The effects of modes of hydrogen input and reactor configuration on reaction rate and H₂ efficiency in the catalytic hydrogenation of alkynol to alkenol

Hydrogenation often involves three phases where hydrogen-on-demand is the typical mode of operation in industrial scale reactors. In research labs and publications, however, continuous hydrogen flow has been used. This paper investigates the effect of such modes of operation on reaction rate using a selective hydrogenation of 3-butyn-2-ol over Pd/Al₂O₃ to obtain 3-buten-2-ol as the model reaction. The two modes of operation were first tested in a commercial PARR stirred tank reactor and then repeated in an oscillatory baffled reactor (OBR) in order to validate the experimental results. Our investigation demonstrates that an enhanced reaction performance and 10 times better H₂ efficiency were obtained when the pressure was maintained constant during the reaction by feeding gas as required, ie hydrogen-on-demand mode. The method of a continuous flow of hydrogen in hydrogenation means that excess hydrogen is vented out when operating at ambient pressures or builds up at elevated pressures. Our work also enables a comparison of reactor designs on reactor performance, and three times higher H₂ efficiency and 2.3 times shorter residence time were achieved when using the OBR instead of the PARR due to its enhanced and uniform mixing, regardless of the mode of operation.