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Venkateswara A Rao - One of the best experts on this subject based on the ideXlab platform.
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methyltriethoxysilane new precursor for synthesizing silica aerogels
Journal of Alloys and Compounds, 2009Co-Authors: Digambar Y Nadargi, Venkateswara A RaoAbstract:Abstract Traditionally, silica aerogels are synthesized using three silicon alkoxides, namely, Tetraethoxysilane (TEOS), tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) for various applications in science as well as in technology. Among all these precursors, only MTMS-based silica aerogels are inherently superhydrophobic with so far reported, the highest contact angle of 173°. In the present paper, we reported a new precursor, namely, methyltriethoxysilane (MTES) for synthesizing the silica aerogels having the novel properties as that of MTMS-based aerogels. The aerogels have been prepared using the MTES by two-stage acid–base catalyzed sol–gel process followed by supercritical drying. The solvent and catalysts used for the synthesis were methanol (MeOH), oxalic acid (C 2 H 2 O 4 ) and ammonium hydroxide (NH 4 OH), respectively. The aerogels of different densities were obtained by varying the molar ratio of MeOH/MTES ( S ) from 6.45 to 19.35. In order to get good quality aerogels in terms of low density, high contact angle and less volume shrinkage, the oxalic acid ( A ) and NH 4 OH ( B ) concentrations were varied from 0 to 1 and from 2 to 13.36 M, respectively. Monolithic aerogels have been obtained for the values of A = 0.01 M and B = 13.36 M. Simultaneously, the aerogels are superhydrophobic with contact angle as high as 163°. Furthermore, the effects of molar ratio of H 2 O/MTES ( W 1), i.e. acidic water and H 2 O/MTES ( W 2), i.e. basic water on the physical properties of the aerogels have also been studied. The molar ratio of MTES:MeOH:acidic water:basic water was optimized at 1:19.35:3.57:3.57, respectively. The aerogel thermal stability was studied by TGA–DTA while the hydrophobicity was quantified in terms of the contact angle measurements and FTIR studies. The as-prepared aerogels have been characterized by bulk density, porosity, volume shrinkage, thermal conductivity, contact angle measurements, transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The physical properties of the aerogels have been explained by taking into account of sol–gel reactions and the gel network formation.
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organic modification of teos based silica aerogels using hexadecyltrimethoxysilane as a hydrophobic reagent
Applied Surface Science, 2006Co-Authors: Nagaraja D Hegde, Venkateswara A RaoAbstract:Abstract The experimental results on the synthesis and characterization of Tetraethoxysilane (TEOS) based hydrophobic silica aerogels using hexadecyltrimethoxysilane (HDTMS) as a hydrophobic reagent by two step sol–gel process, are described. The molar ratio of Tetraethoxysilane (TEOS), methanol (MeOH), acidic water (0.001 M, oxalic acid) and basic water (10 M, NH 4 OH) was kept constant at 1:55:3.25:1.25 and the molar ratio of HDTMS/TEOS (M) was varied from 0 to 28.5 × 10 −2 . The organic modification was confirmed by infrared spectroscopic studies, and the hydrophobicity of the aerogels was tested by the contact angle measurements. The maximum contact angle of 152° was obtained for M = 22.8 × 10 −2 . The aerogels retained the hydrophobicity up to a temperature of 240 °C and above this temperature the aerogels became hydrophilic. The aerogels were characterized by the thermal conductivity, density, contact angle measurements, optical transmission and scanning electron micrographs.
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effect of precursors methylation agents and solvents on the physicochemical properties of silica aerogels prepared by atmospheric pressure drying method
Journal of Non-crystalline Solids, 2001Co-Authors: Venkateswara A Rao, Elin Nilsen, Mariann EinarsrudAbstract:Abstract The properties of aerogels dried at ambient pressure prepared using different synthesis conditions have been studied. The effect of type of precursor (tetramethoxysilane (TMOS), tetraethoxysiliane (TEOS) and sodium silicate (`water glass')), solvent ( n -hexane, n -heptane and toluene) and surface chemical modification agent (trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDS) and hexamethyl-disilazane (HMDZ)), on transparency, monolithicity and physical properties of the aerogels are studied. Low density aerogels (0.14–0.3 g/cm 3 ) could be prepared by using the present TMOS and `water glass' based recipes and TMCS or HMDZ as surface modifiers. Using the TMOS precursor, TMCS; and for `water glass' precursor, HMDZ resulted in the best quality silica aerogels in terms of monolithicity, visual transparency and lowest density. HMDS surface modification resulted in hydrophilic high density aerogels for all the precursors, hence this chemical is not recommended for effective surface modification. The `water glass' based gels experienced the highest degree of monolithicity because these wet gels had the highest stiffness and the largest pore size of the three precursors studied. Using n -hexane or n -heptane as the solvent resulted in aerogels with the lowest density and the best visual transparency.
Cleide Mara Faria Soares - One of the best experts on this subject based on the ideXlab platform.
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characterization of biocatalysts prepared with thermomyces lanuginosus lipase and different silica precursors dried using aerogel and xerogel techniques
Applied Biochemistry and Biotechnology, 2014Co-Authors: Carlos Eduardo Barao, Leandro Daniel De Paris, Joao Henrique Dantas, Matheus M Pereira, Lucio Cardozo Filho, Heizir F De Castro, Gisella Maria Zanin, Flávio Faria De Moraes, Cleide Mara Faria SoaresAbstract:The use of lipases in industrial processes can result in products with high levels of purity and at the same time reduce pollutant generation and improve both selectivity and yields. In this work, lipase from Thermomyces lanuginosus was immobilized using two different techniques. The first involves the hydrolysis/polycondensation of a silica precursor (tetramethoxysilane (TMOS)) at neutral pH and ambient temperature, and the second one uses Tetraethoxysilane (TEOS) as the silica precursor, involving the hydrolysis and polycondensation of the alkoxide in appropriate solvents. After immobilization, the enzymatic preparations were dried using the aerogel and xerogel techniques and then characterized in terms of their hydrolytic activities using a titrimetric method with olive oil and by the formation of 2-phenylethyl acetate in a transesterification reaction. The morphological properties of the materials were characterized using scanning electron microscopy, measurements of the surface area and pore size and volume, thermogravimetric analysis, and exploratory differential calorimetry. The results of the work indicate that the use of different silica precursors (TEOS or TMOS) and different drying techniques (aerogel or xerogel) can significantly affect the properties of the resulting biocatalyst. Drying with supercritical CO2 provided higher enzymatic activities and pore sizes and was therefore preferable to drying, using the xerogel technique. Thermogravimetric analysis and differential scanning calorimetry analyses revealed differences in behavior between the two biocatalyst preparations due to the compounds present.
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characterization of biocatalysts prepared with thermomyces lanuginosus lipase and different silica precursors dried using aerogel and xerogel techniques
Applied Biochemistry and Biotechnology, 2014Co-Authors: Carlos Eduardo Barao, Leandro Daniel De Paris, Joao Henrique Dantas, Matheus M Pereira, Lucio Cardozo Filho, Heizir F De Castro, Gisella Maria Zanin, Flávio Faria De Moraes, Cleide Mara Faria SoaresAbstract:The use of lipases in industrial processes can result in products with high levels of purity and at the same time reduce pollutant generation and improve both selectivity and yields. In this work, lipase from Thermomyces lanuginosus was immobilized using two different techniques. The first involves the hydrolysis/polycondensation of a silica precursor (tetramethoxysilane (TMOS)) at neutral pH and ambient temperature, and the second one uses Tetraethoxysilane (TEOS) as the silica precursor, involving the hydrolysis and polycondensation of the alkoxide in appropriate solvents. After immobilization, the enzymatic preparations were dried using the aerogel and xerogel techniques and then characterized in terms of their hydrolytic activities using a titrimetric method with olive oil and by the formation of 2-phenylethyl acetate in a transesterification reaction. The morphological properties of the materials were characterized using scanning electron microscopy, measurements of the surface area and pore size and volume, thermogravimetric analysis, and exploratory differential calorimetry. The results of the work indicate that the use of different silica precursors (TEOS or TMOS) and different drying techniques (aerogel or xerogel) can significantly affect the properties of the resulting biocatalyst. Drying with supercritical CO2 provided higher enzymatic activities and pore sizes and was therefore preferable to drying, using the xerogel technique. Thermogravimetric analysis and differential scanning calorimetry analyses revealed differences in behavior between the two biocatalyst preparations due to the compounds present.
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characterization of sol gel encapsulated lipase using Tetraethoxysilane as precursor
Journal of Molecular Catalysis B-enzymatic, 2006Co-Authors: Cleide Mara Faria Soares, Heizir F De Castro, Flávio Faria De Moraes, Onelia Dos A Santos, Gisella Maria ZaninAbstract:Abstract Lipase from Candida rugosa was encapsulated within a chemically inert sol–gel support prepared by polycondensation of the precursor Tetraethoxysilane (TEOS) in the presence of polyethylene glycol (PEG) as additive. The properties of silica and their derivatives with regard to mean pore diameter, specific surface area, mean pore size, weight loss upon heating (thermogravimetric analysis, TGA) and 29 Si and 13 C NMR are reported. The pH optimum shifted from 7.8 to 6.7 and optimum temperature jumped from 36 to 60 °C upon enzyme encapsulation. Encapsulated lipase in presence of PEG (EN-PEG) exhibited higher stability in the range of 37–45 °C, but from 50 to 65 °C the EN-PEG was inactivated after seven cycles. Hydrolytic activity during long-term storage at room temperature decreased to 50% after 94 days. High diffusional resistance was observed for large oil concentration reducing hydrolytic effectiveness by 60% in the case of the encapsulated lipase. NMR, pore size and specific surface area data suggested an active participation of the lipase enzyme during gelling of the silica matrix. This lead to reduction of available Si–OH groups, larger pores and smaller surface area. Larger pores increase substrate diffusion that correlates well with higher hydrolytic activity of the TEOS–PEG sol–gel matrix encapsulated enzyme in comparison with other sol–gel supports.
Hee Taik Kim - One of the best experts on this subject based on the ideXlab platform.
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low density teos based silica aerogels prepared at ambient pressure using isopropanol as the preparative solvent
Journal of Alloys and Compounds, 2009Co-Authors: Askwar Hilonga, Jongkil Kim, Pradip B Sarawade, Hee Taik KimAbstract:Abstract In this paper, we report experimental results on the synthesis of low-density Tetraethoxysilane (TEOS)-based silica aerogels prepared with different solvents via an ambient pressure drying (APD) route. Tetraethoxysilane was hydrolyzed and condensed in different solvents (methanol, ethanol, butanol, and isopropanol) using oxalic acid and NH 4 OH as the catalysts. To minimize shrinkage due to drying, the surfaces of the gels were modified using trimethylchlorosilane (TMCS) before the APD via a one-step solvent exchange/surface modification process. The effects of different solvents on the physical and textural properties of the resulting aerogels was investigated. It was observed that solvents containing longer chains of alkyl groups (–CH 2 –CH 3 ) formed high silica polymerization in the alcogels which enhanced a distinct “spring-back effect” during APD, and consequently preserved the highly porous silica network by preventing collapse. Silica aerogels with very low bulk densities (0.041 g/cm 3 ), extremely high specific surface areas (1150 m 2 /g), and large cumulative pore volumes (5.2 cm 3 /g) were successfully synthesized using isopropanol as a preparative solvent.
Gisella Maria Zanin - One of the best experts on this subject based on the ideXlab platform.
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characterization of biocatalysts prepared with thermomyces lanuginosus lipase and different silica precursors dried using aerogel and xerogel techniques
Applied Biochemistry and Biotechnology, 2014Co-Authors: Carlos Eduardo Barao, Leandro Daniel De Paris, Joao Henrique Dantas, Matheus M Pereira, Lucio Cardozo Filho, Heizir F De Castro, Gisella Maria Zanin, Flávio Faria De Moraes, Cleide Mara Faria SoaresAbstract:The use of lipases in industrial processes can result in products with high levels of purity and at the same time reduce pollutant generation and improve both selectivity and yields. In this work, lipase from Thermomyces lanuginosus was immobilized using two different techniques. The first involves the hydrolysis/polycondensation of a silica precursor (tetramethoxysilane (TMOS)) at neutral pH and ambient temperature, and the second one uses Tetraethoxysilane (TEOS) as the silica precursor, involving the hydrolysis and polycondensation of the alkoxide in appropriate solvents. After immobilization, the enzymatic preparations were dried using the aerogel and xerogel techniques and then characterized in terms of their hydrolytic activities using a titrimetric method with olive oil and by the formation of 2-phenylethyl acetate in a transesterification reaction. The morphological properties of the materials were characterized using scanning electron microscopy, measurements of the surface area and pore size and volume, thermogravimetric analysis, and exploratory differential calorimetry. The results of the work indicate that the use of different silica precursors (TEOS or TMOS) and different drying techniques (aerogel or xerogel) can significantly affect the properties of the resulting biocatalyst. Drying with supercritical CO2 provided higher enzymatic activities and pore sizes and was therefore preferable to drying, using the xerogel technique. Thermogravimetric analysis and differential scanning calorimetry analyses revealed differences in behavior between the two biocatalyst preparations due to the compounds present.
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characterization of biocatalysts prepared with thermomyces lanuginosus lipase and different silica precursors dried using aerogel and xerogel techniques
Applied Biochemistry and Biotechnology, 2014Co-Authors: Carlos Eduardo Barao, Leandro Daniel De Paris, Joao Henrique Dantas, Matheus M Pereira, Lucio Cardozo Filho, Heizir F De Castro, Gisella Maria Zanin, Flávio Faria De Moraes, Cleide Mara Faria SoaresAbstract:The use of lipases in industrial processes can result in products with high levels of purity and at the same time reduce pollutant generation and improve both selectivity and yields. In this work, lipase from Thermomyces lanuginosus was immobilized using two different techniques. The first involves the hydrolysis/polycondensation of a silica precursor (tetramethoxysilane (TMOS)) at neutral pH and ambient temperature, and the second one uses Tetraethoxysilane (TEOS) as the silica precursor, involving the hydrolysis and polycondensation of the alkoxide in appropriate solvents. After immobilization, the enzymatic preparations were dried using the aerogel and xerogel techniques and then characterized in terms of their hydrolytic activities using a titrimetric method with olive oil and by the formation of 2-phenylethyl acetate in a transesterification reaction. The morphological properties of the materials were characterized using scanning electron microscopy, measurements of the surface area and pore size and volume, thermogravimetric analysis, and exploratory differential calorimetry. The results of the work indicate that the use of different silica precursors (TEOS or TMOS) and different drying techniques (aerogel or xerogel) can significantly affect the properties of the resulting biocatalyst. Drying with supercritical CO2 provided higher enzymatic activities and pore sizes and was therefore preferable to drying, using the xerogel technique. Thermogravimetric analysis and differential scanning calorimetry analyses revealed differences in behavior between the two biocatalyst preparations due to the compounds present.
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characterization of sol gel encapsulated lipase using Tetraethoxysilane as precursor
Journal of Molecular Catalysis B-enzymatic, 2006Co-Authors: Cleide Mara Faria Soares, Heizir F De Castro, Flávio Faria De Moraes, Onelia Dos A Santos, Gisella Maria ZaninAbstract:Abstract Lipase from Candida rugosa was encapsulated within a chemically inert sol–gel support prepared by polycondensation of the precursor Tetraethoxysilane (TEOS) in the presence of polyethylene glycol (PEG) as additive. The properties of silica and their derivatives with regard to mean pore diameter, specific surface area, mean pore size, weight loss upon heating (thermogravimetric analysis, TGA) and 29 Si and 13 C NMR are reported. The pH optimum shifted from 7.8 to 6.7 and optimum temperature jumped from 36 to 60 °C upon enzyme encapsulation. Encapsulated lipase in presence of PEG (EN-PEG) exhibited higher stability in the range of 37–45 °C, but from 50 to 65 °C the EN-PEG was inactivated after seven cycles. Hydrolytic activity during long-term storage at room temperature decreased to 50% after 94 days. High diffusional resistance was observed for large oil concentration reducing hydrolytic effectiveness by 60% in the case of the encapsulated lipase. NMR, pore size and specific surface area data suggested an active participation of the lipase enzyme during gelling of the silica matrix. This lead to reduction of available Si–OH groups, larger pores and smaller surface area. Larger pores increase substrate diffusion that correlates well with higher hydrolytic activity of the TEOS–PEG sol–gel matrix encapsulated enzyme in comparison with other sol–gel supports.
Haruhiko Sakurai - One of the best experts on this subject based on the ideXlab platform.
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Acute and subchronic inhalation toxicity of Tetraethoxysilane (TEOS) in mice
Archives of Toxicology, 1994Co-Authors: Hiroshi Nakashima, Kazuto Yamazaki, Tohru Sakai, Kazuyuki Omae, Haruhiko SakuraiAbstract:To clarify the acute and subchronic inhalation toxicity of Tetraethoxysilane [TEOS, Si(OC_2H_5)_4], groups of ten male ICR mice (SPF grade) were exposed to 1000 ppm TEOS for 1, 2, 4 or 8 h (acute inhalation study), or to 200 ppm of TEOS for 6 h/day, 5 days/week, for 2 or 4 weeks (subchronic inhalation study). The numbers of mice that died during 2 weeks of observation were 0, 1, 1 and 6 in the 1-, 2-, 4- and 8-h inhalation experiments and zero in the subchronic inhalation study. In the acute inhalation study, body weight decreased after TEOS exposure and did not reach the level of control mice during 2 weeks of observation except in the 1-h inhalation study. In the subchronic exposure study, weight gain was suppressed during the exposure period. Body weight in mice exposed for 2 weeks reached the level of non-exposed mice during the 2-week observation period, but did not do so in mice exposed for 4 weeks. Acute tubular necrosis (ATN) and acute splenic atrophy (ASA) were observed in all dead mice in the acute inhalation study, and tubulointerstitial nephritis (TIN) was frequently found in the surviving mice in both the acute and subchronic studies. However, blood biochemical examinations revealed no evidence of renal dysfunction. The olfactory epithelium was necrotic in all dead mice. In the subchronic inhalation study, infiltration of polymorphonuclear neutrophils in the nasal mucosa was observed in all mice killed 1 day after exposure. These results indicate that the LCL_0 for 1-h exposure to TEOS and LC_50 for 4-h exposure are greater than 1000 ppm, and that the kidney and nasal mucosa are the target organs for TEOS inhalation.
