I n d . E n g . C h e m . R e s . 1987, 26, 1910-1916
1910
Oxidation Catalysis in a Supercritical Fluid Medium Kerry M. Dooley* and F. Carl Knopf D e p a r t m e n t of Chemical Engineering, Louisiana S t a t e U n i v e r s i t y , B a t o n Rouge, Louisiana 70803
T h e supercritical fluid (SCF) extraction technique was extended by its application to SCF-solidcatalyzed reactions that otherwise take place in a two-fluid-phase reactor. By use of COz as a solvent, toluene was contacted with air in the presence of redox or acid catalysts and underwent partial oxidation t o primarily benzaldehyde. A screening study of several oxide and mixed-metal oxide catalysts revealed that supported COO,partly oxidized to Co(III), was the most active (10-5/s turnover number) and selective (no multiring condensation products) catalyst. The relative activity of the supported COOcatalyst was not unexpected, because the Co2+/Co3+redox couple is the most effective catalyst for this reaction in solution. The oxide was capable of duplicating the liquid-phase behavior, although at lower activity levels than promoted (for example, with Br- ions) homogeneous cobalt catalysts. The application of supercritical fluids (SCFs) as solvents for the extraction of relatively nonvolatile compounds has been thoroughly explored, especially as a method of coal liquefaction (Blessing and Ross, 1978; Fong et al., 1983; Vasilakos et al., 1985). Volatiles are liberated from the coal matrix by the thermolysis of bridge bonds and then hydrogen abstraction; in other words, the “extraction” is a complex reaction, as supported by the observation that temperature increases often increase the amount of extract. These findings have prompted interest in reactions using supercritical fluid media; the subject has been recently reviewed (Subramaniam and McHugh, 1986). It is well-known that there is a general increase in the rate constants of most bimolecular homogeneous reactions with increasing pressure (Eckert, 1972). The increase may be explained in terms of transition-state theory; for the concentration-based rate coefficient, the theory predicts
AV’
d In Z
The positive bracketed term on the right-hand side of eq 1 often compensates for the large negative activation volumes ( A V ’ ) of heavy solutes near the mixture’s critical point. This limits the increase of k with increasing pressure (Dooley et al., 1987). Therefore, the dependence of h on pressure is usually a minor consideration in whether to use a SCF reaction medium. Previous experiments demonstrate SCF utility in the following cases: (I) where homogenization of the reaction mixture removes diffusion limitations for a key reactant, catalyst, or promoter, as in the SC paraffin isomerization process using H2 and metal halide catalyst (Kramer and Leder, 1975); (2) where the SCF separates unstable products of reaction from a solid or liquid phase, as in the removal of polymerizable products of thermolysis in coal extraction (Squires et al., 1983); (3) where the SCF plays a direct role in the reaction, say, in the stabilization of free radicals or the participation of the solvent in addition reactions (Lawson and Klein, 1985; Abaham and Klein, 1985); (4) where the SCF aids in the solubilization of reaction products that could result in catalyst deactivation, for example, coke precursors in an acid-catalyzed reaction (Tiltscher et al., 1981). Several of these conditions appear to exist for the catalyzed partial oxidation of alkyl aromatic hydrocarbons to phenols, aldehydes, and acids. These reactions depend critically upon the correct amount of O2 at a catalytic site. In the SC state, it is possible for both the hydrocarbons and O2 to exist as a single phase with viscosiy and diffusivities intermcdiate to those properties for liquids and 0888-5885/87/2626-1910$01.50/0
gases. The SC solvent may affect reaction pathways also, because some elementary reactions involve free radicals and can take place in a fluid phase. Finally, the desired products are unstable relative to the ultimate products of combustion, and the rapid quenching of the highly exothermic (-580 kJ/mol) partial oxidation, which is made possible by pressure letdown from SC conditions, should reduce the yield loss to combustion products. Toluene oxidation is therefore a model for an important, complex class of reactions that may benefit from application of SCFs as reaction media. The partial oxidation/cracking reactions of multiring aromatic and alkyl aromatic compounds may prove important in future coal processing; this is a further reason to examine the high-pressure oxidation of a model alkyl aromatic compound such as toluene. The hydroxy or carboxyl groups resulting from alkyl aromatic partial oxidations can facilitate the rupture of ortho or para methylene bridges. The half-life of the thermal cleavage reaction is reduced by over an order of magnitude by allowing formation of a lower energy keto-enol tautomer intermediate (McMillen et al., 1981). An example of these reactions applied to actual coal liquids is the autoxidation followed by anaerobic thermolysis of solvent-refined coal in quinoline (Hazlett et al., 1978). The yield to combustion products a t 388-453 K was less than 10%. It is generally agreed upon (Kaeding et al., 1965; Heiba et al., 1969; Kamiya and Kashima, 1972; Sheldon and Kochi, 1974; Gates et al., 1979; Schuit and Gates, 1980; Parshall, 1980; Lyons et al., 1980) that the metal-catalyzed oxidations of alkyl aromatics exhibit certain common features. The metal ion catalysts must have stable oxidation states separated by +1,because chain initiation and other steps involve one-electron transfers. A correlation exists between an ion’s thermodynamic stability and its catalytic activity (at least in solution), in that ions with high standard potentials (E,) such as Co3+are powerful oxidizers, while those with lower E, values are more useful in facile oxidation steps. Once a reaction sequence or chain is initiated, thermal reactions involving free radicals occur in parallel with metal-catalyzed steps. Four possible networks that can account for the product distributions in toluene oxidation observed under different reaction conditions are given in Figure 1. The initiation step in Figure 1 involves electron transfer to Co”+, suggesting that the higher valent ion is absolutely necessary for catalysis. However, the divalent ion can be involved in several benzoperoxy decomposition steps, and in practice only a small ratio of Co3+to Co2+proves necessary or desirable. All of the free-radical reactions shown in Figure 1 are nonbranched; they do not result in net C 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1911 $5 m
I
+-;
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#-CHO+H*C
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Table I. Catalysts Tested for Toluene Partial Oxidation temp, K, for designation comaosition. w t % calcination reduction I-1 5% coo 670 1-4 5% coo 470 1-5 5% C00/10% Moo3 670 1-6 5% C00/10% MOOS 670 670 1-9 10.5% MOO^ 670 1-13 19% W, 6% Ni 670 Y-52 Si/Al = 2.37 770
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F i g u r e 1. Reaction networks for toluene oxidation to benzaldehyde/benzoic acid, catalyzed by Co2+/Co3+in solution. 30O-60ODC
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Figure 3. Schematic of apparatus for SCF reaction studies. 300-6OO'C
Figure 2. Transitions of the cobalt oxides, supported on A1203, in air.
radical production, with the possibility of temperature runaway. With increasing temperatures in the 470-520 K range, branching reactions do take place. The networks of Figure 1 also exclude termination reactions and peroxybenzoate decompositions. The decompositions are rapid and unimportant kinetically, but the termination reactions can partly determine overall rates of partial oxidation. Most are of the type CH 2 0 0
CH2OH
I
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These reactions proceed through tetroxide [R(O),R] intermediates and are therefore sensitive to the solvent's effect on dipole-dipole interactions. These solvent effects can be extreme; for example, in acidic solvents, copper ions often catalyze substitution, rather than O2 addition, with phenols as the ultimate products. For this reason, it was expected that the nonpolar medium of SC C02 and toluene would result in a different product distribution than is observed in some liquid-phase processes. In adapting alkyl aromatic partial oxidations to a SCF medium a t high pressure, it is easier to work with a heterogeneous catalyst that can duplicate the action of the redox pair in solution. Cobalt oxides and Co-Mo and Ni-W mixed-metal oxides supported on A1203were the first catalysts tested in this work. The transitions of Co on A1203in air have been categorized as shown in Figure 2 (De Beer et al., 1974; Gajarda et al., 1980; Gates et al., 1979; Lycourghiotis, 1983). The Co is oxidized to a divalent or mixed-valent oxide; the COOphase is metastable and not always observed. At higher temperatures, Co ions diffuse into the support lattice to form epitaxial layers of CoA1204and ultimately tetrahedrally coordinated Co2+ ions. In Co-Mo mixed oxides, the Mo promotes this stabilization and is largely unaffected (Gajardo et al., 1980).
Some Co3+ always remains, for example, as the spinel Co"Co"',Al,~,04. It is seen that the Co oxides meet at least the minimum requirements for alkyl aromatic partial oxidation-an accessible supply of ions in reduced and oxidized states, with facile interconversion ability. Minor components of typical homogeneous catalysts often include free-radical initiators, but for our experiments these are easily dissolved in the hydrocarbon/C02/air feed. Initiators such as peroxides and "accelerators" (actually, reactive cooxidants) such as benzoic acid are soluble in both toluene and C 0 2 at typical usage levels of less than 0.5 wt % of the hydrocarbon.
Experimental Section The catalysts tested for toluene partial oxidation activity are given in Table I. 1-13 is a commercial Ni-W hydroprocessing catalyst, Harshaw Ni-4303 16-in.extrudate. Y-52 is a commercial Y-zeolite powder supplied by Union Carbide as Linde LZ-Y-52. The Na' ions of the zeolite were exchanged for ammonium ions in a 5-fold excess aqueous solution of NH4N03held at reflux for 2 h followed by thorough washing with deionized water. The Co- and Co-Mo-supported oxides were prepared by impregnation of Vista ,6-in. A1203 extrudate with freshly prepared solutions of C O ( N O ~ ) ~ - (Baker ~ H ~ Oreagent) or molybdic acid (85%, Fisher reagent) in enough deionized water to completely wet the support. The water was slowly evaporated at 270 K, and the fines were removed through a 40-mesh sieve. All the catalysts were calcined for 6 h in air (and in one case reduced for an additional 2 h as well in flowing 10% H,) at various temperatures in order to generate surfaces of different ratios of oxidized (Co3+)to reduced (Co2+) metal. A schematic of the high-pressure reactor system appears in Figure 3. C 0 2 is compressed to slightly above critical conditions (T,= 304 K and P, = 72.8 atm) by a Superpressure Model 46-13421 compressor and at a flow rate of 1g/s is mixed with toluene (Baker reagent grade, further purified by passage over activated alumina, distillation, and storage over 4A molecular sieve) delivered by a Ruska
1912 Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987
Model 2252BI metering pump and air from a cylinder. All flow rates were controlled by metering valves. The readion mixture was ca. 1.5 wt % (0.7 mol %) toluene, 1.5 wt % (2.0 mol %) oxygen, 5 wt % nitrogen, and the balance COz, and its temperature was controlled at 400-500 f 1 K by encasing the tubular reactor (l/z-in.-o.d. tube with fritted disk) in an insulated heat source (4-in.-o.d. heated aluminum block). The reactor pressure was 80 f 2 atm. After the reactor, the air/C02 ratio and toluene/C02 ratios were checked by on-line sample injection into a Varian 2700 gas chromatograph with a thermal conductivity detector. The air and COz were separated at 340 K by a 3.2-mm-o.d., 0.61-m-long column of 100/120-mesh Carbosieve G (Supelco). The C 0 2 and toluene were separated at 450 K by a 3.2-mm-o.d., 3.05-m-long column of 23% SP-1700 on 80/100-mesh Chromasorb P (Supelco). The reactor effluent was reduced in pressure and cooled by expansion through two micrometering valves. To prevent ice formation, the valve bodies were heated. Acetone/dry ice traps were used to collect the toluene and products for further analysis. The uncondensed COPwas passed through a Singer Model DTM-200 dry test meter. The products of reaction were analyzed by syringe injection into an HP-5880 gas chromatograph with a flame ionization detector. They were separated a t 353-453 K (at 10 K/min and a 3-min initial hold) by using a 0.32mm-i.d., 30-m-long RSL-300 fused silica capillary column (Alltech). Tentative product identifications from coinjections were confirmed by subsequent analysis on a Finnigan 1020 mass spectrometer operated in the electron impact mode at 70 eV. The six most abundant ions in the unknown spectrum were compared to the most similar standard from the NIH/EPA spectral library. The water content of the collected products was estimated by Karl Fischer titration. The product samples (1.5-3.0 mL) were dissolved in dried n-propyl alcohol and titrated using a Hydranal Composite Karl Fischer reagent (Riedel de Haan) with capacity 2.0 mg of H20/mL.
Results and Discussion Phase Equilibria for the Process. The phase diagrams for the solute toluene in equilibrium with the SCFs COz or COz/air were constructed in order to provide insight into the solubility relations that would exist a t the mixing point and in the reactor. Most theoretical treatments take the SC phase as a dense phase in equilibrium with either a solid or liquid. The fugacity of each component in the SCF is equated to the product of mole fraction, fugacity coefficient, and pressure. The fugacity coefficients are calculated generally from a cubic equation of state. This has been typical of studies on SCF extraction, and there are detailed reviews containing procedures and examples (Paulaitis et al., 1982; Groves et al., 1985). In this work, liquid-, vapor-, and dense-phase fugacities were calculated by using the Peng-Robinson (1976) cubic equation of state. The critical locus ( V = L ) for a binary mixture described by this equation was calculated by the algorithm of Heideman and Khalil (1980). Figure 4 gives the phase diagram for the toluene-COz mixture as constructed from the data of King et al. (1983) and the Peng-Robinson equation. The curves TP,-CP, (CO,) and TP,-CP, (toluene) are the vapor pressure curves for the pure liquids, from their triple points to their critical points. The vapor-liquid critical locus beginning a t CP, ends at the critical end point K due to the formation of two liquid phases in equilibrium with the gas. This three-phase line extends over a few degrees and ends a t a second critical end point, 0. At 0, the liquid phases become identical. From 0, the locus extends to CPz of
-
P
200
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b
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..
E
-'D
100
a
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I 200
300
500
400
600
700
T(K)
Figure 4. Pressure-temperature diagram for toluene and COP.
0
02
04 06 x Z o r y2
08
10
Figure 5. Pressure-composition diagram for toluene and C 0 2 a t 473 K, calculated from Figure 4 and the Peng-Robinson equation of state.
pure toluene. Point a is in the dense region, point b is on the critical locus, and point d is the saturation P of toluene at 473 K. Point c represents operating conditions for the reaction studies. The P-x diagram a t a typical reaction temperature of 473 K is given in Figure 5. There is a two-phase region bounded on the left by either a vapor (V) phase at low P or a SCF (F)phase at high P, in equilibrium with a toluene-rich phase. These results imply that the feed mixture is a single dense phase at the reactor entrance. The ternary system COz-toluene-air is a better approximation to the reacting system, but the picture is incomplete because the reaction products have not been taken into account. Without ternary or high-pressure toluene-air equilibrium data, any ternary diagram for this
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1913 Table 11. Summary of Toluene Partial Oxidation Experiments run catalyst cooxidant 1 1-5 benzaldehyde 2 1-5 benzaldehyde 3 1-6 benzaldehyde 4 1-5 benzaldehyde 5 1-5 benzaldehyde 6 1-6 benzaldehyde 7 1-6 benzoic acid 8 1-4 benzoic acid
run catalyst 9 1-9 10 Y-52 11 1-6/Y-52 (1:l) 12 1-6/Y-52 (l:l, crushed) 13 1-4 15 1-1 16 1-13
r
cooxidant benzoic acid benzoic acid benzoic acid benzoic acid benzoic acid benzoic acid benzoic acid A
A
A AIR(31
C
B TOLUENE(21
Figure 6. Triangular diagram, C02-toluene-air a t 473 K and 80 atm, calculated from Figure 4 and the Peng-Robinson equation of state.
system is less exact than Figure 4,but a triangular phase diagram (Figure 6) a t reaction conditions of 473 K and 80 atm is useful a t least in indicating what phases occur as the mixture is processed. The left-hand boundary of the two-phase region gives the solubility of toluene in a dense phase of various C02/air ratios. The solubility of toluene in a dense phase consisting predominantly of Cop is almost the same as that in pure Cop.The right-hand boundary of the two-phase region shows the solubilities of mixtures of varying C02/air ratios in toluene. The results of the ternary diagram in no way alter the conclusions on reactor phase behavior that were drawn from just the binary Cop-toluene diagrams. Partial Oxidation Catalysis. The results of toluene oxidation experiments using an SC feed of CO,, toluene, and air reveal that purely acidic catalysts such as the H-Y zeolite are inactive and that oxides whose active component is mostly COOare the most active catalysts tested. The reaction rates and product distributions are typical of ion-catalyzed liquid-phase reactions. The turnover numbers for partial oxidation, based on moles of metal (Co, Mo, Ni, or W), are in the 10-’-104/s range, with the reactor operated such that conversions were kept to less than 1%. The turnover numbers were calculated assuming a differential reactor, that is, (3) N = (4 - FL,)/(WS) No initiator was used, although 0.12 X mol % benzoic acid (0.17 mol % of the toluene) of a reactive~cooxidant, or “accelerator”, was added in order to rapidly build up the concentration of radical intermediates. The observed turnover numbers correspond roughly to those of previous studies where Co(I1) homogeneous catalysts were used without initiators and electron-transfer promoters. For example, Zakharov and Geletii (1979) measured a turnover number of 8.3 X lo4/, for this reaction at 343 K catalyzed by cobaltous(I1) acetate in acetic acid. The calculated turnover numbers probably constitute true kinetics information, because standard calculations of the generalized Weisz modulus (Froment and Bischoff, 1979), using the largest turnover number and an estimated minimum diffusivity of loT5cm2/s, show that these reactions were not transport-limited. Three reaction pathways were observed with the metal oxide catalysts. The predominant reactions, partial oxidations, include the formation of benzaldehyde, benzyl alcohol, and the cresol isomers; benzaldehyde was the most
0 O 002 10
200
2 2 04
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180
160 T.
2 40
2W 50
140
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Figure 7. Turnover number for partial oxidation, for a Co-Mo mixed oxide catalyst, using two different cooxidants.
abundant product of reaction. The second family is of condensation reactions, the polymerization to multiring compounds, sometimes accompanied by oxidation. The observed products of these reactions were o-methyldiphenylmethane and a difficult to identify benzofuranone or benzofurandione. The third path is total oxidation to CO,; this may also have been in series with partial oxidation. A listing of the kinetics experiments is given in Table 11. It was realized that benzaldehyde was a poor choice of cooxidant since it was the most abundant product of toluene oxidation, while the cooxidized benzaldehyde was partly reacted to Cop and water. For this reason, the benzaldehyde mass belance was sensitive to small errors in composition measurements. Further analysis is confined to runs 7-16. Benzoic acid, on the other hand, proved to be an ideal cooxidant. It is almost totally oxidized in the reactor even in the absence of catalyst, it was not a significant product of partial oxidation or of any other pathway, and its use resulted in slightly higher partial oxidation rates and less condensation. Figure 7 demonstrates this graphically, showing results for the Co-Mo catalyst 1-6. Benzoic acid is typically one of the most stable products of catalyzed toluene oxidation in the liquid phase. The present results suggest that the benzoic acid is unstable a t the experimental conditions. The results of catalyst screening experiments at 413-493 K suggest that Moos and Bransted acid sites do not promote electron transfer in toluene oxidation; their presence in fact reduces the catalytic activity of the Co oxides. The H-Y zeolite was inactive by itself (run lo), and when mixed in 1:l ratio with the standard Co-Mo catalyst (1-6) there resulted only slight partial oxidation activity (run 11). After we crushed the physical catalyst mixture, the activity increased somewhat, but the partial oxidation rates were still too low to be meaningful (run 12). The results of three runs involving the supported MooB catalyst (Figure 8,I-9))a standard Co-Mo catalyst (Figures 8 and 9, I-6), and a standard Ni-W catalyst (Figure 9,1-13)
1914 Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987
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Figure 9. Turnover number for partial oxidation and for condensation, for Co-Mo and Ni-W mixed oxide catalysts.
demonstrate that these catalysts exhibit low activity and poor selectivity for partial oxidation. The pure Co oxides, in contrast, exhibit higher activities and are selective as well; no condensation products were observed with these catalysts (Figure 10,I-4 and 1-1). The turnover number for catalyst 1-4 approaches 10-5/s, higher than for all other catalysts. It was kept on-line for over 130 ks at 473 K (exceeding a turnover) with minimal deactivation. It is concluded that the presence of cobalt oxide in some form is necessary for partial oxidation at these conditions. The weakly acidic oxides MOO, and WO, [the isoelectronic points of hydrous WO, and MOO, are both C0.5, while that of COO is greater than 10 (Parks, 1965)] promote the formation of condensation, relative to partial oxidation, products. The Co2+/Co3+redox pair, on the other hand, is typically the most active catalyst for toluene partial oxidation to single-ring products in solution. This is true whether the solvent is polar or nonpolar; however, the activities of the homogeneous catalysts are solvent and additive (electron-transfer promoters and initiators) dependent. Similarly, Figures 9 and 10 suggest that the activities of solid Co oxide catalysts, contacted by SCFs,
for partial oxidation depend upon the specific surface chemistry. The COO catalyst calcined at 400 "C (1-1)is less active than the one calcined at 200 "C (I-4), with a higher apparent AE for benzaldehyde formation (38 kcal/mol for 1-1vs. 5.1 kcal/mol for 2-41. Catalyst 1-6, calcined at 400 "C but subsequently reduced at 400 "C with 10% Ha, exhibits partial oxidation activity and an apparent AI3 (8.6 kcal/mol) intermediate to those of 1-4 and 1-1. Catalyst 1-4 is the least oxidized of all the catalysts in the supported oxide series (Table I), in fact the only Co catalyst calcined so as to preserve the metastable COO phase. The kinetics results suggest that this phase, presumably with interspersed Co3+ions and instead of bulk Co304or Co2+/Co3+diffusion into the Al,03 lattice (see Figure 2), constitutes the optimum toluene partial oxidation catalyst a t these conditions. The rate of total oxidation was indicated by the amount of water present in the samples of reactor effluent collected in sealed traps. The amount calculated to be present as a result of the combined partial oxidation reactions was subtracted from the total amount of water to ascertain the amount formed by total oxidation. The blank (no catalyst) runs and also the samples collected with the reactor operating at 453 K and below showed essentially no water by total oxidation. For runs 8 and 9, however (catalysts 1-4 and 1-9), at 473-493 K, the total axidation rates were at least 20% of the partial oxidation rates. The yield to combustion products was significantly less, below lo%, for the more oxidized Co catalysts 1-1and 1-6. If we speculate on future developments, the toluene partial oxidation catalytic activity can probably be enhanced without loss of selectivity by paralleling the development of the homogeneous-catalyzed processes, as follows: (1)oxidize more of the active Co to Co3+without forming bulk Co304; (2) incorporate into the catalyst functions that promote electron transfer in alkyl aromatic systems, such as halide ions; (3) use more cooxidant or use a small amount of initiator. A 2-3 order of magnitude activity enhancement may be expected from step 1alone, as indicated by the results of van der Ploeg et al. (19681, who employed soluble manganese(II1) acetate catalysts, and Kamiya and Kashima (1972), who used cobaltic(II1)
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1915 FLUID-PHASE OXIDATION ( Z O O ' C ) H O C ~0
HCiO
VAPOR-PHASE
ncc c i
O X I D A T I O N (400'C) OH
0
0
Figure 11. Reaction pathways for toluene oxidation.
acetate. Steps 1and 2 together should result in a 3-4 order of magnitude increase; Borgaonkar et al. (1984) measured 2.5 X lO-,/s a t 383 K in 0.02 M cobaltic(II1) acetate and 0.16 M NaBr. Steps 1 and 3 together could result in turnover numbers of 10-2-10-1/s at ca. 370 K (Kaeding et al., 1965; Namie et al., 1975). In these last two studies, benzoic acid was the most effective cooxidant, while C02 production was negligible. If a more nonpolar, liquid-like (more concentrated in COz) environment in the pores will enhance the electrontransfer activity, then a higher pressure would result in a significant activity increase, even greater than that predicted by eq 1. The findings of Hayhurst and Lee (1983) and Jones et al. (1959) indicate that molecules physically adsorbed at pressures above the mixture critical point can behave almost liquid-like in terms of density and heat of adsorption. By use of Figure 4, it appears that a transition from a predominantly liquid toluene to a predominantly C02/air pore environment would take place over a range of pressures from the current 80 atm to 160 atm. Finally, it should be noted that these reaction results are atypical in several instances of previous work with vapor-phase toluene at higher temperatures. The detailed experiments of Germain and Laugier (1972) gave turnover numbers in the 10i/s range at 673 K for a series of oxides including MOO,, WO,, and Vz05.About 20% of this activity was by the pathway of sequential oxidative coupling and decoupling (condensation) to anthraquinone and phthalic anhydride, as shown in Figure 11, which compares the reaction pathways observed in the SCF and the vapor-phase processes. In the present work the condensation pathway was not observed when pure Co oxide catalysts were used (as indicated by the dotted line in the figure). Also, the activities of the Co oxide 1-4 reported here are too high to be consistent with the vapor-phase mechanism, which entails lattice addition of 0'- ions to benzylic intermediates, as in the oxidation of propylene to acrolein by the Mars-van Krevelen mechanism (Sachtler and De Boer, 1965). Below about 520 K, O2dissociation is minimal on most metal oxides (Keulks et al., 1978), and the product distributions obtained here suggest oxidation pathways on a t least the Co oxide catalysts similar to those depicted in Figure 1,involving O2 addition to free radicals and then decomposition of peroxides to stable products. The magnitudes of the apparent AI3's for partial oxidation support this conclusion. For 1-4 and 1-6, these were 5.1 and 8.6 kcal/mol, respectively. Such low AE values are characteristic of free-radical, catalytically assisted reactions
in solution with mechanisms such as those of Figure 1; the data of Kaeding et al. (1965) [mixed Co(I1) and Co(II1) salts; 110-180 "C] and Namie et al. (1975) (mixed Co-Mn salts; 160-190 "C) for toluene partial oxidation also indicate that almost-zero apparent AI3 values characterize these reactions a t these conditions. These data stand in contrast to the data for high-temperature, vapor-phase toluene oxidation. The overall apparent AI3's for these reactions were found to be (1)26 kcal/mol with a Vz05/ K2S04/A120,catalyst at 300-350 "C (Downie et al., 1961) and (2) 27.4 kcal/mol with a Mo03/A1203catalyst at 400-450 "C (Trimm and Irshad, 1970).
Conclusions It was possible to dissolve 1.5 wt % toluene and 6.5 wt % air in supercritical C 0 2 at 80 atm and 293-493 K. By use of this mixture, the toluene could be oxidized at low rates and conversions to benzaldehyde, benzyl alcohol, the cresol isomers, and a lesser amount of condensation products and carbon oxides. A 5% CoO/A1203catalyst, calcined at 200 "C in order to preclude oxidation to Co304 and the cobalt "aluminates", proved to be an active and selective (for partial oxidation) catalyst. Its turnover number for partial oxidation was about 10-5/s at 473 K. Benzoic acid in trace levels successfully accelerated the reaction and was cooxidized to C 0 2 and water. This observation, the product distribution obtained, the moderate rates of reaction measured at low temperatures of 473-493 K, and apparent activation energies for partial oxidation as low as 5.1 kcal/mol taken together indicate the participation of free radicals such as the benzoperoxy radical in the reactions. Similar phenomena are observed in catalytically assisted, free-radical, homogeneous oxidations in the liquid phase. Alkyl aromatic partial oxidation may therefore be feasible in SCF solvents at high pressures, catalyzed by conventional supported metal oxides, but without the poor selectivities to partial oxidation products characteristic of low-pressure, vapor-phase processes which proceed by mechanisms of the Mars-van Krevelen type. Acknowledgment This research was funded by the Louisiana Center for Energy Studies. We acknowledge the assistance of S. Brodt and C.-P. Kao. Nomenclature CP = critical point AE = activation energy F = supercritical phase or molar flow rate, mol/s G = gas phase K = critical end point or binary interaction constant k = reaction rate constant, cm3/(mol.s) L = liquid phase n = order of reaction N = turnover number, mol of product/(s.mol sites) 0 = critical end point P = pressure R = gas constant S = metal site density, mol of metal/g of catalyst T = temperature TP = triple point V = vapor phase AV * = activation volume, difference between partial molar volumes of activated complex and reactants W = catalyst weight X = liquid mole fraction Y = supercritical phase mole fraction 2 = compressibility Subscripts c = value at critical point i = component designation
I n d . E n g . Chem. R e s . 1987, 26, 1916-1923
1916
Registry No. PhMe, 108-88-3; COO, 1307-96-6; Moo3, 131327-5; W03, 1314-35-8; Ni, 7440-02-0; COz, 124-38-9.
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Received f o r reuiew December 10, 1986 Accepted July 7, 1987
Controlled Microcrystalline Growth Studies by Dynamic Laser -Light-Scattering Methods? Charles H. Byers,* Michael T. Harris, and David F. Williams* Chemical Technology Division, Oak Ridge N a t i o n a l Laboratory,§ Oak Ridge, T e n n e s s e e 37831
A method for studying the dynamics of crystallization by using an in situ crystallizer in a dynamic laser-light-scattering system has been developed a t the Oak Ridge National Laboratory. Results for growing systems in the size range 0.005-1 pm may be monitored nondestructively with this method. In a series of current tests, homogeneous precipitation techniques provided reproducible, controlled nucleation and growth of oxides such as hematite and silica. The hydrolysis of tetraethoxysilicate in ethyl alcohol, n-butyl alcohol, and tert-butyl alcohol was investigated as a function of initial water and ammonia concentrations. Ethyl alcohol concentration, particle size, and number concentration were studied as a function of time. In the presence of excess water, the growth rate of silica particles was found t o have a first-order dependence on tetraethoxysilane concentration. The fundamental aspects of nucleation and growth kinetics in crystallization form the basis for the understanding of a broad class of processes that are important Research sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Present address: Department of Chemical Engineering, University of Washington, Seattle, WA 98105. 8 Operated by Martin Marietta Energy Systems, Inc., for the U.S. Department of Energy under Contract DE-AC05-840R21400.
in the chemical and energy-related industries. For instance, crystallization and precipitation are a significant part of typical processes in the nuclear fuel cycle (Lurch and Norman, 1983; General Electric, 1978). In conventional chemical operations, crystallization is an important purification method, while the production of ceramics requires an in-depth knowledge of both nucleation and crystal growth. A central focus in the development of a new generation of ceramics for use under extreme conditions has been on gaining a better understanding of the chemical factors that control the generation of powders
0888-5885/87/2626-1916$01.50/0 0 1987 American Chemical Society
