Adsorption of Nitrogen and Sulphur Organic-Compounds on Titania Nanotubes
Abstract
Anatase TiO2 nanoparticles and TiO2 nanotubes were used as adsorbents to determine their selective adsorption properties on liquid-phase pure S- or N-organic compounds (dibenzothiophene (DBT), 4,6- dimethyl DBT (4,6-DMDBT), pyrrole and quinoline), representatives of those contained in Diesel fuels. As well, model Diesel blends, added with these compounds, and were tested in order to emulate a real Diesel composition. Adsorption isotherms were determined at room temperature in the case of the pure compounds and were fitted to either Langmuir or Tempkin models. In all cases, TiO2 nanotubes showed a higher adsorption performance, either at breakthrough or saturation capacity. For instance, at breakthrough adsorption, TiO2 nanotubes adsorbed 230 and 41 times more DBT and 4,6-DMDBT, respectively, than those in TiO2 nanoparticles. As well, at breakthrough point, TiO2 nanotubes adsorbed 22 and 7.8 times more pyrrole and quinoline, respectively, than those in TiO2 nanoparticles. Saturation adsorption capacity of TiO2 nanotubes is 1.7-1.8 times higher for S-compounds and, 1.4-1.9 times higher for N-compounds than that of TiO2 nanoparticles. In a model Diesel blend, selectively N-compounds were lowered considerably, 50 and 81% for quinoline and pyrrole, respectively, while Scompounds remained almost unchanged. These results confirm that TiO2 nanotubes have a strong preference for Ncompounds when exposed to Diesel blends having competing S-compounds.
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Introduction
Air pollution has been a matter of great concern because of the serious environmental and health problems, which derived from it. An important air pollution source is the vehicle emissions produced during the combustion of fuels. In order to control these emissions, the governments of many countries have established strict regulations for the content of different compounds in transportation fuels. Particularly, sulphur levels are controlled to prevent deactivation of catalytic converter in gasoline vehicles; eventually SOx emissions contribute to acid rain. In the US, the maximum allowable sulphur content in diesel is 15 ppmw (parts per million by weight) meanwhile in the EU is 10 ppmw [1-6]. In addition, interest in ultra-low sulphur fuels is motivated by the need of using new emission-control technology and fuel cells [2,7-10].
Oil Refineries traditionally employ hydrodesulphurization process (HDS) to remove organosulphur compounds from feedstocks for diesel production; but achieving ultra-low sulphur levels solely on this process has become a difficult task, because of the alkylsubstitued dibenzothiophenes (i.e. refractory sulphur) which are not easily eliminated [11,12]. Therefore, several alternatives have been applied to improve the efficiency of HDS: catalysts activity enhancement, increase of HDS process severity, use of complementary non-catalytic process, among others [9,13-16]. An additional challenge that must be solved is the removal of HDS catalysts inhibitors, specially nitrogen compounds which have comparable or lower reactivity than refractory sulphur species [9,17-20]. Basic and non-basic nitrogen molecules naturally occurring in oil feedstocks for fuel production are eliminated by hidrodenitrogenation (HDN); however, this process is significantly more difficult to carry out than HDS [11,21]. Both reactions occur simultaneously. so any reduction in nitrogen prior to the desulphurization will enhance efficiency of the later [22,23]. Denitrogenation process is also important to prevent NOx emissions upon fuel combustion [21,24].
Conclusion
Titanate nanotubes are a promising adsorbent of organosulphur and organonitrogen compounds contained in diesel fuel, according to our results obtained in equilibrium and dynamic adsorption tests. Equilibrium adsorption experiments revealed favorable adsorption of sulphur (DBT, 4,6-DMDBT) and nitrogen (pyrrole, quinoline) molecules. The DBT adsorption experimental data fitted to a Langmuir and Temkin models, while for 4,6-DMDBT the Langmuir model had the best fit. Calculated parameters of Langmuir isotherm indicated that titanate nanotubes adsorbed higher higher amounts of DBT, however 4,6-DMDBT had a stronger interaction with them. In the case of nitrogen compounds, the Freundlich model fit better the pyrrole adsorption isotherm, reflecting heterogeneous adsorption due to energetically different adsorption sites meanwhile Temkin model better describe quinoline equilibrium adsorption data which takes into account some indirect interactions between adsorbate and adsorbent.
In fixed-bed adsorption experiments, titanate nanotubes had a much better performance than anatase nanoparticles in the adsorption of sulphur and nitrogen compounds. The difference observed in adsorption capacity may be related to specific surface area, morphology and/or surface chemical properties, such as surface functional group type. Also, the accessibility of the adsorption sites might play an important role especially in the adsorption of molecules with steric hindrance as in 4,6- DMDBT adsorption capacity values for anatase nanoparticles and titanate nanotubes. Particularly, the results of sulphur compounds adsorption on titanate nanotubes suggest the existence of a direct interaction between the heteroatom of the adsorbate and the surface OH groups in the adsorbent.
Results of fixed-bed nitrogen adsorption pointed out nanotubes have higher affinity for quinoline than for pyrrole, since at breakthrough point the adsorption capacity was higher for the former compound. According to previously reported experiments [71], which suggest that the main interaction between quinoline and OH groups on titanate nanotubes occurred via H-bonding.
In addition, a negligible uptake of toluene on titanate nanotubes was observed in equilibrium and dynamic adsorption experiments. This fact represents an advantage of the nanotubes in desulphurization and denitrogenation of diesel fuels, which contain high amounts of aromatic compounds.