Ag- and Cu-Promoted Mesoporous Ta-SiO2 Catalysts Prepared by Non-Hydrolytic Sol-Gel for the Conversion of Ethanol to Butadiene

Abstract

The direct catalytic conversion of bioethanol to butadiene, also known as the Lebedev process, is one of the most promising solution to replace the petro-based production of this important bulk chemical. Considering the intricate reaction mechanism—where a combination of acid-catalyzed dehydration reactions and metal-catalyzed dehydrogenation have to take place simultaneously—tailor-made bifunctional catalysts are required. We propose to use non-hydrolytic sol-gel (NHSG) chemistry to prepare mesoporous Ta-SiO2 materials which are further promoted by Ag via impregnation. An acetamide elimination route is presented, starting from silicon tetraacetate and pentakis(dimethylamido)tantalum(V), in the presence of a Pluronic surfactant. The catalysts display advantageous texture, with specific surface area in the 600–1000 m² g−1 range, large pore volume (0.6–1.0 mL g−1), an average pore diameter of 4 nm and only a small contribution from micropores. Using an array of characterization techniques, we show that NHSG allows achieving a high degree of dispersion of tantalum, mainly incorporated as single sites in the silica matrix. The presence of these monomeric TaOx active sites is responsible for the much higher dehydration ability, as compared to the corresponding catalyst prepared by impregnation of Ta onto a pristine silica support. We attempt to optimize the butadiene yield by changing the relative proportion of Ta and Ag and by tuning the space velocity. We also demonstrate that Ag or Cu can be introduced directly in one step, during the NHSG process. Copper doping is shown to be much more efficient than silver doping to guide the reaction towards the production of butadiene.

Ethanol-to-butadiene: the reaction and its catalysts

“…A non-hydrolytic sol-gel process was also used by Dochain et al to produce Tacontaining mesoporous silica, which was subsequently doped with silver or copper to promote activity in the Lebedev process. 233 A large surface area and monomeric species of TaO x were obtained with this procedure.…”

“…Dochain et al recently highlighted this by tuning the Ag : Ta ratio of a Ag-Ta-SiO 2 catalyst prepared by a non-hydrolytic sol-gel procedure using silica and a tantalum precursor, followed by impregnation. 233 Both Ag and Cu can enable high butadiene formation under the right conditions. The same can be said for zinc, which has successfully been combined with group 4 and 5 transition metals to prepare catalysts highly active in the Lebedev process.…”

Mildly acidic aluminosilicate catalysts for stable performance in ethanol dehydration

“…Non-hydrolytic sol-gel (NHSG) is a technique that allows attaining high level of homogeneity and excellent textural properties in mixed metal oxides. [38,39] It has been already used for the preparation of various types of porous mixed oxide catalysts including aluminosilicates with low Si/Al ratios [40,41], but also Ta promoted silica, [42] Co doped aluminosilicates, [43] [45], and other metallosilicates and metalloaluminates (M = Ti, Zr, W, etc.). [38,39] The versatility of this preparation route prompted us to explore the potential of NHSG-prepared amorphous aluminosilicate in the dehydration of ethanol.…”

Recent Advances in Catalysts for the Conversion of Ethanol to Butadiene

“…A comparison of acidic ZrO 2 /SiO 2 and basic MgO/SiO 2 catalysts suggested that the strong basic sites in MgO/SiO 2 are more reactive in condensation reactions but suffer fast deactivation, in contrast to the acidic sites of both ZrO 2 /SiO 2 and MgO/SiO 2 . [81] Table 3 summarizes recent ETB acidic catalytic systems with SiO 2 [82][83][84][85][86][87][88][89][90][91][92][93] and ZrO 2 [94][95][96] as support. Some of these systems are used for the ethanol/acetaldehyde to butadiene conversion (step 2 of the two-step process).…”

“…Dochain et al prepared a bimetallic TaÀ Cu/SiO 2 system using a non-hydrolytic sol gel method to give a highly homogeneous acidic mixed oxide. [84] However, the catalytic performance of 30.9% selectivity to butadiene at 75.3% conversion may not justify the complexity of the catalyst synthesis.…”