The isosynthesis reaction converts syngas to isobutene and isobutane under relatively extreme reaction conditions using a thorium or zirconium-based catalyst.
Syngas is a gaseous mixture consisting of mostly of carbon monoxide (CO) and hydrogen (H2). Syngas is produced from the gasification of feedstocks at temperatures in excess of 1100°F under conditions where the amount of oxygen (from air, pure oxygen, or steam) is less than what is needed for complete combustion. Syngas can be produced from many hydrocarbon feedstocks including natural gas, petroleum products, coal, and biomass.
Isosynthesis reactions involve 2 chain growth mechanisms - a step-wise CO insertion reaction and a condensation reaction involving surface adsorbed oxygenates. Isosynthesis reactions generate large quantities of CO2.
The first catalysts used were thorium based (ThO2) which are good alcohol dehydration catalysts and are the most active isosynthesis catalysts. They have long lifetimes because they can be regenerated. Thoria catalysts are not poisoned by sulfur impurities in the syngas and have high resistance to other poisons as well. However, they are radioactive which precludes their commercial use.
Zirconium-based catalysts also have high activity for isosynthesis (although lower than thorium-based catalysts) and with much higher selectivity to isobutene compared to the thorium-based catalysts. Various promoters (Al, Zn, Cr and alkali metals) have been investigated to improve the activity and selectivity of the catalysts. The selectivity of the isosynthesis reaction depends on the nature of the active catalyst sites and the balance between acidic and basic sites on the catalyst. Enhancing the number of acidic sites increases activity and selectivity for linear C4 hydrocarbons. Increasing the number of basic sites increases the yield of iso-C4 hydrocarbons
The isosynthesis process is not currently commercial. Laboratory studies have been conducted in gas-solid fixed bed reactors, and slurry reactors are also being investigated. Selectivities to C4 products are reported to be higher in slurry reactors compared to fixed bed reactors.