Molecular interactions on porous solids under magnetic field

J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger (Eds.) Characterization of Porous Solids I11 Studies in Surface Scicncc and Cahlysis, Vol. 87 0 1994 Elsevicr Scicncc B.V. All rights reserved.

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Molecular interactions on porous solids under magnetic field Sumio Ozeki,* Hiroyuki Uchiyama, Shinji Ono, Chihiro Wakai, Junichi Miyamoto and Katsumi Kaneko Department of Chemistry, Faculty of Science, Chiba University, 1-33Yayoi-cho, Chiba 260,Japan

Abstract The chemical and physical interactions of a paramagentic NO and a diamagnetic water with solid surfaces were enhanced and depressed by static magnetic field. The micropore filling of a supercritical NO onto microspaces of zeolites and pitch-based activated carbon fibers a t 303.2 K was enhanced by a 7.6 kG static magnetic field via magneto-micropore filling (MMF), for which preferential pore sizes were 0.5 and 1.0 nm in diameter or width. An NO dimer seems to be formed via a magnetic effect on a radical pair on a NO dimer in microspaces which fit mono- and bimolecular layers of NO dimer. Water on a carbon black, a pitch-based activated carbon fiber and a zeolite was attracted to the surfaces under a 9.6 kG magnetic field. Only water weakly interacting with the surfaces, such as water in multilayers and cluster and on hydrophobic surfaces, was apt to respond the magnetic field. The magnetic micropore filling of water in slit-like micropores of P-10 was depressed stepwise with an increase in amount of adsorption, as if the adsorption process of water into micropores may occur by multilayer adsorption. 1. INTRODUCTION The chemisorption of NO on metal oxides was enhanced and depressed by the external magnetic field [ll. The magnetoad- and magnetodesorption related intimately to porosity of solids and adsorption state (surface sites) rather than the magnetism of solid. Using activated carbons (AC) and activated carbon fibers (ACF), we examined an role of porosity and surface functional groups in magnetoadsorption 121. Super critical gases usually cannot be adsorbed by physisorption onto porous materials. However, a supercritical NO near room temperature can be adsorbed extraordinally much amount on ACs and ACFs [31.

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When NO molecules are adsorbed in micropores of ACFs and zeolites, NO dimers are formed in the micropores [31. In the previous papers [21, we reported that NO adsorption onto activated carbons are enhanced by magnetomicropore-filling (MMF) in slit-like micropores less than 1.1nm in width and MMF should relate close to micropores, i.e., formation of NO dimer. At the same time, however, MMF is also subject to amount of surface functional groups of carbons. Therefore, the role of micropore in MMF might be somewhat ambiguous. Zeolites have cylindrical micropores with no distribution of pore size, unlike activated carbons having slit-shaped micropores which will be accompanied with some pore size distribution. In addition, there are little functional groups on zeolites. Therefore, zeolites would be suitable for examination of pore effect on magnetomicropore-filling of NO, distinguishing from the effect of surface functional groups on it. Generally, water adsorbed on surfaces are immobilized in the first layer and approaches monotonously to bulk liquid water with amount of water adsorbed. Thus, adsorbed water experiences a variety of states which result in both solid-water and water-water interactions, as water adsorption progresses. Recently, we examined the magnetic effect on water adsorption on oxides as a function of amount of intensity of magnetic field and water adsorbed and found that only water in multilayer, not in first layer, responds to magnetic field [41. In this paper, we discuss the role of micropores in magnetomicropore-filling of paramagnetic NO and diamagnetic water for various zeolites and pitch-based activated carbon fibers (relatively free from surface functional groups) having different pore sizes. 2. EXPERIMENTAL

Samples used here, zeolites (MS3A, 4A, 5A, 13X, mordenite, TSZ-500 (Toso Co.)), pitch-based activated carbon fibers (PIT: P-10, 15, 25, and P-10-1173), and a nonporous carbon black (NPC) and a carbon black (PC) having slight amount of micropores, are relatively free from surface functional groups. P-10-1173 was prepared in order to remove surface functional groups by heating P-10 in air at 1173 Kfor 1h “21. N, adsorption on zeolites, which was pretreated at 573 K and 1mPa for 1h, was carried out at 77 K. The amount of NO and water adsorbed was determined volumetrically at 303.2 f 0.1 K, as described in the previous paper 111. The sensitivity of the adsorption measurement was about 1pg/(g-adsorbent). The changes of gas pressure were measured upon applying the static magnetic field of 7.6 kG for NO and 9.6 kG for water t o the gas-adsorbent systems which had been equilibrated for

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40 h or more. The homogeneities of the magnetic field at the position of the adsorption cell is within 2 %. The samples were pretreated at 1mPa for 1h prior to the adsorption experiments.

3. RESULTS The specific surface area, pore volume, and pore width of the samples were analyzed by the t-plot of N, adsorption isotherms at 77 K using the standard thickness for N, film on the graphitized nonporous carbon [21. Figure 1 illustrates changes of amount of NO adsorbed on various zeolites by application of 7.6 kG which was applied at zero of the time axis and removed at 60 min. A nonporous carbon black as a reference showed no MMF. Magnetoadsorption of NO onto the adsorbents occurred just after application ON

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Figure 1. Examples of the increment of adsorption amount of NO due to 7.6 kG static magnetic field which was applied with the permanent magnet to the quasi-adsorption equilibrium at 303.2 K and 15 Torr NO. The time at which the magnetic field is applied to (ON) and removed from (OFF) are 0 and 60 min. respectively. Adsorbents: A, MS5A, B, MS13X.

Figure 2. Magnetic-field-induced adsorption of NO at 303.2 K and 7.6 kG as a function of pore size (diameter for cylinder and width for lamellael of solids. Samples: 0 , zeolites (MS3A, MS4A, MS5A, mordenite, TSZ-500, MS13X);o,pitch-based activated carbon fibers (P10, P15, P25, P10-1173); 0, carbon blacks (PC, NPCl

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of the magnetic field, reached a constant value or a maximum value (Au)within a few minutes, and in the latter case exponentially decreases during application of the magnetic field. After removal of the magnetic field, NO was reversibly desorbed. The characteristic time for the exponential decrease was, e.g., 28 min for MS5A and 38 min for MS13X. Figure 2 shows the relationship between Au and pore size of the adsorbents. Here Au for the carbons are reduced by multiplying a factor 0.27 so that the point for P-10 may fall on the plot for zeolites in the figure. The pore sizes of the zeolites are effective size and those for the carbons were estimated from the t-plot analysis of N, adsorption isotherms [21. In the plot, two preferential pores for MMF appear markedly at 0.5 and 1.0 nm. The amount of water adsorbed on NPC, P10 and MS5A changed just after application of magnetic field, as in the case of NO, and reached to a constant value within a few minutes. A 9.6 kG magnetic field promoted water adsorption. The Au value for MS5A reached up to 5 ?4 of total amount of adsorbed water. When the magnetic field was removed, the adsorbed amount recovered the initial value. The magnetic effect appeared when water was adsorbed beyond 110 (0.3) mglg (Torr) on MS5A or at the apparent surface coverage 8 > 1(Figure 3). On the other hand, the amounts of water adsorbed on NPC and P10 increased by a 9.6 kG magnetic field even in the monolayer region (if water covers the surfaces which N, molecules access) (Figure 4). 4. DISCUSSION 4.1. Porosiry effect on MMF of NO Very weak interaction of NO with graphite 151 causes small adsorptivity of the nonporous carbon black (NPC) for NO. However, when a carbon black has micropores, such as PC, typically AC and ACF, adsorbs more NO. This dependence of NO adsorptivity on microporosity is parallel t o that in MMF 121. The relationship between Au due to 7.6 kG magnetic field and pore size (width of slitlike micropore) of ACs and ACFs (Fig.10 in Ref.3) suggests that there are two preferential pore regions, 0.7-0.85 nm and near 1.1 nm, which we did not point out previously [21. Since the discrete micropore size distribution of ACFs, e.g., 0.70-0.84 and 0.98-1.18 nm for cellulose-based ACF [61 may be smeared by contributions from surface functional groups, it is difficult to conclude what size of pores is most preferable for MMF of NO. In the current results (Figure 21, obtained by using adsorbents relatively free from surface functional groups, the preferential pore size for MMF appeared at 0.5 and 1.0 nm. Both zeolites and PITS seem to prefer 1.0-nm micropores for

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Figure 5. Magnetoadsorptivity of H,O as a function of total amount of water adsorbed on P-10 under 9.6 kG at 30 3.2K.

Figure 4, Magnetoadsorption of H,O as a function of total amount of water adsorbed on hydrophobic surfaces of NPC (upper) and P-10 (lower) under 9.6 KG at 303.2 K.

MMF, on the other hand, the preference of 0.5-nm pore appears only in zeolite systems. This may be only because activated carbon fibers (ACFs) have little micropores of less than 0.70 nm in width [61. Therefore, consulting with the results obtained here, we may deduce th at most preferential pores for MMF are 0.5 and 1.0 nm in diameter (width). The pore shape (cylinder and slit) effect on MMF may be expected but the experiments showed no desicive indication about it from the comparison of MMF between ACFs and zeolites, Kaneko and coworkers [31 found th at in the micropores of zeolites and ACFs

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some NO molecules are dimerized even above room temperature via the enhancement in the intermolecular interaction of NO, about 10 kJ/mol. A paramagnetic NO having a n unpaired electron forms a diamagnetic dimer (NO), in the condense phase at low temperature [71 and the adsorbed layers on a flat surface below 90 K [8]. MMF was promoted with decrease in pore size and particularly preference of 0.5 nm micropores in MMF agrees with that in micropore filling of NO L31. The dimers are most strongly stabilized in 0.5 nm micropores which just fits the geometry of the dimer (0.53 x 0.41 x 0.30 nm' for the trans 191 and cis 1101 forms). MMF was also enhanced in 1.0 nm micropores, although the micropore filling of NO was not specifically enhanced in 1nm pores of zeolites and ACFs [31. Since the excess stabilization energy of the dimer steeply decreases with pore size, it is inferred that magnetic field promotes the dimer formation in 0.5 nm and, especially, 1.0 nm micropores through a specific effect on adsorbate-adsorbent and/or adsorbate-adsorbate interactions, which were neglected in the micropore filling theories. The 1.0 nm micropores can just accept bilayers of NO dimer. Considering that MMF waa observed in zeolites having small channels and cages, short range interactions between NO molecules, rather than long range interactions as in cluster, are important for MMF. Most plausible species in the MMF process is (NO),. Since NO dimer has a boiling point in contrast with a supercritical NO, further micropore filling of NO may occur. The diamagnetic NO dimer has a weak chemical bond which arises from electron pairing between two NO (n2) molecules, but the coupling between two interacting NO molecules will be still weak and the unpaired electron will mainly be localized on each NO molecule ill]. Then, one may regard a n (NO), molecule as a two spin system. The ground state of (NO), is a singlet (S)H21. The radical pair theory successfully explains the kinetics and the production yield for radical reactions L131. 4.2. Porosity Effect o n Magnetoadsorption of Water Generally, water in first layers on solids is strongly adsorbed, e.g., by hydration around exchangeable cations and interations between water dipole and electric field of zeolite, while condensed water in pores and water in multilayers interact weakly with each other via hydrogen bonds. Thus, the small magnetic energy (cO.1 cm.') seems to affect only the weakly interacted water molecules in the multilayers, subject to the adsorption field. In fact, the magnetic response from water in the narrow pores of MS5A was weaker than that from the chrysotile and silica gel 141, while water which interacts very weakly with hydrophobic

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surfaces of NPC and P10 responded to magnetic field even in the first layer (Figure 4). Figure 5 shows magnetoadsorptivity of P10 for water as a function of total amount of water adsorbed on P10. There seems to be two steps on the curve:the onsets of the first and second decrease corresponds, reffering to the micropore size distribution, roughly to half-filled adsorption onto walls of the micropores with ca. 0.7 nm width and t o complete monolayer adsorption onto both walls of the micropores with 1.0 nm width, respectively, in which water molecules must be loosely adsorbed on the hydrophobic inner surfaces. Further adsorption of water into the half-filled 7 nm-pores and completely-covered 1.0 nm-pores give rise to hydrogen bonding between water molecules on both walls to make a kind of large cluster (a tightly-bonded two dimentional cluster). This may bring about steep reduction of magnetic responce of water adsorption and subsequent constancy. Since both H,O and all solids used are diamagnetic, water on the surfaces may be removed by free energy loss under magnetic field. On the contrary, the magnetic promotion of physical adsorption of water is a remarkable phenomenon. The apparent stabilization of the adsorbed water under magnetic field should be ascribed to a kind of magnetic transition or a structural change. The magnetoadsorption of NO seems to occur via NO dimer formation due to the singlet(S)-triplet(T) transition of a radical pair on a (NO), molecule. It is well known that a magnetic field affects the pardortho (p/o) conversion of H, on solids 114-161:a nuclear spin S / T transition via the interaction with paramagnetic center on surfaces [ E l . The mechanism for the both cases is quite analogous to each other [161. We presume that the magnetic effect may come up via a hypothetical p/o-water conversion due to extrinsic magnetic field. Then, the two nuclear spins in a water molecule must interact with each other and should experience the inhomogeneous dipolar magnetic field due to the paramagnetic spin to change their relative spin alignment via different precession 115,161. The p/o conversion requires energy exchange (-100 cm-') with the translational freedom during a collision with paramagnetic center [16,171. This seems consistent with the fact that the magnetic water adsorption onto the chrysotile at 9.6 kG was enhanced with increasing temperature, despite an exothermic adsorption process, and also that a calf thymus DNA, which have probably zero paramagnetic centers showed no magnetoadsorption at 9.6 kG. The ohter adsorbents here may have some paramagnetic centers on their surfaces 114,181. When the two nuclear spins in a water molecule are coupled via an oxygen atom, one might expect that the p/o conversion may propagate through hydrogen bond networks.

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The experiments suggest that water under adsorption field from solids may be so different from bulk water. as demonstrated by magnetic-field-induced adsorption.

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