SAPOs spectroscopy

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. 

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). 

Catalysts Characterization. Following pretreatment of the SAPO molecular sieves, the catalysts were characterized by temperature programmed desorption (TPD) of ammonia and infrared spectroscopy. To assess the acidity of the samples, the desorption of ammonia from the catalysts was performed in a manner similar to that described by van Hooff et. al. [11]. For the ammonia TPD experiments, typically 0.1 gram of the molecular sieve sample was supported on quartz wool inside a 9 mm O.D. quartz reactor equipped with axial thermowell which contacted the top of the... 

Characterization of the catalyst acid sites by infrared spectroscopy correlated well with the results of ammonia desorption experiments. The transmission spectra of HZSM-5 (Figure 2a), and SAPO-34 (Figure 2b) following in vacuo pretreatment at 650 K all showed absorbance bands near 3610 cm. Since these two molecular sieves were the only samples to show high-temperature NH3 desorption peaks... 

The main applications of Al NMR spectroscopy in connection with zeolitic matmals has been (anoong others) the monitoring of dealumination processes, structure elucidation (for AIPO4 and SAPO molecular sieves) and the detection of extra-framework aluminium. Extra-lattice aluminium is octahedrally coordinated and gives rise to resonance lines at about 0 ppm for zeolites and between -7 and -23 ppm for AIPO4 and SAPO molecular sieves, whereas framework aluminium is tetrahedrally coordinated and resonates between ca. SO and 65 ppm for zeolites and 29 to 46 ppm for AIPO4 and SAPO molecular sieves. 

Solid-state NMR spectroscopy has been demonstrated as a well established technique for characterization of zeolites and other porous materials with respect to structure elucidation, pore architecture, catalytic behaviour and mobility properties. The latest progress in the development of NMR techniques, both with respect to software and hardware improvements, has contributed to the present state of the art for NMR within the field of characterization of zeolitic materials. Furthermore, the introduction of NMR imaging (110), two-dimensional quintuple-quantum NMR spectroscopy (111) and transfer of populations in double resonance (TRAPDOR) NMR (112,113) will extent the horizons of zeolite characterization science. As a final example, the Al => Si TEDOR experiment directly proves, for the first time, that silicon substitutes for phosphorous atoms in the framework of SAPO-37 (114). The Al... 

SAPO-31 molecular sieve was synthesized using dibutylamine as the organic template and was characterized by XRD, SEM, TPD of NHy, DTA-TGA, sorption measurements and IR spectroscopy. Its catalytic activity in m-xylene isomerization at various temperatures and different space velocities was studied. The influence of adding Pt as the promoter on its activity was also examined. Besides its activity was compared with those of the medium pore SAPO-11 and large pore SAPO-5. SAPO-31 showed less isomerization activity than SAPO-11 at higher conversion levels. The isomerization activity of SAPO-31 was more in the presence of Pt and Hz as the carrier gas. 

In MAS-NMR spectroscopy a broad P peak was observed centered near --30.7 ppm for the calcined SAPO-11 sample. For the hydrated form two peaks centered at -23 and -29 ppm and designated B and C in the following were obtained with a ratio close B C to 1 4 but decreasing with Si content as shown in table 1 from ref. 37. In addition a very broad signal (-20 to -23ppm) was also observed for some samples and is designated A in the following. 

Broensted and Lewis acid sites in crystalline phosphates, silicates and in gels with moleculeu sieve properties were studied by IR-spectroscopy. Two types of bridged hydroxyls were fovmd in SAPO-5 which were accessible to adsorbed molecules and were able to interact with ethylene. Lewis sites in metallophosphates, zirconosilicate and in Ti-containing silica gel were observed which did not interact with weak bases (CO, hydrogen) but formed strong complexes with acetonitrile. They were supposed to be framework metal ions in tetrahedral coordination. 

B MAS NMR yields quantitative information about the incorporation of boron into zeolite frameworks. H MAS NMR and IR spectroscopy show that OH groups introduced into the framework by boron substitution are non-acidic. 2D proton spin diffusion measurements of the zeolite SAPO-5 reveal that defect OH groups are adjacent to acidic bridging hydroxyl groups and do not exist in an amorphous phase. Strongly adsorbed water molecules in mildly steamed zeolites H-Y can be explained by Lewis sites. 

Basically, there are two approaches for the use of R NMR spectroscopy of sapo-nins which consist either in working on the underivatized (in deuterated MeOR, pyridine or DMSO) or on derivatized material (peracetates). Each of the methods has its advantage, and inconveniences, which will be discussed below. 

Sinha et al. [281] investigated the framework vibrations of silicoalumino-phosphate SAPO-39 and of magnesium aluminophosphate MAPO-39 by DRIFT spectroscopy and interpreted them according to the approach of Flanigen et al. [112]. The OH stretching region was studied by FTIR transmission spectroscopy (vide infra. Sect. 5.4.1). 

Characterization of the OH stretching region by DRIFT spectroscopy besides TPD measurements was reported by Chen et al. [535] for a series of SAPOs (SAPO-5, SAPO-17, SAPO-18 and SAPO-34), prepared by these authors. Besides... 

Diffuse reflectance IR spectroscopy (DRIFT) was, furthermore, successfully employed in studying molecular sieves of the new generation [957] such as aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs). Thus, Zscherpel et al. and Loeffler et al. [563,564] studied by DRIFT the range of fundamental OH stretching, OH deformation and framework vibrations of AIPO4-18 (upon decomposition of the template, tetraethylanunonium hydroxide), the fundamental and combination vibrations of hydroxy groups of SAPO-5 and SAPO-17 as well as the framework region of SAPO-34. In the latter case a comparison was made with the transmission spectrum. 

Titanium siHcoaluminophosphate with MFI-structure, designated as TAPSO, was prepared by Tuel [573], and characterized, inter aha, by transmission IR spectroscopy using the KBr technique. The author found OH stretching bands at 3745, 3670,3620 and 3530 cm similar to those observed with SAPO-5. Also, a band around 970 cm was detected and related to the presence of Ti in the framework (cf. Sect. 5.2). 

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