SAPO-34 composition

The entrapment-type nanocomposites can be prepared from zeolites and they are of two types zeolite-inorganic and zeolite-organic. Zeolite crystals are three-dimensionally linked network structures of aluminosilicate, aluminophosphate (ALPO), and silicoaluminophosphate (SAPO) composition and are porous, the pores being in the range of 2.8 to 10 A. Many of the highly siliceous, ALPO, and SAPO zeolites have been synthesized using organic templates such as tetrapropyl... 

Thirteen stmctures of various compositions, as AlPO SAPO, MeAPO, and MeAPSO, are available from UOP. 

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. 

The SAPO anhydrous composition can be expressed as 0-0.3R(Si, lyPJ02, where x, y and z are the mole fraction of the respective framework elements. The mole fraction of silicon, x, typically varies from 0.02 to 0.20 depending on synthesis conditions and structure type. Martens et al. have reported compositions with the SAPO-5 structure with x up to 0.8 [32]. Van Nordstrand et al. have reported the synthesis of a pure silica analog of the SAPO-5 structure, SSZ-24 [33]. 

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. 

The MeAPSO family further extends the structural diversity and compositional variation found in the SAPO and MeAPO molecular sieves. These quaternary frameworks have Me, Al, P and Si as framework species [27]. The MeAPSO structure types include framework topologies observed in the binary AIPO4 and ternary (SAPO, MeAPO) compositional systems and the novel structure -46 with a 0.7 nm pore. The structure of -46 has been determined [34]. 

Wu, X. and Anthony, R.G. (2001) Effect of feed composition on methanol conversion to light olefins over SAPO-34. Appl Catal A, 218, 241-250. 

The impact of the structural differences of ZSM-5 and SAPO-34 on representative MTO product compositions is shown in Figure 15.7. The medium-pore sieve... 

In addihon to shape selechvity and acid-site strength, other catalyst characteristics that influence the catalyhc performance of SAPO-34 have also been idenhfied. Variahon in the SAPO-34 gel composition and synthesis condihons have been were used to prepare samples with different median particle sizes and Si contents (Tables 15.3 and 15.4) [104]. In these samples the median parhcle size was varied from 1.4 to 0.6 xm, and the Si mole frachon in the product was varied from 0.14 down to 0.016. A comparison of samples B and E (which have similar parhcle size distributions) shows that reducing Si content decreases propane formation and increases catalyst life. A comparison of samples B and C (which have similar Si contents) illushates an increase in catalyst life with a reduchon in parhcle size. 

Table 15.4 Influence of SAPO-34 particle properties and composition on initial catalyst performance.

It is an appropriate means to determine the composition of a mixture of SAPO-37 and SAPO-40. Such an estimation is rather difficult to achieve by X-ray diffraction because most of their respective diffractogram peaks overlap. 

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) ... 

Along with A1P0 and SAPO, the MeAPO molecular sieves have extended the structural and compositional variety found among the growing numbers of AlPO,-based molecular sieves. Both the metal and the organic template exert a primary influence on... 

The MTO reaction on a calcined SAPO-34 with a unit cell composition of (Si2.88Ali8Pi5.12)072, supplied by SINTEF Materials Chemistry, Norway, has been investigated (90). The catalyst particles (52-140 mesh) were dried at 773 K for more than 3 h. Quartz particles (52-140 mesh) were placed between the quartz wool and the catalyst particles (Fig. 1) to minimize temperature gradients and improve the distribution of the flowing gas in the catalyst bed. 

The classical method of investigation of effects of diffusion on reactions is typically to run a reaction with catalyst particles of various sizes. For zeolites, the resistance of intracrystalline diffusion is normally much larger than that characteristic of molecular diffusion or Knudsen diffusion that could occur in the spaces between the zeolite crystals in a catalyst particle. Thus, the crystal size of the zeolite has to be varied instead of the particle size to determine the effects of diffusion on zeolite-catalyzed reactions. Kinetics of the MTO reaction has been measured with SAPO-34 crystals with identical compositions and sizes of 0.25 and 2.5 pm 89). The methanol conversion was measured as a function of the coke content of the two SAPO-34 crystals in the TEOM reactor. 

Silicoaluminophosphates (SAPO s) (1) are molecular sieves which contain tetrahedra of oxygen surrounded silicon, aluminum, and phosphorus. These microporous solids not only exhibit properties characteristic of zeolites but also show unusual physiochemical traits ascribable to their unique chemical compositions (1,2). 

Bulk and Surface Compositions. The chemical compositions of the molecular sieves used in this study are given in Table I in terms of tetrahedral atom (T-atom) fractions, and are grouped according to structure type. The bulk compositions of AIPO4-5, AlPO -20 and VPI-5 show the ideal 1 1 ratio of A1 and P characteristic of aluminophosphate molecular sieves. The SAPO materials have frameworks consisting of Si, A1 and P T-atoms. 

The two SAPO-20 materials have very different bulk concentrations. SAPO-20A has a composition consistent with a "silicon-substituted aluminophosphate", while the high bulk concentration of Si in SAPO-20B is more consistent with a "phosporous-substituted aluminosilicate". The other SAPO samples all have sufficiently low concentrations of Si to be considered as silicon-substitued ALPO s. 

Superficial (surface) compositions are also listed in Table I as determined by XPS with atomic sensitivity factors characteristic of the electron energy analyzer used for these studies. The superficial compositions agree reasonably well with the bulk values for all samples with the exceptions of SAPO-5 and the two Si-VPI-5 samples which show a significant enrichment of Si at the surface. Note that samples which were examined with and without deposited gold (the reference for the binding energy scale) showed little difference in measured compositions with XPS. 

The result given for Si-VPI-5B appears to be anomalous. However, notice that the superficial composition of this sample gives Al/P - 1. Thus, the aluminum environment near the surface of Si-VPI-5B is more like an AlPO rather than a SAPO. 

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