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An investigation into the mechanism and kinetics of dimethoxymethane carbonylation

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An investigation into the mechanism and kinetics of dimethoxymethane carbonylation over FAU and MFI zeolitesIntroductionExperimentalCatalyst preparationCollection of FTIR spectraSteady-state and transient-response rate dataResults and discussionFTIR spectra of DMM and DMM-like adsorbed speciesFTIR spectra of DME, MF, MMAc, and their derivativesMechanisms of DMM carbonylation and disproportionationDMM carbonylation and disproportionation over MFIDerivation of the kinetic rate expression and the plug-flow reactor modelDetermination of kinetic rate parameters and reactor model resultsConclusionsAcknowledgmentsReferencesAn investigation into the mechanism and kinetics of dimethoxymethanecarbonylation over FAU and MFI zeolitesFuat E. Celik, Taejin Kim, Anton N. Mlinar, Alexis T. Bell*Department of Chemical Engineering, University of California, Berkeley, CA 94720, USAarticle infoArticle history:Received 24 May 2010Revised 22 June 2010Accepted 24 June 2010Available online 2 August 2010Keywords:CarbonylationAcidZeoliteFTIRInfraredFaujasiteCarbon monoxideDimethoxymethaneMethyl methoxyacetateDisproportionationabstractIn situ IR spectroscopy was used to observe the intermediates formed on zeolites FAU and MFI during thesynthesis of methyl methoxyacetate (MMAc) via carbonylation of dimethoxymethane (DMM) and thedisproportionation of DMM to dimethyl ether (DME) and methyl formate (MF). Both reactions are initi-ated by the reaction of DMM with the Brønsted acid protons of the zeolite to form methanol andmethoxymethoxy groups (MMZ). The latter species then undergoes one of two processes – carbonylationto form methoxyacetyl species, the precursors to MMAc, or reaction with DMM, resulting in DMM dispro-portionation. Surface intermediates for both DMM carbonylation and disproportionation respond tochanges in reaction conditions in a manner consistent with observed steady-state kinetics. DMM carbon-ylation occurred more rapidly in the presence than absence of physisorbed DMM, a phenomenon attrib-uted to solvation of the carbocationic transition state involved in the addition of CO to MMZ predicted byDFT calculations. The surface concentration of the methoxyacetyl species at steady state was 10 timessmaller on FAU than on MFI, consistent with the higher rate of DMM carbonylation on FAU. Rate expres-sions for the formation of each product, based on the proposed mechanisms, in combination with a suit-able set of rate coefficients, give a good description of the experimentally observed dependences of therates of product formation on temperature and the feed partial pressures of CO and DMM.Ó 2010 Elsevier Inc. All rights reserved.1. IntroductionMonoethylene glycol (MEG) is a commodity chemical widelyused as antifreeze and as a monomer in the synthesis of polyesterfibers. The current production of MEG is by epoxidation of ethyleneand subsequent hydration of the resulting ethylene oxide [1].While this technology is highly developed, the rising cost of ethyl-ene derived from petroleum or natural gas has motivated consider-ation of alternative routes. The lower cost of carbon derived fromsynthesis gas (CO and H2), produced by gasification of coal or otherlow hydrogen content feed stocks, relative to that derived fromethylene, has led to an interest in identifying routes to MEG fromsynthesis gas. While the direct production of MEG from synthesisgas has been investigated, such processes suffer from low yieldsand require very high pressures (1300–7000 atm) [2,3]. An alter-nate approach to forming MEG from synthesis gas is to begin withmethanol, which can then oxidized to obtain formaldehyde or itsdimethyl acetal, dimethoxymethane (DMM) [4,5]. Since these areC1compounds lacking a CAC bond, they can be converted to C2compounds by carbonylation. Several attempts to carry out theacid-catalyzed carbonylation of formaldehyde in the liquid-phasehave been reported [6–9], with efforts being made to achieve highselectivity at low pressure [10–12]. However, the rate of formalde-hyde carbonylation in all cases has been limited by the low solubil-ity of CO in the solvent used, which results in poor selectivity forreactions carried out at low pressures due to the reactivity of theformaldehyde [12].We have recently shown that the vapor-phase carbonylation ofDMM can catalyzed by acidic zeolites [13,14]. The product ofDMM carbonylation is methyl methoxyacetate (MMAc), whichcan then be converted to MEG in two steps. In the first step,MMAc is hydrogenated to ethylene glycol monomethyl ether,and in the second step, this intermediate is hydrolyzed to produceMEG. Using FAU, 79% selectivity to MMAc was achieved at 393 Kand a total pressure of 3 atm [13]. DMM disproportionation,which produces dimethyl ether (DME) and methyl formate (MF),was the only side-reaction, in contrast to liquid-phase reactions,which produced a large number of by-products [7,9,12]. The high-est selectivity to MMAc was achieved with FAU, whereas smallerpore zeolites such as MFI, BEA, MOR, and FER were significantlyless selective. MMAc formation rates increased with increasingCO partial pressure and were nearly independent of DMM partialpressure over both MFI and FAU. Both catalysts exhibited a max-imum MMAc formation rate as a function of reaction temperature,whereas DMM disproportionation rates increased monotonically0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.jcat.2010.06.015* Corresponding author. Fax: +1 510 642 4778.E-mail address: [email protected] (A.T. Bell).Journal of Catalysis 274 (2010) 150–162Contents lists available at ScienceDirectJournal of Catalysisjournal homepage: www.elsevier.com/locate/jcatwith temperature. DMM disproportionation rates increased withincreasing DMM partial pressure and were independent of COpartial pressure on FAU. The activity of MFI for DMM dispropor-tionation was severely inhibited by the presence of CO [14].Attainment of high MMAc selectivity required a high ratio ofCO/DMM partial pressures and a low DMM partial pressure(0.01–0.02 atm).The aim of present study is to provide evidence for the mecha-nism proposed in our earlier work [14] and to identify the factorslimiting the rates of reaction and effecting the activity and selectiv-ity of the catalysts. In situ FTIR spectroscopy was used to identifyreaction intermediates and to probe key elementary processes in-volved in the carbonylation of DMM to MMAc. The spectroscopicevidence supports our


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