Porous monolithic polymeric supports with uniform diameter and tailored functional groups

Abstract

Porous materials play a crucial role in everyday life. Due to their low density and high stability, they are often applied in nature as a stabilizing framework, e.g., in bones or wood. The basic structure of these materials consists of a solid cavity (pores), which is filled with a liquid or gaseous medium (adsorbent). The volume of these pores in relation to the volume of the solid determines the porosity of the material and the specific surface area, which describes the area accessible to an adsorbent and includes both the outer and inner surface of the material, as well as the inner surface of the pores. The multitude of possible applications are based on noteworthy properties such as large pore volume and surface area, a hydrophobic or hydrophobic surfaces, high chemical and thermal stability, electrical conductivity, ease of handling, and low manufacturing costs. Significantly, mesoporous structured materials, which the International Union of Pure and Applied Chemistry (IUPAC) defines as having pores with a diameter between 2 nm and 50 nm, have received much attention because of their high surface areas and volumes. They include well-known materials such as the M41 group (MCM-41, MCM-48, MCM-50) and the SBA series, as well as mesoporous materials composed of inorganic, organic, or hybrid frameworks and covalent organic frameworks (COFs). Mesoporous materials can be ordered or disordered, characterized by pore size distribution. The unique features of ordered mesoporous materials (OMMs), including high specific surface areas (up to 1,000 m2 g-1) and well-defined, uniform pore sizes, make them suitable for specific practical applications such as catalysis, energy storage systems, photocatalysis, photo electrocatalysis, lithium-ion batteries, heterogeneous catalysis, the extraction of metals, the extraction of lanthanide and actinide species, chiral separations, capturing, and the mode of binding of carbon dioxide (CO2), optical devices, and magneto-optical devices. Of these applications, heterogeneous catalysis is predominant. In heterogenous catalytic processes, the immobilization of the catalysts on solid supports allows for easy separation and reuse of the catalyst and prevents the contamination of the product with catalyst traces. Since work-up and purification, as well as the synthesis of the catalyst itself, are often laborious and expensive, immobilized catalysts are a desirable goal, both ecologically and economically. Mesoporous materials have several properties that make them excellent carrier material for immobilized catalysts. These materials are mechanically stable and inexpensive, do not swell in organic solvents, and allow for high catalyst loadings due to its large surface area. For a good carrier material, not only the area of the surface is important, but the pores must also be easily accessible. The structure of mesoporous materials combines high surface area with optimal accessibility of immobilized catalyst molecules by substrates and regeneration reagents. Compared with heterogeneous batch catalysis, reactions in continuous flow have additional advantages. As major benefits, catalyst separation and recycling are replaced by a continuous process, which simplifies work-up. A reaction in continuous flow is scalable and allows for the extension of a continuous multistep synthesis through the addition of different reactor columns in one setup. At the same time, immobilized catalysts in high-throughput reactors need a carrier material that is suitable for high flow rates and does not cause high pressures inside the column. Based on the above, the following scientific questions/problems emerge as the main objectives of this dissertation. The first chapter includes a brief theoretical survey of the historical background and the recent developments of polymeric monolithic supports. The text also briefly describes the fundamentals, classification, and synthesis of OMMs. In addition, ionic liquid (IL)-based supported ionic-liquid phase (SILP) technology is a topic in the theoretical part of this study. The structural design and characterization of mesoporous polymeric, monolithic support materials derived from poly(urethane) (PUR) and poly(norborn-2-ene) are addressed in the following section of this thesis. An entire process was designed to synthesize polymeric monolithic supports with defined mesoporosity and flow-through porosity, which allows for fast mass transfer. The synthesis of the monolithic support was accomplished by solvent-induced phase separation (SIPS), which derives from the Flory-Huggin’s theory. By varying the ratio and nature of chemicals for the monolith synthesis, such as monomer, crosslinker, initiator, and porogenic solvents, the porosity could be precisely controlled. Such a designed system allows for the preparation of polymeric monolithic supports with the desired mesoporosity. The monolithic support material allows high throughputs without causing excessive back pressures (<10 bar), which is critical for specific applications. In addition, the fabrication of mesoporous channels was accomplished by utilizing the hard template-assisted approach. In regard to the preparation of OMMs, the precursors and synthetic experimental conditions play a major role in the properties of the final product. Here, we report on the first ring opening metathesis polymerization (ROMP)-derived, poly(norborn-2-ene)-based monolithic supports with tailored mesoporosity using a SiO2 nanowire (SNW)-based hard templating approach, which strongly differs from other hard templating approaches in that it provides access to defined mesopores. We hence address two main parameters: First, the SNWs must be compatible with the polymerization mixture such that agglomeration is fully suppressed, and the individual SNWs are fully dispersed within the polymerization mixture. This could be accomplished using a treatment such as (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane. Second, a suitable chemical etching method is required to remove the SNWs from the monolith's surface without mechanical deformation. Removal of the SNWs was accomplished with in-situ generated hydrofluoric (HF) acid under continuous flow at room temperature, allowing for high linear flow rates (>2mms-1) at low backpressure (< 6 bar). This study also focused on confinement effects using a biphasic liquid-liquid system. In this manner, PUR- and ROMP-derived monolithic supports were prepared under SIPS conditions. Then the hydroxyl (OH)-surface-functionalized monolithic supports were surface grafted with [NCO-C6H4-NMe3+][BF4-] for the subsequent immobilization of the IL, 1-butyl-3-methylimidazolium tetrafluorborate [BMIM+][BF4-], containing a new cationic Rh-NHC catalyst. Subsequent immobilization of the IL, [BMIM+][BF4-], contain-ing a new cationic Rh-NHC catalyst, created the supported ionic liquid phase SILP catalyst. Hydrosilylation of terminal alkynes were carried out under batch and continu-ous conditions using a Rh-NHC complex on PUR- and ROMP-derived monolithic supports.