Ammonium Nitrate vs. Ammonium Carbonate: Impact on ZSM -۵ Structure and MTP Performance

Various ammonium salts have been proposed for converting Na-form zeolites to their protonic form via ion exchange. While all these salts can effectively remove Na ions, their pH, ionic strength, and decomposition behavior can significantly impact the zeolite structure. Ammonium nitrate is the most popular salt. However, due to safety concerns and difficulties in handling and storage, some sources suggest using ammonium carbonate. This study investigates the performance of these two ammonium salts in the ion exchange of high-silica ZSM-۵, a suitable zeolite for the Methanol-to-Propylene (MTP) process, and evaluates the catalytic behavior of the resulting zeolites in the MTP reaction. High-silica ZSM-۵ (Si/Al=۲۰۰) was prepared and subjected to ion exchange using ۱ M NH₄NO₃ (ZN) and (NH₄)₂CO₃ (ZC) solutions at ۸۰°C via repeated treatments. BET analysis showed ZC had ~۴% higher specific and external surface areas than ZN, with similar micropore volume. FTIR results indicate a significantly higher I۵۵۰/I۴۵۰ ratio for ZC compared to ZN, suggesting more intact D۵R and better crystallinity. A shift towards lower wavelengths in the ~۱۱۰۰ cm-۱ peak for ZC is also observed, indicating a lower Si/Al ratio compared to ZN. Furthermore, ZC shows a sharp peak at ~۳۷۰۰ cm-۱, characteristic of free, isolated Si–OH groups. (NH۴)۲CO۳ is Mildly alkaline which selectively etches the outer surface (desilication), creating more isolated Si–OH on crystals exterior, particularly for zeolites with low Al content. The increased peak corresponding to free silanols, reduced Si/Al ratio, and increased external surface area in ZC support this argument. NH۴NO۳, being slightly acidic, poses a low risk of dealumination in high Si/Al zeolites, although proton release over time at ۸۰°C can lead to slight dealumination, possibly explaining the decreased intensity of the ~۵۵۰ cm-۱ peak in ZN. ZC also shows a peak at ~۲۸۰۰ cm-۱ indicating C-H stretching vibrations and suggesting that the carbonate anion is more difficult to remove from the structure compared to nitrate. This retention is likely attributable to the formation of insoluble precipitates via the reaction between carbonate ions and residual metal cations in the zeolite. The performance of both samples in the MTP reaction was evaluated under harsh conditions: ۴۸۰°C, ۸ h-۱ WHSV of methanol, and a feed of ۹۰ wt% methanol in water. ZC showed twice the initial selectivity for light olefins and a shorter induction period compared to ZN, attributed to its higher surface area and lower Si/Al ratio. However, it deactivated in less than ۶ hours, while ZN exhibited more stable performance and a ۲.۵ times longer lifetime. The presence of free silanols, increased acidity due to the lower Si/Al ratio, and incomplete removal of carbonate from the structure likely contribute to the inferior performance of ZC in MTP. Although the instability and decomposition of (NH۴)۲CO۳ at ۸۰°C might reduce its effectiveness in ion exchange, the initial catalytic performance and FTIR results for both samples suggest effective Na removal and Brønsted acid site generation by both nitrate and carbonate solutions.

کلیدواژه ها:

High-Silica ZSM-۵ ، Ion Exchange ، Ammonium Salts ، MTP