Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Abstract

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extraframework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Stable and Uniform Extraframework Cations in Faujasite Zeolites

“…Further, the TMI-containing zeolites can behave as supermolecules including the host multidentate metallic complex sites and the guest incoming reactants as well as outgoing products . The successful construction of well-defined extraframework TMIs makes TMI-containing zeolites coordination complex, and the versatile regulation of the TMI-surrounding local electric field contributes to the understanding of active sites and the mechanism. , The tactful utilization of these analogues may lead to suitable candidates for adsorption and catalysis.…”

“…Transition metal sites can be incorporated into zeolites by CVD through loading degassed zeolite materials with vapors of organometallic compounds followed by thermal treatment under reductive or inert conditions. , This strategy is suitable for transition metals that are available only in the form of anions by using zerovalent organometallic compounds as metal precursors and can overcome the limitation of ion-exchange capacities within various zeolites. For CVD, the kinetic size of organometallic precursors should be carefully screened on account of the zeolite pore windows, preventing the formation of clusters or nanoparticles on the outer surface.…”

“…Uniform bare TMIs at site II can be constructed through the ligand-protected in situ hydrothermal route, as confirmed by spectroscopic analyses and theoretical simulations ( vide infra ). It should be noted that great effort has been devoted to control the location and sitting of TMIs in zeolites mainly by ion-exchange, and the results are somewhat controversial due to the vexing question of zeolite compositions (Si/Al and TMI/Al ratios). ,,, For the ligand-protected in situ route, the synthesis parameters (choice of ligand, Si/Al ratio, TMI/Al ratio, crystallization temperature, duration, etc .) should also be carefully optimized for the successful construction of TMIs confined in FAU zeolites.…”

“…5 The successful construction of well-defined extraframework TMIs makes TMI-containing zeolites coordination complex, and the versatile regulation of the TMIsurrounding local electric field contributes to the understanding of active sites and the mechanism. 9,10 The tactful utilization of these analogues may lead to suitable candidates for adsorption and catalysis.…”

Novel Composite Catalysts Efficiently Catalyze C8 Aromatics by Controlling the Reaction Pathway

“…9−13 It is worth mentioning that the ZSM-5 zeolite has three-dimensional pores (5.1 Å × 5.5 Å and 5.3 Å × 5.6 Å) close to the dynamic diameter of PX, which are smaller than MX and OX. 12,14 Therefore, ZSM-5 has high shape selectivity and provides a high p-xylene/o-xylene ratio in the xylene isomerization reaction. 15 As the most suitable catalyst support for the isomerization of C8 aromatics, the low Si/Al ratio HZSM-5 has strong acidity 12,16 and shows high catalytic activity.…”