Kitano Masaaki, Yamaguchi Daizo, Suganuma Satoshi, Nakajima Kiyotaka, Kato Hideki, Hayashi Shigenobu, Hara Michikazu
Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan.
Langmuir. 2009 May 5;25(9):5068-75. doi: 10.1021/la8040506.
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.
通过水溶性β-1,4-葡聚糖的水解反应，对含磺酸基、羧基和酚羟基的石墨烯片层无定形碳基固体酸水解纤维素的反应机理进行了详细研究。虽然一系列固体强布朗斯特酸催化剂（含酸性羟基的无机氧化物、含磺酸基的树脂以及碳基固体酸）均可水解纤维二糖（最短的水溶性β-1,4-葡聚糖）中的β-1,4-糖苷键，但除碳材料外的其他受试固体酸（由传统固体酸组成），无法有效催化纤维六糖（一种长链水溶性β-1,4-葡聚糖）的水解反应。然而，该碳材料对纤维六糖的水解表现出显著的催化性能：碳材料中磺酸基的转化频率（TOF）超过传统固体酸约20倍，达到活性最高的催化剂硫酸的水平。实验结果表明，含酸性羟基的无机氧化物酸性不足以分解由强氢键聚集的纤维六糖分子以及纤维素中的氢和β-1,4-糖苷键，且不吸附β-1,4-葡聚糖的树脂中的磺酸基无法有效进攻纤维六糖分子中的氢和β-1,4-糖苷键。相比之下，碳材料能够通过其酚羟基或羧基吸附β-1,4-葡聚糖，因此与碳相连的磺酸基成为分解氢键以及水解吸附的长链水溶性β-1,4-葡聚糖聚集体中β-1,4-糖苷键的有效活性位点。这些结果表明，与无定形碳相连的高密度官能团的协同作用使得碳材料上包括纤维素在内的β-1,4-葡聚糖能够高效水解。
