METHANE VALORIZATION OVER NOVEL CATALYST SYSTEMS VIA DIRECT PATHWAYS

Abstract

Methane, when converted to higher hydrocarbons, promises a great future as the substituent for liquid petroleum in petrochemical and fine chemical industries. Methane conversion via direct pathways such as oxidative coupling of methane (OCM) to ethylene and direct non-oxidative methane conversion (DNMC) to C2 (acetylene, ethylene and ethane) and aromatics have attracted much attention given their unique capability in circumventing the intermediate energy-intensive steps found in indirect processes. In the OCM process, the more reactive nature of C2 products leads to the sequential oxidation of C2 to COx (CO or CO2). Selective catalysts that favor C2 formation are desired. The DNMC is challenged by low equilibrium conversion, high endothermicity, and high coke selectivity. Catalysts or reaction systems that concurrently solve these challenges are required.

This dissertation aims to develop novel catalyst systems to conquer limitations in OCM and DNMC to realize efficient and effective C2 production. For OCM reaction, hydroxyapatite (HAP), a bioceramic material with the capability of cation and/or anion substitutions, was innovatively employed as a catalyst. The effects of cation and/or anion substitutions in HAP on OCM reaction were studied. The rigorous description of the reaction kinetics of OCM in HAP-based catalysts was conducted. Finally, the selective control of exposed crystalline plane of HAP was realized to further understand the catalytic behaviors of HAP-based catalysts in OCM reactions. It is shown that cation and/or anion substitution can change the physicochemical properties of the HAP catalysts, and as consequences, the OCM catalytic performances. The c-surface (i.e., (002) crystalline plane) of HAP-based catalysts exhibited significant enhancement in areal rate in OCM reaction.

The single iron sites confined in the lattice of silica matrix (Fe/SiO2) is an emerging type of methane activation catalyst in DNMC. We innovated a millisecond catalytic wall reactor made of Fe/SiO2 catalyst to enabling stable and high methane conversion, C2+ selectivity, low coke yield, and long-term durability. These effects originate from initiation of DNMC by surface catalysis on reactor wall, and maintenance of the reaction by gas-phase chemistry in reactor compartment. Autothermal operation of the catalytic wall reactor is potentially feasible by coupling and periodical swapping of endothermic DNMC and exothermic oxidative coke removal on opposite side of the reactor. High carbon and thermal efficiencies and low cost in reactor materials are realized for the techno-economic process viability of the DNMC technology. In addition, we created a process of tailoring product selectivity towards to C2 hydrocarbons by employing a mixture of Fe/SiO2 catalyst and mixed ionic-electronic conductive perovskite (SrCe0.8Zr0.2O3−δ) oxide in the presence of hydrogen co-feed in methane stream. The unprecedentedly high C2 yield was realized in DNMC reaction to maximize its potential as a feedstock for ethylene production in chemical industries.