Deactivation by carbonaceous deposits

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... 

The deactivation of catalysts used in hydrotreating and hydroconversion of heavy petroleum feedstocks is associated with coking and metals deposition. Deactivation by metals has been thoroughly studied [1], but, little is known about deactivation by carbonaceous deposits. The initial decline in activity has often been attributed to this coke formation [2, 3, 4], However, in a recent study [51 it has been shown that coke deactivation can account for more than 50% of the deactivation in resid upgrading. 

Figure 8 Schematic of the four possible modes of deactivation by carbonaceous deposits in HZSM5.

The acidity and pore structure of zeolites play significant roles in their deactivation by carbonaceous deposits ("coke"). This is not surprising, as the formation of coke involves reactions cataly by acid sites located inside the pores and also requires the retention of coke molecules by adsorption on the acid sites or by condensation or by steric blockage in the pores. Although it is often difficult to estimate quantitatively and separately the impacts of the acidity and of pore structure, it is clear that it is the latter characteristic which plays the greater role. 

DMN in the DMN isomers being greater (84% ) on the precoked catalyst than on the fresh catalyst (70%). This is a well known effect in zeolite catalysis due to the partial deactivation of the acid sites by carbonaceous deposits. The authors explain these results by a product selectivity mechanism inside the pore system of ZSM-5 or ZSM-11. Even though the diffusion pathways for the product molecules decreased by the formation of carbonaceous deposits, the narrow molecules such as 2-MN, 2,6-DMN, and 2,7-DMN were less affected than bulkier ones such as 1-MN and 1,6-DMN. 

Methanol adsorption and decomposition on noble metals have been the subject of many surface-analytical investigations (e.g., References 94,171,320,350,378, 478 94). CH3OH dehydrogenation on palladium catalysts could be a valuable source of synthesis gas or hydrogen, but unfortunately catalyst deactivation by carbon deposits (coking) seriously limits this process (495-498). In this respect, the probability of O H vs. C O bond scission is important, the first path resulting in CO and H2, and the second in carbon or carbonaceous species (CH x = 0-3), CH4, and H2O (see scheme in Fig. 49 details are discussed below). 

The thermal degradation of waste HDPE can be improved by using suitable catalysts in order to obtain valuable products. However, this method suffers from several drawbacks. The catalysts are deactivated by the deposition of carbonaceous residues and Cl, N compounds present in the raw waste stream. Furthermore, the inorganic material contained in the waste plastics tends to remain with the catalysts, which hinders their reuse. These reasons necessitate a relatively high purity of waste plastics, containing very low concentrations of a contaminant. Thus, various pretreatments are required to remove all the components that may negatively affect the catalyst. 

Roles of Acidity and Pore Structure in the Deactivation of Zeolites by Carbonaceous Deposits... 

Deactivation of the Mo/HZM-5 catalyst by carbonaceous deposits is a serious drawback for the possible commercialization of the MDA process, and therefore, great efforts have been devoted to improving the stability of the catalyst during the reaction by reducing the formation of coke. This has been achieved through the addition of small amounts of CO, CO2, O2, or NO into the reactant stream [10,11,12,13,14] and by reducing the density of Bronsted acid sites of the HZSM-5 zeolite by steam dealumination followed by acid washing before incorporation of Mo... 

Catalysts in this service can deactivate by several different mechanisms, but deactivation is ordinarily and primarily the result of deposition of carbonaceous materials onto the catalyst surface during hydrocarbon charge-stock processing at elevated temperature. This deposit of highly dehydrogenated polymers or polynuclear-condensed ring aromatics is called coke. The deposition of coke on the catalyst results in substantial deterioration in catalyst performance. The catalyst activity, or its abiUty to convert reactants, is adversely affected by this coke deposition, and the catalyst is referred to as spent. The coke deposits on spent reforming catalyst may exceed 20 wt %. 

Under the conditions used in this study, the catalytic activities were stable for NO reduction for all catalysts. However, in NOj reduction, deactivation was observed. For catalyst 1-7, there was a rapid, reversible deactivation that was more noticeable at lower temperatures. The activity could be restored by removing propene from the feed. Therefore, it was likely due to carbonaceous deposits on the catalyst. In addition, there was slow deactivation. For example, afto the experiment in Table 2 and cleaning in a flow ofN0/O2/H20 (0. l%/4.7%/1.5%, balance He) at SOOT, the catalyst showed an NO conversion of 33% and propene conversion of 42% at 450°C, versus 53 and 99%, respectively, before deactivation. For catalyst 1-5, only slow deactivation was observed. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

Although the FTS is considered a carbon in-sensitive reaction,30 deactivation of the cobalt active phase by carbon deposition during FTS has been widely postulated.31-38 This mechanism, however, is hard to prove during realistic synthesis conditions due to the presence of heavy hydrocarbon wax product and the potential spillover and buildup of inert carbon on the catalyst support. Also, studies on supported cobalt catalysts have been conducted that suggest deactivation by pore plugging of narrow catalyst pores by the heavy (> 40) wax product.39,40 Very often, regeneration treatments that remove these carbonaceous phases from the catalyst result in reactivation of the catalyst.32 Many of the companies with experience in cobalt-based FTS research report that these catalysts are negatively influenced by carbon (Table 4.1). 

For example, the most noteworthy disadvantage of catalytic wet oxidation is the severe catalyst deactivation (Larachi el al., 1999). Hamoudi el al. (1998, 1999) systematically studied the deactivation of Mn02/Ce02 catalyst during wet catalytic oxidation of phenol and the catalyst-surface modifications. It was observed that deactivation was induced mainly by the formation of carbonaceous deposits on the catalyst surface. Ohta et al. (1980) reported that the size of the catalyst particles affected the stabilization of catalytic activity. For granular particles of supported copper oxide, the catalytic activity was decreased after each inn, even after six successive experiments. In contrast, for larger particles the catalytic activity was stabilized after the first three runs. 

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