分子氧为氧化剂磷钒酸盐为催化剂超深度氧化脱除柴油中的二苯并噻吩


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	唐南方

	赵小平

	蒋宗轩

	李灿

Oxidation of dibenzothiophene using oxygen and a vanadophosphate catalyst for ultra-deep desulfurization of diesels

Nanfang Tang^a,b, Xiaoping Zhao^c, Zongxuan Jiang^a , Can Li^a

^a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China;
^b University of Chinese Academy of Sciences, Beijing 100049, China;
^c Shaanxi Yanchang Petroleum (Group) Co., Ltd., Xi'an 710075, Shaanxi, China

Received: 2014-5-30. Accepted: 2014-7-18. Published: 2014-9-20.

* Corresponding author. Zongxuan Jiang, Tel: +86-411-84379990; Fax: +86-411-84694447; E-mail: zxjiang@dicp.ac.cn ;Can Li, Tel: +86-411-84379070; Fax: +86-411-84694447; E-mail: canli@dicp.ac.cn

Foundation Item: This work was supported by the National Natural Science Foundation of China (21173214).

Abstract: A bicapped Keggin structure vanadophosphate showed high catalytic activity in the oxidation of dibenzothiophene (DBT) to sulfone using molecular oxygen as oxidant under mild reaction conditions. This is a promising approach to the ultra-deep desulfurization of fuels (e.g. diesel) as sulfones can be removed from the fuel by extraction or selective adsorption.

Key words: Oxidative desulfurization Molecular oxygen Vanadophosphate Dibenzothiophene

Sulfur-containing compounds are undesirable in fuels because they contribute to acid rain and cause deactivation of catalysts in automotive applications. The Environmental Protection Agency regulations and EU Euro V standards require the S level in diesel fuels to be less than 15 and 10 ppm,respectively. The current technology is conventional hydrodesulfurization (HDS),which shows high efficiency for the removal of thiols,sulfides,and disulfides. However,it is difficult to reduce refractory sulfur-containing compounds such as dibenzothiophene (DBT) and its derivatives,especially 4,6- dimethyldibenzothiophene (4,6-DMDBT),to an ultra-low level using only conventional HDS. This process requires severe operating conditions such as high temperatures,high pressures,and high hydrogen consumption to achieve the ultra-deep desulfurization of diesel. From an economic and environmental point of view,it is necessary to develop an alternative ultra-deep desulfurization processes ^{[ 1, 2 ]}. One promising strategy to remove sulfur-containing compounds from fuel oils (e.g. diesel) is oxidative desulfurization (ODS). ODS avoids the use of hydrogen and allows the process to be conducted under ambient conditions. Moreover,refractory compounds,mainly 4,6-DMDBT,can be removed efficiently,which are difficult to remove with conventional HDS ^{[ 3 ]}.

H₂O₂ is the highly reactive oxidant mostly used for ODS. It has been used with catalysts like Ti molecular sieves ^{[ 4 ]},vanadium oxide ^{[ 5 ]},WO_x/ZrO₂ ^{[ 6 ]},polyoxometallates ^{[ 7, 8 ]},and amphiphilic catalysts in emulsions ^{[ 9, 10 ]}. Other oxidants like organic and inorganic peroxyacids,NO₂,tert-butyl- hydroperoxide,and O₃ have also been reported for ODS ^{[ 11, 12, 13 ]}. Taking into account environmental and economic concerns,the utilization of O₂ as the oxidant for ODS is most desirable ^{[ 14, 15 ]}. In this communication,we report a highly efficient oxidation of refractory compounds DBT using molecular oxygen catalyzed by tetradecavanadophosphate [C₈H₁₇N(CH₃)₃]₅H₄PV₁₄O₄₂ (Q₅H₄PV₁₄O₄₂) under mild conditions.

Q₅H₄PV₁₄O₄₂ was isolated by treating a mixture of metavanadate and orthophosphate (V:P = 4:1,pH = 2.3) with C₈H₁₇N(CH₃)₃Cl. The Fouriter transform infrared (FT-IR) spectrum of Q₅H₄PV₁₄O₄₂ showed four intense bands from 700 to 1100 cm^-1,which served as the signature of the Keggin anion. The band at 1060 cm^-1 was assigned to the stretching of the P-O bond of the central PO₄. According to studies by Rocchiccioli-Deltcheff et al. ^{[ 16, 17 ]},the vibrational motion of the phosphate group is independent of the remaining “cage” of the Keggin anion. The bond at 946 cm^-1 was assigned to the stretching of the terminal V-O_t bond. The two bands at 866 and 804 cm^-1 were the asymmetrical V-O_b-V stretching vibrations. Figure 1 shows the ³¹P and ⁵¹V NMR spectra of tetradecavanadophosphate,which were well explained by the “bicapped Keggin” structure of the anion. The ³¹P NMR spectrum showed a single peak at 0.12 ppm. The ⁵¹V NMR spectrum showed three broad peaks with the relative intensity ratio of 2:8:4. The peak at -529 ppm was assigned to the “capping” 5-coordinated vanadium. The two peaks at -580 and -596 ppm were assigned to the 6-coordinated vanadium in the Keggin unit ^{[ 18, 19 ]}.

Fig. 1. ³¹P and ⁵¹V NMR spectra of tetradecavanadophosphate.

The oxidation of the DBT-containing model oil was conducted in a flask at 90 °C with O₂ gas bubbled in. Samples were periodically taken by a pipette and quantified using an external calibration curve. The sulfur-containing compounds present in the diesel were analyzed by a gas chromatograph coupled to a flame photometric detector (GC-FPD). A white needle-shaped crystal was obtained when the reaction system was cooled down to room temperature. The product was determined to be crystallized dibenzothiophene sulfone (DBTO₂) by FT-IR (Fig. 2) and ¹H NMR (δ 8.20 (2 H),7.98 (2 H),7.80 (2 H),7.65 (2 H)). The band at 1293 cm^-1 corresponded to the asymmetrical stretching vibration of S=O while the symmetrical stretch was at 1165 cm^-1. The bands at 571 and 540 cm^-1 were assigned to the bending vibration of S=O. The mass spectrum of product showed the molecular ion peak at 216 (m/z),which agreed with the molecular weight of DBTO₂ ^{[ 20 ]}.

Fig. 2. FT-IR spectra of DBT before (a) and after (b) reaction.

Figure 3(a) shows the DBT conversion and ln(C_t/C₀) vs. reaction time,which revealed the catalytic reaction kinetics. C_t/C₀ was defined as the ratio of the concentration of final and initial sulfur-containing compounds. Increasing reaction temperature from 80 to 100 °C led to a remarkable enhancement of the conversion of DBT. At 100 °C,the conversion of DBT was 100% in 3 h. The linear fit of ln(C_t/C₀) against reaction time demonstrated pseudo-first order kinetics for the oxidation of DBT on Q₅H₄PV₁₄O₄₂. The apparent activation energy for the ODS of DBT on Q₅H₄PV₁₄O₄₂ was 97.7 kJ/mol (Fig. 3(b)). At 100 °C,the conversion of 4,6-DMDBT was 100% in 4 h.

Fig. 3. (a) Conversion of DBT and lnC_t/C₀ vs. reaction time at different temperatures; (b) Apparent activation energy for the ODS of DBT. Reaction conditions: Q₅H₄PV₁₄O₄₂ 40 mg,DBT (S: 500 ppm) in 20 ml decalin,oxidant O₂ 1 atm.

To study the mechanism of the activation of sulfur- containing compounds,an ESR spin-trap technique with DMPO as a trap for O₂^·- was employed. Figure 4 shows the ESR signals obtained from the in situ ODS process. No ESR signals were observed at 25 and 60 °C for the reaction system without Q₅H₄PV₁₄O₄₂. With the introduction of Q₅H₄PV₁₄O₄₂,no ESR signal was observed at 25 °C. When the reaction system was heated to 60 °C,a broad,8-line ESR signal appeared. The ESR signal was centered at g = 2.0088,and can be assigned to oxygen species ^{[ 21 ]}. The 8-line ESR signal was attributable to the superposition of O₂^·- and O^·- ^{[ 22 ]}. These results indicated that active oxygen species were produced through the reaction of Q₅H₄PV₁₄O₄₂ with O₂. Therefore,we proposed a mechanism where first,O₂ was activated to form an active species when O₂ coordinated to Q₅H₄PV₁₄O₄₂; then,the active species oxidized DBT to DBTO₂.

Fig. 4. In situ ESR spectra obtained in the oxidative desulfurization process. (1) Sample tested without catalyst at 60 °C; (2) Sample tested without catalyst at 25 °C; (1’) Sample tested with catalyst at 60 °C; (2’) Sample tested with catalyst at 25 °C.

In conclusion,we synthesized a “bicapped Keggin” structure vanadophosphate which showed high catalytic activity in the oxidation of DBT to sulfone using oxygen. This provides a new pathway for the ODS of diesel with oxygen as oxidant.

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