No abstract

Olefins' reactions lead to the formation of soluble and insoluble coke at the near-surface region of a SAPO-34 crystal.

SAPO-34 crystals were prepared via a high-low-high temperature crystallization procedure with triethylamine (TEA) as template. Three-stage crystallization is beneficial for the nu- [a] ConclusionsIn summary, we have introduced a high-low-high temperature three-stage crystallization process for reducing the crystal size Eur.

The insoluble coke in SAPO-34 catalyst is investigated by mass spectrometry and scanning electron microscopy (SEM). Results indicate the insoluble coke in SAPO-34 catalyst is a reticulate structure composed of carbon-chain bridged cycloalkanes and aromatics. This reticulate structure located near the surface of SAPO-34 crystal. It is proposed that the continue alkylation and cyclization of cycloalkanes or aromatics with olefins is the main cause of the coke formation. Moreover, the coke preformed by olefins in downstream catalyst bed reduces the active region for methanol conversion.Methanol to olefins (MTO) process provides an alternative route to produce ethene and propene from the abundant natural gas and coal and the renewable biomass. [1] The CHA topology SAPO-34 zeolite consists of 1.1 nm 3 0.65 nm nanosized cavities which are connected with 0.38 nm 3 0.38 nm windows. [2] This special structure, combined with moderate acid strength, favours the selective conversion of methanol to ethene and propene. Thus SAPO-34 becomes the optimum catalyst for the MTO process.The cavity structure with narrow windows also favours the formation of aromatics within the pores. The structure and the amount of confined aromatics affect the performance of catalyst greatly. Generally, it is believed that aromatics smaller than pyrene derivatives can be formed in SAPO-34 cage. [3] The small size methylbenzenes and methylnaphthalenes act as "hydrocarbon pool" species that promotes the methanol conversion after the induction period. [2b,4] However, they further transform into polycyclic aromatics (coke) which block the pore and reduce the diffusion of methanol and products. Therefore, the catalyst deactivates rapidly. [2b,4f,5] Typically, after being dissolved by HF and extracted with chlorohydrocarbons, the soluble aromatic species confined in SAPO-34 cages can be analyzed by gas chromatography. [3,6] The coke composition can also be directly investigated with mass spectrometry. LDI-TOF MS study on coked of ZSM-5 indicated that the polycyclic aromatic with mass signals between 100 and 300 Da was first formed inside the crystal. With the extending of TOS, heavier compounds with masses of 401, 413 and 429 Da were formed on the outer surface. [7] Differently from that of ZSM-5, the deactivation of SAPO-34 is caused by the coke formed within cavities. [5e] In additional to the soluble coke, there are also certain amount of insoluble coke (Ref [3c] and Figure S1). The amount of insoluble coke is even more than that of soluble one. [3c] The structure of insoluble coke might provide important information for discussing the deactivation mechanism of SAPO-34. Therefore, the molecular structure and morphology of insoluble coke were investigated by direct sampling temperature program mass spectrometry and SEM, respectively, in this work. Two typical samples were studied. Coked SAPO-34 A was obtained from the top section of catalyst bed which is almost fully deactivated. Coked SAPO-34 B was taken from the bottom section of catalyst bed...