Recent trends and applications of evacuated tube solar collector in food processing and air heating: a review.

Solar energy demand is growing for future energy needs in different sectors to replace fossil fuels, which leads to a reduced carbon footprint and global warming. Evacuated tube solar collectors (ETSC) harness solar thermal energy for air heating, water heating, and drying in domestic and industrial sectors. The review paper comprises ETSC technology categorization, influencing factors like fin arrangement, integration of phase change material, tilt angle, solar radiation, and airflow rate on the performance of ETSC-based solar air heaters and dryers. The thermal performance parameters, like the collector efficiency, dryer efficiency, energy and exergy efficiency, thermal profile, zone temperature, relative humidity, heat loss during operations, etc., are reviewed. The developed ETSC-based air heating systems and solar dryers for drying agricultural products are performed effectively. However, research progress on improving the thermal performance integrated with nanofluids and phase change materials was discussed. CO2 mitigation analysis and global standards for ETSC-based air heaters and dryers are compiled. A large scope exists by use of solar air heaters (SAH) for food commodity drying with a suitable drying chamber and improving the designs of ETSC-based solar dryers. The work accomplished by various researchers has been analyzed in this study for prospective research gaps in the context of future design and development.

Comparative studies on the rheological characteristics, functional attributes, and baking stability of xanthan and guar gum formulated honey gel matrix.

The research aims to enhance the characteristics of honey by incorporating xanthan gum (XG) and guar gum (GG) at various concentrations (0.5-2.0% w/w) and preparing a honey gel matrix (HGM) through high-shear homogenization. This approach serves as a substitute for fat-based filling materials commonly used in bakery products. The study encompassed an investigation of the rheological characteristics (steady and dynamic), total phenolic content (TPC), antioxidant activity, and baking stability of the HGMs. The concentration of the gums used significantly influenced the transformation of honey into the HGM and its stability. Notably, the XG-HGM demonstrated greater shear thinning behavior and higher consistency compared to the GG-HGM. Herschel Bulkley and power law models were found to be the best-fitted models for XG-HGM and GG-HGM, respectively. Furthermore, both XG-HGM and GG-HGM exhibited a higher viscous component (G″) than an elastic component (G') at low concentrations, up to 1% (w/w) for XG-HGM and 1.5% (w/w) for GG-HGM; however, this behavior reversed beyond those concentrations (G' > G″). The XG-HGM exhibited lower temperature sensitivity compared to GG-HGM, indicating better stability under varying heat conditions. Moreover, both TPC and antioxidant activity decreased with increasing concentrations of both gums. The XG-HGM achieved the highest baking stability index, reaching 95.23% at a 2% concentration. This modified HGM formulated with XG demonstrated superior consistency, color retention, and exceptional baking stability, making it a promising candidate for application as a filling material in the bakery sector. Its improved stability and quality can facilitate the development of a wide range of baking products in the food industry.

Recent advancement in ultrasound-assisted novel technologies for the extraction of bioactive compounds from herbal plants: a review.

Herbal plants comprise potent bioactives, and they have a potential for the development of functional foods. Ultrasonication technology can be used to enhance the efficiency and quality of these bioactivities. The present review discussed the ultrasound-assisted novel extraction technologies (supercritical carbon dioxide (CO2) and high pressurized liquid), including mechanistic understanding, influencing factors, extract process efficiency, and the recovery of bioactives with an industrial perspective. The strong observations of this study are the novel ultrasound-induced extraction process variables, such as ultrasound amplitude, sonication time, temperature, solid-solvent ratio, and pressure, are significantly influenced and must be optimized for maximum recovery of bioactives. The novel green technologies (ultrasound and assisted) could remarkably improve the extraction efficiency and enhance the quality of green extract. This review will support technological understanding about the impact on process parameters for the extraction of bioactives for the development of functional foods and nutraceuticals.

Application of novel pretreatment technologies for intensification of drying performance and quality attributes of food commodities: a review.

Drying is an energy-intensive process that can be reduced by the application of pretreatment prior to drying to enhance mass transfer and minimize energy consumption. This review summarizes the mechanistic aspects and applications of emerging pretreatment approaches, namely ohmic heating (OH), ultrasound (US), high pressure processing (HPP), and pulsed electric field (PEF), with emphasis on the enhancement of mass transfer and quality attributes of foods. Novel pretreatments significantly improved the drying efficiency by increasing mass transfer, cavitation, and microchannel formation within the cell structure. Various processing parameters have great influence on the drying performance and quality attributes of foods. Several studies have shown that novel pretreatments (individual and combined) can significantly save energy while improving the overall drying performance and retaining the quality attributes. This work would be useful for understanding the mechanisms of novel pretreatment technologies and their applications for future commercial research and development activities.

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru-P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies.

The reaction of [(p-cymene)RuCl2]2 with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (tBuPHPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged ("H-P/Ru-Cl exchange") to give the (chlorophosphine)ruthenium hydride complex (tBuPClPP)RuHCl [1Cl-HCl; tBuPClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (tBuPHPP)RuCl2 (1H-Cl2), undergoes an H-P/Ru-Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (tBuPPP)RuHCl2, followed by Ru-to-P α-Cl migration to give the observed product 1Cl-HCl (crystallographically characterized). Dehydrochlorination of 1Cl-HCl under a H2 atmosphere gives (tBuPClPP)RuH4 (1Cl-H4), which then can undergo a second dehydrochlorination and addition of H2 to give (tBuPHPP)RuH4 (1H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1H-Cl2, i.e., loss of H2 from 1Cl-H4 to give 1Cl-H2, which could undergo Cl-P/Ru-H exchange to give (tBuPHPP)RuHCl (1H-HCl). Accordingly, the thermodynamics of Cl-P/Ru-H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (RPXPP)RuHCl (X = H, Cl; R = Me, tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d6 complexes, both pincer- and nonpincer-ligated.

Mechanisms of Electrochemical N2 Splitting by a Molybdenum Pincer Complex.

Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N2 binding and splitting by a molybdenum(III) pincer complex, (pyPNP)MoBr3 (pyPNP = 2,6-bis(tBu2PCH2)-C5H3N)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (pyPNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction-diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e- and generates a bimetallic molybdenum(I) Mo2(µ-N2) species capable of N-N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N2 cleavage. The comproportionation reactions reveal the surprising intermediacy of dimolybdenum(0) complex trans,trans-[(pyPNP)Mo(N2)2](µ-N2) in N2 splitting pathways. The same "over-reduced" molybdenum(0) species was also found to cleave N2 upon addition of lutidinium, an acid frequently used in catalytic reduction of dinitrogen.

Is silver a mere terminal oxidant in palladium catalyzed C-H bond activation reactions?

In the contemporary practice of palladium catalysis, a molecular understanding of the role of vital additives used in such reactions continues to remain rather vague. Herein, we disclose an intriguing and a potentially general role for one of the most commonly used silver salt additives, discovered through rigorous computational investigations on four diverse Pd-catalyzed C-H bond activation reactions involving sp2 aryl C-H bonds. The catalytic pathways of different reactions such as phosphorylation, arylation, alkynylation, and oxidative cycloaddition are analyzed, with and without the explicit inclusion of the silver additive in the respective transition states and intermediates. Our results indicate that the pivotal role of silver salts is likely to manifest in the form of a Pd-Ag heterobimetallic species that facilitates intermetallic electronic communication. The Pd-Ag interaction is found to provide a consistently lower energetic span as compared to an analogous pathway devoid of such interaction. Identification of a lower energy pathway as well as enhanced catalytic efficiency due to Pd-Ag interaction could have broad practical implications in the mechanism of transition metal catalysis and the current perceptions on the same.

Formation of Enamines via Catalytic Dehydrogenation by Pincer-Iridium Complexes.

Di-isopropylphosphino-substituted pincer-ligated iridium catalysts are found to be significantly more effective for the dehydrogenation of simple tertiary amines to give enamines than the previously reported di-t-butylphosphino-substituted species. It is also found that the di-isopropylphosphino-substituted complexes catalyze dehydrogenation of several ß-functionalized tertiary amines to give the corresponding 1,2-difunctionalized olefins. The di-t-butylphosphino-substituted species are ineffective for such substrates; presumably, the marked difference is attributable to the lesser crowding of the di-isopropylphosphino-substituted catalysts. Experimentally determined kinetic isotope effects in conjunction with DFT-based analysis support a dehydrogenation mechanism involving initial pre-equilibrium oxidative addition of the amine α-C-H bond followed by rate-determining elimination of the ß-C-H bond.

Polar molecules catalyze CO insertion into metal-alkyl bonds through the displacement of an agostic C-H bond.

The insertion of CO into metal-alkyl bonds is the key C-C bond-forming step in many of the most important organic reactions catalyzed by transition metal complexes. Polar organic molecules (e.g., tetrahydrofuran) have long been known to promote CO insertion reactions, but the mechanism of their action has been the subject of unresolved speculation for over five decades. Comprehensive computational studies [density functional theory (DFT)] on the prototypical system Mn(CO)5(arylmethyl) reveal that the polar molecules do not promote the actual alkyl migration step. Instead, CO insertion (i.e. alkyl migration) occurs rapidly and reversibly to give an acyl complex with a sigma-bound (agostic) C-H bond that is not easily displaced by typical ligands (e.g. phosphines or CO). The agostic C-H bond is displaced much more readily, however, by the polar promoter molecules, even though such species bind only weakly to the metal center and are themselves then easily displaced; the facile kinetics of this process are attributable to a hydrogen bonding-like interaction between the agostic C-H bond and the polar promoter. The role of the promoter is to thereby catalyze isomerization of the agostic product of CO insertion to give an [Formula: see text]-C,O-bound acyl product that is more easily trapped than the agostic species. This ability of such promoters to displace a strongly sigma-bound C-H bond and to subsequently undergo facile displacement themselves is without reported precedent, and could have implications for catalytic reactions beyond carbonylation.

A quantification scheme for non-covalent interactions in the enantio-controlling transition states in asymmetric catalysis.

The origin of enantioselectivity in asymmetric catalysis is attributed to the energy difference between lower and higher energy diastereomeric transition states, which are respectively responsible for the formation of major and minor enantiomers. Although the increase in the number of transition state models emphasizes the role of weak non-covalent interactions in asymmetric induction, the strength of such interactions is seldom quantified. Through this article, we propose a simple and effective method of quantifying the total non-covalent interaction in stereocontrolling transition states belonging to a group of three representative asymmetric catalytic reactions involving chiral phosphoric acids. Our method relies on rational partitioning of a given transition state into two (or three) sub-units, such that the complex network of intramolecular interactions can be ameliorated to a set of intermolecular interactions between two sub-units. The computed strength of interaction obtained using the counterpoise (CP) method on suitably partitioned transition states provides improved estimates of non-covalent interactions, which are also devoid of basis set superposition error (BSSE). It has been noted that catalysts decorated with larger aromatic arms provide cumulative non-covalent interactions (C-Hπ, N-Hπ and ππ) to the tune of 10 to 15 kcal mol-1. Fine-tuning of the magnitude and nature of these interactions can provide valuable avenues in the design of asymmetric catalysts.

H2 Addition to Pincer Iridium Complexes Yielding trans-Dihydride Products: Unexpected Correlations of Bond Strength with Bond Length and Vibrational Frequencies.

R4PONOP-Ir-Me (R1) and R4POCOP-Ir-CO (R2), R = tBu or iPr, are known to undergo acid-catalyzed oxidative addition of H2 that yields octahedral products with two hydrides in a trans-configuration. We use density functional theory to study the free energies (Δ Gtrans) and equilibrium isotope effects (EIEtrans) for H2/D2 addition to R1, R2, and related complexes for R = tBu, iPr, and Me. For a given R, reaction of R1 is ∼5 kcal/mol more exergonic than R2. For a given subclass of complexes, Δ Gtrans is more exergonic for the smaller R. The computed values of Δ Gtrans vary between +5.1 and -17.4 kcal/mol. EIEtrans varies between 0.78 and 1.22. Counterintuitively, it is the less-exergonic reactions that afford products with shorter Ir-H bonds, greater symmetric and asymmetric trans-Ir-(H)2 stretching vibrational frequencies, and more inverse EIEtrans. This disparity is amplified in Me4PONOP-Os-CO, where Δ Gtrans is -35.2 kcal/mol, yet the Os-H bonds are long, and the Os-H vibrational frequencies are low as compared with the Ir-H bonds, and EIEtrans is high (1.20). Attempts are made to account for the inverted bond strength-bond length correlation based on the hydricity of the products and the total negative charge on the trans-Ir(H)2 unit as computed using the Quantum Theory of Atoms in Molecules.