Phosphotungstic acid H3[PMo12O40](PTA) with the Keggin structure has become well known as a solid... more Phosphotungstic acid H3[PMo12O40](PTA) with the Keggin structure has become well known as a solid superacid with pKa≈-13. Such a strong acidity is caused by delocalization of the negative charge of the anion on many oxygen atoms over the surface of the Keggin structure. High acidity of PTA and its good solubility in water and other polar solvents enables its use as a highly active homogeneous catalyst. However, in spite of relatively higher reaction rate, homogeneous catalysis has various drawbacks that limit its practical application. The main drawback is the difficult and expensive removal of the used catalyst from the reaction mixture and its recycling. PTA also demonstrated good catalytic activity as a heterogeneous catalyst of various organic reactions, e.g. hydrolysis, hydration and polymerization. Wide application of a pure superacid in catalysis is limited by its low surface area and solubility in polar solvents. The objective of this work is the synthesis and study of insoluble superacidic catalysts covalently embedded into the silica matrix. The catalyst PTA/SiO2 was synthesized by the sol-gel method. Tetraethoxysilane was co-condensed with PTA in acidic media in the presence of Pluronic P123 surfactant as a pore-forming agent. The obtained gel was air-dried and calcined at 500 °C producing a mesoporous material with a significant fraction of micropores in its structure. Isotherms of adsorption/desorption of nitrogen indicated cylindrical shape of the pores with necks that is typical for materials obtained with Pluronic P123 as a template. Cs-exchanged material was prepared by mixing PTA/SiO2 with a solution of CsCl. The cation exchange on cesium decreased the total pore volume due to a much higher volume of cesium ions as compared to protons. In addition, partial pore blocking by these ions restricted access to small pores thus reducing accessible surface area. Heteropolyacids are unstable in alkaline media that makes direct solid-state titration impossible. Surface acidity of the samples was determined by reversed titration. Dry samples were dispersed in a solution of pyridine in tetrahydrofuran. After equilibration, the solid phase was filtered, and the filtrate was titrated by HCl. PTA/SiO2 has a very high adsorption capacity on pyridine, which corresponds to 15 molecules of pyridine per [PMo12O40]3- anion. This number exceeded the number of available protons. The catalysts were successfully tested in the alkylation of mesitylene by alkenes. The use of superacidic materials in catalytic reactions can significantly improve the effectiveness of the processes
An adsorbent for Cs removal from contaminated water based on phosphotungstic acid (PTA) embedded ... more An adsorbent for Cs removal from contaminated water based on phosphotungstic acid (PTA) embedded in SiO2 network was synthesized and granulated with γ-Al2O3. PTA/SiO2 had a high adsorption capacity towards Cs while the binder provided excellent mechanical characteristics of the material. It was shown that small particles of PTA/SiO2 with the sizes of 0.1–1 µm occupied space between larger particles of the binder (up to 5 µm). Chemical interaction between PTA and γ-Al2O3 during the adsorbent preparation also took place. The obtained porous material with the specific surface area of 286.9 m2/g contained 4.73% of PTA. Presence of Keggin units in the structure was confirmed by solid state NMR spectroscopy. Study of the adsorbent in Cs+ adsorption from solutions demonstrated its high adsorption capacity. The concentrations of Cs+ in the solutions after the column tests decreased by 3.3–5.2 times. The presence of Na+ and K+ as competing ions did not affect the adsorption. The material was tested in clean-up of radioactive water from the shelter of Chernobyl nuclear power plant (Ukraine). A significant decrease of 137Cs radioactivity was detected in all samples of radioactive water, especially in acidic solutions. Thus the adsorbent can be used for water treatment after incidents resulting in release of radioactive isotopes 134Cs and 137Cs.
The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using eth... more The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using ethanol and n-hexane, and investigated for the presence of secondary metabolites. Both ethanol and n-hexane extracts revealed the presence of alkaloids. Flavonoids, glycosides and saponins were present in only the ethanol extract whereas tannins were present in the n-hexane extract. The bioactivities of the leaf extracts were attributed to their phytochemical constituents. Antimicrobial activity of the extracts were determined against some human pathogenic bacteria and fungi such as Staphylococcus aureus, Escherichia coli, Streptococcus pneumoniae, Bacillus subtilis and Candida albicans using the agar well diffusion and broth dilution methods with the polar extract being more effective. The ethanol extract demonstrated a significant broad-spectrum antimicrobial activity against both gram-positive and gramnegative bacteria, with the highest activity having a zone of inhibition of 10 mm. Anti...
The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using eth... more The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using ethanol and n-hexane, and investigated for the presence of secondary metabolites. Both ethanol and n-hexane extracts revealed the presence of alkaloids. Flavonoids, glycosides and saponins were present in only the ethanol extract whereas tannins were present in the n-hexane extract. The bioactivities of the leaf extracts were attributed to their phytochemical constituents. Antimicrobial activity of the extracts were determined against some human pathogenic bacteria and fungi such as Staphylococcus aureus, Escherichia coli, Streptococcus pneumoniae, Bacillus subtilis and Candida albicans using the agar well diffusion and broth dilution methods with the polar extract being more effective. The ethanol extract demonstrated a significant broad-spectrum antimicrobial activity against both gram-positive and gram-negative bacteria, with the highest activity having a zone of inhibition of 10 mm. Antioxidant activity was determined using the DPPH assay method and the absorbance measured using UV-visible spectrophotometer with ascorbic acid as control. The antioxidant activities of solo and solomix showed IC50 of 1.465x10-2 and 1.364x 10-2 respectively. This study demonstrates the efficacy of ethanolic leaf extracts of C. Papaya as an alternative antibiotic for the development of newer antibacterial agents.
Phosphotungstic acid H3[PMo12O40](PTA) with the Keggin structure has become well known as a solid... more Phosphotungstic acid H3[PMo12O40](PTA) with the Keggin structure has become well known as a solid superacid with pKa≈-13. Such a strong acidity is caused by delocalization of the negative charge of the anion on many oxygen atoms over the surface of the Keggin structure. High acidity of PTA and its good solubility in water and other polar solvents enables its use as a highly active homogeneous catalyst. However, in spite of relatively higher reaction rate, homogeneous catalysis has various drawbacks that limit its practical application. The main drawback is the difficult and expensive removal of the used catalyst from the reaction mixture and its recycling. PTA also demonstrated good catalytic activity as a heterogeneous catalyst of various organic reactions, e.g. hydrolysis, hydration and polymerization. Wide application of a pure superacid in catalysis is limited by its low surface area and solubility in polar solvents. The objective of this work is the synthesis and study of insoluble superacidic catalysts covalently embedded into the silica matrix. The catalyst PTA/SiO2 was synthesized by the sol-gel method. Tetraethoxysilane was co-condensed with PTA in acidic media in the presence of Pluronic P123 surfactant as a pore-forming agent. The obtained gel was air-dried and calcined at 500 °C producing a mesoporous material with a significant fraction of micropores in its structure. Isotherms of adsorption/desorption of nitrogen indicated cylindrical shape of the pores with necks that is typical for materials obtained with Pluronic P123 as a template. Cs-exchanged material was prepared by mixing PTA/SiO2 with a solution of CsCl. The cation exchange on cesium decreased the total pore volume due to a much higher volume of cesium ions as compared to protons. In addition, partial pore blocking by these ions restricted access to small pores thus reducing accessible surface area. Heteropolyacids are unstable in alkaline media that makes direct solid-state titration impossible. Surface acidity of the samples was determined by reversed titration. Dry samples were dispersed in a solution of pyridine in tetrahydrofuran. After equilibration, the solid phase was filtered, and the filtrate was titrated by HCl. PTA/SiO2 has a very high adsorption capacity on pyridine, which corresponds to 15 molecules of pyridine per [PMo12O40]3- anion. This number exceeded the number of available protons. The catalysts were successfully tested in the alkylation of mesitylene by alkenes. The use of superacidic materials in catalytic reactions can significantly improve the effectiveness of the processes
An adsorbent for Cs removal from contaminated water based on phosphotungstic acid (PTA) embedded ... more An adsorbent for Cs removal from contaminated water based on phosphotungstic acid (PTA) embedded in SiO2 network was synthesized and granulated with γ-Al2O3. PTA/SiO2 had a high adsorption capacity towards Cs while the binder provided excellent mechanical characteristics of the material. It was shown that small particles of PTA/SiO2 with the sizes of 0.1–1 µm occupied space between larger particles of the binder (up to 5 µm). Chemical interaction between PTA and γ-Al2O3 during the adsorbent preparation also took place. The obtained porous material with the specific surface area of 286.9 m2/g contained 4.73% of PTA. Presence of Keggin units in the structure was confirmed by solid state NMR spectroscopy. Study of the adsorbent in Cs+ adsorption from solutions demonstrated its high adsorption capacity. The concentrations of Cs+ in the solutions after the column tests decreased by 3.3–5.2 times. The presence of Na+ and K+ as competing ions did not affect the adsorption. The material was tested in clean-up of radioactive water from the shelter of Chernobyl nuclear power plant (Ukraine). A significant decrease of 137Cs radioactivity was detected in all samples of radioactive water, especially in acidic solutions. Thus the adsorbent can be used for water treatment after incidents resulting in release of radioactive isotopes 134Cs and 137Cs.
The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using eth... more The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using ethanol and n-hexane, and investigated for the presence of secondary metabolites. Both ethanol and n-hexane extracts revealed the presence of alkaloids. Flavonoids, glycosides and saponins were present in only the ethanol extract whereas tannins were present in the n-hexane extract. The bioactivities of the leaf extracts were attributed to their phytochemical constituents. Antimicrobial activity of the extracts were determined against some human pathogenic bacteria and fungi such as Staphylococcus aureus, Escherichia coli, Streptococcus pneumoniae, Bacillus subtilis and Candida albicans using the agar well diffusion and broth dilution methods with the polar extract being more effective. The ethanol extract demonstrated a significant broad-spectrum antimicrobial activity against both gram-positive and gramnegative bacteria, with the highest activity having a zone of inhibition of 10 mm. Anti...
The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using eth... more The bioactive compounds of the leaves of Carica papaya; solo and solomix were extracted using ethanol and n-hexane, and investigated for the presence of secondary metabolites. Both ethanol and n-hexane extracts revealed the presence of alkaloids. Flavonoids, glycosides and saponins were present in only the ethanol extract whereas tannins were present in the n-hexane extract. The bioactivities of the leaf extracts were attributed to their phytochemical constituents. Antimicrobial activity of the extracts were determined against some human pathogenic bacteria and fungi such as Staphylococcus aureus, Escherichia coli, Streptococcus pneumoniae, Bacillus subtilis and Candida albicans using the agar well diffusion and broth dilution methods with the polar extract being more effective. The ethanol extract demonstrated a significant broad-spectrum antimicrobial activity against both gram-positive and gram-negative bacteria, with the highest activity having a zone of inhibition of 10 mm. Antioxidant activity was determined using the DPPH assay method and the absorbance measured using UV-visible spectrophotometer with ascorbic acid as control. The antioxidant activities of solo and solomix showed IC50 of 1.465x10-2 and 1.364x 10-2 respectively. This study demonstrates the efficacy of ethanolic leaf extracts of C. Papaya as an alternative antibiotic for the development of newer antibacterial agents.
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Papers by Esther Jemima Alorkpa