Lewis Acid Zeolites Research Articles

The ‘Open’ Site of Lewis Acid Zeolites for Meerwein‐Ponndorf‐Verley‐Oppenauer Reactions in Biomass Catalysis

AbstractHeteroatom zeolites are efficient catalysts for activating oxygenates in Meerwein‐Ponndorf‐Verley‐Oppenauer (MPVO) reactions, due to the unique Lewis acid character of their tetrahedral heteroatom metal sites. Such Lewis acid zeolites, especially heteroatom Beta zeolites, are intensively investigated with increasing attention in the valorization of biomass‐derived oxygenates. Herein, we first briefly review the recent development and advantage of Lewis acid zeolites for MPVO reactions involved in biomass valorization, majorly based on the selected case studies of the most representative heteroatom Beta zeolites. Increasing recognization of the open framework sites as the intrinsic active sites for Lewis acid zeolites in MPVO reactions has been discussed, with static and dynamic viewpoints. Furthermore, the recent progress on the development of characterization technologies applied in recent the state‐of‐the‐art showcases of identification and quantification of open sites, has been assessed, with a special focus on indirect detection using probe molecules and direct detection approaches. Future opportunities and challenges regarding the development of complimentary in situ characterization technologies and in‐depth understanding of the true nature of active sites and correlative mechanisms are examined for the ultimate goal of providing stable MPVO catalysts in biomass valorization.

Selective recovery of light olefins from polyolefin catalyzed by Lewis acidic Sn-Beta zeolites without Brønsted acidity

Lewis acidic Sn-Beta zeolites can produce light olefins selectively during LDPE catalytic cracking. The produced light olefins were protected from being converted to other hydrocarbons by the absence of Brønsted acid sites.

Heteroatom Lewis acid zeolites: synthesis, characterization and application in the conversion of biomass-derived oxygenates

This review summarizes recent advances in the synthesis, characterization and application of heteroatom (Ti, Zr, Sn, and Hf) Lewis acid zeolites in the conversion of biomass-derived oxygenates.

Selective linear ethylene oligomerization over nickel-containing zeotypes with tetravalent framework heteroatoms

Nickel-containing Lewis acidic zeolites as highly selective ethylene oligomerization catalysts.

Water structures on acidic zeolites and their roles in catalysis.

The local reaction environment of catalytic active sites can be manipulated to modify the kinetics and thermodynamic properties of heterogeneous catalysis. Because of the unique physical-chemical nature of water, heterogeneously catalyzed reactions involving specific interactions between water molecules and active sites on catalysts exhibit distinct outcomes that are different from those performed in the absence of water. Zeolitic materials are being applied with the presence of water for heterogeneous catalytic reactions in the chemical industry and our transition to sustainable energy. Mechanistic investigation and in-depth understanding about the behaviors and the roles of water are essentially required for zeolite chemistry and catalysis. In this review, we focus on the discussions of the nature and structures of water adsorbed/stabilized on Brønsted and Lewis acidic zeolites based on experimental observations as well as theoretical calculation results. The unveiled functions of water structures in determining the catalytic efficacy of zeolite-catalyzed reactions have been overviewed and the strategies frequently developed for enhancing the stabilization of zeolite catalysts are highlighted. Recent advancement will contribute to the development of innovative catalytic reactions and the rationalization of catalytic performances in terms of activity, selectivity and stability with the presence of water vapor or in condensed aqueous phase.

Ketonization of propionic acid over TS-1 and Ti-Beta zeolites: Mechanism and effects of topology and hydrophobicity

Ketonization of propionic acid has been investigated on Lewis acidic TS-1 and Ti-Beta zeolites to understand the reaction mechanism and effects of pore topology and hydrophobicity. Characterizations showed that TS-1 and Ti-Beta (Si/Ti = 40) exhibited a similar acidity whereas TS-1 was more hydrophobic than the silanol-rich Ti-Beta. TS-1 showed higher activity, selectivity, stability and was more resistance to H2O than Ti-Beta. A combination of infrared spectroscopic, isotopic and kinetic studies showed that the molecularly adsorbed monodentate propionic acid is the most abundant reactive intermediate, and C-C coupling is the rate-determining step. The mechanism of ketonization of propionic acid at isolated tetrahedral Ti centers can be described by a Langmuir-Hinshelwood model. The turnover rate on TS-1-40 is ∼ 1.3 times higher than that on Ti-Beta-40, which is consistent with the lower fitted free activation energy, i.e., the energy difference between C-C coupling transition state and two co-adsorbed propionic acids at a Ti center. The slightly lower activation enthalpy on Ti-Beta-40 than TS-1-40 is over-compensated by much larger entropy penalty, due to more silanol groups and hydrogen bonded propionic acids in close proximity to the Ti center for Ti-Beta-40, which impose additional steric hindrance to bulkier C-C coupling complex at transition state and outweigh the difference of steric hindrance of pore confinement between MFI and BEA frameworks. This work provides insight into the ketonization mechanism on Lewis acidic zeolites, and indicates that moderate pore size with hydrophobic property promotes ketonization of carboxylic acid at isolated tetrahedral Ti centers confined in zeolites.

The effect of the active site and substrate structure in preparation of substituted tetrahydropyrans via intramolecular cyclization

Substituted tetrahydropyrans (THPs) are important biologically active compounds and synthetic intermediates. Their prevalence and diverse applications make them desirable targets in synthetic organic chemistry. Current synthetic routes for THP synthesis involve homogeneous catalysts and usually multiple steps and the use of expensive heavy metal catalysts, which pose environmental concerns. Therefore, there is a need for alternative approaches that can enhance efficiency, eco-friendliness, and cost-effectiveness This study focuses on the acid-catalyzed synthesis of THPs through the intramolecular cyclization of substituted (E)− 5-phenylpent-4-en-1-ols over zeolites as heterogeneous catalysts. The study particularly focused on investigating the influence of functional groups, as well as substrate size, on the reaction outcome. The results demonstrated that electron-donating groups enhance the conversion of the reactants and increase selectivity towards THPs, while electron-withdrawing groups have the opposite effect. Furthermore, the cyclization reaction also occurred on substrates containing different aromatic cycles. However, the conversion of a bulky substrate containing naphthyl group decreased due to mass transfer limitations. The catalytic performance of different types of zeolites with diverse acid sites was also examined. Brønsted acidic zeolites such as Al-BEA and Ga-BEA exhibited promising catalytic performance and 94 % and 83 % selectivity towards THPs, respectively. In contrast, Lewis acidic zeolites such as Sn-BEA, Ti-BEA, and Zr-BEA showed conversions below 6.5 % and poor selectivity, showing that Brønsted acidic zeolites are more suitable for the cyclization reaction than Lewis acidic zeolites.

Counting Sites in Lewis Acid Zeolite Sn-Beta: Connecting Site Quantification Experiments and Spectroscopy To Investigate the Catalytic Activity for the Alcohol Ring Opening of Epoxides

Counting Sites in Lewis Acid Zeolite Sn-Beta: Connecting Site Quantification Experiments and Spectroscopy To Investigate the Catalytic Activity for the Alcohol Ring Opening of Epoxides

Approaching High-Performance TS-1 Zeolites in the Presence of Alkali Metal Ions via Combination of Adjusting pH Value and Modulating Crystal Size.

Lewis acid zeolites play an important role in industrially important green reactions closely related to fine chemical and biomass conversion. Titanium-doped TS-1 zeolite is a milestone Lewis acid zeolite widely used in industrially significant green oxidation processes with hydrogen peroxide as an oxidant under mild conditions. TS-1 zeolites are normally synthesized in basic conditions under hydrothermal treatment. Up to now, there has still been no success in synthesizing active TS-1 Lewis acid zeolites by using inorganic alkali, e.g., NaOH or KOH as base, which is cheaper and more stable compared to the quaternary ammonium hydroxide or organic amines used in traditional synthesis. Here, an inorganic base of NaOH was employed in synthesizing active TS-1 zeolites for the first time. The crucial factor was the control of adverse effects of sodium cations on the incorporation of active titanium cations. Higher catalytic activity was achieved by further reducing the size of the TS-1 crystal by using the seed-added strategy, which uses the catalytic activity of a commercial catalyst, the production cost being much lower than commercial TS-1 catalysts, indicating great commercial potential and the possibility of preparing other cheap Lewis acid catalysts by using inorganic alkali.

Seed-Assisted Interzeolite Conversion Strategy for Fabricating Lewis Acid Sn-CHA Zeolites.

Small-pore Lewis acid zeolites have been showing increasing potential in shape-selective reactions regarding small-molecule conversion. In this study, Sn-CHA with tunable framework Sn contents was facilely prepared via a fluoride-free, seed-assisted interzeolite conversion (IZC) pathway. Commercially available dealuminated USY functioned as the parent sample, and seeding played a vital role in accelerating the transformation process, promoting the target zeolite yield, and guiding the attached-growth pathway. Notably, a proto-zeolite phase with a semi-constructed pore structure was captured during the IZC process, which represents a crucial intermediate stage for developing the complete CHA structure and ensuring a well-defined Sn status. The detailed synthesis mechanism was explored in multiscale by a series of techniques. The obtained Sn-CHA and proto-Sn-CHA exhibited excellent catalytic performance in converting 1,3-dihydroxyacetone to methyl lactate. Proto-Sn-CHA was proven to be a highly effective glucose isomerization catalyst owing to its larger pore size and Lewis acidic nature.

A Comprehensive Review on Zeolite Chemistry for Catalytic Conversion of Biomass/Waste into Green Fuels

Numerous attempts have been made to produce new materials and technology for renewable energy and environmental improvements in response to global sustainable solutions stemming from fast industrial expansion and population growth. Zeolites are a group of crystalline materials having molecularly ordered micropore arrangements. Over the past few years, progress in zeolites has been observed in transforming biomass and waste into fuels. To ensure effective transition of fossil energy carriers into chemicals and fuels, zeolite catalysts play a key role; however, their function in biomass usage is more obscure. Herein, the effectiveness of zeolites has been discussed in the context of biomass transformation into valuable products. Established zeolites emphasise conversion of lignocellulosic materials into green fuels. Lewis acidic zeolites employ transition of carbohydrates into significant chemical production. Zeolites utilise several procedures, such as catalytic pyrolysis, hydrothermal liquefaction, and hydro-pyrolysis, to convert biomass and lignocelluloses. Zeolites exhibit distinctive features and encounter significant obstacles, such as mesoporosity, pore interconnectivity, and stability of zeolites in the liquid phase. In order to complete these transformations successfully, it is necessary to have a thorough understanding of the chemistry of zeolites. Hence, further examination of the technical difficulties associated with catalytic transformation in zeolites will be required. This review article highlights the reaction pathways for biomass conversion using zeolites, their challenges, and their potential utilisation. Future recommendations for zeolite-based biomass conversion are also presented.

Toward Industrially Relevant Sn-BETA Zeolites: Synthesis, Activity, Stability, and Regeneration

Lewis acidic Snβ zeolites have emerged as highly interesting catalysts for a variety of biomass-related reactions. During the last decades, a vast body of research has focused on unravelling the identity of the (most) active Sn sites. Recent research has shifted its focus from fine-tuning one single type of active Sn-site toward optimizing the overall catalytic activity and stability of the Snβ zeolite, which is more suitable from an industrial point of view. After delving into a discussion of the Sn active sites, this Perspective highlights the recent developments that are essential for industrial implementation of Snβ zeolites, with an emphasis on the most recent insights and findings for improving the catalyst’s productivity and stability.

Insights into glyphosate adsorption on Lewis acidic zeolites from theoretical modelling

The removal of glyphosate via the adsorption process is one of the essential routes for decreasing its harmful impact. Here, we investigate the glyphosate adsorption on the Lewis acidic zeolites through density functional theory (DFT) calculations. We found that Hf-BEA provides the highest adsorption energy (−44.1 kcal/mol) as compared to ones of other zeolites. This result corresponds to its highest ordered Lewis acidity calculated by ammonia adsorption. There is a strong correlation between glyphosate and ammonia adsorption energies, as shown in a linear scaling relationship. This result implies the significant role of the Lewis acidity strength of zeolite in glyphosate adsorption process. The adsorption activity which related to zeolite pore structures (BEA, FAU, ZSM-5 and MOR) are found in the order of Hf-BEA > Hf-ZSM-5 ∼ Hf-MOR > Hf-FAU. The effect of water on the glyphosate adsorption is also investigated. Water is adsorbed on Sn-BEA zeolite much weaker than glyphosate and thus the active site of zeolite cannot be blocked by it. The trend of adsorption energies remains the same as in vacuum. Finally, the hydrolysis of metal-substituted active sites to form open sites strengthens the adsorption of glyphosate.

Catalytic Performances of Sn-Beta Catalysts Prepared from Different Heteroatom-Containing Beta Zeolites for the Retro-Aldol Fragmentation of Glucose

Beta zeolites with different heteroatoms incorporated into the lattice at two loadings (Si/M = 100 or 200, where M = Al, Fe, Ga, B) were hydrothermally synthesised and used as starting materials for the preparation of Sn-Beta using Solid-State Incorporation. 119Sn CPMG MAS NMR showed that various Sn species were formed, the distribution of which depended on the identity of the initial heteroatom and the original Si/M ratio. The final Sn-Beta materials (with Si/Sn = 200) were explored as catalysts for the retro-aldol fragmentation of glucose to α-hydroxy-esters in the continuous regime. Amongst these materials, B-derived Sn-Beta was found to exhibit improved levels of selectivity and stability, particularly compared to Sn-Beta catalysts synthesised from commercially available Al-Beta materials, achieving a combined yield of methyl lactate and methyl vinyl glycolate > 80% at short times on the stream. Given that B atoms can be removed from the Beta lattice in mild conditions without the use of highly concentrated acidic media, this discovery demonstrates that B-Beta is an attractive starting material for the future post-synthetic preparation of Lewis acidic zeolites.

Investigating the Impact of Synthesis Conditions to Increase the Yield and Tin Incorporation Efficiency for Lewis Acid Nano-Sn-MFI Zeolites

Advances in the synthesis of Lewis acid zeolites have been bolstered by the synthesis of nano-zeolites to overcome diffusion limitations. Key challenges remain for the synthesis of Lewis acid nano-zeolites since current methods tend to result in a low tin incorporation efficiency (25%) and a low material yield (∼25%). In this work, insights on how to overcome these limitations are investigated through synthesizing nano-zeolite MFI with different tin precursors (n-Sn-MFI) and different ratios of water to silica and/or structure directing agent (SDA) to silica. The crystallization process is monitored using dynamic light scattering, and it is determined that the tin precursor has a minor impact on the final particle size, but the water and SDA concentrations did affect the final particle size. Compared to the standard synthesis (SiO2/SDA/H2O of 1:0.4:23), reducing the amount of SDA/SiO2 to 0.2:1 results in a larger particle size but increases the yield and tin incorporation efficiency. Reducing the amount of water (H2O/SiO2 of 19 and SDA/Si of 0.2) improves the tin incorporation efficiency over the standard case and results in small particles while achieving a yield of up to 78%. The resulting materials are characterized using several standard techniques to demonstrate that the materials are of high quality. The materials are tested for catalytic activity in the alcohol ring opening reaction of epoxide as a probe reaction and are found to have comparable activity. The rate of conversion of 1,2-epoxyhexane can be increased through synthesizing small crystals in high yield through decreasing the SDA/SiO2 ratio to 0.2:1 and reducing the H2O/SiO2 ratio to 19:1. Overall, the work demonstrates that the synthesis conditions can be tuned to increase the tin incorporation efficiency and increase the yield of the Lewis acid nano-zeolites while producing a highly active catalyst.

Identification and quantification of distinct active sites in Hf-Beta zeolites for transfer hydrogenation catalysis

Despite the significant progress made in characterizing different framework heteroatom sites that exist in Lewis acidic zeolites with probe molecule adsorption and spectroscopy, methods to reliably quantify site counts remain indefinite and have been primarily limited to Sn and Ti Lewis acid sites. Here, methods to quantify framework Lewis acidic Hf4+ sites in zeolite Beta (Hf-Beta) with two Lewis base titrants (pyridine, deuterated acetonitrile) were developed using infrared (IR) spectroscopy. Lewis acid site counts for Hf-Beta zeolites were validated by measuring integrated molar extinction coefficients (IMECs; ε, cm μmol−1) on Sn-Beta zeolites using identical Lewis base titrants to benchmark site counts with established literature procedures to quantify Lewis acid sites in Beta zeolites from IR spectra. Highlighting the importance of benchmarking active site counts against well-established experimental protocols, IMECs of CD3CN bound to open (ε(Sn; 2316 cm−1): 1.80 ± 0.25) and closed (ε(Sn; 2308 cm−1): 3.76 ± 0.33) Sn sites were ∼ 1.8x larger than those previously reported while total Lewis acid site counts agreed with those measured by pyridine (ε(Sn; 1451 cm−1): 1.58 ± 0.16) on six different Sn-Beta zeolites. IMECs measured for IR peaks reflecting pyridine bound to Lewis acidic Hf sites (ε(Hf; 1448 cm−1): 1.54 ± 0.21) and CD3CN bound to open (ε(Hf; 2313 cm−1): 2.40 ± 0.22) and closed (ε(Hf; 2307 cm−1): 3.55 ± 0.41) Hf sites, gave similar counts for the total number of Lewis acidic sites across six Hf-Beta zeolites (Si/Hf = 100–413). Consistent with previous reports with Sn-Beta catalysts where open Sn sites are responsible for catalytic turnover, apparent first and zero-order MPVO rate constants (0.01–1 M cyclohexanone in 2-butanol; per total Hf, 373 K) correlated with the total number of open Hf sites, per total Hf, but not with the total number of closed Hf sites or total Lewis acid site counts. Measured initial MPVO rates (0.1 M cyclohexanone in 2-butanol, per open Hf, 373 K) were ∼ 25x higher on hydrophobic Hf-Beta-F than on hydrophilic Hf-Beta-OH zeolites. Overall, the apparent first-order MPVO rate constants (2-butanol solvent, per open Hf, 373 K) were ∼ 6x higher on Hf-Beta-F than on Hf-Beta-OH zeolites. The characterization methods reported here enable normalization of MPVO turnover rates on Sn- and Hf-Beta zeolites by their number of open sites. This enables performing quantitative rate comparisons across Lewis acid zeolites of varying active site identity, solvation, and pore topology used in liquid-phase catalysis.

Tailoring Distinct Reactive Environments in Lewis Acid Zeolites for Liquid Phase Catalysis

ConspectusLewis acidic zeolites are microporous crystalline materials that offer promise as catalysts for the activation and conversion of biomass-derived precursors in the liquid phase due to their unique water tolerance and synthetic versatility. The active site environment in zeolite catalysts is multifaceted in nature and is composed of a primary catalytic binding site, the secondary pore structure that confines such binding sites, and occluded solvent and reactant molecules that interact with adsorbed species. Moreover, Lewis acidic heteroatoms can adopt structurally diverse coordination that selectively catalyze different classes of chemical transformations and can be difficult to control synthetically or characterize spectroscopically. Thus, precise mechanistic interpretation of liquid-phase zeolite catalysis necessitates the development of synthetic, spectroscopic, and kinetic methods that can decouple such complex active site structures and probe the interactions that occur between confined active sites, solvent and reactant molecules, and adsorbed intermediates and transition states.In this Account, we describe the development and application of synthetic, spectroscopic, and kinetic methods to investigate chemically distinct Lewis acid zeolite environments in siliceous zeolites for liquid-phase catalysis. Identification of unique Lewis acidic active site structures relied on the development of direct and indirect solid-state nuclear magnetic resonance (NMR) methods that probe the number and connectivity of framework Lewis acid sites for a diverse range of metal heteroatoms. Such methods enabled the quantitative comparison of catalytic turnover rates, on a per active site basis, measured on different catalysts in order to establish structure–function relationships between active site structure and reactivity. Rigorous normalization of turnover rate further permits comparison of catalytic turnover rates across materials of varying topology, metal heteroatom identity, solvent, and framework polarity to extract salient thermodynamic descriptors of catalysis through kinetic probes. Ex situ interrogation of alcohols adsorbed within hydrophobic and hydrophilic Sn-containing zeolites revealed that hydrophobic voids induce structural order on confined alcohol hydrogen-bonding networks, which give rise to enhanced turnover rates of liquid-phase transfer hydrogenation catalysis. This acceleration of turnover rates arises because ordered alcohol networks occluded within the pores of hydrophobic zeolites stabilize adsorbed transfer hydrogenation intermediates and transition states to a greater extent than liquidlike solvent networks observed in hydrophilic zeolites. The effects of confined solvent molecules can also influence catalysis, independent of framework polarity, due to differences in solvent polarity and substituent effects, which alter turnover rates via changes in how different solvent molecules interact with adsorbed intermediates and transition states. These observations underscore new opportunities to leverage specific interactions between active sites and solvent molecules to influence solvent organization and transition state stability at confined solid–liquid interfaces.This work illustrates the importance of quantitative methods that count distinct active site structures in order to compare catalytic materials on a per active site basis. This information can be used to develop new synthetic procedures that predictably manipulate the functionalization of both the primary binding site and the secondary reaction environment to tailor catalytic function for a desired chemistry. Collectively, these advances highlight strategies to engineer and characterize microporous catalysts with unique reaction environments in order to capture salient mechanistic features and navigate the complex free energy landscape of catalysis in condensed solvent systems.

The influence of cation exchange and tetravalent metal substitutions in Lewis acidic BEA zeolites for phenol adsorption and Tautomerization: A computational study

The effect of cation exchange and tetravalent metal substitution in Lewis acidic BEA zeolites for phenol adsorption and tautomerization was investigated by deriving the relevant reaction profiles with density functional theory calculations. The strongest adsorption interaction is when phenol chemisorbed to Li-Sn-BEA, due to the highest charge transfers from phenol to zeolites. Based on overall activation energies, the catalytic activity for phenol tautomerization on Sn-BEA can be improved with the exchange of cations. The catalytic activity of tetravalent metal centers substituted into BEA is compared and found in the order of Zr ~ Hf > Sn > Ti.

In-Situ-Generated Active Hf-hydride in Zeolites for the Tandem N-Alkylation of Amines with Benzyl Alcohol

In this work, we have studied the catalytic activity of different silicates (MFI, MCM-41, and Beta) containing Lewis acid sites (including Sn, Ti, Zr, and Hf) for the tandem N-alkylation reaction of aniline with benzyl alcohol. The Hf- and Zr-Beta were the most active catalysts for this transformation, showing in both cases selectivities toward the corresponding N-benzylaniline higher than 97%. FTIR and DFT analyses confirm that the active sites in the Hf-Beta catalyst for this process are the open sites where one of the four Hf–O bonds is hydrolyzed. Moreover, the amount of these active species could be notoriously increased with previous thermal treatment of the Lewis acid zeolite with benzyl alcohol. Isotopically labeled experiments and theoretical mechanistic studies reveal that the N-alkylation reaction occurs through a hydrogen borrowing pathway, in which in situ Hf-hydride species were generated. Finally, the Hf-Beta zeolite was reused several times in the N-alkylation reaction without any appreciable deactivation detected. This catalytic system could be expanded to a variety of amines, including aliphatic and biomass-derived amines.

Ketonization of Propionic Acid on Lewis Acidic Zr-Beta Zeolite with Improved Stability and Selectivity

Ketonization reactions provide a feasible approach to remove oxygen and increase carbon chain length for conversion of biomass derived carboxylic acids. However, the reaction suffers from fast catalyst deactivation and low ketone selectivity. In this work, Lewis acidic heteroatom Ti-, Zr-, Ce-, and Sn-Beta zeolites were prepared using a two-step post-synthesis method and applied in vapor phase ketonization of propionic acid at 350 °C. Among these zeolites, Zr-Beta shows both the highest activity and selectivity. Characterizations indicate that Zr prefers the vacant tetrahedral site when the Zr content is 96% at a space time of 2 h. The amount of coke deposition on Zr-Beta is about 1/3 of that on H-Beta, and the structure of Zr-Beta is preserved after 60 h of reaction. The results of this work indicate that Zr-Beta zeolite is a promising catalyst for ketonization with good stability and high selectivity toward ketone.