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Coordination unsaturation of vanadium nitride quantum dots boosts low-temperature aerobic oxidation of thiophenic sulfides

Aerobic oxidative desulfurization (AODS) promises a sustainable alternative technology for diesel desulfurization, which necessitates the efficient aerobic oxidation of thiophenic sulfides under mild conditions to minimize energy input, yet being longstandingly plagued by the grand challenge in low-temperature activation of triplet oxygen. Here we synthesize vanadium nitride quantum dots on graphene to controllably create coordination-unsaturated edge/corner V sites for boosting the AODS reaction. The catalyst activates the reaction at 70 °C, and is two orders of magnitude more active than the best V-based catalysts. We demonstrate through computational studies that the low-coordinated edge/corner V sites can effectively activate oxygen and adsorb sulfides to lower the activation barrier, dramatically enhancing the activity. The catalyst achieves deep AODS of real diesel at 80 °C with negligible attenuation in successive reuses, which highlights its attractive industrial potential. These findings provide scientific and practical insights to develop high-performance catalysts for a sustainable AODS process.


Catalyst testing
Aerobic oxidation of sulfides was studied at atmospheric pressure in a 50 mL three-neck flask equipped with a gas injection, a reflux tube and a sampling port for liquid. The sulfides were dissolved in decahydronaphthalene to reach 500±5 ppm of sulfur content. In a general test, 30 mL of model oil and 2 mg of catalyst were charged into the flask and sonicated for 15 min. The flask was then immersed in an oil bath to control the desired reaction temperature (60-110 °C) and magnetically stirred at 500 rpm, while high purity N 2 was bubbled into the flask before the temperature of the reaction system reached the set value. To start the reaction, N 2 was replaced by air, which was pretreated by a silica gel column and a 4 A molecular sieve column in series to remove moisture. The flow rate of air was controlled at 40 mL/min with a precision flow meter. Samples were taken from the liquid phase at 30 minute intervals, and GC-FID (Agilent 7890A; column: HP-5, FID: Agilent) was used to quantify the conversion of sulfides without considering the selectivity to sulfones or sulfoxides. Each experiment was repeated at least three times. GC-MS (Agilent 7890A; HP-5ms, MS: Agilent 5975C) was used to identify the products in the reaction. The conversion rate of the sulfides was calculated as follows: where S conv is the conversion of sulfides (%), C 0 and C t were mass concentrations of sulfides before and after reaction began for certain time (mg/L), respectively.

Desulfurization of real diesel
The reaction procedure for real diesel was similar to that for model fuel. The catalyst dosage was 20 mg instead of 2 mg in order to achieve rapid conversion of sulfides. The reaction temperature was set as 80 °C, since a reaction at 80 °C can be facilely realized in the petroleum industry by using waste heat without consuming extra energy. The total sulfur content was quantitatively analyzed by using multi EA 5000 elemental analyzer (Analytical Jena). The concentration and evolution of sulfur species were determined by a GC-7820 instrument (Huifen; column: HP-5; detector: FPD).

Reuse of catalysts
After each recycle run, the reaction system was transferred into a centrifugal tube, and the catalyst was separated by centrifugation. Then, fresh diesel was added into the centrifugal tube to redisperse the catalyst, and the dispersion was transferred into the reaction flask to start a new cycle. The reaction flask and centrifuge tube were not washed during the entire recycle experiment to minimize catalyst loss.

XAFS measurement and data analysis
XAFS spectra data (V K-edge) were collected at the BL14W1 station in Shanghai Synchrotron Radiation Facility (SSRF, operated at 3.5GeV with a maximum current of 300 mA). The data were S4 collected at room temperature (25 °C) in transmission mode using a N 2 -filled ionization chamber). All samples were pelletized as disks of 13 mm diameter and 1 mm thickness using graphite powder as a binder.
The acquired EXAFS data were processed according to standard procedures using the ATHENA module implemented in the IFEFFIT software package. The EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, the χ(k) data were Fourier-transformed to real (R) space using a Hanning window (dk = 1.0 Å −1 ) to separate the EXAFS contributions from different coordination shells.
To obtain the quantitative structural parameters around the central atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of the IFEFFIT software package. The following EXAFS equation was used: where S 0 2 is the amplitude reduction factor; F j (k) is the effective curved-wave backscattering amplitude, N j is the number of neighbours in the j th atomic shell, R j is the distance between the X-ray absorbing central atom and the atoms in the j th atomic shell (backscatterer), λ is the mean free path (in Å), ϕ j (k) is the phase shift (including the phase shift for each shell and the total central atom phase shift) and σ j is the Debye-Waller parameter of the j th atomic shell (variation of distances around the average R j ). The functions F j (k), λ and ϕ j (k) were calculated with the ab initio code FEFF8.2.

DFT calculation
Spin polarized DFT calculations were conducted using the projector augmented-wave method ( Then, the activation the reaction was fitted, and the reaction rate constants at different temperatures were calculated. The apparent activation energy was calculated to be 72.9 kJ/mol.  Mass specific activity (mmol g -1 h -1 ) and turnover frequency (TOF, h -1 ) are calculated as follows:  After extraction 98.8 ± 1.9 7.7 ± 0.9 The extraction of diesel was performed using methanol with a volume ratio of 1:2 (diesel / methanol).
The sulfur content of the oxidized hydrogenation diesel was reduced to 7.7 ppm after extraction, which meets the most stringent standards for transportation fuels (≤ 10 ppm).