786
J. Phys. Chem. C 2008, 112, 786-793
The Condensation Process of Alcohol Molecules in the Nanochannel of MCM-41: A Spin-Probe ESR Study Masaharu Okazaki,* Shinpei Iwamoto,† Yoshimi Sueishi,† and Kazumi Toriyama Research Institute of Instrumentation Frontier, National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98, Shimoshidami, Moriyama-ku, Nagoya, 463-8560, Japan, and Department of Chemistry, Faculty of Science, Okayama UniVersity, 3-1-1, Tsushima, Naka, Okayama 700-8530, Japan ReceiVed: September 6, 2007; In Final Form: October 9, 2007
The solution of di-tert-butylnitroxide (DTBN, a spin probe) in a low molecular weight alcohol was condensed in the MCM-41 nanochannel with changing its volume successively. When the solution volume is much smaller than that of nanochannel, the broad ESR spectrum of immobilized DTBN is observed. The spectrum becomes sharp abruptly when the solution volume exceeds a critical value that is necessary to cover the whole surface of the nanochannel as the monomolecular layer. With a further increase in the solution volume, little change is observed in the line width. From these observations a model is derived for the physicochemical state of alcohol: (1) When the dose is below the critical value, alcohol molecules are adsorbed on the surface sporadically as isolated molecules or as clusters and have strong interactions with the silica wall. (2) At the critical value the alcohol molecules form the monomolecular layer on the surface. They interact with each other by forming a network structure and become free from the nanochannel surface. (3) The DTBN molecule behaves as the solute of alcohol and the ESR spectrum reflects well the physicochemical state of alcohol. To support the above model, the oxygen gas effect on the ESR spectrum was observed and a geographical consideration for the alcohol molecules in the nanochannel is made. The collective molecular flow of alcohols through the nanochannel, which was reported in a previous paper, is discussed with the obtained model.
Introduction MCM-411 is a mesoporous silica with hexagonally stacked nanochannels2 and has been employed as a basic material of catalysts.3,4 The cylindrical nanochannels have a smooth inner wall and a constant radius in the longitudinal direction and serve as completely new space for the solution systems, which show unexpected unique behaviors.5-10 For example, a large magnetic field effect is observed in the reaction of an alcoholic solution of xanthone, a ketone, which is let flow continuously in a quartz column packed with MCM-41 and is irradiated by a UV laser.9-11 The radical pair model has been confirmed for the mechanism of this magnetic field effect.10 It was concluded that the solution mainly flows in the nanochannel12 and the reaction occurs there, since the magnetic field effect is larger when the MCM-41 with smaller channels is employed, and no appreciable magnetic field effect was detected when the column packed with silica gel is employed. The effect of pore diameter cannot arise in this system if the solution does not flow through the nanochannel. Poiseulle’s law predicts that the solution does not flow actually through the nanochannel at a pressure generated by a pump for liquid chromatography. In addition, the observed magnetic field effect indicates that the intermediate pair of radicals without charges must not diffuse away for a long time, say 10 µs, during the flow.9 The rapid solution flow without the translational diffusion of solutes is novel in the solution dynamics and must be important in chemistry.10 For example, unimolecular reactions must be conducted in the * Address correspondence to this author at the National Institute of Advanced Industrial Science and Technology. E-mail: masa-okazaki@ aist.go.jp. † Okayama University.
nanochannel with suppressing the intermolecular side reactions. Thus, (1) the alcohol molecules flow collectively to realize the long pairing time for the two radicals without chemical bonding. This inevitably means that, (2) the molecules flow with slipping on the surface of nanochannel.13 In a previous study, we have shown that the spin exchange between the nitroxide radicals is actually quenched but their rotation is rapid in the alcoholic solutions existing in the nanochanenl of MCM-41.14 Therefore, the translational diffusion of the solute spin probe is quenched but the rotational diffusion is not in the nanochannel of MCM41. This result supports (1). In the present study the ESR spectrum of the alcoholic solutions of di-tert-butylnitroxide (DTBN), a spin probe,15,16 is observed with changing the filling ratio of the nanochannels to obtain information on the interaction between the solution molecules and the surface of the nanochannel. This is important in understanding the interaction of the alcohol molecules with the nanochannel surface, which is closely related to the possibility of (2), the slip flow. DTBN was employed as the spin probe, since its vapor pressure is rather high and the condensation proceeds smoothly in the nanochannel accompanied by the solvent alcohol. A high concentration of 30 mM was chosen, because the solution condensed outside the nanochannel gives a very broad ESR spectrum and can be discriminated easily from the sharp signal due to the solution in the nanochannel.14 Two MCM-41s with different channel diameters, 3.0 and 3.8 nm, are employed to obtain information on the effect of channel diameter. As a result, the solute DTBN molecule makes a rapid rotation as soon as the monomolecular layer of solvent alcohol is formed. This observation indicates that the interaction between the alcohol molecules and the nanochannel surface becomes weak at this stage. The DTBN radical must be on the monomolecular layer of alcohol and its
10.1021/jp077154m CCC: $40.75 © 2008 American Chemical Society Published on Web 01/01/2008
Distribution of Solution Molecules in Nanochannel
J. Phys. Chem. C, Vol. 112, No. 3, 2008 787
rapid tumbling motion indicates the alcohol molecules are also dynamic enough on the nanochannel surface. So, the alcohol molecules may slip on the surface of the nanochannel. Observation on the effect of the oxygen gas on the ESR spectrum and consideration of the packing mode of the alcohol molecules in the nanochannel are made to support the above condensation model. Experimental Section Two MCM-41s with different pore radii were synthesized following the method in the literature,17,18 with dodecyl- and n-hexadecyltrimethylammoniumbromides as the surfactants. These MCM-41s are indicated as MCM-41(12)17 and MCM41(16),18 respectively. The pore radius, 3.0 and 3.8 nm, and the pore volume, 0.74 and 0.92 mL/g, respectively, were calculated from the X-ray diffraction data (d100: 3.1 and 3.7 nm, respectively), BET surface area (1000 and 980 m2/g, respectively), and the density of silica (2.25 g/cm3).14 The sample preparation was made as follows.8 A measured amount (about 100 mg) of MCM-41 powder was set in a glass vessel and dried at 403 K in vacuum for more than 1.0 h. The sample solution was injected through a septum with a microsyringe and degassed by the freeze and thaw method. The coagulated MCM41 powder was warmed and shaken vigorously to distribute the solution uniformly among the nanochannels and transferred to the ESR cell. In the case of the experiment to observe the oxygen gas effect, oxygen gas diluted by nitrogen gas to a partial pressure of 1/5 was introduced through the vacuum line.19 The quantity of O2 added to the system was estimated precisely (Supporting Information, I). ESR observation was made at 293 K with a JEOL RX-1 spectrometer (X-band) equipped with a temperature control unit at a modulation amplitude of 0.01 mT and a microwave power of 1.0 mW. The signal output was fed into a personal computer via an AD converter. DTBN was purchased from Aldrich chemicals and other chemicals including the solvent alcohols and those employed in the synthesis of MCM-41 were from Wako Pure Chemicals (Tokyo, Japan). Results And Discussion 1. ESR Spectra of the DTBN Solution of Alcohol Partially Filling the Nanochannel of MCM-41. In the present study the ESR spectra of DTBN dissolved in a few alcohols were observed to study their condensation process in the nanochannel of MCM41. Most of the experiments were done at a rather high concentration of 30 mM to discriminate the solution condensed outside the nanochannel from that condensed inside the nanochannel. At this concentration the ESR spectrum of DTBN is very much broadened outside the nanochannel by the spin exchange between radicals but remains sharp inside the nanochannel, where translational diffusion of the solutes is quenched.14 Figure 1 shows the ESR spectrum for the methanol solution of DTBN condensed in the nanochannel of MCM-41(12). The solution volumes are given in units of mL (10-6 m3) per 1.0 g of MCM41, and this value is called hereafter the “solution dose”. The left column shows the usual first derivative spectra and the right column shows the integrated ones.20 The latter is given to make the relative quantities of the several different components clearer on the spectrum. At the solution dose of 0.2 mL/g, the ESR spectrum in the left column is broadened as if the DTBN radical is in a very viscous solution, such as glycerol. At 0.3 mL/g the spectrum becomes much sharper and the DTBN molecule must rotate “rapidly”.21 The heights of both the MN ) +1 (low field) and -1 (high field) lines are considerably lower than that of the central line (MN ) 0), indicating that the rotation of the
Figure 1. ESR spectra for the methanol solution of DTBN at 30 mM condensed in the nanochannel of MCM-41(12) at various doses. Both the usual first derivative (A) and the integrated (B) spectra are shown. The latter spectra are normalized to give the same total area. Observations were made at 293 K without adding other gas molecules.
Figure 2. ESR spectra for the 2-propanol solution of DTBN at 30 mM condensed in the nanochannel of MCM-41(12) at various doses. Both the usual first derivative (A) and the integrated (B) spectra are shown. The latter spectra are normalized to give the same area. Observations were made at 293 K without adding other gas molecules.
molecule is still hindered in an anisotropic environment.11 At 0.4 mL/g the spectrum is sharpened a little more and this pattern is not changed until the maximum dose of the solution. At the dose of 0.7 mL/g the integrated spectrum in the right column contains a very broad component. Although the MCM-41 powder appears dry, the surface of the glassware becomes easily clouded when the sample is warmed by hands.22 Thus, we can ascribe this broad component to the DTBN solution existing outside the nanochannel. Therefore, the volume of the nanochannel is less than 0.7 mL/g. Figure 2 shows the results obtained for the system with 2-propanol as the solvent. Since the molecular volume (excluded volume) of 2-propanol is around 1.9 times of that of methanol, the effect of molecular volume should appear in the condensation process in the nanochannel whose diameter (ca. 3.0 nm) is only five times the molecular length of 2-propanol (ca. 0.6 nm). The ESR spectrum at the dose of 0.2 mL/g is much broader than the corresponding one in Figure 1 and the spectrum at 0.3 mL/g is still that of immobilized DTBN radical. At the dose of 0.4 mL/g the ESR pattern shows a rapid rotation of the DTBN molecule and is similar to that of the methanol system at 0.3 mL/g. A little broader spectra at 0.4-0.6 mL/g compared with those in Figure 1 must be due to a larger viscosity of 2-propanol than that of methanol.14 At 0.7 mL/g the broad component due to the bulk solution may be overlapped, since the height of the integrated spectrum is shorter by around 13% than that at 0.6 mL/g. These observations in Figures 1 and 2 indicate that the total volume of nanochannel of MCM-41(12) is about 0.6 mL/ g, which is 0.811 times as large as that determined by the simple method described in the Experimental Section. We employ the
788 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Okazaki et al.
Figure 4. ESR spectra for the methanol (A) and 2-propanol (B) solutions of DTBN at 30 mM condensed in the nanochannel of MCM41(16) at various doses. The spectra in each column have been normalized to the same doubly integrated values. Observations were made at 293 K without other gas molecules. The asterisk in each column indicates the spectrum of DTBN making rapid rotation at the lowest solution dose.
Figure 3. Line width (∆Hpp) (A) of the first derivative ESR spectrum and peak height (P.H.) (B) of the integrated ESR spectrum for the DTBN/MCM-41(12) systems in various solvents as functions of the solution dose. The error bars are given for more than two independent observations.
above value of 0.6 mL/g as the true volume of MCM-41(12) nanochannels for these alcohols.23 Figure 3 shows the peak-to-peak line width of the central line (∆Hpp; upper) in the first-derivative ESR spectrum and the peak height of the integrated ESR spectrum (P.H.; lower), whose total areas are normalized to the same value, as functions of the solution dose for the systems with MCM-41(12). In the present study both the line width and the line height were measured only for the central line. The data on the ethanol system are also included in the figure (Supporting information, II). The ESR line width for the methanol system becomes sharp suddenly at 0.3 mL/g upon increasing the solution dose. Since this change in the ESR spectrum appears like a critical phenomenon, we call these doses “critical dose”, hereafter. The critical dose becomes larger, 0.4 mL/g, for the 2-propanol system. The line width does not change much at a dose larger than the critical dose. The results for the ethanol system appear in the midst of those for the methanol and 2-propanol systems. The peak heights (P.H.) of the integrated ESR spectra also change systematically for the three systems. We measure the P.H. value because in a mixed system the existence of a very broad component can be detected as a decrease in the height of the normalized spectrum. In the methanol system the P.H. value becomes largest at around 0.35 mL/g. The decrease at a dose higher than the critical dose indicates that the spectrum is gradually broadened by the interaction between radicals. This may be due to the fact that the solvent viscosity decreases with a decrease in the S/V value, where S and V are the surface area and the volume of the solution, respectively. In the ethanol and 2-propanol systems the P.H. value becomes largest at around
0.4 and 0.6 mL/g, respectively. Since spin exchange in the last system is quenched rigorously in the nanochannel of MCM41(12), the signal height continues to increase until a part of the solution is condensed outside the nanochannel.14 Therefore, the clear peak at P.H. ) 0.6 mL/g for the 2-propanol system indicates that the volume of the nanochannel is very close to this value. Figure 4 shows the ESR spectra (derivative) for both the methanol (A) and 2-propanol (B) solutions of DTBN condensed in the nanochannel of MCM-41(16) at various volumes. The qualitative changes of the ESR spectrum upon increasing the solution volume appear similar to those in Figures 1 and 2. The critical doses at which the DTBN radical starts rapid rotation21 are 0.3 mL/g for the methanol system and 0.4 mL/g for the 2-propanol system. The line widths in these systems appear a little broader compared with that in the MCM-41(12) system. This is due to the larger interactions between the solute radicals in the wider nanochannel, where the collective nature of the solution molecules14 decreases and the averaged distance between the solutes decreases (distance ∝ 1/πR2, where R is the nanochannel radius). In addition, the very broad component due to the solution condensed outside of the nanochannel does not appear clearly even at the dose of 0.8 mL/ g for both systems. This is because of the larger pore volume of MCM41(16). Figure 5 shows the same type of plotting as those in Figure 3 for the spectra shown in Figure 4 and their integrated spectra (Supporting Information, III). The gradual increase in the line width and the simultaneous decrease in P.H. for the methanol system with an increase in the dose over 0.4 mL/g may be caused by a decrease in the viscosity of methanol at a smaller S/V (see above). In the 2-propanol system the same type of increase in line width is not observed below the dose of 0.6 mL/g. The peak height of the 2-propanol system reaches the top value at 0.7 mL/g and decreases a little at a dose of 0.8 mL/g, but the existence of the very broad component due to the solution condensed outside the nanochannel is not clear. Considering the above, we assume that the volume of the nanochannel of MCM-41(16) is 0.75 mL/g for alcohols hereafter. A smaller value may also be employed for the area of nanochannel surface in discussing the phenomena on alcohols. There may be some imperfections in these MCM-41 samples where the nitrogen gas molecules are adsorbed in the BET experiment but the alcohol molecules do not condense easily. For MCM-41(10), which is made by using decyltrimethylammonium bromide as the surfactant, the experimentally deter-
Distribution of Solution Molecules in Nanochannel
J. Phys. Chem. C, Vol. 112, No. 3, 2008 789
Figure 7. The condensation process for the methanol solution of DTBN in the nanochannel of MCM-41. Schematic models are shown for three different doses: upper, 0.2 mL/g; middle, 0.3 mL/g; and lower 0.4 mL/g. The ESR spectra are taken from Figure 1 for the MCM-41(12) system. The same model is applicable to the 2-propanol system, where each dose may be changed to 0.2-0.3, 0.4, and 0.5 mL/g, respectively. Figure 5. Line width (∆Hpp) (A) of the first derivative ESR spectrum and peak height (P.H.) (B) of the integrated ESR spectrum as functions of the solution dose for the DTBN/MCM-41(16) systems in methanol or in 2-propanol. Measured from Figures 4 and the integrated spectra. Error bars are obtained from at least two independent observations.
Figure 6. ESR spectra for the DTBN solutions of methanol (upper two) and 2-propanol (lower two) condensed at the dose of 0.3 mL/g in the nanochannel of MCM-41(16) (thick lines) or MCM-41(12) (thin lines). Observations were made at 293 K without adding other gas molecules.
mined value of 0.5 mL/g was reported, which is 0.794 times the calculated volume14,24 (Supporting Information, IV). 2. Model for the Condensation of Alcohols in the Nanochannel of MCM-41. Figure 6 compares the ESR spectra for the two solution systems, methanol (upper) and 2-propanol (lower), at the same dose of 0.3 mL/g for the two different mesoporous silica: thick line for MCM-41(16) and thin line for MCM-41(12). The ESR spectrum is very much dependent on the solvent but not so much on the diameter of the nanochannel or the total volume of the nanochannel. The shapes of the ESR spectra for the methanol systems are those for “rapid” rotation21 and the corresponding ones for the 2-propanol system are those for severely restricted rotation. This difference cannot be explained by the difference in the bulk parameters such as viscosities. Therefore, we have to consider that the difference in the ESR spectrum comes from an essential difference in the physical states of the alcohol molecules between these two systems. Our model is that 2-propanol molecules cannot cover the whole area of the nanochannel at this solution volume (0.3 mL/(g MCM41)) but methanol can. From the density and the molecular weights of these solvents the volume of one molecule (VM, excluded volume) is calculated: VM ) 0.0673 and 0.126 nm3 for methanol and 2-propanol, respectively.25 If we simply approximate the molecule as a sphere of diameter d, the exclude volume for
one molecule is V ) d3/x2 and the excluded area on a surface is expressed as S ) d2x3/2 in the closest packing model. We obtain the d values of 0.563 and 0.456 nm respectively for 2-propanol and methanol. The surface of MCM-41(12) (1000 m2/ g) is covered as the monomolecular layer with 0.372 mL/g of methanol or 0.426 mL/g of 2-propanol. These values are rather close to the critical values of 0.3 and 0.4 mL/g, respectively. As discussed in the previous section the pore volume may be about 19% less than the value calculated in the Experimental Section. So, the area of the nanochannel surface covered by the alcohol molecules may also be reduced considerably from the above value. If the same ratios are employed in calculating the area of surface on which the alcohols can adsorb,23 the values are modified as 0.30 and 0.35 mL/g, respectively,26 which make the above model more plausible. Therefore, we propose the following: (1) An essential change occurs in the rotational dynamic of the solute DTBN molecule at the “critical dose” mentioned in the previous section. (2) At this solution dose the nanochannel surface is covered completely as the monomolecular layer. (3) Since the spectral change at the critical dose is drastic and the additional change with further increase in the solution dose is gradual and small, the DTBN molecule is always accompanied by alcohol and works as a good probe for the solvent alcohol. This model is depicted as Figure 7 for the three steps of deposition. (A) When there are not enough solution molecules to complete the first molecular layer on the surface of nanochannel, the alcohol molecules are adsorbed on the silica surface individually or as independent clusters. Under these circumstances the spin probe molecule is also adsorbed on the surface and gives the signal of the immobilized radical. (B) When enough solution is added to the system to cover up the surface a little more than as a monomolecular layer, the interaction network among the alcohol molecules is made in a cooperative way. At the same time the H-bonding with the silica surface almost disappears. The DTBN molecule is solvated completely and starts “rapid tumbling motion”21 simultaneously. Since the rapid rotation is possible only if the alcohol molecules are not adhered rigidly on the silica surface, the monomolecular layer of alcohol is very dynamic. (C) When most of the channel space is filled with alcohol, the DTBN molecules are fully solvated and exist deep in the solvent in the nanochannel. In the figure the typical ESR spectrum of the methanol system with MCM-41(12) is shown beside the nanochannel model. In the case of 2-propanol solution
790 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Figure 8. ESR spectra of the methanol solution of DTBN under the environment of oxygen gas at various pressures: (A) a methanol solution of DTBN at 30 mM condensed in the nanochannel of MCM41(12) at the dose of 0.2 mL/g; (B) a methanol solution of DTBN at 3.0 mM in the bulk. The spectra in the same column are normalized to give the same doubly integrated values.
Figure 9. Integrated ESR spectra of the methanol solution of DTBN at 30 mM condensed in the nanochannel of MCM-41(12) at various doses: (A) 0.20 mL/g; (B) 0.30 mL/g; (C) 0.40 mL/g under the coexistence of the oxygen gas, whose pressure is given in units of kPa. The spectral area is normalized within the same column.
with the same MCM-41, the corresponding dose may be
