Article pubs.acs.org/IECR
Kinetics Study of Hydrogenation of Dimethyl Oxalate over Cu/SiO2 Catalyst Siming Li, Yue Wang, Jian Zhang, Shengping Wang, Yan Xu, Yujun Zhao,* and Xinbin Ma Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China S Supporting Information *
ABSTRACT: Gas-phase hydrogenation of dimethyl oxalate (DMO) on a copper-based catalyst is one of the crucial technologies in the production of ethylene glycol (EG) from syngas. Even though Cu/SiO2 catalyst is widely used in ester hydrogenation reactions, a kinetics study considering multiple active sites has not yet been reported. In this study, a series of experiments were carried out to investigate the heterogeneous catalytic reaction kinetics of the hydrogenation of DMO over Cu/SiO2 catalyst. Considering different situations of ester adsorption, H2 adsorption, and active sites, 34 possible kinetics models were proposed and screened to identify the one most appropriate to describe the hydrogenation of DMO over Cu/SiO2 catalyst. With the help of relevant thermodynamic theories and statistical evaluations, the optimal model was found to fit well to our experimental data. This model proved that the hydrogenation of DMO depends on the synergistic effect of two active sites, wherein hydrogen and the ester were adsorbed on two different active sites with dissociative states. The dissociative adsorption of the ester was found to be the rate-controlling step in the hydrogenation of DMO over Cu/SiO2 catalyst prepared by an ammonia-evaporation method.
1. INTRODUCTION
CH3OOCCOOCH3 + 4H 2
Ethylene glycol (EG) is widely used in the manufacture of polyester and other industrial products, such as antifreeze additives or brake fluids and solvents.1,2 Most EG in the market is produced by ethylene oxidation. In recent years, EG production from syngas has increasingly attracted interest because it is a way to reduce the heavy dependence on petroleum resources.3 This approach includes two steps: the coupling of CO with methanol to form dimethyl oxalate (DMO) and the subsequent hydrogenation of DMO to EG (eqs 1 and 2).4−6 The production of EG from syngas is economical and environmentally friendly because the CH3OH that is formed in the hydrogenation step can be returned to the DMO synthesis process. Both homogeneous and heterogeneous systems have been investigated with regard to the hydrogenation of DMO to EG with noble metal catalysts such as Ru7 and Ag.8 However, considering the cost of noble metal catalyst and the subsequent separation issue in homogeneous catalytic systems, the inexpensive heterogeneous catalytic systems, in particular copper-based catalyst for gas-phase reactions, have recently attracted great attention.9−12 Different supports for Cu-based catalysts, such as SiO2, Al2O3, ZnO, and La2O3, have been investigated in this reaction system;13,14 among these, Cu/ SiO2 catalyst shows the highest yield of EG in the hydrogenation of DMO or diethyl oxalate, resulting from the neutral properties of SiO2. Various synthesis methods, including ammonia evaporation (AE), ion exchange, sol−gel, deposition precipitation, and impregnation, have been reported for the preparation of silicasupported copper catalyst.15−17
→ HOCH 2CH 2OH + 2CH3OH
(2)
Our group has successfully developed a novel Cu/SiO2 catalyst with an AE method. It exhibits several advantages, namely, highly dispersed copper species, appropriate structure, and the desired ratio of Cu0 to Cu+. However, its hydrogenation mechanism and reaction kinetics have not yet been investigated. Great attention has been paid to the mechanisms of the hydrogenation of the CO bond on a Cu-based catalyst, but the roles of copper species with different valences are not yet clear. Controversial arguments can be easily found in the literature, and they can be summarized into two groups: single active site and synergy of Cu0−Cu+. In some reports in the literature, metallic copper is believed to be the active site.18−20 However, He et al.21 claimed that the active sites were Cu+ and Cu0 when the hydrogenation of a fatty acid ester occurs on a Cu-based catalyst. Similarly, Dandekar et al.22 investigated crotonaldehyde hydrogenation on a carbon-supported copper catalyst. They found that crotonaldehyde adsorbed on Cu+ sites with hydrogen that was spilled over from Cu0 sites. In addition to the ambiguity of the active sites, the adsorption species on the surface of a copper-based catalyst is also not yet clear. The adsorption of hydrogen on a Cu-based catalyst was also argued in two cases: dissociative adsorption and molecular adsorption.23,24 Similarly, ester adsorption is also a controversial issue. Some researchers claimed that the ester was molecularly adsorbed.24,25 However, the opinion that an ester could be dissociatively adsorbed on the surface of a catalyst was also proposed Received: November 2, 2014 Revised: January 12, 2015 Accepted: January 12, 2015
1 2CO + 2CH3OH + O2 → CH3OOCCOOCH3 + H 2O 2 (1) © XXXX American Chemical Society
A
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The catalyst sample was degassed at 573 K for 3 h before analysis. The specific surface area was found to be 380.4 m2/g, as calculated by the BET method using the absorption isotherms. 2.2. Catalyst Activity Tests. Gas-phase hydrogenation of DMO was carried out in a stainless-steel fixed-bed reactor with an internal diameter of 7 mm, and the data were analyzed under the assumption of an isothermal integral plug flow reactor (PFR). The catalyst pellets were crushed, sieved, and loaded into the reactor between two quartz-sand zones. The catalyst was reduced in pure H2 at 623 K for 4 h. After reduction, the catalyst was cooled down to the reaction temperature under a hydrogen atmosphere. The DMO solution (20 wt % in methanol) was injected continuously from the top of the reactor through a highpressure pump and was vaporized in the stream of H2 gas at the selected H2/DMO ratio. The reaction temperature was controlled with a variation of ±1 K. The intrinsic kinetics measurements were done under broad reaction conditions, with the reaction temperature varied from 453 to 483 K, the weight liquid hourly space velocity (LHSV) varied from 1.5 to 11 h−1, the reaction pressure varied between 1.0−3.0 MPa, and the ratio of H2/DMO varied from 20 to 70 mol/mol. The reaction products were analyzed using a gas chromatograph (Agilent Micro GC 6820) with an HP-INNOWAX capillary column (Hewlett−Packard Company, 30 m × 0.32 mm × 0.50 μm) equipped with a flame ionization detector (FID). Several samples were taken from under the same experimental conditions and analyzed by GC to ensure repeatability.
in some reports. By using microcosmic calorimetry, Santiago et al.26 verified that n-alkyl acetates were adsorbed on a catalyst via dissociative states. Ju et al.27 investigated the kinetics and mechanism of hydrogenation of butyl butyrate; on the basis of the D2 isotope studies, they believed that butyl butyrate was dissociatively adsorbed on the surface of a catalyst to produce C3H7CO and C4H9O fragments.27 On the basis of these findings, acyl adsorbed species and alkoxy adsorbed species might be formed after the dissociative absorption of the ester, which could be further hydrogenated to form the corresponding alcohol. Most kinetics studies on the hydrogenation of esters only consider a single active site. Although silica-supported Cu catalyst has been extensively investigated for the hydrogenation of DMO or diethyl oxalate to ethylene glycol, the kinetics study of this catalyst was only carried out by Xu et al.28 They considered a single active site and molecular adsorption of the ester on the surface of the catalyst. However, some research indicates that the high activity of a Cu-based catalyst is ascribed to the synergy between Cu0 and Cu+ in the DMO hydrogenation system. Yin et al.29 expressed that the synergetic effect between Cu0 and Cu+ was responsible for high hydrogenation activity. They also believed that Cu+ could work as either an electrophilic or a Lewis acid site to polarize the CO bond via the lone electron pair on oxygen so that the reactivity of the ester group in DMO is improved. Using in situ Fourier transform infrared spectroscopy, Hui et al.30 proved that the dissociative absorption of DMO occurs on a catalyst via the cleavage of the C−O bond. In our previous work on Cu/SiO2 catalyst in the production of ethanol via syngas, we also proposed that the Cu0 and Cu+ active sites play roles similar to those found by Yin et al.31 To provide guidelines for catalyst design aimed at better performance, it is necessary to investigate the kinetics of DMO hydrogenation while also considering the different situations of ester adsorption and active sites to reveal the reaction mechanism. In this work, we focus on the intrinsic kinetics of the hydrogenation of DMO on a silica-supported copper catalyst prepared by an AE method. A series of kinetics model equations derived from different assumptions are proposed and evaluated, and the “synergy mechanism” kinetics model for the hydrogenation of DMO is established.
3. RESULTS AND DISCUSSION 3.1. Elimination of the External and Internal Diffusion. The criteria for no external diffusion−limited reaction relies on the independence of the conversion on the gas linear velocity at any space velocity.32,33 Herein, we measured the conversion of DMO at various gas linear velocities at a fixed LHSV, and the results are plotted in Figure S1. When the gas linear velocity exceeds 2.63 × 10−3 m/s, the DMO conversion is nearly the same, which implies that the effect of external diffusion is negligible. Therefore, a gas linear velocity larger than 2.63 × 10−3 m/s was later used in the kinetics experiments. To prevent further falling of the catalytic reaction in an internal diffusion regime, the conversion of DMO was measured with catalyst of three different particle-size ranges (20−40, 40−60, and 60−80 mesh) and was found to almost the same, as shown in Figure S2. This implies that the effect of internal diffusion is also negligible.33 Catalyst with a diameter of 0.25−0.35 mm (i.e., 40− 60 mesh) was later used in the kinetics studies. 3.2. Effect of Reaction Conditions. According to the gas kinetic theory, the possibility of the simultaneous hydrogenation of two CO functional groups is very low. Therefore, the hydrogenation of DMO is believed to involve two serial reactions: the hydrogenation of DMO to form methyl glycolate (MG) and the subsequent hydrogenation of MG to produce EG, represented as
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. Cu/ SiO2 catalyst was prepared by an AE method, described as follows. About 15.4 g of Cu(NO3)2·3H2O was dissolved in deionized water, and 52 mL of aqueous ammonia solution (25 wt %) was then added at room temperature. Then, 45 mL of silica solution (30 wt %) was added to the copper−ammonia complex solution and stirred for 4 h. The initial pH of the suspension was 11−12. The suspension was heated to 353 K to evaporate the ammonia, allowing the copper species to deposit onto the silica. This process was terminated when the pH decreased to 6−7. The precipitate was then separated by filtration, washed with deionized water three times, and dried at 393 K for 4 h. The catalyst was calcined at 673 K for 4 h, pressed, crushed, and sieved to desirable size. The actual load of copper was found to be ∼18.5 wt %, as determined by inductively coupled plasma optical-emission spectroscopy (ICP-OES). Briefly, samples were dissolved in a mixture of HF and HBO3 and then analyzed by ICP. The textural properties of the prepared catalyst were measured by the N2 adsorption method using a Micromeritics Tristar II 3000 analyzer instrument at the boiling temperature of liquid nitrogen.
CH3OOCCOOCH3 + 2H 2 → CH3OOCCH 2OH + CH3OH
(3)
CH3OOCCH 2OH + 2H 2 → HOCH 2CH 2OH + CH3OH (4)
MG and EG are the two main products, and their overall selectivity (EG and MG) is larger than 95%. Figure 1a shows catalyst performance at different LHSVs. For a given reaction B
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Figure 1. Effect of reaction conditions on reaction activity. (a) Effect of liquid hourly space velocity (LHSV), T = 473 K, P = 2.5 MPa, H2/DMO = 80 mol/mol. (b) Effect of temperature, P = 2.5 MPa, H2/DMO = 80 mol/mol, LHSV = 2.5 h−1. (c) Effect of pressure, T = 473 K, H2/DMO = 80:1, LHSV = 2.5 h−1. (d) Effect of the molar ratio of H2/DMO, T = 473 K, P = 2.5 MPa, LHSV = 2.5 h−1. XDMO stands for percent conversion of DMO, and SEG and SMG stand for selectivity for EG and MG (represented as percent of total product), respectively.
catalyst as nondissociative molecules, the hydrogenation of the ester occurs with the addition of a hydrogen to the carbonyl group and the drop-off of an alkoxy group.27 For a diester such as DMO, its hydrogenation involves two consecutive reactions: DMO reacts with H2 to produce MG, which is further hydrogenated to become EG. When an ester follows the molecular adsorption model, the pathway below is suggested for the hydrogenation of DMO (eqs 5−8): hydrogen first attacks one of the two carbonyls of DMO to form intermediate product A, which is unstable and quickly reacts with another hydrogen molecule to form MG and methanol (ME). In a similar way, the hydrogen molecules continue their reaction with the left carbonyl group on MG to produce EG and methanol (through the formation of intermediate product B).
temperature (473 K), pressure (2.5 MPa), and H2/DMO ratio (80 mol/mol), the selectivity of EG declines when LHSV is larger than 4 h−1, and the selectivity of MG starts increasing rapidly. This suggests that the formation of EG is easier than that of MG with longer residential time. The effect of reaction temperature over the range of 453 to 493 K on catalyst performance was also investigated. As shown in Figure 1b, the EG selectivity increases quickly when raising the reaction temperature, and high temperature seems beneficial for the production of EG. However, if the reaction temperature is too high (e.g., >473 K), further hydrogenation of EG occurs, which leads to the formation of ethanol. The product distribution of DMO hydrogenation is also sensitive to reactor pressure. Figure 1c shows that high pressure benefits the formation of EG, resulting from its common promotion effect on hydrogenation reactions. However, it has almost no influence on DMO conversion because DMO has been almost 100% converted under the given reaction conditions. A high molar ratio of H2/DMO is also favorable for the hydrogenation of DMO. As shown in Figure 1d, both DMO conversion and EG selectivity increase with H2/DMO ratio until it reaches 80, at which point both DMO conversion and EG selectivity are nearly 100%. Because the product distribution for the hydrogenation of DMO is heavily dependent on the actual reaction conditions, the appropriate conditions were chosen for later intrinsic kinetics studies (T = 453−483 K, P = 1.0−3.0 MPa, LHSV = 1.5−11 h−1, H2/DMO = 20−70 mol/mol). 3.3. Reaction Pathway and Kinetics Modeling. In this heterogeneous reaction system, H2 molecules could adsorb on the active sites of the catalyst in either dissociative or nondissociative states. Controversial arguments exist with regard to the adsorption of an ester on a copper-based catalyst. Some researchers believe that ester molecules follow a molecular adsorption model, whereas others treat them as following a dissociation adsorption model.23,26,27,30 When an ester is adsorbed on a
If DMO follows the dissociative adsorption model, the hydrogenation pathway is very different (as shown in eqs 9−13): the C
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active sites adsorb one dissociative ester group of DMO to form methoxyl and acyl species M, and the acyl species M is then gradually hydrogenated to produce MG. MG then splits into acyl species N and methoxyl, and the former (i.e., acyl species N) reacts with hydrogen to form EG. The methoxyl groups produced during the dissociative adsorption of DMO or MG also react with hydrogen to produce ME.
RSS =
acyl species M
methoxyl
To obtain appropriate kinetics model, all 34 models were examined with the same criterion: (i) The reaction rate constant and the adsorption equilibrium constant must be positive numbers; if one or some of them are found to be negative, that particular model is excluded. (ii) The reaction rate constant and adsorption equilibrium constant should follow the Arrhenius equation and Van’t Hoff equation, respectively. (iii) The model should have a sufficiently low residual sum of squares between the experimental and calculated rates. After fitting all 34 kinetics models with the experimental data, models 2, 3, 5−7, 9, 10, 12−14, 17, 18, 20−22, 24−30, 33, and 34 were found to have negative parameters and/or high RSSmin and were thus excluded. The remaining models were plotted in Figure 2.
(9)
Cu/SiO2
CH3OOCCO + 1.5H 2 ⎯⎯⎯⎯⎯⎯⎯⎯→ CH3OOCCH 2OH MG
(10)
Cu/SiO2
CH3OOCCH 2OH ⎯⎯⎯⎯⎯⎯⎯⎯→ HOCH 2CO + CH3O acyl species N
methoxyl
(14)
=1
Cu/SiO2
CH3OOCCOOCH3 ⎯⎯⎯⎯⎯⎯⎯⎯→ CH3OOCCO + CH3O
∑ (ri ,exp − ri ,cal)2
(11)
Cu/SiO2
HOCH 2CO + 1.5H 2 ⎯⎯⎯⎯⎯⎯⎯⎯→ HOH 2CCH 2OH EG
(12)
Cu/SiO2
CH3O + 0.5H 2 ⎯⎯⎯⎯⎯⎯⎯⎯→ CH3OH ME
(13)
Because mechanisms having gaseous reactants reacting with adsorbed reactants are not believed to occur for ester hydrogenation in most studies, we assume that both DMO and hydrogen are adsorbed on the surface of the catalyst. Because the kinetics of many other ester hydrogenation systems have been well described by Langmuir−Hinshelwood (LH) and HougenWatson (HW) models,22,24,26,27 they are also used here to establish the kinetics models of DMO hydrogenation. As previously mentioned, the hydrogenation of DMO involves two consecutive reactions, each reaction is composed of a series of steps: (i) reactants are adsorbed on the surface of the copperbased catalyst with or without dissociation, (ii) the surface reaction occurs between adsorbed reactants on a single adsorption site or on two different adsorption sites, and (iii) products are desorbed from the catalyst surface. The slowest step among all of these controls the overall reaction rate. We explored six different possible combinations of absorption on active sites and the various reaction pathways mentioned above: (1) Molecular adsorption of the ester and dissociative adsorption of hydrogen occur on the same active site. (2) Molecular adsorption occurs for both the ester and hydrogen on the same active site. (3) Dissociative adsorption occurs for both the ester and hydrogen on the same active site. (4) Molecular adsorption of the ester and dissociative adsorption of hydrogen occur on different active sites. (5) Molecular adsorption occurs for both the ester and hydrogen on different active sites. (6) Dissociative adsorption occurs for both the ester and hydrogen on different active sites. To establish the kinetics model, we also assume the following: (1) All adsorption sites are equivalent and the activation energy of adsorption and desorption is independent of the surface coverage. (2) The rate controlling step is from involved surface reaction, adsorption of reactants or desorption of products. A total of 34 possible kinetics models were derived and are listed in Table S1. 3.4. Parameter Estimations and Model Discrimination. The kinetics parameters (rate constants and adsorption equilibrium constants) are obtained by fitting data obtained under different reaction conditions using the Powell method.34 They were determined through a nonlinear regression method using eq 14, in which ri,exp represents the experimental rate and ri,cal stands for the calculated result of rates.
Figure 2. Residual sum of squares for the remaining model.
On the basis of the reaction rate constants needing to satisfy the Arrhenius relationship and the adsorption equilibrium constants needing to satisfy the Van’t Hoff equation at the given reaction temperature (i.e., criterion ii), models 4, 11, 15, 16, 19, and 23 were further excluded. To further validate the best of the remaining models, 30 more experiments were carried out under the kinetics measuring conditions. The relative deviation of the measured DMO conversion and its calculated value from models 1, 8, 31, and 32 are shown in Figure 3. With its near-zero relative deviations, model 31 seems more accurate than the other three models at predicting the experiment data. On the basis of this model, the following reaction pathway is suggested: hydrogen and the ester are adsorbed with dissociative states on two different active sites, respectively, and the slowest step is the ester dissociative adsorption step (eq 9). The expression of this model is represented as
r1 =
⎛ k1⎜pDMO − ⎝ 1 + KEGpEG + KMEpME +
pMG pME ⎞ ⎟ K p1pH2 ⎠
KDMOpMG pME K p1pH2
+
KMGpEG pME K p2pH2
+ KHPH (15)
r2 =
⎛ k 2⎜pMG − ⎝ 1 + KEGpEG + KMEpME +
pEG pME ⎞ ⎟ K p2pH2 ⎠
KDMOpMG pME K p1pH2
+
KMGpEG pME K p2pH2
+ KHPH (16)
D
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ΔHads,i (kJ/mol)
Ki0 (MPa−1)
KDMO KMG KEG KME
−56.28 −48.09 −34.09 −74.42
2.72 × 10−5 1.15 × 10−3 1.36 × 10−3 1.51 × 10−7
Table 3. Activation Energy and the Pre-Exponential Arrhenius Factor of DMO Hydrogenation reaction rate constants
Eai (kJ/mol)
ki0 (mol/(g·h))
k1 k2
36.38 44.84
1.82 × 106 2.81 × 107
Figure 3. Relative deviation distribution of DMO conversion in 30 more experiments. Xc represents the calculated conversion of DMO, and Xe denotes the experimental conversion of DMO. Figure 4. Comparison of the calculated conversion of DMO and the experimentally measured value with model 31.
The reaction rate constants (ki) and adsorption equilibrium constants (Ki) obtained from the reactions at four different temperatures are listed in Table 1. On the basis of the Van’t Hoff equation, we find K i = K i0e−ΔHads,i /(RT )
were compared, and the result is shown in Figure 4. It illustrates the very good agreement between the experimental data and what model 31 predicted. The F test was used to evaluate the significance, and the result is listed in Table 4. For a nonlinear model, it is generally accepted
(i = DMO, MG, EG , and ME) (17)
ki = ki0e
−Eai /(RT )
(i = 1, 2)
(18)
Table 4. Analysis of F Test
The heat of adsorption ΔHads,i and adsorption pre-exponential factors Ki0 were calculated (Figure S3) and listed in Table 2. The reaction activation energy Eai and the pre-exponential Arrhenius factors ki0 were similarly calculated using the Arrhenius equation (Figure S4) and are listed in Table 3. The obtained activation energy of MG hydrogenation to EG is larger than that of DMO hydrogenation to MG. This is consistent with our experimental observation that high temperature is beneficial for the formation of EG. 3.5. Model Verification. 3.5.1. Statistical Evaluation. The kinetics parameters obtained above were substituted into model 31 and used to calculate the conversion of DMO using the fourth-order Runge−Kutta method. The calculated DMO conversion (XDMO,cal) and experimentally measured value (XDMO,exp)
F test statistic value XDMO
YMG
YEG
Fα
157.57
77.80
32.09
1.94
that the models are significant when the F-test statistic value is larger than 10 times the critical statistical value Fα. The kinetics model 31 is significant at a confidence level of 99%. 3.5.2. Thermodynamic Consistency of Parameters. The signs of activation energy Ei and that of heat of adsorptions ΔHads,i should follow relevant thermodynamic law, that is, their values must satisfy eqs 19 and 20. The parameters of kinetics model 31 that are listed in Tables 2 and 3 were further checked
Table 1. Reaction Rate Constants and Adsorption Equilibrium Constants at Different Temperatures temperature (K)
k1 (mol/(g h))
k2 (mol/(g h))
KDmo (Mpa−1)
KMg (Mpa−1)
KEg (Mpa−1)
KMe (Mpa−1)
453 463 473 483
118.63 141.06 170.67 216.99
194.13 243.10 302.57 410.80
84.39 60.69 43.28 33.73
387.08 318.83 246.25 174.46
11.51 9.70 7.72 6.65
56.56 36.61 26.99 15.96
E
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Industrial & Engineering Chemistry Research for their thermodynamic validity. All of the activation energies have positive values, whereas all of the heats of adsorptions are negative.
Table 6. Synergy Mechanism of DMO Hydrogenation reaction
1
H 2 + 2# ↔ 2H#
ΔHads, i < 0
(19)
2a
CH3OOCCOOCH3 + 2∗ → CH3OOCC∗O + CH3O∗
Eai > 0
(20)
3
CH3OOCC∗O + H# ↔ CH3OOCC∗OH + #
K i = exp
ΔadsSi −ΔadsHi ·exp R RT
(21)
ΔSads, i = Sads, i − SG, i < 0
(22)
|ΔSads, i| < SG, i
(23)
|ΔSads, i| ≥ −R ln
ν̅ ≈ 41.8 J/(mol· K) νcr,̅ i
(24)
|ΔSads, i| < 51 J/(mol · K) + 0.0014ΔadsHi
(25)
The adsorption constants Ki were also checked for their thermodynamic consistency via the criteria of Boudart.35 These criteria are listed in eqs 22−25, where Sads and SG stand for the adsorption entropy and entropy of gas, respectively; R is the universal gas constant; and νcr represents the critical molar volume. As listed in Table 5, the calculated values of all of the substances satisfy the criteria of Boudart.
Sads,i SG,i 51 + 0.0014ΔHads,i
DMO
MG
EG
ME
−87.39 364.84 129.83
−56.25 345.12 118.37
−54.86 323.55 98.76
−130.57 239.70 155.23
4
CH3OOCC∗OH + H# ↔ CH3OOCC∗HOH + #
5
CH3OOCC∗HOH + H# ↔ CH3OOCC∗H 2OH + #
6
CH3OOCC∗H 2OH ↔ CH3OOCCH 2OH + ∗
7b
CH3OOCCH 2OH + 2∗ → HOCH 2C∗O + CH3O∗
8
HOCH 2C∗O + H# ↔ HOCH 2C∗OH + #
9
HOCH 2C∗OH + H# ↔ HOCH 2C∗HOH + #
10
HOCH 2CH ∗OH + H# ↔ HOCH 2C∗H 2OH + #
11
HOCH 2C∗H 2OH ↔ HOCH 2CH 2OH + ∗
12
CH3O∗ + H# ↔ CH3OH ∗ + #
13
CH3OH ∗ ↔ CH3OH + ∗
a
Rate-determining step of DMO hydrogenation to MG. determining step of MG hydrogenation to EG.
b
Rate-
activity with regard to ester hydrogenation, which provides some further guidelines for catalyst design.
4. CONCLUSIONS The intrinsic kinetics of gas-phase catalytic hydrogenation of DMO on Cu/SiO2 catalyst was investigated in a fixed-bed reactor. Thirty-four possible kinetics models were screened to identify the model most appropriate to predict DMO hydrogenation and to satisfy the thermodynamic and statistical criterions. The optimal kinetic model was derived from the classical Hougen-Watson mechanism, which considers two different active sites. A synergy mechanism is established in which hydrogen and the ester adsorb on their active sites in dissociative states. Specifically, the hydrogen molecules dissociate to hydrogen atoms on Cu0 sites, whereas DMO is dissociated to produce acyl and methoxyl species on Cu+ active sites. The surface acyl species gradually react with dissociated hydrogen atoms to form compounds containing hydroxyl (MG or EG), and methoxyl species are eventually used to produce methanol. The dissociative adsorption of the ester was found to be the rate-determining step in the hydrogenation of DMO to EG over Cu/SiO2 catalyst prepared by an AE method. The kinetics model parameters were established at different reaction temperatures, and the activation energy for the hydrogenation of DMO to MG (36.38 kJ/mol) was found to be lower than that in the further hydrogenation of MG to EG (44.84 kJ/mol), which is consistent with our experimental observation that high temperature benefits the formation of EG.
Table 5. Calculated Result of Model 31 for Criteria of Boudarta
a
step
Values are represented in joule per mole-kelvin (J/(mol·K)).
3.6. Reaction Mechanism. On the basis of all of the discussions above, model 31 seems to best describe the hydrogenation of DMO. A “synergy mechanism” with two different active sites can be deduced from this kinetics model, the steps of which are listed in Table 6. As evidenced by the results from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) (Figures S5 and S6), two copper species exist in the Cu/SiO2 catalyst: Cu0 and Cu+. Therefore, two different active sites can be ascribed to Cu0 and Cu+ species, respectively, as has been proposed in our previous work.31,36 On these two different active sites, the “#” active site represents the Cu0 sites that adsorb hydrogen, and the “*” active site stands for the Cu+ sites that adsorb the ester.22,29 Hydrogen molecules are adsorbed on Cu0 sites on the catalyst surface (i.e., the “#” active sites) to form two dissociated hydrogen atoms, and DMO molecules in the dissociative state are adsorbed on Cu+ sites (i.e., the “*” active sites). The dissociation of DMO produces adsorbed acyl and methoxyl species, which is consistent with other ester hydrogenation systems.26,27,30 The dissociated hydrogen atoms then gradually attack the acyl groups to form MG. Similarly, the acyl species formed by MG dissociation react with the hydrogen atoms to produce EG, and the adsorbed methoxyl species react with the hydrogen atoms to produce methanol. The overall reaction rate of DMO hydrogenation is controlled by the dissociative adsorption of the ester. This suggests that increasing the number of Cu+ active sites or improving ester adsorption capability may enhance the catalytic
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ASSOCIATED CONTENT
S Supporting Information *
Table S1 shows 34 kinetics model equations for the hydrogenation of DMO, Figures S1 and S2 illustrate that both internal and external diffusion can be excluded under the selected conditions, Figure S3 represents the relation between adsorption equilibrium constants and temperature, Figure S4 shows the relation between reaction rate constants and temperature, and Figures S5 and S6 illustrate the presence of both Cu0 and Cu+ in the Cu/SiO2 catalyst. This material is available free of charge via the Internet at http://pubs.acs.org. F
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +86-22-87401818. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the National Nature Science Foundation of China (21276186, 21325626, 91434127), the National High Technology Research and Development Program of China (2011AA051002), the PetroChina Innovation Foundation (2012D-5006-0503), and the Tianjin Natural Science Foundation (13JCZDJC33000). We also thank Professor Dr. Shengnian Wang (Louisiana Tech. University) for his constructive advice on this paper.
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NOMENCLATURE ri,exp = experimental reaction rate of step i, mol/(g·h) ri,cal = calculated reaction rate of step i, mol/(g·h) r1 = reaction rate of DMO hydrogenation to MG, mol/(g·h) r2 = reaction rate of MG hydrogenation to EG, mol/(g·h) PDMO = partial pressure of DMO, MPa PMG = partial pressure of MG, MPa PEG = partial pressure of EG, MPa PME = partial pressure of ME, MPa PH = partial pressure of hydrogen, MPa ki = reaction rate constants of step i, mol/(g·h) k1 = reaction rate constants of DMO hydrogenation to MG, mol/(g·h) k2 = reaction rate constants of MG hydrogenation to EG, mol/ (g·h) Ki = adsorption equilibrium constants of compound i, MPa−1 KDMO = adsorption equilibrium constants of DMO, MPa−1 KMG = adsorption equilibrium constants of MG, MPa−1 KEG = adsorption equilibrium constants of EG, MPa−1 KME = adsorption equilibrium constants of ME, MPa−1 Ki0 = adsorption pre-exponential factor of compound i, MPa−1 ΔHads,i = heat of adsorption of compound i, kJ/mol ki0 = pre-exponential Arrhenius factor of step i, mol/(g·h) Eai = activation energy of step i, kJ/mol R = universal gas constant, J/(mol·K) T = temperature, K X = conversion, % Y = selectivity, % Sads = adsorption entropy, J/(mol·K) SG = entropy of gas, J/(mol·K) νcr = critical molar volume, L/mol
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