Research On Heterogeneous Hydrolysis Of Cellulose With Carbon-based Solid Acid Prepared From Cellulose, Lignin

Lignocellulose is the most abundant renewable resource on earth, and its main components are cellulose, lignin and hemicellulose. It has been paid more attention to cellulose hydrolysis to produce glucose, and further to produce liquid fuels and industrial chemicals. Importantly, it achieves the multiple use of renewable resource that using carbon-based solid acid(CSA) prepared from lignocellulose to catalyze the cellulose hydrolysis, which has become a hot topic in recent years. However, it is found from the published literatures that the choice of lignocellulosic feedstock is blind and the study of structure-function relationship is simple. The lignocellulosic feedstocks have different structure due to the different component contents, and the structure of raw materials has a significant influence on structure and performance of catalyst. The research defect will inevitably limit the further development and application of biomass carbon-based solid acid(BCSA).In view of this, the CSAs were prepared from separate carbonization and mixed carbonization of cellulose and lignin, both which have the higher content and distinct structure in lignocellulose, and then were applied to cellulose hydrolysis. The pyrolysis process of raw materials and structure of carbon precursors(CPs) and CSAs were characterized by TG-DTG, XRD, FT-IR and XPS. The influence of preparation conditions, relative content of cellulose and lignin, heating rate and pre-oxidation of lignin on structure and performance of CSAs was investigated. The main results are concluded as follows:1. The cellulose-based solid acid(CCSA), lignin-based solid acid(LCSA) and mixed-based solid acid(MCSA) all are polycyclic aromatic carbon structure bearing -SO3H,-COOH and phenolic -OH, and contain some alkyl side chains(-CH3) and bridge linkage(-O-,-C=O, etc).2. Compared to LCSA, CCS A contains more aliphatic side chains and bridge linkage, the aromatic carbon sheets are more flexible and disordered, and the densities of -SO3H,-COOH and phenolic -OH are generally higher than LCSA. The adsorption capacity of CSA is mainly determined by the content of phenolic -OH and the structure of aromatic carbon sheets. The hydrolytic activity of CCSA is higher than LCSA under the synergistic effect of -SO3H group and adsorption capacity of catalyst.3. The carbonization temperature has a considerable effect on structure and performance of catalyst. With the increase of temperature, the structure of catalyst becomes more regular and orderly, the acidic groups density decreases, and the adsorption capacity and hydrolytic activity of catalyst have the highest point. Due to the different structure of cellulose and lignin, the best carbonization temperatures of CCS A and LCSA are different and are 410℃ and 380℃, respectively. The carbonization time and sulfonation time only have much effect on -COOH and phenolic -OH density, and a catalyst with higher activity and stability can be prepared at carbonization time of 6 h and sulfonation time of 1 h.4. With the increase of cellulose proportion, the co-pyrolysis of cellulose and lignin changes from mutual inhibition to mutual promotion, and it has a turning point at the cellulose proportion of 50%. This interaction makes the aromatic carbon sheets of MCSA arranged more disorderly, which reduces the steric hindrance, and significantly improves the adsorption capacity and hydrolytic activity of MCSA. The adsorption capacity and hydrolytic activity of MCSA are highest at the cellulose proportion of 50%, and the hydrolytic activity is almost comparable to CCSA.5. With the decrease of heating rate, the polymerization degree of LCSA increases, the sulfonated reactivity of LCP and the stability of -SO3H are enhanced, and so the adsorption capacity and hydrolytic activity of LCSA are improved. The co-pyrolysis interaction of cellulose and lignin is enhanced with the decrease of heating rate, which results in more obvious structural change of MCSA under different cellulose proportions. Importantly, the adsorption capacity and hydrolytic activity of MCSA are higher than CCSA and LCSA, while they are still highest at the cellulose proportion of 50%.6. It has a significant effect of oxidating lignin on the structure of LCSA. The arrangement of aromatic carbon sheets is more flexible and disordered. The sulfonated reactivity of LCP, and the adsorption capacity and hydrolytic activity of LCSA are improved immensely. Moreover, lignin need moderate oxidation, and the oxidation degree is most appropriate at 40℃.7. The alkyl side chains are the source of -COOH and favor the sulfonation of CP. However, the excessive side chains on CSA increase the steric hindrance and create the hydrophobic micro-environment, which is not conducive to adsorption and hydrolysis of cellulose.8. The arrangement of aromatic carbon sheets is more flexible and disordered with larger interlayer spacing, more bridge linkage and appropriate carbon sheet size, which can reduce the steric hindrance, increase the sulfonated reactivity of CPs, enhance the stability of -SO3H, and significantly improve the adsorption capacity and hydrolytic activity of CSAs.