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On the mechanism of interaction between TiCl4 vapour and surface OH groups of amorphous SiO2
Journal of Non-Crystalline Solids 105 (1988) 107-113 North-Holland, A m s t e r d a m
107
ON T H E M E C H A N I S M OF I N T E R A C T I O N B E T W E E N TiCI 4 V A P O U R AND SURFACE OH G R O U P S OF A M O R P H O U S SiO 2 D. D A M Y A N O V , M. VELIKOVA, Iv. I V A N O V and L. VLAEV Higher Institute of Chemical Engineering, 8010 Bourgas, Bulgaria Received 8 September 1987 Revised manuscript received 16 February 1988
Chemical analysis data concerning CI and Ti contents in various samples of amorphous SiO 2 treated with TiCI 4 vapour have shown that the C I / T i ratio cannot be a reliable criterion in the interpretation of the mechanism of surface reactions. An attempt is made to explain the mechanism of surface processes on the basis of infrared spectroscopy studies. It is shown that the hydrogen chloride evolved during the reaction takes part in a competitive reaction which leads to partial halogenation of the surface and formation of water molecules. Depending on the nature of the substrate, the water molecules are coordinated on the surface silicon or titanium ions and can interact with TiCI 4. For this reason, the reaction of the SiO 2 surface with the TiC14 vapour cannot be used for determining the concentration of silanol groups. The differences observed in the reactions of TiC14 with the silanol groups of silica gel and aerosil are due to differences in their coordination ability which is less pronounced with aerosil but changes upon deposition of molecular titanium oxide layers,
1. Introduction
The reaction of TiC14 with the surface of SiO 2 is of great interest in association with the preparation of a large number of products of practical importance: catalysts, adsorbents, various modifications of fillers to be used in polymerization processes etc. Irrespective of the large number of investigations [1-17] dedicated to the effect of the nature of SiO 2 and the packing density of silanol groups on the composition and structure of surface compounds resulting from the interaction between SiO 2 and TiC14 as well as to the mechanism of this interaction, these problems have not yet been elucidated. According to data from the literature [1-9], the C1 and Ti contents determined by chemical analysis of samples modified by TiC14 as well as the C1/Ti ratio vary in a very wide range. The data obtained by infrared spectroscopy studies [3-10,15], which have confirmed the formation of new surface compounds, have no explicit explanation. For example, the absence of a band at 3550 cm - t after treatment of SiO 2 with 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
TiCI 4 indicates, according to Kunavitz et al. [10], the higher reactivity of hydrogen-bonded O H groups compared to the "free" ones. Tchuiko et al. [3] assumed this fact to be evidence of an interaction between TiC14 and the coordinatively bound water. In refs. [16,17] an attempt is made to explain the difference in structure of the titanium oxide layers with differences in structural peculiarities of the carriers. The probability for a reaction to proceed between the hydroxyl coverage and the hydrogen chloride, which is the product of the main chemical reaction, is an important problem which has been neglected by researchers in their attempts to elucidate the interaction between TiC14 and surface O H groups. In this connection it should be noted that there is even no generally accepted opinion on the direct interaction of HC1 with the silanol groups. For instance, according to Peri [18], calcination of SiO 2 at a temperature as high as 700 ° C in a dry HC1 flow provides evidence of no noticeable continuation of the reaction under consideration. A weak interaction of DC1 with an aerosil surface calcined at 500 ° C was established
108
D. Damyanov et al. / Interaction between T i C l 4 and SiO 2 surface O H groups
by Tanaka and Ogasawara [19]. On the other hand, Tertykh et al. [20] found an increasing rate of interaction between HC1 and the aerosil surface at 350-500°C, 75% of the hydroxyl groups participating in the reaction at 500 ° C. The studies of Baverez et al. [21] have shown that 86 mg C1/g SiO 2 are fixed on the surface at a SiO 2 dehydration temperature of 200 ° C and a reaction with HC1 taking place at the same temperature. These results are confirmed in ref. [22] where it is reported that thermal treatment at 200 ° C in vacuo of a series of aerosil and silica gel samples and interaction with HC1 at 30 and 5 0 ° C lead to different degrees of irreversible adsorption depending on the structure of the hydrate coverage. The purpose of the present paper is t o throw light on the mechanism of interaction between TiC14 and the surface O H groups using chemical analysis and IR data on various SiO 2 samples treated with TiC14 vapour.
2. Experimental Three pre-treated SiO 2 samples differing in texture were investigated: silica gel (KSK, USSR, A = 239 mZ/g), vulcasil-s (Bayer, FRG, A = 158 m2/g) and aerosil (Degussa, FRG, A = 278 mZ/g). The pre-treatment of the samples consisted in acid-activation in a water bath with HC1 (1 : 1) for 20 days, washing with distilled water till the establishment of a negative reaction for chlorine ions (10-12 days), blowing-through water vapour for 8 h at 100 ° C, removal of the intraglobular water and achieving maximum surface hydration by cycles of dehydration at 750 ° C and rehydration at 90 ° C [23]. The reaction of the utmost hydrated or partially dehydrated surface of SiO 2 with TiC14 vapour (Titangesellschaft, F R G ) was materialized at 150 °C for 2 h in a vacuum apparatus [24]. The concentration of chlorine ions in the modified samples was determined by the ammetric titration method [25], and that of titanium, by atomic absorption analysis after treating the samples with 2n H2SO 4. The content of O H groups was estimated both thermogravimetrically, (OH)tg , and on the basis of
data from chemical analyses of the TiC14-modified samples, (OH)oh, according to the formula (OH)ch = 4x -- y,
(1)
where x and y are the Ti and C1 amounts, respectively, in mg at./g. The maximum relative error of determination of O H groups in both cases did not exceed 5%, which permitted comparison of the results obtained. The IR studies were performed on plates (30 x 100 mm 2, thickness 10-15 m g / c m 2) obtained without a binder by pressing a fine silica gel or aerosil powder under a pressure of 120-200 k g / c m 2. The heat treatment of the plates, the partial replacement of their surface O H groups by heavy water vapour, their modification by TiC14 and hydrolysis with water vapour were carried out in a high-temperature vacuum quartz cell. The infrared spectra were taken with a UR-20 (Carl Zeiss, DDR) apparatus at room temperature using a diaphragm in the path of the reference beam.
3. Results and discussion In view of the fact that the TiC14 molecule would hardly participate in a reaction with all of its chlorine ions, one can assume the following possible reactions between it and the surface OH groups: -OH - O H + TiC14 -OH -OH + TiC14 -OH - O H + TiC14
-O - O ~ T i C 1 + 3HC1,
(2)
-O ) TiC12 + 2HC1,
(3)
-0 - O - T i C 1 s + HC1.
(4)
It is obvious that the quantitative ratios between chlorine and titanium will vary within a certain range depending on the extent of participation of the different reactions. For this reason, the C1/Ti ratio is used by many authors as a criterion in the estimation of the mechanism of surface processes and hence, of the structure of surface compounds obtained [1,3,4,8].
D. Damyanov et al. / Interaction between TiCl 4 and SiO: surface OH groups
109
Table 1 Dependence of the CI and Ti contents and the CI/Ti ratio on the nature of SiO 2 and the temperature of preliminary dehydration No support
Dehydration temperature
Specific surface
Elements (rag.at/g)
C1/Ti ratio
Ref.
( o C)
area of support (m2/g)
Chlorine
Titanium
230 160 280 250 200 200 150 116
1.62-1.73 0.81-0.88 1.94 1.12
0.88 0.63 0.78 1.1 0.48 0.9 -
1.84-1.96 1.28-1.39 2.49 1.02 2.4 -2.6 0.46 1.17 2.46 2.2
[11 [2] [31 [41 [51 [61
0.79-0.58 0.55-0.42 0.62-0.47 (I.42-0.48 -0.9 -0.65 0.45-0.7
2.07-2.16 1.4 2,11-1.94 1,60-2.53 2,2-2.3 2 1.7 -1.28
[31 [71 [1] [4]
0.45-0.36 0.34-0.30 0.36 0.34 0.40 0.35-0.12
2.2 1.5 2.4 3.0 3.4 3.0 1.74-2.33
[3] [8] [6] [4]
(a) Low-temperature region (180- 300 o C) 1 silica gel 2 vulcasil 3 aerosil 4 silica gel 5 silica gel 6 aerosil 7 silica gel 8 aerosil 9 silica gel
200-300 200-300 200 180 180 180 200 200 300
0.22 1.05
(b) Medium temperature region (400- 600 o C) 1 2 3 4 5 6 7
silica gel vulcasil aerosil aerosil aerosil silica gel silica gel
400-600 400-600 400-600 400 400 400-600 400-600
230 160 280 200 250 200
1.64-1.25 0.77-0.59 1.31-0.91 0.67-1.22
230 160 280 200 160 116 200
0,99-0.79 0,51-0.45 0,86 1.02 1.37
1.8 -1.3 0.75-0.9
(c) High temperature region (700- 900°C) 1 2 3 4 5 6 7
silica gel vulcasil aerosil aerosil cobosil silica gel silica gel
700-800 700-800 800 700 800 800 700-900
0.61-0.28
It should be expected that with the decrease in packing density of the silanol groups the contribution of the reaction of eq. (4) would increase. However, comparison of data from the literature with those obtained by us shows a not clearly expressed dependence of the increase in the C1/Ti ratio from the low (180-300°C) to the high (700-900 ° ) temperature region, the number of titanium and chlorine ions bonded to the surface decreasing with the rise of temperature of precalcined SiO 2. Even at the same temperature of dehydration of samples different in nature, the C1/Ti ratios, and hence, the C1- and Ti 4+ contents, substantially differ from one another. The boundary values of the C1/Ti ratio in table 1, which were observed by Tchuiko et al. [3] and Hair and Hertl [8], are worth noting. The quite
low value (C1/Ti = 0.46) is ascribed in ref. [3] to hydrolysis of the Ti-C1 bond in the groups bonded to the surface by coordination water tightly adsorbed on aerosil. The rather high value (C1/Ti = 3.4) is attributed by other authors to direct chlorination of the surface by TiC14 molecules. Our results show that the C1/Ti ratio for vulcasil over the whole temperature range (200-800°C) varies within the narrow interval 1.28-1.50, while for silicon gel and aerosil this ratio ranges from 1.8 to 2.5. Besides, no change in interaction mechanism due to the change in valent state of titanium is to be expected since the EPR studies performed showed no EPR signal. Table 2 presents a comparison between the values of the OH groups found thermogravimetrically and those calculated according to eq. (1).
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D. Damyanov et al. / Interaction between TiCl 4 and SiO 2 surface OH groups
Table 2 Estimation of the content of OH groups in n u m b e r / n m 2 from chemical analysis data, (OH)oh, and thermogravimetric determinations (OH),g. Ratio of the two kinds of values depending on the temperature of preliminary dehydration and the nature of SiO 2 Support
OH groups, number O H / n m 2 200
400
600
800
1 silica gel
OHch OHtg ch/tg
4.98 4.79 1.04
4.04 3.45 1.17
2.81 1.95 1.44
2.23 1.41 1.58
2 vulcasil
OHCh OH ~ ch/tg
6.55 7.36 0.89
5.15 5.48 0.94
4.19 4.03 1.04
3.3 2.64 1.25
3 aerosil
OHch OHtg ch/tg
2.57 5.46 0.47
2.68 3.43 0.78
2.16 1.91 1.13
1.52 1.15 1.32
The results from the thermogravimetric measurements show that the packing density of the silanol groups depends on the temperature of preliminary dehydration of SiO 2 and, to some extent, on its nature. For silica gel samples the OHch/OHtg ratio is in all cases more than 1 and increases with the dehydration temperature. A similar dependence is observed for vulcasil and aerosil at a calcination temperature above 600°C. The fact that the above ratio exceeds 1, indicates that the OH groups participating in the reaction are more than those determined thermogravimetrically. That is why calculation of the amount of OH groups on the basis of chemical analysis data is not correct. Probably a secondary reaction of part of the evolved hydrogen chloride with the surface silanol groups proceeds. For this reason, the C1/Ti ratio is not reliable enough in establishing the interaction mechanism of the vapour of transition element halides with the silica surface. We tried to confirm our assumption by IR spectroscopy, all samples being pre-treated, modified and analysed under identical conditions. The spectrum of the sample out-gassed at 200 ° C for 8 h (fig. 1(1)) shows pronounced bands at 3750 and 3580 cm -1, the former belonging to stretching modes of free OH groups, the latter representing a combined signal of hydrogenbonded OH groups and water molecules coordina-
tively bonded to the surface. In the medium frequency area there is a band with a maximum at 1630 c m - t due to the overlap of the vibrations of the silicon oxide frame and the bending modes of the coordination water molecules on the surface. Saturated TiC14 vapour coming in contact with the SiO 2 surface at 1 5 0 ° C (fig. 1(2)) causes the disappearance of the bands at 3750 and 3580 cm-1 and the appearance of a distinct new band with a maximum at about 3660 cm-1. Therefore, it can be assumed that both OH groups and coordinatively bonded water molecules on the surface are reacting. In accordance with ref. [26], the appearance of a band at 3660 cm l can be attributed to the stretching modes of unreacted OH groups in narrow pores or at the sites of contact of the globules. Simultaneously the presence in the low-frequency region of an absorption band at 920-950 cm-1 confirms the formation of both bridge ((SiO2)2TiCI2) [13] and linear ( S i - O - T i ) [27] surface structures. In the medium frequency region a considerable drop in band intensity is observed at 1630 cm -1. Since at 2 0 0 ° C the probability of formation of siloxane bridges is negligible, we should neglect the possibility of their participation in a surface reaction. Therefore, the decrease in band intensity at 1630 cm-1 can be considered as confirming the
i
3800
3600
~' tSO0 ' ' 1600' ~ ~ tlO0' ' 900" ' 800 ' Frequmncy, crn"I
Fig. 1. IR spectra of silica gel: (1) out-gassed for 8 h at 200 o C; (2) treated with saturated TiC14 vapour for 2 h at 1 5 0 ° C and then out-gassed for 30 min; (3) hydrolyzed with water vapour for 16 h and out-gassed at 200 o C for 6 h; and (4) treated with saturated TiC14 vapour for 2 h at 150 ° C and out-gassed for 30 min.
D. Damyanou et aL / Interaction between TiCI 4 and SiO, surface O H groups
1
b
o-
t
.m
E
I
|~,
1
.
t
A
ii
.
i
.
,
I
,
I
.
A
1
,
t
2700 2500 Frequency, cm-~ Fig. 2. IR spectra of samples partially deuterated with D20 and out-gassed for 6 h at 500 o C. (a) silica gel: (1) silica gel; (2), (3), (4), silica gel modified at 1 5 0 ° C with the following amounts of TiC14 vapour: (2), 2.4 mk mol; (3), 4.8 mkmol; and (4), 7.2 mkmol (5) saturated with TiCI 4. (b) aerosil: (1) aerosil; (2), (3), modified with the following amounts of TiCI 4 vapour: (2), 2.4 mkmol, and (3), complete saturation. 3800 3600 3400s2~00 2600
3700 3500 v
continuation of a reaction of TiC14 with the coordinatively bonded water molecules which, according to Golovanova et al. [28], play the role of proton donors on the surface. The hydrolysis of the surface complexes with water vapour (fig. 1. (3)) followed by out-gassing and calcination of the sample at 200 ° C leads to broadening and an intensity increase of bands with maxima at about 3580 and 1630 cm 1. This provides evidence of the additional coordination of water molecules on the surface. Repeated modification with TiC14 vapour results in changes analogous to those of spectrum 2. The effect of the nature of SiO 2 on the surface reactions was also of interest. In this connection, comparison is made in fig. 2 between the behaviours towards TiC14 of silica gel and aerosil samples which were (i) not completely deuterated with heavy water vapour and (ii) preliminary dehydrated at 500 ° C. In the case of preliminary dehydration, the effect of coordination water " a priori" bonded to the surface was to be eliminated, and the reactivity and coordination ability of the initial samples were to be elucidated. As is evident from fig. 2a, the band of free OH groups at 3750 c m - 1 and that of hydrogen-bonded OH groups at 3700-3400 cm -~ for silica gel are barely discernible. At the same time, the corresponding band for free OD groups is quite distinct
111
at 2760 cm -1. The presence of successively increasing amounts of TiC14 produces considerable changes. After the first dose of TiC14 (2.4 mk moles, spectrum 2.2), two new bands at 3540 and 2620 cm-1 are observed which, according to our opinion, are due to the stretching modes of OH(OD) groups connected with neighbouring chlorine ions by hydrogen bonds and to coordinatively bonded water molecules. The changes of spectra can be explained on the basis of the above assumption that part of the nascent hydrogen chloride formed as a result of the reaction of TiCI 4 with silanol groups (reaction schemes 2-4) interacts with the unreacted OH groups (a competitive reaction) according to the equation: S i - O H + HC1 ~ Si-C1 + H 2 0 .
(5)
Thus, simultaneously with the surface halogenation (formation of Si-C1 bonds), water molecules also appear, which additionally complicates the mechanism of surface reactions. Subsequent treatments with TiC14 lead to a decrease in band intensities for both the isolated OD(OH) groups and the hydrogen-bonded ones. After complete saturation of the surface with TiC14 vapour (spectrum 2.5), there is a weak band at 2720 cm ~ solely due to unreacted OD groups situated in inaccessible sites. Aerosil shows a different behaviour. After modification with TiC14 vapour there is no distinct band at 3500-3700 cm-1 for hydrogen-bonded OH groups. The band for hydrogen-bonded OD groups is almost imperceptible. This can be ascribed to the much less pronounced coordination ability of aerosil in comparison with that of silica gel, which was established by us in spectroscopy studies [29]. As a result, the coordination number of surface silicon atoms of silica gel increases up to 6 due to coordinatively bound water molecules whereas with aerosil this process takes place to a smaller extent. Modification of SiO 2 with TiC14 followed by hydrolysis with water vapour [29] shows that the surface structure of the SiO 2 carrier is preserved after the interaction: in the case of silica gel, surface structures consisting of TiO6 octahedra are materialized whereas with aerosil the coordination number of deposited titanium ions exhibits a
112
D. Damyanov et aL / Interaction between
smaller increase. The results obtained are in agreement with the investigations of Pak [16,17] which confirm the dependence of the surface structures on the nature of the SiO 2 carrier. On the basis of scheme (5) it would be easy to explain the OHch/OHtg ratio values of more than 1. At low temperatures of SiO2 pre-calcination not only the "own" coordination water but also water molecules additionally formed during the interaction with TiC14 vapour, react. Part of the water molecules additionally formed react directly with the TiC14 vapour, others are coordinated on the surface. In the absence of coordination water on the SiO 2 surface (calcination temperatures above 400 o C) water molecules are formed during the reaction. Depending on the coordination ability of the SiO 2 carrier, part of these molecules remain on sites inaccessible to TiC14 molecules. The assumption about the formation of water molecules during the reaction and their remaining on the surface as coordinated water is supported by the analogous behaviour of the changes in spectrum of A1203 after its treatment with TiC14 vapour (fig. 3). Initially the spectrum of the 3,-A1203 sample calcined at 5 0 0 ° C shows, in agreement with the model of Peri [30], distinct bands of the stretching modes of free OH groups at 3780, 3744, 3733 and 3700 cm -1. Upon coming in contact with TiC14 vapour, the sample shows a new band at 1630 cm 1 (spectrum 3.2) due to deformational vibrations of unreacted coordinatively bonded water
, g t~ C
O I ~
~ t
Q m
j ~'~ 3800 3600
1 7 0 0 1500
-(
Frequency, cm
Fig. 3. IR spectra of "t-A1203: (1) outgassed for 6 h at 500 o C; (2) treated with 4.8 mkmol TIC14 at 1 5 0 ° C ; and (3) after adsorption of water vapour.
TiC/4 and
SiO e surface OH groups
3800 3600 3400 Frequency, cm q Fig. 4. IR spectra of aerosil: (1), dehydrated for 8 h at 400 ° C; (2), (3), (4), after treatment with TiCI 4 and hydrolysis with water vapour resulting in the formation on the aerosil surface of one (2), two (3) and three (4) titanium oxide layers.
molecules (reaction prevented by the inaccessibility to TIC14 of coordinatively bonded water molecules). The presence of the unreacted water molecules is also evidenced by the band at 3560 cm-1 which is broadened and shifted to lower frequencies after hydrolysis with water vapour. Under these conditions the band at 1630 cm -1 becomes more pronounced. Some information on the coordination ability of titanium ions fixed on the aerosil surface was obtained by performing cycles of hydrolysis of the surface titanium complexes with water vapour followed by treatment of the sample with TiCI4 vapour, i.e. the so-called reactions of molecular deposition were materialized [31]. In this connection, fig. 4 illustrates the changes in the IR spectrum of the titanium oxide layers hydrolyzed by water vapour. It should be pointed out that with the increase in number of the hydrolyzed layers (spectra 2-4), the absorption bands are broadened and their shoulder shows an increasingly pronounced shift to lower frequencies. This can be explained by the more clearly expressed coordination abilities of the surface titanium ions. Hence, it may be assumed that with the deposition of a second and a third layer the structural type of the initial-coordination TiC14 polyhedra is not completely pre-
D. Damyanov et al. / Interaction between TiCl 4 and SiO: surface OH groups
served. On the contrary, there is a trend towards formation of TiO 6 polyhedra, i.e. the same as on the silica gel surface. This indicates the strongest effect of the SiO 2 substrate on the first titanium oxide layer.
4. Conclusion The simultaneous decrease in intensity of the bands of free and hydrogen-bonded OH groups as a result of treatment of SiO 2 with TIC14 vapour indicates the continuation of surface reactions which cannot provide evidence of the higher reactivity of one or the other type of OH groups. The coordinatively bonded water molecules also interact with the TiC14 vapour irrespective of whether they are a priori on the SiO 2 surface or are obtained as a result of a side reaction. The topography of the hydrate coverage on silica gel differs from that on aerosil, which determines the difference in mechanisms of formation and coordination ability of the surface phase. The complex mechanism of surface interactions between the silanol groups and the TiCI 4 vapour depends on the additional interaction with the hydrogen chloride formed. As a result, the SiO 2 surface is additionally halogenated, while the water obtained is partially coordinated on surface silicon or titanium ions depending on the substrate nature. For this reason, the interaction of TiC14 with the silanol groups cannot be used for a quantitative determination of the latter. The CI/Ti ratio cannot be considered as a reliable criterion in the elucidation of the mechanism of surface processes. It can only serve as tentative information.
References [1] S.I. Kol'tsov and V.B. Aleskovskii, Zh. Fiz. Khim. [Russ. J. Phys. Chem.] 42 (5) (1968) 1210; Zh. Prikl. Khim. [Russ. J. Appl. Chem.] 40 (4) (1967) 907. [2] L.M. Sharygin and S. Ya, Tret'yakov, Khemosorbtsiya Chetyrekh-Khloristogo Titana na Silikagele (Khimiya, Moscow, 1973) p. 13. [3] A.A. Tchuiko, V.A. Tertykh, K.P. Kazakov, V.V. Pavlov, S.O. Shimanovskii and R.V. Sushko, Adsorbtsiya i Adsorbenty No. 8 (1980) 39.
113
[4] G. Bliznakov, M. Majdraganova-Khrussanova, Comm. Dept. Chem. Bulg. Acad. Sci, 9 (4) (1976) 668. [5] C.G. Armistead, A.J. Tyller, F.H. Hambleton, S.A, Mitchell and J.A. Hockey, J. Phys. Chem. 73 (1969) 3947, [6] H.W. Kohlschuetter von and U. Boegel, Fortschr. Kolloide u. Polym. 55 (1971) 29. [7] K.V. Nel'son, I,V. lkonitskii and V.A. Ryzhikov, Zh. Prikl. Spektr. [Russ. J. Appl. Spectrosc.] 12 (1) (1970) 80. [8] M.U Hair and W. Hertl, J. Phys. Chem. 77 (1973) 2070. [9] M. Velikova, D. Damyanov and D. Mehandjiev, lzv. Khim. BAN 12 (4) (1979) 647. [10] 3. Kunawicz, P. Jones and J.A. Hockey, Trans. Farad. Soc. 67 (3) (1971) 848. [11] J. Murroy, M.J. Sharp and J.A. Hockey, J. Catalysis 18 (1) (1970) 52. [12] J.A.. Hockey, J. Phys. Chem. 74 (12) (1970) 2570. [13] B.A. Morrow, C.P. Tripp and K.A. McFarlane, J. Chem. Soc. Chem. Commun. No. 19 (1984) 1282. [14] C.F. Smith, R.A. Condrate and W.E. Votava, Appl. Spectrosc. 29 (1) (1975) 79. [15] J.B. Kinney and R.H. Staley, J. Phys. Chem. 87 (19) (1983) 3735. [16] V.N. Pack, Zh. Fiz. Khim. [Russ. J. Phys. Chem.] 50 (5) (1976) 1266. [17] V.N. Pack and Yu.P. Kostikov, Kin. i. Katal. 18 (2) (1977) 475. [18] J.B. PerL J. Phys. Chem. 70 (1966) 2937. [19] M. Tanaka and S. Ogasawara, J. Catalysis 16 (1970) 157. [20] V.A. Tertykh, V.V. Pavlov, Yu.I. Gorlov and K,I. Tkachenko, Teor. Eksp. Khim. 15 (4) (1979) 400. [21] M. Baverez, B. Hatier, M. Bastick and J. Bastick, Bull. Soc. Chim. France No. 6 (1964) 1298. [22] E.V. Makhonina, I.M. Zimmer, V,G. Aristova and A.I. Gorbunov, Kolloid. Zh, 42 (5) (1980) 980. [23] L.R. Shyder and J.P. Ward, J. Phys. Chem. 70 (12) (1966) 3941. [24] D. Damyanov and Iv. Obretenov, Ann. Higher Inst. Chem. Eng. Bourgas 12 (1977) 159. [25] I. Genov and S. Stamov, Khimiya i Industriya 55 (2) (1983) 67. [26] B.P. Laskorin, V.V. Strelko, D.D. Strazhesko and V.I. Denisov, Sorbenty na Osnove Silikagelya v Radiokhimii (Atomizdat, Moscow, 1977). [27] A.V. Kiselev, A.1, Kuznetsov, V.I. Lygin, Yu.I. Rostorguev, B.Z. Shalumov and K.L. Shchepalin, Kolloid Zh. 42 (5) (1980) 964. [28] G.F., Golovanova, V.I. Kvlividze and V.F. Kiselev, Coll. "Svyazannaya Voda v Dispersnykh Sistemakh', No, 4 (University of Moscow, 1977) p. 178. [29] M, Velikova and D. Damyanov, Comm. Dept. Chem. Bulg. Acad. Sci. 1987 (in press). [30] J.B. Peri, J. Phys. Chem. 69 (1965) 211; 69 (1965) 220. [31] V.B. Aleskovskii, Stekhiometriya i Sintez Tverdykh Soedinenii (Nauka, Leningrad, 1976).
107
ON T H E M E C H A N I S M OF I N T E R A C T I O N B E T W E E N TiCI 4 V A P O U R AND SURFACE OH G R O U P S OF A M O R P H O U S SiO 2 D. D A M Y A N O V , M. VELIKOVA, Iv. I V A N O V and L. VLAEV Higher Institute of Chemical Engineering, 8010 Bourgas, Bulgaria Received 8 September 1987 Revised manuscript received 16 February 1988
Chemical analysis data concerning CI and Ti contents in various samples of amorphous SiO 2 treated with TiCI 4 vapour have shown that the C I / T i ratio cannot be a reliable criterion in the interpretation of the mechanism of surface reactions. An attempt is made to explain the mechanism of surface processes on the basis of infrared spectroscopy studies. It is shown that the hydrogen chloride evolved during the reaction takes part in a competitive reaction which leads to partial halogenation of the surface and formation of water molecules. Depending on the nature of the substrate, the water molecules are coordinated on the surface silicon or titanium ions and can interact with TiCI 4. For this reason, the reaction of the SiO 2 surface with the TiC14 vapour cannot be used for determining the concentration of silanol groups. The differences observed in the reactions of TiC14 with the silanol groups of silica gel and aerosil are due to differences in their coordination ability which is less pronounced with aerosil but changes upon deposition of molecular titanium oxide layers,
1. Introduction
The reaction of TiC14 with the surface of SiO 2 is of great interest in association with the preparation of a large number of products of practical importance: catalysts, adsorbents, various modifications of fillers to be used in polymerization processes etc. Irrespective of the large number of investigations [1-17] dedicated to the effect of the nature of SiO 2 and the packing density of silanol groups on the composition and structure of surface compounds resulting from the interaction between SiO 2 and TiC14 as well as to the mechanism of this interaction, these problems have not yet been elucidated. According to data from the literature [1-9], the C1 and Ti contents determined by chemical analysis of samples modified by TiC14 as well as the C1/Ti ratio vary in a very wide range. The data obtained by infrared spectroscopy studies [3-10,15], which have confirmed the formation of new surface compounds, have no explicit explanation. For example, the absence of a band at 3550 cm - t after treatment of SiO 2 with 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
TiCI 4 indicates, according to Kunavitz et al. [10], the higher reactivity of hydrogen-bonded O H groups compared to the "free" ones. Tchuiko et al. [3] assumed this fact to be evidence of an interaction between TiC14 and the coordinatively bound water. In refs. [16,17] an attempt is made to explain the difference in structure of the titanium oxide layers with differences in structural peculiarities of the carriers. The probability for a reaction to proceed between the hydroxyl coverage and the hydrogen chloride, which is the product of the main chemical reaction, is an important problem which has been neglected by researchers in their attempts to elucidate the interaction between TiC14 and surface O H groups. In this connection it should be noted that there is even no generally accepted opinion on the direct interaction of HC1 with the silanol groups. For instance, according to Peri [18], calcination of SiO 2 at a temperature as high as 700 ° C in a dry HC1 flow provides evidence of no noticeable continuation of the reaction under consideration. A weak interaction of DC1 with an aerosil surface calcined at 500 ° C was established
108
D. Damyanov et al. / Interaction between T i C l 4 and SiO 2 surface O H groups
by Tanaka and Ogasawara [19]. On the other hand, Tertykh et al. [20] found an increasing rate of interaction between HC1 and the aerosil surface at 350-500°C, 75% of the hydroxyl groups participating in the reaction at 500 ° C. The studies of Baverez et al. [21] have shown that 86 mg C1/g SiO 2 are fixed on the surface at a SiO 2 dehydration temperature of 200 ° C and a reaction with HC1 taking place at the same temperature. These results are confirmed in ref. [22] where it is reported that thermal treatment at 200 ° C in vacuo of a series of aerosil and silica gel samples and interaction with HC1 at 30 and 5 0 ° C lead to different degrees of irreversible adsorption depending on the structure of the hydrate coverage. The purpose of the present paper is t o throw light on the mechanism of interaction between TiC14 and the surface O H groups using chemical analysis and IR data on various SiO 2 samples treated with TiC14 vapour.
2. Experimental Three pre-treated SiO 2 samples differing in texture were investigated: silica gel (KSK, USSR, A = 239 mZ/g), vulcasil-s (Bayer, FRG, A = 158 m2/g) and aerosil (Degussa, FRG, A = 278 mZ/g). The pre-treatment of the samples consisted in acid-activation in a water bath with HC1 (1 : 1) for 20 days, washing with distilled water till the establishment of a negative reaction for chlorine ions (10-12 days), blowing-through water vapour for 8 h at 100 ° C, removal of the intraglobular water and achieving maximum surface hydration by cycles of dehydration at 750 ° C and rehydration at 90 ° C [23]. The reaction of the utmost hydrated or partially dehydrated surface of SiO 2 with TiC14 vapour (Titangesellschaft, F R G ) was materialized at 150 °C for 2 h in a vacuum apparatus [24]. The concentration of chlorine ions in the modified samples was determined by the ammetric titration method [25], and that of titanium, by atomic absorption analysis after treating the samples with 2n H2SO 4. The content of O H groups was estimated both thermogravimetrically, (OH)tg , and on the basis of
data from chemical analyses of the TiC14-modified samples, (OH)oh, according to the formula (OH)ch = 4x -- y,
(1)
where x and y are the Ti and C1 amounts, respectively, in mg at./g. The maximum relative error of determination of O H groups in both cases did not exceed 5%, which permitted comparison of the results obtained. The IR studies were performed on plates (30 x 100 mm 2, thickness 10-15 m g / c m 2) obtained without a binder by pressing a fine silica gel or aerosil powder under a pressure of 120-200 k g / c m 2. The heat treatment of the plates, the partial replacement of their surface O H groups by heavy water vapour, their modification by TiC14 and hydrolysis with water vapour were carried out in a high-temperature vacuum quartz cell. The infrared spectra were taken with a UR-20 (Carl Zeiss, DDR) apparatus at room temperature using a diaphragm in the path of the reference beam.
3. Results and discussion In view of the fact that the TiC14 molecule would hardly participate in a reaction with all of its chlorine ions, one can assume the following possible reactions between it and the surface OH groups: -OH - O H + TiC14 -OH -OH + TiC14 -OH - O H + TiC14
-O - O ~ T i C 1 + 3HC1,
(2)
-O ) TiC12 + 2HC1,
(3)
-0 - O - T i C 1 s + HC1.
(4)
It is obvious that the quantitative ratios between chlorine and titanium will vary within a certain range depending on the extent of participation of the different reactions. For this reason, the C1/Ti ratio is used by many authors as a criterion in the estimation of the mechanism of surface processes and hence, of the structure of surface compounds obtained [1,3,4,8].
D. Damyanov et al. / Interaction between TiCl 4 and SiO: surface OH groups
109
Table 1 Dependence of the CI and Ti contents and the CI/Ti ratio on the nature of SiO 2 and the temperature of preliminary dehydration No support
Dehydration temperature
Specific surface
Elements (rag.at/g)
C1/Ti ratio
Ref.
( o C)
area of support (m2/g)
Chlorine
Titanium
230 160 280 250 200 200 150 116
1.62-1.73 0.81-0.88 1.94 1.12
0.88 0.63 0.78 1.1 0.48 0.9 -
1.84-1.96 1.28-1.39 2.49 1.02 2.4 -2.6 0.46 1.17 2.46 2.2
[11 [2] [31 [41 [51 [61
0.79-0.58 0.55-0.42 0.62-0.47 (I.42-0.48 -0.9 -0.65 0.45-0.7
2.07-2.16 1.4 2,11-1.94 1,60-2.53 2,2-2.3 2 1.7 -1.28
[31 [71 [1] [4]
0.45-0.36 0.34-0.30 0.36 0.34 0.40 0.35-0.12
2.2 1.5 2.4 3.0 3.4 3.0 1.74-2.33
[3] [8] [6] [4]
(a) Low-temperature region (180- 300 o C) 1 silica gel 2 vulcasil 3 aerosil 4 silica gel 5 silica gel 6 aerosil 7 silica gel 8 aerosil 9 silica gel
200-300 200-300 200 180 180 180 200 200 300
0.22 1.05
(b) Medium temperature region (400- 600 o C) 1 2 3 4 5 6 7
silica gel vulcasil aerosil aerosil aerosil silica gel silica gel
400-600 400-600 400-600 400 400 400-600 400-600
230 160 280 200 250 200
1.64-1.25 0.77-0.59 1.31-0.91 0.67-1.22
230 160 280 200 160 116 200
0,99-0.79 0,51-0.45 0,86 1.02 1.37
1.8 -1.3 0.75-0.9
(c) High temperature region (700- 900°C) 1 2 3 4 5 6 7
silica gel vulcasil aerosil aerosil cobosil silica gel silica gel
700-800 700-800 800 700 800 800 700-900
0.61-0.28
It should be expected that with the decrease in packing density of the silanol groups the contribution of the reaction of eq. (4) would increase. However, comparison of data from the literature with those obtained by us shows a not clearly expressed dependence of the increase in the C1/Ti ratio from the low (180-300°C) to the high (700-900 ° ) temperature region, the number of titanium and chlorine ions bonded to the surface decreasing with the rise of temperature of precalcined SiO 2. Even at the same temperature of dehydration of samples different in nature, the C1/Ti ratios, and hence, the C1- and Ti 4+ contents, substantially differ from one another. The boundary values of the C1/Ti ratio in table 1, which were observed by Tchuiko et al. [3] and Hair and Hertl [8], are worth noting. The quite
low value (C1/Ti = 0.46) is ascribed in ref. [3] to hydrolysis of the Ti-C1 bond in the groups bonded to the surface by coordination water tightly adsorbed on aerosil. The rather high value (C1/Ti = 3.4) is attributed by other authors to direct chlorination of the surface by TiC14 molecules. Our results show that the C1/Ti ratio for vulcasil over the whole temperature range (200-800°C) varies within the narrow interval 1.28-1.50, while for silicon gel and aerosil this ratio ranges from 1.8 to 2.5. Besides, no change in interaction mechanism due to the change in valent state of titanium is to be expected since the EPR studies performed showed no EPR signal. Table 2 presents a comparison between the values of the OH groups found thermogravimetrically and those calculated according to eq. (1).
110
D. Damyanov et al. / Interaction between TiCl 4 and SiO 2 surface OH groups
Table 2 Estimation of the content of OH groups in n u m b e r / n m 2 from chemical analysis data, (OH)oh, and thermogravimetric determinations (OH),g. Ratio of the two kinds of values depending on the temperature of preliminary dehydration and the nature of SiO 2 Support
OH groups, number O H / n m 2 200
400
600
800
1 silica gel
OHch OHtg ch/tg
4.98 4.79 1.04
4.04 3.45 1.17
2.81 1.95 1.44
2.23 1.41 1.58
2 vulcasil
OHCh OH ~ ch/tg
6.55 7.36 0.89
5.15 5.48 0.94
4.19 4.03 1.04
3.3 2.64 1.25
3 aerosil
OHch OHtg ch/tg
2.57 5.46 0.47
2.68 3.43 0.78
2.16 1.91 1.13
1.52 1.15 1.32
The results from the thermogravimetric measurements show that the packing density of the silanol groups depends on the temperature of preliminary dehydration of SiO 2 and, to some extent, on its nature. For silica gel samples the OHch/OHtg ratio is in all cases more than 1 and increases with the dehydration temperature. A similar dependence is observed for vulcasil and aerosil at a calcination temperature above 600°C. The fact that the above ratio exceeds 1, indicates that the OH groups participating in the reaction are more than those determined thermogravimetrically. That is why calculation of the amount of OH groups on the basis of chemical analysis data is not correct. Probably a secondary reaction of part of the evolved hydrogen chloride with the surface silanol groups proceeds. For this reason, the C1/Ti ratio is not reliable enough in establishing the interaction mechanism of the vapour of transition element halides with the silica surface. We tried to confirm our assumption by IR spectroscopy, all samples being pre-treated, modified and analysed under identical conditions. The spectrum of the sample out-gassed at 200 ° C for 8 h (fig. 1(1)) shows pronounced bands at 3750 and 3580 cm -1, the former belonging to stretching modes of free OH groups, the latter representing a combined signal of hydrogenbonded OH groups and water molecules coordina-
tively bonded to the surface. In the medium frequency area there is a band with a maximum at 1630 c m - t due to the overlap of the vibrations of the silicon oxide frame and the bending modes of the coordination water molecules on the surface. Saturated TiC14 vapour coming in contact with the SiO 2 surface at 1 5 0 ° C (fig. 1(2)) causes the disappearance of the bands at 3750 and 3580 cm-1 and the appearance of a distinct new band with a maximum at about 3660 cm-1. Therefore, it can be assumed that both OH groups and coordinatively bonded water molecules on the surface are reacting. In accordance with ref. [26], the appearance of a band at 3660 cm l can be attributed to the stretching modes of unreacted OH groups in narrow pores or at the sites of contact of the globules. Simultaneously the presence in the low-frequency region of an absorption band at 920-950 cm-1 confirms the formation of both bridge ((SiO2)2TiCI2) [13] and linear ( S i - O - T i ) [27] surface structures. In the medium frequency region a considerable drop in band intensity is observed at 1630 cm -1. Since at 2 0 0 ° C the probability of formation of siloxane bridges is negligible, we should neglect the possibility of their participation in a surface reaction. Therefore, the decrease in band intensity at 1630 cm-1 can be considered as confirming the
i
3800
3600
~' tSO0 ' ' 1600' ~ ~ tlO0' ' 900" ' 800 ' Frequmncy, crn"I
Fig. 1. IR spectra of silica gel: (1) out-gassed for 8 h at 200 o C; (2) treated with saturated TiC14 vapour for 2 h at 1 5 0 ° C and then out-gassed for 30 min; (3) hydrolyzed with water vapour for 16 h and out-gassed at 200 o C for 6 h; and (4) treated with saturated TiC14 vapour for 2 h at 150 ° C and out-gassed for 30 min.
D. Damyanou et aL / Interaction between TiCI 4 and SiO, surface O H groups
1
b
o-
t
.m
E
I
|~,
1
.
t
A
ii
.
i
.
,
I
,
I
.
A
1
,
t
2700 2500 Frequency, cm-~ Fig. 2. IR spectra of samples partially deuterated with D20 and out-gassed for 6 h at 500 o C. (a) silica gel: (1) silica gel; (2), (3), (4), silica gel modified at 1 5 0 ° C with the following amounts of TiC14 vapour: (2), 2.4 mk mol; (3), 4.8 mkmol; and (4), 7.2 mkmol (5) saturated with TiCI 4. (b) aerosil: (1) aerosil; (2), (3), modified with the following amounts of TiCI 4 vapour: (2), 2.4 mkmol, and (3), complete saturation. 3800 3600 3400s2~00 2600
3700 3500 v
continuation of a reaction of TiC14 with the coordinatively bonded water molecules which, according to Golovanova et al. [28], play the role of proton donors on the surface. The hydrolysis of the surface complexes with water vapour (fig. 1. (3)) followed by out-gassing and calcination of the sample at 200 ° C leads to broadening and an intensity increase of bands with maxima at about 3580 and 1630 cm 1. This provides evidence of the additional coordination of water molecules on the surface. Repeated modification with TiC14 vapour results in changes analogous to those of spectrum 2. The effect of the nature of SiO 2 on the surface reactions was also of interest. In this connection, comparison is made in fig. 2 between the behaviours towards TiC14 of silica gel and aerosil samples which were (i) not completely deuterated with heavy water vapour and (ii) preliminary dehydrated at 500 ° C. In the case of preliminary dehydration, the effect of coordination water " a priori" bonded to the surface was to be eliminated, and the reactivity and coordination ability of the initial samples were to be elucidated. As is evident from fig. 2a, the band of free OH groups at 3750 c m - 1 and that of hydrogen-bonded OH groups at 3700-3400 cm -~ for silica gel are barely discernible. At the same time, the corresponding band for free OD groups is quite distinct
111
at 2760 cm -1. The presence of successively increasing amounts of TiC14 produces considerable changes. After the first dose of TiC14 (2.4 mk moles, spectrum 2.2), two new bands at 3540 and 2620 cm-1 are observed which, according to our opinion, are due to the stretching modes of OH(OD) groups connected with neighbouring chlorine ions by hydrogen bonds and to coordinatively bonded water molecules. The changes of spectra can be explained on the basis of the above assumption that part of the nascent hydrogen chloride formed as a result of the reaction of TiCI 4 with silanol groups (reaction schemes 2-4) interacts with the unreacted OH groups (a competitive reaction) according to the equation: S i - O H + HC1 ~ Si-C1 + H 2 0 .
(5)
Thus, simultaneously with the surface halogenation (formation of Si-C1 bonds), water molecules also appear, which additionally complicates the mechanism of surface reactions. Subsequent treatments with TiC14 lead to a decrease in band intensities for both the isolated OD(OH) groups and the hydrogen-bonded ones. After complete saturation of the surface with TiC14 vapour (spectrum 2.5), there is a weak band at 2720 cm ~ solely due to unreacted OD groups situated in inaccessible sites. Aerosil shows a different behaviour. After modification with TiC14 vapour there is no distinct band at 3500-3700 cm-1 for hydrogen-bonded OH groups. The band for hydrogen-bonded OD groups is almost imperceptible. This can be ascribed to the much less pronounced coordination ability of aerosil in comparison with that of silica gel, which was established by us in spectroscopy studies [29]. As a result, the coordination number of surface silicon atoms of silica gel increases up to 6 due to coordinatively bound water molecules whereas with aerosil this process takes place to a smaller extent. Modification of SiO 2 with TiC14 followed by hydrolysis with water vapour [29] shows that the surface structure of the SiO 2 carrier is preserved after the interaction: in the case of silica gel, surface structures consisting of TiO6 octahedra are materialized whereas with aerosil the coordination number of deposited titanium ions exhibits a
112
D. Damyanov et aL / Interaction between
smaller increase. The results obtained are in agreement with the investigations of Pak [16,17] which confirm the dependence of the surface structures on the nature of the SiO 2 carrier. On the basis of scheme (5) it would be easy to explain the OHch/OHtg ratio values of more than 1. At low temperatures of SiO2 pre-calcination not only the "own" coordination water but also water molecules additionally formed during the interaction with TiC14 vapour, react. Part of the water molecules additionally formed react directly with the TiC14 vapour, others are coordinated on the surface. In the absence of coordination water on the SiO 2 surface (calcination temperatures above 400 o C) water molecules are formed during the reaction. Depending on the coordination ability of the SiO 2 carrier, part of these molecules remain on sites inaccessible to TiC14 molecules. The assumption about the formation of water molecules during the reaction and their remaining on the surface as coordinated water is supported by the analogous behaviour of the changes in spectrum of A1203 after its treatment with TiC14 vapour (fig. 3). Initially the spectrum of the 3,-A1203 sample calcined at 5 0 0 ° C shows, in agreement with the model of Peri [30], distinct bands of the stretching modes of free OH groups at 3780, 3744, 3733 and 3700 cm -1. Upon coming in contact with TiC14 vapour, the sample shows a new band at 1630 cm 1 (spectrum 3.2) due to deformational vibrations of unreacted coordinatively bonded water
, g t~ C
O I ~
~ t
Q m
j ~'~ 3800 3600
1 7 0 0 1500
-(
Frequency, cm
Fig. 3. IR spectra of "t-A1203: (1) outgassed for 6 h at 500 o C; (2) treated with 4.8 mkmol TIC14 at 1 5 0 ° C ; and (3) after adsorption of water vapour.
TiC/4 and
SiO e surface OH groups
3800 3600 3400 Frequency, cm q Fig. 4. IR spectra of aerosil: (1), dehydrated for 8 h at 400 ° C; (2), (3), (4), after treatment with TiCI 4 and hydrolysis with water vapour resulting in the formation on the aerosil surface of one (2), two (3) and three (4) titanium oxide layers.
molecules (reaction prevented by the inaccessibility to TIC14 of coordinatively bonded water molecules). The presence of the unreacted water molecules is also evidenced by the band at 3560 cm-1 which is broadened and shifted to lower frequencies after hydrolysis with water vapour. Under these conditions the band at 1630 cm -1 becomes more pronounced. Some information on the coordination ability of titanium ions fixed on the aerosil surface was obtained by performing cycles of hydrolysis of the surface titanium complexes with water vapour followed by treatment of the sample with TiCI4 vapour, i.e. the so-called reactions of molecular deposition were materialized [31]. In this connection, fig. 4 illustrates the changes in the IR spectrum of the titanium oxide layers hydrolyzed by water vapour. It should be pointed out that with the increase in number of the hydrolyzed layers (spectra 2-4), the absorption bands are broadened and their shoulder shows an increasingly pronounced shift to lower frequencies. This can be explained by the more clearly expressed coordination abilities of the surface titanium ions. Hence, it may be assumed that with the deposition of a second and a third layer the structural type of the initial-coordination TiC14 polyhedra is not completely pre-
D. Damyanov et al. / Interaction between TiCl 4 and SiO: surface OH groups
served. On the contrary, there is a trend towards formation of TiO 6 polyhedra, i.e. the same as on the silica gel surface. This indicates the strongest effect of the SiO 2 substrate on the first titanium oxide layer.
4. Conclusion The simultaneous decrease in intensity of the bands of free and hydrogen-bonded OH groups as a result of treatment of SiO 2 with TIC14 vapour indicates the continuation of surface reactions which cannot provide evidence of the higher reactivity of one or the other type of OH groups. The coordinatively bonded water molecules also interact with the TiC14 vapour irrespective of whether they are a priori on the SiO 2 surface or are obtained as a result of a side reaction. The topography of the hydrate coverage on silica gel differs from that on aerosil, which determines the difference in mechanisms of formation and coordination ability of the surface phase. The complex mechanism of surface interactions between the silanol groups and the TiCI 4 vapour depends on the additional interaction with the hydrogen chloride formed. As a result, the SiO 2 surface is additionally halogenated, while the water obtained is partially coordinated on surface silicon or titanium ions depending on the substrate nature. For this reason, the interaction of TiC14 with the silanol groups cannot be used for a quantitative determination of the latter. The CI/Ti ratio cannot be considered as a reliable criterion in the elucidation of the mechanism of surface processes. It can only serve as tentative information.
References [1] S.I. Kol'tsov and V.B. Aleskovskii, Zh. Fiz. Khim. [Russ. J. Phys. Chem.] 42 (5) (1968) 1210; Zh. Prikl. Khim. [Russ. J. Appl. Chem.] 40 (4) (1967) 907. [2] L.M. Sharygin and S. Ya, Tret'yakov, Khemosorbtsiya Chetyrekh-Khloristogo Titana na Silikagele (Khimiya, Moscow, 1973) p. 13. [3] A.A. Tchuiko, V.A. Tertykh, K.P. Kazakov, V.V. Pavlov, S.O. Shimanovskii and R.V. Sushko, Adsorbtsiya i Adsorbenty No. 8 (1980) 39.
113
[4] G. Bliznakov, M. Majdraganova-Khrussanova, Comm. Dept. Chem. Bulg. Acad. Sci, 9 (4) (1976) 668. [5] C.G. Armistead, A.J. Tyller, F.H. Hambleton, S.A, Mitchell and J.A. Hockey, J. Phys. Chem. 73 (1969) 3947, [6] H.W. Kohlschuetter von and U. Boegel, Fortschr. Kolloide u. Polym. 55 (1971) 29. [7] K.V. Nel'son, I,V. lkonitskii and V.A. Ryzhikov, Zh. Prikl. Spektr. [Russ. J. Appl. Spectrosc.] 12 (1) (1970) 80. [8] M.U Hair and W. Hertl, J. Phys. Chem. 77 (1973) 2070. [9] M. Velikova, D. Damyanov and D. Mehandjiev, lzv. Khim. BAN 12 (4) (1979) 647. [10] 3. Kunawicz, P. Jones and J.A. Hockey, Trans. Farad. Soc. 67 (3) (1971) 848. [11] J. Murroy, M.J. Sharp and J.A. Hockey, J. Catalysis 18 (1) (1970) 52. [12] J.A.. Hockey, J. Phys. Chem. 74 (12) (1970) 2570. [13] B.A. Morrow, C.P. Tripp and K.A. McFarlane, J. Chem. Soc. Chem. Commun. No. 19 (1984) 1282. [14] C.F. Smith, R.A. Condrate and W.E. Votava, Appl. Spectrosc. 29 (1) (1975) 79. [15] J.B. Kinney and R.H. Staley, J. Phys. Chem. 87 (19) (1983) 3735. [16] V.N. Pack, Zh. Fiz. Khim. [Russ. J. Phys. Chem.] 50 (5) (1976) 1266. [17] V.N. Pack and Yu.P. Kostikov, Kin. i. Katal. 18 (2) (1977) 475. [18] J.B. PerL J. Phys. Chem. 70 (1966) 2937. [19] M. Tanaka and S. Ogasawara, J. Catalysis 16 (1970) 157. [20] V.A. Tertykh, V.V. Pavlov, Yu.I. Gorlov and K,I. Tkachenko, Teor. Eksp. Khim. 15 (4) (1979) 400. [21] M. Baverez, B. Hatier, M. Bastick and J. Bastick, Bull. Soc. Chim. France No. 6 (1964) 1298. [22] E.V. Makhonina, I.M. Zimmer, V,G. Aristova and A.I. Gorbunov, Kolloid. Zh, 42 (5) (1980) 980. [23] L.R. Shyder and J.P. Ward, J. Phys. Chem. 70 (12) (1966) 3941. [24] D. Damyanov and Iv. Obretenov, Ann. Higher Inst. Chem. Eng. Bourgas 12 (1977) 159. [25] I. Genov and S. Stamov, Khimiya i Industriya 55 (2) (1983) 67. [26] B.P. Laskorin, V.V. Strelko, D.D. Strazhesko and V.I. Denisov, Sorbenty na Osnove Silikagelya v Radiokhimii (Atomizdat, Moscow, 1977). [27] A.V. Kiselev, A.1, Kuznetsov, V.I. Lygin, Yu.I. Rostorguev, B.Z. Shalumov and K.L. Shchepalin, Kolloid Zh. 42 (5) (1980) 964. [28] G.F., Golovanova, V.I. Kvlividze and V.F. Kiselev, Coll. "Svyazannaya Voda v Dispersnykh Sistemakh', No, 4 (University of Moscow, 1977) p. 178. [29] M, Velikova and D. Damyanov, Comm. Dept. Chem. Bulg. Acad. Sci. 1987 (in press). [30] J.B. Peri, J. Phys. Chem. 69 (1965) 211; 69 (1965) 220. [31] V.B. Aleskovskii, Stekhiometriya i Sintez Tverdykh Soedinenii (Nauka, Leningrad, 1976).