Abstract The structure of zeolite SSZ‐43 was determined by 3D electron diffraction, synchrotron X‐ray powder diffraction, and high‐resolution transmission electron microscopy. The SSZ‐43 framework forms one‐dimensional, sinusoidal 12‐ring channels from 5461 butterfly units commonly found in other zeolites, but with unique 6.5×6.5 Å apertures and 12‐ring 6.5×8.9 Å windows perpendicular to the channels. SSZ‐43 crystals are intergrowths of two polytypes: ≈90 % orthorhombic polytype A with ABAB stacking of the 12‐rings, and ≈10 % monoclinic polytype B with ABCABC stacking. Molecular modeling performed on the idealized Si‐SSZ‐43 structure along with empirical relationships for zeolite selectivity in boron‐ and aluminum‐containing synthesis gels were used in a combined approach to design new di‐quaternary ammonium organic structure‐directing agents (OSDAs). Experimental trials demonstrated that the new OSDAs produced SSZ‐43 over a broader range of compositions than previous mono‐quaternary OSDAs.

Structure Elucidation and Computationally Guided Synthesis of SSZ‐43: A One‐Dimensional 12‐Ring Zeolite with Unique Sinusoidal Channels

Zeolite synthesis can be a tricky process due to the interactions between inorganic species, organic structure-directing agents, and processing conditions. Our understanding of the underlying mechanisms at work is further obscured by the complex interplay between thermodynamics and kinetics. Molecular modeling can help guide the direction of syntheses, although true a priori prediction of zeolite products is still an elusive goal. These synthesis variables are often interconnected, and modification to one must be made with consideration to trends identified in others. Important inorganic conditions include heteroatom type and content, reagent sources, alkali and total hydroxide content, mineralizing agent, and the role of water. The use of organics in the synthesis gel creates additional opportunities for directing a specific zeolite structure, templating, space-filling, and charge compensation, and the enormous variety of shapes and sizes of these organics can be both daunting and exciting. Molecular modeling is especially useful for predicting the interaction energies between these organics and the inorganic framework, and it has helped shape our interpretation of observed experimental results. Finally, the way in which zeolite research is carried out using high throughput, automation, and data mining will be critical to making step-change advancements in the field of zeolite synthesis. 

1The Confluence of Organo‐Cations, Inorganic Species, and Molecular Modeling on the Discovery of New Zeolite Structures and Compositions

The potential of computational modeling in the design of organic structure directing agents (OSDAs) for target small-pore zeolites was demonstrated some three decades ago by the use of the ZEBEDDE program for an aluminophosphate zeotype with the CHA topology. In this structure type, d6r units are the only secondary building units (SBUs) and there is a single type of cage, making the approach relatively straightforward, but in other target structures of catalytic interest there may be more than one cage type. The use of a co-templating strategy, in which OSDAs are computationally-designed specifically for each cage type, has therefore been explored. This has been successful, both for AlPOs, where organic cations alone are required as templates, and also for zeolites, where inorganic cations required for aluminosilicate gel crystallization also perform templating functions, favoring specific SBUs. The successful use of co-templates in a synthesis requires that each has similar templating efficacy, to avoid either formation of a dominant single-template phase or a simple mixture of phases. Examples of computation-led new material syntheses that demonstrate these principles include SAPOs with the SAV, AFX, and SFW structure types and aluminosilicates of the RHO family of embedded isoreticular zeolites. Where structural considerations permit, for example in polytypes in the ABC-6 or other structural families, intergrown systems with either simple or statistically-complex patterns of intergrowths can result. These have characteristic pore size distributions and physical properties, and here a deep understanding and control over the ratio of intergrowth phases can only be achieved by detailed characterization and modeling as reported, for example for the JMZ-11 family. 

1Co‐templating in the Designed Synthesis of Small‐pore Zeolite Catalysts

The structure of zeolite SSZ-43 was determined by 3D electron diffraction, synchrotron X-ray powder diffraction, and high-resolution transmission electron microscopy. The SSZ-43 framework forms one-dimensional, sinusoidal 12-ring channels from 5461 butterfly units commonly found in other zeolites, but with unique 6.5×6.5 Å apertures and 12-ring 6.5×8.9 Å windows perpendicular to the channels. SSZ-43 crystals are intergrowths of two polytypes: ≈90 % orthorhombic polytype A with ABAB stacking of the 12-rings, and ≈10 % monoclinic polytype B with ABCABC stacking. Molecular modeling performed on the idealized Si-SSZ-43 structure along with empirical relationships for zeolite selectivity in boron- and aluminum-containing synthesis gels were used in a combined approach to design new di-quaternary ammonium organic structure-directing agents (OSDAs). Experimental trials demonstrated that the new OSDAs produced SSZ-43 over a broader range of compositions than previous mono-quaternary OSDAs. 

Two dimensional zeolites have drawn a lot of attention due to their structural diversity and chemical composition, which can be used to obtain 3D zeolites unable to be direct synthesized. Here, a new layer silicate zeolite L was synthesized using the N, N-dimethyl-(2-methyl)-benzimidazolium as the structure-directing agent (SDA) under fluoride condition. Structure determination from the single-crystal X-ray diffraction reveals that the pure silica precursor with five-ring pores in the crystalline sheets is composed of the rth layer stacking along the (001) direction in an …AAAA… sequence with SDA + cations and F - residing within the interlayer spaces. Variable temperature powder X-ray diffraction results showed the new layer could transform into 3D RTH topology structure at 350 °C via 2D-3D topotactic transformation. Furthermore, a new 3D zeolite material is obtained by treating the original layer with diethoxydimethylsilane agent under hydrochloric acid condition (HCl-DEDMS). Based on the powder X-ray diffraction (PXRD) and the original layer structure, the new 3D zeolite structure expanding the rth layer linking with another Si atom is constructed, which possesses a 10×8×6 channel system. It displays a high BET surface area of 188 cm 3 /g with an external surface area of 130 cm 3 /g. The structure and textural properties pave a way for the potential catalytic application. The research not only provides a new layer zeolite, broadening the 2D zeolite framework types, but also allows for the discovery of a new stable 3D zeolite expanding the RTH structure with Si atom, which hasn't been reported yet.

Dan Xie

Papers

Rational Design and Targeted Synthesis of Small-Pore Zeolites with the Assistance of Molecular Modeling, Structural Analysis, and Synthetic Chemistry