Impact of the aluminum species state on Al pairs formation in the ZSM-5 framework

The aluminum (Al) pairs in the ZSM-5 zeolite framework are recognized as an important acid site for a variety of reactions. However, knowledge of how to prepare ZSM-5 zeolites with adjustable Al pairs contents is still lacking. In this work, the impact of the Al species state and concentration on the content of the Al pairs in the preparation of ZSM-5 zeolite was studied. First, we prepared ZSM-5 zeolites with the same SDA (TPAOH), Si source (TEOS), and crystallization condition but with different Al sources (aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum hydroxide, and sodium aluminate). The Raman spectra indicated that the state of the Al species in the silicon aluminum mixture differs for Al[(OH2)6]3+ and Al(OH)4–. Although the prepared ZSM-5 zeolites have similar Si/Al molar ratios, the silicon aluminum mixture with Al(OH)4– is more likely to form Al pairs than the silicon aluminum mixture with Al[(OH2)6]3+ (74% vs. 52%). This conclusion was further proven by well-designed control experiments. For experiments with the same Al source (aluminum sulfate), SDA (TPAOH), Si source (TEOS), and crystallization condition, the conversion of the Al species from Al[(OH2)6]3+ to Al(OH)4– was achieved by the addition of NaOH. Under similar Si/Al molar ratios, ZSM-5 zeolites with Al pairs contents ranging from 44% to 82% were prepared. In addition, ZSM-5 zeolites were synthesized under different water contents. The lower the water content, the more Al pairs were formed (68% vs. ∼78%). In summary, the state of the Al species is an important factor in determining the content of the Al pairs when preparing ZSM-5 zeolites.