Efficient Reduction of Carbon Tetrachloride in an Electrochemical Reactor with a Three-Dimensional Electrode

Abstract

A selective electrochemical synthesis of chloroform from carbon tetrachloride (CT) in a laboratory-scale electrochemical reactor using a carbon felt three-dimensional electrode is studied. The characterization of the electrochemical reactor from the point of view of material transport was carried out, obtaining good correlations both in the adjustment to a simple bath reactor model and the adjustment to a piston-flow model, and the operating parameters were obtained, such as the material transport coefficient or the limiting intensity. The galvanostatic electrolysis of CT in the filter-press reactor obtained good values for current efficiency and selectivity, so that only hydrogen was obtained as a by-product. With respect to the use of flat electrodes, the three-dimensional carbon felt electrode improves the results in all the studied parameters under identical experimental conditions.

1. Introduction

Because of the growing social awareness about the environment and pollution, the chemical industry has had to adapt its processes in recent decades to make them more environmentally sustainable. In this sense, several strategies have been developed, including substituting chemical processes with less polluting ones, the on-site recycling of waste, avoiding any type of landfill, and the safe disposal of polluting waste [1,2].

In this new scenario, electrochemistry can play an important role as an environmentally friendly technique, and in fact, in recent decades, numerous processes in industrial chemistry have been redesigned using electrochemistry as a synthetic route instead of classical homogeneous chemistry [3]. The advantage of electrochemical processes is that they offer direct control over the rate and conditions of the reaction, and thus over the progress of the reaction.

Carbon tetrachloride is a chemical compound that was widely used in the past, e.g., for dry cleaning. Today, however, its use and industrial synthesis has been banned in most countries because of its toxicity and because it is on the Montreal Protocol’s list of compounds considered harmful to the ozone layer [4]. Nevertheless, this compound still appears as a by-product in the synthesis of other chloromethanes, which is a problem for industry, as it has to take care of the safe disposal of this waste, with the added costs that this entails.

Numerous studies have addressed different routes for the disposal of carbon tetrachloride by multiple means, which are not only chemical but also physical, such as pyrolysis [5,6,7,8,9]. However, its disposal through recycling it into a useful product for the chemical industry has hardly been addressed. Only a few studies have addressed the elimination of CT by electrochemical means [10,11,12,13], but reductive electrolysis proved to be not very selective, leading to mixtures of several products, even with two or three carbon chains [10]. In other cases, the process was carried out in aqueous media, leading to low current efficiencies, as low concentrations of carbon tetrachloride had to be used [13].

In the present work, an electrochemical route for the selective transformation of CT into chloroform and dichloromethane is presented. This is an efficient procedure, carried out under mild conditions and in a filter-press-type electrochemical reactor with a three-dimensional carbon felt electrode.

Three-dimensional electrodes have the capacity to considerably increase the effective area in which the electrochemical reaction can take place, which results in the use of lower current densities, as well as a considerable reduction in the time required for the reaction to be completed [14,15]. Numerous studies have demonstrated the effectiveness of this type of electrode in industrial use, particularly carbon felt electrodes, due to their physico-chemical properties and low reactivity under normal operating conditions [16]. Previous studies have shown that the optimum conditions for the electrolysis of TC to chloroform were an ethanol–water mixture (4:1) as the solvent and NaCl as the supporting electrolyte [17].

Therefore, the aim of this work is to study the feasibility of an electrochemical process for the selective transformation of CT into chloroform through its reduction in a laboratory-scale filter press reactor using a three-dimensional electrode. Previous work has demonstrated the feasibility of this process using flat metal electrodes. In this work, the aim is to demonstrate the improvement in this process when using three-dimensional electrodes. This is the first step towards the development of an industrial process that allows for the selective transformation of carbon tetrachloride into a useful compound in an efficient and selective way.

2. Materials and Methods

2.1. Electrochemical Reactor

All experiments were carried out in a homemade split filter press reactor model UA-63.10 (Figure 1 and Figure 2). A Nafion 450 membrane from Nafion^TM, New Castle, DE, USA, was used as the separator of the anodic and cathodic compartments. The reactor has a geometrical area of 63 cm². A graphite plate in the cathode compartment was used as a current collector. The cathode compartment was completely filled with a three-dimensional carbon felt electrode when needed. Two P022 magnetic pumps from Plastomec, Crevalcore (BO), Italy, were used to drive the solutions.

2.2. Mass Transport Characterization Experiments

For these experiments, a power supply unit mod. AL 924A from ELC, Annecy, France was used, together with a coulometer. A 0.05 M cystine and 0.5 M H₂SO₄ solution was used as the catholyte and a 0.5 M H₂SO₄ solution was used as the anolyte. The cathode potential could be monitored by means of a luggin capillary inserted in the cathode compartment and connected to an Ag/AgCl reference electrode. A PbO₂ plate deposited on Pb was used as an anode and the compartment was filled with plastic grids as turbulence promoters. An initial sample of 3.5 mL was taken from the catholyte. All experiments were carried out by applying a constant current of 6 A in order to ensure that the current was clearly above the limiting current. Samples were taken at regular intervals for analysis by UV–visible spectroscopy at 260 nm with a Shimadzu UV-120-02 spectrophotometer from Izasa Scientific, Madrid, Spain, to determine the cystine concentration at each time. The analysis of the samples was immediate to avoid the reoxidation of L-cysteine to cystine. Five experiments were carried out at five different volumetric flow rate values in the range from 0.9 to 2.7 cm s⁻¹. Each experiment was repeated three times to verify the reproducibility of the results.

2.3. Electrolysis of Carbon Tetrachloride with a Three-Dimensional Electrode

For these experiments, the reactor described in Section 2.1 was used. A carbon felt cathode (RVC 4002 from Carbone-Lorraine, Gennevillers, France) with a thickness of 10 mm, 96% carbon richness, 100 kg m⁻³ density, 1.1 m² g⁻¹ specific surface area (N₂), and electrical resistivity in the long direction of 0.2 Ω m, and in the short direction of 0.02 Ω m, was used in the catholyte frame. A dimensionally stable anode for O₂ evolution (DSA-O₂), made of titanium coated with PtO₂/TiO₂, was used as an anode. All the experiments were carried out in galvanostatic mode using a current source mod. FAC-303 from Promax test & measurement, SLU, L’Hospitalet de Llobregat, Spain, together with a coulometer. A 0.25 M solution of TC in ethanol–H₂O (4:1) and 0.2 M NaCl was used as the catholyte. The anolyte was an aqueous solution of H₂SO₄ 3.75 g L⁻¹. The catholyte and anolyte reservoirs were thermostated at 25 °C. A reflux at 0 °C was connected to the catholyte tank to avoid TC and chloroform losses, as well as a gas trap with a NaOH solution and, finally, a graduated gas collector to quantify the possible formation of hydrogen as a by-product.

The experiments were carried out at a reproducible volumetric flow rate of 160 L h⁻¹. When 120% of the theoretical charge corresponding to the TC quantity of the catholyte was circulated, the experiment was terminated. Samples of 4 mL were taken from the catholyte reservoir before the start of the experiment and at regular intervals thereafter, at 25%, 50%, 75%, 100%, and 120% of the circulated theoretical charge. The samples were placed in a vial, which was immediately sealed with a septum for subsequent gas chromatographic analysis. Samples were headspace-analysed by GC using an HP mod. 5890 Series II GC, from Hewlett Packard, Madrid, Spain, equipped with an FID detector. An HP Pora Plot Q column (30 m × 0.53 mm, 40 µm film thickness) was used. The head pressure was 35 kPa, and He at a flow rate of 1 mL/min and a split of 1:7 was used as carrier gas. The oven temperature was held at 150 °C for 5 min and ramped (2 °C/min) to 177 °C. The sample was prepared in a 2 mL flask containing 1 mL of electrolysis sample and 50 µL of 0.61 M bromochloromethane (from Aldrich, Darmstadt, Germany), which was used as the internal standard. The flask was hermetically sealed and held at 25 °C for 30 min just before injection. A headspace sample of 50 µL was then injected. Using the same method, calibration curves for CT, chloroform, and dichloromethane were obtained.

3. Results

3.1. Mass Transport Caracterization

In the design of any reactor, it is essential to achieve an elevated, uniform, and efficient mass transport. The parameter that is directly related to the mass transport is the mass transport coefficient, k_m. Through this coefficient, we can learn other fundamental parameters in electrochemical reactors, such as the limiting intensity I_L. In this case, a method was used in which the conversion of a reactant in a test reaction is followed, and the parameters obtained were adjusted to two classic models of operation for this type of reactor: the simple bath model or the recirculating piston flow model.

In the simple bath model, the relationship between the reactant concentration at time t with respect to the initial concentration follows the following law:

$$\ln \frac{\mathrm{c}\left(\mathrm{t}\right)}{\mathrm{c}\left(0\right)}=-\frac{{\mathrm{V}}_{\mathrm{e}}{\mathrm{k}}_{\mathrm{m}}{\mathrm{A}}_{\mathrm{e}}}{{\mathrm{V}}_{\mathrm{R}}}\mathrm{t},$$

(1)

where V_e is the geometrical volume of the three-dimensional electrode, A_e is the specific surface of the electrode, and V_R is the volume of the test solution. Therefore, in this model, a linear relationship between the logarithmic ratio of the concentrations and the electrolysis time is specified.

On the other hand, in the recirculating piston flow model, the following relationship must be fulfilled:

$$\ln \frac{c\left(t\right)}{c\left(0\right)}=-\left[1-exp\left(\frac{{-V}_e{k}_m{A}_e}{Q_V}\right)\right]\frac{t}{{\tau }_T},$$

(2)

where Q_V is the volumetric flow rate and τ_T is the mean residence time. Here, this is a representation of the logarithmic ratio of the concentration to the initial concentration with respect to the standardized time, which must be linear if the reactor operation fits this model.

The test reaction chosen was the reduction in L-cystine in sulphuric acid medium:

RSSR + 2e + 2 H⁺ → 2 RSH

This reaction was chosen because it involves the reduction in an organic compound, as it is the reduction in the CT that is to be studied, in addition to the fact that the reagent can easily be analytically monitored and the product is not deposited on the electrode.

Both of the above models are only valid if the process is controlled by convection-diffusion, and, for this, the applied current density must be above the limiting intensity I_L. For this purpose, all experiments were carried out with a constant current of 6 A.

Experiments were carried out at five different flow rates. In all cases, good correlations could be obtained, which allowed for the identification of the different control zones of the process. On the other hand, in all cases, the current efficiencies remained constant and above 90% during most of the experiment, and only at the end was a drop observed, as a consequence of reagent depletion and increased hydrogen production as a side reaction (Figure 3). This indicates the efficient behaviour of this electrochemical reactor with the carbon felt electrode.

Regarding the flow model, good linear fits were obtained, with correlation coefficients above 0.999 in all cases (Figure 4) for both models.

Table 1. Values of k_mA_e and k_m obtained from the fits for the simple bath and piston flow reactor models under mass transport control.

v (cm s−1)	kmAe (s−1)		km (m s−1)
	Piston Flow	Simple Bath	Piston Flow	Simple Bath
0.90	4.1 × 10−2	4.1 × 10−2	1.8 × 10−6	1.8 × 10−6
1.50	5.6 × 10−2	5.7 × 10−2	2.5 × 10−6	2.5 × 10−6
1.80	6.0 × 10−2	6.2 × 10−2	2.6 × 10−6	2.7 × 10−6
2.10	6.6 × 10−2	6.8 × 10−2	2.9 × 10−6	3.0 × 10−6
2.70	7.6 × 10−2	7.9 × 10−2	3.3 × 10−6	3.5 × 10−6

shows the values obtained for the product k_mA_e, as well as for the mass transport coefficient.

The specific area previously determined for the carbon felt electrode using Ergun’s method was used to obtain the k_m value. The k_m values allowed for the prediction of the limiting intensity at which the process becomes controlled by mass transport. This parameter is fundamental for the efficiency of the process and was used in the electrolysis with carbon tetrachloride.

3.2. Electrolysis of Carbon Tetrachloride

After the mass transport characterisation of the electrochemical reactor with carbon felt as a cathode, the electrolysis of carbon tetrachloride was carried out. The reduction in carbon tetrachloride leads to the formation of chloroform, which, in turn, can be reduced to dichloromethane:

CCl₄ + H⁺ + 2e → CHCl₃ + Cl⁻

The experiments were carried out with a 0.25 M CT solution and, from the results of the reactor characterisation, the limiting current for that concentration was estimated to be 2.6 A. This current value was calculated according to the expression that allows for the calculation of the limiting current for an electrochemical system under the control of convection–diffusion mass transport:

$$I_L=-k_{m}AnF{C}_0$$

(3)

where k_m is the mass transport coefficient, A is the electrode area, n is the number of electrons transferred in the reaction, F is the Faraday constant, and C₀ is the initial concentration. Therefore, the electrolytic experiments were carried out in galvanostatic mode, applying this current value.

Table 2. Most significant results of the electrolysis of 0.25 M CT in a filter-press reactor with a three-dimensional flat electrode. S_P: product selectivity.

Electrode	Efficiency CT (%)	Efficiency H2 (%)	CT Removed (%)	Initial Potential (V)	Final Potential (V)	Product Yield (%)	SP Chloroform	SP Dichloromethane
Planar	73	27	86.9	−21.3	−13.6	93	94	6
Carbon felt	88	7.0	100	−18.9	−12.3	90	97	3

and Figure 5, Figure 6 and Figure 7 show the main results obtained in the electrolytic experiments compared with those obtained under identical experimental conditions but using a flat electrode instead of a three-dimensional electrode [14].

As can be seen in Table 2, the improvement obtained with the use of the carbon felt electrode, with respect to the flat electrode, was significant. It is worth noting that, with the same total current being used in both cases, the three-dimensional electrode achieved a complete reduction in all the initial TC in solution, with a current efficiency that remained at values above 90% during most of the experiment, and which finally turned out to be 88% (Figure 6). It should also be noted that the selectivity also improved, with dichloromethane production being reduced to only 3%, and chloroform production approaching 100%. This is, therefore, a highly selective process. The production of the main by-product, hydrogen, was also considerably reduced, to only 7%. The amount of dichloromethane that was synthesised was very small and only occurred during the first moments of the electrolysis as, during the rest of the experiment, the amount remained constant, as shown in Figure 5.

The improvement in the performance parameters under identical experimental and operating conditions is probably due to the large surface development of the carbon felt electrode. The main consequence of the increase is a large increase in the system’s limiting current. An increase in the limiting current means that the system can operate at higher current values, without the transport of reagent matter being limited, which translates into high current efficiencies. By applying the same current intensity to the carbon felt electrode as the flat electrode, the system is actually working well below the limiting current that corresponds to the carbon felt, with a consequent improvement in efficiency [15].

The improvement in the current efficiency and the rate of carbon tetrachloride removal is even higher than that described in previous studies on catalysed processes, and also considerably reduces the reaction times [11]. In the same way, the efficiencies for the removal of carbon tetrachloride in the three-dimensional electrode filter-press reactor are similar and even better than those obtained previously by other authors using a dual-chamber reactor [18].

It is also important to note that the potential difference between the anode and cathode was reduced, probably due to the closer proximity of the two electrodes compared to the use of two flat electrodes (Figure 7). This reduction in the potential is very important in order to reduce the energy requirements of the reactor with a view to its industrial use on a larger scale.

Finally, some operating parameters were calculated to provide a first approximation of the productivity, performance, and profitability of the process. These results are shown in

Table 3. Energy consumption and production data for the electrolysis of CT in the filter-press reactor with flat electrodes and a carbon felt cathode.

Electrode	Energy Consumption for CT (kWh kg−1)	Energy Consumption for Chloroform (kWh kg−1)	CT Removal (kg m−2 day−1)	Chloroform Production (kg m−2 day−1)
Planar	0.081	0.11	20.5	14.0
Carbon felt	0.059	0.090	25.3	16.1

.

The results shown in Table 3 reinforce the previous comments regarding the significant improvement in the process with the use of a three-dimensional electrode compared to a two-dimensional electrode. Chloroform production is increased, as well as the amount of CT removed. It is also important to note that the energy consumption was considerably reduced.

4. Summary and Conclusions

It has been demonstrated that it is possible to synthesise chloroform from the electrochemical reduction in CT in a laboratory-scale reactor using an ethanol–water mixture (4:1) and NaCl as a supporting electrolyte. The mass transport characterization of the reactor obtained good fits to both a simple bath and a piston flow operating model. Characteristic parameters of mass transport, such as the mass transport coefficient and the limiting intensity, were obtained. The use of a three-dimensional carbon felt electrode allows for electrolysis to take place with a high current efficiency and selectivity. Compared to the use of flat electrodes, the use of a three-dimensional electrode leads to an improvement in all physical–chemical and industrial parameters. In particular, it results in an increase in current efficiency, an improvement in selectivity, and a reduction in the formation of by-products. Thus, it seems possible to consider the use of an industrial process under these conditions for the reduction of CT to chloroform.