Zeolite-supported Metal Catalysts Research Articles

Construction of a Novel Cascade Electrolysis-Heterocatalysis System by Using Zeolite-Encaged Ultrasmall Palladium Catalysts for H2 O2 Generation.

In situ generation of hydrogen peroxide (H2 O2 ) has attracted extensive attention, especially in water treatment. However, traditional anthraquinones can only produce high-concentration H2 O2 and its transportation and storage are not convenient and dangerous. Herein, an in situ and on-demand strategy to produce H2 O2 by using a cascade water electrolysis together with a heterocatalysis system is provided. Beginning with water, H2, and O2 can be generated via electrolysis and then react with each other to produce H2 O2 immediately on efficient zeolite-encaged ultrasmall Pd catalysts. Significantly, the H2 O2 generation rate in the optimized cascade system reaches up to 0.85mol L-1 h-1 gPd -1 , overcoming most of the state-of-the-art catalysts in previous literature. The confinement effect of zeolites is not only beneficial to the formation of highly dispersed metal species, promoting the H2 O2 generation, but also inhibits the H2 O2 decomposition, enhancing the production yield of H2 O2 . In addition, the effect of electrolytes, sizes of Pd species, as well as zeolite acidity are also systematically studied. This work provides a new avenue for H2 O2 generation via a highly efficient cascade electrolysis-heterocatalysis system by using zeolite-supported metal catalysts. The high catalytic efficiency and green process for H2 O2 generation make it very promising for further practical applications.

Metal-loaded zeolites in ammonia decomposition catalysis.

The viability of using ammonia as a hydrogen storage vector is contingent on the development of catalytic systems active for ammonia decomposition at low temperatures. Zeolite-supported metal catalysts, unlike systems based on supports like MgO or carbon nanotubes (CNTs), are crystalline and lend themselves to analytic techniques like synchrotron X-ray powder diffraction (SXRD) and Rietveld refinement, allowing precise characterisation of catalytic active sites, and therefore mechanistic elucidation. This study focuses on characterising and optimising novel zeolite-supported Ru catalysts for ammonia decomposition, with a focus on the effects of N-substitution on catalyst structure and activity. Characterisation focuses on an unsubstituted and N-substituted Ru-zeolite Y pair with NMR, FTIR, TEM, XRD, XAS, ICP, and BET, demonstrating the successful incorporation of N into the zeolite framework and an enhancement in metal dispersion upon N-substitution. A series of 18 monometallic and bimetallic catalysts is then synthesised on X and USY supports and screened for catalytic activity. Ru is identified as the most active metal for ammonia decomposition. Observed trends suggest catalyst dispersion can be increased with substantially lower metal loadings, and in particular via the formation of stably anchored oligonuclear metal clusters within the zeolite framework, as opposed to much larger nanoparticles (NPs) on its exterior, following N-substitution of the framework. DFT modelling proposes a prismatic Ru6N6 cluster fitted to XAS data. High-activity catalyst Ru-β (N) 2.4% demonstrates comparable or better ammonia conversion by Ru wt% than recently reported catalysts in the literature at 450 °C and 30 000 WHSV.

Environmental assessment of carbon dioxide methanation process using mixed metal oxide and zeolite-supported catalysts by life cycle assessment methodology

Carbon dioxide methanation process is a well-known carbon dioxide utilization technology, not only on account of its ability to subside carbon dioxide in the atmosphere but also to produce methane, which is of serious industrial significance. Although this process is promising in terms of tackling greenhouse gases and global warming, it can, on the other hand, release toxic emissions into the atmosphere, rivers and soil during the process. At this point, life cycle environmental assessment emerges as a crucial tool to reveal the overall effects of this technology. This paper presents a life cycle assessment case study for carbon dioxide methanation process to evaluate all aspects of its environmental impacts. Different scenarios for this purpose were considered by changing catalyst types, namely, mixed metal oxide and zeolite-supported metal catalysts. The results showed that the toxic wastes formed and emissions released when using the Ni/Al 2 O 3 catalyst were less compared to the other cases. Not only did the change in material type in the catalyst affect the total emissions, but the catalyst conversion and selectivity had an influence on the life cycle impact of the system as well. Various power generation alternatives considering renewable and non-renewable sources were evaluated, while a combination of natural gas and wind turbines for the initial sources of power generation was found to perform better in terms of environmental impact. • CO 2 methanation on Ni/Al 2 O 3 catalyst is more environmentally-friendly. • Using natural gas for CO 2 capture emits fewer pollutants than using coal. • Conversion and selectivity factors can affect LCA results of a reaction-based process. • A mixture of natural gas power plants and wind turbines can reduce environmental impacts. • Perovskite solar cells show toxic effects on climate change and metal depletion.

A durable Ni/La-Y catalyst for efficient hydrogenation of γ-valerolactone into pentanoic biofuels

Zeolite-supported metal catalysts containing hydrogenation centers and acid sites are promising in the chemoselective hydrogenation of biomass platform molecules into value-added chemicals and fuels. The primary challenge of employing such bifunctional catalysts for biomass conversion lies in catalyst stability in the liquid phase under harsh conditions. Herein, we have prepared a Ni/La-Y nanocatalyst via an improved wet impregnation method. Compared with Ni nanoparticles on H-Y, La addition shows a significantly enhanced stability and performance in the continuous liquid-phase hydrogenation of γ-valerolactone (GVL) into ethyl pentanoate (EP) at 200 °C for 1000 h. Complementary characterization studies reveal that La addition in the metal/zeolite catalyst not only efficiently modulates the acid property of the zeolite to alleviate coke formation, but also suppresses zeolite dealumination and metal agglomeration and leaching upon catalysis over a 1000 h period. These findings provide an efficient approach for improving the stability of zeolite-supported bifunctional catalysts, leading to potential application in hydrogen-assisted biomass valorization under the liquid-phase conditions.

Combined in situ XAS and DFT studies on the role of Pt in zeolite-supported metal catalysts for selective n-hexane isomerization

In this study, we used several in situ, and spectroscopy analyses to investigate the roles of platinum (Pt) in zeolite-supported Pt catalysts for selective n-alkane isomerization. HY zeolites were synthesized using the rice husk silica and impregnated with Pt precursor by incipient wetness impregnation. Typically, HY zeolites have proved to be active hydrocarbon cracking catalysts. The particular challenge is to find an effective catalyst, which favors the isomerization of n-alkanes without too much cracking. In this work, adding Pt atoms to HY zeolite provides an enhanced multifunctional catalyst for converting n-alkanes to branched hydrocarbons. To understand the effect of Pt, we further investigated the reaction mechanism of n-hexane isomerization to 2–methylpentane (2MP) and 3-methylpentane (3MP) by DFT simulations, using a cluster of 30T HY zeolite modeled through B3LYP + 3D calculations. We found that, at 450 °C, the isomerization on Pt-HY gives higher cracking products. Decreasing temperature to 300 °C yielded greater selectivity of branched hydrocarbons. In addition, DFT calculations demonstrate that the 2MP production via route A1 (C3-C4 bond activation) and a rate-determining step of 0.97 eV proved more thermodynamically and kinetically favorable than the 3MP product. This agrees well with our experimental observations. Consequently, the presence of Pt on the HY zeolite plays an essential role in both CC forming and breaking. Finally, the Pt–HY zeolite is an efficient catalyst for petroleum production, improving the octane number for catalytic performance and product selectivity by isomerizing straight-chain alkanes to their branched chain isomers.

Advances in zeolite-supported metal catalysts for propane dehydrogenation

Propane dehydrogenation (PDH) is one of the most promising on-purpose technologies to produce propylene. In this review, we present a comprehensive summary of the latest research progress on zeolite-supported metal catalysts for PDH reactions.

Advances in Catalytic Applications of Zeolite-Supported Metal Catalysts.

Zeolites possessing large specific surface areas, ordered micropores, and adjustable acidity/basicity have emerged as ideal supports to immobilize metal species with small sizes and high dispersities. In recent years, the zeolite-supported metal catalysts have been widely used in diverse catalytic processes, showing excellent activity, superior thermal/hydrothermal stability, and unique shape-selectivity. In this review, a comprehensive summary of the state-of-the-art achievements in catalytic applications of zeolite-supported metal catalysts are presented for important heterogeneous catalytic processes in the last five years, mainly including 1) the hydrogenation reactions (e.g., CO/CO2 hydrogenation, hydrogenation of unsaturated compounds, and hydrogenation of nitrogenous compounds); 2) dehydrogenation reactions (e.g., alkane dehydrogenation and dehydrogenation of chemical hydrogen storage materials); 3) oxidation reactions (e.g., CO oxidation, methane oxidation, and alkene epoxidation); and 4) other reactions (e.g., hydroisomerization reaction and selective catalytic reduction of NOx with ammonia reaction). Finally, some current limitations and future perspectives on the challenge and opportunity for this subject are pointed out. It is believed that this review will inspire more innovative research on the synthesis and catalysis of zeolite-supported metal catalysts and promote their future developments to meet the emerging demands for practical applications.

Stable Palladium Oxide Clusters Encapsulated in Silicalite-1 for Complete Methane Oxidation

Zeolite-supported metal catalysts are widely employed in a number of chemical processes, and the stability of the catalytically active species is one of the most critical factors determining the reaction performance. A good example is the Pd/zeolite catalyst, which provides high activity for methane oxidation but deactivates rapidly under the reaction conditions due to palladium nanoparticle sintering. Although coating the metals with thin shells of porous materials is a promising strategy to address the sintering of metals, it is still challenging to fix small metal particles completely inside zeolite crystals. Here, using an amine-based ligand to stabilize palladium during the zeolite synthesis, we realize the exclusive encapsulation of highly dispersed palladium oxide clusters (1.8–2.8 nm) in the microporous channels and voids of the nanosized silicalite-1 crystals. The synthesis conditions of the zeolite-supported catalyst influence the encapsulation degree and the size distribution of metal particles. Thanks to the encapsulation effect of small palladium oxide clusters, together with the inherent properties of silicalite-1 such as low acidity, high hydrophobicity, and high hydrothermal stability, the optimized Pd@silicalite-1 catalyst outperforms the traditional Pd-based catalysts prepared by wetness impregnation, exhibiting both high activity and better stability in the lean methane oxidation reaction.

Metal encapsulation in zeolite particles: A rational design of zeolite-supported catalyst with maximum site activity

Zeolite-supported metal catalysts have been proven effective in many important catalytic reactions, such as hydrogenation, Fisher-Tropsch synthesis, automobile exhaust catalysis, selective catalytic reduction and many others. Despite the successful preparation of the catalyst through widely adopted methods, including ion exchange and impregnation, the metal dispersion over the zeolite is lack of control with high randomness. This renders the so-called “catalytic performance” an overall contribution from the metal sites located inside the zeolite micropores and those located on the external surface. This is exceptionally true for small to medium pore zeolites with typical free apertures of 0.3–0.6 nm (such as LTA and MFI). A more rational design of zeolite-supported metal catalysts is by encapsulating the metal nanoparticles or clusters within zeolite pores prior to the zeolite formation. Encapsulation of metals in zeolite prevents them from sintering and sulphur poisoning by cage confinement and molecular exclusion (via well-defined pore size and shape), respectively. This paper gives a new perspective on using metal clusters and nanoparticles as catalysts and the design of an effective zeolite-supported catalytic system.

A General Strategy for Fabricating Isolated Single Metal Atomic Site Catalysts in Y Zeolite.

Exploring high-performance zeolite-supported metal catalysts is of great significance. Herein, we develop a strategy for fabricating isolated single metal atomic site catalysts in Y zeolite (M-ISAS@Y, M = Pt, Pd, Ru, Rh, Co, Ni, Cu) by in situ separating and confining a metal-ethanediamine complex into β-cages during the crystallization process followed by thermal treatment. The M-ISAS are stabilized by skeletal oxygens of Y zeolite, and the crystallinity, porosity, and large surface area are well inherited in M-ISAS@Y. As a demonstration, acidic Pt-ISAS@Y is used for n-hexane isomerization involving consecutive catalytic dehydrogenation/hydrogenation on Pt-ISAS and isomerization on Brønsted acid sites. The turnover frequency value of Pt-ISAS reaches 727 h-1, 5 times more than Pt nanoparticles (∼3.5 nm), with a total isomer selectivity of more than 98%. This strategy provides a convenient route to fabricate promising zeolite-based M-ISAS catalysts for industrial applications.

Zeolite-supported metal catalysts for selective hydrodeoxygenation of biomass-derived platform molecules

Key concepts and challenges of zeolite-supported metal catalysts for hydrodeoxygenation of selected biomass-derived platform molecules.

Catalytic Valorization of Furfural under Methane Environment

The technical feasibility of developing a novel process for coupling bioderived furans (furfural) with methane was investigated at mild reaction conditions (400 °C and 2.5 MPa) over modified zeolite catalysts. Monoaromatics are the main liquid products with high yield, and the oxygen of furfural is removed in the form of CO and CO2. Compared to other zeolite-supported metal catalysts developed in this study, Zn–Ga/ZSM5 shows better catalytic performance on furfural conversion in terms of a higher yield of aromatics, particularly benzene, toluene and xylene (BTX), slightly lower gas yield and higher methane conversion. The presence of methane provides carbon atoms that are effectively utilized in the formation of aromatics, and further significantly enhances the yield of aromatics under the facilitation of the charged catalyst. Direct evidence of methane participation in the formation of aromatic products is witnessed by liquid 1H, 2H and 13C NMR investigations, which reveal that methane tends to be incorp...

Hydrogenation of 2-methylindole using supported metal catalysts

The hydrogenation of 2-methylindole to 2-methylindoline in toluene was studied at 60°C and 40bar hydrogen pressure in a batch mode using a range of carbon, alumina, and zeolite supported metal catalysts were performed in toluene. The results were compared with a more conventional catalytic system containing besides 5% Pt/C also 1.2eq. of p-toluenesulfonic acid in ethanol:water (50:50v/v). The highest turnover frequency (TOF) was obtained a zeolite supported iridium catalyst.

Hydrodeoxygenation of Lignin-Derived Phenolic Monomers and Dimers to Alkane Fuels over Bifunctional Zeolite-Supported Metal Catalysts

A bifunctional catalyst of Ru supported in zeolite HZSM-5, Ru/HZSM-5 (Si/Al = 25), exhibited excellent hydrodeoxygenation activity toward the conversion of lignin-derived phenolic monomers and dimers to cycloalkanes in aqueous solution. The oxygen-containing groups in mono- and binuclear phenols were removed through a cleavage of C–O bonds in phenolics followed by an integrated metal- and acid-catalyzed hydrogenation and dehydration. As a bifunctional catalyst Ru/HZSM-5, the presence of both the Bronsted acid site in the pores of HZSM-5 for dehydration and a metallic function of Ru for hydrogenation was indispensible for the formation of alkanes from lignin-derived phenolics. Our findings also reveal that the Ru/HZSM-5 with the lowest Si/Al ratio of HZSM-5 proved to be most selective to cycloalkanes, indicating that more acid sites over zeolite are favorable for the dehydration of cyclohexanol during hydrodeoxygenation process, which leads higher selectivity to hydrocarbons. This approach for the construc...

Transformation of Isobutyl Alcohol to Aromatics over Zeolite-Based Catalysts

One-step transformation of isobutyl alcohol to aromatics (benzene, toluene, and xylene) has been studied in a gas phase, fixed-bed reactor system over several purely acidic zeolites and zeolite-supported metal catalysts. ZSM-5 zeolites give higher aromatics yields (∼42 wt %) among the evaluated zeolites, and the Si/Al ratios (Si/Al = 13–43) of ZSM-5 slightly influence their catalytic performances. During the transformation of isobutyl alcohol, large amounts of short alkanes (mainly propane and butane isomers) are also generated on the acidic ZSM-5. To improve the conversion to aromatics, several metal species (Zn, Ga, Mo, La, Ni, Ag, and Pt) are supported on the ZSM-5. The enhancements in aromatics yields (∼60 wt %) are observed only on the Zn/ZSM-5 catalysts. The incorporation of Zn species preferentially decreases the strong-strength Bronsted acidity and, thus, suppresses the cracking to C3 fragments. Moreover, mainly the Zn species at the exchange sites facilitate the recombinative desorption of H2 and...

Hydrodeoxygenation and coupling of aqueous phenolics over bifunctional zeolite-supported metal catalysts

Pt supported on HY zeolite is successfully used as a bifunctional catalyst for phenol hydrodeoxygenation in a fixed-bed configuration at elevated hydrogen pressures, leading to hydrogenation-hydrogenolysis ring-coupling reactions producing hydrocarbons, some with enhanced molecular weight.

Dehydrocyclization of Alkanes Over Zeolite-Supported Metal Catalysts: Monofunctional or Bifunctional Route

The direct catalytic conversion of alkanes into aromatics has found potentially important industrial applications. Initially only alkanes with 6 and more carbon atoms in the chain were concerned. Supported platinum catalysts were found active for the aromatization of alkanes; the drawbacks of these catalysts were their deactivation with time on stream and the existence of simultaneous parallel reactions. Much discussion has been published on the aromatization of C6+ alkanes. A bifunctional mechanism which involves both the metal and the acid sites of the support and a monofunctional mechanism involving only the metallic sites operate over, respectively, Pt supported on acidic support and Pt supported on nonacidic support. In the present review the mechanisms proposed for the aromatization of alkanes are described. Over monofunctional Pt catalysts two possible mechanisms prevail: 1,6 ring closure on the Pt surface involving primary and secondary C-H bond rupture, followed by dehydrogenation of the cycloalkanes into aromatics (1,5 ring closure to a lesser extent also contributes to aromatic production); or dehydrogenation of the alkanes into olefins, dienes, and trienes followed by thermal ring closure. Zeolites were found most suitable as support for preparing catalysts more active and more selective in the alkane aromatization. In addition catalysts based on noble metals supported on zeolite appeared more resistant against deactivation by coke. In this review the aromatization of hexane, heptane, and octane over Pt-zeolite catalysts is discussed in detail. Comparisons between different zeolite structures and different dehydrogenation sites are given. In particular a critical analysis of the results and interpretation concerning Pt-KL catalysts strongly suggests that the exceptional high selectivity towards aromatization of n-hexane exhibited by Pt-KL could not be explained by only the nest or constraint effect exerted by the channel dimension and morphology, not by only the terminal cracking properties, not by only the partial electron transfer from the zeolite support to the Pt particles, and not by only the Pt particle size. Zeolite structure also affects the aromatic product distribution, in particular when the alkane contains more than 7 carbon atoms. It is shown how Pt on medium-pore zeolites such as In-ZSM-5, silicalites will favor the aromatization of C8 alkane isomers into ethylbenzene-styrene with respect to other C8 aromatics. Aromatization of light alkanes, C2-C5, requires the increase of the hydrocarbon chain length up to 6 carbon atoms and higher, followed by cyclization reaction. Recently new processes to convert C2-C5 alkanes into aromatics have been disclosed, M2-forming from Mobil, Cyclar from BP-UOP, and Aroforming from IFP-Saluted. In general these processes use bifunctional catalysts possessing a dehydrogenating and an acid function. The catalysts consist of a metal ion or metal oxide supported on a microporous acid solid. In this review we analyze the results concerning mainly platinum supported on pentasil-type zeolite. It is shown that althoug Pt has better dehydrogenating properties as compared with gallium and zinc, the efficiency of catalysts based on Pt-ZSM-5 for light alkane aromatization is less because undersirable reactions such as hydrogenolysis and ethene (olefins) hydrogenation occur on the platinum surface, resulting in the production of unreactive alkanes, CH2, C2H6. These drawbacks could be partially suppressed by alloying Pt and by increasing the reaction temperature.

Zeolite-supported transition metal catalysts by design

Zeolite supported metal Catalysts are prepared by ion exchange or incipient wetness impregnation, or by deposition of metal complexes, followed by calcination and reduction. Hydrogen reduction of encaged transition metal ions yields metal dusters or isolated atoms and protons of high Brønsted acidity. The elementary steps in the genesis of mono- and bimetallic metal dusters have been unravelled by a combination of dynamic and spectroscopic methods. Small metal clusters can be anchored to zeolite cage walls by transition metal ions or protons; however, adsorption of a CO which can displace the protons from the metal, initiates migration of the primary carbonyl dusters leading to their coalescence. At low temperature, this process is limited by the geometry of the metal core vis-à-vis the cage window. Protons of high Brensted acidity can reoxidize metals at elevated temperature, even selectively “leach” the less noble metal atoms out of bimetal clusters. Large metal particles, present in zeolite voids or at the external surface, can be transformed into small particles, exposing virtually all their metal atoms, by either of two novel in situ techniques which are based on oxidative dispersion with oxygen or chlorine gas.

Probing metal cluster formation in NaY zeolite by xenon-129 NMR

The performance of zeolite-supported metal catalysts is known to depend upon preparatory treatments. Homogeneous and reproducible metal dispersion can be achieved only through careful control of calcination and reduction conditions, since migration and agglomeration of cluster precursors can occur with subsequent loss of catalytic activity. In spite of considerable research, the transformations undergone by metal guest species during calcination and reduction have not been established, primarily due to difficulties in characterizing chemical intermediates which exist as cluster precursors. By using the /sup 129/Xe NMR spectroscopy technique pioneered by Fraissard and co-workers and applied to recent advantage by the authors and others, they gain insight into the detailed chemistry of metal-zeolite catalyst preparation. They report for the first time changes in the chemical environment of NaY-supported platinum guest species by monitoring shifts in the /sup 129/Xe resonance signal induced by different chemical and thermal treatments.

Effect of [formula omitted] ratio on secondary reactions during CO hydrogenation on zeolite-supported metal catalysts

The effect of zeolite acidity on the product distribution in CO hydrogenation over ion-exchanged zeolite-supported Ru catalysts was investigated using zeolites with different Si Al ratios. CO hydrogenation over RuNaX, RuNaY, RuKL, and RuNa-mordenite and the transformation of 1-butene on the zeolites without the metal, under conditions similar to those used for CO hydrogenation, were investigated in order to understand the effect of zeolite acidity on possible secondary reactions of the primary olefinic products of Fischer-Tropsch (FT) synthesis. The results of this study establish the importance of the bifunctional nature of the zeolite-supported FT catalysts in modifying catalyst selectivity. This modification is achieved as a result of secondary reactions of the primary olefinic FT products on the acid sites of the zeolite, generated during catalyst preparation. Depending on the strength of the acid sites, a function of the Si Al ratio, various competitive reaction paths can be observed for the transformation of the olefinic compounds.