Zeolite Catalysis Research Articles

From Catalytic Test Reaction to Modern Chemical Descriptors in Zeolite Catalysis Research

AbstractFor the successful implementation of catalysts in industrial processes, three overall goals are optimized: activity, selectivity, and stability. This review will address the role of chemical descriptors in aiding and guiding the development of optimal zeolite catalyst designs with the above performance criteria in mind. It will focus on both gas‐phase and liquid‐phase catalysis with a special focus on methanol to hydrocarbon, and biomass valorization reactions. By way of preface, the research on constraint and spaciousness indices is discussed. These indices can be considered the original chemical descriptors of catalysis research.

Zeolite-Based Catalysis for Phenol Production

Phenol is an important chemical commodity utilized for several applications (e.g., polycarbonate, epoxy–resins, phenolic resins, nylon). Its conventional production starts from benzene and propylene and is based on a series of reaction steps most of which are catalyzed by acid catalysts. Besides this technology other ones have been considered during the last decades, starting from other reactants and catalysts. Looking at the catalyst types both already in use or under evaluation, it is interesting to note that zeolites, as acidic catalysts or red-ox catalysts, were widely investigated. Therefore, the production of phenol represents a fascinating example of how zeolite catalysis contributes to improving the sustainability of these chemical processes by displacing the previous large use of corrosive and dangerous liquid acids and improving overall selectivity, which reduces the side production of wastes. Aromatic alkylation, transalkylation, and disproportionation, oxidations, rearrangements, oligomerization, and cracking are the reactions occurring in phenol production technologies, but also key unit operations in refineries and the chemical industry. A review of the zeolite catalysts utilized and proposed by major licensors and research groups in these technologies is provided as well as a discussion of key process differences and recent advances.

Hierarchically Structured Zeolites: Nature‐Inspired, Computer‐Assisted Optimization of Hierarchically Structured Zeolites (Adv. Mater. Interfaces 4/2021)

Introducing broad pores could decrease diffusion limitations in zeolite catalysis – but what is the optimal structure? In article number 2001409 by Marc-Olivier Coppens, Guanghua Ye, and co-workers, a systematic, optimal design methodology is discussed. It borrows lessons from nature to guide hierarchically structured zeolite synthesis, aided by multiscale modelling. Design rules of thumb are derived and illustrated. Controlling surface barriers is a new challenge, with more scope for inspiration from nature.

Dynamic Intermediate Profiles of Zeolite Catalyzed Methanol to Olefins Revealed by Reactive Molecular Dynamics

Taking the unique advantage of large-scale ReaxFF MD simulation in unravelling varied reaction pathways within one simulation, the process of methanol-to-olefin (MTO) was investigated by the combin...

Silanol defect engineering and healing in zeolites: opportunities to fine-tune their properties and performances.

Zeolites have been game-changing materials in oil refining and petrochemistry over the last 60 years and have the potential to play the same role in the emerging processes of the energy and environmental transition. Although zeolites are crystalline inorganic solids, their structures are not perfect and the presence of defect sites - mainly Brønsted acid sites and silanols - influences their thermal and chemical resistance as well as their performances in key areas such as catalysis, gas and liquid separations and ion-exchange. In this paper, we review the type of defects in zeolites and the characterization techniques used for their identification and quantification with the focus on diffraction, spectroscopic and modeling approaches. More specifically, throughout the review, we will focus on silanol (Si-OH) defects located within the micropore structure and/or on the external surface of zeolites. The main approaches applied to engineer and heal defects and their consequences on the properties and applications of zeolites in catalysis and separation processes are highlighted. Finally, the challenges and opportunities of silanol defect engineering in tuning the properties of zeolites to meet the requirements for specific applications are presented.

Kinetic effects of molecular clustering and solvation by extended networks in zeolite acid catalysis.

Reactions catalyzed within porous inorganic and organic materials and at electrochemical interfaces commonly occur at high coverage and in condensed media, causing turnover rates to depend strongly on interfacial structure and composition, collectively referred to as “solvent effects”. Transition state theory treatments define how solvation phenomena enter kinetic rate expressions, and identify two distinct types of solvent effects that originate from molecular clustering and from the solvation of such clusters by extended solvent networks. We review examples from the recent literature that investigate reactions within microporous zeolite catalysts to illustrate these concepts, and provide a critical appraisal of open questions in the field where future research can aid in developing new chemistry and catalyst design principles.

Innovations in the field of zeolite catalysis

Innovations in the field of zeolite catalysis

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.

Nature‐Inspired, Computer‐Assisted Optimization of Hierarchically Structured Zeolites

AbstractZeolite catalysis is often affected by transport limitations, which significantly influence overall performance. Introducing wide pores as molecular transport highways can reduce transport limitations, control the product distribution, and mitigate effects of catalyst deactivation. Nevertheless, the importance to rationally design the meso‐ and macropore space remains underappreciated. This article reviews multiscale modelling approaches to optimize overall catalytic performance. It provides a general methodology and rules of thumb to guide catalyst synthesis with optimal pore network characteristics. Inspiration is taken from nature, such as the structure of leaves and tissues, with similar requirements and associated features. In optimal hierarchically structured zeolites, the added macro‐/mesopore volume fraction, connectivity, crystal size, and minimum wide pore size are crucial. The broad pore size distribution is secondary. No uncontrolled diffusion limitations should exist within the zeolite crystals. Surface barriers, however, can significantly affect, even dominate overall transport. Understanding their origin and ways to control them is an emergent research area. Synthesis methods to realize hierarchically structured zeolites are briefly reviewed. Significant gaps exist between laboratory synthesis methods and industrial requirements. Zeolite catalysis could benefit from computer‐assisted design of their hierarchical pore network, embracing principles used by natural transport networks for scalable efficiency, selectivity, and robustness.

Control of Surface Barriers in Mass Transfer to Modulate Methanol‐to‐Olefins Reaction over SAPO‐34 Zeolites

AbstractMass transfer of guest molecules has a significant impact on the applications of nanoporous crystalline materials and particularly shape‐selective catalysis over zeolites. Control of mass transfer to alter reaction over zeolites, however, remains an open challenge. Recent studies show that, in addition to intracrystalline diffusion, surface barriers represent another transport mechanism that may dominate the overall mass transport rate in zeolites. We demonstrate that the methanol‐to‐olefins (MTO) reaction can be modulated by regulating surface permeability in SAPO‐34 zeolites with improved chemical liquid deposition and acid etching. Our results explicitly show that the reduction of surface barriers can prolong catalyst lifetime and promote light olefins selectivity, which opens a potential avenue for improving reaction performance by controlling the mass transport of guest molecules in zeolite catalysis.

Control of Surface Barriers in Mass Transfer to Modulate Methanol-to-Olefins Reaction over SAPO-34 Zeolites.

Mass transfer of guest molecules has a significant impact on the applications of nanoporous crystalline materials and particularly shape-selective catalysis over zeolites. Control of mass transfer to alter reaction over zeolites, however, remains an open challenge. Recent studies show that, in addition to intracrystalline diffusion, surface barriers represent another transport mechanism that may dominate the overall mass transport rate in zeolites. We demonstrate that the methanol-to-olefins (MTO) reaction can be modulated by regulating surface permeability in SAPO-34 zeolites with improved chemical liquid deposition and acid etching. Our results explicitly show that the reduction of surface barriers can prolong catalyst lifetime and promote light olefins selectivity, which opens a potential avenue for improving reaction performance by controlling the mass transport of guest molecules in zeolite catalysis.

Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation.

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods. Using a combination of state-of-the-art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)-Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal a site-specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H-ZSM-5.

Ethylene Dehydroaromatization over Ga‐ZSM‐5 Catalysts: Nature and Role of Gallium Speciation

AbstractBifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra‐framework gallium species on enriched aromatics production in zeolite ZSM‐5. We compare three distinct methods of preparing Ga‐ZSM‐5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post‐synthesis methods. Using a combination of state‐of‐the‐art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)–Ga(extra‐framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra‐framework Ga at framework sites comprised of either Al or Ga reveal a site‐specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H‐ZSM‐5.

Investigation of Photocatalytic Efficiency of Supported CuO Nanoparticles on Natural Zeolite Particles in Photodegradation of Methyl Orange.

In the current work, CuO nanoparticles were deposited on natural zeolite's particles to resolve the drawbacks of zeolite catalysis. The synthesized composites were characterized by XRD, SEM, BET, and DRS analyses. The results illustrated that in the 15% CuO composite, CuO nanoparticles with a size of 21 nm are deposited on the surface of the zeolite particles. Deposition of CuO nanoparticles on zeolite's particles decreased the specific surface area from 35 m²/g (pure zeolite) to 28 m²/g (20% CuO composite), and causes a red shift in the absorption edge of the sample to 796 nm for 20% CuO composite. In order to compare the samples' performances in eliminating water pollutants, methyl orange dye removal was investigated. The analyses indicated that the optimum efficiency (85% in 120 min) belongs to zeolite-15% CuO composite with a band gap of 1.70 eV.

Impact of acid site speciation and spatial gradients on zeolite catalysis

This mini-review provides an overview of the current state of acid site control in zeolite catalysts, including methods of synthesis, advanced characterization, and measured effects of acid properties (speciation, concentration, proximity, siting, and spatial distribution) on a variety of commercially-relevant reactions. The diversity of aluminum species is described with respect to their location at specific sites in zeolite crystals as well as mesoscopic gradients in elemental composition that give rise to zoned or core-shell architectures. Challenges in the identification of acid siting are highlighted within the context of trial-and-error synthesis methods for a range of aluminosilicate frameworks, which hinder a priori design of catalysts, and limitations in techniques to suitably characterize active sites. Emphasis is also placed on knowledge gaps in zeolite catalysis wherein broad development of structure-performance relationships relies on future advancement of synthesis and analytical methods in parallel with atomistic modeling.

Zeolite Catalyzed Friedel-Crafts Reactions: A Review

Friedel-Crafts reaction is one of the most useful synthetic tools in organic chemistry, mainly in the synthesis of aromatic ketones. The active catalysts for this reaction are modified zeolites and are preferable catalysts when shape selectivity affects the formation of the expected product. In this review, our aim is to corroborate recent literature available on zeolite catalyzed Friedel-Crafts alkylation and acylation reaction.

Methanol Conversion to Dimethyl Ether in Catalytic Zeolite Membrane Reactors

In this work, two ZSM-5 type zeolite supported membranes were used as catalytic membrane reactors for dimethyl ether (DME) synthesis via MeOH dehydration. The membranes, both commercial and tubular...

Mesopore-selective incorporation of strong Brønsted acid catalytic sites via aluminium grafting on hierarchically porous siliceous MFI zeolite

Abstract Post-synthetic Al grafting onto the surface silanols in mesoporous MFI zeolite has been investigated in an attempt to obtain a model catalytic material that possesses strong Bronsted acid sites at mesopore surfaces of the zeolite, but not inside the microporous framework. To this end, a siliceous MFI zeolite with a mesopore/micropore hierarchy was synthesized using a meso-micro dual structure-directing diammonium surfactant. The hierarchically porous zeolite was treated with anhydrous AlCl3 for the formation of Si–O–Al bonding with silanols on the mesopore surfaces. According to solid-state 31P NMR spectroscopy of the surface-adsorbed phosphine oxides and catalytic activity for decalin cracking, the Al-grafting treatment performed in the conventional manner resulted in the formation of weak Bronsted acid sites, similar to Al grafting on SBA-15 mesoporous silica. To obtain strong Bronsted acid sites, we have developed a method of heating the AlCl3-grafted zeolite in a controlled-humidity chamber before calcination. The resultant zeolite was catalytically active for cracking of hydrocarbons requiring strong acid sites, but not for methanol-to-hydrocarbon conversion. The results support that the moisture treatment caused a local reconstruction of the initially grafted Al–O–Si framework portion to a strong Bronsted acid site. In addition, our model catalytic zeolite material unveiled the catalytic activity of Bronsted acid sites at the mesopore surfaces in hierarchically porous zeolite in n-octane cracking. Furthermore, we confirmed that this method was effective for other mesoporous MFI zeolites obtained through a post-synthetic desilication route.

Efficient heterogeneous Fenton-like catalysis of Fe-doped SAPO-44 zeolite synthesized from bauxite and rice husk

In this article, the Fe-containing silicoaluminophosphate (MeSAPO-44-RH) zeolite is successfully prepared by natural bauxite and rice husk as materials in super-concentrated system. The zeolite structure improves the dispersity of Fe3+ ions, and rice husk promotes Fe incorporation of skeleton, which effectively enhance the redox activity and stability of the skeleton-Fe. MeSAPO-44-RH displays excellent Fenton-like reaction performance, which achieves degrade reactive bright red (X-3B) nearly 100% within 60 min. Meanwhile, it exhibits superior cyclic stability without deactivation for 10 runs experiment. Our strategy provides a clean, green and efficient process route for bauxite and biomass waste.

How Accurately Do Approximate Density Functionals Predict Trends in Acidic Zeolite Catalysis?

Density functional theory (DFT) is increasingly used for computational screening procedures with the aim of finding new catalysts. To achieve this, it is critical that relative differences between materials are predicted with high accuracy. How DFT at the generalized gradient approximation (GGA) level performs in this respect is investigated here for catalytic reactions employing acidic zeotypes using highly accurate DLPNO-CCSD(T) calculations as the reference. This is studied for 65 reaction energies and 130 reaction barriers related to zeolite catalysis. Our results obtained for the PBE-D3 and BEEF-vdW functionals show that while these functionals are prone to large errors, they predict trends occurring from one catalyst to another with an accuracy of about 5 kJ/mol, strongly supporting the widespread use of DFT calculations for the computational screening and design of new catalytic materials.