Hydrogenation

Hydrogenation of aldehydes

Supported cluster catalysts were applied in the selective hydrogenation of aldehydes to alcohols [46, 47]. Catalytic reactions were carried out in water. It was found that the activity of the clusters was largely influenced by the oxide supports; the aldehyde conversion was improved from 27.9% [using Au₉₉(SPh)₄₂/Si0₂ as the catalyst] to 69.8% [Au₉₉(SPh)₄₂/Ti0₂] and then to 93.1% [Au₉₉(SPh)₄₂/Ce0₂] [23]. Next, the Au₉₉(SPh)₄₂/Ce0₂ catalyst was investigated for a range of substrates with aldehyde and nitro groups (Table 8.4). The effects of the nitro group at the ortho-, meta-, and para-position were examined, and the nitro group at the meta-position (m-nitrobenzaldehyde) gave the best activity. Further, the effect of the electronic factor (including methyl, hydroxyl, and chloro groups) of substrates also was explored; the hydroxyl group on the substrate largely affected the catalytic activity. However, the Au₉₉(SPh)₄₂/Ce0₂ catalysts had limited capacity to activate other aldehyde reactants (e.g., benzaldehyde), as most of the protecting ligands were capped on the surface of the particles.

Thus, Lewis acid [such as Cr(N0₃)₃, Co(OAc)₂, NiCl₂, Cu(OAc)₁, Cu(OAc)₂, and Cu(N0₃)₂] was introduced to the reaction system to peel the surface ligands [38]. The partial removal of the "-Au(SR)-” unit from the parent Au₂₅(SR)₁₈ nanoclusters was clearly supported by ESI-MS (electrospray ionization mass spectrometry) analysis and simulated by DFT calculation {i.e., [Au₂₅(SCH₃)₁₈]‘ + n M(NH₃)₄^Z+ + n NH₃ -»[Au_2S._n(SCH₃)₁₈_„]- + n [M(NH₃)₃(SCH₃)p-H⁺ + n Au(NH₃)₂⁺, where n = 1-4). It is very interesting that the catalytic activity of Au₂₅(SR)₁₈/Ce0₂ was largely improved in the presence of Lewis acid at relatively low temperature (Table 8.5). The degree of promotion of the catalytic activity of Au₂₅(SR)₁₈/Ce0₂ by M^z+ is in the order Co²⁺ > Ni²⁺ » Cr³⁺ > Cu⁺ > Cu²⁺. Of note, the anions acted merely as a spectator in the catalytic reactions. Further, Au₂₅(SR)₁₈/Ce0₂ with Co(OAc)₂ was applied in the hydrogenation of benzaldehyde and its derivatives and it showed good recyclability.

Table 8.4 Chemoselective hydrogenation of a range of substrates with nitro and aldehyde groups over Au₉₉(SPh)₄2/Ce0₂

Entry	' Substrate	Product <	:onv. (%)
1			93.1
2			73.2
3			98.5
4			84.1
5			90.0
6			98.9
7			99.5

Source: Adapted with permission from Ref. [38], Copyright 2015, American Chemical Society.

Table 8.5 Influence of different Lewis acids (M²⁺X_z) on the selective hydrogenation of 4-nitrobenzaldehyde catalyzed by Au₂₅(SR)₁₈/ Ce0₂

Table 8.5 (continued)

n.r. = no reaction

Semihydrogenation

Next, spherical Au₂₅(SC₂H₄Ph)₁₈ (noted as Au₂₅ sphere) and rodshaped Au₂s(PPh₃)₁₀(C=CPh)_sCl₂ clusters (noted as Au₂₅ rod) also showed good catalytic activity in the semihydrogenation of alkynes [48, 49]. The semihydrogenations catalyzed over the “ligand- on” Au₂₅ were examined in a combination of solvents, bases, and temperatures. The ethanol/H₂0 as solvent, pyridine as base, and reaction temperature of 100 °C were chosen as the optimized conditions for the catalytic semihydrogenation. Next, the cluster catalysts were examined in various terminal alkynes and it was found that they gave rise to high conversion (up to >99%) and ~100% selectivity for alkenes (Table 8.5). Notably, the "ligand- on” Au₂₅ catalysts were silent in the case of internal alkynes (<1% conversion). However, over the “ligand-off” Au₂₅ catalysts (Au₂₅ clusters were thermal treated at 300 °C), the internal alkynes can undergo semihydrogenation to give Z-alkenes (Table 8.6), which is similar to the conventional gold nanoparticle catalysts. Based on these catalytic results, a unique activation pathway of terminal alkynes by "ligand-on" gold nanoclusters was identified, which should follow a deprotonation activation pathway via a R'-C=C-[Au„L_m], in contrast with the activation mechanism on the conventional gold nanocatalysts. The activation pathway was strongly supported by the observation of R'-C=C-[Au„(SR)_m] via FTIR and by observing the deprotonated -C=CPh as the protecting ligand on rod-shaped Au₂₅(PPh₃) ₁₀(C=CPh)₅X₂ nanoclusters [49].

Table 8.6 Comparison of catalytic performance in semihydrogenation of the alkynes over the "ligand-on" and "ligand-off" cluster catalysts

Finally, the tentative mechanism of semihydrogenation was proposed: a deprotonation activation of terminal alkyne and bridging adsorption of R-C=C on the catalytic site should be similar in “ligand-on" Au₂5 clusters (Fig. 8.6). The reactants (i.e., hydrogen and alkyne) should be absorbed on the triangular Au₃ site of Au₂s(SR)₁₈ and the "waist” site of Au₂₅(PPh₃)₁₀(C=CPh)_sX₂. Then both H- bond and -C=CH triple bond would be activated in the presence of a base (e.g., pyridine). Finally, the alkene products should be formed after the addition process with the aid of gold atoms of active sites.

Figure 8.6 Proposed mechanism for the "ligand-on" Au₂₅ cluster-catalyzed semihydrogenation of terminal alkynes to alkenes using H₂ as the reducing agent. Left panel: Au₂₅(SR)₁₈ nanoclusters. Right panel: Au₂₅(PPh₃)₁₀(OCPh)₅X₂ (X = Cl/Br) nanoclusters. Color codes: Au, green; S, yellow; C, grey; P, pink; X, cyan. The areas marked in orange are the Au₃ active sites (left panel) and the waist active sites (right panel) in the alkyne semihydrogenation reactions. Reprinted with permission from Ref. [49], Copyright 2014, American Chemical Society.

One-Pot Cascade Coupling

The above discussions showed that ultrasmall clusters exhibited high activity in the chemical transformation when partial ligands were removed. Of note, the small particles will grow to big ones when all the thiolate/phosphine ligands are detached during the annealing process (e.g., 300 °C). Thus, the direct synthesis of ultrasmall clusters, supported on oxides, may largely improve the catalytic performance [50-52]. Recently, the preparation of Au/oxide catalysts via a wet synthetic process of Au:PVA colloids (PVA: polyvinyl alcohol) and oxides was achieved. The PVA ligands could be easier to be removed at relative temperatures (e.g., 200 °C in air) and the gold clusters would not be aggregated.

Fang et al. [51] synthesized gold clusters of 2-3 nm (using NiO as support) and applied it in the one-pot conversion of furfural with cellulosic ethanol to generate C₅-Cy hydrocarbon, which is a high- energy density fuel. The one-pot reactions were performed at 130 °C under 1 MPa air, stirring at 600 rpm for 6 h using ethanol, K₂C0₃, and a variety of aldehyde derivatives. First time furyl chemicals via the one-pot cross-aldol condensation was obtained, which was mainly due to the natural basic property of NiO supports (evidenced by the C0₂-TPD). Further, it was found that the substitution of hydrogen at the position 5 of furfural with a methyl or a nitro group exerts a considerable influence on the aldehyde conversion (Table 8.7).

Table 8.7 Au/NiO catalysts for aldehydes conversion

Entry	Aldehyde *“	lonversior (%)	, Selectivity (%)
			A	В	C	D
1		96	7	55	4	34
2		73	9	74	1	16
3 '		54	50	46	2	n.d.
4		65	55	28	6	11
5		93	54	29	17	n.d.

Reaction conditions: 10 mg Au/NiO catalysts; 0.42 mmol aldehydes; 10 mg K₂C0₃; 3 mL ethanol; 130 °C for 6 h under 1 MP of air; n.d. = not detected.

Finally, products of В and D can be fully (i.e., 100%) converted to C„H₂„₊2 ^or C„H_2n_₆ hydrocarbons when the atmosphere of the reaction changes to H₂ (1 MPa) after the completion of the hydrogenolysis process (Scheme 8.3). This hydrogenolysis should occur on the synergistic effect of gold particles and Ni° species on the surface of NiO supports.

Further, the one-pot cascade coupling was investigated in the ethanol conversion with cinnamaldehyde and it finally gave rise to the C_n-C₁₃ hydrocarbon (liquid biofuels) [52]. A 70% selectivity for Сц-С!з hydrocarbon is achieved over Au/NiO. Chemo-adsorption and in situ infrared spectroscopy investigations showed that the cascade reactions should process a CH₃CHO* intermediate, which reacts with cinnamaldehyde at the interfacial perimeter of Au/NiO composite to give the final products (Fig. 8.7).

Scheme 8.3 The conversion pathway of furfural to C₇ and C₉ hydrocarbons via the oxidative esterification and cross-aldol condensation with ethanol catalyzed over Au/NiO catalysts in the presence of 0₂ and K₂C0₃, followed by a hydrogenolysis process using H₂. Reprinted with permission from Ref. [51], Copyright 2019, American Chemical Society.

Figure 8.7 The catalytic pathway of the cascade aldol condensation and Michael addition of ethanol and cinnamaldehyde in the presence of oxygen. Color codes: Au, orange; O, light yellow; Ni, cyan. Reprinted with permission from Ref. [52], Copyright 2019, Royal Society of Chemistry.