Alkylation of Naphthalene with T-Butanol Over Zeolite Y

Indian Journal of Chemical Technology Vol. 11, May 2004, pp 351-356

Alkylation of naphthalene with t-butanol over zeolite Y: Influence of reaction environment and reaction kinetics R Nagotkar, S S Khaire, S Mayadevi* & S Sivasanker National Chemical Laboratory, Pune 411 008, India Received 7 January 2003; revised received 3 October 2003; accepted 8 January 2004 The t-butylation of naphthalene with t-butanol over different Y-zeolites and at different environments is reported. The zeolites investigated are H-Y, RE-Y and three US-Y samples with different Si/Al ratios. Though, as expected, increasing the reaction temperature (160-180 ºC) increases naphthalene conversion, increasing the TBA/naphthalene mole ratio beyond 2 results in a decrease in conversion, presumably due to poisoning by the larger amount of water produced during dehydration. Both activity and yield of dialkylnaphthalenes increase on RE exchange and dealumination of H-Y. Higher conversions are obtained when the reaction is conducted under neat conditions than in the presence of the solvent cyclohexane or the added gas, N2. IPC Code: C07B 37/00 Keywords: Alkylation, naphthalene, t-butanol, zeolite Y reaction kinetics

2,6-dialkylnaphthalenes are important inter- selectivities under mild conditions3,4. Most of the mediates in the manufacture of novel polyester resins. liquid phase studies reported have used an organic 2,6-naphthalenedicarboxylic acid (NDCA) is made by solvent such as cyclohexane as the reaction medium. the oxidation of 2,6-dimethylnaphthalene, which is A study of the t-butylation of naphthalene over REY 8 prepared by dialkylation of naphthalene with in supercritical CO2 has been recently reported . The methanol. Naphthalene alkylation produces a variety results indicated that carbon dioxide could replace of products; two monoalkylates and ten dialkylates organic solvents as a medium for butylation of are produced in equilibrium amounts. This makes naphthalene over REY without loss of its shape product separation and purification quite challenging. selective characteristics. In this paper the t-butylation Monoalkyl naphthalenes can be separated by of naphthalene over different Y-zeolites and in fractionation. Separation of the closely boiling dialkyl different environments is reported. isomers is a difficult task and has led to the development of expensive separation strategies1,2. Experimental Procedure

As 2,6-isomer(β,β’-isomer) is smaller in size HY zeolite was prepared from commercial NH4Y compared to the other isomers, alkylation using shape (Zeolyst) by calcination at 450 °C for 5 h. RE selective zeolites can lead to its preferential exchange was carried out over NH4Y using a 5% formation, especially in the case of alkylation with solution of RECl3 (didymium chloride, supplied by bulky groups like t-butyl. Liquid phase t-butylation of Indian Rare Earths Ltd., Cochin, India; composition: 3-6 naphthalene over a number of zeolites viz. HY, Hβ, Ce < 2%, Nd ~ 35%, La/Y ~ 45%, Pr ~ 10%, other 7 HM, SAPO-5, HZSM-5, MCM-41 and clays has lanthanides rest; pH of solution ~ 5.2) at 80 °C thrice been reported. Among these catalysts, HY and Hβ are (10 mL solution per gram of zeolite). The sample was reported to exhibit high activity and high β- and β,β’- dried (110 °C, 6h) and calcined (450 °C, 4 h). The RE2O3 content of the sample was 15% on dry basis. ______The REY was blended and extruded with alumina * For correspondence (30%) and calcined at 500 °C for 6 h. The extrudates (E-mail: [email protected]; Fax: 91-20-5893041/91-20-5893355) were sized to 0.1 cm length and activated at 450 °C

352 INDIAN J CHEM TECHNOL, VOL 11, MAY 2004

for 4h in air before use. USY(12), USY(30) and Table 1 — Si/Al ratios and surface areas of the Y-type zeolites USY(80) were dealuminated zeolite(Y) with different Si/Al ratios, procured from P. Q. used in t-butylation of naphthalene Corporation, Holland. The zeolite powder was made Zeolite Si/Al ratio Surface area into pellets (2 cm dia., 0.3 cm thick), crushed and m2/g sieved to obtain particles of 0.08-0.2 cm, which were REY 5.1 530 used for the experiments. Sized zeolites were activated at 450° C for 4 h in the furnace before use. HY 5.2 750 The catalysts were characterized by XRD Rigaku USY (12) 12 730 miniflex diffractometer (Model: D/Max – VC, Japan) and Cu Ká radiation (λ = 1.5406 Å). The surface area USY (30) 30 780 (BET, N adsorption, Nova 1200 Qunatachrome high- 2 USY (80) 80 780 speed gas sorption analyzer) and the Si/Al ratio (estimated by chemical analysis) of the zeolite samples are presented in Table 1. Naphthalene (99%) from 0.5 to 1 g. resulted in an increase in conversion and t-butyl alcohol (TBA > 99%) were procured from from 54.8 to 62.3%. This was accompanied by an S.D. Fine Chemicals, India. TBA was further purified increase in MTBN and 2,6-/2,7-DTBN selectivity by distillation and dried over 4A molecular sieves ratio and a small decrease in DTBN% and β/α ratio in before use. the product. The naphthalene conversion was not The experiments were carried out in batch mode significantly affected by increasing the catalyst in a 300 mL stainless steel Parr autoclave. The quantity beyond 1 g. Hence, further data were catalyst (1 g) enclosed in a stainless steel wire mesh collected using 1 g of catalyst charge. was attached to the stirrer blades to ensure good Influence of reaction parameters contact with the reacting fluids. After loading the The influence of reaction parameters reactor with naphthalene and TBA, it was filled with -2 (temperature, pressure, time, reactants mole ratio, N (initial pressure 60 kg.cm in the case of 2 catalyst particle size) on the isobutylation of experiments in nitrogen atmosphere) or 50 mL of naphthalene was studied using REY as the catalyst. cyclohexane for experiments in solvent medium. In The results are presented in Table 2. all experiments, except where noted otherwise, 0.02 mole of naphthalene and a TBA : naphthalene (mole) When reaction temperature was increased from ratio of 2 were used. 160 to 180° C, the naphthalene conversion increased After the reaction, the reactor and its contents from 42 to 80.7%. This was accompanied by an were cooled in ice bath and the product mixture was increase in MTBN and DTBN formation. The MTBN dissolved in diethyl ether. Generally, mass balance of selectivity ratio (β/α) increased on increasing the >95% with respect to naphthalene and alkyl reaction temperature as the 2-(β-) isomer is naphthalenes was obtained. The products were thermodynamically more stable than the 1-(α-) analyzed by GC [Perkin-Elmer Auto System XL; isomer. The DTBN selectivity ratio 2,6-/2,7- column: Ultra 2 (cross-linked 5% PHME siloxane), decreased due to an increase in the formation of other film thickness: 0.52 μm, length: 25 m, i.d.: 0.32 mm] isomers. with FID detector. Similarly, the influence of reaction time was examined. It was noticed that the conversion Results and Discussion increased rapidly up to 2 h and then slowly (62.3% at The composition and surface area of the zeolites 2 h and 72.6% at 3 h). All the data reported here were used in the studies are presented in Table 1. collected at a reaction time of 2 h. Both (β/α) MTBN Preliminary experiments on the influence of catalyst and (2,6-/2,7-) DTBN selectivities increased with quantity were conducted at 170 °C and increase in time. Naphthalene conversion is maximum TBA/naphthalene mole ratio of 2 (naphthalene = 0.02 at a reactants mole ratio (TBA/naphthalene) of 2. On mole) by varying the catalyst quantity from 0.5 to 2 g. increasing the mole ratio from 2 to 4, the conversion of REY catalyst. An increase in the catalyst quantity decreases, presumably due to poisoning by the larger

NAGOTKAR et al.: ALKYLATION OF NAPHTHALENE WITH T-BUTANOL OVER ZEOLITE Y 353

amount of water produced during dehydration. Effect of Si/Al ratio and RE exchange Adsorption of excess alcohol molecules onto the A comparison of the performance of REY and Y catalyst also can lead to the passivation of active zeolite with different Si/Al ratios is presented in catalyst sites leading to lower conversion of Table 3. Comparing REY with HY, it is noticed that naphthalene. Other probable reasons could be a REY is only slightly more active than HY in spite of counter diffusion effect of TBA, which may reduce the anticipated greater acidity of REY. This may be the diffusion rate of naphthalene or its products in or due to the fact that the REY catalyst used was in a out of the pores of the zeolite, decrease in the initial bonded form and contained only 70% zeolite (30% naphthalene concentration due to the presence of alumina binder). With increase in the Si/Al ratio of excess alcohol, and catalyst deactivation by zeolites, the strength of the Brönstead acid sites oligomerization of the olefinic species produced by increases and the total number of the acid sites alcohol dehydration. β/α and 2,6-DTBN/2,7-DTBN decreases. The naphthalene conversion increases to a ratio decrease with increase in the reactant mole ratio. maximum of 78.2% at a Si/Al ratio of 12. At higher

Table 2 ⎯ Influence of reaction parameters on t-butylation of naphthalene over REY (Conditions: Temperature (oC): 170; naphthalene (mol): 0.02; t-butanol (mol): 0.04; catalyst weight (g): 1.00; reaction time (h): 2; pressure: self generated)

Parameter Value Naphthalene MTBN α+β DTBN Selectivity Selectivity ratio conversion (%) (%) for DTBN (%) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯– (%) β/α 2,6-/2,7- Temperature (ºC) 160 42.10 33.36 8.74 20.76 35.22 5.68

170 62.32 40.11 22.21 35.64 40.23 5.90

180 80.70 37.85 42.85 53.09 71.93 3.36

Duration of run (h) 1 17.24 12.11 5.13 29.73 36.43 5.88

2 62.32 40.11 22.21 35.64 40.23 5.90

3 72.62 40.80 31.82 43.85 44.80 6.30

TBA/Naph (mole) ratio 1 58.02 28.79 29.23 50.38 70.19 5.95

2 62.32 40.11 22.21 35.64 40.23 5.90

3 59.35 39.07 20.28 34.17 35.75 5.30

a N2 pressure (Initial) MTBN = Monotertiary-butylnaphthalene DTBN = Ditertiary-butylnaphthalene

Table 3 — Comparison of different Y-zeolites for their activity in t-butylation of naphthalene (Conditions: Temperature (ºC): 170; pressure: self generated; naphthalene (mol): 0.02; t-butanol (mol): 0.04; catalyst weight (g): 1.00; reaction time (h): 2)

Zeolite Naphthalene MTBN α+β DTBN Selectivity Selectivity ratio conversion (%) 2,6-+2,7- for DTBN ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ (%) (%) (%) β/α 2,6-/2,7- REY 62.32 40.11 22.21 35.64 40.23 5.90

HY 61.21 33.89 27.32 44.63 40.44 6.01

USY (12) 78.20 22.20 56.00 71.61 52.00 4.19

USY (30) 75.76 25.78 49.98 65.97 65.32 2.82

USY (80) 67.32 22.47 44.85 66.62 33.95 4.59

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Si/Al ratios, the conversion decreases. This indicates reaction period). To find out which of the reasons is that the naphthalene conversion is dependent upon the responsible for the decrease in conversion in the strength of acid sites as well as the number of acid presence of N2 or cyclohexane, a kinetic analysis of sites. At higher Si/Al ratios, the increase in the the data obtained at different temperatures under number of strong acid sites is offset by a larger different reaction environments was carried out. decrease in the total number of acid sites, leading to a Influence of reaction environment on the kinetics of alkylation decrease in the naphthalene conversion. In general, Naphthalene alkylation to mono- and di-alkylated the larger conversion over the dealuminated zeolites is products is a second order reversible, series reaction probably due to the greater hydrophobicity and that can be represented by the general equations: consequent larger resistance to poisoning by H2O generated in the reaction. A + B R + D ...(1) On increasing the Si/Al ratio, the concentration of R + B S + D ...(2) MTBN in the product decreases while that of DTBN passes through a maximum at the Si/Al ratio of 12 Estimation of equilibrium concentration for this (USY(12)). The larger amount of DTBN in the reaction based on free energy calculations revealed dealuminated zeolites is a result of the creation of the equilibrium conversion of naphthalene to be above mesoporosity in the zeolite crystals during 99% when equimolar quantities of the reactants are dealumination. As DTBN is bulkier than MTBN and used. Hence, the reaction rate constant for the first the reaction of MTBN to DTBN requires a bulkier step was calculated based on second order irreversible transition state, it appears that the pore size reaction kinetics as given below: restrictions are responsible for the lower -dC /dt = k C C DTBN/MTBN ratio on HY and REY. A 1 A B 2 Influence of reaction environment = k1CA0 (1-XA) (M-XA) ...(3) The influence of reaction environment (i.e. no M = CB0 /CA0 solvent (neat), with cyclohexane as solvent, and in = 2 ...(4) nitrogen pressure) on naphthalene conversion, and the selectivity ratios of MTBN and DTBN, β/α and 2,6- Here, CA, CA0 and CB, CB0 are the concentrations /2,7- DTBN ratio are presented in Table 4. For similar of reactants A and B respectively at time t and at the reaction conditions, naphthalene conversion is the start of the experiment. All the experiments were highest when the reaction is carried out under self carried out with a t-butanol/naphthalene mole ratio of generated pressure. The presence of cyclohexane or 2, for a period of 2 h. nitrogen results in the dilution of reactants i.e., a -dCA/dt = – (CA –CA0) /(2-0) decrease in the reactants’ partial pressure, which could result in a decrease in the conversion. = CA0 XA /2 ...(5) Introduction of organic solvents like cyclohexane can Equating equations (3) and (5) and simplification also lead to the development of mass transfer gives resistance around the catalyst surface which can also result in the lower observed conversions (for the same 2CA0 k1 = XA /[(1-XA) (2-XA)] ...(6)

Table 4 — Effect of reaction environment on the t-butylation of naphthalene over REY (Conditions: Temperature (ºC): 170; naphthalene (mol): 0.02; t-butanol (mol): 0.04; catalyst weight (g): 1.00; reaction time (h): 2)

Solvent Naphthalene MTBN α+β DTBN Selectivity Selectivity ratio conversion (%) 2,6-+2,7- for DTBN ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯– (%) (%) (%) β/α 2,6-/2,7- No Solvent 62.32 40.11 22.21 35.64 40.23 5.90

N2, 900 psig (Initial) 26.68 24.32 2.36 8.84 26.22 4.14

Cyclohexane (50 mL) 21.97 12.17 9.8 44.59 19.42 5.16

NAGOTKAR et al.: ALKYLATION OF NAPHTHALENE WITH T-BUTANOL OVER ZEOLITE Y 355

Calculation of the rate constant for different environments are presented in Table 5. The monoalkylation of naphthalene at 170 °C using the values are comparable for neat reaction and for integrated expression of Eq. (3) reaction in nitrogen environment. It is lower for the reaction in cyclohexane. Also the activation energy ln[(M-XA)/M(1-XA)] = CA0k1t ...(7) for the second step is slightly higher than the first, for neat reaction and reaction in nitrogen atmosphere. For and Eq. (6) gave comparable results. Hence. Eq (6) these two cases a higher reaction temperature favours was used for the estimation of the rate constant k 1 the formation of dialkylate. from the experimental data at different temperatures. When the reaction is carried out in the presence of The second order reaction rate constant for the a solvent or gas (N ) under pressure, the conversion second step (Eq. 2) was obtained from the standard 2 can reduce due to a dilution effect or due to increased expression: mass transfer resistance in the catalyst pores. A

dCs/dt = k2 CRCB ...(8) dilution effect will result in similar reaction rate constants but decreased conversion for the same CR and CS at time t were obtained from the reaction time. Decrease in conversion due to increase experimental data and CB, estimated by material in mass transfer resistance will be reflected in the balance. The activation energy for mono- and dialkylation Table 6 — Effect of reaction environment on activation energy values in t-butylation of naphthalene over REY (Conditions: same of naphthalene was calculated from the slope of the as Table 5) E1, E2: activation energies for the first and second graph ln k versus 1/T using the Arrhenius’ Expression, steps of the series reaction)

(-E/RT) Reaction medium E1 E2 k = koe ...(9) (kcal.mol-1) (kcal.mol-1) Reaction rate constants for the alkylation of naphthalene at different temperatures and the Neata 30.47 42.32

activation energy for alkylation in different b environments are presented in Tables 5 and 6. Nitrogen 34.59 37.44 c The reaction rate constants (k1) for MTBN Cyclohexane 26.53 25.50 formation (Eq. 1) and (k ) for DTBN formation have 2 a Selfgenerated pressure higher values for neat reaction, at all temperatures, b Initial pressure: 900 psig compared to other environments. The activation c 50 mL of cyclohexane energy values for the two steps of the reaction in

Table 5 — Effect of reaction environment on the kinetics of t-butylation of naphthalene over REY (Conditions: Temperature varied; other conditions same as in Table 4) k1, k2: second order reaction rate constants for the first and second step of the series reaction) Temperature (ºC) Neata Nitrogenb Cyclohexanec

k1 k2 k1 k2 k1 k2

(l.mol-1.h-1) (l.mol-1.h-1) (l.mol-1.h-1) (l.mol-1.h-1) (l.mol-1.h-1) (l.mol-1.h-1)

150 1.02 3.18 ⎯ ⎯ ⎯ ⎯

160 1.47 ⎯ 0.26 0.14 0.16 ⎯

170 3.26 3.58 0.79 0.51 0.18 2.59

180 6.53 10.42 1.46 1.46 0.60 4.92

a Self-generated pressure b Initial pressure: 900 psig c 50 mL of cyclohexane

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values of the rate constants and the activation energy. water produced during dehydration. RE exchanged Y A decrease in conversion and the rate constants is is slightly more active than HY and a larger observed when the reaction is carried out in presence conversion is observed over the dealuminated of solvent or gas. But the activation energies zeolites, probably due to the greater hydrophobicity estimated (Table 6) are higher than those for either a and consequent larger resistance to poisoning by H2O mass transfer limited reaction or a reaction in the generated in the reaction. transition region indicating clearly that the reaction is Naphthalene conversion is the highest when the not in the mass transfer limited regime. The 2,6-/2,7- reaction is carried out under self generated pressure. ratios (Table 4) are also not significantly affected by Rate constants of the reaction were found to be changes in the reaction medium, which again different in different environments. Activation energy indicates that the reactions are not limited by mass values for cyclohexane and nitrogen mediated transfer. Further research is necessary to find out the reactions were comparable to that of the neat reaction, reason for the observed difference in reaction rate the values indicating that the reaction is not in mass- constants in different environments. transfer controlled regime in any of the cases. The ratio of the observed reaction rates k2/k1 is References higher at lower temperature, for the neat reaction. In 1 Santilli D S & Chen C Y, US Patent 6, 015, 930 (2000). the case of nitrogen environment, the ratio initially is 2 Motoyuki M, Yamamoto K, Sapre A V & McWilliams J P, less than one and reaches one at a reaction US Patent 6, 011, 190 (2000). temperature of 180 °C. Hence, a nitrogen environment 3 Liu Z, Moreau P & Fajula F, Appl Catal A: Gen, 159 (1997) favours the formation of MTBN at lower 305. temperatures. In the case of cyclohexane, dialkylate 4 Liu Z, Moreau P & Fajula F, Chem Commun, (1996) 2653. 5 Kamalakar G, Ramakrishna P M, Kulkarni S J & Raghavan formation is high, the k2/k1 ratio being 14.4 and 8.2 at K V, Proc Int Zeolite Conf, 12th Meeting, Vol. 2, edited by 160 and 170 °C. Treacy M M J, Materials Research Society: Warrendale, Pa., Conclusion 1999, 897. The t-butylation of naphthalene over different 6 Armengol E, Corma A, Garcia H & Primo J, Appl Catal A, 149(2) (1997) 411. Y-zeolites and different environments has been 7 Kitabayashi S, Shindo T, Ono K & Ohnuma H, Kagaku studied. An increase in TBA/naphthalene mole ratio Kaishi, 7 (1996) 624. beyond 2 results in a decrease in conversion, 8 Marathe R P, Mayadevi S, Pardhy S A & Sivasanker S, presumably due to poisoning by the larger amount of J Molecular Catal, 181 (2002) 201.