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

Abstract

Methanol-to-olefin (MTO) catalysis is av ery active field of researchb ecause there is aw ide variety of sometimes conflicting mechanistic proposals.A ne xample is the ongoing discussion on the initial C À Cb ond formation from methanol during the induction period of the MTOprocess.Byemploying ac ombination of solid-state NMR spectroscopyw ith UV/Vis diffuse reflectance spectroscopya nd mass spectrometry on an active H-SAPO-34 catalyst, we provide spectroscopic evidence for the formation of surface acetate and methyl acetate,aswell as dimethoxymethane during the MTOp rocess.A saconsequence,new insights in the formation of the first C À Cbond are provided, suggesting ad irect mechanism may be operative,a t least in the early stages of the MTOr eaction.

The Effect of Co-feeding Methyl Acetate on the H-ZSM5 Catalysed Methanol-to-Hydrocarbons Reaction

“…Understanding how the first C-C bonds are created may help optimisation of catalyst composition in order to both maximise efficiency but also tune selectivity. One recent proposal independently reported by the groups of Lercher [5] and Weckhuysen [6] is that carbon-carbon bonds are created via methyl acetate (MeOAc) as the first intermediate. [5,6].…”

Transient kinetic studies and microkinetic modeling of primary olefin formation from dimethyl ether over ZSM‐5 catalysts

“…Various direct mechanisms have been proposed for the conversion of methanol/DME to primary olefins. 12 These direct mechanisms are known by their intermediates and include oxonium ylide, 13,14 carbene, 15 methane-formaldehyde, 16,17 carbon monoxide, [18][19][20] methoxymethyl, 21,22 and surface methoxy groups. [23][24][25][26] Using density functional theory (DFT) calculations, Lesthaeghe et al [27][28][29] refuted some direct mechanisms based on high activation energy barriers and highly unstable intermediates.…”

“…Recently, there has been a surge in the evidence for the direct mechanisms leading to primary olefin formation. 18,19,21,22 Li et al 22 gave evidence, using DFT calculations for the formation of propylene from methanol through methoxymethyl cations. In this pathway, 1,2dimethoxyethane or 2-methoxyethanol was proposed as key intermediates propagating the direct formation of propylene.…”

“…This was also evidenced by Liu et al 18 and Wei et al 21 In this paper, we investigate the induction period during the formation of primary olefins from DME at 300 • C by conducting novel step response experiments in a temporal analysis of products (TAP) reactor. Nine reaction schemes obtained from the literature [18][19][20][21][22] were compared and used as a basis for a microkinetic model. The experimental data were simulated to extract kinetic parameters that describe the formation of primary olefins from DME over fresh ZSM-5 catalysts.…”

Highly Oriented Growth of Catalytically Active Zeolite ZSM‐5 Films with a Broad Range of Si/Al Ratios

“…To demonstrate the utility of the highly b-oriented zeolite ZSM-5 films under realistic reaction conditions and provide secondary confirmation of their Brønsted acidity, operando UV/Vis diffuse-reflectance micro-spectroscopy with on-line mass spectrometry (MS) was utilized on an EtOH-directed, highly b-oriented zeolite ZSM-5 film with Si/Al = 45 during the MTH process conducted at 673 K. [35] Thea bsorption maxima that developed with time-on-stream at around 410, 520, and 620 nm were assigned to neutral methylated benzenes,d ienylic carbocationic/methylbenzenium ions,t rienylic and methylated poly-arenium ions,r espectively (Figure 4a and S16). [35,36] Thec learly separated bands,c ompared to the broad signal normally found from bulk crystals, [36] can be used to easily explore the evolution of the reaction intermediates,a nd their contribution to the activity and deactivation of the MTH process demonstrates that the oriented films are promising for use as am odel system to study catalytic reactions.M oreover,t he on-line MS data reveals the existence of dimethyl ether (DME) and ethylene, along with propylene,a st he reaction products during the MTH process (Figure 4b and Figure S17), meaning that the films are catalytically active through their Brønsted acid sites. [37,38] Summarizing,highly b-oriented zeolite ZSM-5 films with ab road range of Al contents have been fabricated using SDAs with hydroxy groups,w hich serve to suppress film overgrowth by properly introducing Al 3+ into the zeolite framework.…”