546667 Mechanistic Insights in the Methanol-to-Hydrocarbon Process Using Advanced and Combined Operando Spectroscopy

Wednesday, June 5, 2019: 3:00 PM
Texas Ballroom D (Grand Hyatt San Antonio)
Sophie H. van Vreeswijk, Joris G. Goetze and Bert M. Weckhuysen, Inorganic Chemistry and Catalysis, Utrecht University, Utrecht, Netherlands

The Methanol-to-Hydrocarbon (MTH) process is currently of great interest as the feedstocks for the production of methanol can be obtained from practically any carbon source. Thereby, an alternative route for the production of more renewable plastics and fuels can be obtained. The MTH process, in which methanol is converted to hydrocarbons, can be catalyzed by zeolites. The amount of Brønsted acid sites, the pore size and structure and the cage size and structure of the zeolites determine which actual products are formed.1,2  The most accepted mechanism of the formation of hydrocarbons within the structures is the hydrocarbon pool mechanism or the dual cycle mechanism, in which aromatic hydrocarbons and olefins function as intermediate products in the zeolite cages. Insights in the formation of these intermediates during the conversion of methanol are required to define the actual mechanism and to gain more knowledge about the tunability of the process within the frameworks of the zeolites. Additionally, the formation of large hydrocarbon species as polyaromatic compounds can block the pores as they occupy the whole cavity opening or large coke species can accumulate on the outside of the zeolites by which the accessibility of the pores and cavities is hindered. In the recent years, our group developed multiple operando spectroscopic techniques to analyze the formation of different species within the cages of the zeolites during the MTH reaction to, in the end, follow the active and deactivating period of the process.

Figure 1: Schematic representation and a picture of the operando UV-vis set-up. A high temperature UV-Vis probe is used to obtain information about the intermediate species formed during the MTH reaction. Multiple UV-vis probes can be used to record the UV-Vis spectra at different positions along the catalyst bed to follow the so called “cigar burn” principle.

Charged and neutral aromatic intermediates absorb visible and ultraviolet light. Therefore, operando UV-vis spectroscopy (UV-vis spectroscopy combined with on-line product analysis) provides us of a way to follow the formation of the aromatics formed within the hydrocarbon pool. Figure 1 represents the schematic of the operando UV-vis set-up used for our group’s research. This technique, combined with chemical analysis of carbonaceous deposits on the zeolites, were used to obtain information about the mechanism of zeolites with different topologies. It was found that the exact size and shape of the cages of a zeolite is significantly influencing the mechanism, deactivation and product distribution of the MTH reaction. To obtain as much as possible information about the evolution of the UV-vis spectra and thereby the formation of the aromatic compounds within the structure, multivariate analysis can be used. With this technique, it was found that, besides the significant influence of the nature of the zeolite, especially for zeolites with 2D pore networks, the reaction temperature also has an important effect on the number and role of certain active and deactivating hydrocarbon pool species.1

Introduction of alkaline earth metals can drastically influence the reaction mechanism and thereby the products formed.2 However, the way of introducing these metals is very important to the final contribution to the mechanism.3,4 With operando spectroscopy, the differences between both the formed products and reactivity as well as the differences between the reaction intermediates can be analyzed and combined to enable the rationalization of the relation of these intermediates and products.

Additionally, operando spectroscopy can provide information about the location of the deactivating carbon deposition on the zeolites and enables us to follow the so called “cigar burn” principle.1,3 Regeneration studies combined with operando spectroscopy can provide information about the nature of the coke species within the pores of the zeolites.

The accumulation of hydrocarbon species in the zeolite framework result in the expansion of the lattice. With operando X-ray diffraction this lattice expansion can be correlated to the activity and selectivity of the zeolites in the MTH process. In our lab the operando XRD set-up and an operando UV-Vis set-up are combined to be able to correlate possible lattice expansion to actual hydrocarbon species. The exact lattice expansion and parameters can be determined using Rietveld refinement. These experiments were done for three small pore 8-ring zeolite frameworks: CHA, DDR and LEV. All three structures showed lattice expansion. The origin of this lattice expansion was correlated to different hydrocarbon species and is illustrated in Figure 2.5

Figure 2: The three zeolite frameworks and the species causing the lattice expansion during MTO observed with operando XRD experiments.5

To conclude, in the work to be presented we will include the operando spectroscopic results of the MTH process on different zeolite frameworks with and without incorporation of hetero atoms to be, in the end, able to contribute to the unraveling of the complicated and promising MTH process.

Reference list:

(1)       Goetze, J.; Meirer, F.; Yarulina, I.; Gascon, J.; Kapteijn, F.; Ruiz-Martínez, J.; Weckhuysen, B. M. Insights into the Activity and Deactivation of the Methanol-to-Olefins Process over Different Small-Pore Zeolites As Studied with Operando UV-Vis Spectroscopy. ACS Catal. 2017, 7, 4033–4046.

(2)       Yarulina, I.; Chowdhury, A. D.; Meirer, F.; Weckhuysen, B. M.; Gascon, J. Recent Trends and Fundamental Insights in the Methanol-to-Hydrocarbons Process. Nat. Catal. 2018, 1, 398–411.

(3)       Goetze, J.; Weckhuysen, B. M. Spatiotemporal Coke Formation over Zeolite ZSM-5 during the Methanol-to-Olefins Process as Studied with: Operando UV-Vis Spectroscopy: A Comparison between H-ZSM-5 and Mg-ZSM-5. Catal. Sci. Technol. 2018, 8, 1632–1644.

(4)       Yarulina, I.; De Wispelaere, K.; Bailleul, S.; Goetze, J.; Radersma, M.; Abou-Hamad, E.; Vollmer, I.; Goesten, M.; Mezari, B.; Hensen, E. J. M.; et al. Structure–performance Descriptors and the Role of Lewis Acidity in the Methanol-to-Propylene Process. Nat. Chem. 2018, 10, 804–812.

(5)       Goetze, J.; Yarulina, I.; Gascon, J.; Kapteijn, F.; Weckhuysen, B. M. Revealing Lattice Expansion of Small-Pore Zeolite Catalysts during the Methanol-to-Olefins Process Using Combined Operando X-Ray Diffraction and UV-Vis Spectroscopy. ACS Catal. 2018, 8, 2060–2070.

 


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