Catalytic carbon formation

Figure 2 shows the effect of temperature and benzene concentration on the kinetics of carbon deposition on a nickel foil. The temperature behavior follows a pattern that has been previously observed in the catalytic carbon formation from non-aromatic hydrocarbons (1 ,2). There are three regions in the Arrhenius plot. At low temperatures, the rate increases with increasing temperature and, thus, a negative slope-line is obtained in the Arrhenius plot. This low temperature region is denoted as Region I. 

Figure 5 shows the rate of carbon formation as a function of the gas flow rate in the reactor as measured by the volumetric residence time. The rate of the catalytic carbon formation, as expected, is nearly independent of the gas residence time in the experimental range of this investigation. 

Lobo, L.S. and Pranco, M.D. Kinetics of catalytic carbon formation on steel surfaces from light hydrocarbons. Catal Today 1990, 7, 247-256. 

By the mid-1930s, catalytic technology entered into petroleum refining. To a greater extent than thermal cracking, catalysis permitted the close control of the rate and direction of reaction. It minimized the formation of unwanted side reactions, such as carbon formation, and overall improved the yield and quality of fuel output. 

Barnett et al. [AIChE J., 7 (211), 1961] have studied the catalytic dehydrogenation of cyclohexane to benzene over a platinum-on-alumina catalyst. A 4 to 1 mole ratio of hydrogen to cyclohexane was used to minimize carbon formation on the catalyst. Studies were made in an isothermal, continuous flow reactor. The results of one run on 0.32 cm diameter catalyst pellets are given below. 

The reformer pressure drop depends on the number of tubes, tube diameter and catalyst selection. The typical pressure drop ranges from 40 to 60 psi. The reforming catalysts are made in a ring or modified ring form. Nickel is the chief catalytic agent. Heavier feedstocks use an alkali promoter is to suppress carbon formation. 

Which has been studied on supported Ni catalysts and on Ni films . Studies such as those described here show that methane can be catalytically synthesized over Ni by an active (carbidic) carbon formed via the Boudouard reaction and its subsequent hydrogenation to methane. However, to demonstrate that this surface carbon route is the major reaction pathway, kinetic measurements of both carbon formation from CO and its removal by H2 were carried out . 

In a natural gas fueled PAFC, water is condensed out of the fuel stream going to the fuel cell to increase the partial pressure of hydrogen. In a coal gasification MCFC, water often is added to the fuel stream prior to the fuel cell to prevent soot formation. The addition of excess steam not only prevents the soot formation, but also causes a voltage drop of approximately 2 mV per each percentage point increase in steam content (45). The use of zinc ferrite hot gas cleanup can aggravate the soot formation problem because of the catalytic effect of the sorbent on carbon formation, and requires even higher moisture levels (46). 

As discussed in previous sections, Cu acts primarily as an electronic conductor within the Cu-based anodes. Because it is a poor catalyst for C—H and C—C bond scission, it is essential to incorporate an oxidation catalyst, ceria, within the anode. While Ni has many attractive properties, its propensity for catalyzing carbon formation prevents its use in dry hydrocarbons at high temperatures. One approach for enhancing the catalytic properties of Cu and stabilizing the tendency of Ni for forming carbon is to use Cu—Ni alloys. Cu—Ni alloys have been used... 

A structured ruthenium catalyst (metal monolith supported) was investigated by Rabe et al. [70] in the ATR of methane using pure oxygen as oxidant. The catalytic activity tests were carried out at low temperature (<800 ° C) and high steam-to-carbon ratios (between 1.3 and 4). It was found that the lower operating temperature reduced the overall methane conversion and thus the reforming efficiency. However, the catalyst was stable during time on-stream tests without apparent carbon formation. 

Similar results were achieved over a Rh/alumina monolith catalyst " using catalytic POX for the reforming of a simulated JP-8 military feed containing 500 ppm of sulfur (as benzothiophene or dibenzothiophene). Stable performance for over 500 h with complete conversion of the hydrocarbons to syngas at 1,050°C, 0.5 s contact time, and LHSV of about 0.5 h was reported. At this high temperature, carbon formation was not reported and the sulfur exited as hydrogen sulfide. 

One of the initial attempts of exhaust gas reforming, as well as onboard H2 generation, was reported by Newkirk and Abel. Their process of high-temperature, non-catalytic SR of gasoline resulted in carbon formation in the reformer however, their objective, to reduce emissions by feeding H2 to the gasoline engine, was achieved. 

In 1982, Curtis and co-workers reported that Vaska s complex promotes the formation of phenylsiloxanes from the reaction of hydridosiloxanes and benzene in a catalytic, albeit low-yield, process.93 Catalytic arylsilane formation has also been reported by Tanaka and co-workers. Under photo-lytic conditions, RhCl(CO)(PMe3)2 catalyzes the C-H bond activation of arenes in reactions with hydrosilanes or disilanes, leading to the formation of new silicon-carbon bonds.94 More recently, C-H bond activation of arenes resulting in arylsilane formation has been observed in the... 

A number of simple and inexpensive materials catalytically promote the cobalt-carbonylation (Reaction 2) in aqueous solution. These include ion-exchange resins, zeolites, or special types of activated carbon. Formation of the active catalyst in a separate reactor is thus economically feasible. The mechanism of this catalysis has not yet been elucidated and seems to differ for each promoter mentioned. After an induction period during which the cobalt fed to the reactor is partially retained by the promoter, fully active materials have absorbed cobalt carbonyl anion Co(CO)4 (ion exchange resins), Co2+ cation (zeolites), or a mixture of Co2+, cobalt carbonyl hydride, and cluster-type cobalt carbonyls (activated carbon). This can be shown by analytical studies (extraction, titration, and IR studies) of active material withdrawn from the reactor. 

The hydrocyanation of alkenes [1] has great potential in catalytic carbon-carbon bond-formation because the nitriles obtained can be converted into a variety of products [2]. Although the cyanation of aryl halides [3] and carbon-hetero double bonds (aldehydes, ketones, and imines) [4] is well studied, the hydrocyanation of alkenes has mainly focused on the DuPont adiponitrile process [5]. Adiponitrile is produced from butadiene in a three-step process via hydrocyanation, isomerization, and a second hydrocyanation step, as displayed in Figure 1. This process was developed in the 1970s with a monodentate phosphite-based zerovalent nickel catalyst [6],... 

Until now, three main F-C transformations have been used for catalytic stereoselective formation of benzylic carbon stereocenters - 1,2-addition of arenes to carbonyl (C=X, X O, NR) moieties, 1,4-addition of arenes to electron-deficient C-C double bonds, and ring-opening reaction of epoxides. 

C-H activation with transition-metal complexes will open a new chemistry of catalytic carbon-carbon bond formation because of its potent ability to generate reactive carbon-metal complexes. The design of a catalytic reaction which involves C-H activation followed by reaction with a reagent such as an electrophile is particularly important to provide an environmentally friendly non-salt process, which proceeds under neutral conditions. 

---