Catalysts product selectivities over

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

A representative comparison of the effect of the catalyst bed geometry on methane conversion and product selectivity over a range of methane/air ratios is shown in Fig 4 Unlike typical supported catalysts, where the catalyst is well-dispersed and submicrometer-sized, the noble-metal catalysts in these methane oxidation reactions were basically films with micrometer-sized surface features (Other tests on both extruded cordiente and alumina foam monoliths with lower catalyst loading resulted in similar carbon monoxide production but lower hydrogen yields than those illustrated in the figure, which provided evidence that the reaction is catalyst-dependent and not initiated by the monoliths or gas... 

Wang et al. [5] reported the dehydrogenation and isomerization of -butane on Cr-supported W03-Zr02 and, in his case, the catalytic activity decreased with reaction time by deposition of carbon. In our study, the catalyst performance for -butane conversion and the C4 products selectivity over Cr/H-SSZ-35 catalyst (Si/Al2=500) remained unchanged at 500°C for 6 h. From these findings on the catalytic activity and lifetime, H-SSZ-35 was expected to be one of the promising supports for the production of isobutene by hydrogenation and isomerization of n-butane. 

Even higher activities and para product selectivities over the La exchanged catalysts were observed for the reactions with the propionyl chloride system, as shown in Figure 2. In contrast to the large difference in activity between the La(26)Y and La(70)Y catalyst for the benzoyl chloride system, only a small difference in activity is observed between the two levels of La exchange for the... 

Product selectivity over copper catalysts is strongly dependent on operating conditions, and for two reasons. First, feed ratios, temperatures, and pressures affect selectivity over a given catalyst state in... 

Although acrylonitrile manufacture from propylene and ammonia was first patented in 1949 (30), it was not until 1959, when Sohio developed a catalyst capable of producing acrylonitrile with high selectivity, that commercial manufacture from propylene became economically viable (1). Production improvements over the past 30 years have stemmed largely from development of several generations of increasingly more efficient catalysts. These catalysts are multicomponent mixed metal oxides mostly based on bismuth—molybdenum oxide. Other types of catalysts that have been used commercially are based on iron—antimony oxide, uranium—antimony oxide, and tellurium-molybdenum oxide. 

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

There are two general temperature poHcies increasing the temperature over time to compensate for loss of catalyst activity, or operating at the maximum allowable temperature. These temperature approaches tend to maximize destmction, yet may also lead to loss of product selectivity. Selectivity typically decreases with increasing temperature faster deactivation and increased costs for reactor materials, fabrication, and temperature controls. 

The activity of CoSx-MoSx/NaY (2. IMo/SC) is shown in Fig.5 for the HYD of butadiene as a function of the Co/Mo atomic ratio. The HYD activity decreased slightly on the addition of Co up to Co/Mo = ca. 1, followed by a steep decrease at a further incorporation of Co. The HYD/HDS activity ratio decreased with increasing Co content and reached the ratio for CoSx/NaY at the Co/Mo atomic ratio of the maximum HDS activity (Fig.3). The product selectivity in the HYD of butadiene shifted from t-2-butene rich distribution to 1-butene rich one on the addition of Co, as presented in Fig.6. It is worthy of noting that at the Co/Mo ratio of the maximum HDS activity, the butene distribution is close to that for CoSx/NaY. It should be noted, however, that these product distributions are not the initial distributions of the HYD over the catalyst but the distributions modified by successive isomerization reactions. It was found that MoSx/NaY showed high isomerization activities of butenes even in the... 

Product selectivities ( 2 mol%) (taking into account 2- and 3-etho entane only) from the reaction of 0-ethanol and S-2-pentanol ( 0) over Nafion-H and HZSM-5 catalysts at 100°C and 1 MPa and in concentrated H2SO4 solution at 100°C and 0.1 MPa, where the true inversion (I) was calculated by using the equation given above. 

Figure 5 Conversion and Product Selectivity for Ni/Re Catalyst over 1700 hours, aqueous hydrogenation condition (unspecified).

Figure 4. Product selectivity in the degradation of HDPE over different catalysts at 400 °C...

Wan et al. [61] also reported the highly effective conversion of methane to aromatic hydrocarbons over Cu, Ni, Fe, and Al catalysts. The effects of the type of catalyst, its configuration, and the microwave irradiation conditions on reaction path and product selectivity were examined under both batch and continuous-flow conditions. 

Although the mechanism proposed for the ZSM-5/methanol system adequately explains the production of the primary C2-C5 products, it is not clear how these are converted into the final gasoline product or indeed why this product should be so rich in aromatics. Production of olefins from methanol over zeolite catalysts has previously been described (110, 112) however, the ZSM-5 system appears to be unique with respect to both product selectivity and catalyst stability. Mobil now has some 140 patents relating to the preparation and use of ZSM-5 zeolites and has stated that "given a favorable economic and political climate a commercial unit could be in operation by the early 1980 s (101). 

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