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Communications to the Editor
can be noted from the data. (I) Strong-acid sites are generated when MoO3 is deposited on alumina (compare samples A and I-I or B and C) in agreement with the results of Kiviat and 1’etrakis.l ( 2 ) When Moo3 is supported on alumina, the atisence or presence of Na+ ions (samples H and C, respectively1 does not alter significantly the concentration of the strong-acid sites (pK, < -5.6). Weak-acid sites, however, are suppressed in the presence of Na+ ions. (3) Co-A1203 samples do not contain strong-acid (pK, < -5.6) sites (samples D and J). (4) When Co and Mo salts are impregnated simultaneously (samples G and I), both strong and medium acidities (2.0 > pK, > -5.6) are suppressed by the presence of‘ Na+. Alkali ions actually increase the proportion of weak-acid sites (5 > pK, > 3.3). It is possible that some 01’ the strong-acid sites are converted to weaker ones In the presence of Na+. ( 5 ) The sample obtained by depositing MoO3 “on top” of preimpregnated Co (sample E) contains a larger number of strong-acid sites than the one wherein Moo3 had been deposited first (sample F). (6) The last and what we consider to be the most significant point is the following. In the absence of Na+, the CoMoA1203 catalyst is more acidic than Mo-A1203 (samples I and HI. When Na+ is present, however, they have similar acidic properties (samples G and C). In other words, the interaction between the cobalt and molybdenum moieties to generate strong-acid sites is inhibited by the Na+ concentration of the alumina support. These results lend additional support to clur earlier findings3 that in the Co-Mo-Al203 system, not only is there a structural interaction between the cobalt and molybdenum constituents but also the nature of this interaction itself is a function of the structural features of the support. Thus, all three constituents (Co, Mo, and ~41203)form a true interacting system and it is not possible to discuss the nature of any one of them without reference to the other two. Acknow1edggr:zent. We thank Drs. M. G. Krishna and K.
M.Bhattacharya for encouragement and support. References arnd Nates (1) F. E. Kiviat and I_. Petrakis, J. Phys. Chem., 77, 1232 (1973). (2) A. E. Hirschlisr a’nd A. Schneider, J. Chem. Eng. Data, 6, 313 (1961). (3) (a) P. Ratnasamy, R. P. Mehrotra, and A. V. Ramaswamy, J. Catal., 32, 63 (1974);(b) 1’. Ratniisamy, A. V. Ramaswamy, K. Banerjee, D. K. Sharma, and N. Ray, submitted for publication in J. Catal.
india Institute d Pketroleurn Wehradun (U. f ?), kdia
P. Ratnasamy* D. K. Sharma L. D. Sharma
Received March 20. 1974
Surface Acidity of Modified Alumina Publication costs assisted by GulfResearch and Development Company
Sir: The preceding communication by Ratnasamy, et al., presents some very interesting data pertaining to the effect of compositional and preparational factors on the acidity of Co-Mo-Al203 systems. Their results are not surprising and they are in general agreement with other finding~.l-~ This communication is not the appropriate vehicle for a discusThe Journal of P i i y s c a l Chemistry, Vol. 78. No. 20, 1974
sion of the merits and limitations of the “indicator titration” method utilized by the authors, but rather we simply refer to the recent review article by Forri4 on the subject. What is interesting are the additional data provided by these authors on the complexity of the system and the undisputed profound effects that dopants can have on the surface characteristics of A1203. It is pleasing, of course, that Ratnasamy and coworkers find, in agreement with our work,’ that strong-acid sites are generated when MOOSis incorporated in alumina. However, one must be very careful in making comparisons, especially when the levels of dopant substitution and manner of preparation are not precisely defined. In a series of paper^^-^ Petrakis and coworkers have demonstrated and quantified aspects of the vital role that the levels of Moos substitution and pretreatment of the system have on the surface properties. For example, it has been shown that the distribution of molybdenum valences depends importantly on the amount initially incorporated. Low levels of Moo3 favor M O W , while at higher levels the Mo(V) is quenched in favor of Mo(1V) and Mo(V1). In fact, there is a dramatic decrease in the Mo(V) levels in going from 9% Moo3 to the 12.5% Moo3 utilized by the authors of the above comm~nication.~ Thus, if it is assumed that molybdenum plays a critical role in the modification of the host surface, the distribution of molybdenum among the various valences and the type of sites they occupy are bound to have a determining effect on the observed surface modifications. Similar results have been obtained8 on the dependence of the distribution of the molybdenum species on the nature of host (alumina) and temperature and time of pretreatment. Thus, we reach the conclusion that cross-system comparisons must be made with the utmost care. Another factor not considered by Ratnasamy, et al., in comparing their results to those presented in ref I is that the alumina substrates differed. It is known that pyridine adsorbs differently on y, 9 - , and &alumina surfaces, the differences being ascribed to differences in the numbers and relative strengths of the Lewis acid sites.l An investigation of Moo3 supported on both an 7- and &alumina indicated that these differences are maintained in the presence of M0.l Hence, a quantitative comparison of surface acidities of systems where the substrate differs is subject to inherent differences in results. Finally, with respect to the above authors’ claim that in the absence of Na+ the Co-Mo-Al203 catalyst i s more acidic than the Mo-A1203, it must be said that that situation could occur without necessarily contradicting our earlier observations. Given the demonstrated great sensitivity of the systems on the factors discussed earlier, the same cautioa must be exercised in comparing ternary systems. As to the relative capacity of sodium and cobalt to quench the molybderrnm-induced acidic sites of the host material, only a very ch eful series of experiments designed in the context discussed here could delineate that point. References and Notes (1)F. E. Kiviat and L. Petrakis, J. Phys. Chem., 77, 1232 (1973). (2)J. H. Ashley and P.C. H. Mitcheli, J. Chem. SOC.A, 2730 (1969). (3)J. M. J. G. Lipsch and G. C. A. Schit, J. Cafal., 15, 174 (1969). (4)L. Forri, Catal. Rev., 8, 65 (1973). (5) K. S.Seshadri and L. Petrakis, J. Phys. Cbem., 74, 4102 (1970). (6) L. Petrakis, et a/., paper presented at the 165th National Meeting of the American Chemical Society, Dallas, Tex., April 1973;to be submitted for publication in J. Phys. Chem.
Communications to the Editor
2071
L. Petrakis and K. S. Seshadri, J. Phys. Chem., 76, 1443 (1972). K. S. Seshadri and L. Petrakis, J. Cat& 30, 195 (1973). L. Petrakis and Y. S,Seshadri, to be submitted for publication in J. Phys. Chem.
Gulf Research and Dsvebpment Company Pittsburgh, Pennsylvania 15230 Received April 19, 1974
L. Petrekis. F. E. Klvial
TABLE I: Products and Yields from 2537-A Mercury-Sensitized Photolysis of c-C~FS
Product CF4 CzFe CaFs C-CaFa n-C4Fl0 CFa-c-CdFs C~F~-C-C~FB c-Cb.i-c-Cd7 12 additional high molecular weight products
Relative yield (c-CIFB 100) 0.170 iO.018 0.147 f 0.018 0.072 f 0.005 0.016 f 0.004 0.029 f 0.006 0.713 f 0.042 0.328 f 0.022 0.629 f 0.048 O r 135
2537-A Mercury-Sensitlzed Photochemical Decomposition of Perfluorocyclobutane
ed compounds, we have tentatively identified the C3Fs as the ring compound. The higher molecular weight products perfluoromethylPublication costs a:;sisted by The Robert A. Welch Foundation cyclobutane ( C F ~ - C - C ~ F ~perfluoroethylcyclobutane ), ~ F ~perfluorobicyclobutyl ), (c-CqF7-c-CqF~) Sir: The decomposition of perfluorocyclobutane, c - C ~ F ~ , ( C ~ F ~ - C - C and were trapped out from the gas chromatograph effluent and has been studied by several methods including y-radiolysisY1X-radiolysis,2 Xe-sensitized photolysis: and hot lSF analyzed mass spectrometrically. The identity assignments of C F ~ - C - C ~and F , C ~ F S - C - Care ~ Ftentative, ~ since no comatom r e ~ o i l . ' ~We , ~ report some results of our work with parison mass spectra or supplementary analysis was avail2537-A mercwy-sensitized decomposition of c - C ~ Ffor ~ able; however, these assignments are reasonable based on comparison. the characteristics of perfluorocarbon mass spectra given The perfluorocyclobutane used was obtained from Penby Majer7 (e.g., the highest mass peaks observed are the insular ChemResearch, Inc., and further purified by bulbm/ e 231(C$g+) and mle 2 8 1 ( C ~ F 1 1 ~respectively, ), and to-bulb distillation until gas chromatograms showed no imboth compounds exhibit additional mle peak yields resempurity peaks. bling that of c - C ~ F ~The ) . identification of perfluorobicyIrradiations (3-min) of c - C ~ were F ~ carried out in a cylinclobutyl was based on a close agreement of the mass specdrical 130 mm X 20 mm 0.d. quartz vessel at pressures tra with that reported by Banks, et aL8 ranging from 100 to 105 Torr and at a temperature of 43 f Individual product yields were normalized to the residulo. These conditions gave an overall decomposition of 5.6 f al c - C ~ Fand S are reported as relative yields in Table I. The 1.9%. During irradiations, the reaction vessel was inserted CF4 and C3Fs yields have been corrected for relative therwithin the coils of EL Hanovia low-pressure mercury-resomal conductivity by direct calibration, whereas C3F6 and nance lamp of spiral configuration. The lamp intensity was n-C4Flo yields were corrected with the use of data from determined by potassium ferrioxalate actinometry to be 2.8 Askew, et aL9 All products of higher molecular weight than x 1818 quanta S P ~ C " - * ~ c - C ~ were F ~ assumed to have the same thermal conductiviThe products were analyzed on a Beckman GC72-5 gas t y as c - C ~ FThe ~ . imprecision reflects one standard deviachromatograph equipped with thermal conductivity detection. Individual relative yields for the 12 higher molecular tors. A standard four stopcock by-pass loop injection sysweight products (grouped together in Table I) were all less tem between a high vacuum manifold and the gas chromathan 0.025. tograph was used for direct injection of the products from a As shown in Table I, we observed only saturated prodgiven sample; therefore, only the volatile products from a ucts to be produced (C2F4, cis- and trans-2-C4Fgt C - C ~ F ~ , given sample were analyzed in this work. The additional and lr3-C4F6are all separated on our columns) with 75% of material not accounted for in the material balance was asthe product yield accounted for by the CF3, C2F5, and csumed to be n f f n ~ ~ l a tpolymeric i~e material since a white C4F7 derivatives of the parent c-CaF8. The fact that we obresidue was observed in the photolysis vessel following irraserved only saturated products does not preclude the possidiations of longer than 3 min. Two columns were employed bility of the transient formation and subsequent loss in the analys,is: a KELF No. 3 oil on Chromosorb W for through internal radical scavenging of unsaturated prodlower molecular weight products (
