Reaction of Formalin with Acetic Acid over VanadiumâPhosphorus

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Reaction of Formalin with Acetic Acid over V-P Oxide Bifunctional Catalyst Dan Yang, Dan Li, Haoyu Yao, Guoliang Zhang, Tiantian Jiao, Zengxi Li, Chunshan Li, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01422 • Publication Date (Web): 17 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015

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Industrial & Engineering Chemistry Research

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Reaction of Formalin with Acetic Acid over V-P Oxide Bifunctional Catalyst

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Dan Yang a,b, Dan Li a,c, Haoyu Yao a,b, Guoliang Zhang b, Tiantian Jiaoa,b, Zengxi Li a,*,

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Chunshan Li b,*, Suojiang Zhang b,*

4

a

College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

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b

Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase

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Complex System, Institute of Process Engineering, University of Chinese Academy of Sciences,

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Beijing 100190, People’s Republic of China c

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CNOOC Energy Technology & Services Ltd. Co. Safety & Environmental Solution Sub-Co, Tianjin 300452, People’s Republic of China

10 11

Abstract:

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A new route to synthesizing acrylic acid from acetic acid and formalin by

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one-step aldol condensation reaction was proposed and developed. V-P/ SiO2 oxide

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bifunctional catalysts were designed and prepared by ultrasonic impregnation method.

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The catalysts were characterized by XRD, BET, TEM, TG/DTA, and XPS, as well as

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NH3, CO2-TPD, and pyridine-FTIR methods. Catalytic performance was evaluated

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using a fixed-bed tubular microreactor operating with a CH3COOH/HCHO (HAc/FA)

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molar ratio of 10 under atmospheric pressure. Influences of the balance between

19

acidic and alkaline sites, as well as ratio of V4+ and V5+, on catalyst activity were

20

further studied. Reaction parameters were systemically optimized. Through a series of

21

comparisons, V-P/SiO2 binary oxide catalyst with a bulk density ratio of 1:2 was

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selected. 1 1 *

Corresponding author: Zengxi Li. TEL/FAX: +86-10-88256322; E-mail: [email protected] Suojiang Zhang. TEL/FAX: +86-10-82627080; E-mail: [email protected] Chunshan Li. TEL/FAX: +86-10-82544800; E-mail: [email protected]

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Keywords: V-P oxide, Acetic acid, Formalin, Acrylic acid, Aldol condensation

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1. Introduction

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Acrylic acid (AA)

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is a raw material for organic synthesis and an important

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monomer of synthetic resin, which was mostly used to manufacture methyl acrylate,

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butyl acrylate, ethyl acrylate, and 2-hydroxyethyl ester, etc. Furthermore, AA and

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esters can undergo homopolymerization and copolymerization, and the resulting

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polymers can be used in synthetic resins, synthetic fibers, absorbent resins, building

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materials, paints, and other industrial sectors. The polymer is currently produced via a

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two-step propylene oxidation method. In this route

2, 3

, propylene is oxidized to form

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acrolein, which is subsequently converted to AA. However, the rising cost and

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declining productivity of propylene limit the application of this reaction route. Hence,

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it is urgently needed to develop an alternative route for AA production.

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A new route to synthesizing AA using acetic acid and formalin is developed,

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which is a typical aldol condensation reaction. Past research works 4-6 mainly focus on

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the aldol condensation reaction of propionic acid and esters with formaldehyde. For

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this reaction, a highly-active catalyst is crucial. As for the aldol condensation of

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formaldehyde with carboxylic acid or its ester, basic oxide catalysts, namely alkali

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metal oxides or alkaline earth metal oxides supported on traditional carriers have been

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claimed to be useful. O.H. Bailey et al. 7 reported a 39% conversion of propionic acid

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and 91% of selectivity to methacrylic acid on a cesium-loaded silica catalyst. Recently,

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J. Yan et al. 8 reported the fabrication of methyl acrylate via the condensation of

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methyl acetate with formaldehyde over a cesium supported SBA-15 catalyst, and the


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catalyst obtained the highest (48.4%) conversion of methyl acetate and 95.0%

2

selectivity for methyl acrylate. On the other hand, G. Albanesi et al. 9, as well as J. J.

3

Spivey et al.

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niobium oxides, tantalic oxides, or zirconium–aluminum oxides were active

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component in this kind of reactions. M. Paulis et al.

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catalyst exhibited an excellent performance in the gas-aldol condensation of acetone

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with formaldehyde to form methyl vinyl ketone. Basic catalysts involved higher

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conversion than acidic catalysts, but the relatively low selectivity of main products

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presented its major disadvantage. In contrast, the reverse is true in acidic catalysts.

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reported that acidic oxide catalysts containing of vanadic oxides,

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found that Nb2O5·nH2O

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In recent years, the acid-base bifunctional catalyst was proposed and became a

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promising method spoor route, which presenting a better performance than the single

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acid or alkali catalyst. Several researchers indicated that the activity and selectivity of

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catalysts for the aldol condensation of formaldehyde with carboxylic acid and its

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esters were governed by the balance of acidic and basic properties. B. Li et al.

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reported that a Zr–Mg–Cs/SiO2 catalyst exhibited moderate activity for aldol

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condensation of methyl propionate with formaldehyde to produce methyl

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methacrylate. M. Ai et al.

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following the patented procedures

19

reaction. As mentioned above ?in article, V2O5-P2O5 binary oxide catalyst possessed

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an enhanced acidic property. At the same time, its certain basic property was effective

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in promoting aldol condensation reactions. Furthermore, M. Ai et al. 4 found that the

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combination of vanadyl pyrophosphate [(VO)2P2O7] with silica gel in the presence of

13

12

found that the V2O5-P2O5 binary oxide catalysts prepared 14

, performed well in the aldol condensation



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a small amount of phosphoric acid obtained an improved catalytic performance in

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aldol condensation reactions. X.Z. Feng et al.

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methyl acetate (including 30–35% acetic acid and 3–12% COx) over a VPO catalyst

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activated in a 1.5% butane–air mixture.

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reported a 84.2% conversation of

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The atomic ratio of P/V of the catalyst, namely the number and the distribution

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of acid and base active centers, and the ratio of V4+ and V5+ were the key influence

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factor of catalytic performance 16. In this study, the supported V-P oxide bifunctional

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catalysts were designed and used in the one-step aldol condensation reaction of acetic

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acid with formalin to produce AA with a fixed-bed tubular micro reactor. The yield

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and selectivity of AA were calculated based on the feeding amount of formaldehyde.

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The strength and number of alkline and acidic sites of V-P bifunctional oxide catalysts

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with different V and P-loading were studied using NH3 and CO2-TPD, and several

13

consistent results were obtained. The valence state of vanadium in the V-P oxide

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bifunctional catalysts was confirmed by XPS.

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2. Experimental

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2.1 Catalyst Preparation

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Acetic acid (≥99.0%), formalin (37.0%), isobutyl alcohol (≥99.0%), ammonium

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metavanadate (≥99.0%) and ammonium phosphate (≥99.0%) used were of analytical

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grade. Oxalic acid was provided by Sinopharm Chemical Reagent Co., Ltd. SiO2 (20

20

to 40 mesh) was purchased from Qing Dao Hailang Silica Gel Drier Factory. In all

21

cases, the mentioned percentage purities refer to mass fraction as reported by the

22

suppliers. All chemical agents were used upon receipt without further purification.


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All catalysts were prepared by equivalent-volumetic ultrasonic impregnation

2

method. Silica (20-40 mesh) was dried at 120 °C overnight, and then calcined at 500

3

°C for 6 h. The precursor salts, ammonium metavanadate and ammonium phosphate,

4

were added sequentially in oxalic acid solution at room temperature. The

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impregnation precursor was maintained in an ultrasonic machine at a frequency of 50

6

kHz for 3h, then dried at 120 °C overnight, and calcined at 500 °C for 6 h under

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flowing air. After natural cooling, activated catalysts were obtained.

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2.2 Catalyst characterization

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The X-ray diffraction (XRD) patterns of catalysts were recorded on a

10

diffractometer (Model: X’Pert PRO MPD, PAN analytical Co., Ltd.) with Cu-Kα

11

radiation (40kV and 50 mA), and the scanned 2θ ranged from 5 to 90 °.

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Surface area and pore size were determined by using Brunauer−Emmett−Teller

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(BET) and Barrett-Jayner-Halenda (BJH) method based on nitrogen adsorption and

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desorption isotherms at liquid nitrogen temperature (Quanta Chrome Instrument

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NOVA 2000), respectively. The catalysts were degassed at 350°C for 6 h prior to

16

analysis.

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Transmission electron microscopy (TEM) was performed with the model: JEOL

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JEM-2100 system under an acceleration voltage of 200 kV. Samples were prepared

19

via ultrasonic dispersion in ethanol, with a drop of the resultant suspension evaporated

20

onto a holey carbon-supported grid.

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X-ray photoelectron spectroscopy (XPS) data were obtained using an ESCA

22

Lab220i-XL electron spectrometer (VG Scientific) at 300 W Al Kɑ radiation. The



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base pressure was approximately 3×10-9 mbar. Binding energies (BEs) were

2

referenced to the Si 2p line at 103.21 eV from silica.

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Total acidity and basicity of the catalysts were measured by thermal programmed

4

desorption (TPD) of NH3 and CO2, performed by using Autochem II 2920 apparatus

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from Micromeritics. Approximately 50 mg samples were thermally treated at 450 °C

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with flowing He for 1h, at a heating rate of 10 °C /min, then cooled to 50 °C prior to

7

adsorption. At 50 °C, a total of 10 % NH3-He or 10 % CO2-He was passed over the

8

samples for 30 min. After purging with pure He for 1 h at 50 °C until the baseline was

9

stable, the desorption profile was measured using the thermal conductivity detector in

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He flow at a heating rate of 10 °C /min to 650 °C.

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The surface acidity of catalyst was investigated by means of studying pyridine

12

adsorption on Fourier transform infrared (FTIR) spectroscopy. Pyridine FTIR

13

spectrum was recorded on a Nicolet 6700 spectrometer in situ equipped with a cell.

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The samples were pressed into a self-supporting plate (7 mg, 13 mm diameter), placed

15

in an IR cell, and then treated at 150 °C under vacuum (0.0018 Pa) for 30 min. After

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the IR cell was cooled to room temperature, pyridine vapor was introduced into the

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cell, adsorbed for 10 min, and reached equilibrium for 30 min. Then, the cell was

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scanned after being vacuumed for 10 min and recorded at 50, 100, 200, and 300 °C

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under vacuum.

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2.3 Reaction system

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The performance of the catalyst was investigated in a fixed-bed reactor under

22

atmospheric pressure. The reactor was fabricated from a stainless steel tube, with


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dimensions 50 cm length and 1 cm i.d., which was mounted vertically at the heating

2

furnace. About 5 mL catalyst was loaded in the middle of the reactor, quartz wool was

3

placed both under and above the sample, and then the remainder was filled with

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stainless steel. The temperature range of this reaction was 300 to 400 °C. Acetic acid

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and formaldehyde (molar ratio HAc/FA: 10/1) were fully mixed, and then the mixture

6

was fed from the top of the reactor by an advection pump with the feed rate of 0.1

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mL/min. As the source of formaldehyde, formalin was unified containing 37%

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formaldehyde. After the reaction reached equilibrium, the reaction products were

9

collected per 4 h. These products were analyzed by GC (Shimadzu GC 2010 plus),

10

with isobutanol as an internal standard substance.

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3. Results and Discussion

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3.1 Catalyst characterization

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3.1.1 XRD Analysis

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Figs. 1 and 2 present the XRD patterns of fresh V/SiO2 and V-P/SiO2 samples.

15

As shown in Fig. 1, an amorphous peak derived from SiO2 was recognized in the 20 ˚

16

area, and no diffraction peak was observed in catalyst V/SiO2 with the V content

17

below 2.0 wt. %. Given that the content of V was extremely small, dispersing VO2

18

or/and V2O5 on the SiO2 surface evenly with small particle form. When the content of

19

V was above of 5.0 wt. %, the characteristic diffraction peaks of V2O5 and VO2 were

20

detected, and the peaks became more evident as the increasing content of V, which

21

indicating that crystal size of V2O5 and VO2 enlarged. The diffractograms of SiO2

22

supported catalysts with an addition of 2 wt. % V and different additions of P are



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shown in Fig. 2. The VOPO4 and (VO)2P2O7 phase 15, 18 exerted an significant impact

2

on catalyst performance, characteristic diffraction patterns were investigated. And the

3

intensity of peaks increased slightly with the increasing of the content of P

4

exponentially. This result indicated that the addition of P promoted the formation of

5

VOPO4 or (VO)2P2O7 phases. However, excess P had no function for generating VPO

6

phase 19.

7

Fig.1. XRD patterns of fresh catalysts V/SiO2 with differen V contents.

8

Fig.2. XRD patterns of fresh catalysts V-P/SiO2 with different P contents.

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3.1.2 BET Specific Surface Area Analysis

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The N2-BET surface area, average pore diameter, and pore volume of catalysts

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V/SiO2 and V-P/SiO2 are summarized in Tables 1 and 2. As can be noted, the N2-BET

12

surface area and total pore volume in both cases decreased with increasing V and P

13

content, whereas average pore diameter gradually increased. Increasing the content of

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V from 0.5 to 15 wt. %, the corresponding specific surface areas decreased from

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463.4 to 224.3 m2/g. Compared with the catalysts in Tables 1 and 2, the addition of P

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notably increased the specific surface area. For example, the surface area of catalysts

17

V2%-P1%/SiO2 and V2%-P2%/SiO2 were 411.9 and 388.9 m2/g, respectively.

18

However, superfluous P rapidly reduced the specific surface areas of catalysts. This

19

result indicated that the amount of P increased the BET surface of V-2%/SiO2. Higher

20

P-loading resulted in the pore of the support and a part of active species were covered.

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Table 1. Texture properties for the series of V/SiO2catalysts with different V contents.

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Table 2. Texture properties for the series of catalysts V2%-P/SiO2 with different P contents.


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3.1.3 TEM Analysis

2

The transmission electron microscopy (TEM) of the samples are shown in Figs.

3

3 and 4, The results revealed the changes in particle size and active constituents of

4

catalysts with different amount of V and P. The morphology of SiO2 was a regularly

5

porous and spongy-like material

6

catalyst with an addition of 2 wt. % V was irregular structure. Figs. 3b, 3c, and 3d

7

were the electron micrographs of the catalysts with 5, 10, and 15 wt. % additions of

8

V, respectively. Where active components presented the same irregular structure, and

9

the particle morphotogies were apparent. With the increasing addition of V, crystal

10

particles size ranged from 6 up to 10 nm, which also confirmed by XRD results. Fig.

11

4 also shown that with greater addition of phosphorus, larger and more irregular

12

structures were obtained 21.

[20]

. It can be seen from Fig. 3a that the supported

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Fig. 3. TEM of V/SiO2: (a) V-2%/SiO2, (b) V-5%/SiO2, (c) V-10%/SiO2, (d) V-15%/SiO2.

14

Fig. 4. TEM of V2%-P/SiO2: V2%-P2%/SiO2, (b) V2%-P4%/SiO2, (c) V2%-P8%/SiO2, (d)

15

V2%-P12%/SiO2.

16

3.1.4 Thermal Analysis (TG/DTA)

17

The thermal decomposition and stability of fresh V-2%/SiO2 (uncalcinated) and

18

V2%-P4%/SiO2 (uncalcinated) samples were investigated by TG/DTA method.

19

TG/DTA curves are presented in Figs. 5 and 6, respectively. For the V-2%/SiO2

20

sample, a weight loss between room temperature and 75 °C in the TG curve alonged

21

with a weak endothermic peak in DTA curve was due to the loss of physical adsorbed

22

water and partial ammonia elimination. The following apparent weight loss between

23

75 and 300 °C was attributed to the decomposition of oxalic acid, as well as the



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1

release of ammonia 22. A slightly weight loss between 300 and 650 °C was dectected,

2

and catalyst V-2%/SiO2 was ultimately stabilized at 650 °C. This finding indicated

3

that when the catalyst V-2%/SiO2 was calcined at 650 °C, the ammonium

4

metavanadate was completely decomposed and the V2O5 phase was formed. As

5

shown in Fig. 6, three inflection points were observed in the TG curve of catalyst

6

V2%-P4%/SiO2, namely 100 °C, 200 °C, and 550 °C. The first weight loss between

7

room temperature and 100 °C was attributed to the loss of physical adsorbed water

8

and partial ammonia elimination. Due to the decomposition of oxalic acid and the

9

release of ammonia, an obvious weight loss was detected between 100 °C and 200 °C.

10

The catalyst V2%-P4%/SiO2 was ultimately stabilized at 550 °C, and VOPO4 phase

11

formed. According to the references, both V4+ (VO2 and (VO)2P2O7) ) and V5+ (V2O5

12

and VOPO4) ) were essential active phase, However, high calcination temperature was

13

not conducive to the formation of V4+, and the kinds of non active phases increased

14

with the increasing of calcination temperature. Hence, the calcination temperature of

15

catalyst was 500 °C. According to the results, after adding P, the minimum

16

temperature required to form stable structure of catalyst V2%-P4%/SiO2 was lower

17

than the required temperature of catalyst V-2%/SiO2.

18

Fig. 5. TG/DTA curve of V-2%/SiO2.

19

Fig. 6. TG/DTA curve of V2%-P4%/SiO2.

20

3.1.5 XPS Studies

21

The chemical state of V on the surface of catalysts V/SiO2 and V-P/SiO2 was

22

investigated by XPS. And they are shown in Figs. 7 and 8, respectively. The

23

corresponding binding energies (BEs) and intensity ratios are summarized in Tables 3

24

and 4. Given the presence of errors in catalyst charge correction, Si 2p was selected as


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a reference with values between 103.19 and 103.21 eV. As reported, binding energy

2

of V4+ 2p3/2 was 516.6 eV, and V5+ 2p3/2 was 517.7 eV

3

(V4+) and V2O5 (V5+), and (VO)2P2O4 (V4+) and VOPO4 (V5+) exsited on catalysts

4

Vx/SiO2 and V2-Py/SiO2 surface simultaneously, which were confirmed by XRD

5

analysis. In such case, the deconvolution of V 2p3/2 peak in V5+ (517.7 eV) and V4+

6

(516.6 eV) contributions are also shown and summarized in Table 3 and 4. In view of

7

Fig. 7 and Table. 3, the suface of samples were composed of VO2 (V4+) and V2O5

8

(V5+). Both VO2 (V4+) and V2O5 (V5+) were effective V oxidative states that

9

contribute to the catalyst performance. Previous research results revealed that a part of

10

VO2 (V4+) promote a synergistic effect of VO2 (V4+) and V2O5 (V5+), which

11

determined the good catalytic performance of catalyst for aldol condensation

12

catalysts Vx/SiO2, a detectable amount of surface (VO2) V4+ was found to be

13

effective. By varying the P/V ratios over a range 0.5 to 6, the ratio of (VO)2P2O7 (V4+)

14

/ VOPO4 (V5+) changed. The general observed trend was the oxidation of V4+ with

15

increasing P/V ratios, and the result are shown in Fig. 8 and Table 4. The ratios of

16

V4+/V5+ decreased from 1.64 to 0.14 as P loading increased from 1 to 12 wt. %,

17

namely the increase in P content was followed by a significant increase in VOPO4

18

(V5+) phase, which was the main catalytic active phase. In conclusion

19

presence of excessive surface P element was favorable for promoting formation of

20

VOPO4 (V5+) phase. However, a slight P-loading was play a positive role in favorable

21

for stabilizing (VO)2P2O7 (V4+), which had a deleterious effect on catalytic

22

performance.

23-25


. The active phases, VO2

26

. For

27, 28

, the


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Fig. 7. V 2p3/2 curve fittings of V/SiO2 catalysts with different V contents

2

Fig. 8. V 2p3/2 curve fittings of V2%-P/SiO2 catalysts with different P contents.

3

Table 3. XPS results for the catalysts V/SiO2 with different V contents.

4

Table 4. XPS results for the catalysts V2%-P/SiO2 with different P contents.

5

3.1.6 NH3–TPD and CO2-TPD Analysis

6

The surface acidity and basicity of catalysts, V/SiO2 and V-P/SiO2 were

7

estimated by NH3 and CO2 adsorption. The desorptions of NH3 and CO2 determined

8

by the TPD experiments are shown in Figs. 9 and 10, respectively. The peak areas

9

represented the desorptions of NH3 and CO2 bound to the acid and base sites of the

10

oxide surface. Higher desorption temperature corresponded to the stronger strength of

11

acid and base of catalysts 29. Fig. 9a shows the desorption profiles of NH3 of different

12

V-loaded catalysts. All catalysts revealed certain affinity for NH3. However, all

13

desorption peaks were found within 300 °C, which indicated that all catalysts only

14

possessed weak acid sites. The main peak of V/SiO2 catalyst enlarged with increased

15

V content from 0.5 to 15 wt. %, and it meant the number of acid site increased. On the

16

other hand, the NH3 adsorptive capacity of V-P/SiO2 sample is presented in Fig. 9b.

17

Compared with catalysts V-2%/SiO2, the desorption peak of V2%-P/SiO2 shifted

18

toward higher temperature. In addition, the amount of acid site was higher on

19

V2%-P/SiO2 samples than that on catalyst V-2%/SiO2, which indicated that the

20

addition of P increased the number and strength of acid site of catalyst V-2%/SiO2.

21

The characteristic CO2-TPD patterns for catalysts V-2%/SiO2 and V2%-P/SiO2

22

are shown in Figs. 10a and 10b. Figure 10a compares desorption of CO2 on SiO2


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loaded with different amounts of V. The desorption peak between 125 and 225 °C

2

was attributed to weak strength base sites. However, the desorption peaks shifted

3

towards higher temperature with the increasing content of V, indicating the gradually

4

strengthened basicity. The desorption peaks between 225 and 380 °C were

5

mid-strength base sites. The onset temperature above of 380 °C was assigned to

6

strong base sites, and catalysts V-5%/SiO2, V-10%/SiO2, and V-15%/SiO2 presented

7

stronger basicity than other catalysts. On the other hand, the comparison of desorption

8

profiles of CO2 from catalyst V-2%/SiO2 with different amounts of P is shown in Fig.

9

10b, which indicate that P-loaded samples present no significant change on

10

V-2%/SiO2, whereas, catalyst V2%-P4%/SiO2 shows greater CO2 uptake at weak base

11

sites than the other samples.

12

In conclusion, the addition of P was related to the changes of surface acidity and

13

basicity of V-2%/SiO2, namely, provided part of middle acid sites and weakened the

14

strong base sites, which agreed with literature previous report 30.

15

Fig. 9a. NH3 -TPD profiles of V/SiO2 samples with different V contents.

16

Fig. 9b. NH3 -TPD profiles of V2%-P/SiO2 samples with different P contents.

17

Fig. 10a. CO2 -TPD profiles of V/SiO2 samples with diferent V contents.

18

Fig. 10b. CO2 -TPD profiles of V2%-P/SiO2 samples with different P contents.

19

3.1.7 Pyridine-FTIR analysis

20

Figs. 11 and 12 show the different FTIR spectra of the pyridine desorped at 50,

21

100, 200, and 300 °C on catalysts V-2%/SiO2 and V2%-P4%/SiO2, respectively. Note

22

that the intensity of the bands was proportional to the concentration of acid sites. As



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

shown in Fig. 11, the V-2%/SiO2 sample showed the typical pyridine adsorption

2

peaks on Lewis acid sites near 1450, and 1598 cm-1 after adsorption at 30 °C and gas

3

phase evacuation for 30 min,. There were weak bands at 1540 cm-1, which indicated

4

that there was a few number of Brønsted acid sites on the surface of V-2%/SiO2

5

sample. The band corresponding to the combination of both Brønsted and Lewis acid

6

sites appeared at 1490 cm-1 19. Both the amount and the strength of Lewis acid sites,

7

and Brønsted acid sites decreased with the increasing of temperature. Generally,

8

pyridine adsorbed on weak acid sites desorbed at low temperature, it adsorbed on

9

strong acid sites desorbed at high temperature. Only Lewis acid sites could be

10

detected on the catalyst above 300 °C. The FTIR spectra of catalyst V2%-P4%/SiO2

11

obtained after pyridine desorption at various temperature is illustrated in Fig. 12.

12

From Fig. 11 and 12, the comparing between the V-2%/SiO2 and V2%-P4%/SiO2

13

catalyst, near 1548 cm-1, the typical band of pyridine on Brønsted acid sites was

14

detected and the intensity of V2%-P4%/SiO2 catalyst was show higher than

15

V-2%/SiO2 catalyst. Notably, the amount and strength of Lewis acid sites were low.

16

The bonds corresponding to Lewis acid sites decreased with the increasing of

17

degassing temperature from 50 to 300 °C. However, the Brønsted acidity did not have

18

significant change. To summarize, the combination of V and P formed a part of stable

19

and strong Brønsted acid sites, which matched with the CO2, and NH3-TPD results.

20

Fig. 11. Pyridine FTIR spectra as a function of temperature for the catalyst V-2%/SiO2.

21

Fig. 12. Pyridine-FTIR spectra as a function of temperature for the catalyst V2%-P4%/SiO2.


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1

3.2 Effects of the amounts of V and P supported on silica in the aldol

2

condensation of AA with formalin

3

The aldol condensation of acetic acid with formalin to form AA was conducted

4

with a series of catalysts V/SiO2 and V2%-P/SiO2, which were calcinated at 500°C

5

for 6 h. The mixture of acetic acid and formalin was fed with a rate of 0.1 mL/min at a

6

mole ratio of 10 to 1. The yields and selectivities of AA conducted on catalysts

7

V/SiO2 at 350 °C are shown in Fig. 13a. Catalyst V-2%/SiO2 presented better

8

catalytic activity. Selectivities did not present a significant difference and stabilized

9

around 98%. As shown in Fig. 13b, the best yield is obtained with the catalyst

10

V2%-P4%/SiO2 at 370 °C, and selectivities are stabilized around 98%. In conclusion,

11

the amount of V and P had great influence on catalyst performance, namely, the yield

12

of AA. On the other hand, it had little effect on selectivity of AA products. This

13

finding may be due to the following reasons. First, the XRD results coincided with the

14

changes of catalytic activity. No obvious characteristic diffraction peak was detected

15

in XRD profile of the V-2%/SiO2 sample. Thus, the active phase of the V-2%/SiO2

16

samples dispersed uniformly on the catalyst surface. This result was also confirmed

17

by the TEM image of catalyst V-2%/SiO2 in Fig. 3a. From the XPS result, tetravalent

18

and pentavalent vanadium coexisted in catalyst V-2%/SiO2 with a ratio of 0.46,

19

namely V2O5 (V5+) was the main active center, which corresponded well with the

20

previous reports of the literatures 16, 31, 32. As mentioned in the TPD of NH3, CO2, and

21

pyridine-IR analysis, V-2%/SiO2 possessed a weak base site, which is essential for

22

aldol condensation, as well as weak Lewis and Brønsted acid sites that improved the



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1

selectivity of catalyst. For catalyst V2%-P4%/SiO2, the characteristic diffraction

2

peaks of (VO)2P2O7 and VOPO4 phase were investigated, which were considerable in

3

catalytic performance. 4 wt. % P that added into catalyst V-2%/SiO2, the V4+/V5+ ratio

4

of catalyst V2%-P4%/SiO2 was 0.58, which promoted the formation of VOPO4 phase

5

and stabilized (VO)2P2O7 phase. Meanwhile, according to the TPD and pyridine-IR

6

analysis, 4 wt. % P effectively regulated the acid-base property of the catalyst surface.

7

The V2%-P4%/SiO2 sample possessed appropriate acidity and basicity. All catalysts

8

were evaluated at 330, 350, and 370 °C, and the results were presented in Figs. 14a

9

and 14b. AA yield increased with the increasing of reaction temperature, which

10

revealed the great influence of reaction temperature on the synthetic reaction.

11

Fig. 13a. Effect of the amount of V on the yields and selectivities of AA.

12

Fig. 13b. Effect of the amount of P on the yields and selectivities of AA.

13

Fig. 14a. Effect of the reaction temperature on the yield of AA.

14

Fig. 14b. Effect of the reaction temperature on the yield of AA.

15

3.3 Catalyst Stability and Reusability

16

The stability and reusability of catalyst V2%-P4%/SiO2 (calcinated at 500°C for

17

6 h) was also evaluated. The reaction was performed at 370 oC; the mixture of acetic

18

acid and formalin was fed with a rate of 0.1 mL/min at a molar ratio of 10/1. Figure

19

15 showed the yield of AA. The activity dropped by nearly 50 % after reacting of 70

20

h. The deactivation mechanism may be due to the formation of carbonaceous deposits

21

or the loss of active components on the surface of the sample. The used catalyst was

22

regenerated by calcination in air for 24 h at 400 oC, The regenerated catalyst was


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1

evaluated according to the same condition with the fresh catalyst by reaction 70 h.

2

The catalyst was repeatedly regenerated 3 times and total operation time was over 300

3

h. And the catalytic properties could be completely restored after regeneration. Fig. 15. Yield of AA on the fresh catalyst.

4 5

4. Conclusions

6

Four series of catalysts prepared by ultrasonic impregnation method had been

7

evaluated for the aldol condensation reaction of acetic acid with formalin to produce

8

AA. The optimal yield and selectivity of AA were 21.9% and 97.8%, respectively.

9

According to TG/DTA analysis, only VOPO4 phase was formed when the catalyst

10

precursor calcinated at or above 550 °C. Hence, calcination temperature of catalyst at

11

500 °C was propitious to form a coexistence of V4+ and V5+. The evaluation result of

12

calcination temperature indicated that catalyst V2%-P/SiO2 calcined around 500 °C

13

had a better effect on activation of catalyst precursor. Around this temperature, both

14

(VO)2P2O7 and VOPO4 phases were obtained. The physicochemical properties and

15

catalytic activity of V-2%/SiO2 and V2%-P/SiO2 were studied. The evaluation results

16

indicated that catalyst V2%-P4%/SiO2 exhibited a better catalytic activity at 370 °C

17

for this reaction, and this result was supported by a series of characterization analyses.

18

According to the evalution and characterization results, the balance between acid and

19

base sites and the ratio of V4+ and V5+ had much more influence on catalytic

20

performance than other properties. Furthermore, the optimal catalytic activity was

21

obtained when weak acid and base sites coexisted in the catalyst at the V4+ and V5+

22

ratio of 0.58.



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1

Acknowledgments

2

The authors gratefully acknowledge financial support from the National Basic

3

Research Program of China (2015CB251401), the National Science Fund for

4

Excellent Young Scholars (No. 21422607) and the National Natural Science

5

Fundation of China (No. 21276267).

6 7


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Literature Cited [1]. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice [M], New York: Oxford University Press. 1998: 11-56. [2]. d’Alnoncourt, R. N. The reaction network in propane oxidation over phase-pure MoVTeNb M1 oxide catalysts. J. Catal. 2014, 311, 369-385. [3]. Jo, B. Y; Kum, S. S.; Moon, S. H. Performance of WOx-added Mo–V–Te–Nb–O catalysts in the partial oxidation of propane to acrylic acid. Appl. Catal. A: Gen. 2010,378, 76-82. [4]. Ai, M.; Fujihashi, H.; Hosoi, S.; Yoshida, A. Production of methacrylic acid by vapor-phase aldol condensation of propionic acid with formaldehyde over silica-supported metal phosphate catalysts. Appl. Catal. A: Gen. 2003, 252, 185-191. [5]. Tai, J. R.; Davis, R. J. Synthesis of methacrylic acid by aldol condensation of propionic acid with formaldehyde over acid-base bifunctional catalysts. Catal. Today. 2007, 123, 42-49. [6]. Ai, M. Formation of methyl methacrylate by condensation of methyl propionate with formaldehyde over silica-supported cesium hydroxide catalysts. Appl. Catal. A: Gen. 2005, 288, 211-215. [7]. Bailey, O. H.; Montag, R. A.; Yoo, J. S. Methacrylic acid synthesis: I. Condensation of propionic acid with formaldehyde over alkali metal cation on silica catalysts. Appl. Catal. A: Gen. 1992, 88, 163. [8]. Yan, J. B.; Zhang, C. L.; Ning, C. L.; Tang, Y.; Zhang, Y.; Chen, L. L.; Gao, S.; Wang, Z. L. Zhang, W. X. Vapor phase condensation of methyl acetate with formaldehyde to preparing methyl acrylate over cesium supported SBA-15 catalyst. J. Ind. Eng. Chem. 2014, 2296, 8. [9]. Albanesi, G.; Moggi, P. Methyl methacrylate by gas-phase catalytic condensation of formaldehyde with methyl propionate. Appl. Catal. 1983, 6, 293-306. [10]. Spivey, J. J.; Gogate, M. R.; Zoeller, J. R.; Collerg, R. D. Synthesis of methyl methacrylate by vapor phase condensation of formaldehyde with propionate derivativesInd. Eng. Chem. Res. 1997, 36, 243-254. [11]. Paulis, M.; Martin, M.; Soria, D. B.; Odriozola, J. A.; Montes, M. Preparation and characterization of nibium oxide for the catalytic aldol condensation of acetone. Appl. Catal. A: Gen. 1999, 180, 411-420. [12]. Li, B.; Yan, R. Y.; Wang, L.; Dao, Y. Y.; Li, Z. X.; Zhang, S. J. Synthesis of methyl methacrylate by aldol condensation of methyl propionate with formaldehyde over acid–base bifunctional catalysts. Catal Lett. 2013, 143, 829–838. [13]. Ai, M. Vapor-phase aldol condensation of formaldehyde with acetic acid on V2O5-P2O5 catalysts. J. Catal. 1987, 107, 201-208. [14]. Ronald, A. Catalyst for a n-butane oxidation to maleic anhydride. [P] US3864280. 1975-04-19. [15]. Feng, X. Z.; Sun, B.; Yao, Y.; Su, Q.; Ji, W. J.; Au, C. T. Renewable production of acrylic acid and its derivative: New insights into the aldol condensation route over the vanadium phosphorus oxides. J. Catal. 2014, 314, 132–141. [16]. Zeidan, R. K.; Davis, M. E. The effect of acid-base pairing on catalysis: an efficient acid-base functionalized catalyst for aldol condensation. J. Catal. 2007, 247, 379-382. [17]. Xu, C. Y.; Pang, M. J. Preparation of VO2 powder by deoxidizing V2O5. J. Mat. Sci. Eng. 2006, 24, 252.



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[18]. Doornkamp, C.; Clement, M.; Gao, X.; Wache, I. E.; Ponec, V. The oxygen isotopic exchange reaction on vanadium oxide catalysts. J. Catal. 1999, 185, 415–422. [19]. Abdelouahad, K. A.; Roullet, M.; Burrows, M.; Kiely, C. J.; Voita, J. C.; Abon, M. Surface alteration of (VO)2P2O7 by α-Sb2O4 as a route to control the n-butane selective oxidation. Appl. Catal. A: Gen. 2001, 210, 121-136. [20]. Wang, F.; Dubois, J. L.; Ueda, W. Catalytic performance of vanadium pyrophosphate oxides (VPO) in the oxidative dehydration of glycerol. Appl. Catal. A: Gen. 2010, 376, 25–32. [21]. Taufiq-Yap, Y. H.; Looi, M. H.; Waugh, K. C.; Hussein, M. Z.; Zainal, Z.; Samsuddin, R. The effect of the duration of n-butane/air pretreatment on the morphology and reactivity of (VO)2P2O7 catalysts. Catal. Lett. 2001, 74, 99-104. [22]. Du, J. M.; Li, Q.; Fu, L. X. V2O5/SiO2 catalyst: synthesis, characterization and investigation of their catalytical property, Journal of Hebei University of Science and Technology. 2010, 31, 166-171. [23]. Reddy, B. M.; Ganesh, I.; Reddy, E. P. Study of dispersion and thermal stability of V2O5 /TiO2-SiO2 catalysts by XPS and other techniques. J. Phys. Chem. B. 1997, 101, 1769-1774. [24]. Rihko-Struckmann, L. K.; Ye,Y.; Chalakov, L.; Suchorski, Y.; Weiss, H.; Sundmacher, K. Bulk and surface properties of a VPO catalyst used in an electrochemical membrane reactor: conductivity-, XRD-, TPO- and XPS-study. Catal. Lett. 2006, 109, 89-96. [25]. Abon, M.; Bere, K. E.; Tuel, A.; Delichere, P. Evolution of VPO catalyst in n-butane oxidation reaction during the activation time. J. Catal. 1995, 156, 28-36. [26]. Zou, Y. N.; Zhao, J.; Ji, W. J.; Chen, Y. Influence of V5+ species on the catalytic performance of the vanadium phosphorous oxide catalysts. Chin. J. inorg. chem. 2003, 19, 497-500. [27]. Cornaglia, L. M.; Lombardo, E. A. XPS studies of the surface oxidation states on vanadium-phosphorus-oxygen (VPO) equilibrated catalysts. Appl. Catal. A: Gen. 1995, 127, 125-138. [28]. Richter, F.; Papp, H.; Wolf, G. U.; Kubias, B. Study of the surface composition of vanadyl pyrophosphate catalysts by XPS and ISS – Influence of Cs+ and water vapor on the surface P/V ratio of (VO)2P2O7 catalysts. Fresenius J. Anal. Chem. 1999, 365, 50-153. [29]. Zhu, M. L.; Li, S.; Li, Z. X.; Lu, X. M.; Zhang, S. J. Investigation of solid catalysts for glycolysis of polyethylene terephthalate. Chem. Eng. J. 2012, 185-186,168-177. [30]. Landi, G.; Lisi, L.; Volta, J. C. Oxidation of propane to acrylic acid over vanadyl pyrophosphate: modifications of the structural and acid properties during the precursor activation and their relationship with catalytic performances. J. Mol. Catal. A: Chem. 2004, 222, 175–181. [31]. Tanner, R.; Gill, P.; Wells, R.; Bailie, J. E.; Kelly, G.; Jackson, S. D.; Hutchinqs, G. J. Aldol condensation reactions of acetone and formaldehyde over vanadium phosphate catalysts: comments on the acid-base propenies. Phys. Chem. Chem. Phys. 2002, 4, 688-695. [32]. Jing, T.; Tian, J. Z.; Zheng, Y. J.; Sun, D. Z. Synthesis of methyl acrylate with Cs-Sb2O5/SiO2 as catalyst. Chem. Eng. 2010, 38, 5.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Table and Figure Captions Table 1. Texture properties for the series of catalysts V/SiO2 with different V contents. Table 2. Texture properties for the series of catalysts V2%-P/SiO2 with different P contents. Table 3. XPS results for the catalysts V/SiO2 with different V contents. Table 4. XPS results for the catalysts V2%-P/SiO2 with different P contents. Fig.1. XRD patterns of fresh catalysts V/SiO2 with differen V contents. Fig.2. XRD patterns of fresh catalysts V-P/SiO2 with different P contents. Fig. 3. TEM of V/SiO2: (a) V-2%/SiO2, (b) V-5%/SiO2, (c) V-10%/SiO2, (d) V-15%/SiO2. Fig. 4. TEM of V2%-P/SiO2: V2%-P2%/SiO2, (b) V2%-P4%/SiO2, (c) V2%-P8%/SiO2, (d) V2%-P12%/SiO2. Fig. 5. TG/DTA curve of V-2%/SiO2. Fig. 6. TG/DTA curve of V2%-P4%/SiO2. Fig. 7. V 2p3/2 curve fittings of V/SiO2 catalysts with different V contents Fig. 8. V 2p3/2 curve fittings of V2%-P/SiO2 catalysts with different P contents. Fig. 9a. NH3 -TPD profiles of V/SiO2 samples with different V contents. Fig. 9b. NH3 -TPD profiles of V2%-P/SiO2 samples with different P contents. Fig. 10a. CO2 -TPD profiles of V/SiO2 samples with diferent V contents. Fig. 10b. CO2 -TPD profiles of V2%-P/SiO2 samples with different P contents. Fig. 11. Pyridine FTIR spectra as a function of temperature for the catalyst V-2%/SiO2. Fig. 12. Pyridine-FTIR spectra as a function of temperature for the catalyst V2%-P4%/SiO2. Fig. 13a. Effect of the amount of V on the yields and selectivities of AA. Fig. 14. Effect of the amount of P on the yields and selectivities of AA. Fig. 14a. Effect of the reaction temperature on the yield of AA. Fig. 14b. Effect of the reaction temperature on the yield of AA. Fig. 15. Yield of AA on the fresh catalyst.



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Page 22 of 42

1 2

Table 1. Texture properties for the series of catalysts V/SiO2 with different V contents. Total pore

Surface area(m g )

Average pore diameter(nm)

volume(cm g )

V-0.5%/SiO2

463.4

8.83

1.067

V-1%/SiO2

455.6

9.06

1.030

V-2%/SiO2

362.7

9.02

0.861

V-5%/SiO2

322.6

9.75

0.808

V-10%/SiO2

252.5

11.38

0.753

V-15%/SiO2

224.3

11.63

0.645

Catalyst

2 -1

3 -1

3

4

Table 2. Texture properties for the series of catalysts V2%-P/SiO2 with different P contents Total pore

Catalyst

Surface area(m g )

Average pore diameter(nm)

V2%-P1%/ SiO2

411.9

8.72

0.972

V2%-P2%/ SiO2

383.6

9.01

0.921

V2%-P4%/ SiO2

282.2

11.89

0.850

V2%-P8%/ SiO2

146.9

22.76

0.745

V2%-P12%/ SiO2

29.3

40.37

0.229

2 -1

5 6 7 8 9 10 11


3 -1

volume(cm g )

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1

Table 3. XPS results for the catalysts V/SiO2 with different V contents.

Catalyst

2 3

Binding energy(eV)

Surface composition

Si 2p

V 2p3/2

V4+/V5+

V-0.5%/ SiO2

103.89

517.74

0.28

V-1%/ SiO2

104.10

517.89

0.39

V-2%/ SiO2

104.07

517.70

0.46

V-5%/ SiO2

104.00

517.63

0.61

V-10%/ SiO2

103.64

517.57

0.49

V-15%/ SiO2

103.56

517.50

0.12

Table 4. XPS results for the catalysts V2%-P/SiO2 with different P contents.

Catalyst

Binding energy(eV) Si 2p

Surface composition

V 2p3/2

V4+/V5+

V2%-P1%/ SiO2

103.17

517.03

1.64

V2%-P2%/ SiO2

103.18

517.12

0.76

V2%-P4%/ SiO2

103.20

517.17

0.58

V2%-P8%/ SiO2

103.28

517.29

0.55

V2%-P12%/ SiO2

103.21

517.34

0.14

4



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Figure 1. Texture properties for the series of catalysts V/SiO2 with different V contents. 64x52mm (300 x 300 DPI)


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Figure 2. Texture properties for the series of catalysts V2%-P/SiO2 with different P contents. 65x54mm (300 x 300 DPI)



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Fig. 3. TEM of V/SiO2: (a) V-2%/SiO2, (b) V-5%/SiO2, (c) V-10%/SiO2, (d) V-15%/SiO2. 54x36mm (300 x 300 DPI)


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Fig. 4. TEM of V2%-P/SiO2: V2%-P2%/SiO2, (b) V2%-P4%/SiO2, (c) V2%-P8%/SiO2, (d) V2%P12%/SiO2. 53x36mm (300 x 300 DPI)



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Fig. 5. TG/DTA curve of catalyst V-2%/SiO2. 58x42mm (300 x 300 DPI)


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Fig. 6. TG/DTA curve of catalystV2%-P4%/SiO2. 58x42mm (300 x 300 DPI)



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Figure 7. V 2p3/2 curve fittings of V/SiO2 catalysts with different V contents 68x58mm (300 x 300 DPI)


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Fig. 8. V 2p3/2 curve fittings of V2%-P/SiO2 catalyst with different P contents. 62x48mm (300 x 300 DPI)



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Figure 9a. NH3 -TPD profiles of V/SiO2 samples with different V contents. 61x47mm (300 x 300 DPI)


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Figure 9b. NH3 -TPD profiles of V2%-P/SiO2 samples with different P contents. 61x47mm (300 x 300 DPI)



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Figure 10a. CO2 -TPD profiles of V/SiO2 samples with diferent V contents. 65x53mm (300 x 300 DPI)


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Figure 10b. CO2 -TPD profiles of V2%-P/SiO2 samples with different P contents. 65x53mm (300 x 300 DPI)



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Figure 11. Pyridine FTIR spectra as a function of temperature for the catalyst V-2%/SiO2. 62x48mm (300 x 300 DPI)


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Figure 12. Pyridine-FTIR spectra as a function of temperature for the catalyst V2%-P4%/SiO2. 63x50mm (300 x 300 DPI)



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Figure 13a. Effect of the amount of V on the yields and selectivities of AA. 56x40mm (300 x 300 DPI)


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Figure 13b. Effect of the amount of P on the yields and selectivities of AA. 57x41mm (300 x 300 DPI)



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Figure 14a. Effect of the reaction temperature on the yield of AA. 62x49mm (300 x 300 DPI)


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Figure 14b. Effect of the reaction temperature on the yield of AA. 62x49mm (300 x 300 DPI)



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Figure 15. Yield of AA on the fresh catalyst. 63x49mm (300 x 300 DPI)


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Reaction of Formalin with Acetic Acid over VanadiumâPhosphorus

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