Selective hydrogenation of citral with transition metal complexes in supercritical carbon dioxide

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.     

Products,pathways, and kinetics for reactions of indole under supercritical water gasification conditions

Indole is commonly reported as a product from hydrothermal processing of algal biomass. The reactions of indole in supercritical water were investigated between 550 and 700 °C in quartz, mini-batch reactors. The indole disappearance rate followed first-order kinetics, and the activation energy was 155 ± 10 kJ/mol. Methane and hydrogen were the most abundant gaseous products under most of the conditions tested, whereas benzene was the most abundant liquid-phase product. Hydrogen and carbon gasification efficiencies (HGE and CGE) exhibited values up to 79% and 20%, respectively. The influence of water density on the yields of H₂, CH₄, and C₂H₆ was negligible at densities above 0.081 g/ml, but the CO₂ yield increased with water density whereas the CO yield decreased. The yield of CH₄ increased significantly as the initial indole concentration increased. The collective results, which showed how the yields of numerous intermediate reaction products responded to changes in the process variables, permitted advancement of a potential reaction network.     

Anion supported TiO2–ZrO2 nanomaterial synthesized by reverse microemulsion technique as an efficient catalyst for solvent free nitration of halobenzene

Mixed oxide like TiO₂–ZrO₂ in nanoscale were synthesized using reverse microemulsion process and characterized by XRD, TEM, FT-IR and surface area measurement. Anion supported mixed oxide systems were evaluated for the solvent free nitration of halobenzene even at low temperature (30 °C) reaction. Nitration yield increases with the increase in reaction temperature up to 70 °C. Mostly 500 °C calcined sample was found to be most active catalyst for nitration reaction and further increase in the calcination temperature decreases the yield. Both the in situ and ex situ sulphated samples show higher yield and para selectivity even at low temperature reaction. Higher para selectivity is explained with a proposed mechanism involving Lewis acid site. It is also marked that the para selectivity decreases with the change in halo group from chloro to iodo keeping the total yield nearly same. The decrease in para selectivity can be explained on the basis of −I effect (Cl > Br > I) of the halo group which effects the nearest ortho position and hence the higher ortho product.     

Reaction coupling of diethylbenzene dehydrogenation with water–gas shift over alumina-supported iron oxide catalysts

Starting from the thermodynamic analysis of the coupling of diethylbenzene (DEB) dehydrogenation to divinylbenzene (DVB) with reverse water–gas shift, the dehydrogenation of DEB in the presence of carbon dioxide over alumina-supported iron oxide catalysts was carried out at 450–650 °C under atmospheric pressure with CO₂/DEB mole ratio of 10–70 and DEB liquid space velocity (LHSV) of 0.2–1.2 h⁻¹. The effects of reaction temperature, LHSV, CO₂/DEB ratio, as well as iron loading and addition of promoters upon DEB conversion and DVB selectivity were investigated. It was revealed that the yield of DVB and ethylstyrene (EST) could be greatly improved by the reaction coupling due to the simultaneous elimination of the hydrogen produced in the dehydrogenation by reverse water–gas shift. A typical result with EST + DVB selectivity of 90.3%, DVB yield of 43.9% and EST yield of 38.0% was obtained at 550 °C with DEB LHSV of 0.4 h⁻¹ and CO₂/DEB mole ratio of 40.     

Controllable synthesis of α-MoC1-x and β-Mo2C nanowires for highly selective CO2 reduction to CO

Transition metal carbides are attractive catalysts because of their similar properties to precious metals. Here, we report the controllable synthesis of α-MoC_1-x and β-Mo₂C nanowires as highly active and selective catalysts for CO₂ reduction to CO (CO₂ + H₂ → CO + H₂O, reverse water-gas shift reaction, RWGS). CO₂ conversion of > 60% together with nearly 100% CO selectivity was achieved at 600 °C, H₂:CO₂ molar ratio of 4:1, and space velocity of 36,000 mL g^− 1 h^− 1. A formate decomposition mechanism for the RWGS reaction was proposed based on the in-situ DRIFTS results.     

Hydrogenation of phenol in scCO2 over carbon nanofiber supported Rh catalyst

A catalyst of Rh nanoparticles supported on a carbon nanofiber, 5 wt.% Rh/CNF, with an average size of 2–3 nm has been prepared by a method of incipient wetness impregnation. The catalyst presented a high activity in the ring hydrogenation of phenol in a medium of supercritical CO₂ (scCO₂) at a low temperature of 323 K. The presence of compressed CO₂ retards hydrogenation of cyclohexanone to cyclohexanol under the reaction conditions used, and this is beneficial for the formation of cyclohexanone, increasing the selectivity to cyclohexanone. But the selectivity to cyclohexanone is very low at the completion of reaction in the absence of CO₂, at low CO₂ pressures, and in the presence of pressurized N₂ instead of CO₂. That is, high selectivity to cyclohexanone can be achieved with CO₂ species at higher pressures but not with the application of an inert hydrostatic pressure on the liquid substrate phase.     

Efficient reduction of nitroarenes using supercritical alcohols as a source of hydrogen in flow-type reactor in the presence of alumina

Reduction of nitroarenes using supercritical alcohols as a source of hydrogen was studied in a flow reactor in the presence of alumina at temperatures 515–615 K and residence times not exceeding 6 min. In the reaction of nitrobenzene reduction by 4-methylpentan-2-ol in supercritical (sc) CO₂, the selectivity for aniline reached 95% with a conversion of 71%. If methanol was used as the reducing agent, the main reaction product was N,N-dimethylaniline. It was shown that the use of scCO₂ co-solvent had a significant impact on the course of the reduction process including secondary and side reactions. The conditions for the reduction of halogenated nitroarenes into respective amines with high conversion and nearly 100% selectivity almost without dehalogenation were found.     

Effect of heteroatom substituted mesoporous support on the selective hydrogenation of cinnamaldehyde in supercritical carbon dioxide

Selective hydrogenation of cinnamaldehyde (CAL) is being carried out in supercritical carbon dioxide over Pt nanoparticle, supported on Al-MCM-48 and Ti-MCM-48. To evaluate the influence of the support, results are compared with Pt-MCM-48 (Si only). Pt-Al-MCM-48 and Pt-Ti-MCM-48, (containing metal particles of similar sizes) exhibits remarkable selectivity for cinnamyl alcohol (COL). A strong influence of heteroatom (Al and Ti) substitution is found in the activity and selectivity of the reaction. The rate of formation of COL follows the order of Pt-Ti-MCM-48 > Pt-Al-MCM-48 > Pt-MCM-48 under the same reaction conditions. We therefore propose that due to the interaction of Ti with CO₂, the support becomes activated, which increases the electron density of Pt and thereby interacts preferentially with the CO bond. Depending on the nature of the heteroatom, a significant change in the CO₂ pressure dependent activity and selectivity is observed. For Pt-Al-MCM-48, the maximum selectivity for COL is observed in the higher pressure (single phase) region. On the contrary, as the interaction of Ti and CO₂ is favorable in the lower pressure region, Pt-Ti-MCM-48 shows highest selectivity for COL at 7–8.5 MPa (biphasic region) and decreases in the single phase region. Conventional Arrhenius behavior is maintained for Pt-Al-MCM-48, whereas Pt-Ti-MCM-48 displays significant decrease in the reaction rate with increasing temperature.     

Phase equilibria for the 2-ethoxyethyl acetate and 2-(2-ethoxyethoxy)ethyl acetate in supercritical CO2 at various temperatures and pressures up to 20 MPa

The (CO₂ + 2-ethoxyethyl acetate) and (CO₂ + 2-(2-ethoxyethoxy)ethyl acetate) systems at 313.2, 333.2, 353.2, 373.2 and 393.2 K as well as pressures up to 20.59 MPa have been investigated using variable-volume high pressure view cell by static-type. The solubility curve of 2-ethoxyethyl acetate and 2-(2-ethoxyethoxy)ethyl acetate in the (CO₂ + 2-ethoxyethyl acetate) and (CO₂ + 2-(2-ethoxyethoxy)ethyl acetate) systems increases as the temperature increases at a constant pressure. The (CO₂ + 2-ethoxyethyl acetate) and (CO₂ + 2-(2-ethoxyethoxy)ethyl acetate) systems exhibit type-I phase behavior. The experimental results for the (CO₂ + 2-ethoxyethyl acetate) and (CO₂ + 2-(2-ethoxyethoxy)ethyl acetate) systems correlate with the Peng–Robinson equation of state using a van der Waals one-fluid mixing rule including two adjustable parameters. The critical properties of 2-ethoxyethyl acetate and 2-(2-ethoxyethoxy)ethyl acetate are predicted with the Joback–Lyderson group contribution and Lee–Kesler method.     

Supercritical CO2 extraction of lutein esters from marigold (Tagetes erecta L.) enhanced by ultrasound

As a novel technique, supercritical CO₂ (SC-CO₂) extraction enhanced by ultrasound was applied to the extraction of lutein esters from marigold and the extraction curves were described by Sovová model. The mass transfer coefficient in the solid phase (k_s) increased from 3.1 × 10⁻⁹ to 4.3 × 10⁻⁹ m/s due to ultrasound. The effect of extraction parameters including particle size of matrix, temperature, pressure, flow rate of CO₂, and ultrasonic conditions consisting of power, frequency and irradiation time/interval on the yield of lutein esters were investigated with single factor experiments. The results showed that the yield of lutein esters increased significantly with the presence of ultrasound (p < 0.05). The maximal yield of lutein esters (690 mg/100 g) was obtained for a particle size fraction of 0.245–0.350 mm, extraction pressure of 32.5 MPa, temperature of 55 °C and CO₂ flow rate of 10 kg/h with ultrasonic power of 400 W, ultrasonic frequency of 25 kHz and ultrasonic irradiation time/interval of 6/9 s.     

Microwave-assisted,rapid cycloaddition of allyl glycidyl ether and CO2 by employing pyridinium-based ionic liquid catalysts

This study investigated the use of pyridinium-based ionic liquids (ILs) as an efficient catalyst for the rapid solvent-free microwave-assisted cycloaddition of allyl glycidyl ether (AGE) and CO₂ to yield allyl glycidyl carbonate (AGC) under moderate reaction conditions. The cycloaddition reaction occurred over a short reaction time of 30 s, resulting in a high turnover frequency (TOF) ranging from 200 to 7000 h^− 1. The effects of alkyl chain length and anion of pyridinium-based catalysts on the cycloaddition reactivity were studied. The effects of reaction parameters such as the amount of catalyst, microwave power, CO₂ pressure, and reaction time were also investigated.     

Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose

Gasification of glucose in near- and supercritical water was investigated at temperature and pressure ranges from 400 to 600 °C and 20 to 42.5 MPa with a reaction time of 1 h. Hydrothermal gasification of glucose was performed in the absence and presence of catalyst (K₂CO₃) in a batch reactor. The influences of temperature and pressure in the supercritical regimes of water, catalyst were examined in relation to the yield and composition of the gases and aqueous products. The product gases were analyzed by gas chromatography, and the aqueous products were analyzed by high performance liquid chromatography. The gases produced were carbon dioxide, methane, hydrogen, carbon monoxide, and C₂–C₄ hydrocarbons and there was significant production of aqueous products and residue. The aqueous products composed of oxygenated compounds, including carboxylic acids (glycolic acid, formic acid, acetic acid), furfurals (furfural, 5-hydroxymethyl furfural, 5-methyl furfural), phenols (phenol, methyl phenols, hydroxy phenols, methoxy phenols), aldehydes (formaldehyde, acetaldehyde, acetone, propionaldehyde), ketones (3-methyl-2-cyclo-pentene-1-one, 2-cyclo-pentene-1-one) and their alkylated derivatives. Carbon gasification efficiencies were improved by addition of K₂CO₃ into the reacting system. Carbon gasification efficiency reached maximum (94%) at 600 °C and 20 MPa. The yield of hydrogen among gaseous products increased with increasing temperature and decreasing pressure.     

Microwave enhanced synthesis of MOF-5 and its CO2 capture ability at moderate temperatures across multiple capture and release cycles

The metal-organic framework, MOF-5 (Zn₄O(BDC)₃), was prepared using solvothermal synthesis under microwave irradiation, followed by solvent exchange to improve molecular stability at high temperatures, and assessed for its ability to capture CO₂ at ambient pressure and temperatures up to 300 °C. The reaction product was characterised by X-ray diffraction, scanning electron microscope, N₂ physisorption, thermogravimetric analysis and CO₂ physisorption. Cyclic CO₂ physisorption showed the capacity of the MOF-5 crystals to be 3.61 wt% when cycled between 30 °C and 300 °C through 10 separate capture and release cycles. Above 400 °C MOF-5 underwent thermal decomposition and was no longer capable of capturing CO₂.     

Enzyme-catalyzed Sequential Reduction of Carbon Dioxide to Formaldehyde

It has been reported that enzymatic-catalyzed reduction of CO₂ is feasible. Most of literature focuses on the conversion of CO₂ to methanol. Herein we put emphasis on the sequential conversion of CO₂ to formaldehyde and its single reactions. It appears that CO₂ pressure plays a critical role and higher pressure is greatly helpful to form more HCOOH as well as HCHO. The reverse reaction became severe in the reduction of CO₂ to formaldehyde after 10 h, decreasing HCHO production. Increasing the mass ratio of formate dehydrogenase to formaldehyde dehydrogenase could promote the sequential reaction. At concentrations of nicotinamide adenine dinucleotide lower than 100 mmol·L^− 1, the reduction of CO₂ was accelerated by increasing cofactor concentration. The optimum pH value and concentration of phosphate buffer were determined as 6.0 and 0.05 mol·L^− 1, respectively, for the overall reaction. It seems that thermodynamic factor such as pH is restrictive to the sequential reaction due to distinct divergence in appropriate pH range between its single reactions.     

Synthesis,characterization and kinetics properties of chromium(III) complex [Cr(3-HNA)(en)2]Cl · H2O · CH3OH

The reaction of chromium(III) chloride, 3-hydroxy-2-naphthoic acid (3-HNA) and ethylenediamine (en) led to the formation of complex [Cr(3-HNA)(en)₂]Cl · H₂O · CH₃OH, Bis(ethylenediamine-κ²N,N′)(3-hydroxy-2-naphthoic acid-κ²O,O′) chromium(III) monochloride monohydrate monomethanol. The kinetics of transfer of Cr(III) from the title compound to the low-molecular-mass chelator EDTA and to the iron-binding protein apoovotransferrin (apoOTf) were carried out by means of UV–Visible (UV–Vis) and fluorescence spectra in 0.01 M Hepes at pH 7.4. The second-order rate constants were calculated, respectively. The results show that Cr(III) can be transferred from the complex to apoovotransferrin.     

The catalytic activities and thermal stabilities of Li/Na/K carbonates for diesel soot oxidation

The alkali carbonates displayed a good catalytic activity for soot oxidation and their catalytic performances follow the order K₂CO₃ > Na₂CO₃ > Li₂CO₃ with soot ignition onset temperatures (IOTs) of 310 °C, 320 °C and 320 °C respectively. Na/K and Li/Na/K carbonate catalysts produced by combinations of the three alkali carbonates displayed the lowest IOT of about 320 °C that was higher than that of pure K₂CO₃. It was found that the variation of Li:Na:K molar ratios has very limited effect on the catalytic activity, but considerable effect on the thermal stability of the catalysts.Thermal treatment at 700 °C caused a limited change of IOT, but the deterioration of catalytic performance. In the Li/Na/K catalyst system, the formation of crystalline phases with low melting temperature was observed.     

Hydroformylation of 1-octene using rhodium–phosphite catalyst in a thermomorphic solvent system

The use of a liquid–liquid biphasic thermomorphic or temperature-dependent multicomponent solvent (TMS) system, in which the catalyst accumulates in one of the liquid phases and the product goes preferably to the other liquid phase, can be an enabling strategy of commercial hydroformylation processes with high selectivity, efficiency and ease of product separation and catalyst recovery. This paper describes the synthesis of n-nonanal, a commercially important fine chemical, by the hydroformylation reaction of 1-octene using a homogeneous catalyst consisting of HRh(PPh₃)₃(CO) and P(OPh)₃ in a TMS-system consisting of propylene carbonate (PC), dodecane and 1,4-dioxane. At a reaction temperature of 363 K, syngas pressure of 1.5 MPa and 0.68 mM concentration of the catalyst, HRh(CO)(PPh₃)₃, the conversion of 1-octene and the yield of total aldehyde were 97% and 95%, respectively. With a reaction time of 2 h and a selectivity of 89.3%, this catalytic system can be considered as highly reactive and selective compared to conventional ones. The resulting total turnover number was 600, while the turnover frequency was 400 h^?1. The effects of increasing the concentration of 1-octene, catalyst loading, partial pressure of CO and H₂ and temperature on the rate of reaction have been studied at 353, 363 and 373 K. The rate was found to be first order with respect to concentrations of the catalyst and 1-octene, and the partial pressure of H₂. The dependence of the reaction rate on the partial pressure of CO showed typical substrate inhibited kinetics. The kinetic behavior differs significantly from the kinetics of conventional systems employing HRh(CO)(PPh₃)₃ in organic solvents. Most notable are the lack of olefin inhibition and the absence of a critical catalyst concentration. A mechanistic rate equation has been proposed and the kinetic parameters evaluated with an average error of 5.5%. The activation energy was found to be 69.8 kJ/mol.     

High pressure plant for heterogeneous catalytic CO2 hydrogenation reactions in a continuous flow microreactor

The CO₂ hydrogenation to methanol is favored by high pressure from the thermodynamic point of view. Mostly experimental work on this reaction is limited at 400 bar due to technical and safety reasons. In this work we present a high pressure plant able to conduct CO₂ hydrogenation reactions at pressures up to 950 bar in a capillary microreactor; we focus on the influence of pressure concerning process intensification.To validate the plant functionality the reverse water–gas shift (RWGS) reaction was conducted over a 1 wt% Pt/CeO₂ catalyst at 450 °C and between 200 and 950 bar. A mass flow controller for hydrogen was developed due to lack of commercial available hydrogen mass flow controller able to work in the micro liter per minute range and up to 1000 bar. Additional to the RWGS reaction two more reactions take place. The first is the CO disproportionation reaction which results in deposited carbon on the catalyst. The second is the subsequent hydrogenation of carbon to methane. The experimentally determined CO₂ conversion is clearly below the equilibrium of the entire reaction network, hence the reaction is kinetically limited. The reaction performance increases with pressure showing process intensification.     

Density and viscosity of the binary polyethylene glycol/CO2 systems

Density of CO₂ saturated solutions of polyethylene glycols (PEGs) of different molecular weight was measured in pressure range from 8.0 MPa up to 47.7 MPa at a temperature of 343 K by a volumetric method. To validate the method density of pure CO₂ was measured at different pressures and a temperature of 293 K. The results were compared to the literature data and the accuracy was better than 2%. The density was between 1.17 g/mL for PEG 1000/CO₂ at 14.5 MPa and 1.78 g/mL for the system PEG 4000/CO₂ at 35 MPa. Further, the data were compared to results, obtained by a gravimetric method using magnetic suspension balance (MSB).Viscosity of CO₂ saturated solutions of polyethylene glycols (PEGs) of different molecular weight at different pressures and at a temperature of 343 K was measured using a high pressure view cell. Also a temperature impact on the viscosity of pure PEGs was observed at ambient pressure. After saturating PEG 1500 with 10 MPa of CO₂ pressure its viscosity decreases from 76.6 mPa s to 2.24 mPa s at 333 K. Further addition of CO₂ and increasing the pressure results in even lower viscosity and the highest viscosity reduction was reached at the highest pressure; at 35 MPa viscosity of the system PEG 1500/CO₂ is only 0.665 mPa s.     

Hydrogen storage using heterocyclic compounds: The hydrogenation of 2-methylthiophene

Alkyl substituted thiophenes are promising candidates for hydrogen carriers, as their dehydrogenation reactions are known to occur under mild conditions. Four types of catalysts, including supported noble metals, bimetallic noble metals, transition metal phosphides and transition metal sulfides, have been investigated for 2-methylthiophene (2MT) hydrogenation and ring-opening. The major products were tetrahydro-2-methylthiophene (TH2MT), pentenes and pentane, with very little C₅-thiols observed. The selectivity towards the desired product TH2MT follows the order: noble metals > bimetallics > phosphides > sulfides. The best hydrogenation catalyst was 2% Pt/Al₂O₃ which exhibited relatively high reactivity and selectivity towards TH2MT at moderate temperatures. Temperature-programmed reaction (TPR) experiments revealed that pentanethiol became the major product, especially with HDS catalysts like CoMoS/Al₂O₃ and WP/SiO₂.