Tuning the porosity of sulfur-resistant Pd-Pt/MCM-41 bimetallic catalysts for partial hydrogenation of soybean oil-derived biodiesel

Abstract Partial hydrogenation of soybean oil-derived fatty acid methyl esters was studied using MCM-41 mesoporous silica-supported Pd-Pt bimetallic catalysts with tunable porosity under mild reaction conditions (100 °C, 0.4 MPa H2, 4 h). This process produced partially hydrogenated fatty acid methyl esters (H-FAME) as a new type of high-quality biodiesel fuel enriched in monounsaturated fatty acid methyl esters (mono-FAME), which is a potential source for formulating high blends of biodiesel fuel with petrodiesel. MCM-41 supports with various structural properties and morphologies were synthesized by self-assembly with different amounts of ammonia solution as a mineralizing agent. Bimetallic Pd-Pt nanoparticles with a Pd/Pt atomic ratio of 4 were stepwise impregnated on three MCM-41 supports, resulting in a series of Pd-Pt/MCM-41 bimetallic catalysts with tunable porosity (0.89–1.79 cm3 g−1), average pore size (3.2–8.5 nm), and particle size (0.12–0.62 μm). The Pd-Pt/MCM-41 catalyst prepared with the least amount of ammonia produced the best partial hydrogenation conversion of polyunsaturated FAME into mono-FAME, ascribed to nano-aggregation resulting in a dual-pore system containing large pores for fast molecular diffusion; a high turnover frequency (1920 h−1), larger k1 rate constant (0.60 gcat−1h−1), and smaller k2 rate constant (0.37 gcat−1h−1) were obtained. Furthermore, this Pd-Pt/MCM-41 catalyst exhibited excellent sulfur resistance in the synthesis of H-FAME, even though the feedstocks contained approximately 5 ppm of sulfur contaminates. These results demonstrate that the sulfur-resistant Pd-Pt/MCM-41 bimetallic catalysts with stable and dual pore system are beneficial for obtaining higher quality BDF to reduce our reliance on fossil fuels.