Silicoaluminophosphates

CAL-4, a structural study of a dual templated chabazite-type silicoaluminophosphate... 

Keywords porous materials, silicoaluminophosphates, solid-state catalysts, structuredirecting agents... 

The silicoaluminophosphate (SAPO) family [30] includes over 16 microporous structures, eight of which were never before observed in zeolites. The SAPO family includes a silicon analog of the 18-ring VPI-5, Si-VPI-5 [31], a number of large-pore 12-ring structures including the important SAPO-37 (FAU), medium-pore structures with pore sizes of 0.6-0.65 nm and small-pore structures with pore sizes of 0.4-0.43 nm, including SAPO-34 (CHA). The SAPOs exhibit both structural and compositional diversity. 

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

Silicoaluminophosphates (SAPOs), along with their crystalline aluminum phosphate counterparts (ALPOs), first discovered by Union Carbide workers in the early 1970s [41, 42], derive their acidity through the substitution of framework phosphorous by silicon thereby creating the charge imbalance which, when compensated for by protons, creates acidic centers. SAPOs in general have seen limited use in bond-breaking applications primarily due to weaker acidity, framework stability, or technoeconomic reasons. Of the rich variety of structures available,... 

In this study, we used the 29xe-NMR technique to examine the behavior of gaseous xenon adsorbed at different pressures on a series of intermediate phases isolated during the crystallization of a Faujasite-type silicoaluminophosphate, SAPO-37. Such a method has already proved successful in defining the different steps that successively occur during the crystaiiization process of zeoiites NaY, ZSM-5 and ZSM-20 [10] gel restructuration, increase of the crystallinity of the... 

VII. Investigations of Chemical Behavior and Local Structure of Surface Sites in Zeolites and Microporous Silicoaluminophosphates... 

In the preceding decade, microporous silicoaluminophosphates have drawn increasing interest as solid catalysts in chemical technology, because of their acidic and shape-selective properties. H-SAPO-34 with the chabasite structure, for example, is a suitable catalyst for the conversion of MTO (210). H-SAPO-37 with the faujasite structure was applied for the isomerization of -decane (211) and the isobutylene/2-butene alkylation (212). 

Investigations performed by Minchev et al (215) indicated that the framework of crystalline silicoaluminophosphates can be damaged upon the rehydration of the template-free material. In the case of rehydrated template-free H-SAPO-5 and H-SAPO-34, for example, a strong loss of the crystallinity occurs in the presence of water. However, the crystallinity can be completely restored after an additional dehydration at 823 K. Hydration of H-SAPO-37 at room temperature causes irreversible structural changes and leads to a material that is totally amorphous to X-ray diffraction (216). At temperatures of more than 345 K, template-free H-SAPO-37 exhibits a high stability toward hydration (216). 

To investigate the hydration and dehydration processes of H-SAPO-34 and H-SAPO-37, H and Al MAS NMR spectroscopy was applied under CF conditions with the equipment shown in Fig. 12 (217). The chemical behavior and the change of the silicoaluminophosphate framework were monitored as nitrogen loaded with water or dry nitrogen was injected into the MAS NMR rotor filled with the silicoaluminophosphates. By this approach, the primary adsorption sites of water in silicoaluminophosphates and the variation of the aluminum coordination were observed. Furthermore, the formation of framework defects and the conditions of water desorption were characterized. 

Fig. 23. H (left) and Al MAS NMR spectra (right) of silicoaluminophosphate H-SAPO-34 recorded during the hydration of the calcined sample in a flow of nitrogen loaded with water. Reproduced with permission from (277). Copyright 2003 Elsevier Science.

Hydration of the NH4-form of SAPO-34 and SAPO-37, that is, of materials that were ammoniated at the bridging OH groups, caused a coordination of water molecules exclusively to Al atoms in =P-O-A1= bridges. This process led to a hydrolysis of the framework (220). No hydrolysis of the silicoaluminophosphate framework occurred, provided that not only the bridging OH groups (SiOHAl), but also the aluminophosphate framework (=P-O-A1=) was covered by ammonia. The latter finding may explain the stabilizing effect of preloaded ammonia on silicoalumino-phosphates toward hydration and weak hydrothermal treatments as recently observed for H-SAPO-34 (227). 

Among the early investigations of methanol adsorption and conversion on acidic zeolites, most of the H and C MAS NMR experiments were performed under batch reaction conditions with glass inserts in which the catalyst samples were fused. Zeolites HZSM-5 76a,204,206,264-272), HY 71,72), H-EMT 273), HZSM-12 274), HZSM-23 275), H-erionite 275), H-mordenite 271,272), and H-offretite 275,276), silicoaluminophosphates H-SAPO-5 271,274), H-SAPO-11 274), and H-SAPO-34 76,277,278), as well as montemorillonite 279) and saponite 279) were investigated as catalysts. 

More recently phosphorus-containing zeolites developed by Union Carbide (alu-minophosphates, silicoaluminophosphates) were shown to be equally effective in methanol condensation.439-444 ZSM-5 was also shown to exhibit high activity and selectivity in the transformation of Fischer-Tropsch oxygenates to ethylene and propylene in high yields.445 Silicalite impregnated with transition-metal oxides, in turn, is selective in the production of C4 hydrocarbons (15-50% isobutane and 8-15% isobutylene).446... 

Other elements, such as Ga and Ge, can substitute for Si and A1 in the zeolitic framework, and there are claims that many other elements can also do so. New classes of nonsilicate zeolite-type crystalline aluminophosphates (31) and silicoaluminophosphates (SAPO) (65) have been reported but relatively little is known about their chemical behaviour. 

The syntheses of novel molecular sieves such as aluminophosphates, silicoaluminophosphates (SAPO), gallosilicates, aluminogermanates, ferro-silicates, borosilicates, and chromosilicates, clearly open new vistas for the... 

Hydrothermal Stability and Cracking Behavior of Silicoaluminophosphate Molecular Sieve-37 with Different Silicon Contents... 

Synthesis of Microporous Silicoaluminophosphates in Hexanol—Water Biphasic Systems... 

Crystalline microporous silicoaluminophosphates have been patented as SAPO-n (1) or MCM-n (2) materials. The SAPO materials crystallize from an aqueous medium in the presence of organic templates, the MCM materials from a biphasic medium, using similar templates. Most of the actually known MCM s and SAPO s are crystallographically different apart from SAPO-34, SAPO-44, SAPO-47 and MCM-2 which have the chabasite topology (2,2) The structure of other MCM materials is presently unknown. 

From the available literature it appears that the Si, A1 and P ordering in the two groups of microporous silicoaluminophosphates should be different. The anhydrous chemical composition of SAPO-n corresponds to (1) ... 

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