Enhancing oxidation resistance of Cu(I) by tailoring microenvironment in zeolites for efficient adsorptive desulfurization

ARTICLE https://doi.org/10.1038/s41467-020-17042-6 OPEN Enhancing oxidation resistance of Cu(I) by tailoring microenvironment in zeolites for efficient adsorptive desulfurization 1234567890():,; Yu-Xia Li 1, Jia-Xin Shen1, Song-Song Peng1, Jun-Kai Zhang1, Jie Wu1, Xiao-Qin Liu1 & Lin-Bing Sun 1✉ The zeolite Cu(I)Y is promising for adsorptive removal of thiophenic sulfur compounds from transportation fuels. However, its application is seriously hindered by the instability of Cu(I), which is easily oxidized to Cu(II) even under atmospheric environment due to the coexistence of moisture and oxygen. Here, we report the adjustment of zeolite microenvironment from hydrophilic to superhydrophobic status by coating polydimethylsiloxane (yielding Cu(I) Y@P), which isolates moisture entering the pores and subsequently stabilizes Cu(I) despite the presence of oxygen. Cu(I) in Cu(I)Y@P is stable upon exposure to humid atmosphere for 6 months, while almost all Cu(I) is oxidized to Cu(II) in Cu(I)Y for only 2 weeks. The optimized Cu(I)Y@P material after moisture exposure can remove 532 μmol g−1 of thiophene and is much superior to Cu(I)Y (116 μmol g−1), regardless of similar uptakes for unexposed adsorbents. Remarkably, Cu(I)Y@P shows excellent adsorption capacity of desulfurization for water-containing model fuel. 1 State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Chemical Engineering, Nanjing Tech University, 30 South Puzhu Road, 211816 Nanjing, China. ✉email: NATURE COMMUNICATIONS | (2020)11:3206 | https://doi.org/10.1038/s41467-020-17042-6 | www.nature.com/naturecommunications 1 ARTICLE E NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17042-6 mission of acidic pollutants like SO2 that originate from the combustion of organosulfur compounds in fuels is a serious environmental issue1–3. Therefore, removal of organosulfur compounds from transportation fuels has attracted worldwide attention4–6. Recently, deep desulfurization becomes more challenging due to even tighter regulations on sulfur contents in commercial fuels7–9. Although hydrodesulfurization (HDS) can eliminate thiols and sulfides efficiently, it is less effective for the removal of thiophenic sulfur compounds such as thiophene, benzothiophene (BT), and their derivatives10–12. In addition, HDS is generally operated at high temperatures (300 − 350 °C) and high hydrogen pressures (2 − 10 MPa), and even at harsher conditions, to meet the regulations with lower sulfur contents13–15. Among alternatives for deep desulfurization, adsorption desulfurization (ADS) receives much attention because it can remove thiophenic sulfur compounds selectively under mild conditions. It is known that the ADS performance is highly dependent on the type of adsorbents16–18, and substantial progresses have been achieved on the preparation of adsorbents for ADS9,19,20. Various adsorbents including activated carbons21, zeolites22,23, and metal-organic frameworks (MOFs)24–26 have been developed for ADS. Cu(I)-containing adsorbents27–30 are of great interests owing to the π-complexation interaction between Cu(I) and thiophenic sulfur compounds. It has been demonstrated that Cu (I)-exchanged Y zeolite, namely Cu(I)Y, exhibits unique faujasite (FAU) pore structure, stable inorganic frameworks, and abundant Cu(I) sites31. These properties endow Cu(I)Y with good ADS performance with regard to uptake and selectivity, making it highly promising for deep desulfurization of transportation fuels32. Nevertheless, the practical application of Cu(I)Y is seriously hindered by the instability of Cu(I) that is easily oxidized to Cu(II) even under atmospheric environment due to the coexistence of moisture and oxygen33,34. Cu(I) can capture thiophenic compounds through π-complexation, a special interaction between Cu(I) and the π-orbital of thiophenics; however, Cu(II) does not show a noticeable adsorption capacity because Cu(II) does not give such a π-complexation interaction27. During the complexation, Cu(I) can form the usual σ bonds based on their sorbitals and, in addition, their d-orbitals can back-feed electrons to the antibonding π-orbitals of the thiophenic compounds. The preparation, storage, and utilization of Cu(I) have to be conducted in the absence of air, which leads to the difficulty in operation and significant increase of costs. It is reported that the oxidation of Cu(I) to Cu(II) by oxygen does not take place at room temperature in a dry environment, but easily occurs when oxygen is adsorbed on hydrated surfaces33. Therefore, in order to stabilize Cu(I), it is necessary to isolate Cu(I) sites contacting with either moisture or oxygen. In contrast to avoiding contact with oxygen, the preclusion of moisture seems earlier to realize. In addition, water is inevitable in commercial fuels. For instance, the water content of BP commercial diesels is in the range of 100 and 500 ppmw (parts per million by weight)35. Such water in fuels not only accelerates the oxidation of Cu(I), but also competes with thiophenic sulfur compounds to interact with active sites in adsorbents36. Hence, from the viewpoint of practical application, it is extremely desirable to develop an approach to tune the nature of Cu(I)Y zeolite, so that the accessibility of Cu(I) sites to moisture can be excluded and the stability of Cu(I) is thus improved. Here we report a strategy of tailoring the Cu(I)Y microenvironment from hydrophilic to superhydrophobic by coating polydimethylsiloxane (PDS), producing the materials denoted as Cu(I)Y@P (Fig. 1). This isolates moisture entering the pores and subsequently stabilizes the Cu(I) despite the presence of oxygen. The results show that Cu(I) in Cu(I)Y@P is stable upon exposure 2 to humid atmosphere with 75% relative humidity (RH) for 4320 h (6 months), while almost all Cu(I) is oxidized to Cu(II) in uncoated Cu(I)Y for only 336 h (2 weeks). The optimized Cu(I) Y@P material after moisture exposure can remove 532 μmol g−1 of thiophene, which is obviously higher than that of Cu(I)Y (116 μmol g−1), regardless of similar uptakes for unexposed adsorbents. It is worth noting that Cu(I)Y@P shows excellent ADS capacity for water-containing model fuel and is superior to all adsorbents reported so far. Furthermore, the adsorbent Cu(I) Y@P can be recycled without any loss in activity, whereas only 3% of adsorption capacity is retained after four cycles for uncoated Cu(I)Y. The good oxidation resistance, adsorption capacity, and recyclability make our adsorbents highly promising in practical ADS application. Results Effect of PDS coating on structural and surface properties. Cu (II)Y was prepared by ion exchange of the zeolite NaY with copper(II) nitrate and reduced to Cu(I)Y selectively via vaporinduced reduction (VIR) in term of our previous reports37,38. To tailor the microenvironment of Cu(I)Y, PDS was chosen as a typical com (...truncated)