Cellulose Depolymerization over Heterogeneous Catalysts

Abstract

Cellulosic biomass is the largest source of renewable organic carbon on our planet. Cellulose accounts for 40-50 wt % of this lignocellulose, and it is a feedstock for industrially important chemicals and fuels. The first step in cellulose conversion involves its depolymerization to glucose or to its hydrogenated product sorbitol. The hydrolysis of cellulose to glucose by homogeneous mineral acids was the subject of research for almost a century. However, homogeneous acids have significant drawbacks and are neither economical nor environmentally friendly. In 2006, our group reported for the first time the ability of heterogeneous catalysts to depolymerize cellulose through hydrolytic hydrogenation to produce sorbitol. Later, we reported the hydrolysis of cellulose to glucose using carbon catalyst containing weakly acidic functional groups. Understanding the reaction between cellulose and heterogeneous catalyst is a challenge as the reaction occurs between a solid substrate and a solid catalyst. In this Account, we describe our efforts for the conversion of cellulose to sorbitol and glucose using heterogeneous catalysts. Sorbitol is produced by sequential hydrolysis and hydrogenation of cellulose in one pot. We reported sorbitol synthesis from cellulose in the presence of supported metal catalysts and H gas. The reducing environment of the reaction prevents byproduct formation, and harsh reaction conditions can be used to achieve sorbitol yield of up to 90%. Glucose is produced by acid catalyzed hydrolysis of cellulose, a more challenging reaction owing to the tendency of glucose to rapidly decompose in hot water. Sulfonated carbons were first reported as active catalysts for cellulose hydrolysis, but they were hydrothermally unstable under the reaction conditions. We found that carbon catalysts bearing weakly acidic functional groups such as hydroxyl and carboxylic acids are also active. Weakly acidic functional groups are hydrothermally stable, and a soluble sugar yield of 90% was achieved in a 20 min reaction. We clarified that the polycyclic aromatic surface of the carbon adsorbs cellulose molecules on its surface by CH-π and hydrophobic interactions driven by a positive change in entropy of the system. The adsorbed molecules are rapidly hydrolyzed by active sites containing vicinal functional groups that recognize the hydroxyl groups on cellulose to achieve a high frequency factor. This phenomenon is analogous to the hydrolysis of cellulose by enzymes that use CH-π and hydrophobic interactions along with weakly acidic carboxylic acid and carboxylate pair to catalyze the reaction. However, in comparison with enzymes, carbon catalyst is functional over a wide range of pH and temperatures. We also developed a continuous flow slurry process to demonstrate the feasibility for commercial application of carbon-catalyzed cellulose hydrolysis to glucose using inexpensive catalyst prepared by air oxidation. We believe that further efforts in this field should be directed toward eliminating roadblocks for the commercial...

Soluble Cello‐Oligosaccharides Produced by Carbon‐Catalyzed Hydrolysis of Cellulose

“…Carbon materials containing oxygenated functional groups are potential heterogeneous catalysts for the synthesis of cello‐oligosaccharides from cellulose . These functional groups are stable under the ball‐milling and hydrothermal conditions required for cellulose hydrolysis .…”

Electrical Double Layer as a Model of Interaction between Cellulose and Solid Acid Catalysts of Hydrolysis

“…The clear benefit of the solid catalysts is the ease of their separation from the reaction medium: this can be made trivial by embedding magnetic particles inside catalyst grains . Solid catalysts of cellulose conversion are actively developed in the direction of bifunctionality and one‐pot processes combining hydrolysis and hydrogenation …”

“…[14] Solid catalysts of cellulose conversion are actively developed in the direction of bifunctionality and one-pot processes combining hydrolysis and hydrogenation. [15][16][17] When using solid acids for cellulose hydrolysis, one should be wary of a possibility that a solid catalyst may decompose, releasing a homogeneous acid into the reaction medium, which then causes catalytic action. For example, this happens when hydrolysis of CÀ Cl bonds occurs in a catalyst modified with chlorine atoms, which leads to formation of hydrochloric acid.…”

Selective Cellulose Hydrogenolysis to Ethanol Using Ni@C Combined with Phosphoric Acid Catalysts

“…). The target CÀCa nd CÀOb onds are efficiently activated and broken under the synergistic hydrogenolysis of Ni@C. Note that the ethanolf ormation over the Ni@C catalyst withoutt he aid of H 3 PO 4 is responsible for the adjacent ÀCOOH groups on the catalysts urface,w hich play as imilar role in formingt he cyclic di-estersw ith glucose in comparison with H 3 PO 4 [36][37][38][39][40]. …”