Carbon deposition, with catalysts

Recently, UV laser stimulation of catalyst samples has been developed to overcome the problem of interference by coke (carbon deposition) on catalysts.Fig. 9 shows a typical Raman data set that was obtained for carbon deposition as a function of temperature. To explore different coke formation behavior, the reaction of propene on a zeolite was performed. The spectra obtained were (A) C3H6/He flow at 773 K for 3 h (B) O2 flow at 773 K for 1 h and (C) O2 flow at 873 K for 1 h. This data shows that most of the carbon, identified as polyaromatic and pregraphite, can be removed at 773 K with oxygen. However there is still carbon present as identified by the broad band at 1610 cm suggesting that carbon is in a more inert form such as coke. Not until the temperate is taken to 873 K with oxygen is that carbon removed. 

Carbonaceous overlayers have always been associated with catalytic poisoning, but it does seem, in some cases, that they can also be associated with catalytic activity. In one sense, this is not suprising, since carbonaceous overlayers can have exactly the same structure as carbonaceous catalysts. Nonetheless the association of carbon deposits and catalyst poisoning is too strong to be easily forgotten. 

TPSR Characterization of Deposited Carbon. In an experiment to analyze the TPSR characteristics of a carbon deposit, 15 to 35 mg of powdered (170/250 Tyler mesh) catalyst was placed on the porous disk of a quartz microreactor. Typically, a fresh sample of catalyst was reduced for 15 hr at 773 K in 1 atm flowing H2 and a previously reduced catalyst was heated to 773 K for at least 1 hr before a TPSR experiment. Following reduction, the catalyst was flushed with pure He and cooled or heated to a predetermined carbon deposition temperature. Carbon was deposited by exposure to a stream of dilute hydrocarbon (typically 0.14-vol% C2H4) in helium using a special 10-port switching valve (Figure 1). Following carbon deposition, the catalyst was cooled to room temperature while the bed was flushed with pure He, and the He carrier (now in reactor by-pass mode) was replaced by the TPSR reactant gas stream (1-atm Hj or 0.03-atm H2O in He). After the... 

Denton et al. describe a procedure for aminating alkyl aromatic hydrocarbons, particularly toluene, which is converted to benzonitrile. Toluene and ammonia, at various reactant ratios and liquid space velocities, are passed at atmospheric pressure over a supported molybdenum trioxide catalyst at about 525-550 C. The conversion per pass is -10 per cent, and the yields are 60-85 per cent based on the toluene consumed. The process, as operated commercially, involves the continuous feed of toluene and NHj to one of two reactors followed by continuous removal of ammonia and toluene, for recycle, from the benzonitrile. Catalyst in one reactor is regenerated over a 3-6 hr period by oxidizing carbon deposits with air diluted with an inert gas, while the other reactor is on stream. 

Figure 1-14. Catalyst made of carbon deposited with platinum particles...

Afterward, the notion of unspecified carbon deposition with an olefin-like composition (CH ) has been gradually transformed to Polymethylbenzenes (PMBs) by many research groups [87,97]. Those PMBs serve as scaffolds/cocatalysts, where methanol is added and olefins are eliminated in a closed catalytic cycle [87,98]. It is therefore indicated that the interplay between the inorganic framework and the organic reaction centers dictates the activity and selectivity. However, according to Ref. [97], the role of PMBs as the major hydrocarbon pool species appears to be independent of the zeotype catalyst chosen. Haw et al. [87] provided both experimental and theoretical evidence in favor of PMBs as the driving force for the hydrocarbon pool mechanism. In 1998, by means of pulse-quench reactions on an H-ZSM-5 catalyst and GC-MS and MAS NMR analysis, it was reported... 

Naphtha desulfurization is conducted in the vapor phase as described for natural gas. Raw naphtha is preheated and vaporized in a separate furnace. If the sulfur content of the naphtha is very high, after Co—Mo hydrotreating, the naphtha is condensed, H2S is stripped out, and the residual H2S is adsorbed on ZnO. The primary reformer operates at conditions similar to those used with natural gas feed. The nickel catalyst, however, requires a promoter such as potassium in order to avoid carbon deposition at the practical levels of steam-to-carbon ratios of 3.5—5.0. Deposition of carbon from hydrocarbons cracking on the particles of the catalyst reduces the activity of the catalyst for the reforming and results in local uneven heating of the reformer tubes because the firing heat is not removed by the reforming reaction. 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

CAMET control catalyst was shown to obtain 80% NO reduction and 95% carbon monoxide reduction in this appHcation in the Santa Maria, California cogeneration project. The catalyst consists of a cormgated metal substrate onto which the active noble metal is evenly deposited with a washcoat. Unlike the typical 20 on titania turbine exhaust catalysts used eadier in these appHcations, the CAMET catalyst is recyclable (52). 

Fig. 11. The loss of carbon rapidly increases with the increase of temperature. Heating of the catalysts in open air for 30 minutes at 973 K leads to the total elimination of carbon from the surface. The gasification of amorphous carbon proceeds more rapidly than that of filaments. The tubules obtained after oxidation of carbon-deposited catalysts during 30 minutes at 873 K are almost free from amorphous carbon. The process of gasification of nanotubules on the surface of the catalyst is easier in comparison with the oxidation of nanotubes containing soot obtained by the arc-discharge method[28, 29]. This can be easily explained, in agreement with Ref [30], by the surface activation of oxygen of the gaseous phase on Co-Si02 catalyst.

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