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Fabrication of Hierarchical Channel Wall in Al-MCM-41 Mesoporous Materials to Enhance Their Adsorptive Capability: Why and How? Fang Na Gu,†,‡ Feng Wei,† Jia Yuan Yang,† Ying Wang,*,‡ and Jian Hua Zhu*,† Key Laboratory of Mesoscopic Chemistry of MOE, College of Chemistry and Chemical Engineering, and Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing UniVersity, Nanjing 210093, China ReceiVed: January 31, 2010; ReVised Manuscript ReceiVed: March 16, 2010
To overcome the inefficiency of mesoporous materials in the adsorption of small molecules, this article reports the effort how to create hierarchical channel wall in Al-MCM-41 and more important, how to distinguish the contribution of the newly formed micropores in adsorption by the mesoporous materials. Fabrication of hierarchical channel wall is realized through extracting framework aluminum of sample by acid leach to create micropores and defects, providing the fine geometric confinement toward tiny targets. The influence of original Al content of Al-MCM-41 on the controlled dealumination was studied, and X-ray diffraction, N2 adsorption-desorption, 27Al and 29Si MAS NMR, Fourier transform IR techniques were employed to characterize the resulting samples. Besides, volatile nitrosamine N-nitrosopyrrolidine (NPYR) was chosen as a probe to assess the adsorption of the resulting samples. Hierarchical channel wall in Al-MCM-41 significantly increased its ability to trap NPYR, and for the first time the adsorptive contribution of newly formed micropores and defects in the mesoporous silica was distinguished by the instantaneous adsorption under the carrier gas with different flow rate, which is beneficial for developing new functional materials to protect environment. 1. Introduction Developing the special adsorbent with high efficiency for environment protection is still a challenge because of the stricter requirement. It is normal for the adsorbents to adsorb the targets with trace amounts (ppm or ppb) among a large numbers of other components, and one noteworthy example is to remove the carcinogens such as nitrosamines in environmental tobacco smoke in the high space velocity aeration system,1-3 cleaning the indoor air and protecting outside environment. Zeolite is the famous selective adsorbent, but its sole microporous structure restricts the transport of airflow and leads to a high airresistance.4 Consequently, mesoporous silica such as SBA-15 and MCM-41 becomes the promising candidate for environmental purification. Wide channel is the advantage of mesoporous materials that overcomes the limitation of pore size in zeolites and other microporous materials, enabling mesoporous materials to be the special adsorbents of large molecules such as proteins.5 For the bulky tobacco specific nitrosamines (TSNA) that are well-known carcinogens in tobacco smoke, mesoporous silica exhibited an excellent adsorption and catalytic ability, ten times higher than zeolite such as NaY.6 Nonetheless, large pore diameter is turned into the disadvantage of mesoporous materials when they capture the small pollutant such as volatile nitrosamines. Mesoporous materials usually lack the fine geometric confinement toward these small molecules hence these tiny targets fleet through the wide channel. Either SBA-15 or MCM41 is inferior to zeolite in adsorbing volatile nitrosamines such as N-nitrosopyrrolidine (NPYR) in gas flow.6,7 There are many efforts devoted to improve the adsorption of mesoporous silica forward small target such as volatile nitro* To whom correspondence should be addressed. (J.H.Z.) E-mail: jhzhu@ netra.nju.edu.cn. (Y.W.) E-mail:
[email protected]; Tel.:+86 13813991318; Fax: +86 25 83317761. † College of Chemistry and Chemical Engineering. ‡ Ecomaterials and Renewable Energy Research Center (ERERC).
samines. Introduction of metallic component such as copper or aluminum into mesoporous silica, through postmodification or one-pot synthesis procedures, could dramatically increase the adsorption capability.6-8 In these cases, the guest species are inserted into the framework or coated onto the surface of channel to play the role of adsorptive site, frequently having synergy with adjacent silanol group to trap the target molecule.8 However, increasing metal content of mesoporous adsorbents inevitably elevates their cost, and moreover, in our opinion, it is hopeless for mesoporous adsorbents to overcome their inherent structural drawback of lacking fine geometric confinement toward the tiny target. These guest modifiers may form some ultrafine particles inside the main channel to increase collision probability with adsorbate molecules, but they cannot provide the microenvironment like that of zeolite in which the adsorbate molecule is tightly surrounded. Thus, a challenge arises from this problem: how do we tailor the channel walls of mesoporous materials to enable them to provide the fine geometric confinement toward small molecules? One strategy is to create a hierarchical channel wall in mesoporous adsorbents so that they can have both wide channels and micropores for fast mass transport and efficient adsorption, respectively. Compared with the efforts of generating “plugs” and/or “constrictions” in channel,9-15 hollowing the pore wall of mesoporous materials has its instinctive advantage, and since these newly formed micropores and/or tiny defects in the channel wall do not hinder mass transition their negative curvature is beneficial for trapping targets.16-18 Recently we tried a route of “nanocasting” to prepare Al-containing mesoporous silica by using ample aluminum salt in synthesis, inserting into the framework and coating on the channel wall. After washing the synthesized products with an acidic solution, most of the aluminum species were removed to create many artificial defects in the pore wall forming a hierarchical structure.19 Nonetheless, the resulting samples lost their ordered mesoporous structure more or less though their adsorption ability was really elevated.
10.1021/jp1009143 2010 American Chemical Society Published on Web 04/16/2010
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In the present work, we succeed this route to develop hierarchical MCM-41 materials. The hierarchical mesoporous silica with various pore wall curvatures was fabricated by acid leaching the aluminum containing MCM-41, forming the abundance of surface flutes and micropores on the channel wall by eluting framework aluminum at a mild condition while keeping the mesostructure of MCM-41. These newly formed micropores and defects would have synergy with the included or adjacent Al sites to promote the adsorption of small molecules. The resulting adsorbents will have various cavities; the regular and straight channels of MCM-41 in axial direction lead airstream passing swimmingly, while the various ultramicropores, bestrewing in channel in the radial direction, will provide a kind of fine geometric confinement for tiny molecules to increase their advantageous collision probability with the adsorbent. Through the controllable mild dealumination of AlMCM-41 by acid leach, we also want to tailor the distribution of Al species and to make many Al sites locating inside the artificial holes with high accessibility, enabling them to exert their optimal function within the special geometrical microenvironment.20 As the result, they will form an efficient “trap” to capture the target such as volatile nitrosamine. To evaluate the contribution of the hierarchical channel wall on the adsorption of resulting samples, two typical environmental pollutants, NPYR and 1,3-butadiene, were selected as probes in instantaneous adsorption. Another aim of the present work is to distinguish and assess the adsorptive function of micropores in the mesoporous materials, which is also the challenge faced by researchers. In principle, these micropores in mesoporous adsorbents can be characterized by using N2 adsorption at 77 K with ultralow relative pressure; however, this method is not only expensive and time-cost, but it also fails to reflect the real performance of these micropores in environment protection because of the different temperature and targets. There are some literature published on the improvement of optical property, electrocatalytic activity and sorption of porous materials by tuning their texture property,21-24 but no report to describe the adsorptive contribution of newly formed micropores and defects in mesoporous materials. Clearly, it is very difficult to use a probe molecule to distinguish the contribution of micropores in the adsorption of mesoporous silica. Nonetheless, recently we observed different behaviors of various zeolites in the instantaneous adsorption of volatile nitrosamines.25 Zeolite NaZSM-5 exhibited a stronger adsorptive ability toward NPYR when the adsorption was performed in faster carrier gas or elevated temperature, which was different from NaY and NaA zeolites, due to its special narrow channel with the diameter closed to the molecular size of NPYR. This phenomenon encouraged us to try similar strategy in probe adsorption. Besides, we also found that adsorption of 1,3-butadiene could reveal the different distribution and function of alumina modifier in four SBA-15 samples prepared in various methods.26 Thus, the function of micropores and defects in the channel wall of Al-MCM-41 samples would be assess by means of instantaneous adsorption with different flow rates of carrier gas and temperature. 2. Experimental Section NPYR was bought from Sigma and dissolved in dichloromethane (A.R.) at the volume ratio of 1:19. Tetraethylorthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) were the products of Shanghai Wulian and Lingfeng (China), and other regents with the AR purity were used as received. 2.1. Preparation of Samples. Al-MCM-41 samples were synthesized according to the literature.27 In a typical synthesis,
Gu et al. 1.5 g of NaOH was dissolved in 45 g of water, followed by addition of 3 g of silica aerosol to yield a silicate gel. Twentyfive milliliters of CTAB aqueous solution containing 4.5 g of CTAB was added dropwise to the silicate gel under stirring at 303 K, and then a certain amount of AlCl3 solution was put into the resulting mixture and the pH value was adjusted to 10 using 2 mol L-1 HCl. After stirring for 6 h at 303 K, the mixture was transferred into an autoclave and statically heated at 373 K for 24 h. Finally, the resulting solid was filtered, washed with distilled water, air-dried, and calcined in air at 823 K for 5 h to give the samples denoted as Mn where n represented the actual molar ratio of Si/Al. Acid leaching of the Mn sample was performed using 0.05 mol L-1 hydrochloric acid for 2 h at room temperature, in which the ratio of liquid to solid was 100 mL g-1; the solid product then was filtrated, washed thoroughly with distilled water, and dried to give the sample of TMn. For comparison, part of M32 sample was washed with water instead of acid solution in the same procedure, and the obtained sample was named as M32-w. 2.2. Characterization. The XRD patterns of the samples were recorded on an ARL XTRA diffractometer with Cu KR radiation in the 2θ range of 0.5-6°; their chemical composition was detected by X-ray Fluorescence (XRF) method using ARL9800 X-ray fluorescence spectrometer. After the acid leach of sample, the filtrate was also measured by inductive coupled plasma-atomic emission spectrometry (ICP-AES) to determine its composition. Fourier transfor IR (FTIR) spectrum of the sample was recorded on a Bruker 22 infrared spectrophotometer in 4000-400 cm-1 with a resolution of 4 cm-1 for which the KBr pellet containing 2 wt % of sample was utilized. Nitrogen adsorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 volumetric adsorption analyzer, and the sample was outgassed at 573 K for 4 h prior to test. The pore size distributions (PSDs) curve was calculated from the analysis of adsorption branch of isotherm using density functional theory method.28 Solid state 27Al MAS NMR measurement, instantaneous adsorption of NPYR or 1,3-butadiene were carried out in the process reported previously.25-27 The amount of NPYR trapped by various samples at the accumulated amount of NPYR of 1.4 mmol g-1 was utilized to represent their adsorptive capacity, and the more NPYR adsorbed by Al-MCM-41 than MCM-41 indicated the contribution of Al content in the sample. To roughly assess the efficiency of Al content in adsorption, RAl is calculated by simply dividing the increased adsorption capacity of NPYR to the Al-content of sample (Table 1). 3. Results 3.1. Fabrication of Hierarchical Channel Wall in AlMCM-41 Mesoporous Materials. Figure 1A illustrates the lowangle XRD patterns of Mn samples before and after acid leaching. All Mn samples exhibited a well-ordered hexagonal structure identical to MCM-41, and most of them remained the original structure after acid-leaching though their regularity declined slightly. One exception was TM16 on which partial collapse of mesostructure emerged due to the removal of large amount (0.60 mmol g-1, Table 1) of framework aluminum. Since TM24 still kept original structure though it lost 52% of Al content and the amount of 0.36 mmol g-1, it is clear that removal of Al below 0.3 mmol g-1 from Mn sample is sustainable for maintaining the mesostructure. Unlike zeolite whose maximum Na/Al ratio usually reaches one,29 Mn samples have the Na/Al ratios near to one (Table 1) since not all of the Al atom incorporated in framework is accompanied by a relevant Na+ or H+ ion to balance the charge.30,31 After acid leaching, Na+ ion was absent in TM64 and TM32 but still survived in TM24
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TABLE 1: Surface Property of Al-MCM41 Samples before and after Acid-Leaching sample
Al content (mmol g-1, A)
Na content (mmol g-1)
SBET (m2 g-1)
Smicro (m2 g-1)
Vp (cm3 g-1)
Vmicro (cm3 g-1)
D (nm)
adsorption capacity of NPYR (mmol g-1, B)a
RAlb
MCM-41 M64 TM64 M32 M32-w TM32 M24 TM24 M16 TM16 NaY
0 0.24 0.06 0.51 0.49 0.24 0.69 0.33 1.00 0.40 4.44
0 0.23 0 0.48 0.47 0 0.62 0.14 0.87 0.30 4.44
1119 1029 790 1023 916 728 1001 649 1148 665 672
0 0 118 0 50 134 0 201 0 159 631
0.94 0.94 0.50 0.92 0.81 0.60 0.97 0.42 1.22 0.54 0.34
0 0 0.05 0 0.02 0.06 0 0.09 0 0.07 0.31
3.4 3.4 3.1 3.4 3.4 3.1 3.3 3.1 3.2 3.1 0.74
0.06 0.24 0.57 0.55 0.58 0.70 0.71 0.78 0.68 0.41 1.16
0.75 8.50 0.96 1.06 2.67 0.93 2.18 0.62 0.93 0.26
a The accumulated amount of NPYR passed through the sample at 453 K was 1.4 mmol g-1. b RAl is calculated by dividing the increased adsorption capacity of Al-MCM-41 toward NPYR to the Al content of the sample.
and TM16 samples (Table 1), indicating that these Al sites still holding Na+ ion might locate inside the channel walls of TM24 and TM16 where they could not contact with solution. Different ion exchange capacity of TMn samples originated from their different distribution of Al. When Al-MCM-41 had high Si/Al ratio, say, Si/Al ) 76-40, Al atoms occupied only the T sites on the surface of channels; as the framework Al content of sample increased, more Al atoms incorporated into the inner channel walls and they did not form cationic sites.30 For the sample with Si/Al ratio of 15, 35% of Al atoms were located inside the channel walls and the counter-cations Na+ could not be exchanged.31 On the basis of these results, it is known that the accessibility of Al in TM64 and TM32 should be larger than that in TM24 and TM16, because some Na+ ions still cannot be exchanged by acid solution (Table 1). Figure 1B shows the 27Al MAS NMR spectra of M32 and TM32 samples. M32 exhibited a single at 52 ppm, attributed to the tetrahedral aluminum (Td-Al).32 The intensity of 52 ppm signal decreased in TM32 sample due to the removal of framework aluminum, and another signal at 0 ppm appeared owing to the formation of octahedral aluminum (Oh-Al).33 The ratio of the Td-Al to Oh-Al was 4 in TM32 sample. It seemed that extra framework aluminum species formed in TM32 sample, because some removed Al species deposited or reincorporated into the sample,34 enhancing the accessibility of Al sites. There were three features to be identified in the 29Si MAS NMR spectrum of M32 (Figure 1C), the peak at -110 ppm originated from Si(4Si) (Q4) structure units, -106 ppm peak came from Si(3Si,1Al) sites, and the shoulder at -100 ppm was due to Q3 silicon on Si(OSi)3 OH sites.35 Acid leaching removed -106 ppm signal from TM32 sample, and the Q3(Si(OSi)3(OH)) to Q4(Si(OSi)4 ratio changed from 0.37 to 1.8, resulting from the broken of Si-O-Al band and formation of surface Si-OH. TM32 had a low Al content so that its signal of Si (3Si, 1Al) sites was overlapped by other signals. Enhancement of Si-OH signal in TM32 sample was confirmed by IR spectra shown in Supporting Information Figure S1 in which TM32 exhibited a strengthened 960 cm-1 band in comparison with M32 sample. Since 960 cm-1 band means the asymmetrical (Si-O) stretching mode of -Si-OH group,36 this phenomenon reflects the existence of more -Si-OH in TM32 sample. Figure 2A depicts the N2 adsorption-desorption isotherm of M64, M32, and their acid-leached analogues. Four samples displayed the type IV isotherm with a sharp inflection at p/p0 ) 0.3, typical for mesoporous materials, and another extraordinarily large hysteresis loop at p/p0 between 0.5 and 1, which ascribed to the structural defects formed in hexagonal channel owing to incorporation of aluminum.26 Acid leaching decreased
Figure 1. Low-angle XRD patterns (A), 27Al MAS NMR (B) and 29Si MAS NMR spectra (C) of the parent and acid-leached samples. (a) M64, (b) M32, (c) M24, (d) M16, (e) TM64, (f) TM32, (g) TM24, (h) TM16.
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Figure 3. HRTEM images of (a) M32, (b) TM32, (c) M24, (d) TM24 samples.
Figure 2. Nitrogen adsorption-desorption isotherms (A) and pore distribution (B, C) of the parent and acid-leached M64 and M32 samples, where the isotherms of M32 were lowered by 100 cm3 g-1 and panel C mainly showed the pore distribution in the micrpore region.
the size of latter hysteresis loop markedly, indicating the reduced number of ink-bottle like pore. Apart form the preserved mesoporous channel of 3.2 nm, large amount micropore emerged on TM64 and TM32 sample with the centers at 0.68 and 1.2 nm, respectively, which would be beneficial for adsorption of small molecules. TM64, TM32, and TM16 samples had a microporous area in the range of 118-159 m2 g-1 (Table 1), and the more aluminum species were removed in acid leach, the larger microporous area would be (Figure 4A). This is not unexpected because removal of Al species in framework must create micropores or tiny defects in the channel wall, resulting in a hierarchical structure in the mesostructured materials.19 TM24 seemed to be an exception because it had a larger microporous area (201 m2 g-1) than TM16 (159 m2 g-1), but less Al species were removed (Table 1). In other words, establishing micropores in TM24 sample through acid leach is much more efficient though the reason is unknown yet. It should be pointed out that
the water in the acid solution causes part of the newly formed micropores. As demonstrated in Table 1, M32-w sample that was washed with water had similar Al-content and pore diameter to M32 (Supporting Information Figure S3), but it possessed a microporous area of 50 m2 g-1 that originated from probably the hydrolysis of some Si-O-Si bands in the sample.37 Figure 3 displays the TEM images of M32 and M24 samples in which hexagonal pores and the smooth straight channel demonstrate the mesostructure of Al-MCM-41 (Figure 3a,c). Contrarily, the pore wall of TM32 and TM24 became distorted (Figure 3b,d). Furthermore, some parts of the channel wall seemed to be broken so that these channels connected each other (the circular area in Figure 3b,d), which made the diffusion between the MCM-41 channels more effective. Unlike these SBA-15 samples with large Al-content lost their mesostructure characteristic after acid leach,19 both TM24 and TM32 still kept the structure of MCM-41 in their TEM images, in good agreementwiththeresultsofXRDandnitrogenadsorption-desorption. The main reason for these differences, in our opinion, is the chosen suitable Al content of Mn samples because either high Al-content sample or MCM-41 itself will loss the ordered mesoporous structure after acid leach (Supporting Information Figures S2A and S3A). 3.2. Adsorption of Volatile Nitrosamines and 1,3-butadiene by Al-MCM-41 Samples. Figure 4B displays the instantaneous adsorption of NPYR by Mn samples and their acid leached analogues at 453 K. The Al content of Al-MCM-41 mesoporous materials actually determined their adsorption since cations provided the electrostatic interaction toward the N-NO group of nitrosamines.7 Consequently, M64 exhibited lower capability than M32 while M16 and M24 were more active throughout the whole adsorption. Nonetheless, M16 possessed the adsorption ability similar to M24 (Figure 4B), implying the limited promotion of adsorption ability just by increasing the Al content of MCM-41. If we roughly assess the efficiency of Al content in various samples by comparing their RAl values (Table 1), it is clear that M32 had the highest RAl value (0.96) but M16 possessed a declined one (0.75) due to the large amount of Al atoms located inside the channel walls of M16, and those aluminum species could not exert their function as long as they failed to contact nitrosamine molecules.31 A similar problem of accessibility was also found on zeolite NaY where only the Al components in the channel surface could contact with NPYR
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J. Phys. Chem. C, Vol. 114, No. 18, 2010 8435 TABLE 2: Freundlich Constants of the Adsorption Isotherms of NPYR on Al-MCM-41 Samples at 453 K sample M64 TM64 M32 TM32 M24 TM24
flow rate of carrier gas (mL min-1)
KF
n
R2
20 20 10 20 30 10 20 30 20 20
0.20 0.47 0.66 0.45 0.35 0.48 0.55 0.64 0.57 0.59
2.81 1.70 1.27 1.65 1.67 1.55 1.52 1.33 1.58 1.50
0.99 1.00 1.00 0.99 1.00 1.00 1.00 1.00 1.00 1.00
removal of framework Al has a small effect, and more important, such small negative effect can be compensated by the fabrication of micropores. For the sample with middle Al content such as M24, the promotion of newly formed micropores is counteracted by the negative effect caused by removal of Al species hence no obvious improvement was observed (Table 1, Figure 4B). To have an overall analysis of adsorption isotherms, we fit the experimental isotherms of M64, M32, M24, and their acid leached analogues by Freundlich adsorption equation that is an empirical model38
ln q ) ln KF + (1/n)ln C
Figure 4. (A) Relation between removal of aluminum in TM-n samples and the resulting microporous area, (B) adsorption of NPYR by Alcontaining MCM-41 at 453 K.
hence its RAl value decreased to 0.26 although NaY was the best adsorbent for NPYR among commercial microporous zeolites. Creation of micropores in TMn samples dramatically promoted the adsorption of NPYR, but the improvement was affected by the Al-content of samples. TM64 showed a 1.5 times higher adsorptive capacity than M64 (Figure 4B), TM32 and TM24 exhibited the adsorption performance 27% and 10% higher than their parents, M32 and M24, respectively. However, TM16 lost about 40% of adsorptive ability in comparison with M16, due to the poor accessibility of Al components TM16 had the largest Al content among four TM-n samples but its RAl value was 0.93 (Table 1), obviously smaller than that of TM24 (2.18) and TM32 (2.67) as well as TM64 (8.5). Another reason for the declined adsorption capability of TM16 is the collapse of mesostructure as aforementioned. To accurately assess the adsorptive ability of samples without the influence of surface area, we also calculate the adsorption capacity in unit area, µmol m-2 instead of mmol g-1. The corresponding value of TM64, TM32, TM24 and TM16 was 0.72, 0.95, 1.20, and 0.62 µmol m-2, exceeded that of their parent M64 (0.23), M32 (0.54), M24 (0.71) and M16 (0.59). Supposing the remained mesopores in TMn sample kept their original adsorptive property, the contribution of newly formed micropores in adsorption of NPYR can be tentatively calculated. As the result, the value of TM64, TM32, TM24, and TM16 was 3.5, 2.8, 2.3, and 0.70 µmol m-2, and TM16 was still the smallest. On the basis of these calculations, it is safe to infer that acid leach optimally promotes the adsorption of low Al content mesoporous sample such as M64 toward NPYR, because Al component has a minor contribution in the adsorption of M64 sample so the
where KF is known as Freundlich constant that is related to adsorbent capacity and 1/n is an exponent associated to adsorptive strength as well as the favorability.38 Table 2 lists the isotherm parameters calculated with the method of leastsquares. The KF value of NPYR in TM64 was doubled in comparison with that of M64, and the smaller n values of TM64 and TM32 than that of M64 and M32 indicated the enhanced adsorptive strength owing to the formation of micropores. As the Al content of sample further rose, such as that in M16, removal of framework aluminum resulted in serious negative influence on the adsorption of NPYR, which was uncompensated by the improvement of newly formed micropores therefore the mean adsorption capability of TM16 was lowered instead. Several experiments were performed to further assess the promotion of newly formed micropores on the adsorption of NPYR by Al-MCM-41. Figure 5A plots the adsorption of NPYR on M-64 and TM64 at different temperatures. As temperature rose from 453 to 473 K, the adsorption of NPYR in both samples were suppressed because of the enhanced desorption at elevated temperature. Nonetheless, at 473 K M64 failed to trap NPYR once the accumulated amount of NPYR reached 0.8 mmol g-1 while TM64 still kept an increasing adsorptive capacity thanks to the existence of micropores that provided the fine geometric confinement to NPYR molecule preventing its desorption.25 Strange phenomenon was found in Figure 5B in which the adsorption of NPYR by M32 and TM32 was carried out at 453 K with different carrier gas flow rates. When the flow rate of carrier gas rose from 10 to 30 mL min-1, the adsorptive capability of M32 gently decreased. This is normal because the contact time between NPYR and adsorbent is dramatically decreased from 0.1 to 0.04 s,25 and the faster gas flow effectively pulls the adsorbed NPYR in sample leading to desorption. However, different situation was observed in TM32 sample, the higher the carrier gas flow rate, the more NPYR trapped (Figure 5B), resulting in the increscent KF and 1/n values in the isotherm parameters (Table 2). Similar phenomenon had been found in
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Figure 6. Adsorption of 1,3-butadiene by MCM-41 mesoporous materials at 298 K.
Figure 5. (A-C) Adsorption of NPYR on Al-containing MCM41samples, (D) Relation between the increased amounts of NPYR adsorbed by Al-MCM-41 in fast carrier gas and the newly formed microporous area in the adsorbent.
the adsorption of NPYR by zeolite NaZSM-5 and attributed to the accelerated diffusion of NPYR in the narrow and sinusoidal channels of NaZSM-5.25 For the acid leached Al-MCM-41 sample, in our opinion this exception should be related to the rough surface of channel wall with various micropores and defects.8 The faster carrier gas enabled NPYR molecule to have a higher collision frequency with the channel wall therefore more adsorbates inserted into the defects or micropores to be adsorbed.
Our inference was supported by experiments. When the rate of carrier gas was increased from 20 to 30 mL min-1, the adsorption of NPYR was also improved on TM24 and TM64 samples (Figure 5C), confirming the contribution of micropores in adsorption of volatile nitrosamines. To explore the flow effect further, the same experiments were performed on M32-w sample. Water wash did not obviously change the Al-content of M32 but created some tiny holes in the surface of the sample so it had a microporous area of 50 m2 g-1. However, M32-w not only exhibited a little bit higher activity than M32 in the adsorption of NPYR (Table 1), but also had different behaviors once the flow rate of carrier gas increased from 20 to 30 mL min-1. More NPYR was captured by M32-w sample in the faster carrier gas (Figure 5C), which clearly originated from the contribution of these micropores. In fact, the increased amounts of NPYR adsorbed by Al-MCM-41 in faster carrier gas had a relation with the newly formed microporous area in the adsorbent (Figure 5D), which may provide one potential method to characterize the micropores in mesoporous materials. Figure 6 displays the instantaneous adsorption 1,3-butadinene on MCM-41 and its analogues at 298 K. MCM-41 only trapped little 1,3-butadinene, same as SBA-15,26 due to lack of active sites. Incorporation of aluminum into mesoporous silica significantly promoted the adsorption of 1,3-butadiene, and the capability of M32 exceeded that of M64 owing to its high Al content. However, TM32 exhibited an adsorption capacity same as M32 though its Al content was half less than M32. Likewise, TM32 adsorbed doubled the amount of 1,3-butadiene in comparison with M64 although they had the same Al content. These results further proved the important contribution of micropores in mesoporous materials toward small adsorbate molecules. Flow effect was also observed in the adsorption of 1,3-butadiene. Both M32 and M64 samples showed declined adsorption ability when the flow rate of carrier gas increased from 20 to 30 mL min-1, but TM32 exhibited the slightly increased adsorptive capability in the faster carrier gas, owing to the existence of micropores and defects in the adsorbent. 4. Discussion MCM-41 has only an array of hexagonally ordered primary channels without micropore, and its pore size exceeds the molecular diameter of NPYR (0.42 nm × 0.54 nm),25 so that MCM-41 cannot provide the elaborate geometric restriction toward NPYR. Monocyclic nitrosamines such as NPYR usually take planar structure because of their large rotational barriers of the N-NO bond,39 and they adsorb on zeolite in the way of inserting their N-NdO group into the channel of zeolites due
Al-MCM-41 Mesoporous Materials to the electrostatic interaction from cations of zeolite. Consequently, two factors are necessary to capture volatile nitrosamines. One is the narrow channel to confine the movement of nitrosamine and another is the electrostatic interaction to induce and anchor the target. Moreover, synergy of geometric confinement and electrostatic action is crucial. The availability of electrostatic interaction is limited in a very short-range,40 hence the active site cannot exert its function until the targets enter its range. Likewise, it is hard to trap nitrosamines in the siliceous micropores only with the assistance of silanol groups because of the weak action. The optimal combination is the narrow micropores or defects with the efficient adsorptive sites, usually played by cations, located in the bottom or side face. Here we provide a feasible route for this purpose. At first, we start our experiments on Al-containing mesoporous silica because framework aluminum can stabilize the structure of sample.37 Otherwise, siliceous MCM-41 easily loses its ordered mesoporous structure in acid leaching (Supporting Information Figure S3A) due to the poor hydrothermal stability and uncontrollable hydrolysis of Si-O-Si bond in aqueous solution.41 Proper Al content of mesostructured sample is important for hollowing of the pore wall by acid leach, because it affects the succeeding dealumination in which process some framework Al atoms will be moved from the channel wall to make tiny holes. Different density and distribution of Al in mesoporous silica caused different dealumination, and moreover the different silica fragments involved, determining the position and shape of newly formed micropores and/or defects. The incorporated Al in Al-MCM-41 usually existed in the form of isolated Al atoms or Al pairs such as NNN Al pairs (AlO-SiO-Al) and NNNN pairs (AlO-(SiO)2-Al).42 At low framework Al content, Al “pairs” represent 95% of Al atoms in MCM-41. With increasing Al content, the number of Al “pairs” decreased, and for the sample with Si/Al ratio of 15, Al “pairs” represent 70% of Al atoms and single Al atoms represent 30% of Al atoms on the channels surface.31 As demonstrated in Supporting Information Table S1, the Al/Si ratios of the filtrate of acid leached TM64 and TM32 were about 2, implying that the domination of NNN Al pairs (AlO-SiO-Al) in M32 sample and participation of silica species in dealumination. However, the Al/Si ratio in the filtrate of TM24 and TM16 samples reached 3.6 and 3.8, respectively. This mirrors the different dealumination occurred in the A1-MCM-41 sample with different Al-content. In the case that Al-MCM-41 had a low Al content, the Al atoms located on the surface preferentially formed NNN or NNNN Al pairs, and the acid leach of Al atoms would result in the formation of Si(OSi)2(OH)2 (Q2) sites; however, these resulting terminal OH groups on the channel surface were easily attacked by water so that they were involved into the solution. A different situation emerged on the Al-MCM41 with high Al content where the number of isolated Al atoms increased with the framework Al content,31 and the Al-O band was more easily attacked by H+ than Si-O band. As the result, more Al atoms were extracted but less Si species were involved, hence the Al/Si of filtrate increased. Both surface area and volume of micropores in the sequence of TM64 < TM32 < TM24 is coincided with the Al-content of Mn sample M64 < M32 < M24. If we roughly assess these newly formed tiny holes by dividing the microporous volume of sample to its microporous surface area, the corresponding values of M32-w, TM64, TM32, TM24 and TM16 is 0.40, 0.42, 0.45, 0.45, and 0.44 µL m-2. These different data implies the formation of various tiny holes in these samples, including the deepness and breadth of holes. M32-w sample should have the
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8437 SCHEME 1: Proposed NPYR Adsorption on the Parent and Acid Leached Al-MCM-41
shallow micropores since water washing cannot remove framework aluminum, and only a few Al species is involved by the hydrolysis of adjacent Si-O-Si bond. However, TM32 can have the micropores deeper than that in M32-w, because acid solution extracts some aluminum cations from framework to make holes or defects with enlarged depth. The shape of micropores in mesoporous samples will have an influence on the succeeding adsorption of NPYR more or less, as discussed later. Creation of tiny holes is at the expense of losing framework Al that is well-known to be adsorptive sites hence the adsorption of final sample is affected by two opposite factors, the improvement resulting from the hollowing of pore wall and the decline caused by dealumination. TM64 showed the most obvious enhanced activity in NPYR adsorption due to the smallest proportion of dealumination and the relatively low adsorption ability of M64, and vice versa, TM16 was inferior to M16. Combining the extent of enhancement and the absolute increased amount in adsorption of NPYR, M32 is the suitable Al-MCM-41 candidate for fabrication of hierarchical channel wall. Apart from the hollowing of the channel wall in mesoporous materials, it is hard to distinguish the contribution of micropores from the adsorption of sample although they really improve the performance of mesoporous adsorbents. For the first time, we find a simple way to overcome this difficulty through examining the behavior of sample in the adsorption of NPYR in faster carrier gas. Usually faster carrier gas is not beneficial for adsorption of nitrosamines due to the shortened contact time between the adsorbent and adsorbate.43 Moreover, the adsorptive sites of mesoporous adsorbent became insufficient to trap the target with higher kinetic energy in wide channel, so that M32 sample exhibited the declined adsorption ability in faster carrier gas. Appearance of shallow tiny holes in the sample of M32-w turned this situation, and a little bit more NPYR was trapped even in faster carrier gas (Figure 5C). Likewise, TM32 showed the obviously increased ability instead (Figure 5B) owing to the existence of more and deeper micropores and defects in channel wall, because more target molecules were pushed inserting into these tiny holes by the faster currier gas, being captured by the adsorptive site inside the hole (Scheme 1). The narrow microenvironment of adsorptive sites is beneficial for capture and anchor of target molecule since the movement of target will be limited. Micropores and rough surface can enhance the number of advantageous orientations collision of adsorbates in the sorption process, as it would allow them to retain a higher flexibility in their degrees of freedom when adsorbing from the
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gas phase.44 Besides, existence of tiny holes in the channel wall of mesoporous sample changed the surface curvature, relating to the possible orientation-change of gas stream, and thus the whirling and swerving of flow would occur in the seam to prolong the contact time of target with adsorbent.45 The promotion of micropores and tiny defects in adsorption of small molecules seem to be indispensable, which cannot be compensated by the high Al content of mesoporous sample. M32 owned more cations than TM32 to offer strengthened electrostatic interaction toward nitrosamines, but it was still inefficient to capture NPYR in faster carrier gas (Figure 5B). This fact indicates the limitation of electrostatic interaction and thus fabrication of hierarchical channel wall in mesoporous materials becomes crucial to enhance their efficiency in adsorption of small molecules. On the other hand, the rough relation between the newly formed tiny holes, characterized by microporous surface area of sample, and the increased adsorption capacity caused by faster carrier gas was found (Figure 5D), which not only further confirmed the vital role played by micropores to elevate NPYR adsorption in faster carrier gas, but also gave a valuable clue to distinguish the contribution of these newly formed micropores in the adsorption. It is feasible to vary the flow rate of carrier gas in the instantaneous adsorption by mesoporous samples, and then to examine their performance in faster carrier gas. Such flow effect was also observed in the adsorption of 1,3-butadiene by TM32 sample (Figure 6), but the increased capability was smaller than that in adsorption of NPYR, because 1,3-butadiene was nonpolar molecule whose olefinic double bond had a special interaction with cation,46,47 and it was easily affected by the acidity of adsorbent.26 So, although micropores have different performance from mesopores in the adsorption of small molecules, further investigation is required to select optimal probe enabling the adsorptive contribution of tiny holes in Al-MCM-41 to be subtly assessed. Different from the thickly docked deep microporous channels in zeolites, these tiny holes established in the channel wall of mesoporous silica are scattered and shallow (Scheme 1). However, these holes made a kind of hierarchical structure to be realized on the channel wall of mesoporous adsorbents, obviously enhancing their performance in adsorption of small environment pollutants such as volatile nitrosamines or 1,3butadiene. 5. Conclusions It is feasible to fabricate hierarchical channel wall in Al-MCM-41 mesoporous materials through acid leach treatment, removing some framework aluminum species and creating micropores and defects in channel wall. In the case that Al-MCM-41 contained aluminum of 0.2-0.6 mmol g-1, more micropores could be created on the high Alcontent sample by acid leach. Fabrication of hierarchical channel wall in these samples can considerably enhance their performance in adsorption of small molecules such as volatile nitrosamines and 1,3-butadiene. It is first time to find the elevated adsorption of NPYR in faster carrier gas by acid leached Al-MCM-41 samples, and this elevation is proportional to the microporous surface area of mesoporous adsorbent, which may provide a method to assess the adsorptive contribution of micropores and defects in mesoporous materials. Acknowledgment. Financial support from 863 Program of MST of China (Grant 2008AA06Z327), NSF of China (20773061, 20873059, and 20871067), Jiangsu Provincial NSF
Gu et al. Industrial Supporting Program (BE2008126), Jiangsu Province Environmental Protection Bureau Scientific Research Program (2008005), and Analysis Center of Nanjing University is gratefully acknowledged. Supporting Information Available: The chemical composition of the acid leaching filtrate of TM-n samples, IR spectra ofparentandacid-leachedM32sample,nitrogenadsorption-desorption isotherms and pore distribution of parent and acid-leached M24, M16 samples, and the low-angle XRD patterns of MCM-41 sample before and after acid leaching. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xu, Y.; Zhu, J. H.; Ma, L. L.; Ji, A.; Wei, Y. L.; Shang, X. Y. Microporous Mesosoporous Mater. 2003, 60, 125. (2) Meier, W. M.; Siegmann, K. Microporous Mesosoporous Mater. 1999, 33, 307. (3) Hoffmann, D.; Hoffmann, I. J. Toxicol. EnViron. Health 1997, 50, 307. (4) Pe´rez-Ramı´rez, J.; Abello´, S.; Bonilla, A.; Groen, J. C. AdV. Funct. Mater. 2009, 19, 164. (5) Vinu, A.; Gokulakrishnan, N.; Balasubramanian, V. V.; Alam, S.; Kapoor, M. P.; Ariga, K.; Mori, T. Chem.sEur. J. 2008, 14, 11529. (6) Xu, Y.; Jiang, Q.; Cao, Y.; Wei, Y. L.; Yun, Z. Y.; Xu, J. H.; Wang, Y.; Zhou, C. F.; Shi, L. Y.; Zhu, J. H. AdV. Funct. Mater. 2004, 14, 1113. (7) Wu, Z. Y.; Wang, H. J.; Zhuang, T. T.; Sun, L. B.; Wang, Y. M.; Zhu, J. H. AdV. Funct. Mater. 2008, 18, 82. (8) Zhou, C. F.; Wang, Y. M.; Cao, Y.; Zhuang, T. T.; Huang, W.; Chun, Y.; Zhu, J. H. J. Mater. Chem. 2006, 16, 1520. (9) Yang, X. Y.; Feng, Y. F.; Tian, G. ; Du, Y. C.; Ge, X.; Di, Y.; Zhang, Y. L.; Sun, B.; Xiao, F. S. Angew. Chem., Int. Ed. 2006, 45, 5729. (10) Choi, M.; Cho, H. S.; Srivastava, R.; Venkatesan, C.; Choi, D. H.; Ryoo, R. Nat. Mater. 2006, 5, 718. (11) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. Chem.sEur. J. 2005, 11, 4983. (12) Kruk, M.; Jaroniec, M.; Joo, S. H.; Ryoo, R. J. Phys. Chem. B 2003, 107, 2205. (13) Celer, E. B.; Kruk, M.; Zuzek, Y.; Jaroniec, M. J. Mater. Chem. 2006, 16, 2824. (14) Van Der Voort, P.; Ravikovitch, P. I.; De Jong, K. P.; Neimark, A. V.; Janssen, A. H.; Benjelloun, M.; Van Bavel, E.; Cool, P.; Weckhuysen, B. M.; Vansant, E. F. Chem. Commun. 2002, 1010. (15) Van Der Voort, P.; Ravikovitch, P. I.; De Jong, K. P.; Benjelloun, M.; Van Bavel, E.; Janssen, A. H.; Neimark, A. V.; Weckhuysen, B. M.; Vansant, E. F. J. Phys. Chem. B 2002, 106, 5873. (16) Bronton, P.; Lu, A.; Schu¨th, F. Carbon 2009, 47, 1005. (17) Xu, J. H.; Zhuang, T. T.; Cao, Y.; Yang, J.; Wen, J. J.; Wu, Z. Y.; Zhou, C. F.; Huang, L.; Wang, Y.; Yue, M. B.; Zhu, J. H. Chem. Asian J. 2007, 2, 996. (18) Zukal, A.; Siklova, H.; Cejka, J. Langmuir 2008, 24, 9837. (19) Gao, L.; Wu, Z. Y.; Yang, J. Y.; Zhuang, T. T.; Wang, Y.; Zhu, J. H. Microporous Mesoporous Mater. 2010, 131, 274. (20) Wang, Y. M.; Wu, Z. Y.; Shi, L. Y.; Zhu, J. H. AdV. Mater. 2005, 17, 323. (21) Braun, P. V.; Rinne, S. A.; Garcı´a-Santamarı´a, F. AdV. Mater. 2006, 18, 2665. (22) Sakai, G.; Yoshimura, T.; Isohata, S.; Uota, M.; Kawasaki, H.; Kuwahara, T.; Fujikawa, D.; Kijima, T. AdV. Mater. 2007, 19, 237. (23) Shu, S.; Husain, S.; Koros, W. J. J. Phys. Chem. C 2007, 111, 652. (24) Reitmeier, S. J.; Gobin, O. C.; Jentys, A.; Lercher, J. A. Angew. Chem., Int. Ed. 2009, 48, 533. (25) Zhou, C. F.; Cao, Y.; Zhuang, T. T.; Huang, W.; Zhu, J. H. J. Phys. Chem. C 2007, 111, 4347. (26) Gao, L.; Gu, F. N.; Zhou, Y.; Yang, J.; Wang, Y.; Zhu, J. H. J. Hazard. Mater. 2009, 171, 378. (27) Gu, F. N.; Zhou, Y.; Wei, F.; Wang, Y.; Zhu, J. H. Microporous Mesoporous Mater. 2009, 126, 143. (28) Ridha, F. N.; Yang, Y. X.; Webley, P. A. Microporous Mesoporous Mater. 2009, 117, 497. (29) Linssen, T.; Cassiers, K.; Cool, P.; Lebedev, O.; Whittaker, A.; Vansant, E. F. Chem. Mater. 2003, 15, 4863. ˇ ejka, J. Collect. Czech. Chem. (30) Deˇdecˇek, J.; Zˇilkova´, N.; Kotrla, J.; C Commun. 2003, 68, 1998. ˇ ejka, J. Microporous Mesoporous Mater. (31) Deˇdecˇek, J.; Zˇilkova´, N.; C 2001, 44-45, 259.
Al-MCM-41 Mesoporous Materials (32) Lin, H. P.; Wong, S. T.; Mou, C. Y.; Tang, C. Y. J. Phys. Chem. B 2000, 104, 8967. (33) Biz, S.; White, M. G. J. Phys. Chem. B 1999, 103, 8432. (34) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. Chem.sEur. J. 2005, 11, 4983. (35) Chen, L. Y.; Horiuchi, T.; Mori, T.; Maeda, K. J. Phys. Chem. B 1999, 103, 1216. (36) Laha, S. C.; Mukherjee, P.; Sainkar, S. R.; Kumar, R. J. Catal. 2002, 207, 213. (37) Shen, S. C.; Kawi, S. J. Phys. Chem. B 1999, 103, 8870. (38) Slejko, F. L. Adsorption Technology: a Step-by-Step Approach to Process EValuation and Application; Marcel Dekker: New York, 1985. (39) Miura, M.; Sakamoto, S.; Yamaguchi, K.; Ohwada, T. Tetrahedron Lett. 2000, 41, 3637. (40) Yang, J.; Ma, L. L.; Shen, B.; Zhu, J. H. Mater. Manuf. Processes 2007, 22, 750.
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8439 (41) Zhao, X. S.; Audsley, F.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 4143. (42) Rice, M. J.; Chakrabortyt, A. K.; Bell, A. T. J. Catal. 1999, 186, 222. (43) Siril, P. F.; Brown, D. R. J. Mole. Catal. A: Chem. 2006, 252, 125. (44) Bhat, Y. S.; Das, J.; Rao, K. V.; Halgeri, A. B. J. Catal. 1996, 159, 368. (45) Gao, L.; Cao, Y.; Zhou, S. L.; Zhuang, T. T.; Wang, Y.; Zhu, J. H. J. Hazard Mater. 2009, 169, 1034. (46) Weinstock, B. A.; Yang, H.; Griffiths, P. R. Vib. Spectrosc. 2004, 35, 145. (47) Armaroli, T.; Finocchio, E.; Busca, G.; Rossini, S. Vib. Spectrosc. 1999, 20, 85.
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