Venkata Narayana Kalevaru

Heterogeneously Catalyzed Ammoxidation: A Valuable Tool for One-Step Synthesis of Nitriles

Ammoxidation is a partial oxidation with selective insertion of nitrogen into a methyl group (aldehyde or alcoholic groups are also able to react) that is in the α‐position to double bonds of olefinic, aromatic or heteroaromatic hydrocarbons to produce corresponding nitriles. The reaction is accelerated by heterogeneous catalysis and carried out in continuous process, in general. The heterogeneously catalyzed ammoxidation of various hydrocarbons to synthesize a wide range of industrially important nitriles has been the subject of great interest in recent times because nitriles are very useful basic chemicals (e.g. acrylonitrile) and organic intermediates (e.g. nicotinonitrile) used in the manufacture of numerous value‐added fine chemicals. The reaction proceeds with H abstraction from the methyl group, leading to an allylic or benzylic intermediate that reacts with catalyst lattice oxygen to form an oxygen‐containing intermediate through the Mars–van Krevelen mechanism. Nitrogen insertion seems to be due to adsorbed N species. Finally, nitrile formation occurs via an imine intermediate. Alkanes could be converted in a similar way, albeit with catalysts that possess a dehydrogenation function to give rise to the intermediate allylic species. Furthermore, ammoxidation is a very “green” reaction because oxygen or air is used as oxidant and water is generally the only byproduct. The ammoxidation reaction is used in the production of large‐scale chemicals, such as acrylonitrile, as well as that of intermediates, specialty chemicals, and fine chemicals in the agrochemical, health, and nutrition industries. The reaction is mostly carried out in the gas phase using supported transition metal oxides as catalysts. The most important advantage is that the ammoxidation is a simple, single‐step, eco‐friendly reaction. Nevertheless, one bottleneck is that only less‐functionalized reactants can be converted; that is, higher substituted molecules are often subject to side‐reactions. However, at present, ammoxidation is indeed a lucrative field of study, which has recently found much wider application in industry and hence will in years to come continue to offer exceptional commercial rewards for the production of industrially important nitriles.

Continuous synthesis of diethyl carbonate from ethanol and CO2 over Ce–Zr–O catalysts

CexZr1−xO2 (x = 0, 0.2, 0.5, 0.8 and 1.0) solids were prepared by a citrate method and characterized by various techniques such as N2-adsorption (BET-SA), XRD, XPS, TEM, H2-TPR, NH3- and CO2-TPD. The catalytic performance of these solids was evaluated for the direct synthesis of diethyl carbonate (DEC) from ethanol and CO2 in continuous mode using a plug-flow reactor (PFR). According to thermodynamic data, the reaction is favourable at low reaction temperatures and high reaction pressures. Thus, the catalytic experiments were carried out at reaction temperatures ranging from 80 to 180 °C and at reaction pressures from 80 to 180 bar. The CexZr1−xO2 catalysts exhibited significant differences in their performance mainly depending on (i) their Ce : Zr ratio and (ii) the different acid–base characteristics. Among the series Ce0.8Zr0.2O2 (C80Z) and Ce0.5Zr0.5O2 (C50Z) catalysts displayed the most efficient performance. Moreover, C80Z, pretreated at 700 °C, yielded DEC at the equilibrium conversion level of YDEC ~ 0.7% at 140 °C and 140 bar at a CO2 : ethanol ratio of 6 : 1 at a LHSV of 42 Lliq kgcat−1 h−1.

Significant Formation of Adipic Acid by Direct Oxidation of Cyclohexane Using Supported Nano‐Gold Catalysts

Adipic acid (AA) is one of the highest volume chemicals in world use with a wide range of commercial applications. This paper demonstrates the possibility of producing significant proportions of AA by the selective oxidation of cyclohexane (CH) in one step using various supported gold‐nanoparticle (AuNP) catalysts. The catalysts were characterized by ICP, BET, XRD, X‐ray photoelectron microscopy, and TEM. The catalytic activity tests were carried out in the liquid phase using autoclaves in the temperature range of 100–170 °C at 10 bar. A CH conversion of over 25 %, with 26% selectivity of AA and 70% selectivity of KA oil (cyclohexanone+cyclohexanol), was achieved over a nano‐gold/TiO2 (anatase) catalyst. The superior performance of this catalyst is attributed to the smaller size of AuNPs, which has an average particle size of 2 nm. Good correlation between activity and Au particle size could be achieved. Overall, the particle size of the AuNPs showed a strong influence on catalytic performance.

Impact of Co-Components on the State of Pd and the Performance of Supported Pd/TiO2 Catalysts in the Gas-Phase Acetoxylation of Toluene

The effect of the co‐components Mn, Co, Au, and Sb with a wide range of standard reduction potentials (Mn2+/Mn: E°=−1.18 V; Co2+/Co: E°=−0.28 V; Sb3+/Sb: E°=+0.2 V; Au3+/Au: E°=+1.52 V) on the catalytic performance of 10 wt % Pd, 8 wt % M/TiO2 (M=Mn, Co, Sb, Au) catalysts in the gas‐phase acetoxylation of toluene to benzyl acetate has been studied. Co‐components with low E° are more active, but less selective than those with high E°. Co‐components with low E° (Mn, Co) stabilize Pd in its oxidized form, whereas those with high E° (Sb, Au) support the formation of metallic Pd. Sb and Au are incorporated into the Pd lattice, whereas Mn and Co are enriched at the surface during time on stream. Carbon is deposited on all catalysts during the reaction. However, for Mn‐ and Co‐containing catalysts, carbon is incorporated into the metal particles, whereas it is deposited on top of Pd particles modified with Sb and Au, which leads to faster deactivation.

Enhanced formaldehyde selectivity in catalytic methane oxidation by vanadia on Ti-doped SBA-15

Vanadia species (2.5 wt% V) were supported on 0.2 wt% Ti-doped SBA-15 (V/Ti-SBA-15) and tested towards the selective oxidation of methane to formaldehyde. V/Ti-SBA-15 shows improved redox properties and higher selectivity towards formaldehyde over all conversions compared to VOx on pure SBA-15 (V/SBA-15).

New Insights into the Nature of Co-components and Their Impact on Pd Structure: X-ray Absorption Studies on Toluene Acetoxylation Catalysts.

Co-components are a powerful tool to tune the performance of catalysts, but their nature and their impact on the catalysts is often controversially discussed. In this study X-ray absorption spectroscopy (XAS) was employed to elucidate the nature of co-components and their impact on the catalytic reaction. In anatase-supported Pd-based catalysts for the gas-phase acetoxylation of toluene, less noble co-components (e.g., Mn, Co, and Sb) spread over the support in their oxidic form and changed their valence state on stream. Incorporated atoms such as C or a small part of the Sb affect the electronic structure of Pd. For the noble Au, only a weak interaction with the support and Pd was observed during time on stream. Only XAS at the K-edges together with investigations of the Pd L-edge for a better understanding of the electronic structure, supplemented by STEM for elemental mapping, allow such detailed insights.

The Impact of Reaction Pressure on the Catalytic Performance of the PdSb/TiO2 Catalyst in the Acetoxylation of Toluene into Benzyl Acetate

The acetoxylation of toluene in the presence of acetic acid and oxygen was performed over a PdSb/TiO2 catalyst at 210 °C and at various reaction pressures (1–10 bar). A remarkable improvement in the catalytic performance and a significant shortening of the induction period were found with an increase in pressure. At a pressure of 6 bar, the highest toluene conversion of 75 % and 100 % selectivity for benzyl acetate were observed. This result could be due to both the restructuring of the catalyst surface and to the formation of active PdSb particles of desired size, composition, and shape during the course of the induction period. Samples of the most‐active and spent catalysts were studied by ex situ and in situ XRD, XPS, and TEM.

Synergy between vanadium and molybdenum in bimetallic ZSM-5 supported catalysts for ethylene ammoxidation

Ammoxidation of ethylene to acetonitrile was studied on V/ZSM-5, Mo/ZSM-5 and V–Mo/ZSM-5 catalysts prepared by a solid-state ion exchange method. The physico-chemical properties were investigated by means of XRD, N2 physisorption, 27Al and 29Si MAS NMR, UV-Vis DRS, XPS, pyridine-IR and FT-IR spectroscopies and H2-TPR/O2-TPO. Based on the characterization results, M–Ox (M = V or Mo) species react with zeolite protons during the exchange process and generate new Lewis acid sites, which act as redox centers. The M–Ox species are essentially, monomeric and dimeric/polymeric species or metal oxide crystallites (less than 4 nm) highly dispersed in the channel and/or on the external surface of zeolite. For the Mo/ZSM-5 sample, the formation of Al2(MoO4)3 nano-crystallites was observed. UV-Vis DRS and TPR results showed that V and Mo species in all catalysts are mainly in the highest oxidation states. The V–Mo/ZSM-5 catalyst exhibited a more reversible behavior of the M–Ox centers throughout the H2/O2 redox cycles than those in V/ZSM-5 and Mo/ZSM-5 catalysts. The best catalytic performance was achieved over the bimetallic V–Mo/ZSM-5 catalyst. These results revealed that the partial substitution of molybdenum with vanadium has a positive effect on the activity and the selectivity to acetonitrile. This implies clearly that a synergetic effect between V and Mo species plays an important role in the ammoxidation reaction. This synergetic effect is related to the existence of electronic interaction at short range order between the V and Mo species, which may influence the catalyst redox properties.

Phosphate Functionalization of CeO2-ZrO2 Solid Solutions for the Catalytic Formation of Dimethyl Carbonate from Methanol and Carbon Dioxide

Phosphate surface groups on CeO2–ZrO2 solid solutions were generated by treating Ce–Zr–hydroxide precursors with phosphoric acid. In the catalytic formation of dimethyl carbonate (DMC) from methanol and CO2, the performance of the P‐modified samples was markedly affected relative to that of the unmodified ones: Phosphate treatment caused remarkable changes in the phase composition, acid–base properties, and the ability to form monodentate methoxy intermediates. The DMC yield (1.6 %) was successfully improved from 0.24 to 1.6 % by phosphate modification of CeO2–ZrO2 (Ce/Zr=4.7, P/Zr=0.13) and by performing the reaction at 170 °C and 6.5 MPa for 1 h.

Supported Gold Nanoparticles as Promising Catalysts

In recent times, gold nanoparticles (AuNPs) either in the form of colloids or as supported nanoparticles are being extensively used as efficient redox catalyst materials. Cataly‐ sis particularly using supported gold nanoparticles (AuNPs) has attracted immense research interest due to their unique properties and greater potentiality that is directly related to their particle size. The primary objective of this chapter is to provide comprehensive overview about gold metal nanoparticles (AuNPs) and their applica‐ tion as promising catalysts. This chapter contains six sections in total. Section 1 starts with a general introduction, recent progress, and brief summary of the application of supported AuNPs as promising catalysts for different applications. Section 2 briefs the properties and stability of gold nanoparticles. Section 3 reviews the preparation methods of supported AuNPs for a wide range of catalytic applications. Section 4 describes briefly some of the most commonly reported supported AuNPs for different applications. Section 5 concentrates on our own results related to the application of supported AuNPs in heterogeneous catalysis. In this section, the oxidation of cyclo‐ hexane (CH) and benzyl alcohol (BA) to adipic acid (AA), benzaldehyde (BAl), and ammoxidation of 2-methylpyrazine to 2-cyanopyrazine are discussed. Finally, Section 6 describes, main points and outlook are summarized.