Hydrogenation of 2,4-Dinitrotoluene Using a Supported Ni Catalyst

Feb 2, 1999 - For the purpose of kinetic modeling, experimental data on the effects of H2 ... SiO2, and TiO2 were obtained from United Catalysts India...
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Ind. Eng. Chem. Res. 1999, 38, 906-915

Hydrogenation of 2,4-Dinitrotoluene Using a Supported Ni Catalyst: Reaction Kinetics and Semibatch Slurry Reactor Modeling Rajashekharam V. Malyala*,† and Raghunath V. Chaudhari Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India

The kinetics of hydrogenation of 2,4-dinitrotoluene (2,4-DNT) using a 10% Ni supported on zeolite Y (10% Ni/HY) powdered catalyst (dp ) 1 × 10-5 m) was studied experimentally in a semibatch slurry reactor over a temperature range of 333-363 K. The effects of reaction temperature, H2 pressure, concentration of 2,4-DNT and catalyst loading on the concentration-time and H2 consumption-time profiles were studied under isothermal conditions. To explain the rate behavior of this complex, consecutive, and parallel reaction, several rate expressions were derived based on Langmuir-Hinshelwood type rate mechanisms. The rate equations that were derived assuming the reaction between competitively adsorbed organic species and dissociatively adsorbed hydrogen as the rate-limiting step were found to represent the kinetics best. Quantitative analysis of the experiments performed under the chosen conditions (temperature, 333-363 K; H2 pressure, 1.3-5.4 MPa; 2,4-DNT concentration, 0.14-0.55 kmol/m3) indicated that the rate data obtained were under the kinetic regime. The rate and equilibrium parameters were evaluated for the different steps involved in the reaction network. A semibatch slurry reactor model has been developed to predict both integral concentration-time and H2 consumption-time profiles and was compared with experimental data at different sets of initial conditions. An excellent agreement between the model predictions and the experimental data was observed. Introduction Hydrogenation of 2,4-dinitrotoluene (2,4-DNT) is an excellent example of a multiphase catalytic reaction which follows a complex, consecutive, and parallel reaction network and is highly exothermic.1 The hydrogenated product 2,4-toluenediamine (2,4-TDA) finds a variety of applications, of which the most important one is in the production of toluenediisocynate (TDI), used in the manufacture of polyurethanes. Hydrogenation of 2,4-DNT is usually carried out in the liquid phase either in batch or in continuous mode in the presence of supported Pd or Ni catalysts.2,3 The kinetics and the reaction engineering aspects have been extensively studied using supported Pd catalysts, and detailed analyses of batch and continuous reactors have been addressed earlier.1,4-12 Also the issues related to catalytic activity, selectivity, and stability of supported Pd and bimetallic Pd-Fe catalysts have been fairly well understood.13,14 Hydrogenation of 2,4-DNT to 2,4-TDA using supported Pd and Ni catalysts is known1,4,15 to proceed via the formation of intermediate hydrogenated products such as 4-(hydroxylamino)-2-nitrotoluene (4HA2NT), 2-amino-4-nitrotoluene (2A4NT), and 4-amino-2-nitrotoluene (4A2NT). The extent of formation of these products depends on the catalyst used as well as the reaction conditions. Janssen et al.4,5 have studied the kinetics of hydrogenation of 2,4-DNT using a 5% Pd/C catalyst and proposed rate equations based on the Langmuir-Hinshelwood type rate mechanism, which also accounts for the deactivation of the catalyst. Rajashekharam et al.1 reported the kinetics and intraparticle diffusion effects during hydrogenation of 2,4-DNT * To whom correspondence should be addressed. † Present address: Department of Chemical Engineering, University of Saskatchewan, Saskatoon, SK S7N 5C9, Canada.

using a 5% Pd/Al2O3 catalyst under both isothermal and nonisothermal conditions and proposed rate equations to explain the kinetic behavior based on a molecular level approach. White15 carried out investigations on the kinetic modeling of hydrogenation of 2,4-DNT using a Ni/SiO2 catalyst and proposed rate equations based on the Langmuir-Hinshelwood type rate mechanism in which the rate-limiting step was assumed to be the reaction between competitively adsorbed hydrogen with the adsorbed liquid-phase components. It was demonstrated earlier15-18 that supported Ni catalysts are highly active for hydrogenation of 2,4-DNT. However, there are no published reports which deal in detail with the issues related to both catalysis and reaction engineering, in spite of the fact that Ni catalysts have potential applications for such reactions on a commercial scale. For example, in the RhonePoulenc, Olin Mathieson, and Bayer AG and Mobay Corp. processes for hydrogenation of 2,4-DNT, an activated Ni catalyst (Raney Ni or Ni supported on a silicabased carrier such as Kieselguhr) is used.2,3 For commercial processes, it is often desirable to develop cheaper catalysts with high activity and selectivity. In this context, supported Ni catalysts are highly promising especially for hydrogenation of 2,4-DNT. Hence, it is the aim of this paper to first compare the performance of several supported Ni catalysts with that of Pd catalysts reported earlier in the literature and to study the kinetics of hydrogenation of 2,4-DNT using the best catalyst in terms of the overall hydrogenation activity. For the purpose of kinetic modeling, experimental data on the effects of H2 pressure, 2,4-DNT concentration, catalyst loading, and agitation speed on the concentration-time and H2 consumption-time profiles were studied in a temperature range of 333-363 K. The initial rate analysis was carried out, and the effect of various operating parameters is discussed. Quantitative criteria proposed earlier in the literature were used to

10.1021/ie980423y CCC: $18.00 © 1999 American Chemical Society Published on Web 02/02/1999

Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999 907 Table 1. Range of Operating Conditions and Specifications of the Catalyst Used for Kinetic Study concn of 2,4-DNT (kmol/m3) temperature (K) H2 pressure (MPa) catalyst loading (kg/m3) agitation speed (Hz) reaction volume (m3)

0.14-0.55 333-363 1.3-5.4 10-40 5-20 1 × 10-4

support surface area of support (m2/kg) Ni content (% w/w) particle size, dp (m) particle density, Fp (kg/m3) porosity, p tortuosity, τ

zeolite Y (HY) 5 × 105 10 1 × 10-5 2.5 × 103 0.58 3.5

assess the significance of gas-liquid, liquid-solid, and intraparticle mass-transfer resistances. Several rate expressions based on the Langmuir-Hinshelwood type mechanism were considered, and the rate model that explains best the integral concentration-time profiles in a semibatch slurry reactor was selected. The rate and the equilibrium parameters are reported. Also, the discrimination of various models and the adequacy of the models have been discussed. Experimental Section Materials. 2,4-DNT obtained from M/s Fluka (Switzerland) was thoroughly dried to remove moisture before use. The solvent used in this study was methanol and was obtained from M/s S.D. Fine Chemicals (India). The solvent was freshly distilled before use in hydrogenation experiments. H2 and N2 gases of purity greater than 99% were obtained from M/s Indian Oxygen Limited (India). 2A4NT, 4A2NT, and 2,4-TDA were obtained from Aldrich (Milwaukee, WI) and were used as GC standards. Ni(NO3)2‚6H2O and (NH4)2CO3 were obtained from S.D. Fine Chemicals (India). The catalyst supports such as HY, HZSM-5, Al2O3, SiO2, and TiO2 were obtained from United Catalysts India Limited (India). Catalyst Preparation Method. The supported Ni catalysts (Ni content in percent w/w) were prepared by a well-known procedure described by Roberts.19 A detailed procedure of the catalyst preparation method along with extensive data on catalyst characterization is given elsewhere.20 The specifications of the catalyst used for kinetic study (10% Ni/HY) are given in Table 1. Experimental Setup. The experimental setup used in this work is shown in Figure 1. The hydrogenation experiments were carried out in a 3 × 10-4 m3 capacity stirred pressure reactor made of SS316 material supplied by Parr Autoclave Instruments (Moline, IL). The reactor had provisions for automatic temperature control, variable stirrer speeds, high-temperature cutoff, safety rupture disk, and sampling of the liquid phase. The reactor had a maximum operating temperature and pressure of 523 K and 10 MPa. A storage reservoir for H2 gas was used along with a constant-pressure regulator. This allowed the determination of H2 consumption as a function of time, while maintaining the reactor at a constant desired pressure. Experimental Procedure In a typical experiment, 1 × 10-2 kg of 2,4-DNT was added to 1 × 10-4 m3 of methanol (solvent), and 2 × 10-3 kg of Ni catalyst was charged to a clean, dry

reactor. The contents in the reactor were flushed with N2 and then with H2 (two to three times) at room temperature, and the heating was turned on. After the desired temperature was attained, the reactor was pressurized with H2 and switching the stirrer on started the reaction. Immediately, the H2 gas absorption was noted by observation of depletion of pressure in the H2 reservoir vessel maintained at a constant temperature. In each experiment, liquid samples were withdrawn at regular intervals of time and were analyzed using a gas chromatograph. The H2 consumption as a function of reaction time was determined, and for each experiment, the H2 consumption vs time plots were generated. The H2 gas loss during the sampling was recorded, and appropriate correction in H2 consumption was made. Experiments were carried out in a temperature range of 333-363 K, and the other operating conditions are given in Table 1. Analysis of Reactants and Products. The analysis of the liquid-phase samples for the quantitative estimation of 2,4-DNT and the products formed during the course of hydrogenation was carried out using a HP 5840A gas chromatograph (GC) with a flame ionization detector (FID). A column packed with Tenax GC 60/80 mesh and 6 ft long was used. The conditions of the analysis are as follows: FID temperature, 523 K; injector temperature, 523 K; oven temperature, 343 K (2 min) to 423 K (6 min) with a temperature programming of 5 K/min. Standard solutions of 2,4-DNT, 4HA2NT, 4A2NT, 2A4NT, and 2,4-TDA and their mixtures were prepared in methanol and were used as GC standards for the purpose of quantitative analyses. A few samples were also analyzed using a Shimadzu QP 2000 A GC-MS (mass spectrometer) to confirm the products. Quantum Chemical Calculations. To understand whether the reactants and the intermediate products formed during the course of hydrogenation of 2,4-DNT were accessible to the Ni sites present inside the framework pore structure of the zeolite Y, quantum chemical calculations were performed. The molecular dimensions of different organic species are estimated using a Silicon Graphics Indigo II workstation supplied by Biosym Technologies (San Diego, CA). The molecules were optimized using Central Valence Force Field (CVFF).21 Results and Discussion The experiments on hydrogenation of 2,4-DNT were carried out with the following specific objectives: (a) performance comparison of supported Pd and Ni catalysts; (b) activity and selectivity studies on various supported Ni catalysts: catalyst selection; (c) kinetic modeling using a 10% Ni/HY catalyst. In the initial phase of this work, the performance of several supported Ni catalysts was compared with that of supported Pd catalysts. For this purpose, literature data4-12 on the conversion of 2,4-DNT, selectivity to 2,4TDA, and the average activity during hydrogenation of 2,4-DNT using different Pd catalysts have been compared with those obtained using supported Ni catalysts. The average activity of each catalyst was calculated based on the total moles of 2,4-DNT consumed per unit weight of the actual metal content present in the catalyst per unit time (kmol/kg of Pd or Ni/h). Figure 2 shows a comparison of different supported Pd and Ni catalysts in terms of the conversion of 2,4-DNT, selectivity to 2,4-TDA, and the average activity. The results

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Figure 1. Schematic of the reactor setup: (1) reactor, (2) stirrer shaft, (3) impeller, (4) cooling water, (5) sampling valve, (6) magnetic stirrer, (7) furnace. TI: thermocouple. PI: pressure transducer. CPR: constant-pressure regulator. PR: pressure regulator. TR1: reactor temperature indicator. TRG: gas temperature indicator. PR1: reactor pressure indicator. PR2: reservoir pressure indicator. TR2: reservoir temperature indicator.

Figure 3. Reaction scheme for hydrogenation of 2,4-DNT.

Figure 2. Performance comparison of supported Pd and Ni catalysts for hydrogenation of 2,4-DNT. Reaction conditions: concentration of 2,4-DNT, 0.55 kmol/m3; catalyst loading, 20 kg/ m3; temperature, 348 K; PH2, 2.6 MPa; solvent, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3; reaction time, 2 h.

on H2 consumption and liquid-phase concentration of reactant/products were used to calculate the average activity of the catalysts and selectivity to different products. These results indicate that the performance of supported Ni catalyst (especially that of 10% Ni/HY catalyst) is comparable to Pd catalysts in terms of conversion of 2,4-DNT and selectivity toward 2,4-TDA. However, the average activity of this catalyst is lower by more than 4-5 times when compared with Pd (5% Pd w/w) catalysts. Considering the cost involved in the

preparation of supported Pd as well as Ni catalysts, this decrease in activity seems to be acceptable. For all of the Ni supported catalysts under investigation, the reaction scheme shown in Figure 3 was well represented. The products formed during the hydrogenation of 2,4-DNT using supported Ni catalysts were found to be 4HA2NT, 4A2NT, 2A4NT, and 2,4-TDA as identified and characterized by GC and GC-MS. Because in most cases the formation of 4HA2NT was less than 1% of the total 2,4-DNT that is converted, the step representing the formation of 4HA2NT has been lumped as shown in the reaction scheme (see Figure 3). Experiments were also performed at the same operating conditions using a 10% Ni/HY catalyst to ensure the reproducibility of the results and to check the consistency of the catalytic activity. Also, there was no significant loss of activity upon recycling this catalyst. The recycle experiments

Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999 909 Table 2. Hydrogenation of 2,4-DNT Using Supported Ni Catalystsa average activity, % conkmol/kg version 4A2NT 2A4NT 2,4-TDA of Ni/h product distribution in % selectivity

catalyst 20% Ni/Al2O3 20% Ni/SiO2 20% Ni/TiO2 20% Ni/HZSM-5 20% Ni/HY 10% Ni/HY

25 85 15 36 100 100

70 52 65 70 10 20

15 18 25 12 5 3

15 30 10 18 85 78

0.026 0.1 0.015 0.037 0.1 0.21

L,b Å 300 202 320 266 152 120

Table 3. Molecular Dimensions and Minimum-Energy Configurational Valuesa molecule

minimum energy, kcal/mol

a

b

c

2,4-DNT 4A2NT 2A4NT 2,4-TDA

59.96 44.99 36.91 23.51

5.36 5.28 5.36 5.08

7.09 6.69 6.76 6.41

1.95 1.85 1.73 1.71

a a, b, and c are the dimensions of the molecule in angstroms along the x, y, and z axes.

a Reaction conditions: concn of 2,4-DNT, 0.412 kmol/m3; catalyst, 20 kg/m3; temperature, 348 K; PH2, 2.59 MPa; solvent, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3; reaction time, 2 h. b Average metal crystallite size (L, Å) calculated from X-ray diffraction line-broadening analysis. L ) kλ/β cos θ, where k ) 0.9 and λ ) 1.5403 Å for Cu KR radiation.

were performed by charging the same catalyst into the reactor after filtration, washing several times with methanol (solvent), and drying the catalyst for 1 h at 373 K. Hence, from this performance comparison study, it can concluded that Ni catalysts are quite promising for commercial applications particularly considering the cost of the catalyst required to produce a unit weight of the desired product. Therefore, the catalysis and reaction engineering aspects of hydrogenation of 2,4-DNT using supported Ni catalysts need to be further investigated. Screening of Supported Ni Catalysts. A few experiments were carried out to understand the role of supports and Ni metal content in 2,4-DNT hydrogenation. The results on the conversion of 2,4-DNT, selectivity toward the different products, and the average activity of different supported Ni catalysts are shown in Table 2. These results indicate that 20% Ni/SiO2, 20% Ni/HY, and 10% Ni/HY catalysts resulted in 85 and 100% conversion of 2,4-DNT, respectively. Furthermore, the selectivity toward 2,4-TDA, the desired product, was higher when the reaction was carried out using Ni supported on zeolite Y catalysts. The conversion of 2,4DNT obtained for supported Ni catalysts with supports such as Al2O3, TiO2, and HZSM-5 was comparatively much less (25, 15, and 36%, respectively). On the basis of a detailed characterization of these catalysts, the experimental results observed on 2,4-DNT conversion were attributed to the nature of the support, surface area, metal dispersion, active metal surface area, and average metal crystallite size.20 The high activity of the 10% Ni/HY catalyst is due to the presence of small metallic Ni sites with an average crystallite size of 120 Å as evidenced from the X-ray diffraction line broadening analysis of the catalyst samples (see Table 2). This, in turn, would have resulted in a better dispersion of the Ni sites on the zeolite Y support when compared to other supported Ni catalysts. On other supports such as Al2O3, TiO2, and HZSM5, the average Ni metal particle size was in the range of 260-320 Å. Also, the high activity could have been due to the presence of two different types of active Ni sites, one where Ni gets exchanged with the protonic sites of the zeolite and forms small clusters of Ni on subsequent calcination and activation22 and the other site which is present on the external surface area of the support. To have an idea about whether the organic species are really accessible to such Ni sites present inside the pores of the framework zeolite Y structure, it was

Figure 4. Selectivity ratio of 4A2NT/2A4NT vs 2,4-DNT conversion at different temperatures. Reaction conditions: concentration of 2,4-DNT, 0.412 kmol/m3; catalyst loading, 20 kg/m3; PH2, 2.6 MPa; solvent:, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3.

thought necessary to evaluate the molecular dimensions of reactant/products that are formed in the reaction. If the molecular dimensions of the reactant/products are less than the structural pore opening of zeolite Y (∼7.4 Å), then there is a high probability of these Ni sites to take part in the reaction. Hence, the molecular dimensions were estimated by performing quantum chemical calculations on a Silicon Graphics Indigo II workstation supplied by BioSym Technologies. These calculations were based on the assumption that the molecule would attain the minimum-energy configuration while interacting with the catalyst. The results on the molecular dimensions along with the minimum-energy configuration values for the liquid-phase reactant/products are given in Table 3. The results indicate that these molecules are definitely accessible to the innermost Ni sites, as their overall dimensions are less than 7.4 Å. Another interesting feature is the low selectivity toward 2A4NT observed for all Ni catalysts under investigation (see Table 2). The range of selectivity observed for different catalysts is between 3 and 25% after a reaction time of 2 h. This resulted in a significant increase in the selectivity ratios of 4A2NT to 2A4NT. Figure 4 shows a plot of selectivity ratio vs 2,4-DNT conversion for all three temperatures under investigation using a 10% Ni/HY catalyst. The value of this ratio was found to be always greater than (between 6 and 7 to be more precise) and was independent of 2,4-DNT conversion with a few exceptions. These results can be explained as follows: The formation of 2A4NT involves the hydrogenation of the nitro group present in the 2nd position, which is sterically hindered, by the methyl group adjacent to it. The presence of the methyl group

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Figure 5. Concentration-time profile at 348 K. Reaction conditions: concentration of 2,4-DNT, 0.412 kmol/m3; catalyst loading, 20 kg/m3; temperature, 348 K; PH2, 2.6 MPa; solvent:, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3.

Figure 6. H2 consumption-time plot for different concentrations of 2,4-DNT at 348 K. Reaction conditions: catalyst loading, 20 kg/ m3; temperature, 348 K; PH2, 2.6 MPa; solvent, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3.

results in a difficulty in hydrogenating this nitro group unlike the nitro group present in the 4th position, which can interact freely with the Ni surface. This may be the reason for the observed low selectivity to 2A4NT. On the basis of the above argument, once the amino group is formed at a position adjacent to the methyl group the reaction proceeds very fast, leading to the formation of 2,4-TDA. The above results support the optimum activity observed for a 10% Ni/HY catalyst during hydrogenation of 2,4-DNT and, hence, this catalyst was chosen for the purpose of kinetic modeling. Typical concentration-time and H2 consumption-time profiles obtained at 348 K using this catalyst are shown in Figures 5 and 6. The material balances of the reactants consumed (H2 and 2,4-DNT) and the products formed (4A2NT, 2A4NT, and 2,4-TDA) were found to agree to an extent of >95%. The reaction scheme using this catalyst was well represented, as shown in Figure 3. Also, no hydrogenation took place in the absence of the catalyst, and the side product (water) formed had no influence on the overall

Figure 7. Effect of 2,4-DNT concentration on initial rates. Reaction conditions: catalyst loading, 20 kg/m3; PH2, 2.6 MPa; solvent, MeOH; agitation speed:, 18 Hz; reaction volume, 1 × 10-4 m3.

rate of hydrogenation. This was verified by performing a few experiments where the water concentration was varied. Analysis of Initial Rate Data. The H2 consumption-time data obtained for different sets of operating conditions (listed in Table 1) were fitted by a linear regression procedure from which the initial rates were calculated. The effects of H2 pressure, concentration of 2,4-DNT catalyst loading, and agitation speed on the initial rates were studied in a temperature range of 333-363 K, and the individual effects are discussed below. The effect of 2,4-DNT concentration on the initial rate of hydrogenation at different temperatures is shown in Figure 7. The results indicate a first-order tending to zero-order dependence at higher concentrations of 2,4DNT in most conditions. This observation using Ni catalysts is clearly different when compared to Pd catalysts, where in most cases a zero-order dependence on the rate was observed.1,4,5,12 However, it should be noted that the data presented in Figure 7 mainly represent the conversion of 2,4-DNT to 4HA2NT, 4A2NT, and 2A4NT (first three steps in the reaction scheme) and, hence, the trend observed cannot be generalized for all of the hydrogenation steps shown in the reaction scheme. The variation of initial rates with H2 pressure is shown in Figure 8. A first-order (at lower partial pressures of H2) tending to zero-order dependence at higher hydrogen pressures for all three temperatures investigated was observed. These results indicate that adsorption of H2 as well as the liquid-phase component is important and needs to be considered in the rate model being developed. The effect of catalyst loading on the initial rates of the reaction is shown in Figure 9. A linear variation of catalyst loading was observed in the range of temperature investigated in this work. Figure 10 shows the effect of agitation speed on the initial rates. Experiments were performed in the agitation range of 5-20 Hz at 363 K and a catalyst loading of 40 kg/m3. There was a mild dependence of the agitation speed on the initial rates at lower agitation speeds; however, beyond an agitation speed of 15 Hz, this dependence was almost negligible. Hence, all experiments were conducted at an agitation speed of 18 Hz. These results

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Figure 8. Effect of partial pressure of hydrogen on initial rates. Reaction conditions: concentration of 2,4-DNT, 0.412 kmol/m3; catalyst loading, 20 kg/m3; solvent, MeOH; agitation speed:, 18 Hz; reaction volume, 1 × 10-4 m3.

Figure 9. Effect of catalyst loading on initial rates. Reaction conditions: concentration of 2,4-DNT, 0.412 kmol/m3; PH2, 2.6 MPa; solvent, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3.

showing the effects of catalyst loading and agitation speed on initial rates indicate that the external masstransfer resistances may not be important, the reliability of the rate data being used to study the intrinsic kinetics of hydrogenation of 2,4-DNT using a 10% Ni/ HY powdered catalyst. Analysis for Mass-Transfer Effects. Before we proceed further, it is necessary to check quantitatively the significance of external and internal mass-transfer effects for hydrogenation of 2,4-DNT even though it was confirmed that the catalyst loading was linear and the agitation speed had no influence, indicating the absence of external mass-transfer resistances. To evaluate the importance of gas-liquid, liquid-solid, and intraparticle mass-transfer resistances, quantitative criteria proposed by Ramachandran and Chaudhari23 were used. The criteria involved the determination of Rl ()RA/klaBA*), R2 ()RA/ksapA*), and φexp [)(dp/6)(FpRA/wDeA*)0.5] defined as the ratios of observed rate of reaction to the maximum rate of gas-liquid, liquid-solid, and intra-

Figure 10. Effect of agitation speed on initial rates. Reaction conditions: concentration of 2,4-DNT, 0.412 kmol/m3; catalyst loading, 40 kg/m3; temperature, 363 K; PH2, 2.6 MPa; solvent, MeOH; reaction volume, 1 × 10-4 m3. Table 4. Typical Values of ks, HA, and De at Different Temperatures T, K

ks × 102, m/s

HA × 103, kmol/m3/atm

De × 109, m2/s

333 348 363

0.9 1 1.41

4.61 5.02 5.31

1.43 1.50 1.56

particle mass transfer, respectively. If the values of these parameters (R1, R2, and φexp) are less than 0.1, 0.1, and 0.2, respectively, then the external and intraparticle mass-transfer resistances are considered to be unimportant. The initial rate data were analyzed for different sets of initial conditions to test these criteria, and the values of Rl, R2, and φexp were found to be in the ranges of 4 × 10-4-7.95 × 10-3, 2.01 × 10-6-2.35 × 10-6, and 4.58 × 10-3-1.88 × 10-2, indicating the absence of external and intraparticle mass-transfer resistances. The various parameters used in performing the calculations are discussed below and are summarized in Table 4. The klaB value of 0.2 s-1 at an agitation speed of 15 Hz determined by Chaudhari et al.24 for the same equipment was used, while the liquid-solid masstransfer coefficient, ks, was calculated from the correlation of Sano et al.25 The effective diffusivity values were evaluated using the relation De ) DM/τ where DM is the molecular diffusivity calculated from the Wilke and Chang equation26 and  and τ are the porosity and tortuosity, respectively of the catalyst particle. The Henry’s constant (HA) values determined by Radhakrishnan et al.27 for H2 solubility in methanol were used. The saturation solubility of H2 in methanol, i.e., the values of A*, were then evaluated from the relation A* ) PH2HA, where PH2 is the partial pressure of hydrogen. To estimate the values of the vapor pressure of methanol at different temperatures, the correlation described earlier in the literature was used.28 The typical values of the liquid-solid mass-transfer coefficient, ks, effective diffusivity values, De, and Henry’s constant, HA, values used in this work are reported in Table 4. The effective diffusivity, De, values were determined by taking into consideration the estimated value of porosity,  ) 0.58, and assuming a tortuosity, τ, value of 3.5. Thus, the De values reported in Table 4 are reasonable considering the range of τ values observed for various supports.29

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Kinetic Modeling. The kinetic modeling of multistep hydrogenation reactions was investigated earlier by Chaudhari et al., Joly Villuemin et al., Benaissa et al., and Zhu et al. among others.30-33 The kinetics of hydrogenation of 2,4-DNT using Pd catalysts was also studied earlier by many investigators.1,4-12 The kinetics and reactivity aspects of the intermediate mono nitro isomers were also investigated by Neri et al.10 It is well accepted that initial rate data alone may not be sufficient to explain the kinetics of such complex reactions and a knowledge of integral rate data is required to interpret the kinetics in detail. Also, it is important to examine that the kinetic model developed explains not only the data for one set of initial conditions but also those for different sets of initial conditions. For the present study the rate data were obtained for a wide range of operating conditions (see Table 1). In all cases H2 consumption-time and concentration-time profiles were obtained. From the concentration-time and H2 consumption-time profiles shown in Figures 5 and 6, some general trends indicated that only after complete consumption of 2,4-DNT was achieved was there a depletion in the concentration of 4A2NT, indicating that adsorption of 2,4-DNT is important and needs to be considered. Also, the amount of 2A4NT formed was lower when compared to the amount of 4A2NT, indicating that the adsorption of 2A4NT may not be important and that adsorption of 4A2NT should be considered in the rate equation. The initial rate analysis discussed in the previous section indicated some general trends that the rate of reaction is first-order tending to zeroorder dependence with respect to both 2,4-DNT concentration and H2 pressure. The above trends indicate that the overall rate of hydrogenation can be explained based on a Langmuir-Hinshelwood type of rate mechanism. Several models have been derived for the present case, and the model derived by assuming competitive adsorption of both organic species and hydrogen (hydrogen being dissociatively adsorbed) as the rate-controlling step is discussed below. Thus, the rate equations considering this model for the four hydrogenation steps occurring can be given as

r1 )

r2 )

r3 )

r4 )

wk1xA*Bl (1 + xKAA* + KBBl + KEEl)2 wk2xA*Bl (1 + xKAA* + KBBl + KEEl)2 wk3xA*Cl (1 + xKAA* + KBBl + KEEl)2 wk4xA*El (1 + xKAA* + KBBl + KEEl)2

(1)

(2)

(3)

(4)

The total rate of hydrogenation can then be given as

RA )

(klBl + k2Bl + k3Cl + k4El)wxA* (1 + xKAA* + KBBl + KEEl)2

(5)

To verify the applicability of the above rate model under integral conditions and to evaluate the kinetic parameters, the experimental data obtained on concentrations of 2,4-DNT, 4A2NT, 2A4NT, and 2,4-TDA as a

function of time were used. On the basis of eqs 1-4, the following set of material balance equations were derived to represent the concentration-time profiles of the liquid-phase reactants/products in a semibatch slurry reactor when operated under isothermal conditions and when external and intraparticle mass-transfer resistances are unimportant.

-

dBl w(kl + k2)BlxA* ) dt (1 + xKAA* + KBBl + KEEl)2

(6)

w(k2Bl - k3Cl)xA* dCl ) dt (1 + xKAA* + KBBl + KEEl)2

(7)

w(k1Bl - k4El)xA* dEl ) dt (1 + xKAA* + KBBl + KEEl)2

(8)

w(k3Cl + k4El)xA* dPl ) dt (1 + xKAA* + KBBl + KEEl)2

(9)

A represents hydrogen, and Bl, Cl, El, and Pl represent the liquid-phase concentrations of 2,4-DNT, 2A4NT, 4A2NT, and 2,4-TDA, respectively. Equations 6-9 were solved using the following initial conditions:

at t ) 0

Bl ) Bl0, Cl ) El ) Pl ) 0

(10)

To evaluate the rate and equilibrium parameters, the experimental concentration-time profiles were fitted to the above model using an optimization routine based on Marquardt’s algorithm combined with a fourth-order Runge-Kutta method. The optimization procedure involved the minimization of a parameter, called the object function or the optimization criterion, φmin, defined as

φmin )

∑(Yi,exp - Yi,mod)2

(11)

where Yi represents the concentration of species i in kmol/m3. The experimental data were simulated for each temperature separately, and the best set of common values of rate and equilibrium parameters was determined. These values along with the optimization criterion values and the % RR values, also known as the “relative residuals” evaluated as

∑(Yi,exp - Yi,mod)/∑Yi,mod] × 100

% RR ) [

(12)

are given in Table 5 for the rate model discussed in the above equations and for other models that were considered. Comparisons of experimental concentration-time profiles at different temperatures and H2 consumptiontime plots at different inlet concentrations of 2,4-DNT are shown in Figures 5, 6, 11, and 12. A good agreement between the model predictions and the experimental data was observed, indicating that the rate model represented in eqs 6-9 explains best the integral concentration-time data and also the H2 consumption profiles. Also, the % RR and the φmin values obtained for this model were lower when compared to other models (see Table 5, model 4). The values reported for all of the models under consideration are in 95% confidence limits. The temperature dependence of the rate and equilibrium parameters is shown in Figures 13 and 14, from which the activation energy values for different hydrogenation steps and the heats of adsorp-

Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999 913 Table 5. Best-Fit Parameters and Optimization Results for Different Modelsa rate constants, (m3/kg) (m3/kmol) s-1 T, K model 1 model 2 model 3 model 4

333 348 363 333 348 363 333 348 363 333 348 363

k1 ×

103

0.53 1.17 2.86 0.363 0.843 0.906 0.716 2.42 4.52 2.59 4.75 6.45

k2 ×

104

-0.55 3.19 4.37 0.68 1.36 2.17 2.11 3.82 5.81 3.44 6.76 9.23

k3 ×

104

09.08 31.80 48.33 1.11 2.48 3.56 3.67 5.75 9.41 4.27 6.70 10.80

k4 ×

equilibrium constants, m3/kmol 104

2.03 2.42 4.02 1.55 -1.39 3.16 2.25 5.08 6.92 2.34 4.85 6.37

KA

KB

KE

φmin × 104

% RR

-11.47 0.22 0.214 0.382 -4.67 2.13 2.33 1.26 0.831 0.963 0.620 0.337

3.7 0.562 0.427 1.11 0.837 1.27 10.91 6.42 2.58 11.06 9.83 7.23

1.41 1.00 0.667 1.75 4.9 2.46 1.53 1.35 0.855 0.98 0.86 0.68

69.5 2.87 2.65 4.84 2.78 3.43 6.48 3.22 3.35 6.54 1.49 2.02

(30 (22 (15 (18 (24 (26 (14 (20 (10 (5 (8 (5

a For model 1 r ) wk B A*/(1 + K A* + K B + K E )2 and similar equations derived as explained in the kinetic model section. For 1 1 l A B l E l model 2 r1 ) wk1Bl(A*)0.5/(1 + (KAA*)0.5)(1 + KBBl + KEEl). For model 3 r1 ) wk1BlA*/(1 + KAA*)(1 + KBBl + KEEl). For model 4 r1 ) wk1Bl(A*)0.5/(1 + (KAA*)0.5 + KBBl + KEEl)2. For each case a semibatch reactor model was developed and solved as shown in eqs 6-12.

Figure 13. ln k vs 1/T. Figure 11. Concentration-time profile at 333 K. Reaction conditions: concentration of 2,4-DNT, 0.412 kmol/m3; catalyst loading, 20 kg/m3; temperature, 333 K; PH2, 2.6 MPa; solvent, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3.

Figure 12. Concentration-time profile at 363 K. Reaction conditions: concentration of 2,4-DNT, 0.412 kmol/m3; catalyst loading, 20 kg/m3; temperature, 363 K; PH2, 2.6 MPa; solvent, MeOH; agitation speed, 18 Hz; reaction volume, 1 × 10-4 m3.

tion values were evaluated. The activation energies and the heats of chemisorption of hydrogen, 2,4-DNT, and 4A2NT are shown in Table 6. The activation energy values for different steps of hydrogenation were in the range of 30-34 kJ/mol. Also, it was observed that the ratio of KB/KE is always >1 (see Table 5, model 4),

Figure 14. ln K vs 1/T.

indicating the strong adsorption of 2,4-DNT when compared with 4A2NT. This resulted in a larger value of heat of chemisorption for 2,4-DNT (see Table 6). These results are consistent with the observed trends in concentration-time profiles shown in Figures 5, 11, and 12, wherein depletion of 4A2NT starts after complete consumption of 2,4-DNT. The agreement between the model predictions and experimental results was excellent, indicating the applicability of the kinetic

914

Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999

Table 6. Activation Energies (kJ/mol) and Heats of Chemisorption (kJ/mol) activation energy for step 1, ∆E1 activation energy for step 2, ∆E2 activation energy for step 3, ∆E3 activation energy for step 4, ∆E4 heat of chemisorption of hydrogen, -∆HA heat of chemisorption of 2,4-DNT, -∆HB heat of chemisorption of 4A2NT, -∆HE

30.69 33.22 31.02 33.75 -34.07 -14.14 -12.19

model over the temperature range investigated. Thus, a single-site model (adsorption of hydrogen and organic species on the same Ni site) seems to represent the experimental data more satisfactorily when compared to other rate models. However, for hydrogenation of aromatic nitro compounds using Pd catalysts, several studies4-12 suggest a dual-site mechanism with a few exceptions.11,34 Pd is known35-37 to interact strongly with H2, leading to the formation Pd hydride species (both mono- and dihydrides are formed on Pd surfaces) even at room temperatures. Such species are not formed on Ni surfaces.38 This may be a plausible reason for the observed difference in the rate models that are reported for hydrogenation of 2,4-DNT using Pd and Ni catalysts. Although, our experimental results suggest a single-site surface rate model, knowledge on the stoichiometric interactions between the catalyst and the reactants/ products would be very useful to address this difference between Pd and Ni catalysts in a more detailed manner. Conclusions The kinetics of hydrogenation of 2,4-DNT was studied using a 10% Ni /HY powdered catalyst in a temperature range of 333-363 K. The performance of this catalyst was comparable to that of reported Pd catalysts in terms of 2,4-DNT conversion and 2,4-TDA selectivity. The high activity of 10% Ni/HY was due to the formation of small Ni metal crystallites with an average metal particle size of 120 Å when compared to other Ni supported catalysts. Quantum chemical calculations confirmed that the reactant and the intermediate products formed are easily accessible to the Ni sites present inside the zeolite pores because the molecular dimensions are much less than the pore opening of the zeolite support. The effects of H2 pressure, concentration of 2,4-DNT, catalyst loading, and agitation speed on the initial rates were discussed. The rate was found to be first order tending to zero order with respect to 2,4-DNT concentration and H2 pressure, linear with respect to catalyst loading, and independent of agitation speed in the conditions chosen for the present study. Quantitative analyses of the initial rate data at different sets of conditions indicated that the rate data were devoid of external and intraparticle mass-transfer resistances. Several rate expressions were derived based on Langmuir-Hinshelwood type rate mechanisms. The rate model derived by assuming that the reaction between competitively adsorbed organic species with dissociatively adsorbed hydrogen as the rate-limiting step was found to represent the experimental data best. The rate and equilibrium parameters were evaluated, and from the temperature dependency of these values, the activation energies for different steps of hydrogenation and the heats of adsorption of hydrogen, 2,4-DNT, and 4A2NT were determined. A semibatch reactor model has been developed, the predictions of which agreed well with the experimental observations. Thus, this study confirms

the potential applicability of supported Ni catalysts as alternatives for Pd-based catalysts for hydrogenation of 2,4-DNT. Acknowledgment R.V.M. thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing him with a research fellowship. Nomenclature ap ) external surface area of the catalyst particle, m2/m3 A* ) concentration of H2 in equilibrium with the liquid, kmol/m3 Bl ) concentration of 2,4-DNT, kmol/m3 B10 ) initial concentration of 2,4-DNT, kmol/m3 Cl ) concentration of 2A4NT, kmol/m3 DM ) molecular diffusivity, m2/s De ) effective diffusivity ) DM/τ, m2/s El ) concentration of 4A2NT, kmol/m3 HA ) Henry’s constant, kmol/m3/atm klaB ) gas-liquid mass-transfer coefficient, s-1 ks ) liquid-solid mass-transfer coefficient, ms-1 k1-k4 ) rate constants (kg/m3)-1 (kmol/m3)-1 s-1 KA, KB, KE ) equilibrium constants, m3/kmol r1-r4 ) rates of reaction, kmol/m3/s RA ) overall rate of hydrogenation, kmol/m3/s Pl ) concentration of 2,4-TDA, kmol/m3 Yi ) concentration of species i, kmol/m3 w ) catalyst loading, kg/m3 Greek Symbols R1 ) ratio of the rate of reaction to the maximum rate of gas-liquid mass transfer R2 ) ratio of the rate of reaction to the maximum rate of liquid-solid mass transfer  ) porosity of the catalyst particle φexp ) ratio of the rate of reaction to the maximum rate of intraparticle diffusion φmin ) objective funtion or optimization criterion defined in eq 11 Fp ) density of the catalyst particle, kg/m3 τ ) tortuosity of the catalyst particle

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Received for review July 2, 1998 Revised manuscript received December 3, 1998 Accepted December 11, 1998 IE980423Y