Molecular Modeling, Multinuclear NMR, and Diffraction Studies in the Templated Synthesis and Characterization of the Aluminophosphate Molecular Sieve STA-2

摘要

Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO4 form of STA-2, bis-diazabicyclooctane-butane (BDAB). This is incorporated as a divalent cation within the large cages of STA-2, as determined via a combination of solid-state ~(13)C and ~(15)N MAS NMR, supported by ~(14)N and ~1H-~(15)N HMQC solution NMR and density functional calculations. As-prepared AlPO4 STA-2 containing cationic SDA molecules achieves neutrality by the inclusion of hydroxide ions bridging between 5-fold coordinated framework Al atoms. Synchrotron X-ray powder diffraction data of the dehydrated as-prepared form indicates triclinic symmetry (Al_(12)P_(12)O_(48)(OH)2·BDAB, P1, a = 12.3821(2) A, b = 12.3795(2) k, c = 12.3797(3) A, α = 63.3585(8)°, β = 63.4830(7)°, γ = 63.4218(7)°) with the distortion from rhombohedral R3 symmetry resulting from the partial order of hydroxide ions in bridging Al-OH-Al sites within cancrinite cages. Upon calcination in oxygen, the organic SDA is removed, leaving AlPO4 STA-2 with a pore volume of 0.22 cm~3 g~(-1) (R3, Al_(36)P_(36)O_(144), a = 12.9270(2) A, c = 30.7976(4) A). Dehydrated calcined AlPO4 STA-2 has two crystallographically distinct P and Al sites: ~(31)P MAS NMR resolves the two distinct P sites, and although ~(27)Al MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D ~(27)Al → ~(31)P MQ-J-HETCOR correlation spectroscopy confirms that each framework Al is linked to the two different P sites via Al-O-P connections in a 3:1 ratio (and vice versa for P linked to different Al). The ~(27)Al and ~(31)P resonances are assigned to the crystallographic Al and P sites by calculation of the NMR parameters using the CASTEP DFT program for an energy-minimized AlPO4(SAT) framework.