Selective Hydrogenation of Cinnamaldehyde to Cinnamal Alcohol over Platinum/Graphene Catalysts

Abstract

IntroductionThe selective hydrogenation of a,b-unsaturated carbonyl compounds is of theoretical and practical significance. [1][2][3] For instance, the hydrogenation of cinnamaldehyde (CAL) can produce cinnamal alcohol (COL), hydrocinnamaldehyde (HALD), and/or hydrocinnamal alcohol (HALC; Scheme 1), but the selective production of COL is difficult because the hydrogenation of the C=C bond is thermodynamically favored over that of the C=O moiety. [1,4] Moreover, acetals and other unidentified high-molecular-weight compounds can also be produced in large quantities in these reactions. [4,5] Although much research has already been focused on this issue, the selective hydrogenation of a,b-unsaturated aldehydes remains a challenge.The selective hydrogenation of a,b-unsaturated aldehydes toward the unsaturated alcohol can be achieved with homogeneous catalysts such as metal hydrides, aluminium isopropoxide, and others. [6,7] However, it is desirable to develop an equal-ly selective heterogeneous catalyst as these are easier to handle and separate from the products. To this end, a large number of studies on supported catalysts based on Pt, Rh, Au, Ru, and Pd active phases have been reported. [8][9][10][11][12] Cordier et al. reported that the selectivity for COL production from CAL hydrogenation follows the sequence Os > Ir > Pt > Ru > Rh > Pd, [13,14] a trend that was correlated with the width of the dband of the metal (Pd < Pt < Ir, Os). Many other studies have targeted supported Pt catalysts because of their high activity and moderate selectivity.In addition to the different metals that can be used to control selectivity in CAL hydrogenation processes, the supports may play an important role to define the selectivity. Indeed, differences in activity and selectivity may be obtained by varying the nature of the interaction between the support and nanoparticles (NPs). For instance, the selectivity toward COL production may be enhanced by using reducible oxides such as CeO 2 , [15] MnO 2 , [16] SnO 2 , [17] TiO 2 , [18] and ZnO [19] because of the electron transfer that takes place between these supports and Catalysts made of Pt nanoparticles dispersed on graphene (X wt %Pt/G, X = 2.0, 3.5, and 5.0) were prepared and characterized by XRD, Raman spectroscopy, BET surface area measurements, TEM, and X-ray photoelectron spectroscopy (XPS), and a 3.5 wt % Pt supported on Vulcan Carbon catalyst (3.5 wt %Pt/ VC) was included as a reference. Although the mean Pt nanoparticle size is approximately 4.4 nm for all X wt %Pt/G and 3.5 wt %Pt/VC catalysts, cinnamal alcohol was produced with high selectivity only with the graphene-supported catalysts:92 % conversion and 88 % selectivity toward cinnamal alcohol were obtained with 3.5 wt %Pt/G. This catalyst also showed good stability in recycling tests. The good selectivity observed with the graphene-based catalysts is attributed to the higher fraction of reduced surface Pt 0 atoms seen on the surface of the Pt nanoparticles (determined by XPS). This interpretation is consistent with D...

Platinum-catalysed cinnamaldehyde hydrogenation in continuous flow

“…23 Guo et al have shown that confinement of Pt nanoclusters within the cavity of metal-organic frameworks also promotes CinnOH selectivity; with steric constraints on CinnALD believed to hinder C=C planar adsorption with consequent preferential C=O activation. 24 Kinetics of CinnALD hydrogenation are also a function of support properties 25 and hydrogenation pressure. We recently reported a detailed mechanistic study of the structural and electronic factors controlling the liquid phase hydrogenation of cinnamaldehyde and related benzylic aldehydes over fumed SiO2 and mesoporous SBA-15 supported Pt nanoparticles.…”

An Effective Pt-Cu/SiO2 Catalyst for the Selective Hydrogenation of Cinnamaldehyde

“…In this work, the enhanced selectivity to COL caused by promoter Cu could be attributed to the increased fraction of the metallic Pt 0 species on the surface of the Pt-Cu/SiO 2 catalyst. According to our previous studies, the amount of Pt 0 on the catalyst has a much larger influence on the selectivity of COL, while the Pt 2+ and Pt 4+ species are responsible for the high-molecular-weight secondary products [13,[20][21][22]. For Pt/SiO 2 , there are more Pt 2+ and Pt 4+ species and less Pt 0 species, so it exhibits less selectivity to COL and higher selectivity to those condensation products.…”

“…The unique advantage of such catalysts is that activity and selectivity can be tuned by controlling the size, shape, and surface species of Pt particles [1,[13][14][15]. In fact, many methods have been developed to enhance the performance of Pt-based catalyst, such as investigating novel support and promoter [15][16][17][18][19].…”

“…In fact, many methods have been developed to enhance the performance of Pt-based catalyst, such as investigating novel support and promoter [15][16][17][18][19]. In our previous works, it was found that graphenesupported Pt catalysts showed high selectivity to COL [13]; and the activity and selectivity of Pt/carbon nanosheets (CNS) catalysts increased with the rise of the graphitization degree of CNS [20]. Moreover, the selectivity of Pt catalyst does not depend on particle size, at least within the 1.5-2.5 nm particle size range over Pt/SiO 2 [21].…”

Improvement Effect of Ni to Pd-Ni/SBA-15 Catalyst for Selective Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde

“…We also have investigated in detail the CAL hydrogenation by using all sorts of catalysts consisting of Pt/Graphene [6], Pt/CNS and Pd/CNS [7], Pt-Mo 2 N/SBA-15 [8], Pd-WN/SBA-15 [9], and Co-Pt/SBA-15 [10]. In general, Pt [11][12][13], Ru [14,15], Au [16], Ir [17,18], Co [19,20], Cu [21,22] are conductive to the C=O bond hydrogenation to generate cinnamal alcohol (COL), while Pd [23,24], Rh [25,26], Ni [27][28][29] prefer a higher selectivity to the C=C bond, thus leading to the production of HALD.…”