(193e) Adsorption of Polycyclic Aromatic Sulfur Heterocycles and Polycyclic Aromatic Hydrocarbons On Activated Carbons

Introduction

Desulfurization of transportation fuels such as diesel has increasingly gained importance since developed countries have implemented stringent legislation to regulate sulfur content of transportation fuels. The new regulations for diesels brought down the S level from about 400-500ppm (parts per million on weight bases) to 10-15ppm S. Near zero sulfur (NZS) diesel fuels of 15 ppm or less allows advanced post engine exhaust clean up devices to effectively decrease emissions and remove particulate matters. Due to the additional requirements in the desulfurization process, there has been a growing concern to the long term economics of petroleum refineries.

Although deep HDS can be readily used to produce ultra-low sulfur diesels (ULSD), alternative desulphurization technologies such as oxidative desulfurization (ODS) [1], and selective adsorption of organic sulfur compounds over various adsorbents [2] have been extensively explored and developed.

Recently, activated carbons (AC) have been recognized and widely used as adsorbents in gas-phase and liquid-phase adsorptions for removal of organic sulfur compounds, mainly due to  ADDIN EN.CITE  ADDIN EN.CITE.DATA their very high surface areas, large pore volumes, and tunable surface properties. However, the competiveness of selective adsorption technologies for sulfur removal could be strongly affected by the competitive adsorption of polycyclic aromatic hydrocarbons (PAHs). In this contribution, we report a comparative study on the adsorption of polycyclic aromatic sulfur heterocycles and polycyclic aromatic hydrocarbons on activated carbons.

Experimental

All activated carbons used in this study are commercial activated carbons obtained from different precursor materials, including pitch and biomass. Samples, before and after adsorption, were characterized using BET and FTIR. Sulfur analysis was performed using XRF and total S analyzer (Mitsubishi Chemical Co. TS-100). Dynamic adsorption experiments were carried out in a fixed-bed adsorption system which allows for the automatic collection of samples at regular time intervals. In addition, batch adsorption experiments were also carried out.

Results and Discussion

Adsorptions of diesels as well as model diesels containing aromatics and sulfur compounds over several activated carbons were conducted in both batch and flow adsorption modes. Adsorption results showed that the adsorption capacities strongly depend on the nature of the activated carbons.

In order to get more insight into the adsorption mechanism, Langmuir adsorption isotherms of PASHs and PAHs were performed. From the isotherms, the parameter (K_L*q_m) was estimated, where K_L represents the adsorption equilibrium constant and q_m the maximum adsorption capacity. The parameter (K_L*q_m) is a characteristic constant related to the intensity of the adsorption and reflects the affinity of each adsorbate to the adsorbent. Figure 1 summarized the results obtained for several model compounds.

The adsorption results showed that the adsorptive affinity of molecules having polycyclic aromatic skeleton structure will be primarily governed by the π-π dispersive interaction between aromatic rings and grapheme structures of the activated carbon. In addition, the electron donor-acceptor mechanism also plays an important role for those molecules containing sulfur atoms. Furthermore, for effective adsorption of large molecules, not only the pore size of the adsorbent should be, at least, larger than the critical diameter of adsorbate, but also the pore size distribution should be sufficiently wide to reduce the diffusion kinetic resistance during the adsorption process. Based on this studies, it can be concluded that the adsorption selectivity of several adsorbates increases in the order of Naphthalene < Fluorene < Dibenzothiphene < 4,6-Dimethyl dibenzothiphene  < Anthracene < Phenanthrene. The adsorption capacity of PASHs decreases significantly in presence of PAHs as a result of the competitive adsorption between PASHs and PAHs, mainly due to the similarity in structure, molecular diameter and adsorption mechanisms.

Figure 1: Parameter (K_L*q_m) for various PASH and PAH compounds

Acknowledgements

The authors are grateful to Dr. P.K. Wong and Dr.
Keith Carpenter for providing constructive comments on this study. The financial support from ETPL (A*STAR) is also gratefully acknowledged.

References

[1] Sampanthar, J.T., Huang Xiao, Jian Dou, Teo Yin Nah, Xu Rong, Wong Pui Kwan, Appl. Catal. B, 85 (2006), 85.

[2] Zhou, A., X. Ma, and C.S. Song., J. of Phys. Chem. B, 110(10) (2006), 4699