One-pot photocatalysis-assisted adsorptive desulfurization of diesel over doped-TiO₂ under ambient conditions

Abstract

One-pot photocatalysis-assisted adsorptive desulfurization (ADS) of diesel using bi-functional doped-TiO₂ adsorbents with molecular oxygen in air under ambient conditions was developed, which achieved a high ADS capacity of 1.89 mg-S per g-Ads. The developed approach provides a new path for ultra-clean fuel production.

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Ultra-deep desulfurization of diesel fuel has attracted widespread attention due to the stringent government regulations for the sulfur content in diesel for protection of the environment and human health.^1,2 The commercial desulfurization technology in refineries is hydrotreating, which operates at high temperatures and pressures (>300 °C, hydrogen pressure of 20–100 atm (ref. 3)) using excess hydrogen. As a complementary technology, adsorptive desulfurization (ADS) preferentially adsorbs organosulfur compounds over a solid adsorbent under ambient conditions without the use of hydrogen.³ To date, progress has been made on ADS adsorbents, including π-complexation adsorbents,^4,5 immobilized π-acceptor adsorbents,^6,7 Ni⁰-based adsorbents,^8,9 O-functional carbon adsorbents,^10–12 metal organic frameworks,^13–15 soft acid-based adsorbents,¹⁶ and aluminosilicates.¹⁷ However, most ADS adsorbents have low efficiency for the ultra-deep desulfurization of real diesel under ambient conditions because of (a) low adsorption affinity to refractory sulfur compounds with steric hindrance effect,¹⁸ and (b) low ADS selectivity with high concentration of polyaromatic hydrocarbons in real diesel fuel.¹⁹ Therefore, it is important to explore new ADS approaches to tackle the above challenges for the ultra-deep desulfurization of real diesel.

Owing to the tunable porosity, versatile functionalities and low cost, titanium oxide has been applied widely in the fields of adsorption, heterogeneous catalysis and photocatalysis.^20–22 Among the applications, TiO₂-based adsorbents show promise for the reactive ADS of fuels. Watanabe et al.²³ developed mixed TiO₂–CeO₂ adsorbent that demonstrated high ADS capacity from jet fuel with a high sulfur content (>1000 ppmw). Xiao et al.²⁴ reported a TiO₂–CeO₂/MCM-48 adsorbent with high desulfurization capacity (1.143 mg-S per g-Ads) of 14.5 ppmw-S diesel involving a two-step ADS approach, where the original diesel was first treated by light irradiation (for in situ peroxide generation in diesel), followed by ADS over the TiO₂–CeO₂/MCM-48 adsorbent under ambient conditions.

In this work, we report the one-pot UV photocatalysis-assisted ADS of diesel using TiO₂–SiO₂ adsorbents under ambient conditions. Photocatalysis and adsorption were coupled into one-pot to achieve highly efficient desulfurization for ultra-clean fuel production. A series of TiO₂–SiO₂ adsorbents with varied Ti/Si ratios were prepared using a sol–gel method.²⁵ The textural properties of TiO₂–SiO₂ adsorbents were characterized using a N₂ adsorption test. The ADS performance was evaluated in a UV photocatalytic batch reactor (with fed air to fuel). The total sulfur concentrations in the fuels were monitored using total sulfur analyzer. The sulfur chemistry during the reactive adsorption was clarified by GC-MS to identify the sulfur species in the initial model fuel/treated fuel/eluent of the spent Ti_0.3Si_0.7O₂ adsorbent. The reactive adsorption pathway was elucidated by a model compound study.

Fig. 1 shows ADS capacity of the prepared Ti_0.9Si_0.1O₂ and Ti_0.9Ce_0.1O₂ adsorbents from real diesel at 25 °C with and without air feeding and UV light irradiation. Noticeably, by introducing UV light irradiation during ADS, the ADS capacity increased significantly to 1.54 from 0.20 mg-S per g-Ads, and to 1.28 from 0.30 mg-S per g-Ads for the Ti_0.9Si_0.1O₂ and Ti_0.9Ce_0.1O₂ adsorbents, suggesting the strong promotion of UV light irradiation on ADS over TiO₂-based adsorbent. Moreover, compared to the Ti_0.9Ce_0.1O₂ adsorbent that demonstrated superior ADS performance,²³ Ti_0.9Si_0.1O₂ shows a higher ADS capacity under UV light irradiation. It should also be noted that by introducing air without UV light irradiation to fuel/adsorbent mixture during ADS, the desulfurization capacity increased only slightly from 0.14 to 0.23 mg-S per g-Ads for the Ti_0.9Si_0.1O₂ adsorbent and from 0.20 to 0.30 mg-S per g-Ads for the Ti_0.9Ce_0.1O₂ adsorbent, which can be attributed to a mild oxidation-enhanced ADS mechanism.³ Nevertheless, the increase in ADS capacity was negligible compared to that under both air and UV light irradiation.

Fig. 1 ADS capacities of Ti_0.9Si_0.1O₂ and Ti_0.9Ce_0.1O₂ adsorbents from real diesel at 25 °C (+air: with air fed to fuel; +UV: under UV light irradiation).

Fig. 2 shows the effects of the Ti/Si ratio in the Ti_1−xSi_xO₂ adsorbent on ADS from the model fuel at 25 °C. It is observed that desulfurization capacities of the prepared Ti_1−xSi_xO₂ adsorbents with varying Ti/Si ratios were much higher than those in the absence of air and UV light irradiation, which verifies the promotion of UV light irradiation on ADS. The maximal ADS capacity (6.1 mg-S per g-Ads) can be achieved at a Ti/Si ratio of 3/7 (Ti_0.3Si_0.7O₂), which can desulfurize the fuel from 320 to 12 ppmw-S (effect of fuel-to-adsorbent ratio on the desulfurization capacity is shown in Fig. S1†). For comparison, commercial Degussa P25 was tested for ADS under UV light irradiation, and achieved ADS capacities of 1.8 mg-S per g-Ads, which is lower than that of TiO₂ synthesized in this work (1.9 mg-S per g-Ads). One commercial Ti/Si zeolite, TS-1, was also tested, and achieved an ADS capacity of 0.8 mg-S per g-Ads, which is much lower than the Ti_0.3Si_0.7O₂ adsorbent synthesized in this work.

Fig. 2 Caption ADS capacities of Ti_1−xSi_xO₂ adsorbents from model fuel at 25 °C (+air: with air fed to fuel; +UV light: under UV light irradiation).

From Fig. 2, under UV light irradiation, the ADS capacity at varying Ti/Si ratios follows the order of Ti_0.3Si_0.7O₂ > Ti_0.5Si_0.5O₂ > Ti_0.7Si_0.3O₂ > Ti_0.1Si_0.9O₂ > Ti_0.9Si_0.1O₂ > TiO₂ > SiO₂, which is different from the trend of S_BET, which increased with increasing Si ratio in the Ti_1−xSi_xO₂ adsorbents. The results suggest that the surface area of the Ti_1−xSi_xO₂ adsorbents may or may not be the only factor governing the ADS capacity over the Ti_1−xSi_xO₂ adsorbents. In addition, although SiO₂ has the highest S_BET of 696.2 m² g⁻¹, its ADS capacity is as low as 0.7–0.8 mg-S per g-Ads under the conditions with and without UV light irradiation, which is similar to the commercial mesoporous MCM-41 (0.9 mg-S per g-Ads). The results suggest that the Ti chemistry in the Ti_1−xSi_xO₂ adsorbents played a key role in the significant promoting effect of UV light irradiation on ADS, which will be elucidated in a future work.

Fig. 3 shows GC-MS chromatographs of the initial model fuel, desulfurized fuel samples at different adsorption times, and the eluent of the spent Ti_0.3Si_0.7O₂ adsorbent. This is helpful for understanding the promotion effect of UV light irradiation on sulfur chemistry during ADS over Ti_0.3Si_0.7O₂ adsorbents. It was noted that the peak intensity of DBT decreased with adsorption time, but no new species were detected in the desulfurized fuel samples, as shown in Fig. 3(b) and (c). To clarify the sulfur chemistry, the spent Ti_0.3Si_0.7O₂ adsorbent was washed with a polar solvent, and the GC-MS chromatograph of the eluent is shown in Fig. 3(d). Interestingly, in addition to DBT, a new species was detected at the retention time of 17.8 min. The new species was confirmed to be dibenzothiophene sulfoxide (DBTO) compared to the standard spectrum, as shown in Fig. 4. The results suggest that UV light irradiation during ADS promoted the oxidation of DBT to DBTO under ambient conditions. Therefore, this strong promotion effect of UV light irradiation on ADS can be attributed to the stronger adsorption of DBTO on the Ti_0.3Si_0.7O₂ adsorbent than the original DBT. A much higher adsorption capacity of DBTO₂ (6.15 mg-S per g-sorb) than that of DBT (0.11 mg-S per g-sorb) over Ti_0.3Si_0.7O₂ adsorbent from a model fuel (100 ppm-S in a mixed solvent of 40% toluene and 60% tetradecane) further suggested the stronger adsorption of oxidized sulfur species including DBTO.

Fig. 3 GC-MS chromatographs of initial model fuel, desulfurized fuel samples, and the eluent of the spent Ti–Si–O adsorbent.

Fig. 4 Mass spectra of (a) species at the retention time of 17.8 min and (b) standard DBTO.

In the absence of Ti_0.3Si_0.7O₂ adsorbent, no oxidized sulfur species, such as DBTO or dibenzothiophene sulfone (DBTO₂) were detected in the fuel under UV light irradiation. This suggests that besides the adsorbent, Ti_0.3Si_0.7O₂ also played a role as a photocatalyst for the transformation of DBT to DBTO. Moreover, molecular oxygen in air, rather than intermediates like organoperoxides formed by photosensitive hydrocarbon–O₂ reactions,²⁶ served as the oxidant for sulfur oxidation, because no peroxides were detected in the model fuel under light irradiation in the absence of the Ti_0.3Si_0.7O₂ adsorbent. The results suggest that the one-pot photocatalysis-assisted adsorption is not limited to photosensitive hydrocarbons-containing diesel, but is applicable to other fuel desulfurization processes as well. Compared to the previously reported two-step ODS approach,²⁷ oxygen generation as a first-step is not required anymore in the study and excellent desulfurization performance was achieved. From the forgoing discussion, the photocatalysis-assisted adsorption mechanism can be summarized as follows: under UV light irradiation with the Ti_0.3Si_0.7O₂ photocatalyst, the organosulfur compounds were oxidized to the corresponding sulfoxides in air under ambient conditions. Simultaneously, the transformed sulfoxide strongly adsorbed on the Ti_0.3Si_0.7O₂ adsorbent, as illustrated in Fig. 5. Ti_0.3Si_0.7O₂ can be regenerated by an oxidative air treatment (Fig. S3†). The bi-functions of Ti_0.3Si_0.7O₂ for ADS will be studied further in future work.

Fig. 5 Illustration of the one-pot photocatalysis-assisted adsorptive desulfurization of fuel over Ti_1−xSi_xO₂ adsorbent under ambient conditions.

Conclusions

In summary, a methodology for the one-pot photocatalysis-assisted ADS of diesel over Ti_1−xSi_xO₂ adsorbent under ambient conditions was developed. The Ti/Si molar ratio was optimized to 3/7 with a high ADS capacity of 1.89 mg-S per g-Ads. Under UV light irradiation with Ti_1−xSi_xO₂ photocatalyst, organosulfur compounds were oxidized to the corresponding sulfoxide using molecular oxygen in air as the oxidant under ambient conditions, and simultaneously, the transformed sulfoxide strongly adsorbed on the Ti_0.3Si_0.7O₂ adsorbent. As a result, organosulfur compounds were removed selectively from diesel. The developed approach provides a new path for the production of ultra-clean fuels.

Acknowledgements

We are grateful to acknowledge the research grants provided by the National Natural Science Foundation of China (21306054), the Guangdong Natural Science Foundation (S2013040014747), the Specialized Research Fund for the Doctoral Program of Higher Education (20130172120018), NSFC-Guangdong Joint Fund Project (U1034005), and PetroChina Innovation Foundation.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and the schematic diagram of the batch reactor setup for photocatalysis-assisted adsorptive desulfurization of fuel (Fig. S1); desulfurization capacities of the Ti_0.3Si_0.7O₂ at different fuel-to-adsorbent ratios (Fig. S2); desulfurization capacities of the Ti_0.3Si_0.7O₂ in the first three regeneration cycles (Fig. S3). See DOI: 10.1039/c4ra09680k

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