•Supported polyethylene glycol stabilized Pt nanoparticles were prepared.•Selectivity of m-iodoaniline reached 99.3% for hydrogenation of m-iodonitrobenzene.•The dehalogenation and the accumulation of intermediates were simultaneously inhibited.•The addition of CO2 remarkably affected the reaction rate and the product selectivity.•The SPPN catalyst was stable and can be reused several times.Polyethylene glycol stabilized platinum nanoparticles were immobilized on solid supports such as γ-Al2O3, SBA-15, TiO2 and active carbon, forming supported polyethylene glycol stabilized platinum nanoparticles (SPPNs). In the hydrogenation of p-chloronitrobenzene (p-CNB) in supercritical carbon dioxide (scCO2), the SPPN showed high selectivity to p-chloroaniline (>99.3%) in the whole range of conversion. Such high selectivity to corresponding haloanilines (HANs) (>99.1%) was also obtained in the hydrogenation of o-CNB, m-CNB, 2-chloro-6-nitrotoluene, p-bromonitrobenzene and m-iodonitrobenzene. The dehalogenation and the accumulation of intermediates were fully inhibited simultaneously in scCO2. The SPPN catalysts could be reused several times without loss of high selectivity in present reaction system.

In this work, we have developed a new method to synthesize a Fe3O4@graphene (Fe3O4@GN) composite. First, the precursor was synthesized through the decomposition of ferric nitrate in the presence of graphene oxide in the mixed solvent of CO2–expanded ethanol. Then, the precursor was converted to the Fe3O4@GN composite via thermal treatment in N2 atmosphere. With the help of the CO2–expanded ethanol, Fe3O4 nanoparticles were coated on the surface of GN completely and uniformly with high loading. However, it is difficult to load Fe3O4 particles onto the surface of GN and most of the Fe3O4 particles were deviated away from GN and aggregated to form larger units in pure ethanol. When used as anode for Li-ion batteries (LIBs), the Fe3O4@GN composite with a graphene content of 25 wt% synthesized in CO2–expanded ethanol manifested excellent charge–discharge cycling stability and rate performance compared with the sample synthesized in ethanol. Such improved electrochemical performances should be attributed to the intimate contact between the GN and Fe3O4 nanoparticles in the composite. Since the present method does not need tedious pre-treatment, surfactant, or precipitate, it is a green or sustainable technology and the solvents could be recycled easily after simple phase separation. This facile method can be extended to the synthesis of other metal oxide composites, which are expected to have good performance as anode materials for LIBs and other applications.

In this work, we report a facile method for the synthesis of a Co3O4–functionalized carbon nanotube (Co3O4–f-CNT) composite via the growth of Co3O4 nanoparticles on the surface of functionalized carbon nanotubes (f-CNTs) by thermal decomposition of cobalt nitrate hexahydrate in ethanol. The composite consists of 13% carbon nanotubes and 87% Co3O4 nanoparticles by weight, and all the Co3O4 particles grew compactly along the carbon nanotube axis with a highly uniform dispersion. When used as an anode material for rechargeable lithium ion batteries, the composite manifested high capacities and excellent cycling performance at high and low current rates. The discharge capacity was 719 mA h g−1 at the 2nd cycle and 776 mA h g−1 at the 100th cycle. Even at a current density of 1 A g−1, the specific capacity still remained at about 600 mA h g−1. This superior electrochemical performance was attributed to the unique nanostructure of the composite. Because almost all of the Co3O4 nanoparticles were immobilized on the surface of f-CNTs, physical aggregation of nanoparticles was avoided during the charge–discharge processes. Furthermore, the good mechanical flexibility of f-CNTs can readily alleviate the massive volume expansion/shrinkage associated with a conversion reaction electrode. Finally, f-CNTs are highly conductive matrices for electrons due to their high conductivity, which can shorten the diffusion path for electrons.

In this work, we have developed a new method to grow NiO nanomaterials on the surface of graphene nanosheets (GNSs). The morphologies of NiO nanomaterials grown on GNSs could be tailored by trace amounts of water introduced into the mixed solvents of CO2-expanded ethanol (CE). Small and uniform Ni-salt nanoparticles (Ni-salt-NPs) were grown on the surface of graphene oxide (GO) through the decomposition of nickel nitrate directly in CE. However, when trace amounts of water were introduced into the mixed solvents, Ni-salt nanoflakes arrays (Ni-salt-NFAs) were grown on the surface of GO with almost perpendicular direction. After thermal treatment in N2 atmosphere, these Ni-salt @GO composites were converted to NiO@GNSs composites. The forming mechanisms of the NiO-NPs@GNSs and NiO-NFAs@GNSs were discussed by series comparative experiments. The presence of the trace amounts of water affected the chemical composition and structure of the precursors formed in CE and the growth behaviors on the surface of GNSs. When used as anode materials for lithium-ion batteries, the NiO-NPs@GNSs composite exhibited better cycle and rate performance compared with the NiO-NFAs@GNSs.Keywords: anode material; CO2-expanded ethanol; growth behaviors; lithium-ion batteries; NiO nanostructures;

Two-dimensional (2D) SnS2 nanoplates were synthesized through a facile hydrothermal method. The influences of reaction conditions such as temperature and pH on the size, crystallinity and the forming process of SnS2 were investigated in detail. At low temperature (160 °C), the SnS2 nanoplates showed poor crystallinity; while at higher temperatures above 200 °C, the crystallinity and thickness of the SnS2 nanoplates tended to increase. In addition, pH had notable impact on the nucleation velocity of SnO2 and the conversion speed from SnO2 to SnS2 in further as well. When used as anode materials in rechargeable lithium ion batteries, the SnS2 nanoplates synthesized at 200 °C and pH = 10.5 (SnS2-200-10.5) showed the best lithium storage capacity, good cycling stability and excellent rate capability. It retained a high reversible capacity of 521 mA h g−1 over 50 cycles at a current of 100 mA g−1, equal to 90.0% of the initial reversible capacity. In addition, the coulombic efficiency increased from 36% in the first cycle to over 97% in the subsequent cycles. Even at high current densities of 1, 2 and 3 A g−1, the electrodes could still delivery as high as 472, 397 and 340 mA h g−1, respectively. The enhanced electrochemical performance of the SnS2-200-10.5 can be attributed to the compact and regular crystal structure with a moderate thickness and crystallinity, which is beneficial for maintaining the stability of the structure and fast ion transport during lithiation/delithiation processes.

•The addition of CO2 remarkably affected the reaction rate and product selectivity.•The decreased reaction rate is due to the formation of solid carbamate in the presence of CO2.•The yield of benzylamine was increased to >94.4% in the presence of CO2.•The presence of CO2 could prohibit the formation of dibenzylamine in organic solvents.•The acidic nature of H2O–CO2 benefited the elimination of NH3.Selective hydrogenation of benzonitrile was studied with Ni/Al2O3 in compressed CO2, hexane–CO2, ethanol–CO2 and H2O–CO2 systems. The phase behavior and the effect of CO2 on the conversion of benzonitrile and the yield of benzylamine were discussed. The reaction rate was retarded in solventless, hexane and ethanol, but accelerated in water in the presence of compressed CO2. The decrease in reaction rate was mainly ascribed to the formation of carbamate from benzylamine and the intermediate 1-aminodibenzylamine reacting with CO2, and it precipitated out to coat on the surface of catalyst in solventless and hexane, and the dilution effect of compressed CO2 in ethanol. But the yield of benzylamine was increased in solventless, hexane and ethanol due to the following nucleophilic addition of benzylamine and benzylimine to N-benzylidenebenzylamine was inhibited. Although, the reaction rate increased in water for the enhanced solubility of H2 and benzonitrile in H2O and decreased mass-transfer resistance in the presence of compressed CO2, the yield of benzylamine decreased because of the acidic nature of H2O–CO2 favorites the elimination of NH3 and the formation of NH4HCO3. Accordingly, the possible reaction pathway of benzonitrile hydrogenation was proposed for all the studied systems.

The reduction of an α,β-unsaturated aldehyde, citral, was investigated over a 10 wt% Pd catalyst under transfer hydrogenation (TH) conditions in a closed system with microwave assistance. Surprisingly, it was found that hydrogen was produced quite fast under the microwave irradiation during the reaction, and the reduction of citral was proved to go mainly through consecutive pathways of hydrogen production – hydrogenation rather than those commonly considered for TH reactions. Similar reaction pathways were also observed with a homogeneous catalyst of [RuCl2(C6H6)]2 and other typical hydrogen donors like formate salts and isopropanol, which are usually used in the typical transfer hydrogenations.Graphical abstractUnusual reaction mechanisms are proved for selective reduction of an α,β-unsaturated aldehyde of citral under typical transfer hydrogenation (TH) reaction conditions. The reaction goes through consecutive pathways of hydrogen production – hydrogenation than those commonly considered for TH reactions.Highlights► Hydrogen is produced promptly from HCOONa and H2O over Pd/C under microwave assistant. ► Citral was hydrogenated by the hydrogen produced in situ efficiently. ► A consecutive hydrogen production and hydrogenation process was confirmed.

A highly active and selective Ni/ZSM-5 catalyst was prepared by a simple method. A selectivity of 91.2% to hexitols was obtained at intermediate conversion in the hydrolytic hydrogenation of cellulose.

In this study, an effective method of slow hydrolization of metal alkoxide (e.g., Ti(C4H9O)4) in an ethanol–water system was systematically investigated and used to finely control the deposition of titania on carbon colloids. A model of adsorption–hydrolization of precursors during the coating process was rationally built for the first time to interpret the usability of the method and facilitate its further extension. Using this strategy, titania in the form of supported nanocrystals or layers on carbon colloids (TiO2/C, C@TiO2) was successfully tailored. Meanwhile, finely dispersed hollow TiO2 nanoparticles with shells consisting of different crystalline structures were also prepared by varying the calcination conditions after removing the carbon cores. More importantly, the effects of the crystalline and nano/macrostructures of the as-prepared TiO2 samples in photocatalysis and lithium-ion battery applications were analyzed in detail. The preliminary results show that anatase–rutile TiO2 hollow particles demonstrate a higher catalytic activity in the photo-degradation of rhodamine B than anatase TiO2 hollow particles, powders, and P25. However, in the case of Li-ion battery applications, the anatase TiO2 hollow particles exhibited better performance as anode materials with high capacities of around 190 mA h g−1, 140 mA h g−1, and 120 mA h g−1 at current densities of 60 mA g−1, 120 mA g−1, and 300 mA g−1, respectively, accompanied by stable cyclability.

An efficient Ru/TiO2 catalyst was successfully designed and synthesized for the deoxygenation of long chain fatty acid esters under mild conditions (200 °C, 3.0 MPa). This work provides an energy-economic route to upgrade the oils with high oxygen content into green biofuels.

Polyureas were synthesized from diamines and carbon dioxide in the absence of any catalyst or solvent, analogous to the synthesis of urea from condensation of ammonia with carbon dioxide. The method used carbon dioxide as a carbonyl source to substitute highly toxic isocyanates for the synthesis of polyureas. FTIR and DFT calculations confirmed that strong bidentate hydrogen bonds were formed between urea motifs, and XRD patterns showed that the PUas were highly crystalline and formed a network structure through hydrogen bonds, which served as physical cross-links. The long chain PUas presented a microphase separated morphology as characterized by SAXS and showed a high melting temperature above 200 °C. The PUas showed high resistance to solvents and excellent thermal stability, which benefitted from their special network structures. The PUas synthesized by this method are a new kind of functional material and could serve some areas where their analogues with similar functional groups could not be applied.

Co(OH)2 coated platinum nanoparticles Pt/Co(OH)2 were prepared by microwave assistance and hydrothermal method, and the prepared samples were composed of Pt nanoparticles with an average size of 1.8 nm coated uniformly in the thin Co(OH)2 leaves based on the results of X-ray diffraction, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The Pt/Co(OH)2 presented excellent catalytic performance in the chemoselective hydrogenation of halonitrobenzenes such as chloronitrobenzenes, bromonitrobenzene and iodonitrobenzene, and above 99.6% selectivity to haloanilines was achieved at complete conversion irrespective of the substrates used, even for iodonitrobenzene to which the dehalogenation is more easily to occur. Co(OH)2 was confirmed to prohibit the dehalogenation effectively, and the Pt/Co(OH)2 catalyst could be recycled for several times.Graphical abstractHighlights► Co(OH)2 coated platinum nanoparticles Pt/Co(OH)2 have been prepared. ► The Pt nanoparticles were high dispersed and embodied into the thin leaves of Co(OH)2. ► Pt/Co(OH)2 was an efficient catalyst for chemoselective hydrogenation of halonitrobenzenes. ► Above 99.6% yield to haloanilines was achieved. ► The Pt/Co(OH)2 catalysts were stable and could be reused for several times.

SnS2/graphene nanosheets (SnS2/GNS) composites were synthesized by a one-step hydrothermal method. The composites exhibit remarkably improved Li-storage ability with a good cycling life and high capability superior to that of the pure SnS2 counterpart due to a synergic effect between the graphene and SnS2 nanosheets.

Selective reduction of phenol to cyclohexanone over the Pd/C catalyst in the presence of a hydrogen source of HCOONa/H2O has been studied. Surprisingly, phenol was transformed efficiently to cyclohexanone in an excellent yield of above 98% under microwave irradiation. The influence of some parameters like reaction temperature, time and amount of hydrogen donor, as well as the reaction pathway has been discussed. The combination of microwave irradiation and HCOONa/H2O was certified to be effective for the reduction of phenol as well as its derivatives to their corresponding cyclohexanones.

Highly concentrated microcrystalline cellulose was directly converted to isosorbide with yields of 35–50%, providing a new approach for producing important fine chemicals from biomass.

In this contribution, monodisperse porous hollow bi-phase γ-/α-Fe2O3 nanoparticles were successfully fabricated based on hard-template method with using carbon colloids as sacrificial templates. A new concept of assembling one kind of metal oxide with different crystalline structures into a single shell was presented for the first time. The critical procedure of coating carbon cores with a uniform layer of oxide was performed in CO2-expanded ethanol, which is a versatile way to produce high-quality hollow oxide nanoparticles. The formation of the novel bi-phase shell was achieved through combining the reduction ability of carbon cores under inert calcination atmosphere and the unique chemical composition of intermediate-shell formed in CO2-expanded ethanol. The porous hollow γ-/α-Fe2O3 nanoparticles with an average diameter of 99 nm not only possess combined properties of γ-Fe2O3 and α-Fe2O3, but also have a large specific surface area of 93.7 m2 g−1 and a high pore volume of 1.056 cm3 g−1, enabling them to have widespread applications in sensors, catalysis, magnetic and electrochemical areas, etc. Herein, such hollow bi-phase γ-/α-Fe2O3 nanoparticles were utilized to prepare a sensor device, and intriguingly it shows higher sensitivity and selectivity to ethanol than γ-Fe2O3 powders and many other porous α-Fe2O3 materials reported recently. The probable sensor mechanism of hollow γ-/α-Fe2O3 nanoparticles was discussed in detail.

In this work, we presented a new way for the functionalization of carbon nanotubes (CNTs) with the use of biomass as starting materials and introduced a novel concept of knitting process in the chemistry of CNTs for the first time. A mixture of aromatic compounds obtained from the hydrothermal treatment of biomass, rather than the traditional polymer monomers, was used as the nanoscale building blocks to knit an oxygenated network-coat on the CNTs layer-by-layer. It is an effective, mild, green and easily-controlled method for the functionalization of CNTs. The obtained f-CNTs were proved to be a promising catalyst support for metal catalysts, such as Ru/f-CNTs, showed high activity and selectivity for the hydrogenation of citral to unsaturated alcohol. More importantly, we opened a pioneering way for the conversion of low-cost, abundant and renewable biomass into a hydrophilic/chemical reactive network-coat on the inert surface of a wide range of sp2carbon materials, such as prevalent fullerene, carbon nanotubes, carbon nanohorns and hot grapheneetc.

In this contribution, we present an efficient, versatile and green strategy for finely controlling the metal (oxide) coating on core particles through in situ reaction of precursors in CO2 expanded ethanol without using any precipitants. It not only avoids the formation of free metal (oxide) and/or naked cores, but also permits individual dispersion of all the resultant particles without aggregation. With this method, the composition, thickness, uniformity, and structure of the metal (oxide) shell could be precisely controlled. A wide variety of unreported high-quality core-shell particles with a shell consisting of highly dispersed metal (oxide) nanocrystals or nanoalloys, such as C@Ni, CoO/C, C@Ni&Co and C@Ni&Pd particles have been fabricated, and the properties of the resultant particles were precisely tailored, such as the promising catalytic performance obtained over Ni/C and C@Ni particles in the hydrogenation of nitrobenzene. The present coating strategy is more simple and precisely controllable compared to the conventional deposition method and it is suitable for most precursors and even for multi-component materials, enabling the fabrication of nanostructured materials more easily and precisely.

A new concept of steaming multiwalled carbon nanotubes (MWCNTs) via acid vapour was presented for controllable nanoengineering of the MWCNTs. This method is more simple, effective, precisely-controllable and environmentally-friendly compared to traditional ones. Moreover, novel porous carbon nanotubes, named carbon nanoflutes, were fabricated based on this strategy.

The hydrogenation of o-chloronitrobenzene over Au/TiO2 was investigated in supercritical carbon dioxide (scCO2), ethanol, H2O, and H2O/CO2 at 140 °C. The reaction rate followed the order of H2O > H2O/CO2 > ethanol > scCO2. Au/TiO2 was deactivated in the systems containing CO2. Changes of Au/TiO2 catalysts were investigated by means of transmission electron microscopy (TEM), in situ infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance ultraviolet-visible spectra (DR-UV/Vis). Aggregation of Au particles was excluded from origins of deactivation by TEM. Carbonate-like species formed on gold in the presence of CO2 and H2 were detected by in situ IR spectroscopy. The results of XPS and DR-UV/Vis reveal that the active species of Au0 was oxidized in the presence of CO2. The possible formation of CO and its influence on the activity of Au/TiO2 were also discussed. Accordingly, the formation of carbonates, the oxidation of surface Au0 by CO2, and the CO formation are proposed to be the possible factors for the deactivation of Au/TiO2.Graphical abstractAu/TiO2 catalyst was found deactivated in scCO2. The results of XPS, TEM and DR-UV/Vis indicated that the formation of carbonates, the oxidation of surface Au0 by CO2, and the CO formation might be the possible factors for the deactivation of Au/TiO2.Research highlights▶ The catalytic performance of Au/TiO2 catalyst was discussed in the presence of supercritical carbon dioxide. ▶ Au/TiO2 is confirmed to deactivate during the hydrogenation of o-chloronitrobenzene in scCO2. ▶ Several factors were proposed to arouse the decrease of activity of Au/TiO2 catalyst: (a) carbonate-like species are formed on gold; (b) the Au0 species is partially oxidized by CO2 even in the presence of H2. (c) CO might be formed in the hydrogenation process and poisons some active sites.

The fabrication of nanostructured materials from hydrous inorganic metal salt processed in supercritical CO2 (scCO2) expanded liquids has advantages to obtain good results. However, the behavior of the inorganic salts and the detailed reaction mechanism are still unknown up to now. In this work, the actual behavior of hydrous inorganic metal salts in CO2 expanded ethanol at the temperature of 50–200 °C, including the phase behavior, deposition mechanism, reaction rate and the effect of templates on the deposition were systematically investigated by using XRD, FTIR, CHN-analysis, TGA, ICP-AES. A series of experimental parameters such as CO2 flow rate, expand procedure, reaction temperature and time have been discussed, as well as the roles of CO2 and H2O (often originated from salts and/or solvent but always neglected) were studied in this contribution. Significantly, a coordination–decomposition (C–D) was proved to be the converting mechanism for the precipitation of the hydrous inorganic metal salts, rather than the fuzzy decomposition.Graphical abstractHighlights► The actual behavior of hydrous inorganic metal salts in supercritical CO2 expanded ethanol was studied at the first time. ► The deposition mechanism of inorganic metal salts, reaction rate and the effect of templates on the deposition were systematically investigated. ► A series of experimental parameters and procedures have been discussed and optimized carefully, which will benefit the readers to repeat the experiment. ► The roles of CO2 and H2O (often originated from salts and/or solvent but always neglected) were discussed in detail. ► Coordination–decomposition (C–D) was proved to be the converting mechanism for the precipitation of the hydrous inorganic metal salts, rather than the fuzzy decomposition.

Solvation behavior of o-hydroxybenzoic acid (o-HBA) and m-hydroxybenzoic acid (m-HBA) in CO2 and methanol mixtures was investigated by molecular dynamics simulation. The results indicated that the distribution of methanol around o-, m-HBA molecules was different, and it was ascribed to the different hydrogen bonding numbers formed between methanol and HBA molecules. Moreover, the interaction or hydrogen bonds between m-HBA and methanol was much stronger than that between o-HBA and methanol, and with the increasing of CO2 pressure, it did not change for the former, but decreased for the latter. In addition, the local mole fraction enhancement was also studied. It was demonstrated that the methanol molecules become less aggregate with increasing CO2 pressure.Graphical abstractHighlights• The influence of co-solvent on the solubility of substrates in CO2 was investigated by molecular simulation. It is very important to understand the properties and the interactions from the molecular level. • It was found that the distribution of methanol around o-, m-hydroxybenzoic acid molecules was different, and it ascribed to the numbers of hydrogen bonding formed between methanol and hydroxybenzoic acids. • Based on the analysis of the total number of hydrogen bonding, the interaction between m-HBA and methanol are much stronger than that between o-HBA and methanol. • The methanol molecules around o-HBA and m-HBA become less aggregate with increasing CO2 pressure due to the increase of the number of CO2 around o-HBA and m-HBA.

An efficient method for dispersing active metal colloidal nanoparticles onto supports uniformly is revealed in this paper. The critical feature of this method is to separate the stabilizer simply with the phase-switch function of scCO2; with it the highly dispersed Pd colloidal catalysts were prepared, and the forming procedure is analyzed and discussed in detail. The present work not only overcame the common difficulties of removing the stabilizers in the preparation of supported colloidal particles, but also provided a facile and green process for preparing supported metal colloidal nanoparticles with using the environmentally benign solvent of scCO2.

The interactions between CO2 and carbonyl compounds at different CO2 pressures have been studied both experimentally and theoretically. In situ high-pressure FTIR on carbonyl compounds, i.e., acetaldehyde, acetone, and crotonaldehyde, in supercritical CO2 have been measured at various CO2 pressures varying from 6 to 22 MPa. In order to get insights into the mechanism, theoretical study has been conducted concerning the effect of CO2 on frequency shift of CO in acetaldehyde, acetone, benzaldehyde, crotonaldehyde and cinnamaldehyde at different CO2 pressures. It has been shown that the experimental frequency shifts can be well simulated by the theoretical model calculations using particular structures, in which a carbonyl compound interacts with a few CO2 molecules, depending on the carbonyl compounds examined, except for acetaldehyde.The interaction energies between CO2 and those carbonyl compounds are also given. In addition, the effect of CO2 on hydrogenation of crotonaldehyde and benzaldehyde has been discussed by means of the local softness (s+) calculated at CO2 pressures of 0–22 MPa, which can explain the reactivity difference in the crotonaldehyde and benzaldehyde hydrogenations in supercritical CO2.

Selective hydrogenation of citral was investigated over Au-based bimetallic catalysts in the environmentally benign supercritical carbon dioxide (scCO2) medium. The catalytic performances were different in citral hydrogenation when Pd or Ru was mixed (physically and chemically) with Au. Compared with the corresponding monometallic catalyst, the total conversion and the selectivity to citronellal (CAL) were significantly enhanced over TiO2 supported Pd and Au bimetallic catalysts (physically and chemically mixed); however, the conversion and selectivity did not change when Ru was physically mixed with Au catalyst compared to the monometallic Ru/TiO2, and the chemically mixed Ru-Au/TiO2 catalyst did not show any activity. The effect of CO2 pressure on the conversion of citral and product selectivity was significantly different over the Au/TiO2, Pd-Au/TiO2, and Pd/TiO2 catalysts. It was assumed to be ascribed to the difference in the interactions between Au, Pd nanoparticles and CO2 under different CO2 pressures.

CO2-in-Water (C/W) emulsion was formed by using a nonionic surfactant of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (P123), and palladium nanoparticles were synthesized in situ in the present work. The catalytic performance of Pd nanoparticles in the C/W emulsion has been discussed for a selective hydrogenation of citral. Much higher activity with a turnover frequency (TOF) of 6313 h−1 has been obtained in this unique C/W emulsion compared to that in the W/C microemulsion (TOF, 23 h−1), since the reaction was taking place not only in the surfactant shell but also on the inner surface of the CO2 core in the C/W emulsion. Moreover, citronellal was obtained with a higher selectivity for that it was extracted to a supercritical carbon dioxide (scCO2) phase as formed and thus its further hydrogenation was prohibited. The Pd nanoparticles could be recycled several times and still retain the same selectivity, but it showed a little aggregation leading to a slight decrease in conversion.

A clean process has been developed for the synthesis of p-menthane-3,8-diols from cyclization of citronellal in CO2–H2O medium without any additives. With the addition of CO2, the reaction rate could be enhanced about 6 times for the cyclization of citronellal in H2O, because CO2 dissolved into water and formed carbonic acid inducing an increase of the acidity. Although, the reaction conversion in CO2–H2O is slightly lower compared to that obtained with sulfuric acid as catalyst, CO2–H2O could replace the sulfuric acid at a relative higher reaction temperature. The reaction kinetics studies showed that the hydration of isopulegols to p-menthane-3,8-diols is a reversible reaction. The equilibrium constant and the maximum equilibrium yield obtained in CO2–H2O at a range of CO2 pressures are similar to that with sulfuric acid catalyst.

Ti70Zr10Co20 containing an icosahedral quasicrystalline phase has been fabricated, and presents high activity and selectivity in catalyzing the oxidation of cyclohexane with oxygen under solvent-free conditions.

In the present work, platinum nanoparticles were prepared by in situ reduction with polyethylene glycols (PEGs). The catalytic performance of Pt nanoparticles immobilized in PEGs (Pt-PEGs) is discussed for the hydrogenation of o-chloronitrobenzene (o-CNB). A high selectivity to o-chloroaniline (o-CAN) of about 99.7% was obtained with the Pt-PEGs catalysts at the complete conversion of o-CNB, which is much higher than that (83.4%) obtained over the conventional catalyst of Pt/C. The Pt nanoparticles could be immobilized in PEGs stably and recycled for four times with the same activity and selectivity. It presents a promising performance in the hydrogenation and its wide application in catalytic reactions is expected.Platinum nanoparticles were prepared and stabilized in polyethylene glycols (PEGs) successfully, and a high selectivity (>98%) to o-chloroaniline in the hydrogenation of o-chloronitrobenzene was obtained.

Hydrogenation of α,β-unsaturated aldehydes (citral, 3-methyl-2-butenal, cinnamaldehyde) has been studied with tetrakis(triphenylphosphine) ruthenium dihydride (H2Ru(TPP)4) catalyst in a poly(ethylene glycol) (PEG)/compressed carbon dioxide biphasic system. The hydrogenation reaction was slow under PEG/H2 biphasic conditions at H2 4 MPa in the absence of CO2. When the reaction mixture was pressurized by a non-reactant of CO2, however, the reaction was significantly accelerated. As CO2 pressure was raised from 6 MPa to 12 MPa, the conversion of citral increased from 35% to 98%, and a high selectivity to unsaturated alcohols (geraniol and nerol) of 98% was obtained. The products were able to be extracted by high pressure CO2 stream and separated from the PEG phase, dissolving the Ru complex catalyst and the catalyst was recyclable without any post-treatment.

The potential of CO2-expanded liquid media for chemical reactions has been examined in this work, using cyclohexane as a solvent and Pd/C as a heterogeneous catalyst for hydrogenation of styrene, citral, and nitrobenzene with H2. The rate of hydrogenation reactions is increased, and the product selectivity is altered in the CO2-expanded cyclohexane phase. In the hydrogenation of citral, the selectivity to citronellal decreases with CO2 pressure, which changes from ∼80% in the neat cyclohexane to ∼65% at 16 MPa. The CO2 dissolved in the cyclohexane phase may accelerate the rate of further hydrogenation of citronellal. In the hydrogenation of nitrobenzene, the selectivity to aniline is large >90% in the absence of dense phase CO2 but it increases up to >95% on CO2 pressurization of the liquid reaction phase.

Hydrogenation of o-chloronitrobenzene (o-CNB) to o-chloroaniline (o-CAN) with Pd/C has been investigated in supercritical carbon dioxide (scCO2) at 308 K. The influences of several parameters such as CO2, H2 pressures, Pd metal particle size and reaction time have been discussed. CO2 pressure presented markedly effects on the reaction rate and product selectivity under the reaction conditions used, the selectivity to o-CAN at CO2 pressure from 8 to 13 MPa (supercritical region) was larger than that at CO2 pressure below 6 MPa (subcritical region). Moreover, the larger selectivity to o-CAN was obtained in scCO2 compared with that in ethanol.In the hydrogenation of o-CNB over Pd/C, the presence of scCO2 could improve the yield of o-CAN. The reactive rate and the selectivity to o-CAN depended largely on CO2 pressure. In addition, the larger Pd particle benefited the formation of o-CAN in scCO2.

•The activity of Ni–Ir/TiO2 catalyst was four times as high as that of the Ni/TiO2 catalyst.•The reducibility of Ni was improved as addition of tiny amount of Ir.•The hydrogen spillover from Ir to Ni occurred in Ni–Ir/TiO2 catalyst.•The electronic structure of the surface Ni atoms was modified upon the addition of Ir.The catalytic performance of Ni/TiO2 and Ni–Ir/TiO2 catalysts in the selective hydrogenation of cinnamaldehyde (CAL) has been investigated. The Ni–Ir/TiO2 presented higher activity than the Ni/TiO2. Here, we studied and gave some insights into the interaction between Ni and Ir species and the role of Ir using X-ray diffraction (XRD), transmission electron microscope (TEM), hydrogen temperature-programmed reduction (H2 TPR), hydrogen temperature-programmed desorption (H2 TPD), and X-ray photoelectron spectroscopy (XPS). The size of Ni particles on the Ni–Ir/TiO2 was 10.1 nm, smaller than that (12.7 nm) on the Ni/TiO2. The reducibility of Ni was improved by addition of a small amount of Ir, as confirmed with the H2 TPR analysis. The Ni–Ir/TiO2 could be reduced at a temperature of 352 °C, which is lower than the temperature for Ni/TiO2 (385 °C); moreover, a new reduction peak appeared at 240 °C due to the stronger interaction of Ni–Ir species, which was certified by XPS analysis. The H2 TPD results indicate that the hydrogen spillover effect may occur in Ni–Ir/TiO2. The electronic structure of the surface Ni atoms was modified upon addition of Ir, resulting in an enhanced activity of the Ni–Ir/TiO2 catalyst, about four times as high as that of the Ni/TiO2 catalyst.Graphical abstractThe activity of Ni/TiO2 catalyst was significantly improved by adding a tiny amount of Ir, and the reaction rate on the Ni–Ir catalyst was four times higher than that on the Ni/TiO2 catalyst. The excellent catalytic performance and good recyclability of Ni–Ir/TiO2 can be ascribed to the high dispersion of Ir and the electronic transfer between Ni and Ir.Download high-res image (46KB)Download full-size image

Water as a green solvent to replace the conventional organic solvent presents many advantages in the organic synthesis. The hydrogenation of nitrobenzene in water has been investigated by using Ni/TiO2 catalyst in this work, and our main purpose was focused on the Ni/TiO2 catalyst activity and its stability improvement. The experimental results and analysis from the data of XRD, XPS, ICP revealed that the formation of nickel hydroxide from metallic nickel reacting with water caused a rapid deactivation of Ni/TiO2 catalyst. Based on these, we designed a catalyst with hydrophobic property to prevent the nickel active species to contact with water; thus, a hydrophobic carbon layer was coated on the surface of Ni/TiO2. As expected, the hydrophobic carbon was successfully coated on Ni/TiO2 catalysts by a hydrothermal method and they presented higher reactivity and improved stability in the present aqueous reaction system; nickel hydroxide was not detected on the used and water treated carbon-coated Ni/TiO2 samples. The improved abilities were attributed to the increased hydrophobicity of catalysts modified by carbon, which not only prevents water to contact with nickel catalytic species, but also protects the metallic nickel to be oxidized as it exposed to air.Graphical abstractThe activity and stability of Ni/TiO2 catalyst were significantly improved after the catalyst was modified by a carbon coating. The surface hydrophilic Ni/TiO2 catalyst changed to hydrophobic (Ni/TiO2)@C after coated with carbon, which inhibited the transformation of active nickel metallic species to Ni(OH)2 in H2O.Download high-res image (109KB)Download full-size imageHighlights► Formation of nickel hydroxide caused Ni/TiO2 deactivation in water. ► A hydrophobic carbon layer was successfully coated on the surface of Ni/TiO2. ► The activity and stability of the catalyst were improved significantly after carbon coating. ► The surface hydrophobicity prevented water to contact with nickel catalytic species forming nickel hydroxide.

The selective hydrogenation of nitrobenzene (NB) over Ni/γ-Al2O3 catalysts was investigated using different media of dense phase CO2, ethanol, and n-hexane. In dense phase CO2, the total rate of NB hydrogenation was larger than that in organic solvents under similar reaction conditions; the selectivity to the desired product, aniline, was almost 100% over the whole conversion range of 0–100%. The phase behavior of the reactant mixture in/under dense phase CO2 was examined at reaction conditions. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions of CO2 with the following reactant and reaction intermediates: NB, nitrosobenzene (NSB), and N-phenylhydroxylamine (PHA). Dense phase CO2 strongly interacts with NB, NSB, and PHA, modifying the reactivity of each species and contributing to positive effects on the reaction rate and the selectivity to aniline. A possible reaction pathway for the hydrogenation of NB in/under dense phase CO2 over Ni/γ-Al2O3 is also proposed.The complete selective hydrogenation of nitrobenzene to aniline can be achieved over conventional supported Ni catalysts under mild conditions in the presence of dense phase CO2.Download high-res image (81KB)Download full-size image

The activity and selectivity of the transition metal complexes formed from Ru, Rh, Pd and Ni with triphenylphosphine (TPP) have been investigated for hydrogenation of citral in supercritical carbon dioxide (scCO2). High activities are obtained with Ru/TPP and Pd/TPP catalysts, and the overall activity is in the order of Pd≈Ru > Rh > Ni. The Ru/TPP complex is highly selective to the formation of unsaturated alcohols of geraniol and nerol. In contrast, the Pd/TPP catalyst is more selective to partially saturated aldehydes of citronellal. Furthermore, the influence of several parameters such as CO2 and H2 pressures, N2 pressure and reaction time has been discussed. CO2 pressure has a significant impact on the product distribution, and the selectivity for geraniol and nerol can be enhanced from 27% to 75% with increasing CO2 pressure from 6 to 16 MPa, while the selectivity for citronellol decreases from 70% to 20%. Striking changes in the conversion and product distribution in scCO2 could be interpreted with variations in the phase behavior and the molecular interaction between CO2 and the substrate in the gas phase and in the liquid phase.High activities are obtained with Ru/TPP complexes for hydrogenation of citral in supercritical carbon dioxide (scCO2). CO2 pressure has a significant impact on the product selectivity. Striking changes in the conversion and selectivity in scCO2 could be interpreted with the variations in the phase behavior and the molecular interaction between CO2 and the substrate.

The hydrogenation of chloronitrobenzene to chloroaniline was investigated over Ni/TiO2 at 35 °C in supercritical CO2 (scCO2), ethanol, and n-hexane. The reaction rate followed the order of scCO2 > n-hexane > ethanol. In scCO2, the selectivity to chloroaniline and to aniline over Ni/TiO2 were 97–99.5% and <1%, respectively, in the conversion range of 9–100%. The high chemoselectivity to chloroaniline cannot be achieved over Ni/TiO2 in ethanol and n-hexane. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions of CO2 with the following reactant and reaction intermediates: chloronitrobenzene, chloronitrosobenzene, and N-chlorophenylhydroxylamine. The molecular interaction modifies the reactivity of each species and accordingly the reaction rate and the selectivity. The influence of Cl substituent on the interaction modes of CO2 with these reacting species is discussed. Possible reaction pathways for the hydrogenation of chloronitrobenzene in scCO2 over Ni/TiO2 are also proposed.The chemoselective hydrogenation of chloronitrobenzene to chloroaniline can be achieved at any conversion levels up to 100% over Ni/TiO2 in scCO2 at 35 °C.Download high-res image (52KB)Download full-size image

The selective hydrogenation of citral was studied with various TiO2-supported monometallic and bimetallic Pd and Au catalysts and their physical mixtures in supercritical CO2 (scCO2). Significant synergistic effects appeared when active Pd species was chemically or physically mixed with less active Au species. The total rate of conversion was greatly enhanced and the selectivity to citronellal (CAL) was improved. The physical properties of those catalysts were characterized by TEM, HRTEM-EDS, XPS, and UV/Vis and their features of H2 desorption were examined by TPD. The physical and chemical characterization results were used to discuss the reasons for the unexpected synergistic effects observed. The same selective hydrogenation was also conducted in a conventional non-polar organic solvent of n-hexane to examine the roles of scCO2. The use of scCO2 was effective for accelerating the hydrogenation of citral and improving the selectivity to CAL.The synergistic effects between Pd and Au or TiO2 make the chemically and physically mixed Pd/TO2 and Au/TiO2 present unexpected high reaction rate and selectivity in citral hydrogenation in scCO2.Download high-res image (64KB)Download full-size image

An efficient Ru/TiO2 catalyst was successfully designed and synthesized for the deoxygenation of long chain fatty acid esters under mild conditions (200 °C, 3.0 MPa). This work provides an energy-economic route to upgrade the oils with high oxygen content into green biofuels.

A new concept of steaming multiwalled carbon nanotubes (MWCNTs) via acid vapour was presented for controllable nanoengineering of the MWCNTs. This method is more simple, effective, precisely-controllable and environmentally-friendly compared to traditional ones. Moreover, novel porous carbon nanotubes, named carbon nanoflutes, were fabricated based on this strategy.

Ti70Zr10Co20 containing an icosahedral quasicrystalline phase has been fabricated, and presents high activity and selectivity in catalyzing the oxidation of cyclohexane with oxygen under solvent-free conditions.

In this work, we have developed a new method to synthesize a Fe3O4@graphene (Fe3O4@GN) composite. First, the precursor was synthesized through the decomposition of ferric nitrate in the presence of graphene oxide in the mixed solvent of CO2–expanded ethanol. Then, the precursor was converted to the Fe3O4@GN composite via thermal treatment in N2 atmosphere. With the help of the CO2–expanded ethanol, Fe3O4 nanoparticles were coated on the surface of GN completely and uniformly with high loading. However, it is difficult to load Fe3O4 particles onto the surface of GN and most of the Fe3O4 particles were deviated away from GN and aggregated to form larger units in pure ethanol. When used as anode for Li-ion batteries (LIBs), the Fe3O4@GN composite with a graphene content of 25 wt% synthesized in CO2–expanded ethanol manifested excellent charge–discharge cycling stability and rate performance compared with the sample synthesized in ethanol. Such improved electrochemical performances should be attributed to the intimate contact between the GN and Fe3O4 nanoparticles in the composite. Since the present method does not need tedious pre-treatment, surfactant, or precipitate, it is a green or sustainable technology and the solvents could be recycled easily after simple phase separation. This facile method can be extended to the synthesis of other metal oxide composites, which are expected to have good performance as anode materials for LIBs and other applications.

In this study, an effective method of slow hydrolization of metal alkoxide (e.g., Ti(C4H9O)4) in an ethanol–water system was systematically investigated and used to finely control the deposition of titania on carbon colloids. A model of adsorption–hydrolization of precursors during the coating process was rationally built for the first time to interpret the usability of the method and facilitate its further extension. Using this strategy, titania in the form of supported nanocrystals or layers on carbon colloids (TiO2/C, C@TiO2) was successfully tailored. Meanwhile, finely dispersed hollow TiO2 nanoparticles with shells consisting of different crystalline structures were also prepared by varying the calcination conditions after removing the carbon cores. More importantly, the effects of the crystalline and nano/macrostructures of the as-prepared TiO2 samples in photocatalysis and lithium-ion battery applications were analyzed in detail. The preliminary results show that anatase–rutile TiO2 hollow particles demonstrate a higher catalytic activity in the photo-degradation of rhodamine B than anatase TiO2 hollow particles, powders, and P25. However, in the case of Li-ion battery applications, the anatase TiO2 hollow particles exhibited better performance as anode materials with high capacities of around 190 mA h g−1, 140 mA h g−1, and 120 mA h g−1 at current densities of 60 mA g−1, 120 mA g−1, and 300 mA g−1, respectively, accompanied by stable cyclability.

Polyureas were synthesized from diamines and carbon dioxide in the absence of any catalyst or solvent, analogous to the synthesis of urea from condensation of ammonia with carbon dioxide. The method used carbon dioxide as a carbonyl source to substitute highly toxic isocyanates for the synthesis of polyureas. FTIR and DFT calculations confirmed that strong bidentate hydrogen bonds were formed between urea motifs, and XRD patterns showed that the PUas were highly crystalline and formed a network structure through hydrogen bonds, which served as physical cross-links. The long chain PUas presented a microphase separated morphology as characterized by SAXS and showed a high melting temperature above 200 °C. The PUas showed high resistance to solvents and excellent thermal stability, which benefitted from their special network structures. The PUas synthesized by this method are a new kind of functional material and could serve some areas where their analogues with similar functional groups could not be applied.

In this contribution, monodisperse porous hollow bi-phase γ-/α-Fe2O3 nanoparticles were successfully fabricated based on hard-template method with using carbon colloids as sacrificial templates. A new concept of assembling one kind of metal oxide with different crystalline structures into a single shell was presented for the first time. The critical procedure of coating carbon cores with a uniform layer of oxide was performed in CO2-expanded ethanol, which is a versatile way to produce high-quality hollow oxide nanoparticles. The formation of the novel bi-phase shell was achieved through combining the reduction ability of carbon cores under inert calcination atmosphere and the unique chemical composition of intermediate-shell formed in CO2-expanded ethanol. The porous hollow γ-/α-Fe2O3 nanoparticles with an average diameter of 99 nm not only possess combined properties of γ-Fe2O3 and α-Fe2O3, but also have a large specific surface area of 93.7 m2 g−1 and a high pore volume of 1.056 cm3 g−1, enabling them to have widespread applications in sensors, catalysis, magnetic and electrochemical areas, etc. Herein, such hollow bi-phase γ-/α-Fe2O3 nanoparticles were utilized to prepare a sensor device, and intriguingly it shows higher sensitivity and selectivity to ethanol than γ-Fe2O3 powders and many other porous α-Fe2O3 materials reported recently. The probable sensor mechanism of hollow γ-/α-Fe2O3 nanoparticles was discussed in detail.

In this work, we presented a new way for the functionalization of carbon nanotubes (CNTs) with the use of biomass as starting materials and introduced a novel concept of knitting process in the chemistry of CNTs for the first time. A mixture of aromatic compounds obtained from the hydrothermal treatment of biomass, rather than the traditional polymer monomers, was used as the nanoscale building blocks to knit an oxygenated network-coat on the CNTs layer-by-layer. It is an effective, mild, green and easily-controlled method for the functionalization of CNTs. The obtained f-CNTs were proved to be a promising catalyst support for metal catalysts, such as Ru/f-CNTs, showed high activity and selectivity for the hydrogenation of citral to unsaturated alcohol. More importantly, we opened a pioneering way for the conversion of low-cost, abundant and renewable biomass into a hydrophilic/chemical reactive network-coat on the inert surface of a wide range of sp2carbon materials, such as prevalent fullerene, carbon nanotubes, carbon nanohorns and hot grapheneetc.

In this contribution, we present an efficient, versatile and green strategy for finely controlling the metal (oxide) coating on core particles through in situ reaction of precursors in CO2 expanded ethanol without using any precipitants. It not only avoids the formation of free metal (oxide) and/or naked cores, but also permits individual dispersion of all the resultant particles without aggregation. With this method, the composition, thickness, uniformity, and structure of the metal (oxide) shell could be precisely controlled. A wide variety of unreported high-quality core-shell particles with a shell consisting of highly dispersed metal (oxide) nanocrystals or nanoalloys, such as C@Ni, CoO/C, C@Ni&Co and C@Ni&Pd particles have been fabricated, and the properties of the resultant particles were precisely tailored, such as the promising catalytic performance obtained over Ni/C and C@Ni particles in the hydrogenation of nitrobenzene. The present coating strategy is more simple and precisely controllable compared to the conventional deposition method and it is suitable for most precursors and even for multi-component materials, enabling the fabrication of nanostructured materials more easily and precisely.

In this work, we report a facile method for the synthesis of a Co3O4–functionalized carbon nanotube (Co3O4–f-CNT) composite via the growth of Co3O4 nanoparticles on the surface of functionalized carbon nanotubes (f-CNTs) by thermal decomposition of cobalt nitrate hexahydrate in ethanol. The composite consists of 13% carbon nanotubes and 87% Co3O4 nanoparticles by weight, and all the Co3O4 particles grew compactly along the carbon nanotube axis with a highly uniform dispersion. When used as an anode material for rechargeable lithium ion batteries, the composite manifested high capacities and excellent cycling performance at high and low current rates. The discharge capacity was 719 mA h g−1 at the 2nd cycle and 776 mA h g−1 at the 100th cycle. Even at a current density of 1 A g−1, the specific capacity still remained at about 600 mA h g−1. This superior electrochemical performance was attributed to the unique nanostructure of the composite. Because almost all of the Co3O4 nanoparticles were immobilized on the surface of f-CNTs, physical aggregation of nanoparticles was avoided during the charge–discharge processes. Furthermore, the good mechanical flexibility of f-CNTs can readily alleviate the massive volume expansion/shrinkage associated with a conversion reaction electrode. Finally, f-CNTs are highly conductive matrices for electrons due to their high conductivity, which can shorten the diffusion path for electrons.