1. Heterogeneous catalysts for hydrolytic hydrogenation of woody biomass

Cellulose, the main ingredient of woody biomass, is the most abundant biomass, and because it is non-edible, it is required to make effective use of it as a renewable resource. We succeeded in synthesizing sorbitol by hydrolytic hydrogenation of cellulose using only solid catalyst (Figure 1). Isosorbide obtained by dehydrating sorbitol is used as a monomer for engineering plastics. However, isosorbide is currently synthesized with sulfuric acid as a catalyst. We are developing a solid catalyst to replace the sulfuric acid.

Figure 1. Hydrolytic hydrogenation of cellulose

We found that H-beta is active for dehydration of sorbitol to isosorbide (Figure 2). The catalytic activity showed a volcanic-type correlation with a Si/Al ratio of the zeolite, and H-beta with the Si/Al ratio of 75 was the most active. This result suggests that the catalyst needs both acid sites and hydrophobicity for this reaction. In addition, kinetic studies and modification of exterior surface revealed that the main active sites are acids inside the pores. We are trying to elucidate the mechanism of selective isosorbide formation in the pore space of beta zeolite by DFT calculation.

Figure 2. Dehydration of sorbitol
  1. A. Fukuoka, P. L. Dhepe
    Catalytic Conversion of Cellulose into Sugar Alcohols, Angew. Chem. Int. Ed., 45 (31) 5161-5163 (2006).
  2. H. Kobayashi, Y. Ito, T. Komanoya, Y. Hosaka, P. L. Dhepe, K. Kasai, K. Hara, A. Fukuoka
    Synthesis of sugar alcohols by hydrolytic hydrogenation of cellulose over supported metal catalysts, Green Chem., 13 (2), 326-333 (2011).
  3. H. Kobayashi, H. Matsuhashi, T. Komanoya, K. Hara, A. Fukuoka
    Transfer hydrogenation of cellulose to sugar alcohols over supported ruthenium catalysts, Chem. Commun., 47 (8), 2366-2368 (2011).
  4. H. Kobayashi, Y. Hosaka, K. Hara, B. Feng, Y. Hirosaki, A. Fukuoka
    Control of selectivity, activity and durability of simple supported nickel catalysts for hydrolytic hydrogenation of cellulose, Green Chem., 16 (2), 637-644 (2014).
  5. H. Kobayashi, Y. Yamakoshi, Y. Hosaka, M. Yabushita, A. Fukuoka
    Production of sugar alcohols from real biomass by supported platinum catalyst, Catal. Today, 226, 204-209 (2014).
  6. H. Kobayashi, H. Yokoyama, B. Feng, A. Fukuoka
    Dehydration of sorbitol to isosorbide over H-beta zeolites with high Si/Al ratios, Green. Chem., 17, 2732-2735 (2015).
  7. H. Yokoyama, H. Kobayashi, J. Hasegawa, A. Fukuoka
    Selective Dehydration of Mannitol to Isomannide over H-Beta Zeolite, ACS Catal., 7, 4828-4834 (2017).

2. Hydrolysis of woody biomass and marine biomass

Hydrolysis of woody biomass containing cellulose and hemicellulose to monosaccharides is a grand challenge in the biorefinery. We are studying to develop solid catalysts to replace conventional enzymes and homogeneous catalysts. We have found that carbon materials with weak acid sites, carboxylic acids, act as good catalysts. Based on the mechanistic study, we are designing a new catalyst for hydrolysis of cellulose and woody biomass.

(1) To verify the high activity of carbon material bearing carboxylic acid and phenolic hydroxyl groups, hydrolysis of cellobiose was tested. We found that salicylic acid and phthalic acid having adjacent functional groups show higher turnover frequency (TOF) than other compounds. The high activity of these compounds are due to a synergistic effect of the hydrolysis by carboxylic acids and the adsorption of another hydrophilic group (carboxylic acid or phenolic hydroxyl group).

(2) The carbon skeleton is oxidized by mixing activated carbon with the persulfate without using solvents, and many hydrophilic carbonyl groups can be added to the carbon. The carbon catalyst after the oxidation treatment showed high catalytic activity for cellulose hydrolysis by a mix-milling treatment of solid substrate and catalyst.

(3) When eucalyptus powder is oxidized in air at 300 C, carbon having many weakly acidic functional groups is obtained. When this carbon catalyst and eucalyptus powder were mix-milled and heated in a diluted acid solution, hydrolysis of eucalyptus proceeded to obtain glucose at 80% yield and xylose at 90% yield (Figure 3). In this reaction, more than 50 g of sugar can be produced from 100 g of eucalyptus. The solid residue after the reaction contains the catalyst and lignin, and the solid can be converted into an active carbon catalyst by the same air oxidation treatment.

Figure 3. Recycling process for saccharification of eucalyptus

Chitin contained in crab and shrimp are polymers of N-acetylglucosamine (NAG), and it is a potential precursors to pharmaceuticals and functional polymers. However, for selective synthesis of NAG from chitin, it is necessary to use a large amount of hydrochloric acid as a catalyst with high environmental impact or a long-time reaction with enzymes. In our study, chitin was impregnated with a small amount of sulfuric acid and then subjected to planetary ball-milling at room temperature, giving water-soluble oligomers of NAG (Figure 4) as a product. Presumably, mechanical force in the ball-milling acts as tensile force against the main chain of chitin molecules to promote selective depolymerization. When this intermediate was further depolymerized in hot water, NAG was obtained in a good yield, and in methanol 1-o-methyl-N-acetylglucosamine (MeNAG) was formed. In this process, it was possible to reduce the amount of acid by 99.8% compared with the conventional method using only liquid acid.

Figure 4. Depolymerization of chitin using mechanical force
  1. H. Kobayashi, T. Komanoya, K. Hara, A. Fukuoka
    Water-Tolerant Mesoporous Carbon-Supported Ruthenium Catalyst for Hydrolysis of Cellulose to Glucose, ChemSusChem, 3 (4), 440-443 (2010).
  2. H. Kobayashi, M. Yabushita, T. Komanoya, K. Hara, I. Fujita, A. Fukuoka
    High-Yielding One-Pot Synthesis of Glucose from Cellulose Using Simple Activated Carbons and Trace Hydrochloric Acid, ACS Catal., 3 (4), 581-587 (2013).
  3. M. Yabushita, H. Kobayashi, J. Hasegawa, K. Hara, A. Fukuoka
    Entropically Favored Adsorption of Cellulosic Molecules onto Carbon Materials through Hydrophobic Functionalities, ChemSusChem, 7 (5), 1443-1450 (2014).
  4. H. Kobayashi, M. Yabushita, J. Hasegawa, A. Fukuoka
    Synergy of Vicinal Oxygenated Groups of Catalysts for Hydrolysis of Cellulosic Molecules, J. Phys. Chem. C, 119, 20993-20999 (2015).
  5. H. Kobayashi, H. Kaiki, A. Shrotri, K. Techikawara, A. Fukuoka
    Hydrolysis of woody biomass by a biomass-derived reusable heterogeneous catalyst, Chem. Sci., 7, 692-696 (2016).
  6. A. Shrotri, H. Kobayashi, A. Fukuoka
    Air Oxidation of Activated Carbon to Synthesize a Biomimetic Catalyst for Hydrolysis of Cellulose, ChemSusChem, 9, 1299-1303 (2016)
  7. M. Yabushita, H. Kobayashi, K. Kuroki, S. Ito, A. Fukuoka
    Catalytic Depolymerization of Chitin with Retention of N-Acetyl Group, ChemSusChem, 8, 3760-3763 (2015).

3. Low-temperature oxidation of ethylene by Pt/mesoporous silica

Ethylene released from fruits and vegetables even in a trace amount promotes aging of fruits, vegetables and flowers. So development of efficient removal of ethylene is required. In particular, it is important to develop a technology that can remove ethylene at low temperature. We have found that platinum nanoparticles in mesoporous silica show excellent catalytic activity to oxidize ethylene (50 ppm) to CO2 at 0 C (Figure 5). Now we are studying to develop more active catalyst and to elucidate the reaction mechanism. The mesoporous silica-supported platinum catalyst was installed in a new refrigerator released by Hitachi Appliances Co., Ltd. in August, 2015.

Figure 5. Low-temperature oxidation of ethylene by mesoporous silica-supported platinum nanoparticles
  1. C. Jiang, K. Hara, A. Fukuoka
    Low-Temperature Oxidation of Ethylene over Platinum Nanoparticles Supported on Mesoporous Silica, Angew. Chem. Int. Ed., 52 (24), 6265-6268 (2013).

4. Single site catalyst for selective oxidation of alkane

Mesoporous organosilica is a unique material having both a crystalline array of organic groups in the silica wall and a periodic mesoporous structure. So it is possible to form a highly dispersed catalytic site having a uniform structure on the solid surface. We have developed a single site catalyst having a uniform active site of ruthenium by forming a ruthenium complex on bipyridine-containing mesoporous organosilica. When this catalyst was used for oxidation of alkanes such as adamantane and cis-decalin, tertiary C-H bonds were selectively oxidized to form alcohols (Figure 6). The homogeneous counterpart catalyst undergoes decomposition due to the interaction between the active sites, and the separation of catalyst needs a large amount of energy. However, our catalyst can be separated from the products by simple filtration and reusable with keeping its activity and selectivity. This facile reusability is due to suppression of the interaction between catalytic active sites with high dispersion on the mesoporous organosilica surface.

Figure 6. Oxidation of alkanes by a single site Ru catalyst on mesoporous organosilica.
  1. N. Ishito, H. Kobayashi, K. Nakajima, Y. Maegawa, S. Inagaki, K. Hara, A. Fukuoka
    Ruthenium-Immobilized Periodic Mesoporous Organosilica: Synthesis, Characterization, and Catalytic Application for Selective Oxidation of Alkanes, Chem. Eur. J., 21, 15564-15569 (2015).