Indi an Journal of Chemistry Vo l. 45A. May 2006. pp. I 13 1- 11:18

Alkylation of phenol with tert-butyl alcohol catalysed by some sulphated titania systems

K R Sunajadev i & S Sugunan*

Depart ment of App li ed Chemi stry. Coc hin University of Science and Technology. Kochi 682 022. India

Ema il: ssg@c usat.ac. in

Recei1·ed 9 Febnwrr 2005: n:~·ised 21 Mlll'ch 20()(5

Titani a sulphated titania and tran siti on metal loaded (9'/o ) sulphat ed titania have been prcp:1rcd by so l gel method and ch:1racteri 1.ed by XRD, F riR , BET surface JreJ. EDX and UV-v is DRS. Surface acidity o f these catal ysts is determined by temperature programmed desorption of ::ll111110nia. Alkylati on of phenol with ten-butanol in the vapour phase over the prepared systems has been studi ed at I atm and 160-2:W0 C. The reaction provides hi gh selec ti vity of alkylati on at the pam

posit ion . The product selec ti vity has been corrc!Jtcd with the surface acid it y of the systems.

IPC Code: Int. Cl. s 1301J2 1/00: BOIJ37/00: C071337/00

Nanocrysta lline material s were first sy nthesized by Gleiter and co-workers in 198 1 and have since become a major focu s of research because of thei r interesti ng and potentially useful properties 1

• The nanoscalc chemistry in volved in sol-gel methods is a more direct way to prepare highl y di vided material s2

.

Sol ge l method is a homogeneous process, which resu lts in a continuous transformation of a solution into a hydrated solid precursor (hydrogel). Titania so l­gel synthes is has been developed from inorganic precursors and from metal organic precursors like titanium isopropox ideJ. The advantages of so l-gel process in general are hi gh purity, homogeneity and low temperature. For a lower temperature process, there is a reduced loss of volatile components and thus the process is more environmental friendly. Sulphated metal oxides are strong solid acids that have recently become the focu s of much interest because of the enhanced chemical properties imparted by the presence of sulphate groups4

. Titania, classified as a solid acidic oxide in both the anatase and rutile crystallographic form s, has long been known to possess catalytic activity, although anatase was found to be more active than rutil e5

. Photocatalytic degradation of phenol using Ti02 immobilized in polyvinyl alcohol has been reported6

. The chemical and catalytic properti es of titania can be modifi ed by the incorporation of metallic ions .

Friedei-Crafts alkylation is an important means of attaching alkyl chains to aromatic rings and hence is a key reaction in organtc chemistry. Alky lation of

phenol provides many industrial intermediates such as agrochemicals and polymers. Alkylation of benzene with isopropanol on mixed oxides7 and benzoy lation of toluene over different sulphated zirconia and iron incorporated sulphated zi rconia systems8 have been reported. The reaction of phenol alkylation with tert­butyl alcohol (TBA) have been studied extensively owing to industrial interest in the products as antioxidants, ultravio let adsorbers and heat stabili zers of polymeric materials. The catalysts used are developed from liquid acid, metal oxide, AI salt catalyst to cation exchange res in'J. However. the reacti on of tert-butlati on of phenol gives numerous products depending on the nature of the catalysts as well as on the react ion temperature 10

• For example. weak acid catalysts such as zeolite- Y favour tert-buty pheny ether and strond acid catalysts like zeolite, lead to meta-tert-butylphenol. On the other hand, moderate acid catalysts like SAPO-I, MCM-41, etc. produce ortli o and para tert-butyi phenol. Sakthivel et a/. 10

reported the vapour phase /ert-butylation of phenol over sulphated zircon ia catalyst with a high select ivity to para isomer. Incorporation of Zn and Fe in AI­MCM-41 framework is found to increase the total acidity of catalysts and the incorporated Fe( lll ) is found to be in tetrahedral co-ordination. Savidha et a/. 11 explained tert butylation of phenol with /-butyl acetate over Al-MCM-41, Zn- and Fc-AI-MCM-41 and found that the phenol conversion and 4-t-butyl phenol selectivity are higher over Zn- and Fe-A I­MCM-41 than that of Al-MCM-41 catalysts .

11 32 INDIAN J CHEM. SEC A. MAY 2006

Dapurkar et a /. 12 reported that mesoporous ga ll os ili cate (GaMCM-48) molecular sieve catalyst is highly ac ti ve for the tert butylation of phenol and shows a much hi gher substrate conversion than the ana logous catalys( systems . Karthik et a/. u reported the effect of various parameters for the tert-butylati on of pheno l over Co-A l-MCM-4 1. The objecti ve of the present study is to demonstrate the feasibility of phenol alkylation with ten-butyl alcohol over transition metal loaded sulphated titania systems. Alkylat ion occurs selec ti vely at the pom position . The observed activities coul d be correlated with the acidi ty of the catal ys t.

Materials and Methods

Catalyst preparation

Cr, M n, Fe, Co, N i, Cu and Zn load eel (9%) sulphated ti tania nano powders were prepared by so l­ge l process using titanium isopropoxide (A ldri ch 98%) as the starting material. Titanium isopropox icle was added to water- nitri c acid mi xture in the rati o 5:60:0.5 with constant st irring. Precipitation occurred immediately and the precipitates were stirred continuously at roo m temperature to fo rm a hi ghl y dispersed sol. To thi s, Cr, Mn , Fe, Co, Ni, Cu and Zn nitrate so lutions (9 wt %) were aclclecl separately and stirred agai n for about 4 h. After keeping the sol for ag ing. it was concentrated and dried at 60°C. Su lphation was clone using 0.5 M sulphuri c ac id so lution (2 mL g·1 of the hyd roxide). The precipitate was dried at l I ooc for 12 h in an air oven and powdered below 100 mi crons mesh size and ca lcined at 500°C fo r 5 h in a muffl e furnace. The ge neral sample notation STX(9) stands for sulphated titani a with 9 wt% of X metal ox ide whereas T and ST denote pure and sulphated titania respecti vely.

Physico-chemical characterization

The catalyst samples were characteri zed by different phys ico-chemica l techniques such as XRD ana lys is, infrared spectra. BET surface area, energy di spersive X-ray (E DX) analysis, diffuse refl ec tance li V-v is spectral analys is and ac idity cle tennination by temperature programmed desorpti on of am monia.

X-ray powder diffracti on (X RD) patterns have been recorded on a Ri gak u D-max C X-ray diffractometer using Ni filtered Cu Ka radiation source ()c = 1.5406 A). ~The XRD phase present in the sa mples was identifi ed with the help of JCPDS data fil es. Simultaneous determination of surface area and pore

volumes of the catalys t samples was clone on a Micromeritics Gemini-2360 surface area analyzer under liquid N2 temperature using N2 gas as the adsorbent. Previously ac ti vated sa mples at 500°C were degassed at 350°C under nitrogen atmosphere for 4 h prior to each measurement. The pore volumes of the samples were measured by the uptake of nitrogen at a relati ve pressure of 0.9. FTIR spectra of the samples were recorded using a agna 550 ico let in strument by the KBr elise method in the range 400-4000 cm·1

• Quantitati ve elemental analys is of the sa mples was clone by EDX measurements using EDX­JEM-35 instrument (JEOL Co. link system AN -1 000 Si-Li detector). Samples were prepared by dusting the titan ia powder onto doubl e sided carbon tape. mounted on a metal stub. Diffuse refl ectance UV-Vis spectra 200 and 800 nm of the sa mp les were recorded at room temperature between using MgO as standard in the Ocean Optics AD 2000 in strument with CC D detector. Temperature programmed desorption of ammonia was clone in the range o r I 00-600°C in a conventi onal flo w-type apparatus at a heating rate or 20°C min-1

Catalytic activity

The alkylation of phenol (Merck) with tert-butyl alcohol (Quali ge ns) was carri ed ou t at atmospheri c press ure in a fixed bed, tubular clown fl ow reactor using 0.5 g of the catalyst placed on a glass woo l bed packed between si li ca beads . The catalyst was ac tivated at 500°C for 2 h prior to catalyti c runs. The reactor was heated to the req ui site temperature using a tubular furnace contro ll ed by a di gi tal temperat ure controll er cum indi cator and the temperature is monitored using a thermocouple. A reaction mixture of phenol and tert-butyl alcohol wi th a molar ratio or I :2 was feel into the reactor by means of a sy rin ge pump, at a flow rate of 4 mL h-1

• The products vverc coll ected with a condenser and the li quid products were anal yzed by gas chromatography (Chemito GC 1000) using a BPI capill ary colu mn (12 m x 0.32 mm) with FID detector, ni troge n a:; carri er gas (injection and detection port temperatu res- i SO"C. temperature programme-60°C- l-l OOC- 1 50°C).

Results and Discussion

Catalyst characterization

All peaks meas ured by XRD analys is could be ass igned to those of Ti02 crys tal. The average crys tallite size is calcul ated (Table I) using Scherrer

SUNAJADEV I & SUGUNAN: ALKYLATION OF PHENOL CATALYSED 13Y SULPHATED TITANIA SYSTEMS 1133

Table I - Su;·face parameters of the prepared syste ms

System Crystal lite size Elemental com[>OSi ti on fro m EDX (%) Pore diameter BET surface area Pore volu me (nm) Ti01 so-~ Metal

T 12.7 1 100

ST 9.62 95.34 4.66

STCr(9) 6.38 81.60 10.58

STMn(9) 19.39 83.52 9.64

STFe(9) 13.56 82.92 10.2 1

STCo(9) 15.07 80. 18 11 .30

STNi(9) 18. 14 79.09 12.85

STCu(9) 16.57 79.42 11.97

STZn(9) 14.86 82.77 10.1 8

equation from the I 0 I reflection of anatase 14• The

average crystallite size L, as given by Scherrer equation is L = 0.9N'~ Cos 0, where A. = wavelength of the X-ray used, 0 = glancing angle and ~ = FWHM (half the width of the peak with maximum intensity). It has been reported that the degree of crystalli zation of the sulphated oxides is much lower than that of the oxides without sulphate treatment 15

. From the XRD patterns (Figs J and 2), it is clear that the rutile phase is completely eliminated in the case of sulphated samples . Sulphation retards the transformation from anatase to rutile in comparison with the sample without sulphat ion. Dispersion of SO/ species hinders agglomerization of the titania particles indicating delayed crystallization . In addition to stab ili zing anatase Ti02 crystallites, su lphate surface spec ies inhibit Ti02 crys tallite sintering leading to lower crystallite than in pure Ti02. Since no titanium sulphate is detected by XRD for the catalys ts examined in the present study, the sulphur may ex ist in a form of sulphate on the surface of Ti02. Choo et

a /. 16 reported that there was no formati on of titanium sulphate for the ST calc ined even at 900°C. In order to identify the state of sulphur species on the surface of Ti02, they examined the sulphated su pports by XPS and found that the su lphur compound exists in a form of sulphate (S04

2. ) on the catalyst surface. The

bulk structure of titania remains virtually unchanged by the incorporation of metal ions, except for a loweri ng in crystallinity. The absence of any characteri stic peaks of the metal ox ides, sugges ts that these ox ides are present in the form of di spersed oxide species, since the total content of them is rather small.

7.82

n.84

6.81

8.52

8.06

8.6 1

7.05

::J

~ .£ (/)

c: Q)

c

10

10

(nm ) (m" g·l) (cc g· 1)

102.8 35 0.09

92.3 91 0. 2 1

53. 1 128 0.1 7

57. 1 98 0.14

49.3 138 0.17

44.9 98 0.11

46.3 95 0. 11

55.0 80 0.11

44.9 98 0.11

20 30 40 50 60 70

28 in degrees

Fig. I - XRD profiles of (a) T and (b) ST.

20 30 40 50 60 70

28 in degrees

Fi g. 2- XRD profiles o f STCr(9) STMn(9) STFe(9) STCo(9 ) STNi (9) STCu(9) STZn(9).

1134 INDIAN J CI-I EM . SEC A. M AY 2006

Energy dispersive X-ray analysis

Energy di spersive X-ray anal ys is yields the chemical composition of the prepared samples. The elemental compositions of individual systems are presented in Table 1. The sulphate content of the metal incorporated sampl e is considerably higher, when compared with simple sulphated system, whi ch indicates that metal doping brings about a considerable reducti on in the ex tent of sulphat e loss from the catalys t surface. It may be assumed that the di spersion of iron particles res tricts the sulphate spec ies more or less to the surface. minimi zin g their mi grati on into the bulk .

FTIR spectroscopy

IR spectrum of pure titania (Fig. 3) contains two maj or absorpti on bands at 3383 and 1630 cm·1

• These bands may be attributed to the hydroxyl groups; the former corresponds to the stretching and the latter to the bending modes of OH group. In compari son with pure titania, the infrared spec tra of sulphated metal ox ides exhibit a strong absorption band at 1375- 1390 cm·1 and a broad peak with shoulders at around I I 00-1200 cm-1 (Fi g. 4 ). The peaks at I 029, I 076 and 1222 cm-1 are typical of the S=O mode of vibrati on of a chelating bidentate sulphate ion coordinated to a metal cation 17. When SO.t is bound to the titani a surface, the symmetry can be lowered to either C," or C2v. The sulphate species modifi ed the el ectroni c environment around the Ti-1+ ion by anchoring SO.t in either bridging biclentate or chelating biclentatc compl exes18

·19

. The bands obtained in the 1200-1100 cm-1 regions are typical of sulphato complexes in a bi clentate configurati on with C2,. sy mmetr/ 0

. Thus, the IR spectral bands of the samples close ly agree to the biclentate sulphate complex structure having bands around 1119 and 11 29 cm-1

• The bands observed in the range of 400-900 em · I are the vibration modes of

0 . 0 "I ,, anatase skeletal -Tt- bonds- ·-- . In the low energy region of the spectrum the bands at 595 and 467 cm-1

are assigned to ben eli ng vibrati ons of Ti-0 boncls2' .

Sw:face area and pore volume measurements

Tabl e I shows the surface parameters of the prepared systems. As evident from the data, sulphated titania showed a hi gher surface area compared to pure titania. Jt is already reported that the retention of surface area by the sulphated samples occurs even aft er hi gh temperature calcination, and is explained on the basis of the higher resi stance to sintering acquired by eloping with sulphate ions2

.J . Addition of transition

4000 3500 3000 2>00 2000 1.' 00 . 000 , 00

Wave Numbers (cnY' )

Fi g. 3 - FT IR spectra ofT and ST.

4000 2000 1000

Wave Numbers (cm ~ l)

Fig. 4 - FTIR spec tra o f STCr(9) ST Mn(9) STFe(9) STCo(9J ST Ni(9) STCu(9) STZn(9 l.

SUNAJADEVI & SUGUNA :ALKYLATION OF PHENOL CATALYSED BY SULPHATED TITANIA SYSTEMS 11 35

metal species causes a further setback to the crystallization and sintering process, which is evident from the higher surface area of the samples in comparison with the simple sulph ated system. The metal oxide species along with the sulphate io ns prevent the agglomerati on of titania particles resulting in a higher surface area. Onl y exception is STCu(9) samples for which there was a s light lowering of surface area compared to ST. Among the different metal incorporated systems, there was no significant variation in the surface area value. Assuming the pores are cy lindrical , the average pore di ameter is calcul ated using the formula: d = 4Vp!S,, where d is the average pore di ameter, V, is the pore vo lume, and SP is the surface area 15

• Decrease in the pore diameter is observed after sulphation. Metal incorporated samples also show a decrease in pore diameter.

UV-vis dijjitSe reflectanc e spectroscopy

UV -vi s DRS is used to probe the band structure, or molecular energy levels, in the materia ls since UV-Vis li ght excitation creates photo generated electrons and holes. Zhang eta/. 25 reported the UV-Vis spectra of titania samples calcined at different temperatures. In all the samples, characteristic band for tetrahedrally coordinated titanium appears at about 300-380 nm. The absorption IS associated to

the 0 2.-1Ti4+ charge transfe r corresponding to

electronic excitation from the valence band to the

conduction band. A111,, and band gap energy calculated for the present catalysts are given in Table 2. The band gap energy is found to increase after sulphate modification and metal incorporation. The presence of the doping ions caused significant absorption shifts into the lower wave length region compared to pure titania.

Temperature programmed desorption of ammonia

It is generally recogni sed that ammonia is an excellent probe mol ecule for testing acidic properti es of solid catalysts as its strong basicity and small molecular size allow detecti on of acidic sites located in very narrow pores a lso20

. Acid site di stributi on in Table 2 shows the presence of weak (I 00-200°C). medium (200-400°C) and strong (400-600°C) acid sites. Pure titania shows only low ac idity and sulphation increase its acidity. It is well established that for sulphated metal oxides, the e lectron withdrawing inductive effect of the sulphate groups through the bridged oxygen atoms generates hi gh surface acidity . The sulph ating agent being acidic , prefe rentially attacks the basic sites, and converts them to acidic sites leading to increase in the total acidity. Incorporati on of metal io ns also changes the acidity considerably. The nature of the acid sites is greatly a ltered by the nature of the ions incorporated into the latti ce. The di stribution change may be a coupled effect of the crystalline and structural changes.

The change in the acid strength di st ribution for the different systems may be rel ated to the inte racti on o f the added metal cations with the Ti02. All meta l incorporated systems show lower acidity compared to sulph ated titania . The increased loading o f sulphate o n Ti02, as ev ident fro m EDX, can form the po lynuclear type of sulphate complex and increase the coverage of the Ti metal ion by the sulphate ion. The po lynuclear sulphate cannot ex tract as many e lec trons as iso lated su lphate, to generate a strong ac idi ty . Samantaray et a/. 15 reported a decrease in surface acidity at high sulphate concentrations. It is reported in lite rature that the generat ion o f to tal and strong acidity is not affected by the type of sulph ate spec ies . such as isolated and po lynuc lear, but by the coverage

Tab le 2 - Band gap energy from UV -Y is DRS and acidity from am mo ni a-TPD

Catal ys t A llla'\ Band gap Amount of ammo ni a desorbed (mmol !:!.1)

(nm ) energy (eY ) Weak Medium Strung Tota l

T ]72 3.25 CUI 0.20 O.O l 0 .52

ST 322 ].76 0 50 0.3 2 0. 09 0 .9 1

STCr(9) ] 10 ].90 0.-+2 0.47 0.02 0.91

STM n(9) ]14 J.X5 0.]7 0.46 0.0 1 0.84

STFe(9) ]20 3.78 0.]6 (l.\4 0.03 0. 73

STCo(9 ) .\21 ]77 0 . .\7 0.35 0.0 I 0.7 3

STN i(9 ) 32: ] 76 0.37 0. 35 0.00 072

STCu(9 ) ' "~ ~')~ ,. :uo 0 . .\X () 35 000 0.73

STZn{9) 31 ~ 3.XX 0.](> 0. 35 o.o:' 0. 74

11 36 INDI AN J C HEM. SEC A. MA Y 2006

of the surface of the Ti by sulphate ions27.

Accordingly, it is concluded that the free Ti 1on surrou nding the sulphate is responsible for the generation of strong ac id sites.

Tert-butylation of phenol

All the prepared systems were tes ted for act1v1ty over a reaction time of 2 h under the reaction conditi ons, temperature of 200°C. fl ow rate of 4 mL h-1 and a reactant ratio of I :2. An attempt to correlate the surface acidity with the product selecti vity has been carried out. The major products obtained during the studi es are 2-tert butyl phenol (2-TBP), 4-tert butyl phenol (4-TBP) and 2,4-di-tert-buty l phenol (2,4-DTBP). 2-tert butyl phenol eas ily isomeri zes to 4-tert bu tyl phenol whereas the reverse reaction is not signi ficant. There is no fo rmation of 3-TBP, which may be formed in the presence of Bronsted ac id sites. ln the case of our samples, the Bronsted sites are weaker, and hence no 3-TBP formati on is ex pected. Trace amount of tert-buty l phenyl ether (TBPE) was detec ted.

Pure titani a ex hibited poor acti vity towards tert­butylation of phenol under the spec ified reacti on conditi ons. The reacti on proceeds very effi cientl y over different sulphated titan ia systems. An attempt to investigate the in fluence of the metal loading on catalytic acti vity is quite reasonable. As ex pected, vari ati on in metal loading had a signi ficant impact on the catalyti c acti vity. In compari son with simple sulphated system, metal incorporated samples are more effici ent for the buty lation reac tion (Tab le 3 ). Chromium loaded samples show the hi ghest acti vity when co mpared with the other systems. In all cases, ort/10 and para isomers were obtained with a high selecti vity fo r the para isomer. Usha et a /.28 reported the same observation for mesoporous aluminophosphate and heteropolyac id supported aluminophosphate molecular sieves towards the tert butylaiton of phenol. The p/o rati o was max imum for chromium and min imum for iron. The ac id base properties of the catalys ts affect the final selecti vity of

"Q 28 h heterogeneous catalysts- . It was reported that. as t e Brci nsted acid sites over the catalyst increases , the selec ti vity fo r pa ra isomer increases. Figure 5 gives the correlation between the product selec ti vity and the acidi ty assessed by ammonia TPD. lt was observed that the p/o rati o was very low in the case of pure titani a and sulphated titania. Metal incorporation increased the selectivity to para isomer.

Table 3 - Cata lyt ic acti vity and prod uct se lec ti viti es over the prepared systems (amount o f cata lys t: 0.5 g. !low rate: -+ mL h·1

•

phcnoi: TB A: I :2. reaction time: 2 h. reac ti on temperature: 200°C)

Systems Conversion Se l ec ti v it ~ ('Yo ) plo of phenol 2-TBP 4- TBP 2.4- DTB P Rati o

(wt%)

T 14.90 2:1.53 50.64 2.54 2. 15

ST 28.45 19.8 1 65 .64 5.44 3.] I

STCr(9) 34. 12 14.47 70.95 4.98 4.83

STMn(9) 33.85 14.94 70.02 3.47 4.68

ST Fe(9) 29.47 20.04 67.32 4.62 3.]6

STCo(9) 32.94 19.79 69 .05 4.25 3.4l)

STN i(9) 32.54 16.72 68.89 2.45 4.12

STC u(9) 3 1.25 17. 17 68.0 1 2.92 3.96

STZn(9) 3 1.09 17.63 67.98 4.09 3.85

:=- --80 OJ

0 E

~-fr----6 .s "0 ~ Q)

€ I >-~ 0 5

L 60 :~

"0 ti I

Q)

a; z (f)

0 c => 0

0 E 40 < f- f- §: Ci) Ci) Ci) §: Ci) Ci)

(f)

0 c Q) 0 z '5 c :::;; u_ u f- u N

f- f- f- f- f- f-(f) (f) (f) (f)

(f) (f) (f)

--...- Medium acidity -tr- 4-TBP selectivity (%)

-~ 0.2 8 0 E .s 6

l "0 Q)

€ c Sl 0.1 4 ·:; Q) ·u "0

Q)

I a; z 2 (f)

0 /' c => 0 0 0 E f- f-

~ Ci) Ci) Ci) §: Ci) Ci) < (f) c Q) 0 '5 c u :::;; u_ u z u N f- f- f- f- f- f- f-(f) ({) (f) ({)

(f) (f) (f)

--...- Strong acidity -tr- 2,4-DTBP selectivity(%)

Fig . 5 - Corre lati on between ac idity and product se lec ti vi ty.

In the present study, the reac tion was promoted by med ium and strong acid sites. Strong acid si tes are necessary to get higher selectivity of 2.4-DTBP while medium acid sites are helpful in enh anc ing the selec ti vity of 4-TBP. Corma et a /.30 reported the same observati on in zeo lites Y. Mediu m ac id sites may promote the isomeri zation or transalky lati on reaction of o-TBP to p-TB P, whil e strong acid sites are helpfu l in forming 2,4-DTBP. The catalytic alky lation of

SUNAJADEVI & SUGUNAN: ALKYLATION OF PI-IE OL CATALYSED BY SULPHATED TITANIA SYSTEMS 11 37

OH C(CH,)3

~

.,&

@] C(CH3) 3

[A) TBPE, [B]2·TBP, [C) 4-TBP, [D)2,4·DTBP

Fig. 6 - General scheme for ten-butylation of phenol.

phenol with alcohols gives ri se to two di stinct classes of products depending on the site where the alkyl group alkylates phenol. The formation o f 0 -alkylated products depends both on the intrinsic properti es of the alcohol and on the structural and acid-base properties of the catalysts. Corma et a/3 0 found that the alkylation of phenol by TBA over the solid acid HNa- Y zeolites at 303 K occurs both at 0 - atoms and C-atoms, but the 0-alkylation had a higher selecti vity, whereas ring alkylation alone occurred at higher temperatures. Zhang el a!.~ already reported that the strong acidity was required for the fo rmation 2.4-DTBP and acid sites of medi um strength were responsible for the fo rmation of 4-TBP. Figure 5 indi cates thar the selectivity to 4 -TBP and 2.4-DTBP li es ni cely with the medium and strong acid sites from the TPD of ammonia of the prepared systems. Presence of high concentration of moderate-to-strong aci d sites over mesoporous H-GaMCM-48 also shows high selectivity towards 4-TBP31

•

Mechanism of the reaction The interes ting aspect of this reaction is the

selecti vi ty of alkylation at the para position. General scheme of the reacti on is given in Fig. 6 . The genera ll y accepted mechani sm for aromatic alkylation is that the terti ary carbenium ion interacts with

adsorbed phenol forming a n-complex. which then

rearranges to 0 -compl cx by the electrophile attacki ng a ring carbon atom. The complex on proton elimination gives tert-butyl phenol. 1t has been

suggested that Bronsted acid sites interact wi th the n-

cloud of aromatic ring bringing the molecule parall el to the surface3c.:n _ This will allow alkylation at the

para position easier as compared to the orrli o pos itions . The para se lectivity of the catalysts can be attributed to the nature of adsorption of pheno l over the catalyst surface. According to Tanabe3~. the phenolate ion is adsorbed such that the orrho pos iti on is very near to the catalyst surface in the case of basic catalysts such as MgO, hence the ortho positi on can be alkylated. However, the interaction of acidic catalysts are different which influence the electron current around the benzene ring such that the aromatic ring lies paralle l to the catalyst surface favouring para alkylation . Also, it has been reported35 that the steric hindrance in the tran sition state due to the substitution of bulkier terr-butyl group at the ortho positi ons. enhances the para selectivity.

Conclusions Transition meta l loaded sulphated titania has

excellent catalytic activity for lert-butylati on of phenol with butyl alcohol. Selectivity towards para product is show n to be hi gh compared to other products . The ac idity plays an important role in thi s reaction. Medium acid sites are helpful to produce 4-TBP while strong ac id sites govern the formati on of 2.4-DTBP.

Acknowledgement The authors are thankful to Regional Sophi sticated

Instrument Facility (RSIC), llT Bombay. India fo r FT-IR analysis. Fi nancial assistance from CSIR . ew De lhi to KRS is gratefull y acknowledged.

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11 38 INDI AN J CHEM. SEC A, MAY 2006

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