Article pubs.acs.org/IECR
Reaction Kinetics and Thermodynamic Equilibrium for Butyl Acrylate Synthesis from n‑Butanol and Acrylic Acid Anna M. Ostaniewicz-Cydzik,† Carla S. M. Pereira,‡ Eugeniusz Molga,† and Alírio E. Rodrigues*,‡ †
Department of Chemical and Process Engineering, Warsaw University of Technology, ul. Waryńskiego 1, 00-45 Warsaw, Poland Laboratory of Separation and Reaction Engineering (LSRE), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
‡
S Supporting Information *
ABSTRACT: The esterification reaction of n-butanol with acrylic acid in the presence of a commercial ion-exchange resin, Amberlyst 15-wet, was carried out in a batch reactor. The reactions were performed at different temperatures (50 to 90 °C), different n-butanol/acrylic acid molar ratios (2 and 3), and different catalyst amounts (1 wt % to 3.5 wt %). Different reaction rate expressions were evaluated. A simplified Langmuir−Hinshelwood−Hougen−Watson kinetic model was found to be the best model to describe the experimental results. This model is given by the following expression: r = Kc·((a1·a2 − (a3·a4)/Keq)/(1 + K4·a4)2), with kc (mol·gcat−1·min−1) = 1.52 × 107 − 66 988/(RT) and K4 = 1.589. Also equilibrium experiments were carried out. The proposed equilibrium equation was Keq = exp((−(1490 ± 577)/T + (7.21 ± 1.67)). From this equation, it was possible to determine the reaction standard enthalpy and entropy values: ΔH° = 12.39 ± 4.80 [kJ/mol] and ΔS° = 59.98 ± 13.87 [J/mol· K].
1. INTRODUCTION Acrylates have important applications in everyday life and in many industries. Butyl acrylate is used in the production of coatings and inks, adhesives, sealants, textiles, plastics, and elastomers. Coating applications include architectural latex coatings, water-based dispersions, automotive original equipment manufacture, and refinish materials. Pressure sensitive adhesives contain n-butyl acrylate. Other adhesive applications are found in the textile and construction industries. The textile industry products that contain n-butyl acrylate are fibers, warp sizings, thickeners, and back coat formulations (adhesives). In the plastics industry, n-butyl acrylate is found in some PVC modifiers and molding or extrusion additives. In 2005, the global demand for butyl acrylate was 1.67 million tonnes. The largest consumer was Western Europe (459 820 tonnes/year), followed by the U.S. (417 900 tonnes/ year), Asia-Pacific (355 900 tonnes/year), Japan (113 460 tonnes/year), and Latin America (94 630 tonnes/year). Figure 1 presents the distribution of the global demand of butyl acrylate in different industries. The main part comes from coating (39%); the next part is a comonomer in polyetylen (25%) and adhesives (17%).1 TranTech data from 2005 show that the global production capacity was about 1.92 million tonnes/year. This includes 622 730 tonnes/year in the U.S., 469 090 tonnes/year in Western Europe, 477 960 tonnes/year in Asia-Pacific (excluding Japan), and 125 450 tonnes/year in Japan.1 The main producers of butyl acrylate, in 2006, were Rohm and Haas incl Stoltaas, BASF, and DOW Chemicals (see Tables S1 and S2 in Supporting Information). Prices of butyl acrylate depend on its purity. Usually, butyl acrylate has a minimum purity of 99.5%. Based on a ICIS pricing report, European prices of butyl acrylate, at the end of © 2014 American Chemical Society
Figure 1. Distribution of the global demand in different industries.1
2012, were between 1.84 and 1.89 €/kg; in the U. S. A., the price range was between 1.94 and 2 USD/kg.1 The production of butyl acrylate involves an equilibrium limited esterification reaction between acrylic acid and nbutanol, having water as a byproduct (Scheme 1). Scheme 1. Reaction Scheme of the Esterification of Acrylic Acid and n-Butanol CH3(CH 2)2 CHOH + CH 2CHCOOH ↔ CH 2CHCOOCH(CH 2)2 CH3 + H 2O Received: Revised: Accepted: Published: 6647
January 16, 2014 March 12, 2014 April 3, 2014 April 3, 2014 dx.doi.org/10.1021/ie5002247 | Ind. Eng. Chem. Res. 2014, 53, 6647−6654
Industrial & Engineering Chemistry Research
Article
properties of butyl acrylate, n-butanol, acrylic acid, and water are presented in Table 1.
This reaction is very slow and can be performed by using homogeneous (as sulfuric acid, hydrofluoric acid, or paratoluenesulfonic acid) or heterogeneous catalysts (as oxides and ion-exchange resins). However, the use of solid catalysts is preferable because of their lower toxicity, lower corrosive effects, and the easiness to recover them from the reaction mixture and reuse. Several solid catalysts were already tested for the esterification of n-butanol with acrylic acid, such as heteropolyacids,2−4 Amberlyst 15,3,5,6 Amberlite 200C,3 Nafion-H,3 Nafion-SiO2,3 Lewatit K 2621,7 Amberlyst 46,8 Amberlyst-131, and Dowex 50Wx-400.6 Butyl acrylate is currently produced in a homogeneously catalyzed multistage process using two reactors, followed by three distillation columns applied for the purification of the product and recovery of the reactants.9 This is a cost intensive process and motivates the search for better alternatives. In this sense, the use of reactive separation technologies, where reaction and separation are integrated into a single equipment, offers an attractive solution for the sustainable synthesis of butyl acrylate. Reactive distillation was already evaluated;8,10,11 nevertheless, the butyl acrylate esterification is a reaction with high risk of polymerization, which is further promoted by the high temperatures used in this technology. A feasible solution might be the use of reactive adsorptive technologies since they usually work at moderate temperatures, as is the case of the Simulated Moving Bed Reactor (SMBR), which was successfully applied on the production of, for instance, an ester (ethyl lactate)12 and some acetals.13−15 The successful implementation of the SMBR, for the previously mentioned compounds, was strongly related with the solid used: a strong acidic ion-exchange resin, the Amberlyst 15-wet that acts as both catalyst and water selective adsorbent. Consequently, the use of Amberlyst 15-wet resin in a reactive adsorptive process seems to be an efficient approach for the synthesis butyl acrylate. In order to develop such a process, the knowledge about reaction thermodynamic equilibrium and kinetics is fundamental. In spite of the number of kinetic studies available in the literature, just one proposed a kinetic model for the esterification of n-butanol with acrylic acid in the presence of Amberlyst 15 (dry form),5 where the model is expressed in concentrations though the highly nonideal behavior of the reaction mixture, which clearly suggests the use of activities for both equilibrium and reaction kinetics equations. Therefore, in this work, the esterification reaction of n-butanol with acrylic acid in the presence of Amberlyst 15-wet was studied. Thermodynamic equilibrium data were measured, and from the chemical equilibrium constant dependence on temperature, the reaction standard enthalpy and entropy were determined considering a nonideal liquid-phase reaction and computing the activity of each species through the UNIFAC group contribution method. Different kinetic models expressed in activities were assessed in order to select the one that better predicts the experimental results.
Table 1. Properties of All Species Involved in the Butyl Acrylate Synthesis properties molecular weight - M (g/mol) density - ρ (g/cm3) melting temperature - Tf (K) normal boiling temperature Tb (K) critical temperature - Tc (K) critical pressure - Pc (bar) critical volume - Vc (cm3/ mol) acentric factor - w
acrylic acid
n-butanol
butyl acrylate
water
72.06
74.12
128.17
18.02
1.05 286.15
0.81 183.85
0.90 208.55
1.03 273.15
414.15
391.90
420.55
373.15
615.00 56.6 208.0
563.10 44.14 273.0
598.00 29.1 428.0
647.10 220.64 55.95
0.538
0.588
0.482
0.345
As catalyst and adsorbent, a commercial strong-acid ionexchange resin named Amberlyst 15-wet (Rohm and Haas) was used. This resin is a bead-form macroreticular polymer of styrene and divinylbenzene, with particle diameters varying between 0.3 and 1.2 mm, an ion-exchange capacity of 4.7 mequiv of H+/g of dry resin, and an inner surface area of 53 m2/g. The Amberlyst 15-wet (A15) resin adsorbs water, which decreases the reaction rate since water is a byproduct. Therefore, in order to guarantee anhydrous resin, prior to use, the A15 resin was washed several times first with deionized water and after with ethanol; then the washed resin was dried at 90 °C until the mass remained constant. 2.2. Experimental Setup. 2.2.1. Equilibrium Experiments. The experiments to measure the equilibrium constant were carried out, in 100 mL glass vessels, at different temperatures (from 60 to 90 °C) and using at each temperature different initial n-butanol/acrylic acid molar ratios (from 1 to 3). All vessels were closed and immersed into a thermostated water bath with agitation in order to control the temperature. Samples were collected, until the equilibrium was reached, with a syringe through a sampling tube immersed in the reaction medium. Before samples were collected, the vessels were placed in an ice bath to avoid n-butanol evaporation. 2.2.2. Kinetic Experiments. The kinetic experiments were carried out in a glass-jacketed 1 dm3 batch reactor (Büchi, Switzerland), equipped with pressure and temperature sensors and with a blow-off valve, and mechanically stirred at 700 rpm. A schematic representation of the experimental setup is presented in Figure 2. The temperature was controlled through a thermostated bath (Lauda, Germany). To keep the reaction mixture in the liquid phase, over the whole operating temperature range, the pressure was set to 6.0 bar with helium. The catalyst was placed in a special basket at the top of the stirrer shaft. When the stirrer starts rotating, the basket falls down and the esterification reaction starts. The basket ensures free movement of the reaction mixture through the ionexchange resin. Samples were taken in registered time intervals and analyzed by gas chromatography. 2.2.3. Analytical Method. All samples were analyzed in a gas chromatograph (Shimadzu, GC 2010 Plus) using 2-propanol as the solvent and as the internal standard. The compounds were separated using a silica capillary column (CPWax57CB, 25 m × 0.53 mm i.d., film thickness of 2.0 μm) and quantified by a
2. EXPERIMENTAL SECTION 2.1. Chemicals and Catalysts. The chemicals used were 2propanol (>99.97%), n-butanol (>99.9%), acrylic acid (>99.5%), and butyl acrylate (>99.9%) from Acros Organics. Butyl acrylate is a colorless liquid ester with a characteristic fruity odor. It is dissolved in organic solvents. The main 6648
dx.doi.org/10.1021/ie5002247 | Ind. Eng. Chem. Res. 2014, 53, 6647−6654
Industrial & Engineering Chemistry Research
Article
The experimental equilibrium compositions, the activity coefficients, and the determined equilibrium constants, for each performed experiment are presented in Table 2. It should be mentioned that during the butyl acrylate synthesis there is a high risk of polymerization of both acrylic acid and butyl acrylate species.17 Indeed, in the equilibrium experiments performed, the formation of two byproducts was observed; the comparison of the retention times shown in Chen et al.’s work3 with the ones in our study suggests that these products are butyl 3-butoxypropanoate and 3-butoxypropionic acid. The amount of these components was more significant for lower reactants’ molar ratio and higher temperatures. Nevertheless, the higher percentage of byproducts’ peak area in the GC measurements was equal to 3.5% (the average percentage, considering all the experiments, was 1.6% of the total peaks area). Therefore, in order to simplify the thermodynamic equilibrium study, the presence of these compounds was neglected. Besides, acrylic acid used in this study contains 180 to 220 ppm of hydroquinone monomethyl ether applied to prevent acrylic acid polymerization, and that was not also taken into account. These simplifications together with some experimental errors and/or deficiencies in the thermodynamic model used to calculate the activity coefficients18 might justify the different equilibrium constant values for different initial molar ratio at the same temperature observed in Table 2. The equilibrium constant is only temperature dependent. The chemical equilibrium constant is generally given as a function of the reaction standard Gibbs free energy (ΔG°) and temperature (T):
Figure 2. Experimental setup for kinetic studies. BR: batch reactor; M: motor; TT: temperature sensor; PT: pressure sensor; PM: manometer; BV: blow-off valve; NV: needle valve; GC: gas chromatograph; TB: thermostatic bath.
thermal conductivity detector (TCD). Helium N50 was used as the carrier gas at a flow rate of 3.3 mL·min−1. The temperature of the injector and of the TCD was set to 250 °C. The initial column temperature was 120 °C for 3 min, the temperature was then increased at a rate of 60 °C/min up to 200 °C and held constant for the following 12 min.
⎛ ΔG° ⎞ ⎟ Keq = exp⎜ − ⎝ RT ⎠
3. THERMODYNAMIC EQUILIBRIUM RESULTS The thermodynamic equilibrium constant, for the reaction between n-butanol and acrylic acid, is given by the following expression: a ·a x ·x γ ·γ Keq = 3 4 = 3 4 · 3 4 a1·a 2 x1·x 2 γ1·γ2 (1)
(2)
where R is the ideal gas constant. Moreover, (3)
ΔG° = ΔH ° − T ΔS°
where ΔH° and ΔS° are the reaction standard enthalpy and entropy, respectively. Combining eqs 2 and 3 gives
where ai is the activity of compound i, xi is the molar fraction of compound i, γi is the activity coefficient of compound i, and 1, 2, 3, and 4 represent n-butanol, acrylic acid, butyl acrylate, and water, respectively. In order to calculate the equilibrium constant (eq 1), the equilibrium composition was experimentally measured and the activities of the compounds were computed by using the UNIFAC method; this method requires the use of relative molecular volume and surface area of pure compounds, and interaction parameters between the different groups of each molecule. The values for these parameters are presented as Supporting Information in Tables S3 and S4.16
⎛ ΔG° ⎞ ⎛ ΔS° ΔH ° ⎞⎟ ⎟ = exp⎜ Keq = exp⎜ − − ⎝ RT ⎠ ⎝ R RT ⎠
(4)
Adjusting the obtained results (Table 2) to eq 4, it was possible to find the thermodynamic equilibrium constant dependency on temperature expressed by Keg = exp(−(1490 ± 577)/T + 7.21 ± 1.67), as well as the reaction standard enthalpy and entropy values, which are 12.39 ± 4.80 kJ/mol and 59.98 ± 13.87 J/(mol·K), respectively. The reaction enthalpy value obtained indicates that this is an endothermic reaction as also concluded in other studies.4,8 The standard
Table 2. Activities for the Equilibrium Composition and Thermodynamic Equilibrium Constant T [K]
RBut/AA
x1
x2
x3
x4
a1
a2
a3
a4
Keq
333 333 333 343 343 343 363 363 363
1 2 3 1 2 3 1 2 3
0.1827 0.4113 0.5011 0.1838 0.4034 0.5461 0.1725 0.3852 0.5301
0.1799 0.0804 0.0681 0.1807 0.0873 0.0473 0.1693 0.0691 0.0313
0.3187 0.2542 0.2154 0.3178 0.2547 0.2033 0.3291 0.2729 0.2193
0.3187 0.2542 0.2154 0.3178 0.2547 0.2033 0.3291 0.2729 0.2193
0.2081 0.4526 0.5429 0.2101 0.4442 0.5853 0.1982 0.4271 0.5709
0.1220 0.0497 0.0388 0.1235 0.0548 0.0287 0.1160 0.0430 0.0188
0.5518 0.4712 0.4064 0.5446 0.4641 0.3916 0.5484 0.4745 0.3998
0.9584 0.7752 0.6743 0.9453 0.7707 0.6219 0.9682 0.8258 0.6733
20.82 16.24 13.01 19.84 14.71 14.51 23.09 21.33 25.09
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dx.doi.org/10.1021/ie5002247 | Ind. Eng. Chem. Res. 2014, 53, 6647−6654
Industrial & Engineering Chemistry Research
Article
state enthalpies of formation of all the compounds involved in the butyl acrylate synthesis found in the DIPPR 801 database19 are presented in Table 3. As can be observed, the uncertainty of Table 3. Standard State of Enthalpy of Formation of Different Species19 n-butanol
acrylic acid
butyl acrylate
water
−274.60
