SAPO-34 CHA

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. 

Figure 10.9 Framework structures of ZSM-5 (MFI) (left) and SAPO-34 (CHA) (right).

Related infrared spectroscopic studies of weak bases such as methanol and water have also been performed on the silicoaluminophosphate SAPO-34 (CHA structure type) and its zeolitic analogue SSZ-13. SAPO-34 is known to possess acidity that is among the highest found in aluminophosphate-based solids. Nevertheless, SAPO-34 is found to possess weaker acidity than the isostructural zeolite. ... 

Recently, Yu and coworkers have successfully controlled the crystal size and morphology of the silicoaluminophosphate zeolite SAPO-34 (CHA) under microwave irradiation in the system of AljOj-P Oj-SiOj-TBAOH-H O by studying the synthetic factors such as the silica source, water content, crystallization time, and aging time [52]. Microwave-assisted synthesis proved to be an efficient approach to produce nanosized zeolite crystals [53]. As a comparison to traditional hydrothermal synthesis at 200°C for 72 h, the microwave-assisted synthesis of SAPO-34 needed only 1 h at 200°C, which greatly reduced the reaction time. 

In the metal aluminophosphate (MeAPO) family the framework composition contains metal, aluminum and phosphorus [27]. The metal (Me) species include the divalent forms of Co, Fe, Mg, Mn and Zn and trivalent Fe. As in the case of SAPO, the MeAPOs exhibit both structural diversity and even more extensive composihonal variation. Seventeen microporous structures have been reported, 11 of these never before observed in zeoUtes. Structure types crystallized in the MeAPO family include framework topologies related to the zeolites, for example, -34 (CHA) and -35 (LEV), and to the AIPO4S, e.g., -5 and -11, as well as novel structures, e.g., -36 (O.Snm pore) and -39 (0.4nm pore). The MeAPOs represent the first demonstrated incorporation of divalent elements into microporous frameworks. 

In the early 1980s researchers at Union Carbide discovered that small-pore size silicoaluminophosphate (SAPO) molecular sieves were effective for converting methanol to ethylene and propylene. The best performances were obtained with SAPO-34 and SAPO-17 catalysts (6). SAPO-34 has the CHA structure with a three dimensional pore system consisting of large cavities (about 9.4 A in diameter) separated by small windows (3.8 x 3.8 A). 

Chabazite CHA 3.8 X 3.8 3 CoAPO-44 CoAPOA SAPO-34 LZ-218 Linde D Linde R MeAPO-47 MeAPSO-47 Willhendersonite ZK-14 ZYT-6... 

Strictly, these catalysts are not zeolites (this name is reserved for aluminosilicates), but aluminumphosphates (AlPOs) or silicon-aluminumphosphates (SAPOs). It is indeed possible to synthesize acidic high-silica (H-SSZ-13) [3] and SAPO catalysts (H-SAPO-34) [4] with the same framework structure (CHA) [5]. 

CHA SAPO-34 (methanol to olefins or MTO process- demonstration unit) ... 

CHA Chabazite (also a known mineral phase), SAPO-34 (silico-aluminophos-phate, DAF-5 (Mg-Aluminophosphate)... 

Li et al. [54] increased the length of their tubular SAPO-34 membranes by a factor of 5 by controlling thoroughly the synthesis conditions. Sato et al. [55] have developed industrial size (40 cm long) chabazite (CHA)-type zeolite membranes by controlling the synthesis conditions and used them in pervaporation showing that continuous research is devoted in this field to be able to compete with polymeric membranes. 

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