Chiranjit Santra

Barium, calcium and magnesium doped mesoporous ceria supported gold nanoparticle for benzyl alcohol oxidation using molecular O2

In the era of sustainable energy, catalysis using gold nanoparticles has drawn considerable attention from world researchers. Oxidation of benzyl alcohol by molecular O2 is an atom efficient path to synthesize benzaldehyde. Nanocrystalline ceria has been proven as a useful support to disperse gold nanoparticles since last few years, however there are a few reports on mesoporous ceria supported gold nanoparticles. In this work a systematic investigation was carried out to improve the activity of Au/CeO2 catalyst by incorporating Ba2+, Ca2+ and Mg2+ cations into the ceria lattice through a sol–gel procedure. Both the doped ceria and ceria supported gold nanoparticles are characterized by BET S.A, XRD, TEM, SAXS, XPS, TPR, CO2-TPD techniques. BET S.A measurements show the mesoporous oxides where H3 hysteresis loops are found. The decrease in the crystallite size of ceria after doping by metal cations is observed in the XRD measurement. The TEM and HRTEM characterization shows the nanocrystalline particle size around 30–50 nm and gold nanoparticles around 10–15 nm in size. Distribution in the particle size for doped ceria have been obtained using SAXS measurements where narrow distributions of ceria particles are found in the 10–20 nm range. The existence of oxide vacancies and the mixture of Ce3+/Ce4+ oxidation states are observed for doped ceria materials in the XPS investigation. The strong gold-support interaction was also evidenced by XPS characterization where oxidic gold was found on the doped ceria surface. Lowering of the reduction peak in ceria after gold nanoparticle deposition was observed from TPR investigation whereas the change in basic site distribution is observed from CO2 TPD experiment, instigating new insights into the surface properties of the catalysts. The catalytic activities of the catalysts were determined for benzyl alcohol oxidation reactions using molecular O2. The catalytic activity was in the order of Au/Ba–CeO2 > Au/Ca–CeO2 > Au/Mg–CeO2 > Au/CeO2. The synergistic effect of gold nanoparticles and dopant cations to the ceria was explained in this work.

Niobium doped hexagonal mesoporous silica (HMS-X) catalyst for vapor phase Beckmann rearrangement reaction

The synthesis of e-caprolactam, a demanding monomer, through a heterogeneous catalytic pathway has remained a key area of research in the last decade. The Beckmann rearrangement reaction in the vapor phase using a solid acid catalyst was found to be very effective for producing e-caprolactam. It is observed that niobium incorporated into mesoporous silica serves as a good catalyst for the Beckmann rearrangement reaction. Recently developed mesoporous silica, having an ordered honeycomb structure, is very useful as it can lead to effective diffusion of reactants and products for several reactions. In this study, Nb-doped mesoporous HMS-X nanocomposite materials with different Nb loadings were prepared by a one step hydrothermal synthesis and characterized by BET surface area and porosity measurements, wide and small angle XRD, SEM, HR-TEM, elemental mapping, FTIR, 29Si-NMR and NH3-TPD techniques. The activity of the Nb–HMS-X catalyst was evaluated for the vapour phase Beckmann rearrangement reaction. The catalyst characterization study shows that Nb is highly dispersed on the HMS-X matrix at lower Nb loadings. At higher Nb loadings it is present in the extra-framework position as revealed from XRD, HR-TEM and 29Si-NMR studies. The NH3-TPD result shows the presence of acidic sites on the catalyst surface, which are active sites for the Beckmann rearrangement reaction. Using the Nb–HMS-X catalyst (Si/Nb = 13) under vapour-phase reaction conditions [temperature = 350 °C, weight hourly space velocity (WHSV) = 15 h−1, cyclohexanone oxime in benzene, cyclohexanone oxime : benzene weight ratio of 1 : 11] gave 100% cyclohexanone oxime conversion with 100% e-caprolactam selectivity, with a space time yield of 1.4–1.6 × 10−3 mol h−1 gcat−1. The catalyst was highly recyclable up to 9 times without significant loss of catalytic activity.

Mesoporous TUD-1 supported indium oxide nanoparticles for epoxidation of styrene using molecular O2

Activation of molecular O2 by metal or metal oxide nanoparticles is an area of recent research interest. In this work, for the first time, we report that indium oxide nanoparticles of <3 nm size dispersed on mesoporous silica (TUD-1) can activate molecular O2 and produce styrene epoxide with a selectivity of 60% and styrene conversion around 25% under mild conditions. It is found that neither indium oxide nor TUD-1 themselves respond to the styrene epoxidation reaction. The computational studies provide evidence that an oxygen molecule is highly polarized when it is located near the interface of both surfaces. The kinetic study shows that the reaction is of pseudo-first order and that the activation energy for styrene conversion is 12.138 kJ mol−1. The catalysts are recyclable for up to four regeneration steps, with the styrene conversion and styrene epoxide selectivity almost unchanged.

Bi doped CeO2 oxide supported gold nanoparticle catalysts for the aerobic oxidation of alcohols

Gold nanoparticles supported on Bi–CeO2 with four different bismuth loadings (2 to 8 mol%) were prepared to determine the role of oxide vacancies in doped ceria in the benzyl alcohol oxidation reaction. The catalytic activity was tested for a liquid phase benzyl alcohol oxidation reaction with molecular oxygen under mild conditions of pressure and temperature. The catalytic activity depends on the optimum composition of bismuth concentration (2 to 8 mol%), the nominal gold loading (1 to 4 wt%) and the preparation method of the gold nanoparticles (DP-NaOH, or DP-Na2CO3, gold concentration, and calcination temperature). Transmission electron microscopy (TEM) results showed that Au(3.5 wt%)/Bi(6 mol%)–CeO2 catalyst had the smallest Au NPs, and the majority of Au particles had diameters in the range of 4.04 ± 0.8 nm. X-ray photoelectron spectroscopy (XPS) revealed both metallic and oxidized gold species on the surface of Bi–CeO2. Au(3.5 wt%)/Bi(6 mol%)–CeO2 catalyst showed superior activity for the oxidation reaction of benzyl alcohol to benzaldehyde (conversion 60% and >99% selectivity). The catalysts exhibited a high turnover frequency (TOF) value (0.144 s−1) for benzyl alcohol oxidation. The strong metal support interactions occur due to the presence of higher amounts of positively charged gold species, and a higher number of surface oxygen vacancy sites was responsible for the high catalytic activity of the Au(3.5 wt%)/Bi(6 mol%)–CeO2 catalyst. Corresponding kinetic measurements indicated that the reaction has an apparent activation energy of 34.1 kJ mol−1. Microkinetic studies showed that there was no mass transfer limitation in the three phase catalytic system. The catalyst exhibited high TOFs for the oxidation of other alcohols, such as 2-octanol, cinnamyl alcohol and geraniol, under similar reaction conditions.

Efficient oxidation of hydrocarbons over nanocrystalline Ce1−xSmxO2 (x = 0–0.1) synthesized using supercritical water

Selective oxidation of hydrocarbons to more functional oxygenated compounds is a challenging task for industrial research. Here we report the synthesis of highly crystalline Ce1−xSmxO2 (x = 0–0.1) using supercritical water and their excellent catalytic activity for selective oxidation of hydrocarbons (ethyl benzene, n-butylbenzene, biphenyl methane, 1,2,3,4-tetrahydro naphthalene, cyclohexene and cyclopentene) to corresponding ketone through the oxidation of activated proton. Materials characterization results revealed the formation of highly crystalline small cube shaped nanoparticles (∼8–10 nm) with highly exposed (100) facet and exhibiting a surface area of 83–96 m2 g−1. The catalytic study revealed that Ce0.95Sm0.05O2 is highly active towards selective oxidation of stable sp3 hybridized C–H bond of different hydrocarbons. The superior activity is most probably due to its high surface area, high degree of crystallinity with exposed high energy active (100) facet and presence of large amount Ce3+. In optimized condition as high as 90% conversion of ethyl benzene with 87% selectivity of acetophenone was observed. Among other different substrates n-butylbenzene and cyclopentene showed 100% selectivity towards corresponding ketone with the conversion of 60% and 73% respectively. The catalyst is re-usable for minimum 5 times without any deactivation of its activity.

Indium oxide nanocluster doped TiO2 catalyst for activation of molecular O2

The In2O3 nanocluster doped faceted nanosize anatase TiO2 can activate molecular O2 for styrene epoxidation reaction. The {001} planes of anatase TiO2 are exposed for 550 °C calcined samples whereas {101} planes are predominantly observed for 450 °C calcined samples. The computational studies highlight that In2O3 is better stabilized on {001} planes of TiO2 resulting in efficient activation of molecular oxygen on In2O3 nanocluster doped {001} faceted TiO2 nanostructures. From the kinetic measurements, it is found that styrene epoxidation reaction is of pseudo-zero order and the corresponding rate constant (k) for the reaction calcined at 450 °C is 0.188 h−1 and at 550 °C it is 0.366 h−1. The activation energy for the reaction is found to be 28.44 kcal mol−1.