of the labyrinth system. Secondly, imperfections in the lattice structure may also cause some tunnels to be shut off from the exterior. Thirdly, the above value of 40 ~ m . ~ / grefers . to an infinite crystal; grinding has the effect of converting internal into exteriial surface, on which the sorptioii of nitrogen a t -78" or of hydrogen a t - 183" would be entirely
negligible a t the pressures used in the present, experimeut,s. Finally, under the conditions used it is unlikely that saturation even of the pore system lvas ever achieved. The authors wish to express their thanks to the Defence Research Board of Canada for generous financial support (Grant No. 356).
HOMOGEYEOUS CATALYTIC HYDROGENATION. 111. ACTlVATION OF HYDROGEN BY CUPROUS AND SILVER ACETATES I N PYRIDINE AND DODECYLAMINE BY LEONWRIGHT,SOLWELLERA T D G . A. MILLS ~ y 0 t i l r i 6 1 i l i from 0~~
tho l € o u d r y Process Co).pomtc'oti, Linuwod, Pennsylvania Received M a y 8 , 1965
Moleciilar hydrogen is activated by cuprous and silver acetate dissolved in pyridine or dodecylemine. Kinetic ineasurements were made a t 78 and 100" for the hvdrogenation of silver and cupric acetate in these solvents. The rate of hydrogenation is expressed by -dH/dt = k p ~ [, M I \ , where [W]represents the concentration of silver or cuprous acetate. The rate of hydrogenation of cupric2 acetate is approximately independent of the concentration of cupric acetate. Molecular weight studies of pyridine solutions of silver and cuprous acetates a t 115", considered in relation to the first order kinetics, demonstrate that the rate-determining step involves the reaction of hydrogen with a metal ion monomer. Possible structures for the metal-hydrogen complex and modes of hydrogen-hydrogen bond scission are discussed. Deuterium exchanges with a hydropen donor in the cuprous acetate-pyridine system in a manner analogous to that previously found in quinoline. No exchange is observed during the reduction of silver acetate in pyridine by deuterium. The reduction of silver acetat,e by deuterium is 30% slower than that by hydrogen.
Introduction The activation of molecular hydrogen i n homogeneous solutions offers an opportunity to define the molecular st,ructure of the catdyst, stlid of the catalyst-hydrogen complex.. A solution of cuprous acet,ate in quinoline is capalde of activating molecular hydrogen for the reductlion of p-benzoquinone and ciipric acetate. La,1h Some work also has hecii reported on the c,liaiige i n cat8alytjicact'ivity as effected 1)y irariatioiis i l l catalyst solveiit for this system.'b The preseiit, paper iiicludes a kinetic study of the catalyzed hydrogenation of cupric acetate by pyridine and dodecylamiiie solutions of cuprous acetate. Since Wilmarth2 reported that quinoline solutions of silver acetate absorbed hydrogen rapidly a t loo", wit'h the formation of silver metal, a kinetic study also was made of the activation of hydrogen by pyridine solutions of silver acetate. These experiments were carried out a t 78 or loo", a hydrogen pressure of 615 mm. and cuprous or silver acetate concentrations from 0.01 to 0.18 molar. Molecular weight determinations, by the method of boiling point elevation, were made for silver and cuprous acetates i n pyridine a t 116". Finally, a few isot'opic rate a i d exchange esperiineiits were made with deuterium as a tracer. Apparatus and Materials The general experimental procedure has becn previously described.lb The cuprous acetate'b contained 51.7% Cu (theor. = 51.6%). C.P. cupric acetate monohydrate was obtained from the J. T. Baker Co. Silver acetate was obtained from the General Chemical Division, Allied Chemical and Dye Corp., as an anhydrous white salt, containing 62.4% _
_
_
~
(1) (a) R I . Calvin. J .
.4m. Chcm. Soc.. 61, 2230 (1939). ( h ) 8. Weller a n d G . .4. hlills. { h i d . , 75, 7139 (1953); L. W. \+'riglit a n d H. b'eller, ibid.. 76, 3345 ( l 9 X ) , iinpera I a n d I1 in this scrim. (2) W.Iion reaction a t 78'; the specific rate constant is within 10-2070 of that obtained with pyridine under similar conditions. An aqueous 0.05 molar solution of silver acetate is not measurably reduced by hydrogen a t loo', although the color of the solution does change from water white to cloudy gray on charging the evacuated reactor with hydrogen. A saturated solution in glacial acetic acid is also not reduced by hydrogen over a one-hour period a t 78". If a solvated silver ion alone is responsible for the observed activation of hydrogen, any soluble monovalent silver salt should, in principle, be capable of activating hydrogen. Although silver chloride is soluble in pyridine, it was found that a 0.05 molar solution does not measurably absorh hydrogen over a two-hour period a t 100". This indicates that the activation of hydrogen by silver acetate is, in some way, related to the nat>ureof the bonding between the silver and acetate ions. The observation that the rate of silver acetat8e reduction decreases with increasing reaction time, Fig. 1, implies that silver nietnl is not a catalyst for the reduction of silver acetate. This point, was further demonstrated by reducing, consecutively, two millimole charges of silver acetate in the same pyridine solution. Within 5-10%, the reduction rat,es were identical. This also demonstrates that the small amount of acet'ic acid produced (2 mmoles) during t,he first reduction does not seriously affect the reduction of the second cha,rge of silver acetate. Hydrogenation of Cupric Acetate.-Typical curves for the hydrogenation of cupric acetate monohydrate, catalyzed by cuprous acetate in pyridine, are shown in Fig. 2 . Similar results are obtained in dodecylamine solution. The reaction is autocatalytic, the induction period almost disappearing when sufficient cuprous acetate is added
initially. In all cases the quantity of hydrogen absorbed is close to the theoretical value for the reduction of cupric acetate to cuprous acetate. After reduction to the cuprous state is complete, further reduction to metallic copper seems to occur very slowly, if at all; this is shoivii by the flatness of the final portions of the hydrogen absorption curves aiid by the absence of metallic copper on filtration of the solution a t the end of an experiment.
increases with increasing time almost up to the point of complete reduction of cupric acetate to cuprous acetate. The individual points in Fig. 3 represent instantaneous rates, calculated from the curves for five experiments iu pyridine solution, as a function of the total cuprous acetate present a t the corresponding time. (Five experiments in dodecylamine solution, covering a range of cuprous acetate concentration up to 0.14 Ad, showed the same linear dependence of rate on cuprous acetate concentration.) The total cuprous acetate was taken as the slim of the cuprous acetate initially added and that produced by reduction of the cupric salt. Inspection of Figs. 1 and 3 shows that the rate of reduction of both silver and cupric acetates by hydrogen in these studies is expressible in the form Rate = k p ~ , [ M ~ ]
2 5 8 MILLIMOLES CuOA
2 32 MILLIMOLES CuOAc
TIME, MIN.
Fig. 2.-Reduction of cupric acetate in ppridine a t 100'; effect of cupric acetate coilcentration.
A study was made of the effect of hydrogen pressure and cupric acetate concentration on the rate of reduction a t 100". Within the limits of experimental error, the initial rate of reduction, expressed as ml. (STP) hydrogen absorbed per unit time, was linearly depeiideii t on hydrogen pressure over t,he range 37-282 mm. for pyridine solutions, and over the range 290-750 mm. for dodecylamine solutions. The reduction is, therefore, first order in hydrogen pressure. As Fig. 2 s h o w , at. fixed cuprous acetate concentration, the same initial rate was observed for 0.05 and 0.10 JI cupric acetate in pyridine. (The rate was actually lower for the highest cupric acetate concentration, 0.20 A I ; this may result from an inhibition by water, added iiidirectly as part of the cupric acetate monohydrate.) Three similar experiments ill tlodecylamiiie solution, a t cupric acetate concentrations of 0.025, 0.050 and 0.075 Ad, showed the same independelice of rate 011 cupric acetate concentration. This result is qualitatively different from that of Dakers and Halpern, mho found the rate of reduction of aqueous cupric acetate solutions, a t 100" and 190 p.s.i. hydrogen pressure, to be proportional to the cupric c~ncentration.~ The curves in Fig. 2 show that the instantaneous rate of hydrogenation, which depends on the instantaneous concentration of cuprous acetate, (3) R . G. Dakers and J. Halpern, Can. J . Chem., 32, 969 (1954).
where [MI] represents the total concentration of silver or cuprous acetate. Molecular Weight Studies.-The first-order dependence of the hydrogenation rate on the total concentration of silver or cuprous acetate can be interpreted on the basis either that the catalytically active species is a monomer, or that it is a dimer, provided, in the latter case, that the dimerization constant js so high that substantially all of the catalyst molecules are present as dimers. In order to resolve this problem, the extent of dimerization in pyridine was studied by determining the apparent molecular weights of cuprous acetate and of silver acetate in pyridine a t 115" by the boiling point elevation method. The results of this study are given in Fig. 4, in which the apparent molecular weights are plotted as a function of the salt concentration. Extrapolation of these data yields an apparent molecular weight a t infinite dilution of 122 for cuprous acetate and 162 for silver acetate, in reasonable agreement with the monomer formula weights of 123 and 168, respectively. The increase in apparent molecular weight with increasing concentration (Fig. 4) may be interpreted as resulting from formation of a dimer or more highly associated ~ o m p l e x . ~The computed dimerization constant is quite small, however, and it varies systematically with concentration; for cuprous acetate it increases from a value of 0.5 mole-1 1. at 0.1 M to 1.2 mole-1 1. a t 0.G iif. This drift in the dimerization constant may be attributed to the increasing formation of nmers (n > 2) at higher coiicentration~.~ It is clear from Fig. 4 that over the concentration range in which kinetic studies were made (up to 0.18 If), the extent of dimerization of cuprous acetate does not exceed 15010, and that of silver acetate does not exceed 25%, Taken in conjunction with the results of the kinetic studies, this means that the catalytic species for both salts in pyridine is the monomer. It was found impractical to study molecular weights in dodecylamine a t temperatures near 100" because of the low vapor pressure of this solvent. As a result, although the kinetic behavior for cupric acetate reduction was similar for solutions in dodecylamine and in pyridine, it was not possible (4) Cf.B. M. Batson and C. A. Kraus, J . Am. Chem. SOC.,S6, 2017 (1934).
0
0.04 0.08 0.12 0.I6 TOTAL CUPROUS ACETATE ,MOLES / L.
0.20
Fig, ~3,-lted11ctioii of cupi,ic acetate; inst~niitniieo~is into us. t o t d cr~prous:acet,nt,ec.oric~ent,i'atiot~ pyi,idiiie, 100'.
t o I-ule out tl)e possibility that iii dodecylamine most, of the cuproim acetate is dimerized and that the dimer is the catalyt'ically active species. Exchange and Isotopic Rate Studies.--When deuterium iis actijrated by the cupi-ous acet.at.equinoline system, isotopic exchange occurs with a hydrogen do no^ in solution.lb h similar exchange rem t,ioii is ohserved when solnt,ioiiii of cuprous wetjute iii tiither pyridine or tlodecylamine are t,reatetl ]\.it11 deuterium at 100". The "halfttimc" for escaliiuige in pyridine solution is approsimutely one-lidf hour. By contrast, 710 exchange OCCIII'S either chiring: or after the redwtiou of silver acetizte iii pyritliiie by either pure deut,erium or a hyrli,oaeii-deuteriiini mixture a t 78". The failure of exchange t,o occur in the presence of metallic is coiwist,eiit8with the inability of tthe metal to cnt8:i,ljrzet8he reduction of sillrer acetate (see : h i x i ) . The iioti-occurreiice of exchange during the i*erliictioti, irhen hydrogen is cei,taitily being tlissorint,ed, results from the fact that the hydrogen, once wt,ivatcd, is rapidly remoIw1 h y t,he irre\w*sitile ieaci ion witli silver acetatme1)efoi.e it can i*etriiwto the gas phme (see ref. l h ) . The silver acetate-pyridirie sy from the cuprous nc.et,nt,e-quiiioliiie s>-stcin iii tJhat' the i*etluctioiiof silISer n,cetnt8en,t '78" tleiiteriiini proceeds at :t rate that is 30y0 slower than the t)y llydrogell' I t preT7ious1ybeet' foutitl t h n t the rate of quinone redriction, catalyzed t)y ( ' u p r o w ncetnte iti qiiitiolitie~is t'he same uit'lliii 5-1O(L/, \vitJi tleiit,ei*iiimns \\.it11 Iiytliq?;eii.I1' Discussion The romhiiied itsiilts of the kinetic and niolecu1ar weight, stirdies i.eportcr1 hew tleinoiist,ixtp that, sil\rei*
a t least over the coiiceiit'ratioti imige st~utlietl,t'he molecule ~esponsiblefor the activation of Iiycl~*ogeii is iiot a dimer of silver 01' cuprous acet,ate i i i pyrkliiie solut,ion. This conclusion is coiitlwy t#o that' 2,0
0
$ 190
< ~3
4 B
$
,-
@
1 1 ~ 7
CONCENTRATION, MOLES /L.
Fig. 4.-.4pp:ll,eiit moleculny weight 1)s. collcelltrntioll of silvev acetate anid criproiis ncetnte.
previously o1)tainetl for cuprous acetnt'e iii qiiinoliiie.la,lb There is no qiiestioii about) the validity of the earlier results; a noli-first-order &e law for the act,ivat>iotiof hyclrogen hy ciiprous acet8atein rliiiiioliiie has beeii iiidepentletit,ly estaldisiied both
10G4
Vol. 59
LEONWRIGHT,SOLWVPLLER AND B. A. MILLS
by Weller and R$illslb and by Wilmarth and B a r ~ h . ~ It then hecomes pertinent to examine the One can only speculate on the reason for the dif- energetics for, and the detailed nature of, the ference between the quinoline and the pyridine splitting of the hydrogen molecules in such systems. systems, It seems probable that the difference Even when the catalyst for hydrogen activation is is related to the different size of the solvent mole- a monomeric species, it is necessary, for reasons of cules. Construction of scale models shows that energetics, that both fragments of the hydrogen for cuprous acetate in pyridine solution, for es- molecule be able to simultaneously form bonds (or a,mple, there is no steric hindrance to the forma- the equivnlent) of considerable stability. For tion of a tetrahedral, four-coordinated monomer of example, Wilmarth's postulated mechanism of the formula CuPy30Ac, Py representing a pyridine hydroxyl ion-catalyzed exchange,5 which involves molecule; sp3 hybrid orbitals would be involved in the transitory formation of a hydride ion, is conthe formation of this complex. In quinoline, ceivable only for a solvent of high dielectric conhowever, a solvated, monomeric molecule of stant, since only in this case will the solvation formula CuQsOAc is sterically much less favorable, energy of the hydride ion be large enough to make as is shown by the interference between the co- the energetics reasonable. ordinated quinoline molecules in a scale model. In the case of cuprous and silver acetates in -4s a result, a three-coordinated dimeric molecule pyridine, both a heterolytic split of hydrogen (into may be preferentially formed in quinoline solu- a proton and a hydride ion) and a homolytic split tion. l b Certainly the apparent dimerization con- (into two hydrogen atoms) should be considered as stant for cuprous acetate in quinoline (-11 mole-' possible rate-determining steps. If two atoms are 1. a t 100') is 10-20 times greater than it is in formed, both may become attached to the metal pyridine. ion; although no metal salt of this nature is known It has become clear during the past two years to exist, there is some evidence that this may be that the homogeneous activation of hydrogen does the case for iron hydrocarbonyl, H2Fe(C0)4. not require the intervention of a dimeric (or Alternately, one hydrogen atom may go to the higher) metal species. Wilmarth and his co- metal ion and one to a pyridine molecule with the w o r k e r ~have ~ ~ ~described base-catalyzed exchanges formation of a radical. If a split into a hydride in which no metal a t all is involved. In other ion and a proton occurs, the hydride ion might cases, such as the systems described in this paper, react with, for example, the silver ion to form a apparently only a monomeric metal species partici- solvated silver hydride molecule, and the proton pates. In this group also fall the solutions of might react with pyridine to form pyridinium ion. cupric acetate3 and mercuric acetate*,g in water, It is worth noting that for the reaction which Halpern and his eo-workers have shown to Ag+ (8s) Hdg) +& W a s ) H*(aq) react homogeneously with hydrogen a t elevated AHo is in the neighborhood of +30-35 kcal. (The pressures. In view of Halpern's results, it also heat of hydration of gaseous silver hydride is seems likely that much of Ipatieff's early work on estimated to be about -10 kcal./mole.) Since the reduction of aqueous solutions of met'al salts by the over-all activation energy must a t least equal hydrogen under pressure will also fall in this class.l0 the (endothermic) heat of the rate-determining ( 5 ) W. K. Wilmarth and M. K. Barsh, J . Am. Chem. Soc., 75, 2237 step, this high AHo value is consistent with the (1953). failure of aqueous silver acetate to be reduced by (6) W. K. Wilmartli, J. C. Dayton a n d J. ill. Flournoy, ibid., 75, hydrogen. The corresponding AHo value for 4549 (1953). pyridine solutions cannot be computed because of ( 7 ) W. K. Wilmarth and J. C. Dayton, ibid., 75, 3353 (1953). ( 8 ) J. Halpern, G. Kounek and E. Petam, Research, I, SG1 (1954). inadequate data, but qualitative considerations of (9) E. Peters and J. Hslpern. personal ~ o ~ n ~ n u n i c n t i o n . the difference in heats of solvation of silver and (10) For a surnniary, see V. N. Ipatieff, "Earlier Work on Hydrohydrogen ion indicate that the value might be 5-10 genation a t High Temperatures and Pressures." i n Dunstan, "The kcal. lon,er than iii water. Science of Petroleiiiii," Vol. ITI, Oxford, E n g l n n d , 1938, 1). 2133.
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