Preparation of CuNaY zeolites with microwave irradiation and their application for removing thiophene from model fuel

Separation and Purification Technology 64 (2009) 326–331

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Preparation of CuNaY zeolites with microwave irradiation and their application for removing thiophene from model fuel Xiaojuan Li, Xingwang Zhang, Lecheng Lei ∗ Institute of Environmental Pollution Control Technologies, Xixi Campus, Zhejiang University, Hangzhou 310028, PR China

a r t i c l e

i n f o

Article history: Received 15 July 2008 Accepted 4 October 2008 Keywords: Thiophene Zeolite Adsorption Microwave Copper

a b s t r a c t To produce ultralow-sulfur fuels, copper ions have been introduced into the framework of Y zeolite by a novel, simple liquid-phase ion exchange method taking advantage of microwaves irradiation. Removal of thiophene from model fuels were studied over activated copper ion-exchanged zeolite samples using column breakthrough experiments at ambient temperature and pressure. The effects of the microwave irradiating power, duration time and the copper ion concentration in aqueous solutions on the ion exchange level and the structure of copper ion-exchanged zeolite samples were investigated by atomic absorption spectrophotometer, X-ray powder diffraction, N2 adsorption and scanning electron microscope and X-ray photoelectron spectroscopy. The results demonstrated that ion exchange under microwave irradiation was a more attractive zeolite preparation method compared with conventional ion-exchange process. The maximum exchange level of 75% could be obtained after only 10 min irradiation while the exchange level was 71% after conventional ion exchange for twice and the copper can be better dispersed in zeolite framework under microwave irradiation. Microwave-irradiated CuNaY zeolites could efficiently remove thiophene from model fuel with a high sulfur removal capacity of 1.22 mmol/g. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Ultra-deep removal of organic sulfur compounds in transportation fuels has become an important research subject due to increasingly stringent regulations and fuel specifications in many countries for environmental protection purpose [1]. Recently, US Environmental Protection Agency has issued regulations that will require the refineries to reduce the sulfur content of gasoline from the current average of 300 to 30 parts per million by weight (ppmw) and diesel from 500 to 15 ppmw by 2006 [2]. European legislation also restricts the sulfur level to less than 10 ppmw for both fuels by 2009 [3]. The hydrodesulfurization (HDS) using sulfided NiMo/Al2 O3 and CoMo/Al2 O3 catalyst at high-temperature (300–340 ◦ C) and highH2 -pressure (20–100 atm of H2 ) is the conventional method being employed by refineries to remove organic sulfur compounds from fuels for several decades [4]. However, achieving deep desulfurization by HDS would require more severe reaction condition, increased reactor size, and paying the penalty of increased hydrogen consumption [5]. Recently, many efforts have been made to develop the non-HDS methods to produce ultralow-sulfur fuels

∗ Corresponding author. Tel.: +86 571 88273090; fax: +86 571 88273916. E-mail address: [email protected] (L. Lei). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.10.016

such as biodesulfurization [6], extraction using ionic liquids [7], selective adsorption [1,5,8–24], and oxidative desulfurization [25], etc. Among these alternative new methods, selective adsorption at ambient temperature and pressure is considered as the most economical approach. A wide variety of porous materials, such as activated alumina [5,8–10], zeolites (including 5A, 13X, various ZSM’s, mesoporous zeolites and ion-exchanged zeolites) [1,11–19], carbon materials (including activated carbons and carbon aerogels) [20–23], and other solid materials [24] have been investigated as adsorbents for removing organic sulfur compounds from model fuel solutions or real fuels. Among these possible materials, Cu (I)–Y zeolites have received much attention due to their relatively higher adsorption capacity and superior selectivity for removal of organosulfur molecules. Hernández-Maldonado and Yang showed that Cu (I)–Y zeolites were capable of removing 0.20 mmol of organo-sulfur species from a commercial diesel fuel (297.2 ppmw total sulfur) per gram of zeolite and the fuel contained a total sulfur content of less than 1 ppmw S. The remarkable adsorption feature of Cu (I)–Y zeolite is obtained because of a ␲-complexation between the cuprous ion (1s2 2s2 2p6 3s2 3p6 3d10 4s0 ) and the thiophenic aromatic rings. It has been shown that ␲-complexation with cuprous ions is stronger with organo-sulfur molecules than with aromatics without sulfur [12]. Cu (I)–Y zeolites can be prepared by solid-phase ion exchange (SPIE) but are mostly prepared by first liquid-phase ion exchange

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(LPIE) to achieve Cu (II)–Y zeolites followed by activated of Cu (II)–Cu (I) [26]. Conventional LPIE is often carried out in batch reactors at a temperature higher than room temperature (60–90 ◦ C) for period days and/or for several times, which frequently involves heat transfer from a bath with oil or water or an electrical oven to the exchange suspension. Recently, microwave irradiation has been used as heat resource in LPIE process [27–29]. Microwave energy interacts with material at the molecular level through different mechanisms, which are ionic conduction and dipole rotation [30]. Thus, microwave can heat the suspension rapidly, uniformly and directly without any problem of heat transfer and the time of ion exchange process can be reduced to a few minutes under microwave irradiation [31]. Moreover, materials treated with microwave radiation might have different properties than ones treated with conventional heating [32]. Kuroda et al. have employed an ion-exchange method under microwave irradiation to prepare Cu ion-exchanged ZSM-5 samples and their results materials exhibited quite a peculiar N2 molecular adsorption property compared with conventional ion-exchanged one [27]. In the present paper, we report the application of this technique to prepare copper ion-exchanged Y zeolites and the use of the activated ones as adsorbents to remove thiophene from model fuel. In addition, to understand the adsorption selectivity of this adsorbent, the influence of aromatics (e.g. benzene) on thiophene adsorption is also investigated. 2. Experimental 2.1. Materials NaY zeolite (Na56 [(AlO2 )56 (SiO2 )136 ]·nH2 O, Si/Al = 2.43) was obtained from Nanjing Heyi Chemical Industrial Co. Ltd., Jiangsu, China. Thiophene (purity, 99%) was purchased from Alfa Aesar, a Johnson Matthey Company. n-Heptane (purity, 99%) used for making model fuel solution and for solvent wash, was obtained from Fisher Scientific Ltd., Hong Kong, China. Other reagents used in the preparation methods were of analytical reagent grade. 2.2. Adsorbents preparation NaY zeolite was used as the starting material for preparation of CuNaY zeolite samples. Each sample of NaY zeolite (≈5.0 g) with a particle size in the range of 165–350 ␮m was contact with cupric nitrate solution with the concentration of 0.1, 0.2, 0.5, 1.0, 2.0 M, and a (solution volume)/(mass of zeolite) ratio of 8, corresponding to 0.5-, 1.0-, 2.5-, 5.0-, and 10.0-fold of ion-exchange capacity of NaY zeolite, under microwave irradiation. The mixtures were placed in a conical flask and put in a microwave oven (2450 MHz) equipped with a condenser pipe. The power of the microwave radiation varied between 119 and 700 W and the duration of the treatment was between 2 and 20 min. After microwave treatment, the mixtures were cooled down to room temperature, then filtered, washed with deionized water and dried at 373 K. The resulting samples were calcined at 773 K for 5 h in air and designated as CuNaY-MX, where the X represents the % exchange of Na+ ions with Cu2+ ions. For comparison, the conventional ion-exchange technique was also used for CuNaY zeolites preparation. 5.0 g fresh zeolite and 200 mL cupric nitrate solution of 0.2 M were placed in a 250 mL conical flask. The suspension was stirred at 333 K for 24 h. Then the solid was filtered, washed, dried and calcined following the same procedure mentioned above. Another sample was prepared by conducting the conventional ion-exchange procedure twice. The samples were designed as CuNaY-CX, where the X also represents the % exchange of Na+ ions with Cu2+ ions.

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2.3. Sorbents characterization Copper contents in the Cu ion-exchanged zeolite samples were determined on a Shimadzu AA-6800 atomic absorption spectrophotometer (AAS). Previously, solid samples were dissolved by acidic digestion using a microwave digestion system (MDS-2003F, Sineo Microwave Chemistry Technology Co., Ltd., China). X-ray powder diffraction (XRD) measurements of the Cu ionexchanged zeolite samples at ambient temperature were carried out using Siemens X-ray diffractometer D5005 (Cu K␣ radiation, 2 = 5–60◦ ). Surface area, total pore volume and average pore diameter of the zeolite samples were determined from the N2 adsorption data at 77 K. The equilibrium nitrogen adsorption at 77 K was measured using a BELSORP-mini. All the samples were degassed at 623 K for 2 h prior to the adsorption experiments. The surface areas (SBET ) of different zeolite samples were determined by applying the BET equation to the measured N2 adsorption data. The total pore volumes (Vp ) were determined using Brunauer MP Method. Average pore radius (rp ) were calculated from BET surface area and pore volume, rp = 2000 Vp /SBET (nm). The surface morphological details of catalysts were studied by scanning electron microscope (SEM, JSM-5600LV). The zeolite samples were mounted directly on the holders and covered with sputtered gold and then observed by SEM. X-ray photoelectron spectroscopy (XPS) spectra were recorded on PHI-5000C ESCA system (PerkinElmer) with a Mg K␣ (h = 1253.6 eV) excitation source. Accurate binding energies were determined with respect to the position of the adventitious C 1s peak at 284.8 eV. The base pressure of the analyzer chamber was about 5 × 10−8 Pa. Peak position and area were determined after satellite and background subtraction and fitting with Gaussian–Lorentzian curves. In order to minimize the photoreduction of copper species on the adsorbent surface during XPS experiments, all the samples were analyzed within a short period of time [33]. 2.4. Column breakthrough experiments All dynamic adsorption experiments were carried out in a stainless steel adsorption column, having an inside diameter of 3 mm and a length of 350 mm. The setup consisted of a low-flow liquid pump, vertical adsorption column and feed solution bottles. Initially, about 0.5 g adsorbents were loaded inside the adsorber and activated at 623 K in flow of inert gas (N2 ) for 2 h. The gas used for activation was pretreated inline prior to contacting the adsorbents using dried silica gel. Then the adsorbent under study was cooled down to room temperature under nitrogen gas. After activation treatment, a sulfur-free n-heptane was allowed to flow through the adsorbent to remove any entrapped gas for about 10 min. The feed dehydrated by Na2 SO4 was switched to the thiophene solutions at a flow rate of 0.28 mL/min. Effluent samples were collected at regular intervals until saturation was achieved. Breakthrough adsorption curves were generated by plotting the transient thiophene concentration normalized by the feed thiophene concentration versus cumulative fuel volume normalized by the adsorbent weight. The adsorption amounts (normalized per adsorbent weight) were calculated from the following equation [11]:



qbreakthrough

or saturation

=

×

Ci madsorbent MWThiophene

 t 0

1−

 C  t

Ci

dt



(1)

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Fig. 1. The exchange levels obtained at different power of irradiation for 10 min (), and for different microwave irradiation time at 119 W (䊉).

where q is the thiophene adsorption capacity (mmol/g),  is the feed volumetric flow rate (L/min), madsorbent is the mass of the activated adsorbent (g), MWThiophene is the molecular weight of thiophene (g/mol), Ci is the thiophene concentration in the feed (mg/L), Ct the effluent thiophene concentration (mg/L) at any time t (min). The sulfur adsorption capacity is the integral on the right hand side of Eq. (1), which is the area above the breakthrough curves at time t. The breakthrough adsorption amounts were obtained at the point where the effluent sulfur concentration was less than 1 ppmw S. 2.5. Analytical methods The thiophene and benzene concentration in the solution was analyzed using a Knauer HPLC unit with a reversed phase C18 column and UV detector. The detecting wavelength was set at 231 nm for only thiophene and at 208 nm for thiophene and benzene. The mobile phase was prepared by methanol (A) and deionized water (B) in 80/20 (v/v) ratio. A flow rate of 1.0 mL/min was chosen. 3. Results and discussion 3.1. Characterization of Cu ion-exchanged zeolites The results obtained from the experiments performed at different irradiation power for 10 min and at different irradiation time under fixed irradiation power (119 W) are depicted in Fig. 1, in the case that the amount of Cu2+ in preparation solution is 5.0-fold of ion-exchange capacity. It can be seen that irradiating at 385, 539

and 700 W did not induce any improvement in the results relative to that obtained at 119 W, which indicates that this copper exchange is not strictly an irradiation power dependent process and 119 W is enough to supply sufficient energy for promoting ion exchange process. Fig. 1 also shows that the maximum exchange level can be obtained after 10 min irradiation, in good agreement with results reported by Lopes [28] and Lei [29]. Therefore, taking into account the ion-exchange efficiency and energy consume, it is clear that irradiation for 10 min is the most appropriate time for preparing high copper content zeolite samples. In order to increase the exchange level, excess loading are often used [11]. Table 1 lists the unit cell composition and the exchange levels for the zeolites resulting from different amount of Cu2+ in the preparation solution under 10 min irradiation at 119 W. The exchange levels reached by 0.5-, 1.0-, 2.5-, 5.0- and 10.0-fold of ion-exchange capacity in preparation solution are 36%, 50%, 64%, 75% and 75%, respectively, so as to these zeolite samples were designed as CuNaY-M36, CuNaY-M50, CuNaY-M64, CuNaY-M75I and CuNaY-M75-II. It is observed that the content of copper exchanged into the zeolite framework increases with the increase of the amount of Cu2+ in preparation solution and the higher content of copper is achieved at 5.0-fold of ion-exchange capacity. As also shown in Table 1, the degree of ion exchange of a level of 75% can be attained under microwave irradiation for only 10 min, which is higher than the level of 64% obtained by conventional ionexchange for 24 h while the amount of Cu2+ in the ion-exchange solution was equivalent to a 5.0-fold cation-exchange capacity. In addition, even after twice ion exchange by conventional ionexchange method, the exchange level (71%) still is lower than that under microwave irradiation. The higher exchange level achieved by microwave irradiation heating may be explained as a consequence of an activation process by hot spots, molecular agitation and improved transport properties of molecules [28]. All ion-exchange level data show that neither conventional ion exchange nor microwave ion exchange treatment cross 75% threshold. It is reported that the liquid-phase ion exchange is often limited by the hydrolysis of the cation species in the aqueous solution [34]. Copper ion aqueous solutions can lead to formation of species such as Cu(OH)+ , Cu(OH)2 , Cu(OH)3 − , Cu(OH)4 2− , and Cu2 (OH)2 2+ [11]. The presence of such species can greatly limit the loading of transition metal being exchanged to the zeolite. Thus, a complete ion exchange in aqueous phase is usually not easily obtained by solution ion exchange. The XRD patterns of NaY and Cu ion-exchanged zeolite samples, recorded at 2 values between 5◦ and 60◦ , are shown in Fig. 2. The XRD profiles reveal that the characteristic peaks for Cu ionexchanged zeolite samples are similar to those of NaY. No shift in the peak positions and no significant diffraction lines assigned to any new phase are observed. These results demonstrate that ion exchange does not damage the NaY zeolite crystal and cupric ion seems to be well dispersed in the zeolite framework by both conventional method and microwave promoted method. Slight

Table 1 Chemical composition and physical characteristics of different Cu ion-exchanged zeolite samples. Cu2+ concentration (fold of ion-exchange capacity)

Adsorbent

Composition

SBET (m2 /g)

Vp (cm3 /g)

Vp (nm)

– 0.5 1.0 2.5 5.0 10.0 5.0 5.0

NaY CuNaY-M36 CuNaY-M50 CuNaY-M64 CuNaY-M75-I CuNaY-M75-II CuNaY-C64 CuNaY-C71

Na56 (Al56 Si136 O384 ) Cu10 Na36 (Al56 Si136 O384 ) Cu14 Na28 (Al56 Si136 O384 ) Cu18 Na20 (Al56 Si136 O384 ) Cu21 Na14 (Al56 Si136 O384 ) Cu21 Na14 (Al56 Si136 O384 ) Cu18 Na20 (Al56 Si136 O384 ) Cu20 Na16 (Al56 Si136 O384 )

608 595 582 580 576 575 562 553

0.279 0.270 0.263 0.259 0.247 0.247 0.250 0.242

0.92 0.91 0.90 0.88 0.86 0.86 0.89 0.88

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Fig. 4. Cu 2p spectra of CuNaY-M36 and CuNaY-75M-I.

Fig. 2. XRD patterns of different Cu ion-exchanged zeolite samples. All the samples were activated at 623 K for 2 h in N2 .

decrease of the peak intensity after exchange process is also found, indicating a certain loss of crystallinity. All the nitrogen adsorption–desorption isotherms of Cu ion-exchanged zeolite samples belonged to the BDDT type I classification, characteristic for microporous material. Both the BET surface area and the pore volume show a tendency to decrease slightly with an increase in the amount of Cu loading for both conventional method and microwave promoted method, but the rates of decrease are not significant (Table 1). In addition, the BET surface area and the pore volume of microwave irradiation samples are a little larger than those of conventional ion-exchanged samples, which might be attributed to better dispersed in zeolite framework of copper by microwave irradiation than conventional heat process. The mean pore radius is around 0.9 nm regardless of the amount of metal ion exchange.

The scanning electron micrographs of the Cu ion-exchanged zeolite samples are given in Fig. 3. It is found that morphology of both conventional ion-exchanged zeolite sample and microwave promoted ion-exchanged zeolite sample is retained and similar to NaY zeolite sample. In order to know the copper species present in the copperexchanged zeolites after activated in inert gas (N2 ), samples of CuNaY-M36 and CuNaY-M75-I have been studied by XPS and the Cu2p spectra are shown in Fig. 4. The characteristics of divalent copper, Cu (II) can be confirmed by the Cu 2p3/2 binding energy within the range of 933–936 eV and the shake-up satellite peak [33]. Monovalent copper can be assigned to the binding energy of the XPS peaks ranging from 932 to 933 eV without any shake-up satellite peaks [33]. By the curve-fitting of the Cu 2p3/2 spectra, two peaks denoted by I and II (932.2 and 934.7 eV) can be anticipated. Fig. 4 also shows a peak III at 943.6 eV, which is 8.9 eV higher than peak II, in agreement with the result reported in the literature [35]. It is obvious that peak II is due to Cu (II) loaded in the Y-zeolite and the appearance of peak I may attribute to the autoreduction of some Cu (II)–Cu (I) in N2 atmosphere. Obviously, as shown in Fig. 4, samples of CuNaY-M36 and CuNaY-M75-I have nearly 65 and 45% Cu (I), respectively. Since short period of time (10 min) were used for X-ray irradiation, the photoreduction of Cu (II) species can be mostly avoided [36]. Therefore, it indicates that about half of the Cu (II) in zeolite sample can be reduced under N2 at 623 K for 2 h and the increase of copper content in Y-zeolite will reduce the autoreduction degree. This degree is slightly lower than that (70%) obtained under argon at 673 K and dramatically lower than that (100%) obtained at 1023 K by Richte et al. [37]. Hence, to get more Cu (II) reduced, high temperature and long pretreatment time are

Fig. 3. SEM images of the Y-zeolites (a) NaY, (b) CuNaY-75M-I, and (c) CuNaY-64C.

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Table 2 Breakthrough/saturation capacity data for different Cu ion-exchanged zeolite samples. Adsorbent

Breakthrough capacity (mmol/g)

Saturation capacity (mmol/g)

NaY CuNaY-M36 CuNaY-M50 CuNaY-M64 CuNaY-M75-I CuNaY-M75-II CuNaY-C64 CuNaY-C71 CuNaY-M75-Ia

– 0.32 0.41 0.45 0.50 0.50 0.31 0.23 0.40

0.93 0.97 1.09 1.19 1.22 1.22 1.07 0.95 0.72

a Breakthrough and saturation capacity for thiophene in the presence of benzene (1000 ppmw).

required. The effects of temperature and time on pretreatment of Cu ion-exchanged Y-zeolites will be done in further research. From the above discussion, it can be concluded that ion exchange under microwave irradiation is a more attractive zeolite preparation method, because it can easily and with quite a short operating time promote the cation exchange process to interesting levels and the copper can be better dispersed in zeolite framework under microwave irradiation than conventional exchange process. Cu (II) can be partially reduced to Cu (I) under inert gas (N2 ) at 623 K for 2 h. 3.2. Column breakthrough experiments After the adsorbents were activated, a model fuel containing thiophene and n-heptane was passed through a fixed-bed column and the sulfur contents in the effluent samples were monitored as a function of time until the effluent sulfur concentration was equal to the influent sulfur concentration. The results of the breakthrough and saturation capacity data for different CuNaY adsorbents used in our experiments are summarized in Table 2. The breakthrough and saturation capacity of Cu ion-exchanged zeolite samples ranges from 0.23 to 0.50 mmol/g and from 0.95 to 1.22 mmol/g, respectively, which are higher than that of the unmodified NaY zeolite. The results demonstrated that all Cu ion-exchanged zeolite samples are found to be good at removing thiophene from n-heptane. The higher Cu loading favors the thiophene adsorption when using the microwave-irradiated zeolite samples, as the breakthrough and saturation capacity increases with increasing amount of copper in zeolite. The maximum breakthrough and saturation capacity is 0.50 mmol/g and 1.22 mg mmol/g, respectively, when the CuNaY-75M is used. Table 2 also shows that the breakthrough and saturation capacity of the conventional ion-exchanged zeolite samples are smaller than that of the microwave-irradiated zeolite samples even of higher Cu loading. And the breakthrough and saturation capacity of CuNaY-71C is lower than CuNaY-64C. These suggest that the adsorption of thiophene by CuNaY zeolite samples is not only dependent on the Cu loading. As we know, a number of possible sites within the framework are available for the location of the cations required to balance the charge on the aluminosilicate framework in faujasite (see Fig. 5). Site I is located inside hexagonal prism and is octahedrally coordinated to the framework. Site II is on the 6-member ring of the sodalite cage, facing the supercavity and has three nearest neighbors. Sites I and II lie on the other sides of the 6-member rings, opposite sites I and II, respectively, inside the sodalite cage and are also threefold-coordinate. Site III is occupied for some zeolites and is located at the center of the square faces of the sodalite unit. Maxwell and de Boer [38] reported that in the dehydrated zeolite copper ions are located at sites I, I , II, II , III. Sites II and III are expose

Fig. 5. Faujasite zeolite framework with cation sites.

to the zeolite supercage and, therefore, accessible to the thiophenic molecules, while thiophenic molecules remain too large to enter the areas where the SI, I , II cations are located. Thus, the location of activated copper may also have significant influences upon thiophene adsorption. Higher thiophene adsorption capacity may be due to more activated copper located at exposed sites (SII and III). The thiophene adsorption on CuNaY zeolite samples is mentioned in a range of studies. Presently, Hernández-Maldonado and Yang have mentioned that Cu (I)–Y(LPIE) zeolites are capable of removing about 1.28 mmol/g thiophene (500 ppmw) from n-octane [14]. Ma et al. show a breakthrough capacity of 0.56 mmol/g when adsorptive removal of thiophene from iso-octane [9]. Then we can make out that our results are similar to those reports. The slightly dissimilarity of the adsorption capacity may be due to the variation in the conversion of Cu (II)–Y to Cu (I)–Y in the activation process, thiophene concentration, different solvent or experimental condition. To study the adsorption selectivity of our microwave-irradiated zeolite sample, benzene was added in n-heptane and the result is also shown in Table 2. It is showed that the saturation adsorption capacity was decreased by 40% while the concentration of the benzene is 1000 ppmw in n-heptane. It is evident that the competition for adsorption by CuNaY zeolites between the aromatics and thiophene is critical for deep desulfurization of real fuels, which is also demonstrated in other research reports. Hernández-Maldonado et al. employed Cu (I)–Y zeolites as sorbents to obtain a ultra-deep desulfurization commercial diesel fuel which contains a great deal of aromatics and a saturation adsorption capacity of 0.20 mmol/g, much lower than that with model fuel system, was observed [12–14]. Ma et al. reported a breakthrough capacity of 0.015 mmol/g when desulfuried a commercial gasoline [9]. Therefore, it is obvious that both the microwave-irradiated CuNaY zeolites and conventional ones are capable of removing thiophene from n-heptane, however, the performance of microwave-irradiated CuNaY zeolites is superior to conventional ones. The presence of aromatics (e.g., benzene) would to some extent influence the thiophene adsorption capacity of the microwave irradiate CuNaY zeolites.

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4. Conclusion Liquid-phase ion exchange taking advantage of microwaves can be effectively applied in preparing copper ion-exchanged zeolites. The characteristics of microwave-irradiated CuNaY zeolites determined by AAS, XRD, BET and SEM suggest that microwave irradiation can easily and with quite a short operating time (10 min) promote the cation exchange process to interesting levels and the copper can be better dispersed in zeolite framework than conventional exchange process. The results of XPS indicate that the pretreatment of Cu ion-exchanged zeolites in N2 will cause part of Cu (II) to autoreduce to Cu (I). Column breakthrough experiments showed that the maximum breakthrough and saturation capacity was 0.50 and 1.22 mmol/g when the CuNaY-75M was used, respectively. Thiophene adsorption by microwave-irradiated CuNaY zeolites can be to some extent influenced by the presence of aromatics (e.g., benzene). The microwave-irradiated zeolites are a promising adsorbent for the removal of sulfur-containing compounds from model fuel and the encouraging results point to a need for investigating the desulfurization performance of real fuels. Acknowledgements The authors would like to acknowledge financial support for this work provided by NSFC (No. 20576120, 90610005, 20836008 and U0633003), Project of Zhejiang province (No. 2007C13061 and Y5080192) and MOST project of China (No. 2007AA06Z339; No. 2008BAC32B06; No. 2007AA06A409). References [1] V.M. Bhandari, C. Hyun Ko, J. Geun Park, S.-S. Han, S.-H. Cho, J.-N. Kim, Desulfurization of diesel using ion-exchanged zeolites, Chem. Eng. Sci. 61 (2006) 2599–2608. [2] C.S. Song, An overview of new approaches to deep desulfurization for ultraclean gasoline, diesel fuel and jet fuel, Catal. Today 86 (2003) 211–263. [3] F.F. Li, L.J. Song, L. Duan, X.Q. Li, Z.L. Sun, A frequency response study of thiophene adsorption in zeolite catalysts, Appl. Surf. Sci. 253 (2007) 8802–8809. [4] H. Topsøe, B.S. Clausen, F.E. Massoth, Hydrotreating Catalysis: Science and Technology, Wiley, New York, 1996. [5] X. Ma, L. Sun, C. Song, A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultra-clean fuels and for fuel cell applications, Catal. Today 77 (2002) 107–116. [6] K.I. Noda, T. Kogure, S. Irisa, Y. Murakami, M. Sakata, A. Kuroda, Enhanced dibenzothiophene biodesulfurization in a microchannel reactor, Biotechnol. Lett. 30 (2008) 451–454. [7] C.P. Huang, B.H. Chen, J. Zhang, Z.C. Liu, Y.X. Li, Desulfurization of gasoline by extraction with new ionic liquids, Energy Fuels 18 (2004) 1862–1864. [8] A.J. Hernández-Maldonado, G.S. Qi, R.T. Yang, Desulfurization of commercial fuels by ␲-complexation: monolayer CuCl/␥-Al2 O3 , Appl. Catal. B 61 (2005) 212–218. [9] X. Ma, S. Velu, J.H. Kim, C. Song, Deep desulfurization of gasoline by selective adsorption over solid adsorbents and impact of analytical methods on ppmlevel sulfur quantification for fuel cell applications, Appl. Catal. B 56 (2005) 137–147. [10] J.C. Zhang, L.F. Song, J.Y. Hu, S.L. Ong, W.J. Ng, L.Y. Lee, Y.H. Wang, J.G. Zhao, R.Y. Ma, Investigation on gasoline deep desulfurization for fuel cell applications, Energy Conver. Manage. 46 (2005) 1–9. [11] A.J. Hernández-Maldonado, F.H. Yang, G. Qi, R.T. Yang, Desulfurization of transportation fuels by ␲-complexation sorbents: Cu (I)-, Ni (II)-, and Zn (II)-zeolites, Appl. Catal. B 56 (2005) 111–126. [12] A.J. Hernández-Maldonado, R.T. Yang, Desulfurization of diesel fuels by adsorption via ␲-complexation with vapor-phase exchanged Cu (I)–Y zeolites, J. Am. Chem. Soc. 126 (2004) 992–993.

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