Electrochemical desulfurization

In this section, a summary of the chemical principles involved with membrane reactors for desulfurization are overviewed. The details will be covered in the following sections. Electrochemical desulfurization technologies assisted by membranes have been extensively explored for the removal of sulfur that exists in sulfur compounds in fossil fuels and in SO2 form in flue gas. In principle, SO2 can be absorbed by an aqueous electrolyte solution and then electrochemically converted into species such as sulfate, hydrogen sulfide, and sulfur, among others, by oxidation or reduction processes, whereas the sulfur compounds in fossil fuels can be similarly removed. The universal reaction mechanism of the electrochemical cathodic reduction of organic sulfur compounds in gasoline and diesel is shown in Eqn (14.1) (Lam et al., 2012) ... 

As aforementioned, electrochemical desulfurization of gases can be achieved by inner-ceU and outer-cell processes, as shown in Figure 14.1. 

The first step of electrochemical desulfurization is the adsorption of SO2 into the liquid phase. To increase the solubility of the gases in the liquid phase, a reaction, which converts the primarily dissolved species to a more soluble one, must be used. This can be achieved by either the direct conversion at the electrode or the indirect conversion via chemical reactions with a mediator. 

The other type of electrochemical desulfurization of gases is the reduction of dissolved SO2 to monomeric sulfur at potentials less than 0.45 V, which is described by Eqn (14.10) (Bard, Parsons, Jordan, 1985) ... 

In recent years, there have been numerous inner-cell systems implemented for electrochemical desulfurization. Some membrane reactors for desulfurization via electrooxidation are listed in Table 14.1. Different types of electrodes and electrolytes have been used in the process of desulfurization of gases. In addition to oxidation, the reduction of the dissolved SO2 can obtain sulfur by controlling the applied potential. 

D Eha Camacho et al. (2011) proposed a novel concept using an assisted electrochemical reaction to produce atomic hydrogen from water electrolysis for different heterorganic compounds conversion. The electrochemical reactor is divided into two compartments by a palladium membrane in which atomic hydrogen is absorbed and permeated. Organic sulfur in the oil can be desulfurized and transformed to H2S in the electrochemical compartment. In addition, Lam et al. (2012) recently presented a review of electrochemical desulfurization technologies for fossil fuels. Various electrodes and electrolytes that have been used for desulfurization accomphshed by oxidation, reduction, or both were summarized by Lam et al. in their paper. Some electrochemical desulfurization processes for transportation fuels were chosen for listing in Table 14.2. 

Table 14.2 Some electrochemical desulfurization processes for transportation fuels...

The electrochemical desulfurization (ECDS) of transportation fuels includes two methods oxidation and reduction. The electrochemical reduction of sulfur-containing fuels was presented in Section 14.1.1 using Eqn (14.1). The organic sulfur compounds will be reduced to hydrogen sulfide form, which can be subsequently removed by a gas/liquid separation process. 

In addition, a mediator can be applied to increase the removal of sulfur in the indirect electrochemical desulfurization processes, in which the mediator can be regenerated at the electrode. Recently, Shu, Sun, Jia, and Lou (2013) proposed a novel integrated process for reductive desulfurization of diesel fuel (shown in Figure 14.7), in which the reductant sodium borohydride (NaBHa) was in situ generated via sodium metaborate (NaB02) electroreduction. 

There are two types of membrane reactor processes adaptable to the electrochemical desulfurization of gases inner-cell process and outer-cell process (Figure 14.1 in Section 14.1.2.1). In an inner-cell process, the electrolyser is usually composed of a proton exchange membrane and two electrodes, in which the cathodic compartment and anodic compartment are divided by membrane. The electrodes are immersed in the electrolyte adjacent to the membrane surfaces. In the outer-cell process, there are various conhgurations able to be integrated into the existing desulfurization units for regenerating desulfurization adsorbent and recovery. 

Both precious and nonprecious metals can be used for electrochemical desulfurization. The best choice of an efficient catalyst of desulfurization depends on weighing and considering the favourable and unfavourable factors, such as low cost and high... 

In summary, the design of membrane reactors for electrochemical desulfurization of transportation fuels should take into consideration the issues shown in Figure 14.10. 

Improvements in membrane reactor performance may also be obtained by further developments on new processes and methods for scaling up. Electrochemical desulfurization of gases by a membrane reactor is a promising and proven technology. 

In this chapter, the chemical principles of electrochemical desulfurization of gases and transportation fuels using a membrane reactor were introduced. Theory, applications, and design in the development of membrane reactors to remove sulfur-containing compounds or sulfur dioxide were described. Lastly, future trends in the developments of membrane reactors for ECDS were covered. We sincerely expect an increasing number of researchers and achievements will contribute to the development of ECDS processes in the near future. 

Modulation of the working potential to -1.4 V revealed, in phosphate buffer pH 7, a new result, since 1 reacted under these conditions with a ring contraction and formation of 1,3,8-trimethylxanthine (10). Further increase of the potential to -1.6 V led to a mixture of 10 and a small amount of 1,3-dimethyllumazine (12) showing that electrochemical desulfurizations of thiolumazines are in principle possible but cannot be considered as the main reaction pathway with preparative applications. Cathodic reduction of 1 at -1.9 V finally generated 10 in 90% yield. A first hint regarding the mechanism of the ring contraction was derived from the electroreduction of l,3,6-trimethyl-7-thiolumazine (9), which was converted at -1.4 V in phosphate buffer into 8-ethyl-l,3-dimethyIxanthine (11) and a smaller amount of 1,3,6-trimethyllumazine (13). 

Organic Electrochemistry IV Electrochemical Desulfurization Reactions of Thiolumazines W. Pfleiderer and B.S. Schulz... 

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