Production of sugar alcohols from real biomass by supported platinum catalyst
Graphical abstract
Introduction
Catalytic conversion of lignocellulosic biomass has attracted great interest for the production of renewable chemicals and fuels, since this abundant material has been largely wasted [1], [2]. Furthermore, the use of lignocellulose in chemical industry does not compete with food production, which is contrastive to current biorefinery using starch and molasses. Lignocellulose consists of cellulose, hemicellulose, and lignin, in which cellulose is a polymer of glucose and hemicellulose is a copolymer of various C5 and C6 sugars. Thus, the hydrolytic hydrogenation of the sugar polymers produces hexitols and pentitols (Fig. 1), and these sugar alcohols are practically used as precursors to plastics, surfactants, and medicines as well as low-calorie and non-cariogenic sweeteners [2]. The annual productions of sorbitol and xylitol have already been 6.5 × 105 and 2–4 × 104 tons per year, respectively, although the current feedstock of sorbitol is starch. Hence, the hydrolytic hydrogenation of lignocellulosic biomass is an attractive issue for the next-generation biorefinery.
Cellulose conversion by heterogeneous catalysts has been intensively studied in the last several years [2], and it has been known that pure cellulose can be converted into sugar alcohols in good yields in one-pot by various supported metal catalysts under H2 pressure [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. In the mechanism of this reaction, it has been found that the hydrolysis step is promoted by some supported metal catalysts [6], [7] in addition to effect of hot-compressed water [28], and the hydrogenation is catalyzed by transition metals. In contrast, only a few reports have addressed the one-pot conversion of raw or less-treated biomass; arabitol [29] and xylitol [30], [31] have been synthesized from beet fiber and bleached birch kraft pulp, respectively. Furthermore, influence of various ingredients such as lignin and ionic compounds on this catalytic reaction has not yet been evaluated. Both acid and reduction catalysis can be affected by these components, and hydrolysis rate of cellulose is indeed declined by sulfate ions [32]. In this study, we conducted the conversion of real biomass and crystalline cellulose by a supported Pt catalyst (Pt/BP2000) to evaluate the effects of lignin and salts. Pt/BP2000 is one of the most active and durable catalysts for the conversion of pure cellulose [7]. Silver grass was mainly used as a substrate because this plant survives in cold climate unsuitable to food crops. Furthermore, silver grass moves nutrients such as K and P from the plant body to rhizomes before winter [33], and hence loss of the elements in soil by collecting the plant for biorefinery is small.
Section snippets
Biomass materials
Silver grass-1 is silver grass (Miscanthus sinensis) collected in summer in Kochi, and silver grass-2 is that mowed in winter in Nakashibetsu (Hokkaido). Other biomass used in this study was Amur silver grass (Miscanthus sacchariflorus) sampled in Nakashibetsu (Hokkaido) and wheat straw (Triticum aestivum) in Memuro (Hokkaido). These samples were coarsely ground using a cutter mill with a 3 mm screen (Horai MAC-0.75kW). Microcrystalline cellulose (Avicel, 1.02331.0500) was purchased from Merck.
Pretreatment of biomass
Analysis of treated silver grasses
Elemental compositions of silver grasses were analyzed by EDX as shown in Table 1. Si of 3.7 wt% was detected in silver grass-1 as gramineous plants use silica for their frameworks (entry 1) [36]. Other ingredients were K (1.0 wt%), Ca (0.14), P (0.13), S (0.09), Fe (0.03), and Cl (0.02). K, P, and Cl were almost completely removed by boiling in water, and the amounts of Si and S decreased by half (silver grass-1W, entry 2). However, the contents of Ca and Fe were unchanged. The Soxhlet
Conclusions
The direct conversion of raw silver grass by a carbon-supported Pt catalyst under H2 pressure produced small amounts of sorbitol (2.8 wt%), xylitol (7.3 wt%), and other sugar alcohols. It has been suggested that lignin reduces the reactivity of cellulose, as lignin presents together with cellulose and both compounds are insoluble in water. Moreover, even weak bases drastically changes the main products from sugar alcohols to by-products such as EG and PG. The major effect of bases is the
Acknowledgements
We would like to thank Prof. Toshihiko Yamada for supplying silver grass-1. This work was supported by a Grant-in-Aid for Scientific Research (KAKENHI, 20226016) and JST-ALCA.
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