A mechanistic study of formation and growth of humins during acid catalyzed hydrolysis of cellulosic sugars
Patil, Sushil K. R.
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Acid catalyzed hydrolysis of biomass is an important process which can produce platform chemicals like levulinic acid and formic acid. Solid particles called humins are formed during this process which reduces the selectivity of feed towards desired products. It is critical to understand how these humins are forming to understand the whole process. Moreover, dilute acid treatment of biomass is widely used as a pretreatment method for cellulose and glucose extraction. A mechanism for the formation of humins during the process of acid catalyzed hydrolysis of sugars (glucose, fructose and HMF) is proposed based on aldol addition/condensation reactions. Selectivity data, FTIR spectroscopy, microscopy studies and DFT calculations on the humins support the proposed theory. According to the theory proposed, humins are produced from glucose or fructose via HMF through 2,5 dioxo-6-hydroxyhexanal (DHH). Results from cellobiose, and by extension cellulose, indicate that formation of humins begins only after DHH appears in the system i.e. the route to the formation of humins is through HMF. Based on the results it is unlikely that there is a direct pathway to humins from glucose or fructose. The idealized polymeric structure of humins, based on this theory, has been proposed. It explains all the FTIR spectral features observed in humins and the variation in their relative intensities in HMF humins, glucose humins and fructose humins. The FTIR spectra indicate the presence of furan, ketone, C=C, carboxyl and C-O groups in humins obtained from all three sources. The proposed theory has been further tested by adding benzaldehyde. Benzaldehyde is an aldehyde similar to HMF and cannot polymerize by itself through aldol reactions. The incorporation of benzaldehyde into the structure of humins confirms that aldol addition/condensation reactions are responsible for the formation of humins. This not only supports the theory but also paves way for a method by which humins can be functionalized in-situ. It has been shown that humins recovered after the reaction can also be functionalized in the same way. Humins were further studied for their ability to disperse in various solvents. It has been found that polar solvents such as DMSO (dimethyl sulfoxide), DMF (dimethylformamide), acetone, pyridine, BMImCl (1-Butyl-3-methylimidazolium chloride), NaOH (sodium hydroxide) and NH4OH (ammonium hydroxide) partly disperse the humins at room temperature. NaOH and NH4OH were found to disperse all the humins recovered from acid hydrolysis of glucose or fructose at 135°C at autogenous pressure. These solvents can be used to functionalize humins to turn them into useful products. Humins are found to be in the dispersed state when the reactions were performed using formic acid as solvent and sulfuric acid as catalyst. Characterization of these humins indicated that C-O bonds are responsible for the growth/agglomeration of humins. When water is used as co-solvent along with formic acid uniform carbon spheres were produced. This is one of the methods by which uniform carbon spheres can be obtained along with platform chemicals.