HYDROGENATION OF BENZENE WITH NICKEL AND PLATINUM * BY WILDER D. BAXCROFT AND AVERY B. GEORGE
Introduction The statement is usually made that a nickel or copper catalyst will do things that a platinum catalyst will not. If we consider only the hydrogen, this is impossible, because the catalyst can apparently only increase the rate by increasing the amount of monatomic hydrogen. This action may be merely a question of rate, or there may be a specific effect of the nickel on the substance to be reduced. The object of this work was to find out the reason for the different action of catalytic nickel and platinum. It was desired to pick a case in which nickel is said to be a better catalyst than platinum. So the reaction, benzene to hexahydrobenzene, was chosen for carrying out this investigation. The use of metallic catalysts has provided a method capable of fairly general application to the hydrogenation of the benzene nucleus. According to Sabatier it is undoubtedly the most important of the operations that reduced nickel has rendered possible in synthetic organic chemistry. At temperatures in the neighborhood of 180°C the aromatic nucleus may readily be hydrogenated in presence of nickel without isomerisation of the products or production of secondary reactions. Early experiments of Lunge and Akunoffl had demonstrated a partial hydrogenation of benzene to cyclohexane, in presence of platinum black, a t the ordinary temperature, or better at 100°C. Palladium sponge, on the other hand, yielded cyclohexane, C6H10. The composition of the products was deduced from the contraction in volume of the gases and is therefore uncertain. The catalytic activity of the metal also rapidly diminished. The work of Sabatier and Senderens demonstrated the efficiency of the reduced nickel catalyst. The direct hydrogenation of benzene to cyclohexane, C6H12, takes place with nickel above 70°C. Its speed increases with the temperature up to Cdb
+ 3Hz
--$
CeHiz
I ~ O ~ - - I ~where O ~ C it, is rapid without any side reaction. Above that, and particularly above 300°C~a part of the benzene is reduced to methane, and carbon is deposited on the nickel.* Recently prepared platinum black can transform benzene into cyclohexane a t 1 8 o T for a time, but its activity diminishes rapidly and soon disappears. Platinum sponge has not this power.3 * This work is done under the programme now bein carried out at Cornell University and supported in part by a grant from the Heckscher soundation for the Advancement of Research established by August Heckscher a t Cornell University.
2220
WILDER D. BANCROFT AND AVERY B. GEORGE
Zelinsky4 affirms that both platinum and palladium are as efficient as nickel in this and other hydrogenations of the aromatic nucleus. Cobalt8 also catalyzes the hydrogenation of benzene at 18o"C, but soon loses its activity. Dougherty and Taylor5 made a study of the mechanism of the catalytic reduction of benzene to hexahydrobenzene. The results indicated that the reaction does not occur a t all according to the stoichiometric equation, as calculated from gas concentrations, but a t rates governed by the distribution of the reacting materials between the catalyst and the gas phase. Pease and Purdum6 have made copper catalysts reactive in this synthesis, contrary to the statements of Sabatier that copper would not reduce benzene to hexahydrobenzene. An excess of benzene was found to inhibit the reaction, and this was explained as due to the hydrogen adsorption being blocked by preferential adsorption of benzene. The reverse of hydrogenation, or the process of dehydrogenation takes place as the temperature becomes more elevated. Cyclohexane, which cannot be formed by the direct hydrogenation of benzene by the aid of nickel above 3oo0C, suffers a partial dehydrogenation to benzene above this temperature, but a part of the benzene is transformed to methane by the liberated hydrogen.
+
3C6H12 + ~ C ~ H S6CH4 The presence of a current of hydrogen stabilizes the molecule to a certain extent so that it is only slightly broken up a t 35OoC. At 4ooOC about 30% of the cyclohexane passing over the nickel with the hydrogen is decomposed into benzene.8
Experimental Procedure Hydrogenation In order to ascertain whether or not the apparently greater action of a nickel catalyst was a question of rate, some experiments were made on the hydrogenation of benzene vapor in the presence of a nickel catalyst and a platinum catalyst. From the work of Sabatier and Senderens,* it was known that benzene can be readily hydrogenated a t 18o'C in the presence of a nickel catalyst. Freshly prepared platinum black can be used for hydrogenating benzene; but its activity diminishes rapidly. This action may be due to the sintering of the platinum black, so we used platinum on an asbestos support, which prevents sintering. The finely-divided nickel and the platinized asbestos catalysts were very kindly furnished by Dr. C. M. A. Stine of the Du Pont Company, to whom we extend our thanks. The procedure of the hydrogenation consisted essentially in passing the hydrogen, admixed with benzene vapor, through a combustion tube containing the catalyst a t the proper temperature, the product being condensed and collected on emergence from the heated tube. Electrolytic hydrogen was used, and this was further purified by passing the gas through a tube containing platinized asbestos a t a dull red heat, then over some solid potassium hydroxide. To dry the hydrogen it was passed through a tube containing soda-lime, and finally through phosphorus pent-
HYDROGENATION OF BENZENE WITH WCKEL A S D PLATISUY
222I
oxide. In order to regulate the flow of gas a flow-meter was interposed in the path of the hydrogen. A thin layer of the catalyst was spread along the reaction tube, the same amount being used in each experiment. Before making a run the furnace was heated to 300'C with a slow stream of hydrogen flowing through the apparatus, and this condition was maintained for thirty minutes. Then the temperature of the tube was lowered to 18o~-18j T , a t which temperature all of the runs were made. A measured amount of the benzene was put into the flask through a funnel a t the top. At the end of a run the amount of liquid left was measured, then the amount of benzene actually used could be determined. The dry hydrogen was then bubbled through the benzene contained in a flask, which was immersed in a water bath kept a t 35OC, a t which temperature the vapor pressure of the benzene is about 145 mm. With the flow of hydrogen a t the desired rate, and the delivery tube extending nearly to the bottom of the flask the run was started. The hydrogen admixed with the vapor of benzene was passed through the catalyst tube, the product being condensed and collected on emergence from the heated tube. The experiments were carried out for periods of three hours, and four and one half hours. The product contained, in addition to the cyclohexane formed, some unchanged benzene that passed over. The cyclohexane is not attacked by fuming sulphuric acid, while the benzene is, so this offers a method of separation of the two. The product was treated with some fuming sulphuric acid, the benzene forming benzene sulphonic acid and this was separated from the cyclohexane by use of a separatory funnel, the top layer being the cyclohexane portion. The cyclohexane was then dried over a little calcium chloride, and the percentage yield of the resulting product was calculated. A series of experiments were carried out using nickel and platinized asbestos as catalysts. h few runs were made employing the same rate of hydrogen in each case. Then some experiments were made with the platinum catalyst, in which the rate of hydrogen flow was diminished to two thirds of the initial rate, and the run was allowed to go one and a half times as long. The time for the initial runs was three hours, while the latter ones were carried out for a period of four and a half hours. The average values of the results obtained are given in the table below, this also includes some physical constants taken on the products. C&*
Catalyst
70Yield
Boiling Point
Nickel Platinum Platinum Platinum
36.8 27.8 51.7
78.0'-79. OOC 75.5'-76. g0C 77.5'-78.5°c 7 7 , 5 0 3 9 . 0°C
52.4
Specific Gravity 0.77820'
0.79520'
0.78720~ 0.78420'
The results given in the above table were obtained under similar conditions, except for varying the rate of hydrogen flow. I n the first two sets of data the hydrogen flow was the same, and while the last two sets were a t the same rate, the flow of hydrogen in these cases was two thirds of that in the
2222
WILDER D. BANCROFT AND AVERY B. GEORGE
first sets of data. This means that the rate of hydrogen flow is the important factor, in comparing the relative amount of hydrogenation of benzene in the presence of a nickel and of a platinized-asbestos catalyst. For by using a nickel and a platinized-asbestos catalyst in the presence of the same rate of hydrogen, a larger percentage yield of the cyclohexane is obtained with the nickel catalyst. However by diminishing the rate of hydrogen flow in the case of the platinized-asbestos catalyst the percentage yield of product was greatly increased. So a t a given rate of hydrogen, there is a larger amount of monoatomic hydrogen present a t the surface of the nickel than a t the surface of the platinized asbestos. In other words hydrogen is activated more strongly by nickel than by platinum. The efficient utilization of a platinized-asbestos catalyst, therefore, hinges on the fact that a slow current of hydrogen must be employed in the hydrogenation. Dehydrogenation The reaction was now approached from the opposite side by employing a much higher temperature, which favors the dehydrogenation process. The dehydrogenation can take place in the presence of an excess of hydrogen, and in some cases the excess of hydrogen, far from hindering the reaction, regulates it by favoring the maintenance of the cyclic structure and diminishing the tendency to the breaking up of the molecule into many fragments. At 3ooOC the cyclohexane first formed is then reduced to methane using a nickel catalyst. 3CsH12 + 2Cs& 6CH4 CsHs 9Hz -+ 6CH4
+
+
The experimental procedure in this dehydrogenation was practically the same as in the hydrogenation of benzene. The furnace was maintained a t a temperature of 3oo0C, and a slow current of hydrogen was bubbled through the benzene in the flask. I n this case the gas issuing from the apparatus was analyzed, a sample being collected after the run had proceeded for about an hour. The analysis of the gas was carried out in the following manner. First any benzene vapor or unsaturated hydrocarbons were removed by absorbing them in fuming sulphuric acid, then the hydrogen was removed by passing the gas over hot copper oxide, a t z7ooC. Finally, the remaining portion of the original sample was burned in a combustion pipette, the total contraction volume and the amount of carbon dioxide formed being determined. This latter portion would consist of saturated hydrocarbons. A series of runs was made using a nickel catalyst a t 3oo0C, and the evolved gas analyzed according to the procedure outlined above. After removing the unsaturated hydrocarbons and hydrogen, the remaining gas was combusted and the average value for n, in the series C, H2n+2,was found to be 1.04which shows that the gas was methane. The percentage composition was calculated after having subtracted the volume of hydrogen from the original volcme. The average composition then of the remaining gas was about 90% methane, with a little ethane present, and 10% of benzene vapor and unsaturated hydrocarbons.
HYDROGENATIOS OF BENZESE WITH NICKEL AND PLATINUM
2223
From these results it was thought that with nickel a t z50°C the amount of methane formed would be less than that obtained a t 3oo0C, so a few experiments mere carried out under these conditions. After analyzing the gas evolved, by the same method as employed above, about 15% of the original volume was found to be of the saturated group, as compared to about 6orC obtained a t 3ooOC. By running a combustion on this remaining gas, the average value of n was found to be 2.1, which showed that there were other gases present rather than just methane. These data did not tell the proportions of the various gases present, so some method of analysis of the mixture had to be devised. The following method was devised and used in this case. The mixture was thought to contain methane, ethane, and probably some propane, for from the first gas analysis the mixture might contain methane and propane, ethane and a little propane, or a combination of all three of these. The solubility of these gases in absolute alcohol is as follows. Methane 52.2 Ethane 46.0 Propane 790.0
CC./IOOCC.
CC./IOOCC. CC./IOOCC.
alcohol alcohol alcohol
So the method employed was to run a combustion on one sample of gas after removing the unsaturated hydrocarbons and hydrogen, thus leaving only the saturated hydrocarboas- Then another sample of gas was taken, and before combusting it, the saturated hydrocarbon gas was shaken up with 4.5 cc. of absolute alcohol. After removing the alcohol, the last traces with water, a combustion was run on the remainipg gas. This amount of alcohol would remove all the propane which might be present, so by finding the value of n, from the combustion data, of the remaining gas the amounts of methane and ethane in this gas can be determined. The solubility of the gases in 4. j cc. of absolute alcohol is as follows. Methane 2 . 2 cc. Ethane 1.6 cc. Propane 36.0 cc. The volume of gas removed by the alcohol was recorded, and on analyzing the remaining gas an average value of 1.3 was obtained for n,which shows the ratio of methane to ethane in this gas. By assuming that the alcohol used became saturated in respect to methane and ethane, the volume of propane present in the ariginal gas could be found by difference, which was actually done. The volumes of methane and ethane in the gas remaining after the absorption with absolute alcohol, was calculated from the combustion data. Then the total volumes of these constituents in the gas were found by adding the volume absorbed by the alcohol to the volumes as calculated above. With these data the percentage composition of the three gases methane, ethane, and propane in the total volume of saturated hydrocarbons was calculated. The amount of oxygen necessary
WILDER D. BANCROFT ALSD AVERY B . GEORGE
2224
to burn this mixture was calculated, and the values agreed very well with the amount of oxygen actually used in the combustion. The average percentage composition of the gases present in the saturated hydrocarbon gas was as follow~s. Methane 6 0 7 ~ Ethane z5yc Propane 157~ S o w to put these values on the same basis as in the above case, that is, the compositivn of the evolved gas after removing the hydrogen, the following results were obtained. Unsaturated hydrocarbons 2 5 % Methane 45% Ethane 20% Propane 10%
More hydrogen was obtained in the gas evolved using nickel at z50°C, than a t 300°C, and as there is a much larger percentage of methane obtained a t the latter temperature, this means that more hydrogen is used for reducing the benzene to methane. At 2go°C the nickel causes a certain decomposit'ion of the benzene, but somewhat partial as compared to that a t 300%. There are more of the unsaturated hydrocarbons, and there being some of the higher members of the saturated series, this means that the reduction has not been carried so far as occurs a t 3oo0C, when almost all of the evolved gas was methane. This action must, be due to the increase in temperature, and the more important factor the increased activity of the nickel catalyst a t the elevated temperature. With these data on the effect of a nickel catalyst, the dehydrogenation was next carried out using a platinized asbestos catalyst. The first series of runs were made a t 3oo0C, in the same manner as in the case of nickel, samples of the issuing gas being collected and analyzed as before. After removing the unsaturated hydrocarbons and hydrogen, there remained only about 3 % of the saturated series as compared to about 6 0 7 ~with nickel under the same conditions. These percentages are based on the original volume of gas as evolved from the apparatus. After removing the hydrogen from this volume, the average percent of unsaturated hydrocarbons was 7 5 7 0 , and of the saturated hydrocarbons 2 5 % . From this marked change in percentage composition, it can be seen that there must have been a large amount of hydrogen present in the evolved gas. These figures can be compared with the data obtained by using a nickel catalyst in the following manner. Catalyst
Temp.
yo Uneaturated
Nickel Platinum
300°C
IO
b: Saturated 90
3ooOC
75
25
This means that nickel is a more active catalyst than platinized asbestos under these conditions, and so causes a much greater decomposition of the
HYDROGENATION OF BENZEXE WITH NICKEL AND PLATINUM
2225
benzene to saturated hydrocarbons. A run was made with the platinized asbestos catalyst a t 4oo0C, and another a t 50o'C; w e n a t these elevated temperatures only a very small amount of decomposition to saturated hydrocarbons takes place. The dehydrogenation of benzene over platinized asbestos at 2 5o°C gave a little less of the unsaturated hydrocarbons, than obtained a t 3oo0C, the amount of the saturated series remaining about the same. The increasing of the temperature of the benzene bath from 35'-47'C, thus increasing the concentration of the benzene in the vapor passing over the catalyst, merely increased the amount of the unsaturated hydrocarbons in the gas formed. The next step was to substitute cyclohexane in place of the benzene. Hydrogen was passed through some cyclohexane, the water bath being a t 4ooC, and the vapor passed over platinized asbestos at 3oo0C, as in the above cases. The results obtained were similar to those given by benzene under the same conditions. Cyclohexane is probably formed first on dehydrogenating benzene in the presence of hydrogen, and then this is dehydrogenated, in which case the same results would be expected whether starting with benzene or with cyclohexane.
Conclusions I. In the hydrogenation of benzene with a nickel and a platinized asbestos catalyst, the rate of hydrogen flow is the important factor. A slow rate of hydrogen is necessary in the case of the platinum. JVith nickel a t 3oo0C, the decomposition of the benzene is more marked 2. than at 2 jo"C. About 90$ methane is formed in the former case, and i5yc of mixed members of the saturated series a t the latter temperature. 3 . Platinized asbestos a t 3 0 0 T gave much less of the saturated hydrocarbons than did nickel a t the same temperature, the activity of the latter catalyst being much greater. 4. At temperatures up to 500'C only a small amount of decomposition to saturated hydrocarbons was obtained with platinized asbestos. j. A t 250°C platinized asbestos gave a little less of the unsaturated compounds than obtained a t 300'C. Increasing the concentration of the benzene in the vapor passing over the catalyst at 3oo0C, merely increased the amount of unsaturated hydrocarbons in the gas. 6. Substituting eyclohesane for benzene with platinized asbestos a t 30oOC gave results similar to those obtained with the benzene.
Cornell CnimrsifU
References
j
*
Lunge and Akernoff: B. anorg. Chem., 24, 191 (1900). Sahatier and Senderens: Compt. rend., 132, 2 1 0 (igoi). Sahatier and Senderens: .4nn. Chim. Phys., (8) 4,368 (1905). Zelinsky: J. Russ. Phys. Chem. Sor., 44,274 (1912). Doupherty antl Taylor: J. Phys. Chem., 27, 533 (1923). Pease and Purdum: J . .4m. Chem. Sor., 47, 1435 (i92j). Sahatier antl Mailhe: Compt. rend., 137, 240 (1903). Sabatier and Daudier: Compt. rend., 168, 670 (19;9).
