NOTES
1312
Vol. 63
quencies in aliphatic and benzene derivatives provide independent measures of the inductive effects and the over-all electronegativities of groups.
area. (2) Heating the catalyst in dry air a t a sufficiently high temperature leads to the same kind of reversible loss in activity without a change in surface area.
T H E CUMENE CRACKING ACTIVITY OF CO-GELLED SILICA-ALUMINA CATALYSTS AS A FUNCTION OF SURFACE AREA BY RUSSELL W. MAATYAN~ AND CHARLES D. PRATER
The conclusion that the available sites cannot lie very far below the surface is based upon the fact that a decrease in surface area implies an increase in the thickness of the malls. Let D be the distance from the pore wall surface beyond which no significant contribution to activity is made. Referring to Fig. 1, consider a catalyst having a wall
Socony Mobil Oil C o m p a n y , I n c . , Research and Development Laboratory,
Paulsboro, N e w Jersey Received October 7, 1968
It has been reported by Dobychin and Tsellinskaya2that the activity per unit surface area of silica-alumina cracking catalysts for the dealkylation of cumene remains constant as the surface area of the catalyst is changed over a wide range by means of a high temperature steam treatment. This work seems to have been done on catalyst samples prepared by impregnation of silica gels with aluminecontaining compounds. In this case such results might be expected. Although such an effect might not be expected with catalysts prepared by co-gelation, in this note we will present data to show that the effect is observed in such catalysts. This constancy of activity per unit surface area as the total surface area is varied implies that significant contributions to the activity are not made by structural material lying very deep within the walls of a pore. This fact can be used t o eliminate some mechanisms which have been postulated to explain the cracking of cumene on silica-alumina. The surface area per gram of the silica-alumina catalyst was changed by high temperature steam treat,ment. The activity per unit avea, as a function of the surface area: 80.1 moles propylene/m.a/sec. ( X IOg) for 499 m.z/ 75.8,440; 80.4,393; 79.8,320. The catalyst of 499 rn.Zyg! surface area contained 10 wt. % Al20$and it was prepared by co-gelation; the others were prepared from it. Similar results were obtained on other silica-alumina catalysts. Their initial surface areas, per cent. surface area remaining after steam treatment, and per cent. activity per gram remaining after steam treatment: 350 m.2/g., 58%, 65%; 350 m.2/g., IO%, 10%; 282 mVZ/g.,76%: 72%; 270 m.2/g., 78%, 78%. All surface areas were obtamed by the B.E.T. nitrogen adsorption method. Catalytic activities were determined in a differential reactor described elsewhere.3 The reaction rate was measured a t 420” a t atmospheric pressure, and with a flow.of reactant over the catalyst sufficient to keep the conversion of reactant to products less than 1%. The catalyst was used as 100-200 mesh powder spread in a 200 p thick layer in a tray. For these conditions the rate of diffusion of reactant into pores or into the bed does not inodify the reaction rate. Steam treatments were carried out a t 550’ with 100% steam at atmospheric pressure for periods up to 2.5 hours. In order to prevent a decrease in catalytic activity caused by dehydration of the catalyst during steam treatment, it is necessary to restore the condition of hydration which these catalysts had before steaming. This was accomplished by placing catalyst in a moist atmosphere (100% relative humidity) a t room temperature for 18 hr., and then heating in a stream of dry air a t 450’ for 1 hour to remove excess water. These facts indicate that the decrease in activity which is restored by this treatment is independent of surface area (1) The restoration of activity by the moist air treatment is not accompanied by an increase in surface (1) Department of Chemistry, The University of Mississippi, Unisity, Mississippi. (2) D. P. Dobychin and T. E’. Tsalliiiekaya, Uuklady A k a d . N a u k USSR,109, 351 (1956). ( 3 ) C. D. Prater and R. L. Lago, Advances in Catalysis, 8, 293 (1950). vet
S U R F A C E
t
I
t
Rw2
v
Rw,
I
I
(I‘)
(B) Fig. 1.-Pore
(C) walls.
half-thickness of RW1. If on steam treatment the half-thickness is increased to R,z, then the activity per unit surface area is increased by the effective activity of the sites between the distances RW1and R,z from the surface. However, if the catalyst pore walls have the thickness Rw3,steam treatment will not increase activity per unit area. The constancy of activity per unit area with varying wall thickness indicates that the nature, energy and total number of sites are not changed by steam treatment. Any other conclusion would indicate that changes (during steam treatment) in the nature, energy, total number of sites and wall thickness accidentally compensate to leave the activity per unit area unchanged. An estimate of an upper limit for the effective depth of activity can be made from an “average” wall helf-thickness (radius) change on steam treatment. The “average” wall radius R for a catalyst of surface area 8 and pore wall volume, V,, was calculated from the equation R = -2 v
x
w
which is derived for a cylindrical or cubic model for the pore wall. The pore wall volume for the catalyst initially 499 m.2/g. was 0.435 ml./g.; thus for this catalyst R is 17.4 A. in the unsteamed material and 28 A. in the sample steamed most. If one assumes that all malls have a thickness equal to the average, our results indicate that with 95% probability the increase in activity per unit area is less than 10% upoii changing the average wall radius from 17.4 to 28 A. Porosimeter measurements indicate that a large ofraction of the pore walls have radii less than 17 A. This makes the above estimate conservative. Thus, mechanisms which allow significant contributions t o the activity by pore wall volume lying 17 A. below the surface can be eliminated from consideration. How much closer to the surface one can go before obseryiiig significant activity contributions is, of course, not known.
August, 1959
NOTES
Tamele4 suggests a mechanism of hydrocarbon cracking by silica-alumina catalysb according to which surface aluminum acts as a Lewis acid. Thomasi proposes surface protons take part in the cracking reaction. However, more general electronic interactions between the hydrocarbon and a solid catalyst are conceivable; the electron acceptors ("acid" centers) need not be surface centers. For example, four coordinated aluminums lying below the surface in the silica lattice could lead to the formation of an electron acceptor (hole). Under appropriate conditions the hole could become freed from the fixed lattice charge and become completely mobile. When this occurs acceptors in a thick boundary layer can participate in the reaction.6 This mechanism is eliminated from consideration. This result is consistent with the results of electrical conductivity studies made by Weisz, Prater and Rittenhouse7 which showed that no conduction by holes could be demonstrated in this catalyst.
The experimental data presented by the authors2*3 do not show the expected convergence. It can be shown in still another way that the hypothesis of constant molar absorptivities is hardly justifiable. The average molar absorptivity,2 a, of a solution of cerous perchlorate, supposed to be partly associated according to equilibrium 1 can be expressed by the equation4
(4) M. W. Tamele, "Heterogeneous Catalysis," Faraday Socicty, April, 1950, p. 270. (5) C. L. Thomas, Ind. Eng. Chem., 41, 2564 (1949). (6) K. Hauffe and H. J. Engell, Z. Elektrochem., 66, 366 (1952); P. Aigrand a n d C. Dugas. ibzd., 66, 363 (1952); P. B. Weisz, J. Chem. Phys., 21, 1531 (1953). (7) P. B. Weisz, C. D. Prater and K. D . Rittonhouse, cbzd., 23, 1965 (1955).
SPECTROSCOPIC STUDIES ON RARE EARTH COMPOUKDS. 111.1 T H E INTERACTION BETWEEN NEODYMIUM AND PERCHLORATE IONS BYP. NRUMHOLZ Contrtbutzo?k from the Research Laboratoru of Orquima S.A . . Sdo l'uulo, Brad Recezved October 1 6 , 1968
Perchlorate ions are commonly used as a standard of non-association. Recently, however, Heidt and Rerestecki2and Sutcliff and Weber3 attributed the observed variations of the absorptivity of the 2960 A. peak of cerous perchlorate solutions with the temperature and with the concentration of the perchlorate anion to the reaction Ce + 3
+ Clod-
= CeClOa + 2
(1)
Molar absorptivities of the free cerous ion, eo, and of the complex, €1, were assumed to be independent of the temperature and of the medium and numerical values were assigned to those constants. If those assumptions and assignments were correct, plots of absorptivities against the perchlorate concentration (or any suitable function of that, concentration) should extrapolate for (C104-) + 0 a t all temperatures to the same value, namely, eo. Similarly, absorptivities plotted a t different perchlorate concentrations against the temperature should converge a t high and low temperatures toward the values assigned to eo and el, respectively (reaction 1was supposed to be strongly exothermic). (1) Preceding communications. (a) Spectrochim Acta, 10, 20c) (1957), (b) 274 (1957) (2) L. J . Heidt aiid J. HaiastaLki, J . A n i P h a n ~ Sw 77, JU4q (1955). (3) L. H. Sutcliff a n d J. R. Weber, Trans. Faradau Soc , 63, 1225 (1956).
1313
a
-
EO
= f(€l
-
(2)
to)
\\.here j is the degree of association of CeC104+2. It follon-s from equation 2 that if eo and €1 are independent of the temperature and of the medium, the ratio -
Ea
ab - . f a
- ac
- f b
f &-
fc
(3)
where the indices a, b aiid c denote any three solutions of cerous perchlorate differing in their analytical composition or/and temperature, should be independent of the wave length. Contrary to what would be expected, the data presented by Heidt and Berestecki* in Fig. 1 reveal a strong dependence of R on the wave length. Thus either molar absorptivities depend,on the temperature or/ and the medium, or a second complex is formed, or both.4 The appearance of but one isosbestic point2 makes the existence of a second complex improbable. Whatever the reason for those discrepancies, not only are the numerical values of the thermodynamic constants reported2,3to be regarded as doubtful, but the very existence of the CeC104+2complex cannot be taken for granted. In order to clarify further the question of a perchlorate complexing of rare earth ions we studied the influence of perchloric acid on the absorption spectrum of the neodymium ion. Experimental Materials.-A 0.1 ti' solution of neodymium perchlorate was prepared by dissolving neodymium oxide of 99.9% purity in slightly less than the necessary amount of 0.3 M perchloric acid, filtering and acidifying to 0.01 M HCIO,. The perchloric acid (Baker analyzed reagent) contained less than O . O O l ~ oof nitrate and sulfate. Five ml. of the neodymium solution was mixcd with the desired amount of perchloric acid and diluted a t 25" to a volume of 25 =t0.02 ml. If necessary, the neodymium solution was rvaporated within the volumetric flask to a smaller volume. Spectrophotometric Measurements.-All measurements were made with a grating specLrophotomcter previously described's at a band width of l A. and a t an optical depth of 50 mm. Temperature was hrld constant during the measurcmentb ~ i t h i n1 0 . 1'. L\lcasurcmcnts were performed by a differential method5 romparing the alisorptivities of ncodymium pcrchlorate solutions xithout arid with the addition of perchloric arid 1)isturhanccs due to accidental inpurities of the solutions were rliminated by comparing absorbancies a t a wave length where the absorptivity of ncodymium perchlorate is negligible and substracting the difference from the main result. We used as ;eferencc wave length 543q A for measurements on the 5200 A. band system and 6050 A. for the 5750 A. band system. S o correction could be applied for the measurements on the 5200 A. band system in 11.8 M perchloric acid due to the strong increase of the background absorption. Differential absorbancy measurements could be reproduced with a precision of about f0.001. Calibration errors and drift of the amplifier introduce it11 :r,ddit8ion:drrror of :thout f l 9 ot t h r measiirrd tliflerrwt.. (4) See T. W . Neutoii arid F. B. Bakci, THE JOLRNAL, 61, 934 (1957). (5) C. F.Hiskey, Anal. Chem., 21, 1440 (1919).
