Porous Hybrid Nanoflower Self-assembled from Polyoxometallate and

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Kinetics, Catalysis, and Reaction Engineering

Porous Hybrid Nanoflower Self-assembled from Polyoxometallate and Polyionene for Efficient Oxidative Desulfurization Luhong Zhang, Shanshan Song, Na Yang, Xiaowei Tantai, Xiaoming Xiao, Bin Jiang, and Yongli Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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Industrial & Engineering Chemistry Research

Porous Hybrid Nanoflower Self-assembled from Polyoxometallate and Polyionene for Efficient Oxidative Desulfurization Luhong Zhang,†,‡ Shanshan Song,† Na Yang,*,† Xiaowei Tantai†, Xiaoming Xiao†, Bin Jiang†, Yongli Sun†,‡ †School

of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

‡Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin

300072, China

Corresponding author: Name: Na Yang E-mail: [email protected]

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ABSTRACT: A novel and precise ionic self-assembly (ISA) construction strategy for the self-assembly of polymeric quaternary ammonium salts (polyionene) and Anderson-type polyoxometalate (β-Mo8O264−) to form a three-dimensional nanoflower structure was developed successfully and used for oxidative desulfurization (ODS) of fuels for the first time. Furthermore, the morphologies of the self-assembled structures could precisely regulated by varying the reaction conditions, including the concentrations of the polyionene and temperature. Additionally, the nitrogen sorption analysis indicated its porosity with a surface area up to 140 m2 g-1. Remarkably, the nanoflowers showed excellent conversion (98.9%) towards oxidative ODS process with six times recycle. This could be ascribed to the large amount of active sites dispersed over the relatively high specific area, providing contact with reactants in (ODS) process. This facile and innovative fabrication method shed new light on the versatile design and construction of nanostructures.

Keywords:

Oxidative

desulfurization,

Self-assembly,

Polyoxometalate, Polyionene

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Hybrid

nanoflower,

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 INTRODUCTION Sulfur-containing compounds in fuel oils produce sulfate particulate matter and exhaust gas, which lead to acid rain as well as PM 2.5, the main causes for air pollution in recent years.1-3 Consequently, deep desulfurization of fuels has attracted global concerns owing to the increasingly strict environmental legislation enacted by governments.4-6. Hydrodesulphurization (HDS) is currently deemed as a major process used in refining industry.7 Nevertheless, HDS requires severe operating conditions such as elevated temperature and high hydrogen pressure. Besides, it is less effective using HDS for the deep desulfurization of aromatic sulfides including dibenzothiophene (DBT) and its derivatives on account of the low hydrogenation activity.6, 8, 9 In addition of many alternative approaches, ODS has attracted much interest for yielding a low sulfur content to achieve ultra-deep desulfurization under milder conditions comparing with HDS.10-12 Complex aromatic sulfides including DBT and alkyl derivatives can also be oxidized easily and then be adsorbed or extracted by suitable absorbent or solvent to achieve high sulfur removal.13 Catalyst plays a significant role in ODS process. Polyoxometalates are proved to be promising due to their highly controllable composition, shape, redox potential and alkalinity or acidity.14,

15

More importantly, the activation of H2O2, a widely used

oxidant in ODS, can produce effective polyoxoperoxo complexes to oxidize the thiophenic compounds by POMs.16 Preparing heterogeneous POM-based catalysts is highly demanded because pure POMs are difficult to separate and recycle for their 3



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solubility in most polar solvents. However, the main limitations for POM-based heterogeneous catalysts are still diffusion, mass transfer, and accessibility of the active catalytic sites.16-18 In order to solve problem of poor processability and complicated fabrication, the development of preparing and improving POM-based heterogeneous catalysts remains a challenge. One method is the immobilization of POMs on different support structures with favorable porosity or dispersion, such as matrices based on silica compound4, 8, 19, 20

and organic polymers.14, 16, 21 This way favors higher surface areas yet the decrease

of accessibility for catalytic sites in holes is inevitable, thus not always improving the catalytic activity. Furthermore, the whole preparation procedures are frequently complex and costly. The other method is the assembly of POMs with multivalent organic cations in a single step. On the one hand, POMs are deemed as outstanding candidates for self-assembly due to the uniform morphologies, rich architectures, and delocalized multiple charges.22, 23 On the other hand, the ISA of POMs with counter ions can also flexibly control the redox properties, solubility or acid base properties of POMs.24,

25

So far, organic-inorganic hybrids have been generated through self-

assembly method yielding morphologies ranging from one-dimensional (1D) to threedimensional (3D) in a number of studies, including nanofibers,26,

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thin films,28

vesicles,29, 30 nanoparticles,31-33 leaf-like aggregates,34 honeycomb structures,35 and so on. Due to their desirable properties, they have shown excellent performances in various fields. In recent advances for POM-based self-assemblies, various ordered or

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disordered construction of hybrids were reported,25,

36-40

but among them, very few

possessed a porous structure with high surface area, thus limiting the heterogeneous catalytic efficiency. To date, few attempts have been made using ISA method to prepare porous structure with desirable pores by linking POM and organic counter ions, in order to improve catalytic activity in ODS. Herein, a kind of polymeric quaternary ammonium salts called polyionenes and a Mo-based Anderson-type POM were selected. Through the cooperative self-assembly of cationic polymer and anionic POM building blocks, a porous 3D nanoflowers was formed by a direct precipitation method, possessing a moderate specific surface area and less limitation for diffusion. This tactic firstly incorporates the catalytic POM building blocks with polyionenes. Polyionenes are a kind of polymer with ion-containing polymeric quaternary amine chain in the main backbones. As the negative charges in the POMs are highly delocalized, cationic groups in polyionenes can adjust their locations around the POMs spontaneously and flexibly based on the combination of steric effect and multiple interactions. Counterion-mediated electrostatic interaction plays as the dominating part in the self-assembly of multi-charged cations and POMs.23 In this manuscript, a kind of polyionene containing aromatic rings and aliphatic fragments in the main chain and polyoxometalate (NH4Mo7O24) was employed to self-assemble into hierarchical nanoflower structures. A probable mechanism for the self-assembly process was proposed after characterization. These characterizations including SEM, TEM, XRD, BET as well as other techniques also indicated that the hybrid nanostructures have

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relatively high specific surface area and uniformly distributed active catalytic sites on the surface. Upon varying the reaction conditions, the morphology changes were investigated. Besides, the hybrid was applied to heterogeneous ODS of model fuel oil and real diesel. The influence factors of desulfurization, catalytic performance in real diesel, recycling performance, stability of the catalyst, contributions of nanoflower structure and catalytic mechanism were also investigated systematically. The equivalent catalytic performance, superior stability and separability indicated it a promising method to facilely fabricate more delicate and functional nanostructures and use for further application. 

EXPERIMENTAL SECTION Materials. 1,4-bis (chloromethyl) benzene was marketed from Shanghai Meryer

Chemical Technology Co., Ltd. N, N, N', N'-Tetramethylethylenediamine (TMEDA, AR grade) and N,N-Dimethylformamide (DMF, AR grade) was marketed from Tianjin Hengshan Chemical Technology Co., Ltd. N-octane and ethyl ether (AR grade) were marketed in Tianjin Jiangtian Chemical Technology Co., Ltd. Ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 99.9%) was marketed from Tianjin Fuchen Chemical Technology Co., Ltd. Deionized water was purchased from Tianjin Yongqingyuan Chemical Technology Co., Ltd. Benzothiophene (BT, 99%), dibenzothiophene (DBT, 99%), 4,6-dimethyldibenzothiphene (4,6-DMDBT, 99%) and H2O2 (30 wt%) were marketed from Aladdin Reagent Co., Ltd. Acetonitrile (CH3CN, AR grade) was purchased from Sinopharm Chemical Technology Co., Ltd.

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Preparation of the POM hybrid. A polyionene denominated Poly (2, pmethylphennyl-ionene) (PMIn)41 was prepared through a Menshutkin reaction of ditertiary amines and alkyl dihalides (Scheme 1A).42 The experiment details are presented in supplementary material. The hybrid nanoflowers were prepared using PMIn and (NH4)6Mo7O24·4H2O by a direct precipitation strategy. Briefly, PMIn (0.44g, n= 2 mmol) was weighed and dissolved into certain amounts of water to prepare clear solution A, and solution B was prepared by dissolving (NH4)6Mo7O24·4H2O (1.41g, 1.14 mmol) in 10 mL deionized water. Using a syringe to add solution B into solution A slowly under vigorous agitation at 40 ° C in a water bath for 2 h. Upon mixing of solutions, a white precipitate immediately formed. Then, the white slurry was moved into the hydrothermal reactor under a certain temperature for 24 h. The obtained white solids were collected by filtration, followed by successively washing with 30 mL deionized water for at least 3 times. Finally, after freeze drying for 24 h, a white powder was obtained. Scheme 1. (A) Synthesis of PMIn and Ion-exchange Reaction. (B) Schematic Illustration of the Assembly Process.

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Characterization method. The catalysts morphologies were carried out by SEM on Hitachi S-4800 spectrometer and by TEM on a JEOL JEM-2100F spectrometer. The FT-IR of the catalyst was record on a Bio-Rad FTS 6000 FT-IR spectrometer. The XRD analysis was obtained on a Bruker diffractometer by Cu Kα radiation. The XPS characterization was carried out with an ULVAC-PHI, PHI 5000 Versa Probe electron spectrometer. Thermograms were characterized using the Mettler Toledo TGA-DSC 1 TGA instrument at a ± 0.1% precision in a temperature ranging between 50 °C and 600 °C at a heating rate of 10 °C/min under air atmosphere. The Nitrogen sorption isotherms characterization was carried out on an ASAP-2420 analyzer. Before testing, the pretreating was under 100 °C for 3 h for outgassing. The structure of the polyionene was identified by 1H NMR spectra carried out on a VARIAN INOVA 500 MHz spectrometer.

Desulfurization and analysis. To prepare model oil at sulfur content of 250, 500 and 1000 ppm, certain amounts of DBT, 4,6-DMDBT, and BT were mixed with n-

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octane. In a typical run, 10 g model oil, 10 ml acetonitrile and certain amount of catalysts were charged into a 50 mL round-bottom flask in a circulator bath under vigorous agitation 400 rpm. Next, according to the O/S proportion calculation, certain dosage of H2O2 aqueous solution (30 wt%) was charged into the reactor. At short intervals, the upper layer of solution was withdrawn periodically and the sulfur compound concentration was detected by GC-FID. (GC: Gas Chromatograph, Agilent 7890A; HP-5; FID: Flame Ionization Detector, Agilent). Desulfurization experiments of real diesel at a sulfur content of 559.7 ppm was also conducted. Detailed sulfur compounds were listed in Supporting Information Table S1. The experimental procedure was similar to ODS of model oil. During the experiment, samples were taken from the upper layer of solution and analyzed by GC-FPD at different time intervals. (GC: Gas Chromatograph, Lunan 7820; HP-5; FPD: Flame Photometric Detector, Agilent). The total sulfur content was analyzed by an elemental analyzer (Analytical jena, multi EA 5000).

 RESULTS AND DISCUSSION Characterization of POM hybrid nanostructures. Figure 1a-b revealed that the hybrid catalyst consisted of regular flowerlike structures with a diameter of 510 μm. Figure 1c showed the nanoflower had densely compacted petals connecting tightly at the surface. In the supporting information, the elements distribution of C, O, N, Mo in EDX analysis and SEM-EDX mapping (Figure S1) also confirmed the successful hybridation of Mo8O264-,15 C and N elements came from the polyionene and 9



were well distributed on the nanoflower, demonstrating the existence of PMIn on the hybrid catalyst. The Mo and O elements came from the POM cluster and were uniformly dispersed in the nanoflower. Br element was barely detected, demonstrating the completion of the ion-exchange reaction. In addition, Figure 1d-f showed the TEM images of local enlarged nanoflowers, the POM cluster could be identified by darker color and lattice fringes, demonstrating the homogeneous dispersion of Mo8O264- part and low-electron-contrast polyionene part in the nanostructured skeleton.

Figure 1. SEM images at different magnifications of (a, b) hybrid nanoflowers, (c) nanopetals (a local enlarged image of panel b), and (d, e, f) TEM images at different magnifications of hybrid nanoflowers. To further understand the formation mechanism of the POM hybrid nanostructure, the morphologies over different hydrothermal incubation time was monitored by SEM. In Figure 2 (a, e, i), after stirring for 2 h at 40 °C without hydrothermal incubation, the catalyst showed blocky structure. The block was composed of small rough 10


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nanoparticles. After the hydrothermal incubation for 6 h, the nanoparticles began to aggregate together to form layer structure which was the embryonic form of the petals. After 12 h, the layers gathered together at a core, which resembled a Rosa multiflora in the nature. After 24 h, the petals on the surface were more tightly aligned and the anisotropic growth of petals contributed to a perfect spherical nanoflower, resembling a peony in its full blossom.

Figure 2. Formation process of POM-PMIn nanoflowers with different hydrothermal incubation time. SEM images at (a, e, i) 0 h, (b, f, j) 6 h, (c, g, k) 12 h, and (d, h, l) 24 h. Panels e-i, f-j, g-k and h-l are the high-resolution images of the samples of panels a, b, c and d, respectively. (Reaction conditions: PMIn concentration, 0.04 mmol/ml; temperature, 80°C) The results showed the evolution of the nanoflowers at different time. This might be explained as the self-assembly process tended to develop spontaneously to a lowest energy state. As time progressed, the electrostatic interaction between the POM and 11



PMIn enhanced, in addition, the π−π stacking and steric interaction due to aromatic backbone of PMIn further led to the tendency to form into anisotropic flower architectures.22

FT-IR, XRD, XPS, and TG analyses. In Figure 3a, characteristic peaks of PMIn was observed at 3416 cm–1 (O-H stretching), 1638 cm–1 due to bound water existed in the ionene,43 while the band at 1485 cm–1 for methylene scissoring vibration (CH2) was also observed.44 The band at 1030 cm–1 corresponde to bending vibration of C-N group.45 Moreover, the band at 3016 cm-1, 1619 cm-1,877 cm-1 were ascribed to the backbone vibration and C-H bending vibration of benzene.46 In Figure 3b, the strong and sharp bands at 912 cm-1and 870 cm-1 were the characteristic peaks of Mo7O246-.47 In Figure 3c, the characteristic peaks for Mo7O246- disappeared, and the bands were likely to be consistent with that of Mo8O264-.15, 48 991 cm-1and 914 cm-1 could be seen as terminal vibration of Mo=O, 886, 857, 779 and 653 cm-1 was assigned to the Mo-OMo vibration.49-51 The band between 600-400 cm-1 was due to the bending vibration of Mo-O.52 The bands at 3016 cm-1, 1485 cm-1 and 1030 cm–1 could still be observed in hybrid nanostructure, indicating the existence and maintenance of the polyionene.

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Figure 3. FT-IR spectra of (a) polyionene, (b) (NH4)6Mo7O24·4H2O, (c) POM hybrid nanoflowers, (d) recycled hybrid nanoflower, and (e) hybrid nanoflower after calcination. Mo7O246- reacts in solution as the following reaction: 8Mo7O624― + 20H + →7Mo8O426― + 10H2O

Figure 4. XRD spectra of (a) (NH4)6Mo7O24·4H2O, (b) POM hybrid nanoflowers and (c) recycled hybrid nanoflowers kept for 60 days after ODS. Figure 4 illustrated the XRD spectra of (NH4)6Mo7O24·4H2O and the hybrid catalyst. 13



The characteristic peaks of (NH4)6Mo7O24·4H2O in Figure 4a disappeared in the case of hybrid catalyst.53 For the hybrid nanoflower (Figure 4b), at 2θ = 9.63°, a new Bragg peak appeared with a d spacing of 0.917 nm, approximately equal to the theoretical value for the primary single unit of the anion building blocks. This indicated that during the self-assembly process, novel crystalline structures formed and POM clusters were uniformly distributed in the whole hybrid structure, which also corresponded to the EDX analysis. This was further demonstrated from the TEM image (Figure 1f), in which the primary units were clearly observed in the secondary structure. On the basis of the analyses, it was inferred that the nanoflower possessed high dispersion of POM clusters, simultaneously improving the accessibility of active sites.

Figure 5. TGA curves of (a) PMIn and (b) POM hybrid nanoflowers. In Figure 5, under 100 °C, the release of moisture and bound water led to slight weight loss. At 600 °C, the weight loss of PMIn almost reached 100%, thus it could be concluded in Figure 5b, the weight loss of the hybrid was ascribed to the complete decomposition of organic PMIn part at temperature over 600 ° C. The residual part 14


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accounted for 47.8% of total weight could be the calcinated product of Mo8O264-, which composed of MoO3.54 The weight loss was slightly greater than the calculated weight (44.2%). This was due to the loss of moisture at relatively low temperature. Under the air atmosphere, the initial decomposition temperature of the hybrid was around 200°C, suggesting its good thermal stability.

Figure 6. XPS spectra of hybrid nanoflower. (a) Survey spectrum and (b) Mo 3d profile. To further investigate the chemical structure of the hybrid, the XPS spectrum (Figure 15



6a) showed four main peaks at 285.0 eV, 398.3 eV, 531.8 eV and 232.7 eV, proving the existence of C, N, O and Mo elements. C elements mainly existed in the benzene ring and carbon skeleton, taking up to 63.31atm.%. The C 1s profile (Figure S2) was separated into binding energy peaks of 284.6, 286.2 and 288.5 eV, which were assigned to the C element in carbon chain (C=C), benzene ring backbone (C=C-C) and quaternary ammonium group.55 O element mainly came from POM cluster, and accounted for 16.77atm.%. As illustrated in Figure S2, the multi-peaks in O 1s profile was probably due to the presence of three kinds of in Mo8O264−, including bridging and terminal oxygen.56

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Figure 4b showed the two peaks at 232.39 and 235.53 eV,

corresponding to the MoVI in Mo 3d profile.58, 59

Figure 7. SEM images of calcinated hybrid nanostructures with different magnification. As shown in Figure 7, after calcination, the nanoflower collapsed into balls stacked by small polygon plates, whose side length was 200-400 nm. According to the XRD spectrum in Figure S3, the presence of molybdenum oxide was clearly observed.60 FI-

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IR spectrum in Figure 3e exhibited one strong band at 579 cm−1, corresponding to the vibrations of the metal-oxygen, further proving the left part was composed of MoO3.54 From the results above, it could be inferred that the PMIn played a part as a glue to adhere the petals together. In addition, this demonstrated the petals were mainly composed of Mo8O264-, π−π stacking and steric repulsion further led to the selfassembly formation.61 According to the results above, one of the most probable mechanisms for the selfassembly process was suggested as following. The formation of nanoflower went through three periods as shown in Figure 2: At an early stage (step 1, Figure 2a, e, i), linear molecular structure of PMIn formed primary rough nanoparticles with POM, which functioned as a cross-linker.15 The main driving force was electrostatic interaction between the multivalent anion and the cationic PMIn. In the second step (step 2, Figure 2b, f, g), during the hydrothermal incubation, the primary particles aggregated as a result of the π−π stacking between benzene rings in PMIn and steric effect caused by rigid aromatic structure. The kinetically controlled growth of the primary particles aggregates led to separate petals to form. In the last stage (step3, Figure 2 c, j, k), PMIn functioned as a glue to adhere adjacent petals together. Finally, anisotropic growth resulted in a full formation of the well-ordered, uniform

and

perfect flower shape62 (Figure 2 d, h, l). Scheme 1B featured the whole evolution process. The Mo8O264- cluster mainly function as the catalytic active parts in the ODS process. 17



Figure S1 showed the presence and uniform distribution of the Mo and O elements throughout the sample, indicating high dispersion of active catalytic sites on surface of the nanostructure. The element composition and chemical states were further proved by XPS (Figure 6a). The structural characterization and morphological observation (Figure S1, Figure 6a, Figure 1d-f and Figure 7) testified that the self-assembly process provided the arrangement and dispersion of active sites.

Incubation temperatures and PMIn concentration on the effect of nanostructure morphology. Figure 8 indicated that the incubation temperature also had great impact on the morphonology of the nanoflowers. At 50 °C, the hybrid showed crumby structure with no regular shape. The microstructure was still particle stacking which resembled the structure without hydrothermal incubation. At 80 °C, a perfect peony nanoflower with an average diameter of 5-8 μm formed. The diameter of nanoflower decreased (3-5 μm), at a higher temperature, 100 °C. Meanwhile, the nanoflower looked flatter with loose petals. At 150 °C, the flower became even smaller (1.5-2 μm), with faulty shape and sparse petals.

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Figure 8. Figure caption SEM images of POM-PMIn nanoflowers at different hydrothermal incubation temperatures. (a, e, i) 50 °C, (b, f, j) 80 °C, (c, g, k) 100 °C, and (d, h, l) 150 °C. Panels e-i, f-j, g-k and h-l are the high-resolution images of the samples of panels a, b, c and d, respectively. (Reaction conditions: PMIn concentration, 0.04 mmol/ml; hydrothermal time, 24 h) This indicated that without enough temperature, the self-assembly reaction rate was extremely slow or even nonreactive. However, excessive temperature might lead to an early termination of the self-assembly process, thus the nanoflower appeared to collapse as the temperature increase.

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Figure 9. SEM images of POM- PMIn nanoflowers at different concentrations of PMIn. (a, e, i) 0.02 mmol/ml (NF-1), (b, f, j) 0.04 mmol/ml (NF-2), (c, g, k) 0.1 mmol/ml (NF3), and (d, h, l) 0.2 mmol/ml (NF-4). Panels e-i, f-j, g-k and h-l are the high-resolution images of the samples of panels a, b, c and d, respectively. (Reaction conditions: reaction temperature, 80 °C; hydrothermal time, 24 h) To investigate the relationship between the morphology and concentration of PMIn, the concentration of (NH4)6Mo7O24·4H2O was kept constant. We denoted the concentration at 0.02 mmol/ml, 0.04 mmol/ml, 0.1 mmol/ml, 0.2 mmol/ml as NF-1 to NF-4. From NF-1 to NF-4. As shown in Figure 9, with the increase of polyionene concentration, the average diameter of nanoflower firstly increased to a top at 0.04 mmol/mol and then decreased. At the same time, as the concentration increased, the petals became tighter at 0.04 mmol/ml and then went sparse. The change of flower size and petal density on the surface caused by concentration variations directly led to a change in surface area and average pore size, of which the trend was consistent with 20


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Figure 10 and Table 1. According to Figure 10, the Nitrogen sorption isotherms were type IV with a clear hysteresis loop, showing that the hybrid catalyst had porous structure. BJH pore size distributions of the catalysts showed a wide distribution of mesopore size ranging from 10 - 100 nm. Brunauer–Emmett–Teller surface areas (SBET), pore volumes (Vp) and average pore sizes (Dav) were listed in Table 1. The result was in line with the compactness of the nanoflowers, as the morphologie comparation shown in Figure 9. The above results indicated that the surface and pore volume could be facilely and precisely controlled through varying the concentration of PMIn.

Figure 10. Nitrogen sorption isotherms (left) and pore size distributions (right) of 21



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hybrid nanoflower prepared at the PMIn concentration of (a, b) 0.02mmol/ml, (c, d) 0.04mmol/ml, (e, f) 0.1mmol/ml, and (g, h) 0.2mmol/ml. Table 1. Textural Properties and Catalytic Performance of Catalysts in ODS Vpb (cm3g-1)

Davc (nm)

Time (min)

Conversion (%)

(NH4)6Mo7O24·4H2O 5

-

-

-

15.2

NF-1

85.0

0.34

15.9

120

95.0

NF-2

140.4

0.43

12.1

120

98.9

NF-3

55.7

0.34

24.4

120

94.3

NF-4

28.8

0.13

18.2

120

90.0

Samples

a

SBETa (m2g-1)

BET surface area. b Pore volume. c Average pore size

Catalytic application of hybrid nanoflower in ODS. To test the catalytic performance of the hybrid nanostructure, the nanoflower was employed as catalyst for ODS of model fuel oil with initial sulfur content of 1000 ppm. As demonstrated in Table 1, the conversion in ODS process for the original POM was rather low because of the limitation of mass transfer. All NF1-4 demonstrated a high conversion in ODS: 95.0%, 98.9%, 94.3% and 90.0%, respectively. Comparing all the above catalysts, the excellent desulfurization performance was related to the large surface area of the NF1-4 catalysts, among which NF-2 showed a best desulfurization efficiency. This could be ascribed to NF-2 had the maximum compactness as well as best dispersion of active sites on the surface, providing more contact with reactants. This explanation further matched the morphology shown in Figure 9 and the BET analysis. Due to the best

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performance of NF-2 in ODS, it was chosen to systematically investigate the influence factors of desulfurization.

Figure 11. Conversion of S-compound with catalyst. (Reaction conditions: (a) mcatal, 40 mg; H2O2/S, 5/1; (b) mcatal, 40 mg; T, 50 °C; (c) H2O2/S, 5/1; T, 50 °C; and (d) mcatal, 40 mg; H2O2/S, 5/1; T, 50 °C) Due to the best performance of NF-2 in ODS, it was chosen to systematically investigate the influence factors of desulfurization. It was noteworthy that the DBT conversion at 120 min increased greatly from 88.0% to 98.9% (Figure 11a) as the reaction temperature increased from 30 °C to 60 °C. This could be due to the desulfurization reaction rate was restricted by reaction kinetics at rather low temperature. At the same time, it was also due to the existence of kinetic competition 23



bewteen H2O2 thermal decomposition and desulfurization reaction while increasing the temperature. Therefore, the optimum temperature was 50 °C. Figure 11b demonstrated the influence of H2O2 dosage on sulfur removal efficiency. The theoretical H2O2/S ratio was 2, but increase the H2O2/S ratio to 6, the desulfurization rate of DBT in 60 min increased remarkably from 72% to 96%, and reached up to 98.9% in 120min, demonstrating an increase of H2O2 dosage elevated the desulfurization rate. This could be explained as the enhancement of interactions between the oxidant and catalyst as the H2O2 dosage increased. Through economically trade-off, H2O2/S= 5 was chosen as the final molar ratio. As expected, the sulfur removal rate was significantly elevated by adding more catalyst into the system due to the increase of contact area and catalytic sites during the reaction (Figure 11c).5 The conversion reached 95% at 120 min with the dosage of 40 mg, which was deemed as the optimal dosage. To study the catalytic reactivity for different thiophenic compounds, DBT, 4,6DMDBT and BT were used to prepare 1000 ppm sulfur content oil. As shown in Figure 11d, under the same reaction conditions, the sulfur removal rate was in the order DBT>4,6-DMDBT>BT. It was wildly approved that the oxidation of sulfur compounds by H2O2 accord with the mechanism of electrophilic addition.63, 64 This explained the reactivities increased with the increases of electron density of S atoms. Although the electron donor effect of alkyl substituents might increase the electron density of S atoms, the steric hindrance effect caused by alkyl substituents led to the difficulty of interaction 24


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between S atom and oxidant. As a result the sulfur removal rate followed the order above.65 Model oil at various DBT concentrations including 250 ppm, 500 ppm and 1000 ppm was also investigated. As revealed by Figure S4, model oil at higher sulfur content showed a slightly higher conversion in the ODS process. For example, in 60 min, the conversion of sulfur for 1000, 500 and 250 ppm model oils were 96.0%, 95.8% and 93.0%, respectively. However, the slight difference among the three model oils is almost negligible. Besides, they all achieved a sulfur removal rate of over 98% after 120 min. In short, initial sulfur content of model had little impact on the efficiency of ODS for this nanostructure catalyst. Catalytic application of hybrid nanoflower in real diesel. The catalytic performance of the hybrid nanostructure in real diesel was also investigated to test its potential in industrial application. Hydrogenation Diesel (HD) with an initial sulfur content of 559.7 ppm was used, the detailed sulfur components were listed in Table S1.

25



Figure 12. Conversion of S-compound with catalyst for real diesel. (Reaction conditions: mcatal, 40 mg; H2O2/S, 5/1; T, 50 °C;) During the catalytic process, samples were taken at certain time intervals and analyzed by GC-FPD, as shown by Figure 12. For the original diesel (0min), the peaks at 14-22 min belonged to BT and its derivatives, the peaks at 22-35 min was ascribed to DBT and its derivatives. As the reaction proceeded, all peaks of the sulfur compounds were decreasing gradually without any appearance of new peaks. This revealed that the original thiophenic compounds in the diesel were converted to sulfones and then extracted by acetonitrile. Figure 12 demonatrated that almost all BT, DBT and their derivatives were completely removed within 120 min, achieving a sulfur content of 7.55 ppm.

Recycling performance and stability of the catalyst. To investigate the recycling performance of the NF-2 nanoflower, repeated experiments were carried out. After each cycle, the catalyst was recycled through filtration, washing and vacuum drying. For the next cycle, fresh H2O2, acetonitrile and model oil were added. Figure 13 showed the nanostructure of the recycled catalyst was still kept. The FT-IR characterization also demonstrated characteristic bands were still preserved after six cycle compared with the original spectrum (Figure 3d). XRD spectrum (Figure 4c) showed no noticeable change in the crystalline structures of the catalyst after reaction. Moreover, XPS fully spectrum (Figure S5) showed no obvious impurities, and the element composition and states showed no noticeable change. Besides, the Mo 3d peak

26


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at 235.53 eV explained that the oxidation state remained unchanged. After six repeated cycles, it could be observed no noticeable decrease in sulfur conversion rate (Figure S6). These all indicated that the catalyst was recyclable.

Figure 13. SEM images at different magnifications of recycled nanoflowers after six cycles.

Contributions of nanoflower structure to desulfurization efficiency. To determine the contributions of flower-like nanostructure on ODS efficiency, pure (NH4)6Mo7O24·4H2O and NF-2 catalyst without hydrothermal treatment was applied in ODS reaction, as shown in Figure 14. Using (NH4)6Mo7O24·4H2O as catalyst, the homogeneous POM catalyst could remove only 15.2% of sulfur contents in DBT. Comparing with (NH4)6Mo7O24·4H2O, the catalysts combining polyionene and POM showed an obvious enhancement of sulfur removal rate. Both NF-2 and NF-2 without hydrothermal treatment showed an enhanced ODS efficiency. This was probably due to the heterogenous catalytic system, as demonstrated by previous studies.66 However, NF-2 without hydrothermal treatment could only reach 78% desulfurization rate in 120 min, inferior to that of NF-2. This result could be ascribed to the amorphous non-porous block structure without self-assembly process. For NF-2 27



nanoflower structure, the porous structure and high dispersion of active sites provided more catalytic sites. Ultimately, the superior performance of NF-2 comparing with the others confirmed the excellent synergistic effect of nanoflower structure and heterogeneous system to enhance the catalytic activity.

Figure 14. Conversion of S-compound with different catalysts. (Reaction conditions: mcatal, 40 mg; H2O2/S, 5/1; T, 50 °C;)

Probable mechanism for the oxidation of DBT. On the basis of the experimental results and previous report,47 one probable mechanism of desulfurization process the hybrid nanostructure was proposed in Scheme 2. The catalysts possessed a porous structure, promoting the accessibility of reactant to active catalytic sites. In the reactor, model oil was on the upper phase, and then catalyst, acetonitrile and H2O2 were added in turn into the catalyst phase. After vigorously stirring, catalyst appeared at the interface, and through the nucleophilic attack of the bridging oxo ligands of Mo8O264− clusters by H2O2, the polyoxoperoxo complex species [Mo8-nO26-3n{MoO2(O2)}n]4− 28


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formed.67 Finally, the sulfur compounds could be oxidized by [Mo8-nO264−

3n{MoO2(O2)}n]

into corresponding sulfones.

Scheme 2. Probable Mechanism for the Oxidation of DBT.



CONCLUSIONS

In this study, a well-ordered hybrid nanostructure with both high porosity and well distribution of catalytic sites was fabricated by a simple ISA method. The multivalent electrostatic attraction played a key role for the formation of POM and PMIn primary unit, and then self-assemblied at a ceratin condition into monolithic structure. The π−π stacking and steric interactions further led to the complete formation of hybrid nanostructure. Moreover, it was proved that the morphologies of POM-PMIn nanoflower could be adjusted by the altering the concentrations of PMIn and the reaction temperature. Because of the relatively high specific area and catalytic active sites distribution, the catalyst exhibited excellent performance and recyclability in ODS 29



of model oil, achieving 98.9% conversion of DBT within 2 h. This work described a novel and facile ISA strategy to synthesize porous POM-polyionene hybrids, and the whole process need no template agents, providing a promising route for the construction of hierarchical architecture in a versatile manner.



ASSOCIATED CONTENT

Supporting Information Figure S1, SEM image of hybrid nanoflowers and SEM element mapping analyses of nanoflowers; Figure S2, XPS curves of hybrid nanoflower; Figure S3, XRD spectra of POM hybrid nanostructures after calcination; Figure S4, conversion of S-compound with NF-2 catalyst at different sulfur content model oil; Figure S5, XPS spectra of POM hybrid nanostructures after recycle; Figure S6, reusability of the catalyst in the ODS of DBT; Table S1, sulfur compounds of the diesel in this work. 

AUTHOR INFORMATION

Corresponding Author *E-mail:

[email protected] Tel./Fax: +86 2227400199.

ORCID Na Yang: 0000-0003-4888-5971 Luhong Zhang: 0000-0001-5190-2918

Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS 30


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We are grateful for the financial support from National Key R&D Program of China (No. 2016YFC0400406)

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Porous Hybrid Nanoflower Self-assembled from Polyoxometallate and

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