Ethanol conversion into 1,3-butadiene over a mixed Hf-Zn catalyst: Effect of reaction conditions and water content in ethanol

Assessment of catalyst performance is necessary for process design in order to evaluate the effect of reaction conditions on process economics and configuration. In this work, the combined effect of reaction conditions and quality of feedstock (i.e. water content) on the performance of a Hf-Zn catalyst, prepared by physically mixing the zinc silicate hemimorphite and HfO2/SiO2, for the conversion of ethanol to 1,3-butadiene is investigated. To this aim, an experimental design was performed with temperature, space velocity, and water content in ethanol as factors. In situ IR spectroscopy unambiguously indicates that the presence of water in the ethanol feed induces the generation of new Brønsted acid sites, most probably by reaction of Zn2+ Lewis acid sites with water, and thus modifies the Brønsted-Lewis acid site balance in the catalyst. This fact results in (i) higher selectivity to dehydration products catalyzed by Brønsted acid sites, and (ii) lower ethanol conversion as some Zn2+ Lewis acid sites, active for the dehydrogenation of ethanol, are transformed into Brønsted acid sites. In addition, higher acetaldehyde yield is observed in experiments feeding hydrous ethanol, indicating that water inhibits aldol condensation reactions to a larger degree than ethanol dehydrogenation. This effect is particularly beneficial at a high operating temperature, where acetaldehyde is so reactive that it is rapidly converted to heavy compounds unless water is present. Therefore, the benefits of water in ethanol under such reacting conditions should lead to savings in operating costs for the energy-intensive removal of water from ethanol and the use of a cheaper ethanol feedstock with high water content.