One of the more innovative low-temperature, low-pressure, thermochem­ical techniques of directly liquefying biomass in water involves the use of 57 wt % aqueous hydriodic acid (HI), the azeotrope boiling at 127°C (Douglas and Sabade, 1985). When treated at 127°C with the azeotrope in a stoichiomet­ric excess of 1.6 to 3.8 of the amount required for complete reduction, cellulose is rapidly hydrolyzed and converted to hydrocarbon-like molecules. The yields reach 60 to 70% at reaction times as short as 0.5 min. The laboratory data are consistent with chemistry in which HI acts to form alkyl iodide intermediates that are then converted to hydrocarbons and molecular iodine by further reaction with HI. The stoichiometry developed from the experimental data with cellulose is

C6H10O5 + 8.28HI -> C6H912I018O0,40 + 4.52H20 + 4.08I2.

Products corresponding to 50% deoxygenation in 1 min, 75% in 30 min, and 92% in 24 h are obtained; charcoal is not formed. Up to 98% of the HI reacted appears as molecular iodine. Ether extraction yields a material that has H: C and I: C ratios of 1.52 and 0.03, and yet there is a 90% reduction in the О: C ratio.

Hydriodic acid is a powerful reducing agent that can even be employed for conversion of benzene to cyclohexane. It is also well known that alcohols can be converted to alkyl iodides on reaction with HI and that alkyl iodides react with HI to form hydrocarbons. Since the experimental data obtained with cellulose shows that most of the HI is ultimately converted to molecular iodine, a cyclic process can be conceptualized in which HI is regenerated. One scheme might use hydrogen sulfide to regenerate HI. Another might use hydrogen. Thus,

ROH 4- HI —» RI 4- H20 RI 4- HI —> RH 4- I2 (I2 4- H2S -> 2HI 4- S)

(I2 + H2 ^ 2HI).

It appears that further research on HI chemistry could lead to processes for direct conversion of biomass to hydrocarbons without the economic penalty associated with operation at high pressure and temperature. The key to the value of such developments resides in the ability to recycle HI. Note that loss of only a small amount of the HI reacted can make the process quite uneconomical, so if it is developed to the point of commercial use, iodine recoveries would have to be substantially improved.