Silicoaluminophosphate molecular sieves of SAPO-5 type are synthesized with variable Si/(AI + P + Si) mole ratios using triethylamine as the organic additive. The crystalline products are characterized by several physico-chemical methods. Incorporation of silicon into the framework of SAPO-5 is demonstrated by the variation in the unit-cell volume with silicon mole fraction, chemical analysis and magic-angle-spinning nuclear magnetic resonance. When Si/(AI + P + Si) < 0.077, silicon substitutes predominantly for phosphorus. For compositions where Si/(AI + P + Si) > 0.077, it appears that a combination of mechanisms, substitution of one silicon for one phosphorus and substitution of two silicons for aluminium-phosphorus pairs, takes place. Surface analysis using depth profiling shows an inhomogeneous distribution of silicon at higher silicon content. Preliminary catalytic results in 0 -and rn-xylene isomerization are discussed. The aluminophosphate-based molecular sieves are an important relatively new class of adsorbents and catalytic material^.'^^ Microporous materials containing framework silicon are denoted by the acronym SAP0.2,3 The generation of SAPOs is considered to follow a mechanism by which silicon substitutes into the corresponding AlPO, framew o r k ~. ~ This substitution3 can involve (1) replacement of one aluminium by one silicon (SM1); (2) replacement of one phosphorus by one silicon (SM2) or (3) replacement of aluminium-phosphorus pairs by two silicons [SM3 Ho (Ho = homogene~us)].~ The substitution of silicon for phosphorus leads to the formation of Brnnsted-acid sites of medium acid strength. Increasing the amount of silicon in the framework of these SAPO materials can lead to high activity in certain acid-catalysed reactions. It is therefore very important to synthesize SAPOs with high silicon content. Additionally, it is highly desirable to control the synthesis so as to aim at SAPO catalysts with silicon content optimized to produce the maximum number of Brnnsted-acid sites of medium strength acidity.Recent papers have appeared on the synthesis and/or characterization of SAPO-55-'2 and a few exist on the incorporation of silicon into the AIPO, framework. Mertens et aL5 studied the incorporation of silicon into a series of SAPO-5 materials with silicon mole fraction, x = 0.01, 0.15, 0.28 and 0.36. These authors suggest silicon substitution by SM2 when x = 0.01 and substitution by combined SM2 and SM3 He (He = heterogeneous) processes when x 3 0.15. It is of interest to know the value of x corresponding to the change in mechanism which occurs when x is increased from 0.01 to 0.15. Moreover, it is important to know whether the build-up of siliceous regions results in composition gradients in the crystals. These questions are addressed in the present work by considering SAPO-5 synthesized to have a silicon mole fraction in the range 0.05-0.17. These materials are characterized using several physical methods and a preliminary catalytic evaluation is made. Simple cluster M O calculations are used t...
lsomorphous substitution of Al by Ga into the framework of zeolites Y, ZSM-20, CSY, USY and beta has been achieved by reaction with aqueous gallium fluoride solution. Characterisation by XRD, solid-state NMR, FTIR, SIMS and sorption studies demonstrates that Ga is incorporated into the framework of the zeolite, except for USY where Ga substitutes for non-framework aluminium. Preliminary catalytic results for the conversion of hexane show that reaction rates increase with increased amounts of framework Ga.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.