A stepwise loading method to magnetically responsive Pt-Fe3O4/MCNT catalysts for selective hydrogenation of 3-methylcrotonaldehyde
© Song et al.; licensee Springer. 2014
Received: 3 October 2014
Accepted: 3 December 2014
Published: 13 December 2014
Pt-loaded multi-walled carbon nanotubes (Pt/MCNTs) and magnetically responsive Pt-Fe3O4/MCNT catalysts were prepared by a stepwise loading of preformed Pt and Fe3O4 nanoparticles onto multi-walled carbon nanotubes (MCNTs). The structure, composition, and magnetism of the catalysts were characterized by X-ray diffraction (XRD), TEM, H2-O2 titration, inductively coupling plasma-atomic emission spectroscopy (ICP-AES), and superconducting quantum interference device (SQUID) techniques. Ascribed to the well-controlled particle size in the preformed Pt colloids, Pt particles in the consequent Pt/MCNT and Pt-Fe3O4/MCNT catalysts are of high uniformity and dispersion. The prepared Pt catalysts show an excellent catalytic performance in the liquid phase hydrogenation of 3-methylcrotonaldehyde, one of typical α,β-unsaturated aldehydes. A very high selectivity to 3-methylcrotonalcohol of 98% at a conversion of about 80% was available on the magnetic Pt-Fe3O4/MCNT catalyst. The magnetic catalyst, with good superparamagnetism, can be easily recovered from the liquid phase system under the external magnetic field. Moreover, both the Pt/MCNT and magnetic Pt-Fe3O4/MCNT catalysts show a good recyclability, confirmed by five cycles of reusage.
Metallic catalysts, e.g., Pt, Pd, Ru, Cu, Co, and Au, have been intensively studied in the selective hydrogenation of α,β-unsaturated aldehydes [4–10]. Among the most active and selective catalysts, Pt has been retained in the majority of the published work for the selective hydrogenation of α,β-unsaturated aldehydes [11–14]. Its selectivity depends critically on the particle size, i.e., the larger the particle size, the higher the selectivity to the unsaturated alcohol [15–18]. Smaller Pt particles would reduce the selectivity to unsaturated alcohol. As a result, heterogeneity in size distribution would induce the descent of the overall selectivity. It is therefore highly desirable to have a narrow-sized distribution of Pt nanoparticles which exhibits decent selectivity in addition to high conversion. Loading a preprepared Pt colloid on support has been practiced to prepare supported Pt particles with uniform size . However, organic stabilizers are often used in the preparation of colloidal Pt solution. After the Pt particle deposition on supports, the stabilizer has to be removed typically by heat treatment, during which, unfortunately, the size of Pt particles varies, depending on the temperature and time applied in heat treatment.
On the other hand, the filtration-based recovery of expensive Pt catalyst from the liquid reaction system is rather time- and energy-consuming. The introduction of magnetically responsive substances to the catalyst could endow the catalyst magnetic properties, which makes it easy to recover the catalyst under a magnetic field and improves the separation efficiency [20–24]. Although magnetically responsive catalyst has been intensively reported [25–31], scarce work has been dedicated to their application in the selective hydrogenation of unsaturated aldehydes .
The Pt nanoparticles were prepared by a two-phase liquid-liquid method (for more details, see Additional file 1). The as-made Pt nanoparticles were dispersed in toluene (15 mL) and then mixed with an appropriate aliquot of MCNT. After ultrasonic treatment for 1 h, the mixture was magnetically stirred overnight. Finally, Pt/MCNT catalyst was recovered by filtration and dried at 80°C overnight. The Pt/MCNT catalysts are referred to as x Pt/MCNT, where x is the stoichiometric loading in weight percent.
Fe3O4 nanoparticles were prepared by a simple organic-phase synthesis (for more details, see Additional file 1) [33, 34]. The prepared Fe3O4 nanoparticles were redispersed in hexane in the presence of oleic acid and oleylamine and mixed with an appropriate aliquot of MCNT. The mixture was ultrasonicated for 1 h and stirred overnight. Then the magnetic composites were recovered by filtration and dried at 100°C and referred to as y Fe3O4/MCNT, where y is the stoichiometric loading in weight percent. The as-made Pt nanoparticles were redispersed in toluene (15 mL) and then the as-made Fe3O4/MCNT magnetic composite was added. After ultrasonic treatment for 1 h and magnetically stirred overnight, the mixture was filtered and dried at 80°C overnight to obtain z Pt-y Fe3O4/MCNT magnetic catalysts. Here, z is the stoichiometric loading in weight percent based on MCNT.
X-ray diffraction (XRD) patterns were recorded on a Philips PW3040/60 diffractometer (Philips Analytical, Almelo, Netherlands) using CuKα radiation (λ = 1.54 Å). The scans were recorded in the 2θ range between 10° and 80° with a step width of 0.03°. Transmission electron microscopy (TEM) observations were carried out on a 2100 JEOL TEM (JEOL Ltd., Akishima, Tokyo, Japan) working at 200 kV. The sample was diluted in ethanol to give a 1:5 volume ratio and sonicated for 10 min. The ethanol slurry was then dripped onto a Cu grid covered with a holey film of carbon. H2-O2 titration measurements were performed on a Micromeritics AutoChem II chemisorption analyzer (Micromeritics Instrument Co., Norcross, GA, USA) to determine the Pt dispersion of the Pt/MCNT catalysts. Prior to the measurement, the catalyst was preheated in a helium flow at 373 K for 30 min and completely reduced in an Ar flow with 10 vol.% H2 at 573 K for 1 h. The concentrations of the effluent gases were monitored by a calibrated thermal conductivity detector (TCD). Pt loading of the Pt/MCNT and Pt-Fe3O4/MCNT catalyst was determined by a Jarrell-Ash 1100 inductively coupling plasma-atomic emission spectrometer (ICP-AES, MOA Instrumentation, Inc., Levittown, PA, USA). Each sample was analyzed three times and the results were averaged. The magnetic measurements were carried out on a superconducting quantum interference device (SQUID, MPMS, Quantum Design, San Diego, CA, USA) at 27°C. About 2.5 g of sample was used in each measurement.
Hydrogenation of 3-MeCal was carried out in a 50-mL Teflon-lined stainless steel autoclave equipped with a hydrogen inlet, a pressure gauge, and a thermocouple. Prior to the first reaction, the catalyst was activated in a N2 flow at 300°C for 1 h. The catalyst (0.1 g), 3-MeCal (2.0 mL), deionized water (2.0 mL), and ethanol (16.0 mL) were mixed and sealed into the autoclave under vigorous magnetic stirring at a speed of 960 rpm. For the sake of safety, N2 was firstly charged to replace the air, and subsequently, H2 was charged several times. Afterwards, the autoclave was heated up to 80°C. The reaction was started by raising H2 pressure up to 1.4 MPa. The reaction solution was periodically sampled and analyzed by a gas chromatography Shimadzu GC-2014 (Shimadzu, Kyoto, Japan) equipped with an FID detector and a DB-5 capillary column. As for the recovery of Pt-Fe3O4/MCNT, a magnet with a magnetic field strength of 10.5 to 11.0 kOe was put near the Teflon container, driving the magnetic catalyst to be attached to the side with the magnet. Then, the liquid phase was poured out. The catalyst was washed with ethanol twice and used for a next reaction without activation.
Results and discussion
The as-prepared Pt/MCNT catalysts exhibit good performance in the hydrogenation (see Additional file 1: Figure S1). On the three catalysts, the molar fraction of 3-MeCol in the reaction system linearly increases with the reaction going on, and more than 95% selectivity to 3-MeCol can be achieved at the initial reaction period. The initial reaction rate increases with increasing of the Pt loadings. Selectivities higher than 92% are always available even at the conversions more than 95%.
Pt particle data of the Pt/MCNT catalysts and TOF values in the hydrogenation of 3-methylcrotonaldehyde (3-MeCal)
Pt particle size (nm)
The catalyst was recovered through filtration after reaction, washed with ethanol twice, and then used for the next reaction without other treatments. The 3Pt/MCNT catalyst, as an example, maintained its original activity and selectivity during the five cycles (Additional file 1: Table S1). The Pt loading was monitored by inductively coupling plasma (ICP), showing that no leaching of Pt occurs.
Magnetic Pt-Fe3O4/MCNT catalysts
The magnetic phase of Fe3O4 was introduced to the catalyst by the stepwise loading method to prepare the magnetic responsive catalyst. Easy catalyst recovery from the reaction system is anticipated under the magnetic field.
Results of the hydrogenation of 3-MeCal over Pt-Fe 3 O 4 /MCNT catalysts
Pt loading (wt.%)
Specific reaction rate(10-2mol·g-1·min-1)
Hydrogenation of 3-MeCol over the 3Pt-5Fe3O4/MCNT was conducted to investigate the hydrogenation of the C = C bond (see Additional file 1: Figure S2). It can be seen that the conversion of 3-MeCol over 3Pt-5Fe3O4/MCNT is only 1.0% even after the reaction for 8 h under the same reaction conditions. The non-preferential hydrogenation of C = C over the catalyst leads to the very high selectivity to 3-MeCol.
3Pt-5Fe3O4/MCNT catalyst, as an example, can be facilely separated from the liquid reaction system with the help of an external magnetic field. During five cycles, the magnetic responsive catalyst maintains a stable performance in terms of both conversion of 3-MeCal and selectivity to 3-MeCol, which is about 80% and 98%, respectively, displaying a good recyclability (Additional file 1: Table S2). The Pt loading of the catalyst after being used for five times was checked keeping about 2.7 wt.%, and no Pt was detected in the reaction solution by ICP technique, indicating that no Pt leaching happens during the hydrogenation.
Pt/MCNT catalysts were prepared by loading the forehanded Pt nanoparticles onto MCNT. Monodispersed Pt particles with high uniformity in size are available during the preparation of Pt colloids. Thus, the Pt particles on the consequent Pt/MCNT catalysts are of good uniformity with size around 2.6 nm and of high dispersion (up to 40%), resulting in an excellent catalytic performance in the liquid phase hydrogenation of 3-methylcrotonaldehyde. Subsequently, Pt-Fe3O4/MCNT magnetic catalysts were prepared by adopting the developed stepwise loading method. The prepared magnetic responsive catalysts exhibit good superparamagnetic behaviors and can be facilely separated from the liquid solvent. The hydrogenation of 3-MeCal results reveal that the magnetic catalysts show excellent hydrogenation properties, achieving a high selectivity to 3-MeCol of 98% at a conversion of more than 80%. Additionally, the magnetic responsive catalysts show a good recyclability. No decay of magnetic and catalytic properties was observed after five-cycle usage.
The financial supports by the Natural Science Foundation of China (21101139, 21371152, and 21471131) are gratefully acknowledged.
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