Adsorption Process of CO2 on Silicalite-1 Zeolite Using Single-Crystal

Mar 16, 2014 - The process of CO2 adsorption on silicalite-1 zeolite (MFI-type) is revealed using a single-crystal X-ray method. The structure of CO2-...
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Adsorption Process of CO2 on Silicalite‑1 Zeolite Using Single-Crystal X‑ray Method Shinjiro Fujiyama,* Natsumi Kamiya, Koji Nishi, and Yoshinobu Yokomori* Department of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan S Supporting Information *

ABSTRACT: The process of CO2 adsorption on silicalite-1 zeolite (MFI-type) is revealed using a single-crystal X-ray method. The structure of CO2-silicalite-1 with a small amount of CO2 in the pore is determined, wherein most of CO2 molecules are located in the straight channel. It indicates the straight channel is the most stable sorption site based on the van der Waals interactions between the CO2 and the framework, and the CO2 molecules initially adsorb in the straight channel in the adsorption process. This is the first report to describe the structure of MFI-type zeolites with the adsorbate molecules occupying only the straight channel.

1. INTRODUCTION Microporous materials such as zeolites, metal−organic frameworks, and carbon nanomaterials are among the most important materials due to the various applications such as catalyst, gas separation, gas storage, and so on. Among the many potential microporous materials, zeolites are promising because of their high thermal, mechanical, and chemical stability. Adsorption properties of various zeolites have been widely investigated. Above all, MFI-type zeolites (e.g., ZSM-5 and silicalite-1) have attracted a lot of interest with their unique channels, which are the straight channel and the sinusoidal channel. There are many previous reports concerning gas adsorption on MFI-type zeolites, and a wide range of information has been reported regarding the adsorption behavior and derivation of a proper adsorption model.1−8 The channel system of the MFI-type zeolites can be divided into three kinds of sorption sites, which are the straight channel, the sinusoidal channel, and the intersection. A lot of structures of MFI-type zeolites loaded with aromatic compounds were determined at several loadings and the adsorption process was almost revealed. The guest aromatic molecules initially adsorb at the intersection until the intersection is fully occupied (4 molecules/u.c.).9−11 And then the additional aromatic molecules adsorb in the sinusoidal channel. Finally, the intersection and the sinusoidal channel are occupied (8 molecules/u.c.).12−15 Benzene is exceptionally located at the intersection and in the straight channel.16 As contrasted with the aromatic compounds, the structures of MFI-type zeolites loaded with chain compounds are not determined, except for CO2.17,18 The key reason why the adsorption structures of chain compounds are not revealed is the twinning problem of the MFI-type zeolites. They exhibit phase transitions between the orthorhombic and the monoclinic phases with varying temperature change, mechanical stress, or presence of guest © 2014 American Chemical Society

compounds. MFI-type zeolites have an orthorhombic single phase with bulky aromatic compounds in their pore. However, they have monoclinic twin phases with small chain compounds like CO2, and it makes crystallographic work quite difficult. Recently, we have overcome this problem using the mechanical stress on the silicalite-1 crystal along the c axis at high temperature and the refinement for pseudomerohedral twinning. And we have determined the location of CO2 molecules on silicalite-1 zeolites under equilibrium condition at 0.080 MPa, 298 K (high-loaded CO2-silicalite-1).18 In this structure, CO2 molecules are located in the both channels and at the intersection. It is still unclear where the most stable sorption site is and where the CO2 molecules initially adsorb in the adsorption process. The contribution of CO2−CO2 interactions obstructs the insight for the CO2-framework interaction. In this study, we carried out single-crystal X-ray diffraction of low-loaded and midloaded CO2-silicalite-1 for the first time and determined the structure in order to clarify the most stable sorption site based on the CO2-framework interaction and the adsorption process of CO2 on silicalite-1.

2. EXPERIMENTAL SECTION 2.1. Preparation of Low-Loaded and Midloaded CO2Silicalite-1. Silicalite-1 single crystals were synthesized according to the method proposed by Kamiya et al.16,19 EDX analysis confirmed that the composition of the obtained crystals was SiO2 with no Al or cation species. Crystals selected for X-ray structure analysis were pressed with a weight of 2.0 g along the crystallographic c axis, while the temperature was changed between room temperature and 473 K. The heating and cooling steps were repeated three times.16 The crystals were exposed to CO2 gas at 0.080 MPa in a closed, vacuum Received: December 26, 2013 Revised: February 18, 2014 Published: March 16, 2014 3749

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instrument (Bell jar-type vacuum oven BV-001, Shibata Science Company) at 298 K for 30 min (low-loaded) or 60 min (midloaded). The adsorption time was shortened compared to the high-loaded structure18 in order to reduce the amount of CO2 molecules in the silicalite-1 pore relative to the high-loaded structure. The CO2 adsorption was cut off before the amount of CO2 reached the equilibrium. 2.2. Structure Analysis of Low-Loaded and Midloaded CO2Silicalite-1. Single-crystal X-ray diffraction data was collected at room temperature using an APEX II X-ray diffractometer (Bruker AXS) with a CCD detector, Mo Kα radiation, and a graphite monochromator. The collected reflections were corrected for Lorentz-polarization and the absorption effect. The structural analysis was conducted in the monoclinic twin in P21/n. The structure was solved using the direct method, and the difference-Fourier synthesis was used for the remaining atoms (SHELXTL20). The pseudomerohedral twinning refinement was performed on F2 and Σw∥Fo| − |Fc∥2 was minimized; w = 1/[σ2(Fo2) + (aP)2 + bP], where P = (Fo2 + 2Fc2)/3 and a and b are the weight parameters. As described in the report,18 the orthorhombic single crystal of silicalite-1 is relaxed into monoclinic twin phases after CO2 adsorption. The reciprocal lattices of the twin phases can be treated as coincident due to the similarity of c* axes of them. So almost all reflections are overlapped, and the pseudomerohedral twinning refinement works successfully. Anisotropic displacement parameters were used, and no restraints were introduced on the framework structures. The refinement with CO2 molecules was conducted as described in the report.17 The structures of the CO2 molecules were constrained as rigid groups (C−O = 1.16 Å and O−C−O = 180°). In the refinement of the midloaded CO2-silicalite-1 structure, some restraints were introduced on the unstable displacement parameters of CO2 atoms and the sum of the occupancy factors of STR2 and STR3 which were the CO2 adsorption sites, as shown in Figure 2. The distance between STR2 and STR3 was 2.6 Å, and it was so close that CO2 molecules would not coexist in these sites. Then the sum of the occupancy factors was constrained to be 1. The experimental data are summarized in Table 1. The full details and reflection data are available as CIF formatted files in the Supporting Information. The structures in Figure 1 and Figure 2 were drawn using the software VESTA.21 2.3. Gravimetric adsorption measurement. The variation of CO2 loading on silicalite-1 with time was measured gravimetrically at

Figure 1. Structure of low-loaded CO2-silicalite-1, with the occupancy factors in parentheses. (a) Along the c axis. (b) Along the b axis.

Table 1. Crystal data and refinement details Crystal Chemical formula Temperature Crystal system Space group Unit cell dimensions

Volume Z Crystal size Independent reflections (Rint) No. of restraints No. of parameters Goodness-of-fit on F2 R (I > 2σ (I))

low-loaded CO2silicalite-1 Si24O48·0.38CO2 296(2) K Monoclinic (twin) P21/n a = 20.00(3) Å b = 19.77(3) Å c = 13.32(2) Å α = 90.03(3)° β = 90° γ = 90° 5267(13) Å3 4 0.21 × 0.12 × 0.08 mm3 12800 (0.1478)

0 664 0.897 0.0636 (6433 reflections) Rall 0.1349 Largest diff. peak and hole 0.63 and −0.76 e Å−3

midloaded CO2silicalite-1 Si24O48·1.18CO2 296(2) K Monoclinic (twin) P21/n a = 20.03(2) Å b = 19.83(2) Å c = 13.339(18) Å α = 90.01(2)° β = 90° γ = 90° 5298(11) Å3 4 0.17 × 0.12 × 0.09 mm3 12659 (0.1103) 6 667 0.815 0.0541 (5450 reflections) 0.1331 0.68 and −0.57 e Å−3

Figure 2. Structure of midloaded CO2-silicalite-1, with the occupancy factors in parentheses. (a) Along the c axis. (b) Along the b axis. 0.080 MPa, 298 K using Bruker AXS TG-DTA 2000SA. Silicalite-1 crystals were synthesized and calcined as reported previously.16,19 The crystals of 10.48 mg were placed in the sample holder and outgassed at 3750

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573 K for 30 min. After the crystals were cooled to 298 K, CO2 gas was introduced into the system to 0.080 MPa quickly and the weight of crystals was measured continuously.

l n[neq /{neq − n(t )}] = (k ad + k de)t

This kinetic model explains the N2 and O2 adsorption on carbon molecular sieves.22 In the case of CO2 adsorption on silicalite-1, the plots in the n(t) range of 0 to 4 (molecules/unit cell) are perfectly consistent with the model. For n(t) values over 4 (molecules/unit cell), the values of n(t) are smaller than the values expected by the model. The tabulated data of Figure 3 are available in Supporting Information.

3. RESULTS 3.1. Structures of low-loaded and midloaded CO2silicalite-1. The packing of CO2 in the low-loaded CO2silicalite-1 is shown in Figure 1 with the occupancy factors in parentheses. STR1 and STR1′, STR2 and STR2′ are related respectively by the center of symmetry in the middle of the straight channel. All CO2 molecules were observed in the straight channel and the locations of STR1 and STR2 were identical to those of the high-loaded CO2-silicalite-1.18 The amount of CO2 calculated using the occupancy factors is 1.5 molecules per unit cell. Figure 2 shows the packing of CO2 in the midloaded CO2-silicalite-1. SIN and SIN’ are related by the screw axis 21 along the a axis. The locations of CO2 labeled as STR2 and SIN were identical to those of the high-loaded CO2silicalite-1.18 STR3 was located relatively outer of the straight channel than STR1, 2. The amount of CO2 calculated using the occupancy factors is 4.7 molecules per unit cell. The atomic coordinates and equivalent isotropic displacement parameters of the low-loaded and midloaded structures are available in Supporting Information. 3.2. Gravimetric CO2 loading and the Langmuir kinetic model. Figure 3 shows the result of the gravimetric

4. DISCUSSION 4.1. Adsorption Process. The structure of the low-loaded CO2-silicalite-1 shown in Figure 1 indicates that the possible closest distance between CO2 molecules is no less than 5.0 Å (O to O). The CO2−CO2 interaction should be negligibly small in this structure, and then the straight channel is clearly the most stable sorption site based on the CO2-framework interaction. The difference of stability of STR1 and STR2 may be subtle, but STR2 is more stable according to the larger occupancy factor. The CO2-framework interaction is based on the van der Waals interaction. STR2 is located at the center of the 10-membered ring of the channel. CO2 molecules at STR2 interact with as many as twenty atoms (ten O atoms and ten Si atoms) in proper distances to minimize their interaction potentials. Predictions of CO2 locations in the MFI-type zeolites using computer simulations have been reported. Selassie et al.23 provided a probability map at 298 K and infinite dilution. It indicated the preferred sites are both channels. Babarao et al.24 mentioned that CO2 are adsorbed mostly in the straight channel at 500 kPa and 300 K. Yue and Yang25 discussed the adsorption behavior in detail. The density distributions of the center-of-mass of CO2 at 0.05 MPa, 318.2 K showed a maximum in the middle part of the straight channel. They predicted the location with high precision, but it is located slightly inward of the straight channel compared to the actual locations determined in this work. Figure 3 shows the linearized Langmuir kinetic plots of the gravimetric adsorption measurement. The Langmuir kinetic model does not take into account interactions between guest molecules. The plots in the n(t) range of 0 to about 4 are perfectly consistent with the model because the CO2−CO2 interaction is negligibly small in this range with a small number of CO2 molecules in the pores. This threshold value of n(t) = 4 clearly corresponds to the number of the sorption sites in the straight channel per unit cell, as there are four sites. The plots are explained by the Langmuir model until the value of n(t) reaches 4, at which time the straight channels are occupied by CO2 molecules at STR1 and STR2. In the equilibrium structure (high-loaded18), the CO2−CO2 interaction acts in addition to the CO2-framework interaction, so the straight channel is not fully occupied, and considerable CO2 molecules are located in the sinusoidal channel (SIN) and at the intersection (INT). The structure of the midloaded CO2silicalite-1 shown in Figure 2 corresponds to an intermediate structure in the adsorption process. The straight channel is completely occupied by CO2 molecules in the midloaded structure. It means the contribution of the CO2-framework interaction is still dominant. However, the CO 2 −CO 2 interaction affects the adsorption structure in the straight channel. STR3 was not observed in the low-loaded structure, and then it should be less stable than STR1 or STR2 based on the CO2-framework interaction. STR3 would be stabilized by the CO2−CO2 interaction.

Figure 3. Variation of CO2 loading with time based on the linearized Langmuir kinetic model at 0.080 MPa and 298 K.

adsorption measurement. We applied the linearized Langmuir kinetic model to the results of the gravimetric adsorption measurements. The model is based on the Langmuir model assumptions. By definition of kad and kde as the adsorption and desorption rate constants, the adsorption rate is given by dn(t )/dt = k ad{(nsat − n(t ))} − k den(t )

(1)

where t is the adsorption time, n(t) is the CO2 loading at time t, nsat is the saturated loading. Rates of adsorption and desorption are equal under equilibrium condition, so the following equation is obtained. k ad{(nsat − neq)} = k deneq

(2)

where neq is the equilibrium loading. The integrated form of eq 1 is n(t ) = neq[1 − exp{−(k ad + k de)t }]

(4)

(3)

Equation 3 is linearized as 3751

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located at the intersection, unlike CO2. This difference in initial adsorption behavior is due to van der Waals interactions between the guest compounds and the framework atoms. This interaction can be approximately expressed using the Lennard− Jones potential model. The adsorption potential energy is assumed to be

Figure 4 shows the adsorption process of CO2 on silicalite-1. The initial adsorption behavior is governed by the CO2-

ULJ(r ) = 4ε{(σ /r )12 − (σ /r )6 }

(5)

where r is the interatomic separation, σ is the separation at which the potential becomes zero, and ε is the depth of the potential well. The value of σ is obtained from the relation σ = (rguest + rframework ) × 2−1/6

(6)

where rguest is the molecular radius of the guest molecules and rframework (1.52 Å) is the atomic radius of oxygen at the surface of the pore. Tjatjopoulos et al.26 integrated the potential over the entire internal surface of the pore cylinder to give Uguest − framework(R, x) =

∫A nULJ(r) dA

(7)

where A is the surface area of the pore cylinder, n is the number density of the surface atoms, R is the radius of the cylinder, and x the closest approach between the guest molecules and the center of the cylinder. Figure 5 shows the adsorption potential energy in a cylindrical pore using the model of Tjatjopoulos et al.26 The

Figure 5. Normalized adsorption potential energy in a cylindrical pore. (a) rguest = 2.0 Å. (b) rguest = 3.0 Å. Figure 4. Adsorption process of CO2 on silicalite-1 at 0.080 MPa and 298 K. (a) Every CO2 molecule is located in the straight channel (lowloaded). (b) The straight channel is fully occupied [n(t) = 4 (molecules/unit cell)]. (c) Under equilibrium condition (highloaded).18

potential energy clearly indicates whether the channel or the intersection is a stable sorption site, depending on the size of the guest molecules. A cylindrical pore with R = 4.0 Å can be considered a model for the MFI-type zeolite channels. As can be seen in Figure 5a, small molecules with around rguest = 2.0 Å prefer the channels, rather than the larger pore. On the other hand, large molecules with around rguest = 3.0 Å are energetically disfavored in the channels compared with the larger pore (Figure 5b). This means that the channels are too small to maintain the preferred distances to the surface for large molecules around rguest = 3.0 Å. The CO2 molecule is about the same size as the former, so the small channels are favorable sorption sites for CO2 molecules based on the van der Waals interactions between the CO2 and the framework atoms. Aromatic molecules fall in the latter category and are more stable at the larger intersection than in the smaller channels based on their interaction with the framework atoms.

framework interaction. The straight channel is the most stable sorption site, so CO2 molecules initially adsorb at STR1 and STR2 (low-loaded, Figure 4a). Eventually, all straight channels are occupied by CO2 molecules [n(t) = 4, Figure 4b]. The additional CO2 molecules adsorb at SIN (midloaded), and CO2−CO2 interactions arise. Then some of the CO2 molecules in the straight channel move to the intersection. Finally, the adsorption structure reaches equilibrium condition (highloaded,18 Figure 4c). 4.2. Pore Size and Sorption Sites Stability. The structures of MFI-type zeolites with low-loaded aromatic compounds have been reported.9−11 The compounds are 3752

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(7) Kolokolov, D. I.; Jobic, H.; Stepanov, A. G. Mobility of n-Butane in ZSM-5 Zeolite Studied by 2H NMR. J. Phys. Chem. C 2010, 114, 2958−2966. (8) García-Pérez, E.; Parra, J. B.; Ania, C. O.; García-Sánchez, A.; van Baten, J. M.; Krishna, R.; Dubbeldam, D.; Calero, S. A Computational Study of CO2, N2, and CH4 Adsorption in Zeolites. Adsorption 2007, 13, 469−476. (9) van Koningsveld, H.; Jansen, J. C.; de Man, A. J. M. Single-Crystal Structure Analysis and Energy Minimizations of a MFI-Type Zeolite at Low p-Dichlorobenzene Sorbate Loading. Acta Crystallogr. 1996, B52, 131−139. (10) van Koningsveld, H.; Jansen, J. C. Single Crystal Structure Analysis of Zeolite H-ZSM-5 Loaded with Naphthalene. Microporous Mater. 1996, 6, 159−167. (11) van Koningsveld, H.; Koegler, J. H. Preparation and Structure of Crystals of Zeolite H-ZSM-5 Loaded with p-Nitroaniline. Microporous Mater. 1997, 9, 71−81. (12) van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J. C. The Location of p-Xylene in a Single Crystal of Zeolite H-ZSM-5 with a New, Sorbate-Induced, Orthorhombic Framework Symmetry. Acta Crystallogr. 1989, B45, 423−431. (13) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. The Location of p-Dichlorobenzene in a Single Crystal of Zeolite H-ZSM-5 at High Sorbate Loading. Acta Crystallogr. 1996, B52, 140−144. (14) Nishi, K.; Hidaka, A.; Yokomori, Y. Structure of Toluene6.4ZSM-5 and the Toluene Disproportionation Reaction on ZSM-5. Acta Crystallogr. 2005, B61, 160−163. (15) Kamiya, N.; Oshiro, T.; Tan, S.; Nishi, K.; Yokomori, Y. Adsorption Process of Phenol on Silicalite-1 and Crystal Structure of Phenol8.0-silicalite-1 using a Single Crystal X-ray Diffraction Method. Microporous Mesoporous Mater. 2013, 169, 168−175. (16) Kamiya, N.; Iwama, W.; Kudo, T.; Nasuno, T.; Fujiyama, S.; Nishi, K.; Yokomori, Y. Determining the Structure of a Benzene7.2silicalite-1 zeolite Using a Single-Crystal X-ray Method. Acta Crystallogr. 2011, B67, 508−515. (17) Fujiyama, S.; Kamiya, N.; Nishi, K.; Yokomori, Y. Location of CO2 on Silicalite-1 Zeolite Using a Single-Crystal X-ray Method. Z. Kristallogr. 2013, 228, 180−186. (18) Fujiyama, S.; Kamiya, N.; Nishi, K.; Yokomori, Y. Reanalysis of CO2-Silicalite-1 Structure As Monoclinic Twinning. Z. Kristallogr. 10.1515/zkri-2013-1663. (19) Kamiya, N.; Torii, Y.; Sasaki, M.; Nishi, K.; Yokomori, Y. Largequantity single crystal synthesis of TPA-ZSM-5 using KOH as a mineralizer. Z. Kristallogr. 2007, 222, 551−554. (20) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr. 2008, A64, 112−122. (21) Momma, K.; Izumi, F. VESTA: a Three-Dimensional Visualization System for Electronic and Structural Analysis. J. Appl. Crystallogr. 2008, 41, 653−658. (22) O’koye, I. P.; Benham, M.; Thomas, K. M. Adsorption of Gases and Vapors on Carbon Molecular Sieves. Langmuir 1997, 13, 4054− 4059. (23) Selassie, D.; Davis, D.; Dahlin, J.; Feise, E.; Haman, G.; Sholl, D. S.; Kohen, D. Atomistic Simulations of CO2 and N2 Diffusion in Silica Zeolites: The Impact of Pore Size and Shape. J. Phys. Chem. C 2008, 112, 16521−16531. (24) Babarao, R.; Hu, Z.; Jiang, J.; Chempath, S.; Sandler, S. I. Storage and Separation of CO2 and CH4 in Silicalite, C168 Schwarzite, and IRMOF-1: A Comparative Study from Monte Carlo Simulation. Langmuir 2007, 23, 659−666. (25) Yue, X.; Yang, X. Molecular Simulation Study of Adsorption and Diffusion on Silicalite for a Benzene/CO2 Mixture. Langmuir 2006, 22, 3138−3147. (26) Tjatjopoulos, G. J.; Feke, D. L.; Mann, J. A., Jr. MoleculeMicropore Interaction Potentials. J. Phys. Chem. 1988, 92, 4006−4007.

CO2 initially adsorbs in the straight channel (STR1 and STR2) rather than the sinusoidal channel. The difference in pore size between the straight channel and the sinusoidal channel is too slight to use the simple potential model described above. The difference in their stability as CO2 sorption sites is due to differences in the structure of the channels, as previously discussed.17 That is to say, the conformation of two 10-membered rings of the straight channel is parallel and that of the sinusoidal channel is angled. Because of this difference between the channel structures, the potential well of the straight channel is deeper than that of the sinusoidal channel.

5. CONCLUSION The structure of low-loaded and midloaded CO2-silicalite-1 is determined. The straight channel is the most stable sorption site based on van der Waals interactions between the CO2 and the framework, so CO2 molecules initially adsorb at STR1 and STR2 in the adsorption process. This is the first report to describe the structure of MFI-type zeolites with the adsorbate molecules occupying only the straight channel. The contribution of CO2−CO2 interactions become significant as the adsorption process approaches equilibrium conditions, and the CO2 molecules begin to occupy other locations. The Lennard−Jones potential model explains why small chain compounds such as CO2 prefer the smaller channels to the larger intersection. However, the effect of the structural difference between the straight channel and the sinusoidal channel on the adsorption behavior of guest molecules remains unclear. Further investigation of the adsorption structures of other small chain compounds in the channels is required.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data (CIF formatted files) and additional materials mentioned within the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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