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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...

Nanosized phosphated TiO2 catalysts with different phosphate contents were synthesized and tested for the conversion of glucose to 5‐hydroxymethylfurfural. The resulting materials were characterized by using N2‐adsorption, XRD, inductively coupled plasma atomic emission spectroscopy, X‐ray spectroscopy, TEM, temperature‐programmed desorption of ammonia, and FTIR spectroscopy of pyridine adsorption techniques to determine their structural, bulk, surface, and acid properties. We found that TiO2 nanoparticles catalyzed this reaction under mild conditions in a water–butanol biphasic system. The incorporation of phosphorus into the TiO2 framework remarkably enhances the target product selectivity, which is ascribed to increased surface area, enhanced acidity, as well as thermal stability resulting from the TiOP bond formation. Under optimal reaction conditions, phosphated TiO2 was found to exhibit excellent catalytic performance, which resulted in 97 % glucose conversion and 81 % HMF yield after 3 h of reaction at 175 °C. More importantly, the catalyst showed good stability and could be reused for several reaction cycles.

Controlling the selectivity of products among CO, methane, and methanol is a challenge in CO2 hydrogenation. Catalysts with oxygen vacancies are helpful for CO2 activation, but they exhibit poor CO selectivity as intermediates stabilized over oxygen vacancies undergo deep hydrogenation to methanol and methane. Here, we report the synthesis of a catalyst with isolated Co atoms in ZrO2 that exhibits oxygen vacant sites near Co atoms owing to charge imbalance between cations. The resulting catalytic site effectively adsorbs CO2 and also achieves more than 95% CO selectivity during hydrogenation. The CO selectivity was independent of other reaction parameters such as reaction pressure, space velocity, and H2/CO2 ratio. Operando DRIFTS analysis showed that CO2 was first hydrogenated to formate, which preferentially decomposed to CO under the reaction condition instead of forming methanol. Furthermore, the adsorption of CO on active sites was less favorable than the adsorption of CO2, limiting its further hydrogenation to methane. The synergy between Co and Zr was crucial for the generation of oxygen vacancy and stabilization of formate species as an intermediate for CO formation. This study shows the importance of strategic design of atomic interface to control the selectivity of a specific product from CO2 hydrogenation.

A carbon catalyst prepared by air oxidation of woody biomass hydrolyses woody biomass, and the reaction residue is transformed back to the catalyst by the same air oxidation method.

A water-THF biphasic system containing N-methyl-2-pyrrolidone (NMP) was found to enable the efficient synthesis of 5-hydroxymethylfurfural (HMF) from a variety of sugars (simple to complex) using phosphated TiO2 as a catalyst. Fructose and glucose were selectively converted to HMF resulting in 98 % and 90 % yield, respectively, at 175 °C. Cellobiose and sucrose also gave rise to high HMF yields of 94 % and 98 %, respectively, at 180 °C. Other sugar variants such as starch (potato and rice) and cellulose were also investigated. The yields of HMF from starch (80-85 %) were high, whereas cellulose resulted in a modest yield of 33 %. Direct transformation of cellulose to HMF in significant yield (86 %) was assisted by mechanocatalytic depolymerization-ball milling of acid-impregnated cellulose. This effectively reduced cellulose crystallinity and particle size, forming soluble cello-oligomers; this is responsible for the enhanced substrate-catalytic sites contact and subsequent rate of HMF formation. During catalyst recyclability, P-TiO2 was observed to be reusable for four cycles without any loss in activity. We also investigated the conversion of the cello-oligomers to HMF in a continuous flow reactor. Good HMF yield (53 %) was achieved using a water-methyl isobutyl ketone+NMP biphasic system.

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Biphasic dehydration of cellulosic biomass into remarkable yield of HMF can be achieved via mechanocatalysis and fractionation treatment.