Direct conversion of carboxylic acid to olefins over Pt-loaded, oxygen-deficient alkali hexatitanate catalysts with ketonization-hydrogenation-dehydration activity

Catalytic deoxygenation of fatty acids via ketonization and α-carbon scissions over layered alkali titanate catalysts under N2

The ketonization of fatty acid with subsequent McLafferty rearrangement of the fatty ketone allows the deoxygenation to hydrocarbons. Here, we report the cascade reaction of palmitic acid (C₁₆) to hydrocarbons (≤C₁₄) over lepidocrocite-type alkali titanate K_0.8Zn_0.4Ti_1.6O₄, K_0.8Mg_0.4Ti_1.6O₄, and K_0.8Li_0.27Ti_1.73O₄ and the reassembled TiO₂ catalysts at ≤400 °C under atmospheric N₂ in a continuous fixed-bed flow reactor. The C₁₆ acid is coupled to C₃₁ ketone prior to the scissions mostly to a C₁₇ methyl ketone and C₁₄ hydrocarbons (i.e., the McLafferty rearrangement). The hydrocarbons yield increases with temperature and is proportional to partial charge at the O atom, suggesting that basic sites are responsible for C₃₁ ketone scissions. The layered alkali titanate catalysts with two-dimensional (2D) space inhibit diffusion of the ketone primarily formed and promote its scissions to hydrocarbons within the confined space. Otherwise, low hydrocarbons yield (but high ketone yield) is obtained over TiO₂ and the Mg/Al mixed oxide catalysts possessing no interlayer space. Meanwhile, the semi-batch experiment with pre-intercalated palmitic acid favors a direct deoxygenation, demonstrating the essential role of reaction mode toward ketone scission reaction pathway. Over K_0.8Li_0.27Ti_1.73O₄, the complete palmitic acid conversion leads to ∼47% hydrocarbons yield, equivalent to ∼80% reduction of the oxygen content in the feed under N₂.

Structural and Compositional Characteristics of Ball-Milled Lepidocrocite Alkali Titanate and the Correlation to Its Surface Acidic-Basic Properties

The studies on mechanical treatments of layered alkali metal oxides are limited despite their diverse compositions/structures and potential for property tuning. In this work, we vibratory mill Cs_0.7Zn_0.35Ti_1.65O₄, K_0.8Zn_0.4Ti_1.6O₄, and Cs₂Ti₆O₁₃ for up to 4 h, during which the lepidocrocite-type structure and the plate-like morphology are well preserved. X-ray diffraction (XRD) indicates a tiny (≤0.6 Å) interlayer expansion accompanied by the enhancement of the preferred orientation along the stacking direction. Chemical analyses across multiple length scales suggest Cs deintercalation, elemental redistributions, and bulk-to-surface (or crystal edge) Cs migration. This ball-milling-induced Cs-rich moiety partially blocks the surface acid sites, although the solids still show a dominating acidic character. The ball-milled samples Cs_0.7-pZn_0.35-qTi_1.65O_4-δ contain vacancies between the sheets (p) and at the sheets (q and δ). It is deduced from Sanderson's electronegativity equalization principle and experimentally verified by X-ray photoelectron spectroscopy (XPS) that ball milling increases (decreases) the partial charge at the surface acidic Ti⁴⁺/Zn²⁺ (basic O^2-) sites. These nonporous solids (≤20 m²·g^-1) contain water sorbed on the external surface as high as 1.1 mol·mol^-1, which is comparable to that in a water-intercalated sample. Our work expands the current understanding of the reactivity vs robustness in layered alkali titanates under physically demanding conditions, complementing knowledge gathered via the soft chemistry approach.

AC Conductivity and Dielectric Properties of Lepidocrocite-type Alkali Titanate Tunable by Interlayer Cation and Intralayer Metal

The lepidocrocite-type layered alkali titanate A_xM_yTi_2-yO₄ has diverse chemical compositions with variation in charge per formula unit x, the interlayer cation A⁺, and the intralayer metal M. Despite this multivariable nature, the composition dependence of physical properties is not well explored. We report herein the AC conductivity and the complementary dielectric properties of Cs_0.7M_0.35Ti_1.65O₄, K_0.8M_0.4Ti_1.6O₄ (M = Zn, Ni), and the mixed-interlayer ion Cs_0.6K_0.1Zn_0.35Ti_1.65O₄. For Cs_0.7Zn_0.35Ti_1.65O₄, the total AC conductivity is ∼7 × 10^-8 to 2 × 10^-6 S·cm^-1 at 200-350 °C, associating with an activation energy E_a ∼ 865 meV. Meanwhile, the conductivity of K_0.8Zn_0.4Ti_1.6O₄ is higher by 1 order of magnitude at much lower temperature (25-150 °C) and a smaller E_a ∼ 250 meV. This difference originates from the compositional robustness of the cesium-containing samples, contrasting with the sintering-induced changes in the potassium analogues. For the latter, the loss of the interlayer K⁺ ion results in (i) generation of carriers due to charge compensation, (ii) reduction of sheet charge density and weakening of electrostatic attraction, and (iii) widening of the interlayer distance, all contributing to a lower E_a in K_0.8M_0.4Ti_1.6O₄. The angular frequency dependence of conductivity, dielectric permittivity (up to a colossal value of 10⁹), and dielectric loss follows the universal power law. Our work demonstrates the potential of simple compositional variation for electrical properties tuning, prompting a more in-depth investigation covering a wider range of possible candidates of x, A⁺, and M in lepidocrocite titanate.