$N$-Vinyl-2-Pyrrolidonmonomere auf Basis biogener Carbonsäuren : Design heterogener Katalysatoren und Prozessbewertung
- $N$-vinyl-2-pyrrolidone monomers from biogenic carboxylic acids: heterogeneous catalyst design and process evaluation
Haus, Moritz Otto; Palkovits, Regina (Thesis advisor); Pich, Andrij (Thesis advisor)
Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2021
The utilization of biomass in chemical production continues to attract scientific, political, and corporate attention due to its potential for sustainable development - most notably regarding CO2 emissions. However, as of today, an integrated network of biomass-based conversion technologies, which could mimic and rival the petrochemical sector, remains under construction. Thus, the search for enabling catalytic materials and effective value chains remains an acute topic of chemical research. In this context, detailed evaluations of the projected economic and environmental impacts associated with bio-based production have also gained traction. They serve as motivation for decision-makers in politics and industry, whilst also highlighting the critical trade-offs in biomass cultivation forchemical applications. Within this broader context, the thesis at hand discusses a two-step value chain from chosen, biomass-based platform chemicals - dicarboxylic acids, such as succinic acid - to N-vinyl-2-pyrrolidone (NVP) monomers. The process starts by converting the acid substrate with monoethanolamine and hydrogen in an aqueous environment to generate an N-(2-hydroxyethyl)-2-pyrrolidone intermediate. The latter undergoes subsequent gas-phase dehydration to NVP so that water appears as sole by-product in an idealized scenario. Together with the use of recyclable, heterogeneous catalysts in both stages, this allows for an efficient access to high-value, nitrogen-containing monomers starting from biomass. First, chapter 5.1 serves as an overview of the entire two-stage conversion, which includes the underlying reaction networks, the performance of commercial catalysts and the influence of reaction conditions. Based on the presented results, ruthenium on carbon (Ru/C, 5 wt. % metal) and sodium-impregnated silica (Na2O/SiO2, molar ratio 1:20) were established as benchmark catalysts for stage one and two, respectively. While total yields around 0.7 molNVP mol-1 acid were achieved with these systems, improvements in catalyst activity and selectivity are desirable. Especially, the reduction of amide/imide intermediates in stage one of the value chain necessitates harsh conditions (150-200 °C, 150 bar H2) and yields by-products from overreduction and oligomerization .As shown in chapter 5.2, these issues were partially overcome by optimized bimetallic catalysts (Pt-Re/TiO2) due to the synergy between platinum and rhenium in the reduction of carboxylic acid derivatives. In this context, it is evident that rhenium oxides deposited on or near Pt0-surfaces are partially reduced after catalyst pretreatments in hydrogen. The density of the resulting Pt0/ReOx-y active sites, which can be modified by the catalyst preparation procedure, correlates with activity. Most notably, the functionalization of the TiO2 surface with ReOx species prior to the impregnation of a positively charged Pt-complex led to the highest dispersion. When rhenium was impregnated on a Pt/TiO2 parent material, on the other hand, the density of active sites was determined by the Pt-dispersion of the parent and the extent of Pt-surface blocking by rhenium species. Given this structure-activity relationship, further attention was devoted to the complementary understanding of Na2O/SiO2 materials in the second stage of the value chain (chapter 5.3). Thus, surface silanols (Si-OH groups) and basic sites caused by sodium impregnation were found to define this catalyst class. In detail, optimal catalyst activity was achieved at low sodium loadings, through the formation of weakly basic Si-ONa following the preferential ion exchange of isolated Si-OH groups. The excessive neutralization of surface silanols at high sodium content was, however, counterproductive, highlighting the role of Si-OH in substrate adsorption and catalysis.Given the combined knowledge of these investigations, chapter 5.4 explores the economic and environmental merit of the proposed value chain based on operating cost and life cycle assessment, respectively. The early-stage evaluation based on realistic, but simplified process simulations underlines the central role of catalyst performance for the outcome in both categories. In detail, improved hydrogenation catalysts (stage one, as compared to Ru/C) alone may reduce the operating costs of biomass-based NVP production by up to 27 % (4.59 kg-1NVP vs. 6.25 kg-1NVP, assuming 2.5 kg-1acid). While this reduction would be necessary to compete with the best-case scenario of fossil-based production at current succinic acid prices, a further commercial development of the biomass-based platform would likely reduce the costs of the proposed value chain. Furthermore, the carbon credit of biomass as compared to fossil feedstocks leads to a more favorable comparison in terms of global warming impact (GWI). Here, the new value chain may reduce the impacts of monomer production by at least 41 % (as compared to the non-ideal fossil case, e.g. 4.49 kgCO2 kg-1NVP vs. 7.60 kgCO2 kg-1NVP), if new catalyst technologies and/or alternative, lignocellulosic feedstocks can be implemented. Thus, further research and scale-up testing is warranted.