Ind. Eng. Chem. Res. 1996, 35, 3257-3279
3257
REVIEWS Catalytic Oxidation in Supercritical Water Zhong Yi Ding, Michael A. Frisch,† Lixiong Li, and Earnest F. Gloyna* Environmental and Water Resource Engineering Program, Supercritical Water Oxidation Project, J. J. Pickle Research Campus (CES), The University of Texas at Austin, Austin, Texas 78758
Recently, catalytic oxidation in supercritical water (SCW) has received considerable research attention. The major thrust of this current research effort is attributable to the rapid development of supercritical water oxidation (SCWO) as an innovative wastewater treatment technology. The incentives of catalyst-enhanced processes may include increased reaction rates, reduced residence times and temperatures, and optimized reaction pathways that are otherwise difficult to achieve through noncatalytic processes. However, the databases associated with the use of catalysts in SCWO are limited. The purpose of this paper is to (1) review catalytic enhancement and technology as related to SCWO; (2) analyze effects of SCW on catalysts and catalytic SCWO processes; and (3) present a catalyst development strategy for SCWO-related applications. Catalyst activity and stability (in terms of reaction kinetics, surface phenomena, and phase behavior of catalysts in SCW) are emphasized. The paper presents a useful database, provides guidelines for catalyst selection, and illustrates how effective use of catalyst may enhance SCWO process development. Contents Introduction Status of Catalytic SCWO Role of Supercritical Water Properties of Supercritical Water Hydrolysis of Organic Compounds Oxidation of Organic Compounds Stability of Inorganic Solids Catalyst and Its Preparation for SCWO Applications Catalyst Activity Catalyst Stability Catalyst Stability/Activity Affected by Preparation Methods Catalytic Kinetics Other Considerations Associated with Catalytic SCWO Conclusion Acknowledgment Literature Cited
3257 3258 3260 3260 3260 3261 3264 3265 3265 3267 3269
3270 3272 3274 3274 3274
Introduction Catalytic oxidation has been used in many wastewater treatment processes. Examples include traditional applications such as wet air oxidation (Mishra et al., 1995) and photolysis (Fox, 1988; Pichat et al., 1993; * Corresponding author. Telephone: 512-471-7053. Fax: 512-471-1720. E-mail:
[email protected]. † Present address: Eco Waste Technologies, 2305 Donley Drive, Suite 108, Austin, TX 78758.
S0888-5885(96)00022-X CCC: $12.00
Teichner et al., 1985). Catalysts are now being applied to enhance supercritical water oxidation (SCWO) operations. SCWO is a rapidly developing technology for the destruction of organic wastes (Gloyna and Li, 1993, 1995). In 1994, the world’s first commercial SCWO facility for treating industrial wastewater became operational (McBrayer, 1995; Svensson, 1995). Ongoing catalytic SCWO studies have demonstrated the benefit of utilizing heterogeneous catalysts for reducing energy and processing costs. Hazardous organic pollutants can be destroyed by SCWO at temperatures around 500 °C and reactor residence times of less than 1 min (Krietemeyer, 1992; Modell, 1989; Tester et al., 1993). However, other researchers of SCWO report the formation of refractory organic intermediates, such as acetic acid (Li et al., 1991; Shanableh, 1990; Tongdhamachart, 1990; Wilmanns, 1990) and ammonia (Dell’Orco et al., 1993; Killilea et al., 1992; Takahashi et al., 1989). Similar refractories are also formed in wet air oxidation (WAO) studies (Baillod et al., 1982; Fisher, 1971; Foussard et al., 1989; Friedman et al., 1988; Keen and Baillod, 1985; Ploos van Amstel and Rietema, 1973; Takahashi and Isobe, 1988; Teletzke et al., 1967; Wu et al., 1987). In addition, SCWO tests involving aromatic and substituted aromatic compounds have indicated the formation of dimers and other condensation products (Ding et al., 1995b; Gopalan and Savage, 1995; Thornton et al., 1991). At higher process temperature (possible 600 °C or higher) and longer reaction residence times, these refractory compounds and condensation products will be further oxidized to end products, such as water, carbon dioxide, molecular nitrogen, and mineral acids. The major incentive for using catalysts in SCWO is the process capacity. A successful catalytic process depends on the optimized combination of catalyst (components, manufacturing process, and morphology), reactants, reaction © 1996 American Chemical Society
3258 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 1. Relationships among various aspects of catalytic oxidation in supercritical water.
environment, process parameters, and reactor configuration. The simplified interrelationships between these aspects are illustrated in Figure 1. Because of the harsh environment created by the SCWO process, the demand on materials is of major importance. Catalysts must be more durable as compared to catalysts used in typical gaseous phase operations, due to water adsorption, sintering, and dissolution of catalyst components. Each of these interrelationships must be understood in order to develop a successful catalytic SCWO process. The purpose of this paper is to evaluate the current status of catalytic SCWO. Specifically, the objectives are to analyze catalytic SCWO processes, including supercritical water (SCW), catalysts, and kinetics. Guidelines for catalyst development and use in SCWO are proposed. Status of Catalytic SCWO As shown in Figure 1, a number of issues associated with catalytic SCWO can be identified. For example, the various interactions involving reaction environment, catalyst, and reactants require attention. To date, several reactants have been studied in conjunction with catalytic SCWO. The compounds have included phenol (Ding et al., 1995a,b,c; Kranjc and Levec, 1994), chlorophenol (Yang and Eckert, 1988), benzene (Ding et al., 1995a), dichlorobenzene (Jin et al., 1990, 1992), and refractory reaction intermediates such as ammonia (Ding et al., 1995d; Webley and Tester, 1991) and acetic
acid (Chang et al., 1993; Frisch, 1992, 1995; Frisch et al., 1994). These compounds are representative of many components found in wastewaters. These studies have focused mainly on the destruction rates of the compounds, reaction pathways, and selectivity to CO2. An important objective of catalytic SCWO studies is to identify the catalysts that are both stable and active in SCW. The selection of many catalysts used in SCWO studies is based on previous WAO research. Both homogeneous and heterogeneous catalysts have been used in WAO of a wide variety of hazardous organic compounds (Mishra et al., 1995). Catalytic WAO has increased reactant conversion and total oxidation efficiency beyond those obtained by the conventional WAO process (the Zimmerman process). As shown in Table 1, when results from all four wet oxidation options (WAO, catalytic WAO, SCWO, catalytic SCWO) are compared, catalytic SCWO can be the most effective hydrothermal oxidation process for the destruction of acetic acid, ammonia, phenol, and other refractory compounds. For example, the destruction efficiency (DE) of acetic acid is greater than 90% in the catalytic WAO and SCWO systems, while the DE is less than 50% in the corresponding noncatalytic processes. Destruction efficiencies of ammonia and phenol show similar trends. Ammonia is normally formed during WAO of nitrogen-containing organic compounds (Ito et al., 1989; Imamura et al., 1985; Mishra et al., 1995). Even under SCWO conditions, the oxidation rate of
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3259 Table 1. Comparison of Catalytic and Noncatalytic Oxidation in Subcritical and Supercritical Water treatment option WAO acetic acid ammonia phenol catalyzed WAO acetic acid ammonia phenol SCWO acetic acid ammonia phenol catalyzed SCWO acetic acid ammonia phenol
concn (ppm)
resid. time (min)
temp (°C)
destruction (%)
reference
5000 1000 1400
60 60 30
248 220-270 250
15 5 98.5
Imamura et al., 1982 Imamura et al., 1985 Pruden and Le, 1976
5000 1000 2000
60 60 60
248 263 200
90 50 94.8
Imamura et al., 1982 Imamura et al., 1985 Imamura et al., 1988
1000 100 480
5 0.1 1
395 680 380
14 10 99
Frisch, 1992 Webley et al., 1993 Thornton, 1991
1000 1000 1000
5 0.1 0.1
395 450 400
97 20-50 99.9
Frisch, 1992 Ding et al., 1995d Ding, 1995
Table 2. Summary of Oxidation Products of Phenol with Oxygen or Ozone products 4-phenoxylphenol 2,2′-biphenol dibenzofuran catechol hydroquinone p-benzoquinone
ozone
Gould & Weber, 1976; Li et al., 1979 Gould & Weber, 1976; Eisenhauer, 1968 Gould & Weber, 1976
o-benzoquinone muconic acid maleic acid
Eisenhauer, 1968; Baillod et al., 1982 Baillod et al., 1982
succinic acid hydracryllic acid fumaric acid propionic acid oxalic acid
Baillod et al., 1982
glyoxal acid glyoxal acetic acid formic acid carbon monoxide
Baillod et al., 1982
oxygen Gopalan & Savage, 1995 Gopalan & Savage, 1995 Gopalan & Savage, 1995 Sadana & Katzer, 1974a,b; Delvin & Harris, 1984; Thornton, 1991; Pintar & Levec, 1994 Sadana & Katzer, 1974a,b; Delvin & Harris, 1984; Thornton, 1991; Pintar & Levec, 1994 Sadana & Katzer, 1974a,b; Walsh et al., 1973; Ahmad et al., 1973; Delvin and Harris, 1984; Thornton, 1991; Pintar & Levec, 1994 Sadana & Katzer, 1974a,b; Walsh & Katzer, 1973; Ahmad et al., 1973 Li et al., 1979 Ahmad et al., 1973; Baillod et al., 1982; Delvin and Harris, 1984; Thornton, 1991; Pintar and Levec, 1994; Kulkarni and Dixit, 1991 Baillod et al., 1982; Delvin and Harris, 1984; Thornton, 1991 Delvin and Harris, 1984; Thornton, 1991 Li et al., 1979; Delvin and Harris, 1984 Delvin and Harris, 1984; Thornton, 1991 Gould and Weber, 1976; Li et al., 1979; Baillod et al., 1982; Delvin and Harris, 1984; Thornton, 1991; Kulkarni and Dixit, 1991 Gould and Weber, 1976; Delvin and Harris, 1984; Thornton, 1991 Gould and Weber, 1976; Delvin and Harris, 1984 Baillod et al., 1982; Delvin and Harris, 1984; Kulkarni and Dixit, 1991; Ding, 1995 Baillod et al., 1982; Delvin and Harris, 1984; Thornton, 1991; Ding, 1995 Ahmad et al., 1970; Delvin and Harris, 1984
ammonia becomes significant only at temperatures above 540 °C (Helling and Tester, 1988). With the participation of a Mn/Ce catalyst, ammonia conversion is increased to 20-50% at a temperature of 263 °C and a 1-h reaction time (Imamura et al., 1985). Under SCWO conditions, ammonia conversion of 30-40% is obtained using packed Inconel 625 beads at a temperature of 680 °C, pressure of 24.6 MPa, and a reactor residence time of 10 s (Webley et al., 1991), and 2040% is obtained with MnO2/CeO2 at a temperature of 450 °C, pressure of 27.6 MPa, and reactor residence time of less than 1 s (Ding et al., 1995d). Under these operating temperatures, the major products from ammonia oxidation are N2 and N2O; the formation of NOx is thermodynamically unfavorable (Golodets, 1983; Killilea et al., 1992), and the presence of excess water readily converts NOx to nitrate or nitrite. The aromatic compounds that have been studied using the catalytic WAO process include phenol (Devlin and Harris, 1984; Levec, 1990; Pintar and Levec, 1992; Sadana and Katzer, 1974a,b), chlorophenol (Ukrainczyk et al., 1993), and chlorobenzene (Yakubovich et al., 1992). The effectiveness of the catalytic SCWO process for destroying aromatic compounds has been reported.
The use of V2O5/Al2O3 and MnO2/CeO2 catalysts not only increase the rate of phenol conversion but also achieve nearly complete conversion of phenol to CO2 (i.e., selectivity ∼1) at 390 °C, 500% excess oxygen, and reactor residence times of less than 10 s (Ding et al., 1995a). High conversion of benzene and high selectivity to CO2 are also obtained on V2O5/Al2O3 and MnO2/CeO2 catalysts. As shown in Table 2, oxidation of aromatic compounds in aqueous environments results in the formation of a variety of partial oxidation products and dimerization products. The high selectivity to CO2, which was obtained in a catalytic SCWO environment, indicates that these partial oxidation and dimerization products are either destroyed or not formed. These data support the potential application of catalytic oxidation in SCW. However, within the SCW environment, the stability of catalyst activity and mechanical structure may be of concern. For example, increased levels of dissolved metal ions from V2O5/Al2O3 and Cr2O3/Al2O3 have been found in effluents following hydrolysis reactions involving inorganic compounds (Ding, 1995; NIST, 1964). Similarly, deactivation of Pt/ ZrO2 and Pt/TiO2 has occurred in a short time (Frisch et al., 1994), partially due to the crystalline growth of
3260 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 3. Properties of Ambient Water, Steam, and Supercritical Water ambient water
fluid temp (°C) pressure (psia)
supercritical water
Typical Conditions 25 450 14.7 4000
Properties and Parameters dielectric constanta 78 1.8 hydrocarbon variableb ∞c solubility (mg/L) oxygen solubility 8e ∞f (mg/L) density, F (g/cm3)g 0.998 0.128 viscosity, µ (cp)g 0.890 0.0298 particle Reynolds 18.5 553 no. Reph effective diffusion 7.74 × 10-6 i 7.67 × 10-4 j coeff., De (cm2/s) Thiele modulus, φ l 2.82 0.0284 a
superheated steam 450 200 1.0 variabled ∞ 0.00419 2.65 × 10-5 622 1.79 × 10-3 k 0.0122
b
Uematsu (1980). Depending on the affinity of its functional group to water. c Connolly (1966). d Depending on the boiling points of HCs. e Metcalf & Eddy (1980). f Pray et al. (1952). g NIST (1980). h Dautzenberg (1989); Rep ) udpF/µ; u ) 10.2 m/s and dp ) 0.386 mm. i De ) DAB2; DAB calculated from Stokes-Einstein equation (Hines and Maddox, 1985). j De ) DAB2; DAB calculated from Fuller et al. (1966). k De ) DAB2; DAB calculated from Green (1984). l Levenspiel (1972); φ ) RA,V (rp/3)2/(De[A]); all parameters constant except De.
platinum particles. Inconsistent activity has been noted in the use of TiO2, which was likely caused by changes in oxidation states and/or pore structure of the catalyst (Frisch, 1995). Softening and swelling of Ni/Al2O3 has been reported in SCW due to the physical strength of the catalyst (Baker et al., 1989; Elliott et al., 1993). Since these phenomena may prevent or limit catalysts used in the SCWO process, it is necessary to understand the interaction between SCW and catalytic materials. Role of Supercritical Water Studies have shown that SCW acts not only as a solvent but also as an important reactant, affecting the rate of organic destruction, catalyst stability, and thereby, the catalytic process. Since water typically occupies 85-99% of total volume of flow in SCWO reactors, its effect on the reaction may be significant. The effect of SCW on the hydrolysis and oxidation of organic compounds as well as on the hydrolysis of inorganic compounds are discussed. Properties of Supercritical Water. Catalytic oxidation in hydrothermal environments can be affected by three types of water properties: (1) thermodynamic properties (P-V-T relationships of water and phase behavior of water-solid mixtures); (2) solution properties (dielectric constant, electrolytic conductance, dissociation constant, and hydrogen bonding); and (3) transport properties (viscosity, heat capacity, diffusion coefficient, and density). All of these properties change drastically near the critical point of water (Tc ) 374.1 °C, Pc ) 22.1 MPa) (Franck, 1973; Uematsu and Franck, 1980). As a result, supercritical water becomes a unique reaction medium (Shaw et al., 1991). Because of these unique properties, the catalytic oxidation mechanism in SCWO may not be correlated directly with the oxidation occurring in liquid phase or gas phase. As shown in Table 3, the properties of SCW are compared to those of ambient water and superheated steam. As indicated by the dielectric constant, a polarity measurement of a solvent, SCW, is a nonpolar
solvent capable of dissolving most organic compounds and gases. This characteristic provides an advantage over traditional WAO processes, where oxidation rates are most likely mass-transfer limited due to low solubility of oxygen and organic compounds in the liquid phase of water. Catalytic reactions are often mass-transfer limited due to high reaction rates, low diffusion rates, and poor fluid flow characteristics. High density and low viscosity of SCW enhance the particle Reynolds number and effective diffusion coefficient. Particle Reynolds number, NRep ) udpF/µ, is the ratio of inertial force to viscous force along the particle. For the correct ratio of bed length to particle diameter (L/dp > 50), a high-particle Reynolds number in a fixed-bed reactor diminishes the chance that an external mass-transfer gradient will exist between the bulk fluid and the surface of catalyst. Dautzenberg (1989) suggested that a particle Reynolds number of 10 would prevent external mass-transfer limitations. Table 3 shows particle Reynolds number values obtained in a 30 g/min fixedbed reactor (Frisch, 1995). The particle Reynolds number criterion was easily met for both SCW and superheated steam scenarios. Similarly, a high effective diffusion coefficient diminishes the chance that a masstransfer gradient exists in the catalyst internal surface area. In this case, the degree of internal or pore diffusion limitation is often represented by the Thiele modulus, φ: much less than unity indicates that porediffusion limitations do not exist in the catalyst. This criterion can be met in SCW and superheated steam. Recently, the effects of SCW on hydrogen bonding (Bennett and Johnston, 1994; Chialvo et al., 1994; Cochran et al., 1992; Gupta and Johnston, 1993; Gupta, 1994; Mizan et al., 1994), acid-base equilibrium constants of organic compounds (Xiang and Johnston, 1994), and inorganic acids (Mesmer et al., 1988) have been studied using UV, fluorescence, and Raman spectroscopy techniques. These in situ experimental techniques along with the molecular dynamics simulation (Balbuena et al., 1994, 1995; Cui and Harris, 1994) and thermodynamic relations help to define the fundamental behavior of organic and inorganic substances in SCW and the properties of SCW. Hydrolysis of Organic Compounds. A number of studies dealing with the hydrolysis of organic compounds in SCW have been reported and are summarized in Table 4. These studies include the hydrolysis of coal model compounds (e.g., dibenzyl ether, benzylphenylamine, dibenzylamine), single-ring aromatic substances, alcohols, heteroatom removal from organic compounds, and special processes such as coal liquefaction. Hydrolysis of many organic compounds exposed to SCW environments reportedly follows a polar or heterolytic nucleophilic substitution mechanism (Boock et al., 1993; Klein, 1992; Penninger and Kolmschate, 1989). The proton can be produced from a dissociation of water
2H2O f H3O+ + OH-
(1)
This proton-catalyzed ionic reaction provides a path parallel to free-radical pyrolysis. Water initiates the reaction and affects reaction selectivity by providing the oxygen and hydrogen required to form alcohol and phenolic compounds. The hydrolysis reaction results in cleavage of the bond between a saturated carbon and a heteroatom-containing a leaving group (Klein et al., 1992; Houser et al., 1993). The selectivity to the hydrolysis pathway increases with an increase in the initial water loading and
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3261 Table 4. Selected Studies of Hydrolysis and Oxidation in Supercritical Water compd category
reference
coal compds aromatic compds alcohols acetamide acetic acid heteroatom-containing org compds coal liquefaction H2 and CO glucose phenol chlorophenol dichlorobenzene pyridine o-cresol acetic acid volatile acids C1-C4 alcohols MEK ammonia H2, CO, and methane wastewaters/sludges glucose
Hydrolysis Abraham and Klein, 1985; Lawson and Klein, 1985; Townsend et al., 1988; Klein, 1992 Penninger and Kolmschate, 1989; Houser et al., 1986, 1989; Li and Houser, 1992 Narayan and Antal, 1990; Narayan et al., 1989; Ramayya et al., 1987; Xu et al., 1990, 1991 Lee and Gloyna, 1992 Meyer et al., 1995 Houser et al., 1993 Amestica, 1986 Helling, 1986; Helling and Tester, 1987; Holgate et al., 1992 Holgate et al., 1995 Oxidation Wightman, 1981; Thornton and Savage, 1990, 1991, 1992; Ding et al., 1995b; Krajnc and Levec, 1994 Yang and Eckert, 1988; Li and Savage, 1993 Jin et al., 1990, 1992; Ding, 1995 Crain et al., 1993; Kanthasamy, 1993 Griffith, 1993; Martino et al., 1995 Wightman, 1981; Lee et al., 1990; Wilmanns, 1990; Frisch, 1992; Kanthasamy, 1993; Boock and Klein, 1993; Rice et al., 1993; Li et al., 1994; Crain, 1994; Meyer et al., 1995; Savage and Smith, 1995 Wilmanns et al., 1989 Dixon and Abraham, 1992; Boock and Klein, 1993; Webley and Tester, 1988 Griffith, 1993 Webley et al., 1991; Helling and Tester, 1988; Killinea et al., 1992 Webley and Tester, 1988, 1991; Webley et al., 1990; Helling, 1986; Holgate et al., 1992, 1993 Li et al., 1993a; Dell’Orco et al., 1993; Shanableh and Gloyna, 1991 Holgate et al., 1995
Table 5. Summary Studies of Catalytic Oxidation in Supercritical Water catalysts
reference
alcohols acetic acid
compd category
CuO/ZnO CuO/ZnO, TiO2, MnO2, KMnO4
ammonia benzene benzoic acid butanol chlorophenol dichlorobenzene 2,4-dichlorophenol MEK phenol 2-propanol pyridine quinoline
Inconel beads, MnO2 V2O5, MnO2, Cr2O3 CuO/ZnO CuO/ZnO Cu2+, Mn2+ V2O5, MnO2, Cr2O3 Pt (supported), TiO2 Pt (supported), TiO2 V2O5, MnO2, Cr2O3, CuO/ZnO CuO/ZnO Pt (supported), TiO2 ZnCl2
Krajnc and Levec, 1994 Frisch et al., 1994; Frisch, 1992, 1994; Krajnc and Levec, 1994; Chang et al., 1993 Webley et al., 1991; Ding et al., 1995d Ding et al., 1995a Krajnc and Levec, 1994 Krajnc and Levec, 1994 Yang and Eckert, 1988 Jin et al., 1990, 1992; Ding, 1995 Frisch et al., 1994 Frisch et al., 1994 Ding et al., 1995a, b; Krajnc and Levec, 1994 Krajnc and Levec, 1994 Frisch et al., 1994 Li and Houser, 1992
dielectric constant, , or through the addition of salts, acids, or bases (Torry et al., 1992). With the addition of acids, the rate of dehydration of alcohols in SCW increases (Xu et al., 1990, 1991). Such results can be interpreted by a proton-promoted reaction mechanism. Similarly, the addition of ZnCl2 increases the removal rate of heteroatoms from aromatic compounds (Houser et al., 1993). Thus, SCW creates a favorable environment for heterolytic hydrolysis. Oxidation of Organic Compounds. Table 4 also summarizes a variety of SCWO studies. The first type involves aromatic compounds such as phenol, chlorinated phenols, dichlorobenzene, pyridine, and o-cresol; the second type deals with aliphatic compounds such as volatile acids, alcohols, methane, and methyl ethyl ketone; the third type includes inorganic compounds such as ammonia, H2, and CO; and the fourth type lists various wastewaters and sludges. Additional data may be found in other review papers (Savage et al., 1995; Subramaniam and McHugh, 1986; Thomason et al., 1990; Tester et al., 1993). As reported by Haber (1992), the oxidation reaction may start by activation of either the dioxygen (electrophilic oxidation) or the hydrocarbon molecule (nucleophilic oxidation). The oxidation reaction in SCW generally follows the free-radical mechanism that dominates
gas-phase oxidation and WAO. The free-radical reaction mechanism often involves an induction period, the generation of a radical pool, and a fast free-radical reaction period. This phenomenon has been observed in WAO of organic compounds (Mishra et al., 1995) and SCWO of acetic acid (Meyer et al., 1995). The induction time and free-radical concentration depend on the oxidizing agent, temperature, catalyst, and reactant. Solvent or “cage” effects may play an important role in the oxidation reaction kinetics and mechanism (Abraham et al., 1985). Antal et al. (1987) explains that water density in a SCW reaction is one of the major facts for the determination of reaction mechanism. At higher water density, associated with a relatively lower temperature, such a reaction may occur through an ionic mechanism. Both homolytic (free-radical) and heterolytic (ionic) reaction mechanisms have been proposed for the oxidation of aromatic compounds resulting in a ring-opening reaction (Yang and Eckert, 1988; Ding, 1995). When following the homolytic reaction pathway, the hydroxyl radical (•OH) is first produced from the initiation reaction of water:
H2O f H• + •OH
(2)
3262 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
The hydroxyl radical concentration may be relatively high if the balancing hydrogen radicals are consumed by oxygen to form peroxy radicals (Williams, 1960):
H• + O2 f •HO2
(3)
Phenol, catechol, hydroquinone, benzoquinone, and dimers are obtained through the following free-radical reactions involving the initiation or the propagation of hydroxyl radicals:
hydrogen abstraction: peroxy formation:
φ• + O2 f φ-OO•
homolytic hydroxylation: condensation:
φ-H + •OH f φ• + H2O (4)
φ• + •OH f φ-OH
φ• + •φ f φ-φ
(5) (6) (7)
The heterolytic reaction mechanism for SCW and oxygen results in the dissociation and the formation of two ionic species (Narayan et al., 1989):
H2O + O2 f OH- + HO2+
(8)
Heterolytic hydroxylation involves an electrophilic substitution step. The heterolytic reaction pathway can be enhanced by the addition of salt, acid, and base. Thus, increasing the water or salt concentrations enhances the heterolytic reaction. Therefore, the coupling mechanism, where aromatic radicals (φ•) react with each other to form a condensation product (φ-φ), can be prohibited. With the substitution of a hydroxyl group, π-electrons become delocalized and the benzene ring is destabilized. Further substitution and oxygen attacks result in the complete destabilization of the benzene ring and lead to a ring-opening reaction. The final result is the formation of relatively stable low molecular weight products (acids, alcohols, carbon monoxide, and carbon dioxide). These reaction mechanisms are aided through the identification of reaction intermediates and products. For example, incomplete oxidation of aromatic compounds (such as phenol, benzene, dichlorobenzene, o-chlorophenol, p-chlorophenol, and pyridine) result in similar intermediate products. In a series of studies involving the SCWO of phenol and chlorinated phenols, Savage and co-workers (Gopalan and Savage, 1995; Li et al., 1993; Thornton and Savage, 1990, 1992) have identified ring-opening reaction products that result from the substitution of a hydroxyl group leading to catechol and hydroquinone as well as dimerization products, which can be interpreted by a coupling mechanism. Both ring-opening and coupling mechanisms are well known in the traditional oxidation chemistry of aromatic compounds (Musso, 1967; Sainsbury, 1992; Williams, 1960). The products of phenol oxidation in the aqueous phase, as shown in Table 2, indicate that these two reaction pathways can occur simultaneously in SCWO. The domain of these reaction pathways largely depends on the reaction temperature, the concentration ratio of oxygen to aromatic compounds, the ionic strength (affected by water density, addition of salts, acids, bases, and catalysts), and the substituted group on aromatic compounds (Ding, 1995). In catalytic SCWO, oxygen may participate in a reaction as an adsorbed species on the catalyst surface
Figure 2. Oxidation pathway of aromatic compounds on V2O5 in supercritical water (Ding, 1995).
or as part of the lattice oxygen present in the metal oxides. Adsorbed oxygen may come from oxides of Cr, Mn, Fe, Co, Ni, and Cu, and the lattice oxygen may come from vanadium oxide (Fierro and Garcia de la Banda, 1986; Novakova, 1971). Thus, the presence of catalysts creates an ionic environment that enhances heterolytic reactions. The mechanism involved in catalytic SCWO may be interpreted similarly to those describing gas-phase or liquid-phase oxidation. However, special factors, such as high concentrations of water and large differences in physicochemical properties between ambient water and SCW, can influence catalytic oxidation pathways, product distributions, and catalyst stability (Tiltscher and Hofman, 1987; Wu et al., 1991). For example, when copper(II) tetrafluoroborate and manganese(II) chloride salts are used as catalysts, modest increases can be observed in the rate of chlorophenol oxidation (Yang and Eckert, 1988). The product distributions from catalytic reactions are similar to those obtained from noncatalytic SCWO, suggesting that both systems follow a similar reaction mechanism. Similarly, catalytic oxidation of 1,4-dichlorobenzene on V2O5/Al2O3 yields products derived from both catalytic gas-phase oxidation and dechlorination reactions with water (Jin, 1991; Jin et al., 1990, 1992). A high selectivity to CO2 is apparent in the catalytic SCWO of phenol, benzene, and 1,3-dichlorobenzene on V2O5/Al2O3 and MnO2/CeO2 (Ding, 1995; Ding et al., 1995a,b). As shown in Figures 2 and 3, the substitution of the hydroxyl groups, according to the ring-opening
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3263
Figure 3. Oxidation pathway of aromatic compounds on MnO2 in supercritical water (Ding, 1995).
concept, is respectively enhanced by the participation of the lattice oxygen in V2O5/Al2O3 and the adsorbed oxygen on MnO2/CeO2 (Ding, 1995). Catalytic oxidation pathways, product distributions, and catalyst stability for aromatic compounds exposed to SCWO environments can be explained through the proposed reaction mechanisms. For example, the lattice oxygen in V2O5 is highly mobile, and aromatic compounds adsorbed on the catalyst may be involved in hydrogen abstractions, formation of OH surface species, incorporation of OH species into the aromatic molecule, and ring-opening reaction. Thus, low molecular weight alcohols, acids, and CO2 are formed. The alcohols and acids can be continuously adsorbed on the active sites until CO2 is formed. Experimental results have demonstrated the participation of V2O5. In these tests, the reactant conversion is increased, a lesser number of partial oxidation products remains, and more CO2 is obtained. However, water is normally adsorbed on catalyst as a hydroxyl group (Baldi et al., 1974; Jahan and Kung, 1994; Takita, 1986; Topsoe et al., 1992; Witko et al., 1993). The high mobility and vacancy of lattice oxygen in V2O5 are susceptible to interaction with the hydroxyl group adsorbed on the catalyst. Such interaction results in the formation of vanadium hydrate and, consequently, a loss of catalyst activity. MnO2 typifies the case where the adsorbed oxygen participates in the reaction and the poorly mobile lattice oxygen remains intact. The stability of MnO2 is maintained because it resists attack by water, oxygen, and
Figure 4. Reaction mechanism of supercritical water oxidation of acetic acid on anatase TiO2.
chloride ions. In this case, the catalyst surface must be regenerated (desorb intermediate and readsorb oxygen) before further reaction can proceed. Thus, catalytic oxidation on MnO2 will involve several adsorption and desorption processes before complete oxidation products are formed. In some cases, partial oxidation products remain in the effluent because of short residence times. The reported experimental results are consistent with the proposed reaction mechanism, as shown in Figure 3. Oxidation of aliphatic compounds in SCW follows both gas-phase and liquid-phase oxidation mechanisms (Boock and Klein, 1993b). This reaction behavior may be described in terms of reaction kinetics developed from a free-radical mechanism. Although there is no experimental evidence to demonstrate the participation of water in the free-radical mechanism, it appears plausible that a relatively large quantity of water in the reaction system might involve free-radical initiation, propagation, chain transfer, and termination. For example, the effect of water density or solvent properties
3264 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 6. Classification of Deactivation Phenomena Due to Solid State Chemical Reactiona phenomena no change of overall catalyst composition change of catalyst composition complex processes a
reasons phase transition, segregation of phase, reaction between solid phases reaction with gas or liquid phase: reduction, oxidation, carbide formation, loss of catalyst component, formation of compound with deposited impurities simultaneous occurrence of solid state chemical reactions with textural change, simultaneous occurrence of two or several solid-state reactions
Source: Delmon and Grange (1980).
Table 7. Summary of Catalysts Used in WAO Studies catalyst
reactant
reference
Pd/TiO2-ZrO2, Ru/TiO2, Pt, Ru
wastewater
Pt/TiO2-ZrO2, Ru/Ce Pt, Ru, Pt/Al2O3, Ru/Ce
alcohols phenol
Noble Metals Mitsui et al., 1989; Yamada et al., 1990; Chowdhury et al., 1975; Oguchi et al., 1992; Imamura et al., 1988 Ishii et al., 1991; Imamura et al., 1988 Oguchi et al., 1992; Imamura et al., 1988; Higashi et al., 1991
Zn/Al, Co Cu/Zn, Cu/Ni, Co/Bi, Ni/Bi, Cu/Mn, Zn/Bi, Sn/Bi, Cu/Al Mn/Ce, Cu2+, Co/Bi, Co/Bi Mn/Ce, Co/Bi, Cu2+
alcohols acetic acid
Metal Oxides Levec et al., 1976; Imamura et al., 1982 Imamura et al., 1982
Cr, Fe, V, Mn, Co, Ce metal salts
phenol PEG amines ammonia wastewater
Katzer et al., 1976; Sadana and Katzer, 1974a, b; Ito et al., 1989 Imamura et al., 1986, 1988 Imamura et al., 1986 Imamura et al., 1986 Chowdhury et al., 1975; Brett and Gurnham, 1973
on a reaction appears to be an example of such a freeradical involvement. Catalytic SCWO of acetic acid has been used to test catalyst activity and oxidation kinetics (Chang et al., 1993; Frisch, 1992, 1995; Frisch et al., 1994). The reaction mechanism of acetic acid under SCWO and WAO conditions are similar. Acetic acid initially adsorbs onto an active site, reacts with adsorbed oxygen, and goes through several intermediate steps. The later involves the formation of formaldehyde, carbon monoxide, formic acid, and eventually carbon dioxide plus water (Imamura et al., 1982; Levec et al., 1976). As an example, TiO2 (anatase) catalyst can be used effectively to destroy acetic acid (Frisch, 1995). Kinetic results indicate that the acetic acid and oxygen reaction orders were not statistically different from 1 and 0, respectively. As illustrated in Figure 4, the LangmuirRideal mechanism and byproduct analyses may be used to postulate an overall reaction sequence. The Langmuir-Rideal mechanism results in first-order kinetics if oxygen is strongly adsorbed (as compared to weakly adsorbed acetic acid and water) to the active site and if acetic acid molecules collide and react with these oxygen-catalyst complexes. Carbon monoxide and small quantities of formic acid, methanol, methane, and hydrogen are detected in the effluents. According to the mechanism, oxygen molecules chemisorb onto the anatase lattice to form two titanium oxide surface intermediates. To initiate the oxidation reaction, acetic acid molecules collide into a titanium oxide surface intermediate and form a second surface intermediate. The surface intermediate desorbs to form methanol, formaldehyde, methane, hydrogen carbon monoxide, carbon dioxide, oxygen, and water. No formaldehyde is detected in the effluent; thus, the mechanism postulated that it undergoes a second surface reaction to form formic acid. A third surface reaction oxidizes formic acid to carbon dioxide and water. Experimental results are consistent with the proposed Langmuir-Rideal mechanism. However, due to the relatively narrow pressure range used in this study, other reaction mechanisms cannot be excluded. Stability of Inorganic Solids. High-temperature
and high-pressure water may promote many reactions. In addition to the hydrolysis and oxidation participation, SCW can hydrolyze metal oxides and metal salts, promote the growth of crystal and phase transformation, reduce solid defect, and accelerate solid uniformity. These effects of SCW have been utilized in studies of mineral formation (Klingsberg and Roy, 1960), thin-film processing (Matson and Smith, 1989), and crystal growth (Hirano, 1988; Laudise, 1987; Nishikawa and Wakao, 1992; Pommier et al., 1990; Rabenau, 1985). Heterogeneous oxidation catalysts are usually made of metal oxides or metals dispersed on metal oxide supports. An active catalyst normally has a high surface area and sometimes may be in a metastable state. Therefore, many potential catalysts exposed to SCWO are highly susceptible to surface area loss. Furthermore, the catalyst oxidation state and crystal size, which affect catalyst activity, may be similarly altered. For gas-phase oxidation, the deactivation of a catalyst is mainly caused by coking, poisoning, and the solidstate transformations of catalysts (Froment and Bischoff, 1990; Rase, 1990). Coking is the result of carbon deposition on active sites. Poisoning is the physical or chemical adsorption of impurities on active sites. The solid-state transformations include phase transition (e.g., γ-Al2O3 to R-Al2O3), solid solution formation (e.g., spinel from Cr2O3/Al2O3), sintering of metals coated on a support, and migration of active components. One advantage of catalytic reaction in SCW is the prevention of coke formation on catalyst. As compared to gas-phase oxidation, catalytic reactions in supercritical fluids often form much less coke on the catalyst surface (Adschiri et al., 1991; Baptist-Nguyen and Subramaniam, 1992; Ding, 1995; Dooley and Knopf, 1987; Fan et al., 1991, 1992; Frisch, 1995; Gabbito et al., 1988; Ginosar and Subramaniam, 1994; Knopf et al., 1987; Monos and Hofmann, 1991; Saim and Subramaniam, 1990, 1991; Tiltscher et al., 1981, 1984; Tiltscher and Hofmann, 1987; Yokota and Fujimoto, 1989, 1991; Yokota et al., 1991). The coke precursor, if it is formed on the catalyst surfaces, can be carried out by SCW because of the high miscibility of organic
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3265 Table 8. Physical and Chemical Properties of Catalytic Materials in Supercritical Water catalysts R-Al2O3, HfO2, ZrO2 (Yttria stabilized) MgO/Al2O3 NiO Pt/TiO2, Pt/Al2O3, Pt/HfO2, Pt/ZrO2 MnO2 MnO2 TiO2 V2O5 CuO/ZnO Cr2O3
test methods
experimental results and comments
batch
physically and chemically stable in SCW; these oxides can be used in SCWO as catalyst supports batch unstable, Mg(OH)2 forms at SCW condition batch unstable, dissolution occurs batch chemically active as catalysts for the oxidation of organic compounds, but fast deactivation occurs due to the aggregation of platinum particles batch active as catalyst, but oxide status changes at low oxygen concn (phase transfer to Mn2O3 as a function of oxygen) continuous flow physically stable and chemically active as catalyst at high oxygen partial pressure batch, continuous flow physically stable and chemically active in SCW batch, continuous flow physically unstable due to the hydrolysis of V2O5; chemically active as catalyst in oxidation of org compds continuous flow trace level of Cu found in effluent due to the low solubility of copper at SCW; chemically active as catalyst in oxidation of organic compounds batch, continuous flow unstable due to the CrOOH formation
reference Frisch et al., 1994 Frisch et al., 1994 Frisch et al., 1994 Frisch et al., 1994 Frisch, 1992 Ding, 1995 Frisch, 1995 Ding, 1995 Kranjc and Levec, 1994 Ding, 1995
Table 9. Melting Points of Selected Metal Oxidesa
a
oxide form
melting point (°C)
Ag2O R-Al2O3 B2O3 BaO BeO Bi2O3 CaO CdO CeO2 Co2O3 Cr2O3 CuO FeO Fe2O3 GeO2 HfO2 HgO In2O3 IrO2 K2O La2O3 Li2O MgO MnO2 Mn2O3
230 (decompd) 2015 450 1918 2530 825 2614 1500 2600 895 (decompd) 2266 1326 1369 1565 1115 2758 500 (decompd) 850 (vol) 1100 (decompd) 350 (decompd) 2307 1700 2852 535 (decompd) 1080 (decompd)
stable form (400-650 °C)
CeO2 Co3O4-CoO Cr2O3 (>500 °C) Cu2O-CuO FeO (O2 30%) GeO2 monoclinic
K2O + liquid
MnO2-Mn2O3 Mn2O3
oxide form
melting point (°C)
stable form (400-650 °C)
Mn3O4 MoO3 Na2O Nb2O5 NiO P 2 O5 PbO PtO2 ReO2 Rh2O3 Sb2O3 SeO2 SiO2 SnO2 SrO TeO2 ThO2 TiO2 Tl2O3 UO2 V 2 O5 WO3 Y2O3 ZnO ZrO2
1564 795 1275 (subl) 1485 1984 580 886 450 1000 (decompd) 1100 (decompd) 656 340 1723 1630 2430 395 (decompd) 3220 1830 717 2878 690 1473 2410 1975 2700
Mn2O3 MoO3 (O2 > 75%) Nb2O5 (O2 >30%) NiO PbO-Pb2O3 ReO2 Rh2O3 Sb2O3 SiO2 SnO-SnO2 ThO2 TiO2 (anatase) UO2-U4O9 V2O5 WO2-WO3 ZnO ZrO2
Sources: Weast and Astle (1981); NIST (1964-1990).
compounds with SCW. As compared to that of liquid and gas, such removal of a coke precursor from catalyst surfaces occurs because of the relatively high mobility (low viscosity and high diffusion coefficient) and high carrying capacity (high density) of SCW. Generally, it appears that solid-state transformations of a catalyst are primarily responsible for catalyst deactivation in laboratory studies. In these studies, high-purity reactants and oxidants are used to avoid catalyst poisoning by unwanted impurities. As compared to typical gas-phase oxidation conditions, the extent of various solid-state transformations in SCW is higher (Adschiri et al., 1992a,b; Rase, 1990). Such is the case for aggregation, phase transition, formation of a solid solution, and dissolution of solid components. Table 6 summarizes the classification of catalyst deactivation phenomena due to solid-state reactions (Delmon and Grange, 1980). The extent of these reactions in SCW depends on the catalyst components, manufacturing process, and reaction conditions. Thus, SCW can enhance the reaction rate through homolytic and heterolytic mechanisms, eliminate possible coke formation, and promote various solid-state
transformations. To fully utilize these unique properties of SCW for the enhancement of the catalytic oxidation process, it is necessary to understand catalyst activity and stability as well as the impact of catalyst preparation methodology. Catalyst and Its Preparation for SCWO Applications A useful catalyst is normally characterized by a balance of its activity and stability. In many instances, substances of high catalytic activity have been discovered only to be discarded because their activity could neither be maintained nor regenerated effectively (Smith, 1981). In SCW, maintaining catalyst activity is even more critical because interactions between the catalyst and water may be extensive and irreversible. Catalyst Activity. Transition metal oxides and noble metals are extensively used as active components in catalytic oxidation (Satterfield, 1991; Spivey, 1987). A significant body of knowledge exists describing metal oxides and their physicochemical properties such as electrical conductivity (Bielanski and Haber, 1991;
3266 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 5. Phase diagram of Cr2O3-H2O. Reprinted with permission from Levin et al. (1964). Copyright 1964 The American Ceramic Society.
Figure 8. Phase diagram of In2O3-H2O. Reprinted with permission from Levin et al. (1964). Copyright 1964 The American Ceramic Society.
Figure 6. Phase diagram of La2O3-H2O. Reprinted with permission from Levin et al. (1964). Copyright 1964 The American Ceramic Society.
Figure 9. Phase diagram of Y2O3-H2O. Reprinted with permission from Levin et al. (1964). Copyright 1964 The American Ceramic Society.
Figure 7. Phase diagram of MgO-H2O. Reprinted with permission from Levin et al. (1975). Copyright 1975 The American Ceramic Society.
Fierro and Garcia de la Banda, 1986; Spivey, 1987), redox potential (Muzykantov, 1987; Sokolovskii, 1981a,b, 1987), acid-base characteristics (Bond, 1974; Seiyama, 1982; Tanaka et al., 1967), heat of formation (Bielanski and Haber, 1991; Morooka et al., 1967; Roiter and
Golodetz, 1968; Vijh and Lenfant, 1971), oxygen adsorption (Fierro and Garcia de la Banda, 1986; Iwamoto et al., 1978; Trifiro and Pasqon, 1968), and surface structure (Marshneva and Boreskov, 1974; Novakova, 1971; Pankratiev, 1982; Winter, 1969). Correlation of catalytic activity with physicochemical properties provides preliminary information for the selection of catalysts and suggests possible catalytic performance for a specific reaction. Oxides of V, Cr, Mn, Fe, Co, Ni, and Cu for a variety of gaseous oxidation processes are the most active single metal oxide catalysts (Spivey, 1987). These metal oxides as well as some noble metals have been used previously as catalysts in WAO treatment operations. Water-insoluble (heterogeneous) catalysts have been preferred in WAO to minimize unwanted contamination of liquid effluents. Table 7 lists a summary of WAO catalysts. Although some of these WAO catalysts may have commercial value (Chornet et al., 1988; Harada et al., 1987; Lichtin et al., 1989), little information is available regarding durability.
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3267 Table 10. Hydrothermal Stability of Metal Oxides compd Al2O3 BeO CaO Cr2O3 Fe2O3 Fe3O4 Ga2O3 Gd2O3 GeO2 HfO2 In2O3 K2O, Ta(Nb)O2 La2O3 MgO MnO MnO2 Nb2O5 NiO PbO Sc2O3 SiO2 Sm Sn Ta2O5 Tb (Eu, Ho, Tm, Yb) TiO2 UO2 V2O5 Y ZnO ZrO2 a
stable form in water at 27.6 MPa (°C) corundum (>400) BeO CaO + Ca(OH)2 CrOOH (92)
1.8 (500, 103.4) 120 (500, 103.4)
500 (10 M NaOH)
550 (103.4)
90 (500, 103.4) β-Ga2O3 (>300) GdOOH GeO2 (rutile)
8700 (500, 103.4)
