ReviewGlycerol hydrogenolysis into useful C3 chemicals
Graphical abstract
Introduction
Biomass is biological material derived from living organisms, and it represents abundant carbon-neutral renewable resources for the production of bioenergy and biochemicals, which can replace the energy and the materials produced from fossil resources. In recent years, applications of the biomass resources have attracted much attention from the view point of CO2 emission. Shifting society’s dependence away from petroleum to renewable biomass resources is essential for the development of sustainable industrial societies and efficient management of greenhouse gas emissions [1]. The bio-derived chemicals are mainly produced by two types of main components of sugars: hexoses and pentoses, which can be obtained from starch, cellulose and hemicellulose [2]. Bioenergy usually means biofuels, which mainly consist of bioethanol and biodiesel, and the production of those fuels has been increasing rapidly in the last decade [3]. The bioethanol production depends heavily on the fermentation of starch obtained from corn and sugar cane [4].
Glycerol is the smallest polyol available from triglycerides, vegetable oil and animal fat, which constitute approximately 10 wt.% of total biomass [5]. Biodiesel is produced from triglycerides by transesterification with short chain alcohols through catalysis by alkali, and a huge amount of glycerol, ca. 10 wt.% of the overall biodiesel production, is generated as the by-product in the process [6]. Consequently, glycerol constitutes ca. 1 wt.% of total biomass. The production of biodiesel is 22.7 million metric tons in 2012, and it increases rapidly and is even forecasted to increase to 36.9 million metric tons in 2020 [7]. Glycerol is also produced as a by-product of ethanol production by fermentation of sugars. Although the extraction of glycerol from this residue is not economically feasible, the fermentation of sugar into ethanol is also a potential additional resource of glycerol [8]. In the cleavage processes of fatty acids, the purity of the crude glycerol is high and ca. 80 wt.% glycerol aqueous solutions can be obtained from most of the conventional processes of biodiesel production, but it also contains water, methanol, traces of fatty acids as well as various inorganic and organic impurities [9], [10]. Crude glycerol has to be purified by distillation prior to further use in most cases, whereas the cost of the distillation is high. Furthermore, although glycerol has been produced at a large quantity with a rapid growth, the market of glycerol is small and the price of glycerol is low [9]. As a consequence, the proportion of refined glycerol is actually steadily decreasing and the unrefined crude glycerol is generally disposed by burning, which must be considered as a waste of a potentially useful organic raw material [10]. Thus, new economical ways of using glycerol must be developed in order to substantially increase the demand and the price of crude glycerol, and also to ensure the sustainability of the biodiesel production. Glycerol can be a starting material for further chemical derivatization, and many useful intermediates and specialty chemicals can be produced by catalytic reactions [11], [12], [13].
The catalytic conversion of glycerol into useful chemicals are mainly performed through stream reforming, oxidation, dehydration, acetalization, esterification, etherification, carboxylation, and chlorination, which have been summarized in many review papers at different periods [2], [11], [14], [15], [16], [17]. Among the various ways for glycerol derivatization, the dehydration of glycerol into acrolein, the oxidation of glycerol into dihydroxyacetone and glyceric acid, and the hydrogenolysis of glycerol into 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) have been intensively investigated because of the wide and important use of the corresponding chemicals (Scheme 1). Dehydration of glycerol into acrolein has been well summarized in some review papers [7], [10], [18], [19], [20], [21]. Although high acrolein selectivity has been obtained over some solid acid catalysts in many reports, the catalysts are deactivated rapidly in most cases and the development of solid catalysts for a stable acrolein formation from glycerol is still required. Because acrolein is mainly used for acrylic acid formation, the direct production of acrylic acid from glycerol is also attractive. In recent 5 years, direct synthesis of acrylic acid from glycerol has been extensively reported [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. In the processes, acrolein is generated as an intermediate and it is further oxidized into acrylic acid under either O2 or air flow conditions.
The oxidation of glycerol into dihydroxyacetone and glyceric acid has been reviewed [38], [39]. Supported precious metals, such as Pt, Pd and Au, are generally used as catalysts for glycerol oxidation into both dihydroxyacetone and glyceric acid. The features of the supported precious metals, such as the particle size and the acid-base conditions, significantly affect the selectivity to the oxidized products. In the latest review paper [39], a detailed summary has been reported on the glycerol oxidation into glyceric acid over Au-based catalysts, which show more advantages than the traditional Pt- and Pd-based catalysts. Lactic acid is another attractive chemical which can be derived via oxidation [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55]. The catalyst system for glycerol oxidation into lactic acid is similar to that into glyceric acid, whereas the reaction conditions are much different: the formation of lactic acid requires much higher reaction temperatures and a basic media is indispensable in most cases. The glycerol oxidation is expected to be applied for further studies and even industrial applications.
In this review, we focused on the glycerol hydrogenolysis into useful chemicals, which contain 1,2-PDO, 1,3-PDO, allyl alcohol, 1-propanol, and propylene. All these chemicals in the glycerol hydrogenolysis are commercially produced from fossil resources now, and the technologies of catalytic transformation make it possible to produce these chemicals from a renewable resource such as glycerol. It is generally accepted that both 1,2-PDO and 1,3-PDO are produced via the dehydration of glycerol followed by hydrogenation, whereas different catalysts are reported to work effectively under different reaction conditions. In 2011, Dam and Hanefeld published a detailed review on glycerol dehydroxylation [5], and Nakagawa and Tomishige also published a review paper summarizing their works on glycerol hydrogenolysis into propanediols [6]. However, the reviews have focused mainly on the liquid-phase reactions, while vapor-phase reactions are less discussed. Although new achievements are summarized in review papers for the recent 5 years [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], the advantages and disadvantages of vapor- and liquid-phase reactions are less discussed. In addition, some important achievements have been reported in the past 5 years. Allyl alcohol is an attractive target chemical, which is also an intermediate in glycerol hydrogenolysis, but it is difficult to be produced selectively because it is easy to be further hydrogenated into 1-propanol under H2 flow conditions. Consequently, efforts have been made to produce allyl alcohol from glycerol by hydrogen-transfer reactions using either monoalcohols or formic acid as the H-donor molecule. Recently, glycerol multi-step hydrogenolysis into propanols and propylene has also attracted much attention. In this review, the new trends in the glycerol hydrogenolysis are also summarized and discussed.
Section snippets
1,2-Propanediol
1,2-PDO is a valuable chemical used widely in the synthesis of pharmaceuticals, polymers, agricultural adjuvants, plastics, and transportation fuel [66], [67], [68]. Depending on its purity, 1,2-PDO can be used as an antifreeze agent, a hydraulic fluid, and a solvent, and it has also usages for cosmetics and food applications [69]. 1,2-PDO is currently produced by the hydration of propylene oxide, which is produced through the selective oxidation of propylene [62]. Because propylene is produced
1,3-Propanediol
1,3-Propanediol (1,3-PDO) is commercially the most valuable product in the hydrogenolysis of glycerol. It is used in resins, engine coolants, dry-set mortars, water-based inks, but most of 1,3-PDO is used in the production of polypropylene terephthalate, which is a polyester synthesized from 1,3-PDO and terephthalic acid [5]. The market for 1,3-PDO is currently over 105 tons per year, and the methods for producing 1,3-PDO are hydroformylation of ethylene oxide followed by hydrogenation,
Monoalcohols and propylene
As described in the previous sections, a large number of efforts have been conducted for producing 1,2-PDO and 1,3-PDO through glycerol hydrogenolysis. On the other hand, the further hydrogenolysis products from 1,2-PDO and 1,3-PDO, such as allyl alcohol, propanols, and propylene, are also attractive and valuable. In the last 5 years, many studies dealing with glycerol hydrogenolysis into these chemicals have been reported. Scheme 8 summarizes the reaction routes of multi-step hydrogenolysis of
Concluding remarks and prospects
Glycerol is the smallest polyol readily available from biomass, and it is now produced as a large amount of waste in the biodiesel production process. Glycerol can be converted to various useful chemicals, and glycerol hydrogenolysis into C3 useful chemicals, such as 1,2-PDO, 1,3-PDO, allyl alcohol, propanols, and propylene, is particularly summarized and discussed in this review.
Glycerol conversion into 1,2-PDO is mostly investigated among the glycerol hydrogenolysis processes, and most of the
Acknowledgement
This research was partly supported by JST, Strategic International Collaborative Research Program, SICORP.
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