Mesoporous inorganic salts with crystal defects: unusual catalysts and catalyst supports

Abstract

We proposed a strategy to synthesize mesoporous inorganic salt particles using the special properties of ionic liquid (IL) mixtures, and hollow mesoporous LaF₃, NdF₃, and YF₃ particles were synthesized and characterized using different techniques. The size of the mesopores in the salt particles was about 4 nm, and the materials were full of crystal defects. The LaF₃, NdF₃ and YF₃ particles were used as the catalysts for the cyanosilylation reaction of benzaldehyde using trimethylsilyl cyanide, and Ru/LaF₃ and Ru/NdF₃, in which Ru nanocatalysts were supported on the LaF₃ and NdF₃ particles with mesopores, were used to catalyze hydrogenations of benzene to cyclohexane and levulinic acid (LA) to γ-valerolactone (GVL). It was discovered that the activities of these catalysts were unprecedentedly high for these reactions. Detailed study showed that both the crystal defects and the mesopores in the salt particles played crucial roles for the extremely high catalytic activity.

Fig. 1. XRD patterns of LaF₃: (a) LaF₃ synthesized at x ₂ = 0.5; (b) LaF₃ synthesized at x ₂ = 0.33; (c) commercial LaF₃; and (d) standard pattern.

Fig. 2. SEM and TEM images of mesoporous LaF₃ synthesized in the OmimCl–N(Bu)₄PF₆ mixtures: (a) SEM image of the LaF₃ synthesized at x ₂ = 0.5; (b) TEM image of the LaF₃ synthesized at x ₂ = 0.5; (c) SEM image of the LaF₃ synthesized at x ₂ = 0.33; (d) TEM image of the LaF₃ synthesized at x ₂ = 0.33; (e) HRTEM image to show the pores on the walls of the LaF₃ synthesized at x ₂ = 0.5; and (f) HRTEM image to show the crystal structure and the SAED pattern (the inset pattern) of the LaF₃ synthesized at x ₂ = 0.5.

Fig. 3. N₂ adsorption/desorption isotherms and pore size distribution (the insets) of the LaF₃ particles synthesized at x ₂ = 0.5 (a) and 0.33 (b).

Fig. 4. Mass fractal dimension (D _m) from SAXS curves of the as-prepared LaF₃ materials synthesized at x ₂ = 0.5 (a), 0.33 (b).

Fig. 5. SAXS curves (a) and normalized pair-distance distribution function curves (b) of the OmimCl–N(Bu)₄PF₆ mixtures at x ₂ = 0.5 (A) and 0.33 (B).

Fig. 6. The schematic diagram for the formation mechanism of mesoporous LaF₃ particles with a hollow core.

Fig. 7. XRD patterns of the as-synthesized fluoride salts (A) synthesized at x ₂ = 0.5 and the corresponding standard patterns (B): (a) NdF₃; and (b) YF₃.

Fig. 8. SEM and TEM images of NdF₃ (a and b) and YF₃ (c and d) materials synthesized at x ₂ = 0.5.

Scheme 1. The reaction mechanism for cyanosilylation of benzaldehyde to cyanohydrin.

Scheme 2. The reaction mechanism for benzene hydrogenation to cyclohexane.

Fig. 9. The reusability of the Ru/LaF₃ catalyst for benzene hydrogenation to cyclohexane at 50 °C, 0.22 h (a) and 25 °C, 0.5 h (b). The other conditions were the same as in Table 1.

Scheme 3. The reaction mechanism for LA hydrogenation to GVL.