Homogeneous Acid Catalysts Research Articles

Fries rearrangement of phenyl acetate over sulfonic modified mesostructured SBA-15 materials

Arenesulfonic modified mesostructured SBA-15 is shown as an active catalyst in the liquid-phase Fries rearrangement of phenyl acetate. The arenesulfonic acid-centers located within the structure of SBA-15 show high catalytic performance as compared to other homogeneous and heterogeneous acid catalysts. This high activity is accompanied with a remarkable stability without leaching of sulphur species during the reaction. Reaction conditions have been investigated in order to optimize production of hydroxyacetophenones. Strong adsorption of reaction products and/or coke deposition on the sulfonic acid sites rapidly deactivates the catalyst. Interestingly, when dichloromethane is used as solvent the deactivation process is slowed down drastically. Finally, this contribution supports a new application of these sulfonated mesostructured materials in the production of fine chemicals.

Claisen rearrangement with a novel solid superacid UDCaT-5: Atom economical and selective conversion of allyl-4-methoxyphenyl ether into 2-allyl-4-methoxyphenol

The Claisen rearrangement of allyl aryl ethers provides a convenient route to obtain allyl phenols which are precursors to a variety of natural products. All uncatalyzed Claisen rearrangements are energy intensive requiring temperatures above 200 °C whereas those using homogeneous acid catalysts leads to a loss in yield and selectivity, in addition to pollution problems. In the current work, the Claisen rearrangement of allyl-4-methoxyphenyl ether to 2-allyl-4-methoxyphenol was investigated at 90 °C using chlorobenzene as a solvent over several solid acid catalysts such as dodecatungstophosphoric acid/hexagonal mesoporous silica (HMS), Cs 2.5H 0.5PW 12O 40/K10 clay, sulfated zirconia and a new mesoporous solid superacid UDCaT-5. 2-Allyl-4-methoxyphenol is used in perfumes, flavors, soaps, detergents, air fresheners, and cosmetic materials. UDCaT-5 was found to be superior, robust and recyclable and 100% selective. A complete theoretical and experimental analysis is also presented. This is a 100% atom economical and efficient process.

Ion-Exchange Resin Catalysis in Benign Synthesis of Perfumery Grade p-Cresylphenyl Acetate from p-Cresol and Phenylacetic Acid

p-Cresylphenyl acetate is a very important perfume that finds wide applications and possesses an organoleptic character similar to those of honey, nuts, and butter. It is produced by mineral acid-catalyzed esterification of p-cresol with phenylacetic acid. Use of homogeneous acid catalysts leads to posttreatment pollution problems apart from the quality-related issues. The current work is focused with an eco-friendly and benign catalytic process, employing the solid acid catalysts such as dodecatungstophosphoric acid (DTP) supported on K-10 clay, ion-exchange resins, sulfated zirconia, etc. for esterification of p-cresol with phenylacetic acid to p-cresylphenyl acetate. The order of catalytic activity was found to be Amberlyst-15 ≈ Indion-125 > 20% w/w DTP/K-10 > sulfated zirconia. Indion-125 was used for further experiments. It was observed that the catalyst has excellent reusability and that the reaction was 100% selective towards p-cresylphenyl acetate. A pseudo-first-order kinetic model was built up t...

Benzylation of benzene to diphenylmethane using zeolite catalysts

The catalytic liquid phase benzylation of benzene to diphenylmethane (DPM) with benzyl chloride (BC) is investigated over a number of zeolite catalysts at 358 K and under atmospheric pressure. Conventional homogeneous Lewis acid catalyst, AlCl3, is also included for comparison. Zeolite H-β is found to be more selective but less active compared to HY and H-ZSM-5 zeolites in the benzylation of benzene. The conversion of BC, rate of BC conversion and selectivity to DPM over H-β after 6 h of reaction time are 33.3 wt%, 4.7×10−3 mmol g−1 h−1 and 89.1 wt%, respectively. For comparison, the conversion of BC, rate of BC conversion and selectivity to DPM over AlCl3, under identical reaction condition, are found to be 100 wt%, 170×10−3 mmol g−1 h−1 and 58 wt%, respectively. Higher amounts of consecutive products are obtained over AlCl3 due to its non shape selectivity. The acidity of the zeolite catalysts is measured by temperature programmed desorption method. The effect of the duration of the run, SiO2/Al2O3 ratio of H-β, catalyst concentration, reaction temperature and benzene to BC molar ratio on the catalyst performance is also investigated in order to optimize the conversion of BC and selectivity for DPM. The conversion of BC using H-β is increased with the increase in the reaction time, catalyst concentration, reaction temperature and molar ratios whereas it decreases with the increase in SiO2/Al2O3 molar ratio of H-β. H-β is recycled two times and a slight decrease in BC conversion is observed after each cycle, which is related to the minor dealumination of the zeolite catalyst by HCl, which is produced during the reaction as by product. The formation of DPM is explained by an electrophilic attack of the benzyl cation (C6H5CH2+) on the benzene ring, which is produced by the polarization of BC over acidic sites of the zeolite catalysts.

An Ionic Liquid Mediated Friedel—Crafts Addition of Arenes to Isothiocyanates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Involvement of solid acid on Al- and Ga-doped porous silica in the Diels–Alder reaction

The Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone using Al- and Ga-doped porous silica, i.e., MCM-41, zeolite, and amorphous silica, was studied focusing on individual effects of types, strength, and amount of acid sites and pore size of solid acid catalysts. MCM-41 and amorphous silica-based catalysts exhibited high activity for the Diels–Alder reaction, while zeolites only showed low activity. Comparison with IR spectra of adsorbed pyridine clearly indicates that solid Lewis acid sites are effective for the reaction. Turnover numbers of each catalyst were compared based on the number of total acid sites measured by NH 3-temperature programmed desorption (NH 3-TPD). Al-MCM-41 and Ga-MCM-41 showed higher turnover numbers than that of AlCl 3, which is a typical homogeneous Lewis acid catalyst. The catalysts having stronger acid sites basically showed higher turnover numbers in the Diels–Alder reaction. The effect of pore size was examined by comparison of Ga-MCM-41 and H-type mordenite (H-MOR) evacuated at 973 K. Through dehydration of H-MOR by evacuation at 973 K, Brønsted acid sites of H-MOR were converted to Lewis acid sites of which acid strength is comparable to Ga-MCM-41. Based on the number of Lewis acid sites, Ga-MCM-41 and H-MOR after evacuation at 973 K showed the same turnover number, although they have different pore sizes. These results clearly demonstrate that the catalytic activity depends on strength and amount of Lewis acid sites, while there is not much effect of pore size on the catalytic activities of the Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone.

Reversible in situ acid formation for β-pinene hydrolysis using CO2expanded liquid and hot water

Large amounts of waste may result from the neutralization of homogeneous acid catalysts following reaction. Here we present examples of in situ acid formation and self-neutralization, thus eliminating waste and offering advantages for product recovery. The formation of α-terpineol (2) from β-pinene (1) is a reaction of commercial significance that is typically run with strong acid. We demonstrate that the reaction can be performed under more environmentally benign conditions using the in situ acid formation capabilities of two different green technologies: CO2 expanded liquids and reactions in hot water (200 °C). This work presents an example of the application of these methods to a reaction that has commercial significance and adds to our knowledge about the benefits and effects of co-solvents. The relative rates and product distributions achieved in each system are presented and discussed.

Chiral rare earth organophosphates as homogeneous Lewis acid catalysts for the highly enantioselective hetero-Diels–Alder reactions

Various trivalent rare earth-chiral phosphate complexes [(R)-1-RE, (R)-3-RE, and (R)-4-Ce] were prepared and evaluated as a Lewis acid catalyst for the asymmetric hetero-Diels–Alder reaction of aldehydes with the Danishefsky's diene. Some of them effectively promoted the reaction at room temperature in the presence or absence of achiral additives under homogeneous conditions to afford the corresponding cycloadducts with high ee's (up to 99% ee). During these reactions, remarkably high asymmetric amplifications (positive nonlinear effects) were observed as the first example in the metal ion–chiral ligand 1:3 catalytic system. A scandium catalyst bearing the H8-BNP ligand, (R)-3-Sc, could be recovered after the reaction and successfully reused for the next round of reactions. In addition, the hetero-Diels–Alder reaction of α-keto esters was effectively catalyzed by the ytterbium complex, (R)-1-Yb, without any additives thus producing the asymmetric quaternary carbon in excellent enantioselectivities (up to >99% ee).

An ionic liquid mediated Friedel-Crafts addition of arenes to isothiocyanates

A new protocol is developed for the synthesis of N-substituted thioamides, employing arenes and isothiocyanates in 1-butyl-3-methylimidazolium chloroaluminate ionic liquid, [bmim]Cl·2AlCl3, as a homogenous Lewis acid catalyst and solvent. The effect of Lewis acidity and the stoichiometry of the ionic liquid on the extent of product formation is studied. Studies reveal that a progressive increase in yields was observed with increasing Lewis acidity, and two equivalents of [bmim]Cl·2AlCl3 was the optimal amount for the reaction. A distinct para selectivity for the incoming thioamido group on activated arenes was observed under ambient conditions.Key words: arenes, isothiocyanates, Friedel–Crafts, ionic liquids, thioamides.

Saucy–Marbet ketonization in a continuous fixed-bed catalytic reactor

Saucy–Marbet ketonizations of two unsaturated alcohols (3,7-dimethyl-oct-6-en-1-yn-3-ol (dehydrolinalool (DLL)) and 2-methyl-3-butyn-2-ol (MB)) with 2-methoxypropene (MP), generally performed in a batch reactor using homogeneous acid catalysts, have been studied in a continuous fixed-bed reactor containing a commercial alkyl sulfonic acid polysiloxane catalyst. The solid acid catalyst showed an initial transient period of several hours till stable operation was reached. Similar products were observed as in corresponding homogeneous catalytic reactions indicating similar reaction pathways. However, the heterogeneous catalytic reactions showed lower selectivity to the desired products than the corresponding homogeneous reactions reported in the literature. This behaviour is traced to the complex transformation the unsaturated ether (2-methoxypropene) undergoes upon interaction with the surface of the solid acid catalyst. Furthermore, both Brønsted and Lewis-acid sites were found to be present on the catalyst surface. These factors are proposed to be the main reasons for the less efficient use of reactants observed in the heterogeneous catalytic reactions. A maximum yield of 54% to the desired product could be achieved in the heterogeneous catalytic ketonization of 3,7-dimethyl-oct-6-en-1-yn-3-ol and 45% for the 2-methyl-3-butyn-2-ol ketonization, respectively. The studies indicate that the continuous ketonization in a fixed-bed reactor is feasible, but for technical application more selective acid catalysts are necessary to compete with the generally used homogeneous catalytic routes.

An in situ NMR study of the reactions of cyclohexanone promoted by solid acid catalysts: acetalization versus deuteration

The effect of different catalysts (NaY, Cr-VPI-5, acidic kaolinite clay, clayfen, and p-toluenesulfonic acid) on the reaction between cyclohexanone and methanol-d 4 was investigated using GC-MS and in situ NMR spectroscopy at room temperature. An in situ IR spectroscopy analysis of pyridine adsorbed on the clay and NaY samples showed that they are both weakly acidic. Depending on the catalyst employed, acetalization, α-carbon deuteration, or both of these reactions occurred. With the clays only acetalization proceeded to yield 1,1-dimethoxycyclohexane with 90% conversion at room temperature. When zeolite Y was used, only deuteration of the cyclohexanone α-carbon was observed, while with the homogeneous p-toluenesulfonic acid catalyst, both deuteration of the α-carbons and the acetalization reaction proceeded.

Mesoporous Al-containing MCM-41 molecular sieves: highly active catalysts for Diels–Alder reaction of cyclopentadiene with α,β-unsaturated aldehydes

We investigated the liquid phase Diels–Alder reaction of cyclopentadiene with α,β-unsaturated aldehydes (at 37 °C) over mesoporous Al-containing MCM-41 molecular sieves, with different Si/Al ratios, prepared by four different methods—sol–gel, hydrothermal, grafting and template cation exchange. The catalysts have been characterized for their phase purity, surface area, average pore size and acidity. The Al-containing MCM-41 catalysts prepared by different methods behaved differently in the acid-catalyzed Diels–Alder reaction. Among the different MCM-41 catalysts (having almost the same Si/Al ratio), Al:MCM-41, prepared by grafting method showed the highest activity in the Diels–Alder reaction of cyclopentadiene with crotonaldehyde, which may be due to its higher acidity. The introduction of Al into the framework of MCM-41 increased the acidity; however, the increase in the acidity up to an optimum level increased the activity. Al:MCM-41 catalyst is more catalytically active in the Diels–Alder reaction than the heterogeneous solid acids—mesoporous SBA-15, microporous H-ZSM-5 and HY zeolites, amorphous silica–alumina, amberlist 15, active alumina, silica gel and homogeneous acid catalysts— p-toluenesulfonic acid and AlCl 3.

Nanoporous Solid Acid Catalyst for the Diels-Alder Reaction of 1,3-Dienes with Acrylates

Abstract Nanoporous aluminosilicate, Al-HMS with a high aluminum content, strong acidity, and nanoporosity was found to catalyze the Diels-Alder reaction of 1,3-dienes with methacrylate and acrylate, and turned out to be a green alternative to homogeneous acid catalysts.

A note on the role of methanol in the homogeneous and heterogeneous acid-catalyzed hydroxymethylation of guaiacol with aqueous solutions of formaldehyde

The acid-catalyzed hydroxymethylation of 2-methoxyphenol (guaiacol) with formaldehyde aimed at the synthesis of benzyl alcohol derivatives (vanillic alcohols) was studied with specific attention being given to the role of methanol which is present in commercial aqueous solutions of formaldehyde. Methanol had a pronounced effect on the catalytic performance with both homogeneous and heterogeneous acid catalysts. Specifically, the conversion of guaiacol decreased considerably with increasing amounts of methanol in the formaldehyde solution, and the distribution of products is also changed. In the absence of methanol the formation of diaryl compounds predominated over the formation of monoaryl by-products.

An FT-IR study on Diels–Alder reactions catalysed by heteropoly acid containing sol–gel silica

Following the general trend of using heterogeneous catalysts whenever possible, the replacement of traditional homogeneous Lewis acid catalysts for the Diels–Alder reaction is currently considered in the literature. In this contribution, we report the successful application of sol–gel derived tungstophosphoric acid–silica composites as catalysts in the reaction of 1,3-cyclohexadiene and 2-propenal. The reaction was monitored by in situ FT-IR spectroscopy following the changes in the fine structure of the ν(CO) band. The catalytic activity of the sol–gel immobilised heteropoly acids seems to be comparable with that of more conventional sol–gel silica–aluminas and their selectivity is even better because Broensted acid sites are destroyed upon calcination of the dry gel.

Selective benzoylation of o-xylene to 3,4-dimethylbenzophenone using various zeolite catalysts

The liquid phase benzoylation of o-xylene with benzoyl chloride (BOC) over various zeolite catalysts is studied in a batch reactor at atmospheric pressure and 411 K. The results obtained over different zeolite catalysts are compared with the homogeneous Lewis acid catalyst, AlCl 3 and amorphous silica–alumina. The protonic form of zeolite beta is found to be an efficient catalyst compared to other zeolites in the benzoylation of o-xylene. The conversion of BOC, turnover rate of BOC conversion (TOF) and selectivity for 3,4-dimethylbenzophenone (3,4-DMBP) over zeolite H-beta are found to be 52.8 wt.%, 69.7×10 −5 s −1 mol −1 Al and 94.7 wt.%, respectively, at the following reaction conditions [H-beta to BOC (w/w)=0.28, o-xylene to BOC (molar ratio)=5; 411 K; 1 atm]. The non-shape selective AlCl 3 catalyst produces higher amount of consecutive products (16.2 wt.%) and hence gives lower selectivity for 3,4-DMBP (76.5 wt.%). The acidity and pore structure of H-beta appeared to be responsible for good performance. Increase in reaction time, catalyst concentration, reaction temperature, o-xylene to BOC molar ratio enhances the conversion of BOC, whereas it decreases with the increase in degree of Na-exchange and SiO 2/Al 2O 3 molar ratios of H-beta. The zeolite H-beta is recycled three times without the loss of 3,4-DMBP selectivity but with a decline in the catalytic activity of H-beta. Additionally, the benzoylation of m-xylene and p-xylene is also investigated using H-beta. The Friedel–Crafts acylation reaction mechanism involves the formation of an electrophile (C 6H 5CO +) over an acidic zeolite catalyst which attacks the xylene ring resulting in the formation of dimethyl benzophenones.

Liquid-phase selective benzylation of o-xylene using zeolite catalysts

Liquid-phase benzylation of o-xylene to 3,4-dimethyl diphenylmethane (3,4-DMDPM) with benzyl chloride (BC) has been investigated in the presence of various zeolite catalysts. Conventional homogeneous Lewis acid catalyst, AlCl 3, is also included for comparison. Under identical reaction conditions, zeolite H-beta is considerably more selective than the other zeolite catalysts and AlCl 3 in the benzylation of o-xylene. The conversion of BC, rate of BC conversion and selectivity to 3,4-DMDPM over H-beta after 1 h of reaction time and at 363 K are ca. 33.5 wt.%, 12.6 mmol g −1 h −1 and 92.2 wt.%, respectively. For comparison, the conversion of BC, rate of BC conversion and selectivity to 3,4-DMDPM over AlCl 3, under identical reaction conditions, are estimated to be 76.6 wt.%, 28.8 mmol g −1 h −1 and 65.5 wt.%, respectively. Higher amounts of consecutive products (32.9 wt.%) are achieved over non-shape-selective AlCl 3 as compared to the zeolite H-beta (6.9 wt.%). The conversion of BC to the products largely depends on the reaction conditions and acidity of the zeolite catalysts. Acidity of the zeolite catalysts is measured by temperature-programmed desorption (TPD) of ammonia. The effect of reaction time, SiO 2/Al 2O 3 ratio of H-beta, catalyst concentration, reaction temperature and o-xylene to BC molar ratio on the catalyst performance is examined in order to optimize the conversion of BC and selectivity to 3,4-DMDPM. The conversion of BC using H-beta is increased significantly with the increase in reaction time, catalyst concentration, reaction temperature and o-xylene to BC molar ratios, however, an increase in the SiO 2/Al 2O 3 molar ratio of H-beta lowered the BC conversion. The benzylation of m-xylene and p-xylene is also investigated over zeolite H-beta catalyst. H-beta is recycled four times and a decrease in BC conversion is observed after each cycle which is related to the minor dealumination of zeolite catalyst by HCl formed during the reaction. The formation of DMDPM is explained by an electrophilic attack of the benzyl cation (C 6H 5CH + 2) on the o-xylene ring whose formation is facilitated by acid sites of the zeolite catalysts.

Application of zeolite titanium Beta in the rearrangement of α-pinene oxide to campholenic aldehyde

Zeolite titanium Beta is found to be an effective catalyst for the industrially relevant rearrangement of α-pinene oxide to campholenic aldehyde, giving selectivities of up to 89% in the liquid phase. In the gas phase, up to 94% selectivity is obtained at conversions above 95%, which exceeds the best values obtained with homogeneous Lewis acid catalysts. The selectivity obtained is profoundly influenced by the solvent or co‐adsorbate applied.

On the Absorption of Isobutene and trans-2-Butene in Sulfuric Acid Solutions

In reactions in which alkenes react in the presence of homogeneous Bronstedt acid catalysts, the protonation step is rate determining. Existing reaction rate correlations for protonation of butenes in sulfuric acid solutions are not consistent and limited to sulfuric acid concentrations below 80 wt %. The absorption of isobutene and trans-2-butene in sulfuric acid solutions was studied in the range of 30-98 wt %. Reevaluation of literature data, completed with new data from this study, revealed a set of kinetic rate data which coincide satisfactorily. Experiments and modeling of the absorption experiments showed a strong dependence of the reaction rate on local mixture composition through the activity coefficient of the protonating agent. This strong dependence prevents the occurrence of a diffusion-limited absorption regime. Application of the usually used criteria for operating in the kinetic regime can lead to erroneous results.

Development of a kinetic model for the esterification of acetic acid with methanol in the presence of a homogeneous acid catalyst

The esterification kinetics of acetic acid with methanol in the presence of hydrogen iodide as a homogeneous acid catalyst was studied with isothermal batch experiments at 30–60°C. The catalyst concentration was varied between 0.05 and 10.0 wt%. The experiments revealed that besides the main reaction, the esterification of acetic acid, a side reaction appeared: the catalyst, hydrogen iodide, was esterified by methanol to methyl iodide. Plausible reaction mechanisms for methyl acetate and methyl iodide formation were proposed. The rate-determining step in the acetic acid esterification was assumed to be the nucleophilic attack of methanol to the carbenium ion formed through proton donation to acetic acid, whereas the rate-determining step in the hydrogen iodide esterification was presumed to be the substitution of the iodide to protonated methanol. Hydrogen iodide and acetic acid act as proton donors; thus the protolysis equilibria of acetic acid and hydrogen iodide were included in the mechanism. Rate equations, concentration-based as well as activity-based with UNIFAC activity coefficient estimations, were derived, and the kinetic and equilibrium parameters included in the rate equations were estimated from experimental data with regression analysis. Simulation of the models with the estimated parameters revealed that the rate equations predict correctly the experimental trends in the acid catalyzed esterification.