Catalysis over coprecipitated nickel-alumina. Effect of nickel content on the hydrogenation of benzene

Abstract

A study has been made of the effect of nickel content on the surface area of the metal and on the kinetics of the hydrogenation of benzene to cyclohexane for five coprecipitated nickel alumina catalysts containing 5.4-75-0 wt % nickel. From a study of the hydrogen adsorption isotherms in the temperature range from -196 to 5 5 T it was assumed the surface of the metal was saturated with dissociatively adsorbed hydrogen at ambient temperature. The specific activity, i.e., the reaction rate per unit area of nickel, the apparent activation energy, and the reaction orders were measured in the temperature range 0-70°C in a differential flow reactor. The orders of reaction were approximately zero with respect to benzene and 0.6 with respect to hydrogen. Cyclohexene was not detected in the effluent gases. The specific activity, apparent activation energy, and preexponential factor for the catalysts were essentially the same. The results are in general agreement with those reported for an evaporated nickel f i l m .

The Role of Hydrogen Partial Pressure in the Gas-Phase Hydrogenation of Aromatics over Supported Nickel

“…As a means of illustrating the difficulties involved in comparing kinetic data, the reported activation energies for the hydrogenation of benzene have, in many instances, been generated from the temperature dependences of the reaction rate rather than the rate constant 5,6,8 while in a number of reports it is unclear how the quoted activation energy was derived. 4,7,[9][10][11] The activation energy generated from rate constants will only equal that obtained from the use of raw rate data when the reaction orders are temperature invariant. This is not the case with the systems under consideration, and the differences in values are marked, as is evident from the information provided in Table 3.…”

“…The heterogeneous hydrogenation of aromatic compounds not only is a useful model reaction to gauge the activity of metal catalysts but also is of commercial importance in the upgrading of coal liquids 1 and of environmental significance in reducing undesired emissions in exhaust gases. 2 The hydrogenation of benzene over a range of transition-metal catalysts is welldocumented, 2,3 and the activity of unsupported nickel [4][5][6] and nickel supported on amorphous 4,[7][8][9][10][11][12][13][14][15][16][17] and zeolite [17][18][19][20] carriers is now established. Briefly, the reported benzene/ hydrogen/nickel reaction systems have been variously characterized by the following features: (a) rate maxima at 423, 15 453, 6,8,14 and 473 K; 16,19 (b) rate inhibition by the product (cyclohexane); 5,6 (c) rapid desorption of cyclohexane with no rate inhibition; 8, 14,16,20 (d) competitive adsorption of benzene and hydrogen; 7 (e) noncompetitive adsorption of benzene and hydrogen; 14,16,17,20,21 (f) a temperature dependence of the reaction order with respect to the benzene and hydrogen partial pressures that varied from 0 to 0.5 [4][5][6][7][8][9]…”

“…Briefly, the reported benzene/ hydrogen/nickel reaction systems have been variously characterized by the following features: (a) rate maxima at 423, 15 453, 6,8,14 and 473 K; 16,19 (b) rate inhibition by the product (cyclohexane); 5,6 (c) rapid desorption of cyclohexane with no rate inhibition; 8, 14,16,20 (d) competitive adsorption of benzene and hydrogen; 7 (e) noncompetitive adsorption of benzene and hydrogen; 14,16,17,20,21 (f) a temperature dependence of the reaction order with respect to the benzene and hydrogen partial pressures that varied from 0 to 0.5 [4][5][6][7][8][9]11,12,14,16,17,22 and from 0.5 to 3, [4][5][6][7][8][9]11,12,[15][16][17]19 respectively; (g) apparent activation energies in the range of 25-94 kJ mol -1 . [4][5][6][7][8][9][10][11]16,19 The source of such discordant reports can be attributed, at least in part, to incomplete ...…”

“…It is, however, difficult to gauge the relative importance of each catalyst variable on performance where benzene hydrogenation, for instance, has been characterized as both structure-sensitive 4,15,17 and insensitive. 7,9,11 Benzene has thusfar been the overwhelming model aromatic reactant, and a limited number of reports have emerged dealing with toluene hydrogenation over nickel catalysts 24 where the focus was placed on the electronic effects induced by the methyl substituent on the degree of ring reduction. The hydrogenation of xylene(s) has received even less attention, and the available data are generally in the form of rate ratios relative to a benzene feed.…”

The activity and stability of nickel/silica catalysts in water and methane reaction

“…Small crystallite size ensures high specific metal area, but sintering results when the nickel loading increases. The maximum nickel surface per unit volume of the catalyst (Dixon and Singh, 1969) usually appears in the range 30-50 wt % Ni. A preparation method via impregnation produces inhomogeneous crystallite size, except for low metal loadings.…”