Cu-Based Catalytic Sites for Methane to Methanol: Comparison
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Direct conversion of methane to methanol is an effective and practical process to improve the efficiency of natural gas utilization. Copper (Cu)-based catalysts have attracted great research attention, due to their unique ability to selectively catalyze the partial oxidation of methane to methanol at relatively low temperatures. Many different catalysts have been studied to achieve a high conversion of methane to methanol, including the Cu-based enzymes, Cu-zeolites, Cu-MOFs (metal-organic frameworks) and Cu-oxides.

  • methane to methanol
  • copper
  • active site
  • structure characterization

1. Introduction

Nowadays, extensive consumption of energy for transportation, electricity and heat are continuously increasing due to the fast growth of global population and industrial production. However, the primary energy resources are still fossil fuels, even if there are also some alternative renewable energy sources [1][2][3][4][5]. With the sustainability and environmental concerns, considerable efforts have been devoted to the progressive exploration of renewable energy sources. Methane, as a greenhouse gas and an earth-abundant carbon feedstock, is mainly reserved in natural gas [6]. However, the wide application and effective utilization of methane meet the access and transportation challenges. Therefore, the effective catalytic conversion of methane to value-added chemicals has drawn considerable attention [7][8][9][10][11][12].
Because the C−H bonds in methane are quite stable (438.8 kJ/mol), the conversion of methane under moderate reaction conditions is a long-standing challenge [13]. However, its valuable products are readily converted to CO and CO2 at high temperatures. The dilemma thus makes the upgrading of methane more complicated [14]. Methanol is thought of as one green raw material for biodiesel and a very important chemical feedstock as well, which can be converted to many different commodities via well-developed techniques [15][16][17]. In the current industrial route, methanol can be formed from methane through a syngas intermediate. However, this indirect process requires high-temperature and high-pressure conditions, requiring large energy consumption [18]. Therefore, the partial oxidation of methane to methanol directly was proposed as the holy grail in chemistry [19]. Many observations and conclusions on the direct conversion of methane to methanol have been reported. Recently, several valuable review articles have already been published, which focused on biological methane oxidation [20] and zeolite-supported metal catalysts for methane conversion [21][22].

2. Cu-Based Enzymes

Methanotrophs, a kind of methane-consuming bacteria living in nature, have the ability to oxidize methane to methanol with their methane monooxygenases (MMOs) at an ambient temperature and pressure. Two kinds of MMOs exist in bacterium; a copper-based particulate membrane-bound enzyme (pMMO) and an iron-based soluble cytoplasmic enzyme (sMMO). The active structure of sMMO is a diiron active site, but the catalytic site of pMMO has remained unclear. Recently, many scientists have devoted their efforts to unveiling the structure of pMMO and its biochemistry on methane oxidation [23][24][25].

2.1. The Trinuclearcopper Sites Found in Enzymes

Extensive efforts have been made to reveal the catalytic center of the pMMO enzyme to understand its working mechanism. Because pMMO is an instable complex membrane protein composite, it is rather difficult to be isolated and purified for characterization. In 2004, Chan et al. proposed that the active sites in pMMO were trinuclear copper clusters, which worked for alkane hydroxylation or electron transfer [26]. They then demonstrated the trinuclear copper clusters by an electron paramagnetic resonance spectroscopic (EPR) test, in which the copper ions for alkane hydroxylation were reduced and the intensity of the CuII EPR signal decreased by increasing the negative potentials [27]. Via a simulation strategy, they confirmed that the EPR signal was assigned to 0.84 CuII ions at the negative potential of −53.0 mV, which consisted of 0.56 type 2 CuII ions and 0.29 trinuclear CuII clusters. At a more negative potential of −121.0 mV, the EPR intensity was only attributed to 0.13 trinuclear CuII clusters. Therefore, these spectroscopic data showed concrete evidence to confirm the presence of trinuclear copper clusters [27]. In addition, they also modeled a minimized trinuclear copper site computationally, considering the side-chain rotomers, hydrogen bonds, and metal–ligand bonds [27]. Via the anaerobic electrospray mass spectrometry, Chen et al. found that the activation of a tri-copper cluster with O2/H2O2 was similar to that of pMMO, also supporting the trinuclear copper cluster theory [28].

2.2. The Dicopper Sites Found in Enzymes

The pMMO comprises three polypeptides that are encoded by pmoB, pmoA and pmoC genes. The isolation and purification process should be much elaborate to keep the original statement of the catalytic centers. It is thus a great challenge to determine the nuclearity, ligation and position of the copper sites in pMMO. Advanced characterization techniques were used to deepen researchers' understanding of the catalytic center of the pMMO enzyme. Besides the above-mentioned EPR technique, the extended X-ray absorption fine structure (EXAFS) spectroscopy was also applied in the study and another proposal on the active site was attributed to a dicopper center in PmoB, based on the obtained evidence [29][30]. In 2003, Rosenzweig and coworkers purified pMMO from methylococcus capsulatus (Bath) and tested its structure by EXAFS [25]. The EXAFS spectra indicated that the purified pMMO contained both CuI and CuII oxidation states and a fitted Cu–Cu bond of 2.57 Å, providing direct evidence for a dicopper-containing cluster in pMMO [25]. Later in 2011, the same group gained more detailed results on different treated pMMO samples [31]. It was seen that the Fourier transform of the EXAFS data for the samples showed two scattering interactions of the nearest-neighbor ligands at around 2 and 2.5 Å. The EXAFS simulations indicate that each sample contains both Cu−O/N and Cu−Cu ligand environments, and the reduced pMMO has an additional Cu−O/N environment. The bond distance of Cu−O/N ligand is a little longer in the reduced sample (2.02 Å) than the isolated (1.97 Å) and oxidized pMMO (1.96 Å). More importantly, the reduced pMMO has a Cu−Cu bond length of 2.64 Å, which is also longer than that in the as-isolated or oxidized pMMO (2.53 Å) and the Cu-reconstituted pMMO (2.52 Å). Excluding the presence of Cu metal, they proposed that the dinuclear metal sites existed in the tested samples as confirmed by the short Cu−Cu interaction, which was also maintained under the reduction treatment [31].

3. Cu-Zeolites

Inspired by enzymology, scientists are taking considerable strides in designing and engineering these similar high-active sites in the catalysis community, which have the ability to convert methane to more valuable products. The crystalline zeolites, which contain various ordered porous frameworks, have been chosen as the potential supports to host the active metal sites [32][33]. In addition, they contain condensing aluminum and silicon tetrahedrons, providing the local coordination environments for copper cations [34]. Therefore, Cu-zeolite is a promising material to mimic the pMMO active site. In this section, researchers provide an overview of the current understanding on the nature of the catalytic Cu sites supported in zeolite for the direct conversion of methane to methanol.

3.1. The Dicopper Sites Supported in Zeolites

In 2005, Groothaert et al. first found that copper supported in zeolites could catalyze methane to methanol, and they observed an obvious decrease in the intensity of a UV–Vis band at 22,700 cm−1 of an O2-activated catalyst during the reaction process of the methane oxidation. This band is best assigned to the bis(µ-oxo)dicopper core ([Cu2-(µ-O)2]2+), which was first proposed as the catalytic center in the copper hosted in zeolites catalysts. Since then, considerable efforts have been taken to analyze the active sites of Cu-zeolite catalytic systems and improve the methanol yield in the reaction of methane upgrading [21]. In 2010, Smeets et al. found a UV–Vis absorption band of ~29,000 cm−1 when pretreating the Cu-ZSM-5 catalyst with an oxidation procedure. However, this band disappeared gradually with the parallel formation of an absorption band at 22,700 cm−1 [35]. Combining the Raman spectra, in which the signals of ν(O-O) and ν(Cu-Cu) were determined at 736 and 269 cm−1, respectively, they thus confirmed that the UV–Vis absorption band of 29,000 cm−1 was assigned to a peroxo species. The UV–Vis absorption changes demonstrated that the side-on bridged [Cu2(O)2]2+ species transferred to the [Cu2O]2+ species. Therefore, they proposed that the active center was the [Cu2O]2+ sites, which was transformed from a precursor of peroxo dicopper(II) species ([Cu2(O)2]2+) in the partial oxidation of methane to methanol [35].

3.2. The Multiple Copper Clusters Supported in Zeolites

In addition to the first discovered dicopper core sites, trimeric ([Cu3(m-O)3]2+) sites and even more copper cores are also proposed as the catalytic sites in Cu-zeolite system. In 2015, exploiting the in situ XAS technique, Grundner et al. unveiled that the trinuclear Cu-oxo clusters ([Cu3(m-O)3]2+) were the exclusively single sites in the activated Cu-MOR catalyst under investigation. In combination with ab initio thermodynamic analysis of DFT results, they confirmed that the trinuclear copper clusters were anchored at the windows of the 8-MR MOR side pockets with the connection to two framework Al atoms [36]. In 2016, Li et al. found that both the binuclear [Cu(µ-O)Cu]2+ sites and trinuclear oxygenated [Cu3(µ-O)3]2+ species were presented in ZSM-5 and the trinuclear copper sites were the most stable clusters in Cu/ZSM-5 under calcination conditions [37]. Mahyuddin et al. suggested that the trinuclear copper clusters in MOR and MAZ differed in reactivity in methane conversion based on DFT calculations [38]. Moreover, Palagin et al. proposed the existence of tetra-/pentamer Cu sites of CunOn2+ and CunOn−12+ in zeolite pores which exhibited higher relative stability than the smaller clusters [39]. Therefore, researchers summarized these works together in this paragraph regarding the catalytic center as the multiple copper core clusters. Researchers believe that these multiple copper clusters could be readily formed and may be preferred in zeolites because of the high mobility of Cu atoms under the thermal treatment.

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