In-Depth Analysis of Key Factors Affecting the Catalysis of Oxidized Carbon Blacks for Cellulose Hydrolysis

Abstract

Carbon black is treated with each of three different oxidizing agents (nitric acid, air, and ozone) to introduce oxygencontaining functional groups (OFGs) and the resulting oxidized carbon blacks are used as model catalysts for the hydrolysis of cellulose. At the same time, they are thoroughly characterized by many techniques, especially in terms of surface chemistry, including the type/number of OFGs and the site where OFGs are formed in a carbon structure. Then, by correlating the catalytic activities toward the cellulose hydrolysis with the characterization results, key factors governing the catalysis are elucidated. As a result, this study reveals that two types of acidic OFGs, carboxyl groups (including those derived from the hydrolysis of acid anhydride and lactone groups in the reaction) and phenol groups (including those from lactone groups), act as catalytic sites with almost equal activity, as long as they are present on the external surface and thereby accessible to cellulose molecules.

Tuning the reactivity of carbon surfaces with oxygen-containing functional groups

“…It has been suggested that the composition in acidic OCFGs (–OH, –COOH, lactone, carboxylic anhydrides) affects the metal particle size distribution by serving as coordination sites for metal cations, greatly dispersing metal particles 13 . Furthermore, several OCFGs can serve as Brønsted acid sites (BAS), e.g., for dehydration, cellulose hydrolysis, etc 14 – 16 ., or base sites. The relative activity of each type of site is unknown.…”

“…Boehm titration 24 , 25 assumes bases of different strength react with a specific type of OCFGs but is challenged when distinct chemical functions possess similar pK a values. Furthermore, it cannot distinguish aprotic acidic groups (lactones, carboxylic anhydrides) that can hydrolyze into –OH or –COOH in water or acid/base solutions and does not account for the porosity and material hydrophilicity/hydrophobicity (in the liquid phase) 16 , 17 , 25 . High-resolution X-ray photoelectron spectroscopy (XPS) is sensitive to the elemental composition and the relative carbon fraction in various oxidation states.…”

Selective Synthesis of Oligosaccharides by Mechanochemical Hydrolysis of Chitin over a Carbon‐Based Catalyst

“…Although strong acids have often been employed for the hydrolysis of polysaccharides, weak acids including carboxylic and phenolic groups can efficiently cleave glycosidic bonds. [34][35][36] Hexagonal boron nitride (h-BN), having lower acidity than AC-Air, [37,38] produced a small amount of products (entry 5, oligomer 9.1 %C). This material became partially soluble in water after the reaction due to exfoliation.…”

“…The difference indicates that surface weak acids produced by air oxidation are active. Although strong acids have often been employed for the hydrolysis of polysaccharides, weak acids including carboxylic and phenolic groups can efficiently cleave glycosidic bonds [34–36] . Hexagonal boron nitride (h‐BN), having lower acidity than AC‐Air, [37, 38] produced a small amount of products (entry 5, oligomer 9.1 %C).…”

Selective hydrogenation via precise hydrogen bond interactions on catalytic scaffolds

“…Controlling the spatial interactions of substrates with the surface functional groups of the scaffold catalysts with precision is crucial. In many traditional catalysts, such as activated carbon, there are many functional groups 32 . Although one could determine their ratio, it is hard to distinguish the role of each.…”