Micromesoporous Monolithic Al-MSU with a Widely Variable Content of Aluminum Leading to Tunable Acidity

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Abstract

We report a new development of the so-called MSU route, the first route to use nonionic surfactants and block-copolymers, for the synthesis of silica with bimodal porosity in the micromesoporous range. Among the different synthesis strategies that have been explored until now, the “two-step” process led to the formation of stable hybrid micelles before silica condensation. This mechanism provided a large freedom in the adjustment of the reaction parameters and consequently of the final material structure. We report here that these hybrid micelles are stable enough to allow them to be concentrated through a new three-step method that involves (i) hybrid micelle assembly, (ii) hybrid micelle concentration (HMC), and (iii) silica condensation that leads to monolithic bodies instead of the usual powder. The silica:surfactant ratio is critical to achieving a well-ordered structure that exhibits a final pore size in between zeolites and mesoporous materials with a bimodal distribution centered at 0.6 and 2.0 nm. This method allowed us to homogeneously insert metal salts that remain trapped between the hybrid micelles after the second step without any disturbance of the silica framework. This is illustrated in this report by the addition of aluminum with different rates. Aluminum was inserted homogeneously up to a Si:Al (respectively, Al:Si %) atomic ratio of 2.6 (respectively, 38%). The Al-MSU obtained by this method exhibits strong acidity with a homogeneous broad range of acidity strength that varies linearly with the aluminum content, as demonstrated by NH3 TPD ranging from 100 to more than 600 °C and preliminary catalytic tests.

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