Catalytic Performance and Characterization of Silica Supported

Sep 23, 2009 - Dehydration of methyl lactate (ML) to methyl acrylate (MA) and acrylic acid (AA) over silica supported sodium phosphates was investigat...
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Ind. Eng. Chem. Res. 2009, 48, 9083–9089

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Catalytic Performance and Characterization of Silica Supported Sodium Phosphates for the Dehydration of Methyl Lactate to Methyl Acrylate and Acrylic Acid Zhiqiang Zhang, Yixin Qu, Shui Wang, and Jidong Wang* Beijing Key Laboratory of Bioprocess, College of Chemical Engineering, Beijing UniVersity of Chemical Technology, Beisuanhuan East Road 15, Chaoyang District, Beijing 10029, China

Dehydration of methyl lactate (ML) to methyl acrylate (MA) and acrylic acid (AA) over silica supported sodium phosphates was investigated in a fixed-bed continuous flow reactor. On NaH2PO4/SiO2 a significantly higher selectivity to MA + AA than on Na2HPO4/SiO2 and Na3PO4/SiO2 was observed. For NaH2PO4/SiO2, there is an optimum loading of NaH2PO4, with which a higher selectivity to MA + AA could be obtained. It was found that the ratio of Na2O to P2O5 of the phosphates influenced the catalyst performance in the conversion of ML. The catalysts were characterized using XRD, Raman, NMR, and NH3-TPD techniques. The higher selectivity for the dehydration of ML to MA + AA on NaH2PO4/SiO2 is assumed to be related to the acidity and the amount of the terminal POH groups on the polyphosphate chains. 1. Introduction Lactic acid (LA) is a commercial fine chemical that is primarily used in the food, medical, and cosmetic industries. LA is now mainly produced via fermentation using a wide variety of raw materials from carbohydrates such as corn syrup, whey, dextrose, cane sugar, and beet sugar as feedstock. The current capacity of worldwide production of LA is estimated to be around 120 000 t y-1.1 Dehydration of LA via the R-hydroxyl group can produce acrylic acid (AA), which is a potential method to manufacture AA with renewable resources. In 1958, Holmen2 reported that LA could be converted to AA using a mixture of Na2SO4 and CaSO4 as a catalyst in a fixed bed tubular reactor. The best yield of AA was 68%. There was no further development of the process since then. Sawicki3 reported that phosphate impregnated metal oxides catalyzed the dehydration of LA to AA with a typical yield close to 40%. They found that pretreatment of phosphate impregnated metal oxides with basic solutions improved the selectivity to AA. Paperizos et al.4 reported that LA and ammonium lactate could be converted to AA in the vapor phase using base-treated aluminum phosphates as catalysts at temperatures from 320 to 375 °C and contact times from 2 to 4 s. The reported best yield to AA was 61%, with a majority of AA yields being below 40%. Gunter et al.5 and Tam et al.6 studied the conversion of LA on sodium phosphates at a pressure of 0.5 MPa. It was found that at elevated pressures the main products were 2,3-pentanedione and AA. The catalytic activity of sodium phosphates toward 2,3-pentanedione and AA was related to the formation of sodium lactate and sodium pyrophosphate, which resulted from the proton transfer from LA to the catalyst surface. Both conversion of LA and selectivity to main products increase with an increase in the basicity of the phosphate salts (Na3PO4 > Na2HPO4 > NaH2PO4). Conversion of LA in supercritical water at 385 °C and 34 MPa has been investigated by Mok et al.7 using H2SO4 and NaOH as catalysts. It was found that decarbonylation to acetaldehyde (AD) was predominant in the presence of H2SO4. * To whom correspondence should be addressed. Tel.: +86 10 6443 4785. Fax: +86 10 6443 6781. E-mail address: jidongwang1963@ yahoo.com.cn.

However, addition of NaOH led to formation of more CO2 and H2. Lira and McCrackin8 studied the reaction of LA under supercritical water using phosphates as catalysts. They found that phosphates suppressed the formation of AD, rather than enhanced the dehydration of LA. Due to its high boiling point and chemical instability, LA presents problems with its separation and purification from fermentation broth. Therefore, LA was generally converted into its metal salts or its esters for the purpose of separation and purification.1 If esters of LA such as ML could be directly used as a feedstock for the production of other chemicals such as AA, then there would be an advantage for simplifying the manufacturing process and for the reduction of production costs. It is possible that dehydration of lactates, rather than LA, may give higher yield of AA and/or acrylates due to the fact that lactates have lower boiling points and better chemical stabilities than LA. Walkup et al.9 developed a process for converting lactate esters to acrylates using CaSO4 as a catalyst in a fixedbed reactor, and they achieved a yield of 61%. Anstrom10 investigated the dehydration of ML over phosphoric acid and sodium phosphates supported on silica. It was found that the extent of phosphate polymerization could be used as a predictor for the selectivity toward acrylates. Most of the previous studies illustrated that sulfates and phosphates, especially their alkali and alkaline-earth salts, were effective catalysts for converting LA to AA. However, there are only a few reports concerning the conversion of ML to methyl acrylate (MA) and AA. In the present work, we studied the conversion of ML on the catalysts of silica supported sodium phosphates with objectives of producing MA and AA as main target products. On the basis of the results of catalyst testing and characterizations, the influence of the polyphosphate chain length and the acidity of the POH groups on the polyphosphates on the dehydration of ML to MA and AA are discussed. 2. Experimental Section 2.1. Catalyst Preparation. Catalysts were prepared by incipient wetness impregnation. The silica support (Qingdao Hanyang Chemical & Special Silica Gel Co. Ltd.) is a commercial product having a spherical diameter of 0.59-0.84

10.1021/ie900065a CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

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mm. All chemicals used for the preparation of catalysts are analytical grade of commercial products. After the silica was impregnated with a solution containing a specific loading of phosphate salt, the sample was dried in an oven at 80 °C for 24 h and then calcined at 450 °C for 6 h in a furnace under static air conditions. Catalyst samples of NaH2PO4-x% H3PO4 were prepared by impregnation of silica in aqueous solutions containing NaH2PO4 (1.7 mmol g-1 of silica) and x mol % (x ) 10, 20, 30) of H3PO4 based on the amount of NaH2PO4. Catalyst samples of NaH2PO4-x% Na2CO3 were prepared by impregnation of silica in aqueous solutions containing NaH2PO4 (1.7 mmol g-1 of silica) and x mol % (x ) 10, 20, 30, 40, and 50) of Na2CO3 based on the amount of NaH2PO4. 2.2. Characterization Methods. The BET surface areas of different catalysts were determined by N2 physisorption using a Micromeritics ASAP 2020. Each sample was degassed in the system at 300 °C for 3 h prior to N2 physisorption. X-ray powder diffraction (XRD) patterns of catalyst samples were obtained on a Rigaku D/MAX2500 diffractometer using Cu KR radiation at 40 kV and 200 mA. The crystalline XRD patterns were compared with JCPDS standards for the purposes of phase identifications. Temperature-programmed desorption of ammonia (NH3-TPD) for different catalyst samples was performed on a Thermo Electron Corporation TPD/R/O 1100 apparatus equipped with a thermal conductivity detector (TCD).11 A 200 mg sample was calcined for 1 h at 500 °C in a flowing helium gas (20 cm3 min-1) and then cooled to 100 °C to adsorb NH3 in a NH3 stream (20 cm3 min-1) for 0.5 h. Then, the sample was flushed in a flow of helium (20 cm3 min-1) for 2 h to remove any loosely bound NH3 on the sample. TPD measurement was then carried out with a heating rate of 10 °C min-1 from room temperature to 550 °C. The amount of adsorbed NH3 was absorbed with HCl solution and quantified by NaOH titration. Raman microprobe spectrometer, LabRam HR 800 (Horiba Jobin Yvon, Edison, NJ, USA), was used to collect Raman spectra of catalyst samples at room temperature. This spectrometer is equipped with a confocal microscope (Olympus BX40), a piezoelectric x/y stage, and a charge-coupled device (CCD) detector. The 514.5 nm line of an argon ion laser was used for excitation at 30 mW output power. 31 P magic-angle spinning nuclear magnetic resonance (MAS NMR) measurements were carried out at 121.5 MHz on a Bruker AV-300 spectrometer that was equipped with a 4 mm crosspolarization (CP)/MAS probe. The spinning frequency was fixed at 10 kHz, and the pulse delay was 10 s. 1H/31P CP/MAS measurements were carried out using a single contact CP from protons with a CP time of 0.7 ms at 121.5 MHz on the same spectrometer. The spinning frequency was also 10 kHz. 31P isotropic chemical shifts are referenced to 85% phosphoric acid. 2.3. Test of Catalysts. Vapor phase dehydration of ML (99%, Jiangxi Musashino Bio-Chem Co., Ltd.) was carried out in a fixed bed tubular reactor (quartz, 14 mm i.d., and 75 cm long) under an atmospheric pressure using nitrogen (99.99%) as a carrier gas. A catalyst sample of 18 cm3 was packed in the middle of the reactor. Empty spaces on both sides of the catalyst bed were filled with quartz sands. The quartz sands in the inlet section were used as a preheating zone for reactant vaporization. The reaction temperature was measured with a thermocouple that was inserted into a thermocouple well (3 mm o.d.) located at the center of the reactor. ML was fed into the reactor via a microsyringe pump. The reaction products passed through a condenser maintained at -10 °C so that the condensable products were collected. The noncondensable products which

were mainly CO and CO2 were vented out after passing through a flow rate meter and a sampling device. During temperature ramping, the catalyst was purged with a flow of nitrogen. When a desirable temperature was reached, ML was introduced. Samples were taken for analysis 1 h after ML introduction. A typical experiment involved evaluations of a fresh catalyst at temperatures of 340 and 380 °C. The liquid volume flow rate of ML was 0.1 cm3 min-1, and the gas volume flow rate of nitrogen was 50 cm3 min-1, thus giving a molar fraction of ML of 0.33. Under these conditions, the contact time is 6.9 s at 340 °C and 6.5 s at 380 °C. Liquid products were analyzed using a gas chromatograph (BF SP-2100) equipped with a flame ionization detector (FID) and a capillary column of DB-FFAP (30 m × 0.32 mm × 0.50 µm). Products identified with GC/MS were mainly AD, methanol, MA, and AA in the condensed liquid streams. There are a number of minor products that are lumped together and reported as “others” in the analysis reports. Response factors of these minor products were assumed to be equal to the response factor of MA for the purpose of evaluating their selectivity. Methods of the internal calibration standard curve were applied for the quantification of main products. CO and CO2 were analyzed by using another gas chromatograph (BF SP-2100) having a thermal conductivity detector (TCD) and a TDX-01 stainless steel column (1 m × 3 mm). The method of external calibration was used to quantitatively determine amounts of CO and CO2. Conversions of ML (XML, percent) were calculated using eq 1: XML )

nML,in - nML,out × 100% nML,in

(1)

Where, nML,in is the molar quantity of ML fed to the reactor, and nML,out is the molar quantity of ML in the effluent. The selectivity (Si, percent) of liquid products was calculated using eq 2: Si )

ni,out × 100% nML,in - nML,out

(2)

Where, ni,out is the molar quantity of the ith component in the liquid products. The selectivity of CO and CO2 was calculated using eq 3: ni,out 4 × 100% Si ) nML,in - nML,out

(3)

Where, ni,out is molar quantity of CO or CO2 in the gas stream. 3. Results and Discussion 3.1. Effect of Active Components. The main chemical reaction pathways of ML are shown in Figure 1. Dehydration of ML forms MA and H2O; dehydration and deesterification of ML form AA and methanol; decarbonylation of ML forms AD, methanol, and CO; decarboxylation of ML forms AD, methane, and CO2. Conversions of ML on the silica support and the supported sodium phosphates with a loading of 1.0 mmol g-1 at 340 and 380 °C are reported in Tables 1 and 2, respectively. The samples were taken after 1 h on stream. The catalysts could maintain the reported conversion for approximately 5 h. After 5 h on stream, a slight decrease in the conversion of ML is noticed. Over the silica support, the conversion of ML is 51.2% at 340 °C, and the selectivity to MA + AA is 7.0%. At 380 °C,

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Figure 2. PO4 tetrahedral groups existing in phosphate.

Figure 1. Main reaction pathways of ML. Table 1. Conversions of ML and Selectivity to Main Products over Silica-Supported Sodium Phosphates at 340 °Ca substrate

support only

NaH2PO4

Na2HPO4

Na3PO4

XML (%) Smethanol (%) SAD (%) SMA (%) SAA (%) SMA+AA (%) Sothersb (%) SCO (%) SCO2 (%)

51.2 36.6 9.9 0.4 6.6 7.0 11.0 6.5 1.1

78.9 43.1 12.3 9.0 30.8 39.8 30.2 4.3 0.4

45.5 65.8 9.8 3.4 7.6 11.0 32.1 3.5 4.5

92.2 66.9 7.3 7.8 1.9 9.8 24.5 1.4 13.4

a Loading of sodium phosphates ) 1.0 mmol g-1, contact time ) 6.9 s. b Sum of the minor products.

Table 2. Conversions of ML and Selectivity to Main Products over Silica-Supported Sodium Phosphates at 380 °Ca substrate

support only

NaH2PO4

Na2HPO4

Na3PO4

XML (%) Smethanol (%) SAD (%) SMA (%) SAA (%) SMA+AA (%) Sothersb (%) SCO (%) SCO2 (%)

73.7 74.9 31.0 2.9 3.2 6.1 25.7 12.1 3.4

99.5 33.3 15.2 27.4 24.6 52.0 19.7 7.2 5.8

88.6 57.5 14.5 8.9 6.7 15.6 24.3 8.2 4.6

99.4 58.4 9.3 17.2 1.7 18.9 15.3 3.8 15.7

a Loading of H3PO4 and sodium phosphates ) 1.0 mmol g-1, contact time ) 6.5 s. b Sum of the minor products.

those values are 73.7% and 6.1%. On NaH2PO4/SiO2 and Na3PO4/SiO2, conversion of ML at 340 °C is significantly increased to 78.9% and 92.2%. In contrast, the conversion of ML on Na2HPO4/SiO2 is decreased to 45.5%. At 380 °C, the conversion of ML on NaH2PO4/SiO2 and Na3PO4/SiO2 is nearly complete, on Na2HPO4/SiO2 the conversion of LM is also increased to 88.6%. The selectivity to MA + AA on these catalysts depends on the compositions of the sodium phosphates. On Na2HPO4/SiO2, selectivity to MA + AA increases by a factor about 1.6 at 340 °C and 2.6 at 380 °C as compared to that observed for the silica support. On Na3PO4/SiO2, selectivity to MA + AA increases by a factor about 1.4 at 340 °C and 3.1 at 380 °C. On NaH2PO4/SiO2, selectivity to MA + AA increases by a factor of about 5.7 at 340 °C and 8.5 at 380 °C. It is obvious that the conversion of ML and the selectivity to MA + AA on the silica supported sodium phosphates depend on the composition of the sodium phosphates. NaH2PO4/SiO2 shows not only a higher activity but also a higher selectivity to MA + AA as compared to those observed for the silica support. The structures of sodium phosphates have been extensively studied.12,13 The basic unit in phosphates is a PO4 tetrahedron, whose characteristics are generally described in terms of a parameter Q(n), where n represents the number of the PO4

Figure 3. Raman spectra of (a) Na3PO4, (b) Na2HPO4, and (c) NaH2PO4 supported on silica (1.0 mmol g-1). Calcination temperature ) 450 °C. Loading of NaH2PO4 ) 1.0 mmol g-1.

tetrahedra which are connected to the concerned PO4 tetrahedron via oxygen atoms (Figure 2). Upon calcination, mono- and disodium phosphates could be converted to sodium polyphosphates14 and sodium pyrophosphates,15 respectively. Raman spectra of the silica supported sodium phosphates with a loading of 1.0 mmol g-1 are given in Figure 3. The Raman spectrum of Na2HPO4/SiO2 (Figure 3b) shows a strong peak at 1028 cm-1 and a weaker peak at 988 cm-1. These peaks are the symmetric stretching mode of PO3 of Na4P2O7. The peak at 1122 cm-1 can be attributed to the asymmetric stretching mode of PO3 of Na4P2O7. The peak at 730 cm-1 can be attributed to the symmetric P-O-P stretching mode Na4P2O7.16 The Raman spectrum of Na3PO4/SiO2 (Figure 3a) shows a strong peak at 1122 cm-1 and a medium strong peak at 1027 cm-1. The peak at 1122 cm-1 can be attributed to the asymmetric PO3 stretching of Na4P2O7,17 and the peak at 1027 cm-1 can be attributed to the symmetric PO3 stretching of Na4P2O7.16 The peak at 938 cm-1 can be attributed to the PO4 asymmetric stretching mode of Na3PO4.18,19 The Raman spectrum of Na3PO4/SiO2 indicates that the catalyst prepared from Na3PO4 contains both Na4P2O7 and Na3PO4. When Na3PO4 is dissolved in water, PO43- anions hydrolyze according to eq 4: PO43- + H2O ) HPO42- + OH-

(4)

An equilibrium is quickly established between PO43- and HPO42-. Therefore, the aqueous solution of Na3PO4 in fact contains both Na2HPO4 and Na3PO4. It is reasonable that after calcination, the Na3PO4/SiO2 sample prepared via the impregnation method contains Na3PO4 and Na4P2O7 which is formed from the condensation of Na2HPO4. The Raman spectrum of NaH2PO4/SiO2 (Figure 3c) shows a strong peak at 1165 cm-1 and two weaker peaks at 662 and 638 cm-1. By comparing with the Raman spectrum of crystalline sodium polyphosphate ((NaPO3)n),20-22 the peak at 1165 cm-1 can be attributed to the symmetric stretching of the PO2 mode on a Q(2) tetrahedron; the two weaker peaks at 662 and 638 cm-1 fall in the region of symmetric P-O-P stretching modes of (NaPO3)n.14,20,22

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Figure 4. 31P MAS NMR spectra of (a) 0.4 mmol g-1 NaH2PO4, (b) 1.0 mmol g-1 NaH2PO4, (c) 2.1 mmol g-1 NaH2PO4, (d) 1.0 mmol g-1 Na2HPO4, and (e) 1.0 mmol g-1 Na3PO4 supported on silica. Calcination temperature ) 450 °C.

The structure of silica-supported sodium phosphates can also be characterized in term of 31P NMR spectra. The 31P NMR spectrum of 1.0 mmol g-1 NaH2PO4/SiO2 is shown in Figure 4b. There are three main peaks with chemical shifts at 2.0, -6.6, and -18.8 ppm. The peak at 2.0 ppm lies in the range of a chemical shift that is normally assigned to free NaH2PO4.23 The peak at -6.6 ppm is assigned to the Q(1)(1H) (a terminal PO4 with one hydroxyl group of polyphosphate chains) groups of (NaPO3)n.24 And the peak at -18.8 ppm can be ascribed to the Q(2) groups of (NaPO3)n.25 Figure 4d shows the NMR spectrum of Na2HPO4/SiO2 with a loading of 1.0 mmol g-1. Both peaks at 2.1 and 3.1 ppm are assigned to Na4P2O7.23 Figure 4e shows the NMR spectrum of Na3PO4/SiO2. It exhibits two peaks at 14.0 and 2.3 ppm that can be ascribed to the resonances of a mixture of Na3PO4 and Na4P2O7.23 Raman and NMR results obtained for the samples of silica supported sodium phosphates with a loading of 1.0 mmol g-1 indicate that sodium phosphates undergo thermal transformations upon calcinations. For Na2HPO4/SiO2, Na2HPO4 has been converted to Na4P2O7. For Na3PO4/SiO2, a mixture containing Na3PO4 and Na4P2O7 has been produced. For NaH2PO4/SiO2, a high degree of condensation of NaH2PO4 has taken place upon calcinations and most NaH2PO4 has been converted to sodium polyphosphate. These observations are consistent with the previous reports.5,14,15 It is generally accepted that the dehydration of alcohols is an acid-catalyzed process.26,27 ML can be considered as an alcohol in the reactions from ML to MA and AA. Therefore, the existence of the acid sites on the catalysts for the dehydration of ML to MA and AA is necessary. Due to the strong basicity of Na3PO4/SiO2,28 a large portion of ML is converted into CO2 via decarboxylation. Conversion of LA in supercritical water has been investigated by Mok et al.7 using NaOH as catalysts. It was found that addition of NaOH promotes the decarboxylation of LA, producing more CO2. On Na2HPO4/SiO2 which does not have strong acidic or basic sites,28 both the conversion of ML and the selectivity of MA + AA are lower. The higher selectivity observed for NaH2PO4/SiO2 implies that the acid strength and the amount of the acid sites on this catalyst are suitable for the catalytic conversion of ML to MA and AA. From the NMR results, it is most likely that the active sites on NaH2PO4/SiO2 are terminal POH groups on the polyphosphate chains.

Figure 5. Influences of the NaH2PO4 loading on the conversion of ML and the selectivity to MA, AA, and MA + AA. Reaction conditions: ML ) 0.1 cm3 min-1, N2 ) 50 cm3 min-1, T ) 380 °C. Table 3. BET Surface Areas (SBET) and Pore Volumes (PV) of Silica Supported Sodium Phosphate Catalysts Calcined at 450 °C catalyst

SBET (m2 g-1)

PV (cm3 g-1)

pure SiO2 1.0 mmolg-1NaH2PO4/SiO2 1.7 mmolg-1NaH2PO4/SiO2 2.1 mmolg-1NaH2PO4/SiO2 2.6 mmolg-1 NaH2PO4/SiO

302 169 155 133 117

0.79 0.66 0.58 0.47 0.30

3.2. Effect of NaH2PO4 Loading. Since the supported NaH2PO4 shows superior selectivity over the supported Na2HPO4 and Na3PO4, a detailed investigation concerning the effect of NaH2PO4 loading was carried out. The conversion of ML and the selectivity to MA and AA with a loading of NaH2PO4 from 0.0 to 2.6 mmol g-1 are shown in Figure 5. Loading 0.4 mmol g-1 NaH2PO4 results in a nearly complete conversion of ML at 380 °C. Such a high conversion of ML remains until the loading of NaH2PO4 reaches 2.6 mmol g-1, the highest loading of NaH2PO4 investigated in this study. The selectivity to MA and AA increases with increasing NaH2PO4 loading from 0.4 to 1.0 mmol g-1 and remains at about 52% in the range from 1.0 to 2.1 mmol g-1. Further increasing the loadings of NaH2PO4 from 2.1 to 2.6 mmol g-1 leads to a decrease in the selectivity to MA and AA. These results suggest that there is an optimal NaH2PO4 loading range (from 1.0 to 2.1 mmol g-1) for an optimum selectivity to MA + AA. A change in the loading of NaH2PO4 also results in changes in the pore properties of NaH2PO4/SiO2. Table 3 shows the pore properties of NaH2PO4/SiO2 catalysts as determined from N2 adsorption. The BET surface area and the pore volume of the silica support are 302 m2 g-1 and 0.79 cm3 g-1, respectively. Loading 1.0 mmol g-1 of NaH2PO4 onto the silica support leads to a decrease in the BET surface area from 302 to 169 m2 g-1 and a decrease in its pore volume from 0.79 to 0.66 cm3 g-1. Further increasing the loading of NaH2PO4 results in further decreases in the BET surface area and the pore volume. When the loading of NaH2PO4 reaches 2.6 mmol g-1, the BET surface area decreases to 117 m2 g-1 and the pore volume decreases to 0.3 cm3 g-1. The decrease in the BET surface area and the pore volume due to the loading of NaH2PO4 can be attributed to the deposition of the phosphates inside the pores of the silica support or the blockage of pore entrances of the silica support.

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Figure 7. Influences of the ratio of Na2O to P2O5 on the conversion of ML and selectivity to main products. Reaction conditions: ML ) 0.1 cm3 min-1, N2 ) 50 cm3 min-1, T ) 340 °C.

Figure 6. XRD patterns of (a) silica and NaH2PO4/SiO2 with different loading: (b) 1.0, (c) 1.7, (d) 2.1, and (e) 2.6 mmol g-1. Calcination temperature ) 450 °C.

Figure 6 gives the XRD patterns of the silica support and NaH2PO4/SiO2, which are calcined at 450 °C. The XRD patterns of the silica support show only one broad peak at about 22°, which suggests that the silica support is amorphous. XRD patterns of the 1.0 mmol g-1 NaH2PO4 do not show any presence of the crystalline phase of NaH2PO4 or its polymerization products. This suggests that the materials derived from NaH2PO4 are well dispersed on the silica surface. Increasing the loading of NaH2PO4 to 1.7 mmol g-1 (Figure 6c) leads to the detection of several small diffraction peaks that correspond to a crystalline phase of (NaPO3)n (five lines at 2θ ) 15.4, 27.1, 28.8, 29.3, and 30.7°, PDF No. 11-0383) and a crystalline phase of (NaPO3)3 (one line at 2θ ) 17.6°, PDF No. 72-1628). The intensity of these peaks increased as the loading amount is raised to 2.1 and 2.6 mmol g-1 (Figure 6d and e). A change in the loading of NaH2PO4 also results in changes in the NMR spectra. The NMR spectrum of 0.4 mmol g-1 NaH2PO4/SiO2 is given in Figure 4a. Although the assignments of the peaks are the same as those for 1 mmol g-1 NaH2PO4/ SiO2 (Figure 4b), some changes in the relative intensity of the peaks assigned to different groups can be noticed. The relative peak intensity of the Q(1)(1H) groups (chemical shift ) -6.6 ppm) of 0.4 mmol g-1 NaH2PO4/SiO2 does not show a significant change as compared to that of 1.0 mmol g-1 NaH2PO4/SiO2; the relative peak intensity of free NaH2PO4 (chemical shift ) 2 ppm) increases from 18% to 27%, while the relative peak intensity of Q(2) groups (chemical shift ) -18.8 ppm) decreases from 46% to 35%. Comparison of the relative intensities of the NMR peaks between 0.4 and 1.0 mmol g-1 loading of NaH2PO4 indicates that the condensation degree of NaH2PO4 increases with an increase in the amount of loading. When the loading amount is raised to 2.1 mmol g-1 (Figure 4c), the peaks assigned to the Q(2) groups of (NaPO3)n split into three peaks, -15.8, -18.4, and -21.8 ppm, which indicate the

formation of crystal phases.25 This is supported by the results observed from XRD. When the NaH2PO4 is loaded on the surface of the silica support, they would disperse and adsorb on the surface of silica. Thus, when the loading of NaH2PO4 is lower, the condensation reaction of NaH2PO4 is obstructed due to the interaction between phosphates and the surface of silica. There is more free NaH2PO4 left and the chain length of polyphosphate is shortened, which is what has been observed from the NMR. Moreover, when the loading of NaH2PO4 is lower, a part of the silica surfaces cannot be covered by NaH2PO4 or its condensation products. The exposed silica surfaces can catalyze the conversion of ML but give low selectivity to MA + AA. When increasing the loading of NaH2PO4, most of the silica surfaces are covered by NaH2PO4 or its condensation products. The conversion of ML is mainly catalyzed by highly dispersed polyphosphates. When the loading of NaH2PO4 is beyond 2.1 mmol g-1, the influence of the silica support on the condensation reactions of NaH2PO4 becomes negligible and multilayer crystalline polyphosphates with a high degree of condensation can be formed. In such a case, not only the surface area and pore volume of NaH2PO4/SiO2 but also the density of the terminal POH groups is reduced. On these catalysts, both conversion of ML and selectivity to MA + AA are lower. 3.3. Effect of Na2O/P2O5 Ratio. The differences in the activity and selectivity to MA and AA among NaH2PO4, Na2HPO4, and Na3PO4 indicate that the initial composition of the phosphates significantly influence the performances of the catalyst. The differences among the above-mentioned sodium phosphates can be traced back to the differences in Na2O to P2O5 ratios in these compounds. Therefore, it is reasonable to assume that the ratio of Na2O to P2O5 in silica-supported sodium phosphates influence the catalytic performances. To investigate such influences, catalysts with different ratios of Na2O to P2O5 from 0.77 to 2.0 were prepared by addition of either H3PO4 or Na2CO3 to the aqueous solution containing NaH2PO4 (1.7 mmol g-1 loading) during the preparation of the catalysts. The test results obtained at 340 °C are shown in Figure 7. The conversion of ML progressively falls from 94.9% to 39.4% with an increase in the ratio of Na2O to P2O5 from 0.77 to 2.0. The selectivity to AD shows a sharp decrease from 48.1% to 9.8% with an increase in the ratio of Na2O to P2O5 from 0.77 to 1.2. Further

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Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009 Table 4. Surface Acidity of Silica-Supported Sodium Phosphates Derived from TPD Calcined at 500 °Ca catalyst

peak temperature (°C)

amount of acidic sites (mmol g-1)

NaH2PO4-20% H3PO4 NaH2PO4 NaH2PO4-20%Na2CO3 NaH2PO4-30%Na2CO3 NaH2PO4-50%Na2CO3

218 208 208 207 176

0.207 0.156 0.121 0.114 0.071

a

Figure 8. 1H/31P CP/MAS NMR spectrum of silica-supported (c) NaH2PO4 and 31P MAS NMR spectra of silica-supported (a) NaH2PO4-20% H3PO4; (b) NaH2PO4; (d) NaH2PO4-20% Na2CO3; (e) NaH2PO4-40% Na2CO3; and (f) NaH2PO4-50% Na2CO3. Loading of NaH2PO4 ) 1.7 mmol g-1. Calcination temperature ) 450 °C.

increasing the ratio of Na2O to P2O5 from 1.2 to 2.0 does not cause a significant variation of the selectivity to AD. With changing the ratio of Na2O to P2O5 from 0.77 to 2.0, a maximum selectivity to MA was observed at 0.91 and a maximum selectivity to AA at 1.0. The maximum selectivity to MA + AA, 39%, was observed at the ratio of Na2O to P2O5 of 1.0. Figure 8 reports the NMR spectra of supported sodium phosphates with different ratios of Na2O to P2O5. The 31P NMR spectrum of 1.7 mmol g-1 NaH2PO4/SiO2 (Figure 8b) shows four peaks at 1.7, -6.4, -14.7, and -20.9 ppm. Assignments of the peaks at 1.7 and -6.4 ppm are the same as those for 1.0 mmol g-1 NaH2PO4/SiO2 (Figure 4b). The peak at -20.9 ppm can be attributed to the Q(2) groups of (NaPO3)n and the small shoulder peak at -14.7 ppm to the Q(2) groups of (NaPO3)3. The relative intensity of these peaks is 22% at 1.7 ppm, 31% at -6.4 ppm, and 47% at -14.7 and -20.9 ppm. The 1H/31P CPMAS spectrum of 1.7 mmol g-1 NaH2PO4/SiO2 is given in Figure 8c. There are three peaks present with a relative intensity of 20% at 2.4 ppm, 50% at -6.2 ppm, and 30% at -19.3 ppm. A significant increase in the relative intensity of the peak at -6.2 ppm from 31% to 51% in the 1H/31P CP/MAS supports the argument that this peak is the resonance of Q(1)(1H) groups. When changing the ratio of Na2O to P2O5 from 1.0 to 0.83 by adding 20% H3PO4, the spectrum of NaH2PO4-20% H3PO4/ SiO2 (Figure 8a) shows some differences in the relative peak intensities as compared to those for NaH2PO4/SiO2. The relative intensity of the peak representing Q(2) groups (chemical shift ) -14.7, -20.9 ppm) increases from 47% to 67%, implying that the chain length of the polyphosphate has been increased. It is noted that there is no peak at about -51 ppm that represents the resonance of Q(3) groups.12 This suggests that addition of 20% H3PO4 does not lead to the formation of cross-linked phosphate networks. The added H3PO4 either condensates to polyphosphoric acid or is incorporated into the (NaPO3)n chains. The NMR spectrum of NaH2PO4-20% Na2CO3/SiO2 (Na2O/ P2O5 ) 1.4) is given in Figure 8d. As compared to the spectrum of 1.7 mmol g-1 NaH2PO4/SiO2, it shows a remarkable decrease in the relative intensity of the peak at -16.6 ppm that represents Q(2) groups of (NaPO3)n. This implies that the chain length of (NaPO3)n in the sample of NaH2PO4-20% Na2CO3/SiO2 is

Loading ) 1.7 mmol g-1.

shortened due to the addition of 20% Na2CO3. The peak at 1.1 ppm is assigned to free NaH2PO4 and/or Q(1) groups. The peak at -6.4 ppm is assigned to Q(1)(1H) groups. Drastic changes in the NMR spectrum are shown in Figure 8e for the sample with a ratio of Na2O to P2O5 of 1.8. The resonances of Q(1)(1H) and Q(2) groups almost disappear, and the resonance at 2.2 ppm becomes a major peak, which can be attributed to the Q(1) groups of (NaPO3)n and NaH2PO4 or the mixture of them. When the ratio of Na2O to P2O5 is further increased to 2.0 (Figure 8f), only one peak at 2.2 ppm is present. It is reasonable to assign this peak to Q(1) groups of Na4P2O7, since the real species in the NaH2PO4 solution with an addition of 50% Na2CO3 is Na2HPO4 which is converted to Na4P2O7 upon calcinations. The NMR results (Figure 8a) indicate that adding H3PO4 into NaH2PO4 increases the chain length of the polyphosphates and produces POH groups in the middle of the polyphosphate chains. Those POH groups have a higher acidity than that of the terminal POH groups. Adding Na2CO3 into NaH2PO4 decreases the chain length of the polyphosphates and some of the terminal POH groups are replaced by PONa. These will cause a change in the acidity and the amount of the acid site on the catalyst surface. Therefore, the acidity and the amount of the acid sites on the supported sodium phosphates have been determined using the NH3 temperature programmed desorption (NH3-TPD) technique. The peak temperature of NH3 desorption can be regarded as a relative measure of the acidity of the acid sites. The NH3TPD profiles of all supported samples show only one NH3 desorption peak. The peak temperature and the density of the acid site are reported in Table 4. It can be seen that the peak temperature of NH3 desorption and the density of the acid site decrease with an increasing molar ratio of Na2O to P2O5. Addition of H3PO4 into NaH2PO4 results in an increase in both the acidity and the density of acid sites. On the other hand, addition of Na2CO3 into NaH2PO4 results in a decrease in both the acidity and the density of acid sites. From the results concerning the effect of the Na2O to P2O5 ratio it can be concluded that conversion of ML and selectivity to MA + AA on the silica supported sodium phosphates are associated to the acid properties of the catalyst surface. An increase in the amount of the strong acid sites on the catalyst surface promotes the conversion of ML via the decarbonylation reaction and decreases the selectivity to MA + AA. In contrast, a decrease in the amount of the medium acidic sites on the catalyst surface suppresses the conversion of ML and also decreases the selectivity to MA + AA. The highest selectivity to MA + AA is obtained with a catalyst having a Na2O to P2O5 ratio equal to about 1.0, which represents the composition of NaH2PO4. 4. Conclusions Catalysts of silica-supported sodium phosphates were prepared and tested for the selective dehydration of ML to MA

Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009

and AA. The NaH2PO4/SiO2 shows a higher selectivity to MA and AA than Na2HPO4/SiO2 and Na3PO4/SiO2. Upon calcinations, NaH2PO4 is converted to sodium polyphosphate chains bearing terminal POH groups which are the active sites for the catalytic conversion of ML to MA and AA. For NaH2PO4/SiO2, there is an optimum loading of NaH2PO4, with which a higher selectivity to MA + AA could be obtained. Addition of H3PO4 to NaH2PO4 results in the formation of stronger acidic POH groups, which promotes the decarbonylation and decreases the selectivity to MA and AA. Addition of Na2CO3 results in the formation of shorter polyphosphate chains and decreases the acidity and amount of the terminal POH groups. As a result, both the conversion of ML and the selectivity to MA and AA decrease. Acknowledgment This work was supported by the National Basic Research Program (973 Program, Grant No. 2007CB714300). Literature Cited (1) Datta, R.; Henry, M. Lactic acid: recent advances in products, processes and technologies - a review. J. Chem. Technol. Biotechnol. 2006, 81 (7), 1119–1129. (2) Holmen, R. E. Production of Acrylates by Catalytic Dehydration of Lactic Acid and Alkyl Lactates. U.S. Patent 2,859,240, 1958. (3) Sawicki, R. A. Catalyst for Dehydration of Lactic Acid to Acrylic Acid. U.S. Patent 4,729,978, 1988. (4) Paperizos, C. D.; Shaw, W. G. Catalytic Conversion of Lactic Acid and Ammonium Lactate to Acrylic Acid. U.S. Patent 4,786,756, 1985. (5) Gunter, G. C.; Craciun, R.; Tam, M. S.; Jackson, J. E.; Miller, D. J. FTIR and 31P-NMR Spectroscopic Analyses of Surface Species in Phosphate-Catalyzed Lactic Acid Conversion. J. Catal. 1996, 164 (1), 207– 219. (6) Tam, M. S.; Gunter, G. C.; Craciun, R.; Miller, D. J.; Jackson, J. E. Reaction and Spectroscopic Studies of Sodium Salt Catalysts for Lactic Acid Conversion. Ind. Eng. Chem. Res. 1997, 36 (9), 3505–3512. (7) Mok, W. S. L.; Antal, M. J.; Jones, M. Formation of acrylic acid from lactic acid in supercritical water. J. Org. Chem. 1989, 54 (19), 4596– 4602. (8) Lira, C. T.; McCrackin, P. J. Conversion of lactic acid to acrylic acid in near-critical water. Ind. Eng. Chem. Res. 1993, 32 (11), 2608–2613. (9) Walkup, P. C.; Rohrmann, C. A.; Hallen, R. T.; Eakin, D. E. Production of esters of lactic acid, esters of acrylic acid, lactic acid, and acrylic acid. U.S. Patent 5,252,473, 1993. (10) Anstrom, M. InVestigation of hydrogen transfer reaction mechanisms oVer supported oxide catalysts; University of Wisconsin-Madison: Madison, 2002. (11) Arcoya, A.; Gonzalez, J. A.; Travieso, N.; Seoane, X. L. Physicochemical and Catalytic Properties of a Modified Natural Clinoptilolite. Clay Miner. 1994, 29 (1), 123–131.

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ReceiVed for reView January 15, 2009 ReVised manuscript receiVed July 23, 2009 Accepted August 30, 2009 IE900065A