Molecular sieves SAPO

MTO [Methanol to olefins] A catalytic process for converting methanol to olefins, mainly propylenes and butenes. Developed by Mobil Research Development Corporation and first demonstrated in 1985. Another version of this process was developed by UOP and Norsk Hydro and has been ran at a demonstration unit at Porsgrunn, Norway, since June 1995. It is based on fluidized bed technology using a SAPO molecular sieve catalyst. It converts 80 percent of the carbon in the feed to ethylene and propylene. 

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. 

There is limited patent literature available on manufacturing techniques for aluminophosphates. Although many patents describe AlPO synthesis, most described examples are small-scale preparations. The fact that at least two catalytic applications have been commercialized for SAPO molecular sieves indicates that they have been scaled-up to large quantities [55, 56]. A large-scale preparation of SAPO-34 is described in a recent patent [57]. 

Because of the variety of Si locations (isolated Si and Si islands) in SAPO molecular sieves, frequently no correlation exists between Si content and the number of acid sites [105]. However, for SAPO with low Si content. Si sites are usually isolated and there is one acid site per Si. In general, the framework charge and, thus, the maximum number of acid sites in a SAPO should be related to the value of framework (Al-P) [106]. This relationship is true of zeolites too, because (Al-P) is equal to A1 if there is no framework P. Based on this relationship, the... 

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). 

The UOP process, developed jointly with Norsk Hydro/Statoil, and has been developed to semi-commercial scale in Norway. The process uses proprietary catalysts based on a SAPO molecular sieve. 

The local environments of T-atoms in SAPO materials were examined using solid-state NMR, a bulk probe, and XPS, a surface sensitive probe. T-a-tom 2 p binding energies in XPS were found to vary in a predictable fashion with changes in NMR chemical shifts. The comparison demonstrates that XPS is sensitive to variations in the second coordination sphere for T-atoms in SAPO molecular sieves. XPS was also found to give a reasonable, quantitative measure of superficial (surface) T-atom fractions thus providing information about elemental homogeneity by comparison to bulk chemical analysis. 

Also, of secondary importance, we will illustrate that XPS can be useful in ascertaining whether elemental homogeneity is achieved in SAPO molecular sieves. 

Syngas conversion to methanol has been shown to take place on supported palladium catalyst [1]. Methanol can in turn be converted to gasoline over ZSM-5 via the MTG process developed by Mobil [2]. In recent work we have reported syngas (CO/H2) conversion to hydrocarbon products on bifunctional catalysts consisting of a methanol synthesis function, Pd, supported on ZSM-5 zeolites [3]. Work on syngas conversion to hydrocarbon products on Pd/SAPO molecular sieves has been published elsewhere [Thomson et. al., J. CataL. in press].Therefore, this paper will concentrate on propylene conversion. 

Propylene conversion over three SAPO molecular sieves (SAPO-5, SAPO-11, and SAPO-34) was conducted at a variety of operating conditions. Catalyst behavior was correlated with the physical and chemical properties of the SAPO molecular sieves. The objective of this work was to determine the relative importance of kinetic and thermodynamic factors on the conversion of propylene and the distribution of products. The rate of olefin cracldng compared to the rate of olefin polymerization will be addressed to account for the observed trends in the product yields. The processes responsible for deactivation will also be addressed. 

Catalysts Preparation. The silicoaluminophosphate (SAPO) molecular sieves employed in this study were synthesized in the laboratory of Professor Mark Davis in the Department of Chemical Engineering of the Virginia Polytechnic Institute, following the methods reported in U.S. Patent 4,440,871. The three different samples, distinguished by their microscopic structure, were the wide-pore SAPO-5, medium-pore SAPO-11, and the narrow-pore SAPO-34. Verification of their microscopic structure (through x-ray diffraction) and micropore diameters (by argon adsorption measurements) was performed at VPI. The SAPO molecular sieves were provided in the ammonium cation form. Ex situ calcination at 873 K for one hour in oxygen was performed on the SAPO samples prior to their use as catalysts for the propylene conversion. 

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... 

Catalyst Characterization. The three SAPO molecular sieves employed in this study represents the three pore sizes of molecular sieves, ranging tom 0.4 nm to 0.8 nm. While SAPO-5 and SAPO-11 have unidimensional pores, SAPO-34 has a multidimensional pore system with supercages. The chemical composition and total ammonia uptake of the tifiree SAPO molecular sieves are listed in Table I. 

Table I. Framework Composition and Quantitative Analysis of Ammonia Desorption Experiments for SAPO Molecular Sieves...

Figure 3. Comparison of Deactivation Trends over SAPO Molecular Sieves (Propylene Inlet pressure= 16.2 kPa, Temp.= 550 K)...

Olefin oligomerization were found to occur on SAPO molecular sieves, though their activity was far less than the of zeolite ZSM-5[17]. While showing very different initial activity, the wide-pore SAPO-5 and the narrow pore SAPO-34 both deactivated severely (Figure 3). Both of these catalysts yielded a wide spectrum of products presumably following the pathway described by Tabak et. al. [5], in which numerous olefin polymerization and scission reactions take place. Strangely, medium pore SAPO-11 showed complete selectivity for olefin dimers... 

In addition to the rates of olefin reactions, mass transfer also plays an important role in determining the extent of propylene conversion and the product distribution from SAPO molecular sieves. Restrictions on molecular movement may be severe in the SAPO catalysts, due to pore diameters (4.3 A for SAPO-34) and structure (one-dimensional pores in SAPO-5 and SAPO-11). The deactivation of SAPO-5 and SAPO-11 catalysts may be more directly related to mass transfer than the coking of SAPO-34. Synthesis of large or highly-branched products, having low diffusivities, inside the pores of SAPO-5 or SAPO-11 essentially block internal acid... 

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