Understanding refinery catalysts under industrial conditions by in-situ XAS

08-06-2015

The activation of hydrotreating catalysts, applied in oil-upgrading processes, was followed by time-resolved XAS. Short EXAFS scans (6 min/scan) revealed that several intermediates lead up to the formation of the catalytically active MoS2 phase, depending on the method of activation. This provides new insight into the structure of these catalysts at industrially relevant conditions.

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L. van Haandel, E.J.M. Hensen and Th. Weber | Eindhoven University of Technology

 

Catalysts are often studied in the lab under model conditions that deviate substantially from those in industry. Consequently there is a need to develop tools to study catalysts under more relevant operating conditions, namely high pressure, temperature and trickle-flow (a mixture of gasses and liquids). We developed a microreactor that allows us to perform XAS experiments on heterogeneous catalysts in corrosive gas and/or liquid media at temperatures up to 350°C and pressures up to 20 bar.

This microreactor was used to study the effect of operating conditions (p, T) on the active phase formation and structure of Co-Mo hydrotreating catalysts. From previous experiments it was known that the time resolution required to observe phase transitions in this system is ~5 mins. Optimization of scan parameters allowed us to decrease the time per scan from 40 to 6 minutes, without losing significant information. The results show the formation of several intermediates at different temperatures, leading up to crystallization of the catalytically active MoS2 phase. This information could not be obtained from spectral interpretation of XANES data and forms a new basis for understanding the catalytic performance of these materials in refinery applications.

The mobile setup can be quickly installed and is equipped with several safety features to ensure that experiments with toxic gasses such as H2S can be carried out safely at elevated pressure. This is the first time that time-resolved EXAFS spectra have been obtained on a solid catalyst exposed to a continuous flow of gas-liquid mixtures.

Fast EXAFS scans were limited to k = 14Å-1 without significant loss of information.

The activation process of hydrotreating catalysts visualized as a contour map constructed from time-resolved Mo K EXAFS scans is shown as a function of temperature (above, left). The intermediates formed during the activation can be clearly observed, but XANES cannot discriminate between the various MoSx phases (right).

 

 

 

Top image: microreactor with continuous flow of toxic gases at elevated pressure to study catalysts under operating conditions