600 8 8C (Supporting Information, Figure S4). [10] On the other hand, the PXRD pattern of calcined PST-24 shows peak broadening in the 2q range from 98 8 to 128 8 (Figure 1a), indicating the presence of structural disorder.T herefore,3 De lectron diffraction (3DED) and high-resolution transmission electron microscopy (HRTEM) were applied for the determination of the PST-24 structure (see the Supporting Information for more details).
Thea verage structure of PST-24 has 11 symmetryindependent tetrahedral sites (T-sites). Its framework is built from [5 4 .6 2 ]( cas)a nd [4 5 .5 2 ]( double 5-ring; d5r)u nits and zigzag chains (Figure 2), typical building units for zeolites. Product [b] Si/Al HF/R = 0.5 [a] The composition of the synthesis mixture is 0.5 ROH·x HF·1.  The cas units are stacked along the b-axis and connected to a zigzag chain on each side to form ac omposite cas-zigzag (cas-zz)c hain ( Figure 2c). The cas-zz chains (shown in yellow) are isolated from one another by d5r columns (blue) (Figure 2a-c). We note here that while the cas-zz chains are periodically distributed in PST-24 crystals,t he two most adjacent d5r units in the average structure are too close to each other and cannot exist simultaneously.T he only possibility of constructing the fully four-connected framework with ar easonable bond geometry is to allow only every second d5r unit in each d5r column. In such aw ay,t he d5r column along the b-axis has aperiodicity twice that of the caszz chain, that is,t he b-parameter in ar eal crystal should be doubled (ca. 10.1696 ). Thesharp diffraction spots observed in the (h 0 l), (h AE 2 l)… layers (Figure 1b;S upporting Information, Figure S5a) also indicate that there are two possible arrangements for each d5r column, which are related by 1/2b (= 5.0848 (2) ). Thediffuse scattering in the (h AE 1 l), (h AE 3 l)… layers (Figure 1b;S upporting Information, Figure S5b) extends in both a-a nd c-axes,i mplying that the disorder is 2D.T his means that each d5r column can adopt any of the two arrangements,and the choice is independent of other d5r columns.Therefore,itcan be concluded that the 3D framework of PST-24 is composed of two distinct types of regions on the molecular scale: cas-zz chains that are ordered throughout the crystal and d5r columns that are disordered in nature.W hile each cas-zz chain connects four d5r columns, each d5r column links two cas-zz chains.R egardless of the presence of the disorder, in consequence,PST-24 has at least a2 Dp ore system, with parallel straight 10-ring (5.8 5.4 ) and 8-ring (4.8 3.1 )channels along the b-axis (Figure 2b) and sinusoidal 8-ring channels along the c-axis (Figure 2g). On the other hand, the two most adjacent d5r columns are connected by a6-ring (marked in Figure 2b). The d5r column pair can adopt two possible arrangements;either at the same height b or at different heights related by 1/2b shift. Here we define these two d5r column arrangements as arrangements S (same) and D( different), respectively (Figure 2d). It should be noted that arrangement Sc reates 10-ring apertures (6.1 3.5 )t oconnect channels running along the c-axis,whereas arrangement Db locks the connection between those channels.This leads to the formation of apocket, but which is still part of sinusoidal 8-ring channels (Figure 2e-g). Therefore, arrangements Sa nd Dc an be metaphorically considered as the open and closed states of atwo-way valve,respectively.In actual PST-24 crystals,arrangements Sand Dare distributed randomly along the a-a nd c-axes (Figure 2g). As ar esult, PST-24 adapts varying intracrystalline channel dimensionality,f rom 2D in arrangement Dt o3 Di na rrangement S. Indeed, random changes in the dimensionality within real PST-24 crystals should then cause notable differences in the intracrystalline molecular diffusion. Such ac ombination of ordered and disordered chain regions renders PST-24 structurally very unique.
To further confirm the real PST-24 structure model with ar andom distribution of arrangements Sa nd D, we performed asimulation of ED and PXRD patterns using amodel with a1 0 a 1b 10c (b = 10.1696 )s upercell that contains major features of the disorder in PST-24 (see the Supporting Information for more details). Despite the limited size of the supercell for representing the complete disorder,t he simulated ED and PXRD patterns match very well with the experimental ones,i ncluding the broad peak region in the PXRD pattern (Supporting Information, Figures S5 and S7), which confirms our model of this novel disordered structure.
HRTEM also supports that PST-24 crystals are 1D ordered (along the b-axis) and 2D disordered (along the aand c-axes). TheH RTEM image of Si-PST-24 (Figure 3) along the c-axis shows periodic features along the b-axis, corresponding to the cas-zz chains and d5r columns (Figure 3b). Features with the brightest contrast correspond to the sinusoidal 8-ring channels where the majority of d5r columns along the c-axis are at the same height (ordered). Depending on the height of d5r columns,the 8-ring channels can have different orientations as shown in Figure 3b.W hen the d5r columns along the c-axis are at different heights,the 8ring channels become more sinusoidal and are less observable in the projection. Thed isorder in the a direction is more obvious with large variations of the contrasts,which is caused by different ratios of the d5r column arrangements in the projection (Figure 3). HRTEM also shows that the arrangement of d5r columns is quite random along both a-and c-axes. This explains why the crystal in the HRTEM image is heavily disordered so that the ordered regions are limited to only afew atomic columns.
Thea verage structure of as-made Si-PST-24 was also solved and refined against the cRED data (Supporting Information, Table S1). [13] While the same framework as that of calcined Si-PST-24 was obtained, the difference Fourier map of the refined average framework showed relatively large residual electron densities within the d5r units with half occupancyc ompared to the other regions (Supporting Information, Figure S8). Therefore,t he PMI + ions appears to be alternate with the d5r units along the d5r column in actual crystals.Infact, we were successful to energy-minimize the configuration of PMI + ions within the sinusoidal 8-ring channel along the c-axis (Supporting Information, Figure S9).
In principle,infinite numbers of ordered structures could be predicted by simply manipulating the arrangements of d5r columns.H owever,w ithin the given a and c unit cell parameters,w ew ere able to construct three ordered PST-24 polytypes with:1 )arrangement Do nly,2 )ana lternation of arrangements Dand Salong the a-axis,and 3) arrangement S only,d enoted as PST-24A (P2 1 /c), PST-24B (P1 ), and PST-24C (P2/c), respectively ( Figure 4; Supporting Information, Figure S10 and Table S6). Of particular interest is that although the channels along the b-a nd c-axes are almost identical to one another, they have different channel dimensionalities,d epending on the status of the valves.P ST-24A with arrangement Dhas closed valves only,resulting in a2D channel system (Figure 4d). In PST-24B with an alternation of the two types of arrangements,half of the valves are open to connect the adjacent channels along the c-axis,f orming ab i-level 2D channel system (Figure 4e). Finally,P ST-24C with arrangement S, where all valves are open, should possess a3 Dc hannel system (Figure 4f). To our knowledge,t herefore,PST-24 is the first example of disordered zeolites whose polytypes have similar overall pore structures but different channel dimensionalities.W hen the framework structures of the three polytypes in the pure-silica form are optimized using the Sanders-Leslie-Catlow potential with P1s ymmetry,t he corresponding framework energies relative to a-quartz were calculated to be similar to one another (11.6, 11.8, and 11.7 kJ (mol Si) À1 ,r espectively). [14] This indicates no preference to ap articular polytype,e xplaining the variation of channel dimensionality in actual PST-24. Their structures were also determined to have the same framework density (18.7;defined as the number of T-atoms per 1000 3 ).
Apart from the disorder of d5r units,f inding this unique type of building units in the zeolite structure is of much interest because the medium-pore borosilicate (Si/B % 30) zeolite SSZ-58 synthesized using 1-butyl-1-cyclooctylpyrrolidinium cation as an OSDAi nh ydroxide media is the only prior case where the d5r unit has been observed. [15] Considering that PST-24 is essentially pure-silica, the occurrence of d5r units during zeolite crystallization is in principle possible without the aid of heteroatoms other than Si. On the other hand, the 19 FM AS NMR spectrum of as-made Si-PST-24 shows ap rominent resonance at À64 ppm (Supporting Information, Figure S11). This led us to first suspect the encapsulation of F À ions within its d5r units.W hile the 6hedral ([4.5 2 .6 2 ]) t-mel units also exist in the PST-24 structure, however, as-made,p ure-silica ZSM-5 was reported to have a 19 Fr esonance at the same chemical shift, due to the encapsulation of F À ions within t-mel units,w hich has also been confirmed by single-crystal X-ray crystallography. [16] Further study is in progress to accurately locate the F À ions in as-made Si-PST-24.
While the dehydration of 1,3-butanediol is of technological interest to produce butadiene,amajor industrial chemical used mainly for the manufacture of synthetic rubbers and resins,high-silica (Si/Al = 130) H-ZSM-5 zeolite has recently been reported to be apromising catalyst for this dehydration at relatively low temperatures (< 400 8 8C). [17] Finally,w e evaluated the catalytic properties of the proton form of Al-PST-24 (H-Al-PST-24) with Si/Al = 200, for 1,3-butanediol dehydration at 300 8 8Cand 1.4 h À1 weight hourly space velocity and compared the results with those of H-ZSM-5 with Si/Al = 95 ( Figure 5). Thef ormer catalyst is always characterized by ah igher butadiene yield. In our view,t he outstanding performance of H-Al-PST-24 for this reaction can be attributed not only to its low acid site density (Supporting Information, Figure S12) but also to unique pore structure ( Figure 2).

Conclusion
We have synthesized anovel medium-pore zeolite PST-24 under high excess fluoride conditions (HF/PMIOH = 3). cRED combined with HRTEM revealed both the structure and the unique d5r columnar disorder of PST-24. The composite cas-zz chains and d5r columns are arranged alternatively extending in two (a and c)d irections and connected to form the 3D framework of PST-24. While the cas-zz chains are ordered throughout the entire crystal, the d5r column pairs can adopt two arrangements,w hich close and open 10-ring channels,r espectively.T his arrangement leads to au nique channel system in actual PST-24 crystals, varying locally from 2D to 3D,w hich can influence the intracrystalline diffusivity of guest molecules.T hree ordered  PST-24 polytypes have been proposed based on the arrangements of d5r columns and are characterized by consecutive changes in the channel dimensionality:2 D, bi-level 2D,a nd 3D.H igh-silica (Si/Al = 200) PST-24 was found to be highly active and selective for the dehydration of 1,3-butanediol to butadiene,mainly due to its unique pore architecture.