ELS DE CANCK, INMACULADA DOSUNA-RODRIGUEZ, ERIC M. GAIGNEAUX, and PASCAL VAN DER VOORT

9.1 INTRODUCTION

The discovery of Periodic Mesoporous Organosilicas (PMOs) [1-3] with organic bridging groups incorporated in their silica framework has been the start of a fascinating research area which provides materials with huge potential [4-6]. Different organic bridges have been employed for very diverse applications, such as heterogeneous catalysts [7,8], bio-sen­sors [9,10], chromatographic packing materials [11,12], low-k materials [13,14], adsorbents of pollutants [15] and controlled drug delivery sys­tems [16-19]. PMOs are highly porous materials with large specific sur­face areas, pore volumes and narrow pore size distributions. Furthermore, they exhibit a high thermal and mechanical stability [20-22], especially in comparison with other porous silica materials [23]. This type of material is

Periodic Mesoporous Organosilica Functionalized with Sulfonic Acid Groups as Acid Catalyst for Glycerol Acetylation. © De Canck E, Dosuna-Rodriguez I, Gaigneaux EM, and Van Der Voort P. Materials 6 (2013), doi:10.3390/ma6083556. Licensed under a Creative Commons Attribution 3.0 Unported License, http://creativecommons. org/licenses/by/3.0/.

synthesized with structure directing agents such as the non-ionic triblock co­polymer P123. Around this template, a silica source is condensed in basic or acid aqueous environment. Usually, an organo bis-silane (R’O)3-Si-R-Si- (OR’)3 is used where R represents the organic bridging group and R’ usually a methyl or ethyl group. Already many reports have appeared on different bridging groups (R) like phenylene, ethylene, ethenylene and ethylbenzene but also more complex and flexible organic functionalities have been de­scribed. Furthermore the bridging group can be modified to fine-tune the material for a specific application such as solid acid catalysis [4].

Concerning this topic, some very promising results have already been published regarding the incorporation of an acid functionality such as a sulfonic acid group and its catalytic activity. Several diverse methods have been applied to prepare sulfonic acid containing PMO materials. These strategies include the direct sulfonation of the phenylene bridge [24-26], as first attempted by Inagaki et al. [27], and the cocondensa­tion of an organo bis-silane with (3-mercaptopropyl) trimethoxysilane (MPTMS) [28-31] followed by an oxidation of the thiol functionality. The latter can also be achieved by the in-situ oxidation of the thiol func­tionality by the addition of H2O2 during the cocondensation process of tetraethoxyorthosilicate (TEOS) and MPTMS [32,33]. Other silanes have been used in cocondensation processes with an organo bis-silane such as 2-(4-chlorosulfonylphenyl)-trimethoxysilane [34] and perfluorinated al — kylsulfonic acid silanes [35-37].

In the specific case of ethenylene bridged PMOs, — SO3H moieties can be acquired by the direct sulfonation of the C=C bond [38]. However, the sulfonic acid group can detach from the material, depending on the environment used during catalysis. Another explored route is the use of a Diels Alder reaction where the ethene bond acts as dienophile. Kondo et al. [39,40] described the cycloaddition of the ethene bond with ben — zocyclobutene and subsequently the resulting phenylene moiety is sulfo — nated to obtain a heterogeneous catalyst. These authors tested this material for several catalytic reactions (esterification of acetic acid with ethanol, the Beckmann and pinacole-pinacolone rearrangement) and the catalyst showed excellent conversion results. This Diels Alder process has been further used to expand the functionalization possibilities [41].

Another example of modifying the surface of an ethenylene bridged PMO material has been reported by the research group of Kaliaguine [42]. First, the surface silanols were end-capped with hexamethyldisilazane af­ter which a Friedel-Crafts alkylation with benzene was made, and sub­sequently the benzene moiety was sulfonated with concentrated sulfuric acid. The — SO3H containing material exhibited a high catalytic activity in the self-condensation of heptanal.

In this study, the pure trans-ethenylene bridged PMO material [43] is chosen as support material. Thiol functionalities were incorporated ac­cording to a procedure previously described by our research group [44] and subsequently oxidized in order to obtain — SO3H. This material was thoroughly characterized and the solid acid was tested in the acetylation of glycerol. Furthermore, the reusability of this catalyst was investigated.