GUNTERR. JOPPIENAND JOHNE. WILLARD
3158
Further Studies of Chemical Reactions on 7-Irradiated Silica Gel and Porous Vycor Glass1 y Gunter R. Joppien and John E. Willard* Departmend of Chemistry, University of Wisconsin, Madison, Wisconsin, 63706
(Received January 24, 1972)
Rublkatwn costs assisted by the U.S. Atomic Energy Commission
Energy deposited in either silica gel or porous Vycor glass by yirradiation can activate adsorbed CH3C1, GzHhCI, C,H&l, C2H4,C2D4, C8Hs, and CH4 with approximately equal effectiveness ((2 (products) = ca. 1 in each case). The products include CHI and CzHs from CHSCl at 77 and 3OO0K, CZHG and ChH, from CzH4 at 77 and 300"K, CHI radicals from CH4 at 77"K, C2D4Hradicals from C2D4 at 77OE(, and peroxy radicals frdm 02-CH3C1 and 02-CH4 mixtures at 77°K. When CH3C1 and CZH4 are present together on the surface they compete for activation by energy absorbed in the gel. 02 admitted to CH3 radicals trapped on silica gel at 77°K converts them slowly to peroxy radicals. Sorbed CH,radicals exposed to C2H4at 77°K yield propane on analysis at room temperature. The yields of stable products from radiolyses of sorbates on the gel decrease sharply for doses above 4 X lo1* eV g-l at 77"K, but not at 300"K, suggesting a preempting of most of the sensitive energy transfer sites by products at the lower temperature, but release of the sites by desorption at the higher temperature. The influence of gel preparation and sorbate concentration on G values and the effect on the esr spectra of sorbed radicals is illustrated.
Introduction The discovery of Caffrey and Allen,2 confirmed and extended by many others, that adsorbed organic molecules can be decomposed by y-radiation energy deposited in the bulk of various adsorbents has posed maiiy unanswered questions as to the mechanisms of energy transfer to the surface and from the surface to the sorbate, and as to subsequent reactions on the surfaces. Silica gel and porous Vycor (silica) glass are among the substrates most investigated. Radiation G values of unity and higher for reaction of adsorbed molecules (based on the energy absorbed by sorbate plus substrate) are common in these materials. They can be obtained relatively pure, and their surface structurps have been investigated in some detail.4 It has been reported that (1) anion formation by transfer of electrons from the surface can occur during r-irradiation for sorbates such as tetracyanoethylene (TCNE) ,6 galvinoxyl,Ganapthalene,6" biphenyl,GE carbon dioxide,6b and sulfur dioxide;6b (2) easily oxidized sorbates such as tetra methyl-p-phenylenediamine can be converted t o the cationic (3) methyl halides yield methyl radicals at 77"KGaand methane at 273QK;7(4) aliphatic hydroc a r b o n ~ ,azoethane,8 ~~~ ethan01,~ and isopropylbenz e d 0 are decomposed; ( 5 ) high yields (G = 4 and 6.5) of benzene and TCXE-, respectively, are obtained when isopropylbenzene1° and TCXE6 are added to 7-irradiated silica after irradiation; (6) certain active surface centeri; stable at 77°K decay at 300"K.lO These observations indicate both the versatility and complexity of silica as a transducer of y energy to produce chemical change. They show that electrons, positive holes, and/or excitons must be able to travel The Journal o f f h y s i c a l Chemistry, Vol. 76, hro. 81,1972
many angstroms through the irradiated silica t o ''find" sorbed molecules and that different types of energy and charge transfer occur at the surface. In the present work we have obtained further evidence on the following: (a) the relative ability of silica undergoing irradiation to initiate reactions of sorbate molecules of similar size but contrasting electron affinity (e.g,, CH&1 and CH,; CH&1 and CzH4; GJ&Cl and CzH4, CaH7CI and CsHa), both when the species are present separately and when they are competing on the same surface; (b) reactions of activated sorbate molecules with surface silanol groups; (c) reactions of sorbed free radicals with added CzH4 and 0,; (d) the effect of temperature on yields, as a function of dose; (e) postirradiation polymerization of ethylene; (f) properties of sorbed free radicals and hydrogen atoms. (1) This work has been supported in part by the U. S. Atomic Energy Commission under Contract No. AT(11-1)-1715, by the W. F. W a s Trust of theuniversity of Wisconsin, and by the Deutsche Forschungsgemeinschaf t. (2) J. M. Caffrey, Jr., and A. 0. Allen, b.Ph,ys. Chem., 62, 33 (1958). (3) See review article by H. W. Kohn, "The Radiation Chemistry of Surfaces" in "Actions Chimiques et Biologiques des Radiations," onzihme sbrie, Masson and Cie, Paris, 1967, and more recent references included in those cited below. (4) (a) J . B. Peri and A. L. Hensley, Jr., J . ?hg8. Chem,, 72, 2926 (1968); (b) M. L. Hair and W. Hertl, ibid., 73, 4269 (1969). (5) P. K. Wong and A. 0. Allen, ibid., 74, 774 (1970). (6) (a) P. K. Wong and J. E. Willard, ibid., 72, 2623 (1968); (b) P. K. Wong and J. E. Willard, ibid., 73, 2226 (1969). (7) N. H. Sagert, J . A. Reid, and R. W. Robinson, Can. J . C'hem., 48, 17 (1970). (8) J. G. Rabe, B. Rabe, and A. 0. Allen, S.Phys. Chem., 70, 1098 (1966). (9) L. Abrams and A. 0. Allen, ibid., 73, 2741 (1969). (10) E. A . Rojo and R. R . Hentz, ibid., 70, 2919 (1966).
Y-IRRAD~ATED SILICAGEL AND POROUS VYCORGLASS Experimental Section Materials. The reagents, and the manufacturers' purity specifications, were >latheson CHaCl (99.5oj,), CaHaC1 (99.7%), CH, (99.99Oj,), and CzH4 (99.7%), = 1.3878); lClerck Sharp and Aldrich C & , C l (nY0~> Dohnie C2D4 (99% is3topic purity). The silica gel was Davison Chemical Co. Grade 950 with a stated BE:T surface area of 900 m2 g-' and a particle size of 60-200 mesh. Vycor porous glass No. 7930 from Corning was used as glass rods of 2-2.6-mm diameter with a stated B E T surface area of 140 m2 g-l. rSampb Preparation. Prior to degassing, the silica gel was healed in air at 200" for 2 hr, and the Vycor was heated in 508 Torr of dry oxygen at 600" for 2 hr, to remove volatile impurities and physisorbed water. For experiments foilowed by stable product analysis, poi*tiona of ea. 7 g of the silica gel were weighed into aniiular Pyrex irradiation cells with break-seals and Torr. For esr degassed for 96 br at 460" and investigations, eit er 0.2 g of the gel or 30-mm lengths of VYCWrod were placed in 3-mm i.d. Suprasil tubes with gmded seals to P-Jrex and degassed for 24 hr at T o r r ~ A grease-free, mercury-free 460" and (: 110-2 mf CzHsCl on silica gel, dose 4 X 1018 eV g-1, power 8 mVV, modulation amplitude, 1.3 G ; signal level 40, T 77'K; 6 >( :LO-a mf CtHa on silica gel, dose 2 X :LO19 eV g-1, p o m r 1.9 m.W, modulation amplitude 1.3 G, signal level 40, 7' 85%; Is, 4.7 X 10-2 mf GzDn on VPG; dose 4 >: 1018 eV g-1, power 1.1 mW, modulation amplitude 1.3 G, signad level 50,T 77°K; E, 3.3 X mf CzD4 on YPG, photolyzed 12 min. with low-pressure mercury lamp, power 1.1 mW; modulation amplitude 1.3 G, signal level 100, 7' 77°K; F, same sample as E after 48-hr storage a t 77'11; .irerticaX arrows indicate g = 2.0028.
by Figure 5E which is a spectrum produced by photolysis of 3.3 X IO-* mf CzD4 on Vycor with a low-pressure mercury arc. A similar spectrum was produced by photolysis of GzD4 on silica gel. Spectrum 5F, which was recorded after 48 hr of storage of sample !;E at 77"K, shows a marked increase in line resolution. This must be due to reorientation of the radicals on the surface, and it suggests that the differences of spectra E and D are due to similar orientation effects, which may involve the extent of interaction with surface OW groups. Figure 5 6 i s an example of the influence of sorbate concentration on radical geometry. The sample used was similar to that of Figure 5A but contained 6 X mf CJL rather than 3.2 X mf. The change from a 1Bline to a 6-line spectrum with equal spacing of these lines by aavH= 23 G and an amplitude ratio dose to the binomial ratio 1:5 : 10: 10:5 : 1 indicates equal coupling of the unpaired electron with the five protons. This observation suggests that at surface coverages above about 2% of a monolayer the added ethylene molecules are sorbed on a different type of site, QIT form sorbate clusters, favoring a differ-
Total ooncn, mf
Expt NO.
3.0 x 2.2 x 2.1 x 1.8 X 2.5 x 1.9 x
1 2 3 4 5 6
10-2 10-2 IO-*
% CxHaClb
0.0 22
lo-*
39 60
10-2 10-2
100
80
VZH61, nf X 106
2.02 1.79 1.68 1.69
1.79 0.77
G(CzHs)
1.22
1.08 1.01 1.02 1.08 0.46
Absorbed y dose was 1.65 X 1019 eV g-1; irradiations and esr measurements carried out a t 77°K. * 100 X [GKsCl]/ ICdL1). ([C2HsC11 Q
+
Radical Production from C3H6 and CsH7CI on Silica Gel. 7-Irradiation of propylene (1.6 X mf) on silica gel at 77°K generates an esr spectrum of eight well-resolved lines with a splitting constant of 23 6. On warming t o room temperature the six inner lines split into six groups of two giving a, 14-line spectrum similar to that reported for the isopropyl radical with equivalent /3 protons.ls A similar but somewhat less resolved spectrum is obtained from y-irradiation of 1.6 X 10-2 mf n.-CsH7C1on silica gel at 77"K, at a dose of 2 X 1019eV g-I. G(radica1s) was determined to be 0.8 f 0.2 from both spectra. Bn each case the presence of n-propyl radicals could not be excluded. Trapped Hydrogen Atoms and Paramagnetic Centers in Silica Matrices. ?-Irradiated silica gel and Vycor samples prepared by the methods used in this work show no evidence of the hydrogen atom esr doublet, indicating that G(Ht,) is The yield of (CH, plus 2C2H6) recoverable on warming t o 300°K (Table 11) is equivalent t o that formed by radiolysis at 3QO"I< ( i e . , ca. 1). If oxygen is present during radiolysis at 77°K trapped peroxy radical8 are produced with a yield of 0.86. These (data suggest that all GH4 production is via free radicals, (1 0.27)/1 of which abstract hydrogen from the surface by Plat reaction at 77°K and 0.27/1 of which do so by thermal reaction during warm-up. I n the presence of the high concentration of O2 used (60 X mf = ea. 0.15 monolayer) they all react with adjacent oxygen molecules to form the peroxy radicals. do not preclude the possibility that artd CH302 in excess of the 0.27 trapped radical yield observed in the absence of O2 are the result oE excited molecule reactions rather than radical reactions (e.g., C113Cl* iO24 CH302 Cl). In the presence of a high concentration of 0 2 the peroxy radical yield fram @H,is double that from CH3C1,suggesting that HCh as wall as CH302is formed. Thc increase in G(CH3) with increase in methyl
+
+
-I
+
The Journal of Physical Chemistry, Vol. 76,No. 22? 1972
halide concentration on the surface (Table I) may result from a lessening of the probability of the freshly formed hot CHa radicals abstracting H from silanol groups before moderation, when the silanol groups are shielded by CH3C1molecules. There are two demonstrations from the present work of the ability of methyl radicals stabilized on silica gel to react with reagents added after irradiation. One is their quantitative conversion to peroxy radicals when the sample is exposed to oxygen, the other is the forrnaLion of propane when C2H4 is added (experiment 7, Table 11). Reaction Initiated by Stable Active Sites in Irradiated mf of C2H4 was added a t Silila. When 6 X 77°K to silica gel containing 0.6 X mf GH3Cl which had received a dose of 4 X 1020 eV g-lJ an amount equivalent to G( - C2HJ = 4 was not recoverable on warming (experiment 7, Table 11). A. similar amount was lost when the C2N4 alone wa8 present during irradiation at 77°K and about three times as much when present during irradiation at 300°K. This suggests that stable sites on the surface initiate polymerization chains which form a nonvolatile product, the chains being longer at the higher temperature. Samples of porous Vycor glass with or without adsorbed C2H4 or C2D4 (4 X 10-3-4 PO-' mf) had a transparent brown color following r-irpadiation a t 77°K with doses of 2.4 X IOzoeV g-l. After wa,rming to room temperature the samples containing adsorbed ethylene became translucent within minutes arid turned white after ca. 30 min. This observation suggests that polymerization occurs during warm-up of samples but not at 77°K. Variables Which A$ect G Values. The variety of variables which influence the G values of reactions induced by irradiation of sorbates on silica gel is a source of difficulty in seeking t o define the reaction mechanisms. In the present work this is illustrated by the following: (1) the sensitive dose dependrnce at 77°K of the G values of stable products and radicals in the range of 1 X eV g-' t o 2 X IOzoeV g-I (Figure 2); (2) the difference in C(CH3) from CN3Br for gels prepared at 200 and 400" (Table I); (3) the concentration dependence of G(CH3) from CIlaCl and CH3Br (Table I) as compared to the independence of concentration observed in previous work6 with similar systems. (33) Yu. A. Sorokin, V. I. Tupikov, E. A. Borisor, and V. I?. Zakharov, Khim. Vys. Energ., 3 (5),457 (1969).
