Adsorption of unsaturated hydrocarbons on zeolites: the effects of the zeolite framework on adsorption properties of ethylene
Introduction
Zeolites are important catalysts for many industrial processes, due mainly to their shape- and size-selectivity and Brønsted acid sites [1]. The adsorption and subsequent reactions of hydrocarbons on zeolites have been investigated by a number of experimental [2], [3], [4], [5] and theoretical methods [6], [7], [8], [9], [10], [11], [12]. Of particular interest in this area of active research is the alkene adsorption on zeolites that is the foundation of several industrially important reactions, namely the polymerization and hydrocarbon cracking processes [9], [10].
The adsorption of ethylene on zeolites has been studied experimentally by using Fourier transform IR spectroscopy (FTIR) [13], proton and carbon-13 solid-state NMR spectroscopy [14], [15]. The observed shifts in the spectral peaks provide information on the strength of the interaction between the Brønsted acid site and ethylene. Theoretical studies using accurate quantum mechanical methods can provide, in addition to the energetic properties, details on the adsorption structure. For the purpose of this study, we focus mainly on the adsorption of ethylene on zeolites as a starting point for a more comprehensive study on hydrocarbon cracking and ethylene epoxidation reactions in zeolites. There have been several previous theoretical studies on the adsorption of ethylene on zeolites using both ab initio [6], [7], [8], [9], [10], [11] and semiempirical [12] electronic structure methods. All of these studies, however, used the cluster models representing a generic tetrahedral sub-unit in an unconstrained environment and thus cannot make any reference on the effects of the zeolite framework. The predicted adsorption energy of ethylene (−4 to −7 kcal/mol) [6], [7], [8], [9], [10], [11] so far are noticeably smaller compared to the experimental adsorption energy of ethylene on the acidic H-Y zeolite (−9 kcal/mol) [5]. Such deviation indicates that the Madelung potential could be an important factor in stabilizing the adsorption complex. This raises the need for a better understanding on the effects of the Madelung potential in the adsorption of unsaturated hydrocarbons.
To accurately include the effects of the extended zeolite framework on the adsorption properties, one can employ the periodic electronic structure methods such as the periodic density functional theory methodology. However, due to the large computational demand, such calculations are limited to rather small unit-cell zeolites that often do not use in the actual industrial processes. The embedded cluster methodology provides a viable alternative with little additional computational cost compared to the bare cluster calculation. In this approach, the effects of the zeolite framework are modeled by a classical molecular mechanics force field. For adsorption properties, the Madelung potential is often the dominant component and it can be represented by a set of point charges.
In this study, we examine the effects of the zeolite framework, particularly the Madelung potential on the adsorption of ethylene in H-FAU and H-ZSM-5 zeolites using the embedded cluster methodology.
Section snippets
Method
We used a 3T cluster, namely SiH3OHAl(OH)2OSiH3, for the bare cluster calculations. For the embedded cluster model, this 3T cluster is embedded in a set of point charges according to the Surface Charge Representation of External Embedded Potential (SCREEP) method [16]. Accuracy of this method for modeling adsorption processes in zeolites has already been addressed in several previous studies [16]. The SCREEP embedded cluster model used in this study consists of three layers. The center layer is
Results and discussion
Before discussing the results for the ethylene adsorption, we first describe how the Brønsted acidic sites in H-FAU and H-ZSM-5 are modeled. For FAU, all tetrahedral sites (T-sites) (Al or Si tetrahedral sites) are equivalent by symmetry. At a given Al T-site, the Brønsted proton can reside on any of the four non-equivalent neighboring oxygen's (O1–O4) atoms as shown in Fig. 1b. Previous experimental [19], [20] and periodic DFT [21] studies have shown that site O1 in which the Brønsted acidic
Conclusion
The adsorption of ethylene on H-Faujasite (H-FAU) and H-ZSM-5 zeolites has been investigated by means of both the quantum cluster and the embedded cluster approaches at both the B3LYP and MP2 levels of theory. The bare quantum cluster approach predicts both the [C2H4]/H-FAU and [C2H4]/H-ZSM-5 complexes to have almost the same binding energies of −4.37 vs. −4.75 kcal/mol, respectively. The effects of the zeolite framework, particularly the Madelung potential, is modeled by the embedded cluster
Acknowledgements
This work was supported in parts by grants from the National Science Foundation to TNT and by the Thailand Research Fund (TRF) to J.L. and the Royal Golden Jubilee Ph.D. Programme to P.K. S.J. and T.N., as well as the Kasetsart University Research and Development Institute (KURDI) and The Ministry of University Affairs under the Science and Technology Higher Education Development Project (MUA-ADB funds). The computational resources from the University of Utah Center for High Performance
References (25)
- et al.
J. Catal.
(1972) J. Mol. Catal.
(1985)- et al.
J. Mol. Struct.
(1982) - et al.
Stud. Surf. Sci. Catal.
(1981) - et al.
J. Phys. Chem. Solids
(1980) - et al.
J. Phys. Chem.
(1991) - et al.
J. Chem. Soc., Faraday Trans.
(1993) - et al.
Trans. Faraday Soc.
(1966) Catal. Today
(1999)
J. Phys. Chem. A
Acc. Chem. Res.
Cited by (68)
Trap efficiency of exhaust gas pollutants in microporous sorbents under representative driving conditions
2022, Applied Catalysis B: EnvironmentalCitation Excerpt :At T25 °C, only two distinct zones are present (with temperature maxima centered at 75 °C, 170 °C and 173 °C, respectively), while at T50 °C only a component centered at 170 °C is present. Finally, the TPD performed after T150 °C presents a low, noisy signal which makes impossible to deconvolute.The affinity of ethylene to the MFI framework is well known [21] when it is not in competition with other molecules. Upon co-feeding, water and ethylene can compete over Brønsted acid sites (B) but, owing to less basic character of the latter and its minor concentration in the stream, it is reported that water adsorption always prevails.
Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites
2018, Catalysis TodayCitation Excerpt :Various studies have subsequently demonstrated that by incorporating the long-range electrostatic effects of the zeolite lattice into their models, the gap between theoretical predictions and experimental findings can be bridged. For example, Truong and coworkers [112] investigated ethene adsorption in H-FAU using T3 cluster models at the B3LYP and MP2 levels of theory. These authors found that the prediction obtained using the T3 cluster underestimates the magnitude of the adsorption energy owing to the exclusion of long-range framework interactions; by embedding the T3 cluster at the MP2/6-31G(d,p) level of theory in a field of point charges obtained using the SCREEP method [113], they were able to predict the adsorption energy for ethene in H-FAU as −8.2 kcal/mol; this prediction which captured the effect of long-range electrostatics on the physisorbed ethene complex in H-FAU, was found to be in excellent agreement with the experimentally measured value of −9.0 kcal/mol.
Acid treated mordenites as adsorbents of C<inf>2</inf>H<inf>4</inf> and H<inf>2</inf> gases
2017, Microporous and Mesoporous MaterialsCitation Excerpt :They found the amount of adsorbed ethylene 3.1799 mmol/g at 92.25 kPa at 283 K. Jang et al. [19] investigated the crystal structures of the ethylene and acetylene sorption complexes of dehydrated fully Ca2+exchanged zeolite X and they found that the bonds between Ca2+ and ethylene/acetylene were relatively long ascribing to the polarization of the molecular π-electron density by the electric field gradient at Ca2+. Limtrakul et al. [20] studied of the adsorption properties of ethylene on H-Faujasite (H-FAU) and H-ZSM-5 zeolites by using both the cluster and embedded cluster approaches. Al-Baghli and Loughlin [21] reported the binary and ternary equilibrium adsorption data of methane, ethane, and ethylene on titanosilicate ETS-10 zeolites at temperatures of 280 K and 325 K and pressures of 350 kPa and 700 kPa.
Low temperature catalytic combustion of ethylene over cobalt oxide supported mesoporous carbon spheres
2016, Chemical Engineering JournalCitation Excerpt :Currently, several strategies have been proposed for the C2H4 removal, including adsorption [1,5], direct oxidation [6,7] and catalytic oxidation [8,9]. Adsorption usually employs adsorbents with well-developed porosity, such as activated carbon [10–12] and zeolites [13], which have been widely used in air purification. However, these adsorbents usually reversibly trap C2H4 by either ion-quadrupole interaction or Van der Waals force, which show very low C2H4 adsorption selectivity, particularly when dealing with very trace C2H4 concentration.
Ethylene control in cut flowers: Classical and innovative approaches
2014, Postharvest Biology and TechnologyCitation Excerpt :Based on these mechanisms, a number of options are available commercially. These include membranes for filtration, small sachets inside the packages, enriched polyethylene films for modified atmosphere, including zeolites (Suslow, 1997; Limtrakul et al., 2001) and activated carbon (Choi et al., 2003; Bailén et al., 2006), as adsorbers. The efficiency of activated carbon as an adsorber is dependent on a wide range of physical and chemical properties as well as the material formulation, granular, powdered or fibre (Aygün et al., 2003).
A comparative adsorption study of C<inf>2</inf>H<inf>4</inf> and SO<inf>2</inf> on clinoptilolite-rich tuff: Effect of acid treatment
2013, Journal of Hazardous Materials