Article pubs.acs.org/IECR
Effect of Methanol on Formation of Levulinates from Cellulosic Biomass Shimin Kang and Jian Yu* Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States S Supporting Information *
ABSTRACT: Levulinates (methyl levulinate and levulinic acid) were produced from cellulose and eucalyptus wood chips in water and/or methanol solution under acid catalysis. In water, only levulinic acid was formed, and its yield was 49−54 mol % of the glucan in biomass or about half of the glucan was wasted forming byproducts including insoluble humins. In a 90 vol % methanol solution, methyl levulinate was the main product, and the yield of levulinates reached 66 mol %. Protecting glucose from humins formation by quick conversion into methyl glucosides was found to be the key to a high yield of levulinates. Methanol had little effect on formation of humins and levulinates from 5-hydroxymethylfurfural. In addition to the main reactions with glucose, 5-hydroxymethylfurfural, and levulinic acid, methanol was also consumed in other reactions that caused a substantial loss of methanol and presented a technical challenge to using methanol in biomass refining.
1. INTRODUCTION Green chemicals and fuel additives derived from renewable biomass have attracted much attention with the growing concerns over climate change. Levulinic acid (LA) is a platform chemical derived from hexoses under acid catalysis1 and has been considered as one of the top 12 value-added chemicals from biomass.2 Conversion of cellulosic biomass into LA involves consecutive reactions including hydrolysis of cellulose into glucose, dehydration of glucose into 5-hydroxylmethylfurfural (HMF), and rehydration of HMF to LA1,3,4 (see Reactions A, B, and C in Scheme 1). In these main reactions, glucose and HMF are the key intermediates, but they can also form a large amount of byproducts, especially insoluble humins in an acidic water solution.5,6 The dark-brownish humins solid not only reduces the yield of LA but also deposits on solid catalysts causing quick catalyst deactivation.7 In an acidic water solution, glucose can be dehydrated into a carbocation via protonation of the C1 hydroxyl group,8 and the carbocation reacts with other glucose to form oligomers and humins.9 HMF also has reactive hydroxyl and aldehyde groups that may lead to formation of humins.5,10,11 One possible way to control humins formation is to protect the active groups of glucose and HMF (e.g., the C1 hydroxyl group) from oligomerization and hence improve LA yield and catalyst lifetime. Water is probably the most environmentally friendly and inexpensive solvent for biomass hydrolysis and processing. Cosolvents (e.g., water/alcohols, water/acetone, and water/ dimethyl sulfoxide) are increasingly used in carbohydrate conversion for valuable chemicals such as HMF and LA.12−16 One study reports that in cosolvents of water and methanol much less humins is formed from glucose, and methyl levulinate (MLA) is the main product under acid catalysis.9,16 In the presence of a large amount of methanol, the main reaction generates methyl glucosides and methoxymethyl-2furaldehyde (MMF) as the key intermediates (see Reactions E, H, I, and J in Scheme 1),9,16−19 which may inhibit formation of byproducts or humins. The detailed reaction mechanisms, © 2015 American Chemical Society
however, are not clear yet. As a derivative of LA, MLA is also a platform chemical with numerous potential applications such as solvent, oxygenated additives in fuels, and flavoring and fragrance additives in various products.20−23 MLA can also be conveniently hydrolyzed into LA. They are collectively referred to as levulinates in this work. Little knowledge, however, is available on the effects of methanol on formation of levulinates and humins from cellulosic biomass in acidic solutions. It is interesting to know how effectively methanol can improve the yield of levulinates and reduce humins formation in comparison with water. It is also interesting to know to what extent methanol can be reused in biomass refining. In this work, cellulosic biomass including cellulose and eucalyptus wood were treated in acidic water or methanol solutions to form levulinates and humins, in comparison with glucose and HMF under the same reaction conditions. The yields of the main reaction products were compared under different reaction conditions. Finally, the challenge to using methanol as a solvent in biomass refining is discussed.
2. EXPERIMENTAL SECTION 2.1. Materials. Medium cellulose fiber (referred to as cellulose thereafter), D-(+)-glucose (99.5%), levulinic acid (LA, 98%), methyl levulinate (MLA, 98%), 5-(hydroxymethyl)furfural (HMF, 99%), methyl beta-D-glucoside (M-b-G, 99%), methyl alpha-D-glucoside (M-a-G, 99%), 5-methoxymethyl-2furaldehyde (MMF, 95%), levoglucosan (99%), and furfural (99%) were obtained from Sigma-Aldrich (MO, USA). Eucalyptus wood chips were provided by a local farmer (Maui, Hawaii) and grounded into small pieces ( cellulose> eucalyptus wood, which might also affect the LA yield. As shown in Figure 2A, an increase of the glucose load resulted in a slight decrease of LA yield and an increase of humins yield. It is interesting to notice that HMF at a low concentration generated little solid humins with a high LA yield (Figure 2B). For example, at a HMF concentration of 7 g/L, a high yield (93 mol %) of LA was obtained with a very low yield of solid humins (0.1 wt %). Since the HMF concentration during the cellulose reaction was quite low (
