Langmuir 1993,9, 716-722
716
Interactions of Chlorosilanes with a Silica Surface Catalyzed by Amines V. M. Gun’ko,* E. F. Voronin, E. M. Pakhlov, and A. A. Chuiko Institute of Surface Chemistry, Kiev 252650, Ukraine Received November 14,1991. In Final Form: September 10,1992 Experimental and theoretical investigations of trimethylchlorosilane (TMCS) interactions with E S i O H groups catalyzed by NR3 (R = C~HS, CH3) have been conducted using Aerosil samples. The potential energy surface (PES) profiles along the reaction pathway were calculated in a cluster approach by the MNDO/H and AM1 methods. The comparison with ClSiH3 reactions showed that the formation of fourcentered stabilized prereaction complexes is possible unlike TMCS reactions with S S i O H groups. The catalytic effect was caused by the enthalpy factor of the rate constants predominantly. Noncatalytic reaction rate constants were calculated and compared with experimental data.
Introduction Effects, analogous to those observed in solutions, have
group (IR absorption band maximum is 3749 cm-’) concentration (COH)on the surface was calculated by a kinetics methoda8TMCS and TEA (Spectranalyzed grade) were subjected to three freezebeen observed at gas-phase catalytic modifications of pump-thaw cycles before being used. dispersed silica surfaces. So, alkylchlorosilanes which Apparatus. Aerosil samples of 30 mg were pressed in thin interact with silica surfaces at temperatures above 570 K plates (8 X 22 mm) with good transmittance in the IR range substitute H atoms in silanol groups at catalysta by 2000-4000 cm-1. The experiments were carried out with a triethylamine (TEA) practically instantaneously at T = combinedspectral-gravimetric quartz dish permanently installed 300 K.l Similar rapid reactions were observed with on the spectrometer UR-20 (Carl Zeiss), which permitted determination of a sorption value and recording of IR spectra. alcoholization,fluorination, and hydrolysis on silica surface Moreover, transference dishes with CaF2 windows were wed, catalyzed by electron-donor compounds: amines, ethers, and in this case IR spectra were obtained with higher quality. alcohols, ketones, HzO, HC1.2 The Si02 samples were heated above 870 K (T,)at 1W Torr for The problem of catalytic effect mechanisms by electronseveralhours for removal of adsorbed water. After that the Aerosil donor compounds at chemosorption processes remains surface contained isolated =SOH groups (COH= 0.6 mmol/g) disputable. There exists t h e opinion that in electrophilic with an average distance between groups above 0.6 nm.7 substitution reactions (SEi) (for example, haloidGravimetric measurements were carried out at 308 and 328 K silanes with silica) the formation of hydrogen bonds ( Tr). In the second case the samples were heated by IR radiation di-OH-XY with electron-donor molecules XY leads of the spectrometer. to polarization of the 0-H bonds and increases its ability Sorption. TMCS is physically adsorbed on the dehydrated to heterolytic splitting of the Si-Hal bond in adsorbate. Aerosil surface and its vapors are removed completely from a At a limitingreaction stage,the sorption complex included surface by freezing-out with liquid nitrogen. TEA is sorbed a silanol group, haloidsilane,and electron-donor molecule, considerably stronger, and after evacuation at 10-1 Torr it is and it may be considered an intermediate of the SEi retained on an Aerosil surface in an amount only somewhat reaction.lP2 This reaction was studied on example of the yielding to concentration of silanol groups 0.4-0.5 mmollg. system TMCS sorption on the Aerosil surface with preadsorbed TEA differs considerably from adsorption on pure silica (Figure 1). =SiOH ClSi(CH,), N(C,H,), (1) The TMCS interaction with such Aerosil surfaces (with TEA) has a specific character.$ There is a set of absorption bands in Noncatalytic reactions the range of 2400-2800 cm-l in the IR spectrum of Aerosil which contains adsorbed TEA and TMCS molecules (Figure 2). This SSiOH X,Si(CH,),, =SiOSiXn-1(CH3)Pn HX set is typical of compounds R3NH+.l0 At degassing to 10-3 Torr the amount of TMCS that remains on the surface is equal X = C1, OCH,; n = 1-3 approximatelyto the quantity of preliminarilysorbed TEA (Table I). The surface complexes formed are resistant in vacuo at room were studied by experimental methods,,-, but activation temperature and begin to decompose slowly when heated above energies obtained were strongly different and the investigations of reaction mechanism (1) were s ~ a n t y . ~ ~ ~ ~ 6 J350 K, with a noticeable rate at 380-390K. In this case a considerable decrease of the absorption band intensity of silanol groups in the IR spectrum occurs in the region 3750 cm-’, while Experimental Section absorption of differentiating methyl groups in the range of 2800Materials. Dispersed pyrogenic SiOzAerosil (A-300,gg.g% ) 3000 cm-I remains (Figure 2). That means that reaction was obtained from PU “Chlorovinil” (Kalush), and its BET surface area of 300 m2/gwas measured by using nitrogen. Silanol N(C2Hsh =SiOH + ClSi(CH,), =SiOSi(CH3), HC1 (2) * Author to whom correspondence should be addressed. (1) Tertykh, V. A.; Pavlov, V. V.; Vatamanyuk, V. I. Adsorpt. Adsorb. proceeds. It is possible to conclude from Table I that the ratio 1976,4, 57. between values of adsorbed TEA, TMCS, and =SiOSi(CH3)3 (2) Voronin, E. F.; Tertykh, V. A.; Ogenko, V. M.; Chuiko, A. A. Teor. groups is equal to approximately 1:1:1and remains at different Eksp. Khim. 1978, 14,658. (3) Evans, B.; White, T. E. J. Catal. 1968, 11, 336. coverages. Hence, the adsorption complexes including =SiOH (4) Hair, M. L.; Hertle, W. J. Phys. Chem. 1969, 73, 2372. ( 5 ) Bascom, W. D.; Timmons, R. B. J. Phys. Chem. 1972, 76, 3192. (6) Tertykh, V. A.; Belyakova, L. A.; Varvarin, A. M. Reakt. Kinet. (8) Pakhlov, E. M.; Pluto, Yu. V.; Voronin, E. F.; Cbuiko, A. A. Teor.
+
+
+
-
+
-
Catal. Lett. 1989, 40, 151. (7) Tertykh, V. A.; Belyakova, L. A. Chemical Reactions with Participation of Silica Surface; Naukova Dumka: Kiev, 1991.
+
Eksp. Khim. 1987, 23, 252. (9) Kiselev, A. V.; Lygin, V. I. IR Spectra of Surface Compounds and Adsorbed Substances; Nauka: Moscow, 1972.
0743-746319312409-0716$04.00/0 0 1993 American Chemical Society
Chlorosilane Interactions with Silica
Langmuir, Vol. 9, No. 3,1993 717
a ,m o l / g
Table I. Amounts of Strongly Sorbed Molecules of TEA,
I
TMCS,a n d Trimethylsilyl Groups on Dehydrated Aerosil
1
Surface (mmol/g)
TMS TEA TMCS groups
1.o
1 2 3a 3b 3c
0.8
0.6
0.4
0.2
20
60
40
80
P, Tor
Figure 1. Isotherms of TEA (1,2)and TMCS (3,4)adsorption on the surface of initial Aerosil and those of TMCS with preadsorbed TEA (0.53mol/g of TEA) (5,6) at 308 (dark points) and 320 K (light points).
0.40 0.53 0.32 0.18 0.21
0.43
0.39 0.45 0.35 0.10 0.06
0.44
0.34 0.09 0.10
notes dehydration a t 1020 K dehydrationat870K dehydration a t 870 K Aa after first repeated sorption Aa after second repeated sorption
out from the PES calculations. Unlike simple molecules for which calculations may be carried out by ab initio methods,10-16 the system (1) sizes permit using only semiempirical methods for which applications to complicated reactions were warranted as the obtained results, for example by the AM1 method, were satisfactory.18 Study of the reaction mechanism 2 was performed by the MNDO/H m e t h ~ d ' and ~ , ~reaction ~ mechanism 3 by AM1.21 A silica surface fragment had been simulated by cluster (O*3Si0)3Si-OH, where O* were one-electron oxygen pseudoatoms.22The PES profiles were calculated by the reaction coordinate (A) method. At differentprofiles of the reaction pathway either ?'H...Cl or Q,i+ were chosen as X and other geometrical parameters were optimized. A step on X accounted to 0.005-0.01 nm. In some cases the hit into a transition state (TS) was controlled by Hessian eigenvalues. The role of NR3 catalyst in eqs 2 and 3 may be of two k i n d s : ' ~ ~firstly, ~ ? ~ ~it is an increase of the proton mobility in a silanol group, Le., a decrease of the H+transfer barrier (E*H+); secondly,it is an increaseof 0 atom electron-donor properties influencing on stabilization of S i c 0 donoracceptor bond in the prereaction complex and in TS
I
/$\ 4A
I
/si\ 48
The structure of the pentacoordinate (Si) complex 4A is analogous for oxysilicates, but the stabilization energy of 4A is less than at formation anionic Surface pentacoordinate complexes (with Si atom in (10)Gordon, A. I.; Ford, R. A. The Chemist's Companion; J. Wiley: New York. 1972. (11)Tachibana, A.;Fueno, H.; Kurosaki, V.;Yamabe, T. J.Am. Chem. 1989,111,806. (12)Gronert, S.;Glaser, R.; Streitwieser, A. J. Am. Chem. SOC.1989, 111,3111. (13)Sini, G.; Ohanessian,G.; Hiberty, P. C.; Shaik, S. S. J.Am. Chem. SOC.1990,112,1407. (14)Davis, L.P.;Burggraf, L. W.; Gordon, M. S.; Baldridge, K. K. J. Am. Chem. SOC.1985,107,4415. (15)Davis, L.P.;Burggraf, L. W.; Gordon, M. S. J. Am. Chem. SOC. 1988,110,3056. (16)Damraner, R.;Burggraf, L. W.; Davis, L. P.; Gordon, M. S. J.Am. Chem. SOC.1988,110,6601. (17) Boudin, A. Bull. Chem. SOC.Jpn. 1988,61, 101. (18)Dewar, M.J. S. Znt. J. Quantum Chem. Symp. 1988,22,557. (19)Dewar, M.J. S.; Thiel, W. J. Am. Chem. SOC.1977,99,4899. (20)Burshtein, K. Ya.; Isaev, A. N. Zh. Struct. Khim. 1986,27,3. (21)Dewar, M.3. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. SOC.1986,107,3902. (22)Gorlov, Yu. I.; Zayata, V. A.; Chuiko, A. A. Teor. Eksp. Khim. 1986,22,533. (23)Gragerov, I. P.;Pogorely, V. K.; Franchuk, I. F. Hydrogen Bond and Rapid Proton Exchange; Naukova Dumb. Kiev, 1978. (24)Hydrogen Bond; Sokolov, N. D., Ed.; Nauka: Moscow, 1981. (25)Johnson, S. E.; Deiters, J. A.; Day, R. 0.;Holmes, R. R. J. Am. Chem. SOC.1989,111,3250. (26)Swamy, K. C. K.; Chandrasekhar, V.; Harland, J. J.; Holmes, J. M.;Day, R. 0.;Holmes, R. R. J. Am. Chem. SOC.1990,112,2341. SOC.
I
y , om-' Figure 2. IR spectra of Aerosil evacuated at 970 K (1) after adsorption (2)and desorption (3)of TEA at 320 K, sorption of TMCS (4)and evacuation at 320 (5)and 720 K (6). 2400
3000
3600
groups, TEA, and TMCS suffer further transformations on heating with almost complete transition on reaction products
(2).
Results and Discussion Due to complexityof the system (l), quantum chemical calculations were simplified: N(CH3)3 (TMA) was considered instead of TEA. Besides (2), the reaction
-
NH3
4 i O H + ClSiH, ZSiOSiH, + HC1 (3) was studied. The reaction mechanism analysis was carried
718 Langmuir,Vol. 9, No.3,1993
Gun'ko et al.
Table 11. Electronic and Structure Parameters of the Clusters and Comdexes ~
1 2 3 4
(O*3SiO)Si(l,OH ClSi(z)(CH3)3 =+%OH. .ClSi(CH&
"H3,
eSiOH 1
by
MNDO/H.
~
~~~
ELUMO -0.45 -0.88 -0.09 -0.24
-5.8 -15.4
-EnoMo 11.02 11.37 10.92 10.46
122.6
9.86
-1.12
0.673
0.325
0.1197
33.8 -12.5 226.8
10.44 10.18 9.77 12.07
-0.35 -0.21 0.32 -1.27
0.816 0.572 0.731
0.235 0.302
0.0956 0.1411
u
t
0.544
0.197
ron 0.0937
0.568 0.565
0.224 0.234
0.0963 0.0962
0.390 0.391 0.402
0.709 0.721 0.717
0.465
0.858
9n
-90
-9c1
QSli
'CISi(CH,),
.N(CH,), :..':cl
5
=.so
*A
:kS,.* (CH33
6 7 8 9 0
=SiOSi(CH3)3 =SiOH. 'N(CH3)3 -io-. .H+N(CH3)3 ClSiH3
0.902 0.358
0.738
utis a variation of total energy relative t o reagents (kJ/mol), EHOMO and ELUMO are energies of boundary orbitals (eV), q is atom charge,
r is bond length (nm).
=SiOH groups) are formed more easily than 4A2' but correspond to the SNi(Si) reactions. Besides, the
donor-acceptor complexes28are more stabilized than the R Si
0 ' '
= r0 exp(Q/RT) where T~ = hfJkBTfg= Tolfa/fg, r0' =
'XY
in which the Si has weaker electron-acceptor properties than the Si in a 4 i O H group, as the S i 4 bond is less ionic than the S i 4 bond.29 The formation of H complexes
....
d i O H ClSiR,, *iOH....NR,
5A
show that Qst with TMCS adsorption on silica gel at a coverage B =0.5 is 33-38 kJ/mol. So,NR3 adsorption heat is conditioned mainly by specific interactions in H complexes (-AHf > 2/3Qst);in the case of TMCS it is a result of dispersion and weak electrostatic interactions (-AHf 5 l/3QSt),i.e., physical adsorption. A 5B adsorbed state lifetime at indoor temperature is TTMA = le5 s according to works39240 using
(5)
5B
is possible depending on the priority of chlorosilane or catalyst molecule interactions with active surface centers. In 5B the H bond is stronger (Table 11),but the energy obtained is lower than the experimental or that calculated by ab initio methods for analogous hydrogen bonds.3G37 The wavenumber VOH decrease in 5B at R = CH3 is about 990 ~ m - ~ , 3 ~ * ~at ~w R h=iHl eit is somewhat lese: 675~m-l.3~ For 5B at AVOH= 990 cm-l, enthalpy -AHf = 40 kJ/mol according to empirical dependen~ies.~~ This value agrees with the results of nonempirical calculation~30~~~ and data.24*36 A shift of AVOHin 5A is not more than 100cm-1 and -AHTMCS= 10 kJ/mol, but Qst = 32 kJ/mol. Data37 (27)Low, M. J. D. J. Phys. Chem. 1981,85,3543. (28)Alpatova, N. M.; Kessler, Yu. M. J. Strukt. Khim.1964,5,332. (29)Luo, Y. R.;Benson, S. W. J.Phys. Chem. 1989,93,3791. (30)Hobza, P.; Zahradnik, R. Intermolecular Complexes; Prague Academia: Prague, 1988. (31)Del Bene, J. E. J. Phys. Chem. 1988,92,2874. (32)Ugliengo, P.; Saunders, V. P.; Carrone, E. Surf. Sci. 1989,224, 498. (33)Davydov, V.Ya.; Kiselev, A. V.; Kuznetsov, B. V. Zh.Fiz. Khim. 1965,39,2058. (34)Platonov, V.V.;Tretijanov, N. E.; Filimonov,V. N. Usp.Fotoniki 1971,2,92. (35)Bunker, B. C.; Haaland, D. M.; Michalske, T. A.; Smith, W. L. Surf. Sci. 1989,222, 95. (36)Tsutsumi, K.; Emori, H.; Takahashi, H. Bull. Chem. SOC.Jpn. 1975,48,2613. (37)Kiselev, A. V.;Kuznetsov, B. V.; Lapin, S. N. Kolloidn. Zh. 1976, 38,158. (38)Hertl, W.;Hair, M. L. J. Phys. Chem. 1968,72,4676. (39)Adamson, A. W.Physical Chemistry of Surfaces; J. Wiley: New York, 1976. (40)Jaycock, M. J.; Parfitt, G. Chemistry oflnterface;Ellis Horwood Chichester, 1981. (41)Brei, V. V.;Chuiko, A. A. Teor. Khim. 1989,25,99.
(6) 8, and f, and fa are statistical sums of molecules in gas phase and in adsorbed state. For SA TTMCS = 10-8-10-" s. The TMCS diffusion data41(obtained by the NMR impulse gradient method) show that TTMCS = lo-" s (T= 300 K),Le., TTMA
>> TTMCS.
A t 5B formation the H and 0 atom charges increase (Table 11, qH, qo),the 0-H bond is weaker: the Wiberg's index (WOH)decreases by 0.04, Le., acidity of the d i O H group and proton mobility increase. These parameters grow considerably higher with H+transfer *iOH0**N(CH3)3 a =SiO-....H+N(CH3)3
7A
(7)
7B
In 7B: Aqo = -0.16, AqH = 0.06,and AWOH= -0.40. Activation energy in eq 7 is high E*H+= 227 kJ/mol but it is 6 times less than in the process eSiOH +
+ H+
(8) A global minimum of PES (7)corresponds to the complex 7A, which is stipulated by a solid-gas boundary character and agrees with spectral data (Figure 2, no. 2 and 3): absorption bands Correspondmainly to the molecular form of TEA adsorption, though a shoulder in the range of 25002700 cm-l (especially without degassing: Figure 2,no. 2) may embracetypicallines of R3NH+,but with low intensity (compareFigure 2,no. 3 and 4). So, variations in (2)and (3) mechanisms do not depend on the appearance of a long-lived prereaction (7B)structure. That is why the TS structure with H+transfer, a limiting stage in (2)and (3) may be presented as a cyclic perturbed complex 4B
I
/Ti 9A
I
/:\ 9B
Chlorosilane Interactions with Silica
In this case a H+ trajectory in (9)may vary considerably relative to (4), e.g. an overbarrier or tunnel H+ transfer to NR3 and then to C1 is possible, as N basicity in NR3 is higher than that of C1 in a Si-C1 bond, but lower than in a Si-Cl- bond. A H+ return to 0 is less probable than return to C1 as redundant electron density with H+ detachment shifts instantaneously toward 06-+Si+C16-, Le., it is located on C1 and a bond similar to Si-C1- is created. As a result of the TTMA >> TTMCS condition the following reaction scheme is the most probable:
Langmuir, Vol. 9, No. 3, 1993 719 A Et
, kJ/mol I R - H
100 ,"\
50
A\.
products
-50
V
'I,
\
R
K\\
producta
-jt\;--\--v, , \ 0.12
0.16
0.2
-
I1 CH3
0.3
reagents
% 0.5
'
rOSi ' nm m
rHCl '
-50
The process according to (9)or (10)may be provided by an order of a reagent or a catalyst leak-in only at low coverage, when the reaction is limited by bimolecular complex (10A)formation,i.e., by diffusion. But structures of TS (9B and 1OC) coincide; besides, the TEA preadsorption was used in the experiments. Hence, the main condition of the reaction occurring according to scheme 10 is prereaction complex (10A or 10B) formation (depending on the TMCS and NR3 leak-in order) and its lifetime ( T ~ is) to be considerably higher than the time of H+ t r a n ~ f e r ~ ~ , ~ ~
Figure 3. PES sections according to the reaction coordinate. I is reaction 3 by standardmethod of the reactioncoordinate(dash line) and modified method (solid line); I1 is reaction 2 with participation of TMS (solid line) and reaction 19 at R = CH3
(dash line).
with motion in TS the O+SiTMCS bond contribution into hEt grows slower than destabilizing components. This circumstance intensifies difficulties occurring in the noncatalytic reaction (2) and it is the reason that Tr is above 500 K. However, differences in the CS and TMCS reactivity are induced not only by different strain contributions into E* (a steric factor when substituting H by CH3) but also by higher acceptor properties of Si in CS (Table 11,ELUMO, Qsi,--,p), which provides higher *Si bond T , = min(TTMcs,Tbim) >> T H + = 10-l~s (11) strength in the prereaction complex and TS. where 7bim is a bimolecular complex (10A)lifetime, i.e., CS complex (5Aand 10A)formation proceeds with an with NR3 preadsorption. The lifetime 7bim may be increase of qsics (Table 111, q s i c s z ) and a decrease of the represented 88 maX(T,Pij,Tjpji),where Pij probability of acceptor level localized on Sics ( ~ L U M O ) . This leads to an inelastic interaction of molecule j with an adsorption increase of the O+Sics binding energy in (4)and 10B.In complex of molecule i. With allowance for the above Tbim 4A maxima of the local density of electronic states (LDS)46 = TNR$"R{,TMCS. This condition corresponds to the of Sics are nearer to forbidden zone boundaries than Si reaction occurring by the LangmuirHinshelwood mechin the silanol group (Figure 4). In reaction 2, QTMCS also anism391~~ with more reliability at lower temperature. With grows in the mentioned complexes (Table 11, qsiz,cs). In Tr increase above 500 K, the upper bound of values TTMCS TS (4B)(R = H) the shoulder A'of C1-LDS shifts toward SlO-l3 s (with allowance for a T~ decrease in (6)),which is a forbidden zone (Figure 4 and Tables I1 and 111,EHOMO) comparablewith TH+. The TTMS decreasesby severalorders and electrostatic and donor-acceptor interactions of C1 at T growth to 500 K. Thus, the probability of the reaction with H increase. But mixed electron states of C1 and H occurring by the Eley-Rideal collision m e c h a n i ~ m ~in~ ~ " are located deeper (Figure 4,peak B), which is provided which the TS is apparently similar to that of (9A)grows. by the effect of a potential created by positively charged Due to these effects the NR3 catalyst exerts no essential H (QH). In TS (1OC)the Qsi charge grows by 0.341,mainly influence on parameters of processes 2 and 3; i.e., the due to electron density transfer to C1 (Aqcl= -0.246)with reactions proceed as noncatalytic that observed experiweakening of the Si-C1 bond, while a charge on 0 grows mentally at T,above 400 K.lJ due to electron density transfer from H (AQH = 0.123) Let us note some peculiarities when the process occurs (Table 111). In this case the NH3 increases polarization with ClSiH3 (CS). A CS molecule forms a metastable and weakening of the O-H bond. As a whole, electron prereaction donor-acceptor complex 4A or 10B (Figure 3, density on CS at interaction with NH3 grows by 0.090 Table 111),while there is no noticeable minimum what(though Q N H ~= 0.001)as there proceeds a transfer on the soever on PES (2)corresponding to 4A (Figure 31, Le., O+SiCs--Cl bond, while without NH3 it increases only by 0.079. A decrease of the distance between H and C1 in TS (42) Nikitin, E.E.Khim. Fiz. 1987, 6, 1603. (43) Freeman, D.L.;Doll, J. D.J. Chem. Phya. 1983, 78, 6002. (44) Zhdanov,V.P.Elementary PhysicochemicalA.ocesses on Surface; Nauka: Novosibirsk, 1988.
(45) Salahub, D.R.;Messmer, R. P.Phys. Reu. B 1977,16, 2526.
720 Langmuir, Vol. 9,No. 3, 1993
Gun’ko et al.
Table 111. Electronic and Structure Parameters of Clusters and Complexes by the AM1 Method -EHOMO 10.92 11.51 10.84 10.44
AEt
ELUMO 0.50 -0.42 -0.41 -0.69
-4a
Psi,
0.0951 0.0959
0.387 0.323
0.811 0.821 0.826
0.267
0.0955
0.371
0.982
0.828
0.348
0.1272
0.499
1.152
-0.15
0.804
0.359
0.1212
0.546
1.155
4H
roH
0.669
0.236
0.0949
0.673 0.677
0.246 0.247
0.24
0.699
9.96
-0.25
83.0
9.87
1 2 3 4
-7.7 -8.7
5
-10.6
10.24
6
73.4
7
-40
8
ESiqSiHJ
‘.
-12.5
10.24
0.30
0.936
0.220
0.2065
0.223
1.173
9
SSiOSiH,
-8.7
10.99
0.15
0.944
0.215
0.2211
0.220
1.193
3.8
10.79 12.33 11.13
0.39 1.86 0.04
0.932
ACI
10 11 12
=SiOSiHa HC1 ClSi(CH& Ire1
1.197 0.168
0.168 0.386
1.279
than in 1OC (Figure 3, Table III), which disagrees with data on the catalytic action by NR3. This result is due to an error of the reaction coordinate m e t h ~ d and ~ ~ to p ~the ~ fact that the H+ transfer is a fast p r o c e s ~Le., , ~an ~ ~atom ~~ configuration of the active subsystem is not equilibrium for each point of the H+ trajectory. But in the reaction coordinate method a configuration is equilibrium, except grows for A, and a proton “drags” NH3, and barrier E*H+ as a result of this. In practice the H+transfer (TH+) = 8) turns the system only to a slope of the postreaction PES pit, the latter being achieved due to the followingrelaxation of the active subsystem (7 1 1WOs). These effectsintensify at tunnel H+ transfers49 which may be presented as subbarrier motion of H+ on the 0 N C1 trajectory with almost “frozen”heavy atoms. It is possible to model the NR3 action in frames of the modification reaction coordinate method: to take into account high velocity of H+and to inhibit the NR3 motion following H+, i.e., to determine a mass-weighed shift of NR3 relative to 10B. So, if in the 4A 4B process the NR3 interaction whose position corresponds to 10B rather than to 1OC will be operational in each point of the PES profile, a 8-20 kJ/ mol gain in hEt will be obtained (Figure 3). A fall of TS against the initial variant relative to reagents and TS of 4B to 15.4and 7.7 kJ/mol, respectively,Le., TS is stabilized and chemical activation32increases due to NR3 action. That is why the obtained drop of E*H+by 10% only partially corresponds to the reaction rate variation due to the catalyst effect. Reaction 2 catalyzed by NR3 occurs already at indoor temperature; Le., we may consider that T,decreases by 300 K. If one assumes that variations of kinetic parameters, except for E*,are insignificant, the required decrease of apparent E* will be over 50% with differencebetween rate constants k ( T )by 7orders (T, 600 300 K). The rate constant may be determined from the theory of absolute rates (TAR)W
--
-
4
-40 -30 -20 - i o
o
i o 20
E , eV
Figure 4. LDS on active Atoms in reaction 13with R = H in the coordination complex (CC) and transition state. is followedby growth of the deformation electrondensity46 on the bond between them and by an increase of bond strength. Design of the PES profile for H+ transfer in (3) without NH3, i.&, in the 4A complex,has shown that E*H+is lower (46) Tsirelson,V. G.; Antipin, M. Yu. In Crystallochemistry Problem; Porai-Koshita, M. A., Ed.; Nauka: Moscow, 1989;p 119. (47)Miller, W.H.;Ruf, B.A. J. Chem. Phys. 1988,89, 6298. (48) Minkin, V. I.; Simkin,B. Ya.;Minyaev, R. M. Quantum Chemistry of Organic Compounds. Reaction Mechanisms; Khimia: Moscow, 1986. (49)Goldansky, V., I.; Trakhtenberg, L. I.; Flyarov, V. N. Tunnel Phenomena in Chemrcal Physics; Nauka: Moscow, 1986. (50) Robinson, P. J.; Holbruk, K. A. Unimolecular Reactions; Wiley-Interscience: London, 1972.
-
where L* is reaction path degeneration, (1 - a) is overbarrier reflection coefficient. If one postulates uni-
Chlorosilane Interactions with Silica
Langmuir, Vol. 9, No. 3,1993 721
molecularity of complexes 4,5,10A and 10B,k ( T ) may be calculated by the RRKMWtheory, k2 and k-1 flows being determined with allowance for the chemical activation in
--
PiR3
0
/si’\
+ HCI
Reaction.efficiency K in (13) is
ipated in process (2) even at low pressures and as a result = 1 (Table I). One more conclusion follows from experimentaldata: The TEA moleculesparticipating in transformations (2), bound in products with HC1 molecules in ClH...NR3 C1-...[HNRJ+ complexes (bands in IR spectrum within the range of 2 m 2 8 0 0 cm-l) (Figure 2) in future take no part in the reaction. Besides, 8TMS = 0.75 and approaches 0.9 with repeated leak-in (Table I), Le., lateral interactions are sufficiently weak and do not inhibit practically the reaction occurring at 8TMS 5 0.5. At tunneling the H+ transfer along the 0 N C1 path is the most probable. At indoor temperature the halfwidth (d) of the tunneling barrier in (3) is about 0.05 nm, while in (2) it is 0.06 nm. Estimate of d49 is 8TEdeTMS
(13)
The H+ tunneling probability (I%+), if considered to be a unidimensional one and a barrier is ~ a r a b o l i cis, ~above ~ 0.5 in 9B at T I 300 K. Proceeding from (18) and PES section (3) the H+ tunneling is possible with PH+> 0.5 already in the complex 10B. A local minimum corresponding to 10B has not been observed in (2) on PES and as a result of the barrier increase the probability PH+2 0.5 only at ( E S- AEt) I30 kJ/mol. So, if the contribution of tunnel H+ transfers is disregarded, energy of TS stabilization by catalyst (e) is a main parameter which gives an exponential increase of catalytic reaction rate
It is possible in (13) to calculate k l as a unimolecular (in (10) bimolcular) adsorption rate constant with allowance for diffusion. But we do not examine reactions 2 and 3 at small Bcs, that is why molecular diffusion on the silica surface may be disregarded for the following reasons. The activation energy of TMCS self-diffusion (ED) in a polymolecular layer on Aerosil is about 26 kJ/mol (in a monolayer by 3-4 kJ/mol higher), Le., ED= Qst - AE1, where AE1 I 6 kJ/mo1.33141 In this case kD of TMCS on e the silicasurfaceis an order higher that in the liquid ~tate.4~ AU exp In 5B -AHf is about 40 kJ/mol and TTMA 1 10-6 s (T = 300 kBT K). If one assumes that EDfor TMA is determined mainly The carried calculations disregard electrostatic contriby specific interactions, i.e., ED = Qat- AEl = -AHf, it is bution of the whole solid, electron correlation and some apparent that diffusion of NR3 will limit transformations others. Calculations of H complexes of HzO, HHal, and of (2) and (3) at small 8 NH3 by the MNDO/H method in the electrostatic field of a silica fragment of 200 atoms (the cluster face area is 2.5 nm2) have shown that energy of H bonds (AEH)near the -1 surfaceincreases1.5-2 times as compared to the gasphase. k ~ =’ m i n ( k ~ , ~~D~, T ~M CyS ) = ~ D , N R , TNR, Electron correlation also increases AEH by 15-20 %, .3°-32 The reaction activation energy with allowance for diffuThese effects, surely, refer to TS and to prereaction is complexes but differently, as polarization of active bonds in TS is higher (Tables I1 and 111). In the case of inelastic collision of the TMCS molecule with silica surface, ft*/ftis about 9.33!F5, i.e., a loss of translational degrees of freedom decreases a prefactor in and rate constant is (12) by more than 3 orders. This entropic inhibition provides low efficiency of reaction 13. When complex 4A is formed with completeinhibition of rotations about active bonds, we obtain where ED’ and kD’ are determined by (15). But E* = (2~)ED,NR,, and kD,NRs > k, then from (17) E*’ = E* and k’ = k, Le., diffusion may be neglected. With 8i 0, when = 0.55T’.5 (20) probability Pij is small, diffusion limits the reaction (10) rate to a considerably higher extent than that of (13). The ratio (20) depends on the NR3 presence slightly, as Besides, as E ((10) < ES(13), diffusion at 8 10.5 also influences in 4A and 10B,rotations about Sil-0 and Si241 bonds highly the catalytic process (10) with allowance for (15may be frozen. For reaction 13 at R = CH3, E* = 144 17). The diffusion influence may be decreasedby variation kJ/mol, which agrees with the experiment E* = 150 kJ/ of the leak-in priority for ClSiRB and NR3 taking into mol (Tp= 870 K).52 The rate constant (Figure 5 ) account that kD,NRs 450K thereaction occurs in the presence of a catalyst though the adsorption mechanism, but probability of bimolecular adsorption complex formation is scanty and as aresult of this, amines exert no catalytic action.
