August 18, 2014
Making benzene from methane may prevent potential shortages. The sudden availability of very low-cost natural gas from shale is prompting research into unconventional process technologies for making petrochemical building blocks. The abundance of gas has rejuvenated shuttered ammonia and methanol businesses in the United States. Old plants that were not dismantled in 2000–2008 are being put back into service, and new plants are under construction.
Likewise, existing naphtha-fed ethylene steam crackers are being retrofitted to handle low-cost ethane feedstocks, and a flurry of new ethane-only crackers is slated for startup in 2016–2018. Chevron Phillips Chemical, ExxonMobil, and Dow crackers have received their environmental permits and are proceeding toward construction. Whereas existing and new ethane-fed ethylene crackers will be cost-competitive, the downside is that smaller amounts of propylene, butadiene, and benzene will be coproduced, possibly leading to shortages of these products.
BASF has recognized this potential problem as an opportunity. In May 2014, it announced that it will build a plant to produce propylene from natural gas (methane). In this process, methane is first converted to methanol, which is then fed to a methanol-to-propylene (MTP) plant. Although this plant will be the first of its kind in the United States, several MTP plants are in operation or under construction in China. (The Chinese plants are based on methanol made from coal, not methane).
BASF also recognizes that converting methane to benzene could also be of significant value. F. Kiesslich and co-inventors disclose catalysts and operating conditions for the dehydroaromatization of methane to benzene with good selectivity. The catalysts are based on zeolites such as ZSM-5 and MCM-22 that are impregnated with molybdenum and other metals, mainly copper and nickel.
In the patent’s examples, methane conversions range from 4.0 to 10.3% with selectivity to benzene from 50 to 73%. On the downside, coke selectivity is moderate to high (3–27%). The inventors show, however, that the catalyst can be regenerated repeatedly by treating it with hydrogen. The regenerated catalyst gave methane conversions of 9.8–10.3% and benzene selectivities of 72–73%. (BASF SE [Ludwigshafen, Germany]. US Patent 8,742,189, June 3, 2014; Jeffrey S. Plotkin)