Shubhangi B. Umbarkar

Catalytic dehydration of lactic acid to acrylic acid using calcium hydroxyapatite catalysts

A series of calcium hydroxyapatite (HAP) catalysts were synthesised with a Ca/P ratio ranging from 1.3 to 1.89 by a co-precipitation method that involved changing the pH of the calcium and phosphorous precursors. The physicochemical characterization by XRD, SEM, BET surface area and CO2 and NH3-TPD techniques confirmed the hydroxyapatite formation. These HAP catalysts were used for the vapour phase dehydration of lactic acid to acrylic acid. The HAP catalyst with a Ca/P ratio of 1.3 was found to be the most efficient catalyst among the synthesised series, which gave 100% conversion of lactic acid and 60% selectivity towards acrylic acid at 375 °C when a 50% (w/w) aqueous solution of lactic acid was used. The higher selectivity towards acrylic acid has been correlated to the increased acidity and reduced basicity of the HAP catalyst with a Ca/P ratio of 1.3 compared to the other HAP catalysts. The catalyst was found to be very stable and no deactivation was observed even after 300 h of reaction time. In situ FTIR studies were performed for understanding the mechanistic aspects and showed the formation of calcium lactate as an intermediate species during the dehydration of lactic acid to acrylic acid.

Synthesis of Catalytically Active Porous Platinum Nanoparticles by Transmetallation Reaction and Proposition of the Mechanism

A facile method for the synthesis of porous platinum nanoparticles by transmetallation reactions between sacrificial nickel nanoparticles and chloroplatinic acid (H(2)PtCl(6)) in solution, as well as at the constrained environment of the air-water interface, using a Langmuir-Blodgett instrumental setup is presented. To carry out the transmetallation at the air-water interface hydrophobized nickel nanoparticles are assembled as a monolayer on the sub phase containing platinum ions. The porous Pt nanoparticles obtained as a result of the reaction are found to act as extremely good catalysts for hydrogenation reaction. The products are well characterized by TEM, HRTEM, EDAX, and STEM. Attempts are made to postulate the plausible mechanism of this reaction to generate this kind of nanoparticle with controllable geometric shape and structure. This simple strategy has the potential to synthesize other nanomaterials of interest too.

Nonstoichiometric calcium pyrophosphate: a highly efficient and selective catalyst for dehydration of lactic acid to acrylic acid

Calcium phosphate catalysts were prepared by co-precipitation method using calcium nitrate and mixtures of ammonium and different sodium phosphates as calcium and phosphate precursors, respectively. Depending on the phosphate precursor, the pH of the synthesis mixture changed during the catalyst precipitation. The catalyst characterisation by XRD and ICP revealed the formation of a calcium pyrophosphate structure with varying Ca/P ratio from 1.02 to 0.76 which could be correlated to the different pH of the synthesis solutions. Vapour phase dehydration of lactic acid to acrylic acid was carried out using these calcium pyrophosphate catalysts. Non-stoichiometric calcium pyrophosphate catalyst with Ca/P ratio 0.76 was found to be the most efficient catalyst among the synthesized series with 100% lactic acid conversion and 78% acrylic acid selectivity at 375 °C. The higher selectivity for acrylic acid has been correlated to the increased acidity and reduced basicity of non-stoichiometric calcium pyrophosphate compared to other stoichiometric pyrophosphates. In situ FTIR studies showed the formation of a higher amount of calcium lactate on non-stoichiometric compared to stoichiometric pyrophosphate leading to higher selectivity for acrylic acid.

One-pot synthesis of ultrasmall MoO3 nanoparticles supported on SiO2, TiO2, and ZrO2 nanospheres: an efficient epoxidation catalyst

Ultrasmall molybdenum oxide (MoO3) nanoparticles supported on various (SiO2, TiO2 or ZrO2) nanospheres were synthesized in one pot using a reverse micelle method. The prepared catalysts were thoroughly characterized by various physico-chemical methods. TEM images showed uniform dispersion of MoO3 nanoparticles (1.5–4 nm) onto silica (∼275 nm). No separate MoO3 particles were identified from TEM for MoO3/TiO2 (∼10.5 nm) and MoO3/ZrO2 (∼6.5 nm) because AHM reacted with titanium and zirconium hydroxides to form solid solution. Among the prepared catalysts MoO3/SiO2 showed excellent catalytic activity (up to 90% conversion and 100% epoxide selectivity) for olefin epoxidation. The catalyst was successfully recycled up to five cycles without losing much activity and selectivity.

Transesterification of Diethyl Oxalate with Phenol over Sol–Gel MoO3/TiO2 Catalysts

The transesterification of diethyl oxalate (DEO) with phenol to form diphenyl oxalate (DPO) has been carried out in the liquid phase over very efficient MoO(3)/TiO(2) solid-acid sol-gel catalysts. A selectivity of 100 % with a remarkable maximum yield of 88 % were obtained, which opens the route to downstream phosgene-free processes for the synthesis of polycarbonates. Interpretation of the results of various acidity measurements (NH(3) and pyridine desorption, methanol oxidation as a probe reaction) allowed us to identify the catalytic sites as Lewis acid sites.

Mechanistic Studies on the Roles of the Oxidant and Hydrogen Bonding in Determining the Selectivity in Alkene Oxidation in the Presence of Molybdenum Catalysts

When the molybdenum oxo(peroxo) acetylide complex [CpMo(O-O)(O)C≡CPh] is used as a catalyst for the oxidation of olefins, completely different product selectivity is obtained depending on the oxidant employed. When tert-butyl hydroperoxide (TBHP, 5.5u2005M) in dodecane is used as the oxidant for the oxidation of cyclohexene, cyclohexene oxide is formed with high selectivity. However, when H(2)O(2) is used as the oxidant, the corresponding cis-1,2-diol is formed as the major product. Calculations performed by using density functional theory revealed the nature of the different competing mechanisms operating during the catalysis process and also provided an insight into the influence of the oxidant and hydrogen bonding on the catalysis process. The mechanistic investigations can therefore serve as a guide in the design of molybdenum-based catalysts for the oxidation of olefins.

Palladium Nanoparticles Supported on Magnesium Hydroxide Fluorides: A Selective Catalyst for Olefin Hydrogenation

A one‐pot synthesis of palladium nanoparticles supported on magnesium hydroxide fluoride has been performed with the fluorolytic sol–gel method. The prepared catalysts were characterized by using various physicochemical techniques. The sol–gel method led to high surface area (>135u2005m2u2009g−1), mesoporous catalysts (pore volume=0.19–0.23u2005cm3u2009g−1, pore diameter=3–5u2005nm) with uniformly dispersed palladium nanoparticles approximately 2u2005nm in diameter on the surface. The catalysts synthesized by using different concentrations of aqueous hydrofluoric acid exhibited changing surface and acidic properties. Very high dispersion of palladium on magnesium fluoride (47u2009%) was obtained with 1u2005wtu2009% palladium loading. The catalysts were used for hydrogenation of various olefins in the presence of other organic functionalities at room temperature and atmospheric hydrogen pressure. Various substituted olefins were hydrogenated with almost 100u2009% conversion and selectivity. The catalysts were recycled efficiently over five cycles without appreciable loss in catalytic activity. There was no palladium leaching under the reaction conditions, which was confirmed by inductively coupled plasma atomic emission spectroscopy analysis. Activation of olefin on the catalyst surface could not be observed by inu2005situ FTIR studies, indicating facile activation of hydrogen on the palladium supported on magnesium hydroxide fluoride.

Silica microspheres containing high density surface hydroxyl groups as efficient epoxidation catalysts

Uniformly sized silica microspheres were synthesized by a hydrolysis–condensation method. The obtained material was etched with a mild aqueous potassium hydroxide solution for different periods of time to break their Si–O–Si bonds and increases the density of hydroxyl groups on their surfaces. The resulting materials were then used as transition metal-free catalysts for oxidation of olefins in the presence of hydrogen peroxide as a green oxidant. The materials were thoroughly characterized using various physicochemical techniques. These highly populated hydroxyl groups on the surface of silica microspheres were proven to be responsible for excellent conversion (up to 93%) and epoxide selectivity (up to 100%) for various olefins. Quantum mechanical calculations also corroborate the experimental findings. Furthermore, both experimental and theoretical studies show that tertiary silanols were present at the active sites of the catalyst surface and were responsible for olefin epoxidation.

Isolation, Characterization, and Identification of Catalytically Active Species in the MoO3/SiO2 Catalyst during Solid Acid Catalyzed Reactions

We report the isolation, characterization, and identification of the catalytically active species formed during various acid‐catalyzed reactions if silica‐supported MoO3 was used as a catalyst. We have reported previously the synthesis and extensive characterization of the silica‐supported MoO3 catalyst prepared by the sol–gel process with ammonium heptamolybdate and ethyl silicate‐40 as molybdenum and silica precursors, respectively. The TEM images showed uniformly distributed MoO3 nanoparticles on the high‐surface area mesoporous silica support and high acidity (0.9u2005mmolu2009g−1) by using temperature‐programmed desorption of ammonia (NH3‐TPD) analysis. This catalyst has already shown high activity for various acid‐catalyzed reactions. To understand the nature of catalytically active species formed during the reaction, the liquid‐phase esterification of acetic acid and ethanol was studied as a probe reaction with very high acid conversion (83u2009%) in 8u2005h. During esterification, the reaction mixture turned blue, which indicated a change in the nature of the catalyst under reaction conditions. These catalytically active species formed in the reaction mixture were isolated and extensively characterized by using FTIR, Raman, powder XRD, BET surface area, NH3‐TPD, energy dispersive X‐ray, and TEM analysis. The characterization results revealed the inu2005situ formation of silicomolybdic acid on the silica surface in the presence of water, which acts as catalytically active species responsible for the acid‐catalyzed reactions.

Selective Oxidation of Nonrefractory and Refractory Sulfides by Cyclopentadienyl Molybdenum Acetylide Complexes as Efficient Catalysts

The synthesis and catalytic properties of molybdenum acetylide complexes CpMo(CO)3(–C≡CR), Rxa0=xa0Ph(1), C6H4–p-CF3 (2) and C6H4–p-CH3 (3) has been studied. The molybdenum acetylide complexes were synthesized from CpMo(CO)3Cl and aryl acetylenes via Stephens–Castro coupling reaction. These complexes were characterized by single crystal X-ray diffraction analysis, FTIR and 1H NMR spectroscopy. These complexes on treatment with hydrogen peroxide, formed corresponding molybdenum oxo-peroxo species. These in situ formed oxo-peroxo species were found very active (up to 100xa0% conversion) and selective (up to 100xa0%) oxidation catalysts for various refractory and nonrefractory sulfides. Interestingly, even though the molybdenum acetylide complexes are homogeneous, they could be recycled very efficiently by extracting the catalytically active molybdenum oxo-peroxo species in aqueous phase.Graphical AbstractMolybdenum acetylide complexes CpMo(CO)3(C≡CR) synthesized from CpMo(CO)3Cl via Stephens- Castro coupling with aryl acetylenes were used as catalyst precursors for selective oxidation of various sulfides including refractory sulfides to sulfoxides or sulfones using H2O2 as an environmentally benign oxidant with excellent conversion under mild reaction conditions.