The hydrolysis of cellulose into saccharides using a range of solid catalysts is investigated for potential application in the environmentally benign saccharification of cellulose. Crystalline pure cellulose is not hydrolyzed by conventional strong solid Brønsted acid catalysts such as niobic acid, H-mordenite, Nafion and Amberlyst-15, whereas amorphous carbon bearing SO 3H, COOH, and OH function as an efficient catalyst for the reaction. The apparent activation energy for the hydrolysis of cellulose into glucose using the carbon catalyst is estimated to be 110 kJ mol (-1), smaller than that for sulfuric acid under optimal conditions (170 kJ mol (-1)). The carbon catalyst can be readily separated from the saccharide solution after reaction for reuse in the reaction without loss of activity. The catalytic performance of the carbon catalyst is attributed to the ability of the material to adsorb beta-1,4 glucan, which does not adsorb to other solid acids.

The reaction mechanism of the hydrolysis of cellulose by a carbon-based solid acid, amorphous carbon containing graphene sheets bearing SO(3)H, COOH, and phenolic OH groups, has been investigated in detail through the hydrolysis of water-soluble beta-1,4-glucan. Whereas a range of solid strong Brønsted acid catalysts (inorganic oxides with acidic OH groups, SO(3)H-bearing resins, and the carbon-based solid acid) can hydrolyze the beta-1,4-glycosidic bonds in cellobiose (the shortest water-soluble beta-1,4-glucan), the tested solid acids except for the carbon material, consisting of conventional solid acids, cannot function as effective catalysts for the hydrolysis of cellohexaose (a long-chain water-soluble beta-1,4-glucan). However, the carbon material exhibits remarkable catalytic performance for the hydrolysis of cellohexaose: the turnover frequency (TOF) of SO(3)H groups in the carbon material exceeds ca. 20 times those of the conventional solid acids, reaching that of sulfuric acid, which is the most active catalyst. Experimental results revealed that inorganic oxides with acidic OH groups are not acidic enough to decompose the hydrogen and beta-1,4-glycosidic bonds in cellohexaose molecules aggregated by strong hydrogen bonds as well as cellulose and that the SO(3)H groups of the resins that do not adsorb beta-1,4-glucan are unable to attack the hydrogen and beta-1,4-glycosidic bonds in cellohexaose molecules effectively. In contrast, the carbon material is capable of adsorbing beta-1,4-glucan by phenolic OH or COOH groups in the carbon material, and SO(3)H groups bonded to the carbon therefore function as effective active sites for both decomposing the hydrogen bonds and hydrolyzing the beta-1,4-glycosidic bonds in the adsorbed long-chain water-soluble beta-1,4-glucan aggregate. These results suggest that the synergetic combination of high densities of the functional groups bonded to amorphous carbon causes the efficient hydrolysis of beta-1,4-glucan, including cellulose, on the carbon material.

The hydrolysis of cellulose with a highly active solid acid catalyst, a carbon material bearing SO3H, COOH, and OH groups, was investigated at 323−393 K using an artificial neural network (ANN) and a response surface methodology (RSM). The ANN models developed for experimental design accurately reflect the novel solid−solid interface catalysis. The ANN models and RSM revealed that the amount of water dominates the hydrolysis reaction as well as cellulose saccharification by concentrated sulfuric acid, a conventional saccharification method. The correlations of the reaction and each parameter are discussed on the basis of the reaction mechanism, ANN, and RSM.

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SO₃H-bearing amorphous carbon prepared from polyvinyl chloride (PVC) is studied as a heterogeneous Brønsted acid catalyst. Sulfonation of partially carbonized PVC produces amorphous carbon consisting of small SO₃H-bearing carbon sheets linked by sp³ -based aliphatic hydrocarbons. This carbon material exhibits much higher catalytic performance in the hydrolysis of cellobiose than conventional heterogeneous Brønsted acid catalysts with SO₃H groups, including SO₃H-bearing amorphous carbon derived from cellulose. This can be attributed to a high density of SO₃H groups and the fast diffusion of reactants and products enabled by a flexible carbon network.

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Bending vibrations in the infrared (IR) spectra of ammonia adsorbed on Lewis acidic metal oxides, i.e., Al 2 O 3 , ZrO 2 and TiO 2 , and zeolite were analyzed with an aid of density functional theory (DFT) calculations. The results by DFT methods reveal the wavenumbers of the vibration modes (ν 4 and ν 2) of NH 4 bonded to Brønsted acid site and the vibration modes (δ s and δ d) of NH 3 species coordinated to a Lewis acidic metal center (M = Al, Zr or Ti). The wavenumbers calculated based on DFT were reasonably in agreement with the experimentally observed values. The estimation of wavenumbers suggests that the δ s vibration of NH 3 hydrogen-bonded is invisible on a zeolite, because it is hidden by an intense absorption due to skeletal vibration. On the other hand, multiple bands of asymmetric bending modes (δ d and ν 2) observed on a zeolite were assigned. A quantification method of Brønsted and Lewis acid sites, and hydrogen-bonded NH 3 is provided based on the peak assignments.