Synthesis of vanadium phosphorus oxide catalysts assisted by deep

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Kinetics, Catalysis, and Reaction Engineering

Synthesis of vanadium phosphorus oxide catalysts assisted by deep-eutectic solvents for n-butane selective oxidation Bin He, Zihang Li, Huiling Zhang, Fei Dai, Kang Li, Ruixia Liu, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06010 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Industrial & Engineering Chemistry Research

Synthesis of vanadium phosphorus oxide catalysts assisted by deep-eutectic solvents for n-butane selective oxidation Bin He a, b, Zihang Li a, Huiling Zhang a, Fei Dai a, Kang Li a, Ruixia Liu a, b *, Suojiang Zhang a, * a Beijing

Key Laboratory of Ionic Liquids Clean Process ,CAS Key Laboratory of Green Process and Engineering,

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Beijing, 100190, P.R. China b University of Chinese academy of sciences, Beijing 100049,PR China *Corresponding

author: E-mail: [email protected] [email protected];

1 ACS Paragon Plus Environment

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Abstract: The effects of choline chloride/oxalic acid deep eutectic solvents (ChCl/OA DES) as a green and effective promoter assists the synthesis of vanadium phosphorus oxides (VPO) catalysts for the selective oxidation of n-butane to maleic anhydride was investigated in detail. A combination of characterizations with the performance was considered to understand the essential effects of DES. DES plays a role of crystal induced agent and structural modifier, facilitating the formation of a single-crystal structure on the surface of precursor, correspondingly, topological transformation to the single-crystal active phase under the activation conditions accompany with the decomposion of DES. It is suggested that ChCl/OA DES can interact with V2O5 and form of a new V complex, which affects the reaction between V2O5 and H3PO4. Meanwhile, the ChCl/OA DES could regular the surface chemical state and redox characteristic, resulting in the enhancement on the catalytic performance of VPO.


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Keywords: Deep eutectic solvents (DES), Vanadium phosphorus oxides (VPO), n-butane selective oxidation, Maleic anhydride (MA)

1. Introduction Selective oxidation of light alkanes (C1~C5) to valuable functionalized chemicals has been widely studied due to their economy and environmental friendly. Up to now, the most successful example is the partial selective oxidation of n-butane to maleic anhydride (MA) by vanadium phosphorus oxides (VPO) catalyst in industry.1-4 As the catalytic system is fairly complicated due to this reaction involves 14 electrons transfer and 3 insertion of oxygen that result the presence of parallel reactions and consecutive reactions.5,

6

Therefore, it is a challenge to achieve high

selectivity of MA under high n-butane conversion and attract enormous attention.7-11 Research shows that the crystal structure and surface chemistry properties (the P/V, valence state, content of lattice oxygen, etc. on the VPO surface) are crucial for VPO catalysts, which have huge impact on the activation of C-H and transfer of lattice oxygen.12-15 Hutchings et al. suggested that doping metal promoters was considered as an effective way to improve the performance, by which it modulates the surface chemistry characteristics.16-20 Based on this argument, a number of metal cations have been added to VPO to enhance the catalytic performance.8, 19, 21-23 The number of defects of active phase was increased by doping Nb.8 Mo as an electronic promotor could possess strong Lewis acid sites and enhance the lability of the lattice oxygen.24 Meanwhile, organic polymer has been applied into the modification of VPO to perfect the crystallization. Polyethylene glycols (PEGs) was used as template agent to obtain high surface area and well crystallized VPO catalyst.10, 25

Bartley and his co-workers used diblock copolymers, poly(acrylic acid-co-maleic acid) (PAAMA)

and poly(styrene alt-maleic acid) (PSMA) as a structure directing agent to prepare highly crystalline 3 ACS Paragon Plus Environment


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VPO. They suggested that PAAMA and PSMA could passivate the (001) plane of the precursor VOHPO4·0.5H2O, leading to very regular rhomboidal crystals and show an improved performance for the selective oxidation of n-butane to MA.12, 13 Inspired by these works, a strategy on developing a green and efficiently modifying agent to adjust the crystal structure and modulate the surface chemistry characteristics simultaneously is a desirable route for the preparation of efficient VPO catalysts. Ionic liquids (ILs) have been widely used in the synthesis of functional materials due to their particular physicochemical properties, such as abundant hydrogen network, strong solubility for organics/ inorganics substances.26-31 Recently, we firstly used Fe-based ILs as additives during the synthesis of VPO catalyst and achieved an excellent catalytic performance for selective oxidation of n-butane to maleic anhydride (MA) due to the synergistic effect existing between the structure-oriented cations and metal anions.32 In order to decrease the cost of VPO catalyst, in this study, we proposed a new strategy for synthesizing efficient VPO catalyst with metal-free deep eutectic solvents (DESs) as additives, which is nascent class of less expensive, more simply synthesis, non-toxic and biodegradable ILs33-38. DESs are multifunctional in assisting material synthesis and can modulate nucleation growth mechanisms by charge neutralization, modification of reduction potentials and passivation of particular crystal faces, dictating growth along preferred crystallographic directions.29, 33, 39 To our knowledge, DES-assisted synthesis of VPO catalysts and its application in partial selective oxidation of n-butane to MA have rarely been reported in the open literature. In attempt to achieve a high selectivity of MA under high n-butane conversion, here, we selected ChCl/OA DES as a green and effective promoter to assist the synthesis of VPO catalysts. 4 ACS Paragon Plus Environment

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The effects of DES to crystallographic structure, morphology and the surface chemical characteristics, including the chemical state and redox characteristic were studied in great detail in this paper. Meanwhile, the performance of VPO catalysts with different amount of DES assisted was combined with their structure. This work presents a novel method to modified the VPO catalysts by tuning their structure and surface properties, which could achieve high conversion with high selectivity.

2 Experimental 2.1 Materials Vanadium pentoxide (V2O5, >99%) was from Xiya Reagent, benzyl alcohol (>99%) and isobutanol (>99%) were from Sinopharm Chemical Reagent Co., Ltd, phosphoric acid (85%, H3PO4) was from Xilong Scientific. Choline chloride (ChCl, 99%) and oxalic acid dehydrate (OA, 99%) were from Sinopharm Chemical Reagent Co., Ltd 2.2 Synthesis of ChCl/OA DES In a typical synthesis process, the ChCl/OA DES was obtained by simply mixing ChCl and OA in the molar ratio of 1:1 at 80 oC~120 oC with magnetic stirring in 20 mL bottle under sealed state until a transparent liquid was formed. 2.3 Preparation of the VPO precursors The precursors VOHPO4·0.5H2O were prepared according to the following procedure. 10 g V2O5 with 20 mL benzyl alcohol, 80 mL isobutanol and specific amount DES were mixed under 135 oC

reflux for 3h. Then 7.53 mL 85%-H3PO4 was added and the solution was heated and refluxed at

135 oC for 16 h. After that, the slurry was collected by filtration and washed using ethanol for 3-4 times. The solid was dried in an oven at 120 oC overnight to obtain the precursor VOHPO4·0.5H2O. 5 ACS Paragon Plus Environment


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For the DES modified materials, V2O5 was reacted in benzyl alcohol / isobutanol with different amounts of ChCl/OA DES. The calculated amount of DES was added with benzyl alcohol / isobutanol. Henceforth the precursors are named PVPO-DES-x, where x representing the additional amount (g) of ChCl/OA DES. The blank sample is named PVPO-Blank. 2.4 Preparation of the VPO catalysts The catalysts VPO, composing of vanadyl pyrophosphate phase (VO)2P2O7, were prepared from the precursor VOHPO4•0.5H2O. The precursors were pressed into pellets, and crushed to 20-40 mesh sheets. After that, 3 mL precursor was enclosed in the reactor tube, and then the feed gas, consisting of n-butane (1.5 vol%), O2 (17 vol%), and N2 was fed under a gas hourly space velocity (GHSV) of 2000 h-1. The temperature was raised to 430 oC at a rate of 2 oC min-1 and heated for 12 h. After that, the precursor was completely transformed the catalysts. Similarly, the corresponding catalysts are denoted VPO-DES-x, where x representing the additional amount (g) of ChCl/OA DES. The blank sample is named VPO-Blank. 2.5 Catalyst testing The testing of different catalysts was carried out in aforementioned fixed-bed reactor. The reaction temperature was 420 oC with 1.5 vol% n-butane under a GHSV of 2000 h-1 until their performance was stabilized. The output gas from the reactor was analyzed via an on-line gas chromatography (Shimadzu GC-2010 plus) with “ two valves and three columns ” system. The n-butane was analysed by FID-GC with capillary column (alumina substrate, 30 m), while CO, CO2 in the gaseous products are analyzed by a TCD detector with TDX-1 column (carbon molecular sieve substrate) and molecular sieve 5A (0.6m) column. The catalytic performance is presented by conversion of n-butane, MA yield, and MA selectivity, which are defined as follows: 6 ACS Paragon Plus Environment

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n-butane conversion =

MA selectivity =

[C 4 H10 ]in  [C 4 H10 ]out [C 4 H10 ]in

[C 4 H10 ]in -[C 4 H10 ]out -[CO x ]out [C 4 H10 ]in -[C 4 H10 ]out

MA yield =[C4 H10conversion]  [MA selecivity]

2.6 Characterization Methods The formation of the ChCl/OA DES was verified by a Fourier Transform Infrared Spectroscopy ( FT-IR, Thermal Electron Corporation ), 1H NMR spectroscopy ( AVANCE III ) and differential scanning calorimetry ( DSC, DSC1, Mettler-Toledo ). The crystal phases of precursors and catalysts were assessed by powder X-ray diffraction ( XRD ) diffractometer (Rigaku Smart Lab X-ray powder diffractometer ) with Cu Kα radiation (9 kW, λ=0.15406 nm). And the scanned 2θ range is 10o ~ 40 o. The surface morphology and the structure of all samples were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). SEM was acquired using Hitachi SUB8020 instrument, operating at voltage of 5 kV. TEM was carried out in a JEOL JEM-2100 apparatus. Weight loss and phase-transition temperature were conducted via thermogravimetry (TG) and differential thermal analysis (DTA) on a Seiko TG/TGA SSC 5000 analyzer. The precursors were heated from ambient temperature to 900 oC at rate of 10 oC·min-1 in a N2 flow of 40 mL·min-1. The chemical environment of V and O were obtained using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi electron spectrometer, Thermo Fisher Scientific), with 300 W Al Kα radiation. For the analysis, the C1 peak position was set at 284.5 eV and used as reference to position the other peaks while the fitting of the XPS peaks was done by least squares using 7 ACS Paragon Plus Environment


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Gaussian-Lorentzian peak shapes. The catalysts phase were also confirmed by Raman spectra, which obtained by a LabRAM HR800 (Horiba Jobin Yvon, France) spectrograph at ambient temperature and normal pressure. The excitation light source of laser beam was from an argon ion laser in 633 nm wavelength. The scan range was from 200 cm-1 to 2000 cm-1. All samples surface area and pore size were measured by N2 adsorption-desorption isotherms at liquid nitrogen temperature by a Micromeritics ASAP2020 machine. The catalysts were degassed at 120 °C for 12h prior to analysis. Total reduction property of the catalysts was measured by H2 temperature-programmed reduction (H2-TPR, AutoChem II 2920). Approximately 45 mg samples were thermally treated from room temperature to 900 °C under a 10% H2/Ar gas at the heating rate of 10°C·min-1. The surface acidity was investigated by NH3 temperature-programmed desorption (NH3-TPD, AutoChem II 2920). Approximately 40 mg samples were thermally treated from room temperature to 700 °C under a 10% NH3/He gas at the heating rate of 10°C·min-1.

3 Results and discussion 3.1 Synthesis of ChCl/OA DES The ChCl/OA DES was obtained by mixing ChCl and OA in the molar ratio of 1:1 at 80~120 oC.

The formation of the ChCl/OA DES was verified by FT-IR (Figure S1a), 1H NMR (Figure S1b)

and DSC (Figure S1c). As shown in Figure S1a, the characteristic absorption peaks of –NH2 and C-N stretching vibration derived from the ChCl and –CO, C-O, -OH and –COOH stretching vibration derived from OA were detected in the ChCl/OA DES. The slight shifts of –OH and –NH2 prove the existence of interaction between the ChCl and OA. Moreove, as described in Figure S1b, the 8 ACS Paragon Plus Environment

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chemical shift of the bond water hydrogen atoms on oxalic acid dehydrate moved from δ 4.71 to δ 4.61, which was ascribe to the hydrogen bonding between the Cl- and bond water hydrogen atoms from ChCl and oxalic acid dehydrate, respectively. The formation of DES was further confirmed by DSC analysis (Figure S1c). The endotherms at 212 °C for the oxalic acid dehydrate and 304 °C for the ChCl showed the onset melting point of the oxalic acid dehydrate and the ChCl, respectively. The onset melting point of the synthesized DES is −20 °C, lower than that of both the oxalic acid dehydrate and the ChCl, which further proved the formation of DES. 3.2 Effects of ChCl/OA DES on crystallographic structure and morphology The ChCl/OA DES was obtained by thermal treatment the ChCl and OA. And then, accordingly, the precursors with different amount of ChCl/OA DES were synthesized. To investigate the crystallographic structure and the intensity of active plane, XRD analysis was conducted and the results were shown in Figure 1 and Table 1. As depicted in Figure 1(a), all the precursors displayed peaks at 2θ=15.6 o, 19.7o, 24.2 o, 27.1 o, 28.7 o and 30.4 o corresponded to the crystal faces of (001), (101), (021), (121), (201) and (130), which confirmed to be vanadyl hydrogen phosphate hemihydrate, VOHPO4·0.5H2O (JCPDS 84-0761). Although, there is no change of the crystalline phase species of all PVPO-DES, the precursors with DES additional gave higher intensity of (001) plane than PVPO-Blank sample, especially PVPO-DES-0.2 and PVPO-DES-0.4 (as shown in Table 1), which is known that the (001) plane of VOHPO4·0.5H2O is transformed to (200) plane of (VO)2P2O7 and is the active face of VPO catalysts. In addition to this, Table 1 also summarized the full width at half maximum (FWHM) and crystallite size of the (001) plane, which calculated by Scherrer equation. Interestingly, as can be seen from this table, the addition of DES gave the about twice time crystallite size (more than 40 nm) at (001) plane than that of blank (20.9 nm). 9 ACS Paragon Plus Environment


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Figure 1(b) shows the XRD patterns of catalysts. The XRD pattern revealed that (VO)2P2O7 is the main crystalline phase in activated catalysts. The characterized diffraction peaks at 2θ=22.9o, 28.4o, which can be well indexed to the (200) and (013) plane of (VO)2P2O7 (JCPDS 50-0380). The intensity ratio of I(200)/I(013), FWHM and crystallite size of (200) were summarized in Table 2. As expected, VPO-DES-0.4 gave the highest I(200)/I(013) ratio, which indicates that more vanadyl groups on the surface of the catalyst on account of the increase of (200) plane. According to the literature, the performance of catalysts can be enhanced by increasing the relative exposure of the (200) plane of (VO)2P2O7.40 In addition, the crystallite size of (200) was slightly reduced to 16.6 nm, compared to VPO-Blank with 22.0 nm. Smaller grain size is conducive to exposing more active surfaces, which is crucial to the selective oxidation of n-butane to MA. Base on XRD results, it can conclude that DES could induce the formation of active plane and decrease the grain size of (VO)2P2O7. It is noteworthy that even though the main phase does not change among different catalysts, the introduction of DES affects the microstructure of samples, and all the impurity phase, like δ-VOPO4, α-VOPO4, were not detected, which may due to the content of impurity phase was rare or high dispersion in main phase (VO)2P2O7.

Figure1. XRD patterns of the precursors (a) and catalysts (b) Table 1 XRD data for precursors


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Catalysts precursors

I(001) /I(130)

FWHM (001) (Å)

Crystallite size (001) (nm)

PVPO-Blank

51.2

0.396

20.9

PVPO-DES-0.2

80.6

0.193

48.5

PVPO-DES-0.4

79.6

0.209

43.6

PVPO-DES-0.6

74.6

0.221

40.7

PVPO-DES-1.0

73.0

0.204

45.2

Table 2 XRD data for catalysts

Catalysts

I(200) /I(013)

FWHM (200) (Å)

Crystallite size (200) (nm)

VPO-Blank

59.3

0.381

22.0

VPO-DES-0.2

58.7

0.451

18.4

VPO-DES-0.4

76.3

0.491

16.9

VPO-DES-0.6

73.4

0.499

16.6

VPO-DES-1.0

60.8

0.476

17.4

To further study the crystal structure, Raman spectroscopy analysis was performed. Figure 2 summarized Raman spectroscopy of precursors (Figure 2a) and catalysts (Figure 2b). All precursors show main Raman bands attribute to VOHPO4·0.5H2O (933 cm-1, 1012 cm-1, 1078 cm-1). It can be clearly seen that the vibration peak at 933 cm-1, corresponding to the asymmetric P-O stretch in the PO4 groups, for PVPO-DES-0.2, PVPO-DES-0.4 and PVPO-DES-0.6 were significantly enhanced, which indicated that the content of VOHPO4·0.5H2O was increased to a certain degree. However, the vibration peak at 933 cm-1 was reduced when the amount of DES increase to 1.0, which is congruent with the XRD results. On the other hand, the spectra of the catalysts PVPO-DES-0.2, PVPO-DES-0.4 and PVPO-DES-0.6 show a stronger band at 599 cm-1 than blank precursor , which might belong to the bending modes, coupled vibrations, and collective modes of the crystal lattice. The different activated catalysts showed marked differences, specifically, all samples showed the characteristic fingerprints of (VO)2P2O7 at 930 cm-1 (Figure 2b) ,41 which belong to the stretching 11 ACS Paragon Plus Environment


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modes of P-O-P bonds. The peak intensity of 930 cm-1 firstly increases and then decreases, with the increasing amount of DES, indicating that the introduction of DES can contribute to formatting the well crystallized (VO)2P2O7, which is in keeping with XRD results. And the signal at 1088, 1015 and 588 cm-1 could be assigned to small amounts of γ-VOPO4 that is detrimental for selectivity.24, 41, 42 It is a remarkable fact that the XRD data did not show the presence of γ-VOPO4, which maybe due to the small amount and high disperse of this phase. Interestingly, the Raman intensity of γ-VOPO4 was gradually decreased after introducing the DES. It can be conclude that the ChCl/OA DES can reduce the generation of γ-VOPO4.

Figure 2 Raman patterns of the precursors (a) and activated catalysts (b)

The detailed morphology and structure of the precursors and catalysts were confirmed by SEM and HRTEM. As depicted in Figure 3, The PVPO-Blank consists of rose-like nanostructures with diameters around 7.2 μm (Figure 3a). The rose-like morphology is composed of interconnected nanosheet radially grown from the center. This angular nanosheet could result in a reduced exposure of (001) VOHPO4·0.5H2O planes and lead to a lower I(001)/I(220) according to Kiely43. Instead, PVPO-DES exhibits interlaced thick slices structure and their surface has a large amount of debris structure (Figure 3b), suggesting that the crystal growth direction of VOHPO4·0.5H2O has been 12 ACS Paragon Plus Environment

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changed significantly. Beyond that, mass of laminar structure occurred on the edge of the platelet of PVPO-DES. It could be speculated that the nucleation and growth of PVPO could change via introducing the ChCl/OA DES, which may due to the coordination of ChCl/OA DES. TEM and SAED analysis of a single layer crystallite from one of the “block rosettes” of PVPO-Blank and PVPO-DES are shown in Figure 3c and 3d. The width of the single layer of PVPO-Blank is at the range of 30 nm to 700 nm. The SAED patterns suggested that the PVPO-Blank is polycrystal and not detected lattice fringe, which may be indicative of a lower crystallinity. pattern of PVPO-DES exhibit thick platy structure and SAED analysis of the PVPO-DES crystallite revealed that the PVPO-DES is single crystalline on its surface, which have a direct impact on the final catalysts.13 The platy had (220)-type termination facets and a normal direction which corresponded to the [001] projection of VOHPO4·0.5H2O43-45, consisting with the XRD analysis. This variation suggested the DES could induce (001) faces exposure as compared to PVPO-Blank. As well known, ChCl with one OH- groups may act as surfactant like PEG in synthesis of shape-controlled nanomaterials, since it can selectivity adsorb to certain faces to decrease their surface energy and further the growth rate of these faces.46 The reason for that should be the DES interact with V and form a novel complex as the following (confirmed by MS as shown in Figure S2). The ChCl/OA DES (A) complex with V4+ and formed the complex compound (B). B cracked into choline ions (C), Cl- and M+ in ESI mass spectrum.39 Dehydrogenation of choline ions was occurred to give D ions, which further reacts with M+ to give the E ion (m/z=243). ( the structure of A, B, C and D were given in Figure S2 )



(a)

(b)

(c)

(d)

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Figure 3 SEM micrograph of PVPO-Blank (a) and PVPO-DES-0.4 (b); HRTEM micrograph and SAED pattern of PVPO-Blank (c) and PVPO-DES-0.4 (d)

Previous studies have shown that precursors is largely influence the final catalysts.12-14 So the final catalysts were also analyzed by SEM and HRTEM. The morphology of the VPO-DES is resemble for that of the corresponding precursor, holding its platy structure (Figure 4b) as would be expected from the a topotactic transformation of VOHPO4·0.5H2O to (VO)2P2O7. The plated-like structure increased the exposure of the basal (200) face of (VO)2P2O7, which was in accord with XRD and TEM.47 On the contrary, the VPO-Blank showed the disordered structure (Figure 4a). So we hold the opinion that the DES could act as a “shape memory” promoter in the process of the topology transformation. In addition, the VPO-DES possesses smaller platelet size about 300 nm width, while the platelet size of VPO-Blank is 800 nm. The DES is soluble in reaction system. In the present study, the VPO-DES was subjected to mixed-alcohol refluxing at 408 K for 19h before activated (at 723K for 12h). The IR confirmed that most of the DES in VPO precursor had been moved (not detected the peaks of DES). And more remarkable, the function of DES in precursor is similar to polyethylene glycols (PEGs). According to Ji’s works10, 25, it may have some modifier in the surface of precursor, and in the following activation procedure, the residual DES would be 14 ACS Paragon Plus Environment

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gradually removed from the precursor surface. This is explained why the VPO-DES have smaller size. The SAED patterns of VPO-DES can be index [100] projection of (VO)2P2O7,48 which is in accordance with XRD results. Beside this, it can be seen that the diffraction spots were more intense for VPO-DES (Figure 4d), which may be indicative of a higher crystallinity.8 In contrast, The corresponding SAED pattern of VPO-Blank show (200) (024) and (032) reflections of (VO)2P2O7 (Figure 4c), indicating that the particles of VPO-Blank are more randomly oriented relative to the VPO-DES.

(a)

(b)

(c)

(d)

Figure 4 SEM micrograph of VPO-Blank (a) and VPO-DES-0.4 (b); HRTEM micrograph and SAED pattern of VPO-Blank (c) and VPO-DES-0.4 (d)

In theory, the crystal growth is diffusion-controlled growth and nonequilibrium growth49,

50,

which are the prerequisites for the formation of thick platy structure. The formation of the complex compound (confirmed by mass spectra, as depicted in Figure 5 and Figure S2) decline quantity of crystal nucleus (confirmed by ICP date, 246.5 and 32.3 ppm for VPO-Blank and VPO-DES, respectively) and lead to forming thick platy structure in the same nucleation concentration. And the complex of DES and V4+ result in a high nucleation concentration after the nucleation stage, since 15 ACS Paragon Plus Environment


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the complexation of DES and V4+ is weakened after adding H3PO4, and the V4+ is released gradually. Then the active V4+ interacts with P to generate new nuclei. On account of releasing the reaction ions V4+ slowly, the thick platy structure along the [001] direction is favored. The DES acts as like “buffer agent” in this process, as depicted in Figure 5.

Figure 5 The process of synthesize VPO in different system

In order to investigate the phase transition temperature of different precursors, thermogravimetric (TG) analysis and differential thermogravimetry (DTA) analysis were conducted (Figure 6). From the data, the conclusion can be drawn that there were three significant stages in thermo-gravimetric curve for both precursors. The first degradation stage occurred below 250 oC ascribed to desorption of water in the surface.9, 51 The second stage from 250 oC to 450 oC due to the conversion of VOHPO4 to (VO)2P2O7 9, 52, 53, and from the removal of alcohols catch in the layers of precursor. On account of gradually dehydrated and formation of crystal imperfection, the further weight loss above 450 oC was occurred.54,

55

The results show that both PVPO-DES and

PVPO-Blank precursors have similar TG-DTA curve, except that the second stage where the central decomposition temperature of the PVPO-DES is 436 oC, while the PVPO-Blank is 446 oC. As for this result, there may be due to the PVPO-DES possess more perfect crystal, which is in good agree


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with TEM, meaning the thermodynamics is more stable, so the PVPO-DES precursor need higher phase transition temperature.

Figure 6 Thermogravimetric (TG) analysis and differential thermogravimetry (DTA) analysis of PVPO-Blank (a) and PVPO-DES-0.4 (b)

3.3 Effects of DES on the surface chemistry properties As the catalysts for redox reaction, the surface properties including chemical states of V, the oxygen species on the surface, the redox properties and the surface acidity are crucial for the catalytic performance. So the surface properties were investigated by X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR) and NH3-temperature programmed desorption (NH3-TPD). The chemical states of V and the oxygen species on the surface of catalysts were investigated by XPS. They are shown in Figure 7. As reported, the binding energies of V 2p3/2 peak ascribe to V4+ and V5+ were detected at 517.4 eV and 518.7 eV, respectively.7, 56-59 The binding energy of the O 1s level was in the range of 531-533.5 eV.57, 60, which related to surface hydroxide ions and carbonates (Sur-O, 533.0-534.1 eV)57,

60, 61

and lattice oxygen ions in VPO (Lat-O, 531.0-533.0 eV)60. The



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percentage of the chemical states of V and the oxygen species were calculated using the deconvolution of the V 2p3/2 and O 1s peak. The results were shown in Table S1. Figure 7(a) revealed that all catalysts possess V4+ and V5+ state in the surface, which is in good agree with the Raman results. Notably, the V5+ have two existence state: bulk VOPO4 or isolated surface V5+, which was reported beneficial to the selective oxidation of n-butane.40 The vanadium oxidation state were find out based on the difference in the BEs of O 1s and V 2p3/2 signals.23 The average oxidation state of vanadium was calculated by the equation as follows:9, 62 The results of average oxidation state were listed in Table S1. Vox =13.82- 0.68  [BE (O 1s)-BE (V 2p3/2)]

This result show that the average oxidation state of V appeared increase and then decreased with the increase amount of ChCl/OA DES. It is obvious that the ChCl/OA DES could adjust the valence state of surface vanadium, playing an important role in n-butane to MA. The ratio of P and V was also shown in Table S1, the VPO-DES were possess different P/V ratios (P/V=1.76, 1.66, 1.68, 1.67), while the VPO-Blank is 1.74. Interestingly, it is well known metal additives, like Co, Au, Mo, Cr, etc.23, 24, 57, 63, 64could lead to phosphorus surface enrichment, which is responsible to isolate the V4+ and stabilize the active phase.65,

66

On the contrary, in the

present work, the addition of ChCl/OA DES leads to its decrease on the surface, ChCl/OA DES was assumed to prompt a phosphorus diffusion into the bulk.


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Figure 7 XPS spectrum of V 2p 3/2 (a) and O 1s (b) for the activated catalysts

Figure 7(b) revealed that three peaks with binding energies of 533.1, 532.1 and 533.2 eV were detected. According to previous reported, the peaks at 533.1 and 532.1 eV are assigned to Lat-O in VPO, whereas the third peak ascribe to Sur-O in VPO.57 As seen in Table S1, the content of Lat-O was increase obviously with increase of ChCl/OA DES addition, similar to the metal promoter, Ce and Fe.67 The Lat-O is the active oxygen for selective partial oxidation and O of MA is from the Lat-O of VPO.68 For better understanding the influence of DES on redox properties, H2-TPR was conducted. The test were carried out in H2/Ar stream (10% H2 in Ar, 1 bar, 50 cm3·min-1) using catalysts of 40 mg and raising temperature from room temperature to 800 oC at a ramp rate of 10 oC min-1. Additional information with respect to the nature and the oxidizing species available from the catalysts could be obtained. Beyond that, the value of the activation energies can be calculated via Redhead equation: 40, 47

Er  RTm ln(

A[H 2 ]m



)

Here, R is the gas constant, Tm is the peak temperature. A is the pre-exponential factor and δis the heating rate. Figure S3 shows the patterns of H2-TPR and total amount of oxygen removed were presented in Table 3. 19 ACS Paragon Plus Environment


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All the catalysts prepared show two different reduction peaks, which implies that exist two type oxygen species in these catalysts.69 The peaks for VPO-Blank were detected at 551 oC and 719 oC, respectively. The first peak at low temperature was corresponded to the removal of oxygen species (O2-) associated with V5+, whereas, the second peak at high temperature was attributed to the removal of oxygen species (O-) associated with V4+.69 Notably, the peak at 719 oC is correspond to the reduction of (VO)2P2O7 to VPO4. The peaks for VPO-DES were appeared at 564 oC and 694 oC, respectively. Obviously, VPO-DES is easier to reduce than VPO-Blank, which is in accordance with the reduction activation energy (135.66, 163.32 KJ·mol-1 for VPO-Blank and 137.80, 153.21 KJ·mol-1 for VPO-DES). The reduce of reduction activation energy is beneficial to catalytic cycle. According to Herrmann,70 the conversion of n-butane and MA selectivity is connected with the amount of oxygen removed from both phases, V4+ and V5+. From Table 3, we can see that the VPO-DES possesses more reactive oxygen than VPO-Blank, which could enhance the performance for the selective oxidation of n-butane to MA. Meanwhile, the total amount of oxygen removed from VPO-Blank and VPO-DES was 16.45 and 19.63 mmol·g-1, respectively. And the ratio of oxygen removal of V5+/V4+ is 0.34 and 0.32, respectively. So it can be conclude that the promotion of VPO by DES significantly increased the amount of oxygen species removed from V4+ phase. TPR results show that the addition of DES result in a decrease in the reduction temperature of lattice oxygen and reduction activation energy, an increase the number of reducible lattice oxygen species at low temperature. As well known, the surface acidity of catalysts can influence the adsorption and desorption of reactants and products, which further influence the conversion of reactants.71, 72 The surface acidity of different VPO catalysts was investigated by NH3-TPD (as shown in Figure S4). The measure was 20 ACS Paragon Plus Environment

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carried out in 10% NH3 / He stream using catalysts of 40 mg and raising temperature from room temperature to 700 oC at a ramp rate of 10 oC min-1. The peak areas represent the desorptions of NH3 to the acid sites of VPO (summarized in Table 4). From this result, we can see that the total acidity of 1.33 mmol /g measured for VPO with ChCl/OA assisted was higher than that of VPO-Blank catalysts (0.59 mmol/g). The increment of acid sites is conducive to the adsorption of n-butane, resulting in higher conversion of VPO-DES (92.23%) compared with VPO-Blank (86.86%).

Table 3 Total amount of O atom removed, the reduction activation and the ration for O removed from V5+/ V4+ calculated by H2-TPR Catalysts peaks

Tm(℃)

Reduction activation

O atom removed

Ratio for oxygen removal

from the

from V5+ relative to V4+

-1

Energy, Er(KJ·mol )

catalysts(mmol/g) VPO-Blank 1

551

135.66

4.15

2

719

163.32

12.30

Total O atom removed

0.34

16.45

VPO-DES-0.4 1

564

137.80

4.81

2

694

153.21

14.82

Total O atom removed

0.32

19.63

Furthermore, considering that a small amount of DES (~1%) exists in precursor that may serve as template and causing the increment of specific surface area and active sites, BET analysis was conducted. The results revealed the surface area of VPO-Blank and VPO-DES is 21.24, 21.71 m2·g-1, 21 ACS Paragon Plus Environment


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respectively, almost no change. So it is believed that the 1% DES did not work as template to increase the surface area. Table 4 The acid sites distribution based on NH3-TPD date for the different activated catalysts Samples

Acid sites distribution, n(NH ) / (mmol/g) 3

Weak

Medium

Strong

Total

VPO-Blank

0.11

-

0.48

0.59

VPO-DES-0.4

0.35

-

0.98

1.33

3.4 Catalytic Performance The catalytic performance of all the VPO catalyst prepared with different amount of DES additives compared with the one without promoter (VPO-Blank) was carried out under 420 oC with 1.5 vol% n-butane and GHSV of 2000 h-1 in selective oxidation of n-butane to MA. The results are shown in Figure 8. As seen, all the introduction of DES into the catalysts had increase the conversion of n-butane and selectivity of MA simultaneously. The VPO-Blank has shown a conversion of n-butane of 86 %. In comparison with the VPO-Blank catalyst, VPO-DES could improve conversion and selectivity simultaneously, i.e 91 %, 92 %, 88 %, 90 % conversion for VPO-DES-0.2, VPO-DES-0.4, VPO-DES-0.6 and VPO-DES-1.0, respectively, which was well correspond with the average oxidation state of V by XPS data. At the same time, the selectivity of MA rise nearly by 5%, which is differ from the results of the previously research that the MA selectivity would reduce with the enhanced conversion40. These results also indicated that the activity of VPO-DES was decreased when the amount of DES was more than 0.4. The COx yield of different catalysts was reduce in different degree accordingly. This tendency may due to that the decline of Vox and P/V would result in the reduced of conversion. In addition, the selectivity towards MA was decreased slightly when DES was more than 0.4, which may because that the (200) crystallinity was reduced slightly. 22 ACS Paragon Plus Environment

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Figure 8 Catalysts performance of various catalysts for n-butane selectivity oxidation. n-butane conversion (a), MA selectivity (b), yield of MA (c) and COx (d)

3.5 Discussion

In the present work, ChCl/OA DES was used as a green and high-efficiency modifier to synthesize VPO catalysts. From the catalytic performance testing, all DES-assisted VPO exhibit higher catalytic activity than VPO-Blank. Among the catalysts studied, the catalyst VPO-DES-0.4 showed the highest n-butane conversion and MA selectivity than others. Obviously, the excellent catalytic properties of VPO-DES results from their unusual crystal structure and the surface chemistry characteristics. The XRD results from Table 1 and 2 have 23 ACS Paragon Plus Environment


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suggested that the introduction of ChCl/OA DES could induce the formation of active plane and decrease the grain size of (VO)2P2O7, which is crucial to the selective oxidation of n-butane to MA by exposing more active surfaces. This is also in keeping with the Raman analysis (Figure 2). What’ more, ChCl/OA DES reduce the generation of γ-VOPO4 and enhance the crystallinity as a structure directing agent, which confirmed by HRTEM (Figure 4) and TG-DTA (Figure 6) results. HRTEM (Figure 4) has demonstrated that single-crystal structure was formed on the PVPO-DES and VPO-DES surface. The increase of crystallinity could give rise to a much faster in suitable phase transformation of the precursor VOHPO4·0.5H2O to active catalyst (VO)2P2O7, which could accelerate the activation process before the amorphous surface forms on the final catalysts.12 The VPO catalysts that have highly crystalline offen have higher catalytic performance. TG-DTA result shows that PVPO-DES has higher phase transition temperature than VPO-Blank, suggesting PVPO-DES possesses higher crystallization. The increase of crystallinity and change of morphology may due to ChCl/OA DES modulate nucleation and growth mechanisms by a “buffer agent” agent, which was confirmed by mass spectra and ICP test . Meanwhile, ChCl/OA DES could adjust the surface chemistry characteristic, which is beneficial to enhance the catalytic performance of VPO. XPS shows that the average oxidation state of V appeared increase and then decreased with the increase amount of ChCl/OA DES (Table S1), which is in accordance with the n-butane conversion. This may due to the V5+ species could play an important in the break of n-butane C-H bond.9 Table 3 shows that the content of lattice oxygen was increase obviously with increase of ChCl/OA DES addition. Abon et.al

67, 73pointed

out that lattice

oxygen is the active oxygen for selective partial oxidation and the increase of surface lattice oxygen could improve the transfer efficiency of oxygen, which has great impact on MA selectivity. Figure 24 ACS Paragon Plus Environment

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S3 and Table 3 suggested that VPO-DES have lower reduction activation energy and more oxygen species removed from V4+ phase than VPO-Blank, resulting in the enhancement of the catalytic performance.

4

Conclusion In summary, the addition of low amount of ChCl/OA DES during the preparation of

VOHPO4·0.5H2O by the reaction of V2O5 and H3PO4 with isobutanol and benzyl alcohol, leads to an significant enhancement in maleic anhydride selectivity and n-butane conversion and, in particular, the intrinsic activity. The detailed crystallographic structure and morphology properties study indicated that ChCl/OA DES worked as a crystal face inductive agent to induce the formation of (001) plane of VOHPO4·0.5H2O, correspondingly to (002) of (VO)2P2O7, meanwhile, prohibited the formation of impurity phases of δ-VOPO4, α-VOPO4 and γ-VOPO4. Furthermore, a single layer crystallite on the surface of the precursor and active phase of VPO and thick platy structure was formed, which is different from the one with polycrystal and rose-like morphology obtained without promoter. That is because of the interaction between the function group of -OH and -COOH in ChCl/OA DES with V2O5, formation of a new V complex, which affects the process of nuclei formation of PVPO by acting as like “buffer agent” in this process (Figure 5). What’s more, the presence of ChCl/OA DES dramatically affect the surface properties as demonstrated by the results of XPS and H2-TPR. The DES also could increase the average oxidation state of V, and the content of lattice oxygen as increase its amount, while reduce the P/V on the surface, different from the metal promoters. All these were assumed to contribute to the good catalytic performance that increase both the conversion of n-butane from 86.86% (VPO-Blank) to 92.23% (VPO-DES-0.4) and the selectivity of MA from 56.74% to 60.80% to overcome the “ trade off effect”, resulting about 25 ACS Paragon Plus Environment


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12% higher mass yield of MA than that of VPO-Blank. Hence, here we have demonstrated a simple, low cost and environmental friendly method by using ChCl/OA DES as promoters to enhance the catalytic performance of VPO catalysts, which is the sole example of a commercial catalyst for the selective oxidation of an alkane. This strategy, although demonstrated for VPO catalysts, could be extend to other mixed oxide or metal phosphate catalyst preparations to tune their structure and surface properties, according its application.

Acknowledgements

This work was supported by National Key Research and Development Program of China (2017YFA0206803), the Key Programs of the Chinese Academy of Sciences (KFZD-SW-413), the One Hundred Talent Program of CAS, National Natural Science Foundation of China (21808223) and CAS/SAFEA International Partnership Program for Creative Research Teams (20140491518). The authors are grateful for the assistance from teachers Wu Hui, Wang Ling and Zhou Na of Analysis and Test Center, Institution of Process Engineering, Chinese Academy of Sciences.

Supporting information: Figure S1 IR spectra (a), 1H NMR spectra (b), and differential scanning calorimetry spectra (c) of ChCl, OA and DES Figure S2 The MS pattern of the supernatant liquid and speculative mechanism Figure S3 H2-TPR profiles for different catalysts Figure S4 NH3-TPD profiles for different catalysts Table S1 XPS data of all the activated catalysts


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Synthesis of vanadium phosphorus oxide catalysts assisted by deep

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