Microporous Aluminophosphate ULM-6: Synthesis, NMR Assignment, and Its Transformation to AlPO4-14 Molecular Sieve

摘要

A pure fluorinated aluminophosphate [A1(8)P(8)O(32)F(4)center dot(C3H12N2)(2)(H2O)(2)] (ULM-6) has been synthesized via an aminothermal strategy, in which triisopropanolamine (TIPA) is used as the solvent together with the addition of propyleneurea and HF. The C-13 NMR spectrum demonstrates that 1,3-diaminopropane, the in situ decomposer of propyleneurea, is the real structure-directing agent (SDA) for ULM-6 crystals. The local Al, P, and F environments of the dehydrated ULM-6 are investigated by 1D and 2D solid-state NMR spectroscopy. The spatial proximities are extracted from F-19{Al-27}, F-19{P-31}, Al-27-{F-19}, and P-31{F-19} rotational-echo double resonance (REDOR) NMR experiments as well as F-19 -> P-31 heteronuclear correlation (HETCOR) NMR and {P-31}Al-27 HMQC NMR experiments, allowing a full assignment of all the F-19, Al-27, and P-31 resonances to the corresponding crystallographic sites. Moreover, it is found that the structure of ULM-6 is closely related to that of AIPO(4)-14. A combination of high-temperature powder XRD, thermal analysis, and F-19 NMR reveals that the removal of fluorine atoms at higher temperature is crucial to the phase transformation of ULM-6 to AlPO4-14. The calcined product shows high CO2/CH4 and CO2/N-2 selectivity with ratios of 15.5 and 29.1 (101 kPa, 25 degrees C), respectively.