Homogeneous Acid Catalysis Research Articles

Cyclodimerization of α-methylstyrene under homogeneous and heterogeneous acid catalysis conditions

The cyclodimerization of α-methylstyrene in the presence of Bronsted and Lewis acids was realized. The dependence of the cyclic α-methylstyrene dimer yield on the catalyst nature and structure was established. It was shown that the activity of the catalysts is determined by the strength of their acid sites, and the selectivity for the cyclic α-methylstyrene dimer depends on the nature of the anion (homogeneous acids) or the porous structure (heterogeneous catalysts). Sulfuric acid and silicophosphate exhibit the highest selectivity in the cyclodimerization of α-methylstyrene into 1,1,3-trimethyl-3-phenylindane.

Self-Neutralizing in Situ Acid Catalysis for Single-Pot Synthesis of Iodobenzene and Methyl Yellow in CO2-Expanded Methanol

Despite widespread use, homogeneous acid catalysis has the drawback of requiring downstream neutralization, resulting in salt waste. Methylcarbonic acid is a self-neutralizing acid that forms in situ in methanol/CO2 systems at mild pressures (10−47 bar) and then decomposes by depressurization. In the work presented here, methylcarbonic acid catalyzes the diazotization of aniline, which is either coupled with N,N-dimethyl aniline to form methyl yellow or reacted with iodide to form iodobenzene. The syntheses of methyl yellow and iodobenzene represent a class of industrially important reactions.

Transformations of diepoxy derivatives of limonene in homogeneous acidic media

Transformations of diepoxy derivatives of limonene under conditions of homogeneous acid catalysis were studied, and their results were compared with those obtained over heterogeneous catalysts. A number of previously unknown compounds were isolated. The most possible transformation pathways were analyzed by computer simulation using molecular-mechanics and quantum-chemical methods.

A comparison of the esterification of acetic acid with methanol using heterogeneous versus homogeneous acid catalysis

To investigate the similarities and differences between heterogeneous and homogeneous catalyzed esterification, the kinetics of acetic acid esterification with methanol were investigated using a commercial Nafion/silica nanocomposite catalyst (SAC-13) and H 2SO 4, respectively. Reactions were carried out in an isothermal well-mixed batch reactor at 60 °C. Organic base titration, TGA, and elemental sulfur analysis were carried out to estimate the acid site density of SAC-13, permitting the assessment of its catalytic performance for esterification on a per site basis. It was found that SAC-13 has comparable site activity for acetic acid esterification as H 2SO 4. SAC-13 and H 2SO 4 also exhibited similar reaction inhibition due to the presence of water. The similar response to the presence of water and the results of pyridine poisoning experiments suggest that esterification with these catalysts occurs through a common reaction mechanism. Consistent with the hypothesized homogeneous-type reaction mechanism, reaction on SAC-13 appears to involve single-site catalysis with a probable rate-controlling Eley-Rideal surface reaction. A mechanistically derived kinetic model successfully predicts the esterification rate of SAC-13 as reaction progresses. The dissimilarities between homogeneous and heterogeneous acid-catalyzed esterification are also discussed.

Transformations of Isocaryophyllene Diepoxide under Conditions of Homogeneous and Heterogeneous Acid Catalysis

Transformations of isocaryophyllene diepoxide (an isomer of widely spread natural sesquiterpene) were studied in various acidic media under conditions of homogeneous and heterogeneous catalysis. A number of previously unknown compounds were thus obtained. The experimental data were compared with the results of molecular-mechanics and quantum-chemical simulation of the most probable transformation paths.

Synthesis of terpineol from α-pinene by homogeneous acid catalysis

Chloroacetic acid was used as catalyst for the hydration of α-pinene using water as hydroxyl donor, which is soluble in aqueous and organic solvents. The highest selectivity was 95.5 with a conversion of 10%, whereas the higher conversion was 99% with selectivity of 70% after 4 h of reaction at 70 °C. Organo-chlorinated compounds were not found in products as in the case of the use of HCl as catalyst, which indicates that the intermediate carbocation formed after alkene protonation is not susceptible to react with the chloroacetic anion.

Synthesis, characterization and evaluation of sulfonic resins as catalysts

Ion-exchange resins have been often used as catalysts especially those based on styrene–divinylbenzene copolymers with sulfonic acid groups in the aromatic rings of polymer chains. That is due to the advantages of heterogenous catalysis over the homogeneous acid catalysis. Moreover, resin catalysts can often lead to high selectivity in organic reactions due to the matrix effects. Therefore, the study of copolymers synthesis conditions to determine the type of polymer structure produced as well as the characterization of sulfonic resins obtained thereof are of great interest. The current paper describes the synthesis, characterization and evaluation as catalysts of sulfonic resins derived from polymer supports synthesized by aqueous suspension polymerization of styrene and divinylbenzene. The reaction conditions were varied and polymer supports with different physical properties and morphological characteristics were obtained. The polymer supports were chemically modified by sulfonation. The resultant sulfonic resins had their catalyst activity evaluated in the esterification of acetic acid with n-butanol.

Activity–acidity relationship in zeolite ZSM-5. Application of Brönsted-type equations

In this paper the relation between activity and acidity in a variety of ZSM-5 zeolite catalysts, with different Si/Al ratios and different protonic content, is analysed and a quantitative correlation is obtained. The acid site strength distribution was estimated using temperature-programmed desorption (TPD) of ammonia by applying a digital deconvolution method to the curves. These data were then correlated with experimental catalytic activity data for the same catalysts towards n-heptane cracking reaction, by means of a Brönsted-type equation similar to the ones used for homogeneous acid catalysis and already used for other zeolites. It can be noticed that the same types of equation that are used for homogeneous acid catalysis also hold for heterogeneous acid catalysis and that the activation energy for ammonia desorption can be used as acid-strength scale for the purpose of correlation with catalytic activity.

Activity–acidity relationship in zeolite Y: Part 2. Determination of the acid strength distribution by temperature programmed desorption of ammonia

In this paper we will have a detailed look at a numerical procedure that allows the estimation of the acid site strength distribution using a single temperature programmed desorption (TPD) experiment. The possibility of estimating these parameters is of paramount importance in the determination of quantitative relationships between activity and acidity. The application of the procedure to a set of simulated thermograms gives a clear view of the applicability of this method. The procedure was tested both in single and multi energy distributions, as well as quasi-continuum distributions. Then the method has been applied on TPD thermograms obtained from catalysts based on two different forms of zeolite Y. In part 3 of this series, the acid site distributions obtained here will be used in an activity–acidity relationship, using a Brönsted type equation, similar to the one used in homogeneous acid catalysis.

Acidity-activity relationship in zeolite Y. A preliminary study for n-heptane transformation

The transformation of n-heptane at 350°C has been studied on a variety of Y zeolite catalysts. The acidities of these samples were determined by temperature-programmed desorption of ammonia and, by using a numerical deconvolution technique, the acid strength distribution for the sites of each catalyst was obtained. Using these results we were able to correlate the catalytic activity with the acid strength distribution for all the catalysts using a Bronsted type equation, similar to the one that is used in homogeneous acid catalysis.

On the real nature of aliphatic carbenium ions as active intermediates of homogeneous and heterogeneous acid catalysis

The structure and properties of active intermediates of acid-catalyzed homogeneous and heterogeneous transformations of olefins and alcohols are discussed, taking into account the hydration in homogeneous systems or the interaction with the surface of heterogeneous zeolite catalysts on the basis of nonempirical quantum-chemical calculations. In both cases carbenium ions do not represent the actual existing active intermediates, but rather the excited transition states. They could be formed from the more covalent and more stable surface esters in heterogeneous acid catalysis or from hydrated alkoxonium ions in acid catalysis in aqueous solutions. This results in concerted reaction pathways with low activation energies due to almost complete compensation of the energy required for the rupture of the reacting chemical bonds in the initial compounds by the energy released by the formation of the new bonds in the final products.

On the nature of hydrated aliphatic carbenium ions and on their role in homogeneous acid catalysis

The structure of hydrated methyl and t-butyl carbocations is discussed on the basis of non-empirical quantum chemical calculations and literature data from high pressure mass-spectrometry. It is concluded that primary carbocations are transformed by hydration into alkoxonium ions solvated by four or five additional water molecules. The methyl fragment in such a complex is essentially covalent. Therefore any mechanisms of acid-catalyzed reaction in aqueous solution involving the participation of free primary carbenium ions are unrealistic. The t-butyl cations are more weakly solvated, but also bind four water molecules in their hydration sphere. A new explanation for the acid-catalyzed S N1 mechanism based on the hydrophilic properties of iso-butyl fragments is proposed.

Homogeneous acid catalysis of the trans—cis photoisomerization reaction of trans-3-(2-hydroxy- benzylidene)-4,5-dihydrofuran-2(3H)-one

The geometrical photoisomerization reaction of the title molecule ( trans-I) has been studied in methanol/water 90/10 wt.% mixtures containing sulphuric or hydrochloric acid. Quantum yields of the process, measured under acidic conditions in the range 0.095 N < [H +] <4.0 N, reach a maximum at pH ≈ 0. The effect of the counterion has also been investigated. On the basis of the fluorescence behaviour of trans-I as a function of pH, experimental quantum yields have been interpreted in the light of prototropic equilibria in the excited state. Mechanistic models are proposed which are able to fit results obtained in the investigated experimental conditions.

Hydride transfer reactions of benzylic alcohols catalyzed by acid faujasites

AbstractReaction of five benzylic alcohols in solution at 78°C has been performed in the presence of a series of HY zeolites at three different levels of Na+ ‐to‐NH4+ exchange (HY‐21, HY‐50 and HY‐100) and framework Si‐to‐Al ratio (HY‐100, HY‐200 and HY‐400). Dehydration to form a CC double bond is the main primary process for 1‐phenyl‐ and 1,1‐diphenylethanol. Nevertheless, the initial alkenes are unstable, giving rise to the corresponding phenylethanes and, in the case of styrene, also to dimers. In contrast, benzyl alcohol and diphenylmethanol gave equimolar amounts of phenylmethanes and phenyl ketones, by an intermolecular hydride transfer not observed under conventional homogeneous acid catalysis. This process was found to be highly demanding from the point of view of the acid strength of the sites involved. Finally, triphenylmethanol showed a remarkable reluctance to undergo any change, although small amounts of triphenylmethane, benzophenone and 9‐phenyl‐9‐hydroxyfluorene were detected after long reaction times.

5‐Hydroxymethylfurfural (HMF). A Review Focussing on its Manufacture

AbstractThe acid‐catalysed dehydration of hexoses results in the formation of 5‐hydroxymethylfurfural (HMF). Fructose and inulin are especially good starting materials. A review is given of the many methods to produce HMF. The reaction kinetics are dealt with in the sections: raw material, hydrolysis and reversion, catalysts, reaction temperature and time, the concentration and the risk of polymerization and the solvent and HMF stability. Manufacturing processes are illustrated for fructose as the starting material: aqueous systems with homogeneous acid catalysis or those using ion‐exchangers as catalyst, systems using dimethyl sulfoxide as the solvent and those using other organic solvents. A short treatment is given on processes starting with glucose, work up procedures and the formation of levulinic acid and other in situ‐formed HMF‐derivatives.

Relation between homogeneous acid catalysis and ion exchange resins using a test reaction

The kinetic analysis of the depolymerization of trioxane with a strong acid resin supplied as commercial catalyst has been performed. Experiments at different speeds of rotation lead to the conclusion that the external mass transfer is not the controlling step, and the influence of the temperature on the reaction rate indicates the chemical reaction as controlling step. A relation between the resin load and the Hammett acidity function can be obtained after comparison between the homogeneous catalyzed reaction and the resin catalyzed reaction. This relation seems to be useful in order to establish an equivalence between catalytic resins and homogeneous catalyzed reactions for practical purposes. Two parameters, ϱ, which describes the quantitative relation between acidity and resin concentration and Δ E/R, which takes account of the small difference between activation energies in homogeneous acid catalysis and heterogeneous acid (ion exchange) catalysis are able to describe the feasibility to use acid resins in homogeneous acid catalyzed reactions. The studied system seems to have an application as a test reaction for the evaluation of catalytic resins.

Kinetic analysis of homogeneous acid catalysis in the chloromethylation of toluene

The catalytic activity of acids in the chloromethylation of toluene has been studied in a complex mixture of toluene, paraformaldehyde, hydrochloric acid, acetic acid and zinc chloride. A relation between the Hammett acidity function and the initial reaction rate has been found: r 0 = h mf(k,C) m = 1.25 where: h = 10 − H 0; H 0 being the Hammett acidity function. This relation describes the influence of three acids, acetic acid, hydrochloric acid and zinc chloride, on the initial reaction rate in the investigated range of variables C T 0 = 0.075 M, C F 0 = 2.5 M; 30% ⩽ C HAc ⩽ 50%, 5% ⩽ C ZnCl 2 ⩽ 15% and 2 M ⩽ C HCl ⩽ 4 M. These results are useful in the evaluation of the reaction rate for the chloromethylation of toluene as a function of the proton activities. Finally, some mechanistic implications are discussed.

Efficient homogeneous acid catalysis of heteropoly acid and its characterization through ether cleavage reactions

Summary A Keggin-type heteropoly acid has revealed high catalytic activities for the cleavage reactions of epoxides, tetrahydrofuran, dibutyl ether and butyl methyl ether with acetic acid, acetic anhydride and butyl acetate in the non-aqueous homogeneous liquid phase at 30–118 °C. The catalytic activities of the heteropoly acid were much higher than those of conventional acid catalysts such as sulfuric acid, p-toluensulfonic acid and boron trifluoride etherate, at the same catalysts concentrations or the same proton concentrations. On the basis of comparative measurements of electrical conductivity, acidity, and softness of anion for the solutions of acid catalysts, the eficcient acid catalysis by heteropoly acid was suggested to be due to the specific properties of the heteropoly anion, which can be characterized by very weak basicity and great softness, togerther with the large size of the polyhedral structure.

Investigation of the kinetics of the synthesis of methylheptenone under conditions of homogeneous acid catalysis

Investigation of the kinetics of the synthesis of methylheptenone under conditions of homogeneous acid catalysis

Kinetics and mechanism of hydration of ethylene oxide in dilute solution in the presence of Wofatit KPS catalyst

The kinetics of catalytic ethylene oxide hydration in dilute solution and in the presence of Wofatit KPS cation exchange resin has been studied. On the basis of kinetic studies it was found that the mechanism of catalytic action does not differ from homogeneous acid catalysis. Taking into consideration the kinetic isotope effect, an A-2 type mechanism of hydration in the presence of Wofatit KPS catalyst containing 8% divinylbenzene (DVB) was suggested.