(709d) Efficient Adsorptive Desulfurization over Heteroatom-Doped Porous Carbons By Carbonization of an Organic Salt

Desulfurization of gasoline and diesel fuels has attracted increasing attention due to detrimental environmental impacts of sulfur oxides present in engine exhaust emissions. The traditional hydrodesulfurization (HDS) performs well in the removal of thiols, sulfides and disulfides, but it suffers from some major problems, such as high energy and hydrogen consumptions as well as resistance to thiophenic compounds. Among other methods, adsorption desulfurization is considered to be a promising approach which has several advantages, including mild operating conditions, hydrogen-free operation, and selective removal of thiophenic compounds. A variety of materials including zeolites, metal oxides, π-complexation sorbents, carbon materials and metal-organic frameworks (MOFs) have been developed for adsorptive desulfurization. Recently, reports have emerged describing the production of porous carbons by the carbonization of organic salts. In this approach, carbon materials with well-developed porosity and doped heteroatoms can be produced in one step, avoiding the use of large amounts of corrosive chemical agents. In this work, a series of oxygen and nitrogen-doped porous carbons were synthesized by direct carbonization of tetrasodium ethylenediamine tetraacetic acid (EDTA-4Na) at various temperatures. The resulting carbons were shown to be promising for adsorptive desulfurization with high maximum adsorption capacity of DBT from model oil (up to 49.1mg∙S/g). In the presence of aromatics, the adsorption of DBT was still significant (up to 38.6 mg∙S/g).

The adsorption isotherms of DBT were measured in the low concentration range of DBT sulfur (<358 ppmw∙S) at 25°C and fitted to Langmuir and Freundlich equation. All the regression coefficients (R²) are close to 1, suggesting that the adsorption data are well fitted with both models. The effect of carbonization temperature on the desulfurization performance was studied in detail. It was found that with an increase in the pyrolysis temperature, the specific surface areas and total pore volumes increased gradually. However, the largest q_max (49.1 mg∙S/g) was obtained with the carbon synthesized at a medium temperature of 700°C. This was attributed to the fact that pore size was more important that surface area or total pore volume in the adsorption process. Moreover, by plotting q_max to the corresponding volume of pores smaller than 1 nm (V_<1nm), a good linear trend with R₂=0.99 was observed. Beside, previous reports indicated that the adsorption performance of carbon materials was also influenced by chemical factors. In this work, the content of heteroatoms decreased and the types of surface oxygen and nitrogen functionalities varied with increased carbonization temperatures, but their influence on adsorption capacity was limited. This was attributed to the lack of acid groups (phenol and carboxylic) as indicated by FTIR and XPS results.

As real fuels contain a large amount of aromatics, effective adsorption of sulfur compounds in the presence of aromatics is an important issue. The sulfur capacity decreased by less than 25% in the presence of 10% para-xylene, and the capacity reduction was the least significant (5.2%) for the carbon with the highest heteroatom content. A possible explanation is that DBT adsorption was through both polar interaction and π–complexation, but only π–complexation existed for aromatics. The higher content of oxygen and nitrogen introduced more polar centers, favoring the adsorption of DBT. Besides, the types of surface functionalities were found to be important as well. For example, the nitrogen atom in pyridine-N-oxide (N-X species) is different due to its connection to oxygen atom. As a result, the electron density of the pyridinic aromatic ring may be decreased due to the electron-withdrawing ability of oxygen atom, thus the interaction of π–complexation may be weakened.

This work may broaden the library of promising adsorbents for desulfurization and shed some light on the mechanism of competitive adsorption of DBT and aromatics.