Acidic Properties of Mixed Tin+ Antimony Qxide Catalysts BY ELIZABETH A. IRVING AND DUNCAN TAYLOR* Chemistry Department, University of Edinburgh, Edinburgh EH9 355 Receiwd 27th Muy, 1977 Mixed oxides of tin and antimony have been used as catalysts in a static system and outgassed both at room temperature and at 698 K in a study of the approximately zero order stages of the isomerization of 3,3-dimethylbut-l-ene (373 K), cyclopropane (41 1 K), but-1-ene (293 K) and cis- but-2-ene (293 K) and of the dehydration of isopropanoI(343 and 408 K). With catalysts outgassed at room temperature, weakly acidic sites are present, and all the reactions probabiy occur by a carbonium ion type of mechanism with Bronsted acid sites as a source of protons. Rates increase to a maximum as the antimony content increases from zero to NN 50 atomic %, and then decline with further increase in the antimony content.Outgassing of the catalysts at 698 K increased the rate of isornerization of 3,3-dimethylbut-l-ene, but for cyclopropane and isopropanol decreased rates were observed due to poisoning by the propene product. For but-1-ene and cis-but-2-eneY the higher temperature outgassing procedure changed the rate against catalyst composition pattern considerabIy in that only catalysts with less than 50 % Sb were active, and a mechanism involving an allyl inter- mediate is proposed. Catalyst activity could be poisoned by treatment with bases or with sodium acetate. It is concluded on the basis of a proposed correlation between rates and acidity, that the catalyst composition corresponding to maximum acidity is different from that for maximum selective oxidation activity.Mixed oxides of tin and antimony, besides possessing selective olefin oxidation properties,l have been shown to exhibit acidic activity in the propene-D,Q exchange reaction provided the oxides were not pre-exposed to the olefin. The carbonium ion mechanism of the exchange changed after pre-treatment of the oxides with propene to one involving an allyl intermediate. Bronsted acidity has been reported to account for butene isomerization reactions also. To clarify the nature and distribu- tion of acidic centres as a function of catalyst composition, a range of mixed oxides outgassed at both ,low and high temperatures has been used in a study of the isomerization of 3,3-dimethylbut-l-eneY cyclopropane and butenes, and of the decomposition of isopropanol.EXPERIMENTAL All reactions were carried out in an electrically heated 270 cm3 cylindrical Pyrex reaction vessel attached to a conventional gas-handling vacuum system. A gas sampling valve allowed ~ 0 . 5 % of the vessel’s contents to be removed periodically for analysis by G.L.C. In experiments with 3,3-dimethylbut-l-ene, a 2 m column of 35 % propylene carbonate on Chromosorb P was used at 293 K ; the same column was used at 303 K in the cyclopropane work. For isopropanol reactions, a 2 m column of 25 % carbowax 1500 on Chromosorb W AW/DMCS was used at 338 K, and for reactions of butenes a 3 m column of 28 % bis-2-methoxyethyladipate on Chromosorb P at 298 K.The range of tin + antimony oxide catalysts was selected from those described previously. All catalyst compositions are given as atomic percent of antimony in the total metal content. For each run a fresh sample of catalyst of weight between 0.2 and 1.0 g was outgassed by one of two procedures. In experiments described below as series I, catalysts were evacuated at 133 x N m-2 at room temperature for 5 h ; in series I1 experiments evacuation was for 16 h at 698 K. After the outgassing process, the reaction vessel was brought to the reaction temperature 206E . A . IRVING AND D . TAYLOR 207 in vacuo before admission of reactant (purity >99 %) to initial pressures of 1.6 kN m-2 for 3,3-dimethylbut-l-ene and cyclopropane, 1.46 kN m-2 for but-lene and cis-but-2-eneY and 0.67 kN m-2 for isopropanol.Except for isopropanol, a very rapid reaction occurred in the &st 1-2 min, during which 5-10 % of the reactant was consumed. The extent of this rapid reaction depended on both the reactant and the cataIyst composition. The rate of the next 10-20 % of the reaction % Sb Frc, ].-Rates of isomerization of 3,3-dimethylbut-l-ene at 373 K on mixed Sn+Sb oxides. (A) series I catalysts ; (B) series I1 catalysts. was close to zero order, and was taken as a measure of catalyst activity to construct the activity patterns of fig. 1-3. Later in the reactions, slower rates, again approximately zero order, occurred but activity patterns using the slower rates did not differ significantly from those in the figures. For isopropanol reactions, the zero order rate of the first 15-20 % of the decomposition was used.the reactant, and were shown diffusion effects. Rates were reproducible within f 4 to f 8 % depending on by use of different weights of catalyst not to be limited by FIG. 2.- % Sb catalysts. -Rates of propene formation from isopropanol at 408 K on mixed Sn + Sb oxides, series II208 ACIDIC PROPERTIES OF Sn+Sb OXIDES In studies of the effects of poisons on rates, catalysts were either treated with 10 % aqueous sodium acetate followed by washing, drying, and outgassing at 698 K, or, after outgassing at room temperature or 698 K, were exposed in the reaction vessel at the reaction temperature to pyridine (0.13 or 0.4 kN m-2 for 30 min) or 2,6-dimethylpyridine (0.13 kN m-2 for 60 min) followed by 30 min evacuation.i0Q- N B E ri I .- 0 20 4 0 2 0 40 10 5 0 % Sb FIG. 3.-Rates of isomerization of but-1-ene (A) and cis-but-Zene (€3) at 293 K on mixed Sn+Sb oxides, series I1 catalysts. RESULTS AND DISCUSSION Since isomerization of 3,3-dimethylbut-l-ene is known to proceed by a carbonium ion mechanism requiring uptake of a proton by the olefin, the results at 373 K shown in fig. 1 indicate the presence on the mixed oxides of Bronsted acid sites. The fact that a 9-fold decrease in rate occurred after the 26.8 % Sb catalyst had been poisoned by the sodium acetate treatment not only provides support for the occurrence of Bronsted acid sites, but also indicates that the distribution of rates across the catalyst composition range runs parallel to the distribution of acid sites.By Benesi’s method the catalysts were clearly shown to possess acidic properties, but quantitative measure- ments of acidity to confirm the rate-acidity correlation were not possible due to the grey-blue colour of the original mixed oxides. However, indirect support for the correlation was provided by the results of the isopropanol dehydration reaction given below, a reaction for which activity against acidity correlations for catalysts have been established.6*7 Comparison of curves A and B in fig. 1 shows that outgassing of the catalysts at 698 K instead of room temperature caused an increase in the number and/or activity of the acid sites, an effect which may be associated with the removal of adsorbed water molecules and the occurrence of surface phase changes.The only isomers formed were 2,3-dimethylbut-2-ene and 2,3-dimethylbut- 1 -ene in proportions ( 5 : 1) slightly greater than the calculated thermodynamic ratio ; hence the acid sites are to be regarded as weak rather than ~ t r o n g . ~ Decomposition of isopropanol at 343 K in series I experiments also indicated the presence of Bronsted acidity since rates of formation of propene gave an activity pattern similar to that of fig. l(A), the only difference being a sharper maximum at 49.5 % Sb. The mole percent propene in the decomposition products ranged from 85 to 100, and of di-isopropyl ether from 2 to 12, depending on the catalyst composi-E. A . IRVING AND D. TAYLOR 209 tion. Never mor2 than 3 % of the alcohol was dehydrogenated to acetone.In series 11 experiments at 408 K, rates of propene formation gave the pattern in fig. 2. In tb,: catalyst composition range 19.6-75 % Sb, propene was the main product (87-100 %), with small amounts of di-isopropyl ether. Tin oxide and the 6.1 % Sb catalyst, however, gave acetone in 90 % yield, but as the antimony content of the catalysts increased above 6.1 %, the acetone selectivity fell sharply. The dehydration mechanism most probably involves uptake of a proton by the alcoholic OH group, but in spite of the conclusion above that in series I1 the catalysts possess higher Bronsted acidity than in series I, propene formation rates in those series I1 experi- ments, where propene was the major product, were some five times less than in corresponding runs in series I.The slower rates in series I1 are believed to be due to preferential poisoning by propene, an effect which has been reported previously with the same catalysts for the propene-D20 exchange reacti0n.l In view of the reported occurrence of Lewis acid sites on SnOz after evacuation at 508 I<,9 the series I1 mixed oxides may also possess Lewis acidity to an extent modified by the antimony content. Accordingly, dehydration by Gentry and Rudham's mechanism lo involving OH- abstraction from the alcohol by a Lewis site may also be occurring, and could explain the small rate of propene formation found with SnO, and the 6.1 % Sb catalyst, since the latter catalysts, in view of the results in fig. 1, appear to have no Bronsted acidity. However, for Sn02 itself, dehydrogenation sites are clearly more numerous and/or active than dehydration Lewis sites.Treatment with pyridine at 408 K of those series I1 catalysts which gave both acetone and dehydration products in significant amounts showed that dehydration sites were poisoned to a much greater extent that those causing dehydrogenation. This illustrates the difference between the two types of site, besides again indicating the existence of acidic sites. Rates of isomerization of cyclopropane to propene at 41 1 K in series I experiments gave an activity pattern similar to that of fig. 1(A) but with a pronounced maximum at 49.5 % Sb. These results are consistent with a carbonium ion type of mechanism, again involving the Bronsted acidity of the catalysts. Parallel to the results for isopropanol, isomerization rates at 411 K for series I1 were lower (between 4- and 100-fold depending on the % Sb) than rates for series I, caused most probably by preferential poisoning of the series II catalysts by the propene product.The isomerization of but-1-ene and of cis-but-2-ene at 293 K over series I catalysts both gave activity trends not significantly different from that in fig. l(A), but with sharp maxima at 49.5 %Sb. But-1-ene reacted about seven times faster than the cis-isomer ; in each reaction the product ratio (cis : trans from but-1-ene, trans: I-ene from cis-but-2-ene) was always approximately four. Treatment of the catalysts with 2,6-dimethylpyridine partially poisoned both reactions, but without change in the product ratios.These results are adequately accounted for by carbonium ion mechanisms involving the Bronsted acidity of the catalysts. Reaction of the butenes at 293 K over series I1 catalysts, gave activity patterns as shown in fig. 3 quite different from those in fig. 1(B) and 2, and, therefore, indicate a change of mechanism from that in series I. Only those catalysts containing less than 50 % Sb were active, the maximum rate occurred at 19.6 % Sb for but-1-ene isomerization and at 8.7 % Sb for cis-but-2-ene ; the product ratios were respectively M 1 and ~ 4 . The but-1-ene reaction was strongly poisoned by 2,6-dirnethylpyridine and by but-1-en@ itself, and also by butadiene which was formed in ~ 0 . 2 % amounts during the isomerization. The much slower reaction of the cis-isomer was poisoned only to a small extent by the base, and no butadiene was formed.In neither reaction was the product ratio affected by poisoning. In view of the inactivity in series I1210 ACIDIC PROPERTIES OF Sn+Sb OXIDES of the 49.5 and 75 % Sb catalysts compared with their pronounced acidic activity shown in fig. 1(B), it is unlikely that the butene isomerizations occur mainly via carbonium ion intermediates involving the Bronsted acidity of the catalysts. Mechanisms involving allyl intermediates are more probable ; it is suggested that the poisoning effects are caused by strong adsorption of the poisons at those metallic ion sites required for adsorption of the allyl species. It is significant that in the selective oxidation of propene over the same series of catalysts,l a reaction known to involve an allyl species, maximum oxidation activity was observed for catalysts with the same range of antimony content as cause maximum rates of isomerization of the butenes.The main indications from this work are that (1) mixed oxides of tin and antimony exhibit Bronsted acidity, particularly in the composition range 25-75 % Sb, (2) the acidity present after degassing at room temperature is increased by degassing at 698 K, and (3) maximum acidity occurs in a composition range different from that where maximum propene selective oxidation activity is observed. Acknowledgements are made to the B.P. Co. Ltd. for the supply of catalysts, to Dr. C . C . McCain for stimulating discussions, and to the S.R.C. for the award of a maintenance grant (to E. A. I.). G. W. Godin, C. C. McCain and E. A. Porter, Proc. 4th In?. Congr. 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