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Synthesis of ethyl tert-butyl ether with tert-butyl alcohol and ethanol on various ion exchange resin catalysts
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Catalysis Communications 9 (2008) 721–727 www.elsevier.com/locate/catcom
Synthesis of ethyl tert-butyl ether with tert-butyl alcohol and ethanol on various ion exchange resin catalysts Muhammad Umar *, Anwar Rasheed Saleemi, Suleman Qaiser Department of Chemical Engineering, University of Engineering and Technology, Lahore 54890, Punjab, Pakistan Received 9 May 2007; received in revised form 14 August 2007; accepted 15 August 2007 Available online 28 August 2007
Abstract Heterogeneous catalytic synthesis of ETBE with tert-butyl alcohol (TBA) and ethanol (EtOH) using seven ion exchange resins has been studied. After characterization of catalysts, batch kinetic studies were conducted to sort out the most suitable catalyst based on TBA conversion and ETBE selectivity. Optimum operating conditions found were 343 K temperature; 1:2 feed mole ratio (TBA:EtOH), and 10% catalyst loading. Kinetic modeling for the most suitable catalyst CT-145H was done using heterogeneous and quasi-homogeneous models, but latter fit well to experimental data. 2007 Elsevier B.V. All rights reserved. Keywords: ETBE synthesis; Ion exchange resins; Heterogeneous catalysis; Kinetic modeling
1. Introduction In last decade, a lot of research have been done in the field of gasoline fuel oxygenates as well as octane rating boosters. The most widely investigated fuel oxygenates comprise alcohols and ethers such as ethanol, tert-butanol, methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME), tert-amyl ethyl ether (TAEE), and ethyl tertbutyl ether (ETBE). MTBE has been mostly used but now there are some regulations being formulated to restrict its use in United States, the reason being it can pollute the underground water because of its high solubility in water (42 mg/L) [1]. ETBE is found to be a better gasoline additive than its competitors due to having better blending characteristics. Its blending Reid vapor pressure (bRvp) is 27.5 KPa as compared to 55 KPa for MTBE, high octane rating of 112 to that of 109 for MTBE and low oxygen content of 15.7% as compared to 18.2% for MTBE [2]. In addition to these properties, ETBE has low solubility in water (23.7 mg/L) [3].
*
Fax: +92 42 9250202. E-mail address: [email protected] (M. Umar).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.08.016
Mostly ETBE has been synthesized by exothermic reversible reaction between iso-butylene (IB) and ethanol (EtOH) [4–7], but the availability of IB is limited. It is only produced in refinery using catalytic and steam cracking operations. Therefore, alternative routes to synthesize ETBE are under substantial consideration. By far the most important substitute of IB is tert-butyl alcohol (TBA), which is a byproduct of polypropylene production in the ARCO process. ETBE can be formed by direct reaction of TBA and EtOH [8]. Due to the importance of this reaction, various catalysts have been used for the direct reaction of TBA and EtOH to produce ETBE, including heteropoly acids [8], Heteropoly acid-polymer composite catalysts [9], potassium hydrogen sulphates [10], amberlyst-15 [11], ion exchange resins D72 and S-54 [2], b-zeolites [1,11], and H-ZSM-5 zeolite [12]. The main aim of this study is to investigate the catalytic efficiency and capacity of some new macroporous and gelular ion exchange resin catalysts for the ETBE synthesis. These new catalysts include Purolite CT-124 (gel based), CT-145H, CT-151, CT- 175 and CT-275 (macro reticular). Comparison is also made between new catalysts and mostly used Amberlyst-15 and Amberlyst-35 wet. The criteria for the selection of the best catalyst are TBA conversion and
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M. Umar et al. / Catalysis Communications 9 (2008) 721–727
ETBE selectivity under identical experimental conditions. The most suitable catalyst found is then used to study the effect of different variables like temperature, feed mole ratio and catalyst loading. Kinetic modeling is also performed on the most suitable catalyst.
tor temperatures were maintained isothermally at 458 K. Iso-propanol was used as internal standard and separation was achieved for all components.
2. Experimental
The synthesis of ETBE on ion exchange resin catalyst occur according to following mechanism
2.1. Materials and methods All chemicals, TBA (99.5% GLC), EtOH (99.8%, GC), ETBE (97% GC), iso-propanol (99.5% GC) were purchased from Fisher UK and their purity was verified by gas chromatography. These chemicals were used without further purification. Ion exchange resin catalysts of CT brand were supplied by courtesy of M/S Purolite UK, while Amberlyst-35 was provided by M/S Rohm and Haas France. Ion exchange resin catalysts were washed thoroughly with de-ionized water and then with methanol. Washed catalysts were dried in vacuum oven at 378 K for six hours to remove any residual moisture. Washed and dried resins were stored in desiccator for further use. 2.2. Apparatus
2.5. Reaction mechanism
ðCH3 Þ3 C OH þ R SO3 H () ðCH3 Þ3 Cþ þ H2 O þ R SO 3
ð1Þ ðCH3 Þ3 Cþ þ CH3 CH2 OH () ðCH3 Þ3 C O CH2 CH3 þ Hþ ðCH3 Þ3 Cþ () CH2 ¼ C ðCH3 Þ2 þ Hþ þ R SO 3 þ H () R SO3 H
ð2Þ ð3Þ ð4Þ
The limiting step in this sequence of reactions is supposed to be the surface reaction of ethanol, adsorbed in pores of catalyst, and TBA to yield ETBE. Under the atmospheric pressure experimental conditions used in this work, IB was not found in liquid phase even up to very less extent. Rates of the reactions for ETBE formation (r1) and TBA dehydration (r2) can be determined respectively by [13,14]. ðaA aB aC aD =K eq Þ 1 þ K w aD ðaA Þ r2 ¼ k 2 1 þ K w aD
r1 ¼ k 1
ð5Þ
A five necked, jacketed; glass reaction vessel of 5.0 · 104 m3 volume was used to carry out experiments. Mechanical stirrer with adjustable speed was fixed in the central neck and other necks were used for condenser, thermocouple, catalyst feeding and sampling. A thermostatic water bath with temperature controller having accuracy of ±1 C was used to keep the contents of reaction vessel at desired temperature.
k1, k2, Kw and Keq are reaction rate constants for ETBE formation and TBA dehydration, water inhibition parameter, and equilibrium constant for activity based model respectively. The equilibrium constant was calculated from the activities of components by following expression
2.3. Procedure
K eq ¼ ðaC aD =aA aB Þeq
For each experimental run, measured molar quantities of each reactant (TBA and EtOH) were fed into reaction vessel and the contents were continuously stirred and heated to the desired reaction temperature with hot water through the jacket. When the reaction mixture reached the desired temperature, catalyst was added as the weight percent of the reaction mixture and sample was taken. This was considered zero time for the reaction. Samples of approximately 1.0 · 106 m3 were taken at regular intervals until the equilibrium was attained. Each sample was then analyzed to determine the concentration of individual components.
ð6Þ
ð7Þ
where ai is the activity of component i (i = A–D, A = TBA, B = EtOH, C = ETBE, D = Water). Activities were used instead of concentrations to cover non-ideal nature of liquid phase reaction system in the presence of polar compounds like water and ethanol. Activity of component i can be calculated by the equation ai ¼ x i c i
ð8Þ
where xi and ci are mole fraction and activity coefficient of component i respectively. Activity coefficients were calculated by using modified UNIFAC group contribution method [15,16]. Calculation details for activity coefficients are given by B.G.Kyle [17].
2.4. Analysis
3. Results and discussions
Samples were analyzed by Pye Unicam 104 gas chromatograph equipped with Supelco Porapak-Q (80/100) column of 1.83 m length and 3.175 · 106 m diameter and thermal conductivity detector (TCD). High purity (99.99%) helium gas was used as mobile phase at 3.0 kg/ cm2 pressure and 0.60 cm3/s flow rate. Injector and detec-
3.1. Catalyst characterization Particle size distribution measurement was carried out using Coulter130 laser sizer to determine the size range of catalyst particles. Particle sizes with 95% confidence limits for each catalyst are shown in Table 1. The data shows that
M. Umar et al. / Catalysis Communications 9 (2008) 721–727
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Table 1 Physical properties and characterization results of various catalysts Property
A-15
A-35
Matrix Physical appearance
Macro-porous Beige spherical beads 381–1010
Black spherical beads 339–1040
32.9936 45.87
41.1655 57.1435
a
0.2665 60.2216 0.64026 1.411 4.7
0.3140 57.800 0.6073 1.504 5.2
a
300
300
Particle size (lm), 95% confidence limits BET surface area (m2/g) Langmuir surface area (m2/g) Pore volume (cm3/g) Porosity % Bulk density (g/cm3) True density (g/cm3) Ion exchange capacityb (meq/g) Average pore diameter (A) a b
CT-124
CT-145H
CT-151
CT-175
CT-275
Gelular Golden spherical beads
Macro-porous Beige spherical beads 392–1030
Macro-porous Dark grey spherical beads 478–969
Macro-porous Black spherical beads 0–350
Macro-porous Black spherical beads 158–822
11.2474 15.0021
25.2011
21.300
20.5100
a
a
a
46.11 0.7813 1.45 4.9
0.1661 48.2557 0.7304 1.413 4.8
0.1705 56.020 0.6156 1.40 5.1
0.3101 72.5604 0.535 1.95 4.9
0.2410 67.0911 0.543 1.65 5.2
a
a
250
a
650
418–947
a
Data not available. Manufacturer date.
particle size for the most of catalysts fall in the range between 350 and 900 lm. BET surface area, Langmuir surface area, pore size and pore volume were determined by using Micromeritics ASAP2000 (Accelerated Surface Area and Porosimetry) instrument with nitrogen adsorption and desorption at 77 K. The pre weighed catalyst sample for this test was de-gassed at 373 K for a period of 24 h. density functional theory (DFT) model was used to interpret the pore size distribution results. This data illustrates that all of these catalysts are macroporous except CT-124. Due to its gelular matrix, CT-124 did not exhibit any porosity with this method because of the fact that gel based resins need complete wetting to swell and exhibit porosity. Hence, the surface area and pore width spaces are left blank in Table 1 for CT-124. 3.1.1. Scanning electron microscopy (SEM) was performed for all catalysts using Cambridge Stereoscan 360 at 15 KV. Micrographs of two best-performed catalysts CT-145H and CT-124 are shown in
Fig. 1a and b. These micrographs suggest that there are pores of different sizes and geometry. Cracks of very minute size are also observed specially for CT-124 and CT145H. Inside of the resin beads show a cauliflower like surface; this suggests that pores are fused together. The thin line cracks and macro pores are responsible for the pore volume and surface area of the catalyst. The reactants may easily penetrate into these pores, which act as active sites to ensue the chemical reaction. 3.1.2. Density and porosity measurement Bulk density was measured by filling the pre weighed known volume density bottle with catalyst and then mass of catalyst was found by difference and hence the density. True density was measured by using micromeretics multivolume pycnometer-1305 with helium gas as expansion medium. In both measurements, experiments were replicated five times so that the results were repeatable. Porosity was measured by using equation e ¼ ðqT qb Þ=qT
Fig. 1. Scanning electron micrographs of catalysts: (a) CT-124 and (b) CT-145H.
ð9Þ
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where e is porosity, qT and qb are true and bulk particle density (kg/m3) respectively. Some of the physical properties and characterization results of catalysts are shown in Table 1. 3.2. Batch kinetic results Experimental studies were undertaken to find out the best catalyst for ETBE synthesis under identical conditions. To overcome the intra-particle mass transfer resistance, agitation speed of 500 rpm was used after observation that there was no appreciable increase in conversion when speed was increased from 500 to 800 rpm. Since higher impeller speed can cause attrition and disintegration of catalyst particles, therefore impeller speed of 500 rpm was considered optimum and maintained throughout this study. 3.2.1. Selection of the most suitable catalyst All seven catalysts were examined under identical conditions of temperature, feed mole ratio of reactants, agitation speed and catalyst loading. TBA conversion and ETBE selectivity were determined for each catalyst as shown in Fig. 2a and b respectively. Following relations were used to calculate conversion and selectivity. C TBA;0 C TBA;t %TBA Conversion ¼ 100 ð10Þ C TBA;0 C ETBE;t %ETBE Selectivity ¼ 100 ð11Þ C TBA;0 C TBA;t where CTBA,0, CTBA,t are TBA concentrations at time zero and time t respectively and CETBE,t is concentration of ETBE at any time t. It is evident from the Fig. 2a and b that CT-175 and CT-275 gave the maximum conversion but they yielded poor selectivity, While CT-145H and CT-124 resulted in conversion up to 70% but they gave selectivity in the range of 60%. Amberlyst-15, Amberlyst-
a
35 and CT-151 were not found suitable from selectivity standpoint. It can be inferred from these figures that CT124 and CT-145H though gave almost similar results but CT-145H can be considered better because it gave more conversion than CT-124 while the maximum selectivity was same for both catalysts. Keeping in view the above results, CT-145H was selected to study the effect of other variables. 3.2.2. Effect of temperature Experiments were carried out at 323, 333 and 343 K to find the TBA conversion and ETBE selectivity using CT145H. Results showed that temperature of 343 K gave best set of conversion and selectivity. Lower reaction temperatures (323 K and 333 K) did not favor the reaction, so both conversion and selectivity declined. It was also observed during experimentation that higher temperature though favors conversion but selectivity decreases after about 4 h. Probable reason of decrease in selectivity at higher temperature can be attributed to more formation of IB that reduces the selectivity towards ETBE. Another contributing factor towards the reduced ETBE selectivity after 4 h can be strong adsorption of water on active catalyst sites due to its higher polarity than other components. This reduces the number of active sites available for reactants and hence ETBE selectivity decreases. Same behaviour was noticed for other catalysts at higher temperatures. 3.2.3. Effect of feed mole ratio (FMR) Three different feed mole ratios 1:1, 1:2 and 1:4 (TBA:EtOH) were investigated to calculate conversion and selectivity. Out of these FMRs, 1:4 yielded the best results in terms of TBA conversion and ETBE selectivity. The difference of ETBE selectivity for 1:2 FMR and 1:4 FMR was not very significant (72% and 77%). The 1:1 FMR did not resulted in appreciable conversion and selectivity as compared to those with higher ethanol concentration. So it can be concluded that excess of ethanol beyond twice
b
100
Amberlyst-15 CT 124 CT 151 CT 275
80
% ETBE Selectivity
80
% TBA Conversion
100
60
40 Amberlyst-15 CT 124 CT 151 CT 275
20
Amberlyst 35 CT 145H CT 175
Amberlyst 35 CT 145H CT 175
60
40
20
0
0 0
1
2
3
4
Time (h)
5
6
7
0
1
2
3
4
5
6
7
Time (h)
Fig. 2. TBA conversion (a) and ETBE selectivity (b) using different catalysts, 1:2 FMR, 5% catalyst loading, 500 rpm stirrer speed and 348 K temperature.
M. Umar et al. / Catalysis Communications 9 (2008) 721–727
725
the limiting reactant is not so much beneficial in terms of selectivity and conversion. Therefore, 1:2 mole feed ratio was considered optimum in this study.
Riedel (E–R) models were applied to the experimental data. Reaction rate expression of limiting reactant for LHHW model can be expressed as
3.2.4. Effect of catalyst loading Three catalyst loadings, 2.5%, 5% and 10% (w/w) of the reaction mixture were used to find their effect on conversion and selectivity. All other experimental conditions were kept unchanged. It was observed that catalyst loading of 2.5% though yielded about 50% ETBE selectivity, gave poor conversion. The 10% catalyst loading was found best in terms of selectivity (74%) but the TBA conversion was of same magnitude for 5% and 10% catalyst loading (80%). As we were more concerned about ETBE selectivity, so 10% catalyst loading was considered the best for this system.
rA ¼
3.3. Kinetic modeling
rA ¼
The two heterogeneous kinetic models namely Langmuir–Hinshelwood–Hougen–Watson (LHHW) and Eley
where rA is reaction rate of TBA (mol/g s), which is limiting reactant, NAo is initial moles of limiting reactant, XA
k 1 ½aA aB aC aD= =K eq ð1 þ K A aA þ K B aB þ K C aC þ K D aD Þ
ð12Þ
The reaction rate for E–R model is written as k 1 ½aA aB aC aD= =K eq ð1 þ K A aA þ K B aB þ K C aC þ K D aD Þ
rA ¼
ð13Þ
where KA, KB, KC and KD are the adsorption equilibrium constants for components A-D, aA, aB, aC and aD are the activities for TBA, EtOH, ETBE and water, respectively and Keq is equilibrium constant. Experimental reaction rates were calculated by using differential method as adopted by Cunnil et al. [18] N Ao ðdX A Þ V ðdtÞ
10.0
ð14Þ
water (Exp)
9.0
EtOH (Exp)
3
Concentration C i (k-mol/m )
2
8.0
TBA (Exp)
7.0
ETBE (Exp)
6.0 IB (Exp)
5.0 water (Q-H )
4.0
EtOH (Q-H)
3.0
TBA (Q-H)
2.0
ETBE (Q-H)
1.0
IB (Q-H)
0.0 0
1
2
3
4
5
6
7
Time (h) Fig. 3. Concentration profile of experimental and Q-H model for CT-145H at 323 K, 1:2 FMR, 5% catalyst loading, and 500 rpm stirrer speed.
water (Exp)
9.0
3
Concentration C i (k-mol/m )
10.0 EtOH (Exp)
8.0 7.0
TBA (Exp)
6.0
ETBE (Exp)
5.0
IB(Exp)
4.0
water (Q-H)
3.0
EtOH (Q-H)
2.0
TBA (Q-H)
1.0
ETBE (Q-H)
0.0 0
1
2
3
4
5
6
7
IB (Q-H)
Time (h) Fig. 4. Concentration profile of experimental and Q-H model for CT-145H at 333 K, 1:2 FMR, 5% catalyst loading, and 500-rpm stirrer speed.
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M. Umar et al. / Catalysis Communications 9 (2008) 721–727
water (Exp)
9.0
EtOH (Exp)
3
Concentration C i (k-mol/m )
10.0
8.0
TBA (Exp)
7.0
ETBE (Exp)
6.0
IB (Exp)
5.0
water (Q-H model)
4.0
EtOH (Q-H)
3.0
TBA (Q-H)
2.0
ETBE (Q-H)
1.0
IB (Q-H)
0.0 0
1
2
3
4
5
6
7
Time (h) Fig. 5. Concentration profile of experimental and Q-H model for CT-145H at 343 K, 1:2 FMR, 5% catalyst loading, and 500 rpm stirrer speed.
rA ¼ k 1 ½aA aB ðaC aD =K eq Þ
ð15Þ
where k1 is reaction rate constant. This model yielded close fit to the experimental data. Experimental conversions as well as those calculated by Q-H model were compared and it was found that both were in good agreement. Experimental concentration profile and those calculated using QH model for all components at three different temperatures are shown in Figs. 3–5. These figures depict that this model describes the system to a close approximation. It was found that Q-H model fit well at lower temperatures (323 and 333 K) but at 343 K, there is difference in model and experimental values. This may be due to the reason that Q-H model do not assume the adsorption of components at the catalyst surface, while actually some adsorption of components is taking place. Therefore, experimental values of concentration for reactants are somewhat less as compared to the model values. Initiation of side reaction at higher temperature might be another reason for the difference between model and experimental values. Nevertheless, trend of the model and experimental profiles is same. Though the system is heterogeneous originally but it can be supposed that liquid and solid catalyst in reaction vessel are intimately mixed so that the system almost approaches
-5
-6
lnk
is conversion of limiting reactant, V is volume of reaction mixture (m3) and t is time (s). When all experimental values were correlated with those calculated by using the above heterogeneous model equations, both models yielded scattered results in terms of some negative adsorption constants as well as negative reaction rate constant which of course is not possible. Multiple linear and non-linear regression techniques were used to find out the constants of Eqs. (12) and (13) [19]. Similar results have already been reported by Yin et al. [7], Gangadwala et al. [20], Patel and Saha [21]. Since both the heterogeneous models were not found coherent, so quasi-Homogeneous (Q-H) model was used to interpret the data. Reaction rate expression for the QH model is
-7
-8
-9 0.0029
0.00295
0.003
0.00305
0.0031
0.00315
-1
1/T (K ) Fig. 6. Arrhenius plot for etherification of ethanol and tert-butanol with CT-145H.
to a homogeneous one. Literature does support the using of Q-H model for liquid phase reaction systems in the presence of ion exchange resin catalysts [22]. Temperature dependency of rate constant was expressed by drawing Arrhenius plot. Fig. 6 shows the plot with correlation coefficient of 0.96. Expression obtained for rate constant k1 is shown in Eq. (16) k 1 ¼ expð26:877 11281=T Þ
ð16Þ
The pre exponential factor and activation energy calculated are 4.70 · 1011 (mol/g s) and 93.79 kJ/mol respectively. These values are in agreement with the published literature [2]. 4. Conclusions Seven commercially available ion exchange resin catalysts were used in this study to find their efficiency and capacity for ETBE synthesis. CT-145H was found to be the most suitable catalyst and was then further investigated at different conditions. Temperatures of 343 K, 10% cata-
M. Umar et al. / Catalysis Communications 9 (2008) 721–727
lyst loading and 1:2 feed mole ratios of reactants were found optimum. Kinetic modeling results showed that experimental data fit well to quasi-homogeneous model rather than heterogeneous models. These results will be utilized for ETBE synthesis via reactive distillation. Acknowledgements The authors are thankful to Higher Education Commission of Pakistan for funding this research. MU is grateful to Dr. B. Saha of Loughborough University UK for kind help and guidance. References [1] S. Assabumrangrat, W. Kiatkittipong, N. Sevitoon, P. Praserthdam, S. Goto, Intl. J. Chem. Kinet. 34 (2002) 292. [2] Bo-Lun Yang, San-Ba Yang, Rui-qing Yao, React. Funct. Polym. 44 (2000) 167. [3] F. Cunnil, M. Vila, J.F. Izquierdo, M. Iborra, J. Tejero, Ind. Eng. Chem. Res. 32 (1993) 564. [4] C. Fite, M. Iborra, J. Tejero, J.F. Izquierdo, F. Cunnil, Ind. Eng. Chem. Res. 33 (1994) 581. [5] B.H. Bisowarno, M.O. Tade, Ind. Eng. Chem. Res. 39 (2000) 1950. [6] M.G. Sneesby, M.O. Tade´, R. Datta, T.N. Smith, Ind. Eng. Chem. Res. 36 (1997) 1855. [7] Y. Li, S. Huang, S. Wu, X. Yuan, Catal. Lett. 87 (2003) 31.
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[8] X. Yin, Bo-lun yang, S. Goto, Int. J. Chem. Kinet. 27 (1995) 1065. [9] G.I. Park, S.S. Lim, I.K. Song, W.Y. Lee, React. Kinet. Catal. Lett. 75 (2002) 157. [10] M. Matouq, A. Quitain, K. Takahashi, S. Goto, Ind. Eng. Chem. Res. 35 (1996) 982. [11] A. Quitain, H. Itoh, S. Goto, J. Chem. Eng. Jpn. 32 (1999) 280. [12] R. Alcantara, E. Alcantara, L. Canoira, M.J. Franco, I. Martin, A. Navarro, React. Kinet. Catal. Lett. 69 (2000) 239. [13] S. Assabumrangrat, D. Wongwattanasate, V. Pavarajarn, P. Praserthdam, A. Arpornwichanop, S. Goto, Korean J. Chem. Eng. 21 (2004) 1139. [14] W. Kiatkittipong, S. Assabumrungrat, P. Praserthdam, S. Goto, J. Chem. Eng. Jpn. 35 (2002) 547. [15] J. Gmehling, J. Li, M. Schiller, Ind. Eng. Chem. Res. 32 (1993) 178. [16] S. Skjold-Jo´gensen, P. Rasmussen, A. Fredenslund, Chem. Eng. Sci. 37 (1982) 9. [17] B.G. Kyle, Chemical and Process Thermodynamics, third ed., Prentice Hall PTR, 2000, p. 270. [18] F. Cunill, M. Iborra, C. Fite, J. Tejero, J.F. Izquierdo, Ind. Eng. Chem. Res. 39 (5) (2000) 1235. [19] M.B. Cutlip, M. Shacham, Problem Solving in Chemical Engineering with Numerical Methods, Prentice Hall International Series, 1999, pp. 77–357. [20] J. Gangadwala, S. Mankar, S. Mahajani, Ind. Eng. Chem. Res. 42 (2003) 2146. [21] D. Patel, B. Saha, Ind. Eng. Chem. Res. 46 (10) (2007) 3157. [22] F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962, pp. 522– 524.
Catalysis Communications 9 (2008) 721–727 www.elsevier.com/locate/catcom
Synthesis of ethyl tert-butyl ether with tert-butyl alcohol and ethanol on various ion exchange resin catalysts Muhammad Umar *, Anwar Rasheed Saleemi, Suleman Qaiser Department of Chemical Engineering, University of Engineering and Technology, Lahore 54890, Punjab, Pakistan Received 9 May 2007; received in revised form 14 August 2007; accepted 15 August 2007 Available online 28 August 2007
Abstract Heterogeneous catalytic synthesis of ETBE with tert-butyl alcohol (TBA) and ethanol (EtOH) using seven ion exchange resins has been studied. After characterization of catalysts, batch kinetic studies were conducted to sort out the most suitable catalyst based on TBA conversion and ETBE selectivity. Optimum operating conditions found were 343 K temperature; 1:2 feed mole ratio (TBA:EtOH), and 10% catalyst loading. Kinetic modeling for the most suitable catalyst CT-145H was done using heterogeneous and quasi-homogeneous models, but latter fit well to experimental data. 2007 Elsevier B.V. All rights reserved. Keywords: ETBE synthesis; Ion exchange resins; Heterogeneous catalysis; Kinetic modeling
1. Introduction In last decade, a lot of research have been done in the field of gasoline fuel oxygenates as well as octane rating boosters. The most widely investigated fuel oxygenates comprise alcohols and ethers such as ethanol, tert-butanol, methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME), tert-amyl ethyl ether (TAEE), and ethyl tertbutyl ether (ETBE). MTBE has been mostly used but now there are some regulations being formulated to restrict its use in United States, the reason being it can pollute the underground water because of its high solubility in water (42 mg/L) [1]. ETBE is found to be a better gasoline additive than its competitors due to having better blending characteristics. Its blending Reid vapor pressure (bRvp) is 27.5 KPa as compared to 55 KPa for MTBE, high octane rating of 112 to that of 109 for MTBE and low oxygen content of 15.7% as compared to 18.2% for MTBE [2]. In addition to these properties, ETBE has low solubility in water (23.7 mg/L) [3].
*
Fax: +92 42 9250202. E-mail address: [email protected] (M. Umar).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.08.016
Mostly ETBE has been synthesized by exothermic reversible reaction between iso-butylene (IB) and ethanol (EtOH) [4–7], but the availability of IB is limited. It is only produced in refinery using catalytic and steam cracking operations. Therefore, alternative routes to synthesize ETBE are under substantial consideration. By far the most important substitute of IB is tert-butyl alcohol (TBA), which is a byproduct of polypropylene production in the ARCO process. ETBE can be formed by direct reaction of TBA and EtOH [8]. Due to the importance of this reaction, various catalysts have been used for the direct reaction of TBA and EtOH to produce ETBE, including heteropoly acids [8], Heteropoly acid-polymer composite catalysts [9], potassium hydrogen sulphates [10], amberlyst-15 [11], ion exchange resins D72 and S-54 [2], b-zeolites [1,11], and H-ZSM-5 zeolite [12]. The main aim of this study is to investigate the catalytic efficiency and capacity of some new macroporous and gelular ion exchange resin catalysts for the ETBE synthesis. These new catalysts include Purolite CT-124 (gel based), CT-145H, CT-151, CT- 175 and CT-275 (macro reticular). Comparison is also made between new catalysts and mostly used Amberlyst-15 and Amberlyst-35 wet. The criteria for the selection of the best catalyst are TBA conversion and
722
M. Umar et al. / Catalysis Communications 9 (2008) 721–727
ETBE selectivity under identical experimental conditions. The most suitable catalyst found is then used to study the effect of different variables like temperature, feed mole ratio and catalyst loading. Kinetic modeling is also performed on the most suitable catalyst.
tor temperatures were maintained isothermally at 458 K. Iso-propanol was used as internal standard and separation was achieved for all components.
2. Experimental
The synthesis of ETBE on ion exchange resin catalyst occur according to following mechanism
2.1. Materials and methods All chemicals, TBA (99.5% GLC), EtOH (99.8%, GC), ETBE (97% GC), iso-propanol (99.5% GC) were purchased from Fisher UK and their purity was verified by gas chromatography. These chemicals were used without further purification. Ion exchange resin catalysts of CT brand were supplied by courtesy of M/S Purolite UK, while Amberlyst-35 was provided by M/S Rohm and Haas France. Ion exchange resin catalysts were washed thoroughly with de-ionized water and then with methanol. Washed catalysts were dried in vacuum oven at 378 K for six hours to remove any residual moisture. Washed and dried resins were stored in desiccator for further use. 2.2. Apparatus
2.5. Reaction mechanism
ðCH3 Þ3 C OH þ R SO3 H () ðCH3 Þ3 Cþ þ H2 O þ R SO 3
ð1Þ ðCH3 Þ3 Cþ þ CH3 CH2 OH () ðCH3 Þ3 C O CH2 CH3 þ Hþ ðCH3 Þ3 Cþ () CH2 ¼ C ðCH3 Þ2 þ Hþ þ R SO 3 þ H () R SO3 H
ð2Þ ð3Þ ð4Þ
The limiting step in this sequence of reactions is supposed to be the surface reaction of ethanol, adsorbed in pores of catalyst, and TBA to yield ETBE. Under the atmospheric pressure experimental conditions used in this work, IB was not found in liquid phase even up to very less extent. Rates of the reactions for ETBE formation (r1) and TBA dehydration (r2) can be determined respectively by [13,14]. ðaA aB aC aD =K eq Þ 1 þ K w aD ðaA Þ r2 ¼ k 2 1 þ K w aD
r1 ¼ k 1
ð5Þ
A five necked, jacketed; glass reaction vessel of 5.0 · 104 m3 volume was used to carry out experiments. Mechanical stirrer with adjustable speed was fixed in the central neck and other necks were used for condenser, thermocouple, catalyst feeding and sampling. A thermostatic water bath with temperature controller having accuracy of ±1 C was used to keep the contents of reaction vessel at desired temperature.
k1, k2, Kw and Keq are reaction rate constants for ETBE formation and TBA dehydration, water inhibition parameter, and equilibrium constant for activity based model respectively. The equilibrium constant was calculated from the activities of components by following expression
2.3. Procedure
K eq ¼ ðaC aD =aA aB Þeq
For each experimental run, measured molar quantities of each reactant (TBA and EtOH) were fed into reaction vessel and the contents were continuously stirred and heated to the desired reaction temperature with hot water through the jacket. When the reaction mixture reached the desired temperature, catalyst was added as the weight percent of the reaction mixture and sample was taken. This was considered zero time for the reaction. Samples of approximately 1.0 · 106 m3 were taken at regular intervals until the equilibrium was attained. Each sample was then analyzed to determine the concentration of individual components.
ð6Þ
ð7Þ
where ai is the activity of component i (i = A–D, A = TBA, B = EtOH, C = ETBE, D = Water). Activities were used instead of concentrations to cover non-ideal nature of liquid phase reaction system in the presence of polar compounds like water and ethanol. Activity of component i can be calculated by the equation ai ¼ x i c i
ð8Þ
where xi and ci are mole fraction and activity coefficient of component i respectively. Activity coefficients were calculated by using modified UNIFAC group contribution method [15,16]. Calculation details for activity coefficients are given by B.G.Kyle [17].
2.4. Analysis
3. Results and discussions
Samples were analyzed by Pye Unicam 104 gas chromatograph equipped with Supelco Porapak-Q (80/100) column of 1.83 m length and 3.175 · 106 m diameter and thermal conductivity detector (TCD). High purity (99.99%) helium gas was used as mobile phase at 3.0 kg/ cm2 pressure and 0.60 cm3/s flow rate. Injector and detec-
3.1. Catalyst characterization Particle size distribution measurement was carried out using Coulter130 laser sizer to determine the size range of catalyst particles. Particle sizes with 95% confidence limits for each catalyst are shown in Table 1. The data shows that
M. Umar et al. / Catalysis Communications 9 (2008) 721–727
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Table 1 Physical properties and characterization results of various catalysts Property
A-15
A-35
Matrix Physical appearance
Macro-porous Beige spherical beads 381–1010
Black spherical beads 339–1040
32.9936 45.87
41.1655 57.1435
a
0.2665 60.2216 0.64026 1.411 4.7
0.3140 57.800 0.6073 1.504 5.2
a
300
300
Particle size (lm), 95% confidence limits BET surface area (m2/g) Langmuir surface area (m2/g) Pore volume (cm3/g) Porosity % Bulk density (g/cm3) True density (g/cm3) Ion exchange capacityb (meq/g) Average pore diameter (A) a b
CT-124
CT-145H
CT-151
CT-175
CT-275
Gelular Golden spherical beads
Macro-porous Beige spherical beads 392–1030
Macro-porous Dark grey spherical beads 478–969
Macro-porous Black spherical beads 0–350
Macro-porous Black spherical beads 158–822
11.2474 15.0021
25.2011
21.300
20.5100
a
a
a
46.11 0.7813 1.45 4.9
0.1661 48.2557 0.7304 1.413 4.8
0.1705 56.020 0.6156 1.40 5.1
0.3101 72.5604 0.535 1.95 4.9
0.2410 67.0911 0.543 1.65 5.2
a
a
250
a
650
418–947
a
Data not available. Manufacturer date.
particle size for the most of catalysts fall in the range between 350 and 900 lm. BET surface area, Langmuir surface area, pore size and pore volume were determined by using Micromeritics ASAP2000 (Accelerated Surface Area and Porosimetry) instrument with nitrogen adsorption and desorption at 77 K. The pre weighed catalyst sample for this test was de-gassed at 373 K for a period of 24 h. density functional theory (DFT) model was used to interpret the pore size distribution results. This data illustrates that all of these catalysts are macroporous except CT-124. Due to its gelular matrix, CT-124 did not exhibit any porosity with this method because of the fact that gel based resins need complete wetting to swell and exhibit porosity. Hence, the surface area and pore width spaces are left blank in Table 1 for CT-124. 3.1.1. Scanning electron microscopy (SEM) was performed for all catalysts using Cambridge Stereoscan 360 at 15 KV. Micrographs of two best-performed catalysts CT-145H and CT-124 are shown in
Fig. 1a and b. These micrographs suggest that there are pores of different sizes and geometry. Cracks of very minute size are also observed specially for CT-124 and CT145H. Inside of the resin beads show a cauliflower like surface; this suggests that pores are fused together. The thin line cracks and macro pores are responsible for the pore volume and surface area of the catalyst. The reactants may easily penetrate into these pores, which act as active sites to ensue the chemical reaction. 3.1.2. Density and porosity measurement Bulk density was measured by filling the pre weighed known volume density bottle with catalyst and then mass of catalyst was found by difference and hence the density. True density was measured by using micromeretics multivolume pycnometer-1305 with helium gas as expansion medium. In both measurements, experiments were replicated five times so that the results were repeatable. Porosity was measured by using equation e ¼ ðqT qb Þ=qT
Fig. 1. Scanning electron micrographs of catalysts: (a) CT-124 and (b) CT-145H.
ð9Þ
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where e is porosity, qT and qb are true and bulk particle density (kg/m3) respectively. Some of the physical properties and characterization results of catalysts are shown in Table 1. 3.2. Batch kinetic results Experimental studies were undertaken to find out the best catalyst for ETBE synthesis under identical conditions. To overcome the intra-particle mass transfer resistance, agitation speed of 500 rpm was used after observation that there was no appreciable increase in conversion when speed was increased from 500 to 800 rpm. Since higher impeller speed can cause attrition and disintegration of catalyst particles, therefore impeller speed of 500 rpm was considered optimum and maintained throughout this study. 3.2.1. Selection of the most suitable catalyst All seven catalysts were examined under identical conditions of temperature, feed mole ratio of reactants, agitation speed and catalyst loading. TBA conversion and ETBE selectivity were determined for each catalyst as shown in Fig. 2a and b respectively. Following relations were used to calculate conversion and selectivity. C TBA;0 C TBA;t %TBA Conversion ¼ 100 ð10Þ C TBA;0 C ETBE;t %ETBE Selectivity ¼ 100 ð11Þ C TBA;0 C TBA;t where CTBA,0, CTBA,t are TBA concentrations at time zero and time t respectively and CETBE,t is concentration of ETBE at any time t. It is evident from the Fig. 2a and b that CT-175 and CT-275 gave the maximum conversion but they yielded poor selectivity, While CT-145H and CT-124 resulted in conversion up to 70% but they gave selectivity in the range of 60%. Amberlyst-15, Amberlyst-
a
35 and CT-151 were not found suitable from selectivity standpoint. It can be inferred from these figures that CT124 and CT-145H though gave almost similar results but CT-145H can be considered better because it gave more conversion than CT-124 while the maximum selectivity was same for both catalysts. Keeping in view the above results, CT-145H was selected to study the effect of other variables. 3.2.2. Effect of temperature Experiments were carried out at 323, 333 and 343 K to find the TBA conversion and ETBE selectivity using CT145H. Results showed that temperature of 343 K gave best set of conversion and selectivity. Lower reaction temperatures (323 K and 333 K) did not favor the reaction, so both conversion and selectivity declined. It was also observed during experimentation that higher temperature though favors conversion but selectivity decreases after about 4 h. Probable reason of decrease in selectivity at higher temperature can be attributed to more formation of IB that reduces the selectivity towards ETBE. Another contributing factor towards the reduced ETBE selectivity after 4 h can be strong adsorption of water on active catalyst sites due to its higher polarity than other components. This reduces the number of active sites available for reactants and hence ETBE selectivity decreases. Same behaviour was noticed for other catalysts at higher temperatures. 3.2.3. Effect of feed mole ratio (FMR) Three different feed mole ratios 1:1, 1:2 and 1:4 (TBA:EtOH) were investigated to calculate conversion and selectivity. Out of these FMRs, 1:4 yielded the best results in terms of TBA conversion and ETBE selectivity. The difference of ETBE selectivity for 1:2 FMR and 1:4 FMR was not very significant (72% and 77%). The 1:1 FMR did not resulted in appreciable conversion and selectivity as compared to those with higher ethanol concentration. So it can be concluded that excess of ethanol beyond twice
b
100
Amberlyst-15 CT 124 CT 151 CT 275
80
% ETBE Selectivity
80
% TBA Conversion
100
60
40 Amberlyst-15 CT 124 CT 151 CT 275
20
Amberlyst 35 CT 145H CT 175
Amberlyst 35 CT 145H CT 175
60
40
20
0
0 0
1
2
3
4
Time (h)
5
6
7
0
1
2
3
4
5
6
7
Time (h)
Fig. 2. TBA conversion (a) and ETBE selectivity (b) using different catalysts, 1:2 FMR, 5% catalyst loading, 500 rpm stirrer speed and 348 K temperature.
M. Umar et al. / Catalysis Communications 9 (2008) 721–727
725
the limiting reactant is not so much beneficial in terms of selectivity and conversion. Therefore, 1:2 mole feed ratio was considered optimum in this study.
Riedel (E–R) models were applied to the experimental data. Reaction rate expression of limiting reactant for LHHW model can be expressed as
3.2.4. Effect of catalyst loading Three catalyst loadings, 2.5%, 5% and 10% (w/w) of the reaction mixture were used to find their effect on conversion and selectivity. All other experimental conditions were kept unchanged. It was observed that catalyst loading of 2.5% though yielded about 50% ETBE selectivity, gave poor conversion. The 10% catalyst loading was found best in terms of selectivity (74%) but the TBA conversion was of same magnitude for 5% and 10% catalyst loading (80%). As we were more concerned about ETBE selectivity, so 10% catalyst loading was considered the best for this system.
rA ¼
3.3. Kinetic modeling
rA ¼
The two heterogeneous kinetic models namely Langmuir–Hinshelwood–Hougen–Watson (LHHW) and Eley
where rA is reaction rate of TBA (mol/g s), which is limiting reactant, NAo is initial moles of limiting reactant, XA
k 1 ½aA aB aC aD= =K eq ð1 þ K A aA þ K B aB þ K C aC þ K D aD Þ
ð12Þ
The reaction rate for E–R model is written as k 1 ½aA aB aC aD= =K eq ð1 þ K A aA þ K B aB þ K C aC þ K D aD Þ
rA ¼
ð13Þ
where KA, KB, KC and KD are the adsorption equilibrium constants for components A-D, aA, aB, aC and aD are the activities for TBA, EtOH, ETBE and water, respectively and Keq is equilibrium constant. Experimental reaction rates were calculated by using differential method as adopted by Cunnil et al. [18] N Ao ðdX A Þ V ðdtÞ
10.0
ð14Þ
water (Exp)
9.0
EtOH (Exp)
3
Concentration C i (k-mol/m )
2
8.0
TBA (Exp)
7.0
ETBE (Exp)
6.0 IB (Exp)
5.0 water (Q-H )
4.0
EtOH (Q-H)
3.0
TBA (Q-H)
2.0
ETBE (Q-H)
1.0
IB (Q-H)
0.0 0
1
2
3
4
5
6
7
Time (h) Fig. 3. Concentration profile of experimental and Q-H model for CT-145H at 323 K, 1:2 FMR, 5% catalyst loading, and 500 rpm stirrer speed.
water (Exp)
9.0
3
Concentration C i (k-mol/m )
10.0 EtOH (Exp)
8.0 7.0
TBA (Exp)
6.0
ETBE (Exp)
5.0
IB(Exp)
4.0
water (Q-H)
3.0
EtOH (Q-H)
2.0
TBA (Q-H)
1.0
ETBE (Q-H)
0.0 0
1
2
3
4
5
6
7
IB (Q-H)
Time (h) Fig. 4. Concentration profile of experimental and Q-H model for CT-145H at 333 K, 1:2 FMR, 5% catalyst loading, and 500-rpm stirrer speed.
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M. Umar et al. / Catalysis Communications 9 (2008) 721–727
water (Exp)
9.0
EtOH (Exp)
3
Concentration C i (k-mol/m )
10.0
8.0
TBA (Exp)
7.0
ETBE (Exp)
6.0
IB (Exp)
5.0
water (Q-H model)
4.0
EtOH (Q-H)
3.0
TBA (Q-H)
2.0
ETBE (Q-H)
1.0
IB (Q-H)
0.0 0
1
2
3
4
5
6
7
Time (h) Fig. 5. Concentration profile of experimental and Q-H model for CT-145H at 343 K, 1:2 FMR, 5% catalyst loading, and 500 rpm stirrer speed.
rA ¼ k 1 ½aA aB ðaC aD =K eq Þ
ð15Þ
where k1 is reaction rate constant. This model yielded close fit to the experimental data. Experimental conversions as well as those calculated by Q-H model were compared and it was found that both were in good agreement. Experimental concentration profile and those calculated using QH model for all components at three different temperatures are shown in Figs. 3–5. These figures depict that this model describes the system to a close approximation. It was found that Q-H model fit well at lower temperatures (323 and 333 K) but at 343 K, there is difference in model and experimental values. This may be due to the reason that Q-H model do not assume the adsorption of components at the catalyst surface, while actually some adsorption of components is taking place. Therefore, experimental values of concentration for reactants are somewhat less as compared to the model values. Initiation of side reaction at higher temperature might be another reason for the difference between model and experimental values. Nevertheless, trend of the model and experimental profiles is same. Though the system is heterogeneous originally but it can be supposed that liquid and solid catalyst in reaction vessel are intimately mixed so that the system almost approaches
-5
-6
lnk
is conversion of limiting reactant, V is volume of reaction mixture (m3) and t is time (s). When all experimental values were correlated with those calculated by using the above heterogeneous model equations, both models yielded scattered results in terms of some negative adsorption constants as well as negative reaction rate constant which of course is not possible. Multiple linear and non-linear regression techniques were used to find out the constants of Eqs. (12) and (13) [19]. Similar results have already been reported by Yin et al. [7], Gangadwala et al. [20], Patel and Saha [21]. Since both the heterogeneous models were not found coherent, so quasi-Homogeneous (Q-H) model was used to interpret the data. Reaction rate expression for the QH model is
-7
-8
-9 0.0029
0.00295
0.003
0.00305
0.0031
0.00315
-1
1/T (K ) Fig. 6. Arrhenius plot for etherification of ethanol and tert-butanol with CT-145H.
to a homogeneous one. Literature does support the using of Q-H model for liquid phase reaction systems in the presence of ion exchange resin catalysts [22]. Temperature dependency of rate constant was expressed by drawing Arrhenius plot. Fig. 6 shows the plot with correlation coefficient of 0.96. Expression obtained for rate constant k1 is shown in Eq. (16) k 1 ¼ expð26:877 11281=T Þ
ð16Þ
The pre exponential factor and activation energy calculated are 4.70 · 1011 (mol/g s) and 93.79 kJ/mol respectively. These values are in agreement with the published literature [2]. 4. Conclusions Seven commercially available ion exchange resin catalysts were used in this study to find their efficiency and capacity for ETBE synthesis. CT-145H was found to be the most suitable catalyst and was then further investigated at different conditions. Temperatures of 343 K, 10% cata-
M. Umar et al. / Catalysis Communications 9 (2008) 721–727
lyst loading and 1:2 feed mole ratios of reactants were found optimum. Kinetic modeling results showed that experimental data fit well to quasi-homogeneous model rather than heterogeneous models. These results will be utilized for ETBE synthesis via reactive distillation. Acknowledgements The authors are thankful to Higher Education Commission of Pakistan for funding this research. MU is grateful to Dr. B. Saha of Loughborough University UK for kind help and guidance. References [1] S. Assabumrangrat, W. Kiatkittipong, N. Sevitoon, P. Praserthdam, S. Goto, Intl. J. Chem. Kinet. 34 (2002) 292. [2] Bo-Lun Yang, San-Ba Yang, Rui-qing Yao, React. Funct. Polym. 44 (2000) 167. [3] F. Cunnil, M. Vila, J.F. Izquierdo, M. Iborra, J. Tejero, Ind. Eng. Chem. Res. 32 (1993) 564. [4] C. Fite, M. Iborra, J. Tejero, J.F. Izquierdo, F. Cunnil, Ind. Eng. Chem. Res. 33 (1994) 581. [5] B.H. Bisowarno, M.O. Tade, Ind. Eng. Chem. Res. 39 (2000) 1950. [6] M.G. Sneesby, M.O. Tade´, R. Datta, T.N. Smith, Ind. Eng. Chem. Res. 36 (1997) 1855. [7] Y. Li, S. Huang, S. Wu, X. Yuan, Catal. Lett. 87 (2003) 31.
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