Understanding sol–gel processing: Hierarchical silica monoliths towards applications in chemical reaction engineering
Hierarchically structured, porous materials in the form of macro–mesoporous silica monoliths represent ideal support materials for a variety of applications such as chemical separation, heterogeneous catalysis, thermal insulation, electrochemical processes and CO2 adsorption. They are well suited fo...
|PDF Full Text
No Tags, Be the first to tag this record!
|Hierarchically structured, porous materials in the form of macro–mesoporous silica monoliths represent ideal support materials for a variety of applications such as chemical separation, heterogeneous catalysis, thermal insulation, electrochemical processes and CO2 adsorption. They are well suited for this purpose since the macropores enable fast, advection-dominated transport through the material whilst the mesoporous skeleton provides a large external surface area for mass transfer between the macro- and the mesoporous domains, as well as a large, internal surface area for possible functionalization. In this context, this thesis focuses on the generation of hierarchy in sol–gel based porous silica materials, as well as the determination and interpretation of their properties with respect to applications in reaction technology. For the entire preparation process each individual step is fine-tuned in terms of practicability, time-effectiveness and simplicity to obtain robust and straightforward synthetic routes towards stable, hierarchically structured sol–gel monoliths. The understanding of the chemical and physical processes involved in these steps allows the precise control of the material characteristics. Novel experimental methods and strategies are presented, which minimize the laboratory workload and additionally highlight the superior properties of these materials. The potential of hierarchically structured sol–gel monoliths is demonstrated by applications in heterogeneous catalysis and biocatalysis. In the following, the respective concepts of this thesis are briefly summarized.
Chapter 1 examines the concept of hierarchy itself and highlights the advantages of a hierarchically structured pore system in comparison with monomodal pore systems, especially with regard to its mass transport properties. A model system based on silica membranes is presented, which discloses the advantages and disadvantages of purely mesoporous, purely macroporous and hierarchical pore structures. The monolithic support materials are synthesized using the sol–gel process, as this technique is considered as highly variable in the generation of different pore structures and sizes and it enables post-synthetic functionalization of the silica surface. In order to prepare mechanically stable and comparable sol–gel membranes, a novel and simplified drying method is presented. By varying the synthesis compositions, synthesis conditions and post-synthetic treatments, a variety of silica membranes are produced, which differ only in their porous properties. Monomodal structured materials with mean pore sizes in the range from 20 to 40 nm (mesoporous) and 350 to 3250 nm (macroporous) are produced as well as hierarchically structured materials combining these pore size ranges. All surfaces are functionalized post-synthetically with the bifunctional reagent 3-(gylcidoxypropyl)-dimethylmethoxysilane to realize the covalent immobilization of the enzyme acethylcholinesterase (AChE) under ring opening of the epoxy group of the silane. In the following, the three different pore systems are compared in terms of their enzyme loading capacity and their response times. Due to the pore-size-dependent specific surface area, the loading capacities of the representative pore systems differ significantly, resulting in a loading of 4.1 µg of AChE per membrane with a macropore size of 3250 nm and an AChE loading of 38.5 µg per membrane for a mesopore size of 20 nm, whereby the hierarchically structured pore system with equivalent pore size ranges has a loading capacity of 15.5 µg AChE per membrane. The response time of the enzyme-catalyzed substrate degradation reaction of acetylcholine to choline and acetic acid is used to determine the apparent reaction rate, which is used to describe the efficiency of the individual pore structure and thus allows conclusions on intrinsic diffusion limitations. These investigations are conducted for all three pore systems at a constant enzyme loading to ensure comparability. At a loading of 4.1 µg AChE per membrane, the purely macroporous pore system exhibits a slight advantage over the hierarchically structured material, as it causes the fastest reaction, which is due to the lowest mass transfer limitations. By increasing the enzyme loading to 12.9 µg per membrane, it is evident that the hierarchically structured pore system shows a significant reduction of the response time, and thus is superior to the purely mesoporous material, as it combines the advantages of both monomodal systems, the improved mass transport and the higher enzyme loading. In conclusion, this study demonstrates systematic investigations to highlight the advantages of pore space hierarchy, which is characterized by combined functionality and transport efficiency.
Chapter 2 presents the urea-controlled synthesis of hierarchically structured silica monoliths, their transfer into a suitable column system and subsequent functionalization of the support surface in order to use them as flow microreactors. The hierarchy is generated by polymer-induced phase separation, which is an important step in the sol–gel process. The silica gels were synthesized using urea to create a mesoporous domain and to control the macroporous system. The influence of urea on the mean mesopore size is based on the base releasing property of urea by decomposition to ammonia and carbon dioxide under elevated temperature and has already been described in the literature. The resulting raise in pH increases the solubility of silica, whereby dissolution and deposition processes ensure that the initially microporous silica skeleton is expanded, resulting in a mesoporous domain. A common scientific approach is to add urea to the aqueous starting solution for a direct incorporation into the hydrogel to ensure a homogeneous pore expansion in the hydrothermal treatment. However, it is found that urea also has a strong influence on the macropore size and skeleton thickness of the obtained sol–gel monolith. With increasing urea content, the average macropore size is significantly reduced down to the submicron range. Urea influences the time sequences of gelation and phase separation due to an increase in the pH of the sol, which already occurs before substantial decomposition and additionally influences its polarity. Therefore, the gelation point occurs at an earlier stage of spinodal decomposition, which fixes a smaller macropore system. The synthesized monoliths with macropores in the submicron range are well-suited for the investigation of intrinsic reaction kinetics, since external and internal diffusion limitations are eliminated and hydrodynamic backmixing is reduced to a degree that a hydrodynamic plug flow behavior can be reached. For the application as flow microreactor, a novel cladding procedure is presented, which enables seamless housing of the sol–gel monolith in stainless steel tubing to withstand column backpressures >100 bar. By a special stop-flow functionalization method, aminopropyl groups are coated onto the silica surface generating a catalytically active column. The acquisition of reaction data for the reaction kinetics of the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate to trans-α-ethyl cyanocinnamate, which is a well-known test reaction for basic catalysts, is realized with a two-dimensional HPLC setup. The first dimension automates the adjustment of the desired reaction parameters for the microreactor and the second dimension allows the complete quantification of the reaction data by online HPLC. The entire reaction kinetics of the amino-catalyzed Knoevenagel condensation for five different reaction times at seven different reaction temperatures each is recorded in only about 400 minutes. The reaction data results in a pseudo first order reaction kinetics, which is due to the two-step reaction mechanism. The reaction data reveal a reaction behavior under quasi-homogeneous conditions, which confirms the absence of any transport limitations. In conclusion, hierarchically structured sol–gel monoliths are synthesized using urea as pore size controlling agent to obtain catalytic microreactors with a high active surface area, which allow for the investigation of intrinsic reaction kinetics without transport limitations.
Chapter 3 describes a new approach for the preparation of hierarchically structured, sulfonic acid modified silica monoliths based on the sol–gel process, whereby the functionalization is introduced into the pore system in situ during gelation. By using the co-condensation method, an alkoxysilane with a propylthiol function, ((3-mercaptopropyl)trimethoxysilane, MPTMS), is added together with the unfunctionalized silica precursor for sol formation. The synthesis of such organic-inorganic hybrid materials is widely used in the literature, but not for hierarchically structured sol–gel materials, where the hierarchy is generated via polymer-induced phase separation. Here, the incorporated polymer is usually removed from the material at high temperatures by an additional step, called calcination, to obtain the pure silica product. This treatment pyrolyzes any organic matter, which would also result in a loss of incorporated functionality. Therefore, this study presents an extraction of the polymer to avoid the loss of the covalently bound functionalization on the support surface, which simultaneously converts the introduced thiol groups into sulfonic acid functions, resulting in a time-efficient synthesis route. For this purpose, an extracting agent consisting of hydrogen peroxide and nitric acid is used. Macropore size control is demonstrated by the variation of polymer and functionalization reagent compositions, whereby the functionalization reagent has a significant influence on the onset of phase separation and consequently on the final macropore size. Furthermore, the widening process to form the mesoporous domain is strongly affected, resulting in very narrow mesopore distributions <10 nm with specific surface areas of up to 576 m2 g–1. The efficiency of the extraction procedure and the successful generation of sulfonic acid modified silica is extensively investigated and characterized to evaluate the presented approach. It is shown that, as the amount of MPTMS increases, the sulfur content and thus the loading of homogeneously distributed sulfonic acid groups is increased up to 1.2 mmol g–1 without significant loss due to the extraction. The polymer, however, is removed to a high degree during the extraction process. The covalent binding of the functionalization and the successful oxidation of the sulfur to form sulfonic acid functions is demonstrated by IR as well as 13C and 29Si MAS NMR spectroscopy. Moreover, investigations using inverse gas chromatography allow to investigate surface interactions and acid strength. The functionalized organic-inorganic hybrid monoliths have significantly higher surface energies, with the specific (polar) component being more dominant as the dispersive component. Furthermore, it is shown that these materials gain acid strength, which is generated by the incorporated sulfonic acid groups. In summary, a novel and efficient synthesis route for the preparation of hierarchically structured sol–gel monoliths with homogeneously distributed sulfonic acid functionalization is introduced.
In conclusion, this thesis improves the understanding of the individual steps of the sol–gel process for the preparation of hierarchically structured silica-based materials. These results are presented in the context of different possible applications, since their variability allows them to be adapted to diverse requirements and problems.