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

Abstract

Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO(4) form of STA-2, bis-cliazabicyclooctane-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 (1)H-(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)center dot BDAB, P1, a = 12.3821(2) angstrom, b = 12.3795(2) angstrom, c = 12.3797(3) angstrom, alpha = 63.3585(8)degrees, 63.4830(7)degrees, gamma = 63.4218(7)) with the distortion from rhombohedral R (3) over bar 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 AlPO(4) STA-2 with a pore volume of 0.22 cm(3) g(-1) (R (3) over bar, Al(36)P(36)O(144), a = 12.9270(2) angstrom, c = 30.7976(4) angstrom). Dehydrated calcined AlPO(4) STA-2 has two crystallographically distinct P and Al sites: (31)P MAS NMR resolves the two distinct P sites, and although 27AI 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 AI-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 AlPO(4)(SAT) framework.