Synthesis, Characterization, and Reaction Screening of Novel Heterogeneous Catalysts for Propylene Epoxidation

Propylene oxide (PO) is a valuable chemical intermediate that generates billions of dollars per year in revenue to the chemical industry, and one of the most formidable challenges in modern catalysis research is the discovery and optimization of selective heterogeneous catalysts for the direct oxidation of propylene with molecular oxygen to synthesize PO. In an attempt to discover new catalysts for this process, a high-throughput pulsed laser ablation (HT-PLA) system was developed to rapidly prepare catalytic nanoparticles of 35 different metals supported on ceramic pellets, which were screened for activity using an array channel microreactor system and gas chromatography. These initial screening experiments led to the discovery of several previously unreported leads for propylene epoxidation catalysts, namely Mn, Ru, Cr, Sn, and Ir. Further investigation of these leads via a combinatorial strategy led to the discovery of Cu / MnOx bimetallic nanoparticle catalysts that exhibited PO yields 5 times greater than their single-component counterparts.;This finding was surprising because Cu / Mn mixed metal oxide (MMO) catalysts, under the trade name hopcalite, have been used since 1920 as combustion catalysts for the destruction of carbon monoxide and volatile organic compounds. Since the Cu / Mn MMO system showed promise for the epoxidation reaction, a novel high-throughput gel combustion synthesis (HT-GCS) technique was developed to systematically vary the synthesis parameters that affect the Cu / Mn catalyst performance. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and isothermal nitrogen sorption analysis (BET). The crystallinity of the catalysts was of particular importance and was influenced by the fuel that was used for GCS, with glycerol giving the most desired characteristics. Amorphous and weakly crystalline catalysts led to combustion while catalysts consisting of a crystalline Mn2O3 phase and a nanocrystalline phase of Cu1+xMn2-xO4 (0<x<1) gave the best PO selectivity. The Cu / Mn MMOs with a composition of 14 mol% Cu, 86 mol% Mn, and Na (from NaCl) in the 0.2 -- 0.6 mol% range exhibited the best results. A peak performance of about 38% PO selectivity and 1.8% propylene conversion was achieved; this represents, to our knowledge, the first ever heterogeneous epoxidation catalysts derived from copper and manganese.;The material performance was consistent with a Mars van Krevelen (MvK) mechanism; the oxygen species leading to epoxidation originates from Cu 2+, which provides the weakly bound lattice oxygen necessary for epoxidation. Mn3+ subsequently promotes lattice oxygen replenishment by lattice oxygen transport and the following redox reaction that occurs in the CuMn2O4 lattice: Cu2+ + Mn3+ ↔ Cu+ + Mn4+. However, the simultaneous presence of Mn2O3 and CuMn2O4 was critical to higher selectivity catalysts, indicating that a phase cooperation phenomenon between Mn2O3 and CuMn2O4 is likely. Finally, Na promotes the epoxidation reaction by selectively blocking unfavorable oxygen binding sites, while Cl increases selectivity by further inhibiting combustion.