Effect of Electron Density of Pt Catalysts Supported on Alkali Titanate Nanotubes in Cinnamaldehyde Hydrogenation

Abstract

Pt nanoparticles of diameter 0.5-1.5 nm were dispersed homogeneously on the inner and outer surfaces of alkali metal titanate nanotubes (M 2 Ti 3 O 7 , M ) Li + , Na + , and K + ) by ion exchange with Pt(NH 3 ) 2 Cl 2 followed by calcination in ambient air at 673 K and reduction in H 2 at 448 K. Both XPS and DRIFTS revealed that these titanate nanotubes donated their electrons to the Pt particles, yielding a negative Pt oxidation state, and that the degree of electron donation increased with the size of the alkali metal cation. The high electron density of the Pt particles promoted the activation of both CdC and CdO functional groups in cinnamaldehyde. Therefore, the order of the catalytic activity in the hydrogenation of cinnamaldehyde was Pt/KTNTs > Pt/ NaTNTs > Pt/LiTNTs. Since the Pt particles were very small, Pt/MTNTs selectively hydrogenated the CdC bond, yielding a hydrocinnamaldehyde with selectivities of 80-87% (at cinnamaldehyde conversions of 27-50%). The alkali metal ions of the titanate nanotubes were able to modify the mode of adsorption of cinnamaldehyde. Therefore, when a KOH promoter was added to the reaction mixture, cinnamyl alcohol became the main product and the cinnamyl alcohol selectivities were 77-83% (at cinnamaldehyde conversions of 49-74%).

Enhancing Metal–Support Interactions by Molybdenum Carbide: An Efficient Strategy toward the Chemoselective Hydrogenation of α,β‐Unsaturated Aldehydes

“…[46] The enhanced electronic density in Ir would increase the repulsivef our-electron interactions with the C=Cm oiety, and favor ad -electronf eedback into the p*o rbitalo ft he polar C=Og roup. [21,28] We anticipate that such electronic metal-support interactions in Ir/Mo 2 Cw ould benefitt he selective hydrogenationo fa,b-unsaturated aldehydes to unsaturated alcohols.…”

“…The nega- tively charged Ir dÀ on Mo 2 C, distinguishedf rom the metallic Ir 0 on inert SiO 2 ,w ould prefer the adsorption and activation of the polar C d + =O dÀ bond due to the enriched de lectrons in Ir, which is commonly accepted in the chemoselective hydrogenation by using Pt-group metals. [21,28,48] Furthermore,t he selective hydrogenation on Ir/Mo 2 Cisi nfluenced by the nature of the Mo 2 Ca nd the Ir loading, as ar esult of the Ir-Mo 2 Ci nteractions. When commercial Mo 2 Ci se mployed as supports [denoted as Ir/Mo 2 C(c)],t he CRAL conversion (51 %) and the CROL selectivity (60 %) are lower than those on Ir/Mo 2 Cu nder the same condition (entry 3i nT able 1), which can be ascribed to the small surface of bulky Mo 2 C ( Figure S6 in the SupportingI nformation).…”

“…In this regard, electron regulation on the Ir is required. For example, the excessive electrons in the Ir species would enhancet he repulsive force with the C=C group, [28] and also promote an electronf eedback to the p*o rbital in the polar C=Og roup, [21] resulting in the selective hydrogenation of the C=Om oiety.S uch regulation is expected to be achieved on metal-carbide supportst hrough electronic metal-support interactions, which drive the charge transfer betweent he carbides and the Ir duet ot heir variation in work functions. [1,29,30] Hydrogenating crotonaldehyde (CRAL) to crotyl alcohol (CROL) remains the main challenge of chemoselective hydrogenation, [21,31] which is limitedb yt he low selectivity associated with the negligible steric hindrance and electronic effects of as mall methyl group at the g-sites.…”

Chemoselective Hydrogenation of Cinnamaldehyde over a Pt-Lewis Acid Collaborative Catalyst under Ambient Conditions

“…In view of the very small Pt particles (2.3 ± 0.55 nm) in Pt/MIL-101, the steric constraint generated by the phenyl group of the cinnamaldehyde for its adsorption on the surface of Pt particles could be eliminated. 10,39 On the other hand, low coordination sites (such as corners, kinks, adatoms, and defective sites) in Pt NPs favor interactions with the CC bond, which increase accordingly as the size of Pt NPs decreases. Thus, the CC bond may adsorb on the Pt NPs with a small size.…”

“…Thus, the CC bond may adsorb on the Pt NPs with a small size. 10,39 It is known that Lewis acids may coordinate to the CO group via one of the oxygen lone pairs. 35,40 Moreover, the ATR-IR spectra also demostrated the coordination of the CO group to the Lewis acidic Cr site of the MIL-101 support ( Figure 10).…”