MESOPOMs · Nanostructured Mesoporous Polyoxometalate and Transition Metal-substituted Polyoxometalate Materials: Applications in Heterogeneous Catalysis

Nanostructured Mesoporous Polyoxometalate and Transition Metal-substituted Polyoxometalate Materials: Applications in Heterogeneous Catalysis

Project context and objectives Although the primary polyoxometalate (POM) clusters have attractive properties, especially in the fields of acid and oxidation catalysis, they lack significant advantages in various catalytic operations comparable to supported POM catalysts. The low surface area and enormous solubility in polar media are the main limitations of POMs for applications in many heterogeneous catalytic reactions. To overcame these drawbacks and therefore to improve the catalytic performance, the POM anions need to be dispersed in a high-surface-area nanoporous support. The feature of a mesoporous POM-based framework can incorporate new functionalities, such as high catalytic activity and recyclability. Nanoporous POM-based materials will provide a cost-effective, regenerable catalytic system for environmental remediation processes, including decomposition of pollutant gases and fine chemical synthesis (e.g. oxidation of hydrocarbon). The overall objectives of the MESOPOMs project are the synthesis of POM and transmission of metal-substituted POM clusters, and the design and development of new nanostructured mesoporous POM-based frameworks with a well-ordered pore structure and high internal surface area. The evaluation of catalytic activity of these mesostructures in heterogeneous catalysis as a 'green' method for organic synthesis is also another goal of this project. Work performed During the project (September 2008-August 2012), significant progress has been made in synthesis and catalytic applications of mesoporous POM-containing materials. In particular, in the first stage we have studied the chemistry of POM clusters in aqueous and alcoholic solutions. Several POM and TM-POM compounds with different molecular and electronic structures have been synthesised, including [P(W,Mo)12O40]-3, [EMo6O24H6]4- (E=Ni+2,Co+2,Cu+2,Zn+2), [EMo6O24H6]-3 (E=Co+3,Cr+3,Fe+3,Co+3) and [PMo12-nVnO40]-(n+3) (n=1-4) anions. These POMs were found to be excellent building blocks for the construction of novel mesoporous POM-based catalysts. We employed different soft- or hard-templated 'nanocasting' self-assembly approaches to prepare these composite structures and, so far, we have succeeded in preparing well-ordered mesoporous metal oxide-POM composite frameworks with different architecture of pore structure (pore geometry and pore size) and chemical composition (Al2O3, ZrO2, Co3O4, Cr2O3). A significant amount of work has been carried out on the characterisation of these materials, including chemical composition, nanoporosity and electronic structure determination. We either used a multitude in solution and solid-state techniques provided by the host institution (University of Crete) or through collaborations with other multidisciplinary laboratories. For example, detailed characterisation of the prepared materials was accomplished by various state-of-the-art techniques and instrumentation such as small angle X-ray scattering, X-ray diffraction, imaging techniques (transmission electron microscope - TEM, scanning electron microscope - SEM/EDS) X-ray photoelectron spectroscopy, X-ray diffuse scattering (synchrotron radiation), TGA/DSC/DTA, infrared (IR) and ultraviolet (UV)/vis spectroscopy and N2 physisorption. Main results Substantial progress has been made on the catalytic application of prepared materials. As we have shown, this new class of mesoporous materials has a great implication in various catalysis fields, including direct decomposition of nitrous oxide (N2O) and isopropanol, as well as selective epoxidation/oxidation of alkenes and alcohols, showing excellent catalytic activity and stability. The interdisciplinary nature of the project enabled the fellow to conduct a number of collaborations with faculty members within the host institution and beyond. So far, the research results have been disseminated through publications in seven high-impact peer refereed journals (at least two further papers are planned for the high-impact journals), including 'Chem. Mater.', 'J. Mater. Chem.', 'RSC Adv.' and 'Chem. Commun.', as well as two national and two international conferences. By participating in this project, the fellow has gained additional funding from other research grants, including the Greek Ministry of Education (GG??) and Special Account for Research - University of Crete. The main objectives of these projects are highly related to the subjects of MESOPOMs and can enhance or add value to the scientific contribution of this programme. Overall, the MESOPOMs project considerably advanced our knowledge of POM-containing composite materials and has made a significant scientific progress in field of mesoporous composite catalysts.

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