A comparative study of direct versus post-synthesis alumination of mesoporous FSM-16 silica

•Long, medium and short range order depends on the manner of Al insertion.•Distribution of Al can be controlled by the method of alumination.•Direct alumination affects skeleton vibrations more than the post-synthesis one.•Post-synthesis alumination is a more efficient means of introducing acid sites.

Al-FSM-16 mesoporous silicas were synthesized either by direct method, from Al-kanemite (Al-FSM-16/D), or by post-synthesis impregnation of purely siliceous FSM-16 with Al(NO3)3 (Al-FSM-16/P) and characterized with XRD, XRF, SEM, TEM, nitrogen sorption isotherms, 27Al and 29Si MAS NMR, FTIR, XPS, NH3-TPD, FTIR of pyridine adsorption and catalytic decomposition of ethanol. Only substitutional Al sites exist in Al-FSM-16/D, while in Al-FSM-16/P some Al remains in extra-lattice positions. Upon transformation of Al-FSM-16/D into hydrogen form a certain amount of extra-framework Al is formed. Direct alumination introduces a higher degree of structural disorder. In Al-FSM-16/D, Al is preferentially accumulated at inner pore walls, while in Al-FSM-16/P external surface is Al-rich. Post-synthesis alumination is more efficient in introducing acid sites into FSM-16. The generated acidity is of Brønsted and Lewis nature, the latter being stronger than the former.

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