Intensification and Selectivity Modulation of Ion-Exchange Resin

ARTICLE pubs.acs.org/IECR

Intensification and Selectivity Modulation of Ion-Exchange Resin Catalyzed Alkylation of Phenol by Microwave Asraf A. Ali and Vilas G. Gaikar* Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai—400 019, India ABSTRACT: Microwave irradiation has been used to intensify the alkylation reaction of phenol with tert-butyl alcohol using ionexchange resin as the catalyst. About 86% conversion of phenol is obtained in 4 min under microwave irradiation. The major products are ortho-tert-butyl phenol (o-TBP), para-tert-butyl phenol (p-TBP) and 2,4-di-tert-butyl phenol (2,4-DTBP). o-TBP predominates in the product mixture as the kinetically favored product in shorter reaction times, but p-TBP is the major product for longer reaction times. The selectivity toward ortho- or para-substituted phenols can be manipulated by controlling the exposure time of the reaction mixture to the microwave irradiation. The LangmuirHinshelwoodHougenWatson (LHHW) surface reaction controlled model was used for the kinetic data analysis.

1. INTRODUCTION Alkylation of phenol with tert-butyl alcohol is an industrially important FriedelCrafts alkylation reaction, as both o- and palkylated phenols find a wide range of applications. o-tert-Butyl phenol (o-TBP) is an intermediate for pesticides and fragrances, while p-tert-butyl phenol(p-TBP) is a raw material for production of resins, printing inks, rubber chemicals, antioxidants, fungicides, and petroleum additives. 2,4-Di-tert-butyl phenol (2,4DTBP) and 2,6-di-tert-butyl phenol(2,6-DTBP) also find use as antioxidants and intermediates in pharmaceuticals.16 Mineral acids,7 and Lewis acids, such as AlCl3 and BF38 were mostly used as catalysts for alkylation of phenol before the advancement of solid acid catalysts such as metal-exchanged zeolites,2,912 and in recent years, ionic liquids such as [bmim]PF6, [omim]BF4, and [hmim]BF4,13,14 clays and modified clays,15 molecular sieves SAPO-115, ion-exchange resins,1619 and sulfated zirconia and nanosulfated titania.15,20 Solid heterogeneous catalysts are now prevalent because of green chemistry principles, and most give selectivity unattainable by homogeneous catalysts. Scheme 1 shows a network of reactions taking place during the alkylation process. The literature on alkylation of phenol shows a wide variation in the rates and selectivity toward o- and p-substitutions on the phenolic ring. For example, FeAl substituted MCM-41 and H-GaMCM-48, reportedly gave 80% selectivity toward o-TBP in the vapor phase butylation of phenol.10 On the other hand, zeolites, prepared from coal fly ash, were highly selective (95%) toward p-TBP.12 In recent years, ionic liquids have gained attention because of their almost zero volatility under reaction conditions and modulation of their activity by modifying either the alkylated structure or the anion.13,14 In the presence of ionic liquid [bmim]PF6, 2,4-DTBP is the major product, having 75% selectivity. However, more than 99% of ether was formed when [omim]BF4 and [hmim]BF4 were used.13 In the presence of sulfonic acid (SO3H) functionalized ionic liquid, o-butylated phenol was preferentially formed.14 Butylation of phenol with methyl tert-butyl ether (MTBE), using 20% ditungstophosphoric acid(DTP)-K10 as catalyst, gives almost r 2011 American Chemical Society

equal selectivity toward o- and p-alkylated phenols, whereas sulfated zirconia and nanosulfated titania were selective toward the formation of p-TBP.15,20 A vapor phase butylation of phenol over Al-SBA-15 also gives higher selectivity toward p-TBP.21 The acid strength of the catalyst also decides if etherification on phenolic OH would take place instead of C-alkylation. O-Alkylation results in formation of phenyl ethers when mildly acidic catalysts are used.1,10,13 o-TBP is formed at a relatively faster rate initially but as the reaction proceeds it is likely to isomerize to a stable p-TBP.1,5,21,22 The alkylation process is reportedly very slow and is conducted for several hours for appreciable conversions.2,4,6,10,1316,18,19 Unconventional methods, such as sonication and/or microwave irradiation, apart from using improved catalysts, have been reported to increase the rates of similar alkylation reactions and to modify the selectivity toward the desired compounds. For example, the rate of alkylation of benzene with cyclohexene has been increased by an order of magnitude in the presence of ultrasound.23 Microwave activation of reactants has emerged in the recent past as a powerful technique to promote a variety of chemical reactions in synthetic chemistry.2426 The catalytic isomerization of o-TBP using montmorillonite-KF under the influence of microwave has been reported.22 Microwave radiation is strongly absorbed by polar or polarizable compounds by dielectric loss. The rapid orientation of the polar and/or ionic species in the fluid phase leads to rapid frictional heat generation at the molecular level when the molecules try to align themselves along the rapidly changing electromagnetic field. The energy is transferred to the molecules before their relaxation, resulting in instantaneous temperature rise at the molecular level that enhances the rates and in many cases the yield by avoiding side reactions. Thus the reactions can be conducted in a very short time period23,2736 and also under solvent-free conditions.35 Received: October 12, 2010 Accepted: April 16, 2011 Revised: April 16, 2011 Published: April 16, 2011 6556

dx.doi.org/10.1021/ie102051k | Ind. Eng. Chem. Res. 2011, 50, 6556–6566

Industrial & Engineering Chemistry Research Scheme 1. Reaction of Phenol with tert-Butyl Alcohol

In solid-catalyzed heterogeneous reactions, the reaction center is the catalytic site on the surface of the catalyst where the reactive species are formed by their association with the catalyst. If the energy is made directly available to these sites, the rate of the catalytic reaction can be increased by several fold. The advantage of such an energy transfer lies in the fact that the reacting molecules are thermally activated enough to participate in the chemical reactions at a much enhanced rate. However, if the temperature and microwave power are not controlled properly in such heterogeneous reactions, where the catalyst is a strong microwave absorber like metals, metal oxides, or carbon (graphite), then it could lead to thermal runaway with disastrous consequences. There are also possibilities of formation of localized hot spots and/or even arcing on these catalysts which could lead to melting of the apparatus or even an explosion.33,34 It is, therefore, essential that such reactions are studied in controlled conditions of microwave energy supply so that the temperature at the surface of the catalyst does not reach to very high levels. This is also important for scaling up the reactors for large scale applications. In the current work, our objective was two-fold: (1) to use microwave to provide energy to the solid catalyst itself and (2) to facilitate selective formation of the desired products by exploiting the temperature dependence of the reaction rates of the two substitution reactions. The experimental data were obtained under controlled microwave conditions and further used for estimation of the LHHW model parameters of the reaction. Also a continuous setup was established for the synthesis of alkylated phenols. It is worth noting that if the microwave can speed up the reaction several fold, the size of the reactor even for large scale applications can be reduced significantly.

2. EXPERIMENTAL DETAILS 2.1. Materials and Reagents. Phenol (∼99.5%), tert-butyl alcohol (∼99.9%), and Amberlyst-15 (Rohm and Hass) were procured from s.d. Fine Chemicals, Mumbai. The standards for analysis of o-TBP, p-TBP, 2,4-DTBP, 2,6-DTBP, and 2,4,

ARTICLE

6-tri-tert-butyl phenol (2,4,6-TTBP) were obtained from Merck India Ltd., Mumbai. All the standards were of analytical grade of 98% assay. 2.2. Experimental Setup. 2.2.1. Catalyst Modification. A 5 g portion of Amberlyst-15 resin was suspended in 50 cm3 of H2SO4(98% w/w) and heated to 160 °C for 48 h. The resin was then filtered and washed with distilled water until the wash solution was completely acid-free. The resin was then calcined at 300 °C in an oven for 4 h to prepare the carbonized sulphonated acid (CSA) catalyst. The SEM and EDAX analysis of the CSA catalyst was carried out on a scanning electron microscope (JEOL JSM 6380 LA). The calcination did not melt the polymer matrix but resulted in a small degree of cracking of the matrix creating a somewhat open porous structure than the original catalyst beads. The EDAX analysis shows 8% increase in S content of the catalyst after the H2SO4 treatment. The recovery of the catalyst was complete without any loss of weight. The experiments were conducted in a batch mode under controlled microwave conditions and also in a continuous mode using a tubular packed-bed reactor in a modified domestic oven. 2.2.2. Batch Experiments. All the batch reactions were conducted in a self-tuning single-mode microwave cavity of Discover Bench Mate System (model: Discover System 908010, CEM Corporation, U.S.A.). The system contains a single mode microwave cavity with the power output from 0 to 300 W, programmable in 1 W increment. The system can be operated at constant power input or under temperature controlled conditions. The system contains a noncontact infrared sensor which monitors the reactor mixture temperature and gives feedback to an on-board computer which controls the input power, so that the temperature of the system can be maintained to the desired limit. A fully baffled mechanically agitated glass reactor of volume 150 cm3 (4.5 cm i.d.) equipped with a six-bladed turbine impeller running at 800 rpm was kept in the microwave cavity of the Discover system. Two overhead condensers in series, both under cooling water circulation, were used to condense the vapors generated in the reactor. The condensate was refluxed continuously back into the reactor, while the gaseous byproduct was allowed to escape to avoid pressurization of the reactor during the operation. In a typical run, known amounts of phenol, tert-butyl alcohol, and the catalyst were charged into the reactor. The initial batch reaction experiments were carried by intermittently passing the microwave energy through the reaction mixture with an exposure time of 5 min followed by a nonexposure time of 1 min, during which a sample was removed for the analysis. Initially for 6090 s, the power supplied to the reaction medium was about 300 W until the set temperature was reached. Thereafter, the microwave power input decreased to 1030 W, and finally the microwave power input settled to just 110 W. The mixture was then cooled to the ambient temperature in a very short time period of 2 min. The liquid samples were withdrawn from the mixture and analyzed by gasliquid chromatography (GLC). 2.2.3. Continuous Alkylation in a Microwave Oven. A domestic microwave oven (LG Electronics, MS-1927C) with a multimode microwave cavity of 30 cm 27 cm 20 cm was modified to develop the experimental setup. The experimental setup consisted of a feed HPLC-pump (JASCO, PU-980), a fixedbed tubular glass reactor of i.d. 5 mm inside the microwave cavity, and a condenser and collector assembly outside the oven cavity. A sintered disk G3 was fitted at the end of the reactor to avoid the catalyst loss and to filter the reaction liquid. A typical run consisted 6557

dx.doi.org/10.1021/ie102051k |Ind. Eng. Chem. Res. 2011, 50, 6556–6566

Industrial & Engineering Chemistry Research

Figure 1. Conversions of phenol versus time at 100, 110, and 115 °C; mole ratio of phenol/tert-butyl alcohol = 1:1, catalyst loading = 4% (w/w).

of pumping a premixed mixture of phenol and tert-butyl alcohol (TBA) through the fixed bed reactor consisting of 1 g of the catalyst. The product mixture was collected in the reservoir and analyzed at regular intervals until the product composition remained constant. The gaseous byproduct was vented out of the condenser/collector assembly to avoid overpressurization of the reactor. The time spent on the catalyst bed (W/F) on the catalyst bed was calculated as a ratio of weight of the catalyst (in g) to flow rate of the feed (in g 3 min-1). W=F ¼

weight of catalyst ðgÞ flow rate of feed ðg 3 min1 Þ

2.3. Method of Analysis. The samples were analyzed using a Chemito 8610 gas chromatograph, with a flame ionization detector, using OV-17(10%) column (3.175 mm diameter 4 m length). Nitrogen was used as the carrier gas. A standard calibration method with synthetic mixtures was used for quantification of GC data. The products were confirmed on PerkinElmer GCMS equipped with Clarus-500 gas chromatograph and Clarus-500 mass spectrometer using a 30 m long BP-1 capillary column.

3. RESULTS AND DISCUSSION Batch Reaction. The initial batch reaction experiments were carried out at three bulk reaction mixture temperatures, 100, 110, and 115 °C, to study the kinetics of the reaction. The temperature of the reaction mixture was maintained in the microwave

ARTICLE

Figure 2. Selectivity of o-tert-butyl phenol and 2,4-di-tert-butyl phenol at 100, 110, and 115 °C; mole ratio of phenol/tert-butyl alcohol = 1:1, catalyst loading = 4%.

system by the onboard computer by varying the power input of the system. The molar feed ratio of phenol to TBA was maintained at 1:1 in all experiments for a catalyst loading of 4% (w/w). 3.1. Effect of Temperature. Figure 1 shows that conversion of phenol increased, in 30 min of reaction time, from 20% at 100 °C to 30% at 110 °C and 39% at 115 °C. The selectivity of the products, o-TBP, p-TBP, and 2,4-DTBP at various temperatures is shown in Figure 2. o-TBP is formed with 63% selectivity initially at 100 °C but settled to 59% at the end of reaction time. The selectivity toward o-TBP, decreased still further but marginally to 56% and 55% at 110 and 115 °C, respectively. The amount of p-TBP increased initially with time but in the final mixture decreased with increase in temperature. At 100 °C, the selectivity toward p-TBP was 38% which decreased to 36% at 115 °C. DTBP, in the product mixture, increased significantly with the increase in the reaction temperature, from 3% at 100 °C to 8% at 115 °C. Thus, increase in the reaction temperature increases the conversion of phenol, but, the selectivity toward formation of monoalkylated products decreases marginally with increase in temperature because of their subsequent alkylation. The conversion of phenol is also higher under the influence of microwave radiation in a much shorter time as compared to reported conversions without microwave irradiation.16,19 Amberlyst-15 is a wellknown acidic sulfonic styrene divinylbenzene resin and has been used as a catalyst for butylation of phenol using tert-butyl alcohol (TBA) or isobutylene. With TBA, 20% (w/v) catalyst loading has been reported to give only 30% conversion of phenol in 10 h at 6558



Figure 3. Conversions of phenol versus time at 1:1, 1:2, and 1:3 mol ratios of phenol/ter-butyl alcohol, T = 110 °C, catalyst loading = 4% (w/w).

56 °C.16 With iso-butylene also the conversion of phenol was only 20%. But the o/p ratio in the product changed from 30:70 at 80 °C to 5:95 at 120 °C.19 Although, no kinetic data was provided for butylation of phenol using Amberlyst-15 by these authors, the change in selectivity with temperature indicates different activation energies of two substitutions and therefore a possible tool for modifying the selectivity The selectivity toward o-TBP is 55% under microwave, comparatively higher than 48% in the conventional heating method.16 However, the selectivity toward the dialkylated product, that is, 2,4-DTBP is lower at 8% under microwave, as compared to 17% without microwave.16 3.2. Effect of Mole Ratio. Figure 3 shows the conversion of phenol with time at three different molar ratios of phenol to tertbutyl alcohol at 4% catalyst loading. The conversion of phenol was 30% for the molar ratio of 1:1 in 30 min at 110 °C. It increased to 40% in 30 min and 42% in 45 min for the molar ratio of 1:2. For the mole ratio of 1:3, the conversion of phenol increased to 81% during the same reaction time. The conversion of phenol, therefore, increases substantially with the increase in molar concentration of the alkylating agent. Apart from alkylation of phenol, tert-butyl alcohol also undergoes a dehydration reaction in the presence of the acidic catalyst to form iso-butylene. The loss of tert-butyl alcohol to iso-butylene was estimated from the unreacted TBA and alkylated product in the reaction mixtures. The loss of TBA was about 42% for 1:1 molar ratio of phenol to tert-butyl alcohol. Thus, the reduced availability of the

ARTICLE

Figure 4. Selectivity of products at 1:1, 1:2, and 1:3 mol ratios of phenol/tert-butyl alcohol, T = 110 °C, catalyst loading = 4% (w/w).

alkylating agent causes lower conversion of phenol. If isobutylene can be retained in the reaction mixture, or made to react before escape from the mixture, the conversion of phenol can be improved. Figure 4 shows the corresponding selectivity toward o-TBP and 2,4-DTBP for 1:1, 1:2, and 1:3 molar ratios of phenol/tertbutyl alcohol, at 110 °C. With the increased alcohol content with respect to phenol, the dialkylated product in the product mixture also increased significantly apart from increased conversion of phenol. The selectivity toward o-TBP was relatively unaffected in the range 5456% with slight decrease at higher molar ratio. The selectivity toward p-TBP also remained in the band 37% to 34% at 115 °C with increased alcohol content of the feed. The 2,4DTBP content in the product mixture, however, increased from 6.6% to 10.2% with increase in tert-butyl alcohol content in the reaction mixture. Only trace amounts of 2,6-DTBP(∼0.06%) was detected in the product mixture, but we did not find any trialkylated product in the product mixtures. Thus, the increase in the mole ratio of tert-butyl alcohol with respect to phenol increased the conversion of phenol and selectivity toward 2,4-DTBP. However, the increased content of tert-butyl alcohol decreased the selectivity toward monoalkylated products because of subsequent alkylation. 3.3. Effect of Catalyst Loading. The butylation of phenol was conducted at different catalyst loadings, that is, at 4%, 10%, and 15% (w/w of phenol), keeping the molar ratio of phenol to tertbutyl alcohol at unity and temperature at 110 °C. Figure 5 shows 6559



Figure 5. Conversions of phenol versus time at 4%, 10%, and 15% catalyst loading; mole ratio of phenol/tert-butyl alcohol = 1:1, T = 110 °C.

an increased conversion of phenol with the increase in the catalyst loading in the given time of reaction. The conversion of phenol at the catalyst loading of 4% was about 30% where as that for 10% catalyst loading the conversion increased to 43%. With the increase in the catalyst loading the number of active sites available for the reaction increases and so does the rate of the reaction. The increase in the catalyst loading from 4% to 10% almost doubled the initial rate of the reaction from 6.9 103 to 14.3 103 gmol g1 min1. As the catalyst loading was further increased to 15% the conversion of phenol increased to 53% in the same time, but the initial rate increased only by a factor of 1.3 as compared to 10% loading. This proportionate increase in the alkylation rates shows kinetic effects of the reaction. With the increased catalyst loading the formation of iso-butylene, however, also increased leading to more loss of tert-butyl alcohol as isobutylene. Therefore, although initial rate indicated almost a proportionate increase, the final conversions were lower. As was observed earlier the loss of isobutylene from the reaction mixture limits the conversion of phenol. Figure 6 shows the selectivity toward different products at these catalyst loadings. The selectivity toward o-TBP decreased with time from 60% at 5 min to 56% in 30 min at catalyst loading of 4%, whereas at 10% catalyst loading it decreased from 58% to 50% and at 15% catalyst loading, the selectivity toward o-TBP decreased significantly, from 51% to 44% during the same time period. However, the selectivity toward 2,4-DTBP and p-TBP

ARTICLE

Figure 6. Selectivity of products at 4%, 10%, and 15% catalyst loading; mole ratio of phenol/tert-butyl alcohol = 1:1, T = 110 °C.

increased with the increase in the catalyst loading. As more of the sites, with increased catalyst loading, capture the monoalkylated phenol, the subsequent reaction also gets promoted. Thus the selectivity toward the dialkylated product, 2,4-DTBP, increases. The isomerization of o-TBP to p-TBP is also possibly taking place at the same catalytic sites, increasing the selectivity toward p-TBP. Thus the time that a reactant or a monoalkylated product spends at the reaction site with respect to reaction time decides the subsequent conversion. A more active catalyst or increased energy density at the point of application of energy, can lead to more dialkylated product or isomerization of o-TBP to p-TBP. A question, therefore, arises about the product composition and conversion when the microwave is passed for comparatively longer time and continuously. Hence, we decided to study the effect on the product composition and conversion of continuously exposing the reaction mixture to microwave to maintain the temperature of the reaction sites for a comparatively longer period. The reactions were carried out at 110 °C at mole ratios of 1:1 and 1:2 (phenol/tert-butyl alcohol). The results obtained were quite different from what had been observed previously under intermittent microwave irradiation. The conversion of phenol was higher and selectivity toward o-TBP was lower. Once the desired bulk temperature was reached in 30 s, the microwave power reduced to a range of 110 W, and after 5 to 7 min of the further irradiation, the power input reduced to just 13 W. The conversion of phenol under the continuous irradiation at 110 °C and 1:1 (phenol/tert-butyl alcohol) mole ratio is 42%, as against 30% obtained under intermittent radiation. Similarly, the 6560



ARTICLE k2

conversion of phenol using the mole ratio of 1:2 (phenol/tertbutyl alcohol) is 60% as against 40% under intermittent microwave radiation. The selectivity toward o-TBP is the highest during intermittent irradiation, whereas the selectivity toward p-TBP is the highest during the continuous microwave irradiation. Since the microwave power decreased to just 13 W after about 5 min, the microwave irradiation reaching to the catalyst surface maintains the thermal conditions prevalent at the reaction sites. We also observe that the p-TBP is formed more selectively when microwave radiation was available continuously. The selectivity for p-TBP under continuous microwave irradiation was 95% under both 1:1 and 1:2 mol ratios of phenol/TBA, respectively, at 110 °C. A similar result was reported in terms of selectivity by Chaudhuri and Sharma19 at 120 °C using conventional heating mode. It clearly indicates that under continuous microwave irradiation the temperature of the reaction sites plays a more important role in the product formation. On the catalyst, phenol combines with tert-butyl alcohol to form the monoalkylated products (o-TBP and p-TBP). As o-TBP is formed initially at the surface, it probably isomerizes on the surface of the catalyst itself to p-TBP, or formation of p-TBP is favored at the reaction sites on the catalyst surface where the major absorption of microwave energy is expected, having higher activation energy than formation of o-TBP. With continuous microwave irradiation, the temperature of the catalyst surface must be rising and promoting either the p-substitution or even isomerization of o-TBP to p-TBP. Amberlyst-15 has already been reported as a catalyst for the alkylation of phenol and various other phenolic compounds under conventional methods.16,17,19 Considering that only 30% conversion of phenol was obtained in 10 h using this catalyst by conventional method,16 81% conversion in 30 min under influence of MW radiation, even at 13W power input is impressive. Similarly, unlike a product composition of 53% o-TBP, 32% p-TBP, 11.5% 2,4-DTBP, and 1.5% 2,6-DTBP using conventional heating,16 the continuous microwave radiation exposure gave p-TBP with 95% selectivity. The enhanced rates and selectivity modulation under microwave radiation indicates possible application of the microwave for production of these chemicals.

ðR-2Þ

B þ S a0 BS k2

(b) Surface reactions at the catalyst sites (i) Formation of monoalkylated products k3

AS þ BS a0 CS þ WS

ðR-3Þ

k3

k4

AS þ BS a0 DS þ WS

ðR-4Þ

k4

(ii) Formation of dialkylated products k5

CS þ BS a0 ES þ WS

ðR-5Þ

DS þ BS a0 ES þ WS

ðR-6Þ

k5 k6 k6

The two dialkylated products are clubbed together to simplify the analysis. In any case 2,4-DTBP is the major dialkylated product and 2,6-DTBP is formed in trace amounts. (iii) Isomerization of monoalkylated product k7

ðR-7Þ

CS a0 DS k7

(iv) Dehydration of tert-butyl alcohol k8

BS a0 B0 þ WS

ðR-8Þ

k8

(c) Desorption of intermediates and products k9

CS a0 C þ S

ðR-9Þ

k9

k10

DS a DþS 0

ðR-10Þ

k10 k11

EþS ES a 0

ðR-11Þ

k11

4. KINETIC MODEL FOR ALKYLATION OF PHENOL The LangmuirHinshelwoodHougenWatson (LHHW) approach was used to generate the rate expression for the reaction scheme. The model considers the adsorption of tertbutyl alcohol and phenol on the active sites of the catalyst. On the catalytic active site (S), tert-butyl alcohol (B) combines with phenol (A) to form first the monoalkylated products (C and D). These monoalkylated products further undergo alkylation to form dialkylated product (E), if more tert-butyl alcohol is available at the catalytic site to combine with the monoalkylated products. In addition, tert-butyl alcohol undergoes dehydration reaction to form isobutylene (B0 ) and water (W). This reaction scheme is represented by the following mechanism: (a) Adsorption of the reactants k1

A þ S a0 AS

k12

WþS WS a 0

ðR-12Þ

k12

The rate of adsorption of various compounds can be written for the reactions R-1, R-2 and R-9R-12, CAS ð r A ÞAd ¼ k1 CA CS ð1Þ K1 CBS ð r B ÞAd ¼ k2 CB CS K2 ð r C ÞAd ¼ k9

CBS CB C S KC

ðR-1Þ

ð r D ÞAd ¼ k10

k1

6561

ð2Þ

CDS CD CS KD

ð3Þ ð4Þ


Industrial & Engineering Chemistry Research ð r E ÞAd ð r W ÞAd

ARTICLE

CES ¼ k11 CE CS KE CWS ¼ k12 CW CS KW

The adsorption of DTBP is assumed to be poorer as compared to others species as it is the largest species in size,12 that is, 1 þ K1CA þ K2CB þ KCCC þ KDCD þ K W C . K E CE . The rate of disappearance of phenol is thus reduced to

ð5Þ ð6Þ

rAS ¼ Ka K1 K2 CA CB =ð1 þ K1 CA þ K2 CB þ KC CC þ KD CD

where 1 1 1 1 KC ¼ KD ¼ KE ¼ KW ¼ K9 K10 K11 K12

þ KW C W Þ 2

The rate of alkylation of phenol(rAS) is given as 0

0

r AS ¼ ðk3 þ k4 ÞCAS CBS k3 CCS CWS k4 CDS CWS

where Ka = (k3 þ k4)CT2 Similarly, the rate equations are obtained for the formation of products as follows: r CS ¼ Km CB ðk 3 K1 CA k 5 KC CC Þ=ð1 þ K1 CA þ K2 CB

ð7Þ

þ KC CC þ KD CD þ KW CW Þ2 Kiso KC CC =ð1 þ K1 CA

The rate of formation of mono- and dialkylated products are as follows: 0

0

þ K 2 CB þ K C CC þ K D CD þ K W CW Þ

0

r CS ¼ k3 CAS CBS þ k5 CES CWS þ k7 CDS k3 CCS CWS k5 CCS CBS k7 CCS ð8Þ 0

þ KC CC þ KD CD þ KW CW Þ2 þ Kiso KC CC =ð1 þ K1 CA þ K 2 C B þ K C CC þ K D CD þ K W CW Þ ð20Þ

r DS ¼ k4 CAS CBS þ k6 CES CWS þ k7 CCS k4 CDS CWS 0

ð9Þ

r ES ¼ Km CB ðk 5 KC CC þ k 6 KD CD Þ=ð1 þ K1 CA þ K2 CB

and 0

þ K C CC þ K D CD þ K W CW Þ 2

0

r ES ¼ k5 CCS CBS þ k6 CDS CBS k5 CES CWS k6 CES CWS

ð10Þ If the reversible nature of all reactive steps are neglected, then eqs 710) can be reduced to eqs 1114, r AS ¼ ðk3 þ k4 ÞCAS CBS 0

ð12Þ r DS ¼ k4 CAS CBS þ k7 CCS k6 CDS CBS r ES ¼ k5 CCS CBS þ k6 CDS CBS

ð13Þ

CAS ¼ K1 CA CS ,

CBS ¼ K2 CB CS ,

CCS ¼ KC CC CS ,

CDS ¼ KD CD CS ,

CES ¼ KE CE CS ,

and

CWS ¼ KW CW CS ð15Þ

Balancing the total number of catalytic sites, CT ¼ CS þ CAS þ CBS þ CCS þ CDS þ CES þ CWS ð16Þ one can get the vacant sites as CS ¼ CT =ð1 þ K1 CA þ K2 CB þ KC CC þ KD CD þ K E CE þ K W CW Þ

r AS ¼

ð17Þ

K a K 1 K 2 CA CB ð1 þ K1 CA þ K2 CB Þ2

ð22Þ

The temperature dependence of the kinetic and adsorption parameters are expressed mathematically as Ea 0 K ¼ K exp ð23Þ RT K1 ¼ K10 exp

ð14Þ

The adsorbed phase concentration can be related to the bulk phase concentration if adsorption is rapid as compared to chemical reaction rates,

ð21Þ

At, t = 0, CC = CD = CE = Cw = 0 Thus the initial rate for the disappearance of phenol is given as

ð11Þ

r CS ¼ k3 CAS CBS k3 CCS CWS k5 CCS CBS k7 CCS

ð19Þ

r DS ¼ Km CB ðk 4 K1 CA k 6 KD CD Þ=ð1 þ K1 CA þ K2 CB

0

k6 CDS CBS k7 CDS

ð18Þ

ΔH ad RT

ð24Þ

The parameters Ka, K1, and K2, were determined by fitting the LHHW model with initial rate data in eq 22. The remaining parameters KC, KD, KW were determined by fitting the LHHW model with experimental rate data over the entire reaction time. The optimum values of these parameters are obtained by minimizing the square of relative difference between the experimental rates and the theoretical rates. The experimental rate was calculated from the concentration profile of phenol with respect to time as follows, 1 dCA ð25Þ r ¼ w dt The rates of formation of products as obtained from eqs 1921 were also fitted with the experimental data and the rate constants for the formation of individual products were estimated. Tables 1 and 2 summarize the values of the rate constants, adsorption equilibrium constants, activation energy, and the pre-exponential factors for the system. 6562



ARTICLE

Table 1. Adsorption and Kinetic Constants for the Alkylation of Phenol in the Presence of Microwave, with LHHW Model 100 °C 1

Ka (g g

1

mol

1

min )

110 °C

115 °C

0.0422

0.1356

0.1565

Km (g g1 mol1 min1) k3 (g g1 mol1 min1)

0.01775 1.1221

0.0355 1.4358

0.07374 1.8692

k4 (g g1 mol1 min1)

0.4931

0.6675

0.8722

k5 (g g1 mol1 min1)

0.3576

0.6252

0.9544

k6 (g g1 mol1 min1)

0.0088

0.0098

0.0099

K1 (dm3 g1)

0.7885

0.6360

0.4106

K2 (dm3 g1)

0.5951

0.4608

0.3197

KC (dm3 g1)

1.3586

1.0829

1.0366

KD (dm3 g1) Kw (dm3 .g1)

1.3586 2.1986

1.2483 1.7892

1.1222 1.6719

Table 2. Activation Energy and Pre-exponential Factors pre-exponential Ea (kJ/mol) ΔHad (kJ/mol)

factor

o-TBP (from phenol) p-TBP (from phenol)

39.17 44.26

3.34 105 7.66 105

2,4-DTBP (from o-TBP)

76.86

2.03 1010

adsorption (phenol)

48.27

1.44 107

adsorption (TBA)

46.87

1. 68 107

adsorption (o-TBP)

23.93

6.07 104

adsorption (p-TBP)

14.53

9.67 103

adsorption (water)

22.82

1.62 103

The products are formed when phenol and TBA are adsorbed on the surface at the vicinity of each other. However, the adsorption constant of the products are comparatively higher than the reactants which indicates that as the reaction proceeds the products get adsorbed on the catalyst surface reducing the adsorption of reactants. Hence, the rate of reaction decreases after a certain degree of conversion. As a result, though the initial rates are high, the conversion gets limited. Also, the adsorption constant of water is the highest among all the species. This indicates that water formed in the reaction remains adsorbed on the surface of the catalyst reducing the catalytic activity. The rate constant for the formation of o-TBP (k3) is about 2.15 times higher than that for formation of p-TBP. This explains the comparatively higher initial selectivity toward o-TBP. The rate constant for formation of 2,4-DTBP from o-TBP is 0.95 g/ gmol min. This is 2 orders of magnitude higher than the rate constant of its formation from p-TBP (0.0099 g/gmol min) at 115 °C. This indicates clearly that the dialkylated product (2,4DTBP) is formed mainly from o-TBP. The isomerization constant kiso is of the order of 106, which is very low. Hence, the isomerization reaction can be neglected. The estimated energy of activation for formation of o-TBP is 39 KJ/mol, whereas for the formation of p-TBP it is 44 KJ/mol. The lower value of activation energy indicates that the formation of o-TBP is kinetically controlled, while a higher activation energy for the formation of p-TBP indicates it is preferential formation at higher temperatures. The rate constant for alkylation of phenol increases from 0.042 at 100 °C to 0.156 at 115 °C, while the adsorption constants decrease with an increase in temperature. Though the reaction rate increases with an increase in temperature, the

Figure 7. Conversions of phenol at various mole ratios of phenol/tertbutyl alcohol in the continuous system at 800 W and 1 g of catalyst.

adsorption of water on the catalytic site limits the overall rate and conversions. Microwave radiations reportedly does not change the activation energy but provides the momentum to overcome this barrier and complete the reaction more quickly. The temperature that is measured in the system is the bulk temperature and not that of the catalyst. Hence, an attempt was made to calculate the temperature of the catalytic site. The activation energy was taken the same as that obtained (110 kJ/mol) and the pre-exponential factor was taken as 2.64 105 from the literature of the similar reaction using zeolite.12 The calculated temperature was about 574 °C when the measured bulk temperature was 100 °C which seems to be quite unrealistic. Because at this temperature the catalyst would have either fused or broken down. Since microwave does not catalyze the reaction by reduction of activation energy, the microwave enhancement effect must be a combination of the increased local temperature and a different preexponential parameter of the rate constant.

5. CONTINUOUS PACKED-BED REACTOR FOR ALKYLATION OF PHENOL In the continuous mode of reaction, although microwave radiation was focused on the catalyst surface, the reaction mixture passing over the catalyst surface carries away a significant amount of heat as the sensible heat. A premixed solution of phenol and tert-butyl alcohol was used as feed. The reactions were carried out 6563



ARTICLE

Figure 8. Selectivities of o-tert-butyl phenol and 2,4-di-tert-butyl phenol at various mole ratios of phenol/tert-butyl alcohol in the continuous system at 800 W and 1 g of catalyst.

at 800 W microwave power input by varying the molar ratio of phenol to tert-butyl alcohol. Figure 7 shows the conversion of phenol at different phenol/tert-butyl alcohol molar ratios. The products formed are o-TBP, p-TBP, 2,4-DTBP, and trace amounts of 2,6-DTBP. The conversion of phenol increased, both with an increase in W/F and an increase in molar ratio with respect to the alcohol. The conversion increased from 54% to 86% at W/F of 4 min when the molar ratio of phenol/tert-butyl alcohol was increased from 1:1 to 1:4. The conversion obtained with molar ratios of 1:2 and 1:3 at W/F of 4 min are intermittent at 66% and 79%, respectively. Similarly, at a constant mole ratio of phenol/tert-butyl alcohol of 1:4, the conversion of phenol increased from 38% to 86% when the W/F was increased from 0.5 to 4 min. Figures 8 and 9 show the selectivity toward various products formed at 800 W at different mole ratios of phenol/tert-butyl alcohol. The selectivity toward o-TBP decreased, whereas the selectivity toward p-TBP and 2,4-DTBP increased with increase in W/F. The selectivity toward o-TBP at mole ratio of 1:1 decreased dramatically from 73% to 39% when the W/F was increased from 0.5 to 4 min. The selectivity toward p-TBP at the mole ratio of 1:1 increased from 24% to 46% and that toward 2,4DTBP increased from 2% to 14% as W/F was increased from 0.5 to 4 min. The selectivity toward 2,4-DTBP reached to 39% at W/ F of 4 min when the molar ratio of phenol/tert-butyl alcohol was 1:4. The higher conversion of phenol in the presence of the carbonized catalyst and continuous microwave irradiation is because of the ability of the catalyst to absorb microwave more

Figure 9. Selectivity of p-tert-butyl phenol at various mole ratio of phenol/tert-butyl alcohol at in the continuous system at 800 W and 1 g of catalyst.

efficiently. Carbon in most of its forms absorbs microwave strongly. Hence the carbonaceous catalyst absorbs microwave radiation more efficiently as compared to the virgin Amberlyst15. The catalyst bed reached to such high temperature that in some instances the glass reactor melted. Hence, the catalyst was well spread along the length of the bed to avoid hot spots. The reactants that are passed over this catalyst thus experience comparatively higher temperature which leads to higher conversions of phenol. However, at high temperatures, the rate of formation of 2,4-DTBP and p-TBP also increased as can be seen from Table 1. The high temperature conditions lead to further alkylation of the monoalkylated product to the dialkylated products. Also, Table 2 shows that the energy of activation for formation of p-TBP is higher than that o-TBP. Hence higher temperatures favor formation of p-TBP. As the W/F is increased, the time that the reactants spend in contact with catalyst also increases. This results in higher conversion of phenol with a lower selectivity toward o-TBP and increased selectivity toward 2,4-DTBP. The selectivity toward the dialkylated product increased significantly as compared to the batch process. Microwave has thus been successfully used to activate the catalyst and increase the conversion of phenol to 86% in just 4 min as against several hours by conventional process. The selectivity of the o-TBP can be increased by passing microwave intermittently using CSA catalyst and that toward p-TBP can be 6564


Industrial & Engineering Chemistry Research increased by passing microwave continuously. The CSA catalyst absorbs microwave more efficiently as compared to the virgin Amberlyst-15 and gives comparatively higher conversion of phenol. A continuous production of the alkylated phenols is also possible by using the CSA catalyst under microwave irradiation.

6. CONCLUSION Microwave can be used for preparation of kinetically controlled product (o-TBP) by intermittent irradiation. Microwave enhances the rate of reaction and hence, decreases the time of reaction. When the reaction is carried out for a long time period under microwave irradiation, the thermodynamic controlled product (p-TBP) is formed as the major product because the power of microwave decreases and the reaction is governed by thermal conditions. The energy of activation for alkylation of phenol under microwave irradiation was found to be 110.24 kJ/ mol. Alkylation of phenol has also been made continuous using the same continuous system. About 86% conversion of phenol was obtained in a short interval of 4 min W/F at 1:4 molar ratio of phenol/tert-butyl alcohol. The selectivity toward 2,4-DTBP was about 39%. The carbonized sulfated catalyst seems to work very well under microwave irradiation. ’ AUTHOR INFORMATION Corresponding Author

*Fax: 91-22-33611020. E-mail: [email protected].

’ NOMENCLATURE A = phenol B = tert-butyl alcohol (TBA) B0 = iso-butylene C = o-tert-butyl phenol (o-TBP) D = p-tert-butyl phenol (p-TBP) E = 2,4-di-tert-butyl phenol W = water S = catalytic surface k1 = rate constant for reaction 1 (g gmol1 min1) k2 = rate constant for reaction 2 (g gmol1 min1) k3 = rate constant for reaction 3 (g gmol1 min1) k4 = rate constant for reaction 4 (g gmol1 min1) k5 = rate constant for reaction 5 (g gmol1 min1) K1 = adsorption constant of A (phenol) (dm3 g1) K2 = adsorption constant of B (tert-butyl alcohol) (dm3 g1) K3 = adsorption constant of C (o-tert-butyl phenol) (dm3 g1) K4 = adsorption constant of D (p-tert-butyl phenol) (dm3 g1) K5 = adsorption constant of E (2,4-di-tert-butyl phenol) (dm3 g1) rAS = rate of reaction of phenol (alkylation) (gmol g1 min1) rBS = rate of reaction of tert-butyl alcohol (TBA) (dehydration) (gmol g1 min1) rB0 = rate of formation of iso-butylene (TBA) (gmol g1 min1) ’ REFERENCES (1) Sakthivel, A.; Badamali, S. K.; Selvam, P. Para-selective t-butylation of phenol over mesoporous H-AlMCM-41. Microporous Mesoporous Mater. 2000, 39, 457. (2) Krishnan, A. V.; Ojha, K.; Pradhan, N. C. Mild acid MCM-41 promotes para-isomer in the vapour phase alkylation of isopropylbenzene with diethyl carbonate. Org. Process Res. Dev. 2002, 6, 132.

ARTICLE

(3) Zhang, K.; Zhang, H.; Xua, G.; Xiang, S.; Xu, D.; Liu, S.; Li, H. Alkylation of phenol with tert-butyl alcohol catalyzed by large pore zeolitesl. Appl. Catal., A 2001, 207, 183. (4) Chandler, K.; Deng, F.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A. Alkylation reactions in near-critical water in the absence of acid catalysts. Ind. Eng. Chem. Res. 1997, 36, 5175. (5) Sumbramanian, S.; Mitra, A.; Satyanarayana, C. V. V.; Chakrabarty, D. V. Para-selective butylation of phenol over silicoaluminophosphate molecular sieve SAPO-11 catalyst. Appl. Catal., A 1997, 159, 229. (6) Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Noncatalytic and selective alkylation of phenol with propan-2-ol in supercritical water. Chem. Commun. 2001, 17, 1566. (7) Carlton, A. A. Alkylation of phenol with t-butyl alcohol in the presence of perchloric acid. J. Org. Chem. 1948, 13, 120. (8) Sartori, G.; Bigi, F.; Casiraghi, G.; Carnati, G.; Chiesi, L.; Adruini, A. Highly selective mono-tert-butylation of aromatic compounds. Chem. Ind. 1985, 22, 762. (9) Vinu, A.; Nandhini, K. U.; Murugesan, V.; Bohlmannc, W.; Umamaheswari, V.; Poppl, A.; Hartmann, M. Mesoporous FeAlMCM41: An improved catalyst for the vapor phase tert-butylation of phenol. Appl. Catal., A 2004, 265, 1. (10) Dapurkar, S. E.; Selvam, P. Mesoporous H-GaMCM-48: A remarkable solid acid catalyst for tertiary butylation of phenol. J. Catal. 2004, 224, 178. (11) Udayakumar, S.; Pandurangan, A.; Sinha, P. K. Mild acid MCM-41 promotes para-isomer in the vapour phase alkylation of isopropylbenzene with diethyl carbonate. Appl. Catal., A 2005, 287, 116. (12) Ojha, K.; Pradhan, N. C.; Samanta, A. N. Kinetics of batch alkylation of phenol with tert-butyl alcohol over a catalyst synthesized from coal fly ash. Chem. Eng. J. 2005, 112, 109. (13) Shen, H. Y.; Judeh, Z. M. A.; Ching, C. B.; Qing-Hua, X. Comparative studies on alkylation of phenol with tert-butyl alcohol in the presence of liquid or solid acid catalysts in ionic liquids. J. Mol. Catal. A 2004, 212, 301. (14) Guia, J.; Ban, H.; Cong, X.; Zhang, X.; Hu, Z.; Sun, Z. Selective alkylation of phenol with tert-butyl alcohol catalyzed by Bronsted acidic imidazolium salts. J. Mol. Catal. A. 2005, 225, 27. (15) Yadav, G. D.; Doshi, N. S. Alkylationof phenol with methyl-tertbutyl ether and tert-butanol over solid acids: Efficacies of clay-based catalysts. Appl. Catal., A 2002, 236, 129. (16) Chandra, K. G.; Sharma, M. M. Alkylation of phenol with MTBE and other tert-butyl ethers: Cation exchange resins as catalysts. Catal. Lett. 1993, 19, 309. (17) Campbell, C. B.; Onopchenko, A.; Young, D. C. Amberlyst-15catalyzed alkylation of phenolics with branched alkenes. Rearrangement of tert-alkylphenols and catechols to sec-alkyl isomers. Ind. Eng. Chem. Res. 1990, 29, 642. (18) Rajadyaksha, R. A.; Chaudhari, D. Alkylation of phenol and pyrocatechol by isobutyl alcohol using superacid catalysts. Ind. Eng. Chem. Res. 1987, 26, 1276. (19) Chaudhuri, B.; Sharma, M. M. Alkylation of phenol with Rmethylstyrene, propylene, butenes, isoamylene, 1-octene, and diisobutylene: Heterogeneous vs homogeneous catalysts. Ind. Eng. Chem. Res. 1991, 30, 227. (20) Sunajadevi, K. R.; Sugunan, S. Para-selective tert-butylation of phenol over nano-sulfated titania catalysts prepared via solgel route. Catal. Lett. 2005, 99 (34), 26. (21) Vinua, A; Devassyb, B. M.; Halligudib, S. B.; Bohlmannc, W.; Hartmann, M. Highly active and selective AlSBA-15 catalysts for the vapor phase tert-butylation of phenol. Appl. Catal., A 2005, 281, 207. (22) Hajek, M.; Radoiu, M. T. Microwave activation of catalytic transformation of tert-butyl phenols. J. Mol. Catal. A 2000, 160, 383. (23) Sato, S.; Nozaki, F.; Zhang, Shi-Jun; Cheng, P. Liquid-phase alkylation of benzene with cyclohexene over SiO2-grafted A1C13 catalyst and accelerating effect of ultrasonic vibration. Appl. Catal., A 1996, 143, 271. (24) Loupy, A. Microwaves in Organic Synthesis; Wiley-VCH: New York, 2002. 6565



ARTICLE

(25) Hayes, B. L. Microwave Synthesis, Chemistry at the Speed of Light; CEM-Publishing: Mathews, NC, 2002. (26) Kappe, C. O. Controlled Microwave Heating in Modern Organic Synthesis. Angew. Chem., Int. Ed. 2004, 43, 6250. (27) Yang, C.; Murray, W. V.; Wilson, L. J. Microwave enabled external carboxymethyl substituents in the ring-closing metathesis. Tetrahedron Lett. 2003, 44, 1783. (28) Bashiardes, G.; Safir, I.; Mohamed, A. S.; Barbot, F.; Laduranty, J. Microwave-Assisted [3 þ 2] Cycloadditions of Azomethine Ylides. Org. lett. 2003, 5 (25), 4915. (29) Katritzky, A. R.; Zhang, Y.; Singh, S. K.; Steel, P. J. 1,3-Dipolar cycloadditions of organic azides to ester or benzotriazolylcarbonyl activated acetylenic amides. ARKIVOC 2003, xv, 47. (30) Alexandre, F. R.; Berecibar, A.; Bessonb, T. Microwave-assisted Niementowski reaction. Back to the roots. Tetrahedron Lett. 2002, 43, 3911. (31) Larhead, M.; Hallberg, A. Microwave assisted high-speed chemistry: A new technique in drug discovery. Drug Discovery Today 2001, 6 (8), 406. (32) Conde, L. D.; Marun, C.; Suib, S. L.; Fathi, Z. Frequency effects in the catalytic oligomerization of methane via microwave heating. J. Catal. 2001, 204, 324. (33) Marun, C.; Conde, L. D.; Suib, S. L. Catalytic oligomerisation of methane via microwave heating. J. Phys. Chem. A 1999, 103 (22), 4332. (34) Conde, L. D.; Marun, C.; Suib, S. L. Oligomerization of methane via microwave heating using Raney nickel catalyst. J. Catal. 2003, 218, 201. (35) Bose, A. K.; Banik, B. K.; Manhas, M. S. Stereocontrol of β-lactam formation using microwave irradiation. Tetrahedron Lett. 1995, 36 (2), 213. (36) Mingos, D. M.P.; Baghurst, D. R. Applications of microwave dielectric heating effects of synthetic problems in chemistry. Chem. Soc. Rev. 1991, 20 (1), 1.

6566


Intensification and Selectivity Modulation of Ion-Exchange Resin

Recommend Documents