(111c) Mechanistic Investigation into the Formation of Humins in Acid Catalyzed Liquid Phase Biomass Reactions.

Humins are carbonaceous, polymeric by-products formed during acid-catalyzed reactions of biomass-derived carbohydrates to furans such as HMF. Such by-products account for 10-50% carbon loss of the feed. Moreover, humins can deposit on the catalytic surface, clogging their pores, and deactivating the catalyst¹. Investigating humins formation is challenging due to their complex and unknown molecular structure. Likewise, during the dehydration of biomass, a great variety of reaction occurs simultaneously, making it difficult to isolate humins formation reactions². IR spectra of humins formed from HMF show that the furan ring and the hydroxyl methyl group of HMF are present in the humins, but the carbonyl group is not. Thus, the absence of the carbonyl group in humins suggests that HMF is polymerized through its carbonyl group. According to this, aldol addition and condensation are proposed as the main reaction pathways to initiate humins formation^3,4. Hence, in this work, Density functional theory (DFT)-based calculations are performed to compute reaction pathways and activation barriers associated with the key steps in the acid catalyzed reactions leading to humins formation, as shown in Fig [1]. Our calculations suggest that aldol addition is the rate-determining step for all the pathways studied, in which the proton release and the bond formation between enol and HMF occurs in a concerted manner. The 4,5-enol is thermodynamically most favorable, because the Zundel structure, which is used to simulate the protonated water cluster, is well stabilized by the hydroxyl group close to the protonated carbonyl oxygen. Whereas, the proton released from the carbonyl group of 1,2-enol is the least favorable compared to other α-hydrogen transfer reactions of enols studied. These findings open new avenues to understand the formation of humins, since the quantification of the energetic barriers allows the modification of the reaction conditions to alter the formation of humins.