Angewandte Communications Chemie

International Edition:DOI:10.1002/anie.201807814 Heterogeneous Hot Paper German Edition:DOI:10.1002/ange.201807814 Coupling of and 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 to form methyl acetate (MeOAc) and acetic for industrial applications. (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

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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. decreased the selectivity for HOAc,MeOAc,and olefins and To further confirm that CO participates in forming increased that for aromatics.These results indicate that the aromatics, 13Cisotope tracing was utilized to examine the carbonyl compounds act as intermediates to form aromatics. reactions,and the catalytic performance is shown in Fig- To illustrate the evolution from carbonyl intermediates to ure S11. TheMSspectra of products in the presence of 12CO aromatics,the reactions at 523 Kwere quenched by liquid or 13CO are displayed in the Supporting Information, Fig- nitrogen and active species were extracted and analyzed by ure S12. The 13Cdistribution in the products can be calculated GC-MS following Guisnetsmethod.[13] Theresults are shown on the basis of the 12Cdistribution. As determined from in Figure 3, and the catalytic performance is shown in the Figure 4a,the amount of 13Cincorporated into benzene and toluene was significantly higher than that incorporated into propene and propane,which is crucial evidence supporting the involvement of CO in the formation of aromatic rings. Moreover,this result indicates that the principal mechanism of aromatization in the coupling reaction of methanol with CO is not conventional olefin dehydrogenation because the amount of 13Cincorporated into aromatics,olefins,and

Figure 3. GC-MS chromatograms of organic materials retained in 1 1 catalysts. Reaction conditions: T =523 K; WSHV= 0.15 ggÀ hÀ ;N2 or

CO/CH3OH = 113; P = 4MPa. The catalysts were removed at 120 min during the methanolconversion in (N2 (bottom) and CO (top)). The enlarged region of retention time from 6.2 to 7.4 min are shown in the inset, where dimethyl-2-cyclopenten-1-one species are labeled 1*, trimethyl-2-cyclopenten-1-one is labeled 2* while 3-methyl-2-cyclo- hexen-1-one is labeled 3*.

Supporting Information, Figures S9 and S6b.The primary organics retained in the catalyst after the MTAreaction under

N2 were polymethylbenzenes,and we observed afew methyl- 2-cyclopenten-1-ones (MCPOs), which have been detected in the conventional MTH reaction.[14] Interestingly,the intro- duction of CO in the reaction strikingly enhanced the amount of MCPOs.Thus we propose that MCPOs act as intermedi- ates during the conversion of carbonyl compounds into aromatics.From the catalysis results in Figure 1and the Supporting Information, Figure S8, it can also be found that Figure 4. The results of the 13CO isotope tracing method. a) 13C 13 the selectivity of aromatics increased with decreasing selec- distribution in products of coupling reaction of methanol and CO. 1 1 tivity for olefins and carbonyl compounds.Therefore,MCPOs Reaction conditions: T =673 K; WHSV= 0.15 ggÀ hÀ ;CO/ CH OH =28; P =1MPa. b) MS spectra of 3-methyl-2-cyclopenten-1- may be generated by the reaction of low olefins and carbonyl 3 one in 12CO (bottom) and 13CO (top) during the coupling reaction of compounds. methanol and CO. Reaction conditions: T =523 K; 1 1 To identify the product from the reaction of olefins with WHSV= 0.15 ggÀ hÀ ;CO/CH3OH= 28; P =1MPa. The catalysts were carbonyl compounds,excess acetic anhydride was absorbed removed at 120 min during the reaction.

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paraffin differed greatly.The 13Cinolefins and paraffin could have derived from the 13Cpolymethylbenzenes,asitiswidely accepted that hydrocarbon pool species could produce light olefins.[15] Because benzene containing one 13Catom was predominant, we consider that one CO molecule took part in forming an aromatic ring and that olefins containing a 13C atom from aromatics led to the inclusion of more than one 13C atom in aromatics.Wealso analyzed the 13Cincorporation into MCPOs intermediates.Asshown in Figure 4b and the Supporting Information, Figure S13, the mass of the base peak of MCPOs increased by 1when 12CO switched to 13CO, indicating that only one CO molecule was involved in the formation of one MCPO intermediate.Considering that only one CO molecule takes part in the formation of aromatics,we believe that MCPOs act as the intermediates to form aromatics. 13 Theproducts of CO/CH3OH feeds and used catalysts were analyzed by 13Cliquid-state NMR spectroscopy.The 13CNMR spectra of the liquid products collected from the 13 CO/CH3OH feed are shown in the Supporting Information, Figure S14. Thesignal at 128 ppm was assigned to the Catom of the aromatics ring.[16] Signals of methyl groups were not observed in the 13CNMR spectra. These results prove that the 13Catoms of 13CO are indeed introduced into the aromatic Figure 5. Proposed mechanism for aromatics formation on CMTAover rings.The used ZSM-5 catalyst was extracted by CH2Cl2 H-ZSM-5. The reaction products and immediate indicated in red have following Guisnetsmethod, and the resultant liquid solution been experimentally detected in this work. was analyzed through 13Cliquid-state NMR spectroscopy (Supporting Information, Figure S15). Thesignal at 209 ppm Acknowledgements was assigned to keto Cwhile the signals corresponding to the other Catoms in MCPOs (2-cyclopenten-1-one was used as Thework was supported by the National Natural Science stand reference) were not observed, which indicates that the Foundation of China (Grant No.21606224). keto CinMCPOs comes from CO. Because CO participates in the formation of aromatics via carbonyl compounds and MCPO intermediates,wepropose Conflict of interest anew mechanism of aromatization, as shown in Figure 5; the reaction formulas are shown in the Supporting Information, Theauthors declare no conflict of interest. Scheme S1. First, carbonyl compounds such as HOAc and MeOAc are produced by aKoch-type carbonylation reaction, Keywords: aromatics ·carbon monoxide · which has been repeatedly confirmed by our results and past heterogeneous catalysis ·H-ZSM-5 ·methanol studies.[7] Our work demonstrates that carbonyl compounds react with olefins to generate MCPOs.AsMCPOs have been Howtocite: Angew.Chem. Int. Ed. 2018, 57,12549–12553 [14] reported to easily transform into aromatic compounds, we Angew.Chem. 2018, 130,12729–12733 propose MCPOs with 5-MR were transformed into aromatics with 6-MR by dehydrolysis.Since the isotope tracing results indicate that aportion of aromatics do not contain carbon from CO,the conventional aromatization mechanism of [1] A. M. Niziolek, O. Onel, C. A. Floudas, AIChE J. 2016, 62, olefin hydrogen transfer co-occurs with the new mechanism. 1531 –1556. In summary,80% aromatics selectivity and 65%BTX [2] a) Y. Ono,H.Adachi, Y. Senoda, J. Chem. Soc.FaradayTrans. 1988, 84,1091 –1099;b)Y.Inoue,K.Nakashiro,Y.Ono, selectivity are achieved in the coupling conversion of Microporous Mater. 1995, 4,379 –383;c)Y.Ni, A. Sun, X. Wu, methanol and CO over the H-ZSM-5 catalyst. Anew G. Hai, J. Hu, T. Li, G. Li, Microporous Mesoporous Mater. 2011, aromatization mechanism that includes the formation of 143,435 –442;d)J.A.Lopez-Sanchez, M. Conte,P.Landon, W. carbonyl compounds and MCPO intermediates and that co- Zhou, J. K. Bartley,S.H.Taylor,A.F.Carley,C.J.Kiely,K. occurs with adrastically decreased hydrogen transfer reaction Khalid, G. J. Hutchings, Catal. Lett. 2012, 142,1049 –1056;e)N. is proposed. Aportion of the carbons in CO molecules are Wang,W.Qian, K. Shen, C. Su, F. Wei, Chem. Commun. 2016, incorporated into aromatics,which is of great significance for 52,2011 –2014;f)Y.Jia, J. Wang,K.Zhang,W.Feng,S.Liu, C. Ding,P.Liu, Microporous Mesoporous Mater. 2017, 247,103 – industrial applications. 115. [3] X. Sun, S. Mueller,Y.Liu, H. Shi, G. L. Haller,M.Sanchez- Sanchez, A. C. van Veen, J. A. Lercher, J. Catal. 2014, 317,185 – 197.

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