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Abstract: Herein, we have reported an efficient synthesis of 2-aryl benzimidazoles by reacting ophenylenediamines and substituting aromatic aldehydes using SO4 2−/ZrO2-TiO2 as a heterogeneous catalyst. This methodology is straightforward to obtain 2-aryl benzimidazoles with good to excellent yields. It has been performed in ethanol as a green solvent. The reported protocol has some advantages such as a safe and reusable heterogeneous catalyst, without any need for column chromatography to obtain desired products. The catalyst can be recovered for up to five catalytic cycles without significant loss in the catalytic activity.

Quinoxaline derivatives have been synthesized in good to excellent yields by the cyclocondensation reaction of o-pheneylenediamine with substituted phenacyl bromides/benzil in the presence of SO42−/ZrO2-TiO2 as an efficient and heterogeneous catalyst. The catalyst can be recovered up to five catalytic cycles without significant loss in catalytic activity. The reported SO42−/ZrO2-TiO2 catalyst has been thoroughly characterized by using infrared spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and powder X-ray diffraction (XRD). Here, we have used ethanol as a green solvent in this cyclocondensation. This new method has several advantages, such as excellent yields, short reaction time, nontoxic, and easily recoverable catalyst.

Schiff bases and their complexes are versatile compounds, which have been synthesized from the condensation of carbonyl compounds with amino compounds and exhibit a broad range of applications in biological, medicinal, catalysis, and industrial purposes. Furthermore, Schiff basemetal complexes have been used as a precursors for the synthesis of different metal oxides, which include oxides of iron, cobalt, copper, nickel, manganese, vanadium, cadmium, zinc, mercury, etc. and ferrites such as Fe3O4, ZnFe2O4, and ZnCo2O4. These metal oxides have been utilized for several applications as a catalyst for several organic transformations and for biological activity. This review encompasses different methods of synthesis of metal oxides using Schiff base metal complexes precursor, their characterization, and various applications in detail.

Background: Different photocatalysts such as TiO2, ZnO and WO3 have been used for the degradation of organic pollutants. However, these materials have some limitations, which have been affected the catalytic efficiency in the various transformations. The composites of these materials with other oxides can produce better results by tuning structural as well as optoelectrical properties. The composite of TiO2 with ZrO2 has attracted attention due to its use in different areas, as ZrO2 and TiO2 have similar physicochemical features. Methods: This research contains the preparation of ZrO2-TiO2 nanocomposites by hydrothermal method and analysis of photocatalytic activity for the degradation of methylene blue and a mixture of dyes under visible light irradiation. Results: Physicochemical characterization of ZrO2-TiO2 nanocomposites has been studied by using different techniques. Prepared catalysts has shown anatase phase of TiO2 and tetragonal phase of ZrO2. XRD, FESEM and HRTEM have supported the nanocrystalline nature of the composites. The photocatalytic activity of composites and bare TiO2 samples were demonstrated for the degradation of methylene blue dye. Enhanced activity has been shown by composite having Ti:Zr 3:1 molar proportion, i.e., Ti3Zr. Effect of concentration of methylene blue, pH of the solution and catalyst loading have been studied by using Ti3Zr. In addition, the degradation of a mixture of three dyes, namely methylene blue, rhodamine B and methyl orange, has been studied. Conclusion: In summary, prepared ZrO2-TiO2 composites found to be nanocrystalline and visible light active. These catalysts have shown activity for photocatalytic degradation of methylene blue and a mixture of dyes.

A quantitative structure–activity relationship (QSAR) model was built from a dataset of 54 peptide-type compounds as SARS-CoV inhibitors. The analysis was executed to identify prominent and hidden structural features that govern anti-SARS-CoV activity. The QSAR model was derived from the genetic algorithm–multi-linear regression (GA-MLR) methodology. This resulted in the generation of a statistically robust and highly predictive model. In addition, it satisfied the OECD principles for QSAR validation. The model was validated thoroughly and fulfilled the threshold values of a battery of statistical parameters (e.g. r2 = 0.87, Q2loo = 0.82). The derived model is successful in identifying many atom-pairs as important structural features that govern the anti-SARS-CoV activity of peptide-type compounds. The newly developed model has a good balance of descriptive and statistical approaches. Consequently, the present work is useful for future modifications of peptide-type compounds for SARS-CoV and SARS-CoV-2 activity.

Hydrothermal technique was employed for the synthesis of N- and B-codoped TiO2 at different atomic percentages and calcination temperatures (350 and 400 °C). Borane tert-butylamine complex was used as a novel precursor for the dopant atoms. The photocatalytic activities of catalysts were evaluated for destruction of bisphenol A (BPA) spiked in clean water and in secondary treated wastewater influent (SWI) collected from the Groundwater Replenishment System GWRS (Orange County, CA, USA), the world's largest water purification system for indirect potable reuse. Results of X-ray photoelectron spectroscopy (XPS) revealed the incorporation of N and B atoms onto TiO2 lattice via substitution of O and/or Ti, and interstitial incorporation mechanisms. Carbon doping (graphite-like carbon; O-C = O) was also evidenced from XPS spectra. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) revealed pristine TiO2 and all doped TiO2 existed in anatase structure. The anatase phase of catalysts was further confirmed by simulation of selected area electron diffraction (SAED) of the catalysts and reported anatase TiO2 using the simulation software CrysTBox (v 1.10). Among all prepared catalysts, 2.0NBT-350 (N + B = 2.0%, calcination = 350 °C) possessed the smallest particle size (12.9 nm), largest BET surface area (108.5 m2/g) and highest visible-light absorption (i.e., λ > 400 nm). Simulated solar light/2.0NBT-350 system exhibited the maximum removal of BPA (0.1 μM) at the optimum catalyst dose of 0.8 g/L in clean water, over three consecutive degradation reuse cycles without any noticeable decrease in its photocatalytic activity. BPA was also successfully removed in SWI water, however with slower kinetics due to hydroxyl radical quenching effect of dissolved organic and inorganic species present in the SWI matrix. Degradation byproducts were identified by LC/Q-TOF-ESI-MS analysis. Six byproducts were found in the reaction mixture, with two compounds were newly identified in the current study. BPA oxidation routes comprised successive BPA hydroxylation, elimination of aromatic ring, ring opening, and cyclization of aliphatic compounds. This study is a promising demonstration for controlled design and synthesis of photocatalysts useful for environmental applications, especially for the treatment of BPA in wastewater effluents.

In the present work, an extensive QSAR (Quantitative Structure Activity Relationships) analysis of a series of peptide-type SARS-CoV main protease (MPro) inhibitors following the OECD guidelines has been accomplished. The analysis was aimed to identify salient and concealed structural features that govern the MPro inhibitory activity of peptide-type compounds. The QSAR analysis is based on a dataset of sixty-two peptide-type compounds which resulted in the generation of statistically robust and highly predictive multiple models. All the developed models were validated extensively and satisfy the threshold values for many statistical parameters (for e.g. R2 = 0.80–0.82, Q2loo = 0.74–0.77). The developed models identified interrelations of atom pairs as important molecular descriptors. Therefore, the present QSAR models have a good balance of Qualitative and Quantitative approaches, thereby, useful for future modifications of peptide-type compounds for anti- SARS-CoV activity.

ZnO-TiO2 nanocomposites have been successfully synthesized by using simple hydrothermal method with various ZnO-TiO2 concentrations (such as 2:1, 1:1 and 1:2 ZnO-TiO2, pure ZnO and TiO2). The XRD, FT-IR, HR-TEM, FE-SEM with EDAX, XPS and UV–Visible spectroscopy were utilized for characterization of the as-prepared products. XRD revealed that samples are in nanocrystalline nature with formation of anatase TiO2, hexagonal ZnO, cubic Zn2TiO4 and other zinc titanates. The average crystallite size of the samples was between 8 and 36 nm. The photocatalytic activity has been demonstrated for the degradation of methylene blue (MB), tetracycline (TC) and mixture of dyes (includes methylene blue, rhodamine B and methyl orange) under sun light irradiation at room temperature. The degradation rate of methylene blue has been significantly enhanced with increase in percentage of ZnO. Nanocomposites with 2:1 concentration of Zn and Ti, has been more efficient than that of other composites. Also, effect of different parameters such as pH of dye solution, concentration of dye and amount of catalyst etc. has been evaluated.

Iodine-doped TiO2 nanoparticles (I@TiO2 NPs), in different mol percentage (mol%) of iodine with anatase phase only, have been prepared by using simple sol-gel method. Samples were characterized by various physico-chemical techniques. Physico-chemical characterization revealed that crystallite size, surface hydroxylation, and tuning of optical band gap towards visible region of composites increases with increase in dopant concentration upto 7.0 mol% in TiO2 host lattice. Thereafter, the samples were used to photocatalytic degradation of textile effluent (TE) as well as Rhodamine B (RhB) under sunlight irradiation directly. The progress of photodegradation reaction was monitored by UV–visible spectroscopy. The chemical oxygen demand (COD) of TE samples was evaluated before and after photocatalytic studies. During photocatalytic reaction, the effect of pH and amount of catalyst on the rate of degradation reaction was also studied. It is clearly shown that I@TiO2 particles result in the enhancement of the degradation ability of RhB as well as TE in sunlight.

ABBV-075 (Mivebresib) analogues were reported to have BET family bromodomain binding affinity Ki in nano-molar (nM) range (0.8 to 3000 nM). But future optimizations are required to achieve a drug-like molecule with retention of high binding affinity and optimum ADMET (Absorption, Distribution, Metabolism, Excretion and Toxicity) profile. This could be achieved by identifying the pharmacophoric features (salient and concealed) using modern techniques like CADD (Computer-Aided Drug Designing). Therefore, in the present work, QSAR (Quantitative structure-activity relationship) analysis, a thriving CADD branch, has been executed to achieve the determined objectives. The developed QSAR model is statistically acceptable with robust fitting, high internal and external predictive ability. The developed model fulfils the threshold values for a good number of statistical parameters like R2 = 0.80, R2CV = 0.77, etc. The analysis reveals that non-ring Carbon/Nitrogen atoms, frequency of occurrence of specific combinations of Carbon/Nitrogen atoms with acceptor/donor atoms are important pharmacophoric features for BET binding affinity. Thus, the developed QSAR has a balance of quantitative and qualitative approaches. The results could be useful for future optimizations of Mivebresib analogues .

Ta2O5, Ta2O5–SrO and Ta2O5–TiO2 photocatalysts were prepared by hydrothermal method and characterized using X-ray diffraction (XRD), Ultraviolet-visible Diffuse Reflectance spectroscopy (UV-Vis DRS) and Thermogravimetry (TGA). In synthesized series of TiO2-Ta2O5 catalysts, formation of anatase TiO2, hexagonal Ta2O5 and titanium tantalates were observed. For Ta2O5-SrO (Ta:Sr= 1:1) formation of mixed oxide and crystallinity was observed at calcinations temperature at 1100 °C (5hr). These synthesized catalysts were used for degradation study of methylene blue. The result demonstrated that the photocatalytic activity of the mixed oxides [Ta2O5-TiO2(1:1), Ta2O5-TiO2(2:1), Ta2O5-TiO2(1:2), Ta2O5-SrO (Ta:Sr= 1:1)] was higher than that of Ta2O5 alone.

Nitrogen-doped TiO 2 nanoparticles (N–TiO 2 NPs) with anatase phase were synthesized by sol–gel method using a single precursor containing titanium (IV) terbutoxide, glacial acetic acid, sodium dodecyl sulphate, ammonia, and urea. X-ray diffraction (XRD) reveals the nanocrystalline nature with anatase phase of all the samples. The particle size of all samples was found in the range of 5–12 nm using transmission electron microscopy (TEM). UV–visible absorption measurements examined that the optical band gap of the doped samples decrease with increase in dopant concentration from 0.0 to 7.0 mol%. Field-emission scanning electron microscopy (FESEM) with energy dispersive atomic X-ray (EDAX) spectroscopy was employed to analyse the morphology and chemical composition of these N–TiO 2 NPs. The photocatalytic activity of bare/doped TiO 2 samples was demonstrated for the degradation of Rhodamine B (RhB) dye under direct sunlight irradiation. The photocatalytic degradation was monitored by measuring the kinetic parameters based on UV–visible spectroscopy as well as the chemical oxygen demand (COD) during the course of the reaction. The effect of dye concentration and pH of the solution on the photocatalytic degradation reaction in the presence of colloidal bare/doped TiO 2 were also studied. The N–TiO 2 catalyst, with a nitrogen concentration of 7.0 mol%, showed the highest activity for photocatalytic mineralization of dye at acidic or alkaline medium than neutral condition under solar light irradiation directly.

Different nickel oxide catalysts have been prepared by simple precipitation method using different nickel precursors and using surfactant sodium dodecyl sulfate (SDS) or without surfactant. The prepared catalysts have been characterized by using XRD and FT-IR. The particle sizes have been calculated by using the Scherrer equation. The nickel oxide catalyst prepared by using nickel acetate as precursor and surfactant SDS has shown less particle size as compared to other catalysts prepared by using other precursors. Also, nickel oxide catalyst prepared using nickel acetate precursor and surfactant SDS has shown higher catalytic activity for reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) using sodium borohydride (NaBH 4 ). Also, for the said reaction effect of concentration of p-nitrophenol on catalytic efficiency has been studied.

Sulfated Zirconia (SZ) has opened up a very interesting area for application predominantly as catalyst for various acid catalyzed organic syntheses and transformation reactions. Catalytic properties of SZ vary with methods of preparation. Lot of efforts is made to modify SZ to increase reactivity and stability of the catalyst. This review focuses on the individual synthesis routes to prepare nano-sized and –crystalline SZ, short discussion on its characterization and exhaustive survey on its utility in organic chemistry for the development of new synthetic methodologies, which provide simple direction having enormous practical significance. As can be realized, the nano-sized and –crystalline SZ as solid acid catalyst exhibits exceptionally better catalytic activity and selectivity for the synthesis of trisubstituted and tetrasubstituted imidazoles, acetyl salicylic acid, dypnone, esterification of acetic acid, caprylic acid and so on.

Sulfated-zirconia is found to be a highly efficient heterogeneous solid catalyst in activating trimethylsilyl cyanide (TMSCN) for facile cyanosilylation of aldehydes. A unifying explanation is provided for the cyanosilylation reactions. The unique role of sulfated-zirconia consists of rendering cyanide anions from TMSCN to C O of aldehydes and providing the corresponding cyanohydrin silyl ethers in quantitative yields under mild and solvent-free conditions.

A straightforward and general method has been developed for the synthesis of α-aminonitriles from the reaction of aldehydes or ketones with amines and trimethylsilyl cyanides in the presence of a catalytic amount of promoted zirconia solid acid catalyst at room temperature. This simple experimental and product isolation procedure combined with easy recovery and reusability of the catalyst are expected to contribute to the development of clean and environmental friendly strategy for the synthesis of α-aminonitriles. A plausible reaction mechanism has been proposed for this multicomponent reaction.

The aza-Michael addition reactions of amines with α,β-unsaturated carbonyl compounds were efficiently carried out at room temperature under solvent-free condition employing sulfated zirconia as a reusable heterogeneous catalyst. The desired products were formed in short reaction times and in high yields. The bulk and surface properties of the synthesized catalyst was examined by X-ray powder diffraction, BET surface area, temperature programmed desorption of ammonia, scanning electron microscopy and thermogravimetric techniques. Characterization results reveal the super acidic nature of the catalyst.

CuI-modified zeolites, especially CuI-USY, have proved to be very efficient catalysts in multicomponent reactions, allowing for a solvent-free synthesis of propargylamines from aldehydes, amines, and terminal alkynes. With a heterogeneous catalyst and in the absence of solvent, this process is among the greenest ever reported. A mechanism has been proposed for this three-component reaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The catalytic efficacy of sulfated zirconia was investigated towards the opening of epoxide rings by aromatic amines under solvent-free conditions to selectively synthesize various β-amino alcohols with high regioselectivity. Interestingly, the SO 4 2 − /ZrO2 catalyst was found to exhibit an excellent catalytic activity for the title reaction. Various advantages associated with this novel and environmental friendly protocol include, solvent-free conditions, short reaction times, high product yields, simple workup procedure, and easy recovery and reusability of the catalyst.

The catalytic performance of molybdate- and tungstate-promoted zirconia catalysts for tert -butylation of phenol with tert -butanol as alkylating agent has been investigated. The reaction was carried out at 433–493 K in a fixed bed micro-reactor at normal atmospheric pressure. The promoted zirconia catalysts were synthesized by immersing the finely powdered hydrous zirconium hydroxide in aqueous ammonium heptamolybdate or ammonium metatungstate solution and subsequent oven drying and calcination at 923 K. The hydrous Zr(OH) 4 was prepared from aqueous zirconium oxychloride by hydrolysis with a dilute aqueous ammonia solution. To investigate the structural and textural properties, we characterized the synthesized catalysts by X-ray powder diffraction (XRD), BET surface area, temperature programmed reduction (TPR), temperature programmed desorption (TPD) of ammonia, scanning electron microscopy (SEM), Raman spectroscopy (RS), and FT-infrared spectroscopy (FTIR) techniques. All the characterization results reveal that the incorporated promoter cations show a significant influence on the surface and bulk properties of the ZrO 2 . In particular, the impregnated cations stabilize the metastable tetragonal phase of zirconia at ambient conditions and enhance the total number and the strength of the acid sites. A good substrate (phenol) conversion and excellent product selectivity were obtained over the promoted zirconia catalysts.

Knoevenagel condensation of various aliphatic, aromatic and heterocyclic aldehydes with malononitrile was carried out in the liquid phase in one step by employing a sulfate-ion promoted zirconia solid acid catalyst. This catalyst facilitates the reaction under solvent-free conditions at moderate temperatures providing excellent yields of the products. The sulfate-ion promoted zirconia catalyst was synthesized by immersing a finely powdered hydrous Zr(OH)4 into the 1 M H2SO4 solution and subsequent drying and calcination at 923 K. The Zr(OH)4 was prepared from an aqueous ZrOCl2·8H2O solution by hydrolysis with dilute aqueous ammonia. The bulk and surface properties of the prepared catalyst was examined by X-ray powder diffraction, BET surface area, temperature programmed desorption of ammonia and Raman spectroscopy techniques. All characterization results revealed that the incorporated sulfate-ions show a significant influence on the surface and bulk properties of the ZrO2. In particular, the impregnated sulfate-ions stabilize the metastable zirconia tetragonal phase at ambient conditions and enhance the total number and strength of acid sites.

A novel SO42−/CexZr1−xO2 catalyst has been employed for Pechmann condensation of phenols under solvent-free conditions. Catalyst loads as low as 10 wt.% could be used leading to high yields with activated phenols at an oil bath temperature of 393 K. To make the catalyst, Ce–Zr–hydroxide gel was prepared by a coprecipitation method and SO42− ions were deposited by treating the gel with sulfuric acid and calcined at 923 K. Surface and bulk properties of the catalyst was investigated by means of X-ray powder diffraction, ammonia-temperature programmed desorption, Raman spectroscopy, and BET surface area methods. Characterization results reveal the super acidic nature of the catalyst. The methodology presented offers significant improvements for the synthesis of coumarins with regard to yield of products, simplicity in operation, and green aspects by avoiding toxic conventional catalysts and solvents.

CuI-modified zeolites, especially CuI-USY, have proved to be very efficient catalysts in multicomponent reactions, allowing for a solvent-free synthesis of propargylamines from aldehydes, amines, and terminal alkynes. With a heterogeneous catalyst and in the absence of solvent, this process is among the greenest ever reported. A mechanism has been proposed for this three-component reaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)