Alkylation of benzene and toluene with lower olefins

ALKYLATION

OF BENZENE AND LOWER OLEFINS R

Department

of Chemical

K

Technology,

TIWARI

and M

Umverslty

M

of Bombay,

TOLUENE

WITH

SHARMA Matunga

Road, Bombay-400

019, India

Abstract-The kmetlcs of alkylation of benzene and toluene with propene, butene-1 and butene-2, m the presence of sulfuric acrd as a catalyst, was studled m an 11 cm I d mechamcally amtated contactor It was possible to eliminate the mass transfer resistance completely The alkylation takes place m the acid phase and IS first order

with respect to the aromatlc compound as well as the protonated olefin The values of the second order rate constant for propene-benzene, propene-toluene, butene-l- benzene, butene-1-toluene, butene-2-benzene and butene-2-toluene were estimated to be 160, 320, 265, 540, 280 and 545 cm’lmole set, respectively, at 25°C

lNTRODUCTION

CHEMICALS AND METBOIS

The alkylatlon of aromatics with lower olefins, m the presence of sulfuric acid as a catalyst, IS of considerable mdustnal importance A fau amount of hterature IS avadable on the chemistry of these reactlons [ l] but there 1s only limited mformatlon m the literature on the kmetics of aikylatlon Recently an attempt has been made to explain the action of strong acid on the kmetlcs of alkylatlon of benzene with the higher olefins[2] In the present mvestlgatlon benzene and toluene were alkylated wth lower olefins-propene, butene-1 and butene-2, to discern the controlling mechanism The alkylatlon of benzene and toluene wdh propene @ves isopropyl benzene (cumene) and isopropyl toluene (cymene), respectively Sunllarly butene-1 and butene-2 yield the correspondmg see-butyl denvatlves which can be converted via oxidation and the subsequent decomposltlon of hydroperoxlde to two important aromatic compounds namely, phenol and cresols with methyl ethyl ketone as a valuable co-product

The alkylatlon reactions were carried out in a Jacketed 11 cm 1 d agtated reactor The capacity of the reactor was about 0 75 1 and was provided with a stamless steel six blade turbine Impeller and a stainless steel internal coohng toll The reaction mvolves two hquld phases and m all the cases the acid phase was the dispersed phase The temperature of the reactor was mamtamed within -+-025°C of the desired value by clrculatmg water m the Jacket and the coolmg cod through a thermostat The aromatic phase contammg the dissolved olefin and aqueous sulphunc acid were mtroduced separately into the reactor as separate streams and the reaction mutture was withdrawn as a single stream The speed of agtatlon was vmed from 500 to 2000 rev/mm The temperature of the reaction was varied from 5” to 25”C, and the acid concentration from 16 to 17 3 M The order of reactlon with respect to the mdlvldual reactants was determined by varying the initial composltlon of the hydrocarbon mixture with a suitable dlluent CES Vol 32 No

10-I

OF ANALYSIS

Benzene, toluene and sulfunc acid were obtamed from firms of repute Propene was supplied by Wmon Carbide (India) and butene-1 (C P grade-99% pure) was obtamed from Matheson Gas Products (U S A) Butene-2 was prepared in the laboratory by the hqmd phase dehydration of see-butanol m the presence of aqueous sulfuric acid A pure sample of p-cymene was obtained from laboratory chemicals manufacturer Propene, butene-1 and butene-2 were analysed by an F and M model 720 dual column programmed temperature gas chromatograph (U S A ) on an 11 m long sliver nitrate (4 g) plus dlethylene glycol (DEG) (16 73 g) column supported on chromosorb-W (5Og) at 25°C wth 10cm3/mm tamer gas flow rate In the case of butenes DEG-AgNO, column was used to find lsobutylene content m the gas mixture since a very good separation of I-butene from lsobutylene and truns-Zbutene ts obtamed However, this column could not separate l-butene from cls-2-butene and hence the mixture was further analysed on a 15 m long dlmethylsulfolane column at 25°C with 110 cm’/mm carrier gas flow rate m order to determine the percentage of butene-1 m the mixture Butene-2 was found to be about 95% pure (rest 5% bemg butene-1) contammg 41 6% CIS- and 58 4% trans-2-butene The products of alkylatlon were ldentfied on a PerkmElmer mode1 21 infrared spectrophotometer (U S A ) by using its thm film between sodium chloride plates Benzene and alkylated benzene, cumene and set-butyl benzene, were analysed on a 3 m long 10% Aplezon-L column supported on chromosorb-W at 150°C with 30 cm’lmm carrier gas flow rate Toluene and alkylated toluene (artho- and para-cymene and ortho- and parasee-butyl toluene) were analysed on a 3 m long 10% dlethylene glycol succmate (DEGS) column treated with 3% H,PO* and supported on chromosorb-W wth 10 cm3/mm tamer gas flow rate In all the GLC analyses hydrogen was used as the carrier gas The concentration of the olefin m the hydrocarbon phase was estimated by using the bromide-bromate method[3] The acid concentration was determmed by titration with standard sodium hydroxide solution

1253

R K TIWARI and M M

1254 THEORY AND PHYSIC&ERMlCAL

DATA

1 Chemistry of alkylatron The general behavlour of olefins m acid media 1s broadly understood m terms of the formation and the fate of carbomum Ion intermediates The most widely accepted mechanism for the alkylatlon of aromatics with olefins m the presence of an acid catalyst postulates the addition of carbomum ion formed from the olefin to a pair of P-electrons of the aromatic nucleus The acid catalyst converts olefins to the correspondmg carbomum ion by the addition of proton from the acid to the extra electron par m the double bond (the a-electron)

B+H+ A+B+

+ 3’

(1)

+ AB+H+

2 Reactron kmetrcs It IS well established that the protonatlon of an olefin under the condltlons employed m this work 1s a fast reactlon[4] Hence it may be assumed that durmg alkylatlon, the acid phase gets quickly saturated with the protonated olefin, B’ Mu-on and Lee[S] have descrtbed some of the characterlstlcs of the acid-soluble hydrocarbons (species I?‘) Thus reactlon 2 1s the rate controllmg step for alkylatlon The problem may now be defined as hydrocarbon A, going to the acid phase and reacting with the protonated olefin, B’, according to reaction 2 If the rate of reaction between dissolved A and B’ IS very much slower than the rate of transfer of A to the acid phase containing B’, then the acid phase will be saturated with the solute A at any moment and the overall process will be controlled by a homogeneous reactlon between dissolved Table 1 Solubdlty

Propene

Propene Propene Butene-1 Butene- 1 Butene-1 Butene-1 Butene-2 Butene-2 Butene-2 Butene-2 Butene 2 Butene-2 Butene-2 Isobutene Isobutene Isobutene Isobutene

A and B+ The rate of reaction, then be given by

of alkenes

Solvent bqmd Benzene Toluene Toluene Benzene Toluene p-Cymene 84% Toluene + 16% pCymene Benzene Benzene Benzene Toluene Toluene m-Xylene p-Xylene Benzene Toluene m-Xylene p-Xylene

R’ m mole/cm3 set, wdl

R’ = jk,,JA*]‘” [B+]”

(3)

where 1 1s fractional liquid hold-up of the aqueous acid phase, L,, 1s m +nth order reaction rate constant, (cm3/m01e)“+“-’ (set)-‘, (mth order with respect to A and nth order with respect to B’), [A*] IS eqmhbrlum concentration of A m acid (or solublllty of benzenejtoluene m acid), mole/cm3 and [B’] 1s concentration of species. B’, mole/cm’ The condltlon to be satisfied for the validity of the dbove mechanism can be exmessed as

R’ kLg[A*] *’

(2)

where A denotes aromatic hydrocarbon species, B 1s olefin, B’ IS protonated olefin, H’ IS proton and AB IS the product of reaction The carbonmm Ion attacks those nuclear posItions m the ring where electron density 1s relatively high

Solute gas

SHARMA

where kL IS true liquid side mass transfer coefficient m the aqueous phase, cm/set If the process 1s governed by the above mechamsm, then the speed of agitation m agtated contactors wdl not affect the reactlon rate above a certam mmlmum speed which is required to ehmmate mass transfer resistance completely and keep the material reasonably well agltated

3 Solubdlty (a) Solubrlrty of alkenes m aromatics

It was thought desirable to experlmentally determme the solublllty of alkenes m aromatlcs These data are expected to be useful for the alkylatlon studies For this purpose a modified design of an old apparatus was used[6] This provided a simple, quick and accurate method for the measurements of solublhty of relatively more soluble gases The solublhty values are given m Table 1 and compared with Ideal solublhty values The saturated solutions of accurately known olefin concentrations were analysed on GLC for cahbratlon This provided a check on the bromide-bromate method for analysis m aromatlc Temp (“C)

hydrocarbons

Solubdlty Experrmental

32 29 32 28 28 28 5

0 56 0 63 0 51 2 63 261 1 86

29 29 5 30 5 31 28 5 29 29 29 28 5 28 5 2.5 5 25 5

24 33 3 1s 3 12 3 24 3 19 2 78 2 78 2 68 2 22 2 475 2 475

(mole/l atm) Ideal 0 845 0 78 1 45 1 24

165 (3O’C) 141 (30°C) 1 24 1 24

Alkylahon

of benzene

(b) Solubrhty of benzene and toluene m sulfunc acrd The solublllty of benzene and toluene m concentrated sulfuric acid has been experlmentally determined [7] The experlmental data show that the solublllty of hydrocarbon m the acid phase increases exponentially wth the acid concentration These experimental data can be correlated wlthm 1% accuracy by the following equation

[A*]= exp (a, + a,X + a2X2 + a3X’)

(5)

where a,, a,, a, and a3 are constants and X IS acid concentration, weight per cent The experlmental data were correlated with the help of CDC 3600 d@al computer The value of constants a,,, a,, a2 and a, are reported m Table 2

sulfuric acid IS present m excess and acts as the reactlon medmm[9] Thus it may be Inferred that the alkylatlon occurs mamly when the acid 1s the medmm of the reaction The products of reactlon were characterlsed by their IR spectra In the case of butylated products no absorption band was observed at 1260, 1230, 1200, 930 and 920 cm-’ frequencies These are characterlstlc bands of tertiary butyl group This observation 1s mdlcative of the fact that only set-butyl denvatlves were formed durmg butylatlon In the case of alkylated toluene, absorption bands at 815 and 756cm-’ suggest the formation of para- and orfho-denvatlves, respectively As explained earlier the carbomum ion 1s responsible

Table 2 SolubWy constants of benzene Benzene-sulfunc Temp (“Cl 5 25 45

a0

01

-3 01285 -80 619175 5 395964

0 82391 3 411233 0 307616

a,x

a,x

102

104

1 130178 2 15806 0 665944

-1 8593 -4 695653 -0 96172

AND DISCUSSiON

has been suggested that the concentrated sulfuric acid not only mltlates the alkylatlon reaction by protonatmg the olefin but it also provides a medium for the lomc reaction that follows[8] Hence some preliminary experiments on alkylatlon were performed with lower concentrations of sulfuric acid The mam reactlon m the range of X0-84% acid concentration was polymerlzatlon while above 85% acid concentration alkylatlon was predominant (Fig 1) The transItion between 84 and 85% acid can be explamed on the basis of equimolar concentration of sulfuric acid-water mixture which corresponds to 84 5% acid Below this concentration water forms the medium for reaction whereas above 84 5% concentration

I 14 I ACID

Rg

1L ! 5 CONCENTRATION,

m acid (eqn 5) Toluene-sulfuric

it

1: 5

and toluene

acid

It 1s likely that the solubdlty of benzene and toluene m the acid phase changes to some extent with an increase m the concentration of the acid soluble hydrocarbons (species B’) REWLTS

1255

with lower olefins

and toluene

15 I

I

15I 5

t6

M

1 Relative rates of alkylatlon and polymerlsation different acid concentrations Butene-Benzene-H,SO,

with

Temp (“C) 5 25 45

acid

a0

n,

0*x 102

-15 43522 -90 8307 -155 3235

1 04365 3 641944 6 013143

-193173 -4 864483 -7 742272

a,x

104

109729 2 187929 3 347475

for the alkylation reactlon It attacks the nuclear posltlons m the rmg where the electron density IS relatively high The presence of a substltuent on the rmg can alter this electron avadabdlty thereby &ectmg the dlstnbutlon of isomers Durmg the present mvestlgatlon It was found that toluene gives ortho- as well as paru-denvatlves It 1s known that the alkylatlon of toluene with set-butyl alcohol m the presence of sulfuric acid IS ortho-para dlrectmg[lO] Aromatic compounds like benzene, toluene and their alkylated derlvatlves are readdy sulfonated by concentrated sulfunc acid The rate of sulfonatlon mcreases urlth mcreasmg methylatlon of the benzene rmg and with an Increase m the number of fused rings Thus the rate of sulfonahon of toluene 1s about five times faster than that of benzene under otherwise similar condltlons [ 1 l] However, the rate of sulfonatlon of toluene at 10°C 1s about five times lower than the rate at 25°C [12] A decrease m acid concentration also reduces sulfonation rate considerably Hence, the alkylatlon of toluene was carried out at lower temperatures and lower acid concentration The dlstrlbutlon of olefins between aromatlc and sulfuric acid depends on the amount and strength of the acid and temperature Since the protonatlon step 1s a fast reaction, the aqueous phase was assumed to be saturated with the protonated olefin This protonated olefin was taken as [B’] The amount of [B’] was calculated from the difference m the nntlal and the final concentration of the olefin m the organic phase and was further corrected for the amount whtch had already reacted This assumptlon IS valid because there was no ohgomerrsatlon of the olefin Moreover, care was taken to see that there was no stnppmg of olefin from the reaction mixture dunng the experiment Under otherwise slmllar condltlons the rate of reactlon

R K

1256

TIWAIU and M

was found to be independent of agitator speed m the range of 700-2OOOrev/mm (Fig 2) This mdlcates that mass transfer effects were completely eliminated and the reaction 1s controlled by the slow chemical reaction between the two species In order to determine the reaction zone, the hold-up of acid phase, 1, was varred from 0 075 to 0 3 For the same concentration of [B’] m the acid phase, the rate of reaction increases linearly with the acid hold-up (Fig 3)

I 500

0

I 1000 AGITATOR

I

SPEED,

I

I

1500

2000

2500

RPM

Fig 2 Effect of astator speed on the rate of alkylatlon (concentratlon of acid, 16 8M, temp 25”C, 1 =0 1, [B’l = 7 5X lo-* mole/cm’) A, Propene-benzene-H,S04, 0, Butene-l-benzeneH,SO,, 0, Butene-Z-benzene-H,SO,, 0, Propene-tolueneH,SO,, V, Butene-1-toluene-H2S0.,, 0, Butene-2-toluene-H,S0,

25

20 x ul -x u 2 Fi I 0 -0 L -LL

15

10

05

0 0

0 05

01

0 15

02

0 25

03

035

I Fig 3 Alkylation of aromatlc hydrocarbons Effect of acid phase hold-up (concentration of acid, 16 8 M, temp 7 5°C [WI= 8 5 x lO+ mole cm’) A, Propene-benzene-H&O., 0, Butene-lbenzeneH,SO,, 0, Buterie-2-benzene-H2S04, 0, PrOPeneV, Butene-l-toluene-HZS04, 0 Butene-2toluene-H,SO,, toluene-H,SO,

M

SHARMA

This indicates that the alkylation reactlon occurs in the acid phase Also, since the solublllty of acid m the orgamc phase IS neghable, the reaction would be expected to take place m the acid phase[2] The contactor used m our work had an 1 d of 11 cm and was provided with a 4cm dla turbine Impeller Sankholkar and Sharma [ 131 have made measurements of a m a 10 6 cm id contactor and have shown unamblguously that with 72% (w/w) H2S04 dispersed in dlIsobutylene-monochlorobenzene system the value of a varies linearly with the speed of agtatlon m the range of 60&15OOrevlmm, even with a dispersed phase hold-up of 0 3 This system IS very close to that employed m our work Since the speed of agrtatlon m the range of 7m 2000 rev/mm has no effect on the rate of alkylatlon it IS clear that effective rnterfacml area has no effect on the rate of alkylatlon On the basis of the data of Sankholkar and Sharma[l31 and Laddha and Sharma[l4] and other published mformatlon It would be reasonable to assume that the value of k,g would be m the range of 0 5 set-’ With this value of kLc_zIt can be shown that the condltlon gven by expression (4) 1s satisfied In case the reaction conformed to the slow reaction regime then R’ should be mdependent of the concentration of [B’] and should be a strong function of the speed of aatation Our data show that this 1s clearly not the case Thus there 1s an overwhelmmg evidence to suggest that m our work dlffuslonal resistance was completely ehmmated Komasawa et al [2] have also reported that m the case of the alkylatlon of benzene with higher olefins, e g hexene to decene, in the presence of concentrated suiphurlc acid as a catalyst, dtiuslonal resistance could be eliminated m a mechanically agtated contactor The order of reaction with respect to the reactants was determmed by changmg the concentration of the reactants m the organic phase An increase m the olefin concentration m the hydrocarbon phase results m an increase m [B’] m the acid phase Slmla.riy, benzene concentration m the organic phase was varied by using a sultable dluent, preferably one of the reaction products Thus dunng propylatlon of benzene, set-butyl benzene was used as the dluent, and during butylation of benzene, cumene was used as the dlluent This arrangement made the analysis on GLC easier, particularly when the amount of dlluent IS slgmficantiy higher than that of reaction products Durmg the alkylatlon of toluene, cumene was used as the dlluent The reaction was found to be first order with respect to the protonated olefin (Fig 4) The lmear dependence of reaction rate on aromatic concentration, [A*], shows that the reaction 1s first order with respect to aromatic concentration The alkylatlon reactions were carried out at various temperatures rangng from 5 to 25°C to get the values of actlvatlon energies The actlvatlon energy was found to be 13 3 kcallmole m these reactions The rate constants for the olefin-toluene system are pseudo rate constants as both ortho as well as para alkyltoluene are formed during the alkylatlon of toluene The ortho-para isomer ratlo was found to be mdependent of time of reactlon

1257

Alkylatlon of benzene and toluene with lower olefins

16 2

16 L

16 6

CONCENTRATION

16 8 OF

17

172

HISOL,M

Ag 5 Alkylatlon of aromatIc hydrocarbon Effect of acid concentration A(rate), A(solubWy), Propene-benzene-H*SO,,

Fig 4 Alkylatlon of aromatic hydrocarbons Effect of the concn of protonated olefin [B’l(concentratlon of acid, 16 6 M, temp 25°C) A, Propene-benzene-H,SO+ 0, Butene-l-benzene0, Butene-2-benzene-H,SO+ 0, Propene-tolueneH,SO,, H,SO., V, Butene-1-toluene-H,SO,, 0, Butene-2-toluene&SO, However, It was slightly dependent on temperature m the range of S-25°C In this temperature range the ortho-para rat10 varies from 43 57 to 47 53 In the work reported here the acid phase was the dispersed phase and It was assumed that the backmixed model holds rather than a combmed model of forward mixing and backmrxmg Komasawa et al [2] have found that even after the phase mverslon the rate of alkylation per umt volume of the acid phase remained constant This shows that even If the acid phase 1s the dispersed phase It 1s essentially backmlxed Mlsek and Rod[lS] suggest that m such cases some forward mlxmg does occur However, the error associated m usmg the backmixed model 1s not likely to be more than 5% It was experimentally observed that the rate of alkylatlon mcreases exponentially with the acid concentratlon This can be explained by considermg the fact that the solubllrty of benzene and toluene increases almost exponentially with the acid concentration (eqn 5) and the rate of reaction, R’, IS proportional to solubdlty of benzene or toluene (Fig 5) Thus, apparently, the mtrmslc rate of the alkylatlon step 1s not substantially affected with the change m acid concentration However, It may be stressed that the effect of the concentration of sulphurlc acid on the rate of alkylatlon has been considered m a sunphfied way In the actual case the formation of the acid-soluble hydrocarbons (species B’) will result m lowering the concentration of free sulphurlc acid which m turn will affect the rate of alkylatlon In our case the maximum concentration of [B+] was around 1 6 M compared to the sulphurlc acid concentration of about 16m The alkylatlon rates of butene-1 and butene-2 are almost equal and the slmllarlty of the product dlstrl-

V(rate), ~(solubihty), Propene-toluene-H,SOI

butlon for the two butenes suggests that a common lomc species alkylates the aromatic hydrocarbon A smular observation was made by Komasawa et al 121m the case of alkylatlon of benzene with the Isomers of hexene, heptene, octene, etc This 1s probably due to the fact that a-olefins are lsomerlsed m the presence of sulfuric acid to gwe a common protonated olefin which further participates m the alkylatlon[16] In the absence of a tertbutyl derivative m the product, set-butyl cation 1s likely to be present as the intermediate ion In all the experiments the level of conversion of the aromatic was restricted to wlthm 35% During the course of this mvestlgatlon neither ohgomers of the olefins nor poly alkyl derlvatlves were obtamed This mdlcates that the rates of these reactions are relatively slow It has been reported that the followmg rate expression 1s vahd for the alkylatlon of benzene with higher

olefins [2] R’ = k exp (- 145 X 1a’lRT) exp W&o;)

where

CA .XS b

6)

h IS a constant expressing the ability of protonation, R IS gas constant, T 1s absolute temperature m OK, C&so, 1s concentration of HZS04 m mole/cm3, C,, 1s concentration of benzene m hydrocarbon phase and C,, IS concentration of olefin m hydrocarbon phase It was assumed that the equihbrmm distrlbutlon coefficient for the hydrocarbons m the two phases, namely Kn = C,.IC, b and KB = CBalCBb are independent of (I) the concentration of acid and (u) temperature (where the subscrrpt II refers to the acid phase) However our work showr that KA and KS are a function of temperature as well as acid concentration It 1s likely that the term m eqn (6) probably accounts for the varyexp (hGso,) atlon m K with the concentration of sulfunc acid The assumption pertammg to K bemg independent of temperature would probably lead to a higher value of activation energy as K increases with an increase m temperature

R K

1258

TIWAR~and M

It 1s quite likely that m systems mvolvmg lsobutylene and related species the effective mterfaclaI area plays a dominant role[l7,18] For instance Shah and Sharma[l7] have shown that m the case of alkylatlon of toluene with lsobutylene m the presence of 17 6-17 8 M sulphurlc acid the alkylatlon conforms to the mechanism where transfer of toluene IS accompanied by fast reaction m the film Therefore, for this case the rate of alkylatlon m a mechanically aatated contactor will be directly affected by the effective mterfaclal area and hence the speed of agitation CONCLUSIONS

In an aetated contactor It was possible to eliminate the dlffuslonal resistance The alkylatton reaction was found to be first order with respect to the protonated olefin as well as the aromatic substance The rate of alkylatron with butene was found to be significantly faster than with propene under otherwise uniform condltlons The rate of alkylatlon of benzene 1s lower than that of toluene Acknowledgement-One of us (RKT) wishes to thank the Umversrty Grants Commlsslon for an award of ScholarshIp which enabled this work to be carried out REFERENCFS

[l] Olah G 1973

A,

Fnedel

Crafts

Chemrstry

Whey,

New

York

M

SHARMA

[2] Komasawa 1, Inoue T and Otake T , J Chem Engng Japan 1972 5(l) 34 [3] Shapiro S and Gurvtch Ya , Analytical Chemrstry, En&h Edn Mu Publishers, Moscow 1972 [4] Sankholkar D S and Sharma M M , Chem Engng Scl 1973 28 49 [5] Moron S and Lee R J , J Chem Engng Data 1%3 8 150 [63 Morrrson T J and Bdlett F , J Chem Sot 1948 2033 [7] Cerfontam H and Telder A , Recuerl 1%5 84 545 [8] Lder M , Reaction Mechanism m Sulfunc Acrd and Other Strong Actd Solutions Academic Press, London 1971 [9] Franks F , Physrco-Chemrcal Processes m Mixed Aqueous Soloents Hememann Educational Books Ltd , London 1967 [lo] Yashlma T , Terlyakl Y and Hara N , Bull Jap Petrol Inst 1971 13(2) 215 [ll] Stubbs F J , Wlllmms C D and Hmshelwood C N , J Chem Sot 1948 1065 [12] Kdpatnk M and Meyer M W , J Phys Chem l%l 65 530 [13] Sankholkar D S and Sharma M M , Chem Engng Scl 1973 28, 2089 [14] Laddha S S and Sharma M M , Chem Engng Scr 1976 31 843 [15] Mlsek T and Rod V , Recent Advances m Ldquld-Liquid Extractron (EdIted by Hanson C ) Pergamon Press, Oxford 1975 [16] Chpplnger E , Ind Engng Chem Prod Res Dev 1964 3 3 [17] Shah A K and Sharma M M , Ind Chem Engr 1969 11 T-122 [18] Albright L F , Chem Engng 1966 73(14) 119