Oxidoreductase enzymes: Characteristics, applications, and challenges as a biocatalyst.

The effective transesterification process to produce fatty acid methyl esters (FAME) requires the use of low-cost, less corrosive, environmentally friendly and effective catalysts. Currently, worldwide biodiesel production revolves around the use of alkaline and acidic catalysts employed in heterogeneous and homogeneous phases. Homogeneous catalysts (soluble catalysts) for FAME production have been widespread for a while, but solid catalysts (heterogeneous catalysts) are a newer development for FAME production. The rate of reaction is much increased when homogeneous basic catalysts are used, but the main drawback is the cost of the process which arises due to the separation of catalysts from the reaction media after product formation. A promising field for catalytic biodiesel production is the use of heteropoly acids (HPAs) and polyoxometalate compounds. The flexibility of their structures and super acidic properties can be enhanced by incorporation of polyoxometalate anions into the complex proton acids. This pseudo liquid phase makes it possible for nearly all mobile protons to take part in the catalysis process. Carbonaceous materials which are obtained after sulfonation show promising catalytic activity towards the transesterification process. Another promising heterogeneous acid catalyst used for FAME production is vanadium phosphate. Furthermore, biocatalysts are receiving attention for large-scale FAME production in which lipase is the most common one used successfully This review critically describes the most important homogeneous and heterogeneous catalysts used in the current FAME production, with future directions for their use.

Biodiesel is one of the biofuels that has been examined the most in the research and has a promising future as a replacement for fossil fuels. A catalyst is often needed to increase the manufacture of biodiesel, which is produced by transesterification or esterification. A number of difficulties with product separation, a significant amount of wastewater effuents released from the downstream filtration, and the unwanted soap created as a result of the reaction between feedstock with a high FFA composition and homogeneous base catalysts are typical problems faced by commercially available homogeneous catalysts. The use of heterogeneous magnetic catalysts in FAME production has become a focus of interest for many researchers due to its efficient catalyst separation, high catalyst recovery rate, and shorter processing time. The development of magnetic acid and base heterogeneous catalysts, as well as their efficiency in producing biodiesel, were the primary subjects of this study. Additionally, several techniques for producing magnetic particles were evaluated. Method of heterogeneous catalysts in promoting biodiesel synthesis through transesterification procedure was also investigated. To generate more self-sustaining biodiesel industries, further investigation is needed to employ waste materials as a basis for the manufacturing of heterogeneous magnetic acid and base catalysts.

Metal-based catalysts, encompassing both homogeneous and heterogeneous types, play a vital role in the modern chemical industry. Heterogeneous metal-based catalysts usually possess more varied catalytically active centers than homogeneous catalysts, making it challenging to regulate their catalytic performance. In contrast, homogeneous catalysts have defined active-site structures, and their performance can be easily adjusted by modifying the ligand. These characteristics lead to remarkable conceptual and technical differences between homogeneous and heterogeneous catalysts. As a recently emerging class of catalytic material, single-atom catalysts (SACs) have become one of the most active new frontiers in the catalysis field and show great potential to bridge homogeneous and heterogeneous catalytic processes. This review documents a brief introduction to SACs and their role in a range of reactions involving single-atom catalysis. To fully understand process-structure-property relationships of single-atom catalysis in chemical reactions, active sites or coordination structure and performance regulation strategies (e.g., tuning chemical and physical environment of single atoms) of SACs are comprehensively summarized. Furthermore, we discuss the application limitations, development trends and future challenges of single-atom catalysis and present a perspective on further constructing a highly efficient (e.g., activity, selectivity and stability), single-atom catalytic system for a broader scope of reactions.

A heterogeneous catalyst containing MoO42- exchanged on layered double hydroxides (Mo-LDHs) is used to produce 1O2 from H2O2, and with this dark 1O2, unsaturated hydrocarbons are oxidized in allylic peroxides. The oxidation kinetics are studied in detail and are compared with the kinetics of oxidation by 1O2, formed from H2O2 by a homogeneous catalyst. A model is proposed for the heterogeneously catalyzed 1O2 generation and peroxide formation. The model divides the reaction suspension in two compartments: (1) the intralamellar and intragranular zones of the LDH catalyst; (2) the bulk solution. The 2-compartment model correctly predicts the oxidant efficiency and peroxide yield for a series of olefin peroxidation reactions. 1O2 is generated at a high rate by the heterogeneous catalyst, but somewhat more 1O2 is lost by quenching with the heterogeneous catalyst than using the homogeneous catalyst. Quenching occurs mainly as a result of collision with the LDH hydroxyl surface, as is evidenced by using LDH supports containing strong 1O2 deactivators such as Ni2+. A total of 15 organic substrates were peroxidized on a preparative scale using the best Mo-LDH catalyst under optimal conditions.

A review of the problem of distinguishing true homogeneous catalysis from soluble or other metal-particle heterogeneous catalysis under reducing conditions

Abstract: Using a magnetron sputtering approach that allows size-controlled formation of nanoclusters, we have created palladium nanoclusters that combine the features of both heterogeneous and homogeneous catalysts. Here we report the atomic structures and electronic environments of a series of metal nanoclusters in ionic liquids at different stages of formation, leading to the discovery of Pd nanoclusters with a core of ca. 2 nm surrounded by a diffuse dynamic shell of atoms in [C4C1Im][NTf2]. Comparison of the catalytic activity of Pd nanoclusters in alkene cyclopropanation reveals that the atomically dynamic surface is critically important, increasing the activity by a factor of ca. 2 when compared to compact nanoclusters of similar size. Catalyst poisoning tests using mercury and dibenzo[a,e]cyclooctene show that dynamic Pd nanoclusters maintain their catalytic activity, which demonstrate their combined features of homogeneous and heterogeneous catalysts within the same material. Additionally, kinetic studies of cyclopropanation of alkenes mediated by the dynamic Pd nanoclusters reveal an observed catalyst order of 1, underpinning the pseudo-homogeneous character of the dynamic Pd nanoclusters.

Heterogeneous and Homogeneous Catalysts in CO2 Direct Conversion to Methanol

Alpha-galactosidases catalyze the hydrolysis of terminal alpha-1,6-galactosyl units from galacto-oligosaccharides and polymeric galactomannans. The crystal structures of tetrameric Saccharomyces cerevisiae alpha-galactosidase and its complexes with the substrates melibiose and raffinose have been determined to 1.95, 2.40, and 2.70 A resolution. The monomer folds into a catalytic (alpha/beta)(8) barrel and a C-terminal beta-sandwich domain with unassigned function. This pattern is conserved with other family 27 glycosidases, but this enzyme presents a unique 45-residue insertion in the beta-sandwich domain that folds over the barrel protecting it from the solvent and likely explaining its high stability. The structure of the complexes and the mutational analysis show that oligomerization is a key factor in substrate binding, as the substrates are located in a deep cavity making direct interactions with the adjacent subunit. Furthermore, docking analysis suggests that the supplementary domain could be involved in binding sugar units distal from the scissile bond, therefore ascribing a role in fine-tuning substrate specificity to this domain. It may also have a role in promoting association with the polymeric substrate because of the ordered arrangement that the four domains present in one face of the tetramer. Our analysis extends to other family 27 glycosidases, where some traits regarding specificity and oligomerization can be formulated on the basis of their sequence and the structures available. These results improve our knowledge on the activity of this important family of enzymes and give a deeper insight into the structural features that rule modularity and protein-carbohydrate interactions.

The biodiesel manufacturing process favors heterogeneous catalysts over homogeneous catalysts. The main drawbacks of using homogeneous catalysts are their non-renewable nature, separation, and washing, which can be avoided by using heterogeneous catalysts. This research looks into the shell of a river snail (Viviparidae) that has been improved with modified activated carbon (MAC) as a heterogeneous solid base catalyst for palm oil transesterification. The waste shell was repeatedly washed to remove any organic impurities attached to it and then dried in an oven. It was calcined in an air atmosphere for 2 h at a high temperature of 900 °C. The calcined sample (calcium oxide: CaO) was powdered and mixed with MAC. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and the Brunauer-Emmet-Teller (BET) method were used to characterize the CaO/MAC (mass fraction 3:1) catalyst. In order to optimize the reaction conditions for biodiesel production, operating parameters such as methanol to oil mole ratio, catalyst amount, reaction time, and microwave electrical power were investigated. As a result, the best reaction parameters were discovered to be 12:1 methanol to oil mole ratio, 2.5 wt.% CaO/MAC based on oil weight, 4 min of reaction time, and 600 W microwave electrical power. After being reused five times, the biodiesel yield could still reach 90%, indicating that the novel catalyst had good stability and recyclability. The biodiesel fuel properties obtained in this study were compared to the international biodiesel standards ASTM D6751 and EN14214. River snail shell can be thought of as a nature-based benign and resourceful material for biodiesel production, opening up a new path for fuel sustainability.

In the chemistry world, the year 1891 was considered as milestone year for the discovery of new type of heterocyclic compounds by Pietro Biginelli. The Biginelli protocol provides 3,4‐dihydropyrimidin‐2(1H)‐ones/thiones in the presence of Bronsted acids and Lewis acids. Multifunctional Dihydropyrimidones (DHPMs) are displayed wide medicinal properties. Several of homogeneous and heterogeneous catalysts have been reported to improve the yield of product. This review includes the progress of ionic liquids, combination of ionic liquids with homogeneous & heterogeneous catalyst, homogeneous, heterogeneous and asymmetric catalyst took place in last five year (2015–2019) for Biginelli reaction. Most of catalysts showed good catalytic activity (more than 80 %) towards the synthesis of Biginelli adducts.

Cobalt-catalyzed sulfate radical-based advanced oxidation: A review on heterogeneous catalysts and applications

Methyl esters derived from coconut oil are very interesting to study because they contain free fatty acids with a medium carbon chain structure (C12-C14), so most methyl esters (70%) can be bio-kerosene and the rest can be biodiesel. The process of preparing methyl ester by reaction of Trans-esterification triglyceride generally using a homogeneous KOH catalyst but this process requires a long catalyst separation process through washing and drying process. The use of heterogeneous catalysts in the production of methyl esters can remove the washing and drying processes, but trans-esterification reactions with heterogeneous catalysts require severe conditions (high pressure and high temperature), whereas at low temperatures and atmospheric conditions, the methyl ester yield is relatively low. Using microwave-irradiated trans-esterification reactions with heterogeneous catalysts, it is expected to be much faster and can obtained higher yields. Therefore, in this study we prepare a heterogeneous catalyst K/Al2O3 using solution KOH that impregnated in catalyst support Al2O3, and catalyst obtained are caracterized by XRD, BET dan SEM. Our objective was to compare the yield of methyl esters obtained through the trans-esterification process of coconut oil assisted by microwave using a heterogeneous K / Al2O3 catalyst with yield obtained using a homogeneous KOH catalyst. Experimental equipment consists of a batch reactor placed in a microwave oven equipped with a condenser, agitator and temperature controller. The batch process was carried out at atmospheric pressure with variation of K/Al2O3 catalyst concentration (0.5, 1.0, 1.5, 2.0, 2.5%) and microwave power (100, 264 and 400 W). In general, the process of producing methyl esters by heterogeneous catalysts will get three layers, wherein the first layer is the product of methyl ester, the second layer is glycerol and the third layer is the catalyst. The experimental results show that the methyl ester yield increases with increasing of microwave power, catalyst concentration and reaction time. The results obtained with K /Al2O3 catalysts are generally slightly lower than those obtained using a homogeneous KOH catalyst. However, the yield of methyl esters obtained by the K / Al2O3 heterogeneous catalyst process are relatively easy to separate rather than using a homogeneous KOH catalyst.

Review: [116 refs.

Fatty acid ethyl ester is one of the most potential alternative energy because it is renewable and environmentally friendly. Fatty acid ethyl ester is usually made by transesterification using heterogeneous alkali catalyst. Chicken bones ash (CaO) is heterogeneous alkali catalyst which is non-corrosive and environmentally friendly. It can also be separated easily from the product by filtration and less removal problem than homogeneous catalyst. This catalyst was made by grinding and calcination the chicken bones. Others variables used in this research were the dosage of the catalyst, molar ratio of ethanol to crude palm oil and reaction temperature. The best yield of the fatty acid ethyl ester was 90.052% at the condition of 17:1 ethanol to crude palm oil molar ratio, 70 °C reaction temperature, 7% of catalyst (w/w), 7 hours of reaction time and 500 rpm stirring speed.

The heterogeneously catalyzed hydroformylation of 1-hexene in supercritical carbon dioxide is demonstrated as an alternative to homogeneous catalysis through the use of a rhodium–phosphine catalyst tethered to a silica support. Reaction over the heterogeneous catalyst in supercritical CO2 is compared with the use of this catalyst in liquid-phase toluene, and toluene expanded with CO2. Likewise, the performance of the tethered catalyst is compared with a homogeneous rhodium–phosphine catalyst, and shown to be equally effective under identical reaction conditions. Comparable reaction rates were obtained using the heterogeneous rhodium catalyst in supercritical CO2 and CO2-expanded toluene, both of which were superior to the reaction rate with the heterogeneous catalyst in liquid-phase toluene. Initial aldehyde selectivity obtained with the heterogeneous species was also comparable to that obtained with the homogeneous catalyst, but decreased over the course of the reaction. These results demonstrate the value of using phase behavior, and the importance of understanding this behavior in the development and analysis of greener solvent/catalyst systems.