The Catalytic Hydrogen Reduction and Deuterium

The Catalytic Hydrogen Reduction and Deuterium Exchange of cyclopentanone on Evaporated Metal Films and Some Observations on cyclohexanone Author(s): C. Kemball and C. T. H. Stoddart Source: Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol. 246, No. 1247 (Aug. 26, 1958), pp. 521-538 Published by: Royal Society Stable URL: http://www.jstor.org/stable/100661 Accessed: 08-04-2018 22:48 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms

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The catalytic hydrogen reduction and deuterium exchange of cyclopentanone on evaporated metal films and some

observations on cyclohexanone BY C. KEMBALL AND C. T. H. STODDART

Department of Chemistry, The Queen's University of Belfast (Communicated by Sir Eric Rideal, F.R.S.-Received 10 December 1957-

Read 17 April 1958) Some catalytic reactions of cyclopentanone vapour with hydrogen and with deuterium on evaporated metal films in a static system have been followed by means of a mass spectro-

meter. Hydrogenation to cyclopentanol and hydrogenolysis with formation of cyclopentane and cyclopentene have been observed to different extents on films of rhodium, palladium, platinum, tungsten and nickel. Kinetic data, activation energies and frequency factors have been obtained for these reduction processes and possible mechanisms are discussed. Similar information has been obtained for the exchange reactions. An order of activity of the metals

for inducing fission of the C-0 bond is given. Preliminary experiments with cyclohexanone have indicated an analogous behaviour to that found for cyclopentanone. A comparison is

drawn between the reactions of the cyclic ketones and those found for acetone (Stoddart & Kemball I956; Kemball & Stoddart 1957).

INTRODUCTION

In investigations of the catalytic reactions of acetone vapour with hydrogen and

with deuterium on evaporated metal films (Stoddart & Kemball 1956; Kemball & Stoddart I957) three reactions were found under the experimental conditions. These were the reduction of the carbonyl group with formation of isopropanol and propane and exchange of the hydrogen atoms of the methyl groups by deuterium. The major reduction product on rhodium, palladium, platinum, nickel, iron,

tungsten and gold was the alcohol, but small amounts of propane were also formed simultaneously on platinum and to a lesser extent on the other metals. The reactions of importance were, therefore, the reduction to the alcohol and the exchange reaction and the main purpose of the work on acetone was to elucidate

the mechanisms of these two reactions. The small amounts of hydrocarbon formed

on most metals (100% yields) and complications in the mass spectral analysis of the system meant that the estimation of hydrocarbon was subject to large errors,

and that consequently no detailed examination of the mechanism of hydrocarbon production from acetone was possible.

Following these investigations of the catalytic reactions of acetone on films it was of interest to consider the reactions of the cyclic ketones, cyclopentanone and cyclohexanone on a variety of films with a view to obtaining information on the catalytic activity of the carbonyl group in new molecular environments, and also

by studying the exchange reactions with deuterium to obtain some picture of the behaviour of the surface complexes involved. Preliminary investigation on the reduction of cyclopentanone with hydrogen on films showed that hydrocarbon yields were greater than in the case of acetone, and that in addition the mass 34

[

521

]

VOl.

246.


A.

522 C. Kemball and C. T. H. Stoddart spectra involved were such as to allow accurate determination of quite small

amounts of hydrocarbon. It appeared likely, therefore, that a detailed investigation of the catalytic reactions of the cyclic ketones with 'hydrogen' on films would enable not only a comparison of the activities of the metals for alcohol formation and exchange of the two types of ketone, but would also allow a more thorough

investigation of the nature of hydrocarbon formation from ketones on films, thereby giving valuable information on the catalytic activation of the C-O bond and of the relative efficiencies of the different metals for its dissociation. Previous investigations on the catalytic reduction of cyclopentanone and cyclo-

hexanone have been largely confined to bulk platinum and nickel catalysts and no detailed examinations of the kinetics of the reactions have been given. No investigation of the catalytic exchange of these compounds with deuteriumn gas

has been reported. No previous studies on evaporated metal films of any of these reactions have been carried out. EXPERIMENTAL

The mass spectrometer, catalytic apparatus and method of preparing metal

films have been previously described (Kemball 195I, 1952; Anderson & Kemball I954a). An AR grade cyclopentanone of at least 99 % purity was refluxed over freshly baked lime and fractionally distilled; b.p. 130-3 'C (767 mm), nD-4 OC 1-4351. The liquid was rigorously degassed in the vacuum apparatus and stored by freezing

in a bath of solid carbon dioxide in acetone to minimize attack on the stopcock

grease. Cyclopentanol, cyclopentane and cyclopentene (95 to 99 % purity) were further purified for calibration purposes in a similar manner to cyclopentanone

and handled in the same way: cyclopentanol, b.p. 138-5 C (740 mm), n248C 1'4523; cyclopentane, b.p. 480 'C (734 mm), n243 'C 14034; cyclopentene, b.p.

43-8 ?C (761 mm), n 25 c 1-4201. A good grade cyclohexanone containing some

2 % cyclohexanol was dried for 2 days over anhydrous sodium sulphate and fractionally distilled and manipulated in the same way as cyclopentanone;

b.p. 154 'C (755 mm), nD ?0 ?C 1-4501. The purity of the compounds was check in the mass spectrometer and the purity of the ketones further checked by means

of gas chromatography. Hydrogen from a cylinder and deuterium prepared by

electrolysis of 99-95 g/100 g heavy water were purified by diffusion through a palladium thimble.

The gas mixtures in the case of cyclopentanone contained fifteen parts of hydrogen or deuterium to one part of ketone and were admitted to the reaction vessel

(200 ml.) at 0 0C; the partial pressure of cyclopentanone was 1I19 mm and the number of ketone molecules in the reaction vessel was estimated as 8-32 x 1018.

For cyclohexanone the gas mixtures contained thirty parts of hydrogen or deuterium to one part of the ketone and were admitted to the reaction vessel at O QC; the partial pressure of cyclohexanone was 0-60 mm and the number of ketone molecules was 4-16 x 1018. The analyses in the mass spectrometer were made by using low-voltage electrons

(17V) to ionize the molecules, thus minimizing corrections for the formation of


Catalytic hydrogen reduction and deuterium exchange 523 fragmentary ions. Cyclopentanone was estimated from the peak corresponding to the parent ion of mass 84 (C51H80) after correcting for a contribution from a

fragmentary ion formed from cyclopentanol amounting to 10 % of its parent peak. Cyclopentanol was estimated from the peak corresponding to the parent ion of

mass 86 (C5H100). Cyclopentane was estimated from the parent peak mass

70 (C5H1O) after correction for small amounts of fragmentary ions from the keto (0.1 % of parent peak, 84) and alcohol (2.5 % of parent peak, 86). The estimation of cyclopentene was made in terms of the fragmentary ion C5H+, mass 67, formed by loss of a hydrogen atom from the parent, mass 68. Correction had to be applied

for contributions due to fragmentary ions formed from ketone (100 of parent, 84)

and alcohol (75 % of parent, 86) and cyclopentane (0. 8 % of parent, 70). The fragmentary ion was chosen as a measure of the cyclopentene, since its magnitude

under the conditions obtaining was some 27 % greater than that of the parent ion and also because the corrections due to alcohol fragmentation was some 40 0 less at mass 67 than at mass 68. The accuracy of hydrocarbon estimation was favoured by the high sensitivity of the mass spectrometer to these compounds relative to

the ketone and alcohol parents. Thus, for example, although the contribution due to fragmentation of the alcohol to the cyclopentene mass 67 peak was 75 0 of the

magnitude of the alcohol parent, the sensitivity of the instrument to cyclopentene was 24 times that to the alcohol, which meant that the actual alcohol correction was usually small compared with the observed peak. Further corrections were made to allow for the presence of naturally occuring heavy carbon, hydrogen and oxygen. In the analysis of reactions between deuterium and cyclopentanone the ketone was estimated from the peaks corresponding to the parent ions of masses 84 (C5H80) to 92 (C5D80). Analyses for cyclohexanone and the products formed from it were made by similar methods. Sensitivities and extents of fragmentation were determined by calibration experiments with the hydrocarbons, ketones and alcohols. RESULTS

Hydrogenation Hydrogenation of cyclopentanone to the corresponding alcohol, cyclopentanol, occurred on all the metals studied. At low temperatures it was the major reaction on rhodium, nickel and tungsten films and was comparable with hydrogenolysis to the hydrocarbon, cyclopentane, on platinum and palladium films. It was found that the reverse reaction, the dehydrogenation of cyclopentanol to cyclopentanone, was also of importance at temperatures above O?C and that, on the more active metals, the equilibrium between cyclopentanone, cyclopentanol, and hydrogen was being established.

The investigation of the kinetics of the hydrogenation of acetone to isopropanol in films (Stoddart & Kemball I956) showed that for all the metals at temperatures and times where the amount of the reverse reaction was small, and where the small amount of simultaneous hydrocarbon production could be neglected the formation of the alcohol followed the equation

-log1o(1 00-x) = /c1t230 3 - log 100 (1i ) 34-2


524 C. Kemball and C. T. H. Stoddart where x is the percentage of alcohol formed at time t and k1 is a constant. This equation was tested in the present case for the hydrogenation of cyclopentanone

to cyclopentanol on nickel and tungsten films where the amount of simultaneous reduction to hydrocarbon could be ignored. Typical plots (figure 1) show that the equation was applicable to the hydrogenation of the cyclic ketone as well as to

acetone. It can be seen, however, that in the experiments on nickel at 25-4 00 and on tungsten at + 35-4 'C the linear relationship between the logarithm of ketone concentration and time only holds after the first few minutes from the initiation of the reaction have elapsed. The reason for this apparently slow initial rate of reaction is believed to be instrumental, and can be attributed to the slow estab-

lishment of the adsorption-desorption equilibrium on the glass of the capillary time (min) 0 A 20 40 60 80 2-00 A

20

-195

0

1.90

1-4-

0 ~~~~~~~~1-85

1-2-

0

@0

20

40

time (min)

FIGURE 1. Plots to determine the rates (k1c) of hydrogenation of cyclopentanone to cyclopentanol according to equation (1). *, 50 mg nickel at + 254 0C; 0, 6'8 mg nickel at 26'6 'C; A, 17-1 mg tungsten at +35-4 'C.

leak and metal parts of the mass spectrometer of the cyclopentanol produced in the reaction. This explanation is borne out by the fact that no such effect exists for a reaction carried out after allowing a small amount of cyclopentanol production to proceed in the film at a lower temperature to establish equilibrium in the appa-

ratus, e.g. in the experiment on nickel at 26-6 'C in figure 1. Further, the magnitude of the effect is found to depend on the condition of the apparatus with regard to adsorbed alcohol remaining from previous experiments.

From plots such as those in figure 1, the true initial rates of reaction k1 expressed as percentage of cyclopentanone converted to cyclopentanol per minute were determined. For palladium, as the amount of hydrocarbon being formed simultaneously was comparable with the alcohol production, the initial rate was determined from a plot of percentage concentration of cyclopentanol against time. These values were then expressed as a rate per 10 mg of catalyst (k) and are shown in


Catalytic hydrogen reduction and deuterium exchange 525 figure 2 plotted according to the Arrhenius equation. The values of the activation energies so obtained and frequency factors defined by X=AeEI

(2)

where X was the initial rate of formation of cyclopentanol in molecules per second

per square centimetre (Mol./s cm2) of catalyst surface, are given in table 1.

0~~~~~ A

IC

05

-

O\A

\e X

0

1.0-

10

27 2-9 31 33 3-5 3.7

103/T OK

FTiURE 2. The effect of temperature on the initial rate (k) of hydrogenation of cyclopentanone to cyclopentanol expressed as percentage of cyclopentanone removed per minute per 10 mg of catalyst. 0, nickel; A, tungsten; O, palladium.

TABLE 1. ACTIVATION ENERGIES AND FREQUENCY FACTORS FOR THEE HYDROGENATION OF CYCLOPENTANONE TO CYCLOPENTANOL N ]EVArORATED METAL FILMS temp. when rate is

log10 A (A in temp. range I %/min 1000 catalyst E (keal/mole) mol./s cm2) (OC) Cm2 (OC)

nickel 11-0 21-2 0 to 27 -9.4 tungsten 10-6 18-7 11 to 87 814 palladium 4-6 14*8 0 to 80 107-2

The

surface

areas

of

the

films

we

1952). The experimental error in and about + 0-5 in logLOA. The a and not to constant pressure of the gases. The temperatures at which the initial rates of hydrogenation were 1 %/min 1000 cm2 of surface are also included and are used as a measure of the activity of the metals for the reaction.


326 C. Kemball and C. T. H. Stoddart The rates of hydrogenation of cyclopentanone to cyclopentanol on rhodium and

platinum films were very rapid even at 0 'C (10-3 %0/min 1000 Cm2 for rhodiu 9*7 %/min 1000 cm2 for platinum) and since the very low vapour pressures of th ketone and alcohol did not permit measurement at temperatures much below 0 'C, it was not possible to obtain data on the activation energies and frequency factors for the reactions on these metals. The difficulties of slow stabilization, low sensitivity to parent ion and low vapour

pressure associated with the analysis of cyclopentanol were even more pronounced with cyclohexanol, but it appeared that its rate of production from cyclohexanone on tungsten and platinum films was comparable with the rate of formation of cyclopentanol from cyclopentanone.

Hydrogenolysis In addition to the hydrogenation of cyclopentanone to cyclopentanol the simul-

taneous hydrogenolysis of the carbon-oxygen bond in both the ketone and the alcohol with formation of hydrocarbon and water occurred on all the metals in the

temperature ranges investigated with the exception of nickel. Both cyclopentane and cyclopentene were produced on tungsten films, but cyclopentane alone was

formed on platinum, palladium and rhodium films. It was not possible to detect the water produced in these reactions owing to the high background of water in

the mass spectrometer. A100

0

~ ~~~~~ CCio-~ ~10

0

0

5

-90

0b

-

20)

40

time (min)

FIGuRE 3. The production of cyclopentanol and cyclopentane from a 15:1 hydrogen-ceylopentanone mixture on 26*4 mg of palladium at 12-0 'C. O, cyclopentanone; 0, cyclopentanol; Fl, cyclopentane.

Figure 3 shows the results of a typical experiment on a palladium film expresse as the variation in the percentage composition of five-membered ring compounds with time. The yields of cyclopentane were almost equal to those of alcohol on this metal even at 12-0 'C. The initial inflexion in the curve representing cyclo-

pentanol production is due to the effects of equilibration of this compound in the apparatus already discussed.


Catalytic hydrogen reduction and deuterium exchange 527 Figure 4 presents the results of an experiment on tungsten at an elevated temperature where the cyclopentanone and the cyclopentanol formed from it were

in equilibrium after the lapse of a few minutes from the start of the reaction, and both compounds were then completely converted to a mixture of the hydrocarbons, cyclopentane and cyclopentene. After the decomposition of ketone and alcohol was virtually complete, the cyclopentene formed in the reaction was converted to cyclopentane, so that the final product of the whole reaction was the saturated hydrocarbon and water. 80

60

40

20

time (min)

FIGURE 4. The relative amounts of cyclopentanone (A), cyclopentanol (A), cyclopentane (EJ) and cyclopentene (V) during the reaction of 1-19 mn cyclopentanone and 17 -85 mm hydrogen on 18-3 mg tungsten film at 136-0 'C.

Figure 5 illustrates in detail hydrocarbon formation on a series of tungsten films at various temperatures. Cyclopentene formation increased with increasing temperature to a greater extent than cyclopentane formation. The formation of both

compounds was characterized by a sharp burst of reaction over the first few minutes at all temperatures and this was followed by a slower and decreasing rate of reaction. Similar curves were obtained for cyclopentane production over platinum,

palladium and rhodium. In the case of platinum the initial rapid burst of activity was very marked, while on palladium and rhodium films the effect was less well defined than on tungsten (cf. figure 3).

From plots such as these the rates of production of each hydrocarbon were obtained, tangents to the curves being taken at times after the initially rapid reaction had ceased and had been succeeded by the slowly decreasing rate. These rates were then converted to rates per 10 mg of catalyst and were plotted according

to the Arrhenius equation. Typical plots are shown in figure 6. The values of the activation energies and frequency factors defined by equation (2), where X was now the rate of removal of cyclopentanone and cyclopentanol to form a, particular


528 C. Kemball and C. T. H. Stoddart hydrocarbon in mol./s cm-2 of catalyst surface are given in table 2. The temperatures at which the rate of production of each hydrocarbon attained a value of

04 I %/min 1000 cm2 are also given and are a convenient measure of the re activities of the metal films for this reaction. 30 Y

0 -

+D

-41 ~ ~ ~ ~ ~ ~~1

O~~~~~

secod sybol epreen0 20loetee 40 60 802 100 85 C ,0 1 ga

89-8 ?C, 0,*, 20-2 mg at 104-5 ?C, O, V, 18-3 mg at 136-0 ?C.

I*5 10

-

-

4

a

20

bo 2 5 _ 20 _mn

23

5

Pd> Rh > W > Ni.

This order is quite different from that for the hydrogenation of the ketone and doe not follow the order of percentage d-character of the intermetallic bonds. The few results obtained for the hydrogenolysis of cyclohexanone agree with the same sequence of activity, and also the only metal in this series which showed any appreciable formation of propane from acetone was platinum. It is difficult to explain the exceptional capacity of platinum for inducing fission of the C-O bond, but it may perhaps be related to the ease with which the chemisorbed

oxygen, hydroxyl radicals or water molecules formed in the reaction can be removed from the surface. Exchange

The detailed kinetics of the exchange of cyclopentanone with deuterium on tungsten and nickel are similar to those found for the exchange of acetone. Figure 7 shows that results over tungsten obeyed equations (4) and (5), but that slightly curved lines were obtained for nickel films. Exactly similar behaviour was observed in the exchange of acetone (Kemball & Stoddart I957) and consequently the explanation already given applies to the present work. In brief, this explanation

suggested that on both metals the carbinol group of the alcohol, produced simuiltaneously with exchange, competes equally for the surface with the carbonyl group of the ketone, but that on nickel the carbinol group interferes to a greater

extent with the dissociative adsorption of the neighbouring C-H bonds than on tungsten which adsorbs hydrocarbons fairly strongly. The activation energy and frequency factor for the exchange of cyclopentanone over tungsten (the only metal over which results were obtainable over a sufficiently large range of temperature to find these parameters) are in good agreement with the corresponding values for the exchange of acetone on that metal. Likewise, the results over nickel and palladium films confirm the order of activity of the

metals already established in the exchange of acetone, i.e. Pd > Ni > W.


Catalytic hydrogen reduction and deuterirum exchange 537 The distribution of the isotopic species formed initially over palladium in the exchange of cyclopentanone is quite different from that observed in the exchange of

acetone. Both the intial distribution and the course of the exchange reaction indicate that four of the eight hydrogen atoms in the molecule are more easily

exchanged than the remaining four. By analogy with the exchange mechanisms

observedfor the cyclic hydrocarbons (Anderson & Kemball I954b) it might have bee expected that the four hydrogen atoms on the side of the ring nearer the surface in the adsorbed cyclopentanone molecule would have been easily exchanged, and that the remaining four hydrogen atoms would only have been exchanged after

desorption and readsorption of the molecule with the second side nearer the surface.

However, the exchange behaviour did not support this view because the d5 to d8compounds were formed slowly and not rapidly as would have been possible on this theory.

It seems clear, therefore, that we must consider the eight hydrogen atoms divided into two groups of four depending on their position relative to the carbonyl group,

i.e. the four on the a-methylene groups and the four on the f8-methylene groups. Since the a-set are similarly situated to the carbonyl group as the hydrogen atoms

in acetone, it was expected that this set was the more readily exchangeable. In order to check this hypothesis, the exchange of cyclohexanone was studied. The

interpretation of the mass spectra was difficult owing to the amount of reduction which occurred simultaneously with exchange, but it appeared that substantial amounts of the d4-, d5- and d6-compounds were iproduced initially as well as smaller

amounts of the more highly deuterated species up to and including the djo-ketone. These results indicate that for the C6-ketone there cannot be a substantial difference

in the reactivity of the four a-hydrogens compared with the four ,l- or the two y-hydrogens, and provide no evidence in favour of the suggested differences in the reactivity of the different hydrogens in the C5-ketone. More experimental work

with compounds labelled in specific places with deuterium will be necessary before the exchange of cyclopentanone is understood.

The high proportion of the d,-cyclopentanone formed initially over tungsten and nickel films (table 3) indicates that, as with acetone, a simple exchange mechanism involving the replacement of only one hydrogen atom during the lifetime of the molecule on the surface is the most important process operating. The results over tungsten are also similar to those found for acetone in that multiple exchange processes are also operating to a minor extent. There are differences, however, in

the type of multiple exchange occurring over nickel for the two ketones. With acetone, exchange of all six hydrogen atoms occurred whereas, with cyclopentanone

the exchange produces no products beyond the d4-compound initially. These differences must be associated with the influence of the geometry of the molecules but more information is required before the results are full understood.

One of us (C. T. H.S.) wishes to thank the Ministry of Education, Northern Ireland, for a research grant.

35

Vol.

246.

A.


538 C. Kemball and C. T.- H. Stoddart REFERENCES

Anderson, J. R. & Kemball, C. I954a Proc. Roy. Soc. A, 223, 361. Anderson, J. R. & Kemball, C. I954b Proc. Roy. Soc. A, 226, 472. Beeck, 0. 1950 Heterogeneous catalysis, p. 118; Disc. Faraday Soc., no. 8. Jenkins, G. T. & Rideal, E. K. 1955 J. Ghem. Soc. p. 2490. Kemball, C. I951 Proc. Roy. Soc. A, 207, 539. Kemball, C. I952 Proc. Roy. Soc. A, 214, 413. Kemball, C. I956 J. Chem. Soc. p. 735.

Kemball, C. & Stoddart, C. T. H. I957 Proc. Roy. Soc. A, 241, 208. Moss, R. L. & Kemball, C. I956 Nature, Lond. 178, 1069. Stoddart, C. T. H. & Kemball, C. I956 J. Colloid Sci. 11, 532.
