Metal Zeolite Catalysts Research Articles

Noble metal-free tandem catalysis enables efficient upcycling plastic waste into liquid fuel components

The lack of effective approaches for upcycling waste plastic has led to severe environmental pollution and squandering of carbon resources. While hydrocracking macromolecular polyolefins over typical metal-zeolite catalyst produces high quality liquid fuel components, the limited activity of base metal and restricted accessibility of acid sites within microporous zeolite renders this process still not in practical scale. A tandem catalyst comprising Ni-WO3/Al2O3 and Beta zeolite was developed for consecutive hydrocracking polyolefins, achieving a yield of 86.2 % of primarily gasoline-jet ranged hydrocarbons (mainly C5-C16 and minor C16+) at 280 °C. The incorporation of tungsten species onto Ni/Al2O3 largely enhances the surface acidity and accelerates the primary cracking process that converts polyethylene (PE) into medium-sized intermediates, which would readily diffuse into the pore networks of Beta and undergo further cracking to yield desired liquid hydrocarbons. In addition, this tandem catalyst also promotes facile hydrocracking of consumer-grade plastic waste and maintains excellent stability over 5 recycling tests. The noble-metal-free catalyst achieves an impressive liquid formation rate of 5.74 g·gcat.−1·h−1, rivaling the noble metal-based catalyst and demonstrating high application prospects.

Tuning the size and spatial distribution of Pt in bifunctional Pt-zeolite catalysts for direct coupling of ethane and benzene

Controlling the dispersion and position of metal active sites in bifunctional metal–acid zeolite catalysts is essential for achieving synergy between the metal and acid sites in tandem catalytic processes, but still challenging nowadays. Herein, we propose an approach to modulate the dispersion and position of Pt particles in ZSM-5 zeolite by employing various types of organic structure-directing agents (OSDAs) in the one-pot synthesis procedure. Systematic characterizations show that the OSDAs mainly influence the encapsulation of the Pt precursor into the ZSM-5 zeolite crystallites during the hydrothermal crystallization process, resulting in discrepancies in the dispersion and spatial distribution of Pt sites in the final Pt-zeolite catalysts. Comparative studies on different Pt-zeolite catalysts show that the formation of subnanometer Pt clusters and the close spatial proximity of Pt and Brønsted acid sites in the zeolite efficiently promote the tandem dehydrogenation-alkylation process and enhance the production rate of ethylbenzene in the direct ethane-benzene coupling reaction. This study offers insights into the design and synthesis of bifunctional metal-zeolite catalysts for tandem catalysis.

The Rôle of Iron in Zeolite Beta for deNOx Catalysis

Selective catalytic reduction with ammonia (NH3-SCR) is a widely used deNOx process, employing zeolitic catalysts. Here, we examine framework substituted and extra-framework exchanged transition metal zeolite catalysts, in the extensively studied Fe – zeolite Beta focusing on the influence of the transition metal cation on the NH3-SCR reaction, and aiming to unravel the underlying mechanisms and their impact on catalytic performance. We use hybrid multiscale density functional theory (DFT)/molecular mechanics to investigate the NH3-SCR at various Fe-BEA active sites including systems with both framework and extra-framework Fe cations. The catalytically active sites under investigation include Fe-FeF-BEA, Fe-AlF-BEA, and Cu-FeF-BEA, (where the subscript F indicates framework species) on which the formation and consumption of key intermediates of both oxidation and reduction half cycles species are analysed. We show the distinctive ability of framework Fe sites to promote the formation of key nitrate and nitrosamine intermediates. Furthermore, we find a strong binding affinity of NH3 with framework Fe systems including Cu(II)-FeF-BEA, Fe(III)-FeF-BEA, and H-FeF-BEA compared to framework Al systems (Fe(III)-AlF-BEA). We explore the reduction potential of the framework Fe3+/Fe2+ in BEA zeolite and establish its feasibility in the presence of a Brønsted acid site. Our calculations clearly predict the bimetallic Cu-FeF system to be the best catalyst which supports a recent experimental report by Yue et al. (Chem. Eng. J. 2020, 398, 125515) for a related FeCu-SSZ-13 (CHA) zeolite. We provide a new understanding of the reactivity of Fe-BEA zeolites, suggesting the advantages of framework Fe over Al sites in NH3-SCR reactions, and establishing a foundation for interpretation of the rôle of framework cations in metal-exchanged zeolites.

Preparation of catalyst for CO2 hydrogenation reaction based on the idea of element sharing and preliminary exploration of catalytic mechanism.

Under the background of the continuous rise of CO2 annual emissions, the development of CO2 capture and utilization technology is urgent. This study focuses on improving the catalytic capacity of the catalyst for CO2 hydrogenation, improving the efficiency of CO2 conversion to methanol, and converting H2 into chemical substances to avoid the danger of H2 storage. Based on the concept of element sharing, the ASMZ (Aluminum Shares Metal Zeolite catalysts) series catalyst was prepared by combining the CuO-ZnO-Al2O3 catalyst with the ZSM-5 zeolite using the amphoteric metal properties of the Al element. The basic structural properties of ASMZ catalysts were compared by XRD, FTIR, and BET characterization. Catalytic properties of samples were measured on a micro fixed-bed reactor. The catalytic mechanism of the catalyst was further analyzed by SEM, TEM, XPS, H2-TPR, and NH3-TPD. The results show that the ASMZ3 catalyst had the highest CO2 conversion rate (26.4%), the highest methanol selectivity (76.0%), and the lowest CO selectivity (15.3%) in this study. This is mainly due to the fact that the preparation method in this study promotes the exposure of effective weakly acidic sites and medium strength acidic sites (facilitating the hydrogenation of CO2 to methanol). At the same time, the close binding of Cu-ZnO-Al2O3 (CZA) and ZSM-5 zeolite also ensures the timely transfer of catalytic products and ensures the timely play of various catalytic active centers. The preparation method of the catalyst in this study also provides ideas for the preparation of other catalysts.

Catalytic Vacuum Gas Oil Oxycracking in Presence of Metal Zeolite Catalysts

Catalytic Vacuum Gas Oil Oxycracking in Presence of Metal Zeolite Catalysts

Isomerization of dihydroxybenzenes over metal-zeolite catalysts through the carbonaceous deposits

Dihydroxybenzenes, including catechol, resorcinol and hydroquinone, have significant commercial value for a variety of applications such as adhesives, resins, pharmaceuticals, coatings etc. However, selective production of required isomers by phenol hydroxylation represents a considerable challenge.Here, we report a new approach for the synthesis of dihydroxybenzenes by their isomerization using a bifunctional Pt/ZSM-5 catalyst. The catalyst successfully facilitates the transformation of catechol and hydroquinone to each other with a selectivity of up to 74 % and yields up to 50 %. The investigation of the mechanism suggests that isomerization proceeds via a carbonaceous deposit (coke) formed by intermediate quinone condensation with subsequent hydrogenolysis to isomers. The proposed mechanism shows the way for the design of the efficient process for isomerization of dihydroxybenzenes.

Boosting the catalytic performance of metal–zeolite catalysts in the hydrocracking of polyolefin wastes by optimizing the nanoscale proximity

Loading Pt exclusively on the surface of USY constructs a consecutive hydrocracking process of polyolefin wastes.

Dynamics-driven tailoring of sub-nanometric Pt-Ni bimetals confined in hierarchical zeolite for catalytic hydrodeoxygenation

Metal-zeolite catalysts are promising for hydrodeoxygenation of bio-oils, but they always encounter sluggish diffusion kinetics due to sole micropores and inferior metal stability. Here, highly stable sub-nanoscale Pt-Ni bimetals confined in a hierarchical ZSM-5 zeolite (Pt-Ni@HZ) are constructed. The formed Pt-Ni@HZ species create framework-bonding metal sites with zeolite in a confined microenvironment, thereby establishing strong interactions and generating uniform distribution of Pt-Ni bimetals. In the Pt-Ni@HZ-catalyzed hydrodeoxygenation of phenolics, sub-nanometric Pt-Ni facilitates the adsorption and transfer of hydrogen, and hierarchical pores promote the diffusion kinetics of reactants. The designed Pt-Ni@HZ with small metal particles and high coordination numbers of Pt-Ni bonds, favors the complete deoxygenation of phenolics, with a higher yield of cyclohexane than conventional catalysts prepared by impregnation. Benefiting from the protection of the zeolite framework, the Pt-Ni@HZ catalyst exhibits high thermal stability and low leaching of metal active sites, with only 4.4 % catalyst deactivation after five reaction cycles.

Atomic Insights into the Cu Species Supported on Zeolite for Direct Oxidation of Methane to Methanol via Low-Damage HAADF-STEM.

Precise determination of the structure-property relationship of zeolite-based metal catalysts is critical for the development toward practical applications. However, the scarcity of real-space imaging of zeolite-based low-atomic-number (LAN) metal materials due to the electron-beam sensitivity of zeolites has led to continuous debates regarding the exact LAN metal configurations. Here, a low-damage high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging technique is employed for direct visualization and determination of LAN metal (Cu) species in ZSM-5 zeolite frameworks. The structures of the Cu species are revealed based on the microscopy evidence and also proved by the complementary spectroscopy results. The correlation between the characteristic Cu size in Cu/ZSM-5 catalysts and their direct oxidation of methane to methanol reaction properties is unveiled. As a result, the mono-Cu species stably anchored by Al pairs inside the zeolite channels are identified as the key structure for higher C1 oxygenates yield and methanol selectivity for direct oxidation of methane. Meanwhile, the local topological flexibility of the rigid zeolite frameworks induced by the Cu agglomeration in the channels is also revealed. This work exemplifies the combination of microscopy imaging and spectroscopy characterization serves as a complete arsenal for revealing structure-property relationships of the supported metal-zeolite catalysts.

Illustrating the overall reaction network of the synthesis-gas-to-hydrocarbons process over iron-zeolite bifunctional catalysis

Since its discovery in the early 20 th century, Fischer-Tropsch synthesis (FTS) has opened a path, as an alternative to crude oil, to produce fuels and chemicals. When classical FTS catalysts are combined with acidic zeolites, the scope of this gas-phase polymerization can be narrowed, thus maximizing the production of value-added commodities and eliminating energy-consuming separation steps. However, from a mechanistic standpoint, even now, little is known about the role of the different reaction intermediates. Here, we present a comprehensive, in-depth, mechanistic investigation using solid-state NMR spectroscopy and well-designed control experiments on combining a classical Fe-based FTS catalyst and zeolites with different topologies to establish the impact of “co-catalytic” key organic carbon-based reaction intermediates, including carbonylated/oxygenated species (ester/ketone/alcohol/ether/epoxide/ketene). Consequently, this work provides experimental evidence supporting the “co-existence” of oxygenate (cf. surface-enol and CO-insertion) mechanisms (together with the traditional carbide-based FTS mechanism). The significance of “supramolecular reactive centers” within zeolite and host-guest chemistry has also been illuminated. • Zeolite topologies altered preferential hydrocarbon product selectivities • Mobility-based solid-state NMR identified crucial carbon-based reaction intermediates • Co-involvement of oxygenated/carbonylated species is evaluated during syngas conversion • Significance of inorganic-organic supramolecular reaction centers has been established To alleviate pressure from increased environmental regulations over carbon emissions, the direct hydrogenation of CO/CO 2 to diverse hydrocarbons over bifunctional catalysts is a priority research topic worldwide. Identifying new catalytic routes to valorize C1-feedstock is a key to combating climate change, whereby Fischer-Tropsch synthesis (FTS) over bifunctional metal-zeolite catalysts offers elegant solutions to diversify global fuel/chemical supply. However, their overall reaction network to deliver preferential final product selectivity remains elusive and controversial. Through this work, we achieve this goal by identifying key carbon-based reaction intermediates and establishing a full reaction network of the synthesis-gas-to-hydrocarbons process over Fe 2 O 3 @KO 2 /zeolite catalyst. This established structure-reactivity relationship can upgrade the current FTS process and improve the fundamental understanding of bifunctional catalysis. Identifying “descriptor”-style carbon-based reaction intermediates is significant in upgrading the current state of metal-zeolite bifunctional “cascade” catalysis. In this work, four different product selectivities (short olefins, light paraffins, long-chain hydrocarbons, and aromatics) were observed during the syngas-to-hydrocarbon process (Fischer-Tropsch type) over potassium superoxide-doped iron oxide/zeolite-based bifunctional catalysts. Here, we demonstrate that the zeolite topology and “co-catalytic” hybrid supramolecular reactive centers (i.e., organic hydrocarbons pool species trapped by the inorganic zeolite) regulate the reaction mechanism and the final product selectivity.

CATALYTIC OXIDATIVE CONVERSION OF LOWER OLEFINS AND PARAFFINS OVER ZEOLITES MODIFIED WITH DIFFERENT METAL CATIONS

With the purpose of development of high efficiency catalysts for reactions of the oxidative conversion of lower olefins and paraffins into the desired products of petrochemical industry it has been synthesized by ion exchange method a range of the metalzeolite catalysts on the basis of synthetic (NaY) and natural (pure and dealuminated clinoptilolite and mordenite) zeolites and the cations of different metals. It has been established that the metalzeolite catalysts prepared on the basis of synthetic zeolite NaY shows relatively high activity in the oxidative conversion of ethylene and propylene into acetaldehyde and acetone accordingly. It has been also synthesized a highly effective metalzeolite catalysts for the processes: oxidative conversion of methane into ethylene; oxidative conversion of propane into acetaldehyde and formaldehyde; oxidative conversion of methane to ethylene and acetylene; oxidative conversion of methane to 1,4-butanediol

Nickel silicate beta zeolite prepared by interzeolite transformation: A highly active and stable catalyst for dry reforming of methane

• Hydrothermal interzeolite transformation of nickel silicate MWW to *BEA zeolite. • Novel synthesis of nickel silicate beta zeolite using bottom-up method. • In-situ formation of highly dispersed metallic nickel clusters in Ni-BEA. • Highly active and stable Ni catalyst for dry reforming of methane. • Platform technique for the preparation of zeolite-metal composite. The preparation of porous nickel silicates has received considerable attention because of its potential as a catalyst for various applications, such as dry reforming of methane (DRM), CO 2 hydrogenation, and water–gas shift. In this study, we first synthesize the nickel silicate *BEA-type zeolite (Ni-BEA) via the interzeolite transformation of nickel silicate MWW within 12 h. The presence of Ni in the tetrahedral framework of the *BEA structure is experimentally demonstrated using diverse analytical methods. The extraction of framework Ni at high temperatures generates uniform metallic Ni clusters within the pores of the *BEA structure. Ni-BEA exhibits the highest conversions of CH 4 and CO 2 and stability for DRM among the employed catalysts owing to the significant quantity of highly dispersed Ni clusters in the three-dimensional large-pore system. The synthesis of metallosilicate *BEA by interzeolite transformation and in-situ generation of uniform metal clusters can be expanded to the other metal-zeolite catalysts.

Ammonia pools in zeolites for direct fabrication of catalytic centers

Reduction process is a key step to fabricate metal-zeolite catalysts in catalytic synthesis. However, because of the strong interaction force, metal oxides in zeolites are very difficult to be reduced. Existing reduction technologies are always energy-intensive, and inevitably cause the agglomeration of metallic particles in metal-zeolite catalysts or destroy zeolite structure in severe cases. Herein, we disclose that zeolites after ion exchange of ammonium have an interesting and unexpected self-reducing feature. It can accurately control the reduction of metal-zeolite catalysts, via in situ ammonia production from ‘ammonia pools’, meanwhile, restrains the growth of the size of metals. Such new and reliable ammonia pool effect is not influenced by topological structures of zeolites, and works well on reducible metals. The ammonia pool effect is ultimately attributed to an atmosphere-confined self-regulation mechanism. This methodology will significantly promote the fabrication for metal-zeolite catalysts, and further facilitate design and development of low-cost and high-activity catalysts.

OPTIMAL DESIGN OF THE OXIDATIVE DEHYDROGENATION OF METHYLCYCLOHEXANE INTO METHYLCYCLOHEXADIENE ON A MODIFIED ZEOLITE CATALYST

Selection and theoretical optimization of the reactor type was carried out on the basis of a kinetic model of the process of selective oxidative dehydrogenation of methylcyclohexane into methylcyclohexadiene on a modified active metal-zeolite catalyst. It was determined that it was more expedient to carry out the process in an ideal tubular ( packed-bed ) type reactor. As a result of theoretical optimization of the process, optimal technological regimes were determined and the optimal design dimensions of the reactor element for a given capacity calculated. A complete mathematical model of the process was developed with regard to the effect of heat and pressure drop.

Metal-zeolite catalysts for the removal of pharmaceutical pollutants in water by catalytic ozonation

Catalytic ozonation of salicylic acid (SA), as model pharmaceutical pollutant, at room temperature was studied in an aqueous phase using mono- (Pd, Cu) and bimetallic (Pd-Cu, Cu-Pd) zeolite catalysts prepared by the ion exchange method, using different zeolite structures: FAU (USY) and MFI (ZSM5). All metal-zeolite catalysts positively affected SA degradation in comparison with the parent zeolites, except for PdCu-HUSY. The mineralization degree was improved in the presence of the heterogeneous catalysts, regardless of the zeolite used. Nevertheless, the catalysts based on different zeolite structures presented distinct performances during SA oxidation, with those based on HUSY zeolite showing lower activity, which indicated that the physicochemical properties of the zeolite structures are very important. Wide-ranging, the catalytic oxidation of SA with ozone on zeolite-based catalysts was found to be affected by a combination of the acidic properties of zeolites and the chemical composition at the surface. The evolution of oxalic acid as a by-product in SA oxidation, as well as its degradation by catalytic ozonation, were also studied. Reutilization runs revealed that the zeolite-based catalysts were very stable.

MATHEMATICAL DESCRIPTION FOR THE OXIDATIVE DEHYDROGENATION OF ISOPROPANOL TO ACETONE

The paper presents the results of the development of the complete mathematical model for the oxidative dehydrogenation of isopropanol to acetone on the metal-zeolite catalyst – CuPd-mordenite. Using the kinetic model of this process have been chosen the optimal type of reactor. Have been identified the most significant physicochemical phenomena significantly affect to the process. The mathematical description of this process consists of the heat balance equation, as well as an equation that takes into account the pressure drop in the system at the gas mixture moving through a fixed-bed catalyst

OXIDATIVE CONVERSION OF LOWER OLEFINS AND PARAFFINS OVER THE MODIFIED ZEOLITE CATALYSTS

With the purpose of development of high efficiency catalysts for reactions of the oxidative conversion of lower olefins and paraffins into the desired products of petrochemical industry it has been synthesized by ion exchange method a range of the metalzeolite catalysts on the basis of synthetic (NaY) and natural (pure and dealuminated clinoptilolite and mordenite) zeolites and the cations of different metals. It has been established that the metalzeolite catalysts prepared on the basis of synthetic zeolite NaY shows relatively high activity in the oxidative conversion of ethylene and propylene into acetaldehyde and acetone accordingly. It has been also synthesized a highly effective metalzeolite catalysts for the processes: oxidative conversion of methane into ethylene; oxidative conversion of propane into acetaldehyde and formaldehyde; oxidative conversion of methane to ethylene and acetylene; oxidative conversion of methane to 1,4-butanediol

One-pot fabrication of metal-zeolite catalysts from a combination of solvent-free and sodium-free routes

The metal-exchanged zeolites as catalysts have been widely applied in many catalytic reactions, but their syntheses are environmentally unfriendly due to the multiple steps such as ion-exchange for the formation of a large amount of wastes. Herein, we for the first time report a method for one-pot fabrication of metal-zeolite catalysts (M-SSZ-13, M-Beta, M-ZSM-5, and M-EU-1; M = Cu, Co, Ni) from a combination of solvent-free and sodium-free routes by mixing, grinding, and heating the raw materials of aluminosilicate gel, organic templates, and metal-amine complexes. As a typical example, Cu-SSZ-13 zeolite is successfully synthesized from heating the mixture of aluminosilicate gel, organic template, and Cu-amine complex. Catalytic tests in selective catalytic reduction of NOx with NH3 (NH3-SCR) show that the synthesized Cu-SSZ-13 zeolite before and after hydrothermal aging at 750 °C for 16 h are completely comparable with the commercial Cu-SSZ-13 zeolite from the conventional hydrothermal synthesis. Very interestingly, the method in this work has obvious advantages such as high yield, reduced pollutants, and simple process, which should be potentially important for practical applications of the metal-zeolites in the future.

Probing cobalt localization on HZSM-5 for efficient methane dehydroaromatization catalysts

A new catalyst family of Co-exchanged HZSM-5 zeolite catalyst without additional active metals was for the first time modeled for the non-oxidative conversion of methane toward aromatics, which provides an alternative and promising way to produce aromatics from huge methane resources compared with the traditional routes from coal and crude oil. The prepared catalyst contains a well-defined Co site in the HZSM-5 zeolite anchored at Brønsted acid sites and is much appropriate for activating CH bond in CH4 for controllable formation of CHx species rather than coke. Over the Co-exchanged HZSM-5 catalyst, the maximum selectivity to total aromatics (mainly benzene and naphthalene) over 80% could be obtained. Compared to almost all non-Mo-based catalysts including V, Cr, Mn, Fe, Zn, Cu, Ga and W investigated in literature, the Co-exchanged catalysts exhibited several times higher TOF values (s−1). To corroborate the distinct Co speciation associated with the ion-exchanged catalyst and illustrate the structure–activity relationship of the well-modeled catalyst, a series of comparative activity experiments, various spectroscopic and microscopic techniques (ICP-AES, XRD, FT-IR, XPS, UV–Vis DR, and TEM) and chemical transient experiments (H2-TPR, CO-TPR, NH3-TPD) and DFT calculations were performed. The exchanged Co(II) species connected with zeolite framework are much stable in reductive gases and could not be reduced and carburized, which should be the active sites for appropriately activating CH4 into CHx species available for subsequent aromatic formation. DFT calculations confirmed that these Co-exchanged sites required much higher energy for the dissociation of CH bond in CH4 than supported Co clusters, suggesting that the formed CHx species have a low possibility of further cleavage of CH bonds and are stable for the subsequent aromatization reactions. The distinct Co locations and chemical states in HZSM-5 showing different catalytic behaviors has actually provided us a new insight into how to construct an active metal-zeolite catalyst for selective conversion of CH4 to value-added chemicals.

Impact of the Spatial Organization of Bifunctional Metal–Zeolite Catalysts on the Hydroisomerization of Light Alkanes

AbstractImproving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one‐dimensional zeolites (ZSM‐22 and mordenite) and a γ‐alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n‐heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the “intimacy criterion” for the rational design of bifunctional catalysts for the conversion of low‐molecular‐weight reactants.