Removal of C2H4 from a CO2 Stream by Using AgNO3-Modified Y

Aug 2, 2006 - Removal of C2H4 from a CO2 stream is very important in ultrapurification of CO2 for its application and utilization. However, it is diff...
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SEPARATIONS Removal of C2H4 from a CO2 Stream by Using AgNO3-Modified Y-Zeolites Jinxia Zhou, Yongchun Zhang, Xinwen Guo,* Anfeng Zhang, and Xiaomeng Fei State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian, China 116012

Removal of C2H4 from a CO2 stream is very important in ultrapurification of CO2 for its application and utilization. However, it is difficult to remove C2H4 completely from a CO2 stream via conventional cryogenic distillation because the volatility of C2H4 is similar to that of CO2. Another choice is the catalytic combustion method, but it is an energy-inefficient process, especially when it is coupled with the cryogenic distillation. Selective adsorption for removing C2H4 might be a promising alternative. A great effort has been made in C2H4/C2H6 separation and recovery of C2H4. There were, however, few reports about the removal of a trace amount of C2H4 from a CO2 stream. In this study, the AgNO3-modified NaY and HY zeolites are prepared and their adsorption performances are evaluated via a series of experiments of fixed-bed adsorption, frontal chromatographic adsorption, and temperature-programmed desorption (TPD). The structure of the sliver salt after being impregnated onto the supports has been characterized by thermogravimetric/differential thermal analysis, X-ray diffraction, and transmission electron microscopy. The experimental results show that both NaY and HY have poor adsorption selectivities of C2H4 from the C2H4/CO2 stream because of a stronger competition of CO2 with C2H4 for the physical adsorption sites, and AgNO3/NaY and AgNO3/HY, favored by their strong chemical adsorption sites, Ag ions, are able to remove C2H4 from the CO2 stream effectively. Introduction Recovery of CO2 is very important for both reduction of greenhouse gases and efficient utilization of carbon resources. However, a captured CO2 stream is not easy to deeply purify because some impurities, such as C2H4, in the CO2 stream are difficult to remove. Ethylene oxide and ethylene glycol processes produce a large amount of CO2 that can be further purified as food-class liquid additive or other useful pure CO2 feed. But C2H4 species present in the CO2 feed cannot be well-removed by cryogenic distillation because of the close relative volatilities. Ideally catalytic combustion can oxidize C2H4 to H2O and CO2, but the operational condition (>300 °C, containing sufficient O2)1 is still an energy-intensive process, especially when combined with cryogenic distillation. Technoeconomic adsorption-separation approaches might be promising alternatives for such a difficult purification. However, despite the great efforts that have been made in C2H4/C2H6 separation or in recovery of C2H4,2-9 there are few reports about removal of a trace amount of C2H4 from a CO2 stream. Since the properties of CO2 are not the same as that of C2H6, it is necessary to carry out a fundamental study of the feasibility of deep removal of C2H4 from CO2 feed by adsorption. The characteristics for an adsorbent to be used for removal of C2H4 from a CO2 feed are as following: first, the active sites of the adsorbent should have a high adsorption selectivity for C2H4 over CO2, even when the sites have been preoccupied by CO2 molecules; second, the adsorbing bonds should be weak enough to be broken by simple engineering operations such as raising the temperature or decreasing the system pressure, so the regeneration of the adsorbent is possible; and third, the active sites of the adsorbent should be relatively stable under operating * To whom correspondence should be addressed. Tel.: 86-41188993908. Fax: 86-411-83689065. E-mail: [email protected].

conditions. As for C2H4 adsorbent, Ag+ and Cu+ have been commonly used as active species.10 But Cu+, which has a poor chemical stability, can be easily oxidized to Cu2+ in the presence of O2 and then loses its adsorption capability. In addition, compared with active carbon, resin or carbon fiber, metal ionmodified zeolites can form stronger adsorptions with adsorbate and are usually used for purification.10 Therefore, AgNO3modified zeolites might be preferred adsorbents. The purpose of this work was to perform a detailed study of an adsorptive approach for removing C2H4 impurity from a CO2 stream using AgNO3-modified NaY and HY zeolites. Moreover, the effects of C2H4 concentration, adsorption temperature, and pretreatment temperature on the performances of the adsorbents were studied via a series of experiments of fixed-bed adsorption, frontal chromatographic adsorption, and temperature-programmed desorption (TPD). Experimental Section Preparation and Characterization of Adsorbents. HY powder (Si/Al ) 2.8; BET surface ) 717.6 m2/g) was prepared by ion exchange of NaY powder (Si/Al ) 2.7; BET surface ) 847.6 m2/g; from Wenzhou Zeolite Co., Zhejiang, China) with NH4NO3. The NaY and HY powder was shaped into cylindrical pellets (1.2 mm × 3 mm) by mixing with colloidal silica, extruding, drying, and calcining at 540 °C. AgNO3 (0.15 g) was dispersed on a 1 g support by an incipient wetness technique. Some samples heated in a temperature no more than 300 °C are termed as AgNO3/NaY and AgNO3/HY, and others calcinated in the air at 540 °C for 6 h are termed as AgNO3/NaY-C and AgNO3/HY-C, respectively. The Si/Al ratio of the zeolite powder and the exchange degree of HY (96%) were measured by the sequential X-ray spectrometer system (SRS-3400, Bruker Co., Rheinstetten, Germany). The BET surface areas of AgNO3/ NaY, AgNO3/HY, AgNO3/NaY-C, and AgNO3/HY-C were

10.1021/ie0605478 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/02/2006

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Figure 1. DTA and TGA curves of the air-dry AgNO3/NaY (a) and AgNO3/HY (b) samples (10 °C/min in N2).

641.5, 602.9, 592.0, and 534.7 m2/g, respectively, measured by physical adsorption apparatus (AUTOSORB-1, Quantachrome Co., Boynton Beach, FL) using N2 adsorption at 77 K.The thermal gravimetric analysis of the adsorbent samples was measured by thermogravimetric/differential thermal (TG/DT) analysis (TGA/SDTA851e, Mettler-Toledo Co., Greifensee, Switzerland). The X-ray powder diffraction analysis (XRD; D/Max 2400 automatic X-ray diffractometer, Rigaku Denki Co. Ltd., Tokyo, Japan) was used to determine the structure of the AgNO3 on the support. Transmission electron microscopy (TEM) was carried out in a JEOL JEM-2000EX TEM at 100 kV. For the TEM analysis, sample powder was dispersed in isopropyl alcohol using a ultrasonic bath and deposited on a carbon-coated 200-mesh Cu sieve. The heats of adsorption were measured in a DSC-910 modulus (TA Instrument Co., New Castle, DE) coupled to a TGA thermal analyzer (TGA/ SDTA851e, Mettler-Toledo). Experimental Equipment and Conditions. The breakthrough curves of the removal of C2H4 from the C2H4/CO2 and C2H4/N2 streams were measured in a fixed-bed adsorption column with 14 mm internal diameter. The adsorption column and the inlet gas line were equipped with a heat-controlling device including a temperature programmer to monitor the operation temperature. The inlet and outlet gas concentrations to and from the adsorbent column were analyzed by a GC-14C gas chromatograph (Shimadzu Co., Kyoto, Japan) equipped with an flame ionization detector (FID) and an accessory for reducing CO2 to CH4. The experiments were done under the following conditions: the adsorbents were heated at 250 °C for 3 h in air to remove water before adsorption; for each test, 5 g of adsorbent was loaded into the fixed-bed adsorption column, and breakthrough curves were measured at the following conditions: at concentration levels of 1.03% (V) C2H4/CO2 and 1.01% (V) C2H4/N2, 25 °C and atmospheric pressure, and the flow rate maintained at 100 mL/min (normal temperature and pressure (NTP)) during the adsorption process. When the effluent concentration of C2H4 reached the specified influent concentration (100% breakthrough), the experiment was stopped. The gases (offered by Guangming Institute, Dalian, China) used in the experiment were ethylene (CP grade, g99.5%), CO2 (CP grade, g9.95%), and N2 (CP grade, g99.99%). The adsorption isotherms and isobars of C2H4 on NaY, AgNO3/NaY, HY, and AgNO3/HY at atmospheric pressure were determined with a frontal chromatographic method.11 The frontal chromatographic adsorption system was a modified gas chromatograph: the chromatographic column was replaced by the sample column (made in our laboratory), and a six-switch valve and a flow control system were installed. The sample column was filled with 1 g of adsorbent, C2H4 was diluted with N2 and

CO2 at different concentration levels, and the N2 was used as purge gas too. Detailed TPD studies on C2H4 saturated adsorptions were performed for the analysis of regeneration. A 1 g sample of 20-40 meshes was first saturated at atmospheric pressure by using a given stream at a given temperature. After each saturation step, the TPD analysis was performed under 40 mL/ min N2 by ramping the temperature at 4 °C/min from 40 °C to the temperature when no C2H4 was detected, and GC was used to analyze the content of C2H4. Results and Discussion Characterization of Samples. The structure of the AgNO3supported Y-zeolite adsorbent was characterized by TGA/DTA, XRD, and TEM. The DTA and TGA curves of the air-dry AgNO3/NaY and AgNO3/HY samples are shown in Figure 1. The sharp peak of the DTA spectra of the AgNO3/NaY sample in the higher temperature range manifests that AgNO3 decomposes near 470 °C, and the peak around 130 °C was caused by water evaporation, as shown in Figure 1a. By analyzing the TG curve, it can be concluded that if the operation temperature is no more than 300 °C, the silver salt supported on NaY is mainly in the form of AgNO3, and if AgNO3/NaY is heated at a temperature above 500 °C, the AgNO3 supported on the NaY will completely decompose. But the TGA/DTA spectra of the AgNO3/HY sample differs greatly from that of the AgNO3/NaYsample, as shown in Figure 1b. Both the DTA spectra and the TG curve of the AgNO3/HY sample show that most of AgNO3 supported on the HY support has changed its state. When HY is impregnated with the AgNO3 aqueous solution, the liquid can penetrate into the zeolite channels, and Ag+ ions can exchange with the H+ ions on the cation sites.12 The exchanged H+ ions will recombine with the nitrates to form HNO3, and the HNO3 is unstable and prone to decompose at a lower temperature. There might be a certain portion of AgNO3 in the zeolite channels which does not exchange with H+ ions and directly decomposes at elevated temperature. Therefore, in the DTA spectra of the AgNO3/HY sample, the peak around 130 °C was caused by the water evaporation, the broad peak around 350 °C was due to the decomposition of HNO3, and the hightemperature peak around 450 °C was attributed to the decomposition of the remaining AgNO3 (Figure 1b). Figure 2 shows the XRD patterns of the adsorbents before and after calcination. The samples “before calcination” are prepared at a temperature no more than 300 °C, and the ones “after calcination” are pretreated in the air at 540 °C for 6 h. The samples are all of white color except AgNO3/NaY-C,

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Figure 2. XRD patterns of the adsorbents before and after calcination.

Figure 4. Breakthrough curves of removal of C2H4 from the 1.03% C2H4/ CO2 and 1.01% C2H4/N2 streams at 25 °C.

Figure 3. TEM images of (a) AgNO3/NaY-C and (b) AgNO3/HY-C. Scale bar: 50 nm.

which is dark gray. As expected, no sharp characteristic peaks of the salt crystallites are detected on the AgNO3/NaY and the AgNO3/HY samples, which suggests that the metal salts are probably dispersed in (or near) a monolayer on the support surfaces.2 However, the characteristic peaks of the two samples after calcination are obviously different. The diffraction peak for metallic silver at 2θ of 38° is clearly observed over AgNO3/ NaY-C, which indicates that with the decomposition of AgNO3 at elevated temperature, metallic silver particles are formed on the surface of NaY. But no diffraction peak for metallic silver particles was observed in the XRD spectra of the AgNO3/HY-C sample. The TEM images of the AgNO3/NaY-C and AgNO3/HY-C samples are shown in Figure 3. Lots of small silver particles are dispersed on the individual NaY particle (Figure 3a), while the AgNO3/HY-C sample does not show large silver particles in the TEM (Figure 3b). These results are in agreement with the XRD result, since the large silver particles give rise to the observed XRD diffraction peak and the finely dispersed silver could not be detected by XRD.13 Breakthrough Curves. The breakthrough curves of C2H4 adsorption on the various adsorbents are depicted in Figure 4, and the breakthrough capacities and saturation capacities on the basis of the breakthrough curves are shown in Figure 5. The shape of the breakthrough curve, or the concentration front, is determined by both hydrodynamic (axial dispersion) and kinetic (finite transport rates) factors. However, the breakthrough behavior is dependent strongly on the nature or type of the adsorption isotherm. The sharp concentration front means the strong curvature of the isotherm. It is found that with the 1.01% C2H4/N2 stream, the NaY and HY adsorbents have some adsorption capacities, but for the 1.03% C2H4/CO2 stream, a large amount of C2H4 break out during the initial adsorbing period, and the purification effect of these two adsorbents is less significant. However, the adsorption capacity and the

Figure 5. Breakthrough capacity and saturation capacity on the basis of the breakthrough curves of removal of C2H4 from the 1.03% C2H4/CO2 and 1.01% C2H4/N2 streams at 25 °C.

purification effect are significantly improved by AgNO3 supported on NaY and HY. As shown in Figure 4, the breakthrough curves of both AgNO3/NaY and AgNO3/HY are s-shaped, and the curves rise sharply at the breakthrough points. The breakthrough times of C2H4 on the two modified adsorbents for the C2H4/CO2 stream are shorter compared with that for the C2H4/ N2 stream, but they can still effectively remove the C2H4 from the CO2 feed with a high purification efficiency (e1 ppmv) as well as a relatively high adsorption capacity. Equilibrium Isotherms. The adsorption isotherm data of C2H4 on NaY and HY are fitted with the Langmuir isotherm shown as

q)

qmpbpp 1 + bp p

(1)

The adsorption of C2H4 on AgNO3/NaY and AgNO3/HY includes both physical adsorption and chemisorption; therefore, a different model2 is used as shown below:

q)

qmpbpp qmc 1 + bcpes + ln 1 + bpp 2s 1 + b pe-s

(2)

c

where qm is the saturated amount adsorbed, and a bigger qm means more adsorption sites; b is a Langmuir constant, and a larger b means a stronger adsorptive affinity; and the subscripts p and c stand for the physical adsorption and chemisorption, respectively. The parameter, s, is a heterogeneity

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Figure 7. Equilibrium isobars of C2H4 on AgNO3/NaY and NaY. The concentration of the gas is 5.60% C2H4 in N2. Figure 6. Equilibrium isotherms of C2H4 on AgNO3/NaY and AgNO3/ HY in N2 and CO2 at 30 °C. Table 1. Summary of Equilibrium Isotherm Parameters

AgNO3/NaY-C2H4/N2 AgNO3/NaY-C2H4/CO2 NaY-C2H4/N2 NaY-C2H4/CO2 AgNO3/HY-C2H4/N2 AgNO3/HY-C2H4/CO2 HY-C2H4/N2 HY-C2H4/CO2

qmp (mL/g)

bp (kPa-1)

qmc (mL/g)

bc (kPa-1)

s

60.7 60.6 73.1 70.0 50.2 50.1 59.6 57.2

0.037 0.012 0.059 0.012 0.028 0.009 0.028 0.006

23.7 23.7

7.749 8.459

7 7

23.5 23.5

7.876 14.404

7 7

Table 2. Summary of Adsorption Energies (kJ mol-1)

C2H4 CO2

AgNO3/NaY

NaY

AgNO3/HY

HY

-73.2 -19.4

-17.7 -20.1

-81.1 -17.0

-16.2 -18.6

parameter indicating the spread of the energy, and the empirical values for s are available from the literature.14,15 Equilibrium isotherms of C2H4, diluted with N2 and CO2, respectively, on the various adsorbents, are shown in Figure 6, and the computational fitting parameters are shown in Table 1. Figure 6 shows that the physical adsorbability of C2H4 on the NaY decreases drastically when the N2 dilution gas is replaced with CO2, from which we know that the presence of CO2 makes it difficult to selectively adsorb C2H4. The same phenomenon is observed on the HY adsorbent. For the AgNO3/NaY and AgNO3/HY adsorbent, however, the low-pressure region of the isotherms are steep, and the adsorption capacity of C2H4 in CO2 was not weakened more drastically than that in the N2, especially in the low-pressure region. Compared with NaY and HY, AgNO3/NaY and AgNO3/HY exhibit excellent adsorption capacity and adsorption selectivity because the Ag+ can form strong π-complexation bonds with C2H4. The strong curvatures of the favorable isotherms of AgNO3/NaY and AgNO3/HY predict the sharp shape of the breakthrough curves of them, as shown in Figure 4. The heats of adsorption obtained from TG/DSC analysis (DSC ) differential scanning calorimetry) are also shown in Table 2. The adsorption heat of C2H4 on AgNO3/NaY or AgNO3/HY is much higher than the others. These trends manifest that the AgNO3-modified NaY and HY, improved by Ag+ active sites, can form strong adsorption with C2H4. Table 2 also shows that the value of adsorption heat of AgNO3/NaY is lower than that of AgNO3/HY. However, Figure 5 shows that AgNO3/NaY-C2H4/N2 has higher adsorptive capacity than that of AgNO3/HY-C2H4/N2. The larger values of bc and adsorption heat of AgNO3/HY mean that the chemical site on it has stronger

Figure 8. Adsorption amounts of C2H4 on the adsorbents at 30 °C. The concentration of the gas is 1.01% C2H4 /N2.

adsorptive affinity. As for adsorption capacity, it is affected not only by Ag+ sites but also by the sites on the support. The larger value of qmp of AgNO3/NaY means that NaY adsorbs more C2H4 than HY when there is no CO2 competition. This could be why AgNO3/NaY has a higher adsorption capacity than AgNO3/HY. Equilibrium Isobars. The isobar profiles of C2H4 adsorption on AgNO3/NaY and NaY are depicted in Figure 7. It is shown that the adsorption capacities of C2H4 on both AgNO3/NaY and NaY decreases with increasing the adsorbing temperature, and the latter decreases more quickly. The empirical value for monolayer dispersion of AgNO3 on a support is 0.083 g of AgNO3/m2 support surface.10 That is to say the maximum dispersion amount of AgNO3 on the shaped NaY (708.8 m2/g) is 0.588 g of AgNO3/(g of NaY), and for the 0.15 g of AgNO3/ (g of NaY), about 74.5% surface is not covered by AgNO3. We suppose that the physical adsorption capacity of C2H4 on the 0.15 g of AgNO3/(g of NaY) is approximately equal to the 74.5% amount of C2H4 on the NaY, and the rest is regarded as the chemical adsorption amount. Thereby, the chemical adsorption isobar of AgNO3/NaY is drawn in Figure 7, which first increases and then decreases with increasing adsorbing temperature. According to the Lennard-Jones model,15 the physical adsorption does not need to be activated and a low temperature is preferred. However, the chemical adsorption that has to be activated is favored by a high adsorption temperature, but too high temperature can result in desorption. This explains why the isobar of the chemical adsorption first increases and then decreases with increasing adsorbing temperature. Active Site in AgNO3/Support Before and After Calcination. During the pretreatment of the adsorbents or a regeneration process, the operation temperature might have a strong effect on the active sites of adsorbents. Figure 8 shows that the adsorption capacity of AgNO3/NaY after calcination decreases drastically compared with that before calcination, while the

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Figure 9. TPD profiles of C2H4 from AgNO3/NaY and AgNO3/HY saturated with 5.60% C2H4/N2, 2.03% C2H4/N2, or 2.95% C2H4/CO2 at 40 °C.

values on the AgNO3-modified HY adsorbent are almost the same. These might be due to the different effects of the temperature on the state of the silver. Before calcination, the silver species can be effectively dispersed on both the NaY and the HY through the incipient wetness method and kept in the form of Ag+ ions (the Ag+ ions of AgNO3/NaY are in the form of AgNO3 and the Ag+ ions of AgNO3/HY are in the form of the cation sites of the zeolite). However after calcination, the AgNO3 species supported on NaY decompose and form metallic silver particles on its surface. It is believed that the new state of the metallic silver particles can decrease the amount of the active sites and then decrease the adsorbing ability. However, the AgNO3/HY sample after calcination can still contain a large fraction of isolated Ag+ in highly dispersed state. This might be due to the good stability of the Ag+ ions after exchanging with the H+ ions in the cation sites of HY. So for the AgNO3/ NaY adsorbent, the preferred operating temperature is no more than 300 °C, and the AgNO3/HY adsorbent can have a wider range of the operation temperature. TPD Studies and Regeneration. Detailed TPD profiles on C2H4-saturated AgNO3/NaY and NaY are shown in Figure 9a. It indicates that the TPD profile for NaY has a single peak at about 50 °C, which is conformed as a physical adsorption peak. However, AgNO3/NaY shows a larger total amount of C2H4 adsorption than NaY at higher temperature range, and there are three kinds of desorption peaks in the TPD curves of AgNO3/ NaY: the position of the peak at low temperature is similar to that of NaY, and it may be ascribed to the presence of physical adsorption species; the shoulder at 80 °C represents the weak chemical adsorption; and the other between 100 and 240 °C is a broad peak, which is attributed to stronger chemical adsorption species. Weakly bonded C2H4 can be desorbed at temperature below 100 °C, and C2H4 adsorbed on the stronger active sites needs a higher desorption temperature. This might be due to the different active sites of AgNO3/NaY adsorbent. Figure 9 also shows that the decrease in the C2H4 concentration (from 5.60% C2H4/N2 to 2.03% C2H4/N2) results in the drastic decrease in the low-temperature region of the TPD curves, but it does not have strong effect on the broad peaks at high temperature, from which it can be concluded that the stronger adsorption sites are first occupied. When only a small amount of C2H4 was employed, the C2H4 was adsorbed only on the stronger active sites and needed a higher desorption temperature. Further study shows that the low-temperature peak of AgNO3/NaY saturated with C2H4/CO2 is much lower, which is due to the competition adsorption of CO2 with C2H4 for the weak adsorption sites. Therefore, the ability of the AgNO3/NaY for the removal of C2H4 from the CO2 stream is mainly from its stronger

Figure 10. TPD profiles of CO2 from the adsorbents saturated with 8.30% CO2/N2 at 40 °C.

chemical adsorption sites. Similar conclusion can be drawn on the AgNO3/HY adsorbent as shown in Figure 9b, except two differences: one is that the complete desorption temperature on AgNO3/HY (which is about 270 °C) is about 30 °C higher than that on AgNO3/NaY, and this trend accords with the values of the adsorption heat in Table 2 and the fitting parameters of bc in Table 1, all of which mean that the chemical adsorption site on AgNO3/HY has stronger adsorptive affinity. The other difference is that the low-temperature peak of AgNO3/HY is smaller than that of AgNO3/NaY, which means that AgNO3/ HY has fewer weak adsorption sides, and this trend agrees with the fitting parameters in Table 1, which shows that the qmp of AgNO3/HY is smaller than that of AgNO3/NaY. For this purification method, the temperature swing adsorption (TSA) is the process of choice, and the regeneration performance of the saturated adsorbent by a purge gas is very important. The repeated TPD profile of AgNO3/NaY saturated with C2H4/ CO2 (which is termed as AgNO3/NaY-2.95% C2H4/CO2(2)) is almost the same as the original one (which termed as AgNO3/ NaY-2.95% C2H4/CO2(1)), as shown in Figure 9a. The readsorption amount of the regenerated AgNO3/NaY under the same condition is 17.7 mL/g, which is very close to that of the fresh sample (17.8 mL/g). Further study shows that the saturated AgNO3/NaY adsorbent can be effectively regenerated in air or CO2 at 230-250 °C. And the complete regeneration of AgNO3/ HY is feasible too, but it needs a higher regeneration temperature around 270 °C. The TPD profiles of CO2 from the adsorbents are shown in Figure 10. The complete-desorption temperature of CO2 from NaY (which is 98 °C) is higher than that of C2H4 from NaY (which is 75 °C), and the temperature of CO2 from HY (which

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Figure 11. TPD profiles of C2H4 from AgNO3/NaY saturated with 5.60% C2H4/N2 at various adsorption temperatures.

is about 84 °C) is higher than that of C2H4 from HY (which is about 65 °C) too. The two results show that the adsorption of CO2 on the bare Y-zeolite surface (the physical-adsorption sites) is stronger than that of C2H4. When the two kinds of adsorbates approach the supports simultaneously, CO2 will compete with C2H4. That is why the adsorption capacity is lower when there is CO2 in the stream for all samples. However, the completedesorption temperature of C2H4 from AgNO3/NaY and AgNO3/ HY (which are 240 and 270 °C, respectively) are much higher than that of CO2 from the two adsorbents (which are 98 and 84 °C, respectively), which means that the AgNO3-supported Y-zeolite has high adsorption selectivity for C2H4 over the corresponding CO2. The complete-desorption temperature, as shown in Figure 9 and Figure 10, can predict the adsorption strength, and the trends are in agreement with the results in Table 2.

Figure 11 shows the TPD profiles with various saturation temperatures. The increase in the saturation temperature results in the drastic decrease in the low-temperature peaks, which indicates that the weak adsorption shows strong temperature dependence. Compared with the low-temperature peaks, the broad peaks in the high-temperature range change slightly and the little changes show that the high-temperature peak increases first and then decreases with increasing saturation temperature. This can be attributed to the chemical adsorption that needs to be activated; namely, some of the adsorbing sites cannot be activated if the temperature is too low, but too high temperature would result in desorption. The AgNO3/HY sample shows a similar trend in the temperature-dependence TPD curves. Figure 11 can also indicate that when the sample was treated with a high temperature, for example, 90 °C, the stronger active sites can still adsorb C2H4, so, under such conditions, the removal of C2H4 from the CO2 stream is possible. The TPD spectra of C2H4 from AgNO3/NaY and AgNO3/ HY before and after calcination are shown in Figure 12. When the adsorbents were calcinated, the intensity of the highertemperature peak of the AgNO3/NaY-C sample decreased drastically, while the profile of the AgNO3/HY-C sample did not show any apparent difference, compared with their original samples before calcination, respectively. In the adsorption experiments, the results show that the calcination of AgNO3/ NaY can decrease the adsorbility drastically, and in the TPD experiment the results further show that the decrease is mainly resulted from the weakening of the stronger adsorption. Since the adsorption of the trace amount of C2H4 depends on the stronger-adsorption sites, the calcination of AgNO3/NaY is not favored for purification. These experiments further suggest that

Figure 12. TPD profiles of C2H4 from the adsorbents saturated with 2.03% C2H4/N2 at 40 °C.

Figure 13. TPD profiles of C2H4 from the adsorbents before and after H2 reduction saturated with 1.01% C2H4/N2 at 40 °C.

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the pretreating temperature of an adsorbent might have great effect on its performance. Does Ag0 (metallic silver) also play a role in the adsorption of C2H4? The TPD spectra of AgNO3/NaY and AgNO3/HY before and after H2 reduction are shown in Figure 13. The adsorbents turned black and lost purification performance drastically when reduced by H2 at 200 °C. When the adsorbents were reduced, nearly all the higher-temperature peaks of the samples disappear. The TPD results show that the contribution of Ag0 for removal of C2H4 is less significant compared with that of Ag+. Conclusion In this article, a series of experiments of fixed-bed adsorption, frontal chromatographic adsorption, and temperature-programmed desorption, combining with the characterization, have been performed to investigate the feasibility of deep removal of C2H4 from a CO2 stream by using AgNO3-modified Yzeolites. The results showed that (1) there is an intense competition between CO2 and C2H4 for the weak-site adsorption, and the AgNO3-modified NaY and HY, through Ag+ active sites forming stronger π-complex with C2H4, can deeply remove C2H4 species from the CO2 stream as well as achieving a relatively high adsorption capacity; (2) the Ag+ ions on AgNO3/HY show better stability than those on AgNO3/NaY when the adsorbents are pretreated in a high temperature; and (3) the weakeradsorption sites show stronger temperature dependence and C2H4-concentration dependence compared with the stronger sites, while the stronger adsorption first increases and then decreases with increasing adsorption temperature, which is in agreement with the Lennard-Jones theory. Acknowledgment We acknowledge Dr. Ma Xiaoliang of The Pennsylvania Sate University and Dr. Zhang Shuguang of Dalian University for their helpful comments.

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ReceiVed for reView April 29, 2006 ReVised manuscript receiVed June 26, 2006 Accepted June 29, 2006 IE0605478