Catalyst long-term aging

A number of reaction variables or parameters have been examined. Catalyst solutions should not be prepared and stored since the resting catalyst is not stable to long term storage. However, the catalyst solution must be aged prior to the addition of allylic alcohol or TBHP. Diethyl tartrate and diisopropyl tartrate are the ligands of choice for most allylic alcohols. TBHP and cumene hydroperoxide are the most commonly used terminal oxidant and are both extremely effective. Methylene chloride is the solvent of choice and Ti(i-OPr)4 is the titanium precatalyst of choice. Titanium (IV) t-butoxide is recommended for those reactions in which the product epoxide is particularly sensitive to ring opening from alkoxide nucleophiles. ... 

Fig. 12.16 Long-term stability of MTO-100 catalyst hydrothermal vs. multiple cycle aging...

The crucial additive is 3A or 4A molecular sieves. Without their addition, products of 39-80% ee are obtained, reactions proceed slowly and to only 50-60% conversion. In control experiments it was determined42 that water irreversibly destroys the catalyst in the long term but that activity can be preserved if molecular sieves are added before or shortly after all the components. As in the case of the stoichiometric method, aging of the catalyst prior to addition of the fourth component gives higher enantioselection. 

Bench-scale studies defined conditions for successful operation of the MTG process. The long-term performance of the DME and ZSM-5 catalysts was demonstrated. Inherent gasoline selectivity was found to be insensitive to catalyst age. The raw gasoline produced is of high quality, and its only unusual aspect is that is contains a much higher concentration of durene than conventional gasolines. 

Currently, the best base-metal catalyst developed by CESI possesses much better activity and stability than the reported Pt-based catalyst and is comparable to commercial catalysts in long term activity. However, while the WGS 92 catalyst shows very good stability, the kj value of 0.0016 still implies a half life of about 433 hours, and if this deactivation rate is maintained throughout 2,000 hours of operation, the catalyst activity would decrease by 25 fold. Work is in progress to develop static aging procedures to measure activity out to 5,000 hours and to accelerate the development of more stable catalyst systems. 

Study. This cycle was chosen to give a first indication of the long term stability of the catalyst. The aging cycle includes low temperature conditions as well as high temperature phases. The diesel fuel used contained 0.15 wt.-% sulfiir, the lubricating oil had 0.1 wt.-% phosphorous. The cetane number, density and all other specifications correspond to the German standard DIN 51601 for automotive diesel fuel [22]. 

Table 5.1 lists a few of the examples reported in the patent application by Andersen et al. [23]. For clarity, NOx conversions at 250 °C on a series of Cu/zeolite SCR catalysts before and after different hydrothermal treatments, are summarized in the table. Data from two sets of hydrothermal aging conditions are included. The 750 °C/24 h and 900 °C/1 h each represents that the catalysts have been hydrothermally aged in a flow of 4.5 % H20/air mixture at 750 °C for 24 h or 900 °C for 1 h, respectively. These two conditions are used to assess the long-term hydrothermal durability and the upper temperature limit of these catalysts. 

The AuATi02 catalyst was also tested concerning its hydrothermal resistivity under authentic operating conditions and its behavior after modeled long-term poisoning by sulfur compounds. In all aging experiments, the catalyst performance even after severe treatment such as hydrothermal aging at up to 850 °C or 200 ppm of SO2 in the gas feed still maintamed acceptable activity for catalytic decomposition of NH3 precursor compounds [113]. 

Moreover, the study of pilot reactor performance data is very important in the understanding of the long-term stabiUty of an industrial catalyst. Plant operation often requires a catalyst lifetime of four years without losing too much PA yield during this period of time. PHTHALIMAX , for example, loses about 0.5-1% PA yield per year. To study aging, catalysts are subjected to special stress tests on the bench-scale, as well as pilot units under high temperature and load conditions for... 

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