Diffusion-Limited Heterogeneous Catalytic Reactions on a Rotating Disk. I I. Hydrogenation of Phenylacetylene over Palladium David E. White and Mitchell Litt* Department ot Chemicai and Biochemicai Engineering. Coiiege of Engineering and Appiied Science. University of Pennsylvania Phiiadeiphia. Pennsylvania 191 74
T h e rotating disk catalytic reactor is extended to the s t u d y of the kinetics of a multiple reaction system, the liquid phase hydrogenation of phenylacetylene over a thin film of palladium in t h e presence of a reactive but less strongly adsorbed solvent, a-methylstyrene. T h e effects of diffusion, of variation in reactor parameters, and of crystallographic orientation on selectivity are discussed. T h e limiting diffusing
reactant is hydrogen, all hydrocarbon species residing in relative abundance on the catalytic surface.
All reactions can be represented by zeroth-order hydrocarbon and first-order hydrogen kinetics. T h e (110) face of palladium is found to b e especially favorable [vis-a-vis t h e (1 11) face] for selective alk-
ene formation in alkyne hydrogenation. Kinetic factors (selectivity order, activation energy) compare favorably with those found by previous investigators in other types of reactors using inert solvents.
Introduction Previous work (White et al., 1974) has demonstrated that the catalytic rotating disk reactor is an excellent tool for the determination of surface kinetics of diffusion-limited heterogeneous catalytic systems. A particular advantage of the rotating disk over other geometries such as the rotating basket reactor is that the flat film catalyst is amenable to surface characterization, allowing correlation between the kinetic parameters of such reaction systems and the crystallographic orientation and crystallite size. This reactor extends to diffusion-limited systems, particularly to liquid-reacting ones, the advantages offered by film catalysts used to study gas systems. Of course, the relevance of such catalyst systems to industrially important supported catalysts may be questioned. However, our previous paper on hydrogenation of a-methylstyrene demonstrated that, a t least for that system, the thin film reaction results could be extrapolated to supported catalyst systems using data available in the literature. Thus, the reactor system used here may be applied to a t least certain reactions of industrial importance to further our understanding of fundamental catalytic mechanisms. In this paper we extend the rotating disk method, with its exactly defined mass transfer characteristics, to the investigation of a multiple-reaction system whose intrinsic kinetic behavior is masked by diffusion. The objectives of this study are: elucidation of the surface chemical kinetics of the individual reactions present; determination of the limiting diffusing species; and observation and quantification of the effects of diffusion, variation of reactor parameters, and crystallographic orientation on selectivity. The model chemical system is the liquid-phase hydrogenation of phenylacetylene (PA) over a thin film of ion-plated palladium in the presence of a reactive but less strongly adsorbing solvent, a-methylstyrene (AMS). The choice of the latter hydrocarbon species as solvent was made for several reasons. The reaction characteristics of AMS on similar Pd disks had been extensively studied (White et al., 1974; White, 1972), allowing us to obtain in each experiment specific catalytic activity data for the AMS-hydrogen reaction before charging phenylacetylene to the reactor. This permitted an unbiased activity comparison of each catalytic disk in use. Furthermore, the structural similarity of a-methylstyrene to phenylacetylene provided the basis for the study of the hydrogenation of mixtures of related aromatic hydro-
carbons. Finally, acetylenic compounds are presumably much stronger catalytic adsorbates than their olefinic counterparts (Bond, 1962), and hence phenylacetylene should associatively adsorb on the catalytic surface to the virtual exclusion of a-methylstyrene. Accordingly, the expected decrease in the production rate of cumene upon the introduction of phenylacetylene to the reactor could serve as an indicator of the availability of the various reacting species a t the catalytic surface. Additional considerations related to the mechanism and the reactivity of the phenylacetylene-hydrogen reaction over palladium. It has been determined (Bal’yan and Borovikova, 1959; Sokol’skii, 1964; Sokol’skii, 1968; Shutt and Winterbottom, 1971) that this reaction follows the series or Type I11 pattern (Wheeler, 1955) and affords an intermediate species (styrene) which is detectable in the bulk phase. Hence, the effect of the diffusion of the olefinic intermediate on the selectivity could be investigated with the rotating disk system. Also, the liquid-phase hydrogenation rates of phenylacetylene and of styrene over palladium have been found (Sokol’skii, 1968) to be slightly higher than that of a-methylstyrene, assuring not only relatively short experiments (despite the small catalytic surface area) but also bulk-phase hydrogen diffusion control. An additional consideration was the sufficiently low heats of hydrogenation of phenylacetylene, styrene and a-methylstyrene, allowing neglect of local heating in the vicinity of the disk surface and the assumption of isothermal conditions. Finally, comparisons could be made of the results with those of the workers cited above.
Experimental Methods Chemicals, Catalytic Disks, and Apparatus. Technical grade phenylacetylene was obtained from the J. T . Baker Chemical Company (Cat. No. M360) and from the Eastman Chemical Company (Cat. No. 2081). The sole purification step involved the passage of phenylacetylene through a bed of basic alumina, employing a procedure similar to that previously used for n-methylstyrene. T w o new ion-plated palladium disks were used in this study. Details of the film preparation, apparatus, and techniques for surface characterization have been given previously (White et al., 1974: White, 1972). The apparatus previously used for the AMS studies was modified in order t o allow the addition of small quantities ( - 100 cm3) of Iiquid phenylacetylene to the reactor. A pressure-tight :{04 Ind. Eng. Chem., Fundam., Vol. 14, No. 3, 1975
183
stainless steel cylinder with a capacity of 300 cm3 was connected in parallel to the reactor sampling line to provide a means of deoxygenating the phenylacetylene and subsequently adding it to the reactor under nitrogen pressure. Experimentation. The phenylacetylene experiments were conducted similarly to those involving a-methylstyrene as the sole hydrocarbon reactant. Data were first obtained at a constant set of standard conditions to characterize the initial specific catalyst activity-AMS as the sole hydrocarbon reactant, 400 rpm disk rotational speed, 70°C reactor temperature, and 4 atm abs hydrogen pressure. Phenylacetylene was then introduced through the reactor sampling line, and the effect of the alkyne on the previous alkene reaction rate was observed. The remainder of the experiment was conducted via a succession of parametric changes in temperature, hydrogen pressure, or disk rotational speed. The liquid reactor samples were analyzed on a Beckman GC-5 gas chromatograph with a hydrogen flame detector. Satisfactory separation of the hydrocarbon species was effected using an Wft, lh-in. 0.d. column packed in series in respective 1:5 proportions with 20% butane diol succinate on 80/100 Chromosorb W and 5% methyl silicone on 60180 Gas-Chrom P.
Theoretical Approach The Kinetic Model. The multiple-reaction system can be represented as follows Ai
+
Hz
kl
-
A2
(1)
A,
(2)
A,
(3)
Pd
A2
+
H2
k2
Pd
a t the catalytic surface and C ~ / O , i = 1,3, respectively, represent the surface concentrations of reactants A,, i = 1-3. Significantly, for each of the three reactions the denominator of the rate expression is identical. Further, the contribution of the common reactant, hydrogen, is also the same, and hence the only difference is the expression for the hydrocarbon in the numerator. Following the assumptions made previously for the AMS-hydrogen system, eq 4-6 can be simplified into power functionalities. For example, it can be assumed that ( ~ H c H / o ) ~< ’< ~ 1, indicating that all three reactions are first order in hydrogen. Further, high surface coverage of phenylacetylene due to its high heat of adsorption implies that the term ( b l c l l o ) supersedes all others in the relevant term in the denominator. The result is to obtain kinetic expressions for the three reactions which are first order in the respective hydrocarbon reactant and negative firstorder in phenylacetylene. Finally, if there is experimental evidence that the hydrogen surface concentration varies much more than that of the unsaturated hydrocarbon species, then all three rate expressions can be assumed to be pseudo-zeroth-order in all hydrocarbon species and pseudo-first-order in hydrogen. With this assumption, to be justified below, the use of this chemical system in conjunction with the catalytic rotating disk would then afford particularly simple mathematical expressions for the diffusional kinetics. The Diffusional Kinetics for the Rotating Disk. Following Frank-Kamenetskii (1969), the steady-state rate of interphase transport is equated to the isothermal surface reaction rate for each of the reactions above
k3
A3 + Hz
where phenylacetylene (AI) and a-methylstyrene (A3) react concurrently on the palladium surface with hydrogen to respectively form styrene ( A d and cumene (As). Styrene in turn either desorbs and diffuses into the bulk, desorbs and then readsorbs and combines with hydrogen to form ethylbenzene (A4), or reacts with hydrogen without first desorbing. Modeling of the reaction set above is aided by assuming that the three unsaturated hydrocarbon species of interest adsorb associatively on palladium surfaces (Bond et al., 1958; Meyer and Burwell, 1963). This fact, in conjunction with the homologous nature of the hydrocarbons under investigation and the similarity of the reaction conditions to those in the previous study, suggests that a suitable reaction rate expression for each of the reactions above would be a Langmuir-Hinshelwood development of the type used previously for the a-methylstyrene-hydrogen reaction over palladium (White et al., 1974). For each of the three reactions above, the controlling step-apart from all diffusional considerations-is considered to be the surface reaction between one molecule of At, i = 1-3, and both parts of a dissociated hydrogen molecule, where it is assumed that hydrogen adsorbs on a different kind of site than A,, i = 1-3. The rate expression describing any one of the three individual reaction steps is hence written as follows
(4-6)
where c 184
~ refers , ~ to the molecular hydrogen concentration
Ind. Eng. Chem.. Fundam., Vol. 14, No. 3, 1975
where k,* are the pseudo-first-order rate constants for each of the reactions 1, 2, 3, and /3i are known mass transfer coefficients for the respective hydrocarbon species, obtained from rotating disk theory. In addition, the overall rate of consumption of hydrogen can be written as follows H ‘ = o r as
P(CH
- cH/o) rH
= (ki* + kz* =
~2
+
2r4
+
+ YS
ks*)~H/o
(IO) (11)
Consequently, rH can be obtained from the experimentally determined hydrocarbon production rates, and the hydrogen surface concentration C H / O is then calculated from the left-hand side of eq 10. The mass transfer coefficient for hydrogen, p, is calculated from the solution of the conservation equations for the rotating disk. An implicit assumption in this calculation is that there is no interaction between the various diffusing species which would alter the mass transfer environment for hydrogen. Due to the small catalytic surface area and the mild reaction conditions, the catalytic reactivity is low enough to allow for this approximation. The pseudo-first-order reaction rate constants, k,*, i = 1-3, can be calculated subsequently from the right-hand sides of eq 7-9 and the activation energies obtained from Arrhenius plots.
Results and Discussion Conversion and Reactivity. Typical experimental results are shown in Figure 1, giving concentration data a t 72°C for one of the disks studied. In this experiment, phenylacetylene (2.3 mol 7’0) was added to the reaction mixture after about 280 min. It is seen that there was immediate production of both styrene and ethylbenzene, with a marked decrease in the rate of production of cu-
Table I. Kinetic Parameters for Phenylacetylene Hydrogenation (Disk Pd-8) k,* x lo’,
P , psia
T , ‘C
w, rpm
cc/sec
121A
61 61 60 61 61 60 60 60 61 60 60 60 60 60
72 72 72 73 73 72 72 73 73 101 101 48 49 76
401 101 400 400 101 401 402 100 401 398 100 40 1 101 400
... ... ...
...
2.47 2.22 2.22 1.85 1.24 1.35 2.28 1.73 0.22 0.13 0.65
0.09 0.08 0.09 0.09 0.06 0.08 0.13 0.11 0.02 0.02 0 -06
J K L M a
N 2.3 mol ’70phenylacetylene added.
k3*
X
Run B C Da E F G H I
10’3 cc/sec
lo’, cc/sec
k2*
... ...
X
0.83 0.53 0.46 0.33 0.22 0.23 0.26 0.10 0.17 0.55 0.30 0.05 0.07
0.16
s2 ...
s 3
...
...
... ... ...
0.97 0.96 0.96 0.95 0.95 0.94 0.94 0.94 0.91 0.89 0.92
0.88 0.91 0.91 0.88 0.93 0.89 0.81 0.85 0.82 0.66 0.80
Table 11. Kinetic Parameters for Phenylacetylene Hydrogenation (Disk Pd-9) k,* x lo’,
k2* x lo‘,
k3* x lo’,
Run
P , psia
T , “C
w , rpm
cc/sec
cc/sec
cc/sec
s2
s3
123A
60 59 59 59 59 59 59 59 59 59 59 60 59 103 148 193 32 61
74 76 77
401 99 400 100 400 99 399 398 99 402 98 98 401 402 402 402 402 403
... ... ... ...
... ... ... ...
... ... ...
... ... ...
0.50 0.60 0.63 0.56 0.67 1.04 0.84 0.37 0.33 0.17 0.23 0.45 1.17 0.65
0.11 0.14 0.17 0.15 0.19 0.27 0.24 0.10 0.11 0.05 0.04 0.04 0.08 0.07
2.45 1.54 1.27 0.89 0.92 1.15 1.06 1.08 1.21 2.38 1.99 0.60 0.46 0.37 0.30 0.23 0.15 0.08
0.82 0.81 0..79 0.79 0.78 0.79 0.78 0.78 0.75 0.78 0.87 0.92 0.94 0.90
0.35 0.34 0 -37 0.34 0.36 0.31 0.30 0.38 0.41 0.32 0.43 0.66 0.89 0.90
B C D Ea
F G
H I
J K
L M N 0
P
Q R
77 77 77 77
94 94 116 115 70
66 65 65 64 64 64
...
...
3.81 mol ’70phenylacetylene added.
mene. Calculated values of pseudo-first-order rate constants and selectivities for various experimental conditions are listed in Tables I and I1 for each of the disks studied, Complete tabulations of all the data and calculated results are available in the thesis of White (1972). For the four experiments investigating the simultaneous hydrogenations of phenylacetylene and a-methylstyrene, the overall percent conversion of the latter species to cumene was extremely low, the maximum value being 0.8%. The percent conversion of phenylacetylene was much higher, ranging between 3.5 and 70% for the four runs over two different ion-plated palladium disks. Nevertheless, there was found to be no effect of the degree of phenylacetylene conversion on reactivity or selectivity. The order of magnitude difference in the values of overall conversion for phenylacetylene and a-methylstyrene is attributable to several items. The stronger adsorption of the former species-resulting in high surface coverage of phenylacetylene and low coverage of a-methylstyrene despite its high bulk concentration-and the higher reactivity of phenylacetylene over palladium were probably key factors in this regard. Also, only small quantities of phen-
I
r
0.2-
a
0
d
-
9
,
s
7,
I
-002
,
TIME, MINUTES
Figure 1. Sample data, product concentration vs. time, disk Pd-8, 60 psia, 72°C: open symbols, 400 rpm; closed symbols, 100 rpm; 0, cumene; 0,styrene; A , ethylbenzene.
ylacetylene, ranging from 0.6 to 3.8 mol %, were charged to the reactor. Because of the latter fact, studies were made in order to determine the minimum concentration of phenylacetylene Ind. Eng. Chem.. Fundam., Vol. 14, No. 3, 1975
185
On a different catalytic disk (Pd-9), however, the relative effect of phenylacetylene addition on a-methylstyrene hydrogenation was much less, and the intrinsic activity of the palladium surface for alkyne hydrogenation was also much lower, although the hydrogenation rate for alkene alone on Pd-9 was comparable to that on Pd-8. We believe that these differences can be ascribed to the crystalline nature of the catalytic surface of disk Pd-9 and specifically to the degree of (110) preferred orientation of that palladium film. This point is pursued in detail in the next section. Selectivity and Diffusion. Selectivity is considered in terms of two factors, 5’2 and Sa, defined as
l
ZG
(13) E
(
-1
x8
03
‘eu- I
8
*
Figure 2. Arrhenius plot of observed reaction rate, 0.6 mol olo phenylacetylene, disk Pd-8, run 120: 0 ,cumene; 0 ,styrene; A, ethylbenzene.
which would be required to preclude the possibility, of phenylacetylene diffusional control in addition to that of hydrogen. An indication of the existence of diffusional control of both species is provided by a break in the Arrhenius plot of the observed reaction rate, as seen in Figure 2, where 0.6 mol ?’% phenylacetylene had been charged to the reactor. In this temperature range under identical reaction conditions, no break was observed in similar plots for the AMS-hydrogen reaction where hydrogen was the only limiting diffusing reactant (White, 1972), giving evidence in this case of resistance due to phenylacetylene diffusion. In later runs involving PA, it was determined that the addition of a t least 2.3 mol 70phenylacetylene to the reactor was sufficient to provide “total availability” of that reactant a t the catalytic surface; i.e., the rate of diffusion of hydrogen under these circumstances becomes rate-limiting before that of phenylacetylene. This conclusion is also supported by calculation of the respective surface concentrations of phenylacetylene and hydrogen using eq 7-9. A t the standard reaction conditions and with a bulk concentration of phenylacetylene of 2.3 mol 70,the phenylacetylene surface-to-bulk concentration ratio was 0.88 and the phenylacetylene surface concentration was 15 times that of hydrogen. It is apparent from Tables I and I1 that each of the two catalytic disks used showed quite different effects of phenylacetylene on the hydrogenation rate of a-methylstyrene. The addition of 2.3 mol % phenylacetylene into the reactor containing palladium disk Pd-8 decreased the AMS hydrogenation rate to about one-half of its immediately previous value (obtained before the charging of phenylacetylene to the reactor), while resulting in a phenylacetylene hydrogenation rate 10 times larger than the new a-methylstyrene hydrogenation rate. An additional observation was the constancy, within experimental error, of the total hydrocarbon hydrogenation rate a t the standard conditions both before and after the addition of phenylacetylene to the reactor. The constancy of this rate (0.1 pmol/cmz sec) indicates that the total number of active sites on the palladium surface available for hydrocarbon (a-methylstyrene, phenylacetylene, and styrene) hydrogenation was constant and that the same sites conceivably catalyzed either reaction (White, 1972). 186
Ind. Eng. Chem., Fundam., Vol. 14, No. 3, 1975
Both factors have a maximum value of unity. Values of Sz near one indicate high catalytic selectivity for styrene production vis-a-vis that of ethylbenzene, while high S3 denotes preferred phenylacetylene hydrogenation over that of a-methylstyrene. The selectivity SZ for styrene production had values greater than 0.9 on disk Pd-8 but much lower values, around 0.8, for most conditions on disk Pd-9. Larger differences were found for the values of S3 on the two disks, S3 being 0.88 f 0.03 on disk Pd-8 and 0.35 f 0.05 on disk Pd-9 a t the standard conditions. In addition, within experimental error SZ and S3 were found to be independent of both temperature and the disk rotational speed when either disk was used, and there was an indication of an inverse pressure functionality, when pressure was varied over disk Pd-9. The high values of S2 on disk Pd-8 agree with those of other investigators (Bond and Wells, 1964; Freidlin and Kaup, 1964; Sokol’skii, 1964; Shutt and Winterbottom, 1971), who found initial selectivities greater than 0.94 during the hydrogenation over palladium of alkynes ranging from acetylene to 1-octyne. Bal’yan and Borovikova (1959) reported a selectivity of 0.977 for the hydrogenation of phenylacetylene in methanol a t ambient conditions over colloidal palladium. As discussed earlier, the large preference of the palladium surface on disk Pd-8 for the hydrogenation of phenylacetylene rather than either of the olefinic species is not surprising in view of the appreciably higher heat of adsorption of acetylenes. Styrene, when produced, was for the most part immediately displaced from the catalytic surface by phenylacetylene molecules. Also, styrene was unable to readsorb because of the extreme paucity of vacant sites. The selectivity S3 was lower than S2 because of the extremely high a-methylstyrene concentration in the liquid phase a t the catalytic surface. However, the selectivity results of disk Pd-9 did not reflect the usual large preference of palladium surfaces for acetylenic rather than olefinic hydrogenation that was found on disk Pd-8. Disk Pd-9 exhibited a much lower catalytic activity for PA hydrogenation and was much less selective for styrene production versus that of either cumene or ethylbenzene. This lower intrinsic activity of disk Pd-9 for alkyne hydrogenation is interpreted in terms of the crystalline nature of that catalytic surface. X-Ray diffraction studies showed a quite pronounced (110) preferred orientation for disk Pd-8 and only a slight (110) preference on disk Pd-9 (relative to the second most
predominant configuration, the (111)plane). This result is significant, since it has been suggested (a) that the acetylenic group adsorbs on only the longer face-centered cubic lattice spacing (3.78 A for palladium), which is available on either the (110) or the (100)-but not the (111)-plane, while (b) the olefinic group is believed to adsorb on only the shorter fcc metal-metal spacing, which is available on all three of the low index fcc planes (Bond, 1962). Furthermore, it has been demonstrated (Bond and Sheridan, 1952) via geometrical considerations that adsorbed acetylenic molecules can pack more tightly on the (110) lattice plane than on the (100)plane. Accordingly, it would be expected-and the present data bear this out-that the larger the degree of (ill), relative to (110), preferred orientation, the lower would be the preference of the catalytic surface for alkyne hydrogenation as opposed to alkene hydrogenation. Disk Pd-9 was characterized by a significantly higher degree of the (111) orientation which may not be amenable for acetylenic adsorption, and hence the selective behavior of disk Pd-9 favored a larger degree of alkene adsorption, resulting in lower selectivity factors Sz and S3. The drop in these selectivity factors is due, in particular, to the drop in the alkyne adsorption/hydrogenation rates, since alkene adsorption occurs on the shorter fcc metal-metal spacing which is present on all three of the low index fcc planes. Investigations were also made of the effect of parametric variation on the magnitudes of the above selectivity factors Sz and Sa, particularly to gain insight into the forms of the kinetic expressions describing the surface reactions in the present system. Inserting the most general Langmuir-Hinshelwood forms (eq 4-6) into the selectivity expressions of eq 12 and 13, the following equations obtain
(14)
(15) The diffusional terms can be introduced into eq 14 and 15 via substitution for the ( C i / o ) terms from eq 7-9. The disk rotational speed was varied to determine whether transport limitation of the intermediate product, styrene, away from the catalytic surface played a role in the selectivity behavior. It was found that, within the experimental error, the selectivity factors SZ and S3 were independent of the disk rotational speed. For example, a fourfold decrease in the rotational speed lowered the original rate of hydrogen consumption by 25%, but did not affect the selectivity. (Cf. Table I, Runs 121 D-F; and White, 1972.) From these results it hence follows that the ratios ( C i / o / C l / o ) ,i = 2,3 in eq 14 and 15, respectively, are not functions of the disk speed. Further, it must be that the surface concentrations of the individual hydrocarbon reactants are also constant with rotational speed change. Examining eq 15, c1/0 and c 3 / 0 must be constant if S3 does not change with rotational speed, since those two surface concentrations would change in opposite directions if the phenylacetylene surface concentration c 1 / 0 were variable. This invariance of the a-methylstyrene and phenylacetylene surface concentrations with disk speed is expected when the bulk concentration of the latter and its collision rate with the catalytic surface are high. Other investigators of the liquid-phase competitive hydrogenation of olefins (Germain, 1969) have also affirmed this constancy of the reaction rate ratio despite change in the amount of catalyst, the agitation rate, and the conversion.
In addition, it must follow from the invariance of S Z with speed that the styrene surface concentration C Z / O also does not change. In other words, the same proportion of styrene molecules react to form ethylbenzene at high or low disk rotational speeds. This implies a lack of dependence of Sz on the diffusion of styrene away from the catalytic surface. Bond and Rank (1964) also concluded that olefinic transport limitation was negligible in their investigation of the liquid-phase isomerization of 1-pentene over Pd/C. Summarizing, the entire decrease in each of the individual production rates for styrene, ethylbenzene, and cumene as the rotational speed was lowered is attributable to only the drop in the surface concentration of hydrogen. The constancy of the percent decrease of the ethylbenzene and the styrene production rates (i.e., of S Z ) and of the cumene production rate (i,e,, of S 3 ) as the speed was lowered leads to the conclusion that the reactions of eq 1-3 can be considered to be pseudo-zeroth order in their respective hydrocarbon reactants. In other words, the surface concentrations of the unsaturated hydrocarbon species were high and therefore approximately constant. In partial support of this conclusion, the production rate of ethylbenzene was found to be independent of the bulk phenylacetylene concentration. Additional data indicate that, aside from the results of Figure 2 (which can be ascribed to the presence of phenylacetylene diffusional control in addition to that of hydrogen), S Z and Sa were independent of the reactor temperature. This lack of dependence is attributable primarily to the fact that the apparent activation energies Ez* and E3* (cf. next section) are not very far removed from El*. This statement is made in view of the rather narrow temperature range investigated (AT = 50°C) and in view of the redefinition of the reaction rate expressions ri in terms of the apparent or pseudo-first-order reaction rate constants of eq 7-9. Other investigators (Bond and Wells, 1964; Mann and Khulbe, 1969) have also reported no effect of temperature on selectivity during the hydrogenation of acetylene over palladium. Arrhenius Parameters. Figure 3 presents the Arrhenius plots of the pseudo-first-order apparent reaction rate constants ki* describing the reactions of eq 1-3. Since quite different results were obtained for El* over each of the two catalytic disks employed, separate least-squares lines are shown for each of the disks for the phenylacetylene-hydrogen reaction, as well as for the reactions producing ethylbenzene and cumene. The apparent Arrhenius activation energy E3* for the hydrogenation of a-methylstyrene in the presence of phenylacetylene on disks Pd-8 and Pd-9 compares quite favorably with that for the hydrogenation of AMS alone (White et al., 1974). The value of E3* was found to be 8.4 f 1.3 kcal/g-mol, compared with 7.4 i 1.2 kcal/g-mol in the earlier study. No attempt was made to relate the activation energy and frequency factor differences to the operation of a compensation effect because of the relatively large experimental error, due in part to the low cumene production rates after the addition of phenylacetylene to the reactor. Nevertheless, some conclusions can be drawn from the above agreement of the a-methylstyrene activation energies with and without the presence of phenylacetylene. The activation energies reported are actually apparent activation energies, since they have not been corrected for the heats of adsorption of the related species. The activation energy for the AMs-hydrogen reaction in the phenylacetylene environment could be higher because of the contribution of the heat of adsorption of PA. That is, if the mechanism of eq 6 were correct and the denominator Ind. Eng. Chem.. Fundam., Vol. 14, No. 3, 1975
187
double bond on the ease of interaction of the adsorbed *-bond with neighboring adsorbed hydrogen atoms. In general agreement with the above result for styrene, apparent activation energies of 5-10 kcal/g-mol have been reported for the same reaction over various platinum (Sokol'skii, 1964) and palladium (Shutt and Winterbottom, 1971) catalysts. The apparent activation energy El* for the hydrogenation of phenylacetylene differed appreciably in experiments over two different catalytic disks, suggesting the operation of a different mechanism in each case. On disk Pd-8, El* was found to be 10.9 f 1.9 kcal/g-mol, whereas the use of disk Pd-9 resulted in a value of 5.8 1.3 kcal/g-mol. The former result agrees with that of Shutt and Winterbottom (1971), who reported apparent activation energies for the hydrogenation of phenylacetylene over reduced palladium oxide of 7.1-11.0 kcal/g-mol depending upon the particular solvent used. The higher values were obtained when the solvent was n-heptane, the dielectric constant of which parallels those of aromatic hydrocarbons with saturated or unsaturated side-chains. In addition, values for the apparent activation energy generally range from 10 to 12 kcal/g-mol for the hydrogenation of acetylenic compounds (Bond, 1962; Sokol'skii, 1964). Reshetnikov and Sokol'skaya (1964), however, reported apparent activation energies of 2-5 kcal/g-mol for the hydrogenation of phenylacetylene over palladium. They attributed their results to the presence of virtually no adsorbed atomic hydrogen on the catalytic surface, the reaction mechanism being described by the combination of negatively charged phenylacetylene ion radicals and positively charged hydrogen ions. This electronic mechanism can be interpreted as a phenomenon whose rate-limiting step is the diffusion of dissolved hydrogen in the form of protons through the palladium lattice to the catalytic surface. Support for this mechanism is provided by the work of several investigators (Lewis, 1967) who have reported activation energies for the diffusion of hydrogen in the a and p phases of palladium hydrides of 6.0 and 5.7 kcal/gmol, respectively. In view of these resuIts and the previously cited value of 5.8 d= 1.3 kcal/g-mol for El* on disk Pd-9 in the present study, there is evidence for the operation of this limiting mechanism over that particular catalytic disk. On the other hand, when the adsorbed surface concentration of hydrogen is low but of sufficient magnitude to account for the entire hydrogenation rate, the lattice-diffusion mechanism is not rate-determining. In this case, the reaction is controlled by the hydrogen adsorptionreaction sequence on the palladium surface, and the corresponding activation energy is 10-12 kcal/g-mol. The reconciliation of this two-mechanism approach with the results of this study is possible, considering the large difference in preferred orientation of disks Pd-8 and Pd-9 and the consequent differences in the relative amounts of the various reacting species, particularly phenylacetylene and hydrogen, adsorbed on the surface. The quite predominant (110) preferred orientation of disk Pd-8 is significant, since the (110) plane of fcc metals is more favorable for hydrogen adsorption than either the (100) or the (111)plane, the latter, in fact, being extremely unconducive to the adsorption of that species (Sokol'skii, 1964; Clark, 1970). Consequently, over disk Pd-8 phenylacetylene hydrogenation is believed to have proceeded via combination with adsorbed hydrogen on the predominant (110) plane, giving rise to the high activation energy. Disk Pd-9, however, showed only very slight (110) preferred orientation and hence presented significantly lower
*
(1IT)X
Id
(OK-')
Figure 3. Arrhenius plot of pseudo-first-order apparent rate con-
stants, hydrogenation of AMS and PA; open symbols, Pd-8, filled symbols, Pd-9: 0.0,cumene; 0,m, styrene; A, A, ethylbenzene.
reduced to the term (blc1,o) because of the stronger adsorption and much higher surface coverage of phenylacetylene, we would have
E3* = E ,
+
AH,
+
AH3
-
AH1
(16)
On the other hand, for the hydrogenation of a-methylstyrene alone the following equation obtains E3* = E3 + AH, (17) No attempt is made to quantify these relations because of the extreme variability of the heat of adsorption with surface coverage, mode of catalyst preparation, and other factors. It is expected, however, that the difference AH3 AH1 between the heats of adsorption of AMS and phenylacetylene is large and accordingly should produce a significant increase in E3* in the combined hydrocarbon system according to eq 16. Since the results do not show any great increase, it is concluded that the only contribution to the a-methylstyrene apparent activation energy in the PA experiments was from the intrinsic activation energy E3 and from the heat of adsorption of hydrogen per eq 17. Phenylacetylene can be assumed to affect the a-methylstyrene reactivity only as it affects the surface coverage of the latter-and hence the frequency factor. Phenylacetylene had no effect on the energetics (Le., the facility) of the combination of a-methylstyrene with hydrogen on the palladium surface. Additionally, from the above result it can be concluded that the pseudo-first-order kinetics in hydrogen obtain for the a-methylstyrene-hydrogen reaction in the presence of phenylacetylene. The apparent activation energy Ez* for the hydrogenation of styrene was found to range from 5 to 8 kcal/mol on either disk, which values are close to those above for AMs. The experimental error in the styrene determinations was rather large, again due to the low reactivity of the olefinic species in the presence of the alkyne. This circumstance precludes the drawing of definitive conclusions regarding the effect of the a-methyl substituent at the 188
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amounts of the longer metal-metal spacings believed to be conducive to acetylenic adsorption. Hence, quite conceivably phenylacetylene could have saturated these active sites on the (110) faces and subsequently reacted with hydrogen ions which had entered the palladium lattice after dissociative adsorption on active sites on the (111) faces. Unfortunately, the results of phenylacetylene hydrogenation as reported by other investigators do not give any indication of the crystallographic structure of their catalysts. Further experimental studies should hence be undertaken in order to confirm the above correlation of the alkyne activation energy differences with the surface concentration of adsorbed hydrogen and with the degree of preferred orientation. For example, the effect of pre-run saturation of the palladium film with hydrogen should be studied on surfaces of widely varying degrees of preferred orientation. Reaction Order. It was found in the present study that a threefold increase in the hydrogen pressure resulted in a like increase in the reaction rate of a-methylstyrene, thus justifying the first-order functionality of the hydrogen concentration for the AMS-hydrogen reaction of eq 3. Quantitative determinations were not made of the individual hydrocarbon reaction orders for any of the three hydrogenation reactions under study. From the discussion of the previous sections, however, it has been seen that pseudo-zeroth-order expressions can be approximated for the unsaturated hydrocarbon reactants under the experimental conditions investigated, in agreement with Matsumot0 et al. (1972), who studied the liquid-phase hydrogenation of phenylacetylene over alumina-supported platinum. At phenylacetylene concentrations high enough to ensure that the species was not diffusion controlling, the PA and a-methylstyrene surface concentrations are effectively constant and are hence adsorbed into the pseudo-first-order reaction rate constant. Also, the disk rotational speed change results indicated that the same simplification procedure could be used for the styrene surface concentration. The hydrogen pressure change data further indicated that a t the low (60 psia) hydrogen pressures employed in this work, relationships first-order in the hydrogen concentration could also be used to describe the reactions of eq 1 and 2. Consequently, under the mild experimental conditions employed here, the pseudo-first-order expressions of eq 7-9 obtain for the chemical system under investigation. Similar results have been reported in other studies of acetylenic and ethylenic hydrogenations over platinum metals (Bond, 1962; Sokol’skii, 1964; Bond and Wells, 1964; Matsumoto et al., 1972). Additional observations relating to the hydrogen pressure change data lead to the conclusion that long periods of exposure of the catalytic disk surface to hydrogen pressures significantly higher than normally employed resulted in a n irreversible poisoning of the palladium surface for a-methylstyrene hydrogenation and a n enhancement in the rate of styrene production (cf. Table 11, Runs 123 M,R). Other investigators have attributed similar activity changes to the onset of the P-palladium-hydrogen phase, which contains much higher concentrations of absorbed hydrogen (Babcock et al., 1957; Scholten and Konvalinka, 1966). In this regard, the theory has been advanced that the dissolved hydrogen donates a sufficient number of electrons to the unfilled d-band of palladium to affect the chemisorption of the olefinic but not of the more strongly adsorbing acetylenic species (Bond et al., 1958). An order of magnitude calculation (White, 1972) sup-,
ports the possibility of the a- to 8-Pd-H phase transition a t the higher hydrogen pressures used in this study. The calculated surface concentration of hydrogen a t 60 psia in the present system fell within the critical region on the 70°C hydrogen-palladium adsorption isotherm. This result, in conjunction with the possible combination between phenylacetylene and hydrogen dissolved in the palladium lattice (as discussed in the previous section), lends support for the onset of the P-Pd-H phase in this study. Further deactivation was noted during instances when the same catalytic disk was used for different experiments. Since this deactivation rendered the catalytic surface less active for the hydrogenation of phenylacetylene but not a-methylstyrene, and because of the residual unsaturation of phenylacetylene in the chemisorbed state, the cause may have been irreversible PA polymerization on a hydrogen-deficient surface a t the start of the run. A polymer test on the hydrocarbon liquid according to the method of Boundy and Boyer (1952) was negative but may have been too insensitive in view of the fact that only 0.02 pmol of a catalytic poison such as oxygen is sufficient for monolayer coverage of the surface of an fcc metal (White, 1972).
Conclusions and Significance The catalytic rotating disk has many advantages for the study of selective reaction behavior in a multi-reaction system, particularly one dependent upon the crystalline nature of the catalytic surface. The feature of the equiaccessibility of the rotating disk surface with respect to all diffusing species offers an unbiased approach for the determination of the limiting diffusing reactant. In the present study, it was found that the limiting diffusing reactant was hydrogen, all other species residing in relative abundance on the catalytic surface. The ease of preparation and characterization of catalytic films provides the opportunity for determination of the relationship between the catalytic activity and selectivity on the one hand and the specific surface properties (degree of crystallinity and preferred orientation, crystallite size, lattice defects) on the other. It was determined in the present study that the (110) face of palladium is especially favorable for selective alkene formation in alkyne hydrogenation. The use of a weaker adsorbate as the reactive solvent is an effective way to gauge the degree of surface coverage of the more strongly adsorbing species. Furthermore, the use of such a solvent does not affect the reactions of the solutes on the catalytic surface; e.g., the results (selectivity, order, activation energy) of the hydrogenation of phenylacetylene over palladium in a-methylstyrene solvent compared favorably with those of other investigators employing n-heptane (Shutt and Winterbottom, 1971) or methanol (Sokol’skii, 1968) as the inert solvent. Conversely, the introduction of a more strongly adsorbing reactant (phenylacetylene) into the bulk phase did not affect the mechanism of the solvent (a-methylstyrene)-hydrogen reaction on the catalytic surface. The individual reaction orders and the activation energy of the latter reaction were found to be the same as those when the solvent alone reacted with hydrogen on the palladium surface. The (110) face of palladium is approximately as active for the hydrogenation of phenylacetylene as it is for the hydrogenation of a-methylstyrene alone. Discounting the probably unimportant steric effect of the a-methyl group, this result implies the operation of a geometric factor in adsorption, since the (110) plane contains equal numbers of the longer and shorter fcc metal-metal spacings which respectively favor acetylenic and ethylenic adsorption. Ind. Eng. Chem., Fundam.. Vol. 14, No. 3, 1975
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The apparent activation energy for the hydrogenation of phenylacetylene to styrene mirrors and depends upon the form of the participatory hydrogen atoms. There is an indication that under certain conditions the rate-limiting step is the diffusion of dissolved hydrogen in the form of protons through the palladium lattice to the catalytic surface. The consecutive liquid-phase hydrogenation of phenylacetylene over palladium can be represented by zeroth-order kinetics in PA and styrene, and first-order kinetics in hydrogen for the series reaction scheme. The a-methylstyrene-hydrogen reaction in the same system was describable by zeroth-order hydrocarbon and first-order hydrogen kinetics. Acknowledgments Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. One of the authors (D.E.W.) was also supported by a Ford Foundation Fellowship and by intradepartmental funds provided by the Department of Chemical and Biochemical Engineering of the University of Pennsylvania; these grants are gratefully acknowledged. We also thank Mr. Donald M. Mattox of Sandia Laboratories far his aid in preparing the catalytic disks. Nomenclature A = hydrocarbon species symbol b = adsorption equilibrium constant, cm3/g-mol c = bulk concentration E = intrinsic Arrhenius activation energy, kcal/g-mol E* = apparent Arrhenius activation energy, kcal/g-mol -AH = heat of adsorption, kcal/g-mol Hz = hydrogen species symbol k = intrinsic reaction rate constant, g-mol/sec k* = apparent or pseudo-first order rate constant, cm3/sec P = system pressure, psia r = rate of reaction, g-mol/sec Sz = selectivity factor, defined in eq 12 S3 = selectivity factor, defined in eq 13 T = temperature, "K
Subscripts 1 = species component phenylacetylene 2 = species component styrene 3 = species component a-methylstyrene 4 = species component ethylbenzene 5 = species component cumene H = species component Hz, hydrogen i = species component, 15 i 5 5 J = species component, 1 5 j 5 5 0 = refers to value a t catalytic surface Literature Cited Babcock. 6. D.. Mejdell, G. T., Hougen. 0. A.. A./.Ch.E. J . , 3, 366 (1957). Bal'yan, K. V . , Borovikova, N. A.. J. Gen Chem. USSR, 29, 2520 (1959). Bond, G . C.. "Catalysis by Metals," Academic Press, New York. N.Y.. 1962. Bond, G. C.. Dowden, D. A.. Mackenzie. N., Trans faraday SOC., 54, 1537 ( 1958). Bond, G. C.. Rank, J. S., Proc. Int. Congr. Cat., 3rd. (1964). Bond. G. C.. Sheridan. J.. Trans. FaradaySoc.. 48, 651 (1952). Bond, G .C., Wells, P. B., Adv. Cat., 15, 91 (1964). Boundy, R. H..Boyer. R . F.."Styrene," Reinhold, New York, N.Y., 1952. Clark. A . . "The Theory of Adsorption and Catalysis," Academic Press, New York, N.Y., 1970. Frank-Kamenetskii. D. A., "Diffusion and Heat Exchange in Chemical Kinetics," 2nd ed, Trans. Ed. J. P. Appleton. Plenum Press, New York, N.Y., 1969. Freidlin. L. K.. Kaup, Y. Y.. Izv. Akad. Nauk SSSR, Ser. Khim., 12, 2047 (1964). Germain. J. E., "Catalytic Conversion of Hydrocarbons." Academic Press, New York, N.Y.. 1969. Lewis, F. A . . "The Palladium-Hydrogen System," Academic Press, New York, N.Y.. 1967. Mann. R. S..Khulbe, K. C.. Can. J. Chem., 47, 215 (1969). Matsumoto, S.. Fukui, H.,Imanaka, T.. Teranishi. S., Nippon Kagaku Kaishi. 8 , 1527 (1972): (Chem. Abstr.. 77, 128,572v, 112 (1972)). 85, 2877, 2881 (1963). Meyer, E. F.. Burwell. R. L., J . Am. Chem. SOC., Reshetnikov, S. M., Sokol'skaya, A. M., Russ. J. Phys. Chem.. 39, 723 (1965) Scholten. J. J. F., Konvalinka. J. A., J. Cat.. 5, 1 (1966) Shutt, E.. Winterbottom. J. M., Plat. Met. Rev.. 15, 94 (1971). Sokol'skii. D. V . . "Hydrogenation in Solutions," trans., Daniel Davey. New York. N.Y.. 1964. Sokol'skii. D. V.. I V In/. Congr. Cat.. Moscow. 1968. Preprint No. 45. Wheeler, A., "Reaction Rates and Selectivity in Catalyst Pores." in "Catalysis," 2nd ed., P. H. Emmett, Ed.. Reinhold, New York. N.Y., 1955. White, D. E.. Ph.D. Thesis, University of Pennsylvania, Philadelphia, Pa., 1972. White. D . E., Lift. M.. Heyrnach, G. J . , I l l , Ind. Eng. Chem , Fundam. 13, 143 (1974).
Creek Letters @ = mass transfer coefficient through liquid film, cm/sec o = rotational speed of disk, radians/sec
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Received for review May 17,1974 Accepted April 3,1975
