Copper-zeolites Prepared by Solid-state Ion Exchange - Characterization and Evaluation for the Direct Conversion of Methane to Methanol

Abstract

Direct conversion of methane to methanol (MTM) over Cu-zeolites is a so-called “dream reaction” for the chemical industry. There is still a lot that can be done in order to optimize the reaction by e.g. achieving a deeper understanding of the reaction mechanism and the nature of the Cu-sites. In this study, we investigated a solid-state ion exchange method to incorporate CuI ions into zeolites (MOR, BEA, ZSM-5 and FAU), as a more scalable technique. The solid-state ion exchange led to a Cu/Al ration of about 0.8, however with a heterogeneous distribution of Cu. Regardless, Fourier transform-infrared spectroscopy still revealed that most Brønsted acid sites were exchanged in all four samples. Further, CH4-temperature programmed reaction experiments showed that some Cu-sites formed were reactive towards CH4, with CuI-MOR and CuI-FAU having the largest CH4 consumption. Ultimately, the CuI-zeolites were tested in the MTM reaction and proved capable of producing methanol, even without the presence of Brønsted sites. A MOR with lower Cu/Al ratio (0.30) was also tested for comparison, and as this sample obtained a much higher productivity than the CuI-MOR with high Cu-loading (0.10 vs. 0.03 molMeOH/molCu), it was demonstrated that some fine-tuning is necessary to obtain the active Cu sites for methane activation.

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

“…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. Our simulations show that on the extreme ends, MOR has the highest fraction of Cu dimers at all compositions, and BEA has the lowest dimer population, which is consistent with the significantly lower PMO yields reported for BEA compared to CHA and MOR with similar compositions. ,,, The mol CH 3 OH/mol Cu values for AFX, FER, and BEA are again near to or below the line demarcating one CH 3 OH per model-predicted binuclear Cu site (Figure S9.2). FER methanol yields for a random Al distribution (Figure S9.3b) are far above this threshold, suggesting these FER materials do not contain Al–O–Si–O–Al configurations at Si/Al > 8, as reported by previous studies. , At a typical composition of Si/Al = 12, Cu/Al = 0.3, the ordering from most to least Cu dimers is MOR(0.92) ≫ CHA(0.45) > FER(0.34) > AFX(0.32) > BEA(0.28), consistent with the ordering of the maximum mol CH 3 OH/mol Cu that is reported for these five zeolites at similar compositions ,, (Si/Al 8–15, Cu/Al 0.3–0.5) following cyclic PMO treatments.…”

“…Our simulations show that on the extreme ends, MOR has the highest fraction of Cu dimers at all compositions, and BEA has the lowest dimer population, which is consistent with the significantly lower PMO yields reported for BEA compared to CHA and MOR with similar compositions. 18 , 19 , 126 , 131 The mol CH 3 OH/mol Cu values for AFX, FER, and BEA are again near to or below the line demarcating one CH 3 OH per model-predicted binuclear Cu site ( Figure S9.2 ). FER methanol yields 132 for a random Al distribution ( Figure S9.3b ) are far above this threshold, suggesting these FER materials do not contain Al–O–Si–O–Al configurations at Si/Al > 8, as reported by previous studies.…”

“…Although the methanol produced in cyclic PMO over different zeolites depends on the Al distribution, activation conditions, ,,,− and Cu-exchange protocol, ,− some zeolite topologies, ,, and specific compositions for a given topology, have generally been reported to have higher methanol per Cu yields. While we emphasize our goal here is not to determine specific active sites and activities for PMO, we next tested if observed trends in methanol yields correlate with our predicted total dimer populations.…”

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

“…16 The mentioned zeolites feature a combination of large and small ring systems, rendering it difficult to determine which zeolite properties are most important for methane activation and methanol formation. However, well-distributed Cu ions, 17 as well as an intermediate loading of Cu have been identied as important requirements to achieve the highest methanol yields. 10,18 This is linked with the need to have Cu ions in suitable proximity to form both accessible and reducible multimeric species, while at the same time retaining a sufficient number of Brønsted acid sites to effectively aid in stabilizing the intermediates.…”

Cu(dppf) complexes can be synthesized from Cu-exchanged solids and enable a quantification of the Cu-accessibility by 31P MAS NMR spectroscopy

“…In particular Cu-MCM-22 (MWW structure) is applied herein, as this material was recently shown to be active in the oxidation of methane to methanol. 13 The amount of external ion exchange sites was investigated using triphenylphosphine, which was applied on the H-form of the material according to literature. 44 In the spectrum in Fig.…”