Quantum mechanical calculations on molecular sieves. 2. Model cluster investigation of silicoaluminophosphates

Abstract

Ab initio molecular orbital calculations have been performed on model clusters simulating silicate, aluminosilicate, and silicoaluminophosphate dimeric and trimeric entities. This investigation was aimed at obtaining preliminary information on the relative stability of the Si-O-AI, Si-0-P, and AI-0-P bridges and on modifications generated by the presence of phosphorus in such and related systems. The results show that the AI-O-P bridge is more stable than Si-0-AI or Si-0-P linkages, which is in agreement with the high stability of framework aluminum phosphate structures. They also indicate a preferential tendency for P to replace Si rather than AI in aluminosilicate structures and agree with the observation that Si incorporation in aluminum phosphate molecular sieves occurs preferentially by replacement of P. Finally, the results also suggest that bridging hydroxyl groups in aluminum phosphates-the proton originating from nonstoichiometry, defects, or Si incorporation-should show acidic properties.

Control of the alumino-silico-phosphate geopolymers properties and structures by the phosphorus concentration

“…It suggests that increasing the amount of water in the reactive mixture leads to the presence of more physisorbed water, however it does not seem to (1206 cm -1 ), its relative intensity diminishes when (nP/nAl) + nH2O increases. This can be explained by the preferential formation of aluminophosphate entities [38], when the species are available. Besides, for higher Al/P ratios, the presence of large amounts of unreacted metakaolin and undissolved silica is higher, and the molecular interactions will thus favor the formation of Si-O-P and Si-O-Al bonds [39].…”

Role of soluble aluminum species in the activating solution for synthesis of silico-aluminophosphate geopolymers

“…The last phase transition was the formation of mullite from residual, fully dehydroxylated MK particles at 980 to 1010 ºC [44], and is especially prominent in the raw MK particles. For all geopolymer samples, such a peak is less significant, indicating that the raw MK particles had reacted with the phosphate activating solution to synthesize new gels: silico-alumino-phosphate (S-A-P) gel [16] and amorphous or semi-crystalline aluminum phosphate compounds [12,17].…”

“…This reaction occurs in an acidic or low-alkaline medium environment in which a phosphate or phosphoric acid solution, rather than the alkali-silicate used in conventional geopolymers, chemically activates the aluminosilicate precursors to yield a compact geopolymer matrix with a condensed structure of silico-alumino-phosphate linkages [14,15]. It has been claimed that the newly formed Al-O-P linkages could balance the charge caused by changes in the chemical environment of aluminum [16], as is also observed in the synthesis of charge-neutral crystalline zeolite-like aluminophosphate and silico-alumino-phosphate frameworks [17]. Thus, a charge balance within the molecular structure is achieved without the involvement of monovalent cations.…”

Reaction mechanism of NO with hydrolysates of NAMI‐A: an MD simulation by combining the QM/MM(ABEEM) with the MD‐FEP method

“…This treatment comes from the fact that a diffuse basis function located on an atom may to some extent cover the regions of the other adjacent atoms leading to overestimating the population of this atom slightly in the Mulliken population analysis. Derouane and his coworkers showed that the partial charges calculated by the 6‐21G basis set are higher than those obtained from the STO‐3G basis set, and thus, STO‐3G charges may be more reliable. Wilson and Ichikawa and Torrent‐Sucarrat and coworkers pointed out that the charge transfer between atoms in a molecule is overestimated by using the polarization basis sets.…”