SAPO

Methanol to Ethylene. Methanol to ethylene economics track the economics of methane to ethylene. Methanol to gasoline has been flilly developed and, during this development, specific catalysts to produce ethylene were discovered. The economics of this process have been discussed, and a catalyst (Ni/SAPO 34) with almost 95% selectivity to ethylene has been claimed (99). Methanol is converted to dimethyl ether, which decomposes to ethylene and water the method of preparation of the catalyst rather than the active ingredient of the catalyst has made the significant improvement in yield (100). By optimizing the catalyst and process conditions, it is claimed that yields of ethylene, propylene, or both are maximized. This is still in the bench-scale stage. 

Ishihara, T., Kagawa, M., Hadama, F. et al. (1997) Copper ion-exchanged SAPO-34 as a thermostable catalyst for selective reduction of NO with C3H6, J. Catal., 169, 93. 

Figure 1. SEM images of the zeolite crystals under investigation a) CrAPO-5 b) SAPO-34 c) SAPO-5 and d) ZSM-5.

In order to illustrate the general applicability of the methodology we have extended our approach to other large zeolite crystals, such as SAPO-34, SAPO-5 and ZSM-5. Our study on the rhombic SAPO-34 crystals reveals a four-pointed star fluorescence pattern at 445 K, which is transformed into a square-shaped feature at 550 K. This is illustrated in Figure 4a. Confocal fluorescence slices, summarized in Figures 4b-d, recorded at different temperatures show the cubical pattern, which proceed from the exterior of the crystal inwards. Both observations are consistent with a model which involves six components of equal tetragonal pyramids as illustrated in Figure 3b. 

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. 

Most of the catalytic interest in the AlP04-based molecular sieves have centered on the SAPOs which have weak to moderate Bronsted acidity, and two have been commerciahzed SAPO-11 in lube oil dewaxing by Ghevron and SAPO-34 in methanol-to-olefins conversion by UOP/Norsk Hydro. Spurred on by the success of TS-1 in oxidation catalysis, there is renewed interest in Ti, Co, V, Mn and Cr substituted AlP04-based materials, for a review of recent developments in the AlP04-based molecular sieves see [35]. 

Synthetic forms AlPO-34, CoAPO-44, CoAPO-47, DAF-5, GaPO-34, Unde D, Unde R, LZ-218, MeAPO-47, MeAPSO-47, (Ni(deta)2)-UT-6, Phi, SAPO-34, SAPO-47, UiO-21, ZK-14, ZYT-6... 

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]. 

Various routes to methane utilization-SAPO-34 catalysis offers the best option. Catal. Today, 141, 77-Si. 

Carreon, M.A., H, S.G., Falconer, J.L., and Noble, R.D. (2008) SAPO-34 seeds and membranes using multiple structure directing agents. Adv. Mater.,... 

Some small-pore zeolite and molecular sieve membranes, such as zeolite T (0.41 nm pore diameter), DDR (0.36 x 0.44nm) and SAPO-34 (0.38nm), have been prepared recenhy [15-21]. These membranes possess pores that are similar in size to CH4 but larger than CO2 and have high CO2/CH4 selechvihes due to a molecular sieving mechanism. For example, a DDR-type zeolite membrane shows much higher CO2 permeability and CO2/CH4 selechvity compared to polymer membranes [15-17]. SAPO-34 molecular sieve membranes show improved selechvity for separation of certain gas mixtures, including mixtures of CO2 and CH4 [18-21]. 

Chen, Y., Zhou, H., Zhu, J., Zhang, Q., Wang, Y., Wang, D., and Wei, F. (2008) Direct synthesis of a fluidizable SAPO-34 catalyst for a fluidized dimethyl ether to olefins process. 

Light alkene selectivities from MTO over SAPO-34 at 400-450 °C (ethylene > propylene butylenes > pentenes) are quite different than those predicted from thermodynamic equilibrium (butylenes > propylene > pentenes > ethylene). Over... 

ZSM-5 propylene and higher alkene selectivities are typically higher and ethylene selectivity is typically lower compared to SAPO-34 [97]. Over Beta light alkene breakdown is closer to thermodynamic equilibrium compared with ZSM-5 and SAPO-34 [97]. While the SAPO-34 pore opening is not large enough to let aromatics and branched alkanes out, di- and trimethylbenzenes and even some tetra-methylbenzenes are observed over ZSM-5 as products in the effluent and hexamethylbenzenes are observed over Beta as products in the effluent [98, 99]. 

Figure 13.50 C CP/MAS NMR spectra of products retained in SAPO-34 and GC analyses of volatile products formed after various reaction times following a methanol pulse (0.053 g/g cat) at 673°K[109],...

Figure 13.51 The 0-0 distances across the eight-ring channel in SAPO-34.

Finally, an additional reaction pathway exists and this does not seem to be operative with SAPO-34 and Beta under regular processing conditions. This path seems to be operative with ZSM-5 and that may involve successive methylations of propene, followed by cracking to yield higher alkenes [111]. A similar mechanism that involves successive methylations of ethylene followed by cracking to yield higher alkenes over ZSM-5 does not seem to be as important [125]. It is conceivable that this mechanism may be partly operative during the MTO experiments over SAPO-34 described above that used co-fed ethylene or co-fed propylene [126]. 

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. 

Dahl, I. and Kolboe, S. (1993) On the reaction mechanism for propene formation in the MTO reaction over SAPO-34. Catal Lett., 20, 329-336. 

S. (2001) Methanol-to-hydrocarbons reaction over SAPO-34. Molecules confined in the catalyst cavities at short time on stream. Catal. Lett., 71, 209-212. 

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