Indian JOUfn,d of Chemical TechnologyVol 7, July 2U()(). pp, 155-1 DO

Effect of sulphate treatment in the alkylation of phenol with methanol over mixedoxides of tin with lanthanum and samarium

T M Iyothi", S Sugunun", K Sreekumarb M B. Talawar' & B S Rao'""Catalysis Division, National Chemical Laboratory. Punc 411 OOS, India

"Department of Applied Chemistry. Cochin University of Science and Technology. Kochi 682 022. India

'High Energy Materials Research Laboratory. Pune 411 021. India

Rcccivcd-! October 1999; accepted 27 April :CO()()

Pathways of phenol alkylation is investigated in detail over mixed oxides of tin with lanthanum and samarium and theeffect of sulphate modification on the product selectivity is highlighted, It is found that unmodified catalysts are selective inthe preparation of (}-cresol and 2.6-xylcnol whereas sulphate modification leads to the formation of all possible productswhich is attributed to the nature of interaction of phenol with the catalyst surface,

The alkylation of phenol is a very important reactionindustrially and alkyl phenols are widely used in avariety of applications such as antioxidants,herbicides, insecticides or polymers':". Numerousstudies have been devoted to the alkylation of phenolwith methanol for the synthesis of cresols andxylenols'-') Depending on the catalysts, reactionconditions, and alkylating agents prevailing, ortho orpara monoalkylated, 2, 4 or 2, 6-dialkylated and 2, 4.6-trialkylated products are formed, The selectivity ofproducts depends on the acid-base properties of thecatalysts, Tanabe et a/.lo reported that basic catalystssuch as l'vIgO selectively alkylate at the orthoposition. It has been reported that catalysts withstrong acidsites favour O-alkylation while weak acidsites or strong basic sites favour C-alkvlation II,

In recent vears, metal promoted sulphated oxidesare receiving increasing interest clue to their higherthermal stability and ~nhanced cutalvtic activi~vI2.Most of the cat:tlysts reported in liter~;ture are ba~eclon sulphated zirconia and little attention has beenpaid to catalysts. which can be prepared from othermetal oxides' (, like SnO,. Fe,O,. I-IrO" TiO, andFe20" ,\Ii these oxide~ exl1ibit str~)ng c~cidicbehaviour upon proper treatment with sulphuric acid,Rare earth oxides are always advisable as promotersSlllce the v arc eusilv dispersible and non-reducible.".Recentl\,' lh"'-';l-l-t~l'''''' propcrt ie s "f ",,_1 't '1Ill-l Sn-~. ,.,,- ll'-.-, .lL)'-..- l)lU -'•...'~I'-~~ u. ,..J,.""-----''- '-~., ,.•

Sm mixed oxides with the selectivity of the products

:;p---~Or corrl2:,;p!.lndC:llll~

in the alkylation of phenol with methanol have beencorrelated". In the present paper, a detailed accountof phenol alkylation employing Sn-La and Sn-Smmixed oxides and a sulphate doped analogue ascatalysts has been reported, giving special emphasison the reaction pathways and acid-base properties ofthe catalysts,

Experimental ProcedureThe Sn-Sm and Sn-La binary mixed oxides were

prepared by co-precipitation method from therequired quantities of stannic chloride solution andrare earth chloride solution using I: I aqueousammonia as the precipitant followed by aging,washing, filtration and drying. The compositions ofthe mixed oxides prepared are.

TSS2 ---SnO,tSO';{) Sm-O. 121l';;) TLS2--- SnO, (SlY;) La20, 12W,)TS)S~-- Sn02 (SOCk) S ui-O 1. (5()!~; ) TLS5 --- SI102 (S(V~'~') Lt2(J.; (5(YX)

TS28 ---Sn(h 120'k) Sm,(), ISO';) TL2S--- 5nO, 120'k) La20, ISWi)

The sulphate-modified oxide (STS) was preparedas-IO g of TS82 catalyst dried at 383 K wasimmersed in 0.3 M sulphuric acid solution (150 ml.)for 3 h and finally evaporated to dryness verycarefully on a water bath. The material was dried at383 K for 6 h to get sulphate modified TS82 catalyst.The catalysts were sieved to a mesh size < 100 urn andcalcined in air at 773 K before each experiment. Thechemical composition was determined by Energ,dispersive X-ray analysis t Stcrcoscan 44-0 Cambridg.:UK), The different oxide phases were detected by X-ray diffraction using a Ni filtered Cu-K" radiation(/,.=15404 A), Ornni sorb 100 CX (supplied

156 I0iDIAN 1. CHE:-'1. TECHNOL JULY :WOO

Table I-A comparison of the physico-chernic.il charactcrisncs (If SnO:-Sm:O, and its sulphated analogue

Catalyst Surface area Pore volume SO,' Acid strength(lT12 g"l) (cnl'~ g-l) ('X)

TS82" 102.7 CUI

srssz' 190.8 027 4.8 pKaS: -13.75

"SnO,-Sm,O,. "Sulphated TS82. From EDX analysis.

Table 2-Average particle size (nrn) calculated from X-raydiffraction peak widths

Catalysts

SnO,TS82STS82

Average particle size t nrn)

9.702.881.98

COULTER corporation, USA) unit was used for themeasurement of nitrogen adsorption to determinesurface areas. The SEM (scanning electronmicroscopy) photographs were also recorded to studysurface morphology of oxides. The acid strength ofsulphated oxide was measured qualitatively after thepretreatment (823 K in air) using a set of Hammettindicators.

The reaction was carried out in a vertical type flowreactor of 2.2 cm I.D. and 30 cm length, kept in acylindrical furnace mounted vertically. The catalyst(3 g as pellets) was loaded in the middle of thereactor and packed with glass beads. Before eachexperiment the catalyst was activated in a current ofdry air at 773 K for 6 h and then brought to thereaction temperature in presence of nitrogen flow.The reactant feed (phenol methanol mixture) wasintroduced at the top of the reactor by means of aninfusion pump (SAGE, USA). The products wereanalysed in a gas chromatograph (GC IS-A) fittedwith SE30 column and FID. The mass balance wasnoted each time and gas products were collectedusing an ORSAT apparatus.

Results and DiscussionA detailed account of the important physico-

chemical characteristics of Sn-La and Sn-Sm mixedoxides has been reported elsewhere. Modification oftin-rare earth oxides with sulphate anion led to anotable variation in the textural properties of theoxide. A comparison of the physico-chemicalcharacteristics of tin-lanthanum mixed oxide and itssulphated analogue is given in Table I. The specificsurface area of sulphated oxide is much greater thanthe unmodified catalyst. which is attributed to the

retardation of crystallization due to sulphtreatnlentl9

. A comparison of X-ray diffractpatterns of sulphated and unmodified cataljrevealed that the intensity of diffraction pedecreased after su Iphate treatment20 It has alre:been reported that the addition of a small amountrare earth oxide hinders the crystallization of Sn02preventing the aggregation of smaller partieHence, both the addition of sulphate and a seeroxide to Sn02 led to the broadening of the Xpeaks. The average particle size of the modi!oxides calculated from the X-ray diffraction piwidth employing Sherrer's equation is presentedTable 2, Moreover, from the SEM pictures, bigcrystallites are seen in the case of mixed oxwhereas in the case of sulphated sample smaparticles with almost uniform size are seen (Fig.This is expected as the addition of sulphate preveagglomeration of smaller particles:". It is found tsulphate treatment resulted 111 the creationsuperacid sites with HI)~-IJ.75. Superacidic characof the catalyst is retained even at a very htemperature (IOn KJ.

Conversion and selectivity of different product~the alkylation of phenol with methanol over Snand Sn-Sm binary mixed oxides of varycomposition are shown in Table J. It can be seen tall the catalysts gave very little O-alkylated produand hence the C-alky lation takes place predominClfover these catalyst systems. The selectivity ofcresol and 2,6-xylenol, which are the main produvaries with the catalyst composition. Maximum 2xylenol selectivity is observed in the case of T~and TL55 catalvsts. Moreover, some amounttrimethylphenols -are also formed over these cawlyIn the case of TS82 and TL82 ()-cresol is the mproduct, whereas TS28 and TL28 afforded ()-creand 2,6-xylenol in nearly equal amountS. 1conversion of phenol over these two cawlystslower. which is clue to lower surface areas of thl

samples.

~--------------------------------~ cd

JYOTI ct al.: PHENOL ALKYLATION OYER MIXED OXIDES 157

Table 3-AlkyiJtion of phenol with methanol over mixed oxides of tin with lanthanum and samarium"

PrOlluct TSX2 TS55 TS28 TL~2 TLS5 TUXdistribution \\'t%

Anisole 0.15 0.31 0.18 0.15 0 ..10 0.17

Phenol 23.86 23.21 40.00 24.6') 24.39 43.82

(i-Cresol 45.(,1 22.74 29.% 43.25 26.97 26.75

2,6-Xylenol 27.04 41.79 27.26 29.14 41.84 26.57

TMr" 2.70 9.05 1.43 2.45 6.50 1.58

Others 0.04 2.90 1.17 0.32 nil 1.11

Conversion 76.14 76.79 60.00 75.31 75.61 56.18

Sel.C-alkylation 99.81 9960 9970 99.80 99.60 99.67Sel. o-cresol 59.90 29.61 49.93 57.42 35.6(, 47.61SeI.2,6-xylenol 36.30 54.42 45.43 38.69 55.33 47.29

n

y

y"Reaction conditions: catalyst wcight=3 g: reaction tcmp.=623 K; molar ratio rphenol.mcthanol)« I :6; feed ratc=4 ml/h;Time on stream= I h: calcination temp= 773 K./' Trimethyl phenol.

j

)

j<;.

Table 4-Comparison ofTS 82" and STS8i' catalysts in the methylation of phenol'

Prod.Temperature (K)

523 S73 623 673 698sel. %b b b b ba a a a a

Anisole 85.6 100 5.80 10.8 0.1 6.5 0.1 20 0.1 0.8o-Cresold 14.3 0 74.7 21.2 59.9 18.7 31.9 23.8 23.6 27.8

2,6-Xylcnul 0 () 19.4 25.8 36.3 30.7 52.1 29.8 54.1 22.7TMP 0 0 35.6 44.9' 427" 45.9Conv. S'r: 2.6 3.4 32.6 30.4 76.1 79.1 83.9 67.8 75.2 61.3

"Sn02 (80%)-Sm,O, (20%)hSulphatc modified TS82'Rcaction conditions: catalyst \Vciglll=] g; molar ratio(phenol:mcthanol)= I :(,; feed rate=4 mUh; Time on stream» I hdlndicate a mixture or all the isomers in the case ofsulphated oxide.'Othcr products include polyalkylatcd phenols and condensation products.

At low temperatures (523 K) anisole and a-cresolare formed at a phenol conversion of only 2.64%,anisole being the main product (selectivity 85.64lfc).It is reported that the activation energy of 0-alkylation is lower than that of C-alkylation"2. As thetemperature IS increased o-cresol selectivity IS

decreased while the selectivity of 2,6-xylen~1 IS

enhanced, along with small amounts oftrimethylphenol. I~ the temperature range 573-698 K,the selectivity of anisole remained practicallyconstant (Table 4). There can be two importantpathways for the formation of 2,6-xylenol and 0-

cresol (Scheme I J. First one is through the formationof anisole formed initially which further react with~ethanol to give '2,6-~~lenol via methvlanisoleI . ~ - ,somenzation. Second one is direct C-alkylation inWhich () , . I t' d . .. I' .. I-CIexo orrnec uunu ty react agam WIt lonemole of I . . ~met iano! to give 2,6-xylcnol.

The ortho selectivity of the catalysts (formation ofa-cresol, 2,6-xylenol) can be attributed to the natureof adsorption of phenol over the catalyst surface. Asdescribed by Tanabe, the phenolate anion is adsorbedsuch that the ortho position is very near to the catalystsurface in the case of basic catalysts such as MgO,hence the ortho position can he methylnted ' .•. To gaininsight into the mechanism by which alkylation takesplace the reaction was carried out by changingdifferent reaction parameters. If the reaction proceedsvia anisole as intermediate, at lower phenolconversions, anisole must be formed, So the feed rateis increased step by step. But even at low contact timeonly a small amount of anisole is formed (Table 5).The selectivity of anisole remained practicallyconstant with increase in phenol conversion, whilethe vield of a-cresol reached a maximum, thendecreased with consequent increase in the yield of2.6-xylenol. These results indicate that the (}-cresol

,

_, . ' ,. lit_ . ~_"'''1''-.'.l':='.~.-A.". ~ . "II'. _ • -c-- -. _, <rr ", ,', " " ,••.• " I, ,', ! .,',::J!IIUll 'i

1\01..••.\ J. CHE\\. TECHNOL, ni.v 201)(1

Feed Lite ImLtll1I' ~'",.I. 4 h ~ ;1)

"Scl ,h b

~l

,I.1 b "

\:muk 1),1 (1.) (;,3 7,9 0,,, ; -' X I)h 21 ;

IJ·Cr':s{ll 59,9 1:0 C()~ 2-1,1 79.5 2J.1) Xil,.' ~ 1.3

2,r,·:\vknul .,r,.3 ,,0.7 27._ 1 2Sl) 22.1 " IS,l)__ . I

],\!I' .L5 -14I)'! ].l '<)9 1J.-1 '2.! nil

C·alk)Litl<lIl l}\),X 'J I.~ ')(),7 SI.G 9'H) i:.) l)l).J 643

('111\"'; i().1 7'1.1 () 1.1 (d.'! -11).2 "0, '(J.! .'2.~

'SnO: I XO(';1.Sm:0, !20r.;.)hSulph:llc ruudificd TS~Q: Reaction cOllJitiollS: C:\t:l!vst wel~ht~3 ". molur ratioipilelloi:meth:II111II

00! :(,: reaction icmp.> 623 K: Time on strcam= l h .. 'Other prnJlIL'I; include puly:\l~yl:lteJ phenols

and condcns.u ion products

undcrgoex secondary reactions to give ::,h-xyknilland trimethylphenol to a greater extent. From theseresults it is concluded that the formation of anisoledocs not occur appreciably over these catalysts andhence the .ilk ylu: ion of phenol proceeds throughdirect C-:tlkylation withou; the inten'emion of anisoleas the intermediate. The react inn of »-crcso' andmethanol in 1:7 molar ratio over these catalystsproduced a large amount of 2.6-xylenol.

When a mix ture of anisole and methanol waspassed over these cltalySl:i 2,6-\ylellol 8()j2c'c-, (1-

cresol S.59«, phenol 'J.2Q' and T\\P 1.43'+ areformed at a ciln\ersion 01'27.21 (:(-. This means thattht? COI1\C\'Sli)\1 of anisole to 2,6-xvlenol is possible,)VeT the-,e c~lt~d\sts \Vhen2-methyl anisole alone\I':IS passed over these c~ltal\'sts at the S~I\I1etCll1per~lturc. ::,(1-xylel1tl1 "'as the 111:1111product.ilongwith sm:ill amounts of crcso] and T1'v1P.

The :ICld-h:ISC propc,rtics of the cat:tiv,\S \\crecnrreialc'd wii h dcllVdr:ttiilll .md dell\'dl'llgell:ttin~.ictivitv e\llplt"'lllg cvl'iohCX:IIl11i (\e·;()\llIW\ltioll as ;1

test reaction::. ill the lkccllnpi)sitil)il Ill' c\cluhl'XiIlWIddl\drll~ellilti()n \\as the 11l:l111 rc'adl(lll". lhc<clcctivitv ill' C\Cli1Ik'xene IS round to he III the order:TL:'5 > TL28 > TU;2 and TS55 > TS2S > TSS2. SllTS.'i5 .uu,l TL.'i.'i 'y,\CIl1' arc more acidic compared ttl.uher S\steIllS. \\hile TS::S .md TL2S <Ire lc.r-t acidic'.:\(1\'" the product sekctlvity In the .ilkvuhion olnhenol \\ Ith meth.m»! ,::111he c'a~,Ih C(lITe\;lted I\'iththe acid-b~l,.;e pn'penic'''; <)1' the c·at~ll\'t\. The Iw;her,e.iectility ;'f,ec'lilJ~I[\ alk: bled prudu..:t 2.6-\ykl1olill the C:be ()t TS.'i5 .uid TL.'i5 mus: be .luc io theirhigher ;\(i·...1Jty \ h..:r i;.:~~a(ld!( ..,\",Li..:iii.' -TSS~ ~l;~l.l

TLS:: ,>-de,.;,)1 h the 1ll:1I1lproJud. QlI:dll.lli\ch itc.m he C'l'IlC!lhkd ;Iut mo-t ()f the ,tele! ,ilc'.s PL'SCllion ihe ,c I.-'.tLih :-'t~ .uc ",1, l:ak ill :">lr-':'I1~lh. F..'rom lb\.,

titration methods It is found that comparati\'ely strongbasic sites are also present on these systems. Theseobservations are supported hy the observations ofBenzouhanavu 1!1 at" that weak acid sites or strongbasic sites favour C-alkybtion. Since these catalystscontain only weak acid sites the electron currentaround benzene ring is unaffected and hence thephenolate ion is adsorbed almost perpendicular [Q thecatalyst surface which results III the selective

ii.: .. -~E\I·'

, '-' ' . ,

1111111111ill III 11111 11111 II 1111 1111 11111 11111 11111 11111 11111 II III IIIIJIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII!IIIIIIIIIIIIIIIIIIIIIIII

JYOTI ct al : PHE;\OL ALKYLATION OVER ~'llXED OX [DES 159

alkylation at ortho positions to form o-cresol and 2,6-

xylenol., Compared to TS82 sulphated catalyst produced an

cl"lbl" 'Imount of Ovalkvlated product, anisoleappre ' v, -,(Table 4). Moreover, large amounts ot polyalkylated

ducts are also formed when sulphated oxide IS~: [oyed as the catalyst. At 623 K, it afforded arni~ture of products including anisole, cresols,xylenols and polyalkylatcd phenols. ThIS IS expectedas sulphate treatment leads to the. creation of strongacid sites, It is reported that acidic catalyst such assilica-alumina and zeolite promote Ovulkylationgiving anisole". In the present case" the electroncurrent around the benzene ring IS influenced bystrong acid sites and hence the arornanc rrng lIe.parallel to the catalyst surface, Since the alignment ofphenol is parallel to the catalyst surface, methylationis possible at all positions, which leads to a mixtureof various C-alkylated products.

The effect of feed rate on the product selectivityand conversion is studied to get an idea about themechanism of the reaction (Table 5), The selectivityof anisole increases with increase in feed rate. Thisindicates that at lower feed rates secondary reactionsof anisole are suppressed, which results in increase inanisole selectivity. The selectivity' of o-cresol isalmost constant whereas the selectivity of xylenolsand polyalkylated products is decreased as the feedrate is increased. The 2,6-xylcnol may be formed bythe direct C-alkylation of phenol via v-cresol or fromanisole, Anisole can undergo an intramolecularrearrangement to gi ve (I-cresol or it can undergofurther methylation to give xyleno!. The selectivity ofo-cresol is almost constant as the feed rate is changedwhich suggests that the formation of o-cresol takesplace by direct C-alkylation. Moreover the anisoleformed initially must be undergoing furthermethylation to give xylenol. This is clear since as thereaction temperature is increased anisole selectivitydecreased from 10.8% at 573 K to 2,00(, at 673 K,From the above discussion it is clear that the phenolmethylation 0\'C[ sulphated oxide is possible via bothpa~hways, (I) direct C-alkybtion and (2) throughanisole as the intermediate.

In summary, alkylation of phenol with methanoloVer Sn-Sm and Sn-La mixed oxides result ill thefonn ti . .a IOn of ()-cresol and 2,6-xvlenol as mainr~OdUcts However, the sulphate' modified oxidea !S) afforded a mixture of. products includingnlsole, cresols, xylenols and polyalkylated phenols.

6' OCH,I

CHPH (rCH,

OH Path 1

0-Y

methylanlsoleCH,OH anisole

lphenol OH

CH,OH OH

Path2CrCH'

Hc"6-CH,

CHpH a '"

I~ • I ",'2.6-xylenol

o-cresol

Fig. 2-Scheme t -Pathways or phenol alkylatiun.

Methylation of phenol over Sn-Sm and Sn-La mixedoxides proceeds through direct C-alkylation whereasin the case of sulphate modified oxide reactionproceeds via both pathways viz., (a) direct C-alkylation and (b) through anisole as intermediate,The acid-base properties of catalysts play animportant role in determining the selectivity ofproducts, The mode of interaction of benzene ring ofphenol molecule with catalyst surface is intluencedby the catalyst acidity. If the catalyst is highly acidic,the interaction with the electron current aroundbenzene ring will be very strong, which result in theformation of all possible products. However, if onlyweak acid sites are present the ortho selectivity willbe high.

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