NaY Zeolites Catalyze Dehydration of Lactic Acid to Acrylic Acid

Sep 2, 2010 - College of Biotechnology and Pharmaceutical Engineering, Nanjing UniVersity of Technology,. Nanjing 210009, China, and State Key Laborat...
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NaY Zeolites Catalyze Dehydration of Lactic Acid to Acrylic Acid: Studies on the Effects of Anions in Potassium Salts Peng Sun,† Dinghua Yu,†,‡ Zhenchen Tang,‡ Heng Li,† and He Huang*,†,‡ College of Biotechnology and Pharmaceutical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China, and State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China

NaY zeolite catalysts with and without potassium salt modifications were studied with respect to dehydration of lactic acid to acrylic acid. The selectivity for acrylic acid could be dramatically enhanced by modification of the NaY catalysts with potassium salts. The KI-modified NaY catalyst exhibited the best catalytic performance for lactic acid dehydration, over which 97.6% conversion and 67.9% acrylic acid selectivity could be gained at 598 K. Characterizations with TGA, XRD, N2 adsorption, 27Al MAS NMR, XPS, NH3TPD, CO2-TPD, and mechanism analysis revealed that the counteranions in potassium salts exerted a significant influence on the catalytic behaviors via an electronic effect. 1. Introduction Acrylic acid and its esters are the primary building blocks of all acrylate polymers and plastics.1 Currently, acrylic acid is manufactured mainly by two-step propylene oxidation. The general disadvantage of this route is the total dependence on a nonrenewable fossil resource. The development of a novel process, which is both renewable and economically viable, would be of great importance. New fermentation-based technologies to produce lactic acid from starch hydrolysates are making this optically active, bifunctional molecule into a viable feedstock for chemical production. With both hydroxyl and carboxylic acid functions, lactic acid and its derivatives offer novel routes to a variety of products.2,3 Dehydration of lactic acid to acrylic acid has long been of interest. Vapor phase conversion of lactic acid to acrylic acid is reported over several salt catalysts such as CaSO4/ Na2SO4, Na2HPO4, and AlPO4.4-6 Unfortunately, acetaldehyde formation via parallel thermal or acid-catalyzed decarbonylation and decarboxylation pathways and hydrogenation of acrylic acid to form propanoic acid directly compete with dehydration to reduce acrylic acid yield. In a recent paper,7 we found that when a NaY zeolite catalyst is modified with potassium nitrate, the lactic acid dehydration can proceed with high selectivity and durability. Cation-exchanged zeolites in particular have found a wide range of commercial uses in zeolite catalysis. The presence of both acidic and basic sites in zeolites makes the material a unique type of catalyst in the hydrocarbon reaction. However, most studies focus on the effect of the cation on the acidity and basicity of cation-exchanged zeolites.8,9 To the best of our knowledge, the influence of anions introduced by impregnation is rarely reported. This study employs NaY zeolites modified with various potassium salts to investigate the acidic catalysis with dehydration of lactic acid. The main objective of this work is to elucidate the roles of anions in potassium salts in the catalysis of NaY-based heterogeneous dehydration of lactic acid via detailed charac* To whom correspondence should be addressed. Telephone/fax: +86 25 83172094. E-mail: [email protected]. † College of Biotechnology and Pharmaceutical Engineering, Nanjing University of Technology. ‡ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.

terizations and mechanistic analysis. Our understanding of the function of the potassium salts in our system will provide new insights into the catalyst requirements for the occurrence of the dehydration of lactic acid and the key factors in controlling the reaction route. 2. Experimental Section 2.1. Catalysts. NaY zeolites (Si/Al ) 2.5 ( 0.03, determined by XRF-1800) were treated with a 1 mol/L NaNO3 aqueous solution to produce the 100% Na ion-exchanged forms before use. Potassium salt was introduced into NaY zeolites by an impregnation method.10 NaY (10 g) after calcination was immersed in an aqueous solution (22 mL) of potassium salt at a given concentration (0.4 mol/L) and stirred for 12 h at room temperature. The slurry was dried at 343 K for 4 h with continuous stirring. The solid sample was further dried in a vacuum at 313 K for 12 h. Then it was calcined at 823 K in air for 6 h to produce the catalysts denoted M/NaY, where M represents potassium salts. The amounts of sodium in NaY, KF/ NaY, KCl/NaY, KBr/NaY, and KI/NaY samples (measured with a Shimadzu XRF-1800 spectrometer equipped with a Rh X-ray tube target) were 9.24, 9.24, 9.23, 9.24, and 9.23 wt %, respectively. 2.2. Catalytic Reaction. The catalytic reactions were conducted using a fixed bed reactor with an inner diameter of 8 mm operated at atmospheric pressure. The catalyst (1.5 g) was charged in the middle section of the reactor, with quartz wool packed in both ends. Quartz sand (2 mL) was placed above the catalyst bed to preheat and vaporize the feed. Before the reaction, the catalyst was pretreated at 598 K for 0.5 h under N2 at a rate of 30 mL/min. Then the feedstock (29 wt % lactic acid aqueous solution) was pumped into the preheating zone, and the vapor was carried through the catalyst bed by nitrogen. The products were condensed and analyzed by gas chromatography (GC) (Agilent 6890 N) equipped with a FFAP (free fatty acid phase) capillary column and FID detector. Liquid reaction products (1 mL) were mixed with 1 mL of n-butanol (an internal calibration standard) and were diluted with ethanol to 5 mL. Samples (0.2 µL) of the mixture were injected. Response factors were determined via injection of calibration solutions with known concentrations of major products and lactic acid. (1) For lactic acid, y ) 9.2143x + 0.0077. (2) For acrylic acid, y )

10.1021/ie101093x  2010 American Chemical Society Published on Web 09/02/2010

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a

Table 1. Catalytic Dehydration of Lactic Acid over NaY Zeolites Modified with Various Potassium Salts selectivity (%) catalyst

conversion (%)

acrylic acid

NaY KF/NaY KCl/NaY KBr/NaY KI/NaY KNO3/NaY K2SO4/NaY K2HPO4/NaY K2C2O4/NaY K2CO3/NaY KOH/NaY

96.3 91.2 97.1 97.1 97.6 98.0 94.8 94.5 94.4 95.1 96.3

35.9 39.9 53.8 59.9 67.9 58.2 51.6 49.7 48.5 46.7 44.2

acetaldehyde

propanoic acid

2,3-pentanedione

coke

othersb

20.2 17.1 10.0 9.2 6.8 20.2 6.0 14.8 14.7 -

3.9 2.6 -

8.0 10.4 8.1 7.9 10.0 6.5 6.5 4.9 -

29.4 19.3 17.4 15.7 13.0 17.8 18.1 20.9 14.8 13.8 13.8

6.5 23.7 4.5 7.1 11.2 4.6 10.1 16.9 15.4 19.9 42.0

a Reaction conditions: lactic acid feedstock, 29 wt %; feedstock flow rate, 4.5 mL/h; N2 flow rate, 30 mL/min; catalyst, 1.5 g; TOS, 120 min; temperature, 598 K. b Selectivity for others equals 100 - the total selectivity for all identified products. Other products mainly include CO, CO2, and unknowns.

8.1648x + 0.0056. (3) For acetaldehyde, y ) 0.7073x - 0.0034. (4) For propanoic acid, y ) 0.7718x + 0.0009. (5) For 2,3pentanedione, y ) 0.2914x + 0.0014. x and y are the concentration of i (grams per milliliter) and the area ratio of i to n-butanol, respectively, and i is lactic acid or a major producs. Lactic acid conversion and product selectivity were calculated as described in our previous study.11 2.3. Characterization. The thermogravimetry analysis was conducted with the NETZSCH STA 409 PC equipment. The samples (∼5 mg) were placed in an Al2O3 cell and heated from room temperature to 1073 K at a heating rate of 10 K/min with a gas feed (air) of 30 mL/min. XRD patterns of the samples were recorded in the 2θ region of 5-60° using a Philips X’Pro X-ray diffractometer with Cu KR irradiation. The X-ray source was operated at 40 kV and 40 mA. N2 adsorption at 77 K was conducted with a Micromeritics ASAP 2020 surface area and porosity analyzer to examine the porous property and the surface area for each sample. The sample was pretreated at 573 K in a vacuum for 4 h before N2 adsorption. 27 Al MAS NMR spectra were recorded on a Bruker Avance 400 D spectrometer using a standard Bruker solid MAS probe, with a spinning speed of 15 kHz and a recycling delay of 0.5 s. X-ray photoelectron spectra (XPS) were recorded with a Thermo ESCALAB 250 apparatus with an Al KR (1486.6 eV) X-ray source. The binding energy was calibrated with a C1s photoelectron peak at 284.8 eV as a reference. Reported values of binding energies were averages of at least three different runs and were accurate to (0.1 eV. NH3 temperature-programmed desorption (NH3-TPD) was conducted with the BEL-CAT-B-82 equipment. Typically, the sample (150 mg) loaded in the quartz tube was first pretreated with He at a rate of 50 mL/min at 823 K for 1 h. The adsorption of NH3 was performed at 373 K in an NH3/He (10 vol % NH3) mixture for 0.5 h, and then the physically adsorbed NH3 was purged with He. TPD of ammonia was performed in the He flow by increasing the temperature to 923 K at a rate of 10 K/min, and the liberated NH3 was detected with an on-line thermal conductivity detector. CO2-TPD was performed like NH3-TPD. 3. Results and Discussion 3.1. Catalytic Reaction. Table 1 shows the catalytic performance of NaY zeolites modified with various potassium salts for the dehydration of lactic acid. As described in our earlier

Figure 1. Lactic acid conversion and the selectivity for acrylic acid vs time on stream: NaY (0), KF/NaY (O), KCl/NaY (4), KBr/NaY (3), and KI/ NaY (]).

paper,7 acetaldehyde is the main byproduct competing with acrylic acid over NaY zeolites without modification, but all the potassium salts could result in an increase in selectivity for acrylic acid, indicating that the K+ played an essential role. However, significant differences in catalytic performance were observed among these potassium salts, and KI was the best for lactic acid dehydration with 97.6% conversion and 67.9% selectivity for acrylic acid. All KX (X ) F, Cl, Br, or I)-modified NaY zeolites showed dramatic performances except for KF. The dependence of lactic acid conversion and selectivity for acrylic acid on time on stream as illustrated in Figure 1 was investigated to confirm the effect of KX on the dehydration performance of NaY zeolites. KX, except for KF, indeed improved the conversion of lactic acid, the selectivity for acrylic acid, and the durability of the catalyst. The selectivity for acrylic acid remained at 40.8% on KI/NaY, while that with NaY parent zeolites was less than 10% after reaction for 6 h. To disclose the quick deactivation, the surface area and pore volume of the spent catalyst have been measured by N2 adsorption. As shown in Table 2, the surface area and pore volume of the spent catalyst after 2 h on stream showed a sharp decline compared with those of fresh catalysts, indicating that

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Table 2. Porous Properties, Acidities, and Basicities of NaY and KX-Modified NaY Zeolites catalyst

SBETa (m2/g)

Vtotala (cm3/g)

amount of total acid (µmol/g)

acidity density (µmol/m2)

total amount of base (µmol/g)

basicity density (µmol/m2)

NaY KF/NaY KCl/NaY KBr/NaY KI/NaY

719 (33) 608 (12) 575 (11) 555 (10) 548 (10)

0.32 (0.01) 0.31 (0.0042) 0.29 (0.0023) 0.27 (0.0022) 0.28 (0.0019)

2040 1563 1580 1520 1100

2.84 2.57 2.75 2.74 2.00

97.2 105.9 161.6 196.5 208.1

0.14 0.17 0.28 0.35 0.38

a

In parentheses are the values of the spent catalysts after 2 h on stream. Table 3. XPS Data of NaY- and KX-Modified NaY Zeolites binding energy (eV)

atomic ratio

sample

Si 2p

Al 2p

O 1s

Si/Al

K/Al

K/Ala

X/Al

X/Ala

NaY KF/NaY KCl/NaY KBr/NaY KI/NaY

102.8 102.8 102.7 102.5 102.2

74.7 74.5 74.5 74.4 74.3

532.4 531.9 531.7 531.5 531.2

2.62 2.65 2.64 2.65 2.62

0 0.40 0.41 0.39 0.38

0 0.29 0.29 0.28 0.27

0 0.40 0.40 0.37 0.10

0 0.28 0.28 0.27 0.07

a

Measured using inductively coupled plasma (ICP).

Figure 2. TG curves of NaY zeolites modified with KF (a), KCl (b), KBr (c), and KI (d) before (s) and after (---) calcination.

serious coke deposition resulted in the blocking and packing of the micro channel. The blocking and packing of the micro channel would reduce mass transfer efficiency and further interrupt the contact between internal surface active sites and reaction species,12 finally leading to the quick deactivation of the NaY-based catalyst. One of the major pathways of lactic acid conversion was the polymerization of lactic acid to poly(lactic acid),13 which might result in coke deposits. It should be noted that acrylic acid also had high polymerization activity. Therefore, the possible coke precursors could include poly(lactic acid) and/or poly(acrylic acid). 3.2. Thermogravimetry Analysis and N2 Adsorption. To disclose the effects of potassium salts on the catalyst structure, we have conducted detailed characterizations of the potassium halogen salts (KX, where X ) F, Cl, Br, or I)-modified samples. Because there is a possibility that KX may decompose during calcination at 823 K, TGA up to a high temperature of 1003 K has been performed to examine the total decomposition temperature of KX and calculate the retention degree of KX on NaY zeolites before and after calcination. Figure 2 shows the TG curves of KX/NaY zeolites before and after calcination. The weight loss before 473 K in all samples could be assigned to the physically adsorbed water. No weight loss after 473 K was observed in NaY zeolites modified with KF, KCl, and KBr, indicating that KF, KCl, and KBr were preserved well after calcination. However, obvious weight loss from 473 to 1003 K appeared in the TG curve of KI/NaY. The quantitative results of weight loss in KI/NaY before and after calcination were 9.78 and 4.48%, respectively, demonstrating that partial decomposition of KI occurred during calcination at 823 K. This partial decomposition would result in the amount of iodine on KI/NaY zeolites being smaller than the amounts of other halogens (lower I/Al value in Table 3).

Figure 3. XRD patterns of NaY zeolites and KX-modified NaY zeolites: (a) NaY, (b) KF/NaY, (c) KCl/NaY, (d) KBr/NaY, and (e) KI/NaY.

The results with regard to porous properties obtained from N2 adsorption at 77 K are presented in Table 2. The BET surface area and pore volume of KX-modified samples decreased with an increase in halogen anion diameter. These decreases suggested that the pores of NaY were filled with KX species. Thus, the KX species in these samples were probably located in the pores of NaY. 3.3. Effect of Modification on NaY Zeolite Structure. Figure 3 shows the XRD patterns for NaY zeolites and KX/ NaY zeolites. The diffraction lines at 2θ of 6.28°, 10.22°, 11.97°, 15.73°, 20.43°, 23.72°, 27.12°, and 31.47° ascribed to the characteristic peak of NaY zeolites11 were observed for all these samples, suggesting that the regularity of the crystal structure of NaY zeolites was sustained after the introduction of KX. In most cases, a large surface area was favorable for the dispersion of species.14,15,29 In our case, neither KX nor new phases were observed on the XRD patterns of KX/NaY zeolites. The identical XRD patterns of these samples and the parent zeolites indicated that the large surface area of the NaY zeolites might contribute to the good dispersion of KX on NaY zeolites. With the strongest diffraction peak at 2θ ) 6.28° as a standard, the quantitative relative crystalline degrees of KF/NaY, KCl/NaY, KBr/NaY, and KI/NaY were 72, 71, 49, and 43%, respectively. For the NaY, KF/NaY, KCl/NaY, KBr/NaY, and KI/NaY samples, the cell parameter (a533) values were 2.4666, 2.4665, 2.4663, 2.4662, and 2.4660 nm, respectively. The relative

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Figure 5. NH3-TPD profiles of (a) NaY, (b) KF/NaY, (c) KCl/NaY, (d) KBr/NaY, and (e) KI/NaY. Figure 4. 27Al MAS NMR spectra of NaY and KX-modified NaY zeolites: (a) NaY, (b) KF/NaY, (c) KCl/NaY, (d) KBr/NaY, and (e) KI/NaY.

standard deviation was 0.01%. The relative crystalline degree and the cell parameters decreased with an increase in halogen atomic radius, indicating a slight distortion of the zeolite crystal cells. 27 Al MAS NMR spectra of NaY and KX/NaY zeolites are presented in Figure 4. None of the spectra showed significant resonances around 0 ppm, which was the expected position for peaks corresponding to nonframework octahedral Al. A broad peak at 59 ppm on the parent NaY was attributed to the tetrahedrally coordinated aluminum in the zeolite framework, which was charge-balanced by a sodium ion. This peak became sharp and shifted to 61 ppm on KX/NaY zeolites. In 27Al MAS NMR, several reasons for shifts in peak position exist. The peak position is a function of (i) the coordination number, (ii) the Al-O-Si angle, (iii) the mean Al-O distance, and (iv) the presence of quadrupolar interactions.16,17 In our case, the quadrupolar interactions were believed to be the most influential factor, because of the strong electronegativity of halogen. Table 3 summarizes the binding energies of KX/NaY zeolites as well as the surface and bulk K/Al and X/Al atomic ratios. Kaliaguine et al.18 have investigated a series of alkali cationexchanged zeolites using XPS. Their results indicate that the larger the electropositivity of the countercation, the lower the framework element (Si 2p, Al 2p, and O 1s) binding energies. In our case, introduced potassium with a larger electropositivity could result in the lower framework element binding energies in KX/NaY compared with NaY zeolites. Meanwhile, a halogen anion with a different electronegativity could cause the distinction in framework element binding energies in KX/NaY. The lower the anion electronegativity, the lower the framework element (Si 2p, Al 2p, and O 1s) binding energies. Besides, the O 1s binding energy evaluated by XPS studies can reflect the electron density of the framework oxygen atoms.19,20 A decrease in O 1s binding energy can be interpreted as an increase in the basicity (electron density) of oxygen.9,21 Thus, using the O 1s binding energy in Table 3, it was shown that the framework basicity of NaY zeolites increased in the following order: F < Cl < Br < I. We noticed that for all samples, the surface atomic Si/Al ratio is slightly higher than that of the bulk (2.5). The result suggested that the surfaces of zeolites were slightly richer in silicon than the bulk, which meant that the surface of these zeolites might have undergone slight dealumination. This result was in accordance with that reported by Kaliaguine et al.22,23 The K/Al

and X/Al surface atomic ratios were also higher than the bulk values. In potassium-exchanged Y-type zeolites, site II was the preferred site for potassium cations.24 The observation that the K/Al and X/Al surface atomic ratios estimated by XPS were considerably higher than the bulk values supported further the likelihood that mainly site II atoms are “seen” by XPS.25 By comparison with the results of the bulk and XPS measurements, we could conclude that the K/F, K/Cl, and K/Br values (atomic ratio) were almost 1, whereas the K/I value was ∼3.8, which indicated that KI partially decomposed and lost some iodine after calcination. This result was in agreement with that of TGA. For the KI/NaY sample, both the surface and the bulk I/Al atomic ratios were below the average, because of the partial decomposition of KI reflected by Figure 2. 3.4. Acidity and Basicity. The strong acid sites are detrimental to selectivity for acrylic acid over the NaY zeolites, leading to decarbonylation and/or decarboxylation to acetaldehyde.26 Here, the influence of the presence of KX on the acidity of NaY zeolites has been investigated by NH3-TPD measurements. NH3-TPD profiles obtained over NaY and KX/NaY zeolites are shown in Figure 5. A desorption of NH3 at 509 K has been observed over NaY zeolites, consistent with the fact that NaY zeolites possess acidic sites. Potassium modification could significantly decrease the acidity of NaY zeolites. After modification with KX, the NH3 desorption peak shifted gradually to low temperatures and the calculated amount of acid (Table 2) decreased, showing that the counteranions in the KX also exerted an influence on the acidity of NaY zeolites. The acidities of zeolites decreased in the following order: Cl > Br > I, except for F. To the best of our knowledge, no report has clearly illustrated the influence of types of acidic sites (Brønsted or Lewis) during lactic acid dehydration. As is well-known, charge-balancing metal cations act as Lewis acid sites and structure hydroxyl groups are Brønsted acid sites in zeolites. In our case, NaY zeolites were treated with a 1 mol/L NaNO3 aqueous solution to produce the 100% Na ion-exchanged forms before use. Thus, the treated NaY zeolites should exhibit only one type of surface hydroxyl group, the SiOH group terminating the outer surface of the zeolite crystallites or located on framework defects. These OH groups are, in general, considered as nonacidic.27 Therefore, although direct experimental evidence is absent, we speculate that the Lewis acid plays an essential role during lactic acid dehydration. It was reported that the Lewis acidity of the cationexchanged faujasite zeolite decreased in the following order: [Li-FAU] > [Na-FAU] > [K-FAU].8 In our case, after the

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Figure 6. CO2-TPD profiles of (a) NaY, (b) KF/NaY, (c) KCl/NaY, (d) KBr/NaY, and (e) KI/NaY.

Figure 7. Selectivity for acrylic acid, O 1s binding energy, and base amount vs the electronegativity of halogens (X ) F, Cl, Br, or I).

introduction of potassium salts into NaY zeolites, the potassium cation could exchange with the original sodium cation and hence weaken the Lewis acid in potassium-modified NaY zeolites. The basicity of KX/NaY zeolites measured by CO2-TPD was displayed in Figure 6. Three peaks defined as weak (400-480 K), moderate (480-530 K), and strong (530-750 K) basic sites

were visible on parent NaY zeolites. After modification with KX, three kinds of basic sites moved to high temperatures except for KF, demonstrating the increase in basicity. The basic strength increased in the following order: Cl < Br < I. It has been reported that the basic strength of framework oxygen increases with a decrease in the electronegativity of the extra-framework cation.22 Therefore, we could deduce that the lower the electronegativity of the extra-framework anion, the greater the basic strength of framework oxygen, when the cations were the same. Note that ∼70% of the iodine in KI/NaY was lost during calcination. The smaller amount of iodine would also impair the electronegativity of the anion and decrease the O 1s binding energy in KI/NaY compared with other KX/NaY zeolites, further improving the basicity of KI/NaY. It should be noted that K2O was generally estimated to be the main basic species.28 The TGA and XPS results showed that KI was partially decomposed to K2O and I2 during calcinations. The resulting K2O species could also cause the increase in the basicity of KI/NaY. The role of basic sites in lactic acid dehydration was to maintain the basic conditions to prevent acetaldehyde formation via decarbonylation.5,26,29 3.5. Discussion. As illustrated in our previous paper,7 the decrease in the number of acidic sites in KNO3-modified NaY zeolites could restrain the formation of acetaldehyde and maintain the high selectivity for acrylic acid. In this paper, we introduced various potassium salts into NaY zeolites to study the effect of counteranions on the catalytic performace of NaY zeolites. As described above, the selectivity for acrylic acid was significantly enhanced after modification of NaY zeolites with potassium salts in the dehydration of lactic acid. We found that obvious differences in catalytic performance existed among these potassium salts. All KX (except for KF)-modified NaY zeolites exhibited better performance than the other potassium salts. Hence, we conducted detailed characterizations of KX/NaY zeolites to disclose the effect of KX on the catalyst structure. XPS revealed that an introduced halogen influenced the electronegativity and basicity of the framework oxygen atoms adjacent to extra-framework cations. The O 1s binding energy, basicity, and selectivity for acrylic acid as a function of the electronegativity of a halogen are depicted in Figure 7. As one can see, basicity and selectivity for acrylic acid increased and O 1s binding energy decreased with a decrease in the electronegativity of a halogen. It effectively revealed the effect of

Figure 8. Mechanism proposed for dehydration of lactic acid to acrylic acid over the KX-modified NaY zeolites.

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electronegativity of an introduced halogen on the surface property and catalytic performance of NaY zeolites. The mechanism of heterogeneous catalytic dehydration of lactic acid to acrylic acid has been widely studied. Gunter and co-workers29 proposed that the phosphate forms a cyclic transition state with the C2 and C3 atoms of the lactic acid molecule via which dehydration occurs. The low activation barrier calculated by AM1 semiempirical molecular orbital calculations for the formation of the phosphate analogue suggests that such a transition state is formed and is the primary route to acrylic acid formation. Frising and Leflaive have reviewed the cation distributions in metal-modified Y zeolites.24 Here, we speculate that the halogen anion adjacent to the extraframework cation affects the electron density of the framework oxygen atoms close to the extra-framework cation. As illustrated in Figure 8, NaY zeolites formed a cyclic transition state with the C2 and C3 atoms of the lactic acid molecule. The dehydration starts with the attraction of the R-hydroxyl group by the acidic site (metal cation) and an attack on a β-hydrogen by a basic site (framework oxygen), followed by the loss of the R-OH group and the β-hydrogen via an E2 elimination mechanism.26 The fact that the acid/base sites simultaneously interacted with R-OH and β-H to form a cyclic transition state that was immediately pyrolyzed to acrylic acid proved the existence of the acid/base synergy. The introduced halogen anions with strong electronegativity could reduce the electron density and promote the basicity of the framework oxygen atoms that interacted with β-hydrogen at C3 atoms of the lactic acid molecule. As illustrated in our previous paper,7 the replacement of Na+ with K+ could reduce the acidity of NaY zeolites. Hence, both increased basicity and decreased acidity of KX-modified NaY zeolites could restrain the formation of acetaldehyde and coke deposits and maintain the high selectivity for acrylic acid. 4. Conclusions After modification of the NaY zeolites with potassium salts, the selectivity for acrylic acid is enhanced dramatically. Among a large variety of potassium salts, KI exhibits the best modifying effect for acrylic acid formation. This work has elucidated that anions introduced by impregnation play important roles in accelerating the acrylic acid formation by decreasing the electron density of framework oxygen atoms to increase the corresponding basicity. After modification, the decreased acidity and increased basicity of KX-modified NaY zeolites could suppress the decarbonylation of lactic acid to acetaldehyde and coke deposits, yielding higher acrylic acid selectivity. Acknowledgment This work was supported by National Natural Science Foundation of China (Grant 20906051), Jiangsu Province Fundamental Research Plan (Grant BK2009478), Key Discipline Fund of Nanjing University of Technology. Literature Cited (1) Varadarajan, S.; Miller, D. J. Catalytic Upgrading of FermentationDerived Organic Acids. Biotechnol. Prog. 1999, 15, 845. (2) Wadley, D. C.; Tam, M. S.; Kokitkar, P. B.; Jackson, J. E.; Miller, D. J. Lactic Acid Conversion to 2,3-Pentanedione and Acrylic Acid over Silica-Supported Sodium Nitrate: Reaction Optimization and Identification of Sodium Lactate as the Active Catalyst. J. Catal. 1997, 165, 162. (3) Gunter, G. C.; Langford, R. H.; Jackson, J. E.; Miller, D. J. Catalysts and Supports for Conversion of Lactic Acid to Acrylic Acid and 2,3Pentanedione. Ind. Eng. Chem. Res. 1995, 34, 974.

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ReceiVed for reView May 13, 2010 ReVised manuscript receiVed August 14, 2010 Accepted August 16, 2010 IE101093X