Article pubs.acs.org/IECR
Preparation and Characterization of Perfluorosulfonic Acid Nanofiber Membranes for Pervaporation-Assisted Esterification Pei-Pei Lu, Zhen-Liang Xu, Xiao-Hua Ma,* and Yue Cao State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Laboratory, Chemical Engineering Research Center, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China S Supporting Information *
ABSTRACT: Multilayer membranes were prepared by the combination of perfluorosulfonic acid/SiO2 nanofibers and a poly(vinyl alcohol) (PVA) pervaporation layer and were used to enhance the esterification of acetic acid (HAc) and ethanol (EtOH). The esterification−pervaporation experiments were carried out in a continuous membrane contactor. The effects of the temperature, the ratio of HAc to EtOH, and the ratio of membrane area to reaction volume were investigated. The results demonstrated that the membranes had good catalytic activities even at low temperature because of the nanofibrous structure of the catalysis layer. The conversion of HAc at 60 °C after 10 h was 10−15% more than the equilibrium conversion and by improved about 45% with respect to the equilibrium conversion after 55 h. The yield of EtAc was higher than 90%, which demonstrates that the difunctional membrane could enhance the esterification process greatly through the in situ removal of water. polymer such as Nafion14 or Amberlyst-1515) with a polymeric or inorganic matrix and casting them onto a suitable support.16,17 To improve the performance of membrane reactors, various membrane configurations have been suggested, prepared, and tested.7,12,18 de la Iglesia et al. compared two different zeolite membranes in the esterification of HAc with EtOH in a continuous membrane reactor packed with catalyst Amberlyst-15 and separated the water from the reaction medium.8 They reported that both membranes were capable of shifting the equilibrium in less than one day. In another study, catalyst-coated PERVAP 1000 (Sulzer) membranes were used for pervaporationassisted esterification in the presence of Amberlyst-15 and -35 catalysts.15 The catalytic membrane showed catalytic properties similar to those of the commercial membranes and catalysts and increased the conversion to 60% in 8 h at 60 °C. Perfluorosulfonic acid (PFSA) resin is a type of solid acid catalyst, and PFSA nanofibers were demonstrated to be efficient catalysts in the batch reaction of HAc and EtOH in our previous work.19 In the present study, we describe a difunctional membrane based on PFSA nanofibers. This type of membrane contains three layers: the PFSA/SiO2 catalysis layer supported by poly(ether sulfone) (PES) nanofibers; the pervaporation layer made up of poly(vinyl alcohol) (PVA) solution; and the supporting layer, which is a poly(acrylonitrile) (PAN) ultrafiltration flat membrane. The PFSA−SiO2/PVA/PAN composite membranes were used to enhance the esterification between EtOH and HAc by catalysis−distillation coupling, and positive results were reported.20 However, the membranes were used simply as packages in the distillation column but were not involved in the pervaporation process. In this study, we couple the esterification and pervaporation processes into one step.
1. INTRODUCTION The esterification of ethanol (EtOH) and acetic acid (HAc) is a typical reversible reaction, and the conversion of the reactants is generally low because of thermodynamic equilibrium. To achieve a high ester yield, a large excess of one of the reactants (usually the alcohol) must be used, whereas the recovery of the product becomes more expensive because of the increased energy consumption in the subsequent separation process (distillation or vacuum stripping). Pervaporation-assisted esterification has been studied experimentally and theoretically since the 1980s in an effort to enhance process performance.1−4 Through the controlled removal of one of the products (usually water, but removal of the ester has also been reported5) from the mixture by a pervaporation membrane, the reaction can be enhanced thermodynamically and kinetically.6 The most frequent configuration for carrying out pervaporation-enhanced esterification consists of a batch reactor, where the reaction takes place with a homogeneous or heterogeneous catalyst, followed by a pervaporation membrane and refluxing to the reactor. This configuration has been used widely in the esterification of HAc with EtOH.3,7,8 The operating parameters of the pervaporation modules and reactors can be adjusted separately to optimize the process performance, improving the conversion of the reactants and reducing the operating costs.9,10 In recent years, batch and continuous membrane reactors integrating esterification and pervaporation in one step have been reported.5,11 Such membrane reactors are equipped with difunctional (catalytic and separative) membranes that usually have one of the following two types of structures: a bilayer structure, in which one layer is a porous support layer and the other is a difunctional layer with both catalytic and pervaporation properties, or a multilayer structure including at least three layers: the catalysis layer, the pervaporation layer, and the supporting layer.12,13 These membranes can be prepared by blending the necessary catalytic component (a zeolite12 such as H-ZSM-5 or zeolite-Y, or a functional © XXXX American Chemical Society
Received: January 7, 2013 Revised: May 17, 2013 Accepted: May 27, 2013
A
dx.doi.org/10.1021/ie400065e | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
The difunctional membrane was evaluated in the membrane reactor, and the effects of the parameters such as temperature, reactant ratio, membrane area (S), and catalyst loading (W) on the reactor performance were investigated.
2. EXPERIMENTAL SECTION 2.1. Membrane Preparation. PVA [1799, from Sinopharm Chemical Reagent Co. Ltd. (SCRC), Beijing, China] was dissolved in water and stirred at 95 °C for 12 h to obtain a 10 wt % PVA solution. The resulting homogeneous solution was cast onto a PAN ultrafiltration membrane (water flux = 80 L·m−2·h−1) and subsequently cross-linked to form the PVA/ PAN pervaporation substrate. The catalysis−pervaporation membrane was prepared by depositing a thin layer of PES/ PFSA/SiO2 nanofibers onto the PVA/PAN substrate, where the nanofibers were fabricated from PES/PFSA/SiO2 (20:20:60, w/w/w) suspensions by the electrospinning method.19 The catalysis−pervaporation membrane was immersed in 1 M HCl solution for 24 h, washed with deionized water, and then dried at 60 °C under a vacuum until the mass remained unchanged. 2.2. Membrane Characterization. The structure of the membrane was observed by scanning electron microscopy (SEM; JSM-6360LV, JEOL, Tokyo, Japan), and the specific surface area (SSA) of the catalysis layer and pore structure were determined by N2 adsorption−desorption (JW-BK112F, JWGB Science & Technology Co., Ltd., Beijing, China). Surface acid site (SAS) concentration of the catalysis layer was measured by the acid−base titration method.19 Thermogravimetric analysis (TGA, Shimadzu Corporation, Kyoto, Japan) was used to analyze the thermal stability of the catalysis−pervaporation membrane at a heating rate of 10 °C/min under a nitrogen atmosphere. 2.3. Swelling Measurements and Sorption Experiments. A piece of the catalysis layer or the pervaporation layer was immersed in an EtOH/HAc mixture (50:50, w/w, for the catalysis layer) or an EtOH/H2O mixture (10:90, w/w, for the pervaporation layer), respectively, at 60 °C for 12 h to achieve swelling equilibrium. The membrane was then removed, the solution on the surface was carefully wiped off with tissue paper, and the membrane was weighed in a tightly closed bottle. The liquid adsorbed in the membrane sample was desorbed in a vacuum environment, collected by a liquid-nitrogen trap, and then analyzed by gas chromatography (GC7980T, Techcomp Ltd., Beijing, China). The swelling ratio (SW) was calculated according to the equation m − md SW (%) = w × 100 md (1)
Figure 1. Schematic of the experimental apparatus: (1) feed liquid, (2) thermometer, (3) feed pump, (4) membrane module, (5) liquid nitrogen trap, (6) vacuum pump.
the permeation side was kept below 1 kPa by a vacuum pump, and the permeation vapor was condensed by a liquid nitrogen trap. The liquid collected in a time interval (30−40 min) was weighed and analyzed by gas chromatography (GC7980T, Techcomp Ltd., Beijing, China). The permeation flux (Ji) of each component was calculated according to m Ji = i (3) At where mi (g) is the permeation mass of component i, A (m2) is the effective area of the membrane in contact with the feed, and t (h) is the permeation time. 2.5. Esterification−Pervaporation Experiments. The esterification−pervaporation experiments were performed in the same continuous operating mode as described for pervaporation experiments. EtOH and HAc were heated to reaction temperature and then added to the feed tank. Meanwhile, the reaction mixture was slowly and continuously pumped across the membrane module. The reaction temperature was kept constant to within ±1 °C by a water heating bath. The reaction mixture and the permeation liquid were analyzed by gas chromatography once per hour.
3. THEORETICAL SECTION The esterification carried out in this work can be described as k1
A+B⇌E+W k2
where A represents acetic acid (HAc), B represents ethanol (EtOH), E represents ethyl acetate (EtAc), and W represents water (H2O). kl and k2 are the rate constants for the forward and reverse reactions, respectively. A simple mathematical model was developed for the esterification−pervaporation process based on the difunctional membrane. In the development of this model, a number of simplifications and assumptions were made: (1) The self-catalytic reaction is negligible. (2) The acid sites are uniformly disturbed on the surface of the nanofibers. (3) Operation is isothermal, and negligible temperature change occurs in the membrane. The material balance in the reactor at any time can be written as
where md (g) and mw (g) are the weights of the membrane sample in the dry and wet states, respectively. The solubility selectivity (αs) was calculated as
αs =
wi /wj Fi /Fj
dci A = −ri − Ji (4) dt V where ci and Ji denote the concentration and permeation flux, respectively, of component i; ri is the rate of disappearance/ appearance of component i in the reactor due to the reaction; and V and A are the volume of the reaction mixture and the effective area of the permselective membrane, respectively. The reactants diffuse into the catalysis layer from the liquid bulk while flowing across the membrane. In the direction perpendicular to the membrane, the concentrations of the reactants
(2)
where wi (g) and wj (g) are the weight fractions of components i and j, respectively, adsorbed in the liquid and Fi and Fj are the corresponding weight fractions in the mixture. 2.4. Pervaporation Measurements. Pervaporation experiments of the reaction mixtures (EtOH, HAc, H2O, and EtAc) through the difunctional membrane were carried out in a laboratory-made apparatus as shown in Figure 1. The membrane contactor (Figure 2a) can be stacked layer upon layer to adjust the effective area of the pervaporation module. The pressure of B
dx.doi.org/10.1021/ie400065e | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 2. Schematics of (a) the membrane contactor and (b) the mass-transfer process over the membrane.
Table 1. Concentration Variations in the Reaction System B↔
A+ t=0 t=0
concentration moles moles
cA V 0 cA V0cA(1 − xA)
concentration
V0 cA(1 − xA) V
V0cA(θ1 − xA) −
∂ 2ci ∂l
2
−
∂ci − ri = 0 ∂l
(5)
l = δc ,
(6)
ci = cb, i , ri = 0
Dm, i
ci = c in, i
Di ci δp
V0cA(θ2 + xA) −
∫0
t
JW S dt
(8)
where Mi and ρi are molar mass and density, respectively, of component i. For the reaction system, consider the concentration of the less-abundant reactant (designated as component A hereafter), for which the initial molar concentration is cA0. Let XA be the conversion at any time t, and define θ1 and θ2 as the ratios of the initial molar concentrations of components B and W, respectively, to that of component A. (Generally, the initial concentration of component E is zero.) Thus, the changes in concentration with the extent of reaction can be predicted, as shown in Table 1. Therefore the, mass balance in the esterification−pervaporation membrane can be described by the equations
−
According to the solution−diffusion model, the permeation flux of component i has a linear relationship with the concentration on the membrane surface;21,22 thus
Ji = −
JB S dt
J Mi A dV = −∑ i dt ρi i
where Ke is the equilibrium constant of the reaction. Reactions with HAc/EtOH = 1:1 were carried out at various temperatures. The equilibrium conversions were 38.5%, 42.8%, and 45.0% at 50, 60, and 70 °C, respectively (Supporting Information, Figure S2). By calculation, the equilibrium constants at these temperatures were 0.392, 0.560 and 0.668, respectively (Supporting Information, Figure S2). The boundary conditions of eq 5 are l = 0,
t
W θ2cA V0θ2cA
where δp is the thickness of the pervaporation layer and ci is the concentration of component i on the interface between the catalysis and separation layers. Di denotes to the solution− diffusion coefficient through the membrane of component i, which was determined by experiments (Supporting Information, Figure S4). The volume change of the reaction mixture in the system is given by
where Dm,i is the molecular diffusion coefficient of component i, which was calculated by the Wilke−Chang equation (Supporting Information, eq S2). ri is the rate of disappearance/appearance of component i, which can be written as ⎞ ⎛ 1 c Ec W ⎟ ri = k1cAc B − k 2c Ec W = k1⎜cAc B − Ke ⎠ ⎝
∫0
0 0 V0cAxA
V0 cAxA V
decrease, whereas those of the products increase due to the reaction. The accumulated EtAc returns to the liquid bulk through molecular diffusion, and the H2O is removed from the reaction system by pervaporation (Figure 2b). The mass balance for component i through a differential thickness, dl, in the membrane is described by Dm, i
E+
θ1cA V0θ1cA
d2cA dl
2
−
⎧ dcA − k1/V 2⎨V0cA[V0cA(θ1 − xA ) − Y ] dl ⎩
⎫ 1 V0cAxA[V0cA(θ2 + xA ) − X ]⎬ = 0 Ke ⎭
(9)
with X=
(7) C
∫0
t
JW S dt dx.doi.org/10.1021/ie400065e | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 3. SEM images of the membrane: (a) single nanofiber of the catalysis layer, (b) nanofibrous structure of the catalysis layer, (c) cross section of the membrane.
and
Y=
∫0
t
JB S dt
4. RESULTS AND DISCUSSION 4.1. Morphology and Structure of the Membrane. The structures of the catalysis−pervaporation membrane are shown in Figure 3, and the physical properties of the catalysis layer and pervaporation layer were measured separately and are summarized in Table 2. Three layers of the composite membrane Table 2. Properties of the Catalysis−Pervaporation Membrane property
catalysis layer
pervaporation layer
thickness (wet state) (μm) swelling ratio (%) solubility selectivity specific surface area (m2·g−1) surface acid site (mmol·g−1)
25−200 760 0.95 80.2 0.106
15 41 52 − −
Figure 4. Pervaporation performance of the difunctional membrane at various water contents (■, H2O; ●, EtOH).
difunctional membrane under various temperature and feed conditions. Generally, the permeation performance of a pervaporation membrane is significantly associated with the gradient difference of the chemical potential between the upstream and downstream sides. Previous research demonstrated that the water flux was practically linear with water variation for small water concentrations in the feed.9,10 In this study, pervaporation experiments were carried out with EtOH/water mixtures with various water contents in a typical esterification system (0−15 wt %), and obviously, the water flux increased with water content in the feed, whereas the EtOH flux decreased. In this range, the water flux showed a good linear relationship with the content, and the diffusion coefficients (Dw) calculated from the slope were 2.04 × 10−9, 2.63 × 10−9, and 3.78 × 10−9 m2/s at 40, 50, and 60 °C, respectively (Supporting Information, Figure S4 and Table S1). However, at high water contents, the water flux was no longer linear with the water content but increased rapidly (Supporting Information, Figure S5).25 Both the water flux and the EtOH flux increased with temperature because of the increase in the motion of the polymer chains and the expansion of the free volume in the membrane.23 During the reaction, the produced water should be removed from the system as much as possible, and therefore, high temperature was applicable because of the high water flux. In this situation, however, more EtOH would be wasted as a result of the high EtOH flux. The difunctional membrane was employed with mixtures of the reaction system H2O/EtOH/HAc/EtAc = 30:30:20:20 (w/w/w/w) at 60 °C for 15 h to evaluate the stability of the
can be clearly observed in Figure 3c: the upper catalysis layer, the middle pervaporation layer, and the bottom supporting layer. The surface of the membrane had a continuous layer of disordered nanofibers that were 150−350 nm in diameter (Figure 3a). The mass-to-area ratio of the catalysis layer was 0.47−3.82 g/100 cm2, and the thickness was 25−200 μm, both of which could be adjusted by electrospinning time. Because of its highly porous structure, the catalysis layer had a large mass swelling ratio but exhibited little geometrical deformation (Supporting Information, Figure S3). Table 2 showed that the catalysis layer had little selectivity for the reactants so that the composition of the liquid taken up inside the layer was considered to be the same as that of the bulk liquid. The pervaporation layer of the difunctional membrane was 15 μm in thickness, and this value was much larger than the thicknesses reported in the literature (1−5 μm).19,23 Generally, the thickness of the PVA layer has a direct effect on the pervaporation performance of a membrane.24 However, this was not the emphasis of this study. The mass swelling ratio of the pervaporation layer was smaller than that of the catalysis layer, which was about 40%, but the solubility selectivity was much higher. The supporting layer had many fingerlike structures and was about 150 μm in thickness. This structure provided sufficient mechanical strength to the composite membrane but did not affect the pervaporation performance of the PVA layer. 4.2. Pervaporation Performance. Figure 4 shows the pervaporation performance of water and EtOH through the D
dx.doi.org/10.1021/ie400065e | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
These conversions were much higher than those obtained in zeolite-catalytic reactions,27 probable because of the highly porous structure of the nanofiber catalysts. The permeate was weighed and analyzed once per hour, and the results are shown in Figure 7. Obviously, the mass of the
Figure 5. Long-time permeation performance of the difunctional membrane. Temperature = 60 °C.
pervaporation performance. As shown in Figure 5, the water flux was 4−5 g·m−2·min−1 and was able to remain on this level throughout the experiment. This flux reached a level similar to those in the literature (4.6 and 6.2 g·m−2·min−1),24,26 even though the PVA layer of the difunctional membrane was quite thick. Moreover, the EtOH flux remained below 0.3 g·m−2·min−1, and the fluxes of HAc and EtAc were even lower (
