Indian Journal of Chemistry Vol. 39A, September 2000, pp.92 1-927

Ethylation of toluene over alurninophosphate based

molecular sieves in the vapour phase

K K Cheralathan, C Kannan. M Palanichamy & V Murugesan** Depanment of Chemistry, Anna University,

Chennai 600 025, India t email: v_murugu@hotmail.com

Received 24 .lanttwy 2000; revised 22 July 2000

Ethylation of toluene wi th ethanol has been examined over MnAP0-46, NAP0-46 and ZAP0-46 catalysts at 300, 350, 400 and 450 oc. The products obtained are benzene, styrene, m-ethyltoluene and diethyl ether. Toluene conversion is maximum at 350 oc. Formati on of diethyl ether is high at 350 °C, but it decreases with increase in temperature due to prevention of ethanol adsorption on active sites. The yield of m-ethyl toluene which is the largely expected product in this investigation increases with increase in the acidity of the catalyst. Among the three catalysts stud ied, toluene convers ion and yield of m-ethyl toluene are more over highly ac idic MnAP0-46 catalyst.

Ethylat ion of toluene with ethanol is an industria lly important reaction, as the subsequent dehydrogenation of ethylto lune gives the corresponding methylstyrene, the monomer fo r the production of polymethylstyrene. The polymer, polymethylstyrene has more commerc ial importance than polystyrene, as it possesses hi gh flash point and glass transition temperature and low specific gravity. Conventional mineral acid catalysts fo r toluene ethylation cause envi ronmental concern and corros ion problems. Zeolites, H-ZSM-5 and modified H-ZSM-5

1-4 catalysts , have been examined for this reacti on. AIPO-n molecular s ieves are material s of recent researc h interest in the area of catalysis and they are thermally more stable than H-ZSM-5 . Eventhough AIPO-n molecular sieves have high thermal stability, they possess less Bronsted ac idity. But the Bronsted acid ity of these molecu lar sieves can be increased by isomorphous substitution of AI or P with lower valent

2+ 2+ elements. With thi s view, Ni and Zn substituted AIP0-5 and -II based molecul ar sieves ha ve been

5 synthesised and examined for ethylat ion of toluene . But these catalysts were shown to exhibit less convers ion and hence in the present investigati on more acidic catalysts li ke MnAP0-46, NAP0-46 and ZAP0-46 have been examined for this reaction . The products formed are styrene, benzene, m-ethyltoluene and diethyl ether. The e ffect of temperature, reactants ratio, time on stream and acidity on the conversion and products distribution are discussed in the following sections.

Materials and Methods

Synthesis

MnAP0-46 was synthes ised by hydrotherma l crystallisation ustng dipropylamine (DPA) as the structure-directing template. MnAP0-46 was crystallised hydrothermally from a gel compos iti on of 0.2 MnO: 4.0 DPA: AI 0 : P 0 : 55H 0 . In the

2 3 2 5 2

synthes is aluminium isopropoxide (27 g) was soaked in 30 ml distilled water for 24 h. Manganese acetate (2.48 g), di ssolved in I 5 g of orthophosphoric acid and diluted with 55 ml water, was added to it dropwise and continued the stirring for 6 h to have uniform composition. DPA (26 g) was added dropwi se and the stirring was continued for another 4 h. Then the pH of the ge l was adjusted to 7 .7 by adding orthophosphoric acid. This gel was crystallised in an autoc lave at I 50 oc for 4 days followed by heating at 180 °C for 5 days. The product was washed repeatedly with di stilled water until the pH of the filtrate was neutra l. It was then dried at II 0 oc for 12 h and then calcined at 550 oc in a flow of air for 8h to remove the template. NAP0-46 and ZAP0-46 molecular sieves were synthesised in the same method from the ge l composit ion of 0 .2 NiO: 4.0 DPA: Al

20

3: PPs : 55 H20 and 0.2 ZnO: 4.0 DPA:

Al20

3: Pp

5 : 55 Hp respectively by using nickel

acetate and zinc ox ide as source materi als for nickel and . 6

ZtnC.

922 INDIAN J CHEM. SEC A, SEPTEMBER 2000

~I ?:' ·;; c .:! £

6

(a)

.I .IIi I (b)

I. .I I. I ' . I \II · • ,I

(c )

(d )

10 14 18 22 26 30 34

29

Fig. 1- X-ray diffraction patterns of (a) AIP0-46 (b) MnAP0-46 (c)NAP0-46 and (d) ZAP0-46

Characterisation X-ray diffraction analysis was carried out employing

a Siemens DSOO diffractometer in the scan range of 28 between 5 and 50 using CuKa radiation . The peaks were identified with reference to compilation of simulated XRD powder patterns . Unit cell parameters were calculated using the standard leas t squares refinement technique. IR spectra of the sample were recorded on a Bmker IFS 66v FT-IR spectrophotometer using KBr pe llets. TG analysis was carried out with Met_~ler TA 3000 system at a scanning rate of 20 oc min in a stream of dry air. Inductive ly coupled plasma (ICP) ARL 34 I 0 with mini torch was used to determine the chemical composition of the samples. BET surface area was measured in a Micromeritics Pulse Chemisorb 2700 using nitrogen as adsorbate at 77K. Initially the Calcined samples were degasified at 200 oc for 2 h in a flow of oxygen . ESR spectra were recorded on a varian E I 2 spectrophotometer at room temperature. Acidity of the samples was measured by temperature programmed desorption (TPD) of pyridine using TGA (Mettler T A

<I)

'E :J

.0 .... 0

Q) (.,)

c

~ .E <I)

c 0 ....

......

4000 3000 2000 1500 1000 500

Wavenumber ( em-• )

Fig.2- FT-IR spectra of (a) AIP0-46 (b) MnAP0-46 (c)NAP0-46 and (d)ZAP0-46

Table !-Unit cell parameters and unit cell volume or the AIP0-46 based molecu lar sieves

Catalyst Unit ce ll parameter A Unit ce ll

volume A

a b c

AIP0-46 13.40 13.40 13.40 484~

MnAP0-46 13.6 1 13.61 13.66 501Sl

NAP0-46 13.55 13.55 13.55 4997

ZAP0-46 13.40 13.40 13.40 4884

.l

CHERALATHAN eta/. : ETHYLATION OF TOLUENE OVER MOLECULAR SIEVES 923

(a)

~ c :J

..ci .... 0

>o -"iii c ~ £

2750 3 150 3550 3950 Magnetic field (gauss)

9 = _2 . 0066 (b)

2750 3150 3550 3950

Magneticf ield (gauss )

Fig.2- FT-IR spectra of (a) AIP0-46 (b) MnAP0-46 (c)NAP0-46 and (d)ZAP0-46

3000). P9or to adsorption, the samples were evacuated to a I 0 · torr at 450 oc. Catalyst was a ll owed to

equilibrate with pyridine at room temperature in a closed vessel. TG analys is was _crarried out up to 500 oc at a scanning rate of I 0 oc mi n . From the weight loss at various temper~rure ranges, acidity and acidic strength in mmol g were determined . The complete data obtained in the above characterisation are compared and di scussed.

Catalytic studies

The reactor system was a fix ed-bed, verti ca l fl ow type reactor made up of quartz tube of 40cm length and

Table 2 -Framework vibrations and 0-H vibrations of the AIP0-46 based molecular sieves

Catalyst Wave number(cm-1)

Framework vibrations 0 -H vibrati om

AIP0-46 11 25 ,750,600,532 & 485 3422

MnAP0-46 1122,737,620,559 & 485 3506

NAP0-46 11 65,7 13,624,547 & 472 3477

ZAP0-46 1124,7 17,623,543 & 475 3464

2cm internal diameter. The quartz reactor was heated to the requisite temperature with the help of a tubular furnace contro lled by a digital temperature contro ller cum indicator. At the bottom of the reactor a thermowell was placed in which a chromel-alumel thermocouple was kept to measure the temperature at the middle of the catalyst bed with an accuracy of ±5 oc. About 2 g of the catalyst was placed in the midd le of the reactor and supported on either side with a thin layer of quartz wool and ceramic beads. Reactants were fed into the reactor from the top by a syringe infusion pump that can be operated at different flow rates. The bottom of the reactor was connected to a co iled condenser and a receiver in which the products were co llected. The liquid products were analysed using Hewlett Packard gas chromatograph 5890A with bentone column , N as carrier gas and flame ionisation

2 detector.

Results and Discussion

Characterisation X-ray diffraction patterns of calc ined MnAP0-46,

NAP0-46 and ZAP0-46 molecular sieves are presented

in Fig. I . The 28 values of the patterns have been found to be the same as reported in the literature. The unit cell parameters a, b, c and unit cell vo lumes are presented in Table I . The increased unit cell volume of substituted molecular sieves is a convincing evidence for the isomorphous substitution of metal ions in the AIPO

framework.

924 INDIAN J CHEM . SEC A, SEPTEMBER 2000

Table 3-Effect of temperature on toluene conversion and products yield - I

[WHSY =5h ; reactams ratio =1:2)

Catalyst Temp.( °C) Conversion Products yield (wt%) (wt%)

Benzene Styrene m-ethyltoluene Diethyl ether

MnAP0-46 300 25 25.0

350 55 2.2 3.0 19.3 30.5

400 46 6. 1 10.3 17.4 19.2

450 38 6.0 10.6 12.5 4.0

NAP0-46 300 10 10.0

350 26 1.5 2.1 10.0 12.4

400 16 1.4 3.0 8.6 3.8

450 9 1.0 3.0 5.0

ZAP0-46 300 20 20.0

350 47 3.1 5.4 18.3 20.0

400 41 7.0 8.3 10.4 15 .3

450 36 10.0 8.5 6.5 I I .0

Table 4 -Et"tect of feed ratio on toluene convers ion and products yield - I

[Temp. =350 oc; WHSV = 5 h )

Catalyst Mole ratio Con version( wt%) Products yield(wt%)

Benzene Styrene 111-ethyltoluene Diethyl ether

MnAP0-46 1:1 30 10.6 10.4

1:2 55 8.2 19.3 30.5

I :3 53 2.0 3.0 10.0 38.0

NAP0-46 1:1 13.5 3.5 1.8 8. 2

1:2 26 1.6 2.1 10.0 12.0

I :3 23 1.8 5.2 16.0

ZAP0-46 1: 1 22 6.0 1.5 12.5 10.0

1:2 47 3.0 5.4 18.3 20.2

1:3 41 2.5 1.0 10.5 27.0

CHERALATHAN et al.: ETHYLATION OF TOLUENE OVER MOLECULAR SIEVES 925

The Ff-IR spectra of the catalysts are present_<yd in Fig. 2. The bands between I 000 and 1200 em are assigned to the asymmetric stretching of T0

4 tetrahedra

and these are characteristics of all zeotype molecular sieves 7. The absorption in the region 500-650 cm-1 is assigned to vibrations in the double ring region . Other bands in the spectra at 700 and 470cm-1 are assigned to symmetric stretching of T04 tetrahedra and T-0 bends respectively. The 0-H vibration in the substituted aluminophosphate molecular sieves is shifted to higher wave number in comparison to the corresponding unsubstituted molecular sieve8 (Table 2). Hence in the substituted molecular sieves 0 -H bond is strong compared to the same in the substituted molecular sieves. This may be due to the substitution of more

electronegative M2+ions in the tetrahedral framework instead of AI sites .

The ESR spectra of MnAP0-46 and NAP0-46 are shown in Fig. 3. The Mn2+ substituted AIP0-46 molecular sieve shows a broad ESR signal with g value of 2.0007. The broad ESR signal without hypetf ine splitting indicates that the Mn2+ ions are strongly interacting with their environment in the tetrahedral framework. The g value of Ni2+ in NAP0-46 is 2.0066. This value agrees with values reported for NAP0-5 and NAPO-li molecular sieves9.

Effect of temperature The effect of temperature on ethylation of toluene

with ethanol over MnAP0-46, NAP0-46 and ZAP0-46

Table 5-EtTect of time on stream on toluene conversion and products yield

[Temp.= 350 "C; reactants ratio= I :2; WHSV=5h- l]

Catalyst Time on Conversion(wl Products yield(wt%) st ream( h) %)

Benzene Styrene m-ethyl toluene Diethyl ether

MnAP0-46 55 2.2 3.0 19.3 30.5

2 32 2.0 2.8 10.0 17.2

3 25 2.5 2.0 5.5 15.0

4 15 2.3 1.5 2.0 6.2

5 8 6.0

NAP0-46 26 1.5 2.1 10.0 12.4

2 20 2.0 3. 1 8.2 7.0

3 18 3.0 3.2 5.2 7.0

4 15 1.5 3.0 4.5 6.0

5 6 6.0

ZAP0-46 47 3.1 5.4 18.3 20.2

2 42 2.5 5.6 12.0 21.0

3 32 2.0 4.3 10.2 12.0

4 20 1.0 7.3 12.0

5 7 7.0

926 INDIAN J CHEM. SEC A, SEPTEMBER 2000

Table 6- Molar composition, BET surface area and total acidity of aluminophosphate based molecular sieves

Catalyst Molar composition BET surface area Total acidi ty

MnAP0-46

NAP0-46

ZAP0-46

0.32 MnO: 0.84AI203:P205

O.IONiO: 0 .95AI203:P205

0.26 ZnO: 0.87AI203: P205

molecular sieves has been examined between 300 and 450 oc. The results obtained are presented in Table 3. The conversion is maximum at 350 °C. This may be due to the predominance of desorption of the reactants from the active sites above 350 °C. Ether is the only product formed at 300 C, but at 350 oc both ether and m­ethy ltoluene are formed. Hence at 350 oc conversion of ether into ethyl cations catalysed by Bronsted acid sites is expected for the alkylation reaction. But at 400 and 450 oc formation of both ether and m-ethyltoluene decreases indicating the flow of ethanol molecule without much adsorption on the active si tes of the catalysts. The formation of benzene increases with increase in temperature is presumed to be the cracking of toluene.

Effect of feed ratio The effect of toluene to ethanol ratio on the

conversion and products yield in the ethylation of toluene is examined at 350 oc over Mn2+. Ni2+ and

zn2+ substituted AIP0-46 catalysts by varying the ratio ,

viz., I: I, I :2 and I :3 with WHSV = 2.5h-l. The results are presented in Table 4. Conversion increases with increase in feed ratio from I: I to I :2, but at I :3 ratio a slight decrease in conversion is observed. Though the yield of ether increases with increase in the feed ratio, the yield of ethyl toluene that is high at I :2 becomes less at I :3. This is because in the feed rat io I: I the number of ethyl carbocations will be less compared to the feed ratio I :2. The formation of these carbocations increases with increase in the feed ratio and hence favours the

(m2g-l) (mmol g- 1)

275 0.890

263 0.558

271 0.820

alkylation. But further increase in the feed ratio to I :3, the yield of m-ethyltoluene decreases because ethanolic

OH coordination is more than the toluene n-electron coordination on the active si tes of the catalysts. Effect of time on stream

The time on stream is studied for ethylation of toluene over Mn2+, Ni2+ and zn2+ substituted AIP0-46 catalysts at 350 oc. The toluene to ethanol mole ratio is maintained at 1:2 and WHSV = 2.5h-1• The results are illustrated in Table 5. The data indicate similar trend for conversion and products yield over all the catalysts. Formation of ether requires existence of both free ethanol and pre-ad orbed e[hanol (or ethyl carbocation) in close proximity. It is certainly more favoured with increase in the alcohol content of the feed. But at the feed ratio I :3, the yie ld of ethyl toluene decreases as toluene is diluted due to high alcohol content. In addition, adsorption of toluene on the Bronsted acid sites in presence of large amount of alcohol is presumed to be less . In the ea rly hours of time on stream, the yield of ether and m-ethyltoluene is large but at the end of 5th hour the yield of m-ethyltoluene and diethyl ether is almost nil. It is due to the blocking of act ive sites by coke deposition at the later stages of time on stream.

Influence of acidity

Acidity, molar composition and surface area of MnAP0-46, NAP0-46 and ZAP0-46 are presented in Table 6. Among the metal substituted a lumino phosphate molecular sieves, Mn2+ substituted catalysts

CHERALATHAN eta!.: ETHYLATION OF TOLUENE OVER MOLECULAR STEVES 927

possess higher acidity than the others. In aluminophosphate molecular sieves generally acidity depends on the extend of metal substitution in the framework. As Mn2+ substitution is more than the other metal ions in the framework, the acidity is also more in MnAP0-46 compared to other -46 molecular sieves. This factor may be attributed to the higher conversion of toluene and higher yield of m­ethyltoluene in MnAP0-46 molecular sieve.

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3. Paparatto G, Moretti E, Leofanti G & Gatti F, J Cared, 205(1987)227.

4. Warren W, Kaeding L, Young & Chin-Chium, J Cared, 89( I 984 )267.

5. Kannan C, Elangovan S P, Palanichamy M & Murugesan V, Indian J chem Tech, 5(1998)65.

6. Kannan C, Aluminophosphate-based molecular sieves: sy11thesis, characterisation and catalytic applications, Ph.D thesis. Anna University, 1999.

7. Zecchina A & Otero Arean C, Chem Soc Rev, ( 1996) 187.

8. Lohse V, Bruckner A, Kintscher K and Pariitz B, J chem Soc Faraday Trans 91(7)(1998)1173.

9. Elangovan S P, Krishnasamy V & Murugesan V, Bull chem Soc Japan, 68(1995) 3659.