Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania During Hydrogenation of Alkenes and Aldehydes

This protocol shows a convenient method for comparing the catalytic properties of supported platinum catalysts. The hydrogenation of cyclohexene serves as a model reaction for determining the catalytic activity. Our colloidal synthesis is a promising approach besides impregnation and calcination methods for the fabrication of heterogeneous catalysts, as this allows the synthesis of nanoparticles in defined size and shape.

Since the colloidal synthesis approach allows for use of different ligands like amines or thiols, platinum nanoparticles with other ligands and their influence on the catalytic properties can be investigated. The choice of a suitable ligand is challenging. An appropriate ligand should have strong absorption at selected absorption sites, so that desorption is prevented but catalytic activity is still present.

To begin, prepare the reduction solution by dissolving 25.4 milligrams of tetrabutyl ammonium borohydride and 46.3 milligrams of di-dodecyl dimethylammonium bromide in one milliliter of toluene at room temperature in 10-milliliter rolled-rim glass. Next, prepare the metal salt solution by first dissolving 8.5 milligrams of the precursor platinum IV chloride in 2.5 milliliters of toluene at room temperature in a 10-milliliter rolled-rim glass. After the platinum IV chloride has dissolved, add 185.4 milligrams of the ligand dodecylamine.

Then, sonicate both solutions at room temperature for one to two minutes in an ultrasonic bath at a frequency of 35 kilohertz. Add the complete metal salt solution with a plunge-operated pipette having a disposable tip in a 10-milliliter round-neck flask. Then, add the entire volume of the reduction solution to the metal salt solution by shock injection, while stirring the solution with a magnetic stir bar for 60 minutes under ambient conditions.

To purify the platinum nanoparticles, transfer the complete reaction solution with a plunge-operated pipette having a disposable tip into an 80-milliliter centrifuge tube and add 14 milliliters of methanol. Then, centrifuge at 2, 561 times G for 10 minutes at room temperature and dispose of the solution after centrifugation. To resolve the nanoparticle residue, add three milliliters of toluene with a plunge-operated pipette with a disposable tip and transfer the nanoparticle solution into a rolled-rim glass for further use.

To remove synthesis residues, transfer three milliliters of the purified platinum nanoparticles in toluene into a 100-milliliter round-neck flask and fill with toluene to a final volume of 50 milliliters. Then heat the solution to 52 degrees Celsius and hold the temperature for 60 minutes while stirring the solution with a magnetic stir bar. Next, dissolve 185 milligrams of dodecylamine in 2.5 milliliters of toluene in a 10-milliliter rolled-rim glass at room temperature and add this solution with a plunge-operated pipette with a disposable tip to the heat-treated platinum DDA nanoparticle solution at 52 degrees Celsius.

Then, heat and stir the solution for a further 60 minutes. After purification, as demonstrated previously, dissolve the platinum nanoparticles in three milliliters of n-Hexane instead of three milliliters of toluene. Then, evaporate the solvent in the fume cupboard overnight at room temperature and ambient pressure and weigh the platinum nanoparticles the next day.

Disperse titania in n-Hexane at room temperature in an appropriately-sized beaker using an ultrasonic bath at 35 kilohertz. Add n-Hexane to the beaker containing titania. After preparing a nanoparticle solution of the previously fabricated particles with a mass concentration of one milligram per milliliter in n-Hexane, add the solution to the dispersed titania at room temperature using a disposable syringe with a needle at a flow rate of 0.016 milliliters per minute using a syringe pump.

Then, dry the loaded powder under ambient conditions overnight in the fume cupboard and subsequently for 10 minutes in a vacuum. Fill 1, 000 milligrams of titania in a crystallizing dish and add water until titania is covered. Then, dissolve three grams of chloroplatinic acid hexahydrate in 20 milliliters of distilled water and add the aqueous solution to the submitted titania with a 20-milliliter volumetric pipette.

Next, heat and maintain the solution at 75 degrees Celsius while stirring with a magnetic stir bar for four hours until the solution is viscous. Afterward, dry the solution in the crystallizing dish for one day at 130 Celsius in an oven under atmospheric conditions. To perform calcination in a temperature-programmed oven, fill the previously dried powder in a porcelain crucible.

Then, heat up to 400 degrees within 30 minutes and hold the temperature for four hours. Next, cool the sample down to room temperature in the oven without using a temperature ramp. To reduce the catalyst in a tube furnace, heat to 180 degrees Celsius with a temperature ramp of four degrees Celsius per minute and hold the temperature for 1.5 hours under a continuous flow of hydrogen.

After filling the heating jacket with a desired heating medium, fill the stirred tank reactor with 120 milligrams of the synthesized catalyst and 120 milliliters of toluene. Then degas the stirring tank reactor by applying a vacuum of around 360 millibars. To remove oxygen, put a rubber balloon filled with one standard atmosphere hydrogen on top of the reflux condenser and flush the stirring tank reactor with hydrogen.

Then, start heating and stirring the reactor tank with a magnetic stir bar under hydrogen atmosphere. Once the constant temperature is attained, inject one milliliter of the reactant cyclohexene via the rubber septum using a disposable syringe with a needle. Using a syringe filter, separate the catalyst from the reaction solution and fill the liquid into an autosampler vial that is properly sealed afterward.

After preparing the stirring tank reactor to test the poisoning effect, inject 5-methyl furfural into the submitted catalyst in toluene and let the mixture stir for 120 minutes. Then, add cyclohexene with a disposable syringe in a molar ratio of 1:1 and 1:10 in 5-methyl furfural. Use a syringe filter to separate the catalyst from the reaction solution and fill the liquid into an autosampler vial sealed properly afterward, as demonstrated previously.

To analyze the products by gas chromatography, inject the samples into the gas chromatographic column and assign the peaks to the different substances by comparison with reference standards. Evaluate the gas chromatograms using the 100%method and calculate the percentage amount of each compound by dividing the measured peak area for this compound by the sum of all peak areas. The TEM imaging revealed a quasi-spherical shape for smaller and partly asymmetrical shape for bigger nanoparticles without changes after deposition on titania.

The size and shape of the impregnated catalysts were comparable. The XP spectra showed two signals at 71.5 and 74.8 electron volts for platinum DDA. No significant shift was observed after ligand exchange and deposition on titania.

However, the impregnated catalyst is downshifted by 0.6 electron volts and exhibits oxidized platinum species. In C1s region, three signals arise between 289.0 and 284.0 electron volt. The N1s spectrum exhibits ammonium, amine, and an additional surface species at 402.6, 399.9, and 398.2 electron volts.

Ammonium is removed by ligand exchange. The amine-stabilized platinum nanoparticles exhibit a higher cyclohexene conversion than the amine-free ones. Small platinum nanoparticles exhibit the highest conversion after ligand exchange, up to 72%In the absence of 5-methyl furfural, conversion of cyclohexene was 72%while increasing the ratio decreases the conversion rate to 30%and 21%respectively.

The platinum IV-F spectra are downshifted by 0.6 electron volts after adding 5-methyl furfural to the hydrogenation of cyclohexene, while the C1s spectra reveal the same three signals as 5-methyl furfural after hydrogenation. The amount of nitrogen decreases in the N1s spectra after hydrogenation, indicating a partial exchange of dodecylamine by 5-methyl furfural. The FTIR spectrum for platinum DDA after adding 5-methyl furfural indicates a partial exchange by 5-methyl furfural as vibration modes for both appear.

Oxygen and presence of hydrogen over metallic catalysts is dangerous. Therefore, we remove any oxygen by purging the reactor multiple times with hydrogen. The hydrogenation of cyclohexe served only as model reaction.

Further, also other alkenes can also be used. Platinum nanoparticles can be synthesized in different sizes with different ligands to influence the catalytic properties. Ligands in heterogeneous catalysts can offer a new catalytic approach to control the activity and selectivity of catalyzed reactions besides the particle size and support effects.