SUBSTITUTION OF ALUMINUM IN ALUMINOPHOSPHATE MOLECULAR-SIEVE BY MAGNESIUM - A COMBINED NMR AND XRD STUDY

A series of MgAPO and MgAPSO molecular sieves with a structure of AlPO 4-5 or chabazite were synthesized with Mg/Al mole ratios ranging from 0.053 to 0.43 and the substitution of magnesium was thoroughly investi gated by P-31, Al-27 magic-angle spinning (MAS) nuclear magnetic reson ance (NMR) in combination with X-ray diffraction (XRD). In the case of MgAPO, pure MgAPO-5 can be synthesized only at a low Mg mole content (y) of 0.1 (corresponding to Mg/Al = 0.053) and the MgAPO-5 product wi ll be associated with a certain amount of MgAPO-34 when y exceeds this value and no pure MgAPO-34 can be formed even at y = 0.6 (correspondi ng to Mg/Al = 0.43). For the MgAPSOs, a mixture of MgAPSO-5 and MgAPSO -34 is present even at y = 0.1 and pure MgAPSO-34 is formed when y exc eeds 0.4 (Mg/Al = 0.25). The preferred formation of MgAPO-34 (or MgAPS O-34) rather than MgAPO-5 (or MgAPSO-5) with increasing the Mg content may partially result from the difference in the bond lengths of Mg-O and Al-O (P-O and Si-O) and partially from the different structural na ture of AlPO4-5 and chabazite. NMR results indicate that magnesium ion s can substitute for aluminum and four local structural units: P(4Al), P(3Al,1Mg), P(2Al,2Mg), and P(1Al,3Mg) can be formed in the framework s of both MgAPO and MgAPSO molecular sieves. It is probably the increa se in the amounts of 2Mg and 3Mg substitutions that favors the formati on of MgAPO-34 (or MgAPSO-34). The P-31 chemical shift differences (De lta delta) between two neighboring resonances are correlated to the Mg -O and Al-O bond lengths, which enables us to understand the subtle ch anges in the framework after the incorporation of magnesium. The signa l arising from P(3Al) unit is obviously split into two lines for the M gAPOs with y = 0.4 and 0.6, and for all the MgAPSO samples. This was a ttributed to the existence of two distinct P(3Al,1Mg) units, probably locating in the same ring (8- or 12-membered ring) of the molecular si eves. The framework compositions of MgAPO molecular sieves were obtain ed from the P-31 MAS NMR spectra. The calculated Mg/Al ratios are clos e to those mixed in the starting gels, which may suggest that most of the magnesium atoms have entered into the framework of MgAPO molecular sieves. The P-31 MAS NMR unambiguously demonstrates that a restricted random distribution of Mg is present in the MgAPO molecular sieves.