Bifunctional catalysts prepared by non-hydrolytic sol-gel for the production of butadiene

In the actual context of petroleum insecurity combined with a growing trend in its independence, environmental awareness and biomass valorisation, bio-based processes have become a focal point in fundamental research. Today, butadiene is mainly obtained through hydrocarbons steam cracking, a highly energy-consuming and polluting petrochemical process. As an alternative, the catalytic conversion of (bio)ethanol is one of the most promising solution to free ourselves from this non-renewable and climate change inducing operation. In the literature, a wide range of composition and synthesis methods for bifunctional heterogeneous catalysts needed for the production of butadiene from ethanol has been investigated. Tantalum-silica mixed oxides doped with silver (Ag Ta2O5•SiO2) have shown interesting performances in the direct conversion of ethanol into butadiene. This formulation displays suitable properties as a bifunctional catalyst, capable of achieving simultaneously dehydrogenation and dehydration reactions. Many synthetic techniques have already been explored and an innovative non-hydrolytic sol gel (NHSG) approach is studied in this work. This synthesis method allows for homogeneous and mesoporous catalysts with high surface areas. Two NHSG synthetic pathways are used: the ether route and the acetamide elimination route. Silver is incorporated via two different ways: one-pot or wet impregnation. After synthesis, ether catalysts display a mesoporous behaviour while acetamide catalysts require the use of a templating agent (Pluronic F127) to obtain a similar texture. IR, XPS and catalytic data discard ether catalysts as a way to obtain effective material for the production of butadiene. Characterization shows a poor tantalum incorporation into the silica matrix as well as insignificant butadiene yields. This is not the case with the acetamide elimination route where tantalum is well incorporated resulting in high acidity and sufficient butadiene yields. Thus, acetamide catalysts are further researched in terms of tantalum/silver loading and incorporation method. Catalytic results demonstrate the benefit of working with a 5 wt% silver loading instead of 2 wt%. Moreover, ideal tantalum loading for butadiene production is dependent on the reactor temperature. At 325 °C, the molar ratio Si/Ta=60 is more effective whereas at 355 °C, a molar ratio Si/Ta=120 will be preferred. The effectiveness of the catalysts is reflected through the butadiene yield and some representative by-product yields (ethylene, diethylether, acetaldehyde). Those by-products offer indications for the improvement of the catalyst. The diversity in by-products obtained proves the importance of a well-balanced catalyst. This rigorous balance is conceptualised with an Ag/Ta weighted ratio taking into account the relative loadings of both active metals as well as the total metal loading in the material. It highlights the fact that lower loadings in metals can give similar catalytic results and bring out lines of thought for future research in order to further optimize the catalyst’s formulation.