Dehydration of ethanol over hydrophobic hybrids of niobium-silica mixed-oxides prepared by non-hydrolytic sol-gel process

The production of bio-sourced compounds has been deeply investigated during the past few decades as an alternative to the petrochemical industry. Currently, the main pathway to produce ethylene, one of the most produced chemical compounds in the world, is through hydrocarbon cracking process. Because of the depletion of fossil fuel reserves and their harmful impact on the global warming, the use of biomass ethanol catalytic dehydration has been developed. Various heterogeneous catalysts for this dehydration reaction have been explored, but poor resistance in high water concentration was reported. However, new niobium-silica mixed-oxides (Nb2O5-SiO2) materials in recent research have displayed high hydrothermal stability while showing good catalytic performance. In line with it, hybridization (i.e.: incorporation of organic moieties) of this promising catalyst has been investigated in this work to make it even more resistant against hydrolysis. Another suggested feature of hydrophobic hybrids is that water, created during dehydration reaction, would desorb easier from the surface of the solid and thus impact the catalytic activity. In this study, novel preparation method, the non-hydrolytic sol-gel (NHSG) process has been applied as it largely improves the homogeneity of the synthesized solids, which is an important property of hybrid mixed-oxides. Furthermore, this particular technique usually produces mesoporous materials with high specific surface areas. Two synthetic NHSG pathways have been studied in this work: (i) the alkyl halide elimination and (ii) the acetate route. Hybridization was carried out during the NHSG process by adding different types of organosilane precursors directly into the reaction mixture. Tested organosilanes include aromatic and aliphatic moieties both terminal (C6H5SiR3, (C6H5)2SiR2, CH3SiR3) and bridging (R3SiC6H4SiR3) (R=Cl (i),OAc (ii)). Different molar ratios of organic groups were used in order to see the influence of their amount on the properties of the material. In order to understand the stability and the catalytic performances of the hybrids, their physico-chemical characterization has been performed. Surface analyses reported that all synthesized materials were mesoporous with high surface areas. Niobium was successfully incorporated in all cases, however samples from the acetate route have shown a slightly better homogeneity of niobium dispersion in silica. The overall homogeneity of Nb-Si mixing in hybrids samples decreased with higher loadings of organic groups. A big part of this master’s thesis has been devoted to the evaluation of the catalytic activities in the ethanol dehydration reaction. Samples produced through the acetate pathway display higher conversions than samples from the alkyl halide elimination route. It was found out that high organic content in hybrid catalysts decrease the conversion of ethanol. All types of organic compounds integrated in hybrids behave similarly, except for methylated samples which have a relatively good conversion but show lower ethylene production. Another objective was to enhance the hydrothermal stability of the samples to make them more economically and ecologically interesting. Indeed, pure ethanol production implies several step of distillation to get rid of the water. If the catalyst stability was improved in presence of water, then these expensive unit operations would be avoided. Unfortunately, when the hybrid samples were exposed to water at 250 °C for 24 hr, significant losses of surface area and pore volumes were observed. Further research proved that the hydrolysis of the silicon-carbon bond takes place under these conditions. All types of organic compounds have been shown unstable against water, however methylated samples seem to be slightly more stable than catalysts with incorporated aromatic groups.