Electrochemical Microreactor Design for Alkoxylation Reactions

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Electrochemical Microreactor Design for Alkoxylation Reactions— Experiments and Simulations Jiri Kristal,†,* Roman Kodym,‡ Karel Bouzek,‡ Vladimir Jiricny,† and Jiri Hanika† † ‡

Institute of Chemical Process Fundamentals AS CR, v.v.i., Rozvojova 135, 165 02, Prague 6, CZ Institute of Chemical Technology Prague, Technicka 5, 166 28, Prague 6, CZ ABSTRACT: Two types of the bipolar electrochemical microreactor (BEMR) were investigated for methoxylation of 4-methylanisole: A block-type BEMR with a fixed number of parallel electrodes and a filter-press-type BEMR with a variable number of parallel electrodes. On the basis of a numerical parametric study, the current utilization factor η was presented for the two microreactors as a function of the dimensionless current I* for different system parameters. Conversion and selectivity of the alkoxylation reaction was studied experimentally and the performance of both BEMR was compared. Even though the optimization of the reactor design is still needed particularly with respect to liquid distribution into the individual parallel microchannels, heat management and gas removal, the presented results indicated that the bipolar arrangement of electrochemical microreactors certainly offers a great potential for the intensification of alkoxylation reactions.

’ INTRODUCTION Electrochemical microreactors represent a promising way for intensification of electro-organic reaction syntheses. In the electro-organic processes, microreactors not only provide very close control of the local conditions due to extraordinarily fast transfer phenomena and well-defined local electrode potential but also their small dimensions (interelectrode distances) are favorable for the processes involving organic solvents whose electric conductivity is low. Application of devices with the characteristic scale in millimeters is not new in electro-organic technology. Already in 1960s, the capillary-gap cells were used for electrochemical methoxylation of 4-methylanisole (4-MA) by BASF.1 However, a more intensive research in this field began together with the onset of microtechnology as we know it today, that is, in early 1990s. Belmont and Girault presented several papers on the application of a microstructured cell for the furan methoxylation2 and propylene epoxidation.3 Reported results were comparable or better that the most efficient industrially used reactors at that time. Ferrigno at al.4,5 investigated similar systems for the seawater electrolysis for the production of hypochlorite. L€owe and Ehrfeld6 reviewed the current status of microtechnology in late 1990s, including electrochemical applications. The methoxylation of 4-MA was identified as a suitable model reaction with respect to the electrochemical microreactors. In a subsequent work, K€upper et al.7 described an original electrochemical microreactor for both mono- and bipolar operation, and tested it on the D-arabinose synthesis. Matlosz8 published an analysis of the potential process intensification by using the microstructured cell with the segmented electrode to control local current load. Results of this theoretical analysis were validated experimentally with the electro-oxidation of sodium gluconate to D-arabinose.9 The idea of a segmented electrode for local control of the potential and current distribution for the 4-MA methoxylation was further studied within the EU project IMPULSE. Rode et al.10 published a theoretical study on the effect of the electrode r 2011 American Chemical Society

segmentation on the reaction performance. The studied microreactor featured a thin (100 μm) interelectrode gap and the model results predicted superior performance compared to industrial technology. Attour et al.11 continued with an experimental study of the methoxylation reaction in a thin-gap microreactor with segmented anode and demonstrated the efficiency of this design. For an unbiased comparison of the different experimental results the authors defined a dimensionless current I* as the ratio of the overall current I passed to the minimum current required for the total conversion of anisole to acetale with ne = 4 electrons: I ¼

I ne FQL CA0

ð1Þ

where QL is the volumetric flow rate of the solution and CA0 is the inlet concentration of 4-MA. One of the bottlenecks identified by this experimental study was the significant effect of gas phase that was generated at the counter-electrode. In a related paper, Kristal et al.12 reported an initial numerical and experimental study assessing the effect of the gas phase on the current density distribution (CDD) in the monopolar thin-gap electrochemical reactor. The numerical results showed that bubbles with diameters larger than 50% of the interelectrode distance significantly disturb the local CDD and may negatively affect the microreactor performance. The hydrodynamic aspects related to the gas evolution during the electrochemical reaction were investigated in details by Kristal.13 In the following work, Rode14 and Attour15 focused on the microreactor operation at elevated pressure. A review of the Special Issue: Nigam Issue Received: March 31, 2011 Accepted: July 19, 2011 Revised: July 18, 2011 Published: July 19, 2011 1515

dx.doi.org/10.1021/ie200654c | Ind. Eng. Chem. Res. 2012, 51, 1515–1524

Industrial & Engineering Chemistry Research microreactor application for electro-organic synthesis was published by Ziogas et al.,16 where the authors also selected the methoxylation of 4-MA as an example demonstrating the advantages of microreactors compared to that of the conventional technology. In our previous work,17 we proposed a concept for a bipolar electrochemical microreactor for the electro-organic synthesis. Potential process intensification was documented on an example of the 4-MA methoxylation and compared to the current industrial BASF process. The obtained results confirmed that the bipolar reactor provided the electrochemical characteristics (process selectivity and substrate conversion) very close to the industrial capillary-gap cell operated by BASF.1 At the same time, it could provide substantial investment and operational cost savings in the subsequent product separation step of the technology due to the reduction of the volume of the treated electrolyte solution. A savings of approximately 50% of the heat energy were estimated. However, the savings can be substantially increased by increasing the substrate concentration in the electrolyte solution provided that a high degree of conversion is maintained during a single-pass of the electrolyte through the series of electrochemical microreactors. The reduction of the investment costs is strongly related to the reduction of the size of the distillation column required for the separation step. In the presented paper, we focus on the design of the bipolar electrochemical microreactor. First, the background behind the mono- and bipolar arrangement is given, and the choice of bipolar arrangement is justified. Then, two possible microreactor constructions are presented and compared by the numerical simulations and experiments. The numerical simulations are focused on the predictions of the qualitative trends of the parameters affecting the reactor design and performance (e.g., the parasitic current effect with respect to interelectrode distance, the length of the electrodes, the number of gaps, and the electrolyte conductivity). In particular, the numerical simulations focus on the description of the local potential and current density, which is not accessible experimentally, but it represents an important aspect in understanding the experimentally observed phenomena. In the experimental part, the performance of both types of the proposed bipolar microreactors is verified by evaluating the effect of electrode material and the number of electrode gaps on the conversion, selectivity and cell voltage. The two proposed BEMRs were manufactured using different technologies with different accuracy of the interelectrode gaps. Therefore the study also focuses on the verification and comparison of the effect of manufacturing method on the length of the parasitic currents pathways and gas generation.

’ ELECTROCHEMICAL MICROREACTOR DESIGN There are two major types of electrolyzers in industrial electrochemistry differing by the contact of the electrodes with the power supply: (i) monopolar and (ii) bipolar cells.18 In the monopolar arrangement each electrode is connected to an external direct current (DC) source so that it is polarized intermittently either as an anode (positively polarized electrode) or cathode (negatively polarized electrode), see Figure 1a. The total voltage of the monopolar electrolyzer (Um) is relatively low, because it is delimited only by the single-cell voltage. However, the total electric current input (Im) is high because the individual monopolar electrolytic cells are connected in parallel.

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Figure 1. Schematic sketches of electrolyzers with (a) monopolar and (b) bipolar electrodes arrangement: I, total current; U, total voltage; m, monopolar; b, bipolar; solid-dotted line, distribution of Galvani potential in the system; x, positive polarity; Q, negative polarity. Black arrows indicate electronic current (effective current) direction; white arrows indicate ionic current (parasitic current) direction.

In the case of the bipolar arrangement, the electric circuitry is significantly simplified. As shown in Figure 1b, only the terminal electrodes are in electric contact with the current source (power supply). The total voltage of the bipolar electrolyzer (Ub) is significantly higher and the total current (Ib) is lower, when compared to the voltage and current of the monopolar cell. This is because the single electrolytic cells in the bipolar arrangement are connected in series. The simplified electric circuitry of the bipolar arrangement significantly reduces the investment costs and makes the device more compact. It is particularly advantageous when the process is operated at an elevated temperature or pressure, or as a microscale process. Moreover, the higher operational voltage and lower current load regime of the bipolar arrangement allow for the reduced ohmic losses (heating) in the external electric circuitry and the lower energy losses due to the rectification of the distribution network current. In practice, the bipolar electrode arrangement represents an important feature of the electrolytic cell construction and the installed capacity prevails over the monopolar cells. As an example, one of the most important electrochemical processes, the brine electrolysis, is operated in the bipolar arrangement.19 Another typical applications of bipolar electrodes are the fuel cells, water electrolysis, and potassium permanganate and potassium dichromate production.2023 The bipolar electrode arrangement is also frequently employed in the field of organic electrosynthesis, for example, in case of methoxylation and hydrodimerization reactions or for oxidations of polynuclear aromatic hydrocarbons.23 Design of electrochemical systems should meet the requirements associated with the supply, circulation, and removal of solutions and products, including heat. In the case of electrolytic cells equipped with bipolar electrodes the ionic conduction paths in solution beyond the interelectrode gap lead to the existence of a so-called “parasitic” current (also referred to as a bypass or shunt current), that is, current flowing between separate electrolytic cells outside the bipolar electrode assembly (e.g., through the manifold system24). The parasitic current pathways and its direction are shown in Figure 1b as white arrows. Black arrows in the same figure represent the charge flux crossing the bipolar electrodes, which is related to the electrochemical processes at the electrode surface. For the purpose of this paper, this charge flux is referred to as the “effective” current. Note that the value of 1516

dx.doi.org/10.1021/ie200654c |Ind. Eng. Chem. Res. 2012, 51, 1515–1524

Industrial & Engineering Chemistry Research

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Figure 2. Filter-press-type bipolar electrochemical microreactor BEMR-FP: (a) scheme; (b) single electrode plate; (c) photograph.

the effective and parasitic current may vary along the electrode stack with the minimum or maximum in its center, respectively. A possibility of the check of the parasitic current is, therefore, the key factor determining the feasibility of the electrochemical process in the bipolar electrode arrangement. In practice the process efficiency loss due to the existence of the parasitic current is often expressed by the current utilization of the bipolar electrolyzer η [%] according to eq 2. It represents the ratio of the total electrical current passing through each bipolar and one terminal electrode, to the cell current load multiplied by the number of bipolar cells. I þ η% ¼

N1

∑

i¼1

NI

I ef , i 100

ð2Þ

In this equation, N denotes the number of bipolar electrochemical cells, N 1 corresponds to the number of bipolar electrodes, I [A] represents the total current load and Ief [A] is the effective current corresponding to each bipolar electrode i. The Ief,i can be obtained by integration of the local current density along either the anodic or cathodic fraction of the bipolar electrode surface. The minimum economically acceptable value of η is around 90 to 95%, depending on the nature of the process. There are few simplified methodologies to predict the magnitude of the parasitic current in dependence on the system properties, which are mainly based on the equivalent electric circuitry analysis.24 The more rigorous approach, necessary for the complex geometries and complex electrochemistry, is based on the solution of the Galvani potentials distribution resulting from the numerical integration of the Laplace’s equation, see eq 3,2022,24,25 0 ¼ ∇ðσ∇ϕÞ

ð3Þ

where σ [S m1] represents the specific conductivity of ionic or electronic conductive media and ϕ [V] is the Galvani potential. The approach built on the solution of eq 3 is also employed in this paper to evaluate the parasitic current values for the two proposed types of the bipolar electrochemical microreactors and their geometrical and operating parameters.

Electrochemical Microreactor Construction. In the presented study, two types of the in-house-made bipolar electrochemical microreactors (BEMR) were investigated: (i) filterpress-type microreactor (BEMR-FP) and (ii) block-type microreactor (BEMR-B). Both BEMRs had the low aspect ratio (depth-to-width) of the interelectrode gap, 0.0109 and 0.0176 for BEMR-FP and BEMR-B, respectively. The difference between the two designs was mainly the ease of reconfiguration, scalability, and possible gas-phase separation. Electrode material also played an important role in the electrochemical process. Glassy carbon (Sigradur G, HTW, Germany) was used in the construction of the BEMR as anode materials for good chemical resistance, corrosion stability, and electro-catalytic properties. For cathode construction, glassy carbon and stainless steel were tested in various BEMR arrangements. Filter-Press-Type Bipolar Electrochemical Microreactor. The BEMR-FP had an arrangement similar to the filter press device. The design of this BEMR has an inlet and an outlet part of each electrode compartment (electrode gap between the electrodes) electrochemically nonactive to minimize the parasitic current value. Construction of BEMR-FP is shown in Figure 2. The 5 mm thick electrode frames of BEMR-FP were made from Hard Resin CR39 (Great Lakes Chemical Co., USA), which is chemically very stable and resistant. The glassy carbon electrode (27.7 mm long and 9.2 mm wide) was placed in the center of the electrode frame. The electrode gap was 0.1 mm deep. The alignment of the electrode and plastic walls in the gap was made to a precision on the level of 0.01 mm. The gap, also 9.2 mm wide, had a conical shape around the inlet liquid distribution channel and on the top around the outlet collector channel for the removal of the mixture of hydrogen and electrolyte from the electrode gap. These two plastic parts of the gap are 10.7 mm long including the conical ends. The nonconductive electrochemically inactive parts of the gap were intended for increasing the ohmic resistance of the gap and hereby decreasing the value of the parasitic current. The terminal electrodes were connected to the power supply. The contact between the glassy carbon electrode surface and the connecting wire was realized using a mercury drop. The set of a 1517



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Figure 3. Block-type bipolar electrochemical microreactor: (a) microreactor assembly; (b) fixed block of electrodes.

Table 1. Geometry of the EMRsa Filter-press

Block

BEMR-FP

BEMR-B

material, anode

glassy carbon

glassy carbon

material, cathode

glassy carbon

glassy carbon

stainless steel number of gaps

14

electrode size w l (mm2)

9.2 27.7

4 6.8 30.5

active electrode area (cm2)

2.548

2.074

10.194b

8.296b

0.1 9.2 0.92

0.12 6.8 0.816

3.68b

3.264b

0.0109

0.0176

Figure 4. Schematic sketch of the 2D and 3D model geometry of the BEMR-FP: (a) cross-section through the center and in parallel with the main current flux, (b) side view perpendicular to main current flux; l = 28.0 mm, d = 0.1 mm, h = 9.8 mm, m = 5.6 mm, t = 0.55 mm (2D), t = Ø = 2.5 mm (3D), w = 9.0 mm (figure not in scale). Gray fields, active electrodes; A, anode; B13, bipolar electrodes; C, cathode; numbering of boundaries bounded by circular points: 1, electric insulation; 1p, electric insulation, however hydraulically permeable boundary (i.e. electrolyte inlet and outlet, flow direction indicated by white arrows); 2, anode feeder; 3, cathode collector; 4, electrode/electrolyte interface.

Abbreviations: w = width, l = length, d = depth (inter-electrode distance). b Four gaps.

very low. The reaction temperature was easily controlled by the inlet temperature of the reaction mixture. The advanced heat management, scale up of the reactor, and comparison with industrial process is subject of further study. The overview of the geometric parameters of both BEMR is given in Table 1.

maximum of five electrode sheets was clamped together with two 10 mm thick stainless steel flanges and four bolts. The flanges ensure uniform pressure on the electrode sheets and provide good tightness of the electrode gaps via O-rings. The BEMR-FP construction enabled us to alternate the number of electrodes and thus to operate the BEMR-FP both as a mono- or bipolar reactor. In the monopolar arrangement all bipolar electrodes are removed and a reactor with a single cell is obtained. In this case, the installation of one electrode manufactured from various materials is possible. Electrode sheets were manufactured by conventional machining technologies, using milling machinery. The accuracy of the gap dimensions was within 0.05 mm. Block-Type Bipolar Electrochemical Microreactor. The BEMR-B (Figure 3a) had four parallel electrode gaps. The glassy carbon 9.8 0.87 30.5 mm3 electrodes (width thickness length) were fixed 0.125 mm apart using spacers made of Kapton. The Kapton spacers in the shape of strips (0.125 mm thick and 1.5 mm wide) were fixed along the long side of the gaps. The active width of each electrode gap was 6.8 mm. The set of five electrodes was manufactured as a fixed block in glass gasket (Figure 3b). The whole length of the electrode channel was equal to the length of the electrochemically active electrode. The electrode and electrode gaps were made with the accuracy of 1 μm. Neither of the proposed BEMR designs included internal heat exchanger. Our experiments with a single gap EMR proved that for the studied reaction the heat generated by the ohmic resistance in the investigated range of operating conditions is

’ MATHEMATICAL MODEL Model geometries corresponding to the proposed experimental block-type microreactor (BEMR-B) and the filter-press type microreactor (BEMR-FP) are shown in Figure 4 and Figure 5, respectively. For the modeling purposes, both of them consisted of three bipolar and two terminal parallel plate electrodes, that is, of four bipolar electrolytic cells. The cells were oriented vertically, the electrolyte was supplied to the bottom of the BEMR, and gasliquid mixture was removed from the top, as shown by arrows in the respective figures. For the calculation, it was assumed, that the outlet gasliquid mixture was primarily formed by the gas phase and as such it was very little conductive. Therefore, the outlet collector channel and the inactive part of the gaps above the electrodes were not taken into account in the model geometry. Due to the consideration of the electrochemically inactive parts the individual bipolar cells in BEMR-FP were separated with an electrically insulating barrier and the absence of the anodecathode polarity transition region along the bottom side of the bipolar electrode. The only available pathway for the parasitic current was through the gaps and inlet distribution channel. The BEMR-FP was modeled in 3D geometry. Figure 4a represents cross-section through the center of the reactor in parallel with the direction of the main electric current flux, while Figure 4b provides the side view perpendicular to the main direction of the electric current flux. However, the modeling of the reactor with a higher number of bipolar cells was not feasible in 3D due to the excessive computational demands. In order to

electrode gap size d w (mm ) electrode gap cross-section (mm2) 2

electrode gap aspect ratio a

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Figure 4 and Figure 5. The boundaries with identical boundary condition are connected together and bounded with circular points. Zero potential gradient in the direction normal to the boundary expressed by eq 6 was considered for insulated boundaries. ∂ϕi ð6Þ ¼0 ∂n b, i

Figure 5. Schematic sketch of the 2D model geometry of the BEMR-B; l = 30.0 mm, d = 0.12 mm, h = 2.0 mm, m = 1.0 mm, a = (l b)/2, b = 5.0 mm, w = 6.8 mm (figure not in scale). Gray fields, active electrodes; A, anode; B13, bipolar electrodes; C, cathode; numbering of boundaries bounded by circular points: 1, electric insulation; 1p, electric insulation, however hydraulically permeable boundary (i.e. electrolyte inlet and outlet, flow direction indicated by white arrows); 2, anode feeder; 3, cathode collector; 4, electrode/electrolyte interface.

observe the effect of the number of bipolar cells on current utilization, the model geometry was reduced from 3D to 2D. The 2D model geometry of the BEMR-FP is represented by Figure 4a. The thickness of the inlet distribution channel t was chosen so as to keep the same relative ratio between the crosssection areas of the horizontal and vertical distribution channels for 2D and 3D model geometry. In case of the BEMR-B in the absence of inactive part of the gap, the bipolar electrodes were placed directly in the electrolyte allowing the parasitic current to flow directly along the bipolar electrode sides (Figure 5). Modeling of this type of bipolar arrangement was significantly more complex, comparing to the BEMR-FP. This was due to the high numerical instability in the polarity transition region. This case was mathematically treated as a 2D assuming symmetry of the system in the direction perpendicular to the main electric current flow and parallel to the main electrolyte flow direction. The model of the secondary current density distribution was used to calculate the local Galvani potential field and parasitic current value in both studied BEMR geometries. The background of this model is the Laplace equation, eq 3, which under the assumption of constant medium (electrolyte, electrode) conductivity was simplified to eq 4. 0 ¼ ∇2 ϕi

ð4Þ

The values of local current density were then obtained using eq 5 assuming that the only migration allowed is the transport of the electric charge both in the electrolyte and the electrode phase. j ¼ σi ∇ϕi

ð5Þ 2

The j [A m ] in eq 5 represents the local current density and subscript i distinguishes electrolyte or electrode phase. Specific conductivities of 0.05144 S m1 and 22.151 S m1, for electrolyte and electrode material, respectively, were used for the calculation. Furthermore, there were four different boundary conditions used in the model designated as (1) electric insulation, (2) anode feeder, (3) cathode collector, and (4) electrode/ electrolyte interface. The boundary conditions are indicated in

Subscripts b,i indicate the boundary adjacent to phase i, and n is the unit vector perpendicular to the boundary. The anode feeder boundary condition is characterized by the constant current density, eq 7, and simulates the part of the anode connected to the electric current supply. ∂ϕs ð7Þ σ s ¼ j0 ∂n b, s

Here, subscript s indicates the solid phase of the electrode and j0 [A m2] is the applied current density calculated from the value of required current load and current feeder area. In the case of the current collector, the zero Galvani potential was applied, see eq 8. ϕs jb, s ¼ 0

ð8Þ

At the electrode/electrolyte interfaces the charge flux continuity was considered, eq 9: ∂ϕs ∂ϕe ¼ σe ð9Þ jse ¼ σs ∂n ∂n se

se

where subscript e refers to electrolyte and se to electrode/ electrolyte interface. This condition represents the continuity of the electronic and ionic charge flux. Furthermore, the electrode/electrolyte interface was characterized by discontinuity of the Galvani potential due to the electrode surface polarization, eq 10: ϕs ϕe jse ¼ E

ð10Þ

where E is the electrode potential that is in close relationship with the local intensity of charge flux (current density) through the interface. This relationship is also called the polarization function. In this case, the experimentally determined Bulter Volmer polarization curve corresponding to the studied system was applied, eq 11.17,26 jse ¼ jEref , A 10ððE Eref , A Þ=bA Þ þ jEref , C 10ððE Eref , C Þ=bC Þ

ð11Þ

The kinetics parameters jEref,A and jEref,C represent the anodic and cathodic current density at formal reference electrode potentials Eref,A and Eref,C, respectively. The concept of the formal electrode potentials was adopted because the corresponding reactions are irreversible and the classic exchange current densities cannot be evaluated.26 Next, bA and bC are Tafel’s slopes of the anodic and cathodic reaction. The values of the individual kinetic parameters used in this study are summarized in Table 2. Different numerical approaches were employed to solve Galvani potential distribution in BEMR-B and BEMR-FP. In the BEMR-B case, where the bipolar electrodes include the anode/cathode transition region, a rectangular nonequidistant 2-dimensional grid was used for the numerical approximation of the Laplace equation by the finite volumes method. The system of the linear and nonlinear algebraic equations was subsequently 1519



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Table 2. Electrode Reaction Kinetic Parameters26 parameter

value

jEref, A

14.93 A m2

description current density at formal anodic potential

jEref, C

1.0 A m2

current density at formal cathodic potential

Eref,A

1.31 V

formal anodic electrode potential

Eref,C

1.6 V

formal cathodic electrode potential

bA

0.107 V dec1

Tafel’s slope of anodic reaction

bC

0.2

Tafel’s slope of cathodic reaction

solved by Newton’s method using MATLAB. This modeling approach was proposed and validated in ref 24. This procedure exhibited sufficient numerical stability even in the vicinity of the anodecathode transition region of the bipolar electrode surface. The absence of the anode/cathode transition region in BEMR-FP allowed the use of the less stable finite-elements method implemented in the COMSOL Multiphysics environment to solve the Laplace equation numerically. Owing to COMSOL, the more accurate 3D model of this microreactor containing numerous curved boundaries could be easily solved. However, the BEMR-FP with a higher number of bipolar cells had to be solved using the simplified 2D version as mentioned above. The partial differential equation was approximated by the second order Lagrange element, and the standard linear system solver “umpfack” was used. A relative tolerance was 1 108. Experimental Section. The following experimental setup was used for the experiments. The solution of the reaction mixture was pumped by a syringe pump (KDS200, KD Scientific) to the BEMR. Samples for analysis were collected to the vials immediately at the BEMR outlet. The terminal electrodes of the BEMR were connected to the power supply (MOTECH 1022). A data acquisition program (Labview, National Instruments) was used to monitor the cell voltage and applied current. HPLC and GC were used for analysis of the reaction products. A solution of the reaction mixture contained 4-methylanisole (purissimum grade, Acros Organics), methanol (HPLC grade, Merck), and potassium fluoride (analytical grade, Acros Organics) as a supporting electrolyte. All experiments were carried out for the following operating conditions: QL = 0.04 mL min1 per one gap; P = 1 bar; T = 25 °C; 4-methylanisole inlet concentration = 0.1 mol dm3; KF concentration = 0.01 mol dm3. For each gap and operating conditions the current was increased in steps to cover the desired range of dimensionless current I*. At each value of the current the BEMR was operated for 30150 min (1 gap) to reach the steady state and then until 2 mL of sample was collected.

’ RESULTS AND DISCUSSION In the following, the results from both the modeling and the experiments are presented. For the unbiased comparison of different results we used the concept of the dimensionless current I* defined by eq 1. The presented model results document the current utilization η in the block and filter press BEMR with respect to (i) interelectrode distance, (ii) electrode length, (iii) electrolyte conductivity, and (iv) number of bipolar electrolytic cells (or interelectrode gaps). The presented experimental results include the effect of (i) cathode material,

Figure 6. Dependence of current utilization η on the dimensionless current I* for various parameters of BEMR-FP; (a) interelectrode distance d = 0.01, 0.10, and 1.00 mm, (b) length of the electrodes l = 10, 28, and 48 mm, (c) conductivity of the electrolyte σ = 0.001, 0.05144, 1.0, and 10.0 S m1, (d) number of the bipolar electrochemical cells (or gaps) N = 2, 5, 10, 20, and 30; arrows indicate direction of studied parameter value growth. Initial input parameters (unless otherwise stated): electrolyte specific conductivity = 0.05144 S m1, electrode specific conductivity = 2.2 104 S m1 (for basic geometrical parameters see Figure 4).

(ii) number of interelectrode gaps, and (iii) performance comparison of the block and filter-press BEMR. Model Results. All results of the mathematical modeling parametric study presented in Figures 6 and 7 show a common behavior. It is the constant value of the current utilization in the low current load range. The extent of this behavior strongly depends on the cell geometry and electrolyte conductivity. It clearly corresponds to the situation, when the bipolar electrodes are not activated and only the terminal electrodes participate in the electrode reaction. In this case, the utilization of current passing through the system is, according to eq 2, constant and independent of the cell load. This explanation is confirmed by the fact that the cell with four bipolar electrodes exhibits in this region a current utilization of 25%. It is particularly well visible in Figures 6c and 7c. The extent of the region of constant initial current utilization is for the given electrochemical process determined mainly by the manifold ohmic resistivity as discussed in theoretical part of this study. In the case of high resistivity this region is extremely short; see Figures 6c and 7c for the lowest electrolyte solution conductivity value. In opposite, with decreasing value of the manifold resistance, extent of this region is increasing (see Figures 6c and 7c as well). It has to be pointed out in this place that in the case of BEMR-B and electrolyte solution conductivity 10 S m1, the bipolar electrodes are not activated within the entire investigated range of the current load. This is due to the specific geometry of this cell characterized by the electrochemically active electrode 1520



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Figure 8. Effect of cathode material on the conversion and selectivity: BEMR-FP 1 gap; QL = 0.04 mL min1; P = 1 bar; T = 25 °C; anisole inlet concentration= 0.1 mol dm3; potassium fluoride concentration = 0.01 mol dm3.

Figure 7. Dependence of current utilization η on the dimensionless current I* for various parameters of BEMR-B; (a) interelectrode distance d = 0.01, 0.12, and 1.00 mm, (b) length of the electrodes l = 10, 30, and 48 mm, (c) conductivity of the electrolyte σ = 0.001, 0.05144, 1.0, and 10.0 S m1, (d) number of the bipolar electrochemical cells (or gaps) N = 2, 5, 10, 20 and 30; arrow indicates direction of studied parameter value growth. Initial input parameters (unless otherwise stated): electrolyte specific conductivity = 0.05144 S m1, electrode specific conductivity = 2.2 104 S m1 (for basic geometrical parameters see Figure 5).

sides, that is, by the absent vertical electrochemically inactive part of the interelectrode gap. The current load corresponding to the activation of the bipolar electrodes is indicted by a rapid increase of the current utilization. However, in the region of η I* curve characterized by activated bipolar electrodes, the additional characteristic behavior can be identified. The rapid increase of the current utilization indicates behavior driven by the equilibrium single-cell voltage and electrode reaction kinetics. As the current load is further increased, the current utilization curve passes through a transition interval to the region, where the drop of the potential on the effective current pathway is limited to a constant value due to the exponentially increasing electrode reaction kinetics. Because of this effect the absolute value of the parasitic current is getting more or less independent of the cell current load. Current utilization thus asymptotically approaches the maximum of 100%. It is worth noticing, that the model results corresponding to BEMR-FP predict high current utilization already at very low dimensionless currents. However, in the case of BEMR-B similar current utilization values are obtained at current loads about 2 orders of magnitude higher. This is due to the absence of the electrochemically inactive part of the interelectrode gap, which significantly increases the electric resistance of the entire hydraulic circuitry providing ionic conductive pathways for parasitic current flow. Figure 6a shows that in the case of the BEMR-FP the utilization of the current increases when interelectrode distance d is reduced. This is because the interelectrode distance

Figure 9. Effect of cathode material on the cell voltage: BEMR-FP 1 gap; QL = 0.04 mL min1; P = 1 bar; T = 25 °C; anisole inlet concentration = 0.1 mol dm3; potassium fluoride concentration = 0.01 mol dm3; dashed lines, linear fit.

determines the effective cross-section area of the interelectrode gap and therefore its ohmic resistance of the distribution channels, i.e. electrolyte manifold system. A similar trend is observed for BEMR-B at a higher current load region, as shown in Figure 7a. However, in the lower current load region the current utilization decreases with the decreasing distance between electrodes of the BEMR-B. This corresponds to the already discussed absence of the electrochemically inactive parts of the interelectrode gap in the BEMR-B. A reduced interelectrode distance thus in the low current load region contributes mainly by the reduced ohmic resistance between the terminal electrodes (reduced distance). First, after the sufficient bipolar electrode activation, the effect of the increased interelectrode cross-sectional area becomes dominant similarly to the BEMR-FP cell. As mentioned above, the selectivity and the current efficiency of the process is significantly affected by the amount of hydrogen accumulated in the interelectrode gap during the single electrolyte pass through the cell. Reduction of the channel length lessens the amount of the produced gas, which, however, may contribute to higher parasitic current. It is due to the lower relative crosssectional area of the single-cell with respect to the cross-section available for parasitic current flow. The calculations for both BEMR revealed that the length of the electrodes not to be a 1521



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Figure 10. Effect of the number of electrode gaps on the conversion and selectivity: BEMR-FP; QL = 0.04 mL min1 per gap; P = 1 bar; T = 25 °C; anisole inlet concentration = 0.1 mol dm3; potassium fluoride concentration = 0.01 mol dm3.

crucial problem (from the current utilization point of view) in practically relevant range of this geometrical parameter, see Figures 6b and Figure 7b. One of the intensively studied problems of the electro-organic synthesis is the high ohmic drop in the electrolyte between the electrodes due to the low ionic conductivity of the reaction solutions. In the case of bipolar arrangement, the increase of the solution conductivity should be carefully analyzed with respect to the current utilization. As demonstrated in Figure 6c and Figure 7c, the advantage of increased electrolyte conductivity is gained at the expense of the loss of the current load utilization. This corresponds to the fact, that at higher conductivity a higher fraction of the input current is needed to reach the activation ohmic drop. Particularly, in the case of the BEMR-FP, the model results confirmed that the variation of electrolyte conductivity is viable in the entire studied range without a noticeable loss of current utilization. However, for BEMR-B already the electrolyte conductivity higher than 0.0514 S m1 would have the detrimental effect on the reactor performance. Since in this particular study the increase in the production capacity of the BEMR is proposed in the form of an increase in the number of bipolar cells, the dependence of cell efficiency on this parameter is of great interest. The model results of the corresponding parametric study for BEMR-FP and BEMR-B are summarized in Figure 6d and Figure 7d. It is evident that the current utilization continuously decreases with increasing number of the bipolar cells up to 20 in both cases. It is due to linear increase of the total cell voltage with the number of bipolar cells. The higher cell voltage results in higher parasitic currents in the horizontal distribution channel. The increase has a limit where the ohmic potential drop in the horizontal distribution channel along a single bipolar electrode side exceeds the limit necessary for its function at given current load. In the present case, this limit is reached in the cell containing approximately 20 bipolar cells. The observed decrease of current utilization with increased number of bipolar cells, however, contradicts the required extension of the BEMR capacity. Nevertheless, the current utilization drops below 95% only at the current loads significantly lower when compared to the operating value. The concept of increased number of bipolar cells is therefore viable in a very broad number of cells. However, a few implications have to be considered, especially the significantly more complex design for ensuring the uniform flow distribution among the individual cells as well as heat management.

Experimental Results. As seen in Figure 8 the results in the substrate conversion obtained for the two cathode materials considered do not differ. Also the maximum reaction selectivity reaches the same value of 72% at I* about 0.9 (approximately 25 mA) for both materials. At the current loads higher than I* = 1 the selectivity on glassy carbon electrode is systematically lower by 10 to 20%. The effect of the cathode material on the cell voltage in BEMRFP with one gap is illustrated in Figure 9. In contrast with the previous figure there is a significant difference between the glassy carbon and the stainless steel cathode. The voltage of the cell utilizing the glassy carbon cathode is higher approximately by 0.6 V. This difference is in agreement with the theory because the hydrogen evolution reaction overvoltage is higher on the glassy carbon than on the stainless steel. The number of the electrode gaps affects also the substrate conversion in an expected manner (Figure 10a). An increase in the number of interelectrode gaps was expected to result in a lower substrate conversion due to the parasitic current flow via the inlet electrolyte distribution channel and the output collector channel. Exclusively this effect, however, cannot explain the observed decrease of the conversion. The inhomogeneous electrolyte flow distribution between the individual channels in the stack seems to play an important role. This effect is chiefly caused by the presence of the gas phase in the interelectrode gap. It not only disturbs the local current density distribution field but also introduces the irregularity into the hydraulic resistance of the individual channels. This can explain the fact that the decrease of the conversion is especially pronounced between the one- and the two-gap arrangement, while the conversion for the two- and three-gap arrangement is practically the same in the whole range of I*. The four-electrode gap again decreases the conversion of EMR due to the variation in the electrolyte flow distribution at the high dimensionless currents I* > 1 and low total electrolyte flow rate. The effect of the number of the interelectrode gaps on the reaction selectivity in Figure 10b is less explicit. The maximum selectivity decreases with increasing number of electrode gaps (from 70, 60, 57 to 52%) but the position of the peak values varies between I* = 0.85 and 1.3, without a clear trend. This can be explained by the combined effect of several simultaneous factors. The low electrolyte flow rate and the short electrode compartment result in the low pressure drop in the interelectrode gap, which adversely affects the uniformity of the electrolyte 1522



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clearly see the good agreement between both designs. Therefore, for the particular application, one needs to consider the advantages of individual designs and to choose the optimal BEMR to meet the specific process requirements.

Figure 11. Effect of the number of electrode gaps on average cell voltage: BEMR-FP; QL = 0.04 mL min1 per gap; P = 1 bar; T = 25 °C; anisole inlet concentration = 0.1 mol dm3; potassium fluoride concentration = 0.01 mol dm3; dashed lines, linear fits.

Figure 12. Comparison of the conversion in BEMR-B and BEMR-FP; QL (BEMB-B) = 0.045 mL min1 per gap; QL (BEMB-FP) = 0.04 mL min1 per gap; P = 1 bar; T = 25 °C; anisole inlet concentration = 0.1 mol dm3; potassium fluoride concentration = 0.01 mol dm3.

distribution between the individual interelectrode gaps. The low flow rate promotes the bubble coalescence and formation of the large bubble resulting in the flow pattern similar to the Taylor flow in the capillaries. The presence of gas in the collector channel causes oscillation of the parasitic current in the manifold and amplifies the potential and current oscillations on the bipolar electrodes resulting in the reduced selectivity. Figure 11 shows the effect of the number of interelectrode gaps on the average cell voltage in BEMR-FP. The cell voltage increases with the increasing number of interelectrode gaps, which is again in agreement with the expectations. As stated above, this is because the individual electrolytic cells in the bipolar arrangement are connected in series. The linear trends of all voltage curves for all cases confirm that the BEMR-FP performs uniformly from the electrochemical standpoint, regardless of the number of interelectrode gaps. A comparison of the conversion achieved in BEMR-B and BEMR-FP is shown in Figure 12. As mentioned earlier, BEMRFP provides the possibility of an easy change of the number of interelectrode gaps, and the top and bottom of each electrode are insulated. This results in weaker parasitic currents. On the other hand, BEMR-B design has fixed and more accurate positions of the electrodes leading to slightly better performance and reproducibility of the results. However, from the comparison one can

’ CONCLUSIONS The presented study summarizes the results of the theoretical and experimental comparison of the monopolar (single gap) and two types of bipolar microstructured electrochemical reactors for electro-organic synthesis. As expected, the best performance was observed for the monopolar arrangement. The reason is based on the well-defined electrochemical, but mainly hydrodynamic, conditions of the process. Unfortunately, the utilization of the monopolar cell is not feasible on the industrial scale owing to the extreme demands of this arrangement on the electric and hydraulic circuitry. From this point of view the bipolar cell represents the only viable solution. In the case of the bipolar arrangement, the BEMR-FP reactor principally shows a higher potential for an industrial application. However, due to the minor irregularities in the BEMR-FP component manufacturing, the BEMR-B shows better results in the investigated range of operating conditions. A comparison of the experimental data with the mathematical model indicates an overestimation of the cell performance by the mathematical model. This was explained in terms of neglecting the evolution of the gaseous phase in the interelectrode gap and possible flow irregularities induced between the individual channels of the bipolar cells in the applied mathematical model. To conclude, if manufactured to a high precision the BEMRFP represents the best solution of the bipolar cell for electroorganic synthesis. The optimization of the flow distribution between individual channels is an important target of further research leading to the improved cell performance. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Authors gratefully acknowledge the financial support of the GACR Project No. GA104/09/0880. ’ REFERENCES (1) Lund, H.; Hammerich, O. Organic Electrochemistry; Marcel Dekker: New York, 2001. (2) Belmont, C.; Girault, H. H. Coplanar interdigitated band electrodes for electrosynthesis. Part 2: Methoxylation of furan. J. Appl. Electrochem. 1994, 24, 719–724. (3) Belmont, C.; Girault, H. H. Coplanar interdigitated band electrodes for electrosynthesis. Part 3: Epoxidation of propylene. Electrochim. Acta 1995, 40, 2505–2510. (4) Belmont, C.; Ferrigno, R.; Leclerc, O.; Girault, H. H. Coplanar interdigitated band electrodes for electrosynthesis. Part 4: Application to sea water electrolysis. Electrochim. Acta 1998, 44, 597–603. (5) Ferrigno, R.; Josserand, J.; Brevet, P. F.; Girault, H. H. Coplanar interdigitated band electrodes for electrosynthesis. Part 5: Finite element simulation of paired reactions. Electrochim. Acta 1998, 44, 587–595. (6) Lowe, H.; Ehrfeld, W. State-of-the-art in microreaction technology: Concepts, manufacturing and applications. Electrochim. Acta 1999, 44, 3679–3689. 1523



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