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-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 1 H-15 N HMQC solution NMR and density functional calculations. Asprepared AlPO 4 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) Å, b ) 12.3795(2) Å, c ) 12.3797(3) Å, R ) 63.3585(8)°, β ) 63.4830(7)°, γ ) 63.4218(7)°) with the distortion from rhombohedral R3 j 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 (R3 j , Al 36 P 36 O 144 , a ) 12.9270(2) Å, c ) 30.7976(4) Å). 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 27 Al MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D 27 Al f 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 AlPO 4 (SAT) framework.

Advances in theory and their application within the field of zeolite chemistry

“…197 The charge of the DABCO-based templates is found to be divalent. Molecular modeling is a useful tool to assist in the design of a new structure directing agent (SDA) for the synthesis of zeolites, or to unravel the mechanistic role of the template in the synthesis process.…”

“…As discussed in Section 3.3 the search for a suitable and efficient template is a long-standing issue in the synthesis community. 197. The experimental approach comprises multinuclear NMR and diffraction studies, but spectra of new materials require a correct assignment of the NMR resonances, and in the exploration phase (different environment of NMR active nuclei results into ''unknown'' chemical shifts) theoretical support is requisite.…”

Retrosynthetic Co-Templating Method for the Preparation of Silicoaluminophosphate Molecular Sieves

“…48 Considering P 5+ substitution by Si 4+ as the mechanism for the incorporation of silicon atoms within the framework, a Si/Al ratio of 0.2 is insufficient to balance the six positive charges of OSDAs per unit cell (considering one To understand the involvement of TrMA during the synthesis of SAPO-56, its concentration was varied within the starting gels while ensuring a starting pH of 7 by adding different amounts of TBAOH. Pure SAPO-56 was only obtained at a TrMA/Al ratio of ca.…”

Broadband excitation in solid-state NMR of paramagnetic samples using Delays Alternating with Nutation for Tailored Excitation (‘Para-DANTE’)

“…On the other hand, if the spinning rate is not too fast, spinning sidebands remain, and the principal components of the interaction tensors can be determined from the intensities of these sidebands [5][6]. This information can then be compared with DFT calculations of NMR parameters to obtain unambiguous spectral assignments and to investigate disorder in position or composition [7][8][9][10][11][12][13].…”