TiO2-Catalyzed n-Valeraldehyde Self-Condensation to 2-Propyl-2

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TiO2-catalyzed n-valeraldehyde self-condensation to 2propyl-2- heptenal: acid catalysis or base catalysis? Lili Zhao, Hualiang An, Xinqiang Zhao, and Yanji Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03424 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 16, 2016

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Industrial & Engineering Chemistry Research

TiO2-catalyzed n-valeraldehyde self-condensation to 2-propyl-2heptenal: acid catalysis or base catalysis? Lili Zhao, Hualiang An, Xinqiang Zhao*, and Yanji Wang Hebei Provincial Key Lab of Green Chemical Technology and Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China

Abstract:

Several

TiO2

samples

with

different

morphologies

and

structures,

nano/micro-composite of TiO2 (anatase), TiO2 whisker (anatase) and nano-TiO2 (anatase), were prepared and characterized by means of N2 adsorption-desorption, CO2-TPD and NH3-TPD, and their catalytic performance for n-valeraldehyde self-condensation was investigated. The results indicated that the conversion of n-valeraldehyde was correlated with their acid amount while the selectivity of 2-propyl-2-heptenal was associated with their base amount. Since the catalytic performance of nano-TiO2 (anatase) was the best, its preparation process was further studied and the suitable preparation conditions were obtained. Then the effect of reaction conditions on the catalytic performance of nano-TiO2 (anatase) for n-valeraldehyde self-condensation was investigated and the suitable reaction conditions were obtained as follows: a weight percentage of TiO2 catalyst of 15 wt.%, a reaction temperature of 190℃ and a reaction time of 10h. Under the above reaction conditions, the conversion of n-valeraldehyde, 2-propyl-2-heptenal yield and selectivity were 94.6%, 93.7% and 99.1%, respectively. The TiO2 catalyst could be reused four times without a significant loss in its catalytic performance, which was different from most of the literatures. The catalytic stability of TiO2 catalyst was associated with the properties of the active sites, especially acid-base property. Not as some of the literatures claimed that their TiO2-catalyzed reactions were base-catalyzed reactions, the TiO2 catalyst used in this work

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possessed much greater acid amount than base amount. To assess the role of acidic and basic sites in n-valeraldehyde self-condensation, ammonia and carbon dioxide were separately used as a probe molecule for poisoning the corresponding active sites. The results confirmed the key role of acid sites in n-valeraldehyde self-condensation. Therefore, we were convinced that the TiO2-catalyzed n-valeraldehyde self-condensation was mainly acid-catalyzed reaction.

1. INTRODUCTION Dioctylphthalate (DOP) is a well-known PVC plasticizers which is produced from 2-ethylhexanol. However, the application of DOP is now restricted in the western countries due to some safety concerns about the PVC products produced using DOP as a plasticizer. So developing a safer and greener plasticizer is extremely crucial and urgent. 2-Propylheptanol (2-PH) is also an important plasticizer alcohol like 2-ethylhexanol. More importantly, the plasticizers derived from 2-PH have the advantages such as relatively high molecular weights, temperature resistance, low volatility, and being non-toxic, eliminating the concerns about the safety of the plastic products. Therefore

2PH-derived

plasticizers

such

as

2-propylheptyl

phthalate

(DPHP)

are

environment-friendly. Besides used as a plasticizer alcohol, 2-PH can be applied to synthesize a surfactant for detergent component, to prepare a binder for steel, etc.

The synthesis of 2-propyl-2-heptenal (2-PHEA) via n-valeraldehyde self-condensation is one of the important chemical processes for industrial manufacture of 2-PH. At present, an aqueous alkali catalyst (NaOH) is used for n-valeraldehyde self-condensation in industry and some drawbacks exist in this process such as equipment corrosion, environment pollution and poor product selectivity (due to the formation of trimers and polymers). If the aqueous alkali catalyst can be replaced by a solid catalyst, these deficiencies will be overcome. Solid base catalysts were firstly


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focused on. Disappointingly they could hardly be used in industry since they are sensitive to air and water and show poor stability.1,2 So, the development of a solid catalyst with both high catalytic performance and high stability is very important. Morris et al.3 investigated the vapor-phase aldol condensation of n-butyraldehyde over TiO2 (rutile and anatase) catalyst in a fixed-bed reactor. Anatase TiO2 showed relatively high catalytic activity; the conversion of n-butyraldehyde was 80% at 150℃. Rekoske et al.4 investigated the vapor-phase aldol condensation of acetaldehyde catalyzed by TiO2 anatase and found that the reaction rate declined quite rapidly for the first 10 min at 373K. The main reason for the deactivation of TiO2 anatase was attributed to the strong adsorption of larger molecular organics on the active sites. Ai et al.5 studied the vapor-phase aldol condensation of formaldehyde and acetaldehyde over various oxide catalysts and obtained a acetaldehyde conversion of 89 % and an acraldehyde selectivity of 37% over TiO2 catalyst. Idriss et al.6 compared the catalytic performance of single-crystal TiO2 (rutile) and polycrystalline TiO2 (anatase) in acetaldehyde self-condensation reaction. They found that some reactions such as aldol condensation, reduction, reductive coupling and Cannizzaro could take place on the surface of single-crystal TiO2 (rutile) and polycrystalline TiO2 (anatase) by means of TPD-MS (Temperature Programmed Desorption-Mass Spectrometer) analysis. The crotonaldehyde selectivity on a polycrystalline TiO2 (anatase) catalyst was better than that over a single-crystal surface of TiO2 (rutile). However, there are different opinions on the catalytic mechanism for TiO2-catalyzed aldol condensation reaction. Idriss et al.6,7 investigated the reaction of acetaldehyde on both single-crystal and polycrystalline TiO2 surface and found that aldolization of acetaldehyde to 2-butenal took place because of the presence of O2- on the surface acting as Lewis base sites. Singh et al.8 studied the reaction



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mechanism of acetaldehyde aldol condensation on TiO2 surface and proposed a mechanism based on Lewis acid-base pair sites and Brönsted acid sites. Thanh et al.9 considered that the liquid-phase aldol condensation of furfural with acetone was catalyzed by base sites of TiO2 and the presence of OH groups played an important role in the aldol condensation reaction. In conclusion, it is necessary for analyzing the acid-base property of TiO2 to elucidate the catalytic mechanism. In this work, TiO2 was used to catalyze n-valeraldehyde self-condensation reaction. We aimed to clarify the role of acidic and basic sites of TiO2 in n-valeraldehyde self-condensation and to answer the question whether the reaction was acid-catalyzed or base-catalyzed. Firstly, we investigated the catalytic performance of TiO2 catalysts with different morphologies and the stability of TiO2 catalysts. Then, ammonia and carbon dioxide were separately used as a probe molecule to study the effect of the variation of acidity and basicity on the catalytic performance. Finally, a clear conclusion was drawn for the catalytic sites of TiO2 in n-valeraldehyde self-condensation.

2. EXPERIMENTAL SECTION 2.1 Materials and Reagents n-Valeraldehyde (AR, J & K Chemical Technology Co., Ltd., China), butyl titanate, ethanol, acetic acid (AR, Tianjin Damao Chemical Reagent Factory, China), potassium carbonate, hydrochloric acid (AR, Tianjin Kemiou Chemical Reagent Co., Ltd., China).

2.2 Catalysts preparation Amorphous nano-TiO2 catalyst was prepared by sol-gel method according to the literatures.10-12 Anatase nano-TiO2 catalyst was prepared by calcinating dry TiO2 gel on a muffle furnace under air


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atmosphere. The temperature was increased from room temperature to 500℃ at a heating-rate of 10℃·min-1 and then kept for 1h. The sample of anatase nano-TiO2 was analyzed by TEM technique and the result is shown in Fig. S1 (Supporting Information). It can be seen that the anatase TiO2 sample was nano-sized and the particle sizes ranged from 10 nm to 25 nm. TiO2 whisker preparation referred to the literature.13 Nano/micro-composite TiO2 preparation was similar to that of TiO2 whisker except for direct ion-exchange without hydration.14

2.3 Catalyst characterization The X-ray diffraction (XRD) analysis was performed on a Rigaku D/max-2500 X-ray diffractometer using Cu Kα radiation and a graphite monochromator at 40 kV and 100 mA. The scan range covered from 10° to 90° at a rate of 8° min-1. Transmission electron microscopy (TEM) analysis was carried out in a FEI Tecnai G2 F20 instrument operating at an accelerating voltage of 200 kV. Samples were ultrasonically dispersed in ethanol and a couple of drops of the suspension were deposited on a standard 3 mm copper grid covered with a holey carbon film. The textural properties of TiO2 samples were measured using an ASAP 2020 specific surface area and porosity analyzer. Prior to the test, 0.2g of the sample was degassed at 150℃ for 4 h in vacuum to remove the impurities adsorbed on the sample surface. Then N2 adsorption-desorption test was performed at -195.8 ℃ . The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method while the pore volume and pore diameter were calculated by the Barrett-Joyner-Halenda (BJH) method. The acidity and basicity of TiO2 samples were measured by a temperature programmed



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desorption using CO2 or NH3 as a probe molecule (CO2-TPD, NH3-TPD) performed on an AutoChem II 2920 chemical adsorption instrument. Taking the test of CO2-TPD as an example, a typical procedure is described as follows. 0.1g of the sample was placed in a quartz sample tube under helium atmosphere with a flow of 25mL·min-1 and then the temperature was increased from room temperature to 500℃ at a heating-rate of 10mL·min-1 and then kept for 1 h in order to remove the impurities adsorbed on the sample surface. Next the temperature was decreased to 110℃. Subsequently the sample was saturated with CO2 with a flow of 25mL·min-1 for 30 min. Then the sample was purged by helium with a flow of 50mL·min-1 for about 1 h to remove the CO2 absorbed physically. After that, the temperature was increased to 500℃ at a heating-rate of 10℃·min-1. The test of NH3-TPD is similar to CO2-TPD.

2.4 Catalysts activity The self-condensation of n-valeraldehyde was conducted in a 100 mL stainless steel autoclave. In a typical procedure, 40 mL (about 32g) n-valeraldehyde and 4.68g catalyst were added into the autoclave and then the air inside was replaced with nitrogen. The reaction was conducted at 180℃ for 8 h with stirring. After the completion of reaction, the mixture was cooled to room temperature. The catalyst was separated by centrifugation and the liquid was quantitatively analyzed by a gas chromatograph.

2.5 Product analysis A quantitative analysis of the product was carried out using a SP-2100 gas chromatograph (Beijing Beifen-Ruili Analytical Instrument Co., Ltd). Nitrogen was used as a carrier gas and its flow rate was 30mL·min-1. The product mixture was separated in a KB-1 capillary column and the components were analyzed quantitatively in a flame ionization detector (FID). The injector


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temperature was controlled at 220℃.The temperature of KB-1 capillary column was controlled according to the following program: an initial temperature of 80℃ and held for 3 min, heated to 160℃ in a ramp of 10℃·min-1 and held for 2 min, then heated to 200℃ in a ramp of 10℃·min-1 and held for 6 min. Cyclohexanol was used as an internal standard to improve the analytical accuracy.

3. RESULTS AND DISCUSSION 3.1 Catalytic performance of TiO2 samples with different morphologies The catalytic performance of TiO2 samples with different morphologies are listed in Table 1.The conversion of n-valeraldehyde, the yield and the selectivity of 2-propyl-2-heptenal were very low in the blank experiment, suggesting that a catalyst was required. Among the three TiO2 samples, nano-TiO2 (anatase) showed the best catalytic performance. Although the selectivity of 2-PHEA was improved over TiO2 whisker, the conversion of n-valeraldehyde was declined significantly. Furthermore the preparation of nano-TiO2 (anatase) was simple while a long time was needed for the preparation of TiO2 whisker. So nano-TiO2 (anatase) was determined as the catalyst for n-valeraldehyde self-condensation reaction. Table 1 Catalytic performance of TiO2 samples with different morphologies Catalyst

XV/%

YP/%

SP/%

none

39.4

5.6

14.2

nano-TiO2 (anatase)

90.1

87.2

96.8

TiO2 whisker (anatase) nano/micro-composite TiO2 (anatase)

83.6

83.7

99.0

78.9

75.3

95.4

Reaction conditions: a weight percentage of catalyst =15%, T=180℃, t =8h. V: n-valeraldehyde; P: 2-propyl-2-heptenal. X: conversion; Y: yield; S: selectivity.

The acid-base properties of TiO2 catalysts with different morphologies were characterized by NH3-TPD and CO2-TPD. The results are separately shown in Fig. S2 and S3 (Supporting



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Information) while the measurement data are summarized in Table 2. The textural structures of the TiO2 catalysts with different morphologies were characterized by N2 adsorption-desorption and the details are enclosed in Table 3. It can be seen that the specific surface area decreased in the following order: TiO2 whisker (anatase) > nano-TiO2 (anatase) > nano/micro-compositeTiO2, suggesting that the specific surface area had not much effect on the catalytic performance. Combined with Table 1, it could be concluded that the conversion of n-valeraldehyde decreased in the following order: nano-TiO2 (anatase) > TiO2 whisker (anatase) > nano/micro-compositeTiO2 (anatase), in accordance with the order of the acid amount of the TiO2 samples. It was inferred that acid amount played a major role in the improvement of the conversion of n-valeraldehyde. On the other hands, the selectivity of 2-PHEA decreased in the following order: TiO2 whisker (anatase)>nano-TiO2 (anatase)>nano/micro-composite TiO2 (anatase), in consistence with the order of the base amount of the TiO2 samples. It was inferred that base amount played a crucial role in the enhancement of selectivity. Ordomsky et al.15 compared the catalytic performance of MgO/SiO2 and ZrO2/SiO2 in acetaldehyde self-condensation reaction and found that MgO/SiO2 had higher base amount (37.4µmol/g) than ZrO2/SiO2 (13.5µmol/g) while their acid amount was matchable. The experimental results showed that the difference in the acetaldehyde conversion over the two samples was little while the crotonaldehyde selectivity over MgO/SiO2 was better than that over ZrO2/SiO2. These results also indicated that the conversion of acetaldehyde was correlated with acid amount while the selectivity of crotonaldehyde was correlated with base amount. It can be inferred that for the same crystal type of TiO2 (anatase), the difference in acid-base property was the main reasons for the different catalytic performance of the TiO2 samples with


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different morphologies. Table 2 Acid and base property of TiO2 samples with different morphologies Acid property Catalyst

nano-TiO2 (anatase) TiO2 whisker (anatase) nano/microcomposite TiO2 (anatase)

CO2 desorption peak at lower temperature Peak top Weak base temperature amount /µmol.g-1 /℃

Base property CO2 desorption peak at higher temperature Peak top Strong base temperature amount /µmol.g-1 /℃

NH3 desorption Peak /℃

Total acid Amount /µmol.g-1

168.8

296.5

174.6

116.1

346.8

43.44

159.5

179.4

241.8

172.8

127.4

324.0

43.88

171.3

163.8

186.1

170.2

71.81

279.2

35.19

107.0

Total base Amount /µmol.g-1

Table 3 Textural properties of TiO2 samples with different morphologies Specific surface area

Catalyst

2

-1

Pore volume 3

-1

Pore size

/m ·g

/cm ·g

/nm

nano-TiO2 (anatase)

53.8

0.103

5.02

TiO2 whisker (anatase)

94.0

0.0859

4.13

31.0

0.0715

6.22

nano/micro-composite TiO2 (anatase)

3.2 Effect of preparation conditions of TiO2 3.2.1 Effect of calcination temperature The effect of calcination temperature on the catalytic performance of TiO2 was studied and the results are shown in Table 4. With the increase of calcination temperature, the conversion of n-valeraldehyde and the selectivity of 2-PHEA rose firstly and then dropped. Since the conversion of n-valeraldehyde and the yield of 2-PHEA attained their maxima over TiO2 prepared at the calcination temperature of 450℃, the suitable calcination temperature was determined to be 450℃. The XRD patterns of TiO2 samples with different calcination temperatures are shown in Fig. 1. The diffraction peaks of TiO2 with an anatase phase were observed and no other diffraction peaks appeared at the calcination temperature of 400℃, indicating the existence of bare anatase phase.



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These results are in consistent with the literatures.16,17 The diffraction peaks were relatively wide, suggesting small grain size and amorphous structures of the sample. The intensity of peaks increased and the diffraction peaks became sharper and narrower with the increase of calcination temperature, indicating the growth of TiO2 grain. Larger grain implied the decrease of defect existed and intergranular disordered structure and the enhancement of crystallinity of anatase phase.18 Phase transformation from anatase to rutile occurred at 550℃ as shown in Fig. 1. The phase composition of the samples was calculated using the following equation9: Rutile phase(%) =

100 [1 + 0.8（IA / IR）]

where, IA is the integrated intensities of (1 0 1) crystal face diffraction peak of anatase phase while IR is the integrated intensities of (1 1 0) crystal face diffraction peak of rutile phase. The result showed that the proportion of rutile phase was 5.88wt.% in the sample calcinated at 550℃. Combined with the catalytic performance in Table 4, it could be concluded that rutile TiO2 was not beneficial for the enhancement of the catalytic activity. Table 4 Effect of calcination temperature on the catalytic performance of TiO2 calcination temperature

crystal type

XV/%

YP/%

SP/%

400

anatase

89.4

84.4

94.4

450

anatase

92.9

87.6

94.3

500

anatase

90.1

87.2

96.8

550

anatase:rutile =16:1

79.9

77.2

96.6

600

anatase:rutile =3:1

63.7

59.3

93.1

/℃


The average grain size was calculated by MDI Jade 5.0 software using XRD patterns of the TiO2 samples calcined at different temperatures and the results are reported in Fig. 2. It can be seen that the grain size gradually increased with the increase of calcination temperature. Especially, the grain size slightly increased in the calcination temperature range from 400℃ to 500℃ and


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dramatically increased with the increase of calcination temperature beyond 500℃. The growth of the grain size was one of the reasons for significant decline of the catalytic activity of TiO2 when calcination temperature exceeded 500℃. ●

o

◆

400 C,2h o 450 C,2h o 500 C,2h o 550 C,2h o 600 C,2h

◆

● ◆ ◆

◆

40

◆ ●

30

●●

●

Grain size/nm

◆

Intensity(a.u.)

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


●● ●

20

10

0 10

20

30

40

50

60

70

80

400

450

o

500

550

600

o

2θ/( )

Temperature/ C

Fig.1 XRD patterns of TiO2 prepared at different calcination temperature ● Anatase phase；◆：Rutile phase

Fig.2 Relationship between grain size and calcination temperature

The textural structure of TiO2 calcinated at different temperatures was characterized by N2 adsorption-desorption and the results are shown in Table 5. Combined with Fig. 2, it could be concluded that the increase of average grain sizes was also accompanied with a significant decrease of specific surface area as the calcination temperature increased. The result was in consistence with the literatures.19,20 Upon calcination in the temperature range of 500-600℃, the specific surface area decreased drastically, possibly due to the grain growth and even sintering. Especially at high calcination temperature, the grain sintering was more prominent, causing pore collapse, pore volume decrease and average pore size increase. Table 5 Effect of calcination temperature on the textural properties of TiO2 calcination temperature

Specific surface area

Pore volume

/℃

2

3

-1

-1

Pore size

/m .g

/cm .g

/nm

400

105

0.180

4.75

450

79.4

0.143

4.99

500

53.8

0.103

5.02

550

2.93

0.006

5.19

600

1.68

0.003

11.3

3.2.2 Effect of calcination time



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The effect of calcination time on the catalytic performance of TiO2 was investigated. As can be seen from Table 6, calcination time had slight effect on the catalyst activity; the conversion of n-valeraldehyde remained around 91%. When the calcination time was more than 1h, the conversions of n-valeraldehyde rose firstly and then dropped while the selectivity of 2-PHEA decreased gradually. Since the selectivity and the yield of 2-PHEA attained their maxima over TiO2 prepared at the calcination time of 1h, the suitable calcination time was determined to be 1h. It also can be seen from Table 6 that the specific surface area slightly decreased while pore volume and average pore size of TiO2 sample gradually increased with the prolonging of calcination time. Compared with the calcination temperature, calcination time had slight effect on the specific surface area, pore volume and pore size of TiO2 catalyst, in agreement with the results of Manshid et al.17 and Zhu et al.21 In addition, the conversion of n-valeraldehyde increased but the selectivity decreased with the increase of pore size. A larger pore volume means more n-valeraldehyde molecules existing in the pore on the premise that pore size was large enough for free diffusion of the reaction species and the conversion of n-valeraldehyde would be enhanced. However, more reactant and product molecules in the pore would favour deep condensation, causing a decline of the selectivity.22 The average grain size was calculated by MDI Jade 5.0 software using XRD patterns of the TiO2 samples calcined at different times and the results are reported in Fig. S4 (Supporting Information). The grain size increased gradually with the extension of calcination time. However, calcination time had little effect on the grain size compared with calcination temperature. An increase in calcination time from 1h to 3h resulted in a slow growth of grain size from 12 nm to 16 nm while an increase in the calcination temperature from 400℃ to 550℃ resulted in a fast


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growth of grain size from 12 nm to 42 nm. It could be clearly seen that the average grain size of TiO2 was sensitive to calcination temperature. Table 6 Effect of calcination time on the catalytic performance and textural properties of TiO2 calcination time

Specific surface area

Pore volume

Pore size

XV

YP

SP

2

3

-1

-1

/h

/m .g

/cm .g

/nm

/%

/%

/%

0.5

81.2

0.090

3.35

90.3

82.2

91.0

1

80.5

0.128

3.84

91.0

89.3

98.1

2

79.4

0.143

4.99

92.9

87.6

94.3

3

78.8

0.158

5.08

91.4

86.2

94.3


3.3 Effect of reaction conditions and catalyst stability TiO2 catalyst was prepared under the suitable conditions mentioned above and used to study the effect of reaction conditions and the catalytic stability.

3.3.1 Effect of catalyst amount The effect of TiO2 amount on n-valeraldehyde self-condensation was investigated and the results are shown in Fig. 3. It can be seen that the conversion of n-valeraldehyde gradually increased with the increase of TiO2 amount. However, the conversion of n-valeraldehyde increased slowly when the TiO2 amount was more than 15wt.%. The selectivity of 2-PHEA slightly decreased with the increase of TiO2 amount. When TiO2 amount was 5wt.%, the selectivity of 2-PHEA was higher but the conversion of n-valeraldehyde was lower. The increase of TiO2 amount meant the increase of the active sites. However, more active sites promoted side reactions such as the Cannizzaro, reducing the selectivity of 2-PHEA. The conversion of n-valeraldehyde was 91% at TiO2 amount of 15wt.% and then leveled off roughly with a continued increase of TiO2 amount. Therefore, the suitable TiO2 amount was 15wt.%.



100

100 98

95 96 94

XV,YP,SP/%

90

XV,YP,SP/%

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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85

92 90 88

80 86

XV 75

YP SP

70

84

XV

82

YP SP

80

0

5

10

15

20

25

30

160

170

Reaction conditions: T=180℃, t=8h. V: n-valeraldehyde; P: 2-propyl-2-heptenal. X: conversion; Y: yield; S: selectivity.

190

200

o

Catalyst amount /%

Fig. 3 Effect of TiO2 amount on n-valeraldehyde self-condensation reaction

180

Reaction temperature/ C

Fig. 4 Effect of reaction temperature on n-valeraldehyde self-condensation reaction Reaction conditions: a weight percentage of catalyst =15%, t=8h. V: n-valeraldehyde; P: 2-propyl-2-heptenal. X: conversion; Y: yield; S: selectivity.

3.3.2 Effect of reaction temperature The effect of reaction temperature on n-valeraldehyde self-condensation reaction was investigated. As can be seen from Fig. 4, the reaction was kinetically controlled at a lower temperature. With the increase of reaction temperature, the conversion of n-valeraldehyde increased gradually while the selectivity of 2-PHEA tended to decrease. According to the results of Idriss et al.6, aldol condensation, reduction, reductive coupling and Cannizzaro reaction could take place on the surfaces of TiO2. However, the reaction temperature for reduction reaction and Cannizzaro reaction were higher than the temperature for aldol reaction. As a result, the higher the reaction temperature was, the more the side reactions took place. When the reaction temperature was 190℃, the yield of 2-PHEA was up to 90.6% and the conversion of n-valeraldehyde was 93.7%. Therefore, the suitable reaction temperature was 190℃. 3.3.3 Effect of reaction time Under the conditions of TiO2 amount of 15wt.% and reaction temperature of 190℃, the effect of reaction time on the n-valeraldehyde self-condensation reaction was investigated. As can be


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seen from Fig. 5, the conversion of n-valeraldehyde rose gradually with the increase of reaction time. The selectivity of 2-PHEA remained around 98% at first and then increased to 99.5% at a reaction time of 10h. The selectivity of 2-PHEA rapidly decreased to 96% afterwards due to the generation of trimers from the aldol condensation of 2-PHEA and n-valeraldehyde.23 When the reaction time was longer than 10h, the conversion of n-valeraldehyde increased slowly and the color of the reaction solution turned dark due to the side reactions of polycondensation. Therefore, the suitable reaction time was 10h. 100

95

XV,YP,SP/%

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90

85

XV YP SP 80 0

2

4

6

8

10

12

14

Reaction time/h

Fig. 5 Effect of reaction time on n-valeraldehyde self-condensation reaction Reaction conditions: a weight percentage of TiO2 =15%, T= 190℃. V: n-valeraldehyde; P: 2-propyl-2-heptenal. X: conversion; Y: yield; S: selectivity.

3.3.4 Reusability of catalyst After the completion of reaction, the recovered TiO2 was washed with ethanol, air-dried and calcinated at 450℃ for 1h and then was reused in the next cycle. The result of the reusability of TiO2 catalyst is shown in Table 7. It can be seen that the catalytic activity of TiO2 did not change after reused for several times. Therefore, TiO2 not only showed a high catalytic performance but also exhibited a good reusability in n-valeraldehyde self-condensation. The fresh and recovered TiO2 samples were analyzed by XRD and result is shown in Fig. S5 (Supporting Information). It can be seen that the recovered TiO2 anatase crystallite phase did not



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change. Table 7 Reusability of TiO2 catalyst run

XV/%

YP/%

SP/%

1

94.6

93.7

99.1

2

93.9

92.4

98.4

3

92.8

92.4

99.6

4

94.7

94.1

99.5

5

94.9

93.2

98.2

Reaction conditions: a weight percentage of catalyst =15%, T= 190℃, t=10h. V: n-valeraldehyde; P: 2-propyl-2-heptenal. X: conversion; Y: yield; S: selectivity.

3.4 Reasons for high stability of TiO2 Rekoske et al.4 studied the stability of TiO2 anatase in the vapor-phase aldol condensation of acetaldehyde and found that TiO2 had good initial activity and high selectivity at 373K. However, the catalyst deactivation occurred during the first 10min on stream due to the accumulation of strongly adsorbed species on the catalyst surface. They considered that these nonvolatile large organic species were produced by continued aldol-type condensation between product molecules and occupied the active surface area of the catalyst. Thanh et al.9 studied the stability of TiO2 in aldol condensation of furfural with acetone and found that the catalyst activity decreased significantly with the repeated use. However, TiO2 catalyst had good stability in the liquid phase n-valeraldehyde self-condensation reaction in our study. Why is the result of our study different from the literatures? Aldol condensation reaction can be catalyzed by an acid or a base from the viewpoint of reaction mechanism. TiO2 catalysts possess different distributions of catalytic active sites due to different precursors and different preparation methods: some shows more base sites and some exhibits more acid sites. Rekoske et al.4 considered that TiO2-catalyzed vapor-phase acetaldehyde self-condensation reaction was base-catalyzed reaction and organic species occupying the active


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surface resulted in catalyst deactivation. Thanh et al.9 used nano-TiO2 synthesized via hydrolysis of TiOSO4 in the liquid phase aldol condensation of furfural with acetone. They thought that the reaction was catalyzed by base sites and the presence of OH groups played an important role in catalysis. The presence of acidic sites could be advantageous for more facile dehydration of condensation products. From the above analyses, different TiO2 catalysts had different acid-base properties, so catalytic active site distributions were different. The comparison of TiO2 acid-base property in our study with that of Thanh et al.9 is shown in Table 8. It can be seen that there were more acid sites and less base sites in our TiO2 catalyst while the TiO2 catalyst of Thanh et al.9 had more base sites and less acid sites. This means that the TiO2 catalyst of Thanh et al.9 acted as a base as they claimed but the role of our TiO2 catalyst was an acid. Hence, the difference in the catalytic active sites was one of the main reasons for the variation of the catalyst stability in our study from that in the literatures. This further confirmed that TiO2-catalyzed n-valeraldehyde self-condensation should be acid-catalyzed reaction in our study. Table 8 Comparison of acid-base property of TiO2 sample in our study with that of literature9 Acid property

Base property

Catalyst

NH3 desorption peak/℃

Total acid amount /µmol.g-1

CO2 desorption peak /℃

Total base amount /µmol.g-1

TiO2 in our study

169

297

175

160

TiO29

274

256

184

344

The deactivation of TiO2 occurred not only in aldol condensation reaction but also in the photocatalytic reaction. Marci et al.24 studied the photocatalytic reaction of toluene in gas-solid and liquid-solid system catalyzed by anatase TiO2 and polycrystalline TiO2 (80% anatase-20% rutile) and found that the catalysts deactivated in the gas-solid system. An FT-IR study gave information on the role of OH groups both on the activity and the deactivation process. Hydroxyl



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groups on the surface of TiO2 were able to react with benzaldehyde and made it convert to other products such as benzoic acid which strongly adsorbed on the TiO2 surface, leading to the progressive deactivation of TiO2 catalyst in the gas-solid system. Arana et al.25 studied photocatalytic degradation mechanism of 2-propanol in gas phase over different TiO2 catalysts by FT-IR analyses and found that the deactivation of the catalyst was mainly due to acrylic acid strongly adsorbed on the TiO2 surface. Cao et al.26 reported that the photocatalytic oxidation of toluene using nano-structured TiO2 catalysts and found a severe deactivation of TiO2 occurred due to the accumulation of benzoic acid on the surface of TiO2. From the above analyses, the deactivation of TiO2 catalyst was influenced by surface active site kinds and reaction phases. Organics adsorbed on the surface of TiO2 could be desorbed easily and did not lead to a significant deactivation in liquid-solid system. However, acidic by-products strongly adsorbed on the TiO2 surface in gas-solid system could be hardly desorbed, resulting in deactivation of TiO2 catalyst easily. To elucidate the nature of the active sites, an acidic or a basic probe molecule was separately introduced to the TiO2 catalyst in order to poison the corresponding active sites, and then the catalytic performance of the TiO2 sample was evaluated in n-valeraldehyde self-condensation. The results are summarized in Fig. 6. Carbon dioxide which was supposed to be adsorbed on the strong basic sites had no effect on the catalytic performance of TiO2 (there are mainly weak basic sites on the surface of TiO2 catalyst). On the contrary, ammonia resulted in significant decrease of the activity and selectivity of TiO2. A dramatic inhibition to the activity of TiO2 by pre-adsorption of ammonia suggested that TiO2-catalyzed n-valeraldehyde self-condensation was acid-catalyzed reaction in our study. In order to explain the effect of pre-adsorption of ammonia or carbon


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dioxide on the catalytic performance of TiO2, the samples were characterized by FT-IR and the results are shown in Fig. S6 and S7 (Supporting Information). The new bands at 1152cm-1 and 1215cm-1 in Fig. S6 were separately attributed to the interaction of ammonia with oxygen atom on the TiO2 surface and to the adsorption of ammonia on Lewis acid sites of TiO2, resulting in the poisoning of the acid active sites.25 However, the only weak band at 1616cm-1 in Fig. S7 was attributed to O–C–O stretching vibration in a bidentate carbonate formed on O2- of TiO2 surface, which suggested that pre-adsorption of carbon dioxide on TiO2 resulted in the slight poisoning of the strong base sites on TiO2 surface.27-29 Nevertheless, pre-adsorption of carbon dioxide on TiO2 did not result in significant decrease of the activity and selectivity. Therefore, we convinced that TiO2-catalyzed n-valeraldehyde self-condensation was mainly acid-catalyzed reaction. The above analyses suggested that acid sites played an important role in the n-valeraldehyde self-condensation. The results obtained were in accordance with the results published by Ordomsky et al.15, who used pyridine and carbon dioxide as probe molecules to poison corresponding active sites of MgO/SiO2 and ZrO2/SiO2 for acetaldehyde condensation. Pre-adsorption of pyridine showed a significant decrease of the catalytic activity of MgO/SiO2 and ZrO2/SiO2 but carbon dioxide had no effect. The results suggested that the acid sites played an important role in the aldol condensation of acetaldehyde. However, the results obtained here were in contradiction with the results published by Zhang et al.30, who used ammonia and carbon dioxide as probe molecules to poison corresponding active sites of MgO in condensation of butyraldehyde. Pre-adsorption of carbon dioxide showed a significant decrease of the activity of MgO but ammonia had no effect, in consistence with their suggestion that the active site was the surface O2-. This contradiction could be accounted for by different aldol condensation mechanisms



on different acidic sites and basic sites of catalysts.15 100 98

TiO2

96 94

NH3-TiO2

92

CO2-TiO2

90

%

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20

10

0

Conversion of n-valeraldehyde

Selectivity of 2-propyl-2-heptenal

Fig. 6 Effect of pre-adsorption of ammonia or CO2 on the catalytic performance of TiO2 a) All molecules adsorbed at 110℃. CO2-TiO2: pre-adsorption of carbon dioxide on the TiO2 catalyst; NH3-TiO2: pre-adsorption of ammonia on the TiO2 catalyst. b) Reaction conditions: a weight percentage of catalyst =15%, T=90℃, t =2h

4. CONCLUSIONS (1) The catalytic performance of TiO2 samples with different morphologies for n-valeraldehyde self-condensation was investigated and the results showed that nano-TiO2 (anatase) was the best one. Characterizations such as N2 adsorption-desorption, CO2-TPD and NH3-TPD were performed and results indicated that the conversion of n-valeraldehyde was correlated with acid amount and the selectivity of 2-propyl-2-heptenal was correlated with base amount. The difference in acid-base property was main reason for different catalytic performance of the different TiO2 catalysts. (2) The suitable preparation conditions were obtained as follows: a calcination temperature of 450 ℃ and a calcination time of 1h. The suitable reaction conditions for n-valeraldehyde self-condensation were obtained as follows: a weight percentage of TiO2 of 15 wt.%, a reaction temperature of 190℃ and a reaction time of 10h. Under the above reaction conditions, the conversion of n-valeraldehyde, the yield and selectivity of 2-PHEA were 94.6%, 93.7% and 99.1%,


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respectively. TiO2 could be reused four times without a significant change in its catalytic performance. (3) The stability of TiO2 catalyst was influenced by the kind of surface active sites and reaction phase. In our study, TiO2 mainly played a role of acid catalyst, so the stability was better than that of the literatures in which TiO2 acted as a base catalyst. To assess the role of acidic and basic sites in n-valeraldehyde self-condensation, ammonia and carbon dioxide were separately used as a probe molecule for poisoning the corresponding active sites. The results confirmed that the TiO2-catalyzed n-valeraldehyde self-condensation was mainly acid-catalyzed reaction.

AUTHOR INFORMATION Corresponding Author *Tel.: +86-22-60202427. Fax: +86-22-60204294. E-mail: [email protected].

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Grant No. 21476058, 21506046). The authors are gratefully appreciative of their contributions.

SUPPORTING INFORMATION STATEMENT (1) The TEM image of nano-TiO2. (2) Characterization of NH3-TPD and CO2-TPD of TiO2 catalysts with different morphologies. (3) Relationship between grain size and calcination time. (4) The XRD patterns of the fresh and recovered TiO2. (5) FT-IR of NH3-TiO2 and CO2-TiO2.

REFERENCES (1) Zhang, X. L.; An, H. L.; Zhang, H. Q.; Zhao, X. Q.; Wang, Y. J. n-Butyraldehyde self-condensation catalyzed



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by sulfonic acid-functionalized ionic liquids. Ind. Eng. Chem. Res. 2014, 53, 16707. (2) Xiong, C.; Liang, N.; An, H. L.；Zhao, X. Q.; Wang, Y. J. n-Butyraldehyde self-condensation catalyzed by Ce-modified γ-Al2O3. RSC Adv. 2015, 5, 103523. (3) Morris, D. L. Preparation of α,β-aldehydes by aldol condensation. U.S. Patent 4,316,990,1982. (4) Rekoske, J. E.; Barteau, M. A. Kinetics, selectivity, and deactivation in the aldol condensation of acetaldehyde on anatase titanium dioxide. Ind. Eng. Chem. Res. 2011, 50, 41. (5) Ai, M. Formation of acryladehyde by vapor-phase aldol condensationⅡ.phosphate catalysts. Bull. Chem. Soc. Jap. 1991, 64, 1346. (6) Idriss, H.; Kim, K. S.; Barteau, M. A. Carbon-carbon bond formation via aldolization of acetaldehyde on single crystal and polycrystalline TiO2 surfaces. J. Catal. 1993, 139, 119. (7) Idriss, H.; Barteau, M. A. Selectivity and mechanism shifts in the reactions of acetaldehyde on oxidized and reduced TiO2(001) surfaces. Catal. Lett. 1996, 40, 147. (8) Singh, M.; Zhou, N. J.; Paul, D. K.; Klabunde, K. J. IR spectral evidence of aldol condensation: acetaldehyde adsorption over TiO2 surface. J. Catal. 2008, 206, 371. (9) Thanh, D. N.; Kikhtyanin, O.; Ramos, R.; Kothari, M.; Ulbeich, P.; Munshi, T.; Kubicka, D. Nanosized TiO2-A promising catalyst for the aldol condensation of furfural with acetone in biomass upgrading. Catal. Today 2016, http://dx.doi.org/10.1016/j.ca ttod.2015.11.027 (10) Rachel, A.; Subrahmanyam, M.; Boule, P. Comparison of photocatalytic efficiencies of TiO2 in suspended and immobilised form for the photocatalytic degradation of nitrobenzenesulfonic acids. Appl. Catal. B: Environ. 2002, 37, 301. (11) Ding, X. Z.; Liu, X. H. Synthesis and microstructure control of nanocrystalline titania powders via a sol-gel process. Mater. Sci. Eng. A. 1997, 224, 210 (12) Jing, L. Q.; Sun, X. J.; Cai, W. M.; Xu, Z. L.; Du, Y. G.; Fu, H. G. The preparation and characterization of nanoparticle TiO2 /Ti films and their photocatalytic activity. J. Phys. Chem. Solids 2003, 64, 615. (13) He, M.; Lu, X. H.; Feng, X.; Yu, L.; Yang, Z. H. A simple approach to mesoporous fibrous titania from potassium dititanate. Chem. Comm. 2004, 10, 2202. (14) Wang, H. Y.; Cheng, X. S.; Wang, C. J.; Xiao, B.; Jiang, F. Hydrodesulfurization performance of nano/micro-composite TiO2 supported catalysts. CIESC J. 2015, 66, 114. (15) Ordomsky, V. V.; Sushkevich, V. L.; Ivanova, I. I. Study of acetaldehyde condensation chemistry over magnesia and zirconia supported on silica. J. Mol. Catal. A: Chem. 2010, 333, 85. (16) Li, B. R.; Wang, X. H.; Yan, M. Y.; Li, L. Preparation and characterization of nano-TiO2 powder. Mater. Chem.


Page 22 of 24

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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 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


Phy. 2003, 78, 184. (17) Manshid, S.; Askari, M.; Ghamsari, M. S. Synthesis of TiO2 nanoparticles by hydrolysis and peptization of titanium isopropoxide solution. J. Mater. Process. Tech. 2007, 189, 296. (18) Agartan, L.; Kapusuz, D.; Park, J.; Ozturk, A. Effect of initial water content and calcination temperature on photocatalytic properties of TiO2 nanopowders synthesized by the sol-gel process. Ceram. Int. 2015, 70, 12788. (19) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Effects of F-doping on the photocatalytic activity and microstructures of nanocrystalline TiO2 powders. Chem. Mater. 2002, 14, 3808. (20) Kurtoglu, M. E.; Longenbach, T.; Reddington, P.; Gogotsi, Y. Effect of calcination temperature and environment on photocatalytic and mechanical properties of ultrathin sol-gel titanium dioxide films. J. Am. Chem. Ceram. Soc. 2011, 94, 1101. (21) Zhu, Y. H.; Zhou, Y. X.; Yang, Z. H.; Lu, X. H. Thiophene hydrodesulfurization of a novel Mo/TiO2 catalyst. Acta Petrolei Sin. (Pet. Process. Sec.) 2007, 23,76 (in Chinese). (22) Li, Y. P. Catalyst and technology for the synthesis of o-phenylphenol. Doctoral Thesis, China University of Petroleum, Beijing, China, 2007(in Chinese). (23) Alan, J. D.; Thomas, F. S.; Norman, H. Hydroformylation process. EP. Patent 0,016,286, 1980. (24) Marci, G.; Addamo, M.; Augugliaro, V.; Coluccia, S.; Garcia-Lopez, E.; Loddo, V.; Martra, G.; Palmisano, L.; Schiavello, M. Photocatalytic oxidation of toluene on irradiated TiO2: comparison of degradation performance in humidified air, in water and in water containing a zwitterionic surfactant. J. Photoch. Photobio. A: Chem. 2003, 160, 105. (25) Arana, J.; Pen-Alonso, A.; Dona-Rodrguez, J. M.; Colon, G.; Navio, J. A.; Pena-Perez, J. FTIR study of photocatalytic degradation of 2-propanol in gas phase with different TiO2 catalysts. Appl. Catal. B: Environ. 2009, 89, 204. (26) Cao, L. X.; Gao, Z.; Suib, S. L.; Obee, T. N.; Hay, S. O.; Freihaut, J. D. Photocatalytic oxidation of toluene on nanoscale TiO2 catalysts: studies of deactivation and regeneration. J. Catal. 2000,196, 253. (27) Morterra, C.; Chiorino, A.; Boccuzzi, F.; Fisicaro, E. A spectroscopic study of anatase properties 4. The adsorption of carbon dioxide. Z. Phys. Chem.(N. F.) 1981, 124, 211. (28) Martra, G. Lewis acid and base sites at the surface of microcrystalline TiO2 anatase: relationships between surface morphology and chemical behaviour. Appl. Catal. A: Gen. 2000, 200, 275. (29) Hertl, W. Surface chemistry of zirconia polymorphs. Langmuir 1989, 5, 96. (30) Zhang, G.; Hattori, H.; Tanabe, K. Aldol addition of butyraldehyde over solid base catalysts. Bull. Chem. Soc. Jap. 1989, 62, 2070.



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8.46cm × 4.76cm


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TiO2-Catalyzed n-Valeraldehyde Self-Condensation to 2-Propyl-2

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