Investigation of direct methane-methanol conversion mechanism over the Cu sites in Cu-exchanged zeolites

From 22.07.2021 till 30.06.2023
Grant holder: Ilia Pankin

The objects under study in this project are copper exchange catalysts with chabazite (CHA) and mordenite (MOR) topology. Copper-exchange zeolites are considered as promising materials for the implementation of nature-like technology of direct methane-methanol conversion. These materials are considered as so-called "single-site" catalysts, where the active sites are represented either by isolated ions or by small groups of atoms, which provides wide variability in the formation of various configurations of copper active sites. The mechanism of operation of such diluted catalytic systems is similar to those of the nature enzyme monooxygenase in methanotrophic bacteria, which are also capable of direct conversion of methane to methanol. In recent works it was shown that copper-exchange zeolites can efficiently cleave the CH bonds of the methane molecule CH4, thereby stabilizing the intermediate forms of methyl, which can later be extracted already in the form of oxygen-containing forms, such as CH3OH, when interacting with water vapour. Selective catalytic direct conversion of methane to methanol (so-called MTM reaction) represents a huge potential for the development of the raw chemicals sector of the chemical industry and the oil and gas industry in general. In this consideration, one of the most important tasks in the field of applied catalysis is the development of effective catalysts for the implementation in the reaction of direct methane-methanol conversion. Due to the wide variability of the framework structure, a number of zeolites of various topologies were investigated for catalytic activity in the methane-methanol reaction, where the best performance was obtained for the following topologies MOR, CHA and MFI. However, in addition to the large variability of the framework structure, no less important is the question of clarifying and establishing the relationship between the local atomic structure of the active metal sites of zeolites and their catalytic properties, i.e. catalytic activity and selectivity. The study of the speciation of active copper sites (or other polyvalent metals) during activation and their further structural evolution during reaction at different temperatures underlie the importance of reaction mechanism understanding at the atomic level and, as a result, further improve the productivity and selectivity of catalysts. Currently, the issue of the role of metal dimers or higher-order oligomers in improving the catalytic properties of a number of catalysts based on copper-exchange zeolites is being actively discussed in the literature. In particular, Grunder et al. proposed a model of a trinuclear copper complex for copper-exchange mordenite MOR, where a similar structural topology of trinuclear sites in the monooxygenase molecule is noted, which is capable of selectively converting methane into methanol. However, in later works by Pappas et al., the significantly higher catalytic activity of copper-exchanged zeolites with CHA (1.25 μGy / mol) and MOR (0.47 mol CH3 / mol Cu, normalized conversion) topology was demonstrated, where monomeric/dimeric and dimeric sites have been proposed as catalytically active for the catalysts based on Cu-CHA and Cu-MOR, respectively. In particular, two possible mechanisms of the formation of copper oxide complexes as a result of high-temperature activation of Cu-CHA in an oxygen atmosphere were proposed. The structural models proposed in this work were proposed on the basis of quantitative or semi-quantitative analysis according to the principle of "fingerprint". At the same time, the conclusions drawn about the reactivity and possible ways of implementing the proposed mechanisms were not supported by studies based on quantum chemical calculations. Also, it should be noted that in few recent works the very important information on the copper sites speciation in copper-exchange zeolites was obtained based on X-ray absorption spectroscopy. In particular, in addition to EXAFS analysis, XANES spectroscopy (X-ray Absorption Near Edge Structure) attracts much attention of researchers, due to its atomic selectivity and sensitivity to both local atomic and charge states of Cu sites in copper-exchange catalysts. The later can be determined directly from the experimental data on the course of the experiment (operando regime). It should be noted that despite the most important results, based on the analysis of experimental XANES data in a number of recent works using methods that allow one to isolate the spectra of the principal components and then reconstruct their concentration profiles often the interpretation of the received signal is carried out on the basis of generalized information about the systems under study, which makes this method of interpretation not straightforward and often ambiguous. The scientific novelty of this project lies in the fact that within the framework of solving the task of establishing the reaction mechanism and the relationship between the configuration of active sites and the productivity of samples, it is planned to simulate a wide selection of structural models of active Cu sites in zeolites with topology CHA and MOR based on DFT methods using genetic algorithms for structure prediction. Afterwards assessing their ability to activate methane molecules can be applied as criteria of ranking. Finally refining the structural parameters selected as the most effective structural models will be performed using XANES simulations by means finite difference method and applying algorithms based on supervised machine learning. Such a systematic and generalized approach for modelling both thermodynamic quantities and refining the structural parameters of Cu sites will be implemented for the first time and, in combination with XANES simulations and comparative analysis with experimental data, should unambiguously establish the relationship between catalyst topology (including stoichiometric Si: Al and Cu: Al) <> active site structure <> catalyst productivity. Moreover, this approach is intended to provide the most complete interpretation of experimental XANES spectra using a direct method based on simulation results. We expect that obtained results will allow developing the most probable mechanisms of the MTM reaction under various regimes i.e. at different temperatures and partial pressure of CH4. In the future, understanding the reaction mechanism of the previously mentioned interrelationships: topology <> structure of an active site <> productivity should lead to more efficient catalysts and the implementation of a natural-like technology for direct conversion of methane-methanol.