Single‐Crystalline MFI Zeolite with Sheet‐Like Mesopores Layered along the a Axis

Abstract

Designing a templating strategy for directing mesopore growth along different crystallographic directions is essential for fabricating two‐ or three‐dimensional single‐crystalline mesoporous zeolites. However, so far, mesopores formed in MFI zeolites by soft templates have mostly been generated by disrupting growth along the b axis; generating mesopores by disrupting growth along the a axis is rare. Herein, a single‐crystalline mesoporous MFI zeolite (SCMMZ) with sheet‐like mesopores layered along the a and b axes was synthesized using a triply branched surfactant with diquaternary ammonium groups connected to 1,3,5‐triphenylbenzene by a six‐ and eight‐carbon alkyl chain (TPB‐6 and 8). The sheet‐like mesopores were embedded in the MFI framework and were retained even after calcination. Molecular mechanics calculations provided evidence of low binding energy configurations of the surfactant that directed the growth of straight and zigzag channels along the b and a axes, respectively. The formation of nanosheets was attributed to the geometric matching of the arrangement of the aromatic groups to the zeolite framework.

π–π Interactions Between Aromatic Groups in Amphiphilic Molecules: Directing Hierarchical Growth of Porous Zeolites

“…The strategy was to introduce a larger hydrophobic center along with triply branched bis(quaternary ammonium)‐containing head groups, and stabilize the micelles with decreasing binding energy and facilitate the formation of mesopores along the a ‐axis. The results showed that a single‐crystalline mesoporous MFI zeolite (SCMMZ) with sheetlike mesopores layered along the a ‐ and b ‐axes was synthesized by using triply branched 1,3,5‐triphenylbenzene, having bis(quaternary ammonium)‐containing amphiphilic molecules, with a formula of C 6 H 3 [C 6 H 4 ‐O‐C 6 H 12 ‐N + (CH 3 ) 2 ‐C 6 H 12 ‐N + (CH 3 ) 2 ‐C 6 H 13 ⋅(2 Br − )] 3 (denoted TPB‐6) …”

“…d–f) Packing configurations of the surfactants in the as‐prepared SCMMZ along the a ‐, b ‐, and c ‐axes. Reproduced with permission from Wiley‐VCH …”

“…As mentioned above,although the introduction of athird branch led to the formation of embedded 3D mesopores, mesopores were mainly generated by blocking the crystal growth along the b-axis,e xposing the a-c plane,w hereas almost no mesopores formed by disrupting the growth of the MFI zeolite along the other axis.T hus,t he fifth question is how to open mesopores along the a-axis.T he strategy was to introduce al arger hydrophobic center along with triply branched bis(quaternary ammonium)-containing head groups,a nd stabilize the micelles with decreasing binding energy and facilitate the formation of mesopores along the aaxis.T he results showed that as ingle-crystalline mesoporous MFI zeolite (SCMMZ) with sheetlike mesopores layered along the a-a nd b-axes was synthesized by using triply branched 1,3,5-triphenylbenzene,h aving bis(quaternary ammonium)-containing amphiphilic molecules,with aformula of C 6 H 3 [C 6 H 4 -O-C 6 H 12 -N + (CH 3 ) 2 -C 6 H 12 -N + (CH 3 ) 2 -C 6 H 13 ·-(2 Br À )] 3 (denoted TPB-6). [16] TEM images taken along the [010] direction of the MFI zeolite (Figures 7a,c) show sheetlike mesopores stacking along the a-axis with al ength of about 5-50 nm. The corresponding SAED pattern (Figure 7b)reveals the formation of asingle-crystalline mesostructured MFI.…”

“…Then, azobenzene was introduced to the hydrophobic tail, giving rise to helical cylindrical assemblies, in contrast to nanosheet assemblies, directing a lamellar‐structured zeolite, which leads to the formation of hierarchical MFI zeolite with a two‐dimensional (2D) square mesostructure . Finally, benzene‐ring‐ and triphenylbenzene‐centered three‐armed amphiphilic molecules, each with bis(quaternary ammonium)‐containing head groups, were synthesized, and they directed the formation of single‐crystalline mesoporous ZSM‐5 with three‐dimensional (3D) pores and sheetlike mesopores layered along the a ‐axis, respectively. We believe that this design concept will shed light on the synthesis of other types of mesoporous zeolites.…”

“…Recently,asingle-crystalline mesoporous beta zeolite was obtained by employing ac ationic polymer as ab ifunctional template,w here the quaternary ammonium groups bonded to the chain of the polymer act as SDAs for the beta zeolite,while the polymer functions as aporogen on the mesoscale. [7g] Furthermore,W u et al synthesized hierarchical MWW (Mobil Composition of Matter-twentytwo) zeolite nanosheets through controllable self-assembly of lamellar zeolite precursors and asimple surfactant, C 16 H 33 -(CH 3 ) 3 N + Br À (cetyltrimethylammonium bromide;CTAB). [11] Although various mesoporous zeolites have been successfully synthesized through amphiphilic templating strategies, to date,i tr emains ag reat challenge to understand the formation mechanism and to use it to construct hierarchical zeolites with different mesostructures or diverse topological frameworks.I nt his minireview,w ef irst describe our investigation into the role of one simple amphiphilic molecule, CTAB,i nt he formation of bulk MFI zeolites to understand the structural-directing mechanism of amphiphilic molecules in zeolite synthesis.…”