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demands of future chemical processes in which efficient utilization of feed resources is a key requirement. Registry No. CHz=CHCH3,115-07-1;CH,=CHCHO, 10702-8; CH,=CHCN, 107-13-1; Moo3, 1313-27-5; Bi2Mo3OI2, 13595-85-2;CcH&H&H=CHz, 300-57-2; C H ~ O P C ~ H ~ C H ~ H = CHz, 140-67-0; BizMo06, 13565-96-3; Biz03, 1304-76-3; leO, 14797-71-8; allyl alcohol, 107-18-6; allylamine, 107-11-9; azopropene, 29771-78-6;acrylic acid, 79-10-7;deuterium, 7782-39-0.
Literature Cited Adams, C. R. Roc. 3rd Int. Cog. Catal., Amsterdam 1985, 240. Burrington, J. D.; Grasselll, R. K. J . Catal. 1979, 59, 79. Burrington. J. D.; Kartlsek, C. T.; Grasselll, R. K. J . Catal. 1980, 63,235. Burrlngton, J. D.; Kartlsek. C. T.; Grasselll, R. K. J . Catal. 1983, 81, 489. Burrlngton, J. D.; Kartlsek, C. T.; Grasselll, R . K. J . Org. Chem. 1981, 4 6 , 1877.
Davydov, A. A.; Mlkhakchenko, V. G.; Sokolovskll, V. D.; Boreskov, G. K. J . Catel. 1978, 55, 299. Grasselll. R. K. "Acrylonltrlle Synthesis and Related Processes by Heterogeneous Catalysis"; Proceedlngs of CIESC/AChE, Jolnt Meeting of Chemlcal Engineering, Beljlng, Chlna, Sept 1982; Vol. 11, p 731. Gfasselll, R. K.ACS Symp. Ser. 222 1983. 332. Gfasselll, R. K.; Burrington, J. D. A&. Catal. 1981, 3 0 , 133. Qrassel, R. K.; Burrington, J. D.; Brazdll, J. F. faraday Discuss. Chem. Soc. 1981, 72, 204. Qrrybowska, 6.; Heber, J.; Janus, J. J . M a l . 1977, 49, 150. Iwasawa, Y.; Nakano, Y.; Ogasawara, S. J . Chem. Soc.,Faraday Trans. 1 1978, 7 4 , 2988. SheRon, J. R.; Llang, C. K. J . Org. Chem. 1973. 38, 2301.
Received for review August 26, 1983 Revised manuscript receiued November 21, 1983 Accepted December 13, 1983
Influence of Acid Strength Distribution on the Cracking Selectivity of Zeolite Y Catalysts Aveilno Corma, Juan 6. Monton,t and Antonio V. Omhll16st Instituto de Cai%lisis y Petroleoqdmlca, C.S. I.C., Serrano, 7 19,MadrM -6, Spain
The cracking of n-heptane on HY uttrastable zeolites has been studied in a continous flow reactor at atmospheric pressure and up to 470 OC. The selectivity curves in the absence of decay have been obtained for each reaction product, the initial SeleCtMtles to the primary products have been calculated, and from them the values of the initial selectivities to the three main reactions, i.e., cracking, isomerization, and disproportionation, have been obtained. The kinetic parameters have been calculated and these values compared with those obtained using a CrH NaY zeolite. The differences in activity and selectivity between the two catalysts have been discussed in the light of differences In mechanisms, the relative amount of Bronsted to Lewis acM sites, and the acid strength distribution in the two catalysts.
Introduction Most of the time, the activity and selectivity of different cracking catalysts have been compared on the bases of one experimental result obtained at a fixed set of conditions. This type of procedure has implicit the assumptions that the mechanism of the reaction and catalyst decay is the same on all catalysts. If the first assumption can be accepted in general, the second one is far from being true in the case of cracking catalysts. Also, it is well-known that besides primary cracking, other parallel and consecutive reactions are also taking place. Then, if these reactions are not either avoided or taken into account, the comparison of the final cracking results can be quite misleading (Corma and Wojciechowski, 1983). In previous work (Corma et al., 1981,1982)a methodology for studying such reactions was presented and applied to the case of nheptane cracking on a chromium exchanged Y zeolite. There, high initial selectivities for n-heptane isomerization and disproportionation were found. The relatively high selectivity of that catalyst for those reactions may be explained either by the presence of chromium on the catalyst, which could make it act as a bifunctional catalyst, or because of the particular acid strength distribution created by exchanging the NaY zeolite by chromium ions. In the present work a systematic and extensive study of the cracking of n-heptane on an HY ultrastable zeolite t Departamento Quimica TBcnica, Universidad de Valencia. Avda. Dr. Moliner, Burjasot (Valencia). 0196-4321/84/1223-0404$01.50/0
has been carried out. The selectivity curves for reaction products have been generated in the absence of decay (Best and Wojciechowski, 1977). Initial selectivity values, kinetic rate constants, activation energies, and acid strength distribution for the catalyst have been obtained and compared with those obtained for the chromium exchanged Y zeolite (CrHNaY). Finally, the possible role of chromium ions and the nature of the active sites involved in the reactions taking place during cracking of n-heptane are discussed. Experimental Methods Materials. The HY ultrastable zeolite (HWS) catalyst used in this study was prepared by a conventional exchange technique. The starting material was NaY from the Na+ was replaced by H+ by repeated Linde (SK-40); ammonium acetate exchange followed by calcination, until the fiial Na+ content of the zeolite was lower than 2% of the initial. The CrH NaY zeolite was prepared by exchange of the sodium zeolite at 55 "C and constant PH 4.5, with aqueous solution of chromium acetate. In the final CrHNaY zeolite catalyst, 45% of the original Na+ has been exchanged, in a 32% by Cr3+ions, and 13% of direct exchange of the Na+ by protons from the aqueous solution. More details on the preparation and characterization of this sample are given elsewhere (Asensio, 1975;Agudo et al., 1982). The n-heptane was a high-purity Carlo Erba reagent. Procedure. The cracking experimentswere carried out in a continuous flow fixed-bed reactor at atmospheric 0 1984 American Chemical Society
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pressure and temperatures of 430,450, and 470 O C . The catalyst was placed in a Pyrex tubular reactor (30 mm i.d.) and pretreated in an N2stream at 350 "C for 1h; then the temperature was raised progressively to 450 "C and held for 1h; 12.5 cm3 of n-heptane (8.625 g) was always fed by means of a positive displacement pump, and in this way a catalyst to oil ratio was fixed. The time-on-stream was changed by feeding the n-heptane at different velocities. The catalyst to oil ratio was modified by changing the amount of catalyst into the reactor. The effluent from the reactor was passed through a water-ice cooled condenser and a acetone-dry ice cold finger condenser. When the run was finished, the reactor system was stripped by a flow of N2at reaction temperature, for 10 min. Then the liquid products were removed, weighed, and analyzed. The gaseous stream was introduced into gas buret, where it was trapped by downward displacement of water saturated with NaC1. The total volume of gas recovered in each run was measured and their composition determined. Mass balance of not less than 95% was obtained on the basis of the liquid, gaseous products, and coke on catalysts. After stripping with N2,the catalyst was regenerated in situ by passing through air at 500 "C for 4 h. It was found that under these conditions the regeneration of the catalyst was completed. During the regeneration the outcoming gases from the reactor were passed through a furnace heated at 350 "C and containing metal copper pellets, where the CO was oxidized to C02. After this, the water generated by the combustion of coke was adsorbed on drierite and the C 0 2 on ascarite, and thus the amount of coke on catalysts was determined. Total conversion was calculated as the total number of carbon atoms of outlet hydrocarbons, other than the feed, divided by the number of carbon atoms in the n-heptane fed. The yield of a product was defined as moles of product divided by moles of n-heptane fed. For the IR spectroscopic measurement a wafer of 10 mg cm-2 of the H W S zeolite was desgassed for 3 h at 450 "C and lob ton in a conventional greasless cell. The spectrum was registered at room temperature in a Perkin-Elmer 5808 spectrophotometer equipped with data station. The n-butylamine titration of the catalysts was carried out following Benesi's method (Benesi, 1956, 1957).
Kinetic Calculations A first-order kinetic equation has been used to fit the overall cracking results -In (1- x ) = K1[S]7= K l [ S O ] ~ ~ (1) where x is the fraction of n-heptane cracked, K l is the overall kinetic rate constant, [SI is the concentration of active sites at each time on stream while [So] is the concentration of active sites at zero time on stream, r is the contact time, and 0 is the fraction of active sites available . decay of the catalyst with time for cracking ( [ S ] / [ S o ] ) The on stream is taken into account by means of a time on stream function (Wojciechowski, 1968)
With eq 1 and 2 and that relating the instantaneous ( x ) and average conversion (a) (3) is is possible to calculate the parameter K l , Kd, and m. The Kctcc5and KGM,values can be calculated by taking into account the initial selectivities (I.S.) for the two
f
3900
3800
3700
3600
3500
3400 cm-l
Figure 1. IR spectrum of the HYUS zeolite in the OH streching region
.
cracking reactions yielding C2 + C5 and C4 + C3 and the following equations (4) (5) KC,+C, = Kl[SOI - KC2+C6 For the isomerization and disproportionation reactions a first- and a second-order kinetic rate equation have been used, respectively, and the kinetic rate constants have been calculated following the equation
which can be further simplified since in this work Co was maintained constant at 1 atm ri Ki -= (7) -rh i=3 C Ki i=l
in which K2 and K3 are the rate constants for isomerization and disproportionation, respectively. Results The IR spectrum of the HYUS zeolite (Figure 1)shows absorption bands at 3560,3600,3675, and 3750 cm-'. This spectrum indicates that indeed an ultrastable Y zeolite of the so-called deep bed type (Jacobs and Uytterhoeven, 1971; Ward, 1976) has been obtained by the preparation procedure described before. In a series of preliminary experiments it was established that in the experimental conditions used in this work, thermal cracking is negligible and it has not been taken into account for corrections during catalytic calculations. No control by external or intraparticle diffusion was observed for the flows and catalyst particle size (0.5-0.75 mm) used in this work. The experimental cumulative average conversion at different catalyst/oil ratio, and time on stream have been fitted to eq 1 and 2, using a sum of squares of residuals as the criterion of fit, with a residual being defined as the difference between the experimental average conversion and that predicted theoretically using eq 1 and 2. The experimental data are well fitted by eq 1and 2 (Figure 2) having the standard deviations 0.0066,0.0056, and 0.0093 at 430,450, and 470 "C, respectively. The product yields at constant catalysts to oil ratio vs. the corresponding total conversion at 430 "C are shown in Figure 3. Broken lines appear where necessary to distinguish parts of the various constant catalyst to oil loops. Solid lines enclosing such loops are the OPES (Best and Wojciechowski, 1977). The
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Table I. Reaction Products from the NH Cracking Reaction over HYUS Zeolite a t 430 "C product methane ethane ethylene propane propylene n-butane isobu tane butenes C, fraction isopentane n-pentane pentenes C, fraction n-hexane 2-methylpentane 3-methylpentane hexenes C, fraction 2-methylhexane 3-methylhexane aromatic fraction toluene xylenes
type secondary stable primary + secondary stable primary + secondary unstable primary + secondary stable primary unstable primary + secondary stable primary + secondary stable primary unstable primary unstable primary + secondary stable primary unstable primary unstable primary + secondary unstable primary unstable primary + secondary stable primary + secondary stable primary + secondary stable primary + secondary stable primary + secondary stable primary + secondary stable secondary stable secondary stable secondary stable
types of products which these OPESrepresent are listed in Table I and the initial selectivities for all primary products are listed in Table 11. The initial selectivities have been calculated by measuring the slope of the tangent to the OPES at zero conversion of each product given in Figure 3, Taking into account the disproportionation reaction (Corma et al., 1981) the corrected initial selectivities are presented in Table 111,energies and frequency factors were calculated and are listed in Table V. Discussion From the behavior of the OPE curves shown in Figure 1 it can be seen that in the range of conversions studied at 430 "C (
