Influence of Sodium on the Physico-chemical and Catalytic Properties of Magnesium Oxide BY JACEK KIJE~SKI AND STANISLAW MALINOWSKI* Institute of Organic Chemistry and Technology, Technical University (Politechnika), Koszykowa 75, 00 662 Warszawa, Poland Received 1 1 th February, 1977 Physico-chemical and catalytic properties of two series of catalysts comprising magnesia and sodium were determined. The first (I) series of catalysts was prepared by doping magnesia with varying amounts of NaOH. The second (11) series of catalysts was obtained by evaporating metallic sodium onto MgO preparations precalcined at different temperatures. The concentration and strengths of basic and acid sites, as well as the amounts of one-electron donor and one-electron acceptor sites, were measured, and the specific surface areas of the catalysts determined.Catalytic activity in isomerization of pent-1-ene, tram-pent-2-ene and the conversion of cumene was studied. It was concluded that the (11) series of catalysts displayed remarkably strong basic and one-electron donor properties. Also, it was proved that basic sites coexist on the surfaces of catalysts with the one-electron donor sites. Catalytic activity in alkene isomerization and cumene dehydrogenation was unambiguously associated with the presence of well-defined surface active sites. In previous papers it was shown that addition of alkali metal to oxide catalysts brings about a considerable change in their physical and catalytic properties, the variation being non-linear. 1-6 The aim of the present study was to investigate the sodium effect on the physico- chemical and catalytic properties of magnesium oxide.Magnesia is known to be one of the strongest solid bases,7* and oxygen anions 0’- of low coordination number are considered to be responsible for its basic properties. Taking into consideration the low first ionization energy of sodium compared with that of magnesium, it was anticipated that the reaction of sodium with the oxygen ions of MgO surfaces would result in an increase in the effective negative charge of oxygen anions. The immediate result of such a reaction should be a significant rise in both the basic and one-electron donor power of the catalytic system. In the present work, acid-base, radical and catalytic properties of two sets of MgO + Na catalysts have been studied.The catalysts were prepared (I) by impregna- tion of MgO with NaOH in aqueous solution, and (11) by evaporation of metallic sodium onto a MgO surface. EXPERIMENTAL PREPARATION OF CATALYSTS Magnesium hydroxide was obtained by hydrolysing Mg(NO& 6H20 with aqueous ammonia. The resulting precipitate was dried at 60 and 120°C and subsequently heat- treated at 550°C for 16 h to give pure magnesia. Both drying and calcination were performed in a flow of argon. Catalysts of the MgO+NaOH series were prepared by impregnating magnesia with aqueous NaOH solutions of varying concentrations. The preparations were treated in the manner described above. The amount of sodium introduced was measured 250J . K I J E ~ S K I AND s. MALINOWSKI 25 1 using an atomic absorption spectrometer.The catalysts under consideration were found to contain 0 ; 0.005 ; 0.01 ; 0.07 ; 0.35 and 0.82 mmol Na per g MgO respectively. Metallic sodium was evaporated onto the magnesia surface at 480°C under Torr pressure (1 Tom = 133.32 N m-2). In one run 200 mg of sodium was deposited on 3 g of MgO. After all the sodium was used up the temperature was raised to 550°C and maintained at this level for 2.5 h, the pressure being Torr. SPECIFIC SURFACE AREA MEASUREMENTS The method of thermal desorption of argon was employed to measure the specific surface areas of catalysts under examination. A gas mixture containing 5 % argon and 95 % hydrogen was used for the determinations. BASE A N D ACID PROPERTIES The number of basic sites at various strengths were determined by titrating the catalysts with benzoic acid solutions in benzene in the presence of appropriate Hammett indicators.The following were applied as basicity indicators : 2,4,6-trinitroaniline (PKa = 12.2) ; dinitroaniline (PKa = 15.0) ; 4-chloro-Znitroaniline (PKa = 17.6) ; 4-nitroaniline (PKa = 18.4) ; diphenylamine (PKa = 22.3) ; 4-chloraniline (PKa = 26.5) ; aniline (pKB = 27) ; triphenylmethane (PKa = 33); diphenylmethane (PKa = 35). Acidity was measured by titrating the surface with a benzene solution of n-butylamine. Dicinnamalacetone (PKa = - 3.0), benzalacetophenone (pKa = - 5.6) and anthraquinone (PKa = - 8.2) were used as zcidity indicators. Since nearly all the indicators used assumed the colour of the corres- ponding anions in the presence of extremely strong surface basic sites, it was impossible to determine the acidity of the catalysts with evaporated sodium.The only finding in this connection was that anthraquinone adsorbed on the MgO+ Namet catalysts does not undergo colour change, this being indicative of the absence of acid sites at a strength of Ho < -8.2. SURFACE FREE RADICAL PROPERTIES The one-electron donating or accepting properties of the catalysts were determined by adsorbing organic molecules, i.e. one-electron acceptors or donors respectively, onto the catalyst surface and recording the e.s.r. signals of the resulting ion radicals. Perylene (ionization energy (IE) 7.22 eV) and pyrene (IE 7.55 eV) are known to form cation radicals on oxide surfaces by donating one electron to the surface accepting s i t e ~ .~ - l l Tetra- cyanoethylene [electron affinity (EA) 2.8 eV], s-trinitrobenzene (EA 1.7 eV), m-dinitrobenzene (EA 1.4 eV) and nitrobenzene (EA 0.7 eV)? are capable of accepting one electron from the surface donor sites to give the corresponding anion radica1s.l’. l3 Tetracyanoethylene is thought to react readily with all one-electron donor centres (including those of low one- electron donating power) in contrast with TNB, DNB and NB which are only able to react with correspondingly stronger sites. Adsorption of electron donors or acceptors from benzene solutions was carried out under argon. The samples were kept under argon while recording e.s.r. spectra. REACTION PROCEDURE Conversion of cuniene was carried out in a conventional flow reactor with a fixed catalyst bed.The space velocity was 1 g cumene per 1 g catalyst per hour. Isomerization of pent-1-ene and trans-pent-2-ene was performed in a batch-type set-up with stirring at 20°C. Reaction products were analysed using a gas chromatograph Chrom 4 with a 50 m capillary Squalane column. Products of isomerizations were analysed at room temperature, whereas the analyses of cumene conversion products required a temperature of 90°C. Mass spectrometry was employed for additional identification of products. The mass spectra were recorded using Varian MAT 11 1 spectrometer. t s-TNB is sym-trinitrobenzene, rn-DNB is metu-dinitrobenzene and NB is nitrobenzene,252 Na INFLUENCE ON MgO PROPERTIES POISONING OF THE ACTIVE SITES The catalyst was suspended in a benzene solution of Hammett indicator or one-electron acceptor, the amount of which was stoichiometric with respect to the number of corres- ponding active sites on the catalyst surface.The suspension was stirred for 4 h at room temperature. Benzene was then distilled off in a vacuum of Torr and dry deoxidized argon was admitted to the flask with the catalyst. E.S.R. MEASUREMENTS The e.s.r. spectra were registered using a Jeol JES-MC3X spectrometer. Preparations with preadsorbed electron donors or acceptors were all studied at room temperature, while those with preadsorbed cumene or pent-1-ene were being examined at - 100°C. RESULTS The acid-base properties of catalysts prepared by impregnating MgO with aqueous solutions of NaOH are close to those of the starting material, i.e. pure MgO.The reaction of magnesium oxide with sodium ions occurring on the MgO surface does not affect the maximum acid-base strength of the system. As a result only the number of centres is modified to an extent dependent on the quantity of NaOH added (table 1). Maximum concentration of both basic and acid surface sites was noted for the catalyst containing 0.35 mmol Na+ per g MgO ; and the lowest concentration of both types of sites was found for the catalyst doped with 0.82 mmol Na+ per g MgO The basic properties of both pure MgO and MgO doped with NaOH weakened with increase in the temperature of calcination. Thus, at 1000°C, the strongest basic sites disappear (27 < H- < 33) and the concentration of basic sites at lower strengths decreases substantially.TABLE CO CONCENTRATIONS OF BASIC AND ACID SITES FOUND FOR CATALYSTS DOPED WITH DEFERENT AMOUNTS OF SODIUM HYDROXIDE concentrations sodium content calcination mmol m-2 of acid centres! in catalyst temperature specific surface mmol m-2 concentrations of basic centres at various strengths/ /(mmol/g MgO) 1°C area/m2 8-1 12.2 < H- 18.4 d H- 27 < H- Ho < -3.0 < 15 <22.3 <33 total 0 0.005 0.01 0.07 0.35 0.082 550 750 lo00 550 750 lo00 550 750 1000 550 750 lo00 550 750 lo00 550 750 1000 64 58 48 72 65 51 81 73 36 58 35 31 29 26 22 28 25 22 0.015 0.006 0.006 0.014 0.008 0.005 0.014 0.006 0.008 0.01 1 0.01 5 0.01 1 0.031 0.019 0.017 0.008 0.010 0.003 0.012 0.007 0.006 0.010 0.014 0.004 0.008 0.008 0.009 0.017 0.020 0.013 0.026 0.018 0.01 6 0.020 0.002 0.002 0.023 0.01 1 0 0.021 0.014 0 0.021 0.01 1 0 0.010 0.013 0 0.01 5 0.014 0 0.007 0.001 0 0.050 0.024 0.012 0.045 0.036 0.009 0.043 0,025 0.017 0.038 0.048 0.024 0.072 0.051 0.033 0.035 0.013 0.005 0.007 0.01 5 0.012 0.005 0.012 0.010 0.007 0.012 0.015 0.006 0.029 0.027 0.014 0.033 0.029 0.010 0.027 0.019J .K I J E ~ S K I AND s. MALINOWSKI 253 An insignificant rise in the concentration of acid sites can be observed when the calcination temperature is raised from 550 to 750°C. Catalysts calcined at 750 and 1 000°C appeared to have practically identical acid properties. The one-electron donating properties of magnesium oxide undergo an insignificant change upon doping with sodium hydroxide. The greatest concentration of one-electron donor centres capable of reducing TCNET was found in the catalyst containing 0.35 mmol Na+ per g MgO (fig.1 ).0 0,005 0.01 037 0.35 0.82 30 25 20 T: k ri 15 \ .f3 8 10 5 mmol Na per g MgO FIG. 1 .-Concentrations of one-electron donor centres on magnesia doped with varying quantities of NaOH and the yields of cumene transformation reactions over theselcatalysts. Since both the electron donor and the basic properties are most pronounced for the same catalyst in the series, i.e. the one containing 0.35 mmol Na+ per g MgO, it was necessary to find out if donating one electron or a pair of electrons can be attributed to one and the same active site on the surface. The fact that the observed concentra- tions of one-electron donor centres are much lower than those of basic sites seems to favour the idea that there must be different types of sites responsible for the two properties.Additionally, no e.s.r. signals of anion radicals are observed upon adsorption of Hammett indicators, which leads to the conclusion that the apparent colour change is caused by the process of donating an electron pair rather than one electron to the Hammett indicator molecule. In order to suppress all basic sites, 2,4,6-trinitroaniline (pK, = 12.2) was adsorbed on the catalyst surface, its amount corresponding stoichiometrically to the number of detected basic sites. TCNE was subsequently adsorbed on such a catalyst. E.s.r. spectroscopy revealed (TCNE)- anion radicals in a quantity equal to that observed for the catalyst containing no Hammett indicator.This result gives support to the supposition that there are two different types of sites : one responsible for one-electron donating properties and the other for the basic properties of the catalyst. t TCNE is tetracyanoethylene.254 The surface reaction between MgQ and metallic sodium results in the catalytic system having salient basic properties. Catalysts with evaporated sodium were found to possess basic sites of extreme strength (superbase properties If- 2 35). Na INFLUENCE ON MgO PROPERTIES 8 N z E X I N totol :.8, /---- MgO pretreatment temp./"(= FIG. 2.-Concentrations of basic sites at various strengths as found for the catalysts (MgO) doped with metallic sodium. ! MgO pretreatment temp /"C FIG. 3 .-Concentrations of one-electron donor centres on catalysts doped with metallic sodium.J .K I J E ~ S K I AND s. MALINOWSKI 255 To date, centres exhibiting such extraordinary strength have not been reported in the literature. The greatest concentration of superbase centres as well as those of strength 27 < H- < 33 were detected for catalysts containing magnesium oxide pretreated at relatively lower temperatures, i.e. 550 and 650°C. Both superbasic and basic centres at 27 4 H- < 33 disappear for catalysts prepared from MgO precalcined at higher temperatures; only the concentration of basic centres at a strength of 18.4 < H- < 22.3 is practically independent of the temperature of MgO pretreatment. The one-electron donor sites concentration against temperature plots for catalysts doped with metallic sodium are given in fig.3. Exceedingly strong sites of a one- electron donor type were found for catalysts belonging to this series ; they can reduce a nitrobenzene molecule. A maximum concentration of one-electron donor sites was noted for the catalytic system Mg0700+Namet. The general conclusion is that catalysts obtained by evaporating sodium onto magnesia pretreated at higher tempera- tures (700-1000°C) have considerably stronger free radical properties than those prepared from MgO activated at lower temperatures, 550 and 650°C. No one-electron accepting centres were found for catalysts prepared by doping magnesia with NaOH and metallic sodium. Paramagnetic cation radicals were not formed even upon adsorption of such strong electron donors as perylene and pyrene.CATALYTIC ACTIVITY I SOMERIZATION Pure magnesia and preparations of MgO doped with sodium hydroxide were completely inactive in isomerization of pent- 1-ene and trans-pent-2-ene under adopted conditions. Under the same conditions magnesia doped with metallic sodium was found to exhibit remarkable isomerizing activity (table 2). It follows from the results summarized in table 2 that increasing the precalcination temperature of MgO substantially influences the isomerizing activity of the catalyst. Both the initial &/trans ratio of pent-2-ene formed through isomerization and the ratio of isomers after 2 and 10 h of reaction undergo a significant change. TABLE 2 . V A L U E S OF PENT-2-ENE CiS/tUanS RATIO AND PENT-1-ENE CONVERSION OBTAINED OVER THE MgO+Na,,t SERIES CATALYSTS cisltrans ratio of pent- 1 -ene cisitruns pent-2-ene conversion initial ratio of pent-1-ene over catalyst over catalyst cisltrans pent-2-ene conversion poisoned with poisoned with specific surface ratio of after 2 after 2 TPM after 2 TPM after 2 catalyst area/m2 g-1 pent-2-ene and 10 h and 10 h/ % and 10 h and 10 h/ % In the case of catalysts with marked superbase properties (Mg0550 +Namet, Mg0650 +Name# the composition of the reaction mixture in steady state conditions approaches equilibrium composition at 20°C and the initial cisltrans ratio of pent-2- ene isomers becomes close to 2.With catalysts comprising MgO precalcined at higher temperatures, i.e. those with dominating radical properties, the selectivity t Mg055o+Namet and MgO65o+Namet etc.are catalysts obtained by evaporation of metallic sodium onto MgO surface calcined at respectively 550, 650°C, etc. 1-9256 Na INFLUENCE ON MgO PROPERTIES towards cis pent-2-ene increases, the initial cisltrans ratio of pent-2-ene isomers reaches - 3 and the composition of the reaction mixture under steady state conditions differs considerably from the equilibrium composition. Since neither pure magnesium oxide nor the MgO specimens doped with NaBH showed any isomerizing activity, it was reasonable to link this activity with active sites existing only on the metallic sodium doped surface of magnesia. Thus it became evident that the sites responsible for the isomerizing activity of the catalysts being studied are the superbase centres (H- 2 35) or the one-electron donor sites able to reduce nitrobenzene molecules.Triphenylmethane (pK, = 33) was adsorbed on the Mg05s0 + Namet catalyst surface (possessing a high concentration of superbase sites) and on the MgO700 + Namet catalyst characterized by the largest amount of one-electron donor centres. The quantity of TPM* corresponded to the amount of strongest basic sites. The results of pent-1-ene isomerization over these catalysts are presented in table 2. In the presence of Mg0550+Nam,t poisoned with TPM the value of the cis- pent-2-eneltrans-pent-2-ene ratio changed from 0.29 (the value being attained with unpoisoned catalyst) to 3.2 after a lapse of 2 h reaction time. The poisoned Mg0700 + Namet catalyst gave a smaller change in cisltrans ratio.Also, the conversion of pent-1-ene decreased to a lesser extent. The observed decrease in pent-1-ene conversion over poisoned Mg05 + Namet catalyst is presum- ably the result of both poisoning the superbase sites and suppressing the remaining active sites (one-electron donor centres) by physically adsorbed triphenylmethane. As the quantity of TPM introduced onto the surface of MgO,OO + Namet catalyst is markedly smaller and the amount of superbase sites on this catalyst is also much smaller, it is felt that the probability of accidentally suppressing the one-electron donor sites with the introduced poison was much lower. Hence we consider the diminishing amount of pent-1-ene conversion to be due to the elimination of superbase centres by the poison.Moreover, it should be emphasized that values of the cis/trmzs ratio attained over poisoned catalyst with TPM are close to those obtained over MgOlooo +Namet catalyst, in which no ionic sites at the basic strength of H- 3 35 were detected. The isomerization of trans-pent-2-ene to cis-pent-2-ene and pent-1-ene was carried out over MgOeSo + Namet and MgOlooo + Namet catalysts, the ones most distinctly varying in physicochemical properties. The catalyst MgOlooo + Namet exhibited no activity towards the isomerization of trans-pent-2-ene, whereas the reaction was observed to proceed over the h4g0650+Namet catalyst to yield, after 2 h, a mixture of cis-(11.5 %) and tuans-(87 7:) pent-2-ene and pent-1-ene (1.5 %). Blocking the superbase centres of this catalyst with a stoichioinetric quantity of TPM rendered the catalyst entirely inactive towards isomerization.The e.s.r. spectroscopic method revealed a signal for the organic radical resulting from pent-1-ene adsorbed on the surface of Mg0700 + Namet catalyst [fig. 4(a)]. The g value of this signal was estimated to be 2.0012. CUMENE TRANSFORMATIONS The yields of cumene transformation reactions over catalysts doped with sodium hydroxide are shown in fig. 1. The maximum conversion of cumene is reached with the catalyst containing 0.35 mmol Na+ per g MgO, the main product being a-methyl- styrene with all the catalysts studied. The only exception is the catalyst comprising 0.01 mmol NaOH over which the formation of a-methylstyrene is accompanied by equivalent yields of toluene.The variation in a-methylstyrene yields with the amount * TPM is triphenylmethaneJ . K I J E ~ S K I AND s . MALINOWSKI 257 (4 (b) FIG. 4.-E.s.r. siguals of organic radicals obtained after (a) pent-1-ene adsorption on the Mg,o,+ Namet surface, (b) cumene adsorption on the Mg0,50 t- Namet surface. I O E - 1-1 : : 553 650 7W 750 MgO pretreatment temp. /"C FIG. 5.-Yields of cumene transformations products obtained over MgO + Namet catalysts.258 Na INFLUENCE ON MgO PROPERTIES of sodium accurately corresponds to the variation in concentration of one-electron donor sites capable of reducing TCNE. The variations in cumene conversion to ethylbenzene and styrene are similar to those of a-methylstyrene yields. The results of the cumene conversion over catalysts with evaporated sodium are demonstrated in fig.5. Catalysts doped with metallic sodium were found to be more active than those doped with sodium hydroxide and the rezction appeared to be more selective, the main product being a-methylstyrene resulting from the dehydrogenation of cumene. Among the reaction products were ethylbenzene, n-propylbenzene, toluene, benzene and styrene. The largest yields of the majority of cumene conversion products are obtained over the hfgB750 + Namet catalyst. This catalyst possesses a relatively large number of one-electron donor sites and an insignificant concentration of saper- base sites (fig. 1 and 3). The formation of n-propylbenzene was favoured over Mg0650+Namet catalyst which was the most abundant in superbase centres (fig.I). Table 3 summarizes the results of the conversion of cumene over MgQ750 4- Namet (the most active) as a function of temperature. TABLE 3 .-CUMENE DEHYDROGENATION YIELDS AT VARIOUS TEMPERATURES reaction temperature/'C 20 250 350 450 500 550 yields* of a-methylstyrene/m2 - 0.03 0.09 0.22 0.27 0.91 other products - I - - benzene, ethyl benzene, toluene, styrene, n-propyl benzene 0.14 0.23 * mole per 100 moles of cumene The first product to appear in conversion of cumene at 250°C was a-methylstyrene. Other products were found only after the temperature was raised to 500°C. The reaction with cumene was carried out in the presence of the MgQ750 + Name* catalyst whose surface was devoid of one-electron donor centres by suppressing them with stoichiometric quantity of TCNE.As a result, a decrease in the yield of a-methyl- styrene was observed (table 4). The products did not contain styrene. The yield of toluene was the least affected. TABLE 4.-TCNE ADSORPTION INFLUENCE ON ACTIVITY OF Mg07;3+ Namet CATALYST IN CUMENE TRANSFORMATIONS cumene reactions products/(mole/ 100 moles of cumene) m2 ethyl- a-methyl- n-propyl- catalyst benzene toluene benzene styrene styrene benzene M g 0 7 5 o + N a m e t 0.03 0.04 0.12 0.01 0.91 0.03 MgO, 5 0 + Namet after TCNE adsorption I 0.01 0.01 - 0.10 - Cumene vapours were adsorbed at 250°C on the Mg0,50 +Namct catalyst surface. It should be remembered that the only reaction product at this temperature was a-methylstyrene (table 3). The e.s.r. spectra were measured for the catalyst with preadsorbed cumene.A strong signal originating from the organic radical was recorded [fig. 4(b), g = 2.0064, AH,,, = 1.3 GI. The narrow signal width isJ . KIJEASKI AND s. MALINOWSKI 259 presumably due to strong exchange interactions among adsorbed radical species. Formation of paramagnetic surface species may be considered sufficient evidence of the radical reaction being initiated at 250°C. DISCUSSION Sodium addition to magnesia brings about alterations in the physicocheinical properties of the oxide. Thus, if sodium is introduced in the ionic form (NaOH) the only apparent change is in the amount of active sites. If, on the other hand sodium is vaporized onto magnesia, the resulting system differs from the parent one in the quantity as well as quality of active sites (superbase or strong one-electron donating properties are generated).We suppose that in both cases the modifying action of sodium consists in its interacting with surface oxygen atom groupings (lattice oxygen anions or oxygen atoms from adsorbed water). We observed that suppressing the basic sites had no effect on the concentration of one-electron donor sites. This observation led us to conclude that these centres exist on the surface of MgO + NaOH catalyst entirely independently. It is however beyond any doubt that in the series of catalysts doped with NaOH there exists a parallelism between basic and one-electron donor activity of the surface, which was confirmed by Cordischi and Indovina who investigated the one-electron donor properties of a set of ionic oxides.14 For catalysts sputtered with sodium vapours we observed no parallelism between basic (maximum for MgO,,, + Namet catalyst) and one-electron donor activity (maximum for Mg0700 + Namet catalyst).A similar differentiation of properties can also be observed for pure MgO. The strongest basic properties were noted for MgO specimens calcined at temperatures from 550-600°C (heating over 600°C results in rapid decrease of catalyst basicity), which was also observed by Tanabe and Hattoris The maximum of one-electron donor properties of MgO falls in the calcination temperature range from 700-800°C.1 Comparison of the results of isomerization reactions with the variations in basic and one-electron donating properties in the series of catalysts being tested, as well as the results of experiments involving poisoning of active sites, warrant the statement that two different reaction mechanisms are operative in isomerization.These are : the ion-type mechanism (involving the superbase sites) and the radical-type mechanism (where one-electron donor sites are responsible for catalytic activity). Superbase sites catalyse isomerization of pent-1-ene to give a mixture of pent-2-eiie isomers and isomerization of cis-pent-2-ene towards trans-pent-2-ene as shown in the diagram : cis-pent-2-ene trans-pent-2-ene. pent- 1 -ene Jr It is most probable that the initial stage of these reactions involves proton abstraction from the alkene molecule by the superbase site (pK, = 37 for pentenes). Therefore a mechanism similar to that proposed by Pines and Schaap l5 can hold in this case a3 0 B + R--CH2--CH=CH2 -+ BH[R-cH-CH=CH2] (2) BH[R--C_H--CH=-CI3,] t) BH[R-CH=CH---C_H,] (3) BH[R-CH=CH-GH,] + B + R-CH=CH-CHS] (4) (3 8 fi3 8 8 e where B is a superbase centre.260 The initial cisltrans ratio of pent-2-ene isomers obtained in our experiments was found to be greater than unity.This result might be accounted for by the higher stability of the intermediate cis-ally1 carbanion as compared with that of trans-ally1 species.16 A further stage would be the removal of a proton from the resulting cis- or trans-pent-2-ene. As a result a new allyl-type carbanion would be formed : Na INFLUENCE ON MgO PROPERTIES el3 B + R-CH=CH-CH, -+ BH[R-CH==CH=-==CHJe ( 5 ) which on addition of one proton would transform into trans- or cis-pent-2-ene or pent-1-ene.The main product of the consecutive reaction is the most thermo- dynamically stable trans-pent-2-ene. The values of the initial cisltrans ratio and the cisltrans ratio after 2 and 10 h measured for Mg0550 +Namet and MgQ650 +Name, catalysts (table 4) support the reaction pathway described above. Superbase sites are considered to activate trans-pent-2-ene in the same manner ; as a result of proton abstraction from the trans-alkene molecule, an ally1 carbanion is formed which on subsequent addition of a proton may be transformed either to cis- and trans-pent-2-ene or to pent- 1 -ene. Over catalysts possessing one-electron donor sites isomerization of pent-1-ene gives a mixture of isomers with the cis-form as the major product of the reaction.In this case the reaction pathway may be envisaged as follows : (one-electron donor centre). + R-CH2-CH=CH2 -+ (one-electron donor centre)-H + R-kH-CH=CH2 (6) R-cH-CH=CH2 0 ++ R-CH=CH--dH2 (7) R-CH=CH-CH2 + (one-electron donor centre)-H -+ R-CH=CH-CH3 + (one-electron donor centre).. (8) According to Walling and Thaler l7 cis-pent-2-ene is the most favoured radical isomerization product from the statistical point of view. As suggested by these authors cis-ally1 radical (the intermediate for cis-pent-2-ene) is formed through hydrogen atom abstraction from one of the two equivalent gauche conformers of pent-1-ene. On the other hand, trans-ally1 radical is formed via hydrogen atom abstraction from one possible trans conformer of pent-1-ene. Hence there is a greater likelihood of cis pent-2-ene formation.A similar mechanism might play a substantial role in the reactions studied in this work. The cisltrans ratio of the pent-2-enes calculated at the end of the reaction does not correspond to the theoretical equilibrium ratio; this is probably the consequence of the absence of an internal double bond isomerization process, i.e. cis-pent-2-ene to trans-pent-2-ene. In favour of this view is the complete lack of trans-pent-2-ene transformation over catalysts possessing exclusively one-electron donor sites (MgO, ooo + Nilmet and Mg8 6 + Name* poisoned with TPM). The correlations found between the yields of cumene conversion products, i.e. a-methylstyrene, ethylbenzene and styrene and the variation in one-electron donating properties of catalysts doped with NaOH and metallic sodium are indicative of a free radical nature of these transformations.The idea of a radical type of reaction pathway is further supported by the fact that, after poisoning the catalyst with TCNE, dehydrogenation is no longer observed. Moreover, paramagnetic organic radicals are formed from cumene adsorbed on Mg0750 + Namet catalysts. The radical mechanism of cumene dehydrogenation may be analogous to that put forward by Krause * for ethylbenzene dehydrogenation on one-electron donor centres :J . K I J E ~ S K I AND s . MALINOWSKI 26 1 (one-electron donor centre). + C6H5C2H5 -+ (one-electron donor centre)-H + C6H5C2H4 + (one-electron donor centre)-H 4- C6H,&H4 (9 (10) 0 C&&CH=CH2 4 H2 + (one-electron donor centre). and n-propylbenzene seems to be formed in the ionic pathway. The greatest amount of this compound was obtained over catalysts with the strongest superbase properties (MgOS5* +Namet and Mg0650 +Name*), possibly formed by realkylation of benzene (or toluene) with propylene (or ethylene). According to the mechanism proposed by Pines and Schaap l5 one may write : %3 e B + C6H5CH3 -+ Bh-I[C6H5_CH2] G3 e J. M. Parera and N. S . Figoli, J. Catalysis, 1969, 14, 303. L. D. Scharme, J. Phys. Chem., 1974,20,2070. T. T. Chuang and I. G. DaIla Lona, J.C.S. Faraduy I, 1972,68,777. H. Bremer, K. H. Steinberg and K. D. Wendlant, 2. anorg. Chem., 1969, 366, 130. H. Pines and W. 0. Haag, J. Amer. Chem. Soc., 1960,82,2471. S . Santhangopalan and C. N. Pillai, Indian J. Chem., 1973, 11, 957. ' J. Take, N. Kikuchi and Y . Yoneda, J. Catalysis, 1971, 21, 164. H. Hattori, N. Yoshii and K. Tanabe, Proc. Fifrrh Jnt. Congress on Catalysis (Amsterdam, 1972), vol. 10, p. 1. B. D. Flockhart, I. A. N. Scott and R. C. Pink, Trans. Faraduy SOC., 1966, 62, 730. BH[C,H5CH2CH2CHZ] -+B + CbH5CW2CH2CHS (1 3) lo J. H. de Boer and J. Weissnian, J. Anzer. Chem. SOC., 1958,80, 4549. l 1 J. Kijehski, S. Malinowski and B. Zielihski, Kinetics Catalysis React. Letters, 1976, 4(2), 251. l2 A. I. Tench and R. L. Nelson, Trans. Faraday SOC., 1967,63,2254. l3 M. Che, C. Naccache and B. Imelik, J. Catalysis, 1972, 24, 328. l4 D. Cordischi and V. Indovina, J.C.S. Farada-v I, 1976, 72,2341. l 5 H. Pines and L. A. Schaap, Adu. CataZysis, 1960, XII, 117. l 6 S. Bank, A. Schriesheim and C. A. Rowe, J. Amer. Chem. SOC., 1965, 87, 3244; S. Bank, l 7 C. Walling and W. Thaler, J. Amer. Chem. Soc., 1961, 83, 3877. J . Amer. Chem. SOC., 1965,87, 3245. A. Krause, Sci. Pharm., 1970, 38, 266. (PAPER 71238)