AbstractAlthough a terminal oxyl species bound to certain metal ions is believed to be the intermediate for various oxidation reactions, such as O−O bond generation in photosystem II (PSII), such systems have not been characterized. Herein, we report a stable ZnII–oxyl species induced by an MFI-type zeolite lattice and its reversible reactivity with O2 at room temperature. Its intriguing characteristics were confirmed by in situ spectroscopic studies in combination with quantum-chemical calculations, namely analyses of the vibronic Franck–Condon progressions and the ESR signal features of both ZnII–oxyl and ZnII–ozonide species formed during this reversible process. Molecular orbital analyses revealed that the reversible reaction between a ZnII–oxyl species and an O2 molecule proceeds via a radical O–O coupling–decoupling mechanism; the unpaired electron of the oxyl species plays a pivotal role in the O−O bond generation process.

AbstractAlthough a terminal oxyl species bound to certain metal ions is believed to be the intermediate for various oxidation reactions, such as O−O bond generation in photosystem II (PSII), such systems have not been characterized. Herein, we report a stable ZnII–oxyl species induced by an MFI-type zeolite lattice and its reversible reactivity with O2 at room temperature. Its intriguing characteristics were confirmed by in situ spectroscopic studies in combination with quantum-chemical calculations, namely analyses of the vibronic Franck–Condon progressions and the ESR signal features of both ZnII–oxyl and ZnII–ozonide species formed during this reversible process. Molecular orbital analyses revealed that the reversible reaction between a ZnII–oxyl species and an O2 molecule proceeds via a radical O–O coupling–decoupling mechanism; the unpaired electron of the oxyl species plays a pivotal role in the O−O bond generation process.

Understanding the exact position and the detailed role of the Al array in zeolites is essential for elucidating the origin of unique properties that can be derived from the metal-ion exchanged in zeolite samples and for designing zeolite materials with high efficiency in catalytic and adsorption processes. In this work, we investigate, for the first time, the important role of the Al array in the reactivity observed on the metal-ion exchanged in zeolites on the basis of the calculation method by utilizing the spontaneous heterolytic cleavage of H2 observed experimentally on the Zn2+-ion exchanged in MFI-type zeolites (Zn2+-MFI) as the model reaction. In the case of calculation, two main types of models for considering the Al positions in MFI-type zeolites were adopted: in the first type, the Al atoms with appropriate distances are aligned in the circumferential direction of the straight channel (abbreviated as a circumferentially arrayed Al–Al site); in the second type, the nearest neighbouring Al atoms with appropriate distances are directed toward the straight channel axis (abbreviated as a channel directionally arrayed Al–Al site). Results indicated that the Al-array direction governs the reactivity of Zn2+-MFI. The former type of array well explains the experimental fact that spontaneous and irreversible heterolysis of H2 takes place on Zn2+-MFI, even at room temperature, whereas the latter type of array is less reactive; high activation energy is required for the heterolytic cleavage of H2 (ca. >70 kJ mol−1). A detailed analysis of the geometric and electronic structures of a series of Zn2+-MFI models with various Al-array directions clarified the following facts: the circumferentially arrayed Al–Al site induces an inevitable environment around the Zn2+ site, with the simultaneous existence of both a Lewis acid point (coordinatively unsaturated and distorted Zn2+) and a Lewis base point (the lattice oxygen atom juxtaposed with exchanged Zn2+, which participates in the activation of H2: OjL). It is the circumferentially arrayed Al–Al atoms that confer acidic and basic nature on the metal ion and the lattice oxygen atom (OjL), and ultimately trigger the heterolytic dissociation of H2, even at 300 K.

Understanding the exact position and the detailed role of the Al array in zeolites is essential for elucidating the origin of unique properties that can be derived from the metal-ion exchanged in zeolite samples and for designing zeolite materials with high efficiency in catalytic and adsorption processes. In this work, we investigate, for the first time, the important role of the Al array in the reactivity observed on the metal-ion exchanged in zeolites on the basis of the calculation method by utilizing the spontaneous heterolytic cleavage of H2 observed experimentally on the Zn2+-ion exchanged in MFI-type zeolites (Zn2+-MFI) as the model reaction. In the case of calculation, two main types of models for considering the Al positions in MFI-type zeolites were adopted: in the first type, the Al atoms with appropriate distances are aligned in the circumferential direction of the straight channel (abbreviated as a circumferentially arrayed Al–Al site); in the second type, the nearest neighbouring Al atoms with appropriate distances are directed toward the straight channel axis (abbreviated as a channel directionally arrayed Al–Al site). Results indicated that the Al-array direction governs the reactivity of Zn2+-MFI. The former type of array well explains the experimental fact that spontaneous and irreversible heterolysis of H2 takes place on Zn2+-MFI, even at room temperature, whereas the latter type of array is less reactive; high activation energy is required for the heterolytic cleavage of H2 (ca. >70 kJ mol−1). A detailed analysis of the geometric and electronic structures of a series of Zn2+-MFI models with various Al-array directions clarified the following facts: the circumferentially arrayed Al–Al site induces an inevitable environment around the Zn2+ site, with the simultaneous existence of both a Lewis acid point (coordinatively unsaturated and distorted Zn2+) and a Lewis base point (the lattice oxygen atom juxtaposed with exchanged Zn2+, which participates in the activation of H2: OjL). It is the circumferentially arrayed Al–Al atoms that confer acidic and basic nature on the metal ion and the lattice oxygen atom (OjL), and ultimately trigger the heterolytic dissociation of H2, even at 300 K.

A nitrogen-doped TiO2 sample was prepared at 413 K by direct hydrothermal treatment of titanium isopropoxide in an aqueous solution of NH3. This new material has a large specific surface area of ca. 220 m2 g−1 because of its tubular structure and it exhibits a prominent absorption feature in the region between 400 and 650 nm. It responds strongly to light in the visible region, which is key to its potential performance as a photocatalyst that may improve the efficiency for utilization of solar energy. Actually, this sample exhibits very efficient activity in the decomposition of CH3COOH under visible light among the samples prepared. This effective photocatalysis of the present sample was substantiated by characteristic spectroscopic features, such as: (1) an optical absorption band with λ > 400 nm because of the doped nitrogen species; (2) the formation of EPR-active, long-lived N˙ and O2– species, as well as N2– species, under visible-light irradiation in the O2 or N2 adsorption process at 300 K by way of the monovalent nitrogen ions in the bulk (both substitutional and interstitial); (3) the existence of IR-active O2 species adsorbed on the nitrogen-doped TiO2 sample even without light irradiation; and (4) an XPS N1s band around 399.6 eV that is assignable to the N− species. The amounts of N˙ and O2− species formed in the nitrogen-doped TiO2 sample under visible-light irradiation correlated well with the levels of reactivity observed in the decomposition of CH3COOH on the samples with varying amounts and types of doped nitrogen species. We conclude that the photoactive N˙ and O2– species created in the present sample are responsible for the decomposition of organic materials assisted by visible light irradiation. These features may be attributable to the interface between the sample's tubular structure and anatase with poor crystallinity, which probably causes the resistance to the recombination of electron–hole pairs formed by irradiation.

Carbon dioxide (CO2) gas is well-known as a greenhouse gas that leads to global warming. Many efforts have been made to capture CO2 from coal-fired power plants, as well as to reduce the amounts of excess CO2 in the atmosphere to around 400 ppm. However, this is not a simple task, particularly in the lower pressure region than 1000 ppm. This is because the CO2 molecule is chemically stable and has a relatively low reactivity. In the present study, the CO2 adsorption at room temperature on MFI-type zeolites exchanged with alkaline-earth-metal ions, with focus on CO2 concentrations <1000 ppm, was investigated both experimentally and by calculation. These materials exhibited a particularly efficient adsorption capability for CO2, compared with other presented samples, such as the sodium-form and transition-metal ion-exchanged MFI-type zeolites. Ethyne (C2H2) was used as a probe molecule. Analyses were carried out with IR spectroscopy and X-ray absorption, and provided significant information regarding the presence of the M2+–O2––M2+ (M2+: alkaline-earth-metal ion) species formed in the samples. It was subsequently determined that this species acts as a highly efficient site for CO2 adsorption at room temperature under very low pressure, compared to a single M2+ species. A further advantage is that this material can be easily regenerated by a treatment, e.g., through the application of the temperature swing adsorption process, at relatively low temperatures (300–473 K).Keywords: acetylide species; BaMFI; CO2 adsorption at 300 K; easy regeneration of the sample; fast adsorption; M2+−O2−−M2+ (M: alkaline-earth-metal ion)

Compared with mercury, the existence of [Zn2]2+ species is rare. We succeeded in preparing a stable [Zn2]2+ species by utilizing an MFI-type zeolite as a nano-reaction pot, which was confirmed using XAFS spectroscopy: the bands at R = 2.35 Å due to the Zn+–Zn+ scattering and at 9660.7 eV due to the 1s–σ* (the anti-bonding orbital comprised of the 4s–4s orbital) transition of the [Zn2]2+ species. This species also gives the characteristic band around 42000 cm−1 due to its σ–σ* transition. Furthermore, UV-irradiation corresponding to the σ–σ* transition causes the bond dissociation, forming two unprecedented Zn+ ions, and detached Zn+ ions were recombined through heat-treatment at 573 K: [Zn+–Zn+] ⇄ 2Zn+. These processes were reproduced by applying the DFT calculation method to the assumed triplet, σ(α)–σ*(α), structure formed on the M7-S2 site with the specific Al array in the MFI-type zeolite. Research into the specific field using zeolites to synthesize “ultra-state ions” is very promising.

We present clear IR and density functional theory (DFT) evidence demonstrating that the electron-accepting nature of Zn2+ ion exchanged in MFI-type zeolite (ZnMFI) plays a dominant role in CH4 activation. The IR study revealed that the heterolytic dissociation of CH4 takes place on the Zn2+ ion exchanged in MFI under a CH4 atmosphere even near room temperature, whereas a similar reaction scarcely occurred on Mg2+ ion exchanged in MFI, although the ionic radius and charge of Mg2+ are almost the same as those of Zn2+. These data indicate that the dissociation reaction of CH4 on Zn2+ in MFI is facilitated not only by the electrostatic interaction but also by the electron-transfer interaction. This interpretation was clearly evidenced by the observed v1 mode of the C–H symmetric stretching vibration, i.e., a larger band shift toward lower wavenumbers, for the molecular CH4 adsorbed on ZnMFI, compared with those for a gaseous CH4 molecule. Additional experiments were also performed by the IR method utilizing CO as a probe molecule that has an electron-donating nature. All experimental data presented were successfully explained in terms of the superior electron-accepting nature of Zn2+ exchanged in MFI. Furthermore, the DFT calculation method completely explained all experimental data by adopting the M7S2 model, which was truncated from the ZnMFI structure; the electron-accepting nature is dominant in the heterolytic activation of CH4 in the Zn2+ ion in MFI in comparison with that of Mg2+ exchanged at the same site. We have thus shown that the electron-transfer interaction between Zn2+ and CH4 plays a key role in the heterolytic CH4 activation process: the σ donation from the σ(C–H) orbital of CH4 toward the Zn 4s orbital through overlapping with each orbital.

For the first time, the paramagnetic Zn+ species was prepared successfully by the excitation with ultraviolet light in the region ascribed to the absorption band resulting from the 4s–4p transition of an atomic Zn0 species encapsulated in an MFI-type zeolite. The formed species gives a specific electron spin resonance band at g = 1.998 and also peculiar absorption bands around 38,000 and 32,500 cm–1 which originate from 4s–4p transitions due to the Zn+ species with paramagnetic nature that is formed in MFI. The transformation process (Zn0 → Zn+) was explained by considering the mechanism via the excited triplet state (3P) caused by the intersystem crossing from the excited singlet state (1P) produced through the excitation of the 4s–4p transition of an atomic Zn0 species grafted in MFI by UV light. The transformation process was well reproduced with the aid of a density functional theory calculation. The thus-formed Zn+ species which has the doublet spin state exhibits characteristic reaction nature at room temperature for an O2 molecule having a triplet spin state in the ground state, forming an η1 type of Zn2+–O2– species. These features clearly indicate the peculiar reactivity of Zn+ in MFI, whereas Zn0–(H+)2MFI hardly reacts with O2 at room temperature. The bonding nature of [Zn2+–O2–] species was also evidenced by ESR measurements and was also discussed on the basis of the results obtained through DFT calculations.

In this work, we used both experimental and density functional theory (DFT) calculation methods to examine the mechanism of CH4 activation taking place on the Zn2+ ion exchanged MFI-type zeolite (ZnMFI). The heterolytic dissociation of CH4 on ZnMFI around 300 K was observed experimentally, causing the appearance of IR bands at 3615, 2930, and 2892 cm–1. The first band can be assigned to the OH stretching vibration associated with the formation of the Brønsted acid site and the latter to the C–H stretching modes ascribable to the −[ZnCH3]+ species. Combining the IR spectroscopy with a DFT calculation, it is apparent that the heterolytic C–H bond dissociation of CH4 has an activation energy of 15 kJ mol–1 and takes place on a monomeric Zn2+ at the M7S2 site. The M7S2 site has a specific Al arrangement in MFI and exhibits a pronounced reactivity for the H–H bond cleavage of H2, even at room temperature. In addition, to our knowledge, we are the first to succeed in explaining the dissociation process of CH4 by applying natural bond orbital (NBO) and interaction localized orbital (ILO) analyses to the present system; the donation interaction from the CH4–σ(C–H) orbital to the Zn–4s orbital triggers the cleavage of the C–H bond of CH4 under mild conditions.

We have recently clarified the following point: a dual-type site, which is composed of a pair of monovalent copper ions (Cu+) formed in a copper-ion-exchanged MFI-type zeolite (CuMFI), functions as the active center for strong ethane (C2H6) adsorption even at room temperature rather than a single-type site composed of a Cu+ ion. However, the character of the dual-Cu+ site in a CuMFI is not yet fully understood. In this study, we have elucidated the nature of the active sites for C2H6 based on infrared (IR) and calorimetric data. On the basis of the results obtained, we came to the conclusion that the dual-Cu+ site composed of Cu+ ions giving the adsorption energy of 100 kJ mol–1 and the absorption band at 2151 cm–1 for carbon monoxide (used as a probe molecule) at room temperature functions as an adsorption site for C2H6. We also evaluated, for the first time, the interaction between the dual-Cu+ site and C2H6 energetically, by the direct measurement of heat of adsorption. The value of 67 kJ mol–1 that we recorded was higher than that for the single-Cu+ site in this sample and also for other samples, such as NaMFI and HMFI.

The catalytic activity of nickel ion-loaded mesoporous silica MCM-41 (Ni-M41) for ethene dimerization was investigated as a function of the pore size and the amount of nickel. In addition, the silica wall and the loading of the nickel species were characterized. The Ni-M41 samples with smaller pore size and higher Si/Ni ratio exhibited greater reaction rate constants. The Fourier transform infrared (FT-IR) spectra indicated the formation of 2:1 nickel phyllosilicate-like species along the pore wall. Furthermore, the IR band at approximately 570 cm–1 and the X-ray absorption fine structure (XAFS) spectra indicated the existence of five-membered rings consisting of Si–O on the M41 pore wall in addition to the typical six-membered ones. On the basis of the UV–vis–NIR diffuse reflectance (UV–vis–NIR DR), FT-IR, and XAFS data, we propose that the three- and four-coordinated Ni2+ ions lie on the five- and six-membered Si–O rings of silica, respectively. Nitrogen monoxide was employed as a probe molecule in the FT-IR and UV–vis–NIR DR experiments and revealed that NO adsorbed as di- and mononitrosyl species on the three- and four-coordinated Ni2+ ions. The intensity of the dinitrosyl species on the three-coordinated Ni2+ ions correlated with the catalytic activity for ethene dimerization. Therefore, the three-coordinated Ni2+ ions are proposed to act as the active site for the reaction.

The role of dual-cation sites in zeolites has received a renaissance in chemistry and industry directed toward fixation and activation of various gases; such sites may be expected to be more efficient than a single-cation site. We aimed to clarify the real active centers in the copper-ion-exchanged MFI-type zeolite (CuMFI) for ethane (C2H6). A peculiar feature was found in the appearance of the characteristic IR bands at 2644 and 2582 cm–1 when C2H6 was adsorbed on Cu+ formed in CuMFI. The existence of dual species composed of two Cu+ ions bridging C2H6 was clearly indicated by extended X-ray absorption fine structure (EXAFS) data. Density functional theory calculations gave clear evidence that the two IR bands are distinctly due to C2H6 adsorbed on the dual-Cu+ site and not on a single site; this agrees with the EXAFS data. These data lead us to conclude that the dual-Cu+ site in the CuMFI sample is indispensable for efficient activation of C2H6 through the simultaneous interaction of C2H6 with two Cu+ ions.

We succeeded in achieving visible-light responsiveness on a tubular TiO2 sample through the treatment of a tubular TiO2 that has a large surface area with an aqueous solution of ammonia or triethylamine at room temperature and subsequent calcination at 623 K, which produced a nitrided tubular TiO2 sample. It was found that the ease of nitridation is dependent on the surface states; washing the tubular TiO2 sample with an aqueous acidic solution is very effective and indispensable. This treatment causes the appearance of acidic sites on the tubular TiO2, which was proved by the following experiments: NH3 temperature-programmed desorption and two types of organic reactions exploiting the acid properties. The prepared samples, TiO2−δNδ, efficiently absorb light in the visible region, and they exhibit a prominent feature for the decomposition of methylene blue in an aqueous solution at 300 K under irradiation with visible light, indicating the achievement of visible-light responsiveness on the tubular TiO2 sample. This type of tubular TiO2−δNδ sample has merit in the sense that it has a large surface area and a characteristic high transparency for enabling photocatalytic reactions because it has a tubular structure and is composed of thin walls.

The interaction of ethyne (C2H2), as well as of carbon dioxide (CO2), with copper–ion-exchanged MFI zeolite (CuMFI) at room temperature was examined. It was found that CuMFI preferentially adsorbs C2H2, while this material does not respond to CO2. To clarify the specificity of CuMFI, a combination of various experimental techniques and theoretical calculations was adopted. Distinctive interaction energies of 140 and 110 kJ mol−1 were clearly observed at the initial stage of C2H2 adsorption on CuMFI, suggesting the presence of two types of adsorbed C2H2. Two distinct IR bands at 1620 and 1814 cm−1 appeared, which were assigned to the CC stretching vibration modes of C2H2 differing in their adsorbed state. Both photoluminescence and X-ray absorption spectra showed that cuprous ions (Cu+) in CuMFI act as efficient sites for a marked C2H2 adsorption. From the analysis of the latter spectra and the calculational results based on the density functional theory, the formation of dual Cu+⋯(C2H2)⋯Cu+ complexes was indicated for the first time for CuMFI, and such a special configuration of the Cu+ sites contributed to the extremely strong adsorption of C2H2. In contrast, it was necessary for the linear CO2 molecule to take a bent structure to be adsorbed on Cu+ in CuMFI. It was concluded that the difference in the adsorption response of Cu+ in CuMFI towards C2H2 and CO2 is due to the chemistry between the nature of electron donation of Cu+ and the hybrid orbitals of the respective molecules. This work promotes further understanding of the states of active centres in CuMFI for C2H2 activation, as well as for N2 fixation.

The interacted species of Xe with metal ions that are stable at room temperature are not known and are a subject of interest for fundamental chemistry. We have experimentally found a new and stable Xe species, XeCu+, which was formed at room temperature in a copper ion-exchanged MFI-type zeolite. The presence of a prominent interaction between Cu+ in MFI and Xe, which has a covalent nature, was for the first time evidenced from experimental in situ synchrotron X-ray absorption fine structure and heat of adsorption measurements: the Cu+−Xe bond length of 2.45 Å and the bonding energy of ca. 60 kJ mol−1. The bonding nature between Xe and Cu+ in the MFI zeolite was discussed utilizing density functional theory; the observed significant stabilization comes from the d(Cu+ in MFI)−p(Xe) orbital interaction. These new findings may pave a new way to developing fundamental chemistry of Xe compounds.Keywords (keywords): 129Xe NMR; adsorption of Xe; bond formation between Xe and Cu+ at 300 K; copper-ion-exchanged MFI-type zeolite; DFT calculation method; inorganic chemistry on the Xe−Cu+ species; synchrotron XAFS measurement;

A peculiar N2 adsorption was found on a copper-ion-exchanged MFI-type zeolite (CuMFI); the N2 adsorption was established within 20 s at 300 K. Related to this fact, the bond dissociation energy of N≡N in a stable Cu+−N≡N species in CuMFI was, for the first time, evaluated to be 9.11 eV from the characteristic bands at 2295, 2654, and 4553 cm−1, which correspond to the fundamental, combination, and overtone vibrations of N≡N adsorbed on Cu+ of CuMFI, respectively. The vibrational frequency of Cu+−N in the Cu+−N≡N formed in CuMFI was also determined to be ∼360 cm−1, together with the energy for the formation of a Cu+−N bond; the Cu+−N≡N species is stable enough to maintain a N2 molecule on MFI at 300 K. DFT calculations reasonably explain the experimental data and also the N2 adsorption model based on the three-coordinate Cu+ site in CuMFI.Keywords (keywords): bond dissociation energy; copper-ion-exchanged MFI zeolite; DFT calculation; diffuse reflectance near-infrared spectra; infrared spectra; N2 adsorbent; N2 fixation;

A silver-ion-exchanged HZSM-5 zeolite sample (Ag(H)ZSM-5) evacuated at 573 K exhibited prominent catalytic behavior in the partial oxidation of CH4 at temperatures above 573 K, exceeding the performance of Ag/SiO2⋅Al2O3 and Ag/SiO2 catalysts. From the infrared (IR) and X-ray absorption fine structure (XAFS) spectra, as well as the dioxygen adsorption measurement, it was concluded that the simultaneous existence of Ag+ ions and small clusters of Ag particles leads to the partial oxidation of methane. Taking the magnitude of the formation enthalpy (per oxygen atom) of Ag2O (ΔH=26 kJ/molΔH=26 kJ/mol) into consideration, we propose the interpretation that the dioxygen activated on small Ag metal clusters formed in ZSM-5 elaborates a surface oxide layer on small Ag clusters and the thus-formed species is simultaneously and easily decomposed at 573 K or above, and the oxygen activated in this way on the Ag metal spills over and can react with methane that has been activated by the Ag+ ions exchanged in ZSM-5, resulting in the high catalytic activity of the Ag(H)ZSM-5 sample in the partial oxidation of methane. This interpretation is also well evidenced by XAFS and IR data. It is anticipated that this material has the potential to be a promising catalyst in the conversion of natural gas into higher value-added chemicals and fuels.AgZSM-5 exhibited prominent catalytic behavior in the partial oxidation of CH4 to CO and H2 above 573 K and has a potential for acting as the catalyst for the conversion of abundant gases into valuable chemicals.

For alkali-metal ion-exchanged ZSM-5 zeolites (MZSM-5; M: Li, Na, K, Rb, Cs) the analysis of ion-exchangeable sites was performed by means of a combined method based on IR spectroscopic and calorimetric measurements using CO as the probe molecule. The heat of adsorption of CO was found to be correlated with an IR frequency of stretching vibration of C–O in the adsorbed species. It was revealed that there exists at least two types of sites capable of ion-exchanging; for the lithium ion-exchanged ZSM-5 (LiZSM-5) CO adsorption on each type of site is evaluated to give a set of IR bands and heats of adsorption, 2195 cm−1 and 49 kJ mol−1, 2185 cm−1 and 39 kJ mol−1 with the aid of the newly developed method utilizing the data obtained from a combined microcalorimetric and IR-spectroscopic study. Such types of data were also obtained for Na- and K-ion-exchanged ZSM-5 samples. Furthermore, a linear relationship between the differential heat of adsorption (qdiff) evaluated and the shift of wavenumber of the C–O stretching vibration from that of a gaseous CO molecule (Δν) was established for the systems of MZSM-5–CO, and the bonding nature of the CO molecule with each site can be explained in terms of the electrostatic force. The model of each adsorption site was also examined by the quantum calculation method (density functional theory: DFT). The trends obtained from the experimental data may be substantially supported by the calculation method even adopting a model as simple as the ZSM-5-type zeolite: the composition of MAlSi4O4H12.

We have found that a copper ion-exchanged MFI (Mobil Five)-type zeolite, having an ion exchange capacity of 112% (CuMFI-112), exhibits a specific interaction with CH4, even at room temperature. The observed properties are very remarkable compared with those found in the adsorbed amount of sodium ion-exchanged MFI and in the interaction with the sodium ion in MFI as well as in the adsorbed amounts for other types of copper ion-exchanged zeolites. The specificity of Cu+ in MFI is clearly evident. The most remarkable feature of the present system is the distinct appearance of infrared bands (IR) at 2661 and 2619 cm−1 due to adsorbed CH4 at 300 K and the change in the X-ray absorption fine structure spectra of CuMFI before and after CH4 adsorption at 300 K, indicating the existence of substantial and specific interaction in this system. In addition, calculations based on the density functional theory were carried out by utilizing a Cu−AlSi91O151H66 cluster model, which is made up of a 10-membered ring of CuMFI. As a result, the calculations gave absorption bands that can be explained reasonably by the observed novel IR bands, indicating the existence of two types of η2(H, H)-type interactions between Cu+ and CH4: (1) hydrogen interacted with a Cu cation coordinating to two oxygen atoms near an intersection of straight and sinusoidal channels, and (2) hydrogen interacted with a Cu cation located above a 5-membered ring on a wall of a straight channel. The nature of such bonding can be described by the formation of Cu−C bonds, which causes the deformation the Td structure of CH4. Such interaction results in a weakening of the C−H bond and thus a lower frequency shift of the C−H bond interacting with Cu+.

Three different approaches have been used to characterize the state of exchanged copper ions in copper-ion-exchanged MFI (CuMFI) samples. (1) Two types of an ion-exchangeable site with different adsorption properties for N2 or CO molecules were identified depending on the pre-treatment temperature (723 or 873 K) of a sample prepared by using an aqueous solution of CuCl2. (2) The state of the active sites formed by the evacuation of a sample at 873 K that had been prepared using a mixture solution of aqueous NH4CH3COO and Cu(CH3COO)2 was analysed utilizing both 13C18O and 12C16O to identify the two types of active adsorption sites for CO molecules. (3) CuMFI samples prepared by the ion-exchange method employing anhydrous CuCH3COO showed a surprising adsorption feature characterized by a single IR band occurring at 2159 cm−1 due to the adsorbed CO molecules, but there was no corresponding IR band due to adsorbed N2 molecules. A successful preparation of CuMFI, in which the monovalent copper ions exclusively occupied another one of the two types of ion-exchangeable sites, was also carried out utilizing the solid-ion exchange method using Cu(CH3COO)2·H2O. This site exhibits an IR band occurring at 2151 cm−1 for CO molecules and also acts as an active site for N2 molecules. These experimental data correlate, and clearly indicate that there are at least two types of exchangeable sites for copper ions in MFI-type zeolites.

Direct ion-exchange of monovalent copper ions into a ZSM-5-type zeolite was carried out using an aqueous solution of diammine–copper(I) ions, [Cu(NH3)2]+, to prepare a copper ion-exchanged ZSM-5 zeolite including only monovalent copper ions, and the effect of monovalent copper ions on the zeolite's adsorption properties for dinitrogen (N2) was examined. Strangely enough, the reoxidation of monovalent copper-ion exchanged in ZSM-5 took place in the evacuation process at around 473 K. The changes in valence and structure of the exchanged copper-ions during the evacuation process and the interaction with N2 molecules at room temperature have been investigated by using spectroscopic techniques such as X-ray absorption fine structure (XAFS), IR and photoemission spectroscopy, as well as by measurements of adsorption isotherms and adsorption heats. On the basis of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses, it has become apparent that the Cu+ species exchanged in ZSM-5 zeolite is oxidized by water to form a divalent species having a CuO-like structure through heat-treatment in vacuo at 473 K. Further heat-treatment at temperatures above 673 K caused a reduction of the divalent species to a monovalent one that exhibits a pronounced adsorption feature for N2 even at room temperature. XANES and photoemission data clearly indicated that the Cu+ species in an 873 K-treated CuZSM-5 sample has a three-coordinate structure with lattice oxygen atoms and interacts strongly with an N2 molecule at room temperature. The strong interaction with N2 was also verified through the adsorption heat and IR data: an initial adsorption energy of 85 kJ mol−1 and an absorption band at 2295 cm−1. A prominent feature of this system is that some of the adsorbed species survives after evacuation at 300 K, indicative of a strong interaction between N2 and the three-coordinate copper ion.

Ion-exchange of ZSM-5-type zeolites with copper ion was carried out by three different methods to obtain information on the states and roles of the effective sites for NO decomposition and N2 adsorption activity of copper-ion-exchanged ZSM-5-type zeolites (CuZSM-5). The first method of preparation is chemical vapour deposition (CVD) using bis(1,1,1,5,5,5-hexafluoroacetylacetonato)copper(II), [Cu(hfac)2], as a volatile complex, and the second one is to utilize CuCl as a vaporizing source. These were compared with the third method, that is, the ordinary ion exchange method using an aqueous solution of CuCl2. It is apparent that in the first method the Cu2+ species deposited on ZSM-5 as [Cu(hfac)2] (i.e., via the hydrogen bonding between a Brønsted acid site and a pseudo-aromatic ring of ligands) is reduced to the monovalent species (Cu+) by evacuation at 573 K, and also that the reducibility of Cu2+ is superior to that in the sample prepared by the conventional ion-exchange in an aqueous solution. As for the sample prepared by using CuCl, Cu+ deposited as CuCl was exchanged with H+ on a Brønsted acid site in the HZSM-5 sample through treatment at temperatures above 573 K with a release of HCl. The CuZSM-5 sample prepared by the CVD method gave a single IR band at 2159 cm−1 due to the adsorbed CO species, while the sample prepared by evaporation of CuCl at 573 K and also the sample ion-exchanged in an aqueous solution gave a broad band at around 2155 cm−1 (composed of two bands at 2159 and 2151 cm−1) for the adsorbed CO species, indicating the existence of at least two dominant types of exchangeable sites in these CuZSM-5 samples. Each sample reveals different features for NO decomposition reactivity and N2 adsorption, and such behaviours are explained by the difference in ratio of the respective sites occupied by copper ions. As a result, it was clearly demonstrated that the simultaneous existence of two types of sites lying close together has important implications for the catalytic activity for NO decomposition by CuZSM-5, and that the site giving the 2151 cm−1 band is the effective site for N2 adsorption.

Copper ion-exchanged ZSM-5 samples, prepared using an easy method that takes advantage of microwaves, exhibit a quite peculiar adsorption feature for dinitrogen molecules, in that a large volume of chemisorbed N2 was detected, even at room temperature, and the specificity of the adsorption properties was clarified by comparing with the properties of samples prepared by an ordinary ion-exchange method.

Using ethylene glycol derivatives of aluminium isopropoxide and ethyl orthosilicate precursors in the sol–gel process, discrete aluminosilicate nanoparticles were produced that had a strong Brönsted acidity, high surface area and high thermal stability; these properties were ascribed to a high dispersion of the aluminium atoms in the silica matrix.

The copper-ion-exchanged ZSM-5 type zeolite, prepared by ion-exchange in an aqueous solution of Cu(CH3COO)2 and evacuation at 873 K, gives a distinctive IR band at 2151 cm−1 due to the adsorbed CO species. More efficient adsorption of N2 was exhibited by this sample, compared with samples prepared by other methods, implying site-selective ion-exchange in the preparation process. On the basis of X-ray absorption near-edge structure (XANES) spectra the exchanged copper ion was proved to be in a monovalent state; one of the splitting strong bands, due to the 1s–4pz transition of the monovalent copper ion, loses its intensity on N2 adsorption. The extended X-ray absorption fine structure (EXAFS) spectral pattern around the copper ion also changed on N2 adsorption and a shoulder appeared at around 1.5 Å (no phase-shift correction), in addition to the strong band at around 1.65 Å (no phase-shift correction). It was concluded that the monovalent copper-ion-exchanged site giving the 2151 cm−1 band due to the adsorbed CO species is the active site for specific N2 adsorption. A first principles calculation was carried out with the object of finding the most appropriate model for the CO species adsorbed on the exchanged copper ions in ZSM-5. The data obtained suggest that a three-coordinate copper ion bonded to three lattice oxygen atoms adsorbs CO to give the 2151 cm−1 band. A pseudo-planar structure including the monovalent copper ion bound to three oxygen atoms is assumed to change to a pseudo-tetrahedral arrangement on N2 adsorption. Such a site-selectively ion-exchanged substance has potential for the development of materials for N2 separation or fixation and activation catalysts, as well as for the analysis of NO-decomposition sites.

For alkali-metal ion-exchanged ZSM-5 zeolites (MZSM-5; M: Li, Na, K, Rb, Cs) the analysis of ion-exchangeable sites was performed by means of a combined method based on IR spectroscopic and calorimetric measurements using CO as the probe molecule. The heat of adsorption of CO was found to be correlated with an IR frequency of stretching vibration of C–O in the adsorbed species. It was revealed that there exists at least two types of sites capable of ion-exchanging; for the lithium ion-exchanged ZSM-5 (LiZSM-5) CO adsorption on each type of site is evaluated to give a set of IR bands and heats of adsorption, 2195 cm−1 and 49 kJ mol−1, 2185 cm−1 and 39 kJ mol−1 with the aid of the newly developed method utilizing the data obtained from a combined microcalorimetric and IR-spectroscopic study. Such types of data were also obtained for Na- and K-ion-exchanged ZSM-5 samples. Furthermore, a linear relationship between the differential heat of adsorption (qdiff) evaluated and the shift of wavenumber of the C–O stretching vibration from that of a gaseous CO molecule (Δν) was established for the systems of MZSM-5–CO, and the bonding nature of the CO molecule with each site can be explained in terms of the electrostatic force. The model of each adsorption site was also examined by the quantum calculation method (density functional theory: DFT). The trends obtained from the experimental data may be substantially supported by the calculation method even adopting a model as simple as the ZSM-5-type zeolite: the composition of MAlSi4O4H12.

Compared with mercury, the existence of [Zn2]2+ species is rare. We succeeded in preparing a stable [Zn2]2+ species by utilizing an MFI-type zeolite as a nano-reaction pot, which was confirmed using XAFS spectroscopy: the bands at R = 2.35 Å due to the Zn+–Zn+ scattering and at 9660.7 eV due to the 1s–σ* (the anti-bonding orbital comprised of the 4s–4s orbital) transition of the [Zn2]2+ species. This species also gives the characteristic band around 42000 cm−1 due to its σ–σ* transition. Furthermore, UV-irradiation corresponding to the σ–σ* transition causes the bond dissociation, forming two unprecedented Zn+ ions, and detached Zn+ ions were recombined through heat-treatment at 573 K: [Zn+–Zn+] ⇄ 2Zn+. These processes were reproduced by applying the DFT calculation method to the assumed triplet, σ(α)–σ*(α), structure formed on the M7-S2 site with the specific Al array in the MFI-type zeolite. Research into the specific field using zeolites to synthesize “ultra-state ions” is very promising.

Three different approaches have been used to characterize the state of exchanged copper ions in copper-ion-exchanged MFI (CuMFI) samples. (1) Two types of an ion-exchangeable site with different adsorption properties for N2 or CO molecules were identified depending on the pre-treatment temperature (723 or 873 K) of a sample prepared by using an aqueous solution of CuCl2. (2) The state of the active sites formed by the evacuation of a sample at 873 K that had been prepared using a mixture solution of aqueous NH4CH3COO and Cu(CH3COO)2 was analysed utilizing both 13C18O and 12C16O to identify the two types of active adsorption sites for CO molecules. (3) CuMFI samples prepared by the ion-exchange method employing anhydrous CuCH3COO showed a surprising adsorption feature characterized by a single IR band occurring at 2159 cm−1 due to the adsorbed CO molecules, but there was no corresponding IR band due to adsorbed N2 molecules. A successful preparation of CuMFI, in which the monovalent copper ions exclusively occupied another one of the two types of ion-exchangeable sites, was also carried out utilizing the solid-ion exchange method using Cu(CH3COO)2·H2O. This site exhibits an IR band occurring at 2151 cm−1 for CO molecules and also acts as an active site for N2 molecules. These experimental data correlate, and clearly indicate that there are at least two types of exchangeable sites for copper ions in MFI-type zeolites.

A nitrogen-doped TiO2 sample was prepared at 413 K by direct hydrothermal treatment of titanium isopropoxide in an aqueous solution of NH3. This new material has a large specific surface area of ca. 220 m2 g−1 because of its tubular structure and it exhibits a prominent absorption feature in the region between 400 and 650 nm. It responds strongly to light in the visible region, which is key to its potential performance as a photocatalyst that may improve the efficiency for utilization of solar energy. Actually, this sample exhibits very efficient activity in the decomposition of CH3COOH under visible light among the samples prepared. This effective photocatalysis of the present sample was substantiated by characteristic spectroscopic features, such as: (1) an optical absorption band with λ > 400 nm because of the doped nitrogen species; (2) the formation of EPR-active, long-lived N˙ and O2– species, as well as N2– species, under visible-light irradiation in the O2 or N2 adsorption process at 300 K by way of the monovalent nitrogen ions in the bulk (both substitutional and interstitial); (3) the existence of IR-active O2 species adsorbed on the nitrogen-doped TiO2 sample even without light irradiation; and (4) an XPS N1s band around 399.6 eV that is assignable to the N− species. The amounts of N˙ and O2− species formed in the nitrogen-doped TiO2 sample under visible-light irradiation correlated well with the levels of reactivity observed in the decomposition of CH3COOH on the samples with varying amounts and types of doped nitrogen species. We conclude that the photoactive N˙ and O2– species created in the present sample are responsible for the decomposition of organic materials assisted by visible light irradiation. These features may be attributable to the interface between the sample's tubular structure and anatase with poor crystallinity, which probably causes the resistance to the recombination of electron–hole pairs formed by irradiation.

The interaction of ethyne (C2H2), as well as of carbon dioxide (CO2), with copper–ion-exchanged MFI zeolite (CuMFI) at room temperature was examined. It was found that CuMFI preferentially adsorbs C2H2, while this material does not respond to CO2. To clarify the specificity of CuMFI, a combination of various experimental techniques and theoretical calculations was adopted. Distinctive interaction energies of 140 and 110 kJ mol−1 were clearly observed at the initial stage of C2H2 adsorption on CuMFI, suggesting the presence of two types of adsorbed C2H2. Two distinct IR bands at 1620 and 1814 cm−1 appeared, which were assigned to the CC stretching vibration modes of C2H2 differing in their adsorbed state. Both photoluminescence and X-ray absorption spectra showed that cuprous ions (Cu+) in CuMFI act as efficient sites for a marked C2H2 adsorption. From the analysis of the latter spectra and the calculational results based on the density functional theory, the formation of dual Cu+⋯(C2H2)⋯Cu+ complexes was indicated for the first time for CuMFI, and such a special configuration of the Cu+ sites contributed to the extremely strong adsorption of C2H2. In contrast, it was necessary for the linear CO2 molecule to take a bent structure to be adsorbed on Cu+ in CuMFI. It was concluded that the difference in the adsorption response of Cu+ in CuMFI towards C2H2 and CO2 is due to the chemistry between the nature of electron donation of Cu+ and the hybrid orbitals of the respective molecules. This work promotes further understanding of the states of active centres in CuMFI for C2H2 activation, as well as for N2 fixation.