Catalytic Performance of Food Additives Alum, Flocculating Agent, Al(SO4)(3), AlCl3, and Other Lewis Acids in Microwave Solvolysis of Hardwoods and Recalcitrant Softwood for Biorefinery

The development of a novel pretreatment system using catalysts with high safety and low cost is pivotal to establish lignocellulosic biorefinery. We evaluated 16 Lewis acid catalysts for microwave solvolysis pretreatment to enhance enzymatic saccharification of woody biomass and found that very cheap and safe food additives, alum, are effective for the pretreatment, giving high sugar yield comparable to that of AlCl3, a strong Lewis acid catalyst effective both for softwood and hardwood. In microwave solvolysis of Japanese cedar, Paraserianthes falcataria and Eucalyptus globulus using alum in ethylene glycol/water (9/1, w/w), the maximum sugar yields after enzymatic saccharification reached 47.8, 51.0, and 59.7% based on the weight of each wood. The same reactions in glycol/water (9/1, w/w) gave the sugar yield, 34.5, 54.1, and 58.5%, indicating differential reactivity of the Lewis acid/solvent system depending on wood species. We found that efficiency and selectivity of pretreatment with a flocculating and astringent agent, Al-2(SO4)(3), was promoted by microwave. Reaction of Japanese beech wood with Al-2(SO4)(3) in 50% aqueous 1-propanol by microwave and conventional heating revealed that microwave irradiation suppressed excessive degradation of carbohydrates into furfural derivatives. Saccharification of the wood pretreated by microwave with 8 and 1 filter paper units of cellulolytic enzymes gave sugar yields per wood of 50.3 and 43.9%, whereas the same reaction in an autoclave resulted in 48.9 and 34.7% yields, demonstrating that microwave irradiation accelerated the saccharification and the effect was remarkable with a lower enzyme dosage.