Article pubs.acs.org/EF
Reaction Kinetics of Vanillin Hydrogenation in Aqueous Solutions Using a Ru/C Catalyst Ankush B. Bindwal and Prakash D. Vaidya* Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Road, Matunga, Mumbai 400019, India ABSTRACT: Aqueous phase processing (APP) of bio-oil represents a candidate technique for the production of renewable hydrogen (H2) in a fast pyrolysis-based biorefinery. Low temperature hydrogenation of the water-soluble portion of bio-oil is a useful intermediate step of APP. In this work, the reaction kinetics of mild aqueous-phase hydrogenation of vanillin (VL), a model compound of the bio-oil aqueous fraction, was studied using Ru/C catalyst. The investigated aromatic aldehyde was converted to vanillyl alcohol (VA) and creosol (CR). Catalytic runs were performed in a slurry reactor in the 318−338 K range using different catalyst loadings (0.2−0.8 kg/m3), initial VL concentrations (32.9 to 65.7 mM) and H2 partial pressures (0.69− 2.07 MPa). The initial rates varied linearly with respect to both initial VL concentration and H2 partial pressure. Four Langmuir− Hinshelwood−Hougen−Watson (LHHW) type kinetic models were proposed assuming that surface reaction was ratedetermining. These models were simplified to a second order reaction model based on bulk concentration of the reacting species. From the temperature dependence of the second order reaction rate constant, the activation energy was found to be 41.2 kJ/mol.
1. INTRODUCTION Fast pyrolysis technology effectively converts biomass into biooil (or pyrolysis oil), which is a liquid rich in oxygenated hydrocarbons. This intermediate renewable energy carrier can be transformed into hydrogen (H2) or alkanes (C1−C6) by a three-step aqueous phase processing (APP) technique.1 In the first stage, bio-oil is extracted with water and separated into a water-insoluble portion (or pyrolytic lignin) and a watersoluble portion. Pyrolytic lignin, which has high energy content, can be upgraded to fuels by hydrotreatment. The water-soluble portion is a complex mixture of several constituents such as alcohols, sugars, aldehydes, ketones, acids, guaiacols, syringols, furans, furfurals and water. In the second stage, this portion is hydrogenated at low temperature. In this way, thermally unstable compounds (e.g., aldehydes, acids, and sugars) that decompose at high temperature and cause catalyst coking during subsequent processing can be converted into stable compounds (e.g., alcohols and diols). Thus, the water-soluble portion of bio-oil with its increased intrinsic hydrogen content becomes suitable for further treatment. Finally, a third stage involves selective conversion to H2 (by aqueous-phase reforming) or alkanes (by aqueous-phase dehydration/hydrogenation). Earlier, Cortright et al.2 and Huber et al.3 investigated catalytic processing of biomass-derived oxygenates in liquid water for the production of H2 and alkanes, respectively. There are two comprehensive works on the mild hydrogenation of bio-oil available.1,4 Vispute and Huber1 reported the formation of C2−C4 diols and sorbitol during hydrogenation of the bio-oil aqueous fraction over Ru/C in the 398− 448 K range. In another study, they reported the conversion of aldehydes, sugars, and unsaturated aromatic compounds in biooil to alcohols, sugar alcohols, and saturated aromatics (T = 398 K, PH2 = 5.2 to 10 MPa).4 As evident from these works, Ru/C catalyst facilitated the intermediate hydrogenation step of the bio-oil transformation process. Besides, Ru/C has high efficacy © 2014 American Chemical Society
for the conversion of oxygen-containing model compounds of bio-oil such as hydroxyacetone, hydroxyacetaldehyde, guaiacol, 2-furanone, and levoglucosan, too.5,6 Such model compound studies are useful as they highlight structure−reactivity of biooil components. They provide a systematic approach for determining how best the targeted C−O bonds can be selectively hydrogenated while minimizing the cleavage of C− C and C−O bonds (which results in the formation of undesired methane). In this work, the aromatic aldehyde vanillin (VL) was selected as a model compound. VL is an important constituent of bio-oil.7 Literature data on VL hydrogenation suggests that, in earlier works, Mahfud et al.,8 Huang et al.,9 and Busetto et al.10 used homogeneous Ru-based catalysts. The application of heterogeneous Ru/C should be advantageous; however, such a study is still missing. In this work, we investigated reaction mechanism and kinetics using Ru/C in a slurry reactor in the 318−338 K range using different catalyst loadings (0.2−0.8 kg/ m3), initial VL concentrations (32.9 to 65.7 mM) and H2 partial pressures (0.69−2.07 MPa).
2. EXPERIMENTAL SECTION 2.1. Materials. Vanillin (purity 99%) was purchased from S. D. Fine Chemicals Pvt. Ltd., Mumbai. Vanillyl alcohol (4-hydroxy-3methoxybenzyl alcohol, purity 98%) and creosol (2-methoxy-4methylphenol, purity 98%) were acquired from Sigma-Aldrich Pvt. Ltd., Mumbai. Hydrogen (H2) and nitrogen (N2) cylinders (purity 99.9%) were purchased from Inox Air Products, Mumbai. A commercial 5% Ru/C catalyst was supplied by Arora-Matthey Ltd., Kolkata, India. 2.2. Methods and Analysis. The experimental setup, procedure and analytical techniques were comprehensively described in a previous work.6 All experiments were conducted in a 0.1 dm3 SS316 high pressure reactor (Parr Instruments Company, Illinois, Received: March 2, 2014 Revised: April 16, 2014 Published: April 16, 2014 3357
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U.S.A.). High pressure liquid chromatography (HPLC) and LC-MS techniques were used to detect vanillyl alcohol or VA (formed by VL hydrogenation), creosol or CR (formed via hydrodeoxygenation of VA) and VL. The reproducibility of experiments was checked and the error in all experimental measurements was less than 3%. Using a method earlier reported by Lee et al.,11 a carbon balance close to 100% was established. Several useful properties of Ru/C, found by using BET, particle size analysis and H2 chemisorption techniques, are represented in Table 1. The results obtained by using SEM and XRD techniques are shown in Figures 1 and 2, respectively. A detailed description on catalyst characterization can be found elsewhere.6
Table 1. Certain Features of the Fresh Ru/C Catalyst property
value
BET specific surface areaa micropore vol.a avg. pore diam.a mean particle size (dp)b metal dispersion (D)c crystallite sizec
793 m2/g 0.4 cm3/g 2.2 nm 50 μm 2.74% 48.2 nm
a
Figure 2. XRD images of fresh and spent Ru/C catalyst.
Determined by N2 adsorption−desorption isotherms at 77.5 K. Determined by particle size analyzer. c Determined by H 2 chemisorption, D = ((2 × H2 adsorption)/reduced Ru) × 100, D = ((number of surface atoms)/(total number of atoms of specified catalytic phase)) × 100. b
From plots of CVL vs t, initial rates (r0) of disappearance of VL were determined using regression technique. Knowing the value for metal dispersion (D), the turnover frequency (TOF) was calculated as follows:
TOF =
⎛ r0MRu ⎞ ⎜ ⎟ ⎝ D ⎠
(1)
In the kinetically controlled regime, the initial TOF is independent of the mass transfer coefficients kL and kSL, and hence, it should not depend on the speed of agitation. This effect was experimentally studied by varying the stirring speed in the range, 300 to 1400 rpm. It was found that there is practically no change in the initial TOF values above a speed of 900 rpm. In all further experiments, a stirring speed of 1200 rpm was used. To investigate any possible influence of intraparticle diffusion on the reaction rates, the Weisz and Prater criterion was used.13 The parameter ηϕ2i was calculated by using the relation:
ηϕi2 =
r0ωL2 CiDei
where
i = H 2 , VL
(2)
Here, r0, ω, and L denote initial reaction rate, catalyst loading and the characteristic length of the spherical catalyst particle (L = dp/6, where dP = 50 μm). The value of CH2 (kmol/m3) was estimated by using the method of Pintar et al.14 whereas the liquid-phase effective diffusivities of VL and H2 were estimated using the Wilke−Chang equation.15 It was found that the value of ηϕ2i for both the reactants was much less than unity under the conditions used for this study (see Table 2). Therefore, the intraparticle diffusion resistance was neglected. Thus, it is clear that the investigated reaction system belongs to the kinetically controlled reaction regime systems. Figure 1. SEM images of the fresh Ru/C catalyst.
3. RESULTS AND DISCUSSION As mentioned in previous works,8−10 VL hydrogenation selectively proceeds through the formation of VA. However, the product distribution depends on the relative rates of cleavage of C−O and C−C bonds. Mahfud et al.8 reported the formation of CR via hydrodeoxygenation of VA (see reaction scheme in Figure 3). Huang et al.9 found that guaiacol is formed via C−C bond cleavage. A typical C vs t profile observed in this study at T = 328 K, CVL,0 = 65.7 mM, PH2 = 1.38 MPa, and ω = 0.5 kg/m3 is shown in Figure 4. In this
2.3. Mass Transfer Considerations. Heterogeneous catalytic hydrogenation is a three-phase reaction system that involves external and internal mass transfer processes, which can influence the rates of reaction.12 To determine the kinetic parameters, it is essential to ensure the absence of mass transfer limitations. The resistance to mass transfer of H2 on the gas-side was deemed negligible, due to its high diffusivity in the gas phase and low solubility in the liquid. The extent of the liquid-phase and liquid−solid mass transfer resistances is determined by the level of turbulence in the liquid phase. 3358
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Table 2. Parameters Used to Validate the Absence of Internal Diffusion and Values of the Weisz−Prater Modulus at 338 K param.
values
ω (kg/m3) CVL,0 (mM) CH2 (mM)
0.5 65.7 5.0
0.5 65.7 15.1
0.8 65.7 5.0
r × 103 (kmol/(kg catalyst min)) L (m) De,VL × 109 (m2/s) De,H2 × 108 (m2/s)
6.6 8.3 × 10−6 1.9 1.2
19.5 8.3 × 10−6 1.9 1.2
6.5 8.3 × 10−6 1.9 1.2
ηϕ2VL ηϕ2H2
3.0 × 10−5 6.3 × 10−5
8.9 × 10−5 6.2 × 10−5
4.8 × 10−5 1.0 × 10−4
are also shown in Table 3. Mahfud et al.8 reported TOF value of 16.9 mol/(mol Ru h)) using homogeneous RuCl3/TPPTS catalyst at T = 333 K; contrarily, our initial TOF value at 338 K was much higher (1458 1/h). The distribution of the reaction products at different temperatures is shown in Table 4. The values of PH2 and CVL,0 were varied over the ranges 0.69 to 2.07 MPa and 32.9 to 65.7 mM, respectively. A constant catalyst loading (ω = 0.5 kg/ m3) was used during these runs. The rise in temperature facilitated the formation of VA and CR. For example, VA concentration after 1 h increased from 19.1 (at 318 K) to 31.0 mM (at 338 K) at PH2= 0.69 MPa and CVL,0 = 32.9 mM. Similarly, CR concentration increased from 4.5 (at 318 K) to 6.3 mM (at 338 K) at PH2= 2.07 MPa and CVL,0 = 65.7 mM after 1 h. Interestingly, the formation of trace amount of an unidentified product was observed at PH2 = 2.07 MPa and T = 338 K. 3.2. Effect of Catalyst Loading. The effect of catalyst loading on the initial TOF values was investigated at 318, 328, and 338 K in the 0.2 to 0.8 kg/m3 range. The values of PH2 and CVL,0 were 0.69 MPa and 65.7 mM, correspondingly. These results are represented in Table 5. With a 4-fold increase in catalyst loading, the initial rate increased from 13 × 10−4 to 52 × 10−4 kmol/(m3 min) at 338 K. Thus, it can be concluded that the reaction rate exhibits first order dependence on catalyst concentration. Since the initial TOF values for varying catalyst concentrations are alike, it is clear that the reaction rates are not influenced by the rates of mass transport. 3.3. Effect of H2 Partial Pressure. The effect of PH2 on the initial TOF values was studied in the 0.69 to 2.07 MPa range at 318, 328, and 338 K. While CVL,0 was kept constant at 65.7 mM, a value of ω = 0.5 kg/m3 was used. The results are shown in Figure 5. A 3-fold increase in the initial TOF was observed at T = 338 K when PH2 was increased from 0.69 to 2.07 MPa. Clearly, the dependence of initial TOF on PH2 was linear. This behavior suggests that H2 is weakly adsorbed on the catalyst surface. A rise in H2 partial pressure facilitated the formation of CR. Thus, VA concentration after 1 h decreased from 63.5 to 58.2 mM whereas CR concentration increased from 1.7 to 6.3 mM when PH2 was increased 3-fold from 0.69 to 2.07 MPa at T = 338 K, CVL,0 = 65.7 mM, and ω = 0.5 kg/m3 (see Table 4). This behavior may be attributed to the hydrodeoxygenation of VA to CR. 3.4. Effect of Initial VL Concentration. The effect of CVL,0 on the initial TOF values was studied at 318, 328, and 338 K in the 32.9 to 65.7 mM range. The results are shown in Figure 6. Certainly, the reaction is of the first order with respect to VL. A
Figure 3. Reaction scheme of vanillin hydrogenation proposed by Mahfud et al.8
section, the effects of reaction variables on VL conversion and product distribution are discussed.
Figure 4. Typical “concentration vs time” profile at T = 328 K, CVL,0 = 65.7 mM, PH2 = 1.38 MPa, and ω = 0.5 kg/m3.
3.1. Effect of Temperature. VL conversion (X) vs temperature data at 318, 328, and 338 K is represented in Table 3. In all experiments, the reaction variables used were as follows: PH2 = 0.69 MPa, CVL,0 = 65.7 mM, and ω = 0.5 kg/m3. As expected, the increase in reaction temperature resulted in increased catalytic activity. For example, VL conversion after 1 h increased from 93 to 100% when T was raised from 318 to 338 K. Clearly, heterogeneous Ru/C has high activity for VL hydrogenation. For the sake of comparison, some useful results of previous works8−10 using homogeneous Ru-based catalysts 3359
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Table 3. Temperature and Fractional Conversion (X) Data for VL Hydrogenation and Comparison with Previous Works8−10 catalyst
temperature and fractional conversion (X) data
reaction conditions
heterogeneous 5% Ru/C
PH2: 0.69 MPa
t=1h T = 318 K T = 328 K T = 338 K t=3h
X = 0.93 X = 0.96 X = 1.00
homogeneous RuCl3/TPPTS
CVL,0: 65.7 mM solvent: water ω: 0.5 kg/m3 PH2: 4.5 MPa
T = 318 K T = 333 K T = 343 K t=6h
X = 0.08 X = 0.79 X = 0.84
homogeneous RuCl2(PPh3)3
CVL,0: 9.33 mM solvent: water−n-hexane VL/catalyst ratio: 65 mol/mol PH2: 3.3 MPa CVL,0: 100 mM solvent: ethyl acetate VL/catalyst ratio: 32 mol/mol
T = 333 T = 343 T = 353 T = 363 t=1h
X X X X
homogeneous Ru-based Shvo
PH2: 1.0 MPa CVL,0: 6.58 mM solvent: toluene VL/catalyst ratio: 200 mol/mol
K K K K
T = 363 K T = 418 K
= = = =
products identified
reference
VA and CR
this work
VA and CR
Mahfud et al.8
VA, CR, and guaiacol
Huang et al.9
VA and CR
Busetto et al.10
0.66 0.72 0.90 0.91
X = 0.82 X = 0.98
Table 4. Product Distribution as a Function of Temperature, Initial VL Concentration (CVL,0), and PH2 (ω = 0.5 kg/m3) concn. (mM) at t = 1 h CVL,0 (mM) 32.9
P H2 (MPa) 0.69
42.7
0.69
52.6
0.69
65.7
0.69
65.7
1.38
65.7
2.07
T (K) 318 328 338 318 328 338 318 328 338 318 328 338 318 328 338 318 328 338
fractional VL conversion (X) 0.59 0.93 0.99 0.67 0.93 0.96 0.80 0.94 0.99 0.93 0.96 1.00 0.94 0.97 1.00 0.95 0.98 1.00
VA
CR
19.1 30.1 31.0 27.6 38.8 39.1 40.3 47.6 49.9 59.4 61.0 63.5 58.9 60.0 59.0 56.7 57.7 58.2
0.0 0.6 1.0 0.7 1.0 1.2 1.2 1.5 1.6 1.6 1.7 1.7 2.7 3.8 5.6 4.5 5.1 6.3
Figure 5. Effect of H2 partial pressure (PH2) on the initial TOF values at 318, 328, and 338 K (CVL,0 = 65.7 mM and ω = 0.5 kg/m3).
to 1.7 mM correspondingly at PH2 = 0.69 MPa and ω = 0.5 kg/ m3. 3.5. Reaction Mechanism and Kinetic Modeling. Langmuir−Hinshelwood−Hougen−Watson (LHHW) kinetics was used for modeling initial rates of VL disappearance. The hydrogenation reaction of VL to VA was represented as
Table 5. Effect of Catalyst Loading (ω) on Initial VL Disappearance Rates (r0) and TOF Values (PH2 = 0.69 MPa, CVL,0 = 65.7 mM) r0 × 104 (kmol/(m3 min)) ω (kg/m ) 0.2 0.5 0.8 3
318 K 4.7 12.0 19.0
328 K 8.0 20.0 31.9
338 K 13.0 33.0 52.0
TOF (1/min) 318 K 8.6 8.9 8.8
328 K 14.8 14.8 14.7
338 K 24.0 24.3 24.0
C8H8O3 + H 2 → C8H10O3
0 ΔH298K = −63.6 kJ/mol
(3)
From knowledge of the sequence of elementary steps for the chemisorption of H2 and the reactant,6 four kinetic models were developed assuming that surface reaction was ratedetermining. These models I to IV are listed in Table 6. It is seen from Figures 5 and 6 that the initial rates vary linearly with respect to both partial pressure of H2 and the
rise in the concentrations of VA and CR was observed with increasing values of CVL,0 (see Table 4). For example, when CVL,0 was increased from 32.9 to 65.7 mM at T = 328 K, VA and CR concentration increased from 30.1 to 61.0 mM and 0.6 3360
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order reaction model reasonably represented kinetic data. The temperature dependence of the reaction rate constant was represented as ⎛ −4.96 ⎞ ⎟ k = 5.8 × 107 exp⎜ ⎝ T ⎠
(5)
Figure 6. Dependence of the initial TOF on initial VL concentration at various temperatures (PH2 = 0.69 MPa and ω = 0.5 kg/m3).
Table 6. Plausible Kinetic Models (I to IV) for VL Hydrogenation model I
r0 =
II
III
IV
r0 =
r0 =
r0 =
initial-rate expression
model characteristics
ksK H2KVLC H2C VL
competitive adsorption of H2 and VL (dissociatively adsorbed H2)
(1 +
K H2C H2 + KVLC VL)3
ksK H2KVLC H2C VL 2
(1 + K H2C H2 + KVLC VL)
ksK H2KVLC H2C VL (1 +
K H2C H2 )2 (1 + KVLC VL) ksK H2KVLC H2C VL
(1 + K H2C H2)(1 + KVLC VL)
Figure 7. Parity plot for the second order reaction model comparing initial TOF values.
competitive adsorption of H2 and VL (molecularly adsorbed H2) noncompetitive adsorption of H2 and VL (dissociatively adsorbed H2)
The activation energy value was found to be equal to 41.2 kJ/ mol. VL concentrations predicted by the second order model were in good agreement with the experimental values (see Figure 8).
noncompetitive adsorption of H2 and VL (molecularly adsorbed H2)
initial concentration of VL. Clearly, such linear dependence can only be achieved if denominators of all model equations represented in Table 6 are equal to one. This implies that the adsorption of both H2 and VL on the catalyst sites is so weak that all models reduce to a simple second order reaction model based on bulk concentration of the reacting species. Hence, it is unnecessary to discriminate among models I to IV. The second order reaction model was represented by r = kC H2C VL
(4)
The second order reaction rate constant was estimated using least-squares regression. The results are represented in Table 7. A parity plot comparing the predicted and experimental values of the initial TOF is shown in Figure 7. Clearly, the second Table 7. Kinetic Parameters for the Second Order Reaction Model (with 95% Confidence Intervals) T (K)
k (m6/kmol kgcat min)
order with respect to H2
order with respect to VL
318 328 338
10.5 ± 0.5 13.7 ± 0.2 26.7 ± 0.1
1.0 ± 0.1 1.0 ± 0.1 1.0 ± 0.1
1.2 ± 0.2 1.0 ± 0.1 1.1 ± 0.1
Figure 8. Comparison of the experimental VL concentrations and model predictions (second order kinetics) at 318, 328, and 338 K (CVL,0 = 65.7 mM, PH2= 0.69 MPa, and ω = 0.5 kg/m3). 3361
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4. CONCLUSIONS In the present work, reaction kinetics of VL hydrogenation was studied in a slurry reactor using commercial Ru/C catalyst. VL was hydrogenated to VA and CR. It was found that in the range of temperatures, 318 to 338 K, H2 partial pressures, 0.69 to 2.07 MPa, initial VL concentrations, 32.9 to 65.7 mM and catalyst loading, 0.2 to 0.8 kg/m3, the investigated system belongs to the kinetically controlled reaction regime systems. The initial rates varied linearly with respect to both partial pressure of H2 and the initial concentration of VL, thereby suggesting that the adsorption of both the reactants on the catalyst sites was weak. Langmuir−Hinshelwood−Hougen−Watson (LHHW) kinetics was used for modeling the initial rates of VL disappearance. These kinetic models were simplified to a second order reaction model based on bulk concentration of the reacting species. From the temperature dependence of the second order reaction rate constant, the activation energy was found to be 41.2 kJ/ mol.
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REFERENCES
(1) Vispute, T. P.; Huber, G. W. Production of hydrogen, alkanes, and polyols by aqueous phase processing of wood-derived pyrolysis oils. Green Chem. 2009, 11, 1433−1445. (2) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002, 418, 964−967. (3) Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Renewable alkanes by aqueous-phase reforming of biomass-derived oxygenates. Angew. Chem., Int. Ed. 2004, 43, 1549−1551. (4) Vispute, T. P.; Zhang, H.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable chemical commodity feedstocks from integrated catalytic processing of pyrolysis oils. Science 2010, 330, 1222−1227. (5) Bindwal, A. B.; Bari, A. H.; Vaidya, P. D. Kinetics of low temperature aqueous-phase hydrogenation of model bio-oil compounds. Chem. Eng. J. 2012, 207−208, 725−733. (6) Bindwal, A. B.; Vaidya, P. D. Kinetics of aqueous-phase hydrogenation of levoglucosan over Ru/C catalyst. Ind. Eng. Chem. Res. 2013, 52, 17781−17789. (7) Wang, H.; Male, J.; Wang, Y. Recent advances in hydrotreating of pyrolysis bio-oil and its oxygen-containing model compounds. ACS Catal. 2013, 3, 1047−1070. (8) Mahfud, F. H.; Bussemaker, S.; Kooi, B. J.; Ten Brink, G. H.; Heeres, H. J. The application of water-soluble ruthenium catalysts for the hydrogenation of the dichloromethane soluble fraction of fast pyrolysis oil and related model compounds in a two phase aqueous− organic system. J. Mol. Catal. A: Chem. 2007, 277, 127−136. (9) Huang, F.; Li, W.; Lu, Q.; Zhu, X. Homogeneous catalytic hydrogenation of bio-oil and related model aldehydes with RuCl2(PPh3)3. Chem. Eng. Technol. 2010, 33, 2082−2088. (10) Busetto, L.; Fabbri, D.; Mazzoni, R.; Salmi, M.; Torri, C.; Zanotti, V. Application of the Shvo catalyst in homogeneous hydrogenation of bio-oil obtained from pyrolysis of white poplar: New mild upgrading conditions. Fuel 2011, 90, 1197−1207. (11) Lee, J.; Xu, Y.; Huber, G. W. High-throughput screening of monometallic catalysts for aqueous-phase hydrogenation of biomassderived oxygenates. Appl. Catal. B: Environ. 2013, 140−141, 98−107. (12) Doraiswamy, L. K.; Sharma, M. M. Heterogeneous Reactions: Analysis, Examples, and Reactor Design, Vol. 2; John Wiley and Sons: New York, 1984. (13) Fogler, H. S. Elements of Chemical Reaction Engineering; PrenticeHall of India: New Delhi, India, 2008. (14) Pintar, A.; Bercic, G.; Levec, J. Catalytic liquid-phase nitrite reduction: Kinetics and catalyst deactivation. AIChE J. 1998, 44, 2280−2292. (15) Wilke, C. R.; Chang, P. Correlation of diffusion coefficients in dilute solutions. AIChE J. 1955, 1, 264−270.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +91 22 33612014. Fax: +91 22 33611020. E-mail: pd.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Ankush B. Bindwal is grateful to University Grants Commission, New Delhi, for the financial assistance
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Article
NOMENCLATURE
CH2 = concentration of H2 in the liquid phase, kmol/m3 CR = creosol CVL = concentration of vanillin in the liquid phase, kmol/m3 CVL,0 = initial concentration of vanillin in the liquid phase, kmol/m3 dP = catalyst particle diameter, m D = metal dispersion, % Dei = effective diffusivity of species i in eq 2, m2/s k = second-order reaction rate constant ks = surface reaction rate constant kL = liquid-side mass transfer coefficient, m/s kSL = solid-liquid mass transfer coefficient, m/s KH2 = adsorption equilibrium constant for H2, m3/kmol KVL = adsorption equilibrium constant for VL, m3/kmol L = characteristic length of catalyst particle (=dP/6), m MRu = molecular weight of ruthenium, g/mol PH2 = H2 partial pressure, MPa r = observed rate of VL disappearance, kmol/(kgcat min) r0 = initial rate of VL disappearance, kmol/(kgcat min) t = time, h T = temperature, K TOF = turnover frequency, 1/min VA = vanillyl alcohol VL = vanillin
Greek Symbols
ω = catalyst loading, kg/m3 η = effectiveness factor ϕ = Thiele modulus ηϕ2i = observable modulus for species i in eq 2 3362
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