4 September 1998

Ž .Chemical Physics Letters 293 1998 541–546

Molecular mechanism of capillary condensation of acetonitrilevapor on MCM-41 with the aid of a time-correlation function

analysis of IR spectroscopy

Hideki Tanaka a, Taku Iiyama a, Naofumi Uekawa a, Takaomi Suzuki a,Akihiko Matsumoto b, Michael Grun b, Klaus K. Unger b, Katsumi Kaneko a,)¨

a Physical Chemistry, Material Science, Graduate School of Natural Science and Technology, Chiba UniÕersity, Inage, Chiba, Japanb Institute for Ingorganic Chemistry and Analytical Chemistry, Johannes Gutenberg UniÕersity, Becherweg 24, D-55099 Mainz, Germany

Received 12 May 1998; in final form 9 July 1998

Abstract

Ž .The adsorption isotherm and IR spectra of acetonitrile adsorbed on MCM-41 pore-widths3.2 nm were measured at303 K. The adsorption isotherm had a sharp jump at PrP s0.3 without adsorption hysteresis. The CN stretching n -band0 2

of adsorbed acetonitrile had two-component a- and b-bands at 2263 and 2254 cmy1, respectively, assigned to hydrogen-bonded molecules on surface hydroxyls of MCM-41 and physisorbed molecules in mesopores whose walls are coated withhydrogen-bonded molecules, respectively. The b-band was analyzed with a time correlation function, giving a reorientationtime t . The t value of the adsorbed molecule before capillary condensation was smaller than that of the bulk liquid,indicating a gas-like state. After capillary condensation, t was the same as for the bulk liquid. q 1998 Elsevier Science B.V.All rights reserved.

1. Introduction

Physical adsorption is divided into multi-layeradsorption on flat surfaces or macropores, capillary

Žcondensation in mesopores 2 nm-pore-width-50. Žnm and micropore filling in micropores pore-width

. w x-2 nm 1,2 . Although capillary condensation hasbeen devoted to the determination of pore size distri-bution of mesopores, adsorption studies on regular

w xmesoporous silica such as MCM-41 3–5 or FSMw x6,7 pointed out that classical capillary condensationtheory cannot explain the dependence of the adsorp-

) Corresponding author. E-mail:kaneko@pchem2.s.chiba-u.ac.jp

tion hysteresis on the pore-width. Recently, it wasproposed that the dependence is associated with theintrinsic atomic structural roughness of the mesopore

w xsurface 8,9 . At the same time, we must examinewhether the condensed state in the mesopores isreally the same as the bulk liquid state from themolecular level. Thus, capillary condensation shouldbe reinvestigated with microscopic techniques usingthe model mesoporous system.

Conventional vibrational spectroscopy, which canmeasure the transitions between various quantumstates, has been helpful to determine the chemisorbed

w xspecies 10,11 . This conventional spectroscopy,however, is not effective for a complex system suchas the condensed phase, because there are so many

0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 98 00819-7

( )H. Tanaka et al.rChemical Physics Letters 293 1998 541–546542

transitions and the absorption lines blend together toform a continuous band. Accordingly vibrationalspectroscopy has not been sufficiently applied toelucidate the physisorbed molecular state. TheHeisenberg picture of quantum mechanics can pro-vide a powerful interpretive tool for spectra of acomplicated system. The Heisenberg picture of spec-troscopy leads to the consideration of a spectrum asthe Fourier transform of an appropriate time-correla-

w xtion function, giving the essential information 12,13 .IR spectroscopy using time-correlation function anal-ysis can provide the reorientational relaxation of

w xmolecules in the liquid phase 14,15 . Also the time-correlation function analysis is quite effective for

w xsimulation of IR bands 16 .The time-correlation analysis of IR spectroscopy

should be effective for the elucidation of confinedmolecular states in mesopores. An acetonitrilemolecule is a symmetric top molecule and has adipole moment of 3.92 D, which plays a key role in

w xthe reorientational relaxation 15 . The vibrationalfrequency of the CN stretching n -band is distant2

from those of MCM-41. The authors applied thetime-correlation function analysis of IR spectroscopyto determine the change in the reorientational state ofacetonitrile molecules upon capillary condensation.

2. Experimental

MCM-41 samples were prepared by JohannesGutenberg University. The detailed preparation pro-

w xcedure will be detailed elsewhere 17 . The porestructure of MCM-41 was determined by N adsorp-2

w xtion at 77 K 18 . The surface area, pore volume andpore width were 940 m2 gy1, 0.74 ml gy1 and 3.2nm, respectively. Acetonitrile of spectroscopic gradereagent was used. The adsorption isotherm of aceto-nitrile was measured at 303 K using the computer-

w xcontrolled volumetric apparatus 19 . The rotation–vibration spectra of acetonitrile adsorbed on MCM-41were measured at 303 K using an in situ IR cell withKRS-5 windows with the aid of a FT-IR spectrome-

Ž .ter JASCO FTrIR-550 . The IR spectrum was mea-sured with the summation of 256 consecutive scansand a resolution of 1 cmy1. The MCM-41 powderwas uniformly coated on the KBr disk under a

pressure of 5 MPa, which does not induce the de-w xstruction of pore structures 20 .

3. Results and discussion

3.1. Adsorption isotherm and FT-IR spectral change

Fig. 1 shows the adsorption isotherm of aceto-nitrile on MCM-41 at 303 K. The adsorption steeplyrises at PrP s0.3, indicating capillary condensa-0

tion in regular mesopores. In this case no adsorptionhysteresis is observed. The FT-IR spectra of aceto-nitrile-adsorbed MCM-41 were examined at 303 Kover 400 to 4600 cmy1 in the capillary condensationregion. The absorption band at 3744 cmy1, which isassigned to the OH stretching band of free surfacehydroxyls, disappeared even with a slight adsorptionof acetonitrile at PrP s0.05. A new strong band0

appeared at 3400 cmy1 in plane of the disappearing3744 cmy1 band, which is caused by adsorption ofan acetonitrile molecule with the hydrogen bond ofSi–OH PPP NCCH . The CN stretching n -band3 2

changed remarkably with the increase of PrP . The0

difference spectra of the rotation–vibration bands,due to adsorbed acetonitrile, were obtained by sub-traction of the spectrum of MCM-41 in vacuo.

Fig. 1. The adsorption isotherm of acetonitrile on MCM-41 at303K. Solid and open symbols denote adsorption and desorption,respectively.

( )H. Tanaka et al.rChemical Physics Letters 293 1998 541–546 543

We focused on the analysis of the CN stretchingn -band, which changes sensitively with adsorption2

and is not influenced by the absorption of MCM-41.Fig. 2 shows the change of the rotation–vibrationabsorption band of n at 303 K as a function of2

PrP of the acetonitrile vapor. Here the n qn -band0 3 4

at 2295 cmy1 is also shown in this region. Theabsorption band at 2263 cmy1 appears even at thebeginning of adsorption. The absorption band be-comes more asymmetric and broad with the increaseof PrP . This band can be assigned to an aceto-0

nitrile molecule adsorbed on a surface hydroxyl ofMCM-41 because of the above-mentioned change ofthe absorption band at 3744 cmy1 due to the freesurface hydroxyls. Above PrP s0.3, a new peak is0

Žexplicitly observed at a lower-frequency side 2254y1 .cm showing the presence of acetonitrile molecules

perturbed more weakly compared with those on sur-face hydroxyls. This band becomes predominant

Žabove the adsorption jump after capillary condensa-.tion . Also the band peak agrees completely with that

of the bulk liquid acetonitrile. The band shape isasymmetric even at PrP s0.05, indicating the0

presence of the minor component at 2254 cmy1.Hence there are two kinds of the molecular states ofacetonitrile, at least from the beginning of adsorp-

Fig. 2. Change of the CN stretching n band of acetonitrile2

molecules adsorbed on mesopores at 303K, as a function ofPrP . Here the n qn band is also shown.0 3 4

tion. The bands at 2263 and 2254 cmy1 are denoteda- and b-peaks, respectively in this Letter.

3.2. Change in molecular reorientation upon capil-lary condensation

The n -band has a- and b-peaks, which are at-2

tributed to molecules adsorbed on the surface hy-droxyl with hydrogen bonding and physisorbedmolecules in mesopores whose pore walls are coatedwith the hydrogen-bonded molecules, respectively.Both contributions change with the progress of ad-sorption. Hashimoto et al. showed the presence oftwo hot bands, which gives rise to the asymmetry ofthe band in the low-frequency side of the bulk liquid

Ž . w xn -band that is, the b-band in this Letter 15 . Then2

observed n -bands were deconvoluted using one2

Gaussian curve for the a-band and three Lorentziancurves for the b-band, as shown in Fig. 3. Weassumed the absence of the hot band for the a-band,because a-band of adsorbed CD CN, which can be3

almost completely separated from other band, wassymmetric according to the preliminary experiments.The fitting procedure including hot bands was simi-

w xlar to that used by Hashimoto et al. 15 . Fig. 3a andb shows the deconvoluted spectra at PrP s0.270

and 0.37, respectively, which correspond to beforeand after the capillary condensation. The a-band is

Žmuch higher than the b-band at PrP s0.27 before0.capillary condensation , while the b-band becomes

Žcomparable to the a-band at PrP s0.37 after0.capillary condensation . Then we determined pre-

cisely the changes in the intensity and half-bandwidth of the a- and b-bands using the deconvolutedspectra. We do not discuss the hot band here, be-cause the band is too weak to get sufficient informa-tion on the confined state. We must be cautiousabout the discussion on the change of the b-band inthe low-PrP region, because the deconvolution of0

the b-band below PrP s0.24 should have in-0

evitable uncertainties. The changes of the a- andb-bands are used for the discussion. Fig. 4a and bshows the changes of the half-band width and ab-sorbance of the a- and b-bands with PrP . The0

half-band width of the a-band is greater than that ofthe b-band. The half-band width of the a-bandincreases slightly until PrP s0.27 and becomes0

constant. However, the half-band width of the b-band

( )H. Tanaka et al.rChemical Physics Letters 293 1998 541–546544

decreases remarkably with PrP and the decreasing0

rate becomes small at PrP s0.32. These changes0

suggest that the molecular state transforms on capil-lary condensation. The absorbance of both of the a-and b-bands increases with PrP , while the rela-0

tionship between the absorbance of the b-band andPrP has a steep upward jump at PrP s0.3. This0 0

steep jump stems from capillary condensation,whereas other gradual increases of the absorbancecorrespond qualitatively to the adsorption increase.These results clearly show that the rotation–vibrationstates of the molecule adsorbed on the surface hy-droxyl and the condensed molecules are explicitlydifferent from each other.

Fig. 3. Changes in the a and b bands of the CN stretching n 2Ž . Ž .band before a and after b capillary condensation. Two weak

hot bands are shown around 2250 cmy1. Circles denote observeddata and solid curves are the spectra calculated from the summa-tion of the components.

Ž . Ž .Fig. 4. Changes in half-band width a and absorbance b of thea- and b-bands with adsorption. Open and solid symbols denotethe a- and b-bands, respectively.

The half-band width of the Lorentzian band pro-Ž .vides the relaxation time through Eq. 1 derived

from the time-correlation function theory.y1

ts 2pcDn , 1Ž .Ž .1r2

where t , Dn and c are the relaxation time, the1r2

half-band width and the velocity of light, respec-tively. As the a-band is Gaussian, we focus ourdiscussion only on the b-band. The change of therelaxation time of the b-band with adsorption givesthe information on the molecular rotational andtranslational states. The obtained t values for the

( )H. Tanaka et al.rChemical Physics Letters 293 1998 541–546 545

b-bands are summarized in Table 1. It is noteworthythat the t values of acetonitrile adsorbed belowPrP s0.27 are much smaller than those of aceto-0

nitrile adsorbed above PrP s0.32, which are the0

same as that of the bulk liquid near the boiling point.Accordingly, acetonitrile molecules adsorbed just be-low the steep rising of the adsorption isotherm, thatis, prior to the capillary condensation, behave freely,just like a two-dimensional gas. These moleculesshould collide with each other so often or reorientatequickly in the allowed two-dimensional space alongthe mesopore-walls, leading to the small relaxationtime. Above PrP s0.32, t values are the same as0

that of the bulk liquid at 303 K. Accordingly, it isconcluded that motional states of molecules con-densed in mesopores are the same as those of thebulk liquid.

Fig. 5 shows the model of the adsorbed states inmesopores. Before the capillary condensation,molecules adsorbed on surface hydroxyls are local-ized, whereas molecules adsorbed on these localizedones can freely reorientate andror migrate; thesemolecules behave like a two-dimensional gas. Theaverage intermolecular distance is estimated to bemore than 1.0 nm for these precondensed moleculesfrom the amount of adsorption, which is greater than

Ž .that of the bulk liquid 0.44 nm . Hence the above-mentioned two-dimensional gas could be realized.After the capillary condensation, the two-dimen-sional gaseous state disappears to give the liquid-likemolecular state. The transformation of the two-di-mensional gas to liquid occurs upon capillary con-densation even from the molecular level, althoughthe relaxation time change with PrP upon capil-0

lary condensation is not so critical as the amount of

Table 1Relaxation time of an acetonitrile molecule

State Temp. t Ref.Ž . Ž .K ps

adsorbed state below PrP s0.27 303 1.2–1.40

adsorbed state above PrP s0.32 303 1.6–1.70

liquid 298 1.7

w xliquid 298 1.8 15w x350 1.3 15w x251 2.4 15

Fig. 5. Model of adsorbed states of acetonitrile molecules inŽ . Ž .mesopores of MCM-41. a and b express the adsorbed states

just before and after capillary condensation, respectively. Openand solid circles denote a molecule adsorbed on the surfacehydroxyl and a physisorbed molecule without the hydrogen bond-ing, respectively. The bar in the solid circle indicates the molecu-lar axis, which is used for the expression of the reorientation.

adsorption. The relaxation of molecular reorientationdepends sensitively on the local nanoenvironmentaround a central molecule; the nanomolecular envi-ronment changes gradually and continuously belowthe capillary condensation and it settles in a new anduniform state after the capillary condensation. Thatis, molecules are preparing the transformation ofcapillary condensation.

We must examine the molecular mechanism oftransformation of the two-dimensional gas to capil-lary condensate for other systems. At the same time,a cooperative molecular simulation study is prefer-able to elucidate the capillary condensation from themolecular level.

( )H. Tanaka et al.rChemical Physics Letters 293 1998 541–546546

Acknowledgements

This work was funded by the Grant-in-Aid forScientific Research B from the Japanese Govern-ment.

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