Pore network modeling of catalyst deactivation by coking, from single site to particle, during propane dehydrogenation

Abstract

A versatile pore network model is used to study deactivation by coking in a single catalyst particle. This approach allows to gain detailed insights into the progression of deactivation from active site, to pore, and to particle-providing valuable information for catalyst design. The model is applied to investigate deactivation by coking during propane dehydrogenation in a Pt-Sn/Al 2 O 3 catalyst particle. We find that the deactivation process can be separated into two stages when there exist severe diffusion limitation and pore blockage, and the toxicity of coke formed in the later stage is much stronger than of coke formed in the early stage. The reaction temperature and composition change the coking rate and apparent reaction rate, informed by the kinetics, but, remarkably, they do not change the capacity for a catalyst particle to accommodate coke. Conversely, the pore network structure significantly affects the capacity to contain coke.

Molecular elucidating of an unusual growth mechanism for polycyclic aromatic hydrocarbons in confined space

“…C 4 + products gradually declined in parallel with the increase of methane, being typical after around 55 min of reaction (that is, after pseudo steady-state stage). Methanol started to appear in the effluent after 55 min of reaction, implying that the fraction of accessible SAPO-34 pores dropped below a critical threshold, unable to sustain full methanol conversion 36 . During the pseudo steady-state stage, both the specific microporous surface area and pore volume were significantly reduced by~80% (Supplementary Table 1) associated with the almost linear increase of the deposited "coke" (Fig.…”

Optimizing catalyst pore network structure in the presence of deactivation by coking

“…Common types of discrete models include Bethe lattices 24,25 and various pore network models 24,26 to describe both morphology and topology of the void space in catalyst particles, and thus the local heterogeneity and dynamic change in pore network structure can be properly accounted for. More importantly, pore network models have been successfully used to simulate some reaction systems with percolation phenomena, such as hydrodesulphurization 27,28 , hydrodemetalation 1,19,20 , deactivation of immobilized glucose isomerase [29][30][31] , benzene hydrogenation 32,33 , and propane dehydrogenation 34 . Therefore, a pore network model is preferable to carry out the optimizations in this work.…”

“…For example, the Pt-Sn/ZnAl 2 O 4 catalyst used in the Uhde STAR process should be regenerated after 7 hours on stream; the CrO x /Al 2 O 3 catalyst used in the Catofin process is significantly deactivated after only 12 minutes 38,39 . Recently, we proposed a two-dimensional pore network model to simulate the process of deactivation by coking in a Pt-Sn/Al 2 O 3 particle for non-oxidative propane dehydrogenation, and we found that the pore network structure strongly affects this deactivation process and the catalyst performance 34 . However, to the best of our knowledge, no literature has reported the optimization of propane dehydrogenation catalyst particles even in the absence of deactivation by coking, let alone in its presence.…”

“…Before introducing the three parts of the pore network model, the following assumptions are made: Pt-Sn/Al 2 O 3 is chosen as the model catalyst, since it is widely used in propane dehydrogenation processes 15 ; the gradients of temperature and total pressure are assumed to be negligible, which has been proven in the literature 34 ; side reactions, except for the coke formation reaction, can be neglected under the reaction conditions in this work 15 ; coke is assumed to be non-catalytic and its formation is irreversible 45 ; all types of coke are assumed to have similar properties and they deactivate the catalyst particle following the same mechanisms 45 .…”

“…In practice, the two assumptions about the intrinsic activity are reasonable to a large extent: the metal loading per unit catalyst weight stays constant when the incipient wetness impregnation method is used; the metal loading per unit internal surface area tends to be constant when the wet impregnation method is adopted 42 . In this work, only the primary features and principal model equations of the three-dimensional pore network model are described, since the model is similar to the two-dimensional one in our previous work, where more details can be found 34 .…”