Sulfur Deactivation of Fatty Ester Hydrogenolysis Catalysts

Abstract

Trace organosulfur compounds present as natural impurities in oleochemical feedstocks may lead to deactivation of copper-containing catalysts applied for hydrogenolysis of esters toward fatty alcohols. In this paper, the sulfur deactivation of Cu/SiO2 and Cu/ZnO/SiO2 catalysts was studied in the liquid-phase hydrogenolysis of methyl palmitate. The rate of deactivation is fast and increases as a function of the sulfur-containing compound present: octadecanethiol ≈ dihexadecyl disulfide < benzyl isothiocyanate < methyl p-toluene sulfonate < dihexadecyl sulfide < di-benzothiophene. The rapid deactivation is caused by the fact that sulfur is quantitatively removed from the reaction mixture and because mainly surface sulfides are formed under hydrogenolysis conditions. The life time of a zinc-promoted catalyst is up to two times higher than that of the Cu/SiO2 catalyst, most likely due to zinc surface sulfide formation. The maximum sulfur coverage obtained after full catalyst deactivation with dibenzothiophene and dihexadecyl sulfide—the sulfur compounds that cause the fastest deactivation—may be as low as 0.07. This is due to the fact that decomposition of these compounds as well as the hydrogenolysis reaction itself proceeds on ensembles of copper atoms. For the most reactive sulfur compounds, surface coverage near the maximum value of θCu=0.5 or—in the presence of zinc—formally in excess of this quantity may be reached at full catalyst deactivation. At that point, still some sulfur uptake occurs. Decomposition of such compounds is even possible in the absence of hydrogen and sulfur is not laid down in a dispersed fashion, as in the case of dibenzothiophene and dihexadecyl sulfide. Catalyst regeneration studies reveal that activity cannot be regained by reduction or combined oxidation/reduction treatments. XRD, TPR, and TPO results confirm that no distinct bulk copper or zinc sulfide or sulfate phases are present.