Chromia-based catalysts

Activity for Cl-VOC oxidation at lab scale of eleven catalysts in monolithic shape from eight different manufacturers has been here reported. When the ko (for Eapp=44 kJ/mol) values are compared with the similar ones for the Pt, Pd, and chromia based catalysts, indicated in Tables 4 and 5 of ref 7, it is concluded that ground (particles) V-W-Ti catalysts are, by average, more active than the competitive (used for the same application) Pt, Pd and Cr203 catalysts. Nevertheless, as monoliths they are not so active, and the activity of commercial V-W-Ti monoliths is similar to the commercial Pt or Pd spheres. Besides, these results about catalytic activity have to be used joint with the deactivation or life time results (for the same catalysts) shown in forthcoming part II of this paper. Their deactivation above 200-250° C for relative high contents of Cl-VOCs (besides their price) will be the key or limiting aspect in their use. 

Padilla A, M., Corella J., Toledo J. M, (1999) Total Oxidation of Some Chlorinated Hydrocarbons with Commercial Chromia Based Catalysts". Applied Catai B Environm., 22, 107-121. 

In 1975, Ohloff etal. studied the gas-phase oxidation of ot-isophorone to KIP over a vanadia/pumice catalyst modified with 1 wt% of hthium phosphate at 230°C. Under these conditions, simultaneous formation of KIP and formylisophorone occurred. More than 20 years later, Baiker et al. revisited the catalytic gas-phase oxidation of isophorone. At 200-250°C, 75% combined yields of KIP and formyhsophorone were obtained at 17% ot-isophorone conversion over vanadia/pumice impregnated with hthium phosphate j6-isophorone was found as a major by-product (18%). Bismuth molybdate or vanadium phosphate showed poor selectivity and rapid deactivation. The Ag/y-alumina-catalysed oxidation was unselec-tive and resulted mainly in isomerisation to j6-isophorone. Chromia-based catalysts led to an increased formation of 3,5-xylenol. To efficiently remove coke deposits and to re-oxidise vanadium oxides to vanacha, temperatures higher than 300°C would be needed however, under these conditions isophorone and KIP are not stable. Thus, highly selective catalysts would be required which are active at lower temperatures. 

Cavani F, Koutyrev M, Trifiro F, Bartolini A, Ghisletti D, lezzi R, Santucci A, Del Piero G (1996) Chemical and physical characterization of alumina-supported chromia-based catalysts and their activity in dehydrogenation of isobutene. J Catal 158 236-250... 

Padilla, A.M., Corella, J., and Toledo, J.M. Total oxidation of some chlorinated hydrocarbons with commercial chromia based catalysts.Catal B Environ. 1999, 22, 107-121. 

The more active copper-promoted, high-temperature shift catalysts can be operated at somewhat lower temperatures. Haldor Tops0e (1992) recommends 610°-660°F as the optimum inlet temperature for their SK-201 copper-promoted iron/chromia-based catalyst and report that it can be operated at temperatures down to 570°F. 

A high temperature water-gas shift reactor 400°C) typically uses an iron oxide/chromia catalyst, while a low temperature shift reactor ( 200°C) uses a copper-based catalyst. Both low and high temperature shift reactors have superficial contact times (bas on the feed gases at STP) greater than 1 second (72). 

Aside from the recently described Cu/Th02 catalysts, copper on chromia and copper on silica have been reported to catalyze methanol synthesis at low temperatures and pressures in various communications that are neither patents nor refereed publications. It is not feasible to critically review statements unsupported by published data or verifiable examples. However, physical and chemical interactions similar to those documented in the copper-zinc oxide catalysts are possible in several copper-metal oxide systems and the active form of copper may be stabilized by oxides of zinc, thorium, chromium, silicon, and many other elements. At the same time it is doubtful that more active and selective binary copper-based catalysts than... 

The derivation of a mechanism for a chemical reaction is by its very nature an uncertain process, being dependent critically on the nature and extent of the experimental evidence. Mechanisms that have at their heart a surface process or processes are even more uncertain and when the constraints imposed by the manipulation of HF are also taken into account, it is not surprising that there have been relatively few mechanistic studies made of heterogeneous catalytic fluorination. However a catalytic process cannot be said to be understood fully without a mechanism based on the experimental evidence available and such studies are helpful in the design of the next generation of catalysts. In most cases the work described below involves chromia or y-alumina based catalysts that have been pretreated according to the methods described above. Studies involving C2 and Q compounds are described in turn. 

There are several types of catalysts which could be useful for this application. Among them, results obtained with chromia and with Pt and Pd-based catalysts have been already reported [refs. 6 and 7, respectively]. This work is devoted only to the effectiveness of Vanadia (V Os)- Tungstena (WO3) catalysts supponed on Titania... 

It is not easy to compare the activity of the V-W-Ti catalysts here tested with the lot of chromia, Pt and Pd based catalysts previously used because they have different shapes (monoliths and spheres) and because very different particle sizes arc involved (having thus very different effectiveness factors). For conqiarison purposes, all X-T curves were adjusted to a simple fust order kinetic model (with rate based on overall volume of catalyst, both for monoliths and for fixed beds). From the kinetic constants so obtained (see details of the method in ref 7), the preexponential factors (ko) of the Arrhenius law and the apparent energies of activation (E, p) were calculated for all catalysts. One example is shown in Figure 17. By the well Imown compensation effect between ko and E,pp, the kg values so obtained were recalculated for a given E.pp value of 44 kJ/mol. Such new ko value was used [7] as an activity index of the catalyst. 

Further papers in this section include a straightforward synthesis of 4-methyl-car-3-ene and some derivatives, another report of cobalt-catalysed air oxidation of car-3-ene cf. Vol. 7, p. 45), anodic oxidation of car-3-ene to yield 2,6,6-trimethylcyclohepta-2,4-dienol predominantly (together with some p-menthane derivatives), a further paper cf. Vol. 8, p. 57) on dehydrogenation of car-3-ene over chromia and chromia-alumina catalysts, the full paper on isomerization of 2,3- and 3,4-epoxycaranes over solid acids and bases, dimerization of (-)-cw-caran-4-one to 4-caranylidenecarane (mostly transoid) using the diamagnetic complex [THF,Cl2Mg2Ti]2, (+)-car-3-ene nitroso-... 

Carbon deposits on the catalyst during the reaction and is burned off rvith air in the regenerator. This combustion reaction generates much heat which is used to preheat fresh feed or to produce steam. The catalyst, usually a chromia-base type, is finely divided and must be cheap, rugged, and not susceptible to poisoning by sulfur or water. In conunercial installations of 40,000 bbl per day capacity, the catalyst circulating from reactor to regenerator and return may amount to as much as 30,000 tons per day. 

Commercially, the oldest dehydrogenation process for the dehydrogenation of propane to propene is the Catofin process (113,114) developed in the late 1980s, which is based on catalysts Houdry developed in the mid-1940s. In this process, the feed is heated to 525-635°C and passed over a chromia-alumina catalyst. 

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