Propylene coke formed from

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. 

Coke deposits were formed from biphenyl over dealuminated HM only in the presence of propylene, although the amount of coke was less.26,27 Coke deposition occurred in a short period after starting the reaction, and by the contact of 4,4 -DIPB with HM even in the absence of propylene.26 These results suggest that the isopropylated biphenyls produce coke by dehydrogenative condensation at their isopropyl groups on acid sites. Propylene oligomers were formed during the reaction 26 They are alternative precursors of deposited coke. 

Figure 6 indicates that amorphous coke was formed from acetylene, ethylene, propylene, and butadiene at 600°C on alonized Incoloy 800 surfaces. These cokes were in all cases nonmagnetic in character and contained no detectable iron. They did contain a trace of aluminum, probably as alumina. 

Detailed kinetic studies performed by Froment s group shows an extrapolation of the propylene yield to initial times [101]. This proves that propylene might be at least one of the primary products in MTO reaction after the initiation step, which is formed directly from MeOH/DME. This result is consistent with [61,103]. According to Froment [119], the propylene formation is very fast, but this olefin is very reactive on the catalyst. With progressive deactivation by coke, the reactor zone in which the propylene yield reaches its maximum moves further downstream [101]. 

At low temperature, propylene is mostly formed by DME propagation as well as ethylene methylation. In conditions of reduced DME conversion, propylene formation occurs nearly exclusively and via methylation of the added ethylene with selectivity close to 100% on SAPO-34 and H-ZSM-5. It was also demonstrated that DME propagation does not proceed via ethylene, on both topologies. The addition of ethylene to the DME feed switches the MTO mechanism toward a mechanism where propylene is a true primary product from DME. At higher temperature, the propylene is a product of the ethylene methylation, aromatic/ coke and olefins cycles. 

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