Control of hydroxyl group content in silica particle synthesized by the sol-precipitation process

This study investigated the control of hydroxyl groups, one of key factors determining the surface properties of silica particles synthesized by the sol-precipitation of tetraethyl orthosilicate (TEOS). Thus, a thermal gravity analysis (TGA) was used to facilitate quantitative measurements of the hydroxyl groups on the silica particles, while BET and FT-IR were used to analyze the specific surface area and functional silane groups on the silica particles, respectively. In the sol-precipitation process, silanes that include various hydroxyl groups are formed as intermediates based on the hydrolysis and condensation of TEOS. Thus, NH₃, as a basic catalyst initiating the nucleophilic substitution of TEOS, was found to accelerate the hydrolysis and increase the hydroxyl group content on the silica particles. Plus, the hydroxyl group content was also increased when increasing the concentrations of TEOS and water as the hydrolysis reactants. However, the hydroxyl group content was reduced when increasing the temperature, due to the promotion of condensation. Based on the weight loss of the particles according to the thermal analysis, the hydroxyl group content on the silica particles varied from 5.6–42.7 OH/nm² under the above reaction conditions.     

Modified Silico-Tungstate: An Easy and Efficient Catalyst for the Acetylation of Amines Under Solvent-Free Condition

Abstract  

In this present work, organo-functionalization of silica gel was carried out using 3-aminopropyltriethoxysilane (APTES) as a reactive surface modifier. Tungstate ion (WO₄ ²⁻) was immobilized onto the modified silica gel and its catalytic activity was investigated for acetylation of amines under solvent-free conditions. The method is highly chemoselective—amine group was selectively acetylated in presence of the hydroxyl group. The above catalyst gave acetylated products in excellent yield in shorter duration of time. The catalyst was characterized by various techniques such as BET surface area measurements, FTIR, CHNS, ¹³C-CPMAS-NMR, ED-XRF and diffuse reflectance UV–vis techniques. The catalyst was reused five times without significant loss of catalytic activity.     

The key role of support surface tuning during the preparation of catalysts from reverse micellar-synthesized metal nanoparticles

Silica-supported cobalt catalysts have been prepared by depositing cobalt nanoparticles synthesized in reverse micellar systems. Two different reverse microemulsions comprising both a neutral (Triton X114) or an ionic (AOT) surfactant were used for the synthesis of nanoparticles. The materials have been characterized by ²⁹Si MAS NMR, XRD, FTIR, H₂-TPR, and TEM. Protection of the hydroxyl groups on the silica surface by silylation prior to loading of the cobalt nanoparticles has been found crucial for attaining the desired metal dispersions in the final catalyst owing to a higher chemical compatibility between the hydrophobic support surface and the microemulsion during the deposition step.     

Interaction of triton X-100 on silica: A relationship between surface characteristics and adsorption isotherms

Adsorption of Triton X-100 on various silica substrates has been investigated. A number of solids, including a natural quartz, this quartz washed with HCl acid and subsequently heated at 1273 K; two aerosils and one Kieselgel silicas were studied. These solids exhibit surface areas in the range of 5 to 430 m² g^?1. All the Triton adsorption isotherms display an S-shape at the adsorption temperatures studied (298 and 308 K). It has been found that the pretreatments of natural quartz (by water washing, impurities removed by acid and/or high temperature calcination) affect considerably the amounts of TX-100 adsorbed. Measurements of surface composition have been made by X-ray photoelectron spectroscopy (XPS) with particular emphasis on the presence of impurities and on the number of OH groups at the surface of the samples. The nature of the surface hydroxyl has also been studied by infrared spectroscopy. Furthermore, the specific number of hydroxyl groups on the surface of the silica samples has been determined by thermogravimetric analysis. Finally an attempt to correlate solid surface characteristics with adsorption isotherms has been developed.     

Fly ash-supported cerium triflate as an active recyclable solid acid catalyst for Friedel–Crafts acylation reaction

An efficient solid Lewis acid, has been synthesized by loading cerium triflate (7 wt%) on the acid activated fly ash with high silica content (81%). The physico-chemical properties of synthesized fly ash-supported cerium triflate catalyst (CFT) were monitored by XRD, FT-IR spectroscopy, FT-IR spectroscopy of the ammonia adsorbed catalyst, SEM-EDAX, TEM, Flame Atomic Absorption Spectrophotometer and TG-DTA study. The increased concentration of silica surface hydroxyl groups on activated fly ash have a major influence on the loading of cerium triflate. The catalytic activity of the catalyst CFT was tested in the acylation of veratrole using acetic anhydride as the acylating agent. The proposed model structure of CFT shows that the triflate species withdraws the electron density from the surface cerium making it electron deficit and generate Lewis acidity on the surface of fly ash as confirmed by NH₃ adsorbed FT-IR spectrum. The activity data indicate that this heterogeneous catalyst is very active, corresponding to high conversion (88%) of veratrole to 3,4-dimethoxyacetophenone. The catalyst could be easily recovered and reused giving similar conversion up to three reaction cycles indicating its stability under experimental conditions. Thus fly ash-supported cerium triflate is a novel and efficient catalyst and is a promising way of bulk utilization of waste fly ash by developing cost effective catalyst system for industrially important Friedel–Crafts acylation reactions.     

Significant variation of molecular weight distribution (MWD) of polyethylene induced by different alkyl-Al co-catalysts using a novel surface functionalized SiO2-supported Ziegler-Natta catalyst

In this work, a novel surface functionalized SiO₂-supported Ziegler-Natta catalyst was developed for ethylene polymerization. A state-of-the-art control of some unique characteristic catalytic properties on the surface functionalized SiO₂-supported catalyst analogized from the previously reported poly[propylene-co-(7-methyl-1,6-octadiene)]-supported Ziegler-Natta catalyst had been successfully realized through a precise molecular level catalyst design on the silica gel surface functionalized with linear long alkene chains through the reaction between unsaturated alcohol 9-decene-1-ol and hydroxyl groups on the silica surface. The active sites of this SiO₂-supported catalyst prepared in this study showed very stable activity without obvious deactivation for more than 50 h and the molecular weight distribution of polyethylenes varied significantly from narrow and unimodal to broad and multimodal by solely changing the type of Al-alkyl cocatalyst from diethylaluminumchloride to triethylaluminum.     

Analysis of the interaction of vinyl and carbonyl silanes with carbon nanofiber surfaces

Carbon nanofibers (CNFs) have been treated with vinyltryethoxy silane (VTS) and 3-metacryloxypropyltrimethoxy silane (MPS). CNF–silane interactions have been analyzed by means of TGA, FTIR-ATR, TEM, HRTEM, SEM and nitrogen adsorption. For similar silane concentration solutions TG analysis has shown that VTS and MPS form one and three silane monolayers, respectively. This has also been corroborated by the presence of an FTIR-ATR band at 1250 cm⁻¹ assigned to Si–O–Si bonds of silica layers. For low silane concentrations, the vinyl group of VTS is bonded to the graphene CNF surface mainly through π–π interactions. However, MPS interacts through the carbonyl group with hydroxyl groups of graphene defect sheets existing probably in micropores. Silanol–CNF hydroxyl interactions are also expected at these silane concentrations. For high silane concentration, when the silica layer is formed, both silanes present vinyl free and carbonyl free groups, as observed by the 1370 and 1686 cm⁻¹ FTIR-ATR bands, respectively. Nitrogen adsorption has shown that while VTS is adsorbed mainly on the defect free graphene surface, MPS is adsorbed on the micropores and, therefore on the hydroxyl defect graphene sites. These results are finally correlated with the dispersion stability of both silanes on water and styrene solutions.     

Preparation of silica‐supported late transition metal catalyst and ethylene polymerization

A Correction has been published for this article in Polymer International 51(6) 2002, 561 The late transition metal catalyst 2,6‐bis[1‐(2,6‐diisopropylphenylimino)ethyl]pyridine iron(II) chloride was supported on silica. Fourier transform infrared spectroscopy, scanning electronic micrograph and X‐ray photoelectron spectroscopy measurements were utilized to examine the process of supporting catalyst on silica and investigate the possible mechanism of support. Furthermore, ethylene polymerizations with the supported catalysts were carried out in various conditions such as different reaction temperatures and Al/Fe molar ratios. The results showed that MAO first reacted with the hydroxyl of silica forming Si? O? Al bonds and then the catalyst was bridged through MAO on the surface of silica. Compared with homogeneous catalysts, the supported catalysts show some decrease in catalyst activity. However, they can show good activity at a lower Al/Fe molar ratio with MAO as co‐catalyst and give rise to higher molecular weight and melting temperature of the polymer. Better morphology of polyethylene was obtained by a supported catalyst than by its corresponding homogeneous catalyst. The late transition metal catalyst 2,6‐bis[1‐(2,6‐diisopropylphenylimino)ethyl]pyridine iron(II) chloride was supported on silica. Fourier transform infrared spectroscopy, scanning electronic micrograph and X‐ray photoelectron spectroscopy measurements were utilized to examine the process of supporting catalyst on silica and investigate the possible mechanism of support. Furthermore, ethylene polymerizations with the supported catalysts were carried out in various conditions such as different reaction temperatures and Al/Fe molar ratios. The results showed that MAO first reacted with the hydroxyl of silica forming Si? O? Al bonds and then the catalyst was bridged through MAO on the surface of silica. Compared with homogeneous catalysts, the supported catalysts show some decrease in catalyst activity. However, they can show good activity at a lower Al/Fe molar ratio with MAO as co‐catalyst and give rise to higher molecular weight and melting temperature of the polymer. Better morphology of polyethylene was obtained by a supported catalyst than by its corresponding homogeneous catalyst. © 2002 Society of Chemical Industry     

Manganese ferrite nanoparticles synthesized through a nanocasting route as a highly active Fenton catalyst

Spinel ferrite MnFe₂O₄ nanoparticles were synthesized by means of a nanocasting technique using a low-cost mesoporous silica gel as a hard template. The magnetic nanoparticles, of <10 nm diameter and with a surface area of around 100 m²/g, were tested as a heterogeneous Fenton catalyst for the decomposition of hydrogen peroxide under neutral and basic conditions. This catalyst shows a much higher activity than previous heterogeneous catalysts reported in the literature, which is mainly ascribed to its small particle size. Furthermore, the magnetic catalyst can be easily separated from the reaction medium by means of an external magnetic field. The effects of residual silica and the purity of the catalyst (hematite formation) on catalytic activity have been studied and correlated. The results obtained show this catalyst to be a suitable candidate for the removal of pollutants in wastewaters by means of the Fenton heterogeneous reaction.     

Purification,surface modification of coal ash silica and its potential application in rubber composites

This article reports the purification and surface modification of coal ash silica and afterwards its utilization as reinforcing filler in solution‐styrene‐butadiene rubber/butadiene rubber (S‐SBR/BR). The coal ash silica free of unwanted metal ions/mineral oxides was obtained using phosphoric acid. The chemical composition of the purified coal ash silica in comparison to impure coal ash indicates the presence of characteristic hydroxyl functional group at 3440 cm^?1 and an increase in the oxygen content as determined with the help of Fourier Transform Infrared and x‐ray photoelectron spectroscopy, respectively. The contact angle analysis shows that after purification the polar part of the surface energy increased to 17.5 from 12 m Jm^?2. While the surface area increased to an order of magnitude, i.e., 11.6 to 110.5 m² g^?1. The modification of purified silica particles with bis(3‐triethoxysilylpropyl) tetrasulfane reveal the functionalization of hydroxyl to ‐Si‐O‐R groups as detected at 1560 cm^?1. As a consequence, the modified silica based S‐SBR/BR composite resulted in improved mechanical properties due to enhanced silica‐rubber interaction. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Studies of the hydrogen held by solids: XXII. The surface chemistry of reduced molybdenaalumina catalysts

The surface hydroxyl concentrations of a fresh molybdenaalumina catalyst (8% Mo) and of the alumina from which it was made have been determined as a function of the temperature of pretreatment. Similar data were obtained for catalysts reduced with H₂ or with CO. In all cases, the hydroxyl concentrations decreased with increasing pretreatment temperatures. The difference between the curves for the parent alumina and the molybdenaalumina preparation made from it provided a measure of the number of hydroxyl groups eliminated as the epitaxial monolayer of molybdena was grown onto the surface. The values obtained (1.7 ± 0.6 OH/Mo) showed that the surface hydroxyl groups of alumina are replaced by molybdate anions. When the catalyst was reduced with CO to about $\text{e}\text{Mo}\text{= 1.5}$ (average valence, Mo^+4.5), the curve obtained was almost identical with that for the unreduced catalyst, but when the catalyst was reduced with H₂, values for the retained hydrogen were higher than for the oxidized catalyst and approached those of the parent alumina as its evacuation temperature was increased to 550 °C. This increase in hydroxyl concentration was in agreement with earlier deductions.The hydroxyl region of the infrared spectra of similar preparations was recorded. Four distinct bands could be characterized for the parent alumina at 3780, 3740, 3705, and 3650 cm^?1 and a shoulder at 3795 cm^?1. The same bands were present on the oxidized catalyst, but with lower intensities and with altered intensity ratios; i.e., some bands were affected more than others as hydroxyl groups were replaced by molybdena species. Spectra from catalysts reduced with CO were indistinguishable in the OH region from those for the unreduced catalyst. No new bands appeared when the catalysts were reduced with H₂, but the intensities of bands attributable to alumina OH increased with the 3795 cm^?1 band strengthening noticeably more than the others. Thus, the new hydroxyl groups introduced on reduction are probably alumina OH rather than MoOH as previously supposed. A form of hydrogen which is chemisorbed but which can be removed from the catalyst as H₂ on evacuation at the reduction temperature also appeared in the OH region, mainly as a continuous contribution to the low frequency edge. From absorption coefficients derived from the present data, it was deduced that about twice as many hydrogen atoms were present in the H₂ formed than were supplied by these OH groups; i.e., the chemisorption appears heterolytic with half the atoms unseen by ir. A search was made for a band attributable to MoH, but without success. A brief study was made of this adsorption process, which was found to be slow but reversible, and to have a positive pressure dependence. When the catalyst was reduced with CO, rather than with H₂, a portion of the CO remained irreversibly chemisorbed in electronically comparable amounts. Infrared spectra of such samples contained a band at about 1585 cm^?1 attributable to a carbonate species. Data for the two reducing gases differed in that no reversibly chemisorbed CO was observed. At room temperature, CO was also chemisorbed as a linear species with the stretching frequency (2190 cm^?1) higher than that of the gaseous molecule (2143 cm^?1).     

Manganese ferrite nanoparticles synthesized through a nanocasting route as a highly active Fenton catalyst

Spinel ferrite MnFe₂O₄ nanoparticles were synthesized by means of a nanocasting technique using a low-cost mesoporous silica gel as a hard template. The magnetic nanoparticles, of <10 nm diameter and with a surface area of around 100 m²/g, were tested as a heterogeneous Fenton catalyst for the decomposition of hydrogen peroxide under neutral and basic conditions. This catalyst shows a much higher activity than previous heterogeneous catalysts reported in the literature, which is mainly ascribed to its small particle size. Furthermore, the magnetic catalyst can be easily separated from the reaction medium by means of an external magnetic field. The effects of residual silica and the purity of the catalyst (hematite formation) on catalytic activity have been studied and correlated. The results obtained show this catalyst to be a suitable candidate for the removal of pollutants in wastewaters by means of the Fenton heterogeneous reaction.     

Die mit Cäsiumphenolat katalysierte Addition von Propylenoxid an Monophenylmonopropylenglykol bei Drücken bis 1 000 at

The addition of propylene oxide to monophenyl monopropylene glycol in the presence of cesium phenolate as catalyst was investigated under pressure up to 1 000 at. Experiments carried out in substance have proved a first order in the catalyst, a pseudo zeroth order with respect to the epoxide and the hydroxyl component, respectively. The activation energy has been found to be 14.8 kcal/mole. The very high activation volume of ?55 cm³/mole rules out the possibility of an ionic mechanism under these conditions. The reaction takes place via a ternary transition state preceded by associates. The results with ¹⁴C-marked phenolate as well as paper chromatographic experiments show that there is no phenolate-glycolate transformation. The phenolate as catalyst maintains its original form in the course of the reaction.     

Comparison of ethylene polymerization catalyzed over Cr(VI)/silica and Cr(II)/silica catalysts

Summary Ethylene polymerization and the morphology of produced high density polyethylene (HDPE) catalyzed over. Cr(VI)/silica and Cr(II)/silica were studied. Cr/silica catalyst with Cr loading of 1 wt % has been prepared by impregnating an aqueous solution of CrO₃ onto silica having specific surface area of 301 m²/g and pore volume of 1.64 ml/g. The rate profiles and the morphologies of polyethylene polymerized over Cr(VI) and Cr(II)/silica were different. The process of active site formation influences the rate profiles. The shape of polyethylene particles polymerized with Cr(II)/silica resembled the original shape of catalyst particles. However, Cr(VI)/silica catalyst particles were fractured inhomogeneously during the polymerization. The variation of molecular weight and molecular weight distribution at various polymerization times indicated that the formation of active sites of Cr(VI)/silica was accomplished gradually during polymerization.     

Propriétés photocatalytiques de Ta2O5/SiO2

The photocatalytic properties of deposited tantalum oxide (Ta₂O₅) on silica, in the reaction of photocatalytic oxidation of cyclohexane by oxygen, have been studied. The properties of this catalyst were compared with those of the previously studied system V₂O₅/SiO₂, and it was concluded that the mobilities of V⁺⁵ and Ta⁺⁵ ions on the surface of silica under radiation arc verymuch different. This study was completed by an analysis of the diffuse reflectance spectra and a thermogravimetric analysis of the used catalysts. The conclusion was that the site active in the photocatalytic reaction, is hydrated during the oxidation of cyclohexane.     

Synthesis of a nano-crystalline solid acid catalyst from fly ash and its catalytic performance

The synthesis of nano-crystalline activated fly ash catalyst (AFAC) with crystallite size of 12 nm was carried out by chemical and thermal treatment of fly ash, a waste material generated from coal-burning power plants. Fly ash was chemically activated using sulfuric acid followed by thermal activation at 600 °C. The variation of surface and Physico-chemical properties of the fly ash by activation methods resulted in improved acidity and therefore, catalytic activity for acid catalyzed reactions. The AFAC was characterized by X-ray diffraction, FT-IR spectroscopy, N₂-adsorption–desorption isotherm, scanning electron microscopy, flame atomic absorption spectrophotometry and sulfur content by CHNS/O elemental analysis. It showed amorphous nature due to high silica content (81%) and possessed high BET surface area (120 m²/g). The catalyst was found to be highly active solid acid catalyst for liquid phase esterification of salicylic acid with acetic anhydride and methanol giving acetylsalicylic acid and methyl salicylate respectively. A maximum yield of 97% with high purity of acetylsalicylic acid (aspirin) and a very high conversion 87% of salicylic acid to methyl salicylate (oil of wintergreen) was obtained with AFAC. The surface acidity and therefore, catalytic activity in AFAC was originated by increased silica content, hydroxyl content and higher surface area as compared to fly ash. The study shows that coal generated fly ash can be converted into potential solid acid catalyst for acid catalyzed reactions. Furthermore, this catalyst may replace conventional environmentally hazardous homogeneous liquid acids making an ecofriendly; solvent free, atom efficient, solid acid based catalytic process. The application of fly ash to synthesize a solid acid catalyst finds a noble way to utilize this abundant waste material.     

Surface Modification of Amorphous SiO2 Nanoparticles by Oxygen-Plasma and Nitrogen-Plasma Treatments

In this study, amorphous silica gels were synthesized and treated by oxygen plasma and nitrogen plasma at a radio frequency power of 60 W, at heating temperature of 300°C, and a treatment period of 400 s. The silica gels were characterized by using X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, Brunauer–Emmett–Teller, and thermal conductivity analyzers. The characterization results show that oxygen-plasma treatment has remarkably reduced the surface hydroxyl groups in silica gels, whereas nitrogen-plasma treatment has less effects. The reduction of the hydroxyl groups is a key factor contributing to reduced particle size from 30 nm to 15 nm, thus resulting in an increased specific surface area from 124 to 420 m²/g. In addition, the reduced particle size has drastically lowered the thermal conductivity of silica gels, from 0.14220 to 0.00014 W/mK. Therefore, oxygen-plasma treatment is a feasible method to enhance the thermal insulating properties of silica gels.     

Crosslinking of polydimethylsiloxane diol blended with poly(dimethylsiloxane-b-styrene-b-dimethylsiloxane)

A variety of blends of poly(dimethylsiloxane-b-styrene-b-dimethylsiloxane) copolymers with hydroxyl terminated polydimethylsiloxane (molecular weight, 29 600 gmol^?1) were prepared. Crosslinking of blends was accomplished at room temperature by reacting hydroxyl chain ends with a tetrafunctional orthosilicate using dibutyltin dilaurate as a catalyst. The crosslink density was evaluated by toluene swelling. Catalyst concentration influences the crosslink density as observed from the amount of extractables in toluene. The tensile strength of crosslinked polydimethylsiloxane was found to increase by blending with block polymers or with silica fillers. Structural uniformity of fractured surfaces of rubbers was also studied by scanning electron microscopy.     

Infrared spectroscopy studies of cyclohexene hydrogenation and dehydrogenation catalyzed by platinum nanoparticles supported on mesoporous silicate (SBA-15). Part 1: The role of particle size of Pt nanocrystals supported on SBA-15 silicate

Platinum nanoparticles were prepared from colloidal solution in the 5–16 nm range. SBA-15 mesoporous silica was impregnated with particles small enough to enter the 10 nm pores until 0.1 wt% loading was reached. Characterization of the catalysts was carried out by XRD, TEM and BET. Cyclohexene hydrogenation/dehydrogenation was monitored using a reaction cell that permitted infrared spectroscopy monitoring of the gas phase as well as the catalyst surface that was pressed in a wafer and inserted in the reactor. The surface hydroxyl groups on the mesoporous silica show shifts in the 3633–3705 cm^–1 range characteristic of the presence of cyclohexene, 1,3- and 1,4-cyclohexadienes. Reaction studies using 10 Torr of cyclohexene and 100 Torr of hydrogen in the 298–473 K range showed that hydrogenation occurs readily at room temperature while dehydrogenation takes place only at higher temperatures as expected. The small platinum nanoparticles carry out reactions at the highest rates while the largest size metal particles of the lowest. There is no apparent change of metal particle size during the reactions.     

Textural and Structural Properties of Mesoporous Silica Synthesized Under Refluxing Conditions

Mesoporous silica with pore sizes of 3–6 nm has been synthesized under refluxing and autogenous pressure conditions of hydrothermal synthesis from precursor gels having different alkaline pH. The mesoporous silica prepared is characterized by powder X-ray diffraction, nitrogen adsorption-desorption measurement and scanning electron microscopy. Thermal stability has been tested by XRD analysis of mesoporous silica after thermal treatment at 823 K, 6 h; 1023 K, 1 h and 1223 K, 1 h. The results indicate that the mesoporous silica prepared under refluxing condition from precursor gel of pH 11 has large surface area (ca.1103 m² g^{− 1}) and pore volume (ca. 0.868 cm³ g^{− 1}) and is thermally stable at 1223 K. The surface area, pore volumes and pore wall thickness increase as the pH of the precursor gel is increased for refluxing condition of synthesis. The comparison of textural properties revealed that the refluxing condition is advantageous over autogenous pressure condition for obtaining mesoporous silica with higher surface area (852 m² g^{− 1}), pore volume (0.894 cm³ g^{− 1}) and pore diameter > 4 nm with wall thickness of 1.59 nm, when synthesized from precursor gel of pH 9.2. The ²⁹Si NMR spectra showed that a great part of the Si atoms exists as silanol groups. The mesoporous silica made at the lower pH (9.2) under refluxing conditions have more condensed framework. In calcined mesoporous silica, the proportion of partly condensed silica (Q³) is higher than fully condensed silica (Q⁴).