at Carnegie-Mellon University. The author is grateful to both Carnegie-Mellon University and Westinghouse R & D Center for supplying the analytical instruments used in this work. Literature Cited (1) Booth, F.. Proc. Roy. SOC.London, Ser. A, 203, 533 (1950). (2) Boscoe, R., Brit. J. Appl. Phys., 3, 267 (1952). (3) Brodngan. J. G.. Kelley, E. L.. J. Colloid Sci., 20, 7 (1965). (4) Brookfield Synchro-Lectric Instruction Manual. Brookfield Eng. Labs., Stoughton, Mass. (5) Bruere, G. M., Constr. Rev., 16-21, 164 (Feb 1964). (6) Bruere, G. M.. "Effects of Mixing Sequence on Morter Consistencies When Using WaterReducing Agents," Symposium on Structure of Portland Cement Paste and Concrete Highway Research Board, 1966. (7) Chan, F. S., Gering, D. A. T.. J. Colloid Sci., 22, 371 (1966). (8) Davis, J. T., Rideal, E. R., "Interfacial Phenomena", p 139, Academic Press, New York, N.Y., 1961. (9) Einstein, A., Ann. Phys., 4, 19 (1906). (10) Eirich, F. R., "Rheology", Vol. 3, pp 9-1 19, Academic Press, New York, N.Y., 1960.
(11) Elton, G. A. H., Proc. Roy. SOC.London, Ser. A, 197, 568 (1949). (12) Ernsberger, F. M., France, W. G., lnd. Eng. Chem., 37, 598 (1945). (13) Fryling, C. F., J. ColloidSci., 18, 7B(1963). (14) Harmsen, G. J., Schoater, J. V., Overbeek, J. Th. G., J. ColloidSci., 8, 64, 72 (1953). (15) Krasny-Ergen, W., Kolloid-Z., 74, 172 (1936). (16) Micromeritics Corp., Norcross, Georgia. (17) Mooney, M., J. ColloidSci., 6, 162 (1951). (18) Petrie, E. M., Westinghouse Research Center, unpublished work, 1974. (19) Schaller, E. J., Humphrey, A. E., J. Colloid lnterface Sci., 22, ST3 (1966). (20) Sennett, P., Olivier, J. P., lnd. Eng. Chem., 57, 33 (1965). (21) Smoluchowski, N., Kolloid-Z., 18, 190 (1916). (22) Street, N., J. ColloidSci., 12, l(1957). (23) Zeta-Meter Manual, Zeta-Meter Inc., New York, N.Y. (24) Zhuravlev, V. F., Tikhonor, V. A,, J. Appl. Chem. USSR, 25, 1317 (1952).
Received for review April 15, 1976 Accepted July 30, 1976
Catalytic Characteristics of a Rhodium Complex Attached to an Aromatic Polyamide Tae H. Kim and Howard F. Rase* Department of Chemical Engineering, The University of Texas at Austin, A'ustin, Texas 787 12
The characteristics of the versatile Wilkinson homogeneous catalyst, tris(triphenylphosphine)chlororhodium(l) bound to a rugged, high-melting polymer, poly( mphenyleneisophthalamide), have been studied in hydrogenation of olefins. Steam treatment of the polymer produced a high surface area which in turn gave the most active polymer-bound catalyst. The bound catalyst exhibited similar selectivities and poison resistance as the homogeneous form, and was less active but more resistant to deactivation at higher temperatures.
In comparison to heterogeneous catalysts, homogeneous catalysts often exhibit improved selectivity and higher activity at modest temperatures. This latter characteristic has caused an even greater interest in homogeneous catalysts as a means for reducing energy consumption in commercial processes. Homogeneous catalysis is employed in manufacturing approximately 15%of the total value of products based on catalytic processes (Heinemann, 1971). One of the reasons preventing further growth in commercial use is the difficulty in recovering the catalyst for reuse in the large majority of cases where the catalyst is too valuable to discard. In highly successful processes such as the Oxo process, the difficulty and cost of recovering the catalyst (Lemke, 1966) are apparently overshadowed by its unique character and efficiency. One major means under investigation for overcoming the separation problem is to attach the homogeneous catalyst, usually a transition metal complex, to an insoluble polymer or silica. Such techniques for preparing so-called heterogenized homogeneous catalysts have been thoroughly reviewed by Burwell (1973), Heinemann (1971), Manassen (1969), Michalska and Webster (1975), and Pittman and Evans (1973). See also Delmon and Jannes (1975). Many problems remain to be solved before widespread commercial use will be
possible. These include in many cases a markedly lower activity of the bound catalyst compared to that of the free catalyst in solution and, in the case of polymer-bound complexes, a low maximum use temperature dictated by the melting point of the polymers generally employed, polystyrenes and polymethacrylates. The purpose of the present study was to develop and evaluate a polymer-bound catalyst using a rugged, high-melting polymer. For this purpose we selected a commercial poly( m -phenyleneisophthalamide), Nomex, which has a melting point of 427 "C and is soluble only in concentrated sulfuric acid at room temperature and in boiling dimethylacetamide with 3% CaC12. Such a support could increase the utility of any bound catalyst by expanding the range of reactants and solvents that might be used with it and increasing the range of operable temperature so that either reactions could be conducted a t higher temperatures when necessary or temperature runaways would not destroy the integrity of the support. The versatile Wilkinson hydrogenation catalyst was selected, tris(tripheny1phosphne)chlororhodium(I), because it has been thoroughly studied in the homogeneous form, and it has also been attached to a chloro-methylated styrene-divinylbenzene with a 1.8% cross-linking by Grubbs and coworkers (1971, 1972). It has Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
249
Table I. Effect of Preparation on Phosphorus Content of Polymer
Run no
Surface area, m2/g
Polymer
Reflux solvent
PhZPCl, m'l
Filament Toluene 9.0 PLM-P-3 Filament Xylene 9.0 PLM-P-4 Rope PLM-P-5 4.32 Toluene 9.0 Rope PLM-P-6 4.32 Toluene 9.0 PLM-P-8 Rope 97.66 Toluene 9.0 PLM-P-20 Rope 97.66 Xylene 9.0 PLM-Rh-20 Rope 97.66 Xylene 9.0 Rope PLM-P-25 97.66 Xylene 9.0 Triethylamine was added. Polymer was pretreated with benzaldehyde. been shown to have an activity of 6% of an equivalent amount of homogeneous catalyst and some interesting selectivity effects due to the pore structure of the cross-linked polymer. Catalyst Preparation The procedure developed for attaching the rhodium complex to commercial poly(m-phenyleneisophthalamide) or Nomex consisted of phosphinating the polymer using diphenylphosphine chloride. H
1
Reactions of this type for both primary and secondary aryl and alky amides have been described (e.g., Sasse, 1963). Great care must be used to exclude air and moisture from the reaction system. The HCl was continuously swept from the reaction system by an inert gas (N2) purge. Then this phosphinated polymer was reacted with RhCl(PPh3):I to produce a chelated polymer-bound rhodium complex. The key to maximum activity was to produce a polymer of high surface area and maximize the phosphorus content of the polymer in the phosphinating step. Much of the initial effort was devoted to these tasks. Details are given in the Experimental Procedure section. Both Nomex rope and staple (T-450,1.5 denier, 1.5 in. cut length) were used. As shown in Table I, the ultimate phosphorus content was found to be a function of the total surface area of the polymer and the temperature of reflux during phosphinating as set by the boiling point of the solvent used. Nomex rope was selected as the preferred form of the polymer since it not only had higher exterior surface area than the filament but also could be steamed in a pressurized autoclave to substantially increase its total area. Pretreatment of the polymer with a softening agent such as dimethylacetamide and benzaldehyde was also tried as a means for increasing the ultimate phosphorus content. Apparently, however, the rhodium complex reacts with strong donor solvents yielding a complex of the form RhCl(PPh312 (solvent) which was a less active catalyst perhaps because this strong bonding with the complex prevented or retarded the olefin coordination and Rh-hydrogen addition processes (see Osborn et al., 1966). 250
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
Reflux time, h
Phosphorus content,
24
0.20 0.40 0.54 0.79 1.36 2.07 2.52
24 24 24
48 48 48 48
wt %
Remarks
1.06
Experimental Procedures and Equipment The most successful and recommended procedures are described in this section. Polymer Pretreatment. Five grams of an uncrystallized and undried form of Nomex rope obtained from the du Pont Co. was placed in a sample holder inside a small stainless steel autoclave and steamed a t 15 psig for 20 min. The sample was then placed in a Schlenck tube and dried under vacuum. The vacuum was adjusted by means of a needle valve to prevent freezing of the sample. After the major portion of water had been removed (approximately 30 min) the needle valve was completely opened so that the highest vacuum (500 F ) could be obtained for final drying (an additional 10 h). This procedure consistently produced polymer with a surface area of about 100 m2/g as measured by a Sor-Bet, Model 5-7300 adsorption apparatus (Aminco). The surface area of the untreated rope was below the lower limit of measurement of the adsorption apparatus (1m2/g). Its exterior surface based on density and denier was 0.0024 m2/g. Phosphination of Polymer. A sample of Nomex polymer (0.3 g = 1.28 mmol of repeating units) was placed in a 100 ml, 3-necked, round-bottom flask with a reflux condenser fitted to the center neck and a dropping funnel in a side neck. The system was evacuated and flushed with nitrogen three times through a connection at the top of the condenser. Then 25 ml of dried and deoxygenated p-xylene (CP grade, bp = 138 OC) was added to the polymer with a syringe inserted in a serum cap on the dropping funnel. Then 9 ml of diphenylphosphine chloride was dissolved in 25 ml of p-xylene in the dropping funnel and slowly added to the polymer-xylene mixture in the flask. The mixture was refluxed under nitrogen for 48 h a t atmospheric pressure and maintained constant by a mercury bubbler. Gaseous HC1 product was purged out of the system by the nitrogen flow. Stirring was accomplished with a magnetic stirrer. After the reaction, the polymer was washed several times in benzene and stored under nitrogen. It was pale yellow in color and contained 2.5% of phosphorus. (See Table I1 for complete elemental analysis.) Rhodium Complex Addition. A sample of phosphinated polymer (0.3 g, 0.2 mmol) was added to RhCl(PPh3)3 (0.3 g, 0.324 mmol), manufactured by Alfa Inorganics, dissolved in 30 ml of benzene under nitrogen. The mixture was equilibrated for 4 weeks after which the polymer was washed repeatedly with benzene until the rinse showed no coloration. Ultraviolet spectrophotometric observations (Beckman ACT AIII) showed no rhodium in these clear rinse solutions at h 500 mp ( t 740 for 0.216 mM solution Rh complex in benzene). The polymer-bound rhodium complex was filtered and dried under vacuum. Elemental analysis is given in Table I1 and an ir spectrum for the polymer-bound complex is given in Figure 1. Hydrogenation Experimentations. Hydrogenations were
Table 11. Elemental Analysis of Catalyst Samples Wt % STOPCOCK
Sample
Sampleno.
C
H
N
P
1
Rh MANOMETER
Dried Nomex
BALL JOINT
PLM-S-11 66.49 4.59 9.84 66.71 4.46 9.66 PLM-P-20 65.82 4.61 6.96 2.07
Phosphinated polymer Rhodium complex PLM-Rh- 68.58 4.95 6.90 2.52 0.95 polymer 20 PLM-Rh0.97 20" PLM-Rh0.80 24 " For PLM-Rh-20, the elemental analysis was performed after the hydrogenation of substrates.
LEVELING
lr
HEATING C O I L
::vw
Figure 2. Hydrogenation apparatus.
40
-
2
0
2
-
10
11
14
t
16
MICRONS
Figure 1. Infrared spectrum of the polymer-bound catalyst. studied using the apparatus depicted in Figure 2. Alternate vacuum-hydrogen cycles were used t o flush air out of the system using hydrogen purified by passing through a Deoxo purifier (Engelhart)for removing trace oxygen which can affect the rate of hydrogenation drastically as shown by Mitchell e t al. (1971) and Van Beekkum e t al. (1969). T h e Deoxo unit was followed by a drier (3-ft column containing a n indicating Drierite) for removing moisture. Benzene was then added t o the reactor flask with a syringe through a serum cap in the reactor side arm. This was followed by three more vacuumhydrogen cycles after which the compound to be reduced was added with a syringe. The reaction mix was stirred magnetically and operated a t atmospheric pressure in hydrogen. Hydrogen uptake readings were observed on the manometer during the course of the reaction. Analytical Procedures. In addition to observed values of hydrogen uptake, the product was analyzed by gas chromatography using a Perkin-Elmer Model 154 vapor fractometer, with a %-in.o.d. X 2-m column packed with 15 wt % polypropylene glycol (UCON LB-550-x) Chromosorb NAW, 8D/100 mesh. At operating conditions of 70 "C and 48 cm"/min helium flow, good separations between cyclohexane, cyclohexene, and benzene were obtained and also between hexene-1, hexane, and benzene. Reaction products, polymer, and final catalysts were also analyzed using a Perkin-Elmer Model R12, NMR spectrometer. A Beckman Model IR-S infrared spectrophotometer and Philips Electronic Model 12215/0 x-ray diffractometer were also used as will be described. Carbon, hydrogen, nitrogen, and phosphorus analyses were made by Chemalytics, Inc., Tempe, Ariz., using a PerkinElmer Model 240 elemental analyzer. T h e procedure for phosphorus was adapted from a method described by Scroggins (1968). Rhodium was analyzed by Galbraith Laboratories, Knoxville,Tenn., using atomic absorption after wet digestion of the samples. ESCA spectra were obtained using a Hewlett-Packard 5959A ESCA spectrometer through the courtesy of Dr. H. Spell, Dow Chemical, Freeport, Texas.
l
i
TIYE
(MI~J-E)
Figure 3. Hydrogenation of hexene-1 and octene-1 by the polymerbound rhodium complex and the equivalent amount of homogeneous rhodium complex (0.019 mmol of Rh for each) at 25 OC. Performance of Catalyst The bound catalyst was tested in various ways and compared to a n equivalent amount of homogeneous catalyst. Activity Characteristics. Activities of the polymer-bound catalyst and the homogeneous catalysts were observed a t 25 "C by measuring the rate of uptake of hydrogen with time in the batch reactor at constant hydrogen pressure. These results are summarized in Figure 3 for hexene-1 and octene-1. Calculated data based on the following equation, originally derived by Osborn e t al. (1966) for the homogeneous catalyst, fit data for both catalyst systems as noted by Grubbs (1973) for polystyrene-bound catalyst.
where (H2), (S),(Rh) = concentrations of Hz, substrate, and rhodium complex in substrate in moles per liter, respectively. This agreement in fit does not necessarily imply exactly similar mechanisms, and eq 1 should be regarded as a useful empiricism. T h e rate constants given in Figure 3 were obtained by fitting the observed data to eq 1.The activity of the polymer-bound catalyst is seen t o be approximately 10% of that of a n equivalent amount of homogeneous catalyst. Effect of Diffusional Resistances. Homogeneous hydrogenations of hexene-1 were conducted using two different concentrations of rhodium complex (1.0 and 0.5 mM) in order t o observe possible mass-transfer limitations for hydrogen Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
251
.
,.
0 ~
HEXEVEE-i
PLY-R',
- 2C
2 1g 3 c 11 '. 2 5 -1
. fl
0 SECOND RUN
$10.0 '-
RHODIUM COMPLEX HEXENE- 1
2 7 7 5 mg 1 2 5 ml
BENZENE
30 n l
ii ;" (1
BENZENE
x
0 F I R S T RUL
s
v
-15.0
0.0' 0 50
150
100
'
"
2
1
4
'
6'
"
8
'
fL'
J
NUMBER O r RUNS
TliYE (MINUTE)
Figure 4. Hydrogenation of hexene-1 by homogeneous rhodium complex (0.03 mmol) a t 25 "C-sequential run.
0 F I R S T RUN w
SECOND RUN
c < RHODIUM t3MPLEX WEXENE- 1 BEYZENE
25
50
75
T I Y E (MINUTE)
Figure 5. Hydrogenation of hexene-1 by homogeneous rhodium complex (0.019 mmol) at 45 OC-sequential run. transfer to the solvent a t the same conditions of temperature, degree of stirring, substrate concentration, and hydrogen pressure. The data for each run yielded the same rate constant when fitted to eq 1,indicating that diffusion of hydrogen in the range of conditions studied was rapid and did not affect the observed rate. In the polymer-bound system, diffusion rates of two different sizes of olefins (hexene-1and octene-1) apparently were not the same, and that of octene-1 was slow enough to affect the observed rate as shown in Figure 3. Rate constants for hexene-1 and octene-1 were identical for the homogeneous catalyst, but on the polymer-bound catalyst that for octene-1 was significantly lower while that for hexene-1 remained the same. Effect of Temperature. Reaction temperatures were varied from 25 to 45 "C. In this temperature range, all runs exhibited the standard Arrhenius temperature dependency, and the activation energies of both systems were the same order of magnitude ( E h o m o = 9.4 kcal/mol and Epoly= 8.3 kcal/mol). Figures 4 and 5 compare the catalytic activity of sequential experiments using the same homogeneous rhodium complex at 25 and 45 "C. A sequential run involves the repeated addition of substrate to the reaction vessel after complete conversion of that substrate previously added. In sequential runs at 25 "C, the catalytic activity of the homogeneous catalyst remains constant, whereas the activity is decreased by almost 252
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
Figure 6. Effect of catalyst service life on activity of polymer-bound rhodium complex (0.0095 mmol of Rh) at 45 "C.
50% a t 45 "C. Catalytically inactive and insoluble dimers or dimeric hydrides are apprently formed a t high temperatures or high concentrations of the complex as described by Osborn et al. (1966). Catalytic deactivation was much less in the polymer-bound catalyst system shown in Figure 6. It is likely that dimerization might be prevented to a large degree by the rigidity of the resin matrix of the heterogeneous system. The observed loss in activity cannot be attributed to loss of rhodium, for as shown in Table I1 no change in rhodium content occurs with use. Selectivity Characteristics. I t has been postulated that the ligands surrounding the metal ions may influence the selectivity of the catalyst (Hamer and Walton, 1974; Houghton, 1973; Morrison and Burnett, 1971; Morrison, 1971; Osborn et al., 1966). The ligand is in fact changed by attachment to the Nomex polymer. A test for possible variations in selectivity, therefore, was made by using a compound which had more than one double bond, d -carvone. This compound has been reported by Birch and Watson (1966) to rapidly and specifically hydrogenate on the isopropylidene group in the presence of tris(tripheny1phosphine)chlororhodium to give carvotanacetone. NMR spectra of the product of d-carvone hydrogenation a t 25 "C with the homogeneous and bound catalyst showed the disappearance of the vinyl methylene signal a t 6 = 4.72 (ppm) but no change in the cyclohexene proton a t 6 = 6.62 (ppm). All other peaks also remained unchanged; and the presence of only carbotanacetone a t complete reaction for both catalysts was confirmed, which supports a conclusion of equal selectivity for this reaction with both forms of catalyst. The tests were conducted using equivalent amounts of rhodium (0.019 mmol) for both catalyst systems in 30 ml of benzene with 1.56 ml of d-carvone added. The activity for the polymer-bound catalyst in this reaction was approximately 8%of an equivalent amount of homogeneous catalyst. Effect of Sulfur Compounds. Although many heterogeneous metal catalysts are deactivated by the poisoning action of sulfur compounds, these effects are not very serious in the homogeneous rhodium complex system (Birch and Walker, 1967). Hydrogenations of hexene-1 in the presence of sulfur compounds were conducted to determine effects of sulfur poisoning in the polymer-bound catalyst system. The data shown in Table I11 indicate that a low concentration of sulfur, 0.8 vol% of the substrate, did not affect the initial rate of the hydrogenation. However, in high concentration, 8.0%, the initial rates decreased markedly in the presence of diethyldisulfide and n-hexanethiol. Thiophene did not affect the initial rate in this concentration. A separate study of thiophene was initiated to determine whether the rhodium complex acts as a desulfurization cata-
Table 111. Hydrogenation of Hexene-1 by the Polymer-Bound Catalyst in the Presence of Sulfur Compoundsa Without sulfur
Diethyl disulfide
compound min
0.01 ml min
ml
ml
N-Hexanethiol
0.1 ml min
0.01 ml ml
min
ml
Thiophene
0.1 ml ‘min
0.01 ml
ml
min
ml
5 1.3 5 1.2 5 0.2 5 1.2 5 0.2 5 1.3 10 2.6 10 2.5 10 0.5 10 2.4 10 0.4 10 2.6 15 4.0 15 3.7 15 3.6 15 0.6 15 4.1 20 5.3 20 5.0 20 1.0 20 4.8 20 0.8 20 5.4 25 6.6 25 6.2 25 25 6.0 25 6.8 30 1.4 30 7.2 30 1.1 30 8.0 30 7.8 30 7.4 40 10.5 40 9.8 40 1.9 40 9.5 40 1.5 40 10.5 50 13.3 50 12.2 50 2.4 50 12.0 50 1.9 50 13.0 2.8 60 14.3 60 2.2 60 15.8 60 15.7 60 14.6 60 a Catalyst: PLM-Rh-22,0.1g (0.0095 mmol); solvent: benzene, 30.0 ml; substrate: hexene-1, 1.25 ml (0.318 M).
PPhj
\
Ph3P,
ph3p/~
/ pph3
nh/c’\ ‘el/
I
Rh\PPh3
Figure 7. Possible chelated forms of polymer-boundrhodium complex.
lyst under mild conditions. A small amount of hydrogen uptake was observed, but NMR analysis indicated that only hydrogenation of thiophene to tetrahydrothiophene occurred. N a t u r e of t h e Polymer-Bound Complex In an effort to determine some details concerning the nature and location of the rhodium complex on the polymer, a number of analytical procedures were invoked. Quantitative analyses of several of the catalysts and the polymer are summarized in Table 11. From these data, calculations were made to determine how many ligands per rhodium atom were displaced from RhCl(PPh.j)Jto obtain the polymeric complex. Based on a molecular weight of Nomex of 50 000 (Lee et al., 1967) and the observed phosphorus and rhodium contents shown in Table 11, 8% of the N-H groups appear to have reacted with diphenylphosphine chloride ( (C6H;i)2PC1),and about 12%of P in the polymer seems to be bonded to rhodium. No differences were detectable in Debye-Scherrer x-ray diffraction patterns for dried Nomex and the polymer-bound catalyst, which indicates that binding of the rhodium complex to the polymer did not alter the polymer crystal structure significantly. Infrared spectra were also similar for the two samples except that absorption in the 3300-cm-’ region, which is due to N H groups, was less for the polymer-bound complex. This suggests that NH groups in the polymer could be involved in coordinating the rhodium complex.
0.1 ml min
ml
5 10 15 20 25 30 40 50 60
2.5 3.8 5.1 6.3 7.6 10.0 12.5 14.9
1.3
Several probable chelated forms of the polymer-bound rhodium complex are shown in Figure 7. The possible structure of a chelated form of the polymer-bound complex may be determined by measuring the magnitude of the binding energy separation between the different types of chioride environment, terminal chlorine (Rh-C1) and bridged chlorine (Rh-C1-Rh), using x-ray photoelectron spectroscopy (ESCA). It has been reported that the measurements of the C1 2p electron binding energies of rhenium and molybdenum complexes containing metal halide clusters showed that in all cases 2p electron binding energy of the bridged chlorine atom was significantly greater (1.5 to 2.4 eV) than the binding energy of the corresponding electron of the terminal atom (Hamer and Walton, 1974). The x-ray photoelectron spectra of RhCl(PPh3)J and the polymer-bound complex indicated that the C12p peak fell about the same position 196.5 and 196.6 eV, respectively. Thus, it is reasonable to conclude that the chelated form of the polymer-bound complex contains terminal chlorine as in the first structure in Figure 7. Based on this probable structure, some conjecture on the possible causes of reduced activity of the polymer-bound catalyst is in order. Osborn et al. (1966) and Montelatici et al. (1968) have discussed the dependence of hydrogenation rate on electronic and steric factors induced by changes in the nature of the ligand. Some initial data (Horner et al., 1968; Mague and Wilkinson, 1966; Montelatici et al., 1968; O’Connor and Wilkinson, 1969; Stern et al., 1967) indicate that electron donors (methyl, methoxy, and dimethylamino) parasubstituted on the aryl phosphine enhance activity, while electron acceptors (halogeno, acetyl, phenyl, and naphthyl) decrease activity. It might be hypothesized that the substitution of the polymer (=N-) for a phenyl in the rhodium complex reduces activity presumably because of a change of electron density. There also might be a tendency toward chelation of Rh by the phosphine-substituted polymeric ligand. In addition, a small portion of the polymeric sites might be too sterically hindered to act as ligands. Conclusions It would seem that the use of Nomex as a means for “hetrogenizing” a homogeneous catalyst is entirely feasible. A reasonably high-area form can be prepared, and the polymer is rugged enough to allow phosphinating at sufficiently high temperature in this case, to maximize the amount of rhodium complex that is chelated. Although the activity of the particular catalyst is slightly higher than that reported for the same rhodium complex bonded to polystyrene, it remains a t a low value that must be improved if commercial use of such catalysts is to result. Improvement must come from additional studies of methods for increasing the degree of initial phosInd. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
253
phinating of the polymer and from attempts to obtain alternate rhodium complexes which will attach to the polymer in a manner to provide the least restriction to free movement of the bonded complex. Literature Cited Birch, A. J., Walker, K. A. M., Tetrahedron. 20, 1935 (1967). Burwell, R. L., Jr., Chem. Techno/., 370 (June 1974). Delmon. E., James. G., "Catalysis, Heterogeneousand Homogeneous," Elsevier, New York, N.Y., 1975. Grubbs, R. H., Kroil, L. E., J. Am. Chem. SOC.,93, 3062 (1971). Grubbs, R. H.,Kroli, L. E., Sweet, E. M., J. Macromol. Sci. Chem., 7 (5), 1047 (1973). Hamer, A. D., Walton, R . A., Inorg. Chem., 13 (6), 1446 (1974). Heinemann, H., Chem. Techno/., 286 (1971). Horner, L., Buthe, H., Siegel, H., Tetrahedron Lett., 4023 (1968). Houghton. R. R., Chem. Ind., (4), 155 (1973). Lee, H., Stoffey, D., Neville, K., "New Linear Polymer", p 134, McGraw-Hill, New York, N.Y., 1967. Lemke, H., Hydrocarbon Process., 45 (2), 27 (1966).
Mague, J. T., Wilkinson, G., J. Chem. SOC.,A, 1736 (1966). Manassen, J., Chim. Ind. (Milan),51, 1058 (1969). Michalska, 2. M., Webster, D. E., Chem. Techno/., 117 (Feb 1975). Mitchell, R. W., Ruddick, J. D.,Wilkinson, G., J. Chem. SOC.,A, 3224 (1971). Montelatici, S.,Osborn, J. A., Wilkinson, G., J. Chem. SOC.,A, 1054 (1968). Morrison, J. D., Burned, R. E., J. Am. Chem. Soc., 93, 1301 (1971). Morrison, J. D., "Asymmetric Organic Reactions", Prentice-Hall, Englewood Cliffs, N.J., 1971. O'Connor, C., Wilkinson, G., Tetrahedron Lett., 1375 (1969). Osborn. J. A., Jardine, F. H., Young, J. F., Wilkinson, G., J. Chem. SOC.,A, 1711 (1966). Pittman, C. U., Jr., Evans, G.O., Chem. Techno/., 560 (Sept 1973). Sasse, K., "Methoden der Organischen Chemie (Houben-Weyl)", p 213, G. Thieme Verlag, Stuttgart, 1963. Scroggins, L. H., Microchem. J., 13, 385 (1968). Stern, R., Chevallier, Y., Sajus, L.. Compt. Rend., 264, 1740(1967). Van Beekkum, H., Van Rantwijk, F., Van de Putte, T., Tetrahedron Lett., 1 (1969).
Received for review October 20, 1915 Accepted June 21,1976
Platinum/Alumina Catalysts in Reforming Methylcyclopentane Bjirn B. Donnis' lnstituttet for Kemiindustri, The Technical University of Denmark, DK-2800 Lyngby, Denmark
Reactions of methylcyclopentane on commercial R-AI2O3-CI reforming catalysts have been studied. Kinetic data (activation energies, reaction orders for methylcyclopentane and hydrogen)are presented for the formation of: (1) cyclohexane and benzene, (2) 2-methylpentane and 3-methylpentane, and (3) n-hexane. The reactions were studied at temperatures from 470 to 515 OC, partial pressures of methylcyclopentane from 0.02 to 0.14 atm, and partial pressures of hydrogen from 6 to 40 atm. The conversion of methylcyclopentane was kept below 10%. The kinetic data combined with results from varying Pt content of the catalyst, water vapor pressure, and catalyst age imply that for the formation of (1) and (3) the rate-determining step is catalyzed by acidic centers, while the formation of (2) is catalyzed by platinum; i.e., there are two different ring opening mechanisms.
Introduction This paper presents a study of the catalytic reforming of methylcyclopentane (MCP) with a conventional platinumalumina-chloride catalyst in a differential fixed-bed reactor. The study appears warranted, since the reforming of MCP should provide information about the dual-function character of this type of catalyst. Moreover, an extensive study of the kinetics of the MCP reactions has not been reported previously. Experimental Section The experimental reactor used in this study was made from a 13-mm i.d. stainless steel pipe, surrounded by a 100-mm 0.d. copper block, surrounded in turn by three independent 2000-W heaters. The reactor was equipped with two axial thermocouple wells, ending 25 mm over and 13 mm below the catalyst bed, respectively. By proper adjustment of three PID regulators, controlling the power of the heaters, the temperatures measured at four different points in the thermocouple wells showed that the reactor during use was isothermal (f0.3 "C) over 300 mm of length. The hydrogen used was electrolytic grade and was deoxygenated over a Pd catalyst and dried by molecular sieves (4A). After having passed a reduction valve, a known amount of
Address correspondence to the author a t Haldor Topsie A D , Nymillevej 55, P.O. Box 213, Dk-2800 Lyngby, Denmark. 254
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
water vapor was added by letting the hydrogen equilibrate with ice in a vessel cooled in a cryostat (normally operated a t -36 OC). The Phillips pure grade MCP used was stored over molecular sieves and refluxed under nitrogen prior to use. The analysis of the MCP after purification is: MCP 99.79%, nHx 0.14%, and CH 0.07%. The product analyses are corrected for the nHx and CH content of the feed (the CH is subtracted from the Bz of the product, because the reaction CH Bz is fast). The MCP was added to the hydrogen by letting the hydrogen bubble through the liquid in a vessel, immersed in a constant-temperature bath, this temperature thus determining the partial pressure of MCP. (By varying the hydrogen flow (to low values) and analyzing samples of constant volume by gas chromatography it was shown that the samples always contained the same amount of hydrocarbon; therefore it seems justified to consider the hydrogen saturated by MCP. After discharge from the reactor, the pressure was relieved by letting the gases pass a reduction valve and a calibrated needle valve, and the gas volume was finally measured in a wet gas meter. Catalysts from Ketjen, Netherlands, were used with 0.3% and 0.6% Pt on y-alumina, originally containing 0.67% chlorine. Before use the catalysts were crushed and screened to 140/170 mesh and mixed with (catalytically) inactive sand. The catalyst bed contained about 2.5 cm3 of this mixture. The start-up procedure including reduction was standardized in such a way that removal of the water vapor evolved occurred a t a rate which kept the water vapor pressure constant. The start-up procedure is listed in Table I.
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