Zeolite Catalysis Research Articles

Reaction rate enhancement by reducing surface diffusion barriers of guest molecules over ZSM-5 zeolites: a structured illumination microscopy study

Mass transfer of guest molecules is crucial for zeolite catalysis. Compared to intracrystalline diffusion, the surface barriers in zeolite catalysis have been less understood. Taking furfuryl alcohol oligomerization catalyzed by ZSM-5 zeolites as model reaction, here we present a single-crystal level study and gain insight into the surface barriers induced spatiotemporal evolution of oligomer products using structured illumination microscopy (SIM). It is shown that, as a result of the enhanced accessibility to zeolite crystals, promoting the surface permeability leads to the increase in catalytic reaction rate that is characterized as the generation rate of oligomer products.

Supercritical ion exchange: A new method to synthesize copper exchanged zeolites

A new technique termed Supercritical Ion Exchange (SCIE) was developed and used to synthesize Cu-mordenite (Cu-MORS). The ion exchange takes place between the Cu complex (Copper(II)trifluoroacetylacetonate) dissolved in supercritical CO2 (scCO2) and the extraframework protons in mordenite zeolite without requiring an aqueous phase. The occurrence of the ion exchange reaction was demonstrated by using 1H NMR analysis of the high-pressure fluid phase samples and by visual inspection of the fluid phase color change during the synthesis. SCIE resulted in selective ion-exchange inferred by the equilibrium isotherm. The prepared catalysts were used for the stepwise direct methane to methanol (sDMTM) process and results showed that methanol productivity increased linearly with increasing Cu loading up to a certain Cu wt%. Cu-MORS displayed 16% higher methanol productivity as compared to Cu-MORA (prepared by aqueous ion exchange) with the same Cu loading. The results demonstrated the importance of site selective ion-exchange for zeolite catalysis.

Molecular Understanding of the Catalytic Consequence of Ketene Intermediates under Confinement.

Neutral ketene is a crucial intermediate during zeolite carbonylation reactions. In this work, the roles of ketene and its derivates (viz., acylium ion and surface acetyl) associated with direct C–C bond coupling during the carbonylation reaction have been theoretically investigated under realistic reaction conditions and further validated by synchrotron radiation X-ray diffraction (SR-XRD) and Fourier transformed infrared (FT-IR) studies. It has been demonstrated that the zeolite confinement effect has significant influence on the formation, stability, and further transformation of ketene. Thus, the evolution and the role of reactive and inhibitive intermediates depend strongly on the framework structure and pore architecture of the zeolite catalysts. Inside side pockets of mordenite (MOR), rapid protonation of ketene occurs to form a metastable acylium ion exclusively, which is favorable toward methyl acetate (MA) and acetic acid (AcOH) formation. By contrast, in 12MR channels of MOR, a relatively longer lifetime was observed for ketene, which tends to accelerate deactivation of zeolite due to coke formation by the dimerization of ketene and further dissociation to diene and alkyne. Thus, we resolve, for the first time, a long-standing debate regarding the genuine role of ketene in zeolite catalysis. It is a paradigm to demonstrate the confinement effect on the formation, fate, and catalytic consequence of the active intermediates in zeolite catalysis.

Emerging applications of zeolites in catalysis, separation and host–guest assembly

Emerging applications of zeolites in catalysis, separation and host–guest assembly

Progressive steps and catalytic cycles in methanol-to-hydrocarbons reaction over acidic zeolites

Understanding the complete reaction network and mechanism of methanol-to-hydrocarbons remains a key challenge in the field of zeolite catalysis and C1 chemistry. Inspired by the identification of the reactive surface methoxy species on solid acids, several direct mechanisms associated with the formation of the first C-C bond in methanol conversion have been recently disclosed. Identifying the stepwise involvement of the initial intermediates containing the first C-C bond in the whole reaction process of methanol-to-hydrocarbons conversion becomes possible and attractive for the further development of this important reaction. Herein, several initial unsaturated aldehydes/ketones containing the C-C bond are identified via complementary spectroscopic techniques. With the combination of kinetic and spectroscopic analyses, a complete roadmap of the zeolite-catalyzed methanol-to-hydrocarbons conversion from the initial C-C bonds to the hydrocarbon pool species via the oxygen-containing unsaturated intermediates is clearly illustrated. With the participation of both Brønsted and Lewis acid sites in H-ZSM-5 zeolite, an initial aldol-cycle is proposed, which can be closely connected to the well-known dual-cycle mechanism in the methanol-to-hydrocarbons conversion.

Microwave synthesis of zeolites and their related applications

Zeolites are an emerging class of crystalline, nanoporous, inorganic material with regular micropores and highly tunable functionalities. Zeolites have been proposed as ideal materials for applications in catalysis, adsorption, and biomedicine due to their notable intrinsic features such as accessible void space, large surface area, and strong acidity. Microwave radiation is a robust technology that can quickly and efficiently prepare uniform and controllable zeolites that enable remarkable functions. We first review the current research progress in, and the fabrication of, zeolites using microwave technologies, and the related application. After a brief introduction to microwave theory and modification of zeolites, the discussion focuses on the five main microwave technologies for the synthesis of zeolites, that include microwave irradiation, microwave assisted hydrothermal synthesis, microwave assisted aging synthesis, microwave assisted ionothermal synthesis, and other microwave related methods. Some critical applications such as catalysis (NO x reduction, CO 2 reduction, and Fenton), gas sensing and CO 2 capture, and biomedicine are selected and discussed in detail. Finally, a summary and perspectives on research that may lead to the future development of this blossoming field. • Discuss the microwave theory and modification of zeolites. • Highlight recent achievements in manufacturing zeolites via microwave synthesis. • Some applications of such zeolites in catalysis, gas sensing and CO 2 capture, and biomedicine follow. • Comment on remaining challenges and provide prospects for further developments in this blossoming field.

The Role of Zeolite Framework in Zeolite Stability and Catalysis from Recent Atomic Simulation

The Role of Zeolite Framework in Zeolite Stability and Catalysis from Recent Atomic Simulation

Confinement in a Zeolite and Zeolite Catalysis.

ConspectusZeolites, accompanied by their initial discovery as natural mines and the subsequent large-scale commercial production, have played indispensable roles in various fields such as petroleum refining and the chemical industry. Understanding the characteristics of zeolites, in contrast to their counterparts with similar chemical compositions and the origin thereof, is always a hot and challenging topic. Zeolites are known as intrinsic confined systems with ordered channels on the molecular scale, and structural confinement has been proposed to explain the unique chemical behaviors of zeolites. Generally, the channels of zeolites can regulate the diffusion of molecules, leading to a visible difference in molecular transportation and the ultimate shape-selective catalysis. On the other hand, the local electric field within the zeolite channels or cages can act on the guest molecules and change their energy levels. Confinement can be simply interpreted from both spatial and electronic issues; however, the nature of zeolite confinement is ambiguous and needs to be clarified.In this Account, we make a concise summary and analysis of the topics of confinement in a zeolite and zeolite catalysis from two specific views of spatial constraint and a local electric field to answer two basic questions of why zeolites and what else can we do with zeolites. First, it is shown how to construct functional sites including Brønsted acid sites, Lewis acid sites, extraframework cation sites, and entrapped metal or oxide aggregates in zeolites via confinement and how to understand the specific role of confinement in their reactivity. Second, the multiple impacts of confinement in zeolite-catalyzed reactions are discussed, which rationally lead to several unique processes, namely, Brønsted acid catalysis confined in zeolites, Lewis acid catalysis confined in zeolites, catalysis by zeolite-confined coordinatively unsaturated cation sites, and a cascade reaction within the confined space of zeolites. Overall, confinement effects do exist in zeolite systems and have already played extremely important roles in adsorption and catalysis. Although confinement might exist in many systems, the confinement by zeolites is more straightforward thanks to their well-ordered and rigid structure, deriving unique chemical behaviors within the confined space of zeolites. A zeolite is a fantastic scaffold for constructing isolated sites spatially and electrostatically confined in its matrix. Furthermore, zeolites containing well-defined transition-metal sites can be treated as inorganometallic complexes (i.e., a zeolite framework as the ligand of transition-metal ions) and can catalyze reactions resembling organometallic complexes or even metalloenzymes. The local electric field within the confined space of zeolites is strong enough to induce or assist the activation of small molecules, following the working fashion of frustrated Lewis pairs. The tactful utilization of structural confinement, both spatially and electronically, becomes the key to robust zeolites for adsorption and catalysis.

Zeolitic imidazolate frameworks as intrinsic light harvesting and charge separation materials for photocatalysis.

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal organic frameworks that have attracted considerable attention in the past years and have found many applications including heterogeneous catalysis due to their highly ordered porous structure, large surface area, and structural flexibility. However, ZIFs are largely utilized as simple hosts or passive media for dispersing other catalytically active species, resembling the roles of zeolites in catalysis. In contrast, our recent findings show that ZIFs not only have broad absorption across the UV-visible and near IR spectral region but also have an exceptionally long-lived excited charge separated state, suggesting that ZIFs may be used as intrinsic light harvesting and photocatalytic materials rather than as inert hosts. This Perspective will focus on the recent progress on the fundamental studies of the intrinsic light absorption, charge separation, and photocatalytic properties of ZIFs and will discuss the outlook for future development.

Induced Active Sites by Adsorbate in Zeotype Materials.

There has been a long debate on how and where active sites are created for molecular adsorption and catalysis in zeolites, which underpin many important industrial applications. It is well accepted that Lewis acidic sites (LASs) and basic sites (LBSs) as active sites in pristine zeolites are generally believed to be the extra-framework Al species and residue anion (OH-) species formed at fixed crystallographic positions after their synthesis. However, the dynamic interactions of adsorbates/reactants with pristine zeotype materials to "create" sites during real conditions remain largely unexplored. Herein, direct experimental observation of the establishment of induced active sites in silicoaluminophosphate (SAPO) by an adsorbate is for the first time made, which contradicts the traditional view of the fixed active sites in zeotype materials. Evidence shows that an induced frustrated Lewis pair (FLP, three-coordinated framework Al as LAS and SiO (H) as LBS) can be transiently favored for heterolytic molecular binding/reactions of competitive polar adsorbates due to their ineffective orbital overlap in the rigid framework. High-resolution magic-angle-spinning solid-state NMR, synchrotron X-ray diffraction, neutron powder diffraction, in situ diffuse reflectance infrared Fourier transform spectroscopy, and ab initio molecular dynamics demonstrate the transformation of a typical Brønsted acid site (Al(OH)Si) in SAPO zeolites to new induced FLP structure for hetereolytic binding upon adsorption of a strong polar adsorbate. Our unprecedented finding opens up a new avenue to understanding the dynamic establishment of active sites for adsorption or chemical reactions under molecular bombardment of zeolitic structures.

The Effect of Zeolite Catalysis Size on Biodiesel Characteristics of Bio-Oil from Tuna Waste

Indonesia is a country with the third largest tuna production in the world. Fish oil as a fishery product processing waste is a potential material for making biodiesel because it has a high fat content. In this research, biodiesel was made from bio-oil from tuna waste through esterification and transesterification processes at 60°C, stirring speed 360 rpm with a reaction time of 2 hours. The purpose of this study was to determine the levels of Free Fatty Acid (FFA) contained in bio-oil from tuna waste and to determine the effect of the size of the natural zeolite catalyst in the transesterification process on the characteristics of the biodiesel produced. The variables used were the size of the zeolite catalyst 10 - 20, 30 - 40, 50 - 60, 70 - 80, 100 - 120 mesh. From the research results, it can be concluded that the smaller the zeolite catalyst size, the resulting biodiesel is closer to the biodiesel standard and the size of 100-120 mesh which is very close to the SNI 7182: 2015 biodiesel standard with a density value of 849.6 kg/m 3 , kinematic viscosity 3,413 cSt, moisture content. 0%, acid number 1.10 mg.KOH /gr, flash point 75 ° C, calorific value 8.240 Cal/gr.

Effect of External Surface Diffusion Barriers on Platinum/Beta‐Catalyzed Isomerization of n‐Pentane

AbstractWe have developed a generalizable strategy to quantify the effect of surface barriers on zeolite catalysis. Isomerization of n‐pentane, catalyzed by Pt/Beta, is taken as a model reaction system. Firstly, the surface modification by chemical liquid deposition of SiO2 was carried out to control the surface barriers on zeolite Beta crystals. The deposition of SiO2 leads to a very slight change in the physical properties of Beta crystals, but an obvious reduction in Brønsted acid sites. Diffusion measurements by the zero‐length column (ZLC) method show that the apparent diffusivity of n‐pentane can be more than doubled after SiO2 deposition, indicating that the surface barriers have been weakened. Catalytic performance was tested in a fixed‐bed reactor, showing that the apparent catalytic activity improved by 51–131 % after SiO2 deposition. These results provide direct proof that reducing surface barriers can be an effective route to improve zeolite catalyst performance deteriorated by transport limitations.

Effect of External Surface Diffusion Barriers on Platinum/Beta-Catalyzed Isomerization of n-Pentane.

We have developed a generalizable strategy to quantify the effect of surface barriers on zeolite catalysis. Isomerization of n‐pentane, catalyzed by Pt/Beta, is taken as a model reaction system. Firstly, the surface modification by chemical liquid deposition of SiO2 was carried out to control the surface barriers on zeolite Beta crystals. The deposition of SiO2 leads to a very slight change in the physical properties of Beta crystals, but an obvious reduction in Brønsted acid sites. Diffusion measurements by the zero‐length column (ZLC) method show that the apparent diffusivity of n‐pentane can be more than doubled after SiO2 deposition, indicating that the surface barriers have been weakened. Catalytic performance was tested in a fixed‐bed reactor, showing that the apparent catalytic activity improved by 51–131 % after SiO2 deposition. These results provide direct proof that reducing surface barriers can be an effective route to improve zeolite catalyst performance deteriorated by transport limitations.

Developing quantitative synthesis-structure-function relations for framework aluminum arrangement effects in zeolite acid catalysis

In this manuscript, we investigated in detail one of the critical parameters that govern the reactivity of the carbonylic group toward hydrogenation and hydrodeoxygenation: the effect of the side chains of aldehydes. We observed that the presence of a π-electron conjugation on the side chains strongly affects the conversion of aldehydes to alcohols and hydrocarbons under the reaction conditions studied. In particular, the presence of a strong π-electron conjugation, such as the aromatic ring, enables the hydrogenation and further hydrodeoxygenation of the carbonylic group, while, on the other hand, molecules with no or weak π-electron conjugation (such as alkyl chains or single conjugated CC double bonds) remained inactive. These results were supported by adsorption analyses as well as theoretical models, which showed that differences in adsorption mode and strength between two model substrates (benzaldehyde and octanal) were not the decisive factors in the conversion of the carbonyl group.

Zeolite Catalysis Enables Efficient Pyrazinone Synthesis in Water

Abstract We employed herein a Linde Type A zeolite as a heterogeneous catalyst to condense amino acid amides with glyoxal affording pyrazinones. The synthesis was conducted in water without using corrosive reagents, organic solvents, or additives. The power of this “aquachemistry” was demonstrated through robust, continuous-flow synthesis.

Electrolytic transesterification of waste frying oil using Na+/zeolite–chitosan biocomposite for biodiesel production

Given the economic and environmental advantages of using Waste Fried Oil (WFO) as a starting material, this investigation explores the conversion of WFO to Fatty Acid Methyl Ester (FAME) via electrolysis for use in waste. In electrolysis, hydroxyl ions are generated from water in close proximity to the cathode. When hydroxyl ions react with methanol, they produce a species of nucleophilic methoxide which is the main actor in converting WFO into FAME. This study specifically investigates the effects of voltage, catalyst concentration, co solvent amount, rotation speed, and molar ratio of methanol to WFO in electrolytic transesterification converting WFO into FAME using graphite electrodes in the presence of a heterogeneous, catalytic zeolite–chitosan composite. With an alcohol to WFO molar ratio of 8:1, 1 wt% zeolite-chitosan composite concentration at 40 V in the presence of 2 wt% H2O of the whole solution at room temperature and stirrer rate of 400 rpm and reaction time of 30 min, a 96.5% yield of FAME was achieved. Characterization of physical and biodiesel fuel properties was performed using American Society for Testing and Materials (ASTM) methods. The biocomposite was characterized using Fourier Transform Infrared (FTIR), X–ray Diffraction (XRD), Transmission Electron Microscopy(TEM), Brunauer Emmett Teller(BET), Thermogravimetric analysis (TG), Scanning Electron Microscopy (SEM) and Energy Dispersive X–ray spectrometry (EDX). Finally, the physical properties of FAME produced under optimal conditions were studied using Gas Chromatography–Mass Spectrometry (GC–MS), FTIR, surface tension, and viscosity.

Dynamic Interconversion of Metal Active Site Ensembles in Zeolite Catalysis.

Catalysis science is founded on understanding the structure, number, and reactivity of active sites. Kinetic models that consider active sites to be static and noninteracting entities are routinely successful in describing the behavior of heterogeneous catalysts. Yet, active site ensembles often restructure in response to their external environment and even during steady-state catalytic turnover, sometimes requiring non-mean-field kinetic treatments to describe distance-dependent interactions among sites. Such behavior is being recognized more frequently in modern catalysis research, with the advent of experimental methods to quantify turnover rates with increasing precision, an expanding arsenal of operando characterization tools, and computational descriptions of atomic structure and motion at chemical potentials and timescales increasingly relevant to reaction conditions. This review focuses on dynamic changes to metal active site ensembles on zeolite supports, which are silica-based crystalline materials substituted with Al that generate binding sites for isolated and low-nuclearity metal site ensembles. Metal sites can become solvated and mobilized during reaction, facilitating interactions among sites that change their nuclearity and function. Such intersite communication can be regulated by the zeolite support, resulting in non-single-site and potentially non-mean-field kinetic behavior arising from mechanisms of catalytic action that combine elements of those canonically associated with homogeneous and heterogeneous catalysis.We discuss recent literature examples that document dynamic active site behavior in metal-zeolites and outline methodologies to identify and interpret such behavior. We conclude with our outlook on future research directions to develop this evolving branch of catalysis science and harness it for practical applications.

Two-dimensional zeolites in catalysis: current state-of-the-art and perspectives

ABSTRACT Two-dimensional (2D) zeolites with modifiable structures, can possess great advantages in catalysis compared to three-dimensional (3D) rigid ones. Three kinds of 2D zeolites, including traditional lamellar ones, 2D zeolites induced by oriented growth with specially designed structural-directing agents, and 2D intermediates derived from germanosilicates, are covered in this review. 2D zeolites derived catalysts with higher surface area, larger pore size or hierarchical pore system release the diffusion constrains and enhance the accessibility of active sites, but they may also exhibit different microenvironments to the traditional 3D zeolites. The present review focuses on the complex relationship among pore architectures, location of active sites and the catalytic properties by comparing the 2D zeolite catalysts to 3D ones in the reactions catalyzed by framework Al or other heteroatoms (Ti and Sn), such as alkylation, selective oxidation and isomerization reactions. Last but not least, the challenges for future progress on both the preparation and application of these 2D zeolite catalysts are addressed.

Jens Weitkamp – in Memoriam

AbstractProf. Jens Weitkamp was a pioneer in the field of zeolite catalysis, especially for applications such as hydrocracking, isomerization, alkylation, but also selective ring opening of polynuclear aromatics. This is an extended version of the closing remarks delivered at the Symposium in honor of Professor Jens Weitkamp on February 11th, 2020, at the DECHEMA house, Frankfurt, Germany.

Dynamic Activation of C1 Molecules Evoked by Zeolite Catalysis.

Direct observation of the activation and transformation of reactant molecules is extremely attractive but very challenging in the study of most chemical processes. Here is reported the first case of dynamic activation of C1 molecules in zeolite-catalyzed chemistry. During the methanol conversion over the HZSM-5 zeolite, a sequence of progressive activation states of dimethyl ether (DME) evoked by the special catalysis from CH3-Zeo, a hybrid supramolecular catalytic system formed by the organic methylic species growing on the inorganic silico-aluminate zeolite framework, has been directly observed by in situ ssNMR spectroscopy at programmed temperatures. Operando simulations visually display the variability of this hybrid supramolecular system of which the C–O bond property goes through a dynamic transition from covalent to ionic with the temperature increase, and thus the gradually enhanced electrophilicity of CH3δ+ and nucleophilicity of Zeoδ− lead to the dynamic activation of DME. This dynamic transition is generally reflected in the alkyl-Zeo system with other alkoxy groups, which linked the alkoxy species and carbocations in zeolite catalysis. Consequently, this work not only sheds light on the key issue of the first carbon–carbon (C–C) bond formation in the methanol to hydrocarbons (MTH) process but also brings a new awareness on the essence of acid catalysis in zeolite mediated chemical processes.