Density functional theory studies on the external OH⁻‐induced barrierless proton dissociation mechanism for the forced hydrolysis reaction of Al³⁺(aq)

Abstract

The forced hydrolysis reaction of aqueous aluminum ion (Al 3+ ) is of critical importance in Al chemistry, but its microscopic mechanism has long been neglected. Herein, density functional calculations reveal an external OH − -induced barrierless proton dissociation mechanism for the forced hydrolysis of Al 3+ (aq). Dynamic reaction pathway modeling results show that the barrierless deprotonations induced by the second-or third-shell external OH − proceed via the concerted proton transfer through H-bond wires connected to the coordinated waters, and the inducing ability of the external OH − decreases with increasing hydration layers between Al(H 2 O) 6 3+ and the external OH − . The OH − -induced forced hydrolysis mechanism of Al 3+ (aq) is quite different from its self-hydrolysis mechanism without OH − . The inducing ability is a unique characteristic of OH − , rather than other anions such as F − or Cl − .aluminium ion, barrierless proton dissociation, density functional theory, forced hydrolysis,The hydrolysis chemistry of aluminum (Al) is central to environmental science, human health and industry activities. [1][2][3] Extensive efforts have been devoted to exploring the self and forced hydrolysis reactions of Al 3+ (aq) in aqueous solution that form various monomeric and polymeric hydrolytic Al species. [4] Unlike the self-hydrolysis of Al 3+ (aq) in acidic solutions whose detailed reaction steps have been carefully studied using both experimental [5,6] and theoretical [7][8][9][10][11] methods, the microscopic reaction kinetics for the forced hydrolysis of Al 3+ (aq) under elevated pH conditions such as base titration is rarely considered, and the role of hydroxyl in the formation mechanisms of the hydrolytic Al species remains poorly understood. This has hindered the precision studies of the hydrolysis-polymerization mechanisms of Al species.Conventionally, the self-hydrolysis of Al 3+ (aq) is viewed as the spontaneous proton removal on Al 3+ inner-shell coordinated waters with the assistance of adjacent solvent water molecules, [12] while the forced hydrolysis of Al 3+ (aq) involves the participation of external OH − . The firstorder forced hydrolysis of Al 3+ (aq), which acts as the first reaction step in forming polymeric hydroxyl Al species upon base titration, is usually expressed as: [13,14] Al 3 + + OH − ! AlðOHÞ 2 +AlðH 2 OÞ 6 3 + + OH − ! AlðOHÞðH 2 OÞ 5 2 + + H 2 O:In aqueous solution, the Al 3+ ion exists mainly in the hydrated form of Al(H 2 O) 6 3+ rather than the naked Al 3+ , thus, Equation (2) is more suitable to describe the reaction in aqueous solution. The reaction may proceed via the pathway that a proton on an Al(H 2 O) 6 3+ inner-shell coordinated water dissociates and moves into OH − , or in other words, the external OH − captures the proton. Herein, this possible reaction pathway is modeled using the density functional theory-quantum chemical cluster model (DFT-CM) method, [15,16] in order to elucidate the molecular mechanism of the forced hydrolysis reaction of Al 3+ (aq) in aqueou...

27Al NMR Chemical Shifts and Relative Stabilities of Aqueous Monomeric Al3+ Hydrolytic Species with Different Coordination Structures

“…This study clarifies the dominant existing forms and transformation processes of the monomeric hydrolytic species in the four stages of Al 3+ hydrolysis reactions, and provides important reference for further study on the hydrolysis-polymerization and transformation mechanisms of aqueous Al species. As an extension of future research, the critical issues such as the proton and hydroxyl transfer mechanism in the Al 3+ hydrolysis reactions, − the interaction between the dehydration and deprotonation processes, and the effect of pH on the distributions of the Al 3+ hydrolysis products can be explored on the basis of our calculation results. The method used in this study to determine the favorable dehydration reactions of the hydrolytic Al 3+ species with appropriate solute–solvent interactions (named as the “one-by-one” method) , is expected to combine with molecular dynamics simulation methods in future studies .…”

Ab Initio Molecular Dynamics Study of the Proton Transfer in Hydroxyl Ion-Induced Hydrolysis of Aluminum Monomers

“…However, the coexistence of multiple aluminum species in solution often causes disturbances in experimental observation, rendering detailed research of the hydrolysis and dehydration reaction processes and identifying different intermediate hydrolytic species quite challenging. In contrast to the difficulties in experimental characterizations, computational studies conducted at the atomic level have gained momentum in recent years. − Density functional theory-based static calculations have been employed to study aluminum monomers’ hydrolysis and dehydration reactions, albeit with limited water molecules and lacking a complete solvent environment. Furthermore, the specific configurations applied in static calculation models have been constructed artificially, making it difficult to obtain all reaction paths and complete energy landscapes.…”

“…6−10 Nanoscale aluminum particles gradually form through hydrolysis and polymerization reactions during the sol process. Despite numerous studies conducted on large polymeric species that exist stably in solution, such as AlO 4 Al 12 (OH) 24 , the mechanisms behind the formation and growth of these aluminum clusters remain unclear. 11−14 Since aluminum oligomers are the basis for the formation of polymeric species, the study of aluminum oligomers, particularly the most fundamental aluminum monomers, contributes to revealing the formation process of polynuclear aluminum clusters and provides guidance for the preparation of aluminum-based materials.…”