Indian Journal of Chemistry Vol. 4 1 A. February 2002, pp. 259-265

Para-selective ethylation of t-butylbenzene with diethyl carbonate over mesoporous molecular sieves

V Umamaheswari , M Palanichamy, Banumathi Arabindoo & V Murugesan* Department of Chemistry, Anna University . Chennai 600 025, India

Received 16 April 2001; revised 8 Oclober 2001

The vapour phase ethylation of I-buty lbenzene (I-BB) wi th diethyl carbonate has been studied over mesoporous AI-MCM-41 (SilAI = 50 and 90) and AI , Mg-MCM-41 [Si /(A I+Mg) = 50] from 200 to 400°C. The products are benzene. ethylbenzene (EB), p-diethylbenzene (P-DEB), p-I-butylethylbenzene (p-IBEB) and p-I-butylvinylbenzene (P-1BVB) . I-Butylbenzene conversion increases with increase in temperature up to 300°C, but above thi s temperature it decreases. Orllio and //lela-products are completely absent. Ethylbenzene is the product of ethylation of benzene and cracking of p­IBEB .The selectivity of para-I-butylated products is fa voured at lower temperature while higher temperature favours ethylbenzene selecti vity . The inOuence of time on stream is examined over AI-MCM-41 (50) and the results are di sc ussed in thi s paper.

Alkylated aromatics are the products of high commercial significance and most of the products such as ethyl benzene, isopropylbenzene, p­dii sopropylbenzene, C 10-C I4 linear alky lbenzenes, isopropyl toluenes and p-I-buty ltoluenes are important chemical intermediates obtained by acid catalysed alkylation of appropriate aromatics I. In many petrochemical plants these alkylations are still performed with mineral acids such as HF, AICI) - HCI etc. , which have drawbacks like safety, handling, cOITosion and waste disposal.A variety of zeolites have been examined and few have been employed for industrial alkylation of aromatics since 1960. Venuto el al. reported that X and Y zeolit~s are more effective catalysts for benzene alkylation with olefins than amorphous si lica-alumina gels2

.) . Alkylation of substituted aromatics has received considerable attention in the last two decades because of the possibility of selective production of either para or orlllO substituted deri vatives. However, problems are encountered in such regioselecti ve alkylation of substituted aromatics when zeolites and zeotypes are used as catalysts. In the ethylation of toluene with ethanol over zeotype molecular sieves, l11-ethyltolu ~ne was also obtained although p-ethyltoluene was principally aimed in this reaction4

. Microporous nature and large acidity of the Bronsted acid sites are shown to be the reasons for the

Correspond ing author:E-mai l : v_murugu @hotmail.com. Fax: 9 1-44-2200660

decrease in the selectivity of desired products in many reactions. The discovery of mesoporous molecular sieves in the recent past by Mobil Researchers has aroused much interest in these materials for catalytic applications5

.6

. Compared to microporous zeolites, these mesoporous molecular sieves are found to be more advantageous for regioselective alkylation of aromatics as they have straight pores, very well-scattered mild Bronsted acid sites and large pore diameter. These features of the materials aid the reactants to feel an environment similar to that in the homogeneous liquid phase. The reactants can freely diffuse through the pores and the aromatic substrates, for example, can easily bring their kinetically controlled carbon sites for electrophilic attack by a counterion remaining as charge compensating cation in the pores. This ha been envisaged in the I-butylation of Ill-cresol over mesoporous AI-MCM-41 molecular sieves7

. [t is wi th these background in the present investigation, ethylation of I-B8 in the vapour phase over Al-MCM-41 (SO), Al­MCM41 (90), AI, Mg-MCM-4l (SO) mesoporous molecular sieves has been studied. The influence of temperature, WHSV, time on stream on I-butyl benzene conversion and product selectivity has been examined and the results are discussed.

Materia[s and Methods

SYlllhesis Hydrotherma[ crystallization procedure was

adopted for the preparation of A[-MCM-4 J molecular

260 INDIAN J CHEM, SEC A, FEBRUARY 2002

sieves with SilAI ratios 50 and 90, and AI, Mg­MCM-41 with Si/(AI+Mg) = 50. In a typical synthesis, 10.6 g of sodium silicate nanohydrate (Merck) in water was combined with appropriate amount of aluminium sulphate and magnesium sulphate (Merck) in solution . It was then acidified with 1M H2S04 to bring down the pH at 10.5 under vigorous stirring. After 30 min of stirring, an aqueous solution of cetyltrimethylammonium bromide (CT AB) (Merck) was added and the surfactant­silicate mixture was stirred for further 30 min at room temperature. The molar composition of the resultant mi xture was Si02: 0 .2 CT AB : x AI 20 3 : y MgO : 0.89 H2S0 4: 120 H20 (y = 0 for AI-MCM-41, x varies with the Si/AI ratio ).The resultant gel was autoclaved and

heated for 12 h at 170°C.The solid obtained was filtered and dried at 80°C in air. The sample was then

calcined at 550°C in air for 5 h in a muffle furnace to expel the template.

Characterization X-ray powder diffraction data for the materials

were collected on a Scintag XDS 2000 diffractometer eq uipped with CuKa radiation source and a liquid nitrogen cooled germanium solid state detector. The samples were scanned from 1 to 30° (28) in steps of 0.02° with a count time of i5 s at each point. The BET surface area measurements were done using the Sorptomatic 1990 instrument. The samples were outgassed for 16 h under vacuum at 350°C prior to use.

Alkylation Ethylation of t-butylbenzene with diethyl carbonate

was carried out in a fixed bed continuous down flow glass reactor of 40 cm length and an internal diameter of I cm. About 0 .5 g of the catalyst was taken in the reactor and the reaction mixture was fed into it by a motor driven sy ringe infusion pump at predetermined flow rates. The liquid product collected for the first 15 min was discarded and only the product collected after this time for I h was analysed. The analysis of the products was done in a Hewlett-Packard gas chromatograph 5890A equipped with a flame ionisati on detector (FlO) using OY 17 column and nitrogen as the carrier gas. After completion of each run , the reactor sys tem was flushed with nitrogen to remove the adsorbed molecules and regenerated at 550°C in a cun'ent of carbon dioxide free air for 6 h.

Results and Discussion X-ray powder diffraction patterns of all the three

as-synthesized and calcined materials are shown in Figs la and 1 b respectively which exhibit a very strong peak at about 2° and weak peaks between 3 and 4° (28). These peaks are indexed for a hexagonal meso phase. These spectral features resemble those reported by Chen el al. 8 The infrared spectra of the as­synthesized and calcined samples are shown in Figs 2a and 2b. The broad envelope in both the figures around 3500 cm·1 is due to O-H stretch ing of water, surface hydroxyl groups and bridged hydroxyl groups. In the spectra of the as-synthesized sam ples there are less intense peaks just below 3000 cm' \ which are assigned to symmetric and asymmetric stretching modes of the -CH2 group of the locked in template. The corresponding bending mode is observed at 1400 cm-I

• The peaks between 500 and 1200 em·1 are assigned to framework vibrations. These spectral

2

2CJeO

u CII .!(!I~

.sa c ::> 8 1040

~ C ::i

u

2900

:}: 2320 .... .sa c § 1740

[10)

2 4

29

t1b)

(e)

(b)

(0)

2 3 4

29 Fig. l-XRD pallerns of( 1 a) as-synthesized and ( I b) calcined materials AI-MCM-41 (50), (b) AI-MCM-41 (90) . (c) AI. Mg­MCM-41 (50)

UMAMAHESWARI el al.: PARA-SELECTIVE ETHYLATION of I-BUTYLBENZENE 26 1

features also resemble those reported by Chen el aZ. The BET surface area of the three calcined AI-MCM-4 1 (50), AI-MCM-41 (90) and AI, Mg-MCM-41 (50) samples measured using nitrogen physisorption methods are 895, 856 and 805 m2/g. The surface area obtained for all the three calcined materials are comparable to those prev iously reported for these type of materials5

.

Ethylation of t-butylbenzene (t-BB) with diethyl carbonate was studied over AI-MCM-41 (50), AI-

MCM-41 (90) and AI, Mg-MCM-41 (50) at 200,250, 300, 350 and 400°C. The products obtained were benzene, EB, p-DEB, p-tBEB and p-tBVB. Table I presents the data for the t-BB conversion and product selecti vity overAI-MCM-41 (50) for the feed ratio I : I (t-BB:diethyl carbonate) and WHSV 2.74 h-I. Conversion increases from 200 to 300°C, but above this temperature it decreases due to blocking of active sites by polybutene and/or coke9

. I-BB undergoes electrophilic substitution with ethy l cation remaining

4000 3000 2000 1000 sao 4000 3000 2000 1000 500

'Voverumber (cm-1 W:lvenumbef (em-l)

Fig. 2 - FTIR spectra of (2a) as-synthesized and (2b) calcined materials (a) AI-MCM-4 1 (50), (b) AI-MCM-4 1 (90) . (cl AI. Mg- MCM-41 (50)

Table I - Catalytic activity of AI-MCM -4 1 (50) fo r ethylation of I-BB Mole rati o of I-BB:Diethyl carbonate = I : I

WHSV Temperature I-BB conversion Selec ti vity of products (0/0 ) Wi ) CC) (%) Benzene EB p-DEB p-IBEB p-IBVB

2.74 200 16.8 16.6 40.5 42 .9

250 33.4 10.0 27.3 32.7 30.0

300 48.9 12.4 30.0 16.7 20.0 20.9

350 35.3 16. 1 52.9 13.0 18.0

400 28 9.3 52.5 13.9 24.2

4.57 200 22.2 13.0 43.2 43 .7

250 35.5 28.5 3 1.2 40.3

300 46.3 33.5 17.3 4. 1 45. 1

350 30 59.0 4 1.0

400 27.7 69 .7 30.3

262 INDIAN J CHEM, SEC A, FEBRUARY 2002

as charge compensating ion on the catalyst surface to yield p-IBEB which inturn undergoes dehydrogenation over alumina to yield p-IBVB '0. Alumina was abtained by framework leaching of aluminium during calcination of the as-synthesized AI-MCM-4 l in air at 550°C to expel the template. This type of origin of alumina has already been proved in the literatures. The decrease in selectivity of p-/BEB with increase in temperature is not due to its decrease in formation but due to its conversion to p­IB VB through dehydrogenation , and to ethyl benzene th rough cracking. The formation of p-IBVB is nota di sadvantage in thi s process as the ultimate aim of this study is to produce thi s compound through dehydrogenation of p-/BEB . The selectivity of ethyl benzene increases with increase in temperature as cracking of p-tBEB is generally more favoured at hi gh temperatures. But the selectivity of benzene decreases with increase in temperature. The decrease in selectivity at high temperature may be attributed to decrease in the cracki ng of I-BB due to blocking of active si tes by coke and partial conversion of benzene to ethylbenzene. The formation of ethy lbenzene from benzene has been separately verifi ed in thi s study by passing benzene and diethyl carbonate over the catalyst. p-/BEB can be cracked better than {-BB as the electron release from ethyl group to aromatic ri ng would build more electron densi ty thus, aidi ng

Tab le 2- lnnuence of mole ratio onl-BB conversion and produc selectivity over AI-MCM-4 1 (50) at 30QoC

Mole I-BB Selecti vity of products (%)

ratio conversion Benzene EB p-DEB p-IBEB (%)

1:2 60 26.7 23 .8 49.5

1:3 53.9 15.4 27.4 22 .4 34.7

1:5 53.7 14.5 16.9 23.1 45 .4

cracking of t-butyl group through protonati on.ln addition t-BB has to go in search of Brbnsted ac id sites for cracking to yield benzene whereas for p­IBEB it is not necessary . l-BB reacts wi th ethyl cation to yield p-tBEB which in turn can undergo cracking on the same Brbnsted acid sites to yield ethyl benzene.

In order to reduce cracking of I-BB without reducing the selectivity of ethylbenzene, p-/BEB and p-/BVB, the reaction was carried out with the increased WHS V (4.57 hOi). The results are presented in Table I. Benzene is completely absent whereas the selectivity of other products is almost retained with little changes thus supporting cracking of p-/BEB by the above route.

There is maximum conversion of I-BB at 300°C and hence the influence of feed ratio was studied with WHSV 2.74 hoi. The results are presented in Table 2. As expected /-BB cracking to benzene is minimum as the selectivity of benzene for all the feed ratios is less than 20%. The higher reactant feed ratio does not show any advantage over the lower feed ratio as indicated by the data.

In the present study, orlho-t-butylethylbenzene is not observed which might be due to mere steric hindrance. The absence of 11I-I-buty lethylbenzene indicates negligence of adsorption and deactivation of aromati c rings of l-BB on the Brbnsted ac id sites of the catalyst. This could be ascribed to the poor electron release from (-butyl group to the ring in I-BB compared to that of ethyl group in ethyl benzen . In addition. adsorption of t-B B through alky l aromatic C-C bond on Brbnsted acid si tes is ex pected to be more important than with aromatic ring. The absence of electronic confinement effect" as observed in zeolites and zeotype molecular sieves and the exi stence of freedom in the molecular sieves for the substrate to orient and bring kinetically controlled carbon site of the aromatic

Table 3-Catalytic activi ty of AI-MCM-4 1 (90) for ethylation of I-BB

Mol e rati o WHS Y (h·l )

1:1 2.74

1:2 1.39

Temperature (0C)

200

250

300

350

400

200

250

300

350

400

I-BB conversion (%) Benzene

33.7 65 .9

40.3 34.2

45.8 17.0

42.3 55. 1

45 .3 17.2

31.7

3 1.0

56.0 20.5

35.8

35.3

Selectivity of products (%)

EB p- DEB p-IBEB p-IBYB

6.8 13.4 13.9

16.6 20.1 29.0

24.0 15.1 16.8 27.1

37.1 7.8

28.9 53.8

58.0 20.2 2 1.8

43 .5 24.8 31.6

26.1 9.3 11.3 32.8

64.8 35 .2

34.2 65.7

264 INDIAN J CHEM, SEC A, FEBRUARY 2002

Table 5-Catalytic activity of AI, Mg-MCM-41 (50) for ethylation of t-BB . Mole ratio M t-BB:Diethyl carbonate = 2: I

WHSY Temperature t-BB conversion Selectivity of Products (%) (h-I ) (ac) (%) Benzene EB p-DEB p-tBEB p-fBYB

2.69 200 14

250 14.1

300 29.9

350 20.1

400 27.5

4.48 200 11.9 26.9

250 23.1 11.7

300 36.2 27.6

350 39.4 45.4

400 12.3

observed as shown by the data in the Table 5. When the WHSV is increased to 4.48 h-I, benzene is observed . As the feed has the tendency to diffuse fast through the pores, I-BB could also get adsorbed on the Brbnsted acid sites and undergo cracking thus producing benzene.

The influence of time on stream on I-BB conversion and product selectivity over AI-MCM-41 (50) was examined at a feed ratio of 1:2 and WHSV 1.5 h-I at 300°C. The plot of time versus t-BB conversion and products selectivity is shown in Fig_ 3. Conversion of I-BB decreases with increase in temperature due to blocking of active sites by polybutenes and/or coke. As benzene is not completely observed throughout the stream, cracking of I-BB in the feed ratio 1:2 to yield benzene by adsorption on Brbnsted acid sites might not be a favourable process. Since the catalyst could adsorb diethyl carbonate better than I-BB , cracking of t-BB could become an impossible process. On the other hand when I-BB in the vapour phase reacts with ethyl cation adsorbed on the surface, the product p-tBEB can have more chances to undergo cracking on the same acid site to yield ethylbenzene. This newly formed ethylbenzene again possibly react with another ethyl cation during its diffusion through the pores to yield p-DEB. The selective formation of p­DEB with stream is an interesting aspect. Though p­IBEB is observed as the product with maximum selectivity, its dehydrogenated product p-tBVB is not observed indicating that at this WHSV (1.39 h-I) the former is not adsorbed on alumina as diethyl carbonate could be better competing. After sixth hour of stream both ethylbenzene and p-DEB are absent in the product. As most of the active sites would have

49.3 50.7

18.4 36.2 45 .4

36.8 23.0 14.0 26.2

66. 1 33.8

61.8 38.2

35.3 37.8

18.2 29.4 40.7

18.8 16.0 14. 1 23-"

36.0 18.5

76.4 8. 1 15. I

been blocked by coke at this stage, there would be more free diethyl carbonate which cou ld better dilute p-/BEB and prevent its cracking. Hence selectivity to p-/BEB increases with increase in stream.

Conclusion Ethylation of t-butylbenzene over AI-MCM-41

could be a convenient route when one aims for p-I­butylethyl or vinylbenzene. However, there is an inconvenience as a result of cracking of the parent [­BB. The use of higher feed ratio (t-BB:diethyl carbonate) is not advisable as it favours largely cracking. Increase in the diethyl carbonate content in the feed significantly reduces cracking.There is difficulty in controlling andlor completely eliminating cracking of p-tBEB, as the reactions, namely, ethylation of t-BB and its subsequent cracking could occur on the same active sites. AI-MCM-41 could be more suitable even for regioselecti ve alkylation than microporous zeolites and zeotypes as they are providing same environment for the substrate as in homogeneous liquid phase thus permitting the substrate to put the kinetically active sites into reaction with any attacking electrophiles. The study of time on stream indicated exclusive formation of p+ butylethylbenzene.

Acknowledgement The authors gratefully acknowledge the financial

support from the Department of Science and Technology, Government of India, New Delhi for this major project (Project No. SP/S I/H-23/96). The authors are thankful to Prof. A. Kalanidhi, Vice­Chancellor, Anna University, Chennai, for hi s constant encouragement and providing all the facilities to carry out the work.

UM AMAHESWARI et al.: PARA-SELECTIVE ETHYLATION of t-BUTYLBENZENE 265

References I Frank H G & Stadelhofer J W, IlIdlistrial aromatic

chemistry, (Spri nger, Berlin), 1988. 2 O' Kelley A A, Kellett J & Plucker J. Illd ElIgng Chem, 39

( 1947) 154. 3 Venuto P B, Hamilton L A, Landi s P S & Wise J J, J Caral, 5

( 1966) 8 1. 4 Kannan C, Elangovan S P, Palanichamy M & Murugesan V,

Illdiall J Chem Tech, 5 (1998) 65. 5 Beck J S, Vartuli C, Roth W J, Leonowicz M E, Kresge C T,

Schmidt K D, Chu C T-W, Sheppard E W, McCullen S B, Hi ggines J B & Schlenker J L, J Am chem Soc, 11 4 ( 1992) 10834.

6 Avelino Corma, Chem Rev, 97 ( 1997) 2373. 7 Umamaheswari V, Palanichamy M, Banumathi Arabi ndoo &

Murugesan V, Indian J Chem, Sec A, 39A (2000) 1241. 8 Cong-Yan Chen, Hong-Xin Li & Mark E Davis.

Microporous Mater, 2 (1993) 17. 9 Zhongmin Liu, Patrice Moreau & Francois Fajula. Appl

Caral A Gen, 159 (1997) 305 . 10 Herman Pines, The chemislly of catalytic hydrocarbon

cOll versiolls, (1981) 208. II Derouane E G, J Catal, 100 (1986) 54 1. 12 Gui snet M & Magnoux P, KIlI IVer academic Pub, ( 1992)

457.