Cation Effects on the Brønsted Acidity of Solid Tungstosilicic Acid Clusters
Bibliographic record
Abstract
Rigorous kinetic assessments, pyridine chemical titration and desorption, together with density functional theory calculations establish the trends in the modulation of chemical identity, valence, site density, and strength of Brønsted acid sites by counter cations (Na +, K +, Cu 2+, Mg 2+, and Al 3+ ) on Keggin-type polyoxometalate tungstosilicic acid clusters (H 4 SiW 12 O 40, POM). Monovalent cations (Na + and K + ) exchange protons and decrease the acid strength of the residual protons, as indicated by the deprotonation energy (DPE) that increases from 1100 to 1175 kJ mol –1 with an increasing extent of proton exchange (decreasing the nominal H + -to-POM ratio). In contrast, di- and trivalent cations preferentially exchange protons in the form of hydroxides (Y z + (OH) m, Y z + = Cu 2+, Mg 2+, or Al 3+, 0 < m < z ), resulting in an average DPE value on both POM clusters and associated hydroxides ranging between 1100 and 1150 kJ mol –1 . A portion of these cations disperse on the silica support, generating Lewis acid sites. The exchanged cations modulate the charge within the W 12 O 36 oxide shell, rather than the central SiO 4 4– tetrahedron, which mainly modifies the ionic component of DPE values. Monovalent cations with smaller electronegativities than di- and trivalent cations donate more electrons, which increases the electrostatic interaction of residual protons with conjugate POM – anions and leads to higher DPE values (weaker acids). This study expands the library of Brønsted acidic catalysts with flexibility in tuning their acid strengths and densities, thus providing a series of samples for constructing structure–reactivity relationships and probing site electrostatic correlations on structurally constrained domains.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".