Stability Investigation of a Supported TiO2âPd

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Kinetics, Catalysis, and Reaction Engineering

Stability investigation of supported TiO2-Pd bifunctional catalyst over the one-pot liquid-phase synthesis of methyl isobutyl ketone from acetone and H2 Hui Duan, Zhihui Wang, Lifeng Cui, Baining Lin, and Yonghua Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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Industrial & Engineering Chemistry Research

Stability investigation of supported TiO2-Pd bifunctional catalyst over the one-pot liquid-phase synthesis of methyl isobutyl ketone from acetone and H2

Hui Duan,a Zhihui Wang,b Lifeng Cui,b Baining Lin,a Yonghua Zhoua,* a

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China b

Shandong Hualu-Hengsheng Chemical Co., Ltd., Dezhou 253024,China *E-mail: [email protected]

ABSTRACT One-pot liquid-phase synthesis of MIBK from acetone and H2 is a significant process in fine-chemicals industry. In this paper, we fabricated a series of TiO2-SiO2/SiO2(F)&Pd/Cor

bifunctional catalysts and optimized the ratio of TiO2 to SiO2 to be 1. The catalyst with the optimal mole ratio of TiO2 to SiO2 was further investigated in a long-term operation in trickle-bed reactor, which delivered a stable MIBK selectivity of 83%-93% at 26%-36% acetone conversion for 300 hours. The characterizations of FT-IR, Raman spectra, TGA, XRD, XPS, CO2-TPD, NH3-TPD and Py-IR spectra reveal that the active sites of acetone condensation and dehydrogenation could be attributed to the Ti-O-Si bonds. The deactivation of catalyst resulted from the decrease of active sites, due to the aggregation of TiO2 and carbonaceous accumulation on the catalyst surface. Keywords: acetone，methyl isobutyl ketone，Ti-O-Si bond，liquid-phase synthesis, trickle-bed reactor

1. INTRODUCTION Methyl isobutyl ketone (MIBK) is one of the most important chemical products used widely in coatings, pharmaceuticals, petrochemicals and other chemical industry fields.1 In addition, it is often used as an intermediate in the preparation of pesticides, rubber antioxidants, surfactants, and antibiotics. With the development of global economy, the future demand for MIBK will increase steadily. Asian regions, especially China and South Korea, will witness a rapid growth in demand for MIBK. This is mainly driven by the requirement for rubber antioxidant 4020 and high solids-containing coatings. At present, MIBK can be manufactured by three-step process or one-step (also named as one-pot) process.2 Traditionally a three-step process consists of aldol condensation, dehydrogenation and hydrogenation. Firstly, the aldol condensation of acetone in the presence of a basic or acidic catalyst, such as NaOH, Ba(OH)2, MgO, MoO, Fe(OH)3 H2SO4, H3PO4 and so on, produces diacetone alcohol 1

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(DAA).3 Secondly, dehydration of DAA form mesity oxide (MO) over an acidic catalyst，for instance H2SO4, H3PO4, ZnCl2, and CaCl2.4 The last step is hydrogenation of MO to form MIBK catalyzed by metal including Ni,5,6 Pd,7,8 Pt,9 and Cu.10 Nowadays, MIBK is produced mainly by one-pot process instead of three-step process due to the virtue of process-intensification. And Pd/resin is widely applied in the mass production of MIBK as the commercial multifunctional catalyst, in which the ion-exchange resin catalyze the aldol condensation and dehydrogenation reactions, and Pd catalyze the hydrogenation reaction. Nicol et al. and Mahajani et al. reported that they obtained acetone conversions varying from 25 to 50% and MIBK selectivities varied between 70% and 90% by using a commercial catalyst Pd/Amberlyst CH28 at pressure 1-10 MPa and temperature 120-200 °C.22-24 Nevertheless, the disadvantages of high pressure operating and poor thermal stability of the resin increase extra commercial costs and limit its lifetime. So, for several decades, other types of multifunctional catalysts with metal oxides as support, such as Pd/CaO-MgO-SrO-Al2O3,18 Pd/Nb2O5,19-21 Pd/ZnO-Cr2O3,11,38 Pd/MgO/SiO2,12 Ni/CaO-C,13 Cu/MgO14 and Pd/Ni-LDH-NO3,15 were extensively investigated for gas-phase reaction in the atmospheric fixed-bed reactors or liquid-phase reaction in the autoclave batch reactors. However, there is few report on the stability investigation of catalyst for liquid-phase reaction in the trickle-bed reactors, which are closer to the industry practice of MIBK mass production than other types of reactors. Previously, a physically mixed catalyst of TiO2/SiO2(F)&Pd/Cor was reported to be a new, efficient bifunctional catalyst for the one-step synthesis of MIBK in the gas phase reaction.16,17,50 However, it was found that the stability of catalyst was below 40 hours in the trickle-bed reactor due to the easy aggregation of TiO2 particles. It was previously reported that TiO2 can be well dispersed and stabilized by SiO2.33 Inspired by this point, to promote the stability, here we fabricated catalysts TiO2-SiO2/SiO2(F)&Pd/Cor with three layers coating of different mole ratio of TiO2:SiO2 and in-depth investigated the stability of this catalyst in the trickle-bed reactor. It was found that our catalyst can keep active in the trickle-bed reactor for at least 300 hours with a MIBK selectivity of 83%-93% at 26%-36% acetone conversion. By comparing the characterizations of the catalyst before and after reaction, the deactivation of catalyst TiO2-SiO2/SiO2(F)&Pd/Cor was mainly attributed to the decrease of catalytic active bonds Ti-O-Si and carbonaceous accumulation on the catalyst surface.

2. EXPERIMENTAL 2.1. Catalyst Preparation. The support SiO2 with particle size of 20-40 mesh was supplied by Qingdao Ocean Chemical Company of China. The support SiO2 treated for 6 hours by 1.0 mol/L of NH4F solution was denoted as SiO2(F). The TiO2-SiO2 coating with different ratio of TiO2 to SiO2 was loaded on SiO2(F) by impregnation technique. The resulted samples were denoted as TiO2-SiO2/SiO2(F)-X. X represented the mole ratio of TiO2 to SiO2 (TiO2:SiO2=2, 1, 0.5, 0.3). For instance, TiO2-SiO2/SiO2(F)-1 was prepared as follows. Firstly, a solution A was prepared by mixing 2


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10 ml of tetrabutyl titanate, 6.55 ml of tetraethyl orthosilicate and 35 ml of ethanol under vigorous agitation. Then, 20 ml of acetic acid was added in the solution A, followed by adding 10 g of support SiO2(F) and stirring for one more hour. The obtained slurry was filtered to separate the solid and recover the solution A. The recovered solution A was transferred into the beaker at room temperature and the solid product was dried at 150 ℃ for 2 hours. The dried solid product was impregnated by the recovered solution A for two more times. Finally, after three times impregnation, the solid product was calcined under air flow for 3 hours at 500 ℃, which was denoted as TiO2-SiO2/SiO2(F)-1 catalyst. It should be noted that as for the preparation of TiO2-SiO2/SiO2(F)-2, the solution A was prepared by mixing 10 ml of tetrabutyl titanate, 3.28 ml of tetraethyl orthosilicate and 35 ml of ethanol. For TiO2-SiO2/SiO2(F)-0.5 preparation, the solution A was prepared by mixing 10 ml of tetrabutyl titanate, 13.1 ml of tetraethyl orthosilicate and 35 ml of ethanol. For TiO2-SiO2/SiO2(F)-0.3 preparation, the solution A was prepared by mixing 10 ml of tetrabutyl titanate, 19.6 ml of tetraethyl orthosilicate and 35 ml of ethanol. Synthesis of Pd/Cor: The Pd/Cor was prepared by conventional impregnation method. The cordierite granule with 60-80 mesh was immersed in H2PdCl4 solution (0.01mol/L) under intense ultrasonic for 20 min. After dying at 110 ℃ and calcination at 350 ℃ for 3 h in air atmosphere, it was further reduced by KBH4 solution for 30 min and washed by deionized water to neutral. Eventually, vacuum dying at 50 ℃ was carried out. The obtained sample was denoted as Pd/Cor. The Pd loading on Pd/Cor in the present work was 0.152 wt% by Inductive Coupled Plasma Emission Spectrometer (ICP). Synthesis of TiO2-SiO2/SiO2(F) & Pd/Cor catalyst：The bifunctional catalysts were obtained by physically mixing TiO2-SiO2/SiO2(F) and Pd/Cor with different mass ratio, which was denoted as TiO2-SiO2/SiO2(F)&Pd/Cor. The mass ratio has been optimized in study as shown in Table 1. After 330 hours reaction, the physically mixed bifunctional catalyst was carefully separated into Pd/Cor and TiO2-SiO2/SiO2(F)-1 again by sieving. Synthesis of TiO2: TiO2 powder was used in the control experiment. It was synthesized by the sol-gel method. Firstly, a solution A was prepared by mixing 10 ml of tetrabutyl titanate and 35 ml of ethanol under vigorous stirring by a magnetic stirrer for 30 min. Then, 20 ml of acetic acid and 2 ml of nitric acid (0.1 mol/L) was added in the solution A and stirring for one more hour to prepare a transparent TiO2 precursor sol and then a uniform translucent gel. After being aged for 24 hours, the gel was dried at 80 ℃ for 2 hours and then was calcined under 500 ℃for 3 hours to obtain the TiO2. TiO2/SiO2(F)&Pd/Cor was prepared according to the literature.50 2.2. Catalyst Characterizations. Fourier transform infrared (FT-IR) spectra were measured on a Nicolet 6700 Fourier transform spectrometer (Nicolet Magana Co.). A structural characterization was obtained by X-rays diffraction (XRD) and Raman spectroscopy. XRD patterns of catalysts were recorded between 5 ° and 80 ° (2θ) on a Bruker D8 Advance A25 diffractometer with a monochromatic Cu Kα radiation (λ=0.154nm). Raman spectra were achieved at room tempareture on a 3



LabRam HR Raman spectrometer (Horiba-Jobin Yvon) using a 514 nm laser with 50% ND filter. Thermogravimetric analysis (TGA) was carried out in a TA Instruments TGA Q500 at the heating rate of 10 ℃/min to 800 ℃ in the air. X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB250Xi instrument with an Al Kα X-ray source (1486.6 eV), under about 2×10−9 mbar at room temperature and a pass energy of 20 eV. The measurements of the acidity and basicity of the catalysts were accomplished by Autosorb-iQ-C equipped with a thermal conductivity detector. MS detector was used to distinguish the signal of NH3 and H2O. The sample was pretreated in helium gas at 400 ℃ for 2 hours and then cooled down to 50 ℃. The surface of the materials was saturated with NH3 (or CO2) for 60 min. TPD profile of NH3 (or CO2) was recorded at the rate of 10 ℃/min from 50 ℃ to 800 ℃ by a He flow. Py-IR spectra analysis for catalysts was carried out in a Nicolet 380 Magna spectrophotometer. Samples of catalysts were dried in-situ at 350 ℃ at vacuum (2×10-3Pa). Pretreated sample was cooled at 50 ℃ to proceed to pyridine adsorption. Excess of pyridine was eliminated by vacuum at 30 ℃. Lewis and Brønsted acidic type was analyzed by Py-IR spectra analysis at 200 ℃ desorption temperatures. Pd mass content was measured with a Perkin-Elmer Inductively Coupled Optical Emission Spectrometer (ICP), model Optima 5300DV, equipped with a peristaltic pump and a cross-flow nebulizer, coupled to a Ryton double pass spray chamber of the Scott type. Before analysis, Pd/Cor was immersed in aqua regia solution (v(HCl+HNO3)/vH2O=20:30) to dissolve Pd into solution. The CO chemisorption measurements were performed in AutoChem1Ⅱ2920 equipment. Before analysis, the catalysts were reduced under H2 (50 cm3/min) flow at 150 ℃ for 1 hour, and subsequently under He (50 cm3/min) flow at 150 ℃ for 1 hour. Then, CO chemisorption analysis using double isotherm methodology was performed at 35 ℃ and metal dispersion (or the particle size) was calculated. 2.3. Catalytic Testing. The catalytic performance was tested in a trickle-bed reactor using 10 ml bifunctional catalyst composed of TiO2-SiO2/SiO2(F) (20-40 mesh) and Pd/Cor (60-80 mesh). Reaction parameters such as temperature, pressure and time on stream were investigated. For a given temperature, pressure and liquid hour space velocity (LHSV), the reactor was allowed to run at specified conditions before the composite sampling commenced to ensure that steady state was achieved. The liquid phase synthesis of MIBK from acetone in a trickle-bed reactor was performed under the following conditions: 0.2 ml/min of acetone flow, 5.0 ml/min of hydrogen flow, 110-200 ℃, 1.0-5.0 MPa and LHSV of 1.2 h-1. A Shimadzu GC-2010 gas chromatograph (GC) equipped with a flame ionization detector (FID) and a 30 m × 0.25 mm × 0.25 µm Insert Cap WAX capillary column was used to determine the amounts of acetone, MIBK, MO, DAA, and so on in the product. Total analysis time was 15 min. The following formulas were used to calculate the conversions of acetone (xA), selectivity to MIBK (SMIBK) and yield of MIBK (YMIBK)

xA (%) = 1-

moles of acetone in the product moles of acetone in the feed

4


×100

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moles of acetone converted to MIBK SMIBK (%) =

moles of acetone converted

×100

YMIBK (%) = xA×SMIBK/100

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance. The liquid-phase synthesis of MIBK in the trickle-bed reactor was performed

at

110-200

°C

and

1.0-5.0

MPa

H2

pressure

in

the

presence

of

TiO2-SiO2/SiO2(F)-1&Pd/Cor catalyst. The effects of reaction temperature and pressure on the

performance of TiO2-SiO2/SiO2(F)-1&Pd/Cor catalyst are shown in Table 1. Increasing the reaction temperature from 100 °C up to 200 °C lead to an increase of acetone conversion from 14.1% to 65.6 % accompanied with a volcanic changed MIBK selectivity. The MIBK selectivity dropped at higher temperature is due to the formation of heavier condensation products (C9+). With the increase of H2 pressure from 1.0 to 5.0 MPa, acetone conversion increases from 29.5% to 50.3%, while MIBK selectivity decreases from 88.9% to 60.1% and MIBC selectivity increases from 3.3%-17%. In addition, isopropanol (IPA) selectivity increases from 3.0%-13.5% as well. It is reported that the increase of H2 pressure led to the formation of MIBC and isopropanol due to the hydrogenation of the C=O group in MIBK and acetone.38 The mass ratio of TiO2-SiO2/SiO2(F)-1 to Pd/Cor and the feed ratio of C3H6O/H2 were also optimized as shown in Table 1. If we fixed the volume of bifunctional catalyst, higher mass ratio of TiO2-SiO2/SiO2(F) to Pd/Cor led to the increase of MO and C9+. Comparatively, lower mass ratio resulted to the increased selectivity to MIBK and decreased acetone conversion. Considering the yield of MIBK, the mass ratio should be maintained 1:1. In addition, the optimal mole ratio (C3H6O/H2) of this reaction is 2:1 in theory, but acetone conversion is proved to be only 32.4 % as shown in Table 1. With the increase of mole ratio (C3H6O/H above 2:1), acetone conversion drops slightly to 31.8 %. Therefore, in the paper, we utilized the mole ratio (C3H6O/H) of 1:1 for further investigation.

Catalytic performance of TiO2-SiO2/SiO2(F)&Pd/Cor catalysts with the various mole ratio of TiO2 to SiO2 at 150 °C and 2.0 MPa is shown in Table 2. The catalyst TiO2/SiO2(F)&Pd/Cor,

which has the single layer TiO2 coating on SiO2 support, is used here as a control criterion. The acetone conversion ranges from 33.5% to 38.4% and the selectivity to MIBK changes between 65.5% and 84.8% for all TiO2-SiO2/SiO2(F)&Pd/Cor catalysts. Among them, the catalyst TiO2-SiO2/SiO2(F)-0, in which no SiO2 component was mixed in the coating, exhibits a low acetone conversion of 25.6% and a selectivity to MIBK of 74.6%. As contrast, with the increase of the mole ratio of TiO2 to SiO2, the acetone conversion and selectivity to MIBK of the catalyst increase firstly and then decrease subsequently. The optimal mole ratio of TiO2 to SiO2 is found to 5



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be 1, which delivers the highest MIBK yield 32.6%. This value is quite near to the performance of the catalyst TiO2/SiO2(F)&Pd/Cor with single layer TiO2 coating.

Table 1. Liquid-Phase Synthesis of MIBK over TiO2-SiO2/SiO2(F)-1&Pd/Cor Catalyst a Selectivity (%)

Acetone Temperature

Pressure

C3H6O/H2

Mass

(°C)

(MPa)

(mol/mol)

ratio*

MIBK yield

conversion

a

b

(%)

MIBK

MIBC b

MO

DAA

IPA

Others c

(%)

110

2.0

1:1

1:1

14.1

80.2

4.7

3.8

5.7

1.0

4.6

11.2

130

2.0

1:1

1:1

29.1

87.1

4.5

1.4

2.1

0.7

4.2

25.3

150

2.0

1:1

1:1

38.4

84.8

6.3

0.6

0.7

0.7

6.9

32.6

170

2.0

1:1

1:1

52.8

65.7

5.4

1.1

2.9

2.8

22.1

34.7

200

2.0

1:1

1:1

65.6

63.4

5.7

1.0

3.5

3.0

23.4

41.6

150

1.0

1:1

1:1

29.5

88.9

3.3

2.0

2.1

0.1

3.6

24.5

150

3.0

1:1

1:1

42.3

74.5

7.9

1.2

1.4

9.7

5.3

31.5

150

4.0

1:1

1:1

47.9

65.3

12.2

0.9

1.1

13.4

7.1

31.3

150

5.0

1:1

1:1

50.3

60.1

17.0

0.8

1.4

13.5

7.2

30.2

150

2.0

1:1

2:1

34.7

72.2

2.3

9.9

0.7

0.2

14.7

25.1

150

2.0

1:1

1:2

23.6

86.1

7.1

0.3

0.3

2.1

4.1

20.3

150

2.0

1:2

1:1

35.6

85.2

6.4

0.4

0.5

0.7

6.8

30.3

150

2.0

2:1

1:1

32.4

86.1

7.1

0.2

0.3

0.5

5.8

27.9

150

2.0

3:1

1:1

31.8

86.7

7.2

0.3

0.3

0.6

4.9

27.6

The volume of TiO2-SiO2/SiO2(F)& Pd/Cor is 10 ml, H2 flow rate=5 ml/min, LHSV=1.2 h-1 and time on stream=2 h.

MIBC is the abbreviation of Methyl isobutyl alcohol.

c

Others is mainly the heavier condensation product ( C9 and above, denoted as C9+).

*

The mass ratio represents the ratio of TiO2-SiO2/SiO2(F)-1 to Pd/Cor.

6


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Table 2. Liquid-Phase Synthesis of MIBK over Various TiO2-SiO2/SiO2(F)&Pd/Cor Catalysts a Acetone

Selectivity (%) MIBK

conversion

Catalyst

yield (%)

(%)

MIBK

MIBC

MO

DAA

IPA

Others

Pd/Cor

10.9

13.5

0.7

0.8

3.7

74.4

6.9

1.47

SiO2(F)

1.3

29.7

14.8

3.3

22.0

3.7

26.5

0.39

TiO2

4.7

31.1

12.8

4.7

19.6

2.3

29.5

1.46

SiO2(F)&Pd/Cor

11.5

14.3

1.3

0

9.3

63.4

11.7

1.64

TiO2&Pd/Cor

15.9

90.1

0.9

2.5

2.7

1.2

2.6

14.3

TiO2/SiO2(F)&Pd/Cor

38.5

85.1

4.3

1.1

2.4

0.5

6.6

32.7

TiO2-SiO2/SiO2(F)-2&Pd/Cor

37.5

80.2

8.1

3.7

0.8

0.7

6.5

30.1

TiO2-SiO2/SiO2(F)-1&Pd/Cor

38.4

84.8

6.3

0.6

0.7

0.7

6.9

32.6

TiO2-SiO2/SiO2(F)-0.5&Pd/Cor

35.5

73.0

7.6

7.8

2.2

1.1

8.3

25.9

TiO2-SiO2/SiO2(F)-0.3&Pd/Cor

33.5

65.5

8.7

6.4

0.8

1.3

17.3

21.9

TiO2-SiO2/SiO2(F)-0* &Pd/Cor

25.6

74.6

5.9

3.2

1.1

0.6

14.6

19.1

*

TiO2-SiO2/SiO2(F)-0 was prepared by adding 10 ml of tetrabutyl titanate and 0 ml of tetraethyl orthosilicate without changing other procedures.

a

The volume of various TiO2-SiO2/SiO2(F)& Pd/Cor is 10 ml (mass ratio = 1), T=150 °C, P=2.0 MPa, H2 flow rate=5 ml/min, LHSV=1.2 h-1 and time on stream=2 h.

In addition, the long-time stability of TiO2-SiO2/SiO2(F)-1&Pd/Cor catalyst in the trickle-bed reactor is investigated (in Figure 1). The catalyst reaches steady state in about 25 hours and operates steadily

for

at

least

300

hours

without

obvious

deactivation.

Before

deactivation,

TiO2-SiO2/SiO2(F)-1&Pd/Cor catalyst keeps acetone conversion of 26%-36% and MIBK selectivity of 83%-93%. So far, the best stability of Pd catalyst loaded on metal oxide for liquid-phase synthesis of MIBK in trickel-bed reactor was reported to be Pd/MCM-56 with 33.5%-29.9% acetone conversion and MIBK selectivity of 80%-83% for 72 h of time on liquid stream.39 The surface component and acid-base property of TiO2-SiO2/SiO2(F)-1&Pd/Cor catalyst before and after reaction would be investigated in details by FT-IR, TGA, XRD, XPS and TPD characterization to reveal its deactivation 7



Page 8 of 20

reason (as below).

Figure 1. The stability of TiO2-SiO2/SiO2(F)-1&Pd/Cor catalyst. Reaction conditions: Volume of catalyst=10 ml, T=150°C, P=2.0 MPa, H2 flow rate=5 ml/min and LHSV=1.2 h-1.

3.2. Characterization of Catalysts. The fresh catalysts are investigated by FT-IR and Raman spectra to detect their surface property. Figure 2a presents the FT-IR spectra of TiO2-SiO2/SiO2(F) 25,26

with various mole ratio of TiO2 to SiO2. According to the related literatures,

the absorption

-1

bands at around 460 cm corresponds to the bending and stretching vibrations of Ti-O-Ti bond. The peaks at around 1065 and 1250 cm-1 could be attributed to the asymmetric Si-O-Si bending vibration and the peak at near 810 cm-1 corresponds to the symmetric stretching of Si-O-Si. The observed peak at 950 cm-1 is associated with the vibrational mode of Ti-O-Si bond. It demonstrates the successful formation of Ti-O-Si linkage on the catalysts. And with the increase of the amount of SiO2, the intensity of the peaks (950 cm-1) attributed to the Ti-O-Si bond become stronger first and then weaker. In order to further confirm the present of TiO2 and SiO2 on different catalysts, the Raman spectra of TiO2-SiO2/SiO2(F) with various mole ratio of TiO2 to SiO2 are shown in Figure 2b. According to literatures, the peaks at 120, 230, 401, 505 and 678 cm-1 can be correlated to the presence of anatase nanocrystallites of TiO2.29,30,50 The peaks at 305 cm-1 can be attributed to the bending modes of Si-O-Si and torsional vibrations.30 In addition, the broad features at 810 cm-1 are due to the stretching modes of Si-O-Si linkage of the SiO2, and the peaks at 613 cm-1 are attributed to the D2 defects modes of SiO2.31

8


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Figure 2.

FT-IR (a) and Raman (b) spectra of various TiO2-SiO2/SiO2(F) with various mole ratio of TiO2:SiO2

To identify the difference of catalyst before and after 330 hours reaction, FT-IR, TGA, XPS, CO2-TPD and NH3-TPD were utilized. The FT-IR spectra of TiO2-SiO2/SiO2(F)-1 catalyst before and after reaction are shown in Figure 3a. No obvious change is observed about the characteristic peaks of Ti-O-Ti bands at 460 cm-1 and Si-O-Si bands at 810 cm−1, 1065 cm−1 and 1247cm-1. However, the peak at 950 cm-1 attributed to the characteristic stretching vibration of Ti-O-Si bands distinctly decreases when the catalyst was continuously performed for 330 hours. In addition, new peaks at 2835 cm-1 and 2930 cm-1 relate to the C-H stretching vibrations of –CH3 and –CH2 groups appeared, indicative of the carbonaceous accumulation on the surface of catalyst.25 The broad peak at 1630 is attributed to the O-H bending vibrations of the absorbed water, and the absorption peak at 3465 cm-1 is associated with the stretching and bending vibrations of O-H of the crystal water as well as the surface hydroxyl groups.27 The TGA curves of the catalyst TiO2-SiO2/SiO2(F)-1 before and after reaction are showed in Figure 3b. It is seen that the weight loss of the fresh catalyst is only 6.6% up to 800 °C. The first weight loss of 5.9 % between 30-250 °C is attributed to the evaporation of the crystal water as well as the absorbed water.28 The second weight loss of 0.7% between 250-460 °C corresponds to the thermal decomposition of residual organic substance on the catalyst before reaction. Meantime we observe the present of carbon element (3.04 % in mole) by XPS in Table 3, which is originated from tetrabutyl titanate and tetraethyl orthosilicate. And the organic substance decomposes completely at the temperature above 460 °C. After 330 hours reaction, the catalyst shows a mass decrease (about 7.2 %) in the temperature range of 250-460 °C, which is attributed to the decomposition of

low-boiling organic substance on the surface of catalyst.26 In addition, the last stage with mass loss of about 1.7 % in the temperature range of 460-800 °C is due to the decomposition of the high-boiling residual organic substance, for example C9+, absorbed on the surface of catalyst after reaction. At the same time, a higher carbon amount (10.61 % in mole) on the used catalyst is observed by XPS in Table 3. 9



Figure 3. (a) FT-IR spectra and (b) TGA curves of TiO2-SiO2/SiO2(F)-1 catalyst before and after reaction.

The XRD analysis (Figure 4a) is used to further determine the phase change of TiO2 crystalline grain of TiO2-SiO2/SiO2(F)-1 catalyst before and after reaction. The broad diffraction peak between 15°-30° is assigned to the amorphous SiO2.32 The XRD pattern of the fresh catalyst show six characteristic peaks at 25.2°, 37.3°, 48.2°, 53.8°, 58.2° and 62.7°, which are strictly related to the anatase phase.16,33 After 330 hours reaction, TiO2-SiO2/SiO2(F)-1 catalyst exhibits enhanced intensity of peaks assigned to the anatase TiO2, indicating the growth of anatase TiO2 crystalline grain.33 In addition, characteristic peaks (26.1°, 27.5°, 28.8° and 32.3°) attributed to rutile TiO216,33,34 appear for TiO2-SiO2/SiO2(F)-1 catalyst after reaction. It’s reported that anatase was only metastable TiO2 crystal phase and rutile was more stable than anatase. This has been confirmed by thermodynamic studies,40,41 which showed that anatase was more stable than rutile under negative pressure. That means high pressure is not beneficial to maintain the anatase phase.

Under 2 MPa pressure for 330 hours, the loaded TiO2 experienced the transition from anatase to rutile phase. Previously, Jose et al. also reported that the pressure induced phase transitions in TiO2 nanoparticles from anatase to rutile phase at an applied pressure of about 2.683 MPa.42 To further obtain information about the interfacial interaction and the chemical state of surface elements, XPS is conducted to analyze our catalysts. As shown in Figure 4b, the surface of the initial catalysts is composed of Ti, O and Si elements. After 330 hours reaction, the amount of carbon element on the surface of catalyst obviously increases from 3.04 % to 10.61 % in Table 3， indicative of the carbonaceous accumulation. Meantime, the ratios of (Ti+Si):O and Ti:F almost unchanged. Figure 5b shows that as for fresh TiO2-SiO2/SiO2(F)-1 catalyst, the binding energy of Ti 2p3/2 and Ti 2p1/2 are 458.8 and 464.5 eV, respectively. The gap between the two peaks is 5.7 eV, which demonstrated that the Ti element exists in the form of Ti4+ in the catalysts.35,43 After 330 hours reaction, the binding energy of Ti 2p shifts towards higher value by 0.4 eV, resulted from the generated carbonaceous accumulation around Ti4+. Since the electronegativity of carbon 10


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is stronger than that of Ti, the binding energy of Ti4+ increases. Jiang et al.43 observed that peaks at 457.73 eV and 463.45 eV can be assigned to Ti 2p3/2 and Ti 2p1/2 in TiO2 powder and peaks at 458.15 eV and 463.88 eV attributed to Ti 2p3/2 and Ti 2p1/2 in g-C3N4/TiO2. That showed g-C3N4/TiO2 had +0.42 and +0.43 eV chemical shift of Ti 2p when Ti element was surrounded by C and N elements. Dolat et al. synthesized nitrogen, carbon co-doped titanium dioxide (N,C-TiO2) photocatalysts within the pressure range of 0.38-0.94 MPa, and the obvious binding energy shift of Ti4+ was observed.51 In this paper, the catalyst was tested continuously for 330 hours under 2 MPa pressure at 150 °C. So we speculate that some carbon element in carbonaceous species probably formed chemical bonding with Ti4+. In the O 1s XPS spectra (Figure 5a), two peaks at 530.0 eV and 532.9

eV on both catalysts are observed due to the presence of Ti-O-Ti and Si-O-Si bonds, respectively.33,36 It is noteworthy that an obvious shoulder peak at 531.0 eV marked on the catalyst before reaction. However, after 330 hours reaction, the shoulder peak disappears completely. According to literature, the shoulder peak could be attributed to the presence of the interfacial Ti-O-Si bonds.37 Together with the results of FT-IR in Figure 3a, the O1s XPS spectra results also prove that Ti-O-Si bonds act as the active sites for the reaction. And XRD results indicate that the disappearance of Ti-O-Si bonds could be caused by the phase transfer of TiO2 under high pressure.

Figure 4. XRD patterns (a) and XPS survey spectra (b) of TiO2-SiO2/SiO2(F)-1 catalyst before and after reaction.

Table 3. Atomic Content in Mole revealed by XPS of TiO2-SiO2/SiO2(F)-1Catalyst before and after Reaction. Atomic content by XPS (%) Catalyst C

O

11


Ti

F

Si


Page 12 of 20

Before reaction

3.04

65.18

2.69

0.64

28.45

After reaction

10.61

60.73

1.7

0.39

26.57

TiO2-SiO2/SiO2(F)-1

Figure 5. O 1s XPS spectra (a) and Ti 2p XPS spectra (b) of TiO2-SiO2/SiO2(F)-1 before and after reaction.

Figure

6

shows

the

deconvolution

of

CO2-TPD

and

NH3-TPD

profiles

of

TiO2-SiO2/SiO2(F)-1 catalysts before and after reaction. We could observe that four types of basic and acid sites present on the catalysts.34 They are weak base (centered in the range 100-125 °C), intermediate base (300-310 °C), strong base (400-500 °C) and very-strong base (550-600 °C) in Figure 6a, and weak acid (centered in the range 120-150 °C), intermediate acid (300-350 °C), strong acid (550-600 °C) and very-strong basic (710-720 °C) in Figure 6b. After reaction, both the intensities of basic and acid sites of the catalysts are weaker than that before reaction. Especially, weak basic, very-strong basic and all acid sites, reduce obviously after reaction. It was reported that the acidic and basic property of TiO2-SiO2 nanocomposites depended largely on TiO2 particle size, Ti-O-Si interface, and the degree of surface hydroxylation.45 And it was also confirmed that smaller TiO2 particle displayed stronger acidity than that with larger size.46 The decrease of the acid sites amount between 300 to 800 °C in Figure 6b confirms the aggregation of TiO2, in agreement with the XRD results in Figure 4a. It is that the aggregation of TiO2 lead to the disappearance of the interface between TiO2 and SiO2 and therefore the reduction of the number of Ti-O-Si bonds, reflected by the FT-IR and XPS results in Figure 3a and Figure 5a. Figure 7 shows the Py-IR spectra of the catalyst before and after reaction. The peaks at 1450, 1452, 1596, 1605, and 1612 cm-1 are due to pyridine coordinated with strong Lewis acid sites and 1575 cm-1 due to weak Lewis acid sites.34,48,49 The peak at 1491 cm-1 can be assigned to pyridine 12


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associated with Lewis-Brønsted acid sites.34,46-49 In addition, the peaks at 1545 and 1640 cm-1 represent Brønsted acid sites.34,46-49 After reaction, the amount of Lewis acid sites drops significantly from 137.5 umol/g to 68.9 umol/g in Table 4, while the amount of Brønsted acid sites has no obvious change, suggesting that Lewis acid on the catalyst surface plays more important role than Brønsted acid in this reaction. It should be mentioned that there is seemly no consensus in the literatures regarding the type and strength of surface acid sites for TiO2-SiO2 mixed oxides.45 For instance, Liu et al. reported in 1994 that new Brønsted acid sites were created when titania and silica form Ti-O-Si chemical bonds.44 However, This viewpoint was objected by Notari et al. and Hu et al. in 2006, who claimed that only Lewis acidity, not Brønsted acidity, was present on the surface of TiO2-SiO2 mixed oxides.46,47 And several recent literatures confirmed that TiO2-SiO2 mixed oxides possessed more Lewis acid sites than Brønsted acid sites.26,29,34,48,49 We speculate that the types of acid sites could be closely related to the preparation technique and the resulted interface surrounding. According to the results of CO2-TPD, NH3-TPD and Py-IR spectra in this paper, we assume that the Ti-O-Si bonds arise from the interface of TiO2 and SiO2. And because of the diversity of the interface, for instance the variety of coordination number of Ti-O-Si species, they act as both base sites and Lewis acid sites, which catalyze the acetone condensation and the dehydrogenation of DAA. Therefore, the deactivation of catalysts is mainly related to the significant reduce of the number of Ti-O-Si bonds. This is well supported by the FT-IR and XPS results in Figure 3a and Figure 5a. And the reduce of the number of Ti-O-Si bonds results from the aggregation of TiO2 and carbonaceous accumulation, confirmed by FT-IR, TGA, XRD, and XPS characterizations. The Pd mass percent and Pd dispersion of fresh and used Pd/Cor were characterized by ICP and

CO chemisorption as shown in Table 4. As for the fresh Pd/Cor, the Pd mass percent is 0.152 % with an average Pd particle size of 8.31 nm. And the Pd dispersion is estimated to be 13.5 %. Comparatively, as for the used Pd/Cor, the Pd mass percent drops to 0.101% accompanied by the aggregation of Pd particle to 20.90 nm (Pd dispersion of 5.3 %). Accordingly, the percentage of MO in product was observed to increase from 0.6 % to 4.9 % after 330 hours reaction due to the loss and aggregation of Pd.

13



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Figure 6. CO2-TPD (a) and NH3-TPD (b) profiles of TiO2-SiO2/SiO2(F)-1catalyst before and after reaction.

Figure 7. Py-IR spectra of TiO2-SiO2/SiO2(F)-1 catalyst before and after reaction.

Table 4. Results of Mass Percent (P), Pd Dispersion (D) and Particle Size (d) of Fresh and Used Pd/Cor. Catalyst

P (wt%)

D (%)

d (nm)

Before reaction

0.152

13.5

8.31

After reaction

0.101

5.3

20.90

Pd/Cor

4. CONCLUSION 14


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In this paper, we fabricated a series of TiO2-SiO2/SiO2(F)&Pd/Cor bifunctional catalysts for liquid-phase synthesis of MIBK from acetone and H2 in trickle-bed reactor. The mole ratio of TiO2 to SiO2 of catalysts was optimized firstly and then tested in a long-term operation in trickle-bed

reactor under the conditions close to industrial production. TiO2-SiO2/SiO2(F)-1&Pd/Cor catalyst kept active for 300 hours with a MIBK selectivity of 83%-93% at 26%-36% acetone conversion. By comparing the characterizations of the catalyst before and after reaction, the active sites of catalyst for acetone condensation and dehydrogenation could be attributed to the Ti-O-Si bonds, and the deactivation of catalyst was mainly attributed to decrease of active sites, caused by the aggregation of TiO2 and carbonaceous accumulation on the catalyst surface. This work will supply new alternatives for the design and utilization of industrial catalysts of MIBK synthesis.

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No.21676303), the Fundamental Research Funds for the Central Universities of Central South University (No.2018zzts382).

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