Qingyun Qian

Mechanistic Studies on Chabazite‐Type Methanol‐to‐Olefin Catalysts: Insights from Time‐Resolved UV/Vis Microspectroscopy Combined with Theoretical Simulations

The formation and nature of active sites for methanol conversion 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 organic species is trapped in zeolite pores. As a case study, microporous materials with the chabazite topology, namely, H‐SAPO‐34 and H‐SSZ‐13, are considered with trapped (poly)aromatic species. 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 acidity. 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 visible region of the spectrum. We have monitored the growth of characteristic peaks and derived activation energies of formation 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 activation 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 obtained by using a combined in situ spectroscopic and theoretical approach for a complex catalyst of industrial relevance.

Identification of Intermediates in Zeolite-Catalyzed Reactions by In Situ UV/Vis Microspectroscopy and a Complementary Set of Molecular Simulations

The optical absorption properties of (poly)aromatic hydrocarbons occluded in a nanoporous environment were investigated by theoretical and experimental methods. The carbonaceous species are an essential part of a working catalyst for the methanol-to-olefins (MTO) process. In situ UV/Vis microscopy measurements on methanol conversion over the acidic solid catalysts H-SAPO-34 and H-SSZ-13 revealed the growth of various broad absorption bands around 400, 480, and 580 nm. The cationic nature of the involved species was determined by interaction of ammonia with the methanol-treated samples. To determine which organic species contribute to the various bands, a systematic series of aromatics was analyzed by means of time-dependent density functional theory (TDDFT) calculations. Static gas-phase simulations revealed the influence of structurally different hydrocarbons on the absorption spectra, whereas the influence of the zeolitic framework was examined by using supramolecular models within a quantum mechanics/molecular mechanics framework. To fully understand the origin of the main absorption peaks, a molecular dynamics (MD) study on the organic species trapped in the inorganic host was essential. During such simulation the flexibility is fully taken into account and the effect on the UV/Vis spectra is determined by performing TDDFT calculations on various snapshots of the MD run. This procedure allows an energy absorption scale to be provided and the various absorption bands determined from in situ UV/Vis spectra to be assigned to structurally different species.

Single‐Particle Spectroscopy on Large SAPO‐34 Crystals at Work: Methanol‐to‐Olefin versus Ethanol‐to‐Olefin Processes

The formation of hydrocarbon pool (HCP) species during methanol-to-olefin (MTO) and ethanol-to-olefin (ETO) processes have been studied on individual micron-sized SAPO-34 crystals with a combination of in situ UV/Vis, confocal fluorescence, and synchrotron-based IR microspectroscopic techniques. With in situ UV/Vis microspectroscopy, the intensity changes of the λ=400 nm absorption band, ascribed to polyalkylated benzene (PAB) carbocations, have been monitored and fitted with a first-order kinetics at low reaction temperatures. The calculated activation energy (Ea ) for MTO, approximately 98 kJ mol(-1) , shows a strong correlation with the theoretical values for the methylation of aromatics. This provides evidence that methylation reactions are the rate-determining steps for the formation of PAB. In contrast for ETO, the Ea value is approximately 60 kJ mol(-1) , which is comparable to the Ea values for the condensation of light olefins into aromatics. Confocal fluorescence microscopy demonstrates that during MTO the formation of the initial HCP species are concentrated in the outer rim of the SAPO-34 crystal when the reaction temperature is at 600 K or lower, whereas larger HCP species are gradually formed inwards the crystal at higher temperatures. In the case of ETO, the observed egg-white distribution of HCP at 509 K suggests that the ETO process is kinetically controlled, whereas the square-shaped HCP distribution at 650 K is indicative of a diffusion-controlled process. Finally, synchrotron-based IR microspectroscopy revealed a higher degree of alkylation for aromatics for MTO as compared to ETO, whereas high reaction temperatures favor dealkylation processes for both the MTO and ETO processes.

Combined Operando UV/Vis/IR Spectroscopy Reveals the Role of Methoxy and Aromatic Species during the Methanol-to-Olefins Reaction over H-SAPO-34

The methanol‐to‐olefins (MTO) process over H‐SAPO‐34 is investigated by using an operando approach combining UV/Vis and IR spectroscopies with on‐line mass spectrometry. Methanol, methoxy, and protonated dimethyl ether are the major species during the induction period, whereas polyalkylated benzenes and polyaromatic species are encountered in the active stage of the MTO process. The accessibility of SAPO‐34 is linked with the amount of methoxy species, whereas the formation of polyaromatic species that block the pores is the main cause of deactivation. Furthermore, the reaction pathways responsible for the formation of olefins and polyaromatics co‐exist and compete during the whole MTO process, and both routes are directly related to the amount of surface polyalkylated benzene carbocations and methoxy species. Hence, a first‐order kinetic model is proposed and comparable activation energies for both processes are obtained.

Single-Particle Spectroscopy of Alcohol-to-Olefins over SAPO-34 at Different Reaction Stages : Crystal Accessibility and Hydrocarbons Reactivity

In situ synchrotron‐based IR and UV/Vis micro‐spectroscopy combined with isotopically labeled reactants have been used to identify the different hydrocarbon species formed as well as to assess the activity and accessibility of individual 50 μm‐sized SAPO‐34 crystals. For the methanol‐to‐olefins process, two reaction stages can be distinguished. The first involves the formation of methoxy species, protonated dimethyl ether, and polyalkylated benzene (PAB) carbocations, which do not affect the accessibility of the SAPO‐34 crystal. In addition, methoxy species are very dynamic during this stage. The second stage is related to the formation of polyaromatic (PA) species concentrated in the outer rim of the crystal, which are bulky and interact with the acid sites and thus alter the overall accessibility of the crystal. In contrast, the ethanol‐to‐olefins process only consists of one major stage, as the formation of PAB and PA species cannot be separated. Furthermore, the formation of these species is more internal, and coke formation is mainly concentrated in a layer located in the inner part of the SAPO‐34 crystal.

Fluoride-assisted synthesis of bimodal microporous SSZ-13 zeolite

The presence of small amount of fluoride in alkaline hydrothermal synthesis of SSZ-13 zeolite yields bimodal microporous particles with substantially improved performance in the methanol-to-olefins (MTO) reaction. Hydrocarbon uptake measurements and fluorescence microspectroscopy of spent catalysts demonstrate enhanced diffusion through micropores at the grain boundaries of nanocrystals running through the zeolite particles. Fluoride-assisted SSZ-13 synthesis is a cheap and scalable approach to optimize the performance of MTO zeolite catalysts.