Catalyst, Emulsion Stabilizer, and Adsorbent: Three Roles In One for

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Catalyst/emulsion stabilizer/adsorbent three-roles-in-one for synergistically enhancing the interfacial catalytic oxidative desulfurization Ran Xia, Wenjie Lv, Kaiqing Zhao, Shihao Ma, Jun Hu, Hualin Wang, and Honglai Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00019 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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Catalyst/emulsion stabilizer/adsorbent three-roles-in-one for synergistically enhancing the interfacial catalytic oxidative desulfurization Ran Xia,a Wenjie Lv*,b Kaiqing Zhao,a Shihao Ma,b Jun Hu*,a Hualin Wang,b Honglai Liu.a a. State Key Laboratory of Chemical Engineering and School of Chemistry and

Molecular Engineering East China University of Science and Technology 130 Meilong Road, Shanghai 200237, China. b. National Engeering Laboratory for Industrial Wastewater Treatment, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China. ABSTRACT: A Pickering emulsion catalytic system was proposed to reduce the transfer limitation between two immiscible reactant phases for enhancing the kinetics of heterogenetic oxidative desulfurization (ODS). By loading phosphotungstic acid (HPW) nanoparticles on a novel pyridine-based porous organic polymer of P[tVPB-VPx], the amphiphilic catalysts were produced and used as the stabilizer for Pickering emulsions. Specifically, an ultra-fast ODS rate was realized in the HPW/P[tVPB-VP1] stabilized Pickering emulsion catalytic system, that just within 15 min, 100 ppm dibenzothiophene (DBT) was completely oxidized by H2O2. Since the obtained hierarchical porous HPW/P[tVPB-VPx] catalysts showed both high adsorption capacity of DBT and excellent catalytic ODS performance, the catalysts assembling at the interface of emulsions provided this fastest reaction dynamics.

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Acted as three roles of catalyst, emulsion stabilizer, and adsorbent in a one, the synergistic functional catalytic emulsions can be a promising approach to significantly boost the heterogeneous catalytic ODS performance. KEYWORDS: Oxidative desulfurization, Pickering emulsion, Amphiphilic porous organic polymers, Adsorption enhanced oxidation, Ultra-fast catalytic efficiency.

INTRODUCTION The mass transfer limitation among immiscible reagents in heterogenetic catalytic reactions has been an extremely difficult bottleneck. Recently, the field of interfacial catalysis has been boosted by introducing the concept of Pickering emulsion system, since it provides a high interfacial reaction area per unit volume, improves the compatibility between hydrophilic and hydrophobic reagents, and facilitates the mass transfer.1 More significantly, when the catalysts themselves act as emulsion stabilizers, the highly dispersed active catalytic sites at the interface of the emulsion droplets can effectively enhance the catalytic reaction dynamics.2,3 Several heterogeneous catalytic reactions such as oxidation,4 etherification,5 acetalization,6 and transesterification reactions7 have demonstrated the success of this Pickering emulsion catalytic system .Typically, the use of amphiphilic particles as a catalyst support to form emulsions to facilitate catalytic reactions has been of great interest. Liu et al. prepared

a

Janus-type

catalyst

for

emulsion

interface

catalysis

8

of

nitro-compounds by selective modification of metal nanoparticles on one or


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both beads of the snowman-like PDVB/PS-SiO2 Janus particles. Cho et al.9 reported a colloid surfactant catalyst by site-specific patching of nanoparticles such as Ag, Pd and Fe2O3 NPs onto the surface of Janus microparticles, exhibiting remarkable catalytic activity and magnetic responsiveness in Pickering emulsion microreactors. However, applications are far from sufficient, since the design of catalysts simultaneously coupling with the stabilizer properties are of great challenges. Herein, we chose a typical heterogeneous catalytic reaction of the oxidative desulfurization (ODS) to facilitate the reaction dynamics by the Pickering emulsion catalytic system. ODS has been proved as a most competitive method for ultra-deep desulfurization of fossil fuels10,11 and attracted increasing concerns due to the stringent environmental regulations regarding the sulfur content in fuel.12-15 Typically, hydrogen peroxide (H2O2) is the most used oxidant in the presence of catalyst, such as ionic liquids (ILs),16, catalysts18, organic acids

19

and polyoxometalates (POMs)20,

21.

17

solid

Andevary et

al.22 synthesized a low viscosity iron-based IL of [Omim]FeCl4 as both catalyst and extractant to achieve a complete removal of BT, DBT within 15 min in a model fuel. Teimouri et al

23

reported that catalytic oxidative desulfurization of

crude fuel containing dibenzothiophene using molybdenum and vanadium oxide as catalysts resulted in complete conversion at 0.13 g and 70 ℃ in 60 min of treatment. Recently, there has been an increasing effort in developing oxidative desulfurization systems based on polyoxometalates (POMs) for


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treating model fuel under sustainable conditions.24-27 Azam et al.28 prepared HPW-GO heterogeneous catalysts by immobilizing different contents of HPW on GO. A complete desulfurization of DBT-containing model fuel could be attained within 30 min using the catalyst with HPW content of 40 wt%. As the ODS reaction is a typical heterogeneous catalytic oxidation, the mass transfer limitation across the interface among water, oil, and solid catalyst triple-phases would usually cause a low oxidation rate. So far, there are a few successful Pickering emulsion catalytic system for ODS have been reported. Banisharif et al.29 combined a vanadium substituted Dawson-type emulsion catalyst with IL and ethylene glycol to present a novel extractive-catalytic oxidative desulfurization ECODS process, which can remove 90% sulfur from 500 ppmw fuel and 87% sulfur from 1500 ppmw fuel under optimized conditions. Song et al.30 prepared ionic-liquid emulsion system to achieve efficient deep desulfurization

by

self-assembling

Lanthanide-containing

POMs

and

surfactants. However, by these modifications, a large portion of the active catalytic sites were covered by surfactants, which would obstruct the entrance of reagents to the catalytic active site. Therefore, except for the catalytic and amphiphilic properties, the accessibility to the catalytic active sites should also be taken into account for designing a Pickering emulsion catalytic system. As we all know, the most convenient way to achieve the amphiphilicty is to assembly the hydrophilic POM catalyst on a hydrophobic organic substrate. To further facilitate the accessibility, porous organic polymers (POPs) attracted our


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particular

interests

because

of

their

extraordinary

inherent

structural

characteristics, such as high specific surface area, adjustable pore structure, and versatile functional organic framework.31, 32 Previously, we successfully applied POPs as adsorbents for deep adsorption desulfurization.33, 34 Recently, Jiang et al.35 prepared a series of ionic porous organic polymers and loaded PW12O403through the ion exchange. The obtained hybrids showed a quite good catalytic ODS performance in bulk system. Accordingly, by combing the synergistic multi-functional roles of adsorbents, catalyst and emulsion stabilizer together, POP-POM hybrids would be highly expected to enhance the ODS efficiency in the catalytic emulsion system. Taking these in mind, we fabricated a novel hierarchical pyridine-based POPs of (P[tVPB-VP]) through the Suzuki coupling copolymerization36a, in which pyridine groups provided a strong affinity to phosphotungstic acid (HPW) nanoparticles. The catalytic ODS efficiency of the obtained catalysts HPW(y)/P[tVPB-VPx] were first investigated in a bulk system to optimize the catalyst structures and ODS conditions. Then, the optimized amphiphilic HPW(50)/P[tVPB-VP1] was further applied as the stabilizer for producing Pickering emulsion catalytic system, in which ODS efficiency was significantly enhanced, that a complete conversion of DBT from 100 ppm model fuel was realized just within 15 min. The mechanism of this ultra-fast Pickering emulsion catalytic system for the ODS was illustrated by the adsorption enhanced catalytic reaction performance, as well as the extraction of polar


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sulfone products by methanol in water droplet. This study provided a first attempt by using POPs-based amphiphilic catalysts as the emulsion stabilizer to enhance the ODS efficiency. As POPs can provide various task-specific properties, this approach could be a potentially promising solution for efficient heterogeneous catalytic reactions.

EXPERIMENTAL SECTION Materials Phosphotungstic acid (HPW, P2O524WO344H2O) was purchased from Macklin Reagent Co., Ltd.. 1,3,5-tribromobenzene (C6H3Br3, 98%), 4-vinylboronic acid (C8H9BO2), Tetra(triphenylphosphine) palladium(0) (C17H60P4Pd, 97%) were obtained from TCI (Shanghai) Development Co., Ltd.. Benzothiophene (BT, 99%), 4,6-dimethyldibenzothiophene(4,6-DMDBT,

98%),

dibenzothiophene

sulfone

(DBTO2, 98%) were purchased from Adamas Reagent Co., Ltd.. 4-vinylpyridine (C7H7N, 96%) was purchased from Aladdin Reagent Co., Ltd.. Dibenzothiophene (DBT, 98%), 2,2-azobisisobutyronitrile (C8H12N4, 99%) were purchased from J&K Chemicals (Shanghai, China). Potassium carbonate (K2CO3), Toluene, petroleum ether, and n-octane were obtained from Shanghai Titan Scientific Co., Ltd.. H2O2 (30% in water) was obtained from Shanghai Chemistry Reagents Co., Ltd.. All reagents were commercially purchased and used without further purification except azobisisobutyronitrile was recrystallized.


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Synthesis Preparation of hierarchical pyridine-based POPs, P[tVPB-VPx] The synthesis of 1,3,5-tris (4-vinylphenyl)-benzene (tVPB) monomer was based on the Suzuki coupling method36b (the details of the synthesis and characterization can be found in supplementary, Figure S1, S2). The P[tVPB-VPx] POPs were prepared by the radical co-polymerization of 1,3,5-tris (4-vinylphenyl)-benzene (tVPB) and 4-vinylpyridine (VP) with AIBN as an initiator, in which x represented the molar ratio of VP to tVPB. In a typical synthesis, tVPB (0.04 mol, 15.36g), certain amounts of VP (0.04, 0.08 or 0.12 mol), and AIBN (8 wt.%) were mixed with DMF (15 mL) in a 50 mL autoclave reactor and reacted at 135 oC for 36 h, respectively. The resulting solid was filtered and washed with methanol for several times to remove the catalyst and reactant residues, then dried under vacuum overnight. The resulted product was denoted as P[tVPB-VPx] . Preparation of HPW(y)/P[tVPB-VPx] hybrid catalysts As-prepared P[tVPB-VPx] (0.1 g), certain amounts of HPW, and H2O (5 mL) were added into a 250 mL flask. The mixture was stirred at room temperature for 24 h. The precipitate was then separated, and washed with water (50 ml, twice) and methanol (50 ml, twice) successively by a centrifugation. The obtained hybrids were dried under vacuum at 80 oC for 24 h and denoted as HPW(y)/P[tVPB-VPx], where y represented the mass content of HPW in the hybrid, and y = 30, 50 and 80 wt.%, respectively. Characterization


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The morphology of samples was characterized by Field-emission scanning electron microscope (FESEM, Nova Nano SEM 450) and transmission electron microscope (TEM, JEOL JEM-2100). The content of P and W was determined by inductively coupled plasma-atomic (optical) emission spectrometry (ICP-AES, Vanan 710). C, N, and H elemental analysis was conducted on an elemental analyzer (Vario EL III). The FTIR spectra were recorded on a Nicolet iS10 FTIR spectrometer using the KBr pellet technique. Solution 1H nuclear magnetic resonance (NMR) spectrum was obtained in a CDCl3 solution with a Bruker 400 MHz Advance spectrometer. N2 adsorption-desorption isotherms at 77 K were measured by volumetric adsorption analyzer ASAP 3020. The thermal decomposition behavior was performed on a NETZSCH STA 499 F3 thermogravimetric analyzer (TGA), and the sample was heated at a rate of 10 oC min-1 under air flow with a rate of 40 mL min-1. The oxidation products were analyzed by gas chromatography mass spectrometry (GC-MS, Agilent, 7890A GC, 5975C MSD). The emulsion droplets were observed with the optical microscope (Nikon DS-U3). The contact angle of samples was measured using a contact angle goniometer (JC2000D1,

Zhong

Chen

POWERREACH)

at

room

temperature. The

three-phase antennae of gas/liquid/solid were tested by suspension drop method. Ultra-pure water and n-octane were used as the liquid phase, respectively. The catalyst powder was pressed into thin tablet. The contact


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angle was calculated by the five-point fitting method for analysing the images of water drop or n-octane drop on the surface of the pressed sample tablets. Formation of stable catalytic Pickering emulsions To be a stabilizer, HPW(50)/P[tVPB-VP1] catalyst powder was placed in a ball mill (QM-3SP04). After milling at 500 rpm for 3 h, the powder changed into uniformed micrometre-scale particles. The formation conditions of the Pickering emulsion system were optimized by varying the volumetric ratios of n-octane, water, and methanol, such as (5:1:1, 10:2:1, 20:4:1, and 25:5:1); and the adding amount of stabilizer of HPW(50)/P[tVPB-VP1] catalyst (1.9, 2.5, 3.2 mg/g fuel). The mixture was emulsified by an emulsifier B25 (B.R.T Co., Ltd.) at a speed of 10000 rpm for 2 min at room temperature. After the optimization, the temperature was increased to desulphurization temperature of 60 oC to investigate the stability of the selected Pickering emulsion systems. Desulfurization Catalytic oxidation desulfurization of model fuels in bulk system Model fuels were prepared by dissolving DBT, BT and 4,6-DMDBT in n-octane with a specific S-content, respectively. The bulk ODS process was conducted in a 50 ml jacked reactor equipped with a circulator bath, a magnetic stirrer and a reflux tube. Certain amounts of catalysts, model fuels and H2O2 aqueous solution were added into the reactor in sequence, and stirred at 400 rpm. For a 100 ppm model fuel, the catalyst dosage was 2.5 mg/g (fuel) and the


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molar ratio of oxidant to sulfur (O/S) was 9:1. Kept the O/S ratio and other conditions unchanged, the ODS performance in a 500 ppm model fuel was also determined. Samples were taken from the oil phase at set intervals and analysed by GC-FPD (GC-950, Haixin Chromatography ), which was equipped with a HP-5 capillary column (15 m × 0.53 mm × 1.5 μm film thickness). Catalytic oxidation desulfurization of model fuels in Pickering emulsion system The ODS performance in Pickering emulsion system was determined based on the optimized formation conditions. Specifically, DBT model fuel (100 or 500 ppm), H2O2 aqueous solution with methanol, HPW(50)/P[tVPB-VP1] catalyst were added into the flask with the molar ratio of O/S kept as 9:1. The mixture was emulsified at a speed of 10000 rpm for 2 min. After forming a stable catalytic Pickering emulsion, it was stirred very slowly. Samples were taken from the upper oil phase at set intervals, and subjected to GC–FPD to analyse the sulfur content. After the completely ODS conversion, the demulsification was conducted by the centrifugation at 8000 rpm (TG16G, Kaida Scientific Instruments Co., Ltd.), and the lower water phase was separated by the separating funnel. Recycle use of the Catalyst After each ODS run, the catalysts were separated by the centrifugation, washed by methanol, and dried in vacuum at 100 oC for 12 h. The recycle use stability of the regenerated catalysts were determined by repeating the ODS test, in


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which the regenerated catalysts, fresh model fuels, H2O2 were added into the reactor to start a new cycle. The sulfur concentration in the model fuel was determined by the GC-FPD chromatograms using a calibration curve, which was obtained by plotting the peak areas in the spectra with the corresponding concentrations. The desulfurization efficiency was calculated by the conversion as eq. 1, where C0 and Ct are the sulfur concentration in the original model fuel, and after ODS in a specific time, t. Conversion = (C0-Ct)/C0 × 100%

(1)

The kinetics study of the ODS for DBT was based on the pseduo-first-order law as eq. (2), by plotting ln C0/Ct against t to yield a linear relation, and the rate constant was obtained from the slope of the plot. lnC0/Ct = -kt

(2)

Arrhenius equation, eq. (3), was used to calculate the apparent activation energy of the ODS for DBT. lnk = -Ea/RT + lnA

(3)

Plotting lnk against 1/T gave a straight line, from its slope, the value of Ea was calculated.

RESULTS AND DISCUSSION Characteristics of catalyst A schematic illustration of the synthesis pathway of pyridine-based POPs of


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(P[tVPB-VP]) and the construction of amphiphilic catalysts of HPW/P[tVPB-VPx] was shown in Figure 1. FTIR spectra (Figure 2a) confirmed the successful copolymerization of P[tVPB-VPx] that the characteristic peaks of pyridine ring at 1500 cm-1 (C=C) and 1415 cm-1 (C=N),37, 38 the alkyl chain at 2924 cm-1 (C-H), and the benzene ring at 1632 cm-1),39 respectively. After loading HPW, the new bands at 1080, 980, and 898, 818 cm-1, corresponding to the characteristic bands of P-Oa, W=Od, W-Ob-W, and W-Oc-W in the Keggin structure indicated the successful assembly of HPW nanoparticles on P[tVPB-VPx]. More importantly, a small red shift of the C=N stretching vibration of pyridine ring to 1390 cm-1 and a small blue shift of W-Oc-W vibration to 835cm-1 suggested pyridine rings provided a coordination with W cations to enhance the stability of loading HPW particles. The elemental and ICP analytical results (Supplementary Table S3) showed that the weight percentage of W was about 33% in HPW(50)/P[tVPB-VP1].

Figure 1. Synthetic pathway of HPW(y)/P[tVPB-VPx] As 3-dimensional stretching tVPB monomer was the molecular resource of the porosity of P[tVPB-VPx], too many vinylpyridine in the copolymer would result in a low porosity, consequently, low catalyst loading and reduced desulfurization efficiency. Therefore, we balanced this trade-off by adjusting the molar ratio x of VP


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to tVPB. Nitrogen adsorption–desorption isotherms of three P[tVPB-VPx] samples measured

at

77

K

(Supplementary,

Figure

S3a)

showed

that

the

Brunauer–Emmett–Teller (BET) surface area (m2/g) and the total pore volume (cm3/g) decreased with x in the sequence of P[tVPB-VP1] (930, 1.41) > P[tVPB-VP2] (751, 1.06) > P[tVPB-VP3] (351, 0.37) (Supplementary Table S1). The pore size distribution (Supplementary Figure S3b) revealed their characteristic mesoporosity, with an average pore size of 30 - 50 nm for P[tVPB-VP1] and P[tVPB-VP2], but significantly decreased to 8 nm for P[tVPB-VP3]. Therefore, we chose P[tVPB-VP1] as the support for loading catalytic HPW. As listed in Table S1, with HPW loadings, an evident decrease of BET surface area was observed from 930 to about 200 m2/g. Further increasing HPW loading resulted in a step reduction of the pore size (Figure 2b), but no significant surface area reduction was observed, suggesting HPW were mainly supported in mesopores of P[tVPB-VP1]. TEM images further provided a visual image of porosity that the P[tVPB-VP1] was an hierarchical POP with an average pore size of about 2 nm of intrinsic micropores (Figure 2c, insert), as well as amorphous mesopores produced by the cross-linked chains (Figure 2c). After loading HPW, the HPW(50)/P[tVPB-VP1] hybrid demonstrated a similar hierarchical network but with more dark dots (stands for HPW) (Figure 2d), manifesting that HPW was highly dispersed in P[tVPB-VP1]; meanwhile, a large amount of mesopores were still remained.

The

SEM

image

(Supplementary,

Figure

S4)

revealed

that

HPW(50)/P[tVPB-VP1] were composed of stacked fine nanoparticles, with an average size of 100 nm, after ball-milling, HPW(50)/P[tVPB-VP1] were composed of stacked


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fine nanoparticles, with an average size of 20 nm. Thermogravimetric analysis (Supplementary, Figure S5) indicated that HPW(y)/P[tVPB-VP1] could be stable up to 350 °C, which would be benefit for thermal recycle treatment.

Figure 2. (a) FTIR spectra of I. P[tVPB-VP1], II. HPW(50)/P[tVPB-VP1], III. Recycled HPW(50)/P[tVPB-VP1] and IV. HPW; (b) pore size distributions of P[tVPB-VP1] before and after different amounts of HPW loading; TEM images of (c) P[tVPB-VP1] and (d) HPW(50)/P[tVPB-VP1]. ODS performance in bulk system Effect of HPW loading. For a 100 ppm model fuel, the desulfurization efficiency of the pristine HPW was so low that DBT conversation was only 5.54% at 60 min (Supplementary Table S2). Compared with this, all the obtained HPW(y)/P[tVPB-VP1] demonstrated much higher desulfurization efficiency under the same ODS conditions


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(Figure 3a). Among three catalysts, HPW(50)/P[tVPB-VP1] exhibited the best catalytic desulfurization performance, that only after 5 min, the conversation of DBT was already as high as 75%, and a complete conversion was obtained within 30 min. When HPW loading was 30%, the reaction time for the complete conversion was similar, but the initial catalytic rate was lower due to the insufficient catalytic sites. Whereas when HPW loading increased to 80%, the desulfurization efficient decreased significantly due to the pore-blockage and the aggregation of HPW particles. Consequently, the HPW(50)/P[tVPB-VP1] was chosen for the subsequent ODS process. Effect of H2O2/DBT molar ratio (O/S). To elucidate the influence of the amount of H2O2 on the desulfurization, the oxidation of DBT in the model oil under different O/S molar ratios were carried out at 60 ℃ (Supplementary, Figure S7). With the O/S ratio of 5, only a 48% sulfur removal was obtained at 30 min. With the increase in the O/S ratio, the interactions between the oxidant and catalyst were enhanced; this promoted the oxidation rate of DBT. Increasing O/S molar ratio up to 8 increased desulfurization efficiency to 99% in 30 minutes. A 100% sulfur removal rate could be gained with an increase in the O/S ratio to 9 in 30 min to achieve a deep ODS. Consequently, the O/S=9 was selected as the optimal molar ratio. Recycling Stability. When we fixed the reaction time at 30 min, the recycle stability of HPW(50)/P[tVPB-VP1] (Figure 3b) showed its sulfur removal was still about 98.6% at the sixth recycle usage, without significant decrease in catalytic performance. Furthermore, the recovered catalyst showed similar FTIR spectrum (Figure 2a) as the


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fresh catalyst; only 1% weight loss compared with the fresh catalyst in their TG curve (Supplementary, Figure S5c) due to the partial leaching of the HPW particles during the recycling process. As the catalytic efficiency of pure HPW was very poor, 1 wt.% leaching of HPW showed neglected contributions for the whole catalytic performance. Therefore, HPW(50)/P[tVPB-VP1] catalyst behaved heterogeneously in the ODS system. More importantly, the porosity of the recycled catalyst showed almost no change (Supplementary, Figure S6), which ensured its excellent recycle stability in bulk ODS. Effect of the type of S-compounds. Besides DBT, HPW(50)/P[tVPB-VP1] also showed good catalytic ODS performance for BT and 4,6-DMDBT (Figure 3c). At 30 min, the efficiency decreased in the order of DBT (100%) > 4,6-DMDBT (95%) > BT (73%) ； For complete conversion, the reaction time prolonged to 60 min for 4,6-DMDBT, and more than 120 min for BT, which could be attributed to the electron density on the sulfur atom of these organic sulfur40 and the steric hindrance of the methyl groups in 4,6-DMDBT.41


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Figure 3. (a) The effect of the loading amount of HPW of HPW(y)/P[tVPB-VP1] catalysts on the ODS efficiency. (b) The recycle stability of HPW(50)/P[tVPB-VP1] in bulk desulfurization process. The reaction conditions are: 10 g model fuel (with DBT of 100 ppm), 25 mg catalyst, O/S ratio of 9:1, at temperature of 60 °C. (c) Catalyst efficiency of HPW(50)/P[tVPB-VP1] for the ODS of different organic sulfur compounds. The reaction conditions are: 10 g model fuel (with DBT, 4,6-DMDBT, BT of 100 ppm), 25 mg catalyst, O/S ratio of 9:1, at temperature of 60 °C. (d) the effect of the reaction temperature on ODS performance and the Arrhenius equation plotting Effect of the reaction temperature and the limitation of the bulk desulfurization system. The catalytic oxidation rate of DBT on HPW(50)/P[tVPB-VP1] significantly increased with the rise of reaction temperature (Figure 3d). Specifically, at 30 min,


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DBT conversion increased from 39% to 100% when the temperature increased from 30 to 60 °C (Supplementary, Figure S8a). Straight plots of ln(CDBT/CDBT0) against time suggested the first-order dynamics of the catalytic ODS for DBT (Supplementary, Figure S8b). The calculated rate coefficients k at different temperatures observed the Arrhenius equation quite well (Figure 3d), and the activation energy Ea was calculated as 44.54 kJ/mol. Being similar with the reported common Ea values 42, 43, the temperature could not be the key parameter to influence the ODS rate, let alone it has a maximum limitation in case of the thermal decomposition of H2O2. To further enhance the reaction rate under the same consumption of H2O2 oxidant, the only way is to improve the collision frequency between DBT and H2O2 molecules. However, DBT and H2O2 are two immiscible phases, the mass transfer limitation across the interface among triple-phases of water, oil, and solid catalyst should be greatly reduced to facilitate the kinetics. Ultra-fast ODS in the Pickering emulsion catalytic system As the Pickering emulsion catalytic system is one of the most efficient methods that can overcome the mass limitation,44-46 we explored its ODS performance by using HPW(50)/P[tVPB-VP1] catalyst as the emulsion stabilizer. Thanks to the chemical compositions

of

hydrophilic

HPW

and

hydrophobic

P[tVPB-VP1],

HPW(50)/P[tVPB-VP1] hybrid showed amphiphilicity, with the contact angels of H2O and n-octane on HPW(50)/P[tVPB-VP1] as 34.6 o and 31.2 o, respectively (Figure 4a). We optimized the formation conditions such as the amount of catalyst, the volumetric ratios of DBT model fuel, H2O2 aqueous solution and methanol to produce stable


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catalytic emulsion system (Table S4, Supplementary). It was found that when the stabilizer/fuel mass ratio was 1:400, the stability increased with the increase of the oil/water volume ratio; and it produced a stable emulsion system as the ratio was 5:1 (entry 5). Further increasing the oil/water volume ratio led to instability. As methanol is a good solvent for the oxidation product of polar sulfone, we added methanol in the water phase. When the volumetric ratio of the model fuel : H2O2 aqueous solution : methanol was 25:5:1 (entry 12), a stable water-in-oil (W/O) catalytic emulsion system was successfully produced. Changing the stabilizer/oil mass ratio, such as 1:514, when the volumetric ratio of the model fuel : H2O2 aqueous solution : methanol was 50:5:2, a stable water-in-oil (W/O) catalytic emulsion system can also be produced (entry 14). In order to keep the catalyst/oil mass ratio as the same as bulk system, we chose entry 12 as the formation conditions of catalytic emulsion system and tested its stability at the reaction temperature of 60 oC. When the emulsion system stood for 5 minutes after the emulsification, the stratification of oil phase and emulsion phase was formed (Figure 4b) the corresponding microscopic image of the bottom emulsion system showed that the average size of emulsion droplets was about 20-30 um in diameter. The formed emulsion system was so stable that after 1 month, there was no significant demulsification occurred (Figure 4c). Under the same reaction conditions as in the bulk catalytic system, as the DBT concentration in model oil was 100 ppm, the Pickering emulsion catalytic system shortened the ODS time by a half, from 30 min of bulk system to only 15 min (Figure


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5a). To further illuminate the effect of the emulsion catalytic system, we evaluated the ODS performance at higher concentration of 500 ppm with the other conditions kept as the same as that at 100 ppm. The complete DBT conversion time prolonged in both system, which were 45 min and 60 min in emulsion and bulk, respectively (Supplementary, Figure S10). Though the DBT conversion time increased with the sulfur content, the comparison of the catalytic performance in emulsion and bulk systems reflected a significant enhancement by the emulsification , that after 5 min, the conversion of DBT already reached as high as 60% in emulsion system, whereas in bulk system, it was only 15% (Figure 5b). And the complete conversion time of the emulsion system was also shorter than that of the bulk system.

Figure 4. (a) Contact angle measurements of H2O and n-octane droplets on HPW(50)/P[tVPB-VP1] surfaces, (b,c) polarizing microscope images of H2O/n-octane


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catalytic emulsion stabilized by HPW(50)/P[tVPB-VP1], insert are the picture of the emulsions, (b) 5 minutes after the emulsification at 60 oC, (c) after 1 month at room temperature. (d) Schematic illustration of Pickering emulsion catalytic system for simultaneous oxidation and extraction desulphurization.

Figure 5. Comparison of ODS efficiency between the emulsion and bulk systems at (a) 100 ppm, (b) 500 ppm. The reaction conditions are: 10 g model fuel with DBT, 25 mg

catalyst, O/S ratio of 9:1, at temperature of 60 °C.

Synergistic roles of catalyst/emulsion stabilizer/adsorbent in the Pickering emulsion catalytic system for ODS When the hierarchical porous HPW(50)/P[tVPB-VP1] catalyst itself acted as the emulsion stabilizer to produce the Pickering emulsion catalytic system, we can see the multi-functional roles made synergistic effects on ODS (Figure 4d). We summarized the contributions as (1) Highly dispersed water droplets in the model fuel provided extremely large interface to improve the compatibility between two immiscible phases and hence to enhance the mass transfer. (2) Well-assembled catalyst nanoparticles at the interface of the emulsion droplets ensured the active catalytic sites of HPW fully


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exposed to the reagents from both the inner water phase and outside oil phase. (3) As expected, the DBT adsorption capacity of porous HPW(50)/P[tVPB-VP1] was determined as high as 14 mgS/g from its 100 ppm model fuel, which provided an assurance for enhancing the following catalytic ODS. (4) It is worthy to point out that methanol in the water droplets is a good solvent for the oxidation product of polar sulfone (Supplementary, Figure S9). As a result, the efficiency of ODS increased even more when sulfone was removed via extraction once it was produced. Therefore, combining the above advantages together, this Pickering emulsion catalytic system realized the ultra-fast ODS. As listed in Table S5 (Supplementary), compared with those HPW-based catalysts loading on the other porous supports, such as inorganic SiO2,47-49 Al2O3,50 TiO2,51, 52 MOFs,53 G-h-BN,54,

55

and r-GO,56 the special structure of HPW(50)/P[tVPB-VP1]

provided a large BET surface area of 234 m2/g and favorable channels for DBT and H2O2 molecules transferring to catalytic active sites. More importantly, the existing pyridine rings and abundant benzene rings in the network of P[tVPB-VP1], as well as HPW particles provided a synergistic affinity with DBT through π-π interaction, π-complex and S-M binding. In this regard, both suitable porosity and active catalytic sites should be taken into account simultaneously when designing porous catalyst for ODS. As a result, the catalyst dosage in this work is very small among all the similar works and the time of a complete conversion of DBT is shorter than other works. By producing the Pickering emulsion catalytic system, DBT and H2O2 molecules can easily react at the interface of the droplets, i.e. the surface of HPW(50)/P[tVPB-VP1]


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catalyst, to accomplish the ODS reaction. Therefore, the mass transfer limitation was reduced to the greatest extent, resulting in the fastest reaction rate. Compare the ODS efficiency of emulsion catalytic systems reported recently (Table S6), the amphiphilic HPW(50)/P[tVPB-VP1] outperformed most surfactant-modified catalytic emulsion system, , since the long tails of surfactants would partially cover some activity sites of catalysts, and also prevented H2O2 molecules from accessing to the inner nucleus of catalytic sites. The exception was by using [(C12H25)N(CH3)3]9LaW10O36/ [omim]PF6, as both catalyst and extractant, in which 99% DBT conversion could be achieved in 14 min at 30 ℃ with H2O2/DBT=4 (Table S6, entry 5). However, the possible retention of organic surfactants in the desulfurization system would cause secondary pollution, let alone high cost of the ILs. Compared with systems do not contain surfactants (Table S6, entry 10, 11), the catalyst dosage in this study was very small, and the time for complete DBT conversion was greatly reduced. In short, the amphiphilic HPW(50)/P[tVPB-VP1] catalyst was a promising stabilizer of the Pickering emulsion catalytic system, which successfully combined the adsorption, catalytic oxidation and extraction together for ultra-deep desulfurization.

CONCLUSIONS In conclusion, we fabricated a novel type of hierarchical pyridine-based POPs of P[tVPB-VPx] through a copolymerization. With large surface area, suitable porosity and pyridine rings, P[tVPB-VP1] was a good support for loading HPW nanoparticles to produce an amphiphilic porous hybrid catalyst of HPW(50)/P[tVPB-VP1]. Taking


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the advantages of the adsorption enhanced ODS, the time of a complete conversion of DBT from 100 ppm model fuel in bulk phase was only 30 min at 60 oC. Besides, the catalyst could be recycled for at least six times without a significant decrease in activity. Notably, when the amphiphilic HPW(50)/P[tVPB-VP1] was used as the stabilizer to produce a Pickering emulsion catalytic system, the time of a complete conversion of 100 ppm DBT in the model fuel even shortened to only 15 min due to the successful elimination of the mass transfer limitation. Moreover, for higher concentrated DBT of 500 ppm model fuel, that after 5 min, the conversion already reached as high as 60% in emulsion system, relatively, it was only 15 % in the bulk system. This amphipathic polyoxotungstate-pyridine POP catalyst provided a good example for ultra-fast and deep ODS through a pickering emulsion catalytic system, which could be a potential generalized solution for enhancing the heterogeneous catalytic reaction. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website

at

DOI:

Including

Synthesis

and

1H

NMR

spectrum

of

1,3,5-tri(4-vinylphenyl)-benzene; Nitrogen adsorption isotherms and pore size distributions

of

P[tVPB-VPx],

HPW(y)/P[tVPB-VP1],

fresh

and

recycled

HPW(50)/P[tVPB-VP1]; SEM images of HPW(50)/P[tVPB-VP1]; Thermogravimetric patterns

of

HPW(80)/P[tVPB-VP1],

HPW(50)/P[tVPB-VP1],


Recycled

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HPW(50)/P[tVPB-VP1], HPW(30)/P[tVPB-VP1] and P[tVPB-VP1]; Influence of reaction temperature, O/S molar ratio on the conversion of the sulfur compound with HPW(50)/P[tVPB-VP1] and Plotting ln(CDBT/CDBT0) against time at different temperatures; Sulfur-specific GC-MS chromatograms of dibenzothiophene sulfone; Comparisons of the catalytic performance of HPW and HPW(x)/P[tVPB-VP1]

in

ODS at 60 min; Elemental and ICP analytical results of P[tVPB-VP1] before and after HPW loading; Optimization of the formation conditions of the Pickering emulsion catalytic system; Comparisons of the state-of-the-art catalysts on the ODS efficiency of DBT removal. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Jun Hu: 0000-0002-3020-0148 Wenjie Lv: 0000-0003-3289-5198 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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This work is supported by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAC04B01) and the National Natural Science Foundation of China (Nos. 21676080, 21776069, 21878076).

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Catalyst, Emulsion Stabilizer, and Adsorbent: Three Roles In One for

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