T H E
J O U R N A L
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PHYSICAL CHEMISTRY Registered i n U. S. Patent Office 0 Copyright, 1978, by t h e American Chemical Society
VOLUME 82, NUMBER 8
APRIL 20, 1978
A Comparative Study of the Pyrolyses of the Tetramethyl Derivatives of SOlicon, Germanium, and Tin Using a Wall-less Reactor Jay E. Taylor” and Thomas S. Miiazzo Department of Chemistry, Kent State University, Kent, Ohio 44242 (Received May 27, 1977; Revised Manuscript Received January 26, 1978) Publication costs assisted by the Petroleum Research Fund
Pyrolyses of the tetramethyl derivatives of tin, germanium, and silicon have been studied under both homogeneous and surface conditions using the wall-less reactor. Under homogeneous conditions all reactions are first order. The products are predominantly methane and ethane + ethene with lesser amounts of butenes and propane or propene. The order of reactivity is Sn(CHJ4 > Ge(CH3)4> Si(CH3)4> C(CH3)4(from previous work). The activation energies are 55, (611, 72, and 81 kcal/mol, and the log A values are 13.9, (13.41, 14.1, and 16.9, respectively. Addition of toluene to the homogeneous pyrolysis of SII(CH~)~ has a very small effect. Upon introducing a stainless steel surface (S/V = 7.8-’ cm), the observed activation energies are lowered significantly for all reactants except Ge(CH3)4for which an increase is noted. The mechanisms and certain anomalies are discussed.
The group 4A metal alkyls have kinetic and thermodynamic properties which, for the most part, vary with the metallic characters of the central atoms. Further, the 4A metal alkyls are relatively unreactive to air and water, particularly as contrasted to the 3A metal alkyls, and they show little tendency to expand the coordination shell as do the 5A metal alkyls. Thus, the tetramethyl derivatives of silicon, germanium, and tin appeared to be appropriate and uncomplicated subjects for studies of their pyrolysis reactions. Earlier studies on neopentane (tetramethylcarbon) are also reviewed to compare four of the 4A alkyls. Kinetic studies of the decompositions of the group 4A metal alkyls are few in number, particularly of the tetramethyl derivatives. The homogenity of these reactions has been assumed on the basis of very limited evidence, and no studies under surface free conditions have been attempted. Helm and Mack1 pyrolyzed tetramethylsilicon (TMS) in a static reactor and concluded that the reaction is homogeneous and unimolecular in the range of 659-717 “C at pressures above 100 Torr. In more recent work QQ22-3654/78/2Q82-Q847$0 l.QQ/ Q
Clifford et al.2arrived at these same basic conclusions using a flow reactor at 537-561 “C. Studies on tetramethyltin (TMT)were conducted in a static reactor by Waring and Horton3at 440-493 “C. They reported that the reaction is “homogeneous” above 80 Torr once the reaction vessel is “conditioned”. Johnson and Price4 made similar conclusions using a flow reactor and toluene carrier gas. Sathyamurthy et al.5 reinterpreted the two earlier papers1v3 as three-halves-order reactions. Although no studies on tetramethylgermanium (TMG) have been reported to date, Geddes and Mack6 compared the effects of packed and unpacked reactors on the decomposition of tetraethylgermanium and concluded that the reaction is 98% homogeneous. An early exception to the practice of packing reactors was voiced by Meinerta7 The intent of this paper is to determine the absolute effects of surface on certain group 4A tetramethyls and to evaluate the kinetic constants pertaining to these reactions. The technique used is the wall-less react~l.8~~ which permits the comparative determination of reaction rates in the total absence of surface and in the presence of a specific surface. 0 1978 American Chemical Society
840
The Journal of Physical Chemistty, Vol. 82, No. 8, 1978
J. E. Taylor and T. S. Milazzo
In this way an absolute comparison of homogeneous and surface reactions is possible. This is a very sensitive method, so that surface effects at very low surface to volume ratios can be measured.
Experimental Section The principle and details of the wall-less reactor have been previously described.*pg Since the previous studies have been with gaseous reactants and since the reactants for this project are liquid a t room temperature, the injection system has been redesigned. TMT was noted to be light sensitive over a moderate period of time requiring a darkened environment during both storage and the period of injection. The same types of design for the reactor, furnace, sampling unit, temperature monitoring, nitogen purification, and plug flow of the nitrogen stream, as previously employed, are used. For the revised unit, the liquid reactant is inserted by syringe through a gas-tight septum into a nitrogen filled, oxygen-free 150-mL specially constructed gas syringe set at its lowest position. The syringe, fabricated from a 1.0000-in. precision bore Pyrex cylinder and a piston fitted with mercury sealed Teflon piston rings, is designed to accommodate, without leakage, temperature changes as great as 150 "C. (The mercury used in the mercury seal is enclosed also to avoid escape of mercury vapor.) The 150-mL syringe, enclosed in a constant temperature air bath, is heated to a temperature above the boiling point of the reactant, and the piston is raised to its maximum height allowing the gaseous reactant to fill the cylinder which is then opened to a heated line connected with the wall-less reactor. The flow rate of the reactant is controlled by a synchronous motor-driven gear box with change gears activating a 12-in. precision ground screw drive. The nitrogen used was Linde high-purity dry nitrogen and was further purified to remove oxygen. The TMS, TMG, and TMT were from Alfa and were classified as NMR grade, Electronic grade (99.998%), and 99.8%, respectively. Each of these compounds was checked for purity in the gas chromatograph before use. Since lateral d i f f u ~ i o nof~ ,both ~ reactants and products in the nitrogen stream effects a dilution of the center stream and since the lower molecular weight products diffuse the more rapidly, it is necessary to determine the concentration gradient of each product across the entire nitrogen stream. This is accomplished by performing a cross sectional analysis from center to edge. Due to the symmetry of the stream, analysis along a single radius only is needed. A three-dimensional plot of the concentrations of a reactant or product at a given downstream distance takes the form of an elongated bell. The volume within this bell (representing the concentration of that reactant or product) is evaluated by the process of summation of segmental cylinders of height h. The distribution curve for product or reactant is highly reproducible, and for this reason a single center stream analysis suffices, once the diffusion factor has been established for each substance. The validity of this procedure has been repeatedly demonstrated.*-12 In this manner a point sample taken at center stream at a specified distance from the injection tube is used to determine the concentrations of all reactants and products in that plane, The system is computerized such that the raw chromatographic data are converted directly to rate constants, activation energies, graphs etc. using the least-squares treatment as appropriate. Activation energies are evaluated by setting the sampling
-0 2 -
0
I
v)
2I
0
-
20
n
TMT
Q
Homogeneous Compd
log A
TMCd 16.9 TMS 14.1 r 0.2 TMG 15.1s 12.1h TMT 13.9 ~t 0.4
SurfaceC Bondb E,,, dissoc Eactr kcal/mol energy log A kcal/moI
80.5 72.0 r 0.7 69g 51h 5 5 . 4 r 1.4
82 74 69
65: 302 12.5 61 17.0 75
65
13.0 48
These constants are calculated from composite data which in each case includes three to five experimental runs as indicated in Figures 2-4. See ref 13. S/V = 7.8 cm-l except TMC (not determined). Neopentane from ref 10. e Above 625 "C. f Below 625 " C . g Above 710 "C. Below 710 "C. a
tube at a distance of 4 cm from the inlet tube and varying the reaction temperature. The reaction time is computed from the diameter of the quartz tube and the flow rate of nitrogen a t ambient temperature and pressure. The corrections to reaction temperature are then applied. Surface effects are evaluated using an insert of oxidized 1/16-in.stainless steel rods set into two 16 mesh stainless steel screens, as previously d e ~ c r i b e d . ~ The kinetic data are calculated from C/Co such that C = number of moles of reactant remaining 4 cm from the inlet and Co = C plus the sum in carbon equivalents of each product. Thus C, represents only that concentration of reactant which forms products identifiable in the gas chromatograph. Since all gaseous decomposition products detected in the gas chromatographic analysis are counted as C and as part of Co, errors resulting from side reactions (e.g., polymerization) which may release gaseous products are minimized. Reaction times vary from 0.12 to 0.18 s depending upon the temperature. Results To determine the order of the reaction, TMS, TMG, and TMT were pyrolyzed at a constant temperature with varying rates of injection of reactant. As seen in Figure 1, the first-order rate constants are invariant within experimental error over a 20-fold change in concentration. The first-order character of each of these reactions is thereby clearly established. All reactions were carried out with the sampling tube located 4 cm from the inlet tube resulting in wide variations in conversion with changes in temperature. Even a t the temperature extremes, the graphs of 1/RT vs. In C show excellent linearity as seen in Figures 2-4. The
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The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 849
Pyrolyses of 4A Tetramethyls
TABLE 11: Product Analysis at Varying Temperature Conversions (Interpolated Data)
"C
% convrn
754 818 876 716 779 842 649 712 761 630 674 714 52I 572 611 474 516 553
1.0 7.0 30.0 1.o 7.0 30.0 1.0 7.0 30.0 1 .o 7.0 30.0 1 .o 7.0 30.0 2.0 7.0 30.0
Temp, TMS-~b
TMS-SC TMG-~b
TMG-SC TMT-~b
TMT-SC
M o l e % yield" f r o m (CH,),M CH,
C,H,
C,H,
0.50 2.5 7.4 0.75 4.5 17.5 0.20 1.5 5.4 0.55 2.9 10.1 0.50 2.0 5.6 1.2 2.4 6.7
0.26 1.7 6.8 0.09 0.63 2.4 0.44 3.0 16.6 0.29 2.4 12.1 0.45 4.5 22.2 0.70 4.4 21.9
0.28 2.8 15.7 0.16 1.8 10.0 0.36 2.5 8.0 0.16 1.7 7.7 0.04 0.41 1.6
C,H,
C,H,
0.16
