Silica nanoboxes from alumina rich zeolites: thermal and chemical stability of the monomodal and bimodal materials
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Catalysis Letters, Vol. 114, Nos. 3–4, April 2007 (Ó 2007) DOI: 10.1007/s10562-007-9053-5
Silica nanoboxes from alumina rich zeolites: thermal and chemical stability of the monomodal and bimodal materials R. Le van Mao,* L. Lu, N. T. Vu, Q. Zhao, H. T. Yan, and N. Al-yassir Industrial Catalysis Laboratory, Department of Chemistry & Biochemistry, Concordia University, 7141 Sherbrooke West, SP 275-09, Montreal, Quebec Canada H4B 1R6
Received 22 November 2006; accepted 16 January 2007
Silica nanoboxes synthesized from alumina-rich zeolites, showed two distinct categories of nano-structured materials. Entirely mesoporous nanoboxes (monomodal Al-nanoboxes) were highly thermally and chemically resistant even in the acidic form whereas calcination at high temperature of materials still containing some zeolite remnants (bimodal Al-nanoboxes), resulted in dramatic loss of surface area and pore volume. Pore closure by mobile and amorphous aluminic species was believed to occur. The high thermal and chemical stability of the monomodal Al-nanoboxes was found to be related to the highly siliceous character of the mesopore walls which contained isolated tetrahedral Al atoms linked to tetrahedral Si atoms. Treatment of the acid form of the bimodal nanoboxes with La or Ce containing solutions resulted in quite thermally stable materials owing to the reduction of the number of protonic sites by the rare-earth metal ions. KEY WORDS: silica nanoboxes from Al-rich zeolites; thermal stability; monomodal; bimodal pore size distribution; structure of the cavity walls.
1. Introduction Novel mesoporous materials, silica nanoboxes [1], were synthesized by controlled dealumination of alumina-rich zeolites (X and A types). In contrast with the synthetic MCM-41 materials [2] which show relatively low thermal and chemical stability [3], the new silica nanoboxes could withstand a temperature of at least 700 °C or the loading of triflic acid – a superacidic compound – up to 20–25 wt without undergoing significant structural collapse [1]. Silica nanoboxes were obtained by pore enlargement of zeolite materials using ammonium hexafluorosilicate (AHFS) in buffered solution which ‘‘selectively’’ removed some zeolite walls in the way that the newly created nanometer-sized cavities exhibited a quite narrow pore size distribution (monomodal Al-nanoboxes) [1]. However, if some zeolite ‘‘clusters’’ were left in these cavities (bimodal Al-nanoboxes), the material underwent a rapid pore closure upon activation at high temperature, resulting in a dramatic decrease in surface area and pore volume. In the present paper, we report the results of our investigations on the chemical composition and structure of the walls of the mesoporous cavities of the monomodal Al-nanoboxes. We also report that in the bimodal Al-nanoboxes, the ‘‘mobile’’ aluminic species resulting from the thermal degradation of the (left-over) zeolite *To whom correspondence should be addressed. E-mail: [email protected]
clusters are the cause of the pore closure. Finally, some methods
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