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R. S. MANNAND E(. C. KHULBE
Hydrogenation of Methylacetylene over Platinum and Iridium Catalysts by R. S. Mann and K. C. Khulbe Department of Chemical Engineering, University of Ottawa, Ottawa-$, Canada
(Received February 86, 1968)
The reaction between methylacetylene and hydrogen over pumice-supported and unsupported platinum and iridium catalysts has been investigated in a static system for a wide range of reactant ratios between 20 and 150". The order of reaction with respect to hydrogen was one and nearly temperature independent. While the order of reaction with respect to methylacetylene for unsupported iridium was zero and temperature independent, it was slightly negative and temperature dependent for all other catalysts. Selectivity was dependent on initial hydrogen pressure and temperature and independent of methylacetylene pressure. The over-all activation energies for platinum-pumice, platinum powder, iridium-pumice, and iridium powder were 12.4, 14.7, 8.6, and 6.2 kcal/mol, respectively.
Introduction The kinetics of acetylene hydrogenation' over metal catalysts have been studied extensively, but much less has been reported on the kinetics of methylacetylene hydrogenation. Bond and SheridanZ studied the kinetics of methylacetylene hydrogenation over supported nickel, platinum, and palladium metals. Recently, Mann and Khulbe2-6 have reported the kinetics of the reaction of methylacetylene with hydrogen over unsupported nickel, copper, cobalt, iron, and nickelcopper alloys. The general features of the kinetics of hydrogenation of methylacetylene over the catalysts were very similar. Iron was found to have the highest selectivity. The kinetics of methylacetylene hydrogenation were studied over pumice-supported and unsupported platinum and iridium catalysts to verify whether the kinetics of hydrogenation and the steric hindrance caused by the methyl group and the extent of polymerization over these catalysts were the same as over other metals (Nil Cu, Co, Fe, and Ni-Cu alloys) investigated previously.
Experimental Section The reaction rates were measured in a static system. Details of the apparatus, purification of reactants, experimental procedure, and method of analysis of the products have been described previously. Platinum and iridium catalysts (5 wt %) were prepared by impregnating and evaporating platinic chloride and iridium ammonium chloride solutions containing the calculated weight of metals over 2 0 4 0 mesh pumice granules. The impregnated materials were dried at 110" for 24 hr, and the required amount of the catalysts were reduced in situ at 200" in a stream of hydrogen (80 cc/min) for 24 hr. A reduction temperature of 200-250" has been suggested previously by Sheridan and Reid6 and Bond, et aL7 The unsupported (powder) catalysts were prepared by reduction of the calculated amounts of the metal compounds, at 200". The Journal of Physical Chemistry
Results and Discussions A . Pressure-Time Curves. The pressure-time curves obtained during methylacetylene hydrogenation (Figure 1) over pumice-supported and unsupported platinum and iridium catalysts consisted of nearly two linear portions (curve AFG, Type I, Figure 1) of different slopes for hydrogen/methylacetylene ratios of less than 2. However, over unsupported platinum and supported and unsupported iridium catalysts curve -4DE (Type 111) was obtained for hydrogen/ methylacetylene ratios of 2 or greater. In case of platinum-pumice, when the hydrogen/methylacetylene ratio was about 2, the rate of pressure fall during the course of reaction was nearly constant until nearly all the methylacetylene had reacted (curve AB). For hydrogen/methylacetylene ratios greater than 2, a different type of curve (curve ABC, Type 11, Figure 1) was obtained. This was similar to the one observed by Bond and Wells* for acetylene hydrogenation over platinum. In the region AB, the reaction was first order with respect to hydrogen, and the main product was propylene, whereas after the rapid acceleration (BC region) the main process occurring was the further hydrogenation of propylene to propane. A very slight acceleration was also observed in case of SUPported and unsupported iridium (curve ADE). B. Orders in Methylacetylene and Hydrogen by the Initial Rate Method. The order of reaction with respect to hydrogen (n) was determined at several temperatures by using a fixed methylacetylene pressure (30 mm) and wide range of hydrogen pressures (25-200 (1) G. C. Bond and P. B. Wells, Advan. Catal., 15, 91 (1965). (2) G. C. Bond and J. Sheridan, Trans. Faraday Soc., 48, 651 (1962). (3) R. S. Mann and K. C. Khulbe, Indian J . Technol., 5 , 65 (1967). (4) R . S. Mann and K. C. Khulbe, Can. J . Chem., 45, 2755 (1967). (5) R. S. Mann and K. C. Khulbe, ibid., 46, 623 (1968).
(6) J. Sheridan and W. D. Reid, J . Chem. SOC.,2962 (1952). (7) G . C. Bond, D , A. Dowden, and N. Mackenzie, Trans. Faraday SOC.,54, 1537 (1968). (8) G. C. Bond and P. B. Wells, J . Catal., 4, 211 (1965).
HYDROGENATION OF METHYLACETYLENE OVER PLATINUM AND IRIDIUM CATALYSTS
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Table I : Order of Reaction and Activation Energies for Catalysts
W t of catalyst, Catalyst
Pt-pumice Pt powder Ir-pumice I r powder a
g
0.025 0.025 0.025 0.025
Temperature range, OC
Specific rate a t 60'
72-118 95-135 85-121 40-70
0.51
m*, order with respect to methylacetylene.
0.16 2.01 6.2
Activation energy, k x 102, kcal/mol-l min-1
12.4 14.7 8.6 6.2
--
Order of reaction-
m* a
n* b
-0.3 f0.3 - 0 . 4 f 0.3 -0.2 f 0 . 1 0
1 . 0 1 i0 . 0 1 1 . 0 1 i 0.01 1.03 i 0.01 1.03 f 0.03
n*, order with respect to hydrogen.
and second stages of the reaction, and shown as -Ap, in Figure 1, was obtained only while hydrogenating methylacetylene (hydrogen/methylacetylene ratios greater than 2) over platinum-pumice. No such acceleration in the rate of reaction was observed over other catalysts. The acceleration point was dependent on temperature, when the initial pressures of methylacetylene and hydrogen remained constant. It also depended on the initial pressure of hydrogen for constant temperature and methylacetylene pressures. It decreased fairly linearly with increasing temperature and hydrogen pressure. Similar results were observed by Mann and Khulbe5 for methylacetylene hydrogenation over nickel-copper alloys and by Bond and Wells* for acetylene hydrogenation over alumina-supported platinum. Because methylacetylene is more strongly adsorbed than propylene or hydrogen, an increase in the initial pressure of the propylene or hydrogen would facilitate the entry of propylene into the reactive layer and further hydrogenate propylene to propane. This would result in an acceleration of the Figure 1. Pressure-time curves: curve ABC, Pt-pumice, reaction. Pc~= H 5~0 m m ; P H = ~ 150mm, T = 85'; curve AFG, Pt powder, P c , ~ = , 50 mm, P H = ~ 50 mm, T = 95'; curve I n the case of unsupported iridium and supported ADE, Ir-pumice, Pcsa4= 50 mm, Paz = 150 mm, T = 115'. and unsupported platinum catalysts conditions favorable for the entry of propylene into the reactive layer mm). Similarly, the order in methylacetylene (m) only arise when most of the methylacetylene present is converted into propylene. Propane formed in the was obtained at several temperatures by using 60 mm early stages of the reaction must have therefore come of hydrogen and a wide range of methylacetylene presfrom the further hydrogenation of methylacetylene sures (30-150 mm). The order of reaction with respect without the readsorption of gaseous propylene on the to hydrogen was always one and independent of t'emsurface. In the case of a supported iridium catalyst perature. it is easier for propylene to get readsorbed on the reThough the order of reaction with respect to methylactive layer than on other catalysts. Hence selectivity acetylene for unsupported iridium was zero and temfalls with increasing conversion (Figure 2). perature independent, it was slightly negative and temperature dependent for iridium-pumice and supD . Temperature Dependence of Rate Constants. Plots ported and unsupported platinum. This negativity of log specific reaction rates against the reciprocal of increased with increasing temperature. The results absolute temperature for all the catalysts satisfied the are summarized in Table I. Arrhenius equation. The apparent activation energies A comparison of the specific reaction rates for difthus calculated for the catalyst are given in Table I. ferent catalysts at 60" showed that iridium was much E . Dependence of Selectivity and the Distribution of more active than platinum, Products on Per Cent Conversion of Methylacetylene. C. Dependence of Acceleration Point (- A p J . The The course of the reaction (50 mm of methylacetylene acceleration point (- ApJ , defined as the pressure oband 50 mm of hydrogen) was followed by analyzing the tained by extrapolating the linear portions of the first reaction products after various pressure falls ( A p ) . TIME, MIN.
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R. S. MANNAND I> k-4(*), the reaction would be first order, which is the same as found experimentally. From eq A, the linear relationship between methylacetylene and the per cent hydrogenation is quite understandable. It is also possible to explain this linearity by assuming that hydrogenation proceeds with zero order with respect to methylacetylene and propylene. A negative order with respect to hydrocarbons has
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