Anal. Chem. 2007, 79, 1425-1432
Design of Efficient Zeolite Sensor Materials for n-Hexane Ping Yang,†,‡,§ Xingnan Ye,† Choiwan Lau,† Zengxi Li,§ Xia Liu,‡ and Jianzhong Lu*,†
School of Pharmacy, Fudan University, 138 Yixueyuan Road, Shanghai 200032, China, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, China, and Graduate University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China
The effectiveness of several zeolite catalysts was investigated using the cataluminescence (CTL) gas sensor system. Trace amounts of n-hexane in air samples were detected by this method. This research establishes that the specific pore size of the zeolite offers designable environment for selective CTL reaction, and “Lewis-type” basic sites appear to contribute to the catalytic nature of the zeolite surface. By incorporating either Cs+ or K+, the velocity and luminescence intensity of these catalytic reactions increase while going from Na to Cs, according to the basic nature of this group of cations in the following order: Cs > K > Na. The proposed sensor shows high sensitivity and selectivity to n-hexane at a mild reaction temperature of 225 °C. Quantitative analysis was performed at a selected wavelength of 460 nm. The linear range of CTL intensity versus concentration of n-hexane was 0.776-23.28 µg/mL (R ) 0.997, n ) 7) on CsNaY, and 0.776-23.28 µg/mL (R ) 0.998, n ) 7) on CsNaX, with a detection limit of 0.155 µg/mL (signal-to-noise ratio 3). Interferences from foreign substances such as methanol, ethanol, 2-propanol, acetone, acetonitrile, chloroform, or dichlormethane and other alkanes, aromatics, and alkyl aromatics such as methane, n-pentane, 3methylpentane, 3,3-dimethylpentane, methylbenzene, ethylbenzene, and sec-butylbenzene were very low or not detectable. Results of a series of GC and GC/MS experiments suggest that the possible mechanism of the reaction is the formation of an unstable transition structure with a four-member ring, and this ring most probably consists of an oxygen atom and a carbonium ion localized on the zeolite suface. In recent years, considerable effort has been made to protect against significant negative effects of air pollution on human health and the environment. n-Hexane, as a major indoor and industrial air pollutant, has been recommended as one of the eight representative indoor VOCs (the pollution of indoor volatile organic compounds).1 Various instruments for managing VOCs were developed, such as GC, GC/MS-based instruments;2 how* To whom correspondence should be addressed. E-mail:
[email protected]. † Fudan University. ‡ Lanzhou Institute of Chemical Physics. § Graduate University of Chinese Academy of Sciences. (1) VanOsdell, D. W. ASHRAE Trans. 1994, 100, 511-511. 10.1021/ac061811+ CCC: $37.00 Published on Web 01/09/2007
© 2007 American Chemical Society
ever, they are typically clumsy, complicated, expensive to manufacture, and difficult to operate and maintain. Recently, chemiluminescence (CL) emission based on the catalytic oxidation of organic analytes has been reported. McCord observed the CL emission on the surface of solid porous materials when porous silicon and nitric acid or persulfate were mixed together.3 Breysse reported a weak CL emission based on the catalytic oxidation of carbon monoxide on the surface of TiO2,4 which is known as “cataluminescence” (CTL). In addition, Nakagawa et al. and Zhang et al. have been engaged in the study of the simple CTL-based gas sensors for direct, robust, and rapid quantitative and qualitative analysis since 1998.5 Sensor materials include MgO, TiO2, Al2O3, Y2O3, LaCoO3, SrCO3, ZrO2, and BaCO3, while the analytes include acetaldehyde,6 ethanol,5b,7 acetone,7a hydrogen sulfide,8 NH3,9 ethylene dichloride,10 propane, n-butane, and isobutane.11 However, few systematic investigations have addressed the modification of selected catalyst for CTL applications. Still, there is a strong research effort under way to meet the challenge that exploits novel sensor material for the selective detection of specific member from a homologous series, thus offering a designable catalyst for a selective CTL reaction. The molecular sieving action of zeolite solids, with their welldefined channels and cavities, has been widely exploited in separation processes and in selective cracking and reforming of hydrocarbons.12 The pore size of the channels effectively determines the molecular size of the reactants and the products for a (2) (a) Santos, F. J.; Galceran, M. T. TrAC, Trends Anal. Chem. 2002, 21, 672685. (b) Nishikawa, H.; Sakai, T. J. Chromatogr., A 1995, 710, 159-165. (3) Mccord, P.; Yau, S. L.; Bard, A. J. Science 1992, 257, 68-69. (4) Breysse, M.; Claudel, B.; Faure, L.; Guenin, M.; Williams, R. J. J. Catal. 1976, 45, 137-144. (5) (a) Nakagawa, M.; Okabayashi, T.; Fujimoto, T.; Utsunomiya, K.; Yamamoto, I.; Wada, T.; Yamashita, Y.; Yamashita, N. Sens. Actuators, B: Chem. 1998, 51, 159-162. (b) Zhang, Z. Y.; Zhang, C.; Zhang, X. R. Analyst 2002, 127, 792-796. (6) Cao, X. A.; Zhang, Z. Y.; Zhang, X. R. Sens. Actuators, B: Chem. 2004, 99, 30-35. (7) (a) Zhu, Y. F.; Shi, J. J.; Zhang, Z. Y.; Zhang, C.; Zhang, X. R. Anal. Chem. 2002, 74, 120-124. (b) Zhang, Z.; Xu, K.; Baeyens, W. R. G.; Zhang, X. R. Anal. Chim. Acta 2005, 535, 145-152. (8) Sun, Z. Y.; Yuan, H. Q.; Liu, Z. M.; Han, B. X.; Zhang, X. R. Adv. Mater. 2005, 17, 2993-2997. (9) Shi, J. J.; Yan, R. X.; Zhu, Y. F.; Zhang, X. R. Talanta 2003, 6, 157-164. (10) Cao, X. A.; Feng, G. M.; Gao, H. H.; Luo, X. Q.; Lu, H. L. Luminescence 2005, 20, 104-108. (11) (a) Cao, X. A.; Zhang, X. R. Luminescence 2005, 20, 243-250. (b) Okabayashi, T.; Fujimoto, T.; Yamamoto, I.; Utsunomiya, K.; Wada, T.; Yamashita, Y.; Yamashita, N.; Nakagawa, M. Sens. Actuators, B: Chem. 2000, 64, 54-58.
Analytical Chemistry, Vol. 79, No. 4, February 15, 2007 1425
given process. Due to the existence of a large variety of natural/ synthetic zeolites having a wide range of charged acid/base groups, zeolites represent a particularly attractive class of catalyst. It is well known that the photocatalyzed oxidation of hydrocarbons could happen in zeolite cages,13 and thus, it can be expected that the molecular sieving may potentially be employed as the new catalyst for the CTL of alkanes. With matrixes that possess well-defined channels and surfaces that are easily modified, there were many options that needed to be explored in order to improve the sensitivity of the sensor material. Zeolites X and Y, an aluminum-rich end member of the faujasite family, have peculiar structures and large numbers of exchangeable cations.14 In order to prepare the zeolite matrix with its most effective catalytic surface, the alkali metal exchange method is recommended. The alkali-metal exchanged zeolites have both basic and shape-selective properties, and this allowed us to improve the sensitivity and selectivity of our sensor. The analytical characteristics were evaluated by the examination of CTL emission on the sensor, such as different molecular pore sizes, flow rate, temperature, and CTL emission wavelength. According to our original idea, a strong CTL emission was generated by the catalytic oxidation of n-hexane on the surface of zeolite in air medium. Importantly, this research shows that the “Lewis-type” basic sites improve the catalytic activity, and the selective detection is obtained by designing their well-defined channels and cavities. The Cs+-exchanged X- and Y-zeolites were chosen as the catalyst for preparing the sensor because of the high activity and selectivity for n-hexane. EXPERIMENTAL SECTION Chemicals. All of the chemicals used in the experiment were of analytical grade. Standard n-hexane (purity g99.5%) was purchased from Chem Service (West Chester, PA). Methylbenzene, ethylbenzene, sec-butylbenzene, n-pentane, 3,3-dimethylpentane, 3-methylpentane, and methane (purity g99%) were purchased from Sigma. The 4A, 5A, and NaX zeolites were obtained from Shanghai BOJ Molecular Sieve Ltd. Three different sizes of Al2O3 were bought from Tianjin Institute of Chemical Technology, China. The mesoporous Al2O3 and NaY were kindly donated by the Department of Chemistry, Fudan University. Apparatus. The CTL detection system employed in this work is shown in Figure 1. A pump with an air cleaner was used to deliver the flow stream at a controlled flow rate from 0 to 1500 mL/min. Sample gas was contained in a volume of a 50-mL flask and could be carried out by the carrier stream directly and then reached the converter based on catalyst materials. Figure 1 shows the schematic diagram of the converter. It can be seen that a cylindrical ceramic heater of 5 mm in diameter was put in a quartz tube of 12 mm in inner diameter. The outside of the quartz tube was a cooling water sleeve to protect the photomultiplier and (12) (a) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: New York, 1978. (b) Serrano, D. P.; Aguado, J.; Escola, J. M. Ind. Eng. Chem. Res. 2000, 39, 1177-1184.(c) Smirniotis, P. G.; Zhang, W. M. Ind. Eng. Chem. Res. 1996, 35, 3055-3066. (13) (a) Blatter, F.; Sun, H.; Frei, H. Catal. Lett. 1995, 35, 1-12. (b) Li, J.; Ma, W. H.; Huang, Y. P.; Cheng, M. M.; Zhang, J. C.; Yu, J. C. Chem. Commun. 2003, 2214-2215. (14) (a) Yang, S. Y.; Navrotsky, A. Microporous Mesoporous Mater. 2000, 37, 175-186. (b) Joshi, U. D.; Joshi, P. N.; Tamhankar, S. S.; Joshi, V. V.; Rode, C. V.; Shiralkar, V. P. Appl. Catal., A: Gen. 2003, 239, 209-220. (c) Chan, B.; Radom, L. J. Am. Chem. Soc. 2006, 128, 5322-5323.
1426 Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
Figure 1. Schematic representation of a CTL-based gas sensor.
decrease the noise caused from heating. This ceramic heater was operated at a required temperature by using a thermocouple, which was controlled by a temperature-programmed controller. Outside the ceramic tube, a 0.5-mm layer of catalytic materials was sintered on the surface. The sample gas can only flow through the outside of the ceramic heater, since the ceramic tube is solid. The main reaction between the catalyst and sample molecules occurred on the surface of the solid powder. The produced CTL signal was detected and recorded with a computerized BPCL CL Analyzer (Biophysics Institute, Chinese Academy of Science). The wavelengths could be selected in the range of 400-640 nm by changing the optical filters. At the beginning of each series of experiments, the sensor was heated at 500 °C for 30 min in air, in order to avoid the influence of previous residues. The resultant gas released from the sensor was introduced in a GC 112A system (Shanghai Precision and Scientific Instrument Factory) equipped with an OV-101 analysis column (30 m, 0.25-mm i.d., 0.25-µm film thickness). The GC/MS assay was carried out with a HewlettPackard 6890/5973 system (Agilent Technologies, Palo Alto, CA) equipped with a HP-5 analysis column (30 m, 0.25-mm i.d., 0.25µm film thickness). Synthesis of Nanosized Materials. A wet chemical method was used to synthesize the ZrO2 nanoparticles. Ammonia was added slowly to 0.1 M Zr(NO3)4 solution with vigorous stirring at 0 °C until pH 10, and then a white precipitate was generated immediately. The mixture was put into ice bath for 24 h. Then the precipitate was filtered and washed 5 times with deionized water. The precursor was dried at 110 °C and calcined at 500 °C in a muffle furnace for 3 h. The size of the ZrO2 nanoparticle was determined by field emission scanning electron microscopy (JEOL S-4300F). The image obtained is shown in Figure 2a. It can be seen that the ZrO2 particle size obtained is ∼100 nm. The images of TiO2, the mesoporous Al2O3, and microporous zeolites NaX and NaY obtained are shown in Figure 2b-e, respectively. Modification of Microporous Zeolite. Microporous zeolites NaX and NaY were used as the starting zeolites for cation exchange. The postsynthesis modification of NaX and NaY was carried out by a conventional ion-exchange technique, using 0.5 M KNO3, 0.4 M CsNO3, or 0.5 M NH4NO3 aqueous solution, respectively. The aqueous salt solution was taken in the proportion of 12.5 mL/g of zeolite (KNO3), 4.0 mL/g of zeolite (CsNO3), or 10.0 mL/g of zeolite (NH4NO3) for each exchange experiment. The solution with the exchanging cations and NaY or NaX powder
Figure 4. Effect of different materials on the CTL intensity. The CTL intensity on the surface of NaY, Al2O3, or ZrO2 was detected when 15.5 µg/mL sample vapors passed through at a flow rate of 200 mL/min, a selected wavelength of 460 nm, and a detection temperature of 225 °C. (A) methanol, (B) ethanol, (C) 2-propanol, (D) acetone, (E) acetonitrile, and (F) n-hexane.
Figure 2. SEM images of nanosized (a) ZrO2, (b) TiO2, (c) mesoporous Al2O3, and (d) microporous zeolites NaX and (e) NaY.
Figure 3. SEM images of microporous zeolites (a) NaY and (b) CsNaY.
were mixed and stirred in different proportions under reflux at 70 °C for 5 h. Different exchange degrees were achieved by repeating cation-exchange treatment for 3 times. The ionexchanged zeolite was recovered by filtration and washing. The solid was then dried in an air oven maintained at 110 °C overnight and calcined at 500 °C in a muffle furnace for 3 h. The HNaY sample was prepared by repeated ion exchange with NH4NO3, followed by thermal treatment at 350 °C. It was important to examine the crystalline nature of these zeolites as well. SEM and element analysis were employed. The SEM image of NaY and the modified zeolites CsNaY are shown in Figure 3a and b, respectively. It can be seen that the final state of the modified zeolite is indeed in the same framework structure of the original state. The decayed formation depends mostly on the size of the exchanged cation, while the effect of the Cs and K ions on the structure of zeolites is much less apparent. The element analysis results show that the KNaY zeolite was obtained by repeating K+-exchange treatment for two times, Na (w/w) decreased from 7.429 to 1.589% while K (w/w) increased from 0.00 to 10.55%. The CsNaY zeolite was obtained by repeating the Cs+-exchange treatment two times; Na (w/w) decreased from 7.429 to 5.355% while Cs (w/w) increased from 0.00 to 7.208%.
RESULTS AND DISCUSSION Selectivity Study of the Catalyst. The nanosized materials mentioned above were often sensitive to other gases besides sample molecules. In order to find out a suitable material for our n-hexane sensor, the influences of many species were investigated by observing the response of the CTL system to a series of 15.5 µg/mL common interferents, including methanol, ethanol, 2-propanol, acetone, acetonitrile, chloroform, and dichloromethane on different materials. The CTL intensity of n-hexane on MgO, TiO2, Al2O3, ZrO2, and NaY was comparatively measured at 225 °C and 460 nm. On the surface of MgO and TiO2, no obvious CTL signal was detected for n-hexane even at ∼300 °C. However, as shown in Figure 4, on the surface of NaY, a strong CTL signal could be detected for n-hexane. Most importantly, the detection system exhibited insignificant interference (less than 2%) from methanol, ethanol, 2-propanol, and acetonitrile. No signal was observed while the chloroform and dichloromethane vapors were passing through the NaY sensor. Note also that acetone caused significant interference (∼20%) for n-hexane determination on the surface of NaY. This suggested that the polar group “carbonyl” in the acetone molecules has a strong effect on the regional electric field caused by the negative charge of the framework and the cations on the surface of the zeolite.15 Overall, these results clearly indicated that the CTL detection system combined with the molecular sieving converter exhibited substantially high selectivity for the detection of n-hexane molecules. Selectivity Study of Microporous Zeolite. The relatively rigid frameworks of aluminosilicate zeolites are composed of TO4 tetrahedra (T ) Si and Al) strongly bonded together via oxygen bridges to form well-defined channels and cavities. Inside the void spaces of the zeolite matrix extraframework species such as cations and water molecules were present. The cations, which balance the negative charge of the framework, were mobile and exchangeable. Water molecules were eliminated by heating or (15) Venuto, P. B.; Landis, P. S. Advances in Catalysis; Academic Press: New York, 1968.
Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
1427
Figure 5. Schematic illustration of the selectivity catalytic action of the zeolite molecular sieving.
Figure 6. CTL spectra of various gases on the microporous zeolite. Other experimental conditions were the same as Figure 4.
evacuating without affecting the framework topology. The dimensions of the channel entrances effectively controlled entry of reactant molecules. The diameter of n-hexane was 4.9 Å. For comparison purposes, different pore sizes of the molecular sieving were employed, such as NaA (4 Å, Si/Al ) 1), CaA (5 Å, Si/Al ) 1), NaX (9-10 Å, Si/Al ) 1-2), and NaY(9-10 Å, Si/Al ) 2-6).15,16 The effect of various large-pore sizes on the selectivity and catalytic activity was also investigated (as shown in Figure 5 and Figure 6). Although the pore size of CaA matched with the n-hexane molecular diameter, the narrow R, β-cage was not favorable for the formation of a four-member-ring transition structure in our CTL reaction, leading to a bad CTL reproducibility on CaA. The observed selectivity can be attributed to a combination of differences in (i) diffusivity, (ii) adsorption behavior, and (iii) intrinsic reaction rate (restrictions on the size of the transition state).17 So NaX and NaY, which bear large supercages, were selected as the catalysts to develop simple, efficient, and rapid sensors to determine trace n-hexane in air samples. Although the dimensions of the channel permitted the entry of methanol, (16) Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds. Handbook of Zeolite Science and Technology; Dekker: New York, 2003. (17) (a) Vos, A. M.; Rozanska, X.; Schoonheydt, R. A.; van Santen, R. A.; Hutschka, F.; Hafner, J. J. Am. Chem. Soc. 2001, 123, 2799-2809.(b) Bates, S. P.; van Well, W. J. M.; van Santen, R. A.; Smit, B. J. Am. Chem. Soc. 1996, 118, 6753-6759.
1428 Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
Figure 7. Effect of alkalinity on the CTL detection system. Temperature, 225 °C; flow rate, 200 mL/min; wavelength, 460 nm; n-hexane concentration, 7.76 µg/mL (-1, -2, -3 stand for the number of times of exchange).
ethanol, and 2-propanol, the hydroxyl group in the alcohol molecules may make it difficult for the reaction of oxygen extraction from the carrier gas to form a four-member-ring transition structure, leading to a decrease of luminescence intensity as shown in Figure 4. Effect of Catalytic Alkalinity. Figure 6 shows that the microporous zeolite has a better selectivity than others, but it has a lower sensitivity, like NaY. Therefore, we have studied the dual effect of acidity and basicity of NaY and NaX zeolite to improve their sensitivity in the CTL of n-hexane. The framework of NaX can be destroyed by acid for the dealumination, so the preparation of HNaY sample was selected.18 Generally, the basic sites on the surface of the solid catalyst (zeolite) are of Lewis type. Lewis basicity is associated with the framework oxygen-bearing negative charge of the lattice and, consequently, the density of negative charge on a given oxygen atom. Therefore, with regard to its proton-abstracting capacity, alkali-exchanged NaX and NaY zeolites have more basic strength due to the formation of alkali-metal oxide during high-temperature calcinations in the presence of oxygen, thus leading to the high electrostatic field inside the supercage.15,19 This important characteristic would favor the carbonium ion evolution from alkanes before the following reaction was carried out in our research. The basicity of NaX and NaY zeolite was systematically varied by varying nonframework K or Cs cations. The acidity character of NaY-type zeolites was systematically altered by incorporating H cations. Correlations between activity and basicity are presented in Figure 7 and Table 1. The velocity and intensity of these reactions increased while going from Na to Cs (as a nonframework cation) directly according to the extent of basicity of cations in the following order: Cs > K > Na. This sequence of the basicity of zeolites was quarried from the literature data. For example, Heidler et al. employed20 the modified electronegativity equalization method and Monte Carlo techniques to study the basicity of faujasite-type zeolites and showed that the main factor influencing the zeolite basicity was the framework composition. By decreasing the Si/Al ratio or exchanging Na and Cs atoms, they found that the negative charge on the oxygen atom was increased and the (18) Caldarelli, S.; Buchholz, A.; Hunger, M. J. Am. Chem. Soc. 2001, 123, 71187123. (19) Ferrari, A. M.; Neyman, K. M.; Rolsch, N. J. Phys. Chem. B 1997, 101, 9292-9298. (20) Heidler, R.; Janseens, G. O. A.; Mortier, W. J.; Schoonheydt, R. A. J. Phys. Chem. 1996, 100, 19728-19734.
Table 1. Effect of the Modified Catalyst on the CTL Intensity (7.76 µg/mL of n-hexane) catalyst NaY CsNaY KNaY HNaY NaX CsNaX KNaX a
exchange times
mensuration times (n)
relative CTL intensity (103)a
RSD (%)
0 1 2 3 1 2 3 2 0 1 2 3 1 2
4 5 5 4 4 4 4 4 4 5 5 5 4 4
171.52 454.64 873.69 255.09 251.24 301.78 200.31 10.38 243.56 383.40 913.45 261.35 235.21 294.31
4.45 5.74 4.91 7.13 5.35 4.04 6.48 >10 >10 4.67 4.38 5.31 6.71 5.83
Integrated for 10 s after subtracting the background.
basicity of zeolites was thus enhanced. Deka et al. also concluded21 that the larger the cation, the more basic the oxygen atoms in the cluster. In other words, the basicity of the zeolite increases in the sequence Cs > K > Na, parallel to that the velocity and intensity of our catalytic reactions increase. Figure 7 gave another result: though the CTL intensity on the third exchange CsNaY and CTL intensity on the “no exchange” NaY were almost the same, the amplified profile in the right upper corner showed that the basic sites could accelerate the velocity of this reaction obviously. The maximum increase in catalyst activity was found when the zeolite was exchanged twice. After the third exchange, the presence of large alkali cations in the zeolite channels may reduce the adsorption of the reactants on the basic sites, which resulted in the decrease of the catalyst activity. Selectivity for a Homologous Series. Various alkanes, branched alkanes, aromatics, and alkyl aromatics such as methane, n-pentane, 3-methylpentane, 3,3-dimethylpentane, methylbenzene, ethylbenzene, and sec-butylbenzene were selected as the sample gases to confirm the dimensions of the channel entrances for effectively controlled entry of reactant molecules, leading to an excellent selectivity for a family of compounds to be obtained. No signal was seen while the methane vapors were passing through the CsNaY sensor. Possibly due to its too small diameter, methane effuses from the zeolite cages quickly. Furthermore, as shown in Figure 8, low signal of the branched alkanes and aromatics and alkyl aromatics was observed. In this case, possibly their large stereostructures make these compounds difficult to enter the zeolite cages. As expected, a relatively high signal of n-pentane came from its almost same structure with n-hexane. But the lesser number of C atom and the less small diameter of molecules resulted in the lower signal as compared with n-hexane. To avoid the adsorption behavior of aromatic compounds that would damage the activity of the catalyst, a lower concentration of 3.88 µg/mL sample gases was employed. Note also that the selectivity for other compounds such as methanol, ethanol, 2-propanol, acetone, chloroform, dichloromethane, methylbenzene, and acetonitrile was unchanged after the modification of the catalyst. (21) Deka, R. C.; Roy, R. K.; Hirao, K. Chem. Phys. Lett. 2000, 332, 576-582.
Figure 8. CTL spectra of 3.88 µg/mL various alkanes, aromatics, and alkyl aromatics on the CsNaY-2 zeolite. Other experimental conditions were the same as in Figure 4 (-2 stands for the number of times of exchange).
Figure 9. Effect of temperature and wavelength on the CTL detection system. Flow rate, 200 mL/min.
Effect of Catalytic Temperature and Wavelength. Temperature is a key factor for catalytic oxidation reactions. In most cases, the activity of the catalyst increases with increasing temperature. The CTL spectra of n-hexane on the CsNaY-2 at different temperatures and wavelengths are shown in Figure 9. The curves corresponded to the working temperature at 165, 195, 225, and 245 °C, respectively. Each curve in the figure showed three peaks, at around 425, 460, and 535 nm. It can be seen that the CTL intensity reached the maximum at about 225 °C at 460 nm, which were selected as an optimum temperature and wavelength for the quantitative detection of n-hexane in the subsequent experiments. Flow Rate of Carrier Gas. The flow rate dependence of CTL intensity was studied in the range of 50-350 mL/min at 225 °C with a band-pass filter of 460 nm. Figure 10 shows that the CTL intensity increased gradually with the increase in flow rate from 50 to 200 mL/min. It saturated above a flow rate of 200 mL/min. As expected, a lower flow rate could bring a longer contact time between the sample vapor and the selected catalyst, resulting in a sufficient reaction. But when the flow rate was too high, the reaction would be insufficient to keep the higher sensitivity of the CTL detection system. Finally, a flow rate of 200 mL/min was selected as an optimal one. Analytical Characteristics. Under the optimal conditions described above, the calibration curve of CTL intensity versus Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
1429
Figure 10. Effect of flow rate of carrier gas on CTL intensity at 225 °C. n-Hexane concentration, 3.88 µg/mL.
Figure 12. Influence of different carrier gases on the various catalyst. Temperature, 225 °C; flow rate, 200 mL /min; wavelength, 460 nm.
Figure 11. Calibration curve between CTL intensity and n-hexane concentration on CsNaY and CsNaX.
n-hexane concentration was linear in the range of 0.776-23.28 µg/mL with a detection limit (3σ) of 0.155 µg/mL (Figure 11). The linear regression equation is described by y ) 118.02x - 50.70 (R ) 0.997) on CsNaY, y ) 122.3x - 66.45 (R ) 0.998) on CsNaX, where y is the relative CTL intensity, x is the concentration of n-hexane, and R is the regression coefficient. Relative standard deviations (n ) 5) were 4.24, 4.91, and 5.05% for 3.88, 7.76, and 19.4 µg/mL n-hexane on CsNaY; and 4.72, 4.38, and 4.89% for 3.88, 7.76, and 19.4 µg/mL n-hexane on CsNaX, respectively. Lifetime of the Sensor. The deactivation observed for all the catalysts can be attributed to the deposition of coke, whose rate depended on the type of cation. The lifetime of the n-hexane sensor based on the modified microporous zeolite was examined. The CTL intensity was detected for intraday by continuously merging 7.76 µg/mL of n-hexane with the air carrier to pass through the sensor. No significant decrease of CTL intensity was observed during the detection. The relative CTL intensity of determination is 840.74 ( 24.4 on CsNaY, 880.43 ( 41.4 on CsNaX and the RSD is less than 5% (n ) 10). Note also that the heater was heated at 500 °C for 30 min in air to avoid the influence of previous residues at the beginning of interday experiments. The relative CTL intensity of determination is 825.74 ( 63.8 on CsNaY, 879.81 ( 71.2 on CsNaX, and the RSD is less than 10% (n ) 10). In contrast, the relative CTL intensity is 106.18 on NaY, 143.25 on NaX for intraday (n ) 10) with more than 20% RSD. The relative CTL intensity is 89.34 on NaY and 99.27 on NaX for interday (n ) 10) with more than 20% RSD. 1430 Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
Figure 13. GC chromatogram from the catalytic product of nhexane by different carrier gases. Capillary column OV-101 (30 m × 0.25 mm × 0.25 µm); constant temperature, 40 °C; N2 flow rate,1 mL /min; detection mode, FID. (a) 0.68, (b) 0.74, (c) 0.96, and (d) 1.05 min. (a), (b), and (d) represent the new products; (c) is n-hexane.
Mechanism of the CTL Reaction. In order to clarify the mechanism, other carrier gases such as N2, He, and CO2 were employed to replace for air, respectively. As shown in Figure 12, no signal could be detected at our optimal experimental conditions. The experiments indicated that the role of oxygen was necessary. However, when the product was detected by GC, a single peak only was obtained and identified as n-hexane by GC/MS. We speculated that possibly the concentration of the product was too low for detection by GC and GC/MS. Thus, the cold trapping was employed to enrich the products when the n-hexane was injected into the sensor continuously and the O2 was selected as carrier gas to avoid the interference caused by the air. The GC chromatogram was obtained and shown in Figure 13. The red spectrum stands for the GC chromatogram from the carrier gas O2 (only carrier gas O2, without n-hexane) collected in the cold trapping. The green spectrum stands for the blank test (carrier gas N2 and n-hexane). The black spectrum stands for the product gas from the CTL reaction (carrier gas O2 and n-hexane). Note that the peaks a, b, and d are the new products, and they are classified as ethanol, 1, 2-butylene, and 2,3- hexylene as shown in Figure 14.
Figure 14. GC/MS chromatogram from the catalytic products of n-hexane. Ethanol (1.52 min); 1,2-butene (1.63, 1.68 min); trans-, cis-2pentene (1.83, 1.92 min); background impurity (2.26 min); n-hexane (2.40 min); 2,3-hexylene (2.56, 2.68 min).
Based on the carboniogenesis theory, i.e., the zeolite framework stabilization of carbonium or carbenium ion-type transition states or intermediates is mainly of electrostatic short-range nature due to the charge screening effect of the zeolite framework,22 a widely accepted CL mechanism (the formation of an intermediate of a highly reactive endoperoxide23) was suggested as shown in Figure 15. The structure of type X,Y zeolite consists of a threedimensional network of molecular-size cages (“supercages”) (a). In the supercages, the collisional pairs of small molecules such as n-hexane and O2 can be formed in high concentrations. The first adsorption behavior has been proposed to proceed via formation of a structure, which resembles a carbonium ion strongly stabilized by the lattice of zeolite (b). The second reaction involves a C-H bond breaking possibly, with formation of a transition-state carbenium-like structure that becomes attached to the zeolitic framework by base medium. This new carbenium ion can actively extract oxygen from the carrier gas to give a fourmember-ring product. This unstable transition structure stays in contact with catalytic groups on the matrix surface (c). Then the four-member ring is broken, emitting strong luminescence (d). The following H radical rearrangement gave the main products, for example, ethanol, 1-butene, etc. (e-h). Although n-hexane has three kinds of C-H bonds, the bond energy of alkane decreases in the order of primary > secondary > tertiary. The terminal methyl group should have been the least reactive. The high selectivity for the 1-butene and ethanol formation agrees with the secondary C-H broken and farther thermal rearrangement results in the formation of the corresponding compound under elimination of H2O. Due to the large number of carbons in the n-hexane, the availability of many reactive C-H bonds has led to more complex mixtures of products. Increased reactor temperature increases unit conversion, primarily through a higher rate of reaction for the endothermic (22) (a) Boronat, M.; Viruela, P.; Corma, A. J. Phys. Chem. A 1998, 102, 98639868. (b) Sauer, J.; Sierka, M.; Haase, F. Transition State Modeling for Catalysis; Truhlar, D. G., Morokuma, K., Eds.; ACS Symp. Ser. 721; American Chemical Society: Washington, DC, 1999; Chapter 24. (c) Gourseot, A.; Arbuznikov, A.; Vasilyev, V. Density Functional Theory, a Bridge Between Chemistry and Physics; Geerlings, P., De Prooft, F., Langenaeker, W., Eds.; Vubpress: Brussels, 1999. (23) (a) Beck, S.; Koster, H. Anal. Chem. 1990, 62, 2258-2270. (b) Kricka, L. J. Anal. Chim. Acta 2003, 500, 279-286.
ring formation reaction. In addition, it also enhances the activity of locus of catalysis. The higher rate of ring cracking maybe result in the higher CTL intensity, but a too high temperature will cause a rapid desorption of the reactants from the surface of the zeolite and lead to a decrease of CTL intensity. It shows a similar reason that optimizes the flow rate of carrier gas to bring a sufficient number of the collisional pairs of small reactant molecules in the supercages and longer contact time to carry out the multistage reaction. The remaining fragment stays in contact with the catalyst in a relatively stable fashion. The radical pair is expected to undergo recombination in the supercages and then carbonize and deposit on the surface of zeolite under continuous heating at 225 °C. At the same time, we observed that the color of the modified zeolite becomes dark and the color change on the modified zeolite was slower than that on NaY and NaX. When we increased the temperature to 500 °C for 30 min, however, the dark zeolite changed to the original color again. We also observed that the CTL intensity was almost constant by continuously purging the same concentration of n-hexane to pass through the modified zeolite sensor, leading to a longer lifetime for the CsNaY- and CsNaX-based sensor. The general reaction profile for the sequence of reactions is suggested in Figure 15. CONCLUSION A strong CTL emission was generated by the catalytic oxidation of n-hexane on the surface of several molecular sieves in air. The microporous zeolite materials-based sensing mode utilizing the phenomenon of CTL reported in this paper shows high sensitivity and better linear response for the determination of n-hexane. This research establishes that the specific pore size of the zeolite offers a designable environment for selective CTL reaction, Cs, K incorporation could improve the catalytic activity of zeolites for these base-catalyzed reactions. The role of the Lewis-type base was found to be as important as the changes in micropore volume of cation-exchanged samples in deciding the catalyst activity in this CTL reaction. This CTL reaction may be attributed to a combination of three major processes: (i) the base medium and the ideal electrostatic field, which favors carbonium ion evolution to initiate reaction; (ii) molecular cages of a solid matrix offer a Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
1431
Figure 15. Possible reaction processes for zeolite-CTL of n-hexane.
natural environment for the formation of unstable transition structures that remains in contact with the catalyst as a complex; (iii) the energy emission-based breakdown on the ring structure was one of other possible luminescence mechanisms. An explanation of the reaction mechanism for the CTL of alkanes is not only of fundamental interest but could also help to optimize reaction conditions and to find better catalysts for the reaction. However, full details of the reaction mechanism are not yet known. In addition, the employment of new catalytic materials and the improvement of this kind of apparatus can be broadened on portable testing instruments and devices applied in biological manufactures, food, medicines, and environment. 1432 Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
ACKNOWLEDGMENT We gratefully acknowledge financial support from National Natural Science Foundation of China (20575014), the Program for New Century Excellent Talents in University, Shanghai Key Basic Research Program (05JC14010) and “Shu Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation.
Received for review September 26, 2006. Accepted November 29, 2006. AC061811+
