Heterogeneous catalytic Wacker oxidation of ethylene over oxide-supported Pd/VOx catalysts: The support effect

This paper concerns the selective oxidation of ethylene (EE) to acetaldehyde (AL) and acetic acid (AA) by oxygen in the presence of steam over non-supported Pd/V2O5 catalyst and over Pd/V2O5 catalysts supported by SiO2, TiO2, γ- Al2O3, and α-Al2O3. A flow-through microreactor was applied at atmospheric pressure in the temperature range of 150-200 oC. The WHSV of EE was 0.17 or 0.84 h-1. The vanadia content of the supported catalysts was 17 wt%, whereas their Pd content was 0.8 wt %. The reducibility of vanadia was determined using temperature-programmed reduction by hydrogen (H2-TPR). Applying ultraviolet-visible (UV-vis) spectroscopy and X-ray diffractometry (XRD) different vanadia species were identified over different supports. In the Pd/V2O5/α-Al2O3 catalyst the vanadia had the same structure than in the Pd/V2O5 preparation. Even the low surface area α-Al2O3 support affects the Wacker oxidation activity of the catalyst. Vanadia, deposited on the surface of TiO2 or γ-Al2O3 forms easily reducible polymeric species. In interaction with Pd this polymeric species is responsible for the total oxidation EE to CO2. Palladium, bound to the surface of bulk V2O5 or to monomeric vanadate-like species on silica, forms Pd/VOx redox pairs, which are active and selective catalytic centers of the Wacker reaction. The Wacker mechanism was verified by test reactions, where one of the four components, such as Pd, V2O5, O2, or H2O, was left out from the reacting system. In absence of any of the components no selective catalytic partial EE oxidation proceeded, indicating that the Wacker mechanism could not operate

Selective hydroconversion of levulinic acid to γ-valerolactone or 2 methyltetrahydrofuran over silica-supported cobalt catalyst

Solvent-free hydroconversion of levulinic acid (LA) was studied over Co/silica catalysts applying flow-through fixed-bed microreactor. Consecutive hydrogenation/hydrogenolysis and dehydration reactions proceeded over the catalyst having Co0 metal and CoOx Lewis acid active sites. As a first step, LA was dehydrated to form angelica lactone (AL) intermediate. Because dehydration of LA is a facile reaction, the selectivity was controlled by the hydrogenation/hydrogenolysis activity of the catalyst. At 200 °C and 30 bar total pressure in the steady state, the catalyst could only saturate the double bond of AL ring. Thus, γ-valerolactone (GVL) was obtained with 98 mol% yield at full LA conversion. However, at temperature 225 °C the hydrogenation activity was high enough to cleave the GVL ring and obtain 2-methyltetrahydrofuran (2-MTHF) with a stable yield of about 70 mol %. FT-IR spectroscopic examination of the adsorbed LA showed the formation of H-bound LA and also surface carboxylate. 4-Hydroxy-3-pentenoate and 4-hydroxypentanoate were substantiated as surface intermediates of lactone formation by dehydration

Structure and activity of Pd/V2O5/TiO2 catalysts in Wacker oxidation of ethylene

Studies reveal that V2O5 supported on TiO2 is more active in gas phase oxidation reactions by O2 than on Al2O3 or SiO2 support. Nevertheless, the Pd/V2O5/TiO2 catalyst was hardly studied in heterogeneous Wacker-oxidation. The present study concerns preparation of Pd/V2O5/TiO2 catalysts and activity of the catalysts in selective gas phase oxidation of ethylene by O2 in presence of water at atmospheric pressure and in the temperature range of 100-200 oC. The influence of palladium and vanadia loading and the partial pressures of the reactants on the yield of oxidation products (acetaldehyde and acetic acid) were examined. The surface-bound vanadia forms were identified by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and FT-Raman spectroscopy. The best activities were achieved with catalysts, having near to monolayer vanadia coverage of the support. Time-on-stream catalytic tests and chemical analysis of the fresh and used catalysts proved the structural and catalytic stability of the Pd/V2O5/TiO2 preparations. It was shown that under the conditions of Wacker-oxidation primary product acetaldehyde could become further oxidized to CO2, whereas no consecutive oxidation of product acetic acid proceeded

Acetone alkylation with ethanol over multifunctional catalysts by a borrowing hydrogen strategy

Step by step alkylation of acetone (A) with ethanol (E) in a ratio of 1 : 2 was investigated. A fixed bed flow-through reactor system was used at a total pressure of 21 bar and in the temperature range of 150-350 [degree]C in inert He or a reducing H2 medium. Following the hydrogen borrowing methodology, two types of catalysts were prepared; using neutral activated carbon (AC) and alkaline hydrotalcite (HT) supports, namely 5 wt% Pd/AC in the presence of alkaline additives (10, 20 and 30 wt% KOH or 20% K3PO4); 9 wt% Cu/HT and 5 wt% Pd/HT. The catalysts were activated in a H2 flow at 350 [degree]C. Different yields of mono- or dialkylated ketones were observed. In a hydrogen medium over the same catalyst systems the ketone products could be reduced to alcohols. In this study the Pd/HT catalyst seems to be the most promising for fuel production based on biomass fermentation

Texture and morphology-directed activity of magnesia-silica mixed oxide catalysts of ethanol-to-butadiene reaction

Magnesia-silica mixed oxide catalysts of different texture and morphology were prepared for the ethanolto-butadiene (ETB) reaction. Magnesia of low and high specific surface area (SSA) were made by thermal decomposition of magnesium nitrate. High-SSA MgO was prepared using hard-templating (HT) method. Mesoporous carbon, obtained by carbonizing a resorcinol-formaldehyde polymer was used as template. The carbon pores were saturated by Mg(NO3 )2 solution and calcined then in order to decompose the nitrate and combust the carbon to get high-SSA MgO. The processes induced by latter calcination were followed by Thermogravimetry-Differential Scanning Calorimetry (TG-DSC) method. To obtain MgO-SiO2 catalysts both magnesia samples were wet-kneaded (WK) with a silica aerogel and a structured mesoporous SBA-15 silica material, having lower and higher SSA, respectively. Morphological and textural properties of these mixed oxide catalysts were characterized by means of N2 physisorption, X-ray powder diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Energy Dispersive X-ray Spectroscopy (EDX). Ranking the catalysts was attempted according to their acidity and basicity, i. e., by the concentration and strength of their acidic and basic sites. Therefore, the samples were characterized by their adsorption interaction with molecules, having either basic or acidic character. The adsorption of CO2 and NH3 was studied by Temperature-Programmed Desorption (TPD) method, whereas that of the pyridine and CDCl3 by Fourier Transform Infrared Spectroscopy (FT-IR). The WK changed the structure of the parent oxides, generated Mg-O-Si bonds, acidity, and basicity. The XPS and EDX results showed that the surface Mg content of the mixed oxide samples made from the low-SSA MgO was higher than that of the bulk phase, while that of the samples made of the high-SSA MgO was lower. This demon- strates that MgO prepared by use of a carbon template proved to be more reactive in the WK process and generated more Mg-O-Si bonds. The mixed oxide catalysts containing high-SSA MgO showed always higher activity and butadiene (BD) selectivity than the corresponding catalyst containing low-SSA MgO. The higher BD selectivity of these catalysts is related to their subtle acidity-basicity balance. Neverthe- less, the measured selectivities did not show correlation with any of the parameters characterizing the acid-base properties of the catalysts. The highest BD yield was obtained at 425 °C achieving 75% ethanol conversion level and 50% BD selectivity

MgO-SiO2 Catalysts for the Ethanol to Butadiene Reaction: The Effect of Lewis Acid Promoters

MgO SiO2 samples, having the composition of natural talc (NT); were obtained by co-precipitation (CP) and wet kneading (WK) methods. The materials were used as catalysts of the ethanolto-1,3-butadiene reaction. ZnO, Ga2O3 and In2O3 were tested as promoters. The catalyst WK gave the highest 1,3-Butadiene (BD) yield among the non-promoted catalysts because of the high specific surface area and strong basicity. Results suggested that over the neat WK catalyst the acetaldehyde coupling to crotonaldehyde was the rate-determining process step. Formation of crotyl alcohol intermediate was substantiated to proceed by the hydrogen transfer reaction between crotonaldehyde and ethanol. The crotyl alcohol intermediate becomes dehydrated to BD or, in a disproportionation side reaction, it forms crotonaldehyde and butanol. The promoter was found to increase the surface concentration of the reactant and reaction intermediates, thereby increases the rates of conversion and BD formation. The order of promoting efficiency was Zn>In>Ga