Propane dehydrogenation (PDH)

Propylene and hydrogen are two valuable products of non-oxidative propane dehydrogenation (PDH) viaheterogeneous catalysis. Therefore, their production from cost-effective sources such as propane has attractedsignificant interest. Among various catalysts used for PDH, Pt-based catalysts offer remarkable propane conversionand propene yields, such that their utilization in commercial PDH processes has already been established atindustrial scale.1 Nevertheless, like all other PDH catalysts, Pt-based catalysts suffer from substantial deactivationas a result of coking. However, factors influencing coking, the composition of the coke, as well as the possiblechanges to the Pt particles and their relation to deactivation, remain poorly understood. Furthermore, efficientregeneration – a recipe to remove coke and at the same time retain activity of the catalyst – remains stillchallenging in PDH.In this project, we study the amounts and nature of the coke formed during PDH, as well as track the changes ofPt morphology and its effect on coking and activity of the catalyst. The general mechanisms of deactivation areinvestigated and subsequently, possible effective regeneration recipes are developed. We address thesechallenges by meticulously examining fresh, coked and regenerated supported Pt catalysts using variouscomplementary multi-scalar techniques for both in-situ and ex- situ analysis and imaging.Among several findings, for instance, it was revealed that the coke formed on the catalyst in the 2nd cycle afterregeneration has a more ordered character (Fig. 1a and b) than the one of the 1st cycle, which is consistent withthe highly decreased catalyst activity in cycle 2. In addition, transmission electron microscopy (TEM) revealed theformation of filamentous carbons initiated at the Pt particles, which led to their detachment from the support(Fig. 1d). The unsupported Pt particles are more likely to suffer changes such as sintering during regeneration,which is of high relevance for a decrease in catalytic activity during subsequent PDH cycles.2 Our results suggestthat several interrelated factors such as formation of highly ordered coke, size and morphology of Pt particles playimportant roles in progressive deactivation of the catalyst within each PDH cycle and after each regeneration.In the PDH project we aim at fundamental understanding of the coking and deactivation mechanisms ofsupported Pt catalysts, which additionally serve to enhance the knowledge of catalyst design for alkanedehydrogenation processes with an optimized efficiency and stability.In Collaboration with Dr. Hannah C. Nerl

Figure 1. Raman analysis of coke formed in a) PDH cycle 1, andb) after first regeneration and use in cycle 2. Peakdeconvolutions and the nomenclature are based on a model,suggested by Sadezky et al., developed for characterization ofsoot and related materials.3 The low ID1/IG indicates a highly-ordered or ‘graphite-like’ character of the coke formed in the2nd cycle. c) Bright-field TEM image of the coke formed duringPDH after >200 h in cycle 1, and d) an exemplary filamentouscarbon formed in cycle 2.

References:[1] Z. Nawaz, Rev. Chem. Eng. 31 (2015) 413-436.[2] C. H. Bartholomew, Appl. Catal. A 212 (2001) 17-60.[3] A. Sadezky et al., Carbon 43 (2005) 1742.

Siamak Nakhaie