Catalytic Depolymerization of Lignin to Value-added Chemicals in Supercritical Ethanol

Abstract

Finding alternative resources is inevitable as the conventional fossil fuels are rapidly depleted due to the increasing demand. In this respect, biomass is the most abundant renewable carbon resources so that it is considered as one of the promising alternative chemicals. Especially, lignocellulosic biomass, the second-generation biomass does not have a food vs. fuel issue like the first-generation biomass. Therefore, the supply of lignocellulosic biomass is relatively easy compared with the other resources. Especially, lignin, one of the constituents of lignocellulosic biomass, is an amorphous co-polymer consisting of various aromatic compounds, cross-linked by C-C and C-O bonds. Therefore, it has considerable potential to be used as value-added chemicals. Meanwhile, the amount of C-O bonds in lignin such as β-O-4 (46 ~ 60 %), α-O-4 (6 ~8 %) and 4-O-5 (3.5 ~ 6.5 %) is much more than that of C-C bonds in lignin such as β-β (7 %), β-5 (6 ~ 12 %), β-1 (4.5 ~11 %) and 5-5 (2 ~3 %). Hence, the selective cleavage of C-O bonds in lignin is considered as an essential process to depolymerize lignin into value-added chemicals. For the decades, various thermochemical processes such as pyrolysis, gasification, catalytic cracking and solvolysis have been applied for depolymerization lignin in order to get the high yield of value-added chemicals. However, the recalcitrant structures of lignin make it hard to be converted into value-added chemicals. Meanwhile, solvolysis has gained attention as one of the promising techniques because yield of products is relatively high and the better quality of products can be obtained by using solvolysis compared with the others. Especially, solvolysis by using alcohol solvents such as methanol or ethanol receives more attention because in-situ hydrogen is generated from supercritical alcohol, which is essential to cleave C-O bonds. Also, the combination of catalyst and supercritical ethanol is recently applied to the depolymerization of lignin since the catalyst is able to selectively cleave C-O bonds. In this work, catalytic conversion in supercritical ethanol was conducted to explore i) reaction pathway of benzyl phenyl ether, containing α-O-4 bonds ii) depolymerization of real lignin sources (concentrated strong acid hydrolysis lignin

CSAHL, Protobind lignin

PL) without supplying external hydrogen. At first, model compound reaction was conducted by using benzyl phenyl ether (BPE, containing α-O-4 bonds) in supercritical ethanol. The yield of phenol and toluene over carbon-supported catalysts was two times higher than that of phenol and toluene over Al2O3-supported catalysts. In addition, Ru catalysts demonstrated the maximum yield of phenol and toluene of 96.4 % among the three different metal catalysts (Ru, Pt and Ni). It was ascribed to the small particle size (about 1.5 nm) with highly dispersed phases. Meanwhile, both the amount of generated H2 and the amount of alkylated phenols were the highest in supercritical ethanol over 5 wt.% Pt/C among the catalysts, implying that the catalyst promoted alkylation more abundantly. Secondly, the concentrated strong acid hydrolysis lignin (CSAHL) was used for depolymerization lignin in supercritical ethanol over various catalysts. In case of MO(30)MgAlOy (M=Co, Ni, and Cu) catalysts, CuO(30)MgAlOy revealed the highest yield of monoaromatic compounds of 18.4 wt.% because the amount of NH3 desorbed from the catalyst was the highest among the catalysts. It can be concluded that acid sites is a critical factor for depolymerization of lignin to produce monoaromatic compounds. Meanwhile, as the amount of Cu loading was changed from 10 wt.% to 40 wt.% in CuO(X)MgAlOy, the trend of the yield of monoaromatic compounds showed a volcano shape. Thus, the maximum yield of monoaromatic compounds displayed in CuO(30)MgAlOy, resulting from the highest number of acid sites. When the amount of Cu loading exceeded over 30 wt.%, the number of acid sites decreased, leading to the decrement of the yield of monoaromatic compounds. The depolymerization of Protobind lignin was conducted in supercritical ethanol over various parameters based on ZSM-5 zeolites such as types of transition metals (Co, Ni and Cu), the amount of Cu loading (5 wt.%, 10 wt.% and 30 wt.%) and the Si/Al2 ratio (30, 50, 80 and 200). As a result, 10 wt.% Cu/ZSM-5(30) showed the highest yield of monoaromatic compounds of 98.2 wt.% due to the highest acid density (3.2 mmol/m2). As the acid density increased, the yield of monoaromatic compounds increased as well. Thus, the linear correlation between the acid density and the yield of monoaromatic compounds was confirmed. On the basis of the results, it can be summarized that the acid density played a key role in depolymerizing Protobind lignin to convert monoaromatic compounds. Meanwhile, through HSQC NMR analysis, it was identified that depolymerization of Protobind lignin was conducted via hydrogenolysis. Moreover, HSQC NMR signals corresponding to ethylated products were observed with weak intensity, which is consistent with the product analysis. It was also found that the addition of Cu metal plays a promotional role in enhancing the lignin depolymerization.