Oct., 1961
INTRINSIC ACIDITY OF
ALUMIN.4s UPON
and this difference becomes greater with decreasing hydrochloric acid concentration. If it is assumed that this difference is due mainly to the difference in the degree of complexing in the case of U(1V) and Pu(IV) and tha,t the actual solubility products for the two salts are about the same, the amount of hexachloro complex in 12 M HC1 would be about 40 times greater in the case of Pu(1V) than in the case of U(1V). This is in qualitative agreement with the spectrophotometric data presented here. In lithium chloride the degree of chloride com-
HYDROGEN-DEUTEKIUM EXCHANGE
1859
plexing of Yu(1V) is considerably less talian in hydrochloric acid. Thus in saturated lithium chloride (19.2 m) the same amount of chloro complexing (approximately 50% hexachloro) is ohserved as in 14 m HCl. Since for 19.2 m LiC1, u& = 1,100 and for 14 m HC1, ai = 380,19 the difference in chloro coniplexing in the two media cannot be explained by differences in mean ionic activity. (19) R . A . Robinson and R. M. Stokes, "Electrolyte Solutions," Butterworth Scientific Publications, London, 1955.
ALUMINA: CATALYST ANI) SUPPORT. XII.' THE EFFECT OF INTRINSIC _$CIDITY OF ALUMINAS UPON HYDROGEN-DEUTERIUM EXCHANGE BY HERMAN PINESAND JEANRAVOIRE~ Ipattcff Hagh Pressure and Catalytac Laboratory, Department of Chemistry, Northwestern Unaversrty, EiiutLston, 111. Receited Apral 1 7 , 1961
The hydrogen-deuterium exchange on ?-alumina has been studied. The catalytic effect of the aluminas was correlated with the intrinsic acidity of the aluminas. Alumina prepared from aluminum isopropoxide and having relatively strong acid sites is about 60 times a s active for the exchange reaction a s a n alumina catalyst prepared from sodium aluminate and containing less than 0.1% by weight of sodium. A mechanism for hydrogen-deuterium exchange is proposed.
Extensive studies carried out in our laboratory have revealed that alumina has intrinsic acidic properties, and that the strength of the acid sites depends upon the methods used for the preparation of the a l ~ m i n a . Alumina ~ prepared by hydrolysis of aluminum iwpropoxide catalyzed the isomerization of cyclohexene to methylcyclopentenes, whereas alumina prepared from potassium aluminate did not affect such isomerization. It also was demonstrated that aluminas of different intrinsic acidities exert a deep influence upon the isornerization of alkenes4and dehydration of alcohols5: they also influence the catalytic properties of molybdena-alumina,6 chromia-al~mina,~~~ and platinumaluminap catalysts. The hydrogen-deuterium exchange over yaluminas has been reported by Holm and Blue'" and by Weller and Hindin.ll These authors relatjed the effects of pretreatments on the activity of aluminas for hydrogen-deuterium exchange. More recently 'Taylor and Kohnl2 have shown that. ionizing radiat)ions enhance the activity of yalumina for the same reaction. The purpose of the present paper is to determine the effect of thc ( I ) For paper XI o f this series see C. N. Pillai and 13. Pines, J . .4m. Chem. Soc.. 8'3, 3274 (1961). (2) Postdoctoral Fellow S ~ I [ J I I I J Iby ' ~ ~a~ grant from the Petroleuni Research Fund administered hy the American Chemical Society. (;ratefill acknowledgment is hprehy made to the donors of said Fund. ( 3 ) H. Pines and W . 0. Haar, ./. A m . Chem. Soc., 82,2471 (1960). (4) W. 0. IIaag and H. Pines, ibid., 82, 2488 (19fiO). ( 5 ) H. Pines and (1. N. Pillai. zbid.. 81, 2401 (19tiO). ( 6 ) H. Pines and G. Benoy. i b i d . , 81, 2483 (1960). (7) C. T . Chen, W . 0 . Haar and H. Pines, Chem. and Ind. (London), 1374 (1959). ( 8 ) H. Pines and (2. T . Chen, J . Org. Chen., 16, 1057 (19Gl). (9) H.Pines and 'r. W. Greenlee, ibid., 26, 1052 (1961). (10) V. C . F. Holm and R . W. Blue, I n d . Eng. Chem., 43, 1506 (1951); 44, 107 (1952). (1 1) S. W. Weller m d S. G. Hindin, J. Phys. Chem., 60,1506 (195ti). (12) (a) E. H. Taylor and H. W. Kohn, J . A n . Chem. SOC..79, 252 (1957): (b) €1. W. Kohn and E. H. Taylor, J . Phys. Chem.. 63, 500
(1959).
intrinsic acidities of aluminas upon the rate of the exchange reaction between hydrogen and d a terium. Experimental 1. Apparatus and Procedure.-The
apparatus is similar
to the one described by Hindin and Wrller." In the present system a magnetic pump was used for circulating the gases. The pump, as described by Watson,13 consists mainly of a glass piston (containing an iron core) moving forward and backward in a glass tube thanks to two selenoids energised by a d.c. current, switched electronically. The gas is drawn in and expelled through four valves. The reactants before reaching the catalyst passed through a coil which was made of one meter long tubing and which surrounded the chamber containing the catalyst. The reactor was surrounded either by a bath or by a furnace. The gases were circulated a t a rate of about 40 cc. per second. A mixture of hydrogen and deuterium was admitted into the reactor by expansion of the gases from a tube in which a certain pressure has been established by means of a mercury bulb. All the volumes being known, the pressure in the reactor could be established in a fraction of a second. The volume of the reactor was 176 cc. After the desired time about 4 cc. of gas a t STP were withdrawn and analyzed by means of mass spectroscopy. Before each set of experiments the reac:t>or was tilled with helium to about 200 mm. of pressure and kept in contact with the catalyst for about 0.5 hour, in ordcr t,o bring the catalyst to the temperature of the bath or the furnarc surrounding the reactor. Helium t,hen was pumped oil1 . Electrolytic hydrogen was used, which was purified by passing it through a palladium cat>alyst,a liquid nitrogc'ri trap, and finally through charcoal kept at, liquid nitrogcii temperature. Deuterium and helium used wercs of w r y high purity. Each of the gases was purified further by passiig through activated charcoal maintained a t t,ho temperature of liquid nitrogen. An equimolal mixture of hydrogen and deuterium was used for the exchange studies. 2. Catalysts.-The catalysts were prepared according to the gcneral method described previously.3 Catal.yst A.-Aluminum isopropoxide was disti1lt.d twice and then hydrolyzed with distilled water. The precipitatf of aluxhurn hydroxide was washed, filtered, dried a t 120 for 7'2 hours, compressed and screened to obtain 8-10 mesh granules. Catalyst B.-Aluminum turnings made from hiqh purity (13) J. S.Watson, Can. J . Technol., 14, 373 (195tr).
HERMAN PIYES- 4 ~ 1JEAN 1 RAVOIRE
1860
Vol. 65
aluminum bar, 99.99% pure, were dissolved in a solution of sodium hydroxide. The solution was neutralized with nitric acid, and alumina precipitated by passing carbon dioxide into the solution. The precipitate was washed eight times m t h distilled water, dried, compressed and screened, ae indicated above. Catalyst C.--The preparation was the same as that for B, but the precipitate was washed four times instead of eight times. The catalystri were degassed and calcined under conditions specified in part C of Results. The areas of the calcined catalysts 8s detrrminect hy adso tions of nitrogen using the BET method were Eatalyst A, 304 m.2/g.; B, 280 m."g.; C 283 m z/g.
pressure (t = 2 min., final [HD] = 0.60). The same catalyst however, degased a t 300' for 15 hours and then at 520' for 2 hours had an activity given by k = 600 a t -95' (t = 30", final [HD] = 0.41). The difference in activity between catalyst A, which was prepared from aluminum isopropoxide and B and C, prepared from potassium aluminate is most striking. In the case of alumina A it was necessary to carry out the reaction a t -95' in order to have a measurable rate with 0.176 g. of the catalyst,; k was found to be equal t.o 600, as Results indicated ahove. Catalyst B and C were less actjive a. Definition of the Activity.-The activity of and therefore it was possible to measure the exthe catalyst is given by the rate of exchange which change reactions a t 20'. The rat,es of reaction IC, can be correlated with the rate of formation of the for catalysts B and C are given in Table I. HD molecules by the first-order law of isotopic exA comparison can he made between the a c t i ~ i t ~ y change reactions. l 4 This relation takes the follow- of cat(a1yst.A and catalysts R and C by accept,ing ing form in the case of the reaction H2 Dz k = 30 as an average value for catalysts R and C 2HD (neglecting the kinetic isotope effect) and by ext:apolating k to -%io, taking 1 kcal. as the apparent activation energy. It thus was found that for cntaiysts B and C k is equa! t,o 10 as comR = pared wit,h k = 600 for catalyst A. 2t N o sharp differences can be detected between in which R == rate of exchange; t = time; [HD] alumina R and C. The differences found are = concentra1,ion in HD a t the time t; [HDIeq = within an experimentd error, the degassing not, concentration in HD a t equilibrium. The [HD],, values a t different temperaturcs being a very reproducible operation. were calculated from the reported data.I5 It lias TABLE I been found more convenient to express the rate of ILATE O F EXCHANGE BETWEEN TIYDROGICX ?LND 1)WtITERIUM exchange in atom-grams of hydrogen per gram of OVER ALUMINA B AND C catalyst per minute multiplied by lo5
+ *
A k=R-105 W
in which .4 is the number of moles of gas in thc reactor; W is the weight of the catalyst. It has been noticed that the activity of a freshly activated catalyst, tested by successive identical experiments, decreased with time, and then became constant. The ratio of k calculated from the first experiment to the k corresponding to the "steady state" was about 3 for catalyst A a t -95'. At this temperature a time of contact of one miiiute was sufficient to reach a constant value of k. The deactivation effect decreased with increasing reaction temperature; a t 300' with catalyst C the deactivation effect was not observed. The activity which will be reported in this paper ~ v d lbe the "Steady State" activity. Once the steady stat,c if3 reached, the first-order law is always obeyed, which permits one to conclude that the circulation of the gas was not a rate-determining factor. b. Compmison of Catalytic Activity us. Intrinsic Acidity .-In line with observations reported by Weller and Hindin the activity of alumina catalysts depend upon the methods of pretreatment. It thus was found that catalyst A, which had been calcined a t 700' for 4 hours, placed in contact with air a t room temperature and then degassed at 280' for 16 hours a t 10-8 mm. pressure, had a relative activity for hydrogen-deuterium exchange equal to k = 1.4 a t 0' and p = 45 mm. (14) 0. E. Myers a n d R. J. Prestwood, "Radioactivity Applled to Chemmtry," ed. by A. C. Walil a n d N. A. Ronner. J o h n Wiley a n d Sons. Jnc., 1951, New York. N. Y . (15) H. C. Uiey and D. Rittenberg, J. Chem. PhUs.. 1, 137 (1833).
IIr. of degassing at 520", mm. Alumina €3: time, SCC. Final [HD] Ii Alumina, C: t.imc, rnin. Final [HD] 12
24 46 30 30 0.083 0.10!) 19 26 1 30 ser. I 3 0.238 0.257 0.340 0.456 18 40 32 29
TARLE I1 1)ETERUINATION O F TIlE ORDER O F ~ ~ Y l ~ R ~ ~ C . E N - ~ ~ l ~ ~ " l . l i : l t l l r M
EXCIIANGE Cntalyst
A
13
Temp,.,
"C.
-95 -95 -95 -95
30 30 30
I3
-23 -23 -23 -23 -23 -23 -23
Pres-
Time
miw,
(' min.) ("see.)
Pin:il
4.5 270 270 45
30" 16" I'
0.41 0.19 .40
3or
.40
2' 5' 10' 2' 7' 15' 5'
.19 .33 .43
nim.
lFIDl
k
no
600 1A90 1640 550
1 \
0.6
j
46 45 450
45 45 45 450 450 450 45
14 ' 13 14
.on
54
.27 .40
65 65
.30
11,
'
0.0
C
a
300 45 800 45 300 270 n = order of reaction.
c. Kinetic Data.-The ordcr of the exchange reaction between hydrogen and deuterium as a
ETHANE RADIOLYSIS 4~ VERYLow CONVERSION
Oct., 1961
function of prmsure of 4.5 to 450 and 270 mm. has been found to be in the range between 0.6 and 0.7, regardless of whether catalysts A or B and C are used (Table 11). The order with respect to the total preswre I S defined by kcup. The artivstion energy a t a pressure of 45 mm. had been fourid to be equal to 1.0 kcal./mole for alumina R between 25 and 100’ and to 1 3 kcal./ mole for alumina C between 26 and 300’. These values arc lower than thc 2 5 to 4.5 kcal.,/mole found by ll’eller and Hindin,’ for an alumina prepared from aluminum isopropoxide and corresponding to our alumina A. This difference agrees with the compensation effect, catalyst €3 and C being less actii-e. It should be noted however that our catalyst A is apparentlv more active than the one used by Hindin and ’VJellerll since we can evaluate from their reculte 0‘:~I)le111, R, identical to their k , = I$), k = 400 a t -78’ and 380 mm. pressure, assuming that thr weight of the catalyst is 0.2 g. This has to bel compared with k = 1640 a t -93’ and 270 mm. pressure in the present work. Discussion of Results The experirnenta! data show that there is a strong correlation iirtween the activity of alumina for hydrogen-deutenirm exchange and its intrinsic acidity. .4luniina .Iwhich has the strongest acidic sites IS GO tim.s as w t i w as aluminas R and C in which some of the acidic sites were neutralized by sodium ions (0.1% n.t. 9; of Na). It seems that Lewis acid sites are responsible for the catalytic action, It had been reported” that the exposure of the alumina to the action of traces of moisture lowers its catalytic activitj. It also was shown’ that exposure of alumina il. to the humidity of the
1861
atmosphere inhibits the development of color in the alumina to certain indicators like crystal violet leuko base or malachite green leuko base. ill1 these phenomena give support to a heterolytic dissociation of hydrogen and deuterium gas, whereby a hydride ion is abstracted by the T,e\vis acid sites of the alumina. The hydrogen4euteriuni exchange on alumina can be presented schematically using for the surface of dehydrated alumina a model suggested by Hindin and lVeller.*6 The hydrogen and deuterium molecules mould he dissociated heterolytically in a way for the hydride or deuteride to be adsorbed on the incompletely coordinated aluminum ions on the surface (Lewis acid sites), while the proton or deuteron will then be preferentially adsorbed on the oxygen. This can bc presented as 6+
usI
OS-
1
a+
-0-AI-0-AI-0-AI4-AI-0-
,‘i‘\
