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Hierarchical Silicoaluminophosphates by Postsynthetic Modification: Influence of Topology, Composition, and Silicon Distribution

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journal contribution
posted on 12.08.2014, 00:00 by Danny Verboekend, Maria Milina, Javier Pérez-Ramírez
AlPO-5, SAPO-5, SAPO-11, and SAPO-34 silicoaluminophosphates (SAPOs) are exposed to various acid and base treatments aimed at mesopore formation and investigation of associated physicochemical modifications. SAPOs amorphize strongly in aqueous NaOH, requiring the use of organic bases (e.g., tetrapropylammonium hydroxide or diethylamine) to preserve the crystallinity during base treatment. In acid media (HCl, H4EDTA, and Na2H2EDTA), SAPO-11 remains fully crystalline, while SAPO-34 strongly amorphize. No clear influence of the framework topology is established. The high resistance in alkaline media and low stability in acid media of SAPO-34 is attributed to its relatively high silicon content. Base treatment of SAPOs leads predominately to the formation of intercrystalline porosity. Still, an up to 4-fold increase in external surface and pore volume in SAPO-11 are achieved. Besides the formation of secondary porosity, base treatment of SAPOs induces a variety of (correlated) physicochemical changes. For example, the silicon distribution clearly influences the dissolution behavior of SAPO-11 in alkaline media, as zeolitic-like Si-domains are more resistant than AlPO domains. As a result, base leaching is selective to phosphorus and leads, depending on the silicon distribution, to either aluminum or silicon enrichment. The resulting changes in bulk composition can be directly related to the secondary porosity, as the Si and Al enrichment takes place predominately on the external surface. Acidity characterization (TPD of ammonia and IR spectroscopy of pyridine or 2,6-di-tert-butylpyridine adsorbed) shows that base treatment of SAPO-11 slightly reduces the concentration of Brønsted sites, while the number of Lewis sites is substantially increased. Moreover, the amount of Brønsted acid sites associated with the external surface is largely enhanced. The behavior of zeotypes, that is, AlPOs, SAPOs, and zeolites, in acid and basic aqueous solutions is generalized, highlighting the role of charge balancing cations. Catalytic evaluation of SAPO-11 shows the potential of base-treated samples in the alkylation of benzyl alcohol with toluene.