Eur. Phys. J. Special Topics 189, 273–277 (2010)c© EDP Sciences, Springer-Verlag 2010DOI: 10.1140/epjst/e2010-01332-x

THE EUROPEANPHYSICAL JOURNALSPECIAL TOPICS

Regular Article

Effect of guest-host interaction on thedynamics of ethylene glycol in H-ZSM5 zeolite

V.K. Sharma1, S. Mitra1,a, P. Maheshwari2, D. Dutta2, and P.K. Pujari2,and R. Mukhopadhyay1

1 Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India2 Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

Received 01 September 2010 / Received in final form 14 September 2010Published online 12 November 2010

Abstract. Dynamics of ethylene glycol (EG) adsorbed in H-ZSM5 zeo-lite as studied using quasielastic neutron scattering (QENS) techniqueis reported here. Analysis of QENS data revealed that observed dy-namics correspond to isotropic rotational motion of EG. Rotationaldiffusion coefficient of EG adsorbed in H-ZSM5 zeolite is found to beslower compared to bulk and comparable to that of benzene adsorbed inthe same host. Positron Annihilation Spectroscopy (PAS) technique isused to understand the nature of guest-host interaction and the resultsof both QENS and PAS are found to be consistent with each other.

1 Introduction

There have been reports in the literature on the effect of confinement on severalproperties of fluids like structure, diffusivity, freezing behaviour, etc. [1]. Diffusion ofliquid in restricted geometries is determined by a combination of two factors: geomet-rical constraint and guest-host interaction. The modification in dynamics of liquidsin confined geometry compared to its bulk state has attracted considerable interest inscientific research, not only for fundamental aspects but also for practical applicationsin many pertinent industrial problems, in petroleum industries, catalysis, water purifi-cation, building optical switches and so forth. Zeolites offer a confined system whichis used in wide range of cation exchange, catalytic and molecular sieving propertiesetc., elucidation of which requires an understanding of diffusivity of the adsorbedmolecules [2]. Diffusivity of guest molecules depend on many factors such as sizeand shape of the guest molecule, temperature, concentration, guest-host interactionsetc [3–5]. Diffusion of several hydrocarbons (acetylene, propylene, butadiene etc.) indifferent zeolites (e.g. ZSM5, Na-Y etc.) had been studied by us using quasi-elasticneutron scattering technique (QENS) and molecular dynamics (MD) simulation tech-niques [6–10]. QENS technique is used to study motions occurring at time scales inthe range 10−9–10−13 s and covering length scale in the range of angstrom (A) [11].EG(HO-CH2-CH2-OH) can form hydrogen bond with the oxygen of host zeolite andit will be interesting to study the effect of guest-host interaction on its dynamical

a e-mail: smitra@barc.gov.in

274 The European Physical Journal Special Topics

parameters. EG is used mainly as an antifreeze material and for the purpose of or-ganic syntheses. H-ZSM5 zeolite has two different kind of channels, straight as wellas sinusoidal (diameter ∼5.4 A) with intersections having diameter of 9 A. Structureas well as dynamics of EG have been studied extensively in the bulk phase usingneutron scattering and MD simulation techniques [12–14]. Here we report rotationaldynamics of ethylene glycol (EG) confined in H-ZSM5 zeolite as studied by QENS.The freezing/melting temperature of a liquid/solid inside the pore decreases from

the normal equilibrium temperature when the fluid-wall interaction is weakly attrac-tive/ repulsive. On the other hand, the transition temperature increases for attractivefluid-wall interaction. The utility of positron annihilation spectroscopy (PAS) as anin-situ and non-destructive technique is well documented in the study of phase tran-sition of confined molecules [15–19]. The ortho-Positronium pick-off annihilationrate with its fractional intensity and Doppler broadened line shape parameter (S-parameter) provide important probe for observation of phase transition in confinedgeometry. To find out the freezing behaviour of EG in H-ZSM5, PAS technique isused here.

2 Experiment

Commercially available H-ZSM5 zeolite (SiO2/Al2O3 = 30, surface area 380m2gm−1

and pore volume 0.22 cm3gm−1) and analytical grade ethylene glycol were usedfor the present study. Neutron scattering measurements were carried out using theQENS spectrometer at Dhruva reactor, Trombay [20]. For incident neutron energyof 5.1meV, this instrument provides an energy resolution of 200μeV and wave vec-

tor transfer (Q) range of 0.6–1.8 A−1. QENS measurements were carried out on the

dehydrated as well as EG adsorbed in H-ZSM5 zeolite at 300K. Saturation loadingwhich amounts to 0.24 gm of EG per gm of H-ZSM5 zeolite is used for the experi-ment. Data recorded for dehydrated H-ZSM5 zeolite was used to take into accountthe contribution of H-ZSM5 zeolite alone. Experimental details of PAS can be foundin Ref. [19].

3 Results and discussion

In a QENS experiment from a hydrogenous sample, the measured intensity is propor-tional to the incoherent scattering law and for localized motion, this can be writtenas [11],

S(Q,ω) = A(Q)δ(ω) + [1−A(Q)]L(ω,Γ) (1)

where the first term represent the elastic part and second term is the quasielasticpart. L(ω,Γ) is a Lorentzian function with half width at half maxima (HWHM) Γ.The fraction of elastic contribution to total spectra, A(Q), is called Elastic IncoherentStructure Factor (EISF). It is convenient to analyse the data in terms of EISF, whichprovides information about the geometry of the molecular motion. It may be notedthat if many dynamical processes simultaneously present in the system, effect ofinstrumental resolution on the EISF may be substantial since all processes that areslower than, tres = �/ΔE where ΔE is the instrumental resolution, will contribute toelastic scattering. However, EG, being a small molecule, is not expected to have anyrotational motion other than whole body reorientation which can contribute towardsEISF.In the present case, the QENS data from EG adsorbed in H-ZSM5 zeolite showed

significant quasielastic broadening whereas dehydrated H-ZSM5 showed no quasielas-tic broadening suggesting the presence of stochastic dynamics due to EG molecules

Progress in Dynamics in Confinement 275

-1.0 -0.5 0.0 0.5 1.00

500

1000

1500

2000

2500

S(Q

, ω)

(arb

.uni

t)

Energy Transfer (meV)

Q=1.08 Å-1

-1.0 -0.5 0.0 0.5 1.00

500

1000

1500

2000T=300 K Q=1.32 Å-1

-1.0 -0.5 0.0 0.5 1.00

200

400

600

800Q=1.66 Å-1

-1.0 -0.5 0.0 0.5 1.0

EG+ HZSM-5

DehydratedHZSM-5

S(Q

, ω)

(arb

.uni

t)

1.32 Å-1

Fig. 1. Fitted QENS spectra from EG adsorbed in H-ZSM5 zeolite at some typical Q values.Solid lines represent the fit while dash-dot and dotted lines represent quasielastic and elasticcomponents respectively. Instrument resolution is shown by dashed line in middle panel. Inset

shows the relative contribution of dehydrated H-ZSM5 in the total spectra at Q = 1.32 A−1.

0.0 0.5 1.0 1.5 2.0

0.0

0.2

0.4

0.6

0.8

1.0

CalculatedExperiment

EIS

F

Q(Å-1)

T=300 K

Fig. 2. Variation of EISF obtained in the QENS experiment.

only. Data from the dehydrated zeolite was subtracted from that of EG adsorbed inH-ZSM5 zeolite. Subtracted QENS data was separated out into elastic and quasielas-tic contributions by fitting the scattering law given in Eq. (1) after convoluting withthe instrumental resolution function, which is determined using a standard vanadiumsample. Typical data and fits are shown in Fig. 1.Analysis of QENS data showed a non-zero value of elastic incoherent structure

factor (EISF), which essentially indicates the presence of localized motion of EGinside the pores. Extracted EISF (Fig. 2) is compared with various models. It isfound that the isotropic rotational diffusion model [21] fits the EISF very well. Thecalculated variation of the EISF is shown in Fig. 2. Radius of gyration r is found tobe 1.9 A, which is equivalent to average distance of all hydrogen atoms from centerof mass of EG molecules.Although, the geometry of molecular motion can be established from the EISF

variation, obtaining the rotational diffusion coefficient involves the fitting of the wholespectra with the model scattering law. Least square fitting method is employed tofit QENS spectra with the model scattering law for isotropic rotation [21]. Fits arefound to be reasonably good which gives an extra support to the reliability of thismodel. Obtained rotational diffusion coefficient, DR = 16 ± 4μeV is compared withthat of bulk EG. It is found that the rotational diffusion coefficient of EG confined

276 The European Physical Journal Special Topics

240 250 260 270 2800.550

0.555

0.560

0.565

0.570

S-p

aram

eter

Temperature (K)

Cooling cycleHeating cycle

Fig. 3. Variation in the S parameter with temperature during cooling and heating cyclesfor ethylene glycol in H-ZSM5.

in H-ZSM5 is less compared to it’s bulk value (39μeV) at room temperature [14].Results can be compared with the dynamical parameters of other hydrocarbons suchas benzene and propylene adsorbed in H-ZSM5 zeolite. It is found that propyleneand EG both show isotropic rotation while six fold rotational jump motion was ob-served in case of benzene. Rotational diffusion coefficient of EG is almost one orderof magnitude less than propylene (370μeV) [7] and almost comparable to benzene(20μeV) [10] in H-ZSM5 in spite of the fact that EG is having a lower moment ofinertia and less symmetry than benzene. Therefore, main cause of hindered rotationmay be the strong attractive interaction between EG molecules and H-ZSM5 zeolite.This should reflect in freezing behavior of EG/benzene confined in H-ZSM5 zeolite, asfreezing temperature of fluids in confined media is strongly affected by the strengthof forces between fluid molecules and the pore walls. Temperature dependent PASstudy was carried out to probe the freezing behavior of EG and benzene confined inH-ZSM5 zeolite. The variation in Doppler broadened S-parameter with temperatureduring cooling and heating cycle for confined ethylene glycol is shown in Fig. 3. Themeasured S-parameter profile shows discontinuity at a temperature (270K) higherthan the bulk freezing point of EG (260K). Moreover, life time of o-Ps and its in-tensity also show discontinuity at 270 and 260K during the cooling cycle [19]. Thetransition at 270K may be attributed to the freezing of ethylene glycol confined inmicropores of H-ZSM5, which is 10K above the bulk freezing temperature. Such anelevation in freezing point of confined molecules signifies an attractive interactionbetween the fluid molecules and the pore-wall, which, in the present case is a con-sequence of the interfacial hydrogen bonding between ethylene glycol and H-ZSM5pore surface. Existence of hydrogen bonding between ethylene glycol and H-ZSM5pore surface is also confirmed from NMR results [19]. However, depression in freez-ing temperature was observed for benzene adsorbed in H-ZSM5, which correspondsto the weakly attractive or repulsive interaction between the benzene molecules andpore-surface [18]. Therefore, hindered rotational dynamics of EG is a consequence ofthe strong hydrogen bonding between ethylene glycol and zeolite surface. A combinedPAS and QENS study thus reveal important insight about the rotational motion ofmolecules adsorbed in zeolites.

4 Conclusion

Rotational dynamics of ethylene glycol (EG) confined in H-ZSM5 has been investi-gated using Quasielastic Neutron Scattering (QENS) technique. Comparison has been

Progress in Dynamics in Confinement 277

made between the dynamical parameters obtained with EG and benzene/propyleneadsorbed in H-ZSM5 zeolite. The nature of guest-host interaction is probed usingPositron Annihilation Spectroscopy (PAS) by observing the freezing/melting behav-iour of the guest molecules inside the host. It is found that the guest-host interactionhas a predominant role in determining the dynamical behaviour of adsorbed mole-cules. The results obtained from the two techniques, QENS and PAS, are thus foundto be consistent with each other.

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