Microwave-assisted pyrolysis of biomass: Catalysts to improve product selectivity

Abstract

This study was intended to evaluate the effects of catalysts on product selectivity of microwave-assisted pyrolysis of corn stover and aspen wood. Metal oxides, salts, and acids including K 2Cr 2O 7, Al 2O 3, KAc, H 3BO 3, Na 2HPO 4, MgCl 2, AlCl 3, CoCl 2, and ZnCl 2 were pre-mixed with corn stover or aspen wood pellets prior to pyrolysis using microwave heating. The thermal process produced three product fractions, namely bio-oil, gas, and charcoal. The effects of the catalysts on the fractional yields were studied. KAc, Al 2O 3, MgCl 2, H 3BO 3, and Na 2HPO 4 were found to increase the bio-oil yield by either suppressing charcoal yield or gas yield or both. These catalysts may function as a microwave absorbent to speed up heating or participate in so-called “ in situ upgrading” of pyrolytic vapors during the microwave-assisted pyrolysis of biomass. GC–MS analysis of the bio-oils found that chloride salts promoted a few reactions while suppressing most of the other reactions observed for the control samples. At 8 g MgCl 2/100 biomass level, the GC–MS total ion chromatograms of the bio-oils from the treated corn stover or aspen show only one major furfural peak accounting for about 80% of the area under the spectrum. We conclude that some catalysts improve bio-oil yields, and chloride salts in particular simplify the chemical compositions of the resultant bio-oils and therefore improve the product selectivity of the pyrolysis process.