Polyoxometalate as an effective catalyst for the

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Apr 11, 2019 - Fuel desulfurization technology can be divided into hydro-desulfurization (HDS) .... with oil-soluble oxidants; and the last one is a gas-liquid.

Rev Chem Eng 2019; aop

Majid Taghizadeh*, Elaheh Mehrvarz and Amirhossein Taghipour

Polyoxometalate as an effective catalyst for the oxidative desulfurization of liquid fuels: a critical review https://doi.org/10.1515/revce-2018-0058 Received August 12, 2018; accepted February 5, 2019

Abstract: In order to meet the stringent environmental and industrial legislation on fuel specifications, sulfur compounds have to be removed efficiently from fuels. The requirement to produce ultralow-sulfur fuels (S  H+ > (VO)+ > (n-Bu4N)+

  Comment

M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate      9

Authenticated | [email protected] author's copy Download Date | 4/11/19 11:01 AM

  MeCN

  H2O2

  4 [email protected]°C

         

Zhu et al. (2007)

Zhu et al. (2011a,b, c)

Zhu et al. (2011a,b, c)

Zhu et al. (2008)

Rafiee and Mirnezami (2014) Ribeiro et al. (2016)

  Reference

PW12, TBA3PW12O40; PW11Zn, TBA4H[PW11Zn(H2O)O39] · 5H2O; (PW9)2Zn4, TBA7H3[(PW9O34)2Zn4(H2O)2] · 9H2O; (ChO)2K0.2Na2.6IMo6, {[(CH3)3N(CH2)2OH]2}[H0.2K0.2Na2.6(H2O)6][IMo6O24]; TBAPWCr, (TBA)4[PW11CrO39] · 3H2O; PW, H3PW12O40; PhPy, 4-phenyl-pyridine; Bs, 1,4-butane sultone; TBA, tetra-n-butylammonium; BMI, 1-butyl-3-methylimidazolium; ODA, octadecyltrimethylammonium; MeCN, acetonitrile; [Bmim]PF6, 1-butyl-3 methylimidazolium hexafluorophosphate; [Omim]BF4, 1-n-octyl-3-methylimidazolium tetrafluoroborate; [Bmim]BF4, 1-butyl-3-methylimidazolium tetrafluoroborate; methyltri-n-octylammonium peroxomolybdate, [(CH3)N(n-C8H17)3]2Mo2O11.

  H2O2

[Bmim]BF4   180 [email protected]°C   [Omim]BF4 O/S = 10   [Bmim]PF6   [Omim]PF6  

  n-Octane   [WO(O2)2‚ Phen‚ H2O] [MoO(O2)2‚ Phen]

DBT

       

  [Bmim]BF4   60 [email protected]°C   O/S = 4        

[Bmim]3PW12O40   H2O2 [Bmim]3PMo12O40 [Bmim]4SiW12O40 [MIMPS]3PW12O40 · 2H2O [MIMPS]3PW12O40 · 2H2O

  n-Octane   Actual   diesel   fuel    

DBT

DBT

  Comment

  The reaction system was homogeneous during the reaction   and then became heterogeneous at lower temperatures, which could be easily recovered and reused 100, 100, 97%  The quaternary ammonium catalysts TBAPW11Zn and   100, 87, 21% ODAPW11Zn showed higher catalytic desulfurization 99, 99, 44% efficiency than the ionic liquid catalyst BMIPW11Zn. 61% TBAPW11Zn behaved as a homogeneous catalyst 67% immobilized in the extraction solvent, while ODAPW11Zn behaved as a heterogeneous catalyst 99%   Polyoxometalate-catalyzed oxidation in conjunction with IL   94.2% extraction could increase the sulfur removal significantly. 98.4% 92.6% Comparing several molybdic compounds in the oxidative 97.9% system, salts yielded higher S removal than acids. These 98.7% results indicated that a higher electrolyte strength of the catalyst may contribute to better reactivity in ionic liquid 99.2%   Removal of DBT increased to 99.2% in the water-immiscible   99.1% ionic liquid [Bmim]PF6 using the peroxo-molybdenum amino 99.2% acid complex MoO(O2)2C2H5NO2 as a catalyst and remained 99.0% in the water-miscible ionic liquid [Bmim]BF4 97.9%   The catalytic activity depended on the type of cations and   95.1% metals in the catalysts. Phosphotungstate catalysts were more 36.7% active than phosphomolybdate due to the formation of the 100% activity species, where the activities are not dictated by the 82.2% original Keggin structure (and its redox potential) but by the oxidative activities of the derived polyoxoperoxo species 98.6, 93%   Four ILs were immiscible with the model oil, which formed   90.5, 68.6% biphasic systems. Peroxotungsten and peroxomolybdenum 95.7, 96.9% complexes had been synthesized and were immobilized in 97.3, 97% ILs

  10 [email protected]°C   98% O/S = 2

  %S removal

  [Bmim]BF4   180 [email protected]°C     O/S = 4      

  Ethyl acetate

  H2O2

  Extraction   Optimum solvent conditions

  [Bmim]PF6   180 [email protected]°C   O/S = 4    

  n-Octane          

DBT

PhPyBs-PW PyBs-PW QBs-PW TBAPW11Zn BMIPW11Zn ODAPW11Zn TBAPW11Zn ODAPW11Zn

  Oxidant

Na2MoO4.2H2O   H2O2 H2MoO4 (NH4)6Mo7O24 · 4H2O H3PMo12O40 · 13H2O (NH4)3PMo12O40 · 7H2O Na3PMo12O40 · 7H2O   n-Octane   MoO(O2)2C2H5NO2   H2O2   MoO(O2)2C3H7NO2 · H2O   MoO(O2)2C5H9NO4 · H2O

  n-Hexane        n-Octane   real   diesel      

  Catalyst

Methyl phenyl sulfide DBT 4,6-DMDBT 1-BT

S-compounds  Oil

Table 1 (continued)

10      M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate

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M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate      11

and DBT were selectively and efficiently oxidized to form products of benzothiophene sulfone (BT-O) and dibenzothiophene sulfone (DBT-O), respectively; (iv) the peroxometal complex compound was reduced and dissociated with PTA, which was brought back to the aqueous phase; and (v) ultrasound or high-shear mixing enhanced the oxidation efficiency as a result of effective mass transfer in the biphasic system. They found that the NaPW/H2O2 system oxidized BT and DBT best under mild operating conditions. Table 1 reports the ODS of sulfur-containing compounds using a number of homogeneous POM catalysts under different experimental conditions, where POMs have been shown to be particularly interesting and useful catalysts for the desulfurization of liquid fuels. It can be seen that these homogeneous catalysts normally exhibit high activity and selectivity in the ODS process. These homogenous catalysts can greatly contribute to the desulfurization of fuel oil. However, their separation and recovery can be quite difficult, especially for large-scale applications.

3.3.3.2 P  OM as heterogeneous catalyst Different obstacles of homogeneously used POMs in the oxidation process, including low specific area and high solubility, specifically in polar liquids, which bring secondary problems such as recyclability and environmental constraints, have motivated researchers to study and develop heterogeneously active forms of POM in recent years (Li et  al. 2016). The research methodology of heterogeneous POM catalysts is quite different from that of homogeneous ones. In heterogeneous catalysis with POMs, emphasis is given to the heterogenization of POM catalysts to achieve good recovery and recyclability besides high activity. Generally, two strategies, namely the “solidification” (formation of insoluble solid ionic materials with appropriate counter-cations) and “immobilization” of catalytically active POMs onto appropriate supports through adsorption, dative, covalent, or electrostatic binding, ion exchange, encapsulation, and substitution, have been applied in the design of POM-based heterogeneous catalysts (Figure 4) (Mizuno et al. 2011). Depending on the type of support, immobilization of POMs can be classified as immobilization on metal surfaces, immobilization on oxide surfaces, and their incorporation into polymeric matrices, mainly with the aim of improving POM-based catalytic systems via their heterogenization. Solubility of the active layer (catalytic region) of the heterogenized catalyst in the feed greatly reduces the activity and, consequently, reusability of the

recovered catalyst. Solubility of POMs (in a heterogeneous system) in the feed can be controlled using insoluble solid ionic materials such as metal ions and alkylammonium ions. The formation of micro/mesoporous materials by cross-linking the copolymer with the POM is another technique for making the prepared catalyst insoluble. The intercalation of POMs in anion-exchange materials is another synthesis technique that has been performed using hydrotalcite-like compounds (HTs). These materials have the anion-exchange ability, and different types of anions, including POMs, can be intercalated by an anionexchange reaction. Modification of the surface of the support is another technique that improves the intermolecular bonds between the solid phase (support) and the POM phase. There are different methods for modifying the support. For example, organic groups can be easily linked with a silanol-containing surface. In this method, surface characteristics such as polarity can be easily controlled by changing the organic groups. The details on the developments of heterogeneous POM catalysts have been summarized in a number of review articles (Mizuno and Misono 1998, Proust et al. 2012, Song and Tsunashima 2012). In POM-based heterogeneous catalysts, homogeneous catalysts are heterogenized at the molecular level with the retention of their active sites, thus preserving the unique catalytic performance of homogeneous POMs. Therefore, the heterogenization of catalysts combines the classic benefits of heterogeneous catalysis (recyclability and easy removal from a reaction mixture) with homogeneous catalysts (high selectivity and reactivity) (Song and Tsunashima 2012). Selection of an appropriate solid support has major effects on the performance of the oxidation process, since the nature of the support will determine many other factors such as the effective surface area, mechanical stability, catalyst recyclability, and cost of the process. There are different kinds of materials that can be used as solid supports, such as TiO2, ZrO2, Al2O3, Ta2O5, siliceous materials, zeolites, carbonaceous materials (carbon nanotubes (CNTs), activated carbon, and graphite carbon), and metal organic frameworks (Yan et  al. 2009, 2014, Ribeiro et  al. 2013a, Xie et al. 2015, Zhang et al. 2016c). In this regard, Gao and coworkers (2018) investigated the ODS of a model fuel (2000  ppm S content) in the presence of molecular oxygen as oxidant over POMbased catalysts supported on CNTs. Two series of catalysts were synthesized with Dawson-type POMs as the active component and CNTs as the carrier through two different methods. One way was to directly impregnate the POM into the channel of pristine CNTs, forming Authenticated | [email protected] author's copy Download Date | 4/11/19 11:01 AM

12      M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate

Figure 4: Strategies for heterogenization of POM-based compounds. Reprinted from Mizuno et al. (2011) with permission from Elsevier (copyright 2011).

[email protected]; another approach was to wrap the CNTs by poly(diallyldimethylammonium chloride, PDDA) followed by functionalization of PDDA-wrapped CNTs by POMs forming POM @[email protected] CNTs. Results of their study revealed that the catalytic performance of the POM @ [email protected] CNT catalyst was better than that of [email protected] and the maximum DBT conversion reached up to 99.4% using a catalyst dosage of 1 g l−1 under 1.5 l min−1 oxygen flow volume at 70°C. In another study, Zhang et  al. (2017) prepared a number of magnetic POM-based mesoporous silica samples as efficient catalysts over a facile hydrothermal and impregnation method and evaluated them in the fast ODS of a fuel containing BT, 3-methylbenzothiphene, DBT, 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiphene (4,6-DMDBT). These hybrid materials

exhibited the interesting feature of rapid separation by external magnets and excellent desulfurization performance to refractory sulfur compounds. Furthermore, the desulfurization performance of various aromatic sulfur-containing compounds decreased in the order DBT > 4-MDBT > 3- MBT > 4,6-DMDBT > BT. Jiang et al. (2017) synthesized a supported heterogeneous catalyst with polyoxometalate-based ionic liquid (POM-IL) serving as the active component and graphite carbon (GC) with a layer structure as support. The supported catalyst was prepared successfully through the hydrothermal/impregnation method. In this work, three typical sulfur-containing compounds, namely BT, DBT, and 4,6-DMDBT, were selected to investigate the desulfurization performance of the supported catalyst using hydrogen peroxide (H2O2) as oxidant. The proposed mechanism Authenticated | [email protected] author's copy Download Date | 4/11/19 11:01 AM

M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate      13

is shown in Figure 5. First, H2O2 donated an oxygen to Mo = O and formed the active peroxo species [Mo(O2)]. After that, DBT was first oxidized into DBTO and eventually to DBTO2. Finally, the active peroxo species Mo(O2) returned to the Mo = O form and continued the reaction until H2O2 was exhausted. Moreover, the appropriate reaction conditions were determined through a series of experiments. The suitable experimental conditions were as follows: m (catalyst) = 0.05 g, V (model oil) = 5  ml, (H2O2/model oil) molar ratio = 3, T = 50°C, and t = 60 min. The removal of DBT, 4,6-DMDBT, and BT under the aforementioned conditions was reported as 100%, 90.3%, and 46.3%, respectively. Zhang et  al. (2014a) investigated the amphipathic [Bmim]3PW12O40/SiO2 and H2O2 heterogeneous system for the ODS of sulfur-containing compounds in a model oil and actual diesel oil. The whole process was carried out at the interface of the water and oil phase because of the hydrophilic and hydrophobic properties of the catalyst. Therefore, the limitation of mass transfer across the interface was eliminated, and the oxidative reaction could proceed at a faster rate in the [Bmim]3PW12O40/SiO2 and H2O2 system. The experimental results demonstrated that the proposed oxidation system was effective in achieving DBT, 4,6-DMDBT, and BT conversions of 100%, 100%, and 71.6%, respectively, at 50°C in 100 min. Furthermore, the sulfur content of the actual diesel oil decreased from 455 to 8 mg l−1 after oxidation and extraction with DMF at 80°C in 180 min. Abdalla and Li (2012) prepared an MCM-41-supported Keggin-type POM encapsulated quaternary ammonium salt (Bu4N)4H3(PW11O39) catalyst and evaluated its performance in the ODS of a model oil and FCC diesel oil using

Figure 5: Proposed mechanism of ODS. Reprinted from Jiang et al. (2017) with permission from Elsevier (copyright 2017).

H2O2 as oxidizing reagent. The prepared catalyst was systematically characterized by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), N2 adsorption/desorption, Fourier transform infrared (FTIR), phosphorus cross-polarization magic angle spinning nuclear magnetic resonance (PCP-MAS NMR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and TEM analyses. Nitrogen adsorption/desorption results revealed that the synthesized catalyst possessed high surface area (805–912  m2  g−1). Wide-angle XRD and 31P CP-MAS NMR results showed that TBA-POM was grafted onto silica surface and its structural integrity was preserved during preparation. Based on the obtained results, the TBA-POM/MCM-41 catalyst was found to be highly efficient for the oxidation of both DBT and thiophene under mild reaction conditions and reduced their sulfur content from 500 and 320 ppm to a level of 1 and 2 ppm, respectively. The catalyst was also applied for the ODS of FCC diesel oil with 974 ppm S, wherein it lowered the sulfur content to a level of 27.3 ppm, corresponding to 97.2% S removal. Li and coworkers (2015) employed the amphiphilic POM-based supported silica ([C4mim]3PW12O40/SiO2) (C4mim = 1-butyl-3-methylimidazolium) in the ODS of a model oil containing BT, DBT, and 4,6-DMDBT compounds. They proposed a mechanism for the catalytic ODS process using [C4mim]3PW12O40/SiO2 catalyst in the presence of H2O2, as shown in Figure 6. Their experimental results indicated that the hybrid material [C4mim]3PW12O40/SiO2 could completely remove DBT in 30  min under optimal conditions without any other organic solvents. In this work, the [C4mim]3PW12O40/SiO2 catalyst possessed a moderately hydrophilic-hydrophobic balanced surface, leading to higher sulfur removal. In the study by Zhang et al. (2013), a series of POMbased hybrid materials, namely phosphotungstic acidsupported ceria (HPW-CeO2) with different HPW loading contents and treatment temperatures, were synthesized through a simple, one-pot sol–gel method. The amount of catalyst, the O/S molar ratio, the reaction time, and the temperature were investigated in detail, and the most favorable operating condition was achieved at. Compared with the other catalysts, 30%HPW-CeO2(400) combined with [C8mim]BF4 as extractant and H2O2 as oxidant was very efficient on the removal of DBT, which could reduce the S content in the model oil from 500 to 3  ppm under optimum reaction conditions. The desulfurization process of DBT is also described in Figure 7. The catalyst, H2O2, [C8mim] BF4, and the model oil were added into the batch reactor in turn (Figure 7A). All the above materials were mixed together by stirring (Figure 7B). At first, DBT was Authenticated | [email protected] author's copy Download Date | 4/11/19 11:01 AM

14      M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate

Figure 6: Proposed mechanism of ODS process using [C4mim]3PW12O40/SiO2 catalyst in the presence of H2O2. Reprinted from Li et al. (2015) with permission from Elsevier (copyright 2015).

extracted into the IL phase, then adsorbed on the surface of CeO2, and reacted with the active peroxo species, which was formed by HPW in the presence of H2O2. As the reaction continued, the peroxo species got transformed to the HPW species, which continually combined with H2O2 (Figure 7D). After oxidation, polarity of the sulfur compound increased, which was easily dissolved in the IL. Finally, with increasing recycle runs, the solubility of sulfones in IL reached saturation, and the sulfones precipitated gradually and accumulated in the IL phase (Figure 7C). Table 2 reports the ODS of liquid fuels using a number of heterogeneous POM catalysts under optimized experimental conditions.

4 Regeneration and recycling Reusability of POM catalysts in the oxidation process is limited by different factors such as the feed properties (polarity and temperature), catalyst phase, extraction technique, etc. Therefore, various studies have been carried out to simplify the separation of these catalysts and increase their reusability in homogeneous and heterogeneous catalytic media. As mentioned earlier, heterogenization of homogeneous POMs have been implemented to reduce the recycling challenges of homogeneous catalytic systems. However, there have also been studies in which the recycling and regeneration challenges of homogeneously used POMs were reduced by inventive methods. The regeneration capability and recyclability of homogeneous and heterogeneous catalytic systems in ODS in the presence of POMs as catalysts are presented below.

4.1 Homogeneous system POMs as homogeneous catalysts in the ODS process have been used frequently with common difficulties, usually related to their separation and recycling processes. So, in most of the recent studies on homogeneous catalytic systems, one or more inventive methods or procedures were presented to decrease the separation and purification challenges of these systems. An oxidative-extractive desulfurization (OEDS) system with H2O2 as oxidant using a functionalized POM catalyst with two PEG (polyethylene glycol) chains and Anderson-type polyoxoanion Imo6O245− was investigated by Yu et al. (2015). The catalyst was formed in situ by the oxidation of heteropoly blue with H2O2 (Figure 8). The prepared homogeneous catalyst was successfully used for the conversion of refractory sulfur compounds (DBT, BT, and thiophene). After the desulfurization reaction, the upper oil phase separated, and the lower phase (heteropoly blue) was dried and reused by adding fresh gasoline and oxidant. The reactivity of catalytic systems did not change significantly even after eight recycling operations. After the eighth run, the catalytic phase was regenerated by back-extraction with water for removing the sulfides. Comparative results with the fresh catalyst showed that the presented catalyst had good recyclability. In another study by Lü et al. (2017), a hybrid Anderson-type POM in deep eutectic solvents (DESs) was synthesized and used for the deep desulfurization of a model diesel in ILs. In the regeneration step, since the IL is miscible with the oil phase, the fuel phase, which formed in the upper layer (for its lower density), was separated by decantation. Then, the water phase and the residual fuel were removed at 60°C under partial vacuum over 60 min. Authenticated | [email protected] author's copy Download Date | 4/11/19 11:01 AM

M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate      15

Figure 7: Mechanism for the oxidation of DBT. (A) Before oxidation, (B) during oxidation, (C) after oxidation, and (D) catalytic oxidation process of DBT. Reprinted from Zhang et al. (2013) with permission from Elsevier (copyright 2013).

The catalyst and IL retained in the reaction system could be recycled. It was claimed that the presented desulfurization system could be recycled 20 times without considerable decrease in its activity. During the recycling process, a white precipitate was formed gradually and accumulated.

4.2 Heterogeneous system The separation of the heterogeneous POM-supported catalyst is much easier and usually carried out by simple

physical separation methods such as filtration. Generally, the recycling process of fresh and/or relatively fresh catalysts follows some physical pretreatments (washing), and then the catalyst is charged for the next run. Usually, there is no appreciable change in the catalyst activity and sulfur conversion using the regenerated catalyst up to a certain number of cycles, after which a sudden deactivation occurs. The catalyst H3PW12O40/mpg-C3N4 (mpg-C3N4 =  mesoporous graphitic carbon nitride) was prepared by Zhu et  al. (2015a) and used in the ODS process. They reported that the catalytic performance of around 100% Authenticated | [email protected] author's copy Download Date | 4/11/19 11:01 AM



DBT 4,6-DMDBT BT –

Toluene diesel oil



         

n-Octane



DBT 4,6-DMDBT BT

n-Octane



n-Octane



DBT



                     

n-Octane



DBT

DBT



Oil

S-compounds 

[Bmim]3PW12O40/SiO2

0.05HSiW-[pmim]Cl/SBA-15 0.1HSiW-[pmim]Cl/SBA-15 0.2HSiW-[pmim]Cl/SBA-15 0.3HSiW-[pmim]Cl/SBA-15 0.2HSiW-[pmim]Cl/SBA-15

[PSPy]PMo/GC

[email protected] [email protected] [email protected] [email protected] [email protected] [email protected]@CNTs [email protected]@CNTs [email protected]@CNTs [email protected]@CNTs [email protected]@CNTs MPMS

Catalyst













H2O2

H2O2

H2O2

H2O2

O2

Oxidant













None

None

None

None

None













Extraction   solvent



100 [email protected]°C    O/S = 3 180 [email protected]°C    O/S = 5

120 [email protected]°C  O/S = 3       120 [email protected]°C  O/S = 4

60 [email protected]°C   O/S = 3    

180 [email protected]°C                    60 [email protected]°C   O/S = 4

Optimum reaction conditions

100%, 100% 71.6% 98.2%

47.6% 82.0% 85.9% 67.2% 97.9%

100% 90.3% 46.3%

87.54% 92.84% 89.43% 87.14% 79.75% 88.58% 94.70% 91.46% 89.76% 88.87% 100%











%S removal  

Table 2: Oxidative desulfurization of sulfur-containing compounds by heterogeneous POM catalysts at optimized conditions.  

The synthesized catalyst showed   superparamagnetism and could be attracted easily by an external magnet, which laid the foundation for magnetic fixation in the separation process A graphite-carbon-supported POM-IL   catalyst was prepared successfully through hydrothermal impregnation. The hydrophobic property of the GC and the hydrophilic property of [PSPy]PMo help the supported catalyst remove DBT efficiently in the solvent-free ODS system When the HSiW was increased from   0.05 to 0.2 g, the removal of DBT also increased. When the loading amount of HSiW was further increased to 0.3 g, the desulfurization activity reduced. This was because too much loading of HSiW blocked the pore channel of SBA-15. Also, the surface area and pore size become smaller, which influences the mass transfer, and is not conducive to the desulfurization reaction   Result revealed that [Bmim]3PW12O40 supported on SiO2 exhibits a high ODS activity by using H2O2 solution as the oxidant, because the catalyst has both hydrophilic and lipophilic properties

By increasing the V load on POM, the   conversion of DBT is enhanced. However, excess V produces the opposite effect and it can be clearly observed that the performance of [email protected]@CNTs is better than that of [email protected], which is maybe caused by the different position of the POM. [email protected]@CNTs showed superior performance

Comment

Zhang et al. (2014a)

Yuan et al. (2016)

Jiang et al. (2017)

Zhang et al. (2017)

Gao et al. (2018)

Reference

16      M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate

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n-Octane



           

n-Octane



DBT

DBT 4,6-DMDBT BT

[C4mim]3PW12O40/SiO2 [C8mim]3PW12O40/SiO2 [C16mim]3PW12O40/SiO2

     

n-Octane



DBT

30% HPWCeO2(400)

m-[C16mim]nSiW-TiO2 0.05-[C16mim]4SiW-TiO2 0.1-[C16mim]4SiW-TiO2 0.2-[C16mim]4SiW-TiO2 0.1-[C4mim]4SiW-TiO2 0.1-[C8mim]4SiW-TiO2

25%TBA-POM/MCM-41

n-Octane   FCC cycle oil



Catalyst

DBT



Oil

S-compounds 

Table 2 (continued)











H2O2

H2O2

H2O2

H2O2

Oxidant













[C8mim]BF4  

[Bmim]BF4  

None

Methanol      

Extraction   solvent



30 [email protected]°C O/S = 6 60 [email protected]°C O/S = 6

           

60 [email protected]°C   O/S = 2          

30 [email protected]°C   O/S = 3    

60 min   @ 70°C, O/S = 4  180 min  

Optimum reaction conditions

99% 97% 75.6% 100% 100% 75.6%

90.7% 95.3% 85.1% 92.3% 93.1%

100% 92.1% 80.6%

99.82% 99.22% 88.5%









%S removal  



  During the ODS reaction, the supported hydrophilic lacunary Keggin unit (PW11O39)−7 accepted active oxygen from the oxidant H2O2 to form an oxoperoxo intermediate, and the hydrophobic tetrabutyl quaternary ammonium cation encapsulated the monovacant Keggin unit transferred these oxoperoxo species to the substrate (thiophene and DBT) in oil, and thus facilitated the oxidation process   The [C4mim]3PW12O40/SiO2 catalyst possessed moderate amphiphilic property, leading to the fast interaction with H2O2 and model oil as well as larger BET surface area, which could accelerate the mass transfer process m represents the molar ratio of   tungsten:titanium, m = 0.05, 0.1, 0.2; n = 4, 8, 16, and TiO2 was the support. As the molar ratio of tungsten:titanium (W: Ti) in the catalysts increased, the removal of DBT increased first and then decreased. Low loading resulted in less active centers, while excessive loading also led to the agglomeration of the SiW-based IL The reactivity of sulfur removal decreased   in the order DBT > 4,6-DMDBT > BT under the same reaction conditions. The S electron density of BT (5.739) was lower than that of DBT (5.758) and 4,6-DMDBT (5.760), which led to the lower sulfur removal. In the case of 4,6-DMDBT and DBT, the main difference in desulfurization performance was caused by the influence of steric hindrance. 4,6-DMDBT has two more methyl groups on the benzene ring, as well as higher steric hindrance

Comment

Zhang et al. (2013)

Xun et al. (2016a,b)

Li et al. (2015)

Abdalla and Li (2012)

Reference

M. Taghizadeh et al.: Desulfurization of liquid fuels by polyoxometalate      17

Authenticated | [email protected] author's copy Download Date | 4/11/19 11:01 AM



     

n-Octane

n-Octane













1-BT DBT 4-MDBT 4,6-DMDBT

DBT

DBT

DBT

DBT

DBT



n-Octane

n-Heptane        

       

n-Octane

      n-Heptane  



Oil

S-compounds 

Table 2 (continued)





PMo/BzPN-SiO2 PMo/PrNH2-SiO2 PMo/iBuPN-SiO2 PMo/iPrPN-SiO2

POM-based nanowires

0.2W-MCM-41 0.1W-MCM-41 0.05W-MCM-41 0.025W-MCM-41

[email protected](Cr)-Diatomite









[email protected][email protected] [email protected]@MCM-41 [email protected]@MCM-41 [email protected]@MCM-41 [email protected]@MCM-41

ODAPW11

Catalyst

H2O2

H2O2

H2O2

H2O2

O2

H2O2

Oxidant















None

None

None

MeCN









[Bmim]BF4  

[Bmim] PF6  

Extraction   solvent



180 [email protected]°C  O/S = 3      

60 [email protected]°C   O/S = 6

20 [email protected]°C   O/S  =  2.5      

150 [email protected]°C              120 [email protected]°C  O/S  =  5

40 [email protected]°C   O/S = 8

Optimum reaction conditions

100% 32% 86% 61%

100%

99.8% 99.6% 93% 77.4%

45.1% 64.9% 87.3% 92.8% 100% 90.5% 98.6%

100%













%S removal  



The heterogenization of the highly active   monovacant polyoxotungstate (PW11) was achieved by preparing the corresponding long-chain quaternary ammonium salt (ODA7PW11). The heterogeneous catalyst showed superior desulfurization performance Because of the wantonly agglomerated   state of heteropolyacid species in the relatively large pores of MCM-41, following leading the self-assembly of the metal organic framework MOF-199, the dually supported catalyst SRL-PMM showed high efficiency on DBT removal Catalyst was prepared by restricting   MIL-101 encapsulating Keggin POMs into mesoporous diatomite, which could overcome the obstacle of poor solubility of POMs in the overall nonpolar environment As the tungsten content increased in the   samples, the oxidative desulfurization performance was improved as a result of the increase in active tungsten species, which could activate H2O2 and oxidize the sulfur to sulfone (W/Si molar ratios = 0.2, 0.1, 0.05, and 0.025) In spite of the nonporous nature of the   catalyst, the surface catalyst exhibited excellent catalytic performance in multiphase reactions. Furthermore, the multivalent electrostatic interaction was assumed to contribute most to the stability of the assembled structure Keggin-type polyoxometalates were   immobilized on RPN-SiO2 supports. (R = benzyl, iso-butyl, iso-propyl). Catalytic activity increased with the size of the phosphazene R group (iPr 

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