Over SAPO molecular sieves

THOMSON ETAL. Propylene Conversion over SAPO Molecular Sieves... 

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

Another important aspect is how to control coke content distribution in the fluidized bed reactor with catalyst circulation. As we know, there exists optimal coke content for catalyst particle which can maximize the selectivity to hght olefins. If there is circulation of catalyst particles, the coke content in catalyst shows a certain distribution. Ideally, if the coke content distribution is uniform such as that encountered in the fluidized bed reactor without circulation, the selectivity to hght olefins can reach 90% over SAPO molecular sieves. Therefore, how to optimize the coke content distribution in MTO fluidized bed reactor to improve the selectivity to hght olefins represents another future direction. 

Product distribution in MTO reaction over SAPO molecular sieves 350°C WHSV= 0.3 h" MeOH = 0.02 bar = 0.98 bar. Adapted from YangSM, WangSI, Huang CS, Holmen KJJA, Kolboe S, editors. Studies in surface science and catalysis, vol. 61. Elsevier 1991. p. 429—35. 

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. 

Sinha, A.K. and Sivasanker, S. (1999) Hydroisomerization of n-hexane over Pt-SAPO-11 and Pt-SAPO-31 molecular sieves. Catal Today, 49, 293-302. 

Meta-diisopropylbenzene is reacted with propylene over the acid form of the molecular sieves SAPO-5, mordenite, offretite, beta, hexagonal and cubic faujasite (EMT and FAU), L, SAPO-37, and an amorphous silica-alumina at temperatures around 463 K in a flow-type fixed-bed reactor. A small amount of cracking is observed. However, the main reactions of meta-diisopropylbenzene are isomerization and alkylation. It is proposed that this alkylation can be used as a new test reaction to characterize the effective size of the voids in larger pore (12 T-atom rings or above) molecular sieves by measuring the weight ratio of 1,3,5- to 1,2,4-triisopropylbenzene formed. In most cases, this ratio increases with die increasing effective void size of the molecular sieves in the order SAPO-5 < mordenite < offretite < beta < EMT FAU < L < SAPO-37 < amorphous silica-alumina. 

The as-synthesized and calcined CrAPO-5 and CrS-1 were characterized by XRD which showed that the samples were pure and had an API and MFI structure respectively. ICP analysis showed that both catalysts contained about 1 % chromium. The results observed in the decomposition of cyclohexenyl hydroperoxide over several redox active moleular sieves are presented in Table 1. CrAPO-5 and CrS-1 displayed rougly equal activity and selectivity in the decomposition of cyclohexenyl hydroperoxide. Blank reactions carried out with Silicalite-1 (S-1) and silicon incorporated Aluminophosphate-5 (SAPO-5) show low conversions confirming that the chromium was responsible for the catalysis. Other transition- metal subsituted molecular sieves showed low conversions. 

Industrially important olefins can be made with the help of molecular sieves. Methanol can be converted into 50% ethylene and 30% propylene with a SAPO catalyst at 350-500X.190 Isobutene can be made by isomerization of n bulcnes over clinoptilolite at 45OX with 91.6% selectivity at 23.5% conversion,191 or with H-ferrierite with 92%... 

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]. 

In line with the idea of reducing the acidity of the zeolites in order to achieve high selectivity and a long catalyst life, the weakly acidic non-zeolitic molecular sieves, for example the medium-pore SAPO-11 or SAPO-41. were used for the Beckmann rearrangement [97]. Over SAPO-11. a 5 % strength solution of cyclohexanoneoxime in acetonitrile reacts at 350 -C. under atmospheric pressure and at a WHSV of 10.8 h to give e-caprolactam with 95 % selectivity and a conversion of 98 %. 

Figure 12.19 In situ C MAS NMR (A) and UV-vis (B) spectra recorded during the conversion of C-enriched methanol over an H-SAPO-34 molecular sieve at different reaction temperatures (a)-(e). (Reproduced with permission from Ref. [139].)...

Mores D, Stavitski E, Kox MHF, Komatowski J, Olsbye U, Weckhuysen BM Space- and time-resolved in-sim spectroscopy on the coke formation in molecular sieves methanol-to-olefin conversion over H-ZSM-5 and H-SAPO-34, Chem Eur J 14 11320-11327, 2008. 

Akolekar [151] found a linear relation between the number of strong acid sites (determined by adsorption of pyridine at 400 °C) and the conversion of cumene over metal-substituted AlPO-ll with the exception of Mg-AlPO-11. A closer look at the acid site distribution, however, showed that this catalyst has the largest concentration of sites with lower add strength (pyridine desorption between 300 and 400 °C). As described above, the activity of these sites must be included. Tian et al. [152] studied SAPO-11 molecular sieves, which were dealuminated by EDTA to achieve different Al contents. After de-alumination the activity for cumene cracking improved, which was attributed to the formation of strong Bronsted add sites around Si domains and to a reduction of the Lewis acidity. 

Prakash et al. [217] compared the activity of SAPO-11, SAPO-31, and SAPO-41 for toluene alkylation. TPD of ammonia showed a decrease in acidity in the order of SAPO-41 > SAPO-31 > SAPO-11, which was the same order as that observed in the activity for toluene alkylation. Ethylbenzene and styrene were not formed over these catalysts, which confirmed that basic sites do not exist on these molecular sieves. 

The influence of equilibrium on the ethylene/propylene product ratio obtained with SAPO-34 catalyst over a range of temperatures and pressures can be seen from (Fig. 35) [189]. The relationship is linear with a slope that is significantly greater than 1, indicating that ethylene is in excess of equilibrium on the small-pore zeolites. An explanation for this relationship is that ethylene and propylene are in equilibrium within the pore structure of the SAPO-34 molecular sieve. However, because the 0.43 nm pore diameter of the SAPO-34 is too small to allow propylene to take a straight-line path through the pore mouth, its diffusion is hindered relative to ethylene. This would imply that diffusion limitations are the key factor controlling the C " / C," ratio [189]. 

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