Gas-Phase Reactions of [VO₂(OH)₂]⁻and [V₂O₅(OH)]⁻with Methanol: Experiment and Theory

Abstract

The gas-phase reactivity of the vanadium hydroxides [VO(2)(OH)(2)](-) and [V(2)O(5)(OH)](-) toward methanol was examined using a combination of ion-molecule reactions (IMRs) and collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Isotope-labeling experiments with CD(3)OH, (13)CH(3)OH, and CH(3)(18)OH were used to confirm the stoichiometry of ions and the observed sequence of reactions. The experimental data were interpreted with the aid of density functional theory calculations, carried out at the B3LYP/SDD6-311++G** level of theory. While [VO(2)(OH)(2)](-) is unreactive, [V(2)O(5)(OH)](-) undergoes a metathesis reaction to yield [V(2)O(5)(OCH(3))](-). The DFT calculations reveal that the metathesis reaction of methanol with [VO(2)(OH)(2)](-) suffers from a barrier of +0.52 eV (relative to separated reactants) but that the reaction of [V(2)O(5)(OH)](-) with methanol readily proceeds via addition/elimination reactions with both transition states being below the energy of the separated reactants. CID of [V(2)O(5)(OCH(3))](-) (m/z 213) yields three ions arising from activation of the methoxo ligand: [V(2), O(6), C, H](-) (m/z 211); [V(2), O(5), H](-) (m/z 183); and [V(2), O(4), H](-) (m/z 167). Additional experiments and DFT calculations suggest that these ions arise from losses of H(2), formaldehyde and the sequential losses of H(2) and CO(2), respectively. The use of an (18)O-labeled methoxo ligand in [V(2)O(5)((18)OCH(3))](-) (m/z 215) showed the competing losses of H(2)C(16)O and H(2)C(18)O and [H(2) and C(16)O(18)O] and [H(2) and C(16)O(2)], highlighting that (16)O/(18)O exchange between the methoxo ligand and the vanadium oxide occurs prior to the subsequent fragmentation of the ligand. DFT calculations reveal that a key step involves hydrogen atom transfer from the methoxo ligand to the oxo ligand of the same vanadium center, producing the intermediate [V(2)O(4)(OH)(OCH(2))](-) containing a ketyl radical ligand and a hydroxo ligand. This intermediate can either undergo CH(2)O loss, or the ketyl radical can couple with an oxo ligand of the adjacent vanadium center, producing [V(2)O(3)(μ(2)-O(2)CH(2))](-), which is a key intermediate in the (16)O/(18)O scrambling and in the H(2) loss channel.

A novel method for determination of inorganic oxyanions by electrospray ionization mass spectrometry using dehydration reactions

“…the order of the reactivity with [Mo 2 O 6 (OH)] – was acetic acid > CF 3– CH 2 OH > CH 3 OH, depending on their acidities . They also proposed the mechanism of the dehydration reaction of [V 2 O 5 (OH)] – and methanol based on their theoretical calculation study, which said that the reaction begins by the formation of an ion–molecule complex between [V 2 O 5 (OH)] – and methanol; then, it is followed by the formation of a bridging oxo ligand deprotonating methanol with concomitant opening of the VO 2 V ring (transition state) . That is, the deprotonation process is one of the most important ones; thus, the acidities of carboxylic acids may affect their reactivities greatly.…”

“…arsenate (AsO 4 3− ) and selenate (SeO 4 2− ), respectively. The higher valency of the central atom of the oxyanion species may cause a higher acidity (Table S2), in other words, a lower gas‐phase basicity, therefore resulting in lower ability to accept proton from carboxylic acids (see above) . Nevertheless, the dehydration products of arsenite (AsO 2 − ) and arsenate (AsO 4 3− ) with IDA gave high sensitivities; particularly, the 1:2 product of arsenite and IDA ( m / z 337) from which two water molecules were lost (As‐2 in Fig.…”

“…The oxyanions, VO 3 − , CrO 4 2− , MoO 4 2− , WO 4 2− , BO 3 3− , SiO 3 2− , SiO 4 4− , AsO 4 3− , AsO 2 − , SeO 4 2− , SeO 3 2− and NO 2 − , reacted with four APCs at the ES interface to produce dehydration products that gave intense ion responses. We assume that these dehydration reactions should take place in the gas phase, considering the works of O'Hair's group . However, it should be noted that we do not have any evidence to eliminate the possibility that the dehydration reactions take place in the liquid phase, and further studies should be necessary to clarify the mechanism of these reactions.…”

“…O'Hair's group has observed dehydration reactions at the ESIMS interface, where group VI oxyanions ([MO 3 (OH)] − and [M 2 O 6 (OH)] − , M = Mo, W) acted as catalysts for a two‐step gas‐phase catalytic cycle for the dehydration of acetic acid to ketene, where at the initial step, the condensation product of acetic acid, [MO 3 (OCOCH 3 )] − or [M 2 O 6 (OCOCH 3 )] − was formed at the ES interface . The aforementioned workers also reported gas‐phase reactions of group VI oxyanions and [VO 2 (OH) 2 ] − and [V 2 O 5 (OH)] − with methanol . However, their interest in these studies appears to be limited to elucidation of reaction pathways and mechanisms of the gas‐phase reactions and not the exploitation of the reactions for analytical purposes.…”

Can Supported Reduced Vanadium Oxides form H2 from CH3OH? A Computational Gas-Phase Mechanistic Study

“…[5][6][7][8][9] Despite its vast use, there is still a lack of understanding on the nature of the reaction mechanisms involved in the catalytic transformations, due mainly to the difficulty for obtaining representative model systems on which to carry out detailed chemical and kinetic analyses. In this context, extensive research efforts have been made both experimentally and theoretically to search for suitable catalysts to improve chemical processes concerning alcohol transformations; 10,11 Likewise, gasphase experiments combined with theoretical works [12][13][14][15][16][17][18] have provided a wealth of information of elementary steps and intermediates for various catalytic reactions, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and provides ideal model systems for local reactive sites of condensed-phase catalysts. Therefore, small transition metal oxides clusters may well be effective models of real surface species or active sites over the bulk transition metal oxides materials, 38 that are capable to provide a conceptual framework for modifying or controlling the chemical properties of mixed-metal oxides and for engineering industrial catalysts.…”