Bimodal mesoporous silicas and materials MCM-41 type were synthesized with the use as a template cationic surfactants distinguished by a length of the alkyl chain. Fumed silica, tetraethyl orthosilicate as well as of mixture these compounds were applied as a silicate precursor. Obtained samples have been characterized by the powder X-ray diffraction technique and adsorption measurements. Specific surface area, pore diameter, pore size distribution was determined from nitrogen ad(de)sorption isotherms at 77 K for the silicas prepared. The products with bimodal pore distribution have not ordered structure and contain macropores and mesopores, whose sizes are typical for MCM-41 materials. Effect of surfactant and precursor nature on the geometrical features of the ordered structure of MCM-41 type and bimodal porous materials was studied

Il'in, V.G.

Leboda, R.

Pavlenko, A.N.

Skubiszewska-Zieba, J.

Tertykh, V.A.

Yanishpolskii, V.V.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

 19EFFECT OF PRECURSOR AND SURFACTANT NATURE ON GEOMETRICAL CHARACTERISTICS OF MESOPOROUS SILICAS  A.N. Pavlenko1, V.V. Yanishpolskii1, V.A. Tertykh1, V.G. Il'in2, R. Leboda3, and J. Skubiszewska-Zieba3  1Institute of Surface Chemistry, National Academy of Sciences, 03680 Kyiv-164, Gen. Naumov Str. 17, UKRAINE 2L.V.Pisarzhevsky Institute of Physical Chemistry, National Academy of Sciences, 03039 Kyiv, Prospect Nauki 31, UKRAINE  3Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, POLAND  Abstract Bimodal mesoporous silicas and materials MCM-41 type were synthesized with the use as a template cationic surfactants distinguished by a length of the alkyl chain. Fumed silica, tetraethyl orthosilicate as well as of mixture these compounds were applied as a silicate precursor. Obtained samples have been characterized by the powder X-ray diffraction technique and adsorption measurements. Specific surface area, pore diameter, pore size distribution was determined from nitrogen ad(de)sorption isotherms at 77 K for the silicas prepared. The products with bimodal pore distribution have not ordered structure and contain macropores and mesopores, whose sizes are typical for MCM-41 materials. Effect of surfactant and precursor nature on the geometrical features of the ordered structure of MCM-41 type and bimodal porous materials was studied.    Introduction The synthesis of periodic mesoporous materials of the M41S type had become available with the use of different surfactants as micellar templates. They play a role of the structure-directing agents [1] in an assembly process of the periodic mesoporous materials because surfactants have ability to self-organize into mesophases with the different dimensional structure: hexagonal, cubic or lamellar. For preparation of the MCM-41 type silica materials a series of the surfactants CnH2n+1(CH3)3N+A-,  where  A- - counterion, n=14, 16 or 18, were mostly used as a micellar template and sodium silicate, tetraethyl orthosilicate (TEOS), fumed silica were applied as a silica source. Synthesis of the material with a hexagonal array of pores is usually carried out for a  few  hours  as  well  as  some  days  [2].  It  is  conceivable  that  the  formation  of  the  silica  framework can come from both as a result of monomeric silicate ions adsorption on the micelle surface and at the expense of using of the more bulky oligomeric silicate species. In the most cases the synthesis of mesoporous silicas was performed under the conditions of hydrothermal treatment, while the use of the condensed silicate species allows one to avoid this stage. The some differences in the structure of surfactant, nature of counterion A-,  value  of  the  surfactant/silica molar ratio, contents of water lead to obtain of the porous materials with the bimodal pore size distribution, where the product obtained contains of mesopores, their sizes being typical for the MCM-41 material and the larger ones [3].  20In  this  work  we  have  continued  a  systematic  research  in  the  field  of  synthesis  of  the  bimodal mesoporous materials with the purpose of study of an effect of the cationic surfactant chain length on geometrical parameters of the mesoporous silicas.  Experimental For preparation of the mesoporous silica matrices the fumed silica (Aerosil, 300 m2/g) and tetraethoxysilane were used as a silicate precursor. Quaternary ammonium salts hexadecyltrimethylammonium bromide (CTMABr), octadecyltrimethylammonium chloride (ODTMACl) and tetramethylammonium bromide (TMABr) were applied as micellar templates. These short- and long-chain surfactants were transformed into hydroxyl form with the use of the ion-exchange column filled by the AB-17-8 anionite in hydroxyl form. Synthesis of aerosilogel – Sample 1. Fumed silica (280 g; 4.67 mol) was suspended in water (1250 ml; 69.44 mol) at the intensive stirring. Obtained suspension is allowed to stand for 24-28 hrs at room temperature, then it was dried up at 150ºC and calcined at 540ºC for 6 h on air. Synthesis of mesoporous silicas with the use of the short- and long-chain surfactants: with TMABr/OH – Sample 2; with ODTMACl/OH – Sample 3; with CTMABr/OH – Sample 4. Weighed portion of the fumed silica (6.4 g, 0.11 mol – for Sample 2; 14.6g, 0.24 mol – for Samples 3 and 4) was dissolved in an aqueous solution of the appropriate quaternary amine hydroxide (15.4 wt % solution TMABr/OH; 8.7% solution ODTMACl/OH; and 9.1% solution CTMABrOH) at stirring. Surfactant/silica molar ratios are indicated in the Table. Obtained homogeneous milk-white mixtures were dried on air at room temperature up to formation of the gel-like products. Finally the samples were calcined at 540ºC for 6 h on air. Synthesis of mesoporous silica with the use of mixture of the fumed silica and TEOS as a silicate precursor – Sample 5. 8.7 wt % aqueous solution ODTMACl/OH was added to the mixture of the fumed silica (7.4 g, 0.123 mol) and TEOS (24 ml, 0.107 mol) at stirring. The obtained caseous suspension is allowed to age on air up to formation of the solid product. The sample prepared was calcined at 540ºC for 6 h on air. Synthesis of mesoporous silica with the use of TEOS as a silica source – Sample 6. TEOS (22.4 ml, 0.1 mol) was added at stirring to 3.48 wt % aqueous solution ODTMACl/OH. Obtained homogeneous milk-white mixture was subjected to aging on air up to the gel-like product formation and then was calcined at 540ºC for 6 h on air. Synthesis of the MCM-41 materials with the use of CTMABr (Sample 7) and ODTMACl (Sample 8) as a micellar template. Samples were synthesized according to the following procedure. Weighed portion of the surfactant CTMABr (8.3 g, 0.023 mol) or ODTMACl (7.9 g, 0.023 mol) was dissolved in 43 ml of concentrated ammonia solution and intermixed to homogeneous milk-white mixture. After addition TEOS (9.67 ml, 0.043 mol) the mixture obtained was stirred up to gelation. The resulting products were dried up at air with following calcination at 540ºC for 6 h on air. Characterization of the synthesized samples was performed by the powder X-ray diffraction (XRD) data (automated diffractometer DRON-3M, CuKa radiation) and nitrogen adsorption isotherm measurements at 77K on ASAP-2000 Micromeritics equipment. Average pore diameter was estimated by Gurvich method (d=4V/S). The specific surface area, SBET, was determined from the linear part of the BET equation (at p/ps=0.05-0.35). Pore volume distribution was calculated from desorption branch of an isotherm of nitrogen adsorption using the Barrett-Joyner-Halenda (BJH) formula [4-6].   21Results and discussion It has been known that XRD technique allows one to reveal a presence of a dimensional self-organization in the mesoporous materials. The X-ray diffraction patterns of the Samples 7 and 8 exhibit distinct small-angle peaks in the range 2q=2-6º, which can be attributed to different hkl reflections and are characteristic of the well-ordered hexagonal structure. The XRD patterns for MCM-41 samples are shown in Fig.1. The XRD diffractograms for the rest synthesized mesoporous silicas are depicted in Fig.2. The most complete and reliable information about mesoporous structure comes from the low-temperature N2 ad(de)sorption isotherms [7]. Adsorption data for the synthesized samples are represented in Table.  Table. Geometrical characteristics of the synthesized mesoporous silicas.    Sample   Micellar template   Silicate precursor, mol   Medium, mol Surfac-tant/ silica molar ratio  Pore diameter  Å BET surface area, m2/g   Fumed silica TEOS     1 - 1.0  H2O, 14.9  - 208 253 2 TMABr/OH 1.0  H2O, 12.0  0.22 209 205 3 ODTMACl/OH 1.0  H2O, 23.2  0.10 98 388 4 CTMABr/OH 1.0  H2O, 22.9  0.10 136 416 5 ODTMACl/OH 1.0 1.2 H2O, 48.3  0.10 93 463 6 ODTMACl/OH  1.0 H2O, 55.6  0.10 63 784 7 CTMABr  1.0 NH4OH, 27.7  0.52 36 1171 8 ODTMACl  1.0 NH4OH, 27.7  0.52 49 977  0204060801001200 2 4 6 8 102Q (degrees)intensity (arb.units)(b)(a) Fig. 1. X-ray diffraction patterns of the MCM-41 type silicas: (a) – Sample 7; (b) – Sample 8.   220204060801001201400 5 10 15 20 25 30 35 402Q (degrees)intensity (arb.units)123456 Fig. 2. X-ray diffraction patterns of the porous silicas (Samples 1-6).  02040608010012014016010 100 1000 10000Pore diameter (Angstrom)DS/ DD, m2 /g/A00,10,20,30,40,5DV/DD, ml/g/A  05010015020025010 100 1000Pore diameter (Angstrom)DS/ DD, m2 /g/A00,10,20,30,4DV/ DD, ml/g/A Fig. 3. Pore volume and pore surface area distributions for the Sample 3. Fig. 4. Pore volume and pore surface area distributions for the Sample 6.  From  the  adsorption  measurements  and  XRD  data  it  follows  that  the  carrying  out  of  syntheses with the use of the fumed silica, TEOS, mixture of the fumed silica and TEOS as a silicate precursor and the cationic long-chain surfactants as a micellar template permits one to obtain mesoporous materials. These samples have specific surface areas, which exceed in different extent of SBET for the aerosilogel. At the use of the short-chain surfactant (TMABr) the amorphous macroporous silicas with low specific surface area are formed (Sample 2). In fact, in line with the data [8, 9] surfactant molecules with a number of carbon atoms in alkyl chain less 7 do not form micelles  23in aqueous solutions, and consequently, supramolecular templates are absent under these synthetic conditions. Bimodal pore distribution and an increase of specific surface area approximately up to 400 m2/g were observed for the Sample 3 (Fig. 3) and Sample 4 (see previous our work [6]), prepared with the use of the long-chain surfactants ODTMACl/OH, CTMABr/OH and the fumed silica as a silicate precursor. For the Sample 3 there are two maxima on a curve of pore distribution at 33 and 434Å. The pore diameter 33Å in this sample corresponds to the pore size of the MCM-41 type materials, however in XRD pattern of this sample we do not find a presence of the reflections at small angles, which are characteristics of the well-ordered hexagonal structure. At the same time the most considerable increase of surface is attributed to these pores. The curve of pore distribution for Sample 4 reveals two maxima at 30 and 300Å and hexagonal dimensional packing is also absent. In view of the fact that mesopore sizes for the Sample 4 are smaller compared with those for the Sample 3, consequently the increment of surface attributed to these mesopores for the former sample is larger as compared with the Sample 3. The difference in mesopore sizes can be related to an effect of the alkyl chain length in the molecules of applied cationic surfactants. Other authors [9, 10, 11] have also detected a similar influence of a chain length in the template molecule on the pore size. Application of a mixture of the fumed silica and TEOS as an inorganic precursor at the molar ratio of surfactant/silica=0.1 does not improve of structural features of the Sample 5, though a small increase of specific surface area is observed. The Sample 6, synthesized from TEOS and ODTMACl/OH at the same molar ratio of surfactant/silica, reveals the bimodal pore distribution (Fig. 4) as well as high specific surface area (784 m2/g). XRD pattern of this mesoporous silica exhibits only the small-angle peak because of the poorly ordered pore system [11]. It may well be that at the realization of the bimodal mesoporous materials synthesis at the lower molar ratio of surfactant/silica the primary particles are small and spherical in the shape due to insufficient silicate polymerization. Thereafter these particles coalesce to form a secondary porous structure.   02004006008001000120010 100 1000Pore diameter (Angstrom)DS/ DD, m2 /g/A00,20,40,60,81DV/ DD, ml/g/A 020040060080010 100 1000Pore diameter (Angstrom)DS/ DD, m2 /g/A00,20,40,60,81DV/ DD, ml/g/A Fig. 5. Pore volume and pore surface area distributions for the Sample 7. Fig. 6. Pore volume and pore surface area distributions for the Sample 8.    24At an increase of a molar ratio of surfactant/silica to 0.52 the organic template molecules self-organize into micellar structures, which are optimum for preparation of the ordered mesopores materials in the basic media. From adsorption data and XRD results it appears that in this case the highly ordered MCM-41 materials, possessing by the uniform pore size structure (35-50Å) and extraordinary high specific surface area (980-1170 m2/g), can be obtained. The curves of pore distributions for the Samples 7 and 8 are represented in Figs. 5 and 6 respectively. It is characteristically that an effect of the alkyl chain length of surfactant on pore size of the synthesized mesoporous materials is also observed. Hence, it  can be stated that in the systems ODTMACl-TEOS and CTMABr-TEOS in the NH4OH media the optimum geometrical and energy accordance is realized for preparation MCM-41 products owing to the rigid silicate framework is formed around supramolecular structures of the cationic surfactant.  Conclusion The obtained results testify about essential effect of the surfactant and silicate precursor nature on the geometrical parameters of mesoporous silicas. An use of tetraethyl orthosilicate and fumed silica as a silica precursor, hexadecyltrimethylammonium bromide and octadecyltrimethylammonium chloride as a micellar template at the molar ratio surfactant/silica is equal 0.10 results in the bimodal mesoporous materials involved macropores as well as mesopores and having non-ordered hexagonal structure. Synthesis of products exhibiting the typical highly-ordered MCM-41 materials structure with the exclusively narrow mesopore distribution,  hexagonal  symmetry  of  the  porous  framework  and  high  specific  surface  was  executed at an increase of the molar ratio surfactant/silica to 0.52.   Acknowledgement This work was supported by Foundation of Fundamental Research of the Ministry for Education and Science of Ukraine (Project N03.07/00099) and in part by NATO grant No.EST.CLG.976890.  References 1.  Zhao  X.S.,  (Max)  Lu  G.Q.,  and  Millar  G.J.  Advances  in  mesoporous  molecular  sieve  MCM-41 // Ind. Eng. Chem. Res. - 1996. - V.35. - P.2075-2090. 2. Koyano K.A. and Tatsumi T. Synthesis of titanium-containing MCM-41 // Microporous Materials. - 1997. - V.10. - P.259-271. 3. Schulz-Ekloff G., Rathousky J., and Zukal A. Controlling of morphology and characterization of pore structure of ordered mesoporous silicas // Microporous and Mesoporous Materials. - 1999. - V.27. - P.273-285. 4. Gregg S.J. and Sing K.S.W. Adsorption, Surface Area and Porosity. - London: Academic Press, 1982. 5. Barrett E.P., Joyner L.J., and Halenda P.P. The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms // J. Am. Chem. Soc. - 1951. – V.73. - P.373-380. 6. Tertykh V.A., Yanishpolskii V.V., Pavlenko A.N., Leboda R., and Skubiszewska-Zieba J. Template-based approach in the preparation of nanoporous silicas and titania-silicas // In: Chemistry, Physics and Technology of Surfaces, Issues 4-6. – Chuiko A.A. (Ed.) – Kyiv: KM Academia. - 2001. - P.59-68.  25 7. Romero A.A., Alba M.D., Zhou W., and Klinowski J. Synthesis and characterization of the mesoporous silicate molecular sieve MCM-48 // J. Phys. Chem. B – 1997.- V.101. - P.5294-5300.  8. Voyutskii S.S. Course of Colloid Chemistry. – Moscow: Khimiya, 1975 (in Russian).  9. Raman N.K., Anderson M.T., and Brinker C.J. Template-based approaches to the preparation of amorphous, nanoporous silicas // Chem. Mater. - 1998. - V.8. - P.1682-1701. 10. Beck J.S., Vartuli J.C., Roth W.J., Leonowicz M.E., Kresge C.T., Schmitt K.D., Chu C.T.W., Olson D.H., Sheppard E.W., McCullen S.B., Higgins J.B., and Schlenker J.L. A new family of mesoporous molecular sieves prepared with liquid crystal templates // J. Am. Chem. Soc. -1992. - V.114. - P. 10834-10843. 11. Sayari A. and Liu P. Non-silica periodic mesostructural materials: recent progress // Microporous Materials. - 1997. - V.12. - P. 149-177. 

Effect of precursor and surfactant nature on geometrical characteristics of mesoporous silicas

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Effect of precursor and surfactant nature on geometrical characteristics of mesoporous silicas

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