Lucio Ronchin

Selective hydrogenation of benzene to cyclohexene using a Ru catalyst suspended in an aqueous solution in a mechanically agitated tetraphase reactor: a study of the influence of the catalyst preparation on the hydrogenation kinetics of benzene and of cyclohexene

The reactivity of Ru based catalysts in the benzene selective hydrogenation to cyclohexene has been studied. The reaction has been carried out in a tetraphase slurry reactor at 423 K, at 5 MPa of pressure, in the presence of two liquid phases: benzene and an aqueous solution of 0.6 mol l 1 ZnSO4. A detailed kinetic measurement has been carried out in order to study the influence of the preparation procedure and of the treatments on the catalyst activity and selectivity. Moreover, a comparison of benzene and cyclohexene hydrogenation kinetics is presented in order to evaluate, which are the influence of the treatments carried out on the unreduced catalyst on the activity, selectivity and yield to cyclohexene. The kinetics of benzene hydrogenation indicates that the catalyst activity is influenced by the time, during which it remains in the mixture of precipitation for its preparation. The longer the time, the lower the activity is. Selectivity and yield remain practically unaffected after 18 h of permanence in the mixture of precipitation. Treatment of the unreduced catalyst with water depresses both activity and selectivity, while treatment with 1 mol l 1 NaOH gives the best results. On the contrary, in the hydrogenation of cyclohexene, the NaOH treated catalyst is three times less active than the catalyst treated with water. These results are in agreement with the higher selectivity achieved using NaOH treated catalysts.

Acidity and reactivity of trifluoromethanesulfonic acid in liquid and solid acid catalysts

Abstract The acidic properties of CF 3 SO 3 H/SiO 2 acid catalysts have been investigated by the protonation of weak bases (B) (B+H + ⇌BH + ) and the proton-transfer process from (H + A − ) to (BH + A − ) has been analysed by a thermodynamic procedure used to account the variation of the activity coefficient terms of the species involved. Acid–base systems with different substituted pyridines as back-titrating agents of BH + (i.e. BH + →B) have also been studied and the changes observed in the acid–base interactions according to basicity of pyridines are discussed. The results in solid phase have been compared with those observed in concentrated aqueous solutions (i.e. CF 3 SO 3 H+H 2 O) where “acidity” and “protonating ability” have been distinguished as parameters of interest in the description of nonideal acid systems. Silica loaded with CF 3 SO 3 H, with H 2 SO 4 and with a mixture of both acids have been tested in acid catalysed reactions and their catalytic effectiveness has been explored towards substrates with high acid requirements for the conversion reagents–products. From the available observations in liquid and in solid phase, CF 3 SO 3 H has been proved to be a less effective acid catalyst than expected from the observed protonating ability of acidic medium. Strong interactions between ionic species and the involvement of ion-pairs in concentrated acid systems have been suggested.

Supported Ru catalysts: a study of the influence of supports, promoters and preparative variables on the catalytic activity and selectivity

The influence of some preparative variables, of the metal loading and of the support on the activity of Ru catalysts for the selective hydrogenation of benzene to cyclohexene has been studied. The reaction has been carried out in a tetraphase reactor (in the presence of an aqueous solution of ZnSO4) at 423 K and 5 MPa pressure. The effect of hydrogen diffusion on the reaction kinetics and on cyclohexene selectivity was studied. The hydrophilicity of the support was related to the observed selectivity. Hydrogen chemisorption indicates that the catalyst activity is not influenced by the Ru dispersion, but mainly by the weakly chemisorbed species on the catalyst surface.

On the acidity of liquid and solid acid catalysts. Part 2. A thermodynamic and kinetic study for acid-catalysed nitrations

Solid acids prepared by adding sulfuric acid on silica gel have been used as catalysts in the nitration of nitrobenzenes and their properties have been tested by kinetic studies at 25°C. Nitration rates in concentrated aqueous solutions of sulfuric acid were also analysed and the catalytic efficiencies of sulfuric acid in liquid and solid phase were compared by using kinetic data of analogous compounds. The results show that the solid acid samples exhibit nitrating properties very similar to those observed in concentrated aqueous solutions of sulfuric acid (range of 90 wt%). The relationship between nitration rates and effective concentration of electrophilic species [NO2+], determined by studying the protonation–dehydration equilibrium of nitric acid in strong acids (HNO3 + H+ ⇌ H2O + NO2+), was tested to better understand the acidity properties of medium.

Palladium catalyzed hydrodechlorination of α-chloroacetophenones by hydrogen transfer from the H2OCO system

Abstract PdCl 2 (PPh 3 ) 2 , in combination with an extra amount of PPh 3 , is an excellent catalyst precursor for the hydrodechlorination of α-chloroacetophenone to acetophenone by hydrogen transfer from the H 2 OCO system. The reaction occurs with concomitant evolution of CO 2 . Under typical reaction conditions (50–70°C, 40–80 atm, substrate/Pd/P = 2000/1/50, H 2 O/substrate = 8–12/1), the reaction occurs in 70–80% yield in 2 h, using ethanol or dioxane as a solvent ([Pd] = 5 · 10 −4 mol · l −1 ). When the catalyst precursor is employed without adding an additional amount of PPh 3 extensive decomposition to metallic palladium occurs. Also Pd C is active in promoting the hydrodechlorination reaction. As expected the reaction rate increases upon increasing concentration of catalyst, carbon monoxide pressure and temperature. The yield is slightly influenced by the concentration of the substrate. The effect of the concentration of H 2 O is the most significant. In ethanol as a solvent at low concentration of water the reaction rate increases to reach a plateau above 6–7 · 10 −2 mol · l −1 of water. On the basis of the fact that it is known that (i) the precursor is reduced to a Pd(0) species by the H 2 OCO system, even in the presence of hydrochloric acid, which is freed during the course of the hydrodechlorination reaction and that (ii) the starting α-chloroacetophenone oxidatively adds to Pd(0) to give Pd(CH 2 COPh)Cl(PPh 3 ) 2 (I) and that (iii) this complex reacts with hydrochloric acid to give acetophenone and PdCl 2 (PPh 3 ) 2 (II), it is proposed that the hydrodechlorination reaction proceeds via the intermediacy of a species analogous to complex (I) and that (II) is reduced to the Pd(0) complex through the intercation of CO and H 2 O with the metal center to give a species having a Pd-(COOH) moiety, which after β-hydride abstraction gives a palladium-hydride species with concomitant evolution of CO 2 . The hydride gives off a proton and reduces Pd(II) returning a Pd(0) species back to the catalytic cycle. We found also that complex (I) is reduced to a Pd(0) complex with formation of acetophenone through the action of H 2 O and CO. It is proposed that this reaction, which may be at the base of a different catalytic path, occurs via the intermediacy of a species having a HPd(CH 2 COPh) which, after reductive elimination of acetophenone give the Pd(0) complex starting a new catalytic cycle. In the case of the Pd C catalyzed hydrodechlorination it is suggested that H 2 O and CO interacts on the surface of the metal to give a hydride and evolution of CO 2 and that this hydride displaces a chloride anion from α-chloroacetophenone absorbed on the catalytic surface to give the hydrodechlorination product.

Phosgene-free synthesis of 1,3-diphenylurea via catalyzed reductive carbonylation of nitrobenzene*

1,3-Diphenylurea (DPU) has been proposed as a synthetic intermediate for phosgene-free synthesis of methyl N-phenylcarbamate and phenyl isocyanate, which are easily obtained from the urea by reaction with methanol. Such an alternative route to synthesis of carbamates and isocyanates necessitates an improved phosgene-free synthesis of the corresponding urea. In this work, it is reported that Pd(II)-diphosphine catalyzed reductive carbonylation of nitrobenzene in acetic acid (AcOH)-methanol proceeds in high yield and selectivity as a one-step synthesis of DPU. We have found that the catalytic activity and selectivity of this process depends on solvent composition and on the bite angle of the diphosphine ligands. Under optimum reaction conditions, yields in excess of 90 molar % and near-quantitative selectivity can be achieved.

On the acidity of liquid and solid acid catalysts. Part 3. Esterification of benzoic and mesitoic acids

Methyl esters of benzoic and mesitoic acid have been prepared with high yields (>98 wt%) from the corresponding carboxylic acids + methanol in aprotic solvents over samples of H2SO4/SiO2 at 60°C. The results show a high catalytic efficiency of the solids but also suggest an acid strength comparable to that observed in concentrated aqueous H2SO4 (range >90 wt%) when the acid requirements for the esterification of analogous compounds in aqueous acid solutions are taken into account. Indeed, different reacting species, i.e., ArC(OH)2+ from benzoic acid and 2,4,6‐triMe‐ArC=O+ from mesitoic acid are involved in the esterification, but the mesitoyl cation can be formed and esterified in the acidity ranges between 92 and 98 wt% H2SO4.

Raman and kinetic evidence of NO2+ ion in solid acid catalysts

Samples of H2SO4=SiO2 loaded with HNO3 and analysed by Raman spectroscopy exhibit the band of the NO a ion, which is formed by the protonation‐dehydration equilibrium of HNO3 in strong acidic media. The eAectiveness of NO a as a nitrating species is tested in the nitration of aromatic compounds towards substrates with high acid requirements for the conversion reagents‐products. The acidic properties of the acid systems used as catalysts are also described, and the thermodynamic parameters related to the acid‐base proton transfer process are discussed. ” 2000 Elsevier Science B.V. All rights reserved.

On the acidity of liquid and solid acid catalysts: Part 1. A thermodynamic point of view

The protonation equilibria of weak bases (B) in solid acids (HClO4/SiO2, CF3SO3H/SiO2, H2SO4/SiO2) were studied by UV spectroscopy and the results were compared to those obtained for analogous compounds in concentrated aqueous solutions of strong acids (HClO4, CF3SO3H, H2SO4). The behaviour of B in liquid (L) and solid (S) phase was analysed by titration curves, log[BH+]/[B] ratios and thermodynamic pKBH+ values. It has been shown that the proton transfer process acid → base (i.e., from (H+A-)(L,S) to (BH+A-)(L,S)} can be described by the relationship observed between the activity coefficient terms that are to be taken into account for acid–base equilibria occurring in nonideal systems ( – log(fBfB+/fBH+)(L,S)= -nBA log(fA–fH+/fHA)(L,S)) and can be estimated by the nBA values. Two “activity coefficient functions” (i.e., Mc(B) = – log(fBfB+/fBH+)and Mc(s) = – log(fA–fH+/fHA)) were used to describe, respectively, the equilibria of B and the equilibria of the acids in concentrated aqueous solutions and the meaning of terms “activity coefficient function” and “protonating ability of an acid” were discussed. The difference between “acidity functions”, determined for solutes (Ac(i)) and solvents (Ac(s)) in aqueous acids, and the Hx acidity functions, the latter developed for solutes in analogous media by the Hammett procedure, was also shown.

Hydrogenation of mandelic acid derivatives to the corresponding phenyl acetic acid derivative catalysed by Pd/C. A kinetic study

The hydrogenolysis of the c~-C-O bond of mandelic acid derivatives catalysed by 5% Pd/C, in the presence of hydrochloric or sulphuric acids as cocatalysts, was carried out in water or ethanol as solvent, under 35-150 kPa of hydrogen pressure, at 343 K. Typically, the substrate/catalyst/cocatalyst ratio was 200 : 1 : 10. The hydrogenation of the ethyl ester of mandelic acid in ethanol as solvent is much faster, ca. 20 times, than that of the acid in water. The influence of the concentration of the reagents, products and cocatalysts on the initial reaction rate was investigated. Upon increasing the concentration of the ester the rate increases to a plateau. The pressure of hydrogen has little influence. The products inhibit the reaction. The rate steeply increases and reaches a maximum upon increasing hydrochloric acid concentration. From equilibrium constant data, the concentration of protonated ester as a function of the hydrochloric cocatalyst concentration has been estimated. The trend of the concentration of the protonated species parallels the trend of the reaction rate, thus suggesting that the protonated species plays a key role in relation to the catalyst activity. It is suggested that from this species, adsorbed on the catalyst surface, a molecule of water is displaced by a hydride formed upon activation of molecular hydrogen by palladium. Though less effective than hydrochloric acid, sulphuric acid acts also as a cocatalyst. However, in the latter case, the initial hydrogenation rate increases to reach a plateau. In addition, when HC1 is introduced in the reaction after the preactivation step of the catalyst, the hydrogenolysis rate is equal to the rate observed when sulphuric acid is used as cocatalyst. It is suggested that in the first case the possible formation of the superficial PdOxCly may be related to the higher activity of the chlorided catalyst, i~ 1997 Elsevier Science B.V.