The goal of this project is to develop a liquid phase photocatalytic process for direct conversion of methane into methanol so that, when applied to flared gas at a well site, the process can generate methanol using a direct photocatalytic route. The goal will be accomplished by completing several research objectives that include:
Initially, the major focus of the project will be to develop and optimize the semiconductor catalyst for the effective conversion of photons into hydroxyl (•OH) radicals which support methane activation and a co-catalyst that selectively, and with high yield, converts the methyl radicals generated from methane activation into methanol and integrate these into a single bifunctional catalyst. Stanford will then shift to process development focusing on testing to identify optimal operating conditions, completing long-term testing at realistic commercial conditions, testing with simulated natural gas compositions and intermittent operation involving on/off and varying flow conditions.
Stanford University, Stanford, California
Susteon, Inc, Cary, North Carolina
Casale, SA, Lugano, Switzerland
The conversion of methane present in natural gas into a liquid or solid product that would enable flared, vented, or otherwise rejected gas to be monetized through conversion to transported, value-added products has the potential to produce a new revenue stream. This project will focus on a process to convert methane to methanol (CH3OH) as an attractive alternative to a wasted commodity. Currently, methanol is synthesized from methane in an indirect, inefficient two-step syngas process requiring high temperature (800°C) and pressure (60 bar), which needs support from a large balance of plant systems that effectively prohibits small-scale and remote application. Direct conversion of methane to methanol is highly desirable but, remains a fundamental challenge due to the large energy barriers required to activate the inert CH4 molecules as well as the higher relative reactivity of the C-H bond in methanol compared to those in methane. While liquid-phase methane conversion has been attempted at room temperature, methane solubility in water limits methane mass transfer to the catalyst surface, significantly lowering total methanol production rates below acceptable commercial rates. Due to these issues, direct methane to methanol conversion remains a fundamental challenge.
This project aims to overcome this challenge by developing novel photocatalytic activation of methane at a gas-water interface such that methanol can be formed at room temperature. Photons are used to excite hydroxyl (OH) radicals in aqueous media, which then excite methane to form methanol on a catalyst surface. As part of this project, Stanford will design and use novel high-throughput screening to identify the best photocatalyst to achieve the highest selectivity and methane conversion efficiency. Based on these values, Stanford will design and build a new photoreactor system that can be modular and scalable for direct methane to methanol conversion. So, when applied to flared gas at a well site, the process can generate methanol using a direct photocatalytic route.
The project offers a potential mechanism to reduce quantities of flared gas at oil and gas production sites, where gas transport options are insufficient or do not exist, by converting the gas to energy-dense liquid products. This process, if successfully developed and deployed, will reduce the volume of natural gas being flared to the atmosphere, thereby having a positive effect on the environment.
The project started on October 1, 2020. Project activities were initiated on October 1, 2020 and an initial project kickoff meeting was held on October 23, 2020. The Technology Maturation plan was submitted on time. All milestones have been completed on time.
In the initial quarter of research, the project has developed and synthesized a series of photocatalysts containing TiO2 as semiconductors and different metal co-catalysts (Au-Pd, Ru, Rh etc.), which is the milestone of Task 2. The Stanford team also developed a multiplex fluorescence array for rapid measurement of •OH radicals on the as-synthesized catalysts. Stanford has investigated the effect of co-catalysts and reaction condition (H2O2 concentration), and we have successfully optimized the •OH radical production on the TiO2 by loading Au-Pd as co-catalysts.
Catalytic testing was conducted in a high pressure photoreactor that was constructed on-site. A proof-of-concept demonstration of the photocatalytic approach by using TiO2 was performed. The integration of Au-Pd on TiO2 improves methanol yield by 5 times and methane conversion by 1.5 times. Additionally, methanol selectivity of TiO2 is improved by more than 10 times after growing an oxide layer to form a composite oxide. Further integration of Au-Pd co-catalyst with this TiO2 composite oxide increases the methane conversion and methanol selectivity simultaneously. After preliminary reaction condition optimization, our current best performing catalysts Au-Pd/TiO2 composite oxides achieved 0.16% methane conversion and ~ 90% liquid product selectivity.