Cation-induced speciation of port-size during mordenite zeolite synthesis.

Mordenite (MOR) zeolite, an important industrial catalyst exists in two, isostructural variants defined by their port-size, small and large-port. Here we show for the first time how a systematic, single-parameter variation influences the synthesis out-come on the final MOR material leading to distinctly different catalysts. The cation identity has a direct impact on the synthesis mechanism with potassium cations generating the more constrained, small-port MOR variant compared to the large-port obtained with sodium cations. This was expressed by different degrees of accessibility ascertained with a combination of toluene breakthrough and temperature programmed desorption (TPD), propylamine TPD, as well as sterically sensitive isobutane conversion. Rietveld refinement of the X-ray diffractograms elucidated the preferential siting of the smaller sodium cations in the constricted 8-ring, from which differences in Al distribution follow. Note, there are no organic structure directing agents utilized in this synthesis pointing at the important role of inorganic structure directing agents (ISDA).

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments.

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH4, C2H6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH4 adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different CuII and CuI components were found. Close correlations were found between the XANES-derived CuII to CuI reduction, CH4 consumption, and CO2 production. A reducibility-activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the CuII components consist of multimeric CuII-oxo sites, albeit with different Cu-Cu distances.

Cu-loaded zeolites enable the selective activation of ethane to ethylene at low temperatures and pressure.

Cu-zeolites are found to activate the C-H bond of ethane already at 150 °C in a cyclic protocol and form ethylene with a high selectivity. Both the zeolite topology and Cu content are found to impact the ethylene yield. Ethylene adsorption studies with FT-IR, demonstrate that oligomerization of ethylene occurs over protonic zeolites, while this reaction does not occur over Cu-zeolites. We postulate that this observation is the origin of the high ethylene selectivity. Based on the experimental results, we propose that the reaction proceeds via the formation of an ethoxy intermediate.

EXAFS wavelet transform analysis of Cu-MOR zeolites for the direct methane to methanol conversion.

Cu-exchanged zeolites have been shown to possess Cu-oxo species active towards the direct methane to methanol (DMTM) conversion, carried out through a chemical-looping approach. Different Cu-zeolites have been investigated for the DMTM process, with Cu-mordenite (Cu-MOR) being among the most active. In this context, an accurate determination of the local structure and nuclearity of selective Cu-oxo species responsible for an efficient DMTM conversion still represents an ongoing challenge for characterization methods, including synchrotron-based X-ray absorption spectroscopy (XAS). Herein, we explore the potential of an alternative analysis of Extended X-ray Absorption Fine Structure (EXAFS) data using wavelet transform (WT) to enhance the technique sensitivity to multimeric Cu species hosted in the MOR framework. Combining ex situ XAS measurements under model red-ox conditions with in situ data collected after the key steps of the DMTM process, we demonstrate how EXAFS-WT enables unambiguous detection of Cu-Cu scattering contributions from multimeric Cu-species. As also confirmed by complementary in situ IR spectroscopy results, these are observed to dynamically respond to the chemical environment over the different conditions probed. We finally report a proof-of-concept EXAFS fit using the WT representation, applied to the structural refinement of O2-activated Cu-MOR. The fitting results reveal a Cu local coordination environment consistent with mono-(µ-oxo) di-copper cores, with Cu-Cu separation of ∼3.1 Å, paving the way to future applications and developments of the method in the field of Cu-zeolite research and beyond.

The Nuclearity of the Active Site for Methane to Methanol Conversion in Cu-Mordenite: A Quantitative Assessment.

The direct conversion of methane to methanol (MTM) is a reaction that has the potential to disrupt a great part of the synthesis gas-derived chemical industry. However, despite many decades of research, active enough catalysts and suitable processes for industrial application are still not available. Recently, several copper-exchanged zeolites have shown considerable activity and selectivity in the direct MTM reaction. Understanding the nature of the active site in these materials is essential for any further development in the field. Herein, we apply multivariate curve resolution analysis of X-ray absorption spectroscopy data to accurately quantify the fraction of active Cu in Cu-MOR (MOR = mordenite), allowing an unambiguous determination of the active site nuclearity as a dicopper site. By rationalizing the compositional parameters and reaction conditions, we achieve the highest methanol yield per Cu yet reported for MTM over Cu-zeolites, of 0.47 mol/mol.