Sulfonated mesoporous silica-carbon nanocomposites for biomass conversion
Over the past decades, researchers have made great efforts in the synthesis and functionalization of ordered mesoporous carbon or silica-carbon nanocomposites. These materials have the merits of mesostructures; that is, a high surface area and large pore volume. Meanwhile, the carbon moieties in the framework provide the advantages of organic component, which is hydrophobicity and the ability to be easily functionalized. These materials are highly interesting for catalysis, adsorption, energy storage, drug delivery, sensors, etc.
Based on the mature synthesis of ordered mesoporous silica, generally two strategies, hard-template and soft-template, were applied for the synthesis of mesoporous carbon or silica-carbon nanocomposites. On one hand, the hard-template synthesis refers to nanocasting of a pre-formed hard template (such as MCM-41 and SBA-15) with a carbon precursor, which involves multi-step synthesis operations and is time-consuming. On the other hand, soft-template synthesis directly utilizes the one-pot co-assembly induced by the soft template co-polymer (such as Pluronic P123 and F127). Though the structure-directing co-polymer is essentially the same as those used for mesoporous silica synthesis, the direct synthesis from organic-organic assembly is more difficult. The introduction of a SiO2 precursor such as TEOS can assist in the co-assembly, forming well-ordered mesostructures. To avoid the macrophase separation of silica and carbon precursors and to allow for the fast constructing of a mesostructure, evaporation-induced self-assembly (EISA) approach is employed. Herein, novel silica-carbon nanocomposites with different silica-carbon ratios were synthesized and careful analysis were performed to ascertain the mesophase ordering as well as the entanglement of nano-sized silica and carbon phases in the mesopore walls. The degree of carbon may be varied between 19 to 62 wt%, and their presence creates additional microporosity in the composite material besides the mesopores, forming a very accessible hierarchical pore architecture; larger carbon contents block the mesopores.
The mesoporous silica-carbon nanocomposite precursors were subjected to pyrolysis in inert atmosphere at 400 or 550 oC. Different amounts of phenolic OH and COOH groups on the carbon surface and different cross-linking degrees of carbon were thus attained. These materials were sulfonated with concentrated H2SO4 producing 0.57-0.15 mmol/g SO3H sites, which are bound to the carbon phase, while the ordered mesoporous structure was kept intact in these circumstances due to the strengthening role of SiO2 in the framework walls.
The sulfonated silica-carbon nanocomposites with accessible strong SO3H acid sites and tunable surface properties were applied for several acid-catalyzed reactions, including classic carbocation hydrocarbon chemistry and those related to biomass conversion. The catalyst performance in terms of product yields, the mechanism and kinetics, as well as stability were investigated. The sulfonated mesoporous silica-carbon nanocomposites showed high selectivity for the dimerization of styrene/α-methylstyrene, and the one with the highest mesopore volume exhibited the highest catalytic activity. However, in the ethanolysis of fructose to furans and levulinate esters, a high micropore volume appeared with the best catalytic performance. A simplified reaction scheme was proposed and fitted fairly well with the experimental results upon kinetic modeling, which deepens the understanding of the reaction cascade. Regarding the practical improvement of the synthesis, a novel rapid rotation-evaporation induced self-assembly (ROT-EISA) was proposed for the first time to avoid the laborious work of the conventional EISA via thin-film evaporation. The material synthesized from ROT-EISA resembled the counterpart from conventional EISA in the textural properties, and displayed comparably high catalytic performances for both fructose ethanolysis and sylvan condensation reactions.
In summary, a novel series of mesoporous silica-carbon nanocomposites have been synthesized successfully from tri-constituent EISA. An alternative practical synthesis method using a rotavap process, which has a better chance for upscale, is presented. The composite materials, after sulfonation, can be used in acid catalysis, and show a large potential for instance in the valorization of biomass. Besides catalysis, the highly porous nanocomposite material may also be promising in other applications.