Scientific paper
Organic-inorganic Hybrid Gels of Diol-TEOS Type. Synthesis and Study on the Chemical Interaction
Oana Stefanescu, Marcela Stoia, Mirela Barbu and Mircea Stefanescu
"Politehnica" University Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, Romania * Corresponding author: E-mail: oanaelenastefanescu@yahoo.com Received: 04-08-2011
Abstract
Hybrid inorganic-organic materials, silica - diol, were prepared by the sol-gel process from mixtures of tetraethyl orthosilicate (TEOS) and two diols (1,2-propanediol and 1,4-butanediol), using different molar ratios TEOS: diol, in acidic catalysis. The resulted materials were studied by thermal analysis, FT-IR spectroscopy, 29Si-NMR and nitrogen adsorption measurements. The thermal analysis of the gels, in air and nitrogen, has clearly evidenced the chemical bonding of the studied diols with the Si-OH groups resulting from the hydrolysis of TEOS, forming hybrid gels. The mass loss registered on TG in the range 250-300 °C, corresponding to the burning of the organic chains from the hybrid network allowed us to calculate the fraction of the bonded diol. This fraction depends on the initial molar ratio TEOS:diol and on the diol's nature. By annealing the hybrid gels at 600 °C we have obtained silica matrices with different textural parameters.
Keywords: Sol-gel, diol, inorganic-organic hybrid, thermal analysis
1. Introduction
Organic-inorganic materials with well defined morphology and controlled structure at nanometric scale represent an interesting class of materials studied in the last years due to their potential applications in different fields of advanced technology.1-3 The possibility of using organic compounds in order to modify the inorganic framework has long been recognized as an interesting instrument for the development of new composite materials. One of the possibilities to synthesize hybrid materials was to modify the inorganic network by introducing selected organic groups leading to organic modified silicates (Or-mosils).4'5
The presence of organic compounds with small molecules, especially diols significantly influences the structure and morphology of the silica matrix, leading to organic-inorganic hybrid materials.6 7
During the getation process, ethylene glycol is capable to form hydrogen bonds with the Si-OH groups result ing from the hydrolysis react ion of Si(OR)4. The strong hydrogen bond network shields the reactive centers (=Si-O-) of the incompletely condensed chains. This prevents the efficient condensation which leads to the forma-
tion of a branched network and produces larger and uniformly distributed micropores within the polymeric network.8 Glycerol is considered as a special chemical additive because of the three hydroxyl groups in its molecule. The main disadvantage of glycerol is that it remains retained in the pores and it is difficult to remove. Even at high temperatures, it tends to decompose to carbonate than to evaporate.9
In literature it is usually considered that the presence of primary - OH groups is responsable for the interaction of organic compounds with silanols containing Si-OH groups resulted during the sol-gel synthesis.10 This interaction between organic compounds and tetraethyl orthosilicate or its hydrolysis products influences the morfo-logy of the silica matrix and thus, the dispersion and size of the embedded oxidic nanoparticles.
Our previous research on the system TEOS - ethyle-ne glycol - H2O, in acidic catalysis,11 has shown that during the formation process of the silica matrix, the diol interacts chemically or by H bonds with the hydrolysis products of TEOS. Furthermore we have shown that independent on the diols nature, the interaction TEOS-diol takes place by the same bond type but the morphotogy of the silica matrix is modified.12 The chemical bonding of
the organic chains within the silica gel is influenced by the molar ralio TEOS-diol.13 The diol-TEOS interaction studies are important for the synthesis of materials with magnetic properties embedded in silica matrix.1415 The molar ratio diol-TEOS is essential due to the involvement of the diol in the formation of the hybrid gel.
In this paper we report the synthesis and characterization of the hybrid gels 1,2-propanediol-TEOS and 1,4-butanediol-TEOS, for molar ratios within the range 0.1 -f 1.5, in order to establish the optimal interaction ratio diol-TEOS. The study was also focused on the influence of the diols structure (nature) and the elucidation of the interaction mechanism TEOS-diol. The hybrid gels could be used for the obtaining of support materials with modified morphology and for embedding of nanomaterials with catalytic, magnetic and environmental sensing properties.
2. Experimental
2. 1. Synthesis
Reagents used for the synthesis of the hybrid gels were of analytical purity (Merck): 1,2-propanediol (1,2PG), 1,4-butanediol (1,4BG), ethanol (EtOH), tei traethyl orthosilicate (TEOS), concentrated HNO3.
The hybrid gels wer e synthesized by adding, at room temperature, under intense magnetic stirring, of an ethanolic TEOS solution to the hydro alcoholic diol solution, previously acidified with HNO3 (cac = 0.01 mol/L). The obtained clear solution was stirred for 30 minutes and was left for gelation at room temperature. Samples were synthesized for different molar ralios diol/TEOS = 0.1, 0.2, 0.5, 1.0, 1.5 and the same molar ratio H2O/TEOS (8:1). The composition of the synthesized gels and their characteristics are presented in Table 1.
After gelation, the gels were grinded and dried at 60 °C for 2 hours. The dried gels were thermally treated at 200 and 600 °C. The obtained samples were studied by
thermal analysis (TG, DTA), FT-IR spectrometry, 29Si-NMR spectrometry on solids. Specific surface areas were measured for the samples thermally treated at 600 °C.
2. 2. Experimental Techniques
Thermal analysis was performed on a Diamond Per-kin Elmer thermobalance, in the temperature range 20-550 °C, with a heating rate of 10 °C /min, using a-Al2O3 as reference, in air and nitrogen and on a 1500 MOM Budapest Derivatograph, in air up to 500 °C. FT-IR spectra were recorded with a Shimadzu Prestige-21 FT-IR spectrometer, in KBr pellets, in the range 400-4000 cm-1. 29Si-NMR spectra were taken on a Bruker MSL300 spectrometer at a frequency of 59.53MHz (7.05 T). Chemical shifts were reported in parts per million (ppm) relative to tetramethylsilane (TMS). Textural characteristics of the out-gassed samples were obtained from nitrogen physi-sorption using a MICROMERITICS ASAP 2020 instrument. Prior to the measurements, the samples were degassed in vacuum at 250 °C. The BET surface area was calculated by using the standard Brunauer-Emmett-Teller (BET) method, while the pore size distributions were calculated applying the Barret-Joyner-Halenda (BJH) method to the desorption branches of the isotherms.
3. Results and Discussion
The hybrid gels synthesized with diols were characterized by thermal analysis, in air and nitrogen, up to 550 °C. Figures 1 and 2 present the TG curves registered in air and in nitrogen, for the hybrid gels G12PG and G11°4SG thermally treated at 200 °C.
The mass losses registered in air up to 250 °C correspond to the elimination of the adsorbed water and volatile compounds of the poly-condensation (EtOH, H2O) as well as to the evaporation of the diol from the matrices po-
Table 1. Characteristics of the synthesized gels (TEOS: Diol: H2O)
Quantity (mols)	Molar ratio
Sample	Diol	TEOS	Diol	H2O	Et OH	TEOS:Diol:H2O	tg (hours)
G1,2PG	1,2 PG	0.05	0.005	0.4	0.12	1:0.1:8	130
G1,2PG	1,2 PG	0.05	0.010	0.4	0.12	1:0.2:8	140
G1,2PG	1,2 PG	0.05	0.025	0.4	0.12	1:0.5:8	144
G1,2PG	1,2 PG	0.05	0.050	0.4	0.12	1:1.0:8	154
g1.5 1,2PG	1,2 PG	0.05	0.075	0.4	0.12	1:1.5:8	168
G1,4BG	1,4 BG	0.05	0.005	0.4	0.12	1:0.1:8	140
G1,4BG	1,4 BG	0.05	0.010	0.4	0.12	1:0.2:8	148
G1,4BG	1,4 BG	0.05	0.025	0.4	0.12	1:0.5:8	130
gi.O G 1,4BG	1,4 BG	0.05	0.050	0.4	0.12	1:1.0:8	264
G1.5 1,4BG	1,4 BG	0.05	0.075	0.4	0.12	1:1.5:8	288
Fig. 1. TG curves in air and nitrogen of gel G02pGthermally treated at 200 °C
Time (miri)
Fig. 2. TG curves in air and nitrogen of gel G\°4bg thermally treated at 200 °C
res. The process that occurs with mass loss in the range 250-300 °C corresponds to the oxidative decomposition (burning) of the organic chains chemically bonded within the silica network. The slow mass loss registered up to 550 °C can be assigned to the evolution of the silica matrix poly-condensation.
In nitrogen, the evolution of the TG curves up to 250 °C is similar to the one registered in air. In inert atmosphere the thermal decomposition of the organic chains takes place in the range 450-500 °C. The shifting of the thermal process towards higher temperatures confirms the chemical inleraclion between the diol and the Si-OH groups. The corresponding mass losses registered in this range are identical with the ones registered in the range 250-300 °C, in air.
From the obtained thermal data, it results that up to ~250 °C we have a hybrid diol-silica gel. According to these data, all initial gels (Table 1) have been thermally treated at 200 °C (in order to eliminate the adsorbed water and the volatiles from the pores). The as obtained hybrid gels have been studied by thermal analysis in air up to 500 °C.
0	100	200	300	400	500
T, °C
Fig. 3. TG and DTA curves of the hybrid gels: G0,2pg (1), G0'2pg (2),
G1.2PG (3), G1,2PG (4), G1.2PG (5)
Figures 3 and 4 present the TG and DTA curves obtained in air for the hybrid gels heated at 200 °C, synthesized with 1,2PG and 1,4BG for different molar ratios di-ol:TEOS (Table 1). The mass losses registered in the range 250-400 °C, assigned to the oxidative decomposition of the organic chains bonded within the network depend on the initial molar ratio diol:TEOS and on the diols nature. At low initial molar ratio diol:TEOS (0.1, 0.2) the oxidative decomposition of the organics is shifted towards higher temperatures (300-400 °C) due to the dispersion of the organic chains bonded within the network and to the difficult air diffusion within the pores. For higher molar ratios diol:TEOS = 0.5, 1.0, 1.5, this process takes place at lower temperatures, in the same range (250-300 °C) and the mass losses are close to each other. For all samples, the oxidative decomposition of the organic chains generates on the DTA curve a corresponding exothermic effect.
Table 2 presents the results of the thermal analysis for the hybrid gels. The amount of the diol bonded within the hybrid gels, was calculated from the mass losses in the temperature range 250 - 400 °C corresponding to the burning of the organic chain Si - O - R - O - Si ^ Si - O - Si + [OR], whe-
0	100	200	300	400	500
T, C
Fig. 4. TG and DTA curves of the hybrid gels: G0'1bg (1), G0^ (2),
G1.4BG (3), G\,4BG (4), G1,'4bG (5)
Table 2. Mass losses registered at thermal analysis of the hyrbid gels
Sample	mols	Amt %	Am %	mresidue SiO2	mols OR	Initial molar	Experimental
	TEOS	20-500 °C	250-400 °C	(%)500°C	chemically bounded	ratio Diol: TEOS	molar ratio Diol:TEOS
G0.1 g1,2PG	1.50	12	7	88	0.12	0.10	0.08
g0.2 G1,2PG	1.40	16	14	84	0.24	0.20	0.17
g0.5 g1,2PG	1.28	23	18	77	0.31	0.50	0.24
G1.0 G1,2PG	1.25	25	20	75	0.34	1.00	0.27
g1.5 1,2PG	1.25	25	20	75	0.34	1.50	0.27
G0.1 G1,4BG	1.40	16	11	84	0.15	0.10	0.10
G0.2 G 1,4BG	1.30	22	17	78	0.23	0.20	0.18
G0.5 G1,4BG	1.21	27	25	73	0.34	0.50	0.28
G1.0 G 1,4BG	1.08	35	31	65	0.43	1.00	0.40
G1.5 1,4BG	1.03	38	34	62	0.47	1.50	0.46
'i-1-1-'-1-'-1-<-1-'-1-'-1-'-1-1—
0,0 0.2 0,1 0.6 O.e 1.0 1.3 1,4 1.6 Initia! molar (atio diol TE OS
Fig. 5. Dependence of bounded diol: TEOS mol ar ratio with the initial diol:TEOS for: 1,2PG (1) and 1,4BG (2)
re R = - CH(CH3) - CH2 - or R = - (CH2)4 - ) and the residual mass at 500 °C (considered as SiO2).
Figure 5 presents the dependence between the amount of chemically bonded diol and the diol amount intro du ced in synthe sis.
The dependences show that the diols are chemical interacting at low molar ratios diol : TEOS (< 0.5). For molar ratios higher than 1.0 the interaction diol-TEOS becomes insignificant.
The hydroly sis-con den sa tion reac tions and the in te -raction of the silanol groups (^Si-OH) with the -OH groups of the diols take place concurrently. By introducing low amounts of diol, we can presume that the interaction process devetops with a higher rate once the gel is formed. At higher diol concentration, the interaction is limited in time by the progress of the hydrolysis-condensation reactions (gelation).
The evolution of the presented curves (Fig.5) shows that the nature of the diol influences the interaction process.
We can propose an interaction mechanism, according to the schemes below, which shows that both hy-droxyl groups (-OH), independent on their nature, participate at the formation process of the hybrid gels.
The interaction results and the proposed mechanism suggest that a linear diol favors the chemical bonding while a branched diol is more sterically hindered to interact with TEOS.
FT-IR spectrometry has also evidenced the presence of the diols chemically bonded within the silica gel. Figu-
Fig. 6. FT-IR spectra of the hybrid gel G4ja2PG thermally treated at 60 (1), 200 (2) and 600 °C (3)
Fig. 7. FT-IR spectra of the hybrid gel G411'0lBG thermally treated at 60 (1), 200 (2) and 600 °C (3)
res 6 and 7 present the FT-IR spectra of the hybrid gels G1'2pg and G1104BG dried at 60 °C and thermally treated at 200 and 600 °C.
The hybrid gels dried at 60 °C (spectrum 1) present the bands characteristic for the silica gel and for the chemically bonded diol: v(C-OH) - H bonds (3400 cm1) and the free diol within the gels pores: v(C-OH) (1100-900 cm-1), v(CH) (3000-2800 cm-1; 1300-1400 cm-1) and
is not well formed and its characteristic bands are overlapped with the bands of the diols.
The FT-IR spectra of the hybrid gels thermally treated at 200 °C (spectrum 2), when the poly-condensation degree of the diols within the matrix is more advanced, the intense bands correspond to the sitica matrix (the bands at 1080 cm-1 (Si-O-Si), 960 cm-1 (Si-OH), 794 cm-1 (SiO4), 570 cm-1 (cyclic Si-O-Si), 450 cm-1 (Si-O)).17-22 v(C-C) 877 cm-1.16 At this temperature, the silica network The bands v(CH) in the range 3000-2800 cm-1 are still
Fig. 8.29Si-NMR spectra of the hybrid gels G1^ (1) and G1104bg (2) thermally treated at 200 °C
Fig. 9. 29Si-NMR spectra of the hybrid gels G1^ (1) and G1;1 (2) thermally treated at 600 °C
0
,4BG
present. In the region 1000-1300 cm-1 the Si-O-C vibrations attributed to unreacted alkoxides which are overlapped with the vibrations of other species from the hybrid gel also exist.23
Spectrum (3) of the gels treated at 600 °C, when the organic chains of the bonded diols have burned, forming the silica matrix, presents only the bands characteristic for the silica matrix.
The studied gels where also characterized by solid state 29Si-NMR spectrometry in order to describe the local surrounding of the Si atom within the network. Figure 8 presents the 29Si-NMR spectra of the hybrid gels G\02PG and G1104SG thermally treated at 200 °C, when the diol is chemically bonded within the matrix.
All NMR spectra have evidenced the presence of the species Q4 (8 = -110 ppm), Q3 (8 = -100 ppm) and Q2 (8 ~ -90 ppm), Q3 being predominant. The presence of the
spe cie Q3 and especially of Q2 points to a low poly-condensation degree of the matrix at this temperature (200 °C), which can be explained by the existence of organic chains bonded within the matrices network concordant to thermal analysis results of the hybrid matrices at 200 °C.
By thermal treatment at 600 °C, the combustion of the organic chains of the hybrid matrices with formation of a silica matrix takes place. The 29Si-NMR spectra of the silica matrices obtained by calcinations at 600 °C are similar. Figure 9 presents the NMR spectra of the matrices G\1pg and G\04BG obtained at 600 °C. The spectra are similar with a shift at -110 ppm corresponding to Q4 species, indicating a high condensation degree (SiO2).
The microporous texture of the samples with different diol content was studied by N2 adsorption isotherm measurements. The textural properties (BET surface area, pore volume and average pore size) of the silica samples, obtained by anneating at 600 °C of the synthesized gels showed that the presence of the diol in the gels leads to silica matrices with narrow mesopores (> 2 nm at the boundary between meso- and micro-pores). The surface area for the gel G\04BG was 277 m2/g, the pore votume was 0.975 cm3/g and the average pore diameter was 1.87 nm. The values of the textural parameters for the gels synthesized with 1,2PG are indicated in Table 3.
adsorption isotherm
Sample	Surface area SBET (m2/g)	Pore volume BJHdes (cm3/g)	Average pore diameter BJHdes Dp (nm)
G1,2PG G1,2PG G1.5 1,2PG	284 396 420	0.161 0.220 0.241	1.97 3.05 3.40
All compositions showed the same type of adsorption-desorption isotherms. Figure 10 presents the adsorp-
Fig. 10. N2 adsorption - desorption isotherms for the gel Gla2PG
Fig. 11. Pore size distribution for the gels: G°1^pg, G112pg, G||Pg
Table 3. Textural characteristics of the SiO2 matrices from the BET
tion-desorption isotherm for the gel G1'2pg. The shape of the isotherm is close to type 1, characteristic for micropo-rous materials.23 The pore size distributions for the gels synthesized with 1,2PG are presented in Fig. 11.
The textural parameters values of the gels synthesized with different diols (G1'2PG and G\04BG) are due to the structure (branched or linear) of the diols. The surface areas values of the gels G0'5PG, G[0pg, G}'5pg depend on the molar ratio 1,2PG - TEOS.
The presence of diols in the system leads to much higher surface areas compared to the silica gel without diol (surface area of ~ 11 m2/g),25 making these materials appropriate for nanocomposites with controlled properties.
4. Conclusions
The paper reports the formation of some hybrid organic-inorganic gels starting from tetraethyl orthosilicate and two diols: 1,2-ethanediol and 1,4-butanediol for a wide range of diol:TEOS motar ratios: 0.1 -f 1.5. By thermal analysis in air and nitrogen of the hybrid gels, we have shown that the diols are chemically bonded within the silica network by covalent bonds (Si-O-C) leading to the obtaining of organic-inorganic gels, thermally stable up to ~ 250 °C for both diols and all initial diol:TEOS molar ratio.
At low molar ratios diol:TEOS, the interaction develops faster while for higher initial molar ratios the interaction is inhibited. Thus, the optimal interaction ratio was established at 0.5.
FT-IR and sotid state Si-NMR studies on the gels obtained at 200 °C have evidenced a low condensation degree in the hybrid gels due to the chemical bonding of the diols. The matrices resulted at 600 °C indicate a more ad-van ced con den sa tion de gree. N2 ad sorp tion-de sorp tion isotherms of the samples obtained at 600 °C have evidenced the formation of microporous silica. The textural properties of the silica matrices depend on the molar ratio diol: TEOS.
The method used by us all ows the obt aining of materials which can be used as templates for nanocom-po si tes.
5. Acknowledgment
This work was partially supported by the strategic grant POSDRU/88/1.5/S/50783, Project ID50783 (2009), co-financed by the European Social Fund - Investing in People, within the Sectoral Operational Programme Human Resources Development 2007-2013.
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Povzetek
Hibridni anorganski-organski materiali v sistemu silicijev oksid - diol so bili pripravljeni po sol-gel postopku iz mešanic silicijevega etoksida (TEOS) in dveh diolov (1,2-propandiol in 1,4-butandiol) v različnih molskih razmerjih TEOS: diol s kislinsko katalizo.
Materiali so bili analizirani s termično analizo, FT-IR spektroskopijo, 29Si-NMR in meritvami adsorpcije dušika. Termična analiza gelov na zraku in v dušiku je potrdila, da je prišlo do nastanka hibridnih gelov. Na osnovi izgube mase v območju od 250 °C do 300 °C, ki ustreza termičnemu razpadu organskih skupin iz hibridne strukture, je bil izračunan delež vezanega diola. Odvisen je od molskega razmerja TEOS: diol in od vrste diola. Z žarenjem hibridnih gelov pri 600 °C smo pripravili silicijev oksid z različnimi teksturami.