Phase Transformation of MCM-41 in the Mother Liquid at Moderate Temperature

A new crystalline phase (via amorphous intermediate), NCUZ-1, was obtained when MCM-41 (Si/Al=9) was under prolonged heating in the mother liquid (which contains NaAlO₂, [N(CH₃)₄]₂SiO₃, SiO₂, and surfactant C₁₆H₃₃N(CH₃)₃OH(aq) at 105°C for more than two weeks. The largest d spacing of NCUZ-1 calculated from X-ray diffraction data is approx. 6 Å, indicating that the long-range order of the mesopores in MCM-41 was not present in NCUZ-1. Nitrogen absorption studies showed that NCUZ-1 contains both mesopores and micropores. The volume ratio of the mesopore to micropore is approx. 10 to 1 and the BET surface area is 400 m²/g. The TEM micrograph of NCUZ-1 revealed a homogenous phase with distorted mesopores. The ¹³C NMR spectrum of NCUZ-1 before calcination is similar to that of uncalcined MCM-41, indicating that the organic templates in both phases have a similar structure. In the phase transformation process, the counteranion (OH⁻) of the surfactant template played an important role. It increased the solubility of the aluminosilicate wall; the breaking and reforming the Si–O and Al–O bonds make the phase transformation possible, although the process is very slow. When C₁₆H₃₃N(CH₃)₃Cl, instead of C₁₆H₃₃N(CH₃)₃OH, was used as a template, no NCUZ-1 phase was obtained under the same reaction conditions. TEM micrograph, nitrogen absorption isothermal, and ¹³C NMR spectra of NCUZ-1 suggested that the mesopores were present in the NCUZ-1 phase, although there is no long-range order of these mesopores.