Journal of Colloid and Interface Science 359 (2011) 9–18

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Journal of Colloid and Interface Science

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Efficient adsorbents of nanoporous aluminosilicate monoliths for organic dyesfrom aqueous solution

Sherif A. El-Safty a,b,c,⇑, Ahmed Shahat a,1, Md. Rabiul Awual a

a National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba-shi, Ibaraki-ken 305-0047, Japanb Department of Chemistry, Faculty of Science, Tanta University, Tanta, Egyptc Graduate School for Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

a r t i c l e i n f o

Article history:Received 13 October 2010Accepted 2 January 2011Available online 12 March 2011

Keywords:Cage poresNanoadsorbentsOrganic compoundsDye adsorptionColorimetric determination

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.01.006

⇑ Corresponding author at: National Institute for MaSengen, Tsukuba-shi, Ibaraki-ken 305-0047, Japan.

E-mail addresses: sherif.elsafty@nims.go.jp, sherif@1 Current address: Chemistry Department, Faculty

University, Egypt.

a b s t r a c t

Growing public awareness on the potential risk to humans of toxic chemicals in the environment hasgenerated demand for new and improved methods for toxicity assessment and removal, rational meansfor health risk estimation. With the aim of controlling nanoscale adsorbents for functionality in molecularsieving of organic pollutants, we fabricated cubic Im3m mesocages with uniform entrance and large cav-ity pores of aluminosilicates as highly promising candidates for the colorimetric monitoring of organicdyes in an aqueous solution. However, a feasible control over engineering of three-dimensional (3D) mes-opore cage structures with uniform entrance (�5 nm) and large cavity (�10 nm) allowed the develop-ment of nanoadsorbent membranes as a powerful tool for large-quantity and high-speed (in minutes)adsorption/removal of bulk molecules such as organic dyes. Incorporation of high aluminum contents(Si/Al = 1) into 3D cubic Im3m cage mesoporous silica monoliths resulted in small, easy-to-use opticaladsorbent strips. In such adsorption systems, natural surfaces of active acid sites of aluminosilicate stripsstrongly induced both physical adsorption of chemically responsive dyes and intraparticle diffusion intocubic Im3m mesocage monoliths. Results likewise indicated that although aluminosilicate strips withlow Si/Al ratios exhibit distortion in pore ordering and decrease in surface area and pore volume,enhancement of both molecular converges and intraparticle diffusion onto the network surfaces and intothe pore architectures of adsorbent membranes was achieved. Moreover, 3D mesopore cage adsorbentsare reversible, offering potential for multiple adsorption assays.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction ple, when dyes and organic compound discharge are flushed into

In recent years, nanoscience and nanotechnology have gainedconsiderable public interest due to the needs and applications ofnanomaterials in almost all areas of human endeavor, includingindustry, agriculture, business, medicine, and public health. Theenvironment pertains to the air, water, land, and structures thatconstitute communities [1]. A healthy environment can enhancethe quality of life of individuals and communities. When the envi-ronment is safe, people tend to be free from diseases.

Among all chemicals, dyes are group of substances appliedwidely in industries, including textile preparation [2] and pharma-ceutical development [3]. As the textile dyeing process involvesvarious chemical complexities, the release of dyes can pollute theenvironment, especially in the form of wastewater [4]. For exam-

ll rights reserved.

terials Science (NIMS), 1-2-1

aoni.waseda.jp (S.A. El-Safty).of Applied Science, Port Said

surface waters, they become highly colored and covered by foam;this is because of the incorporation of the dyes with auxiliariesused in the dyeing process. Foam reduces oxygen diffusion throughthe water surface, thus limiting depuration by natural oxidation.Dyes render water untenable for drinking. Furthermore, certaindyes are carcinogenic and mutagenic [5]. As a number of organicdyes are harmful to human beings, removal of color from processor waste effluents becomes environmentally important. However,due to the sizeable degree of organics present in molecules as wellas the stability of modern dyes, conventional physicochemical andbiological treatment methods have proved to be ineffective for dyeremoval [6].

Among the possible techniques for water treatment, adsorptionby solid adsorbents shows promise in being among the most attrac-tive and efficient methods for purification and separation of traceorganic contaminants in wastewater treatment. Activated carbonis commonly used as adsorbents for dyes due to its immense activesurface area [6]. For example, a highly mesoporous activated carbonfiber (Y-ACF) has been used to adsorb various types of dyes such asacid dyes (acid blue 9, acid blue 74, acid orange 10, and acid orange51), direct dyes (direct black 19, direct yellow 11, and direct yellow

Scheme 1. Aluminosilica adsorbents (disk-like monoliths) before (A) after (B) theadsorption of organic H2DSPy dye (D) inside the mesocage cavity and onto poresurfaces of 3D cubic Im3m structures (C): Note that 3D TEM image (C) was recordedwith disk-like monolithic adsorbents with a Si/Al ratio of 4.0.

10 S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18

50), and basic dyes (basic brown 1 and basic violet 3) [7]. Activatedalumina is used for dye adsorption as well (crystal violet, malachitegreen, rhodamine B, methylene blue), but its surface area is smallleaded for low-performance removal of dyes [8]. The activated alu-mina, however, can adsorb cationic dyes easily and obtain the equi-librium state within several minutes.

Recently, mesoporous silica-based materials have been em-ployed to adsorb dyes due to their high specific surface [9–11]. Themodification of the silica surface by a cationic polyelectrolyte (chito-san) enabled to produce composed materials, which may be capableof adsorbing both cationic (rhodamine B) and anionic (tectilon blue)dyes. Mesoporous silica treated with chitosan manifested higheradsorption capacity for tectilon blue compared with rhodamine Bdue to difference in electric charge. Dynamic adsorption of organicdye (methylene blue) in different structured mesoporous silicas(MCM-41, MCM-48, and MCM-50) was investigated by Wang andLi [10]. Results revealed that depending on the structure of mesopor-ous silica, dye can adsorb reversibly or irreversibly on mesoporoussilica. In addition, natural aluminosilicate adsorbents have also beeninvestigated to adsorb dyes based on colorimetric adsorption meth-ods, in which the monitoring process of the dye removal could beeasily controlled through the colorimetric recognition [12–15].

To date, many reports have investigated the synthesis of or-dered mesoporous aluminosilicates, including direct synthesis[16] or post-synthesis approaches [17]. For example, direct synthe-sis offers the advantage of varying the Al/Si ratio, but it faces diffi-culty in controlling the reaction due to different hydrolysis andcondensation rates of aluminum and silicon precursors. Mean-while, in the post-synthesis route, the alumina incorporates intothe inner wall of the silica without destroying the ordered struc-ture of the silica framework; however, it is limited by its loadingamount and contains certain by-products in the pore and on theexternal surface. Control over the local aluminum structure inframeworks as well as structural integrity with increasing the alu-minum content in the mesopore frameworks remains a challenge[16,18–22]. The present research efforts focus on this endeavor.However, the successful fabrication of 3D nanoscale monolithicdisks as membrane platforms enables the design of easy-to-useadsorbents for removal of organic dyes from aqueous solution.

Here, we present a general yet relatively simple pathway syn-thesis of cubically ordered mesocage Im3m aluminosilica mem-branes (disk-like monoliths measuring 1.2 mm in thickness and�12 mm in length). In this approach, casting of microemulsion li-quid crystalline phases of triblock copolymer F108 (EO141-

PO44EO141) surfactant assists significantly in the fabrication ofwell-defined mesostructures with condensed and rigid disk-likemembranes. Functional acid sites, multidirectional (3D) pore con-nectivity, high aluminum content, and ordering structures of alu-minosilica adsorbents exhibit efficient removal of organic dyessuch as N,N0-disalicylidene-4,5-diamino-6-hydroxy-2-mercapto-pyrimidine (H2DSPy), to a maximum of 100% within a short periodfrom the aqueous solution phase. It is worth noting that our ap-proach likewise permits easy-to-use removal assays of organiccompounds, and portable and reusable chemical adsorbents with-out any mesostructural damage even under rigorous chemicaltreatment for a number of reuse/cycles (Scheme 1).

2. Experimental

2.1. Chemicals

All materials were analytical grade and used as purchased with-out further purification. Tetramethylorthosilicate (TMOS), alumi-num nitrate enneahydrate (Al(NO3)3�9H2O), dodecane (C12H26),and the triblock copolymers of poly(ethylene oxide-b-propylene

oxide-b-ethylene oxide), Pluronic F108, designated as F108 (EO141-

PO44EO141) were obtained from Sigma–Aldrich Company Ltd., USA.In addition, 4,5-diamino-6-hydroxy-2-mercaptopyrimidine wasobtained from Wako Company Ltd. (Osaka, Japan).

2.2. Synthesis and characterization of the H2DSPy dye

The H2DSPy dye was prepared by dissolving 25 mmol (2.372 g)of 4,5-diamino-6-hydroxy-2-mercaptopyrimidine in 20 mL of DMFand stirred for 3 h. After that, 50 mmol (6.106 mL) ofsalicylaldehyde was mixed in 150 mL of ethanol. The 4,5-diami-no-6-hydroxy-2-mercaptopyrimidine solution was added to thesalicyla-ldehyde solution using an overhead stirred for completemixing, and the reaction mixture was refluxed for 2 h. The yellowprecipitate was filtered, washed with small amount of ethanol, andthen dried at 60 �C. The purity of the (H2DSPy) dye product wasanalyzed by CHNS elemental analyses as follows: C, 58.99%; H,3.81%; N, 15.35%; S, 8.70%, as consistent with the C18H14N4O3Smolecular formula, which requires C, 59.02%; H, 3.82%; N,15.30%; S, 8.74%. The product was characterized by 1H NMR and13C NMR spectroscopies. The 1H NMR spectra were shown withthe following signal resonances: d 12.15 (H, s, SH), 11.53 (H, s,OH Pyr), 5.35 (2H, s, OH Ph), 7.02 (2H, m, CH Ph), 7.08 (2H, m,CH Ph), 7.52 (2H, m, CH Ph), 7.66 (2H, m, CH Ph), 8.59 (2H, s, CHAzo), where the Ph, Pyr, and Azo are the phenyl, pyridine, and azo-methine groups, respectively. The 13C NMR spectra were shownwith the following signal resonances: d 172.3 (C–SH), 166.7 (Pyr,C–OH), 174.7 (Pyr, C), 134.1 (Pyr, C), 161.8 (Azo, CH), 160.0 (Azo,CH), 120.5 (2Ph, C), 117.8 (2Ph, CH), 132.4 (2Ph, CH), 132.1 (2Ph,CH), 121.4 (2Ph, CH), 161.1 (2Ph, C–OH). In addition, the dyeproduct was recorded by UV–vis spectroscopy and showed anabsorbance band at kmax = 369 nm (e, dm3 mol�1 cm�1 = 40,650).

2.3. Synthesis of cage cubic Im3m aluminosilica monolithic-diskadsorbents

Simple synthesis process of monolithic aluminosilica adsor-bents was based on direct-templating of microemulsion liquid

Table 1Synthesis conditions and structural and textural parameters of cage cubic Im3m aluminosilica adsorbents fabricated using microemulsion phases of F108 (EO141PO44EO141) as softtemplates and with a wide range of Si/Al ratios; unit lattice constant (a)�, BET surface area (SBET), and NLDFT pore size (R), and pore volume (Vp).

Membrane withSi/Al ratios

Synthesis conditions Structure parameters

102 Ta (mol) (103) Al (NO3)3 (mol) F108 (TMOS w/w) Si/Al mole (mol) ab (nm) SBET (m2/g) R (nm) Vp (cm3/g)

19 1.31 0.7 0.8 19 19.5 770 8.4 1.169 1.31 1.5 0.8 9 19.5 756 8.9 1.154 1.31 3.4 0.8 4 20.1 600 9.4 0.872.3 1.31 5.8 0.8 2.3 20.8 550 9.8 0.81.5 1.31 9.1 0.8 1.5 21.1 450 9.9 0.651 1.31 13.6 0.8 1.0 21.1 400 10.0 0.56

a T, TMOS.b a, and unit lattice constant (aIm3m = d110

p2).

S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18 11

crystalline phase of F108 surfactant. In this direct synthesis of cu-bic Im3m aluminosilica monoliths (i.e., disk-like membrane), suchas at the Si/Al ratio of 9.0 (mol/mol) and at F108/TMOS ratio of 0.8(w/w), the precursor solution [1.6 g of F108, 0.5 g dodecane, 2 gTMOS, 0.569 g Al(NO3)3, 2.5 g H2O–HCl (at pH = 1.3), and 10 g ofethanol] was stirred for 30 min to form a homogenous sol–gelsolution. The resulting optical gel-like mixture was placed in agraduate ingot, allowing it to acquire the shape and size of thecylindrical casting vessel. ‘‘Disk-like’’ membranes were gentlydried at room temperature for 2 h and subsequently allowed tostand in a tightly closed ingot for 1 d to complete the drying pro-cess. To obtain monolithic samples with various aluminum con-tents at the Si/Al ratio of 19, 10, 4.0, 2.3, 1.5, and 1.0, the molarcomposition of Al(NO3)3 was varied from 0.7 � 10�3 to 13.6 �10�3 mol (Table 1). Organic moieties were removed by calcinationat 550 �C under air for 5 h. Calcined cage cubic Im3m monolithsclearly possessed stable and tough disks measuring 1.2 mm inwidth and �12 mm in length. In turn, transparency of the disk-likemonoliths was lost by calcination, as previously reported on thosewith silica monolithic types [23–26].

2.4. Batch adsorption studies

H2DSPy adsorption on cubic cage Im3m disk-like monoliths wasdetermined from batch experiments as a function of contact time,initial H2DSPy concentration, temperature and adsorbent dose, andits amount. Experiments were performed using a 50-mL bottle. Adifferent amount of monolithic adsorbent was added to 25 mL ofH2DSPy of different concentrations ranging from 0.02 to 0.08mmol L�1. The mixture was agitated in a temperature-controlledwater bath shaker for 24 h at a constant agitation speed of90 rpm to achieve equilibrium. The pH solution of the H2DSPydye in this batch experiments was ca. 7. Thus, the H2DSPy dye doesnot bear any positive/negative charges. Concentration of adsorbatesolutes was measured on a Shimadzu 3700 model solid-state UV–vis spectrophotometer at k = 369 nm. The adsorbed H2DSPy wasobtained from the difference between the initial H2DSPy concen-tration and final detected H2DSPy concentration. Kinetic studieswere conducted at five different concentrations (0.02, 0.03, 0.05,0.07, and 0.08 mmol L�1) of the H2DSPy at 40 �C. Samples werewithdrawn at regular intervals to plot the amount adsorbed versustime. Kinetic studies were conducted as well at four different tem-peratures, ranging from 35 �C to 50 �C, using an initial dye concen-tration of 0.05 mmol L�1.

2.5. Analysis

Small/wide angle powder X-ray diffraction (XRD) patterns weremeasured by using an 18 kW diffractometer (Bruker D8 Advance)with monochromated Cu Ka radiation and with scattering reflec-tions recorded for 2h angles between 0.1� and 6.5� corresponding

to d-spacing between 88.2 and 1.35 nm. First, the powder sampleswere ground and spread on a sample holder. The samples werescanned in the range from 2h = 0.1� to 6.5� with step size of0.02�. To confirm the resolution of the diffraction peaks with stan-dard reproducibility in 2h (±0.005), the sample measurement wasrecorded by using both graphite monochromator and Göbel mir-rors detector. Both detectors were used to generate focusing beamgeometry and parallel primary beam. The sample measurementwas repeated three times under rotating at various degrees (15�,30�, and 45�). The diffraction data were analyzed using DIFRAC plusEvaluation Package (EVA) software with PDF-2 Release 2009 dat-abases that provided by Bruker AXS. The standard diffraction datawere identified according to the databases of the InternationalCentre for Diffraction Data (ICDD). For the poor-quality diffractiondata, TOPAS package program that provided by Bruker AXS 2009for profile and structure analysis was applied to integrate varioustypes of X-ray diffraction analyses by supporting all profile fitmethods currently employed in powder diffractometry.

Transmission electron microscopy (TEM) was operated at 200-kV electron microscope (JEOL 2000 EX II). The accelerating voltageof 200 kV also permits the observation of thick specimens. Thisinstrument has a lattice image resolution of 0.14 nm, a point imageresolution of 0.21 nm, and a spherical aberration of 0.7 nm. Thesamples for TEM were prepared by placing a drop of a dilute solu-tion of the sample in ethanol on a copper grid (400 mesh, agar). Theexcess solvent allowed drying completely at room temperature.Fourier transform diffraction (FTD) patterns were recorded by aslow scan charge-coupled device (CCD) camera (Gatan Model 694).

N2 adsorption–desorption isotherms were measured using aBELSORP MIN-II analyzer (JP. BEL Co. Ltd.) at 77 K. The pore sizedistribution was determined from the adsorption isotherms byusing nonlocal density functional theory (NLDFT). Specific surfacearea (SBET) was calculated using multipoint adsorption data fromlinear segment of the N2 adsorption isotherms using Brunauer–Emmett–Teller (BET) theory. Before the N2 isothermal analysis,all membrane samples were pretreated at 300 �C for 8 h under vac-uum until the pressure was equilibrated to 10�3 Torr.

FESEM images were measured by a field-emission scanningelectron microscopy (Hitachi S-4300). Before insertion into thechamber, the disk-like monolith substrates were fixed on a SEMstage using carbon tapes. The Pt films were deposited on the sub-strates at room temperature by using an ion-sputter (Hitachi E-1030). The distance between the target and the disk-like monolithssubstrate was 5.0 cm. The sputtering deposition system used forthe experiments consists of a stainless steel chamber, which wasevacuated down to 8 � 10�5 Pa with a turbomolecular pumpbacked up by a rotary pump. Before sputtering deposition, the Pttarget (4 in. diameter, purity 99.95%) was sputter cleaned in pureAr. The Ar working pressure (2.8 � 10�1 Pa), the power supply(100 W), and the deposition rate were kept constant throughoutthese investigations. Moreover, to better record the SEM images

Fig. 1. XRD patterns of cubic Im3m aluminosilica disk-like monoliths with Si/Alratios of 19 (a), 9.0 (b), 4.0 (c), 2.33 (d), 1.5 (e), and 1.0 (f).

12 S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18

of disk-like monoliths, the SEM micrographs were operated at20 keV.

Energy dispersive X-ray microanalyzers (EDX) were recorded byemploying Horiba EDS-130S, which directly connected with Hit-achi FE-SEM S-4300. Elemental mapping of all the aluminosilicamonoliths was carried out with the energy dispersive X-ray micro-analyzers with an acceleration voltage of 30 kV. Ten distinct spotswere analyzed per sample, which resulted in 99% confidencebounds of ±0.01 in the molar fraction of each cation (with theirsum normalized to unity). The experimentally analyzed composi-tions were for each element very close to those of the batches.The analyzed and nominal cation contents generally agreed within5% (relative), which for the present glass compositions are withinthe uncertainties of the EDX measurements.

27Al MAS NMR spectra at room temperature were also recordedusing a Bruker AMX-500 spectrometer. 27Al NMR spectra weremeasured at a frequency of 125.78 MHz with a 90� pulse lengthof 4.7 ls. For all samples, the repetition delay was 64 s with a rotorspinning at 4 kHz for 27Al NMR. The chemical shift scale was exter-nally set to zero for 27Al signal by using aqueous solution (1 N) ofAl(NO3)3.

NH3 temperature-programmed desorption (NH3-TPD) was mea-sured by using a BEL-Japan TPD-1S system with a quadrupole massspectrometer. After being pretreated at 500 �C under a He flow for4 h, the monolithic sample was treated with a flow of 10% NH3/Hefor 20 min. To completely remove the NH3 molecules physisorbedon the surface, helium carrier gas subsequently flowed through thesample at 100 �C for 4 h. Ammonia desorption between 100 �C and650 �C (heating rate 10 K min�1) was analyzed using an on-lineconductivity cell with a HCl solution for trapping the effluentgases. Desorption of physically adsorbed NH3 molecules was per-formed by interchanging the supply of ammonia with the originalflow of helium carrier gas at the same temperature range. Calibra-tion of the areas under the thermal peaks was carried out by dissi-pating a known amount of energy in the cell under the sameflowing conditions and integrating the related exothermic peak.The thermal conductivity (TC) detector calibration factor wasdetermined from the injection of 500 mL of pure ammonia intothe stream of helium carrier gas percolating through the detector.

Fig. 2. N2 isotherms of cubic Im3m aluminosilica disk-like monoliths with Si/Alratios of 19 (a), 9.0 (b), 4.0 (c), 2.33 (d), 1.5 (e) and 1 (f).

3. Results and discussion

3.1. Characterization of cage cubic Im3m aluminosilica disk-likemonoliths

For all synthesis designs, X-ray diffraction (XRD) patterns ofaluminosilica disk-like monoliths exhibited well-resolved Braggpeaks characteristic by highly ordered cubic Im3m nanophase do-mains, particularly at aluminum content ratios of 19 6 Si/Al 6 2.3(Fig. 1). Such systematic Bragg reflections were broadly resolvedand clearly indicated topological 3D nanophases with cubicIm3m symmetry. On the contrary, structural ordering of disk-likemonoliths was crucially influenced by aluminum contents of theframeworks. However, with high aluminum amounts (2.3 6 Si/Al 6 1), XRD patterns exhibited broad and poorly resolved diffrac-tion peaks. The broadening and low-resolution peaks with lessintensity indicated the formation of disordered aluminosilica mes-ostructures. However, the addition of a high amount of aqueousaluminum salt to the synthesis composition domains played a rolein the increase in micellar aggregate sizes and volume fractions;this leads to increase in unit cell constants of cubic structures[29]. Fig. 1 presents all assigned X-ray diffraction hkl of disk-likemonoliths with Si/Al ratio of 19. However, peak positions werespecifically determined using DIFRAC plus Evaluation Package(EVA) software with PDF-2 Release 2009 databases. The hkl

reflections (Fig. 1, insets) were uniquely consistent with the dis-tinctive cubic structure of Im3m symmetry.

N2 adsorption/desorption isotherms (Fig. 2) of aluminosilicadisk-like monoliths feature a broad hysteresis loop of H2 type fortypical cage mesopores with uniform entrances. Such cage struc-tures have been previously discussed in many reports (e.g., Refs.[23–28]). The evaporation delay and sudden perpendicular closureof the desorption branch of the N2 isotherms indicate that the cageopenings are of �5 nm. The lower closure point of the hysteresisloop was observed in a relative pressure of 0.45, indicating the cav-itation-induced stepwise desorption in the ink-bottle pores [23–28]. Adsorption isotherms of disk-like monoliths shifted toward ahigher relative pressure (P/Po), but the adsorption volume uptakewas significantly decreased with the increase in aluminum con-

S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18 13

tent. Based on the N2 isothermal results, cage aluminosilica mono-liths observed the appreciable textural parameters of specific sur-face area (SBET), mesopore volume (Vp), and tunable pore diametersof 8.9–10 nm. Increase in the spherical cavity of aluminosilicasmay be attributed to the instability of aluminum in the frameworkof disk-like monoliths. Due to the degradation of structural order-ing with high aluminum content of monoliths, decrease in thesetextural parameters was observed (see Table 1).

HRTEM images of cage cubic Im3m aluminosilicas (disk-likemonoliths) recorded along [1 1 1] and [1 1 0] indices revealedwell-defined and regular mesopore channels arranged along alldirectional arrays (Fig. 3). Presented in Fig. 3 insets are the FTDanalyses, which typically confirms the general observed trends ofTEM images for these samples. FTD patterns provide more detailedinsights into nanostructure symmetries, according to the abun-dance of lattice fringes. Arrangement of planes in these specific lat-tice fringes along the zone axes provided further evidence of theformation of such cubic Im3m lattice symmetry [26]. In general,TEM images revealed that the mesoscopic qualities of aluminosili-ca structures continue to retain their long-range structural order-ing over a large area of cubic lattices even for monolithicsamples with Si/Al ratios as high as 4.0. However, with low silicacontents (2.3 6 Si/Al 6 1), representative TEM images (data notshown) revealed short-range ordering or even worm-like meso-pore channels interconnecting in large domains. On the basis of ob-tained TEM results, it can be noted that aluminum contentssubstantially affected the uniformity of mesopore framework inthe present experimental conditions, consistent with XRD patterns.

112

220

202

A

Fig. 3. Representative HRTEM micrographs and FTD (insets) patterns of cubic Im3m alumrecorded along the [1 1 1] (A), and [1 1 0] (B) zone axes, respectively.

A

10 µm1 µm

a

C

Fig. 4. Low (A) and high (a) particle surface magnification of HFSEM micrographs, and thea Si/Al ratio of 1.5: Inset (B) shows the calculated and analyzed values of atomic abund

HRSEM micrographs revealed that cage cubic Im3m aluminosil-icas (disk-like monoliths) possessed large particle sizes (�5–10 lmin diameter), containing plate-like particles with smooth fine lineson their surface morphology for aluminosilica samples with Si/Alratios as high as 4.0. This is in perfect agreement with our previousSEM data on pure silica monoliths [24–26]. On the other hand,morphology of aluminosilicas fabricated with high aluminum con-tents of Si/Al ratios 62.3 is intrinsically different; however, HRSEMmicrographs (Fig. 4A) reflect dense macroporous voids. These voids(open holes) are attributed to structural defects (Fig. 4Aa). The sizeof these morphological defects is not uniform (distributes between100 and 800 nm), and their shapes are irregular. Our finding indi-cated that the amount and size of structural defects increasedalong with rising aluminum content in disk-like monoliths. This in-crease in macroporous voids of aluminosilicas may be attributed tothe instability of aluminum in the framework of disk-like mono-liths. However, during the dealumination process with high-tem-perature calcination, aluminum may migrate to the frameworksurface to form a separate aluminum species (i.e., structural de-fects) [31].

EDX and elemental analysis data (Fig. 4B) reveal that a minimalamount of Al constituent was lost during calcinations at 550 �Ccompared with the amount added with the synthesis compositiondomains of cage cubic Im3m aluminosilicas (disk-like monoliths).To obtain high-degree precision of quantification measurements,10 distinct spots were analyzed per sample, resulting in �99% con-fidence bounds of ±0.01 in the molar fraction of each element (i.e.,Al, Si, and O). Results (Fig. 4B, inset) revealed that Si/Al values were

112004

220

B

inosilica disk-like monoliths with a Si/Al ratio of 9.0: HRTEM and FTD patterns were

B

0.5 1.0 1.5 2.0 2.5keV

SiKa

AlKa

O Ka

Ka

Adsorbent

Si/Al=1.5

Element

OAlSi

Weight (wt %)

Calculated Analyzed59.016.725.3

58.9815.7924.23

corresponding EDX spectrum of cubic Im3m aluminosilica disk-like monoliths withance of O, Al, and Si species that existed in the bulk solid monoliths.

14 S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18

compatibly averaged with the bulk quantities of synthesis gel sam-ples despite the heterogeneity that may be produced with the self-aggregation of aluminum during aluminosilica synthesis.

Fig. 6. NH3-TPD spectra of cage cubic Im3m aluminosilicas (disk-like monoliths)with Si/Al ratios of 19 (a), 9.0 (b), 4.0 (c), 1.5 (d), and 1.0 (e).

3.2. Feature of aluminosilica (disk-like monoliths) adsorbents

Coordination state of aluminum species was investigated by27Al NMR (Fig. 5). With all disk-like aluminosilica monoliths(19 6 Si/Al 6 1), two 27Al peaks centered on the chemical shift of�1 and 58 ppm, indicating the existence of octahedral (AlVI, AlO6,extra-framework) and tetrahedral (AlIV, AlO4, framework) alumi-num sites, respectively. Results revealed that tetrahedrally coordi-nated aluminum sites were significantly increased with the rise inaluminum contents of mesocage monoliths, as evidenced by in-crease in AlIV/AlVI ratio from 1.08 to 2.33 (Table 2). Coordinationand location of aluminum sites in the frameworks were a keydeterminant in the generation of surface acidity of disk-like alu-minosilica membranes [30].

Surface acidity of disk-like aluminosilica monoliths was deter-mined using NH3-TPD (Fig. 6). Two types of acid sites were charac-terized by disk-like aluminosilica monoliths with all Si/Al ratios, asevidenced by desorption curves (Fig. 6a–e). However, incorporatedaluminum into the cubic Im3m mesostructured monoliths gener-ated the acid sites. The TPD profiles revealed two main peaks ofNH3 desorption around 200 �C and a small, broad intensity peakwithin the range of 400 �C–500 �C (Fig. 6). Results here clearly indi-cated that the ammonia desorbed from weak and mildly strongacid sites of disk-like membranes. To determine quantitativelythe amount and strength of acid sites of mesoporous aluminosili-cas, peaks around 200 �C and 450 �C were deconvoluted usingGaussian function with temperature as variant (Table 2). Increasein the intensity of NH3 desorption peak at lower temperaturearound 200 �C indicated that the number of acid sites increased

Fig. 5. 27Al MAS NMR spectra of cage cubic Im3m aluminosilicas (disk-likemonoliths) with Si/Al ratios of 9.0 (a), 4.0 (b), 1.5 (c), and 1.0 (d).

Table 2Analyzing data of cubic Im3m aluminosilica adsorbent monoliths using EDX, TPD, and 2

environments was calculated by analyzing the peak areas at chemical shift �1 and 58 ppm

EDX TPD

Calculated Si/Al Analyzed Si/Al (±1) % Peak �200 �C

19 19.2 62.349 9.11 66.644 4.05 67.012.3 2.32 69.451.5 1.53 69.931.0 1.02 70.67

with the amount of aluminum in the disk-like membranes[31,32]. Surface acidity creates binding sites to organic dye mole-cules, resulting in high accessibility and removal of dye moleculesfrom aqueous solution. However, the increase in the acidity withincorporation of Al atoms enabled the disk-like aluminosilicaadsorbent to retain (organic dyes) substances that have basic prop-erties. Thus, this acidic adsorbent could only show the adsorptionbehavior of neutral (such as H2DSPy) or basic dyes.

3.3. Removal of H2DSPy dye onto monolithic adsorbents

It is important to note that the H2DSPy dye structures based on2-mercaptopyrimidine compounds have shown biological activi-ties, which are involved in the composition structure of nucleicacid in living systems. For example, the derivatives of this dyecould be used as selective adenosine receptor antagonists [33].Thus, the successful design of condensed and rigid (disk-like)monoliths with active acid sites, multidirectional (3D) pore con-nectivity, well-defined cage cavities, and mechanical stability ispromising for easy-to-use adsorbents for H2DSPy dye within ashort period.

Scheme 1B shows that the uniformity of pore surface not onlyleads to a facile accessibility of H2DSPy dye to the cavity sitesbut likewise increases the homogenous transport and diffusion ofdye molecules from the aqueous phase to the binding sites ofnanoadsorbent surfaces [34]. Despite the lack of homogenous dif-fusion of H2DSPy dye into the disordered pores of disk-like mono-liths with low silica contents, these adsorbents manifest significantadsorption capacity and fast mass transport. Therefore, the effect ofhigh aluminum contents in the adsorption assays is overshadowed(Table 3).

In this study, we performed a series of batch contact-timeexperiments to define and evaluate systematically the suitableconditions of H2DSPy dye adsorption on the mesocage adsorbents.Concentration of adsorbate solutes was continuously detected and

7Al NMR techniques: The amount percentage of four- and six-coordinate aluminum, respectively, from 27Al MAS NMR spectra.

27Al NMR

% Peak �450 �C AlVI (%) AlIV (%) AlIV/AlVI (ratio)

37.66 52 48 1.0833.36 53 47 1.1332.99 56 44 1.331.54 61 39 1.5630.07 65 35 1.8629.33 70 30 2.33

Table 3Effect of the cage aluminosilica-based adsorbents (0.2 g) with different Si/Al ratios onadsorption performance of (0.5 mmol L�1) H2DSPy dye at 40 �C.

Al/Si 1017 D (cm2/min) fc 102 (g/m2) qm (mmol g�1) KA (L/mmol)

19 0.166 1.62 – –9 5.64 1.69 0.00776 230.04464 15.1 2.99 0.008451 238.51442.3 20.6 5.02 0.008953 260.47411.5 36.7 9.13 0.009279 275.08671 104.0 12.72 0.009365 432.4862

Fig. 7. Time sequence of the adsorption of (0.5 mmol L�1) H2DSPy using 0.3 g cubicIm3m disk-like aluminosilica monoliths with a Si/Al ratio of 1.0, and at 40 �C.

Fig. 8. Time dependence of adsorption amount of H2DSPy onto mesoporous aluminosilH2DSPy on the adsorption process using (0.2 g) disk-like membrane with a Si/Al ratio ofadsorption of (0.5 mmol L�1) H2DSPy using (0.2 g) disk-like membrane with a Si/Al ratio oof (0.5 mmol L�1) H2DSPy at 40 �C.

S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18 15

monitored using UV–vis absorption at 369 nm as a function of time(Fig. 7) and under specific conditions (i.e., temperature, shakingrate, and adsorbate concentration). High performance of alumino-silica nanoadsorbents in terms of adsorption capacity of dye mole-cules in an aqueous solution depended on such key factors astemperature, amount of nanoadsorbents, and dye concentration(Fig. 8A–C). However, adsorption amount (qe, mol g�1) of dye mol-ecule at the equilibrium step was determined using the followingequation:

qe ¼ ðCo � CeÞV=m ð1Þ

where V is the solution volume (L), m is the mass of monolithicadsorbents (g), and Co and Ce are the initial and equilibrium adsor-bate concentrations, respectively.

Fig. 8D demonstrates that the adsorption amount (qt, mol g�1)of the H2DSPy dye molecule was effectively appended to the load-ing amount of active function sites of the aluminum species intothe (disk-like membrane) adsorbents. Adsorption capacity andequilibrium for adsorbate molecules on the monolithic adsorbentsto a high extent depended on the aluminum surface chemistry ofcage aluminosilica structures (Fig. 8D). However, functional alumi-num active sites of four- or six-coordinate species contributed tobinding with accessible adsorbate molecules. Notably, such syner-gistic interaction did not display a significant alteration of chemicalproperties of the active aluminum species, as evidenced by the 27AlNMR spectra recorded after adsorption assays (Fig. 9). Results (seeTable 3) revealed that increase in AlIV/AlVI ratios led to an enhance-ment of adsorption capacity and affinity of H2DSPy dye. This find-ing indicated that the organic molecules may readily adsorb to

ica monoliths (disk-like membrane): (A) study the effect of initial concentration of1.0, at 40 �C; (B and C) effect of temperature and amount of disk-like membrane onf 1; and (D) effect of different Si/Al ratios of (0.2 g) disk-like membrane of adsorption

Fig. 9. 27Al MAS NMR spectra of cage cubic Im3m aluminosilicas (disk-likemonoliths) with Si/Al ratios of 4.0 (a) and 1.0 (b) that recorded after adsorptionof (0.5 mmol L�1) H2DSPy dye.

16 S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18

monoliths that exhibit highly tetrahedral (AlIV, AlO�4 , framework)aluminum sites.

The Langmuir isotherm is the widely applied theoretical modelfor monolayer adsorption. Adsorption characteristics of a H2DSPydye molecule onto the adsorbent can be studied through the Lang-muir isotherm [35,36]:

Ce=qe ¼ 1=ðKLqmÞ þ ð1=qmÞCe ð2Þ

where qm (mmol g�1) is the amount of H2DSPy dye adsorbate ad-sorbed to form a monolayer coverage, and KL is the Langmuiradsorption equilibrium constant. On the plot of Ce/qe against Ce,qm and KL can be determined from the slope and intercept. Fig. 10presents the adsorption isotherms of H2DSPy dye. Results (Fig. 10)indicate the formation of a monolayer of H2DSPy dye on nanoadsor-bent monoliths (Scheme 1B). From the linear plot of Langmuir iso-therm (Fig. 10B), the monolayer adsorption capacity, qm, andLangmuir coverage constant, KL, were obtained (Table 3). The qm

and KL values increased with the loading amount of active functionsites of the aluminum species into the (disk-like membrane) adsor-bents. This tendency is consistent with the intraparticle diffusion(D) and coverage surfaces (fc) of the H2DSPy dye (Table 3).

Fraction of the coverage mesocage adsorbent surfaces (fc, g/m2)occupied by the pollutant molecules was calculated according tothe following equation:

fc ¼Mb=S ð3Þ

whereM is the molecular area of the H2DSPy dye molecule, whichwas determined by the computer program applying MOPAC PM3method with COSMO salvation in aqueous medium [34], S (m2/g)is the surface area of monolithic adsorbents, and b is the number

Ce (µmol/L)

q e (µ m

ol/g

)

0

2

4

6

8

Si/Al= 1Si/Al= 1.5Si/Al= 2.3Si/Al= 4Si/Al= 9

(A)

0 5 10 15 20 25

Fig. 10. Langmuir adsorption isotherms (A) and the linear form of the Langmuir plot (B) oSi/Al ratios of 9.0, 4.0, 2.3, 1.5, and 1.0, respectively, at 40 �C.

of molecules adsorbed per unit area of mesocage adsorbents. How-ever, b can be calculated as follows: b = (qe/S) � NA, where NA per-tains to Avogadro’s number (6.02 � 1023 mol�1).

Coverage surfaces (fc) of H2DSPy dye molecules were signifi-cantly affected by active functional acid sites (see Table 3). How-ever, (fc) value was increased with high Al-doped the disk-likemonoliths, despite the location of Al particles inside or outsidethe pore frameworks [16,21,22]. Although monoliths (disk-likemembrane) with low Si/Al ratios exhibited a distortion in poreordering and a decrease in surface area and pore volume (see Ta-ble 1), enhancement of both molecular coverage of H2DSPy dyemolecules on the network surfaces and in the pore architecturesof monolithic adsorbents, respectively, was achieved (Table 3). Itis important to note that the orientational dimension of theH2DSPy dye is very small (�12–14 Å) compared with large poreopenings of the cubic cage (�5 nm). Thus, the adsorption of thedye was mainly occurred into the cage pores, but this does not ex-clude the possibility of adsorption onto the adsorbent surfaces[35]. Furthermore, intraparticle diffusion of H2DSPy dye moleculeinto mesocage adsorbents may be determined by plotting the frac-tional attainment of equilibrium (Fe) = qt/qf against t12 according toFick’s second law relationship [36–38]:

Feðqt=qf Þ ¼ 6=rffiffiffiffi

Dp

t=p ð4Þ

where qtqf is the ratio of adsorbed quantity of H2DSPy dye moleculeat time (t) to the amount adsorbed at saturation time, r is the poreradius of mesocage adsorbents, p is constant, and D is the intrapar-ticle diffusion coefficient.

Fig. 11 shows the fractional attainment of equilibrium (Fe)against t12 of disk-like membranes with different loading ofaluminum contents. Evidently, Fig. 11 can be classified into threeportions. The first linear portion reflects the instantaneous adsorp-tion stage. During this step, monolithic particles of adsorbents areconsidered surrounded by a boundary layer of fluid film throughwhich the adsorbate H2DSPy dye molecule must diffuse prior toexternal adsorption on adsorbent surfaces. The second curve por-tion signifies the intraparticle diffusion step, and the third portionis the final equilibrium stage. However, the intraparticle diffusioncoefficient D is calculated from the slope of the second portion ofFig. 11 (Table 3) [21]. Moreover, Fig. 11 plateau provides evidencethat intraparticle diffusion may control the mass transport of pol-lutant molecules from aqueous phase to the binding sites of thealuminosilica monoliths.

These results indicated that insertion of aluminum into the so-lid mesocage monoliths led to the development of active acid sitesat the internal mesopore surfaces of adsorbents [19,20,35]. Thesenatural surfaces of acid sites strongly induced the acid/base

Ce (µmol/L)0 5 10 15 20 25

Ce/

qe (g

/L)

0

1

2

3

4Si/Al= 1Si/Al= 1.5Si/Al= 2.3Si/Al= 4Si/Al= 9

(B)

f (0.5 mmol L�1) H2DSPy onto cage aluminosilicate-based adsorbents with different

Fig. 11. Illustration of fractional attainment of equilibrium of (0.5 mmol L�1)H2DSPy onto cage aluminosilicate-based adsorbents with different Si/Al ratios of9.0, 4.0, 2.3, 1.5, and 1.0, respectively, at 40 �C.

S.A. El-Safty et al. / Journal of Colloid and Interface Science 359 (2011) 9–18 17

interactions. Such interactions were occurred through bothH-bonding and dispersive feature of the adsorbents with H2DSPydye molecules. In general, our findings indicated that aluminosilicamonoliths may act as effective adsorbents of H2DSPy dye mole-cules from aqueous solutions.

3.4. Reusability of the nanoadsorbents

A major advantage of nanostructured adsorbents is their retain-ing functionality in terms of adsorption activity after multipleregeneration/reuse cycles of adsorbents. Although the improve-ment of reducibility and reversibility of adsorbents remains a chal-lenge, aluminosilica nanostructured adsorbents can extend controlof adsorption and removal of H2DSPy dye from aqueous water evenafter several cycles. However, after finishing one cycle set of dyeremoval, aluminosilica nanostructured adsorbents were collectedand washed by NaOH solution (0.005 mol L�1) for several timesand subsequently dried at 200 �C for 12 h under air to removeH2DSPy dye adsorbate on the surfaces. After multiple regenera-tion/reuse cycles (i.e., P5), although the regenerated adsorbentsrevealed a relatively lower activity in terms of particle diffusionD (cm2/min) and fraction of coverage mesocage adsorbent surfaces(fc, g/m2) by H2DSPy adsorbate, well-controlled removal efficiencywas exhibited (Fig. 12). Results revealed that the monolithic adsor-

Fig. 12. Reusability study of up to six times the removal assay of (0.5 mmol L�1)H2DSPy in terms of particle diffusion D (cm2/min) and fraction of the coveragesurfaces (fc, g/m2) using cage aluminosilicate-based adsorbents with both Si/Alratios of 4.0 and 1.5, respectively, at 40 �C.

bents lost approximately 6% of its original efficiency after fiveregeneration/reuse cycles (Fig. 12). In general, monolithic adsor-bents exhibited high nanostructural stability, active function sites,and physical parameters such as surface area and pore volume.Measurements of textural properties of the regenerated (disk-likemembrane) adsorbents revealed no significant influences on thespecific surface area, and mesopore volumes were evident (de-crease in parameters was 65% from the original data, Table 1). Inaddition, retention of shapes and sizes of adsorbent cage-like poreswas observed even after several reuse cycles. Such long-termreversibility renders the (disk-like membrane) adsorbents particu-larly applicable for further removal process.

4. Conclusion

With the aim of controlling nanoscale adsorbents for function-ality in molecular sieving of organic pollutants, we fabricated cubicIm3m mesocages with uniform entrance and large cavity pores ofaluminosilicates as highly promising candidates for the colorimet-ric monitoring of organic dyes in an aqueous solution. In our syn-thetic approach, casting of microemulsion liquid crystalline phasesof triblock copolymer F108 surfactant assists significantly in thefabrication of well-defined mesostructures with condensed and ri-gid disk-like membranes. Functional acid sites, multidirectional(3D) pore connectivity, high aluminum content, and orderingstructures of aluminosilica adsorbents exhibit efficient removalof organic dyes such as N,N0-disalicylidene-4,5-diamino-6-hydro-xy-2-mercaptopyrimidine (H2DSPy), to a maximum of 100% withina short period from the aqueous solution phase. In such adsorptionsystems, natural surfaces of active acid sites of aluminosilicatestrips strongly induced both physical adsorption of chemicallyresponsive dyes and intraparticle diffusion into cubic Im3m meso-cage monoliths. It is worth noting that our approach likewise per-mits easy-to-use removal assays of organic compounds, andportable and reusable chemical adsorbents without any meso-structural damage even under rigorous chemical treatment for anumber of reuse/cycles.

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