MFI Zeolite Research Articles

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes.

The selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)-based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization-reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd-C4 structure and, meanwhile, facilitate the desorption of alkenes by forming the Si-O-C structure with zeolite framework, as validated by the state-of-the-art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H2 ), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite-encapsulated carbon-free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite-confined Pd-carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes

AbstractThe selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)‐based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization‐reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd−C4 structure and, meanwhile, facilitate the desorption of alkenes by forming the Si−O−C structure with zeolite framework, as validated by the state‐of‐the‐art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H2), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite‐encapsulated carbon‐free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite‐confined Pd‐carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.

Propane dehydrogenation over Pt and Ga-containing MFI zeolites with modified acidity and textural properties

A variety of oxides (titanium, tin, calcium, magnesium, and gallium) were supported over nano-crystalline ZSM-5 zeolite (n-ZSM-5) by wet impregnation, characterized and evaluated for propane dehydrogenation (PDH) reaction. To enhance the catalytic performance of the oxide-modified n-ZSM-5, Pt nanoparticles were also dispersed over the oxides-supported zeolite catalysts by wet impregnation. Finally, Ga-containing MFI zeolites were used as catalysts in the PDH. Ga was incorporated into the zeolite by two different methods, via hydrothermal synthesis and via wet impregnation. In the PDH reaction, Pt-containing samples exhibited a high initial catalytic activity although they suffered a fast deactivation by coke deposition. On the contrary, Ga-containing MFI catalysts showed a remarkable stability in the PDH reaction. In particular, the catalyst in which Ga was incorporated into the MFI structure by hydrothermal synthesis (Ga-MFI (nSH)) achieved the highest catalytic performance in PDH (9% conversion and 80% propylene selectivity) due to the synergy between the Brønsted and Lewis acid sites (BAS and LAS) and the optimal strength of its LAS sites. These results denote the great potential of Ga-MFI zeolites as catalysts in PDH reactions.

Efficient Catalytic Production of Hydrogen Peroxide Using Tin‐containing Zeolite Fixed Palladium Nanoparticles with Oxidation Resistance

AbstractThe metal surfaces tend to be oxidized in air through dissociation of the O−O bond of oxygen to reduce the performances in various fields. Although several ligand modification routes have alleviated the oxidation of bulky metal surfaces, it is still a challenge for the oxidation resistance of small‐size metal nanoparticles. Herein, we fixed the small‐size Pd nanoparticles in tin‐contained MFI zeolite crystals, where the tin acts as an electron donor to efficiently hinder the oxidation of Pd by weakening the adsorption of molecular oxygen and suppressing the O−O cleavage. This oxidation‐resistant Pd catalyst exhibited superior performance in directly synthesizing hydrogen peroxide from hydrogen and oxygen, with the productivity of hydrogen peroxide at ≈10,170 mmol gPd−1 h−1, steadily outperforming the catalysts tested previously. This work leads to the hypothesis that tin is an electron donor to realize oxidation‐resistant Pd within zeolite crystals for efficient catalysis to overcome the limitation of generally supported Pd catalysts and further motivates the use of oxidation‐resistant metal nanoparticles in various fields.

Efficient Catalytic Production of Hydrogen Peroxide Using Tin-containing Zeolite Fixed Palladium Nanoparticles with Oxidation Resistance.

The metal surfaces tend to be oxidized in air through dissociation the O-O bond of oxygen to reduce the performances in various fields. Although several ligand modification routes have alleviated the oxidation of bulky metal surfaces, it is still a challenge for the oxidation resistance of small-size metal nanoparticles. Herein, we fixed the small-size Pd nanoparticles in tin-contained MFI zeolite crystals, where the tin acts as an electron donor to efficiently hinder the oxidation of Pd by weakening the adsorption of molecular oxygen and suppressing the O-O cleavage. This oxidation-resistant Pd catalyst exhibited superior performance in directly synthesizing hydrogen peroxide from hydrogen and oxygen, with the productivity of hydrogen peroxide at ~10,170 mmol gPd-1 h-1, steadily outperforming the catalysts tested previously. This work leads to the hypothesis that tin is an electron donor to realize oxidation-resistant Pd within zeolite crystals for efficient catalysis to overcome the limitation of generally supported Pd catalysts, and further motivates the use of oxidation-resistant metal nanoparticles in various fields.

Oxidative adsorption mechanism-based screening of zeolites for deep purification and recycling of NOx from humid gases

Adsorption is a promising technology for deep purification of nitrogen oxides (NOx) from flue gases and simultaneously recycling NO2 as an economically valuable chemical. Zeolite as a robust NOx adsorbent provides the key step for success in practical applications, of which the screening remains challenging regarding the NO-NO2 oxidation and adsorption combined process. In this work, experimental evaluations on practical NOx sorption performances on series of zeolites were conducted. Effects of the zeolite typological structure, compensating cation, and SiO2/Al2O3 ratio on deep purification efficacy, water resistance, and regenerability at varying humidity were obtained, revealing the importance of kinetics of the NO oxidation and NO2 physisorption consecutive process. The H-typed 10-membered-ring straight-channel zeolites (MFI and FER) were demonstrated to be the preferred NOx adsorbents qualified for industrial needs, as compared to other caged-channel, hybrid-channel, larger-sized straight-channel, or Na-typed counterparts. Deeper insights into the key role of Brønsted acid sites (BAS) in driving the NO-NO2 catalytic cycle and promoting the adsorption competitiveness of NOx over water vapor were revealed based on in-situ diffuse reflectance Fourier transform infrared spectroscopy characterizations. The MFI zeolite with improved performance at the low-silica version could benefit from the efficient utilization of BAS within the highly accessible medium-sized straight channel, while the FAU zeolite (representative caged-channel zeolite) requiring appropriate SiO2/Al2O3 ratio for optimal BAS utilization may suffer from the limited site accessibility. The findings clarify the relationships between NOx oxidative adsorption performances and zeolite properties, which provides a fundamental perspective and scientific screening strategy for NOx adsorbents.

Dehydrogenation–cracking synergistic pathway of n–pentane by core–shell MFI zeolite to produce light olefins

A core@shell MFI type zeolite supporting PtZn alloys catalyst was prepared by dry gel conversion method, which is used for catalytic cracking of n–pentane. The shell and core of catalyst is composed of nanosheet silicalite–1 and microporous ZSM–5, respectively. A proposed dehydrogenation–cracking pathway was demonstrated, which facilitate the generation of light olefins. At 600 °C with WHSV of 6.26 h−1, the yield of ethylene and propylene in the product attained up to 35.3 wt% and 33.4 wt%, respectively.

A simple strategy for synthesis of b-axis-oriented MFI zeolite macro-nanosheets

Zeolite nanosheets with shortened diffusion paths and high external surface areas have drawn significant attention. This unique morphology of zeolites can endow them with reduced diffusion resistance and high accessibility of active sites, and thereby good catalytic performance. Exploring simple synthesis strategies for zeolite nanosheets is mostly desired in zeolite chemistry. Herein, MFI zeolite nanosheets with a thickness of 50-100 nm along the b -axis and a length of 1 μm along the c -axis, namely zeolite macro-nanosheets, have been successfully synthesized via the strategy of combining pre-aging and pH regulation without the use of complex templates/additives and fluorine media. Pre-aging at a relatively high temperature of 90 °C and low alkalinity for crystallization (pH = 7.0~8.5) are found to play key roles in controlling macro-nanosheet synthesis, which can promote the formation of MFI zeolite precursors and then dissociate them into smaller ones for the following oriented aggregation during hydrothermal treatment. Crystallization kinetic studies suggest that the two-dimensional crystal growth is the intrinsic behavior of zeolite crystallization under employed conditions. This synthesis strategy can be easily extended to the synthesis of heteroatom-containing MFI macro-nanosheets, such as ZSM-5 and TS-1.

Comparison of the Mechanisms of deNOx and deN2O Processes on Bimetallic Cu–Zn and Monometallic Cu–Cu Dimers in Clinoptilolite Zeolite—A DFT Study Simulating Industrial Conditions

This paper presents two mechanisms for the deNOx process and for the deN2O process (in two variants). The processes were carried out on a clinoptilolite zeolite catalyst with a deposited Cu–Cu monometallic dimer and Cu–Zn bimetallic dimer with bridged oxygen between the metal atoms. Analyses were performed for hydrated forms of the catalyst with a hydrated bridging oxygen on one of the metal atoms. Calculations were performed using DFT (density functional theory) based on an ab initio method. The analyses included calculations of the energies of individual reaction steps and analysis of charges, bond orders and bond lengths as well as HOMO, SOMO and LUMO orbitals of selected steps in the mechanism. Based on the results obtained, it was determined that the most efficient catalyst for both processes is a Cu–Zn bimetallic catalyst with a bridged hydroxyl group. It shows higher efficiency in the limiting step (formation of the -N2H intermediate product) than the previously studied FAU and MFI zeolites with a Cu–Zn bimetallic dimer. In addition, the possibility of using the catalytic system from the deNOx process in the deN2O process was presented, which can benefit SCR installations. In addition, it was proved that the order of adsorption of NO and N2O has significance for further steps of the deN2O process. In order to improve the comparison of FAU, MFI and CLI zeolite catalysts with a Cu–Zn dimer, further studies on the deN2O mechanism for the first two zeolites are needed. This study allows us to propose a bimetallic catalyst for the deNOx and deN2O processes.

Tailoring the pore structure、morphology and acidity of MFI zeolites by the regulation of Si/Al ratio in the synthetic gel

In this work, we report a facile and low-cost approach for controllable preparation of hierarchical mesoporous MFI zeolites via simply manipulating the Si/Al ratio in the initial synthetic precursor. The results demonstrate that Si/Al ratio significantly influences not only the acidity but also the pore structure as well as morphology of MFI zeolites. With the increase of Si/Al ratio, the mesoporous structures are weakened due to the increasingly severe crystal fusion leading to the formation of bulk zeolite crystal and the acid amounts decrease as well. The optimal sample Z-55 with spherical morphology is composed of numerous superfine nanocrystals. These primary results are quite beneficial to guiding reasonable design of the zeolites with other topologies. In LDPE cracking, the sample Z-55 with tri-modal mesopores distributions performs enhanced catalytic activity compared with convention MFI zeolite and the T50 is reduced to 391 °C, mainly ascribed to the luxuriant mesoporous structures.

A theoretical study on the separation of CO2/CH4 through MFI zeolite

The separation of carbon dioxide (CO2) and methane (CH4) is important in addressing global warming and energy shortages. Porous materials play a large role, of which MFI zeolite has received much attention. To better analyse mechanisms or guide experiments, the adsorption and diffusion properties of CO2 and CH4 in MFI zeolite have been studied by Monte Carlo and molecular dynamics simulations. Due to stronger interactions, the loading of pure CO2 is significantly higher than that of CH4. For mixtures (biogas or natural gas), CO2 dominates in competitive adsorption. Lower temperatures and higher pressures correspond to higher adsorption selectivity. In addition, the self-diffusion coefficients of guest molecules in MFI zeolite were also calculated. The diffusion performance of pure CH4 is superior to CO2 under the same conditions. And their self-diffusion coefficient increases with increasing temperature and decreasing load. For mixed components, CO2 greatly limits the diffusion of CH4.

Products and coke shape-selectivity during anisole disproportionation over HZSM-5

Anisole conversion on an MFI zeolite (SiAl = 43) at atmospheric pressure and 673 K is a model for the upgrading of bio-oils produced by catalytic fast pyrolysis (CFP) of biomass. Catalyst activity and selectivity are measured experimentally, while deactivation is studied experimentally and by DFT calculations. Anisole dismutation is a Friedel-Crafts-type alkylation between two anisole molecules. Consecutive reactions produce phenol and methyl anisoles, cresols and xylenols, as primary, secondary, and tertiary products respectively. The resonance effect of the hydroxyl and methoxy groups, combined with the "product shape selectivity" of the zeolite, results in a high selectivity for p-methyl-anisole, o-cresol and 2,4 xylenol. Coke deposition consists exclusively of methyl phenols with 0–4 methyl groups. Their retention is due to steric hindrance for the larger ones and strong adsorption on stronger acid sites for the smaller ones, as confirmed by DFT calculations. Catalyst deactivation occurs by progressive fouling of the micropores rather than by poisoning.

Frequency Response Analysis of FAU, LTA and MFI Zeolites Using UV-Vis and Electrochemical Impedance Spectroscopy

Zeolites are porous materials that have cavities interconnected by channels. These crystalline materials are composed of Si-O tetrahedral structures, and according to the assembly of such tetrahedral structures, specific crystalline structures are obtained. Until now, it has been said that there are more than 245 different zeolitic frameworks, and since each one has a specific distribution of pores and cavities, each kind of zeolite has a specific area-to-volume ratio. As a result of the type of zeolite structure, the zeolite can exhibit specific properties, i.e., electrical or optical. Moreover, the physical properties of zeolites can be modified after the inclusion of another chemical species in their structure or in their voids, which can result in tuning a zeolite for specific applications. In this work, synthetic zeolites of types LTA, FAU and MFI are characterized by a number of methods. In particular, the data from UV-Vis spectroscopy are analyzed, and the effect of crystalline structure on properties such as optical bandgap, refractive index, absorption coefficient, incident photon frequency, and extinction coefficient is studied.

Synthetic methods to vary crystallite properties of TON zeolites and their consequences for Brønsted-acid catalyzed propene oligomerization

Brønsted acid zeolites catalyze light alkene oligomerization to higher molecular weight alkenes. Rates and selectivities of propene oligomerization in MFI zeolites are regulated by intrazeolite diffusional constraints imposed by heavier alkene products that become occluded within zeolitic micropores during reaction, and by active site properties that influence kinetic rate constants. Propene oligomerization rates and selectivity are also reported to vary with zeolite topology; however, evaluating the relative influences of kinetic and diffusional factors on oligomerization rate and selectivity and their dependence on zeolite framework topology is challenged by a dearth of synthesis methods to crystallize zeolite topologies with independently varied active site and crystallite properties. Herein, we use combinations of different structure directing agents to crystallize TON zeolites, which comprise medium-pore unidirectional straight channels, with varied crystallite sizes (0.9–5.4 μm) at similar H+-site density (∼0.3 H+/unit cell (u.c.)). Propene dimerization rates (per H+, 503 K) decrease systematically with increasing TON crystallite size over a wide range of propene pressures (16–607 kPa C3H6), reflecting the strong influence of intrazeolite diffusion limitations. The effective diffusivity of propene, estimated through the effectiveness factor formalism, systematically decreases (by ∼5×) with increasing propene pressure, indicating that the higher concentrations of heavier alkene products formed at higher propene pressures inhibit intrazeolite diffusion; however, this decrease is less severe compared to that for MFI samples of similar H+-site density. An analysis of product selectivity on zeolite topologies of independently varied pore size and dimensionality (TON, MFI, MOR) reveals that the smaller pore size of TON (∼5 Å diam.) leads to higher rate constants for propene dimerization, but also restricts the growth of larger oligomer products (e.g., ≥ C9), resulting in a lighter composition of occluded products that is also more weakly influenced by changes in propene pressure compared to those present in MFI (∼5–7 Å diam.). Together, these findings reveal the distinct roles of zeolite pore size and interconnectivity in governing kinetic factors and the composition of occluded products that regulate intrazeolite diffusion during propene oligomerization catalysis in medium-pore zeolites, and thereby rate and selectivity.

Green hydrogen production from ammonia water by liquid–plasma cracking on solid acid catalysts

In this study, a method for producing green hydrogen from ammonia water using plasma cracking was proposed. Solid acid catalysts, FAU and MFI zeolites, were used as catalysts. Zeolites were prepared by varying the Si/Al molar ratio to investigate the relationship between the acid sites of the catalyst and the reaction activity. The plasma was directly discharged into the ammonia water to generate hydrogen from the decomposition of ammonia. The gaseous products from the ammonia decomposition by plasma were mostly hydrogen with some nitrogen. No CO2 was produced. Even when the catalyst was not injected, approximately 100 L/h of hydrogen was produced only by plasma discharge. The injection of the FAU or MFI zeolite catalysts improved the hydrogen production rate. The rate of hydrogen production obtained from the decomposition of ammonia water by catalyst and plasma irradiation was the highest for the FAU zeolite with the highest number of acid sites. The hydrogen production rate tended to increase in proportion to the number of acid sites in the catalyst.

Design of bi-functional Ni-zeolites for ethylene oligomerization: Controlling Ni speciation and zeolite properties by one-pot and post-synthetic Ni incorporation

The design of diverse Ni-zeolites has been performed attempting to control different physico-chemical properties of the zeolite supports as well as the Ni speciation to study their influence on the industrially relevant ethylene oligomerization reaction. In this sense, Ni-containing medium-pore MFI and large-pore BEA zeolites have been synthesized with different particle sizes, Si/Al molar ratios and following different Ni-incorporation strategies (one-pot versus post-synthesis). Characterization techniques reveal the preferential presence of cationic Ni2+ and NiO nanoclusters inside the zeolitic pores when Ni has been incorporated by post-synthesis impregnation or one-pot approaches, respectively. The presence of Ni2+ species, rather than other factors, mainly governs olefin oligomerization under near-ambient pressures, guiding preferentially towards C4-C6 olefins as main products. Crystal size, acidity and/or Ni incorporation methodology, on the other hand, play key roles under high-pressure reaction conditions, resulting in the formation of heavier olefins, especially in zeolites containing less cationic nickel, larger proportion of NiO clusters and higher Brønsted acid site density.

Conversion of polyethylene waste to short chain hydrocarbons under mild temperature and hydrogen pressure with metal-free and metal-loaded MFI zeolites

Catalytic hydrocracking on supported metal zeolites promisingly converts waste plastics into more valuable hydrocarbons. Although the presence of metal sites on solid acids is typically considered necessary for C-C bond cleavage of alkanes at modest temperatures, we show that polyethylene (PE) depolymerization on metal-free MFI zeolites proceeds at higher rates than analogous metal-loaded MFI zeolites (Pt, Ni) under mild reaction conditions. Higher rates on metal-free MFI are consistent with higher ratios of alkenes to alkanes, leading to subsequent alkene-mediated beta-scission events that form smaller molecules. Under varied reaction conditions (e.g., temperature, pressure, H2 versus N2), both metal-free and metal-loaded MFI catalysts demonstrate high selectivity to C3-C7 gaseous hydrocarbons, likely due to the 10-MR channel structure. Collectively, these findings demonstrate the importance of deconvoluting metal and acid sites contributions for PE hydrocracking, which has significant implications on rates and product selectivities of plastic upcycling reactions.

Unveiling the elusive roles of Cu species in determining the hydrocarbon trap performance during cold start in Cu-impregnated MFI type zeolites

High-performance zeolite-based hydrocarbon (HC) traps were developed by impregnating MFI zeolites with Cu species. The corresponding cold-start test (CST) results revealed that the optimal Cu loading (ca. 5–7 wt%) led to high CST performance. To the best of our knowledge, our unprecedented complementary investigation based on both experimental and theoretical approaches revealed that Cu+ ions rather than Cu2+ ions were key for securing high HC adsorption (especially propene) in the presence of copious amounts of water vapor. Moreover, tiny CuO nanoparticles (≤ 4 nm) on the exterior zeolite surface resulted in effective HC oxidation. This preferential HC adsorption and oxidation synergistically contributed to effective hydrocarbon trapping, making the Cu-impregnated MFI zeolites active HC traps. However, after hydrothermal treatment, a lower Cu content (3 wt% instead of 5–7 wt%) was rather desirable for preserving the marked CST performance, indicating that the original physicochemical properties were more damaged at higher Cu content.

Three-dimensional inhomogeneity of zeolite structure and composition revealed by electron ptychography.

Structural and compositional inhomogeneity is common in zeolites and considerably affects their properties. Thickness-limited lateral resolution, lack of depth resolution, and electron dose-constrained focusing limit local structural studies of zeolites in conventional transmission electron microscopy (TEM). We demonstrate that a multislice ptychography method based on four-dimensional scanning TEM (4D-STEM) data can overcome these limitations. Images obtained from a ~40-nanometer-thick MFI zeolite exhibited a lateral resolution of ~0.85 angstrom that enabled the identification of individual framework oxygen (O) atoms and the precise determination of the orientations of adsorbed molecules. Furthermore, a depth resolution of ~6.6 nanometers allowed probing of the three-dimensional distribution of O vacancies, as well as the phase boundaries in intergrown MFI and MEL zeolites. The 4D-STEM ptychography can be generally applied to other materials with similar high electron-beam sensitivity.

Structural Evolution of Copper-Oxo Sites in Zeolites upon the Reaction with Methane Investigated by Means of Cu K-edge X-ray Absorption Spectroscopy

The structure of copper sites formed under an oxidative environment and their evolution in the course of the reaction with methane at elevated temperature was investigated by means of Cu K-edge X-ray absorption spectroscopy for a series of copper-containing MFI, MOR, and FAU zeolites. The pretreatment in oxygen at 723 K leads to the formation of copper(II)-oxo sites, whose nature depends on the framework type. Dimeric species are formed in CuMFI material, dimeric and monomeric sites coexist in CuMOR, and agglomerated copper-oxo nanoclusters are found in large-pore copper-containing faujasite (CuFAU). For all studied materials, the reaction with methane resulted in the exclusive formation of copper(I) species; no formation of metallic copper was detected even at 748 K. The nature of formed copper(I) species is governed by the structure of corresponding copper(II) centers. In particular, monomeric and dimeric copper(II)-oxo sites hosted in CuMOR and CuMFI are transformed into isolated copper(I) cations coordinated to ion-exchange positions of the zeolite. Contrarily, copper(II)-oxo clusters present in CuFAU undergo restructuring with only a partial loss of extra-framework oxygen and form aggregated species with a structure similar to that of bulk copper(I) oxide.