Initial catalyst deactivation, process

This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

Although the process is of significance, it has not well studied. Since the initial development of the CTA hydropurification process in 1960s , only a few papers have been published, mainly regarding catalyst deactivation [2]. Recently, Samsung Corporation, in collaboration with Russian scientists, developed a novel carbon material-CCM supported palladium-ruthenium catalyst and its application to this process [3]. However, pathways and kinetics of CTA hydrogenation, which are crucial to industrialization, are not reported hitherto. 

The long-term stability of the Ru/Ti02 catalyst was studied under various reaction conditions and the spent catalysts were characterized for assessing the reasons of deactivation. It was observed that the rate exhibits a rapid reduction at the initial several hours of reaction, followed by a slow and continuous deactivation. Analysis of the spent catalyst, by H2 adsorption after removing surface carbon, showed that the initial rapid reduction of activity is mainly due to metal sintering, while the continuous and slow deactivation is related to the occurrence of the SMSl phenomenon at the later part of the catalyst bed, where reducing conditions prevail. In order to avoid these processes which lead to catalyst deactivation, Ti02... 

Previous reports on FMSZ catalysts have indicated that, in the absence of added H2, the isomerization activity exhibited a typical pattern when measured as a function of time on stream [8, 9], In all cases, the initial activity was very low, but as the reaction proceeded, the conversion slowly increased, reached a maximum, and then started to decrease. In a recent paper [7], we described the time evolution in terms of a simple mathematical model that includes induction and deactivation periods This model predicts the existence of two types of sites with different reactivity and stability. One type of site was responsible for most of the activity observed during the first few minutes on stream, but it rapidly deactivated. For the second type of site, both, the induction and deactivation processes, were significantly slower We proposed that the observed induction periods were due to the formation and accumulation of reaction intermediates that participate in the inter-molecular step described above. Here, we present new evidence to support this hypothesis for the particular case of Ni-promoted catalysts. 

The results for PtSn-BM and PtSn-OM catalysts (Figure 6.15) indicate that the addition of tin substantially improves their stability, almost inhibiting the deactivation processes. In the case of PtSn-OM, no deactivation is observed and only a slight loss in the conversion level is observed in the case of PtSn-BM. Nevertheless, in the latter case, catalyst regeneration in air at 773 K allows the original catalytic phase to be obtained, since it recovers its initial activity and selectivity. 

Upon heating in air the TGA-DTA measurement showed that the spent catalyst sample lost 5% weight and two exothermal peaks (at 300°C and 400°C) were observed. Similar measurements with spent alumina (without CuO) showed only one peak in the DTA diagram (405°C). A fresh and spent Cu0/A1203 sample were characterized with EDAX. The analysis showed that chlorine was present on the spent catalyst, whereas it was totally absent in the fresh sample. The origin of chlorine was from a chlorine containing impurity in the hydroxy ketone I which irreversibly adsorbed on the catalyst. Adsorbed chlorine is known to increase the acidity of the alumina support and thereby may enhance cracking or polymerisation processes which finally lead to catalyst deactivation. The dark-yellow colour of the initially white supports after use in the reaction indicated that residues were retained on the catalyst. The yellow colour disappeared after calcination in air at 500°C,... 

Processing aids are used to directly influence the synthesis process function as reaction controllers. Depending on their chemical state they can function as reaction accelerators (the actual catalysts and starters or initiator substances), crosslinkers and/or hardeners, reaction inhibitors or catalyst deactivators, molecular weight controllers, chain splitters or lengtheners. From a chemical standpoint (structure and method of function) the radical builders, mainly peroxides and azo compounds, are treated separately from the catalysts which are mainly metals, metal oxides, salts (redox systems) and organo-metal compounds. The carrier substances, promoters and deactivators are placed in the catalyst class of substances. 

Like all catalysts the F-T catalysts deactivate with time on line. The deactivation process is typical of most catalysts in that studies reveal a rapid initial loss in activity followed by activity stabilization with a subsequent gradual decrease in activity (6). The latter decline in activity has important ramifications for the actual lifetime of the catalyst. It is therefore important to identify and minimize the factors that influence this part of the deactivation profile in order to maximize the catalyst lifetime. 

The decrease in activity of heterogeneous Wacker catalysts in the oxidation of 1-butene is caused by two processes. The catalyst, based on PdS04 deposited on a vanadium oxide redox layer on a high surface area support material, is reduced under reaction conditions, which leads to an initial drop in activity. When the steady-state activity is reached a further deactivation is observed which is caused by sintering of the vanadium oxide layer. This sintering is very pronounced for 7-alumina-supported catalysts. In titania (anatase)-supported catalysts deactivation is less due to the fact that the vanadium oxide layer is stabilized by the titania support. After the initial decrease, the activity remains stable for more than 700 h. 

The initial rapid decrease in activity (stage 1) is not caused by deactivation processes but is a result of the reduction of the catalyst under reaction conditions. The reduction of the catalyst follows from the reaction equations, which represent the catalytic cycle during 1-butene oxidation ... 

Catalyst deactivation was investigated in the range of 1 -6 h by virtue of the assumption that the main part of deactivation process was governed by the initial stage. Coke was formed by n-hexane conversion. Experiments were run in a throug- flow reactor under the following conditions T = 425° C, LHSV = 7 h catalyst amount, 3 ccm. The reaction products were determined by gas chromatography in a 3 m column of 10 % squalane on Chromosorb NAW,... 

Catalyst deactivation was first identified as a percolation-type process by Sahimi and Tsotsis (2,3,41). The process studied by these authors was relatively simple. A porous catalyst, inside a differential isothermal reactor (with the catalytic active material uniformly distributed in its pores at an initial concentration CQ) is reacting while simultaneously undergoing slow deactivation. The overall reaction rate r in a single pore of radius R and length L is given by... 

Deactivation, or "passivation , behavior has been found to be a feature of the oxygen evolution process on these electrodes [284], While apparently not related to oxide dissolution, the loss of activity for oxygen evolution is evidently related to the formation of a hydrated Co-rich oxide multilayer film over a significant portion of the LaosBojCoOg electrode surface [284]. Lowering of the electrode potential restored the initial activity for oxygen evolution. Further study of this deactivation process in warranted since similar deactivation processes may occur at other oxide catalysts. 

Swingler [307] has also investigated the Y-TiClj/AlEtj Cl system for ethylene polymerization. In this investigation, contrary to the earlier work [133], the initial stages of the polymerization show considerable complexity. The rate falls sharply in the first stage of the polymerization and then slowly accelerates to a maximum value at temperatures below 70°C. At higher temperatures the rate declines slowly from the maximum rate, due to a deactivation process which is second order in active centres. At sufficiently, high Al/Ti ratios the maximum rate declines, consistent with competition between metal alkyl and monomer for the catalyst surface. (Table lOA.)... 

---