X‐Ray Imaging of SAPO‐34 Molecular Sieves at the Nanoscale: Influence of Steaming on the Methanol‐to‐Hydrocarbons Reaction

Abstract

The effect of a severe steaming treatment on the physicochemical properties and catalytic performance of H‐SAPO‐34 molecular sieves during the methanol‐to‐hydrocarbons (MTH) reaction has been investigated with a combination of scanning transmission X‐ray microscopy (STXM), catalytic testing, and bulk characterization techniques, including ammonia temperature programmed desorption and 27Al and 29Si magic angle spinning nuclear magnetic resonance. For this purpose, two samples, namely a calcined and a steamed H‐SAPO‐34 catalyst powder, have been compared. It has been found that calcined H‐SAPO‐34 displays a high selectivity towards light olefins, yet shows a poor stability as compared to a zeolite H‐ZSM‐5 catalyst. Moreover, in situ STXM at the carbon K‐edge during the MTH reaction allows construction of nanoscale chemical maps of the hydrocarbon species formed within the H‐SAPO‐34 aggregates as a function of reaction time and steam post‐treatment. It was found that there is an initial preferential formation of coke precursor species within the core of the H‐SAPO‐34 aggregates. For longer times on stream the formation of the coke precursor species is extended to the outer regions, progressively filling the entire H‐SAPO‐34 catalyst particle. In contrast, the hydrothermally treated H‐SAPO‐34 showed similar reaction selectivity, but decreased activity and catalyst stability with respect to its calcined counterpart. These variations in MTH performance are related to a faster and more homogeneous formation of coke precursor species filling up the entire steamed H‐SAPO‐34 catalyst particle. Finally, the chemical imaging capabilities of the STXM method at the Al and Si K‐edge are illustrated by visualizing the silicon islands at the nanoscale before and after steaming H‐SAPO‐34.

Conversion of methanol to olefins: Stabilization of nanosized SAPO-34 by hydrothermal treatment

“…This behavior was already reported in the literature for more conventional SAPO-34 samples [22][23][24][25]. Briend et al [22] related the lack of long-term stability to the presence of silicon islands, favored by the use of morpholine as template.…”

“…Recently, Aramburo et al [25] reported no difference with the purely thermally treated samples [31,34,35], i.e., a reduction in acidity with migration of silicon species to silicon islands. However, in an earlier contribution Park et al [37] showed that SAPO-34 thermally treated at 750ºC loses more crystallinity after reaction Theoretical calculations on the role of steaming on clusters of isomorphous zeolite SSZ-13 and SAPO-34 [38] also to support our hypothesis on the effect of steaming on SAPO-34 stability.…”

“…In fact, while steaming at high temperature generates wide dealumination in ZSM-5 crystals with severe loss of acidity [47][48][49], in the case of standard SAPO-34, both, textural and acidity properties are preserved (see Table 1 and ref. [25]). Moreover, if the SAPO-34 sample presents high mesoporosity, as it is the case of the nano-SAPO-34 sample, steaming at 400ºC strongly affects the stability.…”

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 physicochemical properties of the H‐SAPO‐34 catalyst material under study have been reported in the previous work of our group 14. Nitrogen physisorption results indicated that the H‐SAPO‐34 sample is mainly microporous with a BET surface area of 594 m 2 g −1 and a micropore volume of 0.22 cm 3 g −1 .…”

Large Ferrierite Crystals as Models for Catalyst Deactivation during Skeletal Isomerisation of Oleic Acid: Evidence for Pore Mouth Catalysis

“…[31][32][33][34][35][36] This multi-technique approach has also been successfully applied to the study of spatiotemporal heterogeneities in for example, individual fluid catalytic cracking catalyst particles of different age, [37] or catalyst powderb efore/after steaming. [38][39][40] UV/Vis and IR micro-spectroscopies are limited in spatialr esolution, but have proven to be valuablet ools for the chemical identification of hydrocarbon deposits;c onfocal fluorescencem icroscopy on the other hand enables their 3D visualisation with ar esolution down to approximately 500 nm. In addition, measurements based on visible absorption with polarised light, [41] or the study of the polarisation of emitted fluorescent light, [42] can be used to reveal the locationa nd specific orientation in the zeolite framework of the chromophore under investigation, and link their formation and presence to specific channel types.…”