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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.60 no.3 Concepción Sept. 2015

http://dx.doi.org/10.4067/S0717-97072015000300006 

 

PHOSPHORUS PROMOTED SO42-/TiO2 SOLID ACID CATALYST FOR ACETALIZATION REACTION

 

SHAOFENG ZHONG1,2*, QIONGRONG OU2, LINJUN SHAO1

1 Department of Textile, Zhejiang Industry Polytechnic College, Shaoxing 312000, People's Republic of China;
2
Department of Light Sources and Illuminating Engineering, Fudan University, Shanghai 200433, People's Republic of China
* e-mail address: sfzhong76@163.com


ABSTRACT

A novel phosphorus modified SO42-/TiO2 catalyst was synthesized by a facile coprecipitation method, followed by calcination. The catalytic performance of this novel solid acid was evaluated by acetalization. The results showed that the phosphorus was very efficient to enhance the catalytic activity of SO42-/TiO2. The solid acid owned high activity for the acetalization with the yields over 90%. Moreover, the solid acid could be reused for six times without loss of initial catalytic activities.

Keywords: solid acid, TiO2, acetalization, coprecipitation, phosphorus.


 

INTRODUCTION

Acid catalyzed reactions played an important role in chemical industry to produce various chemicals.1,2 Over 15 million tons of homogeneous acid (sulfuric acid, phosphorus acid, etc) is annually consumed as an unrecyclable catalyst, which requires costly and inefficient separation of the catalyst from homogeneous reaction mixture. Thus, heterogeneous catalyst has received tremendous interest because catalyst reuse and recycling can further improve the overall productivity and cost-effectiveness, thereby minimizing the waste generation and catalyst contamination, leading to a greener and more sustainable chemical transformation process.3-5 Solid acid could be easily separated from the reaction mixture by simple filtration or centrifugation and do not require the neutralization procedure. During the past decades, many solid acids have been developed for the replacement of homogeneous acid catalyst.6,7 However, till now, preparation of a solid acid as active, stable and inexpensive as sulfuric acid is still great challenging for the researchers.

TiO2 based solid acid are quite promising because they are environmentally benign, easy to prepare, low cost, excellent thermal stability and strong surface acidity.8,9 As mixed oxides usually show better physicochemical and acidic properties than individual component oxides, TiO2 mixed oxides were synthesized to further increase the catalytic performance of TiO2.10,11 Herein, phosphorus was firstly incorporated into the TiO2 particles and its catalytic performance was evaluated by the acetalization reaction (Scheme 1).

 

Scheme 1. Scheme representation of acetalization reaction.

 

EXPERIMENTAL

Materials

All organic reagents were commercial products of the highest purity available (>98%) and were used for the reactions without further purification. TiO2 (anatase) particles was purchased from Shanghai Reagent Plant.

Catalyst preparation

The phosphorus modified SO42-/TiO2 (SO42-/TiO2-P) was prepared as follows: 2 g titanium butoxide was dissolved in 10 ml anhydrous ethanol and 2 g H3PO4 was added to the titanium butoxide/ethanol solution. A white precipitate was produced and the suspension was stirred at room temperature for another 3 hrs. The prepared phosphorus modified TiO2 was separated by centrifugation and then treated at 500 oC (TiO2-P) in air for 5 hrs.

Sulfated TiO2-P (SO42-/TiO2-P) was prepared as follows: 2 g TiO2-P powder was added into 10 mL of H2SO4 aqueous solution (0.5M) and stirred magnetically for 2 hrs. The SO42-/TiO2-P catalyst was separated by centrifugation and then treated at calcined at 200 oC in air for 2 h before catalysis use.

Sulfated TiO2 was also prepared for comparison and the sulfation process of TiO2 particles was the same as that of TiO2-P.

General procedure for acetalization reaction

Aldehyde or ketone (20 mmol), diol (24 mmol), 5 mL cyclohexane, and the catalyst (20 mg) were mixed together in a three necked round bottomed flask equipped with a magnetic stirrer and a thermometer, and a Dean-Stark apparatus which was constituted with manifold and condenser to remove the water continuously from the reaction mixture. The reaction was refluxed for 2 h to complete the reaction.

Characterization

The acidity was determined according to the literature.12 The quantitative analysis was performed on a Shimadzu (GC-14B) gas chromatograph. The morphologies of electrospun fiber mats were recorded with a scanning electron microscope (SEM) (Jeol, jsm-6360lv, Japan). Samples for SEM were coated with a 2-3 nm layer of Au to make them conductive. The elemental analysis was taken on the EuroEA 3000 from Leeman, USA. FT-IR/ATR spectra were recorded on a FT-IR spectrometer (Nicolet, Nexus-470, USA) with the accessories of attenuated total reflection. Phase composition of samples was determined by means of X-ray powder diffraction (XRD) (Rigaku D, max-3BX, Japan). Surface area was measured by TriStar II 3020 from Micromeritics, USA.

RESULTS AND DISCUSSION

Characterization of the catalyst

The surface morphologies of SO42-/TiO2-P and TiO2 powder were shown in Figure 1. It can be found that the SO42-/TiO2 catalyst was spherical particle with diameters about 1 μιη, while the SO42-/TiO2-P powder was irregular. The BET measurements also showed that the surface specific area of SO42-/TiO2 catalyst (134 m2/g) was a little larger than that of SO42-/TiO2-P (125 m2/g).

 

Figure 1. SEM images of SO42-/TiO2-P (a) and SO42-/TiO2 (b).

 

The FT-IR spectra of TiO2-P before and after thermal treatment were shown in Figure 2. After thermal treatment, the intensities of absorption at 3300 cm-1 and 1626 cm-1 assigned to -OH group were significantly decreased. The strong absorbance band at 1015 cm-1 was attributed to the P-O group. It can be concluded that the phosphorus has been successfully incorporated into the TiO2 structure.

 

Figure 2. FT-IR spectra of TiO2, TiO2-P befor and
after calcination.

 

The crystal phases of the two catalysts were analyzed by the XRD pattern. The diffraction peaks at 25o was the characteristic peaks of the (101) planes of anatase phase of TiO2, which is known for its high catalytic activities. The treatment of sulfuric acid had no influence on the crystal phase of TiO2.

The elemental analysis catalyst showed that the sulfur contents in SO42-/ TiO2 and SO42-/TiO2-P catalysts were 6.51% and 6.72%, respectively. This result demonstrated that the amounts of absorbed H2SO4 in the two catalysts were comparable. The acidity measurement showed that the acidities of SO42-/ TiO2 and SO42-/TiO2-P were 1.95 mmol/g and 2.67 mmol/g, respectively, which indicated the phosphorus could increased the acid centre in the surface of SO42-/ TiO2 catalyst.

The catalytic activities for the acetalization

A comparative study on the catalytic activities of the SO42-/TiO2-P with heterogeneous and homogeneous acid catalyst was carried out (Table 1). It is interesting to found that the phosphorus species could significantly increase the catalytic activity of SO42-/TiO2 catalyst (Entry 1-3 in Table 1). Yang et al have reported that the sulfate species and silica species could work together to increase the acidity and catalytic activity of SO42-/TiO2 catalyst.13 Thus, it can be concluded that the sulfate species and phosphorus species may also have the similar synergetic effects, which is responsible for the high catalytic activity of SO42-/TiO2-P catalyst. Due to the disfavored kinetics of the biphasic catalytic system, the heterogeneous catalyst usually was not as active as the homogeneous catalyst. Although the catalytic activity of SO42-/TiO2 catalyst could be obviously enhanced by incorporation of phosphors, it is still slightly lower than that of homogeneous H2SO4 catalyst.

 

Table 1. The catalytic activity comparison of different catalysts.a

a: Catalytic condition: 20 mmol cyclohexanone; 24 mmol
1,2-ethenediol; 10 mg catalyst in 5 mL cyclohexane under reflux
for 2 h; with Dean-Stark apparatus.
b: The synthesis and sulfation processes of TiO2 were the same
with that of SO42-/TiO2-P but with H2O as the replacement of H3PO4.

 

Effect of catalyst amount on the acetalization

Table 2 summarizes the dependence of the acetalization with the catalyst loading. It can be found that the yields for the acetalization of cyclohexanone with 1,2-ethenediol gradually increased with the increment of SO42-/TiO2-P catalyst. Excellent conversion and yield could be obtained even by just using 10 mg catalyst. When the amount of SO42-/TiO2-P catalyst increased to 20 mg, the yield was up to 99% and the further addition of catalyst has no obvious effect on the yield of acetalization, indicating that 20 mg SO42-/TiO2-P catalyst was enough for the acetalization.

 

Table 2. Effect of catalyst amount on the conversion and yield
of acetalization of cyclohexanone with 1,2-ethenediol.a

a:Reaction condition: 20 mmol cyclohexanone; 24 mmol
1,2-ethenediol; 5 mL cyclohexane under reflux for 2 h.

 

Figure 3. XRD patterns of TiO2, SO42-/TiO2 and SO42-/TiO2-P
catalysts.

 

Acetalization catalyzed by SO42-/TiO2-P catalyst

A series of carbonyl compounds and diols were employed to examine the scope of SO42-/TiO2-P catalyst (Table 3). Examination of entry 1-8 in table 2 showed that the aliphatic aldehydes were more reactive than the aromatic aldehydes due to the electronic effect. Due to the steric hindrance, the cyclohexanone acted more effective than the 2-butanone (Entry 11, 12, 15, and 16 in Table 3). As the five- and six- membered ring were more stable than seven-membered ring, 1,2-ethanediol, 1,2-propylenediol and 2,2-dimethyl-1,3-propanediol worked better with carbonyl compound than 1,4-butanediol.

 

Table 3. The SO42-/TiO2-P catalyst catalyzed acetalization.a

a: Catalytic condition: 20 mmol carbonyl compound; 24 mmol diol; 20 mg catalyst in 5 mL
cyclohexane under reflux for 2 h; with Dean-Stark apparatus.

 

Reuse of SO42-/TiO2-P catalyst

As the SO42-/TiO2-P catalyst is immiscibility with organic compound and solvent, the catalyst could be recovered by centrifugation. The reusability of the SO42-/TiO2-P catalyst was evaluated by the acetalization of cyclohexanone with 1,2-ethanediol. After the reactions, the SO42-/TiO2-P catalyst was recovered by centrifugation and reused in the second run directly. Figure 4 clearly showed that the catalytic activity of the SO42-/TiO2-P was unchanged even after six runs.

 

Figure 4. The reuse of the SO42-/TiO2-P catalyst.

 

CONCLUSION

In summary, a novel phosphorus modified SO42-/TiO2 solid acid was prepared. The phosphorus species could increase the catalytic activity of SO42-/TiO2 solid acid. The SO42-/TiO2-P catalyst was very efficient for the acetalization. Moreover, the SO42-/TiO2-P solid acid could be recovered conveniently and reused for six times without loss of its initial activity. This environmentally benign heterogeneous SO42-/TiO2-P solid acid will have potential for large-scale industrial applications.

ACKNOWLEDGEMENT

The author gratefully acknowledges financial supports from Professional Leaders Leading Project of Education Department of Zhejiang Province, China (No. lj2013131) and Science and Technology Project of Education Department of Zhejiang Province, China (No.Y201432680).

 

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