Periodic Reporting for period 2 - Pores4Olefins (Engineering Pores for Sustainable Catalytic Olefins Upgrading)
Período documentado: 2023-07-01 hasta 2024-06-30
The research project of this fellowship deals with chemical reactions to upgrade alkenes (hydrocarbons with C-C double bonds, also known as olefins). Reactions that form new C-C bonds are particularly significant among those involving alkenes to obtain value-added chemicals. For instance, metathesis reactions of alkenes create new molecules by breaking and rearranging the C-C double bonds of the initial alkenes, while the dimerization and hydroarylation of alkenes allow C-C coupling of two molecules to yield one heavier molecule. Currently, all of these reactions are paramount, taking place on an industrial scale, for the production of a plethora of chemicals, ranging from small alkenes used in the manufacturing of plastics and polymers to alkylbenzenes used for the synthesis of surfactants and detergents. These reactions are carried out in the industry in the presence of catalysts. Nevertheless, in most cases, although the processes are practical from an industrial point of view, the understanding of how the reactions occur is still very poor. Increasing the understanding of these reactions is therefore key to developing new catalytic processes that are not only more economically efficient but also environmentally friendly (less energetically demanding, avoiding or forming fewer side-products and residues, using non-toxic catalysts, etc.). Following this line, this project of fundamental research is focused on gaining new knowledge regarding the heterogeneous catalysis of metathesis, dimerization, and hydroarylation of alkenes. The research is mainly centered on better understanding what the active sites that catalyze these reactions are and how they are formed. With the benefit of this knowledge, it will be possible to develop new catalysts and processes that perform these types of reactions more efficiently and even combine more than one of these reactions in a single stage (tandem catalysis). Specifically, this research studies the metathesis and dimerization reactions of light alkenes to develop a single-stage process for the production of propene from ethene using catalysts based on Mo and Ni. Developing new routes for the production of propene is very important because, with the current demand for propene, existing technologies may not meet market needs in the coming decades. The alkene hydroarylation catalyzed by Ni or Co is also examined for the production of linear alkylbenzenes that might be used, especially, for the synthesis of more biodegradable surfactants and detergents. Furthermore, a deeper understanding of these reactions is required to develop new efficient processes for the upgrading of platform molecules obtained from biomass feedstock, which are more complex (because of O2 presence) than those obtained from fossil resources that are ubiquitously found in the current chemical industry.
During the outgoing phase of this fellowship (first and second years) in the University of California, Berkeley (USA), the studies focused on the alkene metathesis reaction, particularly on the propene metathesis catalyzed by Mo-based materials (Mo oxides supported on SiO2 MCM-41) as a model system. Different catalysts were used, varying the preparation methods and Mo surface densities. Thermal and chemical pretreatments of the catalysts were performed with different molecules (O2, He, H2, CO, alkenes), allowing for new insights into the formation of active sites for the alkene metathesis reaction and high metathesis rates (0.1 Mo s-1, 278 K) similar to those higher ever reported but at much higher reaction temperatures (473 K). Kinetic measurements enabled an understanding of the elementary steps. A reaction model consistent with the kinetic measurements and elementary steps was developed, enhancing the understanding of the reaction network. This model highlights the importance of reaction inhibition by products and provides a method for the dynamic measurement of surface intermediates that act as surrogates for active sites.
For the returning phase of this fellowship (third year), the studies focused on alkene hydroarylation using molecular catalysts for the selective formation of linear alkylbenzenes. This investigation found that Ni(0) N-heterocyclic carbene (NHC) molecular catalysts can be formed in situ starting from Ni(II) salts, NHC substrates, and a reducing agent without the need to form and isolate Ni(0)-NHC in previous steps, as was the state-of-the-art until now. This procedure allows high conversions and selectivity to the linear alkylbenzene isomer for a model reaction involving benzene as the arene and 3,3-dimethyl-1-butene as the alkene (120 ºC, 6 h, >95% conversion, >92% yield). This finding is promising for the stability and immobilization of Ni-NHC complexes, which would provide the corresponding heterogeneous catalysts because the Ni center could be preserved in the form of Ni(II) until the moment of reaction (reducing it to active Ni(0)). The attempt to immobilize Ni-NHC molecular catalysts was unsuccessful, as the resulting materials were inactive for alkene hydroarylation. The use of Co instead of Ni for the metal salts to form metal-NHC complexes did not result in active species for alkene hydroarylation catalysis. The work was carried out in collaboration between the University of Oviedo (Spain) and the Leibniz Institute for Catalysis (Germany) through a six-month secondment of the researcher at the latter organization.
Two manuscripts are in preparation to be published in scientific journals related to the catalysis field. The most interesting results in alkene metathesis were presented as an oral contribution at a scientific conference (28th North American Meeting of the North American Catalysis Society). The project was also presented at an event at the Faculty of Chemistry of the University of Oviedo.
For the returning phase of this fellowship (third year), the studies focused on alkene hydroarylation using molecular catalysts for the selective formation of linear alkylbenzenes. This investigation found that Ni(0) N-heterocyclic carbene (NHC) molecular catalysts can be formed in situ starting from Ni(II) salts, NHC substrates, and a reducing agent without the need to form and isolate Ni(0)-NHC in previous steps, as was the state-of-the-art until now. This procedure allows high conversions and selectivity to the linear alkylbenzene isomer for a model reaction involving benzene as the arene and 3,3-dimethyl-1-butene as the alkene (120 ºC, 6 h, >95% conversion, >92% yield). This finding is promising for the stability and immobilization of Ni-NHC complexes, which would provide the corresponding heterogeneous catalysts because the Ni center could be preserved in the form of Ni(II) until the moment of reaction (reducing it to active Ni(0)). The attempt to immobilize Ni-NHC molecular catalysts was unsuccessful, as the resulting materials were inactive for alkene hydroarylation. The use of Co instead of Ni for the metal salts to form metal-NHC complexes did not result in active species for alkene hydroarylation catalysis. The work was carried out in collaboration between the University of Oviedo (Spain) and the Leibniz Institute for Catalysis (Germany) through a six-month secondment of the researcher at the latter organization.
Two manuscripts are in preparation to be published in scientific journals related to the catalysis field. The most interesting results in alkene metathesis were presented as an oral contribution at a scientific conference (28th North American Meeting of the North American Catalysis Society). The project was also presented at an event at the Faculty of Chemistry of the University of Oviedo.
This research project has contributed to deepening the understanding of the nature and formation of alkene metathesis active sites and the reaction pathways using propene metathesis as a model reaction on Mo-based catalysts with silica as the support. The knowledge obtained from these alkene metathesis studies will help develop a strategy for the production of propene from ethene in a single-stage process through tandem catalysis. Additional experimentation on alkene hydroarylation enabled the synthesis of selective Ni-based molecular catalysts for the formation of linear alkylbenzenes. These molecular catalysts were formed in situ under reaction conditions and activated in the presence of a reducing agent, which is promising for the immobilization of these catalysts on modified-silica supports, allowing for heterogeneous catalysts that are more interesting for catalysis practice (catalyst separation and recovery, environmental issues).
The knowledge obtained from this fundamental research could encourage other applied research projects that would improve current catalytic processes used in the industry, as well as lead to the emergence of new catalytic processes for the production of many chemicals in compliance with more environmentally friendly practices as demanded by society.
The knowledge obtained from this fundamental research could encourage other applied research projects that would improve current catalytic processes used in the industry, as well as lead to the emergence of new catalytic processes for the production of many chemicals in compliance with more environmentally friendly practices as demanded by society.