Tuning nano-nickel selectivity with tin in flow hydrogenation of 6-methyl-5-hepten-2-one by surface organometallic chemistry modification

Article abstract of DOI:10.1016/j.cattod.2017.08.062

Chemoselective flow hydrogenation of 6-methyl-5-hepten-2-one was performed over nano-nickel catalysts. The parent catalyst composed solely of nickel nanoparticles grafted on the polymeric resin exhibited high activity and selectivity towards C[dbnd]C bond saturation but its modification with small quantities of tin significantly increased its ability to perform C[dbnd]O bond hydrogenation. The post-synthetic modification of the parent catalyst was achieved by surface organometallic chemistry approach and performed in the same flow micro-reactor, which was used for the catalytic studies, providing a methodology for online modification of parent catalysts.

Full text of DOI:10.1016/j.cattod.2017.08.062

CatalysisTodayxxx(xxxx)xxx–xxx
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Catalysis Today
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Tuning nano-nickel selectivity with tin in flow hydrogenation of 6-methyl-5-
hepten-2-one by surface organometallic chemistry modification
Małgorzata Zienkiewicz-Machnika,⁎, Ilona Goszewskaa, Anna Śrębowataa, Adam Kubasa,
Damian Gizińskia, Grzegorz Słowikb, Krzysztof Matusc, Dmytro Lisovytskiya, Marcin Pisareka,
⁎
Jacinto Sáa,d,
a
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Faculty of Chemistry, Department of Chemical Technology, Maria Curie-Skłodowska University, 20-031 Lublin, Poland
Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland
b
c
d
Department of Chemistry, Ångström Laboratory, Uppsala University, Uppsala 75120, Sweden
A R T I C L E I N F O
A B S T R A C T
Keywords:
Chemoselective flow hydrogenation of 6-methyl-5-hepten-2-one was performed over nano-nickel catalysts. The
parent catalyst composed solely of nickel nanoparticles grafted on the polymeric resin exhibited high activity
and selectivity towards C]C bond saturation but its modification with small quantities of tin significantly in-
creased its ability to perform C]O bond hydrogenation. The post-synthetic modification of the parent catalyst
was achieved by surface organometallic chemistry approach and performed in the same flow micro-reactor,
which was used for the catalytic studies, providing a methodology for online modification of parent catalysts.
Chemoselective flow hydrogenation
6-methyl-5-hepten-2-one
Nano-Ni grafted on resin
Tin surface modification
1. Introduction
but also a decrease in conversion, which is expected if the added Sn is
added is located on the catalyst surface because Sn is inactive metal in
Continuous-flow processes offer significant advantages over batch
reactions in terms of safety, efficiency, reproducibility and waste re-
duction [1–5]. It allows for practical, efficient organic transformations,
which comply with the green chemistry’s twelve principle [6,7].
Nevertheless, the replacement of conventional batch processes with
flow mode remains a major challenge in modern organic chemistry, not
only in research laboratories but also in the chemical and pharmaceu-
tical industries. The development of continuous-flow systems for the
catalytic hydrogenation with heterogeneous catalysts is a rapidly
growing research topic [2,8–12].
Selective hydrogenation of α,β-unsaturated carbonyls has been in-
tensively studied for the production of unsaturated alcohols, which are
highly desirable molecules for the fine chemical industry [13]. Nickel
based catalysts display selectivity towards C]C saturation rather than
C]O bond, the desirable reaction [12,14–16]. A possible strategy to
improve nickel-based systems chemoselectivity toward C]O group
hydrogenation is the modification of the parent catalysts by addition of
more electropositive metal [17], such as cerium [18], iron [19] and tin
[13–15,20,21]. Tin’s low cost and significant impact in reaction se-
lectivity with low dopant levels made it a very popular choice. Gen-
erally, Sn modification of catalysts composed of Pt [24–27], Ru
[28–31], etc, caused a significant increase in C]O bond hydrogenation
hydrogenation reactions.
This study reports on nano-nickel based polymer-supported catalyst
active in the chemoselective flow hydrogenation of 6-methyl-5-hepten-
2-one to 6-methylheptan-2-one, a precursor employed in vitamin E
synthesis [32]. The parent catalyst selectivity can be modified with the
addition of tin to produce the unsaturated alcohol (6-methyl-5-hepten-
2-ol), which is widely used as a fragrance ingredient in cosmetics,
fragrances, shampoos, toilet soaps and other toiletries as well as
household cleaners and detergents [33]. Tin surface doping was
achieved via post-synthetic surface organometallic modification
chemistry (SOMC) performed in situ in the same reactor where catalytic
tests were performed, opening the prospectus for online and on demand
tuning of catalytic performance. This post-synthetic modification
methodology seems to be a promising tool especially for polymer sup-
ported catalysts with low thermal stability. Furthermore, our pre-
liminary quantum chemical calculations revealed the role of surface tin
atoms in selectivity tuning via van der Waals interactions. In our sim-
plified analysis, we considered only local energy minimum structures
(intermediates) present in the assumed reaction mechanism. Such
analysis bases on the Brønsted–Evans–Polanyi principle [34,35] that
relates linearly the activation barrier difference of a process with the
enthalpy of this transition. Such approach was recently very successful
⁎
Corresponding authors at: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.
E-mail addresses: mzienkiewiczmachnik@ichf.edu.pl (M. Zienkiewicz-Machnik), jacinto.sa@kemi.uu.se (J. Sá).
http://dx.doi.org/10.1016/j.cattod.2017.08.062
Received 8 July 2017; Received in revised form 4 August 2017; Accepted 30 August 2017
0920-5861/©2017ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Zienkiewicz-Machnik,M.,CatalysisToday(2017),http://dx.doi.org/10.1016/j.cattod.2017.08.062

M. Zienkiewicz-Machnik et al.
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in prediction of new catalysts via quantum chemical calculations
[36,37]. In this way we found the steric hindrance caused by the tin ad-
atoms leads to switch of the thermodynamic preference from C]C bond
hydrogenation in the case of pure Ni catalyst into exclusive selectivity
towards carbonyl group hydrogenation upon tin addition.
employing Bragg-Brentano configuration. This type of arrangement was
provided using PANalytical Empyrean diffraction platform, powered at
40 kV × 40 mA and equipped with a vertical goniometer, with the-
ta–theta geometry using Ni filtered Cu Kα radiation. Data were col-
lected in range of 2θ = 5–95°, with step size of 0.008° and counting
time 60 s/step.
2. Experimental
The chemical composition and chemical state of the catalyst sam-
ples’ were characterized by the X-ray photoelectron spectroscopy (XPS).
XPS were measured in a Microlab 350 spectrometer (Thermo Electron)
using AlKα (hν = 1486.6 eV, 300 W) as a source. Survey spectra and
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. Sample charging was corrected
using C 1s sp3 signal (284.6 eV).
2.1. Materials
Analytical grade reagents and solvents were purchased from com-
mercial sources and used as received. TentaGel-S-NH2 was purchased
from Rapp Polymer GmbH, trioctylophosphine oxide from Sigma-
Aldrich Co., Ni(II) acetylacetonate from Acros Organics, tetrabuthyltin
from ABCR, ethanol from POCH S. A. Gliwice and 6-methyl-5-hepten-2-
one from TCI Co.
2.4. 6-Methyl-5-hepten-2-one hydrogenation
2.2. Preparation of the catalysts
Catalytic hydrogenation was performed using ThalesNano H-Cube
Pro continuous-flow micro-reactor. 6-methyl-5-hepten-2-one solution
in ethanol (5 × 10−2 M) was flown through 0.15 g of catalyst with a
HPLC pump. The catalyst was placed in CatCart®70 cartridge with an ID
of 4 mm. The hydrogen was generated in situ via water electrolysis. In
order to obtain the best reaction parameters, reactions were conducted
with NiTSNH2 over a wide range of temperatures (45–100 °C) and
pressures (5–60 bar) but constant reactant flow rate (0.5 ml/min). The
LHSV (liquid hourly space velocity) for the conducted reactions was
estimated and it was find to be 34. All the other reactions were carried
out using the optimum conditions established with parent catalyst.
Substrate conversion and products formation were analysed by gas
chromatography (GC), namely a Bruker 456 GC equipped with FID
detector and non-polar BP-5 0.25 μm (5% phenyl, 95% dimethyl
polysiloxane) column.
2.2.1. Synthesis of NiTSNH2
Nickel catalyst (NiTSNH2) was obtained at room temperature con-
ditions, during one-pot, two-step synthesis. In the first step, Ni nano-
particles (Ni NPs) were collected by chemical reduction of Ni(II) acet-
ylacetonate (0.001 mol) with NaBH4 as a reducing agent in 65 ml of
ethanol. The molar ratio of Ni(acac)2: NaBH4 was found to be 1:2. To
prevent agglomeration of Ni NPs, tenfold excess of trioctylophosphine
oxide (TOPO) in relation to Ni was used. Obtained NPs were subse-
quently immobilised on the commercial −NH2 terminated polymeric
resin (TentaGel-S-NH2, 5 g) with pore volume of 0.13 cm3/g and bead
size of 90 μm.
2.2.2. Synthesis of NiSnTSNH2
Due to the fact that the parent catalyst is supported on resin with
low thermal stability, classical preparation method, such as salt im-
pregnation or post-grafting of organometallic precursors on pre-formed
catalyst nanoparticles followed by high temperature treatments
(> 300 °C), can not be used. Therefore parent catalyst modification
with tin was performed in the H-Cube Pro™ continuous-flow system
using surface organometallic modification chemistry methodology,
adapted from a previous work [38]. Additionally we assumed that the
changing of the synthesis method from batch mode to continuous flow
improves sample homogeneity and process scalability. Briefly, 0.15 g of
NiTSNH2 was loaded in the 70 mm long stainless-steel cartridge (4 mm
i.d.) and pretreated for 1 h with 40 bar of hydrogen and ethanol flow.
2.5. Quantum chemical calculations
All calculations were carried out using ORCA 4.0 package [39]
within the density functional theory (DFT) framework. Spin-unrest-
ricted formalism was used throughout the study and metallic clusters
were assumed to have multiplicity of 11 [40,41]. In the case of mole-
cular complexes of 6-Methyl-5-hepten-2-one with Ni13 and Ni13Sn the
starting geometries of the interacting organic molecule were obtained
with Avogadro program [42] by generating large number of rotamers of
the parent molecule and subsequent random placement in the vicinity
of the metallic clusters. Initial pre-optimisations were performed with a
PBE functional [43] augmented with the recent D3BJ dispersion cor-
rection of Grimme [44,45] (denoted with ‘ + D3′ suffix) using a com-
pact def2-SVP basis set [46]. Lowest energy isomers were subsequently
subject to geometry refinement with a larger def2-TZVP basis set [46].
All reported structures were submitted to Cartesian second derivative
calculations and were confirmed as true local minima (no imaginary
modes). Final reported single-point energies were evaluated at PBE
+ D3/def2-TZVP level of theory and include zero-point energy cor-
rection. All calculations employed the resolution-of-the-identity ap-
proximation (RI) [47] along with the corresponding auxiliary basis set
[48].
Afterwards
a
solution of tetrabuthyltin (SnBu4) in ethanol
(2.3 × 10−2 M) was flown through the cartridge at 100 °C under 40 bar
of hydrogen with a flow rate of 0.5 ml/min for 10 min leading to the
preparation of NiSn0.12TSNH2. The obtained bimetallic catalyst was
used in 6-methyl-5-hepten-2-one hydrogenation immediately after the
preparation.
2.3. Catalysts characterisation
The metals concentration, both in monometallic and bimetallic
catalysts was determined by atomic absorption spectrometry (AAS) and
inductively coupled plasma − optical emission spectrometry (ICP-OES)
after dissolution of the metals in aqua regia.
Transmission electron microscopy (TEM) experiments were carried
out on the electron microscope Titan G2 60–300 kV (FEI, Japan)
equipped with EDAX EDS (energy dispersive X-ray spectroscopy) de-
tector. Microscopic studies of the catalysts were carried out at an ac-
celerating voltage of the electron beam equal to 300 kV. The samples
were prepared by their dispersing in pure alcohol using ultrasonic
cleaner and putting a drop of this suspension on carbon films on copper
grids.
3. Results
The nickel content was estimated by AAS and ICP-OES, and found to
be 0.68 wt% for NiTSNH2 and 0.66 wt% for NiSn0.12TSNH2. The ab-
breviation of the doped catalyst corresponds to the Ni/Sn atomic ratio,
corroborated by XRF measurements. Fig. 1 shows HR-TEM images and
nickel nanoparticles size distribution for NiTSNH2. Metal nanoparticles
are well dispersed on the polymeric resin with an average Ni particles
size ca. 6.8 nm.
Powder X-ray diffraction (PXRD) measurements were performed
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Fig. 1. TEM images and metal nanoparticle size
distribution of NiTSNH2.
Fig. 2. PXRD patterns for the polymeric resin, mono- and bi-metallic catalysts.
Fig. 3. Ni 2p3/2 XPS region of NiTSNH2.
PXRD diffraction patterns of the pure support (TentaGel-S-NH2) and
monometallic nickel catalyst (NiTSNH2) were largely the same (Fig. 2),
suggesting that metal loading did not change the structure of the resin,
which is in agreement with our previous report concerning PdTSNH2
catalyst [10]. However small changes in the polymer terminations after
doping with Sn were revealed by FTIR measurements. The comparison
study of the IR spectra for the parent and Sn-modified catalysts show
some changes in region of 1900–1600 and 1250–1150 cm−1 (Fig. S1).
However these identified changes did not affect the catalytic behaviour
of NiSnTSNH2 catalyst.
XPS was carried out to determine the oxidation state of the grafted
Ni NPs. Fig. 3 shows Ni 2p region fitted using constrains for area in-
tensity, full maximum half width and doublet energy separation tabu-
lated in the NIST database. The dominant signal of Ni 2p3/2 located at
854.8 eV (51%) relates Ni(OH)2 [49]. The other two peaks at 860.0 eV
(33%) and 852.2 eV (16%) are ascribed to metallic Ni shake-up satellite
and Ni0 [50], respectively.
Continuous-flow hydrogenation of 6-methyl-5-hepten-2-one was
carried out over NiTSNH2 under different conditions of the temperature
and pressure, in order to optimize reaction conditions. Figs. 4 and 5
show the relationship between the reaction conditions (temperature
and H2 pressure) in the substrate catalytic activity and the amounts of
desired product, respectively. While temperature seems to affect mini-
mally the substrate activity, this is not the case with H2 pressure, where
higher H2 pressures led to a significant increase in activity. It should be
mentioned that measurable activity was only detected above 65 °C and
20 bar of H2 pressure, and thus only values above those limiting con-
ditions are reported.
In respect to selectivity, the increase in reaction temperature
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respect to the parent catalyst. However, there is a significant increase of
the metallic nickel component and a shift of 0.6–0.8 eV to higher
binding energy of all the components, when tin is present. Fig. 8 shows
Sn 3d region fitted using constrains for area intensity, full maximum
half width and doublet energy separation tabulated in the NIST data-
base. The signal was successfully fitted with a single component; the Sn
3d5/2 peak at 485.2 eV is ascribed to Sn0 [51]. The fact that Sn is me-
tallic is also a good indication that the modification involves catalytic
reduction of the Sn salt on the surface of Ni, where it most likely stays
because it needs Ni to stabilize it’s oxidation state. Surface segregation
would lead almost inevitably to Sn oxidation.
The addition of small amounts of tin to the parent catalyst resulted
in a significant increase in selectivity towards unsaturated alcohols
(Fig. 9), which is in agreement with previous reports on unsaturated
aldehydes and ketones hydrogenation with tin modified Ni-based cat-
alysts [14,15]. A 32% selectivity toward unsaturated alcohol was
achieved with NiSn0.12TSNH2. The activity of the modified catalysts
decreased from 0.021 mmol/min (NiTSNH2) to 0.012 mmol/min (what
corresponds to 42% conversion) for NiSn0.12TSNH2, in agreement with
previous findings reported for tin doped materials used in the hydro-
genation of α, β-unsaturated aldehydes and ketones [24–31,52,53]. It
should be mentioned that monometallic SnTSNH2 did not show any
activity.
Fig. 4. Effect of hydrogen pressure and reaction temperature on 6-methyl-5-hepten-2-one
catalytic activity over NiTSNH2.
As with the parent catalyst, the tin modified version maintained
catalytic performance for 5 h.
EDXRF analysis of the post-reaction solutions had no measurable
amount of metals (i.e., no detectable metal leaching) and the XPS of the
spent catalyst was qualitatively the same as before reaction. All this
suggests that the system is rather stable but more should be done to
ascertain catalyst stability. Note that extensive stability tests are out of
the scope of this communication, and should be performed only when
the desired selectivity to unsaturated alcohol is attained.
In order to gain further insights into the selectivity alternation
mechanism upon tin addition to the nickel catalyst we carried out series
of quantum chemical simulations. According to Fig. 10, C]C hydro-
genation of the 6-methyl-5-hepten-2-one is the most thermo-
dynamically plausible route without a catalyst. Alternative C]O hy-
drogenation is less exothermic by about 15 kcal/mol. The situation is
somehow different when the reaction involves a nickel or nickel-tin
catalyst. We used a simple, approximately icosahedral Ni13 molecule
[54–56] to represent a nickel nanoparticle. According to recent litera-
ture reports this cluster is the smallest nickel cluster that possess
properties similar to those of nanometre regime [55,57,58]. A tin-
doped system was represented with a Ni13Sn cluster where Sn atom
occupies a vertex site [59]. 6-methyl-5-hepten-2-one interacts with Ni13
and Ni13Sn most favourably via both functional groups, ketone and
double bond. We found the interaction energy of 6-methyl-5-hepten-2-
one with a Ni13 cluster to be higher by 9.8 kcal/mol as compared to
Ni13Sn bimetallic cluster (-70.4 vs. −60.6 kcal/mol, respectively). The
difference is particularly due to steric hindrance caused by the tin ad-
atom (depicted in the inset). Subsequent H2 addition leads to C]C or
C]O hydrogenation products depicted in Fig. 10 in red or blue, re-
spectively. The energy difference between possible reaction products
still interacting with the catalyst is noticeably smaller than in the case
of the free products. Interestingly, hydrogenation in the presence of
Ni13Sn catalyst switches the order of thermodynamically favoured
products. Here, the ketone hydrogenation product in a complex with
the Ni13Sn is the most stable while the Ni13Sn ligated with 6-methyl-
heptan-2-one was found to be 2.1 kcal/mol higher in energy. Thus, the
energy profile calculations support experimentally observed change in
selectivity switch of the nickel catalyst upon tin donation.
Fig. 5. Effect of hydrogen pressure and reaction temperature on the selectivity of 6-me-
thyl-5-hepten-2-one hydrogenation towards 6-methylheptan-2-one with NiTSNH2.
promotes product (6-methylheptan-2-one) formation, while the op-
timum H2 pressure for saturated product was found at the intermediate
value, which suggests that higher pressure and temperature resulted in
loss of selectivity. According to these tests, the optimum values were
found to be 100 °C and 40 bar H2 pressure, equating to a rate of sub-
strate conversion of ca. 0.021 mmol/min while preserving 100% se-
lectivity to the saturated ketone. These conditions were selected as the
best conditions and used subsequently to evaluate the effect of tin
doping. It is worth mentioning that during the reaction with TSNH2
there was no product of hydrogenation what excludes catalytic activity
of the polymeric resin. Additionally, the parent catalyst maintained the
reported performance for the entire time on stream (ca. 5 h).
Tin surface modification was carried out to modify catalytic se-
lectivity. XPS analysis was performed to establish i) Sn presence on Ni
surface, ii) what is Sn oxidation state. To answer the first point, the N 1s
area normalized Ni 2p3/2 area measured in the XPS survey scans; the N
signal originates solely from the resin termination groups and is con-
stant throughout. By doing this normalization, it is possible to perform
direct comparison of Ni surface concentration between parent and
doped catalysts. Addition of Sn led to a significant decrease of Ni sur-
face signal, roughly by 70%. A similar conclusion follows from STEM
with EDS-elemental mapping investigation (Fig. 6). TEM analysis re-
vealed no significant modification of Ni particle size. Moreover ob-
tained results confirmed that tin is evenly distributed on the nickel
surface.
4. Discussion
XPS was used to establish the oxidation state of Ni and Sn after
doping. Fig. 7 shows the Ni 2p3/2 region for the sample NiSn0.12TSNH2.
In terms of chemical species present, the signal does not vary much in
Although the hydrogenation of 6-methyl-5-hepten-2-one towards 6-
methylheptan-2-one might have a technological significance as a
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M. Zienkiewicz-Machnik et al.
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Fig. 6. STEM images with EDS elemental mapping
for NiSn0.12TSNH2.
selectivity to 6-methylheptan-2-one. Despite showing lower activity
when compared to the palladium system [10]; the high selectivity to
the desired product, low metal loading (NiTSNH2 (0.68 wt%) rather
than PdTSNH2 (2.16 wt%)) and low-cost of the active site, makes the
results very exciting especially from technological point of view, paving
the way for development of chemoselective hydrogenation reactions in
flow with low cost and abundant 3d transition metals, such as Ni, Cu
and Fe.
Probably, the most important factors affecting catalytic perfor-
mance is the substrate structure and metal electronic properties.
Literature data, based on theoretical results, suggests that competitive
adsorption of the functional groups is dependent on the radial expan-
sion of the metal d orbitals [60]. Hence, the electronic properties of Ni
may allow the substrate molecules to approach the metallic surface
through C]C group, resulting very high selectivity to saturated ketone,
as reflected in the present study.
Fig. 7. Ni 2p3/2 XPS region of NiSn0.12TSNH2.
In order to modify the natural aptitude of nickel to hydrogenate
C]C bonds, tin was added to the parent catalyst. XPS analysis revealed
that tin species are located on the surface of nickel since the Ni 2p XPS
signal decrease significantly. They are in agreement with elemental
mapping (Fig. 6). According to Sordelli et al. [53] the formation of
unsaturated alcohols needs an initial induction period during which the
conjugated double bond is predominantly attached.
The addition of tin to the nickel catalyst (NiTSNH2) affected the
nickel properties towards saturation of C]C bond hydrogenation.
However, the effect of tin was largely unclear and a topic of intense
debate. XPS analysis revealed that tin affects nickel electronic structure
because it shifted the Ni 2p3/2 signal by 0.6–0.8 eV to higher binding
energy. Shift to higher binding energy is often associated to an increase
in electron donation from the parent catalyst active metal, somehow
expected based on Allred-Rochow electronegativity scale, which states
that Sn and Ni have an electronegative of 1.72 and 1.75, respectively.
The Allred-Rochow electronegativity values are estimated by taking the
electrostatic force exerted by effective nuclear charge on the valence
electron. The result however contracts with previous study that re-
ported that adding tin increases nickel electron density, which was
confirmed by XANES [61] and in accordance with Pauli’s electro-
negativity scale. If the activity is normalized by the available Ni surface
sites, the Sn addition caused a little but positive effect in catalytic ac-
tivity, thus suggesting some new active sites were formed, which are
more prone to hydrogenate C]O group due to change of the electronic
properties of nickel site.
Fig. 8. Sn 3d XPS region of NiSn0.12TSNH2.
Our preliminary quantum chemical investigations suggest that the
effect of tin is dual − on one hand side it decreases the binding energy
of the α, β-unsaturated ketone to the metal nanoparticle, thus in-
creasing the kinetic control over the process while, on the other hand
the tin-doped nickel nanoparticles seem to cause steric hindrance that
favours C]O hydrogenation products over otherwise thermo-
dynamically more stable C]C hydrogenation products. Note that the
prefer mode of adsorption of C]C bond on metallic surfaces is parallel
to the surface, which requires at least two adjacent metal atoms.
Fig. 9. Selectivity toward C]C and C]O hydrogenation of 6-methyl-5-hepten-2-one for
the obtained catalysts. Reaction conditions: T = 100 °C, pressure = 40 bar.
precursor for vitamin E synthesis [32], the literature data concerning
this substrate hydrogenation is scarce [10]. The results concerning the
chemoselective hydrogenation of 6-methyl-5-hepten-2-one on Ni NPs
grafted on the resin are consistent to our previous results using palla-
dium nanoparticles anchored on TentaGel-S-NH2 [10], namely 100%
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Fig. 10. Relative energies of intermediates and pro-
ducts in the hydrogenation of 6-methyl-5-hepten-2-
one. Virtually, the reaction can proceed in an un-
catalyzed way (left) to yield thermodynamically
more stable product of C]C bond hydrogenation
(red) or C]O bond hydrogenation (blue). In the
presence of a catalyst (pure nickel or nickel doped
with tin modelled as Ni13 or Ni13Sn clusters, re-
spectively) initially a reactive intermediate complex
is formed (see structures in the insets). In the case of
Ni13Sn catalyst the steric hindrance between Sn atom
and the −CH3 group of the substrate is marked in a
usual way. For both catalysts, the energy difference
between two products is lower than in the case of
uncatalyzed reaction what eventually leads to switch
in thermodynamic stability order for Ni13Sn. In this
case C]O hydrogenation product is more stable than
the C]C hydrogenation product. (For interpretation
of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Finally, note that the proposed flow doping methodology ensures
high level of reproducibility, high control of amount of metal dopant
added, and a strategy for online modification of parent catalysts,
comparatively to batch-mode modification. All the mentioned ad-
vantages can effectively decrease time and costs with doping optimi-
zation because doping is performed on a very small amount of catalyst
at the time and the effect on catalytic performance is evaluated straight
after modification. The later opens even the possibility for modification
with air unstable modifiers.
Acknowledgments
The National Science Centre in Poland financially supported this
work under the project OPUS 8, grant no. UMO-2014/15/B/ST5/02094
and the Polish Ministry of Science and Higher Education for support
from the budget for science in 2016–2019 under grant IDEAS Plus II
IdPII 2015000164.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.cattod.2017.08.062.
5. Conclusions
Catalytic performance of the polymeric resin supported nano-nickel
and Ni-Sn catalysts have been evaluated in continuous flow chemose-
lective hydrogenation of fine chemicals. Monometallic Ni catalyst with
only 0.68% weight loading showed very high activity and 100% se-
lectivity toward C]C bond saturation. The obtained results revealed
that the addition of small quantities of tin significantly increased the
amount of unsaturated alcohol formation, which was found dependent
on amount of tin added. The effect of tin additions was examined both
experimentally and theoretically. On the basis of XPS analysis it seems
that in very low amounts, tin addition affects nickel electronic struc-
ture, while at higher concentrations it behaves as geometric modifier,
effectively decreasing adsorption of C]C bond on the catalyst surface.
Quantum chemical calculations revealed that the tin decreases the
binding affinity of the unsaturated substrate towards metal nano-
particle and leads to switch in selectivity so that C]O hydrogenation
product interacting with the catalyst is more thermodynamically stable
as compared to the C]C hydrogenation product. The post-synthetic
modification of the parent catalyst was achieved by surface organo-
metallic chemistry approach and performed in the same flow micro-
reactor, which was used for the catalytic studies, providing a metho-
dology for online modification of parent catalysts.
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