大孔硅胶的制备及其在多糖衍生物手性固定相上的应用

为制备孔径为100 nm的大孔硅胶,考察了热液法和焙烧法对球形硅胶（粒径5 μm,孔径10 nm）的扩孔效果。采用热液法扩孔时,在水溶液中加入22 g/L氟化钠,可以有效增强扩孔效果,在高压釜内160℃加热48 h便可扩孔至100 nm,但孔径不均匀。采用焙烧法扩孔时,通过调节焙烧温度、时间以及复盐LiCl-NaCl的加入量可以方便地控制扩孔速度与效果;在每10 g硅胶中加入1.125 g LiCl·H₂O和0.75 g NaCl,于500℃焙烧3~5 h,可得到100 nm大孔硅胶;该方法简单、高效,扩孔后的硅胶孔径分布均匀,表面形态与商品化的Fuji-1000硅胶相似。将两种扩孔方式得到的硅胶经氨基修饰后,涂覆纤维素-三（3,5-二甲基苯基氨基甲酸酯）制得了相应的手性固定相。结果表明,采用焙烧法扩孔得到的硅胶制备的固定相明显具有较好的分离选择性及分离度。     

Experimental evidence of the influence of the surface chemistry of the packing material on the column pressure drop in reverse-phase liquid chromatography

The permeabilities of six columns packed with different packing materials (neat silica, C(1) endcapped silica at 3.92 micro mol/m(2), C(18) bonded and endcapped silica with 0.42, 1.01, 2.03, and 3.15 micro mol/m(2) of C(18) bonded chains) were measured. All these materials derive from the same batch of spherical particles, 5 micro m in diameter. The columns have the same tube inner diameter (phi=0.460+/-0.003 cm) and length (L=15.000+/-0.003 cm). The experimental conditions were the same, flow-rate (F(v)=1.000+/-0.003 mL/min) and temperature (295 K). Nevertheless, it was found that the column permeability decreases significantly, by about 25%, from the neat silica column to the one packed with the highest density of C(18)-bonded silica (3.15 micro mol/m(2)). The results measured on two duplicate columns were very reproducible. Accurate (+/-0.5 %) measurements of the hold-volumes with concentrated and dilute solutions of NO(3)(-) showed that the columns had all nearly the same external porosity. The result cannot be explained by the error made on the volume of the column tube either as it was measured accurately for all the columns. The residual explanation is that the interstitial velocity distribution between the packed particles depends on the chemical nature of the external surface of these particles.     

Structure Design of Double-Pore Silica and Its Application to HPLC

Utilizing the concurrence of polymerization-induced phase separation and sol-gel transition in the hydrolytic polycondensation of alkoxysilanes, a well-defined macroporous structure is formed in a monolithic wet gel. By exchanging the fluid phase of the wet gel with an appropriate external solution, the nanometer-range structure of the wet gel can be reorganized into structures with larger median pore size essentially without affecting the macroporous framework. The double-pore structure thus prepared is characterized by open pores distributed in discrete size ranges of micrometers and nanometers. A new type of chromatographic column (silica rod) has been developed using monolithic double-pore silica instead of packed spherical gel particles. Typical silica rod columns had significantly reduced pressure drops and improved analytical efficiencies which do not deteriorate even at higher sample flow rates, both arising from a greater macropore volume than particle packed columns.     

A particle size distribution analysis of used HPLC column packing material

Summary Particle size distribution analysis and scanning electron microscopy (SEM) were carried out on eight used HPLC columns containing either irregular silica based, spherical silica based or spherical polymer based packing material. Particle size distributions of the used irregular silica based columns were at least bimodat at the outlet ends and either biomodal or log-normal at the inlet ends with regular progressions between the two extremes through the column. A new ODS-3 column showed log-normal size distributions from the inlet to the outlet ends. Spherical silica based column particle size distributions showed distinct shoulders on large central distribution peaks in most column sections with various degrees of shoulder erosion. The spherical resin based column showed a broader inlet particle size distribution progressing to a very narrow outlet distribution. SEMs of both irregular and spherical silica based columns revealed a larger number of undersized particles and debris at the outlet than inlet ends which could have resulted from stationary phase degradation, since this was not seen in the new ODS-3 column. While several SEMs of the spherical silica based columns revealed hollow spheres and twins, the spherical resin based column packing showed stress fractures or wrinkle lines resulting from use or dehydration.Presented in part as a poster at the HPLC '92, 16^th Symposium on CLC, Baltimore, MD, USA.     

Spherical ordered mesoporous silicas and silica monoliths as stationary phases for liquid chromatography

Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (approximately 1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 microm could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC.     

Synthesis of spherical porous silicas in the micron and submicron size range: challenges and opportunities for miniaturized high-resolution chromatographic and electrokinetic separations

Classical silica technology has reached its limit with respect to an ultimate minimum particle size of about 2 microm in diameter. Here, a novel process is presented which allows one to synthesize porous silica beads and control their particle diameter in situ, within the range of 0.2-2.0 microm. As a result, no sizing is required and losses of silica are avoided. Furthermore, the process enables one to control in situ the pore structural parameters and the surface chemistry of the silica beads. Even though surface funtionalized silicas made according to this process can principally be applied in fast HPLC the column pressure drop will be high even for short columns. In addition, the column efficiency, expressed in terms of the theoretical plate height is about H-2d(p) in the best case and limited by the A and C term of the Van Deemter equation. In other words the gain in total plate number when using 1-2 microm silica beads in short columns is minimal as compared to longer columns packed with 5 microm particles. Capillary electrochromatography (CEC) as a hybrid method enables the application of micron size as well as submicron size particles. This consequently enhances column efficiency by a factor of 5-10 when compared to HPLC. The use of short CEC columns packed with submicron size silicas provides the basis for fast and efficient miniaturized systems. The most significant feature of CEC as compared to HPLC is that the former allows one to resolve polar and ionic analytes in a single run. An alternative method for miniaturization is capillary electrophoresis (CE) which generates extremely high efficiencies combined with fast analysis. Its application, however, is limited to ionic substances.     

Towards the ultimate minimum particle diameter of silica packings in capillary electrochromatography

Porous silica beads with an average particle diameter between 0.2 and 3 microm have been applied as packing material in capillary electrochromatography (CEC). The experiments were directed to investigate whether it is really feasible and as promising as expected to use such small particles. In CEC, plate heights of H approximately/= 1-2 d(p) can be achieved which is smaller than the plate heights usually attained in high-performance liquid chromatography. Using a capillary packed with 0.5 microm silica beads we achieved a plate height of H = 3 d(p) indicating the presence of dispersive effects like Joule heating. Calculations demonstrate that at a field strength of about 800 V cm(-1) one third of the plate height can be lost by Joule heating effects if the heat is not removed by a cooling system. Additionally, the H(u) curve is still descending at the maximum electroosmotic flow (EOF) velocity we generated with the modified capillary electrophoresis instrument. To fully exploit the potential of submicron size silicas higher field strengths, i.e., higher EOF velocities, must be attained. To study the influence of the kind of packing on the EOF porous as well as nonporous silicas have been applied. The experiments clearly indicate that the EOF of porous and nonporous silicas is the same. Since the EOF is more or less exclusively generated by the packing material the zeta potential of n-octyl bonded 0.5 microm silica has been determined. The dependence of the zeta potential on the pH is identical to the dependence of the EOF on the pH in a packed capillary. The point of zero charge of the silica is at pH 2-3.     

Chromatographic evaluation of poly (trans-1,2-cyclohexanediyl-bis acrylamide) as a chiral stationary phase for HPLC

Chiral stationary phases (CSPs) based on polymeric (R,R)- or (S,S)-1,2-diaminocyclohexane (DACH) derivatives are synthesized. When bonded to 5 microm porous spherical silica gel, the poly (trans-1,2-cyclohexanediyl-bis acrylamide) based poly-cyclic amine polymer (P-CAP) stationary phases is proved to be effective chiral stationary phases that could be used in the normal-phase mode, polar organic mode and with halogenated solvents mobile phases, if desired. Since these are entirely synthetic CSPs, the elution order of all enantiomers can be reversed between the (R,R) P-CAP and (S,S) P-CAP columns. Because of the high loading of chiral selectors, the columns exhibit very high sample capacities. Thus, P-CAP columns are useful for preparative and semi-preparative enantiomeric separations. The application of these CSPs and optimization of their separations are discussed.     

Benzene Adsorption Isotherms on MCM-41 and their Use for Pore Size Analysis

This work is focused on the elaboration of methodology for adsorption characterization of porous silicas by using benzene adsorption isotherms measured on good quality MCM-41 materials. Three MCM-41 samples were synthesized by using tetraethyl orthosilicate (TEOS) as silica source and surfactants, octyltrimethylammonium (C8), decyltrimethylammonium (C10) and cetyltrimethylammonium (C16) bromides as templates. A characteristic feature of this synthesis was relatively long hydrothermal treatment (5 days) at 373 K, which gave well ordered samples as evidenced by powder XRD analysis. Benzene adsorption isotherms measured on these MCM-41 samples were used to evaluate such standard quantities as the BET specific surface area, total pore volume, external surface area and the volume of ordered mesopores, and to obtain the statistical film thickness (t-curve) as well as the Kelvin-type relation, which describes the dependence between pore width and condensation pressure for benzene on silica at 298 K. The latter relations were incorporated into the Barrett-Joyner-Halenda algorithm to extend its applicability to calculate the pore size distributions from benzene adsorption data.     

Optimization of the phase system in the analysis of polynuclear aromatics (PNA) from diesel engine exhaust by high performance liquid chromatography (HPLC)

Summary Polynuclear aromatics (PNA) were collected and enriched from diesel engine exhausts by means of a probe and an absorber system. Group separation of PNA from other constituents of exhaust was accomplished through elution with n-heptane on a silica column. A variety of phase systems ranging from unpolar to strong polar packings and eluents, respectively, were tested to resolve PNA into single species. Of these a n-octadecyl bonded silica packing and methanol/water was found to be the most optimal with regard to resolution for real PNA mixtures. Separation was completed within 2–3 h at isocratic conditions.Paper presented at Euroanalysis III, Dublin. August 20–25, 1978.     

Influence of the degree of coverage of C18-bonded stationary phases on the mass transfer mechanism and its kinetics

The influence of the degree of coverage of a silica surface with bonded C18 alkyl chains on the mass transfer mechanism in RPLC was investigated. Five packing materials were used, prepared with the same batch of silica particles (5 microm diameter, 90 A average pore size): one column was packed with the silica derivatized by trimethylchlorosilane (TMS) (C1, 3.92 micromol/m2), and the other four with the silica first derivatized with octadecyl-dimethyl-chlorosilane (C18, 0.42, 1.01, 2.03, and 3.15 micromol/m2), and then endcapped with TMS. A solution of methanol and water (25/75, v/v) was used as the mobile phase. The experimental HETP curves were acquired for each column by measuring the first moment and the second central moment of phenol and correcting them for the influence of the temperature increase due to the heat generated by the friction of the stream against the bed. The different kinetic parameters of the mass transfer in these packed chromatographic columns were identified (longitudinal diffusion, eddy diffusion, film mass transfer, and transparticle mass transfer) and quantified by fitting the experimental data to a new general HETP equation recently derived [F. Gritti, G. Guiochon, Anal. Chem., in press (AC-060203R).]. The agreement was excellent and allowed the comparison of the kinetic parameters among the six columns used. The highest column efficiency measured at conventional or fast flow rates (>0.5 ml/min) is obtained for the most retentive column, which has a surface coverage of 2.03 micromol/m2. The smallest HETP measured is as low as 10 microm, only twice the average particle diameter dp, due to the large contribution of surface diffusion (90%) to the particle effective diffusivity. However, no significant difference was observed between the efficiencies of the columns packed with C1 and C18 derivatized silica.     

A particle size distribution analysis of stressed HPLC column packing material

Summary A particle size distribution analysis has been completed on three different HPLC column packing materials including silica gel (Si60) and two bonded phases (RP8 and RP18). The stationary phases were subjected to 18 hours stress with 1 N or 3 N KOH and found to have quantitatively different distribution patterns initially, at 13 hours and finally at 18 hours although the average particle diameters for the Si60 and RP8 were the same or higher at 18 hours as initially. Thirteen hoursstress with sodium octanesulfonate, tetrabutylammonium phosphate and ammonium acetate at exaggerated conditions also resulted in distributional changes with the Si60 and RP8 decreasing in average particle diameter when exposed to ammonium acetate and tetrabutylammonium stressing respectively.     

Influence of the pore structure on the properties of silica based reversed phase packings for LC

This paper describes the preparation and investigation of new, highly loaded, monomeric, silica based, reversed phase C18 and C30 packings. The influence of pore structure and endcapping on the properties of C18 and C30 packings is described. Using hydrothermal procedures, silicas with predictable pore size (9.3-25.5 nm) and surface area have been prepared. Silylation with long chain silanes substantially alters the pore structure of the silica: pore size and pore volume decrease. A new parameter, the volumetric surface coverage [mm3 x m(-2)] has been introduced. This parameter--calculated from on-column measured porosity data--indicates the pore volume portion occupied by the hydrocarbon chains. Endcapping does not significantly change the pore structure of the bonded phases. The reduced retentions (reduced with respect to unit area: [k/m2])--a good measure for comparing the retention behaviour of packings with different surface areas--are similar for most of the phases, demonstrating good accessibility of the pores for the solutes. Slightly lower retentions were found on the endcapped than on the non-endcapped phases for probes with dense pi-electron system (e.g. polyaromatic hydrocarbons) demonstrating the contribution of silanophilic interactions to the retention. The phases had been successfully used for various demanding separations, e.g. for the separation of flavonoids, carotenoids, resveratrol, and tocopherol isomers, fullerenes, and anions.     

Packing density of slurry-packed capillaries at low aspect ratios

The average interparticle voidage or porosity (epsilon(inter)) in cylindrical capillaries is studied in dependence on the column diameter (d(c)) to particle diameter (d(p)) ratio for 5 < d(c)/d(p) < 50. Using optimized slurry and packing solvents, high pressure and ultrasonication, 5 mum-sized porous C18-silica particles were slurry-packed into fused-silica capillaries having ids from 30 to 250 mum. Packing densities are assessed by a polystyrene standard which is size-excluded from the intraparticle pore space of the packings. For d(c)/d(p) > 35 densely packed beds are realized (epsilon(inter) = 0.36-0.37), while for decreasing aspect ratios an exponential increase in epsilon(inter )is observed reaching epsilon(inter ) approximately 0.47 at d(c)/d(p) = 5. This behaviour is ascribed to a combination of the geometrical wall effect operating in the direct vicinity of the column wall, caused by the inability of the particles to form a dense packing against the hard surface of the column wall, and particle characteristics like the size distribution, shape and surface roughness. Results are compared with the literature data to address also the importance of absolute particle size in studying structure-transport relations in packed beds in dependence on the aspect ratio d(c)/d(p).     

High efficiency,narrow particle size distribution,sub-2 μm based macrocyclic glycopeptide chiral stationary phases in HPLC and SFC

State of the art chiral chromatography still employs 3–5 μm bonded or immobilized chiral selectors in 10–25 cm columns. With the availability of 1.9 μm narrow particle size distribution (NPSD) silica, it is now possible to make ever shorter, high efficiency columns practical for sub-minute chiral separations. Three macrocyclic glycopeptides (teicoplanin, teicoplanin aglycone, and vancomycin) were bonded onto 1.9 μm NPSD particles. Such packed columns had ∼80% lower backpressure as compared to polydisperse (PD) 1.7 μm silica materials when using the same mobile phase. The decreased backpressure allowed for diminution of frictional heating and allowed for the use of the 1.9 μm NPSD particle based columns at high flow rates. The 1.9 μm NPSD particle based columns showed up to 190,000 plates m⁻¹ for chiral molecules and 210,000 plates m⁻¹ for achiral probes. Representative enantiomeric separations are shown for wide classes of compounds, including different types of amino acids, β-blockers, and pharmaceutically important heterocyclic compounds such as oxazolidinones. Applications in three liquid chromatography modes, namely, reversed phase, polar organic mode and normal phase chiral separations were shown with resolution values ranging from 1.5 to 5.7. Additionally, the same columns were used with supercritical fluid chromatography (SFC) for ultrafast separations.     

聚烯烃催化剂载体硅胶研发进展

以Ziegler-Natta催化剂和茂金属催化剂为例,介绍了聚烯烃均相催化剂的负载化技术以及载体选择的影响因素,明确了载体的强度、密度、总孔容、孔分布、孔径、粒度和颗粒形状等物理性质是选择载体要考虑的主要因素。综述了聚烯烃催化剂载体硅胶的结构特征和性能特征,并对聚烯烃催化剂载体硅胶的合成机理、影响因素进行了详细介绍。此类载体硅胶比表面积280~320m2/g、堆密度0.28~0.32g/cm3、孔容1.40~1.70cm3/g、平均孔径18~22nm。建议通过开展对不同性能硅胶(如更小粒径、不同元素改性等)的研究开发,以满足不同催化剂对载体硅胶的特殊要求。     

Physical properties and structure of fine core–shell particles used as packing materials for chromatography: Relationships between particle characteristics and column performance

The recent development of new brands of packing materials made of fine porous-shell particles, e.g., Halo and Kinetex, has brought great improvements in potential column efficiency, demanding considerable progress in the design of chromatographic instruments. Columns packed with Halo and Kinetex particles provide minimum values of their reduced plate heights of nearly 1.5 and 1.2, respectively. These packing materials have physical properties that set them apart from conventional porous particles. The kinetic performance of 4.6 mm I.D. columns packed with these two new materials is analyzed based on the results of a series of nine independent and complementary experiments: low-temperature nitrogen adsorption (LTNA), scanning electron microscopy (SEM), inverse size-exclusion chromatography (ISEC), Coulter counter particle size distributions, pycnometry, height equivalent to a theoretical plate (HETP), peak parking method (PP), total pore blocking method (TPB), and local electrochemical detection across the column exit section (LED). The results of this work establish links between the physical properties of these superficially porous particles and the excellent kinetic performance of columns packed with them. It clarifies the fundamental origin of the difference in the chromatographic performances of the Halo and the Kinetex columns.     

Sorption and diffusion of phenols onto well-defined ordered nanoporous monolithic silicas

The sorption of phenol, and o-, m-, and p-aminophenol (o-, m-, and p-AP) onto highly ordered mesoporous silicas (HOM) with cubic Im3m (HOM-1), hexagonal H(I) (HOM-2), 3-D hexagonal p6(3)/mmc (HOM-3), cubic Ia3d (HOM-5), lamellar L( infinity ) (HOM-6), and solid phase S (HOM-8) materials has been investigated kinetically. Nanostructured silica molecular sieves have been prepared at 25 and 60 degrees C with lyotropic liquid-crystalline phases of the nonionic surfactant (Brij 76) that was used as a structure-directing agent. Such nanostructured silicas have been studied by 29Si nuclear magnetic resonance (29Si NMR), powder X-ray diffraction (XRD), the Brunauer-Emmett-Teller (BET) method for nitrogen adsorption and surface area measurements, and transmission electron microscopy (TEM) techniques after synthesis and sorption. It was found that all materials exhibit well-defined long-range porous architectures without significant loss of the ordered texture during phenol sorption. The kinetics of phenol sorption has been studied spectrophotometrically at different temperatures (25-40 degrees C; +/-0.1 range). The sorption rate is zero order in all phenols sorbed, and increases directly in the pattern P >m-AP > o-AP > p-AP, which reflects the mobility of the phenol compounds on the particle pores. The isothermal sorption and the kinetic parameters were discussed and it was established that a diffusion-controlled process characterizes phenol sorption. Furthermore, the mechanism of phenol sorption was deduced to be predominantly particle diffusion. The diffusion coefficients were determined using Fick's equation. The trend of diffusion of all phenols onto nanoporous silica was HOM-8 > HOM-2 > HOM-6 > HOM-5 > HOM-1 > HOM-3, reflecting the effect of the uniform pore size distribution and the internal surface area of the nanostructured silicas on the diffusion process.     

以EDTANa2为矿化剂近中性条件下室温合成介孔二氧化硅

利用EDTANa2作为矿化剂, 在近中性条件下于室温合成了厚壁(约3 nm)蠕虫介孔分子筛. 在反应物配比为n(CTAB):n(TEOS):n(EDTANa2):n(H2O)=(0.05~0.2):(0.1~2):(0.1~0.5):(100~500)范围内都能得到介孔二氧化硅. 以XRD、N2吸附、FTIR、SEM和TEM详细考察了体系配比和温度对介孔二氧化硅结构和形貌的影响, 发现CTAB用量越大, 介孔d值相应增大. 温度对介孔二氧化硅的结构和形貌有很大的影响, 温度在2~15 ℃范围内都能生成孔径分布较规则的介孔, 介孔材料的形貌随温度的升高由空心管变成小颗粒聚集体; 合成的介孔分子筛中的模板剂可以通过乙醇萃取的方式除去. 提出了介孔的生成遵从(S+-E-)0-I0中性模板机理.     

Preparation of MCM-41 silica using the cationic surfactant blend

A series of samples of MCM-41 silica was synthesized using surfactant blends of 1-alkyl-3-methylimidazolium and alkyltrimethylammonium salts or blends of two different 1-alkyl-3-methylimidazolium salts (alkyl denotes octyl or hexadecyl) as structure-directing agents. The precipitation of solid particles from a homogeneous water solution of sodium metasilicate and surfactant blend was achieved by lowering the pH due to the hydrolysis of ethyl acetate added. The molecular sieves were characterized by scanning as well as transmission electron microscopy, X-ray powder diffraction, and nitrogen adsorption using a proper nonlocal density functional theory approach for calculations of the textural parameters. All the prepared silicas were of MCM-41-type; they differ in the integral breadth of the pore size distribution curve and the presence of secondary mesopores. The best quality MCM-41 silica of spherical particle morphology was synthesized by using of optimized blend of hexadecyltrimethylammonium bromide and 1-methyl-3-octylimidazolium chloride. The results obtained showed that spherical particles are composed of domains of perfectly ordered hexagonal porous structure. Some samples prepared by using 1-alkyl-3-methylimidazolium salts featured a narrow pore size distribution. However, they contained a small volume of secondary mesopores.