Elucidating the role of NiMoS-USY during the hydrotreatment of Kraft lignin

Abstract

Major hurdles in Kraft lignin valorization require selective cleavage of etheric and C–C linkages and subsequent stabilization of the fragments to suppress repolymerization reactions to yield higher monomeric fractions. In this regard, we report the development of efficient NiMo sulfides and ultra-stable Y zeolites for the reductive liquefaction and hydrodeoxygenation of Kraft lignin in a Parr autoclave reactor at 400 °C and 35 bar of H2 (@25 °C). Comparing the activity test without/with catalyst, it is revealed that NiMo sulfides over ultra-stable Y zeolites (silica/alumina = 30) achieved a significant reduction (∼50 %) of the re-polymerized solid residue fraction leading to a detectable liquid product yield of 30.5 wt% with a notable monocyclic and alkylbenzenes selectivity (∼61 wt%). A physical mixture counterpart, consisting of hydrothermally synthesized unsupported NiMoS and Y30, on the other hand, shows lower selectivity for such fractions but higher stabilization of the lignin fragments due to enhanced access to the active sites. Moreover, an extended reaction time with higher catalyst loading of the impregnated NiMoY30 facilitated a remarkable alkylbenzene (72 wt%) selectivity with an increased liquid yield of 38.9 wt% and a reduced solid residue of 16.4 wt%. The reason for the high yield and selectivity over NiMoY30, according to the catalyst characterization (H2-TPR, XPS, TEM) can be ascribed to enhanced stabilization of depolymerized fragments via H2-activation at a lower temperature and high hydrodeoxygenation ability. In addition, the better proximity of the acidic and deoxygenation sites in NiMoY30 was beneficial for suppressing the formation of polycyclic aromatics.