Vapor phase alkylation of toluene with 2-propanol to cymenes with a novel mesoporous solid acid UDCaT-4

Microporous and Mesoporous Materials 103 (2007) 363–372 www.elsevier.com/locate/micromeso

Vapor phase alkylation of toluene with 2-propanol to cymenes with a novel mesoporous solid acid UDCaT-4 Ganapati D. Yadav *, Suraj A. Purandare Department of Chemical Engineering, University Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai 400 019, India Received 2 September 2006; received in revised form 1 January 2007; accepted 4 January 2007 Available online 23 January 2007

Abstract Friedel–Crafts alkylation of toluene with propylene, isopropyl alcohol, and isopropyl halides leads to cymenes and other dialkylated products, among which p-cymene (4-methyl-1-isopropyl benzene) is used in pharmaceutical industries and for the production of fungicides, pesticides, etc. The current work deals with the development of a novel catalyst named UDCaT-4 which is hexagonal mesoporous silica as an inert support for persulfated alumina and zirconia. Its superior activity and selectivity for p-cymene was tested in the vapor phase alkylation of toluene with isopropyl alcohol. An insight it provided by developing mathematical model for the complex chemical reaction. The catalyst is robust and recyclable and can be advantageously used in a number of other reactions.  2007 Elsevier Inc. All rights reserved. Keywords: Alkylation; Toluene; p-Cymene; Isopropyl alcohol; Mesoporous catalyst; Heterogeneous catalysis; UDCaT-4; Persulfated alumina; Zirconia selectivity; Kinetics

1. Introduction Friedel–Crafts alkylation and acylation reactions involve use of a variety of acid catalysts. Some of the well established processes still employ homogeneous acid catalysts in batch reactors using large excess of the substrate or solvent causing problems of corrosion and pollution, loss of selectivity of the desired product, etc. In most of the industrial alkylation processes different catalysts are used such as mineral acids, anhydrous AlCl3, etc. and solvents like nitrobenzene, carbon disulphide and halogenated hydrocarbons are used. Relatively high concentration of catalyst is needed; often the amount being more than stoichiometric and these problems make most alkylation reactions highly polluting [1–5]. In addition, since the reagents are mixed with acids, separation of the products from the catalyst is often a difficult and energy consuming process.

*

Corresponding author. Tel.: +91 22 2410 2121; fax: +91 22 2414 5614. E-mail addresses: [email protected], [email protected] (G.D. Yadav). 1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.01.013

The alkylation of toluene with 2-propanol is industrially important to make p-cymene (4-methyl-1-isopropyl benzene) which is extensively used for the production of pesticides, fungicides, flavors and heating media [6,7]. o-Cymene and m-cymene are also produced depending on reaction conditions and they have industrial applications as well. Indeed, all the cymenes can be advantageous converted into corresponding phenols or acids, opening up new vistas in specialty applications. Selective production of one over other cymene isomers is a challenging task and choice of a right catalyst has been a subject of intense speculation and studies. Solid acids, amongst which catalysts based on clays and zeolites [8–12], have proven excellent alternatives for traditionally polluting catalysts in view of their ease of handling, robustness, activity and remarkable shape selectivity. Unfortunately, slow diffusion of bulky reactants through micro-pores of zeolites render them relatively poor catalysts for reactions of such bulky molecules which are encountered in fine chemical and pharmaceutical industries. The stability of zeolites in reactions where acids are produced as co-products or byproducts is susceptible and rapid catalyst deactivation due to coke formation still

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Nomenclature A C D E FAo Kij kSR T Pi r0i

IPA concentration of a species (mol l1) cymene diisopropyl ether molar flow rate of the toluene (mol h1) adsorption equilibrium constant surface reaction constant (cm3/g-cat s) toluene partial pressure of species i (atm) rate of reaction of species i (atm/s g-cat)

poses a question [13]. Parikh et al. [14] have studied toluene alkylation over zeolites varying in pore system, crystal size and silylation extent and proposed a more realistic mechanism for ZSM-5. Kinetic studies of toluene isopropylation using Al-ZSM-5 have been reported [15]. Recently isopropylation under gaseous nitrogen, supercritical CO2 and supercritical toluene over parent and modified HZSM-5 pellets has been carried out in fixed beds [16]. Substantial progress has been made towards the development of environmentally friendly processes in the alkylations of aromatics with olefins, particularly using solid catalysts, which are adequate to substitute mineral or Lewis acids and free bases. Alkylation of benzene and toluene for the production of ethylbenzene (EB), cumene, linear alkyl benzene and cymene is extensively studied [17]. The isomerization and disproportionation of m-xylene, and the alkylation of toluene with either ethanol or isopropanol is reported with acidic zeolite ITQ-7 (ISV structure) [18], and with the large pore zeolites Y, beta, mordenite and ZSM-12 and medium pore MFI silicates with isomorphously substituted Al and Fe in the framework. More open structures (Y, beta, AlCl3, H3PO4) to give the formation of branched isomers is preferred whereas for synthesis of p-cymene, ferrisilicates of MFI structure possessing less acidic Si–OH–Fe groups are better [19]. Molecular sieves possessing different acidity (Al- and Fe-silicates) and structural type (Y, mordenite and MFI structure) are also used to elucidate factors playing a decisive role in iso-/n- and para-selectivity in propyltoluenes. The desorption/transport of bulky propyltoluenes from the zeolite channel systems was found to be the reaction rate controlling step. Of the propyltoluenes only cymenes were found with H-mordenite, and H–Y yielded n-propyltoluenes only at temperatures above 550 K, while up to 520 K cymenes were exclusively produced [20]. The highest yield of p-cymene was obtained with molecular sieves having a MFI structure possessing a low number of bridging OH groups of a lower acid strength, (H–(Fe)ZSM-5), and by employing short contact times and a reaction temperature below 570 K. A highly siliceous, large pore, crystalline aluminosilicate has been employed for isopropylation of toluene to cym-

W Xi

weight of the catalyst (g) fractional conversion of species i

Subscripts ij species j adsorbed on site i t  Si total sites S of type i V  Si vacant sites S of type i Greek letter qp density of catalyst particle (g/cm3)

enes. Beta zeolite was found to be selective for cymenes and diisopropyl benzene formation [21]. Al-MCM-41, zinc and iron containing Al-MCM-41 molecular sieves with Si/ M (M = Al, Zn, and Fe) as well as commercial zeolites b, mordenite and ZSM-5, for vapor phase isopropylation of toluene with isopropyl acetate (IPA) was carried out over 200–275 C in the production of p-cymene from toluene, using 2-propanol as the alkylating reagent [22]. The catalytic activity of macroreticular cation-exchange resin (Amberlyst-15) was evaluated for the reaction of toluene with isopropanol, 1-octanol, 2-octanol and 1-octene at 80 C, in the liquid phase [23] whereas MCM-41/c-Al2O3 catalyst resulted into isopropyltoluenes fraction containing more para isomer [24]. Alkylation of toluene with isopropanol and methanol was carried out over alkali cationexchanged b, Y and ZSM-5 zeolites. Over Li exchanged zeolites isopropylation takes exclusively place at the nucleus, resulting in cymenes while methylation gives xylenes and ethylbenzene, the former being the major product. Over Na, K, and Cs exchanged zeolites both isopropylation and methylation occurs at the side chain, resulting in isobutylbenzene from the former and ethylbenzene and styrene from the latter at 648 K for isopropylation and 698 K for methylation [25]. There are better alternatives to zeolites in a number of reactions such as sulfated zirconia, titania, solid supported heteropolyacids, ion exchange resins, acid treated clays, etc. in several reactions which are yet to be commercialized [26]. Mesoporous solid acids and superacids have been vigorously pursued by us to extend their utility for niche applications. The alkylation of toluene was thus investigated using a novel catalyst UDCaT-4 in the current studies and is the subject matter of this paper. Our laboratory has been engaged in the development of several novel solid acid catalysts and the practice of green chemistry which eliminates the use or generation of hazardous substances in the design, manufacture and makes the process ecofriendly [27–35]. This work deals with the effectiveness of a novel mesoporous solid acid catalyst named UDCaT-4 [36] which is the combination of persulfated alumina and zirconia and hexagonal mesoporous silica. The acronym UDCaT stands for our institute which was

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popularly known as UDCT (University Department of Chemical Technology) until recently. The catalytic activity of above mentioned catalyst is also compared with persulfated alumina and zirconia. 2. Experimental 2.1. Chemicals Zirconium oxychloride, aluminum nitrate (AR grade), aqueous ammonia solution, ammonium persulfate (AR grade), isopropyl alcohol (IPA), toluene, commercial grade (95%) ethanol were procured from M/s. s. d. fine Chemicals Ltd., Mumbai, India. Tetraethyl orthosilicate (TEOS) (Fluka) was taken as the neutral silica source and hexadecyl amine (Spectrochem Ltd.) as the neutral amine for the template.

365

The liquid feed containing the desired proportion of toluene and isopropanol (IPA) was fed by a double piston pump (Well Chrom HPLC-pump K-120) to the vaporiser. The gaseous feed was transported to the reactor by using N2 as a carrier gas at a known flow rate (20 ml/min) at reaction temperature. Typically 1:1 molar ratio of toluene and IPA was used. In a typical run, 1 g of catalyst was charged to the reactor, packed in a layer of ceramic wool and supported by glass beads. The reactor was maintained under isothermal conditions during the isopropylation of toluene and effects of various parameters like mole ratio, WHSV, reaction temperatures and W/FA0 were studied, where W is the mass of the catalyst and FA0 is the molar feed rate of the limiting reactant. A steady state was typically achieved within 30–45 min and the samples were collected typically after 1 h. Samples were condensed after the steady state and analysed by GC. All data reported here are averaged over 3–5 runs for each parameter.

2.2. Experimental procedure 2.3. Analysis A downflow fixed bed hastealloy HC-276 reactor with 25.4 mm ID and 300 mm length at atmospheric pressure, equipped with an upstream vaporiser and downstream condensor, supplied by Chemito Instruments Pvt. Ltd., Mumbai, India was used for all experimental investigations (Fig. 1).

Samples were collected at regular intervals and analyzed on GC (GC1000 Chemito, Toshniwal Instruments, India) equipped with a capillary column of 0.22 mm · 25 m and FID detector. Products obtained were confirmed by GC-MS and authentic samples.

Fig. 1. Schematic flow diagram of fixed bed catalytic reactor. 1. Pump 2. Vaporiser 3. Reactor 4. Condenser 5. Phase separator 6. Receiver.

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2.4. Catalyst preparation Persulfated modified alumina and zirconia (PAZ) was prepared according to a method described elsewhere [36]. Aqueous ammonia was added dropwise to a mixed solution of ZrOCl2 Æ 8H2O and Al(NO3)3 Æ 9H2O until pH was in the range of 9–10. Mixed hydroxide of zirconia and alumina were washed with deionized water until a neutral filtrate and the absence of chlorine ion was detected by phenolphthalein and AgNO3 tests. The mixture of zirconium hydroxide and aluminum hydroxide was dried in an oven for at 110 C for 24 h and crushed to 100 mesh size. It was immersed in 0.5 M ammonium persulfate solution for 30 min to get persulfated Al(OH)3–Zr(OH)4. It was filtered off, dried at 110 C for 24 h and thereafter calcined at 650 C for 3 h to get the catalyst persulfated alumina and zirconia (PAZ) with 0.6% w/w of alumina (3 mol%). The ordered hexagonal mesoporous silica (HMS) was prepared according to our earlier work [34]. Desired quantities of zirconium oxychloride (2.39 g) and aluminum nitrate (0.11 g) were dissolved in an aqueous solution and added to 5 g of precalcined HMS by the incipient wetness technique. 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 neutral filtrate was obtained and the absence of chlorine ion in the filtrate was detected by phenolphthalein and silver nitrate tests. It was then dried in an oven for 24 h at 110 C. Persulfation was carried out by immersing the above solid material in 0.5 M aqueous solution of ammonium persulfate for 30 min. It was dried at 110 C for 24 h and calcined at 650 C for 3 h to get catalyst called UDCaT-4 with 0.6% w/w of alumina. 3. Results and discussion 3.1. Catalyst characterization UDCaT-4 is completely characterized by XRD, BET surface area, FTIR, SEM and EDAX and the details were published recently by us [36]. Only a few salient features are reported here. XRD, BET surface area and pore size analysis provided an explanation for entrapment of nanoparticles of PAZ (<3.6 nm) in mesoporous of HMS. XRD data of UDCaT4 suggested that the structural integrity of HMS is retained even after converting it into UDCaT-4. The diffractogram of UDCaT-4 further revealed that the introduction of small amount of alumina (0.16% w/w) and sulfate ion (1.17% w/ w) stabilized the tetragonal phase of the zirconia, which is an ideal phase conducive for superacidity in sulfated zirconia, into the pores of HMS. Furthermore, the pore volume of UDCaT-4 (0.21 cm3 g1) is much less than that of pure HMS (0.78 cm3 g1) indicating that large amount of crystalline zirconia (9.01% w/w), and alumina must be present inside pores of UDCaT-4. FTIR spectroscopy and EDAX analysis further support the assumption drawn on introduc-

tion of sulfate ion in UDCaT-4. The sulfur Ka1 and zirconium La1 distribution spectra determined by EDAX analysis shows the incorporation and homogeneous distribution of zirconia and sulfur atoms in UDCaT-4. The TPD profiles of PAZ and UDCaT-4 show that UDCaT-4 possesses weak medium acid and some strong acid sites over a large surface area created by nano-particles and thus total acid strength of UDCaT-4 is 0.56 mmol/g, which is much greater than that of PAZ (0.09 mmol/g), which is a known superacid. SEM of UDCaT-4 revealed that similar to the morphology of HMS, UDCat-4 is made up of sub-micrometer sized free standing or aggregated sphere shaped particles. SEM analyses further supports the argument that active centers of the PAZ are successfully embedded in HMS and the structural integrity of HMS is unaltered even after it is converted to UDCaT-4. The nano-particles of PAZ on HMS give rise to large surface area which is responsible for almost six times greater total acidity. 3.2. Comparison of catalysts In order to see the efficacy of UDCaT-4 in the alkylation of toluene, its activity was compared with the reference cata;yst persulphated alumina–zirconia (PAZ) without support. BET analysis showed that UDCaT-4 possesses higher surface area than PAZ, is mesoporous and also superacidic in nature [34,35]. PAZ is a microporous material and there is a diffusion resistance. Thus, the turn over number (TON) for UDCaT-4 is much greater (Table 1). Alkylation of toluene with isopropanol (IPA) involves a complex reaction mechanism in that there is dehydration of IPA leading to propylene which is the actual alkylating species. Indeed, the formation of water as a co-product suppresses coking in such reactions at high temperature. Propylene can also undergo etherification with IPA in situ leading to disisopropyl ether which is also an alkylating agent as shown in Scheme 1 depending on temperature [38,39]. The possibilities of oligomerization of propylene and dialkylated products depend on the type of catalyst, its acidity and pore size distribution. In the current studies, no diisopropyl ether formation was witnessed when IPA was cracked on the same catalyst under identical reaction conditions, the reason being use of high temperatures. The alkylation products were identified under all sets of conditions used in this study and the reaction products which were confirmed suggested the reaction paths given in Scheme 2. In the current case, there was no diisopropyl Table 1 UDCaT-4 vs. PAZ performance in toluene alkylation with IPA Parameters 2

BET surface area m gm Pore volume cm3 g1 Turnover number

1

HMS

PAZ

UDCaT-4

833 0.78 0

81 0.11 30

233 0.21 157

(Turn over number) = moles of product/moles of zirconium.

G.D. Yadav, S.A. Purandare / Microporous and Mesoporous Materials 103 (2007) 363–372

367

OH

UDCaT-4 OH

2

O

-H2O

-H2O

Scheme 1. IPA cracking in situ: possible products and alkylating species.

UDCaT-4 +

OH

kp

p-cymene ko

+ H2O OH

+ H2O

km

o-cymene Dialkylated toluenes isomerization

m-cymene

Scheme 2. Products identified in alkylation of toluene with IPA over UDCaT-4.

ether detected under the reaction conditions, which was the case for low temperature dehydrations. However, there was a formation of monoalkylated and dialkylated products of toluene. m-Cymene formation occurs by three independent routes, namely, alkylation of toluene and isomerization of o- and p-cymene at higher temperatures. A typical product distribution is seen in Table 2 at various temperatures. The per pass conversion at the standard temperature of 523 K is 45% at W/FA0 of 23 g h mol1 giving 16.4% o-cymene, 35.4% p-cymene and 32% m-cymene with 15.6% of the mixed dialkylated toluenes. This suggests that although per pass conversion has increased from 8.5% at 433 K, the selectivity of p-cymene has decreased to 35.4% due to isomerization of o-cymene m-cymene along with that of p-cymene, which are all temperature dependent reactions. This will be discussed further in later part of the paper. 3.3. Effect of mole ratio Mole ratio of toluene and isopropanol was varied by keeping the same total gaseous flow rate to see its effect

Table 2 Products distribution with UDCaT-4 at different temperatures Temperature K

Conversion (single pass) %

oCymene %

pCymene %

mCymene %

Dialkylation %

433 463 493 523 573

8.5 25 26 45 54

32.2 24 23.6 16 16.4

42.3 34 35.3 30.5 35.4

15.3 13 16.3 20.7 32.6

10.2 29 29.8 32.8 15.6

Reaction conditions: IPA:toluene = 1:1, W/FA0 = 23 g h mol1.

on conversion and p-cymene selectivity. At 523 K with W/FA0 23 g-cat h mol1, as the mole ratio of toluene to isopropyl alcohol is increased from 1:1 to 1:5, the conversion is increased since more of alcohol gets dehydrated to propylene. However, this also leads to dialkylated product and cymenes selectivity decreases, because of the subsequent reaction of cymenes (Fig. 3). Highest p-cymene selectivity is obtained for 1:1 mol ratio.

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3.5. Effect of temperature

% Conversion / Selectivity

50

40

30

20

10

0 0

0.2

0.4

0.6

0.8

1

1.2

Mole ratio (Toluene:IPA)

Fig. 3. Effect of mole ratio (toluene:IPA). (r Conversion, j p-cymene) W/FA0 23 g-cat h mol1, temperature 523 K.

3.4. Effect of W/FA0 on conversion W/FA0 was varied from 19 to 51 g-cat h mol1 by keeping temperature and mole ratio constant. With increasing W/FA0 from 19 to 51 g-cat h mol1 there is increase in the conversion, which is due to the increased residence time of the reactants (Fig. 4). However, beyond 23 g-cat h mol1 there was marginal increment in the conversion. This is due to the formation of more dialkylated products and appearance of external mass transfer resistance. In order to justify this conclusion and whether internal resistance was limiting reaction rates, the Weitz-Prater criterion was used [40], according to which the value of ðrobs qp R2p =De ½As Þ should be far less that unity for the reaction to be intrinsically kinetically controlled. The calculated value was found to be 2.2 · 104, which suggested that for a mesoporous UDCaT-4, there was no intra-particle diffusion limitation.

As stated earlier while discussing the nature of products, temperature had a pronounce effect on the conversion of toluene. In order to study the effect of temperature on conversion and selectivity, the W/FA0 was maintained constant at the given temperature. Temperatures were varied from 433 to 573 K (Fig. 5). It was found that conversion increased with increasing temperature, there was a loss in selectivity of p-cymene. A maximum p-cymene selectivity as high as 48% was obtained at 463 K and W/FA0 19 g-cat h mol1 (Fig. 6). A maxima and minima of conversion were observed for W/FA0 of 19 and 23 g-cat h mol1. For a W/FA0 from 19 gcat h mol1 the conversion increases with temperature upto 548 K and then it decreased at 573 K, thereby bringing into picture intra-particle diffusion limitation for the same residence time, which results into formation of substantial dialkylated products which are likely to block the pore channels. No experiments were conducted beyond 573 K. For a W/FA0 of 23 g-cat h mol1, the conversion increases

60 50

% Conversion

60

40 30 20 10 0 400

425

450

475

500

525

550

575

600

o

Temperature ( K)

Fig. 5. Effect of temperature. (IPA:toluene = 1:1) (r 19, j 23, m 27, · 51) (g-cat h mol1).

0

60 50

Ln k

% Conversion

-2 40 30

y = -5081.1x + 6.0838

-4

R2 = 0.9616

20

-6

10 0 0

10

20

30

40

50

60

-8

W/FA0 (g-cat. h.mol-1)

Fig. 4. Effect of IPA:toluene = 1:1.

W/FA0

on

conversion.

0.002

Temperature

523 K,

0.0021

0.0022

0.0023

1/T

Fig. 6. Arrhenius plot of ln (k) vs. 1/T.

0.0024

G.D. Yadav, S.A. Purandare / Microporous and Mesoporous Materials 103 (2007) 363–372

with temperature upto 528 K due to higher reaction rates and then decreased at 548 K due to presence of intra-particle resistance; however, a further increase to 573 K increased the conversion to almost the same level as that at 528 K. This shows the temporary drop was due to substantial formation of dialkylated products and a diffusion limited situation giving the same conversion had arisen. The experiments at higher W/FA0 of 27 and 51 g-cat h mol1 show that the conversions at 548 and 573 K were nearly equal in each case, respectively. Further the increase of W/FA0 by almost 2 times increased the conversion only marginally, which suggests strong intra-particle diffusion limitation. Table 2 shows the cymene isomer distribution with variation in temperature. There were no noticeable n-propyltoluenes at the given conditions. The standard reaction conditions of 523 K do not lead to n-propyltoluenes. It is in consonance with the literature; for instance, only cymenes were found with H-mordenite, and H–Y yielded npropyltoluenes only at temperatures above 550 K, while up to 520 K cymenes were exclusively produced [20]. In order to see that there was no mass transfer limitation, the rate of reactions, calculated on toluene conversion, at steady state were used to make Arrhenius plot under otherwise similar conditions. It should be noted here that this is an apparent activation energy and not a true value. The activation energy was found to be 10.1 kcal mol1(Fig. 7), which suggested that there was no diffusion limitation. 3.6. Selectivity over UDCaT-4

ene. The highest yield of p-cymene can be obtained by employing short contact times and a reaction temperature below 523 K. An increased selectivity to cymene in the vapor phase alkylation of toluene with isopropanol was reported using different large pore zeolites (i.e. Beta, mordenite and ZSM-12) at a lower temperature (453 K) using toluene to IPA molar ratio of 3 [41]. Under such conditions, ZSM-12 and Beta showed greater stability. Moreover, beta-zeolite was more selective to cymene at 22.4% conversion and 98% selectivity to cymenes, at 508 K at WHSV of 5.5 h1 and toluene to IPA ratio of 4. Beta was demonstrated to give higher performances than ZSM-5 [21]. When compared with these materials, our catalyst UDCaT-4 also gives, with a mole ratio of 1:1 of toluene to IPA, only cymenes and some diisopropyltoluenes with a selectivity of 48% to p-cymene at 463 K (Fig. 7). 3.7. Time on stream Stability of the catalyst was studied at 523 K and mole ratio of 1:1 of toluene to IPA for 48 h. It was found to be stable (Fig. 8). This was for different periods using the same catalyst. The catalyst is robust. 3.8. Mechanism and kinetic model The reaction proceeds due to the in situ generation of propylene by dehydration of isopropanol (IPA). This reaction has been studied by us independently on two different solid acids by us [29,30], according to which dehydration is a second order reaction. Model based on two catalytic sites is proposed according to which IPA (A) gets adsorbed on to two different sites S1 and S2 (Fig. 9): A þ S1 A þ S2

55

50

50

45

45

40

40

35

Conversion %

% Selectivity

The series reaction of cymenes to dialkylated products is responsible for drop in selectivity, which is again a strong function of temperature. This is also due to the mesoporosity of the catalyst as well as higher acid strength. Unfortunately, the material is not a molecular sieve and thus a proper combination of temperature, W/FA0, and mole ratio of toluene to IPA will result into maximum yield of p-cym-

35 30 25 20

369

K 1A

ð1Þ

! AS1

K 2A

ð2Þ

! AS2

30 25 20 15

15 10

10

5

5

0

0

400

420

440

460

480

500

520

540

560

580

600

o

Temperature ( K)

Fig. 7. Effect of temperature on p-cymene selectivity. (IPA:toluene = 1:1) (r 19, j 23, m 27, · 51) (h mol1).

0

10

20

30

40

50

60

Time (Hrs.)

Fig. 8. Time on stream. W/FA0 23 g-cat h mol1, temperature 523 K, IPA:toluene = 1:1.

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Fig. 9. Conceptual model for: (a) dehydration of IPA (b) alkylation of toluene with IPA. (A = IPA, D = cymene, E = ether, T = toluene, S1 = site 1, S2 = site 2 and W = water.) A is adsorbed on two adjacent sites S1 and S2 and the product ether (E) is formed which is undergoes dehydration instantaneously to propylene (P) in the case of first reaction. In the second case, E from site S2 reacts with T from the adjacent S1 sites leading to product.

These two adsorbed species participate in the reaction. Two different cases are considered, (1) dehydration of IPA give propylene and water, and (2) alkylation of toluene with IPA. 3.8.1. (1) Dehydration of IPA In this case when the rates determining step is the reaction of AS1 and AS2 to form diisopropyl ether (E) and water (W) as the surface complexes (ES1) and (WS2), respectively. ES1 subsequently decomposes instantly in to propylene (P) in the gas phase as shown below. k SR1

AS1 þ AS2 ! ES1 þ WS2 ES1 ! 2P þ WS1

ð3Þ ð4Þ

The site balance for this case is: C tS1 ¼ C VS1 þ C AS1 þ C ES1 þ C WS1 C tS2 ¼ C VS2 þ C AS2 þ C WS2

ð5aÞ ð5bÞ

The following adsorption equilibria for different species hold: W þ S1 W þ S2 E þ S1

K 1W

! WS1

K 2W

! WS2

K 1E

! ES1

ð6aÞ ð6bÞ ð6cÞ

Thus, the rate of formation of propylene, r0P (mol/s g-cat), in terms of partial pressures of different species is: r0P ¼

K SR1 K 1A P A K 2A P A C TS1 C TS2 ð1 þ K 1A P A þ K 1W P W Þð1 þ K 2A P A þ K 2W P W þ K 1E P E Þ

ð7Þ When the adsorption constants of all species are very weak, Eq. (7) is reduced to the following:

r0P ¼ k P P 2A where;

ð8Þ

k P ¼ K SR1 K 1A K 2A C TS1 C TS2

ð9Þ

For a fixed bed catalytic integral reactor containing a catalyst mass W and with molar flow rate of AðF A0 Þ, the above equation can be integrated to get the following:   W XA ð10Þ k P P 2A0 ¼ F A0 ð1  X A Þ   XA W Thus, a plot of ð1X vs. was made to get an excellent FA AÞ 0

fit thereby supporting the model. Dehydration of iso-propanol has been studied on the same catalyst and it was observed the reaction followed overall second order kinetics for weak adsorption of isopropanol [37]. 3.8.2. (2) Alkylation of toluene The second order dehydration model was taken as a basis. First and second order kinetics with weak adsorption of isopropyl alcohol and weak/strong adsorption of toluene respectively did not fit to the observed data well. Toluene is adsorbed on sites S2, and it reacts with ES1, which is formed due to the surface reaction (3), to give the cymene (D) as follows: T þ S2

K 2T

! TS2 k SR2

ð11Þ

TS2 þ ES2 ! 2DS2 þ WS2 þ P C tS1 ¼ C VS1 þ C AS1 þ C ES1 þ C WS1

ð12Þ ð13Þ

C tS2 ¼ C VS2 þ C AS2 þ C WS2 þ C TS2 þ C DS2

ð14Þ

The following adsorption equilibrium is defined: D þ S2

K 2D

! DS2

ð15Þ

G.D. Yadav, S.A. Purandare / Microporous and Mesoporous Materials 103 (2007) 363–372 4 3.5

y = 0.0621x 2 R = 0.7766

2.5

y = 0.0253x 2 R = 0.9794

2

[1/(1-XT) -1]

3

2

371

the complex chemical reaction. UDCaT-4 showed better activity than persulphated alumina–zirconia. The time on stream studies were conducted up to 48 h, over different periods, and absolutely no drop in catalytic activity was observed which indicates the robustness of UDCaT-4 even in presence of water and resistance towards the coke formation. The apparent activation energy was found to be 10 kcal mol1.

y = 0.0159x 2 R = 0.9957

1.5

Acknowledgements

1

G.D.Y. acknowledges receipt of a research Grant from Darbari Seth Professorship Endowment. S.A.P. acknowledges receipt of Junior Research Fellowship from University Grants Commission.

0.5 0 0

10

20

30

40

60

50

W/FA0(hr.mol-1) 523

493

463

433

y = 0.0086x 2 R = 0.8983

Fig. 10. A plot of [1/(1  XT)2  1] vs. W/FA0.

For alkylation of toluene  r0M ¼

k SR2 C TS1  C 2TS2 K 1A K 2A P 2A K 2T P T ð1 þ K 1A P A þ K 1W P W þ K 1E P E Þð1 þ K 2A P A þ K 2W P W þ K 2T P T þ K 2D P D Þ2

ð16Þ

for weak adsorption of all species:  r0T ¼ k SR2 C TS1  C 2TS2 K 1A K 2A P 2A K 2T P T ¼ k T P 2A P T ð17Þ where

k T ¼ k SR2 C TS1 S 2TS2 K 1A K 2A K 2T

ð18Þ

For equimolar quantities of IPA and toluene at the inlet, the following solution for an integral reactor can be obtained: #   " W 1 3 2k T P A0 1 ð19Þ ¼ 2 F A0 ð1  X T Þ h i   Thus, a plot of ð1X1 Þ2  1 vs. FWA can be made to get a T

0

straight line. This is shown in Fig. 10 to see that the model is valid for the experimental conditions. 4. Conclusion Many solid acids have been investigated to replace homogeneous acids for industrially important reactions but their small surface area and non-uniform pore size may limit their potential applications for preparation of bulky molecules like cymene. The theme of current work deals with the development of a novel catalyst named UDCaT-4 which is hexagonal mesoporous silica as an inert support for persulfated alumina and zirconia. Its superior activity and selectivity for p-cymene was tested in the vapor phase alkylation of toluene with isopropyl alcohol. An insight it provided by developing mathematical model for

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