Use of Short Time-on-Stream Attenuated Total Internal Reflection Infrared Spectroscopy To Probe Changes in Adsorption Geometry for Determination of Selectivity in the Hydrogenation of Citral

Abstract

A new experimental procedure based on attenuated total reflection infrared spectroscopy has been developed to investigate surface species under liquid phase reaction conditions. The technique has been tested by investigating the enhanced selectivity in the hydrogenation of α,β-unsaturated aldehyde citral over a 5% Pt/SiO 2 catalyst toward unsaturated alcohols geraniol/nerol, which occurs when citronellal is added to the reaction. The change in selectivity is proposed to be the result of a change in the citral adsorption mode in the presence of citronellal. Short time on stream attenuated total internal reflection infrared spectroscopy has allowed identification of the adsorption modes of citral. With no citronellal, citral adsorbs through both the CC and CO groups; however, in the presence of citronellal, citral adsorption occurs through the CO group only, which is proposed to be the cause of the altered reaction selectivity.In chemoselective hydrogenations, such as the hydrogenation of α,β-unsaturated aldehydes, there is the desire to understand and control the reaction pathway so as to obtain a specific product with high selectivity. The hydrogenation of citral is of particular interest because of the complex reaction network resulting from the preferential adsorption geometry and strength of interaction of different functional groups within the molecule, namely the CO, conjugated CC, and isolated CC bonds the hydrogenation and further reaction of which can form a wide range of products illustrated in Scheme 1. The hydrogenation of citral has been studied extensively with the choice of metal, support, reaction conditions, and solvent all reported to affect the selectivity. 1 With unsaturated alcohols being important fine chemical and pharmaceutical intermediates, there is significant interest in improving the selectivity to these desired products. Modification of the catalyst through addition of a second metal, such as Sn, Fe or Ge, has been reported to alter reaction selectivity toward CO hydrogenation. 2 For other chemoselective hydrogenations, modification of the catalyst surface via the adsorption of organic compounds has also been reported, with n-alkanethiol self-assembled-monolayer-coated Pd catalysts and thiol-modified Pt/TiO 2 catalysts giving excellent selectivity enhancements in hydrogenation of unsaturated epoxides to saturated epoxides 3 and a complete switch in the products formed for 4-nitrostyrene hydrogenation following modification, respectively. 4The attainment of higher selectivity upon addition of an organic compound into the liquid phase reaction has also been reported for the hydrogenation of citral. The addition of thiophene to Ru/KL catalysts gave a maximum selectivity to geraniol and nerol of 46% (at 15% citral conversion with 3 ppm thiophene); in the absence of the thiophene, the Ru catalysts typically favor CC hydrogenation. 5 Therein, it was also reported that the adsorption mode of the thiophene (relating to thiophene concentration/surface coverage) altered whether the citr...

A Control over Hydrogenation Selectivity of Furfural via Tuning Exposed Facet of Ni Catalysts

“…A comparative study was implemented between these two catalysts, and detailed knowledge on catalytic reaction pathway can be acquired based on the accumulation and decline of various surface species. Figures B and C display in situ FT-IR spectra of reaction process of FAL hydrogenation on Ni/MMO-NO 3 and Ni/MMO-CO 3 , respectively, with purging H 2 at time point t = 0, 30, 90, and 180 s. For the Ni/MMO-NO 3 sample (panels (B1) and (B2) in Figure ), the band at 1653 cm –1 (the characteristic band of CO) declines gradually along with continuous flushing of H 2 , accompanied by observation of a new band at 1105 cm –1 (alcohol C–O group), which illustrates the hydrogenation process of the CO bond. Moreover, the characteristic signals of ring breath (1578 cm –1 ) and CC (1474 cm –1 ) do not display marked variation from t = 0 to t = 90 s, excluding the hydrogenation progress on the furan ring.…”

Breaking Trade‐Off between Selectivity and Activity of Nickel‐Based Hydrogenation Catalysts by Tuning Both Steric Effect and d‐Band Center

“…In the case of CAL, two inseparable characteristic peaks assigned to –CC and –CHO appear for Ni 2 P, meaning cinnamaldehyde is adsorbed flatly on Ni 2 P via the di‐σ (CO) ‐σ (CC) . While SPhF‐Ni 2 P presents strong signals assigned to v (CO) and the v (CC) peak becomes faint, which means SPhF‐Ni 2 P only absorb –CHO via a vertical configuration (di‐σ (CO) ), suggesting an inhibited flat adsorption on SPhF‐Ni 2 P.…”

Palladium Supported on Calcium Decorated Carbon Nanotube Hybrids for Chemoselective Hydrogenation of Cinnamaldehyde

“…Catalytically selective hydrogenation of α, β-unsaturated aldehydes via C=O or C = C bond to α, β-unsaturated alcohols or saturated aldehydes is considered as an important fine chemical process for producing intermediates for food additives and pharmaceuticals (Gallezot and Richard, 1998; Mäki-Arvela et al, 2005; Wu et al, 2012; Daly et al, 2014; Kahsar et al, 2014; Tian et al, 2015). However, the C = C and C=O bond are conjugated in the typical substrate of α, β-unsaturated aldehydes-cinnamaldehyde (CAL), which result in a mixture of hydrogenation products such as cinnamyl alcohol (COL), hydrocinnamaldehyde (HCAL), and 3-phenyl propanol (PPL).…”