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Ultra-deep desulfurization is a major requirement for upgrading the quality of fuel and power sources for fuel-cells. A series of mesoporous TiO 2 —SiO 2 adsorbents were prepared and investigated for ultra-deep adsorption of benzothiophene BT and dibenzothiophene DBT from model fuel at ambient conditions. The high desulfurization performance of the adsorbent was attributed to its large specific surface and surface acidity.
It also achieved a high sulfur adsorption capacity of 7. The kinetics of the adsorption of organic sulfur was studied and the results indicated that the pseudo-second order model appropriately fitted the kinetics data. Furthermore, the used adsorbent can be easily regenerated and the desulphurization efficiency of the recovered adsorbent after five regeneration cycles was still maintained at Deep desulfurization of traditional fuels has been explored in the past decades.
Hydrodesulfurization HDS is a conventional process for removing sulfur from fossil fuels. Moreover, the operating temperature and pressure of HDS is very high, which makes its operation dangerous.
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Oxidative desulfurization ODS and extractive desulfurization EDS are new technologies for deep desulfurization that have become popular because of their mild operating conditions. The sulfones and sulfoxides could easily be removed by extraction and adsorption processes because of their high polarity. The removal rate of Ccoral reached The extraction of DBT reached Therefore, developing new methods for removing the sulfur in fuels is a persistent challenge faced by researchers.
Adsorptive desulfurization ADS is considered as one of the most promising methods to obtain ultra-low sulfur fuels.
ADS can be carried out at atmospheric pressure and room temperature, without consumption of hydrogen. The most commonly-used adsorbents include activated carbons, 19—21 zeolites, 22—24 ionic liquids, 25—27 metal oxides 28—30 and other mesoporous materials.
In addition, TiO 2 —SiO 2 complex oxides have considerably high specific surface areas and surface acidities, which can significantly improve the desulfurization performance of ADS. It was also pointed out that specific surface area and surface acidity in the adsorption process could influence the overall adsorption performance of different types of adsorbents. According to Lewis acid—base theory, most thiophene-based sulfur compounds in commercial fuels appear to be Lewis bases, which are easily adsorbed by Lewis acidic sites.
The unique surface effects of TiO 2 ensured good low-temperature desulfurization activity, but its thermal stability and mechanical stability were poor.
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It has been reported that the thermal stability and the crystalline stability of TiO 2 could be remarkably enhanced by SiO 2 modification. The amount of Lewis acid would increase when TiO 2 was modified by SiO 2and the Bronsted voral center was produced at the same time, which could promote the desulfurization performance. Compared with hydrodesulfurization, Corap 2 —SiO 2 complex oxides-based adsorbents can achieve deep desulphurization at room temperature and atmospheric pressure without consuming hydrogen.
Compared with oxidative desulfurization and extraction desulfurization, the adsorption desulfurization process can operate without oxidants, thus reducing the cost and improving the stability. Furthermore, the TiO 2 —SiO 2 adsorbent can be easily regenerated. Mesoporous TiO 2 —SiO 2 complex oxides were prepared by a sol—gel method, which is an effective way to synthesize homogeneous metal oxide materials.
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The preparation process was as follows: A mixture of ethanol: HNO 3 with a molar ratio of The mixed solution was further stirred for several hours until a wet sol was obtained and the temperature was maintained for another two hours. Dynamic breakthrough experiments were carried out using a self-designed fixed-bed test. For each test run, 2 g of pre-weighted adsorbent was put in a stainless tube containing model fuel with a measured initial sulfur concentration of ppm at room temperature and atmospheric pressure.
To improve the quality of results, the amount of adsorbent was increased by tenfold compared to the static saturation tests. The fuel flowed vertically upward at a constant flow rate of 0. The schematic of the fixed-bed sulfur adsorption system is shown in Fig. The desulfurization performances of the adsorbents were characterized by measuring the residual sulfur concentration in the fuel.
The breakthrough curves were obtained by plotting the instantaneous sulfur concentration and the initial sulfur concentration by the mass of adsorbent used in this study.
The breakthrough sulfur capacity is defined according to eqn 3: As the adsorption reaction proceeds, the adsorbents turned gradually yellow and became saturated as the content of adsorbed thiophene increased. Spent adsorbents can be regenerated by heating at high temperature.
The detailed steps are as follows: The desulfurization performance of the regenerated adsorbent was measured in a new reaction. In static saturation tests, the saturated sulfur capacity in the case of DBT could reach All of the TiO 2 —SiO 2 adsorbents were more efficient and have higher sulfur removal rates than silica or titania for the ADS reaction.
It was found that the mesoporous TiO 2 —SiO 2 adsorbent had a significantly improved sulfur capacity compared with traditional adsorbents. As shown in Fig. They are dramatically different from the surface morphologies of TiO 2 —SiO 2 powder adsorbents prepared by extrusion, which show a porous coral-like structure. There are no diffraction peaks observed for the crystalline silica phase, indicating that pure SiO 2 is amorphous.
Moreover, pure titania showed typical diffraction peaks at around Anatase and rutile crystalline phases also coexist simultaneously. For the TiO 2 —SiO 2 complex oxides, only the anatase crystalline phase was observed.
The rutile crystalline phase disappeared with the increase in silica content. This confirmed that highly dispersed crystalline phases of titania on silica could be found in the TiO 2 —SiO 2 complex oxides.
In addition, the difference in intensities of peaks for various TiO 2 —SiO 2 adsorbents indicates a variation in the corresponding amounts of each adsorbent.
It can be verified that silica can largely improve the thermal stability of titania. A previous report has shown that anatase is better than rutile for sulfur adsorption from liquid fuels.
It is generally known that adsorption—desulfurization performance is heavily dependent on the specific 81 areas of the adsorbents. Hsd order to investigate the microscopic surface texture, N 2 adsorption—desorption isotherms were measured and pore size distribution for TiO 2 —SiO 2 complex oxides was calculated see Table 1. Considering the raw material composition, TiO 2 —SiO 2 adsorbents could achieve and maintain large surface areas on addition of silica component.
In addition, it has been confirmed that direct sulfur-adsorbent interaction plays an important role in the adsorptive desulfurization process.
This indicates that TiO 2 —SiO 2 complex oxides were the main active component and not the pure silica. However, their desulfurization efficiencies were still less than that of the Ti—Si complex oxide.
These results indicated that specific surface area was not the only factor influencing the desulfurization performance and that it is dominated by other factors. This indicated that the microstructure of Ti—Si is large enough to accommodate DBT molecules and other bulky sulfur compounds.
The critical diameters of the DBT molecules were less than 1 nm, which are smaller than the pores size of the Ti—Si adsorbent. Thiophene and its derivatives gds easily diffuse into the pores, where most of the active sites for ADS adsorption were located. The peak becomes more intense with the increase in Si content, clearly indicating that changing the silica content can influence the atomic scale structure of the TiO 2 —SiO 2 adsorbents.
It can be speculated that the formation of Ti—O—Si linkages are crucial to the desulfurization performance of different TiO 2 —SiO 2 binary oxides. It has been reported that the surface acidity of adsorbents can play an important role in the adsorption capacity of thiophene and its derivatives.
Pure silica and titania have slight acidity, which are easily desorbed, resulting in their poor desulfurization performances. On the contrary, the acidic properties of TiO 2 —SiO 2 complex oxides were quite different from those of titania and silica. With the increase in TiO 2 content, the number of acidic sites increased monotonously, which can be ascribed to an increase in exposed Ti species. According to Tanabe’s hypothesis, a binary oxide with TiO 2 as the major oxide component exhibits Lewis acidity.
Therefore, the large specific surface area and the Lewis acidity of mesoporous TiO 2 —SiO 2 binary oxides will remarkably promote adsorption capacity of thiophene sulfurs and its derivatives at low temperatures. An empirical kinetic model 32 was expressed as follows: The correlation coefficient was over 0. In other words, the adsorption desulfurization reaction follows pseudo-first order kinetics. The pseudo-second order model 50,51 is presented as follows: The integrated form of eqn 7 yields: The intra-particle diffusion IPD model proposed 46 is expressed in the form of: The results of the pseudo-second order kinetic model are shown in Fig.
It can be observed that this model perfectly fits the experimental data. The correlation coefficient and other parameters of the three kinetic models are listed in Table 3. It can be observed that the pseudo-second order model has a high correlation coefficient 0. The intra-particle diffusion model was applied to further measure the adsorption of DBT.
This indicates that two or more steps occur in the adsorption process.
According to our knowledge of adsorption, the following hypothesis is proposed: In second stage, intra-particle diffusion controls the overall adsorption rate. The lager specific surface area and the uds Lewis acidity of the mesoporous TiO 2 —SiO 2 binary oxide remarkably enhanced adsorption capacity of thiophene sulfurs and its derivatives at low temperatures.
Kinetic studies showed that the pseudo-second order model could be used to describe the adsorption kinetics satisfactorily. The adsorbent could be regenerated by high temperature treatment with negligible loss in activity, indicating that the mesoporous titania—silica adsorbents are highly stable. The desulfurization experiment for crude oil will be carried out in the future. Received 5th JanuaryAccepted 29th January Table 1 Physical properties of titania—silica adsorbents.
Table 2 Effect of calcination temperature on sulfur removal performance. Table 3 Kinetic analysis results of two different models.