Catalytic Transformations of Carbohydrates - American Chemical

Chapter 5

Catalytic Transformations of Carbohydrates

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: January 12, 2006 | doi: 10.1021/bk-2006-0921.ch005

Pierre Gallezot, Michèle Besson, Laurent Djakovitch, Alain Perrard, Catherine Pinel, and Alexander Sorokin Institut de Recherches sur la Catalyse-CNRS, 69626 Villeurbanne, France

Catalytic processes for converting starch, starch derivatives and glycerol to valuables chemicals or polymeric materials are considered. A first approach was aimed at using molecularly pure feedstocks such as glucose or glycerol to convert them by selective catalytic reactions to specialties or fine chemicals. A second approach consisted of converting native starch via one step catalytic processes to a mixture of products fulfilling required physicochemical specifications to be incorporated in the formulation of end-products such as paint, paper or cosmetics. A l l the catalytic routes considered follow the principles of green chemistry.

Introduction Biosynthesis in plants and trees using sun radiation, atmospheric carbon dioxide, water, and soil nutrients produces huge amounts of biomass estimated up to 200 Gt/y, a figure to be compared to 7 Gt/y of extracted fossil fuels. Increasing use of biomass for energy, chemicals and material supply is one of the key issues of sustainable development because bio-based resources are renewable and C O neutral unlike fossil fuels. Presently, only 7% of the annual biomass is harvested for food, feed and non-food applications. Food and feed will remain priority number one, but improved agricultural techniques and genetic modification of crops will increase yields substantially. Renewables dedicated to non-food applications could come from specialized crops or 2

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© 2006 American Chemical Society

In Feedstocks for the Future; Bozell, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.


53 forestry products giving much higher dry matter per cultivated area and a more reproducible and specific content of useful molecules for chemical applications. Moreover, large streams of crop and forestry wastes can be subject to upgrading. Most of the non-food applications of bio-based resources fall into four categories: (i) traditional uses in timber, paper, fiber, rubber, fragrance industries, etc., (ii) thermal power generation (bio-power) by direct combustion or after catalytic or fermentative gasification, (iii) biofuels, such as ethanol produced by fermentation of carbohydrates, bio-diesel by transesterification of vegetable oils, and hydrogen by steam reforming/WGS of biomass, (iv) bioproducts, i . e., chemicals or materials produced by chemo-catalytic and/or enzymatic conversion of carbohydrates, triglycerides, and terpenes. According to the US roadmap for biomass technologies-2020 vision goals (7), bio-power will meet 5% of the total industrial and electric generator energy, bio-fuels 10% of the transportation fuels, and bio-based chemicals will attain 18% of the US market. The economy and ecology of the various processes to produce bio-fuels (hydrogen, ethanol, biodiesel) should be precisely evaluated for each particular situation. In contrast, producing chemicals from renewable feedstocks generally represent a more sound and sustainable approach. The molecules extracted from bio-based resources already contain functional groups so that the synthesis of chemicals generally requires a lower number of steps than from alkanes. Synthesis could be achieved by alternative processing routes adapted to biobased feedstocks combining catalytic and enzymatic steps, rather than by employing conventional flow sheets front hydrocarbons. Synthesis of polylactate (Dow-Cargill) and 1,3-propanediol (DuPont-Genencor) from carbohydrates are successful industrial examples of this approach. Ideally, biorefineries should produce chemicals in the first place and fuels as byproducts. A t any rate, research in chemistry, biochemistry and engineering is needed to decrease the cost of processing bio-based resources to produce cheaper chemicals. This article deals with various catalytic processes that were investigated during the last few years in our laboratory. Two strategies were pursued namely: (i) highly selective catalytic conversions of pure glucose and glycerol to pure compounds that can be used as building blocks for chemistry or incorporated in the formulation of end-products and (ii) chemical modifications by one-pot catalytic reaction of native starches extracted from various cereals to obtain mixtures of polysaccharides with hydrophilic or hydrophobic properties suitable to meet specification for direct incorporation in marketable end-products. Figure 1 shows the different carbohydrate transformations investigated.

Selective conversion of glucose and derivatives Hydrogénation of glucose In view of the large amounts of sorbitol 3 produced batchwise by hydrogénation of glucose 2 on Raney-nickel catalysts, continuous processes In Feedstocks for the Future; Bozell, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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ÇOOH

Figure 1. Catalytic conversion of starch and glucose

would be preferable and Raney-nickel would be advantageously replaced by ruthenium catalysts which are more active, selective and less prone to leaching. Hydrogénation of 40 wt% aqueous solutions of glucose were carried out in a trickle-bed reactor on Ru/C catalyst obtained by impregnation of Norit carbon extrudates (2). A 99.3 % yield of sorbitol was obtained even after 596 h on stream. No leaching of ruthenium was detected. The hydrogénation was also conducted on Ru-Pt/C bimetallic catalysts of different composition prepared by co-exchange of Pt and Ru amino cations (3). Interestingly, the activity passed through a maximum at 1470 mmol/h/g for the specific atomic composition Ru Pt 4. Pt-Ru catalysts were also more selective to sorbitol because the sorbitol epimerization to mannitol decreased. The contact time with the catalyst can be increased without loss of selectivity thus allowing operation at total Ru

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4


55 conversion of glucose and more than 99% selectivity over a large domain of liquid flow rates. The bimetallic catalysts loaded in the trickle-bed reactor gave a productivity of 2 tons/day/kg of sorbitol at 99.5% purity. Ru

Hydrogénation of arabinonic acid There is a great interest to convert C carbohydrates available in large supply from starch or sucrose into C5 and C polyols that are little present in biomass but find many applications in food and non-food products. Thus, glucose can be converted to arabitol 7 by an oxidative decarboxylation of glucose to arabinonic acid 6 followed by a hydrogénation step. The main pitfall is to avoid dehydroxylation reactions leading to deoxy-products not compatible with the purity specifications required for arabitol. Aqueous solutions (20 wt%) of arabinonic acid were hydrogenated on Ru catalysts in batch reactor (4). The selectivity was enhanced in the presence of small amounts of anthraquinone-2sulfonate (A2S) which decreased the formation of deoxy by-products. A2S acted as a permanent surface modifier since the catalyst was recycled with the same selectivity without further addition of A2S. The highest selectivity to arabitol was 98.9% at 98% conversion with a reaction rate of 73 mmol h" g at 80°C. 6


4

1

_ I

R u

Hydrogenolysis and dehydroxylation of sorbitol This study was aimed at converting starch into mixtures of polyols that could be used in the manufacture of polyesters, alkyd resins, and polyurethanes employed in paints, powder coatings, and construction materials. Deoxyhexitols consisting of C diols, triols, and tetrols 4 are suited to replace polyols derived from petrochemistry such as pentaerythritol. Sorbitol, which is easily derived from starch, was taken as starting feedstock for the hydrogenolysis studies on metal catalysts (J). To improve the selectivity of sorbitol hydrogenolysis towards deoxyhexitols, catalysts and reaction temperatures were optimised to favour the rupture of C-OH bonds (dehydroxylation reactions) rather than C - C bond rupture. Copper-based catalysts, which have a low activity for hydrogenolysis of C - C bonds, were employed to hydrogenolyse a 20 wt% aqueous sorbitol solution in the temperature range 180-240°C. Reactions carried out in the presence of a 33% CuO-65% ZnO catalyst at I80°C under H -pressure gave a 73% yield of C polyols, and more specifically, 63% of deoxyhexitols. In contrast, operating in the presence of palladium catalysts at 250°C under 80 bar hydrogen pressure cyclodehydration reactions of sorbitol and mannitol occurred with formation of cyclic ethers (isosorbide 5, 2,5-anhydromannitol, 2,5-anhydroiditol, and 1,4-anhydrosorbitol) (6). Up to 50% and 90% yield of isosorbide were obtained from sorbitol and mannitol, respectively. These 6

2

+

4


56 mixtures of polyols were effectively employed by ICI paints to synthesize alkyd resins and make decorative paints which performed comparably to the commercial ones.


Oxidation of glucose to gluconate Gluconic acid 8, used as a biodegradable chelating agent or as an intermediate in the food and pharmaceutical industry, is produced by enzymatic oxidation of glucose. A n alternative route employing oxidation with air in the presence of palladium catalysts, was investigated (7). Unpromoted palladium catalysts were active in glucose oxidation, but the rate of reaction was low because of the over-oxidation of Pd-surface, and side oxidation reactions decreased the selectivity. The beneficial effect of bismuth on the activity and selectivity was clearly demonstrated with Pd-Bi/C catalysts of homogeneous size and composition (5 wt% Pd, Bi/Pd = 0.1) prepared by deposition of bismuth on the surface of 1-2 nm palladium particles. The rate of glucose oxidation to gluconate was 20 times higher on Pd-Bi/C (Bi/Pd = 0.1) than on Pd/C catalyst, and the selectivity at near total conversion was high on the fresh and recycled catalysts (Table 1). These results were interpreted in terms of bismuth acting as a co-catalyst protecting palladium from over-oxidation because of its stronger affinity for oxygen. This oxidation reaction is a nice example of green chemistry (one pot catalytic conversion of renewables, mild conditions, water as solvent, air as

Table 1 Product distribution in glucose oxidation Yield/mol% Catalysf (run) PdBi/C PdBi/C PdBi/C PdBi/C PdBi/C Pd/C

(%)

8

9

12

13

Selectivity (%)

99.6 99.7 99.8 99.9 99.9 82.6

99.4 98.9 98.5 98.5 99.1 78.1

Catalytic Transformations of Carbohydrates - American Chemical

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