Consuelo Montes de Correa

Kinetics of limonene epoxidation by hydrogen peroxide on PW-Amberlite

Abstract Selective limonene epoxidation was carried out over a heteropolyacid anchored on Amberlite IRA-900, using hydrogen peroxide as the oxidant. The effects of various parameters such as the concentration of catalyst, H 2 O 2 , limonene, and water on the rate of reaction have been studied. A mechanism similar to that proposed under the so-called Ishii–Venturello chemistry was used to develop a kinetic model based on the Langmuir–Hinshelwood formalism. This includes the adsorption of reactants and solvent on the catalyst. Although the kinetic model showed to be in agreement with the experimental results, water did not appear to adsorb on the catalytic sites. Furthermore, the influence of epoxide addition to the reaction mixture was examined. Epoxide deactivates the catalyst and the reaction practically stops when the amount of epoxide added is equivalent to about 160 turnovers. Notwithstanding, the catalyst is reactivated after being washed with acetone.

The Effect of NaOH on the Liquid-Phase Hydrodechlorination of Dioxins over Pd/γ-Al2O3

The effect of sodium hydroxide on the-liquid phase hydrodechlorination (LPHDC) of polychlorinated dibenzo- p-dioxins/polychlorinated dibenzofurans (PCDD/Fs) over 2% Pd/gamma-Al 2O 3 was evaluated. Reactions were carried out using 2-propanol both as a hydrogen donor and as a solvent. Fresh and used catalyst samples were characterized by BET, hydrogen chemisorption, TEM/EDS, XPS, and TPR. When the reaction mixture contained no NaOH, active-phase leaching and Pd-C formation were observed even after 10 min of reaction. Therefore, sodium hydroxide appears to be required to maintain surface metal clusters on the support and avoid binding of carbon species to the active metal. On the other hand, excess NaOH in the reaction mixture led to deposition of organic and inorganic solid residues on the catalyst surface, blocking the active sites. Under the conditions of this study, the addition of 30 mg of NaOH maintained the basicity of the system and diminished deposition of solid residues on the catalyst samples, and almost 100% detoxification was reached after a 3 h reaction.

Synthesis and characterization of sol-gel Cu-ZrO2 and Fe-ZrO2 catalysts

Fe-ZrO2 and Cu-ZrO2 xerogels were prepared by a sol-gel method. The effect of the hydrolysis catalyst during the gelation step, namely H2SO4 or NH4OH, on the properties of the resulting materials was investigated by XRD, BET, TGA/DTA, TPD of ammonia, FTIR, and TPR. Fe-ZrO2 and Cu-ZrO2 xerogels, with sulfuric acid introduced as the hydrolysis catalyst, mainly crystallyzed in the tetragonal phase and exhibited larger surface area and acid amount than those obtained with NH4OH. Ammonia TPD shows that copper promoted sulfated zirconia is the most acidic material. TGA and FTIR reveal that under oxidizing conditions sulfated zirconia promoted with iron and copper retains more sulfate species than unpromoted sulfated zirconia. Regardless of the hydrolysis catalyst employed, copper promoted catalysts calcined at 600°C, contain a large fraction of copper oxide specieseasily reduced at low temperatures. These copper oxide species are believed to have different environment and interactions with the surface oxygen vacancies of the zirconia support. A FeO-like phase appears to be the most probable one after reduction of Fe-ZrO2 catalysts prepared with NH4OH as the hydrolysis catalyst. The formation of Fe° species may be hindered by the high dispersion and interaction of Fe2+ ions with the zirconia support. On the other hand, the reduction peaks of iron oxide and sulfate species exhibit a considerable overlap in the TPR profiles of sulfated Fe-ZrO2 samples. Hence, the nature of the supported phase in the latter samples is rather uncertain.

Decomposition of nitrous oxide in excess oxygen over Co- and Cu-exchanged MFI zeolites

The decomposition of nitrous oxide on several Co- and Cu-ZSM-5 zeolite catalysts was studied in the absence and presence of excess oxygen. Also, the effect of methane addition, as well as catalyst steaming in dry and wet feeds is reported. N2O decomposition with no oxygen in the feed was proportional to metal loading on both catalysts. Co-ZSM-5 was much more resistant than Cu-ZSM-5 in excess oxygen. The tolerance of Co-ZSM-5 catalysts to excessive amounts of oxygen is high when Co2+ is stabilized in the zeolite framework and depends on the catalyst method of preparation. The presence of methane with no oxygen in the feed enhanced N2O decomposition while the addition of both methane and oxygen to the feed decreased N2O conversion on all catalysts tested. Co2+ ions stabilized by ZSM-5 framework have high hydrothermal stability in comparison to Cu2+ -exchanged ZSM-5.

Combustion of methane over palladium ZSM-5 and mordenite catalysts

Abstract The effect of Pd-loading on Pd-NaZSM-5 and Pd-NaMordenite catalysts prepared by ion exchange was studied for methane combustion with excess oxygen (1% CH4, 18% O2, balance N2) in the temperature range 40–500°C. Fresh and calcined samples (3 h, 450°C) showed methane conversions proportional to Pd-loading on Pd-NaZSM-5 catalysts, while conversions decreased with Pd-loading on calcined Pd-NaMordenite catalysts. TOF (number of methane molecules converted per second per Pd2+ ion) for over exchanged Pd-NaZSM5-116 was low as compared to under exchanged Pd-NaZSM5-80 and Pd-NaZSM5-58 samples. Close TOFs were found for the last two samples at 330°C. TOF differences in Pd-NaMordenite catalysts demonstrate the heterogeneity of Pd+2 sites due to structurally nonidentical locations of cations. TOFs appear to be related to Na/Pd ratios in both catalyst types. Apparent activation energies for Pd-NaZSM-5 materials are higher than those for Pd-NaMordenite catalysts.

Lean NOx reduction with dodecane over cerium and palladium loaded mordenite

The selective catalytic reduction (SCR) of NOx by dodecane in excess oxygen-containing gas mixtures was studied on Ce and/or Pd-loaded HMOR. Under dry conditions the best fresh bimetallic catalyst, 6.2 wt.% Ce–0.08 wt.% Pd-HMOR, displayed a maximum of about 70% NOx conversion to N2 at 350 ◦ C and GHSV = 30,000 h −1 , while either Ce-HMOR or Pd-HMOR exhibited low activity for the SCR reaction. The presence of both Ce and Pd in the zeolite was crucial for high deNOx activity. The dodecane concentration is very important for the reaction. The catalyst is not active in the absence of the reducing agent and inhibition of NOx reduction is observed at dodecane concentrations higher than 440–500 ppm. The presence of 40 ppm SO2 in the gas feed suppresses the reaction. However, the coexistence of 15% H2O and 40 ppm SO2 has no appreciable effect on the catalyst activity. Enhanced activity and a broader temperature window was observed in the NO2 + dodecane + O2 reaction in the presence of 15% H2O and 100 ppm SO2. In this reaction mixture, the catalyst was capable of retaining a stable NO2 conversion to N2 for a period of ten days. Characterization by XPS and UV–VIS–diffuse reflectance spectra (UV–VIS–DRS) of selected fresh and aged catalyst samples indicates that Pd exists mainly as Pd 2+ cations in Ce–Pd-HMOR. Cerium is mostly present as CeO2 on the surface of the zeolite particles. However, part of the cerium in 6Ce–Pd-HMOR, exists as stable Ce 3+ species.

The role of zeolite type on the lean NOx reduction by methane over Pd loaded pentasil zeolites

Abstract The lean selective catalytic reduction of NO x by methane over protonic palladium loaded ZSM-5, FER and MOR, as well as, on bimetallic Pd–Pt-HMOR was examined. Special emphasis was paid on the combined effects of water and SO 2 in the feed stream. Under dry conditions and in the absence of SO 2 , the degree of NO x conversion at 450°C decreases as follows: Pd-HZSM-5>Pd-HMOR>Pd-HFER. Sulfur dioxide alone has no apparent effect on the activity for NO x reduction, but the coexistence of water and SO 2 inhibits both NO x and methane conversions. The extent of inhibition by water and SO 2 on NO x reduction is Pd-HFER>Pd-HZSM-5>Pd-HMOR. Acid mordenite doped with low levels of Pt and Pd leads to an active catalyst that is more tolerant to the presence of either water or SO 2 than the corresponding monometallic Pt- and Pd-HMOR. Nevertheless, NO x reduction is also inhibited at temperatures below 450°C when SO 2 and water are both present. TPD experiments of water over calcined samples of protonic Pd supported pentasil zeolites, Pd/γ-Al 2 O 3 and Pt–Pd-HMOR with and without pretreatment in SO 2 +O 2 indicate that sulfation of the surface increases water chemisorption by the support. Therefore, the observed decrease of NO x reduction on Pd-loaded zeolite catalysts when SO 2 and H 2 O coexist in the feed stream may be due to enhanced water inhibition and presumably active site poisoning.

Lean selective catalytic reduction of NO2 by methane over Co‐zeolites

The reaction of NO2, CH4 and O2 was studied using low levels of methane compared to NO2 and O2 over protonic and cobalt‐exchanged ferrierite, ZSM‐5 and mordenite zeolites. Results suggest that two reaction pathways at low and high temperatures may be involved in the lean selective catalytic reduction (SCR) of NO2 by methane. At low temperatures, the reduction of NO2 to NO and N2 might be the initial reaction step. It is likely that NO2 or its adsorbed precursors initiate the reaction of methane at low temperatures. At high temperatures, the oxidation of NO and combustion of methane with oxygen might be involved. No appreciable differences were observed in the reduction of NO2 over Co‐zeolites as compared to known results of NO reduction over these materials. However, enhanced N2 formation rate was observed on H‐zeolites starting from NO2 instead of data reported for NO. Furthermore, it appears that the active sites for SCR are both acid and metal sites.The reaction of NO2, CH4 and O2 was studied using low levels of methane compared to NO2 and O2 over protonic and cobalt‐exchanged ferrierite, ZSM‐5 and mordenite zeolites. Results suggest that two reaction pathways at low and high temperatures may be involved in the lean selective catalytic reduction (SCR) of NO2 by methane. At low temperatures, the reduction of NO2 to NO and N2 might be the initial reaction step. It is likely that NO2 or its adsorbed precursors initiate the reaction of methane at low temperatures. At high temperatures, the oxidation of NO and combustion of methane with oxygen might be involved. No appreciable differences were observed in the reduction of NO2 over Co‐zeolites as compared to known results of NO reduction over these materials. However, enhanced N2 formation rate was observed on H‐zeolites starting from NO2 instead of data reported for NO. Furthermore, it appears that the active sites for SCR are both acid and metal sites.

Effect of the substrate and catalyst chirality on the diastereoselective epoxidation of limonene using Jacobsen-type catalysts.

Chiral and achiral Jacobsens catalysts in their homogeneous form or immobilized on Al-MCM-41 exhibit similar catalytic activity during diastereoselective epoxidation of limonene when in situ generated dimethyldioxirane is used as oxidizing agent. Experimental observations suggest that not only the catalyst chiral center but also the substrate chiral center participates in the preferential formation of most diastereomers. Remarkable turnover numbers (TON), up to 288, was achieved over the heterogeneous catalysts in comparison to their homogeneous counterparts (TON up to 46). Catalyst leaching rather than catalyst oxidative degradation was identified as the main source of catalyst deactivation during reutilization tests.

Theoretical and experimental study of NO/NO2 adsorption over Co-exchanged type-A zeolite

Abstract Adsorption of NO and NO2 on cobalt exchanged type-A (Co-A) zeolite, has been studied by combining experimental and theoretical techniques. Adsorption energies were determined from temperature programmed desorption (TPD) measurements and ab initio calculations at the B3LYP/6-31G(d) level of theory. TPD profiles suggest that NO is disproportionated upon desorption on Co-A while NO2 desorbs without transformation. An eight-atom cluster model of the transition metal and its closest environment in zeolite Co-A appears to qualitatively reproduce basic features of NOx adsorption. Results from geometrical optimizations confirm that nitrogen monoxide (NO) is preferentially adsorbed through its nitrogen atom, whereas the two oxygen atoms of nitrogen dioxide (NO2) appear to interact more favorably with Co-A. Upon NO adsorption on Co-A, a bent cobalt-mononitrosyl complex is formed. However, a negative charge is not developed on the nitrosyl and cobalt is not oxidized. Similarly, adsorption of nitrogen dioxide over Co-A does not change the oxidation state of cobalt. Theoretical calculations appear to reproduce the experimental adsorption energies within expected limits of error.