MTO reaction over SAPO

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. 

The present work deals with the effects of coke deposition in the MTO reaction over SAPO-34 using the TEOM reactor. The effects of the external and internal coke on the activity and selectivity on SAPO-34 are studied by controlled coke formation from... 

The effect of coke formation during the MTO reaction over SAPO-34 at 425°C can be summarized as follows ... 

The coke formation is critical for MTO reaction over SAPO-34 catalyst. The influence of coke formation is twofold a certain amount of coke deposition can prompt the selectivity to light olefins, while it also makes the catalyst deactivate rapidly. Thus, the understanding of the coke formation at microscale is extremely important for controlling coke distribution in the reactor. The influences of zeofite structure and reaction temperature on coke formation have been discussed to illustrate the essence of the mesoscale researches. However, there is still a lot of work to be explored at mesoscale concerning the coke formation. These results are eventually expected to benefit the reactor design and operation. 

Original scheme of hydrocarbon pool mechanism. Adapted from Dahl IM, Kolboe S. On the reaction mechanism for propene formation in the MTO reaction over SAPO-34. Catal Lett 1993 20 329-36 Dahl IM, Kolboe S. On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34 1. Isotopic labeling studies of the co reaction ofethene and methanol. J Catal 1994 149 458—64. 

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. 

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]. 

Figure 13 Deactivation process in MTO reaction over ZSM-5 (A) (Guisnet et al., 2009) and SAPO-34 (B) (Haw et al., 2003) zeolites.

As discussed above, coke formation affects the selectivity to Hght olefins in MTO process over SAPO-34 catalyst. It has been found that at a given temperature, the ethylene-to-propylene ratio in MTO reaction is increased when coke content in catalyst increases (Barger, 2002 Song et al., 2001). Figure 18 shows the typical results in a microscale fluidized bed reactor at temperature of 450 °C and weight hourly space velocity (WHSV) of... 

Hereijgers BPC, Bleken F, Nilsen MH, et al Product shape selectivity dominates the methanol-to-olefins (MTO) reaction over H-SAPO-34 catalysts, J Catal 264 77-87, 2009. 

Chen and co-workers have studied the role of coke deposition in the conversion of methanol to olefins over SAPO-34 [111]. They found that the coke formed from oxygenates promoted olefin formation while the coke formed from olefins had only a deactivating effect The yield of olefins during the MTO reaction was found to go through a maximum as a function of both time and amount of coke. Coke was found to reduce the DME dilfusivity, which enhances the formation of olefins, particularly ethylene. The ethylene to propylene ratio increased with intracrystal-line coke content, regardless of the nature of the coke. 

Despite the development of microscale modeling for reaction—diffusion in zeolite, the complex of MTO reaction mechanism impedes the application of microscale modeling to MTO process. Up to now, the reliable reaction kinetics based on element reactions in MTO process is still under development (van Speybroeck et al., 2014). However, a reduced or simplified microscale model could be applied. Basically, the diffusion effect is negligible if the crystal radius is small enough. Then mass equation, i.e., Eq. (1), could be simplified by neglecting the species ffux term. In this case, MTO processes over ZSM-5 and SAPO-34 catalyst could be simulated by use of the single-event kinetics by Alwahabi and Froment (2004a) as an input. 

Methanol transformation to olefins over SAPO-34 catalyst is featured by high exothermicity (Alwahabi and Froment, 2004b) and rapid deactivation (Chen et al., 1999 Parket al., 2008). Considering these, circulating fluidized bed is used for MTO process in industry. The model describing reaction-diffusion process in this macro reactor is called macroscale model. However, it should be pointed out that the focus of macroscale model on reaction and diffusion is far different from microscale model. 

MeXTH [Methyl halides to Hydrocarbons] (X = Cl or Br) A general name for processes for converting methanol (from methane) to hydrocarbons via methyl halides. This has been proposed as an alternative to the MTO process, requiring less energy, but it has not yet been commercialized. An oxychlorination step is followed by reaction over a zeotype catalyst such as H-SAPO-34. 

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