Catalytic dehydroaromatization of methane over zeolite-supported molybdenum nanoparticles.

摘要

140 billion m3 of natural gas, which is mostly composed of methane, are wastefully flared or vented worldwide annually because there are no efficient technologies for methane conversion. Direct methane conversion into liquid aromatic hydrocarbons over catalysts with molybdenum nanostructures supported on shape selective zeolites is a promising new chemistry that can solve this problem and dramatically improve natural gas utilization. In this work, a combination of spectroscopic measurements, transient reaction kinetic studies and quantum chemical calculations based on density functional theory determines the identity and anchoring sites of the initial molybdenum structures in such catalysts as isolated Mo(=O)2 species on double Al-atom sites and Mo(=O)2OH species on single Al-atom sites in the zeolite framework. During the reaction with methane, the initial isolated Mo oxide species agglomerate and convert into Mo oxycarbide or carbide nanoparticles. This work determines that this process is reversible and that the initial isolated Mo oxide species are restored by a treatment with gas-phase oxygen. These findings will be helpful in extending catalyst lifetime and in optimization of catalyst formulations and reaction conditions. The developed methodology of combining vibrational spectroscopies with quantum chemical calculations for molecular-level understanding of reaction mechanisms on catalytic surfaces was extended to characterization of hydrocarbon adsorption and reactivity of platinum-tin alloy catalysts. A new preferential catalytic mechanism for the transformation of acetylene (CH-CH) to vinylidene (C-CH2) was identified. Unlike the direct H shift along the C-C bond in organometallics, this mechanism has a geometric site requirement of three adjacent Pt atoms in a threefold arrangement. The same geometric site requirement is identified for C-H bond cleavage. In the absence of three-fold Pt sites, the reaction mechanism changes, and reactions of H transfer and C-H bond cleavage are suppressed. As synthesis methods for custom-tailored alloy surfaces are being rapidly developed to control both the composition and geometric arrangements of elements, this information on the geometric requirements for hydrocarbon reactivity is critical for realizing the full potential of rational design for new bimetallic catalysts.