692
J. H. SINFELT, H. HCRWITZ AND J. C. ROHRER
acid, and the pure acid resonance is about 100 c.p.s. to higher field from the calculated 6 ~ . In the formation of hydrogen-bonded polymers larger than the dimer it generally has been observed that the OH stretching frequency in the infrared is shifted a larger amount than in the dimer, indicating a stronger hydrogen bond in the higher polymers.21 On this basis it mould be expected that the stronger hydrogen bonds in the higher polymers would also cause a corresponding shift of the n.m.r. resonance to lower field than in the dimer. This has been observed in the alcohols and p h e n ~ l s . ~Reeves and SchneiderZ2have suggested that the hydrogen bonds in the higher polymers of the acids are not as strong as in the dimer, but such an argument does not seem reasonable qince the polymers presumably have lower entropy per moiiomer unit but are formed preferentially at hipher concentrations in competition with the dimer. Also it is known that the hydrogen bonds in the polymer in crystals are shorter than the bonds in cj-clic dimer.6 It seems to us much more likely that the n.1n.r. shift is not a reliable indicator of hydrogen bond strength because this shift is influenced by factors which have no effect (or may even have a reverse effect) on the bond strength. Thus in the higher polymers the 0-0 distance may be small enough for the unbonded oxygen electrons to contribute significantly to a diamagnetic shielding of the carboxyl
Tol. 64
proton.o The 0-0 distance in formic acid dimer is 2.75 -4.;while that in $he polymer present in the solid is reduced to 2.58 A. The actual 0-0 distance in the polymers in solution may also be of this magnitude. Sei-era1molecules are known in which this distance is extremely short because of steric reasons. Examples are malgic acid (2.46 A , ) and Si-dimethylglyoxime (2.44 -4.). It has not been possible to observe the i1.m.r. shift of these compounds in non-hydrogen-bonding solvents because of their limited solubilities. It would be desirable to do so, since then only the hydrogen bonds of the molecule of interest would be observed. Several solutions of maleic acid in acetone have been measiired, and an extrapolation of the shifts toward pure maleic acid indicates that the shift in the absence of bonding with acetone is about 250 C.P.S. below benzene. This is a little higher than that usually observed for hydrogen-bonded acids (300400 C.P.S.below benzene), but not appreciably so. Since this average shift includes the internally bound hydrogen as well as the hydrogen bond in the dimer, the higher observed shielding may reflect a higher shielding in the internal bond. Acknowledgment.-The authors wish to express their appreciation to Dr. George C. Pimentel for many helpful discusqions and to Dr. Power Sogo for aid with electronic problems. A portion of this work was supported by the Atomic Energy Commission.
KINETICS OF ?7-PESTA4NEISOJIERIZATIO?;' OYER Pt-Al,O,
CAIT,lLYST
BY J. H. SINFELT, H. HCRTVITZ AND J. C. ROHRER Esso Research and Enganeering Company, Linden, S.J . Received .January 16,I960
The kinetics of n-pentane isomerization over a Pt-.U201 catalyst were investipated a t 372". The reaction vas carried out in a flow reactor in the presence of added hydrogen a t pressures ranging from i.7 to 27.7 atm. and hydrogen to n-pentane ratios ranging from 1.4 to 18. The rate of isomerization was found to correlate with the n-pentane to hydrogen mole ratio and to be independent of total reactor pressure a t a fixed n-pentane to hydrogen ratio. These results can be explained in terms of the postulated mechanism by which isomerization proceeds via an olefin intermediate present in equilibrium concentration. According to this mechanism n-pentane dehydrogenates on platinum sites to n-pentene, which in turn migrates to acidic sites to isomerize, presumably by a carbonium ion mechanism. The rate-controlling step is the isomerization of the intermediate olefin on acidic sites.
Introduction Isomerization of n-paraffins over dual-function catalysts is becoming increasingly important in the petroleum industry. The dual-function catalysts consist of an active hydrogenation-dehydrogenation component such as platinum supported on an acidic oxide such as alumina or silica-alumina. il mechanism involving olefin intermediates has been proposed by several investigators for isomerization of paraffins over these According to this mechanism the dehydrogenation component of the catalyst generates the intermediate olefin which migrates to acidic sites to isomerize, presumably via a carbonium ion mechanism. In general, either the dehydrogenation or acidic func(1) G. A. Mills, H. Heinemann, T. H. Milliken and A . G. Oblad, I n d . Eno. Chcm.. 46, No. 1, 134 (1953). (2) F. G. Ciapstta and J. B. Hunter, { b i d . , 46, 147 (1953). (3) P. B. Weisz and E. W. Swegler, Science, 196, 31 (1957).
tion may be rate controlling, depending on catalyst and reaction conditions. With these thoughts in mind it was decided to investigate the kinetics of n-pentane isomerization over a Pt-A120s catalyst to check the proposed mechanism. Experimental Materials.-Phillips pure grade n-pentane (>99 mole 70 purity) u-as used in these esperiments. The n-pentane was dried with Drierite (CaSOa) t o less than 5 p.p.m. H,O before using. The hydrogen mer! was gassed through a Deoxo cylinder containing palladium catalyst to convert trace amounts of ovvgen to water, and then dried over Linde 5.4 molecular sieves. The catal) s t contained 0.3% platinum Pupported on alumina and was prepared by impregnation of alumina with aqneous chloroplatinic acid. The surface area of the catalyst mas 155 m.2/g. The ratalynt was used in the form of pellets a i t h an equivalent spherical diamptrr of about l/g inch. Procedure.--The reaction studies were carried out in the inch presence of added hydrogen in a flow system using a i.d. stainless steel reactor containing 15 g. of catalyst. The
July, 1960
KINETICS OF WPENTANE ISOMERIZATION OVER P T A L ~ OCATALYST ~
893
TULE I 372'-PRODUCT COMPOSITION DATA 11 9 10 8 7.7 24.4 24.4 11.0 7.7 5.0 18.0 18.0 1.4 5.0 9.1 26.7 23.4 9.1 26.6
?&-PENTANEISOMERIZATIOS A T
Period no. Pressure, atm. H2/n-Cb
5 27.7 5.0 26.4
LHSV" Composition, mole C1
0.1 .2 .2
cz
6 27.7 5.0 9.3 0.1 .3 .3 .1
rn
0.1
0.1 .2 .2 .1
0.1 .3 .2 .1
0.2 0.1
12 21.0 5.0 26.5
0.1 0.1
0.1 0.1
13 11 .o 1.4 9.4
0 .2 .1
9 C:, .1 n-C4 .1 .1 i-C4 8". 1 94.0 85.8 95 1 87.4 90.7 95.1 n-Cj 95.9 92.2 i-Cs 5.5 13.4 4.7 3.4 7.1 17.5 12.0 8.8 4.5 0.2 0.2 Cyclo-C6 0.2 0.2 0.2 0.2 0.2 0.1 Where blank spaces occur the amount present is less than a Liquid hourly space velocity, g. n-pentane/hr./g. catalyst. 0.1%. .I
reactor was surrounded by an electrically heated aluminum block to maintain isothermal operation. The catalyst was pretreated with hydrogen for three hours a t 527" prior to introducing the n-pentane fwd. Reaction products were analyzed by a chromatographic column coupled directly to the outlet of the reactor. Reaction prriods of 30 minutes were employed throughout this %ark. This was found to be more than adequate for attainment of steady-state conditions piior to sampling. Reaction temperature was maintained a t 372" for all the n-pentane runs. Total pressure was varied from 7.7 t o 27.7 atm., and the hydrogen to n-pentane mole ratio from 1.4 to 18. Liquid hourly space velocities ranged from 9.1 to 26.7 g./hr./g. catalyst. The n-pentane conversion levels ranged from 4.1 to 17.9%.
where the constants k and n at 372" take the values 0.040 and 0.5, respectively, when the rates are expressed in g. moles/hr./g. catalyst.
The n-pentaiie isomerization experiments were made a t low- conversion levels to minimize the effects of secondary reactions. The reaction rate r is defined by the expression
These results support the postulated mechanism by which paraffin isomerization proceeds via an olefin intermediate, with the rate-controlling step being the reaction of the olefin on acidic sites.
TABLE I1 n-PEhTANE ISOMERIZATION RATES A T 372' Pressure, atm. 7 7 210 Total 244 277 110 n-Cs 1 3 1 3 3 5 4 6 4 6 6 4 6 4 175 231 231 Hz Reaction rate" 0 011 0 018 0 017 0 020 0 034 a Initial rate (zero conversion), g. mole/hr./g. catalyst.
SCXhlARY OF
Discussion
r =
Pt
F dx d TV
where I" is the n-pentane feed rate in g. niole/hr. and dx is the fractional conversion obtained in an element of catalyst dW. Thus, the slope a t the origin of a plot of fractional conversion 2's. W / F is the initial reaction rate. The quantity W / F is the reciprocal of the space velocity. By making the experiments at low conrersioii levels the initinl rates can be determined satisfactorily. The reaction kinetim were investigated by varying the n-pentane and hydrogen partial pressures individually and noting the effects on initial isomerization rates. Detailed product composition data from which the rates n-ere calculated are shown in Table I. Small amounts of hydrocracking and cyclization were noted in addition to isomerization, but the selectivity to isonlerization was 90-95%. Isomerization rates are summarized in Table 11. The rate was found to increase with increasing npentane partial pressure but to decrease with iiicreasing hydrogen partial pressure. At a constant n-pentane to hydrogen mole ratio, the rate was found to be indcpendent of total pressure. The reaction rates c u i be rorre1:ited with the n-pentane to hydrogen mole ratio. The rate data are fitted satisfactorily by nn equation of the form
n-Cb
n-C6=
n-Cb-
+ HP
Acid
-+i-Cj' Site
(fast) (slow)
Pt
Hz
+ i-Cs- I_ i-C5
(fast)
The n-pentene which is formed on the platinum is in equilibrium with n-pentane and hydrogen in the gas phase, so that the partial pressure of the n-pentene is given by (3)
where K is the thermodynamic equilibrium constant and pncS-,pncoand p ~ are * the partial pressures of n-pentene, n-pentane and hydrogen, respectively. The n-peiitene migrates to an acidic site where it is adsorbed. The adsorbed n-pentene then isomerizes. presumably by a carbonium ion mechanism. The rate of this step is controlling and therefore the over-all rate is given by T =
k'[n-Ca"],
(4)
where k' is the rate constant and [n-C,=],is the concentration of adsorbed n-pentene. The concentration of adsorbed n-pentene may be related to the a-pentene partial pressure in the gas phase by :I I'reundlich type relation [n-C,=], = bp",c,-
(5)
J. H. SINFELT, H. HURWITZAND J. C. R ~ H R E R
10 100 1000 P e n k n e partial pressure, atm. x 104. Fig 1.-Isomerization rate us. pentene partial pressurr.
I
where b and n are constants. Substituting equations 3 aiid 5 into 4 the rate expression becomes I
=
k'bK"
= 1;
(E:)"
((i)
(;;)'I
which is identical with the form of equation 2 found by experiment. The rate equation could also he expressed in the form r =
k" p d p a , 1 4-m p d m
(7)
which follows from the assumption that the Langmuir adsorption isotherm applies in relating the concentration of adsorbed n-pentene to its partial pressure in the gas phase. However, the simple Langmuir equation is strictly applicable to a homogeneous surface, which is very likely not the case for the type of catalyst used here. To apply the Langmuir equation rigorously it is necessary to account for differences in avtivity of the various sites. If it is assumed that the active sites are distributed exponentially with respect to the energy of adsorption, it has been hhowi that the Freundlich type isotherm is a reasonable approximation.4 The assumption of olefin intermediates in isomerization is supported by the observation of olefins in the reaction products. However, a t 372" and at the hydrogen pressures used in this work the equilibrium concwitration of olefins is very low (
