Carbene-like reactivity of methoxy groups in a single crystal SAPO-34 MTO catalyst

Minova, Ivalina B.; Bühl, Michæl; Matam, Santhosh Kumar; Catlow, C. Richard A.; Frogley, Mark D.; Cinque, Gianfelice; Wright, Paul A.; Howe, Russell F.

doi:10.1039/d1cy02361f

Cited by 7 publications

(5 citation statements)

References 87 publications

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“…After that, Chowdhury et al again identified surface carbenoid species, using dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS), on the post-MTH reacted zeolite ZSM-5 (Figure d) . In recent years, Howe and colleagues also studied the fate of SMS to understand initial events in MTH catalysis over zeolites (ZSM-5 and SAPO-34) with the help of synchrotron infrared microspectroscopy. , Upon their investigations with labeled methanol (CD 3 OH), they detected the formation of zeolite OD groups (Z-O-D), which was indicating the CD bond breakage of labeled SMS and formation of carbene-like species. In general, during investigations on the presence of carbenes in MTH, one should pay special attention to zeolite topologies, and their pore/cage dimensions as well, because confinement effects can change the stability of formed carbenes and their contribution to the whole mechanism.…”

Section: Carbene Chemistry In Zeolite Catalysismentioning

confidence: 99%

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Gong

Çağlayan

et al. 2022

Chem. Rev.

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Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical “technical bottleneck” that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such “short-lived and highly reactive” intermediates at the interface (gas–solid/solid–liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the “circular carbon economy” initiatives.

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Section: Carbene Chemistry In Zeolite Catalysismentioning

confidence: 99%

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Gong

Çağlayan

et al. 2022

Chem. Rev.

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“…Several mechanisms have been used to describe the induction period of the methanol-to-olefin conversion over zeolite catalysts. These include: (1) carbene, − (2) carbon monoxide, − (3) methane-formaldehyde, − and (4) methoxymethyl cation. , Each mechanism involves a similar set of steps, namely: adsorption and desorption of methanol, dimethyl ether, and water, olefin (ethylene and/or propylene) formation, and release. These adsorption and desorption steps occur as the catalysts evolve from fresh catalysts to working catalysts at steady-state.…”

Section: Resultsmentioning

confidence: 99%

Site-Specific Activity Maps Observed during Methanol-to-Olefin Conversion over ZSM-5 Catalysts

Omojola

2024

Ind. Eng. Chem. Res.

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The Sabatier principle has been applied to nonuniform site ensembles of ZSM-5 catalysts of different compositions (Si/Al = 25, 36, and 135) during the induction period of the conversion of methanol and dimethyl ether to propylene. For the first time, site-specific volcano-plots and site-specific activity-maps have been observed following microkinetic simulations of the temperature-programmed surface reaction of methanol and dimethyl ether over ZSM-5 catalysts in a temporal analysis of products reactor. The microkinetic simulations are constructed using coupled 1D nonlinear partial differential equations that connect reactions at the active site to convection and dispersion through the reactor. The methoxymethyl cation pathway predicts the formation of propylene from methanol and dimethyl ether via dimethoxyethane. Site-specific scaling relations are observed between the acid site density and the barriers of the dissociative desorption of dimethyl ether with the barrier of methoxymethyl cation formation during the induction period of propylene formation from methanol and dimethyl ether, respectively. During the induction period of methanol conversion to propylene, depending on the barrier to methoxymethyl cation formation, the active site density should not be too low such that site cooperation is inhibited and not too high such that site competition is promoted. The optimum site density is ca. 5 × 10–4 mmol g–1 of active sites over the high-temperature active sites of ZSM-5 (Si/Al = 36) catalysts. Over the high-temperature active sites on ZSM-5 (25) catalysts, we observe a maximum rate of propylene formation at an optimum site density of 7 × 10–4 mmol g–1 active sites. During the induction period of dimethyl ether conversion, the binding energies should not be too weak to activate dimethyl ether and not too strong to inhibit its release into the gas phase. The binding energy of methoxymethyl cation should be between −80 kJ mol–1 and −100 kJ mol–1 for maximum propylene formation over the low-temperature active sites of ZSM-5 (Si/Al = 36) catalysts. Over the medium-temperature active sites of ZSM-5 (Si/Al = 36) catalysts, the maximum rate constant of propylene formation is observed between a binding energy of dimethyl ether of −135 kJ mol–1 and −145 kJ mol–1. The site-specific activity maps show that the catalytic activity of methanol-to-olefin conversion varies across the different site ensembles. In each site-specific activity map, the direction of the maximum rate constant of propylene formation changes with zeolite composition and site-ensemble. The precision given by this site-specific control shows that the catalytic activity can be tailored to the site characteristics.

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“…Monitoring the IR spectra of samples obtained during the reaction of surface-bound deuterated methoxy (OCD 3 ) with ethene at 250 °C over ZSM-5 indicated that the intensities of the C-D stretching bands decreased while a peak related to an acidic O-D group appeared at 2655 cm -1 and increased in intensity, demonstrating the attachment of a D atom to a neighboring O [42] . Additional evidence for this process was obtained by reacting OCD 3 with DME at 200 °C [43] . In 2016, Chowdhury et al provided further support for the carbene mechanism on the basis of analyses by two-dimensional magic angle spinning 1 H- 13 C and 13 C- 13 C solid-state NMR spectroscopy [36] .…”

Section: Carbene Mechanismmentioning

confidence: 91%

Fundamentals of the catalytic conversion of methanol to hydrocarbons

Liu¹,

Huang²

2022

Chem Synth

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For more than four decades, the methanol-to-hydrocarbons (MTH) reaction has been a successful route to producing valuable fuels and chemicals from non-petroleum feedstocks. This review provides the most comprehensive summary to date of recent research concerning the mechanistic fundamentals of this important reaction, covering different reaction stages. Mechanisms that have been proposed to explain the initial C-C bond formation during the induction stage of the MTH reaction are introduced, including the methoxymethyl cation, Koch carbonylation, carbene and methane-formaldehyde processes. At present, there is no consensus regarding these hypothetical mechanisms as a consequence of the limited amount of conclusive experimental evidence. The steady state of the MTH reaction is also examined with a focus on the widely accepted indirect hydrocarbon pool mechanism and the dual cycle concept that provides a mechanistic basis for the effects of zeolite structures and reaction conditions on product distribution. In the following section, advanced characterization techniques capable of providing new insights into the formation of coke species during the MTH reaction and innovative approaches effectively inhibiting coke formation are introduced. Finally, a summary is provided and perspectives on current challenges and the future development of this area are presented.

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Carbene-like reactivity of methoxy groups in a single crystal SAPO-34 MTO catalyst

Abstract: Synchrotron FTIR microspectroscopy coupled with mass spectrometric analysis of desorbed products has been used to investigate the initial stages of the methanol to olefins (MTO) reaction in single crystals of...

Cited by 7 publications

References 87 publications

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Site-Specific Activity Maps Observed during Methanol-to-Olefin Conversion over ZSM-5 Catalysts

Fundamentals of the catalytic conversion of methanol to hydrocarbons

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