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

Abstract

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.

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

“…In contrast, the lower ratio of mono‐aromatics versus poly‐aromatics in the methanol (A) and (E) feedstocks (Figure 5) suggests that catalyst deactivation likely occurs by pore blockage by poly‐cyclic molecules throughout the H‐SAPO‐34 crystal. Figure 7 shows a schematic representation of the hypothesized effect of feedstock impurities on catalyst deactivation, which is in line with previous findings by our group 43. This envisaged mechanism of deactivation is also supported by the relatively low absorbance of the active species (characterized by the absorption bands at 350 and 390 nm) if a methanol (E) feedstock is used compared with a methanol (U) feedstock (Figure 1 versus Figure 3, summarized in Figure 4).…”

“…In general, it can be said that fewer gas‐phase impurities lead to longer induction and active periods. However, the elongated induction period for methanol (E) with respect to methanol (A), containing less impurities, also suggests a competing pathway for ethanol‐to‐olefins conversion, which was previously noted by our group 43. That is, the ethanol‐to‐olefins (ETO) process leads to the selective formation of aromatics, or coke deposits, over olefins 2, 49, 50.…”

“…followed this approach by suggesting that the initial bond formation proceeds not through a direct route, but rather by the presence of organic impurities 41. External “impurities”, such as toluene, ethanol, and acetone, are proven to have an effect on the MTO process 3, 41, 42, 43. The complexity of the MTO reaction undoubtedly allows the (simultaneous) existence of both a direct and an indirect mechanism, in which the prevailing mechanism is dependent on the system and the tools used to observe it.…”

“…The extensive number of publications dealing with the effect of feedstock impurities41, 42, 43, 44, 45 is in contrast with the lack of fundamental understanding on the influence of internal impurities (i.e., hydrocarbon deposits) in reactions with zeolites or zeotypes. To the best of our knowledge, there are no studies providing detailed information about their effect on the zeolite's catalytic performance and the role of these impurities on the origin of the first active HCP species.…”

Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy

“…30,31 Finally, secondary reaction products including less active poly-aromatic compounds are formed as can be seen in UV-Vis spectra of catalyst samples under MTO conditions. [32][33][34][35] This coke formation eventually leads to catalyst deactivation by pore 36 and/or site 37 blocking. It has been reported that the deactivation rate was substantially decreased by water addition to the feed.…”