Close Intimacy between PtIn Clusters and Zeolite Channels for Ultrastability toward Propane Dehydrogenation

Propane dehydrogenation (PDH) serves as a pivotal intentional technique to produce propylene. The stability of PDH catalysts is generally restricted by the readsorption of propylene which can subsequently undergo side reactions for coke formation. Herein, we demonstrate an ultrastable PDH catalyst by encapsulating PtIn clusters within silicalite-1 which serves as an efficient promoter for olefin desorption. The mean lifetime of PtIn@S-1 (S-1, silicalite-1) was calculated as 37317 h with high propylene selectivity of >97% at 580 °C with a weight hourly space velocity (WHSV) of 4.7 h-1. With an ultrahigh WHSV of 1128 h-1, which pushed the catalyst away from the equilibrium conversion to 13.3%, PtIn@S-1 substantially outperformed other reported PDH catalysts in terms of mean lifetime (32058 h), reaction rates (3.42 molpropylene gcat-1 h-1 and 341.90 molpropylene gPt-1 h-1), and total turnover number (14387.30 kgpropylene gcat-1). The developed catalyst is likely to lead the way to scalable PDH applications.

Bimetallic CoCu-modified Pt species in S-1 zeolite with enhanced stability for propane dehydrogenation

Propane dehydrogenation (PDH) has been an outstanding technique with a bright prospect, which can meet the growing global demand for propylene. However, undesired side reactions result in the deactivation of the Pt-based catalysts, which contribute to the insufficient lifetime of the catalysts. Herein, we describe a novel catalyst by encapsulating bimetallic CoCu-modified Pt species in S-1 zeolite for efficient dehydrogenation of propane, which synergizes the confinement of zeolites and the geometric and electronic effects on Pt species for enhancing the catalyst stability. The introduction of bimetallic additives efficiently promotes the dispersion of platinum and the electron transfer between Pt species and the additives, which greatly prolongs the lifetime of the catalysts. Particularly, no obvious deactivation is observed on 0.2Pt0.3Co0.5CuK@S-1 after 93 h on stream with a weight hourly space velocity (WHSV) of 5.4 h-1, revealing an ultralow deactivation constant of 0.0011 h-1 (t = 909 h). The formation rate of propylene still maintains at a high value of 407 mol gPt-1 h-1 (WHSV = 21.6 h-1) at 580 ℃ even after on pure propane stream for 42 h. The catalyst with the bimetallic CoCu-modified Pt species in S-1 zeolite reveals ultra-high activity and stability for PDH, which is ascribed to the highly dispersed Pt species and the stabilization effect of bimetallic additives on Pt species.

Zinc Oxide Nanoclusters Encapsulated in MFI Zeolite as a Highly Stable Adsorbent for the Ultradeep Removal of Hydrogen Sulfide

Often, trace impurities in a feed stream will cause failures in industrial applications. The efficient removal of such a trace impurity from industrial steams, however, is a daunting challenge due to the extremely small driving force for mass transfer. The issue lies in an activity-stability dilemma, that is, an ultrafine adsorbent that offers a high exposure of active sites is favorable for capturing species of a low concentration, but free-standing adsorptive species are susceptible to rapidly aggregating in working conditions, thus losing their intrinsic high activity. Confining ultrafine adsorbents in a porous matrix is a feasible solution to address this activity-stability dilemma. We herein demonstrate a proof of concept by encapsulating ZnO nanoclusters into a pure-silica MFI zeolite (ZnO@silicalite-1) for the ultradeep removal of H2S, a critical need in the purification of hydrogen for fuel cells. The Zn species and their interaction with silicalite-1 were thoroughly investigated by a collection of characterization techniques such as HADDF-STEM, UV-visible spectroscopy, DRIFTS, and 1H MAS NMR. The results show that the zeolite offers rich silanol defects, which enable the guest nanoclusters to be highly dispersed and anchored in the silicious matrix. The nanoclusters are present in two forms, Zn(OH)+ and ZnO, depending on the varying degrees of interaction with the silanol defects. The ultrafine nanoclusters exhibit an excellent desulfurization performance in terms of the adsorption rate and utilization. Furthermore, the ZnO@silicalite-1 adsorbents are remarkably stable against sintering at high temperatures, thus maintaining a high activity in multiple adsorption-regeneration cycles. The results demonstrate that the encapsulation of active metal oxide species into zeolite is a promising strategy to develop fast responsive and highly stable adsorbents for the ultradeep removal of trace impurities.

The Synergistic Impact of Crystal Seed and Fluoride Ion in the Synthesis of Silicalite-1 Zeolite in Low-Template Systems

Silicalite-1 zeolites are widely applied in gas adsorption, catalysis, and separation due to their excellent hydrothermal stability and unique pore structure. However, traditional preparation methods have inherent drawbacks such as high pollution, high cost, etc. Therefore, this work proposed a green and efficient route for preparing Silicalite-1 zeolite by adding NH4F (F/Si = 0.1) and seeds (10 wt%) in a much shorter time (8 h) in a low-template system (TPA+/Si = 0.007). It was found that NH4F is beneficial for inhibiting the formation of SiO2. The S-1 seeds could drastically induce the formation of the zeolite skeleton structure. Noteworthy, the morphology of zeolites was determined by the relative content of NH4F and seeds. The crystal morphology is determined by the higher content of the two substances; however, when the content is similar, the crystal morphology is determined by NH4F. The results showed that simultaneous control of NH4F and seeds can suppress SiO2 formation, can improve the relative crystallinity of products, and can be precisely regulated via the synergistic effect of both in zeolite morphology. This work not only provides new ideas for regulating the morphology of silicate-1 crystals but also offers a new path for industrial large-scale production of low-cost and efficient zeolites.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

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 extra-framework 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.