Cyclohexanone Selectivity Research Articles

Regulating the electronic state of Pd nanoclusters and Lewis acid density by the carbon doping for synergistically enhancing phenol hydrogenation in green solvent

Pd-based catalysts have been widely applied in selective hydrogenation of phenol. However, avoiding excessive hydrogenation of C=O functional groups is still challenging. Here, we synthesized Pd nanoclusters supported by N-doped C-Al2O3 (Pd/CN-Al2O3) catalyst for phenol hydrogenation. Remarkably, the cyclohexanone yield shows a volcano trend with the doping of carbon content, where Pd/CN-Al2O3 with carbon content of 6.76 % (Pd/CN-Al2O3-2) exhibits the highest cyclohexanone yield. Under the optimized conditions, the phenol conversion and cyclohexanone selectivity over Pd/CN-Al2O3-2 are 99.5 % and 96.7 %, respectively. The corresponding TOF value is 1731.7 h−1, which is 5.8 times higher than that of Pd/Al2O3 (299.7 h−1). Mechanism studies reveal the excellent catalytic performance of Pd/CN-Al2O3-2 is mainly attributed to the synergistic effect of electron-deficient Pd clusters and Lewis acid sites. Pd nanoclusters with moderate electron-deficient state and moderate Lewis acid density are beneficial to the adsorption of phenol and the desorption of cyclohexanone, thereby exhibiting the best phenol conversion and cyclohexanone selectivity. This work provides a carbon modification strategy to regulate the structure of Pd-based catalysts, which is a helpful guide for the hydrogenation reactions.

Selective oxidation of cyclohexanol to cyclohexanone with peroxydisulfate over Co/N doped porous carbon

Cyclohexanone serves as a vital chemical intermediate in nylon production and as a solvent in the manufacturing of resins and paints. The quest for facile, environmentally-friendly, and cost-effective methods to achieve selective oxidation of cyclohexanol is highly desirable. In this study, we successfully achieved the oxidation in water solution using peroxydisulfate (PDS) activation. This was accomplished by employing a Co/N doped porous carbon catalyst synthesized through the one-step carbonization of zeolite-imidazolate frameworks (ZIFs). The catalysts derived from ZIFs with various Co/Zn ratios were extensively characterized using XRD, SEM, TEM, N2 adsorption–desorption, and XPS techniques. The catalyst's performance was assessed under different reaction conditions, including reaction temperature, PDS dosage, and water quality. Notably, the results revealed that under mild conditions, a high efficiency was achieved with over 55% cyclohexanol conversion and 80% cyclohexanone selectivity within 3 h. To gain a greater understanding of the reactive oxygen species involved, we conducted further quenching experiments and EPR analysis. Our findings suggested that·SO4-, ·OH, and mediated electron transfer played a dominant role in the oxidation process. The insights obtained from this work have the potential to offer a green and economically viable method for the selective oxidation of cyclohexanol to cyclohexanone.

SnO2 modified intermetallic PdSn for selective hydrogenation of phenol to cyclohexanone with hydrogen donor

Lignin oil consists of a large amount of phenolic compounds, and how to economically and efficiently utilize its phenolic compounds remains challenging while chitin is the second largest natural organic compounds in quantity, and finding its more application is highly desired. In this work, we demonstrate to use chitin as supports to prepare SnO2 modified intermetallic PdSn bifunctional catalysts for hydrogenation of phenolic compounds to cyclohexanone derivatives. Under mild reaction conditions and HCOONa as hydrogen donor, the synthesized Pd1Sn2Ox/chitin exhibits 99.3 % of phenol conversion and 99.8 % of cyclohexanone selectivity, and illustrates a wide adaptability for selective hydrogenations of various methylphenols and dihydroxybenzenes. Theoretic calculations and experimental results suggest that the significant enhancement can be attributed to the synergistic effect between SnO2 and the formation of PdSn intermetallic phase, where the Lewis acid SnO2 promotes the activation of the benzene rings of phenolic compounds and intermetallic PdSn accelerates HCOONa in water medium to release H2 for hydrogenation.

The Deposition of Isolated Fe(3+) Species in Mesoporous Silicon for Oxidation of Cyclohexane

AbstractThe balance of activity and selectivity in liquid alkane oxidation is challenging for the design of supported Fe catalysts. When Fe species leach to the solvent, uncontrolled free radical chain reactions happen. Herein, isolated Fe(3+) species were constructed by forming a strong Fe−O−Si bond for the selective oxidation of cyclohexane to cyclohexanone by H2O2. Compared to the supported FeOx clusters, the strong Fe−O−Si bond between isolated Fe(3+) species and SiO2 prevents the leaching of Fe in strong oxidation (H2O2) reaction conditions and dominates the non‐free radical mechanism. The turnover frequency over 10FeOx/SBA‐15 reached 15.2 h−1, higher than for reported Fe‐based catalysts. High selectivity of cyclohexanone is maintained at different conversions. Moreover, the (SiO)xFe3+(OH)3‐x−OOH active species were detected by Raman and FTIR and are generated from the oxidation of isolated Fe species. The strong Fe−O−Si bond and non‐free‐radical mechanism by (SiO)xFe3+(OH)3‐x−OOH active species induce high activity and selectivity for the oxidation of many other alkanes.

Ruthenium incorporation into hydrotalcites-derived mixed oxides for phenol hydrogenation: Role of Mg/Al molar ratio

In this work the catalytic behaviour of Ru supported on mixed oxides derived from non-commercial hydrotalcites in phenol hydrogenation was studied in a batch reactor working at 30 bar and 200 °C. To this end, a set of catalysts with 2 wt% Ru and a Mg/Al molar ratio of 1, 2, 3 and 4 was synthesized. The catalysts were tested in phenol hydrogenation to assess the influence of Mg/Al ratio on the catalytic performance in terms of conversion and selectivities to cyclohexanol and cyclohexanone. Physicochemical characterization was performed by X-ray Diffraction (XRD), N2 adsorption-desorption isotherms, dispersive X-ray spectroscopy in scanning transmission electron microscopy (EDS-STEM), CO chemisorption at 35 °C, CO2 and NH3 thermoprogrammed desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). The Mg/Al molar ratio employed determined the catalytic response of the resulting catalysts, obtaining the best catalytic performance (95.9 % conversion and 47.4 % cyclohexanone selectivity) with the sample with the lowest Mg/Al ratio, RuMA1. XRD results showed that in RuMA1 catalyst the hydrotalcite structure was completely transformed into the corresponding mixed oxide after thermal treatment and was also the only one in which MgAl2O4 spinel was not formed. In addition, RuMA1 presented the highest specific surface area, the greatest Ru dispersion, as evidenced by CO-chemisorption and EDX-STEM analysis, as well as a good balance between basic and acid sites and a greater proportion of Bronsted acid sites that also explain it greater selectivity to cyclohexanol.

Pd Nanoparticles Supported on N-Incorporated Hybrid Organosilica as an Active and Selective Low-Temperature Phenol Hydrogenation Catalyst

A heterogeneous Pd-NPMO hybrid-silica catalyst is synthesized and its application for aqueous phase selective hydrogenation of phenol to cyclohexanone at near ambient temperature (40 °C) and under atmospheric hydrogen pressure is demonstrated. The homogeneously distributed Pd nanoparticles on N-bridged hybrid mesoporous organosilica showed remarkable activity and selectivity for cyclohexanone compared to the unmodified Pd-SBA-15 catalyst. Control experiments strongly claim the role of nitrogen domains in the organic framework of hybrid silica support in stabilizing small Pd nanoparticles and possibly modifying the Pd sites responsible for catalysis to activate the substrate molecules in water. The hybrid silica catalyst was stable and reused several times without any significant drop-in activity, proving the heterogeneity of the bifunctional Pd catalyst. Based on the density functional theory study and experimental interventions, a possible reaction mechanism for the low-temperature phenol hydrogenation explaining the role of organic domains in the hybrid-silica framework is proposed.

Selective hydrogenation of phenol to cyclohexanone over small amount of Ru-promoted Pd/ZrHP catalysts

Selective hydrogenation of phenol to cyclohexanone is a vital step in the manufacture of fiber (nylon), which is currently carried out over expensive Pd catalyst with low productivity. In this work, a series of Ru-promoted Pd catalysts were synthesized in a facile microwave-assisted method and tested in the hydrogenation of phenol to cyclohexanone. It was found that small amount of Ru in Ru-Pd alloy can increase the conversion of phenol obviously without loss of cyclohexanone selectivity. The selectivity of cyclohexanone over Ru0.15Pd4.85/ZrHP remained higher than 99%, while the conversion of phenol increased from 20.5% (over Pd5/ZrHP) to 97.9% at 100 °C and 1 MPa. And the highest turnover frequency (TOF) of surface metal in Ru0.15Pd4.85/ZrHP at 100 °C and 2 MPa reached 1605 h−1. Characterization results revealed that the addition of Ru can improve the dispersion of Ru-Pd alloy and enhance its electron density. In addition, the activated hydrogen on Ru-Pd alloy and acid sites in ZrHP played a synergistic effect for the selective formation of cyclohexanone from phenol.

Metal nanoparticles with a clean surface via clay-nanosheets as stabilizers and supports for catalytic hydrogenation reactions in aqueous-phase

The clean surface of the metal nanoparticles (NPs) plays an important role for their catalysis applications due to more active catalytic sites being available. However, the organic stabilizer is difficultly completely removed for the metal NPs by colloidal preparation. The synthesis of metal NPs without stabilizers requires a suitable solid support as an immobilizer and a proper template to control their shapes. Herein, a general route via clay-nanosheets as both a stabilizer and a support for colloidal synthesis of aqueous soluble metal NPs with a clean surface was proposed. Due to its clean surface and two-dimensional (2D) structure for a great promotion on hydrogenation activity, and Na+ ions for promotion on cyclohexanone selectivity, the Pd-NPs showed superior catalytic performance for hydrogenation of phenol to cyclohexanone in water. Ru, Pt and Rh NPs showed superior catalytic performance for hydrogenation of toluene at room temperature and atmospheric pressure.

Activity enhancement of core–shell Pd@mCeO2 catalysts for phenol hydrogenation to cyclohexanone by tuning metal-support interactions

A core–shell catalyst composed of mCeO2 shell and Pd metal core was prepared by one-pot wet chemical method and used for the hydrogenation of phenol to cyclohexanone. The phenol conversion and cyclohexanone selectivity were 96.3 % and 95.3 % at reaction conditions of 50 min, 80 °C and 1 MPa H2. The effects of Ce species and Br species in the preparation of catalysts was studied, and the synthesis mechanism of core–shell Pd@mCeO2 catalysts was preliminarily revealed. By discussing the phenol adsorption mechanism and the redox properties of the catalyst, the activation of H2 by Pd sites and phenol by cerium oxide (CeO2) sites of the surface were studied. H2-TPR and XPS characterization were used to explain the effect of Ce species and Br species for the catalytic performance. These results indicated that the electronic structure of Pd was regulated by the interaction between Pd and Ce, and different O sites were generated around the Pd sites. In situ IR spectra indicated that phenol was adsorbed and dissociated on cerium to form phenoxy species and water, this synergistic catalytic pattern of carrier activated substrate and hydrogenation site promoted the hydrogenation of phenol and improved the catalytic efficiency.

Highly efficient conversion of phenol to cyclohexanone on Pd-based catalysts by cobalt doping

Efficient catalysts are still the key to practical industrial applications. In this work, Pd-based catalyst was modified by doping metal Co to obtain Pd-Co@mSiO2 bimetallic core–shell catalyst. The modified bimetallic core–shell catalyst was used in the hydrogenation of phenol, and the catalytic results showed that the Co modification effectively improved the catalytic performance of the Pd-based catalyst, especially, it reduced the activation energy of the reaction and greatly improve the conversion of phenol. Combined with characterization, it was found that Co doping reduced the particle size of Pd metal particles and affected the adsorption capacity of substrate phenol on the surface of Pd metal. The conversion of phenol was close to 99.9 % within 1 h at 80 °C, while the selectivity of cyclohexanone reached 95.4 %. In addition, the core–shell structure provided a stable environment for the catalyst, which still maintained high activity after being recycled for 5 times.

Electrocatalytic hydrogenation of phenol by active sites on Pt-decorated shrimp shell biochar catalysts: Performance and internal mechanism

The development of highly-active electrocatalysts and identification of active sites contribution are necessary for electrocatalytic hydrogenation (ECH) of bio-oil. Herein, the Pt-decorated shrimp shell biochar (SSB) catalysts with high nitrogen content was prepared successfully. The biochar-based catalysts were first used for the ECH of bio-oil model compounds. Results showed that the Pt/SSB catalysts exhibited excellent electrocatalytic activity and stability, realizing 100 % conversion of phenol and 98 % total selectivity of cyclohexanone and cyclohexanol within 5 h. By correlation analysis and density functional theory (DFT) calculations, the Pt, Pt-Nx, and CO sites in Pt/SSB catalysts were found to play an important role in the stepwise hydrogenation of phenol. Furthermore, the Pt-Nx sites were identified as the main contributor to the high selectivity of cyclohexanol generation. This study lays the foundation for the controllable preparation of electrocatalysts with high activity and stability, which is significant for large-scale upgrading of bio-oil in the future.

Metal-acid interface encapsulated in hybrid mesoporous silica for selective hydrogenation of phenol to cyclohexanone

The traditional multi-functional catalytic system is to physically mix each catalyst to meet the multi-step reaction process, so the multi-point synergy is insufficient. Here, we obtained the hybrid mesoporous silica shell by aluminum doping and coated it on Pt particles, showing a synergistic mode between different sites. This strategy realized the shortest geometric contact distance between metal and acid, formed a stable metal-acid interface and enhanced the catalytic efficiency. In the hydrogenation of phenol, Pt@Al-mSiO2 bifunctional catalyst showed 96% selectivity for cyclohexanone. Hybrid silica stabilized the metal-acid interface and showed good stability in application, which was better than general supported catalysts.

Poly(ionic liquid)s hollow spheres nanoreactor for enhanced cyclohexane catalytic oxidation

Hollow structure nanoreactors take advantage of catalyst loading and enhanced activity by shell construction. In this work, functionalized poly(ionic liquid)s (PILs) with hollow spherical structures were synthesized by polymerization- and quaternization-based approaches. These hollow structure PILs (HPILs) could be controllable designed from IL monomer easily, giving HPILs various properties. In the subsequent selective oxidation of cyclohexane, the HPILs acting as nanoreactors could efficiently improve the activity of metal salt catalysts (CoCl2) and displayed adjustable selectivity for cyclohexanone, cyclohexanol (KA) or adipic acid (AA) by different types of HPILs. In kinetic study, it follows first-order reaction kinetics which CoCl2/HPIL-C12 had a lower the activation energy of 29.1 kJ/mol. Importantly, these nanoreactors storing the catalysts could be recovered and reused more than 7 times without significant loss of activity. A theoretical model for the hollow spherical reactor was further established to support the advance of the HPILs reactor, which could also help to predict reaction process in other core-shell catalytic system. Due to controlled substrate transfer and enhanced radical generation in HPILs, the reaction process could be adjusted to enhance the conversion of cyclohexane and selective oxidation to AA.

Highly dispersed Pd clusters/nanoparticles encapsulated in MOFs via in situ auto-reduction method for aqueous phenol hydrogenation

In this work, a novel in situ auto-reduction strategy was developed to encapsulate uniformly dispersed Pd clusters/nanoparticles in MIL-125-NH2. It is demonstrated that the amino groups in MIL-125-NH2 can react with formaldehyde to form novel reducing groups (-NHCH2OH), which can in situ auto-reduce the encapsulated Pd2+ ions to metallic Pd clusters/nanoparticles. As no additional reductants are required, the strategy limits the aggregation and migration of Pd clusters and the formation of large Pd nanoparticles via controlling the amount of Pd2+ precursor. When applied as catalysts in the hydrogenation of phenol in the aqueous phase, the obtained Pd(1.5)/MIL-125-NH-CH2OH catalyst with highly dispersed Pd clusters/nanoparticles with the size of around 2 nm exhibited 100% of phenol conversion and 100% of cyclohexanone selectivity at 70 °C after 5 h, as well as remarkable reusability for at least five cycles due to the large MOF surface area, the highly dispersed Pd clusters/nanoparticles and their excellent stability within the MIL-125-NH-CH2OH framework.

Palladium (Pd0) Loading-Controlled Catalytic Activity and Selectivity for Chlorophenol Hydrodechlorination and Hydrosaturation.

Reductive catalysis by zero-valent palladium nanoparticles (Pd0NPs) has emerged as an efficient strategy for promoting the detoxification of chlorophenols (CPs) via hydrogenation. Most studies achieved hydrodechlorination of CP to phenol for detoxification, but it requires considerably high energy input and harsh conditions to further hydrosaturate phenol to cyclohexanone (CHN) as the most desired product for resource recovery. This study documented 4-CP hydrodechlorination and hydrosaturation catalyzed by Pd0NPs deposited on H2-transfer membranes in the H2-based membrane catalyst-film reactor, which yielded up to 99% CHN selectivity under ambient conditions. It was further discovered that the Pd0 morphology and size, both determined by Pd0 loading, were the key factors controlling the catalytic activity and selectivity: while sub-nano Pd particles catalyzed only 4-CP hydrodechlorination, Pd0NPs were able to catalyze the subsequent hydrosaturation that requires more Pd0 reactive sites than hydrodechlorination. In addition, better dispersion of Pd0, caused by lower Pd0 loading, yielded higher activity for hydrodechlorination but lower activity for hydrosaturation. During the 15 day continuous tests, the substantial product from 4-CP hydrogenation was constantly phenol (>98%) for 0.2 g-Pd/m2 and CHN (>92%) for 1.0 g-Pd/m2. This study opens the door for selectively manipulating desired products from Pd0-catalyzed CP hydrogenation under ambient conditions.

Kinetics of the in−situ hydrogenation of phenol with formic acid as a hydrogen source

Highly active phenolic compounds in biomass pyrolysis oil are an important factor for limiting the utilization of the biofuel. The catalytic hydrogenation of phenolic compounds is considered to be an effective method for reforming bio−oil. Pd/CB (carbon black), which was synthesized by using a facile impregnated method, is found to be an effective catalyst for the in-situ hydrogenation of phenol (a representative model compound of bio−oil) using FA as a hydrogen source to produce cyclohexanone. A 98.99% of the conversion of phenol and 90.20% of the selectivity of cyclohexanone were obtained under the optimized reaction conditions. The catalyst also showed excellent stability after three recycled process. The catalytic kinetics study of the in−situ hydrogenation of phenol was investigated using Power−Rate Law model and Langmuir−Hinshelwood model. The Langmuir−Hinshelwood model fit well to the experimental data and the apparent activation energies (Ea) was 50.96 kJ mol−1.

Liquid phase hydrogenation of phenol catalyzed by silica supported palladium to yield cyclohexanone

In the work, liquid phase hydrogenation of phenol reaction was carried out using an isothermal fixed bed catalytic reactor. The catalyst was prepared by wet impregnation of palladium precursor onto silica gel support. The effect of reaction parameters such as catalyst particle size in the range of 1–2 mm, time factor (in terms of contact time of catalyst with reactant feed) in the range of 60–250 g.min/mole, and reaction temperature in the range of 80–130 °C on the conversion of phenol and selectivity towards cyclohexanone was studied. From the results, it was observed that the conversion of phenol and selectivity of cyclohexanone were increased with the decrease of catalyst particle size. Further it was observed that the phenol conversion increased with the increase in reaction temperature from 80 to 110 °C and time factor. However, the conversion was decreased with increase in reaction temperature beyond 110 °C. A decreased trend was observed for selectivity of cyclohexanone with the increase of reaction temperature and time factor. The obtained maximum conversion of phenol was around 92% with a cyclohexanone selectivity of more than 99% at an optimum temperature of 110 °C and contact time of 250 g.min/mole in presence of Pd/SiO2 catalyst at hydrogen-to-phenol mole ratio of 1:4. It was observed furthermore that the conversion of phenol and selectivity of cyclohexanone was higher for 1 mm catalyst particle size when compared to 2 mm catalyst particle size. From the results, it can also be concluded that the developed empirical models to predict conversion of phenol and selectivity towards cyclohexanone were good in agreement with the experimental data.

Mesoporous SBA-15 supported gold nanoparticles for solvent-free oxidation of cyclohexane: superior catalytic activity with higher cyclohexanone selectivity.

Surface modification of mesoporous SBA-15 with (3-mercaptopropyl) trimethoxysilane greatly enhances its capability to adsorb the tetrachloroauric anion (AuCl4-). The calcination of the sample after the adsorption experiment led to the generation of homogeneously dispersed, spherical, single crystalline gold nanoparticles (Au0 NPs) of less than 5 nm size, embedded on SBA-15 as observed from the TEM images. The as-prepared SBA-15/Au0 nanohybrid material has offered excellent catalytic activity for the selective oxidation of cyclohexane using TBHP as the oxidant in the absence of any solvent. A maximum of 48.7% cyclohexane conversion was achieved and surprisingly, cyclohexanone (K) has much higher selectivity (>95%) than cyclohexanol (A). The hot-filtration study confirmed the leach-resistant characteristics as well as the true heterogeneous catalytic activity of the SBA-15/Au0 nanohybrid catalyst. The catalyst was recycled up to four times without significant loss in its catalytic activity.

In situ hydrogenation of phenol to cyclohexanone on Pd/C with phosphotungstic acid at open atmospheric condition

The production of cyclohexanone, of industrial importance, is at a distance to environment friendly and efficiency. As a solution, a mild sustainable catalytic hydrogenation is presented herein, of phenol into cyclohexanone on Pd/C with potassium formate the hydrogen donor under open atmosphere. Some important reaction conditions, such as temperature, reaction time, catalyst, amount of potassium formate, etc. were investigated and analyzed. The positive growth was found to accelerate the conversion rate of phenol while reducing cyclohexanone selectivity by weakening the protection of phosphotungstic acid. Phosphotungstic acid was confirmed to be the co-catalyst exhibiting the highest activity under given conditions. Based on experiments, we have made reasonable assumptions about the reaction mechanism, while the reaction kinetics proves high reaction rates within a wide temperature range, indicating the reaction to be quick process. Furthermore, the good reusability and recyclability of Pd/C catalyst is verified to benefit future industrial applications.

Carbon supported copper catalyst prepared in situ by one-pot pyrolysis of Bougainvillea glabra: An efficient and stable catalyst for selective oxidation of cyclohexane

So far, the poor stability of Cu2O is still the major challenge in heterogeneous catalysis. Herein, Bougainvillea glabra (B. glabra) derived carbon supported copper composite (Cu2O/BGC) was synthesized in situ by one-pot pyrolysis method. In the synthetic strategy, B. glabra was used as reductant and carbon source simultaneously, and copper nitrate as copper precursor. Cu2O/BGC exhibited a significant enhancement in both catalytic activity and stability for cyclohexane oxidation with tert-butyl hydroperoxide as oxidant under solvent-free condition. Cu2O/BGC with a Cu/C ratio of 0.8 calcined at 800 °C showed the highest catalytic cyclohexane conversion efficiency (77.1%) and cyclohexanone selectivity (68.5%). Its turnover number (TON) is 12.2- and 1.3-fold compared to pure Cu2O and Cu2O/BGC prepared by deposition-reduction method, respectively. Hot filtration experiment confirmed that the sample was a heterogeneous catalyst which could be reused five times without losing its activity.