Production of Hydrogen Peroxide in Liquid CO2. 2. Catalytic

Pd-catalyzed hydrogenations of fluoroether-functionalized anthraquinones (FAQs) were conducted in liquid CO2 (P = 235 bar) at room temperature. The ki...
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Ind. Eng. Chem. Res. 1999, 38, 2833-2841

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Production of Hydrogen Peroxide in Liquid CO2. 2. Catalytic Hydrogenation of CO2-philic Anthraquinones Dan Haˆ ncu and Eric J. Beckman* Chemical Engineering Department, University of Pittsburgh, 1249 Benedum Hall, Pittsburgh, Pennsylvania, 15261

Pd-catalyzed hydrogenations of fluoroether-functionalized anthraquinones (FAQs) were conducted in liquid CO2 (P ) 235 bar) at room temperature. The kinetics of the hydrogenation of the FAQs in liquid CO2 was investigated in a high-pressure batch reactor under a 10-fold excess of hydrogen, while varying the catalyst loading and catalyst particle size. The pressures employed were such that H2, CO2, and FAQ formed a single phase. The 1H NMR analysis of the FAQs after a hydrogenation-oxidation cycle showed no indication for “deep” hydrogenation or degradation of the linker. True kinetic constants, diffusion coefficients, and effective diffusivities were determined by simultaneous regression of the kinetic data. The diffusion coefficients of the FAQs decreased as the length of the fluoroether tail increased. Small catalyst particles and high stirring rates totally eliminated the external transport limitations. The nature of the linking group and the spacer (between the tail and the anthraquinone block) affected the reactivity of FAQs in the hydrogenation process. Using FAQs with relatively short fluoroether tails, we could readily achieve conditions where hydrogenation in CO2 was kinetically controlled. Introduction Carbon dioxide has emerged as a potentially environmentally benign substitute for volatile organic chemicals (VOC) in many applications. CO2 is environmentally safe, exhibits low density, low viscosity, low critical temperature (31 °C), and moderate critical pressure (72.9 atm), and is readily available in large quantities. Reactions in sub- or supercritical fluids, especially in carbon dioxide have received increasing interest in the past decade. Subramaniam and McHugh,1 Tiltscher and Hofmann,2 and Savage et al.3 have reviewed early work on reactions in supercritical fluids (SCFs). In a special issue of Chem. Rev. dedicated to supercritical fluids, Baiker, Jessop et al., and Savage synthesized the most recent developments in the heterogeneous and homogeneous catalysis in SCFs.4 For processes involving gaseous reactants, such as hydrogenation5,6 or oxidation of organics, the enhanced solubility of gases in CO2 at elevated pressures provides the opportunity to overcome interphase transport limitations. Low values of density and viscosity of sub- or supercritical fluids increase the diffusivity of the reactants inside the catalyst pores, which produces potentially higher effectiveness factors and thus higher throughputs. The relatively higher solubility of coke precursors in SCFs as compared to those in the gas phase offers potentially better catalyst maintenance in processes such as Fischer-Tropsch synthesis7 or olefin isomerization.8 Hydrogen and CO2 are completely miscible over a wide range of temperature and pressure.9 In hydrogenation processes conducted in organic solvents at ambient pressure, the reaction can be limited by the low solubility of hydrogen in the working solution (see, for example, the review by Valderrama et al.10) and hence by the mass transfer of hydrogen through the gas* To whom correspondence should be addressed. Telephone: (412) 624-9630. Fax: (412) 624-9639. E-mail: [email protected].

Scheme 1. Conventional Anthraquinone Process to Produce H2O2

liquid (g-l) interface.11 Even though hydrogenations conducted in CO2 can benefit from both low masstransfer resistance and high solubility of hydrogen in the reaction mixture, there are only a few literature examples of homogeneous12 or heterogeneous3,4a hydrogenations carried out in CO2. Kinetic data on heterogeneous hydrogenation of R-unsaturated ketones in CO2 were recently reported by Bertucco et al.5 A kinetic model correlated with vapor-liquid equilibrium data was developed to study the influence of pressure, temperature, and concentration on the reaction rate of hydrogenation. H2O2 Synthesis in Liquid Carbon Dioxide. Hydrogenation of anthraquinone represents the key step in the anthraquinone (AQ) process for generating hydrogen peroxide (Scheme 1).13,14 Because the anthraquinone process for generation of hydrogen peroxide is considered to be mature, relatively few scientific papers have been devoted to the kinetics of the hydrogenation of AQ to form anthrahydroquinone (AQH2), with the available literature consisting mostly of patents.15 Berglin and Scho¨o¨n,16 for example, studied the kinetics of the hydrogenation of 2-ethyl-9,10-anthraquinone (eAQ)/2-ethyltetrahydro-9,10-anthraquinone (H4eAQ)

10.1021/ie9807396 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/06/1999

2834 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

in a slurry reactor in the presence of a Raney nickel catalyst. In agreement with the previous work of Kirdin et al.,17 they found that the rate of hydrogenation was limited by the transport of hydrogen and that the reaction was first order with respect to hydrogen and zero order with respect to eAQ. In a series of papers, Drelinkiewicz18 studied the Pd-catalyzed hydrogenation of eAQ in a batch reactor under an atmospheric pressure of hydrogen. Under these conditions, the reaction order with respect to eAQ was found to vary during the three stages distinguished in the process: stage 1, no reaction order determined; stage 2, zero order; stage 3, fractional order (0.5). It was also shown that the reaction was conducted under a “mixed diffusion-kinetic regime” in which the gas-liquid (g-l) and/or liquid-solid (l-s) mass transport resistances of hydrogen played a significant role. In a separate study of the Pd-catalyzed hydrogenation of H4eAQ in a slurry semibatch or continuous reactor, Santacesaria et al.19,20 found that the process could not be conducted under kinetic control because of fast reaction and slow mass transport of hydrogen through the three-phase system. However, the reaction was found to be zero order with respect to hydrogen and first order with respect to H4eAQ, contradicting the previous findings of Drelinkiewicz. Production of hydrogen peroxide represents a potentially efficient process for application of liquid or supercritical CO2. As discussed in our companion paper,21 there are significant advantages to replacing a typical organic solvent with CO2 in this application. Also, there are significant differences between the use of liquid or supercritical CO2 as a working solvent. On the basis of the lower values of viscosity of supercritical CO2 (as compared to liquid), one could achieve higher rates of diffusion. As one moves from liquid to supercritical CO2, the ability to solubilize functionalized anthraquinones diminishes significantly because of a lower density of supercritical CO2 as compared to that of liquid. Also, as one increases the temperature, the “true” kinetic constant will rise, but this is only important if one can retain kinetic control over the reaction. Thus, it is not clear whether the advantages of working in the supercritical regime outweigh the disadvantages, but it is certainly an issue to consider if the process has to be optimized. If one understands the dependence of the rate constant on temperature (simple Arrhenius description), the dependence of the cloud-point curves on temperature and fluid density, and the dependence of the diffusivity on fluid density, then this optimization can be performed. The study of the phase behavior of functionalized anthraquinones in CO2 has revealed that fluoroetherfunctionalized anthraquinones (FAQ; Figure 1), synthesized by attaching CO2-philic tails to anthraquinone blocks via either ester or amide linkers, exhibits liquidliquid-phase behavior in CO2 with a minimum miscibility pressure between 170 and 210 bar.21 Our work suggested that small to medium CO2-philic tails attached through non-H-bonding spacers to the AQ block in a nonsymmetrical configuration would lower the minimum miscibility pressure in CO2. In the present work, we have studied the kinetics of Pd-catalyzed hydrogenation of FAQs in liquid CO2 at 25 °C and P ) 235 bar. Kinetic data in liquid CO2 are interpreted considering the influence of both masstransfer and chemical reactions. A conventional mathematical model is adopted to determine true kinetic

Figure 1. General formula of functionalized anthraquinone (FAQ) (the general formula for the amide FAQ is x-(Kr-CONH)AQ and for the ester FAQ is x-(Kr-COO-CH2)AQ, where Kr is a poly(perfluoropropylene oxide) polymer).

Figure 2. Experimental setup.

constants (kc), diffusion coefficients of FAQs in bulk CO2 (De), and effective diffusivities (Deff) of FAQs inside the pores of the catalyst. The calculated values of kc and Deff allow us to evaluate the effect of structural parameters such as the length of the CO2-philic tail and the nature of the spacer/linker on the performance of FAQ in the hydrogenation process. Experimental Section Materials. All functionalized anthraquinones were synthesized according to the procedures outlined previously.21 Structural characterization of FAQs was performed by 1H NMR on a Bruker DMX 300 spectrometer and by IR on a Mattson Genesis II FTIR spectrometer. Palladium, 1 wt % on alumina powder (Aldrich), and supercritical grade CO2 (Praxair) were used as received. Pd catalysts with larger particle sizes were prepared by compressing the powdered catalyst into pellets which were sieved into three fractions: 20 < dp < 40 mesh, 40 < dp < 60 mesh, 60 < dp < 80 mesh. Hydrogenation of Functionalized Anthraquinones in Liquid CO2. Hydrogenation experiments in liquid CO2 were performed using the apparatus shown in Figure 2. The experimental setup consists of two independent sections. A H2-CO2 mixture was prepared in section A, which consists of a syringe pump (High Pressure Equipment) and a sample injection valve (Rheodyne) connected to both a vacuum and a venting line. The H2-CO2 mixture was prepared in the syringe pump, and the required amounts of H2 were injected to the reactor through the sample injection valve. The amount of H2 transferred to the syringe pump was calculated using a virial equation of state22 at the

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2835

pressure indicated by the regulator mounted on the H2 tank. The hydrogenation process was carried in section B, which consists of a 35 cm3 stainless steel highpressure cell with two sapphire windows (produced at University of Pittsburgh) and equipped with a mechanical stirrer (modified Parr stirring unit). An internal filter at the output recirculating port was mounted on the wall of the reactor vessel to prevent entrainment of the catalyst particles in the system. In a typical run, the reactor was loaded with known amounts of FAQ and Pd/Al2O3 catalyst. Subsequently, both sections of the system were evacuated for 15 min to remove any traces of oxygen. After the syringe pump in section A was filled with H2, both sections A and B were filled with CO2 and pressurized to a pressure 6065 bar below the target reaction pressure using a Haskell gas booster and an Eldex piston pump. The magnetic stirrer was started in the reactor, and the mixture was stirred for 10 min to allow enough time for FAQ to dissolve. Meanwhile, the UV absorbance of pure CO2 was measured to be used as a reference for the UV measurements of FAQ. After equilibration, the high-pressure gear pump (Micropump) was turned on and valve T4 (left) was opened to start the circulation of fluid through the UV spectrometer. After stabilization of the UV spectrometer, the initial spectrum of FAQ was recorded in a range between 290 and 370 nm. Subsequently, the CO2-H2 mixture was injected, and the sample injection valve was washed with CO2 until the target pressure (P ) 235 bar) was reached. The hydrogenation reaction was followed in time by the disappearance of the peak at 320 nm (functionalized esters) or 330 nm (functionalized amides). After the reaction was completed, the high-pressure reactor was slowly depressurized, and then FAQH2 was exposed to air to produce H2O2 and regenerate the initial FAQ. Residual FAQ in the vessel was washed with 1,1,2-trichlorotrifluoroethane, and the solution was analyzed by 1H NMR. Results and Discussion General Procedure for Evaluation of MassTransfer and Kinetic Parameters. Previous studies of the efficiency of CO2-philic chelating agents in metal extractions showed that the direct attachment of the perfluoroether tail to the chelating head decreased the effectiveness of metal binding because of the electronwithdrawing effect of the tail.23 The kinetic studies of hydrogenation in liquid CO2 were designed to evaluate the influence of the mode of attachment of the CO2philic tail upon the reactivity of FAQ during the catalytic hydrogenation. In a previous study of hydrogenation in CO2,5 the rate of reaction was followed by on-line GC analysis after depressurization of the reaction mixture. The main disadvantage to this approach is that the compositions in the CO2 phase in the reactor have to be predicted from the data using vapor-liquid equilibrium values that are in many cases not straightforward to calculate. In the case of hydrogenation of FAQ, the instability of FAQ:H2 in air makes it even more difficult to follow the rate by this method. Therefore, we chose to monitor the rate of hydrogenation by UV spectroscopy by following the disappearance of the FAQ peak. The same method was used previously to monitor the photoreduction of AQ derivatives.24,25 In this way, we can measure the real concentration of FAQ in liquid CO2 vs time without the need to calculate phase equilibrium data and without exposing the reaction system to air.

For a given FAQ, a series of experiments were run while varying particle size and catalyst loading. The procedure allowed us to identify the controlling regimes in the process and determine the true kinetic constant (kc), diffusion coefficient (De) of FAQ in bulk CO2, and effective diffusivity (Deff) of the reactant inside the pores of the catalyst. Kinetic data were analyzed assuming the following: (1) The reaction is zero order with respect to hydrogen, because of the large excess of hydrogen (in a typical run, FAQ:H2 (molar ratio) ) 1:10). (2) The reaction is first order with respect to FAQ based on a good fit of the rate data (cFAQ ) f(t)) to an exponential function and Santecesaria’s previous work.20 (3) The only transport resistances considered in the kinetic expression are due to liquid-solid mass transfer and intraparticle diffusion, given that there is no gasliquid interface in our system. In hydrogenations conducted in organic solvents, the low solubility of hydrogen and small difference in diffusion coefficients between hydrogen and the organic substrate create significant concentration gradients in the system. Therefore, in previous kinetic studies hydrogen was considered the limiting reagent and the mass-transfer and kinetic expressions were written as a function of hydrogen concentration and hydrogen diffusivity. In these cases, the test introduced by Ramachardran and Chaudhari26 to ascertain whether the concentration of FAQ is uniform throughout the system

DeFAQcFAQ >10 DeH2cH2

(1)

is satisfied for high concentrations of organic reactant. In liquid or supercritical CO2, the situation is reversed. The high solubility of H2 in CO2 makes it easily possible to conduct the reaction under an excess of hydrogen, and the high molecular perfluoroether tails lower FAQ’s diffusion coefficient compared to eAQ. Under these conditions, the aforementioned criterion (eq 1) suggests that the hydrogen concentration is uniform. Thus, the following apply: (4) The concentration of H2 throughout the system is uniform. (5) The limiting reactant within the catalyst pores is FAQ. Under these assumptions, the rate of the reaction (R) is given by

R ) keffwcFAQ

(2a)

or -

cFAQ ) cFAQoe kgt

(2b)

The global rate constant (kg) can be written in terms of the liquid-solid mass-transfer coefficient (ks), the true kinetic rate constant (kc), the catalytic effectiveness factor (η), and the external surface area of the catalyst particle (ap):26

1 1 1 ) + kg ksap wηkc

(3)

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Consider the expression for the external area (ap) for a spherical particle26

ap )

6w Fpdp

(4)

and the expression for ks written for the case of the mass transport to particles in agitated tanks27

( )

edp4FL3 ksdp ) 2 + 0.4 DeFc µL3

( )

1/4

µL FLDe

1/3

(5)

where Fc is the shape factor, De is the diffusion coefficients (cm2/s), FL is the density of the liquid phase (g/ cm3), µL is the viscosity of the liquid phase (g/(cm s)), and e is the energy supplied per unit mass (erg/(g s)). Equation 3 can therefore be expanded as

((

) )

Fp 1 1 1 1 1 ) ) + kg w keff w 6Fcω(De,dp,FL,µL,e) ηkc

(6)

The expression of the catalytic effectiveness factor (η) for a first-order reaction26 is

η)

1 1 coth(3φ) φ 3φ

(

)

Figure 3. Hydrogenation of 2-(5000-COO-CH2)AQ followed by UV spectroscopy (0.95 mM FAQ; 1.69 g/L Pd (1%)/Al2O3; P ) 235 bar; FAQ:H2 ) 1:10 (molar)). The chemical formula is defined in Figure 1. The number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail.

(7a)

where φ is the Thiele modulus:

φ)

( )

dp Fpkc 6 Deff

1/2

(7b)

written as a function of the average particle diameter (dp, cm), the density of the catalyst particle (Fp, g/cm3), and effective diffusivity (Deff, cm2/s). Hence, the expression of keff in eq 6b is given by

Fp φ(Deff,dp) 1 ) + keff 6Fcω(De,dp) kc

1

(

coth(3φ) -

1 3φ

)

(8) Figure 4. Concentration vs time for 2-(5000-COO-CH2)AQ monitored at 320 nm (the same conditions as in Figure 3): (b) experimental data; (s) simulated data (eq 2b).

where ω(De,dp) is defined as

ω(De,dp) )

2De + 0.4QdpDe2/3 dp2

(9)

In eq 9, Q is a constant given by

Q ) e1/4FL5/12/µL5/12

(10)

where the density (FL) and viscosity (µL) of liquid CO2 at the experimental conditions were determined using a multiparameter virial equation of state.28 The following steps were followed to determine kc and De’s/Deff’s for a certain group of functionalized anthraquinones having the same AQ block, spacer, and linker but different CO2-philic tail lengths: (1) Values of kg were determined by fitting the data cFAQ ) f(t) with an exponential function (eq 2b), for different catalyst loadings and different particle size catalysts. (2) Values of keff were determined for different particle size catalysts, by plotting 1/kg vs 1/w (eq 6). (3) For all FAQs in a group, kc, De’s, and Deff’s were derived from the simultaneous nonlinear regression of

the data 1/keff ) f (dp) with the function given in eq 8 (parameters kc, De, and Deff). We assumed that, within the same group, all FAQs exhibit the same kc but different De and Deff. The procedure is illustrated in Figures 3-7 for 2-(5000-COO-CH2)AQ. UV spectra recorded on-line exhibit isosbestic points (290 and 345 nm), demonstrating the quantitative conversion of the anthraquinone to the corresponding anthrahydroquinone (Figure 3). The reaction was monitored by measuring the rate of disappearance of the absorption peak at 320 nm-1, a peak that has been previously assigned to the anthraquinone block.24 Global rate constants (kg) were determined by fitting the experimental data (cFAQ ) f(t)) with the exponential function given in eq 2b (Figure 4). In Figure 5, the global rate constant (kg) was plotted vs catalyst loading (w) for different catalysts of varying particle size. For all sieved fractions of the catalyst, the linear dependence of kg vs w was observed. Interestingly, all curves in Figure 5 exhibit a positive intercept on the w axis, suggesting that a minimum loading of catalyst (wmin) is required for the reaction to occur. wmin

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Figure 5. kg vs w for various particle size catalysts 2-(5000-COOCH2)AQ (0.5 mM; P ) 235 bar; FAQ:H2 ) 1:10 (molar)): (2) 0.06 cm; (b) 0.04 cm; (9) 0.02 cm; (1) 0.0032 cm.

Figure 6. Specific adsorption of FAQ (∆cFAQ/w) vs particle size of the catalyst for (O) 2-(2500-CONH)AQ and (9) 2-(5000-CONH)AQ (∆cFAQ ) cw)0 - cw; mmol/L). The chemical formula is defined in Figure 1. The number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail.

increases with the particle size of the catalyst and appears to be inversely proportional to the length of the FAQ fluoroether tail. According to Minder et al.,29 one possibility for the partial deactivation of the catalyst described above would be to consider that the Pd active sites of the catalyst are partially covered with CO molecules formed in a reverse water shift reaction, i.e., reaction between CO2 and H2 forming CO and water. However, in a study of the surface interaction of CO2 and H2 over Pd/Al2O3, Solymosi et al.30 showed that no CO was formed on the surface of the catalyst below 100 °C, and the hydrogenation of CO2 occurred at a measurable rate only at temperatures above 250 °C. We consequently do not consider that our experimental conditions (25 °C, P ) 235 bar) favor the formation of CO on the surface of the Pd catalyst, and therefore we rule out this possibility.

Figure 7. 1/kg vs 1/w for various particle size catalysts (the same conditions as in Figure 5): (9) 0.06 cm; (b) 0.04 cm; (2) 0.02 cm; (1) 0.0032 cm.

Another possibility is that a portion of FAQ is adsorbed irreversibly onto the catalyst surface and thus prevents access to some of the active sites. To evaluate this possibility, the UV absorbance of a given amount of FAQ (in CO2) was measured before and after addition of the Pd/Al2O3 catalyst. The drop in absorbance could be correlated to the drop in concentration of FAQ due to the presence of the catalyst. Figure 6 shows the dependence of the specific adsorption of FAQ on Pd/ Al2O3 as a function of the particle size catalyst for two amide FAQs. We consider that FAQ might be able to adsorb on the active sites of the catalyst in two ways: through its AQ block or through its fluorinated tail. While the first type of interaction leads to a reaction, the second one blocks the active sites of the catalyst. Therefore, some of the FAQ molecules might act as a poison for the catalyst. Increasing the CO2-philicity of the molecule might lead to a shift of the equilibrium toward desorption of FAQ and, therefore, to a smaller specific adsorption. As suggested by other authors (Clarke et al.31 and Drelinkiewicz18), we correlated 1/kg with 1/w, where w is the active fraction of the catalyst calculated as w wmin (Figure 7). For all sieved fractions of the catalyst, the dependence 1/kg vs 1/w is linear, which confirms the assumption that the reaction is first order with respect to FAQ.26 Experimental values of keff given by eq 8 were obtained at different particle size catalysts, and the values for kc, De, and Deff were obtained from simultaneous nonlinear regression. The effect of diffusion (both liquid-solid and intraparticle diffusion) was evaluated from the values of the overall effectiveness factor (β) calculated as the ratio between the experimental rate constant (keff) and the true kinetic constant (kc) determined from regression. Values of β calculated for both amide and ester FAQs are given in Table 1. External Mass-Transfer Effects. In Figure 8, the individual contribution of liquid-solid diffusion to the mass-transfer limitations was studied by varying the mixing rate in the hydrogenation of 2-(5000-CONH)AQ catalyzed by two different catalyst sizes. For the powdered catalyst the global rate constant (kg) is not influenced by the stirring rate, showing that liquid-

2838 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 1. Effective Kinetic Constant (keff) and Overall Effectiveness Factor (β) Determined during Hydrogenation of Functionalized Anthraquinones (FAQs) (P ) 235 bar, T ) 25 °C) entry

FAQa

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d 4a 4b 4c 4d 5a 5b 5c 5d 6a 6b 6c 6d

2-(700-COO-CH2)AQ

2-(2500-COO-CH2)AQ 2-(5000-COO-CH2)AQ

2-(2500-CONH)AQ

2-(5000-CONH)AQ

2-(7500-CONH)AQ

particle size keff catalyst (cm) [cm3/(g s)] 0.06 0.04 0.02 0.0032 0.06 0.04 0.02 0.0032 0.06 0.04 0.02 0.0032 0.06 0.04 0.02 0.0032 0.06 0.04 0.02 0.0032 0.06 0.04 0.02 0.0032

1.2 2.1 2.7 3.8 0.8 1.6 2.4 3.5 0.4 1.0 1.4 2.1 0.5 0.7 1.5 1.8 0.3 0.5 0.8 1.8 0.2 0.3 0.6 1.1

βb 0.32 0.55 0.71 ∼1 0.21 0.42 0.84 0.92 0.11 0.26 0.37 0.55 0.25 0.35 0.75 0.9 0.15 0.25 0.4 0.9 0.1 0.15 0.3 0.65

Figure 9. Effect of dp on 1/keff for (0) 2-(700-COO-CH2)AQ, (2) 2-(2500-COO-CH2)AQ (b) 2-(5000-COO-CH2)AQ, and (s) simulated data (eq 8). The chemical formula is defined in Figure 1. The number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail. Table 2. Physical Constants/Parameters of the Mathematical Model

a

The chemical formula is defined as in Figure 1. The number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail. b β ) keff/kc, where kc is true kinetic data determined by regression (see Table 3).

a

Figure 8. Effect of the stirring rate on reaction rate for 2-(5000CONH)AQ (0.5 mM) and different particle size catalysts: (2) 0.0032 cm (w ) 1.4 g/L); (9) 0.04 cm (w ) 2.5 g/L). The number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail.

solid mass-transfer limitations are negligible. However, for 0.04 cm catalyst, the rate of reaction depends on the stirring rate and, therefore, the mass-transfer limitations are due to both liquid-solid and intraparticle diffusion. Internal Mass-Transfer Effects. (a) Hydrogenation of Ester-Functionalized Anthraquinones. The procedure presented in the previous section was performed for ester-functionalized anthraquinones with 700, 2500, and 5000 FW fluoroether chains (2-(700COO-CH2)AQ, 2-(2500-COO-CH2)AQ, and 2-(5000-

parameter

value

units

N dI FLa µLa Q Fcb Fp

1400 1 0.94 1.026 × 10-2 ∼21 0.55 1.5

rpm cm g/cm3 g/(cm s)

Calculated at T ) 25 °C and P ) 235

g/cm3 bar.28 b

Reference 27.

COO-CH2)AQ), and the results are presented in Figure 9. For all three FAQs, keff depends on dp, indicating that the reaction is under a mixed diffusional-kinetic regime due to a very fast kinetic process and relatively slow diffusion of the FAQ molecules. As shown in Table 1, the weight of each type of mechanism, expressed by the overall effectiveness factor (β ) keff/kc), depends on the size of the CO2-philic tail and on the particle size of the catalyst. For a 700 FW tail and powdered catalyst, the diffusional mechanism is less important and the global effectiveness factor (β) approaches 1. When the particle size is increased from 0.0032 cm (powdered catalyst) to 0.02 cm and further to 0.06 cm, β drops dramatically from 1 to a value of 0.71 and 0.3, respectively. For a catalyst with a large particle size, the diffusion of FAQ controls the hydrogenation process. The effect of diffusion is even more obvious for the 5000 FW fluoroether tail. Here, even for the powdered catalyst, β is close to 0.5, dropping to 0.1 for the largest particle size (0.06 cm). The values for the diffusion coefficients of FAQs in CO2 (De’s), effective diffusivities of FAQs inside the catalyst (Deff’s), and the true kinetic rate constant (kc) are obtained from simultaneous fitting of the experimental values 1/keff ) f(dp) for the three ester-functionalized anthraquinones with eq 8 under the assumption that kc is the same for all three FAQs. The values for the physical constants used in the simulations are listed in Table 2. The resulting diffusion coefficients (De’s) are in the range of 10-3-10-4 cm2/s, in good agreement with other values reported in the literature for diffusion of organic substances in supercritical

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2839 Table 3. Kinetic and Mass-Transfer Parameters Determined by Regression (T ) 25 °C, P ) 235 bar) 2-(Kr-NHCO)AQa b

c

2-(Kr-COO-CH2)AQ

FW

kc

De

De/Deff

r2

700 2500 5000 7500 eAQd

2.0 ( 0.2 2.0 ( 0.2 2.0 ( 0.2 0.47e

4.8 ( 0.3 2.1 ( 0.1 1.1 ( 0.1

5.1 4.7 4.4

0.99 0.98 0.95

a

FW

kc

Dec

De/Deff

r2

700 2500 5000 7500

3.8 ( 0.4 3.8 ( 0.4 3.8 ( 0.4

13.5 ( 1.1 7.0 ( 0.4 2.5 ( 0.3

4.3 3.5 3.8

0.96 0.99 0.93

The chemical formula is defined as in Figure 1. b [cm3/(g s)]. c [cm2/s × 104].

b

d

2-Ethyl-9,10-anthraquinone (eAQ). e Reference 20.

Figure 10. Calculated diffusion coefficients for 2-(Kr-NHCO)AQ and 2-(Kr-COO-CH2)AQ in liquid CO2 at P ) 235 bar and T ) 25 °C. The chemical formula is defined in Figure 1. FW indicates the molecular weight of the poly(perfluoropropylene oxide) tail.

CO2.32,33 In agreement with other studies (see, for example, the work of Satterfield34), diffusion inside the catalyst is almost 4 times slower than that in the liquid phase, as shown by the ratio Deff/De in Table 3. The fact that the diffusion coefficients vary inversely with the molecular weight (and thus with the size) of FAQs is not surprising (Figure 10). In summary, the kinetically controlled regime can be accessed for small particle size catalysts and small CO2philic tails where diffusion is faster than the kinetic process. When the length of the CO2-philic tail is increased, the diffusion coefficients decrease and the process becomes diffusionally controlled. (b) Hydrogenation of Amide-Functionalized Anthraquinones. The analysis outlined previously was performed for 2-(Kr-CONH)AQ with 2500, 5000, and 7500 FW fluoroether tails. As shown in Figure 11, the UV spectrum of an amide FAQ recorded as the hydrogenation proceeded shows two isosbestic points at 314 and 368 nm. Kinetics was followed by measuring the disappearance in time of the FAQ peak at 330 nm for different particle size catalysts and different catalyst loadings. The effect of dp on 1/keff is illustrated in Figure 12, while the values of the effective rate constants (keff) for different particle size catalysts are listed in Table 1. The values for the global effectiveness factor (β; see Table 1) show that the process is in a mixed diffusionalkinetic regime for all of the FAQs considered. Values of true kinetic rate constants for amide- and ester-functionalized AQs are compared in Table 3 with the corresponding value calculated for eAQ in organic solvents.20 Inspection of the data presented in Table 3 shows that amide anthraquinones are less reactive than the ester variants. We suspect that the electronic effect of the group attached to the aromatic rings plays an important role. The global electronic effect of the group

Figure 11. Hydrogenation of 2-(7500-CONH)AQ followed by UV spectroscopy (0.9 mM FAQ; 2.95 g/l Pd (1%)/Al2O3; H2:FAQ ) 10:1 (molar); P ) 235 bar). The number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail.

Figure 12. Effect of dp on 1/keff for (2) 2-(2500-CONH)AQ, (O) 2-(5000-CONH)AQ, (1) 2-(7500-CONH)AQ, and (s) simulated data (eq 8). The chemical formula is defined in Figure 1. The number in the chemical formula indicates the molecular weight of the poly(perfluoropropylene oxide) tail.

attached to the AQs block is the result of the individual electronic effect of the spacer, linker, and perfluoroether tail. The acyl-amino group (NHCOR) is a mildly electron-donating group that stabilizes the quinone in the redox equilibrium quinone T hydroquinone. The

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also followed by 1H NMR spectroscopy, which showed that under the experimental conditions no deep hydrogenation products were detected. Conclusions

Figure 13. Performance of 2-(5000-COO-CH2)-AQ during a hydrogenation-oxidation cycle in liquid CO2 followed by 1H NMR.

ester group separated by a methylene spacer from the aromatic rings has the opposite effect, stabilizing the hydroquinone by its electron-withdrawing effect. Therefore, the methylene spacer introduced between the linker and the AQ block increases the reactivity of the FAQs in the hydrogenation reaction by minimizing the electron-donating effect of the linker toward AQs aromatic rings. Besides the effects of the spacer and linker, the attachment of the perfluoroether tail also increases the reactivity of FAQs compared to the conventional eAQ, as shown in Table 3. This can be the result of the strong electron-withdrawing effect of the perfluoroether polymer. Side Reactions. Hydrogenation of an alkyl AQ can form a wide range of products, depending on the amount of H2 reacted, the temperature, and the catalyst loading. Drelinkiewicz35 and Santacesaria et al.36 showed that, for the case of hydrogen reacting with eAQ over Pd/ Al2O3, if the ratio H2 consumed by eAQ was less than or close to 1, the only product formed in the process was eAQH2. If more H2 was allowed to react, hydrogenation proceeded at a lower rate and two types of products were detected: products of aromatic ring hydrogenation (H4eAQ, 2-ethyltetrahydro-9,10-anthraquinone; H6eAQ, 2-ethylhexahydro-9,10-anthraquinone; H8eAQ, 2-ethyloctahydro-9,10-anthraquinone) and products of hydrogenolysis (anthrone and anthracene derivatives). This overhydrogenation process has been called “deep hydrogenation”,35 and it is not desired because among the deep hydrogenation products only H4eAQ can generate hydrogen peroxide after a hydrogenation-oxidation cycle. The performance of 2-(5000-COO-CH2)AQ during a hydrogenation-oxidation cycle in liquid CO2 was followed by 1H NMR spectroscopy, as shown in Figure 13. The ratio of the integrated peaks in the aromatic region remained unchanged after the reactions, which indicates that no measurable deep hydrogenation took place under the experimental conditions. The aliphatic region (1-2.5 ppm) did show an additional peak, but this was found to correspond to the aliphatic additives of the silicone grease used for the sealing of the highpressure reactor. The performances of the amide FAQs in a hydrogenation-oxidation cycle in liquid CO2 were

Hydrogenation of functionalized anthraquinones in liquid CO2 shows that the reactivity can be tuned by varying the length of the CO2 tail and the nature of the spacer and linker. Diffusion coefficients vary inversely to the molecular weight of the CO2-philic tail, and masstransfer limitations become negligible for FAQs with shorter tails. Kinetically controlled hydrogenation can thus be achieved by using small particle size catalysts and FAQs with short CO2-philic tails. On the other hand, linkers with an electron-donating effect bonded directly to AQ blocks decrease the intrinsic reactivity of FAQ in the hydrogenation process. A remedy to this problem is to introduce a methylene spacer between the AQ block and the linker that mitigates the electrondonating effect of the linker. While longer fluoroether tails allow complete miscibility at lower pressures, they also decrease the effective diffusivity of FAQ, making the achievement of kinetic control more difficult. Nomenclature aP ) external surface area of catalyst particle [cm2/cm3] cA ) molar concentration of A [mol/cm3] De ) diffusion coefficient [cm2/s] Deff ) effective diffusivity, De/τ [cm2/s] dp ) average particle diameter [cm] dI ) diameter of the impeller [cm] e ) energy supplied per unit mass, P/(FLVL) [erg/(g s)] Fc ) shape factor kc ) true kinetic constant [cm3/(s g)] keff ) effective rate constant [cm3/(s g)] kg ) global rate constant [s-1] ksap ) liquid-solid mass-transfer coefficient [s-1] N ) rps for the impeller [rad/s] Np ) power number, P/(N3dI5FL) P ) power consumption [erg/s] R ) reaction rate [mol/(L s)] Re ) Reynolds number, NdI2FL/µL t ) time [s] VL ) reactor volume [cm3] w ) catalyst loading [g/cm3] wmin ) minimum catalyst loading [g/cm3] Q ) constant, e1/4 FL5/12/µL5/12 Greek Letters β ) overall effectiveness factor, keff/kc  ) porosity [cm3/cm3] η ) catalytic effectiveness factor µL ) viscosity of liquid [g/(cm s)] φ ) Thiele modulus, φ ) dp/6(Fpkc/Deff)1/2 FL ) density of the liquid [g/cm3] Fp ) density of the catalyst [g/cm3] ω(De,dp) ) function, d2p/(2De + 0.4QdpDe2/3) τ ) tortuosity factor

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Received for review November 23, 1998 Revised manuscript received March 25, 1999 Accepted April 7, 1999 IE9807396