Stoichiometric adsorption complexes of boron trichloride and antimony pentafluoride in H-ZSM-5

This work demonstrates that inorganic metal halide compounds such as BCl₃ and SbF₅ can be used to prepare adsorption complexes with a stoichiometry of one molecule per Brønsted acid site in the zeolite H-ZSM-5. BCl₃-acid site complexes are prepared by saturating the zeolite with BCl₃ and then evacuating. The BCl₃complex desorbs above 400 K, and ~ 50% of the BCl₃ remains adsorbed at temperatures above 750 K. SbF₅-acid site complexes are prepared by saturating the zeolite, evacuating, and heating to 700 K in vacuum. The SbF₅ complex is shown to polarize coadsorbed acetone to the same degree as magic acid. The stability of the metal halide complexes is further characterized with adsorption and reaction of propene and ethene to for moligomers. Small amounts (<10%) of the metal halide desorb during the oligomer desorption experiment.     

Unraveling the cooperative effects of acid sites and kinetics for pyrolysis of CHF3 to C2F4 and C3F6 on SO42−/ZrO2-SiO2

The treatment or upgrading of waste trifluoromethane (CHF₃, R23), which has a significant greenhouse effect, is of great importance in industry. Herein, series of SO₄²⁻/ZrO₂-SiO₂ catalysts with different Brønsted and Lewis acid site densities and ratios were prepared for pyrolysis of R23 to tetrafluoroethylene (C₂F₄, TFE) and hexafluoropropylene (C₃F₆, HFP). The effects of impregnation concentration of (NH₄)₂SO₄ on specific surface area, crystal phase, and Brønsted and Lewis acid site densities and ratios were respectively demonstrated. The Brønsted and Lewis acid sites were observed to have cooperative effects on R23 transformation and up to 94.6% selectivity of (TFE + HFP) could be achieved at 750°C. The kinetic studies revealed the decomposition of R23 into CF₂ carbene and HF was the rate-determining step, and a deactivation behavior was found due to the site coverage and pore blockage by the oligomers of TFE and HFP.     

Efficient Addition Reaction of Dibutylphosphane Oxide with Alkynes: New Mechanistic Proposal Involving a Duo of Palladium and Brønsted Acid

The addition reaction of dibutylphosphane oxide [Bu₂P(O)H] with alkynes proceeds efficiently in the presence of palladium‐chelating phosphane–Brønsted acid catalyst systems. Terminal alkynes afford branched‐structured products selectively. On the other hand, the same reaction using monodentate phosphane ligands or the reaction run in the absence of a Brønsted acid affords a much lower yield. A mechanistic study has revealed that Brønsted acids (XOH) interact with oxygen in M P(O)R₂ species (M=Pd, Pt) through hydrogen bonding to transform them to ionic M⁺←PR₂(OH⋅⋅⋅O⁻X) species, which was confirmed by NMR spectroscopy and X‐ray crystallography. The phosphane‐like PR₂(OH⋅⋅⋅O⁻X) moiety is coordinatively labile, as substantiated by the ligand exchange reaction with tert‐butyl isocyanide. A new mechanism that accommodates these observations has been proposed to rationalize the enhancement of catalytic activity and the regioselectivity induced by the Brønsted acid.     

Synthesis of Glycerol Triacetate Using a Brønsted–Lewis Acidic Ionic Liquid as the Catalyst

Brønsted–Lewis acidic ionic liquids (IL) were used in the esterification of glycerol and acetic acid to produce glycerol triacetate. The results show that the IL (3–sulfonic acid)–propyltriethylammonium chloroironinate [HO₃S–(CH₂)₃–NEt₃]Cl–[FeCl₃]_x (molar fraction of FeCl₃, x = 0.67) was an efficient catalyst for the esterification reaction. The yield of glycerol triacetate and its content were greater than 98 % when reacted under reflux for 4 h. It was observed that a synergistic effect of Brønsted and Lewis acid sites enhanced the catalytic performance of IL. The reusability of IL was good. After six reaction cycles, the glycerol triacetate yield and concentration were still greater than 98 %. Likewise, the Brønsted–Lewis acidic IL was an efficient catalyst for esterification reactions of high boiling points alcohols with acetic acid.     

Improving bifunctional zeolite catalysts for alkane hydroisomerization via gas phase sulfation

Pt-containing H-BEA zeolites were functionalized with sulfate anions via H₂S chemisorption followed by oxidation. This treatment generates sulfate groups and Brønsted acidic sites, modifies the supported metal particles, and leads to higher activity and selectivity for pentane hydroisomerization. The strength and the accessibility of the acid sites present in the parent material were not affected by this procedure. The concentration of Brønsted acidic sites and the catalytic activity for light alkane isomerization varied sympathetically. The parallel increase in isomerization selectivity indicates that either the residence time of the alkoxy intermediates decreases for the modified samples (suppressing undesired cracking reactions on the acid sites) or the selective decoration of the Pt particles with sulfur reduces hydrogenolysis on the metal particles.     

Alkane isomerization over sulfated zirconia and other solid acids

Some solid acids, including sulfated zirconia and certain industrial isomerization catalysts, catalyze two types of n-butane isomerizations, avoiding primary carbenium ions or carbonium ions: (1) an internal rearrangement of the C atoms in n-butane and (2) skeletal isomerization of n-butane to iso-butane. No superacid sites are required for these reactions. The skeletal isomerization is an intermolecular reaction, involving a C₈ intermediate. Easily accessible Brønsted acid sites and small amounts of olefin are crucial. Spectroscopic examination of the acid sites on sulfated zirconia shows that they are not stronger than the acid sites in zeolites such as HY. The butane isomerization rate is suppressed by CO, even when no CO is adsorbed on Lewis sites; formation of oxocarbenium ions is likely. The decisive role of Brønsted acid sites is demonstrated by results on deuterated catalysts.     

Acid centers in sulfated, phosphated and/or aluminated zirconias

On sulfated ZrO₂, the comparison of the effects of adsorbing water or ammonia on the infrared bands between 1400 and 1000 cm^?1 suggests that besides structural Lewis sites on the surface of ZrO₂, strong Lewis sites are made from chemisorbed SO₃. Upon adsorption of water, SO₃ is converted, partially, into a surface sulfated species which may act as strong Brønsted sites. At moderate surface hydration, both types of sites may coexist. The catalytic activity in the isomerization of isobutane is a function of the overall nominal surface density in SO₄. The acid sites on the surface of phosphated mesoporous zirconia are attributable to surface P–OH groups working as weak Brønsted sites. On both sulfated and phosphated zirconia, surface coating of alumina stabilizes the porosity, but it does not modify the nature of their acid centers.     

Discrepancy between TPD- and FTIR-based measurements of Brønsted and Lewis acidity for sulfated zirconia

Temperature-programmed desorptions (TPD) of isopropylamine (IPA), NH₃, and pyridine were compared with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine to determine the effect of H₂O on the Brønsted and Lewis acidities of two sulfated zirconia (SZ) catalysts. Although the traditional interpretation of pyridine infrared spectra showed an apparent increase in Brønsted acidity upon treating SZ with H₂O, TPD spectra showed that H₂O displaced IPA from approximately one-fifth of the Lewis sites with no corresponding increase in Bronsted acidity. Water treatment prior to TPD displaced similar amounts of both NH₃ and pyridine. The primary effect of H₂O is displacement of weakly adsorbed basic probe molecules from Lewis sites, rather than the conversion of Lewis sites to Brønsted sites. Finally, different types of analyses (e.g. infrared or TPD) of catalyst acidity yield dramatically different conclusions regarding Brønsted and Lewis acidity.     

Promoting Effect of Lewis Acid on the Olefin Hydroesterification Catalyzed by Triphenylphosphine–Palladium Complex

A number of Lewis acids obtained by a reaction of metal oxides with Brønsted acids such as para-toluenesulfonic acid (p-TsOH) and methanesulfonic acid (CH₃SO₃H) have been employed as promoter for styrene hydroesterification catalyzed by triphenylphosphine–palladium (PPh₃–Pd) complex. The acidities of the Lewis acids were analyzed by potentiometric titration and the PPh₃ consumption during the hydroesterification was characterized by ³¹P NMR. The results indicated that the use of Lewis acid with apparent pH similar to p-TsOH can bring about a marked enhancement in the reaction rate and a significant depression in the PPh₃ consumption. Such kind of Lewis acids may be more suitable as promoter for the olefin hydroesterification than the conventional Brønsted acids.     

Metathesis of 1‐Hexene over Heterogeneous Tungsten‐Based Catalysts

1‐Hexene metathesis was performed over standard and potassium‐doped WO₃/SiO₂ catalysts. The samples were tested at various reaction temperatures, molar feed compositions, and space times. Under the applied reaction conditions, doping with potassium reduced the isomerization and cracking activity of the catalyst by at least half and improved the yield of detergent‐range alkenes twofold. However, increasing the potassium loading to a higher amount resulted in a significant reduction in the metathesis activity as both Brønsted and Lewis acid sites were affected. Optimum operating conditions for the yield of detergent‐range alkenes were identified using response surface methodology.     

Study of alkylation on a Lewis and Brønsted acid hybrid catalyst and its industrial test

A novel catalyst ZnBr₂ modified bentonite is prepared. Pyridine adsorbed FTIR spectra demonstrated the amount of Brønsted and Lewis acid was improved to 2 and 5 times, respectively. Thermal analysis indicated the thermal stability of ZnBr₂-bentonite could reach 350 °C. Mechanism research led to an interesting hypothesis that in the former stage, the active position is Brønsted acid sites and in the later stage, both Lewis acid and Brønsted acid sites act as active positions. Industrial test showed the ability of ZnBr₂-bentonite to remove olefins is 17 times higher than that of bentonite, and the industrial application prospect is good.     

Brønsted acidity of amorphous silica–aluminas studied by 1H NMR

¹H broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica–aluminas interacting or not with adsorbed water. The study is more difficult than for zeolites, because the acidic SiO(H)Al bridges are reversibly destroyed by dehydration. However, an acidity coefficient value (H₃O⁺ concentration per Brønsted acid site when one water molecule interacts with each Brønsted site) of 0.34±10%; has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al ratios high enough for maximum acid strength.     

Synthesis and application of a novel strong and stable supported ionic liquid catalyst with both Lewis and Brønsted acid sites

Poly(4-vinylpyridine)-supported ionic liquid with both Lewis and Brønsted acid sites was easily prepared from its starting materials and used as a novel and highly efficient heterogeneous catalytic system for the synthesis of biscoumarins by two-component one-pot domino Knoevenagel-type condensation/Michael reaction between various aliphatic and aromatic aldehydes with 4-hydroxycoumarin. The Lewis and Brønsted acidic sites loading in [P₄VPy-BuSO₃H]Cl-X(AlCl₃) were found to be 2.15 and 0.9 mmol per gram of catalyst, respectively. The effect of the simultaneous presence of Lewis and Brønsted acid sites was evaluated. The catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), elemental analysis, and atomic absorption technique. The catalyst is stable (as a bench top catalyst) and reusable.     

Activity of modified SnO2 catalysts for acid-catalysed reactions

The effect of Lewis and Brønsted acid modification on the catalytic behaviour of SnO₂ was studied. Modified AlCl₃, FeCl₃, ZnCl₂ and H₂SO₄ were used for this purpose. Catalytic activity was tested for Friedel–Crafts alkylation, acylation and Pechmann condensation. It was observed that the alkylation activity of SnO₂ was improved upon treatment with Lewis acids and Brønsted acids. However the acylation activity was observed only when SnO₂ was treated with H₂SO₄. Moderate improvement in the activity for Pechmann condensation reaction was observed in the case of all the modified catalysts. It was inferred that this method of modification resulted in an increase in acid strength as well as acidity of the parent oxide catalyst. © 1998 SCI     

Catalytic hydrogenation of alkenes on acidic zeolites: Mechanistic connections to monomolecular alkane dehydrogenation reactions

Brønsted acid sites in zeolites (H-FER, H-MFI, H-MOR) selectively hydrogenate alkenes in excess H₂ at high temperatures (>700 K) and at rates proportional to alkene and H₂ pressures. This kinetic behavior and the De Donder equations for non-equilibrium thermodynamics show that, even away from equilibrium, alkene hydrogenation and monomolecular alkane dehydrogenation occur on predominantly uncovered surfaces via microscopically reverse elementary steps, which involve kinetically-relevant (C–H–H)⁺ carbonium-ion-like transition states in both directions. As a result, rate constants, activation energies and activation entropies for these two reactions are related by the thermodynamics of the overall stoichiometric gas-phase reaction. The ratios of rate constants for hydrogenation and dehydrogenation reactions do not depend on the identity or reactivity of active sites; thus, sites within different zeolite structures (or at different locations within a given zeolite) that favor alkane dehydrogenation reactions, because of their ability to stabilize the required transition states, also favor alkene hydrogenation reactions to the exact same extent. These concepts and conclusions also apply to monomolecular alkane cracking and bimolecular alkane–alkene reaction paths on Brønsted acids and, more generally, to any forward and reverse reactions that proceed via the same kinetically-relevant step on vacant surfaces in the two directions, even away from equilibrium. The evidence shown here for the sole involvement of Brønsted acids in the hydrogenation of alkoxides with H₂ is unprecedented in its mechanistic clarity and thermodynamic rigor. The scavenging of alkoxides via direct H-transfer from H₂ indicates that H₂ can be used to control the growth of chains and the formation of unreactive deposits in alkylation, oligomerization, cracking and other acid-catalyzed reactions.     

Variable temperature FTIR study on the surface acidity of variously treated sulfated zirconias

The acidity of three related sulfated zirconias has been compared by IR spectroscopy using CO and H₂ as probe molecules. The parent material (SZ) was obtained by calcination of a commercial sulfated zirconium hydroxide. The other two samples, SZ-WW and SZ-SO₃, were obtained from SZ by water washing and by sulfation with gaseous SO₃, respectively. The labile sulfate groups responsible for alkane activation and associated with an IR band at 1406 cm⁻¹ are removed by water washing and increased by SO₃-sulfation. No remarkable differences in the strength of either Lewis or Brønsted acid sites have been observed between SZ and SZ-SO₃. Water washing strongly weakens Brønsted acidity but only slightly weakens Lewis acidity.     

Solid acid catalysts based on supported tungsten oxides

Supported WO _x clusters are active and stable catalysts for isomerization, dehydration, and cracking reactions. Brønsted acid sites form on WO _x clusters when a lower valent element replaces W^{6 +} or when W^{6 +} centers reduce slightly during catalytic reactions. WO _x clusters of intermediate size provide the balance between reducibility and accessibility required to maximize the number of surface H⁺ species in WO _x –ZrO₂, zirconium tungstate, and oxygen-modified WC catalysts. H₂ is involved in the generation and maintenance of Brønsted acid sites during catalytic reactions on WO _x clusters.     

Cyclisation of citronellal over zirconium zeolite beta— a highly diastereoselective catalyst to (±)-isopulegol

The catalytic cyclisation of citronellal was studied over Zr-zeolite beta, micro/mesoporous Al-MSU-S_FAU, and microporous HY catalysts. All samples showed good activity in the cyclisation of citronellal to form isopulegols with >97% selectivity. A high diastereoselectivity for (±)-isopulegol of ∼93% was observed over Zr-zeolite beta, whereas Al-MSU-S_FAU and HY showed a lower selectivity of ∼65%. Zr-zeolite beta was synthesised in a range of Si/Zr of 75–200 with the use of fluoride and zeolite beta seeds. Zeolite beta with Al and Ti substitution was less active and selective than Zr-zeolite beta. The rate of reaction strongly depended on the type of solvent used, but the reaction could also be carried out without any solvent. A hydroxylated surface is important for good activity. This is consistent with the proposed mechanism, where both Lewis and Brønsted acid sites are essential for the reaction.     

Cobalt-exchanged mordenites: preparation,characterization and catalytic activity for the abatement of NO with CH<Subscript>4</Subscript> in the presence of excess O<Subscript>2</Subscript>

The abatement of NO with methane in the presence of oxygen was studied on various commercial MOR in the Na-form (Na-MOR) and H-form (H-MOR), or exchanged to various extents with cobalt (Co-MOR). The sodium and cobalt contents were determined by atomic absorption. Samples were characterized by FTIR and volumetric measurements of CO adsorption. Chemical analysis indicated that one cobalt species replaced two Brønsted acid sites in H-MOR and two Na⁺ ions in Na-MOR. The IR analysis of the OH stretching region, evidencing an unexpected presence of Brønsted acid sites (band at 3610 cm^?1) in Co-MOR, indicated that the exchange process had a more complex stoichiometry. The adsorption of CO at RT on Co-MOR, in addition to the bands of the corresponding H-MOR and Na-MOR matrices, yielded two types of Co^II-carbonyls, the first type occupied the?mordenite main channels, and the second one the mordenite smaller channels. Brønsted acid sites in mordenites were active for the selective catalytic reduction of NO with CH₄. Co-MOR samples were far more active than Na-MOR and H-MOR samples, showing that acid protons play a negligible role when Co is present. Co-MOR catalysts showing the highest activity had the largest amount of Co^II-carbonyls in the main channels. This result strongly suggests that Co^II in the main channels of MOR are the active sites for the CH₄ + NO + O₂ reaction.     

Vapour phase hydrogenolysis of biomass‐derived diethyl succinate to tetrahydrofuran over CuO?ZnO/solid acid bifunctional catalysts

BACKGROUND: Catalytic upgrading of fermentation‐derived succinic acid or its derivates (succinic acid esters and succinic anhydride) to value added chemicals has received great attention recently. The aim of this work is to provide a process for the production of tetrahydrofuran from succinic acid esters. RESULTS: The hydrogenolysis of biomass‐derived diethyl succinate was investigated over CuO? ZnO and CuO? ZnO/solid acid (HY, HZSM‐5, SAPO‐11 and Al₂O₃) catalysts in a fixed‐bed reactor. Over CuO? ZnO, gamma‐butyrolactone and 1,4‐butanediol can be selectively produced under appropriate reaction conditions, while the selectivity of tetrahydrofuran is relatively low due to the weak acidity of CuO? ZnO. Over CuO? ZnO/HZSM, both the formed 1,4‐butanediol and ethanol can be further converted to tetrahydrofuran and diethyl ether, while tetrahydrofuran is selectively produced over CuO? ZnO/HY. CuO? ZnO/Al₂O₃ and CuO? ZnO/SAPO exhibit slight improvements in terms of selectivity to tetrahydrofuran when compared with CuO? ZnO. CONCLUSION: CuO? ZnO/HY is an appropriate catalyst to produce tetrahydrofuran from biomass‐derived diethyl succinate with high activity, selectivity and stability. Furthermore, Brønsted acid sites with appropriate acid strength are responsible for the selective formation of tetrahydrofuran under the applied reaction conditions. Copyright © 2010 Society of Chemical Industry