Cs(NH4)xH3âxPMo11VO40 Catalyzed Selective Oxidation of

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Cs(NH4)xH3-xPMo11VO40 Catalyzed Selective Oxidation of Methacrolein to Methacrylic Acid: Effects of NH4+ on the Structure and Catalytic Activity Yun-Li Cao, Lei Wang, Li-long Zhou, Guangjin Zhang, Bao-Hua Xu, and Suo-Jiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04133 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Industrial & Engineering Chemistry Research

Cs(NH4)xH3-xPMo11VO40 Catalyzed Selective Oxidation of Methacrolein to Methacrylic Acid: Effects of NH4+ on the Structure and Catalytic Activity Yun-Li Cao ab, Lei Wanga, Li-Long Zhouab, Guang-Jin Zhang a, Bao-Hua Xu a, Suo-Jiang Zhanga,* a

Beijing Key Laboratory of Ionic Liquids Clean Processes, Key Laboratory of Green Processes and

Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, 100190, Beijing, People’s Republic of China. b

University of Chinese Academy of Sciences, 100190, Beijing, P. R. China

*Corresponding author, Email: [email protected] (S. Zhang), Fax: +8682544875.

Keywords: ammonium, polyoxometalates, methacrolein, methacrylic acid, selective oxidation

Abstract A series of Cs(NH4)xH3-xPMo11VO40 with different x value were synthesized to catalyze the selective oxidation of methacrolein to methacrylic acid. The effects of ammonium on both structure and catalytic activity were explored in this study. Compared with CsH3PMo11VO40, the surface area, amount of acid sites and active species (V4+/VO2+) were found strongly dependent on the content of ammonium. The optimum structure of Cs(NH4)1.5H1.5PMo11VO40 shows comparable large surface area of 50.33 m2/g, large amount of acid sites and active species (V4+/VO2+), consequently offering a higher methacrolein conversion (83%) and methacrylic acid selectivity (93%). Furthermore, the steady activity of Cs(NH4)1.5H1.5PMo11VO40 was not affected much by the decomposition of ammonium, and the catalyst exhibited good stability.

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1. Introduction Methacrylic acid (MAA), as an important chemical intermediate for synthesis of methyl methacrylate (MMA), was normally synthesized by selective oxidation of methacrolein (MAL) over Keggin-type polyoxometalates catalysts1-3. Among those screened, the cesium salts of 11-phosphomolybdovanadyl acids, behaved with a good catalytic efficiency in the selective oxidation of MAL to MAA4-10. In recent years, ammonium modified polyoxometalates received increasing attentions11-19 because they exhibited enhanced catalytic performance in the production of MAA16. For instances, Areán et al. found two extra framework molybdenum ions coexisted in Csx(NH4)yHzPMo12O40, thereof offering the conversion of 50.7% from MAL to MAA11. Marchal-Roch et al. found a new cubic phase of (NH4)6(VO) [PMo11VIVO40][PMo12O40], in which two types of heteropolyanions coexist: [PMo11VIVO40]5− and [PMo12O40]3− has been obtained by thermal treatment of (NH4)5[PMo11VIVO40]. The catalyst showed about twice more active than that of the pure cesium salts in the oxidative dehydrogenation of isobutyric acid with leading to an enhanced selectivity to MAA12. Paul et al. found that (NH4)3HPMo11VO40 loaded on the carrier showed better performance in the selective oxidation of isobutane to MAA13. Other studies were almost focused on the syntheses12,

14

, surface morphology15, structural evolution16, thermal

stability17 and the interaction of Cs+ and NH4+ cations18 in Csx(NH4)yHzPMo11VO40 or Csx(NH4)yHzPMo12O40 system during the process of the oxidation of isobutene or MAL to MAA. To our knowledge, however, the detailed investigation on the inherent effect of the NH4+ on the catalytic activity was seldom further researched.


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To better understand this issue, a series of Cs(NH4)xH3-xPMo11VO40 (Cs(NH4)xH3-xPVA)) with different ammonium content were synthesized and their performance in the oxidative transformation of MAL to MAA was deeply studied. The existence of cesium aimed to stabilize and moderate the property of catalyst based on the previous studies12, 20. To our surprise, the reaction efficiency was strongly x value dependent. The optimum one was obtained by comparing the catalytic performance of the newly prepared catalysts. The positive effect of introducing an appropriate amount of ammonium to the catalyst structure was proposed on the basis of various advanced detection methods. Finally, the long-term stability test of the selected catalyst was carried out for the next real application.

2. Materials and methods

2.1. Preparations of the catalysts

The hydrated 11-molybdo-1-vanadophosphoric heteropolyacid (H4PMo11VO40, HPVA) was prepared according to the method described previously21. A series of samples Cs(NH4)xH3-xPMo11VO40 (x = 0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0) were prepared by co-precipitation method as described below. According to the molar ratio of Cs+ and NH4+, 0.0025 mol CsNO3 (0.49 g, A) and a certain amount of NH4HCO3 (B) were dissolved in 5 mL deionized water, respectively. 0.0025 mol hydrated H4PMo11VO40 (4.46 g, C) was dissolved in 15 mL deionized water. The above aqueous solutions of A and B were concurrently added dropwise to the aqueous solution of C under stirring at 40 °C. Then the mixture was strongly stirred for about 2 h at 80 °C. The yellow suspension was then formed and evaporated the solvent at 80 °C, further dried at 80 °C in an oven for another 24 h to obtain precursor sample (uncalcined sample). The finally catalysts (calcined sample) were obtained after thermal treatment by further calcination at



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350 °C under air flow for 12 h to decompose the nitrate and bicarbonate precursors. The above precursors and catalysts were tableted, crushed, and sieved to a mesh size of 0.2–0.4 mm for further use. The prepared samples were designated according to the ammonium and cesium stoichiometry. For example, Cs(NH4)1.5H1.5PVA denoted the catalyst with the stoichiometry Cs (NH4)1.5H1.5PMo11VO40. And in the case of x = 0, the sample corresponds to the CsH3PMo11VO40 (CsH3PVA) was prepared as a reference catalyst.

2.2. Characterization of the catalysts

Thermogravimetry analysis(TGA) of the uncalcined samples (about 10 mg) was performed using a thermogravimetric analyzer (LabsysEvo, France).The samples were heated from room temperature to 650 °C or 700 °C at a rate of 5 °C/min under air ﬂow

Fourier transform infrared (FT-IR) spectroscopy was executed on a FT-IR spectrometer (Nicolet 380, Thermal Electron Corporation, Japan). The samples were compressed into a KBr pellet (1 wt.%) for FTIR spectroscopy measurement, and the spectra were recorded from 400 to 4000 cm-1 with a spectral resolution of 4 cm-1 .

Powder X-ray diffraction (XRD) was performed on an X-ray diffractometer (Rigaku Smartlab X-Ray diffractometer, Rigaku Corporation, Japan). X-ray powder diﬀractometer operated at an accelerating voltage of 40 kV and an emission current of 200 mA with Cu Kα radiation (λ = 1.5418 Å). A 2θ range of 5–70 degrees was scanned with steps of 0.02°/s and a 2 s acquisition time.


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The nitrogen sorption of the prepared series samples Cs(NH4)xH3-xPVA was evaluated by surface area and pore analyzer (Micromeritics ASAP2460, US). All samples were degassed under vacuum at 120 °C for 6 h before adsorption measurements. Surface areas and pore volumes of the prepared samples were calculated by the BET equation and the BJH model, respectively. The morphology and the pore structure of Cs(NH4)xH3-xPMo11VO40 samples were observed by the scanning electron microscopy (SEM, SU8020, Hitachi electronic electric appliance company, Japan)with 10 kV energy and 90mA of beam current and the transmission electron microscope (TEM, JEM-2100F, JEOL, Japan) with an acceleration voltage of 300 kV.

Ammonia temperature-programmed desorption (NH3-TPD) was carried out in order to elucidate the amount and the strength of the acid sites over the series catalysts. About 50 mg calcined sample loaded in a U-shaped quartz reactor was pretreated in a He flow (50 mL/min) at 120 °C for 60 min and then cooled to 50 °C. Then, the catalyst was exposed to a NH3 and He gas mixture (10% NH3, 15 mL/min) at 100°C for 60 min. The TPD profiles were obtained in a He flow (25 mL/min) by heating up to 650 °C at a rate of 10 °C/min from the thermal conductivity detector (TCD) signal.

X-ray photoelectron spectroscopy (XPS) spectra of the series samples were performed on a Kratos Axis Ultra DLD (Manchester, UK) spectrometer with monochromatic Al Kα radiation, using a pass energy of 20 eV (0.05 eV/step). The binding energy of each element was calibrated by using the cabon peak as standard (C1s=284.5 eV).

Electron paramagnetic resonance (EPR) measurements were performed at room temperature with a Bruker EleXsys E500 EPR spectrometer (Swiss, Bruker) in the X-band mode. The microwave power,



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resonance frequency, modulation width, and time constant were 10 mW, 9.85 GHz, 0.5 mT, and 0 s, respectively.

2.3. Evaluation of the catalysts in selective oxidation of methacrolein

The catalytic oxidation of MAL to MAA was performed in a conventional fixed-bed reactor at 320 °C under atmospheric pressure. About 0.5 g (0.5-0.6 mL) catalyst was loaded into the fixed bed reactor, When the temperature increased to 320 °C, the reactants (MAL/O2/H2O/N2 = 4.4/11.1/17.8/66.7, volume ratio) were fed into the reactor. Since the contact time was the volume of catalysts divided by the flow of all materials under the standard condition. Then the contact time was about 2 s.

After the reaction

being steady, the highly volatile byproducts such as CO and CO2 were analyzed by a gas chromatograph (Agilent technologies 7890B GC System) with TDX-01 packed column and a thermal conductivity detector (TCD). The products, such as MAL and MAA, and one less volatile byproduct (acetic acid (AA)) were collected and analyzed using the gas chromatograph equipped with a flame ionization detector (FID) and a DB-FFAP packed column.

Method for the calculation of conversion of MAL, selectivity of MAA and the yield value:

C (%) =

n MAL ,F − n MAL ,P × 100% n MAL ,F

(1)

S(%) =

n MAA ,P × 100% n MAL ,F − n MAL ,P

(2)

Y (%) = C (%) × S (%)

(3)

C: conversion of MAL (%)


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nMAL,P: mole of MAL detected in the liquid phase product nMAL,F: mole of MAL fed into the reactor S: selectivity to MAA (%)

nMAA,P: mole of MAA detected in the liquid phase product Y: yields of MAA (%)

3. Results and discussion

3.1. Characterization of catalysts

3.1.1. Thermostability (TG/DTG)

The TG/DTG curves of uncalcined Cs(NH4)xH3-xPVA (x=0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0) in the 30-650 °C range are shown in Figure S1 and the continuous mass loss at each step of the catalysts with different x value along 30-450 °C range are gathered in Table 1.

As shown in Figure S1, two positive peaks were observed from the DTG curve of Cs(NH4)xH3-xPVA (x=0.5, 1.0, 1.5 and 2.0) at about 80 °C and 440 °C and three positive peaks were shown from the DTG curve of Cs(NH4)xH3-xPVA (x=2.5 and 3.0) at about 80, 230 and 440 °C, also there was one negative peak at about 450 °C for all Cs(NH4)xH3-xPVA (x≠0) samples. Based on the previous literature13, the loss of absorbed water and crystal water was observed at 30-250 °C temperature range (DTG signal, Figure S1), respectively. Afterwards, the decomposition of ammonium ion with release of ammonia and the elimination of residual/acidic protons with constitutional water form started from 250 °C to 450 °C



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( DTG signal centered at about 440 °C (x≠0) and 300°C (x=0), Figure S1), leading to the formation of lacunary Keggin-type heteropolycompounds22-24. In addition, the obvious mass gain (about 490 °C, when x>1.5) was ascribed to the reoxidation of V4+ to V5+ during the heating process in air13, which could not observed in the TG/DTG curve of CsH3PVA as the existence of V4+ was mainly due to the previous removal of ammonium ion (cf. 3.1.5).When temperature went up (>450 °C), the V4+ could easily be reoxidized into more stable V5+ under air. As outlined in Table 1, the amount of mass loss (250-450 °C) increased with the content of ammonium ion in series of Cs(NH4)xH3-xPVA samples (1.39% for CsH3PVA, 1.68% for Cs(NH4)0.5H2.5PVA, 2.25% for Cs(NH4)1.0H2.0PVA, 3.26% for Cs(NH4)2.0H1.0PVA, 3.56% for Cs(NH4)2.5H0.5PVA and 3.86% for Cs(NH4)3PVA ), which were all consistent with the corresponding theoretical value. Taken Cs(NH4)1.5H1.5PVA as a sample, the total mass loss was 2.6% between 250-450 °C range (Table 1, Figure S1), and the calculated value is 2.7%. It also suggested that almost complete loss of proton and ammonium ion at 450 °C. Compared with CsH3PVA, it can be inferred that NH3 can be partially released at 350 °C (the calcination temperature of catalysts) resulting in the protonated heteropolycompound species. Furthermore, the experimental mass loss of Cs(NH4)1.5H1.5PVA (250-350°C) was the smallest (0.39%) in all samples (Table 1). Table 1 Decomposition step and mass change of uncalcined Cs(NH4)xH3-xPVA during heating process


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Absorbed water

Ammonia/

Ammonia/ ★

x value Crystal water

★

constitutional water

Mass gain

constitutional water

T (°C)

∆m/m(%)

T (°C)

∆m/m(%)

T (°C)

∆m/m(%)

T (°C)

∆m/m(%)

0

30-250

2.79

250-350

1.16*

350-450

0.23*

-

-

0.5

30-250

2.60

250-350

0.85

350-450

0.83

490

+0.03

1.0

30-250

1.92

250-350

0.68

350-450

1.57

490

+0.05

1.5

30-250

2.05

250-350

0.39

350-450

2.41

490

+0.20

2.0

30-250

1.42

250-350

0.55

350-450

2.71

490

+0.37

2.5

30-250

4.97

250-350

0.74

350-450

2.82

470

+0.30

3.0

30-250

7.00

250-350

1.00

350-450

2.86

470

+0.35

Mass gain: see the text. * Mass loss of constitutional water.

In order to illustrate the thermal decomposition process of the uncalcined sample completely, the TG/DTG curves of uncalcined Cs(NH4)1.5H1.5PVA between 30-700°C temperature range are shown in Figure 1. It was found out that the Keggin structure of Cs(NH4)1.5H1.5PVA finally decomposed to the corresponding meta oxides, such as P2O5, MoO3 and V2O5 when heating up to 650°C, and P2O5 began to sublimate, leading to further mass loss as shown in TG curve (Figure 1). The phenomenon of thermal



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decomposition was consistent with the previous study13.The decomposition scheme during heating process under air is shown in Figure. 2.

Figure 1. TG and DTG curves of the uncalcined Cs(NH4)1.5H1.5PVA (TG, solid line, DTG, dot line)

Figure 2. The decomposition scheme of the uncalcined Cs(NH4)1.5H1.5PVA during heating process under air


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From the above TG results, it can be deduced that: (1) The active phase can retain a large proportion of ammonium and constitutional water at the studied calcination temperature (namely 350 °C), leading to a protonated heteropolycompound species, which is related to the property changes of samples, such as acidity and redox ability, and can further influence the catalytic performance of catalysts (see below). (2) When the temperature exceeds 650 °C, the Keggin structure in the catalyst can be decomposed. The selective reaction temperature (namely 320 °C) is fall far below this decomposition temperature.

3.1.2. Structural features (FT-IR, XRD)

The FT-IR spectra of the catalysts before and after calcination are given in Figure 3. All the catalysts showed the typical bands of the Keggin unit at about 1062, 965, 870 and 785 cm-1, which were assigned to ναѕ P-O of PO4 tetrahedron, ναѕ Mo=Od , ναѕ Mo-Ob-Mo and ναѕ Mo-Oc-Mo, respectively26. For uncalcined catalysts of Cs(NH4)xH3-xPVA (x≠0), two bands could be observed at about 1385 and 1410 cm-1 , which were attributed to the NO3- and NH4+ vibration18. The relative intensity of two bands increased with the content of ammonium. The band at 1385 cm-1 disappeared after calcination and the relative intensity of band at about 1410 cm-1 decreased after calcination. Two small new bands at about 1035 and 595 cm-1 coincidently emerged after calcination, which can be assigned to the isolated vanadium oxide (V5+Ox) and MoO324, 27, respectively. The band at about 3200 cm-1 can also confirm the N-H vibration (Figures S2, S3).



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Figure 3. FT-IR spectra of uncalcined and calcined catalysts changed with x values. The above results can be attributed to the following reasons: (1) The band at about 1385 cm-1 disappeared after calcination illustrated the elimination of nitrate ion under thermal treatment. (2) The appearance of new bands at 1035 and 595 cm-1 illustrated that the migration of vanadium species from the primary structure (Keggin) to the secondary structure and a partial decomposition of Keggin anions during calcination, resulting in the formation of lacunary structure3. (3) After calcination the relative intensity of N–H vibration decreased due to the partial decomposition of the ammonium during the calcination process.

Figure 4. XRD patterns of the uncalcined and calcined catalysts changed with x values.


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The XRD patterns of the uncalcined and calcined catalysts are shown in Figure 4. The observed characteristic peaks at about 10.5, 26.2 and 35.7° are assigned to the (110), (222) and (332) planes of the Keggin-type heteropolycompounds13. However, the exact peak position of the catalyst showed some dependence on x values. For example, the most intense peak of the uncalcined catalysts was presented at 26.25° for Cs(NH4)0.5H2.5PVA, 26.27° for Cs(NH4)1H2PVA, 26.52° for Cs (NH4)1.5H1.5PVA, 26.31° for Cs (NH4)2H1PVA, 26.33° for Cs (NH4)2.5H0.5PVA and 26.30° for Cs (NH4)3PVA, respectively (Figure 4). Meanwhile the same trends of the calcined catalysts were also observed (Figure 4). These shifts were related to the changes in the cubic cell unit (Figure 4). Then, the cubic cell parameter (noted α) and the crystalline size p (noted p) were calculated based on the selected reflection planes (110), (222), (400) and (332), which were the characteristic peaks of the Keggin type catalysts. The results are collected in Table S1. When x

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