Impact of Al Distribution in the Cu-Exchanged CHA-Type Zeolite on the Catalytic Performance in CH4 Conversion

1. Introduction

Methane is a highly abundant and inexpensive source of fuel and chemicals. The development of novel technologies that can convert methane easily into chemicals has strongly been desired. However, its kinetic inertness and low reactivity limit the industrial utilization of methane. Recently, it was reported that Cu-exchanged zeolites are a promising catalyst for the catalytic conversion of methane. We are also tackling the development of novel zeolite-based catalysts for direct conversion of methane into methanol followed by lower olefins.

Meanwhile, recently, we have developed a facile method for preparing the CHA-type zeolite with controlling the Al distribution, i.e., proportion of Q⁴(2Al)/Q⁴(1Al) ratio, where Q⁴(nAl) is Si(OSi)_4-n(OAl)_n. In this work, the impact of Al distribution in the Cu-exchanged CHA-type zeolite on the catalytic performance in CH₄ conversion was investigated.

2. Experimental

The CHA-type aluminosilicate zeolites with different proportions of Q⁴(2Al)/Q⁴(1Al) ratios were synthesized in the presence of N,N,N-trimethyl-1-adamantammonium cation (TMAda⁺) from the different starting materials including fumed silica, aluminum hydroxide, and the FAU-type zeolite (JRC-Z-Y5.5, Si/Al = 2.8), with their proportions varied. Cu-ion exchange was carried out using Cu(NO₃)₂・3H₂O.

The catalytic reaction was performed in a fixed bed reactor. The flow rates of the reactants were CH₄/O₂/Ar = 16/4/5 (SCCM). The catalyst amount was 100 mg. The reaction temperature was varied ranging from 400 to 600 °C, and the reaction time at each temperature was 1 min. The products including CO and CO₂ were analyzed by GC-TCD, and other hydrocarbon products were analyzed by GC-FID.

3. Results and discussion

The ²⁹Si MAS NMR spectra clearly showed that thus obtained CHA-type zeolites had different Al distributions depending on the proportion of the FAU-type zeolite as the starting material. The use of the FAU-type zeolite with a high proportion led to the high proportion of Q⁴(2Al). Here, representative catalysts were designated as “Q⁴(1Al)-rich” and “Q⁴(2Al)-rich”, respectively. The Si/Al ratios of Q⁴(1Al)-rich and Q⁴(2Al)-rich were 9 and 13, respectively, and the Cu content for both catalysts were almost similar, ca. 1.4 wt%. The Cu state was evaluated by UV.-vis technique; both catalysts gave only one peak at 220 nm, indicating Cu species were highly isolated.

Both catalysts showed similar CH₄ conversion, and it was increased along with the temperature. However, there was a significant difference in the products selectivities between Q⁴(1Al)-rich and Q⁴(2Al)-rich catalysts. The yield of CO₂ on Q⁴(2Al)-rich catalyst was higher than that on Q⁴(1Al)-rich. The yields of target products, CH₃OH, DME, C₂H₆ and C₂H₄ on Q⁴(1Al)-rich were higher than those on Q⁴(2Al)-rich. These results clearly suggest that the Al distribution affected the catalytic performance in CH₄ conversion; Q⁴(1Al)-rich catalysts are advantageous in terms of the production of target products from CH₄.

The Cu state was also investigated by NO-adsorbed FT-IR technique as follow [4]. First, the catalysts were evacuation at 723 K, and then NO molecules was adsorbed onto the catalysts at 293 K. The peaks around 1900-1950 cm^-1 are derived from to NO adsorbed on Cu²⁺ species, and ones around 1810 cm^-1 are derived from NO adsorbed on Cu⁺ species. There was a marked change in the peaks derived from NO adsorbed on Cu⁺ species; Q⁴(1Al)-rich first gave a peak at 1809 cm^-1, but another peak at 1816 cm^-1 was observed when NO loading was increased. However, Q⁴(2Al)-rich did not exhibit such change. Thus, we have considered that the Al distribution would affect the state of Cu species, resulting in different catalytic performance.

References

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