Re-Pd catalysts supported on TiO2 (P25 or DT51) prepared by successive impregnation (SI) or by Re deposition on Pd/TiO2 by catalytic reduction (CR) were characterized by XANES and EXAFS at the Pd and Re edges. These samples were shown to be efficient for the selective hydrogenation in aqueous solution of succinic acid (SUC) to 1,4-butanediol (BDO) (T = 160 degrees C, P(H-2) = 150 bar). This study clearly highlights the need for in situ reactivation of Re-based systems before their characterization or use, since Re species are fully re-oxidized (Re7+) by contact with air. The XANES quantitative analysis at the Re L-III-edge reveals that after in situ reduction, the Re average reduction state is in most cases inferior or equal to +3, indicating the presence of a mixture of Re3+ and Re(0 )species in all reduced catalysts. EXAFS study indicates that the Re species are mainly located at the Pd-support interface for the CR sample, whereas a part of Re entities is also deposited as isolated forms on the support in the case of the SI catalyst. The Re-Pd/DT51-SI system leading to the best BDO yield displays a specific arrangement of the Pd and Re species (Re deposit on Pd sites of high coordination). In this sample, the Re(0 )species, present in none negligible proportion, would contribute similarly as Pd metal to the efficient transformation of SUC towards GBL, which is further converted to BDO on sites involving Pd-0 and oxidized Re3+ species nearby.

TiO2-supported Re-Pd catalysts were evaluated for the selective hydrogenation of succinic acid (SUC) to 1,4-butanediol (BDO) in aqueous solution at 160 degrees C under 150 bar of H-2. Rhenium immobilized on Pd/TiO2 catalysts by ex situ preparation methods, namely successive impregnation (SI) or deposition by catalytic reduction (CR), was partially leached after handling in air and introduction into the SUC aqueous solution in the batch reactor; it was then re-deposited under H-2 pressure. It was demonstrated in this study that Re can be deposited by CR on a monometallic Pd catalyst directly in situ in the autoclave under hydrogen pressure before the catalytic test. The Re loading and the activation time under reducing atmosphere in the autoclave were optimized. Finally, the best bimetallic catalysts synthesized in situ demonstrated an intermediate selectivity to BDO compared to the systems prepared ex situ from the same parent monometallic catalyst, in the following order: ex situ CR < in situ CR < ex situ SI.

ReOx-Pd/TiO2 catalysts prepared from different 2wt%Pd/TiO2 catalysts using two protocols for the deposition of the Re promoter (successive impregnation and catalytic reduction) were characterized by different techniques to better understand the nature of the active and selective sites implied in the aqueous-phase hydrogenation of succinic acid to 1,4-butanediol. Regardless of the support and Re introduction method, it was established that varying amounts of Pd and Re were in very close proximity without electronic interaction in the reduced catalysts. A high fraction of Re always remained partially oxidized to generate a bimetallic catalyst that can provide the necessary bifunctional sites to enable the selective hydrogenolysis of the intermediate -butyrolactone to 1,4-butanediol. Depending on the method of promotion, the ReOx species that interact with Pd were deposited as clusters with different spatial Re-Re interactions.

Bimetallic Re-Pd catalysts were prepared by addition of Re onto a Pd/TiO2 monometallic catalyst, either by a catalytic reduction method or by successive impregnation. The physical and chemical properties of the bimetallic catalysts were evaluated by several characterization techniques, including analysis by transmission electronic microscopy, hydrogen chemisorption, temperature programmed reduction and X-ray photoelectron spectroscopy to explain their different catalytic performances in the selective hydrogenation of succinic acid to 1,4-butanediol in aqueous solution. The localization of the Re deposit on the parent Pd catalyst surface varied according to the Re introduction mode. The catalytic reduction method induced a selective Re deposit at the Pd-TiO2 interface compared to a random distribution obtained by successive impregnation. The oxidation state of the Re species (Re3+, Re-0) on the reduced bimetallic samples was also linked to their preparation mode. (C) 2014 Elsevier B.V. All rights reserved.

Supported noble-metal catalysts (Ru, Pd or Pt) and the corresponding Re-promoted catalysts exhibit a high activity for the hydrogenation of biobased carboxylic acids. Levulinic acid and succinic acid are converted into the lactones or the diols depending on the nature of the catalyst and the reaction conditions. The highest selectivity to 1,4-pentanediol of 82 % is achieved at 140 °C in the presence of the 1.9 % Ru-3.6 % Re/C catalyst.

A series of 2 wt % Pd/TiO2 monometallic catalysts were prepared by varying some parameters, such as the nature of the precursor salt, the titania support, and the preparation method. The structural and textural properties of the catalytic systems were fully characterized by several physical and chemical techniques (inductively coupled plasma optical emission spectrometry, N-2 physisorption, H-2 chemisorption, transmission.electron microscopy coupled with energy dispersive X-ray spectroscopy, powder X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, and gas phase reaction of cyclohexane dehydrogenation). The catalytic performances were further estimated for the hydrogenation of an aqueous solution of succinic acid (SUC) performed in a batch reactor at 160 degrees C and under 150 bar total pressure. The results showed that all the Pd catalysts are very Selective to produce gamma-butyrolactone, the first hydrogenated product. However, the rate of succinic acid conversion is a function of both the Pd dispersion and the preparation method. The deposition-precipitation method allows one to obtain the highest performing 2 wt % Pd/TiO2 samples during SUC hydrogenation in terms of activity and stability.

Hydrogenation of succinic acid aqueous solutions was performed using TiO2-supported 2 wt% Pd and 2 wt%Pd–x wt%Re catalysts, using either impregnation method or surface redox reduction of the monometallic catalyst. The former catalysts were superior in terms of activity and selectivity to 1,4-butanediol than the latter ones. However, higher Re loadings (3.4–3.6 wt% compared to 0.6–0.8 wt%) were necessary to initiate this synergy.