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

Al-FSM-16 mesoporous silicas were synthesized either by direct method, from Al-kanemite (AI-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, Al-22 and Si-29 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 Bronsted and Lewis nature, the latter being stronger than the former. (C) 2016 Elsevier Ltd. All rights reserved.