The development of new and improved processes for the synthesis of bio-based chemicals is one of the scientific challenges of our time. These new discoveries are not only important from an environmental point of view, but also represent an important economic opportunity, provided that the developed processes are selective and efficient. Bioethanol is currently produced from renewable resources in large amounts and, in addition to its use as biofuel, holds considerable promise as a building block for the chemical industry. Indeed, further improvements in production, both in terms of efficiency and feedstock selection, will guarantee availability at competitive prices. The conversion of bioethanol into commodity chemicals, in particular direct 'drop-in' replacements is, therefore, becoming increasingly attractive, provided that the appropriate (catalytic) technology is in place. The production of green and renewable 1,3-butadiene is a clear example of this approach. The Lebedev process for the one-step catalytic conversion of ethanol to butadiene has been known since the 1930s and has been applied on an industrial scale to produce synthetic rubber. Later, the availability of low-cost oil made it more convenient to obtain butadiene from petrochemical sources. The desire to produce bulk chemicals in a sustainable way and the availability of low-cost bioethanol in large volumes has, however, resulted in a renaissance of this old butadiene production process. This paper reviews the catalytic aspects associated with the synthesis of butadiene via the Lebedev process, as well as the production of other, mechanistically related bulk chemicals that can be obtained from (bio)ethanol.

The preparation method greatly influences morphology, acid–base properties and performance of SiO2–MgO catalysts. Wet-kneaded catalysts possess an improved distribution, proximity and strength of acidic-basic sites, thus leading to higher butadiene yields.

Silica-magnesia (Si/Mg=1:1) catalysts were studied in the one-pot conversion of ethanol to butadiene. The catalyst synthesis method was found to greatly influence morphology and performance, with materials prepared through wet-kneading performing best both in terms of ethanol conversion and butadiene yield. Detailed characterization of the catalysts synthesized through co-precipitation or wet-kneading allowed correlation of activity and selectivity with morphology, textural properties, crystallinity, and acidity/basicity. The higher yields achieved with the wet-kneaded catalysts were attributed to a morphology consisting of SiO2 spheres embedded in a thin layer of MgO. The particle size of the SiO2 catalysts also influenced performance, with catalysts with smaller SiO2 spheres showing higher activity. Temperature-programmed desorption (TPD) measurements showed that best butadiene yields were obtained with SiO2-MgO catalysts characterized by an intermediate amount of acidic and basic sites. A Hammett indicator study showed the catalysts' pK(a) value to be inversely correlated with the amount of dehydration by-products formed. Butadiene yields could be further improved by the addition of 1 wt% of CuO as promoter to give butadiene yields and selectivities as high as 40% and 53%, respectively. The copper promoter boosts the production of the acetaldehyde intermediate changing the rate-determining step of the process. TEM-energy-dispersive X-ray (EDX) analyses showed CuO to be present on both the SiO2 and MgO components. UV/Vis spectra of promoted catalysts in turn pointed at the presence of cluster-like CuO species, which are proposed to be responsible for the increased butadiene production.

Dehydrogenation promoters greatly enhance the performance of SiO 2 −MgO catalysts in the Lebedev process. Here, the effect of preparation method and order of addition of Cu on the structure and performance of Cupromoted SiO 2 −MgO materials is detailed. Addition of Cu to MgO via incipient wetness impregnation (IWI) or coprecipitation (CP) prior to wet-kneading with SiO 2 gave similar butadiene yields (∼40%) as when Cu was added to the already wet-kneaded catalyst. In contrast, the catalyst prepared by impregnation of Cu on SiO 2 first proved to be the worst catalyst of the series. TEM, XRD, and XPS analyses suggested that, for all catalyst materials, Cu 2+ forms a solid solution with MgO. This was confirmed by UV−vis, XANES, and EXAFS data, with Cu being found in a distorted octahedral geometry. As a result, the acid−base properties, as determined by Pyridine-and CDCl 3 −IR as well as NH 3 -TPD, are modified, contributing to the improved performance. Operando XANES and EXAFS studies of the evolution of the copper species showed that Cu 2+ , the only species initially present, is extensively reduced to a mixture of Cu 0 and Cu + , leaving only a limited amount of unreduced Cu 2+ . This formation of Cu 0 is the result of the reducing environment of the Lebedev process and is thought to be mainly responsible for the improved performance of the Cu-promoted catalysts.