Crystal Engineering for Catalysis

Abstract

Crystal engineering relies upon the ability to predictively control intermolecular interactions during the assembly of crystalline materials in a manner that leads to a desired (and predetermined) set of properties. Economics, scalability, and ease of design must be leveraged with techniques that manipulate the thermodynamics and kinetics of crystal nucleation and growth. It is often challenging to exact simultaneous control over multiple physicochemical properties, such as crystal size, habit, chirality, polymorph, and composition. Engineered materials often rely upon postsynthesis (top-down) processes to introduce properties that would otherwise be challenging to attain through direct (bottom-up) approaches. We discuss the application of crystal engineering to heterogeneous catalysts with a focus on four general themes: ( a) tailored nanocrystal size, ( b) controlled environments surrounding active sites, ( c) tuned morphology with well-defined facets, and ( d) hierarchical materials with disparate pore size and active site distributions. We focus on nonporous materials, including metals and metal oxides, and two classes of porous materials: zeolites and metal organic frameworks. We review novel synthesis methods involving synergistic experimental and computational design approaches, the challenges facing catalyst development, and opportunities for future advancement in crystal engineering.

Time-Resolved Dynamics of Intracrystalline Mesoporosity Generation in USY Zeolite

“…3 More recently, the surfactant-templating approach has also been extended to the synthesis of hierarchical zeolites in which the surfactant can be used to generate intracrystalline mesoporosity in the structure of these microporous materials. 4 Among the different procedures developed to generate secondary porosity within zeolites, [5][6][7][8][9][10] surfactant-templating in alkaline media allows for the incorporation of mesoporosity with tailored dimensions, while simultaneously maintaining the strong acidity and hydrothermal stability of the original zeolite. [11][12][13][14][15] The structure of surfactant-templated zeolites has been resolved by a combination of advanced gas adsorption, rotation electron diffraction (RED), and electron tomography (ET), which unambiguously confirmed the presence of intracrystalline mesoporosity within the zeolites.…”

Organic Mesopore Generating Agents (OMeGAs) for Hierarchical Zeolites: Combining Functions on Multiple Scales

“…In the study of zeolite growth modifiers, Lupulescu and Rimer first reported in 2012 the use of ZGMs, such as functional organic molecules found in silica proteins, termed silicateins or silaffins, to alter the anisotropic rate of step growth on the surface of crystals, thereby regulating the morphology of the synthesized silicalite‐1 zeolite . Zeolite growth modifiers can be added to the crystallization medium in small amounts without significantly altering the growth environment, while they can be designed to have specific binding to a particular crystal surfaces to enhance exposure of the desired crystallographic faces . Various zeolite growth modifiers, including alcohols, amines, organophosphine oxides, crown compounds, cationic polymers, and peptoids have achieved a high degree of control over zeolite crystal morphology for silicalite‐1, ZSM‐5, zeolite L, SSZ‐13, and ZSM‐22 beyond what is attainable in the absence of ZGMs.…”

“…[21] Zeolite growth modifiers can be added to the crystallization medium in small amounts without significantly altering the growth environment, while they can be designed to have specific binding to a particular crystal surfaces to enhance exposure of the desired crystallographic faces. [2] Various zeolite growth modifiers, including alcohols, amines, organophosphine oxides, crown compounds, cationic polymers, and peptoids have achieved a high degree of control over zeolite crystal morphology for silicalite-1, [21] ZSM-5, [24] zeolite L, [25] SSZ-13, [26] and ZSM-22 [27] beyond what is attainable in the absence of ZGMs. Some zeotype molecular sieves have been tuned with ZGM, such as the modification of silicoaluminophosphate SAPO-35 mophology through the addition of glucosamine.…”

“…Among them, zeolites are currently the most important solid catalysts used in petrochemical industries owing to their inherent orderly distributed nanopores of molecular dimensions that host the active sites. In terms of composition and structure, zeolites are crystalline aluminosilicates, in which the framework silicon and aluminum atoms are arranged in the form of AlO 4 and SiO 4 tetrahedra, with inherent micropores, typically <1 nm . Such a microporous transport system imposes diffusion limitation on guest molecules, prevents access of bulky molecules, and adversely affects the catalytic performance with consequences of decreased accessibility of active sites, lowered overall reaction rate, increased undesirable side reactions, and pejorated catalyst deactivation due to coke formation and/or pore blockage .…”

“…In terms of composition and structure, zeolites are crystalline aluminosilicates, in which the framework silicon and aluminum atoms are arranged in the form of AlO 4 and SiO 4 tetrahedra, with inherent micropores, typically < 1 nm. [2] Such a microporous transport system imposes diffusion limitation on guest molecules, prevents access of bulky molecules, and adversely affects the catalytic performance with consequences of decreased accessibility of active sites, lowered overall reaction rate, increased undesirable side reactions, and pejorated catalyst deactivation due to coke formation and/or pore blockage. [3] Therefore, the zeolitic materials need be engineered to improve their structural and transport properties, thereby lending them the potential for a wider range of applications.…”