acetic acid modification

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Kinetics, Catalysis, and Reaction Engineering

Production of IPA from MX catalyzed by Co(II) and HPW@C modified with acetic acid Xinzhi zhou, Zhihao Wang, Zhouwen Fang, Huajie Liu, and Xiangli Long Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01820 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Industrial & Engineering Chemistry Research

acetic acid modification

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

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Production of IPA from MX catalyzed by Co(II) and HPW@C modified with acetic acid Xin-zhi Zhou, Zhi-hao Wang, Zhou-wen Fang, Hua-jie Liu and Xiang-li Long∗ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, People’s Republic of China

∗

Corresponding author. Tel.: +86 21 6425 3267; fax: +86 21 6425 3528; E-mail: [email protected] (X. L. Long)

1


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Abstract Isophthalic acid produced from the oxidation of m-xylene by air is catalyzed by H3PW12O40 (HPW) loaded on carbon and cobalt. In this paper, acetic acid solution is used to treat the carbon to ameliorate the catalytic ability of HPW@C catalyst. The experiments indicate that the best supporter is obtained by immersing the carbon in 3.5 mol l-1 HOAc for 24 h followed being carbonized at 500 ℃ for 4 h. The surface characterization proves that the modification with HOAc augments acidic groups and amplifies specific surface area on the carbon surface.

The surface chemistry plays a more vital role than the

physical property in determining the catalytic ability of HPW@C. The IPA produced after 180 min reaction got by the HPW@C made of the carbon modified in the best condition is 76.7% over that acquired by the HPW@C made of the original carbon. Keywords: H3PW12O40; activated carbon; m-xylene; isophthalic acid; catalysis

2



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1. Introduction Isophthalic acid(IPA) is a vital chemical material utilized more and more widely, such as producing unsaturated polyester resin and comonomer, improving the comfort and softness of polyester fiber material and being utilized to synthesize refractory materials1-4. The commercial production technologies of IPA and TPA( terephthalic acid) are from the oxidation of p-xylene (PX) and m-xylene (MX) catalyzed by the homogeneous Co-Mn-Br system in HOAc solution.

Many scholars have paid attention to

optimizing the Co-Mn-Br system to improve IPA and TPA selectivity or yield. Some experts attempted to add a fourth active catalyst constituent, such as lanthanide or alkali metal, to accelerate the oxidation of PX and upgrade the selectivity of TPA5-9. Partenheimer5,6 reported that the oxidation of PX to TPA was improved by the addition of Zr4+ and Hf4+ into the Co-Mn-Br system. Cheng et al.7 reported that the introduction of Zr and Hf accelerated the side reactions more conspicuously than the main reactions. Jhung et al.8,9 investigated the role of alkali metals in the oxidation of xylenes. 10

Replacement of acetic acid

11

with water or utilization of carbon dioxide as a co-oxidant has been tried in the Co–Mn–Br system for the oxidation PX to TPA. Cheng et al.12 indicated that guanidine was able to enhance the transformation of PX to TPA catalyzed by Co–Mn–Br system and control the side reactions. Some scholars are looking for catalytic systems without bromide to engender TPA and IPA13 because bromide not only corrode the expensive reactors but also form CH3Br(g) that may consume O3 in the stratosphere.

Ishii et al.14 put forward a bromide-free catalyst system consisting of Co(OAc)2 and

N-hydroxyphthalimide(NHPI) to turn PX into PTA. Saha et al.15 acquired satisfactory PX oxidation rate and PTA yield by utilizing NHPI and its derivatives, such as 3-F–NHPI, as accelerators with Co(OAc)2 for the autoxidation of PX and other methyl arenes.

Wentzel et al.16 investigated the autoxidation of

ethylbenzene catalyzed with Co2+/NHPI. Kim et al.17 oxidized PX partially without a catalyst in sub and supercritical water at 513-673 K and 220-300 atm.

Acharyya et al.18 tried to apply three dimensional

hierarchical Ag/WO3 flower-like catalyst materials for the selective oxidation of m-xylene to isophthalic acid. But none of these technologies has been applied commercially due to the fast deactivation of the catalysts. Heteropoly acid (HPA) has been utilized as a catalyst in a lot of reactions due to its excellent properties.

HPA owns strong Brønsted acidity similar to super acid. The Hammett acidity H0 of

H3PW12O40 is -13.2, which is much greater than that of 100% H2SO4 (H0 = -11.9) 19-21.

Long et al.22 put

forward the oxidation of NMST to NMSBA with air under the catalysis of homogeneous H3PW12O40/ Co(OAc)2/Mn(OAc)2/Br system. Lv et al.13 turned MX to IPA with a catalyst system composed of H3PW12O40/ Co/Mn. HPAs supported hold a lot of advantages, such as easy separation from liquid, large surface area and less corrosion to the device.

Activated carbon has been found wide application in industry as catalyst

supporter for its big surface area, gigantic porous structure and characteristic flexibility. HPA loaded on activated carbon is thought to be a potential catalyst for liquid-solid catalytic reactions due to its extraordinary stability toward HPA leaching from the supporter.

Rafiee et al.23 loaded heteropoly acids on

activated carbon to catalyze the one-pot synthesis of β-acetamido ketones.

Zhao and Wang24 supported 3


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phosphotungstic acid on mesoporous carbon with bimodal pore to catalyze Friedel-Crafts alkenylation of aromatics with phenylacetylene.

Xiao et al25 investigated the oxidative desulfurization by hydrogen

peroxide catalyzed with a phosphotungstic acid/activated carbon. Long et al.26 supported H3PW12O40 on activated carbon to catalyze the oxidation of MX to IPA. The activated carbon owns complex porous structure and abundant nitrogen and oxygen-containing groups, such as amine, quinine, hydroxyl, phenol and carboxylic. Its physical structure and surface groups play an important part in adsorption and chemical reactions. The modification of activated carbon may improve its performance in adsorption and reactions by altering its chemical characteristics and porous structure. Guo et al.27 used humic acid to modify biomass-derived activated carbon to remove Pb(II) from aqueous solutions quickly and efficiently. Fu et al.28 removed NO with the activated carbon oxidized at 300℃ with a gas stream composed of 1.5% oxygen and 98.5% nitrogen. Wan et al.29 treated the activated carbon with non-thermal plasma to enhance its adsorption capability of Copper ions.

Lee et al.

30

ameliorated the adsorption of ammonium ion from aqueous solution by modifying the activated carbon with anionic surfactant. Tyagi et al.

31

improved the catalytic activity of activated carbon with acid

treatment in the conversion of microcrystalline cellulose to 5-hydroxymethyl furfural. Wang et al.32 enhanced the catalytic ability of HPW@C in the transformation of MX to IPA by treating the carbon with HNO3 solution and found that the IPA produced after 180 min operation gained by the HPW@C prepared from the carbon modified with HNO3 was 31.6% over that catalyzed by the HPW@C prepared from the original carbon. In this article, H3PW12O40 loaded on the coconut activated carbon treated with acetic acid solution and cobalt acetate function as the catalyst for the oxidation of MX to IPA. The modification condition with HOAc solution is explored systematically.

The chemical characteristics and physical

structure of carbon samples were determined by means of FT-IR, XPS, Boehm titration and BET specific surface analysis.

The relation of the carbon surface characteristics to the catalytic properties of

H3PW12O40@C is expounded by discussing the MX oxidation results and characterization data. 2 Experimental 2.1 Materials The activated carbon of coconut utilized in this study was produced by Shanghai Activated Carbon Co., Ltd. The following reagents got commercially were not purified before being used: Co(OAc)2.4H2O, H3PW12O4·34H2O, glacial acetic acid, isophthalic acid(IPA), m-xylene(MX), m-tolualdehyde (3-IMA), M-Totuic acid (M-TA), 3-carboxybenzaldehyde (3-CBA) and Potassium Phosphate Monobasic. These reagents are of analytical grade. CH3CN and CH3OH of chromatographic grade were supplied by J&K SCIENTIFIC LTD. 2.2 Modification of activated carbon The carbon of 20-40 mesh purged with deionized water till constant pH of effluent solution was dried at 110 ℃ under vacuum for 48 h before being treated with CH3COOH. The carbon was modified in the process described as follow : (1) 20 g activated carbon was immersed in 250 ml CH3COOH solution for some h at 40 ℃; (2) the carbon samples were dried under vacuum at 110℃ for 12 h after being separated from the CH3COOH solution; (3) the sample obtained was carbonized in nitrogen at scheduled temperature 4



Page 6 of 21

for some h. 2.3 Preparation of H3PW12O40@C catalyst The H3PW12O40@C catalyst was prepared by impregnating 15 g carbon sample in 60 ml HPW solution of 18.75g l-1 at 60 ℃ for 4 h followed by heating the mixture at 100℃ to evaporate the water. Then the sample was desiccated under vacuum at 110℃ for 12 h. The H3PW12O40@C catalyst was obtained after the carbon immobilized with H3PW12O40 being calcined under nitrogen at 220℃ for 4 h26. 2.4 Characterization Boehm titration was utilized to test the acid and alkaline groups quantitatively on the carbon surface33. An ASAP2020 Surface Analyzer (Micromeritics Co. USA) was used to detect the physical structure with adsorption-desorption isotherms of N2 at 77 K. BET method was applied to compute the overall surface FTIR34 was adopted to analyze the chemical groups on the carbon surface

of the carbon sample.

qualitatively. The parameters of the micropores and mesopores were acquired with t-plot method and BJH method, respectively. An ESCALAB 250 electron spectrometer from Thermo Corporation with 300 W AlKa radiation was used to test XPS spectra. The base pressure was sustained at 3×10-12 bar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The XPS peaks were deconvoluted with a non-linear least-squares curve fitting program (XPSPEAK4.1 software, version 2.0) with a Gaussian Lorent-zian mix function and Shirley background subtraction. The carbon 1s electron binding energy corresponding to graphitic carbon was referenced at 284.6 eV for calibration. Atomic proportion was reckoned in terms of the relative peak areas in the XPS spectra. 2.5 Analytical methods Based on the simplified reaction route from MX to IPA depicted in Figure 1, the components in the reaction liquor are MX, IPA, 3-IMA, PIA and 3-CBA. These components were separated in a C18 stainless steel column.

IPA, 3-IMA, M-TA and 3-CBA were tested by a UV detector at 230 nm, and MX

was tested at 220 nm. The mobile phase for HPLC is a mixture composed of 80% CH3CN solution and 0.03 mol l-1 KH2PO4 aqueous solution with a pH of 3.3. After the mobile phase having flowed for 40 min at a rate of 1.0 ml min-1 with a CH3CN/KH2PO4 ratio of 1:4 (volume), the CH3CN/KH2PO4 ratio was changed to 4:1. The concentrations of these constituents were calculated by external standard method. The standard equations gained are listed in Table 1, Xi means peak area and Yi stands for the mass concentration with unit of g g-1. Table 1 Standard equations Sample IPA 3-CBA m-TA

R2

Standard equations YIPA=1.58796×10-10 XIPA+1.26058×10-6 -10

Y3-CBA=1.74589×10

-10

Ym-TA=1.27313×10

X3-CBA -1.28407×10

Xm-TA-6.25067×10

-10

0.99931

3-IMA

Y3-IMA=4.49980×10

MX

YMX=3.43066×10-10 XMX-3.52656×10-6

-6

-7

X3-IMA -3.49858×10

0.99936 0.99966

-6

0.99986 0.99937

5


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Figure 1 Simplified reaction route for the production of IPA from MX oxidation

2.6 Experimental setup and procedure The oxidation of MX to IPA was carried out in the experimental setup as described in literatures13,23 (Figure 2). The oxidation reaction took place in a 1.0 L titanium autoclave. A condenser, a gas-liquid separator and a liquid recycle tank were installed to ensure the complete condensation and circulation of the volatile constituents. A cooling coil is fitted in the reactor to take the reaction heat off. The liquor is agitated at 300 rpm continuously. The deviation of the temperature in the reactor was kept by an electric heating jacket with a thermocouple less than 0.2 ℃. The fluctuation of the pressure in the reactor under the control of a regulating valve is within 2.0 KPa. In a typical experimental operation, the proposed amount of HOAc, H2O, Co(OAc)2·4H2O, MX and catalysts were added into the reactor in turn. N2 was introduced into the autoclave to displace O2 that may affect the accuracy of the experiment before heating the reactor. Then the pressure in the autoclave was lifted to 3.0 MPa with nitrogen. The autoclave was stirred at 300 rpm while it was warmed by an electric heating jacket. The MX oxidation was began with air flowing into the autoclave at a rate of 1000 ml min-1 when 200 ℃ reached. Liquid sample was taken every 30 min to determine the IPA yield and MX conversion.

Figure 2 Schematic of the experimental setup

6



3 Results and discussion 3.1 Effect of acetic acid concentration The acetic acid concentration is a vital parameter influencing the modification of activated carbon. Five carbon samples were soaked in CH3COOH solutions of 1.0 mol l-1, 2.0 mol l-1, 3.0 mol l-1, 3.5 mol l-1 and 4.0 mol l-1, respectively for 24 h at 40 ℃. Then the samples were carbonized in nitrogen at 500 ℃ for 4 h.

The modified carbons as well as the original carbon were immobilized with 7.5% (wt)

H3PW12O40 followed being calcined at 220℃ for 4 h at nitrogen. The HPW@C obtained and 0.29%(wt) Co(OAc)2 were used to accelerate the production of IPA from MX. The concentrations of MX, 3-IMA, M-TA, 3-CBA and IPA in the liquor were determined by HPLC every 0.5 h. The experimental results shown in Figure 3 manifest the effect of HOAc concentration on the catalytic performance of HPW@C catalyst. 100

3-IMA concentration/wt%

MX conversion/%

80 70 60 50

HOAc concentration HOAc concentration HOAc concentration HOAc concentration HOAc concentration Original carbon

40 30 20 10 0


0.28

90

0

20

40

60

80

100

1.0 2.0 3.0 3.5 4.0

120

-1

mol l -1 mol l -1 mol l -1 mol l -1 mol l

0.24 0.20

-1

1.0 mol l -1 2.0 mol l -1 3.0 mol l -1 3.5 mol l -1 4.0 mol l

0.16 0.12 0.08 0.04

140

160

0.00

180

0

20

40

60

80

100

120

140

160

180

time(min)

time(min)

(a)

(b) 0.18

2.1

0.16

3-CBA oncentration/wt%

1.8

m-TA concentration/wt%

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

Page 8 of 21

1.5 HOAc concentration HOAc concentration HOAc concentration HOAc concentration HOAc concentration Original carbon

1.2 0.9 0.6

1.0 2.0 3.0 3.5 4.0

-1

mol l -1 mol l -1 mol l -1 mol l -1 mol l

0.3 0.0

0.14 0.12 0.10 -1

HOAc concentration 1.0 mol l -1 HOAc concentration 2.0 mol l -1 HOAc concentration 3.0 mol l -1 HOAc concentration 3.5 mol l -1 HOAc concentration 4.0 mol l Original carbon

0.08 0.06 0.04 0.02

0

20

40

60

80

100

120

140

160

180

0.00

0

20

40

60

time(min)

80

100

120

140

160

180

time(min)

(c)

(d)

7


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


1.6


1.4

IPA concentration/wt%

Page 9 of 21

1.2 1.0

1.0 2.0 3.0 3.5 4.0

mol mol mol mol mol

-1

l -1 l -1 l -1 l -1 l

0.8 0.6 0.4 0.2 0.0

0

20

40

60

80

100

120

140

160

180

time(min)

(e) Figure 3(a,b,c,d,e). Effect of acetic acid concentration on MX oxidation 200 ℃, 3.0 MPa, Air fluent: 1.0 L min-1, Stiring speed: 300 r/min, HOAc: 400ml, HPW@C: 16.0 g l-1, MX: 3.53 %(wt), Co2+: 0.064 %(wt), H2O: 4.0 %(wt)

It can be concluded from Figure 3(a) that the activated carbon treated with acetic acid is able to enhance the MX conversion dramatically. The HPW@C catalyst prepared with the carbon modified with 3.5 mol l-1 HOAc obtained the highest MX oxidation rate.

For example, after 3.0 h operation, the MX

conversion got by the HPW@C catalyst prepared with the original carbon is 91.26% while that gained by the HPW@C catalyst prepared with the carbon soaked in 2.0 mol l-1 HOAc is 96.73%.

The MX

-1

conversion goes up from 95.30% to 97.44% as HOAc concentration is raised from 1.0 mol l to 3.5 mol l-1. However, the MX oxidation rate decreases if the HOAc concentration rises further above 3.5 mol l-1. The MX conversion obtained with the carbon impregnated in 4.0 mol·l-1 HOAc is 95.86%, which is lower than that gained by the carbon soaked in 3.5 mol·l-1 HOAc. Figure 3(c) manifests that the formation rate of m-TA catalyzed by the HPW@C catalyst prepared from modified carbon is quicker than that of the catalyst prepared from original carbon within the 120 min operation. 120 min later, the depletion rate of m-TA got by the catalyst prepared with the modified carbon exceeds the generation rate of m-TA and the m-TA concentration begins to drop. But the m-TA got by the HPW@C catalyst prepared with the original carbon still keeps rising.

A conclusion can be drawn that

the 3-IMA oxidation rate and the m-TA oxidation rate acquired by the modified samples are faster than those gained by the original one. Figure 3(e) manifests that the modified samples can produce more IPA than the original carbon. The highest IPA concentration is got by the catalyst prepared with the carbon modified with 3.5 mol l-1 HOAc. For instance, after 3.0 h run, the IPA produced by the original carbon is 0.896%(wt) while that engendered by the one modified with 2.0 mol l-1 HOAc is 1.35%(wt).

The IPA concentration obtained increases

from 1.26%(wt) to 1.59%(wt) as the HOAc concentration rises from 1.0 mol l-1 to 3.5 mol l-1. But the IPA concentration decreases from 1.59%(wt) to 1.38% (wt)as the HOAc concentration increases from 3.5 mol l-1 to 4.0 mol l-1. In a word, the optimum HOAc concentration is 3.5 mol l-1. The explanation that the modification with CH3COOH improves the catalytic ability of HPW@C may be given in terms of the changes in the physical structure and functional groups on the carbon surface resulted from the treatment with acetic acid. 8



100 Original carbon

95

% Transmittance

1700

1600

90

1550 870

Carbon m odified by HOAc

85

80 1100

75 3400

4000

3500

3000

2500

2000

1500

1000

Wavenumber(cm -1 )

500

Figure 4 FT-IR spectra of the original carbon and the carbon modified by acetic acid

The carbon samples were detected by FTIR to observe the change of their surface chemistry caused by HOAc modification. The FTIR spectrum of the original carbon and the one immersed in 3.5 mol l-1 CH3COOH are demonstrated in Figure 4.

According to literatures35-37, the peak around 3400 cm -1 in the

FTIR is owing to hydroxyl group O-H and chemical adsorbed H2O on the carbon surface. The bands at 1700 cm -1 is attributed to the symmetric and asymmetric stretching vibration of -C=O in lactone and carboxyl groups. The peak at 1100 cm-1 is ascribed to the phenolic -OH group and C-O group. The band at 870 cm-1 around comes from C-O-C symmetric and asymmetric stretching vibration in the ether.

It

can be deduced from Figure 4 that no new groups come into being on the carbon surface after being modified with CH3COOH. XPS analysis has been completed for the original carbon(OC) and the carbon(AC) soaked in 3.5 mol -1

l CH3COOH. 36000 28000

32000 28000

24000 1

24000

Intensity

20000

Intensity

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

Page 10 of 21

16000 12000

1

20000 16000 12000

8000

2

8000 2

4000

3 4

5

3

4000

6

0

4

5

6

0 280

284

288

292

296

280

Electron binding energy / eV

284

288

292

296


Figure 5 C 1s fitting curve of original carbon (left) and carbon treated with CH3COOH (right)

The C1s spectra illustrated in Figure 5 are resolved into six individual component peaks38. The molar percentages based on the peak resolution are shown in Table 2. The data in Table 2 suggest that graphitic carbon reduces from 56.64% to 46.78% after being treated with CH3COOH. The carbon in phenolic, alcohol or ether groups increases from 8.50% to 24.73%. The carbon in carbonyl or quinine groups 9


Page 11 of 21

decreases from 19.64% to 13.48%. The carbon in anhydride and ester group decreases from 6.71% to 3.91%. The CH3COOH modification leads to an increase from 1.80% to 7.88% in the molar percentage of carboxyl group. Table 2 Fitted C 1s peak parameters deduced from XPS for carbon samples Peaks

B.E(eV)

Assignment

OC(%)

AC(%)

1

284.1

C=C

2

284.8

C-OH, C-O-C, C-O-R

56.64

46.78

8.5

24.73

3

285.8

C=O (carbonyl)

19.64

13.48

4

288.3

COOR

6.71

3.91

5

289.4

COOH

1.80

7.88

5500

5000 5000

4500

2

2

Intensity

4500

Intensity

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


3

4000 4

3500

3

4000

3500

1 4

1

3000

3000

2500

2500 524

526

528

530

532

534

536

538

540

524

526

528

530

532

534

536

538

540



Figure 6 O 1s fitting curve of the original carbon (left) and the carbon treated with CH3COOH (right) Table 3 Fitted O 1s peak parameters deduced from XPS for carbon samples Peaks

B.E (eV)

1

530.6

2

531.9

3

533.2

4

534.7

Assignment

OC (%)

AC (%)

carbonyl and quinone group (C=O)

16.63

21.89

33.52

36.20

43.30

28.33

6.55

13.58

C=O in mide, anhydride,ester group and oxygen in alcohol, ether group(C-OH、 C-O-C) C-O in anhydride and ester group (CO-O-R) carboxyl (COOR)

The O 1s spectra resolved into four separated constituent peaks are depicted in Figure 6. The molar percentages in terms of the peak resolution are listed in Table 3. Table 3 manifests that the proportion of the oxygen in the carbonyl group and the quinone group increases from 16.63% to 21.89% and that of the oxygen in the carboxyl group goes up from 33.52% to 36.20%. The oxygen in the ester, anhydride, alcohol, ether decreases from 43.30% to 28.33%. And the oxygen in carboxyl rises from 6.55% to 13.58%. The XPS characterization exhibits that the HOAc treatment brings about great alteration in the surface chemistry on the carbon. The amount of carboxyl and lactonic groups increases greatly after acetic acid treatment. 10



Page 12 of 21

The Boehm titration data listed in Table 4 show that the HOAc modification brings about great change in acidic groups and basic groups on the carbon surface. The amount of total acidic groups of the carbon sample immersed in 3.5 mol l-1 HOAc is 1.76×10-4 mol g-1 while those of the original one is only 6.67×10-5 mol g-1. The carboxyl on such modified sample is 8.1 times that on the original one. The lactone on this modified carbon is 9.33×10-5 mol g-1 while that on the original one is 1.62×10-5 mol g-1. But the phenolic hydroxyl group decreases from 4.53×10-5 mol g-1 to 4.02×10-5 mol g-1 after being treated with 3.5 mol l-1 HOAc.

It can also be found that the gross alkaline groups on the modified samples reduce obviously.

The alkaline groups on the original carbon are 8.69×10-4 mol g-1 while those on the carbon modified with 3.5 mol l-1 HOAc are 6.44×10-4 mol g-1. The Boehm results agree well with the XPS analysis data. Table 4 demonstrates that the carboxylic groups increase with HOAc concentration. The carboxylic groups rises from 3.86×10-5 mol g-1 to 4.88×10-5 mol g-1 as HOAc concentration ranges from 1.0 mol l-1 to 4.0 mol l-1.

The acidic groups increase from 1.45×10-4 mol g-1 to 1.76×10-4 mol g-1 with HOAc

concentration increasing from 1.0 mol l-1 to 3.5 mol l-1. The lactone increases from 1.62×10-5 mol g-1 to 9.33×10-5 mol g-1 when HOAc concentration is raised from 1.0 mol l-1 to 3.5 mol l-1. But the acidic groups and the lactone reduce when HOAc concentration rises further to 4.0 mol l-1. The phenolic hydroxyl decreases from 4.53×10-5 mol g-1 to 4.02×10-5 mol g-1 as HOAc concentration is raised from 1.0 mol l-1 to 3.5 mol l-1. The phenolic hydroxyl increase to 4.28×10-5 mol·g-1 as HOAc concentration goes up to 4.0 mol l-1. The overall alkaline groups on the modified sample reduce with HOAc concentration. The shifting of carboxyl groups from HOAc to the carbon surface in the treatment with CH3COOH is the reason for the increase of the carboxyl group with HOAc concentration. The carboxyl group transferred on the carbon may combine with the phenolic hydroxyl when carbonized at high temperature, which leads to the augment of lactonic and the reduction of phenolic hydroxyl.

However, more acidic groups will

-1

disintegrate if HOAc concentration rises above 3.5 mol l because the carboxyl is apt to be turned into CO2 and H2O at the temperature above 400℃39-41. Table 4 Chemical groups of carbon samples (HOAc concentration) HAc concentration

Acidic groups 10-4 mol·g-1

Lactone 10-5mol g-1

Phenolic hydroxyl 10-5mol g-1

Carboxyl 10-5mol g-1

Basic groups 10-4 mol g-1

0 mol·l-1

0.667

1.62

4.53

0.519

8.69

1.0 mol·l-1

1.45

6.80

3.88

3.86

6.77

3.5 mol·l-1

1.76

9.33

4.02

4.20

6.44

4.0 mol·l-1

1.65

7.36

4.28

4.88

5.75

It can be seen from the physical data presented in Table 5 that the HOAc treatment results in the augment in total surface area and micropores. The SBET of the HPW@C catalyst prepared with the original carbon is 690.8 m2 g-1 while that of the catalyst prepared with the one soaked in 3.5 mol l-1 HOAc is 755.1 m2 g-1. The micropore area augments from 670.4 m2 g-1 to 692.2 m2 g-1. The reason for this phenomenon is that CH3COOH is liable to dissolve some soluble metal salt on the activated carbon, which 11


Page 13 of 21 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


is able to form some micropores on the carbon surface. The decomposition of HOAc transferred on the carbon surface may lead to an augment in the pores on the carbon. It can also be seen from Table 5 that SBET and mesopores increase with HOAc concentration. SBET increases from 719.0 m2 g-1 to 790.7 m2 g-1 and the mesopore area increases from 11.6 m2 g-1 to 99.6 m2 g-1 as HOAc concentration is raised from 1.0 mol l-1 to 4.0 mol l-1. Such phenomena are due to the fact that more pores are produced and some micropores are transformed into mesopores as HOAc concentration goes up. Table 5 Physical characteristics of HPW@ C catalysts (HOAc concentration) HAc

SBET

Smic

SBJH-A

Vt

Vmic

PBET-W

concentration

m2 g-1

m2 g-1

m2 g-1

cm3 g-1

cm3 g-1

nm

0 mol l-1

690.8

670.4

20.4

0.3651

0.3423

2.11

1.0 mol l-1

719.0

707.4

11.6

0.3671

0.3548

2.04

3.5 mol l-1

755.1

692.2

62.9

0.4197

0.3500

2.22

4.0 mol l-1

790.7

691.2

99.6

0.4513

0.3443

2.28

The experiments indicate that the catalytic ability of the HPW@C prepared with the modified carbon is advantageous over that of the HPW@C prepared with the original carbon. The augment of SBET and acidic groups brings about the improvement of catalytic performance of the HPW@C catalyst. Though the SBET of the HPW@C prepared with the carbon treated with 3.5 mol l-1 HOAc are smaller than that of the HPW@C prepared with the one treated with 4.0 mol l-1 HOAc, the former obtains more MX oxidation rates and IPA yields than the latter due to the former having more acidic groups. The acidic groups on the sample surface play a more vital part than SBET. The HPW@C catalyst prepared with the carbon soaked in 1.0 mol l-1 CH3COOH is inferior to that of the HPW@C catalyst prepared with the carbon soaked in 4.0 mol l-1 CH3COOH owing to the former holding smaller SBET and acidic groups than the latter. 3.2 Effect of impregnation time Five carbon samples were carbonized in nitrogen at 500 ℃ for 4 h after having been immersed in a 3.5 mol l-1 HOAc at 40 ℃ for 12 h, 16 h, 20 h, 24 h and 28 h, respectively. The HPW@C catalysts prepared with these carbon samples were utilized to speed up the production of IPA from MX by air. The MX conversion and IPA concentration acquired by these catalysts are depicted in Figure 7. It can be found from Figure 7 that there exists small difference in the MX conversions got by the HPW@C catalysts prepared with the samples impregnated for different time. After 3.0 h min operation, the MX conversions obtained by the HPW@C catalysts prepared with the carbon samples impregnated for 12 h, 16 h, 20 h, 24 h and 28 h are 96.92%, 96.35%, 97.06%, 97.44% and 97.45%, respectively. The HPW@C catalysts prepared with the carbon samples impregnated for 24 h produces more IPA than any other samples.

The IPA concentration obtained increases from 1.44%(wt) to 1.59%(wt) as the

impregnation time prolongs from 12 h to 24 h. However, the IPA engendered decreases from 1.59%(wt) 12



to 1.53%(wt) if the immersion time extends from 24 h to 28 h. A conclusion can be made that the best immersion time is 24 h. 100 1.6

90 Impregnation time 12 h Impregnation time 16 h Impregnation time 20 h Impregnation time 24 h Impregnation time 28 h

1.4


80

MX conversion/%

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

Page 14 of 21

70 60 Impregnation Impregnation Impregnation Impregnation Impregnation

50 40 30

time 12 time 16 time 20 time 24 time 28

h h h h h

20

1.2 1.0 0.8 0.6 0.4 0.2

10 0 0

20

40

60

80

100

120

140

160

180

0.0 0

20

40

60

time(min)

80

100

120

140

160

180

time(min)

Figure 7 Effect of acetic acid impregnation time on MX oxidation 200 ℃, 3.0 MPa, Air fluent: 1.0 L min-1, Stiring speed: 300 r/min, HOAc: 400 ml, HPW@C: 16.0 g l-1, MX: 3.53 %(wt), Co2+: 0.064 %(wt), H2O: 4.0 %(wt)

The Boehm data listed in Table 6 show that the extension of immersion time from 12 h to 24 h brings about an increase in the acidic groups on the carbons. However, the acidic groups reduce with the immersion time extending to 28 h. The increase of immersion time is helpful to shifting the carboxyl of CH3COOH to the carbon, which aids to augment the acidic groups on the carbon surface. More carboxyl groups will transfer on the carbon with the extension of the immersion time to 28 h, but the disintegration of carboxyl group at high temperature to CO2 and H2O leads to the reduction of acidic groups. The physical data shown in Table 7 manifest that SBET alters little as the impregnation time prolongs from 12 h to 24 h. The SBET reduces from 755.1 m2 g-1 to 709.3 m2 g-1 when the immersion time extends from 24 h to 28 h in the consequence of the reduction of micropores and mesopores resulted from the chemical erosion caused by acetic acid. Table 6 Chemical groups of carbon samples (impregnation time) Impregnation time

Acidic groups 10-4mol g-1

Lactone 10-5mol g-1



Basic groups 10-4 mol g-1

12 h

1.25

6.60

4.30

1.56

6.32

24 h

1.76

9.33

4.02

4.20

6.44

28 h

1.65

9.94

3.50

3.10

6.32

The HPW@C catalyst prepared with the carbon impregnated for 24 h manifests better catalytic activity than the catalyst prepared with the carbon impregnated for 28 h because the former has more acidic groups and larger surface area than the latter. The carbon sample impregnated for 12 h owns greater total surface area than the one impregnated for 28 h, but the catalytic activity of the HPW@C catalyst prepared with the latter is superior to that of the former because the latter has more acidic groups than the former. The order of the catalytic capability of these catalysts concurs with the order of their acidic groups on the carbon surface. 13


Page 15 of 21

Table 7 Physical characteristics of HPW@C catalysts (impregnation time) Impregnation time

SBET m2 g-1

Smic m2 g-1

SBJH-A m2 g-1

Vt cm3 g-1

Vmic cm3 g-1

PBET-W nm

12h

751.8

713.2

38.6

0.3962

0.3545

2.11

24h

755.1

692.2

62.9

0.4197

0.3500

2.22

28h

709.3

667.5

41.8

0.3900

0.3441

2.20

3.3 Effect of activation temperature To test the impact of carbonization temperature on the catalytic capability of HPW@C, three carbon samples were carbonized under N2 atmosphere at 450 ℃, 500 ℃and 600 ℃ respectively for 4 h after having been immersed in a 3.5 mol l-1 CH3COOH at 40 ℃ for 24 h. Figure 8 depicts the IPA yield and MX conversion engendered by the HPW@C prepared with the carbon obtained. 100

1.6

90


70 60 50 o

Carbonization temperature 450 C o Carbonization temperature 500 C o Carbonization temperature 600 C

40 30 20

Carbonization temperature 450 C o Carbonization temperature 500 C o Carbonization temperature 600 C

1.2 1.0 0.8 0.6 0.4 0.2

10 0

o

1.4

80

MX conversion/%

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


0

20

40

60

80

100

120

140

160

180

0.0 0

20

40

60

80

100

120

140

160

180

time(min)

time(min)

Figure 8 Effect of calcination temperature on MX oxidation 200 ℃, 3.0 MPa, Air fluent: 1.0 L min-1, Stiring speed: 300 r/min, HOAc: 400ml, HPW@C: 16.0 g l-1, MX: 3.53 %(wt), Co2+: 0.064 %(wt), H2O: 4.0 %(wt)

It can be concluded from Figure 8 that 500℃ is the optimal activation temperature.

After 3.0 h

reaction, the MX conversions got by the HPW@C catalysts made of the carbon calcined at 450 ℃, 500 ℃ and 600 ℃ are 96.65%, 97.44% and 96.67%, respectively. The IPA concentration produced increases from 1.42%(wt) to 1.59%(wt) as the carbonization temperature is raised from 450℃ to 500℃. But the IPA concentration decreases to 1.48%(wt) if the calcination temperature goes up further to 600℃. MX conversion varies little with the calcination temperature.

The highest IPA yield is acquired by the

HPW@C made of the carbon calcined at 500 ℃. Table 8 manifests that lactonic groups augment obviously as the carbonization temperature rises from 450℃ to 600℃ because the production of lactonic groups resulted from the combination between carboxyl and phenolic hydroxyl is enhanced at higher temperature. The phenolic hydroxyl and carboxyl groups reduce remarkably with the rise of the carbonization temperature. The consumption of carboxyl 14



Page 16 of 21

and phenolic hydroxyl increases because these two acidic groups react with each other quickly at higher temperature. Table 9 illustrates that SBET augments from 713.1 m2 g-1 to 755.1 m2 g-1as the carbonization temperature is raised up from 450℃ to 500℃ due to more micropores and mesopores produced with the rise of calcination temperature. The SBET alters little if the carbonization temperature increases from 500℃ to 600℃. Table 8 Chemical groups of carbon samples (calcination temperature) Calcination temperature

Acidic groups 10-4mol g-1

Lactone 10-5mol g-1


450℃

1.68

4.39

6.72

5.70

5.87

500℃

1.76

9.33

4.02

4.20

6.44

600℃

1.66

12.6

1.68

2.29

7.14


Basic groups 10-4mol g-1

Table 9 Physical characteristics of carbon samples (calcination temperature) Calcination temperature

SBET m2 g-1

Smic m2 g-1

SBJH-A m2 g-1

Vt cm3 g-1

Vmic cm3 g-1

PBET-W nm

450℃

713.1

680.9

32.1

0.3813

0.3467

2.14

500℃

755.1

692.2

62.9

0.4197

0.3500

2.22

600℃

758.7

721.8

36.9

0.3997

0.3596

2.11

Though the carbon samples calcined at 500℃ and 600℃ have similar total surface area, the HPW@C catalyst prepared with the former exhibits better catalytic activity than the catalyst prepared with the latter because the former possesses more acidic groups than the latter. The carbon samples calcined at 450℃ and 600℃ own similar acidic groups, but the catalytic ability of the HPW@C catalyst prepared with the latter is superior to that of the catalyst prepared with the former because the latter holds greater surface area. The chemical properties and the physical structure affect the catalytic ability of HPW@C catalyst cooperatively.

3.4 Effect of calcination time To test the influence of activation time on the catalytic capability of HPW@C, three carbon samples were calcined in nitrogen at 500 ℃ for 3 h, 4 h and 5 h respectively after having been impregnated in a 3.5 mol l-1 CH3COOH at 40 ℃ for 24 h. The MX oxidation results catalyzed by the HPW@C made of the carbon samples obtained are demonstrated in Figure 9. Figure 9 demonstrates that the difference of the MX conversions got with these carbon samples is little. The HPW@C catalyst prepared with the carbon calcined for 4 h obtains the greatest IPA. After 3.0 h operation, the conversion acquired by the HPW@C catalysts prepared with the carbon calcined for 3 h, 4 h and 5 h are 96.64%, 97.44% and 97.13%, respectively. The IPA increases from 1.42%(wt) to 1.59%(wt)

15


Page 17 of 21

as calcination time prolongs from 3 h to 4 h. But the IPA produced reduces to 1.54% (wt) when the calcination time increases to 5 h. 100 90

1.5 Carbonization time 3 h Carbonization time 4 h Carbonization time 5 h


80

MX conversion/%

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


70 60 50 Carbonization time 3 h Carbonization time 4 h Carbonization time 5 h

40 30 20

1.2

0.9

0.6

0.3

10 0 0

20

40

60

80

100

120

140

160

180

0.0

0

20

40

60

80

100

120

140

160

180

time(min)

time(min)

Figure 9 Effect of calcination time on MX oxidation 200 ℃, 3.0 MPa, Air fluent: 1.0 L min-1, Stiring speed: 300 r/min, HOAc: 400ml, HPW@C: 16.0 g l-1, MX: 3.53 %(wt), Co2+: 0.064 %(wt), H2O: 4.0 %(wt) Table10 Chemical groups of carbon samples (calcination time) Calcination time

Acidic groups 10-4 mol g-1

Lactone 10-5mol g-1



Basic groups 10-4mol g-1

3h

1.63

6.95

6.19

3.14

6.26

4h

1.76

9.33

4.02

4.20

6.44

5h

1.70

10.8

1.63

4.55

6.27

The Boehm results listed in Table 10 demonstrates that the acidic groups augment as the carbonization time prolongs from 3 h to 4 h but reduce when the carbonization time is 5 h. There may be some halogen elements on activated carbon. The halogen elements may react with the α -H of CH3COOH when the carbon is carbonized at high temperature, which helps to fix the carboxyl of CH3COOH on activated carbon. The carboxyl increases with the prolongation of carbonization from 3 h to 5 h due to longer carbonization time benefiting to transferring the carboxyl group of HOAc to the carbon. It can also be noticed from Table 10 that the extension of carbonization time is conducive to the production of lactonic groups because phenolic hydroxyl may react with carboxyl sufficiently at longer carbonization time.

The phenolic

hydroxyl decreases with the extension of calcination time due to the consumption of phenolic hydroxyl resulted from the combination between phenolic hydroxyl and carboxyl. The decomposition of some acidic groups at 500℃ leads to the decrease of the acidic groups. Therefore, the carbonization time extending from 3 h to 4 h helps to increase acidic groups but the calcination time of 5 h leads to a decrease of acidic groups. The physical characteristics of the HPW@C catalysts presented in Table 11 shows that SBET enlarges as the calcination time extends from 3 h to 4 h because more pores are turned into mesopores because of the reaction between carbon and HOAc at high temperature.

SBET alters little if the

carbonization time extends further to 5 h. 16



Page 18 of 21

Table 11 Physical characteristics of HPW@C catalysts (calcination time) Calcination time

SBET m2 g-1

Smic m2 g-1

SBJH-A m2 g-1

Vt cm3 g-1

Vmic cm3 g-1

PBET-W nm

3h

711.8

705.6

6.1

0.3655

0.3589

2.05

4h

755.1

692.2

62.9

0.4197

0.3500

2.22

5h

751.5

678.4

73.1

0.4279

0.3487

2.28

Though the HPW@C catalysts prepared with the carbon samples calcined for 4 h and 5 h own similar SBET, the former manifests more excellent catalytic performance than the latter owing to the former having more acidic groups. The HPW@C catalyst prepared with the carbon calcined for 5 h is able to obtain higher IPA yield than the HPW@C catalyst prepared with the carbon calcined for 3 h because the former owns more acidic groups and greater SBET than the latter. 4 Conclusions A study has been made to ameliorate the catalytic performance of HPW@C catalyst by treating the carbon with CH3COOH. Some conclusions based on the experiments are given as follows: 1 The catalytic capability of HPW@C in the production of IPA from MX oxidation by air has been ameliorated after the activated carbon having been modified with CH3COOH solution. 2 The best sample is acquired by soaking the carbon in 3.5 mol·l-1 HOAc solution for 24 h followed being carbonized at 500 ℃ in nitrogen for 4 h. 3 The surface characterization proves that the treatment with acetic acid increase the acidic groups and reduce the alkaline groups on the carbon. The specific surface area of activated carbon also augments after being treated with acetic acid solution. The increase of acidity is of benefit to the catalytic activity of the HPW@C catalyst in the oxidation of MX to IPA. The catalytic ability of the HPW@C catalyst relies on its surface chemical characteristics and physical property. The surface chemistry plays a more vital part than the physical property.

4 The MX conversion after 180 min reaction got by the HPW@C prepared from activated carbon modified in best condition is 6.77% over that acquired by the HPW@C catalyst prepared from the original one. The IPA produced by the former is 76.7% over that produced by the latter. Acknowledgements The present work is supported by the NSFC(No. 21176081).

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20


acetic acid modification

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