Elimination of Product Inhibition by Ethanol Competitive Adsorption on

Research Article pubs.acs.org/journal/ascecg

Elimination of Product Inhibition by Ethanol Competitive Adsorption on Carbon Catalyst Support in a Maleic Acid Electrochemical Hydrogen Pump Hydrogenation Reactor Shishui Liu,† Wu Xiao,† Shaofeng Zhang,† Xuemei Wu,*,† Shiqi Huang,† Lin Ma,† Wei Chen,† Dongxing Zhen,† and Gaohong He*,†,‡ †

State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Linggong Road, Dalian, China 116024 ‡ School of Petroleum and Chemical Engineering, Panjin Campus of Dalian University of Technology, Dagong Road, Panjin, China 124221 ABSTRACT: In the present work, a special competitive adsorbent, ethanol, is proposed for in situ release product inhibition of a hydrophobic product system from a hydrophobic carbon catalyst support. Product inhibition is serious in electrochemical hydrogen pump hydrogenation reactors (EHPRs) due to slow molecular diffusion of the products through the microporous flow channels (1− 10 μm). The hydrogenation rate of maleic acid decreases rapidly with reaction time, only 37.4% retention after 10 h reaction. By introducing ethanol in the aqueous solution, both reaction rate and conversion after 10 h reach 1.46 folds of those without ethanol. Evidenced by competitive adsorption, the adsorption capacity of the product amber acid is decreased by 98% on carbon particles compared with that without ethanol. Ethanol takes most of the place of amber acid due to much better affinity with carbon support, which releases more amber acid molecules into the bulk solution and relieves product inhibition. Kinetics parameters are fitted with modified Langmuir−Hinshelwood kinetics by considering the influence of ethanol competitive adsorption on maleic acid hydrogenation performance. KEYWORDS: Biomass hydrogenation, Electrochemical hydrogen pump hydrogenation reactor, Product inhibition, Catalyst layer, Adsorption effect

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INTRODUCTION Recently, biofuel has attracted worldwide interest because it is renewable and generates less greenhouse gases and pollution compared to fossil fuels.1,2 Hydrogenation is a key step in the conversion of biomass derivatives into biofuel due to the unsaturated nature of biomass.3,4 For instance, through hydrogenation and the following fast pyrolysis, biomass could produce biofuel including C4, C5, or aromatic hydrocarbons.5 After hydrolysis and heterogeneous catalytic hydrogenation, polyhydric alcohol can be converted into liquid fuels.6 Most studies on hydrogenation of biomass derivatives have been done in conventional three-phase reactors. Due to the low solubility of hydrogen in the liquid phase, the adsorption of hydrogen on the surface of the catalyst is restricted over the adsorption competition with the organic reactants. Therefore, high pressure is essential for three-phase reactors to increase the concentration of the chemisorbed hydrogen (Hads) on the catalyst surface. With an electric power supply, electrocatalytic hydrogenation of biomass delivers chemisorbed hydrogen to the cathodic catalyst surface.7−9 However, relatively high proton transport resistance in the electrolyte solution from anode to cathode often results in considerable energy consumption.10 An electrochemical hydrogen pump hydrogenation reactor (EHPR), with the same structure as the proton exchange © 2017 American Chemical Society

membrane fuel cell, uses a polymer electrolyte (much thinner than electrolyte solution) to fast transport proton to the cathode to create in situ chemisorbed hydrogen (Hads) on the catalyst surface. Compared with the conventional three-phase reactors, Benziger et al. observed negligible hydrogen transport resistance for the hydrogenation of volatile biomass model compounds, olefin, and acetone in EHPR.11 Green et al. achieved nearly 2 folds of the hydrogenation rate of acetone with EHPR compared with a stirred stainless-steel autoclave three-phase reactor and monometallic platinum catalyst.12 In our previous works, mass transport resistance of nonvolatile reactants such as maleic acid in aqueous solution was reduced dramatically by replacing a commercial hydrophobic diffusion layer with hydrophilic stainless-steel welded mesh.13 Different hydrogen sources, such as an H2/CO2 mixture and organic 2-propanol, are proposed to replace the high cost pure hydrogen or water. Hydrogenation has been successfully coupled with separation or dehydrogenation in one EHPR (performed independently at anode and cathode, respectively) owing to a good barrier of proton exchange membrane.14 With a high reaction rate and mild operation Received: May 15, 2017 Revised: August 4, 2017 Published: August 23, 2017 8738

DOI: 10.1021/acssuschemeng.7b01520 ACS Sustainable Chem. Eng. 2017, 5, 8738−8746

Research Article

ACS Sustainable Chemistry & Engineering

folds. Based on the adsorption experiment, a mechanism is proposed to illustrate the elimination of product inhibition by the competitive adsorption between ethanol and the reaction components on the carbon support. Assuming Langmiur adsorption, a semiempirical hydrogenation kinetics model could be fitted well with the experimental hydrogenation rate.

conditions, hydrogenation biomass with EHPR has a promising future. Despite the advantages of EHPR, product inhibition is observed in most reported hydrogenation with EHPR, which means the hydrogenation rate declines fast with the increase in reaction time or reactant concentration.12−14 Product inhibition of CC or CO hydrogenation is more serious in EHPR than in conventional three-phase reactors, which greatly correlates to the microporous flow channels in the catalyst and diffusion layers. Even with very high flow rate, products could only transport through the microporous flow channels (1−10 μm) by molecular diffusion, which limits rate and conversion of hydrogenation and becomes a challenging issue for EHPR.15,16 However, to the best of our knowledge, there are few or no reports in the literature on elimination of product inhibition in EHPR. Methods from other areas may be used for reference. For instance, removing products from the reaction mixture as far as possible by reactive distillation17 and membrane reactor18 are most likely to be complicated and energy intensified because of the analogous structure of the hydrogenation products and reactants. From the literature, the sorption property of the catalyst support has been proved to greatly influence the diffusion behavior of fluid flowing through microchannels because the catalyst support usually has a large surface area; for instance, around 65% of the microchannels surface is composed of carbon particles in a Pt/C (70% Pt loading) catalyst layer. Hydrophilic modifications of carbon support by amination and mechanochemical oxidation are employed to the cellulose hydrolysis catalyst to adsorb more reactant water on the surface.19,20 Hydrophobic treatment of the catalyst support is often applied during preparation of the catalyst layer to help drain the product water out of the microchannels, for instance, in ethyl acetate synthesis, oxidation of volatile organic compounds, and the fuel cell diffusion layer.21−23 While for the hydrogenation of biomass derivatives where products have a hydrophobic nature, hydrophobic treatment of the catalyst support could not help drain of the products. The above hydrophilic chemical modifications of the carbon support increase the difficulty in manufacture and might influence conductivity and reactivity of the catalyst layer. In the present work, a special competitive adsorbent, ethanol, is proposed for in situ release product inhibition of a hydrophobic product system from a hydrophobic carbon catalyst support. As the hydrophobic hydrogenation product produces in situ on the Pt catalyst surface, its diffusion process to the bulk solution is controlled by concentrated driving through the microchannels in the catalyst layer and is sluggish due to the strong adsorption ability and hydrophobic nature of the microscale carbon support. As ethanol is introduced into the reaction solution, it competitively adsorbs onto the carbon particle surface, thus reducing the sorption of product and promoting diffusion of the product through microchannels. As shown in eq 1, maleic acid is chosen as the model compound of biomass derivatives because its hydrogenation

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RESULTS AND DISCUSSION Effects of Ethanol on Performance of Maleic Acid Hydrogenation in EHPR. Product inhibition is a serious issue in hydrogenation with EHPR.12−14 As shown in Figure 1a, the hydrogenation rate of maleic acid decreases rapidly with reaction time, and the average reaction rate of the last 2 h is only around 37.4% of that of the initial 2 h, which is much smaller than the reaction rate predicted by the hydrogenation kinetics model of Benziger for alkene hydrogenation on a Pt/C catalyst (above 73.8% of the initial reaction rate).11 As ethanol is introduced into the reaction aqueous solution, the reaction rate decreases much slower. After 10 h, hydrogenation conversion with ethanol reaches 38.3% The average reaction rate of the last 2 h is about 59.0% of that of the initial 2 h, and the average reaction rate of 10 h still keeps a high value of about 241.1 nmol−1 cm−2 s−1. Both the hydrogenation conversion and the average reaction rate of 10 h are about 1.46 folds of that without ethanol. It is indicated later that the addition of ethanol releases the product inhibition and promotes the reaction. It is noticed from Figure 1b that there is no obvious difference of reaction rate with or without ethanol in the initial 2 h because the accumulation of product is small in a short period of time. The effect of ethanol is further investigated with different operating parameters to improve the performance of EHPR as much as possible. Figure 2a depicts the influence of ethanol concentration on the performance of hydrogenation of maleic acid. It is seen that only a small amount of ethanol (0.1 M) could enhance the conversion, reaction rate, and current efficiency by about 18.3%. With the increase in the concentration of ethanol, the conversion, reaction rate, and current efficiency of maleic acid hydrogenation increase to maximum values of about 41.5%, 218.1 nmol cm−2 s−1, and 74.2%, respectively, at 2 M ethanol, which are about 1.36 folds of those without ethanol. This is related to the competitive adsorption between ethanol and the product amber acid, which is discussed in the Mechanism of Eliminating Product Inhibtion section. Figure 2b depicts the effect of 1 M ethanol on the conversion and reaction rate at different maleic acid concentrations. When the initial concentration of maleic acid is 1 M, the conversion, reaction rate, and current efficiency with ethanol are increased by about 46.2%. The maximum reaction rate with ethanol reaches 241.1 nmol−1 cm−2 s−1, which is around 1.18 folds of the maximum reaction rate without ethanol. Figure 2c depicts the effect of ethanol on the conversion and reaction rate with different current densities. With ethanol, the current efficiency, reaction rate, and conversion achieve higher values at different current densities. For instance, the conversion, reaction rate, and current efficiency at 132.3 mA cm−2 are increased by about 83.1%. The reaction rate and conversion with ethanol go through a maximum with current density as a result of the competition between hydrogenation and hydrogen evolution. At higher current density, more adsorption sites on the catalyst are taken by chemisorbed hydrogen (Hads) that enhances the hydrogen evolution side reaction. The reaction rate goes to a maximum value and then decreases, which indicates that after addition of ethanol the reaction still goes through a typical Langmuir−

conversion is restrained to less than 30% after 10 h based on our previous work.13 With the addition of ethanol, it is found that both hydrogenation conversion and rate increase by about 1.46 8739


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Figure 1. Conversion and reaction rate for the hydrogenation of maleic acid in EHPR as functions of reaction time: (a) 2−10 h and (b) 20 min to 4 h. Conditions: current density 56.7 mA cm−2, 20 sccm hydrogen 15 mL min−1, circulation fluid 15 mL min−1, concentrations of maleic acid and ethanol of 1.2 and 1 M, respectively, temperature 40 °C.

Figure 2. Conversion, current efficiency, and reaction rate for the hydrogenation of maleic acid in EHPR as functions of (a) ethanol concentration, (b) maleic acid concentration, (c) current density, and (d) temperature. General reaction conditions: current density 56.7 mA cm−2, hydrogen flow rate 20 sccm, reaction solution circulation fluid 15 mL min−1, maleic acid concentration 1 M, ethanol concentration 1 M, temperature 40 °C, reaction time 10 h, unless otherwise mentioned.

work that over 90% hydrogenation conversion could be retained after four batches of reaction.14 Therefore, the decrease in hydrogenation performance is most likely caused by the accumulation of product in the catalyst layer. Ethanol enhances hydrogenation performance; however, it might influence morphology of the catalyst layer at higher temperature (60 °C) and concentration (maleic acid concentration 1.8M) because it is a good suspensor for the Nafion binder in the catalyst layer. Figure 3b−d shows the surface SEM images of the Pt/C catalyst layer, corresponding to the experimental conditions shown in Figure 2. It indicates that ethanol or maleic acid with higher concentration or temperature causes a crack of the catalyst layer and might be a possible reason for the sluggishness of the conversion and rate (as shown in Figure 3b−d). As a good suspensor for the Nafion binder in the catalyst layer, ethanol may cause swelling of the catalyst layer at higher concentration or temperature. Concentrated maleic acid is likely crystallized from the solution and causes the catalyst layer to be unstable, which has been discussed in our previous work.13

Hinshelwood−Hougen−Watson (LHHW) kinetic expression.11,13 Figure 2d depicts the effect of ethanol on the conversion, reaction rate, and current efficiency at different temperatures. With the addition of ethanol, the reaction rate, conversion, and current efficiency reach the maximum values at around 40 °C, while without ethanol the maximum values are observed at around 50 °C. This might be related to the evaporation of ethanol (boiling point: 78 °C) at higher temperature and the probable instability of the catalyst layer with hot ethanol. From data in Figure 1, it is seen that conversion and rate of maleic acid hydrogenation become sluggish with the increasing reaction time. There are two possible reasons for the sluggishness: damage of the catalyst layer and product inhibition. Morphology of the catalyst layer is investigated through SEM to distinguish the difference before and after the hydrogenation reaction. As shown in the surface SEM image in Figure 3a, there is no obviously morphological difference with low ethanol concentration and temperature. It is also proved in our previous 8740


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Figure 3. SEM images of the surface of the cathode catalyst layer after hydrogenation of maleic acid with different (a,b) ethanol concentration, (c) maleic acid concentration, and (d) temperature. General reaction conditions are the same as those in Figure 2.

Mechanism of Eliminating Product Inhibtion. From the experimental data above, it is seen that ethanol could relieve product inhibition and improve performance of maleic acid hydrogenation. Since the solubility of amber acid in ethanol is smaller than that in water, ethanol should not be a solubilizer. It is suggested to have a great relationship with the competitive adsorption of ethanol on the carbon particle catalyst support. The competitive adsorption among ethanol, maleic acid, and amber acid is investigated to confirm that ethanol reduces the interaction between the product amber acid and the surface of

the carbon support. As a result, amber acid is easier to drain away from the catalyst layer. Ethanol has better affinity with the hydrophobic carbon surface compared with amber acid. Figure 4a depicts the contact angle of water on the surface of the catalyst layer, which is 139° and indicates high hydrophobicity of the catalyst layer. By contrast, ethanol has such a strong interaction with the hydrophobic catalyst layer surface that it infiltrates the porous surface immediately as it is dropped onto the surface. Figure 4b depicts the dispersibility of carbon particles in the solution of 0.3 8741


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Figure 4. (a) Contact angle of water on the surface of catalyst layer. (b) Distribution of carbon particles in 0.3 M amber acid (left) and 0.3 M ethanol (right) solutions.

Figure 5. (a) Adsorption capacity of maleic acid and amber acid (1:1) with varying total concentrations; adsorbent 1 g of carbon (30 nm). (b) Adsorption capacity of maleic acid and amber acid with varying amber acid proportions; adsorbent 0.7 g of carbon (30 nm). General condition: adsorption temperature 40 °C.

The intersection point roughly indicates that at a relatively high molar ratio of amber acid (greater than 50% and corresponding to higher conversion) most of the adsorption sites are taken by amber acid. The strong interaction between amber acid and the surface of a carbon particle would cause sluggishness of amber acid diffusion through the microchannel, as a result leading to product inhibition. The weaker adsorption of the reactant maleic acid on a carbon support would lead to little impact of the carbon support on the concentration driving diffusion to the catalyst surface. With the addition of ethanol, the adsorption behavior of the amber acid and maleic acid binary solution changes a lot. As shown in Figure 6, in a ternary solution, with the same

M amber acid (left) and 0.3 M ethanol (right) after standing for 24 h. Carbon particles precipitate out of the amber acid solution while still suspended well in the ethanol solution, which means ethanol, as a competitive adsorbent, has better affinity with the hydrophobic carbon surface comparing with amber acid. In the amber acid and maleic acid binary solution with a molar ratio of 1:1, the competitive adsorption of amber acid and maleic acid on carbon particles with different total concentrations is shown in Figure 5a. It is seen that more amber acid is adsorbed in the competitive adsorption at a certain concentration. With the increase in total concentration, the adsorption capacity difference between maleic acid and amber acid increases. The adsorption ability is a positive correlation with the difference between the adsorption energy and the dissolution energy.24 Biomass reactants often have smaller water solubility compared to its hydrogenation product (for instance, the solubility of maleic acid is 78 g/100 g but that of amber acid is 11 g/100 g at 25 °C), indicating much smaller dissolution energy for amber acid. Meanwhile, the adsorption energy of amber acid and maleic acid on carbon particles is similar due to the similar molecular polarization and relative molecular mass. This results in less hydrophobicity of maleic acid and thus weaker interaction with the hydrophobic carbon particles. Therefore, amber acid is more likely to adsorb on the surface of carbon particles. Given a constant total concentration of 0.6 M and varying the ratio of amber acid to maleic acid, competitive adsorption between them is investigated and shown in Figure 5b. With the increase in amber acid molar ratio, the adsorption capacity of amber acid increases from 0.532 to 2.59 mmol g−1, but the adsorption capacity of maleic acid decreases from 2.07 to 0.31 mmol g−1.

Figure 6. Adsorption capacity of maleic acid and amber acid with varying ethanol concentrations. Adsorption conditions: adsorbent 0.7 g of carbon (30 nm), temperature 40 °C. 8742


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Figure 7. Illustration of the competitive adsorption between ethanol and the reacting components and the effect of ethanol on the release of product inhibition.

atmospheric pressure and temperature.25 Therefore, hydrogen gas is much easier to be released from the microchannels of the catalyst layer. Kinetics Analysis for Maleic Acid Hydrogenation with Ethanol. A semiempirical kinetics model is proposed to correlate competitive adsorption of ethanol to hydrogenation performance. According to the experimental data, hydrogenation of maleic acid is fitted to the typical Langmuir−Hinshelwood mechanism. In the literature, Benziger proposed a Langmuir− Hinshelwood kinetics model for olefin bond hydrogenation with EHPR.11 Based on this model, two additional important factors, competitive adsorption and reverse reaction, are considered in this work to deduce the relationship between ethanol concentrate and reaction rate.

concentration of amber acid and maleic acid (both of them are 0.3 M), ethanol greatly restrains the adsorptions of both amber acid and maleic acid by competitive adsorption on carbon particles. When the ethanol concentration reaches 2 M, the adsorption capacity of amber acid is only 0.0427 mmol g−1, decreasing by 98% compared to that without ethanol. This indicates that the competitive adsorption of ethanol decreases the interaction between carbon particles and amber acid and helps to increase diffusion of amber acid through the microchannels of the catalyst layer. The competitive adsorption of ethanol also decreases the adsorption capacity of maleic acid on carbon particles. Based on the adsorption investigation, a schematic mechanism is proposed to illustrate the effect of ethanol on the release of amber acid product inhibition. As shown in Figure 7, amber acid produces in situ on a Pt catalyst and would diffuse through the microchannels of the catalyst layer to the bulk solution by concentration driving. The strong adsorption ability of amber acid on a carbon support (composition of the most microchannels surface) hinders its diffusion and leads to product accumulation in the catalyst layer, resulting in a slump of the hydrogenation rate. When ethanol is added into the reaction solution, it tends to take the place of amber acid by competitive adsorption onto the carbon particles. The weaker interactions between ethanol and amber acid release more amber acid molecules into the bulk solution, eventually relieving product inhibition. In the above schematic mechanism for releasing product inhibition, the adsorption property of the Pt catalyst is not considered for the following two reasons. One reason is that product desorption from the Pt catalyst is not the rate-limiting step according to hydrogenation kinetics for EHPR.11 The surface of the carbon catalyst support takes up at least 60% of the total surface of the microchannel in the catalyst layer. Therefore, adsorption property of the carbon particles should have a great influence on the diffusion of hydrogenation products. The emphasis of this work is the effects of the adsorption property of the carbon particles catalyst support on product inhibition. Also, the effect of ethanol on hydrogen diffusion is not considered due to the low solubility of hydrogen in an aqueous solution and much less adsorption ability of hydrogen on carbon particles at

Maleic acid hydrogenation: i

H+ + e− + ∗ ⎯→ ⎯ H*(hydrogen adsorption) KU

←⎯⎯⎯

U + ∗ → U*(maleic acid adsorption) k

0 H* + U* → UH* + ∗ (first hydrogen addition)

UH* + H* → UH 2 + 2*(second hydrogen addition) (2)

Product amber acid dehydrogenation: K UH 2

←⎯⎯⎯⎯⎯⎯

UH 2 + ∗ ⎯⎯⎯⎯→ UH*2 (amber acid adsorption on the catalyst) k

−1 UH*2 ⎯→ ⎯ U* + H 2(amber acid dehydrogenation)

(3)

Hydrogen evolution: k

d 2H* → H 2 + 2*(hydrogen recombination)

The rate-limiting step of hydrogenation is assumed as the addition of the first hydrogen,11 as given by eq 2. To reflect the 8743


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density.11 The concentration of the adsorbed hydrogen can be calculated and is given by eq 6.

influence of production inhibition on the reaction rate, the dehydrogenation process is also considered. According to the literature, the dehydrogenation process of alkane to olefin is divided into three steps, namely, adsorption on the catalyst, reaction process, and desorption from the catalyst. Because the reaction order is usually one, only one platinum atom is involved in the rate-determining step;26 therefore, an overall dehydrogenation equation is employed in this work, as given by eq 3. The hydrogenation rate is calculated by the difference between the forward and backward rates of the rate-limiting reactions, as given by eq 4. In which, [UH2*], [U*], and [H*] are concentrations of the adsorbed amber acid, maleic acid, and hydrogen atom and can be calculated from the maleic acid adsorption and amber acid desorption, respectively. CU and CUH2 are concentrations of maleic acid and amber acid in the catalyst layer, respectively. Assuming adsorption equilibrium for both maleic acid and amber acid on the carbon support, the empty adsorption site, [∗], is obtained by the total active site concentration, N, subtracting the adsorbed site concentration. Due to the high adsorption heat of Pt, U and UH2 would be strongly adsorbed and will be the principal species adsorbed on the surface.11

(5)

⎛ i ⎞1/2 [H *] = ⎜ ⎟ ⎝ 2kd ⎠

(6)

In the conventional mathematic model of EHPR, the concentration of the product in the catalyst layer was assumed to be the same as that in the bulk solution.11 However, due to the hydrophobicity of the catalyst support carbon particle, the product amber acid tends to accumulate in the microchannels of the catalyst layer and leads to product inhibition. Therefore, in this work, the concentration of amber acid in the catalyst layer is calculated by consideration of the adsorption effect. The Langemuir adsorption equation is applied to estimate the amount of amber acid in the catalyst layer, as given by eq 7. In the catalyst layer, the amber acid concentration, CUH2, is the ratio of the molar amount of amber acid, QUH2, to the free volume of the catalyst layer, V. QM represents the maximal adsorption capacity of the catalyst layer; C UH 2 b and C EtOHb represent the concentration of amber acid and ethanol in the bulk solution, respectively; bUH2 and bEtOH represent the adsorption coefficient of amber acid and ethanol in water, respectively. F is defined as the ratio of maximal adsorption capacity over the free volume of the catalyst layer (reflecting the influence of the adsorption nature of the catalyst layer on the product concentration), and combined with eqs 4−7, the hydrogenation rate can be expressed as eq 8.

Rate = k 0[U*⎤⎦[H*] − k −1[UH*2 ] [U*] = KUC U[∗] [UH*2 ] = K UH2C UH2[∗] [∗] = N − [U*] − [UH*2 ] =

i = 2kd[H*]2 + 2k 0[U*][H*]

N 1 + KUC U + K UH2C UH2 (4)

The current density in EHPR is equal to hydrogen evolution plus electroreduction of maleic acid, as given by eq 5. According to Benziger, the proton reduction reaction is assumed to be infinitely fast compared with hydrogenation of organics; therefore, the hydrogenation reaction rate is neglected to give more easy insight into the dependence of rate on current

C UH2 =

Q UH

2

V

=

Q MbUH2C UH2b (1 + bUH2C UH2b + bEtOHC EtOHb) × V QM

F=

V (7)

1/2 ⎧ ⎛ ⎞⎫ i ⎪ FKUH2C UH2b⎜k 0KUC U k + k −1(1 + C UKU)⎟ ⎪ 1/2 ⎛ ⎞ d ⎪ ⎝ ⎠⎪ N i ⎨k 0KUC U⎜ ⎟ − ⎬ Rate = FKUH2C UH2b 1 + KUC U ⎪ ⎝ kd ⎠ ⎪ (1 + bUH2C UH2b + bEtOHC EtOHb) + 1 + C K U U ⎪ ⎪ ⎩ ⎭

()

By consideration of the sorption effect of catalyst support, eq 7 correlates ethanol concentration to hydrogenation rate. It could be rewriten as a function of ethanol concentration, as given by eq 9, where A is the ideal reaction rate without product inhibition, B is related to the reverse reaction, i.e., amber acid dehydrogenation, and D is related to the adsorption ability of amber acid and ethanol on the carbon support. Rate = A −

B D + C EtOHb

B=

D=

⎛ NFKUH2C UH2b⎜k 0KUC U ⎝

1/2

() i kd

(8)

⎞ + Nk −1(1 + C UKU)⎟ ⎠

(1 + KUC U)bEtOH

(1 + KUC U)(1 + bUH2C UH2b) + FKUH2C UH2b (1 + KUC U)bEtOH

Based on the experimental data, reaction rate is replotted as a function of ethanol concentration at a given conversion of 20%, as shown in Figure 8. A small amount of ethanol addition improves the reaction rate. For ethanol concentration greater than 1 M, the reaction rate increases much slowly and then stays stable because most of the carbon particle surface has been taken up by ethanol at higher ethanol concentrations, and increasing ethanol concentration has little effect on the reaction rate. Ethanol of 3 M is not considered in this fitting due to the

(9)

where, ⎛ i ⎞1/2 N k 0KUC U⎜ ⎟ A= 1 + KUC U ⎝ kd ⎠ 8744


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diffusion layer (GDL) for an anode (SGL Tech., Germany, 39BC), and a hydrophilic stainless-steel welded mesh diffusion layer for a cathode (Anping, P.R. China, 100 meshes) were used to guarantee duplication of the experiments. The membrane electrode assembly (MEA) for EHPR consisted of a CCM sandwiched between two pieces of diffusion layers. Electrocatalytic Hydrogenation of Maleic Acid. Electrocatalytic hydrogenation of maleic acid was performed in EHPR,13 as shown in Figure 9, which was a standard graphite single cell with serpentine

Figure 8. Fitting curve of reaction rate at conversion of 20% with varying ethanol concentration. Reaction conditions: current density 56.7 mA cm−2, hydrogen flow rate 20 sccm, reaction solution circulation fluid 15 mL min−1, maleic acid concentration 1 M, current density 56.7 mA cm−2, temperature 40 °C. Fitting parameters: A = 185.8 nmol cm−2 s−1, B = 5.7 nmol2 cm−2 s−1, and D = 0.13 nmol.

observed unstable catalyst layer. With a multiparameter simulation below 2 M of ethanol, the perimeters in eq 9 are fitted as A = 185.8 nmol cm−2 s, B = 5.7 nmol2 cm−2 s, and D = 0.13 nmol. R-square is 0.90, indicating the validity of the semiempirical kinetics model. It also suggests the rationality of the competitive adsorption mechanism by introducing ethanol to release product inhibition.

Figure 9. Schematics of the device for the electrocatalytic hydrogenation of bio-oils in EHPR. Temperature control is employed at the humidifier, EHPR, and cathode circulating tank.13

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channels, and the geometric area was 5.29 cm2. A flow of 20 mL min−1 dry hydrogen maintained by a mass flow controller (Sevenstar D07− 19B Anlog Mass Flowmeter) inlet was allowed into the anode after being humidified. A GWInSTEK GPD-3303S DC power supply was used to apply the current of EHPR. With the supplied voltage, hydrogen on the anode catalyst layer was oxidized into protons, which were transferred from anode to cathode and reduced to chemisorbed hydrogen (Hads), ready for hydrogenation. The hydrogenation reaction was operated in a constant-current and batch-recycle mode. The total volume of the reaction solution was 100 mL, with a circulation flow rate of 15 mL min−1, unless otherwise stated. Because the stability of the catalyst layer was indeterminate, a new CCM was used after every reaction to guarantee comparability and repeatability, unless otherwise stated. Adsorption Experiment. The adsorption experiment was conducted in a 40 °C water bath. A 100 mL solution was stirred for 20 min to get a sufficient contact between the carbon particle and the aqueous solution. After stirring, the mixture stood for 90 min to precipitate the carbon particles. The clear solution from the top of the standing solution was centrifuged twice (1500 r min−1, 1 min; 11,000 r min−1, 1 min) to get the sample for analysis. The adsorption balance was reached before taking the sample. Analytic Methods. 1H NMR spectroscopy (Bruker Avance II 400M) was used to detect the concentration of maleic acid and amber acid after the hydrogenation reactions. Amber acid was the only product detected through 1H NMR spectroscopy. Reaction rate, conversion, and current efficiency were used as the evaluation criteria for the hydrogenation reactions. Conversion can be easily obtained from 1H NMR spectrum. The reaction rate was the average value during the reaction time and obtained on the basis of the MEA geometric area (5.29 nmol cm−2 s−1). Current efficiency is defined by eq 10, in which n is the number of electrons transferred, F is the Faraday constant, I is the current observed in the experiment, and t is the reaction time. The equivalent adsorbed capacity is calculated by eq 11, in which moladsorbed is the capacity of adsorbent on the carbon particles. The weight of the carbon adsorbent is represented by mcarbon. Ca and Cb are the concentration of solution after and before the experiment, respectively. V is volume of the adsorbed solution.

CONCLUSION In the hydrogenation of maleic acid in aqueous solution with EHPR, hydrophobic product amber acid tends to absorb on the surface of the hydrophobic carbon particle catalyst support, leading to product inhibition. The reaction rate of the last 2 h is only 37.4% of those of the initial 2 h. By introducing a third adsorbent, ethanol, into the reaction solution, the adsorption of amber acid is restrained due to the better affinity of ethanol to the surface of the carbon particle. With the addition of 1 M ethanol, both reaction rate and conversion are about 1.46 folds of that without ethanol. The influence of operation conditions, namely, temperature, ethanol concentration, maleic acid concentration, and current density are investigated. When the initial concentration of maleic acid is 1 M, the conversion, reaction rate, and current efficiency with ethanol are all increased by about 34.2%. The conversion, reaction rate, and current efficiency at 132.3 mA cm−2 are increased by 83.1%, indicating that with the addition of ethanol EHPR has a much better performance with high current density. The adsorption experiment confirms the strong interaction between amber acid and the surface of a carbon particle. The addition of ethanol decreases the interaction between amber acid and the surface of the carbon surface and thus promotes the diffusion of amber acid through the catalyst layer. A semiempirical kinetics model is proposed to correlate ethanol concentration to hydrogenation. By multiparameter simulation below 2 M of ethanol, R-square is 0.90, indicating the validity of the semiempirical kinetics model and the rationility of the competitive adsorption mechanism by introducing ethanol to release product inhibition.

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EXPERIMENT SECTION

Materials. Maleic acid (AR, Kefeng, P.R. China), succinic acid (AR, Guangfu, P.R. China), ethanol (AR Fuyu, P.R. China), formic acid (AR, Bodi, P.R. China), and carbon particles (99.5% Macklin, 30 nm) were purchased. A catalyst-coated membrane (CCM, Sunrise Power, P.R. China, 70 wt % Pt/C, Nafion/PTFE membrane), hydrophobic gas

Current efficiency = 8745

mol productnF It

× 100%

(10)


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ACS Sustainable Chemistry & Engineering mol adsorbed =

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(C b − Ca)V mcarbon

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AUTHOR INFORMATION

Corresponding Authors

*Xuemei Wu. E-mail: [email protected]. *Gaohong He. E-mail: [email protected]. ORCID

Wu Xiao: 0000-0003-1810-7562 Xuemei Wu: 0000-0002-0930-7602 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Joint Funds of the National Natural Science Foundation of China (U1663223), National Science Foundation of China (Grant 21476044), Fundamental Research Funds for the Central Universities (DUT16TD19), and Changjiang Scholars Program (Grant T2012049) for financial support of this work. The authors also thank Sunrise Power Co. for kindly supplying the catalyst-coated membrane.

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REFERENCES

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