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Coupling of Methanol and Carbon Monoxide Over H‐ZSM‐5 to Form

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Coupling of Methanol and Carbon Monoxide Over H‐ZSM‐5 to Form Angewandte Communications Chemie International Edition:DOI:10.1002/anie.201807814 Heterogeneous Catalysis Hot Paper German Edition:DOI:10.1002/ange.201807814 Coupling of Methanol and Carbon Monoxide over H-ZSM-5 to Form Aromatics Zhiyang Chen, Youming Ni, Yuchun Zhi, Fuli Wen, Ziqiao Zhou, Yingxu Wei, Wenliang Zhu,* and Zhongmin Liu* Abstract: The conversion of methanol into aromatics over via hydrogen transfer.[3] Although modifications with metals unmodified H-ZSM-5 zeoliteisgenerally not high because the such as Zn,[2a,c,e] Ga,[2d] and Ag[2b] increase aromatics forma- hydrogen transfer reaction results in alkane formation. Now tion by enhancing the dehydrogenation of alkanes via circa 80%aromatics selectivity for the coupling reaction of catalysis by these metal species,[4] some methanol decompo- methanol and carbon monoxide over H-ZSM-5 is reported. sition and the formation of unrecoverable catalyst structure Carbonyl compounds and methyl-2-cyclopenten-1-ones owing to metal evaporation, segregation, and aggregation are (MCPOs), which were detected in the products and catalysts, inevitable.[2a,5] respectively,are considered as intermediates.The latter species Recently,Cheng et al. reported that aromatization could 13 can be synthesized from the former species and olefins. C be enhanced by CO over the bifunctional ZnZrOx/H-ZSM-5 isotope tracing and 13Cliquid-state NMR results confirmed that catalysts in the conversion of syngas or intermediate meth- the carbon atoms of CO molecules were incorporated into anol because CO plays arole in the removal of Hspecies and MCPOs and aromatic rings.Anew aromatization mechanism subsequent formation of methanol. As aresult, it can be that involves the formation of the aboveintermediates and co- deduced that ZnZrOx is indispensable to the catalysis occurs with adramatically decreased hydrogen transfer mechanism.[6] reaction is proposed. Aportion of the carbons in CO molecules In the presence of CO,H-zeolites catalyze methanol are incorporated into aromatic,which is of great significance carbonylation to form methyl acetate (MeOAc) and acetic for industrial applications. acid (HOAc) through the Koch reaction.[7] It has been proposed that the formation of the first carbon–carbon Aromatics,especially benzene,toluene,and xylene (BTX), bond occurs through methanol carbonylation during the are important bulk chemicals that are primarily produced methanol to olefin reaction.[8] from petroleum by catalytic reforming or cracking.[1] Obtain- Herein we report an aromatics selectivity of 80%along ing aromatics from non-petroleum resources,such as coal, with 65%BTX for the coupling reaction of methanol and CO natural gas,orbiomass,isvery important because of the (CMTA) over H-ZSM-5 zeolite.Anew aromatization increase in market demand for such products and the mechanism in which the hydrogen transfer reaction is sharply depletion of petroleum resources.Methanol can be derived decreased is proposed. from these alternative resources via syngas chemistry,and TheCMTAand MTAreactions were performed at 673 K aromatics can be readily synthesized from methanol by under 4.0 MPa over Z-25 (H-ZSM-5 with the SiO2/Al2O3 amethanol-to-aromatics (MTA) reaction.[2] Generally, ratio = 25). Theresults are shown in Figure 1a,anapprox- acidic ZSM-5 zeolites are selected as MTAcatalysts owing imate 40%initial aromatics selectivity with a53% C2–C4 to their unique topology and three-dimensional micropore paraffin selectivity was obtained under N2,which agree with systems.However,acidic H-form ZSM-5 zeolite (H-ZSM-5) the typical hydrogen transfer mechanism.[3] Interestingly,the without metal modifications usually has low aromatics initial aromatics selectivity was dramatically increased to selectivity because of the corresponding formation of alkanes 80%along with 65%BTX when methanol was coupled with CO,while the CH4 and C2–C4 paraffin selectivity was only 3% and 20%, respectively (Figure 1b). Thedetailed selectivity of [*] Z. Chen, Dr.Y.Ni, Dr.Y.Zhi, F. Wen, Z. Zhou, Prof. Dr.Y.Wei, aromatics is shown in the Supporting Information, Figure S1. Prof. Dr.W.Zhu, Prof. Dr.Z.Liu National Engineering Laboratory for Methanol to Olefins As presented in the Supporting Information, Figure S2, Dalian National Laboratory for Clean Energy increasing the reaction temperatures of the CMTAreaction Dalian Institute of Chemical Physics was advantageous to aromatics generation because C2–C4 Chinese Academy of Sciences, Dalian 116023 (P. R. China) paraffin formation was suppressed;however amuch higher E-mail:[email protected] temperature (723 K) led to lower aromatics selectivity owing [email protected] to sharp increases in CH4. Z. Chen, F. Wen, Z. Zhou H-ZSM-5 zeolites with SiO2/Al2O3 ratios ranging from 25 UniversityofChinese Academy of Sciences to 175 were employed to study the effect of acidity on Beijing 100049 (China) aromatization in the CMTAreaction. TheXRD,N absorp- Supportinginformation (including experimental details, catalyst 2 tion–desorption, and NH -TPD results are depicted in the characterization, enlarged part of GC-MS chromatograms) and the 3 ORCID identification number(s) for the author(s) of this article can Supporting Information, Figure S3, Table S1, and Figure S4, be found under: respectively.Alower SiO2/Al2O3 ratio results in more acidic https://doi.org/10.1002/anie.201807814. sites.[9] As seen in Figure 2, the aromatics selectivity increased Angew.Chem. Int.Ed. 2018, 57,12549 –12553 2018 Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim 12549 Angewandte Communications Chemie 1 1 Figure 1. a) MTAand b) CMTAperformance over Z-25. Reaction conditions: T = 673 K; WHSV=0.15 ggÀ hÀ ;N2 or CO/CH3OH= 113; P = 4MPa. remained at approximately 70%asmethanol was completely converted into products in 100 honstream. Theenhanced aromatization in the CMTAreaction could not be explained by the dehydrogenation of alkanes[4] or the synthesis of methanol[11] owing to the absence of metal species.Wealso confirmed that propane was slightly con- verted into products over Z-25 at 673 Kand 4MPa in the presence of CO.COwas inactive when fed alone or syngas (H2 :CO= 2:1) over Z-25 at 673 Kand 4MPa. In the presence of CO,carbonyl compounds including MeOAc and HOAc were also detected at the beginning of deactivation at 673 K(Figure 1b). When the reaction temper- ature dropped to 493 K, methanol conversion remained at 11%with a91% selectivity for MeOAc and HOAc (Sup- Figure 2. Effect of SiO2/Al2O3 ratio of H-ZSM-5 on CMTAperformance. 1 1 porting Information, Figure S6a). Theother products were Reaction conditions: T = 673 K; WHSV= 0.5 ggÀ hÀ ;CO/ C2H4 and C4H8.Whenthe temperature rose to 523 K, the CH3OH = 20; P = 5MPa time on stream = 4h;PS:C5+ excludes aro- matics. selectivity for MeOAc and HOAc reached 92.4%with circa 40%methanol conversion in the beginning,which then rapidly dropped to avery low value with the generation of as the SiO2/Al2O3 ratio decreased (or the number of acidic hydrocarbons and complete methanol conversion. Upon sites increased). Notably,1.1–1.2%COwas converted, catalyst deactivation, the aromatics selectivity decreased whereas approximately 65%CO2 selectivity from CO was and the carbonyl compounds appeared again (Supporting obtained. As shown in the Supporting Information, Table S2, Information, Figure S6b). It is widely accepted that the MTH an almost equivalent CO2 amount were detected when co-fed reaction is an autocatalytic reaction, and critical levels of CO-MeOH over H-ZSM-5 and CO-H2Oover quartz sand; species such as polymethylbenzenes in the hydrocarbon pool [12] therefore we consider that CO2 was primarily derived from are necessary to initiate autocatalytic reactions. Thus we the water-gas-shift reaction(CO + H O CO + H ,HOwas propose that carbonyl compounds could be transformed into 2 Ð 2 2 2 formed through the MTH reaction) caused by the materials species in the hydrocarbon pool to initiate the autocatalytic (such as Fe,Ni, and Cr) of the stainless steel tubular reactor. MTH reaction, which can explain the decrease in carbonyl Therefore,approximately 35%ofthe carbon atoms in the compounds with the increase in methanol conversion. Over- consumed CO are incorporated into the hydrocarbon prod- all, we suppose that carbonyl compounds take part in the ucts.AtaCO/MeOH ratio = 20, the hydrocarbons yield will formation of aromatics. increase ca. 8% compared to the yield from methanol To clarify the effect of the carbonyl compounds,methanol conversion, which is of great significance to industrial was co-fed with MeOAc over Z-25 under N2 at 0.1 MPa. As applications.Considering that the nanostructure of H-ZSM- listed in the Supporting Information, Table S3, increasing the 5isbeneficial to prevent coke formation,[2c,f,10] we chose MeOAc concentration enhanced the selectivity for aromatics. ananosized H-ZSM-5 with aSiO2/Al2O3 ratio = 41 to study Theselectivity for aromatics increased with increasing CO the catalytic stability of the CMTAreaction. As shown in the partial pressure (shown in the Supporting Information, Fig- Supporting Information, Figure S5, the aromatics selectivity ure S7), which agrees with previous works showing that the 12550 www.angewandte.org 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem. Int. Ed. 2018, 57,12549 –12553 Angewandte Communications Chemie carbonylation rates increase linearly with the CO partial onto the ZSM-5 catalyst, which was then sealed together with [7b,c] pressure. This relationship suggests that carbonylation C3H6 (5%C3H6,95% N2)orN2.The products formed at could have aclose relationship with aromatization. We also 523 Kwere analyzed by Guisnetsmethod. As shown in the investigated the intermediates formed at areaction temper- Supporting Information, Figure S10, MCPOs were hardly ature as high as 673 Kbyshortening the contact time.Itis detected when acetic anhydride alone was converted over the obvious from the Supporting Information, Figure S8 that the ZSM-5 catalyst. However,asharp increase in MCPOs carbonyl compounds were detected when the contact time occurred after the addition of propene,indicating that was less than 0.15 s, and that prolonging the contact time MCPOs formed from olefins and carbonyl compounds.
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