Several situations, including when takeaway capacity is limited at remote oil-well sites, result in methane flaring. To avoid wasting that resource, several strategies have been explored to convert methane onsite into commercially useful liquid chemicals. One of the approaches is to convert methane into aromatic hydrocarbons, such as benzene, toluene and xylenes (BTX), along with hydrogen gas, using microwave-assisted methane dehydroaromatization (MDA).
A team of researchers from the National Energy Technology Laboratory (NETL; Morgantown, W.Va.; www.netl.doe.gov), led by Swarom Kanitkar, recently investigated the deactivation of a molybdenum-supported H-ZSM-5 zeolite catalyst in the MDA reaction under microwave-heated conditions compared to conventional heating. While traditional processes rely on indirect heat transfer from fossil-fuel combustion, microwave heating generates heat directly through the interaction between microwaves and the solid catalyst. This direct approach offers improved heating rates and efficiency, potentially leading to more cost-effective chemical production.
However, the benefits of the microwave heating are complicated by rapid deactivation. The work by NETL scientists improved understanding on why the catalyst deactivation was so fast when it was exposed to microwaves. This has helped identify ways to improve catalysts’ ability to absorb microwave energy and improve yields, while reducing coke formation and enabling lower reaction temperatures.
One set of experiments examined microwave irradiation of a silicon-carbide (SiC) monolith with the Mo/H-ZSM-5 catalyst packed into the channels, which has shown improved temperature distribution in prior research. The researchers found that the SiC monolith effectively heated the catalyst, leading to enhanced activity and energy efficiency, and that the deactivation was controlled by the overall electric-field distribution, local temperature gradients and coke formation during the reaction.
“Preliminary regeneration results suggest that the in-situ reduction of large amounts of Mo oxides by CH4 resulted in rapid coke formation that may irreversibly deactivate the catalyst due to hot-spot formation,” the researchers said.
The project suggests that “New activation methods are needed to reduce Mo prior to reaction to better control carbon formation rates and improve catalyst stability under microwaves.”