Synthesis of zeolite from industrial wastes: a review on characterization and heavy metal and dye removal

Increasing world population, urbanization, and industrialization have led to an increase in demand in production and consumption, resulting in an increase in industrial solid wastes and pollutant levels in water. These two main consequences have become global problems. The high Si and Al content of solid wastes suggests that they can be used as raw materials for the synthesis of zeolites. In this context, when the literature studies conducted to obtain synthetic zeolites are evaluated, it is seen that hydrothermal synthesis method is generally used. In order to improve the performance of the hydrothermal synthesis method in terms of energy cost, synthesis time, and even product quality, additional methods such as alkaline fusion, ultrasonic effect, and microwave support have been developed. The zeolites synthesized by different techniques exhibit superior properties such as high surface area and well-defined pore sizes, thermal stability, high cation exchange capacity, high regeneration ability, and catalytic activity. Due to these specific properties, zeolites are recognized as one of the most effective methods for the removal of pollutants. The toxic properties of heavy metals and dyes in water and their carcinogenic effects in long-term exposure pose a serious risk to living organisms. Therefore, they should be treated at specified levels before discharge to the environment. In this review study, processes including different methods developed for the production of zeolites from industrial solid wastes were evaluated. Studies using synthetic zeolites for the removal of high levels of health and environmental risks such as heavy metals and dyes are reviewed. In addition, EPMA, SEM, EDX, FTIR, BET, AFM, and 29Si and 27Al NMR techniques, which are characterization methods of synthetic zeolites, are presented and the cation exchange capacity, thermodynamics of adsorption, effect of temperature, and pH are investigated. It is expected that energy consumption can be reduced by large-scale applications of alternative techniques developed for zeolite synthesis and their introduction into the industry. It is envisaged that zeolites synthesized by utilizing wastes will be effective in obtaining a green technology. The use of synthesized zeolites in a wide variety of applications, especially in environmental problems, holds great promise.

Upcycling of polyethylene to gasoline through a self-supplied hydrogen strategy in a layered self-pillared zeolite

Conversion of plastic wastes to valuable carbon resources without using noble metal catalysts or external hydrogen remains a challenging task. Here we report a layered self-pillared zeolite that enables the conversion of polyethylene to gasoline with a remarkable selectivity of 99% and yields of >80% in 4 h at 240 °C. The liquid product is primarily composed of branched alkanes (selectivity of 72%), affording a high research octane number of 88.0 that is comparable to commercial gasoline (86.6). In situ inelastic neutron scattering, small-angle neutron scattering, solid-state nuclear magnetic resonance, X-ray absorption spectroscopy and isotope-labelling experiments reveal that the activation of polyethylene is promoted by the open framework tri-coordinated Al sites of the zeolite, followed by β-scission and isomerization on Brönsted acids sites, accompanied by hydride transfer over open framework tri-coordinated Al sites through a self-supplied hydrogen pathway to yield selectivity to branched alkanes. This study shows the potential of layered zeolite materials in enabling the upcycling of plastic wastes.

Theoretical study of fructose adsorption and conversion to trioses on metal-organic frameworks

The conversion of chemically modified biomass into more valuable chemicals has recently gained significant attention from industry. In this study, we investigate the adsorption of fructose and its conversion into two trioses, glyceraldehyde (GLA) and dihydroxyacetone (DHA), on metal-organic frameworks using density functional theory calculations. The reaction mechanism proceeds through two main steps: first, the opening of the fructose ring; second, the retro-aldol fragmentation, which is favored over intramolecular hydrogen shifts. The substitution of a tetravalent metal in the metal-organic framework leads to different adsorption strengths in the order Hf-NU-1000 > Zr-NU-1000 > Ti-NU-1000. The catalytic activities of Hf-NU-1000 and Zr-NU-1000 are found to be similar. Both are more active than Ti-NU1000, corresponding to their relative Lewis acidity. It was found that functionalization of the organic linkers of the Hf-NU-1000 MOF does not improve its catalytic activity. The catalytic activity follows the order Hf-MOF-808 > Hf-NU-1000 > Hf-UIO-66 when based on either the overall activation energy or the turnover frequency (TOF).

Polymorphism and Structural Variety in Sn(II) Carboxylate Coordination Polymers Revealed from Structure Solution of Microcrystals

The crystal structures of four coordination polymers constructed from Sn(II) and polydentate carboxylate ligands are reported. All are prepared under hydrothermal conditions in KOH or LiOH solutions (either water or methanol-water) at 130-180 °C and crystallize as small crystals, microns or less in size. Single-crystal structure solution and refinement are performed using synchrotron X-ray diffraction for two materials and using 3D electron diffraction (3DED) for the others. Sn2 (1,3,5-BTC)(OH), where 1,3,5-BTC is benzene-1,3,5-tricarboxylate, is a new polymorph of this composition and has a three-dimensionally connected structure with potential for porosity. Sn(H-1,3,5-BTC) retains a partially protonated ligand and has a 1D chain structure bound by hydrogen bonding via ─COOH groups. Sn(H-1,2,4-BTC) contains an isomeric ligand, benzene-1,2,4-tricarboxylate, and contains inorganic chains in a layered structure held by hydrogen bonding. Sn2 (DOBDC), where DOBDC is 2,5-dioxido-benzene-1,4-dicarboxylate, is a new polymorph for this composition and has a three-dimensionally connected structure where both carboxylate and oxido groups bind to the tin centers to create a dense network with dimers of tin. In all materials, the Sn centers are found in highly asymmetric coordination, as expected for Sn(II). For all materials phase purity of the bulk is confirmed using powder X-ray diffraction, thermogravimetric analysis, and infrared spectroscopy.

Synthesis and Catalytic Applications of Advanced Sn- and Zr-Zeolites Materials

The incorporation of isolated Sn (IV) and Zr (IV) ions into silica frameworks is attracting widespread attention, which exhibits remarkable catalytic performance (conversion, selectivity, and stability) in a broad range of reactions, especially in the field of biomass catalytic conversion. As a representative example, the conversion route of carbohydrates into valuable platform and commodity chemicals such as lactic acid and alkyl lactates, has already been established. The zeotype materials also possess water-tolerant ability and are capable to be served as promising heterogeneous catalysts for aqueous reactions. Therefore, dozens of Sn- and Zr-containing silica materials with various channel systems have been prepared successfully in the past decades, containing 8 membered rings (MR) small pore CHA zeolite, 10-MR medium pore zeolites (FER, MCM-56, MEL, MFI, MWW), 12-MR large pore zeolites (Beta, BEC, FAU, MOR, MSE, MTW), and 14-MR extra-large pore UTL zeolite. This review about Sn- and Zr-containing metallosilicate materials focuses on their synthesis strategy, catalytic applications for diverse reactions, and the effect of zeolite characteristics on their catalytic performances.

The Lewis superacidic aluminium cation: [(NHC)Al(C6F5)2]. Chem Commun (Camb) 2024;60:698-701. [PMID: 38111304 DOI: 10.1039/d3cc05440c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The aluminium salt [(NHC)Al(tol)(C6F5)2][B(C6F5)4], (NHC = C3H2(N(iPr2C6H3))2) is shown to behave as a Lewis superacid as it abstracts fluoride from [SbF6]-. It also acts as a Lewis acid catalyst for hydrosilyation, hydrodefluorination and Friedel-Crafts reactions.

A Theoretical Study on the Mechanisms Involved in Catalytic Dehydrogenation and Dehydration of Isopropanol on SrTiO3

The dehydrogenation and dehydration of isopropanol on the SrO and TiO2 terminated surfaces, of the SrTiO3 perovskite, is addressed by periodic DFT calculations in order to shed light on the involved mechanisms. The results show that the dehydrogenation occurs through a mechanism involving the dissociative adsorption of the alcohol on the SrO terminated surface, followed the nucleophilic attack of a hydride species on the previously adsorbed hydrogen atom to form molecular hydrogen and the corresponding carbonyl compound. The dehydration instead occurs by the molecular adsorption of the alcohol on the TiO2 terminated surface, followed by various possible E1 elimination pathways leading to the formation of the corresponding alkene and a water molecule. The article reports a thorough study on the involved mechanisms, including identification of the transition states and intermediates along the reaction paths, and evaluation of the respective activation barriers, as well. Thus, this article provides significant insights about the mechanisms of dehydrogenation and dehydration of isopropanol on the SrTiO3 , not reported earlier in literature. The calculated barrier energies are in good agreement with experimental values.

Effects of a commercial whitening toothpaste containing hydrogen peroxide and citric acid on dentin abrasion and erosion

BACKGROUND

Hydrogen peroxide (HP) and citric acid (CA), key contributors to toothpaste acidity, can lead to dental loss. This study aimed to compare the amount of abrasion or loss of dentin based on pH, buffering, and concentration of HP and CA in commercial and experimental toothpastes after toothbrushing or immersion.

METHODS

Bovine dentin specimens were randomly assigned to nine solutions. The prepared solutions included two commercial toothpastes (whitening toothpaste [WT] with HP and CA; conventional toothpaste [CT] without HP and CA), reference slurry (RS), two CA solutions (1.92%, CAS1; 0.001%, CAS2), basic solution (7.16% sodium phosphate dibasic [SPDS]), CA phosphate buffer solution (3.58% SPDS and 0.96% CA [CAPB]), HP solution (4%, HPS), and distilled water (DW). Dentin specimens were performed in two treatments: one with only abrasion (10,000 brushings) and one with only immersion (1 h). After treatments, the amount of dentin loss and surface images were measured and observed using noncontact profilometry. Data were analyzed using an one-way analysis of variance and the Tukey test as a post hoc analysis (p < 0.05).

RESULTS

WT with pH 5.0 had lower dentin abrasion than CT and RS after brushing but had higher dentin loss than both after immersion. The dentin surfaces of CAS1, CAPB, and WT were damaged after immersion, whereas HPS, CAS2, CT, SPDS, RS, and DW remained intact after soaking. CAS2 and HPS, which had a pH of 5.0 like WT, did not significantly differ from those of DW after brushing.

CONCLUSIONS

WT containing HP and CA did not cause significant dentin abrasion but may cause additional dentin loss even without brushing. After brushing or immersion, the CA concentration may affect the dentin surface more than the HP concentration included in WT. The amount of abrasion or loss of dentin after brushing or soaking can vary based on the composition, concentration, and buffer in the solution, even if the pH of the solution is similar to pH 5.0.

A Comparative Review of Binder-Containing Extrusion and Alternative Shaping Techniques for Structuring of Zeolites into Different Geometrical Bodies

Zeolites are crystalline metallosilicates displaying unique physicochemical properties with widespread applications in catalysis, adsorption, and separation. They are generally obtained by a multi-step process that starts with primary mixture aging, followed by hydrothermal crystallization, washing, drying, and, finally, a calcination step. However, the zeolites obtained are in the powder form and because of generating a pressure drop in industrial fixed bed reactors, not applicable for industrial purposes. To overcome such drawbacks, zeolites are shaped into appropriate geometries and the desired size (a few centimeters) using extrusion, where zeolite powders are mixed with binders (e.g., mineral clays or inorganic oxides). The presence of binders provides good mechanical strength against crushing in shaped zeolites, but binders may have adverse impacts on zeolite catalytic and sorption properties, such as active site dilution and pore blockage. The latter is more pronounced when the binder has a smaller particle size, which makes the zeolite internal active sites mainly inaccessible. In addition to the shaping requirements, a hierarchical structure with different levels of porosity (micro-, meso-, and macropores) and an interconnected network are essential to decrease the diffusion limitation inside the zeolite micropores as well as to increase the mass transfer through the presence of larger auxiliary pores. Thus, the generation of hierarchical structure and its preservation during the shaping step is of great importance. The aim of this review is to provide a comprehensive survey and detailed overview on the binder-containing extrusion technique compared to alternative shaping technologies with improved mass transfer properties. An emphasis is allocated to those techniques that have been less discussed in detail in the literature.

Recent Advances in Tetra- (Ti, Sn, Zr, Hf) and Pentavalent (Nb, V, Ta) Metal-Substituted Molecular Sieve Catalysis

Metal substitution of molecular sieve systems is a major driving force in developing novel catalytic processes to meet current demands of green chemistry concepts and to achieve sustainability in the chemical industry and in other aspects of our everyday life. The advantages of metal-substituted molecular sieves include high surface areas, molecular sieving effects, confinement effects, and active site and morphology variability and stability. The present review aims to comprehensively and critically assess recent advances in the area of tetra- (Ti, Sn, Zr, Hf) and pentavalent (V, Nb, Ta) metal-substituted molecular sieves, which are mainly characterized for their Lewis acidic active sites. Metal oxide molecular sieve materials with properties similar to those of zeolites and siliceous molecular sieve systems are also discussed, in addition to relevant studies on metal-organic frameworks (MOFs) and some composite MOF systems. In particular, this review focuses on (i) synthesis aspects determining active site accessibility and local environment; (ii) advances in active site characterization and, importantly, quantification; (iii) selective redox and isomerization reaction applications; and (iv) photoelectrocatalytic applications.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Defect-Guided Synthesis of Hierarchical Sn-B-Beta Zeolite with Highly Exposed Sn Sites

Selectively anchoring active centers on the external surface for forming highly exposed acid sites is a highly desirable but challenging task in zeolite catalyst synthesis. Herein, a defect-guided etching-regrowth strategy is rationally designed for facilely positioning Sn Lewis acid sites on the outer surface of the Sn-B-Beta while fabricating a bifunctional hierarchical structure. The synthesis was conducted by hydrothermal treatment of the as-made B-Beta (uncalcined), which has intrinsic defects of the BEA structure, with Sn source and basic organic structure directing agent (SDA). Under a moderate SDA concentration, with blocked micropore channels, such SDA-triggered etching-regrowth will proceed along the defect defined pathway, which ensures Sn selectively anchored on the external surface. Moreover, this methodology has exclusively introduced tetrahedrally coordinated framework Sn with open Sn sites as the predominated species. Mono- and disaccharide isomerizations in ethanol over different Sn-Beta catalysts proved the prominent advantages of the hierarchical structure with highly exposed and synergetic acid sites.

Sn-Beta Catalyzed Transformations of Sugars—Advances in Catalyst and Applications

Beta zeolite modified with Sn in the framework (Sn-Beta) was synthesized and introduced as a heterogeneous catalyst for Baeyer–Villiger oxidations about twenty years ago. Since then, both syntheses strategies, characterization and understanding as well as applications with the material have developed significantly. Remarkably, Sn-Beta zeolite has been discovered to exhibit unprecedented high catalytic efficiency for the transformation of glucose to fructose (i.e., aldoses to ketoses) and lactic acid derivatives in both aqueous and alcoholic media, which has inspired an extensive interest to develop more facile and scalable syntheses routes and applications for sugars transformations. This review survey the progress made on both syntheses approaches of Sn-Beta and applications of the material within catalyzed transformations of sugar, including bottom-up and top-down syntheses and catalyzed isomerization, dehydration, and fragmentation of sugars.

Direct Propylene Epoxidation with Molecular Oxygen over Cobalt-Containing Zeolites

Direct propylene epoxidation with molecular oxygen is a dream reaction with 100% atom economy, but aerobic epoxidation is challenging because of the undesired over-oxidation and isomerization of epoxide products. Herein, we report the construction of uniform cobalt ions confined in faujasite zeolite, namely, Co@Y, which exhibits unprecedented catalytic performance in the aerobic epoxidation of propylene. Propylene conversion of 24.6% is achieved at propylene oxide selectivity of 57% at 773 K, giving a state-of-the-art propylene oxide production rate of 4.7 mmol/g_cat/h. The catalytic performance of Co@Y is very stable, and no activity loss can be observed for over 200 h. Spectroscopic analyses reveal the details of molecular oxygen activation on isolated cobalt ions, followed by interaction with propylene to produce epoxide, in which the Co²⁺-Co^δ+-Co²⁺ (2 < δ < 3) redox cycle is involved. The reaction pathway of propylene oxide and byproduct acrolein formation from propylene epoxidation is investigated by density functional theory calculations, and the unique catalytic performance of Co@Y is interpreted. This work presents an explicit example of constructing specific transition-metal ions within the zeolite matrix toward selective catalytic oxidations.

Facile and fast synthesis of highly active Lewis acid MWW zeolite from a pure silica ITQ-1

A facile hydrothermal etching-healing strategy is proposed to fabricate Sn-MWW zeolite based on a pure silica ITQ-1 precursor, which avoids using any supporting agents and additives, such as B3+ and...

Dealumination-controlled strategy mediates Ti–Y zeolite with cooperative active sites for selective oxidations

Hierarchical Ti–Y zeolite, with dual active sites containing framework-substituted Ti and tri-coordinated Al species, was successfully constructed via a dealumination-controlled strategy, which synergistically catalyzed selective oxidations.

Bimetallic Zeolite Beta Beads with Hierarchical Porosity as Brønsted-Lewis Solid Acid Catalysts for the Synthesis of Methyl Lactate

Bimetallic zeolite Beta in bead format and containing Al sites with Brønsted acid behavior and Sn, Zr or Hf sites with Lewis acid character, were prepared using a two-step synthetic route. First, zeolite Beta in the format of macroscopic beads (400 to 840 μm) with hierarchical porosity (micropores accessed through meso- and macropores in the range of 30 to 150 nm) were synthesized by hydrothermal crystallization in the presence of anion-exchange resin beads as hard template and further converted into their H-form. Next, the zeolite beads were partially dealuminated using different concentrations of HNO3 (i.e., 1.8 or 7.2 M), followed by grafting with one of the above-mentioned metals (Sn, Zr or Hf) to introduce Lewis acid sites. These bimetallic zeolites were tested as heterogeneous catalysts in the conversion of dihydroxyacetone (DHA) to methyl lactate (ML). The Sn-containing zeolite Beta beads treated by 1.8 M HNO3 and grafted with 27 mmol of SnCl4 (Sn-deAl-1.8-Beta-B) demonstrated the best catalytic activity among the prepared bimetallic zeolite beads, with 99% selectivity and 90% yield of ML after 6 h at 90 °C. This catalyst was also tested in combination with Au-Pd nanoparticles supported on functionalized carbon nanotubes (CNTs) as multifunctional catalytic system for the conversion of glycerol to ML, achieving 29% conversion of glycerol and 67% selectivity towards ML after 4.5 h at 140 °C under 30 bar air. The catalytic results were rationalized by means of a thorough characterization of the zeolitic beads with a combination of techniques (XRD, N2-physisorption, SEM, XRF, TEM, UV-vis spectroscopy and pyridine-FT-IR).

On the location of Lewis acidic aluminum in zeolite mordenite and the role of framework-associated aluminum in mediating the switch between Brønsted and Lewis acidity

Lewis acidic aluminum in zeolites, particularly acidity that is inherent to the framework, is an indeterminate concept. A fraction of framework aluminum changes geometry to octahedral coordination in the proton form of zeolite mordenite. Such octahedrally coordinated aluminum is the precursor of a Lewis acid site and its formation is accompanied by a loss in Brønsted acidity. Herein, we show that such Lewis acid sites have a preferred location in the pore structure of mordenite. A greater proportion of these Lewis acid sites resides in the side-pockets than in the main channel. By reverting the octahedrally coordinated aluminum back to a tetrahedral geometry, the corresponding Brønsted acid sites are restored with a concomitant loss in the ability to form Lewis acid sites. Thereby, reversible octahedral-tetrahedral aluminum coordination provides a means to indirectly switch between Lewis and Brønsted acidity. This phenomenon is unique to Lewis acidity that is inherent to the framework, thereby distinguishing it from Lewis acidity originating from extra-framework species. Furthermore, the transformation of framework aluminum into octahedral coordination is decoupled from the generation of distorted tetrahedrally coordinated aluminum, where the latter gives rise to the IR band at 3660 cm-1 in the OH stretching region.

First-principles microkinetic analysis of Lewis acid sites in Zn-ZSM-5 for alkane dehydrogenation and its implication to methanol-to-aromatics conversion

Stabilities and dehydrogenation activities of butane and cyclohexane on four different Zn sites in ZSM-5 zeolite were theoretically revealed. ZnOH⁺ was identified as the most active site at low temperature and the activity increases with the sequence of dehydrogenation.

New (and Old) Monomers from Biorefineries to Make Polymer Chemistry More Sustainable

This opinion article describes recent approaches to use the "biorefinery" concept to lower the carbon footprint of typical mass polymers, by replacing parts of the fossil monomers with similar or even the same monomer made from regrowing dendritic biomass. Herein, the new and green catalytic synthetic routes are for lactic acid (LA), isosorbide (IS), 2,5-furandicarboxylic acid (FDCA), and p-xylene (pXL). Furthermore, the synthesis of two unconventional lignocellulosic biomass derivable monomers, i.e., α-methylene-γ-valerolactone (MeGVL) and levoglucosenol (LG), are presented. All those have the potential to enter in a cost-effective way, also the mass market and thereby recover lost areas for polymer materials. The differences of catalytic unit operations of the biorefinery are also discussed and the challenges that must be addressed along the synthesis path of each monomers.

Resolving the Acid Site Distribution in Zn-Exchanged ZSM-5 with Stimulated Raman Scattering Microscopy

Zeolites are widely used acid catalysts in research and in industrial processes. The catalytic performance of these materials is affected by the nature and concentration of Brønsted and Lewis acid sites. The balance between these types of active sites—and thus the activity and selectivity of the zeolite—can be altered by the introduction of metal species, e.g., by ion exchange. Although the acidic properties of zeolites are routinely characterized by bulk-scale techniques, this ensemble-averaged approach neglects the local variations in the material. Insights into the distribution of active sites at the single-particle level are thus critical to better understand the impact of post-synthetic modifications on the zeolite acidity. In this contribution, we spatially resolve Brønsted and Lewis acid sites in protonated and Zn-exchanged ZSM-5 crystals. To this end, the vibrational modes of pyridine chemisorbed on active sites are mapped with stimulated Raman scattering (SRS) microscopy. The SRS images reveal sharp inter- and intra-particle heterogeneities in the distribution of Lewis acid sites introduced upon ion exchange, ascribed to local variations in the Al content. Besides assessing the impact of Zn exchange on the active site distribution in ZSM-5 crystals, this approach enables uniquely to map the distribution of Lewis acid sites in catalysts at the single-particle level.

gem-Diol-Type Intermediate in the Activation of a Ketone on Sn-β Zeolite as Studied by Solid-State NMR Spectroscopy

Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl-containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem-diol-type species in the activation of acetone on Sn-β zeolite. ¹³ C, ¹¹⁹ Sn, and ¹³ C-¹¹⁹ Sn double-resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)₃ Sn-OH) on Sn-β is responsible for the formation of the surface species. ¹³ C MAS NMR experiments together with density functional theory calculations suggest that the gem-diol-type species exhibits high reactivity and can serve as an active intermediate in the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem-diol-type species offers an energy-preferable pathway for the direct carbon-to-carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl-containing molecules catalyzed by Lewis acid zeolites.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

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.

Biomass valorisation over metal-based solid catalysts from nanoparticles to single atoms

Heterogeneous catalysts are vital to unlock superior efficiency, atom economy, and environmental friendliness in chemical conversions, with the size and speciation of the contained metals often playing a decisive role in the activity, selectivity and stability. This tutorial review analyses the impact of these catalyst parameters on the valorisation of biomass through hydrogenation and hydrodeoxygenation, oxidation, reforming and acid-catalysed reactions, spanning a broad spectrum of substrates including sugars and platform compounds obtained from (hemi)cellulose and lignin derivatives. It outlines multiple examples of classical structure sensitivity on nanoparticle-based materials with significant implications for the product distribution. It also shows how the recently emphasised application of metals in the form of ultrasmall nanoparticles (<2 nm), clusters and single atoms, while fulfilling superior metal utilisation and robustness, opens the door to unprecedented electronic and geometric properties. The latter can lead to facilitated activation of reactants as well as boosted selectivity control and synergy between distinct active sites in multifunctional catalysts. Based on the analysis conducted, guidelines for the selection of metals for diverse applications are put forward in terms of chemical identity and structure, and aspects that should be explored in greater depth for further improving the exploitation of metals in this research field and beyond are highlighted.

Hydrothermal synthesis of boron-free Zr-MWW and Sn-MWW zeolites as robust Lewis acid catalysts

An innovative strategy based on dual structure-directing agent-facilitated crystallization was proposed to hydrothermally synthesize boron-free Zr-MWW and Sn-MWW metallosilicates that bear great structural diversity for potential pore engineering. The metallosilicates show distinctive features in Lewis acid-catalyzed reactions as efficient heterogeneous solid catalysts.

Effect of Stannic Species Modification on the Acidity of Silicalite-1 and Its Enhancement in Transforming Ethylenediamine to Heterocyclic Amines

In this study, a series of SnO2 modified zeolite catalysts (Snx-S-1; x is the weight percentage of Sn) were prepared with SnCl2 and a defective Silicalite-1 (S-1) zeolite via facile deposition–precipitation method. It was found that the stannic species modified all-silica zeolite catalysts were active for the intermolecular condensation of ethylenediamine (EDA) to 1, 2-Diazabicyclo [2, 2, 2] octane (TEDA) and piperazine (PIP). The best catalyst Sn6-S-1 (6 wt.% Sn loading) showed 86% EDA conversion and 93% total selectivity to TEDA and PIP. By contrast, the defective S-1 zeolite parent showed only approximately 9% EDA conversion under the same conditions. With the help of catalyst characterization techniques including hydroxyl vibration and pyridine adsorption FT-IR spectroscopy (transmission mode), the enhancement of the catalytic activity of the SnO2 modified zeolite catalysts (Snx-S-1) was mainly attributed to the formation of mild Lewis acid sites in the siliceous zeolite. Both the hydroxyl nests of the defective S-1 zeolite and the dispersed SnO2 clusters should be the important factors for the formation of mild Lewis acid sites on the modified zeolite. Based on the catalytic performance of the modified zeolite in the conversion of EDA to PIP and TEDA, it is inferred that the mildly acidified defective S-1 zeolite by the SnO2 deposition modification might become a very active and durable catalyst for reactions involving strongly alkaline reactants and products.

Relating structural disorder and melting in complex mixed ligand zeolitic imidazolate framework glasses

We report the formation of zeolitic imidazolate framework glasses incorporating three organic linkers, from their corresponding novel crystalline structures [Zn(Im2-x-ybImxmbImy)]. Structure-property relationships between chemical compositions and thermal properties are analysed, in addition to the effect on the nanoscale porosity of the glasses formed. A probabilistic model is used to explain melting and the glass transition temperatures of the obtained glasses and link to the nanoscale structural disorder of their crystalline starting structures.

Synergetic effect of Lewis acid and base in modified Sn-β on the direct conversion of levoglucosan to lactic acid

The synergetic effect of Lewis acid and base in modified Sn-β boosts the yield of lactic acid produced from levoglucosan.