Selective methane chlorination to methyl chloride by zeolite Y-based catalysts

Abstract The CH 4 chlorination over Y zeolites was investigated to produce CH 3 Cl in a high yield. Three different catalytic systems based on Y zeolite were tested for enhancement of CH 4 conversion and CH 3 Cl selectivity: (i) HY zeolites in H + -form having various Si/Al ratios, (ii) Pt/HY zeolites supporting Pt metal nanoparticles, (iii) Pt/NaY zeolites in Na + -form supporting Pt metal nanoparticles. The reaction was carried out using the gas mixture of CH 4 and Cl 2 with the respective flow rates of 15 and 10 mL min −1 at 300–350 °C using a fixed-bed reactor under a continuous gas flow condition (gas hourly space velocity = 3000 mL g −1  h −1 ). Above the reaction temperature of 300 °C, the CH 4 chlorination is spontaneous even in the absence of catalyst, achieving 23.6% of CH 4 conversion with 73.4% of CH 3 Cl selectivity. Under sufficient supplement of thermal energy, Cl 2 molecules can be dissociated to two chlorine radicals, which triggered the C-H bond activation of CH 4 molecule and thereby various chlorinated methane products (i.e., CH 3 Cl, CH 2 Cl 2 , CHCl 3 , CCl 4 ) could be produced. When the catalysts were used under the same reaction condition, enhancement in the CH 4 conversion was observed. The Pt-free HY zeolite series with varied Si/Al ratios gave around 27% of CH 4 conversion, but there was a slight decrease in CH 3 Cl selectivity with about 64%. Despite the difference in acidity of HY zeolites having different Si/Al ratios, no prominent effect of the Si/Al ratios on the catalytic performance was observed. This suggests that the catalytic contribution of HY zeolites under the present reaction condition is not strong enough to overcome the spontaneous CH 4 chlorination. When the Pt/HY zeolite catalysts were used, the CH 4 conversion reached further up to 30% but the CH 3 Cl selectivity decreased to 60%. Such an enhancement of CH 4 conversion could be attributed to the strong catalytic activity of HY and Pt/HY zeolite catalysts. However, both catalysts induced the radical cleavage of Cl 2 more favorably, which ultimately decreased the CH 3 Cl selectivity. Such trade-off relationship between CH 4 conversion and CH 3 Cl selectivity can be slightly broken by using Pt/NaY zeolite catalyst that is known to possess Frustrated Lewis Pairs (FLP) that are very useful for ionic cleavage of H 2 to H + and H − . Similarly, in the present work, Pt/NaY(FLP) catalysts enhanced the CH 4 conversion while keeping the CH 3 Cl selectivity as compared to the Pt/HY zeolite catalysts.