Thanks go to Edith Kieselhorst and Erhard Rhiel for support at the TEM and to Carsten Dosche for support at the XPS. Thanks to Stefan Petrasz for support with the gas chromatograph. The funding of the XPS device by DFG (INST: 184/144-1FUGG) and funding from DFG-RTG 2226 is acknowledged.

The authors have nothing to disclose.

Pt nanoparticles capped with DDA were successfully synthesized in two different sizes and shapes12,14. The small Pt nanoparticles (1.6 nm) show a quasi-spherical form while the bigger particles (2.4 nm) are more asymmetrical exhibiting partly tripodal or ellipsoidal structures. The possibilities are limited to gain larger quasi-spherical platinum nanoparticles, since a formation of elongated structures occurs by further increasing the size of the particles by seeded growth14. The size and shape of the particles can also be influenced by the ligand, reaction time, and temperature. Besides DDA, other ligands can be used in the synthesis, but the capping agent influences the growth and, therefore, the size and shape of the nanoparticles, as has already been shown for the synthesis of gold nanoparticles39. After adding the reduction solution to the metal salt solution, the solution is stirred for 60 min (90 min for the synthesis of larger particles) to ensure that the growth process of the Pt nanoparticles is completed. The transport of monomers to the particle surface can be a limiting factor. Furthermore, the temperature can influence the critical radius, which describes the minimal required particle size, at which seeds are stable in solution. By increasing the temperature, the critical radius decreases, resulting in a faster formation of seeds and consequently a faster decrease of the monomer concentration55. After synthesis, ammonium and bromide impurities can still be observed in XPS which can be eliminated by performing a ligand exchange with DDA. Furthermore, all synthesized nanoparticles were deposited on P25 powders without any changes in form, size, or loss of the ligand. For comparison, a ligand-free Pt catalyst was generated by using the impregnation method, which exhibits a Pt nanoparticle size of 2.1 nm and a quasi-spherical shape. XPS further reveals that not only metallic Pt species were present on the surface but also oxidized species. This indicates that in the absence of amine ligands the platinum nanoparticles interact with the support, which may result in a partial encapsulation of the metal into the support10. As a consequence, the particles partially lose their ability to split hydrogen56. However, such encapsulation is favored by high temperature reduction of the metal salt precursor. The temperature used here for the reduction (180 &#176;C) is far below those mentioned in the literature for encapsulation (600 &#176;C)57. Another more likely explanation would be an incomplete reduction of the used Pt source. However, both explanations result in a partial deactivation of the catalyst.
In literature ligands such as amines or ammonia are often considered as catalyst poison in the classical understanding of heterogeneous catalysis15,16. However, the investigations on the liquid phase hydrogenation of cyclohexene demonstrate that Pt/DDA/P25 is still catalytically active and showed an even higher conversion compared to the amine-free catalyst. Amines are known to systematically block terrace adsorption sites on Pt(111)11,58. Results in the literature have already shown, that this promising active site selecting effect of ligands can be used to improve the selectivity for the hydrogenation of acetylene in ethylene-rich streams by diluting the adsorption sites59. This active site selecting effect was also observed for thiols binding on Pd(111)22,23. For the hydrogenation of cyclohexene, these sites are thereby already blocked by amines, however, highly active undercoordinated reaction centers are still available. In addition to the site selection effect of the ligand, attention should also be paid to other properties of the ligand. When selecting the ligand, care should be taken to ensure that the ligand stabilizes the particles during synthesis and protects them from agglomeration. Furthermore, the ligand should exhibit a strong adsorption on the metal surface and a sufficiently high thermal stability so that the ligand is not desorbed or decomposed under reaction conditions. Results show, that DDA generally seems to be suitable for this catalytic approach. No size effect could be observed in the model reaction. Interestingly, the catalyst containing Pt nanoparticles that did not undergo a ligand exchange exhibited a lower conversion (50%) than Pt particles deposited on P25 after ligand exchange (72%). Therefore, a blocking of active sites by ionic compounds may have to be considered under these conditions. Performing a ligand exchange is crucial to increase the activity of the platinum nanoparticles by removing co-adsorbed ionic compounds such as bromide and ammonium, as XPS before and after ligand exchange shows.
Additionally, the influence of the extra amine surface species on the catalytic activity of platinum nanoparticles remains ambiguous, as this species can potentially serve as an additional, localized hydrogen source. XP spectra and FT-IR spectra seem to indicate a hydrogen abstraction of the amine group by platinum leading to an extra amine surface species. This offers the opportunity to serve hydrogen additionally to the dissolved hydrogen in toluene, which can affect the catalytic activity. A hydrogen donor effect from toluene can be excluded here since toluene is not known to dehydrogenate under low hydrogen pressure and temperature60. However, the influence of hydrogen abstraction on the catalytic activity still needs to be further investigated. The hydrogenation of acetophenone on l-proline-modified platinum nanoparticles has already shown that the amine group can accelerate the hydrogenation by a hydrogen transfer from the amine to the reactant15. Therefore, a possible influence of the amine and the surface species on the hydrogenation should be considered.
Despite the successful use of Pt/DDA nanoparticles for the hydrogenation of simple alkenes, no turnover for the more demanding reactant 5-MF could be observed. Therefore, different possibilities for this may be discussed in the following: one explanation would be that no reaction takes place because of the low reaction temperature and hydrogen pressure. The reaction temperature was limited to 160 &#176;C. As thermogravimetric analysis showed ligand desorption and decomposition of Pt/DDA nanoparticles of comparable sizes take place at these temperatures13. Due to the used reactor, no higher pressures than 1 atm of hydrogen could be used. The lower hydrogen pressure in contrast to literature experiments might be the reason why the hydrogenation of carbonyl compounds, such as 5-MF, was not feasible. Several studies have further shown that strong metal support interactions (SMSI) are crucial for the selectivity of the gas-phase hydrogenation of furfural61,62,63. The SMSI leads to the formation of O-vacancies, which enables the adsorption of furfural via the carbonyl group on the titania surface. A furfuryl-oxy-intermediate is formed that can be hydrogenated. However, this hypothesis is countered by the fact that, in contrast to the gas phase experiments, no evidence for an influence of SMSI could be found for liquid phase hydrogenation of furfural in methanol. Platinum particles on different oxides (MgO, CeO2, and Al2O3) had shown comparable catalytic properties64. This indicates that the hydrogenation could take place undergoing different mechanisms in the liquid and gas phase, which needs to be further investigated. The SMSI effect of the Pt particles and the support were only observed for the ligand-free catalyst, which also does not show any conversion of 5-MF under the used reaction conditions. Therefore an impact of the SMSI effect seems unlikely. As poisoning of the catalyst by 5-MF or a surface intermediate seems more probable under the applied reaction conditions, the catalysts were further analyzed before and after ligand exchange with 5-MF under reaction conditions by XPS and FT-IR. These measurements confirmed the hypothesis of catalyst poisoning by 5-MF as both methods show a decrease in the peaks corresponding to the amine on the Pt surface. FT-IR spectroscopy further hints that 5-MF acts as catalyst poison since bands appear in the wavenumber region below 1,200 cm-1, which are consistent with the bands assigned to 5-MF. A nearly flat adsorption geometry is suggested taking surface selections rules into account. A schematic drawing for the proposed surface restructuring is shown in Figure 8.
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/63936/63936fig08.jpg" /> 
Figure 8: Schematic drawing of structural changes by adding 5-MF to the hydrogenation of cyclohexene at the surface of amine-stabilized platinum nanoparticles. Results from FT-IR and XPS show a partial exchange of DDA by 5-MF at the platinum surface and blocking of active sites for the hydrogenation of cyclohexene. Results of FT-IR data suggest an adsorption of the ring of 5-MF nearly parallel to the surface. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/63936/63936fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
To conclude, amine-capped Pt nanoparticles on P25 are promising candidates for new hydrogenation catalysts as the Pt nanoparticles show a higher conversion than the ligand-free catalyst in the model reaction. However, no conversion of 5-MF was observed on either catalyst. This results from poisoning of the Pt by the reactant and not by the ligand as often considered in the literature under the investigated reaction conditions. For future applications, further understanding of the influence of ligands on the adsorption behavior of reactants and their interaction with metal nanoparticles are needed. A 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 the use of different ligands, for instance, amines, amides, thiols, or alcohols, Pt nanoparticles with other ligands should be investigated and compared. This offers the possibility to use ligands, which show a specific ligand-reactant interaction, such as &#960;-&#960; interactions to control the adsorption geometry and thus also the selectivity of the reaction. This approach could be used for the selective hydrogenation of &#945;,&#946;-unsaturated ketones and aldehydes, as has already been shown for the hydrogenation of cinnamaldehyde21. Furthermore, controlling the stereoselectivity in heterogeneous catalyzed reactions is still a challenging task; however, an appropriate chiral ligand could be used to control the chirality of the product as in homogeneous catalyzed reactions. Besides the ligand-reactant interactions, the stabilizing effect of ligands might be used for protecting metal nanoparticles from strong metal support interaction. The strong metal support interaction would lower the chemisorption of hydrogen by encapsulation of the particles with an oxide layer. For a better understanding of the influence of ligands, XPS and FT-IR can provide useful information about the selective poisoning effect and the binding modes of ligands. Furthermore, CO shall be considered&#160;as a sensor molecule to identify available surface sites of the Pt nanoparticle. Additionally, the adsorption behavior and possible surface reactions of ligands and reactants can be investigated on Pt single crystals under ultra-high vacuum conditions to get a fundamental understanding of the surface processes. All in all, ligands in heterogenous catalysis can offer a new catalytic approach, which can be used to control the activity and selectivity of a catalyzed reaction besides the particle size and support effects. Therefore, the traditional way of thinking for heterogeneous catalysis of ligands as catalyst poison should be reconsidered.

Catalysts in the size of a few single atoms up to larger nanoparticles with high surface-to-volume ratios and defined sizes are promising materials for a wide range of heterogeneous catalyzed reactions, such as hydrogenation, dehydrogenation and photocatalytic reactions1. Platinum nanoparticles are widely used in industrial processes, due to the high activity for hydrogenation of olefins. Besides, platinum nanoparticles are promising catalysts for the selective hydrogenation of &#945;,&#946;-unsaturated ketones and aldehydes1,2,3,4. Here, several parameters such as size, shape, and support are able to affect the catalytic properties1,5,6.
The size influences the morphology of nanoparticles, especially in the range of 1 to 5 nm7. Specifically, the size influences the available adsorption sites (for instance: edges, steps, or terraces) and thereby the catalytically active surface, which further influences the catalytic activity7,8,9. Furthermore, the support is able to interact with the metal. These interactions vary and range from charge transfer or spillover processes to a change in the morphology or encapsulation of nanoparticles6,10. While the effect of size, shape, and support on the catalytic properties is well known, a possible effect of adsorbates not directly involved in the reaction, so-called spectator molecules or surface modifiers, is less evolved1,5,6,11. In case of a colloidal approach for catalyst preparation, using colloidal metal nanoparticles that are subsequently deposited onto the support, ligands stabilize the nanoparticles and thus may potentially influence the reaction.
The big advantage of the colloidal synthesis is that nanoparticles of a certain size and shape can be produced in a targeted manner helping to control the catalytic performance via the synthesis route12,13,14. The function of the ligand is to control the size, shape, and morphology of the nanoparticles. However, ligands similar to amines are often considered as catalyst poison, as ligands block available adsorption sites15,16. Therefore, to increase the catalytic activity of the catalysts, ligands are commonly removed by pre-treatment, for instance, calcination or UV-light induced decomposition17,18.
This stands in contrast to homogeneous catalysis, where ligands are essential for stabilizing the transition metal complexes and tuning their reactivity15,19. The interaction between ligand and reactant enables to control the chemoselectivity, regioselectivity, and stereoselectivity of the homogeneously catalyzed reaction. Since the separation of homogenous catalysts from the products is not trivial, heterogeneous catalysts are more common although these are less selective and the question then arises whether ligands also have a positive effect on heterogeneous catalysis.
A promising approach for ligands in heterogeneous catalysis is the use of self-assembling monolayers containing aromatic and aliphatic thiols to improve the selectivity for the hydrogenation of &#945;,&#946;-unsaturated aldehydes and polyunsaturated fatty acids on Pt and Pd nanoparticles. The enhancement of the selectivity is based on several effects. Specific interactions between reactant and modifier, selectively blocking of certain unwanted&#160;active sites as well as steric and electronic effects play a role in the selectivity enhancement20,21,22,23. A distinction is made between ligands and spectators. Spectators do not participate, but influence the reaction by steric effects, while ligands are involved in reactions24,25. A spectator can be formed during a catalytic reaction or by prior chemical processes11,26.
The choice of a suitable ligand and solvent for a successful liquid phase hydrogenation is a challenging task. The solvent must have a high solubility for hydrogen as well as for the reactant. Furthermore, there should not be any following or side reactions with the solvent, which can lower the selectivity of the reaction. An appropriate ligand should have a strong adsorption at selected adsorption sites so that the desorption of the ligand under reaction conditions is prevented, but catalytic activity is still present. Ideally, the ligand blocks adsorption sites, which favor side reactions or steer the selectivity of the reaction by the sterically demands of the ligand and by interactions with the reactant15,21.
This work elucidates whether steric and electronic effects of dodecyl amine (DDA) influence the hydrogenation of cyclohexene and 5-methylfurfural (5-MF) or not. DDA does not interact directly with the reactants, which implies a spectator-directed hydrogenation. 5-MF, a non-toxic derivate of furfural, was used as a more complex and commercially interesting reactant, compared to the hydrogenation of cyclohexene. The selective hydrogenation of furfural, a side product from the production of bio petroleum, and derivatives of furfural are of industrial interest as these compounds can be gained from the biomass and represent promising starting components for the production of several fine chemicals27,28.
However, selective hydrogenation is challenging, since the hydrogenation of the carbon double bonds, and the carbonyl group are competing. Thermodynamically, the hydrogenation of the carbon double bonds is favored against the hydrogenation of the carbonyl group29.

<table><tbody><tr><td>2-propanol</td><td>Sigma Aldrich</td><td>59300-2.5L</td><td>puriss. p. a., ACS reagent, &gt;99.8%</td></tr><tr><td>4-methyl-2-pentanol</td><td>Carl Roth</td><td>4371.2</td><td>purity: &gt;99%, for synthesis</td></tr><tr><td>5-methylfurfural</td><td>Sigma Aldrich&amp;nbsp;</td><td>137316-100G</td><td>ReagentPlus, 99 %</td></tr><tr><td>acetone</td><td>Sigma Aldrich</td><td>32201-2,5L-M</td><td>puriss. p. a., ACS reagent, &gt;99.5%</td></tr><tr><td>cannula</td><td>B Braun</td><td>4665643</td><td>diameter: 0.80 mm, length: 120 mm</td></tr><tr><td>CasaXPS</td><td>Casa Software</td><td></td><td>software, version 2.3.15</td></tr><tr><td>centrifuge</td><td>Heraeus</td><td></td><td>model: Multifuge 1s</td></tr><tr><td>centrifuge tube</td><td>Schott Duran</td><td>163-9315026</td><td>volume: 80 mL, diameter: 44 mm, length: 100 mm</td></tr><tr><td>chloroplatinic acid hexahydrate</td><td>Merck</td><td>8073400001</td><td>amount of platinum: 40 %</td></tr><tr><td>column</td><td>Agilent Technologies</td><td>19091 S-001</td><td>model: HP-PONA, film: dimethyl polysiloxane, film thickness: 0.2 &amp;micro;m, length: 50 m</td></tr><tr><td>CRYSTAL 17</td><td>CRYSTAL Theoretical Chemistry Group Torino</td><td></td><td>software, version: v1.0.2</td></tr><tr><td>crystallizing dish</td><td></td><td></td><td>volume: 50 mL</td></tr><tr><td>cyclohexene</td><td>Acros Organics</td><td>154840010</td><td>purity: 99 %</td></tr><tr><td>desposable syringe</td><td>Henke Sass Wolff</td><td></td><td>Norm-Ject, volume: 1, 2, 5 mL</td></tr><tr><td>didodecyldimethylammonium bromide</td><td>Acros Organics</td><td>407120250</td><td>purity: 99 %</td></tr><tr><td>diisopropyl ether</td><td>Carl Roth</td><td>T899.1</td><td>purity: 98%, for synthesis</td></tr><tr><td>dodecyl amine</td><td>Sigma Aldrich</td><td>D222208-500ML</td><td>purity: 98 %</td></tr><tr><td>double walled tank reactor</td><td>processed by glass blower</td><td></td><td>Standard ground glass joint sleeves: 2 x 14/23, 1 x 19/26, 1 x 29/32, reactor volume: 150 mL, material: quartz glas, with outer heating jacket</td></tr><tr><td>Fourier-transform infrared spectrometer</td><td>Bruker</td><td></td><td>model: Equinox 55</td></tr><tr><td>rubber balloon</td><td>Deutsch &amp;amp; Neumann</td><td>163-7652667</td><td>volume: 4 L, material: latex,</td></tr><tr><td>gaschromatograph</td><td>Agilent Technologies</td><td></td><td>model: 7820A</td></tr><tr><td>HP-PONA-column</td><td>Agilent Technologies</td><td>19091S-001</td><td>length: 50&amp;nbsp;m, film thickness: 0.5&amp;nbsp;&amp;micro;m, inner diameter: 0.2&amp;nbsp;mm</td></tr><tr><td>hydrogen</td><td>Air Liquide</td><td>P0231L50R2A001</td><td>purity: 5.0</td></tr><tr><td>ImageJ</td><td>Wayne Rasband</td><td></td><td>software, version 1.52</td></tr><tr><td>methanol</td><td>Sigma Aldrich</td><td>32213-2,5L-M</td><td>puriss. p. a., ACS reagent, &gt;99.8%</td></tr><tr><td>n-hexane</td><td>VWR Chemicals</td><td>24577298</td><td>purity: 99 %</td></tr><tr><td>Opus</td><td>Bruker</td><td></td><td>software, version 5.5</td></tr><tr><td>pasteur pipette</td><td>Brand</td><td>747715</td><td>material: glass, length: 145 mm, inside diameter: 1 mm</td></tr><tr><td>pipette ball</td><td>Technikplaza</td><td>89005517</td><td>diameter: 94 mm, material: PVC</td></tr><tr><td>platinum(IV) chloride</td><td>Acros Organics</td><td>195400010</td><td>purity: 99 %</td></tr><tr><td>plunge operated pipette</td><td>LLG Lab Logistics Group</td><td>9.280 005</td><td>volume: 100-1000 &amp;micro;L</td></tr><tr><td>plunge operated pipette</td><td>LLG Lab Logistics Group</td><td>9.280 001</td><td>volume: 0.5-10 &amp;micro;L</td></tr><tr><td>potassium bromide</td><td>Carl Roth</td><td>9252.1</td><td>purity:&amp;nbsp; &gt;98%</td></tr><tr><td>reflux condenser</td><td>neoLab</td><td>LZ-1197</td><td>length: 160 mm, NS 14/23</td></tr><tr><td>rolled rim glass</td><td>VWR Chemicals</td><td>548-0625</td><td>volume: 10 mL</td></tr><tr><td>round neck flask</td><td>Carl Roth</td><td>HY50.1</td><td>volume: 10 mL, NS 14/23</td></tr><tr><td>rubber septum</td><td>Carl Roth</td><td>EE04.1</td><td>material: silicone, NS 14/23</td></tr><tr><td>syringe filter</td><td>Agilent Technologies</td><td>5190-5267</td><td>Captiva Econofilter, pore size 0.2 &amp;micro;m, PTFE menbrane</td></tr><tr><td>syringe pump</td><td>Landgraf Laborsysteme HLL</td><td>106720180</td><td>model: LA180A</td></tr><tr><td>TEM grid</td><td>Plano</td><td></td><td>diameter: 3.05 mm, 300 mesh, covered with formvar and coal</td></tr><tr><td>temperature programmed oven</td><td>Nabertherm</td><td></td><td>model: L5, voltage: 230 V, power: 2.4 kW, controler: C6</td></tr><tr><td>tetrabutylammonium borohydride</td><td>Sigma Aldrich</td><td>230170-10G</td><td>purity: 98 %</td></tr><tr><td>three neck round bottom&amp;nbsp; flask</td><td>Carl Roth</td><td>KY19.1</td><td>volume: 100 mL, NS 14/23, 14/23</td></tr><tr><td>Titania P25</td><td>Acros Organics</td><td>384292500</td><td>purity: 99 %</td></tr><tr><td>toluene</td><td>VWR Chemicals</td><td>32249-1L-M</td><td>puriss. p. a., ACS reagent, &gt;99.7%</td></tr><tr><td>transition piece</td><td>Carl Roth</td><td></td><td>with core and stop cock, straight tubing olive, 29/32</td></tr><tr><td>transmission electron microscope</td><td>Zeiss</td><td></td><td>model: 900N</td></tr><tr><td>ultrasonic bath</td><td>Bandelin</td><td>305</td><td>model: RK 156,&amp;nbsp; volume: 6 L</td></tr><tr><td>volumetric pipette</td><td>Brand</td><td>29718</td><td>volume: 50 mL</td></tr><tr><td>X-ray photoelectron spectrometer</td><td>Thermo Fisher</td><td></td><td>model: ESCALAB 250 xi</td></tr></tbody></table>

catalytic reactions at amine-stabilized and ligand-free platinum nanoparticles supported on titania during hydrogenation of alkenes and aldehydes

Ligands like amines are used in the colloidal synthesis approach to protect platinum nanoparticles (Pt NP&#39;s) from agglomeration. Normally, ligands like amines are removed by diverse pre-treatment procedures before use in heterogeneous catalysis as amines are considered as a catalyst poison. However, a possible beneficial influence of these surface modifiers on hydrogenation reactions, which is known from spectator species on metal surfaces, is often neglected.
Therefore, amine-stabilized Pt nanoparticles supported by titania (P25) were used without any pre-treatment in order to elucidate a possible influence of the ligand in liquid phase hydrogenation reactions. The catalytic activity of amine-stabilized Pt nanoparticles of two different sizes was investigated in a double-walled stirring tank reactor at 69 &#176;C to 130 &#176;C and 1 atm hydrogen pressure. The conversion of cyclohexene to cyclohexane was determined by gas chromatography (GC) and was compared to ligand-free Pt particles. All catalysts were checked before and after reaction by transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS) for possible changes in size, shape, and ligand shell. The hydrogenation of cyclohexene in liquid phase revealed a higher conversion for amine-stabilized Pt nanoparticles on titania than the ligand-free particles. The hydrogenation of 5-methylfurfural (5-MF) was chosen for a further test reaction, since the hydrogenation of &#945;, &#946;-unsaturated aldehydes is more complex and exhibits various reaction paths. However, XPS and infrared spectroscopy (IR) proved that 5-MF acts as catalyst poison at the given reaction conditions.

Watch this Scientific Journal Video about Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania During Hydrogenation of Alkenes and Aldehydes at JoVE.com

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, synthesized by deposition of nanosized colloids or by impregnation. The hydrogenation of cyclohexene serves as a model reaction to determine the catalytic activity of the catalysts.

Ligands like amines are used in the colloidal synthesis approach to protect platinum nanoparticles (Pt NP&#39;s) from agglomeration. Normally, ligands ...

catalytic-reactions-at-amine-stabilized-ligand-free-platinum

Universidad San Sebastian

Research

JoVE Journal

Chemistry

3.4K Views.  Carl von Ossietzky University Oldenburg. This protocol shows a convenient method for comparing the catalytic properties of supported platinum catalysts, synthesized by deposition of nanosized colloids or by impregnation. The hydrogenation of cyclohexene serves as a model reaction to determine the catalytic activity of the catalysts.

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

Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium Under Mild Reaction Conditions

Dichloro-bis(aminophosphine) complexes of palladium with the general formula of [(P{(NC5H10)3-n(C6H11)n})2Pd(Cl)2] (where n = 0-2), belong to a new family of easy accessible, very cheap, and air stable, but highly active and universally applicable C-C cross-coupling catalysts with an excellent functional group tolerance. Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium [(P(NC5H10)3)2Pd(Cl)2] (1), the least stable complex within this series towards protons; e.g. in the form of water, allows an eased nanoparticle formation and hence, proved to be the most active Heck catalyst within this series at 100 &#176;C and is a very rare example of an effective and versatile catalyst system that efficiently operates under mild reaction conditions. Rapid and complete catalyst degradation under work-up conditions into phosphonates, piperidinium salts and other, palladium-containing decomposition products assure an easy separation of the coupling products from catalyst and ligands. The facile, cheap, and rapid synthesis of 1,1',1"-(phosphinetriyl)tripiperidine and 1 respectively, the simple and convenient use as well as its excellent catalytic performance in the Heck reaction at 100 &#176;C make 1 to one of the most attractive and greenest Heck catalysts available.
We provide here the visualized protocols for the ligand and catalyst syntheses as well as the reaction protocol for Heck reactions performed at 10 mmol scale at 100 &#176;C and show that this catalyst is suitable for its use in organic syntheses.

Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-phosphinetriyltripiperidine]}palladium Under Mild Reaction Conditions

Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium [(P(NC5H10)3)2Pd(Cl)2] (1) is an easy accessible, cheap, and air stable, but highly active Heck catalyst with an excellent functional group tolerance that efficiently operates under mild reaction conditions to give the coupling products in very high yields.

Dichloro-bis(aminophosphine) complexes of palladium with the general formula of [(P{(NC5H10)3-n(C6H11)n})2Pd(Cl)2] (where n = 0-2), belong to a new family ...

Synthesis and Testing of Supported Pt-Cu Solid Solution Nanoparticle Catalysts for Propane Dehydrogenation

A convenient method for the synthesis of bimetallic Pt-Cu catalysts and performance tests for propane dehydrogenation and characterization are demonstrated here. The catalyst forms a substitutional solid solution structure, with a small and uniform particle size around 2 nm. This is realized by careful control over the impregnation, calcination, and reduction steps during catalyst preparation and is identified by advanced in situ synchrotron techniques. The catalyst propane dehydrogenation performance continuously improves with increasing Cu:Pt atomic ratio.

A convenient method for the synthesis of 2 nm supported bimetallic nanoparticle Pt-Cu catalysts for propane dehydrogenation is reported here. In situ synchrotron X-ray techniques allow for the determination of the catalyst structure, which is typically unobtainable using laboratory instruments.

A convenient method for the synthesis of bimetallic Pt-Cu catalysts and performance tests for propane dehydrogenation and characterization are ...

Synthesis of Bimetallic Pt/Sn-based Nanoparticles in Ionic Liquids

We demonstrate a method for the synthesis of bimetallic nanoparticles consisting of Pt and Sn. A synthesis strategy is used in which the particular physico-chemical properties of ionic liquids (ILs) are exploited to control both nucleation and growth processes. The nanoparticles form colloidal sols of very high colloidal stability in the IL, which is particularly interesting in view of their use as quasi-homogeneous catalysts. Procedures for both nanoparticle extraction in conventional solvents and for nanoparticle precipitation are presented. The size, structure and composition of the synthesized nanocrystals are confirmed using inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDX). By this, we show that the nanocrystals are random-type alloy and of small (2-3 nm) size. The catalytic activity and selectivity in the hydrogenation of &#945;,&#946;-unsaturated aldehydes is tested in a semi-continuous batch-type reactor. In this context, the bimetallic Pt/Sn-based nanoparticles reveal a high selectivity towards the unsaturated alcohol.

A protocol for the synthesis of bimetallic nanoparticles in ionic liquids and the procedure of their catalytic testing in the selective hydrogenation of unsaturated aldehydes are described.

We demonstrate a method for the synthesis of bimetallic nanoparticles consisting of Pt and Sn. A synthesis strategy is used in which the particular ...

A Continuous-flow Photocatalytic Reactor for the Precisely Controlled Deposition of Metallic Nanoparticles

In this work, a novel photocatalytic reactor for the pulsed and controlled excitation of the photocatalyst and the precise deposition of metallic nanoparticles is developed. Guidelines for the replication of the reactor and its operation are provided in detail. Three different composite systems (Pt/graphene, Pt/TiO2, and Au/TiO2) with monodisperse and uniformly distributed particles are produced by this reactor, and the photodeposition mechanism, as well as the synthesis optimization strategy, are discussed. The synthesis methods and their technical aspects are described comprehensively. The role of the ultraviolet (UV) dose (in each excitation pulse) on the photodeposition process is investigated and the optimum values for each composite system are provided.

For a continuous and scalable synthesis of noble-metal-based nanocomposites, a novel photocatalytic reactor is developed and its structure, operation principles, and product quality optimization strategies are described.

In this work, a novel photocatalytic reactor for the pulsed and controlled excitation of the photocatalyst and the precise deposition of metallic ...

Synthesis of Metal Nanoparticles Supported on Carbon Nanotube with Doped Co and N Atoms and its Catalytic Applications in Hydrogen Production

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented herein. The novel strategy is based on a facile one-pot pyrolysis treatment of cobalt (II) acetylacetonate and nitrogen-rich organic precursors under Ar atmosphere at 800 &#176;C, resulting in the formation of Co- and N- co-doped carbon nanotube with earthworm-like morphology. The obtained catalyst was found to have a high density of defect sites, as confirmed by Raman spectroscopy. Here, cobalt (II) nanoparticles were stabilized on the atomically dispersed cobalt- and nitrogen-doped carbon nanotubes. The catalyst was confirmed to be effective in the catalytic hydrolysis of ammonia borane, in which the turnover frequency was 5.87 mol H2&#183;molCo-1&#183;min-1, and the specific hydrogen generation rate was determined to be 2447 mL H2&#183;gCo-1&#183;min-1. A synergistic function between the Co nanoparticle and the doped carbon nanotubes was proposed for the first time in the catalytic hydrolysis of ammonia borane reaction under a mild condition. The resulting hydrogen production with its high energy density and minimal refueling time could be suitable for future development as energy sources for mobile and stationary applications such as road trucks and forklifts in transport and logistics.

Here, we present a protocol to synthesize Co nanoparticles supported on carbon nanotubes with Co- and N- dopants for hydrogen productions.

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented ...

Introduction to Catalysis

Source: Laboratory of Dr. Ryan Richards &#8212; Colorado School of Mines
Catalysis is among the most important fields of modern technology and presently accounts for approximately 35% of the gross domestic product (GDP) and sustenance of approximately 33% of the global population through fertilizers produced via the Haber process.1 Catalysts are systems that facilitate chemical reactions by lowering the activation energy and influencing the selectivity. Catalysis will be a central technology in addressing the energy and environmental challenges of modern times.

Catalysis: Homogeneous and Heterogeneous Catalysts

Source: Laboratory of Dr. Ryan Richards &#8212; Colorado School of Mines
Catalysis is among the most important fields of modern technology and presently ...

Education

JoVE Science Education

Organic Chemistry

Hydrogenation

Source: Vy M. Dong and Zhiwei Chen, Department of Chemistry, University of California, Irvine, CA
This experiment will demonstrate the hydrogenation of chalcone as an example of an alkene hydrogenation reaction (Figure 1). In this experiment, palladium on carbon (Pd/C) will be used as a heterogeneous catalyst for the process. A balloon will be used to supply the hydrogen atmosphere.
<img alt="Figure 1" src="/files/ftp_upload/10350/10350fig1.jpg" /> 
Figure 1: Diagram showing the hydrogenation of chalcone to 3-phenylpropiophenone.

Catalytic Hydrogenation of Alkene: Applications in Chemistry

Source: Vy M. Dong and Zhiwei Chen, Department of Chemistry, University of California, Irvine, CA
This experiment will demonstrate the hydrogenation of ...

Organic Chemistry II

Catalytic Reactor: Hydrogenation of Ethylene

Source: <a href="https://www-jove-com.remotexs.ntu.edu.sg/author/Kerry+M._Dooley">Kerry M. Dooley</a> and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
The hydrogenation of ethylene (C2H4) to ethane (C2H6) has often been studied as a model reduction reaction in characterizing new metal catalysts.1-2 While supported nickel is not the most active metal catalyst for this reaction, it is active enough that reaction can take place at &#60; 200&#176;C.
The reaction typically involves adsorbed, dissociated hydrogen (H2) reacting with adsorbed ethylene. In other words, both hydrogen-atoms and ethylene molecules form bonds with a metal site (here denoted &#34;S&#34;). The strong bonding of ethylene with S weakens the double bond sufficiently to allow hydrogen atoms to add to ethylene, forming ethane, which is not adsorbed.
The purpose of this experiment is, first, to convert raw composition measurements to limiting reactant fractional conversions.3 These conversions can then be used in a plug-flow reactor (PFR) to fit the data to a standard power-law kinetics model by the &#34;Integral Method&#34;.3 A comparison of the experimental orders of reaction for both ethylene and hydrogen with the theoretical orders reveals in this case that the reaction is kinetically controlled rather than mass-transfer controlled.

Heterogeneous Catalytic Reactor and Hydrogenation of Ethylene

Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
The hydrogenation of ...

Engineering

Chemical Engineering

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

<h4>Introduction</h4>
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
<img alt="Figure1" src="/files/ftp_upload/11841/PT_Figure_1.svg" style="text-align: center; height: 100px;" />
<h4>Thermodynamic Stability</h4>
Catalytic hydrogenation reactions help evaluate the relative thermodynamic stability of hydrocarbons. For example, the heat of hydrogenation of acetylene is &#8722;176 kJ/mol, and that of ethylene is &#8722;137 kJ/mol. The higher exothermicity associated with the addition of hydrogen to acetylene suggests that it is thermodynamically less stable than ethylene.
<img alt="Figure2" src="/files/ftp_upload/11841/PT_Figure_2.svg" style="text-align: center; height: 45px;" />
<img alt="Figure3" src="/files/ftp_upload/11841/PT_Figure_3.svg" style="text-align: center; height: 45px;" />
<h4>Modified Catalyst</h4>
Catalytic reduction of alkynes can be stopped at the cis-alkene stage using a modified or poisoned catalyst such as Lindlar or P-2 catalyst. The Lindlar catalyst consists of palladium metal deposited on calcium carbonate and modified using lead acetate and quinoline; the P-2 catalyst is a nickel-boride complex.
<img alt="Figure4" src="/files/ftp_upload/11841/PT_Figure_4.svg" style="text-align: center; height: 100px;" />
A modified catalyst lowers the activation energy for the reduction of the first &#960; bond; however, it is not powerful enough to catalyze the reduction of the second &#960; bond. For example, hydrogenation of 2-pentyne over Lindlar catalyst gives cis-2-pentene.
<img alt="Figure5" src="/files/ftp_upload/11841/PT_Figure_5.svg" style="text-align: center; height: 95px;" />
<h4>Hydroboration-Protonolysis</h4>
Hydroboration-protonolysis is a non-catalytic method for the conversion of internal alkynes into cis-alkenes. The reaction involves treating an internal alkyne with borane to form a trialkenylborane intermediate followed by treatment with acetic acid to yield the desired cis-alkene.
<img alt="Figure6" src="/files/ftp_upload/11841/PT_Figure_6.svg" style="text-align: center; height: 150px;" />

Catalytic Hydrogenation of Alkynes: Formation of cis-Alkenes

Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves ...

JoVE Core

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

Reduction of Alkenes: Catalytic Hydrogenation

Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms &#8212; fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of the metal catalyst. It begins with the adsorption of the hydrogen onto the metal surface, followed by the cleavage of the H&#8211;H bonds to give individual metal&#8211;hydrogen bonds. The alkene then complexes with the catalyst surface by using its p orbitals to overlap with the empty metal orbitals of the catalyst. The two hydrogen atoms then insert into the &#960; bond sequentially through syn addition (addition to the same face of the &#960; bond) to give the reduced product &#8212; the alkane. The alkane formed is no longer bound to the metal and diffuses away from the catalyst&#39;s surface.
The process of hydrogenation is exothermic. The heat released is called the heat of hydrogenation (&#916;H&#176;), and it helps predict&#160;the relative stabilities of alkenes. For example, although the hydrogenation of both cis-2-butene and trans-2-butene gives the same product &#8212; butane, trans-2-butene is more stable than cis-2-butene. This can be explained based on the heat of hydrogenation of the two isomers. The cis isomer (&#916;H&#176;&#160;= &#8722;28.6 kcal/mol) has a slightly higher heat of hydrogenation compared to the trans isomer (&#916;H&#176;&#160;= &#8722;27.6 kcal/mol). In cis-2-butene, the steric repulsion between the two methyl groups lying on the same side of the double bond makes it less stable, which is reflected in its larger heat of hydrogenation.