Current Catalysis - Volume 6, Issue 2, 2017

10.2174/221154470602170502200331

Background: Ceria (CeO2) is the focus of constant and diverse research interest due to its wide industrial applications. In particular, ceria-based mixed oxides with nanostructures exhibit better catalytic properties owing to high specific surface area, improved sintering properties and high oxygen storage-release characteristics in comparison to the individual bulk materials. This work is aimed at analyzing the significance of doping in the ceria lattice to enhance CO and soot oxidation activity over a series of CeO2-La2O3, CeO2-Sm2O3, CeO2-Eu2O3 and CeO2-Gd2O3 mixed oxides. Methods: The investigated CeO2 and Ce0.8M0.2O2-δ (M= La, Sm, Eu and Gd) mixed oxides were prepared by a simple and environmental friendly co-precipitation method, characterized in terms of composition, crystalline structure, particle size, hydrogen consumption and oxidation state by the stateof- art techniques, namely, XRD, RS, TEM, TPR, UV-vis DRS, BET SA and OSC and evaluated for CO and soot oxidation reactions. Results: Systematic characterization of the synthesized mixed oxides by various techniques provided interesting information. XRD results confirmed that doped materials exist as single-phase fluorite structured oxides with significant changes in the lattice parameter. Raman spectroscopy results established the presence of oxygen vacancies in various proportions. The incorporation of trivalent cations into the ceria lattice greatly enhanced the OSC of the materials. H2-TPR results confirmed that doped samples are more reducible than pure ceria. Catalytic studies revealed an enhanced activity for both the reactions in contrast to pure ceria. Conclusion: Among various catalysts investigated, the Ce0.8Sm0.2O2-δ combination exhibited better catalytic activity owing to a considerably high OSC and facile reduction at significantly lower temperatures. The catalytic activity results confirmed that CO and soot oxidation activity could be improved by doping the ceria with appropriate trivalent cations.

Background: Acid catalysis of alkanes is very important as it is the basis of refining. They can be converted into other alkanes or alkenes by isomerization, cracking or alkylation. Heteropolyacids have unique acid - base and redox properties, which render them valuable in a number of applications. Their strong acidity makes them attractive candidates to tackle the current challenges in alkanes. The aim of this review is to describe the state of the art in the transformations of alkanes catalyzed by heteropolyacids with and without addition of noble metal. Methods: We undertook a structured search of bibliographic databases for peer-reviewed research literature using a focused review question. The quality of retrieved papers was appraised using standard tools. The characteristics of screened papers were described, and a deductive qualitative content analysis methodology was applied to analyze the interventions and findings of included studies using a conceptual framework. Results: More than one hundred and fifty papers were included in the review. After a general introduction describing the polyoxometalates and their properties, the generally admitted mechanisms of activation of alkanes by acid catalysts were described. The review was then presented by type of alkanes, starting from propane to long chain paraffins. The activation of methane by heteropolyacids was also described as it can be considered as the first step of the oxidation of methane, which is actually a very important challenge. Conclusion: Polyoxometalates are active for the isomerization/cracking of alkanes. If the first studies were made on light hydrocarbons the most recent works are done on wax or heavy oil, in order to valorize them in gasoline. The polyoxometalate reacts easily with the alkane and this reaction is not the kinetically limiting step.

Background: The development of theoretical insights into heterogeneous catalysis is hindered by the lack of knowledge on the detailed reaction mechanisms. In order to gain such knowledge, it is important to collect information on the nature of intermediates, and their reaction ability Methods: The spectrokinetic method is successfully applied to the studies of intermediates in heterogeneous catalytic reactions. Results: In the present work, this method was used for the studies of intermediates in low-temperature oxidation of ammonia over oxide catalysts; CO – NO interaction over supported metals and oxide catalysts; NOx reduction by hydrocarbons in excess of oxygen over oxide, zeolite, pillared clay catalysts; methanol transformations over supported oxide catalysts. Conclusion: Studies of intermediates formed in the course of heterogeneous catalytic reactions revealed the common patterns which assist the understanding of the catalytic action. It was found that the main function of the catalyst lies in transforming the reactant from its extant gas phase form into an activated form in the course of adsorption, drastically different from its source. The latter transforms into the products by different route than in the case of the initial reactant in the absence of the adsorption step. It was found also that if the initial adsorption forms of the reactant are the same over a certain number of different catalysts of the case reaction, then the sequence of steps in the reaction mechanism is the same for all catalysts. Individual properties of different catalysts are revealed through the ratio of rates of individual steps, most obviously for the limiting step.

Background: Selective oxidation of chemical warfare agent (CWA) simulants including the 2-chloroethyl ethyl sulfide (2-CEES) and other analogs through a green method in the presence of keplerate Cu20-Polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- supported on magnetic nanoparticles was investigated. Keplerate nano-polyoxometalates, a new class of polyoxometalates which commonly known as “Giant”, are desirable materials, used for rapid selective oxidation bis (2 chloroethyl) sulfide to sulfoxid due to the toxic nature of mustard gas, the less toxic but structurally similar analogue 2-chloroethyl ethyl sulfide (2-CEES) is commonly used to study this reaction. Methods: The was catalyst characterized by different techniques, such as XRD, FT-IR, TGA, ICP-AES, TEM and SEM. Herein we reported the first used of this Nano-catalyst as selective and recoverable heterogeneous catalysts for the sulfoxidation of the CWA stimulant 2-CEES and other analogs using H2O2 as the oxidant under mild conditions. It was observed that the [WPCu@ASMNP] shows the highest catalytic activity on sulfoxidation of thioether mustard (HD) analogs with water as green solvent. Influence of various reaction parameters on the sulfoxidation reaction was studied. The catalyst was reused up to 5 times after separation with an external magnet without observable loss of activity and selectivity (According to the results of the FT-IR and XRD). Finally, plausible mechanism for the sulfoxidation thioethers mustard is discussed. Results: After optimizing the reaction conditions, oxidation yields of ten representative thioethers, two diaryl sulfide, two aryl alkyl sulfide and six dialkyl sulfides using H2O2 in the presence of [WPCu@ASMNP] at 25°C was obtained. After reaction completion, the catalyst was easily separated by using an external magnet and tested for five subsequent runs under the optimized reaction conditions. The yield and selectivity of the corresponding sulfoxide was found to be almost similar during these experiments. The recovered catalyst after five cycles was characterized by using FTIR and XRD which indicated results in good agreement with the fresh one, confirming the stability as well as structural integrity of the catalyst. Conclusion: We describe an environmentally friendly protocol for the oxidation of thioether mustard (HD) analogs into sulfoxide at 25°C using 30% aqueous H2O2 under mechanical stirring. In the oxidation system we also observed only one product (sulfoxide) without sulfone. The transformation worked well with both the liquid and solid sulfides in spite of a heterogeneous reaction mixture, and no difficulties with stirring were observed. The FT-IR and XRD of the catalyst show retention of [WPCu@ASMNP] after reaction. At the moment, we are investigating the activity of other giant POMs in oxidation chemical warfare agents. We think that these results open a whole new family of active catalysts in oxidation CWAs.

Background: Dehydrozingerone is a biosynthetic intermediate and structural analog of curcumin. It is a flavoring ingredient and has antioxidant and antimutagenic properties. Dehydrozingerone can be efficiently synthesized from vanillin and acetone by cross-aldol (or Claisen-Schmidt) condensation using heterogeneous base catalysts. Vanillin can be sourced from the valorization of biomass. Green chemistry aspects such as 100 % atom economy and low E factor are very important in any synthesis which could be achieved by use of heterogeneous catalysis. Methods: Hydrotalcites with different Mg: Al ratio were synthesized and calcined in air at 450 ºC and 500 ºC to understand the effect of catalyst composition and calcination temperature on activity and selectivity in the synthesis of dehydrozingerone from vanillin and acetone. All catalysts were characterized by different techniques such as XRD, FTIR, ASAP, TPD, TEM and SEM-EDXS. Effects of various process parameters were studied to establish reaction mechanism and kinetics. The kinetic model was validated by experimental data. The product was confirmed by GC-MS and 1H NMR. Results: Among all catalysts, hydrotalcite of Mg: Al of mole ratio 3:1, calcined at 500 ºC for 6 h (31CHT500) was found active, selective and reusable. Hydrotalcite calcined at 500 ºC gave mixed Mg- Al metal oxides. Dehydrozingerone was efficiently obtained with 100 % selectivity at vanillin conversion of 88 % over 31CHT500 at 130 ºC after 4 h. A pseudo first order kinetics was found to fit the data well. Conclusion: Catalyst composition and calcination temperature were affecting the activity of catalysts. Temperature, catalyst loading and vanillin to acetone mole ratio were influencing the reaction rate. Maximum yield of dehydrozingerone was obtained at following reaction conditions: temperature, 130 ºC; amount of catalyst, 0.015 g mL-1; mole ratio of vanillin: acetone, 1:15; autogenous pressure, 10 atm. All species were weakly adsorbed on catalytic sites. The reaction was pseudo first order with apparent activation energy of 9.4 kcal mol-1. Dehydrozingerone was synthesized from vanillin and acetone by a green process.

Background: There are various conventional ways for the synthesis of aromatic ketones but suffer from disadvantages like the use of toxic reagents, stoichiometric reagents producing huge amount of byproducts, thus causing hazard to the environment, whereas homogeneous protocols or the noble metal which support catalytic processes are not economically viable. On this ground, an effort was made to develop new green catalytic protocol to overcome these environmental concerns. The catalytic activity of manganese dioxides prepared by different recipes was investigated for the oxidation of allylic and benzylic alcohols. Methods: MnO2 B sample was prepared by calcinations of manganese nitrate while sample ANMnO2 was prepared by the reduction of potassium permanganate using triethanolamine as a reductant. All the samples were well characterized by XRD, SEM, EDX and TEM techniques. All the prepared catalyst samples along with commercial MnO2 were tested for the oxidation of alcohols using TBHP as a clean oxidant, where % conversion and % selectivity were determined by Gas Chromatography. The products were further confirmed by the GC-MS and NMR techniques. Results: Of all the oxides, nano amorphous manganese dioxide exhibited significant catalytic activity and selectivity for the corresponding carbonyls. Change from bulk to nano structure enhanced the catalytic activity because of its higher surface area and change in Mn3+/ Mn4+ ratio. The nano amorphous MnO2 (ANMnO2)/TBHP in acetonitrile solvent catalytic system was found to be most efficient with substrate compatibility. In mechanistic investigations, it was observed that from the bulk to the nano structure, Mn3+ species content in an oxide increased which may play a crucial role in the activity. It was also confirmed by hydrogen peroxide decomposition studies. The catalyst ANMnO2 was found to be reusable for five consecutive cycles with no significant loss in catalytic activity. Conclusion: In conclusion, change in the catalyst’s preparation recipe not only alters the particle size but also affects the ratio of Mn3+ to Mn4+ species on the surface as well as on the bulk and thereby catalytic activity. On comparison of initial rate m-2g of all the catalyst samples for the said reaction, there was no significant difference observed which clearly proved the role of Mn3+ in catalytic activity.

Background: V2O5 is significantly soluble in water so the use of this oxide or mixed oxides systems based on V2O5, in photocatalytic processes in water, may release vanadium species in the aqueous medium, generating a certain amount of additional contamination. The present report is devoted to study the physical-chemical properties of V2O5-TiO2 composition and its use in photocatalytic processes in aqueous media. The focus is on the possibility of V2O5 dissolution and influence of this phenomenon on the photocatalysis. The photocatalytic activities of these mixed pigments have been investigated on the discoloration of a dye (Methyl Orange) in aqueous medium under two selected chemical (pH) and photochemical illumination (UV and UV-vis) conditions. Materials and Methods: Vanadium pentaoxide, V2O5, was prepared by a conventional nonhydrothermal process, and mixed oxides TiO2(P25)/V2O5 were also prepared by using parental oxides of commercial TiO2(P25) and the synthesised V2O5. The thermal stability of the prepared mixed system has been studied by XRD, using an acquisition of X-ray diffractograms obtained during the process of continuous heating from room temperature to 800°C and subsequent cooling at room temperature. Portions of the mixed system TiO2(P25)/V2O5, were calcined, in static conditions, at three different temperatures, 300°C, 500°C and 700°C for 2h, studying the viability to release vanadium, from the use of these systems in aqueous suspensions. The catalysts were well characterized using various techniques such as, X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), Field-Emission -SEM, Transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS). Results: Experimental results indicated that the photo-discoloration of Methyl Orange on TiO2(P25), depends on the pH and the illumination conditions (UV or UV-vis). In addition, a part of the V2O5 component from the mixed oxides TiO2(P25)/V2O5 was totally dissolved having thus two toxic species in the medium, the dye and the released vanadium species, which are progressively removed by the photocatalytic action of the TiO2(P25) component. Conclusion: The fact that V2O5 based materials, from which V2O5 (depending of the method of preparation and of its thermal stability) may be released and dissolved, wholly or partly, in the aqueous reaction medium systems, generating species of vanadium which are toxic, can be directed to its recovery by controlled photocatalytic processes, as described in this paper by using TiO2. Titanium dioxide, TiO2(P25), can at pH=2.0, not only act as a good photocatalytic oxidant in the UV, but under the same conditions vanadium (V) species present in the aqueous medium can be recovered. The synergistic effect of other organic pollutants, such as dyes or isopropanol, acting as sacrificial agents, resulted to be in favor in speeding up the photocatalytic reduction of vanadium (V).

Background: Utilization of crude glycerol obtained during the bio-diesel production for the synthesis of fine chemicals and fuel additives by catalytic routes is very important from cost-effective and environmental perspectives. Among all alkylpyrazines, the 2,6-DMP is one of the valuable compounds for the production of various agro-chemicals, food flavoring agent and as a ligand for catalyst synthesis. In this study, 2,6-dimethylpyrazine (2,6-DMP) has been synthesized by dehydrocyclization of crude glycerol and 1,2-propanediamine (1,2-PDA) over CuO-CuCr2O4 (Cu-Cr-O) catalyst and the surface active sites are analyzed by adsorption and spectroscopic techniques. Methods: The Cu-Cr-O sample was prepared by simple co-precipitation method in order to obtain a Cu- Cr hydrotalcite precursor. The dried Cu-Cr sample was calcined at 400, 550, 650 and 750 °C for 5 h in a static air. The alkali metal (Na, K, Cs) modified Cu-Cr-O calcined at 550 °C samples were prepared by a simple wet impregnation method. Results: The Cu-Cr-O calcined at 550 °C demonstrated higher 2,6-DMP yield when compared to other samples. The rate of 2,6-DMP is in good correlation with the Cu metal surface area and the NH3 uptakes of the catalysts. The 2,6-DMP selectivity is improved over potassium modified CC550. A structure activity correlation is established based on the pyridine and formic acid adsorbed IR spectra. Conclusion: At a calcination temperature of 400 °C; the Cu-Cr-O demonstrated a lower dehydrocyclization activity due to a poor dispersion of Cu. The experimental results showed that the dehydrocyclization rate is higher in Cu-Cr-O calcined at 550 °C, and at high temperature, calcination leads to loss of Cu metal surface area occurred as a result decline in the overall activity.