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Preparation of mesoporous silica particles with controlled morphology from sodium silicate solutions and a non-ionic surfactant at pH values between 2 and 6 ARTICLE in MICROPOROUS AND MESOPOROUS MATERIALS · OCTOBER 2000
Impact Factor: 3.45 · DOI: 10.1016/S1387-1811(00)00227-4
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Preparation of mesoporous silica particles with controlled morphology from sodium silicate solutions and a non-ionic
surfactant at pH values between 2and 6
Ligia Sierra a ,Betty Lopez a ,Jean-Louis Guth b,*
a
Departamento de Quimica,Universidad de Antioquia,Apartado Aereo 1226,Medellin,Colombia
b
Laboratoire de Mat e
riaux Min e raux,Universit e de Haute Alsace,3,rue Alfred Werner,F-68093Mulhou Cedex,France Received 31January 2000;received in revid form 27April 2000;accepted 27April 2000
Abstract
Isometric particles of mesoporous silica could be synthesized between 25°C and 45°C from reaction mixtures con-taining silicic acids,sodium chloride and a non-ionic surfactant (Triton X100).The size decreas from some tens of
micrometers to less than 1l m when the pH increas from 1.85to 6.At pH lower than 3.5,glassy material forms a cement between the particles.The particle size distribution narrows at higher pH and at long reaction times (veral days).A spheroidal shape is favored by low pH and high temperature,and a more polyhedral shape appears at high pH whatever the temperature.This behavior could be related to the polycondensation rate of the silicic acids,to the lifetime and disorder of the micelles and to the micelles±silicic acid interactions.Ó2000Elvier Science B.V.All rights re-rved.
Keywords:Mesoporous silica;Particle shape;Particle size;Non-ionic surfactant;Micelle lifetime
1.Introduction
中国有哪些山In order to increa chromatographic lectivity and to extend the analytical capability of normal and rever pha liquid chromatography,there is much interest in the preparation of new packing types with various porous adsorbants.Among the,silica is one of the most important supports [1].
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The interest for silica supports incread in the last years with the discovery of mesoporous silica exhibiting regular pores,narrow pore size distri-butions and pore diameters,which can be adjusted between 1.5and 30nm (MCM-41and related materials)[2±4].However,the disadvantage of this kind of materials is that they are compod of loo agglomerates,which cannot be packed as such in columns for chromatographic u [5].Di erent methods have been tested to synthe-size mesoporous silica particles with spherical shape in a size ranging between 2and 12l m with a
narrow size distribution.Gr u
n et al.[6]prepared the mesoporous silica according to the procedure of Beck et al.[7]using
cetyltrimethylammonium
Microporous and Mesoporous Materials 39(2000)
519±527
www.elvier.nl/locate/micromeso
*
Corresponding author.Tel.:+33-3-89-33-68-80;fax:+33-3-89-33-68-85.
E-mail address:jl.guth@univ-mulhou.fr (J.-L.Guth).
1387-1811/00/$-e front matter Ó2000Elvier Science B.V.All rights rerved.PII:S 1387-1811(00)00227-4
cations in alkaline medium.The loo agglomer-ates,compod of primary particles of about50±100nm,were ground in a mortar.A fraction with a particle diameter of5±10l m was obtained by dimentation in2-propanol and ud as packing material in normal pha HPLC for the paration of acid,neutral and basic compounds.The same team[8]succeeded in the®rst synthesis of spherical particles with the MCM-41structure.They mod-i®ed the St o ber synthesis[9]of monodisperd den silica spheres(hydrolysis of an alkoxysilane in a mixture of alcohol and aqueous ammonia)by adding an alkylammonium surfactant.But the particle size ranged from0.4to1.1l m with an average size clo to0.6l m,which is too small for HPLC applications.The average particle size could be incread up to2.3l m when the alkyl-ammonium surfactant was replaced by a long-chain n-alkylamine[10].
According to Stucky et al.[11]strong and den interface interactions between the cationic surfactant and the silicate anions during the for-mation of the mesoporous silica in alkaline media (S IÀmechani肺腑之言
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sm)give ri to highly organized and structurally well de®ned materials.But this occurs at the expen of a long length scale control,which would be necessary to adjust the shape and the size of the particles.
Weaker interactions mediated by anionic spe-cies located between the cationic surfactant and the cationic silicic species in acidic media(S XÀI mechanism)allow one to adjust the parameters of the silica polycondensation chemistry in order to de®ne di erent shapes and sizes of the particles
[11].This was the method followed by Yang et al.
[12]who reported the synthesis of materials from cetyltrimethylammonium chloride and tetraethoxy-silane with control of the morphology(®bers,®lms, gyroids,spheres)under quiescent aqueous acidic conditions 0`pH` 1 .Gallis et al.[13]ud acid-prepared mesoporous spheres(S XÀI mech-anism)as a matrix material for chromatographic parations.
Similar results can be obtained under particular conditions if the cationic surfactant is replaced by a non-ionic alkyl-polyethylene oxide surfactant. Pinnavaia and co-workers[14]®rst ud the surfactants with alkoxysilanes to prepare disor-dered mesoporous silica in neutral media.But despite the weak hydrogen-bonding interactions (N°I°mechanism),the rapid polycondensation of the hydroly
zed alkoxysilane led to looly aggre-gated small particles without a de®ned shape.The u of silicic acid generated at pH2from silicate solutions,which were then polycondend between pH6and10in the prence of sodium cations and Triton X100(polyethyleneglycol4-ter-octylphenyl ether)allowed us to prepare a ries of materials formed by the aggregates of submicrometric par-ticles with decreasing size when the pH was in-cread[15].The concomittant increa of their pore size from 15to 45#A could be related to the higher polycondensation degree of the walls,to a change of the conformation of the surfactant and to a new mechanism(N°Na IÀ),which adds to the N°I°interactions at higher pH.When the materi-als were prepared at pH lower than6,the particles became more spheroidal and their size incread (b1l m),owing to a slower polycondensation rate. But thermal stability problems appeared due to a lower polycondensation degree of the silica form-ing the walls.
When the silicic species are polycondend in the prence of a polyethylene oxide-bad surf-actant at very low pH(concentrated acid)a mech-anism comparable to the S XÀI mechanism is responsible for the formation of a ries of meso-porous phas with di erent structures and mor-phologies[16].Protonated water molecules are hydrogen bonded to the ethoxy oxygens of the surfactant,which then plays the role of a cationic surfactant(N°H3O XÀI mechanism).
The aim of this study was to®nd the conditions for the preparation of mesoporous silica with a non-io
nic surfactant(Triton X100)and silicic ac-ids at pH between2and6,uful as a HPLC column packing support(spherical particles with an average size in the2±12l m range)with a sta-bility high enough to allow the thermal removal of the template and the modi®cation of the pore surface by silanization.Beside the pH,the other synthesis parameters were the temperature and the duration of the reaction and the concentration of the sodium cations.
520L.Sierra et al./Microporous and Mesoporous Materials39(2000)519±527
2.Experimental part
2.1.Preparation of the materials
2.1.1.Reactants
Sodium silicate solution(Aldrich,analytical grade,composition in wt.%:SiO227,Na2O10.9). Polyethyleneglycol4-ter-octylphenyl ether with9±10ethoxy groups(Triton X100,Aldrich).Analyt-ical grade hydrochloric acid,sodium hydroxide and sodium chloride were ud.
2.1.2.Synthesis procedure
4.62g of sodium silicate solution(20.8mmol SiO2)was diluted with46ml of water and the Na2O/SiO2molar ratio adjusted to0.42by addi-tion of0.05g of NaOH.In some experiments, NaCl and more water was added to the silicate solution(NaCl/SiO2ratio between0.15and0.8; H2O/SiO2ratio between270and960).3.23g of Triton X100(5mmol)were dissolved in50ml of 0.4M hydrochloric acid,and the silicate solution was added under stirring(®nal pH between1.6and 2).The pH of the clear solution of metastable polysilicic acids was immediately adjusted to the value desired for the synthesis(between1.8and
5.8)with a1M sodium hydroxide solution.The resulting solution was kept at a temperature of 25°C or45°C without agitation in a clod poly-ethylene bottle until the end of the synthesis(1±6 days).The solid was parated by®ltration, washed with distilled water and dried at60°C.
2.1.
3.Calcination
The samples were heated in air at a heating rate of1.5°C/min up to480°C and held at this tem-perature for2h.
2.2.Characterization of the materials
2.2.1.Optical and scanning electron microscopy All samples were examined with an optical mi-croscope to determine the shape and size of the particles.Micrographs of some lected samples were performed with a SEM(Philips XL30).2.2.2.X-ray di raction
The XRD patterns of the as-synthesized and calcined samples were recorded in the2h range from1°to10°with a Philips PW1130di ractom-eter.
2.2.
3.Thermal analys
TGA of the as-synthesized samples were per-formed in air from25°C to600°C with a heating rate of10°C/min using a Dupont2000Hi-Res TGA2950apparatus.
2.2.4.Nitrogen adsorption/desorption
The isotherms were measured with an ASAP-2010apparatus(Micromeretics)on calcined sam-ples after activation at300°C under vacuum for6 h.The calculation of the speci®c surface area(BET method),pore volume and pore size distribution (BJH method)was performed with the software of the apparatus.
3.Results
3.1.In¯uence of the synthesis parameters on the particle size
All materials are synthesized at a pH between 1.85and5.80contain particles,which are more or less isometric(Table1).When the pH increas, the average size of the particles decreas from veral tens of micrometers to less than two mi-crometers,mainly for pH values higher than4 (Fig.1).At the same time,the size distribution narrows.The aggregation degree of the particles is more important at low pH.Glassy material is generally prent when the pH value is lower than 3.2±3.5and acts like a cement between the parti-cles(Fig.1a).A high concentration of sodium cations limits its formation.The temperature has an e ect similar to that of the pH.
Ostwald ripening probably caus the shift to larger particles when the synthesis time is in-cread.The biggest particles continue to grow at the expen of the smallest ones,which dissolve and disappear.
L.Sierra et al./Microporous and Mesoporous Materials39(2000)519±527521
T a b l e 1P r e p a r a t i o n c o n d i t i o n s ,m o r p h o l o g y a n d p h y s i c o -c h e m i c a l c h
a r a c t e r i s t i c s o f t h e m e s o p o r o u s s i l i c a (m o l a r c o m p o s i t i o n o f t h e s y n t h e s i s m i x t u r e :1S i O 2;0.24T r i t o n X 100;0.42N a 2O ;x N a C l ;0.96H C l ;y H 2O )
S a m p l e x y p H
T (°C )d (d a y s )S h a p e a s i z e r a n g e (l m )
X R D (a s -s y n t h e s i s e d )b X R D (c a l c .)b P o r o s i t y c
d
I w d I w S
V
D
102701.85453g b s 2±304.430017202701.85455s b g 5±404.5400203.4120028920
0.40`1.7
302703.15253n s b g 3±104.25001040.82703.15253n s )g 3±124.2900550.82703.15453s )g 1.5±124.660015609603.90453s 2±104.47001470.252704.00454s 2±104.21200103.721001813040.671.780.152704.00256n s 4±204.2110073.3
1200
20
746
0.37
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902704.00453s 4±124.490014100.152704.20452s 2±104.2800153.910002012010.541.7110.152704.55451n s 3±54.1150063.638001111790.611.7120.152704.55255n s 4±104.3150053.4
2500
11
977
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`1.7
130.82704.55251n s 2±64.2130051405904.70451s b n s 1±34.2140081502705.20452s b n s 1±44.41000103.911001712390.571.81604105.30452n s 1±34.2180063.7
5500
9
1153
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0.51
1.7
1702705.80453n s 0.5±24.215007
a
g ,t h e g l a s s -o r g e l -l i k e m a t e r i a l s ;s ,t h e s p h e r o i d a l p a r t i c l e s ;n s ,t h e n o n -s p h e r o i d a l a n d m o r e o r l e s s i s o m e t r i c p a r t i c l e s .b
d ,t h
e i n t e r p l a n a r d i s t a n c e i n n m ;I ,t h e r e l a t i v e i n t e n s i t y i n a r b i t r a r y u n i t s ;w ,t h e p e a k w i d t h a t h a l
f h i
g
直笛h
i n a r b i t r a r y u n i t s .c S ,t h e s p e c i ®c s u r f a c e a r e a (B E T m e t h o d )i n m 2/g ;V ,t h e t o t a l p o r o u s v o l u m e i n c m 3/g ;D ,t h e p o r e d i a m e t e r (B J H m e t h o d )i n n m .
522
L.Sierra et al./Microporous and Mesoporous Materials 39(2000)519±527
