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Ind. Eng. Chem. Res. 1999, 38, 4594-4599
Adsorption of Reaction Species on Pd-Carbon for Liquid-Phase Hydrogenation of o-Nitrophenol Vasant R. Choudhary* and Mukund G. Sane Chemical Engineering Division, National Chemical Laboratory, Pune 4110088, India
Adsorption isotherms for the single-component and multicomponent adsorption of o-nitrophenol hydrogenation reaction species (viz. o-nitrophenol, o-aminophenol, and water) from their methanol solution on Pd-carbon (4.6 wt % Pd) catalyst have been measured at the temperatures (278-308 K) at which the hydrogenation reaction is carried out. Both the single and binary adsorption data could be fitted well to Langmuir type adsorption equations. However, the Freundlich-Langmuir equation gives a better fit to the binary adsorption data. The adsorption of reaction species occurs in the order o-nitrophenol > o-aminophenol . water and it is strongly influenced by the solvent used. Introduction
Experimental Section
In any solid catalyzed process, adsorption of reactants (or at least one reactant) on the catalyst, reaction between adsorbed reactants [or reaction between at least one adsorbed reactant and other reactant(s) in the bulk phase], and desorption of product(s) occur in successive steps. The overall catalytic process (in the absence of mass-transfer resistances) is controlled by one of the steps or a combination of the steps. The adsorption of reaction species plays a vital role in the catalytic process. Hence, to understand the catalytic process and its controlling mechanism, it is necessary to investigate the adsorption of reaction species at the catalytic conditions or at least at the temperatures at which the catalytic reaction occurs (Tamaru, 1964). Liquid-phase hydrogenation of o-nitrophenol to oaminophenol over Pd-carbon catalyst is an industrially important process. The kinetics and controlling mechanism of this catalytic process is reported in our earlier publication (Choudhary et al., 1998). Because of the practical importance of this process, it is interesting to study the adsorption of reaction species from their methanol solution. Methanol is the best solvent among C1-C3 alcohols for the hydrogenation process (Sane, 1986). A few studies have been reported earlier for the adsorption of hydrogen on palladium wires (Aldag and Schmidt, 1971) and films and supported Pd (Beeck, 1950; Schuit and van Rejjen, 1958) and also on Pdcarbon (Gentsch et al., 1972; Zakumbseva et al., 1977). However, no study on adsorption of the other reaction species (viz. o-nitrophenol, o-aminophenol, and water) from their methanol solution on Pd-carbon has been reported so far. In the present investigation, the single-component and multicomponent adsorptions of the reaction species (viz. o-nitrophenol, o-aminophenol, and water) for the hydrogenation of o-nitrophenol on Pd-carbon (4.62 wt % Pd) from methanol at the catalytic reaction conditions (temperature: 278-308 K) have been studied.
Materials and Catalyst. Methanol (99.8% dried over 3A molcular sieves), o-nitrophenol (98%), o-aminophenol (99%), nitrogen (>99.9%), and water (deionized distilled) were used in the study. A Pd-carbon catalyst (4.62 wt % Pd) was used, and properties are listed in Table 1. Experimental Procedure. Single-component adsorption of o-nitrophenol (ONP), o-aminophenol (OAP), and water from their respective solution in methanol and also simultaneous two -component (or binary) adsorption of ONP and OAP from their mixture in methanol in Pd-carbon (4.62 wt % Pd) have been measured experimentally at the temperatures (278-308 K) at which the catalytic hydrogenation reaction occurs. Methanol has a high vapor pressure at the highest adsorption temperature (308 K), and hence an adsorption tube provided with a Teflon stopper and an injection port arrangement with a Teflon-lined rubber septum was used for measurement of adsorption. A known weight (0.5-1.5 g) of the Pd-C catalyst (particle size: 30 µm) of each of eight adsorption tubes mounted on a stand kept immersed in a constanttemperature water bath was introduced. The catalyst in the adsorption tube was equilibrated with 25 cm3 of the solution of an adsorbate in methanol at different concentrations for a period of 6 h, which was found to be more than sufficient to establish adsorption equilibrium. During this period, the solution with the catalyst inside the water bath was shaken manually at intervals of 5 min. The catalyst was allowed to settle, and liquid samples were taken out with a syringe for chemical analysis. The amount of solute adsorbed on the catalyst was obtained from the initial and final concentrations of the adsorbate in the solution using the relation
* To whom all correspondence should be addressed. E-mail:
[email protected]. Fax: (+91) 20-5893041/5893355. Tel.: (+91) 20-5893300 (ext. 2163).
Q ) V(CI - Ce)/W
(1)
Analysis of ONP and OAP in their methanol solution was done by gas-liquid chromatography using SE 30 (5%) on chromosorb W using nitrogen as the carrier gas (25 cm3 min-1). A Perkin-Elmer (Sigma 3B) gas chromatograph fitted with a flame-ionized detector was used for this purpose. The column temperature was programmed from 398 (initial period: 8 min) to 423 K at a
10.1021/ie990055d CCC: $18.00 © 1999 American Chemical Society Published on Web 10/19/1999
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4595 Table 1. Catalyst Properties Pd concentration, wt % Pd particle size, µm surface area, m2 g-1 solid phase density, g cm-3 pore volume, cm3 g-1 porosity average pore radius, nm
4.62 30 616 1.87 0.65 0.55 2.1
Figure 2. Isotherms of the single-component adsorption of o-aminophenol at different temperatures.
Figure 1. Isotherms of the single-component adsorption of o-nitrophenol at different temperatures.
heating rate of 30 K min-1. The analysis for water in methanol was performed by a Karl Fischer method. Experimental data for the adsorption of water from methanol at different concentrations were determined only at 308 K, whereas the adsorption data for ONP and OAP at different concentrations were obtained at 278, 293, and 308 K. In all of the above adsorption experiments, the temperature was controlled within 0.1 K. To measure the simultaneous binary adsorption of ONP and OAP from their mixture in methanol on the catalyst, the experimental procedure followed was very similar to that described earlier for the single component except that in the binary adsorption measurement. The catalyst was equilibrated with the methanol solution containing equimolar ONP and OAP. Concentrations of ONP and OAP at the equilibrium were determined by GC analysis. The amount of ONP and OAP adsorbed simultaneously at their different equilibrium concentrations was evaluated from the knowledge of their initial and final concentration using eq 1. Binary adsorption of ONP and OAP was measured at 278, 293, and 308 K. The initial concentration of ONP and OAP taken together in a two-component system was varied from 0.06 to 0.36 mmol cm-3. Results and Discussions Single-Component Adsorption. Adsorptions of ONP, OAP, and water from methanol on the Pd-carbon are presented in Figures 1-3. The use of concentrations of ONP and OAP higher than 0.7 and 0.34 mmol cm-3, respectively, was not possible because of the solubility of these compounds in methanol. The solubilities of ONP and OAP in methanol at 378 K are 1.0 and -0.4 mmol cm-3, respectively. It was not possible to collect precise adsorption data at the temperature above 308 K because of the high vapor pressure of methanol at the high temperatures. According to the classification of isotherms for adsorption from solutions suggested by Giles et al. (1960), the isotherms of ONP, OAP, and water are of type L1.
Figure 3. Isotherm of the single-component adsorption of water at 308 K.
In of all the cases adsorption increases continuously with the increase an the equilibrium concentration of the adsorbate. A compression of adsorption data for the three reaction species indicates that their adsorption on the Pd-carbon catalyst occurs in the following order:
ONP > OAP . water Adsorption data for ONP, OAP, and water were fitted to the following Langmuir and Freundlich adsorption equations.
Langmuir equation q ) qm[KC/(1 + KC)]
(2)
or
C/q ) [1/(qmK)] + (1/qm)C where q is the amount adsorbed; qm, the monolayer adsorption capacity of the adsorbent or catalyst; K, the adsorption equilibrium constant; and C, the equilibrium concentration of the adsorbate.
Freundlich equation q ) kCn
(3)
or
log q ) log k + n log C where k is the adsorption constant and n the exponent. The CN/qN vs CN plots according to the Langmuir equation (eq 2) for the adsorption of o-nitrophenol at different temperatures are shown in Figure 4. The plots
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Figure 6. Freundlich plots (according to eq 3) for the adsorption of o-aminophenol and water. Figure 4. Langmuir plots (according to eq 2) for the adsorption of o-nitrophenol.
Figure 7. Langmuir plots (according to eq 2) for the adsorption of water at 308 K. Table 3. Freundlich Parameters (k and n, Eq 3) for the Adsorption of OAP and Water adsorbate OAP water Figure 5. Langmuir plots (according to eq 2) for the adsorption of o-aminophenol. Table 2. Langmuir Parameters (qm and K, Eq 2) for the Single-Component Adsorption of ONP, OAP, and Water at Different Temperatures adsorbate ONP OAP water
temp (K)
qm (mmol g-1)
K (cm3 mmol-1)
278 293 308 278 293 308 308
5.33 4.15 2.69 2.94 2.94 1.38 3.41
9.28 9.63 11.01 19.60 7.55 14.49 0.82
are linear, and this indicates a good fit of the adsorption data at all of the temperatures to the Langmuir equation. The values of the adsorption parameters (qm and K) obtained from eq 2 are given in Table 2. When the ONP adsorption data are plotted according to the Freundlich equation (eq 3), the log qN vs log CN plots are not linear at all of the temperatures. This indicates that the adsorption of ONP does not follow the Freundlich isotherm. Figure 5 shows that the data for the adsorption of OAP (except at the very low OAP concentration at 293 and 308 K), a good fit to the Langmuir equation, indicate adsorption follows this isotherm. Values of the Langmuir parameters (qm and K) for the adsorption of OAP are included in Table 2. Figure 6 indicates that the OAP adsorption data at 293 and 308 K also fit to the Freudlich equation. The
temp (K)
k (cm3n m mol1-n g-1)
n
293 308 308
4.02 1.63 1.41
0.52 0.31 0.65
adsorption data at 278 K, however, show a break (Figure 6), suggesting OAP data at 278 K do not follow this isotherm. The values of the Freundlich parameters (k and n) for the adsorptions at 293 and 308 K are presented in Table 3. The linear Langmuir and Freundlich plots in Figures 7 and 6, respectively, for the adsorption of water at 308 K show that the adsorption follows both Langmuir and Freundlich isotherms. The values of the Langmuir and Freundlich parameters for the adsorption of water are included in Tables 2 and 3. Adsorption from solution on a solid surface is, in general, a complex phenomenon (Schuit and van Kejjen, 1958) because of competition between adsorption of the solute and that of the solvent on the solid surface. The polarity of the molecules plays an important role in the adsorption. In the case of adsorption from dilute solutions, as in the present case, adsorption of solute can occur to an appreciable extent only if its polarity is much higher than that of the solvent. If this is not so, the adsorption of the solvent is expected to occur in preference to that of the solute as the concentration of the solvent is very high compared to that of the solute. In the present case, the dipole moment of the various reaction species is in the following order: o-nitrophenol (3.13 D) > o-aminophenol (2.78 D) > water (1.71 D) > methanol (1.63 D). The observed order of adsorption (ONP > OAP > water) is quite consistent with these dipole moments. The value for OAP is predicted (Owen, 1969).
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4597
Figure 8. Isotherms of the adsorption of o-nitrophenol from methanol in the presence and absence of o-aminophenol.
According to the classification of isotherms (Giles et al., 1960; Kipling, 1965), the adsorption isotherms of all of the reaction species are of type L1 and the limit of saturation was not attained in any case. The value of qm was found to vary with the temperature of the adsorption (Tables 2 and 3). However, the adsorption equilibrium constant (K) for (ONP and OAP) does not show the expected trend with a temperature increase. Generally, K should decrease with an increase in the temperature. These facts indicate that either the adsorption of the reaction species does not strictly follow the Langmuir isotherm or the solvent, which is at a very high concentration (about 24 mmol cm-3) as compared to that of the reaction species, strongly influences adsorption. The heat of immersion of the catalyst in methanol at 303 K was found to be 63 J g-1. This indicates that the solvent-catalyst interactions are fairly strong. Recently, Augustine et al. (1984) have observed that methanol is readily adsorbed on active Pd catalysts, resulting in the blockage of active sites and also in the conversion of some active sites into unreactive sites. The unexpected trend in the adsorption variation of qm and K with temperature is, therefore, attributed most probably to adsorption of methanol, occurring simultaneously with adsorption of ONP and OAP. Binary Adsorption of ONP and OAP. The individual isotherms of adsorption of ONP and OAP from their mixtures in methanol are shown in Figures 8 and 9, respectively. For comparison single-component adsorption data for ONP and OAP are also included in Figures 8 and 9, respectively. Binary adsorption data are represented by solid lines, whereas single-component adsorption data are given by dotted lines. The comparison clearly shows that adsorption of ONP is decreased very significantly because of the presence of OAP and vice versa. Because single-component adsorption of ONP and OAP could be described by the Langmuir equation, it is expected that binary adsorption of ONP and OAP would also follow the Langmuir isotherm. The binary adsorption data were therefore fitted to the following
Figure 9. Isotherms of the adsorption of o-aminophenol from methanol in the presence and absence of o-nitrophenol.
Figure 10. qN vs R plots (according to eq 4) for the adsorption of o-nitrophenol in the presence of o-aminophenol.
Langmuir equations:
qN ) qm[(KNCN)/(1 + KNCN + KACA)]
(4)
qN ) qmR and
qA ) qm[(KACA)/(1 + KACA + KNCN)]
(5)
qA ) qmβ where the suffixes N and A represent o-nitrophenol and o-aminophenol, respectively. According to the Langmuir theory of adsorption, the adsorption equilibrium constant is expected to be the same for single-component and multicomponent adsorption. Therefore, values of KN and KA obtained from single-component adsorption for ONP and OAP, respectively, are used in eqs 4 and 5 and in qN vs R and qA vs β plots (Figures 10and 11, respectively). Values of qm (obtained from the slopes of the linear plots in Figures 10 and 11) for the adsorption of ONP and OAP from their mixtures in methanol are given in Table 4.
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Figure 11. qN vs R plots (according to eq 5) for the adsorption of o-aminophenol in the presence of o-nitrophenol. Table 4. Langmuir-Freundlich Parameters (qm, K, and n, eqs 6 and 7) for the Binary Adsorption of ONP and OAP qm KN KA (cm3 (cm3 temp (mmol g-1) mmol-1) mmol-1) adsorbate (K) ONP OAP
a
278 293 308 278 293 308
5.8 3.2 3.4 2.4 2.8 1.8
7.4 7.2 7.0 3.8 3.1 1.2
15.4 7.3 5.3 12.8 2.7 2.3
nN
nA
SDa
0.97 0.89 0.99 0.51 0.94 1.00
1.00 1.00 1.00 0.85 0.69 0.55
0.071 0.073 0.048 0.097 0.069 0.075
Standard deviation for estimated qA and qN from the model.
Figure 13. Comparison of the experimental and estimated (from eqs 6 and 7) values of qN and qA for the binary adsorption ONP and OAP.
of eqs 6 and 7 along with the value of qm (obtained from eqs 4 and 5) are given in Table 4. Estimated and experimental values of qN and qM are compared in Figure 13. The standard deviation (Table 4) of the estimated values of qA and qN from the corresponding observed ones is very small. A comparison of the results in Figure 12 with those in Figure 13 indicates that the Langmuir-Freundlich equations (eqs 6 and 7) give a better fit to the binary adsorption data than the Langmuir equation. It may be noted that the better fit may also be because of the increased model parameters. Conclusions
Figure 12. Comparison of the experimental and estimated (from eqs 4 and 5) values of qN and qA for the binary adsorption ONP and OAP.
A comparison between the estimated and experimental qN and qA shown in Figure 12 indicates a fairly good fit of the binary adsorption data to the Langmuir equations (eqs 4 and 5). To obtain a better fit of the experimental data, efforts were also made to fit the binary adsorption data to the following Langmuir-Freundlich equations:
qN ) qm(KNCNnN)/(1 + KNCNnN + KACAnA)
(6)
qA ) qm(KACAnA)/(1 + KACAnA + KNCNnN)
(7)
and
The values of qm were taken as the same as those obtained from eqs 4 and 5, and values of KN, KA, nN, and nA were determined by a nonlinear analysis. The optimized values of the parameters (KN, KA, nN, and nA)
Studies on the single and binary adsorptions of o-nitrophenol, o-aminophenol, and water from methanol on Pd-carbon catalyst at 278-308 K indicate the following: (1) Adsorption of different adsorbates occurs in the following order: o-nitrophenol > o-aminophenol . water. (2) Single-component adsorption isotherm data for all adsorbates fit the Langmuir adsorption isotherm. However, the adsorption of o-aminophenol at higher temperatures (>298-308 K) and water (at 308 K) also follows the Freundlich isotherm. (3) Binary adsorption data for o-nitrophenol and o-aminophenol follow both the Langmuir and Langmuir-Freundlich equation for multicomponent adsorption. Howeve, the Freundlich-Langmuir equation gives a better fit to the binary adsorption data. (4) Adsorption of o-nitrophenol and o-aminophenol from methanol on Pd-carbon catalyst does not strictly follow the Langmuir adsorption, suggesting that monolayer adsorption on the catalyst for both these adsorbates decreases with increasing temperatures. Adsorption of o-nitrophenol and o-aminophenol seems to be strongly influence by the solvent (methanol). Notations CA ) concentration of o-aminophenol (mmol cm-3) Ce ) equillibrium concentration (mmol cm-3) CI ) initial concentration of adsorbate (mmol cm-3) CN ) concentration of o-nitrophenol (mmol cm-3) CW ) concentration of water (mmol cm-3)
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4599 q ) amount of adsorbate adsorbed on catalyst (mmol g-1) qA ) amount of o-aminophenol adsorbed on catalyst (mmol g-1) qN ) amount of o-nitrophenol adsorbed on catalyst (mmol g-1) qW ) amount of water adsorbed on catalyst (mmol g-1) k ) Freundlich parameter (m3n mmol1-n) K ) equilibrium adsorption constant (cm3 mmol-1) V ) volume of solution (cm3) W ) mass of catalyst (g) R ) [KNCN/(1 + KNCN + KACA)] β ) [KACA/(1 + KACA + KNCN)]
Acknowledgment The authors are grateful to Dr. A. P. Singh for his help in the GLC analysis. Literature Cited Aldag, A. W.; Schmidt, L. D. Interaction of Hydrogen with Palladium. J. Catal. 1971, 22, 260. Augustine, R. L.; Warner, R. W.; Melnik, M. J. Heterogeneous Catalysis in Organic Chemistry. 3. Competitive Adsorption of Solvents during Alkane Hydrogenations. J. Org. Chem. 1984, 49, 4. Beeck, O. Hydrogen Catalysts. Discuss. Faraday Soc. 1950, 8, 118. Choudhary, V. R.; Sane, M. G.; Tambe, S. S. Kinetics of Hydrogenation of o-Nitrophenol to o-Aminophenol on Pd/Catalyst in
a Stirred Three Phase Slurry Reactor. Ind. Eng. Chem. Res. 1998, 37, 3879. Gentsch, H.; Guillen, N.; Koepp, M. Isotere Adsorption senthalpiess Wasserstoff anatmor Verteillem Pallidium und Platin auf Khole. Z. Phys. Chem. (Frankfurt am Main) 1972, 82, 49. Giles, C. H.; Mac Evan, H.; Nakhwa, S. N.; Smith, D. J. Studies in AdsorptionsPart XI. A system of Classification of Solutions Isotherms and its use in Diagnosis of Adsorption Mechanisms and in Measurement of Surface Area of Solids. J. Chem. Soc. 1960, 3973. Kipling, J. J. Adsorption of Solids from Solutions. Adsorption from Solutions of Nonelectrolytes. Academic Press: London, 1965; p 90. Owen, A. J. The HMO Treatment and Configuration of some Substituted Phenols. Tetrahedron 1969, 25, 3693. Sane, M. G. Studies on Catalytic Hydrogenation of o-Nitrophenol to o-Aminophenol in Three Phase Slurry Reactor. Ph.D. Thesis, University of Poona, 1986. Schuit, G. S. A.; van Rejjen, L. S. The Structure and Activity of Metal-on-Silica Catalyst. Adv. Catal. Relat. Subj. 1958, 3103, 242. Tamaru, K. Adsorption Measurements during Surface Catalysis. Adv. Catal. 1964, 155, 65. Zakumbseva, G. D.; Zahargna, N. A.; Toktabaera, N. F.; Masalk, N. V. Sorption of Hydrogen by Palladium Catalysts in Solutions. Kinet. Katal. 1977, 18, 1007.
Received for review January 22, 1999 Revised manuscript received August 11, 1999 Accepted August 18, 1999 IE990055D
