Coke Formation during the Methanol‐to‐Olefin Conversion: In Situ Microspectroscopy on Individual H‐ZSM‐5 Crystals with Different Brønsted Acidity

Abstract

Coke formation during the methanol-to-olefin (MTO) conversion has been studied at the single-particle level with in situ UV/Vis and confocal fluorescence microscopy. For this purpose, large H-ZSM-5 crystals differing in their Si/Al molar ratio have been investigated. During MTO, performed at 623 and 773 K, three major UV/Vis bands assigned to different carbonaceous deposits and their precursors are observed. The absorption at 420 nm, assigned to methyl-substituted aromatic compounds, initiates the buildup of the optically active coke species. With time-on-stream, these carbonaceous compounds expand in size, resulting in the gradual development of a second absorption band at around 500 nm. An additional broad absorption band in the 600 nm region indicates the enhanced formation of extended carbonaceous compounds that form as the reaction temperature is raised. Overall, the rate of coke formation decreases with decreasing aluminum content. Analysis of the reaction kinetics indicates that an increased Brønsted acid site density facilitates the formation of larger coke species and enhances their formation rate. The use of multiple excitation wavelengths in confocal fluorescence microscopy enables the localization of coke compounds with different molecular dimensions in an individual H-ZSM-5 crystal. It demonstrates that small coke species evenly spread throughout the entire H-ZSM-5 crystal, whereas extended coke deposits primarily form near the crystal edges and surfaces. Polarization-dependent UV/Vis spectroscopy measurements illustrate that extended coke species are predominantly formed in the straight channels of H-ZSM-5. In addition, at higher temperatures, fast deactivation leads to the formation of large aromatic compounds within channel intersections and at the external zeolite surface, where the lack of spatial restrictions allows the formation of graphite-like coke.

Coke Formation in a Zeolite Crystal During the Methanol‐to‐Hydrocarbons Reaction as Studied with Atom Probe Tomography

“…[5,6] Despite ongoing investigations and debates,there is aconsensus that coking occurs due to the formation of alkylated mono-and polycyclic aromatics near internal channel intersections,followed by an increase in surface coke from polycyclica renes near pore openings, which finally form agraphitic layer and block pore access. [7][8][9][10][11][12][13] In order to more fully elucidate the material properties that promote the detrimental formation of coke during the MTH reaction on ZSM-5, it would be beneficial to study the carbon deposits on the sub-nm length scale.P revious coking studies on ZSM-5 in the MTH reaction have concentrated on the bulk, [7][8][9]13] or on micrometer length scales, [10,11,14,15] to gain some spatial insight, but none have been capable of delivering sub-nm resolution.Theo nly characterization method currently capable of spatially resolving 3D element distributions at the sub-nm scale is atom probe tomography (APT), which was first envisioned in the 1930s,b ut has recently experienced rapid growth due to improvements in instrumentation. [16][17][18][19][20] APT is able to create atom-by-atom 3D compositional reconstructions of materials within afabricated needle-shaped specimen (tens of nanometers in size), across which ahigh electric field is applied that causes atoms to evaporate during pulsing events.The ions are then spatially detected and their identity determined using time-of-flight mass spectrometry.T his technique has traditionally been used to study conductive materials, [19] but owing to experimental advances,A PT can also now be applied to non-conductive materials.A PT was recently applied to zeolites for the first time to study the distribution of Al atoms in parent and severely steamed, large,coffin-shaped ZSM-5 crystals,where anon-uniform Al distribution was observed.…”

“…TheM TH reaction was performed using 13 C-labelled methanol so that coke species could be unambiguously assigned, with a90min reaction time at 623 Kp urposefully chosen to yield crystals that contained sufficient coke for analysis,while still retaining catalytic activity (see Figure S1 and the Supporting Information for further explanation). [13,14] Using advanced cluster analysis we provide an atomic scale visualization of hydrocarbon deposits within as ingle ZSM-5 crystal, and demonstrate acorrelation between the Brønsted acid site density on ananoscopic level and the formation of 13 Cclusters (the term cluster refers to ag roup of closely positioned atoms).TheZ SM-5 crystals employed in this work contain ac omplex internal architecture,s hown in Figure 1a.[22] For APT analysis across-section of the ZSM-5 crystal used in the MTH reaction was prepared via focused ion beam (FIB) milling from ap ortion of the crystal that contained several different subunits.T he locations of the cross-section and needles prepared for APT are shown in Figures 1b and S2, and images of all analyzed needles are displayed in Figure S3. In total four needles were successfully analyzed by the APT technique (see the Supporting Information for details about the APT experiment and related data analysis).…”

“…[5,6] Despite ongoing investigations and debates,there is aconsensus that coking occurs due to the formation of alkylated mono-and polycyclic aromatics near internal channel intersections,followed by an increase in surface coke from polycyclica renes near pore openings, which finally form agraphitic layer and block pore access. [7][8][9][10][11][12][13] In order to more fully elucidate the material properties that promote the detrimental formation of coke during the MTH reaction on ZSM-5, it would be beneficial to study the carbon deposits on the sub-nm length scale.P revious coking studies on ZSM-5 in the MTH reaction have concentrated on the bulk, [7][8][9]13] or on micrometer length scales, [10,11,14,15] to gain some spatial insight, but none have been capable of delivering sub-nm resolution.…”

Effect of Feedstock and Catalyst Impurities on the Methanol‐to‐Olefin Reaction over H‐SAPO‐34

“…According to this mechanism, methanol reacts onto a pool of organic intermediates to yield a wide range of hydrocarbons 16. Active species in the HCP are poly‐methyl‐substituted benzeniums,17 and deactivation is likely caused primarily by coke deposition 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29…”

Highly Oriented Growth of Catalytically Active Zeolite ZSM‐5 Films with a Broad Range of Si/Al Ratios

“…The absorption maxima that developed with time‐on‐stream at around 410, 520, and 620 nm were assigned to neutral methylated benzenes, dienylic carbocationic/methylbenzenium ions, trienylic and methylated poly‐arenium ions, respectively (Figure 4 a and S16) 35, 36. The clearly separated bands, compared to the broad signal normally found from bulk crystals,36 can be used to easily explore the evolution of the reaction intermediates, and their contribution to the activity and deactivation of the MTH process demonstrates that the oriented films are promising for use as a model system to study catalytic reactions. Moreover, the on‐line MS data reveals the existence of dimethyl ether (DME) and ethylene, along with propylene, as the reaction products during the MTH process (Figure 4 b and Figure S17), meaning that the films are catalytically active through their Brønsted acid sites 37, 38…”