The synthesis of novel hybrid thiol-functionalized nano-structured SBA-15 This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2010 Adv. Nat. Sci: Nanosci. Nanotechnol. 1 035011 (http://iopscience.iop.org/2043-6262/1/3/035011) Download details: IP Address: 128.148.252.35 The article was downloaded on 17/03/2013 at 11:32 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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The synthesis of novel hybrid thiol-functionalized nano-structured SBA-15
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
The synthesis of novel hybridthiol-functionalized nano-structuredSBA-15Van Duc Hoang1, Tuyet Phuong Dang2, Quang Khieu Dinh3,Huu Phu Nguyen4 and Anh Tuan Vu3
1 College of Pedagogy, Hue University2 Institute of Chemistry, Vietnam Academy of Science and Technology3 College of Sciences, Hue University4 Vietnamese Society of Catalysis and Adsorption
Received 14 September 2010Accepted for publication 14 October 2010Published 4 November 2010Online at stacks.iop.org/ANSN/1/035011
AbstractMesoporous thiol-functionalized SBA-15 has been directly synthesized by co-condensationof tetraethyl orthosilicate (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) withtriblock copolymer P123 as-structure-directing agent under hydrothermal conditions.Surfactant removal was performed by Soxhlet ethanol extraction. These materials have beencharacterized by powder x-ray diffraction (XRD), nitrogen adsorption/desorption (BETmodel), transmission electron microscopy (TEM), thermal analysis, infrared spectroscopy (IR)and energy-dispersive x-ray spectroscopy (EDX). The main parameters, such as the initialmolar ratio of MPTMS to TEOS, the time of adding MPTMS to synthesized gel and theSoxhlet ethanol extraction on the thiol functionalized SBA-15 with high thiol content andhighly ordered hexagonal mesostructure, were investigated and evaluated. The adsorptioncapacity of the thiol-functionalized and non-functionalized SBA-15 materials for Pb2+ ionfrom aqueous solution was tested. It was found that the Pb2+ adsorption capacity of thethiol functionalized SBA-15 is three times higher than that of non-functionalizedSBA-15.
Since the beginning of mesostructure science a decadeago [1], the preparation of mesoporous silica-based materialshas been a research focus due to their unique largesurface area, well-defined pore size and pore structures,well-modified surface properties and controlled surfacereactivity [2]. Among mesoporous materials, SBA-15 hasreceived much attention because it possesses a highly orderedhexagonal structure, thermal stability and thicker walls incomparison with MCM-41(Mobil Composition of Matter No.41) materials. Recently, the preparation of organic–inorganichybrid silicon materials has attracted much attention [3].Surface modification by organic functional groups can
improve the physical and chemical properties of the silicasurface and extend practical applications [4, 5]. The functionalgroups can be used in the selective removal of cations fromsolution, or to act as tethers for catalytic complexes or ascatalysts themselves in acid- or base-catalysed reactions.Among all of the surface modifications, thiol-functionalizedmesoporous silica [6, 7] has much application potential.Liang et al [8] have reported its wide availability and highcapacity to fix Pb2+ and Cd2+. Thiol groups can also beincorporated and used to synthesize carbon replicas of thesilicas [9]. These groups can be included by post-syntheticgrafting or by co-condensation during synthesis [10]. Inpost-synthetic grafting, a pre-calcined mesoporous silica,being partially re-hydrated to generate surface hydroxyl
Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 035011 V D Hoang et al
groups, is reacted with the appropriate alkoxysiloxane,whereas co-condensation involves the addition of bothtetraethyl orthosilicate (TEOS) and the functionalisedsiloxane group (MeO)3–Si–X to the synthesized mixture.
In the present work, thiol-functionalized SBA-15 wasprepared through co-condensation. Also, the effect of solventsand the time of extraction for removing the template fromthiol-functionalized SBA-15 were discussed.
2. Experimental
Organically modified molecular sieves were prepared bythe co-condensation of trialkoxysilanes and tetraethylorthosilicate. The adopted procedure was similar tothat reported for SBA-15 [11]. The non-ionic triblockcopolymer Pluronic P123 (EO20–PO70–EO20, MW: 5800,Aldrich) was used as a template and tetraethyl orthosilicate(TEOS, 98%, Aldrich) was the silica source. 3-mercapto-propyltrimethoxysilane (MPTMS) (MeO)3Si(CH2)3SH(95%, Merck) was added at a molar ratio of S to Si in therange from 0.053 to 0.308. The surfactant was dissolvedin the hydrochloric acidic media and stirred at 313 K.TEOS and MPTMS were added simultaneously, and themixture was stirred for 24 h for mesostructure formationto take place. Finally,the mixture was transferred to aTeflon bottle and hydrothermal treatment was applied at373 K for 24 h, enabling further condensation of the silicaframework. The resulting solids were filtered. The sampleswere denoted as-synthesized HS-SBA-15(x) with x being0.053, 0.087, 0.121, 0.174 and 0.308, implying a molar ratioof MPTMS/TEOS. The general procedure was similar to thatmentioned above. The molar ratio of MPTMS/TEOS waskept at 0.174 while the TEOS was allowed to prehydrolyzefor a determined number of minutes. After specifici timespans (t = 0, 5, 10, 20 and 30 min) since TEOS and P123were mixed, MPTMS was added to the synthesized gel. Thesesamples were denoted as as-synthesized HS-SBA-15-t, witht being 0, 5, 10, 20 and 30 min.
The template removal from as-synthesizedHS-SBA-15-30 was conducted by Soxhlet ethanol extraction.The part of the as-synthesized HS-SBA-15-30 samplewas placed into the Soxhlet apparatus and extracted withethanol for 24, 48 and 72 h. The samples of the removedtemplate were denoted as extracted HS-SBA-15(0.174) andHS-SBA-15(0.121) with the numbers in brackets implyingthe ratio of MPTMS/TEOS in the initial synthesized gel.
X-ray diffraction (XRD) patterns were recorded ona VNU-D8 Advance Instrument (Bruker, Germany) usingCuKα radiation (λ = 1.5418 Å). N2 adsorption desorptionisotherm measurement was performed at 77 K with asurface area and porosimetry analyzer (Ommisorp-100) inthe relative pressure (P/P0) range from about 10−3 to0.99. Samples were degassed at 250 oC for 2 h beforesetting the dry mass and data collection. Specific surfaceareas were calculated using the Brunauer–Emmett–Tellermodel (BET model). Pore size distributions were calculatedusing the Barret–Joyner–Halenda model (BJH model) onthe adsorption branches. Thermal behaviours of sampleswere recorded by thermal analysis ((Thermal Gravimetry andDiffrential Scanning Calorimetry—TG/DSC) using a Labsys
0 2 4 6 8 10
(100)
HS-SBA-15(0.308)
HS-SBA-15(0.174)
HS-SBA-15(0.121)
HS-SBA-15(0.087)
HS-SBA-15(0.053)
Inte
nsity
(cps
)
2θ (degree)
Figure 1. XRD patterns of as-synthesized thiol-functionalizedSBA-15 with different molar ratio of MPTMS/TEOS.
TG/DSC SETARAM. Transmission electron microscopy(TEM) micrographs were taken using a JEOLE-3432operating at 80 keV. Infrared spectra (IR) were recorded on anFT–IR spectrometer using an IR-Prestige-21(SHIMADZU) ina KBr matrix in the range 4000–400 cm−1. Element analysisof S and Si were conducted by energy-dispersive x-rayspectroscopy (EDX/SEM) using a JED-2300, JEOL.
All experiments were carried out at 24 ◦C and theadsorption capacity qe (mg/g) was calculated by the followingequation:
qe =(C0−Ce) V
m,
where C0(mg/l) and Ce(mg/l) are the initial and theequilibrium Pb2+ concentration, respectively, V (ml) is thevolume of the adsorption system and m (g) is the mass of theadsorbent.
3. Results and discussion
Figure 1 presents x-ray diffraction patterns of theas-synthesized thiol-functionalized SBA-15 with differentmolar ratios of MPTMS/TEOS. Most of the samples havea single intensive reflection at a 2θ angle around 0.8◦, asin the case for typical SBA-15 materials, and the reflectionis generally related to a regular pore size and an orderedpore arrangement [11]. These reflections decrease graduallyin intensity as the relative amount of MPTMS in the initialmixture increases, which indicates a decrease in the orderinghexagonal. The broad peak of (100) and low intensity indicatethe very poor hexagonal mesostructure as the molar ratio ofMPTMS/TEOS increases to 0.174. With further increase inthe molar ratio of MPTMS/TEOS up to 0.308, the hexagonalmesostructure was almost collapsed as the (100) diffractionpeak was not observed.
It is clear that the simultaneous addition of thiol MPTMSand TEOS to the synthesized gel resulted in a decrease inhexagonal mesostructure. There arises the question that if theTEOS is prehydrolyzed and MPTMS is added latter, then thequality of the thiol-functionalized SBA-15 hexagonal orderingmay be improved.
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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 035011 V D Hoang et al
0 2 4 6 8 10
(100)
as-synthesized HS-SBA-15-30
as-synthesized HS-SBA-15-20
as-synthesized HS-SBA-15-10
as-synthesized HS-SBA-15-5
as-synthesized HS-SBA-15-0
Inte
nsity
(cp
s)
2θ (degree)
Figure 2. XRD patterns of as-synthesized thiol-functionalizedSBA-15 with different times of addition of MPTMS to thesynthesized gel.
0 2 4 6 8 10
(100)
extracted by ethanol Soxhlet for 72h
extracted by ethanol Soxhlet for 48h
extracted by ethanol Soxhlet for 24h
as-synthesized HS-SBA-15-30
Inte
nsity
(cp
s)
2θ (degree)
Figure 3. XRD patterns of as-synthesized thiol-functionalizedSBA-15 and thiol-functionalized SBA-15 extracted by ethanolSoxhlet.
Figure 2 shows XRD patterns of as-synthesizedthiol-functionalized SBA-15 with different times of addingMPTMS to the synthesized gels. As can be seen, the intensityof peak (100) characterized for the mesostructure increaseswith the prolongation of time of the TEOS prehydrolysis.After 30 min of TEOS prehydrolysis, the addition of MPTMSto the synthesized gel was favorable for the formation ofa highly ordered hexagonal mesostructure. The sample withan initial molar ratio of MPTMS/TEOS = 0.174, which wassynthesized under conditions of TEOS prehydrolysis for30 min, was used for the study of the template removal.
Figure 3 shows XRD patterns of the thiol-functionalizedSBA-15 removed template by the Soxhlet ethanol extraction.As can be seen, the d100 value of 10.8 nm for extractedHS-SBA-15-30 decreases in comparison with that of 11.3 nmfor as-synthesized HS-SBA-15-30, indicating that the removalof the template may cause shrinkage of the hexagonalcell. The prolonged extraction time led to an increase inthe intensity of peak (100), which is characterized for themesoporous hexagonal structure, and both d100 of samples
extracted during 48 and 72 h are the same, suggesting that theextraction process could end at 48 h.
The introduction of thiol groups into SBA-15 isconfirmed by the results of TG-DSC. As seen from figure 4,for the as-synthesized pure SBA-15 materials, a weight lossof about 2.3 wt% was observed at a temperature lower than120 oC, which was due to the evaporation of adsorbed water,and the further weight loss (about 43.7 wt%) at temperatureshigher than 180 oC and less than 300 oC correspondingto a large exothermic peak around 210 oC is attributedto the surfactant decomposition. For the as-synthesizedthiol-functionalized SBA-15, the broad exothermic peakshould be attributed to surfactant decomposition while theendothermic peak at 450 oC and the exthothermic peak at500 oC are attributed to decomposition of silanol grounds toform siloxan and thiol groups. The total weight loss wasaround 56.4 %. Besides the weight loss due to adsorbedwater at 100 oC, the extracted functionalized samples lostweights in one step at 350 oC (approximately 30%), whichcorresponds to the loss of thiol groups. The fact that theshapes of TG-DSC for samples extracted for 48 and 72 h aresimilar further confirms the extraction process ending after48 h. The result of elemental analysis by EDX indicates theexistence of thiol groups in the functionalized materials, andthe ratio of MPTMS/TEOS estimated by elemental analysisis 0.124 higher than that of 0.174 in the initial mixture, whichmight result from the effect of the surfactants left after theSoxhlet treatment. The content of MPTMS incorporated intothe silica framework SBA-15 in the present work is rather highin comparison with the results of other works [12, 13].
The existence of a thiol group is further confirmed bythe results obtained from IR spectroscopy. The FT-IR spectraof the HS-SBA-15-30 sample are shown in figure 5. A weakvibration band observed at 2571 cm−1 is assigned to the S–Hstretching vibration. The sharp absorption band at 3446 cm−1
is attributed to stretching of O–H on the surface silanol groupswith the hydrogen bond and the remaining adsorbed watermolecules. The absorption bands at 2976 and 2931 cm−1
are ascribed to the asymmetric stretching and symmetricstretching of (–CH2–), respectively, while the absorption bandat 1460 cm−1 is ascribed to the bending of the (–CH2–)group. The absorption bands at 1051 and 800 cm−1 are dueto Si–O–Si and Si–O stretching vibrations, respectively. Theband at 954 cm−1 is assigned to Si–OH stretching [8]. Theadsorption band at 1635 cm−1 is due to deformation vibrationsof adsorbed water molecules (δH–O–H).
The mesoporous ordered hexagonal structure of theobtained thiol-functionalized SBA-15 was also investigatedby TEM. The pure SBA-15 material and extractedHS-SBA-15-30 shows a typical hexagonal pore array, asshown in figures 6(a) and (b). These results are in agreementwith the ordered pore structure, as indicated in the XRDresults.
The mesoporous ordered hexagonal structure is furtherconfirmed by isotherms of adsorption/desorption for nitrogen.The isotherms of the functionalized SBA-15, extractedHS-SBA-15(0.174) and extracted HS-SBA-15(0.121) areshown in figure 7. It can be seen that all of the samples exhibittype IV isotherms with an apparent hysteresis loop caused bycapillary condensation, indicative of the existence of defined
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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 035011 V D Hoang et al
mesopores in the frameworks. The adsorbed amount at thesaturated pressure decreases as the concentration of MPTMSincreases, indicating that the greater the amount of MPTMSincorporated, the lower the porosity that may be obtained.The capillary condensation shifts gradually to a lower vaporpressure with the higher MPTMS content, which is related tothe decrease in mesopore diameter. The textural properties ofthe samples are presented in table 1.
The pore diameter ranging from 5.0 to 6.4 nm anddependent on the relative content of MPTMS is consistentwith that which is implied in the isotherms. As shown intable 1, the surface area decreases dramatically from 587 to543 m2 g−1 and the pore volume from 0.65 to 0.57 cm3 g−1
as the molar ratio of MPTMS/TEOS increases from 0 to0.174. The decrease in pore structure with the addition of thiolgroups can be described as follows. The methoxy terminalof the MPTMS may co-condense with the TEOS to forman norganic framework and the mercaptopropyl branch mayreact with the hydrophilic groups of the P123 template viaH-bonding to form an organized structure. Upon removalof the P123 template, the mercaptopropyl chains may berearranged and act as linked organic functionalities protrudingfrom the inorganic walls into the pore channels [14], thusresulting in the occupation of space inside the pore and thegradual increase in pore wall thickness. The more MPTMScontent is introduced, the more pore space may be packedand the smaller the pore diameter that can be obtained.The evolution of pore structure might also result from theco-surfactant effect of the MPTMS, which interacts with thetemplates and reduces the diameter of the micelles [15].
Although some textural properties of the thiol-functionalized SBA-15 decrease in comparison with
Table 2. Adsorbability of Pb2+ ions of different adsorbents.
Solution Adsorbent AdsorptionCo Ce volume mass capacity
those of the SBA-15 (table 1), the adsorption capacity ofthe functionalized materials for the adsorption of Pb2+ ionfrom aqueous solution was improved dramatically, as shownin table 2. SBET is the specific surface area, dp is the porediameter, tw is the thickness of the wall and Vt is the porevolume.
From the result of table 2, it can be seen that the Pb2+
adsorption capacity for the SBA-15 is low (reaches to about31 mg g−1), while it is very high for the thiol-functionalizedSBA-15 silica. The result also shows that the adsorptioncapacity of Pb2+ ions increases with the increasing number ofthiol groups at the surface and reaches 105.34 (mg g−1) as themolar ratio of MPTMS/TEOS is 0.174. It is found that thePb2+ adsorption capacity of the thiol-functionalized SBA-15is three times higher than that of the non-functionalizedSBA-15.
4. Conclusions
The functional thiol groups were successfully introducedinto the surface of the SBA-15 by the co-condensationof 3-mercaptopropyltrimethoxysilane and tetraethyl ortho-silicate. The functionalized SBA-15 materials with a highlyordered hexagonal structure can be obtained for a molar ratioof MPTMS/TEOS even as high as 0.174 in the reactionmixture when the time of adding MPTMS to synthesizedgel is 30 min. The Soxhlet ethanol extraction with 48 h isfavorable for the complete removal of the template from thethiol-functionalized SBA-15. The addition of thiol groupsresults in a decrease in both surface area and pore size,as well as pore volume, of the functionalized SBA-15silicas. The porosity decreases dramatically when the ratioof MPTMS/TEOS is around 0.174, with a loss of ∼20% forboth the surface area and the pore volume. the Pb2+ adsorptioncapacity of the thiol-functionalized SBA-15 is three timeshigher than that of the non-functionalized SBA-15.
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