Hydrolysis of Cellulose by a Mesoporous Carbon-Fe2(SO4)3/γ-Fe2O3 Nanoparticle-Based Solid Acid Catalyst

A highly active, carbon-based solid acid catalyst has been reported1,2,3. This material is a solid Brønsted acid composed of randomly distributed graphene sheets bearing SO3H-, COOH- and phenolic OH- functional groups. Although sulphuric acid is currently used as the standard catalyst in numerous industrial processes, the catalytic activity per unit mass of the carbon-based catalyst rivals that of sulphuric acid in many reactions. In addition, given the low effective acid density (4.3 mmol g−1) and low surface area (2-20 m2 g−1) of this catalyst, the reactivity of its Brønsted acid sites is at least 10 times greater than that of sulphuric acid2,3. This catalyst performs as well as mineral acid catalysts in the hydrolysis of both pure crystalline cellulose and natural lignocellulosic substrates into water-soluble β-1,4 glucan with subsequent hydrolysis to glucose3,4,5. The catalyst itself is readily prepared by incomplete carbonization of sulphopolycyclic aromatic hydrocarbons or by sulphonation of incompletely carbonized organic compounds2. While it is true that carbon-based solid acid catalysts can also be synthesized by sulphonation of carbon nanotubes6 or activated carbon materials7, the density of SO3H groups in these materials is much lower than in the carbon-based catalysts.

Magnetically separable mesoporous carbon materials have enormous potential for applications as catalytic supports, in separation technology and for the adsorption of biomolecules8,9,10,11,12,13,14. Despite this, a solid acid catalyst which is also a magnetically separable material has not previously been developed, since no suitable precursor compounds were known. Much effort has been expended in attempts to prevent the leaching and oxidation of deposited metal nanoparticles during complicated multi-step synthetic procedures to produce a precursor, a magnetic mesoporous carbon material. The necessity of using a complicated synthetic procedure involving an expensive carbon source and high temperatures to generate sp3 carbon structures in graphite has discouraged the production of magnetically separable solid acid catalysts.

Recently, a new, simple and robust method for the preparation of a mesoporous carbon-γ-Fe2O3 nanoparticle composite (MCNC) has been reported15. This material can be directly synthesized in a one-pot, non-templating and self-sustaining reaction using carboxymethylcellulose as an inexpensive carbon source. Herein, a mesoporous magnetically separable MCNC-based solid acid catalyst (MCNC-SA) bearing SO3H, COOH and OH surface functional groups and containing Fe2(SO4)3/γ-Fe2O3 nanoparticles in its carbon matrix is reported. This new catalyst offers significant performance advantages since its concentration of acid density and its catalytic activity, in the hydrolysis of microcrystalline cellulose, is equal to that of the cellulose-derived conventional catalyst. This paper discusses the measurement and analysis of the catalytic properties and activities of this novel material.