Lewis Acid Zeolites Research Articles

A Chromium Hydroxide/MIL‐101(Cr) MOF Composite Catalyst and Its Use for the Selective Isomerization of Glucose to Fructose

AbstractA metal–organic framework (MOF)‐based catalyst, chromium hydroxide/MIL‐101(Cr), was prepared by a one‐pot synthesis method. The combination of chromium hydroxide particles on and within Lewis acidic MIL‐101 accomplishes highly selective conversion of glucose to fructose in the presence of ethanol, matching the performance of optimized Sn‐containing Lewis acidic zeolites. Differently from zeolites, NMR spectroscopy studies with isotopically labeled molecules demonstrate that isomerization of glucose to fructose on this catalyst, proceeds predominantly via a proton transfer mechanism.

A Chromium Hydroxide/MIL-101(Cr) MOF Composite Catalyst and Its Use for the Selective Isomerization of Glucose to Fructose.

A metal-organic framework (MOF)-based catalyst, chromium hydroxide/MIL-101(Cr), was prepared by a one-pot synthesis method. The combination of chromium hydroxide particles on and within Lewis acidic MIL-101 accomplishes highly selective conversion of glucose to fructose in the presence of ethanol, matching the performance of optimized Sn-containing Lewis acidic zeolites. Differently from zeolites, NMR spectroscopy studies with isotopically labeled molecules demonstrate that isomerization of glucose to fructose on this catalyst, proceeds predominantly via a proton transfer mechanism.

Distinguishing Active Site Identity in Sn-Beta Zeolites Using 31P MAS NMR of Adsorbed Trimethylphosphine Oxide

The identity of the active sites of hydrophobic Lewis acid zeolites was elucidated by solid-state 31P nuclear magnetic resonance (NMR) spectroscopy following adsorption of trimethylphosphine oxide (TMPO) probe molecules. The adsorption of TMPO on these materials resulted in distinct 31P NMR resonances between δiso = 59.9 and 54.9 ppm that correspond to unique chemical environments of the Lewis acidic heteroatoms in the Beta zeolite framework. The 31P NMR resonances were assigned to sites in Sn-Beta zeolites by correlating the variation of 31P NMR spectra during TMPO titration experiments with the corresponding changes in the 119Sn NMR spectra. This method allowed us to establish quantitative relationships between the assignments for each site and the catalytic activity for the glucose isomerization and aldol condensation reactions. The rate of glucose isomerization directly correlated with the combined integrated intensities of 31P MAS NMR resonances at δiso = 55.8 and 54.9 ppm, which amounted to 12–33% o...

Lead-containing Beta zeolites as versatile Lewis acid catalysts for the aminolysis of epoxides

Pb-Beta, a heteroatom zeolite containing out-size atoms of lead, has been successfully prepared via a scalable post-synthesis strategy composed of dealumination of H-Beta to create associated silanol and subsequent dry impregnation of the resulting Si-Beta with lead nitrate. The incorporated lead species exist in the form of well-isolated tetra-coordinated Pb2+ ions with open configurations, as revealed by the characterization results from high angle annular dark filed scanning transmission electron microscopy (HAADF-STEM), UV-Vis, X-ray photoelectron spectroscopy (XPS) and 29Si solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Sufficient Lewis acid sites with moderate acid strength are created by lead incorporation into zeolite framework, as confirmed by the temperature-programmed desorption of ammonia (NH3-TPD). The as-prepared Pb-Beta Lewis acid zeolites exhibit remarkable catalytic activity in the ring-opening aminolysis of epoxides to β-amino alcohols with high regio-selectivity. The presence of wide-open lead sites can promote the simultaneous activation of epoxide and amine substrates and, therefore, catalyze the aminolysis of epoxides with strong basic amines, e.g. butylamine.

Theoretical study of CO2 hydrogenation into formic acid on Lewis acid zeolites

Conversion of carbon dioxide (CO2) to more valuable chemicals is nowadays receiving increasing attention from an environmental and industrial point of view. Herein, we computationally investigated CO2 hydrogenation to formic acid on Lewis acid zeolites by means of density functional theory (DFT) with the M06-L functional. The reaction proceeds in two steps, hydrogenation of CO2 to form the formate intermediate and hydrogen-abstraction to form formic acid. A defect zeolite seems to be favored over a perfect one, leading to its low rate determining step barrier of 5.2 kcal mol-1. We also considered the effect of the zeolite frameworks and found that the catalytic activities are in the order Sn-ZSM-5 > Sn-BEA > Sn-FAU. Finally, we performed catalytic activity screenings of tetravalent metals (Ge, Zr and Hf) substituted into the defect Sn-ZSM-5 zeolite. The order Hf > Zr > Sn > Ge was found based on the rate determining step activation energy. The difference in activation energy can be explained by the difference in charge transfer from the catalytic site to the reacting molecules.

Glucose Conversion to Furans in Alcohols Catalyzed by Lewis Acidic Beta Zeolites and Brønsted Acidic Resins

AbstractGlucose was converted to 5‐(hydroxymethyl)furfural (HMF) and the potential biodiesel additives 5‐(ethoxymethyl)furfural (EMF) and 5‐(isopropoxymethyl)furfural (IMF) by Lewis acidic zeolites and Brønsted acidic resins. The conversion occured by sequential isomerization and dehydration reactions in semiaqueous mixtures containing ethanol or isopropanol. Sn‐beta and Amberlyst® 15 catalyzed the conversion of glucose to HMF and IMF in 53% yield in isopropanol containing 7.5 weight % water. The yield of undetected byproducts including humins was 7%. The moderate yield is attributed to the increased selectivity of the isomerization reaction and the suppression of further reactions of HMF to byproducts such as levullinic acid, formic acid, and humins in the alcohols.

Zeolite-catalyzed Transformation of Glucose: A Review

Background: Isomerization of glucose to fructose is recognized as one of the key reactions for the transformation of cellulosic biomass, a potent substitution for the depleting petroleum resources. Zeolites are among the most useful catalysts for the isomerization process and substantial achievements have been achieved thus far. A state-of-the-art review is given within this content, and catalytic mechanisms and competitive reactions are discussed as well. Methods: We performed a comprehensive search of literatures and the qualities of retrieved papers were appraised using standard tools. Literatures that represent the substantial findings are highlighted reflecting the progresses made with regard to the isomerization of glucose to fructose catalyzed by zeolites. Results: Substantial achievements have been made with regard to the zeolite-catalyzed isomerization of glucose to fructose. The joint experimental-computational studies provide insightful clues to reaction mechanisms; e.g., the isomerization reaction proceeds in sequence of pyranose ring opening, hydride shift from C2 to C1 (rate-decisive) and furanose ring closure while the epimerization reaction is characterized by the intramolecular 1,2-carbon shift; Both of the defects SnOH and proximate silanol promote significantly the isomerization reaction while proximate silanol disfavors the epimerization reaction. Conclusion: On basis of the fruitful catalytic results obtained thus far, two outlets are put forward that should be given priority with regard to the utilization of cellulosic biomass, as the combination of the isomerization reaction with other catalytic processes and the exploitation of the direct transformation of cellulosic biomass to platform chemicals. Keywords: Cellulosic biomass, Lewis acidic zeolites, catalysis, isomerization, reaction mechanism, transformation.

Catalysis of the Diels–Alder Reaction of Furan and Methyl Acrylate in Lewis Acidic Zeolites

Diels–Alder (DA) reactions of furans yield oxanorbornene derivatives which can be converted to a variety of molecules, ranging from molecules of biological interest to naturally occurring organic compounds, and to aromatics via dehydration, a promising alternative for the synthesis of aromatics from renewables. With furan being one of the less reactive dienes, the development of Lewis acidic heterogeneous catalysts, without the shortcomings of the traditional homogeneous catalysts, is critically important. Herein, we use computational chemistry to study the DA reaction of furan and methyl acrylate in three zeotypic Lewis acids, Sn-, Zr-, and Hf-BEA. We find that all three exhibit the same ability to enhance the electrophilic character of the dienophile and promote modest charge transfer from the diene. Despite being moderately Lewis acidic, they still achieve a reduction of about 12.5 kcal/mol in the activation energy relative to the reaction in the absence of catalyst.

Theoretical study on the reaction mechanism of hydrogenation of furfural to furfuryl alcohol on Lewis acidic BEA zeolites: effects of defect structure and tetravalent metals substitution.

Furfural acquired from agricultural sources is receiving extensive attention in the petrochemical industry as it offers an alternative route to generate more valuable hydrocarbon compounds. Herein, we investigate the furfural hydrogenation to furfuryl alcohol catalyzed by Lewis acidic BEA zeolites at the molecular level by means of the M06-L density functional theory. The mechanistic pictures in the catalytic procedure are revealed. The possible reaction pathways are considered to proceed via either concerted or stepwise mechanisms. With the contribution of zeolite oxygen bridging for the H-H splitting, the rate determining step activation barrier for the stepwise mechanism is 14.7 kcal mol-1 lower than that for the concerted mechanism. The stepwise reaction therefore seems to be favored compared to the concerted one. The catalytic effect of the defect zeolite framework on the stepwise mechanism is also investigated. The activation energy for the stepwise rate-determining step over this site is significantly lower than the corresponding step over the perfect one by 14.1 kcal mol-1. Finally, the catalytic activity of tetravalent metal centers (Sn, Ge, Zr and Hf) substituted in BEA is also preliminarily compared and it is found to follow the order of Hf > Zr > Sn > Ge based on activation energies and the reaction rate. The difference in the activation energy can be traced back to the difference in the charge transfer from the catalytic site to the adsorbed molecules.

Production of liquid fuel intermediates from furfural via aldol condensation over Lewis acid zeolite catalysts

Aldol condensation reactions between furfural and acetone can be used to produce liquid fuel intermediates.

Interrogating the Lewis Acidity of Metal Sites in Beta Zeolites with 15N Pyridine Adsorption Coupled with MAS NMR Spectroscopy.

The Lewis acidity of isolated framework metal sites in Beta zeolites was characterized with 15N isotopically labeled pyridine adsorption coupled with magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The 15N chemical shift of adsorbed pyridine was found to scale with the acid character of both Lewis (Ti, Hf, Zr, Nb, Ta, and Sn) and Brønsted (B, Ga, and Al) acidic heteroatoms. The 15N chemical shift showed a linear correlation with Mulliken electronegativity of the metal center in the order Ti < Hf < Zr < Nb < Ta < Sn < H+. Theoretical calculations using density functional theory (DFT) showed a strong correlation between experimental 15N chemical shift and the calculated metal-nitrogen bond dissociation energy, and revealed the importance of active site reorganization when determining adsorption strength. The relationships found between 15N pyridine chemical shift and intrinsic chemical descriptors of metal framework sites complement adsorption equilibrium data and provide a robust method to characterize, and ultimately optimize, metal-reactant binding and activation for Lewis acid zeolites. Direct 15N MAS NMR detection protocols applied to the Lewis acid-base adducts allowed the differentiation and quantification of framework metal sites in the presence of extraframework oxides, including highly quadrupolar nuclei that are not amenable for quantification with conventional NMR methods.

1,2-H- versus 1,2-C-Shift on Sn-Silsesquioxanes

Lewis acidic zeolites such as Sn-Beta catalyze glucose isomerization to fructose via an intramolecular 1,2-H-shift reaction, a key step for converting lignocellulosic biomass into renewable chemicals. Na-exchange of Sn-Beta titrates the neighboring SiOH group in the open Sn site and shifts catalyst selectivity to mannose formed by a 1,2-C-shift reaction. To probe structure/activity relationships in the zeolite, tin-containing silsesquioxanes with (1a) and without (1b) a neighboring SiOH group were recently synthesized and tested. These molecular catalysts are active for glucose conversion, and the presence (absence) of the SiOH favors fructose (mannose) selectivity by intramolecular H(C)-shift reactions. Using density functional theory, we investigated numerous H/C-shift pathways on these tin-silsesquioxane catalysts. On both 1a and 1b, the H-shift reaction occurs through a bidentate binding mode without participation of the SiOH, while the bidentate binding mode is not favored for the C-shift due to ster...

A periodic-DFT study of retro-aldol fragmentation of fuctose on MoO3

We use Density-Functional Theory calculations to investigate the mechanism of the retro-aldol fragmentation of fructose to glyceraldehyde (GLA) and dihydroxyacetone (DHA) on MoO3. We find that the reaction follows the classic mechanism of CC bond activation by proton abstraction from the βOH group of the sugar and stabilization of the transition state and resulting enolate intermediate by charge delocalization through coordination of the sugar’s carbonyl group to a Lewis acidic Mo atom. The deprotonation of fructose and the CC bond scission take place in a concerted fashion. On MoO3, fructose can also undergo epimerization to d-hamamelose through a 1,2-carbon shift along a pathway that is as probable kinetically as the one that leads to retro-aldol fragmentation but is somewhat more favourable thermodynamically. We see that hamamelose formation does not have an adverse effect on the overall yield of alkyl lactates, because hamamelose can itself undergo retro-aldol fragmentation to GLA, which can further be converted to alkyl lactates through a cascade of thermodynamically favorable reactions involving isomerization via 1,2-hydrogen shift, dehydration and ketalization, all catalyzed by the Lewis acidic zeolite Sn-MFI.

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability.

Zeolites containing Sn, Ti, Zr, Hf, Nb, or Ta heteroatoms are versatile catalysts for the activation and conversion of oxygenated molecules owing to the unique Lewis acid character of their tetrahedral metal sites. Through fluoride-mediated synthesis, hydrophobic Lewis acid zeolites can behave as water-tolerant catalysts, which has resulted in a recent surge of experimental and computational studies in the field of biomass conversion. However, many open questions still surround these materials, especially relating to the nature of their active sites. This lack of fundamental understanding is exemplified by the many dissonant results that have been described in recent literature reports. In this review, we use a molecular-based approach to provide insight into the relationship between the structure of the metal center and its reactivity toward different substrates, with the ultimate goal of providing a robust framework to understand the properties that have the strongest influence on catalytic performance for the conversion of oxygenates.

Synthesis of Itaconic Acid Ester Analogues via Self-Aldol Condensation of Ethyl Pyruvate Catalyzed by Hafnium BEA Zeolites

Lewis acidic zeolites are used to synthesize unsaturated dicarboxylic acid esters via aldol condensation of keto esters. Hafnium-containing BEA (Hf-BEA) zeolites catalyze the condensation of ethyl pyruvate into diethyl 2-methyl-4-oxopent-2-enedioate and diethyl 2-methylene-4-oxopentanedioate (an itaconic acid ester analogue) with a selectivity of ca. 80% at ca. 60% conversion in a packed-bed reactor. The catalyst is stable for 132 h on stream, reaching a turnover number of 5110 molEP molHf–1. Analysis of the dynamic behavior of Hf-BEA under flow conditions and studies with Na-exchanged zeolites suggest that Hf(IV) open sites possess dual functionality for Lewis and Bronsted acid catalysis.

Scaling Relationships for Molecular Adsorption and Dissociation in Lewis Acid Zeolites

Linear scaling relationships between adsorption energies of selected C1 and C2 oxygenates, as well as sulfur-containing compounds, are developed for Lewis acid-substituted zeolites in the chabazite framework. The relationships are shown to hold for dissociative adsorption of molecules cleaving similar classes of bonds, including O–H scission in CH3OH and H2O, C–H scission in CH3OH and CH4, S–H activation in CH3SH and H2S, C–H cleavage in CH2O and CH3CHO, and C–H activation in CH2 and CHOH, where charge is conserved in the zeolite framework. These correlations are explained in terms of bond order conservation arguments that have been previously developed for metal and simple oxide surfaces, and extension of the scaling relationships to Bronsted acid sites is briefly discussed. For adsorbates with nonzero net spin, use of a high accuracy hybrid functional is shown to improve the accuracy of the scaling relationships, while such accuracy does not substantially affect the results for adsorption complexes with...

A low-deactivation-rate Lewis acid zeolite prepared in an alkali metal ion-containing system for alkene epoxidation

The best catalytic performance among the currently reported TS-1 Lewis acid zeolites is achieved due to its durable catalytic life.

Lewis acid zeolites for tandem Diels–Alder cycloaddition and dehydration of biomass-derived dimethylfuran and ethylene to renewable p-xylene

p-Xylene was synthesized by [4 + 2] cycloaddition of biomass-derived ethylene and dimethylfuran over Lewis acid zeolite catalyst, Zr-BEA, with 90% selectivity in products.

Lewis acidic zeolite Beta catalyst for the Meerwein–Ponndorf–Verley reduction of furfural

Tin-, zirconium- and hafnium-containing siliceous Beta zeolite were investigated in the Meerwein–Ponndorf–Verley reduction of furfural under continuous flow conditions.

Switching off H2O2 Decomposition during TS-1 Catalysed Epoxidation via Post-Synthetic Active Site Modification

Despite its widespread use, the Lewis acidic zeolite, TS-1, still exhibits several unfavourable properties, such as excessive H2O2 decomposition, which decrease its overall performance. In this manuscript, we demonstrate that post-synthetic modification of TS-1 with aqueous NH4HF2 leads to modifications in epoxidation catalysis, which both improves the levels of epoxide selectivity obtained, and drastically minimises undesirable H2O2 decomposition. Through in situ spectroscopic study with UV-resonance enhanced Raman spectroscopy, we also observe a change in Ti site speciation, which occurs via the extraction of mononuclear [Ti(OSi)4] atoms, and which may be responsible for the changes in observed activity.