Aug ., 1956
THEISOTOPIC EXCHANGE BETWEEN DEUTERIUM AND HYDROCARBONS
1043
sulfates, perchlorates, nitrates or hydroxides beTABLE XI come reversible in chloride solutions. The chloTRANSIENT ENNOBLING BY CHLORIDEOR THIOCYANATE ride ion was assumed to provide a favorable bridge IONS Total increments Z ( A E ) , mv.
Inhibitor
2 . 4 x 10-3fc14.2 X 1 0 - a f S C N 2 . 2 X 10-3fSCNTCOd-, 4 . 7 X f 1 . 2 X 10-af c11 . 6 X 1 0 - a jC13 . 1 x 10-4fciMOO4-, 10-f 8.0 x 10-4jc144 3.1 X 10-4fSCN76 9 . 4 X 10-4fSCN6 . 2 x 10-4fc1MOO4', 10-4f 15" WO,', lo-* f 22d 1 . 0 x 10-*fC153,50,60 2 . 0 X 1 0 + f C1WOr', 10-8 f 2 0 , i i , i ~ 2 . 5 x 10-3fc129 5 . 0 X 10-afC1Cr04-, 10-3 f 4, 6 2.5 x 10-3jcia Electrode pre-stabilized overnight. Electrode not entirely stabilized. Subsequently debased. d Potential fell a t one point but recovered. TCOl-, 1
x
Concn., f
10-3f
125 35a.d 68b 23, 28 25, 15 3, 6 4) 10
cording to an adsorption isotherm. This effect was attributed to a decrease in the activation energy for the transfer of electrons between the ions as a result of adsorption of chloride ions. I n the case of iron passivated by nitric acid, Vetter'g showed that the corrosion-equivalent current increases approximately linearly upon addition of f. chloride ions in concentrations of 1 to 5 X It was also shown by Heyrovskylg that a number of cathodic reductions a t the mercury electrode that are "oscillographically irreversible" in solutions of (18) K. J. Vetter, Z . Eleklrochem., 68, 274 (1951). (19) J. Heyrovsky, Far. Sac. Disc.,1, 212 (1947).
.
for transfer of electrons by virtue of its polarizability. It was suggested8 previously that, according to the hypothesis of electrostatic polarization by inhibitors, adsorption of chloride ions should lead to activation of electrons a t the interface so that cathodic processes would be stimulated. This effect will be considered in more detail in a subsequent paper, but it may be pointed out that the ennobling observed is in complete accordance with the hypothesis. Since both the chloride and thiocyanate ions accelerate corrosion, the ennobling cannot arise from an increase in anodic polarization. Without needing to known specifically which of several possible interfacial processes determines the observed potential, it may be seen that preferential stimulation of any cathodic process would be associated with a rise of potential. That the observed ennobling is associated with readily reversible effects was shown by the rapid loss of excess nobility when the electrolyte was removed and replaced by solution containing no chloride or thiocyanate. Thus, both the inhibitor ions and the chloride or thiocyanate ions are involved in labile adsorption processes by which the corrosion rate and potential are altered. The results indicate that all four of the inhibitors investigated act a t least in large part by an adsorption mechanism, under conditions of aeration. The possibility of blocking of anodes by precipitation of insoluble salts may be significant with the tungstate and molybdate inhibitors in absence of air.
ISOTOPIC EXCHANGE BETWEEN DEUTERIUM AND HYDROCARBONS ON NICKELSILICA CATALYSTS BY ROBERTL. BURWELL, JR.,AND RICHARD H. TUXWORTH Contribution from the Department of Chemistry, Northwestern University, Evanston, Illinois Received January SO, 1066
The effect of temperature, of the ratio of deuterium to hydrocarbon, and of nickel crystallite size upon the isotopic exchange reaction between hydrocarbon and deuterium is reported for nickel-silica catalysts which have been characterized by the magnetic method of Selwood. Cyclohexane and, to a lesser extent, heptane, cyclopentane and (+)3-methylhexane have been employed as test hydrocarbons. Increasing multiple exchange of the hydrocarbon results from increased temperature, decreased ratio of deuterium to hydrocarbon, and increased crystallite size. I n large measure, the increased multiple exchange probably results from increased availability of the free surface sites which are required for the propagation reaction leading to multiple exchange. At 60-15O0, the concentrations of the exchanged species of ,cyclohexane exhibit a and ~ CEHSD~.At higher temperatures no such discontinuity is apparent. The propagation discontinuity between C B H E D reaction does not pass readily from one set of six hydrogen atoms to the other a t lower temperatures. Cyclopentane behaves similarly. In the hydrogenation of l-hexene with deuterium a t about looo, decrease in partial pressure of olefin results in increasing multiple introduction of deuterium, presumably owing to the increased availability of free sites with the decreased partial pressure of the strongly adsorbed olefin.
In a study of the mechanism of interaction of hydrogen and hydrocarbons, we investigated the isotopic exchange between alkanes and deuterium on a nickel-kieselguhr catalyst, on reduced nickel oxide2 and on evaporated nickel films.2 The latter two (1) R. L.Burwell, Jr., and W. 8. Briggs, J . A n . Chcm. Soc., '74,5096 (1952). (2) H.C. Rowlinson, R. L. Burwell, Jr., and R. H. Tuxworth, THIS JOURNAL, 69, 225 (1955).
catalysts exhibited rather similar behavior in the temperature range investigated, 160-200°. The nickel-kieselguhr catalyst, which was studied a t temperatures in the vicinity of lOO", diverged in certain details from this behavior. It appeared desirable to determine whether the origin of these differences lay in differences in temperature or in differences in catalytic characteristics or in both. We have also investigated the effect of variation
1044
ROBERT L. BURWELL, JR., AND RICHARD H. TUXWORTH
in the ratio of the partial pressures of deuterium and hydrocarbon since, in our previous work, this ratio had always been about 2. Selwood, Phillips and Adler3 have recently developed a magnetic method for the determination of particle size distributions of the crystallites in nickel catalysts. We have used the opportunity provided by this work to determine how variation in particle size of the nickel in nickel catalysts affects the exchange reaction. Variation in isotopic distribution patterns of a variety of hydrocarbons is likely t o be of greater utility in studying variations among catalysts than mere determination of the relative rates of some particular reaction. Experimental Procedure Analytical procedures and hydrocarbon preparation have been described.2 We are indebted to S. F. Adler for the nickel-silica catalysts which were prepared by mixing solutions of sodium silicate and high purity nickel nitrate."S The resulting gel was dried a t 105", powdered, pelletted and crushed. Material of 20-40 mesh was used. The material contained about 42% nickel. Reduction regimens were chosen to yield catalysts of known characteristics.6 Reduction was in hydrogen for 48 hours a t 350' save where otherwise stated. When indicated, the catalyst was then sintered in helium for one hour a t 600". Catalytic activity would sometimes decline with use. It was usually possible t o restore original behavior by retreating the catalyst with hydrogen a t 350". The feed unit of the apparatus, shown in Fig. 1, permits the immediate establishment of any desired feed rates of ceuterium and hydrocarbon, it insures the feeding of an oxvgen-free hydrocarbon and it permits changing the type of hydrocarbon being fed without the danger of oxygen introduction.
Fig. 1.-Schematic
diagram of the feed apparatus.
The liquid hydrocarbon feed rate is determined by the stroke and by the reciprocation frequency of the piston at E which operates in a cylinder of inch precision bore tubing. The lower portion of the piston is of stainless steel and of a diameter slightly smaller than that of the cylinder. This portion is surmounted by a portion of reduced diameter over which was forced a piece of Kel-F (polytrifluorochloroethylene) which was then machined to a close fit to the cylinder. Enough mercury was placed above the piston to extend into the 1 mm. capillary joined to the top of the precision bore tubing. The piston moved readily in the cylinder yet would hold pressures in excess of 1/4 atmosphere without mercury being forced into the annular space between piston and cylinder. The stroke of the piston is deter.-
(3) P. W. Selwood, T. R. Pliillips and S. Adler, J . A m . Chem. S O ~ . , 76, 2281 (1954). (4) J. J. B . van Eijk van Voorthuijsen and P. Fransen, Rec. Irau. c h i n . , 70, 793 (1951), catalyst CLA-5421. (5) P. W. Selwood, S. Adler and T. R . Pliilli~s,J . A m . Chem. Soc., 77. 1462 (1955).
Vol. 60
mined by the hole in the lever arm F into which the pivot bearing bolt is inserted. The lever arm is actuated by the cam G (an Archimedean spiral) rotated by a 3 r.p.m. motor through a Metron variable ratio speed changer (4to 1 to 1 t o 4). Valves C and D were made by constricting a section of 3 mm. capillary to somewhat less than 1 mm. A section of drill rod machined a t one end into a 60" cone was ground into the constricted portion with the use of aqueous suspensions of abrasives. This operation was repeated with three new sections of drill rod and the grinding was terminated with the use of polishing alumina. The drill rod was then cut off about 6 mm. above the ground cone. The rate of liquid feed is measured by volume decrement in the buret H which is made of precision bore tubing of a cross-sectional area of 0.3195 sq. em. Hydrocarbon in the storage bulb K and that in H are deaerated by a stream of hydrogen introduced at the upper right. In operation, as the piston E moves up, mercury rises in the capillary tube over the cylinder. Displaced hydrocarbon flows out through valve C and enters the evaporator tube A a t B. The evaporator tube, of 10 mm. inside diameter, encloses a closely fitting Nichrome helix, one section of which touches the entrance a t B. The hydrocarbon flows down along the interface between helix and tubing. The evaporator tube is warmed by a winding of Nichrome ribbon. Deuterium gas is fed to the bottom of the evaporator tube through a flow meter. The current in the Nichrome ribbon is adjusted so that the hydrocarbon thread penetrates about five turns of the helix. Very smooth evaporation results. When the cam G comes to the step, the lever arm F falls back abruptly about 4 mm. closing valve C, opening D and sucking in a unit of hydrocarbon from H . Disturbance to the composition of the mixture of deuterium and hydrocarbon vapor issuing from the top of the evaporator is small. The hydrocarbon thread retreats slightly momentarily and then resumes it usual position. The vapor mixture leaving the evaporator is passed over the catalyst and then through a Dry Ice trap in which the hydrocarbon is recovered for analysis.
Results The experimental conditions, the % of the hydrocarbon molecules which suffer isotopic exchange and the % of the total hydrogen in the hydrocarbon which undergoes isotopic exchange are shown in Table I. The original data for these and other runs are available in the doctoral thesis of R. H. Tuxworth, Northwestern University, 1955. To avoid smearing the distribution pattern as a result of isotopic dilution of the deuterium we have worked a t low total conversions. This results in low accuracy for the singly exchanged species since is large comthe correction for the species C612C13H12 pared to the contribution of C 6 1 2 H l l D . The effect of temperature upon isotopic exchange between cyclohexane and deuterium is shown in Fig. 2. Arbitrary multiplying factors have been used to facilitate presentation. Cyclopentane also exhibits a discontinuity a t half-deuteration a t lower temperatures. For example, a t 68" (run E), the concentrations of the exchanged species in per cent. are: D,, 0.59; Dz, 0.97; D3, 0.62; D4, 0.37; D6, 0.27; De, 0.09; D,, 0.07; Ds, 0.04; D9, 0.03 and D,o, 0.02. At 150' and a t 200' on evaporated nickel film2and a t 172' on a sintered and rather inactive nickel-silica catalyst, the isotopic distribution patterns of cyclopentane exhibit a minimum between Dq and Da and a maximum at Dlo. Multiple exchange also increases with temperature with heptane, but, with the one catalyst investigated, sintered nickel-silica, the effect is less marked than with cyclohexane.
.
Aug., 195G
THEISOTOPIC EXCHANGE BETWEEN DEUTERIUM A N D HYDROCARBONS
10-25
TABLE I ISOTOPIC EXCHANGE RUNS Run
Cat.,a
cc.
Hydrocarbon b
Temp., OC.
x lo', mole/hr.
LHC
x lo', mole/hr.
LDz
Exchange, 0
D,d
%
%
1.05 4.0 A 60 1.75 1.77 0.5A cH 1.6 0.46 B 1.77 100 1.72 2.5B cH 1.8 0.77 1.77 159 1.82 0.5B cH C 4.3 2.78 2.62 187 1.44 D 2.OB cH 0.83 3.0 4.66 68 2.10 E CP .2A 1.32 1.31 2.7 145 2.59 F .5B S 1.19 4.66 2.7 145 0.69 G .5B S 0.66 1.6 1.77 159 3.57 H .5B cH 1.8 4.66 0.47 159 0.89 .5B cH I 2.12 2.66 156 2.32 3.0 J 2.OE S 4.68 0.52 1.2 139 1.80 0.5B K cH 1.21 4.66 2.7 124 1.74 L 1.OB cH 3.93 1.37 3.3 131 2.44 M 0.5A cH 2.80 2.56 5.8 N 96 2.08 2.OF S 2.38 6.1 2.68 86 0.75 2.OG 0 S 0.99 2.7 4.64 2.5B P 99 0.71 S 0.89 4.0 96 1.74 1.06 0.5D S Q 2.12 2.62 5.4 2.OA R 93 1.43 3MH 5.01 11.8 4.35 3MH 0.5C 141 2.75 S B is A sintered in helium for 1 hr. at 600'. C is like A but with reduction a A is nickel-silica reduced at 350' for 48 hr. a t 250'. D is like C but with reduction for 15 hr. E is nickel oxide reduced a t 300" (ref. 2). F is Harshaw nickel-kieselG is Universal Oil Products Co. nickel-kieselguhr reduced a t 300'. Run 0 is from ref. 1. * cH guhr reduced at 325-350 70of total hydrocarbon molecules which is cyclohexane CPis cyclopentane, S is heptane, 3MH is (+)3-methylhexane. % of total hydrogen atoms in hydrocarbon which have been replaced by D. have suffered ikotopic exchange.
.
At 307" with inactive nickel catalyst prepared by impregnation, cyclohexane desorbed as deuterated benzenes. There was little if any deuterated cyclohexane. Such a result would have been thermodynamically impossible a t the lower temperatures of the reactions on the other catalysts. At 277", cyclopentane yielded deuterocyclopentanes characterized by a continuous increase in concentration from D1 to Dlo. At about 150", increase in p ~ J p (the ~ c ratio of the partial pressures of deuterium and hydrocarbon) a t a total pressure of one atmosphere changes the shape of the exchange distribution pattern so as to favor a somewhat larger proportion of the very extensively exchanged species. At lower temperatures, the effect of change in PD*/PHC is smaller. Curves F (PDJPHC = 0.5) and G (6.8) in Fig. 3 exhibit this effect for heptane at 145". An intermediate pattern results for PD,/PHC = 1.0 (run C). At 99", the exchange pattern of heptane is nearly independent of PD~/PHC. Curves H and I (0.5 and 5.2) exhibit the effect for cyclohexane a t 159". Curve B in Fig. 2 for cyclohexane was obtained a t 100" with PD~/PHC = 1.0. Proceeding from a ratio of 5.0. to 0.5, relative increases in Dll and Dlz and relative losses in Dz to D4 resulted. A similar but smaller effect was observed a t 60"; PD~/PHC also affects %D8/%D7. At loo", as ~ D ? / ~ H exceeds C 0.7, %?6/%D7 is progressively greater than unity and vzce versa as the ratio falls below 0.7. Figures 4 and 5 exhibit exchange distribution patterns on several catalysts which have been characterized magnetically3 as to size of nickel crystallites. Reproducibility in rates with different samples of catalyst prepared by reduction a t 250" or by sintering operations was only fair.
0.4 0.2
0 '
0.2 0
4 6 8 10 12 No. of deuterium atoms. Fig. 2.-The effect of temperature on exchange of cyclohexane. The curve a t -34' is on evaporated nickel film from ref. 6.
2
The sintered catalyst (catalyst B) gives rates of exchange of cyclohexane which are 0.14 to 0.4 those of the unsintered (catalyst A). Sintering reduces the activity for the exchange reaction much less than for the hydrogenation of benzene.6 Catalyst reduced at 250" is 2 to 4 times more active than
ROBERT L. BURWELL, JR.,AND RICHARD H. TTJXWORTH
1046
Vol. 60
TABLE I1 Run
Hydrocarbonc Temp., "C. LCeH12, moles/hr.d LDZ,moles/hr. % D in product' % D in exit hydrogen'
HYDROGENATION OF ~-HEXENE WITH DEUTERIUM Ta
Ua
v4
1-CiJL 105 0.0123 0.0425 22 78 4.9 15.2 23.2 22.0 15.7 9.3 4.9 2.3 1.2 0.6 .4 .2 .1
I-CaHia 5 CH 104 0.00234 0.0422 28 94.3 1.4 6.1 16.3 21.3 21.0 16.8 8.0 4.5 2.2 1.1 0.9 .4
l-CaH12 30 CH 105 0,00058 0.0422 33 98.2 1.8 2.8 9.4 15.7 19.6 21.4 12.1 8.7 3.8 2.5 1.4 0.7
+
Wb
+
n-C& 95 0.028 0.0431 2.8 87.3 % Do 91.2 % Di 1.42 1.91 % D1 % D3 1.16 % D4 0.86 % Ds .68 % .55 % Dr .45 % Da .38 .32 % Ds .29 % Dio % D11 .25 % D12 ... ... .21 % Dl3 ... ... ... .18 % D14 ... ... ... .09 Catalyst: 2 cc. of sintered nickel-silica. * Catalyst: 0.5 cc. unsintered nickel-silica. l-C6H1zis 1-hexene, cH is cyclohexane. Actual net flow rate of olefin or, in run W, of hexane. e yo deuterium in roduct hexane. f Computed from flow rates and analysis OF hydrocarbon product. I n run T, the exit hydrogen was anakzed mass spectrographically, % D = 74.
- 0.8 -
- 0.4
0.2
0.0
6 12 14 16 No. of deuterium atoms. Fig. 3.-The effect of the ratio of the partial pressures of h.ydrocarbon and deuterium. The observed concentrations of F and I are multiplied by 1.07 and 1.28, respectively. I n 1, D1 is 1.47. H and I are CYclohexane, F, G and J are heptane,
that reduced at 350" in contrast to nickel-kieselguhr in which reduction a t the lower temperatures led t o a much less active catalyst.1 For comparison with isotopic exchange patterns, we examined the product of the addition of deuterium to On a as shown in Table 11. The deuterium-olefin ratio was
increased by diluting the olefin with 5 and 30 parts of cyclohexane. Since the diluent is of lower molecular weight yet of higher boiling point than hexane, hexane could be concentrated for mass spectroscopy by distillative topping, yet residual cyclohexane would not interfere with the parent peak of hexane. As shown by the absence of any C-D band a t 4.6 p in the still pot residue from the distillation of run V, cyclohexane was negligibly exchanged although olefin was fully hydrogenated. Isotopic exchange of (+)3-methylhexane on the nickel-silica catalyst gives results similar to those on catalysts studied previously. For comparison, the following data should be added to Table I1 of ref. 2. I n run R at 93", the ratio of exchange to racemization was 2.0 f 0.4, the fraction D1 Dz D3was 0.36. In run S at 141", on a catalyst reduced at 250", the ratio was 2.1 f 0.2 and the fraction, 0.33. Discussion Anderson and KembalP have reported that isotopic exchange between cyclohexane and deuterium on films of evaporated palladium (18.5 and 44") and evaporated rhodium (-48 and -28') leads to exchanged cyclohexanes in which marked discontinuities separate the concentrations of C6H6Daand C6HgD,. Similar discontinuities separate CgH6D6 and C6H4D6 when cyclopentane is used* At 160200" no such discontinuities appear when evaporated nickel films or reduced nickel oxide are used as the catalyst.2 Under these conditions, the perdeuterohydrocarbon is the most abundant species. On the other hand, at 34" on evaporated nickel films,'6 the decline in concentration with increasing deuterium content is so rapid that species beyond C6HsD4 have negligible c.oncentrations. Thus the possibility of a discontinuity could not be examined.
+ +
-
( 6 ) J. R. Anderson and 472 (1954).
C. Kemball, Proc. Roy.
Soc. (London), 226,
THEISOTOPIC EXCHANGE BETWEEN DEUTERIUM AND HYDROCARBONS
Aug., 1956
The effect of temperature upon the distribution patterns in the isotopic exchange of cyclohexane on nickel catalysts in the temperature zone between the two ranges previously reported is, therefore, of particular interest. Actually, as shown in Fig. 2, two different effects of temperature are evident. Although one can observe no discontinuity between C ~ H ~ Dand G C6H6D7a t -34" (from 'Anderson and KembalP) a distinct discontinuity appears a t 60" (run A). However, above loo", the discontinuity largely disappears into the experimental error. Run D a t 187" closely resembles those reported earlier2 for reduced nickel oxide and evaporated nickel films a t these temperatures. The other effect is that increased temperature results in a very marked increase in extensive multiple exchange. Comparison of the relative numbers of species D7 to Dlz in curves A to D shows this particularly clearly. The effect of temperature upon the exchange of cyclopentane resembles that of cyclohexane both in regard to discontinuity and to extensive multiple exchange. The production of multiply exchanged species must involve steps equivalent to the following
k
A
&
1047
1 .o
4
0.8
z39k c
.e
0.6
.e
a)
9
-a
82
#
.s
0.4
E
Ly
€s 0.2
1
2
4 5 6 7 8 9 1 0 1 1 1 2 No. of deuterium atoms. effect of sintering on theexchange patterns of 3
Fig. 4.-The cyclohexane as determined at about 130". The observed concentrations in K are multiplied by 2.00. I n K and ibT, D, is 1.20 and 1.11.
*
I
* * I , *I
I
I
I
I
I
1 .o
*
*
Step (2) can be multiply repeated. I n this formud lation the exact details of the processes and the -$ 0.8 method of binding to the surface are left open. h Rapidly established equilibrium is assumed to exist .B D). between gaseous and adsorbed hydrogen (H .e The average % deuterium in the desorbed hydrog0.6 carbon is determined by the relative probabilities of the propagation step (2) and the desorption step 2 (3). From the increasing predominance with increasing temperature of multiply exchanged species, % the propagation step is increasingly favored and 3 0.4 must possess a higher net activation energy than the desorption step. €s I n cyclohexane, the hydrogen atoms exist in two sets of six atoms each. As shown in curve A, step 0.2 ( 2 ) cannot easily lead from one set to the other a t lower temperatures. With increasing temperature, the probability of passing from one set to the other increases markedly. The Effect of Pressure.-As exhibited in Fig. 3 2 4 6 8 10 12 14 and elaborated in the Experimental section, the No. of deuterium atoms. ratio of the partial pressures of deuterium and hydrocarbon a t a total pressure of one atmosphere Fig. 5.-Exchange of heptane on various catalysts. The observed concentrations in P are multiplied by 1.41. affects the shape of the exchange distribution pattern, but to no large degree. An increased ratio L In ( 0 1 0 / 0 1 ) .z p ~ ~ * . 6 p , c o ~ S 8 leads to relative decreases in the most extensively exchanged species. The effect of the ratio is more where L is the liquid flow rate. One replaces the pronounced a t 150" than a t 60 to 100". The term a t the left by L ln(l/Z) where Z is the fraction change in over-all rate is large and is consistent of molecules without deuterium substitution. A with the kinetics which obtain in the racemization relation of similar form was found in the first study of (+)3-methylhexanel of alkane exchange and, in particular, for the ex-
+
(R
(R
i7
Ly
1048
ROBERT L. BURWELL, JR., AND RICHARD H. TUXWORTH
Vol. 60
change of propane on a nickel-kieselguhr catalyst.' 3. Sintered nickel-silica gives less multiple exEffect of Crystallite Size.-Before reduction, the change than might be expected from its particle coprecipitated nickel-silica catalyst forms a layer size alone, but, in catalysts in which various cryslattice with sheets of a silicon-oxygen network tal faces are exposed in unknown degree, one could separated by nickel Nickel-kieselguhr cata- hardly expect the isotopic exchange distribution lysts are prepared by adding a solution of sodium patterns to correlate perfectly with but one varicarbonate to a slurry of kieselguhr in a solution able. of nickel salt. Chemical reaction of the kieselguhr Variation in these patterns is larger for heptane occurs and a material rather similar to coprecipitated (see Fig. 5) than for cyclohexane as might be exnickel-silica results? pected since compact molecules such as cyclohexane During reduction of any of these materials, nickel and 2,3-dimethylbutane exhibit more extensive atoms agglomerate into small cubic close-packed multiple exchange than do extended molecules such crystallite^.^^^ The adsorption of hydrogen on as heptane and 3-methylhe~ane.~ catalysts of both types is similarSg The silica skeleAt identical temperatures, palladium catalysts ton of the reduced catalysts retards ~intering.~ favor the formation of relatively more of the extenThe particle size distributions of both types of sively exchanged species than do nickel catalysts. l1 catalysts have been characterized by Selwood and At 183" a Fischer-Tropsch catalyst (cobalt, thoria, co-workers by magnetic means6J0: Catalyst A magnesia and kieselguhr) even more heavily favors (Table I), nickel-silica reduced at 350", consists extensively exchanged species; butane forms perhe average deuterobutane nearly exclusively.l2 mainly of very small crystallites. althouf diameter is probably less than 10 Mechanism.-One would expect the propagation there are some with diameters as large as 50 step to require a free site adjacent to the site or Catalyst B, like A but then sintered at 600°, shows sites by which a hydrocarbon molecule is bound a large loss of the very small cryst@lites and a to the surface. Reduction in crystallite size marked gain in crystallites of 50+ A. Catalysts should reduce the number of neighboring free sites C and D, nickel-silica reduced at 250" for 48 and particularly with crystallites as small as 10 A.; 15 hr., respectively, exhibit an increasing prepond- thus, the relative extent of the propagation reerance of very small crystallites. Reduction is action would be decreased. probably incomplete. Catalyst E, nickel oxide reThe increased surface coverage by hydrogen duced a t 300") is magnetically massive n i ~ k e l . ~which ~ ~ accompanies increase in its partial pressure The magnetically very similar catalysts F and G are would relatively depress multiple exchange both by nickel-kieselguhr, Harshaw (68% Ni) and Universal augmenting the rate of the desorption reaction and Oil Products (53%). Upon reduction at 350°, a by reducing the number of free sites. A substantial much larger fraction of the nickel crystallites are part of the increasing multiple exchange with inrelatively large than is the case with nickel-silica. creasing temperature may result from decreased As shown in Fig. 4,sintering (runs K and L) a coverage by hydrogen. However, this simple geonickel-silica catalyst reduced a t 350" (run M) metric explanation may be oversimplified. Adsorpleads to increased multiple exchange. Reduction tion of hydrogen on nickel increases the number of a t 250" leads to less multiple exchange, and reduc- electrons in the d-band6Js and the changing election for 15 hr. to less multiple exchange than re- tronic states of the nickel may affect the isotopic duction for 48 hr. It was previously observed' distribution patterns. with Universal Oil Products Company nickelAt lower temperatures, in view of the discontikieselguhr catalyst that reduction at 250" as com- nuity between C6HSD6 and CbH4D6, the propagation pared with 300" resulted in much less multiple ex- reaction (step 2) with cyclopentane must proceed change of 3-methylhexane. The difference was preferably to an adjacent cis-position. The procmuch more drastic than that observed with nickel- ess which proceeds to a trans-position and permits silica because the higher temperature reduction exchange of more than 5 deuterium atoms is very leads to much more extensive multiple exchange probably closely related to that which leads to with nickel-kieselguhr than with nickel-silica (runs racemization of (+)3-methylhexaneW The species N and 0 vs. P) whereas the patterns obtained with A which we earlier proposed as a possible one for reduction at 250" are relatively similar for both racemization112 may not be satisfactory for transcatalysts (run Q). Nickel oxide reduced a t 300" migration in cyclopentane. (Here, Rz = H.) gives much more multiple exchange tha; does Species B, proposed by Anderson and KembalP for sintered nickel-silica (run J vs. runs F and G II-I Fig. trans-migration cannot accommodate racemization. 3). Evaporated nickel film which is also massive Species C seems to be the simplest of this general magnetically6 results in a curve of the type o f Ja type which can accommodate both reactions. at 130". By and large, multiple exchange 1s faWhat one needs for either reaction is a carbon vored by large crystallites. Thus, increase in crys- atom in threefold coordination symmetrically lotallite size seems relatively to favor the propagation cated with respect to surface hydrogen atoms. A step, equation 2, over the desorption step, equation radical adsorbed perpendicularly to the surface or in other symmetrical ways2 would also accommodate (7) K. Morikaws, N. R . Trenner and H. 8. Taylor, J . Am. Chsm. Soc., 69, 1103 (1937). both reactions.
1.
.
(8) J. J. de Lsnge and G. H. Visser, De Znesnisur, 68, 0.25 (1946). (9) G. C. A. Schuit and N. H. de Boer, Rec. trau. chim., 70, 1067 (1951). (10) J. A, Sabatka and P. W. Selwood, J . Am. Chew. Sspc., 77, 5799
11951h
(11) R. L. Burwell, Jr., and B. Shim. unpublished results. (12) 5. 0. Thompson, J. Turkeviah and A. P. Irsa, J . Am. Cham, Soc., 78, 5213 (1951). (13) L,E,Moore and P. W.Selwsod, ibid., 78, 697 (1956).
U
POLYVINYL ALCOHOLAND ITS PARTIALLY SUBSTITUTED ACETATES
Aug., 1956
A
B
1049
tion pattern to higher deuterium content. Such correction would be small in runs U and V. We suggest that the increase in deuterium content which results from decrease in the partial pressure of olefin is consequent to the strong adsorption of olefin, At high partial pressures of olefin, migration of the point of attachment is inhibited by the paucity of free sites. At low partial pressures, more free sites are available and more multiple exchange occurs. I n run W with hexane, the virtual pressure of olefin resulting from hexane = hexene hydrogen is about 10-lo atm. Here, multiple exchange is more abundant and would be even more abundant if correction t o the conditions of run V was made for: (a) a 10" lower temperature, (b) replacement by sintered catalyst of the unsintered catalyst which was used t o get adequate activity, (e) isotopic dilution of the deuterium. Similarly, in the hydrogenation of (+)CC-C-C=C-C, one would expect the propaga-
+
C
Addition of Deuterium to 1-Hexene.-We have assumed that isotopic exchange involves partial reversal of olefin hydrogenation. Accordingly, one might expect that hydrogenation of the optically active olefin, C-C-C-C=C-C, would
I
C result in racemization since identical intermediates would arise in the hydrogenation of the olefin and in exchange of (+)3-methylhexane. However, hydrogenation with nickel-kieselguhr a t 70" and 120 atm. or at 45" and 1atm. leads to (+)3-methylhexane which is hardly racemized at a11.l It appeared desirable to determine the isotopic distribution pattern resulting from addition of deuterium to an olefin under conditions more closely approaching those of exchange between deuterium and alkanes. We chose 1-hexene for this purpose with the results shown in Table 11. With increasing deuterium-olefin ratio, the most abundant species moved from Dz (run T) to D3 (run U) to Ds (run V), the amount of multiple exchange increased and the total deuterium content of the product hexane increased. I n run T with undiluted olefin, correction for isotopic dilution of deuterium would shift the isotopic distribu-
C tion reaction to be depressed and therefore racemization to be reduced. The propagation reaction would also be relatively reduced by reduction in temperatures from those of exchange of alkanes to those employed in hydrogenation. Results generally similar to run T with undiluted hexene were reported'4 for hydrogenation of cis-2butene on a nickel-kieselguhr catalyst. Acknowledgment.-This research was supported in part by the Office of Naval Research. One of us (R. H. Tuxworth) was the Visking Company Fellow, 1953-54. We are indebted to Professor P. W. Selwood and Dr. S. Adler for their kind cooperation in the supply of catalysts and of information on their characterization. (14) C. D. Wagner, J. N. Wilson, J. W. Otvos and D. P. Stevenson, J . Chem. Phye., 20, 338 (1952).
MEASUREMENT OF THE AMOUNT OF BOUND WATER BY ULTRASONIC INTERFEROMETER. 11. POLYVINYL ALCOHOL AND ITS PARTIALLY SUBSTITUTED ACETATES BY HAZIME SHIIOAND HIROSHI YOSHIHASHI Chemical Institute, Facultg of Science, Nagoya University, Nagoya, Japan Received February 7 , 1866
Ultrasonic velocities in ethanol-water solutions of polyvinyl alcohol and its partially substituted acetates have been measured by an ultrasonic interferometer and, thence, the amount of bound water and compressibility of the solute molecule determined, with the following results: the amounts of bound water of polyvinyl acetates of 99.8,90,90.6 and 70.9% sa onification degree are 0.45, 0.43, 0.48 and 0.29 cc./g., respectively. The compressibility of the solute molecule of many 0 8 r a d i cals is decreased by the increasing ethanol ratio, but with t,he solute of sa onification degree 70.970, the compressibility has a smaller value in water, and increases with the addition of ethanol to soyvent. These results suggest that the materials of saponification value 99.8Y0 or 96% are swollen to large extent in water owing to their OH radicals, but that the one of saponification 70.9% is in a compressed state in water, through the hydrophobic acetyl radical.
Introduction A series of investigations concerning ultrasonic velocities through aqueous solution has been carried out in our l a b ~ r a t o r v land *~ I n the (1) Y.Miyahara and H. Shiio, J. Chem. SOC.Japan, 72,876 (1951). (2) Y. Miyahara and H. Shiio, ibid., 73, 1 (1952). (3) T. Sasaki, T. Yasunaga and H. Fujiwara, ibid., 73, 181 (1952). (4) A. Passynsky, Acta Physicochim. U.R.S.S.22, 263 (1947).
previous report' we deduced a general formula which enables us to evaluate the amount of bound water of non-electrolyte, and applied it to the solutions of saccharides. (5) B. Jaoobson. Arkiu Far K s m i , 2 , 117 (1950). (6) F. T. Tucker, F. W. Lamb, 0.A. Marsh and R. M. Haag, J. Am. Chem. SOC.,72, 310 (1950). (7) H. Shiio, T. Ogawa, and H. Yoshihashi, J . Am. Chem. SOC.,77 4980 (1955).
