Applied Catalysis A: General 194 –195 (2000) 359–363

Selective synthesis ofpara-ethylphenol over pore size tailored zeolite

Jagannath Das, Anand B. Halgeri∗Research Centre, Indian Petrochemicals Corporation Limited, Baroda 391 346, Gujarat, India

Received 31 March 1999; received in revised form 21 July 1999; accepted 26 July 1999

Abstract

Selective ethylation of phenol with ethanol topara-ethylphenol, over pore size tailored ZSM-5 zeolite, has been studied. Finecontrol of pore openings of ZSM-5 was achieved by chemical vapour deposition of silica employing tetra-ethyl orthosilicate.Selectivity forpara-ethylphenol increased with the extent of silica deposition and under a comparable phenol conversion level,pore size controlled ZSM-5 exhibited about 88% selectivity forpara-ethylphenol, as compared to only 37% with unmodifiedZSM-5. The influences of various reaction parameters, e.g., temperature, WHSV and phenol to ethanol mole ratio were alsostudied. Addition of water to reactants improved the selectivity for ethylation, as well as selectivity forpara-ethylphenol, andalso stability of the catalyst. A comprehensive scheme for selective phenol ethylation topara-ethylphenol has been proposed.©2000 Elsevier Science B.V. All rights reserved.

Keywords:Phenol; Ethylation;para-ethylphenol; ZSM-5; Pore size tailoring

1. Introduction

Para-ethylphenol has important commercial use asintermediates for the production of synthetic resinsand antioxidants. The tedious, multi-step conventionalprocess for makingpara-ethylphenol comprises of(i) sulphonation of ethylbenzene, (ii) separation ofpara-ethylbenzene sulphonic acid from the ethylben-zene sulphonic acid mixture thus formed, and (iii)alkali fusion of para-ethylbenzene sulphonic acid.Although the process is being practiced in indus-try, however, it is associated with disadvantages likehandling of dangerous sulphuric acid and sodium hy-droxide at high temperature, corrosion of equipment,disposal of waste water containing sulphuric acidand/or alkali, and also the by production ofortho-

∗ Corresponding author. Fax:+91-265-372098.E-mail address:ipcl@giasbm01.vsnl.net.in (A.B. Halgeri).

andmeta-ethylbenzene sulphonic acids. On the otherhand, direct alkylation of phenol with an alkylat-ing agent, over zeolite catalyst, c.a. ZSM-5, for thepreparation of a mixture of alkylphenol isomers isbeneficial and environment friendly as compared toconventional method in respect of safe and minimumoperational steps, avoiding corrosion and waste dis-posal problem. However, in such cases, the productmixture contains thermodynamic equilibrium compo-sition of para-, meta- and ortho-alkylphenol. More-over, the boiling points ofpara- and meta-isomersare so close, that it is virtually impractical to separatepara-alkylphenol from the mixture by distillation.

One way to circumvent the above problems is to de-sign a zeolite catalyst, which will produce selectivelypara-ethylphenol during ethylation of phenol withethanol. Fine control of pore size opening by silana-tion [1–3] offer a means for enhancing concentrationof para-disubstituted aromatics in the product mixture

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360 J. Das, A.B. Halgeri / Applied Catalysis A: General 194 –195 (2000) 359–363

of alkylation of mono-alkylbenzene, or disproportion-ation of mono-alkylbenzene or in aromatization oflight hydrocarbons [4–10]. In the present paper, selec-tive ethylation of phenol over pore size tailored ZSM-5zeolite to obtain preferentiallypara-ethylphenol, hasbeen studied. The performance of such pore size tai-lored zeolite has been compared with those of theunmodified zeolite. The studies has been extended toeffect of composition of the unmodified zeolite, extentof pore size regulation, and also to the influence ofthe reaction parameters as well as addition of water tothe reactants, on the performance of the tailor madezeolite. A comprehensive scheme for the formationof para-ethylphenol though ethylation of phenol overthe pore size tailored zeolite has been proposed.

2. Experimental

Zeolite ZSM-5, with different silica to alumina ra-tio were prepared following the standard procedure[11] and were characterized by the XRD (for phasepurity), SEM (for crystal size and morphology), wetchemical analysis (for bulk chemical composition ofthe zeolite). The as-synthesized zeolite were trans-formed to proton form by calcination in air, (540◦C,8 h), repeated ammonium ion exchange using ammo-nium nitrate solution, and calcination in air (540◦C,8 h). Acidity measurements of the proton zeolite sam-ples were carried out by temperature programmeddesorption of ammonia.

Pore size tailoring of the zeolite ZSM-5 was car-ried out by in situ chemical vapour deposition of silicaemploying tetraethyl orthosilicate. The detailed pro-cedure is described elsewhere [6].

Vapour phase reaction of phenol and ethanol werecarried out in an all-glass, fixed bed, continuousdown-flow reactor, feeding a premixed reactants withthe help of a feed pump. Reaction products wereanalyzed by a Hewlett Packard Gas Chromatographusing HP Innowax capillary column and flame ion-ization detector. Results are reported here on alcoholfree basis.

3. Results and discussion

The results of phenol ethylation over H-ZSM-5samples having different silica to alumina ratio, are

Table 1Effect of silica to alumina ratio of ZSM-5 catalytic performancefor phenol ethylationa

SiO2/Al2O3 ratio 75 300 500

Lighters & other aromatics 3.53 1.24 0.67Phenol 64.45 72.61 80.26

o-Ethyl phenol 8.61 6.16 4.7p-Ethyl phenol 5.00 5.09 4.52m-Ethyl phenol 13.08 9.37 6.48

Heaviers 5.63 5.43 3.37Performance

Phenol conv. (wt.%) 35.55 27.39 19.74Total EtPhOH yield (wt.%) 26.69 20.62 15.7Total EtPhOH selectivity (wt.%) 75.1 75.3 79.5Distribution of EtPhOH isomers

para- 18.7 24.7 28.8meta- 49.0 45.4 41.3ortho- 32.3 29.9 29.9

a Temp.= 400◦C, WHSV= 12 h−1, PhOH : EtOH (mole)=2 : 1.

shown in Table 1. It can be seen that with increase insilica to alumina ratio, the conversion of phenol de-creases. This is expected in line with the decreasingacidity of the samples with increase in silica to alu-mina ratio (Table 2). It is quite interesting to observethat selectivity for ethylphenols remains almost same,in spite of large variation of the acidity of the samples.However the selectivity forpara-ethylphenol (amongthe ethylphenol isomers) was found to increase, whichmay be attributed to the increase in crystal size of thezeolite samples with increase in silica to alumna ratio.

The results of ethylation of phenol over fresh as wellas pore size tailored ZSM-5 are shown in Table 3, atcomparable yield of ethylphenols. The selectivity forpara-ethylphenol (p-EtPhOH) is only 37% with fresh(unmodified) ZSM-5, while with pore size controlledZSM-5 the selectivity for the said isomer reached to88%.

The selectivity enhancement forp-EtPhOH with re-spect to extent of silanation is depicted in Fig. 1. Withincrease in silanation from 0 to 7.5%, the selectivity

Table 2Acidity and crystal size data of the H-ZSM-5 samples

SiO2/Al2O3 ratio Acidity (mmol NH3/g) Crystal size (m)

75 0.40 0.1–1290 0.15 1–3500 0.010 3–6

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Table 3Comparison of fresh and pore size tailored ZSM-5 for phenolethylationa

Parent Pore sizeZSM-5b tailored ZSM-5

Lighters & other aromatics 2.74 0.64Phenol 92.44 94.60

o-Ethyl phenol 1.76 0.15p-Ethyl phenol 1.43 3.90m-Ethyl phenol 0.66 0.37

Heaviers 0.97 0.34Performance

Phenol conv. (wt.%) 7.56 5.3para-EtPhOH selectivity (%) 37.1 88.2

a SiO2/Al2O3 ratio= 75, Temp.= 400◦C, WHSV= 12 h−1,PhOH : EtOH (mole)= 2 : 1.

b WHSV= 400 h−1.

of para-ethylphenol in the ethylphenol product mix-ture increased steadily from about 18% in case of theparent zeolite, to about 88% with the silanated zeo-lite. Such an enormous increase in the concentrationof thepara-isomer in the product can be explained inthe following way. The size of the tetraethyl orthosil-icate (TEOS) molecule is larger than that of the poreopening of ZSM-5 zeolite. The chemical vapour de-position of this molecule results in finely narrowingdown of the pore openings with the silica depositedon the external surface. The internal structure remainsunaffected and only the pore entrance is narrowed.The deposition of silica also covers the acid sites on

Fig. 1. Para-selectivity enhancement of H-ZSM-5 with sila-nation during ethylation of phenol. SiO2/Al2O3 ratio= 75,Temp.= 400◦C, WHSV= 12 h−1, PhOH : EtOH (mole)= 2 : 1.

Fig. 2. Schematic presentation of selective formation ofpara-ethylphenol through ethylation of phenol over pore size tai-lored ZSM-5 zeolite.

Fig. 3. Effect of temperature on pore size tailored ZSM-5 cat-alyst performance for phenol ethylation. SiO2/Al2O3 ratio= 75,WHSV= 12 h−1, PhOH : EtOH (mole)= 1 : 1.

Fig. 4. Effect of WHSV on catalytic performance of pore sizetailored ZSM-5 zeolite. SiO2/Al2O3 ratio= 75, Temp.= 400◦C,PhOH : EtOH (mole)= 1 : 1.

362 J. Das, A.B. Halgeri / Applied Catalysis A: General 194 –195 (2000) 359–363

Fig. 5. Influence of reactant mole ratio on catalyst performanceof pore size tailored ZSM-5 zeolite. SiO2/Al2O3 ratio= 75,Temp.= 400◦C, WHSV= 12 h−1.

the external surface of the crystallites, leading to re-duction in non-shape-selective reactions. According toHibino et al. [3], silica deposits on the external sur-face of ZSM-5 zeolite as a thin layer. The depositedsilica layer consists of siloxane bonds where as zeoliteis composed of siloxane and Si–O–Al bonds. This re-sults in the variation in bond length or angles betweensilica layer and zeolite framework. The siloxane bondof the silica layer protrudes into the pore and due tothis, pore opening is reduced.

Fig. 6. Time of stream behaviour of pore size tailored ZSM-5 zeolite for phenol ethylation. SiO2/Al2O3 ratio= 75, Temp.= 400◦C,WHSV= 6 h−1, PhOH : EtOH (mole)= 1 : 1 (I) without water, (II) with water.

The plausible scheme for the selective forma-tion of para-ethylphenol through ethylation of phe-nol over pore size tailored ZSM-5 zeolite catalystis shown in Fig. 2. It is proposed that withinthe channels of ZSM-5, ethylation of phenol takeplace preferably at thepara- position, givingpara-ethylphenol as primary product, which equi-librates to the thermodynamic composition withinthe channels. However,para-ethylphenol, being thesmallest, comes out faster while the bulkiermeta-and ortho-isomer cannot. This results in a depletionof para-isomer within the channels, and consequentre-equilibration of ethylphenol isomers. Ultimatelythis results in high product selectivity of thepara-ethylphenol.

3.1. Effect of reaction temperature

Results of experiments for studying the influenceof temperature on the catalyst performance is shownin Fig. 3. With increase in temperature from 375 to425◦C the conversion of phenol as well as yield ofEtPhOH enhanced from 5 to 9% and 3.6 to 6.7%, re-spectively; but the selectivity forp-EtPhOH remainedalmost same in the range 85.6–87.2%.

J. Das, A.B. Halgeri / Applied Catalysis A: General 194 –195 (2000) 359–363 363

3.2. Influence of WHSV

The effect of feed rate in terms of WHSV (weighthourly space velocity) on the catalyst performance isstudied in the range from 3–20 h−1 and is shown inFig. 4. It can be seen that with change in WHSVfrom 3 to 20 h−1, the conversion of phenol decreasesfrom 13.9 to 7.03%; consequently the yield of to-tal ethylphenol was nearly halved (from 10 to 5.1%),while the selectivity forpara-ethylphenol increasesfrom 80 to 89%.

3.3. Effect of mole ratio

The influence of phenol to ethanol mole ratio onthe yield of ethylphenols andpara-ethylphenol se-lectivity is presented in Fig. 5. With increase in theratio of phenol to ethanol mole ratio from 0.5 to 2,the PhOH conversion increased from 5.3 to 11.7%with concomitant enhancement of EtPhOH yieldfrom 4.4 to 8.3, without affecting the selectivity forp-EtPhOH.

3.4. Effect of water in feed stream

Keeping in view of the fact that commercial alcoholalways contains certain amount of water, the effect ofwater in the feed mixture (phenol and ethanol) wasstudied. The results of two sets of experiments viz.(I) without addition of water and (II) with addition ofwater in the feed stream are presented in Fig. 6. Inthe second set of experiment, 20 wt.% water (basedon phenol) was added. It is interesting to note that theselectivity for thepara-ethylphenol in experiment setII was higher than that in case of Case I. Also the phe-nol conversion and yield of ethylphenols are better inCase II as compared to Case I. It appears that the pres-ence of water arrests some side reactions of ethanolto other aromatics, thereby increasing the selectivitytowards ethylation of phenol, which results in betteryield of EtPhOH. It is also worth mentioning thataddition of water to reactants enhances the selectivityfor para-ethylphenol. Apart from these, the stability

of the catalyst is also improved. A similar observationwas obtained in case of ethylation of ethylbenzenewith ethanol [9].

4. Conclusions

Ethylation of phenol has been studied over pore sizecontrolled ZSM-5 zeolite. A very high concentrationof para-isomer was observed in the ethylphenol prod-uct mixture, as compared to near thermodynamic com-position value in case of the parent zeolite. The ob-served selectivity ofpara-ethylphenol in the productis dependent on the extent of silanation and is a clearcase of product selectivity. The effects of reaction con-ditions (temperature, WHSV, feed composition) havealso been studied. Increase in reaction temperature, ordecrease of WHSV or increase in ethanol content inthe feed mixture, enhanced the yield of ethylphenolsmaintaining the high selectivitypara-ethylphenol inthe product. Addition of water to reactants providesbeneficial effects on the performance of the catalystin terms of total ethylphenol yield,para-ethylphenolselectivity and stability. A comprehensive scheme forthe selective formation ofpara-ethylphenol has beenproposed.

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