Active solid acid catalysts prepared by sulfonation of carbonization-controlled mesoporous carbon materials

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Abstract

A novel sulfonic acid group-functionalized mesoporous carbon was prepared through vapour-phase transfer sulfonation of CMK-3 type mesoporous carbon with the controllable carbonization degree. The method of vapour-phase sulfonation, that is, through contacting the sample powders with the vapor from fuming sulfuric acid in a closed autoclave, proved to be more effective than both the direct dipping of the carbon in concentrated or fuming H2SO4 and the treatment with pure SO3 gas. The influence of carbonization temperature on the sulfonation was investigated, suggesting an optimum temperature of 823 K. The SO3H group-functionalized mesoporous carbon, CMK-3-SO3H, was characterized by means of XRD, SEM, TEM, N2 adsorption–desorption, 13C NMR, FTIR and elemental analysis, which indicated that it had uniform mesopores together with an extremely high surface area, and also contained an amount of acid sites as high as 1.3 mmol g−1. CMK-3-SO3H catalyzed efficiently the liquid-phase Beckmann rearrangement of cyclohexanone oxime and condensation reactions involving bulky aromatic aldehydes.

Introduction

The discovery of mesoporous materials with extremely high surface areas, large-pore volumes and uniform nanosized pores has opened up new opportunities for converting large molecules into valuable petrochemicals, fine chemicals and pharmaceuticals from the viewpoint of catalysis [1], [2], [3], [4], [5], [6]. It is of particular importance and research interests to develop new heterogeneous solid acid catalysts applicable to Beckmann rearrangement, alkylation, acylation, esterification, condensation, oligomerization, hydration, dehydration, etc., because most of which are currently operated in environmentally unfriendly ways owing to using liquid acid catalysts such as H2SO4, HF and H3PO4 [3], [7].

To prepare mesoporous solid acid catalysts, many works in last decade have focused on the functionalization and modification of silica-based and organosilica hybrid materials with trivalent aluminum cations and sulfonic acid groups. The incorporation of Al into mesoporous silica can be realized by direct hydrothermal synthesis or postsynthesis, while the SO3H group-functionalized mesoporous silicas are usually prepared from the oxidation with H2O2 of SH groups post-grafted or directly incorporated onto the silica walls [8], [9], [10], [11]. However, the silica walls are organically modified only by a limited fraction of SH groups due to the hydrophobic nature of silanes agents. Furthermore, a possible Si–C bond cleavage is needed to avoid during the direct synthesis and surfactant removal processes. Accordingly, the SO3H-functionalized mesoporous silica materials have relatively low SO3H densities, resulting in restricted utility in practical acid-catalyzed reactions. Mesoporous organosilicas with a high functionality, on the other hand, are possibly subject to a direct sulfonation [12]. They, however, suffers the problem of using costly organic silica sources.

Another family of pure carbon-based mesoporous materials, the replication of mesoporous silicas with nanocasting method, are promising in energy storage or supporting for noble metals [13], [14] but they are catalytically inactive owing to a neutral framework. To introduce sulfonic acid groups into these mesoporous carbons would obtain new solid acids of high surface area, stable framework, high hydrophobicity and activity.

CMK-type carbons are generally prepared by introducing carbon sources into the mesopores of silica scaffolds followed by a complete carbonization at 1173 K [13] The high temperature-carbonization is helpful to form an interconnected rigid carbon framework but lacking the functionality. We report here with CMK-3 as a first example that it is possible to develop by controlling carbonization temperature the polycyclic aromatic carbon atoms available for the sulfonation. Partially carbonized mesoporous carbons then are converted to catalytically active solid acids successively through a prudent sulfonation.

Section snippets

Synthesis of mesoporous CMK-3-S3OH

The synthesis of the CMK-3 mesoporous carbon was performed according to the procedure reported elsewhere [13]. In a typical synthesis of SBA-15, the hard template for the preparation of CMK-3 mesoporous carbon, 15 g of TEOS was dissolved in a solution containing 7 g of triblock copolymer (Pluronic 123), 47 g of hydrochloric acid and 187 g of H2O. The mixture was stirred with magnetic stirring at 308 K for 20 h, followed by further aging statically at 363 K for 24 h. The product was filtered, dried

Preparation and characterization of samples

Fig. 1 illustrates graphically the procedures for preparing sulfonic acid-functionalized mesoporous carbon materials, which consist of the preparation of carbonization-controlled CMK-3 replica from SBA-15 and the vapour-phase sulfonation. Following a previous method [13], SBA-15 mesoporous silica was impregnated with the carbon precursor by infiltrating sucrose into its mesopores twice. The resultant composite was subject to a thermal treatment in a quartz tuber under vacuum to carbonize

Conclusions

The new solid acid mesoporous carbon materials with sulfonic acid group loading up to 1.2 mmol g−1 have been prepared readily through the effective vapour-phase sulfonation of CMK-3 mesoporous carbons prepared by controlled carbonization. CMK-3-823-SO3H could serve as an active, selective and stable catalyst for a number of acid-catalyzed reactions particularly involving bulky molecules.

Acknowledgments

This work was financially supported by National Natural Science Foundation of China (Grants Nos. 20473027 and 20602012), 973 Program (2003CB615801) and Science and Technology Commission of Shanghai Municipality (05DZ22306, 06DJ14006). P.W. thanks the Program for New Century Excellent Talents in University (NCET-04-0423) and Pujiang Program (05PJ14041).

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