used to enhance the performance of commercial catalysts, with the promise of becoming a generalized platform for the rational design of zeolites across a broad range of framework types.

Micropore topology and crystal size are two independently adjustable properties that govern the internal mass transport limitations of zeolite catalysts. Deciphering the relative impact of each factor on catalyst performance is often nontrivial owing to the inability to synthesize zeolites with predetermined physicochemical properties. In this study, a series of ZSM-11 (MEL) and ZSM-5 (MFI) catalysts of equivalent acidity, but differing pore architecture, are prepared with well-defined crystal sizes to elucidate the effects of diffusion path length versus topology on catalyst lifetime and selectivity. For these studies, we selected the methanol to hydrocarbons (MTH) reaction to assess the impact of design variables on the hydrocarbon pool (HCP) mechanism. Operando UV−vis microspectroscopy is used to investigate the evolution of active HCP species and heavier aromatic coking species during the transient start up period over both catalysts. Our findings reveal that slight variations in framework topology between MEL and MFI zeolites lead to marked differences in their catalytic performance as well as the evolutionary behavior of HCP species within the zeolite pores. We report that the diffusion limitations imposed by the tortuous channels in ZSM-5 catalysts are analogous to increasing the channel length in ZSM-11 catalysts via larger crystal sizes. Notably, we observe similar (albeit slightly offset) trends in MTH selectivity and HCP speciation for both zeolite framework types; however, differences in pore topology and catalyst size exact different effects on the evolution of intracrystalline hydrocarbon species. Collectively, these findings provide evidence that ZSM-11 is an effective medium-sized pore zeolite catalyst for reactions encumbered by rapid coking that often elicits premature deactivation.

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The ammonia slip catalyst (ASC) is an essential final step in the emission control system and involves the selective oxidation of NH3 to N2. The state-of-the-art ASC has a dual-layer architecture composed of a Pt/Al2O3 (PGM) bottom layer and a metal (Fe, Cu)-exchanged zeolite (M-Z) top layer. The PGM layer provides high NH3 oxidation activity; however, the desired N2 product is achieved over a narrow temperature range just above light-off, whereas the reaction byproducts N2O and NO x (i.e., NO and NO2) appear at intermediate and high temperatures, respectively. An advantage of the M-Z catalyst is the selective lean reduction of NO via conversion of NH3 to N2 over a broad temperature range. Although recent studies demonstrate the effectiveness of the dual-layer design, further advances are needed to reduce the PGM loading and ASC volume while enhancing low-temperature activity. In this study, the dual-layer concept is scaled down to the level of a single core–shell (CS) catalyst particle, Pt/Al2O3@Cu/ZSM-5, composed of a PGM core and a M-Z shell, with the intent to meet the aforementioned challenges. The CS catalyst was realized by rational design of key synthesis steps, the most critical being the initial growth of an intermediate silicalite-1 layer to prevent Al leaching during the secondary growth of the ZSM-5 shell. Characterization of the CS spherical catalyst reveals a mesoporous PGM core (ca. 40 μm diameter) that is active and a nearly dense zeolitic shell (ca. 1 μm thick). Evaluation of the CS catalyst in a fixed-bed reactor shows excellent NH3 oxidation activity and N2 selectivity. In addition, we obtained an unanticipated enhancement of the Pt/Al2O3 performance within the CS configuration that gives an exceptional light-off of the NH3 oxidation. Our findings reveal that the CS catalyst has an equivalent activity to that of a conventional Pt/Al2O3 catalyst containing 3 times higher Pt loading. Further, a dual-layer ASC composed of a bottom layer containing the seeded core Pt/Al2O3 and a Cu-SSZ-13 top layer achieves the same performance as a dual-layer ASC having 3 times higher Pt loading but with unmodified Pt/Al2O3. The enhanced activity of the Pt/Al2O3 catalyst is attributed to a modification of the reducibility of oxides of Pt crystallites owing to the overgrowth of silicalite-1 and ZSM-5 layers in the CS configuration. Finally, the separate impacts of H2O in the feed and of hydrothermal aging (HTA) on catalyst performance are reported. H2O in the feed is shown to have a negligible impact on conversion and product distribution. The silicalite-modified Pt/Al2O3 catalyst is more resilient to HTA treatment than conventional Pt/Al2O3.

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.