Effect of Hydrothermal Treatment on Adsorption Properties of Silica

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Effect of Hydrothermal Treatment on Adsorption Properties of Silica Gel Alfred A. Christy Department of Chemistry, Faculty of Engineering and Science, University of Agder, Serviceboks 422, 4604 Kristiansand, Norway ABSTRACT: It has been proven by coupling near infared spectroscopy to second derivative processing of the spectra that the free and hydrogen bonded silanol groups attached to water molecules absorb at 5314 and 5270 cm 1, respectively. The same method was employed to probe the surface of the hydrothermally treated silica gel samples after pretreatment and evacuation at three different temperatures. The chemical and physical properties of silica gel depend on the nature of the surface silanol groups. Free silanol groups and hydrogen bonded silanol groups play an important role in adsorption of water molecules. Any change or variation in the silanol groups is expected to change the adsorption properties of any silica gel. Two silica gel samples with varying surface characteristics were used in this study. After heating and evacuating at 200, 750, and 1100 °C, the samples were hydrothermally treated in steel bombs and their surface functionalities were probed using near-infrared spectroscopy. The same samples were also dehydroxylated at 450 and 650 °C and analyzed by near-infrared spectroscopy. Furthermore, effectiveness of the treated samples in adsorbing water molecules was followed by monitoring the mass of water adsorbed by silica gel. These investigations form the basis for understanding the relationship between adsorption effeciveness and types of silanol groups on silica gel surface. The results clearly indicate that the hydrothermal treatment of silica gel pretreated at temperatures up to 750 °C increases the concentration of hydrogen bonded silanol groups on the silica gel surface. The sample treated at 1100 °C did not undergo any change under hydrothermal treatment. The concentration of free silanol groups increases in the dehydroxylated samples. The results also show that the adsorption rates of water molecules on the surface decrease significantly after pretreatment of these samples at a particular humidity.

’ INTRODUCTION Silica gel is one of the most important materials in separations science and industry. Its adsorption properties have given silica gel a unique place in several different disciplines in chemistry such as separation science, catalysis, and polymer science, etc. The use of silica gel in these different applications is possible because of the presence of hydoxyl groups (silanol groups) on its surface and their adsorption properties. Much work has been done on the nature of silanol groups on the surface of silica gel and silicious materials since the discovery of silanol groups on silica surface by Kiselev in 1936.1 The silanol groups on the silica gel surface have varying degrees of polarization. There is a general agreement among researchers that the surface of silica gel contains free silanol groups and hydrogen bonded vicinal silanol groups. The silanol groups that are close to each other form strong hydrogen bonding,2 9 and silanol groups that are relatively far apart form weak hydrogen bonding.2 Some of the silanol groups are hydrogen bonded in a chain. The silanol surface may also contain some siloxane groups. Dry silica gel adsorbs water molecules readily when exposed to air. The adsorbed water molecules can be removed by heating or subjecting the silica gel sample under vacuum. Silica gel loses water molecules at 200 °C and then undergoes different degrees of dehydroxylation of hydrogen bonded silanol groups from 200 to 750 °C. The surface then loses all the silanol groups above 1100 °C.10 The silica gel sample surface can also be modified in its concentration of hydroxyl groups. The siloxane bondings can be open to react with water molecules to form new silanol groups. r 2011 American Chemical Society

Practically, this is done by subjecting silica gel sample with water or steam at a temperature above 100 °C. The technique is called hydrothermal treatment and has been used in preparing silica gel with various adsorption and structural properties.11 13 The characteristic parameters such as pore size and particle size have been determined for the hydrothermally treated silica gel samples by using physical methods. The chemical nature of the surface had been characterized by wet chemical methods.11,14 Based on the results from wet chemical methods Okkerse and De Boer14 suggested that the hydrothermal treatment of silica gel increased the surface OH groups by 3-fold. In contrast to this, Chertov15 concluded that the hydrothermal treatment increased the concentration of internal hydroxyl groups and the concentration of surface hydroxyl groups remained the same. However, the works reported in the literature after this earlier disagreement concluded in favor of the increase of surface hydroxyl groups at the surface.16,17 Infrared spectroscopy both in the mid- and near-infrared region has been used in the characterization of surface functionalities of silica gel surfaces and the type of bonding taking place when water molecules adsorb onto the silica gel surface. However, the use of infrared spectroscopy in analyzing OH groups on silica gel samples in the mid-infrared region is difficult because of the broad absorption arising from OH stretching vibrations. The OH Received: September 3, 2010 Accepted: March 16, 2011 Revised: January 21, 2011 Published: March 24, 2011 5543

dx.doi.org/10.1021/ie1018468 | Ind. Eng. Chem. Res. 2011, 50, 5543–5549

Industrial & Engineering Chemistry Research

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Table 1. Silica Gel Samples Used in the Experiments silicagel type

surface

particle

pore size

silanol number

area m2/g

size μm

Å

(OH-groups/nm2)

sample 1

471

37 63

52

3.6

sample 2

600

210 500

31

2.9

bending vibrations give even broader peak in the fingerprint region. Because of this problem, the infrared measurements on silica gel samples were frequently made in the fundamental stretching region. The nature of the broad peaks arising from different OH stretchings of the silanol groups on silica gel surface makes it difficult to characterize the functional groups. Most of the measurements were made using the traditional transmission sampling methods which required very thin discs. In this respect near-infrared spectroscopy offers an advantage because the intensities of the overtones of the OH stretching and combination frequencies are relatively smaller and thick samples can be employed for measurements. However, transmission measurements of dry samples of silica gel needed special setups and vacuum conditions.17 Recent developments of sampling techniques for infrared spectroscopy facilitated the measurement of of solid samples. The transflectance accessory for near-infrared spectroscopy is an excellent sampling technique in the measurements of nearinfrared spectra of silica gel samples. Recently, Christy10 has proven for the first time that the free and hydrogen bonded silanol groups on silica gel samples can be identified by using near-infrared spectroscopy and monitoring water adsorption on the surface. Second derivative techniques were useful in achieving this task. The use of second derivative techniques in interpreting the absorption bands in the near-infrared spectra can be found elsewhere.10,19,20 The aim of this paper is to show that the hydrothermal treatment increases the hydrogen bonded silanol groups on the surface of silica gel and to study the effect of hydrothermal treatment on adsorption properties of silica gel samples. Furthermore, it was also an intention to study the relationship between adsorption properties and surface hydroxyl groups concentration at the surfaces of the silica gel samples. To accomplish this goal, an approach followed by Christy10 in his recent report was used.

’ EXPERIMENTAL SECTION Materials, Equipment, and General Experimental Procedure. Two different silica gel samples that have been used in

previous studies in the characterization of surface functionalities10 were used in these experiments. These samples were bought from Sigma-Aldrich. The samples have different surface areas, particle sizes, and pore sizes. These particle parameters were determined by Brunauer, Emmett and Teller (BET) technique with a TriStar 3000 gas adsorption analyzer (Micromeritics Instrument Corporation, USA). The particle size distributions of the samples are reported as given in the sample bottles (Table 1). The experiments on adsorption evolution of water were carried out in four different sets. In each set the adsorption evolution of water on each sample was followed by NIR spectrometry and separately by gravimetry. In the first set, the shelf silica gel samples were dried (evacuated at 200 or 750 °C or heated at 1100 °C) and their adsorption evolutions of water vapor were followed. In the second set, the dry samples from the first set

Figure 1. Photograph showing the setup used for (a) evacuating the sample at elevated temperatures, (b) hydrothermal treatment of silica gel, and (c) NIR sample cup for measuring spectra.

(evacuated at 200 or 750 °C or heated at 1100 °C) were hydrothermally treated, dried (at 200 °C), and then their adsorption evolution of water vapor was followed. In the third set the samples as received from the shelf were heated in sealed glass ampules, dried (at 200 °C), and then their adsorption evolutions of water vapor were followed. In the fourth set the samples as received from the shelf were heated and evacuated at 450 and 650 °C and their adsorption evolutions of water were followed. The experiments in the fourth set were carried out to reduce the concentration of the hydrogen bonded vicinal silanol groups and increase the concentration of free silanol groups by dehydroxylation. Apart from these, the hydrothermally treated samples were also dehydroxylated and their NIR spectra were measured to study their modified surface structure. The adsorption experiments were carried out in a closed room where the NIR spectrometer was placed. The room had a relative humidity of 22%((1%) during the period the experiments were carried out. The humidity measurements were made by using a small thermometer hygrometer (Termometerfabriken Viking AS, Sweden). Sample Drying, Hydrothermal Treatment, and Dehydroxylation. Heating of the samples was carried out in a ceramic heater (BA electric Bunsen from Electrothermal, UK) controlled by an external power supply (Figure 1). A powerful vacuum pump (Edwards, UK) was used in the evacuation of the samples. The samples were evacuated at the temperatures required (200, 450, 650, and 750 °C) for 2 h (Figure 1). Heating of the samples at 1100 °C was carried out in an oven that had a temperature limit of 1100 °C. The ceramic heater used in heating and evacuating the samples at lower temperatures could not be used in heating the samples at 1100 °C because it could not reach 1100 °C. Hydrothermal experiments were carried out in steel bombs of 25 cm long and 1 cm internal diameter (Figure 1). One gram of a dry silica gel sample (from the first set above) and 0.4 cm3 of distilled water were added to the bomb and sealed. The bomb was then placed in a chromatographic oven set at 160 °C and left for 2 h. The hyrothermally treated sample was removed and evacuated at 200 °C to remove physically adsorbed water from the surface. The same procedure was followed for the sample evacuated at 750 °C and sample heated at 1100 °C. These temperatures were selected because it has been reported in the literature that silica gel samples lose hydrogen bonded silanol groups at 750 °C16 and they lose both free silanol and hydrogen bonded silanol groups at 1100 °C.6,17 Experiments with the samples in the third set were carried out in glass ampules (at 160 °C). These samples contained preabsorbed water on the surface and the heating was carried out without any addition of water. The purpose of these experiments 5544

dx.doi.org/10.1021/ie1018468 |Ind. Eng. Chem. Res. 2011, 50, 5543–5549

Industrial & Engineering Chemistry Research is to study whether the vapor pressure created by the release of the preadsorbed water from silica gel surface was enough to effect any changes in the structure of silica gel surface. Dehydroxylation of the samples hydrothermally treated and dried at 200 °C was carried out to prove again that the band appearing at 5270 cm 1 is due to the combination band of OH stretching and bending vibrations of water molecules hydrogen bonded to vicinal groups of the hydrogen bonded silanol groups on the silica gel surface. The adsorption properties of the samples toward the adsorption of water molecules before and after hydrothermal treatment were also compared. Portions of dry samples (around 0.15 g) were placed in the NIR transflectance sample cup (the cup can be seen in Figure 1c) and the adsorption of water on the surface was followed by recording the increase in mass of the sample by a Mettler electronic balance connected to a computer through an RS232 port for data collection. The data collected at the computer were plotted using an Excel spreadsheet. The balance used for this purpose was in the same room as the NIR spectrometer and the humidity was the same for both experiments. Near-Infrared (NIR) Measurements. The samples from the experiments were subjected to evacuation for 2 h at 200 °C whenever it was necessary to remove physically adsorbed water. A small portion of the sample (around 0.15 g) was placed in the NIR transflectance sample cup with an IR transparent window (see Figure 1c). The near-infrared measurements were made using a Perkin-Elmer Spectrum One NTS FT-NIR spectrometer (Perkin-Elmer Ltd., UK) equipped with a transflectance accessory and deuterated triglycine sulfate detector. The sample cup containing the silica gel sample was placed directly on the crystal of the transflectance accessory and allowed to equilibrate with the surrounding air. The near-infrared measurements, whether a single spectrum of a sample or spectra of a sample during the process of adsorption of water molecules, were made in the region of 10 000 4000 cm 1 at a resolution of 16 cm 1. A total of 30 scans were made during the NIR spectral measurements. All the NIR spectra were transformed to log (1/R) format and its second derivative profiles were saved for detailed analysis and interpretation.

’ RESULTS AND DISCUSSION Near-Infrared Spectra of the Silica Gel Samples and Hydrothermally Treated Silica Gel Samples. As mentioned

in the Experimental Section, the near-infrared spectra acquired from the silica gel samples and the hydrothermally treated silica gel samples were converted to log(1/R) format. Their second derivative profiles were also used often to follow the evolution of bands10 due to changes taking place on the surface of silica gel particles during adsorption of water. Silica gel surface contains free silanol groups and silanol groups that are hydrogen bonded. There may be differences in the hydrogen bonds. Some of the silanol groups are close and are strongly hydrogen bonded2 4 and some are relatively far apart from each other and form weak hydrogen bondings.2 Some silanol groups are hydrogen bonded in a chain. Dry silica gel samples generally give rise to two absorption peaks in the near infared region. The spectra contain one asymmetric peak at 7300 cm 1 and another at 4500 cm 1. The untreated and undried silica gel sample contains one additional peak at 5300 cm 1. The details of the absorptions that give rise to these peaks and the bands that compose these peaks are detailed

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Figure 2. Near-infrared spectra of silica gel sample 1 after heating and evacuating at different temperatures.

Figure 3. Near-infrared spectra of hydrothermally treated silica gel samples.

in Christy.10 The bands that give rise to much variation during the adsorption of water are in the region at 5300 cm 1. By using second derivatives to enhance the resolution of overlapping bands, Christy10 has shown that the peak at 5300 cm 1 is composed of three bands at 5314, 5270, and 5119 cm 1 for a silica gel sample adsorbing water molecules. Furthermore, Christy10 has also shown that the NIR band appearing at 5314 cm 1 is a combination band of OH stretching and bending fundamentals of water molecules involved in hydrogen bonding with free silanol groups. The band at 5270 cm 1 is due to the same combination band of water molecules involved in hydrogen bonding with vicinal OH (OH groups arising from hydrogen bonding between the silanol groups) groups. The broadness of the band at 5270 indicates hydrogen bonding between silanol groups on the surface. These two bands reveal information regarding the free silanol groups and hydrogen bonded silanol groups on the silica gel surface. Therefore, attention will be given to these two bands during the discussions of NIR spectra. The NIR spectra of one of the untreated silica gel samples (sample 1) and evacuated at different temperatures are given in 5545



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Figure 4. Processes taking place during the heating and hydrothermal treatment of silica gel.

Figure 2. All the spectra of the heated samples exhibit an asymmetric peak at 7300 cm 1. The peak is due to the absorptions arising from the overtones of the free and hydrogen bonded silanol groups. However, the peak sharpens with an increase in drying temperature from 200 to 750 °C. The heat treatment at 750 °C leads to the condensation of hydrogen bonded silanol groups to form siloxane bondings and therefore, the peak becomes sharper. Contributions to this peak also come from the hydrogen bonded silanol groups within the pores of the silica gel particles. The asymmetry of the peak is still evident in the peak because of this contribution. The sample heated at 1100 °C exhibits only a small broad peak at this wavenumber indicating hydrogen bonded silanol groups within the pores of the silica gel sample. The weak peak exists because the pores close before all the hydroxyl groups in the pores are dehydroxylated at this temperature.17 The near-infrared spectra of the dried, evacuated (at 200 °C), and hydrothermally treated silica gel samples (sample 1) after their pretreatment at different temperatures are given in Figure 3. The changes are obvious in the figures. The near-infrared spectra of the hydrothermally treated samples that were pretreated at 200 and 750 °C show a relative decrease in the concentration of free silanol groups and an increase in the concentration of hydrogen bonded silanol groups. The hydrothermally treated sample that was pretreated at 1100 °C shows only the hydrogen bonded OH groups that were present in the sample before the hydrothermal treatment. The sample treated at 1100 °C has not been affected by hydrothermal treatment indicating the unreactive nature of the relaxed siloxane bondings. These results suggest that the siloxane bondings of the samples heated at 200 and 750 °C can be chemically modified to silanol groups (Figure 4). The heating of the sample at 750 °C dehydroxylates the hydrogen bonded silanol groups10 on the surface. According to Brinker and Scherer17 the siloxane bondings are strained in the samples treated at temperatures less than 750 °C. These strained bonds react with water molecules during hydrothermal treatment. However, the sample treated at 1100 °C showed no reaction with water when hydrothermally treated. Here, the siloxane groups are relaxed and did not react with water.

Figure 5. Second derivative spectra of the silica gel sample (dry), and hydrothermally treated and dried sample acquired in the region 5500 5000 cm 1 during adsorption of water molecules.

The second derivative profiles of the infrared spectra measured during the adsorption of water molecules are given in Figure 5. The evolution of the bands at 5314 and 5270 cm 1 are very clear in the figure. The second derivative profiles of the NIR spectra show clear differences between these two bands. The band at 5270 cm 1 in the hydrothermally treated silica gel sample evolves into an intense band compared to the band at 5314 cm 1 for the untreated sample (Figure 5). The intense band at 5270 cm 1 clearly shows that hydrothermal treatment increases the hydrogen bonded silanol groups on the surface. Furthermore, a careful observation of the hydration profiles shows that the adsorption process 5546



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Figure 6. Plots of mass of water adsorbed by silica gel samples 1 and 2 against time.

starts from the hydrogen bonded silanol groups and then continues with the free silanol groups. This trend again supports a model of adsorption presented by Christy10 for the adsorption of water molecules on the silica gel surface. Hydrothermal treatment of the silica samples was carried out in glass tubes to check whether the preadsorbed water molecules could cause the cleavage of the Si O Si bonds at the surface and form silanol groups. The results were negative. There was no change in the NIR spectra of the samples before and after the heat treatment. The trial clearly shows that the pressure inside the glass tube is not enough to induce the cleavage of the above bonding. There should be enough water in the reaction mixture to create sufficient pressure inside the vessel during the heating. The pressure increase inside the glass tube or vessel used in the hyrothermal treatment provides high energy water molecules that can easily react with the siloxane bonds and form hydroxyl groups. Effect of Hydrothermal Treatment/Thermal Treatment on Adsorption Properties of Water Molecules on Silica Gel Surface. Plots of the adsorption of water with respect to time for samples untreated and hydrothermally treated silica gel samples are given in Figure 6. The rate of adsorption of water molecules is slower on the hydrothermally treated silica gel surface compared to the adsorption on the untreated silica gel sample. The hydrothermal treatment introduces several silanol groups when the water molecules react with siloxane bonds (Figure 4). The newly formed silanol groups engage in hydrogen bondings with neighboring silanol groups. These newly formed groups increase the proportion of the hydrogen bonded silanol groups 2- to 3-fold. This trend is apparent in the spectrum in Figure 2 for sample 1.

The relative intensity of the OH overtone absorption is small in the hydrothermally treated sample compared to the intensity of the OH overtone of the dry sample. Christy’s10 adsorption model requires almost equal concentration of the hydrogen bonded vicinal OH groups and free silanol groups for effective water adsorption. Disproportionate concentrations of any of these groups will affect the adsorption of water molecules. The absorption band at 5314 cm 1 for sample 1 in Figure 5 seems to be more intense than the band at 5270 cm 1. This increase is caused by the double derivative that enhances the intensity of a narrow band compared to a broad band. The adsorption rate of water molecules on sample 2 is greater than on sample 1 irrespective of the smaller surface area of sample 2. However, sample 2 has a higher silanol number compared to sample 1 and the total number of silanol groups per gram of silica gel is higher for sample 2 compared to sample 1. This explains the effectiveness in water molecular adsorption by sample 2 compared to sample 1. The pretreatment temperatures before hydrothermal treatment can also affect the adsorption properties. The adsorption rates of water molecules by sample 2 pretreated at 750 °C and subsequently hydrothermally treated are less than the samples pretreated at 200 °C and subsequently hydrothermally treated. The heating of the sample 1 at 750 °C and subsequent hydrothermal treatment did not affect the adsorption rate. Both temperatures had the same adsorption rate (Figure 6). The samples heated at 750 °C lose hydrogen bonded OH groups and the hydrothermal treatment was able to create most of the lost OH groups by opening the siloxane bonds. The adsorption properties of water molecules are also affected by heat treatment of the samples. The heat treatment of the 5547



Figure 7. Near-infrared spectra of thermally treated silica gel sample 1.

samples at 450 and 650 °C partially dehydroxylates the hydrogen bonded silanol groups and increases the concentration of free silanol groups on the surface. In these samples the concentration of free silanol groups are in several fold compared to the hydrogen bonded vicinal groups. The adsorption profiles of these samples are shown in Figure 6. The adsorption profiles clearly show that the thermal treatment has a negative effect on the adsorption of water molecules. Dehydroxylation of Hydroxyl Groups from Hydrothermally Treated Silica Gel Samples. The hydrothermally treated silica gel samples were subjected to thermal treatment to investigate whether these newly formed hydroxyl groups can be removed from the surface. Portions of the hydrothermally treated silica gel samples were heated to temperatures of 750 and 900 °C for 2 h and their water adsorption profiles were monitored by near-infrared spectroscopy in the region of 5000 5600 cm 1. The sequentially acquired spectra of the samples show the appearance of bands at 5270 cm 1 after 30 min standing. The intensity of the band remained constant even after 6 months of exposure to the air in the room (Figure 7). A hump near 7000 cm 1 indicates overtones of the silanol groups involved in hydrogen bonding. These observations suggest that a few vicinal groups created by the newly formed hydroxyl groups are still present on the surface of silica gel particles. It is not very clear why these groups remain at this high temperature.

4. CONCLUSION Near-infrared spectroscopic evidence has been presented in this paper to prove that the hydrothermal treatment of silica gel samples increases the concentration of hydroxyl groups on the surface many fold. The results from the water molecular adsorption experiments with the silica gel samples subjected to different treatments reveal that the disproportionate change in the concentration of either free silanol groups or hydrogen bonded vicinal groups reduces the adsorption properties of silica gel surface toward water molecules. Equal proportions of free silanol groups and hydrogen bonded vicinal groups are detected in the spectrum of the samples (dried at 200 °C) used in these experiments. This fits well with the water adsorption model presented by Christy in ref 10. However, the model presented in

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the above reference has to be tested on different types of silica gel samples. The OH groups in the hydrothermally treated samples that are involved in hydrogen bonding to yield the active vicinal groups can be 2- to 3-fold that of the vicinal OH groups. The hydrothermal treatment reduces the short-term adsorption of water molecules of sample 1 by 80% and sample 2 by 65%. The heating of the samples before hydrothermal treatment also affects the adsorption of water by sample 2. The adsorption is reduced by 80%. There was no change in the adsorption of water molecules by sample 1. The dehydroxylation of the samples both at 450 and 650 °C also affects the adsorption properties of sample 1 and 2 drastically. The dehydroxylation changes the concentration of free silanol groups many fold compared to the hydrogen bonded vicinal groups. The experiments clearly show that there should be a balance between the two types of groups for effective water adsorption. The heating experiments with the hydrothermally treated samples indicate that some of the hydrogen bonds between the silanol groups are so strong that they need even higher temperatures for their elimination (Figure 7). Furthermore, it appears that when a sample is hydrothermally treated, some of the free silanol groups are lost forever. They cannot be reformed by chemical reaction. Heating can reduce the concentration of hydrogen bonded OH groups of the hydrothermally treated silica gel samples and increase the concentration of free silanol groups by eliminating water from these goups.18 The above experiments and findings clearly demonstrate that there is a strong relationship between adsorption and the concentrations of hydrogen bonded and free silanol groups on the silica gel surface. Furthermore, these findings pave the way for exploring ways to alter the OH concentrations on the surface so that new silica gel materials can be produced with enhanced adsorption properties.

’ AUTHOR INFORMATION Corresponding Author

E-mail: [email protected].

’ REFERENCES (1) Kiselev, A. V. Structure of silica-acid gels. Kolloidin, Zh. 1936, 2, 17–26. (2) Boehm, H. P. Chemical Identification of Surface Groups. In Advances in Catalysis and Related Subjects; Eley, D. D., Pines, H., Weiz, P. B., Eds.; Academic Press: New York, 1966; Vol. 16, pp 179 274. (3) Hoffman, P; Knozinger, E. Novel aspects of mid and far IR Fourier spectroscopy applied to surface and adsorption studies on SiO2. Surf. Sci. 1987, 188, 181–198. (4) Morrow, B. A.; I.A. Cody, I. A.; Lee, L. S. M. Infrared studies of reactions on oxide Surfaces. 7, Mechanism of the adsorption of water and ammonia on dehydroxylated silica. J. Phys. Chem. 1976, 80, 2761–2767. (5) Iler, R. K. The Chemistry of Silica, Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; John Wiley & Sons: New York, 1979. (6) Unger, K. K. Porous Silica, Its Properties and Use As Support in Column Liquid Chromatography; Elsevier Scientific Publishing Company: Amsterdam, 1979. (7) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface. Studies in Surface Science and Catalysis; Elsevier Science B.V: Amsterdam, 1995; pp 59 131. 5548



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(8) Wan, Q.; Ramsey, C.; Baran, G. Thermal pretreatment of silica composite filler materials. J Therm. Anal. Calorim. 2010, 99, 237–243. (9) Zhuravlev, L. T. The Surface Chemistry of Silica-The Zhuravlev Model. In Collidal Silica Fundamentals and Applications; Bergna, H. E., Roberts, W. O., Eds.; Taylor & Francis Group, LLC: Boca Raton, FL, 2006; pp 261 275. (10) Christy, A. A. New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative Near Infrared Spectroscopy. Vib. Spectrosc. 2010, 54, 42–49. (11) Akshinskaya, N. V.; Beznogova, V. E.; Kiselev, A. V.; Nikitin, Y. S. Geometric modification of the skeleton of xerogels 0.1. production of macroporous silica gels with small surface for gas chromatography. Zhur. Fiz. Khim. 1962, 36, 2277–2280. (12) Akshinskaya, N. V.; Kiselev, A. V.; Nikitin, Y. S. Geometric modification of the xerogel skeleton 0.3. effect of preparation conditions and consequent hydrothermal treatment on the structure of silica gel pores. Zhur. Fiz. Khim. 1964, 38, 488–490. (13) Kondo, S.; Tomoi, K.; Pak, C. The characterization of the hydroxyl surface of silica gel. Bull. Chem. Soc. Jpn. 1979, 52 (7), 2046–2050. (14) Okkerse, C.; De Boer, J. H. Reactivity of amorphous silica in aqueous solutions. In Reactivity of solids; De Boer, J. H., Burgers, W. G., Gorter, E. W., Huese, J. P. F., Schuit, G. C. A., Eds.; Elsevier Publishing Company: Amsterdam, London, New York, Princeton, 1961; pp 240 248. (15) Chertov, V. M.; Dzhambaeva, D. B.; Plachinda, A. S.; Neimark, I. E. The hydroxy-groups on the surface of and within silica gel globules obtained by the hydrothermal treatment of hydrogels. Russ. J. Phys. Chem. 1966, 40 (3), 282–285. (16) Leboda, R.; Mendyk, E.; Tertykh, V. A. Effect of the hydrothermal treatment method in an autoclave on the silica-gel porous structure. Matter Chem. Phys. 1995, 42, 7–11. (17) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990; p 633. (18) Bush, S. G.; Jorgenson, J. W. Confirmation and Application of Transmission Near Infrared Absorption Technique for Absolute Quantitation of Functional Groups on Silica Gel Quantitation of Functional Groups on Silica Gel. J. Chromatogr. 1990, 503, 69–91. (19) Christy, A. A.; Egeberg, P. K.; Østensen, E. T. Simultaneous quantitative determination of isolated trans fatty acids and conjugated linoleic acids in oils and fats by chemometric analysis of the infrared profiles. Vib. Spectrosc. 2003, 33, 37–48. (20) Christy, A. A.; Xu, Z.; Harrington, P. B. Thermal Degradation and Isomerisation Kinetics of Triolein studied by Infrared Spectrometry and GC-MS Combined with Chemometrics. Chem. Phys. Lipids 2009, 158, 22–31.

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Effect of Hydrothermal Treatment on Adsorption Properties of Silica

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