Catalytic activation of CO2: Use of secondary CO2 for the production of synthesis gas and for methanol synthesis over copper-based zirconia-containing catalysts

Consumption of fossil fuel resources throughout the industrial era has resulted in an enormous increase in carbon dioxide concentration in the atmosphere. Developed countries have committed to reducing the atmospheric load of greenhouse gases and ratified the Kyoto Protocol. Chemical utilization of carbon dioxide captured from large scale stationary sources is one possible pathway to decrease the rate of emissions. Catalysis plays a crucial role in these carbon dioxide utilization reactions. In this paper, the production of synthesis gas from carbon dioxide-containing secondary gases and carbon dioxide hydrogenation to methanol over copper-based zirconia-containing catalysts have been investigated. Pathways of carbon dioxide utilization are outlined, research done on carbon dioxide hydrogenation over copper-based zirconia-containing catalysts is reviewed, and the challenges of these reactions are reported. It is argued that direct utilization of secondary carbon dioxide from industrial sources can be a significant step toward developing sustainable industrial practices and a critical part in sustainable energy strategies.     

Energy saving in a CO2 capture plant by MEA scrubbing

The most commonly used process for the CO₂ capture is absorption by means of chemical solvents such as alkanolamines. This consolidated technology can be applied to CO₂ removal from natural gas, refinery gas, and exhaust gas of power plants.This paper focuses on CO₂ capture from exhaust gas by absorption with monoethanolamine (MEA).A commercial simulation software, namely Aspen Plus^®, is used, with Electrolyte-NRTL thermodynamic package where ad hoc parameters, obtained from regression of experimental solubility data for the system CO₂-MEA-H₂O, have been implemented.The comparison among different schemes is based on energy saving. Both the reboiler heat duty and the total equivalent work (which sums up every work and heat contribution in the purification section) are considered as criteria for comparison.     

CO2 absorption and regeneration of alkali metal-based solid sorbents

Potassium-based sorbents were prepared by impregnation with potassium carbonate on supports such as activated carbon (AC), TiO₂, Al₂O₃, MgO, SiO₂ and various zeolites. The CO₂ capture capacity and regeneration property were measured in the presence of H₂O in a fixed-bed reactor, during multiple cycles at various temperature conditions (CO₂ capture at 60 °C and regeneration at 130–400 °C). Sorbents such as K₂CO₃/AC, K₂CO₃/TiO₂, K₂CO₃/MgO, and K₂CO₃/Al₂O₃, which showed excellent CO₂ capture capacity, could be completely regenerated above 130, 130, 350, and 400 °C, respectively. The decrease in the CO₂ capture capacity of K₂CO₃/Al₂O₃ and K₂CO₃/MgO, after regeneration at temperatures of less than 200 °C, could be explained through the formation of KAl(CO₃)₂(OH)₂, K₂Mg(CO₃)₂, and K₂Mg(CO₃)₂·4(H₂O), which did not completely converted to the original K₂CO₃ phase. In the case of K₂CO₃/AC and K₂CO₃/TiO₂, a KHCO₃ crystal structure was formed during CO₂ absorption, unlike K₂CO₃/Al₂O₃ and K₂CO₃/MgO. This phase could be easily converted into the original phase during regeneration, even at a low temperature (130 °C). Therefore, the formation of the KHCO₃ crystal structure after CO₂ absorption is an important factor for regeneration, even at the low temperature. The nature of support plays an important role for CO₂ absorption and regeneration capacities. In particular, the K₂CO₃/TiO₂ sorbent showed excellent characteristics in CO₂ absorption and regeneration in that it satisfies the requirements of a large amount of CO₂ absorption (mg CO₂/g sorbent) and fast and complete regeneration at a low temperature condition (1 atm, 150 °C).     

Activity of carbided molybdena–alumina for CO2 hydrogenation

The relationship between the catalytic activity of carbided molybdena–alumina and the methane desorption from carbidic carbon through temperature-programmed surface reaction (TPSR) were studied. The effects of passivation and hydrogen treatment on the catalytic activities of molybdenum carbides for CO₂ hydrogenation were determined. When the 973 K-carbided catalyst was reduced at 773 K with hydrogen, the catalyst exhibited the highest activity for the reaction, the activity decreasing with increasing H₂ pretreatment temperature. Passivation of this catalyst decreased the reaction rate by 20%. TPSR results were correlated with the activity to reveal that molybdenum carbide with slightly deficient carbidic carbon (Mo₂C_0.962C_1.0) serves as an active site for CO₂ hydrogenation.     

Hydrocarbon synthesis from CO2 over Fe–Cu catalysts

Hydrocarbons and carbon monoxide were produced from carbon dioxide and hydrogen at 400°C over copper-promoted iron catalysts containing sodium. The major surface phases of the catalysts were FeO and/or FeCO₃ although iron carbide is believed as an active phase of the hydrocarbon synthesis from carbon monoxide. Copper was present as metal on the surface and the surface density was considerably high even if the content of copper was less than 1 wt%. The surface density of sodium was not negligible in the catalysts containing sodium less than 0.1 wt% and the sodium ion can affect the surface basicity of the catalysts. The olefin content in the products is believed to relate to the surface basicity.     

CO2 hydrogenation over Pd-modified methanol synthesis catalysts

The effect of palladium incorporation on the performance of Cu–ZnO(Al₂O₃) during the hydrogenation of carbon dioxide has been assessed. Temperature-programmed reduction profiles and X-ray photoelectron spectra of copper revealed that Pd enhances copper oxide reduction. Carbon dioxide conversion and methanol yield were found to increase on Pd-loaded catalysts. The importance of the palladium incorporated to the base Cu–ZnO(Al₂O₃) catalyst in determining the catalytic activity is discussed in terms of the relative ease with which hydrogen is dissociated on the Pd particles and then spilt over the Cu–ZnO phase of the base catalyst.     

Ionic liquids for CO2 capture—Development and progress

Innovative off-the-shelf CO₂ capture approaches are burgeoning in the literature, among which, ionic liquids seem to have been omitted in the recent Intergovernmental Panel on Climate Change (IPCC) survey. Ionic liquids (ILs), because of their tunable properties, wide liquid range, reasonable thermal stability, and negligible vapor pressure, are emerging as promising candidates rivaling with conventional amine scrubbing. Due to substantial solubility, room-temperature ionic liquids (RTILs) are quite useful for CO₂ separation from flue gases. Their absorption capacity can be greatly enhanced by functionalization with an amine moiety but with concurrent increase in viscosity making process handling difficult. However this downside can be overcome by making use of supported ionic-liquid membranes (SILMs), especially where high pressures and temperatures are involved. Moreover, due to negligible loss of ionic liquids during recycling, these technologies will also decrease the CO₂ capture cost to a reasonable extent when employed on industrial scale. There is also need to look deeply into the noxious behavior of these unique species. Nevertheless, the flexibility in synthetic structure of ionic liquids may make them opportunistic in CO₂ capture scenarios.     

Rate-based modelling of SO2 absorption into aqueous NaHCO3/Na2CO3 solutions accompanied by the desorption of CO2

A rate-based model of a counter-current reactive absorption/desorption process has been developed for the absorption of SO₂ into NaHCO₃/Na₂CO₃ in a packed column. The model adopts the film theory, includes diffusion and reaction processes, and assumes that thermodynamic equilibrium among the reacting species exists in the bulk liquid. Model predictions were compared to experimental data from literature. For the calculation of the absorption rate of SO₂ into NaHCO₃/Na₂CO₃ solutions and concomitant CO₂-desorption, it is important to take into account all reversible reactions simultaneously. It is clear that the approximate analytical based model cannot be expected to predict the absorption rates under practical conditions because of the complicated nature of the reactive absorption processes. The rigorous numerical approach described here only requires definition of the individual reactions in the system, and subsequent solution is independent of specific assumptions made, or operational variables like pH or compound concentrations. As an example of the flexibility of this approach, additional calculations were conducted for SO₂ absorption in a phosphate-based buffer system.     

Extraction of nimbin from neem seeds using supercritical CO2 and a supercritical CO2–methanol mixture

Nimbin, a component found in neem seeds, which is reported to have several valuable medicinal properties including: anti-inflammatory, anti-pyretic, anti-fugal, antihistamine and antiseptic was extracted from neem seeds using supercritical CO₂ and CO₂ with a methanol modifier.Nimbin extraction yields using supercritical carbon dioxide were found to be approximately 85% at 308 K, 23 MPa and a CO₂ flow rate of 0.62 cm³/min for a 2-g sample of neem. An optimum extraction pressure appears to exist at ≈23 MPa and 328 K. Although extraction using a methanol modifier did improve the extraction somewhat, methanol was not found to be an effective modifier for extracting nimbin.Dynamic extraction curves were predicted using three empirical models and a theoretical model. The three empirical models were: a Langmuir gas adsorption model, a first order plus dead time (FOPDT) model and a so-called tⁿ cyclone model used to incorporate sigmoidal curves. The parameters in the empirical models were fitted to the experimental data. The Goto et al. [J. Chem. Eng. Jpn. 31 (1998) 171] theoretical model was compared to the experimental results and was found to fit the data well. The theoretical model shows that the extraction yield depends strongly on the solvent flow rate, that is, external mass transfer or equilibrium is the controlling step of this process.     

Thermodynamic modeling of CO2 solubility in aqueous solutions of NaCl and Na2SO4

A thermodynamic model based on the electrolyte NRTL activity coefficient equation and PC-SAFT equation-of-state is developed for CO₂ solubility in aqueous solutions of NaCl and Na₂SO₄ with temperature up to 473.15 K, pressure up to 150 MPa, and salt concentrations up to saturation. The Henry's constant parameters of CO₂ in H₂O and the characteristic volume parameters for CO₂ required for pressure correction of Henry's constant are identified from fitting the experimental gas solubility of CO₂ in pure water with temperature up to 473.15 K and pressure up to 150 MPa. The NRTL binary parameters for the CO₂-(Na⁺, Cl⁻) pair and the CO₂-(Na⁺, SO₄²⁻) pair are regressed against the experimental VLE data for the CO₂-NaCl-H₂O ternary system up to 373.15 K and 20 MPa and the CO₂-Na₂SO₄-H₂O ternary system up to 433.15 K and 13 MPa, respectively. Model calculations on solubility and heat of solution of CO₂ in pure water and aqueous solutions of NaCl and Na₂SO₄ are compared to the available experimental data of the CO₂-H₂O binary, CO₂-NaCl-H₂O ternary and CO₂-Na₂SO₄-H₂O ternary systems with excellent results.     

Utilization of carbon dioxide as soft oxidant for oxydehydrogenation of ethylbenzene to styrene over V2O5–CeO2/TiO2–ZrO2 catalyst

Vanadium oxide and cerium oxide doped titania–zirconia mixed oxides were explored for oxidative dehydrogenation of ethylbenzene to styrene utilizing carbon dioxide as a soft oxidant. The investigated TiO₂–ZrO₂ mixed oxide support with high specific surface area (207 m² g⁻¹) was synthesized by a coprecipitation method. Over the calcined support (550 °C), a monolayer equivalent (15 wt.%) of V₂O₅, CeO₂ or a combination of both were deposited by using wet-impregnation or co-impregnation methods to make the V₂O₅/TiO₂–ZrO₂, CeO₂/TiO₂–ZrO₂ and V₂O₅–CeO₂/TiO₂–ZrO₂ combination catalysts, respectively. These catalysts were characterized using X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature preprogrammed reduction (TPR), CO₂ temperature preprogrammed desorption (TPD) and BET surface area methods. All characterization studies revealed that the deposited promoter oxides are in a highly dispersed form over the support, and the combined acid–base and redox properties of the catalysts play a major role in this reaction. The V₂O₅–CeO₂/TiO₂–ZrO₂ catalyst exhibited a better conversion and product selectivity than other combinations. In particular, the addition of CeO₂ to V₂O₅/TiO₂–ZrO₂ prevented catalyst deactivation and helped to maintain a high and stable catalytic activity.     

Effect of CO2, HCO3 and CO3 on oxygen reduction in anion exchange membrane fuel cells

The effect of carbonate and bicarbonate anions on the oxygen reduction reaction was investigated in four alkaline solutions (pH ∼ 14) on a Pt disk type electrode with varying concentrations of carbonate and bicarbonate. The addition of carbonate and bicarbonate had two primary effects on the observed voltammetric behavior: i) The Tafel slope shifts positive with increasing carbonate/bicarbonate concentration, indicating that the carbonate anions may compete for surface adsorption sites; and ii) The dissolved oxygen concentration and diffusion coefficient are depressed with increasing anion concentration. Finally, adding CO₂ to the cathode stream of an anion exchange membrane fuel cell caused an improvement in the device performance under fully hydrated conditions, suggesting that the fuel cell was operating at least partially under the carbonate cycle.     

CO and CO2 hydrogenation study on supported cobalt Fischer–Tropsch synthesis catalysts

The conversion of CO/H₂, CO₂/H₂ and (CO+CO₂)/H₂ mixtures using cobalt catalysts under typical Fischer–Tropsch synthesis conditions has been carried out. The results show that in the presence of CO, CO₂ hydrogenation is slow. For the cases of only CO or only CO₂ hydrogenation, similar catalytic activities were obtained but the selectivities were very different. For CO hydrogenation, normal Fischer–Tropsch synthesis product distributions were observed with an of about 0.80; in contrast, the CO₂ hydrogenation products contained about 70% or more of methane. Thus, CO₂ and CO hydrogenation appears to follow different reaction pathways. The catalyst deactivates more rapidly for the conversion of CO than for CO₂ even though the H₂O/H₂ ratio is at least two times larger for the conversion of CO₂. Since the catalyst ages more slowly in the presence of the higher H₂O/H₂ conditions, it is concluded that water alone does not account for the deactivation and that there is a deactivation pathway that involves the assistance of CO.     

Dissolution rates of alkaline rocks by carbonic acid: Influence of solid/liquid ratio,temperature, and CO2 pressure

Dissolution rates of alkaline rocks, including wollastonite (CaSiO₃), olivine (Mg₂SiO₄), and phlogopite (KMg₃AlSi₃O₁₀(OH)₂), with high pressure aqueous CO₂ solution were measured to examine the feasibility of CO₂ fixation via carbonation. Influence of solid/liquid ratio (1.0–10 g/250 mL), temperature (303–353 K), and CO₂ pressure (1.0–3.0 MPa) on the extraction rates of calcium or magnesium ions was investigated. Under the experimental conditions studied, the calcium ion extraction rate from wollastonite was the highest among the three rock samples studied. The calcium concentration reached about 120 mg/L, and about 12% of the calcium in wollastonite sample was extracted after 60 min at 353 K with 1.0 MPa CO₂. The calcium and magnesium extraction ratios from the alkaline rocks were much lower than those from waste concrete powder. Increasing the extraction time and temperature would be an effective way to promote calcium extraction from wollastonite.     

Effect of anti-scale agents on the solubility of CO2 in seawater at temperatures of 60 to 90ºC and pressures of 1–2 bar

The objective of this work is to present experimental data that would show the effect of temperature, salinity, pressure and the presence of anti-scale additives on CO₂ solubility in seawater. The paper examines the solubility of CO₂ in real seawater and real seawater dosed with two different anti-scale additives. The study has been performed at temperatures between 60ºC and 90ºC and at pressures of 1 and 2 bar. To assess the effect of each anti-scale additive on CO₂ gas solubility varying doses, 2–10 ppm of anti-scale additive have been used. For the purpose of the experimentation an experimental rig has been designed to ensure adequate contact between the gas phase and the aqueous phase. The continuous quantitative analysis of CO₂ concentration flowing from the experimental rig was measured using a CM-5011 Carbon Coulometer. A mass balance was carried out to calculate the amount of CO₂ absorbed into solution; Henry’s law constant was then calculated.     

Mathematical modelling of mass-transfer and hydrodynamics in CO2 absorbers packed with structured packings

This paper presents a mechanistic model that can predict mass-transfer performance and provide an insight into dynamic behavior within structured packings used for CO₂ absorption. The model was built upon the kinetics and thermodynamics of the absorption system, as well as the liquid irrigation features and the geometry of packing elements. A computer program (Fortran 90) was written to simulate CO₂ absorption into aqueous solutions of sodium hydroxide (NaOH) and monoethanolamine (MEA) in a column packed with Gempak 4A, Mellapak 500Y and Mellapak 500X. The simulation gave essential information, including the concentration of CO₂ in gas-phase, concentration of reactive species in the liquid-phase, system temperature, mass-transfer coefficients (k_G and k_L), and effective interfacial area (a_e) for mass-transfer at different axial positions along the absorption column. The simulation also provided liquid distribution plots representing the quality of liquid distribution or maldistribution across the cross-section of the column. Verification of the model was achieved by comparing simulation results with experimental data. Very good agreement was found for wide ranges of operating and design parameters, including liquid load and initial liquid distribution pattern.     

Effect of ceria on the structure and catalytic activity of V2O5/TiO2–ZrO2 for oxidehydrogenation of ethylbenzene to styrene utilizing CO2 as soft oxidant

The present study was undertaken to investigate the influence of ceria on the physicochemical and catalytic properties of V₂O₅/TiO₂–ZrO₂ for oxidative dehydrogenation of ethylbenzene to styrene utilizing CO₂ as a soft oxidant. Monolayer equivalents of ceria, vanadia and ceria–vanadia combination over TiO₂–ZrO₂ (TZ) support were impregnated by a coprecipitation and wet impregnation methods. Synthesized catalysts were characterized by using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature programmed reduction, transmission electron microscopy and BET surface area methods. The XRD profiles of 550 °C calcined samples revealed amorphous nature of the materials. Upon increasing calcination temperature to 750 °C, in addition to ZrTiO₄ peaks, few other lines due to ZrV₂O₇ and CeVO₄ were observed. The XPS V 2p results revealed the existence of V⁴⁺ and V⁵⁺ species at 550 and 750 °C calcinations temperatures, respectively. TEM analysis suggested the presence of nanosized (<7 nm) particles with narrow range distribution. Raman measurements confirmed the formation ZrTiO₄ under high temperature treatments. TPR measurements suggested a facile reduction of CeO₂–V₂O₅/TZ sample. Among various samples evaluated, the CeO₂–V₂O₅/TZ sample exhibited highest conversion and nearly 100% product selectivity. In particular, the addition of ceria to V₂O₅/TZ suppressed the coke deposition and allowed a stable and high catalytic activity.     

Supercritical CO2 extraction of turmerones from turmeric and high-pressure phase equilibrium of CO2 + turmerones

This work studies the enrichment of turmeric α-β-Ar turmerone using supercritical fluid extractions followed by solid–liquid column partition fractionation. The high-pressure vapor–liquid phase equilibrium of the α-β-Ar turmerone with carbon dioxide was also examined. The purity of α-β-Ar turmerone was increased from 67.7% in extracted oil up to 91.8% in enriched oil, as determined by HPLC analysis. An equilibrium apparatus that comprises two vibrating tube densitometers was then adopted to obtain vapor and liquid equilibrium data for this asymmetric system of α-β-Ar turmerone and carbon dioxide mixture at 313.15 K and 333.15 K. Pressures ranged from 2.82 MPa to 20.80 MPa. Experimental data at elevated pressures were successfully correlated with theoretical methods of Peng–Robinson, Soave–Redlich–Kwong and Patel–Teja equations of state, individually combined with quadratic, Panagiotopoulos–Reid and Adachi–Sugie mixing rules.     

Hybridized synergy effect among TiO2, Pt and graphite silica on photocatalytic hydrogen production from water–methanol solution

The photocatalytic production of hydrogen gas from aqueous methanol solution was performed using powdered mixtures of graphite silica (GS) and platinized TiO₂ (Pt/TiO₂) or those of GS, Pt and TiO₂. The addition of GS to Pt/TiO₂ resulted in the decrease of the amount of H₂ gas, whereas the addition of GS to mixtures of TiO₂ and Pt led to the incremental production of H₂ gas. This effect is attributed to the aggregation of GS, TiO₂ and Pt in suspension. The new additional electron transfer process of TiO₂ → Pt → GS caused the increment of the amount of hydrogen gas.