Stabilizing the framework of SAPO-34 zeolite toward long-term methanol-to-olefins conversion

Abstract

As a commercial MTO catalyst, SAPO-34 zeolite exhibits excellent recyclability probably due to its intrinsic good hydrothermal stability. However, the structural dynamic changes of SAPO-34 catalyst induced by hydrocarbon pool (HP) species and the water formed during the MTO conversion as well as its long-term stability after continuous regenerations are rarely investigated and poorly understood. Herein, the dynamic changes of SAPO-34 framework during the MTO conversion were identified by 1D 27Al, 31P MAS NMR, and 2D 31P-27Al HETCOR NMR spectroscopy. The breakage of T-O-T bonds in SAPO-34 catalyst during long-term continuous regenerations in the MTO conversion could be efficiently suppressed by pre-coking. The combination of catalyst pre-coking and water co-feeding is established to be an efficient strategy to promote the catalytic efficiency and long-term stability of SAPO-34 catalysts in the commercial MTO processes, also sheds light on the development of other high stable zeolite catalyst in the commercial catalysis.

Dynamic Evolution of Zeolite Framework and Metal-Zeolite Interface

“…Very recently, a similar work about the dynamic evolution of the SAPO-34 framework in MTO was reported by Dai and co-workers that the hydrolysis of P−O−Al bonds was found in the spent SAPO-34 after a short time stream treatment at a high reaction temperature of ∼400 °C. 49 With the proceeding of MTO, hydrocarbon pool species would be formed in the cages and pores of SAPO-34, which could cover some BAS and P−O−Al bonds and thereby prevent the SAPO-34 framework from hydrolysis by H 2 O. Contrast experiments about SAPO-34 with/without precoking illustrated that the organic deposits in SAPO-34 zeolite could act as a protective agent to enhance the stability of SAPO-34 zeolite.…”

“…This ship-in-a-bottle strategy is an effective way to illustrate the dynamic and reversible evolution of zeolite, to characterize the zeolite acidity, and to functionalize the zeolites for various applications including MTO. Very recently, a similar work about the dynamic evolution of the SAPO-34 framework in MTO was reported by Dai and co-workers that the hydrolysis of P–O–Al bonds was found in the spent SAPO-34 after a short time stream treatment at a high reaction temperature of ∼400 °C . With the proceeding of MTO, hydrocarbon pool species would be formed in the cages and pores of SAPO-34, which could cover some BAS and P–O–Al bonds and thereby prevent the SAPO-34 framework from hydrolysis by H 2 O.…”

Light olefin synthesis from a diversity of renewable and fossil feedstocks: state-of the-art and outlook

“…Recently, Structure Illumination Microscopy (SIM) has been used as a powerful tool to examine the spatiotemporal evolution of carbonaceous species inside molecular sieves. Using a combination of SIM, DFT calculations and molecular dynamics (MD) 249 simulations, Gao 248 et al and Yang 249 et al explored the deactivation phenomena for the SAPO-34 catalysts. In particular, they focused on defining the coke composition and on localizing the regions of affected crystals.…”

“…Generally, SAPO-34 shows complete conversion and a combined ethene + propene selectivity of around 80-85%. [239][240][241][242] In particular, this material allows obtaining high LO selectivities due to its mild acidity, and the presence of eight-membered ring pore openings, which limits the product molecular size and introduces shape selectivity. On the other hand, a small zeolite pore diameter causes fast catalyst deactivation.…”

“…Yang 249 et al studied coke evolution in order to define a way to extend the SAPO-34 catalyst lifetime. They investigated the long-term effects of MTO conversion on the SAPO-34 zeolite exploiting 1D 27 Al or 31 P MAS NMR and 2D 31 P- 27 Al HETCOR MAS NMR spectroscopy, as well as XRD and DFT calculations.…”