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CO2 and N2 Adsorption in Nano-porous BEA Type Zeolite with Different Cations
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 446 – 455
5th International conference on Advanced Nano Materials
CO2 and N2 Adsorption in Nano-porous BEA Type Zeolite with Different Cations Renjith S. Pillaia,b*, Elby Titusc a
CICECO, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal, bDiscipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research, Bhavnagar-364 002, India, cTEMA, Departamento de Mechanica, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Abstract Adsorption of CO2 and N2 in nano-porous BEA having SiO2/Al2O3 of 25, BEA(25), was studied by combining equilibrium adsorption measurements and Grand Canonical Monte Carlo (GCMC) simulation. CO2 sorption is observed to show higher sorption capacity than N2 in all cation exchanged zeolite samples. On exchanging the BEA(25) with various cations, Li+, K+, Cs+, Ca2+, Sr2+, and Ba2+, the adsorption capacity of CO2 and N2 are increased in these type of zeolites. The isosteric heat of sorption data shows stronger interactions of both CO2 and N2 molecules in BEA(25) on decreasing size of the extra-framework cation. Simulation of the CO2 and N2 sorption in cation exchanged BEA(25) clearly shows that the adsorbed CO2 and N2 molecules sit closely to the extra-framework cations accessible through the large cage. Simulation of adsorption isotherms and heats of adsorption of CO2 and N2 in cation exchanged BEA(25) match reasonably well with the experimental results. © The Authors. Ltd. All rights reserved. © 2014 2015 Elsevier Ltd. AllElsevier rights reserved. Selection andpeer-review peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. Selection and under responsibility of TEMA - Centre for Mechanical Technology and Automation. Keywords: Grand Canonical Monte Carlo Simulation; BEA; Cations; Adsorption; CO2; N2
1.
Introduction
Recently, the increase of global warming gases in atmosphere is become serious apprehension due to the global climate change. In this industrial era, the combustion fossil fuels are main source for CO2 [1]. Flue gas from coalburning plants is the major source point for generation of CO2 in the atmosphere [2, 3]. Currently, all commercial
* Corresponding author. Tel.: +351 911923545; fax: +351 234401470. E-mail address: [email protected] / [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. doi:10.1016/j.matpr.2015.04.054
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power plants use chemical absorption for CO2 capture using solvents like alkanolamines. However, it has disadvantages such as additional processing and corrosion controls [4, 5]. In fact, pressure swing adsorption (PSA)[6] is a very promising separation and recovery process for CO2 due to its proven success in many gas separation processes like oxygen and N2 production from air; hydrogen recovery from hydrocarbon rich gaseous streams, paraffin- olefin separation etc.[7-14] In the design of an adsorption based separation process, the choice of the adsorbent is the most crucial design consideration [15]. The zeolites are very promising adsorbent for selective adsorption and separation of carbon dioxide[6, 16]. For CO2 and N2 adsorption on hydrophobic zeolite, only limited numbers of combined experimental and theoretical studies have been published [17-22], which are mainly report adsorption isotherm in zeolites such as ZSM-5, faujasites [23-28], zeolite A [29, 30] at different temperature. Notably, Maurin et al.[27] has shown that microcalorimetry combined with atomistic simulation is a powerful tool able to understand the interaction between the adsorbent surface and adsorbate molecules and claimed that the first time that a force field developed from ab initio calculations for CO2 has given good results in zeolite systems across a wide range of pressure. In this paper, the adsorption of CO2 and N2 on various monovalent and divalent cation exchanged zeolite-BEA having SiO2/Al2O3 ratio of 25 are studied for CO2 selective adsorbent for pressure swing adsorption processes with the aim of separating CO2 from industrial flue gas. 2.
Experimental
The extra framework cations were introduced into the highly crystalline sodium form of Zeolite BEA with SiO2/Al2O3 ratio of 25, henceforth named as BEA(25), obtained from Zeochem LLC, Switzerland, by the conventional cation exchange from aqueous solution. Typically, the zeolites were repeatedly treating with 0.05 M aqueous solution of the exchanging cations with a solid/liquid ratio 1:80 at 353 K for 4 hours. The repetitions of the cation exchange were carried up to replace all sodium cations in the zeolite sample. The residue was filtered, washed with hot distilled water, until the washings were free from chloride ions and dried in air at room temperature for 12 h. The extent of the different cation exchange was determined by ICP analysis (Perkin Elmer Instruments, Optima 2000DV) of sodium and exchanged cation Prior to adsorption measurements, the samples were activated in situ by heating to 673 K, at a rate of 1 K min -1 under vacuum (5u10-3 mm Hg) for 8 hr. N2 and CO2 adsorption isotherms were measured at 288.0 and 303.0 K using a static volumetric system (Micromeritics ASAP 2010). Adsorption temperature was maintained (r0.1 K) by circulating water from a constant temperature bath (Julabo F25, Germany). Requisite amount of the adsorbate gas was injected into the volumetric set up at volumes required to achieve a targeted set of pressures ranging from 0.1 to 850 mmHg. Three pressure transducers of capacities 1 mmHg (Accuracy within 0.12 % of the reading); 10 mmHg (Accuracy within 0.15 % of the reading) and 1000 mmHg (Accuracy within 0.073 % of full scale) were used for the pressure measurements. A minimum equilibrium interval of 50 seconds with a relative target tolerance of 5.0 % of the targeted pressure and an absolute target tolerance of 5.000 mmHg were used to determine equilibrium for each measurement point. The adsorption and desorption was completely reversible and it is possible to remove the adsorbed gases by simple evacuation. The pure component selectivity of gas A over gas B was calculated by using the equation, α A/B
ª VA º «V » ¬ B ¼ P,T
(1)
where VA and VB are the volumes of gas A and B respectively adsorbed at any given pressure P and temperature T. Isosteric heats of adsorption were calculated from the adsorption data collected at 288.0 and 303.0 K using Clausius-Clapeyron equation, ' ads H 0
ª º R «w ln P » 1 w «¬ T »¼T
(2)
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where R is the universal gas constant, T is the fraction of the adsorbed sites at a pressure P and temperature T. The errors in adsorption selectivity and heat of adsorption estimated from propagation of error method were 0.4 %, and 0.5% respectively. 3.
Computational Methodology
The framework for BEA zeolites were built by the crystallographic data obtained from Meier et al. [31]. To obtain the BEA(25) with desired SiO2/Al2O3 ratio of 25, Si atoms were randomly converted to Al atoms by applying Lowenstein’s rule (Al-O-Al linkages are not allowed). To balance the charge in BEA(25) zeolite, Na ions were located in the framework by using the ‘cation locator’ available in MS Sorption,[32] according to the number of Na ions required. The force field used to locate cations in the framework was the commercially available cvff_aug force field [32], which contains only ionic atom- type and nonbonded parameters. All structures were energy minimized, upon starting a simulation with low energy structures, the Forcite Plus using the force field cvff with the constraints that unit cell parameters does not change, only extra framework cations were considered to be movable during energy minimization [33, 34]. The force field used for the gas adsorption simulations was modified version of the Cerius2 Watanabe-Austin potential energy model [35-38]. The total energy of the zeolite framework and adsorbed molecules (U) is expressed as the sum of the interactions energy between the adsorbate and zeolite (UAZ) and that between the adsorbates (UAA ) molecules. U
uij
U AZ U AA
ª§ V ·12 § V ·6 º § q q · 4H ij «¨ ij ¸ ¨ ij ¸ » ¨ i j ¸ «¨© rij ¸¹ ¨© rij ¸¹ » ¨© rij ¸¹ ¼ ¬
(3) (4)
Both UAZ and UAA are written as the sums of pair wise additive potentials, uij in the form where the first term is the repulsion-dispersion Lennard-Jones (LJ) potential, with εij and σij corresponding to the parameter sets for each interacting pair that is obtained from εi and σi of each species by using the Lorentz Berthelot mixing rule (i.e., a geometric combining rule for the energy and an arithmetic one for the atomic size: εij=(εiεj)1/2 and σij = (σi + σj)/2). The second term is the Coulombic contribution between point charges qi and qj separated by distance rij. N2 molecule was treated as a three site model; i.e., two outer sites are separated by a distance, l = 1.098 Å, as point charges of equal magnitude, q = -0.40484 e and the third site is located at the middle of the outer sites with a point charge -2q, ensuring molecular charge neutrality [39]. CO2 were considered an atomic point charge model in which the charges assigned to the carbon C (+0.72 e) and the oxygen O (-0.36 e) as in Plant et al.[28] The adsorbateadsorbent LJ parameters for CO2 were taken from the literature [13, 27, 28, 37]. The zeolite framework oxygen, silicon and aluminium and the extra framework cations served as both LJ interaction site and the location of point charge. The LJ parameters used for the interactions of adsorbate-adsorbate and adsorbate- zeolite framework in this work are given in Table 1. The calculation requires LJ parameters for the oxygen of the framework, extra framework cations and adsorbates because of considering that the polarizability of silicon and aluminium are much lower than those of oxygen atoms, therefore the non-bond interactions are assigned only for framework oxygen and extra framework cations of the adsorbent in all GCMC simulation. The interaction parameter for cations were taken from Maurin et al. [21], they suggested a relationship between ionic radius and polarizability of cation to deduce the LJ parameter for each cation. The cations in framework were assumed to be partially ionized and their charges were treated as adjustable parameters. The partial charges on silicon (+2.4e) and oxygen (-1.2e) of the zeolite framework system were fixed at the usually considered values.[40] The charges on aluminium and cations were allowed to change in order to take into account the partial charge transfer from the framework, with the constraint of a global charge of zero for the zeolite system. The partial charges for monovalent and divalent cation were taken as +0.7e and
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+1.7e, respectively, which is usually considered for extra framework cations for the faujasite type zeolites [27, 37, 41]. Table 1. Pair potential parameters used for adsorbates (CO2, N2) and adsorbent in the simulation. Interacting pairs
ε (kcal mol-1)
σ (Å)
Interacting pairs
ε (kcal mol-1)
σ (Å)
Oco2-Oco2 Cco2-Cco2 Oco2-C co2 Cco2-Oz Oco2-Oz Cco2-Li Oco2-Li Cco2-Na Oco2-Na
0.1520 0.0927 0.0364 0.0837 0.1386 0.2583 0.3307 0.1751 0.0623
3.36 3.83 3.31 3.90 3.48 2.46 2.23 3.35 2.95
Cco2-K Oco2-K Cco2-Ca Oco2-Ca Cco2-Sr Oco2-Sr Cco2-Ba Oco2-Ba
0.0911 0.1166 0.1260 0.1614 0.1539 0.1971 0.1615 0.2068
3.06 2.82 2.77 2.54 2.91 2.67 3.04 2.80
NN2-N N2 N N2-Oz N N2-Li N N2-Na
0.0724 0.1555 0.2282 0.0851
3.318 3.179 2.208 2.516
N N2-K N N2-Ca N N2-Sr N N2-Ba
0.0804 0.1114 0.1360 0.1427
2.803 2.517 2.653 2.781
The absolute isotherms were then computed using Grand Canonical Monte Carlo (GCMC) algorithm via the sorption module in the Materials Studio software suite [42], which allows displacements, creations and destructions of adsorbate species. All these simulations were performed with fixed pressures at 303.0 K using one unit cell of each model with a typical 1×104 equilibrium and 1×105 production steps. The evolution of the total energy over the Monte Carlo steps were plotted in order to monitor the equilibration conditions. The zeolite structure was assumed to be rigid during the sorption process and the extra framework cations were maintained fixed in their initial optimized positions. Ewald summation method with accuracy 0.001 kcal/mol was used for calculating the electrostatic interactions and the short range interactions, with a cutoff distance of 12.0 Å [37]. The Metropolis Monte Carlo method allows for the insertion of molecules throughout the zeolite framework regardless of the physical diffusion pathways. 4. Results and Discussion The X-ray powder diffractions of cation exchanged BEA(25) samples are shown in Fig.1. The diffraction patterns of all cation exchanged BEA(25) shows the highly crystalline samples showing reflections in the range of 5 – 65q typically observed for zeolites. (a) (b)
Fig. 1. X-ray powder diffraction pattern of a) monovalent and b) divalent cation exchanged zeolite-X
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Adsorption isotherm of CO2 and N2 were measured at 288.0 and 303.0 K in cation exchanged BEA(25), the isotherm at 303.0 are shown in Fig. 2. The equilibrium adsorption capacities for the adsorption of CO2 and N2 on cations exchanged BEA(25) are determined from the adsorption isotherms and the number of molecules of CO2 and N2 adsorbed per unit cell at 101.3 kPa and 303.0 K are given in Table 2. The CO2 adsorption capacity is higher than N2 in all BEA(25) samples. BEA(25) having bigger potassium cations has maximum adsorption capacity at 303.0 K for CO2 and N2 among the monovalent exchanged BEA(25). The higher CO2 adsorption capacity in KBEA(25) could be due to the acid base interaction between framework and CO2 molecules. It can be seen from Fig. 2 that both the adsorption capacity and the initial concave shape of the CO2 isotherm is higher as compared to N2, which is due to the basic nature of the BEA(25). Additionally, the effective charge density of the cations inside the zeolite pores could also be a reason for higher adsorption in bigger cation exchanged BEA(25). On the other hand, the adsorption capacity of N2 increases linearly on increasing the extra framework cation at all range of pressures. However, the molecule per unit cell of both CO2 and N2 slightly increases with increase of the cation size. The adsorption capacities for both CO2 and N2 in BEA(25) increases on increasing the cation size of the divalent cations. In divalent cation exchanged BEA (25), it has only half number of cation that of monovalent exchanged BEA(25). Therefore, the population of cation at the pore entrance of zeolite framework is very less than the monovalent exchanged BEA(25). Indeed, both CO2 and N2 can move freely in divalent exchanged BEA(25), even though the number of extra-framework cations are very few. (a)
(b)
Fig. 2. The adsorption isotherm of CO2 (left panel) and N2 (right panels) in BEA(25) with monovalent (a) and divalent (b) cations at 303.0 K.
The pure component adsorption selectivity for CO2 over N2 at different equilibrium pressures was calculated at 303.0 K, which are given in Table 2. The CO2/N2 increases on increasing cation size in BEA(25) with monovalent cations. The CO2 selectivity value is higher in the low pressure region in a particular cation exchanged BEA(25), and the selectivity is decreased on increasing adsorption pressure. Again, the CO2 selectivity over N2 is higher at 10 kPa for all divalent cation exchanged BEA(25), and the selectivity values are slightly decreased at 101.3 kPa. CaBEA(25) showed the highest CO2/N2 selectivity for BEA(25) with divalent cations.
Renjith S. Pillai and Elby Titus / Materials Today: Proceedings 2 (2015) 446 – 455
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Simulation of both CO2 and N2 adsorption Table 2. CO2 and N2 Adsorption Capacity and Selectivity in Monovalent Cationsin BEA(25) with various cations were Exchanged Zeolite BEA(25) and A at 101.3 kPa and 303.0 K. carried out at 303.0 K. Fig. 3 shows both Adsorbent Adsorbed amounts at 1 Selectivity (D DCO2/N2) at experimental and simulated adsorption atm. and 303.0 K 303.0 K isotherm of CO2 and N2 in cation exchanged (Molecules/ u.c) BEA(25), respectively. The simulation of N2 CO2 10 kPa 101.3 kPa CO2 and N2 predicts the experimental results LiBEA(25) 0.9 8.9 13.4 9.89 reasonably well as shown in Fig. 3. The NaBEA(25) 1.0 10.5 15.6 10.5 higher simulated adsorption isotherm could KBEA(25) 1.5 12.4 40.2 10.4 be due to several reasons, such as decrease CaBEA(25) 1.0 10.2 13.4 10.2 of crystallinity in vacuum activation, SrBEA(25) 1.3 10.3 13.8 8.0 inaccurate parameters for the simulation BaBEA (25) 1.6 10.6 16.0 7.2 studies, and cation locations are used in the simulations that may not exactly match the cation location inside the adsorbent samples used for the adsorption experiment. (a)
(b)
(c)
(d)
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(e)
(f)
Fig. 3. Simulated and Experimental adsorption isotherm of CO2 and N2 in (a) LiBEA(25), (b) NaBEA(25), (c) KBEA(25), (d) CaBEA(25), (e) SrBEA(25) and (f) BaBEA(25) at 303.0 K.
The isosteric heats of adsorption of CO2 and N2 in BEA(25) with different alkali metal and alkaline earth metal, calculated from both experimental and simulation data at initial adsorption coverages are given in Table 3. The simulated heat of adsorption reproduces the experimental data reasonably well. The slight difference in heat of adsorption data could be due to the comparison of simulation in a unit cell of BEA(25) with the experimental data in the bulk zeolite powder sample.
Table 3. Heats of adsorption of CO2 and N2 in cation exchanged zeolite BEA(25) Adsorbent Heat of adsorption (kJ mol-1) Experimental LiBEA(25) NaBEA(25) KBEA(25) CaBEA(25) SrBEA(25) BaBEA(25)
Simulation
N2
CO2
N2
CO2
18.4 16.4 16.1 24.8 24.0 23.6
42.8 38.1 39.2 46.3 44.2 40.4
16.8 15.2 13.1 32.0 30.8 32.0
50.1 42.9 40.3 62.4 60.3 59.4
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The main driving force for the adsorption of (a) (b) (c) CO2 and N2 in zeolite BEA(25) are electrostatic and van der Walls interactions. The electrostatic interactions between the sorbate molecules and the zeolite BEA(25) depend on the quadrupole moments of the sorbate molecules (CO 2>>N2). Both experimental and simulated heat of adsorption of CO2 and N2 in various cations containing zeolite-BEA(25) shows the similar trends, even though there is slight difference in magnitude. The initial heat of adsorption is obtained in the order of LiBEA(25) > NaBEA(25) > KBEA(25) and CaBEA(25) > SrBEA(25) > BaBEA(25) for both CO2 and N2. The cations located in the cavity determine the (d) (e) (f) adsorption capacity as well as heat of adsorption because the positive charge density is around these cations. At lower pressure, the number of N2 molecules adsorbed per unit cell is less than compared to the CO2. The electrostatic interaction is dominant between N2 molecule and zeolites, and the adsorbate-adsorbate interaction are negligible. It is clearly established that the extra framework cations have different polarizing power in the zeolite cavity, which decreases on increasing the cation size. The aluminium and silicon atoms are located in the centre of tetrahedra of the zeolite Fig. 4. Snap shot of a final configuration for CO2 adsorbed in BEA(25) having BEA(25), therefore the principal interaction of monovalent, (a) LiBEA(25), (b) NaBEA(25), and (c) KBEA(25), and divalent, the adsorbates is due to the charge density of (d) CaBEA(25), (e) SrBEA(25), and (f) BaBEA(25), cations at 101.3 kPa and extra framework cations occupied in the cages 303.0 K. Tubes and spheres represent the framework and the extra-frmework of the BEA(25). The heat of adsorption for both cation and/or sorbates, respectively, with yellow for silicon, red for oxygen and CO2 and N2 decreases on increasing the cation pink for aluminium, grey for carbon, yellowish-green for lithium, violet for size of the extra framework cation. Fig. 4 shows sodium, maroon for potassium, cyan for calcium, bluish-green for strontium and the unit cell structure of cation exchanged green for barium. BEA(25) with adsorbed CO2 at 101.3 kPa and 303.0 K. The CO2 molecules are sitting close to the cations inside the cavity, the CO 2 density around the cations increases on increasing the cation size. Fig. 5 shows the unit cell structure of BEA(25) with adsorbed N2 at 101.3 kPa and 303.0 K.
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(a)
Renjith S. Pillai and Elby Titus / Materials Today: Proceedings 2 (2015) 446 – 455
(b)
(c) Conclusions
(d)
(e)
(f)
Fig. 5. N2 adsorbed snap shot of a final configuration of BEA(25) having monovalent, (a) LiBEA(25), (b) NaBEA(25), and (c) KBEA(25), and divalent, (d) CaBEA(25), (e) SrBEA(25), and (f) BaBEA(25), cations at 101.3 kPa and 303.0 K. Tubes and spheres represent the framework and the extra-frmework cation and/or sorbates, respectively, with yellow for silicon, red for oxygen and pink for aluminium, blue for nitrogen, yellowish-green for lithium, violet for sodium, maroon for potassium, cyan for calcium, bluish-green for strontium and green for barium.
Equilibrium adsorption measurements for CO2 and N2 were carried out in cation exchanged BEA(25) and compared with its simulated isotherms utilizing grand canonical Monte Carlo (GCMC) simulations. CO2 sorption is observed to show higher adsorption capacity than N2 in all these zeolite samples. The adsorption capacities of cation-exchanged zeolites are increased from parent zeolites. The pure component CO2/N2 adsorption selectivity increases on increasing cation size in case of monovalent cation exchanged BEA(25), whereas it decreases on increasing divalent cation radius. Isosteric heat of sorption data show stronger interactions of CO2 and N2 molecules with BEA(25) having either monovalent or divalent cations, which decreases on increasing the size of the cation. Simulation of adsorption isotherms and heats of adsorption of CO2 and N2 in metal ion exchanged zeolite match reasonably well with the experimental results. Simulation of CO2 and N2 in cation BEA(25) clearly shows that the adsorbed CO2 molecules sit closely to the extraframework cations accessible through the bigger cavity.
Acknowledgments RSP gratefully acknowledges a post-doctoral fellowship from FCT with reference SFRH/BPD/70283/2010. Reference: [1] J. Wilcox, Carbon Capture Springer, New York, 2012. [2] L. Wallquist, V.H.M. Visschers, S. Dohle, M. Siegrist, Hum. Ecol. Risk Assess., 18 (2012) 919-932. [3] S. Rackley, Carbon Capture: Sequestration and Storage, Elsevier Inc.: 2010. [4] N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, C.S. Adjiman, C.K. Williams, N. Shah, P. Fennell, Energ. Environ. Sci., 3 (2010) 1645-1669. [5] D. Singh, E. Croiset, P.L. Douglas, M.A. Douglas, Energ. Convers. Manage., 44 (2003) 3073-3091. [6] D.M. Ruthven, S. Farooq, K.S. Knaebel, Pressure Swing Adsorption, Wiley-VCH; New York, 1994. [7] R.V. Jasra, N.V. Choudary, S.G.T. Bhat, Sep. Sci. Technol., 26 (1991) 885-930. [8] R.V. Jasra, N.V. Choudary, S.G.T. Bhat, Ind. Eng. Chem. Res., 35 (1996) 4221-4229.
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ScienceDirect Materials Today: Proceedings 2 (2015) 446 – 455
5th International conference on Advanced Nano Materials
CO2 and N2 Adsorption in Nano-porous BEA Type Zeolite with Different Cations Renjith S. Pillaia,b*, Elby Titusc a
CICECO, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal, bDiscipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research, Bhavnagar-364 002, India, cTEMA, Departamento de Mechanica, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Abstract Adsorption of CO2 and N2 in nano-porous BEA having SiO2/Al2O3 of 25, BEA(25), was studied by combining equilibrium adsorption measurements and Grand Canonical Monte Carlo (GCMC) simulation. CO2 sorption is observed to show higher sorption capacity than N2 in all cation exchanged zeolite samples. On exchanging the BEA(25) with various cations, Li+, K+, Cs+, Ca2+, Sr2+, and Ba2+, the adsorption capacity of CO2 and N2 are increased in these type of zeolites. The isosteric heat of sorption data shows stronger interactions of both CO2 and N2 molecules in BEA(25) on decreasing size of the extra-framework cation. Simulation of the CO2 and N2 sorption in cation exchanged BEA(25) clearly shows that the adsorbed CO2 and N2 molecules sit closely to the extra-framework cations accessible through the large cage. Simulation of adsorption isotherms and heats of adsorption of CO2 and N2 in cation exchanged BEA(25) match reasonably well with the experimental results. © The Authors. Ltd. All rights reserved. © 2014 2015 Elsevier Ltd. AllElsevier rights reserved. Selection andpeer-review peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. Selection and under responsibility of TEMA - Centre for Mechanical Technology and Automation. Keywords: Grand Canonical Monte Carlo Simulation; BEA; Cations; Adsorption; CO2; N2
1.
Introduction
Recently, the increase of global warming gases in atmosphere is become serious apprehension due to the global climate change. In this industrial era, the combustion fossil fuels are main source for CO2 [1]. Flue gas from coalburning plants is the major source point for generation of CO2 in the atmosphere [2, 3]. Currently, all commercial
* Corresponding author. Tel.: +351 911923545; fax: +351 234401470. E-mail address: [email protected] / [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. doi:10.1016/j.matpr.2015.04.054
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power plants use chemical absorption for CO2 capture using solvents like alkanolamines. However, it has disadvantages such as additional processing and corrosion controls [4, 5]. In fact, pressure swing adsorption (PSA)[6] is a very promising separation and recovery process for CO2 due to its proven success in many gas separation processes like oxygen and N2 production from air; hydrogen recovery from hydrocarbon rich gaseous streams, paraffin- olefin separation etc.[7-14] In the design of an adsorption based separation process, the choice of the adsorbent is the most crucial design consideration [15]. The zeolites are very promising adsorbent for selective adsorption and separation of carbon dioxide[6, 16]. For CO2 and N2 adsorption on hydrophobic zeolite, only limited numbers of combined experimental and theoretical studies have been published [17-22], which are mainly report adsorption isotherm in zeolites such as ZSM-5, faujasites [23-28], zeolite A [29, 30] at different temperature. Notably, Maurin et al.[27] has shown that microcalorimetry combined with atomistic simulation is a powerful tool able to understand the interaction between the adsorbent surface and adsorbate molecules and claimed that the first time that a force field developed from ab initio calculations for CO2 has given good results in zeolite systems across a wide range of pressure. In this paper, the adsorption of CO2 and N2 on various monovalent and divalent cation exchanged zeolite-BEA having SiO2/Al2O3 ratio of 25 are studied for CO2 selective adsorbent for pressure swing adsorption processes with the aim of separating CO2 from industrial flue gas. 2.
Experimental
The extra framework cations were introduced into the highly crystalline sodium form of Zeolite BEA with SiO2/Al2O3 ratio of 25, henceforth named as BEA(25), obtained from Zeochem LLC, Switzerland, by the conventional cation exchange from aqueous solution. Typically, the zeolites were repeatedly treating with 0.05 M aqueous solution of the exchanging cations with a solid/liquid ratio 1:80 at 353 K for 4 hours. The repetitions of the cation exchange were carried up to replace all sodium cations in the zeolite sample. The residue was filtered, washed with hot distilled water, until the washings were free from chloride ions and dried in air at room temperature for 12 h. The extent of the different cation exchange was determined by ICP analysis (Perkin Elmer Instruments, Optima 2000DV) of sodium and exchanged cation Prior to adsorption measurements, the samples were activated in situ by heating to 673 K, at a rate of 1 K min -1 under vacuum (5u10-3 mm Hg) for 8 hr. N2 and CO2 adsorption isotherms were measured at 288.0 and 303.0 K using a static volumetric system (Micromeritics ASAP 2010). Adsorption temperature was maintained (r0.1 K) by circulating water from a constant temperature bath (Julabo F25, Germany). Requisite amount of the adsorbate gas was injected into the volumetric set up at volumes required to achieve a targeted set of pressures ranging from 0.1 to 850 mmHg. Three pressure transducers of capacities 1 mmHg (Accuracy within 0.12 % of the reading); 10 mmHg (Accuracy within 0.15 % of the reading) and 1000 mmHg (Accuracy within 0.073 % of full scale) were used for the pressure measurements. A minimum equilibrium interval of 50 seconds with a relative target tolerance of 5.0 % of the targeted pressure and an absolute target tolerance of 5.000 mmHg were used to determine equilibrium for each measurement point. The adsorption and desorption was completely reversible and it is possible to remove the adsorbed gases by simple evacuation. The pure component selectivity of gas A over gas B was calculated by using the equation, α A/B
ª VA º «V » ¬ B ¼ P,T
(1)
where VA and VB are the volumes of gas A and B respectively adsorbed at any given pressure P and temperature T. Isosteric heats of adsorption were calculated from the adsorption data collected at 288.0 and 303.0 K using Clausius-Clapeyron equation, ' ads H 0
ª º R «w ln P » 1 w «¬ T »¼T
(2)
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where R is the universal gas constant, T is the fraction of the adsorbed sites at a pressure P and temperature T. The errors in adsorption selectivity and heat of adsorption estimated from propagation of error method were 0.4 %, and 0.5% respectively. 3.
Computational Methodology
The framework for BEA zeolites were built by the crystallographic data obtained from Meier et al. [31]. To obtain the BEA(25) with desired SiO2/Al2O3 ratio of 25, Si atoms were randomly converted to Al atoms by applying Lowenstein’s rule (Al-O-Al linkages are not allowed). To balance the charge in BEA(25) zeolite, Na ions were located in the framework by using the ‘cation locator’ available in MS Sorption,[32] according to the number of Na ions required. The force field used to locate cations in the framework was the commercially available cvff_aug force field [32], which contains only ionic atom- type and nonbonded parameters. All structures were energy minimized, upon starting a simulation with low energy structures, the Forcite Plus using the force field cvff with the constraints that unit cell parameters does not change, only extra framework cations were considered to be movable during energy minimization [33, 34]. The force field used for the gas adsorption simulations was modified version of the Cerius2 Watanabe-Austin potential energy model [35-38]. The total energy of the zeolite framework and adsorbed molecules (U) is expressed as the sum of the interactions energy between the adsorbate and zeolite (UAZ) and that between the adsorbates (UAA ) molecules. U
uij
U AZ U AA
ª§ V ·12 § V ·6 º § q q · 4H ij «¨ ij ¸ ¨ ij ¸ » ¨ i j ¸ «¨© rij ¸¹ ¨© rij ¸¹ » ¨© rij ¸¹ ¼ ¬
(3) (4)
Both UAZ and UAA are written as the sums of pair wise additive potentials, uij in the form where the first term is the repulsion-dispersion Lennard-Jones (LJ) potential, with εij and σij corresponding to the parameter sets for each interacting pair that is obtained from εi and σi of each species by using the Lorentz Berthelot mixing rule (i.e., a geometric combining rule for the energy and an arithmetic one for the atomic size: εij=(εiεj)1/2 and σij = (σi + σj)/2). The second term is the Coulombic contribution between point charges qi and qj separated by distance rij. N2 molecule was treated as a three site model; i.e., two outer sites are separated by a distance, l = 1.098 Å, as point charges of equal magnitude, q = -0.40484 e and the third site is located at the middle of the outer sites with a point charge -2q, ensuring molecular charge neutrality [39]. CO2 were considered an atomic point charge model in which the charges assigned to the carbon C (+0.72 e) and the oxygen O (-0.36 e) as in Plant et al.[28] The adsorbateadsorbent LJ parameters for CO2 were taken from the literature [13, 27, 28, 37]. The zeolite framework oxygen, silicon and aluminium and the extra framework cations served as both LJ interaction site and the location of point charge. The LJ parameters used for the interactions of adsorbate-adsorbate and adsorbate- zeolite framework in this work are given in Table 1. The calculation requires LJ parameters for the oxygen of the framework, extra framework cations and adsorbates because of considering that the polarizability of silicon and aluminium are much lower than those of oxygen atoms, therefore the non-bond interactions are assigned only for framework oxygen and extra framework cations of the adsorbent in all GCMC simulation. The interaction parameter for cations were taken from Maurin et al. [21], they suggested a relationship between ionic radius and polarizability of cation to deduce the LJ parameter for each cation. The cations in framework were assumed to be partially ionized and their charges were treated as adjustable parameters. The partial charges on silicon (+2.4e) and oxygen (-1.2e) of the zeolite framework system were fixed at the usually considered values.[40] The charges on aluminium and cations were allowed to change in order to take into account the partial charge transfer from the framework, with the constraint of a global charge of zero for the zeolite system. The partial charges for monovalent and divalent cation were taken as +0.7e and
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+1.7e, respectively, which is usually considered for extra framework cations for the faujasite type zeolites [27, 37, 41]. Table 1. Pair potential parameters used for adsorbates (CO2, N2) and adsorbent in the simulation. Interacting pairs
ε (kcal mol-1)
σ (Å)
Interacting pairs
ε (kcal mol-1)
σ (Å)
Oco2-Oco2 Cco2-Cco2 Oco2-C co2 Cco2-Oz Oco2-Oz Cco2-Li Oco2-Li Cco2-Na Oco2-Na
0.1520 0.0927 0.0364 0.0837 0.1386 0.2583 0.3307 0.1751 0.0623
3.36 3.83 3.31 3.90 3.48 2.46 2.23 3.35 2.95
Cco2-K Oco2-K Cco2-Ca Oco2-Ca Cco2-Sr Oco2-Sr Cco2-Ba Oco2-Ba
0.0911 0.1166 0.1260 0.1614 0.1539 0.1971 0.1615 0.2068
3.06 2.82 2.77 2.54 2.91 2.67 3.04 2.80
NN2-N N2 N N2-Oz N N2-Li N N2-Na
0.0724 0.1555 0.2282 0.0851
3.318 3.179 2.208 2.516
N N2-K N N2-Ca N N2-Sr N N2-Ba
0.0804 0.1114 0.1360 0.1427
2.803 2.517 2.653 2.781
The absolute isotherms were then computed using Grand Canonical Monte Carlo (GCMC) algorithm via the sorption module in the Materials Studio software suite [42], which allows displacements, creations and destructions of adsorbate species. All these simulations were performed with fixed pressures at 303.0 K using one unit cell of each model with a typical 1×104 equilibrium and 1×105 production steps. The evolution of the total energy over the Monte Carlo steps were plotted in order to monitor the equilibration conditions. The zeolite structure was assumed to be rigid during the sorption process and the extra framework cations were maintained fixed in their initial optimized positions. Ewald summation method with accuracy 0.001 kcal/mol was used for calculating the electrostatic interactions and the short range interactions, with a cutoff distance of 12.0 Å [37]. The Metropolis Monte Carlo method allows for the insertion of molecules throughout the zeolite framework regardless of the physical diffusion pathways. 4. Results and Discussion The X-ray powder diffractions of cation exchanged BEA(25) samples are shown in Fig.1. The diffraction patterns of all cation exchanged BEA(25) shows the highly crystalline samples showing reflections in the range of 5 – 65q typically observed for zeolites. (a) (b)
Fig. 1. X-ray powder diffraction pattern of a) monovalent and b) divalent cation exchanged zeolite-X
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Adsorption isotherm of CO2 and N2 were measured at 288.0 and 303.0 K in cation exchanged BEA(25), the isotherm at 303.0 are shown in Fig. 2. The equilibrium adsorption capacities for the adsorption of CO2 and N2 on cations exchanged BEA(25) are determined from the adsorption isotherms and the number of molecules of CO2 and N2 adsorbed per unit cell at 101.3 kPa and 303.0 K are given in Table 2. The CO2 adsorption capacity is higher than N2 in all BEA(25) samples. BEA(25) having bigger potassium cations has maximum adsorption capacity at 303.0 K for CO2 and N2 among the monovalent exchanged BEA(25). The higher CO2 adsorption capacity in KBEA(25) could be due to the acid base interaction between framework and CO2 molecules. It can be seen from Fig. 2 that both the adsorption capacity and the initial concave shape of the CO2 isotherm is higher as compared to N2, which is due to the basic nature of the BEA(25). Additionally, the effective charge density of the cations inside the zeolite pores could also be a reason for higher adsorption in bigger cation exchanged BEA(25). On the other hand, the adsorption capacity of N2 increases linearly on increasing the extra framework cation at all range of pressures. However, the molecule per unit cell of both CO2 and N2 slightly increases with increase of the cation size. The adsorption capacities for both CO2 and N2 in BEA(25) increases on increasing the cation size of the divalent cations. In divalent cation exchanged BEA (25), it has only half number of cation that of monovalent exchanged BEA(25). Therefore, the population of cation at the pore entrance of zeolite framework is very less than the monovalent exchanged BEA(25). Indeed, both CO2 and N2 can move freely in divalent exchanged BEA(25), even though the number of extra-framework cations are very few. (a)
(b)
Fig. 2. The adsorption isotherm of CO2 (left panel) and N2 (right panels) in BEA(25) with monovalent (a) and divalent (b) cations at 303.0 K.
The pure component adsorption selectivity for CO2 over N2 at different equilibrium pressures was calculated at 303.0 K, which are given in Table 2. The CO2/N2 increases on increasing cation size in BEA(25) with monovalent cations. The CO2 selectivity value is higher in the low pressure region in a particular cation exchanged BEA(25), and the selectivity is decreased on increasing adsorption pressure. Again, the CO2 selectivity over N2 is higher at 10 kPa for all divalent cation exchanged BEA(25), and the selectivity values are slightly decreased at 101.3 kPa. CaBEA(25) showed the highest CO2/N2 selectivity for BEA(25) with divalent cations.
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Simulation of both CO2 and N2 adsorption Table 2. CO2 and N2 Adsorption Capacity and Selectivity in Monovalent Cationsin BEA(25) with various cations were Exchanged Zeolite BEA(25) and A at 101.3 kPa and 303.0 K. carried out at 303.0 K. Fig. 3 shows both Adsorbent Adsorbed amounts at 1 Selectivity (D DCO2/N2) at experimental and simulated adsorption atm. and 303.0 K 303.0 K isotherm of CO2 and N2 in cation exchanged (Molecules/ u.c) BEA(25), respectively. The simulation of N2 CO2 10 kPa 101.3 kPa CO2 and N2 predicts the experimental results LiBEA(25) 0.9 8.9 13.4 9.89 reasonably well as shown in Fig. 3. The NaBEA(25) 1.0 10.5 15.6 10.5 higher simulated adsorption isotherm could KBEA(25) 1.5 12.4 40.2 10.4 be due to several reasons, such as decrease CaBEA(25) 1.0 10.2 13.4 10.2 of crystallinity in vacuum activation, SrBEA(25) 1.3 10.3 13.8 8.0 inaccurate parameters for the simulation BaBEA (25) 1.6 10.6 16.0 7.2 studies, and cation locations are used in the simulations that may not exactly match the cation location inside the adsorbent samples used for the adsorption experiment. (a)
(b)
(c)
(d)
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(e)
(f)
Fig. 3. Simulated and Experimental adsorption isotherm of CO2 and N2 in (a) LiBEA(25), (b) NaBEA(25), (c) KBEA(25), (d) CaBEA(25), (e) SrBEA(25) and (f) BaBEA(25) at 303.0 K.
The isosteric heats of adsorption of CO2 and N2 in BEA(25) with different alkali metal and alkaline earth metal, calculated from both experimental and simulation data at initial adsorption coverages are given in Table 3. The simulated heat of adsorption reproduces the experimental data reasonably well. The slight difference in heat of adsorption data could be due to the comparison of simulation in a unit cell of BEA(25) with the experimental data in the bulk zeolite powder sample.
Table 3. Heats of adsorption of CO2 and N2 in cation exchanged zeolite BEA(25) Adsorbent Heat of adsorption (kJ mol-1) Experimental LiBEA(25) NaBEA(25) KBEA(25) CaBEA(25) SrBEA(25) BaBEA(25)
Simulation
N2
CO2
N2
CO2
18.4 16.4 16.1 24.8 24.0 23.6
42.8 38.1 39.2 46.3 44.2 40.4
16.8 15.2 13.1 32.0 30.8 32.0
50.1 42.9 40.3 62.4 60.3 59.4
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The main driving force for the adsorption of (a) (b) (c) CO2 and N2 in zeolite BEA(25) are electrostatic and van der Walls interactions. The electrostatic interactions between the sorbate molecules and the zeolite BEA(25) depend on the quadrupole moments of the sorbate molecules (CO 2>>N2). Both experimental and simulated heat of adsorption of CO2 and N2 in various cations containing zeolite-BEA(25) shows the similar trends, even though there is slight difference in magnitude. The initial heat of adsorption is obtained in the order of LiBEA(25) > NaBEA(25) > KBEA(25) and CaBEA(25) > SrBEA(25) > BaBEA(25) for both CO2 and N2. The cations located in the cavity determine the (d) (e) (f) adsorption capacity as well as heat of adsorption because the positive charge density is around these cations. At lower pressure, the number of N2 molecules adsorbed per unit cell is less than compared to the CO2. The electrostatic interaction is dominant between N2 molecule and zeolites, and the adsorbate-adsorbate interaction are negligible. It is clearly established that the extra framework cations have different polarizing power in the zeolite cavity, which decreases on increasing the cation size. The aluminium and silicon atoms are located in the centre of tetrahedra of the zeolite Fig. 4. Snap shot of a final configuration for CO2 adsorbed in BEA(25) having BEA(25), therefore the principal interaction of monovalent, (a) LiBEA(25), (b) NaBEA(25), and (c) KBEA(25), and divalent, the adsorbates is due to the charge density of (d) CaBEA(25), (e) SrBEA(25), and (f) BaBEA(25), cations at 101.3 kPa and extra framework cations occupied in the cages 303.0 K. Tubes and spheres represent the framework and the extra-frmework of the BEA(25). The heat of adsorption for both cation and/or sorbates, respectively, with yellow for silicon, red for oxygen and CO2 and N2 decreases on increasing the cation pink for aluminium, grey for carbon, yellowish-green for lithium, violet for size of the extra framework cation. Fig. 4 shows sodium, maroon for potassium, cyan for calcium, bluish-green for strontium and the unit cell structure of cation exchanged green for barium. BEA(25) with adsorbed CO2 at 101.3 kPa and 303.0 K. The CO2 molecules are sitting close to the cations inside the cavity, the CO 2 density around the cations increases on increasing the cation size. Fig. 5 shows the unit cell structure of BEA(25) with adsorbed N2 at 101.3 kPa and 303.0 K.
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(b)
(c) Conclusions
(d)
(e)
(f)
Fig. 5. N2 adsorbed snap shot of a final configuration of BEA(25) having monovalent, (a) LiBEA(25), (b) NaBEA(25), and (c) KBEA(25), and divalent, (d) CaBEA(25), (e) SrBEA(25), and (f) BaBEA(25), cations at 101.3 kPa and 303.0 K. Tubes and spheres represent the framework and the extra-frmework cation and/or sorbates, respectively, with yellow for silicon, red for oxygen and pink for aluminium, blue for nitrogen, yellowish-green for lithium, violet for sodium, maroon for potassium, cyan for calcium, bluish-green for strontium and green for barium.
Equilibrium adsorption measurements for CO2 and N2 were carried out in cation exchanged BEA(25) and compared with its simulated isotherms utilizing grand canonical Monte Carlo (GCMC) simulations. CO2 sorption is observed to show higher adsorption capacity than N2 in all these zeolite samples. The adsorption capacities of cation-exchanged zeolites are increased from parent zeolites. The pure component CO2/N2 adsorption selectivity increases on increasing cation size in case of monovalent cation exchanged BEA(25), whereas it decreases on increasing divalent cation radius. Isosteric heat of sorption data show stronger interactions of CO2 and N2 molecules with BEA(25) having either monovalent or divalent cations, which decreases on increasing the size of the cation. Simulation of adsorption isotherms and heats of adsorption of CO2 and N2 in metal ion exchanged zeolite match reasonably well with the experimental results. Simulation of CO2 and N2 in cation BEA(25) clearly shows that the adsorbed CO2 molecules sit closely to the extraframework cations accessible through the bigger cavity.
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