Hydrocarbon-Hydrogen Reactions

Reactions of hydrocarbons on Pt, and to a lesser extent on other Group VIII metals as well, have already been the subject of three excellent reviews in this series (155-157), each review reflecting the views of the particular author(s). It is not this author s intention to repeat information which is available elsewhere (155-157), but rather to focus on particular points namely, those which help us to rationalize the data obtained with alloys, or vice versa, those which have been established by studies with alloys. Of course, the selection of data presented below, or the evaluation of discussions which have already taken place in the literature, is again unavoidably influenced by the author s personal views. 

It is practical to discuss certain groups of reactions separately. Conveniently, the subdivision of reactions as presented in Table I may be used. A notation 3C-, 5C- has been used in Table I to indicate the number of carbon atoms which form the essential part of the transition state complexes of the reactions mentioned. A more detailed definition of other terms will be given below. 

The three types of C—C bond fission concern the following reactions  

Information on the intermediates operating in the reactions of HC/H2 mixtures has been mainly obtained by the three following ways  

It is a serious but frequently neglected problem that the analysis of the data obtained with the method (2) or (3) above is only straightforward when each molecule undergoes only a one-step reaction upon one adsorption sojourn on the catalyst surface. If several consecutive reactions (e.g., isomerization combined with hydrogenolysis or two isomerization steps in combination) follow each other before the molecules leave the surface, useful information is still gained (167, 168), but the discussion of data is more complicated. Metals like Pt or Pd do not seem to be a problem in this respect, as is the case with other metals at the lowest possible reaction temperatures. However, metals like Ir or Rh are apparently very active in performing several consecutive steps during one residence of the molecules on the surface, and at temperatures above 200°C it is difficult to avoid the multiple reactions (167). 

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The roughened surfaces prepared by the above method provide the higher specific surface areas for catalyst dispersion to be effective (Gryaznov. 1992]. Acid-treated Pd-Zn membranes appear to be very effective in improving both conversion and selectivity of a number of hydrocarbon hydrogenation reactions, resulting in essentially complete conversions in several cases [Mishchenko et al.. 1987]. 

In Chapter 7 we discuss the unique seven-atom surface-ensemble cluster on the Fe(lll) surface (shown in Fig. 2. IOC) that is optimum for N2 activation. Early suggestions that surface ensembles with a particular number of atoms are necessary for a particular reaction to occur are deduced from alloying studies of reactive transition-metal surfaces, with catalytically inert metals such as Au, Ag, Cu or Sn . For example, the infrared spectrum of CO adsorbed on Pd shows the characteristic signature of CO adsorbed one-fold, twofold or three-fold to surface Pd atoms . Alloying Pd with Ag, to which CO only weakly coordinates, dilutes the surface ensembles. One observes a decrease of the three-fold and the two-fold coordinated CO and the one-fold coordinated CO becomes the dominant species. The effect of alloying a reactive metal with a more inert metal is especially dramatic when one compares hydrocarbon hydrogenation reactions with hydrocarbon hydrogenolysis reactionst . 

The presence of an electron donor causes the equiHbrium to shift to the left. The acidity represented by this mechanism is important in hydrocarbon conversion reactions. Acidity may also be introduced in certain high siHca zeoHtes, eg, mordenite, by hydrogen-ion exchange, or by hydrolysis of a zeoHte containing multivalent cations during dehydration, eg,... 

Titanium carbide may also be made by the reaction at high temperature of titanium with carbon titanium tetrachloride with organic compounds such as methane, chloroform, or poly(vinyl chloride) titanium disulfide [12039-13-3] with carbon organotitanates with carbon precursor polymers (31) and titanium tetrachloride with hydrogen and carbon monoxide. Much of this work is directed toward the production of ultrafine (<1 jim) powders. The reaction of titanium tetrachloride with a hydrocarbon-hydrogen mixture at ca 1000°C is used for the chemical vapor deposition (CVD) of thin carbide films used in wear-resistant coatings. 

Industrially, chlorine is obtained as a by-product in the electrolytic conversion of salt to sodium hydroxide. Hazardous reactions have occuned between chlorine and a variety of chemicals including acetylene, alcohols, aluminium, ammonia, benzene, carbon disulphide, diethyl ether, diethyl zinc, fluorine, hydrocarbons, hydrogen, ferric chloride, metal hydrides, non-metals such as boron and phosphorus, rubber, and steel. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

Besser, R. S., Ouyang, X., Suranga-LiKAR, H., Hydrocarbon hydrogenation and dehydrogenation reactions in micro-fabricated catalytic reactors, Chem. Eng. Sci. 58 (2003) 19-26. 

The analysis of published data on reactions of ozone with low molecular hydrocarbons shows that double bonds react with ozone more quickly than saturated bonds (12). Ozone reacts with saturated hydrocarbons in reactions in which hydrogen abstraction s followed by re-hydridization of the carbon atom form sp to sp state (43,44) ... 

Fig. 9. Pulse microreactor system for use with 13C-labeled hydrocarbons. D, E, and J are microreactors J contains the catalyst to be used for hydrocarbon skeletal reaction D and E are used, when necessary, to generate the required reactant hydrocarbon from a non-hydrocarbon precursor (e.g., alcohol dehydration in D and olefin hydrogenation in E) reactant injected at C. F is a trap which allows the accumulation of products from several reaction pulses before analysis G is a G.P.C. column, K a katharometer. Traps H collect fractions separated on G for subsequent mass spectrometric study. When generating reactant hydrocarbon in D and E, a two-step process is preferable in which, with J below reaction temperature, the purified reactant hydrocarbon is collected in H, and this is recycled as reactant with D and E below reaction temperature but with J at reaction temperature. After C. Corolleur, S. Corolleur, and F. G. Gault, J. Catal. 24, 385 (1972).

Fig. 5. Apparent activation energies of the ethane hydrogenolysis and cyclopropane hydrogenation reactions on the group VIII noble metals. The activation energies were determined at hydrogen and hydrocarbon partial pressures of 0.20 and 0.030 atm, respectively (63).

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