M. A. Aramendia

Influence of substituents on the reduction of aromatic nitrocompounds over supported palladium catalysts

Abstract This work reports the activity of several catalysts of Pd supported on both SiO 2 -AlPO 4 and a sepiolite in the reduction of aromatic nitrocompounds, both with gaseous hydrogen and by hydrogen transfer. The reaction is insensitive to structure in the absence of diffusion phenomena. The kinetic expression found for the hydrogenation of nitrobenzene by hydrogen transfer shows zero order for both the donor and the acceptor. The order with respect to hydrogen increases with temperature in the reduction of nitrobenzene with gaseous hydrogen. The reduction of variously substituted nitrobenzenes yields, selectively, their corresponding anilines. This reaction is favoured by electron-withdrawing groups attached to the ring. The results obtained conform the Hammett equation.

Optimization of the selective semi-hydrogenation of phenylacetylene with supported palladium systems

Abstract The influence of factors such as the solvent, temperature, initial hydrogen pressure, addition of amines of different basic strength and steric hindrance, type of catalyst used and procedure followed in its synthesis on the selectivity of the liquid-phase, low-pressure semi-hydrogenation of phenylacetylene with molecular hydrogen by using supported palladium catalysts in a Parr reactor was studied. The optimum experimental conditions found were as follows: 0.01 g of catalyst Pd3-PS400−(4)A, a palladium catalyst containing 3% metal deposited over Pansil sepiolite calcined at 400°C, and containing sodium in an Na+-to-Pd ratio of 4:1, 0.46 M phenylacetylene in cyclohexane, 298 K, 0.138 MPa, and 5·10−4M quinoline. On the other hand, by applying the theory of competitive hydrogenations (procedure 1) and the Sporka equation (procedure 2), the relative reactivities, RT,D, and the relative coefficients of adsorption, DT,D, under various reaction conditions were calculated. Under the optimum conditions, the RT,D values found by the two procedures were the largest possible.

Synthesis and characterization of various MgO and related systems

Various magnesium oxide catalysts have been prepared by thermal treatment of two different precursors: Mg(OH)2 and Mg5(OH)2(CO3)4. An additional system based on MgO and doped with B2O3 was also prepared. The textural and acid–base properties of the catalysts were investigated. The synthesized solids were characterized from adsorption isotherms, X-ray diffraction (XRD), 1H MAS NMR, diffuse reflectance IR Fourier transform spectroscopy (DRIFT) and temperature-programmed desorption–mass spectrometry (TPD–MS) of adsorbed probe molecules (pyridine, 2,6-dimethylpyridine and carbon dioxide). The surface properties of the solids were found to be strongly influenced by the magnesium oxide preparation conditions (viz. the precursor and calcination method used). Use of Mg(OH)2 as the precursor and in vacua calcination provided highly basic solids (those with the highest proportions of strong basic sites). Various types of Lewis-acid sites were observed in the catalysts prepared in vacuo, probably as the result of the presence of Mg2+ cations of low coordination after the calcination.

Transformation of cyclohexene on palladium catalysts: activity and deactivation

The transformation of cyclohexene on palladium catalysts can take place via two competitive processes: disproportionation and dehydrogenation. A study conducted over a broad temperature range revealed that disproportionation prevails at low temperatures and dehydrogenation at high temperatures. When the reaction develops under continuous-flow conditions (viz. in a tubular reactor connected online to a mass spectrometer), a transition temperature exists for each catalyst above which hydrogen is formed in detectable amounts. Metal catalysts are strongly deactivated during the transformation of cyclohexene. This led us to examine the process by performing thermal programmed experiments, desorptions and thermal oxidations, which showed catalyst deactivation in this process to be the result of surface adsorption of benzene and carbon deposition. Finally, desorption and subsequent oxidation were found to effectively regenerate the catalysts, which thus regained their initial activity.

Synthesis and characterization of MgO-B2O3 mixed oxides prepared by coprecipitation; selective dehydrogenation of propan-2-ol

The synthesis of MgO-B n 2 nO n 3 n mixed oxides by coprecipitation from Mg(OH) n 2 n and B(OH) n 3 n (0-10 wt%) led to solids with textural and acid-base properties rather different from pure magnesium oxide. DRIFT and 11 nB MAS NMR studies showed that boron atoms are in a flat trigonal environment, as part of the magnesium oxide structure. Titration of acid and basic sites by TPD-MS of probe molecules (pyridine and 2,6-dimethylpyridine for acid sites, and carbon dioxide for basic ones) indicates that the presence of boron leads to an increase in the acid sites surface density as well as to a decrease in surface density of basic ones, together with a change in strength distribution profile of the later. Propan-2-ol conversion over MgO-B n 2 nO n 3 n mixed oxides showed an increase in the selectivity to the dehydrogenation process with an increase in the amount of boron in the catalyst.

Gas-phase hydrogen-transfer reduction of acrolein using 2-propanol over MgO/B2O3 and SiO2/AlPO4 catalysts

Two MgO/B2O3 and SiO2/AlPO4 catalysts designated BM50 and PM2, respectively, were used in the gas-phase hydrogen transfer between acrolein and 2-propanol to obtain allyl alcohol and propanal. The acid-base properties and catalytic activity of the two systems were found to be rather different. Thus, the MgO/B2O3 catalyst is more selective towards allyl alcohol than is the SiO2/ AlPO4 catalyst (conversion to the alcohol was 28% with the former and 0% with the latter). This special selectivity is discussed in terms of the different ways in which acrolein can be adsorbed on the catalytic surface as revealed by temperature-programmed desorption profiles and DRIFT spectra for pre-adsorbed acrolein.

Reduction by hydrogen transfer processes. IV - Reduction of unsaturated α,β-ketones with formic acid-triethylamine and palladium supported catalysts

Abstract The results obtained for the hydrogenation of unsaturated α,β-ketones and of methyl cinnamate with HOOC/Et 3 N on palladium catalysts supported on AlPO 4 , SiO 2 and SiO 2 -AlPO 4 are reported. The catalysts were synthesized by ion exchange, taking into account the isoelectric point of the supports. The reduction of unsaturated α,β-carbonylic compounds leads to the exclusive hydrogenation of the olefinic double bond. The ease of reduction appears to depend on the steric and electrical effects caused by the substituents of the compound as much in the case of carbonylic carbon as in that of β-olefinic carbon, which suggests that the mechanism of hydrogenation takes place through an adsorbed 1,4-intermediate of the dienic system on the metal.

Hydrodehalogenation of aryl halides by hydrogen gas and hydrogen transfer in the presence of palladium catalysts

Catalytic hydrodehalogenation of halobenzenes over palladium supported catalysts was studied. Two different hydrogen sources were used: molecular hydrogen gas and hydrogen transfer from HCOOK and 2-propanol. HCOOK hydrogen transfer competitive reactions confirm the strong adsorption of iodobenzene over the catalyst which inhibits the adsorption of the other aryl halides and the donor. However, when using propan-2-ol as donor, iodobenzene is easily hydrodehalogenated and even fluorobenzene can be reduced. The basicity of the support plays an important role in the final performance of the catalytic system. Thus Pd/MgO is the best catalyst among all solids tested. Finally, hydrogen gas liquid-phase catalytic hydrodehalogenation indicates that reactivity order (BrBz>ClBz>IBz>FBz) depends on both the C-X bond dissociation energy and the adsorption strength of the halobenzene.

Palladium/sepiolite systems as hydrogenation catalysts, III. Hydrogenation of variously substituted olefins

The application of a Spanish sepiolite variety known as “Pansil” (commercialized by Tolsa S.A.): as a Pd support in the reduction of variously substituted carbon-carbon double bonds, both with gaseous hydrogen and by hydrogen transfer with cyclohexene as donor is described.AbstractПриводится использование испанского сепиолита, известного под названием “Pansil”, производимого фирмой Tolsa S.A., для носителя Pd при восстановлении соединений с различно замещынными двойными связями C=C, как с помощью газообразного водорода, так и с помощью переноса водорода (циклогексан в качестве донора).

Reduction kinetics of benzylideneaniline over Pd/SiO2−AlPO4 and Pd/sepiolite catalysts

The liquid-phase hydrogenation of benzylideneaniline over various Pd catalysts, using gaseous hydrogen has been studied. We have synthesized several Pd catalysts (3 wt.%) over SiO2−AlPO4 and sepiolite supports, and determined their metal surface area by X-ray diffraction. The study performed shows that the process follows a Langmuir-Hinshelwood kinetics with zero order in benzylideneaniline, ri=kokPH2 (1+KPH2)−1.AbstractИсследовали Зидкофазнуй гидрогенизацию бенилиденанилина на различных катализаторах Pd, используя газовый водород. Синтезировали несклько Pd катализаторов (3 вес.%) на носителях SiO2−AlPO4 и сепиолите. Рентгено-диффракционным методом определяли площадь поверхности металла. Показано, что процесс подчиняется кинетике Лангмюра-Хиншельвуда с нулевым порядком по бензилиденанилину, ri=kokPH2 (1+KPH2)−1.