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Ceramics International 40 (2014) 9701–9707www.elsevier.com/locate/ceramint

Functionalization with amine-containing organosilane of mesoporous silicaMCM-41 and MCM-48 obtained at room temperature

Héctor Iván Meléndez-Ortizn, Yibran Perera-Mercado, Jesús Alfonso Mercado-Silva,Yeraldin Olivares-Maldonado, Griselda Castruita, Luis Alfonso García-Cerda

Centro de Investigación en Química Aplicada, Boulevard Enrique Reyna Hermosillo #140, C.P. 25294 Saltillo, Coahuila, México

Received 7 January 2014; received in revised form 11 February 2014; accepted 12 February 2014Available online 20 February 2014

Abstract

Functionalized ordered MCM-41 and MCM-48 materials have been obtained by a post-grafting method using 3-aminopropyltrimethoxysilane(APS) as modifying agent. The dependence of APS concentration, and reaction time on average pore size, grafted APS content, pore volume,superficial area, and morphology was studied. The modified silicas were characterized by powder X-ray diffraction (XRD), infrared spectroscopy(FT-IR), transmission electron microscope (TEM) and nitrogen adsorption–desorption experiments. MCM-41 appeared to be more prone to poreblockage than MCM-48 in the grafting procedure. According to the TEM results, no morphological changes were observed in the modifiedMCM-41 and MCM-48 silicas by comparing with bare silicas. These amine-grafted silicas could have potential application to be used asadsorbents of CO2 in the natural gas sweetening process or as fillers in the preparation of mixed matrix membranes.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Mesoporous silica; MCM-41; MCM-48; Organosilane; Post-grafting method

1. Introduction

Intensive interest has been shown by worldwide researchersfor mesoporous materials because of their high potentialapplications, e.g., CO2 adsorbents [1,2] heterogeneous cataly-sis [3,4], drug delivery [4–6], and membranes for gas separa-tion [7–9]. These applications have been possible by means ofthe functionalization of mesoporous materials with organiccompounds [10,11]. The modified mesoporous materials canbe classified into two categories: the first one corresponds tomaterials with weak bonding between organic and inorganicphases, while the second one corresponds to materials whereboth phases are chemically grafted. The second one results tobe more attractive because a covalent Si–C bond is formed,which prevents the detachment of the organic groups whenmesoporous materials are in solution.

10.1016/j.ceramint.2014.02.05114 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ52 844 4389830x1335; fax: þ52 844 4389839.ss: ivan_melendez380@hotmail.com (H.I. Meléndez-Ortiz).

In general, these mesoporous materials can be functionalizedin two ways: direct synthesis or co-condensation and post-grafting (also known as post modification) procedures [2,4,12–14]. Direct co-condensation consists essentially the co-polymerization of a silica precursor (usually tetraethylorthosi-licate, TEOS) and an organosilane precursor in the presence ofa template [12–14]. The advantages of the co-condensationmethod include one-step synthesis, control of the loading anddistribution of the organic groups. However, the materialsprepared via direct co-condensation possess less orderedmesoporous structures, since adding the organosilane precursorinto the synthesized gel significantly disrupts the process ofsynthesis. On the other hand, modified mesoporous materialscan alternatively be prepared by the post-grafting method[2,4,15]. This procedure is most commonly used in performingsurface modification by covalently linking organosilane spe-cies with surface silanol groups. In this procedure, the silica isreacted with an appropriate organosilane in a suitable solventat reflux. This method allows an effective inclusion of denselypopulated or high concentrations of covalently bound organic

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functional groups. Nevertheless, it has several disadvantagessuch as difficulties in controlling the loading and position ofthe organic modifying agent, low loading of attached com-pound, two-step process, etc. [16].

MCM-41 and MCM-48 mesoporous materials possess alarge number of –OH groups which can react to yield a hybridmaterial, it means, a material with both inorganic and organicparts. Surface functionalization of mesoporous materials withseveral types of functional groups has been recently reported[17–21]. Particularly, MCM-48 silica is found to be attractivefor potential applications in adsorption, membranes for gasseparation, and heterogeneous catalysis due to its three-dimensional interconnected cubic pore structure. The structureof the MCM-48 silica is expected to be less prone to poreblocking than the hexagonal arrangement unidimensional inMCM-41 silica, and it should permit a faster diffusion throughthis structure [19].

Some authors have reported the modification of thesemesoporous materials with amine-containing compounds.Bhagiyalakshmi et al. [2] reported the functionalization ofMCM-41 and MCM-48 silicas with a solution of 3-chloropropylamine (CPA) in water neutralized with potassiumhydroxide. Grafting process was determined by FT-IR and themodified materials were tested for CO2 chemisorption. Kimet al. [19] investigated the pore structures of MCM-48 silicasfunctionalized with four different types of amine-containingcompounds. The N2 adsorption/desorption studies showed adecrease in the pore volume and surface area of the modifiedmesoporous materials. Also, these silicas were used to separateCO2 of nitrogen (N2). Walcarius et al. [22] prepared variousmesoporous materials modified with different amounts of NH2

and SH groups and examined the influence of structure andporosity on the access rates of Hþ , HgII and CuII to thebinding sites in the modified mesoporous materials.

Hence, the objective of this work is to study the dependenceof the initial organosilane concentration, and reaction time onthe mesostructure, pore size distribution, surface area, porevolume, organosilane loading and morphology of both MCM-41and MCM-48 silicas when they are modified with APS by usingthe post-grafting method. These modified materials could havepotential application in CO2 adsorption processes, as drugdelivery systems, and as fillers in the preparation of mixedmatrix membranes used for gas separation.

2. Experimental

2.1. Materials

Tetraethylorthosilicate, TEOS (98%, Aldrich), cetyltri-methylammonium bromide, CTAB (98%, Aldrich), deionizedwater obtained from a system of two ionic interchangecolumns, Cole-Parmer Instruments, ethanol (99.8%) and aqu-eous ammonia solution, NH4OH (29 %, Fermont) were used toprepare mesoporous MCM-41 and MCM-48 materials.Besides, 3-aminopropyltrimethoxysilane, APS (97% Aldrich),isopropanol (99% Aldrich) and toluene (99%, Baker) wereused to modify the mesoporous materials.

2.2. Preparation of mesoporous silica

In a typical synthesis of mesoporous MCM-41 molecularsieve, 0.5 g of CTAB was added to 96 mL of deionized H2Owith stirring. After the solution turned clear, 34 mL of ethanolwas added under stirring. Then 10 mL of aqueous ammoniasolution was added to the system and it was allowed to mix for5 min. After that, 2.0 mL of TEOS was poured into thesolution immediately under stirring. Stirring was continuedfor 3 h at room temperature. MCM-48 was prepared accordingto the procedure reported by our research group [23]. Briefly, asolution containing 5.2 g of CTAB, 240 g of H2O, 100 mL ofethanol and 24 mL of NH4OH were added to 6.8 g of TEOSand the solution was stirred for 16 h at room temperature.Then, the solid products were recovered by filtration and driedat room temperature overnight. The CTAB was removedfrom the composite materials by calcining the samples at540 1C for 9 h.

2.3. Functionalization of mesoporous silicas

200 mg of silica powder (MCM-41 or MCM-48) waspretreated at 100 1C to remove physisorbed water. After that,silica was added to a mixture containing the grafting agentAPS (0.05 or 0.17 g), and toluene (5 mL previously dried).The reaction mixture was vigorously stirred under refluxtemperature at various periods of time (3.5, 7 and 15 h). Then,the solution was decanted and the remaining solid was washedthree times with 5 mL of isopropanol. The functionalized solidwas dried at room temperature overnight.

2.4. Characterization

The powder XRD patterns were recorded on SIEMENSD5000 diffractometer using CuKα radiation. The diffractiondata were recorded in the 2θ range of 2–101. TGA analyseswere carried out under nitrogen flow using a TGA Q500apparatus (TA Instruments, New Castle, DE). Nitrogen adsorp-tion–desorption isotherms were obtained on QuantachromeAS1Win equipment at �196 1C. Before the experiments, themodified samples were degassed under vacuum at 120 1C. Thespecific surface area of the sample was calculated using a BETmethod, and the pore size distribution was calculated usingdesorption branches of nitrogen isotherms and a DensityFunctional Theory (DFT) method. Transmission electronmicroscopy (TEM) was performed using an HRTEM Titanoperated at 300 kV.

3. Results and discussion

3.1. XRD studies

The powder XRD patterns of the calcined mesoporous silicaMCM-48 and MCM-41 before and after modification with APSat different reaction times are shown in Fig. 1. For MCM-48silica (Fig. 1(i)), the XRD diffractograms displayed the typicalreflections at 2θ values of 2.8 (211) and 3.2 (220) and signals

Fig. 1. XRD diffratograms of the mesoporous silicas (i) MCM-48 and (ii) MCM-41 modified with APS (0.19 M) at different reaction times: (a) 3.5 h, (b) 7 h, and (c) 15 h.

Fig. 2. XRD diffractograms of the mesoporous silicas (i) MCM-48 and (ii) MCM-41 modified with different initial APS concentrations: (a) 0.06 M and (b) 0.19 M.

Fig. 3. FT-IR spectra of amine-functionalized (i) MCM-48 and (ii) MCM-41 modified at different reaction times: (a) 3.5 h and (b) 15 h. Initial APS concentration:0.19 M.

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between 4.4 and 5.8 corresponding to the planes (321), (400),(420), (332), (422) and (431) while MCM-41 silica showedthree characteristic diffraction peaks at 2θ values of 2.6, 4.5 and5.1 which correspond to the planes (100), (110), and (200),respectively (Fig. 1(ii)). After modification with APS, the XRDpatterns of both MCM-48 and MCM-41 silicas did not showsignificant changes. However, the peaks between 4.4 and 5.8(for the case of MCM-48) and the peaks at values of 4.6 and 5.3(for the case of MCM-41) almost completely disappeared. Thisbehavior is because the peak intensity depends on the scatteringcontrast between the pore channels and silica walls and, usually,decreases with decreasing scattering contrast after attachment of

organic groups to the pore surface for this kind of materials [24].Thus, the pore filling by aminopropyl groups could cause theobserved decrease in the XRD peak intensity but not changes inthe mesostructure. Previous studies related to the modification ofmesoporous silicas have reported similar behavior [25,26].Fig. 2 shows the XRD diffractograms of these silicas

modified with different initial APS concentrations. In all cases,the XRD patterns of modified silicas presented a decrease inthe peak intensities as it was observed in the above results. Thedecrease in the intensities indicates amine grafting. Also, it wasobserved that when the initial APS concentration increased thedecrease in the peak intensities was more evident.

Table 1Structural properties and APS content of silica materials determined by N2

adsorption–desorption and TGA studies respectively.

Sample Reactiontime (h)

APS content(mmol/g silica)

BET (m2/g) Porevolume(cm3/g)

MCM-48 – – 1912 0.876MCM-48-APS 3.5 1.3 358 0.203MCM-48-APS 7 1.4 495 0.272MCM-48-APSn 7 0.67 1043 0.532MCM-48-APS 15 1.2 454 0.245MCM-41 – – 1460 0.653MCM-41-APS 3.5 1.0 52 0.04MCM-41-APSn 3.5 0.59 620 0.316MCM-41-APS 7 1.2 35 0.038MCM-41-APS 15 0.95 68 0.049

nModified silicas with initial APS concentration of 0.06 M.

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3.2. FT-IR studies

The FT-IR spectra of bare and amine-modified silicas atdifferent reaction times are shown in Fig. 3a and b respectively.Bare MCM-48 and MCM-41 silicas exhibited a signal at3740 cm�1 due to the O–H bond of the silanol groups and peaksat 1058 cm�1 and 800 cm�1 corresponding to the Si–O–Sistretching and bending vibrations ,respectively. The FT-IR spectraof modified silicas showed bands between 2990 and 2890 cm�1

due to the CH2 stretching modes of the hydrocarbon chain of APS,and a signal at 1560 cm�1 assigned to the NH2 scissor [12]. It isinteresting to note that when reaction time was increased, theintensity of the signal at 1560 cm�1 also increased. The C–Nstretching vibration is normally observed in the wavelength rangeof 1000–1200 cm�1. However, this peak was not resolved due tooverlaying with the Si–O–Si band present in the range of 1050–1150 cm�1. Nevertheless, the peak of the modified silicas in thisregion is broader, indicating a possible overlap of peaks. Inaddition, the band of silanol groups (3740 cm�1) completelydisappeared after amine group attachment. The disappearance ofthis peak evidences the reaction of Si–OH groups and APS. Thus,the covalent grafting of APS on MCM-48 and MCM-41 silicaswas clearly established by comparing bare and modified silicas'FT-IR spectra.

3.3. Nitrogen adsorption–desorption studies

The N2 adsorption–desorption isotherms for both MCM-48and MCM-41 silicas modified with APS at different reaction

Fig. 4. N2 adsorption–desorption isotherms and average pore size distributions fodifferent reaction conditions. Solid line for bare silicas, (▲) for modified silicasconcentration of 0.19 M at different reaction times: (□) 3.5 h, (△) 7 h and (○).

conditions are shown in Fig. 4a and b respectively. The N2

adsorption–desorption isotherms of unmodified silicas weretypical reversible type IV which is a characteristic of themesoporous materials, while those for the APS-modified(obtained from initial APS concentration of 0.19 M) weretype I (microporous character) [19,21]. This result suggeststhat the mesoporous character of the bare silicas was lost afterthe aminopropyl group attachment. However, it can be seenthat when an initial APS concentration of 0.06 M is used, theisotherms remains type IV although the content of N2 adsorbeddecreases significantly by comparing with unmodified silicas.

r unmodified and modified MCM-48 (a, c) and MCM-41 (b, d) obtained atwith initial APS concentration of 0.06 M. Modified silicas with initial APS

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In agreement with these results, the average pore sizedistribution graphics for APS-modified silicas (Fig. 4c and d)shows the presence of microporosity (average pore diameterless than 2 nm) when the APS initial amount increased. Thisresult corroborates the effect of the aminopropyl groupattachment on the pore structure. Also, it can be seen thatMCM-41 silica was more susceptible to pore blockage thanMCM-48 at APS concentration of 0.19 M.

The attached APS content, the specific BET surface area andpore volume of the amine-modified silica samples are sum-marized in Table 1. For both silicas, the APS content obtainedfrom TGA analysis was in the range from 0.95 to 1.4 mmol/gwhen it was used as an initial APS concentration of 0.19 M.However, the MCM-48 silica showed higher amount of graftedAPS than MCM-41 silica.

On the other hand, the organosilane attaching was reduced whenthe initial APS concentration was decreased in both MCM-41 andMCM-48 silicas. This is because at low concentrations of APS,less molecules of organosilane are available to react with the silanolgroups in the mesoporous material. In addition, it can be observedthat modified MCM-41 shows lower BET surface area and porevolume values than APS-MCM-48 under the same reactionconditions. This significant decrease in the values of theseparameters for APS-MCM-41 indicates that the pore entranceswere blocked by the aminopropyl groups corroborating the results

Fig. 5. TEM micrographs of (a) MCM-48, (b) APS-MC

obtained in the pore size distribution study. In the case of MCM-48silica, its three-dimensional interconnected cubic pore structureavoided the pore blocking and permitted a better distribution of thegrafted APS. Also, such behavior may be explained in terms of thedifference in concentration of reactive SiOH groups on the poresurface of silicas. Zhao et al. [27] reported that MCM-41 contained0.7–1.9 SiOH/nm2 which reacts with the organosilane agent whileKumar et al. [28] reported that the concentration of the surfaceSiOH groups was about 1.8 SiOH/nm2 on the MCM-48 surface.Since this value includes both reactive single SiOH groups and alsounreactive hydrogen-bonded SiOH groups, it can be concluded thatMCM-41 shows more reactive Si–OH groups than MCM-48 silicaand therefore it is more prone to pore blockage than MCM-48under the same reaction conditions.

3.4. TEM studies

The structure of both MCM-48 and MCM-41 silicas was alsocharacterized by high-resolution transmission electron microscopyand the micrographs are shown in Fig. 5a and c respectively. TEMimage for MCM-48 silica showed spherical particles with sizebetween 200 and 300 nm and uniform channel system thatmatched well with reported MCM-48 images [29,30]. On theother hand, the TEM image of MCM-41 (spherical particle size

M-48, (c) MCM-41 and (d) APS-MCM-41.

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between 200 and 300 nm) provided evidence for a hexagonalmesostructure which is representative of this mesoporous silicaprepared with CTAB. Also, it can be seen from these figures thatthe pore structure is regular over the whole particle in both cases.Fig. 5b and d shows the TEM images for modified silicas with aninitial APS concentration of 0.19 M and reaction time of 15 h.TEM images did not reveal changes in the morphology when theywere compared with bare silicas. These results are in agreementwith those found by XRD technique.

4. Conclusions

Room-temperature synthesized MCM-48 and MCM-41 weremodified with APS via the post-grafting method at refluxtemperature. According to XRD results, synthesized silicasshowed a high ordered structure even after functionalization withAPS. The amine-attaching process was proved with FT-IR andTGA studies. The APS content in MCM-48 and MCM-41 was inthe range from 1.0 to 1.4 mmol/g and it was strongly dependenton the initial APS concentration. It means that the content of APSin the MCM-48 and MCM-41 silicas can be easily controlled byvarying only the initial APS concentration. The reduction of poresize, pore volume and surface area in modified silicas corrobo-rated the attaching of APS. MCM-41 silica was more susceptibleto pore blockage when compared with MCM-48 under the samereaction conditions. No differences in morphology were found inamine-modified silicas which suggests that grafting process is agood method to modify this kind of materials without loss ofstructural order. These amine-modified silicas could be able toadsorb CO2 and/or H2S from natural gas streams due to chemicalinteractions between the basic amine groups and the acidic gases.

Acknowledgments

This work was funded by CONACYT-México (FondoSENER-Hidrocarburos) under Grant no. 127499. The authorsare grateful to G. Mendez, and H. Saade for their technicalassistance in the TGA and N2 adsorption–desorption studiesrespectively. We also thank J. Sanchez and B. Puente for theirtechnical assistance.

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