Ethyl palmitate decarboxylation using colloidal SiO2- templated mesoporous Ni-ZrO2 catalysts

Abstract

A series of 10 wt% NiO-xSiO2-ZrO2 (x = 0–30 wt%) catalysts were synthesized using a one-pot coprecipitation method with different of colloidal SiO2 contents, as a hard template, in order to imprint mesoporous cavities after a post-synthesis selective alkaline NaOH/H2O lixiviation. The final catalysts were characterized by XRD, HRTEM, textural measurements, 29Si-MAS NMR, FTIR, and XPS; the lixiviated, calcined and H2-reduced powders, were evaluated in the decarboxylation (DCO2) of ethyl palmitate as a liquid-solid phase model reaction at 300 °C, and 20 kg/cm2 H2 pressure. Optimal amount of colloidal SiO2 was 10 wt%. SiO2 effect (as colloid in fresh preparations and/or residual SiO2, after lixiviation) on ZrO2 is twofold. (i) to upgrade mesoporosity and (ii) to stabilize the porous structure. Residual SiO2 (from incomplete lixiviation) plays a beneficial role as examined by 29Si-NMR, being chemically bonded to ZrO2 and acting as props of the mesopores. Total pore volume and specific surface area values increased 5 and 6 times, respectively, when using a 30 wt% SiO2 in comparison with a non-silicated NiO-ZrO2 preparation. DCO2 of ethyl palmitate benefit from better mesoporosity and a great number of oxygen vacancies from Ni isomorphic substitution into cubic ZrO2 (as indicated by XPS); being at the origin of the reaction mechanism. For instance, a 10 wt% SiO2 NiO-ZrO2 catalyst is 1.6 times more selective (to n-pentadecane) and 1.2 times more active than a non-silicated NiO-ZrO2, yielding decarboxylated (i.e., deoxygenated) renewable linear hydrocarbons similar to those in a Diesel composition. However, DFT calculations indicate that adsorbed CO2 (as a formate species) desorbs, once fully hydrogenated, as methane regenerating the oxygen vacancies but consuming H2 as well, in agreement with the high methane concentration detected in gas phase.