ZrO2–MoO3 for the Acetalization of 1,3-Propanediol from Dilute

Article pubs.acs.org/IECR

ZrO2−MoO3 for the Acetalization of 1,3-Propanediol from Dilute Solutions Min Wu,* Chun-long Li, Jin Zhang, Chun-cun Miao, Ying-ping Zheng, and Yue-ming Sun School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China ABSTRACT: A reactive isolation approach for the recovery of 1,3-propanediol (1,3-PD) from dilute aqueous solutions was performed on ZrO2−MoO3 solid heterogeneous catalysts through a cyclic reaction with aldehyde to form acetals (2-methyl-1,3dioxane, 2MD). The effects of catalyst composition, reaction temperature, reaction time, and optimal dose on the conversion rates of acetalization and hydrolysis were investigated. For ZrO2−10 wt % MoO3 prepared by the precipitation−impregnation method, the conversion rate of 1,3-PD in acetalization reached 95.7% at 60 °C for 2 h, and the acetals conversion in the hydrolysis reaction reached 97.0% at 100 °C for 10 h. The stability test showed that the 1,3-PD conversion rate still reached 87.3% after five cycles of use. In terms of the catalytic activity in acetalization, the 10 wt % catalyst exhibited much higher selectivity in the simulated fermentation liquid for 1,3-PD than ethanol, 2,3-butanediol, and glycerol. This indicates that ZrO2−10 wt % MoO3 mixed oxide has the best characteristics for the extraction of 1,3-PD from dilute aqueous solutions.

1. INTRODUCTION Environmentally friendly solid acid catalysts not only can exhibit high activities and high selectivities at low temperature, but also can be easily separated from products and reused. With these benefits and advantages, great efforts have been directed toward the fabrication and application of solid acid catalysts. Many types of solid acid catalysts, such as SO42−/TiO2, SO42−/ MxOy, and SO42−/ZrO2-MCM-41, have been widely used in heterogeneous catalysis, esterification, selective oxidation, asymmetric catalysis, condensation, hydration, dehydration, hydrolysis, and so on.1−4 However, the loss of SOx in aqueousphase reactions results in unfavorable catalyst inactivation and environmental pollution. In contrast, composite oxide solid acids with no sulfur acid have many attractive properties, such as high activity, high selectivity, good stability without leaching of active components, and absence of pollution.5 In particular, composite oxide solid acid catalysts used for the reaction of dibasic alcohol extraction are low in cost, provide high yields, and are technically feasible. 1,3-Propanediol (1,3-PD) is an important product for biomass fibers and biochemical raw materials.6 The current synthesis method is a biotechnological method rather than a chemical route, considering the environmental benefits and utilization of glycerol.7,8 However, the 1,3-PD fermentation broth obtained from fermentation is a dilute solution containing a variety of polar alcohols, such as 1,3-PD, 2,3butanediol, residual glycerol, and ethanol. Thus, it is a challenge to separate polyhydroxy 1,3-PD efficiently from this mixture of multiple components. Recovery of this diol from dilute solutions has been the subject of numerous publications.9−11 The conventional evaporation and distillation techniques used in the purification of 1,3-PD suffer from the problems of high energy consumption and low recovery.6 Therefore, much attention has been focused on an effective method for the recovery of 1,3-PD by cyclic acetal formation. A relatively well-developed approach is the acetalization of 1,3-PD © 2012 American Chemical Society

to form highly hydrophobic acetals (2-methyl-1,3-dioxane, 2MD), followed by hydrolysis to 1,3-PD (Scheme 1). This cyclic reaction has been reported previously.12,13 Scheme 1. Recovery of 1,3-Propanediol from Dilute Aqueous Solutions through Reversible Reaction with Aldehydes

Scheme 1 provides recovery of 1,3-propanediol from dilute aqueous solutions through a reversible reaction with aldehyde. In this work, Zr(OH)4 was used as a precursor for the preparation of ZrO2−MoO3 by the precipitation−impregnation method. The catalyst was then characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy. For the recovery of 1,3-PD from a model mixture containing ethanol, 2,3-butanediol, glycerol, and 1,3-PD, which are the typical metabolites from microbial fermentation, the catalyst activities and selectivities of the reactive extraction route, combining the Received: Revised: Accepted: Published: 6304

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The progress of the reactions was monitored by a GC9890A gas chromatograph equipped with a flame ionization detector (Nan Jing Ren Hua, China).

reversible reaction of 1,3-propanediol with acetaldehyde catalyzed by ZrO2−MoO3 mixed oxides, were completely evaluated. The results showed that ZrO2−MoO3 exhibits high activity and good stability in the reversible reaction.

3. RESULTS AND DISCUSSION 3.1. XRD. It was reported in the literature17 that the particle size, dispersion, and crystalline phase have significant influences on the catalytic behavior of the synthesized complex oxide catalyst. Zr(OH)4 consists of only an amorphous phase and can be converted into the tetragonal (t, ZrO2) and monoclinic (m, ZrO2) forms of ZrO2 when calcined at different temperatures.18 High-temperature calcination facilitates the transformation of ZrO2 to the monoclinic phase. When molybdenum oxide is doped over zirconia, its distribution can affect the overall catalytic performance. As shown in Figure 1, the diffractograms showed that the samples were all formed in the tetragonal phase and well-

2. EXPERIMENTAL SECTION 2.1. Materials. Aldehyde, ethanol, 2,3-butanediol, and glycerol were purchased from the Chemical Reagent Company of Shanghai (Shanghai, China). Ammonium hydroxide (NH4OH in H2O, 28%), ammonium heptamolybdate, 1,3-PD (99%), and Zr(NO3)4·5H2O were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China). 2.2. Catalyst Preparation. The ZrO2−MoO3 catalysts were prepared by the precipitation−impregnation method.14−16 According to this method, 10.0 g of Zr(NO3)4·5H2O was dissolved in 50 mL of deionized water, and the solution was maintained at 100 °C under vigorous stirring. Ammonium hydroxide was added dropwise until the solution maintained a constant pH at 9−10. The obtained white slurry was stirred at 100 °C for 24 h. The resulting solids were vacuum filtered, washed with deionized water three times, and dried at 90 °C overnight to form the precipitate Zr(OH)4. Thereafter, the samples were impregnated with 10 mL of 0.003−0.05 mol/L ammonium heptamolybdate [(NH4)6Mo7O24·4H2O] for 24 h, dried at 90 °C overnight, and calcined at 800 °C for 3 h. The resulting mixed oxides are denoted as ZrO2−1 wt % MoO3, ZrO2−5 wt % MoO3, ZrO2−10 wt % MoO3, and ZrO2−15 wt % MoO3. The obtained powder was formed into pellets under 10 MPa for 15 min, which were then crushed and sieved. The fraction of 1.0−2.0-mm particle size was used for the characterization and screening experiments. 2.3. Catalyst Characterization. The crystallographic phase of the samples was identified by X-ray powder diffractometer (XRD). The XRD patterns were recorded on a D8-DISCOVER X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany), employing Cu Kα radiation (35 kV, 40 mA) and 2θ of 10−90° at a scanning rate of 5°/min. Transmission electron microscropy (TEM) was performed on a Tecnai G2 transmission electron microscope (FEI Co., Eindhoven, Netherlands), working at a 200 kV accelerating voltage. Samples for TEM were prepared by dispersing the powder in ethanol by sonication and then drop drying the dispersion on a copper grid. FT-IR data were recorded with a Paragon 500 FT-IR spectrometer (Perkin-Elmer, Shelton, CT) in the midinfrared range (4000−400 cm−1). 2.4. Catalytic Reactions. 2.4.1. Acetalization of 1,3-PD. For the acetalization of 1,3-PD, 0.1 g of ZrO2−MoO3 and a relative quantity of aldehyde were added to 5−40 g/L 1,3-PD dilute solutions (molar ratio of acetaldehyde to 1.3-PD was 2:1), which were stirred in a 50 mL round-bottom flask equipped with a magnetic stirrer, a reflux condenser, and a thermometer. After reaction at 40−80 °C and atmospheric pressure for 2 h, the catalyst was filtered. The reaction products were collected, and the organic phase was filtered to obtain 2methyl-1,3-dioxane (2MD), as mentioned before.3,13 2.4.2. Hydrolysis of 2MD. The hydrolysis reaction of 2MD was carried out in a 50 mL round-bottom flask equipped with a reflux condenser. For this reaction, 20.0 g of 2MD, 4.0 g of H2O, and 0.4 g of ZrO2−MoO3 were stirred magnetically at 100 °C for the entire reaction time of 10 h.

Figure 1. X-ray diffraction (XRD) patterns of ZrO2−MoO3 powders calcined at 800 °C.

crystallized. The peaks detected at 2θ = 30.3°, 35.1°, 50.4°, and 60.2° corresponding to the characteristic peaks of the presence of tetragonal phases. The tetragonal phase is one of the important factors in the acid strength of solid acids.19,20 The X-ray diffraction patterns of the loaded support after calcination indicated no sign of MoO3 in Figure 1; this might due to the good dispersion of MoO3 on ZrO2. The addition of MoO3 suppressed the growth and sintering of ZrO2 particles, as well as the transformation of zirconium from the tetragonal phase to the monoclinic phase, and it also enhanced the changing temperature of ZrO2. This means that the appropriate amount of MoO3 results in a single-layer dispersion on the surface.21,22 3.2. TEM. Transmission electron micrographs of ZrO2− MoO3 particles are shown in Figure 2. After being calcined at 800 °C, ZrO2−MoO3 particles were still loosely dispersed and showed small uniform particles with a range of 10−50 nm, as well as a small amount of aggregates. Although the particles grew with increasing MoO3 loading, they were still evenly distributed, and also maintained the original structure. This indicated the active components were evenly distributed in the mixed oxides. More dispersed active component is useful to prevent the carrier particle from occurring secondary aggregation when calcined at high temperature, and this is very beneficial to the formation of superacid structure.23,24 This method can be used to prepare catalyst particle with uniform distribution and small size. 6305

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Figure 4. Effects of ZrO2−MoO3 on the acetalization of 1,3propanediol.

1 wt % ZrO2−MoO3 system to 95.7% with a 10 wt % ZrO2− MoO3. The catalyst composition has a great influence on the activity in the acetalization reaction, and the optimal composition was determined to be ZrO2−10 wt % MoO3. However, there was a decreasing conversion rate after reaching a maximum with ZrO2−10 wt % MoO3. The decrease in the 1,3-PD conversion rate with increased MoO3 loading might be due to the dispersion of active oxide components such as the coating of excess MoO3 covering the active site.31,32 Figure 5 shows the catalyst activities of ZrO2−MoO3 materials with different MoO3 loadings on the hydrolysis of

Figure 2. Transmission electron micrographs of ZrO2−MoO3 samples.

3.3. FT-IR Spectroscopy. FT-IR spectrum (Figure 3) showed the formation of the ZrO2−MoO3 mixed oxides calcined at 800 °C.

Figure 5. Effects of ZrO2−MoO3 on the hydrolysis of 2-methyl-1,3dioxane.

Figure 3. FT-IR spectrum of ZrO2−MoO3 mixed oxides calcined at 800 °C.

2MD. It is worth noticing that ZrO2−10 wt % MoO3 showed the best catalytic performance. The conversion of 2MD in the hydrolysis reaction reached 97.0% when ZrO2−10 wt % MoO3 was used in the reaction. Previous work indicated that the composition and phase of active oxide components are important to 2MD conversion.3,7 A reversible hydrolysis reaction can also reach a state of chemical equilibrium, a steady stage in which no further changes in concentrations of reactants and products occurs, and the catalyst is the substance that affects the rate of the reaction. The hydrolysis time required to reach equilibrium was analyzed in the process. It can be seen from Figure 5 that the total time of equilibrium conversions was about 10 h. 3.4.2. Effects of Temperature on the Conversion of 1,3-PD in Dilute Solutions. For the recovery of a dilute product, the equilibrium constant is considered to be the most important process parameter determining operational feasibility. As can be seen in Scheme 1, the process for the recovery of 1,3propanediol is a reversible reaction. The acetalization reaction

The broad band at 3430 cm−1 and the peak at around 1630 cm−1 are attributed to the O−H stretching vibration and the bending vibration of physisorbed water associated with the metallic oxide. The peak at 900−1100 cm−1 was assigned to the ionic character of the MoO stretching vibration.25−27 The small band at 950 cm−1 indicates the terminal MoO stretching of the surface molybdenyl species.28,29 The peak at 930 cm−1 represents Mo−Zr hetero linkage, 882 cm−1 for the vibration of the Mo−O−Mo linkage species and 825 cm−1 for the vibration of the MoO3 bulk phase.25 The peak at 596 cm−1 represents an OMo3 vibration mode.30 3.4. Effects of Preparation and Reaction Conditions on Catalytic Performance. 3.4.1. Effects of the Addition of Mo on the Catalytic Performance in the Isolation of 1,3PD. Figure 4 shows the effects of ZrO2−MoO3 with different MoO3 loadings on 1,3-PD conversion. As can be seen, an increasing Mo amount led to an increasing conversion rate of 1,3-propanediol in the acetalization reaction from 57.9% over a 6306

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is exothermal, and the hydrolysis reaction is endothermal; that is, a high temperature is favorable for the equilibrium constant of the hydrolysis of 2MD. To maintain the hydrolysis, reflux conditions are conducive to the hydrolysis reaction.33 For the acetalization reaction, temperature not only affects the equilibrium constant, but also favors the volatilization of acetaldehyde and the production of unwanted byproducts. Thus, constituents at different temperatures were analyzed in the acetalization reaction. Figure 6 shows that the 1,3-PD

Figure 7. Catalytic activities of ZrO2−10 wt % MoO3 for the acetalization of 1,3-PD at different concentrations.

fell. However, it still reached 84.5%, and this might be associated with the lack of active sites on ZrO2−MoO3. In light of these results, the ZrO2−MoO3 heterogeneous catalysts showed the best selectivity in the extraction of 1,3-PD from dilute aqueous solution. 3.4.4. Catalytic Activity of ZrO2−10 wt % MoO3 in the Isolation of 1,3-PD in the Simulated Microbial Fermentation Components. Yet, as noted in Table 1, the selectivity of ZrO2−

Figure 6. Effects of temperature on 1,3-PD conversion.

Table 1. Selectivities of ZrO2−10 wt % MoO3 for Components in Simulated Microbial Fermentation Broth

conversion was reduced normally when the temperature was much higher and much lower, and the proper temperature was 60 °C. The polymerization of acetaldehyde is closely related to 2MD formation, because of the exothermic nature of these two reactions. The acetalization reaction was relatively slower than the polymerization of acetaldehyde. Figure 6 demonstrates that temperature affected the kinetics and the thermodynamic properties of the acetalization reaction in competing with the polymerization reaction. The conversion rate of acetalization of 1,3-PD increased with increasing temperature, reaching a maximum of 95.7% at 60 °C. Below or above this temperature, unwanted side reactions occurred and thus decreased the 1,3propanediol conversion. Complete 1,3-propanediol acetals formation is possible at the proper temperature. With an increase in temperature, the 2MD formation rate exceed the polymerization rate of acetaldehyde. When the temperature reached 80 °C, the concentration of product decreased. This might be due to the disadvantage of higher temperature for the exothermal reaction of condensation, and also might be due to some other side reactions following with the increasing temperature.34 Moreover, higher temperatures are impractical because of the low boiling point of acetaldehyde.35 In connection with the limitations of the kinetics and the thermodynamic properties of the acetalization reaction, the optimal temperature is 60 °C. 3.4.3. Catalytic Activity of ZrO2−10 wt % MoO3 in Dilute Solutions. Figure 7 shows the catalytic activities of the ZrO2− 10 wt % MoO3 for the acetalization of 1,3-PD at different concentrations. The concentrations were 5, 10, 20, and 40 g/L, and the corresponding catalyst and acetaldehyde were added into the solution. ZrO2−10 wt % MoO3 showed good catalytic effects for 1,3PD acetalization in the range from 5 to 40 g/L. The 1,3-PD conversion reached 98.2%, 97.4%, and 92.7%, respectively, in 60 min, and total conversion was achieved in 2 h. When the concentration was increased to 40 g/L, the 1,3-PD conversion

component

selectivity (%)

ethanol 2,3-butylene glycol 1,3-PD glycerol

− 3.48 90.01 6.36

10 wt % MoO3 for components in a simulated 1,3-PD fermentation was investigated. The polyhydroxy aqueous feed in these experiments contained 80 g/L 1,3-PD, 10 g/L ethanol, 20 g/L glycerol, 10 g/L 2,3-butanediol, and water, which corresponds to the major metabolite products from the biotechnological production process. Moreover, 1.0 g of catalyst and an appropriate acetaldehyde were added to 100 g of simulated fermentation broth, and the products of the acetalization were analyzed as before. As shown in Table 1, comparing the catalytic activity in the acetalization, the 10 wt % ZrO2−MoO3 catalyst exhibited much higher selectivity for 1,3-PD in the simulated fermentation liquid than for ethanol, 2,3-butanediol, and glycerol. Furthermore, similar tests were performed by mixing acetaldehyde with ethanol, 2,3-butanediol, and glycerol separately; these components showed little reaction activity with acetaldehyde. Taking into account of these results, the catalyst can be used reliably to separate 1,3-PD from the other three polar alcohols.36 3.4.5. Stability of ZrO2−10 wt % MoO3. To study the stability of the catalyst, the separated catalyst was washed with deionized water and dried, and then the obtained catalyst was used for the next reaction cycle. It can be seen from Figure 8 that ZrO2−10 wt % MoO3 showed good catalytic activities in subsequent catalytic cycles and 1,3-PD conversion in acetalization could still reach 87.3% at the fifth cycle. This indicates that the active components of the solid catalyst leached little in the liquid-phase reaction and the solid catalyst is stable. 6307

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(5) Reddy, B. M.; Sreekanth, P. M.; Yamadab, Y.; Xu, Q.; Kobayashi, T. Surface characterization of sulfate, molybdate, and tungstate promoted TiO2−ZrO2 solid acid catalysts by XPS and other techniques. Appl. Catal. A 2002, 228, 269−278. (6) Liu, H. J.; Ou, X. J.; Zhou, S.; Liu, D. H. Microbial 1,3propanediol, its copolymerization with terephthalate, and applications. Microbiol. Monogr. 2010, 14, 405−425. (7) Wu, M.; He, Q.; Shao, Q. F.; Zuo, Y. G.; Wang, F.; Ni, H. M. Preparation and characterization of monodispersed microfloccules of TiO2 nanoparticles with immobilized multienzymes. ACS Appl. Mater. Interfaces 2011, 3 (9), 3300−3307. (8) Ü lgen, A.; Hoelderich, W. Conversion of glycerol to acrolein in the presence of WO3/ZrO2 catalysts. Catal. Lett. 2009, 131, 122−128. (9) Liu, J.; Bian, S. G.; Xiao, M.; Wang, S. J.; Meng, Y. Z. Highly active SO42−/xTiO2−ZrO2 catalysts for the esterification between terephthalic acid and 1,3-propanediol. Catal. Lett. 2009, 131, 305−311. (10) Hao, J.; Lin, R. H.; Zheng, Z. M.; Liu, H. J; Liu, D. H. Isolation and characterization of microorganisms able to produce 1,3-propanediol under aerobic conditions. World J. Microbiol. Biotechnol. 2008, 24, 1731−1740. (11) Matsumoto, M.; Kado, A.; Shiraki, T.; Kondo, K.; Yoshizuka, K. Reactive extraction of diols with phenyl boronic acid and trioctylmethylammonium chloride as coextractants and quantitative structure−property relationship of their extraction behaviors. J. Chem. Technol. Biotechnol. 2009, 84, 1712−1716. (12) Ni, J. B.; Wu, M.; Yang, Z. H.; Bu, C. F.; He, Q. Effective SO42−/ TiO2−ZrO2 for preparation and hydrolysis of 1,3-propanediol acetals. React. Kinet., Mech. Catal. 2010, 100, 337−346. (13) Malinowski, J. J. Reactive Extraction for Downstream Separation of 1,3-Propanediol. Biotechnol. Prog. 2000, 16, 76−79. (14) Kenney, C.; Maham, Y.; Nelson, A. E. Characterization of monofunctional ZrO2−MoO3 catalysts for methylcyclopentane conversion. Thermochim. Acta 2005, 434, 55−61. (15) Kurosaka, T.; Maruyama, H.; Naribayashi, I.; Sasaki, Y. Production of 1,3-propanediol by hydrogenolysis of glycerol catalyzed by Pt/WO3/ZrO2. Catal. Commun. 2008, 9, 1360−1363. (16) Cortés-Jácome, M. A.; Angeles-Chavez, C.; López-Salinas, E.; Navarrete, J.; Toribio, P.; Toledo, J. A. Migration and oxidation of tungsten species at the origin of acidity and catalytic activity on WO3− ZrO2 catalysts. Appl. Catal. A 2007, 318, 178−189. (17) Zhao, B. Y.; Wang, X. Y.; Ma, H. R.; Tang, Y. Q. Raman spectroscopy studies on the structure of MoO3/ZrO2 solid superacid. J. Mol. Catal. A: Chem. 1996, 108, 167−174. (18) López, T.; Alvarez, M.; Gómez, R.; Aguilar, D. H.; Quintana, P. ZrO2 and Cu/ZrO2 sol−gel materials spectroscopic characterization. J. Sol−Gel Sci. Technol. 2005, 33, 93−97. (19) Huang, Y. Y.; Zhao, B. Y.; Xie, Y. C. Preparation of zirconiabased acid catalysts from zirconia aerogel of tetragonal phase. Appl. Catal. A 1998, 172, 327−331. (20) Gao, Z; Chen, J. M.; Tang, Y. Studies on the formation of ZrO2/ SO42− superacid system. Chem. J. Chin. Univ. 1992, 13, 1498−1502. (21) Zhao, B. Y.; Xu, X. P.; Ma, H. R.; Sun, D. H.; Gao, J. M. Monolayer dispersion of oxides and salts on surface of ZrO2 and its application in preparation of ZrO2-supported catalysts with high surface areas. Catal. Lett. 1997, 45, 237−244. (22) Sun, W. D.; Zhao, Z. B.; Guo, C.; Ye, X. K.; Wu, Y. Study of the alkylation of isobutane with n-butene over WO3/ZrO2 strong solid acid. 1. Effect of the preparation method, WO3 loading, and calcination temperature. Ind. Eng. Chem. Res. 2000, 39, 3717−3725. (23) Naik, M. A.; Mishra, B. G.; Dubey, A. Combustion synthesized WO3−ZrO2 nanocomposites as catalyst for the solvent-free synthesis of coumarins. Colloids Surf. A 2008, 317, 234−238. (24) Zhao, B. Y.; Xu, X. P.; Ma, H. R.; Gao, J. M.; Wang, R. Q.; Sun, D. H.; Tang, Y. Q. A new way to prepare some supported catalysts with highly specific surfaces. Acta Phys.-Chim. Sin. 1993, 9, 8−11. (25) Sarkar, A.; Pramanik, S.; Achariya, A.; Pramanik, P. A novel sol− gel synthesis of mesoporous ZrO2−MoO3/WO3 mixed oxides. Microporous Mesoporous Mater. 2008, 115, 426−431.

Figure 8. Catalytic activities of ZrO2−10 wt % MoO3 in subsequent catalytic cycles.

4. CONCLUSIONS A cyclic acetal reaction catalyzed by ZrO2−MoO3 solid heterogeneous catalysts in the separation of 1,3-PD with an aldehyde was studied. The results in this work demonstrated that catalyst composition and reaction temperature are important factors resulting in a high 1,3-PD conversion in the reaction. In connection with the limitations of the kinetics and the thermodynamic properties of the acetalization reaction, the optimal temperature of acetalization was found to be 60 °C. With a Mo amount of 10 wt %, the highest conversions of 1,3PD in acetalization and of 2MD in the hydrolysis reaction were 95.7% in 2 h and 97.0% in 10 h, respectively. Meanwhile, ZrO2−10 wt % MoO3 showed good catalytic stability and higher selectivity for 1,3-propanediol than other components in the simulated fermentation liquid. The results indicated that the process coupling the reversible reaction of acetalization of 1,3PD with acetaldehyde to 2-methyl-1,3-dioxane by ZrO2−10 wt % MoO3 solid acid could effectively separate 1,3-PD from dilute solutions.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-52090619. Fax: +86-25-52090621. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by National Natural Science Foundation of China (NSFC, Grant 51073035), National Natural Science Foundation of Jiangsu Province (BK2009293), and Educational Commission of Jiangsu Province (JHB20112).

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REFERENCES

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