Silicon-substituted aluminophosphate molecular sieves: Studies on SAPO-5, -11, -34 and -44 (original) (raw)
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Gels
The formation of silicoaluminophosphate gels using boehmite, Al isopropoxide, and di-n-propylamine as a template of silicoaluminophosphate gels as well as their subsequent crystallization into SAPO-11 molecular sieves was studied in detail using X-ray fluorescence spectroscopy (XRF), powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption–desorption methods. The effect of the chemical and phase composition of silicoaluminophosphate gels on the physicochemical properties of SAPO-11 molecular sieves was shown. The secondary structural units that the AEL lattice is composed of were found to be formed at the initial stage of preparation involving aluminum isopropoxide. Several approaches to control their morphology and secondary porous structure are also proposed.
Mechanisms of silicon incorporation in aluminophosphate molecular sieves
Journal of Molecular Catalysis A: Chemical, 1997
Static lattice simulations are presented on the microporous aluminophosphate A1PO4-5. Minimisation methods together with lattice dynamical calculations are able to identify stable minima for the structure. We show, in agreement with the recent work of Henson et al., that reduction of the symmetry of the structure from P6cc to P6 leads to the relaxation of A1-0-P angles from the linear structure reported in earlier crystallographic structures. We find that the orthorhombic space group Pcc2 suggested recently by Mora et ul. has only a very slightly higher energy than that calculated for the P6 structure. Differences in bond lengths and angles between the two structures are correspondingly small. We find that the inclusion of a representation of the polarisability of the oxygen in the potential model is essential in removing the linear A1-0-P angles in the simulated structures.
Microporous and Mesoporous Materials, 2009
Low silicon content SAPO-34 was successfully synthesized using the conventional hydrothermal crystallization in the static condition. Effects of different synthesis conditions including crystallization temperature and the silicon source were investigated through X-ray diffraction patterns. It was concluded that the silicon source had a significant effect on silicon incorporation into the alumino-phosphate building blocks in the course of crystallization. Using precipitated silica instead of silica sol resulted in the formation of impurities of alumino-phosphates phases whose crystalline structures collapse at the high temperature of calcination. For the low silicon SAPO-34 synthesis, the optimum crystallization temperature, as a less critical parameter than silicon source, was about 190°C. Acidity of successfully synthesized low silicon content SAPO-34 has been compared with those of medium and high silicon content SAPO-34s through NH 3 -TPD, TG and in-situ FTIR. The measured amounts of adsorbed ammonia on the samples, using a thermogravimetric (TG) instrument, indicated that the total acidity of the low silicon content sample was only slightly lower than those of higher silicon content SAPO-34s, and this was in agreement with in-situ FTIR data. The lack of acidity increase by increasing the silicon content of the synthesis gels was explained by the formation of silicon islands as revealed by the 29 Si NMR data.
Synthesis novel aluminophosphate molecular sieves at atmospheric pressure
Current World Environment, 2006
Novel small pore aluminophosphates molecular sieves AlPO 4-Atm1, AlPO 4-Atm2 and AlPO 4-Atm3 have been synthesized using hexamethyleneimine template at atmospheric pressure (373K) for the first time. Gel composition Al 2 O 3 : P 2 O 5 : 1.16HEM: 45H 2 O was taken as the standard one which gives AlPO 4-Atm12. Change in water molar ratio to 67.5 gives AlPO 4-Atm2. On changing the aluminium source from catapal B to aluminium isopropoxide in same molar gel composition gives AlPO 4-Atm3. All the materials were characterized by XRD, SEM, TG/DTA, C & N analysis, FT-IR and MASNMR analysis. Elemental analysis shows that Al and P are in equal molar composition. XRD analysis shows that the synthesized samples are highly crystalline and new. SEM shows the morphology change with structure. TG/DTA analysis reveals the presence of maximum four stage elimination of templates. Carbon and nitrogen analysis gives the amount of template present in the sample. 27 Al MASNMR shows the presence of single type tetrahedrally coordinated aluminium atoms in AlPO 4-Atm1. 31 P MASNMR of the same sample shows the presence of two type of tetrahedrally coordinated phosphorous atoms.
Topics in Catalysis, 2000
The skeletal isomerization of 1‐butene was performed over a series of silicoaluminophosphate molecular sieves with AEL structure (SAPO‐11). The results were compared with those obtained over an aluminophosphate molecular sieve (AlPO4‐11). The three SAPO‐11 samples, with different acidic properties, were synthesized by either varying the chemical composition of the synthesis gel or by varying the preparation time of the aforementioned gel. The catalytic results indicate that irrespective of how the acidity of the SAPO‐11 samples is changed (viz., independently of the method elected to modify the acidic properties of the samples), a close parallelism between the selectivity towards the skeletal isomerization and the number of (moderate + strong) Brønsted acid sites (sites retaining pyridine above 623 K) was observed for the SAPO‐11 solids. These results, definitively, indicate the participation of these acid sites in the skeletal isomerization process.
Advances in the synthesis of pure aluminophosphates molecular sieves and derivatives
Keywords: molecular sieves-aluminophosphates-synthesis-properties-catalysts) AIP04 molecular sieves and their SAPO, MeAPO, MeAPSO, EIAPO and EIAPSO derivatives are novel non-zeolitic microporous materials. In the present paper, we discuss about the advances in the synthesis of these pure materials. The purity of the synthesized materials depends on many factors such as the type and the amount of the template involved, the nature of the reagents, the basicity and the chemical composition of the initial gel, the temperature and the time of crystallization. The template and temperature often play the most predominant role in obtaining a given pure structure. The NMR techniques provide sufficient details for detennining the framework topologies and the way a given element is incorporated. The incorporation of silicon and/or a metal generally induces a modification of the acid properties and, consequently, of the catalytic perfonnances. The nature of the active sites, the existence of 'both Bronsted and Lewis acidity, the acid strength were discussed on the basis of an exhaustive bibliography. A wide application field, especially in acid-base catalysis was also summarized.
2001
Mesoporous aluminophosphate (AIPO) and silicon substituted aluminophosphate molecular sieves (SAPO) have been synthesized using ordered array of cetyltrimethylammonium bromide as structure-directing agent at room temperature. The characteristics of these new materials are ascertained by low angle XRD, TGA, Ff-IR, SEM and DRlFf spectra. Vapour phase ethylation of toluene has been attempted over these molecular sieves as a probe reaction. The principal product is pethyltoluene in this reaction. This study also indicates more free uncondensed -OH groups in pure AI PO.
MRS Proceedings, 1991
ABSTRACTAluminophosphate (AlPO4–5, AlPO4–11, AlPO4–17, AlPO4–20) and silicoaluminophosphate (SAPO-5, SAPO- 11, SAPO-17, SAPO-20) molecular sieves of varying pore sizes (3–8 Å) were synthesized and their water adsorption and desorption properties were studied. Water sorption isotherms of AlPO4 molecular sieves were characterized by unusual isotherm shapes, that is, little or no initial adsorption followed by extreme adsorption leading to volume filling by hydrogen bonding and cooperative interaction in micropores, apparently due to the nonpolar nature of pore surfaces coupled with weak (reversible upon evacuation) chemisorption of water, and hysteresis loops extending to very low pressures. Although micropore filling in AlPO2's and isostructural SAPO's was completed almost at the same relative pressure (p/po), SAPO's exhibited less extreme adsorption isotherms as a result of their slightly more polar nature of pore surfaces compared to AlPO4's. Neither AlPO4apos;s nor...