Combined Ex and In Situ Measurements Elucidate the Dynamics of Retained Species in ZSM‐5 and SAPO‐18 Catalysts Used in the Methanol‐to‐Olefins Reaction

Abstract

The dynamics of the retained species on ZSM‐5 and SAPO‐18 catalysts are studied by using a combination of temperature‐programmed desorption/oxidation, ex situ analysis, and in situ FTIR spectroscopic measurements over the entire conversion range, using fixed‐bed and spectroscopic cell reactors, in continuous and discontinuous mode. The results point to the appropriateness of the combined methodologies to track the interconversion of active into deactivating species. A statistically relevant (supported by linear regression and multivariate analysis) association of the observations is found by using the different complementary methodologies. The kinetics of this interconversion depends on the initial conversion (tuned by acidity and space time) and microporous topology, and involve: (i) in the ZSM‐5 catalysts, the diffusion of monocyclic aromatics toward the exterior of the zeolite to form coke, and (ii) in the SAPO‐18 catalysts, the obstruction of the cavities by aromatics that grow into tetracyclic aromatic islands.

Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

“…In general, the bands in the υ C‐H and υ C=C bond regions present less absorbance intensities for the feed of methanol‐water, whereas the bands in the υ O‐H and δ O‐H regions are more intense due to the high water concentration in the reaction medium. In a previous works, [40,59] we observed that the bands in the υ C=C and δ C‐H regions are more reliable to study the retained species as they are less affected by gaseous phase or physisorbed species. The results clearly show that these bands (representative of retained species) are less intense for the feed of methanol‐water, indicating that co‐feeding water disfavors the formation of retained species as also observed in the experiments carried out using UV‐vis spectroscopy.…”

“…Interestingly, the intensity of the 1390 (δ C-H ) and 1591 (υ C = C ) cm À 1 bands decreases so much for the methanol-water feed that it seems to be practically absent, whereas the 1570 cm À 1 band becomes more dominant in the region of retained species. According to the literature, [59,60] the simultaneous rise of the 1390 and 1591 cm À 1 bands can be related to absorbed olefins. The presence of olefins within the retained species in SAPO-34 catalysts was demonstrated using Raman spectroscopy by Rojo-Gama et al, [61] being a finding applicable to our results since the SAPO-18 structure is relatively similar to that of SAPO-34.…”

Alumina‑Embedded Hzsm-5 with Enhanced Behavior for the Catalytic Cracking of Biomass Pyrolysis Bio-Oil: Insights into the Role of Mesoporous Matrix in the Deactivation by Coke

“…In the case of HT-SC coke species released at higher temperature (maximum rate at 500 ºC) greater amount was observed for Cat− Z30 in comparison with Zeo− Z30 catalyst. The species removed during the TPD of zeolite catalysts deactivated in the cracking of hydrocarbons [56] and methanol-to-olefins (MTO) reactions [57] have been previously ascribed to coke precursors. However, these species could be also the consequence of the aging (dehydrogenation and decomposition reactions) that hard coke may undergo under N 2 atmosphere at high temperature [58].…”

Evaluating catalytic (gas–solid) spectroscopic cells as intrinsic kinetic reactors: Methanol-to-hydrocarbon reaction as a case study

“…The use of operando spectroscopy partnering with ab initio calculations has revealed steps of initiation, autocatalysis, and deactivation [12][13][14][15][16][17][18][19] during MTH/MTO reactions. In brief, acid sites are prone to form methoxy and carbocation species that dehydrate, oligomerize or alkylate/dealkylate (methylate), cyclize, and aromatize to form a whole range of products, including deactivating species [20][21][22][23][24][25][26][27][28][29]. Thus, the active site is the combination of the acid site and the adsorbed carbocation.…”

“…Our previous experience has demonstrated that two commercial gas-solid spectroscopic cells are suitable to study the formation kinetics of species on the catalyst surface during the MTH reaction with different catalysts [22,24,50] or cofeeding water [23,51] using FTIR or UV-vis spectroscopies. Accordingly, we verified that the formation rates of species decrease with a decreasing acid concentration in the catalyst and an increasing water concentration in the feed, which are the expected kinetic observations.…”