Understanding the Early Stages of the Methanol-to-Olefin Conversion on H-SAPO-34

Abstract

Little is known on the early stages of the methanol-to-olefin (MTO) conversion over H-SAPO-34, before the steady-state with highly active polymethylbenzenium cations as most important intermediates is reached. In this work, the formation and evolution of carbenium ions during the early stages of the MTO conversion on a H-SAPO-34 model catalyst were clarified via 1 H MAS NMR and 13 C MAS NMR. Several initial species (i.e., three-ring compounds, dienes, polymethylcyclopentenyl, and polymethylcyclohexenyl cations) were, for the first time, directly verified during the MTO conversion. Their detailed evolution network was established from theoretical calculations. On the basis of these results, an olefin-based catalytic cycle is proposed to be the primary reaction pathway during the early stages of the MTO reaction over H-SAPO-34. After that, an aromatic-based cycle may be involved in the MTO conversion for long times on stream.

Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy

“…These are predominant species during the induction time (see Scheme 1) and can further act as methylating agents during the MTO reaction. 19,26,27,29,90 In this view the work of Hunger et al is worth mentioning, as they pretreated zeolite catalysts to cover the acid sites with methoxides to demonstrate the role and high reactivity of surface methoxides during the induction period and product formation of the MTO process. 19,91,92 From the thermodynamic point of view, water can impact the formation of methoxide species by shifting the equilibrium towards the formation of methanol.…”

“…6,[20][21][22][23][24] So far there is no full consensus concerning the mechanism governing the formation of such HP species. [25][26][27] One possibility concerns condensation reactions between initially formed ethene and/or propene, which might undergo oligomerization and cyclization reactions. 25,27 Previous experimental studies on the effect of feed composition on the MTO process suggested that water might alter the adsorption/desorption behavior of these primary products and as such impact their reactivity.…”

“…[25][26][27] One possibility concerns condensation reactions between initially formed ethene and/or propene, which might undergo oligomerization and cyclization reactions. 25,27 Previous experimental studies on the effect of feed composition on the MTO process suggested that water might alter the adsorption/desorption behavior of these primary products and as such impact their reactivity. 10 Furthermore, as methoxides are believed to play a crucial role during the induction period 19,[27][28][29] it is probable that the aforementioned water induced equilibrium shift also affects the second stage of the MTO reaction.…”

Initial Carbon–Carbon Bond Formation during the Early Stages of the Methanol‐to‐Olefin Process Proven by Zeolite‐Trapped Acetate and Methyl Acetate

“…[9] Interestingly,Hunger and co-workers showed already in 2006 that traces of organic impurities neither have any significant influence on product distribution, nor do they govern the formation of HCP species. [10] Since then, the research groups of Hunger, [10][11][12][13][14][15] Kondo, [16] Fan, [17,18] CopØret and Sautet, [19] and Lercher [20] have provided both experimental and theoretical evidences in support of the direct mechanism during the initial stages of the MTO process. Thee arlier proposed direct MTO routes include the a) oxonium ylide,b )carbene,a nd c) methane-formaldehyde mechanisms.…”

“…Theo bserved bands at 297, 350, 479, and 640 nm are attributed to neutral methylated benzenes,dienylic carbocationic/methylbenzenium (up to three methyl groups) ions, trienylic and methylated poly-arenium ions,r espectively. [10][11][12][28][29][30] Similarly,t he 387 nm band is characteristic for ah examethylbenzenium ion (HMB + )a nd its bathochromic shift to 414 nm with increasing time-on-stream can be explained by the further methylation of HMB + to,f or example,h epta-methylbenzenium ions. [31] We propose that the cracking of polyaromatics into trienylic carbenium species and olefins are responsible for the concomitant increase and decrease of the 479 and 640 nm bands,r espectively,a fter 10 minutes of reaction (Figures S3b).…”