Synthesis And Solid-state NMR Spectroscopy Of Anionic Framework Aluminophosphates

Since the first discovery of microporous aluminophosphates AlPO4-n (n denotesa specific structure type), the synthesis of aluminophosphates has attracted greatattention because of their potential application in catalysis, adsorption, andseparation. The aluminophosphate family displays rich structure chemistry,encompassing neutral zeolite-like open-frameworks and anionic frameworks withthree-dimensional (3D) open-framework, two-dimensional (2D) layer,one-dimensional (1D) chain, and zero-dimensional (0D) cluster structures.In contrast to neutral framework AlPO4-n with an Al/P ratio of exclusively unity,which are constructed from the strict alternation of tetrahedrally coordinated Al and Patoms, anionic framework aluminophosphates show vast structural andcompositional diversities with an Al/P ratio of less than unity, owing to theoccurrence of coordination numbers greater than four for Al atoms and the existenceof terminal P-O bonds. The Al polyhedra include AlO₄, AlO5 and AlO6, and the Ptetrahedra include PO_4b, PO_3bO_t, PO_2bO_2t and PO_bO_3t (with b representing bridgingoxygens and t terminal oxygens). The diverse linkages of Al polyhedral units and Ptetrahedral units via Al–O–P bonds result in various stoichiometries, includingAlPO4(OH)-, AlP4O169-, AlP2O83-, Al2P3O123-, Al3P4O163-, Al3P5O206-, Al4P5O203-,Al5P6O243-, Al11P12O483-, Al12P13O523-, Al13P18O7215-, and so forth.Although open-framework compounds are highly crystalline and their structuresare often highly symmetrical, there are still difficulties in determining their crystalstructures only by diffraction techniques. When they are microcrystalline withparticle dimensions of only a few micrometers, the application of single-crystaldiffraction analysis is limited. High-resolution solid-state NMR spectroscopy, whichis sensitive to the local ordering and topology of nuclei environments, can givevaluable complementary information about the framework structure, and has beenproven to be an efficient complementary method to X-ray diffraction. It has beenextensively used in the structural investigations of zeolite structures.AlPO-CJ19, with an Al/P ratio of 4/5, has been synthesized solvothermally inthe system of Al(OPri)3?H3PO4?2-aminopyridine?pyridine. Its structure is based onstrictly alternating Al units including AlO4, AlO5 and AlO6 and P tetrahedra includingPO4b and PO3bOH to form a 3D open-framework with 8-MR channels along the [100]direction. Solid-state NMR techniques confirm the single-crystal X-ray diffractionanalysis results. 27Al MAS NMR spectrum indicates four Al signals with the ratio of2AlO4:1AlO5:1AlO6. 31P MAS NMR spectrum presents five tetrahedral P signalswith the ratio of 1:1:2:1. Sixteen kinds of combination of different Al and Pcoordinations are summarized, which may lead to the [Al4P5O20]3-stoichiometry. Upto now, three kinds of combination have been known. It is believed that many morenew aluminophosphates with [Al4P5O20]3-stoichiometry will be continuouslysynthesized.A series of anionic framework aluminophosphates, with different Al/P ratios,have been studied by various solid-state NMR techniques, and the combination of27Al MAS, 27Al{31P} REDOR, 31P MAS, 27Al → 31P CP/MAS NMR, and 31P{27Al}TRAPDOR techniques can provide more detailed structural information on both theAl coordinations (AlO4b, AlO5b, and AlO6b) and P coordinations (PO4b, PO3bOt,PO2bO2t, and PObO3t). The number of Al (or P) atoms in the second coordinationsphere for P (or Al) atoms can be unambiguously determined by using TRAPDORand REDOR techniques. In terms of the relationship between the coordination statesand Al/P ratios found for open-framework aluninophosphates, a new method todetermine the Al/P ratio of open-framework aluminophosphates based on NMR hasbeen established, which is useful for the understanding of unknownaluminophosphate structures, and this is our principal motivation to determine thecoordinations of Al and P of open-framework aluminophosphates by solid-stateNMR techniques. We are currently developing a computational method to solve thealuminophosphate structure combined with NMR and XRD analyses. The specifiedAl and P units determined by NMR can be assembled by AASBU (automatedassembly of secondary building units) computational method under possible spacegroups suggested by XRD analysis. Solid-state NMR techniques have demonstratedtheir powerful strength in structural investigation of zeolite and relatedopen-framework materials.The methods to assign the 27Al/31P NMR signals of neutral frameworkaluminophosphates have been summarized.On the basis of these, the methods toassign the 27Al/31P NMR signals of anionic framework aluminophosphates have beenstudied. Layer aluminophosphate[Al2P3O12H]2-·2[C6H11NH3+] (UT-4) is presented asan example.Solid-state NMR is a powerful and useful technique for structural identificationof microporous materials. Further, it will be a challenging task to investigate thecrystallization process and phase transformation by advanced NMR techniques.