Capturing Nitrosamines by Zeolite A: Molecular Recognition in

Tobacco-specific nitrosamines (TSNA) and volatile nitrosamines are ..... to the characteristic bidentate nitrates,32 −CH2, and pyrrolidine,15 respec...
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J. Phys. Chem. C 2008, 112, 6740-6748

Capturing Nitrosamines by Zeolite A: Molecular Recognition in Subnanometer Space Jing Yang, Yu Zhou, Hong Ji Wang, Ting Ting Zhuang, Yi Cao, Zhi Yu Yun, Qing Yu, and Jian Hua Zhu* Key Laboratory of Mesoscopic Chemistry, College of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: December 20, 2007; In Final Form: February 21, 2008

The adsorption and catalytic degradation of nitrosamines by zeolite A with cesium, potassium, sodium, or calcium cation was systemically studied in the research reported in this article. The results of instantaneous adsorption, temperature-programmed surface reaction, and in situ Fourier transform infrared spectroscopy measurements proved the adsorption of the bulky nitrosamines, N′-nitrosonornicotine (NNN) and Nnitrosopyrrolidine, on the zeolite A with a pore diameter of only 0.2-0.5 nm. Moreover, thermogravimetricmass spectrometry analysis revealed the different adsorption and degradation manners of NNN on various samples of zeolite A, which will be discussed in terms of electrostatic attraction and geometric confinement provided by the zeolite.

Introduction Life science is a potential application of zeolite; for example, in slow release drugs, enzyme mimetic drugs, anti-tumor drugs, and so forth.1-3 Among these new efforts, a noteworthy example is the removal of carcinogens such as nitrosamines from cigarette smoke.4,5 Smoking is a global issue related to millions of cancer deaths; therefore, reducing the harm of smoking is of practical significance for public health. Often, it is necessary to remove the carcinogenic compounds in the airflow of an air recirculation system for cleaning the indoor air. However, there are more than 4000 currently identified compounds in cigarette smoke, and to selectively adsorb nitrosamines among them is a challenge for zeolites. Exploring the mechanism how zeolite adsorbs and locks nitrosamines is beneficial not only for design and synthesis of new multifunctional materials, but also for improved understanding of the unique selectivity of zeolite. Tobacco-specific nitrosamines (TSNA) and volatile nitrosamines are well-known tobacco carcinogens,6,7 and they are characterized with the nitroso group of N-NO. Among N′-nitrosonornicotine (NNN), N′-nitrosoanatabine (NAT), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), and N′-nitrosoanabasine (NAB),8 NNN was the first TSNA for proved tumorigenicity, and tumor induction was found to occur by way of 2′-hydroxylation of NNN.9 Volatile N-nitrosamines also have carcinogenic potential; for instance, the ethanol with 40 ppm N-nitrosopyrrolidine (NPYR) caused a 5.5-fold increase in lung tumor multiplicity.10 Zeolites are considered as the candidates because of their ability of selective adsorption. As expected, zeolites could efficiently trap the volatile nitrosamines in gas stream at ambient temperature even though the contact time was shorter than 0.1 s,11,12 in which the pore size was assumed to be the main factor governing the adsorption of zeolites and their isotherm could be fitted with the Freundlich equation. Modification of zeolites with copper oxide, cobalt oxide, or zirconia could enhance their adsorptive capability because the modifier strengthened the electrostatic attraction * Corresponding author. Tel: +86-25-83595848. Fax: +86-25-83317761. E-mail: [email protected].

Figure 1. Instantaneous adsorption of NPYR in gas stream by zeolite at (A) 338 K and (B) 453 K.

toward nitrosamines.13-17 Zeolites also adsorbed nitrosamines in aqueous solution and the adsorption isotherms could also be fitted with the Freundlich equation, too.18,19 Creation of mesopores in zeolites through alkaline treatment evidently promoted their adsorption in solution because of the enlarged accessibility

10.1021/jp7119729 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008

Capturing Nitrosamines by Zeolite A

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6741

Figure 2. FTIR spectra of (A) NPYR, (B) dichloromethane solution of NNN, and (C) dichloromethane alone adsorbed on zeolite at 423 K.

Figure 3. NOx desorption in the TPSR process of (A) NNN and (B) NPYR on zeolites.

of the adsorptive sites to nitrosamines.20 In addition, zeolites exhibited a considerable activity in decomposition of nitrosamines at elevated temperature, from the rupture of the N-NO bond in nitrosamine to form nitrogen oxides and amines.14,21-23 Introduction of metal oxide such as copper oxides or cobalt oxides in zeolite dramatically increased the catalytic activity of zeolite and more nitrosamines could thus be decomposed at relatively lower temperature.14,16,17 Because of the promotion of proton, acidic zeolites such as Hβ were able to catalyze the degradation of N-nitrosodiphenylamine (NDPA) in aqueous solutions even at room temperature.24 Apart from the laboratory experiments, zeolites were added to cigarettes to examine their actual function in reducing the nitrosamine levels in tobacco smoke, and some positive results were reported relating to the significant decrease of nitrosamines.4,5 Unlike the traditional application of zeolite in the petrochemical industry, however, reducing the nitrosamines in cigarette smoke is a challenge for the selective adsorption of zeolite because of the complex chemical environment. There are more than 400 compounds in the vapor phase of tobacco smoke and 3500 in the particle phase,25 and it is hoped that the zeolite candidate could possess the unusual ability of adsorbing nitrosamines among the various constituents. Moreover, the zeolite added to the tobacco rod will experience high temperature while the cigarette is being puffed when the hot zone approaches, and the zeolite can strongly adsorb the nitrosamines and catalytically decompose them instead of desorbing them at the relatively high temperature.21 Hence, both the pore structure and the electrostatic attraction of the zeolite candidate need to

be optimized to selectively adsorb nitrosamines in the complex chemical environment of the smoke. On the basis of the results reported previously involving the specific adsorption manner of nitrosamines in zeolite by inserting the N-NO group into the channel,11,12,14,15,22,26 the zeolite candidate should have a suitable pore size to perform the sieving effect and the strong electrostatic attraction to attract the nitrosamines. However, the cation plays the important role in zeolite because it provides the electrostatic affinity on the N-NO group of nitrosamines and occupies the space within the narrow channel of zeolite hindering the entering and diffusion of the adsorbate. Thus, understanding the impact of the cation on the adsorption of nitrosamines by zeolite is valuable not only for the selection of zeolite candidate, but also for the design and preparation of new functional zeolitic material in environment protection. Zeolite A was chosen for the study because it has the smallest micropore among common commercial zeolites, and its pore diameter can be adjusted through exchange with different alkali metal cations or alkaline-earth metal cations because of the highest cation exchange capacity27 that enables zeolite A to be utilized for the adsorption of radioactive elements such as cesium and iodine.28 Thus, it is valuable to explore the different adsorption capabilities of CsA, KA, NaA, and CaA for the bulky nitrosamines. The four samples are basic zeolites, and they have similar particle diameter and structure and pore size ranging from 0.2 to 0.5 nm; the only difference is the alkali metal ion. Two nitrosamines, NPYR and NNN, are chosen as the targets because they are the typical volatile nitrosamine and tobaccospecific nitrosamine, respectively.

6742 J. Phys. Chem. C, Vol. 112, No. 17, 2008 SCHEME 1: Simulation-Optimized Structures of (A) NPYR and (B) NNN

Experimental Section Reagents and Chemicals. Zeolites NaA, KA, and CaA were commercially available powders. Sample CsA was obtained from KA by ion exchange: 5-g samples of KA were stirred in 50 mL of 0.1 M solution of CsCl for 12 h at room temperature. Then the solids were filtrated, washed with deionized water, and dried at 413 K overnight. NPYR was bought from the Sigma Company. For the experiments of instantaneous adsorption and temperature-programmed surface reaction (TPSR), NPYR was mixed with dichloromethane in a volume ratio of 1:19. NNN was purchased from Toronto Research Chemicals Company. To prepare the solution used in FTIR and TG-MS tests, 7 mg of NNN was dissolved in 5 mL of CH2Cl2 solvent. Experimental Methods. Instantaneous adsorption of nitrosamines was carried out by gas chromatography (GC) method.12 A 5-mg sample (20-40 meshes) was filled in a stainless steel microreactor, with a 3-mm diameter and a 150-mm length, whose one end inserted deeply into the injector port of Varian 3380 GC and another end connected with the separation column (10% Carbowax 20 M + 5% KOH with a 3-mm diameter and a 3000-mm length) in the GC. The reactor was sealed by glass wool to fix the position where the temperature could be accurately controlled by the injector port of GC. The sample was directly heated to the given temperature without activation in the flow of carrier gas with a rate of 30 mL min-1. The solution of nitrosamine was pulse injected with the amount of 2 µL each time. Thermal conductivity detector of GC was used to analyze the gaseous effluent, and the decrement in the ratio of solute to solvent was utilized to calculate the adsorbed amount.14

Yang et al. A TPSR test13 was employed to assess the degradation of nitrosamines on zeolites. A certain mass of sample (20-40 meshes) was first activated at 773 K in the nitrogen gas flow with the rate of 30 mL min-1 for 2 h, and then cooled to 433 K to contact 100 µL of dichloromethane solution of NNN or NPYR. After the sample was purged to remove the physically adsorbed adsorbate, the temperature was raised to 773 K at the rate of 10 K min-1 while the nitrogen oxide products formed in the decomposition of nitrosamines were detected by a spectrophotometric method. The amount of NOx determined in the TPSR procedure represents the amount of nitrosamines decomposed on zeolite.21 To determine the amounts of nitrosamines adsorbed by zeolites in the TPSR test, the sample, which had contact with about 0.02 mmol NPYR at 433 K, was put into a test tube containing 10 mL of dichloromethane. Then 0.5 mL of HBr in glacial acetic acid was then added to chemically denitrosate the nitrosamines adsorbed by zeolite and liberate the denitrosated gaseous product. The formed gaseous product was purged in a N2 stream and purified through three consecutive traps containing 10 mL of 5 mol L-1 of NaOH solution and oxidized to NO2 by passing through a CrO3 oxidation tube. Finally, colorimetric method was utilized to detect the released NO2, which represents the total adsorbed NPYR in the zeolites samples.21 For the TG-MS experiment, 30-mg samples (20-40 mesh) were activated at 773 K in N2 flow with a rate of 30 mL min-1 for 2 h at first, and then contacted with 0.1 mL of NNN solution at 313 K. After the sample was transferred into the crucible of Netzsch STA449C TG/DSC-MS instrument, the physically adsorbed NNN was removed by carrier gas at 303 K for 0.2 h. After that, the TGA was operated from 303 to 773 K at the rate of 10 K min-1 with argon flow. Meanwhile, the released components from the sample were detected by the mass spectrometer. In situ FTIR investigation on the adsorption of nitrosamines on zeolite was performed in a purpose-built IR cell with CaF2 windows,26 and a Bruker 22 FTIR spectrometer was used for FTIR measurement. Typically, the spectrometer operated at 2 cm-1 resolution in a single-beam mode. For the IR measurements of the adsorbed species, the zeolite powder was pressed into a disc with an area density of 20 mg cm-2, then carefully put into the sample holder to be slowly heated to 773 K in N2 flow for activation for 2 h. After the sample was cooled to a given temperature to take a background spectrum, the adsorbate, 2 µL of NPYR or the solution containing 0.03 mg of NNN, was injected into the IR cell through an inlet held at 423 K and then purged with nitrogen flow for 20 min to remove the physical adsorbed nitrosamines before recording the FTIR spectrum. To exclude the overlap of infrared adsorption features that originate from the zeolite structural vibrations and the adsorbed surface species, the recorded spectrum was deducted from the background to give the FTIR difference spectrum of nitrosamines adsorbed on zeolite. Results Instantaneous Adsorption of NPYR in Gas Stream by Zeolite A. Figure 1A illustrates the instantaneous adsorption of NPYR by zeolite at 338 K. Among the four samples, NaA zeolite exhibited the highest ability to capture NPYR in gas stream and its adsorption capacity kept increasing in the experiment, whereas both KA and CsA presented the inferior capability and reached the adsorption equilibrium sooner. When 0.17 mmol g-1 of NPYR passed through the zeolite, NaA

Capturing Nitrosamines by Zeolite A

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TABLE 1: Freundlich and Langmuir Constants of the Adsorption Isotherms of NPYR in Zeolites Aa Freundlich equation T (K)

KF

n

R2

KL

qm (mmol/g)

R2

CsA

338 453 338 453 338 453 338 453

0.021 0.005 0.059 0.025 0.136 0.136 nc nc

1.22 1.42 3.91 4.47 2.15 2.15 nc nc

0.83 0.87 0.94 0.68 0.99 0.99 ncb nc

0.008 0.001 0.001 0.004 0.001 0.001 nc nc

11.4 11.7 113.9 28.4 288.8 288.8 nc nc

0.372 0.832 0.997 0.997 0.966 0.966 nc nc

KA NaA CaA a

Langmuir equation

adsorbent

Conditions: 1 atm; the rate of carrier gas was 30 mL min-1. b nc: cannot be calculated.

TABLE 2: TPSR Results of NNN or NPYR Adsorbed on Zeolites NNN sample

NOx/µmol g

NPYR

-1

Tmax/K

8.1 ( 0.8 3.7 ( 1 3.1 ( 1 2.2 ( 1

CsA KA NaA CaA

493, 693 533 593 553

TABLE 3: Determination of NPYR Adsorbed on Zeolite and Relative Materials

sample

by HBr and photometric method (µmol g-1, A)

by TPSR method (µmol g-1, B)

A/B

CsA NaA CaA NaZSM-516 NaY16 SBA-15

0.84 0.72 1.48 28.5 577.6 236.0

4.2 ( 1 3.7 ( 1 6.0 ( 0.6 24.0 ( 1 265.4 ( 1 15.4 ( 1

0.20 0.19 0.25 1.19 2.18 15.3

adsorbed 11%, KA trapped 6%, CsA captured 0.6%, but CaA was inactive. As the accumulated amount of NPYR reached 0.6 mmol g-1, the proportions adsorbed by zeolites NaA, KA, and CsA were 16.8, 8.4, and 1.7%, respectively, but CaA was still inactive. Elevating the adsorption temperature from 338 to 453 K had no impact on the behavior of NaA and CaA but suppressed the adsorption of KA and CsA (Figure 1B). To qualitatively assess the adsorption of zeolite, their isotherms were fitted with Freundlich and Langmuir equations. As shown in Table 1, most of the adsorption isotherms were more suitable to be fitted to the Freundlich equation and the exponent of 1 < n < 10 indicated the favorable nature of the adsorbent NPYR system. The smallest value of n, meaning a strong affinity existed between zeolites and NPYR, was observed on the CsA sample. In case the temperature rose to 453 K, the isotherm of KA seemed suitable to be fitted to the Langmuir equation and the qm value indicated the amount of NPYR to form a monolayer over adsorbents. Judging from the variation of the Freundlich constant KF that relates to the adsorbent capacity,29 it is clear that the pore size of zeolite A governs the adsorption of nitrosamines. As the pore size shrinks to below 0.3 nm, the confinement effect is significant; hence, the difference between KA and CsA in the adsorption was larger than that between NaA and KA. Zeolite CaA is the exception failing to adsorb NPYR (Figure 1) because of the competitive adsorption of solvent.22 The contact time between NPYR and zeolite was less than 0.1 s in the instantaneous adsorption experiment;12 hence, the competition of solvent badly restrained the adsorption of NPYR. FTIR Study on Adsorption of Nitrosamines on Zeolite A. Figure 2 delineates the FTIR difference spectra of NPYR and NNN adsorbed on zeolites CsA, KA, NaA, and CaA. Figure 2A depicts the adsorption of NPYR alone on the zeolite at 423

NOx/µmol g 4.2 ( 1 4.3 ( 1 3.7 ( 1 6.0 ( 0.6

-1

Tmax/K 613 533 553 513

K. Although these zeolites had a small pore size in the range of 0.2-0.5 nm, the IR bands of adsorbed NPYR still appeared in spectra and some seemed similar to the standard spectrum. The bands at 2982 and 2889 cm-1 were attributed to the C-H vibration, the band of 2202 cm-1 was due to the isolated nitrosonium ion, and the 2140 cm-1 band was presumed to be either from NO2+ or the N2O4 species located on zeolite A.14 The 1666 and 1590 cm-1 bands could be assigned to the adsorbed NO2-,30 other bands at 1457, 1418, and 1319 cm-1 originated from the ν3(NO2), the compound with nitryl group,31 and C-N vibration. As shown in Figure 2A, both 2202 and 2140 cm-1 bands were absent on the spectrum of CsA but appeared on that of KA. Moreover, the intensity of IR bands increased as the pore size of adsorbent rose, indicating the improved adsorption of NPYR by the zeolite. A different situation was observed in Figure 2B where the adsorbate is changed as the dichloromethane solution of NNN. The bulky nitrosamine could be captured by zeolites CsA, KA, and NaA, but CaA failed. Zeolite CaA has a relatively larger pore size, 0.5 nm, which should be beneficial to adsorb NNN. However, the bands of NNN adsorbed in CaA were rather faint, and only two very weak bands emerged around 1558 and 1453 cm-1 (Figure 2B). One reason for this phenomenon is the strong competitive adsorption of solvent in CaA (Figure 1C). Adsorption of NNN in zeolite NaA is special because the pore size of NaA is 0.4 nm, close to the five-member ring of NNN (0.42 nm, Scheme 1). In the case of adsorbing NPYR, NaA exhibits a good capacity (Figure 1). However, a pyridine ring linked to the five-member ring in the structure of NNN (Scheme 1); hence, the movement of NNN toward the inner channel was thus hindered.22 Consequently, the molecule of NNN was clipped at the pore mouth of zeolite and thus easily divided. As a result, the characteristic IR bands of NNN disappeared on the spectrum of NaA. Meanwhile, some nitrogenous compounds along with hydrocarbons appeared at 423 K (Figure 2B), giving the bands of 1566, 1456, and 1386 cm-1 that were assigned to the characteristic bidentate nitrates,32 -CH2, and pyrrolidine,15 respectively. Degradation of Nitrosamines on Zeolite A in the Thermal Process. Figure 3 demonstrates the degradation of NPYR or NNN trapped in zeolite A during the TPSR experiment. In the case of NNN (Figure 3A), CsA exhibited the best activity to degrade the bulky nitrosamines. Though the pore size of CsA was 0.2 nm, 8.0 µmol g-1 of nitrogen oxides was detected. There

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Figure 4. MS signals of NPYR, PYR, and pyridine detected from the degradation of NNN on zeolites.

were two desorption peaks of NOx in the profile: one appeared near 493 K and another emerged around 693 K. Zeolite KA showed an activity similar to that of NaA to decompose NNN, forming the NOx products (5.0 µmol g-1) similar to those of NaA (4.6 µmol g-1). The climaxes of NOx desorption on KA and NaA appeared around 613 and 573 K, respectively. The least amount of nitrogen oxides, 2.9 µmol g-1, was collected on a CaA zeolite accompanied with the lowest peak temperature of 513 K that indicates the weak adsorption of NNN on a CaA zeolite. For the four samples of zeolite A with the same pore structure but different cations, their activity to decompose NNN was in the inverse order with their pore size, and CsA showed the highest activity in the TPSR experiments. When the reactant of the TPSR test was changed from NNN to the volatile nitrosamine NPYR, the amount of NOx products detected on zeolite CaA increased while CsA exhibited a relatively weak activity similar to that of KA (Table 2), and this variation was confirmed by repeated experiments. However, the climax temperature of NOx desorbed from CsA zeolite was still the highest (613 K), while that from CaA was the lowest (513 K). For KA and NaA, their climaxes of adsorption were 533 and 553 K, respectively. To determine how many nitrosamines were actually adsorbed by the zeolite in the TPSR test, the sample contacted with NPYR at 433 K was treated by HBr in glacial acetic acid to chemically denitrosate the adsorbed nitrosamines and to liberate the gaseous product NOx. However, the amount of NPYR trapped by CsA, NaA, and CaA zeolites, as detected by the HBr denitrosation method, was much less than that detected by the TPSR method as demonstrated in Table

3. In general, the former value represents the NPYR adsorbed by zeolite, whereas the latter indicates the NPYR decomposed; no doubt the former should be larger than or equal to the latter since not all of the nitrosamines could be degraded or decomposed as the temperature increased and some part of them would be covered by coke and thus located in the channels, as that proved in zeolite NaZSM-5 and NaY.16 However, opposite results were found on zeolite A where the amount of NPYR detected by HBr denitrosation was about one-fourth of that by the TPSR method. To confirm this observation was not caused by the experimental error, mesoporous silica SBA-15 was employed for the test, and it exhibited a different trend in which the NPYR adsorbed by SBA-15 was 10 times more than that decomposed in the TPSR procedure (Table 3). TG-MS Investigation of the Degradation of NNN in Zeolite A. Figures 4-6 present the MS signals of some compounds detected in gaseous phase during the TPSR procedure of NNN on zeolites and the intensity of the compound released from different samples. They are compared because each compound (ion) detected in the mass spectrometer has its own response factor.33 As is evident from these figures, the bulky nitrosamine was trapped and then degraded by all of the zeolites in the TPSR experiment to produce various fragments, and the rupture manner of NNN on zeolite NaA differed from that on the other zeolites. The first, the fragments of NPYR and pyridine as well as PYR, was detected only on zeolite NaA (Figure 4B) but absent on the other three analogues. This originates from the special rupture manner of NNN on NaA sample determined by the special pore structure of the zeolite

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Figure 5. MS signals of (A) N2, (B) NO, (C) O2, and (D) N2O detected from the degradation of NNN on zeolites.

Figure 6. MS signals of NO2 detected in the degradation of NNN from zeolites.

as aforementioned.22 Part of the five-member ring entered the channel together with the nitroso group, while the rest of the NNN molecule was clipped on the pore mouth because of the C-C bond between the five-member ring and the pyridine ring (Scheme 1). Because of the electrostatic interaction from the cations inside the channel of NaA, the clipped NNN molecule would be divided into two parts in the joint bond as the temperature rose,22 consequently forming the fragments such as NPYR and pyridine. The second, Figures 5 and 6 show the MS signal of other products, was detected on N2 (m/e ) 28), NO (m/e ) 30), O2 (m/e ) 32), N2O (m/e ) 44), and NO2 (m/e ) 46) in the degradation of NNN that was adsorbed by zeolites. However, the MS signal intensity of NO from the NaA sample

kept decreasing with only a small peak near 475 K, while an obvious peak emerged on the profile of the other three zeolites (Figure 5B). A similar situation was also observed in the profiles of other products. There was no obvious peak in the MS signals of N2, O2, N2O, and NO2 detected from NaA zeolite (Figures 5 and 6), which was different from that observed on the other samples. All of these phenomena confirm the different rupture manner of NNN in zeolite NaA. Degradation of NNN adsorbed on CaA began by the rupture of the N-N bond because the N-NO bond was the weakest one in the structure of nitrosamine;34 hence, the MS signal of NO around 473 K was relatively strong (Figure 5B). Another MS signal of N2O (370 and 494 K) and NO2 (369K) was also found on CaA. NO2 was produced in the decomposition of nitrosamines mainly through two ways. Heterolytic cleavage of N-NO bond in nitrosamine molecule could produce NO+ then formed NO2 in succedent complex reactions.11,35 NO could also form N2O and NO2 in zeolite through disproportionation under the catalysis of metal cation: 3NO f N2O + NO2.14,28 NO was the primary product that emerged on the MS spectrum of KA at 428 K, while on zeolite CsA the NNN degraded to form the products N2 (487 K) and NO2 (324 K). Discussion Scheme 1 delineates the structure of NPYR and NNN. The structure of NNN looks like one of the H atoms in the fivemember ring of NPYR is replaced by pyridine, consequently the transect size of NNN (0.80 nm) is larger than that of NPYR (0.42 nm). Both NNN and NPYR have the functional group of N-NO in which the oxygen atom possesses the negative charge.

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SCHEME 2: Model of Adsorption Angle for Nitrosamines

TABLE 4: Adsorption Angle of N-NO Group of Nitrosamines in the Channel of Zeolite R (nm) 0.18a/R θ (deg) a

Owing to this feature, nitrosamines will be attracted by zeolite when they are close to the adsorbent because the N-NO group is strongly pulled by the electrostatic interaction from the cations inside the channel of zeolite.12,14,15 However, the large size of the nitrosamines limits their penetration into the relatively narrow channel of zeolite; hence, the matching degree between the molecular structure of NNN or NPYR and the pore size of zeolite determines the adsorption rate and the quantity. At first glance, the adsorption of NNN should be more difficult than that of NPYR by the same zeolite because of its larger size; nonetheless, the volatility of NPYR makes desorption easier. It is necessary for zeolite adsorbent to have strong electrostatic attraction, while the pore diameter of zeolite plays a duplex role for the adsorption. Narrow pores enable zeolites to have the sieve effect but limit the entering and succedent diffusion of nitrosamines into the channel. For zeolite A, the type of cation determines the pore size and the electrostatic interaction to nitrosamines; namely, the cation of zeolite A controls the adsorption. Three factors affect the adsorption of nitrosamines by zeolite A: the pore diameter, the attraction provided by cations, and the surface area related to the number of pores. It can be explained tentatively by the following equation:

N ∝ SθP

(1)

where S is the surface area that depends on the density of the sample. For zeolite A, the larger the surface area of the sample, the more pores the sample can provide. In general, the hydrated zeolite A possesses a chemical composition of Na95(H2O)39[Al96Si96O384],36 and the dehydrated zeolite A is Na91.7[Si96Al96O384].37 As the cation changed from Na+ (M ) 22.98) to Cs+ (M ) 132.9), the mole weight of the zeolite, which consisted of 6.02 × 1023 crystal cells, was changed from 14 311 to 24 761 g in hydrated form and from 13 533 to 23 620 g in the dehydrated form. That is to say, the density of CsA is larger than that of NaA by about 73 or 74.5% in the hydrated or dehydrated form. In other words, in the case taking 1 g of hydrated sample for adsorption, the number of crystal cells to be contained in the sample of CsA is about 58% of that of NaA. Consequently, the number of pores opening in the CsA sample is about 40% less than that of the NaA zeolite. In the dehydrated form, the value is about 42%. Likewise, the number of pores in KA is about 9.6% less than that of the NaA zeolite, in either hydrated or dehydrated forms; the adsorption difference between CsA and NaA was larger than that between NaA and KA (Figure 1). The difference between the density of CaA and NaA is negligible; the former possesses 2% more pore number than the latter. Although this is just a theoretical calculation, the measured surface area of KA (388 m2 g-1) was in fact 15% smaller than that of NaA (458 m2 g-1), and the CaA sample possessed a surface area (471 m2 g-1) similar to that of NaA.38

CaA

NaA

KA

CsA

0.5 0.36 68.9

0.4 0.45 63.3

0.3 0.6 53.1

0.2 0.9 25.8

The minimum length ensured N-NO inserting into the channel.

Thus, the difference in the surface area and the pore number of per gram sample among CsA, NaA, KA, and CaA should be considered for evaluating their adsorption and catalytic function. The second parameter in eq 1 is θ, the angle of adsorption (Scheme 2) that involves the pore size of the zeolite. Because of the small pore size of zeolite A, NPYR or NNN could not entirely enter the channel but partially penetrated the opening of the pore. For KA and CsA zeolites, NPYR or NNN probably only inserted the N-NO group into the channel, whereas it might penetrate the pore of NaA and CaA samples with the N-NO group plus five-member ring. Owing to the attraction of the cation inside the channel of the zeolite and the hindrance of the pore wall, the nitrosamine molecule tended to seek a suitable angle as demonstrated in Scheme 2 for penetrating the N-NO group in the narrow channel of zeolite, and clearly the range of the adsorption angle was proportional to the pore size of zeolite. For the zeolite CsA with a pore size of about 0.2 nm, it was 0-52° while on NaA it could be 0-72°. That is to say, both NNN and NPYR had more possibility to enter their N-NO group inside the channel of NaA than CsA in case they moved close to the zeolite, which seemed crucial especially in the instantaneous adsorption experiment where the contact time was limited within 0.1 s and NaA zeolite captured more nitrosamines than CsA indeed (Table 4). One may worry that the relatively larger pore of adsorbent will permit the nitrosamines to desorb easier. Nonetheless, the strong interaction between zeolite A and nitrosamines, as discussed later, prevents the escape of nitrosamines from the channel. The third parameter in eq 1 is P, the positive electrostatic attraction toward the N-NO group of nitrosamines that relates to the electronegativity and the radius of metal cation in zeolite. This electrostatic force, which is provided by the cation of zeolite and pulls the N-NO group of nitrosamines toward the channel,14 is proportional to the ionic charge but inverse proportional to the electronegativity of the cation, promoting the collision between zeolite and nitrosamines. For the Cs+, K+, Na+, and Ca2+ cation in zeolite, the electrostatic force increases in sequence so that their adsorption probability for nitrosamines should be on the order of CaA > NaA > KA > CsA, as demonstrated in the FTIR spectra of NPYR adsorption. Nonetheless, some conflicting results of CaA zeolite are observed in the instantaneous adsorption of NPYR along with the FTIR measurement of NNN with CH2Cl2 solvent, in which the actual sequence is NaA > KA > CsA > CaA. Two reasons cause this confliction. One is the competitive adsorption of solvent in CaA zeolite as shown in Figure 2C, which hinders the instantaneous adsorption of NNN in CaA. Another involves the distribution of Ca2+ cation in zeolite A where no Ca2+ cation is located in the window of an eight-member ring (Scheme 3), which makes the electrostatic force of CaA weaker than other analogues; hence, it cannot capture nitrosamine efficiently in gas stream. For other static adsorption with longer contact time such as the liquid adsorption, however, nitrosamines could be attracted by the Ca2+ cation located in the six-member ring. Therefore, CaA zeolite exhibited a considerable capability.24 Different TPSR results of NNN and NPYR by zeolite, as aforementioned, reveal the influence of electrostatic interaction

Capturing Nitrosamines by Zeolite A

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6747

SCHEME 3: Possible Adsorption Manner of NNN in Zeolite A

on the decomposition of nitrosamines. Different from NPYR, NNN has one pyridine with a π electron conjugated system (Scheme 1). Such a π electron conjugated system in the structure of nitrosamines will lower the negative charge of the N-NO functional group, weakening the electrostatic interaction of the N-NO group with the cation of zeolite, and yet the N-NO group becomes easier to be broken.34 Consequently, relatively more NNN was trapped and decomposed by the CsA zeolite because the sequence of electronegative property is Cs+ < K+ < Na+ < Ca2+, and the CsA zeolite had the smallest pore size that provided a relatively higher confinement to hold the penetrated N-NO group of NNN; therefore, the N-NO bond would be more easily ruptured because of the thermal movement of the rest of NNN outside the channel at the elevated temperature. Thus, the degradation ability of CsA for NNN was the highest among the samples assessed. Table 3 delineates the unusual adsorptive property of zeolite NaA and its alkali metal cation-exchanged analogues toward nitrosamines. Because of the limitation of the small pore, it is impossible for NNN or NPYR to enter the channel of zeolite A.16 Subsequently, the nitrosamines will adsorb into the zeolite in the way through inserting the N-NO group inside the pore, while the rest of the molecule retains on the external surface of the zeolite.15 Compared with common zeolites such as ZSM-5 or Y, however, zeolite A has the smallest pore size and the largest density of cation to provide the confinement and attraction to the carcinogen. Thus, it is difficult for the nitrosamines to desorb from the zeolite. Contrarily, degradation

of NNN or NPYR happened at the elevated temperature in the TPSR test. Moreover, we found for the first time that the HBr denitrosation method fails to determine the total amount of nitrosamines adsorbed by CsA, NaA, or CaA zeolites, probably because of two reasons. First, the adsorbed NNN or NPYR was held by the zeolite very tightly to block the pore mouth (Scheme 3); hence, HBr could not contact with the N-NO group to denitrosate it. In fact, the narrow pore of KA and CsA was easily plugged by the N atom in the N-NO group (Scheme 1); therefore, the NNN trapped by KA or CsA zeolite kept its integrality (Figure 2B) most of whose bands were close to the standard spectrum of NNN.22 Second, the adsorbed nitrosamines degraded to form nitrate, and nitrate cannot be detected by the HBr denitrosation method21 but decomposed at elevated temperature to release NOx. As was evidenced in Figure 2B, the NNN adsorbed in zeolites NaA began to degrade, forming nitrosonium ion and NO2+ or N2O4 species. CaA zeolite could catalyze degradation of NDPA at room temperature.24 It should be more active at elevated temperature such as 433 K to promote the decomposition of nitrosamines. Different degradation manners of NNN on various zeolite A samples with different pore sizes (Figures 4-6) indicate the delicate impact of zeolite structure on the decomposition of nitrosamines. The NNN molecule could partially enter the channel of zeolite CaA, and it would be degraded from the rupture of the N-N bond in the nitroso group,34 producing nitrogen oxides and the residual carbon-containing parts.22 For zeolite KA and CsA with smaller pores, only the nitroso group

6748 J. Phys. Chem. C, Vol. 112, No. 17, 2008 of NNN inserted into the channel so that the degradation of NNN still began from cleavage of N-NO band. Zeolite NaA possessed a pore size that clips NNN in the pore mouth,22 enabling NNN to decompose through the rupture of the C-C bond linking the five-member ring and pyridine ring. Existence of NPYR and pyridine fragments in the MS spectrum of NNN on NaA zeolite (Figure 4B) confirmed the specific degradation manner of nitrosamines. The high cation density in the zeolite helps to anchor the nitrosamines through strong interaction. Otherwise, the adsorbate will desorb at increased temperature as that on mesoporous silica MCM-41.39 Because of the combination of narrow pore and strong interaction as well as the large surface area where many pores locate, zeolite A can efficiently capture and destroy nitrosamines. Conclusions Some conclusive remarks can be derived from these experimental results. (1) Zeolites CsA, KA, and NaA could capture NPYR in the instantaneous adsorption experiment at 338 or 453 K, and their adsorption capacity depended on their pore diameter. NaA zeolite presented the highest adsorption capacity among three samples, whereas CaA failed to selectively adsorb NPYR because of the competitive adsorption of dichloromethane solvent. (2) Either NPYR or NNN could be degraded by zeolites CsA, KA, NaA, and CaA in the TPSR procedure, forming nitrogen oxides and other residual fragments. However, the decomposition manner of NNN on NaA zeolite differed from the other three samples because of the impact of the zeolite structure on the reaction, starting from the rupture of the C-C bond linking the five-member ring and pyridine ring. (3) With small pore size and plenty of cations inside the channel, zeolite A could strongly adsorb nitrosamines; hence, the adsorbed NNN or NPYR could not be removed by the HBr in acetic acid. Acknowledgment. Financial support from NHTRDP973 (2007CB613301), NSF of China (20773601 and 20673053), Grant 715-04-0120 and 2008AA06Z327 from the 863 Program of the Ministry of Science and Technology of China, the Scientific Research Foundation of Graduate School of Nanjing University, and Analysis Center of Nanjing University is gratefully acknowledged. References and Notes (1) Dyer, A.; Morgan, S.; Wells, P.; Williams, C. J. J. Helminthol. 2000, 74, 137. (2) Mitchell, P. C. H. Chem. Ind. 1991, 308. (3) Weiner, H. C. Immunol. Today 1997, 18, 335. (4) Meier, M. W.; Siegmann, K. Microporous Mesoporous Mater. 1999, 33, 307.

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