Efficient liquid-phase hydrogenation of cinnamaldehyde to cinnamyl alcohol with a robust PtFe/HPZSM-5 catalyst

Unlocking the catalytic potential of heterogeneous nonprecious metals for selective hydrogenation reactions

Selective hydrogenation has been employed extensively to produce value-added chemicals and fuels, greatly alleviating the problems of fossil resources and green synthesis. However, the design and synthesis of highly efficient catalysts, especially those that are inexpensive and abundant in the earth's crust, is still required for basic research and subsequent industrial applications. In recent years, many studies have revealed that the rational design and synthesis of heterogeneous catalysts can efficaciously improve the catalytic performance of hydrogenation reactions. However, the relationship between nonprecious metal catalysts and hydrogenation performance from the perspective of different catalytic systems still remains to be understood. In this review, we provide a comprehensive and systematic overview of the recent advances in the synthesis of nonprecious metal catalysts for heterogeneous selective hydrogenation reactions including thermocatalytic hydrogenation/transfer hydrogenation, photocatalytic hydrogenation and electrocatalytic reduction. In addition, we also aim to provide a clear picture of the recent design strategies and proposals for the nonprecious metal catalysed hydrogenation reactions. Finally, we discuss the current challenges and future research opportunities for the precise design and synthesis of nonprecious metal catalysts for selective hydrogenation reactions.

Mild Hydrogenation of 2-Furoic Acid by Pt Nanoparticles Dispersed in a Hierarchical ZSM-5 Zeolite

Hydrogenation of biobased compounds can add value to platform molecules obtained from biomass refining. Herein, we explore the hydrogenation of 2-furoic acid (2-furancarboxylic acid, FCA), a derivative of furfural, with H2 generated in situ by NaBH4 hydrolysis at ambient conditions. Nearly complete conversion of FCA was obtained with tetrahydrofuroic acid (THFA) and 5-hydroxyvaleric acid (5-HVA) as the only two reaction products over Pt nanoparticles supported on hierarchical ZSM-5. Small Pt nanoparticles (2 to 3 nm) were stabilized by ZSM-5 nanosheets. At an optimized Pt loading, the Pt nanoparticles can catalyze the hydrolysis of NaBH4 and the subsequent hydrogenation of FCA with the assistance of Brønsted acid sites. Nanostructuring ZSM-5 into nanosheets and its acidity contributes to the stability of the dispersed Pt nanoparticles. Deactivation due to NaBO2 deposition on the Pt particles can be countered by a simple washing treatment. Overall, this approach shows the promise of mild hydrogenation of biobased feedstock coupled with NaBH4 hydrolysis.

Enhanced Activity of Alkali-Treated ZSM-5 Zeolite-Supported Pt-Co Catalyst for Selective Hydrogenation of Cinnamaldehyde

A bimetallic Pt8Co1 supported on alkali-treated ZSM-5 zeolite (ZSM-5-AT) was prepared through the impregnation method. The structure and surface properties of the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂-sorption and X-ray photoelectron spectroscopy (XPS) as well as temperature-programmed desorption of NH₃ (NH₃-TPD) and temperature-programmed reduction of H₂ (H₂-TPR). The TEM images present that the bimetallic Pt8Co1 nanoparticles with a mean particle size of 4-6 nm were uniformly dispersed on the alkali-treated ZSM-5 zeolite. The bimetallic Pt8Co1/ZSM-5-AT catalyst exhibited an extraordinary COL selectivity of 65% at a >99% CAL conversion efficiency, which showed a much higher catalytic performance (including the activity and selectivity) than the monometallic Pt/ZSM-5-AT and Co/ZSM-5-AT catalysts in the selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) using hydrogen as reducing agent. The high catalytic activity of the bimetallic catalyst was attributed to the higher electron density of Pt species and more acidic sites of the alkali-treated ZSM-5 zeolite support. The recovery test showed no obvious loss of its initial activity of the Pt8Co1/ZSM-5-AT catalyst for five times.

Reaction Kinetics of Cinnamaldehyde Hydrogenation over Pt/SiO2: Comparison between Bulk and Intraparticle Diffusion Models

The liquid-phase hydrogenation of cinnamaldehyde over a Pt/SiO2 catalyst was investigated experimentally and theoretically. The experiments were conducted in a 300 cm3 stainless steel stirred batch reactor supplied with hydrogen gas and ethanol as a solvent. Five Langmuir–Hinshelwood kinetic models were investigated to fit the experimental data. The predictions from the bulk model were compared with predictions from the intraparticle diffusion model. Competitive and non-competitive mechanisms were applied to produce the main intermediate compound, cinnamyl alcohol. Reaction rate parameters for the different reaction steps were calculated by comparing between the experimental and mathematical models. All rate data utilized in the present study were obtained in the kinetic regime. The kinetic parameters were obtained by applying a nonlinear dynamic optimization algorithm. Nevertheless, the comparison between the methodology of the present model and these five models indicated that the non-competitive mechanism is more acceptable and identical with the single-site Langmuir–Hinshelwood kinetic model including mass transfer effects and it mimicked the reactant behavior better than the other models. In addition, the observed mean absolute error (MAE) for the non-competitive mechanism of the present model was 2.3022 mol/m3; however, the MAE for the competitive mechanism was 2.8233 mol/m3, which is an increase of approximately 18%. The prediction of the intraparticle diffusion model was found to be very close to that of the bulk model owing to the use of a catalyst with a very small particle size (<40 microns). Employing a commercial 5% Pt/SiO2 catalyst showed a result consistent with previous research using different catalysts, with an activation energy of ≈24 kJ/mol.

Induction Heating of Magnetically Susceptible Nanoparticles for Enhanced Hydrogenation of Oleic Acid

Radio frequency (RF) induction heating was compared to conventional thermal heating for the hydrogenation of oleic acid to stearic acid. The RF reaction demonstrated decreased coke accumulation and increased product selectivity at comparable temperatures over mesoporous Fe3O4 catalysts composed of 28-32 nm diameter nanoparticles. The Fe3O4 supports were decorated with Pd and Pt active sites and served as the local heat generators when subjected to an alternating magnetic field. For hydrogenation over Pd/Fe3O4, both heating methods gave similar liquid product selectivities, but thermogravimetric analysis-differential scanning calorimetry measurements showed no coke accumulation for the RF-heated catalyst versus 6.5 wt % for the conventionally heated catalyst. A different trend emerged when hydrogenation over Pt/Fe3O4 was performed. Compared to conventional heating, the RF increased the selectivity to stearic acid by an additional 15%. Based on these results, RF heating acting upon a magnetically susceptible nanoparticle catalyst would also be expected to positively impact systems with high coking rates, for example, nonoxidative dehydrogenations.

Vapour phase hydrogenation of cinnamaldehyde using cobalt supported inside and outside hollow carbon spheres

The hydrogenation of cinnamaldehyde is usually performed in the liquid phase in batch mode. In this study, a vapour phase flow system has been used to evaluate the use of cobalt catalysts supported inside and outside hollow carbon spheres (HCSs). The influence of temperature, hydrogen flow rate, and catalyst mass on the hydrogenation reaction was investigated. The catalysts generally showed modest conversion to the required products, hydrocinnamaldehyde, 3-phenyl propanol, cinnamyl alcohol, together with formation of various decomposition products. The data revealed that the Co@HCS showed better conversion and product selectivity compared with the Co/HCS. The catalysts with smaller particle sizes (ca. 6 nm) were more efficient than those with larger particles (30–40 nm). An increase in reaction temperature (200–300 °C) resulted in a lower cinnamaldehyde conversion and a poor product selectivity. TPR studies revealed that the Co@HCSs had a stronger metal-support interaction than the Co/HCSs catalysts. Catalyst recycling studies revealed that only the Co/HCSs could be regenerated (four cycles) and post reaction analysis of the catalysts revealed that this was due to HCS pore blockage and not Co sintering.

Defect engineering and spilt-over hydrogen in Pt/(WO3–TH2) for selective hydrogenation of CO bonds

The effect of defects in pre-treatment WOx on the catalytic performance of selective hydrogenation of cinnamaldehyde to cinnamyl alcohol has been revealed.

Palladium-doped hierarchical ZSM-5 for catalytic selective oxidation of allylic and benzylic alcohols

Hierarchical zeolites have the potential to provide a breakthrough in transport limitation, which hinders pristine microporous zeolites and thus may broaden their range of applications. We have explored the use of Pd-doped hierarchical ZSM-5 zeolites for aerobic selective oxidation (selox) of cinnamyl alcohol and benzyl alcohol to their corresponding aldehydes. Hierarchical ZSM-5 with differing acidity (H-form and Na-form) were employed and compared with two microporous ZSM-5 equivalents. Characterization of the four catalysts by X-ray diffraction, nitrogen porosimetry, NH₃ temperature-programmed desorption, CO chemisorption, high-resolution scanning transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy allowed investigation of their porosity, acidity, as well as Pd active sites. The incorporation of complementary mesoporosity, within the hierarchical zeolites, enhances both active site dispersion and PdO active site generation. Likewise, alcohol conversion was also improved with the presence of secondary mesoporosity, while strong Brønsted acidity, present solely within the H-form systems, negatively impacted overall selectivity through undesirable self-etherification. Therefore, tuning support porosity and acidity alongside active site dispersion is paramount for optimal aldehyde production.

Fe-Co Alloyed Nanoparticles Catalyzing Efficient Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol in Water

Selective hydrogenation of C=O against the conjugated C=C in cinnamaldehyde (CAL) is indispensable to produce cinnamyl alcohol (COL). Nonetheless, it is challenged by the low selectivity and the need to use organic solvents. Herein, for the first time, we report the use of Fe-Co alloy nanoparticles (NPs) on N-doped carbon support as a selective hydrogenation catalyst to efficiently convert CAL to COL. The resultant catalyst with the optimized Fe/Co ratio of 0.5 can achieve an exceptional COL selectivity of 91.7 % at a CAL conversion of 95.1 % in pure water medium under mild reaction conditions, ranking it the best performed catalyst reported to date. The experimental results confirm that the COL selectivity and CAL conversion efficiency are, respectively promoted by the presence of Fe and Co, while the synergism of the alloyed Fe-Co is the key to concurrently achieve high COL selectivity and CAL conversion efficiency.

Unusual behavior of bimetallic nanoparticles in catalytic processes of hydrogenation and selective oxidation

Recent results obtained in studying mono- and bimetallic catalysts for selective hydrogenation of unsaturated carbonyl compounds, even unsaturated ones, acetylenic and nitro compounds as well as CO and bio-available alcohols oxidation are reviewed from the standpoint of the strong interaction between the metal nanoparticles, on the one hand, and two metals in the composition of bimetallic nanoparticles, on the other hand. Such interactions were demonstrated to result in partial positive or negative charging of metal nanoparticles, which, in turn, changes their adsorption and catalytic properties, especially with respect to the reactions involving hydrogen. Among the systems studied, Au–Pt, Au–Pd, Au–Cu, Au–Fe, Pt–WO x , Fe–Pd, Fe–Pt, Fe–Cu nanoparticles prepared by the redox procedure are considered to be most perspective in diverse catalytic applications because of the proper combination of the particle size and the electronic state of the metals.

Facile hydrogenation of cinnamaldehyde to cinnamyl ether by employing a highly re-usable “dip-catalyst” containing Pt nanoparticles on a green support

We report the fabrication of a ‘dip-catalyst’ based on Pt nanoparticles on jute stalks as a green support and its catalytic application in cinnamyl alkyl ether synthesis.