Characterization of covalently bonded and adsorbed polymer coatings on silica, alumina and zirconia by means of physico-chemical and chromatographic methods

Polymer coatings on porous silica, alumina and zirconia were prepared by covalent bonding of a copolymer of styrene and vinylsilane on the oxidic surface with or without subsequent cross-linking of the immobilized polymeric layer. Polybutadiene and polychloromethylstyrene were adsorbed on the surface of porous alumina without covalent bonding and were cross-linked after deposition. Analysis of the pore structure of coated oxides by means of nitrogen adsorption, mercury porosimetry and inverse size-exclusion chromatography revealed different polymer distributions across the modified surface and different changes of the porosity of the initial oxides depending on the method of polymer immobilization. By covalent bonding of the copolymer of styrene and vinylsilane to alumina and zirconia with subsequent cross-linking the modifying polymeric layer, highly hydrolytic stable packings were synthesized and their application in separations with aggressive eluents were demonstrated.     

Evaluation of the porous structures of new polymer packing materials by inverse size-exclusion chromatography

A highly cross-linked porous polymer resin based on styrene-divinylbenzene matrix with pores created by the use of micellar imprinting technique was used as chromatographic packing material. Its performance as a column packing material in inverse size-exclusion chromatography was compared with a non-imprinted resin of the same polymer matrix. The porous structures (the pore size and the porosity) of the resins in the dry and wet states and their relationships with the elution volume of probe solutes (alkanes and polystyrene standards) were established. Characteristic properties of the resins such as specific pore volume, specific surface area and porosity are compared with results obtained by other methods of characterization such as mercury intrusion porosimetry, solvent regain and nitrogen sorption. The results show that the new porous resin can be used in the separation of small molecules. The separation is based on the size of the molecules, and the larger pores (meso- and macropores) in the porous resin can provide a much easier access to the smaller pores (micropores) which are useful in the chromatographic separations.     

Synthesis and characterization of surface modified SBA-15 silica materials and their application in chromatography

Hexagonally ordered SBA-15 mesoporous silica spheres with large uniform pore diameters are obtained using the triblock copolymer, Pluronic P123, as template with a cosurfactant cetyltrimethylammonium bromide (CTAB) and the cosolvent ethanol in acidic media. A series of surface modified SBA-15 silica materials is prepared in the present work using mono- and trifunctional alkyl chains of various lengths which improves the hydrothermal and mechanical stability. Several techniques, such as element analysis, nitrogen sorption analysis, small angle X-ray diffraction, scanning electron microscopy (SEM), FTIR, solid-state (29)Si and (13)C NMR spectroscopy are employed to characterize the SBA-15 materials before and after surface modification with the organic components. Nitrogen sorption analysis is performed to calculate specific surface area, pore volume and pore size distribution. By surface modification with organic groups, the mesoporous SBA-15 silica spheres are potential materials for stationary phases in HPLC separation of small aromatic molecules and biomolecules. The HPLC performance of the present SBA-15 samples is therefore tested by means of a suitable test mixture.     

Porosity and surface properites of SBA-15 with grafted PNIPAAM: a water sorption calorimetry study

Mesoporous silica SBA-15 was modified in a three-step process to obtain a material with poly-N-isopropylacrylamide (PNIPAAM) grafted onto the inner pore surface. Water sorption calorimetry was implemented to characterize the materials obtained after each step regarding the porosity and surface properties. The modification process was carried out by (i) increasing the number of surface silanol groups, (ii) grafting 1-(trichlorosilyl)-2-(m-/p-(chloromethylphenyl) ethane, acting as an anchor for (iii) the polymerization of N-isopropylacrylamide. Water sorption isotherms and the enthalpy of hydration are presented. Pore size distributions were calculated on the basis of the water sorption isotherms by applying the BJH model. Complementary measurements with nitrogen sorption and small-angle X-ray diffraction are presented. The increase in the number of surface silanol groups occurs mainly in the intrawall pores, the anchor is mainly located in the intrawall pores, and the intrawall pore volume is absent after the surface grafting of PNIPAAM. Hence, PNIPAAM seals off the intrawall pores. Water sorption isotherms directly detect the presence of intrawall porosity. Pore size distributions can be calculated from the isotherms. Furthermore, the technique provides information regarding the hydration capability (i.e., wettability of different chemical surfaces) and thermodynamic information.     

Dynamics and isotherms of water vapor sorption on mesoporous silica gels modified by different salts

The mesoporous silica gels impregnated with different metal salts were prepared and studied. The pore structure and specific surface area of adsorbents were evaluated using nitrogen adsorption. Then, the sorption isotherms and dynamics of water vapor were carried out at 303 K and different relative humidity (RH). The temperature programmed desorption experiments were conducted to estimate the activation energy (E _d) of water desorption on the silica gels. The results showed that the sorption capacity for water decreased with the increase of the ionic radius (except the calcium ion) and that CaCl₂ and LiCl were particularly suitable for use in modification of the mesoporous silica gel to improve their sorption rates and capacities for water vapor at the lower and medium RH (RH < 80%). The larger the average pore diameter and pore volume of the initial silica gels, higher the accrual rates of the water vapor sorption rate and capacity were after modification with hygroscopic salts. The activation energy of the water desorption on the mesoporous silica gel modified by CaCl₂ were much higher than that on the silica gel modified by LiCl, because the polarizability of the Ca²⁺ was higher than that of Li⁺.     

Preparation and Characterization of Porous Titania by Modified Sol-Gel Method

Mesoporous titania, especially anatase, is attractive due to its potential applications. A novel method to control pore structure of titania, surfactant- or polymer modification, is proposed. The wet gels and gel films, prepared from Ti(O-nC₄H₉)₄ were dried at 90°C and annealed at 500°C after immersion in surfactant or polymer solutions, and mesoporous anatase was obtained. The pore size, pore volume and specific surface area of the surfactant-modified bulk gels, estimated from N₂ absorption-desorption curves, are more than twice larger than those of the gels without modification. The pore size of the surfactant-modified gel films, observed by SEM, are similar to that of the bulk gels. The pore size obviously depended on the size of micelles. The pore size of the gels modified with hydrophilic polymers hardly increased, but the pore volume and the specific surface area increased.     

Templated Synthesis of Tailored Clays (Based on Sol-Gel Techniques) to Yield Increased Pore Volumes for Sorption and Exchange

Synthesis and modification of Laponite-related materials by surfactants and globular cations, leading to increased pore volume and enhanced specific surface area, is reported. Materials produced using cationic and neutral surfactants, as well as those with varying tail length and head size are compared. The characteristics of these tailored materials have been determined via nitrogen sorption porosimetry and X-ray powder diffraction. The appropriate choice of surfactant leads to markedly enhanced total pore volumes and modified textures. Most significantly, an increase in total pore volume from 0.27 cm³ g^?1 (Laponite RD without templating) to 0.94 cm³ g^?1 (using a non-ionic surfactant as template/modifier) is reported.     

Surfacial,liquid sorption and monolayer-forming properties of hydrophilic and hydrophobic Stöber silica particles

The surface of silica spheres with a diameter of 500 nm was modified with ethoxysilane. Hydrophilic and partially hydrophobic silica spheres were obtained, suitable for the preparation of two-dimensional monoparticle films at the liquid-air interface. The tendency of these particles to self-assemble is basically dependent on surface hydrophobicity. Liquid sorption excess isotherms were studied in ethanol-cyclohexane and ethanol-chloroform mixtures with the aim of characterizing the adsorption capacity of the particles. Specific surface area and porosity were measured by nitrogen adsorption. The specific surface area determined by liquid sorption was considerably larger than determined by gas adsorption. This is ascribed to penetration of ethanol into the pores and the swelling of the silica particles in ethanol. Surface modification of hydrophilic particles changed the film-forming properties of the particles. The compressibility and the lift-off area of the monolayer films of hydrophobic particles on water were higher than for the films of hydrophilic particles.     

Effect of Candida antarctica lipase B immobilization on the porous structure of the carrier

A series of poly(GMA-co-EGDMA) resins with identical composition but varying particle sizes, pore radii, specific surface areas and specific volumes are studied to assess how Candida antarctica lipase B immobilization affects the porosity of the copolymer particles. Mercury porosimetry reveals a significant change in the average pore size (up to 6.1-fold), the specific surface area (up to 3.2-fold) and the specific volume (up to 2.1-fold) of the epoxy resin. A similar behaviour is observed for glutaraldehyde-modified epoxy resins. The influences of the resin porosity properties on the loading of Candida antarctica lipase B during immobilization and on the hydrolytic activity (hydrolysis of p-nitrophenyl acetate) of the immobilized lipase are studied.     

二乙烯苯交联大孔聚甲基丙烯酸缩水甘油酯的合成及其结构性能的研究

选用反应性单体甲基丙烯酸缩水甘油酯（ＧＭＡ），以二乙烯苯（ＤＶＢ）作为交联剂。在致孔剂甲苯和正庚烷存在下，用悬浮聚合的方法制得了一系列大孔ＧＤ共聚物。通过测定树脂的孔结构性能及化学组成，讨论了不同交联剂用量和配比对共聚物结构的影响。     

New polymer materials with controlled nanoporous structure based on N-vinylpyrrolidone

The polymer networks with nanoporous structure were obtained by the crosslinking free-radical copolymerization of N-vinylpyrrolidone with triethylene glycol dimethacrylate in bulk in the presence of amphiphilic copolymer and its fractions as templates. The templating agents consisted of copolymer or their fragments with similar monomer units and different molecular weight. Macromolecular templates were shown to be removed from the polymer composite by PrⁱOH leaving the pores. The values of the specific surface areas, the total pore volumes, pore size, and pore size distribution were measured by the method of low-temperature nitrogen absorption. The maximum value of the specific surface area was calculated to be ~26 m² g^–1. The value was significantly higher than that for the usual copolymer network. The relationship between specific surface area, parameters of pores, and macromolecular structure of template has been established. It is shown by Brunauer—Emmett—Teller method that the macromolecules having a branched architecture are more effective for the preparation of the polymer network with more developed specific surface area and narrow pore size distribution.

     

Changes in the pore character of silicas caused by surface bonding and polymer coating

The pore character of packings for liquid chromatography, especially reversed-phase (RPLC) packings, has been studied by means of the nitrogen adsorption method (BET method). Micro-spherical silica gels with 9, 12, 30, 40 and 50 nm average pore diameter have been used as carriers. These silica gels have been modified with monochlorodimethyloctadecylsilane and several polymers (polyoctadecylmethacrylate-methylmethacrylate co-polymer, polyacrylamide gel, polyvinyl alcohol, poly-2-hydroxyethyl-methacrylate). A larger decrease in the specific surface area values was observed in the case of the polymer coating with the polyoctadecylmethacrylate-methylmethacrylate co-polymer compared with the derivatization by silanes or the modification with polymers without C18-groups. A new approach has been suggested to explain some questions concerning the interpretation of the data obtained during the measurements of the pore characteristics of the derivatized packings. An attempt has been made to reveal peculiarities connecting values of the measured surface of RP-packings with the specific surface area values of the initial silica, as well as with the chromatographically accessible surface.     

Synthesis of micelle templated silico-aluminas with different alumina contents

The computer aided analysis of the EPR spectra of radical surfactant probes inserted in cetyltrimethylammonium bromide micelles provided information on the kinetics of formation of micelle templated silico-aluminas (MTSA) at 343 K, obtained by means of silica and alumina alkaline solutions at different Si/Al ratios (from infinity to 4). Mainly two spectral components were analyzed and relatively quantified in the EPR spectra: (1) the micellar component, due to probes inserted in the surfactant aggregates, whose mobility decreases over the synthesis time, thus reporting on the progressive modification of the micelle structure and the solid condensation; (2) the interacting component, mainly arising from the electrostatic interactions between the surfactant heads and the charged surface sites. This last component increases its relative intensity over the synthesis time, informing about condensation and structuration of the silico-alumina at the micelle surface. X-ray diffraction (XRD), nitrogen sorption isotherms at 77 K, thermogravimetric analysis, TEM and chemical analysis were performed to characterize both as-synthesized and calcined MTSA materials. Nitrogen sorption isotherms allowed us to evaluate the pore diameter, the specific surface area and the pore volume. At Si/Al<15 a decrease in pore volume and specific surface area was interpreted as due to the contemporaneous presence of a hexagonal MTSA and an amorphous material, which was ascertained by means of XRD as the only present at Si/Al=4. The amorphous structure at Si/Al<15 used Na+ as contraions, whereas the surfactants are no more needed to neutralize the negatively charged groups at the solid surface. The hypothesis of a "break" at Si/Al=15 was supported by EPR: the interactions between the surfactant probe heads and the negatively charged surface groups are drastically reduced at Si/Al<15. On the contrary, at Si/Al>15, increasing amounts of alumina slow the kinetics of the synthesis but enhance electrostatic interactions between the surfactant heads and the negatively charged surface groups. Dilution of the synthesis mixture decreased the extent of the interactions, due to partial protonation of the silanol groups, and slowed the synthesis process.     

超细煤粉的物理特性及其对燃烧性能的影响

采用英国Ｍａｌｖｅｍ公司的马尔文粒度仪,美国Ｍｉｃｍｍｅｒｉｔｉｃｓ公司的ＡＳＡ２０００型比表面及孔径分布分析仪,对合山 煤的各４种没粒径的细化与超细化煤样进行了试验研究。结果表明,通过煤偻的细化、超细化煤粉颗粒的比表面积、孔隙容积与空隙面积显著的提高,对其燃烧过程带来有益的影响。煤中无水无灰基元素碳含理越高则其比表面积、孔只与空隙面积受颗粒粒度的影响也就越大。     

Effect of pore structure on surface characteristics of zirconium phosphate-modified silica

Three samples of silica of different pore structure-predominantly microporous, S1; mesoporous, S2; and nonporous, S3-were modified with zirconium phosphate and examined. Pore structure analysis showed that modification had taken place in wider pores of S1 leaving a totally microporous sample, and in large pores of S2 giving a mesoporous sample of narrower pore size distribution. The modification of the nonporous sample decreased the surface area and pore volume to a lower extent than in the other two samples, but resulted in a surface of lower energy toward N2. The different distribution of surface silanol groups on the surfaces of different porosity may result in variable pictures on the modified surfaces as reflected in the differences observed in Br?nsted acidity of modified surfaces. The use of these modified silica samples for amino acid adsorption (L-glutamic acid and L-alanine) indicated that both the isoelectric point of the amino acid and the distribution of surface groups on modified solids are controlling the adsorption process.     

Preparation of Mesoporous Titania by Templating with Polymer and Surfactant and its Characterization

Mesoporous titania was obtained by gelation from Ti-alkoxide in alcoholic solutions with addition of polymer and/or surfactant. The structure and surface morphology of the gels were characterized by N₂-adsorption measurements, scanning electron microscopy and X-ray diffraction. The specific surface area and pore volume of the gels can be increased with addition of hydrophilic polymer such as polyethyleneglycol. Surfactants like cetyltrimetylammoniumchloride are effective to control the pore size and to increase the pore volume and surface area. The surface morphology of the gels can be modified by the mixing method with polymer and/or surfactant. The effects of the templating on pore size distribution, pore shape, surface area and crystallization behaviors have been discussed.     

Nanoporous polymer networks based on <Emphasis Type="Italic">N</Emphasis>-vinylpyrrolidone

A method of preparing nanoporous polymer networks containing N-vinylpyrrolidone units via the crosslinking radical copolymerization in bulk performed in the presence of amphiphilic N-vinylpyrrolidone copolymers with the branched morphology and different physicochemical characteristics is developed. It is shown that macromolecular nanoobjects may be extracted from polymer composites using good solvents, such as chloroform and isopropyl alcohol. The physicomechanical, thermal, and diffusion–sorption properties of polymer composites before and after their extraction are compared. SEM and low-temperature nitrogen adsorption measurements reveal that nanosized pores are contained in the network copolymers after extraction of the polymer additives. The specific surface area, total pore volume, pore size, and pore-size distribution are determined. The maximum specific surface area of polymer networks attains ~26 m²/g, and mesopores compose the main type of pores.     

高比表面积有序介孔氧化铝的制备与表征

采用溶胶-凝胶法以非离子表面活性剂PEO-PPO-PEO三嵌段共聚物F127为模板剂, 以异丙醇铝为铝源, 以异丙醇为溶剂, 成功地制备出比表面积为485 m2/g、孔径分布窄(2~20 nm)、孔容在1.2 cm3/g以上和孔道呈蠕虫状且具有一定有序性的介孔氧化铝. 采用BET, TEM, XRD和TG多种测试技术对产物性能进行了表征. 探讨了水铝比、醇水混合溶液的滴加速度、反应时间、水浴温度、陈化温度及陈化时间等条件对合成的有序介孔氧化铝结构的影响.     

胶溶法制备改性氧化铝的结构及织构特点

对胶溶法制备的镧和聚乙二醇(PEG)改性的氧化铝载体进行了深入研究.     

Surface Properties of Impregnated Mixed Oxide Catalysts

The results of a comparative study of nickel oxide-alumina and nickel oxide-silica alumina catalysts in the light of the characteristics of the support systems alumina and silica-alumina are presented. The impregnation of NiO does not produce any significant change in the textural characteristics like surface area and pore volume of the support oxides. However the deposited nickel oxide leads to the modification of the surface acidity determined by Hammett indicators. In the case of parent oxides the ammonia adsorption is strong (from heat of adsorption measurement) for alumina and silica-alumina, though for the former it is slightly stronger at comparable surface coverages. But the adsorption of ammonia on NiO? Al₂O₃ is weak while it is strong on NiO/SiO₂? Al₂O₃ at the same comparable surface coverages. The reduction in acidity is reflected in the poor dehydration and absence of skeletal isomerisation on NiO impregnated catalysts.