Silica poisoning, catalyst deactivation

Steam Stability. Steam stability of SOx removal agents is strongly affected by temperature. We have seen previously that at 1350 F deactivation of cerium/alumina additive, caused by silica poisoning, was influenced by how long the additive was steamed and whether the additive was steamed in the presence or absence of cracking catalyst. These results were extended to other temperatures. 

In our test, steaming five commercial SOx additives in the presence of cracking catalyst, shown in Table VI, indicated that deactivation by silica poisoning is important. 

The influence of titania can be isolated by selectively poisoning the titania-associated sites. When the Cr(VI)/silica-titania is treated with CO at 100-150 °C, the more reactive, low-MW producing sites are preferentially reduced first (i.e., those associated with titania). If the catalyst is cooled in CO, those same reduced sites adsorb CO and are deactivated, while other sites are unaffected (Scheme 15). By analysis of the polymer from such partially poisoned catalysts, the influence of titania becomes more apparent. 

Vanadia supported on silica(with titania) promoted by Fe and CTu oxides was studied for SCR of NOx by Bjorklund et al.28 Both Fe and Cu (as oxides) enhanced the activity however, the resistance to deactivation by SO2 differed as the Fe-promoted catalyst became slightly more active with time on stream and the Cu-promoted catalyst decreased in activity to less than half the initial activity with time on stream. The activity for SCR was related to the concentration of as inferred from the solid electrical conductivity. In this case different promoters were shown to change dramatically the ability of a potential poison to deactivate the SCR catalyst Nikolov, Klissurski, and Hadjiivanov29 also studied the deactivation of vanadia/titania. A combination of ESCA, XRD, and IR were employed to characterize the surface and bulk compositions. They concluded that deactivation involved the transformation of the active anatase titania to inactive rutile. Further, there was a concomitant decrease in total surface area and a loss of phosphate promoters for the selective oxidation of xylenes to phthalic anhydride. 

There are many different ways to combine the three main ingredients (metallocene molecule(s), MAO, and the silica) chemically to obtain the final supported metallocene catalysts. One of the most recommended procedures in the patent literature is to first treat the silica with MAO and then combine it with metallocene molecules. No matter which procedure is used, one must be aware that the resulting final heterogeneous metallocene catalyst is very reactive towards moisture, leading to its destruction, and to polar molecules that act as a very effective poison and deactivate the catalyst in an irreversible manner. The long-term storage of different batches of supported catalyst under inert atmosphere, in a temperate environment that excludes excessive heat, and with no exposure to light is essential for its constant activity and reliability. 

SOx emissions from FCCU s can be reduced by the use of SOx catalysts, especially SOx additives which can be added to the FCCU independently of the cracking catalyst. The effectiveness of these catalysts is favored by lower regenerator temperatures, the presence of combustion promoter, and higher oxygen concentrations. Deactivation of these catalysts occurs by loss of surface area and poisoning by silica. We believe that SOx additives will eventually be used by most refiners to control SOx emissions from FCCU s, either on a spot or continuous basis. 

Acid catalysts such as zeolites can be readily poisoned by basic organic compounds. One of the earlier studies of the deactivation of silica-alumina cracking catalysts by organic nitrogen compounds such as quinoline, quinaldine, pyrrole, piperidine, decylamine and aniline was done by Mills et al (6). The results of their partial poisoning studies showed an exponential dependence of the catalyst activity for cumene cracking reaction or... 

Sodium on fluid cracking catalyst, FCC, comes from the raw materials used in the catalyst manufacturing process as well as salt contamination in the feedstock. Sodium can deactivate cracking catalysts by poisoning the acid sites on the matrix and zeolite and by promoting sintering of silica-alumina (1). Sodium can act synergistically with vanadium to accelerate the destruction of zeolite (2). 

From a scientific standpoint, there is an interesting relationship between the fouling and poisoning mechanisms that are typical of silica deactivation, and the poisoning mechanism that more accurately describes phosphorous deactivation. It is of interest to carefully review the dependence and interplay of these processes as a function of the reactant, catalyst support, pore structure, metal loading, and oxidation conditions. 

Siloxanes also have a poisoning effect on SOFCs performances. Segregated silica can deposit in porous cermet anodes [7, 38, 39], also on steam reforming catalysts and FC anodes, causing their silication and consequent deactivation. [17]. Moreover it also affects many other components of the fuel cell system, such as heat exchangers, catalysts, and sensors [3,40]. 

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