Mixed Oxide Catalyst Operation

The oxidative dehydrogenation process is highly exothermic and the reactor temperatme must be controlled to maintain selectivity. The reactor consists of mat r tubes cooled by the circulation of a heat transfer fluid such as Dowtherm. The catalyst tubes have a very small diameter to improve heat transfer and the 

Although the stoichiometric ratio of molybdenum to iron in ferric molybdate is 1.5, the maximum activity is obtained at an atomic ratio of 1.7. However, the presence of free ferric oxide in the catalyst is known to reduce considerably the selectivity of the catalyst to the formation of formaldelyde. For this reason, excess molybdenum is usually added to the catalyst formulation to maximize the yield of the product. The optimum ratio is about 2.0.  

The catalyst is prepared by precipitation from solutions of ferric chloride and ammonium molybdate. The precipitate may not be homogeneous, with significant variations within a single batch. Hydrothermal aging of the precipitate may be necessary to provide a more uniform composition. Precipitation of the catalyst as a gel provides a more uniform ferric molybdate compositioa 

Additives such as chromium or cobalt oxides can stabihze the catalyst. In Table 4.5, catalyst compositions and operating conditions in modem formaldehyde processes are shown. 

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The development of a complex mixed-oxide catalyst brought the activity and selectivity of the catalyst into a range in which the process may become economically feasible and it has been announced that pilot and demonstration plants are to be brought into operation, partly working with a mixed feed of olefin and alkane... 

For most relevant industrial applications, TOP values in the range of 10 -10 s have been observed. For enzyme-catalyzed reactions, TOP levels are much higher at 10 -10 s . TOF is limited by the difficulty in determining the number of active centers for multimetallic, nonmetallic, and mixed oxide catalysts used more frequently in large-scale operations. 

Industrial production of formaldehyde is by catalytic oxidation of methanol using either a silver or an iron-molybdenum mixed-oxide catalyst. The two processes differ in the temperature of operation and the methanol-air ratio in the feed, the process using silver catalyst being at a higher temperature of 600700 versus 270350°C in the other process and employing a methanol-air ratio less than stoichiometric... 

Among various mechanisms of the action of microwave field on solids, several classes of catalysts and processes that offer promise for use in practice can be identified. For instance, the Maxwell-Wagner interphase polarization mechanism can likely operate in mixed oxide catalysts (such as catalysts of partial oxidation based on vanadium and molybdenum oxides). For such catalysts, nontrivial and nonthermal effects can probably be expected. 

By the 1930s, Adkins, working with the Bakehte Corporation, introdnced a mixed oxide catalyst for the direct oxidation of methanol. Dnring the development he found that pare molybdenum oxide gave about 60% conversion to formaldehyde at 400°C, although activity fell after 12-24 h to 30% conversion. Pure iron oxide, on the other hand, was not selective and prodnced only carbon dioxide. However, a mixed iron/molybdate catalyst converted more than 90% of the methanol to formaldehyde. Operation was relatively stable and by 1952 DuPont had built a plant using iron/molybdate in a process similar to that described in Adkins patent. ... 

Titanium dioxide catalysts were first described in the 1940s and 1950s, when mixed oxide catalysts were being investigated and used in a number of oxidation reactions. Mixtures of vanadium pentoxide with titanium dioxide gave better operation and longer life as phthalic anhydride demand increased. An early catalyst that did not sinter and clearly increased the stability of vanadium pentoxide was described in a patent as Ti0(V03)2. At about the same time vanadium pentoxide/phosphorous pentoxide mixtures were also being developed for use in maleic anhydride processes. 

It was suggested that the relatively unstable mixed oxide catalysts could be supplied in the pre-reduced form that was, effectively, pre-shrunk. While these catdysts were offered for use commercially, there is no evidence that operation was actually improved." In any case, the maximum operating temperature in a high-pressure methanol plant was limited to 390°C by the onset of methanation so that a stable activity was more important than high activity in the multibed reactors used. 

Bismuth Molybdates. Bismuth molybdates are used as selective oxidation catalysts. Several phases containing Bi and/or Mo may be mixed together to obtain desired catalytic properties. While selected area electron diffraction patterns can identify individual crystalline particles, diffraction techniques usually require considerable time for developing film and analyzing patterns. X-ray emission spectroscopy in the AEM can identify individual phases containing two detectable elements within a few minutes while the operator is at the microscope. 

Three well known examples of processes employing fluidised-bed operations are the oxidations of naphthalene and xylene to phthalic anhydride using a supported V2O5 catalyst and ammoxidation of propylene utilising a mixed oxide composition containing bismuth molybdate. Typically, this latter reaction is executed by passing a mixture of ammonia, air and propylene to a fluidised bed operating at about 0.2 MPa pressure, 400—500°C and a few seconds contact time between gas and fluidised catalyst peirticles. 

Chain-growth can be initiated with high operating temperatures and pressures, in the presence of alkaHnized zinc-chromium mixed oxides (1 2) possibly modified with other metals (3)- Such promoted catalysts allowed industrial scale production of alcohol mixtures containing up to 30 wt of C alcohols. 

Solid acid catalysts such as mixed oxides (chalcides) have been used extensively for many years in the petroleum industry and organic synthesis. Their main advantage compared with liquid acid catalysts is the ease of separation from the reaction mixture, which allows continuous operation, as well as regeneration and reutilization of the catalyst. Furthermore, the heterogeneous solid catalysts can lead to high selectivity or specific activity. Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. Up until now, the most useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions. 

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