Kinetics Study of Hydrogenation of Dimethyl Oxalate Over Cu/Sio<Sub

Article

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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. ff Considering di erent 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 CH332 OOCCOOCH+ 4H Ethylene glycol (EG) is widely used in the manufacture of →+HOCH CH OH 2CH OH (2) polyester and other industrial products, such as antifreeze 22 3 1,2 additives or brake fluids and solvents. Most EG in the market is Our group has successfully developed a novel Cu/SiO2 catalyst produced by ethylene oxidation. In recent years, EG production with an AE method. It exhibits several advantages, namely, highly from syngas has increasingly attracted interest because it is a way dispersed copper species, appropriate structure, and the desired 0 + to reduce the heavy dependence on petroleum resources.3 This ratio of Cu to Cu . However, its hydrogenation mechanism and approach includes two steps: the coupling of CO with methanol reaction kinetics have not yet been investigated. Great attention has been paid to the mechanisms of the to form dimethyl oxalate (DMO) and the subsequent hydro-  4−6 hydrogenation of the C O bond on a Cu-based catalyst, but the genation of DMO to EG (eqs 1 and 2). The production of EG roles of copper species with different valences are not yet clear. from syngas is economical and environmentally friendly because Controversial arguments can be easily found in the literature, and the CH3OH that is formed in the hydrogenation step can be they can be summarized into two groups: single active site and returned to the DMO synthesis process. Both homogeneous and synergy of Cu0−Cu+. In some reports in the literature, metallic − heterogeneous systems have been investigated with regard to the copper is believed to be the active site.18 20 However, He et al.21 hydrogenation of DMO to EG with noble metal catalysts such as claimed that the active sites were Cu+ and Cu0 when the hydro- Ru7 and Ag.8 However, considering the cost of noble metal genation of a fatty acid ester occurs on a Cu-based catalyst. 22 catalyst and the subsequent separation issue in homogeneous Similarly, Dandekar et al. investigated crotonaldehyde hydro- catalytic systems, the inexpensive heterogeneous catalytic sys- genation on a carbon-supported copper catalyst. They found that crotonaldehyde adsorbed on Cu+ sites with hydrogen that was tems, in particular copper-based catalyst for gas-phase reactions, 0 9−12 ff spilled over from Cu sites. have recently attracted great attention. Di erent supports for In addition to the ambiguity of the active sites, the adsorption Cu-based catalysts, such as SiO2,Al2O3, ZnO, and La2O3, have species on the surface of a copper-based catalyst is also not yet 13,14 been investigated in this reaction system; among these, Cu/ clear. The adsorption of hydrogen on a Cu-based catalyst was SiO2 catalyst shows the highest yield of EG in the hydrogenation also argued in two cases: dissociative adsorption and molecular of DMO or diethyl oxalate, resulting from the neutral properties adsorption.23,24 Similarly, ester adsorption is also a controversial of SiO2. Various synthesis methods, including ammonia evapora- issue. Some researchers claimed that the ester was molecularly 24,25 tion (AE), ion exchange, sol−gel, deposition precipitation, and adsorbed. However, the opinion that an ester could be disso- impregnation, have been reported for the preparation of silica- ciatively adsorbed on the surface of a catalyst was also proposed − supported copper catalyst.15 17 Received: November 2, 2014 1 2CO++→ 2CH OH O CH OOCCOOCH + H O Revised: January 12, 2015 323322 Accepted: January 12, 2015 (1) Published: January 12, 2015

© 2015 American Chemical Society 1243 DOI: 10.1021/ie5043038 Ind. Eng. Chem. Res. 2015, 54, 1243−1250 Industrial & Engineering Chemistry Research Article in some reports. By using microcosmic calorimetry, Santiago The catalyst sample was degassed at 573 K for 3 h before analysis. et al.26 verified that n-alkyl acetates were adsorbed on a catalyst The specific surface area was found to be 380.4 m2/g, as calcu- via dissociative states. Ju et al.27 investigated the kinetics and lated by the BET method using the absorption isotherms. mechanism of hydrogenation of butyl butyrate; on the basis of 2.2. Catalyst Activity Tests. Gas-phase hydrogenation of fi the D2 isotope studies, they believed that butyl butyrate was DMO was carried out in a stainless-steel xed-bed reactor with an dissociatively adsorbed on the surface of a catalyst to produce internal diameter of 7 mm, and the data were analyzed under the 27 fi fl C3H7CO and C4H9O fragments. On the basis of these ndings, assumption of an isothermal integral plug ow reactor (PFR). acyl adsorbed species and alkoxy adsorbed species might be The catalyst pellets were crushed, sieved, and loaded into the formed after the dissociative absorption of the ester, which could reactor between two quartz-sand zones. The catalyst was reduced be further hydrogenated to form the corresponding alcohol. in pure H2 at 623 K for 4 h. After reduction, the catalyst was Most kinetics studies on the hydrogenation of esters only cooled down to the reaction temperature under a hydrogen consider a single active site. Although silica-supported Cu cata- atmosphere. The DMO solution (20 wt % in methanol) was lyst has been extensively investigated for the hydrogenation of injected continuously from the top of the reactor through a high- DMO or diethyl oxalate to ethylene glycol, the kinetics study of pressure pump and was vaporized in the stream of H2 gas at the 28 this catalyst was only carried out by Xu et al. They considered a selected H2/DMO ratio. The reaction temperature was single active site and molecular adsorption of the ester on the controlled with a variation of ±1K. surface of the catalyst. However, some research indicates that the The intrinsic kinetics measurements were done under broad high activity of a Cu-based catalyst is ascribed to the synergy reaction conditions, with the reaction temperature varied from between Cu0 and Cu+ in the DMO hydrogenation system. 453 to 483 K, the weight liquid hourly space velocity (LHSV) Yin et al.29 expressed that the synergetic effect between Cu0 and varied from 1.5 to 11 h−1, the reaction pressure varied between + − Cu was responsible for high hydrogenation activity. They also 1.0 3.0 MPa, and the ratio of H2/DMO varied from 20 to believed that Cu+ could work as either an electrophilic or a Lewis 70 mol/mol. acid site to polarize the CO bond via the lone electron pair The reaction products were analyzed using a gas chromato- on oxygen so that the reactivity of the ester group in DMO is graph (Agilent Micro GC 6820) with an HP-INNOWAX cap- improved. Using in situ Fourier transform infrared spectroscopy, illary column (Hewlett−Packard Company, 30 m × 0.32 mm × Hui et al.30 proved that the dissociative absorption of DMO 0.50 μm) equipped with a flame ionization detector (FID). occurs on a catalyst via the cleavage of the C−O bond. In our Several samples were taken from under the same experimental previous work on Cu/SiO2 catalyst in the production of ethanol conditions and analyzed by GC to ensure repeatability. via syngas, we also proposed that the Cu0 and Cu+ active sites play roles similar to those found by Yin et al.31 3. RESULTS AND DISCUSSION To provide guidelines for catalyst design aimed at better 3.1. Elimination of the External and Internal Diffusion. performance, it is necessary to investigate the kinetics of DMO ff − ff The criteria for no external di usion limited reaction relies on hydrogenation while also considering the di erent situations of the independence of the conversion on the gas linear velocity at ester adsorption and active sites to reveal the reaction mecha- any space velocity.32,33 Herein, we measured the conversion of nism. In this work, we focus on the intrinsic kinetics of the DMO at various gas linear velocities at a fixed LHSV, and the hydrogenation of DMO on a silica-supported copper catalyst results are plotted in Figure S1. When the gas linear velocity prepared by an AE method. A series of kinetics model equations × −3 ff exceeds 2.63 10 m/s, the DMO conversion is nearly the derived from di erent assumptions are proposed and evaluated, same, which implies that the effect of external diffusion is neg- “ ” − and the synergy mechanism kinetics model for the hydro- ligible. Therefore, a gas linear velocity larger than 2.63 × 10 3 m/s genation of DMO is established. was later used in the kinetics experiments. To prevent further falling of the catalytic reaction in an internal 2. EXPERIMENTAL SECTION diffusion regime, the conversion of DMO was measured with 2.1. Catalyst Preparation and Characterization. Cu/ catalyst of three different particle-size ranges (20−40, 40−60, − SiO2 catalyst was prepared by an AE method, described as and 60 80 mesh) and was found to almost the same, as shown in · ff ff follows. About 15.4 g of Cu(NO3)2 3H2O was dissolved in Figure S2. This implies that the e ect of internal di usion is also deionized water, and 52 mL of aqueous ammonia solution negligible.33 Catalyst with a diameter of 0.25−0.35 mm (i.e., 40− (25 wt %) was then added at room temperature. Then, 45 mL of 60 mesh) was later used in the kinetics studies. silica solution (30 wt %) was added to the copper−ammonia 3.2. Effect of Reaction Conditions. According to the gas complex solution and stirred for 4 h. The initial pH of the kinetic theory, the possibility of the simultaneous hydrogenation suspension was 11−12. The suspension was heated to 353 K to of two CO functional groups is very low. Therefore, the hydro- evaporate the ammonia, allowing the copper species to deposit genation of DMO is believed to involve two serial reactions: the onto the silica. This process was terminated when the pH de- hydrogenation of DMO to form methyl glycolate (MG) and the creased to 6−7. The precipitate was then separated by filtration, subsequent hydrogenation of MG to produce EG, represented as 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, CH332 OOCCOOCH+ 2H and sieved to desirable size. →+CH OOCCH OH CH OH The actual load of copper was found to be ∼18.5 wt %, as 32 3 (3) determined by inductively coupled plasma optical-emission CH OOCCH OH+→ 2H HOCH CH OH + CH OH spectroscopy (ICP-OES). Briefly, samples were dissolved in a 32 2 223 (4) mixture of HF and HBO3 and then analyzed by ICP. The textural properties of the prepared catalyst were measured by the N2 MG and EG are the two main products, and their overall adsorption method using a Micromeritics Tristar II 3000 ana- selectivity (EG and MG) is larger than 95%. Figure 1a shows lyzer instrument at the boiling temperature of liquid nitrogen. catalyst performance at different LHSVs. For a given reaction

1244 DOI: 10.1021/ie5043038 Ind. Eng. Chem. Res. 2015, 54, 1243−1250 Industrial & Engineering Chemistry Research Article

ff ff Figure 1. E ect of reaction conditions on reaction activity. (a) E ect of liquid hourly space velocity (LHSV), T = 473 K, P = 2.5 MPa, H2/DMO = 80 ff −1 ff mol/mol. (b) E ect of temperature, P = 2.5 MPa, H2/DMO = 80 mol/mol, LHSV = 2.5 h . (c) E ect of pressure, T = 473 K, H2/DMO = 80:1, LHSV = −1 ff −1 2.5 h . (d) E ect of the molar ratio of H2/DMO, T = 473 K, P = 2.5 MPa, LHSV = 2.5 h .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. temperature (473 K), pressure (2.5 MPa), and H2/DMO ratio catalyst as nondissociative molecules, the hydrogenation of the (80 mol/mol), the selectivity of EG declines when LHSV is larger ester occurs with the addition of a hydrogen to the carbonyl than 4 h−1, and the selectivity of MG starts increasing rapidly. group and the drop-off of an alkoxy group.27 For a diester such as This suggests that the formation of EG is easier than that of MG DMO, its hydrogenation involves two consecutive reactions: with longer residential time. DMO reacts with H2 to produce MG, which is further hydro- The effect of reaction temperature over the range of 453 to genated to become EG. When an ester follows the molecular 493 K on catalyst performance was also investigated. As shown in adsorption model, the pathway below is suggested for the Figure 1b, the EG selectivity increases quickly when raising the hydrogenation of DMO (eqs 5−8): hydrogen first attacks one of reaction temperature, and high temperature seems beneficial for the two carbonyls of DMO to form intermediate product A, the production of EG. However, if the reaction temperature is which is unstable and quickly reacts with another hydrogen too high (e.g., >473 K), further hydrogenation of EG occurs, molecule to form MG and methanol (ME). In a similar way, the which leads to the formation of ethanol. The product distribution hydrogen molecules continue their reaction with the left of DMO hydrogenation is also sensitive to reactor pressure. carbonyl group on MG to produce EG and methanol (through Figure 1c shows that high pressure benefits the formation of EG, the formation of intermediate product B). 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 dissoci- If DMO follows the dissociative adsorption model, the hydro- ation adsorption model.23,26,27,30 When an ester is adsorbed on a genation pathway is very different (as shown in eqs 9−13): the

1245 DOI: 10.1021/ie5043038 Ind. Eng. Chem. Res. 2015, 54, 1243−1250 Industrial & Engineering Chemistry Research Article active sites adsorb one dissociative ester group of DMO to form 2 2 methoxyl and acyl species M, and the acyl species M is then RSS=−∑ () rii,exp r ,cal gradually hydrogenated to produce MG. MG then splits into acyl =1 (14) species N and methoxyl, and the former (i.e., acyl species N) To obtain appropriate kinetics model, all 34 models were reacts with hydrogen to form EG. The methoxyl groups pro- examined with the same criterion: (i) The reaction rate constant duced during the dissociative adsorption of DMO or MG also and the adsorption equilibrium constant must be positive react with hydrogen to produce ME. numbers; if one or some of them are found to be negative, that

Cu/SiO2 particular model is excluded. (ii) The reaction rate constant and CH33 OOCCOOCH⎯→⎯⎯⎯⎯⎯⎯⎯ CH3 OOCCO + CH3 O adsorption equilibrium constant should follow the Arrhenius acyl species M methoxyl (9) equation and Van’tHoff equation, respectively. (iii) The model ffi Cu/SiO2 should have a su ciently low residual sum of squares between CH32 OOCCO+⎯→ 1.5H⎯⎯⎯⎯⎯⎯⎯ CH32 OOCCH OH the experimental and calculated rates. After fitting all 34 kinetics (10) MG models with the experimental data, models 2, 3, 5−7, 9, 10, Cu/SiO 12−14, 17, 18, 20−22, 24−30, 33, and 34 were found to have CH OOCCH OH⎯→⎯⎯⎯⎯⎯⎯⎯2 HOCH CO + CH O 32 2 3 negative parameters and/or high RSSmin and were thus excluded. acyl species N methoxyl (11) The remaining models were plotted in Figure 2.

Cu/SiO2 HOCH22 CO+⎯→ 1.5H⎯⎯⎯⎯⎯⎯⎯ HOH22 CCH OH EG (12)

Cu/SiO2 CH32 O+⎯→ 0.5H⎯⎯⎯⎯⎯⎯⎯ CH3 OH 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 Hougen- Watson (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 copper- based catalyst with or without dissociation, (ii) the surface reaction occurs between adsorbed reactants on a single Figure 2. Residual sum of squares for the remaining model. adsorption site or on two different adsorption sites, and (iii) products are desorbed from the catalyst surface. The slowest step On the basis of the reaction rate constants needing to satisfy among all of these controls the overall reaction rate. We explored the Arrhenius relationship and the adsorption equilibrium con- six different possible combinations of absorption on active sites stants needing to satisfy the Van’tHoff equation at the given and the various reaction pathways mentioned above: (1) reaction temperature (i.e., criterion ii), models 4, 11, 15, 16, 19, Molecular adsorption of the ester and dissociative adsorption of and 23 were further excluded. To further validate the best of the hydrogen occur on the same active site. (2) Molecular adsorption remaining models, 30 more experiments were carried out under occurs for both the ester and hydrogen on the same active site. (3) the kinetics measuring conditions. The relative deviation of the Dissociative adsorption occurs for both the ester and hydrogen on measured DMO conversion and its calculated value from models the same active site. (4) Molecular adsorption of the ester and 1, 8, 31, and 32 are shown in Figure 3. With its near-zero relative dissociative adsorption of hydrogen occur on different active sites. deviations, model 31 seems more accurate than the other three (5) Molecular adsorption occurs for both the ester and hydrogen models at predicting the experiment data. On the basis of this on different active sites. (6) Dissociative adsorption occurs for model, the following reaction pathway is suggested: hydrogen both the ester and hydrogen on different active sites. and the ester are adsorbed with dissociative states on two dif- To establish the kinetics model, we also assume the following: ferent active sites, respectively, and the slowest step is the ester (1) All adsorption sites are equivalent and the activation energy dissociative adsorption step (eq 9). of adsorption and desorption is independent of the surface The expression of this model is represented as coverage. (2) The rate controlling step is from involved surface ⎛ p p ⎞ kp⎜⎟− MG ME reaction, adsorption of reactants or desorption of products. 1⎝ DMO 2 ⎠ Kpp1 H A total of 34 possible kinetics models were derived and are r1 = Kpp Kpp 1 ++Kp Kp +DMO MG ME +MG EG ME + KP listed in Table S1. EG EG ME ME Kp2 Kp2 HH 3.4. Parameter Estimations and Model Discrimination. p1 H p2 H (15) The kinetics parameters (rate constants and adsorption equili- fi ⎛ pp ⎞ brium constants) are obtained by tting data obtained under kp⎜⎟− EG ME ff 34 2⎝ MG 2 ⎠ di erent reaction conditions using the Powell method. They Kpp2 H r2 = Kpp Kpp were determined through a nonlinear regression method using 1 ++Kp Kp +DMO MG ME +MG EG ME + KP EG EG ME ME Kp2 Kp2 HH eq 14, in which ri,exp represents the experimental rate and ri,cal p1 H p2 H stands for the calculated result of rates. (16)

1246 DOI: 10.1021/ie5043038 Ind. Eng. Chem. Res. 2015, 54, 1243−1250 Industrial & Engineering Chemistry Research Article

Table 2. Heat of Adsorption and the Adsorption Pre-Exponential Factors of DMO Hydrogenation

Δ −1 adsorption equilibrium constants Hads,i (kJ/mol) Ki0 (MPa ) − × −5 KDMO 56.28 2.72 10 − × −3 KMG 48.09 1.15 10 − × −3 KEG 34.09 1.36 10 − × −7 KME 74.42 1.51 10

Table 3. Activation Energy and the Pre-Exponential Arrhenius Factor of DMO Hydrogenation · reaction rate constants Eai (kJ/mol) ki0 (mol/(g h)) × 6 k1 36.38 1.82 10 × 7 k2 44.84 2.81 10

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 The reaction rate constants (ki) and adsorption equilibrium ff experimentally measured value with model 31. constants (Ki) obtained from the reactions at four di erent temperatures are listed in Table 1. On the basis of the Van’tHoff equation, we find were compared, and the result is shown in Figure 4. It illustrates the very good agreement between the experimental data and −ΔHRTads,i/( ) KKii==0e ( i DMO, MG, EG , and ME) what model 31 predicted. (17) 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 −Eai /( RT ) kkii==0e(1,2) i (18) Δ Table 4. Analysis of F Test The heat of adsorption Hads,i and adsorption pre-exponential factors Ki0 were calculated (Figure S3) and listed in Table 2. F test statistic value The reaction activation energy Eai and the pre-exponential XDMO YMG YEG Fα Arrhenius factors ki0 were similarly calculated using the Arrhenius equation (Figure S4) and are listed in Table 3. 157.57 77.80 32.09 1.94 The obtained activation energy of MG hydrogenation to EG is larger than that of DMO hydrogenation to MG. This is con- that the models are significant when the F-test statistic value is sistent with our experimental observation that high temperature larger than 10 times the critical statistical value Fα. The kinetics is beneficial for the formation of EG. model 31 is significant at a confidence level of 99%. 3.5. Model Verification. 3.5.1. Statistical Evaluation. The 3.5.2. Thermodynamic Consistency of Parameters. The kinetics parameters obtained above were substituted into model signs of activation energy Ei and that of heat of adsorptions Δ 31 and used to calculate the conversion of DMO using the Hads,i should follow relevant thermodynamic law, that is, their fourth-order Runge−Kutta method. The calculated DMO con- values must satisfy eqs 19 and 20. The parameters of kinetics version (XDMO,cal) and experimentally measured value (XDMO,exp) 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

−1 −1 −1 −1 temperature (K) k1 (mol/(g h)) k2 (mol/(g h)) KDmo (Mpa ) KMg (Mpa ) KEg (Mpa ) KMe (Mpa ) 453 118.63 194.13 84.39 387.08 11.51 56.56 463 141.06 243.10 60.69 318.83 9.70 36.61 473 170.67 302.57 43.28 246.25 7.72 26.99 483 216.99 410.80 33.73 174.46 6.65 15.96

1247 DOI: 10.1021/ie5043038 Ind. Eng. Chem. Res. 2015, 54, 1243−1250 Industrial & Engineering Chemistry Research Article for their thermodynamic validity. All of the activation energies Table 6. Synergy Mechanism of DMO Hydrogenation have positive values, whereas all of the heats of adsorptions are negative. step reaction 1 H22 +#↔ 2H# ΔHads,i < 0 (19) a 2 CH333 OOCCOOCH+∗→ 2 CH OOCC ∗ O + CH 3 O∗ > Eai 0 (20) 3 CH33 OOCC∗+#↔ O H CH OOCC ∗ OH +# 4 CH OOCC∗+#↔ OH H CH OOCC ∗ HOH +# ΔadsSii−Δ adsH 33 Ki = exp· exp 5 ∗+#↔∗ +# R RT (21) CH332 OOCC HOH H CH OOCC H OH 6 CH OOCC∗↔ H OH CH OOCCH OH +∗ ΔSSS=−<0 32 32 ads,iii ads, G, (22) b 7 CH32 OOCCH OH+∗→ 2 HOCH 2 C ∗ O + CH 3 O∗ |Δ|

ν 9 HOCH22 C∗+#↔ OH H HOCH C ∗ HOH +# |Δ|≥−SRln̅ ≈· 41.8 J/(mol K) ads,i 10 HOCH CH∗+#↔ OH H HOCH C ∗ H OH +# νcr,̅ i (24) 222 11 HOCH22 C∗↔ H OH HOCH 22 CH OH +∗ |Δ|

The adsorption constants Ki were also checked for their 13 CHOH33∗↔ CHOH +∗ 35 thermodynamic consistency via the criteria of Boudart. These a b − Rate-determining step of DMO hydrogenation to MG. Rate- criteria are listed in eqs 22 25, where Sads and SG stand for determining step of MG hydrogenation to EG. the adsorption entropy and entropy of gas, respectively; R is ν the universal gas constant; and cr represents the critical molar volume. As listed in Table 5, the calculated values of all of the activity with regard to ester hydrogenation, which provides some substances satisfy the criteria of Boudart. further guidelines for catalyst design. 4. CONCLUSIONS Table 5. Calculated Result of Model 31 for Criteria of a Boudart The intrinsic kinetics of gas-phase catalytic hydrogenation of fi DMO on Cu/SiO2 catalyst was investigated in a xed-bed DMO MG EG ME reactor. Thirty-four possible kinetics models were screened to − − − − Sads,i 87.39 56.25 54.86 130.57 identify the model most appropriate to predict DMO hydro- SG,i 364.84 345.12 323.55 239.70 genation and to satisfy the thermodynamic and statistical crite- Δ 51 + 0.0014 Hads,i 129.83 118.37 98.76 155.23 rions. The optimal kinetic model was derived from the classical aValues are represented in joule per mole-kelvin (J/(mol·K)). 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. 3.6. Reaction Mechanism. On the basis of all of the dis- Specifically, the hydrogen molecules dissociate to hydrogen cussions above, model 31 seems to best describe the hydro- atoms on Cu0 sites, whereas DMO is dissociated to produce acyl genation of DMO. A “synergy mechanism” with two different and methoxyl species on Cu+ active sites. The surface acyl species active sites can be deduced from this kinetics model, the steps of gradually react with dissociated hydrogen atoms to form which are listed in Table 6. compounds containing hydroxyl (MG or EG), and methoxyl As evidenced by the results from X-ray photoelectron species are eventually used to produce methanol. The dissoci- spectroscopy (XPS) and X-ray diffraction (XRD) (Figures S5 ative adsorption of the ester was found to be the rate-determining 0 and S6), two copper species exist in the Cu/SiO2 catalyst: Cu step in the hydrogenation of DMO to EG over Cu/SiO2 catalyst and Cu+. Therefore, two different active sites can be ascribed to prepared by an AE method. The kinetics model parameters were Cu0 and Cu+ species, respectively, as has been proposed in our established at different reaction temperatures, and the activation previous work.31,36 On these two different active sites, the “#” energy for the hydrogenation of DMO to MG (36.38 kJ/mol) active site represents the Cu0 sites that adsorb hydrogen, and the was found to be lower than that in the further hydrogenation “*” active site stands for the Cu+ sites that adsorb the ester.22,29 of MG to EG (44.84 kJ/mol), which is consistent with our Hydrogen molecules are adsorbed on Cu0 sites on the catalyst experimental observation that high temperature benefits the surface (i.e., the “#” active sites) to form two dissociated hydro- formation of EG. gen atoms, and DMO molecules in the dissociative state are adsorbed on Cu+ sites (i.e., the “*” active sites). The dissociation ■ ASSOCIATED CONTENT of DMO produces adsorbed acyl and methoxyl species, which is *S Supporting Information consistent with other ester hydrogenation systems.26,27,30 The Table S1 shows 34 kinetics model equations for the hydro- dissociated hydrogen atoms then gradually attack the acyl groups genation of DMO, Figures S1 and S2 illustrate that both internal to form MG. Similarly, the acyl species formed by MG disso- and external diffusion can be excluded under the selected ciation react with the hydrogen atoms to produce EG, and the conditions, Figure S3 represents the relation between adsorption adsorbed methoxyl species react with the hydrogen atoms to equilibrium constants and temperature, Figure S4 shows the produce methanol. The overall reaction rate of DMO hydro- relation between reaction rate constants and temperature, and genation is controlled by the dissociative adsorption of the ester. Figures S5 and S6 illustrate the presence of both Cu0 and Cu+ in + This suggests that increasing the number of Cu active sites or the Cu/SiO2 catalyst. This material is available free of charge via improving ester adsorption capability may enhance the catalytic the Internet at http://pubs.acs.org.

1248 DOI: 10.1021/ie5043038 Ind. Eng. Chem. Res. 2015, 54, 1243−1250 Industrial & Engineering Chemistry Research Article ■ AUTHOR INFORMATION (5) Gao, Z.; Liu, Z.; He, F.; Xu, G. Combined XPS and in situ DRIRS α study of mechanism of Pd-Fe/ -Al2O3 catalyzed CO coupling reaction Corresponding Author to diethyl oxalate. J. Mol. Catal. A: Chem. 2005, 235, 143−149. *E-mail: [email protected]. Fax: +86-22-87401818. (6) Zhao, L.; Zhao, Y.; Wang, S.; Yue, H.; Wang, B.; Lv, J.; Ma, X. Notes Hydrogenation of dimethyl oxalate using extruded Cu/SiO2 catalysts: ﬁ mechanical strength and catalytic performance. Ind. Eng. Chem. Res. The authors declare no competing nancial interest. 2012, 51, 13935−13943. (7) Boardman, B.; Hanton, M. J.; van Rensburg, H.; Tooze, R. P. A ■ ACKNOWLEDGMENTS tripodal sulfur ligand for the selective ruthenium-catalysed hydro- − ﬁ genation of dimethyl oxalate. Chem. Commun. 2006, 21, 2289 2291. We are grateful for nancial support from the National Nature (8) Teunissen, H. T.; Elsevier, C. J. Ruthenium catalysed hydro- Science Foundation of China (21276186, 21325626, 91434127), genation of dimethyl oxalate to ethylene glycol. Chem. Commun. 1997, 7, the National High Technology Research and Development 667−668. Program of China (2011AA051002), the PetroChina Innovation (9) He, Z.; Lin, H.; He, P.; Yuan, Y. Effect of boric oxide doping on the − Foundation (2012D-5006-0503), and the Tianjin Natural stability and activity of a Cu SiO2 catalyst for vapor-phase hydro- Science Foundation (13JCZDJC33000). We also thank genation of dimethyl oxalate to ethylene glycol. J. Catal. 2011, 277,54− Professor Dr. Shengnian Wang (Louisiana Tech. University) 63. for his constructive advice on this paper. (10) Yin, A.; Guo, X.; Fan, K.; Dai, W. Ion-exchange temperature effect on Cu/HMS catalysts for the hydrogenation of dimethyl oxalate to ethylene glycol. ChemCatChem 2010, 2, 206−213. ■ NOMENCLATURE (11) Brands, D. S.; Poels, E. K.; Bliek, A. 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1250 DOI: 10.1021/ie5043038 Ind. Eng. Chem. Res. 2015, 54, 1243−1250