Comparing the Catalytic Activity of Silica-Supported Vanadium Oxides

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Environmental and Carbon Dioxide Issues

Comparing the catalytic activity of silica-supported vanadium oxides and polymer nanofiber-supported oxidovanadium(IV) complex towards oxidation of refractory organosulfur compounds in hydrotreated diesel. Tendai Dembaremba, Rina van der Westhuizen, Werner Welthagen, Ernst Ferg, Adeniyi Sunday Ogunlaja, and Zenixole Richman Tshentu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01579 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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Comparing the catalytic activity of silica-supported vanadium oxides and polymer nanofiber-supported oxidovanadium(IV) complex towards oxidation of refractory organosulfur compounds in hydrotreated diesel Tendai O. Dembarembaa*, Rina van Der Westhuizenb, Werner Welthagenb, Ernst Ferga, Adeniyi S. Ogunlajaa* and Zenixole R. Tshentua* a

Department of Chemistry, Nelson Mandela University, P.O. Box 77000, Port-Elizabeth 6031,

South Africa b

Analytical Technology, Sasol Technology (Pty) Limited, P.O. Box 1, Sasolburg 1947, South

Africa ABSTRACT: Silica supported vanadium oxides (VxOy-silica 600⁰C) and polymer nanofiber [2(2’-hydroxy-5’-ethenylphenyl)imidazole (PIMv) and styrene (ST) copolymer] supported oxidovanadium(IV) ([VIVO-p(PIMv-co-ST)]) were synthesized and employed as catalysts for the oxidation of refractory organosulfur compounds in fuels in a continuous flow system. Conversion of dibenzothiophene (DBT) to dibenzothiophene sulfone (DBTO2) increased as flow rate

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decreased, reaching 100% at flow rates of 0.1 and 0.2 mL/h for (VxOy-silica 600⁰C) and [VIVOp(PIMv-co-ST)], respectively. This was attributed to improved contact time between catalyst and substrate, which allowed further oxidation to take place. However, catalytic activity of VxOy-silica 600⁰C dropped by 33% after the first oxidation cycle at a flow rate of 0.1 mL/h at 60⁰C unlike [VIVO-p(PIMv-co-ST)] which maintained its activity at 100% after 3 cycles. Optimized conditions were employed in the oxidation of a hydrotreated fuel sample (Sasol diesel 500) followed by extraction of the resulting sulfones using acetonitrile. GC × GC-SCD and GC × GC-HRT analysis confirmed the oxidation of sulfur and the removal of resulting sulfone in the fuel. This study revealed that [VIVO-p(PIMv-co-ST)] was a more robust and more efficient catalyst for the oxidation of organosulfur compounds compared to (VxOy-silica 600⁰C). Monitoring the 51V EPR signal from the catalysts upon adding oxidant and then substrate, showed that the catalysis is redox nature, involving the V4+ sites.

KEYWORDS: vanadium oxides, polymer nanofibers, dibenzothiophene, hydrotreated fuel, oxidation


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1. INTRODUCTION Global demand for energy in modern civilizations, especially transportation fuel, has been escalating over the past few decades, and crude oil has become indispensable as a primary source of energy.1 Consumption of crude oil rose from around 70 million barrels per day (Mb/d) in 1995 to 80 and 95 Mb/d in 2005 and 2015, respectively.2 The 2018 report already shows crude oil consumption to be over 99.8 Mb/d.2 This rise in demand translates to increasing emission of sulfur oxides resulting from the combustion of sulfur containing compounds (SCCs) found in crude oil derived fuels. These sulfur oxides have negative health and environmental implications prompting most governments to advocate for more stringent limits for sulfur levels in fuel, less than 10 ppm.1 Hydrodesulfurization (HDS) has been the most prominent technique employed due to its efficiency on removing a wide spectrum of SCCs in fuel.1 However, steric hindrance posed by polycylic SCCs to the catalysts employed have made the continued drive to achieve ultra-low sulfur levels through HDS formidable.1,3 Biodesulfurization (BDS) is an attractive green option that has been proposed but is also limited in achieving the prospective mandatory levels since the bacteria employed cannot utilize sulfur sources below 50-200 ppm.1 Adsorption and extraction of SCCs have also been reported, with recent techniques proposing the oxidation of SCCs into their respective sulfoxides and sulfones to improve selectivity, a technique known as oxidative desulfurization (ODS).4,5 Although attempts are being made to develop approaches for aerobically oxidizing SCCs in fuel6-8, generally ODS requires considerable amounts of oxidant as well as a highly active catalyst to achieve high oxidation yield. Vanadium is considered a suitable catalyst to make the process affordable and efficient.9


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One crucial factor to consider when designing catalysts for industrial use is their recyclability. In that regard, heterogeneous forms of the catalysts are preferred.10 However, choice of support material is important since it may influence the performance of the catalyst.1 The most common supports are inorganic oxides such as Al2O3, TiO2, ZrO2, silica, zeolites and carbon.9-11 Silica has become an extremely popular choice due to the low cost; chemical and mechanical stability, and high porosity.9,10,12-14 Vanadium oxide crystallites can be uniformly dispersed over fumed silica which also effectively increases exposed active sites for catalysis.15 The active sites are on the surface where they are accessible giving fast kinetics and high yields. On the other hand, organic polymer supports provide a backbone with active sites inside the matrix. The rate of catalytic reaction using polymer support is therefore largely dependent on the rate of diffusion through the polymer. Organic polymers are desirable since their properties can be easily manipulated through chemical modifications to produce unique materials with desirable functionalities, for example; they can be tuned to increase surface area more easily compared to inorganic materials.16,17 In that regard, polymer support materials can be presented for use in the form of microspheres or nanofibers.18-20 Due to better performances reported for nanometric sized catalysts, vanadyl functionalized copolymer nanofibers were targeted.21,22 Therefore, the study seeks to explore the unique properties offered by these materials [VxOy-silica 600⁰C and (VIVO-p(PIMv-co-ST)]. We present the catalytic oxidation of SCCs in hydrotreated diesel using two vanadium catalysts with representative characteristics of organic and inorganic supports; vanadium oxides supported on silica (VxOy/SiO2-600⁰C) and copolymer nanofiber supported oxidovanadium(IV) ([VIVOp(PIMv-co-ST)]),

to provide insights to choosing between inorganic and polymer support

materials. Reusability and recyclability properties of both catalysts were determined as a function of the total conversion achieved.


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2. EXPERIMENTAL 2.1 Materials and instrumentation. All chemicals were used as supplied from commercial sources (ACE, Sigma-Aldrich and Merck). The real fuel sample used was Sasol diesel 500 (standardization showed 385 ppm S content). 1

H NMR spectra were determined using a Bruker Topspin 300 MHz spectrometer and reported

relative to tetramethylsilane (δ 0.00 ppm). FT-IR spectra were acquired using a Bruker Platinum ATR Tensor 27 FT-IR spectrophotometer in the mid-IR range (4000–380 cm-1). Elemental composition of the catalysts was determined using either a Bruker S1 Titan handheld XRF analyzer or a Vario Elementary ELIII Microcube CHNS elemental analyzer. EPR spectra were obtained using a Bruker ESP 300E X-band spectrometer. XPS measurements were performed with a Kratos Axis Ultra XPS equipped with a monochromatic Al Kα source (1486.6 eV) and the data was processed using Kratos version 2 programs. Elemental analysis of samples using EDS was also carried out on a carbon grid without any modifications of the samples before they were gold coated for SEM analysis and viewed using a TESCAN Vega TS 5136LM SEM operated at 20 kV at a working distance of 20 mm. TGA-DSC analysis was performed on a Perkin-Elmer TGA 7 thermogravimetric analyzer. The samples in platinum pans were heated at a rate of 10 °Cmin-1 from 30ºC to 600ºC under a constant stream of nitrogen gas. Powder XRD studies were performed using a D2 Phase Bruker with Cu-Kα radiation in the range 10 < 2θ degrees < 70. Topas version 4.1 software was used to analyze the reflections. The amount of vanadium in the catalysts was determined using a Thermo Electron (iCAP 6000 Series) ICP spectrometer equipped with an OES detector at the following wavelengths; 290.88, 292.40, 309.31, and 311.07 nm. Model fuel samples were analyzed using an Agilent 7890A GC equipped with a 30 m × 0.25 mm × 0.25 μm DB-5 capillary column, a flame ionization detector and a mass spectrometer. Analysis of real fuel


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samples was carried out using a LECO Pegasus GC × GC-HRT; Injection: Split Injection (100:1) at 250˚C; Primary Column: Stabilwax (Restek), 30 m x 250 µm (0.25µm); Secondary Column: Rxi-5 (Restek), 1.5 m x 100 µm (0.1 µm); Carrier Gas: Helium, 1.2 ml/min constant flow; Primary Oven Program: 40˚C (0.1 min) to 260˚ C (78.4 min) at 3 ˚C/minute; Secondary Oven Program: 45 ˚C (0.1 min) to 265 ˚C (56 min) at 3 C/min; Modulator Offset: 15˚C; Modulation Frequency: 8 seconds; Hot Time: 2 sec; MS: LECO Pegasus 4D GC × GC-TOFMS; Ionization: Electron Ionization at 70 eV; Source Temperature: 250 ˚C; Stored Mass Range: 30 to 500 u; Acquisition Rate: 100 spectra/sec. 2.2 Preparatory work. Both catalysts, fumed silica supported vanadium oxides (VxOy/SiO2600⁰C) and polymer nanofiber [2-(2’-hydroxy-5’-ethenylphenyl)imidazole (PIMv) and styrene (ST) copolymer] supported oxidovanadium(IV) ([VIVO-p(PIMv-co-ST)]), were prepared using literature procedures15,22-24 with minor modifications. The synthesis methods and accompanying characterization data are presented in supplementary information. 2.3 Catalytic oxidation studies. Oxidation studies were carried out using an improvised flow set-up (Figure 1) consisting of a pump driving a 20 mL syringe, containing the reaction mixture, with an outlet connected to PTFE tubing which leads to a makeshift catalyst chamber loaded with the catalyst under investigation, where the mixture reacts before exiting into a collection vial. The catalyst chamber is enclosed with a heating blanket maintained at 60⁰C during the reaction. For the model fuel sample (DBT in acetonitrile), 100 µL aliquots were collected in triplicate at each flow rate and mixing with 0.5 mL acetonitrile before analysis using GC-FID. In the case of the real fuel sample (Sasol diesel 500) 100 µL aliquots were mixed with 1500 µL of heptane and analyzed using GC × GC-SCD and GC × GC-HRT. The oxidized SCCs in 100 µL aliquots of


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Sasol diesel 500 were extracted using 3 × 100 µL volumes of acetonitrile followed by mixing of extracted samples with 1500 µL heptane and analysis using GC × GC-SCD and GC × GC-HRT.

Figure 1. Improvised flow reaction set-up applied in the oxidation of model and real fuel samples using VxOy/SiO2-600⁰C and VIVO-p(PIMv-co-ST) nanofibers as catalysts.

3. RESULTS AND DISCUSSION 3.1 Fourier transform infra-red spectroscopy (FT-IR) 3.1.1 VxOy/SiO2-600⁰C. Since fumed silica was used as a support for the VxOy/SiO2-600⁰C catalyst, we compared the FT-IR spectra of fumed silica and the catalyst to confirm covalent hosting of the vanadium oxides on the support material. Intense unsymmetrical bands were observed at 1096 cm-1 and 474 cm-1 in the FT-IR spectrum of fumed silica (Figure 2a). These, respectively, are characteristic of the anti-symmetric motion of silicon atoms and the rocking


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motion of oxygen atoms bridging silicon atoms in siloxane bonds (Si-O-Si). A much weaker band was observed around 840 cm-1 and was ascribed to silanol (Si-OH) stretch vibrations.25 Corresponding peaks were observed in the spectrum for the silica-supported catalyst around 1095 cm-1, 463 cm-1 and 831 cm-1. Bands observed around 952 cm-1 and 576 cm-1 and were attributed to the formation Si-O-V linkages and V-O stretching, respectively, confirming the presence of vanadium oxide on silica.15,26,27 3.1.2 VIVO-p(PIMv-co-ST) nanofibers. In the FT-IR spectrum of the copolymer (Figure 2b), the band at 1494 cm-1 can be attributed to the azomethine stretch [ν(C=N)].28 Coordination of oxidovanadium(IV) to the metal center results in a shift of the azomethine band to 1530 cm-1.29 The band at 1263 cm-1 was tentatively assigned to the phenolic ν(C-O) stretches in the copolymer, and the peak shifted to 1273 cm-1 upon coordination.30 Bands which emerged at 964, 447 and 415 cm-1 which are positions typical for ν(V=O), ν(V-N) and ν(V-O) stretches, respectively, also confirmed successful hosting of oxidovanadium(IV) on the copolymer.31,32

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3.2 Electron paramagnetic resonance (EPR) spectroscopy. The EPR spectrum of VxOy/SiO2-600⁰C, the polymer alone [p(PIMv-co-ST)] and [VIVO-p(PIMv-co-ST)] are presented in Figure 3. Anisotropic EPR spectra were obtained for both catalysts which could be due to restrictions in isotropic tumbling.33 Spectra for VxOy/SiO2-600⁰C and [VIVO-p(PIMv-co-ST)] showed hyperfine interaction lines typical for vanadium, derived from the interaction of unpaired electron with the nuclear magnetic moment (51V, I = 7/2) with g-factors = 2.008. According to studies of supported V2O5 samples by Gupta et al.34, the response to EPR of the samples can be attributed to electron super-exchanges in the V4+-O-V5+ chains and these can manifest as a Zeeman’s effect signal with an underlying hyperfine structure of isolated V4+.34 The additional signal in the spectrum of VIVO-p(PIMv-co-ST) corresponds to signal from the polymer alone [p(PIMv-co-ST)] (g-factor = 1.996) (Figure S4). The EPR activity observed in the polymer is probably due to free organic radicals as reported by Zhang et al. 35

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Figure 3. First derivative X-band EPR spectra of (a) VxOy/SiO2-600⁰C and (b) VIVO-p(PIMv-coST) (51V, I = 7/2). 3.3 X-Ray photoelectron spectroscopy (XPS). In the XPS spectrum of VxOy/SiO2-600⁰C, binding energies of the expected elements were confirmed by peaks at 103.0 eV (silicon) and 532.08 eV (vanadium), and the 3 peaks 516.8, 523.3 and 975.4 for oxygen (Figure 4a). The effect of spin orbit coupling is noticed from the V 2p3/2 and V 2p1/2 peaks at 516.8 and 523.3 eV, respectively. A vanadium Auger electron peak is also observed at 925.4 eV.36 XPS analysis of VIVO-p(PIMv-co-ST) also confirmed the presence of all the expected elements; carbon, nitrogen, vanadium, and oxygen. These appeared at 279.8, 597.6, (514.6/521.5) and 527.3 eV, respectively (Figure 4b). The position of the V 2p3/2 signal confirms that vanadium is in its +IV oxidation state and the additional V 2p1/2 signal shows the effect of spin orbit coupling.36 40000

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Figure 4. Wide scan XPS spectrum of (a) VIVO-p(PIMv-co-ST) nanofibers and (b) VxOy/SiO2600⁰C. The inserts show the high resolution XPS scan for V 2p and N 1s. 3.4 Energy dispersive spectroscopy (EDX). Analysis of the EDX spectra for VxOy/SiO2600⁰C and VIVO-p(PIMv-co-ST) showed that vanadium was successfully hosted onto fumed silica


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and fibers, respectively. The peaks at ~0.5, ~5 and ~5.5 keV correspond to theoretical Lα, Kα and Kβ values of vanadium, respectively (Figure 5).37

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Figure 5. EDX spectra for (a) VxOy/SiO2-600⁰C and (b) VIVO-p(PIMv-co-ST). 3.5 Scanning electron microscopy (SEM). The SEM micrographs of fumed silica (a) and VxOy/SiO2-600⁰C (b) showed amorphous morphology and irregularly sized particles (Figure 6). The images further confirmed the loose nature of VxOy/SiO2-600⁰C similar to the fumed silica particles used as support. In contrast, p(PIMv-co-ST) nanofibers gave uniform fibers with diameters in the range 2-5 µm (Figure 7). Upon coordination of oxidovanadium(IV) to form the polymer pre-catalyst, VIVO-p(PIMv-co-ST), the integrity of the fibers was maintained with a slight increase in the diameter range 2-10 µm.


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Figure 7. SEM micrographs of (a) p(PIMv-co-ST), (b) VIVO-p(PIMv-co-ST). Scale = 50 µm. 3.6 Powder X-ray diffraction (P-XRD). PXRD analysis of [VxOy/SiO2-600⁰C] indicated that it is composed of two mixed phases, 96.13 % V2O5 phase and 3.87 % VO2 phase (Figure 8). An amorphous halo characteristic of adding an amorphous silica support was observed between 2θ = 15 and 35 in the diffractogram. Significant broadening in reflections after 2θ = 40 and merging of


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several reflections which were close to each other (e.g. at 2θ = 41 and between 2θ = 46 and 60) was observed in the VxOy/SiO2-600⁰C catalyst. The weak reflections observed on the XRD pattern further confirmed the amorphous nature of the catalyst as observed on the SEM images (Figure 7 and Figure S5). On adding an amorphous material to a crystalline material, the final pattern is expected to be a convolution of the amorphous and crystalline phase. The amorphous phase was detected as a broad reflection due to scattering from a wide range of directions compared to lattice scattering in crystals. Crystalline phase reflections are intense and narrow; hence the reflections for VxOy appeared to be sitting on top of the amorphous silica phase. Peaks of a crystalline phase can also shift or broaden if the amount of amorphous phase added is enough to influence the lattice distance, the micro-strain or the "crystal size" itself. Broadening of reflections can also be attributed to the wider variety of sizes of the crystallites due to immobilization on silica.38 8000

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mass below 350⁰C which can be attributed to loss of solvents. The loss was more apparent for VIVO-p(PIMv-co-ST), amounting to about 10%. Major losses in mass of about 95% and 80%, which can be attributed to the collapse of the polymer backbone, were observed in the ranges 350475⁰C and 350-450⁰C, for p(PIMv-co-ST) and VIVO-p(PIMv-co-ST), respectively. The fibers are quite stable with Tmax for p(PIMv-co-ST) and VIVO-p(PIMv-co-ST) about 410 and 385 ⁰C, respectively.22 After the decomposition of p(PIMv-co-ST) fibers, 0 % mass remained, whereas ~5% mass, which can be attributed to residual vanadium oxides, remained for VIVO-p(PIMv-coST) fibers. The DSC profile of p(PIMv-co-ST) showed one sharp endotherm at T = 426°C for polymer backbone loss, while VIVO-p(PIMv-co-ST) had two sharp overlapping endothermic peaks at T= 378°C and 406°C. The slight shift to a lower temperature in the endotherm of VIVO-p(PIMvco-ST) could probably be due to the change in the coordination geometry in the polymer. The endotherms are immediately followed by a broad exothermic peak at T=480°C due to metal oxide formation. (a)

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Figure 9. TGA and DSC curves of (a) p(PIMv-co-ST) and (b) VIVO-p(PIMv-co-ST) nanofibers. 3.8 Probing the oxidation mechanism using EPR spectroscopy. The EPR signal attributed to V4+ sites in VxOy/SiO2-600⁰C gradually disappears on addition of oxidant (t-BuOOH) (Figure 10a). The EPR signal originating from oxidovanadium(IV) centers in VIVO-p(PIMv-co-ST) also gradually disappears on addition of oxidant (t-BuOOH) (Figure 10b). This suggests oxidation of the V4+ to the diamagnetic EPR silent V5+ in both cases. In both cases, the signal is recovered gradually after introduction of the substrate into the mixture indicating reduction of the V5+ back to V4+. However, there was a slight variation in the recovered EPR signals. This may be due to slight changes in the coordination geometry of the V4+-O-V5+ system for VxOy/SiO2-600⁰C and possibly a different coordination environment in the case of VIVO-p(PIMv-co-ST).


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Figure 10. First derivative X-band EPR signals for (a) VxOy/SiO2-600⁰C; (i) pre-catalyst, (ii-iv) on gradually adding oxidant, (v and vi) on addition of DBT and allowing to stand for 1 h, and (vii) after standing the mixture overnight; and (b) VIVO-p(PIMv-co-ST); (i) pre-catalyst, (ii-iv) after addition of oxidant aliquots, and (v) after standing the mixture overnight. Neat (0.526M) tBuOOH was used as oxidant in both cases.

From the EPR data, we propose the catalytic oxidation using VxOy/SiO2-600⁰C to be redox in nature where the V4+ sites within the catalyst are the ones directly involved in the catalysis. The reaction involves oxidation of the V4+ sites (a) into V5+ (b) by the oxidant (t-BuOOH) and a possible formation of a highly reactive peroxido species by another t-BuOOH molecule (c) which then oxidizes the substrate (d) to regenerate the V4+ in the catalyst (Scheme 1).


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Substrate

Oxidized substrate

d t-BuOH O O

OH

O

HO

V 5+

c

Si

O

O O

V Si

Si

4+ O

O O

Si

a

Si

Si

t-BuOOH (Oxidant) OH O

t-BuOOH

V Si

5+ O

O

Si

O

Si

t-BuOH

b

Scheme 1. Proposed catalytic oxidation reaction mechanism when using VxOy/SiO2-600⁰C.

It is proposed that catalytic oxidation using VIVO-p(PIMv-co-ST) nanofibers first involves oxidation (by t-BuOOH) of the (a) oxidovanadium(IV) centers into (b) dioxidovanadium(V), and further into (c) oxidoperoxido, a highly active species which then, (d) oxidizes the substrate resulting in the recovery of the oxidovanadium(IV) centers (Scheme 2).


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ON

m

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NH

O V4+ O N (a) HN

t-BuOOH

n t-BuOH

oxidized substrate

substrate

m

m OO O V5+ O N NH O N HN (c)

O O N O V5+ NH O N HN (b) n t-BuOOH

t-BuOH

n

Scheme 2. Proposed catalytic oxidation reaction mechanism when using VIVO-p(PIMv-co-ST) nanofibers. 3.9 Catalytic oxidation studies 3.9.1 Optimization of flow rate using a model fuel sample. We prepared a model fuel sample by dissolving 100 mg (0.543 mmol = 1740 ppm S) of DBT in 10 mL of acetonitrile. We then optimized the flow rate to achieve complete oxidation of the DBT in the model fuel sample using 10.5 mmol t-BuOOH and either 10 mg (11.99 wt% vanadium content = 23.5 µmol) VxOy/SiO2600⁰C or 150 mg (0.725 wt% vanadium = 21.3 µmol) at 60⁰C in a continuous flow set-up. DBTO2 was the only product identified during the oxidation (Figure S6-S9). The flow rate was varied down from 0.6 mL/h to increase contact time of the feed with the catalyst until 100% conversion of DBT to DBTO2 was realized. 100% conversions were realized at 0.1 and 0.2 mL/h for VxOy/SiO2-600⁰C and VIVO-p(PIMv-co-ST), respectively (Figure 11).


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% conversion of DBT to DBTO2

Page 19 of 30

100

VO-p(ST-co-VPIM) VxOy/SiO2-600oC

80 60 40 20 0 0.6

0.5

0.4

0.3

0.2

0.1

Flow rate (mL/h) Figure 11. Change in % conversion of DBT to DBTO2 as flow rate is varied when using VxOy/SiO2-600⁰C and VIVO-p(PIMv-co-ST) as catalysts. We also carried out reusability studies, where we repeated experiments at the optimal flow rate of each catalyst three times after pumping acetonitrile through the flow setup to wash the catalysts after each 1 h cycle. VIVO-p(PIMv-co-ST) retained its activity as witnessed from the 100% conversion of DBT to DBTO2 after three 1 h cycles at a flow rate of 0.2 mL/h. However, the activity for VxOy/SiO2-600⁰C reduced drastically after the first cycle from 100% conversion of DBT to DBTO2 at 0.1 mL/h down to 67%, after which it dropped insignificantly (62 and 60 % conversion on the 3rd and 4th cycles). ICP analysis of the model fuel samples collected during the first 1 h (0.1 mL model fuel in total) showed 19.5 ppm (1.00 % RSD) and 613.6 ppm (2.96% RSD) vanadium in VIVO-p(PIMv-co-ST) and VxOy/SiO2-600⁰C, respectively. These vanadium concentration (ppm) values translate to 0.179% and 5.12 % loss in vanadium from the two catalysts, respectively, during that period. A total loss of 0.324% and 17.8% in vanadium levels of


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

VIVO-p(PIMv-co-ST) and VxOy/SiO2-600⁰C, respectively, after four oxidation cycles confirmed that more vanadium leached out of the silica based catalyst, VxOy/SiO2-600⁰C. This was probably due to disruption of the weakly bonded vanadium oxides on the silica support. 3.9.2 Oxidation of Sasol diesel 500 using the optimized flow conditions. The optimal flow rates (0.1 and 0.2 mL/h) were adopted in the oxidation of 10 mL of the real fuel sample (Sasol diesel 500) for 10.0 mg (23.5 µmol V) VxOy/SiO2-600⁰C and 150 mg (21.3 µmol V) of VIVOp(PIMv-co-ST), respectively. Analysis of Sasol diesel 500 before and after oxidation was carried out using a GC × GC-SCD and GC × GC-HRT and the results are presented in Figures 12, S10S15 and Table 1. The peaks for the aromatic sulfur compounds are in the region 3600 to 4100 s retention time (RT). Considering the complexity of diesel, an array of sulfur compounds can be found and even nitrogen compounds that are also prone to oxidation; we placed our focus on dibenzothiophene and dimethyl dibenzothiophene.1 We consider them the representative compounds since they are the bulk of sulfur compounds that remain in hydrotreated diesel. GC × GC-HRT data showed that their presence in Sasol diesel 500. There were no traces of these compounds after oxidation but rather their respective sulfones (DBTO2 and DMDBTO2), which were previously not present in Sasol diesel 500. There were no sulfoxide peaks for the compounds. Although the relative peak areas of the oxidized products were not an exact match with the starting materials, we tentatively resolved that oxidation was completed since the previously observed peaks of DBT and DMDBT completely disappeared. 3.10 Extraction of oxidized sulfur compounds from Sasol diesel 500 using acetonitrile. After oxidation of the fuel sample (Sasol diesel 500), we applied the solvent extraction technique to remove the oxidized sulfur compounds from samples of Sasol diesel 500 which were oxidized using either 10 mg (235 µmol V) VxOy/SiO2-600⁰C and 150 mg (213 µmol V) of VIVO-p(PIMv-


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co-ST) under optimal conditions. Acetonitrile, a dipolar aprotic solvent commonly used for work requiring general extraction of sulfur compounds (or their sulfones) from fuel was employed.1,39 Preliminary analysis of the samples after solvent extraction using GC × GC-SCD showed the presence of sulfur compounds in the fuel samples before and after oxidation and their disappearance after solvent extraction (Figure 12).

(a-1)

(b-1)

6

6

4

0 0

4 6000

2

2

4000 0

2000

4000

4000

6000 0

4000 2000 0 0

(b-2)

2000

6000

6000 0

2000

0 0

0

(a-2)

0

(c-1)

(c-2)

2000

4000

6000 0

2000

4000

6000

Figure 12. GC × GC-SCD chromatograms of (a) Sasol diesel 500, (b) Sasol diesel 500 after oxidation, and (c) after extracting the oxidized compounds using acetonitrile. Encircled are the regions for sulfur peaks (3600 to 4100 s retention time) and the intensities of the SCD peaks represent their relative amounts. Further detailed analysis of the oil was carried out by using GC × GC-HRT data40 (Figures 13, S16-21 and Table 1). Based on the sum of all the peak areas for the automatically generated compound formulae containing sulfur (matching base masses to known sulfur compounds) in Sasol diesel 500 and after solvent extraction of the oxidized sample using acetonitrile, we calculated the overall removal of sulfur to be 94.46% for VxOy/SiO2-600⁰C and 98.22% for VIVO-


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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p(PIMv-co-ST) nanofibers. Please note that several base masses are not allocated to compound formulae where there is no exact match to expected masses, a phenomenon common in mass spectroscopy. Nevertheless, considering that the Sasol diesel 500 used contained 385 ppmS (standardized), the 94.46% and 98.22% sulfur removal translate to 21.33 ppmS and 6.85 ppmS remaining in the ODS treated fuel. However, these values are still tentative due to several variables that were not taken into consideration, for example optimizing the solvent extraction process. Table 1. Results for amount of DBT, DMDBT and their respective sulfones after analysis of GC × GC-HRT data of Sasol diesel 500 and after oxidation using VIVO-p(PIMv-co-ST) nanofibers and oxidation using VxOy/SiO2-600⁰C. No DBT or DMDBT or their respective oxidized products were detected in both fuel samples after solvent extraction using acetonitrile.

Compound

Retention time (s)

Formula

Base mass

Relative peak area Original fuel

After oxidation

After SE

184.12

34477

ND

ND

216.19

ND

32251

ND

VIVO-p(PIMv-co-ST) nanofibers 3234.86, 2.52696 2962.95, 3.11143

Dibenzothiophene

C12H8S

Dibenzothiophene sulfone

C12H8O2S

Dimethyl dibenzothiophene

C14H12S

4314.47, 2.32286

212.06

32733

ND

ND

C14H12O2S

3404.04, 3.10499

244.21

ND

30494

ND

184.12

34 477

ND

ND

216.19

ND

32 553

ND

Dimethyl dibenzothiophene sulfone VxOy/SiO2-600⁰C Dibenzothiophene

C12H8S

Dibenzothiophene sulfone

C12H8O2S

3234.86, 2.52696 2946.96, 3.13313


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Energy & Fuels

Dimethyl dibenzothiophene Dimethyl dibenzothiophene sulfone *ND: not detected.

C14H12S

4314.47, 2.32286

212.06

32 733

ND

ND

C14H12O2S

3393.20, 3.16469

244.21

ND

32 311

ND

Figure 13. GC × GC-HRT chromatograms of (a) Sasol diesel 500, (b) Sasol diesel 500 after solvent extraction using acetonitrile, and (c) acetonitrile containing the extracted compounds.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Encircled are the regions where we find the majority of sulfur compounds. Most of the peaks in that region disappeared, those belonging to the sulfur compounds. Poor selectivity is one of major challenges commonly observed with the solvent extraction technique.1 It is also important to note that nitrogen compounds found in the fuel were not immune to the oxidation and extraction, but their analysis was beyond the scope of this work. Of note is also possible lack of selectivity during oxidation that may have led to oxidation vulnerable compounds in the system and their subsequent extraction. These compounds appear in the same region as aromatic sulfur compounds.40 However, the use of oxidative desulfurization is still a viable prospect considering that it can be successfully implemented in conjunction with several other alternative adsorption methods are being developed, such as polymer imprinted nanofibers11,35 and metal organic frameworks.41,42 4. CONCLUSIONS Both, silica supported vanadium oxides (VxOy-silica 600⁰C) and nanofiber supported oxidovanadium(IV) complex [VIVO-p(PIMv-co-ST)] showed good catalytic oxidation activity when tested on a model fuel sample (dibenzothiophene in acetonitrile) and real fuel sample (Sasol diesel 500) in a continuous flow set-up. Coupled with solvent extraction of the resulting sulfones using acetonitrile, 94.46% and 98.22% removal of sulfur containing compounds in Sasol diesel 500 (i.e. 21.33 ppm and 6.85 ppm S remained) when using VxOy-silica 600⁰C and VIVO-p(PIMvco-ST), respectively, was achieved. Unfortunately, extraction of the oxidized sulfur containing compounds was accompanied by other hydrocarbons. There is a need to apply specialized adsorbent materials or solvents for improved selective removal of the oxidized sulfur compounds. This study revealed that [VIVO-p(PIMv-co-ST)] was more robust compared to (VxOy-silica


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Energy & Fuels

600⁰C). The catalytic activity of [VIVO-p(PIMv-co-ST)] was maintained at 100% after 3 cycles whereas it dropped by 33% after the first oxidation cycle for [VxOy-silica 600⁰C] mainly due to leaching of vanadium. Although, synthesis of the polymer nanofiber supported catalyst [V IVOp(PIMv-co-ST)] is difficult and expensive, it is reusable due to its robustness. There is also room for further modifications to improve catalytic activity. Studying the mechanistic aspects also provided insights around aspects to consider in the development of the silica supported catalyst. It may be essential to vary the synthesis approaches to obtain different quantities of the V4+-O-V5+ sites and investigate the role of each oxide in the catalytic oxidation. Choosing between the use of inorganic and polymer supports is a matter of weighing between efficiency and cost, where inorganic supports are easy and cheaper to synthesize while on the other hand polymers can be easily engineered to achieve high activity. Overall, the development of such viable catalysts which reduce the amounts of oxidant required for the oxidation of refractory sulfur compounds in fuel make oxidative desulfurization a worthwhile consideration for application as a complimentary technique to hydrodesulfurization to obtain low sulfur level fuels. ASSOCIATED CONTENT Supporting information Synthesis procedures and characterization data for the catalysts (Schemes 1 and 2, and Figures S1S4), and characterization of the oxidation product (Figures S5-S8, and Tables S1 and S2) . AUTHOR INFORMATION Corresponding authors


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*Emails: [email protected] (T.O. Dembaremba), [email protected] (A.S. Ogunlaja) and [email protected] (Z.R. Tshentu). Tel +2741 504 2074. Notes The authors declare no conflict of interests with any other work. ACKNOWLEDGEMENTS Great appreciation goes to Sasol and the National Research Foundation (NRF) of South Africa for funding the project. The authors also acknowledge Bolo Lukanyo for thermogravimetric services rendered as well as Henk Schalekamp, Kina Muller and Olivier Francois for technical assistance. REFERENCES (1) Srivastava, V.C. RSC Adv., 2012, 2, 759-783. (2) BP Statistical Review of World Energy, 2019, 68th edition. www.bp.com, retrieved on 08/07/19. (3) Stanislaus, A.; Marafi, A.; Rana, M.S. Catalysis Today 2010, 153, 1-68. (4) Al-Shahrani, F.; Xiao, T.C.; Llewellyn, S.A.; Barri, S.; Jiang, Z.; Shi, H.H.; Martinie, G.; Green, M.L.H. Appl. Catal., B 2007, 73, 311-316. (5) Xun, S.; Zhu, W.; Chang, Y.; Li, H.; Zhang, M.; Jiang, W.; Zheng, D.; Qin, Y. and Li, H. Chem. Eng. J., 2016, 288, 608-617. (6) Zhu, W.; Wang, C.; Li, H.; Wu, P.; Xun, S.; Jiang, W.; Chen, Z.; Zhao, Z. and Li, H. Green Chem., 2015, 17, 2464-2472.


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Comparing the Catalytic Activity of Silica-Supported Vanadium Oxides

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