Valorization of Biomass-streams via Catalytic Reductive Amination: The challenge of Vicinal Aldehydes and Alcohols

The start of the industrial revolution has led to great economic welfare and population growth, with an increasing demand for energy and commodity goods largely fed by the exploitation of crude oil. Today, we see the first negative effects of this economic model: climate change, depletion of cheap oil reserves and environmental pollution as result of single-use consumer goods. General consensus is growing to switch to a circular economy with minimal waste production. It is important to consider the availability of CO2-neutral technologies for the future demand in energy and transportation, such as wind and solar power. But commodity goods, such as detergents and plastics, will always have the need for a decent carbon source to create their chemical structure. Recycling and bio-degradation are important aspects to lower our environmental footprint, but these goods cannot be labeled as 'sustainable' through the continued use of crude-oil as carbon source. Amines are an important class of daily consumer goods, chemical compounds containing one or more nitrogen atoms. They are common intermediates for agrochemicals, pharmaceuticals, polymers, detergents or CO2 adsorbents, currently produced from petrochemical platforms such as ethylene. The emissions of these amine synthesis routes are however a major contribution to global warming and production commonly involve toxic and explosive intermediates. Considering these issues don't comply with many green chemistry principles, it might be time to look into alternative chemical pathways. In this research, novel processes and catalysts were investigated to obtain our commodity amine chemicals in a safe and sustainable manner. The choice for biomass as renewable carbon feedstock is herein promising to drastically lower the net CO2-emissions and waste production that accompanies the conventional production routes. Biomass however contains a vast amount of heteroatoms, such as vicinal carbonyl and hydroxyl groups, which is in sharp contrast to the non-functional hydrocarbon composition of crude oil. In essence, catalysis that has been historically developed in function of petrochemistry, needs to be re-invented in order to cope with this new resource and will be of major importance to implement for instance a sustainable biorefinery concept. From an economic perspective, sustainability will always have to compete with the low production cost, high product yield, separation- and product specifications of proven fossil-based standards. Petro-refineries achieve these outstanding results through some stronghold aspects such as an integrated platform approach and its large scale, which makes it affordable for consumers. This platform approach allows to steer the process between multiple end products according to market fluctuations whilst the process can be run continuously at large-scale. It will be highly essential to apply these ideas to the biorefinery concept as well to achieve the goal of a circular economy in a price-competitive manner. With these terms in mind, the first challenge was to define a suitable platform molecule derived from biomass that can be obtained via safe processes. An investigation of the state-of-the-art towards structural and sustainable analogues of ethylene oxide (EO) revealed monoethylene glycol (MEG) as potential candidate to obtain the same C2 product scope. More interestingly, renewable MEG is obtained via a retro-aldol fragmentation of carbohydrates, and glycolaldehyde (GA) frequently occurred as key intermediate in renewable pathways to the EO-derived products. Being a reactive C2 molecule that contains both an vicinal aldehyde and alcohol group, it has potential to deliver many new opportunities for our chemical industry. A holistic biorefinery concept was therefore proposed based on glycolaldehyde as sustainable and safe platform molecule for C2 chemistry. New technologies to produce GA are arising, in particular the hydrous thermolysis of Haldor Topsoe that currently runs at demo-scale. Here, the potential of this vicinal aldehyde was investigated as substrate for reductive amination reactions, for the sustainable production of commodity C2 amine chemicals. With selectivity control being the main challenge, choice of solvent such as methanol (MeOH) was key to achieve a high mass balance, in the presence of noble metal catalysts. Theoretical findings via solvent-sensitive DFT calculations elucidated a solvent effect on reaction rate and selectivity in term of energetics and kinetics, with quantitative yields to alkanolamines such as N,N-dimethylaminoethanol (DMAE). Interestingly, sub-stoichiometric amine to GA ratio allows for high yield formation of higher (consecutive) alkanolamines such as N-methyldiethanolamine (MDEA) and triethanolamine (TEOA), for which a peculiar cyclic 5-membered oxazolidinic precursor was analyzed. A shift from alkanolamines to diamines such as N,N,N',N'-tetramethylethylenediamine (TMEDA) could be realized by switching to a two-step one-pot approach. With MEG as solvent, high yields of an unsaturated C2-ene-diamine precursor could be obtained in the first step under inert atmosphere. The successful preparation and sensory assessment of a diester quat as fabric softener demonstrates the viability of this drop-in production route, starting from glycolaldehyde. The second part of this research unraveled process and catalyst composition requirements for the liquid amination of (fatty) alcohols. These components are commonly found in palm- and coconut oil and represents the hydroxyl group functionality in biomass. In contrast to vicinal aldehydes, this functionality requires a higher reaction temperature, which brings along new selectivity issues. Literature suggested a heterogeneous Cu0-NiIIO composition of the catalyst to allow for high product selectivity towards alkyldimethylamines (ADMA). Through intense catalyst parameterization, the optimal catalyst composition was found to be a 20 wt.% Cu/Ni deposition on a 13X zeolite support, with highest activity at a metal ratio of 4. Three fractional effects were elucidated during catalyst synthesis, provoking a different precipitation rate between Copper and Nickel. The ion-exchange properties of the zeolite framework, a dropwise addition of Na2CO3 to induce metal precipitation, and a difference in thermal decomposition of the resulting metal carbonates, proved essential to obtain the proposed catalyst composition. A hypothesis was suggested that Nickel acts a structure-directing agent to allow a high Copper dispersion, essential for catalyst activity. Simultaneously, presence of Nickel in the Copper matrix is capable of being reduced more easily through a hydrogen-spillover between the metals. This may create a secondary active site (Ni0) on the surface supposedly responsible for catalyzing the amine disproportionation. Since this unwanted side-reaction is bimolecular, suppression can be achieved by keeping the partial substrate-to-amine surface coverage near this Nickel site high. This can be done via tuning of the catalyst properties (limit accessibility to secondary active site) and optimization of process conditions (substrate ratio control on catalyst surface). For example with high metal loadings on the catalyst it is possible that these Nickel sites are partially encapsulated by the bulky precipitation of Copper. From a process perspective, a variable DMA feed is demonstrated to further enhance the ADMA selectivity. The screening of other primary alcohol substrates from biomass demonstrated this to be a versatile catalyst. As a path forward, follow-up experiments are needed to fully demonstrate the catalyst potential in terms of recycling and stability. This is preferably done at larger scale, for which little issues are expected. Although our hypothesis on the dual role of Nickel holds for this research, additional proof could still be delivered. Additional characterization such as direct Cuo site measurement and its link with the catalytic activity will be necessary, as well as testing of additional reference catalysts. In order to reach quantitative yields in the near future, conceptual improvements to the catalyst composition are suggested in combination with additional process and reactor optimization.status: publishe