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The direct formation of lactic acid (C3) from acetaldehyde (C2) and CO2 (C1) as a coupling agent catalyzed by heterogeneous Ni/MCM-41 provides an excellent CO2 incorporation route toward the synthesis of α-hydroxy acid from various aldehydes.

The in situ UV-vis-diffuse-reflectance spectroscopy system enables the observation of continuous FADH with a high S/N ratio on a gas/liquid mixture.

To suppress CO2 emissions, technologies that incorporate CO2 capture, utilization, and storage are being actively developed. Particularly, batteries using CO2 redox reactions are one of the most promising systems that combine energy conversion and storage. However, the stoichiometric reactions of CO2 and metal restrict the CO2 storage capacity of previously proposed systems. Applying catalyst-mediated CO2 redox is expected to provide flexible control of the CO2 storage capacity without dependence on the metal content. Herein, we report novel aqueous flow battery using CO2–formate redox with a bifunctional homogeneous Ir catalyst. Using of an Ir catalyst bearing a 4-hydroxy-N-methylpicolinamidate ligand was demonstrated for 50 cycles, with a maximum discharge capacity of 1.5 Ah L-1, capacity decay of 0.20% cycle-1, and total turnover number of 3,500. Furthermore, as the design concept, the CO2 storage capacity of the catalyst-based flow battery was improved more than 1000 times compared to the previously proposed system.

To mitigate environmental concern and energy issues, the conversion of carbon dioxide (CO2) to valuable and useful carbon chemicals offers a promising strategy for the development of a carbon neutral economy. The utilization of inert CO2 as a building block in the synthesis of multi-carbon (>2) oxygenated compounds, specifically propylene glycol methyl ether (PGME; C4H10O2), produced annually at a multi-million-ton scale from petroleum-based propylene oxide is of particular interest because of its multifaceted industrial and commercial applications. Herein, we present a simple and straightforward system that uses CO2 in compressed form as the C1 feedstock for the synthesis of PGME by direct hydrogenation and subsequent C–C coupling without any other sacrificial reagents. In addition, we combine experimental results with DFT calculations to elucidate the synergistic contributions of two catalytic metals (Ru and Ir) to activation of CO2 for hydrogenation and consequent mediation of the C–C bond formation leading to the generation of PGME. Taken together, our findings suggest that the strategy presented herewith may serve as a starting point for the development of a sustainable chemical synthesis platform for multi-carbon oxygenates by utilizing CO2 as the starting material.

PI fine particles encapsulating a large number of TiO2 nanoparticles (PI FPs/TiO2 NPs) were successfully fabricated rapidly and continuously by the emulsion re-precipitation method using a multistep flow synthetic system. The fabricated material, PI FPs/TiO2 NPs, was spherical in structure with a diameter of 214 nm, and the mean size of TiO2 NPs was 5.2 nm. Line scan elemental analysis with SEM-EDX showed that the TiO2 NPs were disproportionately embedded near the surface of the PI FPs. UV-vis transmission spectra revealed high UV shielding efficiency of the PI FPs/TiO2 NPs as the NPs are located near the surface.

A series of Cp*Ir (Cp* = pentamethylcyclopentadienyl anion) complexes with amino-functionalized ligands were developed for the production of high-pressure H2 via catalytic dehydrogenation of formic...

High-pressure H2 was produced by the selective dehydrogenation of formic acid (DFA) using ruthenium complexes at mild temperatures in various organic solvents and water. Among the solvents studied, 1,4-dioxane was the best candidate for this reaction to generate high gas pressure of 20 MPa at 80 °C using the Ru complex having a dearomatized pyridine-based pincer PN3P* ligand. This complex shows reusability for the high-pressure DFA in 1,4-dioxiane while maintaining the catalytic performance, however, deactivation occurred in other solvents. In dimethyl sulfoxide, its decomposition products may cause catalytic deactivation. The gas pressure generated in 1,4-dioxane was lower than that in water due to the high dissolution of 1,4-dioxane into CO2 according the vapor-liquid equilibrium calculations. The role of solvent is crucial since it affected the catalytic performance and also the generated gas pressure (H2 and CO2) from FA.

PI fine particles encapsulating a large number of TiO

Recently, the utilization of formic acid (FA) or formate as promising hydrogen carriers through the interconversion between CO2 and HCOOH or HCO3– and HCOO–, respectively, has attracted increasing research interest. In this work, a PdAu bimetallic catalyst supported on phenylenediamine-alkalized reduced graphene oxide (Pd0.50Au0.50/PDA-rGO) was developed for catalyzing bicarbonate hydrogenation under basic conditions as well as FA/formate dehydrogenation under acidic and basic conditions. Without any additives, a very high yield (94%) of potassium formate (PF) can be achieved from the hydrogenation of potassium bicarbonate at 50 °C for 16 h. On the other hand, initial TOFs of 1.63 × 103 and 6.98 × 103 h–1 were accomplished in the dehydrogenations of 6 mol/L PF and 8 mol/L FA, respectively, at 80 °C. This work successfully demonstrates highly efficient CO2 hydrogenation and is the first report of a Pd-based heterogeneous catalyst for the additive-free dehydrogenation of concentrated (>6 mol/L) PF or FA sol...

The development of a sustainable methodology for catalytic transformation of biomass-derived compounds to value-added chemicals is highly challenging. Most of the transitions are dominated by the use of additives, complicated reaction steps and large volumes of organic solvents. Compared to traditional organic solvents, alternative reaction media, which could be an ideal candidate for a viable extension of biomass-related reactions are rarely explored. Here, we elucidate a selective and efficient transformation of a biomass-derived aldehyde (furfural) to the corresponding alcohol, promoted in compressed CO2 using a Pt/Al2O3 catalyst. Furfural contains a furan ring with CC and an aldehyde group, and is extremely reactive in a hydrogen atmosphere, resulting in several by-products and a threat to alcohol selectivity as well as catalyst life. The process described has a very high reaction rate (6000 h−1) with an excellent selectivity/yield (99%) of alcohol, without any organic solvents or metal additives. This strategy has several key features over existing methodologies, such as reduced waste, and facile product separation and purification (reduced energy consumption). Combining the throughput of experimental observation and molecular dynamics simulation, indeed the high diffusivity of compressed CO2 controls the mobility of the compound, and eventually maintains the activity of the catalyst. Results are also compared for different solvents and solvent-less conditions. In particular, combination of an effective Pt catalyst with compressed CO2 provides an encouraging alternative solution for upgradation of biomass related platform molecules.

Herein, decarbonylation of biomass-based 5-hydroxymethylfurfural (HMF) in compressed CO2 with an unexpected acceleration of the reaction rate and excellent catalytic activity is reported. Without any additive, CO surrogates, or any organic solvents, via the developed method, an excellent conversion of 99.8% and highest selectivity of furfuryl alcohol (99.6%) in 4 h at 145 °C were achieved using an alumina-supported Pd catalyst (Pd/Al2O3). The superior activity is due to the unique characteristics (miscibility of reactant gases and high diffusivity) of compressed CO2 and the synergy between CO2 and Pd/Al2O3, where CO2 plays an interesting role in accelerating the reaction by enhancing the diffusion of CO and furfuryl alcohol (both products have high solubilities in CO2), consequently shifting the equilibrium to the forward direction. Characterisation of the catalyst suggested its direct interaction with the substrate and provided an indication of the possible reaction path. Thus, a mechanism was outlined. Compared to the results obtained using organic solvents, the results obtained using compressed CO2 were superior in terms of activity, selectivity, and reaction rate. This strategy highlights easy product separation, improved catalyst life, and a simple sustainable process. The efficiency of this protocol is confirmed by its potential application to a series of aldehydes with various substituents to produce decarbonylated products in good to excellent yields.

Hydrogen (H2) production from formic acid (FA) is highly attractive as a sustainable energy source from the interconversion between CO2 and FA. Dehydrogenation of FA at high pressures has advantages over a reaction at atmospheric conditions for the separation of H2 and CO2 due to the reaction and the volumetric energy density of H2. We demonstrated the continuous production of high-pressure H2 by catalytic decomposition of FA, and subsequent separation of H2 and CO2 from FA decomposition gas (H2 : CO2 = 1 : 1) using the phase change phenomenon at low temperatures while maintaining high pressure. An iridium aqua complex coordinated with a bidentate pyridyl-imidazoline ligand catalyzed the dehydrogenation of FA with high efficiency at a pressure as high as 153 MPa. The Ir catalyst was found to be stable under continuous addition of neat FA at high pressures. The generation time and rate of high-pressure H2 were controlled by feeding neat FA to the aqueous reaction system. Using our combined system, more than 99 mol% of H2 (96 mol% of purity) and 94 mol% of CO2 (99 mol% of purity) were separately obtained from FA as a gas and liquid, respectively, under the high-pressure conditions without any mechanical compression.

A changeable ligand, which involves in activation of a catalyst or assists a reaction, draws an increasing attention, in contrast to a classical ligand as spectator. Proton-responsive catalysts, which are capable of undergoing changes of properties on gaining/losing one or more protons, provides interesting features as follows: (i) catalyst activation by electronic effect, (ii) pH-tuning of water-solubility, and (iii) second-coordination-sphere interaction. On the basis of this catalyst design concept, we developed several highly efficient proton-responsive catalysts for CO2 hydrogenation as H2 storage, formic acid (FA) dehydrogenation as H2 production, and transfer hydrogenation. The transformable ligands of proton-responsive catalysts in promoting effective catalysis have aroused our interest. In this account, we summarize our efforts for the development and application of proton-responsive catalysts. Specifically, the important role of pH-dependent proton-responsive complexes will be discussed.

Catalytic hydrogenolysis of the C O bond of diphenyl ether (a lignin model compound) was investigated as a function of hydrogen pressure in scCO2 medium in the presence of water. Using commercially available Rh/C catalyst, the C O bond cleavage of diphenyl ether mainly results phenolic monomer at 80 °C. Hydrogen pressure is one of the key parameters because (i) C O bond cleavage and the hydrogenation of aromatic rings are two competitive reactions; very sensitive to hydrogen pressure and (ii) hydrogen has complete solubility in scCO2. Therefore, a critical control of hydrogen pressure was essential to reach the targeted cleavage of the C O bond when the reaction was conducted in scCO2 medium under pressurized condition. Depending on the hydrogen pressure, a significant change in the ratio of monocyclic:bicyclic products from 91:9 (0.2 MPa) to 58:42 (2 MPa) was revealed in the shortest reaction time of 5 min. Thus, low hydrogen pressure was the effective choice for the scission of the C O bond, whereas higher hydrogen pressure hydrogenate the aromatic ring due to the higher coverage of hydrogen on the catalytic surface. Amount of the catalyst (catalyst:substrate ratio) displayed a subtle effect on the breakage of the C O bond. A threshold ratio of 1:5 was preferred under the present reaction condition as the increased amount hampered the substrate:water ratio and hydrogenation of the aromatic ring occurred. In addition, as the change in temperature is associated with the change in the physical properties of scCO2, hence, the effect on the transformation of DPE was complicated and difficult to explain. Furthermore, different organic solvents as neat, along with CO2 and with water also has substantial impact on the rapture of C O bond. The obtained results from the solvent studies again proved that scCO2 along with water was the best choice for C O bond breakage and water is the driving force to mediate the reaction. In addition, a combination catalyst (Ni+Rh) was also tested for the same reaction under the similar working condition. Preliminary results suggested a synergistic effect in terms of the selectivity of monocyclic compounds.

The dehydrogenation of biomass-based 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) was achieved utilizing an activated carbon supported rhodium (Rh/C) catalyst under mild reaction conditions. The developed method successfully afforded complete conversion and the highest selectivity of DFF (>99%) without any additive, conventional hydrogen acceptor and oxidant. The efficiency of the method is achieved by the addition of compressed carbon dioxide (scCO2) and the synergistic effect of scCO2 and Rh/C, where scCO2 plays a pivotal role in accelerating the reaction by removing hydrogen, and consequently shifting the equilibrium to the forward direction. Optimization of different reaction parameters ensures the achievement of high conversion and selectivity. Characterization of the catalyst using different spectroscopic techniques suggests an interaction between the substrate and the catalyst and provides an indication of the possible reaction pathway, thus a mechanism would be outlined. The rate determining step of the reaction was calculated through mechanistic investigations involving theoretical calculations together with experimental analysis. One of the most attractive features of the method developed in this study is the reverse reaction of DFF, which can be achieved in one-pot without the addition of any external hydrogen. This process has successful application to the dehydrogenation of a variety of alcohols with different substituents.

High-pressure hydrogen is of great interest in the industrial utilization of hydrogen energy, especially for hydrogen fuel cell vehicles. In this work, a method of automatic high-pressure H2 generation by the decomposition of formic acid, a recently renowned hydrogen storage material, in the presence of a heterogeneous catalyst (palladium nano-particles on a graphene oxide catalyst (Pd/PDA–rGO)) was proposed. This catalyst can effectively catalyze the decomposition of formic acid to produce high-pressure H2 and CO2 over 35 MPa without any detectable formation of CO or other by-products. For example, a 36.3 MPa total gas pressure was successfully achieved using an aqueous solution of 6.7 mol L−1 formic acid and 6.7 mol L−1 sodium formate at 80 °C. This research provided a preliminary study on the automatic high-pressure hydrogen gas generation by the decomposition of formic acid without any compression facilities in the presence of a heterogeneous catalyst, which can be easily separated from the reaction process, for hydrogen energy utilization.

We report a ruthenium complex containing an N,N′-diimine ligand for the selective decomposition of formic acid to H2 and CO2 in water in the absence of any organic additives. A turnover frequency of 12 000 h–1 and a turnover number of 350 000 at 90 °C were achieved in the HCOOH/HCOONa aqueous solution. Efficient production of high-pressure H2 and CO2 (24.0 MPa (3480 psi)) was achieved through the decomposition of formic acid with no formation of CO. Mechanistic studies by NMR and DFT calculations indicate that there may be two competitive pathways for the key hydride transfer rate-determining step in the catalytic process.

A simple and continuous high-pressure (>120 MPa) hydrogen production system was developed by the selective decomposition of formic acid at 80 °C using an iridium complex as a catalyst, with a view to its application in future hydrogen fuel filling stations. The system is devoid of any compressing system. The described method can provide high-pressure H2 with 85 % purity after applying an effective gas–liquid separation process to separate the generated gas obtained from the decomposition of formic acid (H2/CO2=1:1). The efficiency of the catalyst lies with its high turnover frequency (1800 h−1 at 40 MPa) to produce high-pressure H2 with a good lifetime of >40 h. Interestingly, only very low levels carbon monoxide (less than 6 vol ppm) were detected in the generated gas, even at 120 MPa.

A high-pressure and high-temperature (HPHT) water microflow chemical process was utilized for the synthesis of benzazole derivatives. The current approach enables the extremely rapid production of various 2-arylbenzazoles including benzimidazoles, benzoxazoles, and benzthiazole in excellent yields.

A simple and highly efficient method was developed for the transformation of furfural (a biomass derived aldehyde) to furfurylamine by reductive amination using an aqueous solution of ammonia and molecular hydrogen as an amine source and a reducing agent, respectively. By choosing a suitable catalyst, such as Rh/Al2O3, and reaction conditions, a very high selectivity of furfurylamine (∼92%) can be achieved within the reaction time of 2 h at 80 °C. A detailed analysis of the reaction system sheds some light on the reaction pathway and provides an understanding about each elementary step. The reaction was believed to proceed via an imine pathway although no such intermediate was detected because of the highly reactive nature. Optimization of different reaction parameters such as hydrogen pressure, temperature and substrate/ammonia mole ratio is shown to be critical to achieve high selectivity of furfurylamine. Time-dependent reaction profiles suggested that a Schiff base type intermediate was in the detectable range, which offers indirect evidence of the formation of imine. Competitive hydrogenation and amination of an aldehyde group were strongly dictated by the nature of the metal used. The studied protocol represents an environmentally benign process for amine synthesis, which can be effectively extended to the other aldehydes also. The studied catalyst could be recycled successfully without any significant loss of catalytic activity.

The development of a sustainable methodology for catalytic transformation of biomass-derived compounds to value-added chemicals is highly challenging. Most of the transitions are dominated by the use of additives, complicated reaction steps and large volumes of organic solvents. Compared to traditional organic solvents, alternative reaction media, which could be an ideal candidate for a viable extension of biomass-related reactions are rarely explored. Here, we elucidate a selective and efficient transformation of a biomass-derived aldehyde (furfural) to the corresponding alcohol, promoted in compressed CO

The use of supercritical carbon dioxide (scCO2)/water for the conversion of diphenyl ether was investigated over Rh/C catalysts to achieve the cleavage of C–O bond under mild reaction conditions. In the studied reaction system, ethereal C–O bond was cleaved by hydrogenolysis and hydrolysis, which was strongly hampered if the reaction was conducted individually in CO2 or in water. A combined effect was desirable to achieve targeted cleavage because of the beneficial effect of a mild acidic environment generated through the dissolution of CO2 in water. Optimization of hydrogen pressure suggested a competition between diphenyl ether and hydrogen as the reaction rate (TOF) was significantly decreased with increased hydrogen pressure. In addition, the role of water was also evaluated and prompted the requirement of a suitable amount of water for cleavage. Mechanistic investigations through theoretical calculations together with experimental analysis illustrated that hydrogenolysis and hydrolysis were two reaction paths responsible for the rapture of the C–O bond.

The hydroxyl groups of a 2,2'-bipyridine (bpy) ligand near the metal center activated the catalytic performance of the Ir complex for the dehydrogenation of formic acid at high pressure. The position of the hydroxyl groups on the ligand affected the catalytic durability for the high-pressure H2 generation through the decomposition of formic acid. The Ir complex with a bipyridine ligand functionalized with para-hydroxyl groups shows a good durability with a constant catalytic activity during the reaction even under high-pressure conditions, whereas deactivation was observed for an Ir complex with a bipyridine ligand with ortho-hydroxyl groups (2). In the presence of high-pressure H2 , complex 2 decomposed into the ligand and an Ir trihydride complex through the isomerization of the bpy ligand. This work provides the development of a durable catalyst for the high-pressure H2 production from formic acid.

Kinetic studies on the catalytic reaction mechanism of formic acid (FA) dehydrogenation were performed in the presence of a water-soluble iridium complex bearing a 4,4′-dihydroxy-2,2′-bipyridine ligand. Determination of kinetic isotope effects revealed that a shift of the rate-limiting step at low and high concentrations of FA can be caused by the pH dependence of the reaction steps. The proposed equation for the reaction rate corresponds well with the experimental results concerning the shift phenomena. Towards industrial application in future hydrogen fueling stations, this will able the design of a dehydrogenation system catalyzed by the iridium complex.

In this work, supercritical carbon dioxide (scCO2), and scCO2/H2O were investigated for the processing of biomass-derived compounds such as 5-hydroxymethylfurfural (5-HMF), furfural and tetrahydrofurfuryl alcohol (THFA) into a vast array of fuel and non-fuel related chemicals. Higher solubility of reactant gasses in scCO2 results in an acceleration of the reaction rate and enhancement of the product selectivity. As a reaction medium, scCO2 offers promise to play a role in the conversion of 5-HMF to linear alkane, which relies heavily on hydrogen concentration. Furthermore, 5-HMF can be successfully converted to 2,5-dimethylfuran (a fuel additive) with the highest selectivity (100%) in ascCO2/H2O mixture. In the presence of H2O, scCO2 creates an acidic environment and contributes to improving the selectivity of 2,5-dimethylfuran. Similarly, furfural also produces 2-methylfuran through the hydrogenation/hydrogenolysis of the C–O bond. 1,5-pentanediol, which is used as a monomer in the polyester industry, could be conveniently obtained with 91% selectivity from THFA in scCO2 under the homogeneous conditions of a CO2–H2-substrate. Hence, the remarkable advantage of the present catalytic system has confirmed the potential utilisation of alternative “green” solvents in the conversion of different biomass based compounds.

The hydrogenation of HMF has been conducted in a neutral aqueous medium. Without any additive, HMF was hydrogenated to 2,5-bis-(hydroxymethyl)furan (BHMF) with complete conversion and selectivity (98.9%) using Pt/MCM-41 as catalyst. A very low temperature of 35 °C and 0.8 MPa of hydrogen pressure was used to accomplish the highest selectivity of BHMF within a reaction time of 2 h. Different reaction parameters such as reaction time, hydrogen pressure and the amount of water was optimized to achieve the highest catalytic activity. In particular, the presence or absence of water and the amount of water played an important role to determine the conversion and product distribution of the reaction. For instance, in the absence of water or a large excess of water, the selectivity of BHMF was decreased. In addition, instead of water the influence of three different groups of organic solvent were also explored to obtain BHMF under the studied reaction conditions. It has been observed that the studied organic solvents strongly influenced the catalytic performance, such as solvents with a negative δ value, which followed a clear trend with the substrate conversion, whereas no impact was observed for solvents with a positive δ value. Catalyst recycling experiments revealed that the catalyst could be recycled several times without any significant loss of catalytic activity.

Aiming to develop a highly effective and durable catalyst for high-pressure H2 production from dehydrogenation of formic acid (DFA), the ligand effect on the catalytic activity and stability of Cp*Ir (Cp*:pentamethylcyclopentadienyl anion) complexes were investigated using 5 different kinds of N,N'-bidentate ligands (bipyridine, biimidazoline, pyridyl-imidazoline, pyridyl-pyrazole and picolinamide). The Ir complex with biimidazoline ligand exhibited the highest catalytic activity, but deactivation occurred readily at high pressure. The pyridine moiety in the ligand can enhance the stability of Ir complex catalysts for the high-pressure reaction. The Ir complex catalyst containing pyridyl-imidazoline ligand showed the high activity and best stability under the high-pressure conditions.

The photocatalytic reduction of CO 2 to CO in single-phase, high-pressure mixtures of supercritical CO 2 and N , N -dimethylformamide (DMF), using fac -ReCl(bpy)(CO) 3 as a catalyst in the presence of triethanolamine as a sacrificial electron donor has been investigated. The catalytic efficiency was found to be strongly influenced by both the CO 2 and DMF concentrations. For example, the turnover number (TON) for CO formation increases linearly with CO 2 pressure up to 60 at 17.8 MPa/60 °C. It also increases dramatically as the [DMF] is increased from 3.8 to 6.3 M, and then remains almost constant with further increases in [DMF]. This resulted in an optimized TON of 62 at 17.8 MPa CO 2 /60 °C and [DMF] = 6.3 M, and an initial turnover frequency (TOF) of ∼56 h −1 at 17.8 MPa CO 2 /60 °C and [DMF] = 11.4 M. These values are 5× and 2.3× higher, respectively than those obtained under biphasic low-pressure conditions (0.1 MPa CO 2 ), demonstrating that significant improvements in catalytic performance are possible through the use of high concentrations of CO 2 . Manipulation of the incident light intensity led to further improvements in both TON (up to 72 under low light levels) and TOF (up to 117 h −1 under the highest light level investigated).

Potential-dependent transformations of surface structures, Pt oxidation states, and Pt–O bondings in Pt/C, Au(core)-Pt(shell)/C (denoted as Au@Pt/C), and Pd(core)-Pt(shell)/C (denoted as Pd@Pt/C) cathode catalysts in polymer electrolyte fuel cells (PEFCs) during the voltage-stepping processes were characterized by in situ (operando) X-ray absorption fine structure (XAFS). The active surface phase of the Au@Pt/C for oxygen reduction reaction (ORR) was suggested to be the Pt3Au alloy layer on Au core nanoparticles, while that of the Pd@Pt/C was the Pt atomic layer on Pd core nanoparticles. The surfaces of the Pt, Au@Pt and Pd@Pt nanoparticles were restructured and disordered at high potentials, which were induced by strong Pt–O bonds, resulting in hysteresis in the structural and electronic transformations in increasing and decreasing voltage operations. The potential-dependent restructuring, disordering, and hysteresis may be relevant to hindered Pt performance, Pt dissolution to the electrolyte, and degra...

A high-pressure and high-temperature (HPHT) water microflow chemical process was utilized for the synthesis of benzazole derivatives. The current approach enables the extremely rapid production of various 2-arylbenzazoles including benzimidazoles, benzoxazoles, and benzthiazole in excellent yields.

Carbon dioxide may constitute a source of chemicals and fuels if efficient and renewable processes are developed that directly utilize it as feedstock. Two of its reduction products are formic acid and methanol, which have also been proposed as liquid organic chemical carriers in sustainable hydrogen storage. Here we report that both the hydrogenation of carbon dioxide to formic acid and the disproportionation of formic acid into methanol can be realized at ambient temperature and in aqueous, acidic solution, with an iridium catalyst. The formic acid yield is maximized in water without additives, while acidification results in complete (98 %) and selective (96 %) formic acid disproportionation into methanol. These promising features in combination with the low reaction temperatures and the absence of organic solvents and additives are relevant for a sustainable hydrogen/methanol economy.

A highly efficient and recyclable Ir catalyst bearing a 4,7‐dihydroxy‐1,10‐phenanthroline ligand was developed for the evolution of high‐pressure H2 gas (>100 MPa), and a large amount of atmospheric pressure H2 gas (>120 L), over a long term (3.5 months). The reaction proceeds through the dehydrogenation of highly concentrated aqueous formic acid (FA, 40 vol %, 10 mol L−1) at 80 °C using 1 μmol of catalyst, and a turnover number (TON) of 5 000 000 was calculated. The Ir catalyst precipitated after the reaction owing to its pH‐dependent solubility in water, and 94 mol % was recovered by filtration. Thus, it can be treated and recycled like a heterogeneous catalyst. The catalyst was successfully recycled over 10 times for highpressure FA dehydrogenation at 22 MPa without any treatment or purification.

An exceptionally rapid hydrogenation of nitrobenzene to aniline [TOF=252,000 h−1] over palladium containing MCM-41 (Pd/MCM-41) with excellent yield of >99% can be achieved in supercritical carbon dioxide at 50 °C and a hydrogen pressure of 2.5 MPa. It has been observed that this promising method preferred a single phase between liquid substrate and carbon dioxide-hydrogen system. The ascendancy of the supercritical carbon dioxide medium is established in comparison with the conventional organic solvent and solvent-less conditions. Changes in the reaction parameters such as carbon dioxide and hydrogen pressure, temperature and the reaction time do not affect the selectivity. A combined experimental and theoretical study has elucidated the mechanism under the studied reaction condition because experimental observations revealed a direct conversion of nitrobenzene to aniline. However, density functional theory (DFT) calculation shows that the direct conversion is energetically unfavourable; hence, a stepwise mechanism has been proposed. Theoretical predictions and experimental observations suggested that the rate-limiting step of nitrobenzene conversion is different from that of the liquid phase hydrogenation. This catalytic process can also be successfully extended to the hydrogenation of other aromatic nitro compounds with different substituents. Easy separation of the liquid product from catalyst and the use of an environmentally friendly solvent make this procedure a viable and an attractive green chemical process.

Hydrothermal treatment was conducted on alginate in the interest of obtaining organic acids. Formic acid, acetic acid, lactic acid, glycolic acid, 2-hydroxybutyric acid, succinic acid, malic acid, mannuronic acid and guluronic acid were obtained by the hydrothermal treatment of alginate. The total yield of the organic acids were 46% at maximum yield 350 °C, 40 MPa and 0.7 s reaction time. The formation of organic acids, suggest that the carboxyl group structure of the alginate was preserved during the hydrothermal decomposition of the alginate. The formation of dicarboxylic acids is evidence that oxidation reactions occur during the hydrothermal treatment, introducing carboxyl groups into the decomposition products. The product distribution indicates that both acid and base catalyzed reactions occurred during the hydrothermal treatment of alginate. Hydrothermal treatment of an uronic acid, glucuronic acid, gave the same organic acids as those obtained from hydrothermal treatment of alginate. The reaction from alginate to organic acids probably proceeds via the formation of hexuronic acid (mannuronic acid and guluronic acid) under hydrothermal conditions.

Hydrogenation of aniline to cyclohexylamine was carried out in supercritical carbon dioxide using a variety of noble metal (Pt, Pd and Rh) catalysts. At 80 °C and 8 MPa of CO2 pressure, >95% of aniline conversion with 93% selectivity to cyclohexylamine was achieved on 5% Rh/Al2O3. A strong influence of phase behaviour related to the CO2 pressure was found on the conversion and selectivity. Optimization of reaction parameters resulted in a higher overall activity in the biphase (liquid substrate + gaseous H2 and CO2) than in the single phase (liquid substrate–CO2–H2) condition. It has been found that the interaction of CO2 with amine leads to the formation of solid carbamic acid, which enhanced the selectivity of cyclohexylamine, but reduced the conversion significantly. Furthermore, reaction temperature played a crucial role in preventing the formation of carbamic acid and also maintained a reasonably high reaction performance in terms of conversion and selectivity.

A new reaction setup equipped with micro tubular reactor with thin catalytic metal inner surface (Pd or Pd–Cu alloy) was developed for the reactions in high-pressure and high-temperature water (HPHT-H2O) flow. To evaluate the catalytic performance of this micro tubular reactor, Sonogashira C–C coupling of iodobenzene and ethynylbenzene was investigated by using this system under alkaline aqueous medium. Under the optimized reaction conditions of 250 °C and 16 MPa of pressure, diphenylethyne was obtained with good yield and 100% selectivity within a very short residence time of ∼1.6 s. The reaction proceeds without any additional promoters, additives or organic solvents. Leaching of catalytic metal from the reactor tube was not found under the studied reaction conditions. Alloying of Pd with Cu remarkably improved the efficiency of the reaction due to co-catalytic effect that is related to the easy electron transfer between two metals.

High-pressure and high-temperature water (HPHT-H2O) microreaction system was proven to be an efficient method to carry out the non-catalytic Claisen rearrangement. Allyl phenyl ether undergoes Claisen rearrangement to obtain o-allylphenol with a very high yield and selectivity of 98% within a short reaction time of 13.4 s at 265 °C and 5 MPa pressure. Compare to the solvent-free microreaction, HPHT-H2O plays an important role to accelerate the reaction as a catalyst by transferring a proton along locally formed hydrogen bond with the substrate. Finally, the obtained product can be separated completely from the aqueous suspension within the time range of 40 min to 24 h after the reaction. Even though the reaction has occurred in aqueous conditions, little side reactions, such as hydrolysis, hydration, or pyrolysis, was found.

Selective catalytic oxidation of geraniol to citral with molecular oxygen in supercritical carbon dioxide (scCO 2 ) has been investigated. The catalyst used was a chromium containing mesoporous molecular sieve catalyst viz. CrMCM-41. Comparison studies were performed with CoMCM-41, PtMCM-41 and PdMCM-41 catalysts. Among the various catalysts studied, CrMCM-41 showed a high conversion of geraniol and an excellent selectivity for citral. In contrast CoMCM-41, PtMCM-41 and PdMCM-41 catalysts exhibited low conversion of geraniol. However all three catalysts compared showed similar citral selectivity to CrMCM-41. The effect of CO 2 pressure and reaction temperature geraniol oxidation was studied with CrMCM-41. Supercritical CO 2 medium was found to enhance the conversion of geraniol and/or yield of citral. It was noticed that the catalyst can be recycled with negligible loss of conversion.