Wt

AAa

b

ARRAA

KApWKL

1

(bpcc[ldcsaTw[

tHei

(

1h

Journal of Molecular Catalysis A: Chemical 378 (2013) 22– 29

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Catalysis A: Chemical

j ourna l ho me pa g e: www.elsev ier .com/ locate /molcata

Ox/ZrO2: A highly efficient catalyst for alkylation of catechol withert-butyl alcohol

run Kumara,∗, Asraf Alia, K.N. Vinoda, Amit Kumar Mondala, Hemant Hegdea,shok Menona, B.H.S. Thimmappab

Innovative Plastics Program, GE India Technology Centre, No. 122, EPIP, Whitefield Road, Bangalore 560066, Karnataka, IndiaDepartment of Chemistry, Manipal Institute of Technology, Manipal University, Manipal 576104, Karnataka, India

a r t i c l e i n f o

rticle history:eceived 4 February 2013eceived in revised form 13 May 2013ccepted 14 May 2013vailable online 22 May 2013

a b s t r a c t

The liquid phase alkylation of catechol with tert-butyl alcohol was carried out over a solid acidcatalyst—WOx/ZrO2. p-tert-Butyl catechol (4-TBC) was the predominant product with 4,6-di-tert-butylcatechol as the minor product. A systematic study has been carried out to understand the effect of vari-ous parameters such as WO3 loading, mole ratio of the reactants, temperature and catalyst loading. Theexperimental results indicated that under optimized condition of 1:1 mole ratio of catechol to tert-butyl

eywords:lkylation-tert-Butyl catecholOx/ZrO2

inetics

alcohol at 413 K within 30 min, both the conversion for catechol and selectivity towards 4-TBC was 99%.The Langmuir–Hinshelwood–Hougen–Watson (LHHW) surface reaction controlled model has been usedfor kinetic studies.

© 2013 Elsevier B.V. All rights reserved.

HHW model

. Introduction

Alkylation reaction of catechol (CAT) with tert-butyl alcoholTBA) is an industrially important reaction. The product, 4-tert-utylcatechol (4-TBC) has widespread applications as antioxidants,olymerization inhibitors and also as stabilizer of many organicompounds [1]. This is a Friedel–Crafts reaction and is generallyarried out using acids such as ZnCl2, HCl and FeCl3 as catalysts2]. The use of these catalysts poses many problems such as hand-ing, safety, corrosion and waste disposal. Thus, it is essential toevelop a suitable alternate eco-friendly and commercially viableatalyst system. Although numerous studies have been reported onelective synthesis of phenol derivatives, there are limited reportsvailable for the selective synthesis of 4-TBC using solid catalyst.he butylation of CAT to form 4-TBC is commonly carried outith TBA [3–11], methyltertiarybutyl ether [12–16] and isobutene

17–20] as alkylating agents.The most commonly employed catalysts when TBA is used as

he alkylating agent includes acidic zeolites like H-Y [3], H-USY,

-ZSM-5 [4] and H� [5], clay-based porus montmorillonite het-rostructures (PMH) solid acid catalysts [6–8], SO3H-functionalizedonic liquid [9] and Co-napthenate and ZnCl2 mixture [10]. Anand

∗ Corresponding author. Fax: +91 80 40122111.E-mail addresses: arun.kumarn@geind.ge.com, arun.kumar@sabic-ip.com

A. Kumar).

381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.molcata.2013.05.010

et al. [3] have studied the vapor phase alkylation in the temper-ature range 393–473 K using a fixed-bed vertical flow reactor at1:3 molar ratio of CAT:TBA over HY and dealuminated HY zeolites,they showed marked increase in conversion of CAT and selectivityof more than 86% towards 4-TBC with moderately steam-treatedHY zeolites (steamed at 823 K and 973 K). Among the variousmicroporus acidic zeolite catalysts like H-USY, H�, �-alumina andH-ZSM-5 studied by Yoo et al. [4], H-ZSM-5 showed a conver-sion of more than 90% and selectivity higher than 84% for 4-TBCat 443 K using fixed-bed vertical flow reactor at 1:3 molar ratioof CAT:TBA. H� zeolite was found to be very efficient catalystfor the alkylation with conversion of CAT about 87% and selec-tivity towards 4-TBC was 99% at 1:4 molar ratio of CAT:TBA [5].Chunhui et al. have reported the liquid phase alkylation of CATwith TBA over meso and nanoporus clay composites [6–8]. Intheir work on H-exchanged montmorillonite and novel meso-porus acidic montmorillonite heterostructure catalysts [6], theyobserved best CAT conversion of 83.3% and 4-TBC selectivity of80.6% under the optimized reaction condition of 410 K, 1:2 molarratio of CAT:TBA for 8 h. Under the same reaction condition overthe synthetic nanoporous silicoaluminum montmorillonite het-erostructured composites [7], CAT conversion of 76.8% and 4-TBCselectivity of 85.2% was observed. Recently, Xiaowa et al. [9] have

studied the catalytic properties of several SO3H-functionalizedionic liquids for CAT alkylation with TBA by combination of experi-mental and computational techniques. They achieved the best CATconversion of 41.5% and 4-TBC selectivity of 97.1% in 8 h under

A. Kumar et al. / Journal of Molecular Cata

Nomenclature

A catechol (CAT)B tert-butyl alcohol (TBA)C tert-butyl catechol (4-TBC)D 4,6-di-tert-butyl catecholW waterk1 rate constant for reaction 1 (g/mol min)k2 rate constant for reaction 2 (g/mol min)k3 rate constant for reaction 3 (g/mol min)k4 rate constant for reaction 4 (g/mol min)k5 rate constant for reaction 5 (g/mol min)K1 adsorption constant of A (CAT) (dm3/g)K2 adsorption constant of B (TBA) (dm3/g)KC adsorption constant of C (4-TBC) (dm3/g)KD adsorption constant of D (4,6-di-tert-butyl catechol)

(dm3/g)Kw adsorption constant of water (dm3/g)

tC

sbtSlsiroepo

aw5mlfcatmsdspaw

2

2

nottu

−rAS rate of reaction of catechol at the surface (alkylation)(mol/g min)

he optimized reaction condition of 423 K at 1:2 molar ratio ofAT:TBA.

In recent years, solid super acid catalysts based on doping tung-ten oxides on zirconia support (WOx/ZrO2) is considered to haveetter application prospects for alkylation of aromatics comparedo many other solid super acids such as, sulfate supported ZrO2,bF5, TiO2 and Fe2O3 super acids. This is because WOx/ZrO2 cata-yst system is stable at very high temperature and in a solid–liquidystem [21,22]. Since, it was first reported by Hino and Arata [21],t has been employed by researchers for many versatile catalyticeactions like alkylation of hydrocarbons, liquid-phase alkylationf phenol with long-chain olefins, acetylation of alcohols and isom-rization of light alkanes and nitration of aromatics [23–29]. Thisrompted us to choose WOx/ZrO2 catalyst for selective alkylationf CAT with TBA to form 4-TBC.

To investigate the influence of WO3 loading on the conversionnd selectivity towards various products during alkylation of CATith TBA, we have prepared pure zirconia and 1, 5, 15, 25 and

0 wt.% tungsten oxide on zirconia. A systematic study has beenade, which includes preparation of the above mentioned cata-

ysts, detailed physicochemical studies and catalytic performanceor the alkylation of CAT with TBA. We also made an attempt toorrelate the physico-chemical properties towards the catalyticctivity. The catalyst with highest activity has been used to studyhe effect of reaction parameters, such as reaction temperature,

olar ratio of the reactants and catalyst loading on the conver-ions and product selectivity in both the reactions. Further, we haveeveloped Langmuir–Hinshelwood–Hougen–Watson (L–H–H–W)urface reaction controlled kinetic model and estimated the modelarameters. From the estimated rate constants at different temper-tures, the activation energy of the CAT alkylation reaction with TBAas determined.

. Experimental

.1. Catalyst preparation

Precipitate of Zr(OH)4 was obtained by adding aqueous ammo-ium hydroxide slowly into an aqueous solution of zirconium

xychloride at room temperature with constant stirring untilhe pH of the mother liquor reached about 10. The precipi-ate thus obtained was washed thoroughly with distilled waterntil chloride ion was not detected by AgNO3 solution, and was

lysis A: Chemical 378 (2013) 22– 29 23

dried at 393 K for 12 h. Catalysts containing series of WOx/ZrO2loading (1–50 wt.% of zirconium oxyhydroxide) were preparedby adding Zr(OH)4 powder into aqueous solution of ammoniummetatungstate [(NH4)6(H2W12O40)·nH2O] (Aldrich) followed bydrying at 393 K for 12 h and calcining at 973 K for 5 h. This series ofcatalysts were denoted by xWZ, where x represents wt.% of tung-sten oxide, W represents tungsten oxide and Z represents zirconia.For instance 5WZ represents ZrO2 that was dried at 393 K and dopedwith 5 wt.% tungsten oxide. ZrO2 is the pure support calcined at973 K for comparison purpose. Further, the catalyst 15WZ was cal-cined at different temperatures (873 K and 1073 K for 5 h) to studythe impact of calcination temperature on the catalyst performance.Tungsten content in all the prepared catalysts was determined byinductively coupled plasma optical emission spectroscopy (ICP-OES). The results are reported in Table 1.

2.2. Catalyst characterization

The following characterization has been made on the catalystmaterials. Total surface acidity of all the catalysts was determinedby immersing 0.1 g of catalyst in known amount of excess n-butylamine in dry benzene and back titrating against standardHCl. Powder X-ray diffraction patterns (PXRD) were measuredon a Philips X-ray diffractometer with Ni filtered Cu K� radi-ation over the range of 2� from 20◦ to 40◦ with the intensitydata for each point being collected at a 4 s interval. Surface area(BET), pore volume, average pore radius and pore size distri-bution of the WOx/ZrO2 samples were determined by nitrogenadsorption–desorption method at liquid nitrogen temperature(77 K) using a Micromeritics ASAP2010 instrument.

2.3. Experimental procedure

All the catalytic activity studies were carried out in a batchglass reactor, equipped with reflux condenser and magnetic stir-rer at atmospheric pressure. The stirrer speed was maintained at500 rpm. The product samples withdrawn at different time inter-vals were analyzed for product compositions by gas chromatograph(Shimadzu 2010), equipped with FID and HP-5, 30 m capillary col-umn. Authentic standard and GC–MS (Shimadzu QP 5500) was usedfor confirmation of products. The conversion of CAT is defined asmoles of consumed CAT to the moles of CAT used for the reaction.The selectivity to specific product is defined as moles of specificproduct divided by moles of consumed CAT.

3. Results and discussion

3.1. Physicochemical properties

Total surface acidity of different tungsten loaded catalyst sam-ples were shown in Table 1. It is evident from the data that there isan initial increase in the acidity and maximum acidity is observedfor the 15WZ containing catalysts and acidity decreases with fur-ther increase in WO3 loading. The generation of strong acidic natureis attributed to the crystal structure of ZrO2 phase, the type of tung-sten oxide species and the interaction between the surface tungstenoxide layer and ZrO2 support [30].

The XRD pattern of the catalysts with different WO3 loadingscalcined at 973 K along with 15WZ calcined at different tempera-tures is shown in Fig. 1. Highly crystalline materials were obtainedafter calcination at 973 K. It can be observed that even a small load-ing of tungsten exhibited mixtures of monoclinic (2� = 28.2◦, 31.5◦)

and tetragonal (2� = 30.3◦) ZrO2 [31]. The fraction of tetragonalzirconia becomes dominant with subsequent loading of tungsten,and under the present treatment conditions tetragonal phase wasclearly dominant for WO3 loadings of 15 wt.% (Fig. 1). Thus, the

24 A. Kumar et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 22– 29

Table 1Surface area, pore size and surface acidity of the catalyst with varying WO3 loadings.

Catalyst Calcination temp. (K) Measured WO3 (%) Surface area (m2 g−1) Pore size (A) Surface acidity (mequiv./g) Surface density (W-atoms/nm2)

ZrO2 0.0 23.02 163.8 0.281WZ 973 1.1 23.37 126.7 0.52 1.225WZ 973 5.2 53.24 81.8 1.11 2.5315WZ 973 15.8 67.66 62.8 2.3 6.0725WZ 973 26.3 44.31 87.8 1.14 15.42

at2iwo

1apitaptt

siptatpfbpzve

t

Fc

50WZ 973 52.5 26.91

15WZ 873 15.8 74.92

15WZ 1073 15.8 61.44

dded WO3 stabilizes the tetragonal phase of zirconia. X-ray diffrac-ion lines characteristic of monoclinic WO3 (2� = 23.2◦, 23.6◦ and4.5◦) are observed for sample containing above 15WZ loading. This

ndicates that WO3 is highly dispersed on the support for samplesith less than 15WZ loading and the crystalline WO3 peaks are

bserved when the WO3 loading exceeds the monolayer coverage.The XRD of the 15WZ catalyst calcined at 873 K, 973 K and

073 K shows that ZrO2 exists mainly in the tetragonal phase atll the calcination temperatures. There was no crystalline WO3eaks observed at calcination temperatures at and below 973 K,

ndicating that WO3 is highly dispersed on the ZrO2 support. Whenhe calcination temperature is increased beyond 973 K new peakst 2� of 23.2◦, 23.6◦ and 24.5◦ was observed. These characteristiceaks of WO3 indicate that WO3 crystallizes at temperatures higherhan 973 K reducing the dispersion of WO3 at higher calcinationemperature [32,33].

The BET surface area and BJH mean pore diameters of WOx/ZrO2amples are also included in Table 1. The results indicate that anncrease in the specific surface area for all the tungsten doped sam-les. Surface area becomes maximum at 15WZ (ca. 67 m2 g−1), andhereafter, further loading of WO3 leads to decrease in the surfacerea. This confirms that the surface area stabilizing effect is dueo strong interaction of WO3 species with ZrO2 crystallites. Theresence of monoclinic WO3 as indicated from the XRD patternor samples with higher concentration of tungsten species coulde correlated to decrease in the surface area due to sintering ofarticles. Further, it is also observed that pore size of un-promotedirconia is little wider compared to the W promoted samples. Thesealues are in good agreement with the results reported in the lit-

rature [31].

The surface density shown in Table 1 of W is calculated as perhe following formula:

20 25 30 35 40

Inte

nsi

ty

ZrO2

1WZ- 973 K

5WZ- 973 K

15WZ- 973 K

25WZ- 973 K

15WZ- 1073 K

15WZ- 873 K

50WZ- 973 K

ZrO

2 (M)

ZrO

2 (M)

ZrO

2 (M)

ZrO

2 (T) `W

O3

ig. 1. X-ray diffraction pattern for WO3–ZrO2 containing different WO3 loadingsalcined at 973 K, T and M indicate tetragonal and monoclinic phases, respectively.

81.2 0.62 50.6979.2 2.6 5.4895.8 3.0 6.69

Surface density of W

= [WO3 loading (wt.%)/100] × 6.023 × 1023

231.8 (formula weight of WO3) × BET surface area (m2 g−1) × 1018.

Surface density of W on the catalyst depends on both the WO3loading and the calcination temperature. When the calcinationtemperature is increased from 873 K to 973 K, the surface densityincreases from 5.48 to ∼6 W-atoms nm−2, which is equivalent tothe surface density of W in the monolayer [32]. Also, when the cal-cination temperature is further increased beyond 973 K to 1073 K,the surface density increased to 6.69 W-atoms nm−2, because WOx

species agglomerate into monoclinic WO3 crystallites and becomesless effective sintering inhibitors of underlying amorphous zirconiasupport. Our observations are similar to other reported literaturevalues [32–35].

3.2. Catalytic activity studies

The alkylation reaction of CAT with TBA is a typical exampleof Friedel–Craft acid-catalyzed electrophilic substitution reaction.Dehydroxylation of tert-butyl catechol by the acid sites of the cat-alyst leads to the formation of tert-butyl cation species. The butylcarbocation reacts mainly at the para-position of the CAT to form 4-TBC, trace amount of 4,6-di-tert-butyl catechol(DTBC) compoundsis produced by further alkylation of 4-TBC (Scheme 1).

3.2.1. Effect of WO3 loadingIn order to investigate the effect of WO3 loading on tert-

butylation of CAT, 0-50WZ catalysts calcined at 973 K are used foralkylation reaction at 413 K using equimolar ratio of TBA and CATfor 30 min. It is clear from Fig. 2, that pure ZrO2 showed the leastconversion of 4%, and among the catalysts with different WO3 load-ings, the conversion was found to increase with WO3 loading, andthe catalyst 15WZ gave the highest conversion of 99%, as the WO3loading is further increased the conversion of CAT decreased to 80%at 25WZ and finally at 50WZ it was found to be 63%. Interestingly,0WZ and 1WZ catalysts under the same reaction condition, not onlyshowed less conversion of CAT, but also produced more of DTBC.These observations are in good agreement with the characteriza-tion data and indicate that reaction is highly sensitive to acidity,surface area and pore size of the catalyst. 15WZ catalyst has thehigher acidity (2.3 mequiv./g), higher surface area (68 m2 g−1) andleast pore size (62 A) compared to other catalysts, and is found tobe most active in the tert-butylation of CAT with a 99% conver-sion of CAT and 99% selectivity towards 4-TBC. From Fig. 2, it canalso be observed that DTBC is formed up to 36% in pure ZrO2 and25% with 1WZ. The reason could mainly be attributed to the poresize, which is found to be almost double for ZrO2 (163 A) and 1WZ(127 A) compared to catalysts with WO3 loading of 5% (81.8 A) andabove (Table 1). Thus, it can be concluded that the formation of

di-alkylated product from the mono-alkylated product is favoredonly when the steric hindrance is less, i.e. the catalyst has largepores to allow the bulky alkylated products to travel out [36]. Atand beyond 5% tungsten loading, the pore size of all the catalysts

A. Kumar et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 22– 29 25

OH

OH

OH

+

OH

OH

OH

OH

+

Catechol tert-Butyl alcohol 4-tert-Butyl catechol 4,6-Di-tert-Butyl

tion o

ritttp

(taststTt

Fa

Scheme 1. tert-Butyla

educes drastically to below 82 A, hence, in spite of higher acidityn the catalysts with WO3 loading of more than 5%, the selectivityowards the di-alkylated product is significantly low (<3%) underhe same reaction conditions. These observations clearly indicatehat addition of WO3 has a significant role in promoting the catalyticerformance of ZrO2 for the tert-butylation of CAT with TBA.

Further for the catalyst 15WZ calcined at different temperatures873 K, 973 K and 1073 K), it was found that calcination tempera-ure also affects the catalytic performance. The conversion of CATt 873 K, 973 K and 1073 K was 92.9%, 99% and 97.2% and theelectivity towards 4-TBC was found to be 93%, 99% and 91%, respec-ively. The 15WZ (calcined at 973 K) catalyst with high specificurface area, good balance of acid strength and pore size distribu-

ion was found to be the most efficient for the synthesis of 4-TBC.herefore, the 15WZ catalyst was used to study the different reac-ion parameters.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

% C

on

ver

sion

& S

elec

tivit

y

% WO3 loading

Conversion 4-TBC Di-TBC

ig. 2. Effect of % WO3 loading on conversion of catechol and selectivity of productst 413 K 15% loading (w.r.t. catechol) catalyst for 30 min.

catechol

f CAT over WOx/ZrO2.

3.2.2. Effect of catalyst loadingTo investigate the optimum amount of catalyst required for the

reaction, reactions were carried out with the most active catalyst15WZ at 413 K for 30 min with 1:1 mole ratio of CAT:TBA. The cat-alyst loading was varied from 0% (without catalyst) to 20% (w/w)with respect to CAT. There was no reaction observed in the absenceof the catalyst (i.e. at 0 wt.%).

Fig. 3 suggests that as the catalyst loading is increased, conver-sion of CAT also increases. The conversion of CAT was found to be83.0% with 5 wt.% catalyst, which increased substantially to 94.0%with 10 wt.% and to about 99.0% with 15% catalyst, and furtherincrease to 20%, had only marginal effect on conversion of CAT.The conversion of CAT at 20% is about 99.5%. Hence, 15% loading

seems to be optimum under the prevailing reaction conditions. Inthe heterogeneous catalysis, the reactants are expected to react onthe surface of the catalyst. Hence, the conversion is expected toincrease with increase in catalyst loading as there is availability of

0

20

40

60

80

100

0 5 10 15 20

% C

on

ver

sion

& S

elec

tivit

y

% Catalyst Loading

conversion 4-TBC Di-TBC

Fig. 3. Effect of catalyst loading (w.r.t. catechol) on conversion of catechol andselectivity of products at 413 K 1:1 mole ratio of catechol:TBA for 30 min.

26 A. Kumar et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 22– 29

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3

% C

on

ver

sion

& S

elec

tivit

y

Moles of TBA:Catechol

conversion 4-TBC Di-TBC

Ft

mcc

omlhdc

3

uwTnCeifouapalmop

3

a

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180

% C

on

ver

sion

Time, min

353 K 363 K 373 K 383 K 393 K 413 K 433 K

ig. 4. Effect of moles of TBA (w.r.t. catechol) on conversion of catechol and selec-ivity of products at 413 K 15% loading (w.r.t. catechol) of 15WZ for 30 mins.

ore surfaces for the reactants. Also, this behavior suggests thatatalyst loading is an effective parameter which can influence theonversion if the catalyst loading is less than 15%.

The catalyst loading does not seem to have any significant effectn the selectivity of the products. This is because though we haveore active surfaces with increased catalyst loading, but we have

imited amount of TBA. The moles of TBA are same as that of CAT,ence it leads to selective formation of 4-TBC (mono product). Thei-alkylated product is less than 1% for all the catalyst loading asan be seen in Fig. 3.

.2.3. Effect of molar ratio of catechol:TBAThe effect of the molar ratios of the reactants has been studied

sing 15WZ as the catalyst and about 15% catalyst loading (w/w)ith respect to CAT, at 413 K for 30 min. The molar ratio of CAT to

BA was varied from 1:1 to 1:3. Fig. 4 shows that there is no sig-ificant effect of increasing the moles of TBA on the conversion ofAT. The conversion of CAT was in the range of 99.0–99.9%. How-ver, it can be seen from Fig. 4 that as the molar ratios (CAT/TBA < 1)s increased, the selectivity towards 4-TBC decreases considerablyrom 99.02% at 1:1 to 73% at 1:2 and finally to 65% at 1:3 mole ratiof CAT:TBA, with corresponding increase in the dialkylated prod-ct. The increase in dialkylated product could be attributed to thevailability of excess TBA. As there is more TBA in the reaction theossibility of unreacted TBA and 4-TBC (mono product) to collidet the active surface is higher, leading to formation of more dialky-ated products. Hence, it can be concluded that the increase in the

olar ratio (CAT/TBA < 1) of TBA does not have a significant effectn conversion but decreases the selectivity towards the desiredroduct 4-TBC.

.2.4. Effect of temperatureIncrease in temperature leads to an increase in rate constants

nd hence we have carried out the experiments at seven

Fig. 5. Effect of temperature on conversion of catechol at 1:1 mole ratio of cate-chol:TBA 15% loading (w.r.t. catechol) of 15WZ for 30 min.

different temperatures ranging from 353 K to 433 K, at equal molesof CAT:TBA using 15WZ catalyst loading. The samples were takenfor GC analysis at 15 min interval for first hour and after an hourit was taken at an interval of 30 min. Fig. 5 shows the conversionof CAT at different temperatures. It can be observed from figurethat the conversion of CAT at lower temperature (353 K) increasesfrom 37.2% at 15 min to 93.8% at 90 min and finally reaches 94.4%at 180 min. Thus, indicating that beyond 90 min the conversiondoes not change significantly. A very similar trend is also seenfor conversion of CAT at temperatures of 363 K, 373 K and 383 K.The conversion of CAT was observed to increases from 15 min to90 min from 55.6% to 95.8%, 74.9% to 96.2% and 84.8% to 96.6%for temperatures of 363 K, 373 K and 383 K respectively. When thetemperature is further increased to 393 K, the conversion of CATis seen to increases from 90.4% at 15 min to maximum of 97.7% at60 min and remains almost constant beyond 60 min, while at 413 K,the conversion at 15 min is 99% and it becomes constant beyond15 min. Further, at reaction temperature of 433 K, no significantincrease in TBA conversion was observed. The temperature doesnot seem to have any significant impact on the selectivity of 4-TBC,as all the CAT was mostly converted to 4-TBC. The di-alkylated by-product was formed at a very low level of 1–2% at all the studiedtemperatures.

It can be hence summarized that the optimum conditions for thesynthesis of 4-TBC is by using 15% loading of 15WZ at 413 K and 1:1mole ratio of CAT:TBA. Under these conditions within 30 min about99% conversion of CAT is observed with 99% selectivity towards4-TBC. The performance of WZ is compared with other reported

catalysts for synthesis of 4-TBC in Table 2. It is evident that catalyticperformance of WZ is better than the other catalyst in terms of bothconversion as well as selectivity.

A. Kumar et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 22– 29 27

Table 2Comparison between the activity and selectivity of catalysts under optimum conditions.

Catalyst % conversion of CAT % selectivity (4-TBC) Reaction conditions Reference

HY 86.0 393–473 K1:3 (CAT:TBA)

[3]

H-ZSM-5 90.0 84.0 443 K1:3 (CAT:TBA)

[4]

H� 87.0 99.0 1:4 (CAT:TBA) [5]Montmorillonite 83.3 80.6 410 K, 8 h

1:2 (CAT:TBA)[6]

SO3H functionalized Ionic liquid 41.5 97.0 423 K, 8 h [9]

99.0

3a

3aefiiht(rCCpdaT

(

(

(

WOx/ZrO2 (current study 15WZ) 99.0

.3. Kinetic model – determination of the rate equation forlkylation of CAT

Kinetic runs were conducted at four different temperatures53, 363, 373 and 383 K. The reaction products were sampledt 15, 30, 45, 60, 90, 120, 150 and 180 min in order to havenough data points for evaluating the kinetic parameters with con-dence. For the reaction studied at temperatures beyond 383 K,

t was found that the reaction reached completion within 45 min,ence the temperatures beyond 383 K were not used for estimatinghe kinetic parameters. Langmuir–Hinshelwood–Hougen–WatsonLHHW) approach was used to generate rate expression for theeaction scheme. The model considers the adsorption of TBA andAT on the active sites of the catalyst. On the catalytic surface,AT (A) combines with TBA (B) to form first the mono-alkylatedroducts (C) 4-TBC. These monoalkylated products further undergoialkylation to form dialkylated product (D), if more TBA is availablet the catalytic site to combine with the monoalkylated products.his model is represented by the following mechanism.

a) Adsorption of the reactants

A + Sk1�k′

1

AS (R1)

B + Sk2�k′

2

BS (R2)

b) Surface reaction(i) Formation of mono alkylated products (4-tert-butyl cate-

chol)

AS + BSk3�k′

3

CS + WS (R3)

(ii) Formation of dialkylated products

CS + BSk4�k′

4

DS + WS (R4)

c) Desorption of the products

CSk5�k′

5

C + S (R5)

DSk6�k′

6

D + S (R6)

WSk7�k′

7

W + S (R7)

1:2 (CAT:TBA) 413 K 30 min

1:1(CAT:TBA)

The rate for the surface reaction can be written as follows.The rate of alkylation (−rAS) is given as:

−rAS = k3CASCBS − k′3CCSCWS (1)

If reversible natures of all reactive steps are neglected, then Eq.(1) can be reduced to:

−rAS = k3CASCBS (2)

The adsorbed phase concentrations can be related to the bulkphase concentrations if adsorption is rapid as compared to chemicalreaction rates, then

CAS = K1CACS, CBS = K2CBCS, CCS = KCCCCS, CDS

= KDCDCS, CWS = KWCWCS (3)

Balancing the total no of catalytic sites, we get

CT = CS + CAS + CBS + CCS + CDS + CWS (4)

Substituting Eq. (3) in (4), we obtain

CT = CS(1 + K1CA + K2CB + KCCC + KDCDKWCW) (5)

Then one can get the vacant sites as:

CS = CT

1 + K1CA + K2CB + KCCC + KDCD + KWCW(6)

The adsorption of dialkylated compound is assumed to be lesseras compared to others species as it is the largest [37–39] i.e.

1 + K1CA + K2CB + KCCC + KWCW � KDCD (7)

Then

CS = CT

1 + K1CA + K2CB + KCCC + KWCW(8)

The rate of disappearance of CAT is reduced to

−rAS = k3K1CACSK2CBCS (9)

−rAS = k3C2T K1K2CACB

(1 + K1CA + K2CB + KCCC + KWCW)2(10)

Let k3C2T = Kr, then

−rAS = KrK1K2CACB

(1 + K1CA + K2CB + KCCC + KWCW)2(11)

At t = 0, CC, CD and CW = 0.Thus the initial rate for the disappearance of CAT is given as:

K K K C C

−rAS = r 1 2 A B

(1 + K1CA + K2CB)2(12)

The rate constant for the surface reaction should followan Arrhenius type temperature dependency. The temperature

28 A. Kumar et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 22– 29

Table 3Adsorption and kinetic constants for butylation of CAT.

Temperature (K) Kr (kg/(kg mmol min)) K1 (dm3/g) K2 (dm3/g) KC (dm3/g) Kw (dm3/g)

353 6.508 2.456 2.123 0.890 0.968363 10.045 2.019 1.490 0.806 0.806

1.184 0.725 0.7351.033 0.683 0.6597

de

K

K

Ltttmmct

−

Tat

toutiTdactt1

0

0.5

1

1.5

2

2.5

3

2.55 2.6 2.65 2.7 2.75 2.8 2.85

Ln

(Kr)

1/T x 103

Fig. 7. Arrhenius plot.

Table 4Activation and pre-exponential factors.

Ea (kJ/mol) �Had (kJ/mol) Pre-exponential factor

Butylation of CAT 33.53 6.27 × 105

Adsorption (CAT) −14.23 1.88 × 10−2

Adsorption (TBA) −26.99 2.05 × 10−4

373 12.841 1.873

383 16.155 1.653

ependence of the kinetic and adsorption parameters can bexpressed mathematically as:

= K0 exp(−Ea

RT

)(13)

1 = K01 exp

(−�Had

RT

)(14)

The parameters Kr, K1 and K2 were determined by fitting theHHW models with experimental initial rate data in Eq. (12) andhe remaining parameters KC, KD and KW were determined by fittinghe LHHW model with experimental rate data over the entire reac-ion time. The optimum values of these parameters are obtained by

inimizing the square of relative difference between the experi-ental rates and the theoretical rates. The experimental rate was

alculated from the concentration profiles of CAT with respect toime as follows:

r = 1w

(dCA

dt

)(15)

he experimental rate calculated by the above formula is plottedgainst the predicted rate in Fig. 6 and it shows that the experimen-al rate matches well with predicted rate.

Table 3 describes the values of adsorption constants for the reac-ants and the products. It can be seen that the adsorption constantf the reactants is comparatively higher as compared to the prod-ct, indicating that the reactants would have more affinity to comeo the surface and form the products. Once the product is formed,t would be desorbed more easily. Also, it can be observed fromable 3 that as the temperature increases the adsorption constantecreases and hence it indicates that adsorption of the reactantsnd products on the catalysts is an exothermic process. The rate

onstant for the alkylation reaction increases with increase in theemperature, the rate constant increases by about 1.5 times whenemperature is increased from 353 K to 363 K further increase by0 K increases the rate constant by 1.2 times. The rate constant

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5 3 3.5 4

r (p

red

icte

d)

r (experimental)

353 K 363 K 373 K 383 K

Fig. 6. Comparison of experimental versus predicted rates.

Adsorption (4-TBC) −10.13 2.81 × 10−2

Adsorption (water) −14.03 7.98 × 10−3

for the reaction increases by further 1.2 when the temperatureis increased from 373 K to 383 K. The Arrhenius plot shown inFig. 7 is used to calculate the energy of activation for the reactionand is found to be 33.53 kJ/mol. The activation energy and pre-exponential factors are shown in Table 4. It can be seen from thetable that the pre-exponential factor for the reaction is higher thanthe adsorption indicating, that the adsorbed reactants can easilyundergo surface reaction leading to products.

4. Conclusion

Thus, we conclude that WOx/ZrO2 is a highly active and selec-tive catalyst for the alkylation of CAT with TBA to produce 4-TBCas compared to conventional zeolites, functionalized ionic liquidsor clay-based porus montmorillonite heterostructures (PMH) solidacid catalysts that are reported in the literature so far. The 15WZcatalyst, with very high specific surface area and good balance ofacid strength and pore size distribution was found to be the mostefficient with conversion and selectivity of 99% for the synthesisof 4-TBC. The LHHW surface reaction controlled kinetic model wasdeveloped based on the product distribution. The model was foundto fit reasonably well with the experimental data. From the esti-mated adsorption and kinetic parameters, the energy of activationwas evaluated and found to be 33.5 kJ/mol.

Acknowledgements

The authors would like to acknowledge profusely to Dr. JothiMahalingam and Dr. Shantaram Naik for their fruitful suggestionsand discussions rendered towards the preparation of this article.

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A. Kumar et al. / Journal of Molecula

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