Catalyst deactivation by fast-coking

Activity and Coking Rate of Catalysts Deactivated by Fast-Coking Species Added to the Feed... 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Side reactions that occur simultaneously with the main reaction cause the formation of some light and heavy hydrocarbons that result in the deposition of a small amount of coke on the catalyst. Two different catalyst systems based on chromium and platinum are used within a temperature range of 500-650° C. Because of the fast deactivation by coke formation, different concepts have been used to enable regeneration of the catalyst. 

Hie results of cond sation runs at steady-activity, the last column frcm Table 1, demonstrate that the conversion is correlated to the catalyst acidity, it increases as the concentraticxi of acid sites on the catalyst surface is increcised, whereas the selectivity to isoprene can be better correlated to the distribution of the acid strength than the concentration of acid sites. HM catalyst shows an apart position, though it has the greatest acidity, the ccmversion at steady- activity is rather low due to its fast deactivation by coking. 

Detailed kinetic studies performed by Froment s group shows an extrapolation of the propylene yield to initial times [101]. This proves that propylene might be at least one of the primary products in MTO reaction after the initiation step, which is formed directly from MeOH/DME. This result is consistent with [61,103]. According to Froment [119], the propylene formation is very fast, but this olefin is very reactive on the catalyst. With progressive deactivation by coke, the reactor zone in which the propylene yield reaches its maximum moves further downstream [101]. 

Fast deactivation rates due to coking and the limited hydrothermal stability of pillared clays have probably retarded the commercial development of these new type of catalysts and prevented (to date) their acceptance by chemical producers and refiners. However, there is a large economic incentive justifying efforts to convert inexpensive (i.e. 40-100/ton) smectites into commercially viable (pillared clay) catalysts (56). Therefore, it is believed that work on the chemical modification of natural (and synthetic) clays, and work on the preparation and characterization of new pillared clays with improved hydrothermal stability are, and will remain, areas of interest to the academic community, as well as to researchers in industrial laboratories (56). 

The scheme proposed for the reaction over HFAU was that PA dissociates in phenol (P) and ketene and that o-HAP, which was highly favoured over the para isomer, results partly from an intramolecular rearrangement of PA, partly from acyl group transfer from PA to P whereas p-HAP results from this latter reaction only. In these experiments, the zeolite deactivation was very fast, as a result of coke deposition and zeolite dehydroxylation. Catalyst stability can be considerably improved by operating at lower temperatures and especially by substituting equimolar mixtures of PA and water or P and acetic acid for PA. Much higher HAP yields were obtained by using the P - acetic acid mixture as reactants.[68]... 

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