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VOx Core-Shell Catalysts for One-Pot Oxidation and Separation of Refractory Multiaromatic Sulfur Compounds in a Model Diesel Ulises Arellano, Zhiqi Wang, Lifang Chen, Jin An Wang, Maximiliano Asomoza, and Alberto Estrella Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02787 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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Industrial & Engineering Chemistry Research

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VOx Core‒Shell Catalysts for One‒Pot Oxidation and Separation of Refractory Multiaromatic Sulfur Compounds in a Model Diesel

U. Arellanoa,c, Z. Wangb, L.F. Chena, J.A. Wanga,*, M. Asomozac, A. Estrellac a

ESIQIE, Instituto Politécnico Nacional, Col. Zacatenco, Col. Zacatenco, 07738 Mexico City,

Mexico. b

Department of Materials Science and Engineering, Cornell University, 210 Bard Hall, Ithaca,

NY 14853-1501, USA. c

Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael

Atlixco No. 186, Iztapalapa, Mexico City, Mexico.

*Corresponding author: Dr. Jin An Wang E-mail: [email protected] Tel. 52 55 57296000 ext. 54276; Fax: 52 55 55862728

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Abstract. Ti-MCM-41 supported vanadia core-shell catalysts were prepared and their catalytic activity was evaluated in a biphasic reactor for simultaneous oxidation/separation of refractory organosulfur compounds (dibenzothiophene (DBT), 4‒methyldibenzothiophene (4‒MDBT) and 4,6‒dimethyldibenzothiophene (4,6‒DMDBT)) in a model diesel fuel. Formation of coreshell structure in the catalyst nanoparticles resulted from the surface diffusion of the vanadium ion in semi-melted state of vanadia during the calcination process. It was found that both the value of V5+/(V4++V5+) and the surface acidity of the catalysts generally increased as the content of V content increased until to 15wt% of vanadium and correlated well with variations in catalytic activity, indicating that the surface V5+ and surface acid sites were chiefly responsible for the oxidation of sulfur compounds. The reactivity of organosulfur compounds was influenced by their steric hindrance. Addition of carboxylic acid (acetic acid or formic acid) promoted the sulfur oxidation efficiency via forming more stable and active oxygen species like peroxometallic and superoxometallic species on the catalyst surface. Approximately 99% of the sulfur compounds were removed with the best catalysts containing 15wt%V at a reaction temperature of 60 °C and within 60 min. This one-pot organosulfur oxidation and separation method was proven to be efficient, economical, and practical for deep desulfurization of diesel fuels containing refractory organosulfur compounds.

Keywords: Vanadium, core-shell catalyst, oxidative desulfurization, organosulfur compounds, diesel.

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1. INTRODUCTION

The petroleum refining industry faces major challenges in the production of ultra-clean fuels to meet more strict specifications of sulfur content in fuels for environmental protection purpose.1,2 In the petrochemical industries, the hydrodesulfurization (HDS) technique has long been widely used for removal of sulfur compounds from fuels. The HDS process operates under moderate temperature (300~500 °C) and high pressure (50~80 atm) in the presence of excess hydrogen, making it a very expensive option for achieving deep desulfurization. Moreover, HDS is not least effective for removing heterocyclic sulfur compounds such as dibenzothiophene (DBT), and its derivatives like 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT).3–5 Steric hindrances obstruct these refractory compounds to dehydrogenation, making them difficult to be removed by HDS.6.7 Several other technologies have been proposed as alternatives to achieve deep desulfurization, for example: biodesulfurization,8-10 selective sulfur adsorption,11–14 extraction, 15

, oxidative desulfurization (ODS),16, 17 and regenerative adsorption and removal of hot gas

containing H2S.18–20. Amongst these routes, ODS has emerged as one of the most efficient and promising options as it has proven effective for removal of refractory organosulfur compounds under mild reaction conditions and without hydrogen consumption. In ODS process, DBT, 4‒MDBT and 4,6‒DMDBT (hereafter collectively known as DBTs) are oxidized to their corresponding sulfones (collectively noted as DBSs), which are subsequently removed by extraction with polar solvents or sorbents.1,12,13 Various oxidants have been investigated for ODS, including molecular oxygen, hydrogen peroxide, tert-butyl hydroperoxide, and 3



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peroxyorganic acids, etc..21–27 Amongst these oxidants, H2O2 has been applied most intensively, due to its high effectiveness compared to molecular oxygen, and relative safety of its use compared to organo-hydroperoxides. An active catalyst is usually used in conjunction with the oxidant for improving the effectiveness of the ODS process. Because heterocyclic sulfur compounds are relatively large molecules, catalyst pore diameter and surface area must be considered as part of catalyst design. Many previous investigations have focused on the transition metals oxides supported on oxide solids including titania, alumina, ceria, niobia, silica, and zirconia.23,28,29 Activated carbon materials are also reported as adsorbents and catalyst support for ODS reactions.11– 13,24,30,31

Some porous materials, including especially mesoporous materials or metal doped

MCM-41 and SBA-15 have been used as catalyst supports due to their uniform structure consisting of large pores and possessing high surface areas.32–35 Usually, transition metals oxides were used as active phases for the ODS process. Fe, Co, Ni, Cu, V, Mo, W oxides based catalysts have been proven to be active for sulfur compound oxidation and their catalytic activities greatly depend on the support, preparation method, and oxidizing agent used.23,24,31,34, 36 Due to the high activity of V2O5 for SO2 oxidation to produce sulfuric acid, and MoO3 or WO3 as precursors of the active phase for HDS process, these three oxides received much attention in the ODS reactions.37–39 In comparison with WO3 and MoO3, vanadia is a cheaper oxide. From the practical and economical points of view, V2O5 has a greater potential to be applied as catalyst active phase for ODS treatment. In the present work, we report some new results of mesostructured VOx supported on TiMCM-41 core-shell catalysts for the simultaneous oxidation/separation of DBT, 4-MDBT and 4


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4, 6-DMDBT mixture in a model diesel fuel. Oxidation/separation tests were performed in a biphasic system consisting of a polar solvent (acetonitrile) and a non-polar phase (nhexadecane) containing 500 ppm DBTs. H2O2 was used as oxidant, and two carboxylic acids (acetic acid and formic acid) were used as oxidant promoter and stabilizer. Special attention was paid to the possible correlations of surface acidity and surface V5+ concentration with the catalytic activity of ODS reactions. The formation of core-shell structure, improvement of ODS efficiency promoted carboxyl acid mixing with hydrogen peroxide, and reaction mechanism of DBTs oxidation were investigated and discussed.

2. EXPERIMENTAL SECTION

2.1. Synthesis of Ti-MCM-41 and VOx/Ti-MCM-41 catalysts. The mesoporous TiMCM-41 support was synthesized by the surfactant-templated method using alkoxides of silicon and titanium as Si and Ti precursors. In brief, 6.5 ml of ammonium hydroxide were added to a mixture of 486 ml deionized water and 33.16 ml cetyltrimethylammonium chloride (CTACl, Aldrich, 25 wt%,); the resulting solution was stirred for 2 h at 900 rpm. First, 42.5 ml tetraethylorthosilicate (TEOS, Sigma-Aldrich, 98%) were added slowly to this mixture followed by 5 ml of propoxide of titanium (IV) (Aldrich, 98%); the resulting mixture was stirred for an additional 24 h while the pH was controlled at 9 with ammonium hydroxide (NH4OH Aldrich, 29%). The mixture was then was refluxed to 40 °C under a pressure approximately 0.80 atm for 72 h until a white gel was obtained. After evaporation, the resulting solid was filtered, washed with deionized water using a Buchner funnel. Excess moisture was 5



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removed from the solid by heating in an oven at 60 °C for 8 h and calcined in a dynamic air flow at 500 °C for 6 h with a temperature increasing rate of 1 °C/min. The resulting solid was termed Ti-MCM-41. The VOx/Ti-MCM-41 catalysts were prepared by impregnation of ammonium metavanadate solution (Sigma-Aldrich, 99.99%) on the Ti-MCM-41 support. The necessary amount of ammonium metavanadate was dissolved into methanol and then impregnated onto the TiMCM-41 support. Catalysts containing vanadium contents of 1, 5, 10, 15, 20, and 25 wt% were prepared. These catalyst solids were oven dried at 70 °C and calcined under static air atmosphere at 600 °C for 8 h in a muffle furnace. The resulting catalysts were named: V/TiM1, V/TiM-5, V/TiM-10, V/TiM-15, V/TiM-20 and V/TiM-25, where the number represents the weight percentage of vanadium and TiM represents the Ti-MCM-41 support.

2.2. Characterization. 2.2.1. X ray diffraction. X-ray diffraction (XRD) patterns of the samples were obtained with a Siemens D500 diffractometer coupled to a tube with a copper anode and Cu Kα radiation (λ = 1.5406 Å) at 35 kV and 20 mA. The XRD patterns of the TiMCM-41 and vanadium loaded catalysts were recorded over the 2θ range between 1° and 70° with a step of 0.02° and a measuring time of 2.67 seconds at each point. 2.2.2. UV-Visible Diffuse Reflectance Spectroscopy (DRS- UV-Vis). The UV–Vis spectra of all samples were obtained in a Cary 100 UV–Vis spectrophotometer. The spectra were recorded at ambient temperature in the wavelength region between 200 and 800 nm. 2.2.3. N2 physisorption. The textural properties of the samples were determined at -196 °C by

N2

adsorption

using

a

Micromeritics

ASAP

2000

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apparatus.

Before

the

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adsorption‒desorption experiments, the samples were evacuated under vacuum at 100 °C for 2 h and subsequently heated to 300 °C for 2 h. Surface area data were treated according to the model of Brunauer-Emmett-Teller (BET). The pore size distribution was calculated by using the Barrett-Joyner-Halenda (BJH) model to fit the measured desorption isotherms. 2.2.4. Temperature-programmed desorption of NH3 (TPD-NH3). TPD-NH3 experiments were performed with a TPD/TPR 2900 (Micromeritics) equipped with a thermal conductivity detector (TCD). Approximately 300 mg sample were preheated in N2 to 500 °C for 15 min and then the sample was cooled to 30 °C. After the sample was degassed, ammonia was introduced the reactor at a flow rate 30 ml/min for adsorption until reached saturation. Afterward it was heated again with a temperature ramp rate of 10 °C/min. The ammonia desorption from the solid sample was recorded with the TCD. 2.2.5. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analysis. TEM images were obtained with a high resolution electron microscope (HRTEM Jeol 2100F), using a light-field emitter operating at a voltage of 200 kv. The resolution was 1.86 Å. The microscope was coupled to a detector of an energy dispersive spectroscopy (EDS) for elemental analysis of selected areas in the catalyst sample. 2.2.6. X-ray photoelectron spectroscopy (XPS). XPS measurements were performed on an Escalab 2500 de Termo VG Scientific instrument equipped with a hemispherical analyzer and a microscope to project the image onto a screen for observing the position of analysis using a constant analyzer energy (CAE) mode. XPS spectra were obtained using monochromatic Al Kα radiation operated at 150 W and a voltage of 15 kv with a beam area 500 µm2. The peak

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deconvolution and curve fitting was made with particular software Spectral Data Processor Version 4.3. 2.3. Catalytic activity tests. The catalytic activity of the catalysts was evaluated for oxidative desulfurization of a model diesel composed of n-hexadecane (Aldrich, 99%) and 500 ppm of heterocyclic sulfur compounds (100 ppm dibenzothiophene (Aldrich, 99.9%), 150 ppm 4-methyldibenzothiophene (Aldrich, 96%) and 250 ppm 4,6-dimethyldibenzothiophene (Aldrich, 97%)). The 30 mg catalyst was added to a three-necked flask equipped with a condenser, stirrer and thermometer. The reactor contained 60 ml of a biphasic mixture consisting of 30 ml of the model diesel fuel and 30 ml of acetonitrile polar solvent. A proper amount of the oxidizing agent (Fermont, 30% H2O2 in water) was added to obtain a H2O2/DBTs molar ratio 10. The oxidant mixture consisted of organic acid as oxidant stabilizer (Formic acid, Sigma-Aldrich, >95%; or acetic acid, Sigma-Aldrich, >99.85%) and the volume ratio H2O2/organic acid was 2:3. All the experiments were carried out at 60 °C under atmospheric condition with a proper stirring rate at 300 rpm. The DBTs oxidation reaction mixture was analyzed by gas chromatography analyzer (Shimadzu GC-8A) which was equipped with a flame photometric detector (FPD) and SE-30 (100% dimethylpolysiloxane) column. The analyses were carried out every 5 minutes during the first half hour of experiment and every ten minutes afterwards. The concentration of DBTs and DBSs in the acetonitrile phase and the residual DBTs compounds in the oil phase were quantified using the GC analyzer. Nitrogen was as gas carrier with a flow rate 33.3 cm3·min-1 and 12.1 psi pressure; the injector temperature was controlled at 290 °C; the FID detector temperature was at 573 K. The hydrogen flow rate was at 45.5 cm3 min-1 and air flow rate was 8


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475 cm3 min-1. The run time was 15 min. A representative chromatogram of DBTs reactants and DBSs products were shown in the Supporting Information as Figure S1 and Figure S2. DBTs conversion was determined as a function of reaction time using the following equation:

XDBTs (%) = (Co-Ct)/Co.

Where XDBTs is DBTs conversion or removal percentage; Co is the DBTs concentration at time zero and Ct is the DBTs concentration at time t. The concentration of the corresponding sulfones in acetonitrile phase was determined using a similar method. The concentrations of DBTs and DBSs were reported as concentration sum of the three compounds. For DBTs, the conversion CDBTs = ∑(CDBT +C4-MDBT + C4,6-DMDBT); similarly, for DBSs, CDBSs = ∑(CDBS +C4MDBS

+ C4,6-DMDBS). Hereafter, the catalytic activity of the catalysts or ODS efficiency was

defined as the DBTs conversion.

3. RESULTS

3.1. X-ray Diffraction. Fig.1a shows the X-ray diffractogram of the V/TiM samples in the low 2θ angle region. Four peaks, at 2θ = 2.2, 3.9, 4.3 and 6.2°, were related to the reflections of (100), (110), (200) and (210) planes of the MCM-41 solid, respectively.40 Because the Ti precursor was introduced in the initial synthesis stage and so most of these Ti ions were incorporated into the framework of MCM-41 to form Ti-MCM-41 for the sample V/TiM-0. After vanadia was impregnated onto the Ti-MCM-41 support, the XRD peak positions slightly 9



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shifted to higher 2θ angle region in comparison with bared support, indicating that some vanadium ions may incorporate into the framework of Ti-MCM41 solid. The intensity of the XRD peaks gradually decreased with increasing of vanadium content, which could be attributed to decrease in regularity of the pore structural arrangements resulted from incorporation of part of vanadium ions in the Ti-MCM-41 structure or a diluting effect due to the deposition of vanadia on the Ti-MCM-41. Fig. 1b shows the wide 2θ angle of XRD patterns. For the V/TiM-0 sample, one broad peak between 15 and 40° and centered at about 20° was attributed to amorphous silica. There is one small peak at around 25.5°, it corresponded to the reflection of the (101) planner of titania anatase (JCPDS 21-1272). For the V/TiM-1 sample, two very small peaks corresponding to vanadium pentoxode (V2O5) phases were present, indicating that vanadium oxide was highly dispersed on the support surface. For samples with vanadium content equal to or greater than 5 wt%, new peaks appeared at 2θ = 15.1, 21.5, 25.1, 35.7, 38.7, 45.5, 49.1, 54.1 and 64.4°, corresponding to the reflections of the (200), (101), (201), (211), (104), (411), (112), (220) and (103) planes in V2O5 crystals (JCPDS 19-387. JCPDS 65-0131). The presence of these peaks confirmed that V2O5 crystallites were formed on the support. In addition to the V2O5 peaks, several peaks at around 17.5°, 27.4°, 37.3, 43.4°, 55.6°, 56.3° appear in the V/TiM XRD spectra; these correspond to VO2 crystalline structure (JCPDS 01-082-0661). Therefore, the catalysts chiefly contain vanadium pentoxide (V2O5) with VO2 as a minor component. In addition, a small amount of TiO2 anatase nanocrystals formed in the synthesis of Ti-MCM-41 was also present.

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3.2. UV-vis Diffuse Reflectance Spectroscopy (DRS-UV-Vis). Fig. 2 shows the DRS-UV-Vis spectra of the V/TiM catalysts. The pure support (V/TiM-0) exhibited a band at 222-225 nm that was attributed to electron transfer in the Ti–O bond involving titanium (IV) in a distorted tetrahedral position. The band at around 400 nm indicated the presence of a small amount of anatase phase dispersed on the surface of support, consistent with the XRD data reported previously.40 The V/TiM catalysts showed two partial overlapped bands at 250-300 nm and 330-450 nm as the main features. These were attributed to various charge transitions between O2- and V5+ in vanadium oxide. The absorption at 250-300 nm was assigned to electron transition from O 2p orbitals to V 3d orbitals in a 4-fold coordination state, corresponding to V ions in the tetrahedral coordinated position (Td) in the MCM-41 framework.41 The strong and wide band between 330-450 nm indicated the presence of V5+ species with octahedral (Oh) coordination in V2O5 nanocrystals present in the surface of the catalyst.42 In addition, UV-vis spectra obtained for samples with higher vanadium content (V ≥ 20 wt%) contained an overlapping band between 500 and 550 nm, indicating the presence of polymeric vanadia species.43 3.3. N2 Physisorption. Fig. 3 shows the loops of N2 adsorption-desorption isotherms and pore distribution profiles of the Ti-MCM-41 support and selected V/TiM catalysts. In all cases, the N2 adsorption-desorption profiles were Type IV (IUPAC classification), as is typical of mesoporous materials.44 The data showed narrow hysteresis loops of triangular and parallelogram forms with H1 type. The Ti-MCM-41 (V/TM-0) support exhibited very narrow and monomodal pore distribution centered at approximately 30 Å.45,46 The profiles of isotherms underwent slight changes after vanadium oxide impregnation; most importantly, the single pore 11



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diameter peak centered between 26.1 and 34.2 Å was conserved, confirming that vanadium impregnation did not disrupt the homogeneous pore diameter distribution of the mesoporous support. The textural properties are presented in Table 1. Surface area, pore volume, and pore size all gradually varied with increasing vanadium loading. For the catalysts with vanadium content lower than or equal to 15wt%, not only pore block did not occur, but also surface area was quite similar. For the catalysts containing vanadium > 15 wt%, excess V2O5 particles remained on surface, partially blocking some pores. As a result, the pore volume decreased from 0.81 cm3/g for V/TiM-0 to 0.41 cm3/g for V/TiM-25 and the average pore diameter decreased from 34.2 to 26.1 Å. 3.4. Surface Acidity. Fig. 4a shows the ammonia desorption curves obtained for the support and the V/TiM catalysts. The TPD-NH3 profile of the support showed two peaks centered at 115 °C and 484 °C, respectively. Observation of two peaks indicates the presence of two types of acid sites in the support. For the pure Ti-MCM-41 solid, the lower temperature peak was assigned to ammonia adsorbed on the surface of TiO2 anatase and SiO2 support, and the higher temperature peak was assigned to ammonia strongly adsorbed on Ti4+ in the framework, respectively.23, 47 For the V/TiM catalysts, increasing V content in the catalysts clearly increased the area of the lower temperature peak, without significantly affecting the intensity of the peak at higher temperature. These results indicate that the low temperature peak was due to desorption of the ammonia adsorbed in acid sites provided by surface V2O5 and TiO2 anatase, and the increment of this peak area resulted from more deposition of surface V2O5. The high temperature peak was assigned to ammonia adsorbed Ti4+ in the support in comparison with the TPD-NH3 profile 12


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of the support. By increasing vanadium content also modified the interaction of NH3 with the catalyst, as evidenced by the shift of the temperature corresponding to the peak maximum from 115° for V/TiM-0 to 132° for the V/TiM-25. Table 2 shows the ammonia desorption data and the temperature corresponding to the peak maximum. With respect to bare support Ti-MCM41, the VOx/Ti-MCM-41 catalysts are more acidic, confirming that the V2O5 on surface could significantly enhance the acidity of Ti-MCM-41. Fig. 4b plots the total acidity as a function of the vanadium content. The surface acidity amount varied with vanadium content, showing the following trend: V/TiM-25 > V/TiM-20 > V/TiM-15 > V/TiM-10 >V/TiM-5 > V/TiM-1 3.5. Morphological Feature. Fig. 5 shows TEM micrographs of the selected V/TiM-1, V/TiM-10, and V/TiM-25 catalysts. Many spherical particles with diameter approximately 10~20 nm consisting mainly of a vanadium oxide shell were observed, similar to the results reported by others.48 Fig. 6 shows the EDS spectra of representative areas for the V/TiM-0, V/TiM-10 and V/TiM-25 catalysts. In the support (Fig. 6a), Si, Ti, and O elements were detected, indicating that SiO2 was the primary surface species along with lesser amounts of Ti in the Ti-MCM-41 support. In Fig. 6b and 6c, V, Si, Ti and O elements were detected in different selected areas of the V/TiM-10 catalyst; most of the V content was present within the shell on the exterior of the particles. Similar elemental distribution was detected in the sample V/TiM-25 (Fig. 6d). The quantitative atomic concentrations obtained from the EDS spectra were reported in Table 3. In central region of the particles, vanadium ions were also detected; however, its concentration was lower in comparison with that present in the outer region. It is emphasized that the support is a porous solid with many mesopore channels. Keeping this in 13



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mind, when vanadium ions were impregnated on the surface of the porous solid, they can distribute on both the outer and inner surfaces of the pore channels. That is why when one analyzes vanadium element using the EDS spectroscopic technique, vanadium can be found in different depth of a particle. But vanadium ions were predominately enriched in outer shell of the particles. 3.6. XPS Analysis. XPS spectra of Ti-MCM-41 support showed two peaks with binding energies (BE) of 458.7~460.3 eV and 465.9~466.6 eV, corresponding to Ti 2p3/2 and Ti2p1/2, respectively (see Supporting Information, Figure S3).49 For pure Ti-MCM-41 support, the Ti 2p3/2 and Ti2p1/2 bands could be deconvoluted into doublet A, with BE = 458 eV and 464.2 eV, and doublet B, with BE = 460.2 eV and 465.9 eV. The presence of two peaks in the Ti XPS spectra indicates that some Ti4+ was incorporated into the MCM-41 framework and the rest was dispersed on the support surface as anatase phase. For all V/TiM catalysts the intensity of the peak maximum of Ti2p was very similar, because Ti4+ content was approximately 10 wt% for the Ti-MCM-41 support. In the V/TiM catalysts, the Ti 2p3/2 and Ti2p1/2 bands were deconvoluted into doublet A, with BE = 458 eV and 464.2 eV, and doublet B, with BE = 459.2 eV and 465.3 eV. Band positions slightly shift to low energy region after vanadium impregnation, indicating the influence resulted from the interaction of dispersed vanadia with the support. Surface Ti4+ presence indicated that vanadia deposition on the surface of the catalysts did not completely cover Ti4+ species, probably due to redistribution and diffusion of the Ti4+ and V5+ in the surface during the calcination procedure. Fig. 7a shows the XPS spectra corresponding to V2p core levels of the V/TiM catalysts. The XPS peaks were deconvoluted into two parts (A and B). Part A, with binding energy BE = 14


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516.3 eV, corresponded to the V4+ species,50 and peak B, with BE = 517.8 eV, was assigned to the V 2p3/2 orbital characteristic of V5+.51-53 For the samples with low vanadium content (V/TiM-1 and V/TiM-5), V4+ was dominant on the surface. These V4+ ions substituted some Si4+ or inserted in the structural defects of the MCM-41 framework. More V4+ than V5+ inserted into the MCM-41 framework as the former carries the same positive charge as Si4+. For V >5 wt%, the peak area of V5+ ions gradually increased and became dominant, indicating formation of more V2O5 clusters or microcrystals. Formation of V4+ was closely related to interaction of V5+ with hydroxyls species in the support via an electron transfer from OH‒ species to V5+, forming V4+‒O‒V5+ bonds, which was also confirmed by using ESR characterization as reported by Gupta et al, with the ESR spectroscopy.54 Fig. 7b plots the calculated V5+/(V4++V5+) ratios as functions of the V loading, showing a linear relationship. Results obtained from XRD, TEM, XPS, DR-UV-Vis, and Raman characterizations provide an internally consistent picture of the V/TiM catalysts. Fig. 8 is a schematic representation of this picture, showing formation of different vanadium oxides on the surface shell on the MCM-41 core; the thickness of the shell depended on the V loading of the catalyst. 3.7. Catalytic Activity. In the present work, we fixed the reaction temperature at 60 °C according to our previous experiments.23, 24, 55 Fig. 9 shows DBTs oxidation performance at 60 °C catalyzed with V/TiM catalysts for 120 min of reaction. For comparison of catalysts with different V loadings, the catalytic activity defined as DBTs conversion increased with increasing V content until 15 wt%, independent of the type of oxidant used. Clearly, the V content of the catalysts significantly influenced their ODS efficiency as most of the reactions 15



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took place on the catalyst surface on which V5+ ions are highly distributed in the shell of the catalysts. As shown in Fig. 9, among the DBTs compounds used in the model fuel, the reactivity according to their conversion followed the trend: DBT> 4-MDBT> 4,6-DMDBT. The observed trend in reactivity indicates that the steric hindrance affected the ODS reaction as compounds with greater steric hindrance showed lower reactivity, which in good agreement with other report.55 The ODS efficiency depended on the oxidant showing an order: H2O2 < H2O2/acetic acid ≈ H2O2/formic acid. The use of carboxylic acid clearly promoted DBTs oxidation. In comparison with H2O2, the H2O2/carbonyl acid mixture had greater oxidative strength and stability, consistent with the formation of peroxometallic complexes on the catalyst surface. Fig. 10 displays DBTs oxidation and acetonitrile extraction as a function of reaction time. In all cases, the extraction of DBTs into the acetonitrile phase was very low (< 3%) and took place in the first 30 min of treatment, with the maximum concentration occurring at after 10 min of operation. In comparison, in the absence of catalyst and oxidant, the concentration of DBT, 4MDBT, and 4,6-DMDBT in acetonitrile phase was measured after 120 min to be approximately 30%, 21%, and 9%, respectively. In the presence of oxidant and catalyst, the majority of the extracted DBTs were in their oxidized sulfone form. This means that before extraction, they were oxidized to sulfones. Furthermore, in the presence of oxidant and catalyst, DBTs oxidation rates were the highest in the first 10 min of reaction; more than 80 % of DBTs were oxidized to sulfones within 30 min. These measurements show that the oxidation capacity was much greater than extraction of DBTs in acetonitrile; and the oxidation rate was much faster 16


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than the extraction rate. This means that before extraction, they were predominantly oxidized to sulfones. In the presence of the H2O2/formic acid oxidant (Fig. 10a) and the H2O2/acetic acid (Fig. 9b), the DBT oxidation was more rapid than either 4-MDBT and 4,6-DMDBT. Overall, the formic acid based oxidant is more effective than the one based on acetic acid. Fig. 10a shows that 99% of DBTs compounds are removed from the oil phase within 20 min, whereas Figure 9b shows that 25 min are required to approach similar removal performance. However, in the absence of carbonyl acids, Fig. 10c, longer reaction time was required to achieve the similar conversion. The catalyst stability was evaluated by recycling 4 times of the ODS reactions (see Supporting Information, Figure S4). Results show that the conversions of DBTs were not obviously changed, indicating the good stability of the catalysts.

4. DISCUSSION

4.1. Formation of Core-Shell V/TiM Catalyst. Both the TEM observation and EDS analysis conformed that a high dispersion of V2O5 on surface of the support was achieved for the V/TiM catalysts (Figs. 5, 6 and 8). The Kirkendall Effect and melting behavior of V2O5, TiO2, and SiO2 may help explain the formation of core-shell morphological feature.56 It is known that silica-based MCM-41 framework contains many lattice defects or cationic vacancies which were confirmed by the 31Si-NMR-MAS spectroscopic characterization.57 The melting point temperature for SiO2, TiO2 and V2O5 was 1710 °C, 1688 °C and 690 °C, respectively. At the same temperature, the diffusion rates of silicon, vanadium and titanium 17



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ions are different during the synthesis procedure. When vanadium ions were introduced on the surface of the Ti-MCM-41 support, in the calcination process at 600 °C in air, V2O5, if formed, was present as a semi-melted or quasi-liquid state. V2O5 particles in a semi-melted liquid state might diffuse from inner layer inside the pores to outer of particle and highly dispersed over the surface of the hard SiO2, forming a vanadium-rich shell. From the viewpoint of Kirkendall Effect,56,58 some V4+ ions which were formed probably by electron transfer V5+ and hydroxyls can diffuse inward the framework of MCM-41 to occupy cationic defects. As a result, catalyst with a core (Ti-V-MCM-41) and V5+ enriching shell was formed. It is noted that in the outer shell, Ti4+ ions were also detected by XPS and EDS analysis. Ti4+ may also diffuse from inner layer to outer shell. Therefore both V5+ and Ti4+ coexisted in the outer shell of the particles. 4.2 Correlation of the Surface V5+/(V4++V5+) Ratio with the DBT Conversion. Fig. 11 shows the correlation of the DBT oxidation with the surface V5+/(V4++V5+) ratio calculated from XPS data. The strong correlation between catalytic activity and V5+/(V4++V5+) suggests that V5+ ions are chiefly responsible for the oxidation of the sulfur compounds in the ODS process, at least for the catalysts with V content less than or equal to 15 wt%. The catalytic activity of the catalysts containing higher V content (20 and 25 wt%) did not further increase in comparison with V/TiM-15, probably due to the near complete DBTs conversion. The amounts of the three sulfur compounds in the 30 mL n-hexadecane were 0.016 mmol DBT, 0.023 mmol MDBT, and 0.035 mmol DMDBT, resulting in a total of 0.074 mmol of DBTs. For the best V/TM-15 catalyst, 30 mg catalyst contained 0.088 mmol V if we propose all the V2O5 were dispersed as monolayer on the surface of the catalysts. It is a slightly stoichiometric amount of catalyst, which explains that further increasing the amount of vanadium beyond that limit does 18


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not produce additional improvement in the reaction. Therefore, vanadium content should be controlled no more than 20 wt% in the catalysts from both the ODS efficiency and economic consideration. It is noted that the ox-red chemical potential EV5+/V4+ = 0.48 eV, which was much greater than the value ETi4+/Ti3+ = 0.10 eV. Theoretically, it is possible that V5+ oxidizes Ti3+ to Ti4+ and itself was reduced to V4+. In our catalysts, the fact that Ti3+ ions were not detectable but V4+ coexisting with V5+ (V4+ in a small amount as the amount of titanium ions are small) may indicate that electron transfer via the V5+ + Ti3+ = V4+ + Ti4+ taking place during the catalysts preparation, which is another possible origin of V4+ formation. 4.3 Correlation of Surface Acidity with DBTs Conversion. Fig. 12 plots DBT oxidation profile, showing that DBTs conversion largely increased with increasing surface acidity, again until the V loading reached 15 wt%. For V loading equal to or greater than 15 wt%, DBTs conversion no longer increased with increasing surface acidity. The linear relationship between DBTs oxidation and surface acidity existed in vanadium content lower than the stoichiometric amount. It is noted that all the VOx catalysts predominantly contained Lewis acid sites (> 90% surface acid sites are Lewis acid sites), as confirmed by in situ FTIR spectroscopic characterizations of pyridine adsorption technique.59 The S atom in DBTs molecule has two pairs of isolated electrons. Therefore, the molecule of DBTs can act as electron donator or Lewis base. During the oxidation reaction, DBTs molecules can donate the isolated electrons in S to the Lewis acid sites of the catalysts. Therefore, DBTs in the oil phase could preferentially adsorb on the Lewis acid sites of the catalysts by acid‒base reaction

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pathway. The adsorbed DBTs in Lewis acid sites reacted with adsorbed oxidant in the catalysts to produce sulfone. For the catalysts with higher vanadium loading, other influencing factors like crystallite size and textural properties should be taken into account, too. As discussed above, higher V content lowered the vanadia dispersion due to vanadia particle size increasing, which did not benefit the sulfur oxidation. So we may conclude that the surface V5+ concentration and dispersion, surface Lewis acidity, as well as the textural properties of the catalysts were all critical factors affecting the catalytic activity of the catalysts in the ODS process. 4.4. Roles of Carboxylic Acids Addition and ODS Reaction Mechanism. Under reaction conditions, carboxylic acids were oxidized by H2O2 to form peroxyacetic acid (R— (C=O)—O—O—H, where R = H or CH3).60 In turn, the in situ produced peroxyacetic acid could coordinate with V5+ ions on the catalyst surface due to its vacancies in the 3d electron configuration, thereby reducing the electron density of the O—O bond and forming active oxidizing species like peroxometallics and superoxometallics.61 These oxidizing species are more active due to the presence of a dissymmetry O—O bond with strong polarization in comparison with the symmetrical hydrogen peroxide.62 They were also more stable than H2O2 as connecting with the surface metallic sites. Formation of peroxometallic complexes effectively inhibited the H2O2 decomposition to release gaseous oxygen before the reaction and thus improved the ODS efficiency. The biphasic system described here comprised of two immiscible liquid phases (nhexadecane and acetonitrile) and a solid (catalyst) phase coupled with oxidizing agents. The DBTs oxidation reactions and extraction primarily take place at liquid-solid and liquid-liquid 20


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interfaces. The reaction mechanism largely consists of four principal steps: (i) in situ formation of peroxyacetic acid in liquid phase which coordinated with V5+ ions to form peroxometallic complexes in the surface of catalysts; (ii) adsorption of DBTs in the Lewis acid sites via acidbase reaction pathway; (iii) surface reaction between V5+ coordinated-peroxometallic oxygen complexes and adsorbed DBTs in Lewis acid sites; and (v) desorption of produced sulfones and separation by acetonitrile extraction. Scheme 1 depicts the hypothesized ODS reaction mechanism. In the surface reaction step, the oxdiation reactions occur via nucleophilic attack of the free electron pairs of the sulfur in DBTs by the positively charged V5+ coordinated-peroxometallic oxygen complexes to form the sulfoxide; subsequently, the formed sulfoxide undergoes a second oxidation by feedback free electrons to attack another peroxometallic complex to form sulfone. The second oxidation step appears to be faster than the first, as sulfoxide species were never detected in the final products. Acetonitrile preferentially extracts the polar oxidation product sulfones over the non-polar DBTs precursors. At the end of the oxidation/extraction process, less than approximately 15 ppm of DBTs remained in the n-hexadecane phase. The solid catalyst can be separated by filtration and acetonitrile can be purified via distillation method to remove the SBSs. Therefore the solid catalysts and solvent can be reused for ODS treatment.

4. CONCLUSION

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VOx loaded core-shell catalysts possessed a typical hexagonal mesostructures with pore diameter between 26.1 and 34.2 Å and surface area between 244 and 491 m2/g. V4+ and V5+ ions were present either on the surface or in the framework of Ti-MCM-41, depending of the vanadium content. When vanadium content was lower than or equal to 5 wt%, most of the V4+ ions were inserted into the MCM-41 framework; as the V content was between 5 and 15 wt%, V2O5 was well dispersed on the surface as nanoparticles to form a shell-core structure and morphological feature. Vwt% ≥ 20, V2O5 polymeric microcrystals and some large agglomerates were formed on the catalyst surface. Catalytic activity correlated well with both the V5+/(V4++V5+) ratios and the measured surface acidity until 15wt% of vanadium in catalyst, indicating that surface V5+ and acid sites served as reaction centers. Carboxylic acids in the reaction mixture served as promoters to help the formation of stable and active oxygen species like peroxometallic and superoxometallic complexes on the catalyst surface. The reactivity order of the organosulfur compounds was: DBT> 4-MDBT> 4,6-DMDBT, independent of oxidant but dependent of steric hindrance effect resulting from methyl groups in aromatic rings. In the presence of both H2O2 oxidant and organic acid (acetic acid or formic acid) promoter and stabilizer, a biphasic reactor coupled with core-shell V2O5/Ti-MCM-41 catalysts containing surface Lewis acidity provides an effective alternative for deep desulfurization of diesel fuels in one pot.

ACKNOWLEDGMENTS.

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Thanks do to Dr. Micheal T. Timko at Worcester Polytechnic Institute in USA for his valuable discussion and comments and to G. Wang in the American School Foundation in Mexico City for his help for preparing the graphic for Table of Content. Financial support from the Instituto Politécnico Nacional (IPN) (Grant No. SIP201601343, No. SIP20171266 and No. SIP20170628) is acknowledged. U.A. thanks CONACyT-Mexico for providing financial support for his postdoctoral studies at IPN.

Supporting Information

**Brief statement in nonsentence format listing the contents of the material supplied as Supporting Information.

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[57] Julio González García. “Desulfuración oxidativa (ODS) de dibenciltiofeno (DBT) presente en el diesel con catalizadores VOx/MCM-41”. Master thesis, 2013, Mexico. [58] Yin, Y. D. Rloux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Formation of hollow nanocrystals through the nanoscale Kirkendall effect. Science 2004, 304, 711–714. [59] González, J.; Chen, L.F.; Wang, J.A.; Manríquez, Ma.; Limas, R.; Schachat, P.; Navarrete, J.; Contreras, J.L. Surface chemistry and catalytic properties of VOX/Ti-MCM-41catalysts for dibenzothiophene oxidation in a biphasic system, Appl. Surf. Sci. 2016, 379 367–376. [60] Filippis, P. D.; Scarsella, M.; Verdone, N. Peroxyformic acid formation: a kinetic study. Ing. Eng. Chem. Res. 2009, 48, 1372–1375. [61] Arends, I. W. C. E.; Sheldon, R.A. Activities and stabilities of heterogeneous catalysts in selective liquid phase oxidations: recent developments. Appl. Catal. A: Chem. 2001, 212, 175– 87. [62] Haw, K. –G.; Bakar, W. A. W. A.; Ali, R.; Chong, J. –F.; KAdir, A. A. A. Catalytic oxidative desulfurization of diesel utilizing hydrogen peroxide and funcationalized-activated carbon in a biphasic diesel-acetonitrile system. Fuel Processing Technology 2010, 91, 11051112.

29



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

30

TABLES CAPTIONS Table 1. Textural properties of TiM and V/TiM catalysts. Table 2. TPD-NH3 data of the V/TiM catalysts. Table 3. Atomic concentration (%) in the surface in different samples obtained from EDS spectra shown in Fig. 6. FIGURES CAPTIONS Fig. 1. X-ray diffraction patterns of the Ti/TiM catalysts. (a) Low 2θ angle XRD patterns of the V/TiM catalysts. (b) Wide 2θ angle XRD patterns of the V/TiM catalysts. Fig. 2. Spectra of UV-vis diffuse reflectance spectroscopy of the V/TiM catalysts. Fig. 3. Loops of N2 adsorption/desorption isotherms and pore distribution of the V/TiM catalysts. Fig. 4. TPD-NH3 profiles and correlation of surface acidity with vanadium content in the V/TiM catalysts. (a) TPD-NH3 profiles of V/TiM catalysts; (b) Correlation of surface acidity with vanadium content in the V/TiM catalysts. Fig. 5. TEM micrographs of the selected V/TiM catalysts. (a) V/TiM-0; (b) V/TiM-10; (c) V/TiM-250. Fig. 6. EDS profiles of the selected area in TEM images of different V/TiM catalysts. (a) Ti-MCM-4V/TiM-0; (b) V/TiM-10; (c) V/TiM-10; (d) V/TiM-25. Fig. 7. XPS spectra of the V2p 3/2 core levels and correlation of the V5+/(V4++V5+) values with the vanadium content in V/TiM catalysts (a) XPS spectra of the V2p 3/2 core levels; (b) Correlation of the V5+/(V4++V5+) values with the vanadium content in V/TiM catalysts. Fig. 8. Formation of the vanadia shell on the Ti-MCM-41 core. Fig. 9. Oxidation/elimination of DBTs using different oxidizing agents and different catalysts.

Reaction temperature; 60 °C; reaction time: 60 min; catalyst mass: 3g per liter oil; H2O2/DBTs molar ratio = 1.5; H2O2/carbonyl acid molar ratio = 1.5. HP: H2O2; HP/FA: H2O2/Formic acid; HP/AA: H2O2/Acetic acid. Fig. 10. Oxidation and solubilization of DBTs as well as DBSs extraction as a function of operation time with different promoters and V/TiM 15 catalyst. Reaction temperature; 60 °C; reaction time:

60 min; catalyst mass: 3g per liter oil; H2O2/DBTs molar ratio = 1.5; H2O2/carbonyl acid molar ratio = 1.5. 30


Page 30 of 46

Page 31 of 46

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


31

(a) H2O2/formic acid; (b) H2O2/acetic acid; (c) H2O2. Fig. 11. Correlation of the DBT oxidation removal with the values of V5+/(V4++V5+) in the

catalysts. Fig. 12. Correlation of the DBT oxidative removal with the surface acidity of the catalysts. Scheme 1. A proposed mechanism for oxidation removal of DBTs in the biphasic system.

31



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

32

Figures

Fig. 1. X-ray diffraction patterns of the V/TiM catalysts. (a) Low 2θ angle XRD patterns; (b) Wide 2θ angle XRD patterns

32


Page 32 of 46

Page 33 of 46

33

V/TiM -1 V/TiM -5 V/TiM 10 V/TiM -15 V/TiM -20 V/TiM -25

F(R)(a.u) F(R)(a.u)

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


200

300

400

500

600

700

800

W avelength (nm ) V/TiM 0 V/TiM-0 200

300

400

500

600

700

800

W avelength (nm )

Fig. 2. Spectra of UV-vis diffuse reflectance spectroscopy of the V/TiM catalysts.

33



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

34

Fig. 3. Loops of N2 adsorption/desorption isotherms and pore distribution of the V/TiM catalysts.

34


Page 34 of 46

Page 35 of 46

35

a

b

115 °C

450

132 °C V/TiM-0 V/TiM-1 V/TiM-5 V/TiM-10 V/TiM-15 V/TiM-20 V/TiM-25

484 °C

0

10

20

30

40

50

60

70

Total acidity (µmoles NH3/g)

TPD Signal (a.u)

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


400 350 300 250 200

80

0

Time (mim)

5

10 15 V(wt.%)

20

25

Fig. 4. TPD-NH3 profiles and correlation of surface acidity with vanadium content in the V/TiM catalysts. (a) TPD-NH3 profiles of V/TiM catalysts; (b) Correlation of surface acidity with vanadium content in the V/TiM catalysts.

35



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

36

a)

b)

c)

(a)

Fig. 5. TEM micrographs of the selected V/TiM catalysts a) V/TiM-1, b) V/TiM-10, c) V/TiM-25. 36


Page 36 of 46

37

Counts (a.u)

a

Sik

b

Tik Ck

Counts (a.u)

Counts (a.u)

Ok Vk

0

1

Ok Vk

Vk 2

3 E (keV)

4

Vk

5

Sik

c Counts (a.u)

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


Tik Ck

Vk Vk 0

1

Ok Vk

2

3 E (keV)

4

5

Sik

d Counts (a.u)

Page 37 of 46

Tik Ck

Vk Vk 0

1

2

3 E (keV)

4

5

Fig. 6. EDS profiles of the selected area in TEM images of different V/TiM catalysts. (a) Ti-MCM-4V/TiM-0; (b) V/TiM-10; (c) V/TiM-10; (d) V/TiM-25.

.

37



38

V2p3/2 B

a

V2p3/2 A

V/TiM -1

V/TiM -5

/s

b 0.9

Counts /s Counts

V/TiM -10 0.8 V 5+ / (V 5 + + V 4+ ) (% )

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

Page 38 of 46

V/TiM -15

V/TiM -20

520 520

5518 18

516 516

514 514

0.6 0.5 0.4

V/TiM -25

522 522

0.7

0

512 512

5

10

15

20

25

V(wt.%)

BindingEnerg y(eV) (eV) Binding Energy

Fig. 7. XPS spectra of the V2p 3/2 core levels and correlation of the V5+/(V4++V5+) values with the vanadium content in V/TiM catalysts (a) XPS spectra of the V2p 3/2 core levels; (b) Correlation of the V5+/(V4++V5+) values with the vanadium content in V/TiM catalysts

38


Page 39 of 46

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


39

5wt% < VOX ≤ 15 wt% VOX /Ti-MCM-41 VOX Ti O2

Fig. 8. Formation of the vanadia shell on the Ti-MCM-41 core.

39



40

Catalysts V/ V/ V/ V/ V/ V/ V/ Ti Ti Ti Ti Ti Ti T M M M M i M M M -2 -1 -2 -1 -1 -0 -5 0 5 5 0

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

Page 40 of 46

HP HP/FA HP/AA HP HP/FA HP/AA HP HP/FA HP/AA HP HP/FA HP/AA HP HP/FA HP/AA HP HP/FA HP/AA HP HP/FA HP/AA

0

4,6-DMDBT 4-MDBT DBT

20

40

60

80

100

DBTs Oxidation (%)

Fig. 9. Oxidation/elimination of DBTs using different oxidizing agents and different catalysts. Reaction temperature; 60 °C; reaction time: 60 min; catalyst mass: 1g per liter oil; H2O2/DBTs molar ratio = 1.5; H2O2/carbonyl acid volume ratio = 2:3. HP: H2O2; HP/FA: H2O2/Formic acid; HP/AA: H2O2/Acetic acid.

40


Page 41 of 46

41

a) 100 DBT DBS Solubility DBT 4-MDBT 4-MDBS Solubility 4-MDBT 4,6-DMDBT 4,6-DMDBS Solubility 4,6-DMDBT

(%)

80 V/TiM-15 H2O2/Formic acid

60 40 20 0

0

10

20

30

40

50

60

Time (min)

c)

b) 100

100

80

80 V/TiM-15 H2O2/Acetic acid

60

(%)

(%)

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


40

V/TiM-15 H2O2

40 20

20 0

60

0

0

10

20

30

40

50

60

0

10

20

30

40

50

60

Time (min)

Time (min)

Fig. 10. Oxidation and solubilization of DBTs as well as DBSs extraction as a function of operation time with different promoters and V/TiM 15 catalyst. Reaction temperature; 60 °C; reaction time: 60 min; catalyst mass: 1g per liter oil; H2O2/DBTs volume ratio = 2:3; H2O2/carbonyl acid molar ratio = 0.75. (a) H2O2/formic acid; (b) H2O2/acetic acid; (c) H2O2. .

41



42

100 90 DBT Oxidation (%)

80 70 60 50 40 30 20 0.4

0.5

0.6 5+

0.7 5+

0.8

0.9

4+

V / (V + V )

Fig. 11. Correlation of the DBT oxidation removal with the values of surface V5+/(V4++V5+) in the catalysts.

100 90 80 DBT Oxidation (%)

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

Page 42 of 46

70 60 50 40 30 20 200

250

300

350

400

450

Total acidity (µmoles NH3/g)

Fig. 12. Correlation of the values of DBT oxidation removal with surface acidity of the catalysts.

42


Page 43 of 46

43

O

H +

O O

CH3

H-O-O-H O

O H

SiH2

O

O

O

OH

S

+

Ti

H-O-O-H O

+

V/TiM

O H

H-O-O-H

+

S

CH3 O

O

O

HO

S

O

CH3

H

+ V

H-O-O-H

S

OH O

O

+

Acetonitrile

O H

V/TiM

H-O-H

H 3C

δ

O

H 3C

−

O

O

δ

+

H

O

O

S

O

S

S

....

+

−

δ

+

H

H-O-H

δ

O O

S

..

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


O

O

O HO S

CH3 +

O

O

H-O-H

Scheme 1. A proposed mechanism for the oxidation removal of DBTs in the biphasic system.

43



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

Page 44 of 46

44

Tables Table 1. Textural properties of TiM and V/TiM catalysts. Sample TiM V/TiM-1 V/TiM-5 V/TiM-10 V/TiM-15 V/TiM-20 V/TiM-25

SA (m2/g) 710 491 483 440 394 293 244

Vp (cm3/g) 0.81 0.54 0.53 0.51 0.50 0.43 0.41

Dp (Å) 34.2 28.1 28.3 28.1 27.1 26.6 26.1

S.A: surface area (BET); Vp: pore volume; Dporo: average pore diameter.

Table 2. TPD-NH3 data of the V/TiM catalysts. Sample TiM V/TiM-1 V/TiM-5 V/TiM-10 V/TiM-15 V/TiM-20 V/TiM-25

Maximum desorption temperature (°C) I II 115 483 124 484 132 484 132 484 132 484 132 484 132 484

Surface acidity (µmoles NH3/g) I II 81 32 186 49 239 57 253 55 285 61 324 64 352 70

44


Total acidity (µmoles NH3/g) 113 235 296 308 346 388 422

Page 45 of 46

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


45

Table 3. Atomic concentration (%) in the surface of different samples obtained from EDS spectra in Fig. 6. Element V/TiM-0 (a)* V/TiM-10 (b) V/TiM-10 (c) V/TiM-25 (d) CK OK Si K VK Ti K

0 76.88 19.22 4.066

2.08 57.03 34.25 0.12 6.53

1.55 50.27 41.91 5.65 0.62

* the letter in brackets indicates the corresponding EDS spectrum shown in Fig. 6.

45


2.79 45.36 35.01 13.06 3.78


46

Table of Content V/TiM

Water

Dibenzotiophene

Clean Diesel S

S S

HO

CH3

O

O H2O2 V/TiM X V/TiM

S

HO

S

CH3

V/TiM

H2O

-

O

O

O

O

O

S

O

.. ..

S

CH 3 O

+

CH3

∆

O

O

2-

Cu

HO

HC 3

HO

O -

∆

O

5+ CVu

-

H

O

O

S

S +

HO

O 3C

H

OH

S

O

O

-

..

HO

O

C H3

O

+

O

∆

∆

-

O

+

V2O5

3 CH

+

S

..

∆

46


Polar phase

S

S

O

O

O

..

Extracting agent (Acetonitrile)

Dibenzosulfone

Hydrogen Peoxide

Acetic acid

Diesel (Hexadecane)

Acetic acid

O-

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

Page 46 of 46

O

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