Synthesis, Characterization, and Catalytic Activity ... - ACS Publications

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J. Phys. Chem. C 2009, 113, 16530–16537

Synthesis, Characterization, and Catalytic Activity of Uniformly Crystalline LaPO4 Nanofiber Catalysts for Ethanol Dehydration Kanaparthi Ramesh,*,† Jia’E Zheng,† Eileen Goh Yi Ling,‡ Yi-Fan Han,† and Armando Borgna† Institute of Chemical and Engineering Sciences (ICES), 1 Pesek Road, Jurong Island, Singapore 627833, School of Materials Science and Engineering, Nanyang Technological UniVersity, Block N4.1, 50 Nanyang AVenue, Singapore 639798 ReceiVed: May 6, 2009; ReVised Manuscript ReceiVed: July 13, 2009

Nanostructured lanthanum phosphates (LaPO4) with different P/La ratios ranging from 0.5 to 2.0 were prepared using aqueous solutions of La(NO)3 · 6H2O and NH4H2PO4 as La and P sources, respectively, by a sol-gel precipitation method. These prepared catalysts were thoroughly characterized by various techniques, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy, temperature-programmed desorption of ammonia (NH3-TPD), laser Raman spectroscopy, and N2 physisorption. XRD results of all the samples showed the crystalline features to be mainly attributed to the hexagonal Rhabdophane type of LaPO4 phase. SEM micrographs showed uniformly crystalline nanofiber structures for the samples with high P/La ratios of 1.0 and 2.0. For a sample with a P/La ratio of 0.5, the acidity measured by NH3-TPD showed two peaks with maxima at 210 and 750 °C. The high temperature peak at 750 °C completely disappeared for the higher P/La ratio samples. These catalysts were tested for selective ethanol dehydration to ethylene in the temperature range of 200-450 °C. LaPO4 catalysts showed higher ethylene selectivities as compared to other metal phosphates, such as AlPO4 and ZrPO4. The catalytic properties were found to be influenced by the P-to-La ratio. Higher P/La ratio catalysts exhibited higher ethanol conversion as well as ethylene selectivity. 1. Introduction Lanthanum phosphate materials are finding applications as high temperature (above 600 °C) protonic conductors1 when La is partially substituted with Sr or Ca, storage material for radioactive wastes,2 and luminescent material.3 Due to the fact that lanthanum phosphates exhibit high thermal stability of up to 1700 °C and other properties, such as high mechanical strength, chemical stabilities, and very low water solubility, these materials are very suitable candidates for catalyst or support material applications. Despite the above properties, reports on the application of LaPO4 as a catalyst or support material is rather limited. Takita et al.4 have applied LaPO4 catalysts for selective oxidative dehydrogenation of isobutane to isobutylene. Other examples include as a chiral Lewis acid catalyst for hetero-Diels-Alder reaction,5 hydrolysis of chlorobenzene to phenol,6 and ortho-alkylation of phenol with methanol.7 Dai et al.8 reported the preparation of ultrastable Au nanocatalysts supported on LaPO4 nanoparticles, and the stability of Au nanoparticles was found to be much better as compared to that on TiO2 support during high-temperature treatment in O2. However, the structure-activity relations for ethanol dehydration on LaPO4 catalysts have not been addressed so far. Several preparation methods have been reported in the literature for the synthesis of nanoscale LaPO4 materials. Recently, Rajesh et al.9 have reported the synthesis of LaPO4 from La(NO3)3 and H3PO4 by a sol-gel method. In this method, postgelation treatment of LaPO4 with alcohols improved the surface area of the LaPO4. Synthesis of mesostructured LaPO4 * Corresponding author. E-mail: [email protected]. † ICES. ‡ Nanyang Technological University.

using organic template synthesis was also reported in the literature.10 Lucas et al. reported that the pH of the rare earth phosphate synthesis influences the crystallinity and amount of synthesis residuals.11 During the synthesis of LaPO4 from LaCl3 and H3PO4, the H3PO4 was adsorbed on the surface, and the complete removal of H3PO4 on the LaPO4 surface requires high temperature calcination treatment of 1400 °C. Colomer et al.12 reported the combustion synthesis of nanopowders of LaPO4 from La(NO3)3 · 6H2O and (NH4)2HPO4 at 1300 °C. This method yielded the powders with crystallite size of ∼60 nm. Addition of urea in the preparation was found to influence the physical properties, such as surface area, pore-size distribution, and particle shape of the LaPO4.13 In this study we established a simple sol-gel preparation method to produce uniformly crystalline nanostructures of LaPO4 catalysts without any stable byproducts. Light olefins, such as ethylene, propylene, and butylenes are basic building blocks in the petrochemical industry. Currently, these are mainly produced by thermal cracking of hydrocarbons, such as naphtha. The conventional catalytic cracking process is highly energy-intensive and accounts for 180 million tons of CO2 emissions worldwide.14 Because of the volatile oil price and concerns for climate change, dehydration of lower alcohols (methanol and ethanol) derived from natural gas or biomass to olefins represents an attractive alternative route. Among various light olefins, ethylene is the most widely produced in the chemical industry. Although ethylene as such does not find any direct applications, it is a key intermediate for the production of important chemicals such as ethylene oxides, polythene, vinyl chloride, and styrene. The high purity of ethylene is important in the industry for polymer grade applications. Various catalysts such as metal oxides,15-17 zeolites,18,19 and heteropoly acids20,21

10.1021/jp904215t CCC: $40.75  2009 American Chemical Society Published on Web 08/20/2009

LaPO4 Nanofiber Catalysts for Ethanol Dehydration

J. Phys. Chem. C, Vol. 113, No. 37, 2009 16531

Figure 1. XRD profiles of various P-to-La ratio LaPO4 catalysts.

TABLE 1: N2 Physisorption Results of Various LaPO4 Catalysts sample

BET surface area, m2/g

total pore vol, mL/g

av pore diam (Å)

crystallite size, nma

LaP (0.5) LaP (1.0) LaP (1.5) LaP (2.0)

90 99 77 81

0.33 0.22 0.17 0.22

145 88 86 108

8.7 5.8 9.5 12.1

a

Crystallite size measured from XRD by the Sherrer equation.

were applied for ethanol dehydration. Alumina requires a relatively high temperature (450 °C) to get an ethylene yield above 90%. Zeolites such as H-ZSM-5 are found to be active for ethanol dehydration; however, poor hydrothermal stability and resistance to coke formation make the process unfeasible for industrial applications.22 In this study, we report an alternative LaPO4-based solid acid catalyst for selective ethanol dehydration. LaPO4 as catalysts has several potential advantages, but these materials have not been fully explored as catalysts until now; especially, the unique structural properties of LaPO4 have not been addressed so far. Moreover, the effect of the P-to-La ratio in the structure and catalytic applications has not been addressed so far. In this study, we report the synthesis of nanostructured LaPO4 with various P/La ratios. These catalysts showed very high catalytic reactivity in selective ethanol dehydration. Thus, efforts in this work are directed at investigating the structural properties of nanocomposites of LaPO4. For the above purpose, the catalysts were thoroughly characterized with SEM, TEM, XRD, and FT-IR. The aim of this report is to study the relationship between the unique structures of lanthanum phosphate (LaPO4) and the yield of ethylene produced. 2. Experimental Section 2.1. Catalyst Preparation. LaPO4 with various ratios of P/La ranging from 0.5 to 2.0 were prepared by the sol-gel precipitation method as described below. In a typical preparation method, the calculated amounts of La(NO3)3 · 6H2O (Alfa Aesar) and (NH4)H2PO4 (Alfa Aesar) were placed in 100 mL of distilled water and continuously stirred to make a homogeneous solution, and the solution was acidified with dilute HNO3. The solution was further diluted with 50 mL of water and heat treated at 70 °C for 1 h. Hydrogel was formed by the addition of NH4OH dropwise until the pH reached above 8. The resultant gel was filtered and thoroughly washed with distilled water, and the solid was dried at 120 °C for 16 h. Finally, the samples were calcined at 500 °C in air for 4 h with 10 °C/min heating and cooling

Figure 2. Adsorption/desorption isotherm of (a) LaP 0.5, (b) LaP 2.0.

rates. The final yields of all the catalysts were above 95% of the theoretical. These catalysts are denoted as LaP 0.5, LaP 1.0, LaP 1.5, and LaP 2.0 for P-to-La ratios 0.5, 1.0, 1.5, 2.0, respectively. In addition, AlPO4 and ZrPO4 with a P-to-metal ratio of 1 were also synthesized using the similar method. The BET surface areas measured for the AlPO4 and ZrPO4 were found to be 212 and 127 m2/g, respectively. The resultant reaction of the above method can be best described as follows:

La(NO3)3 + NH4H2PO4 + 2NH4OH f LaPO4 · nH2O + 3NH4NO3 2.2. Catalyst Characterization. The specific surface area, pore volume, and average pore diameter results of catalysts were measured by the adsorption and desorption N2 isotherms that were collected on Autosorb-6 at liquid N2 temperature. Prior to the measurements, all samples were degassed at 200 °C until a stable vacuum of ∼5m Torr was reached. Powder X-ray diffraction (XRD) measurements were collected over a 2θ range of 20-80° using Cu KR with a Bruker D8 X-ray diffractometer with a step size of 0.017° and a step time of 72.1 s. The Rietveld method was employed using the fundamental parameters approach, and the XRD data were analyzed by software TOPAS, version 3.0. High resolution transmission electron microscopy (TEM) and bright field images were captured using a JEOL

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JEM-2100F TEM instrument operated at 200 keV. Each sample was lightly crushed with an agate mortar and pestle and thereafter dispersed ultrasonically in ethanol for 10 min. Several drops of the dispersion were deposited on holey-carbon-coated copper grids, allowing ethanol evaporation. The electron microscopy measurements were performed at a JEOL JSM-6700F field emission scanning electron microscope (SEM). About 200 particles were selected when the average particle size was estimated based on SEM images. FT-IR spectra were recorded on a Bio-Rad FTIR 3000 MX spectrometer in the range of 4000-400 cm-1. In a typical analysis, ∼25 mg of the sample and 1 g of KBr was weighed and milled and ground in an agate mortar until a fine powder with even particle size was obtained. About 250 mg of sample and KBr mixture was then pressed with a steel die into a pellet. The FT-IR spectra were recorded in transmission mode with a resolution of 4 cm-1. In a typical temperature-programmed desorption (TPD) experiment, ∼160 mg of oven-dried sample was placed in a quartz sample tube and was supported on a quartz wool bed. The sample was pretreated in He flow at 500 °C for 2 h, the temperature was lowered to 80 °C, and the sample was treated at this temperature with anhydrous ammonia gas. The sample was then flushed with He (50 mL/min) for 100 min to remove the physisorbed ammonia. The TPD operation was carried out from 50 to 900 °C. The dispersive Raman microscope employed in this study was a JY Horiba LabRAM HR equipped with a confocal microscope; liquid-nitrogen-cooled, charge-coupled device (CCD); and a multichannel detector (256 pixels × 1024 pixels). The visible 514.5 nm argon ion laser was selected to excite the Raman scattering. The laser power from the source is around 20 mW, but the laser output was reduced to around 6-7 mW by passing through filtering optics and the microscope objective before reaching the samples. The Raman shift range acquired was in the range of 50-1200 cm-1 with spectral resolution 1.7-2 cm-1. 2.3. Activity Measurements. The ethanol dehydration reaction was carried out in a vertical fixed bed, continuous down flow, steel microreactor under atmospheric pressure. In a typical experiment, ∼500 mg of the catalyst was diluted with an equal volume of silicon carbide and packed between two layers of quartz wool in the reactor. The upper portion of the reactor was filled with glass beads that served both as a preheater and a mixer for the reactants. Prior to introducing the reactant, the catalyst was treated in nitrogen flow at a rate of 100 mL/min for 5 h at 500 °C. The reaction temperature was monitored by a thermocouple with its tip located in the catalyst bed and connected by a PID-type temperature indicator-controller. Catalytic tests were performed by injecting ethanol (HPLC grade) and its diluted solutions with a HPLC infusion pump (Agilent 1100 series). The reaction was carried out at various temperatures ranging from 200 to 450 °C. Prior to entering into the reactor, the ethanol was mixed with helium at 175 °C. The gaseous products, after the catalyst had attained a steady state, were analyzed by an online gas chromatograph (GC) (Agilent 6890) equipped with a flame ionization detector using a HP-5 capillary column and thermal conductivity detector using a Hayesep D column. Liquid products were condensed at 0 °C and analyzed using GC/MS (Agilent 6890). 3. Results and Discussion 3.1. Physicochemical Properties of LaPO4 Catalysts. Crystalline structures of LaPO4 samples were determined by powder XRD. Figure 1 shows the XRD profiles of various LaPO4 catalysts that were calcined at 500 °C. All the LaPO4 samples

Ramesh et al. with different P/La ratios showed crystalline features. The XRD profile of LaP 0.5 showed the peaks due to the hexagonal phase of Rhabdophane LaPO4 · 0.5H2O (ICDD PDF-46-1439) with a minor quantity of lanthanum oxide (La2O3) phase, La4(P2O7)3 · 0.5La2O3 (ICDD PDF-21-0446). At higher (above 1.0) P/La ratios, the XRD profiles indicate the presence of mainly hexagonal Rhabdophane-type LaPO4 · 0.5H2O. Due to the broad nature of the peaks, it is difficult to rule out the presence of monoclinic phases. However, no characteristic peaks due to monazite (monoclinic) LaPO4 phase were observed. It was reported that phase transformation of rhabdophane to monazite can be achieved only above 600 °C.23 Crystallite sizes obtained by using the Scherrer equation assuming spherical crystals were found to be in the range of 6-12 nm (Table 1). N2 physisorption results of specific surface area (SBET), pore volume (Vp), and the average pore diameter of various LaPO4 catalysts with varying P/La ratios are listed in Table 1. The nitrogen adsorption/desorption isotherms of LaP 0.5 and LaP 2.0 are shown in Figure 2a and b, respectively. LaP 0.5 catalyst showed the H1-type hysteresis that is indicative of a narrow pore size distribution. LaP 2.0 catalyst showed H3-type hysteresis, which indicates slit-shaped pores for aggregates of platelike particles. The specific surface area was decreased from 90 m2/g for LaP 0.5 to 77 m2/g for LaP 1.5, followed by an increment to 81 m2/g for LaP 2.0. Similarly, pore volume also decreased with an increase in the P/La ratio up to LaP 1.5 and marginally increased for LaP 2.0. Average pore size values also followed a similar trend. FT-IR profiles of LaPO4 with various P/La ratios are shown in Figure 3a and b. The band at ∼1630 cm-1 was observed for LaP 0.5, indicating the presence of small amounts of surface adsorbed water. A prominent peak centered at around 1046 cm-1 can be assigned to the asymmetric PO4 stretching mode. For LaP 0.5, the peak observed at 1379 cm-1 can be attributed to NO3- species. The complexed nitrates on La are found to be stable, and high temperatures (800 °C) are required to remove them completely from a LaPO4 sample.9 The presence of the nitrates originated from the excess La(NO3)3 in LaP 0.5, since a nonstoichiometric La-to-P ratio is used. This suggests the presence of La(NO3)3 precursor, even after calcination at 500 °C. FT-IR spectra of LaP 0.5 also showed characteristic PO4 bands at 541, 569, and 611 cm-1. In addition, the spectra of LaP 0.5 showed a peak at 859 cm-1 due to the presence of HPO42-, and these species can yield pyrophosphates by condensation to form P2O74-. Figure 3b shows the FT IR spectra of LaP 1.0, 1.5, and 2.0 catalysts. The peak at 1630 cm-1 for the samples with higher P/La ratios is due to partially hydrated hexagonal LaPO4 · nH2O. FT-IR spectra of all the samples showed bands at 543, 569, and 615 cm-1 due to the vibration of PO4 groups in the ν4 region. These peaks are characteristic of symmetric and asymmetric stretching of PO4 groups attached to a metal.11 The prominent peak centered at 1045 cm-1 is due to the asymmetric stretching of the P-O group in phosphate species. The absence of bands due to water in the wavelength region of 700-900 cm-1 was attributed to the zeolitic nature of water in metal phosphates.24 The peaks at 1380 and 859 cm-1 observed for LaP 0.5 are not seen on other samples. This suggests the complete consumption of La when the precursor is converted into LaPO4. Raman spectra of various calcined LaPO4 samples with P-toLa ratios are shown in Figure 6. The bands at 538, 573, and 617 cm-1 are attributed to typical vibrations of PO4 groups. The peaks at ∼1082 and 1053 cm-1 correspond to asymmetric



Figure 3. FT IR spectra of (a) LaP 0.5, (b) LaP 1.0, LaP 1.5, and LaP 2.0.

Figure 4. Raman spectra of various P-to-La ratio LaPO4 catalysts.

stretching (ν1) of the P-O group in the PO4. The peaks at 965 and 992 cm-1 can be attributed to the symmetric stretching mode (ν1) of the P-O bond in the PO4. The peaks at 617, 538, 465, and 393 cm-1 are due to the O-P-O bending modes.26 In the spectra of LaP 0.5, two additional bands at 1082 and 318 cm-1, respectively, can be observed, which are not present for the higher P-to-La ratios. These two peaks can be assigned to the symmetric PO2 stretching and δPOP modes for pyrophosphate groups.27,28 This result is in good agreement with the XRD data, which indicates the presence of La4(P2O7)3 · 0.5La2O3 and the FT-IR results. Scanning electron microscopy gives the information regarding the surface morphology of LaPO4 catalysts. Figure 5a-d shows the SEM images of various P/La ratios of LaPO4 catalysts. At low P/La ratios, they show a one-dimensional nanofiber structure; however, LaP 1.5 and LaP 2.0 exhibit a nanorodlike crystalline structure with a typical length of 100 nm and width of 10 nm. These images revealed LaPO4 to be homogeneously obtained with a unique nanofiber structure. As the P/La ratio increases, the nanostructure becomes more obvious, which indicates the increase in the number of active sites. The TEM images of various LaPO4 catalysts are shown in Figure 6a-d. The LaP 0.5 catalyst showed one-dimensional nanorods with an average length of 85 nm. From the TEM images, it can be deduced that the aspect ratio of the nanorods decreases with the increase in the P/La ratio. When the P/La ratio is low (i.e., at 0.5 and 1.0), the Rhabdophane-type LaPO4 is more crystalline and homogeneous in its morphology.

NH3-TPD profiles of various P-to-La ratios of LaPO4 catalysts are shown in Figure 7. LaP 0.5 showed two clearly distinguishable peaks at 210 and 750 °C, and the deconvolution of these peaks yielded peak area percents of 32 and 57, respectively. The ammonia concentration during low-temperature desorption, high-temperature desorption, and total desorptions, which are related to the number of low-strength acid sites, high-temperature acid sites, and total acidic sites, are presented in Table 2. The high-temperature peak at 750 °C, which is absent for the other higher P to La ratios, might originate from the decomposition of nitrates on LaPO4. This result is in accordance with results of FT-IR spectra. For a P/La ratio of 1.0 and above, the high-temperature desorption peak completely diminished, resulting in an increase in the low-temperature peak area. The low temperature peak around 210 °C increased from LaP 0.5 to LaP 1.5 and slightly decreased for LaP 2.0; however, the total peak area decreased with increasing P/La ratio until 1 and slightly increased for a P/La ratio of 2.0. It is generally agreed that the weak acidic sites are present below 200 °C. Moderate acidic sites are present between 200 and 400 °C, and temperature maxima (Tmax) above this temperature indicate the presence of strong acid sites.25 On the basis of this classification, the broad desorption peaks centered at 210 °C can be attributed to a combination of weak and moderate strength acid sites. It is also interesting to note that peak maxima shifted from 196 °C for LaP 0.5 to 209 °C, indicating the presence of a higher number of moderate acid sites for higher P/La ratio catalysts. It is also important to note that moderate acid strength sites exhibit very high dehydration activities. 3.2. Catalytic Activity. For comparison, we tested various metal phosphates, such as AlPO4, ZrPO4, and LaPO4 catalysts, with a P-to-metal ratio of 1 for ethanol dehydration reaction at 350 °C and 0.1 MPa. The weight hourly space velocity (WHSV) of the reaction is 18.3 h-1. The conversion of ethanol was found to be higher for ZrPO4 and AlPO4 as compared to LaPO4 (Table 3). However, the selectivity toward ethylene was found to be in the order of LaPO4 > ZrPO4 > AlPO4. An ethylene yield of 55% for LaPO4 catalyst is much superior as compared to ZrPO4 and AlPO4, with 34 and 10%, respectively. Reaction rate measured per unit area of LaPO4 was found to be 2.8 × 103 mol h-1 m-2 and higher than that of ZrPO4 and AlPO4 catalysts. All the above catalysts formed ethylene as the main product and diethyl ether as the byproduct in ethanol dehydration, as

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Figure 5. SEM images of LaPO4 catalysts (a) LaP 0.5, (b) LaP 1.0, (c) LaP 1.5, and (d) LaP 2.0.

shown in eqs 1 and 2. Other products, such as propylene and aromatics, are found to be less than 1%.

C2H5OH f C2H4 + H2O + 45.62 kJ/mol

(1)

2C2H5OH f C2H5-O-C2H5 + H2O - 22.63 kJ/mol (2) These initial results prompted us to carry out a systematic study of LaPO4 catalysts for ethanol dehydration. Ethanol dehydration activity of LaP 1.0 with respect to reaction temperature is shown in Figure 8. Each point in the curve is the average of five or more analyses. On the basis of the GC response factors, the mass balance of the reaction in forming ethylene was found to be 100% for over 60 h. At 200 °C, the conversion of ethanol was almost zero, increased with reaction temperature, and reached a maximum of 80% at 450 °C. The selectivity toward ethylene was also increased with reaction temperature and reached 89% at 450 °C. To understand the influence of the P/La ratio on the catalyst performance, all the catalysts with various P/La ratios between

0.5 to 2.0 were screened for ethanol dehydration in the temperature range from 200 to 450 °C. For comparison, the ethanol conversion and ethylene selectivity of various P/La ratios of LaPO4 catalysts at 400 °C are shown in Figure 9. Obviously, the P/La ratio influenced the catalytic activity, and an increase in the conversion and selectivity was observed with increasing P/La ratio. At 400 °C, LaP 2.0 showed 100% ethanol conversion and about 95% ethylene selectivity, as compared with LaP 0.5 with 16% conversion and 37% ethylene selectivity. In addition, at all reaction temperatures, the major product was ethylene, and diethyl ether is the only byproduct. As can be seen from Figures 9 and 10, the LaP 2.0 catalyst showed the highest ethanol dehydration activity as compared to other P/La ratios. Therefore, LaP 2.0 was tested for ethanol dehydration in the temperature range of 200-450 °C with 50 °C intervals. As the reaction temperature approached 400 and 450 °C, the selectivity toward ethylene was 95% and 97% respectively, with a complete conversion of ethanol. At a reaction temperature above 350 °C, the major product is ethylene, whereas for reaction temperatures below 350 °C, the major product was diethyl ether. This clearly suggests that the intramolecular dehydration to produce ethylene is taking place



Figure 6. TEM images of LaPO4 catalysts (a) LaP 0.5, (b) LaP 1.0, (c) LaP 1.5, and (d) LaP 2.0.

TABLE 3: Ethanol Dehydration Activity at 400 °C over Various Metal Phosphate Catalystsa % selectivity

sample

% conversion of ethanol

ethylene

DEE

reaction rate X103, mol/h/m2

ZrPO4 (1.0) LaPO4 (1.0) AlPO4 (1.0)

82 67 50

41 82 21

59 14 78

2.65 2.8 0.97

a Reaction conditions: 0.5g catalyst, 100% ethanol, 0.1 MPa pressure, WHSV 18.36 h-1.

Figure 7. NH3-TPD profiles of various P-to-La ratio LaPO4 catalysts.

TABLE 2: NH3 Temperature-Programmed Desorption Results of Various LaPO4 Catalysts

sample

LT NH3 desorption, µmol/g

HT NH3 desorption, µmol/g

total acidity, µmol/g

LaP (0.5) LaP (1.0) LaP (1.5) LaP (2.0)

65.8 218 226 160

172 9.4 9.4 60

238 227 236 221

at higher temperatures, whereas the intermolecular dehydration to produce diethyl ether is predominant at low temperatures. Therefore, it can be concluded that with the increasing ratio of P/La, it provides more active sites for the reaction as well as better reaction results due to the more obvious nanostructure of LaPO4. Apparent activation energies calculated from Arrhenius plots for the reaction are on the order of 18-38 kJ/mol. These values

Figure 8. Effect of reaction temperature during ethanol dehydration on LaP 1.0 (WHSV 18.36 h-1 and He flow rate 100 mL/min).

are considerably lower for the ethanol dehydration, as compared to other catalysts reported in the literature.29 The reaction rates of the various catalysts in terms of the amount of the catalyst (g) as well as the surface areas (per unit area) are presented in the Figure 10. Reaction rates in moles per hour per amount (g) or area (m2) of the LaPO4 showed an increasing trend with increasing P/La ratio. An increasing trend can be observed for conversion of ethanol, selectivity of ethylene, and yield of

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Ramesh et al. the LaP 2.0, the catalyst was tested for about 60 h, and no sign of deactivation was observed. 4. Discussion

Figure 9. Dependence of ethanol dehydration activity on various P-toLa ratios at 400 °C (WHSV 18.36 h-1 and He flow rate 100 mL/min).

Figure 10. Dependence of the reaction rate during ethanol dehydration on the P to La ratio of LaPO4 catalysts at 400 °C.

On the basis of the above results, it can be understood that LaPO4 catalysts with higher P/La ratios are highly selective in ethanol dehydration by suppressing other side reactions, such as polymerization, decomposition, and dehydrogenations. In our earlier publication,30 we reported the selective dehydration of ethanol to ethylene by modifying H-ZSM-5 with H3PO4 acid. It was shown that P-modified catalysts suppressed the side reactions by selectively forming ethylene. The modification P on ZSM-5 tuned the acidic properties of catalysts by forming a larger number of moderate acid sites. These sites are responsible for the selective dehydration of ethanol to ethylene. The strong acid sites are said to be responsible for the polymerization to form higher olefins and aromatics. Upon readsorption of these coke precursors, it can lead to the quick deactivation of the catalysts. At this moment, the selective dehydration of ethanol to ethylene over LaPO4 catalysts can be attributed to the generation of moderate acid sites by increasing the P-to-La ratio. It can be concluded that the LaPO4 catalysts can be used as alternative dehydration catalysts for ZSM-5. To produce light olefins selectively on H-ZSM-5, various approaches have been proposed in the literature. ZSM-5 doping with metals such as Mn and Zn also improves the selective formation of ethylene.31 Postsynthesis P-modified ZSM-5 catalyst has been proposed for use during methanol to olefin reactions. Kaeding et al.32 reported a P-modified ZSM-5 catalyst to synthesize C2-C4 olefins selectively. The P modification dramatically improved the selectivity toward propylene and butylenes. However, the selectivity toward ethylene was not significantly affected. Inaba et al.33 recently reported that treating H-ZSM-5 with Fe can significantly decrease the acidity, which leads to a decrease in the formation of aromatics and carbon deposition. H-ZSM-5 framework stability in the presence of water produced in the reaction is another concern using these catalysts. However, the study of the exact role of excess P in the metal phosphates and the reaction mechanisms is currently underway in our group. 5. Conclusions

Figure 11. Ethanol dehydration activity on LaP 2.0 with reaction temperature between 200 and 450 °C (WHSV 18.36 h-1 and He flow rate 100 mL/min).

ethylene as the reaction temperature increases when using LaP 2.0 as the catalyst (Figure 11). From Figure 11, it can be observed that there was no significant conversion of ethanol between 200 to 300 °C. Therefore, it can be concluded that higher P/La ratios generated more moderate acid sites which participated in the selective ethanol dehydration to ethylene and that the optimum reaction temperature for this reaction is between 400 and 450 °C. To study deactivation behavior of

In this work, a method for the synthesis of nanosize fibers of LaPO4 catalysts by sol-gel precipitation method was established. The XRD profile of LaP 0.5 catalyst indicated the presence of hexagonal Rhabdophane LaPO4 · nH2O along with minor quantities of La2O3 and La4P2O7 · 0.5H2O. As the P/La ratio increases, the nanostructure of LaPO4 becomes more obvious by SEM and TEM analysis. LaPO4 exhibited better catalytic activities compared to other metal phosphates, such as ZrPO4 and AlPO4, with the same P/metal ratio. Furthermore, the P/La ratio of LaPO4 proved influencial in the both ethanol conversion and ethylene selectivity. A P/La ratio of 2.0 exhibited very high activity during ethanol dehydration by selectively forming ethylene with a selectivity around 96% at 400 °C. Due to the increase in the medium strength acid sites, a high yield of ethylene was achieved. Hence, it can be concluded that the unique structure and the increase in surface area helps to provide more active sites, which improved the catalytic activity. Relatively low apparent activation energies between 18-38 kJ/ mol were observed for these catalysts as compared to zeolites and other catalysts. Acknowledgment. This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science Technology and Research), Singapore.

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