佐藤 智司

Vapor-phase intramolecular aldol condensation of 2,5-hexanedione (2,5-HD) to 3-methyl-2-cyclopentenone (MCP) was performed over several rare-earth zirconate catalysts, with yttrium zirconate (YZrO) emerged as the most efficient catalyst. Several parameters, such as Y content, calcination temperature, reaction temperature, and contact time, highly influenced the catalytic performance of the YZrO catalyst: YZrO calcined at 700 °C exhibited the highest catalytic activity but still suffered from catalyst deactivation due to coke formation. Modifying the YZrO with Ag and employing the modified catalyst in an H2 flow effectively reduced the coke accumulation, thus improving the catalytic stability during the aldol condensation of 2,5-HD at 350 °C. The poisoning experiment with the co-fed of acidic CO2 gas and basic molecules of 2,6-, and 3,5-dimethylpyridine suggested that the aldol condensation proceeded via an acid-base concerted mechanism.

Selective doping of pyridinic nitrogen in carbon materials has attracted attention due to its significant properties for various applications such as catalysts and electrodes. However, selective doping of pyridinic nitrogen together with controlling skeletal structure is challenging in the absence of catalysts. In this work, four precursors including four fused aromatic rings and pyridinic nitrogen were simply carbonized in the absence of catalysts in order to attain mass synthesis at low cost and a high percentage of pyridinic nitrogen in carbon materials with controlled edges. Among four precursors, dibenzo[f,h]quinoline (DQ) showed an extremely high percentage of pyridinic nitrogen (96 and 86%) after heat treatment at 923 and 973 K, respectively. Experimental spectroscopic analyses combined with calculated spectroscopic analyses using density functional theory calculations unveiled that the C-H next to the pyridinic nitrogen in DQ generated gulf edge structures with controlled pyridinic nitrogen after carbonization. By comparing the reactivities among the four precursors, three main factors required for maintaining the pyridinic nitrogen in carbon materials with controlled edges, such as (1) high thermal stability of the pyridinic nitrogen, (2) the presence of one pyridinic nitrogen in one ring, and (3) the formation of gulf edges including pyridinic nitrogen to protect the pyridinic nitrogen by the C-H groups on the gulf edges, were revealed.

A silica-supported copper (Cu/SiO2) catalyst containing highly dispersed Cu nanoparticles was prepared via a crown-ether-assisted impregnation method. A 12-crown-4-ether-assisted Cu/SiO2 catalyst outperformed several Cu/SiO2 catalysts prepared with various organic additives in the dehydrogenation of 2,3- and 1,4-butanediol. It was found that the catalytic activity, i.e., the formation rate of acetoin from 2,3-butanediol and that of γ-butyrolactone from 1,4-butanediol, was proportional to the copper surface area.

Selective doping of nitrogen-containing functional groups of carbon materials is the most essential technique for enhancing properties significantly for various applications such as catalysts and electrodes. Reported carbon materials with controlled pyridinic nitrogen are generally synthesized on substrates, leading to low productivity and high cost. Thus, simple preparation methods for selective doping of nitrogen in carbon materials without substrates are indispensable for mass production. This work discovered that simple heat treatment of 1,7-phenanthroline at 973 K formed carbon material with exceptionally high pyridinic nitrogen content (92%). The selective doping of pyridinic nitrogen was attained by the protection of pyridinic nitrogen due to the steric hindrance by the neighboring C-H group on armchair edges to avoid the formation of N-H bonding on pyridinic nitrogen on armchair edges. Screening techniques of precursors for preparing carbon materials with high pyridinic nitrogen content are also essential to minimize research costs. Screening by 1) formation energy of hydrogenation on pyridinic nitrogen, 2) molecular dynamic simulation with reactive force field, and 3) comparison of structures of precursors revealed that 1,7-phenanthroline was the best precursor to maintain the high percentage of pyridinic nitrogen after carbonization. Thus, strategic synthesis is now possible using screening techniques.

X-ray photoelectron spectroscopy (XPS) is among the most utilized analytical methods for nitrogen-doped carbon materials. Clarifying the assignments of nitrogen-doped carbon materials with different degrees of carbonization, which relates to conjugated systems, is essential to correlate structures with the performance of various applications, but such precise assignments were challenging. In this work, precise analyses were conducted to overcome the difficulty to assign the peaks of carbon materials with different degrees of carbonization, different edges, and various nitrogen-containing functional groups in graphene nanoribbons (GNRs), such as nitrile, pyridinic, primary with/without charges, secondary with/without charge, tertiary without charges (graphitic nitrogen), and quaternary nitrogen. Electrons donated from hydrogen and Madelung potentials showed a higher correlation to the peak shift of C1s and N1s XPS spectra than Mulliken charges on nitrogen. Zigzag edges showed a greater influence on the peak shift of tertiary amine (graphitic nitrogen) than armchair edges on XPS spectra. Besides, as the degree of carbonization is increased from aromatic compound-like structures to GNRs, the peak positions of C–N in C1s and N1s XPS spectra shifted to higher binding energies. This research proved that XPS could be applied to the precise structural analyses of nitrogen-containing carbon materials.

Vapor-phase catalytic dehydration of 2,3-butanediol (2,3-BDO) to form 1,3-butadiene (BD) via 3-buten-2-ol (3B2OL) was studied over various single metal oxide catalysts. Among the catalysts, Sc2O3 prepared under hydrothermal (HT) conditions at 200 °C followed by 800 °C calcination showed the most excellent catalytic activity. The crystallization of precursor ScOOH during HT aging noticeably enhances the catalytic activity of the resulting Sc2O3 for the formation of 3B2OL in the dehydration of 2,3-BDO. The formation rate of 3B2OL from 2,3-BDO over the HT-aged Sc2O3 was twice as high as Sc2O3 without HT aging. Calcination temperatures of Sc2O3 are also important: calcination at 800 °C is efficient for the selective formation of 3B2OL from 2,3-BDO. The HT-aged Sc2O3 also showed an excellent catalytic activity for the formation of BD with the yield higher than 80% in the dehydration of 2,3-BDO at 411 °C.

X-ray photoelectron spectroscopy (XPS) is among the most powerful techniques to analyse structures of nitrogen-doped carbon materials. However, reported assignments of (1) graphitic nitrogen (N)/substitutional N, quaternary N (Q–N), or tertiary amine (T–N) and (2) pyrrolic N/secondary amine or T–N are questionable. Most reports assign peaks at ca. 401 eV as Q–N or graphitic N, whereas raw materials in most of those works contain neither counter anion nor halogen. Besides, the peak at ca. 400 eV has been assigned as pyrrolic N, but the presence of N–H is generally not confirmed. In this work, it was clarified that one of the reasons for the prevailing ambiguous assignments is the presence of N in heptagonal and pentagonal rings. The peaks at 400.1–401.2 eV were determined to be T–N, but not Q–N by analyzing graphitized polyimide (with the oxygen content of 0.01 at% or lower and the hydrogen content of 0 at%) using Raman spectroscopy, XPS, X-ray diffraction, total neutron scattering, elemental analysis, and molecular dynamics simulation. Besides, it was revealed that the peak at 400.1 eV originated from T–N on 5-membered rings or 7- and 5-membered rings, but not pyrrolic N because graphite including no hydrogen was used for analysis. Graphical abstract: [Figure not available: see fulltext.].

Oxygen-containing carbon materials have been studied extensively because of their excellent dispersibility, absorptivity, separability, and supportability of catalysts. However, structural control by existing top-down methods is almost impossible. Our group has demonstrated that phloroglucinol (PG, 1,3,5-trihydroxybenzene) can be a promising raw material to synthesize structurally controlled oxygen-containing carbon materials. In this study, in addition to PG, hexahydroxybenzene (HHB), which has more oxygen and high symmetry, was used as the raw material, and a Lewis acid catalyst, tris (pentafluorophenyl) borane (TPB), was used to enhance the structural control rate and the removability of catalysts from the carbonized samples. The solubility of heat-treated HHB was lower than that of heat-treated PG, but the oxygen content of heat-treated HHB was higher than that of heat-treated PG even at 673 K. By adding TPB to PG, dibenzofuran-like structures formed, and the structural control rate increased up to 93.6%. Besides, the content of fluorine in the catalyst was reduced to 0%, indicating that TPB can be a promising recyclable catalyst to promote the structural control rate of carbonized PG. Graphical abstract: [Figure not available: see fulltext.].

Vapor-phase catalytic dehydration of 1,4-butanediol (1,4-BDO) was investigated over Y2O3-ZrO2 catalysts. In the dehydration, 1,3-butadiene (BD) together with 3-buten-1-ol (3B1OL), tetrahydrofuran, and propylene was produced depending on the reaction conditions. In the dehydration over Y2O3-ZrO2 catalysts with different Y contents at 325°C, Y2Zr2O7 with an equimolar ratio of Y/Zr showed high selectivity to 3B1OL, an intermediate to BD. In the dehydration at 360°C, a BD yield higher than 90% was achieved over the Y2Zr2O7 calcined at 700°C throughout 10 h. In the dehydration of 3B1OL over Y2Zr2O7, however, the catalytic activity affected by the calcination temperature is roughly proportional to the specific surface area of the sample. The highest activity of Y2Zr2O7 calcined at 700 °C for the BD formation from 1,4-BDO is explained by the trade-off relation in the activities for the first-step dehydration of 1,4-BDO to 3B1OL and for the second-step dehydration of 3B1OL to BD. The higher reactivity of 3B1OL than saturated alcohols such as 1-butanol and 2-butanol suggests that the C=C double bond of 3B1OL induces an attractive interaction to anchor the catalyst surface and promotes the dehydration. A probable mechanism for the one-step dehydration of 1,4-BDO to BD was discussed.

The vapor-phase catalytic reaction of crotyl alcohol (CRO) was investigated over metal oxide-modified silica catalysts. The isomerization of CRO to 3-buten-2-ol (3B2OL) was found to progress in the reaction of CRO over V2O5-modified SiO2 (V2O5/SiO2) at 200 °C. V2O5/SiO2 showed high 3B2OL yield and stable catalytic activity. The isomerization between CRO and 3B2OL reached an equilibrium without the dehydration to 1,3-butadiene. Poisoning experiments using dimethylpyridines suggested that Lewis acid sites acted as the active centers.

Graphene nanoribbon (GNR) has attracted attention because of the adjustable band gap, depending on the width and functional groups. The introduction of sp3C–H on edges is one of the choices to reduce the agglomeration between GNRs and to change their various properties. Infrared spectroscopy is among the powerful tools to analyze the edge structures of carbon materials, but the number of detailed reports is almost nonexistent for sp3C–H in carbon materials. In this work, the influence of the presence of sp3C–H on the peak position of sp2C–H on zigzag and armchair edges of GNR was revealed by comparing experimental and computational infrared spectra of aromatic compounds. The introduction of methylene and methyl groups next to sp2C–H affected peak positions of in-plane stretching and out-of-plane bending vibration of sp2C–H. The peak position of sp2C–H was further shifted by introducing methylene and methyl groups on both sides of sp2C–H. The presence of either methylene or methyl groups can be clearly distinguished from the difference in coupled vibration of out-of-plane vibration of sp2C–H and quadrant stretching vibration of C=C because the presence of methylene groups affects the conjugated system significantly, whereas methyl groups did not affect the conjugated system.

Carbonization process of pyromellitic dianhydride (PMDA)-4,4′-diaminodiphenyl ether (ODA)-type polyimide has been studied for decades. However, various reaction mechanisms have been proposed and the detailed mechanisms are still controversial. It is essential to understand the carbonized structures of PMDA-ODA-type polyimide before analyzing the defect structures in graphite. In this work, the carbonization mechanisms of polyimide heated at 1273 K or lower were unveiled and the methodology to analyze carbon materials containing nitrogen, oxygen, pentagons, and heptagons using computational spectral analysis combined with molecular dynamics simulation (ReaxFF) are exhibited. For example, the formations of isoimide and cyclic ether were estimated by ReaxFF, and the presence was supported by experimental and calculated X-ray photoelectron spectroscopy (XPS) and carbon-13 nuclear magnetic resonance (13C NMR) spectra of polyimide heated between 813 and 873 K. The formations of pentagons and heptagon, suggested by ReaxFF, were also supported by Raman and 13C NMR spectra of polyimide heated between 833 and 1273 K. This work also revealed that the previously reported structures were unstable and that amine in the basal plane was the most plausible structure in polyimide heated at 1073 K or higher, as clarified by XPS, ReaxFF, and the energy calculation.

The bromination reactivity of various types of polycyclic aromatic hydrocarbons (PAHs) with oxygen atoms and graphene with oxygen atoms was estimated by density functional theory calculation and experimentally clarified by analyzing bromination of PAHs using gas chromatography–mass spectrometry. In the experimental and theoretical bromination reactivity of PAHs, the presence of hydroxyl group increased the reactivity of PAHs because of electron-donating nature of the hydroxyl group but the other oxygen-containing functional groups such as lactone, ether, and ketone decreased the reactivity due to the electron-withdrawing nature of those groups. These effects of functional groups on the reactivity were also confirmed in graphene. The tendency of theoretical bromination reactivity of graphene was graphene with hydroxyl group > graphene with no group > graphene with lactone group > graphene with ether group > graphene with ketone group. Our study on the estimation of bromination reactivity of graphene edges provides the groundwork for the bromination of graphene edges.

Abstract: Pentagons in carbon materials have attracted attentions because of the potential high chemical reactivity, band gap control, and electrochemical activity. However, it is challenging to prepare a carbon film with high pentagon density because of the curvature and the high reactivity caused by the presence of pentagons, and it is also challenging to estimate the percentage of pentagons in carbon materials because of the limitation of current analytical techniques. In this work, the percentage of pentagons in carbon materials was experimentally estimated for the first time using experimental and calculated C1s X-ray photoelectron spectroscopy and elemental analysis. Carbon films with 7% of pentagons (40% of pentagons compared to the raw material) with electrical resistivity of 1.1 × 104 Ω meter were prepared by heat treatment of corannulene at 873 K. On the other hand, fluoranthene and fullerene remained as non-film solid and powder without forming films at 873 K. Experimental and calculated Raman and IR spectra revealed the peaks of different types of pentagons. Decrement of pentagons in corannulene and fluoranthene heated at high temperatures can be explained mainly by the scission of C=C bond in pentagons, as suggested by the results of reactive molecular dynamics simulation (ReaxFF). Graphic abstract: [Figure not available: see fulltext.].

Oxygen-containing carbon materials such as graphene oxide have been extensively studied because of their high dispersibility. However, the oxygen-containing functional groups in most carbon materials are not controlled. Uncontrollability of the synthesis is also one of factors that prevent industrialization. Carbon materials derived from phloroglucinol (PG), which show high solubility/dispersibility and controllability of functional groups, have been developed recently by our group. The high performance of carbonized PG originates from the thermally stable backbone structure of the benzene ring with hydroxy groups of PG. However, the degree of carbonization was low. In this study, five heteropoly acids (HPAs), which are thermally stable homogeneous strong acid catalysts, were used to promote carbonization of PG without losing the controllability of functional groups and the dispersibility. HPAs promoted etherification of hydroxy groups followed by C=C coupling reactions (furan cyclization) at 523 K. Furthermore, it was confirmed that particularly furan structures, which contribute to solubility/dispersibility in solvents, and thermal stability in air, could be maintained at 673 K as suggested by spectroscopies and thermogravimetric-differential thermal analysis. Among five HPAs, phosphotungstic acid worked as the excellent catalyst to promote carbonization of PG containing furan structures, exhibiting high solubility/dispersibility and high thermal stability in air.[GRAPHICS].

The aim of this work is to develop an effective catalyst for the conversion of butanediols, which is derivable from biomass, to valuable chemicals such as unsaturated alcohols. The dehydration of 1,4-, 1,3-, and 2,3-butanediol to form unsaturated alcohols such as 3-buten-1-ol, 2-buten-1-ol, and 3-buten-2-ol was studied in a vapor-phase flow reactor over sixteen rare earth zirconate catalysts at 325 °C. Rare earth zirconates with high crystallinity and high specific surface area were prepared in a hydrothermal treatment of co-precipitated hydroxide. Zirconates with heavy rare earth metals, especially Y2Zr2O7 with an oxygen-defected fluorite structure, showed high catalytic performance of selective dehydration of 1,4-butanediol to 3-buten-1-ol and also of 1,3-butanediol to form 3-buten-2-ol and 2-buten-1-ol, while the zirconate catalysts were less active in the dehydration of 2,3-butanediol. The calcination of Y2Zr2O7 significantly affected the catalytic activity of the dehydration of 1,4-butanediol: a calcination temperature of Y2Zr2O7 at 900 °C or higher was efficient for selective formation of unsaturated alcohols. Y2Zr2O7 with high crystallinity exhibits the highest productivity of 3-buten-1-ol from 1,4-butanediol at 325 °C.