Abstract
Thermochemical processing of lignin ends up with a major problem which is the high yield of char remained from lignin conversion, causing low yields of desired products. The ReS2/Al2O3 catalyst, used in this work, exhibited a high char-suppressing potential and high hydrodeoxygenation efficiency in the reductive liquefaction of kraft lignin. Compared to NiMo/Al2O3, as a conventional sulfide catalyst, ReS2/Al2O3 showed significantly better catalytic performance with 72.4 % lower char yield, due to its high efficiency in stabilizing the lignin-depolymerized fragments. The remarkable catalytic performance of ReS2/Al2O3 is attributed to its high oxophilicity, the metal-like behavior of rhenium sulfide and sufficient acidity. The effects of reaction temperature and different catalyst supports (Al2O3, ZrO2 and desilicated HY zeolite) were also studied. In an alkali (NaOH)-assisted depolymerization of lignin, it was revealed that ReS2/Al2O3-to-NaOH (stabilization-to-depolymerization) ratio plays a crucial role in determining the reaction pathway toward either solid char residues or liquid monomeric products.
Highlights
Lignin, the most abundant renewable aromatic material on Earth, can be potentially exploited for the sustainable supply of fuels and chemicals which are currently derived from rapidly depleting and greenhouse gas emitting fossil resources [1,2,3,4,5]
ReS2/Al2O3 was a highly efficient catalyst for high-yield production of monocyclic hydrocarbons in the reductive liquefaction of a sulfurcontaining lignin due to: (i) high stabilizing efficiency via hydrogena tion of the free radicals of lignin-depolymerized fragments, resulting in significant suppression of char-forming condensation reactions; (ii) high hydrodeoxygenation activity leading to an efficient oxygen removal from lignin-derived phenolic compounds; (iii) resistance to sulfur poisoning
Compared to NiMo/Al2O3 as a conventional sulfide catalyst, ReS2/Al2O3 led to a significantly lower char yield and higher monocyclic product yield; in the reductive liquefaction of kraft lignin at 340 ◦C, the alumina-supported nickel-molybdenum and rhenium sulfide catalysts resulted in char yields of 40.6 and 11.2 wt%, total monocyclic product yields of 4.6 and 21.5 wt%, and monocyclic hydrocarbon yields of 4.6 and 7.3 wt%, respectively
Summary
The most abundant renewable aromatic material on Earth, can be potentially exploited for the sustainable supply of fuels and chemicals which are currently derived from rapidly depleting and greenhouse gas emitting fossil resources [1,2,3,4,5]. Lignin, constituting 15–30 % of the biomass weight and up to 40 % of the biomass energy, is an amorphous and highly cross-linked macromolecule composed of the three primary phenylpropane monomers of p-coumaryl, coniferyl and sinapyl alcohols [6,7,8,9,10] It can be processed via various depolymeriza tion techniques for the production of high-value platform chemicals such as phenolics, aromatics and alkanes [11]. There is a major problem in the processing of lignin, which is the high formation of char solid residues remaining from the conversion of lignin, causing a low yield of desired target products [19,20,21,22,23,24] This happens since lignin fragments from degradation of lignin polymer are highly reactive and undergo rapid repolymerization to form large amounts of char [25]. This necessitates the development of the catalytic systems which can effectively suppress char-forming conden sation reactions
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