Article pubs.acs.org/JPCC
Adsorption of Methyl Tertiary Butyl Ether and Trichloroethylene in MFI-Type Zeolites Eren Güvenç and M. Göktuğ Ahunbay* Department of Chemical Engineering, Istanbul Technical University, 34469 Istanbul, Turkey S Supporting Information *
ABSTRACT: We used Monte Carlo simulations to understand how the Si/Al ratio of MFI-type zeolites affects their adsorption performance in the removal of VOCs from water. We considered adsorption of methyl tertiary butyl ether (MTBE) and trichloroethylene (TCE) to zeolites because they have different interactions with water, with MTBE being hydrophilic and TCE being hydrophobic. We carried out adsorption simulation in MFI-type zeolites of three different Si/Al ratios (∞, 191, and 95). Our results showed that because MTBE molecules can adsorb only to the channel intersection, the presence of Al sites close to the intersections would promote water clustering and prevent the access of MTBE molecules to these sites. However, if the hydrophilic sites are far from the intersections, MTBE adsorption capacity is less impeded. Therefore, the distribution of Al sites in ZSM-5 zeolites is also an important factor determining the MTBE removal efficiency of these materials. The reduction in TCE removal capacity of the zeolites due to increasing Al content is less severe because of the strong affinity of the zeolites to hydrophobic TCE with respect to water. Therefore, increasing TCE amount in the feed mixture leads to water exclusion in the zeolites.
1. INTRODUCTION Volatile organic compounds (VOCs), which are important water pollutants that can cause serious risk to human health, have received increasing attention in the recent years due to the difficulty in their removal from water systems. Among the various existing VOC removal technologies such as advanced oxidation processes, air stripping, reverse osmosis, and ultrafiltration, adsorption is of particular interest. Hydrophobic zeolites with high silica content (i.e., high Si/Al ratio), such as β, mordenite, Y, and ZSM-5, were recently demonstrated to be successful adsorbents for the removal of a wide variety of organic compounds from water.1−11 Among various VOCs, we focused on methyl tertiary butyl ether (MTBE) and trichloroethylene (TCE) in this study. Recent studies on the removal of MTBE2−8 and TCE9−11 from water have demonstrated the potential of zeolites as successful adsorbents, in addition to adsorption studies from pure vapor TCE12,13 and MTBE feeds.14 MTBE is a polar species containing an ether functional group and is hydrophilic. In contrast, TCE is hydrophobic while being slightly polar. Adsorption studies of these two species of different characteristics would provide insight into the effect of hydrophilic sites in zeolites on their VOC removal performances. The effect of hydrophilic sites in zeolites has been also of interest in the removal of other organic components from water such as alcohols. Xiong et al.15,16 studied in great detail the interactions between water and alcohols in adsorption to silicalite and other MFI-type zeolites using molecular simulation methods. They concluded that the presence of © 2012 American Chemical Society
water can promote the adsorption of hydrophilic alcohols to silicalite and hydrophilic aluminum, and hydroxyl sites in the zeolites can significantly affect alcohol adsorption capacity. Similarly, in a recent experimental study, Zhang et al.17 evaluated separation performance of MFI-type zeolites for ethanol−water mixtures and reported that other factors such as the zeolite synthesis route and the type of the extraframework cations are also important factor to determine the adsorption selectivity of the zeolites. There have been a number of molecular simulation studies in the past investigating the single-component vapor-phase adsorption of MTBE and TCE in MFI-type zeolites.14,18,19 In this study, we extended our previous work14,19 to the adsorption of these VOCs from their mixtures with water. For this purpose, we investigated the interactions of MTBE and TCE with water in adsorption to high-silica MFI-type zeolites to gain insight into the contribution of hydrophilic aluminum sites on VOC removal performances.
2. METHOD An accurate molecular model for water is essential to reproduce the hydrophobic behavior of high silica MFI-type zeolites. Hence, the semiempirical rigid model (SPC/exp-6)20 that had been previously used to study water adsorption in these zeolites21 was also employed in this study. The advantage of Received: July 6, 2012 Revised: September 18, 2012 Published: September 25, 2012 21836
dx.doi.org/10.1021/jp3067052 | J. Phys. Chem. C 2012, 116, 21836−21843
The Journal of Physical Chemistry C
Article
Figure 1. Vapor-phase MTBE adsorption to ZSM-5 zeolites: (a) isotherms (lines are guide for the eye only), (b) heats of adsorption, and (c) adsorbate−adsorbate (AA) and adsorbate−zeolite (AZ) interaction energies.
former, decreasing for the latter) but finally converge with the increasing water loadings. In our previous work, water adsorption isotherms for silicalite were calculated as a function of water fugacity. However, a more useful insight can be gained if the isotherms are represented as a function of pressure. Below the saturation pressure (Psat), 3.5 kPa at 298 K, the vapor phase can be assumed ideal gas, the fugacity being equal to the vapor pressure. Above Psat, the liquid-phase fugacity ( f) can be connected to the pressure (P) through the Poynting pressure correction
this model was very accurate estimation of the vapor pressure (3.5 kPa at 298 K with respect to the experimental value of 3.1 kPa).21 When combined with the optimized zeolite−water interactions, the model allowed reproducing the hydrophobic behavior of MFI-type zeolites, which was confirmed by the recent simulation and experimental studies. Xiong et al.16 reported in agreement with our previous work that the water molecules are preferentially adsorbed in the sinusoidal channels, and these channels are filled first, followed by further adsorption in the straight channels in the aluminum-free silicalite. Water molecules preferentially adsorb near the aluminum atoms, followed by adsorption in the sinusoidal and straight channels, when aluminum atoms are present in the framework. Zhang et al.17 reported in their experimental study that the ratio of the number of adsorbed water molecules to the number of Al sites in one MFI unit cell, which can be estimated from the adsorption uptakes for H-form ZSM-5 samples, lies between 1.8 and 2. This finding agrees very well with the result of our previous study revealing that the Al sites in Na-ZSM-5 zeolites coordinate two water molecules per site at low coverage, which correspondingly promotes water clustering in the vicinity of these sites. They also reported that the adsorption heats of defect-free silicalite and Al-containing NaZSM-5 exhibit an opposite variation trend (increasing for the
f=f
sat
⎛ P ⎞ ⎜ ∫P sat VdP ⎟ exp⎜ ⎜ RT ⎟⎟ ⎝ ⎠
(1)
sat
where f is the saturation fugacity of water, which is practically equal to the saturation pressure at 298 K. The integral term corresponds to the chemical potential difference between P and Psat and can be evaluated numerically from the PV data obtained from the molecular dynamics (MD) simulations in the NPT ensemble. For this purpose, we carried out MD simulations with 1000 water molecules by increasing the pressure gradually from 3.5 kPa to 300 MPa at 298 K using the 21837
dx.doi.org/10.1021/jp3067052 | J. Phys. Chem. C 2012, 116, 21836−21843
The Journal of Physical Chemistry C
Article
LAMMPS simulation software.22 Figure S1 in the Supporting Information shows the variation of the fugacity as a function of pressure in comparison with the experimental data from the NIST database.23 It can be seen that the liquid-phase fugacity remains close to the saturation fugacity over a wide range of pressure up to 10 MPa. When the fugacity−pressure relationship is linked to the water adsorption isotherms in silicalite, as shown in Figure S2 in the Supporting Information, it can be considered that the hydrophobic behavior of the silicalite is well reproduced above Psat, although the pore filling takes place above the experimental pressure.24 Since the isotherm is more sensitive to the fugacity than the pressure and we have been interested in the water adsorption at the saturation point, these results show that the performance of our water−zeolite interaction model is satisfactory. For the adsorption simulations, we used the MATERIALS STUDIO 5.0 simulation package.25 The MFI framework was modeled based on the orthorhombic structure reported by van Koningsveld.26 The structures of Na-ZSM-5 zeolites with Si/Al ratio 191 and 95 were taken from the previous work,21 where the position of the cations was fixed after Monte Carlo optimization. Simulations were performed in a simulation box of 2 × 2 × 2 unit cells. A cutoff distance of 10 Å was adopted for the calculation of pairwise van der Waals interactions, and Ewald summation was used to calculate electrostatic interactions. All interaction sites present in the system and their potential energy parameters are listed in the Supporting Information. The MTBE molecules were represented using the flexible united-atoms (UA) model,27 and the interaction parameters with the zeolite were taken from the literature.28 The configurational-biased−grand canonical Monte Carlo (CBGCMC) algorithm29 was used in the adsorption simulations. Constant chemical potential was imposed indirectly through the fugacity of the adsorbates, which was assumed to be equal to feed pressure in the vapor phase. Adsorption isotherms of MTBE in ZSM-5 zeolites were calculated up 0.14 kPa, which yields the experimentally determined saturation capacity.14 MTBE pressure was increased gradually from 5.0 × 10−5 kPa over 12 steps gradually with 1.5 × 106 iterations for the equilibration run and 3 × 106 iterations for the production run. An all-atoms (AA) representation was used for TCE, which was shown to reproduce successfully adsorption isotherms in ZSM-5 zeolites in a prior work,19 where the van der Waals interaction parameters were taken from the CVFF force field.30 However, in this study, the zeolite−TCE interaction parameters were replaced with those from the PCFF force field31 because they reproduced the low-pressure range of the isotherm more accurately while slightly underpredicting the saturation capacity with respect to the CVFF force field. TCE molecules were kept rigid in the simulations, and the Metropolis algorithm was employed in the GCMC simulations. TCE isotherms were calculated up 1.0 kPa, which is the saturation pressure at 298 K. TCE pressure was increased gradually from 1.0 × 10−4 kPa over 16 steps gradually with 50 × 106 iterations for the equilibration run and 50 × 106 iterations for the production run. To investigate the adsorption of both VOCs from their mixtures with water, we carried out binary adsorption simulations by fixing the fugacity of water at its saturation value 3.5 kPa at 298 K and increasing the fugacity of the VOCs in the mixture to their pure component saturation values, as explained above.
3. RESULTS AND DISCUSSION Adsorption isotherms of MTBE in all three MFI structures at 298 K are presented in Figure 1a in comparison with previously reported experimental vapor-phase adsorption isotherm.14 The isotherm agrees very well with the Langmuir fit to the experimental data. As discussed in the previous simulation studies,14,18 MTBE molecules are adsorbed only in the intersection of straight and sinusoidal channels due to their large size. Consequently, the saturation capacity is equal to the number of these intersections (four) in the unit cell (u.c). The presence of the Na+ sites increases the adsorption capacity at lower pressures, which may be explained by the preferential adsorption of the MTBE molecules containing polar ether groups on the ionic sites. Consequently, the MTBE loading at 5.0 × 10−5 kPa is equal to the Na+ loading in ZSM-5/191 and ZSM-5/95 (0.5 and 1.0 molec./u.c., respectively), whereas no MTBE molecule was adsorbed in aluminum-free silicalite. However, the ionic sites have no effect on the saturation capacity of the zeolites at higher pressures. Figure 1b shows the change in heat of adsorption (Q) as a function of MTBE loading in the zeolite structures. Whereas the heat of adsorption is constant for silicalite because all channel intersections are equally preferred by the MTBE molecules, it decreases with loading for the Na+ containing zeolites, reflecting the shift of the occupied sites from the highly preferential ones to the less preferential ones. The adsorbate−adsorbate and adsorbate−zeolite interaction energies in Figure 1c also indicate that MTBE molecules do not interact with each other because they are separated with each other by a distance equal to that between two channel intersections. The effect of water on MTBE adsorption is shown in Figure 2. Because silicalite is highly hydrophobic, the low water loading (
