Generating selective adsorptive sites on activated carbon

Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) 9 2005 ElsevierB.V. All rights reserved

595

Generating selective adsorptive sites on activated carbon Y. Cao, L.Y. Shi, C.F. Zhou, T.T. Zhuang, Y. Wang and J.H. Zhu* Department of Chemistry, Nanjing University, Nanjing 210093, China. E-mail: [email protected], cn

Micro-control of activated carbon surface by metal oxides was performed, in order to eliminate nitrosamines in environment, especially in cigarette smoke for the first time. Coating alumina and zinc oxide can promote the adsorption of nitrosamines in activated carbon, opening new avenues to generating selective adsorption sites on amorphous material through the modification on the polarity of surface.

1. INTRODUCTION Developing new adsorptive materials is necessary for environment protection, because it is crucial to separate pollutants from environment and thus selective adsorbents are desirable which, as well known, usually consist of molecular sieves such as zeolites with inherent unique selective adsorption features. However, there are many fields where common adsorbents are used and it seems a long way, if not impossible, to replace them by the selective adsorbents due to economic or technical reasons. For example, activated carbons (AC) with their high specific surface area, well-developed transitional porosities, and various oxygen-containing complexes [1], are widely used in cigarette filter though it cannot selectively adsorb carcinogenic component such as nitrosamines [2]. To reduce the smoking risk on health, it is important to modify the AC filters and generate the selective adsorptive sites on them. Recently, "insert adsorption model" of nitrosamines is reported [2-4] and incorporation of copper ion thus adopted to promote the adsorption of nitrosamines on zeolite [3];' through these reports an important feature of nitrosamines is revealed, the - N - N - O functional group in the carcinogenic compounds easily interacts with the metal ions located within the channel of zeolite, which fastens the adsorption of nitrosamines in zeolites. Consulting this discovery, generation of selective adsorptive sites on AC seems feasible provided some metal oxides are planted into the pore of the porous material, and here we introduce the latest progresses on this subject. Apart from oxidation treatment of AC to increase its mesoporous volume and adsorption capacity [5], chemical deposition of inorganic oxides was effective to alter the surface polarity of AC, on which silica and titania formed both hydrophilic and hydrophobic patches [6, 7], loading MnO2 enhanced the adsorption capacity of water vapour on AC [8]. In addition, impregnation of Mg(NO3)2 and Ba(NO3)2 [9] could change the surface charges of AC to promote its adsorption of polar or non-polar materials. Here alumina and zinc oxide were chosen, along with copper ion and hydrotalcite, to decorate AC through impregnation. These resulting composites were assessed in gaseous phase adsorption of volatile nitrosamine Nnitrosopyrrolidine (NPYR) or liquid adsorption of tobacco specific nitrosamines N-

596 nitrosonornicotine (NNN), some of them were also tested as the filter component in mainstream smoke of Virginia type cigarette. 2. EXPERIMENTAL

Commercial activated carbon (AC), with a surface area of 921 m2g-I and average pore size of 1.6 nm, was produced by Liyang Factory (China). AI(NO3)3 "9H20, Zn(Ac)2"2H20 and other reagents used here were all AR grade while NPYR and NNN obtained from Sigma, and dissolved in dichloromethane at the volume ratio of l:19. Chinese Virginia type cigarette, with a tar value of 15 mg cigarette -I and a nicotine value of 1.2 mg cigarette -~ were purchased from the market. Raw AC (20-40 mesh) was stirred in water at 313 K for 2h and dried at 353 K then calcined in nitrogen at 773 K for 3h prior to modification. To prepare 2 g sample, 0.539g Zn(Ac)2-2H20 and a calculated amount of AI(NO3) 3 -9H20 were put into 50 ml distilled water, followed by addition of pretreated AC and stirring at 313 K overnight. After evaporation of water at 353 K, resulting material was calcined in nitrogen at 773 K for 3h to get the modified AC sample denoted as xMC where x represents the content of alumina, for instance 1 or 10 (mass percentages) while the decorated amount of zinc oxide fixed to 1 wt.-%. In a similar way the copper oxide or hydrotalcite modified sample was prepared. XRD patterns were recorded on a Shimadzu XD-3A X-ray Diffractometer with Cu Ks radiation in the 2 Theta range from 5 to 80 degrees. Nitrogen adsorption and desorption isotherms of sample at 77 K were measured using a Micromeritics ASAP 2000 system, and sample was evacuated for 10 h at 573 K [ l 0]. BET specific surface area was calculated using adsorption data in the relative pressure range from 0.04 to 0.2, while the total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. The mesoporous pore size distribution (PSDs) curves were calculated from the analysis of the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) algorithm. Adsorption of NPYR at 453 K was performed in a fixed-bed micro-reactor filled with 5 mg of sample (20-40 meshes) in the manner reported previously [3], and dichloromethane solution of nitrosamines pulse injected with 2 lal each time. Gaseous effluent was analyzed by an on-line Varian 3380 gas chromatograph (GC), the decrease in NPYR-solvent ratio was utilized to calculate the amount of NPYR adsorbed. Liquid adsorption of NNN was executed at 277 K [4]. 20 mg NNN was dissolved in 1 ml dichloromethane and 50 mg sample (20-40 mesh) added in solution. After adsorption of 24h, residual NNN in solution was analyzed by improved spectrophotometric method [2]. To evaluate the efficiency of modified AC sample to eliminate nitrosamines in cigarette smoke, 20 mg 5MC sample, in 20-40 meshes, were carefully added into the filter to replace part of cellulose matrix with a same volume. 20 cigarettes were smoked 1MC in the glass-made chamber designed by Miyake [ 11], and mainstream was pulled through 100 ml citrate-phosphate buffer containing 0.02 mole of ascorbic acid to absorb nitrosamines. Then the 2o 6o solution was extracted with dichloromethane and 2 Theta ! degrees the nitrosamines content were determined by Figure 1. XRD patterns of modified samples. spectrophotometric technique [2, 3].

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Figure 2. (A) N2 adsorb-desorption isotherms and (B) BJH mesopore size distribution of samples. Table 1 Physicochemical parameters of modified activated carbon samples

...................

SBET

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Sample

/ (mZg-1)

/ (m2 g-l)

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921 865 822 754 642

381 378 352 393 401

0.499 0.466 0.438 0.395 0.326

0.173 0.172 0.160 0.182 0.187

1.57 1.57 1.54 1.53 1.49

3. RESULTS AND DISCUSSION 3.1. Surface modification of activated carbon Figure 1 plots XRD patterns of the samples modified with alumina and zinc oxide, revealing the impact of decoration on the structure of AC. All patterns are characterized with two broad peaks centered at 20 of 25 ~ and 43 ~ which can be assigned to nano-sized carbon [12]. A sharp line superimposed on the peak near 25 ~ was proved to be silica that was perhaps an unavoidable impurity in AC [ 13] and survived in pretreatment. Loading alumina and zinc oxide did not cause any new phase on XRD spectra as seen in Fig. 1, even though the loading amount of alumina reached 10 wt.-%, and the color of sample still kept black. Identical XRD patterns of modified sample to the parent indicate high dispersion of metal oxides on the porous support, due to the high specific surface area of AC. It is very likely that most of the dispersed guest species exist as amorphous materials or nano-particles. Otherwise, amount of 10 wt.-% was enough to form XRD-visible new crystallites phase. Through careful comparison of the patterns of modified samples with the original one, it appeared that the intensity of impurity silica was decreased during the modification, though no regularity was observed relate to the loading amount of metal oxides. That is, the interaction between impurity silica and added metal species occurs indeed, probably during the calcination procedure. Although available data do not allow a discussion on the dispersion state in detail, it is safe to infer that all of the guests equably locate over the surface of AC, but no evidence can confirm or exclude the formation of monolayer with nanometer crystallites yet, and further study is thus desirable. Figure 2A presents influence of modification on the nitrogen adsorption-desorption

598 isotherms of AC at 77 K. Parent AC possessed an isotherm with the shape close to type IV but a hysteresis loop occurred at the relative pressure about 0.45, reflecting the existence of microporous and mesoporous pores mixed in this material. Modification with metal oxides did not change the shape of isotherm, but both the nitrogen adsorption volume and the amplitude of hysteresis loop were lowered. The more oxides loaded, the more obvious the change would. All isotherms of modified sample were type IV, in which the adsorption and desorption branches of AC looked nearly horizontal and parallel over a long-range relative pressure, which was usual the characteristic behavior of microporous materials. At the same time, the isotherms became more and more smooth as the loading amount raised, which means the reduced pore diameter of the composite due to decoration of high-dispersed alumina and zinc oxide on the inner wall of AC pores. Besides, declined hysteresis loops in nitrogen adsorption-desorption isotherm indicated decrement of mesopores in the composite. Table 1 lists the particular physicochemical properties of AC before and after modification. Increasing amount of modifier from 1 wt-% to 10 wt-% made the BET specific surface area and total pore volumes of sample decreased simultaneously. For the sample decorated with alumina of 1%, 3%, 5% and 10% (mass percentage), their surface area decreased 6.1%, 10.7%, 18.1%, and 30.3%, respectively. It is clear that the loss of surface area by adding the first 1 wt.-% modifier on AC is significantly larger than that caused by further adding more modifier, if the loss was calculated according to per wt.-% of modifier. Nonetheless, micropore area of sample did not exhibit same trend; it kept constant in 1MC and decreased in 3MC but increased in 5MC or 10MC, even larger than parent AC. This phenomenon suggests, in our opinion, that a variation in modifier distribution state may occur when the coated alumina species accumulated to a threshold, the metal oxide nano particles coated on the inner wall of micropores or mesopores in AC probably form some new structures that are analogous to micropore. According to the data listed in Table l, for the alumina modified AC the threshold may rely in about 4 wt.-%. Apparently the increase of micropore area in 5MC and 10MC samples induced consentaneous enhancement of micropore volume, which would be beneficial for adsorption appropriate molecule, especial for those volatile nitrosamines with a relative smaller molecular volume. Figure 2B illustrates the pore size distribution of sample; the similar shape of curves means that no destruction of pore wall occurs during modification of sample. Nonetheless, the pore volume between the pore diameters at 2 to 3 nm decreased clearly as the amount of modifier increased, indicating that the modification of nano particles of metal oxides is not limited to the microporous pore below 1 nm but rather interspersed the pore with 2-3 nm sizes. Decoration of these metal oxides crystallites on inner wall of micropores in AC is expected to generate some selective adsorption sites by changing the surface polarity. To explore the impact of surface acidic-basicity on adsorption property of AC, hydrotalcite (HT) was decorated on the porous support. It is amusing to find that the modified AC exhibits an enhanced adsorption capability in both CO2-TPD and NH3-TPD experiments, as seen in Fig. 3. Appearance of new CO2 desorption near 450 K and 800 K (Fig. 3A) and new NH3 desorption around 480 K and 800 K (Fig. 3B) meant that some new sites formed on AC to strongly adsorb these probes. One may argue that variety of TPD profiles on modified samples cannot simply be assigned to the changes in surface acidic-basicity because variation in pore structure of AC itself such as narrowing pore size may hinder the desorption of probe molecules due to geometric effect, this argument however, is not justified by comparing the profiles of 10%HT/AC and 20%HT/AC. No dramatic difference is observed in two spectra although the amount of modifier on the former is only half of that on the latter. So it is safe to infer that the composition along with the morphology of AC surface has been changed more or less by coating the basic guest, like that modified by alumina and zinc oxides.

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Figure 4. Selective adsorption of NPYR on modified activated carbons at 453K. 3.2. Selective adsorption of NPYR in gas phase Figure 4 depicts adsorption of NPYR at 453 K. All modified samples except 1MC showed better adsorptive ability for NPYR than parent AC, and 10MC was the best one. When the accumulative amount of NPYR reached 2.8 mmol g-l, about half could be adsorbed on AC while 67% or 61% were trapped by 3MC or 5MC. Higher adsorptive ability appeared on 10MC sample in which nearly 90% NPYR enmeshed when 1.5 mmol g-INPYR passed (Fig. 4A), this capability was closely matching to that of zeolite NaY [4]. As the adsorption prolonged, adsorptive capacity of NaY decreased to 61% when the accumulated NPYR achieved 2.8 mmol g-1 [4], whereas 10MC exhibited a capacity of 75% under the similar conditions. However, taking into mind that the BET surface area of 10MC (642 m2g1) is much smaller than NaY (766 m2g-l), the actual adsorbing ability of per m2 surface area of 10MC extremely exceeds that of NaY. This may be available to design and prepare special functional materials to adsorb and recycle nitrosamines in factory. Modification on surface acid-basic properties of AC seems not beneficial for promoting nitrosamines adsorption. Although coating hydrotalcite on AC increased the adsorption of both ammonia and CO2 in TPD experiments, however, 20%HT/AC sample exhibited a lowered adsorption capacity of NPYR at 453 K (Fig. 4B). Various metal ions loaded on porous materials usually produce different function of generating selective adsorptive sites, for instance ZnO, ZrO2 and MgO showed different effects on volatile nitrosamines adsorption on zeolite NaY at 593 K [14], and only magnesia could slightly accelerate this selective

600 adsorption. However, the promotion function of single oxide magnesia is usually different from those components in hydrotalcite when they are dispersed on the surface of AC, although they all possess basicity. And the main reason, as we pointed out previously [ 15], is the different microenvironments around the magnesia. Only in the suitable geometric and chemical microenvironments, metal ions can exert the affinity to the -N-N=O functional group of nitrosamines and expedite adsorption [3], which is of key importance for design the functional composites to adsorb nitrosamines in environment. In GC analysis, the broad peak of NPYR appeared when it passed through the adsorptive bed of AC, 1MC or 3MC while that through 5MC or 10MC was relative sharp (figure not show), which reflects the different interactions between NPYR and adsorbent. Some adsorbate molecules entered the pores of AC, but the non-polar surface could not provide an affinity strong enough to pull them, hence they quickly released again and to be detected by GC so that a broad peak formed. With the amount of alumina modifier increased, the surface polarity of AC became strong enough to adsorb NPYR, therefore no desorption of NPYR occurred and thus a sharp GC peak emerged. 50 C0=1.37mmol/L

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Figure 5. Adsorption of NNN on modified activated carbons in dichioromethane solution at 277K 3.3. Liquid adsorption of NNN Figure 5 shows adsorption of NNN, one of the bulky tobacco specific nitrosamines (TSNA), in liquid phase at 277 K. Activated carbon showed a faint ability to adsorb NNN less than 10 ~tg g-i and a similar adsorption emerged on 1MC (13 ~tg g-l). However, a strikingly enhanced adsorptive capacity appeared on 3MC and 5MC as well as 10MC samples and they could adsorb 4 times more NNN than the parent AC. For liquid adsorption, competition of solvent is serious. Surface of adsorbent was saturated by dichloromethane, so NNN molecules collided each other and diffused into the pore. If the polarity of pore wall was strong enough, adsorption of polar molecule must be underlying; of course which was hopeless in AC because it lacked surface polarity. Decoration of metal oxide nano-particles enhanced the surface polarity of AC, causing a remarkable improvement in adsorption of NNN. It is normal to have non-consistent results between liquid and gaseous phase adsorption, because the former is a longtime equilibrium procedure at 277 K to show the accumulative adsorption capacity of sample in which even a slowly adsorption process can still accomplished, while the latter is dynamic procedure at 453 K in which the adsorbent should have a powerful affinity for the adsorbates to catch and trap them when they pass by.

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Figure 6. Removal of nitrosamines from mainstream smoke by adding samples into the filter in different positions (A) rearward and (B) preceded cellulous acetate filters.

3.4. R e m o v a l of nitrosamines from m a i n s t r e a m s m o k e

In order to assess the selective adsorption function of modified AC sample in actual application, they were added into cigarette filter as a triple segment model to trap nitrosamines in mainstream smoke. It is well known that about 4800 compounds are contained in cigarette smoke [ 16], thus selective adsorption of nitrosamines is very difficult. Figure 6A displays the efficiency of modified AC composites in reducing the nitrosamines content of smoke when they are put rearward cellulous acetate filter. There was a faint function observed on AC alone but 29% of the nitrosamines could be removed by 1MC while 50% by 10MC. To examine the impact of copper, aluminum or zinc ions on the adsorption over activated carbon, several samples loaded with 1 wt.-% of metal oxide was thus carefully tested. As seen in Fig. 6A, 1%ZnO/AC possessed the highest adsorptive capability (14%) than 1%AI2Oa/AC (7%) and 1%CuO/AC (2%), which should be attributed to the especial surface properties of AC because modification with zinc oxide on siliceous SBA-15 for adsorption of nitrosamines was not superior to that with copper oxide [4,17]. Combination of zinc and aluminum ions to decorate activated carbon exhibited a much better effect in purification of smoke, because 1MC trapped more nitrosamines, most of them were volatile nitrosamines, than the sum of 1%ZnO/AC and 1%A12Oa/AC. Figure 6B demonstrates the effect of modified AC composites in trapping nitrosamines of smoke when they are put preceded cellulous acetate filter. As a non-selective adsorbent, AC adsorbed 18.6% of nitrosamines from mainstream smoke. Modification with metal oxides made 1MC or 3MC sample to trap about one fifth or one third of the carcinogens. Further addition of modifier on AC was no useful to promote the adsorption of nitrosamines, because the adsorption ability of 5MC or 10MC was obviously decreased. Two reasons, in our opinion, can account for this phenomenon. One is their lowered surface area and pore volume that was easily filled by competitive adsorbates in smoke. Another one is the oxide shell covered the pore probably alters the adsorptive property. It was easy to disperse metal oxides over AC due to the excellent adsorptive properties of the amorphous materials. Nonetheless, distribution of guest metal oxides was affected by the curvature of host surface and the modifier preferably deposited inside the small pore to fill them or form a shell wrapped them. These guest shells, especial those with multi-layers, was not beneficial for this dynamic adsorption because they kept the characteristic of bulk oxides instead of the highly dispersed nano-particles or monolayer. That is, these composites could exhibit a high adsorptive capacity in liquid adsorption through a longtime process, but fail to catch the carcinogens in dynamic procedure.

602 Faintish adsorptive ability of 5MC or 10MC in mainstream validates the complex and difficulty of selective adsorption in cigarette smoke. Some samples may possess excellent properties in the test of laboratory, but fail in real application. When cigarettes were smoked, thousands of resultants passed through and adsorbed in filter. No doubt intensely competitive adsorption occurred, and other components would fill in the pores and interfere adsorption of nitrosamines. In case the modified AC was put rearward cellulous acetate filter, these competitive adsorbates could be trapped by the cellulous adsorbent, while in the tests showed in Fig. 6B, modified AC was faced to thousands compounds that competed for adsorptive sites and hindered the succeeding adsorption of nitrosamines. Together these results with the consideration of additive cost, 3MC will be the best candidate for the potential application in filter matrix while 5MC or 10MC is suitable for trapping nitrosamines in other environments with simper chemical composition or longer equilibrium time. 4. CONCLUTIONS It is feasible to generate selective adsorptive sites on common porous materials, which will be helpful to control the smoking pollution by use of selective adsorption techniques. Coating metal oxides such as alumina and zinc oxide on activated carbon can adjust primary pore diameter and polarity of surface to provide the affinity for nitrosamines and to fasten trapping nitrosamines in the porous material. Modified activated carbon shows an enhanced capacity in adsorption of both volatile nitrosamines such as NPYR and bulky tobacco specific nitrosamines (TSNA) like NNN. ACKNOWLEDGEMENT Financial support from the NSF of China (20273031 and 20373024), Ningbo Cigarette Factory and Analysis Center of Nanjing University is gratefully acknowledged. REFERENCES [ 1] H.P. Boehm, Carbon, 40 (2002) 145. [2] Y. Xu, J.H. Zhu, L.L. Ma, A. Ji, Y.L. Wei and X.Y. Shang, Microporous Mesoporous Mater., 60 (2003) 125. [3] Y. Xu, H.D. Liu, J.H. Zhu, Z.Y. Yun, J.H. Xu and Y.L. Wei. New J. Chem., 28 (2004) 244. [4] C.F. Zhou, Z. Y. Yun, Y. Xu, Y. M. Wang, J. Chen and J. H. Zhu, New. J. Chem., 28 (2004) 807. [5]' C. Jiang, Y. Liu, X. Sun, F. Tian, F. Sun, C Liang, W. You, C. Han and C. Li, Langmuir, 19 (2003) 171. [6] W.P. Hoffman, H. T. Phan and A. Groszek, Carbon, 33 (1995) 509. [7] Matsumoto, K. Tsutsumi and K. Kaneko, Langmuir, 8 (1992) 2515. [8] J. L. Hudson, E. H. Johnson, D. F. S. Natush and R. L. Solomon, Environ. Sci. Technol., 8 (1974) 238. [9] H.L. Chiang, C. P. Huang, P. C. Chiang and J. H. You, Carbon, 37 (1999) 1919. [10] Y.L. Wei, Y.M. Wang, J.H. Zhu and Z.Y. Wu. Adv. Mater., 15 (2003) 1943. [ 1l] T. Miyake and T. Shibamoto, J. Chromatogr. A., 693 (1995) 376. [12] Manivannan, M. Chirila, N.C. Giles and M. S. Seehra, Carbon, 37 (1999) 1741. [13] L. J. Kennedy, J. J. Vijaya, and G. Sekaran, Ind. Eng. Chem. Res., 43 (2004) 1832. [ 14] L.L. Ma, B. Shen, J.H. Zhu, J.R. Xia and Q.H. Xu. Chin. Chem. Lea., 11 (2000) 649. [ 15] J.H. Zhu, Y. Wang, Y. Chun and X.S. Wang. J. Chem. Soc. Faraday Trans., 94 (1998) 1163. [ 16] C. Andreoli, D. Gigante and A. Nunziata, Toxicology in Vitro, 17 (2003) 587. [17] Z.Y. Yun, Y. Xu, J.H. Xu, Z.Y. Wu, Y.L. Wei, Z.P. Zhou and J.H. Zhu, Mieroporous Mesoporous Mater., 72 (2004) 127.