(264i) Design and Synthesis of Zeolite Catalyst for Production of Hydrocarbons from Methane Via Methanol

The study on the methane to methanol (MTM) reaction has extensively been conducted, while, the catalytic performance is still insufficient in terms of CH₃OH yield and selectivity. Apart from of the high stability of CH₄ molecule, one of the reasons is that produced CH₃OH is very easily over-oxidized to carbon dioxide under the oxidizing conditions; there is a trade-off between CH₄ conversion and CH₃OH selectivity. Besides, the production of hydrocarbons from CH₄ under mild conditions has not fully been investigated to date; oxidative coupling of methane is well-recognized as a catalytic process for producing hydrocarbons from CH₄, this reaction is exothermic, requiring high temperature, more than 973 K.

Here, we report a novel catalytic process for selective and effective production of the hydrocarbons production from CH₄ under mild conditions. Our strategy is that produced CH₃OH from CH₄ is immediately converted to hydrocarbons before over-oxidation of CH₃OH; methanol to olefins (MTO) reaction involves in the MTM reaction. For this strategy, we have tackled the design and synthesis of zeolite catalyst that effectively catalyzes both MTM and MTO reactions under oxidizing atmosphere; both active sites for MTM and MTO reaction are created in single zeolite catalyst. Besides, in order to enhance the MTM reaction, oxidative conversion of CH₄ was carried out in the presence of steam and highly active oxidant N₂O.

In this sequential reaction, the adjacency between the active Cu site for MTM and acid site for MTO reactions would be important; acid sites should remain after the introduction of Cu species by ion-exchange method. Therefore, Al-rich aluminosilicate-type CHA zeolite (Si/Al atomic ratio = 3.8) was applied here. The H-form CHA (H/CHA) was prepared via typical procedure, and then Cu species were introduced into the zeolite by ion-exchange method (Cu-H/CHA). Note that Cu /Al ratio was low (ca. 0.04) to keep acid sites for the MTO reaction. As a control, Na-form CHA zeolite without acid sites (Na/CHA) was prepared and Cu-exchanged one was also prepared (Cu-Na/CHA). The XRD patterns of all the obtained samples showed a typical CHA-type structure. In addition, the XRD diffraction patterns of Cu-H/CHA and Cu-Na/CHA did not show any diffraction pattern originated from Cu species due to the high dispersion and/or small amount of Cu species (metal cation and oxides). The high dispersibility of the Cu species were also supported by their UV-vis. spectra.

Figure 1 shows the catalytic performance of Cu-H/CHA and Cu-Na/CHA, indicating that there were clear differences among the samples. Interestingly, both catalysts exhibited almost similar CH₄ and N₂O conversions along with time on stream (TOS). However, there was a marked difference in the products’ distributions. Cu-H/CHA mainly produced CH₃OH and hydrocarbons, while, for Cu-Na/CHA, the main product was CO₂. As a control, the CHA zeolite without Cu and acidity (Na/CHA) did not show any catalytic performance (Figure S2). These results imply that the cooperative effect of Cu and H⁺ is crucial to the production of hydrocarbons, and that the formations of CO and CO₂ are inhibited by the presence of acid sites. For Cu-H/CHA, the selectivity to CH₃OH was decreased but that to hydrocarbons was increased along with TOS, suggesting that the CH₄ is converted to the hydrocarbons via CH₃OH and/or CH₃OH derivates. This process of hydrocarbons production from CH₃OH over zeolite catalyst is very similar to the MTO reaction. Regarding the distribution of the hydrocarbons produced, lower hydrocarbons such as C2-C4 were selectively produced. This is a typical feature of the MTO reaction over CHA-type zeolite; the hydrocarbon are produced by dissociation and desorption of the side chains of aromatic hydrocarbons formed by the “carbon pool” and/or “dual cycle” mechanism.^{27, 54, 60} Hence, these results strongly suggest a new route to the production of the hydrocarbons from CH₃OH, which is produced by oxidative conversion of CH₄. Our findings will contribute to the effective utilization of CH₄ as carbon resource, and to the development of a catalytic process for highly difficult and selective reaction.