Acid Sites In Zeolites Research Articles

Transformation of Dilute Ethylene at High Temperature on Micro- and Nano-Sized H-ZSM-5 Zeolites

Ethylene dehydroaromatisation (EDA) was investigated at 700 °C under 1 bar of ethylene (5 mol% in N2) over a micro-(M) and a nano-sized (N) H-ZSM-5. On the M zeolite an induction period followed by deactivation was observed, which could be related to the presence of long diffusion path lengths in this sample, leading to mass transfer resistance. During the induction step, the aromatics yield increases, despite a significant loss of the acid site concentration as a result of coking. This induction period corresponds to the formation of an active hydrocarbon pool (HCP) composed of units of 2 to 5 aromatic rings with a molecular weight ranging from 130 to 220 g mol−1 (light coke). A kinetic study revealed that the developing HCP species is two times more active than Brønsted acid sites in the fresh zeolite. Diffusion limitations yet impact the product desorption by promoting coke growth and, therefore the deactivation of the HCP and hence of the catalyst. From MA-LDI/LDI-TOF MS (Matrix Assisted Laser Desorption Ionization—Time of Flight Mass Spectroscopy) characterisation was deduced that even after complete catalyst deactivation, the as-deposited coke continues growing at the external surface of the zeolite by condensation reactions, thus leading to heavy coke composed of more than 100 carbon atoms and a molar mass exceeding 1300 g mol−1. Unlike the micro-sized zeolite, the nano-scaled zeolite features a short diffusion path length and promotes fast formation of the active HCP. As a result, higher activity and selectivity into benzene were observed, whilst catalyst deactivation was significantly mitigated.

Mechanistic insight into the catalytic cracking mechanism of α-olefin on H-Y zeolite: A DFT study

In the bifunctional catalyst for Fischer-Tropsch synthesis, the distribution of products in the secondary catalytic reaction of intermediate products is affected by the Brønsted acid sites in zeolites. Linear α-olefins are the intermediates of Fischer-Tropsch synthesis. In this work, density functional theory (DFT) calculation was carried out to understand further Brønsted Acid Sites’ mechanism in the secondary reaction of Fischer-Tropsch synthesis. The adsorption energy of linear α-olefins on H-Y zeolite decreased with the increase of the carbon chain. Besides, four essential reactions were simulated by the DFT method. The four elementary reactions' energy barriers are: β bond-breaking reaction > skeleton isomerization reaction > intramolecular hydrogen transfer reaction > protonation reaction. Then, a complete reaction network of straight-chain α-Nonylene on H-Y zeolite was designed. The β bond-breaking reaction is a key step to determine the type and distribution of products.

Design of a ternary 3D composite from hydrochar, zeolite and magnetite powder for direct conversion of biomass to gasoline

Recyclable zeolite-based composites were prepared using a simple one-step hydrothermal liquefaction (HTL) process. The synthesis process introduced magnetite and commercial zeolite into/onto the meso-macro structured algal hydrochar. The composites were analyzed for their N2 adsorption porosity, surface functional groups, total acid sites, and scanning electron microscopy morphology. The crystalline phase-change during the HTL and chemical interaction of hydrochar/zeolite and hydrochar/magnetic powder were confirmed by XRD and SEM-EDS analyses. The preparation of five catalytic systems along with possible reaction mechanisms were investigated in HTL of a local freshwater green alga topic using Design Expert 11 software to plan the experiments. Py-GCMS of the obtained bio-oil indicated that using two-dimensional (A2D) and three-dimensional (A3D) catalysts could catalyze the reactions toward the production of heavy gasoline (C7-C12) and biodiesel (C8-C21), respectively. It was shown that in 2D architecture, ketones undergo a series of aldol condensation reactions, and alkenes experience oligomerization, whereas in the 3D structure, the increase of the local pressure, expels monomers in the form of gasoline from the catalyst pores. Our newly developed 3D ternary composite not only had no negative effect on total acid sites of commercial zeolite but also, boosted the strong acidic sites due to the accommodation of Zn4.00Fe16.00Ni4.00O32.00 inside the composite.

Obtaining High-Molecular hydrocarbons from synthesis gas and Physico-Chemical characteristics of the catalyst

The article studied the reaction of obtaining high-molecular hydrocarbons from the synthesis gas and the physicochemical characteristics of the catalyst containing Co-Fe-Ni-ZrO2/HSZ used in this process. The reason for the increase in selectivity in methane formation is, firstly, the acceleration of the hydrogenation reaction of CO in the methane formation centers with increasing synthesis temperature and secondly, the diffusion of CO and H2 at different speeds through zeolite pores. In all fractions of hydrocarbons synthesized in the presence of hybrid catalysts, secondary changes: isoparaffins and olefins are formed due to cracking and isomerization. A large amount of isoparaffins is recorded for the composite catalyst-the iso/n parameter is 0.6, which indicates an increase in its activity relative to the catalysts absorbed in the hydrogenation reactions. For catalysts with low cobalt content, the concentrations of olefins will be high, which is due to their low hydrogenation ability compared to unsaturated hydrocarbons. In absorption catalysts, the hydrocarbon synthesis centers and the zeolite acid sites are in close contact with each other, which contributes to the intensification of secondary processes. However, the content of liquid hydrocarbons (C5-C18) in C5+ hydrocarbons is about 83%, which is 12% less than in the composite catalyst. No oxide-oxide interaction was found for the composite catalyst prepared by mixing the components, the pores of the zeolite are not blocked by cobalt, which allows the reagents to be efficiently delivered to the active part of the hydrocarbon synthesis.

High Conversion of Syngas to Ethene and Propene on Bifunctional Catalysts via the Tailoring of SAPO Zeolite Structure

Direct conversion of syngas to light olefins using bifunctional catalysts containing both oxides and zeolites represents a promising alternative in comparison with Fischer-Tropsch synthesis. It is still a great challenge to achieve highly selective production of ethylene and propylene at a satisfying CO conversion for potentially industrial applications. Here, we report that a high selectivity to ethylene and propylene can be achieved by tailoring silicoaluminophosphate zeolite structure and acid site strength. By comparing bifunctional catalysts composed of ZnCr oxide and different zeolites with 8-membered ring openings, we achieve the total selectivity of 76.4% in ethylene and propylene at a CO conversion as high as 38.2% by catalysts containing SAPO-17 zeolite (ERI framework). The detailed hydrocarbon distribution is nearly identical to that in the methanol-to-olefins reaction. The high selectivity can be rationalized due to both smaller 8-membered ring openings and the weaker Brønsted acid strength of SAPO-17 zeolites.

Zeolite catalyzed hydroarylation of alkenes with aromatic amines under organic ligand-free conditions

The hydroarylation of alkenes with aromatic amines is recognized as the most atom-economical and straightforward approach to obtain functional aromatic amines, which are versatile building blocks in organic synthesis and material chemistry. However, controllable synthesis of single hydroarylation product is still a significant challenge because hydroarylation reaction often delivers four hydroarylation products and hydroamination products are also produced during the reaction. Herein, we report the first example of heterogeneous zeolite catalyzed hydroarylation of styrene and norbornene with aniline derivatives under organic ligand-free conditions. With the USY zeolite as catalyst, a wide scope of alkenes and aromatic amines with various functional groups are smoothly converted into the corresponding products in 48–95% yields with high regioselectivity. Detailed characterizations revealed that Lewis acid can promote Hofmann-Martius rearrangement of hydroamination products toward hydroarylation products, resulting in high selectivity for hydroarylation products. In addition, it could be found that the weak acid sites of zeolite play a key role in forming hydroarylation products. Furthermore, the catalyst can be reused at least 10 times without obvious deactivation. This work may promote the development of heterogeneous catalyst system for alkene hydroarylation.

Mechanistic Understanding of Catalytic Conversion of Ethanol to 1-Butene over 2D-Pillared MFI Zeolite

Ethanol is an important C2 platform molecule for producing value-added chemicals and distillate hydrocarbon fuels (e.g., jet and diesel). Among these, catalytic upgrading of ethanol to butenes can generate valuable commodity chemicals (e.g., 1-butene) and provide C4 olefin intermediates that can be further upgraded to jet/diesel fuels. Two-dimensional (2D) zeolites offer hierarchical mesoporous structures, leading to improved mass transport and reduced diffusion length, which can help to address the coking challenges faced by ethanol conversion to hydrocarbons over three-dimensional (3D) zeolites. In this study, we investigate the acid-catalyzed conversion of ethanol to 1-butene over the Brønsted acid sites (BAS) in 2D-pillared MFI zeolite (2D-PMFI) using ab initio molecular dynamics (AIMD) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and calorimetric measurements. A detailed thermodynamic analysis, using quasi-harmonic approximation (QHA), on the Gibbs free-energy pathway of ethanol conversion shows that the consideration of entropy is critical to accurately capture the detailed thermodynamic profiles. Employing the Blue Moon ensemble method, the formation of framework-bound butoxide from ethoxy and ethene is found to be the likely rate-determining step (RDS), proceeding via a stepwise mechanism. The reactivity of 2D-PMFI can be further tuned by manipulating RDS through careful control of the number of BAS and operating temperatures. The calculated vibrational density of states (VDOS) validate the structural models of adsorbed ethanol by comparing with the experimental DRIFTS measurements. Overall, our study provides mechanistic insights into ethanol upgrading over the 2D-PMFI and shows the importance of evaluating entropic effects in such a confined system.

Hierarchy in zeolite catalysis: The influence of enhanced mesoporosity on the synthesis of renewable fuels and bio-based platform chemicals

Faujasite (FAU), ZSM-5 (MFI), beta (BEA) and mordenite (MOR) zeolites were admitted to a variety of chemical treatments accompanied by surfactant templating strategy, aiming to introduce the intracrystalline mesoporosity effectively. The resulting materials were tested as solid acid catalysts for esterification of the oleic acid as a common model impurities found in bio-oil feedstoks. It was found that the esterification of oleic acid can be enhanced by the presence of strong acid sites in zeolites and their improved accessibility. Overall, mesostructured FAU zeolite demonstrated an improved catalytic performance as a result of increasing accessibility of the zeolite active sites.

Two-in-One Catalyst Turns Carbon Dioxide in Base Chemicals

Cascading chemical reactions can be realized when the local structure and composition of two catalytically active phases are fine-tuned and combined in one material. In this issue of Chem, Fan et al. report an innovative approach for the conversion of carbon dioxide into light olefins with a metal oxide-zeolite tandem catalyst.

Promoting Effect of Mn on In Situ Synthesized Cu-SSZ-13 for NH3-SCR

The effect of Mn impregnation on the NH3-SCR (selective catalytic reduction of NOx by NH3) activity of in situ synthesized Cu-SSZ-13 was investigated in this work. It was found that Mn addition could efficiently improve the low-temperature activity of Cu-SSZ-13. The optimal amount of Mn was 5 wt.%, and NOx conversion was improved by more than 20% over a temperature range of 120 °C to 150 °C. SEM (scanning electron microscopy), XRD (X-ray diffraction), N2 adsorption-desorption, H2-TPR (temperature programmed reduction of H2), NH3-TPD (temperature programmed desorption of NH3) and in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) experiments were conducted to investigate the changes in the zeolite structure, active sites, acid sites and reaction mechanism. The impregnated MnOx species caused a decline in the crystallinity of Cu-SSZ-13 but markedly improved the redox ability. Nitrate and nitrite species were observed in the Mn-modified Cu-SSZ-13, and the formation of these species was thought to cause the observed increase in low-temperature NH3-SCR activity. The results show that the addition of Mn is a promising method for promoting the low-temperature catalytic activity of Cu-SSZ-13.

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts for n-Heptane Hydroisomerization.

In this study, Pt nanoparticles on zeolite/γ-Al2O3 composites (50/50 wt) were located either in the zeolite or on the γ-Al2O3 binder, hereby varying the average distance (intimacy) between zeolite acid sites and metal sites from “closest” to “nanoscale”. The catalytic performance of these catalysts was compared to physical mixtures of zeolite and Pt/γ-Al2O3 powders, which provide a “microscale” distance between sites. Several beneficial effects on catalytic activity and selectivity for n-heptane hydroisomerization were observed when Pt nanoparticles are located on the γ-Al2O3 binder in nanoscale proximity with zeolite acid sites, as opposed to Pt nanoparticles located inside zeolite crystals. On ZSM-5-based catalysts, mostly monobranched isomers were produced, and the isomer selectivity of these catalysts was almost unaffected with an intimacy ranging from closest to microscale, which can be attributed to the high diffusional barriers of branched isomers within ZSM-5 micropores. For composite catalysts based on large-pore zeolites (zeolite Beta and zeolite Y), the activity and selectivity benefitted from the nanoscale intimacy with Pt, compared to both the closest and microscale intimacies. Intracrystalline gradients of heptenes as reaction intermediates are likely contributors to differences in activity and selectivity. This paper aims to provide insights into the influence of the metal–acid intimacy in bifunctional catalysts based on zeolites with different framework topologies.

Steering polymer growth by molding nanochannels: 1,5-hexadiene polymerization in high silica mordenite

Zeolites are known as scaffolds for the assembly of molecules via non-covalent interactions yielding organized supramolecular materials. Yet their potential in driving the growth of low-dimensional systems requiring covalent bond formation is still unexplored. We incorporated 1,5-hexadiene in the channels of a high-silica mordenite and analyzed the material by infrared spectroscopy, X-ray powder diffraction, thermogravimetry and modeling techniques. Due to the few zeolite acid sites, 1,5-hexadiene experiences a slow conversion to a polymer mainly formed by cyclopentane units and featuring short side chains that are able to fit the channels. The shape-directing abilities of zeolite frameworks play a two-fold role, involving first the organization of the monomers inside the void-space and then the linear growth of the chain dictated by the channel geometry. These findings highlight the molding action of zeolites in directing transformations of covalent bonds under ambient conditions and may provide insights for obtaining confined polymers with intriguing prospective applications.

A DFT Study for Catalytic Deoxygenation of Methyl Butyrate on a Lewis Acid Site of ZSM-5 Zeolite

The catalytic deoxygenation mechanism of fatty acid esters on a Lewis acid site of ZSM-5 zeolite was elucidated via density functional theory (DFT) by using a methyl butyrate (MB) as the model compound for fatty acid esters. The configurations of the initial reactant, transition states, and products together with the activation barrier of each elementary reaction were determined. The activation barrier of different initial cracking reactions decreases in the order of α-C–C > β-C–C > α-C–O > β-C–O. The best reaction path for catalytic deoxygenation of methyl butyrate over Lewis acid site is CH3CH2CH2C(OCH3)=O⋯Lewis → CH3CH2⋯Lewis⋯C(=CH2)OCH3 → CH2=CH2 + CH3COOCH3 + Lewis. The oxygen of methyl butyrate is mainly removed as CO2, methyl acetate, formaldehyde, and butyraldehyde, while ethylene, propylene, and butane are the main hydrocarbon products. In addition, the group generated by cracking of methyl butyrate form a bond with the Lewis acid site, promoting the transformation between a Lewis acid and a Brønsted acid. The corresponding intermediates have a high single point energy, but the poor stability leads to further deoxygenation and cracking reactions. This work provides a theoretical basis for the modification in the number of Brønsted acid and Lewis acid sites in the ZSM-5 zeolite.

Local environment and catalytic property of External Lewis acid sites in hierarchical lamellar titanium Silicalite-1 zeolites

Abstract Hierarchical two-dimensional (2D) lamellar zeolites represent a young emerging field and a novel expression of the zeolite frameworks. Accompanied with synthesis and catalytic performance investigation, assessment on the local environment and catalytic behavior of acid sites in 2D zeolites are needed. Here, we described the local environment of Lewis acid sites and evaluated catalytic activities of external acid sites (i.e. sites on external surface and in mesopores) in 2D lamellar titanium silicalite-1 (TS-1) zeolites. For comparison purpose, four TS-1 zeolites, including 3D TS-1, multilamellar 2D TS-1 without pillarization and with pillarization by silica and silica/titania, respectively, were prepared. The local environment of Ti-sites, including coordination status of Ti-atom, speciation of hydroxyl (-OH) group that hints degree of confinement, acid site strength and accessibility, was collectively assessed by spectroscopic and organic base titration measurements. Kinetic studies of cyclooctene (C8H14) epoxidation with hydrogen peroxide (H2O2) discerned catalytic activity of external Ti-sites that decreased with increasing mesoporosity due to decrease in site confinement.

China to dominate Asian refinery fluid catalytic cracking units capacity growth by 2024

HZSM-5 was prepared by the hydrothermal method using TPAOH as a structure directing agent without Na+ addition. The calcination process for removing the organic templates was studied and particularly the influence of calcination temperature on zeolite structure and acid property was investigated. It was found that dehydrogenation of Brønsted acid sites in zeolite framework occurred with the elevation of calcination temperature, leading to a decrease of catalytic activity for n-decane cracking. To recover the Brønsted acid sites in zeolite lost during calcination, the calcinated zeolite was treated with various reagents, including H2O, H2O2, NH3 and NH4Cl, to turn Lewis acid sites back to Brønsted acid sites. Among them, the NH4Cl treatment was found to be the most effective way, which increased conversion of n-decane from 6% to 77% and increased the B/L acid sites ratio from 0.503 to 3.95. This enhancement of catalytic cracking ability is probably related to the formation of an intermediate active component ([AlO4]0) during high temperature calcination, and these intermediate sites promoted the protonation of Lewis acid sites in the NH4Cl treatment process.

Dichloromethane catalytic combustion over Co3O4 catalysts supported on MFI type zeolites

Three different MFI type zeolites Silicalite-1, ZSM-5 and TS-1 as supports were synthesized by steam-assisted crystallization method. Then, the as-synthesized zeolites were used to prepare Co 3 O 4 supported MFI type zeolite catalysts by wet impregnation. The as-prepared Co 3 O 4 supported MFI type zeolite catalysts possessed high specific surface area, abundant pore structure and surface acidity. Finally, the as-prepared catalysts were used for dichloromethane (DCM) catalytic combustion and the results showed that Co 3 O 4 supported MFI type zeolite catalysts had better performance than pure Co 3 O 4 . The synergistic catalytic effects between MFI type zeolite and Co 3 O 4 effectively reduced the formation of polychlorinated by-products and suppressed the deactivation of catalyst during the catalytic combustion of DCM. And Co 3 O 4 /ZSM-5 catalyst exhibited the best performance due to its proper Brønsted acid sites amount and the T 90 (the reaction temperature at which the dichloromethane conversion reached 90%) was 370 °C at 1000 ppm DCM, 10% O 2 /N 2 and the gas hourly space velocity of 30,000 mL/(g∙h). • The Co 3 O 4 supported MFI type zeolites were synthesized by impregnation method. • The zeolite as support restrained the deactivation of Co 3 O 4 catalyst during DCM catalytic combustion. • The surface acid sites of zeolite restrained the polychlorinated byproducts formation. • Co 3 O 4 /ZSM-5 exhibited the best performance for the catalytic combustion of DCM.

The Effect of Ni-ZSM-5 Catalysts on Catalytic Pyrolysis and Hydro-Pyrolysis of Biomass.

With the demand of energy and re-utilization of wastes, the renewable lignocellulosic biomass, has attracted increasing and significant attention for alleviating the growing energy crisis and environment problems. As main components of lignocellulosic biomass, lignin, cellulose, and hemicellulose are connected by hydrogen bond to form a compact skeleton structure, resulting the trenchant condition of biomass pyrolysis. Also, pyrolysis products of above three main constituents contain a large amount of oxygenates that cause low heating value, high corrosiveness, high viscosity, and instability. Meanwhile, zeolites are of considerable significance to the conversion of lignocellulosic biomass to desirable chemical products on account of fine shape selectivity and moderate acid sites and strength. Among numerous zeolites, ZSM-5-based catalysts have been most extensively studied, and the acidity and porosity of ZSM-5 can be tuned by changing the content of Si or Al in zeolite. Beyond that, doping of other metal elements, such as Mn, Co, Ni, Ga, Ce, Pt, into ZSM-5 is also an efficient way to regulate the strength and density of acid sites in zeolite precisely. This review focused on the recent investigation of Ni-modified microporous ZSM-5 used in catalytic pyrolysis of lignin and cellulose. The application of metal-modified hierarchical ZSM-5 is also covered.

Towards a better understanding of Lewis acidic aluminium in zeolites.

Zeolites are a class of materials that are of great relevance for industrial catalysis. Several fundamental questions relating to the structure and role of the Lewis acid sites in these materials remain unanswered. Proposals for the origin of such species can broadly be classified into three categories, which have distinct structures: extra-framework, framework-associated and framework aluminium. In this Perspective, we review each of these proposals and proceed to analyse their suitability to understand experimental results. Contrary to traditional belief, the number of Lewis acid sites does not always correlate to extra-framework aluminium content. As a result, we highlight that the terms 'extra-framework' and 'framework-associated' aluminium should be used with caution. We propose how the usage of different characterization techniques can enable the closure of knowledge gaps concerning the strength, multiplicity, localization and structure of catalytically active Lewis acid sites in zeolites.

Controlled direct synthesis of single- to multiple-layer MWW zeolite.

The minimized diffusion limitation and completely exposed strong acid sites of the ultrathin zeolites make it an industrially important catalyst especially for converting bulky molecules. However, the structure-controlled and large-scale synthesis of the material is still a challenge. In this work, the direct synthesis of the single-layer MWW zeolite was demonstrated by using hexamethyleneimine and amphiphilic organosilane as structure-directing agents. Characterization results confirmed the formation of the single-layer MWW zeolite with high crystallinity and excellent thermal/hydrothermal stability. The formation mechanism was rigorously revealed as the balanced rates between the nucleation/growth of the MWW nanocrystals and the incorporation of the organosilane into the MWW unit cell, which is further supported by the formation of MWW nanosheets with tunable thickness via simply changing synthesis conditions. The commercially available reagents, well-controlled structure and the high catalytic stability for the alkylation of benzene with 1-dodecene make it an industrially important catalyst.

The Influence of Residual Chlorine on Pt/Zeolite Y/γ-Al2O3 Composite Catalysts: Acidity and Performance

In previous studies H2PtCl6⋅6H2O or Pt(NH3)4(NO3)2 were used to control the location of Pt nanoparticles in Zeolite/γ-Al2O3 composites for the synthesis of metal-acid bifunctional catalysts. Use of H2PtCl6⋅6H2O led to Pt nanoparticles located on the γ-Al2O3 binder, in a ‘nanoscale’ intimacy with zeolite acid sites being beneficial for performance in the conversion of hydrocarbons, compared to catalysts with Pt nanoparticles located in zeolite Y (‘closest’ metal-acid intimacy) obtained by use of Pt(NH3)4(NO3)2. A side effect of using H2PtCl6∙6H2O was a higher NH3 uptake in NH3-TPD experiments, that suggested a higher acidity possibly affecting catalysis. In this study the effect of residual chlorine on Pt/Zeolite Y/γ-Al2O3 catalysts (0.1-0.6 wt.% Cl) was investigated by (pyridine) FTIR, NH3-TPD and n-alkane hydro-isomerization. The coordination of chlorine to γ-Al2O3 was observed and led to a higher NH3 uptake, but no effect of chlorine on the catalytic performance of the catalysts was detected.