Diffusion of propylene adsorbed in Na-Y and Na-ZSM5 zeolites: Neutron scattering and FTIR studies

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1 PRAMANA c Indian Academy of Sciences Vol. 71, No. 5 journal of November 2008 physics pp Diffusion of propylene adsorbed in Na-Y and Na-ZSM5 zeolites: Neutron scattering and FTIR studies S GAUTAM 1,, A K TRIPATHI 2, V S KAMBLE 2, S MITRA 1 and R MUKHOPADHYAY 1 1 Solid State Physics Division; 2 Chemistry Division, Bhabha Atomic Research Centre, Mumbai , India Corresponding author. sidgau@barc.gov.in Abstract. Here we report the quasielastic neutron scattering and FTIR studies on the dynamics of propylene adsorbed in Na-Y and Na-ZSM5 zeolites. QENS data show that although the mechanism of translational motion of propylene is jump diffusion in both the cases of Na-Y and Na-ZSM5 zeolites, the diffusivity is affected by the host size and is hindered in the case of Na-ZSM5. FTIR studies showed that guest host interaction in Na-ZSM5 is stronger than that in Na-Y zeolite corroborating the QENS results. Keywords. Neutron quasielastic scattering; porous materials; adsorption. PACS Nos Nx; Cd; Mb 1. Introduction Zeolites are microporous crystalline aluminosilicates represented by the general formula Mx/n[(AlO 2 ) x (SiO 2 ) y ] mh 2 O, where M is a cation of valence n. Because of their extensive industrial applications as molecular sieves and as catalysts, they have attracted a lot of attention in research [1]. Of particular interest has been the problem of the dynamics of hydrocarbons adsorbed in zeolites [2]. This has been an attractive field of research because an understanding of diffusion helps in an understanding of catalysis as well as sieving properties of these minerals. An understanding of diffusivities of guest molecules in the zeolitic cages can thus help in synthesizing tailor-made zeolites suited for catalytic and sieving applications. This field has also flourished for a pure interest in the fundamental aspects of molecular motion in confined geometries. Zeolites, because of their regularly arranged porous structure offer a unique advantage for studying such problems experimentally in a systematic way. We have already reported studies of dynamics of several hydrocarbons differing in the number of carbons and the type of bonds. For example, propane [3,4] (a saturated hydrocarbon with three carbons), acetylene [5,6] (an unsaturated hydrocarbon with 2 carbons) and 1,3-butadiene (an unsaturated 1153

2 S Gautam et al hydrocarbon with 4 carbons) [7] adsorbed in Na-Y zeolite have been studied using quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulation. A QENS study on the diffusion of benzene adsorbed in H-ZSM5 zeolite has also been reported [8]. Here we report the QENS studies on the diffusion of propylene (an unsaturated hydrocarbon with three carbons) in Na-Y (with void spaces in the form of cages) and Na-ZSM5 (with void spaces in the form of channels) two zeolites with a different void structure. 2. Experiment The zeolite samples used in the study were obtained from Sud-Chemie. These samples were pre-treated at 300 C for about 48 h in vacuum in an aluminium sample cell and kept for QENS measurement to record the bare zeolite spectra. The samples were then de-gassed at 10 5 torr and propylene gas was loaded at saturation loading. The propylene-loaded samples were then kept for QENS measurement. Experiments were carried out using the QENS spectrometer at Dhruva [9]. This spectrometer has an energy resolution of 200 µev with incident energy of 5.1 mev. QENS spectra were recorded in the wave vector transfer range of Å 1. In situ FTIR studies were carried out in transmission mode using a stainless steel IR cell and a JASCO model 610 FTIR spectrophotometer equipped with a DTGS detector. Three hundred scans were co-added at a resolution of 4 cm 1 for plotting each spectrum. 3. Results and discussion In a neutron scattering experiment on hydrogenous samples, the scattered intensity is predominantly proportional to the incoherent scattering law S(Q, ω). In general, this scattering law consists of two parts and can be written as [10] S(Q, ω) = A(Q)δ(ω) + [1 A(Q)]L(ω, Γ). (1) Here the first term is the elastic scattering due to hydrogen atoms, which are practically immobile within the energy range of the spectrometer, and the second term is the quasielastic contribution from those hydrogen atoms, which are moving with energies in the range of the spectrometer. L(ω, Γ) is a Lorentzian function with a half-width at half-maxima (HWHM) Γ. The coefficient of the delta function δ(ω) gives the fraction of the intensity which is elastic and is called the elastic incoherent structure factor (EISF). It provides information about the geometry of motion. In the case of localized motion or rotational motion, this term is expected to have a non-zero value. The variation of Γ provides information about the time scale of the motion. In practice the two components scattering law given by eq. (1) has to be convoluted with the instrumental resolution function. The experimental spectra can then be fitted with the resulting convolution product with EISF and the Γ as fitting parameters. In the present case, the spectra for bare zeolites did not show any quasielastic broadening whereas propylene-loaded zeolite spectra showed considerable quasielastic broadening. The bare zeolite spectra were subtracted from 1154 Pramana J. Phys., Vol. 71, No. 5, November 2008

3 Neutron scattering and FTIR studies Table 1. Fitting parameters obtained by fitting the behaviour of HWHM with C E model. Host system l (Å) τ (ps) D ( 10 5 cm 2 /s) Na-Y ±0.2 Na-ZSM ±0.1 the propylene-loaded zeolite spectra. The resulting spectra were then fitted with scattering law in eq. (1) convoluted with the instrumental resolution by the method of least squares fitting. The resulting fits for both the systems of propylene in Na- Y as well as Na-ZSM5 zeolites are shown in figure 1a. The elastic components obtained in the fits for both systems were negligible indicating the absence of any localized motion. The Γ values obtained from the fits are plotted in figure 1b as a function of Q 2. In the case of simple Brownian diffusion the variation of Γ with Q 2 is expected to be linear. Here Γ values seem to be saturated at higher Q values, which is an indication of jump diffusion mechanism. One such model describing diffusion by jumps is the Chudley and Elliott (C E) model in which the molecules spend some time called the residence time (τ) on a site and then jump instantly to another site at a distance called jump length (l) from the first site. The jump lengths remain constant between the jumps. It was found that the C E model described above fits the experimental data quite well. The values of residence times (τ) and jump lengths (l) obtained by fitting the experimental HWHM with C E model of jump diffusion, as also the diffusion coefficients calculated using Einstein relation (D = l 2 /6τ) for both the systems and are listed in table 1. FTIR studies were performed on propylene loaded Na-Y and Na-ZSM5 zeolites. The FTIR spectra are shown in figure 2. Spectrum of propylene gas (curve a) is included for comparison. As can be seen in this figure, upon adsorption in Na-Y (curve b) and Na-ZSM5 (curve c) zeolites, most of the vibrational bands observed for the gaseous propylene shifted to lower frequency with marked change in their relative intensities. This effect was more pronounced on Na-ZSM5. Similarly, the amount of propylene adsorbed was found to be more on Na-ZSM5 as can be seen from the intensity of various adsorption bands (curves b and c). Also adsorbed species were found to be more strongly bound on Na-ZSM5 as the spectra after evacuation (curves d and e) show. Thus, bands appearing in νc H region of gas phase spectrum in cm 1 were red-shifted by 3 10 cm 1 in both the zeolites. However, this red-shift was higher on Na-ZSM5. The νc=c vibrations appearing at 1664, 1650 and 1637 cm 1 in the gas phase spectra were red-shifted by cm 1 in the adsorbed state. Similarly, absorption bands in δ(c H) region appearing in gas phase spectra at 1473 and 1443 cm 1 were also influenced in intensity and frequencies. Increased intensity of a CH 3 symmetric deformation vibrational band at 1376 cm 1 in curves b and c is attributed to increased saturation in propylene molecule apparently due to interaction with hydroxyl groups. IR results thus show that propylene is molecularly adsorbed over both the zeolites. However, extent of adsorption (amount and strength) is greater on Na-ZSM5 in spite of the surface area in Na-ZSM5 (380 m 2 /g) being smaller than that in Na-Y zeolite (556 m 2 /g). This indicates that the guest host interaction is stronger in Na-ZSM5 Pramana J. Phys., Vol. 71, No. 5, November

4 S Gautam et al Figure 1. (a) Fitted QENS spectra at different Q values and (b) variation of Γ obtained from QENS experiment. The symbols represent experimental values in the two zeolites while the solid lines are the fits of C E model to the respective Γ variations. as compared to Na-Y zeolite. It may be noted that the value of diffusion coefficient obtained from QENS experiment for propylene in Na-Y zeolite is about a factor of 3 larger than that in Na-ZSM5. This can be seen as a consequence of (i) a smaller jump length and (ii) a larger residence time in the case of Na-ZSM5 in comparison to that in Na-Y zeolite. A smaller jump length in Na-ZSM5 is expected because of a small void volume thereby resulting in stronger confinement. Large residence times can be attributed to stronger guest host interaction in Na-ZSM5 as shown by FTIR studies. Thus FTIR studies corroborate the results of QENS. 4. Conclusion Dynamics of propylene adsorbed in Na-Y and Na-ZSM5 cages has been studied using quasielastic neutron scattering and the adsorption by FTIR. The diffusion 1156 Pramana J. Phys., Vol. 71, No. 5, November 2008

5 Neutron scattering and FTIR studies Figure 2. IR spectra of propylene in gaseous state (a) and when adsorbed over Na-Y (b) and Na-ZSM5 (c) zeolites at room temperature and 100 mbar pressure. Spectra d and e respectively refer to Na-Y and Na-ZSM5 zeolites exposed to propylene followed by 5 min of evacuation. coefficient of propylene in Na-Y zeolite has been found to be a factor of about 3 greater than that in Na-ZSM5. This is a consequence of the interplay of the dependencies of diffusivity on the free space available and the interaction of the host with the zeolites. FTIR experiments show that the guest host interaction is stronger in Na-ZSM5 than in Na-Y zeolite thereby supporting the QENS result about a smaller diffusivity of propylene in Na-ZSM5 vis-à-vis Na-Y zeolite. References [1] G Tomlinson, Modern zeolites, structure and function in detergents and petrochemicals (Traw Tech Publications Ltd., Switzerland, 1998) [2] H Jobic et al, Phys. Rev. Lett. 82, 4260 (1999) [3] Ahmed Sayeed et al, J. Phys. Chem. B107, 527 (2003) [4] R Mukhopadhyay et al, Phys. Rev. E66, (2002) [5] S Mitra et al, Pramana J. Phys. 63, 449 (2004) [6] Siddharth Gautam et al, Phys. Rev. E74, (2006) [7] S Gautam et al, Chem. Phys. Lett. 442, 311 (2007) [8] S Mitra et al, Appl. Phys. A74, S1308 (2002) [9] R Mukhopadhyay et al, Nucl. Instrum. Methods A474, 55 (2001) [10] M Bee, Quasielastic neutron scattering (Adam Hilger, Bristol, 1988) Pramana J. Phys., Vol. 71, No. 5, November