Ind. Eng. Chem. Res. 1991,30, 89-95
89
Low-Temperature Catalytic Oxidation of 1,4-Dichlorobenzene Lei Jin and Martin A. Abraham* Center for Environmental Research and Technology, Department of Chemical Engineering, University of Tulsa, Tulsa, Oklahoma 74104
Chlorinated aromatics, of which 1,4-dichlorobenzene (DCB) is a specific example, represent a class of compounds that are especially challenging to destroy during hazardous waste treatment. Within this paper, it has been demonstrated that oxidation over vanadium oxide catalyst a t 288,315, and 343 "C can be used to selectively convert DCB to CO and C02. At the lowest temperature of 288 "C, the presence of a DCB-rich liquid phase hindered the reaction but increased the selectivity to C02. A consistent set of reaction pathways was developed in which gaseous oxygen reacts with adsorbed DCB to produce the observed CO product, which then either desorbs from the catalyst or reacts further to produce C02. The assumptions of rapid and irreversible adsorption of DCB and the surface reaction as the rate-limiting step provided a consistent rate expression that indicates that increasing the DCB concentration inhibits the rate a t high concentration but accelerates the rate a t low concentration.
Introduction Chlorinated organic compounds are present in many process streams and hazardous waste sites. In the former case, the chlorinated materials must be removed from the stream prior to discharge to the environment. In the latter, the chlorinated compounds must be separated from the carrier, which may be either solid (e.g., soil) or liquid (e.g., groundwater). In either case, these chlorinated compounds must be converted to nonhazardous materials after separation. Catalytic oxidation to C02,HCl, and H20 provides a suitable (although certainly not the only) means of reducing the toxicity and controlling the emissions of these materials to the environment. Although catalytic oxidation is well-known, it is only in recent years that complete catalytic oxidation of low molecular weight hydrocarbons has been investigated for use as a waste treatment technique. Metal oxide catalysts supported on alumina have been shown to be the most active for the oxidation of low molecular weight hydrocarbons, and the catalysis of these compounds has been reviewed (Spivey, 1987). In general, the reaction mechanism has been found to be similar to that of CO oxidation, which involves adsorption of gaseous CO followed by oxidation on the surface and desorption of the C02. However, empirical power-law rate expressions are frequently employed to describe the kinetics of specific species (Spivey, 1987). Several chlorinated low molecular weight materials containing one or two carbon atoms, including chloromethane (Weldon and Senkan, 1986), methylene chloride (Lester, 1989), and 1,l-dichloroethane (Ramanathan and Spivey, 1989), have been oxidized over supported metal oxide catalysts. These low molecular weight compounds were nearly completely converted to C02,HC1, and H 2 0 at temperatures as low as 300 "C. The usual catalyst for this process was Cr203on A1203.Water was added in many cases to promote the formation of HCl relative to C12via the Deacon reaction. Low concentrations of aromatic chlorides, including 30 ppmv 1,2-dichlorobenzene in air, have also been oxidized at 300-400 "C with supported Crz03on A1203 and with a proprietary halohydrocarbon destruction catalyst (Lester, 1989). In experiments with PCB's, destruction efficiencies ranged from 67% for supported Cr20Rto 97% for supported CuO catalyst at 600 "C (Subbanna et al., 1988). While previous efforts have concentrated on identifying highly active catalysts for the complete destruction of volatile chlorinated compounds, little attempt has been
made to identify the reaction kinetics, pathways, and mechanism for the reaction. Within this paper, we describe our efforts to oxidize 1,4-dichlorobenzene (DCB) over commercially available vanadium oxide catalyst. The results are described in terms of the kinetics and pathways for the catalytic oxidation of the chosen system, which provides insight into the opportunities for process improvement in latter experiments. Thermodynamic calculations utilizing the Peng-Robinson equation of state allowed calculation of the region in which the reactants were present in a single vapor phase. DCB was chosen as a reasonably nontoxic representative of the class of chlorinated aromatic hydrocarbons similar to those that may actually be found in a hazardous waste site. The results of this study are presented as follows. The experimentalprocedures and conditions are described first. Next, the thermodynamic calculations are described and the region in which vapor-phase experiments could be accomplished is delineated. This is followed by the experimental results, first as the temporal variation of products' yields and then considering the effect of reactant concentration on the rate of reaction as measured by C02 production. The results are discussed in terms of a set of reaction steps, which involves irreversible adsorption onto the catalyst followed by slow reaction on the surface.
Experimental Section Reactions were accomplished in small batch reactors comprised of 316 stainless steel Gyrolok tube fittings, as described elsewhere (Leavitt and Abraham, 1990). A ball valve allowed input and analysis of gaseous materials. The valve was closed during the reaction to maintain a constant reactor volume of approximately 1.3 mL. A typical experiment was accomplished as follows. The desired weight of vanadium oxide catalyst, available as a powder in pure form from Aldrich Chemical, and 1,4-dichlorobenzene (Aldrich) were added to the reactor, which was then sealed. The ball valve was opened and oxygen added to the required pressure, as calculated by assuming ideal gases and converting from moles to partial pressure through the ideal gas law. Nitrogen was then added so that a constant reaction pressure could be maintained as the oxygen pressure was varied. All materials were commercially available and used as received. The reactor was then placed in a holder within a fluidized sandbath (Tecam SBS-4), which had been preheated to the desired temperature. The reaction time was measured from this point and an agitator turned on so that
0aaa-58a5/91/2630-0089~02.50/0 0 1991 American Chemical Society
90 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 Table I. Summary of Experimental Conditions run 1 2 3 4
temp, OC 288, 315, 343 315 315 315
bar 27.6 0 24.1 0-27.6
c ~ B ,mol/L ~ ,
0.18 0-0.22 0-0.22 0.18
\
reaction time, min
Dew point pressure
0-60
10 10 10
the reactor was moved vertically within the sandbath. Heat-up time was less than 1 min, as determined by pressure measurements within the reactor during previous testing (Sodhi, 1989). This heat-up time was considered negligible compared to ultimate reaction times of 60 min. In any case, the heat-up time was consistent across the series of experiments and would not effect kinetic analyses comparing one set of data against another. After an appropriate reaction time, the agitator was turned off, the reactor was removed from the sandbath, and the reaction was quenched in water. Product analysis was accomplished on a Hewlett-Packard 5890A gas chromatograph equipped with a 30-ft Hayesep D packed column and a thermal conductivity detector. The observed peaks were identified by comparison of retention times obtained from injection of commercially available (Scott Specialty Gases) standard gas samples. Molar yields were determined by comparison of peak areas with standard samples to obtain volume percent; conversion to mole percent was accomplished with the ideal gas law and the measured total pressure within the reactor (before GC injection). Residual components remaining in the reactor were extracted with methylene chloride and analyzed on the H P 5890A by using a 10-m HP-17 (cross-linked 50% phenylmethlysilicon) capillary column and a flame ionization detector. Analysis included the addition of a known quantity of naphthalene to the liquid samples as an internal standard, and peak areas were compared with those obtained from standards produced within the lab from pure component materials. Chloride ion concentration was measured with a Cole-Parmer Chemcadet 5986-50 selective ion monitor to verify that all chlorine atoms were present as a liquid-phase product. The experimental conditions are summarized in Table I. The temporal variation of product yield and conversion for the oxidation of DCB over vanadium oxide catalyst was determined a t 288, 315, and 343 "C and a total loading pressure of 69.0 bar (run 1). A blank experiment in the absence of vanadium oxide catalyst confirmed that no reaction occurred without the catalyst present while reactions in the absence of oxgen were used to quantify the rate of adsorption of DCB onto the catalyst. A second set of experiments (runs 2,3, and 4) at 315 OC and P,,,o = 27.6 bar was accomplished over a range of initial DCB and O2concentrations. C02was a major product in all cases. CO was also a major product under certain conditions. Chlorine atoms were detected as HC1 in the extracted liquid phase; however, chloride ion concentration was not consistently monitored. A small yield of 1,2,4-trichlorobenzene was identified as a liquid-phase product based on GC-mass selective detector (HP 5970B) analysis but was not quantified on a regular basis. The analytical procedures did not permit quantification for water; however, small quantities were always detected in the liquid phase through GC analysis.
Thermodynamic Calculations Because of the high critical temperature for DCB, the temperature and pressure in the current experiments allow multiple fluid phases at relatively high DCB concentrations. In order to determine the limits at which a single
40
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Operating pressure
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.
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was employed to estimate the initial pressure inside the reactor a t the reaction temperature and the dew point pressure a t that temperature. The BASIC computer program VLMU.BAS described in Sandler (1989) was used. For these calculations, the critical constants for dichlorobenzene, oxygen, and nitrogen were used (Reid et al., 1976), the interaction parameters between DCB and the permanent gases were estimated as 0.16 (from data in Sandler (1989) for the benzene-nitrogen system), and the interaction between N2 and O2 was assumed to be ideal. The pressure within the reactor a t the temperature of the experiment was estimated by using the overall molar volume (total moles fed to the reactor/total volume). This pressure, a t the known reaction temperature, could then be compared to the dew point pressure to determine if the reaction was in a single vapor phase. The results of these calculations are summarized in Figure 1,where the initial reaction pressure and the dew point pressure are plotted as a function of the concentration of DCB in the reactor, = 20.7 bar and p~~ = for a constant loading pressure 6.9 bar. From this graph, the maximum DCB concentration, which provides a single vapor phase, was determined to be slightly less than 0.25 mol/L; the maximum DCB concentration actually employed in this study was 0.22 mol/L. Although not indicated, similar calculations were accomplished at all experimental conditions to determine the phase behavior of the reacting mixture.
Results The effect of reaction temperature on the conversion of DCB and yields of products is reported for c ~ B= 0.37 mol/L, ptotal,o = 69.0 bar, and p ~ ,=, 27.6 ~ bar. ?he low carbon balance obtained in these experiments suggested an investigation of the adsorption of DCB onto the catalyst by accomplishing experiments in the absence of added oxygen. Finally, the effect of the initial concentration of both oxygen and DCB on the reaction rate measured by C02 production is reported. The temporal variations of DCB disappearance, COP yield, and CO yield are shown in Figures 2, 3, and 4, respectively, for the oxidation of 0.37 mol/L DCB a t 288, = 27.6 bar. To account for the 315, and 343 "C and stoichiometry of the reaction, the yield was calculated based on carbon atoms, i.e., yCol= nco /6nm~,oand similarly for CO. A t all 3 temperatures, disappearance was
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 91 0.4 -
1
0.3
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A A
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0.2
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Table 11. Estimated Pseudo-First-Order Rate Constants for the DiSaDDearanCe of DCB temp, OC kl, min-l temp, "C kl, m i d 288 2.34 x 10-3 343 3.37 x 10-3 2.92 x 10-3 315 ~
0
~~
0
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n
288'C 319C 50 A 343OC 60 0
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40
20 30 Reaction time (minutes)
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Figure 2. Temporal variation of DCB disappearance at 288, 315, and 343 O C and po2 = 27.6 bar. 2.0
288°C 0 315°C 0
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7 ,
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IO
0
,
,
, , , 30 40 Reaction time (minutes)
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20
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Figure 3. Temporal variation of COzyield at 288,315, and 343 O C and PO, = 27.6 bar.
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315°C A 343F
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8
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0.3 0.2 0.1
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1.70
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Figure 5. Arrhenius plot for the disappearance of DCB derived from pseudo-first-order analysis for the apparent rate constant.
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Reaction time (minutes) Figure 4. Temporal variation of CO yield at 288,315, and 343 OC and pol = 27.6 bar.
initially very rapid, with 13-2070 loss (by weight) of DCB in the first 5 min. The rate of disappearance decreased dramatically after 5 min; after 60 min at 288 "C, DCB loss merely increased to approximately 20%. Increasing the reaction temperature to 315 "C placed the reactants in a single vapor phase and led to an increase in DCB disappearance. At the higher temperatures, nearly 40% weight loss was observed after 60 min. The yield of COPdid not display the rapid initial change that was observed in the DCB disappearance data. At 288 "C, the COz yield increased steadily to 0.1 a t 60 min. When the reaction temperature was increased to 343 "C, a much greater COP yield was observed, increasing to 0.4 after 60 min. No CO was observed a t 288 "C, but a t 315 "C, CO was detected a t 20 min and its yield increased to 0.3 a t 40 min and decreased to 0.2 a t 60 min. The temporal variation of CO
yield at 343 "C followed a similar pattern to that a t 315 "C, the yield being 0.9 a t 10 min but less than 0.1 a t 60 min. Thus, CO production was increased by higher reaction temperatures, but this product was further oxidized to COPat longer reaction times. The initial disappearance of DCB was correlated based on a pseudo-first-order analysis; the values of the rate constants are indicated in Table 11. In the estimation of the pseudo-first-order rate constants, irreversible adsorption of DCB was assumed and discounted from the conversion, based on the information described below. By using these estimated rate constants, the activation energy was estimated from the Arrhenius plot of Figure 5 as 24.9 kJ/mol. The loading conditions for the above experiments correspond to a molar oxygen to DCB ratio of 3.0. On the basis of the stoichiometry of the reaction indicated in eq 2, the maximum DCB conversion that should be expected
-
13 C6H4Cl2+ 202 6COz + 2HC1+ H 2 0
(2)
is approximately 0.465, if all DCB is converted to COP At 343 "C, nearly 40% disappearance of DCB was observed, suggesting that this reaction may have proceeded to its stoichiometric limit. However, only 0.4 yield of COz was obtained, which is substantially below the level of 2.4 (6 mol of COP produced per mole of DCB reacted), which would be expected from the 40% DCB disappearance. A total carbon balance was calculated as Z(yc0, + yco + 6yDcB)/6yDcB,o, which accounted for all carbon materials initially loaded in the reactor. Because of the small amount of DCB that actually reacted, the total carbon balance always remained near 1. A second carbon balance was calculated as Z(yco + yCO)/6xDCB, which only considered the amount of DbB that was lost due to reaction. This is shown in Figure 6 for reaction a t both 315 and 343 "C and reveals that the observed products accounted for less than 50% of the DCB which reacted. The carbon balance indicated in Figure 6 also includes an estimate of the amount of 1,2,44richlorobenzene, which was observed as a minor product. The low observed carbon balance must indicate that either substantial quantities of material were lost during
92 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 0.8 0.7
0.6
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Figure 8. Langmuir plot for DCB adsorption on vanadium oxide catalyst at 315 "C.
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4 5 6 7 8 9 1 0 1 1 1 2 Reaction time (minutes) Figure 7. Temporal variation of DCB disappearance resulting from adsorption onto catalyst during heating to 343 OC without oxygen.
0
1
2
3
the reaction due to the reactor leaking or that some carbon-containing compounds remained within the reactor in a nonidentifiable form. Since the total carbon balance was always near 1 and detectable quantities of products were always obtained, the probability of reactor leaking was slight. In addition, a leaky reactor would result in a systematic decrease in the carbon balance, which was not observed. Thus, it was concluded that the material must be present in some form that could not be easily identified. Upon completion of the reaction, both liquid- and gasphase analyses were completed. Although the solid catalyst remained in the liquid after extraction from the reactor, no separate analysis was accomplished on that phase. It is possible that the DCB, which should adsorb onto the catalyst in order for the reaction to take place, remained adsorbed on the catalyst after reaction, thus substantially decreasing the amount of material that was recovered. In order to confirm the above hypothesis, an adsorption study for DCB on the Vz05catalyst was undertaken. DCB was loaded into the reactor and the reactor heated without any oxygen present; under these circumstances, no oxidation reaction should be possible. Analysis of both the liquid and the gas phase after heating revealed no reaction products; however, the concentration of DCB remaining in the reactor (as measured by GC analysis) was decreased compared to the initial DCB loading, as indicated in Figure 7. A t 343 "C, the disappearance of DCB increased rapidly over the first 3 min of heating, reaching an ultimate value of approximately 0.0075 mol of DCB/g of catalyst. This loss of DCB was attributed to irreversible adsorption of the reactant onto the catalyst.
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20 30 40 Reaction time (minutes)
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Figure 9. Temporal variation of overall reacted carbon balance at 315 and 343 "C and po2 = 27.6 bar, which accounts for unreacted DCB adsorbed on the catalyst.
For adsorption onto the catalyst, the disappearance of DCB should be correlated in terms of the Langmuir adsorption isotherm CDCB.~ KDCBCDCB ODCB = -(3) Ctotal sites 1 + KDCBCDCB By material balance, cDCb = cDCB,O - cNB;thus, the initial DCB concentration was varied at 315 "C and the equilibrium adsorption measured. Rearranging eq 3 reveals that a plot of the inverse of the disappearance of DCB vs the inverse of the final DCB concentration should yield a straight line. This is indicated in Figure 8, in which the straight line represents the best-fit approximation to the data. The slope of the straight line in Figure 8 provides an estimate for the adsorption equilibrium constant of KDCB.=0.136 L/mol at 315 "C. Using the above data, it was possible to estimate the amount of DCB that remained adsorbed on the catalyst after reaction and include this value in the material balance calculations. With this inclusion, the reacted material balance described previously was generally greater than 0.8, as indicated in Figure 9. The small deviation from 1 (indicating complete carbon balance) may result either from formation of a carbonaceous char as a minor product or from inconsistencies in the analytical techniques. The consistency of the carbon balance with time at a value of 0.8 and failure to observe solids accumulation within the reactor support the latter postulate. In order to develop a kinetic rate expression, the initial DCB concentration in the reactor was varied from nearly
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 93
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c o2(molL)
c,,,(mol/L)
Figure 10. Effect of initial DCB concentration on the initial rate of COSproduction at 315 "C and pol = 24.1 bar.
0 to approximately 0.22 mol/L (the thermodynamic vapor-phase limit) and the rate of reaction estimated. The reaction rate was estimated based on the production of COP after 10 min of reaction time; thus reo, = ccoz after 10 min/l0 min (4) Although the disappearance of DCB could have been used, the ratio of adsorbed DCB to reacted DCB was large; thus, the rate of reaction based on the production of CO, was chosen as a more precise measure. Figure 9, a plot of rco as a function of DCB concentration for reaction at 315 and po, = 24.1 bar, reveals a maximum initial rate of 0.0072 mol/(L min) at C ~ =B0.05 mol/L, or approximately 9OOOO ppmv. The rate of production tends toward zero a t the limit of low DCB concentration as expected since DCB is the only carbon-containing species in the reactor. For environmental applications, the region of interest is that of a low initial concentration, under which conditions the reaction is approximately first order in DCB concentration. At higher concentrations, the rate decreases and approaches an assymptote of less than mol/(L min). The decreasing rate at high DCB concentrations is consistent with the assumption of a strongly adsorbed species on the surface of the catalyst, which inhibits further reaction by limiting the amount of oxygen that may adsorb, as was suggested previously in Figure 7. The effect of initial oxygen concentration on the rate of COz production is indicated in Figure 10, at 315 "C and constant CDCB = 0.18 mol/L. At low co , increasing the oxygen concentration increased the rate o$COp production, with the limit of no reaction at co = 0 mol/L. A t coz > 0.5 mol/L, the initial rate of Cd, production was approximately constant at 0.0015 mol/(L min). These results combine to indicate that oxygen was required for the conversion of DCB but was adsorbed in its equilibrium amount at higher gas-phase partial pressures.
"e
Discussion The influence of temperature below 315 "C may be attributed to the presence of a two-phase system under those conditions. DCB disappearance was initially very rapid even at the lowest temperature, indicating that adsorption of the DCB was not significantly impacted by the presence of a liquid phase. However, the oxygen was primarily in the vapor space above the liquid and present in a low concentration within the DCB-rich liquid, which contained the bulk of the catalyst. Thus, the reaction rate was limited by the low concentration of oxygen within the liquid phase. The low CO yield, which was observed, was more likely a result of mass-transfer effects within the catalyst pores, in particular, the low diffusivity of gases within the
Figure 11. Effect of initial oxygen concentration on the initial rate of C02 production at 315 "C and cDCB = 0.18 mol/L.
liquid. Any CO produced in the catalyst pores would be likely to undergo additional reaction to C02before it could diffuse to the bulk liquid and then into the vapor. A set of detailed reaction pathways can be described that better explains the results presented in Figures 10 and 11. The first step in the pathway must be the rapid adsorption of DCB onto the catalyst surface, as evidenced by Figures 7 and 8 when no oxygen was added to the reactor. DCB(g) + S DCB(a) (5) The reaction of the adsorbed DCB may occur either with oxygen that has dissociatively adsorbed onto the catalyst DCB(a) + O(a) CO(a) + C5(a) (6) or with gas-phase oxygen CO(a) + C,(a) (7) DCB(a) + O,(g) In either case, the adsorbed C5 species reacts with additional oxygen to produce the observed product spectrum C5(a) + 302 5CO(a) + HzO(g) + 2HCl(g) (8) The CO that is formed as the first oxidation step may undergo further oxidation to produce the major product CO,,or it may desorb from the catalyst, resulting in the observed minor product, CO(g). 1 CO(a) + -0, COz(g) (9) 2 CO(a) CO(g) + S (10) Information regarding the identification of the ratelimiting step for the above pathway is located within the experiments occurring in the vapor phase. According to the Peng-Robinson calculations, this occurred a t temperatures above 315 "C. When the reaction temperature was increased from 315 to 343 "C, the conversion of DCB was not greatly effected; however, the yield of CO and COz increased dramatically. Separate experiments indicated that rapid adsorption occurred at 315 "C with little product generation. The increase in temperature substantially accelerated the rate-limiting step in the reaction, which could not be DCB adsorption since it occurred rapidly and irreversibly at 315 "C. Since the desorption step is not substantially influenced by temperature, this analysis suggests that the reaction on the surface of the catalyst was likely the rate-limiting step. Also, the slowest of the surface reaction steps should be the initial cleavage of the aromatic ring to produce CO and an adsorbed C5 species, indicated as either of step 6 or step 7. In either case, the identity of the C5 species is not stated: however, it is not required for kinetics analysis. While neither pathway explicitly accounts for the presence of small amounts of 1,2,4-trichlorobenzene, it
-
-
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94 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991
-
could be formed through disproportionation of DCB, CGH4C12 + C6HdC12 C6H3C13 + C,jH&1 (11) either as a catalyst-initiated free-radical reaction in the gas phase or through interaction on the surface of the catalyst. However, the small amounts of trichlorobenzene formed reveal that neither pathway is kinetically significant in the disappearance of DCB and the formation of c02. The reaction steps of eqs 5-10, with the assumption of reaction of adsorbed DCB with adsorbed oxygen on the surface as the rate-limiting step (eq 6), permits estimation of the rate expression as
0
0.05
0.10
0.15
0.20
0.25
c,,,(molL)
Figure 12. Test of expected linearity in DCB concentration for postulated rate expression providing the slope K ~ ~ / ( k ' ) l / ~ .
If all DCB is converted to COP,water, and HC1, the rate of COP production would be 6 times the rate of DCB disappearance, or rco2 = ~ ( - ~ D c B ) . Equation 12 predicts that increasing c X B will increase the reaction rate at low concentration but decrease the rate a t high concentration. This is in agreement with the experimental results indicated in Figure 10. The effect of oxygen concentration on the reaction rate is predicted by eq 12 to be approximately the same as that of DCB, with an increase in the concentration leading to an increased ' rate at low concentration and a decrease in the rate at high concentration. This is consistent with the low coz results in Figure 11 but disagrees with the asymptotic rate that is reached a t cop> 0.5 mol/L. If the reaction proceeds through an Eley-Rideal mechanism, as described through eq 7, the rate expression is written as
This suggests identical behavior for DCB as was developed from eq 12, which was seen to be consistent with the experimental results described by Figure 10. However, eq 13 indicates that the rate is independent of cog a t high concentration of oxygen, as expected from the results of Figure 11. This suggests that the reaction may proceed through reaction with gas-phase oxygen, as indicated by eq 7. This hypothesis is further supported by the adsorption experiments conducted in the absence of gasphase oxygen during which no reaction occurred. If the mechanism involved reaction with adsorbed or lattice oxygen, a small but measurable conversion would have been expected from the reaction of the oxygen that is present within the V205catalyst. Rearranging eq 13 reveals that groups ( c ~ ~ c O ~ / ~ C should be linear in DCB concentration with a slope of KmB/ (k'Y2and should also be proportional to the square root of the oxygen concentration with a slope of (K%/k')1/2. Figure 12 is the appropriate plot for DCB concentration and reveals good agreement with the expected linearity. By using the previously determined value for KmBof 0.136 L/mol (from the experiments in the absence of oxygen, see Figure 81, the current plot provides a value of the constant k ' = 4.88 X lo4 L/ (mol min). A similar plot for oxygen concentration is indicated in Figure 13 and permits calculation of the adsorption equilibrium constant for L/mol. oxygen as KO, = 3.66 X The calculated values for the adsorption constants suggest that DCB is adsorbed more rapidly in higher concentration than is oxygen. This is consistent with the previous analysis based on the observation of rapid DCB
20
1
0 0
0.2
0.6 0.8 {co,(mol/lit)} 1/2
0.4
Figure 13. Test of expected linearity in c expression providing the slope (Ko,/k
1.0
1.2
~ for postulated ~ ~ / rate~
disappearance with small production of COP product, which suggested adsorption of DCB as occurring rapidly and irreversibly. Also, the rate constant is quite small compared to the adsorption constant for both oxygen and DCB, suggesting that the surface reaction is the rate-limiting step as postulated previously based on the observed temperature effect. Conclusions The oxidation of 1,4-dichlorobenzene over vanadium oxide catalyst produced COPas the major product and CO and 1,2,4-trichlorobenzeneas minor products. Under some conditions, the yield of CO exceeded that of COP. The amount of carbon in the reaction products was generally less than that expected based on the weight loss of DCB during reaction. Further analysis suggested that the DCB adsorbed irreversibly on the catalyst; however, the surface reaction proceeded more slowly than the adsorption, providing lower than expected yields of COP. Analysis in ~ terms ~ ) ~ of / ~the reaction pathways provided a kinetic expression that was first order in both DCB and oxygen concentration, although adsorption of both species inhibited the rate at higher concentrations. Mathematical manipulation of the postulated rate expression and experiments at multiple temperatures suggested that the rate of the surface reaction was the limiting step in the mechanism. Acknowledgment This material is based upon work supported in part by the National Science Foundation under Grant No. CBT8909940. Nomenclature ci = concentration of species i, mol/L K i= adsorption equilibrium constant for species i, L/mol k' = reaction rate constant, L/(mol min)
Ind. Eng. Chem. Res. 1991,30,95-100 ni = number of moles of species i pi = partial pressure of species i, bar ptotal= total pressure in the reactor, bar r = reaction rate, mol/(L min) t = reaction time, min Oi = fraction of total sites on which species i is absorbed yi = yield of species i, ni/6nme,o x = conversion or disappearance of DCB, 1 - CDCB/CDCB,~ Subscripts
0 = loading value at room temperature (24 "C) DCB-s = DCB adsorbed on the catalytic site i = arbitrary species
Literature Cited Leavitt, D. D.; Abraham, M. A. Acid Catalyzed Oxidation of 2,4Dichlorophenoxyacetic Acid by Ammonium Nitrate in Aqueous Solution. Environ. Sci. Technol. 1990, 24, 566-71. Lester, G. R. Catalytic Destruction of Hazardous Halogenated Organic Chemicals. Presented at the 82nd Annual Meeting of the
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Supercritical-Phase Fischer-Tropsch Synthesis Reaction. 2. The Effective Diffusion of Reactant and Products in the Supercritical-Phase Reaction Kohshiroh Yokota and K a o r u Fujimoto* Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, J a p a n
A Fischer-Tropsch synthesis reaction was conducted in a supercritical fluid medium with use of a fixed bed reactor. n-Hexane was used as the supercritical fluid. The typical reaction conditions utilized were 240 "C and 45 bar, which consisted of 10 bar for syn-gas and 35 bar for n-hexane. Although the rate of the reaction and the diffusion of reactants were slightly lower than those in the gas-phase reaction, the removal of reaction heat and waxy products from the catalyst surface was much more effective than in the gas-phase reaction. The olefin content in the hydrocarbon product was much higher in the supercritical-phase reaction than in the liquid-phase reaction or in the gas-phase reaction. It was attributed to the short residence time of the product in the catalyst bed for the supercritical-phase reaction, which was caused by the well-balanced desorption and diffusion of the products. Introduction The Fischer-Tropsch (F-T) synthesis reaction, which produces liquid hydrocarbons from synthesis gas, is a solid-catalyzed gas-phase reaction, which is accompanied by a large heat of reaction. Consequently, local superheating of the catalyst bed easily occurs. At the same time, the reaction produces high molecular weight hydrocarbons (wax),which do not vaporize under the reaction conditions, cover the catalyst surface, and plug the micropores of the catalyst. In order to promote the heat transfer and the wax removal from the catalyst, a slurry-phase process, in which a sluny of the catalyst particles in mineral oil is used as the reaction medium, has been developed (Koelbel and Ralek, 1980; Satterfield and Stenger, 1985; Stern et al., 1985; Deckwer et al., 1986). However, in the case of the slurry-phase reaction, the diffusion of synthesis gas in the micropores is so slow that the overall reaction rate is lower than that in the gas-phase reaction (Stern and Bell, 1983; Fujimoto, 1987; Huang et al., 1988). Supercritical fluid has been utilized for a number of extraction processes because of its unique solubility character and its high self-diffusion efficiency (Hoyer, 1985; Debenedetti and Reid, 1986). We have reported that the F-T synthesis reaction in a supercritical fluid has unique 0888-5885/91/2630-0095$02.50/0
characteristics for either the reaction rate or the product distribution (Yokota and Fujimoto, 1989). In this study, we applied the unique characteristics of a supercritical fluid as the reaction medium for the Fischer-Tropsch synthesis reaction. The results were compared with those in the gas phase and in the liquid phase, in terms of material transfer, heat transfer, and in situ extraction. Experimental Section Catalyst Preparation. Silica-supported cobalt catalysts were prepared from cobalt nitrate, lanthanum nitrate, and commercially available silica gels (Fuji Davison ID gel, RD gel, and Aerosil aerosil silica, whose specific surface areas were 270, 720, and 380 m2/g, respectively) by a conventionalimpregnation method (Fujimoto et al., 1983). The composition of the catalysts was 206:87 Co-La/Si02 and 10:3:100 Co-La/Si02 by weight, respectively. The catalyst precursors were dried in air at 120 O C and then calcined at 450 "C for 3 h to form the supported oxides. They were then subjected to hydrogen treatment at 400 "C for 12 h. The activated catalyst was passivated while in contact with 1% oxygen at room temperature. Carbon monoxide adsorption measurement indicated that the 0 1991 American Chemical Society
