Synthesis, Characterization, and Catalytic Activity of Phosphorus

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Ind. Eng. Chem. Res. 2010, 49, 4080–4090

Synthesis, Characterization, and Catalytic Activity of Phosphorus Modified H-ZSM-5 Catalysts in Selective Ethanol Dehydration Kanaparthi Ramesh,* Chang Jie, Yi-Fan Han, and Armando Borgna Institute of Chemical and Engineering Sciences (ICES), 1, Pesek Road, Jurong Island, Singapore 627833

Phosphorus (P) modified H-ZSM-5 catalysts were prepared by wet impregnation method by varying P loadings from 0 to 7.43 wt % using phosphoric acid (H3PO4) as the P source. The catalysts were tested for ethanol dehydration in the temperature range of 523-723 K. The P-modified catalysts were found to be highly active and selective toward ethylene at 673 K and atmospheric pressure. In addition, the P-modified catalysts were found to be extremely stable more than 200 h without any sign of deactivation. However, the selectivity was found to be strongly dependent on several factors such as P content, reaction temperature, and space velocity (WHSV). The P-modified ZSM-5 catalysts were also found to be highly active for the dehydration of aqueous ethanol solutions (10 wt %) showing very high ethylene selectivity (above 98%) at significantly lower temperature 623 K. The catalysts were thoroughly characterized using various methods, including N2 physisorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric differential thermal analysis (TG-DTA), 1H, 27Al, and 31P magic angle spinning nuclear magnetic resonance (MAS NMR), and amonia temperature programmed desorption (NH3-TPD). 27Al MAS NMR spectra suggest that P addition facilitate the breaking of Si-O-Al bond that lead to a partial dealumination. NH3-TPD results indicate that total acidity as well as density of high strength acid sites were decreased with P loading. 1. Introduction Dwindling oil supplies and a volatile price combined with environmental concerns have driven much effort to develop alternative green technologies based on renewable feed stocks. Currently, bioethanol is being widely produced by the fermentation process from biomass feed-stocks like corn, sugar cane, and cellulose which are renewable and independent of fossil fuel. Bioethanol can be used as fuel or fuel additive in automotive engines. Alternatively, production of value-added chemicals from ethanol is also currently attracting considerable interest.1,2 In particular, production of either aromatic hydrocarbons or light olefins through the dehydration of ethanol (ethanol to olefins, ETO) has been popularly studied.3-6 Light olefins such as ethylene, propylene, and butylenes are important intermediates in the petrochemical industry and are mainly produced by cracking higher hydrocarbons, such as naphtha,7 gas oils, and liquid petroleum gas (LPG). Their production primarily depend on cost of feed stocks and continuous supply of fossil fuels. The conventional cracking process to obtain such olefins is energy intensive and accounts for 180 million tons CO2 emissions worldwide annually.8 Therefore, alcohol dehydration is potentially an important alternative route to produce ethers and polymer grade olefins.9 Ethylene is one of the most widely produced light olefins in the chemical industry. Although, ethylene itself does not have any direct applications, it is a key intermediate in the production of important chemicals such as ethylene oxides, polyethylene, polyethylene terephthalate, vinyl chloride, and styrene. Ethylene is also produced by the dehydration of lower alcohols such as methanol and ethanol. Due to the ever increasing demand for ethylene, volatile crude oil price, and the concerns associated with continuous supply, the search for an efficient alcohol dehydration method to produce ethylene is gaining industrial importance. Bioethanol is produced by fermentation in water, and the separation of water by distillation to go beyond 92% * To whom correspondence should be addressed. E-mail: [email protected].

ethanol is costly. If bioethanol could be used directly, the process will be much more cost-effective. Thus the chemical or polymer grade ethylene could be directly obtained by selective ethanol dehydration without any additional separation step. Van Mao et al reported the bioethanol to ethylene (BETE) process on TFA modified ZSM-5 catalysts.10 Catalysts such as metal oxides,11-13 zeolites,14,15 and heteropoly acids16,17 have been applied for ethanol dehydration. Particularly, H-ZSM-5 zeolites are found to be highly active for ethanol dehydration. However, poor hydrothermal stability and a strong tendency to form coke strongly restrict their industrial application.18 Several studies have been devoted to improve the H-ZSM-5 stability by either treatment with water or basic probe molecules such as pyridine and 4-methylquinoline.19 Doping ZSM-5 with metals such as Mn and Zn also improves the selectivity toward ethylene.9 Postsynthesis Pmodified ZSM-5 catalysts have been proposed for methanol to olefins. Kaeding et al.20 reported a P-modified ZSM-5 catalyst to synthesize C2-C4 olefins selectively. The P-modification dramatically improved the selectivity toward propylene and butylenes, while, the selectivity toward ethylene was not significantly affected. Although modification of H-ZSM-5 by P was studied for methanol dehydration, to the best of our knowledge this effect was rarely addressed for the dehydration of other alcohols. We recently reported the beneficial effect of P addition in the selective dehydration of ethanol to ethylene.21 Furthermore, the selective synthesis of ethylene on solid acid catalyst with high stability and potentially using diluted ethanol solutions still needs further developments. In this study, we investigate the influence of P-modification on H-ZSM-5 for the selective dehydration of ethanol to ethylene. The catalysts are thoroughly characterized by different techniques; including N2 physisorption, thermogravimetric differential thermal analysis (TG-DTA), amonia temperature programmed desorption (NH3-TPD), solid state magic angle spinning nuclear magnetic resonance (MAS NMR), and Fourier transform infrared spectroscopy (FT-IR). The main aim of this

10.1021/ie901666f  2010 American Chemical Society Published on Web 04/08/2010

Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010

study is to develop a simple, robust, and effective catalytic system for ethanol dehydration toward selective production of ethylene. In addition, the structural properties will be correlated with the catalytic activity. 2. Experimental Section 2.1. Catalyst Preparation. The parent zeolite, NH4-ZSM-5 (Si/Al 30), was obtained from Zeolyst CBV 3024E. NH4-ZSM-5 zeolite was initially calcined in static air at 773 K, at a heating rate of 10 K/min followed by an isothermal period of 4 h, leading to the formation of H-ZSM-5 zeolite. H3PO4-modified H-ZSM-5 catalysts were prepared through a dry impregnation method, using the required amounts of H3PO4 (85%). The required amounts of H3PO4 (Kanto Chemical Co., Inc.) were dissolved in deionized water and used as impregnating solution to prepare various H3PO4 (5-20 wt %) modified ZSM-5 catalysts. The starting materials were mixed thoroughly and rolled overnight to achieve good homogeneity. The obtained precursors were dried in an oven overnight for 16 h at 393 K. Finally, the samples were calcined at 773 K for 4 h in static air. The samples were pelletized, crushed and sieved to produce catalyst particles with an average size of ca. 300 µm. The P-modified samples were denoted as 1.86P-ZSM-5, 3.72-ZSM5, and 7.43P-ZSM-5 containing 1.86, 3.72, and 7.43% of P, respectively. 2.2. Catalyst Characterization. The specific surface area, pore volume, and average pore diameter of the catalysts were measured by N2 adsorption-desorption isotherms at liquid N2 temperature using an Autosorb-6 system. Prior to the measurements, all samples were degassed at 573 K until a stable vacuum of ca. 5 mTorr was reached. Thermal gravimetric analyses experiments were conducted using a Setaram Setsys Evolution 12 thermo balance using nitrogen dynamic flow of 140 mL (NTP)/min. The temperatures and sample weights were measured as a function of time. Approximately, 10-30 mg of catalyst was used for each experiment. NH3-TPD experiments were conducted on a Quantachrom Autosorb-1 instrument. In a typical TPD experiment, about 160 mg of a dried sample was placed in a quartz sample tube and supported using quartz wool. The sample was pretreated in Helium flow at 773 K for 2 h. Then, the temperature was decreased to 353 K, 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 physisorbed ammonia. The temperature programmed desorption of ammonia (NH3-TPD) experiments were carried out from 323 to 1173 K. In order to get more insight into the nature of the acid sites n-propylamine temperature programmed desorption (TPD) was conducted. In a typical experiment, 100 mg of sample was pretreated at 773 K for 2 h in continuous flow of He (30 mL/min). The sample was cooled down to ambient temperature and then saturated with n-propylamine under helium flow for 20 min. Prior to desorption, the sample was treated at 353 K for 2 h to remove physisorbed npopylamine. Then, the sample was heated up to 773 K with a heating rate of 10 K/min. The evolution of various desorbing

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compounds such as n-propylamine (m/z 30), propylene (m/z 41), and ammonia (m/z 17) was followed using an online mass spectrometer (Hiden MS HPR20 QIC). FT-IR spectra were recorded using a Bio-Rad FT-IR 3000 MX spectrometer in the range of 4000-400 cm-1. In a typical analysis, approximately 25 mg of the sample and 1 g of KBr were weighed, milled, and ground in an agate mortar, until a fine powder with uniform particle size was obtained. It was then pressed with a steel die to obtain a thin wafer. The FT-IR spectra were recorded in transmission mode with a resolution of 4 cm-1. In situ diffusion reflectance infrared fourier transform spectroscopy (DRIFTS) analysis was carried out on a BIO-RAD spectrometer (Excalibur series, FTS 3000) equipped with an mercury cadmium telluride (MCT) detector and a high temperature DRIFT cell fitted with KBr windows. The spectra were acquired with a resolution of 4 cm-1. Typically 256 scans were recorded, averaged and transformed by Kubelka-Munk using KBr as a reference material. In a typical experiment, about 20 mg of sample was loaded into the DRIFTS cell without any further dilution. The sample was initially treated at 673 K for 2 h in the dynamic flow of He (UHP, 99.999%) at 20 mL/min. Subsequently, spectra were recorded in helium flow from ambient temperature to 673 K. Solid state NMR spectra were obtained at room temperature on a Bruker AVANCE 400WB spectrometer using a BBO MAS probe, 4 mm ZrO2 rotors with a spinning rate of 5 kHz. 1H MAS NMR spectra were obtained at a resonance frequency of 400.13 MHz using single-pulse experiments, with 10 µs pulse, 3 s recycle time, and 64 scans. 27Al MAS NMR spectra were recorded at a resonance frequency of 104.26 MHz with a 1 µs pulse, 1 s recycle time, and 512 scans. 27Al chemical shifts were reported relative to Al(NO3)3. 29Si MAS NMR measurements were performed at a resonance frequency of 79.49 MHz. Q8M8 (cubic octamer silic acid trimethylsilyl ester) was used as secondary standard. 29Si MAS spectra were recorded with proton high power decoupling using 4 µs pulse, 60 s recycle time, and 512 scans. 31P MAS NMR measurements were performed at a resonance frequency of 161.98 MHz. NH4H2PO4 was used as secondary standard. 31P MAS spectra were recorded with proton high power decoupling using 3 µs pulse, 60 s recycle time, and 64 scans. 2.3. Activity Measurements. Ethanol dehydration was carried out in a plug flow fixed bed stainless steel microreactor under atmospheric pressure. In a typical experiment, about 500 mg of catalyst crystals were diluted with equal volume of quartz grains and packed between two layers of quartz wool. The upper portion of the reactor was filled with quartz beads, serving both as preheater and a mixer for the reactants. Prior to introduce the ethanol, the catalyst was treated in nitrogen flow (100 mL/ min) for 5 h at 773 K. The reaction temperature was monitored by a thermocouple located inside the catalyst bed. Catalytic tests were performed by injecting ethanol (HPLC grade), and its diluted solutions with an HPLC pump (Agilent 1100 series). The reaction was carried out at various temperatures ranging from 523 to 723 K. Ethanol was mixed with helium (100 mL/

Table 1. Surface Characteristics of Bulk H-ZSM-5 and P-Modified ZSM-5 Catalystsa catalyst

surface area, m2/g

pore volume, cm3/g

micropore volume, cm3/g

micropore area, m2/g

external surface area, m2/g

pore width, Å

H-ZSM-5 1.86P-ZSM-5 3.72P-ZSM-5 7.43P-ZSM-5

366 276 199 74

0.26 0.21 0.16 0.12

0.12 0.10 0.082 0.073

224 196 153 14

142 80 47 60

14.5 14.6 14.5 15.5

a

Surface area and pore volume were derived from BET and BJH methods. Micropore volume, micropore area, and external surface areas were derived from the t method. Pore width was derived from the Horvath-Kawazoe method.

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min) at 448 K before entering into the reactor. The gaseous products, after attaining steady state, were analyzed by an online gas chromatography (Agilent 6890) equipped with a flame ionization detector (FID) using a HP-5 capillary column and thermal conductivity detector (TCD) using a Hayesep D column. Liquid products were condensed at 273 K and analyzed with a GC-MS (Agilent 6890). The mass balance of the reaction was carried out on carbon basis by using argon as an internal standard. The conversion of ethanol and the selectivity to ethylene were calculated as follows. XEtOH )

NEtOH,i - NEtOH,j × 100 NEtOH,i

SE )

Nethylene,j

∑ N, j

× 100

(1)

(2)

Where NEtOH,i is moles of ethanol injected to the reactor, NEtOH,j is moles of unreacted ethanol, Nethylene,j is moles of ethylene observed, ∑N, j moles of all products. 3. Results and Discussion 3.1. Catalyst Characterization. 3.1.1. N2 Physisorption. Surface characteristics of P-modified and pure H-ZSM-5 catalysts were determined by N2 adsorption/desorption isotherms. Textural properties including surface area, pore volume, micropore volume, micropore area, external surface area, and pore size are presented in Table 1. The N2 adsorption and desorption isotherms of all samples (not shown here) were typically type I. BET surface area (m2/g) and pore volume (mL/ g) of H-ZSM-5 decreased with P loading, clearly suggesting the progressive blocking by P species inside the zeolite channels. The external surface area of bulk H-ZSM-5 was found to be 142 m2/g and decreased to 47 m2/g for 3.72P-ZSM-5 and showed a slight increase to 60 m2/g for 7.43P-ZSM-5 catalyst. It clearly suggests that the surface area for 7.43P-ZSM-5 is mainly contributed by the external surface area compare to other samples. However, the pore sizes measured by H-K method remains unchanged until 3.72% of P and showed an increase for 7.43P-ZSM-5, probably due to the formation of wider pores, especially at the entrance of pore channels. Impregnation of P resulted in a decrease of both micropore volumes and micropore areas, particularly at higher phosphorus contents. At higher loadings, the decrease in the surface areas and pore volume can be ascribed to the formation of polymeric phosphates species, especially at the entrance of pore channels. These results are in good agreement with findings in the literature.22 3.1.2. X-ray Diffraction. Powder XRD profiles of H-ZSM-5 and P-modified H-ZSM-5 samples showed a well-crystalline MFI phase.23 The crystallite size, calculated using the Scherrer equation from the full width at half-maximum (fwhm) of the strongest reflections, indicates that crystal size is 48.2 nm and slightly decreased to 47 nm for the 7.43P-ZSM-5 catalyst. The decrease in intensity of the XRD peaks of P-modified ZSM-5 indicates crystallinity decrease, probably due to partial dealumination of H-ZSM-5, this effect being more prominent for 7.43P-ZSM-5 catalyst. AlPO4 formation, even for 7.43P-ZSM-5 was not detected by XRD. However, it cannot be completely ruled out the formation of ultrafine AlPO4 particles with crystal size lower than 40 Å. 3.1.3. Thermal Gravimetric Analysis. Prior to calcinations, thermal gravimetric analysis of the parent H-ZSM-5 and P-modified H-ZSM-5 catalysts were carried out from ambient

to 1173 K (Figure 1). These profiles indicate that, primarily, three zones of weight loss can be identified: zone 1 (below 398 K), zone 2 (398-498 K), and zone 3 (above 498 K). The weight loss in zone 1 is related to the loss of water as it takes place around 373 K for all the samples. Profiles indicate that weight loss in zone 1 and 2 is more prominent for H-ZSM-5 and no significant weight loss was observed above 498 K. However, it appears that for P-modified catalysts, there is a gradual weight loss which is proportional to P content up to 873 K, and above this point, it levels off up to 1173 K. A gradual weight loss in zone 2 can be attributed to condensation of OH groups attached to P. Interestingly, the total weight loss in the temperature range 298-773 K was higher for pure H-ZSM-5 (9.5%) as compared to that of 1.86P-ZSM-5 (8.25%) and 3.72P-ZSM-5 (9.4%). 7.43P-ZSM-5 exhibited slightly higher weight loss compared to H-ZSM-5; this is probably due to the decomposition of H3PO4 at higher contents. However, it is important to point out here that thermal analysis above 773 K is not relevant for the present study since the maximum operating temperature during reaction and pretreatments is 773 K. For P-modified samples, the gradual weight loss in the region from ambient to 773 K can be attributed to the condensation OH groups of H3PO4 along with removal of physisorbed surface water. 3.1.4. IR Spectroscopy. Figure 2 shows the FT-IR spectra of H-ZSM-5 and various P-modified H-ZSM-5 catalysts in the range 400-1400 cm-1. The characteristic vibrational bands due to the symmetric and asymmetric stretching vibrations of Si-O-Si groups of H-ZSM-5 at 800 and 1064 cm-1 can be clearly observed in Figure 2. A band at 1385 cm-1, assigned to the PdO (phosphoryl) group,24 can be observed for the 7.43PZSM-5 sample. In addition, a band around 878 cm-1 due to HPO42- was observed. These HPO42- species could result in the formation of pyrophosphates by condensation, forming P2O74- species. DRIFT spectra of H-ZSM-5, 1.86P-ZSM-5, and 7.43P-ZSM-5 samples in the region of 4000-3200 cm-1 (OH stretching region) are shown in Figures 3a-c, respectively. H-ZSM-5 showed a broad band centered at 3640 cm-1 with a weak band at 3742 cm-1. After heat treatment at 473 K, several peaks at 3742, 3660, and 3603 cm-1 appeared and their intensities increased when temperature was raised to 673 K. The DRIFT spectrum of 1.86P-ZSM-5 showed a broad band around 3600 cm-1, which is shifted to 3552 cm-1 after heating treatment. The peak intensity at 3742 cm-1 for 3.72P-ZSM-5 decreases compared to H-ZSM-5. The DRIFT spectra of 7.43P-ZSM-5 catalyst showed a similar trend, but the band at 3742 cm-1 was much less obvious. The bands at 3740 and 3610 cm-1 can be attributed to nonacidic isolated silanol (Si-OH) and strongly acidic bridging hydroxyl groups (Si-OH-Al), respectively. Another band at 3660 cm-1 attributed to hydroxyl groups on extra framework alumina. The broad band in the region of 3600-3300 cm-1 is assigned to bridged hydroxyl groups, which are perturbed by hydrogen bond interactions with the zeolite framework. The above result suggests that the surface modification of silanol groups occurs upon P impregnation. 3.1.5. Temperature Programmed Desorption. Figure 4 shows NH3-TPD profiles of the H-ZSM-5 and P-modified ZSM-5 catalysts in the temperature range of 377-1000 K. H-ZSM-5 showed two peaks at 520 and 760 K, clearly suggesting the existence of corresponding weak and strong acid sites, respectively. The deconvolution of these peaks resulted in two peaks having a percent area of 72 and 28, respectively. The total acidity of H-ZSM-5 is slightly increased with P addition for 1.86P-ZSM-5 and decreased with further increase


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Figure 1. Thermal gravimetric analysis profiles of H-ZSM-5 and P-modified ZSM-5 catalysts.

Figure 2. FT-IR spectra of H-ZSM-5 and P-modified ZSM-5 catalysts.

in P content. The high temperature peak at 760 K decreased as the P content is increased. The peak maximum of low temperature desorption peak showed a shift to lower temperatures, indicating that P addition generates new weak acid sites by suppressing the strong acid sites of H-ZSM-5. 7.43P-ZSM-5 catalyst exhibited one broad peak centered at 476 K. The total acidity of H-ZSM-5 was found to be 0.56 mmol/g, increased to 0.81 mmol/g for 1.86P-ZSM-5 and decreased to 0.22 mmol/g for 7.43P-ZSM-5. This suggests that as the P content on H-ZSM-5 increases, the total acidity is decreased. The major decrease in acidity is due to the decrease of the strong acidic

sites present on H-ZSM-5. The acidity due to low acidic strength sites measured between the temperature region 400 and 650 K also decreased from 0.32 to 0.22 mmol/g for H-ZSM-5 and 7.43P-ZSM-5 catalysts, respectively. The peak shift to low desorption temperatures at higher P loadings indicate generation of comparatively low/medium strength acid sites. Figures 5a and b show the evolution of propylene and n-propylamine desorption during TPD of n-propylamine, respectively. While the formation of propylene is strongly decreased at high P loadings, the evolution of n-propylamine is significantly less affected. For 7.43 P ZSM-5, the evolution of propylene relative to the one obtained for unmodified ZSM-5 is close to 0.12. However, the relative amount of desorbed n-propylamine is significantly higher, close to 0.5. The same trend was observed for all P-modified samples. 3.1.6. Solid State MAS NMR. The local environment of Si, P, and Al atoms in the calcined samples were studied by solid state MAS NMR spectroscopy. Figure 6 shows the 29Si MAS NMR spectra of P-modified and pure H-ZSM-5 catalysts. H-ZSM-5 shows a broad peak with a peak maxima around -111 ppm and a shoulder at -119 ppm, which are assigned to Si(4Si) units. In addition, another peak with a maximum around -105 ppm is observed, which can be attributed to Si(3Si1Al).25 For 1.86P-ZSM-5, no significant modification of the Si(4Si) peak is detected. The signal at -119 ppm can be related to crystallographically inequivalent Si (0Al) sites,26 which obviously increase with P content. A very broad low intensity peak in the range of -125 to -98 ppm is observed for the 7.43PZSM-5 sample.

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Figure 3. DRIFT spectra of H-ZSM-5 and P-modified ZSM-5 catalysts at various temperatures (a) H-ZSM-5, (b) 1.86P-ZSM-5, and (c) 7.43P-ZSM-5.

Figure 7 shows the deconvolution of 27Al MAS NMR spectra of P-modified and pure H-ZSM-5 catalysts. The 27Al MAS NMR spectrum of the H-ZSM-5 sample shows a peak at 55 ppm, along with a small peak at 0.33 ppm. The peak at 55 ppm indicates tetrahedral Al into the zeolite framework (Altet-f) while the peak at 0.33 ppm is due to octahedrally coordinated Al species (Aloct) in an extra framework position. The intensity of the peak at 55 ppm decreases with increasing the P content, and the peak at 0.33 ppm completely disappears upon P modification. A new peak appeared at -32 ppm for samples 1.86P-ZSM-5 and 3.72P-ZSM-5, which can be attributed to extra framework aluminum phosphate. For 7.43P-ZSM-5, two new peaks are observed at 36.4 and -17 ppm, respectively. The broad peak at 36.4 ppm can be attributed to Al in distorted environment,22 whereas the peak at -17 ppm is attributed to terminal groups of polyphosphates attached to Al.27 For the H-ZSM-5 sample, the peaks at 36 and 55 ppm yielded 8.6 and 82% of total peak area (see the Supporting Information for details). The Altet-f peak area decreased from 82% for H-ZSM-5 with P loading, and 7.43P-ZSM-5 yielded only 18% of total areas. On the other hand, the peak around 36 ppm increased

Figure 4. NH3-TPD profiles of H-ZSM-5 and P-modified ZSM-5 catalysts.


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Figure 5. Temperature programmed desorption of n-propylamine (a) propylene and (b) n-propylamine.

Figure 6. catalysts.

29

Si MAS NMR spectra of P-modified and unmodified ZSM-5

from 8 to 37%. The deconvolution of peak areas due to tetrahedral Al (55 ppm) and extra framework Al species (36.4 ppm) ppm clearly showed that tetrahedral Al sites are decreasing with P content whereas the extra framework Al sites area is increasing. With P addition, it can also be observed from the spectra that the cumulative peak area of Altet and Aloct is decreased indicating the formation Al-O-P species. Figure 8 shows the deconvolution of 31P MAS NMR spectra of P-modified and pure H-ZSM-5 catalysts. For the 1.86PZSM-5 sample, several peaks appear at 22.9, -7.28, -13.73, and -39 ppm. The intensities of all peaks are increased with P content. Between 0 and 2 ppm chemical shift, a shoulder is observed due to monomeric [PO4]3- groups. The peak at -7.28 ppm is attributed to P in pyrophosphoric acid or terminal groups not attached to Al,28 while the one at -13.73 is attributed to terminal groups of polyphophates attached to Al. The peak at -39.08 ppm can be attributed to P atoms that had reacted with impurities.29 These results are in good agreement with BET surface area and FT-IR results, indicating the presence of polyphosphate chains blocking the pore channels.

Figure 7. 27Al MAS NMR spectra of P-modified and unmodified ZSM-5 catalysts (a) H-ZSM-5, (b) 1.86P-ZSM-5, (c) 3.72P-ZSM-5, and (d) 7.43PZSM-5. 1

H MAS NMR spectra of calcined H-ZSM-5 and P-modified ZSM-5 catalysts are shown in Figure 9. H-ZSM-5 shows a broad peak at 3.9 ppm with a small peak at 0 ppm. For sample 3.72PZSM-5, the broad peak is shifted to 4.7 ppm. The broad peaks at 3.9 and 4.7 ppm are attributed to the bridging AlOHSi groups associated to Bro¨nsted acid sites.30,31 This band is also attributed to bridging hydroxyl groups influenced by additional electrostatic interaction with zeolite framework.32 Isolated silanol (Si-OH) groups exhibit a characteristic chemical shift at 2.0 ppm. Due to broad nature of the peak around 4.0 ppm, it is difficult to clearly observe the Si-OH signal. The chemical shift in this region is attributed to silanol groups of H-ZSM-5. This result further corroborates the DRIFTS results, which indicate that silanol groups are being consumed upon P modification. 7.43P-ZSM-5 exhibited a new peak at 6.26 ppm with much

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Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010 Table 2. Results of Ethanol Dehydration Activity on H-ZSM-5 and Various P-Modified ZSM-5 Catalysts at 673 Ka product selectivity (mol %) sample

ethylene

propylene

butylenes

DEE

aromatics


4.5 36 97.8 99.4

12.2 17.2 0.87 0.0

20.4 10.5 0.66 0.29

0.15 4.8 0.11 0.3

58.9 31.3 0.56 0.0

a Reaction conditions: 0.5 g catalyst, reaction temperature 673 K, 1 atm pressure.

Table 3. Results of Ethanol Dehydration Activity on H-ZSM-5 and Various P-Modified ZSM-5 Catalysts at 573 Ka

sample

mol/h · g

mmol/h · m2

surface acidityb (T, 400-650 K) (mmol/g)


2.055 1.66 0.731 0.201

5.61 6.02 1.99 2.73

0.32 0.62 0.24 0.22

reaction rates Figure 8. 31P MAS NMR spectra of P-modified ZSM-5 catalysts (a) 1.86PZSM-5, (b) 3.72P-ZSM-5, and (c) 7.43P-ZSM-5.

total acidity (mmol/g) 0.56 0.81 0.24 0.22

a Reaction conditions: 0.1 g catalyst, 100% ethanol, 1 atm pressure, 573 K. b Weak/medium acidity determined from the low T desorption peak (T ) 400-650 K).

Table 4. Results of Ethanol Dehydration Activity with Reaction Temperature over 7.43P-ZSM-5 Catalystsa

b

Figure 9. 1H MAS NMR spectra of P-modified ZSM-5 catalysts.

lower intensity as compared to the other samples. The peak at 6.5 ppm is assigned to H2O adsorbed on the Lewis acid sites.33 3.2. Catalytic Activity. Prior to catalytic tests, a blank reactor test was conducted by flowing pure ethanol as well as diluted ethanol solutions. No appreciable ethanol conversion is observed up to 673 K. While ethanol dehydration into ethylene is endothermic, the production of diethyl ether on the other hand is a slightly exothermic reaction. C2H5OH f C2H4 + H2O + 45.62 kJ/mol 2C2H5OH f C2H5-O-C2H5 + H2O -22.63 kJ/mol The results of ethanol dehydration on H-ZSM-5 showed a wide range of products under steady state reaction conditions. Besides ethylene, formation of propylene, butylenes, aromatics, and alkylated aromatics are also observed during reaction at 673 K (Table 2). The variation in selectivity upon P-modification with different contents of P on H-ZSM-5 at 673 K is shown in Table 2. It is clear that H-ZSM-5 and lower P-content samples result in the formation of a wide range of products, while mainly ethylene is produced at higher P contents. Conversions of ethanol remain unchanged with P content at 673 K. However, the selectivity toward ethylene is increased, being higher than 98% for 7.43P-ZSM-5 catalyst. In order to understand the kinetic behavior of these catalysts, the reaction was carried out at 573 K and atmospheric pressure using 0.1 g of catalyst. The reaction

RT (K)

% conversion

% selectivity of ethylene

% selectivity of DEEb

523 573 623 673 723

25 75 85 99 99

3 35 88 99 98

96 64 11 0 0

a Reaction conditions: 0.5 g catalyst, 100% ethanol, 1 atm pressure. DEE, diethyl ether.

rates in terms of mol/h · g and mmol/h · m2 of the catalysts are reported in Table 3. The highest reaction rate is observed for the H-ZSM-5 catalyst, upon P-modification, the reaction rate decreases as the P content increased. In order to find a relationship with catalytic activity and surface acidity the integrated values obtained for the low temperature peak in the region between 200-650 K is presented in Table 3. Interestingly, the reaction rates measured in mmol/h · m2 of the catalysts has shown similar trend of surface acidity in the above temperature range. Ethanol conversion over H-ZSM-5 and various P-modified catalysts were studied at different reaction temperatures. All catalysts show an increasing ethanol conversion as the temperature is increased. The catalytic performance as a function of the reaction temperature during ethanol dehydration under steady state conditions over 7.43P-ZSM-5 is presented in Table 4. The conversion of ethanol as well as the selectivity toward ethylene is increased with reaction temperature. For high content Pmodified catalysts, diethyl ether is the only byproduct observed at lower temperatures (e623 K); while the formation of diethyl ether is higher than ethylene at low temperatures, the selectivity toward diethyl ether decreases as the reaction temperature increases. The selectivity of ethylene increases with the increase in reaction temperature, reaching 99% at 673 K. A slight decrease in ethanol conversion is observed by further increasing the reaction temperature. In order to investigate the deactivation behavior, all catalysts were tested for about 220 h at 673 K using a space velocity of 18.36 h-1. As shown in Figure 10, H-ZSM-5 exhibits a high ethanol conversion (above 99%) and remains unchanged up to


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Figure 10. Ethanol dehydration activity on H-ZSM-5 and 7.43P-ZSM-5 catalysts at 673 K and atmosphere pressure: (a) H-ZSM-5 and (b) 7.43P-ZSM-5. Table 5. Results of Ethanol Dehydration Activity Using Dilute Ethanol Solutions on 7.43P-ZSM-5 Catalysta concentration of ethanol

% conversion

% selectivity of ethylene

10 25 50 100

99.5 99.8 99.1 98.7

99.2 99.4 99.1 98.2

a Reaction conditions: 0.5 g catalyst, reaction temperature 673 K, 1 atm pressure.

25 h. But, the conversion drops to 90% after 60 h on stream (Figure 10a). However, the yield of ethylene is significantly increased after 8 h, reaching a maximum value of 90% at 30 h. Then, it decreases to 74% after 60 h on stream. It can be observed that the deactivation is more significant after 50 h. The quick deactivation of H-ZSM-5 catalyst can be understood in terms of its nonselective formation of various olefins and aromatics, resulting in the formation of coke precursors on the catalyst. This is in good agreement with the reported deactivation behavior of H-ZSM-5 in the literature.18 In contrast, the 7.43PZSM-5 catalyst shows an outstanding stability in the selective production of ethylene, without showing sign of any deactivation even after 200 h on stream (Figure 10b). In the above test, the mass balance of the reaction to form ethylene from ethanol calculated on carbon atom basis is found to be close 100% during the whole test. A decrease in catalytic activity was observed during ethanol dehydration over heteropoly acid based catalysts34 when aqueous ethanol solutions were used as a reactant. Since bioethanol is obtained as diluted ethanol solution in water, it is important to ascertain the dehydration activity of these catalysts using aqueous solutions of ethanol. Aqueous ethanol solutions were prepared and tested in the range between 10 and 100 vol %. The results obtained for 7.43P-ZSM-5 catalysts at 673 K using various diluted solutions of ethanol are presented in Table 5. The above result shows that there is no significant change in both conversion and selectivity when various diluted solutions of ethanol were used. Figure 11 shows a comparison in the ethylene selectivity for sample 7.43 P-ZSM-5 using pure ethanol and 10% ethanol in water as feedstock. With pure ethanol a selectivity of 97.5% is observed while the selectivity is in fact improved to 99.7% at 673 K for 10% ethanol in deionized (DI) water. In order to optimize the reaction temperature for 10%

ethanol solution, the reaction was carried out at 673, 623, and 573 K. The catalyst shows stable activity (100%) with selectivity toward ethylene above 98 mol % at 623 K. At 573 K, the conversion drops to 95%, whereas the selectivity remains higher than 99%. The water being formed in the reaction is contributing to this effect of gradual building of the ethylene selectivity. In other words, the water formed in situ in the reaction is playing positive role in the generation of suitable acidic sites. The influence of steaming effect on P-modified ZSM-5 catalysts was reported in the literature.22 Authors have attributed the improvement in butenes conversion to the removal of over doped P, or incorporation of P in zeolite framework. However, the steam treatment in the above study was conducted prior to the catalytic test at 1073 K. A similar effect to modify the present catalysts might be responsible to improve the gradual improvement in the ethylene selectivity. For comparison, PO43- modified metal oxides such as PO43-/ ZrO2, PO43-/Al2O3, PO43-/TiO2, and PO43-/SiO2 were synthesized by wet impregnation method. The above catalysts showed the following trend PO43-/Al2O3 > PO43-/ZrO2 > PO43-/TiO2 > PO43-/SiO2 in ethylene yield at 673 K. The maximum yield of 92.3 mol % was obtained for PO43-/Al2O3 catalyst, which is comparatively less than PO4 modified H-ZSM-5 catalyst. Obviously, the impregnation of P on H-ZSM-5 can lead to following effects: modification in the number and strength of acid sites and blockage of pores. The interaction of phosphoric acid on ZSM-5 has been a subject of interest.22,25,28 Corma et al.25 demonstrated a decrease in the acidity upon phosphorus modification, being higher for H3PO4 as compared to NH4H2PO4. It was proposed that P impregnation preferentially reduces the number of strong acid sites. Caeiro et al.27 also observed a decrease in total acidity by the introduction of phosphorus on MFI zeolite by their TPD measurements using propylamine as a probe molecule. This contribution clearly showed that P-modified H-ZSM-5 exhibited high selectivity toward ethylene by suppressing other products such as higher olefins and aromatics. NH3-TPD results clearly indicate a reduction in number of acid sites and strength of acidity. Generation of new acid sites with medium acid strength seem to play a role in determining the selectivity in ethanol dehydration. Lercher et al.35 observed sharp decrease in the strength of Bro¨nsted acid sites at higher H3PO4 loadings. Temperature programmed desorption of n-propylamine has been proposed

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Figure 11. Influence of diluted ethanol solutions during ethanol dehydration over 7.43P-ZSM-5.

to distinguish between Bro¨nsted and Lewis acid sites.36 Only Bro¨nsted acid sites catalyze the decomposition of n-propylamine to produce ammonia and propylene via Hoffman elimination reaction. n-Propylamine adsorbed on the other acid sites, including Lewis acid sites and acid sites not strong enough to produce propylene, desorbs without dissociation. From Figure 5a, it is clearly evident that the decomposition of n-propylamine to propylene exhibit three peaks which could be associated with acid sites of different strengths. These peaks are centered at 485, 543, and 585 K. For H-ZSM-5 catalyst, the peak centered at 543 K shows maximum intensity. The total area of these peaks strongly decreases with P loading. For 7.43 P ZSM-5, the catalyst shows only one peak centered at 585 K. It is therefore clear that P-modification induces a significant decrease in the acid strength by generating medium acid strength sites. Although this leads to a lower catalytic activity, the selectivity toward ethylene is strongly enhanced by suppressing any side reaction. Figure 5b shows the n-propylamine desorption on ZSM-5 and P-modified ZSM-5 catalysts. n-propylamine desorption is associated with Lewis acid sites and weak/medium acid strengths. Although, Figure 5b shows a decrease in total area upon P modification, this decrease is much smaller as compared to one observed for propylene desorption (Figure 5a). However, it should be noted here that the samples with high P loadings, 3.72P-ZSM-5 and 7.43P-ZSM-5, the surface area dropped 54 and 20%, respectively, to the parent ZSM-5 catalyst. The blockage of pores is clearly evident from our N2 physisorption results and this effect could result in reducing the multiple adsorptions of ethanol, diethyl ether, and/or ethylene. The reduction in multiple adsorptions could effectively improve the selectivity as well as stability of the catalyst. The formation of diethyl ether at low temperatures below 623 K suggests that it might play as an intermediate in the formation of ethylene. In order to improve the stability of H-ZSM-5, methods such as selective poisoning and external dealumination were reported previously.19,37 Van Mao et al.38 doped H-ZSM-5 with fluorine in low concentrations to improve the surface acidity by forming the new Bro¨nsted acid sites and also increasing the strength of acidic sites. Catalytic activity was inhibited by poisoning of H-ZSM-5 with both acids and bases.39 In the present study, we explored the possibility of using P-modified H-ZSM-5 catalysts. The promotional effects of P might result from the surface modification of H-ZSM-5. It was reported that the number of Bro¨nsted acid sites of zeolite framework decreased with phosphoric acid addition.40 The tetrahedral aluminum is mainly responsible for the surface acidity of zeolite. The decrease in

the peak intensity of Al (55 ppm) in the 27Al MAS NMR spectra indicates the decrease of total acidity for P-modified catalysts. Furthermore, the significant decrease in the surface area and pore volume suggest that the blockage of the zeolite channels by P at the entrance of the pores of H-ZSM-5. However, it is worthy to note that a 100% conversion can be achieved on 7.43P-ZSM-5 sample under our reaction conditions. The present study clearly indicates that it is important to modify the surface of H-ZSM-5 in order to achieve high selectivity toward ethylene. From 1H MAS NMR results, it is evident that new type of hydroxyl groups is being formed for 7.43P-ZSM-5 catalyst. Alcohol dehydration proceeds through producing ethers, olefins, and successive formation of products due to dehydrogenation, cracking, oligomerization, and aromatization on acidic sites of zeolites.41,42 Ethanol dehydration reaction on H-ZSM-5 catalyst resulted in the formation of ethylene, diethyl ether, propylene, butylenes, aromatics, and alkyl aromatics. In contrast, P-modified H-ZSM-5 catalysts selectively produced ethylene and diethyl ether. The formation of diethyl ether at low temperatures, below 623 K, suggests it might be the intermediate in the formation of ethylene. The formation of diethyl ether is predominant at temperatures below 573 K, and above this temperature, ethylene become the main product. The P-modified catalysts clearly inhibits the polymerization of ethylene which resulted in suppressing the formation of other products. The above effect might be responsible for the high stability of the catalysts without any deactivation for 200 h. From 1H MAS NMR and NH3-TPD, results indicate that P-modification influences the type and number of acid sites which determine the catalytic activity. Our reaction results in combination with characterization results suggest the following reaction mechanism for ethanol dehydration. In the case of H-ZSM-5, ethanol dehydrated to form diethyl ether on Bro¨nsted acid sites which results in forming C2, C3, and C4 olefinic species to yield aromatics in the zeolite pores by multiple adsorptions. C2H5OH f C2H5OC2H5 f C2, C3, and C4 olefinic species f aromatics In the case of P-modified catalysts, we postulate that only C2 olefinic species are produced from diethyl ether to selectively yield ethylene as follows: C2H5OH f C2H5OC2H5 f C2 species f ethylene


At the same time, the possibility of forming ethylene selectively by intramolecular dehydration of ethanol in a single step cannot be completely ruled out. Reaction results in combination with NH3-TPD make evident that strong acid sites in fact favor the formation of aromatics and C3+ olefins. The selective ethylene formation on P-modified catalysts is occurring on new acid sites generated as evidenced from our 27 Al and 1H NMR as well as NH3-TPD results. Particularly, the improvement in the selectivity and stability thus related to the generation of new type of acid sites by suppressing the high acid strength sites which may often lead to the formation of oxygenate compounds initially. This improved stability implicates that the common regeneration of the catalyst in air, which may often lead to the formation of oxygenate compounds initially, can be avoided by using P-modified ZSM-5 catalysts. On the other hand, H3PO4 is an inexpensive mineral acid compared to metals or mixed metals exchanged in H-ZSM-5 catalysts to achieve high selectivity toward lower olefins. Therefore, the cost for catalyst preparation could be greatly lowered. Then, the use of P-modified zeolite catalysts may offer new opportunities for the ethanol dehydration process. 4. Conclusions Compared to bulk H-ZSM-5, P-modified ZSM-5 catalysts prepared by the wet impregnation method exhibited very high ethylene selectivity during selective bioethanol dehydration. Among various P-modified ZSM-5 catalysts, 7.43P-ZSM-5 catalyst exhibited very high ethanol conversion as well as ethylene selectivity. P-modified catalysts also exhibited very high catalysts stability as compared to H-ZSM-5 catalysts due to their improved coke resistance properties. Thus, the developed catalyst minimizes the common problems associated with catalyst regeneration. The TPD profiles indicate that the strong acid sites are being suppressed by the addition of P by generating mainly weak acid sites. Apart from the strength of acidity, the total acidity of the catalyst was also decreased with P content. 7.43P-ZSM-5 catalyst was also found to be highly active when using diluted ethanol solutions or bioethanol as feedstocks. Interestingly, using diluted ethanol solutions as feedstocks allow the decrease of the optimum reaction temperature to 623 K. Thus, P-modified ZSM-5 catalysts offers several advantages as compared to bulk H-ZSM-5 or Al2O3 catalysts. Acknowledgment This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science Technology and Research), Singapore. Authors also like to thank Ms. Ong Li Li and Dr. Chacko Jacob for their help in running solid state NMR experiments. Supporting Information Available: XRD and NMR plots and detailed peak fit results. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Rass-Hansen, J.; Falsig, H.; Jorgensen, B.; Christensen, C. H. Perspective Bioethanol: fuel or feedstock. J. Chem. Tech. Biotech. 2007, 82, 329. (2) Machado, N. R. C. F.; Calsavara, V.; Astrath, N. G. C.; Neto, A. M.; Baesso, M. L. Hydrocarbons from ethanol using [Fe, Al] ZSM-5 zeolites obtained by direct synthesis. Appl. Catal. A Gen. 2006, 311, 193.

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ReceiVed for reView October 27, 2009 ReVised manuscript receiVed March 10, 2010 Accepted March 17, 2010 IE901666F

Synthesis, Characterization, and Catalytic Activity of Phosphorus

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