Synthesis and properties of SAPO-5 molecular sieves. Silicon incorporation into the framework

Abstract

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...

One-Dimensional Molecular Sieves for Hydrocarbon Cold Start Emission Control

“…They must be in H ? form with the density of Bronsted acid sites directly related to the isomorphous substitution of silicon in the AIPO lattice [10][11][12][13][14].…”

Synthesis and physical properties of single crystals of zeolite-like aluminophosphates of the AFI type

“…However, in the IR spectra of the AFI (AlPO 4 -5) aluminophosphate crystals, this band is shifted toward higher frequencies by ~100 cm -1 (curve 2 ) as compared to the position of the corresponding band in the IR spectrum of the aluminosilicate zeolite NaA (curve 1 ). This appreciable high-frequency shift is associated with the difference between the T -O bond lengths: the P-O distance in the structure of the AFI (AlPO 4 -5) aluminophosphate crystal is equal to 1.54 Å, which is substantially less both than the Si-O bond length (1.61 Å) and than the Al-O bond length (1.75 Å) in the NaA zeolite [10].…”

“…The broad IR band observed near 1000 cm -1 is attributed to the antisymmetric stretching vibrations of the T -O bonds in the T O 4 tetrahedra of the framework and is characteristic of all zeolites and zeolite-like materials [8][9][10]. However, in the IR spectra of the AFI (AlPO 4 -5) aluminophosphate crystals, this band is shifted toward higher frequencies by ~100 cm -1 (curve 2 ) as compared to the position of the corresponding band in the IR spectrum of the aluminosilicate zeolite NaA (curve 1 ).…”

Impact of Conventional and Microwave Heating on SAPO‐5 Formation and Brønsted Acidic Properties

“…With x Ն 0.15, the substitution mechanism is a combination of SM IIa and SM IIIhet. Ojo et al [14] investigated the influence of the Si mole ratio x for 0.05 Յ x Յ 0.20 in the gel on the mechanism of the Si incorporation into SAPO-5. The authors carried out hydrothermal syntheses by conventional heating.…”

“…But also in this case, Brønsted acid sites can be generated at Si-O-Al linkages at the boundary of the Si islands. [12,14] It is obvious that incorporation of silicon atoms according to SM IIa converts the AlPO-n framework into an acidic SAPO-n material. Furthermore, it is desirable to replace as many P as possible to gain a high concentration of acid sites.…”