Performance In Terms Of Conversion Research Articles

Ruthenium incorporation into hydrotalcites-derived mixed oxides for phenol hydrogenation: Role of Mg/Al molar ratio

In this work the catalytic behaviour of Ru supported on mixed oxides derived from non-commercial hydrotalcites in phenol hydrogenation was studied in a batch reactor working at 30 bar and 200 °C. To this end, a set of catalysts with 2 wt% Ru and a Mg/Al molar ratio of 1, 2, 3 and 4 was synthesized. The catalysts were tested in phenol hydrogenation to assess the influence of Mg/Al ratio on the catalytic performance in terms of conversion and selectivities to cyclohexanol and cyclohexanone. Physicochemical characterization was performed by X-ray Diffraction (XRD), N2 adsorption-desorption isotherms, dispersive X-ray spectroscopy in scanning transmission electron microscopy (EDS-STEM), CO chemisorption at 35 °C, CO2 and NH3 thermoprogrammed desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). The Mg/Al molar ratio employed determined the catalytic response of the resulting catalysts, obtaining the best catalytic performance (95.9 % conversion and 47.4 % cyclohexanone selectivity) with the sample with the lowest Mg/Al ratio, RuMA1. XRD results showed that in RuMA1 catalyst the hydrotalcite structure was completely transformed into the corresponding mixed oxide after thermal treatment and was also the only one in which MgAl2O4 spinel was not formed. In addition, RuMA1 presented the highest specific surface area, the greatest Ru dispersion, as evidenced by CO-chemisorption and EDX-STEM analysis, as well as a good balance between basic and acid sites and a greater proportion of Bronsted acid sites that also explain it greater selectivity to cyclohexanol.

Effects of Operating Parameters and Feed Gas Compositions on the Dry Reforming of Methane over the Ni/Al2O3 Catalyst

The effects of operating parameters such as reaction temperature, space velocity, and feed gas composition on the performance of the methane dry-reforming reaction (DRM) over the Ni/Al2O3 catalyst are systemically investigated. The Ni/Al2O3 catalyst, which is synthesized by conventional wet impregnation, showed well-developed mesoporosity with well-dispersed Ni nanoparticles. CH4 and CO2 conversions over the Ni/Al2O3 catalyst are dramatically increased as both the reaction temperature is increased, and space velocity is decreased. The feed gas composition, especially the CO2/CH4 ratio, significantly influences the DRM performance, catalyst deactivation and the reaction behavior of side reactions. When the CO2-rich gas composition (CO2/CH4 > 1) was used, a reverse water gas shift (RWGS) reaction significantly occurred, leading to the consumption of hydrogen produced from DRM. The CH4-rich gas composition (CO2/CH4 < 1) induces severe carbon depositions followed by a reverse Boudouard reaction, resulting in catalytic activity drastically decreasing at the beginning followed by a stable conversion. The catalyst after the DRM reaction with a different feed ratio was analyzed to investigate the amount and structure of carbon deposited on the catalyst. In this study, we suggested that the optimal DRM reaction conditions can achieve stable performances in terms of conversion, hydrogen production and long-term stability.

Metal cocatalyst mediated photocatalytic dehydrogenative-condensation and direct condensation cross-coupling of aniline and alcohol

Cross coupling of aniline and alcohol for the synthesis of imine and secondary amines represents a highly atom efficient processes, yet achieving high selectivity requires specialized catalysts and harsh reaction conditions to avoid over-hydrogenation of as-produced imines and dehydrogenation of secondary amines. Here we employ metal cocatalysts to mediate photocatalytic activation of alcohols, thus separating the stepwise dehydrogenative-condensation process to imine and the direct condensation process to secondary amine. The Rh/TiO2 promotes the photocatalytic dehydrogenation of alcohol into aldehyde and molecular hydrogen, resulting in rapid condensation of aldehyde with aniline to yield imine. In contrast, Fe/TiO2 activates the alcohol mildly without forming aldehyde and hydrogen atoms under irradiation, allowing a direct dehydrative coupling of the activated alcohol with aniline to produce secondary amine. Both Rh/TiO2 and Fe/TiO2 photocatalysts show high performance in terms of conversion, selectivity, stability and a wide substrate scope, rendering the strategy promising for applications.

Plasma-Based CH4 Conversion into Higher Hydrocarbons and H2: Modeling to Reveal the Reaction Mechanisms of Different Plasma Sources.

Plasma is gaining interest for CH4 conversion into higher hydrocarbons and H2. However, the performance in terms of conversion and selectivity toward different hydrocarbons is different for different plasma types, and the underlying mechanisms are not yet fully understood. Therefore, we study here these mechanisms in different plasma sources, by means of a chemical kinetics model. The model is first validated by comparing the calculated conversions and hydrocarbon/H2 selectivities with experimental results in these different plasma types and over a wide range of specific energy input (SEI) values. Our model predicts that vibrational–translational nonequilibrium is negligible in all CH4 plasmas investigated, and instead, thermal conversion is important. Higher gas temperatures also lead to a more selective production of unsaturated hydrocarbons (mainly C2H2) due to neutral dissociation of CH4 and subsequent dehydrogenation processes, while three-body recombination reactions into saturated hydrocarbons (mainly C2H6, but also higher hydrocarbons) are dominant in low temperature plasmas.

Low-cost fabrication of a polydimethylsiloxane (PDMS) microreactor using an improved Print-and-Peel (PAP) method and its performance testing in silver nanoparticle synthesis

Abstract A polydimethylsiloxane (PDMS) microreactor was fabricated using an improved Print-and-Peel (PAP) method. Its performance in terms of conversion was studied by observing the formation of silver (Ag) nanoparticles within the microreactor. Using an improvised LEGO® microsyringe pump, 0.001 M silver nitrate (AgNO3) and 0.002 M sodium borohydride (NaBH4) were introduced in the microchannels. Microreactor length and pumping speed were varied, and their effects on conversion were determined. Results showed that conversion increases with reactor length. Conversions in low speed (130 rad/s) were found to be higher than those in high speed (269 rad/s). Highest conversion of 32.52% was obtained for the 16-mm microreactor at low speed. The mean nanoparticle size of 24 nm measured through Scanning Electron Microscopy (SEM) is smaller than the range of 30 to 60 nm in the macroscale set-up. Spectrophotometric analysis showed that the particle size distribution in the microscale is narrower compared to that of the macroscale indicating the formation of more homogeneous and monodispersed nanoparticles in the microreactor. From the cost analysis, the cost of fabrication is fifty-fold less than that of commercial fabrication. Hence, the fabricated microreactor provides a competitive alternative for conventionally-produced microreactors

Benign-by-Design Orange Peel-Templated Nanocatalysts for Continuous Flow Conversion of Levulinic Acid to N-Heterocycles

In this work, two different strategies have been employed to explore the potential valorization of biomass waste. A TiO2-based sample was prepared by a dry-milling strategy, involving orange peel valorization toward nanostructured materials. Subsequently, ruthenium deposition was accomplished by a chemical reduction method to obtain different ruthenium loadings on the titania support. The prepared catalysts were characterized using a multitechnique approach in terms of chemical, structural, and morphological properties. Levulinic acid, a typical model molecule associated with lignocellulosic biomass, was subsequently converted into N-heterocycles in a continuous flow reactor. The prepared Ru-TiO2 systems exhibited outstanding catalytic performance in terms of conversion and selectivity in comparison with Ru/P25 and Ru/C catalytic references. Maximum activity (79% conversion, 85% selectivity to 1-ethyl-2-(ethylideneamino)-5-methylpyrrolidin-2-ol) was achieved for the sample containing 3 wt % Ru, homogeneou...

The effect of Ce, Zn and Co on Pt-based catalysts in propane dehydrogenation

The effect of the addition of different promoters, such as, Ce, Zn and Co (0.3, 0.5 and 0.7 wt%) to the trimetallic Pt–Sn–K/Al2O3 catalyst in order to enhance performance of propane dehydrogenation was investigated and characterized with several analytic techniques, including, TPR, XRF, UV analysis, SEM and TGA–DTA. The catalysts were prepared by sequential impregnation of γ-Al2O3 technique. It was found that suitable addition of Zn (0.7 wt%), Ce (0.5 wt%) and Co (0.3 wt%) to the Pt–Sn–K/γ-Al2O3 catalyst can improve the catalytic performance more than the commercial Pt–Sn/γ-Al2O3 catalyst. The best results on Sn reduction was obtained by adding Zn (0.7 wt%) and Co (0.3 wt%) to the trimetallic Pt–Sn–K/Al2O3 catalyst. Ce (0.5 wt%) due to its ability on Pt reduction peak temperature was able to reveal better result. The performance of Pt–Sn–K–Ce0.5–Zn0.7/γ-Al2O3 and Pt–Sn–K–Co0.3–Zn0.7/γ-Al2O3 catalysts were improved by a suitable amount of Ce, Zn and Co Pt–Sn–K–Co0.3–Zn0.7/γ-Al2O3 showed better performance in terms of conversion and propylene selectivity. The synergic effect of Co and Zn can enlarge the Pt reduction peak areas than Pt–Sn–K–Co0.3/γ-Al2O3 and Pt–Sn–K–Zn0.7/γ-Al2O3, so can suppress the coke formation and improve the catalyst performance.

First Attempt of Glycidol-to-Monoalkyl Glyceryl Ethers Conversion by Acid Heterogeneous Catalysis: Synthesis and Simplified Sustainability Assessment.

The selective preparation of monoalkylglyceryl ethers (MAGEs) is a task for researchers owing to their broad range of applications. In this work, green feedstocks such as glycidol and alcohols were used to prepare MAGEs under mild reaction conditions (80 °C, 3 h, 0.5 mol % catalyst) in the presence of acid heterogeneous catalysts. Nafion shows the best performances in terms of conversion and selectivity to MAGES and also high stability. A comparison of the environmental performances with the most consolidated pathway from glycerol has shown that the usage of glycidol (recovered as a value-added product from Epicerol process) and Nafion leads to a lower impact on ecosystems. In addition, results achieved from a simplified socio-economic analysis show that the innovative route here proposed has potential (at the laboratory scale) of enhancing potential gains and of reducing the social implications resulting from externalities associated with environmental impacts (e.g., CO2 equivalents).

Selective liquid phase oxidation of benzyl alcohol to benzaldehyde by tert-butyl hydroperoxide over γ-Al2O3 supported copper and gold nanoparticles

Benzyl alcohol oxidation to benzaldehyde was performed by tert-butyl hydroperoxide (TBHP) in the absence of any solvent using γ-Al2O3 supported copper and gold nanoparticles. Li2O and ionic liquids were used as additive and stabilizers for the synthesis of the catalysts. The physico-chemical properties of the catalysts were characterized by atomic absorption spectroscopy (AAS), X-ray diffraction spectroscopy (XRD), N2 absorption/desorption (BET), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and temperature programmed reduction (TPR), whereas, the oxidation reaction was followed by gas chromatography with a flame ionization detector (GC-FID). The as prepared catalysts exhibited good catalytic performance in terms of conversion and selectivity towards benzaldehyde. The performance of the Au-based catalysts is significantly higher than that of the Cu-based catalysts. For both Cu and Au catalysts, the conversion of benzyl alcohol increased as the reaction proceeds, while the selectivity for benzaldehyde decreased. Moreover, the catalysts can be easily recycled and reused with neither significant loss of activity nor selectivity. A kinetic study for the Cu and Au-catalyzed oxidation of benzyl alcohol to benzyldehyde is reported. The rate at which the oxidation of benzyl alcohol is occurring as a function of catalyst and oxidant amounts was investigated, with the apparent rate constant, kapp being proportional to the amount of nano catalyst and oxygen present in the system.

Vapor Phase Oxidation of Benzyl Alcohol over Nano Au/SBA-15 Catalysts: Effect of Preparation Methods

Au/SBA-15 nano catalysts were synthesized from four different methods, viz., homogeneous deposition–precipitation, micro-emulsion, impregnation and polyol, and their catalytic activities were investigated for the vapor phase oxidation of benzyl alcohol to benzaldehyde. The physico-chemical properties of the catalysts were characterised by XRD, TEM, BET surface area, pore size distribution, CO-chemisorption and XPS techniques. The structural data of the catalysts along with their catalytic studies indicate that the presence of very small metallic Au0 species with particle size of 7–8 nm, was responsible for the higher activity observed in the vapor phase oxidation of benzyl alcohol reaction. The title reaction, though industrially important, was used as a test reaction to investigate firstly, the influence of different preparation methods on the uniform dispersion of gold particles on the support SBA-15 and to understand the metal-support interaction in Au/SBA-15 catalysts, and secondly, to study the catalytic performance of the catalyst (Au/SBA-15) in terms of activity, selectivity and stability over a period of reaction time. The conversion of benzyl alcohol was found to increase with decrease in the size of gold particles. Smaller gold particles with higher percentage of dispersion on the support SBA-15 had a beneficial effect on the catalytic activity. Among the four methods used for the preparation of gold on SBA-15 support, the catalyst prepared by homogeneous deposition–precipitation method showed the best performance in terms of conversion, selectivity for benzaldehyde and longer catalyst life. Vapor phase oxidation of benzyl alcohol over nano Au/SBA-15 catalysts.

Production of Light Olefins Through Catalytic Cracking of C5 Raffinate Over Surface-Modified ZSM-5 Catalyst.

Surface modification of phosphorous-containing porous ZSM-5 catalyst (P/C-ZSM5-Sil.(X)) was carried out by a chemical liquid deposition (CLD) method using tetraethyl orthosilicate (TEOS) as a silylation agent. Different amount of TEOS (X = 5, 10, 20, and 30 wt%) was introduced into P/C-ZSM5il.(X) catalysts for surface modification. The catalysts were used for the production of light olefins (ethylene and propylene) through catalytic cracking of C5 raffinate. It was found that external surface acidity of P/C-ZSM5-Sil.(X) catalysts significantly decreased with increasing TEOS content. In the catalytic reaction, both conversion of C5 raffinate and yield for light olefins showed volcano-shaped curves with respect to TEOS content. Among the catalysts tested, P/C-ZSM5-Sil.(20) catalyst exhibited the best catalytic performance in terms of conversion of C5 raffinate and yield for light olefins. Thus, an optimal TEOS content was required for CLD treatment to maximize light olefin production in the catalytic cracking of C5 raffinate over P/C-ZSM5-Sil.(X) catalysts.

Simulated Moving Bed Reactor for the Synthesis of 2-Ethylhexyl Acetate. Part I: Experiments and Simulations

Reactive chromatography (RC) is a promising technology, wherein equilibrium limitations are overcome by simultaneous reaction and adsorptive separation in a single unit. This operation, on a large scale, can be performed conveniently in a simulated moving bed reactor (SMBR) in continuous mode. In this work, we demonstrate the use of an SMBR for the synthesis of 2-ethylhexyl acetate through experiments and simulations. The kinetic model and Langmuir adsorption isotherm, which serve as inputs to the simulator, were taken from an earlier study (GyaniMahajani Sep. Sci. Technol. 2008, 43, 2245). A parametric study was performed using the experimentally validated simulator by varying one of the parameters at a time, to yield desired performance in terms of conversion, purity, productivity, and desorbent consumption. The trends in the results are explained with the help of variations in the solid and liquid concentrations. The simulation results indicate that an SMBR unit containing eight columns is able to offe...

Core–shell structured zeolite–zeolite composites comprising Y zeolite cores and nano-β zeolite shells: Synthesis and application in hydrocracking of VGO oil

Abstract Zeolite–zeolite composites composed of Y zeolite cores and polycrystalline β zeolite shells were prepared, in which Y zeolite was overgrown with a thin layer of nano-β zeolite. The incompatibility of the cores and the shells with different zeolite structure types, chemical compositions and crystallization conditions was circumvented by using Y zeolite as the nutrients for the growth of β zeolite crystals since the dissolution of the core zeolites was inevitable during the formation of the shell layers. The structural, crystalline, and textural properties of the as-synthesized samples were characterized by XRD, SEM, TEM, EDS, SAED, N2 adsorption–desorption, FT-IR, in situ IR spectra of pyridine and di-tert-butyl pyridine. The performances of the as-synthesized zeolite–zeolite composite supported by Ni–Mo active components were investigated during the hydrocracking of Iran VGO oil. The results displayed that the core–shell structured composites with a high-silicon β zeolite shell displayed a better performance in terms of conversion of VGO oil and yield of jet fuel, middle distillate yields and middle distillate selectivity than the reference catalyst and the composite catalyst with a low-silicon β zeolite shell.

Influence of CO2 co-feeding on Fischer–Tropsch fuels production over carbon nanofibers supported cobalt catalyst

Nowadays, the syngas which is obtained from the reforming of coal, biomass or natural gas contain significantly amounts of CO2 that cannot be separated and consequently, it can take part into the Fischer–Tropsch (FTS) catalytic activity. Therefore, the presence of CO2 in the syngas flow should be taken into account. In the present study, the FTS CO hydrogenation process was compared to that of CO2 on a carbon nanofibers supported Co catalyst. The influence of CO2 content in the feed stream (H2/CO/CO2 ratio) on the reaction performance in terms of conversion and selectivity to the different products was described. Both the support and the prepared catalyst were characterized by nitrogen adsorption–desorption, temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Results showed that CO hydrogenation was controlled by a Fischer–Tropsch regime, whereas CO2 hydrogenation was controlled by a methanation process. When feed was composed of CO and CO2 mixtures, the catalytic activity decreased with respect to that obtained with a CO2-free feed stream. Moreover, the presence of CO2 in feed stream favored the formation of lighter hydrocarbons and could block the production of further CO2 via Water-Gas-Shift (WGS) reaction.

Hydrogenation of CO to Methane Over Mesoporous Nickel–Iron–Alumina Xerogel Nano-Catalysts

Mesoporous nickel-iron-alumina xerogel ((40-x)Ni(x)FeAX) nano-catalysts with different iron content (x = 0, 2.5, 5, 7.5, and 10) were prepared by a single-step sol-gel method for use in the methane production from carbon monoxide and hydrogen. The effect of iron content on the catalytic performance of (40-x)Ni(x)FeAX catalysts was investigated. In the methanation reaction, yield for CH4 decreased in the order of 35Ni5FeAX > 32.5Ni7.5FeAX > 30Ni10FeAX > 37.5Ni2.5FeAX > 40Ni0FeAX. This indicated that optimal iron content of mesoporous nickel-iron-alumina xerogel nano-catalyst was required for maximum production of CH4 in the methanation reaction. Experimental results revealed that optimal CO dissociation energy and large H2 adsorption ability of the catalyst were favorable for methane production. Among the catalysts tested, 35Ni5FeAX catalyst, which retained the most optimal CO dissociation energy and the largest H2 adsorption ability, exhibited the best catalytic performance in terms of conversion of CO and yield for CH4 in the methanation reaction. CO dissociation energy and H2 adsorption ability of the catalyst played a key role in determining the catalytic performance of (40-x)Ni(x)FeAX in the methanation reaction.

Methanation of Carbon Dioxide Over Mesoporous Nickel–M–Alumina (M = Fe, Zr, Ni, Y, and Mg) Xerogel Catalysts: Effect of Second Metal

Mesoporous nickel (35 wt%)–M (5 wt%)–alumina xerogel (denoted as 35Ni5MAX) catalysts with different second metal (M = Fe, Zr, Ni, Y, and Mg) were prepared by a single-step sol–gel method for use in the methane production from carbon dioxide and hydrogen. In the carbon dioxide methanation reaction, yield for CH4 decreased in the order of 35Ni5FeAX > 35Ni5ZrAX > 35Ni5NiAX > 35Ni5YAX > 35Ni5MgAX. This indicated that the catalytic performance was greatly influenced by the identity of second metal in the carbon dioxide methanation reaction. Experimental results revealed that CO dissociation energy and metal-support interaction of the catalyst played key roles in determining the catalytic performance of 35Ni5MAX catalysts in the reaction. Among the catalysts tested, 35Ni5FeAX catalyst, which retained the most optimal CO dissociation energy and the weakest metal-support interaction, exhibited the best catalytic performance in terms of conversion of CO2 and yield for CH4. In the carbon dioxide methanation over mesoporous nickel–M–alumina xerogel catalysts with different second metal (M = Fe, Zr, Ni, Y, and Mg), yield for CH4 increased with decreasing TPSR peak temperature and with decreasing metal-support interaction of the catalyst.

Hydrogenation of carbon monoxide to methane over mesoporous nickel-M-alumina (M = Fe, Ni, Co, Ce, and La) xerogel catalysts

Mesoporous nickel(30 wt%)-M(10 wt%)-alumina xerogel (30Ni10MAX) catalysts with different second metal (M = Fe, Ni, Co, Ce, and La) were prepared by a single-step sol–gel method for use in the methane production from carbon monoxide and hydrogen. In the methanation reaction, yield for CH 4 decreased in the order of 30Ni10FeAX > 30Ni10NiAX > 30Ni10CoAX > 30Ni10CeAX > 30Ni10LaAX. Experimental results revealed that CO dissociation energy of the catalyst and H 2 adsorption ability of the catalyst played a key role in determining the catalytic performance of 30Ni10MAX catalyst in the methanation reaction. Optimal CO dissociation energy of the catalyst and large H 2 adsorption ability of the catalyst were favorable for methane production. Among the catalysts tested, 30Ni10FeAX catalyst with the most optimal CO dissociation energy and the largest H 2 adsorption ability exhibited the best catalytic performance in terms of conversion of CO and yield for CH 4 in the methanation reaction. The enhanced catalytic performance of 30Ni10FeAX was also due to a formation of nickel–iron alloy and a facile reduction.

Utilization of Greenhouse Gases through Dry Reforming: Screening of Nickel‐Based Bimetallic Catalysts and Kinetic Studies

A series of bimetallic catalysts containing nickel supported over MgO-ZrO2 were tested for activity in the dry reforming of carbon dioxide. A nickel-cobalt bimetallic catalyst gave the best performance in terms of conversion and coke resistance from a range of Ni-X bimetallic catalysts, X=Ca, K, Ba, La, and Ce. The nitrogen-adsorption and hydrogen-chemisorption studies showed the Ni-Co bimetallic supported catalyst to have good surface area with high metal dispersion. This contributed to the high catalytic activity, in terms of conversion activity and stability of the catalyst, at an equimolar methane/carbon dioxide feed ratio. The kinetics of methane dry reforming are studied in a fixed-bed reactor over an Ni-Co bimetallic catalyst in the temperature range 700-800 °C by varying the partial pressures of CH4 and CO2. The experimental data were analyzed based on the proposed reaction mechanism using the Langmuir-Hinshelwood kinetic model. The activation energies for methane and carbon dioxide consumption were estimated at 52.9 and 48.1 kJ mol(-1), respectively. The lower value of CO2 activation energy compared to the activation energy of CH4 indicated a higher reaction rate of CO2, which owes to the strong basicity of nanocrystalline support, MgO-ZrO2.

Optimal Design of A CPO-Reformer of Light Hydrocarbons with Honeycomb Catalyst: Effect of Frontal Heat Dispersions on the Temperature Profiles

This paper extends a previous investigation on the thermal behavior of CH4-CPO reformers with honeycomb catalysts. Modeling and experimental studies on the short contact time catalytic partial oxidation (CPO) of CH4 to syngas from our and from other groups have shown that Rh-catalysts rapidly deactivate at the very high temperatures, close to 1000 °C, that establish in the inlet zone of the reactor. We have previously shown that a significant reduction of the surface hot-spot temperature can be obtained by properly designing the catalyst: beneficial effects are observed at increasing opening of the honeycomb channels, which decreases the rate of O2 inter-phase mass transfer, and at increasing catalyst activity, which promotes the rate of the endothermic reactions. In this work, we explore the effect of the reactor configuration, namely the effect of heat dispersion from the glowing front face of the monolith. Three reactor configurations were compared in CH4-CPO experiments: (i) a configuration with perfect continuity between the front heat shield (FHS) and the catalytic module, which behaved close to an ideal adiabatic reactor, (ii) a configuration where the FHS was separated from the catalytic monolith and (iii) a configuration where the FHS was at large distance from the catalytic module. State of the art experimental tools, including the spatially resolved measurement of temperature and concentration profiles were used to characterize the thermal behavior of the various configurations. Detailed kinetic modeling supported the analysis of data. The results showed that, at the expense of a small loss of thermal efficiency, a very moderate loss of performance in terms of conversion and selectivity, but, remarkably, an important reduction of the surface inlet temperatures were achieved. Preliminary experiments with propane/air mixtures suggest that the adoption of a moderately dispersive reactor can represent a promising solution for the stable operation of catalytic units treating heavier fuels than methane.

Hydrogenation of aniline to cyclohexylamine in supercritical carbon dioxide: Significance of phase behaviour

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 CO 2 pressure, >95% of aniline conversion with 93% selectivity to cyclohexylamine was achieved on 5% Rh/Al 2O 3. A strong influence of phase behaviour related to the CO 2 pressure was found on the conversion and selectivity. Optimization of reaction parameters resulted in a higher overall activity in the biphase (liquid substrate + gaseous H 2 and CO 2) than in the single phase (liquid substrate–CO 2–H 2) condition. It has been found that the interaction of CO 2 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.