The discovery of ordered mesoporous materials has opened great opportunities for new applications in heterogeneous catalysis, thanks to their hitherto unprecedented intrinsic structural features. Evidence shows that, however, these materials have not met the researchers' expectations mainly because of the severe limitations related to the strength of acid sites and to the thermal/hydrothermal stability, significantly lower than those of zeolites and due to the amorphous nature of the mesostructured materials. These features are highlighted in the first part of this review, where the peculiarities of mesostructured materials are compared with those of zeolite catalysts in some reactions of industrial interest. New synthesis strategies, especially designed for preparing materials with improved physico-chemical and textural properties, together with the catalytic features of the resulting materials, are described and discussed in the second part of the review.

Porous materials in catalysis: challenges for mesoporous materials

Incorporation of gallium into zeolites: syntheses, properties and catalytic application.

Mesoporous templated silicates: an overview of their synthesis, catalytic activation and evaluation of the stability

A pure silica MCM-41 and two aluminosilicates with Si/Al = 1.25 and 30 have been synthesized hydrothermally under basic conditions in the presence of cationic surfactants. Organic molecules could be completely extracted from the mesopores by treating as-made compounds with an ethanolic solution of ammonium nitrate at 333 K. The procedure is particularly efficient for the low-silica compound:  all surfactant molecules are removed within less than 15 min. After treatment, NH4+ cations, which balance the charge of aluminum atoms in aluminosilicates, can be removed by heating extracted Al-MCM-41 at moderate temperature. In the pure-silica material, the in-situ decomposition of NH4+ cations directly forms silanol groups on the surface of the mesopores. Extracted materials possess highly ordered structures with properties similar to those of directly calcined materials. The procedure reported here is fast, cheap, and efficient to remove completely surfactant molecules from the pores of MCM-41 materials with var...

A Fast and Efficient Ion-Exchange Procedure To Remove Surfactant Molecules from MCM-41 Materials

New trends in the design of supported catalysts on mesoporous silicas and their applications in fine chemicals

The influence of the aluminum contents of MCM-41 (Si/Al ratio varied between 2.7 and 69) on the coordination of Al, on the acidity, and on the catalytic properties is studied by 27Al MAS NMR, temperature programmed desorption of ammonia, and the conversion of acetone. Based on TPDA results, an assignment of the peaks of desorption of ammonia is proposed. With low Al contents, the concentration of strong Bronsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al contents, however, the number of strong acid sites decreases again. Besides usual strong Bronsted sites, TPDA reveals the existence of weak Bronsted and Lewis sites and of Lewis sites of a high acidic strength. Separation between the two types of sites of weak acidity is incomplete. In the temperature programmed decomposition of NH4-exchanged MCM-41 samples, strong Lewis acid sites temporarily re-adsorb ammonia generated by the thermal decomposition of the NH4+ ions located at the Bronsted sites. Ammonia re-adsorption at Lewis sites results in a shift of the maximum of the TPDA peak to a higher temperature. Calcination of samples leads to the formation of strong Lewis sites at the expense of Bronsted sites. To evaluate the acidity of Al-MCM-41, recording of decomposition profiles has been extended to Ga- and Fe-MCM-41. Catalytic activity in the conversion of acetone reaches its maximum with the sample based on Al-MCM-41 with the molar Si/Al ratio of 6.85. Highly favored formation of isobutene points to a comparably high acidic strength of the active sites.

W Storek

Papers

Acidity and active sites of Al-MCM-41