DOI: 10.1002/chem.200801606

Functional Mesoporous Material Derived from 3D Net-Linked SBA-15

Yu Zhou,[a] Ling Gao,[a] Fang Na Gu,[a, b] Jia Yuan Yang,[a] Jing Yang,[a] Feng Wei,[a]

Ying Wang,*[b, c] and Jian Hua Zhu*[a]

Introduction

Since the discovery of the M41S family in 1992, a great dealof effort has been made to develop new functional mesopo-rous materials through tuning the pore structure and surfacefunctionalization of the channels.[1–4] But little attention hasbeen paid to the morphological control of mesoporous ma-

terials both at the microscale and macroscale levels[5] thoughspecific morphologies are required, for example, for drugdelivery, as well as photonic, electronic, and magnetic devi-ces.[6] However, certain requirements of society demand newfunctional materials with specific morphologies, thus chal-lenging the design and synthesis of mesoporous molecularsieves. One challenge comes from the fight against cancer inwhich it is of key importance to remove trace amounts ofcarcinogens from the environment. One difficult battle is tocontrol the pollution caused by smoking, which has becomea global problem. Tobacco smoke consists of more than4000 individual components in the vapor and particulatephases;[7] therefore, it is difficult to capture a target such asnitrosamines that are well known as strong carcinogens.[7,8]

Because of their unique selective adsorption abilities, zeoliteand modified mesoporous silica can effectively capture vola-tile nitrosamines in smoke,[8–10] but these ordered porous ma-terials are hardly able to specifically trap tobacco-smoke ni-trosamine, another type of strong carcinogen that exists inthe particles suspended in smoke, because these particlesare usually micrometers in size,[7] thus exceeding the poresize of these adsorbents, which is in the nanometer range.

Abstract: A novel, versatile functionalmesoporous material with the structureof SBA-15 but a specific 3D net-linkedmorphology has been prepared byusing a facile method. This materialhas been assessed for its ability toreduce the level of nitrosamines in to-bacco smoke so that the actual effectof this material in the reduction of car-cinogenic environmental pollutants in acomplex chemical system can be veri-fied. By adjusting the force field duringthe hydrolysis and condensation pro-cesses of tetramethyl orthosilicate(TMOS) in an aqueous reactionsystem, the SBA-15 silica, which has aspecial morphology in that it is long

and fiberlike from a microscopic viewbut large and block-like from a macro-scopic angle, has been successfully syn-thesized. Moreover, the synthetic prod-uct is no longer a powder but a blockbecause of its specific 3D net-linkedmorphology, which enables the materi-al not only to trap particles in smokebut also to potentially be applied as adevice. To strengthen the adsorptivecapability of this porous material, adry-impregnation procedure was ap-

plied in which zirconia was coated onthe silica wall without changing themorphology. As expected, this func-tional material shows a high activity inthe adsorption of N’-nitrosonornicotine(NNN), a bulky tobacco-specific nitros-amine, in solution and of N-nitrosopyr-rolidine (NPYR), a volatile nitrosa-mine, in the gas stream. Furthermore,this 3D net-linked SBA-15 material canefficiently decrease the amount of ni-trosamines and total number of parti-cles in mainstream smoke when it istested as an additive in the filter of acigarette, thus providing powerful pro-tection for the environment and publichealth.

Keywords: environmental protec-tion · mesoporous materials · nitros-amines · zirconia

[a] Y. Zhou, L. Gao, F. N. Gu, J. Y. Yang, J. Yang, F. Wei,Prof. Dr. J. H. ZhuKey Laboratory of Mesoscopic Chemistry of MOESchool of Chemistry & Chemical EngineeringNanjing University, Nanjing 210093 (China)Fax: (+86) 25-83317761E-mail : jhzhu@netra.nju.edu.cn

[b] F. N. Gu, Prof. Dr. Y. WangDepartment of ChemistrySchool of Chemistry & Chemical EngineeringNanjing University, Nanjing 210093 (China)E-mail : wangy@netra.nju.edu.cn

[c] Prof. Dr. Y. WangEcomaterials and Renewable Energy Research Center (ERERC)Nanjing University, Nanjing 210093 (China)

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Likewise, these molecular sieves are also incapable of re-moving the micrometer particles in the air stream, thus fail-ing to clean the environmental tobacco smoke in the ventpipes of multistorey buildings.

Unlike these powder-like molecular sieves, the celluloseacetate fibers in the filters of cigarettes can efficiently trapthe particles in smoke,[11] which gives us a valuable clue forthe development of new adsorbents for environmental pro-tection. Clearly, apart from the high surface area and or-dered pore structure, new mesoporous candidates shouldpossess a fiberlike or more delicate netlike morphology tocapture the particulates that are micrometers in size, in asimilar manner to using a fishing net to hold fish, thus reme-dying the inherent limitation of pore size and enhancingtheir adsorptive efficiency through specific morphology. Oneaim of this study is to synthesize such a novel functional ma-terial, that is, the SBA-15 fibers crosslink to form a 3D net-like aggregation that can effectively filter a stream of parti-cles that are micrometers in size. Also, we aimed to obtainthe synthesized product of mesoporous silica as a nonpow-der to extend potential applications toward the constructionof devices.

The morphological control of mesoporous materials is athermodynamic as well as kinetic process that is influencedby several complex factors, such as the initial synthetic com-position, the rates of hydrolysis and condensation of the in-organic precursor, reaction temperature, and the presence ofadditives.[12–18] Also, the field effects, for example, gravita-tional, electric, magnetic, hydrodynamic, shear, and stressforces,[19] can influence the final morphology to a highdegree. Until now a variety of mesoporous materials withparticular micro- and macromorphologies, such asspheres,[20,21] rods,[22] fibers,[23] tubes,[24] thin films,[25] mem-branes,[26] monoliths,[27] and crystals,[28] have been preparedby adjusting the synthetic conditions. Nonetheless, some spe-cial techniques are necessary in these preparations, for in-stance solvent evaporation,[29] two-phase systems,[30,31] orspecial templates, such as a single-strand spider silk.[32] Formonoliths that are ordered over a macroscopic length scale(greater than millimeter scale), various nonionic tem-plates[33–35] or microemulsions[36] are utilized. Although thesesynthesized materials have many potential applications, suchas laser materials, optic, or electronic waveguides, simplebut effective synthetic routes are still desirable.

Herein, we report a new facile strategy for the synthesisof a SBA-15 monolith that is ordered on a scale from nano-meters to millimeters or even centimeters. Based on thegrowth mechanism of mesoporous silica during thermal syn-thesis,[14] we planned to adjust the force field during hydroly-sis to create a selective adsorptive ability and a netlike tex-tural property within one material. The obtained product iscentimeters in three dimensions, which is beneficial for thedirect application of the SBA-15 material without furthermolding. In addition, through a dry-impregnation procedure,zirconia can be coated onto the channel wall of the mono-lithic mesoporous silica, thus incorporating the necessarymetal component to generate active sites for adsorbingsmall molecules, such as volatile nitrosamines, and enablingthe modified SBA-15 monolith to be a versatile functionalmesoporous material in environmental protection. To assessthe actual adsorptive function of the 3D net-linked SBA-15material, two nitrosamines with different molecular sizes,that is, N-nitrosopyrrolidine (NPYR; a typical volatile ni-trosamine) and N’-nitrosonornicotine (NNN; a tobacco-spe-cific nitrosamine) were employed in gas-stream or solutionadsorption under laboratory conditions. Moreover, the SBA-15 monolith was directly added into the filter of a cigaretteto assess its performance in trapping the micrometer-sizedparticulates in smoke and the nitrosamines in the complexsystem.

Results and Discussion

Preparation of the support materials : Various SBA-15 sam-ples were synthesized by using a conventional procedure[2]

from different silica sources and by using a finely adjustedstirring rate to obtain products with altered morphologies.These resulting samples are denoted as MxSy, where M rep-resents the silica source and S the stirring rate. Figure 1 Adisplays the scanning electron microscopy (SEM) image of afragment of the large monolithic M2S1 with a diameter of40 mm and a height of 30 mm, which was synthesized byusing tetramethyl orthosilicate (TMOS) as the silica sourceand a slow stirring rate of 75 rmin�1. As examined from thetop and sides of the fragment, this monolith has a net-linkedmorphology that consists of numerous flexural fibers thatconnect to each other to form a block. Close inspection ofthe SEM images shows that the long fiberlike micromor-phology was obtained in a length of hundreds micrometers,is composed of a bundle of wires with a diameter of aroundabout 350 nm, and consists of rodlike particles that run par-allel to the long axis from one end to the other (Fig-ure 1 B, C).[14] Unlike common fiberlike SBA-15 silica,[13] theM2S1 sample comprises fibers that are arranged parallel toand cross-linked with each other to form netlike aggrega-tions (Figure 1 C). For comparison, Figure 2 depicts the mor-phology of common SBA-15, which has many rope-like do-mains that aggregate into wheat-like microstructures; there-fore, it is very different from our 3D net-linked sample. Thelattice parameter (a0) of these two samples (M2S1 and

Abstract in Chinese:

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FULL PAPER

common SBA-15) is shown in Table 1. The smaller stirringrate relates to the larger a0 value regardless, thus indicatingthat tuning the stirring rate leads to no change in the natureof the surfactant agent but in the different hydrolysis andcondensation rates of the silica precursor.

Apart from the difference in morphology, our 3D net-linked SBA-15 sample is considerably tougher than common

SBA-15. For example, theformer can resist a pressure of2.5 N cm�2 and is pulverizedpartly as the pressure rises to10 N cm�2 ; whereas, the latterimmediately becomes a finepowder, even when the pres-sure is applied at less than0.5 N cm�2. This specific tough-ness enables the net-linkedSBA-15 to be a potential candi-date for application in a specificdevice, that is, the large blockof product can be divided intoparticles with a natural textualproperty.

Figure 2 shows the SEMimages of M1S1, M2S2, M2S3,and M3S1, thus revealing thecrucial role played by the stir-ring rate in controlling the mor-phology of SBA-15. In the syn-thetic procedure for makingcommon SBA-15, in which tet-raethyl orthosilicate (TEOS)was used as the silica source, itwas feasible to obtain M1S1with a net-linked morphologyprovided that the stirring ratewas very low (e.g., 75 rmin�1).In contrast, in the synthesis ofM2S1 with TMOS as the silicasource, the morphology of theproduct could be changed fromnet-linked to closely connectedfibers of combined wires (seethe images of M2S2 and M2S3),whereas the macromorphologychanges from a block to apowder when the stirring ratebecame higher (i.e. , from 75 to150 or 300 rmin�1). However,varying the stirring rate did notaffect the pore structure of theresulting composites. When thestirring rate was lowered from600 to 75 rmin�1, two sampleshad the same low-angle X-raydiffraction (XRD) patterns(Figure 3 A). The XRD patterns

both show high-intensity reflections and two higher-angle re-flections indexed as (100), (110), and (200), respectively, andthe values of d ACHTUNGTRENNUNG(100):d ACHTUNGTRENNUNG(110):d ACHTUNGTRENNUNG(200) are 1:31/2:2, which indicatea well-ordered mesostructure with a 2D hexagonal symme-try (P6mm). Also, the nitrogen adsorption–desorption iso-therm of M2S1 was similar to that of common SBA-15 (Fig-ure 3 B), and both of these two samples yielded type-IV ni-

Figure 1. a–c) SEM and d) TEM images of M2S1.

Figure 2. SEM images of the SBA-15 samples with different morphologies.

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J. H. Zhu, Y. Wang et al.

trogen adsorption/desorption isotherms with a well-resolvedtype-H1 hysteresis loop, thus indicating they are all typical(1D) cylindrical channel mesoporous materials. Similarly,they yielded a narrow pore-size distribution with a meanvalue of around 9.0 nm from the adsorption progress(Table 1). Figure 1 D delineates the TEM image of M2S1,thus confirming the existence of channels and the hexago-nally ordered pores of SBA-15 in this composite. The silicasource was another factor that influenced the morphologyof the mesoporous silica sample. When Na2SiO3 replacedTMOS or TEOS in the synthesis, a sample with a tight com-bined fiberlike micromorphology and a hard thin bar macro-morphology was obtained (see the image of M3S1 inFigure 2).

Figure 4 delineates the textural property of SBA-15 withvarious morphologies to indicate that the slow stirring ratedid not change the pore structure of the samples made fromTMOS. Four samples have similar Brunauer–Emmett–Teller(BET) and micropore surface areas (Figure 4 A), and theyall possess the most probable pore size of around 9.0 nm(Figure 4 B). However, the wall thickness slightly increasedwhen the stirring rate decreased, probably because a lowerstirring rate leads to a different rate of hydrolysis and con-centration of the silica source. Figure 4 C shows a linear rela-tionship between the BET surface area and the total porevolume of the SBA-15 samples synthesized with differentstirring rates. The ratio of pore volume to surface area, de-noted as Rt, is constant (i.e. , 1.3 �10�3) among the four sam-ples. For the aforementioned sample, M3S1 with structuraldistortion, the Rt value was slightly lower (i.e., 9.0 � 10�4),but still in the range of reported values (i.e. , 1.4 � 10�3 toabout 8.1 � 10�4[2b]). Another linear relationship was ob-served between another two parameters, that is, the micro-pore surface area and micropore volume, and the constantwas around 4.0 � 10�4 (Figure 4 D). These results and theSEM images (Figure 2) demonstrate that the variation ofthe stirring rate during the synthesis only changed the mor-phology but had no influence on the inherent structuralcharacter of SBA-15.

Formation mechanism of the large monolithic SBA-15 mate-rial : The morphology of M2S1 is completely different fromcommon SBA-15 (Figure 1 and Figure 2). The syntheticproducts were blocks instead of powders in the macroscopicview, and these blocks consisted of fibers cross-linked withrods, as concluded from the SEM image. Figure 5 describesthe possible formation process of this unusual morphology.The growth of particular forms of mesoporous silica can beinitiated and directed by different kinds of topological de-fects that exist in a silicate liquid-crystal seed, thus resultingin the formation of mesoporous silica fibers, films, andcurved shapes.[19] These soft primary particles coalesce toform large secondary particles by the polymerization of sur-face silanol groups,[37] and all the fibers obtained from thetwo-phase acidic system under static conditions follow thesame mechanism as the formation process: from a smallnumber of long micelles to loops then to tight coils,[38] per-haps undergoing a four-step mechanism for the aggregategrowth of mesoporous particles.[39] In the colloidal phaseseparation mechanism (CPSM), phase separation is criticalfor the formation of mesoporous materials.[40]

From these mechanisms, it is obvious that the macromor-phology is possibly formed after phase separation and con-trolled by various external influences, such as shearing flow.In our synthetic system that contains block copolymer P123,TMOS, HCl, and water, the rodlike primary particles firstformed after phase separation,[39, 40] then as the size of thesesmall particles reached a relatively large range, they were in-clined to move to the bottom of the Bunsen beaker undergravity. During this falling process, seven kinds of forcessynchronously act on the formed rodlike particles. In the

Table 1. Physicochemical properties of the synthesized samples.

Samples a0[a]

[nm]SBET

[b]ACHTUNGTRENNUNG[m2 g�1]Smic

[c]ACHTUNGTRENNUNG[m2 g�1]Vp

[d]ACHTUNGTRENNUNG[mL g�1]Vmic

[e]ACHTUNGTRENNUNG[mL g�1]Dp

[f]

[nm]hw

[g]

[nm]

SBA-15 10.35 885 157 1.16 0.062 9.10 1.25M2S1 11.02 832 141 1.10 0.056 9.09 1.931% ZrO2/M2S1

11.02 824 185 1.04 0.077 8.98 2.04

3% ZrO2/M2S1

11.02 777 156 0.99 0.065 9.05 1.97

5% ZrO2/M2S1

10.90 705 149 0.91 0.061 8.87 2.03

10% ZrO2/M2S1

10.67 640 117 0.81 0.048 7.32 3.35

[a] The lattice parameter calculated from a0 =d100 � 2/31/2. [b] BET surfacearea. [c] Micropore surface area. [d] Total pore volume. [e] Microporevolume. [f] BJH mesopore diameter calculated from the adsorptionbranch. [g] Wall thickness calculated from hw =a0�Dp.

Figure 3. A) Low-angle XRD patterns and B) nitrogen sorption iso-therms of a) SBA-15, b) M2S1, and their analogues c) 1% ZrO2/M2S1,d) 3% ZrO2/M2S1, e) 5% ZrO2/M2S1, and f) 10 % ZrO2/M2S1.

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FULL PAPERA Net-Linked Mesostructure as an Absorbate

vertical direction, the gravity action (G) causes the formedparticles to fall to the bottom, yet buoyancy (Fb) and anupward friction force (fup) counteract this downward move-ment. In the horizontal direction, the shearing force (Fs),which originates from the shearing flow of the stirring, leadsto flowing of the rodlike particles during which these rodlikeparticles are apt to run parallel from one end to theother,[15,16, 40] and a transverse frictional force (ft) contraba-lances the shearing force. In the centripetal direction, thecentripetal force (Fcp), which results from the interaction ofthe rodlike particles with the wall of the Bunsen beaker orthe solution, supplies the necessary acting force to balancethe centrifugal force (Fcf). Among these forces, the shearingforce is affected by the stirring rate, the sharpness of thestirrer, and the properties of the solution. When the same

stirrer was used, the shearingforce was only determined bythe stirring rate in the uniformsolution. All of the forces arenot independent but correlatewith others: G= mg, Fb = 1wgV,fup =mvFcp, ft =Fs =mtFcp, andFcp =Fcf = mw2r, in which m isthe mass, V the volume of thesingle rodlike particle, g the ac-celeration of gravity, 1w thedensity of the solution, and mv

and mt are the frictional param-eters in the vertical and hori-zontal directions, respectively.At a certain stirring rate, theformed particles are suspendedin solution while circumvolvinground the center of the Bunsenbeaker with a very slow trans-verse rate, so G=Fb + fup.

Under the effect of the shear-ing flow, these soft rodlike pri-mary particles run parallel tothe long axis from one end to

the other and are apt to form the fiberlike particles.[16, 40]

Also, in this specific mild force field, the fiberlike particlescan freely crook and connect in all directions, whereas theycoalesce to form a net-linked morphology as a result of theconsiderable number of silanol groups on the surface and in-sufficient polymerization of the silicate groups. In general,the shearing force is repugnant in the Bunsen beaker: it isstronger at the bottom than the top, though the difference isvery small. This tiny difference leads to the vertical move-ment of various particles, from the primary rodlike particlesto net-linked fragments, during which some particles pile upin solution. This overlapping step, in our opinion, is necessa-ry to form a blocklike synthetic product because these fibersaggregate with each other by cross-linking the vertical rods.At the end of the process, all the primary particles are

Figure 4. A,B) The textural property of the samples synthesized at different stirring rates and the relationshipbetween the C) BET surface area and total pore volume and D) micropore area and volume.

Figure 5. A) The forces acting on the primary rodlike particles. B) Formation process of 1) the cross-linked net and 2) rope-like domains.

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cross-linked in three dimensions to form the block-shapedproduct in solution.

Among various factors, the stirring rate and silica sourceplay essential roles in this formation process.[39, 40] As dis-cussed above, the stirring rate, which directly determines theshearing force, not only indirectly decides the other forcesthat act on the particles but also supplies the necessaryshearing flow for the formation of a long-fiber morpholo-gy.[40] The relatively slow stirring rate allows the fiberlikeparticles to bend and connect to each other in three dimen-sions, thus resulting in the block product of SBA-15 with a3D net-linked morphology. Otherwise, the primary particlesmainly connect linearly and the large aggregation is easilydestroyed by vigorous stirring (Figure 5). A slightly fasterstirring rate cannot lead to a blocklike macromorphology(Figure 1), but a stirring rate that is too slow only results ina sand-like morphology because the stirrer stops stirringduring in the synthesis as the viscosity of the solution in-creases along with the rate of hydrolysis and condensationof the silica source.[41]

A magnetic stirring apparatus was used to control the ro-tating speed of the stirrer in the Bunsen beaker containingthe reaction solution. During the synthetic process, theshape and rotating speed of the stirrer needed to be careful-ly chosen to provide the proper shearing flow. Close investi-gation indicated that a stirrer with a thin-bar shape of a suit-able length could supply the necessary shearing force for theformation of monolithic SBA-15. A stirrer that was too longdestroyed the morphology during the synthesis, whereas astirrer that was too short was unable to yield a strongenough shearing force to keep the solid particles suspendedin solution. Thus, by choosing the stirrer with a propershape for the Bunsen beaker and carefully tuning the rotat-ing speed of the stirrer by utilizing the magnetic-stirring ap-paratus, the stirring rate could be subtly controlled. In addi-tion, although the induction time from the addition of thesilica source to the emergence of milky white precipitates inthe synthesis was about 15 minutes and not dependent onthe stirring conditions, a period of vigorous stirring was stillneeded at the beginning of the synthetic process to com-pletely mix the silica source with the solution containingP123, HCl, and water. Once a homogeneous solution wasobtained, usually about two minutes after the addition ofthe silica source, the stirring rate was tuned to the necessaryvalue that was suitable for the formation of monoliths.

The selection of the silica source was also crucial, andamong the three silica sources used only TMOS producedthe large monolithic mesoporous silica material. The ratesof hydrolysis and condensation, which varied with the silicasource, influenced the formation, growth, arrangement, andconnection of the initial rodlike particles and subsequent fi-berlike particles to a great degree.[14,40] Relative to TEOS,TMOS resulted in a faster condensation rate of the silicatespecies,[14] thus yielding a high local curved energy pathwaythat was beneficial for the formation of the fiberlike mor-phology.[14] When sodium silicate was used, it acted as botha silica source and an inorganic salt that also influenced the

rates of hydrolysis and condensation of the silicate species.It is obvious that the proper rates of hydrolysis and conden-sation, together with the external forces, cause the rodlikeparticles to form well-ordered net-linked frameworks; nei-ther the lower nor higher rates that were observed withTEOS and sodium silicate, respectively, were suitable.

Preparation of zirconia-functionalized M2S1 samples : Zirco-nia-modified 3D net-linked SBA-15 was prepared by usingthe dry-impregnation method to keep both the macro- andmicromorphologies of the composite unchanged becauseother methods did not meet our specific requirements. Aone-pot synthesis did not lead to a 3D net-linked morpholo-gy and solid-state grinding no doubt destroys the morpholo-gy of host, hence we adopted a new way of simply dippingthe zirconium nitrate precursor into the as-synthesized mon-olithic SBA-15 to coat zirconia onto the surface of this mes-oporous material while keeping the morphology unchanged.Figure 3 B presents the nitrogen adsorption/desorption iso-therms of the zirconia-modified samples. The isotherms ofthe modified sample were still type IV and the steep hyste-resis of type H1 exhibited capillary condensation at P/P0 =

0.8, thus implying the reservation of the 1D cylindrical chan-nels. Also, the relative narrow mesopore distribution wasaround 9.0 nm, which confirms that the original structure re-mained, especially in the case of a low-coating amount(Table 1). As the amount of zirconia coating increased, boththe BET surface area and pore volume of the resulting com-posites decreased progressively (Table 1) because the guestmainly coated the inner surface of the host, as revealed pre-viously.[9]

Low-angle XRD patterns (Figure 3 A) demonstrate thatafter impregnation of zirconia, the mesoporous materialskept the original order of M2S1, onto which three well-re-solved peaks with high intensity are indexed as P6mm. Asthe amount of zirconia rose from 1 to 10 % (w/w), the inten-sity of the (100) peak progressively declined, in a similarmanner to the reported CuO/SBA-15 sample from a one-pot synthesis,[9] but the decrease in intensity of our samplesmay have been because of the change in X-ray scatteringrather than weakening of the mesoporous order from the in-troduction of the metal oxide. Figure 6 illustrates the wide-angle XRD patterns of the ZrO2/M2S1 series. The wide

Figure 6. The wide-angle XRD patterns of M2S1 loaded with zirconia:a) 1, b) 3, c) 5, and d) 10 %.

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FULL PAPERA Net-Linked Mesostructure as an Absorbate

peak in the 2q range 20–308 corresponds to the existence ofamorphous silica in the ZrO2/M2S1 series samples. No obvi-ous characteristic peak for the ZrO2 crystalline phase wasvisible in the XRD patterns, even for the 10 % ZrO2/M2S1composite, thus indicating the absence of an aggregation ofZrO2 crystals and well-dispersed ZrO2 on the surface of themesoporous silica. Moreover, the morphology of the meso-porous host was well retained after the dry impregnation, asdemonstrated in Figure 2, which is of key importance forthe composite to maintain a high performance in succeedingtests.

Adsorption performance of large monolith SBA-15 :Figure 7 illustrates the results of the liquid adsorption ofNNN in dichloromethane by the SBA-15 samples with dif-

ferent morphologies. SBA-15 captured 86.8 % of the N’-ni-trosonornicotine (NNN) in solution, M2S1 adsorbed 96 %,M1S1 with a similar morphology to M2S1 trapped 95.6 %,and M2S2 with the same morphology as normal SBA-15 ad-sorbed 86.8 %, thus demonstrating the morphology–perfor-mance relationship. Sample M3S1 with the macromorpholo-gy of a hard, thin bar captured 73.5 % of the NNN, possiblybecause of its small surface area and pore volume. These re-sults reveal the increased tendency of the 3D net-linkedmorphology in SBA-15 toward the adsorption of bulky ni-trosamines in organic solution, though the mechanism is notunderstood yet. The modification of zirconia did not seemto promote the adsorption of NNN in the liquid phase be-cause 5 % ZrO2/M2S1 adsorbed 93.3 % under the same con-ditions, probably as a result of the decreased proportion ofthe silicate structure in the composite.

Figure 8 A displays the instantaneous adsorption of N-ni-trosopyrrolidine (NPYR) in a gas stream by the ZrO2/M2S1series. The normal SBA-15 and M2S1 adsorbed NPYR at453 K, but M2S1 was appreciably inferior probably becauseof its small surface area, which might counter the adsorptionof volatile nitrosamines, such as NPYR, in a gas streamwithin a very short contact time (i.e., less than 0.1 s).[42] Theincorporation of zirconia dramatically improved the adsorp-tion capacity of the 3D net-linked SBA-15, especially in the

initial period of adsorption in which the accumulatedamount of NPYR was below 0.2 mmol g�1. All the zirconia-modified M2S1 samples trapped all the adsorbate. When1 mmol g�1 of NPYR passed the adsorbent, 45.7 % was cap-tured by 1 % ZrO2/M2S1 but 12.4 % by M2S1; that is to say,a coating of 1 wt % zirconia in the 3D net-linked SBA-15was twice as able to adsorb the volatile nitrosamine NPYR.Coating more zirconia further enhanced the adsorption ca-pacity, and 5 % (w/w) was proven to be the optimum value(Figure 8 A).

Figure 8 B plots the FTIR spectrum of NPYR adsorbedon ZrO2/M2S1. There are only several weak absorptionbands observed for M2S1-adsorbed NPYR, such as bands atl=2972 and 2892 cm�1 for nas ACHTUNGTRENNUNG(CH2) and ns ACHTUNGTRENNUNG(CH2) vibrations,respectively, from the pyrrolidinyl unit in NPYR[43] andbands at l=1458 and 1359 cm�1 for n3ACHTUNGTRENNUNG(NO2)

[44] from thecompound containing a nitryl group. Coating 1 % zirconiaon M2S1 strengthens the intensity of all the absorptionbands, whereas bands at l=1673 and 1593 cm�1 from theNO2

� ion[45] appear, thus implying the occurrence of a cata-lytic reaction. As the loading amount of ZrO2 on the poroushost increased, the intensity of the IR absorption bandsgrew continually and reached a maximum in the spectrumof 5 % ZrO2/M2S1. This result is in good agreement withthe NPYR gaseous adsorption mentioned previously.

Figure 9 A demonstrates the reduction of the tar contentin the mainstream smoke of the Chinese Virginia-type ciga-rettes by SBA-15 samples, thus reflecting their ability tofilter the particles from a gas stream with a complex chemi-cal composition. As the control, common SBA-15 did not ef-

Figure 7. Adsorption of NNN by 1) SBA-15, 2) M1S1, 3) M2S1, 4) 5%ZrO2/M2S1, 5) M2S2, 6) M2S3, and 7) M3S1 in CH2Cl2 at room tempera-ture.

Figure 8. A) Instantaneous adsorption and B) FTIR of NPYR in situ ona) common SBA-15, b) M2S1 and its analogues c) 1% ZrO2/M2S1,d) 3% ZrO2/M2S1, e) 5% ZrO2/M2S1, and f) 10% ZrO2/M2S1 at 453 K.

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J. H. Zhu, Y. Wang et al.

ficiently trap the tar in the mainstream smoke, and the de-crease was about 6 % because tar is related to the particlesin smoke and the size of the main particles were in micro-meters,[46] which exceeds the pore size of SBA-15. On thecontrary, the 3D net-linked SBA-15 decreased the tar con-tent in mainstream smoke by 14 % because of its specialmorphology and its analogue modified by 5 % zirconia cap-tured 24 % from the mainstream smoke (Figure 9 A). Tocheck the function of morphology, M2S1 was ground andpressed into small particles (20–40 mesh). The obtainedsample, assigned as M2S1-P, lost the ability to trap tar inmainstream smoke (Figure 9 A). It should be pointed outthat the ability to decrease the amount of tar in mainstreamsmoke by the SBA-15 additive is affected by many factors,especially the type of tobacco; for example, in another testthat used the Blend-type cigarette, common SBA-15 was in-active, M1S1 trapped 20.8 %, and M2S1 captured 42.3 % ofthe tar in smoke. Because of their powder-like morpholo-gies, similar to common SBA-15, both M2S2 and M2S3failed to trap the tar in mainstream smoke as expected.

Figure 9 B illustrates the function of SBA-15 samples indecreasing the level of nitrosamines in the mainstreamsmoke of Chinese Virginia-type cigarette. Common SBA-15cannot trap nitrosamines in smoke for two reasons: 1) thegeometric limitation of the mesoporous channels for the to-bacco-specific nitrosamines exists in particles that are micro-meters in size, as mentioned previously;[7] 2) the lack of cat-ions to provide strong adsorptive sites for volatile nitrosa-mines in the vapor phase of smoke.[9] The smoke of ChineseVirginia-type cigarettes mainly contains volatile nitrosa-mines,[10] therefore, the 3D net-linked M2S1 cannot effi-ciently decrease the level of nitrosamines in smoke either

because of its pure siliceous composition. On the contrary,5 % ZrO2/M2S1 showed an excellent ability to removeabout 37 % of the nitrosamines from mainstream smoke. Inanother test utilizing Blend-type cigarettes, whose smokecontains a lot of tobacco-specific nitrosamines, in the parti-cle phase, M2S1 lowered the content of nitrosamines from 9and 42 % as the amount of M2S1 additive increases from 15to 30 mg per cigarette, respectively, which will be reportedelsewhere in future.

Conclusion

A facile procedure has been developed to prepare versatilefunctional mesoporous materials in a conventional SBA-15synthetic system, through adjusting the force field in the hy-drolysis and condensation of the silica source. This proce-dure allowed the primary particles to connect in all direc-tions to form a 3D net-linked state. As the result, a largeblock of the product SBA-15, with toughness, was success-fully obtained with a well-ordered 2D hexagonal mesostruc-ture (P6mm) with 1D cylindrical channels.

In addition, coating zirconia onto the silica wall by usinga dry-impregnation method enhanced the adsorption capaci-ty of the 3D net-linked SBA-15 toward volatile nitrosa-mines. Owing to its special morphology and incorporatedzirconia guest, the resulting composite exhibited an excel-lent capacity for the adsorption of tobacco-specific nitrosa-mines NNN in organic solution and the volatile nitrosaminesNPYR in the gas stream. Moreover, when the compositewas added to a cigarette filter, it efficiently trapped tar andnitrosamines in the mainstream smoke because the cross-linked net structure provided the necessary adsorption sitesto capture the large particles and the guest zirconia promot-ed the adsorption ability of SBA-15 toward nitrosamines.

Experimental Section

General : Poly(ethylene oxide)-block-poly(propylene oxide)-block-poly-(ethylene oxide) triblock copolymer (EO20PO70EO20, P123; Mav =5800)was obtained from Aldrich. N-nitrosopyrrolidine (NPYR) and N’-nitroso-nornicotine (NNN) were purchased from Sigma. The sample cigaretteswere purchased from market.

SBA-15 was synthesized according to a reported procedure.[2b] Typically,tetraethyl orthosilicate (TEOS, 6.38 g) was added with vigorous stirringto P123 block copolymer (3.0 g) in water (22.5 g) and HCl (2 m, 90 g) at313 K. The reaction mixture was stirred at 313 K for 24 h. The gel solu-tion was transferred into a teflon bottle and heated at 373 K for 24 hwithout stirring. The solid product was recovered by filtration, washedwith deionized water, and air-dried. Different silica sources and stirringrates were employed to control the morphology of SBA-15 (Table 1),thus producing various samples denoted as MxSy, in which Mx indicatesthe silica source (M1 = TEOS, M2= tetramethyl orthosilicate (TMOS),and M3=Na2SiO3) and Sy indicates the stirring rate (S1 =75, S2=150,S3=300 r min�1). To remove the template, M3S1 (0.6 g) was heated toreflux in ethanol (180 mL) for 24 h,[47] and the others were calcined at773 K at a rate of 2 Kmin�1.

The zirconia-functionalized mesoporous silica was prepared as follows:calculated zirconium nitrate (Zr ACHTUNGTRENNUNG(NO3)4·9 H2O) in deionized water was

Figure 9. Reduction of A) tar content and B) nitrosamines in the main-stream smoke of cigarettes by the SBA-15 samples.

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uniformly dipped on the as-synthesized SBA-15. The products were driedin air at room temperature and calcined at 773 K at a rate of 2 K min�1.

XRD patterns of the samples were recorded on an ARL XTRA diffrac-tometer in the 2q range 0.6–58.[4] Transmission electron microscopy(TEM) analysis was performed on a FEI Tecnai G2 20 S-TWIN electronmicroscope at 200 kV. SEM images were obtained with HITACHI S4800microscopes. The samples were coated with a gold film to improve theconductivity prior to imaging. The mechanical strength of M2S1 was in-vestigated on an INSTRON 4466 instrument. Nitrogen adsorption iso-therms were measured at 77 K on a Micromeritics ASAP 2020 volumetricadsorption analyzer.[48] Before the test, the sample was outgassed in thedegas port of the apparatus at 573 K for 4 h. The BET specific surfacearea was calculated by using adsorption data acquired at a relative pres-sure (P/P0) range of 0.05–0.22:1 and the total pore volume was deter-mined from the amount adsorbed at a relative pressure of about 0.99:1.The micropore area (Smic) and micropore volume (Vmic) were calculatedby the t-plot method, according to statistical film thickness (t) in therange 0.35–0.50 nm. The thickness equation used was developed by Har-kins and Jura:[49] t= ACHTUNGTRENNUNG[13.99/(0.034�log ACHTUNGTRENNUNG(P/P0))]0.5.[49] The pore-size distribu-tion (PSD) curves were calculated from the analysis of the adsorptionbranch of the isotherm using the Barrett–Joyner–Halenda (BJH) algo-rithm.

Liquid adsorption of NNN was carried out at 277 K,[43] and 20-mg sampleparticles (20–40 meshes) were in contact with NNN in dichloromethane(1.1 mL) for 24 h. The residual nitrosamines in solution were detected byspectrophotometric analysis.[8, 50] For example, we used NNN in solution(1.1 mL) in the liquid adsorption experiment to check the adsorption bythe porous material. After the adsorption experiment had finished, wedissolved the residual solution (0.1 mL) in dichloromethane to obtain anitrosamine sample solution (10 mL) for further detection. The samplesolution was put into a test-tube with a branch in it, through which thecarrier could pass to the bottom of the test-tube, HBr in glacial aceticacid (0.5 mL) was added to chemically denitrosate the nitrosamines andliberate the denitrosated gaseous products. The formed gaseous productswere purged from the test-tube by an N2 stream for 0.5 h, and the liberat-ed products were purified through three consecutive traps containingNaOH (10 mL, 5 mol L�1), oxidized to NO2 by passing through a tube ofCrO3 (0.4 g) and sea sand (7.6 g), and dried at 383 K prior to use. Theflow rate of the N2 purge gas was controlled at 200 mL min�1. After beingabsorbed in the solution of sulfanilamide and N-1-naphthylethylenedia-mine dihydrochloride, NO2 was converted into NO2

� ions and a digitalvisible spectrophotometer was used to detect the amount of NO2

� ions at540 nm. Finally, the mean amount of nitrosamines in the sample solutionwas determined through the NaNO2 concentration by using an absorben-cy curve. To avoid the loss of reactivity that occurs as the chromate isconverted into chromia, the CrO3/sea sand mixture should be replacedbefore its color changes from red/brown to dark green, which was usuallydone after every two cycles.

The instantaneous adsorption of nitrosamines was carried out by usinggas chromatography.[42] The sample (5 mg) was directly heated to thegiven temperature without activation in the flow of the carrier gas at arate of 30 mL min�1, and the solution of nitrosamine was pulse-injected(2 mL). The gaseous effluent was analyzed by an online Varian 3380 gaschromatograph to assess the adsorption of the nitrosamines by thesample.[51]

The FTIR investigation was performed in situ in a purpose-built IR cellwith CaF2 windows, and a Bruker 22 FTIR spectrometer with a resolu-tion of 2 cm�1 was used for the FTIR measurements. A sample (area den-sity= 10 mg cm�2) was slowly heated to 433 K in the N2 flow. After thetemperature stabilized, a background spectrum was collected. Subse-quently, the adsorbate NPYR (1mL) was injected into the IR cell andpurged with nitrogen for 15 min to remove the physically adsorbed ni-trosamines before recording the FTIR spectra.

To evaluate the ability of the sample to filter the particles in the gasstream, SBA-15 (15 mg, 20—40 meshes) was put into the filter instead ofthe cellulose matrix. After conditioning at 295 K and 60% relative hu-midity for 48 h, 20 cigarettes were machine smoked in the standard inter-national organization for standardization (ISO) machine-smoking

regime,[52] and the amount of tar captured by cambridge pads were de-tected. To assess the ability of the sample to trap the nitrosamines insmoke, 20 cigarettes were continually smoked in a glass chamber inwhich citrate-phosphate buffer containing ascorbic acid (100 mL,0.02 mol) was used to absorb the nitrosamines.[53] The solution was ex-tracted with dichloromethane and the mean amount of the nitrosamineswas determined as previously reported.[8]

Acknowledgements

Financial support from NHTRDP973 (2007CB613301), NSF of China(20773061, 20873059, and 20871067), and Grant 2008AA06Z327 from the863 Program of the Ministry of Science and Technology of China, the Sci-entific Research Foundation of Graduate School of Nanjing University,and Grant CX08B_009 from the Jiangsu Province Innovation for a doc-toral candidate is gratefully acknowledged. We also thank the AnalysisCenter of Nanjing University.

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Received: August 4, 2008Revised: January 19, 2009

Published online: May 28, 2009

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