259 research outputs found

    Characterization of boria-alumina mixed oxides prepared by a sol-gel method. Part 1: NMR characterization of the xerogels

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    While boria-alumina mixed oxides have been widely used for various catalytic reactions, their structure has never been satisfactory elucidated and is still a matter of debates. The present paper deals with the elucidation of the structure of boria-alumina prepared by a sol-gel method with B / Al atomic ratio varying from 0.013 to 1.643. The powders were prepared by hydrolysis of aluminum tri-sec-butoxide in presence of (NH4)2B4O7.4H2O. Then, the solid-state magic angle spinning NMR spectroscopy was used to characterize the obtained solids in the dried state (xerogels), the calcined state being the object of a next paper. Both 27Al and 11B MAS-NMR spectra were recorded with subsequent simulation of these last ones. This allowed to build a consistent structural model of these xerogels, taking into account the evolutions of calculated parameters such as the quadrupolar interaction, the real chemical shift, the relative quantity of respectively BO3, BO4, tetrahedral, pentahedral and octahedral aluminum species as a function of the B / Al atomic ratio. As a result, it was found that for B / Al 0.06, BO3 chains attached to these tetrahedral aluminum species are formed and start to grow through the matrix. Further, for B / Al > 0.15, when the boron loading increases, the BO3 chains progressively emerge outside of the matrix, crossing it over. Furthermore, for B / Al > 0.26, some pentahedral aluminum species are formed supposedly due to the considerable steric strains afforded by some particular aluminum atoms. In brief, the presented model elucidates the structure of the dried alumina-based xerogels and is the key starting point to explain the structure of the oxides obtained after calcination and presented in the following paper

    Characterization of boria-alumina mixed oxides prepared by a sol-gel method. Part 2: Characterization of the calcined xerogels

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    Sol-gel boria-alumina mixed oxides with a wide range of B / Al atomic compositions have been characterized as dried xerogels in the preceding paper in which a structural model with four domains of composition was proposed. The present paper presents the results obtained on the same series of solids after calcination at 500°C. It was found that the structure of the dried xerogels has a strong influence on the structure of the final oxides. In particular, in the dried xerogel state from B / Al ~ 0.15, BO3 chains crossed over the solid matrix and this ratio was also a crucial limit for the calcined solids structure. Indeed, during calcination some BO3 chains were volatilized, giving a measured B / Al ratio lower than the theoretical one; the oxides were dislocated and their SSA brutally increasing from ~ 500 m2.g-1 up to ~ 650 m2.g-1. Cleavage resulted in the exposition of new external surfaces that exhibited BO3OH species incrusted in the host alumina matrix, conferring then an epitactic character to the solids. Further, the 11B MAS-NMR spectra of the solids exhibited a feature constituted of BO3 and BO4 (hydrated surface BO3 species) components that were resolved by a software simulation. While for B / Al 0.15. This increase originated in a strain on the new BO3 species trapped along the cleaved surfaces. Further, XRD and XPS results showed that while for B / Al 0.15 a mixed phase was progressively formed. In good agreement, the 27Al MAS-NMR spectra of the solids with high B / Al ratio were similar to that expected for model boria-alumina mixed phases. In addition, the BO4 / BO3 ratios calculated by simulation of the 11B MAS-NMR spectra simulation were consistent with the XPS results. We calculated the proportion of B species (in wt.%) present on the surface of the solids (i.e. BO4 species), which can be potentially involved in catalytic reactions. It was remarkable that the tendency observed for the BO4 wt.% as a function of the B / Al ratio was consistent with a boria-alumina phase diagram previously proposed by Gielisse et al.

    Characterization and hydrodesulfurization activity of CoMo catalysts supported on sol-gel prepared Al2O3

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    A series of CoMo/Al2O3 catalysts was prepared by impregnation on a series of alumina powders synthesized by the sol-gel method with different hydrolysis ratios R (defined as [H2O] / [aluminum-tri-sec-butoxide (ASB)]; R = 3, 4, ..., 12, 13). The oxide precursors were characterized and subsequently tested in the hydrodesulfurization (HDS) of thiophene, dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT). Mainly due to their large pore diameters of ca. 6 nm, the catalysts prepared from the alumina with hydrolysis ratio R = 7 ~ 10 showed higher HDS activity compared with the activities of the other prepared catalysts. The effect of the pore diffusional limitation was more significant than expected, due to the ink-bottle shape of the pores of the prepared catalysts with hydrolysis ratio R 10. Due to highly dispersed CoMo active phase, the HDS activity of the prepared catalysts with hydrolysis ratio R = 8 and 9 for thiophene was similar to that of a reference industrial catalyst that was designed and manufactured for deep HDS of diesel fuel fractions. Furthermore, the HDS activity of the prepared catalysts with hydrolysis ratio R > 5 for DBT was higher than that of the reference industrial catalyst. For 4,6-DMDBT, however, the reference industrial catalyst showed higher HDS activity compared with the activities of the prepared catalysts. Relatively high HDS activity was observed for the prepared catalysts with R = 9 and 10 with cylindrical pore shape and with a high proportion of strong acid sites. The strong acidity supposedly enhanced the hydrogenation activity of the catalysts that was essential for the HDS of 4,6-DMDBT

    Recent developments in maleic acid synthesis from bio-based chemicals

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    This review paper presents the current state of the art on maleic acid synthesis from biomass-derived chemicals over homogeneous or heterogeneous catalysts. It is based on the most recent publications on the topic, which are discussed in details with respect to the observed catalytic performances. The recent developments and the technical drawbacks in the gas and the liquid phases are also reported. In addition, recent results on the mechanistic aspect are discussed giving insights into the probable reaction mechanisms depending on the starting molecule (furan, furfural and 5-hydroxymethylfurfural)

    Description of coordinatively unsaturated sites regeneration over MoS2-based HDS catalysts using 35S experiments combined with computer simulations

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    By combining experimental results and computer simulations, we previously showed that the coordinatively unsaturated sites (CUS) formation over MoS2 is most likely to occur on the MoS2 metallic edge through the departure of an H2S molecule. In the present paper, we aimed at examining the H2S departure from MoS2 catalysts promoted with Co and Ni. The [35S]DBT HDS experiments results showed that over CoMoS/Al2O3 and NiMoS/Al2O3 catalysts, the activation energy of the H2S release reaction is essentially the same with respective values of 7.4 kcal.mol-1 and 7.9 kcal.mol-1. Considering the H2S departure activation energy in the case of the non-promoted MoS2 surface (10 ~ 12 kcal.mol-1), this result illustrates the synergetic effect between Mo and Co or Ni in terms of CUS regeneration easiness. Further, preliminary computer simulations results showed that for S atoms bridged between Co atoms, a mechanism implying H2S departure from the metallic edge cannot be reasonably envisaged. Moreover, on the sulfur edge the H2S release activation energy is too high (~ 13.5 kcal.mol-1) if we consider experimental results on CoMo/Al2O3 catalysts but not incompatible with the experimental value obtained over Co/Al2O3 catalysts (ca. 10 kcal.mol-1), which suggests that the mechanism on the promoted catalyst differs from that on the un-promoted one

    Characterization and Hydrodesulfurization Activity of CoMo Catalysts Supported on Boron-Doped Sol-Gel Alumina

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    A series of hydrodesulfurization (HDS) catalysts was prepared by impregnation of Co and Mo on sol-gel B-Al2O3 supports with B/Al ratios of 0, 0.02, 0.04, 0.08, 0.20, 0.32, 0.49, and 0.61. The thiophene HDS and dibenzothiophene (DBT) HDS activities were both maximal for the catalyst with B/Al = 0.04, with respective values 70% and 42% higher than those for an industrial reference catalyst. These maxima in HDS activity correlated with the previously reported presence of isolated BO4 surface species. These BO4 species were responsible for a local maximum in the acidity of the B-Al2O3 supports when B/Al = 0.04. In contrast, the formation of mixed oxides (A9B2 and A2B) or B2O3 that also resulted in enhanced acidity of the B-Al2O3 supports had a detrimental effect on the HDS activity. The 4,6-dimethyldibenzothiophene (4,6-DMDBT) HDS activity over the CoMo/B-Al2O3 catalysts decreased when the B/Al ratio was increased. This was attributed to the strong direct desulfurization character of the CoMo catalysts supported on the B-Al2O3 supports, because high hydrogenation ability toward the C=C double bonds is essential prior to sulfur removal from 4,6-DMDBT. The excellent performance in the thiophene and DBT HDS of the CoMo/B-Al2O3 catalysts is particularly useful for ultra-deep HDS of light fractions

    Catalytic Dehydration of Glycerol to Acrolein in a Two-Zone Fluidized Bed Reactor

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    The gas-phase catalytic dehydration of glycerol to acrolein was carried out in a Two-Zone Fluidized-Bed Reactor (TZFBR) using a 20 wt. % phosphotungstic acid (H3PW12O40) catalyst supported on CARIACT-Q10 commercial silica. In the first step, a hydrodynamic study of the reactor was performed. A quality of fluidization of more than 80% was obtained. In the second step, the mechanical stability of the catalyst was studied. It was found that only the external layer of active phase is eliminated under the conditions of operation whereas the global composition of the catalyst was not significantly affected after 44 h of fluidization. Finally, in a third step, the influence of the main operating parameters on the overall catalytic performances (glycerol/oxygen molar ratio and relative volumes of the reaction and regeneration zones) was investigated, showing notably the importance of the O2/glycerol ratio, resulting in an inverse trend between conversion and selectivity. Increasing O2/glycerol ratio led to higher conversion (lower coke deposit as shown by TGA analysis), but to the detriment of the selectivity to acrolein, supposedly due to the presence of O2 in the reaction zone causing the degradation of glycerol and acrolein

    Hybrid Catalysis as a New Topical Concept Bridging Biotechs and Chemistry

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    Types of biorefineries & sourcing

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    Témoignage concernant la progression de carrière

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