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

Abstract

Temperature-programmed-reduction studies combined with advanced spectroscopy and data analysis methods shed light into the dynamic changes of Cu-speciation during light alkane selective oxidation over Cu-mordenite zeolites.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper‐Containing Mordenite: Insights into Reaction Mechanism and Product Protection

“…The results show a pronounced difference between the reaction of the copper(II) sites with methane and ethane/propane and are in line with the recent report by Kvande et al. showing that Cu II reduction with ethane occurs at lower temperature as compared to the reduction with methane [63] …”

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

“…3650 cm –1 ) using in situ FT-IR, and a concomitant increase in Raman features assigned to Z 2 Cu 2 O from 523 to 673 K in their CHA materials. Similar observations and proposals of monomeric sites converting to dimers have been invoked for MOR using XANES, CO-TPR, electron paramagnetic resonance, operando UV–vis, in situ photoluminescence, and FT-IR . Moreover, the proximity of ZCuOH sites, altered by varying Si/Al and Cu/Al for a given zeolite, has been identified as an important factor for achieving higher methanol yields (per mol Cu). , This proximity requirement is consistent with our finding that only 2Al configurations within Al–Al separations of <8.2 Å (CHA) and <9.4 Å (MOR) convert to dimers (Figure S5.1) at high temperatures.…”

“…Therefore, the probability ( p i , s ) for species s to form at 2Al site i is p i , s = exp ( − Δ G i , s R T ) ∑ s A l l .25em C u .25em s p e c i e s .25em exp ( − Δ G i , s R T ) This approach does not assume any Al distribution and gives Cu speciation probabilities for all 2Al configurations in each zeolite. To begin, we chose CHA and MOR (other zeolite topologies are discussed later) due to their topological differences (Figure a) and because previous studies have shown that Cu-dimers are the majority Cu motif in MOR at a wide range compositions, ,− whereas prior experimental and computational results show Cu preferentially occupies the 6MR in CHA forming Z 2 Cu monomers. ,,,,, …”

“…These results suggest that Z 2 Cu 2 O is a reasonable computational probe for estimating the equilibrium total dimer fraction at high temperature dry conditions. Moreover, previous experimental studies have shown spectroscopic evidence for Z 2 Cu 2 O as the majority Cu dimer site in Cu-MOR, 61,66,68,83 Cu-MFI, 53,84,85 and zeolite-Y 86 following exposure to high temperature dry air treatments. Therefore, for the next set of widely used zeolites we studied, BEA, AFX, and FER, we used computed free energies for only Z 2 Cu, ZCuOH, and Z 2 Cu 2 O species.…”

“…This result suggests that in most samples only a fraction of the total binuclear Cu sites react with CH 4 to form CH 3 OH at the reported O 2 and CH 4 activation conditions, consistent with the observations of many experimental reports. 8,57,61,68,130 To compare across MOR, CHA, BEA, AFX, and FER, we used the most common Al distribution biases described in the literature for each zeolite along with Loẅenstein's rule. Specifically, we prohibited Al−O−Si−O−Al configurations for MOR, BEA, and FER, used the Al T-siting bias described above for commercial MOR materials, and random Al distributions for CHA and AFX.…”