Feb 10, 1975 - Because of its high content of silanol groups, sepiolite forms organomineral compounds having a relatively high organic matter content.
Clays and Clay Minerals, Vol.24, pp. 25-30. PergamonPress1976. Printedin GreatBritain
ORGANOMINERAL DERIVATIVES OBTAINED BY REACTING ORGANOCHLOROSILANES WITH THE SURFACE OF SILICATES IN ORGANIC SOLVENTS E. RUIz-HITzKY* and J. J. FRIPIAT~ University of Louvain, Laboratoire de Physico-Chimie Min6rale, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium (Received 10 February 1975; and in final form 20 August 1975)
Abstract--Stable organomineral derivatives are formed by reaction of organochlorosilanes with certain phyllosilicates. Organosiloxyl functions are grafted on silanol groups present at external mineral surfaces. Water molecules adsorbed on external mineral surfaces may cause hydrolysis of the reactant organosilicon products, with liberation of HC1. This, in turn, may react with the silicate: octahedral cations are extracted from the lattice and fresh Si-OH groups, capable of further grafting, are formed on the mineral surface. On the other hand, when difunctional reagents such as methylvinyldichlorosilane are used and if the ratio of adsorbed water to added reactive is adequate, then polymeric species with polysiloxane chains are grafted on the mineral. Because of its high content of silanol groups, sepiolite forms organomineral compounds having a relatively high organic matter content. With chrysotile, the amount of organic matter grafted to the silicate, is considerably smaller, but it increases appreciably if water is added to the reacting products. This is attributed to hydrolysis of the organic reactant and subsequent destruction of external "brucitic" layers by acid attack.
INTRODUCTION
Numerous attempts at synthesizing organomineral derivatives have been carried out by the direct action of very active organic reagents with some natural silicates. Using reactants such as diazomethane, acyl chlorides, anhydrides and epoxides, reactions have been carried out with pre-dried montmorillonites (Berger, 1941; Gieseking, 1949; Deuel, 1950). The grafting of the above organic substances can be explained by their reactions with the silanol groups of the mineral; nevertheless, the bond between the organic entity and the mineral is not very stable (Brown et al., 1952; Greenland and Russell, 1955). The interaction of organochlorosilanes with certain lamellar silicates (montmorillonite, vermiculite) produces complexes relatively unstable with the organosiloxanes which result from the hydrolysis of the reagents. This latter reaction probably occurs in the interlamellar spaces (Arag6n de la Cruz et al., 1972). Attempts to synthesize organomineral derivatives by direct reactions in organic solvents have been carried out by Edwards (1970) beginning with chrysotile asbestos fibers and organochlorosilanes. In studying the resulting products by i.r. spectroscopy, Edwards concluded that it is possible to obtain organomineral derivatives of chrysotile only under extreme experimental conditions. * Instituto de Edafolog~a y Biologia Vegetal, C.S.I.C., c/Serrano 115 dpdo., Madrid 6. t Centre de Recherche sur les Solides /t Organisation Cristalline Imparfaite, C.N.R.S., Orleans-Cedex, 45045, France. C.C.M.24/1--o
25
On the other hand, several organomineral derivatives of silicates have been obtained by simultaneous hydrolysis of some minerals with organosiloxanes or organochlorosilanes in a mixture of iso-propanol and concentrated hydrochloric acid (Lentz, 1964; Frazier et al., 1967; Fripiat and Mendelovici, 1968; Zapata et al., 1972). Under these conditions, the mineral undergoes a profound alteration resulting from extraction of the metallic cations by the acid in the reaction medium. The present work concerns the preparation of stable organomineral derivatives by direct grafting of organochlorosilanes on the surface of several silicates. The presence of superficial silanol groups allows the grafting of organic groups without the need of extracting the cations, thus avoiding destruction of the mineral lattice. Some molecules of water absorbed on the surface can bring about the hydrolysis of organochlorosilanes into HC1 and organosilanols; the HC1 extracts the metallic cations in producing new silanol sites exposed on the surface of the mineral. Sepiolite is a mineral very well adapted for this kind of study because one part of the mineral has a very high density of superficial silanol groups and the other part has easily controlled numbers of differing water species available to the reagent.
EXPERIMENTAL
Minerals
Sepio!ite, the starting mineral which has been used to advantage in this work, is a magnesium hydrated
26
E. RUIZ-HITZKYand J. J. FRIPIAT
I
9
Si
9
Mg
#
O0
|
OH
I~1 H2Oerysr ~) H20zeol
it b
,
-
Fig. 1. Schematic structure of sepiolite, after Brauner and Preisinger (1956). silicate of fibrous morphology whose octahedral layers are discontinuous; the open channels extend longitudinally in the direction of the fiber axis. According to Brauner and Preisinger (1956) the ideal formula of the half-cell corresponds to: Si12Mg803o (OH)4 (H2Oh'8 HzO. Each magnesium having access to a channel (Figure 1) is bound to two water molecules (co-ordinated water); other water molecules are located inside the channels (zeolitic water). Although the co-ordinated water is totally removed only by heating at elevated temperature (around 500~ at atmospheric pressure), the zeolitic water is eliminated very easily either by heating at ll0~ or by high vacuum. The loss of this latter species of water is reversible, but the loss of the co-ordinated water produces structural modifications which apparently are irreversible (formation of "anhydrous sepiolite"). The sepiolite used in this work originates from Vallecas (Spain). Chemical analysis (~): SiO/55.5; AlzO 3 3.25; FezO3 1.4; MgO 21.75; CaO 0.7; K 2 0 1.2; N a 2 0 0.6; H 2 0 (weight loss from 20~ to 600~ 15.15. Specific surface area (B.E.T., N2): 276m2/g. Granulometric fraction used: less than 200 mesh. Some tests have been carried out starting with chrysotile (Si2OsMg(OH),0 and kaolinite (Si2OsA12(OH)4). The chrysotile came from Vancouver (Canada) and is the Cassiar A. K. type; the surface area is 14m2/g The kaolinite came from Zettlitz (Czechoslovakia); fraction used: 1 to 2/~m. Here the B.E.T. surface area is 18 mi/g.
Orqanochlorosilanes The following reagents were used: trimethylchlorosilane: (CH3)3SiCl, methylvinyldichlorosilane: (CH3) (CHe~--------CH)SiC12 and allyldimethylchlorosilane: (CH3)i(CH2~------CH---CHa)SiC1. These reagents, supplied by Fluka, were used in solutions of anhydrous analytical grade organic solvents, supplied by Merck.
Experimental The mineral is treated with the solution of an organochlorosilane in an anhydrous or~,anic solvent.
The mixture is stirred continuously for 30min at 20~ to assure a homogeneous dispersion. The resulting solid is recovered by centrifugation and it is washed with methanol, in order to remove MgCI2 which can be formed in the course of the reaction. These washings are continued until the C1- ions are no longer detected. The solid is then dried at ll0~ for 24 hr. Some reactions have been carried out in the presence of excess organochlorosilane. The quantities used in these cases are: Mineral: 5.00g.; Solvent: 100 ml; Organochlorosilane: 3.00 ml. This amount of organochlorosilane corresponds to the "plateau" in Fig. 2 (both for the amount of organic matter grafted and for magnesium extracted).
E
0_
(a)
.,
..
g~e,
o.---
g, O 0~
40 o~ 20
E
j
w
I
I
I
I00 200 300 mmoLes of o r g a n o c h l o r o s i l a n e
I
400 500 added/lOOg
Fig. 2. Variation in the quantity of magnesium extracted (a) and in the grafted organic groups (b) as a function of the amount of the organochlorosilane added to sepiolite in petroleum ether at 20~ 9 Trimethylchlorosilane 9 Methylvinyldichlorosilane.
27
The surface of silicates in organic solvents A series of experiments has been carried out starting with sepiolites with varying degrees of hydratation which result from heating in air for various times. For the samples pretreated at ll0~ a 200ml. PVC container was used which could be hermetically sealed. After determining the weight loss, the organochlorosilane dissolved in petroleum ether (B.P, 50-70~ Merck) was added to the dehydrated sepiolite. Some samples were pretreated at 500~ the water adsorbed physically during cooling of the samples was eliminated by heating at ll0~ Experiments have been carried out in the presence of excess water on the mineral surface; the addition of water was made in a dispersion of the silicate (sepiolite, chrysotile) in dry petroleum ether; agitation was continued for some minutes before the addition of the methylvinyldichlorosilane in the same organic solvent.
(per 100g sepiolite) the quantity of magnesium extracted is constant (120 m-equiv./100 g). On the other hand, when the reactions are carried out with methylvinyldichlorosilane, the maximum amount of magnesium (about 100m-equiv./100 g) is extracted when around 100mmoles of the reagent are added, but when more than this quantity is added, the extraction is less, being only about 60m-equiv./100 g, while the amount of material grafted continues to increase up to about 65 mmoles/100 g. Difunctional organochlorosilanes may be hydrolyzed totally (equation 1) or partially (equation 2) following the amount of water present (Noll, 1968): CH 3
CH 3
Cl- i-Cl
+.Cl
.).
Chemical titrations and physical determinations The magnesium cations extracted during the course of the grafting reaction with sepiolite and chrysotile were determined from the methanol washings by E.D.T.A. complexometry. The percentage of carbon (and chlorine in some cases) was determined after total rehydrafion of the samples. The i.r. spectra were obtained from two double beam instruments (Beckman IR 12 and Perkin Elmer 180). The samples were dispersed in KBr pellets. Specific surface measurements (B.E.T., N2) were carried out in the apparatus described by Cahen et al. (1965); the samples were previously degassed for 24 hours (10 .3 Torr at 100~ RESULTS AND DISCUSSION
Synthesis of the organic derivatives of sepiolite The surface of the sepiolite is covered very quickly by the organic groups. The products after 5 min and 120min of reaction showed identical i.r. spectra; the chemical composition of the two samples also was identical. The reaction was terminated after a short time of contact between the reagent and mineral no matter what organochlorosilane was used. However certain organic solvents can reduce the extent of the grafting process, because of reaction with ~Si-C1 groups of the reagent. Such is the case for those syntheses carried out in the presence of alcohols. On the other hand, the use of inert hydrocarbon solvents, diethylether and carbon tetrachloride always leads to organic derivatives of similar composition. The extraction of magnesium is also diminished when the reactions are performed with alcohols present. The amounts of material grafted and of magnesium extracted are plotted in Fig. 2 as a function of the amount of the organochlorosilane added. It may be seen that the extraction of magnesium differs appreciably according to the mono- or di-functional character of the silane. Indeed, for additions of trimethylchlorosilane greater than around 240mmoles
Cl--~i--C'
.:o> Cl~- ~ i 2 0
~i--C1
+ HC' (2)
In either case "molecular" hydrogen chloride is formed, and for the same amount of water, reaction 2 yields double the HC1 as compared to reaction 1. If it is admitted that water filling the micropores of the silicate is not accessible for hydrolysis, then it seems reasonable to affirm that the addition of less than 100mmoles of methylvinyldichlorosilane (per 100 g sepiolite) gives rise to formation of methylvinylsilanediol via equation (1), which will be grafted immediately in monomeric form, onto the silanol groups of the mineral surface. By contrast when more than 100 mmoles of methylvinyldichlorosilane (per 100 g) are added, the hydrolysis will proceed according to 30C
\ Z F~
20C
m
toi 0 Grafted
I
20
I
40
species,
I
60
80
mmoles /lOOg
Fig. 3. Variation in the specific surface area of the vinyl derivatives of sepiolite as a function of the number of (CH3) (CHz~------CH)SiO groups grafted/100 g mineral. Synthesis carried out in petroleum ether at 20~
28
E. RUIz-HITZKYand J. J. FRIPIAT
equation (2) and the reagent will be grafted in polymeric form as polymethylvinylsiloxyl groups. The bulkiness of these polymeric grafted species is such that some of the micropores of the mineral are blocked as shown by the decrease in specific surface area measured by nitrogen adsorption (Fig. 3): Since, according to these interpretations, the addition of 100mmoles of methylvinyldichlorosilane (per 100 g) leads to the grafting of around 45 mmoles of methylvinylsiloxyl groups in "monolayers", it is easy to calculate that the surface of the covered mineral is of the order of 175 m2/g (each grafted species covers 65 A2). This value is of the same order of magnitude as the specific surface of organomineral derivatives measured by nitrogen adsorption (176 m2/g). Further240
more, by comparing the quantities of material grafted in the presence of excess methylvinyldichlorosilane, one can easily deduce that the average number of fragments which constitute the polysiloxanyl grafted species is about 1.7. Other types of grafting experiments with trimethylchlorosilane and methylvinyldichlorosilane have been tried starting with sepiolites in which the amount of water is varied either by heating or by the addition of water. Because sepiolite is hydrophylic, the water becomes an interphase between the organic solvent and the external surface of the mineral. These experiments show (Fig. 4) that the extraction of magnesium increases as a function of the amount of water in the sepiolite, but that the
I
I (a)
Water extracted
I
Wateradded
I 0 0
9
I ze~ I i f
180
/
/
~)
/,/
~20
4-X
O~
60 EY
E 0 0 0
(b)
9
O
I I
6C
u
I I
ql
40D
"3
0~0
I 2C--
o j O -o-~
-20
-I0
I
I
9
S E E
II
Water content,
0
I
I0 % ~n w e i g h t
Fig. 4. Variation in the quantity of magnesium extracted lal and in the grafted organic groups (b) as a function of the relative amount of water in the sepiolite. Experiments carried out in petroleum ether at 20~ 10% is the hydratation state of natural sepiolite). 9 Trimethylchlorosllane. 9 Methylvinyldichlorosilane.
20
The surface of silicates in organic solvents quantity of grafted organic matter becomes constant for relative amounts of water greater than -7~o (by weight). Even when the synthesis is attempted starting with anhydrous sepiolite (pretreated at 500~ the fixing of methylvinylsiloxy groups (30 mmoles/100 g) occurs only with silanol groups already present on the surface of the mineral, probably according to reaction
(3):
29
io, ,o
c
(lOx)
o
~.Si--OH Minera
C1 +
~--~Si--OH ~Si--O Minera ~Si--O
Si C#
\CH~-----CH2 CH 3
\/ /\
,D
CH 3
\/
-
Si
+ 2HCI
I
I
I
I
3600 34oo 3200 3000 (31
CH~-------CH:
On the basis of this equation, the grafting occurs in monomeric units, with the grafted species covering a surface area of 117m2/g. This value is in good agreement with the surface area of anhydrous sepiolite as determined by nitrogen adsorption (120-140 m2/g). It is thus shown that the grafting in the total absence of water occurs in monolayers even in the presence of excess difunctional organochlorosilane. For the same type of experiments beginning with trimethylchlorosilane, approximately 13mmoles of trimethylsilyl groups (per 100 g) can be grafted. This value, which is lower than in the case of methylvinyldichlorosilane, supports the idea that blocking is more important with the (CH3)3Si= species as compared to the > Si(CH3) (CH--CH2) species.
Characterization and properties of the organic derivatives of sepiolite The X-ray powder diffraction diagrams of the organomineral derivatives described in the preceding paragraph do not show any modification as compared to the diagrams of the corresponding starting mineral. Furthermore electron microscope observation shows that the sepiolite fibers treated with organochlorosilane retain the same morphology as the starting sepiolite. These observations agree with the interpretation that only the external surface of the mineral (that which is accesible to the reagent) is involved in the grafting reaction. The presence of the S%CH3 groups grafted to the sepiolite, is revealed by i.r. absorption bands at 2970 and 1275 crn- 1. The vinyl group is indicated by bands at 3058 cm -1, v(C-H) of ~ C H 2 ; 3020cm 1, v(C-H) o f - C H - - ; and 1598 cm -1 v(C C) of S~CH--CI-I z. Also noticeable (Fig. 5) is the development of a band at 2932 cm -1 which is assigned to the addition of an HC1 molecule to the vinyl double bond. This agrees with the chlorine content of the samples. These
II
I
iroo
,
L
I~oo
,
I
,300
cm q
Fig. 5. I.r. spectra of the starting sepiolite (A) and of the methylvinyl derivative of sepiolite (B) synthesized with an excess of methylvinyldichlorosilane dissolved in petroleum ether. reactions are a secondary phenomenon (17~ of the Si-CH--CH2 being so reacted). For synthesis carried out with allyldimethylchlorosilane, no absorption typical of the allyl group can be seen; only absorptions characteristic of the -CH3 and -CH 2 groups are observed. By chlorine analysis, 80~o of this reaction is HC1 addition, but this reaction can be avoided by adding an organic base to the reaction mixture which controls the quantity of HC1 present. The unsaturated grafted Si-CH--CH2 groups give the mineral surface a reactivity characteristic of these groups. Thus it is possible to add bromine and osmium tetroxide to the vinyl derivatives of the sepiolite. The yield for the bromine addition is on the order of 50~o, while the yield of the OsO~ addition is 70~o. It may be possible that a fraction of the vinyl groups is located in narrow pores; this could explain why not all of them react with either bromine or OsO4. The i.r. spectra of the resulting products show a decrease in the absorption bands due to double bonds, and at the same time an absorption band is observed at 2932 cm 1 due to v(C-H) o f _ C H 2 . The addition of a hea~2r atom such as osmium allows one to determine the arrangement of the vinyl groups on the sepiolite fibers by electron microscope observation (Barrios-Neira et al.. 1974).
Attempted synthesis with other silicates If the area available to the reagents determines the quantity of material grafted, then experiments conducted with chrysotile and kaolinite should lead to derivatives with only very small numbers of methyl and vinyl groups. In these minerals, as compared to sepiolite, the quantity of accessible silanol groups is relatively small. The reaction of trimethylchlorosilane and methylvinyldichlorosilane with kaolinite produce derivatives containing about 1~, carbon. Even when reactions were conducted under reflux conditions for
30
E. RUIZ-HITZKYand J. J. FRIPIAT Table 1. Effect of adding water during the reaction of chrysotile with methylvinyldichlorosilane in petroleum ether. Results corrected to 100 g of mineral H20 added (% in weight)
Mg 2+ extracted (m-equiv.)
Species grafted (mmoles)
0 2 5 10 15
54 70 102 114 156
3 4.5 5 10 8
a long time (up to 100 hr) similar amounts of grafting were always attained. Comparable results are obtained when starting with chrysotile, although it is possible to force the reaction by the addition of water to the reaction medium. Four grafting reactions were carried out with excess methylvinyldichlorosilane dissolved in petroleum ether in the presence of 2, 5, 10 and 15~o by weight of water. The number of methylvinylsiloxy groups grafted (Table 1) increased regularly as a function of the amount of water present. At the same time, there was an increase in the magnesium extracted. The situation here is quite different from the grafting of sepiolite under the same experimental conditions. Actually at the start of the reaction, chrysotile does not have accessible S i - O H functions on its surface. In order to make these groups available, it is necessary to eliminate at least the first magnesium layer which is exposed on the external surface of the fibers. According to the results of Pundsack and Reimschussel (1956) it is possible to calculate that about 140m-equiv. HC1 (per 100g chrysotile) are needed to extract all of the Mg 2+ cations in this layer. The Mg 2+ cations are extracted on the addition of water to the reaction mixture because of the formation of HC1 resulting from the hydrolysis of the chlorosilane. This can explain the relation between the removal of a fraction of the magnesium layer and the amount of material grafted. CONCLUSIONS 1. The amount of organic matter grafted during the reaction of organochlorosilanes with superficial silanol groups of silicates depends on the extent of the external surface exposed to the reagents and on the number of accessible silanol sites. 2. The reaction of a difunctional reagent such as methylvinyldichlorosilane leads to a grafted polymeric species w h e n the quantity of hydration water on the surface is not sufficient to assure complete hydrolysis of the reagent. 3. The reaction of difunctional reagents in the total absence of hydration water (i.e. starting with "anhydrous sepiolite'?) results in a monolayer, probably ,by the direct grafting of the reagent to the Si O H pairs located on the surface, 4. With silicates whose external layers are not covered by S i - O H groups but rather by hydroxyls bonded to other cations (magnesium in the case of
chrysotile), one can expose the silanol groups and force the grafting reaction by adding water to the reaction mixture. Acknowledgement~Helpful discussions with, and criticism by Dr. A. Van Meerbeek are gratefully acknowledged. The authors are also grateful to Dr. M. della Faille, Director of Research and Development of Eternit S.A. (Kapelle op den Bos, Belgium) for his financial and technical help, and to Mr. H. Seto for reviewing the manuscript. REFERENCES
Aragdn de la Cruz, F., Esteban, J. and Vitdn, C. (1972) Interaction of chlorosilanes with montmorillonite and vermiculite: Proc. lnt. Clay Conf, Madrid 1972, pp. 705 710. Barrios-Neira, J., Rodrique, L. and Ruiz-Hitzky, E. (1974) Mise en 6vidence de groupements organiques insatur+s greffds sur la s6piolite: J. Microscopie 20, 295 298. Berger, G. (1941) The structure of montmorillonite: Chem. Weekblad 38, 42~43. Brauner, K. and Preisinger, A. (1956)Structure of sepiolite: Miner. Petr. Mitt. 6, 120-140. Brown, G., Greene-Kelly, R. and Norrish, K. (1952) Organic derivatives of montmorillonite: Clay Min. Bull. 1 (7), 214-220. Cahen, G., Marechal, J. E. M., della Faille, M. and Fripiat, J. J. (1965) Pore size distribution by a rapid continuous flux method: Anal. Chem. 37, 133-137. Deuel, H., Huber, G. and Iberg, R. (1950) Organische Derirate von Tonmineralien: Helv. Chim. Acta 33, 1229-1232. Edwards, H. (1970) Study of the reactions of surface hydroxyl groups of a chrysotile asbestos with organic silanes by means of i.r. spectroscopy: J. appl. Chem. 20, 76-79. Frazier, S. E., Bedford, J. A., Hower, J. and Kenney, M. E. (1967) An inherently fibrous polymer: lnorg. Chem. 6, 1693 1696. Fripiat, J. J. and Mendelovici, E. (1968) D6riv6s organiques des silicates--I: Le d6riv~ m6thyl6 du chrysotile: Bull. Soc. Chim. pp. 483492. Gieseking, J. E. (1949) The clay minerals in soils: Advan. Agron. 1, 59-204. Greenland, D. J. and Russell, E. W. (1955) Organoclay derivatives and the origin of the negative charge on clay particles: Trans. Farad. Soc. 51, 1301~t307. Lentz, C. W. (1964) Silicate minerals as sources of trimethylsilyl silicates and silicate structure analysis of sodium silicate solutions: lnorg. Chem. 3, 574 579. Noll, W. (1968) Chemistry and Technology of Silicones: Academic, New York. Pundsack, F. L. and Reimschussel, L. (1956) The properties of asbestos--III: Basicity of chrysotile suspensions: J. Phys. Chem. 60, 1218 1222. Zapata, L., Castelein, J., Mercier, J. P. and Fripiat, J. J. (1972) D6riv6s organiques des silicates II: Les d6riv6s vinyliques et allyliques du chrysotile et de la vermiculite: Bull. Soc. Chim. pp. 54-63.
