Adsorption of benzothiophene on Y zeolites investigated by infrared spectroscopy and flow calorimetry
Introduction
Deep removal of organosulfur compounds in fuels has been mandated by government legislations. The Environmental Protection Agency (EPA) of USA requests a reduction of sulfur content of gasoline from current 300 to 30 parts per million (ppm) by weight and diesel from 500 to 15 ppm by 2006 [1], [2], [3]. European legislation mandated a reduction of sulfur compounds in fuels to less than 50 ppm in 2005 [4]. The production of hydrogen for fuel cells also requires sulfur-free fuels since trace amount of sulfur compounds in the fuels will poison fuel cell catalysts [2], [3], [5]. Conventional hydrodesulfurization has been used to remove reactive sulfides, disulfides, mercaptans and light thiophenic sulfur compounds, but it is difficult to remove refractory sulfur compounds such as dibenzothiophene and their alkyl derivatives that are abundant in gasoline and especially in diesel. On the other hand, hydrodesulfurization also significantly reduces the octane number of fuels because of the saturation of alkenes and arenes by hydrogenation reactions at high temperatures and high pressures [6]. An alternative to hydrodesulfurization is adsorptive removal of sulfur compounds from fuels. Adsorption can be performed at ambient temperature and pressure and the content of sulfur in fuels can be reduced to a very low level. Compared to the conventional hydrodesulfurization process, the adsorptive removal technique is very promising in the removal of refractory sulfur compounds to produce ultra-clean fuels. So far, various types of adsorbents, which include metal oxides, active carbon, clays, zeolites and mesoporous materials, have been reported for the adsorptive removal of sulfur compounds in fuels [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Among them, zeolites have been found to be very effective for the adsorption of sulfur compounds [7], [8], [9], [10], [11], [12], [13], [14], [15], [18], [19], [20]. The adsorption of sulfur compounds on metal ion exchanged zeolites such as NaY, ZnY, NiY, CuY and CoZSM and on acidic zeolites such as USY, HY, HZSM and H-Beta have been investigated by Yang and co-workers [7], [8], [9], [10], [11], [12], [13], [14], [15], Song and co-workers [18] and Iglesia and co-workers [21]. The adsorption mechanisms have also been explored by different research groups. Yang and co-workers [7], [8], [9], [10], [11], [12], [13], [14], [15] proposed that the interaction of thiophenic compounds with transition metal ions exchanged into zeolites is through the π-complexation of the heterocyclic rings of the thiophenic sulfur compounds with metal ions. Interaction of thiophene with acidic protons of zeolites has been investigated by using different techniques such as infrared spectroscopy, UV–vis spectroscopy and quantitative adsorption measurements [20], [21], [22], [23], [24], [25]. Recently, we have employed flow calorimetry to the study of adsorption of different thiophenic sulfur compounds in alkane solvents on zeolites and found that the adsorption of sulfur compounds depends on the types of the sulfur compounds, adsorbents, solvent used for dissolving sulfur compounds and the conditions under which the adsorption is performed [26], [27].
This paper reports the adsorption and thermal desorption of benzothiophene on acidic and metal ion exchanged Y zeolites, USY, NaY, NiY and CuY, monitored by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The apparent heat of adsorption of benzothiophene dissolved in normal octane on the Y zeolites was determined by using flow calorimetry. The adsorption on the different Y zeolites was compared and interpreted based on the results obtained from the infrared spectroscopic and flow calorimetric study to provide an understanding of the interaction of the thiophenic sulfur compounds with the zeolites.
Section snippets
Experimental
Zeolites of NaY and ultra-stable Y (USY) were obtained from Strem Chemicals and Grace Division Chemicals, respectively, and used as supplied. NiY and CuY zeolites were prepared by twice aqueous ion exchange of NaY in 0.2 M Cu(NO3)2 and Ni(NO3)2 solution at about 50 °C. The obtained samples were washed and dried in an oven at about 100 °C for 12 h.
DRIFT spectra were obtained on a Bio-Rad FTS 3000 FTIR spectrometer recorded with a resolution of 4 cm−1. The DRIFT cell equipped with a dome ZnS window
DRIFT spectra of benzothiophene adsorbed on the zeolites
Zeolites have well-defined pore structure, high surface area and sites for cation exchange. The pore size of the supercage of Y zeolites is the largest among the different types of conventional zeolites such as A, ZSM and mordenite. Thiophenic sulfur compounds with two or more aromatic/heterocyclic rings can penetrate into the supercage, which makes Y zeolites potential adsorbents for the adsorptive removal of thiophenic sulfur compounds from fuels [7], [8], [9], [10], [11], [12], [13], [14],
Conclusion
From the DRIFT study, benzothiophene can be adsorbed molecularly on NaY, NiY and CuY. The conjugated π-electrons of the sulfur compounds interact with the sodium cations of NaY to form an adsorption adduct. In NiY and CuY, due to the presence of d-electrons, the sulfur compounds interact with the cations through π-complexation. The σ–π donations increase the interaction of the sulfur compounds with NiY and CuY, which gives a higher apparent heat of adsorption than on NaY.
For the acidic zeolite
Acknowledgements
Financial support from Sciences and Engineering Research Council of Canada (NSERC) and Imperial Oil Research Grant are gratefully acknowledged.
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