M. Ali Haider

Catalytic and mechanistic insights into the production of ethyl levulinate from biorenewable feedstocks

The importance of ethyl levulinate (EL) as a fuel additive and a potential biomass-derived platform molecule is noteworthy. EL is obtained from the esterification of levulinic acid (LA) in the presence of ethanol. Besides LA, the acid-catalyzed ethanolysis reaction to produce EL can be carried out on a variety of biomass-derived substrates including furfuryl alcohol (FAL), chloromethyl furfural, monosaccharides, polysaccharides and lignocellulosic biomass. The acid catalysts employed for such conversions cover a wide range of structure and properties. The nature of the acid catalysts and the key intermediates formed during the reaction dictate the overall yield of the desired product. For example, in the ethanolysis reaction of FAL to produce EL, diethyl ether (DEE) and ethoxymethylfuran (EMF) produced as side products are suggested to influence the selectivity of EL. Similarly, in the ethanolysis of glucose, formation of ethyl-D-glucopyranoside (EDGP) results in a slow conversion to product EL. The review, therefore, focuses on highlighting the importance of catalyst structure, acidity and reaction mechanism and the role of key intermediates in the production of EL from biorenewable resources.

Evidence for Two Activation Mechanisms in LSM SOFC Cathodes

Dense La 0.8 Sr 0.2 MnO 3 (LSM) film electrodes with an average thickness of 600 nm were fabricated on yttria-stabilized zirconia and cerium gadolinium oxide by ultrasonic spray pyrolysis. LSM was studied for initial nonstationary behavior by activating with current density for short duration (5 min) and long duration (16 h). The polarization resistance at zero dc bias was reduced upon activation irrespective of the electrolyte, with the reduction more significant after long-duration activation. Short-duration activation was removed by deliberate introduction of La 2 Zr 2 O 7 impurities into the LSM phase or by surface doping with La 0.6 Sr 0.4 FeO 3 nanoparticles. However, long-duration activation still occurred in these samples. Scanning electron micrographs of short-duration-activated films showed no changes in morphology while long-duration activation resulted in a significant bulk pore formation in the LSM phase. Two distinct mechanisms for LSM activation in a solid oxide fuel cell (SOFC) are proposed. Short-duration activation results in changes in the film surface chemistry while long-duration activation leads to the reconstruction of the LSM phase.

Mechanistic Insights into Ring-Opening and Decarboxylation of 2-Pyrones in Liquid Water and Tetrahydrofuran

2-Pyrones, such as triacetic acid lactone, are a promising class of biorenewable platform chemicals that provide access to an array of chemical products and intermediates. We illustrate through the combination of results from experimental studies and first-principle density functional theory calculations that key structural features dictate the mechanisms underlying ring-opening and decarboxylation of 2-pyrones, including the degree of ring saturation, the presence of C═C bonds at the C4═C5 or C5═C6 positions within the ring, as well as the presence of a β-keto group at the C4 position. Our results demonstrate that 2-pyrones undergo a range of reactions unique to their structure, such as retro-Diels-Alder reactions and nucleophilic addition of water. In addition, the reactivity of 2-pyrones and the final products formed is shown to depend on the solvent used and the acidity of the reaction environment. The mechanistic insights obtained here provide guidance for the selective conversion of 2-pyrones to targeted chemicals.

Mechanistic insights into the ring-opening of biomass derived lactones

Deoxygenation of biomass-derived lactone molecules such as γ-valerolactone (GVL) by catalytic ring-opening and decarboxylation facilitate the production of a variety of fuels and chemicals. Density functional theory (DFT) calculations were performed to reveal the mechanism of the ring-opening step. In order to elucidate the effect of substituents and ring-size on the rate of the ring-opening step, lactone molecules such as γ-butyrolactone (GBL), γ-caprolactone (GCL), δ-valerolactone (DVL) and e-caprolactone (ECL) were included. DFT calculations show that the ring-opening reaction proceeds via the formation of a stable oxocarbenium intermediate. Subsequently, the ring-opening occurs through a two-step mechanism to yield unsaturated acids. The intrinsic barriers for the two-step reaction in GVL were estimated to be 69 and 48 kJ mol−1 respectively which are comparable to the experimentally observed apparent activation energies. On changing the ring-size from 5 to 7 member ring lactones (GBL, DVL and ECL), the activation energy for the ring-opening steps was observed to follow the trend predicted by the strain theory. In contrast, on changing the substituent at C4 of the 5-member ring lactones (GBL, GVL and GCL), the activation energies for ring-opening are dictated by a combination of inductive and steric effects. It is expected that the stability of the oxocarbenium ion formed will have a significant role on the rate of the ring-opening. The estimated rates for the ring-opening step with respect to the ring-size show a direct correlation with the enthalpy of oxocarbenium ion formation in the gas phase. Further investigation by ab initio molecular dynamics simulations in implicit and explicit aqueous environment indeed show stable oxocarbenium ions formed which were observed to remain intact for greater than 2 ps of simulation time.

Integrated Bio- and Chemocatalytic Processing for Biorenewable Chemicals and Fuels

Biomass as an alternative to fossil fuels has shown significant potential to produce fuels and chemicals. Analogous to a petrochemical refinery, a biorefinery can be envisaged, where a diverse variety of products will be synthesized. These products can be made either through biological or chemical catalytic routes. Technologies relying solely on biological or chemical conversion methods have shown limitations in achieving high yield of a desired product. As an alternative, an integrated processing strategy can be applied where an intermediate chemical (e.g. lactic acid) or platform molecule is produced through biological transformation, which can be upgraded into useful compounds (e.g. propylene glycol) by chemical catalysis. This study is focused on such integrated methods, which have been applied to produce commodity chemicals and fuels. For the development of an integrated process, heterogeneous catalysts are required, which may be deactivated by the interaction of biogenic impurities, present in the fermentation media. A detailed overview of the available knowledge is presented on the mechanism leading to the deactivation of the catalyst surface by biogenic impurities, which will aid in the better design of the catalyst and purification process. In this regard, electrodialytic purification of the fermentation media is explored as an option for providing economic extraction of the product suited for subsequent catalytic processing. While studies on biogenic impurities are limited only to the hydrogenation reaction on metal surfaces, similar understandings can be applied to other catalytic reactions. Fundamental concepts introduced in this chapter can thus be applied for the development of an integrated process for the conversion of biomass-derived novel platform molecules into high-value chemicals and fuels.

Reactivity descriptor for the retro Diels–Alder reaction of partially saturated 2-pyrones: DFT study on substituents and solvent effects

The retro-Diels–Alder (rDA) reaction of partially saturated 2-pyrone molecules to form the 1,3-butadiene backbone and CO2 was studied using density functional theory (DFT) calculations in vapor-phase, polar and non-polar solvents. The activation barriers for the ring-opening and decarboxylation of the molecules were correlated to the type of substituent present on the 2-pyrone ring. In the vapor-phase, the electronic effect of substituents led to a linear scaling relationship between the calculated activation barrier and corresponding frontier molecular orbital (FMO) gap of the product diene and CO2. A new descriptor was proposed as the average of the ionization potential (IP) of the diene and the electron affinity (EA) of the dienophile to describe the activation energy trend. Solvents were calculated to reduce the activation barriers by stabilizing the polar transition state by as much as 40 kJ mol−1, wherein polar solvents were calculated to reduce the barrier more than the non-polar solvents. The rDA reaction activation barrier in the solvent decreases in the following order: vapor-phase > n-hexane > benzene > acetone > methanol > water. The effect of solvents in rDA reactivity trends was successfully described for the first time through a single descriptor, the FMO gap. The existence of a Bronsted–Evans–Polanyi (BEP) relationship was established for the rDA reaction over different solvents. In solvents, the FMO gap, (IPdiene + EAdienophile)/2 and BEP relationship were proposed as the reactivity descriptors for the rDA reaction of 2-pyrones.

Biological Routes for the Synthesis of Platform Chemicals from Biomass Feedstocks

Development of processes to produce fuels and chemical from biomass offers an exciting opportunity to achieve a sustainable supply from renewable sources as compared to fossil fuel-based methods. Technologies relying only on chemical catalytic routes have shown limitations in achieving the desired yield of a product molecule. Therefore, recent developments in research have emphasized the importance of integrating a biocatalytic route to a chemo-catalytic route to produce commodity chemicals with high conversion and selectivity. Microorganisms including bacteria, fungus and algae are versatile in nature and have potential to yield platform molecules which can be upgraded to produce their petrochemical counterparts. Genetic engineering techniques combined with metabolic flux analysis are employed to further enhance the productivity. On subsequent purification, the platform molecule may be used as a reactant for chemo-catalytic processing to produce a range of high-value chemicals. In this way, a novel integrated fermentation and catalytic processing strategy is envisaged, which will open avenues for producing chemicals from renewable sources. The chapter covers the progress made in this direction by summarizing the routes for producing a platform molecule via biocatalytic transformations.

Field and frequency modulated sub-THz electron spin resonance spectrometer

260-GHz radiation is used for a quasi-optical electron spin resonance (ESR) spectrometer which features both field and frequency modulation. Free space propagation is used to implement Martin-Puplett interferometry with quasi-optical isolation, mirror beam focusing, and electronic polarization control. Computer-aided design and polarization pathway simulation lead to the design of a compact interferometer, featuring lateral dimensions less than a foot and high mechanical stability, with all components rated for power levels of several Watts suitable for gyrotron radiation. Benchmark results were obtained with ESR standards (BDPA, DPPH) using field modulation. Original high-field ESR of 4f electrons in Sm3+-doped Ceria was detected using frequency modulation. Distinct combinations of field and modulation frequency reach a signal-to-noise ratio of 35 dB in spectra of BDPA, corresponding to a detection limit of about 1014 spins.

Non-oxidative conversion of methane into higher hydrocarbons over Mo/MCM-22 catalyst

Molybdenum impregnated zeolite catalyst has been well-known for methane conversion into higher hydrocarbons under non-oxidative condition. HZSM-5 & HMCM-22 zeolites are the effective supports for this purpose. However, the catalytic performance of HMCM-22 supported molybdenum catalyst is considered suitable than that for HZSM-5 catalyst with high aromatic selectivity due to unique pore structure and framework of MCM-22 zeolite support. Effect of Mo loading over MCM-22 zeolite has been studied for the activity test and observed that 5 wt% metal content over the support (MCM-22) is optimum for the proper tuning of acidic & metallic sites of the catalyst. Effect of silica/alumina ratio (SAR, molar) of MCM-22 zeolite has also been studied and observed that lower SAR (30) is suitable (