H-SAPO

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. 

Figure 23 shows H and Al CF MAS NMR spectra recorded during the hydration of calcined H-SAPO-34. The H MAS NMR spectrum of the calcined material recorded before the start of the hydration is dominated by a signal of... 

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.

At the beginning of the hydration of calcined H-SAPO-34 (i.e., after the start of the injection of nitrogen loaded with water vapor into the spinning MAS NMR rotor), only a change in the H MAS NMR spectra was observed (Figs 23b and c, left). After a water adsorption of 0.8 mmol/g, a significant decrease of the signal of... 

As found for H-SAPO-34, the hydration of H-SAPO-37 is separated into two successive steps. At a water adsorption of more than 6mmol/g, only a hydration of Bronsted acidic bridging OH groups occurred, whereas upon further hydration, a coordination of water molecules to Al atoms was found (277). In contrast to the adsorption of water in H-SAPO-34, the adsorption of water at framework Al atoms... 

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. 

To shed more light on this issue, the steady state of methanol conversion on zeolites HZSM-5, H-SAPO-34, and H-SAPO-18 was characterized by CF MAS NMR spectroscopy under CF reaction conditions (49,261). 

It proved to be possible to identify ordered Si-O-Al environments in molecular sieves by 27Al -29Si REAPDOR (i.e. rotational echo adiabatic passage double resonance) NMR techniques.350 NMR studies have been reported for the following silicoaluminophosphate molecular sieves SAPO-5 351 SAPO-11 and -31 352,353 H-SAPO-34 and -37 354 and SAPO-44.355... 

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]. 

While these computational studies [34] were awaiting publication, a neutron diffraction study on H-SAPO-34 provided evidence for a protonated water molecule [38]. Whereas comments in the more popular press [39] stressed the apparent disagreement with previous calculations ( much of the confusion about how zeolites work stems from quantum calculations ) and used the entertaining title Quantum mechanics proved wrong , a comment to the original paper in the same issue... 

Figure 22.5 CPMD simulation (PW 91 functional) of two HjO molecules per two Bronsted sites in H-SAPO (the other cell contains only one Bronsted site and one HjO molecule) [7], One of the Bronsted protons is residing on the SAPO-framework all the time, the distance of the other Bronsted proton to O of the nearest HjO molecule shows large variations (upper curve). The lower curve...

Yashima et al. compared the catalytic performance of H-ZSM-5, H-FER, H-MOR, Ca-A, H-B-MFI, and H-SAPO-5 in the Beckmann rearrangement of cyclohexanone oxime [28]. H-B-MFI was calcined at 603 K only and tetra-n-propylammo-nium remained in the pore. The conversions obtained with Ca-A (molecular sieve 5 A, 8MR) and H-B-MFI were low. As shown in Figure 3, however, the selectivity for e-caprolactam was higher over Ca-A, H-FER (10 MR) and H-B-MFI and lower over H-SAPO-5 (12-MR), H-ZSM-5, and H-MOR (12-MR), which could accommodate cyelohexanone oxime in their pores. It was concluded that the selective formation of e-caprolactam proceeded on the active sites on the external surface of zeolite crystallites rather than in the narrow space of the zeolite pore [28]. They even deduced that at higher reaction temperature cyclohexanone oxime would enter the pore, producing undesirable products such as cyclohexanone and 1-cyanopentene, which are smaller than e-caprolactam, because of the size effect. On the other hand, Curtin and Hodnett reported that caprolactam selectivity was lower over the zeolites with smaller pore diameters [29]. 

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