the last two decades, the synthesis of new mesoporous materials has been one of the most active and successful areas of research in chemistry and materials science.1 The use of surfactants to template the structure of mesoporous materials has produce a myriad of new solids, with narrow and controllable pore size distribution and tunable pore structure.2 Also, different bio-inspired strategies have helped to further expand the palette of porous materials that scientists and industry have at their disposal.3
In most cases, however, mesoporous materials are used only as catalysts supports. Their large surface area and open pore structure are used to accommodate a wide variety of nanoparticles or to anchor homogeneous catalysts to the porous surface. The benefits of supporting catalytically active phases on porous materials are well-known, but also their current limitations, such as partial pore blocking, inhomogeneous distribution of the active phase throughout the particle, and numerous preparation steps.4 Although smartly tuning the synthesis conditions can greatly limit these drawbacks, an ideal catalyst should incorporate the active phase in their structure, which will dramatically improve the dispersion and homogeneity of the active phase while minimizing pore blocking.
Functional mesoporous materials provide many opportunities in a wide variety of fields, from optics to sensing and from drug delivery to adsorption, but also in catalysis.5 They represent the evolution of catalysts from a composite material made of a support plus a supported phase to a new functional solid, which comprises the right chemistry and an optimized porous structure for a given chemical reaction. In this sense, functional mesoporous catalysts can be compared to natural bio-catalysts (enzymes), which possess an optimized tridimensional structure and chemical functionality, defined by their molecular composition (amino acid sequence). The use of building blocks that contain both of these components to produce new catalysts using a bottom up strategy, as in nature, is a new avenue that is still largely unexplored.
Indeed, there are many previous and well-known examples of mesoporous materials that contain an active phase in their structure. A good example of this would be Al-MCM-41 in which the incorporation of aluminum in the structure of the otherwise catalytically inert silica-based MCM-41 makes it active for some acid-catalyzed reactions.6 However, in the last few years the field of functional mesoporous materials—many examples of which have been prepared with other applications in mind—has grown considerably opening-up new opportunities for some challenging and important catalytic chemical process.
This Special Issue of ChemCatChem provides an up-to-date and comprehensive vision of the new synthetic strategies for the incorporation of chemical functionalities in mesoporous materials, their characterization and some relevant, and in some cases surprising examples of the use of functional mesoporous catalysts in important chemical reactions.
Periodic mesoporous organosilicas (PMOs) are excellent examples of a functional mesoporous material with many catalytic applications. Some of the organic moieties that can be incorporated into the framework of PMO present good catalytic activity, especially sulfonic acid groups, which have been widely used in a wide range of acid-catalyzed reactions (figure 1).7 Another, example of the potential of PMO in catalysis is the amination of phenylene moieties in crystal-like mesoporous silica hybridized with phenylene. The modified PMO successfully catalyzed the Knöevenagel condensation reaction as a solid-base catalyst recently reported by S. Inagaki et al. (Figure 2).8 The same group described the synthesis of a mesoporous biphenyl-silica followed by coordination of a rhenium precursor. The PMO containing a rhenium(I) in its framework was then used for the enhancement of photocatalytic CO2 reduction, which displays the potential of modified PMOs as a light- harvesting antenna in photoreaction systems.9
Various metal complexes with catalytic activity have been recently used as building blocks for the preparation of functional mesoporous materials, by using ligands with alkoxysilane terminal groups, which are hydrolyzed in the presence of a surfactant, to produce silica materials with controlled porosity and the metal complex in their framework. The accessibility of the active phase and reusability of the catalyst have been confirmed for same commercially relevant reactions.10
A recent milestone in this field has been recently achieved by the commercialization of hierarchical zeolites as a new class of FCC catalysts, overcoming one of the major limitations of zeolites, i.e. the diffusion of large molecules to their interior, in their most significant commercial application. These functional mesoporous materials, which combine in the same phase controlled mesoporosity (introduced by surfactant-templating) and zeolitic crystallinity, are being used in refineries in USA, and display the practical and enormous potential of more accessible catalysts with chemical functionality, strong Brönstead acidity in this case, in their framework.11
The new bottom-up synthetic strategies, widely used in the preparation of nanomaterials, can be now applied to the fabrication of better functional catalysts, which should not be limited to the deposition of an active phase on a support (a strategy with well-known limitations), but thought in a different more holistic way, in which both chemical function and pore structure should be integrated in the same material. The creativity shown in the last years by synthetic chemists in the preparation of new mesoporous materials can be now further expanded to create new catalysts with both optimized porous structures and chemical functionality.
Please enjoy this Special Issue.
Rafael Luque and Javier Garcia Martinez