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Dehydrocyclodimerization of conjugated dienes catalyzed by solid bases
Journal of lkfolecular Catalgszs,
63 (1990) 371-385
371
Dehydrocyclodimerizatlon of conjugated dienes catalyzed by solid bases Hiroyasu
Suzuka and Hideshi Hattori*
Department of Chemwtry, Sappm-o 060 (Japan)
Facdtg
of Sconce,
Hokkazdo
Unzverszty,
(Received January 26, 1990, revlsed July 14, 1990)
Abstract 1,3-Buta&ene and 2-methyl-1,3-buta&ene undergo dehydrocyclodunenzatlon to form aromatics over sohd base catalysts such as ZrOz, MgO, CaO, SrO, L%O,, and NdzO, below 473 K Among the catalysts exammed, ZrOz exhIbIted the h&est actlvlty In the reaction of 1,3-butadlene, the mam products were ethylbenzene, o- and p-xylenes Over MgO catalyst, o-xylene was most selectively formed, wNe over the other catalysts ethylbenzene was selectively formed In the reaction of 2-methyl-1,3-butadlene, whch 1s more reactive than 1,3-butad~ene, the mam products were m- and p-cymenes Over MgO and CaO catalysts, m-cymene was predommant, while over ZrOz catalyst, p-cymene was pnmanly formed On the basis of product -butIons and polsonmg expenments with carbon &oxide, water, pyndme and ammoma, it 1s suggested that basic sites on the catalysts partlclpate m the reaction, and the reactlon mechamsms are &cussed Over MgO, it IS proposed that the mam reactlon mvolves amomc mtermedlates for dehydrocyclodnnenzatlon of both 1,3-butadlene and 2-methyl-1,3-butatiene Over ZrOz, base-catalyzed Die&Alder reaction followed by double bond Isomenzation and dehydrogenatlon are proposed
Introduction
1,3-Buta&ene undergoes various reactlons, such as non-catalytic Duels-Alder reaction and catalytic dunenzatlon to aliphatlc olefins and cychc olefins Among the reactions of 1,3-butadlene, dehydrocyclodunenzatlon to yield aromatics has been reported to occur over acl&c metal oxide catalysts [ 1 ] and metal-acid blfunctlonal catalysts [Z-6] as one of the steps m the conversion of butane and butenes to aromatics The reactlons was suggested to be associated urlth ac&c sites on the catalysts A reaction temperature above 703 K was needed Recently, we found that dehydrocyclodunenzatlon of 1,3-buta&ene to yield aromatics was effectively catalyzed by non-acldlc metal oxide catalysts below 473 K, and have bnefly reported the results elsewhere [ 71 In particular, zu-comum oxide and magnesmm oxide showed kugh actlvltles The type of 1,3-buta&ene reactlon takmg place on zlrcomum oxide and magnesium oxide *Author to whom correspondence should be addressed
0304-5102/90/$3
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0 Elsevler Sequo&Prmted 111The Netherlands
372
is merent from those catalyzed by acidic metal oxldes and metal-acid blfunctlonal catalysts. In the present paper, we msh to report the 1,3butadlene dehydrocyclo&menzatlon catalyzed by metal oxide catalysts, mcludmg ZVCONUIII oxide and magnesmm oxide, 111detti In ad&tion, the results of 2-methyl-1,3-buta&ene dehydrocyclodunenzatlon are mcluded to exanune the vah&ty of the base-catalyzed dehydrocyclodunerlzatlon of con~ugated dlenes Experimental
MgO was purchased from Merck Zr02 was prepared from 10% aqueous solution of ZrOCl:, by hydrolysis vvlth 28% aqueous ammoma to precipitate Zr(OH),, followed by washmg w&h delomzed water, drymg at 373 K and calcmmg at 773 K [ 81 L&O3 and NdzO, were prepared from 10% aqueous solution of mtrates [9] as described for ZrO, These catalysts were outgassed at several Merent temperatures before use CaO, SrO and BaO were obtamed by thermal decomposltlon of commercially available Ca(OH),, SrC03 and BaCO,, respectively, m vacuum at elevated temperatures [lo] The reactions were carned out m an all-glass H-shaped batch type reactor The two branches were separated by a breakable seal A catalyst sample, normally 200 mg for 1,3-buta&ene and 500 mg for 2-methyl-1,3-butaaene, was placed m one branch, pretreated at an elevated temperature and sealed The reactant, 0 75 mmol for 1,3-buta&ene and 1 3 mm01 for 2-methyl-1,3butacllene, was pu&ied by passage through 4A molecular sieves and stored m the other branch u&l it was introduced through the breakable seal by &Watlon mto the branch contammg the catalyst thermostatted at hqmd mtrogen temperature The reaction was begun by rapld meltmg of the reactant at reaction temperature Most of the reactions were run at 473 K for 1,3butahene, and at 393 K for 2-methyl-1,3-butahene for 17 h Durmg the reaction, the reaction nuxture was gaseous, and thus the catalyst was not stirred For the reaction of 1,3-butahene, the products were collected m a hquld mtrogen trap, dissolved m 1 cm3 of benzene and analyzed by gas chromatography Calculation of converslon was based on the products &ssolved m the solvent, no gaseous products bemg taken mto account For the reaction of 2-methyl-1,3-buta&ene, the products collected m hquld mtrogen were analyzed by gas chromatography wthout solvent. Polsomng experunents usmg carbon &oxlde, water, pvdme and ammoma as poisons were performed on MgO catalyst pretreated at 1073 K Followmg pretreatment of 200 mg of the catalyst, the poison was adsorbed at room temperature for 10 mm, followed by evacuation at the same temperature for 10 mm Reaction of 1,3-butadiene was then carned out at 473 K for 17 h The amounts of poisons adsorbed on the catalyst were measured as follows. The poisoned catalyst was heated to 1073 K, wNe the evolved poison was trapped m hqmd mtrogen The trapped poison was evaporated and measured volumetncally
373
Results
The products distributions m the reaction of 1,3-butadiene over various catalysts under the same reaction conditions are summarized m Table 1 The mam products were 4-vmylcyclohexene, ethylbenzene, o- and p-xylenes The other products mcluded double bond isomers of 4+mylcyclohexene, toluene, cycloheptatnene, 3-ethyhdenecyclohexene, 3-methyl-1,4-heptadiene, 2,5-dunethyl-2,4-hexadrene and 2,3-dunethyl-1,3-butadrene Ethylbenzene, and o- andp-xylenes resulted from dehydrocyclodunerizatlon of 1,3-butadiene The active catalysts for the dehydrocyclodimenzation were MgO, ZrOB, CaO, SrO, L&O3 and Ndz03 In particular, MgO and ZrOz showed high a&n&es Among the dehydrocyclodunenzatlon products, ethylbenzene was donunant over ZrOz, CaO, SrO, L%03 and Ndz03, wlule o-xylene was predominantly formed over MgO. No catalysts have ever been reported to yield primarily o-xylene from 1,3-butadiene m one step Over acidic catalysts such as &02-A1203, Mg-Y zeohte and Na-Y zeohte, a variety of gaseous and liquid products were formed m smali amounts, but dehydrocyclodimellzation products were not found Without catalyst, 3 9% of 1,3-butadiene was converted to 4-vmylcyclohexene, but not to aromatics Non-catalytic Diels-Alder reaction occurred to a small extent under the reaction conditions. The tune dependence of the product composition m the reaction of 1,3-butadiene over MgO, pretreated at 1173 K, is shown m F’ig 1 p-Xylene was primarily formed at the lrut1a.lstage of the reaction, but its concentration did not mcrease as the reaction went on. Ethylbenzene and o-xylene were produced contmuously with the reaction time The selcctnnty to o-xylene mcreased as the reaction time became longer. Even m the lIutlal stage of the reaction, ahphatic tiers were not found as mam products. TABLE 1 Dehydrocyclodunenzatlon of 1,3-butadlene at 473 K’ Catalyst
=% MgO
CaO sro BaO Lazo3 Nd303
none
Pretreatment temperature 0
Conversion (%)
773 1073 773 1073 1073 1073 1073 -
29 2 20 4 6 7d 2 Id 2 Od 45 45 39
Composltlon of products (96) 4-VCHb
ethylbenzene o-xylene
07 10 28 0 57 0 36 46 93 8
97 25 93 100 43 88 92 0
1 1 3 0 3 3
13 53 0 0 0 0 0 0 0
p-xylene
othersc
0 88 0 0 0 0 0 0
12 1 39 0 0 81 31 62
09
‘Catalyst we&t 200 mg, reactant 0 75 mmol, reactlon tune 17 h b4-Vmylcyclohexene ‘Other products were mamly vmylcyclohexene isomers other than I-vmylcyclohexene dGaseous compounds were produced, but not mcluded m the calculation of converslon
374
100
8 0’ -E E
50
“‘m 0
5
10
Reactlon
01
15
time / h
673
073
Pretreatment
1073 1273
temperature
I K
F% 1 Tune dependence of the product composltlon m the reaction of 1,3-buta&ene over MgO pretreated at 1173 K (0) 1,3-butadiene, (0) o-xylene, (A) p-xylene, (V) ethylbenzene I”lg 2 Vanatlons of actn@ of MgO (0) and ZrOz (A) for 1,3-butatiene dehydrocyclodunenzatlon as a function of catalyst pretreatment temperature Reactant 0 75 mmol, weight of catalyst 500 mg for MgO, 200 mg for ZrOz, reactlon temperature 473 K, reaction tune 17 h Surface areas of MgO are 55, 54 and 33 m2 g-’ followmg pretreatment at 873, 973 and 1173 K, respectively [lo] Those of ZrOo2are 109, 65, 32, 21, and 11 m2 g-l followmg pretreatment at 673, 773, 873, 973 and 1073 K, respectively [lo] TABLE 2 Dehydrogenation of 4-vmylcyclohexene at 393 K” Catalyst
Zfi, MgO
Pretreatment temperature (K)
Converslon (%)
Composition of products (46) ethylbenzene
other hydrocarbonsb
823 1173
95 4 83 9
64 6 96 3
35 4 37
‘Catalyst weight 500 mg, reactant 0 40 mmol, reaction tune 17 h bOther hydrocarbons were composed of 28 lands, mamly vmylcyclohexene isomers
Vanatlons of the converslon wrth the catalyst pretreatment temperature for MgO and Zr02 are shown m Fig 2 The actlvlty of MgO appeared at the pretreatment temperature of w 700 K, and mcreased monotomcally as the pretreatment temperature was rased upto 1273K The actlvlty of ZrOz showed a maxunum at the pretreatment temperature of 773 K and dunu-ushed at 1073 K On a umt surface area baa=, the maxunum actlvlty of ZrOz was obtamed by pretreatment at 873 K. The product selectlvltles remamed essentially unaffected by the pretreatment temperature, not only for MgO and ZrO, but also for the other catalysts The results of the reactlons of 4-vmylcyclohexene, whch IS the product of the Die&Alder reaction of 1,3-buta&ene, are given m Table 2 Even at the reactlon temperature of 393 K, 4-vmylcyclohexene converted to ethylbenzene at a rate much faster than the dehydrocyclodnnenzatlon of 1,3buta&ene over both MgO and ZrOp
376
The results of the reactron of 2-methyl-1,3-butadrene are summarized m Table 3. 2-Methyl-1,3-butadrene 1s much more reactive than 1,3-butadrene for the dehydrocyclodunerrzatlon over the catalysts used. The products consrsted of lu-nonene, p- and m-cymenes, menthadtene isomers and 2methylbutenes. For the reactron of 2-methyl-1,3-butadtene too, ZrOa showed the highest actrvrty MgO and CaO showed considerable actlvrtles. Over BaO, all the reactant was consumed, but the products detected were only hght hydrocarbons contammg carbon numbers of 1 to 3 Hrgh-borlmg compounds and carbonaceous residues may be formed, which remamed on the surface of BaO and were not collected m the liqmd mtrogen trap. Without catalyst, 2-methyl-1,3-butadiene converted 1% under the reaction condrtrons However, at 473 K, 2-methyl-1,3-butadrene converted at almost the same level as those wrth catalysts The reactron products wrthout catalysts at 473 K were drfferent from those wrth catalysts The reactron products wrthout catalysts consrsted of lunonene and menthadrenes, wluch are the compounds If Duels-Alder reactron occurs, but drd not mclude aromatics Obviously Duels-Alder reaction of 2-methyl-l,&butadrene took place to a consrderable extent at 473 K wrthout catalyst. Product selectrvlty depended on the catalyst type. p-Cymene was the mam product over ZrOz and &Oa, whrle m-cymene was mainly produced over MgO With MgO, CaO and SrO, Cs-olefins resultmg from hydrogenation of 2-methyl-1,3-butadrene were formed to a considerable extent. The results of the reactron of hmonene over MgO and ZrOa at 473 K are grven 111Table 4. At tlus reaction temperature, the conversron of 2-methyl-1,3-butadrene over MgO was 47%. Lunonene underwent dehydrogenatron to form pnmarrly p-cymene at a rate much faster than the dehydrocyclodunerrzatron of 2methyl-l ,3-butadrene. Tables 5 and 6 summarrze the polsonmg expenments for the reaction of 1,3-butadrene over MgO and ZrOz, respectrvely. For MgO catalyst, the conversron of 1,3-butadrene was reduced to about half the origmal value by addmon of carbon droxrde. The product consisted exclusrvely of 4-vmylcyclohexene; no aromatms were formed. Carbon droxrde completely mhrblted both the conversion of 4-vmylcyclohexene to ethylbenzene and dehydrocyclodunerrzatron of 1,3-butadrene to aromatms The addrtron of water reduced the conversron of 1,3-butadrene to about one quarter its ongmal value, wrth 4-vmylcyclohexene mamly bemg formed The for-matron of o- and p-xylenes was completely mhibrted by water With pyruhne and ammoma, the conversron of 1,3-butadrene decreased to half its orrgmal value The product drstnbutrons were different from those obtamed for the catalysts poisoned by carbon droxrde and water. Ethylbenzene and o-xylene were formed. As compared wrth non-porsoned catalyst, the selectrvny to ethylbenzene mcreased, but that to o-xylene decreased. For ZrOa catalyst, decreases m the conversion of 1,3-butadiene on add&on of any of the parsons were observed, as m the case of MgO catalyst. The products consisted mamly of 4-vmylcyclohexene with all of the poisons except pyndine, whose addrtron produced o-xylene to a considerable extent.
823 1073 773 1073 1073 -
ZrOz MgO CaO sro BaO none
27 4 47 22 15 100 0 10
Converslon (%) 0 0 0 0 1OOd 0
C,-C, 0 192 62 0 514 0 0
methylbutenes
CornposItIon of products (96)
20 5 17 1 62 16 2 0 18 5
menthdenes 53 83 23 10 7 0 815
hmoneneb
“Catalyst weight 500 mg, 2-methyl-1,3-butadlene 1 3 mmol, reaction tune 17 h bLunonene 1s counted separately from other menthadlenes, though lunonene IS a menthahenes ‘Others mclude 3 to 13 kmds of hydrocarbons, each amount small compared to the mdlcated products dThe other h@-bodmg products were not analyzed
Pretreatment temperature (K)
Catalyst
2-Methyl-1,3-butadlene dehydrocyclodimenzation at 393 K”
TABLE 3
133 303 16 7 48 0 0
m-cymene 49 6 17 1 93 66 0 0
p-cymene
113 80 35 10 3 0 0
others’
377 TABLE 4 Dehydrogenatlon of hmonene at 473 K” catalyst
MN ZrG
Pretreatment temperature (K)
Conversion (%)
1073 923
67 7 98 9
Composltlon of products (%) p-cymene
menthahenes
Others
98 7 74 3
13 172
0 8 5b
‘Catalyst we&t 200 mg, reactant 0 80 mmol, reactlon tune 17 h bComposed of 3 lands of hydrocarbons contammg 10 carbons, the compounds were not ldenttied
TABLE 5 Polsonmg effect of 1,3-but.a&ene dehydrocyclodunenzatlon over MgO catalyst’ Poison
Amount of poison (mol g-l)
Conversion Composition of products (%) (%) 4-VCHb ethylbenzene o-xylene m-xylene p-xylene others’
none CO2 Hz0 pyndme NH3
0 2 3x10+ 1 ox 10-3 1 1X10m4 8 4~10-~
204 119 56 93 10 2
10 908 673 32 33
25 1 0 37 61 7 80 6
530 0 0 351 41
0 0 0 0 0
88 0 0 0 0
12 1 92 29 0 0 12 0
“catalyst weight 200 mg, reactant 0 75 mmol, reaction temperature 473 K, reaction tune 17 h b4-Vmylcyclohexene ‘Others mcluded 4 to 5 lands of hydrocarbons, mostly vmylcyclohexene isomers
TABLE 6 Polsonmg effect of 1,3-butad~ene dehydrocyclodunenzatlon over ZrOz catalyst* Poison
Amount of poison (mol g-l)
Conversion Composition of products (46) (%) 4-VCHb ethylbenzene o-xylene m-xylene p-xylene others”
none CO, H2O pyndme NH3
0 2 7x10-4 8 3x10-4 15X 10m4 2 7x lo+
33 5 75 14 7 13 9 96
0 970 907 672 872
98 8 0 0 0 0
06 0 0 185 0
0 0 0 0 0
0 0 0 0 0
06 30 93 14 3 12 8
“Catalyst weight 200 mg, reactant 0 75 mmol, reamon temperature 473 K, reaction time 17 h b4-Vmylcyclohexene ‘Others comprised 3 to 8 lands of hydrocarbons, mostly vmylcyclohexene isomers
378
The addition of carbon dioxide, water and ammoma retarded the formation of 4-vmylcyclohexene and completely mhiblted the dehydrogenatron to aromat1cs
Discussion
Dehydrocyclodunenzation of butane and butenes has been reported to proceed over metal oxide catalysts and bifunctlonal catalysts Bhatla and Phrlhps have reported that conversion of I-butene to aromatics over a crystallme molecular sieve AlPOd-11 catalyst mvolves dehydrogenation of butenes to butadrene, followed by cyclodunenzation of a Die&Alder type [ 11 The Diels-Alder type reaction was suggested to be assrsted by Lewis acid sites on the catalyst. Csmsery has reported the dehydrocyclodunenzatlon of butane to form aromatics over bifunctional catalysts, and suggested that the reaction proceeds through several consecutive and parallel steps, such as metal-catalyzed dehydrogenation, acid-catalyzed dimerrzation, aroma& zatron, lsomenzation and transal.Qlatlon [Z-6]. Conversron of butadiene to aromatics was suggested to mvolve a Diels-Alder-type thermal cychzatron to vmylcyclohexene, followed by metal-catalyzed dehydrogenatron. The types of reactron observed 111the present study are merent m several aspects from those reported for the acidic metal oxide catalysts and brfunctronal catalysts. The maJor drfferences are the reaction temperature, the product distribution and the surface properties required for the catalysts These differences are compared below. Concerning the reaction temperature, reactions over the reported catalysts were appreciable above 703 K [l-6]. In the present study, however, the reaction proceeded below 473 K The difference m product distribution between the reported catalysts and the present catalysts is distinct A wade variety of aromatics ranging from benzene to ethylbenzene, mcludmg toluene, o-, m- and p-xylenes, were produced over the reported catalysts On the other hand, the aromatics of carbon number eight were the mam products m the present study. In partmular, the absence of m-xylene m the reactron products is a characteristic feature of the catalysts m the present study. The difference m the surface properties between the reported catalysts and the present catalysts is also distinct The surface propeties of ZrOz [ 81, MgO [ll], CaO [ll], L&O3 [9, 12, 131, and NdaOs [9, 12, 131, which were found to be active m the present study, have been studred by various methods such as titration wnh benzom acid, temperature-programmed desorption of COz and catalytic test reactrons. The charactenstms common to the catalysts active for 1,3-butadiene dehydrocyclodrmenzatlon m the present study are then basic propeties [ 12, 13 1. Therefore, it is suggested that basic sites on the catalysts are mvolved m the reactron mechanisms in the present study Exceptrons were found with SrO and BaO; these catalysts showed nummal actnnty, and their basic strengths are stronger than those of MgO and CaO
379
[ 12, 141. Probably basic sites wluch are too strong do not act efficiently as catalytic sites. The results of poisonmg expenments support the idea that the basic sites are active for the dehydrocyclodunenzatlon. In general, basic sites on sohd base catalysts are poisoned by carbon dioxide and water The strong polsonmg effects of carbon dioxide and water observed with MgO and ZrOz support thrs view As carbon dioxide and water mhibited both the formation of aromatics and conversion of 4+mylcyclohexene, basic sites are considered to participate m both the activation of 1,3-butadiene, leadmg to aromatics, and the dehydrogenation of 4-vmylcyclohexene to ethylbenzene So far, although a number of reactions are known to be base-catalyzed, dehydrocyclodunerization of comugated dienes has not been recognized as a basecatalyzed reaction either m homogeneous or m heterogeneous catalytic systems The poisonmg effect of ammoma on MgO was drlferent from that on ZrOz The difference is renumscent of the polsonmg effects of ammoma on 1-butene isomerization [8, 1 l] For 1-butene isomellzation, it was reported that ammoma completely poisoned the active sites on ZrOz [8], but slightly mcreased the activity of MgO [ll]. Over ZrOz, it was reported that l-butene isomerization was base-catalyzed reaction, though a basic molecule of ammonia retarded the reaction The reaction 1s llutiated by the abstraction of a H+ by a basic site to form an allyhc amon The Zr4’ cation achacent to the basic site plays a role m stabilrzmg the allyhc amon. The polsonmg effect of ammoma IS thought to block the Zr4+ cation, so that the allyhc amon cannot be stabilized on the surface [8] In the present study with ZrOz, ammoma decreased the conversion of 1,3-butadiene to one third its ongmal value and completely poisoned the formation of ethylbenzene. With MgO, although the conversion of 1,3-butadiene decreased to half its original value, the formation of ethylbenzene mcreased. The formation of ethylbenzene and Isomenzation of l-butene respond smularly toward ammonia pre-adsorption both on ZrOp and MgO As for the reaction mechamsm, two Werent mechanisms are considered, m both of wluch basic sites are mvolved. The first 1s an amo~c mechanism m wluch the reactron is uutiated by H+ abstraction from the reactant by basic sites to form amo~c mtermedlates The second ISa Dlels-Alder reaction, followed by double bond lsomenzation and dehydrogenation. The anionic mechanism proposed is shown m Scheme 1. Amon 1 attacks the 1,3-butadiene molecule at carbon 2 to form anion 2. The amon 2 will undergo both cychzation to the amon 3 by route I and double bond transfer to the anions 4 and 6 by route II. Amens 4 and 5 are pnmary anions, whereas amon 3 IS a less stable secondary amon. Therefore, route II 1s likely to proceed faster than route I By this amo~c mechanism, the expected mam products are o- and p-xylenes. As double bond migration of oletlns proceeds rapidly over solid base catalysts [ 12,13,15 1,the substituted cyclohexadienes which are the precursors of ethylbenzene, o- and p-xylenes are considered to be composed of a mixture
380
381
of double bond isomers The alternative reaction mechamsm mvolvmg a Duels-Alder reaction 1s shown m Scheme 2 By the reaction Scheme 2, ethylbenzene 1s the final product, and the formation of o- and p-xylenes cannot be explamed Over MgO catalyst, the anionic mechamsm 1s suggested to operate because the mam products are o- and p-xylenes It 1s not defirute whether the amomc mechamsm or Duels-Alder reaction IS mvolved m the formation of ethylbenzene over MgO However, as mentloned above, the formatlon of ethylbenzene through the amomc mechamsm seems mcult In ad&tlon, It 1s hard to explam the tierent polsomng effects of ammoma on the formation of ethylbenzene and xylenes if ethylbenzene and xylenes were formed through common amoluc mtermedates. Therefore, it is more hkely that the formation of ethylbenzene on MgO proceeds through a Duels-Alder reaction, as shown m Scheme 2 Over ZrOz and CaO, the selective formatlon of ethylbenzene suggests that the reactlon proceeds through a Duels-Alder reaction Without catalyst, Duels-Alder reaction of 1,3-buta&ene to vmylcyclohexene proceeded to only a small extent. Therefore, Zr02 and CaO catalysts should promote the Duels-Alder reactlon It is well known that the general acid catalysis of the Dlels-Alder reactlon 1s attnbuted to coordmatlon of the acid vvlth the denophile, drawmg electron density away from the reaction site and thus makmg the &enopme more electroph&c [16] Dlenes vvlth electron-urlthdrawmg groups react reatiy urlth tienophiles contammg electron-releasmg groups [ 171 Base catalysts may promote Die&Alder reaction m such a way that the basic sites donate electrons to lenes and thus make the dlenes more reactive urlth &enopties Follow-mg the DIebAlder reaction, double bond lsomenzatlon and dehydrogenation occur to form ethylbenzene With SrO, BaO, L%O, and Ndz03, the conversion levels were close to those vvlthout catalyst, but the product dLstnbutlons wered These catalysts barely promoted the Duels-Alder reaction, but catalyzed dehydrogenatlon of 4-vmylcyclohexene. The basic sites are suggested to partlclpate m the Duels-Alder reaction, the abstraction of H+ from 1,3-butadlene, the lsomenzatlon of vmylcyclohexenes, and the dehydrogenatlon of substituted cyclohexa&enes However, active basic sites may not be common to all these reactlons The existence of Merent basic sites ansmg from the merent coordmatlon numbers of surface 02- ions IS reported on MgO [ 131 In the polsomng expernnents urlth ammoma, the total conversion of 1,3-butadlene decreased but the formation of ethylbenzene mcreased Tlus suggests that ammoma retards the abstraction of H+ from 1,3-buta&ene to form the ~IUONC mtermehate, but does not retard the Duels-Alder reaction and the followmg lsomenzatlon and dehydrogenatlon As ammoma IS a basic molecule, it 1s generally beheved that it poisons the acl&c sites However, it 1s reported that on MgO ammoma 1s &ssoclatlvely adsorbed on basic sites to form NH2- and H+ [18]. The formed H’ blocks basic sites, therefore, ammoma may poison basic sites.
382
383
384
Identification of the active basic sites for each reaction, however, has not been carried out m the present study yet For 2-methyl-1,3-butadiene dehydrocyclodunerrzation, the formation of m- and p-cymenes can be explamed both by a Diels-Alder reaction followed by double bond isomenzation and dehydrogenation, and by the amoruc mechanism Two reaction schemes for 2-methyl-1,3-butadlene dehydrocyclodunenzation are shown below Scheme 3 depicts the reaction mvolvmg a Diels-Alder mechanism and Scheme 4 the amomc mechanism In the amomc mechanisms, the allyhc H seems to be abstracted first because of the ease to liberate as H+. The position of the carbon atom of Z-methyl-1,3-butadlene at which the amon 1' attacks determines the final product Attack of the amon 1' at carbon atom 3 of 2-methyl-1,3-butadiene (route I) yield m-cymene via formation of amon 2’, rmg closure, addition of H+ and dehydrogenation Attack of the amon 1' at carbon atom 4 (route II) yields lunonene and p-cymene, and at carbon atom 2 (route III) yields, 2,4-dunethyl-4-ethyhdenecyclohexene Route III does not lead to dehydrocyclodunenzation for aromatics unless C-C bond cleavage occurs Since the primary atuon 2’ is more stable than the secondary anion 3’, it is considered that route I is more facile than route II Over MgO catalyst, more m-cymene was formed than p-cymene Therefore, it is suggested that over MgO catalyst the amomc mechanism mamly operates m 2-methyl-1,3butadlene dehydrocyclodimerization, as 111the case of 1,3-butadlene Over CaO catalyst, more m-cymene was produced than p-cymene, suggestmg that the amomc mechanism IS dommant. This 1s m contrast to the results with 1,3-butadiene m which the Diels-Alder reaction mechanism 1s suggested to be donunant The change 111mechanism depending on the reactant is considered to be due to the existence of allyhc H m 2-methyl1,3-buta&ene The formation of the amon is easier for 2-methyl-1,3-butadrene than for 1,3-butadiene Over ZrOa catalyst, p-cymene was predonunantly formed The reaction IS suggested to proceed through the Duels-Alder reaction, similar to the case of 1,3-butadrene Although novel aspects of sohd base catalysts are disclosed, there are many questions left unsolved at the present time, such as the activity- and selectivity-governmg factors of catalysts, and applicability of base-catalyzed dehydrocyclodunenzatlon to diolefins other than the substituted butalenes exanuned m the present work To solve these, further studies are required.
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110 (1988)
150
385 7 H Suzuka and H Hatton, Appl Cutal, 47 (1989) L7 8 Y Nakano, T Ilzuka, H Hatton and K Tanabe, J Cutal, 57 (1973) 1 9 H Hatton, H Kumm, K Tanaka, G Zhang and K Tanabe, Proc 8th Nutzonal Symp Catal In&u, S&n, 1987, p 243 10 Y Kakuno and H Hatton, J Cat&, 85 (1984) 509 11 H Hatton, N Yoshu, and K Tanabe, Proc 5th Int Congr Catal, Mzamz Beach, 1972, North Holland-Elsevler, Amsterdam-New York, 1973, p 10-233 12 K Tanabe, M Muono, Y Ono and H Hatton, NaoSolzdAczds andBuses, Kodansha-Elsevler, Tokyo-Amsterdam, 1989 13 H Hatton, Mat Chem Phys, 18 (1988) 533 14 G Zhang, H Hatton and K Tanabe, Appl Cutal, 36 (1988) 189 15 H Hatton, m M Che and G C Bond (eds ), Adsorptwn and Caialyszs on Oxzde Surfaces, Elsevler, Amsterdam, 1985, p 319 16 E S Gould, Mechunwna and Structure zn Organzc Chemzstr/, Henry Holt, New York, 1960, p 537 17 T W G Solomons, Organzc Chazsh?/, Wiley, New York, 1976, p 363 18 E Garrone and F S Stone, Proc 8th Int Congr Catal, Berhn, 1984, III-441
63 (1990) 371-385
371
Dehydrocyclodimerizatlon of conjugated dienes catalyzed by solid bases Hiroyasu
Suzuka and Hideshi Hattori*
Department of Chemwtry, Sappm-o 060 (Japan)
Facdtg
of Sconce,
Hokkazdo
Unzverszty,
(Received January 26, 1990, revlsed July 14, 1990)
Abstract 1,3-Buta&ene and 2-methyl-1,3-buta&ene undergo dehydrocyclodunenzatlon to form aromatics over sohd base catalysts such as ZrOz, MgO, CaO, SrO, L%O,, and NdzO, below 473 K Among the catalysts exammed, ZrOz exhIbIted the h&est actlvlty In the reaction of 1,3-butadlene, the mam products were ethylbenzene, o- and p-xylenes Over MgO catalyst, o-xylene was most selectively formed, wNe over the other catalysts ethylbenzene was selectively formed In the reaction of 2-methyl-1,3-butadlene, whch 1s more reactive than 1,3-butad~ene, the mam products were m- and p-cymenes Over MgO and CaO catalysts, m-cymene was predommant, while over ZrOz catalyst, p-cymene was pnmanly formed On the basis of product -butIons and polsonmg expenments with carbon &oxide, water, pyndme and ammoma, it 1s suggested that basic sites on the catalysts partlclpate m the reaction, and the reactlon mechamsms are &cussed Over MgO, it IS proposed that the mam reactlon mvolves amomc mtermedlates for dehydrocyclodnnenzatlon of both 1,3-butadlene and 2-methyl-1,3-butatiene Over ZrOz, base-catalyzed Die&Alder reaction followed by double bond Isomenzation and dehydrogenatlon are proposed
Introduction
1,3-Buta&ene undergoes various reactlons, such as non-catalytic Duels-Alder reaction and catalytic dunenzatlon to aliphatlc olefins and cychc olefins Among the reactions of 1,3-butadlene, dehydrocyclodunenzatlon to yield aromatics has been reported to occur over acl&c metal oxide catalysts [ 1 ] and metal-acid blfunctlonal catalysts [Z-6] as one of the steps m the conversion of butane and butenes to aromatics The reactlons was suggested to be associated urlth ac&c sites on the catalysts A reaction temperature above 703 K was needed Recently, we found that dehydrocyclodunenzatlon of 1,3-buta&ene to yield aromatics was effectively catalyzed by non-acldlc metal oxide catalysts below 473 K, and have bnefly reported the results elsewhere [ 71 In particular, zu-comum oxide and magnesmm oxide showed kugh actlvltles The type of 1,3-buta&ene reactlon takmg place on zlrcomum oxide and magnesium oxide *Author to whom correspondence should be addressed
0304-5102/90/$3
50
0 Elsevler Sequo&Prmted 111The Netherlands
372
is merent from those catalyzed by acidic metal oxldes and metal-acid blfunctlonal catalysts. In the present paper, we msh to report the 1,3butadlene dehydrocyclo&menzatlon catalyzed by metal oxide catalysts, mcludmg ZVCONUIII oxide and magnesmm oxide, 111detti In ad&tion, the results of 2-methyl-1,3-buta&ene dehydrocyclodunenzatlon are mcluded to exanune the vah&ty of the base-catalyzed dehydrocyclodunerlzatlon of con~ugated dlenes Experimental
MgO was purchased from Merck Zr02 was prepared from 10% aqueous solution of ZrOCl:, by hydrolysis vvlth 28% aqueous ammoma to precipitate Zr(OH),, followed by washmg w&h delomzed water, drymg at 373 K and calcmmg at 773 K [ 81 L&O3 and NdzO, were prepared from 10% aqueous solution of mtrates [9] as described for ZrO, These catalysts were outgassed at several Merent temperatures before use CaO, SrO and BaO were obtamed by thermal decomposltlon of commercially available Ca(OH),, SrC03 and BaCO,, respectively, m vacuum at elevated temperatures [lo] The reactions were carned out m an all-glass H-shaped batch type reactor The two branches were separated by a breakable seal A catalyst sample, normally 200 mg for 1,3-buta&ene and 500 mg for 2-methyl-1,3-butaaene, was placed m one branch, pretreated at an elevated temperature and sealed The reactant, 0 75 mmol for 1,3-buta&ene and 1 3 mm01 for 2-methyl-1,3butacllene, was pu&ied by passage through 4A molecular sieves and stored m the other branch u&l it was introduced through the breakable seal by &Watlon mto the branch contammg the catalyst thermostatted at hqmd mtrogen temperature The reaction was begun by rapld meltmg of the reactant at reaction temperature Most of the reactions were run at 473 K for 1,3butahene, and at 393 K for 2-methyl-1,3-butahene for 17 h Durmg the reaction, the reaction nuxture was gaseous, and thus the catalyst was not stirred For the reaction of 1,3-butahene, the products were collected m a hquld mtrogen trap, dissolved m 1 cm3 of benzene and analyzed by gas chromatography Calculation of converslon was based on the products &ssolved m the solvent, no gaseous products bemg taken mto account For the reaction of 2-methyl-1,3-buta&ene, the products collected m hquld mtrogen were analyzed by gas chromatography wthout solvent. Polsomng experunents usmg carbon &oxlde, water, pvdme and ammoma as poisons were performed on MgO catalyst pretreated at 1073 K Followmg pretreatment of 200 mg of the catalyst, the poison was adsorbed at room temperature for 10 mm, followed by evacuation at the same temperature for 10 mm Reaction of 1,3-butadiene was then carned out at 473 K for 17 h The amounts of poisons adsorbed on the catalyst were measured as follows. The poisoned catalyst was heated to 1073 K, wNe the evolved poison was trapped m hqmd mtrogen The trapped poison was evaporated and measured volumetncally
373
Results
The products distributions m the reaction of 1,3-butadiene over various catalysts under the same reaction conditions are summarized m Table 1 The mam products were 4-vmylcyclohexene, ethylbenzene, o- and p-xylenes The other products mcluded double bond isomers of 4+mylcyclohexene, toluene, cycloheptatnene, 3-ethyhdenecyclohexene, 3-methyl-1,4-heptadiene, 2,5-dunethyl-2,4-hexadrene and 2,3-dunethyl-1,3-butadrene Ethylbenzene, and o- andp-xylenes resulted from dehydrocyclodunerizatlon of 1,3-butadiene The active catalysts for the dehydrocyclodimenzation were MgO, ZrOB, CaO, SrO, L&O3 and Ndz03 In particular, MgO and ZrOz showed high a&n&es Among the dehydrocyclodunenzatlon products, ethylbenzene was donunant over ZrOz, CaO, SrO, L%03 and Ndz03, wlule o-xylene was predominantly formed over MgO. No catalysts have ever been reported to yield primarily o-xylene from 1,3-butadiene m one step Over acidic catalysts such as &02-A1203, Mg-Y zeohte and Na-Y zeohte, a variety of gaseous and liquid products were formed m smali amounts, but dehydrocyclodimellzation products were not found Without catalyst, 3 9% of 1,3-butadiene was converted to 4-vmylcyclohexene, but not to aromatics Non-catalytic Diels-Alder reaction occurred to a small extent under the reaction conditions. The tune dependence of the product composition m the reaction of 1,3-butadiene over MgO, pretreated at 1173 K, is shown m F’ig 1 p-Xylene was primarily formed at the lrut1a.lstage of the reaction, but its concentration did not mcrease as the reaction went on. Ethylbenzene and o-xylene were produced contmuously with the reaction time The selcctnnty to o-xylene mcreased as the reaction time became longer. Even m the lIutlal stage of the reaction, ahphatic tiers were not found as mam products. TABLE 1 Dehydrocyclodunenzatlon of 1,3-butadlene at 473 K’ Catalyst
=% MgO
CaO sro BaO Lazo3 Nd303
none
Pretreatment temperature 0
Conversion (%)
773 1073 773 1073 1073 1073 1073 -
29 2 20 4 6 7d 2 Id 2 Od 45 45 39
Composltlon of products (96) 4-VCHb
ethylbenzene o-xylene
07 10 28 0 57 0 36 46 93 8
97 25 93 100 43 88 92 0
1 1 3 0 3 3
13 53 0 0 0 0 0 0 0
p-xylene
othersc
0 88 0 0 0 0 0 0
12 1 39 0 0 81 31 62
09
‘Catalyst we&t 200 mg, reactant 0 75 mmol, reactlon tune 17 h b4-Vmylcyclohexene ‘Other products were mamly vmylcyclohexene isomers other than I-vmylcyclohexene dGaseous compounds were produced, but not mcluded m the calculation of converslon
374
100
8 0’ -E E
50
“‘m 0
5
10
Reactlon
01
15
time / h
673
073
Pretreatment
1073 1273
temperature
I K
F% 1 Tune dependence of the product composltlon m the reaction of 1,3-buta&ene over MgO pretreated at 1173 K (0) 1,3-butadiene, (0) o-xylene, (A) p-xylene, (V) ethylbenzene I”lg 2 Vanatlons of actn@ of MgO (0) and ZrOz (A) for 1,3-butatiene dehydrocyclodunenzatlon as a function of catalyst pretreatment temperature Reactant 0 75 mmol, weight of catalyst 500 mg for MgO, 200 mg for ZrOz, reactlon temperature 473 K, reaction tune 17 h Surface areas of MgO are 55, 54 and 33 m2 g-’ followmg pretreatment at 873, 973 and 1173 K, respectively [lo] Those of ZrOo2are 109, 65, 32, 21, and 11 m2 g-l followmg pretreatment at 673, 773, 873, 973 and 1073 K, respectively [lo] TABLE 2 Dehydrogenation of 4-vmylcyclohexene at 393 K” Catalyst
Zfi, MgO
Pretreatment temperature (K)
Converslon (%)
Composition of products (46) ethylbenzene
other hydrocarbonsb
823 1173
95 4 83 9
64 6 96 3
35 4 37
‘Catalyst weight 500 mg, reactant 0 40 mmol, reaction tune 17 h bOther hydrocarbons were composed of 28 lands, mamly vmylcyclohexene isomers
Vanatlons of the converslon wrth the catalyst pretreatment temperature for MgO and Zr02 are shown m Fig 2 The actlvlty of MgO appeared at the pretreatment temperature of w 700 K, and mcreased monotomcally as the pretreatment temperature was rased upto 1273K The actlvlty of ZrOz showed a maxunum at the pretreatment temperature of 773 K and dunu-ushed at 1073 K On a umt surface area baa=, the maxunum actlvlty of ZrOz was obtamed by pretreatment at 873 K. The product selectlvltles remamed essentially unaffected by the pretreatment temperature, not only for MgO and ZrO, but also for the other catalysts The results of the reactlons of 4-vmylcyclohexene, whch IS the product of the Die&Alder reaction of 1,3-buta&ene, are given m Table 2 Even at the reactlon temperature of 393 K, 4-vmylcyclohexene converted to ethylbenzene at a rate much faster than the dehydrocyclodnnenzatlon of 1,3buta&ene over both MgO and ZrOp
376
The results of the reactron of 2-methyl-1,3-butadrene are summarized m Table 3. 2-Methyl-1,3-butadrene 1s much more reactive than 1,3-butadrene for the dehydrocyclodunerrzatlon over the catalysts used. The products consrsted of lu-nonene, p- and m-cymenes, menthadtene isomers and 2methylbutenes. For the reactron of 2-methyl-1,3-butadtene too, ZrOa showed the highest actrvrty MgO and CaO showed considerable actlvrtles. Over BaO, all the reactant was consumed, but the products detected were only hght hydrocarbons contammg carbon numbers of 1 to 3 Hrgh-borlmg compounds and carbonaceous residues may be formed, which remamed on the surface of BaO and were not collected m the liqmd mtrogen trap. Without catalyst, 2-methyl-1,3-butadiene converted 1% under the reaction condrtrons However, at 473 K, 2-methyl-1,3-butadrene converted at almost the same level as those wrth catalysts The reactron products wrthout catalysts at 473 K were drfferent from those wrth catalysts The reactron products wrthout catalysts consrsted of lunonene and menthadrenes, wluch are the compounds If Duels-Alder reactron occurs, but drd not mclude aromatics Obviously Duels-Alder reaction of 2-methyl-l,&butadrene took place to a consrderable extent at 473 K wrthout catalyst. Product selectrvlty depended on the catalyst type. p-Cymene was the mam product over ZrOz and &Oa, whrle m-cymene was mainly produced over MgO With MgO, CaO and SrO, Cs-olefins resultmg from hydrogenation of 2-methyl-1,3-butadrene were formed to a considerable extent. The results of the reactron of hmonene over MgO and ZrOa at 473 K are grven 111Table 4. At tlus reaction temperature, the conversron of 2-methyl-1,3-butadrene over MgO was 47%. Lunonene underwent dehydrogenatron to form pnmarrly p-cymene at a rate much faster than the dehydrocyclodunerrzatron of 2methyl-l ,3-butadrene. Tables 5 and 6 summarrze the polsonmg expenments for the reaction of 1,3-butadrene over MgO and ZrOz, respectrvely. For MgO catalyst, the conversron of 1,3-butadrene was reduced to about half the origmal value by addmon of carbon droxrde. The product consisted exclusrvely of 4-vmylcyclohexene; no aromatms were formed. Carbon droxrde completely mhrblted both the conversion of 4-vmylcyclohexene to ethylbenzene and dehydrocyclodunerrzatron of 1,3-butadrene to aromatms The addrtron of water reduced the conversron of 1,3-butadrene to about one quarter its ongmal value, wrth 4-vmylcyclohexene mamly bemg formed The for-matron of o- and p-xylenes was completely mhibrted by water With pyruhne and ammoma, the conversron of 1,3-butadrene decreased to half its orrgmal value The product drstnbutrons were different from those obtamed for the catalysts poisoned by carbon droxrde and water. Ethylbenzene and o-xylene were formed. As compared wrth non-porsoned catalyst, the selectrvny to ethylbenzene mcreased, but that to o-xylene decreased. For ZrOa catalyst, decreases m the conversion of 1,3-butadiene on add&on of any of the parsons were observed, as m the case of MgO catalyst. The products consisted mamly of 4-vmylcyclohexene with all of the poisons except pyndine, whose addrtron produced o-xylene to a considerable extent.
823 1073 773 1073 1073 -
ZrOz MgO CaO sro BaO none
27 4 47 22 15 100 0 10
Converslon (%) 0 0 0 0 1OOd 0
C,-C, 0 192 62 0 514 0 0
methylbutenes
CornposItIon of products (96)
20 5 17 1 62 16 2 0 18 5
menthdenes 53 83 23 10 7 0 815
hmoneneb
“Catalyst weight 500 mg, 2-methyl-1,3-butadlene 1 3 mmol, reaction tune 17 h bLunonene 1s counted separately from other menthadlenes, though lunonene IS a menthahenes ‘Others mclude 3 to 13 kmds of hydrocarbons, each amount small compared to the mdlcated products dThe other h@-bodmg products were not analyzed
Pretreatment temperature (K)
Catalyst
2-Methyl-1,3-butadlene dehydrocyclodimenzation at 393 K”
TABLE 3
133 303 16 7 48 0 0
m-cymene 49 6 17 1 93 66 0 0
p-cymene
113 80 35 10 3 0 0
others’
377 TABLE 4 Dehydrogenatlon of hmonene at 473 K” catalyst
MN ZrG
Pretreatment temperature (K)
Conversion (%)
1073 923
67 7 98 9
Composltlon of products (%) p-cymene
menthahenes
Others
98 7 74 3
13 172
0 8 5b
‘Catalyst we&t 200 mg, reactant 0 80 mmol, reactlon tune 17 h bComposed of 3 lands of hydrocarbons contammg 10 carbons, the compounds were not ldenttied
TABLE 5 Polsonmg effect of 1,3-but.a&ene dehydrocyclodunenzatlon over MgO catalyst’ Poison
Amount of poison (mol g-l)
Conversion Composition of products (%) (%) 4-VCHb ethylbenzene o-xylene m-xylene p-xylene others’
none CO2 Hz0 pyndme NH3
0 2 3x10+ 1 ox 10-3 1 1X10m4 8 4~10-~
204 119 56 93 10 2
10 908 673 32 33
25 1 0 37 61 7 80 6
530 0 0 351 41
0 0 0 0 0
88 0 0 0 0
12 1 92 29 0 0 12 0
“catalyst weight 200 mg, reactant 0 75 mmol, reaction temperature 473 K, reaction tune 17 h b4-Vmylcyclohexene ‘Others mcluded 4 to 5 lands of hydrocarbons, mostly vmylcyclohexene isomers
TABLE 6 Polsonmg effect of 1,3-butad~ene dehydrocyclodunenzatlon over ZrOz catalyst* Poison
Amount of poison (mol g-l)
Conversion Composition of products (46) (%) 4-VCHb ethylbenzene o-xylene m-xylene p-xylene others”
none CO, H2O pyndme NH3
0 2 7x10-4 8 3x10-4 15X 10m4 2 7x lo+
33 5 75 14 7 13 9 96
0 970 907 672 872
98 8 0 0 0 0
06 0 0 185 0
0 0 0 0 0
0 0 0 0 0
06 30 93 14 3 12 8
“Catalyst weight 200 mg, reactant 0 75 mmol, reamon temperature 473 K, reaction time 17 h b4-Vmylcyclohexene ‘Others comprised 3 to 8 lands of hydrocarbons, mostly vmylcyclohexene isomers
378
The addition of carbon dioxide, water and ammoma retarded the formation of 4-vmylcyclohexene and completely mhiblted the dehydrogenatron to aromat1cs
Discussion
Dehydrocyclodunenzation of butane and butenes has been reported to proceed over metal oxide catalysts and bifunctlonal catalysts Bhatla and Phrlhps have reported that conversion of I-butene to aromatics over a crystallme molecular sieve AlPOd-11 catalyst mvolves dehydrogenation of butenes to butadrene, followed by cyclodunenzation of a Die&Alder type [ 11 The Diels-Alder type reaction was suggested to be assrsted by Lewis acid sites on the catalyst. Csmsery has reported the dehydrocyclodunenzatlon of butane to form aromatics over bifunctional catalysts, and suggested that the reaction proceeds through several consecutive and parallel steps, such as metal-catalyzed dehydrogenation, acid-catalyzed dimerrzation, aroma& zatron, lsomenzation and transal.Qlatlon [Z-6]. Conversron of butadiene to aromatics was suggested to mvolve a Diels-Alder-type thermal cychzatron to vmylcyclohexene, followed by metal-catalyzed dehydrogenatron. The types of reactron observed 111the present study are merent m several aspects from those reported for the acidic metal oxide catalysts and brfunctronal catalysts. The maJor drfferences are the reaction temperature, the product distribution and the surface properties required for the catalysts These differences are compared below. Concerning the reaction temperature, reactions over the reported catalysts were appreciable above 703 K [l-6]. In the present study, however, the reaction proceeded below 473 K The difference m product distribution between the reported catalysts and the present catalysts is distinct A wade variety of aromatics ranging from benzene to ethylbenzene, mcludmg toluene, o-, m- and p-xylenes, were produced over the reported catalysts On the other hand, the aromatics of carbon number eight were the mam products m the present study. In partmular, the absence of m-xylene m the reactron products is a characteristic feature of the catalysts m the present study. The difference m the surface properties between the reported catalysts and the present catalysts is also distinct The surface propeties of ZrOz [ 81, MgO [ll], CaO [ll], L&O3 [9, 12, 131, and NdaOs [9, 12, 131, which were found to be active m the present study, have been studred by various methods such as titration wnh benzom acid, temperature-programmed desorption of COz and catalytic test reactrons. The charactenstms common to the catalysts active for 1,3-butadiene dehydrocyclodrmenzatlon m the present study are then basic propeties [ 12, 13 1. Therefore, it is suggested that basic sites on the catalysts are mvolved m the reactron mechanisms in the present study Exceptrons were found with SrO and BaO; these catalysts showed nummal actnnty, and their basic strengths are stronger than those of MgO and CaO
379
[ 12, 141. Probably basic sites wluch are too strong do not act efficiently as catalytic sites. The results of poisonmg expenments support the idea that the basic sites are active for the dehydrocyclodunenzatlon. In general, basic sites on sohd base catalysts are poisoned by carbon dioxide and water The strong polsonmg effects of carbon dioxide and water observed with MgO and ZrOz support thrs view As carbon dioxide and water mhibited both the formation of aromatics and conversion of 4+mylcyclohexene, basic sites are considered to participate m both the activation of 1,3-butadiene, leadmg to aromatics, and the dehydrogenation of 4-vmylcyclohexene to ethylbenzene So far, although a number of reactions are known to be base-catalyzed, dehydrocyclodunerization of comugated dienes has not been recognized as a basecatalyzed reaction either m homogeneous or m heterogeneous catalytic systems The poisonmg effect of ammoma on MgO was drlferent from that on ZrOz The difference is renumscent of the polsonmg effects of ammoma on 1-butene isomerization [8, 1 l] For 1-butene isomellzation, it was reported that ammoma completely poisoned the active sites on ZrOz [8], but slightly mcreased the activity of MgO [ll]. Over ZrOz, it was reported that l-butene isomerization was base-catalyzed reaction, though a basic molecule of ammonia retarded the reaction The reaction 1s llutiated by the abstraction of a H+ by a basic site to form an allyhc amon The Zr4’ cation achacent to the basic site plays a role m stabilrzmg the allyhc amon. The polsonmg effect of ammoma IS thought to block the Zr4+ cation, so that the allyhc amon cannot be stabilized on the surface [8] In the present study with ZrOz, ammoma decreased the conversion of 1,3-butadiene to one third its ongmal value and completely poisoned the formation of ethylbenzene. With MgO, although the conversion of 1,3-butadiene decreased to half its original value, the formation of ethylbenzene mcreased. The formation of ethylbenzene and Isomenzation of l-butene respond smularly toward ammonia pre-adsorption both on ZrOp and MgO As for the reaction mechamsm, two Werent mechanisms are considered, m both of wluch basic sites are mvolved. The first 1s an amo~c mechanism m wluch the reactron is uutiated by H+ abstraction from the reactant by basic sites to form amo~c mtermedlates The second ISa Dlels-Alder reaction, followed by double bond lsomenzation and dehydrogenation. The anionic mechanism proposed is shown m Scheme 1. Amon 1 attacks the 1,3-butadiene molecule at carbon 2 to form anion 2. The amon 2 will undergo both cychzation to the amon 3 by route I and double bond transfer to the anions 4 and 6 by route II. Amens 4 and 5 are pnmary anions, whereas amon 3 IS a less stable secondary amon. Therefore, route II 1s likely to proceed faster than route I By this amo~c mechanism, the expected mam products are o- and p-xylenes. As double bond migration of oletlns proceeds rapidly over solid base catalysts [ 12,13,15 1,the substituted cyclohexadienes which are the precursors of ethylbenzene, o- and p-xylenes are considered to be composed of a mixture
380
381
of double bond isomers The alternative reaction mechamsm mvolvmg a Duels-Alder reaction 1s shown m Scheme 2 By the reaction Scheme 2, ethylbenzene 1s the final product, and the formation of o- and p-xylenes cannot be explamed Over MgO catalyst, the anionic mechamsm 1s suggested to operate because the mam products are o- and p-xylenes It 1s not defirute whether the amomc mechamsm or Duels-Alder reaction IS mvolved m the formation of ethylbenzene over MgO However, as mentloned above, the formatlon of ethylbenzene through the amomc mechamsm seems mcult In ad&tlon, It 1s hard to explam the tierent polsomng effects of ammoma on the formation of ethylbenzene and xylenes if ethylbenzene and xylenes were formed through common amoluc mtermedates. Therefore, it is more hkely that the formation of ethylbenzene on MgO proceeds through a Duels-Alder reaction, as shown m Scheme 2 Over ZrOz and CaO, the selective formatlon of ethylbenzene suggests that the reactlon proceeds through a Duels-Alder reaction Without catalyst, Duels-Alder reaction of 1,3-buta&ene to vmylcyclohexene proceeded to only a small extent. Therefore, Zr02 and CaO catalysts should promote the Duels-Alder reactlon It is well known that the general acid catalysis of the Dlels-Alder reactlon 1s attnbuted to coordmatlon of the acid vvlth the denophile, drawmg electron density away from the reaction site and thus makmg the &enopme more electroph&c [16] Dlenes vvlth electron-urlthdrawmg groups react reatiy urlth tienophiles contammg electron-releasmg groups [ 171 Base catalysts may promote Die&Alder reaction m such a way that the basic sites donate electrons to lenes and thus make the dlenes more reactive urlth &enopties Follow-mg the DIebAlder reaction, double bond lsomenzatlon and dehydrogenation occur to form ethylbenzene With SrO, BaO, L%O, and Ndz03, the conversion levels were close to those vvlthout catalyst, but the product dLstnbutlons wered These catalysts barely promoted the Duels-Alder reaction, but catalyzed dehydrogenatlon of 4-vmylcyclohexene. The basic sites are suggested to partlclpate m the Duels-Alder reaction, the abstraction of H+ from 1,3-butadlene, the lsomenzatlon of vmylcyclohexenes, and the dehydrogenatlon of substituted cyclohexa&enes However, active basic sites may not be common to all these reactlons The existence of Merent basic sites ansmg from the merent coordmatlon numbers of surface 02- ions IS reported on MgO [ 131 In the polsomng expernnents urlth ammoma, the total conversion of 1,3-butadlene decreased but the formation of ethylbenzene mcreased Tlus suggests that ammoma retards the abstraction of H+ from 1,3-buta&ene to form the ~IUONC mtermehate, but does not retard the Duels-Alder reaction and the followmg lsomenzatlon and dehydrogenatlon As ammoma IS a basic molecule, it 1s generally beheved that it poisons the acl&c sites However, it 1s reported that on MgO ammoma 1s &ssoclatlvely adsorbed on basic sites to form NH2- and H+ [18]. The formed H’ blocks basic sites, therefore, ammoma may poison basic sites.
382
383
384
Identification of the active basic sites for each reaction, however, has not been carried out m the present study yet For 2-methyl-1,3-butadiene dehydrocyclodunerrzation, the formation of m- and p-cymenes can be explamed both by a Diels-Alder reaction followed by double bond isomenzation and dehydrogenation, and by the amoruc mechanism Two reaction schemes for 2-methyl-1,3-butadlene dehydrocyclodunenzation are shown below Scheme 3 depicts the reaction mvolvmg a Diels-Alder mechanism and Scheme 4 the amomc mechanism In the amomc mechanisms, the allyhc H seems to be abstracted first because of the ease to liberate as H+. The position of the carbon atom of Z-methyl-1,3-butadlene at which the amon 1' attacks determines the final product Attack of the amon 1' at carbon atom 3 of 2-methyl-1,3-butadiene (route I) yield m-cymene via formation of amon 2’, rmg closure, addition of H+ and dehydrogenation Attack of the amon 1' at carbon atom 4 (route II) yields lunonene and p-cymene, and at carbon atom 2 (route III) yields, 2,4-dunethyl-4-ethyhdenecyclohexene Route III does not lead to dehydrocyclodunenzation for aromatics unless C-C bond cleavage occurs Since the primary atuon 2’ is more stable than the secondary anion 3’, it is considered that route I is more facile than route II Over MgO catalyst, more m-cymene was formed than p-cymene Therefore, it is suggested that over MgO catalyst the amomc mechanism mamly operates m 2-methyl-1,3butadlene dehydrocyclodimerization, as 111the case of 1,3-butadlene Over CaO catalyst, more m-cymene was produced than p-cymene, suggestmg that the amomc mechanism IS dommant. This 1s m contrast to the results with 1,3-butadiene m which the Diels-Alder reaction mechanism 1s suggested to be donunant The change 111mechanism depending on the reactant is considered to be due to the existence of allyhc H m 2-methyl1,3-buta&ene The formation of the amon is easier for 2-methyl-1,3-butadrene than for 1,3-butadiene Over ZrOa catalyst, p-cymene was predonunantly formed The reaction IS suggested to proceed through the Duels-Alder reaction, similar to the case of 1,3-butadrene Although novel aspects of sohd base catalysts are disclosed, there are many questions left unsolved at the present time, such as the activity- and selectivity-governmg factors of catalysts, and applicability of base-catalyzed dehydrocyclodunenzatlon to diolefins other than the substituted butalenes exanuned m the present work To solve these, further studies are required.
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