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

Abstract

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 and Textural Characterization of Mesoporous and Meso-/Macroporous Silica Monoliths Obtained by Spinodal Decomposition

“…For a constant composition of the mixture (1 Si:14.21 H 2 O:0.60 EO unit:0.26 HNO 3 ) increasing the size of the polymer increases the size of the macropore and in the same time of the skeleton thickness. For PEO of 20,35, and 100 kDa, macropore sizes of 4, 10, and 16 microns and skeleton thickness of 3, 6, and 7 microns have been obtained, respectively [9]. Lower molecular weight PEO polymers interact more strongly with silica oligomers via multiple interactions, they adsorb at the surface of the silica oligomer and cover the particle, leading to a faster condensation of the silica oligomers and favor the sol-gel transition, resulting in smaller pores.…”

“…A high mass transfer is reached for a low C parameter of the Van Deemter equation, which is proportional to the square of the diffusion length (particle diameter for a packed-bed or the skeleton thickness for the monolith) [1,20]. We have shown previously that mass transfer in the mesoporosity is also controlled by the size of the mesopores (or more exactly the ratio mesopore diameter/molecule diameter), the homogeneity of the mesopore size distribution (cylindrical pores and pores with constrictions assimilated to spherical pores) and the interconnectivity of the mesopore network [1,20]. The C parameter of the Van Deemter equation is inversely proportional to the diffusion D iff into the pore:…”

Capillary microreactors based on hierarchical SiO2 monoliths incorporating noble metal nanoparticles for the Preferential Oxidation of CO

“…To prepare the SiO 2 monoliths, this solution is flowed through the capillaries by means of a vacuum pump. Once the capillaries were filled with the precursor solution, they were sealed and the gels were aged by submitting the samples to a thermal treatment of 40ºC overnight and 120ºC for 3 h. At 120ºC, the urea is known to decompose generating ammonia and this process is responsible for the formation of mesopores, as other authors have reported [21]. A final calcination step was performed under vacuum at 300ºC for 24 h (heating rate of 3ºC/min) in order to remove the surfactant, with this decomposition being responsible for the formation of the macroporous structure of the monoliths.…”

Synthesis of Robust Hierarchical Silica Monoliths by Surface-Mediated Solution/Precipitation Reactions over Different Scales: Designing Capillary Microreactors for Environmental Applications

“…This urea decomposition takes place in the silica enriched phase, producing the mesoporores inside the silica spheres. The combination between the basic medium (pH around 9-10), the temperature and the pressure enhances the solution/precipitation equilibrium giving rise to the final porous inorganic structure 10 .…”