Catalytic processes are employed in various spheres of life, undoubtedly increasing its quality. Catalysis is involved in over 80% of all chemical reactions [1
], making it a ubiquitous technology for sustainable development as well as a continuous challenge for researchers working on ways to improve it. Various hydrogenation processes are of great importance since they are routinely used in the production of bulk and fine chemicals, with special emphasis on the chemical synthesis of pharmaceutical [2
] and high-end cosmetics [5
An attractive group of reactants is represented by unsaturated carbonyls, which are used extensively by the food, flavor, drugs, and other fine chemicals and pharmaceutical industries [6
]. They are usually produced from unsaturated aldehydes or ketones via the hydrogenation process, thus taking control of the process chemoselectivity of vital importance [13
]. Different active metals are employed in the catalytic process depending on the target bond to be successfully hydrogenated. Traditionally, noble metal (e.g., Pt, Pd, Ru) catalysts are still widely used for hydrogenation reactions, wherein palladium and platinum-based catalysts have been the most frequently studied case in selective hydrogenation towards the preferential formation of C=C hydrogenated product [15
]. Increasingly, for environmental and economic reasons, there is a high demand for the earth-abundant, inexpensive metals like Cu, Ni, Co, Fe. Transition metal catalysts account for several chemoselective hydrogenation processes showing them as valuable alternatives to the noble metals [19
Nickel proficiency on hydrogenations was shown by Sabatier [26
], for which he was awarded the noble prize in 1912. The most notable liquid-phase hydrogenation nickel catalyst is the Raney nickel catalyst [27
] but several other nanosystems have been proposed [28
]. According to Kishida et al. [31
], nickel hydrogenizes ketones via a Langmuir–Hinshelwood mechanism, when the catalyst surface is fully covered with the adsorbed species of hydrogen and ketone. The rate constant and the adsorption strength of ketone were found to become larger as the increasing number of alkyl carbon atoms in the molecule.
In the case of unsaturated aldehydes and ketones, nickel exhibits high selectivity towards C=C bond saturation [32
] but relatively low reaction rates compared to Pd [33
]. Nickel catalyst hydrogenation catalytic properties can be promoted by the addition of a second metal achieved via doping with a second metal. In terms of selectivity enhancement, the most common dopants are less active in the catalysis than the parent metal. Thus, this enhances selectivity related to a suppression of undesirable reactions utilizing geometric and/or electronically blocking of active sites [34
]. Nickel activity in hydrogenation can be promoted by alloy formation with a second metal, such as Co [36
] and Pd [22
]. The enhancement obtained when doping the catalyst with Pd is rationalized on the basis of improved hydrogen activation [38
], and the hypothesis that is yet to be demonstrated experimentally in liquid processes.
Currently, liquid-phase hydrogenations are carried out on large-volume batch reactors [39
] that present several disadvantages, such as low atom efficiency, complicated reaction schemes, low versatility, problems with catalyst separation, and overall product quality and specificity. It is not surprising that flow catalytic hydrogenation has received renewed interest by industry and academia in the last decade [40
]. Continuous flow processes in micro-reactors enable savings in chemicals (reagents and solvents) and energy consumption, reduce reaction times, improve mass transfer and process safety, simplify workflows and reaction schemes, increasing control and quality of the entire process [41
]. However, continuous high-pressure flow processes require innovative solutions concerning active metal morphology, catalyst support, and reactors [42
The Thalesnano company was the first to release “plug-and-play” capable of performing both homogeneous and heterogeneous hydrogenations in flow and pressure, with H2
, as reductant generated via electrolysis [44
]. This type of reactor paved the way for the development of novel heterogeneous hydrogenation processes that comply with the green chemistry exigencies and sustainable goals. In our research group, we have used the system not only to test the catalysts catalytic proficiencies but to modify them, using a technique called on-the-fly modification in which we can modify the surface with other metals via a chemical reduction of organometallic complexes and metal salts [32
]. With respect to support, they need to have suitable nominal sizes and swelling properties that enable high-pressure flow processes. Polymeric resins with specialized functional groups are particularly suitable for this purpose [47
] and allow for the study of nanometal catalysts without support interference.
This study examines the hypothesis that hydrogen activation remains the rate-limiting step in nickel hydrogenations even when the reactions are carried in flow and high pressure. Using isotopic experiments and theoretical calculations, it was possible to confirm and establish that Pd is a suitable dopant to improve catalytic activity. The nickel parent catalyst surface was subsequently modified with small amounts of Pd via surface organometallic modification, resulting in the formation of nickel nanoparticles with Pd atoms on its surface and no alloy structures. Pd doping dramatically improved parent catalyst activity in sulcatone (6-methyl-5-hepten-2-one) flow hydrogenation to levels that adding 0.2 wt% Pd to a 0.88 wt% Ni catalyst even surpass the values obtained with 2.16 wt% Pd catalyst. Most importantly, the addition of Pd did not affect the overall system selectivity that remained 100% to 6-methyl-2-heptanone, confirming Ni appetency for the C=C bond. It should be mentioned that the 6-methyl-2-heptanone is a strategic intermediate in the production of vitamin E [49
]. The catalysts were found to be stable under optimal reaction conditions, showing their potential for high-pressure liquid-phase flow reactions, which to the best of our knowledge, is the first of its kind.
3. Experimental Section
Analytical grade organic salts, polymeric resin, and solvents were used as received without further purification. The chemicals were purchase from Sigma-Aldrich (Poznan, Poland) except the resin that was purchased from Rapp Polymere (Tübingen, Germany).
3.1. Preparation of the Catalysts
3.1.1. Synthesis of NiTSNH2
Parent material (NiTSNH2
) was synthesized at ambient temperature and under an argon atmosphere in two steps, as described elsewhere [21
]. Initially, Ni nanoparticles were obtained by chemical reduction of nickel (II) acetylacetonate (0.001 mol) with 0.01 g of NaBH4
(reducing agent) and 3.5 g of trioctylophosphine oxide (TOPO, capping agent) in 100 mL of ethanol. Since Ni NPs are prone to aggregation, we used tenfold excess TOPO compared to Ni in solution. After synthesis, the as-prepared Ni NPs were immobilized on amino groups terminated polymeric resin (TentaGel-S-NH2
, 5 g) with a pore volume of 0.13 cm3
/g and bead size of 130 m.
3.1.2. Synthesis of PdTSNH2
To prepare palladium nano-catalyst we adopted a similar procedure as for NiTSNH2
synthesis, which was described in detailed elsewhere [10
]. Briefly, the palladium precursor (Pd(acac)2
) was dissolved in ethanol and mixed with 1 g of TOPO (capping agent). 0.03 g of NaBH4
(reducing agent) was added to the mixture at ambient conditions and under argon atmosphere and vigorous stirring. The prepared Pd NPs were centrifuged and purified with ethanol and hexane and finally were immobilized on commercially available polymeric resin TentaGel-S-NH2
(130 μm of bead size, a pore volume of 0.13 cm3
/g). The obtained catalyst PdTSNH2
was dried at 90 °C and then used in the hydrogenation as prepared.
3.1.3. Synthesis of Pd Modified NiTSNH2
Palladium surface modification was carried out via a modified surface redox reaction method, described by Barbier et al. [59
] and Sá et al. [61
]. Scheme 2
depicts a representation of the reactor used to modify the parent Ni catalyst with Pd. As-prepared 0.5 g NiTSNH2
was placed in the sealed batch reactor. The catalyst was reduced at 150 °C for 1 h in a stream of hydrogen and subsequently cooled to room temperature in an argon atmosphere. Once cooled, 15 mL of purified ethanol was introduced with a syringe, followed by 10 mL palladium acetylacetonate (Pd(acac)2
) solution in ethanol. The obtained catalyst was dried in the flow of argon (40 min) and afterwards in the flow of mixture Ar + H2
. The modified catalysts are labeled NiPdy
, y being the Pd atomic ratio.
3.2. Catalysts Characterization Technics
The metals concentration in monometallic and palladium-modified nickel catalysts were determined by atomic absorption spectrometry (AAS) (Thermo Fisher, Waltham, MA, USA) after the dissolution of the metals in aqua regia.
Optical images of were collected on digital USB microscope (RS Pro, Fort Worth, TX, USA). The samples were supported on a microscopic glass and imaged without a glass cover.
Catalysts were characterized by FEI transmission electron microscopy (TEM) with the electron microscope Titan G2 60–300 kV (FEI company, Tokyo, Japan) equipped with EDAX EDS (energy-dispersive X-ray spectroscopy) detector. Microscopic studies of the catalysts were carried out at an accelerating voltage of the electron beam equal to 300 kV. The samples were prepared by dispersing the catalysts in ethanol and sonication until clear solutions were obtained. The TEM specimen consisted of a drop of the prepared suspensions on carbon films on copper grids.
Powder X-ray diffraction (PXRD) measurements were performed in a X-Ray Diffractometer D5000 (Siemens, Munich, Germany) employing Bragg–Brentano configuration. This type of arrangement was provided using PANalytical Empyrean diffraction platform (Malvern, Germnay), powered at 40 kV × 40 mA and equipped with a vertical goniometer, with theta-theta geometry using Ni filtered Cu Kα radiation. Data were collected in the range of 2θ = 5–95°, with a step size of 0.008° and counting time 60 s/step.
X-ray absorption spectroscopy (XAS) measurements were performed in air in a home built laboratory X-ray setup [63
]. The setup uses a von Hamos geometry-based detection system. The source was XOS X-Beam Superflux PF X-ray tube (XOS, New York, NY, USA) operated at the voltage of 40 kV and the current of 0.9 mA. The photon beam left the X-ray tube through an integrated focusing optics at about 3° divergence and illuminated the sample. The sample position was fixed at the beam focal point with the nominal beam spot size of about 100 µm. The radiation transmitted through the target was diffracted by a cylindrically bent Si(440) crystal with 25 cm-radius of curvature at the Bragg angle of 65.2° and registered by an Andor Newton CCD camera (Oxford Instruments, Abingdon, UK) sealed with 250 µm-thick Be window. The camera’s CCD chip, a front-illuminated sensor composed of 1024 × 256 with 26 µm-sized pixels, was operated at the pressure of about 10−7
mbar and the temperature of −40 °C controlled by a thermoelectric cooler built-in device.
The chemical surface composition of the series of Ni-Pd catalysts was characterized by the XPS spectroscopy using a Microlab 350 spectrometer (Thermo Electron, Abingdon, Germany). XPS spectra were excited using AlKα (hν = 1486.6 eV, 300 W) radiation as a source. Survey spectra and also high-resolution spectra were recorded using 100 and 40 eV pass energy, respectively. A linear or Shirley background subtraction was made to obtain XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function. The measured binding energies were revised referring to the energy of C1s at 285.0 eV. For data processing was used an Avantage software version 4.88 (Thermo Fisher, Waltham, MA, USA).
3.3. 6-Methyl-5-Hepten-2-One Hydrogenation
Hydrogenation reactions were conducted using commercial high-pressure micro-reactor H-Cube Pro equipment from Thales Nano (Thales Nano, Hungary) with built in H2 generator (by water electrolysis). Deuterium experiments were performed by using D2O instead of H2O electrolysis. The mixture of 0.05 M 6-methyl-5-hepten-2-on in ethanol was continuously injected to the system using an HPLC pump and it flowed through the disposable cartridge CatCart70 with an ID of 4 mm where was preloaded required 0.15 g monometallic or bimetallic catalyst. To determine the optimal reaction parameters, in every case was performed screening reaction with a wide range of temperatures (25–100 °C) and pressures (10–60 bar) with reactant flow rate 0.5 mL/min (25 mol/min). After that, 5-h hydrogenation reactions were conducted with previously experimentally determined the best reaction parameters. The reported activity and selectivity values were extracted after reaching steady-state performance, normally within 25–30 min of operation. Time zero in every reaction was stipulated to be 3 min of after starting the substrate solution flow as this is the estimated residence time of a solution in the reactor at a flow rate of 0.5 mL/min. The reaction progress and concentrations of the products were verified by gas chromatography (GC), i.e., Bruker 456 GC (Bruker, Bremen, Germnay) equipped with FID detector and non-polar BP-5 0.25 µm (5% phenyl, 95% dimethyl polysiloxane) column.
Note that the gas pressure and flow in the reactor are monitored continuously, and thus providing a direct measurement of the amount of gas present in the system. Additionally, the experiments were performed only after equilibration of the system, meaning reactions were performed after reaching stable gas pressure and flow. Finally, the electrolysis cell produces excess gas to avoid instabilities in the system throughout the reaction. This ensures that KIE results due to the parent catalyst abilities, not experimental limitations.
3.4. Quantum Chemical Calculations
Set-up for the density functional theory (DFT) calculations was described in great details in our previous works on Ni-Sn [34
] and Ni-Zr [35
] systems. Briefly, the final reported energies for icosahedral Ni13
Pd, and Pd13
model nanoparticles were computed with the PBE functional [64
] augmented with D3BJ dispersion correction [65
] along with the def2-TZVP basis set [67
] at the multiplicity of 9. Zero-point energies were obtained after numerical frequency run. ORCA 4.2.0 program [68
] was used for all DFT computations. In this work, however, we used a different method to generate the initial candidates for the lowest energy structures due to multiple possible configurations of H2
and H atoms adsorbed at the nanoparticle models. Here, the pre-optimization was performed using the extended tight binding method (GFN2-xTB) of Grimme et al. [69
] using their stand-alone code. Fermi smearing (300 K) of the orbital occupation numbers was applied to ensure proper electronic structure convergence. Molecular dynamics simulations at 100 ps was run to confirm stability of obtained structures. In all cases, end-on H2
binding mode was obtained and H atoms were decorating the nanoparticles. The obtained structures were then used as the starting point for DFT computations. XYZ coordinates of key structures are reported in the Supplementary Materials