Origin of boron mobility over boron-impregnated ZSM-5. A combined high-resolution–solid-state 11B nuclear magnetic resonance/infrared spectral investigation

Abstract

Boron-impregnated ZSM-5 samples have been shown by ¹¹B n.m.r. spectroscopy to have different phases, which are dependent on the zeolite pre-treatment conditions. While the original ZSM-5 sample reveals a resonance associated with tetrahedral boron (incorporated during synthesis in the zeolite lattice at a typical location of zeolitic aluminium), the other impregnated samples show a variety of spectral patterns associated with different phases of boron. For non-pretreated impregnated samples, boron exists as H₃BO₃, as indicated by a very broad resonance at lower field. Pretreatment at 0.1 mPa/673 K effects the condensation of H₃BO₃ with the host zeolite Brønsted sites into an Si—O—B(OH₂) species. If excess of boron is present it exists as amorphous B₂O₃. Pretreatment at 0.1 mPa/1073 K assists in a further dehydration into a surface-annealed Pyrex-like structure. This structure does not exhibit the Pyrex stability, where it reverts to the less-structured phase upon zeolite rehydration. This suggests a role for water in mobilizing boron on ZSM-5 surfaces. An i.r. investigation of methanol reaction (static conditions) over samples pre-treated at 0.1 mPa/673 K reveals and confirms the boron mobility for the impregnated samples. Methyl borate ester is shown to form at 293 K in proportions increasing linearly with the impregnated boron and is identified in the gas phase. Similar vaporization of boron or boron ester is not evident for the original sample which contains framework boron.