Microporous and Mesoporous Materials 131 (2010) 274–281

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Optimization of mesoporous silica through nano-casting to capture nitrosaminesin environment

Ling Gao a,b , Zheng Ying Wu a, Jia Yuan Yang a, Ting Ting Zhuang a, Ying Wang b, Jian Hua Zhu a,*

a Key Laboratory of Mesoscopic Chemistry of MOE, College of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Chinab Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing University, Nanjing 210093, China

a r t i c l e i n f o

Article history:Received 24 August 2009Received in revised form 4 January 2010Accepted 5 January 2010Available online 11 January 2010

Keywords:Mesoporous materialsEndotemplatingModification with aluminum speciesNitrosaminesEnvironmental protection

1387-1811/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.micromeso.2010.01.002

* Corresponding author. Tel.: +86 25 83595848; faxE-mail address: jhzhu@nju.edu.cn (J.H. Zhu).

a b s t r a c t

A new preparative route of Al-containing mesoporous silica is reported in this paper in order to elevatethe capability of mesoporous materials in adsorbing small pollutant molecules such as volatile nitrosa-mines. A mass of aluminum salt additive was used in the synthesis of mesoporous silica for the purposesof both the insertion of Al into the framework and coating on the channel wall via evaporation. Afterwashing products with an acidic solution, most of the aluminum species were removed to create manytiny flaws and artificial defects in the pore wall, whilst some aluminum species, forming acidic sites, sur-vived in the resulting samples. Both the acidic sites and the hierarchical structure can be formed in themesoporous silica through this nano-casting process, and the resulting samples exhibit a high perfor-mance in the adsorption of volatile nitrosamine N-nitrosopyrrolidine (NPYR).

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Environment protection is not only a new application of but alsothe challenge for molecular sieves. In many situations adsorbentsought to adsorb the targets with trace amounts (ppm or ppb)among a large numbers of other components, and one noteworthyexample is to remove the carcinogenic compounds such as nitrosa-mines in environmental tobacco smoke [1–5]. There are 5200 indi-vidual components in tobacco smoke [6], and among the harmfulconstituents nitrosamines can cause teratogens and carcinogensin laboratorial animals even in trace amount [7]. Two kinds ofnitrosamines exist in environmental tobacco smoke, one is volatilenitrosamine with the relatively small molecular diameter, and an-other is tobacco specific nitrosamine (TSNA) with the bulky size.The complex composition of tobacco smoke makes the selectiveadsorption of nitrosamines difficult. Zeolites are traditional selec-tive adsorbents [8], but their function is limited. The narrow chan-nel of zeolites not only hinders the adsorption of bulkynitrosamines such as TSNA [9,10], but also relates to large pressuredrop. Consequently, zeolites cannot purify a huge mass of gas flowin the aeration system within a short time. Therefore, mesoporousmaterials become the candidates because of their large pore sizes,and as expected, they exhibit a high activity in eliminating tobac-co-specific nitrosamines indeed [11]. However, pure siliceous mes-oporous materials lack cations to exert electrostatic interactions

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toward nitrosamines, resulting in a weak ability to adsorb volatilenitrosamines [9]. To overcome this problem, it is necessary to intro-duce metal compounds such as copper or aluminum species intomesoporous silica through either one-pot synthesis or post-modifi-cation, forming active sites to strengthen the adsorption capabilityof the composite [12,13]. Nonetheless, the dispersion and distribu-tion of guest component affect the actual adsorptive function of thefinal composites, involving not only the accessibility of active sites,but also the synergy between guest species and adjacent silanolgroups [14]. Thus, a question arises from these modifications:how to establish the adsorptive sites with high efficiency insidethe channel of mesoporous silica? In another word, how to maxi-mize the utilization of the modifier? One strategy is to create hier-archical structures in mesoporous composites so that theadsorbents can have both wide channels and micropores for fastmass transport and selective adsorption, respectively. Apart fromthe efforts in generating ‘‘plugs” and/or ‘‘constrictions” in the chan-nel of SBA-15 [15–21], modulating the surface morphology of thepore wall of mesoporous materials was reported [22,23], since themicropores and/or tiny defects in channel wall do not hinder themass transition, but play an important role in adsorbing the harm-ful contents in tobacco smoke [6,22]. Here, a new preparative routeof Al-containing SBA-15-like mesoporous materials with a hierar-chical structure is reported with three characters. The first, for thestarting mixture without acid catalysts, ample aluminum nitratewas utilized to provide the weak acidity for the formation of meso-porous structures through hydrolysis of the salt. It is infeasible toincorporate aluminum into the framework of SBA-15 in traditional

L. Gao et al. / Microporous and Mesoporous Materials 131 (2010) 274–281 275

strong acidic synthesis, because most of aluminum precursors willdissolve in strong acidic media (2 M HCl) [24,25]. In weak acidicsynthetic system, however, aluminum can be incorporated intothe framework of mesoporous silica [26]. The second, Al-containingcomposites ASx (x presents the Si/Al ratio of sample) are synthe-sized by evaporation method. Evaporation of the aged sample withmother solution makes all aluminum compounds retained in prod-uct, inserting into framework or coating on the pore wall of result-ing sample. The third, washing ASx samples with dilute HCl acid toextract most of the aluminum species occupied in framework,forming many tiny flaws in the pore wall of ASx(H+) samples.Through the insertion and removal of aluminum species in meso-porous material, artificial defects are created in its channel walls(Scheme 1), and actually the majority of aluminum salt additiveacts as a template for nano-casting in this synthesis. At the sametime, some aluminum guests survived in the mesoporous host toform acid sites. The resulting samples with hierarchical structurewere characterized by XRD, N2 adsorption–desorption, TEM, andSEM methods, and the accessibility of newly-formed acidic sitesis assessed with traditional NH3-TPD technique. Also, adsorptionof N-nitrosopyrrolidine (NPYR), the monocyclic nitrosamine witha five-member ring [1,27], is utilized to evaluate the function ofAl-modified samples in capturing volatile nitrosamines. Liquidadsorption of N0-nitrosonornicotine (NNN), one of tobacco-specificnitrosamines, is also used to assess the ability of samples in trap-ping bulky nitrosamines.

2. Experimental

2.1. Chemicals and reagents

The triblock poly(ethyleneoxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer Pluronic P123 (Mav = 5800, EO20-

PO70EO20) was the product of Aldrich. Tetraethyl orthosilicate(TEOS), hydrochloric acid and Al(NO3)3�9H2O were obtained fromShanghai Chemical Reagent (China). N-nitrosopyrrolidine (NPYR)and N0-nitrosonornicotine (NNN) were purchased from Sigma [1].All chemicals were used as received without further purification.

SBA-15 was synthesized using P123 as template according toliterature [28]. In a typical synthesis, 4.0 g of P123 copolymerwas dissolved in 150 g of 1.6 M HCl followed by addition of8.50 g TEOS at 313 K. And the resulting mixture was stirred for24 h and then placed in an oven at 373 K for another 24 h understatic condition. The siliceous product was filtered, washes, driedand finally calcined at 823 K in air for 5 h to get template-freesample.

To synthesize the SBA-15 contained small amount of aluminum,named as AS80, 2 g triblock copolymer P123 and a calculatedamount of Al(NO3)3�9H2O were dissolved in 60 g 2 M HCl and15 g H2O, then 4.25 g TEOS was added whilst stirring at 313 K.The solution was stirred for 24 h at 313 K and transferred into a

Scheme 1. The scheme of creating hierarchical structure in mesoporous silica by insertion

Teflon bottle and heated at 373 K for 24 h under static condition.After then, the solvent was evaporated with stirring at 353 K, andthe solid was dried and calcined at 823 K for 5 h.

To prepare Al-containing mesoporous composites without acidcatalyst, 2 g of P123 was dissolved in 75 g H2O, then a calculatedamount of Al(NO3)3�9H2O was added and stirred for 0.5 h. Afterthat, the mixture was heated to 318 K and 4.25 g TEOS was added.The molar composition of mixture, TEOS:P123:Al(NO3)3�9H2O:H2O,was 1:0.017:y:192, where y varied from 1 to 4 to obtain the sam-ples with various aluminum content. Thereafter, the mixture wasstirred under 318 K for 24 h, aged at 373 K for 24 h, and then sol-vent was evaporated with stirring at 353 K. In order to preparemesoporous adsorbent, the as-synthesized sample was dividedinto three parts. The first part was directly calcined at 823 K for5 h to remove the template and to give the composite denoted asASx (x = 1/y = 1, 0.5, 0.33 and 0.25). The second part was washedwith water, for which 1 g of as-synthesized sample was thoroughlywashed by 100 ml distilled water, followed by drying and calcina-tions at 823 K to obtain the sample named as ASx(H2O). However,some ASx samples (x = 0.5, 0.33 and 0.25) need to be eluted by di-luted HCl solution because they could be hydrolyzed by water. Forthe third part to be washed by acidic solution, 1 g of as-synthesizedsample was eluted by 100 ml of 0.2 M HCl then dried and calcinedat 823 K for 5 h, giving the sample denoted as ASx(H+).

2.2. Characterization

XRD patterns of sample were recorded on a set of D/MAX-RAX-ray diffractometer with Cu Ka radiation, in which the X-ray tubewas operated at 40 kV and 100 mA, and the measurement was per-formed in the 2h range from 0.5� to 5� or from 5� to 80�. N2 adsorp-tion and desorption isotherms were measured at 77 K using aMicromeritics ASAP 2020 system, and every sample was outgassedat 573 K for 4 h prior to test. The Brunauer–Emmett–Teller (BET)specific surface area of sample was calculated using adsorptiondata in the relative pressure range from 0.04 to 0.2, and the totalpore volumes were determined from the amount adsorbed at a rel-ative pressure of about 0.99. Pore size distribution curve of samplewas calculated from analysis of the adsorption branch of isotherm,using Barret–Joyner–Halenda (BJH) algorithm. Metal content ofsample was measured by inductive coupled plasma–atomic emis-sion spectrometry (ICP-AES), for which solid sample was dissolvedinto hydrofluoric acid solution at first, and then the solution wasanalyzed by OPTIMA 5300 inductively coupled plasma-atomicemission spectrometry (ICP-AES).

Temperature-programmed desorption of ammonia (NH3-TPD)was performed in a conventional flow-type micro-reactor. Onehundred milligram of samples were activated at 823 K for 2 h ina flow of nitrogen, and then cooled to 323 K to adsorb NH3 [9].After the sample was purged by nitrogen flow at 298 K to removethe physically adsorbed NH3, it was heated at a rate of 8 K min�1

of aluminum species in weak acidic synthetic conditions combined with acid wash.

276 L. Gao et al. / Microporous and Mesoporous Materials 131 (2010) 274–281

up to 823 K while the liberated NH3 was detected by an ‘‘on line”Varian 3380 chromatograph [9].

Solid-state 27Al NMR measurements were performed on a Bru-ker MSL-400 NMR spectrometer equipped with a magic-angle-spinning (MAS) unit. 27Al MAS NMR spectrum of sample was re-corded at a frequency of 72.22 MHz and a spinning rate of 6 kHz,with a pulse length of 1.0 ls, a delay time of 0.2 s, a spectral widthof 330 ppm, and 1024 scans. The line broadening was 50 Hz, and27Al chemical shifts were reported relative to a liquid solution of1 mol L�1 Al(NO3)3.

Instantaneous adsorption of volatile nitrosamines was carriedout by use of gas chromatography (GC) method [27]. Five milli-gram sample (20–40 meshes) was filled in a stainless steel mi-cro-reactor and directly heated to 453 K, and then the solution ofnitrosamine was pulse injected with the amount of 2 lL each time.Thermal conductivity detector of GC was used to analyze the gas-eous effluent and the decrement in the ratio of solute to solventwas utilized to calculate the adsorbed amount [27]. Liquid adsorp-tion of NNN was completed at 277 K [9], and 20 mg sample parti-cles (20–40 meshes) contacted with 1.1 ml dichloromethanesolution of NNN for 24 h, and the residual nitrosamines in solutionwere detected by spectrophotometer [1,29].

3. Results and discussion

3.1. Textural properties of acid washed Al-containing mesostructuredsamples

Fig. 1A depicts the XRD patterns of ASx samples. AS1 showedan ordered mesostructure with p6mm symmetry that was charac-terized with three diffraction peaks indexed to (1 0 0), (1 1 0) and(2 0 0) reflections of hexagonal structure. Since ample aluminumsalt had been used to synthesize AS1 sample, characteristic ofSBA-15 was weakened in the XRD pattern due to block and/orcollapse of channel. Likewise, the ordering of structure was seri-ously reduced in other ASx samples. Fig. 2a shows the SEM imageof AS1 sample. AS1 consisted of many fine grain-like domainsthat aggregated to unordered particle, different from the wheat-like macrostructures of SBA-15 [28].

N2 adsorption–desorption isotherms of ASx samples presentconsistent results to XRD experiments. AS1 sample gave the dis-

1 2 3 4

AS0.5

AS1

AS0.33

AS0.25

2 Theta (degree)

A

Fig. 1. Low-angle XRD patterns of Al-con

torted type IV isotherms with sharp capillary condensation at highrelative pressure, along with H1-type hysteresis loop, indicative oflarge channel-like pores in a narrow range of size (Fig. 3A). Owingto the ample aluminum salt retained in this sample, plugs and/orconstrictions were abounded in mesopores and the pore size distri-bution became wide (Fig. 3B). Moreover, the pore size distributionof AS1 calculated from desorption branch showed that the size ofprimary mesopores declined from 6.3 nm to 3.7 nm (Fig. S1a).Other three ASx samples had similar phenomena, originating fromthe obstruction of mesopores (Fig. 3A and B). As illustrated in TableS1, both BET surface area and pore volume of ASx samples dramat-ically decreased when ample salt was used in their synthesis. Forexample, the surface area and pore volume of AS1 decreased about60% in comparison with SBA-15 (Table 1). Also, both microporousarea and volume of ASx samples declined distinctly, either (TableS1).

Washing AS1 sample by water released some pores so thatAS1(H2O) sample exhibited a XRD pattern similar to SBA-15(Fig. 1B). A majority of aluminum species was removed fromAS1(H2O) hence its Al content reduced from 9.0 to 1.82 mmol g�1.However, AS1(H2O) still had the value of Smic and Vmic similar toAS1 (Table 1), implying the main location of remained aluminumspecies inside micropores. As illustrated in Fig. S2, AS1(H2O) sam-ple possessed the N2 adsorption–desorption isotherms of type IV,H1-type hysteresis loops and narrower pore size distributions,indicative of its highly ordered mesostructures. The two-step cap-illary evaporation of AS1(H2O) was indistinct, implying the re-moval of plugs and/or constrictions in this sample. AS1(H2O)sample consisted of many rope-like domains, aggregated towheat-like macrostructures, similar to SBA-15 [28], as the SEMimages in Fig. 2B showed.

Washing AS1 sample with 0.2 M HCl solution could liberatemore mesopores and micropores. AS1(H+) sample possessed bettertextural properties than AS1 or AS1(H2O), as shown in Table 1. Asmall quantity of aluminum species (1.2 wt.%, 0.24 mmol g�1) re-mained on AS1(H+) sample, while larger BET surface area, micro-pore area and pore volume were revived. Same phenomena wereobserved on other three ASx(H+) samples, AS0.5(H+), AS0.33(H+)and AS0.25(H+) all had large surface areas and pore volumes (Table1), especially the proportion of micropore dramatically increasedin these samples. As the result, they had the larger microporoussurface area and micropore volume than AS1(H+). However, acidic

1 2 3 4

SBA-15

AS80 AS1

AS1(H2O)Inte

nsity

/ a.

u.

AS0.25(H+)

AS0.33(H+)AS0.5(H+)

AS1(H+)

2 Theta (degree)

B

taining mesoporous silica samples.

Fig. 2. SEM image of (a) AS1, (b) AS1 (H2O), (c) AS1(H+), (d) AS0.5 (H+) samples.

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

AS0.25 AS0.33

AS1

AS0.5

Qua

ntity

ads

orbe

d (c

m3 g-1

STP

)

Relative pressure (p/p0)

A

0 5 10 15 20 25

0.0

0.5

1.0

1.5 AS1

AS0.5

AS0.33

AS0.25

Pore diameter (nm)

Pore

vol

ume

(cm

3 g-1)

AdsorptionB

Fig. 3. N2 adsorption–desorption isotherms (A) and the pore size distribution curves calculated from the analysis of the adsorption branch of the isotherm (B) of the Al-contained mesoporous samples synthesized by evaporation method. (The distribution curves calculated from the analysis of adsorption branches for AS0.33, AS0.5 and AS1are offset vertically by 0.2, 0.4 and 0.6 cm3 g�1, respectively.)

L. Gao et al. / Microporous and Mesoporous Materials 131 (2010) 274–281 277

wash caused some structural variations in AS1(H+) thereby it lost(1 1 0) and (2 0 0) character reflections of SBA-15 (Fig. 1B). Moreserious situation was observed on other three ASx(H+) samplesand their XRD patterns seemed no longer the SBA-15 type. Onthe other hand, tailing emerged in the N2 physisorption isotherms

of AS0.5(H+) and AS0.33(H+) samples hence their hysteresis loopswere deviating from H1-type. Moreover, the hysteresis loop ofAS0.25(H+) was almost H2-type (Fig. 4A), indicative of blocked orink-bottle pores. As demonstrated in Fig. 4B, the size and volumeof primary pore decreased in the sequence of AS1(H+) < A-

Table 1Physical properties of Al-containing mesoporous silica samples and their adsorption capacity toward ammonia or NPYR.

Sample Al content(mmol g�1)

SBET

(m2g�1)Smic

(m2g�1)Vp

(cm3g�1)Vmic

(cm3g�1)Vmic/Vp

(%)Dpa

(nm)NH3-TPD(mmol g�1)

Trapped NPYRb

(mmol g�1)

SBA-15 0 902 187 1.09 0.08 7.3 6.3 0.07 0.16AS80a 0.20 861 145 1.14 0.06 5.3 6.4 0.24 0.32AS1a 9.0 339 23 0.42 0.01 2.4 7.3 0.38 0.49AS1(H2O) 1.80 495 25 0.70 �0.01 0 8.9 0.49 0.60AS1(H+) 0.24 835 170 0.86 0.07 8.1 7.3 0.23 0.65AS0.5 (H+) 0.14 721 222 0.62 0.10 16.1 6.2 0.25 0.59AS0.33(H+) 0.16 664 249 0.54 0.11 20.4 6.3 0.28 0.54AS0.25(H+) 0.26 500 213 0.35 0.10 28.6 4.9 0.36 0.51

a The content of Al was theoretically calculated.b This indicated the amount of NPYR captured by the sample when the accumulated amount of NPYR reached 1.2 mmol g�1.

278 L. Gao et al. / Microporous and Mesoporous Materials 131 (2010) 274–281

S0.5(H+) < AS0.33(H+) < AS0.25(H+) sample, mirroring their tailoredpore structure.

AS1(H+) had a lower Al content but a larger amount of microp-ores than AS1(H2O) (Table 1), and its Smic and Vmic values wereclosed to that of SBA-15. Due to different synthetic procedures,AS1(H+) had a larger ratio of Vmic/Vp than AS80 sample thoughtwo samples had similar Al content (Table 1). Unlike AS1(H2O) thathad smooth rope-like domains, AS1(H+) possessed an irregularmorphology with rough surface where a lot of micropores formed(Fig. 2C). Similar image was observed on AS0.5(H+) sample onwhich more micropores were formed as if the surface sufferedfrom erosion (Fig. 2D).

Fig. 5 illustrates the TEM image of Al-containing mesoporoussamples. AS80 had an image identical to SBA-15 (image A inFig. 5), but the channel wall of AS1(H2O) sample became roughwhile some pore-openings were somewhat asymmetrical (imageB in Fig. 5). For AS1(H+) sample the wall of channel looked so harshas if these channels were distorted (image C in Fig. 5), resultingfrom the artificial defects in channel walls (Scheme 1). Worm-hole-like structure appeared in these high Al contents samples,coincided with the result of XRD experiment (Fig. 1B). Long-rangepacking order was absent in the image of AS0.5(H+), in which onlysome channel fragments were remained and accompanied withwormhole-like framework (D in Fig. 5). Referring these phenomenait is safe to infer that artificial defects can be effectively made inthe channel walls of mesoporous silica through special nano-cast-

0.0 0.2 0.4 0.6 0.8 1.0

100

200

300

400

500

600

700

Qu

anti

ty a

dso

rbed

(cm

3 g-1 S

TP

)

Relative pressure (p/p0)

A

AS1(H+)

AS0.5(H+)

AS0.33(H+)

AS0.25(H+)

Fig. 4. N2 adsorption–desorption isotherms (A) and the pore size distribution curves calcevaporated then washed by H2O and HCl aqueous solution. (The isotherms for AS0.5(Hdistribution curves calculated from the analysis of adsorption branches for AS0.33(H+), A

ing process, but using too much aluminum salt in synthesis willcause the disordered structure in sample.

Table 1 lists the amount of NH3 desorbed from SBA-15 and itsAl-containing analogues during TPD process. There were desorp-tions around 416 K and 500–583 K on the profile of AS1 sample(Fig. S3), and the amount reached 0.38 mmol NH3 g�1, larger thanthat on AS80 (Table 1). The value of R that meant the adsorbed NH3

divide by the Al content of sample, was only 0.04, much smallerthan that of AS80. Desorption of NH3 appeared at 393 K and545 K on AS1(H2O) sample and the amount increased to 0.49 mmolNH3 g�1 (Table 1), so its R value rose to 0.27. Contrarily, only asmall desorption of NH3 emerged on AS1(H+) at 695 K with theamount of 0.23 mmol NH3 g�1 (Table 1), but its R value jumpedto 0.96 because the sample had a low Al content. On the NH3-TPD profiles of other three ASx(H+) samples, weak desorption peakin the range of 363–407 K became the primary but their R valuesfurther increased. Judged on these results, it is clear that the alumi-num species in ASx(H+) sample can efficiently form acidic sites toadsorb ammonia.

Fig. 6 demonstrates the 27Al MAS NMR spectra of Al-contain-ing mesoporous samples. Fifty-four ppm peak in the spectrumof AS80 confirmed the insertion of Al into framework of SBA-15,but there was no signal near 0 ppm, indicating the absence ofoctahedrally coordinated Al [30]. Contrarily, a strong signal near0 ppm emerged on the spectrum of AS1(H2O) whereas only ashoulder peak appeared around 54 ppm, meaning that only a

0 5 10 15 20 250

1

2

3

4

5AS1(H+)

AS0.5(H+)

AS0.33(H+)

AS0.25(H+)

Pore diameter (nm)

Po

re v

olu

me

(cm

3 g-1)

AdsorptionB

ulated from the analysis of the adsorption branch of the isotherm (B) of the sample+) and AS1(H+) are offset vertically by 70 and 150 cm3 g�1 STP, respectively. TheS0.5(H+) and AS1(H+) are offset vertically by 0.5, 1.0 and 1.8 cm3 g�1, respectively.)

Fig. 5. TEM image of (A) AS80, (B) AS1(H2O), (C) AS1(H+) and (D) AS0.5(H+) samples.

c

L. Gao et al. / Microporous and Mesoporous Materials 131 (2010) 274–281 279

minority of aluminum species inserted into framework. Acidwash removed the extra framework Al, only those Al speciesinserting in framework survived [17] therefore the peak near54 ppm became primary in the spectrum of AS1(H+) sample.Accurately, the ratio of 4- and 6-coordinated Al in AS1(H2O)and AS1(H+) samples, calculated with graphic integral method, in-creased remarkably from 1.1 to 14.2; This provides a clear evi-dence for the function of acidic wash in adjusting thedispersion and distribution of aluminum in mesoporous silica.Since only the Al incorporating into channel walls can be retainedin acid-washed sample (Scheme 1), optimized modification ofaluminum on mesoporous silica is thus achieved because theseAl species can form acidic sites to promote the adsorption ofnitrosamines as illustrated later.

-200 -100 0 100 200Chemical Shift / PPm

a

b

Fig. 6. 27Al MAS NMR spectra of Al-containing mesoporous silica (a) AS80, (b)AS1(H2O) and (c) AS1(H+) samples.

3.2. Adsorption of nitrosamines by Al-containing mesoporous samples

Fig. 7 displays the instantaneous adsorption of NPYR in gasstream by Al-containing composites. When the amount of NPYRaccumulated to 1.2 mmol g�1, 26% of them (0.32 mmol g�1) wascaptured by AS80 while 13% trapped by SBA-15 (Table 1). It is evi-dent that incorporation of aluminum in SBA-15 significantly pro-motes the adsorption of NPYR. AS1 sample could adsorb about40% of NPYR (Fig. 7A), but other three ASx samples had very lowability due to their nonporous properties. Under the same condi-tions, AS1(H2O) sample could adsorb 50% of NPYR (0.60 mmol g�1)while AS1(H+) sample adsorbed 54%, much more than pure sili-ceous SBA-15 (13%) or AS80 composite (26%). It should be notedthat other three ASx(H+) samples also exhibited the capabilityhigher than AS(80) throughout whole experiment as shown inFig. 7. And the plenty of micropore in ASx(H+) should be taken intoaccount for this high performance because the micropore in meso-porous silica or activated carbon is most efficient for removal ofsmoke vapor phase compounds [6,22,26].

For an overall analysis of adsorption isotherms, we fit the exper-imental isotherms of SBA-15, AS80 and AS1(H+) by Freundlichadsorption equation that is an empirical model [29,31].

ln q ¼ ln KF þ 1=n ln C;

0.0 0.4 0.8 1.2 1.60.0

0.1

0.2

0.3

0.4

0.5A

dsor

bed

NPY

R (m

mol

g-1)

Total amount of NPYR (mmol g-1)

AS1 AS0.5 AS0.33 AS0.25

A

0.0 0.4 0.8 1.2 1.60.0

0.2

0.4

0.6

0.8 AS1(H2O)

AS1(H+)AS0.5(H+) AS0.33(H+)AS0.25(H+)AS80SBA-15

Ads

orbe

d N

PYR

(mm

ol g

-1)

Total amount of NPYR (mmol g-1)

B

Fig. 7. Adsorption of NPYR by Al-containing mesoporous silica samples in gas stream at 453 K.

280 L. Gao et al. / Microporous and Mesoporous Materials 131 (2010) 274–281

where q is the amount of nitrosamine adsorbed on per gram sample,and C is substituted by the total amount of nitrosamines passed pergram adsorbent. KF is known as Freundlich constant, which is re-lated to adsorbent capacity and 1/n is an exponent associated toadsorptive strength as well as the favorability [31]. Table 2 liststhe isotherm parameters calculated with the method of leastsquares. The KF value of NPYR in AS1(H+) was 10% larger than thatin SBA-15, and the largest value of 1/n was also observed onAS1(H+).

If we subtly calculate the adsorptive efficiency of samplesthrough dividing the amount of adsorbed NPYR by their Al content,high values are found on ASx(H+) samples. Both AS0.33(H+) andAS0.25(H+) possessed the Al content close to AS80, but they exhib-ited a higher ability to capture NPYR (Table 1). To accurately assessthe adsorptive ability of samples without the influence of surfacearea, we also calculate the adsorption capacity in unit area,lmol m�2 instead of mmol g�1. The corresponding value of SBA-15, AS80 and AS1(H+) is 0.18, 0.37 and 0.78 lmol m�2, andAS1(H+) sample is also the champion.

AS1(H+) had an Al-content similar to AS80 sample but adsorbeddouble amount of NPYR under the same conditions (Table 1),which resulted from its hierarchical structure. AS1(H+) samplewas evaporated with mother solution, Al species were apt to inter-act with silanol group during the evaporation of water [32], form-ing small hole, kink and vacancy in the surface of sample [33]. Inthe succeeding acid washing stage, only the aluminum species lo-cated in these defects could resist the purge of solution, becausethe negative curvature of defects slowed the flush of liquid [22].These defects also played an important role in the adsorption ofNPYR (Fig. 7B). When NPYR molecules moved to these defects,the negative curvature of defects slowed down the gas flow sothe adsorptive sites in the defects could effectively capture theNPYR. On the other hand, AS1(H2O) sample had a larger Al content

Table 2Freundlich constants of the adsorption isotherms of SBA-15 and Al-containingcomposites.

Sample KF n R2

SBA-15 1.024 9.14 0.95AS80 1.057 3.81 0.95AS(H+) 1.100 2.14 0.95

but trapped 8% less NPYR than AS1(H+), since the former had a pooraccessibility of Al sites. Water washing could not remove the alu-minum species blocked in the micropore of sample (Scheme 1)therefore these species still formed multi-layers. As a result, onlythe species on the surface could contact with NPYR while thoseto be covered could not exert their function [22].

The high accessibility of Al acidic sites in AS1(H+) sample resultsfrom the special synthetic route. We use ample aluminum nitratein one-pot synthesis, not only providing the modifier to functional-ize siliceous host, but also offering the necessary weak acidity toform mesoporous framework. Besides, aluminum salt plays therole of special endotemplating to make new pore and/or defectin channel walls. We inserted the aluminum species inside thechannel walls during evaporation step, and removed most of themthrough acidic wash step to form defects. Through this specialnano-casting process three advantages can be achieved. The first,hierarchical structured pore walls are produced; the second, mostof the aluminum compounds can be recycled. The third, only theseAl species, forming acidic sites in tiny defects, can survive in thefinal sample, and they will exert an optimal function to adsorbNPYR as aforementioned.

Fig. 8 exhibits the adsorption of NNN in organic solution byAS1(H+) sample and its analogues at 277 K. NNN is a bulky nitrosa-mine with a molecular diameter of 0.75 nm � 0.80 nm [11], hencezeolite NaY hardly adsorbed NNN but mesoporous silica SBA-15showed a three times higher capacity than NaY [9]. Among the fivemesoporous samples evaluated in Fig. 8, however, SBA-15 was theworst because it adsorbed 54 lmol g�1 of NNN whilst other fourAl-containing mesoporous samples captured 66–71 lmol g�1 un-der the same conditions. Based on these results, it is evident thatmodification with aluminum and creation of hierarchical structurein mesoporous silica also promote the adsorption of bulky nitrosa-mines in environment.

4. Conclusion

In summary, novel Al-containing mesoporous adsorbents areprepared via one-pot method combined with acid washing, andthey have hierarchical structure and high performance to adsorbvolatile nitrosamines. We utilize aluminum salt additive to createmicropore or flaws/defects in channel walls and to optimize themodification of Al in mesoporous silica. These new mesoporous

40

50

60

70

80

SBA-15

AS1(H2O)AS0.25(H+)

AS0.33(H+)

Amou

nt o

f NN

N a

dsor

bed

( µm

ol g

-1)

Samples

AS1(H+)

C0=1.13 mmol L-1

at 277 K

Fig. 8. Adsorption of NNN by mesoporous materials in CH2Cl2 solution at 277 K.

L. Gao et al. / Microporous and Mesoporous Materials 131 (2010) 274–281 281

functional materials are also efficient to trap bulky nitrosamines insolution, implying a potential application to protect environment.

Acknowledgements

Financial support from 863 Program of MST of China (Grant2008AA06Z327), NSF of China (20773061, 20873059 and20871067), Jiangsu Provincial NSF Industrial Supporting Program(BE2008126), Jiangsu Province Environmental Protection BureauScientific Research Program (2008005), and Analysis Center ofNanjing University is gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2010.01.002.

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