Factors affecting activity and selectivity during cyclohexanone hydrogenation with colloidal platinum nanocatalysts
Graphical abstract
Research highlights
▶ Catalytic activity for the aqueous-phase hydrogenation of cyclohexanone using colloidal platinum nanocatalysts increases with reaction temperature, reactant concentration, hydrogen pressure, and nanoparticle size. ▶ Catalytic activity for the aqueous-phase hydrogenation of cyclohexanone using colloidal platinum nanocatalysts does not depend on the capping agent molecular weight, regardless of nanoparticle size. ▶ Under these mild conditions, the reaction proceeds via a Langmuir-Hinshelwood mechanism with 100% selectivity to cyclohexanol.
Introduction
Liquid-phase hydrogenations play a valuable role in many chemical industries, ranging from fine chemicals to pharmaceuticals [1], [2]. For example, the liquid-phase hydrogenation of phenol over platinum catalysts may be used to produce cyclohexanone, an intermediate important in the production of ɛ-caprolactam [3]. Both cyclohexanone and cyclohexanol were observed as products, with activation energies for cyclohexanone and cyclohexanol formation being reported as 35.55 kJ/mol and 21.93 kJ/mol, respectively. At present it is unknown if cyclohexanol formation occurs through hydrogenation of the phenyl ring or through cyclohexanone hydrogenation. Evidence indicating that cyclohexanone hydrogenation may be the preferred pathway for cyclohexanol formation during phenol hydrogenation comes from the observation of an oxocyclohexadienyl species formed at around 200 K as an intermediate during the adsorption and dissociation of phenol on a Pt(1 1 1) single crystal surface [4]. Cyclohexanone hydrogenation on platinum catalysts has been studied in the gas phase [5], [6] and in the liquid phase [7], [8], [9]. However, only detailed kinetic parameters were determined for the gas-phase reaction over a Pt(1 1 1) single crystal catalyst, where an apparent activation energy of 67.7 kJ/mol and reaction orders of −0.6 and 0.5 for cyclohexanone and hydrogen, respectively, were reported [6]. High selectivity (>90%) to cyclohexanol was observed over the 325–400 K temperature range, while cyclohexene and cyclohexane were the only products detected at 425 and 500 K.
In catalysis, selectivity optimization is important to minimize waste from unwanted byproducts, thereby streamlining processes through the elimination of separation steps [10]. Control of selectivity may be gained by tuning activation energy barriers for reaction pathways [11]. Fundamental to this capability is knowledge of turnover rates as a function of reaction conditions (such as temperature, pressure, and concentration) and catalyst properties (such as particle size and support type). In order to gain a better molecular-level understanding of how particle size and structure influences catalytic activity and selectivity, materials with well-controlled particle sizes and shape must be employed. Preparation of catalytic materials using synthetic methods from nanoscience allows for this control [12], [13]. For example, transition metal nanoparticles may be prepared using solution-based colloidal chemistry [14], [15]. These nanoparticles were previously employed in reaction studies examining the effect of size [16], [17], [18], shape [19], [20], [21], and capping agent [22] on reaction activity and selectivity. Colloidal Pt nanocatalysts stabilized by poly(vinylpyrrolidone) (PVP) have been shown to perform several selective hydrogenations, particularly those involving carbonyl bonds [23], [24]. One possible advantage of using PVP as a capping agent is its solubility in water, which gives rise to the possibility of creating more environmentally friendly catalytic processes. Nonetheless, reports of hydrogenations involving PVP-capped transition metal nanoparticles dispersed in water are limited [25]. Still, from these studies it is evident that results from experimental work done using these prepared nanomaterials increase the molecular-level knowledge of catalyst properties that influence reaction selectivity.
In this work, colloidal platinum nanocatalysts, stabilized by poly(vinylpyrrolidone) to prevent aggregation in solution, were synthesized in the 1–10 nm size range by solution-based methods that give excellent control of particle size. The nanocatalysts were characterized by transmission electron microscopy (TEM) and CO chemisorption via attenuated total reflectance infrared (ATR-IR) spectroscopy. The nanocatalysts dispersed in water were subsequently utilized in the hydrogenation of cyclohexanone, shown in Scheme 1. Product yields were monitored by gas chromatography/mass spectrometry (GC/MS), which allowed turnover frequencies (TOFs) to be determined. By studying the effect of varying reaction conditions (temperature, hydrogen pressure, and cyclohexanone concentration), apparent activation energies and reaction orders were calculated. Through the correlation of measured turnover rates and the spectroscopic identification of adsorbed species, catalytic activity and reaction selectivity were connected with catalyst properties (particle size and PVP molecular weight) and reaction conditions (temperature and reactant concentrations). Taken together, these results may be used to enhance the knowledge of designing environmentally friendly and efficient routes for manufacturing desired chemical commodities.
Section snippets
Nanocatalyst synthesis
Poly(vinylpyrrolidone) of various molecular weights (10,000 g/mol, 29,000 g/mol, and 55,000 g/mol) was obtained from Aldrich and used without further purification. Hexachloroplatinic acid (H2PtCl6·6H2O) was obtained from Alfa Aesar and used without further purification.
PVP-stabilized Pt nanocatalysts in the 1–10 nm range were synthesized according to literature methods [17], [26]. Nanocatalysts of a selected size were obtained by using a specific alcohol as a reducing agent and by varying certain
Results and discussion
Displayed in Fig. 1a is the TEM image, and the corresponding histogram as an inset, for platinum nanocatalysts synthesized using ethylene glycol as the reductant and 29,000 MW PVP as the capping agent. The average size of the nanoparticles was determined to be 1.7 ± 0.2 nm. As indicated in Section 2, other nanoparticle sizes were prepared by varying reaction conditions, such as the alcohol used as the reductant. Additional TEM images and histograms are found in Fig. 1b–d. Results indicate these
Conclusions
PVP-capped colloidal platinum nanocatalysts were synthesized in the 1–10 nm size range and characterized using TEM and CO adsorption studies, which indicate that terminal sites are open on the nanocatalyst surface. The prepared nanocatalysts were active in the aqueous-phase hydrogenation of cyclohexanone and displayed 100% selectivity for the formation of cyclohexanol. Experiments in which the temperature-dependence of the reaction rate was investigated yielded apparent activation energies in
Acknowledgments
This material is based upon work supported by the National Science Foundation under Grant No. 0719160. The authors would like to thank Mr. Roland Myers for his help in obtaining TEM images.
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Activity and selectivity of colloidal platinum nanocatalysts for aqueous phase cyclohexenone hydrogenation
2011, Applied Catalysis A: GeneralCitation Excerpt :For the second pathway, the hydrogenation of cyclohexenone to cyclohexenol, a TOF of 0.068 molCHEOL molsurface Pt−1 s−1 was predicted. This TOF for CO hydrogenation is lower than that determined in the case of cyclohexanone hydrogenation [10], which likely indicates that the presence of the CC bond slows down the rate of CO hydrogenation. The sum of the two calculated values is higher than the experimental value of 0.79 molCHEO reacted molsurface Pt−1 s−1 for the total hydrogenation calculated at 40% conversion of cyclohexenone.