DOI: 10.1002/cctc.201200580 Mechanistic Studies on Chabazite-Type Methanol-to-Olefin Catalysts: Insights from Time-Resolved UV/Vis Microspectroscopy Combined with Theoretical Simulations Veronique Van Speybroeck,* [a] Karen Hemelsoet, [a] Kristof De Wispelaere, [a] Qingyun Qian, [b] Jeroen Van der Mynsbrugge, [a] Bart De Sterck, [a] Bert M. Weckhuysen,* [b] and Michel Waroquier [a] Introduction Light olefins are crucial compounds in today’s petrochemical industry and are traditionally obtained through the refinement of crude oil. In view of the waning oil reserves, the develop- ment of new technologies for the production of chemicals and transportation fuels based on alternative natural sources is very topical. [1] Of these processes, the zeolite-catalyzed metha- nol-to-hydrocarbon (MTH) technology is one of the most promising and several MTH processes have currently been commercialized. [2, 3] The ability of acid zeolites to convert meth- anol to hydrocarbons (in the range C 2 –C 10 ) and water was dis- covered in 1976. [4] Industrial processes, such as methanol-to- gasoline and methanol-to-olefin (MTO) conversions, have been developed since then. Methanol can be obtained from synthe- sis gas (CO + H 2 ), which can be produced from any gasifiable carbonaceous species, such as natural gas, coal, biomass, and waste. The reaction is catalyzed with protonated zeolites or zeotype materials, and after a kinetic induction period, hydro- carbons are formed. [2] To date, there is a consensus that the active site for MTO catalysis consists of a Brønsted acid site to- gether with an organic reaction center also present in zeolite pores, which acts as a cocatalyst. [5–7] Thus, the active site has a hybrid organic–inorganic nature, which leads to an additional level of complexity in designing the catalyst. Each active site of the MTO process can be regarded as a well-defined supra- molecular complex in which an organic component is trapped in the inorganic host (Scheme 1). This concept was described in the seminal papers of Haw and co-workers. [5, 6] A thorough understanding of the interplay between the organic compo- nent and its host is of utmost importance to obtain molecular The formation and nature of active sites for methanol conver- sion over solid acid catalyst materials are studied by using a unique combined spectroscopic and theoretical approach. A working catalyst for the methanol-to-olefin conversion has a hybrid organic–inorganic nature in which a cocatalytic organ- ic species is trapped in zeolite pores. As a case study, micropo- rous materials with the chabazite topology, namely, H-SAPO-34 and H-SSZ-13, are considered with trapped (poly)aromatic spe- cies. First-principle rate calculations on methylation reactions and in situ UV/Vis spectroscopy measurements are performed. The theoretical results show that the structure of the organic compound and zeolite composition determine the methylation rates: 1) the rate increases by 6 orders of magnitude if more methyl groups are added on benzenic species, 2) transition state selectivity occurs for organic species with more than one aromatic core and bearing more than three methyl groups, 3) methylation rates for H-SSZ-13 are approximately 3 orders of magnitude higher than on H-SAPO-34 owing to its higher acid- ity. The formation of (poly)aromatic cationic compounds can be followed by using in situ UV/Vis spectroscopy because these species yield characteristic absorption bands in the visi- ble region of the spectrum. We have monitored the growth of characteristic peaks and derived activation energies of forma- tion for various sets of (poly)aromatic compounds trapped in the zeolite host. The formation–activation barriers deduced by using UV/Vis microspectroscopy correlate well with the activa- tion energies for the methylation of the benzenic species and the lower methylated naphthalenic species. This study shows that a fundamental insight at the molecular level can be ob- tained by using a combined in situ spectroscopic and theoreti- cal approach for a complex catalyst of industrial relevance. [a] V. Van Speybroeck, ++ K. Hemelsoet, ++ K. De Wispelaere, J. Van der Mynsbrugge, B. De Sterck, + M. Waroquier Center for Molecular Modeling (Member of the QCMM Ghent-Brussels Alliance) Ghent University Technologiepark 903, 9052 Zwijnaarde (Belgium) E-mail : veronique.vanspeybroeck@ugent.be [b] Q. Qian, B. M. Weckhuysen Inorganic Chemistry and Catalysis Debye Institute for Nanomaterials Science Utrecht University Universiteitsweg 99, 3584 CG Utrecht (The Netherlands) E-mail : b.m.weckhuysen@uu.nl [ + ] Current address: LANXESS Rubber N.V. BTR—Competence Center Technology Haven 1009, Canadastraat 21, 2070 Zwijndrecht (Belgium) [ ++ ] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200580.  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 173 – 184 173 CHEMCATCHEM FULL PAPERS