Hydrothermal stability, SAPO

The spectrum of adsorption pore sizes and pore volumes and the hydrophilic surface selectivity of the MeAPOs are similar to those described for the SAPOs. The observed catalytic properties vary from weakly to strongly acidic and are both metal- and structure-dependent. The thermal and hydrothermal stability of the MeAPO materials is somewhat less than that of the AIPO4 and SAPO molecular sieves. 

SAPO-37 molecular sieve which has the crystalline structure of faujasite differs from this zeolite by the presence of phosphorus in the structure (1). It was shown that this element increases the thermal and hydrothermal stability of the structure (2). With regards to acidity, the SAPO-37 materials have acidic properties (1,3,4) with two OH groups very similar to those of faujasites (1,4). It was also observed that the SAPO-37 materials have besides acid centers of medium strength a small number of protonic sites stronger than in HY or even than those of an ultrastable LZY-82 (4). 

On the other hand, Briend et al. investigated the thermal stability of SAPO-5, -34, -37 by using XRD, IR, and Si MAS NMR. Thermal and hydrothermal stability of SAPOs strongly depends on the type of crystal structure. Figure 15 shows Si MAS NMR spectra of SAPO-34 after various treatments. Although the sharp peak of Si(4Al) at -91.6 ppm is broadened by hydration at room temperature, the intensity of these peaks was restored by the evacuation at 573 K. A similar decrease in the crystallinity of SAPO-34 by hydration at room temperature was also detected by XRD. However, this does not suggest that thermostability of SAPO-34 was poor in a wet... 

To further demonstrate the improved hydrothermal stability of the small-pore zeolite supported Cu SCR catalyst, the NOx conversion efficiencies in the temperature range from 150 to 550 °C on a Cu/SAPO-34 catalyst, before and after hydrothermal aging at 670 °C in a flow of 4.5 % H20/air mixture for 16 and 64 h. 

AFX), SAPO-18 (AEI), SSZ-39 (AEI), [Ga]SSZ-13 (CHA), ZK-5 (KFI), and STA-7 (SAV) have also been reported to exhibit the same properties [58-63], In addition, catalysts with Cu being incorporated into the small-pore zeolite matrix during the zeolite synthesis step, instead of a post ion-exchange step, have also been found to show excellent SCR activity and hydrothermal stability [63-66],... 

Figure 6.2 shows NH3-SCR activity over the hydrothermally pretreated copper-exchanged smaU-pore zeolites (Cu-SSZ-13, Cu-SSZ-16 and Cu-SAPO-34) and the medium-pore Cu-ZSM-5. It can be concluded that after hydrothermal treatment at 750 °C the copper-exchanged smaU-pore zeolites can still maintain the high SCR activity, while the activity of Cu-ZSM-5 deteriorates significantly [53]. The restricting dimension of the smaU-pores in Cu-SSZ-13, Cu-SSZ-16, and Cu-SAPO-34 is the most prominent reason for the high hydrothermal stability of these materials as well as their exceptional NH3-SCR activity [53]. 

Barger PT, Lesch DA. Hydrothermal stability of SAPO-34 in the methanol-to-olefins process. Arab J Sci... 

The thermal and hydrothermal stabilities of the MeAPO, MeAPSO, ElAPO, and ElAPSO families identified in Tables II and III are more diverse than those of the AIPO4 and SAPO families. Here, the stability depends on both chemical composition and crystal form. Many species display stability similar to that of AlP04 s and SAPO s, while others show significantly reduced crystal retentions when exposed to high temperatures or severe hydrothermal conditions. 

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). 

Ni-modified SAPO-5 molecular sieves have been prepared by addition of NiO or Ni acetate to the reaction mixture used for hydrothermal crystallization of SAPO-5. The crystallinity and thermal stability of the materials obtained are comparable with those of pure SAPO-5. 

Small-pore molecular sieve SAPO-34 is known to exhibit exceptional thermal stability and maintain its crystalhne structure up to 1,000 °C [71]. Since the acid/ exchange sites in a SAPO-34 are associated with the Si tetrahedral sites, they are less susceptible than tetrahedral Al sites to H2O interaction and hence are more stable to hydrothermal aging [72]. 

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