Friedel-Crafts Alkylation of Xylenes With Tert-Butanol Over Me So Porous Superacid UDCaT-5

8/3/2019 Friedel-Crafts Alkylation of Xylenes With Tert-Butanol Over Me So Porous Superacid UDCaT-5

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Friedel-Crafts alkylation of xylenes with tert-

butanol over mesoporous superacid UDCaT-5

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Abstract

Friedel-Crafts alkylation of xylenes with tert-butanol in the presence of novel

mesoporous superacidic catalysts named as UDCaT-4, UDCaT-5 and UDCaT-6 wasinvestigated. The catalysts are modified versions of zirconia showing high catalytic

activity, stability and reusability. The catalytic activity is in the order: UDCaT-5 (mostactive) > UDCaT-6 > UDCaT-4 > sulfated zirconia (least active). Synergistic effect ofvery high sulfur content present (9% w/w S) and preservation of tetragonal phase in

UDCaT-5, in comparison with sulfated zirconia (4% w/w S), were responsible for higher

catalytic activity. The performance of UDCaT-5 in alkylation of xylenes was studied withtert-butanol with reference to selectivity and stability. Alkylation of m-xylene overUDCaT-5 gives 96% conversion of tert-butanol with 82% selectivity towards 5-tert-

butyl-m-xylene (5-TBMX) under optimum reaction conditions. The formation of productsis correlated with the acidity of the catalyst. The reactions were conducted in liquid phaseat relatively low reaction temperatures (130160 C). A systematic investigation of theeffects of various operating parameters was done to describe the reaction pathway. Thereaction was carried out without any solvent in order to make the process cleaner andgreener. An overall second order kinetic equation was used to fit the experimental data,under the assumption that both xylene and tert-butanol are weakly adsorbed. Anindependent dehydration study oftert-butanol (TBA) was also done.

Keywords: Friedel-Crafts alkylation; tert-Butyl xylenes; Mesoporous superacidiccatalysts UDCaT-5; Sulfated zirconia; Green chemistry.

IntroductionAlkylation processes normally require FriedelCrafts acid catalysts such as AlCl3,

BF3, TiCl4, liquid HF, and AlCl3 with elemental iodine.1,2 Several problems are associatedwith these catalysts such as toxicity, corrosiveness, low reaction selectivity, and disposalof effluents.35 Relatively high concentration of catalyst is needed; often the amounts aremore than stoichiometric making the reactions inherently polluting.6 Due to everincreasing societal, environmental, and economic pressure, efforts are devoted to thedevelopment of environmentally friendly catalysts for the production of industriallyimportant chemicals and intermediates.79

Alkylation of xylenes with tert-butanol (TBA) is an important reaction both inorganic synthesis and chemical manufacturing. Some dimethyl alkylbenzenes have

assumed practical significance. In particular, 1,2-dimethyl-4-tert-butylbenzene or 4-tert-butyl-o-xylene (4-TBOX) was proposed as the starting substance for the production of

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novel phthalocyanine pigments, plasticizers, photographic materials and other valuableproducts.1013 tert-Butylated xylenes are usually manufactured by reacting xylenes in the presence of liquid acid catalysts, with pure isobutylene or C4 fraction from naphthacrackers containing isobutylene, giving wide product distribution. These processes sufferfrom problems associated with the use of highly corrosive liquid acids and also the source

of isobutylene. The development of a technologically efficient, highly productive andenvironmentally safe method for the synthesis of dimethyl-tert-butylbenzene (DMTBB)is challenging. There is a tremendous scope for devising a new catalytic process for thesynthesis of tert-butylated xylenes to replace conventional homogenously catalyzed,highly polluting processes. Besides, due to the problems associated with unavailability,transportation and handling of isobutylene, particularly for usage in low-tonnage fine andspeciality chemical industry (typically 10-100 TPA production), it is advantageous togenerate isobutylene in situ. Dehydration of tert-butanol is an attractive source for thesame. Further, tert-butanol is available as a by-product in the ARCO process for

propylene oxide which could be used effectively for this purpose. We have successfullycarried out tert-butylation of several aromatic compounds by using tert-butanol, methyl-tert-butyl ether (MTBE) and isobutene as alkylating agents using ecofriendly solid acidcatalysts.1417 The only problem which needs to be considered was activity and stability ofsolid acid catalysts in the presence of water, which is evolved during the reaction.

For the past few years, our laboratory is engaged vociferously in the synthesis,characterization and application of selective, ecofriendly and active catalysts such asUDCaT series catalysts, sulfated zirconia, heteropolyacids and their modified versionssupported on clays. In particular, sulfated zirconia has been extensively studied in anumber of reactions1820 and it should be modified to bring in shape selectivity,mesoporosity and better acidity. Recently we prepared novel mesoporous superacidsnamed as UDCaT-4, UDCaT-5 and UDCaT-6, which are modified versions of zirconiaand have found a great potential for industrially important reactions.2123 We havereported, for the first time, that a sulfated zirconia, with sulfur content as high as 9% w/w,was produced with preservation of tetragonal phase by using chlorosulfonic acid as a newsource for sulfate ion. It was designated as UDCaT-5. The acronym UDCaT symbolizesour research institute, formerly called University Department of Chemical Technology(UDCT), which is now renamed as Institute ofChemical Technology (ICT). The worksummarizes the investigation of activity and selectivity of these new breed of catalyticmaterials in alkylation of xylenes with tert-butanol, including reaction kinetics. Thecatalyst can be used as the basis for a high performance and environmentally safe methodfor the synthesis of DMTBB. The model studies were done with m-xylene and thenextended to other xylene isomers.

Experimental Section

Chemicals and Catalysts

Pure xylene isomers, tert-butanol, zirconium oxychloride, aluminum nitrate,ammonium persulfate and aqueous ammonia solution were procured from Aldrich, USA.Hexadecyl amine and chlorosulfonic acid was purchased from Spectrochem Ltd.Mumbai, India. Tetraethyl orthosilicate (TEOS) was procured from Fluka, Germany. Allchemicals were of analytical reagent (A.R.) grade. These were used as received withoutany further purification.

Preparation of Catalysts

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The following catalysts were prepared by well-developed procedures andcharacterized in our laboratory: (i) sulfated zirconia (S-ZrO2), (ii) UDCaT-4, (iii)UDCaT-5, and (iv) UDCaT-6.

Preparation of Sulfated Zirconia (S-ZrO2)

Sulfated zirconia (S-ZrO2) was prepared by adding aqueous ammonia solution tozirconium oxychloride solution at a pH of 10, as detailed elsewhere.20 The precipitate wasthoroughly washed with distilled water and made free from ammonia and chloride ions. Itwas dried in an oven at 120 C for 24 h. The sulfation of the zirconia was done using 15cm3 g-1 of 0.5 M sulfuric acid. It was dried at 110 C and calcined at 650 C for 3 h.

Preparation of UDCaT-4

The ordered hexagonal mesoporous silica (HMS) was prepared according to ourearlier work.24 Desired quantities of zirconium oxychloride and aluminum nitrate weredissolved in aqueous solution and added to precalcined HMS by incipient wetnesstechnique. After addition the solid was dried in an oven at 110 C for 3 h. The dried

material was hydrolyzed by ammonia gas and washed with deionized water until a neutralfiltrate was obtained and the absence of chlorine ion in the filtrate was detected by

phenolphthalein indicator and silver nitrate tests. It was then dried in an oven for 24 h at110 C. Persulfation was carried out by immersing the above solid material in to 0.5 Maqueous solution of ammonium persulfate for 30 min. It was dried at 110 C for 24 h andcalcined at 650 C for 3 h to get active catalyst called UDCaT-4 with 0.6% w/w ofalumina.21


UDCaT-5 was prepared by adding aqueous ammonia solution to zirconiumoxychloride (ZrOCl2.8H2O) solution at pH of 9-10. The precipitated zirconium hydroxideso obtained was washed with deionized water until a neutral filtrate was obtained. Theabsence of chlorine ion was detected by the AgNO3 test. A material balance on chlorideions before and after precipitation and washing shows no retention of Cl on the solid.Zirconium hydroxide (Zr(OH)4) was dried in an oven for 24 h at 100 C and was crushedto 100 mesh size. Zr(OH)4 then immersed in 15 cm3 g-1 of 0.5 M solution ofchlorosulfonic acid and ethylene dichloride. All materials were immersed for 5 min in thesolution and then without allowing moisture absorption were kept in an oven and theheating was started slowly to 120 C after about 30 min. These materials were kept inoven at 120 C for 24 h and calcined at 650 C for 3 h to get the active catalysts UDCaT-5.22


UDCaT-6 was prepared by adding an aqueous solution of 2.5 g zirconiumoxychloride to 5 g precalcined HMS by incipient wetness technique and it was dried in anoven at 120 C for 3 h. The dried material was hydrolyzed by ammonia gas and washedwith distilled water until no chloride ions were detected which was confirmed by theAgNO3 test. It was further dried in an oven for 2 h at 120 C. Zr(OH)4/HMS wasimmersed in 15 cm3 g-1 of 0.5 M chlorosulfonic acid in ethylene dichloride. It was soakedfor 5 min in the solution and then without allowing moisture absorption, it was oven driedto evaporate the solvent at 120 C after about 30 min. The sample was kept in the oven at120 C for further 24 h and calcined thereafter at 650 C for 3 h to get the active catalyst

UDCaT-6.23

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Characterization of Catalysts

UDCaT-4,21 UDCaT-5,22 UDCaT-6,23 and sulfated zirconia20 were completelycharacterized by ammonia-temperature programmed desorption (NH3-TPD), X-ray

powder diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area and Fouriertransform infrared (FTIR) and the details were published recently by us. Only a few

salient features which thought be important are reported here.

Characterization of UDCaT-4The XRD, BET surface area and pore size analyses provided an explanation for

the entrapment of nanoparticles of persulfated alumina zirconia (PAZ) (


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converting it into UDCaT-6. Furthermore, the pore volume of UDCaT-6 (0.7 cm 3 g-1) ismuch less than that of pure HMS (1.2 cm3 g-1) indicating that large amount of crystallinenano-particles of zirconia must be present inside the pores of UDCaT-6. The sulfur Ka 1and zirconium La1 distribution spectra determined by EDAX analysis shows theincorporation and homogeneous distribution of zirconia and sulfur atoms in UDCaT-6.

The SEM of UDCaT-6 revealed that similar to the morphology of HMS, UDCaT-6 ismade up of sub-micrometer sized free standing or aggregated sphere shaped particle andthat active centers of zirconia are successfully embedded in HMS and the structuralintegrity of HMS is unaltered even after it is converted to UDCaT-6. 23

Apparatus and Procedures

The reactions were carried out in a 100 cm3 capacity Parr autoclave reactor withan internal diameter of 5 cm, equipped with four bladed pitched turbine impeller. Thetemperature was maintained at 1 C of the desired value with the help of an in-built

proportional integral differential (PID) controller. Specific quantities of desired reactantsand catalyst were charged into the reactor and the temperature was raised to the desired

value. Then, an initial sample was withdrawn and agitation started. Further samples werewithdrawn at periodic time intervals up to 2 h to monitor the reaction. All catalysts weredried in an oven at 120 C for 1 h before use.

In a typical reaction, 0.342 mol xylene isomer was reacted with 0.0855 mol tert-butanol (TBA) (4:1 mole ratio of xylene to TBA) with 2 g of catalyst; this makes thecatalyst loading as 0.04 g cm-3 of liquid phase. The total volume of the reaction mixturewas 50 cm3. The reaction was carried out at 150 C at a speed of agitation of 1000 rpmunder autogenous pressure. The reaction was carried out without any solvent. Isobutyleneformed in situ was not allowed to escape from the reaction vessel.

Method of Analysis

Clear liquid samples were withdrawn at regular time intervals by reducing thespeed of agitation momentarily to zero and allowing the catalyst to settle at the bottom ofthe reactor. Analysis of the samples or compounds were performed by GasChromatograph (Chemito, India: Model 8610 GC) equipped with a 10% SE-30 (liquidstationary phase) stainless steel column (3.175 mm diameter 4 m length) with flameionization detector (FID). Products were isolated and confirmed through gaschromatography-mass spectrometry (GC-MS) and their physical properties and retentiontimes were recorded and compared with authentic samples. Calibrations were done withauthentic samples for quantification of data. The conversions were based on thedisappearance of tert-butanol (TBA), which is the limiting reactant in the reaction

mixture.

Results and Discussion

Choice of Catalysts

The activity and stability of UDCaT-4, UDCaT-5 and UDCaT-6 catalysts weretested by carrying out separate reactions which produced HCl or water as the co-products.Thus, benzylation of toluene with benzyl chloride and esterification ofp-cyclohexanolwith acetic acid, where HCl and water are by-products, respectively, were studiedsystematically, whose details are given elsewhere.2025 The catalysts showed higheractivity and good reusability as compared to sulfated zirconia. It indicated that thecatalysts were stable in presence of HCl and water. Moreover all catalysts were reusable

in all these reactions, without loss of activity. So it was thought desirable to investigatethe efficacy of these catalysts in the liquid phase tert-butylation of xylene isomers with

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TBA, where water is one of the co-products. Effects of various operating parameters suchas effect of speed of agitation, catalyst loading, mole ratio of xylene to TBA and reactiontemperature on rates and product distribution are discussed with m-xylene as the modelisomer and then extended to other xylene isomers to deduce the kinetics of the reaction.

Efficacies of Various CatalystsThe activities of UDCaT-4, UDCaT-5, UDCaT-6 and sulfated zirconia were

evaluated at 150 C. It was found that UDCaT-5 showed higher conversion of limitingreactant, TBA (96%) as compared to the other solid acid catalysts with selectivity of 82%towards the desired product, 5-tert-butyl-m-xylene (5-TBMX). The order of activity was:UDCaT-5 (most active) > UDCaT-6 > UDCaT-4 > sulfated zirconia (S-ZrO2) (leastactive) (Figure 1). All other catalysts contain less number of acidic sites as compared toUDCaT-5 and thus the results are in this order.

The purpose of using several different solid acid catalysts was to study the effectof nature, strength and distribution of acidity, pore size distribution and stability of thecatalyst on conversion and selectivity in a complex network of reactions involving

dehydration, etherification and alkylation. Since the substrate is bulky and involvesgeneration of water, it is essential that sulfated zirconia which gets deactivated ismodified. Besides, our group was the first one to report the highest superacidity in solidsuperacids. This has been already published by us.22 The catalyst properties, selectivity of5-TBMX and final conversion of TBA are given in Table 1, which clearly indicated thatUDCaT-5 is the most active and selective catalyst in terms of conversion and selectivity.Hence further experiments were conducted with UDCaT-5 due to its excellent activity,reusability and stability. The observed concentration profile of reactants and several

products for this reaction at 150 C is depicted in Figure 2, which clearly shows that theselectivity of 82% for 5-tert-butyl-m-xylene was achieved.

Effect of Speed of Agitation

To ascertain the influence of external resistance to mass transfer of the reactants tothe catalyst surface, the speed of agitation was varied over the range of 800 to 1200 rpmunder otherwise similar conditions (Figure 3). At all speeds, solid catalyst particles aresupposed to be completely in suspension. It was observed that the final conversion ofTBA was practically the same in all these cases and thus there was no limitation ofexternal mass transfer of reactants from bulk liquid phase to the outer surface of thecatalyst. Also there was no significant effect on the selectivity of the desired product.Theoretical analyses were also done to establish that there was no effect of external masstransfer limitations as delineated in our earlier work.18,25 Hence, all further reactions were

carried out at 1000 rpm.

Effect of Catalyst Loading

The catalyst loading was varied over a range of 0.010.05 g cm-3 on the basis oftotal volume of reaction mixture under otherwise similar conditions. Figure 4 shows theeffect of catalyst loading on the conversion of TBA. The initial rates of reaction, in theabsence of external mass transfer resistance, are plotted against catalyst loading in Figure5. It demonstrates that the rates are directly proportional to the catalyst loading based onthe entire liquid phase volume. The conversion of TBA increased with an increase incatalyst loading (Figure 4), which is obviously due to the proportional increase in thenumber of active sites. The final conversion of TBA was practically the same in case of

0.04 g cm-3

and 0.05 g cm-3

loading. This suggests that the number of sites available were

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more than the number of molecules. Hence all further experiments were conducted byusing 0.04 g cm-3 loading and at this loading, the intraparticle diffusion resistance sets in.

Proof of Absence of Intraparticle Resistance

Because the average particle size of UDCaT-5 was found to be in the range of 40

50 m and the catalyst is amorphous in nature, it was not possible to study the effect ofcatalyst particle size on the rate of reaction. The average particle diameter of UDCaT-5used in the reactions was 0.0045 cm, and thus a theoretical calculation was done based onthe Weisz-Prater criterion to assess the influence of intraparticle diffusion resistance.According to the Weisz-Prater criterion,26,27 the dimensionless parameter CWP, whichrepresents the ratio of the intrinsic reaction rate to the intraparticle diffusion rate, can beevaluated from the observed rate of reaction (-robs = 3.3410-5 mol gcat-1 s-1), density ofcatalyst particle ( p = 1.255 g cm-3), the particle radius (Rp = 2.2510-3 cm), the effectivediffusivity of the limiting reactant (De = 4.066 10-6 cm2 s-1), and the concentration of thereactant at the external surface of the particle ([As] = 1.7110-3 mol cm-3). Thedimensionless parameterCWP is given by:

][

2

se

ppobs

WPAD

RrC

= (1)

Here, (i) ifCWP >> 1, then the reaction is limited by severe internal diffusion resistance,and (ii) ifCWP


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Effect of Temperature

Intrinsically kinetically controlled reactions show significant increase in theconversion profile with temperature. Since almost all mass transfer limitations wereeliminated, the effect of temperature was studied on two reaction steps.

Dehydration of TBA

CH3 CH3

OH

CH3

2UDCaT- 5

- H2O

TBA

O

CH3

CH3 CH3

CH3

CH3 CH32

CH3 CH2

CH3

DTBE Isobutylene

UDCaT- 5

- H2O

Scheme 1. Dehydration of TBA

An independent dehydration study of TBA (Scheme 1) was studied in thetemperature range of 130160 C (Figure 7) to ensure no coke formation on the catalyst.Isobutylene and di-tert-butyl ether (DTBE) were the products formed in this reaction.DTBE was initially formed and then cracked to isobutylene. This can be explained as,tert-butyl group is bulky and hence ether which was formed is unstable and breaks intoisobutylene. The rate of dehydration increased with increase in temperature. Becauseisobutylene is difficult to sample and quantify, the concentrations of TBA and DTBEwere first quantified by gas chromatography (GC) and then a mass balance wasestablished to calculate the concentration of isobutylene.

Alkylation of m-Xylene with TBACH3

CH3

+

CH3

CH

3

CH3

CH3

CH3

UDCaT-5

150C

5-TBMXm-Xylene

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

2,5-DTBMX

CH3 CH3

OH

CH

3

TBA

UDCaT-5

150C

Scheme 2. Alkylation ofm-xylene with TBA

The alkylation of m-xylene with TBA (Scheme 2) is highly temperaturedependent. The temperature effect was studied at temperatures from 130160 C (Figure8). With an increase in temperature, both the rate of reaction as well as selectivity formonoalkylated product increased. The final conversion of TBA was increased from 74%at 130 C to 96% at 150 C and the selectivity for 5-tert-butyl-m-xylene (5-TBMX) was82% at 150 C after 2 h. There is no significant difference in the final conversion of TBA

between 150 C and 160 C. Therefore, 150 C is the optimum temperature for thisreaction. Spectroscopy evidences proved that UDCaT-5 contains three types of acidicsites, namely, intermediate, strong and very strong. Strong acidic sites present in UDCaT-5 are in more number and hence monoalkylation was dominant. The formation ofdialkylated product i.e. 2,5-di-tert-butyl-m-xylene (2,5-DTBMX) increases with anincrease in temperature.

Alkylation of Xylene Isomers with TBA

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CH3

CH3

+

CH3

CH3

CH3

CH3

CH3

UDCaT-5

150C

2-TBPXp-Xylene

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

2,5-DTBPX

CH3 CH3

OH

CH3

TBA

UDCaT-5

150C

Scheme 3. Alkylation ofp-xylene with TBACH3

CH3

+

CH3

CH3

CH3 CH3

CH3

UDCaT-5

150C

4-TBOXo-Xylene

CH3

CH3

CH3 CH3CH3

CH3

CH3

CH3

4,6-DTBOX

CH3 CH3

OH

CH3

TBA

UDCaT-5

150C

Scheme 4. Alkylation ofo-xylene with TBA

Alkylation ofp-xylene (Scheme 3) and o-xylene (Scheme 4) was also carried outusing TBA as an alkylating agent over UDCaT-5 as a catalyst under otherwise similaroperating conditions as were used for m-xylene. The observed concentration profile ofreactants and several products for these reactions at 150 C are depicted in Figure 9 & 10.These both the reactions gives maximum conversion of 96-97% of the limiting reactant,TBA with 80-82% selectivity towards the desired product; 2-tert-butyl-p-xylene (2-TBPX) in case ofp-xylene with TBA and 4-tert-butyl-o-xylene (4-TBOX) in case ofo-xylene with TBA reaction. It was also observed that the alkylation ofo-xylene was fasterrelative to the alkylation ofp-xylene and m-xylene due to the fact that o-xylene is a more

active species than other xylene isomers (Table 2).Reaction Kinetics

We have recently reported the alkylation of several substituted aromatics usinglinear and branched alcohols, and ethers using a variety of solid acid catalysts, wherein afree olefin and water are generated in situ as a co-product, along with the C or/and Oalkylated products.25,29,30 In condensation reaction of two alcohols it was found that thesymmetrical or asymmetrical ethers are formed (e.g. methyl-tert-butyl ether frommethanol and tert-butanol) in which sulfated zirconia, ion exchange resin, 20% w/wCs2.5H0.5PW12O40/K-10 clay i.e. 20% w/w Cs-DTP/K-10 clay were employed as catalysts.In some cases there was formation of alkenes along with C or/and O alkylated

products; whereas isopropylation using isopropanol gave diisopropyl ether (DIPE) andthis ether further act as an alkylating agent. The rate of alkylation was higher by usingether than alcohol.30 Thus, the alkylation with alcohols does not follow a simple reaction

pathway and our current rate data using a new catalyst needed testing of differenthypotheses.

Several models were tried to fit the experimental data which were collected in theabsence of any external mass transfer and intraparticle diffusion limitations. The resultwas expected to obey a first order kinetics for weakly adsorbed species, but contrary tothis, the model followed second order kinetics. Further, it was found that the alkylation ofm-xylene also followed a second order reaction. The analysis of earlier published reportson the dehydration of ethanol and isopropanol31,32 suggests that there is a production of

ether at lower temperature and also two types of sites should be involved in the reaction.Solid superacids have both Lewis and Bronsted sites and thus the mechanism involves

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bifunctional sites S1 and S2. These two species participate in the reaction. Thusmechanistic models were needed to describe TBA dehydration to isobutylene and waterand alkylation of m-xylene with TBA. These two cases are considered here: (i)dehydration of TBA, and (ii) alkylation ofm-xylene with TBA.

Dehydration of TBAIn this case, a model based on two catalytic sites was proposed according to which

TBA (A) gets adsorbed on to two different sites S1 and S2. These two adsorbed speciesparticipate in the reaction. Assuming that the rate determining step is the reaction of AS 1and AS2 to form di-tert-butyl ether and water as the surface complexes (ES1) and (WS2)respectively and ES1 being unstable subsequently decomposes instantly into isobutylene(P) in the gas phase as shown below:

11

1 ASSA AK

+ (2)

22

2 ASSA AK

+ (3)

1

1 2 1 2

SRk

AS AS ES WS + + (4)

2

1 12SRk ES P WS + (5)

The site balance in this case is

11111 SWSESASVSTCCCCC

+++= (6)

2222 SWSASVSTCCCC

++= (7)

The following adsorption equilibria for different species hold

11

1 WSSW WK

+ (8)

22

2 WSSW WK

+ (9)

11

1 ESSE EK

+ (10)

Thus the rate of formation of isobutylene, -rP'(mol gcat-1 s-1) is:

( ) ( )1 1 21 2

1 1 1 2 2

'1 1

SR A A A A T S T S

P

A A W W E E A A W W

k K C K C C C r

K C K C K C K C K C

=

+ + + + +(11)

When the adsorption of all species are very weak, equation (11) is reduced to2

'AP

kwCr = (12)

Where,211 21 STSTAASR

CCKKkk

= (13)

Writing in terms of conversion, and further integration results into the following equation:

tkwCX

XA

A

A

01=

(14)

Thus a plot of A

A

X

X

1 against t (Figure 11) was made to get an excellent fitthereby supporting the model. This is an overall second order reaction for weakadsorption of TBA (A).

Alkylation of m-Xylene with TBAVarious models were tried, including typical first order kinetics (weak adsorption

of TBA and strong adsorption of m-xylene) and overall second order kinetics (weakadsorption of TBA and m-xylene). The overall second order dehydration model was takenas a basis. As is validated above, TBA dehydration follows second order kinetics byadsorption of TBA on two adjacent sites S1 and S2 and the product di-tert-butyl ether (E)

is formed, which is decomposed instantaneously to isobutylene (P). Thus in thetemperature range studied, the rate of alkylation is not controlled by the dehydration rate,

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but the alkylation ofm-xylene adsorbed on site S2 with TBA adsorbed on adjacent site S1,to give the monoalkylated desired product (D), which is formed due to the surfacereaction as shown below:

11

1 ASSA AK

+ (15)

22

2 BSSB BK

+ (16)

12122 WSDSASBS

SRk ++ (17)Analogously, the site balance can be written to obtain:

( )( )DDBBEEWWAA

STSTBBAASR

A

CKCKCKCKCK

CCCKCKkr

+++++

=

22111

21

11' 212 (18)

With weak adsorption of all species, equation (18) is reduced to

BASRACwCkr = ' (19)

Where,212 21 STSTBASRSR

CCKKkk

= (20)

Writing in terms of conversion, and further integration results into the following equation:

( ) ( )tMwCk

XM

XM

ASR

A

A

11ln 0 =

(21)

Thus, a plot of( )

A

A

XM

XM

1ln against t is shown in Figures 1214 in the

alkylation reaction of m-xylene, p-xylene and o-xylene respectively with TBA as analkylating agent. It is seen that the data fit very well, thereby supporting the model. Thisis an overall second order reaction for weak adsorption of TBA (A) and xylenes (B). Thevalues of rate constants (k or kSR) at different temperatures were calculated and anArrhenius plots (Figure 15) was used to estimate the frequency factor (k0) and apparentactivation energy (E) of each reaction and is tabulated in Table 2. The values of activation

energy also supported the fact that the overall rate of reaction is not influenced by eitherexternal mass transfer or intraparticle diffusion resistance and it is an intrinsicallykinetically controlled reaction on active sites of the catalyst. The values of rate constantsalso shows that o-xylene is most active isomer thanp-xylene and m-xylene (Table 2) andtheir activity is in the ordero-xylene (most active) >p-xylene > m-xylene (least active).

Reusability of Catalyst

Reusability of UDCaT-5 was verified by conducting three runs. After each run thecatalyst was filtered, and then refluxed with 50 cm3 TBA for 30 min in order to removeany adsorbed material from catalyst surface and pores. The catalyst was then dried at 120C for 2 h and weighed before using in the next batch. There was some attrition of

catalyst particle during agitation. In a typical batch reaction, there were inevitably lossesof particles during filtration due to attrition. Although the catalyst was washed afterfiltration to remove all adsorbed reactants and products, there was still a possibility ofretention of small amount of adsorbed reactants and products species which might causethe blockage of active sites of the catalyst. These are apparent factors for the loss inactivity, which have been taken into account because no fresh catalyst was added into thereaction mixture to maintain the same catalyst loading during reusability tests. The actualamount of catalyst used in the next batch, was almost 5% less than the previous batch. Itwas observed that there is only a marginal decrease in conversion (Figure 16); even therewas no effect on selectivity of the products. When make-up quantity of the catalyst wasadded, almost similar conversions were found to suggest that the catalyst is stable.

Conclusion

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The liquid phase alkylation of xylenes with TBA as an alkylating agent wasexamined by using various modified version of zirconia named as UDCaT-4, UDCaT-5and UDCaT-6. The UDCaT-5 catalyst was found to be the best catalyst in terms ofactivity and selectivity, which lead to 96% conversion of the limiting reactant, TBA, with82% selectivity toward the desired product, dimethyl-tert-butylbenzene. No

oligomerisaton of isobutylene was formed at standard reaction conditions andmonoalkylated products were obtained exclusively. The rate of reaction increased withtemperature, catalyst loading and xylene to TBA mole ratio. The reaction should becarried out at 150 C with catalyst loading 0.04 g cm -3 with mole ratio of xylene:TBA, 4:1to obtain dimethyl-tert-butylbenzene with maximum yield. We believe that this is thenovel method to synthesize dimethyl-tert-butylbenzene or in other words tert-butylatedxylenes at such a low temperature and with high activity and selectivity usingmesoporous superacidic UDCaT-5 catalyst. Comprehensive mathematical models weredeveloped and validated with experimental results. The overall second order kineticequation fits the data very well and the activation energy was calculated for all tert-

butylation reactions including dehydration of TBA. The values of activation energy also

supported the fact that the overall rate of reaction is not influenced by either externalmass transfer or intraparticle diffusion resistance; it is an intrinsically kineticallycontrolled reaction on active sites of the catalyst. The reaction is solvent free which could

be advantageous as a green process and the conditions are optimized in a way to getdimethyl-tert-butylbenzene selectively at low temperatures. The present study also showsthat UDCaT-5 has a potential as a solid superacid in a number of reactions.

Acknowledgement

Professor G. D. Yadav acknowledges receipt of a research grant and chair fromDarbari Seth Professorship Endowment. Dr. S. B. Kamble acknowledges receipt ofSenior Research Fellowship (SRF) from University Grants Commission (UGC),Government of India, New Delhi. We also wish to thank anonymous reviewer and editorfor their critical and useful comments which refined the manuscript a lot.

Nomenclature

A limiting reactant species A, tert-butanolB excess reactant species B, m-xylene or xylene isomersD monoalkylated desired product, dimethyl-tert-butylbenzeneP isobutyleneE di-tert-butyl etherW water

M mole ratio of xylene to tert-butanolCi concentration of species i, mol cm-3

CA concentration of A, mol cm-3

CB concentration of B, mol cm-3

CV concentration of vacant sites of catalyst, mol cm-3

CT concentration of total sites of catalyst, mol cm-3

CA0 initial concentration of A at solid catalyst surface, mol cm -3

K1 surface reaction equilibrium constant, k1/k1'k1 surface reaction rate constant for forward reactionk1' surface reaction rate constant for reverse reaction

KA adsorption equilibrium constant for A, cm3 mol-1

KB adsorption equilibrium constant for B, cm3

mol-1

kSR second order rate constant, cm6 gcat-1 mol-1 s-1

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k second order rate constant, cm6 gcat-1 mol-1 s-1

k0 frequency factor, cm6 gcat-1 mol-1 s-1

-ri' rate of reaction of species i, mol gcat-1 s-1

E apparent activation energy, kcal mol-1

Si site of type i

SR surface reactioni-j species j adsorbed on site iT-Si total sites S of type iV-Si vacant sites S of type iw catalyst loading, g cm-3 of liquid phaset reaction time interval, min.

XA fractional conversion of A

References

(1) Olah, G. A.Friedel-Crafts and Related Reactions; Wiley-Interscience: New York,1963; Vol. 1-4.

(2) Olah, G. A.; Krishnamuri, R.; Suryaprakash, G. K. Comprehensive OrganicSynthesis; Pergamon: Oxford, 1991; Vol. 3, Chapter 1.8.

(3) Clark, J. H.; Macquarrie, D. J. Org. Proc. Res. Develop.1997, 1, 149.(4) Corma, C.; Garcia, H. Chem. Rev.2003, 103, 4307.(5) Corma, A. Chem. Rev.1995, 95, 559.(6) Lorenc, J. F.; Lambeth, G.; Scheffer, W.KirkOthmer Encyclopedia of Chemical

Technology; WileyInterscience: New York, 1992; Vol. 2.(7) Sheldon, R. A.; Downing, R. S.Appl. Catal. A: Gen.1999, 189, 163.(8) Sheldon, R. A.; van Bekkum, H. Fine Chemicals through Heterogeneous

Catalysis; Wiley-Interscience, Weinheim: Toronto, 2001.(9) van Bekkum, H.; Hoefnagel, A. J.; Vankoten, M. A.; Gunnewegh, E. A.; Vogt, A.

H. G.; Kouwenhoven, H. W. Stud. Surf. Sci. Catal.1994, 83, 379.(10) Isakov, Y. L.; Minachev, K. M.; Kalinin, V. P.; Isakova, T. A.Russian Chemical

Bulletin1996, 45, 2763.(11) Derfer, J. M.; Derfer, M. M.KirkOthmer Encyclopedia of Chemical Technology;

WileyInterscience: New York, 1978; Vol. 22.(12) Fiege, H.; Bayer, A.; Leverkusen, G. Ullmanns Encyclopedia of Industrial

Chemistry; 5th ed.; Wiley-VCH Verlag GmbH, Weinheim: Germany, 1991.(13) Franck, H. G.; Stadelhofer, J. W.Industrial Aromatic Chemistry; Springer: Berlin,

1988.(14) Yadav, G. D.; Doshi, N. S.J. Mol. Catal. A: Chem.2003, 194, 195.

(15) Yadav, G. D.; Doshi, N. S.Appl. Catal. A: Gen.2002, 236, 129.(16) Yadav, G. D.; Thorat, T. S. Tetrahedron Lett.1996, 37, 5405.(17) Yadav, G. D.; Pujari, A. A.; Joshi, A. V. Green Chem.1999, 1, 269.(18) Kumbhar, P. S.; Yadav, G. D. Chem. Eng. Sci.1989, 44, 2535.(19) Yadav, G. D.; Krishnan, M. S.; Doshi, N. S.; Pujari, A. A.; Mujeebur Rahuman,

M. S. M. Highly Acidic Mesoporous Synergistic Solid Catalyst and itsApplications; US Patent 6,204,424, 2001.

(20) Yadav, G. D.; Nair, J. J. Microporous Mesoporous Mater.1999, 33, 1.(21) Yadav, G. D.; Murkute, A. D.Langmuir2004, 20, 11607.(22) Yadav, G. D.; Murkute, A. D.J. Catal.2004, 224, 218.(23) Yadav, G. D.; Murkute, A. D.J. Phy. Chem. A2004, 108, 9557.

(24) Yadav, G. D.; Manyar, H. G. Microporous Mesoporous Mater.2003, 63, 85.(25) Yadav, G. D.; Salgaonkar, S. S.Ind. Eng. Chem. Res.2005, 44, 1706.

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(26) Fogler, H. S.Elements of Chemical Reaction Engineering; 2nd ed.; Prentice-Hall,New Delhi: India, 1995.

(27) Reid, R. C.; Prausnitz, M. J.; Sherwood, T. K. The Properties of Gases andLiquids; 3rd ed.; McGraw-Hill: New York, 1977.

(28) Song, C.; Kirby, S. Microporous Mater.1994, 2, 467.

(29) Yadav, G. D.; Bokade, V. V.Appl. Catal. A: Gen.1996, 147, 299.(30) Yadav, G. D.; Salgaonkar, S. S. Microporous Mesoporous Mater.2005, 80, 129.(31) Rouge, A.; Spoetzl, B.; Gebauer, K.; Schenk, R.; Renken, A. Chem. Engg. Sci.

2001, 56, 1419.(32) Golay, S.; Wolfrath, O.; Doepper, R.; Renken, A. Dynamics of Surface and

Reaction Kinetics in Heterogeneous Catalysis; Elsevier: Amsterdam, 1997; Vol.109, pp. 295.

Table 1. Properties of catalysts and conversion of TBAa

catalyst source

average

pore diameter()

surface

area(m2 g-1)

acidity by

NH3-TPD(mmol g-1)

selectivityof 5-TBMX

after 2 h(%)

conversion of

TBA after 2 h(%)

UDCaT-4 this work 30 233 0.562 73 60UDCaT-5 this work 40 83 0.584 82 96UDCaT-6 this work 32 877 0.508 66 74sulfated zirconia (S-ZrO2) this work 40 100 0.433 78 36

a Reaction conditions: speed of agitation 1000 rpm, catalyst loading 0.04 g cm-3, moleratio ofm-xylene:TBA 4:1, temperature 150 C, total reaction volume 50 cm3, autogenous

pressure.

Table 2. Kinetic parameters of dehydration of TBA and alkylation reactions

reactionrate constant, korkSR(cm6 gcat-1 mol-1 s-1) at 150 C

frequency factor, k0(cm6 gcat-1 mol-1 s-1)

activationenergy,E(kcal mol-1)

dehydration of TBA 1.16 35.7 1015 33.8

alkylation ofm-xylene with TBA 2.11 45.8 108 19.3

alkylation ofp-xylene with TBA 2.53 10.3 1010 22.4

alkylation ofo-xylene with TBA 3.17 15.4 1011 24.3

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0

20

40

60

80

100

0 20 40 60 80 100 120 140

Time (min)

Conversion(%)

Figure 1. Effect of different catalysts on conversion of TBA

Reaction conditions: speed of agitation 1000 rpm, catalyst loading 0.04 g cm-3, mole ratioofm-xylene:TBA 4:1, temperature 150 C, total reaction volume 50 cm3, autogenous

pressure. () UDCaT-5, () UDCaT-6, () UDCaT-4, () sulfated zirconia (S-ZrO2).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 20 40 60 80 100 120 140

Time (min)

Concentration(molcm

-3)103

Figure 2. Concentration profile of various products in alkylation ofm-xylene with

TBA

Reaction conditions: catalyst UDCaT-5, speed of agitation 1000 rpm, catalyst loading0.04 g cm-3, mole ratio ofm-xylene:TBA 4:1, temperature 150 C, total reaction volume50 cm3, autogenous pressure. () TBA, () 5-tert-butyl-m-xylene, () isobutylene, ()

di-tert-butyl ether, () 2,5-di-tert-butyl-m-xylene.

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0

20

40

60

80

100

0 20 40 60 80 100 120 140

Time (min)

Conversion(%)

Figure 3. Effect of speed of agitation on conversion of TBA

Reaction conditions: catalyst UDCaT-5, catalyst loading 0.04 g cm-3, mole ratio ofm-xylene:TBA 4:1, temperature 150 C, total reaction volume 50 cm3, autogenous pressure.

() 800 rpm, () 1000 rpm, () 1200 rpm.

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Time (min)

Conversion(%)

Figure 4. Effect of catalyst loading on conversion of TBA

Reaction conditions: catalyst UDCaT-5, speed of agitation 1000 rpm, mole ratio ofm-

xylene:TBA 4:1, temperature 150 C, total reaction volume 50 cm3, autogenous pressure.() 0.01 g cm-3, () 0.02 g cm-3, () 0.04 g cm-3, () 0.05 g cm-3.

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y = 335.14x

R2 = 0.9963

0

2

4

6

8

10

12

14

16

18

0 0.01 0.02 0.03 0.04 0.05 0.06

Catalyst loading (g cm-3

)

Initialrates(molcm

-3s

-1)

107

Figure 5. Plot of initial rate of reaction as a function of catalyst loading in liquid

phase

Reaction conditions: catalyst UDCaT-5, speed of agitation 1000 rpm, mole ratio ofm-xylene:TBA 4:1, temperature 150 C, total reaction volume 50 cm3, autogenous pressure.

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Time (min)

Conversion(%)

Figure 6. Effect of mole ratio ofm-xylene:TBA on conversion of TBA

Reaction conditions: catalyst UDCaT-5, speed of agitation 1000 rpm, catalyst loading

0.04 g cm

-3

, temperature 150 C, total reaction volume 50 cm

3

, autogenous pressure. (

)1:1, () 3:1, () 4:1, () 5:1.

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0

20

40

60

80

100

0 20 40 60 80 100 120 140

Time (min)

Conversion(%)

Figure 7. Effect of temperature on the cracking of TBA

Reaction conditions: catalyst UDCaT-5, speed of agitation 1000 rpm, catalyst loading0.04 g cm-3, total reaction volume 50 cm3, autogenous pressure. () 130 C, () 140 C,

() 150 C, () 160 C.

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Time (min)

Conversion(%)

Figure 8. Effect of temperature on alkylation ofm-xylene with TBA


0.04 g cm-3, mole ratio ofm-xylene:TBA 4:1, total reaction volume 50 cm3, autogenouspressure. () 130 C, () 140 C, () 150 C, () 160 C.

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 20 40 60 80 100 120 140

Time (min)

Concentration(molcm

-3)10

3

Figure 9. Concentration profile of various products in alkylation ofp-xylene with

TBA

Reaction conditions: catalyst UDCaT-5, speed of agitation 1000 rpm, catalyst loading0.04 g cm-3, mole ratio ofp-xylene:TBA 4:1, temperature 150 C, total reaction volume50 cm3, autogenous pressure. () TBA, () 2-tert-butyl-p-xylene, () isobutylene, ()

di-tert-butyl ether, () 2,5-di-tert-butyl-p-xylene.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 20 40 60 80 100 120 140

Time (min)

Con

centration(molcm-3)103

Figure 10. Concentration profile of various products in alkylation ofo-xylene with

TBA

Reaction conditions: catalyst UDCaT-5, speed of agitation 1000 rpm, catalyst loading0.04 g cm-3, mole ratio ofo-xylene:TBA 4:1, temperature 150 C, total reaction volume50 cm3, autogenous pressure. () TBA, () 4-tert-butyl-o-xylene, () isobutylene, ()

di-tert-butyl ether, () 4,6-di-tert-butyl-o-xylene.

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y = 0.0152xR

2= 0.9798

y = 0.0356x

R2

= 0.9897

y = 0.0544x

R2

= 0.9925

y = 0.2261x

R2

= 0.9866

0

2

4

6

8

10

12

0 20 40 60 80 100 120 140

Time (min)

XA/1-XA

Figure 11. Validation of mathematical model for dehydration of TBA

() 130 C, () 140 C, () 150 C, () 160 C.

y = 0.0805x

R2

= 0.9914

y = 0.0437x

R2

= 0.9916

y = 0.0277x

R2

= 0.9972

y = 0.0186x

R2

= 0.9948

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20 25

Time (min)

ln[(M-XA)/M(1-XA)]

Figure 12. Validation of mathematical model for alkylation ofm-xylene with TBA

() 130 C, () 140 C, () 150 C, () 160 C.

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y = 0.0937x

R2

= 0.9847

y = 0.0525x

R2

= 0.9801

y = 0.0281x

R2

= 0.9839

y = 0.0183x

R2

= 0.9948

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20 25

Time (min)

ln[(M-XA)/M(1-XA)]

Figure 13. Validation of mathematical model for alkylation ofp-xylene with TBA

() 130 C, () 140 C, () 150 C, () 160 C.

y = 0.1045x

R2

= 0.9698

y = 0.0657x

R2

= 0.9594

y = 0.0334x

R2

= 0.9715

y = 0.0172x

R2

= 0.9839

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20 25

Time (min)

ln[(M-XA)/M(1-XA)]

Figure 14. Validation of mathematical model for alkylation ofo-xylene with TBA() 130 C, () 140 C, () 150 C, () 160 C.

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-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

2.12E-03 2.16E-03 2.20E-03 2.24E-03 2.28E-03 2.32E-03

1/T (K-1

)

lnk

Figure 15. Arrhenius plots

() dehydration of TBA, () alkylation ofm-xylene with TBA, () alkylation ofp-xylene with TBA, () alkylation ofo-xylene with TBA.

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Time (min)

Conversion(%)

Figure 16. Reusability of the catalyst


0.04 g cm-3

, mole ratio ofm-xylene:TBA 4:1, temperature 150 C, total reaction volume50 cm3, autogenous pressure. () fresh catalyst, () first reuse, () second reuse.

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Friedel-Crafts Alkylation of Xylenes With Tert-Butanol Over Me So Porous Superacid UDCaT-5

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