Cyclohexane dehydrogenation for the evaluation of metallic sites accessibility in naphtha reforming bimetallic catalysts.
Advancing the chemical engineering fundamentals
Catalysis (T2-13P)
Keywords: Characterisation, bimetallic catalysts, test reaction, naphtha reforming.
Catalysts characterisation is of paramount importance for any catalytic process. Furthermore, if the catalysts undergo deactivation, the monitoring of catalysts properties becomes the key for the process performance. Naphtha reforming process is one of the most important operation of petroleum treatments. The evolution of catalysts, from classic Pt/Al2O3 to Pt-Re/Al2O3 or Pt-Sn/Al2O3, and the changes in operation units, from cyclic units to units with continuous catalyst regeneration, implies the necessity of a close catalyst evaluation and has lead to the development of new characterisation techniques.
When evaluating metallic sites accessibility, the use of hydrogen chemisorption, or hydrogen-oxygen titration, is well established for monometallic Pt/Al2O3 catalysts. However, classical methods do not provide reliable values for bimetallic catalysts and the use of pilot plants is expensive and time-consuming. Problems are mostly related with the uncertainties of the oxidation state of the second metal and its interaction with platinum.
In order to solve this problem, we propose the use and standardisation of test reactions for the characterisation of catalysts. Cyclohexane dehydrogenation is a structure-insensitive reaction that can be easily used for the evaluation of metallic sites accessibility.
In this work, a wide range of purposely prepared catalysts (Pt/Al2O3 Pt-Re/Al2O3 and Pt-Sn/Al2O3) and industrial fresh an aged catalysts (Pt-Re/Al2O3 and Pt-Sn/Al2O3) have been studied with classical techniques and tested in cyclohexane dehydrogenation.
Results demonstrate that chemisorption show a low sensitivity to Sn/Pt or Re/Pt. However, catalytic tests reveal important changes in the catalyst behaviour. While addition of tin decreases catalysts activity, addition of rhenium enhances it. The assumption of a constant value of turn over frequency, equal to the value obtained with monometallic catalysts, allows to recalculate metallic dispersion due to the insensitiveness of the reaction.
For industrial catalysts, tin-containing fresh and aged catalyst show the similar metallic dispersion evaluated by hydrogen chemisorption while their cyclohexane dehydrogenation reaction rate show a clear and progressive diminution with the use in the industrial process. For rhenium containing catalysts, all aged samples show the same metallic dispersion evaluated by hydrogen chemisorption, lower than that of the fresh catalysts. Catalytic test results however show a progressive diminution of the reaction rate with the ageing of the catalyst.
Cyclohexane dehydrogenation test reactions can be concluded to be a powerful tool to achieve cheap, quick and reliable information for the evaluation of metallic sites accessibility of industrial naphtha reforming catalysts in order to improve catalyst regeneration and process operation.
Presented Wednesday 19, 13:30 to 15:00, in session Catalysis (T2-13P).
