J.C.S. Faraday I, 1980, 76,92-100Catalytic Conversion of AlcoholsPart lO.-T-fluence of Pretreatment on the Selectivity of MgO and CaOBY BURTRON H. DAVIS?Potomac State College of West Virginia University,Keyser, West Virginia 26726, U. S . A.Received 31st January, 1979With both CaO and MgO catalysts, the hydrogen pretreated material is a selective dehydroge-nation catalyst while the oxygen pretreated material has about the same activity for both dehydro-genation and dehydration. Initially the air (or oxygen) pretreated MgO sample produces an alkenedistribution from 2-01s that resembles that obtained with alumina ; as the reaction temperature isincreased a similar alkene distribution, different from the equilibrium value, is obtained with bothpretreatments. The three temperatures used with CaO yield an alkene distribution, with either pre-treatment, that resembles the highest temperature run with MgO. Pure cis-2- or trans-2-methylcyclo-hexanol undergoes extensive cis-trans isomerization ; the different alkene distributions from the twoalcohols suggest that a trans elimination pathway contributes to the dehydration mechanism.Group I1 metals form the most basic oxides of the group A elements that are notsoluble in water.A knowledge of the activity and selectivity of these group IIAoxides is necessary if we are to obtain comprehensive ideas and theories for thecatalytic action of metal oxides for the conversion of alcohols.Studies with these oxides suggest that the number of basic sites depends on thecalcination temperature; the maximum number appears to be present in a materialcalcined at 775-825 K.lm3 These basic sites may determine the catalytic ~electivity.~Most studies of alcohol conversion over MgO and CaO indicate that both oxides arevery selective dehydrogenation cataly~ts.~-l Only those materials that are con-taminated with COz (or carbonate) or which result from an incomplete decompositionof the carbonate appear to be active for alcohol dehydrati~n.~.~~ Propan-2-01 hasbeen employed for the majority of the studies of alcohol conversion but it can onlyform one dehydration product.In a study with a higher carbon number alcohol,butan-2-01, it was observed that, for the small amount of dehydration, the terminalalkene was greatly favoured.In another study, Canesson and Blanchard reportedthat the selectivity for hex-1-ene from hexan-2-01 paralleled the base strength reportedby Iisuka et aL3 Two recent studies have employed the microreactor technique(pulse method) to obtain the activity and selectivity for butan-2-01 conversion.Vinek et aZ.14 found that the dehydrogenation selectivity depended strongly upon thestorage time and calcination temperature; the increase in hydroxide layer anddecreasing basicity with storage changed the selectivity from dehydrogenation intodehydration. Thomke l 5 reported that the dehydration selectivity, at 50 % totalconversion, was 8 % for MgO and 70 % for CaO. The but-1-ene selectivity withMgO changed from =SO % at 673 K to 50 % at 773 K but with CaO the selectivityonly varied from 20 to 30 %.7 Present address : Kentucky Centre for Energy Research Laboratory, University of Kentucky,P.O.Box 13015, Lexington, Kentucky 40583, USA.9B. H. DAVIS 93These limited studies suggest that group IIA metal oxides may resemble otherAlumina has traditionally been viewed as a very selective dehydration catalyst.’selective a-olefin forming catalysts such as thoria.However, pretreatment in oxygen may impart a dehydrogenation activity that is atleast as great as the dehydration activity.17 Even though the pretreatment may playa major role in determining the activity and selectivity, it has not received muchattention for most of the metal oxide catalysts.The present investigation defined the influence of the hydrogen or oxygen pre-treatment on the dehydrogenation activity and the alkene selectivity of CaO and MgO.Hopefully, a unified picture of the catalytic selectivity will emerge as more oxides arestudied.EXPERIMENTALCATALYSTSMagnesium oxide was suspended in distilled water and heated near the boiling point for6 h.The solid was collected, dried at 390 K in air and then heated in air for G h at 1270 K.The CaO used for the runs with octan-2-01 [footnote (c), tables 1 and 21 was prepared byheating Fisher calcium hydroxide in situ in flowing oxygen or hydrogen to 825 K and holdingit at this temperature in the flowing gas. The CaO used for the other runs was prepared byadding ammonium carbonate to a calcium nitrate solution. The resulting calcium carbonatewas collected by filtration, dried at 390 K and then calcined in air at 875 K for four days.PROCEDUREA syringe pump was used to charge the reactant to a fixed bed glass reactor fitted with athermowell.The liquid products, after passing through a water condenser, were collectedat time intervals. The liquid was analysed for conversion using temperature programmedg.c. with a Carbowax 20M column. Alkenes were analysed using g.c. with the columnappropriate for the particular alkene mixture : UC-W, Carbowax 20M or P,P’-oxydipropio-nitrile. cis- And trans-2-methylcyclohexanol content was determined by g.c. with a di-glycerol column.Initially, the catalyst, in powder form, was pretreated in the reactor ; thereafter, a standardregeneration was used prior to the next pretreatment in air, hydrogen or oxygen.Theregeneration consisted of stopping the reactant flow and cooling the catalyst from the runtemperature to near room temperature. A flow of air was passed over the catalyst while itwas slowly heated (for 20-30 min) to 525 K and held at this temperature for M 3 h. Thetemperature was then increased to 790-825 K and held at this temperature for 3-6 h. Foran air pretreatment, the catalyst was then cooled to the reaction temperature in flowing air.For the other pretreatments the air flow was replaced by the pretreatment gas flow and thecatalyst was heated at 790-825 K for 3-6 h. In all cases, the catalyst was cooled to the re-action temperature in the pretreatment gas flow.RESULTSThe conversion data in fig.1 for 4-methylpentan-2-01 over MgO clearly show theinfluence of pretreatment on the dehydration m d dehydrogenation activity. Thecatalyst was about equally active for dehydration and dehydrogenation following anair pretreatment at 775 K [fig. l(a)]. After completing the run represented in fig. l(a),the catalyst was regenerated by the standad procedure and then pretreated withflowing hydrogen for 4 h. The sample now had about the same dehydration activitybut the dehydrogenation activity was two to three times greater than that of the airpretreated sample [fig. l(b)]. The decrease in dehydrogenation activity with time-on-stream for the air pretreated sample is not representative ; in most cases the activitychanged much more slowly (e.g., see the pentan-2-01 results in table 1).In comparin94 CATALYTIC CONVERSION OF ALCOHOLSthe data in table 1 it should be noted that the conversion range represents the high andlow conversion for the 4-6 samples collected during the duration of the run SO thatthere was little change in the activity during a run. Following the run fur fig. l(b),the sample was again given the standard air regeneration and air pretreatment ; thedehydrogenation and dehydration activity of the catalyst repeated the original airpretreatment [compare fig. l(a) and (c)].air-613K-eneI I I I I I I I 1 1 1100 200 300 400 100 200 300 400time/minFIG. 1 .-Influence of pretreatment on the conversion of 4-methylpentan-2-01 to methylpentenes and4-methylpentan-2-one over a magnesium oxide catalyst.TABLE DE DEHYDRATION SELECTIVITY FOR THE CONVERSION OF ALCOHOLS OVER MAGNESIUMAND CALCIUM OXIDE~ ~ ~length of conversionb dehydrationbalcohol pretreatment LHSV T/K runlmin range/mol % selectivityoct an-2-01pentan-2-01 air 0.130.260.51H2 0.130.260.51air 0.260.260.510.511.03H2 0.260.511.99590 360613 180633 105588 335608 160641 120523 230578 380598 325618 240633 175588 365608 29063 3 7029-2824-2425-2238-3939-4045-498-1015-1 812-1516-2317-2226-2727-3023-240.55-0.580.47-0.550.3 5-0.3 80.16-0.200.15-0.170.08-0.090.97-0.980.88-0.900.88-0.890.68-0.860.62-0.710.29-a. 3 10.24-8.250.1 5-0.1B.H.DAVIS 95length of conversionb dehydrationbalcohol pretreatment LHSV T/K run/min range/mol % selectivity4met hyl-pent an-2-01(cis + trans)-2-methylcyclo-hexanoltrans-2-methyl-cyclohexanolpentan-2-01octan-2-014met hyl-pentan-2-01(cis+ trans>-2-methycyclo-hexanoltrans-2-met hyl-cydo hexanolcis-2-met hyl-cyclohexanolairH2airairH2airH2air0 2332airH2airH2H2airJ32air0.13 587 2700.26 613 4800.51' 638 2050.13 589 3750.26 613 3400.51' 633 1950.51' 633 1300.13 589 4950.28 617 1700.51 633 2950.13 593 3700.26 613 4150.51 638 1300.26 613 1650.26 608 190CaO0.130.260.510.250.250.130.260.260.250.130.610.260.260.510.130.260.510.260.260.2660362364356356359361 363355860363359161864360362364362362362329520513043530538031030027036590305330245360370175808590-19-2419-214332-4238-401825-3 123-2516-2224-2541-4830-321 6-2229-3222-3518-2918-3221-4412-1628-4422-4328-5011-1430-5210-2915-2918-2528-3322-2 81 1-2421-331 1-2439-4527-420.58-0.730.46-0.620.43-0.520.22-0.250.13-0.150.83-0.870.71-0.740.62-0.630.58-0.620.20-0.260.29-0.370.8 1-0.860.28-0.290.290.600.54-0.550.36-0.480.45-0.520.11-0.140.47-0.500.36-0.620.46-0.630.09-0.170.43-0.620.3 5-0.740.07-0.140.18-0.240.17-0.210.52-0.660.58-0.690.55-0.630.52-0.600.3 3 -0.5 60.78-0.790.37-0.57~ ~ ~-~ ~a Liquid hourly space velocity (cm3 reactant per cm3 catalyst per h).b Values given are theextremes in conversion and selectivity for the 4-6 samples collected during the course of the run.C Runs with pretreatments of air, hydrogen and air (with regenerations between the runs) in succession.d Calcium hydroxide heated to 500 "C in flowing oxygen. e Sample given standard regenerationfollowed by hydrogen reduction. f Same as (d) except heated in flowing hydrogen96 CATALYTIC CONVERSION OF ALCOHOLSTABLE 2.-ALKENE SELECTIVITY FROM THE CONVERSION OF 2-OLS WITH CALCIUM AND MAGNES-IUM OXIDESalkene/mol %time conversionalcohol pretreatment LHSV T/K /min /mol % 1- trans-2- cis-2-pentan-2-01 air 0.130.260.51H2 0.130.260.51octan-2-01 H2 0.26pentan-2-01 air 0.130.260.51oc tan-2-01 0 2 0.13H2 0.13588 95205280360613 180633 105588 335608 160643 120608 215290CaO293230302422384045323022 24 5326 25 4924 31 4523 34 4322 32 4622 35 4316 55 2924 43 3329 34 3725 48 2728 42 30603 195295623 205643 130563 435563 30525221818231327 37 3626 36 3829 35 3629 35 3634 31 3534 32 34The selectivities for the conversion of pentan-2-01 and 4-methylpentan-2-01 overMgO are very similar for each temperature and pretreatment.Octan-2-01 and2-methylcyclohexanol also have similar selectivities at each temperature and pre-treatment ; however, MgO appears to be more selective for the dehydration of thesehigher molecular weight alcohols than for the pentanols.The CaO results in table 1 are similar to the corresponding runs using MgO andthe dehydration selectivity for each alcohol corresponds closely to that obtained withMgO.However, the conversion varies over a much wider range during the course of arun showing that CaO ages more rapidly than does MgO. The two materials appearto have a similar initial activity when compared on an equal catalyst weight basis. TheB.E.T. surface area of the catalyst was determined after use and storage for a monthor more; it is unlikely that the area measured is the same as it would be if it wasmeasured during use as a catalyst. The area, after evacuation at 473 K, was :MgO, 56 m2 8-l; CaO, 4.2 m2 g-l.This indicates that CaO, on an equal area basis,may be even more active than the MgO catalyst.With MgO the dehydration selectivity decreased with increasing temperature forboth pretreatments ; this trend was not as clear-cut with CaO.The alkene selectivity may also depend on the pretreatment. At 591 K, hydrogenpretreated MgO produced an alkene distribution that was constant with time and wasessentially the equilibrium distribution (fig. 2). The air pretreated sample yielded analkene distribution with cis-pent-2-ene as the major product ; as the reaction progressedthe alkene selectivity changed to produce more of the trans-2-isomer at the expense oB. H. DAVIS c- 6050X 40-E8 30-40\4 cd2010----air, 591 K pentan-2-01.H, 591 K1 I I 1 I 1 I I I 1 1r b A - I0 --- 1 I 197time/minFIG. 2.-Comparison of the alkene distribution from the conversion of pentan-2-01 over air andhydrogen pretreated magnesium oxide ; @, I-pentene ; A, trans-pent-Z-ene ; M, cis-pent-2-ene [opensymbols represent the equilibrium n-pentene composition calculated from data in ref. (26)].600 625 575 600 625 575 600 625FIG. 3.-AIkene composition from the conversion of pentan-2-01 over hydrogen and oxygen pre-treated magnesium oxide catalyst [(-) represents the equilibrium value calculated using datafrom ref. (26) ; the open symbols are for the hydrogen pretreated sample ; solid symbols are for theair pretreated sample].11-98 CATALYTIC CONVERSION OF ALCOHOLSthe cis-2-isomer. Initially, the alkene selectivity of MgO resembled that of alumina.l*Thus, the alkene selectivity of the air pretreated sample changed with time-on-streameven though the dehydration selectivity remained nearly constant with time. Thealkene distribution changed with increasing temperature to approach an alkenecomposition that was the same for both pretreatments ; this composition differs fromthe equilibrium value (fig.3). The alkene distribution from pentan-2-01 was represen-tative of the results obtained with MgO with the other acyclic 2-01s.CaO yielded essentially the same alkene distribution with the hydrogen or theoxygen pretreated material with octan-2-01 and 4-methylpentan-2-01. 0cta.n-2-01produced about equal amounts of the three alkenes allowed by /?-elimination.Thealkene distributions obtained with pentan-2-01 and 4-methylpentan-2-01 did notappear to change with temperature as was the case with MgO; the distribution withCaO was approximately the one that was obtained at the highest temperature usedwith the MgO catalyst.Calcium and magnesium oxide catalysts were similar for the conversion of 2-methylcyclohexanol. CaO may have been slightly more selective for dehydrationthan MgO was. With both catalysts the hydrogen pretreated material was 2-6 timesmore active for the cis-trans isomerization of the charged alcohol than the air pre-treated sample (table 3). With the hydrogen pretreated samples, the amount ofisomerized alcohol in the liquid product was as great as the total conversion by bothdehydration and dehydrogenation.trans-2-Methylcyclohexanol conversion over thehydrogen or air pretreated MgO led to the same alkene composition ; with CaO therewas only a slight difference in the alkene compositions from the two pretreatments.The &ene composition from tr~nst2-methylcyclohexanol with both catalysts and bothTABLE 3 .-PRODUCTS FROM THE CONVERSION OF PURE 2-METHYLCYCLOHEXANOL ISOMERS WITHCALCIUM AND MAGNESIUM OXIDE CATALYSTSalcohol* methylcyclohexenealcohol pre- time conversionisomer treatment T,K LHSV /min /mol % cis trans 4-b 3- 1-MgOtrans-2- air 613 0.26 40 22 4.7 95.3 3 42 5585 18 5.3 94.7 - 47 53165 16 - - - 48 52Hz 608 0.26 95 36 33 67 - 43 57160 32 33 67 - 46 54190 29 29 71 - 47 53CaOtrans-2- air 623 0.26 60 33 14 86 6.2 37 5780 22 9.3 90.7 5.4 37 57Hz 623 0.26 35 24 32 68 6.7 43 5185 21 22 78 6.3 42 52cis-2- air 618 0.26 50 45 87 13 1.8 23 7595 39 93 7 2.2 24 74a Composition of the alcohol in the liquid reaction products ; the alcohol reactant containedc 0.1 % of the other isomer.b May contain some methylcyclohexaneB. H. DAVIS 99pretreatments differed significantly from that with the cis-alcohol over CaO. Thealkene composition from the &-alcohol over CaO is similar to that obtained withother oxides in this temperature range l9 and is probably the equilibrium com-position (considering only methylcyclohex-1- and -3-ene).The kinetics for alcohol conversion with MgO 6 g 8 * 1 0 ~ 1 1 show an alcohol reactionorder varying between zero and one. In most cases our data gave reasonableArrhenius-type plots if the rate was assumed to be zero order in alcohol.However,the temperature coefficients calculated from these plots were only z 10 kcal mol-l.This low value could be accounted for by a rate with some order other than zero.DISCUSSIONWith both CaO and MgQ, the hydrogen pretreated material was a selectivedehydrogenation catalyst while the oxygen pretreated sample had about the sameactivity for both dehydrogenation and dehydration. The selectivity was imparted bythe pretreatment and the catalyst could be changed from one selectivity to the otherby the appropriate pretreatment. Hydrogen or oxygen pretreatment determines theselectivity for many catalysts 2o but in most cases we fuund a selectivity that was justopposite to that of CaO and MgO.For example, hydrogen pretreated alurnina is avery selective dehydration catalyst but oxygen pretreated alumina is as active fordehydrogenation as for dehydration.17The sites created by the pretreatment are catalytic. For example, a run with amixture of cis- and trans-2-methylcyclohexanol using air pretreated MgQ resulted inthe conversion of 50 mmole over 6.3 g of MgO having a total surface area of 345 m2.Estimating the minimum dimension of the catalytic site as 10 A2, a size only slightlygreater than that of an oxide ion, enables one to calculate that a minimum of 10molecules of alcohol were converted per site. Since the conversion remained nearlyconstant during the run it seems clear that sites are catalytic and not transient innature.The results from the conversion of trans-2-methylcyclohexanol suggest thatdehydration and dehydrogenation may occur on different sites.The alkene fractionfrom the conversion of the trans-alcohol over both the air and the hydrogen pre-treated MgO at 613 K contained the same amount of methyIcyclohex-3-ene. Thetotal alcohol conversion was also similar. However, with the air pretreated sample81-86 % of the conversion was dehydration while only 28-29 % of the total conversionwas dehydration over the hydrogen pretreated sample. At the same time, thedehydrogenation conversion was nearly equal to the amount of the cis-trans isomer-ization of the alcohol charge for both pretreatments, suggesting that the isomerizationoccurs on the same site that is active for dehydrogenation.An oxygen species, 0; or OOH-, may serve as a site for dehydrogenation.21Tench et aZ.22 observed that 0- on MgO abstracted the a-hydrogen from bothethanol and propan-2-01. The air pretreated sample should have a higher concentra-tion of oxygen species than the hydrogen pretreated sample and should be moreselective for dehydrogenation. Since we observed the opposite effect, it appears thatthe oxygen ion is not responsible for dehydrogenation with CaO and MgO catalysts.Derouane and Gieseke 23 found that the paramagnetic centres formed byevacuation at 653 K were decreased by the adsorption of hydrogen at 293 K.In-creasing the temperature to 443 K restored many of the paramagnetic centres ashydrogen was desorbed.These paramagnetic centres were active for H2-D2exchange 24 and dehydrogenation of alcohols could be viewed as a special H2-D2exchange. Since hydrogen adsorption decreased with increasing ternperat~re,~ 9 2100 CATALYTIC CONVERSION OF ALCOHOLSwe would expect alcohol dehydrogenation to likewise decrease; this is contrary towhat was observed.The alkene distribution from the acyclic alcohols suggests that the number andkind of dehydration sites change with temperature for MgO. At lower temperatures,an equilibrium distribution was obtained with the hydrogen pretreatment while thedistribution from the air pretreated sample resembled that obtained with alumina.Thus, the hydrogen pretreated material adsorbs the alkene product more strongly soit has time to isomerize or the dehydration reaction occurs through a differentintermediate on the two pretreated materials.The different alkene distribution from pure cis- and pure trans-2-methykyclo-hexanol rule out a mechanism with a common intermediate and is consistent with ananti elimination mechanism. However, the 50 % methylcycloliex-3-ene from truns-2-methylcyclohexanol suggests a contribution from a syn elimination or an isorneriza-tion of the alkene during formation or as a secondary reaction.In summaxy, the hydrogen or air (oxygen) pretreatment determined the dehydra-tion selectivity. The selectivity is imparted by the pretreatment and can be altered bythe next pretreatment.The ability to change from one selectivity to another suggeststhat the range of selecticities reported by earlier workers probably resulted from thepretreatment. It will require more work to decide which of the variety of paramag-netic sites, if any, determine the selectivity.The author thanks the Donors of The Petroleum Research Fund, administered bythe American Chemical Society, for the support of this research.K. Tanabe, Solid Acids and Bases (Academic Press, London, 1970).K. Saito and K. Tanabe, Shokubai (Tokyo), 1969,11,2068.T. Iisuka, H. Hattori, T. Ohno, J. Sohma and K. Tanabe, J. Catalysis, 1971,22, 130.P. Canesson and M. Blanchard, J. Catalysis, 1976, 42, 205.Y. Schachter and H. Pines, J. Catalysis, 1968, 11, 147.W.F. N. M. DeVleesschauwer in Physical and Chemical Aspects of Adsorbents and Catalysts,ed. €3. G. Linsen (Academic Press, London, 1970), p. 225. ' I. M. Hoodless and G. D. Martin, Canad. J. Chem., 1975,53,2729. * N. Takezawa, C. Hanamaki and H. Kobayaski, J. Catalysis, 1975,38,101.H. Vinek, H. Noller, M. Ebel and K. Schwarz, J.C.S. Faraday I. 1977,73,734.Z. Szabo, B. Jover and R. Ohmacht, J. Catalysis, 1975, 39, 225.l 1 E. R. McCaffrey, T. A. Micka and R. A. Ross, J. Phys. Chem., 1972,76,3372.l2 P. Sabatier, Catalysisin Organic Chemistry, translated by E. E. Reid (Van Nostrand, New York,l 3 0. V. Krylov and E. A. Fokiva, Kinetika i Kataliz., 1960, 1,421.l4 H. Vinek, J. Latzel, H. Noller and M. Ebel, J.C.S. Faraday I. 1978, 74, 2092.K. Thomke, Z. phys. Chem. (Frankfurt), 1977,106,225.l6 H. Pines and J. Manassen, Ado. Catalysis, 1966, 16,49.B. H. Davis, J. Catalysis, 1972, 26, 348.B. H. Davis, J. Org. Chem., 1972, 37, 1240.l9 B. H. Davis, unpublished results.2o €3. H. Davis, Colloid Interface Sci., 1976, 3, 115.21 D. G. Klissurski, E. F. McCaffrey and R. A. Ross, Canad. J. Chem., 1971,49, 3778.22 (a) A. J. Tench, T. Lawson and J. F. J. Kibblewhite, J.C.S. Faraday I, 1972,68,1169. (b) A. J.23 E. G. Derouane and W. Gieseke, J. Mol. Catalysis, 1975/76, 1, 411, and references therein.24 M. Boudart, A. Delboville, G. E. Derouane, V. Indovina and A. B. Waters, J. Amer. Chem.25 R. Martens, H. Gentsch and F. Freund, J. Catalysis, 1976, 44, 366.26 J. E. Kilpatrick, E. J. Prosen, K. S. Pitzer and F. D. Rossini, J. Res. Nat. Bur. S t u d , 1946,1923).Tench, J.C.S. Faraday I, 1972, 68, 1181, and references therein.SOC., 1972, 94, 6622.36, 559.(PAPER 91157