From a Molecular Single-Source Precursor to a Selective High-Performance RhMnOx Catalyst for the Conversion of Syngas to Ethanol

Article abstract of DOI:10.1002/cctc.201801978

The first molecular carbonyl RhMn cluster Na₂[Rh₃Mn₃(CO)₁₈] 2 with highly labile CO ligands and predefined Rh-Mn bonds could be realized and successfully used for the preparation of the silica (davisil)-supported RhMnO_x catalysts for the conversion of syngas (CO, H₂) to ethanol (StE); it has been synthesized through the salt metathesis reaction of RhCl₃ with Na[Mn(CO)₅] 1 and isolated in 49 % yields. The dianionic Rh₃Mn₃ cluster core of 2 acts as a molecular single-source precursor (SSP) for the low-temperature preparation of selective high-performance RhMnO_x catalysts. Impregnation of 2 on silica (davisil) led to three different silica-supported RhMnO_x catalysts with dispersed Rh nanoparticles tightly surrounded by a MnO_x matrix. By using this molecular SSP approach, Rh and MnO_x are located in close proximity on the oxide support. Therefore, the number of tilted CO adsorption sites at the RhMnO_x interface increased leading to a significant enhancement in selectivity and performance. Investigations on the spent catalysts after several hours time-on-stream revealed the influence of rhodium carbide RhC_x formation on the long-term stability.

Full text of DOI:10.1002/cctc.201801978

Accepted Article
Title: From a Molecular Single-Source Precursor to a Selective High-
Performance RhMnOx Catalyst for the Conversion of Syngas to
Ethanol
Authors: Matthias Driess, Phil Preikschas, Julia Bauer, Shenglai Yao,
Raoul Naumann d’Alnoncourt, Ralph Kraehnert, Annette
Trunschke, and Frank Rosowski
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10.1002/cctc.201801978
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From a Molecular Single-Source Precursor to a Selective High-
Performance RhMnOx Catalyst for the Conversion of Syngas to
Ethanol
Phil Preikschas,[a] Julia Bauer,[a] Xing Huang,[b] Shenglai Yao,[c] Raoul Naumann d’Alnoncourt,[a] Ralph
Kraehnert,[a] Annette Trunschke,[b] Frank Rosowski,[a,d] and Matthias Driess*[a,c]
Abstract:
The first molecular
carbonyl
RhMn cluster
alternative fuel or as fuel additive in automobiles.[1,2] The use of
bioethanol produced by fermentation of biomass-derived sugars
could lead to rising food prices and ethical problems.[3]
Na2[Rh3Mn3(CO)18] 2 with highly labile CO ligands and predefined
Rh-Mn bonds could be realized and successfully used for the
preparation of the silica (davisil)-supported RhMnOx catalysts for the
conversion of syngas (CO, H2) to ethanol (StE); it has been
synthesized through the salt metathesis reaction of RhCl3 with
Na[Mn(CO)5] 1 and isolated in 49 % yields. The dianionic Rh3Mn3
cluster core of 2 acts as a molecular single-source precursor (SSP)
for the low-temperature preparation of selective high-performance
RhMnOx catalysts. Impregnation of 2 on silica (davisil) led to three
different silica-supported RhMnOx catalysts with dispersed Rh
nanoparticles tightly surrounded by a MnOx matrix. By using this
molecular SSP approach, Rh and MnOx are located in close proximity
on the oxide support. Therefore, the number of tilted CO adsorption
sites at the RhMnOx interface increased leading to a significant
enhancement in selectivity and performance. Investigations on the
spent catalysts after several hours time-on-stream revealed the
influence of rhodium carbide RhCx formation on the long-term stability.
A promising alternative would be a process to synthetic
ethanol, which could also be used as a feedstock for synthesis of
a wide range of industrial chemicals, polymers, or as fuel in direct
alcohol fuel cells.[4,5] The direct conversion of syngas to ethanol
(StE) is a promising process utilizing coal, natural gas, or
preferably biomass as carbon source for syngas generation. To
make this process technically and commercially feasible, a
selective high-performance catalyst for StE is highly desired.
As supported rhodium is the only monometallic catalyst that
has demonstrated selectivity toward ethanol,[6,7] a wide range of
promoters were tested, e. g. containing Fe,[8,9] Mn,[10] Li,[11] and
even rare earth oxides, such as CeO2 or Pr6O11.[12] From these
studies, it turned out that Mn-promoted Rh catalysts are among
the best systems known as yet for the conversion of syngas to
ethanol and other C2+ oxygenates (e. g., acetaldehyde, acetic
acid).[10] By using Mn as a promoter, a higher selectivity toward
ethanol as well as an increase of CO conversion can be achieved.
According to a recent review by Luk et al.,[2] the most performant
Mn-promoted Rh catalysts in the conversion of StE showed an
ethanol selectivity of 17.7 % and an overall C2+ oxygenate
selectivity of 42.0 % at a CO conversion of 17.0 %. These
catalysts were prepared by incipient wetness impregnation of the
respective metal nitrates and subsequent calcination.[13]
Introduction
Depletion of fossil fuel resources and rising crude oil prices
lead to one of the most important scientific challenges of the 21st
century – finding a promising alternative to petroleum-derived
fuels. In this manner, ethanol is being considered as a potential
Despite these promising results, the production of
undesirable methane is still present with selectivities up to
43.7 %.[13] A theoretical study has shown that methane formation
is inevitable on supported RhMn catalysts due to a low activation
barrier for methanation.[14] However, the number of active sites for
the methane formation decreases, if Mn is in close proximity to
Rh.[15,16] These close interactions not only decrease the selectivity
toward methane but also promote the formation of ethanol by
modifying the surface structure for the conversion of StE. One
promising strategy to bring about Rh and Mn in close proximity
could be achieved by the molecular single-source precursor
(SSP) approach with a well-defined Rh:Mn ratio and Rh-Mn bonds.
Encouraged by the recent progress in using molecular SSPs for
solid catalyst design and preparation,[17] we synthesized the first
molecular RhMn SSP and investigated its suitability to produce
more selective RhMn-based StE catalysts. Here we present the
synthesis and characterization of the carbonyl heterobimetallic
Na2[Rh3Mn3(CO)18] cluster 2 that acts as a suitable SSP for the
[a]
P. Preikschas, J. Bauer, Dr. R. Naumann d’Alnoncourt,
Dr. R. Kraehnert, Dr. F. Rosowski, Prof. Dr. M. Driess
BasCat – UniCat BASF JointLab
Technische Universität Berlin
Hardenbergstraße 36, Sekr. EW-K01, 10623 Berlin (Germany)
E-Mail: matthias.driess@tu-berlin.de
[b]
[c]
Dr. X. Huang, Dr. Annette Trunscke
Department of Inorganic Chemistry
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin (Germany)
Dr. S. Yao, Prof. Dr. M. Driess
Metalorganic Chemistry and Inorganic Materials, Department of
Chemistry
Technische Universität Berlin
Strasse des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
Dr. F. Rosowski
[d]
BASF SE, Process Research and Chemical Engineering
Heterogeneous Catalysis
Carl-Bosch-Strasse 38, 67056 Ludwigshafen (Germany)
low-temperature preparation of
a silica-supported RhMnOx
Supporting information for this article is given via a link at the end
of the document.
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10.1002/cctc.201801978
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catalyst, which shows the highest selectivity for the conversion of
syngas to ethanol reported to date.
Results and Discussion
Synthesis and Characterization of a Suitable Molecular
Single-Source Precursor to Give RhMnOx Catalysts
The novel Rh3Mn3 cluster 2 was synthesized from the salt
metathesis reaction of RhCl3 with Na[Mn(CO)5] 1 in THF as a fine
black powder in 49 % yields (Scheme 1). The composition of 2
was confirmed by elemental analysis; 2 is sensitive towards air
and readily soluble in protic and nonprotic polar solvents (ethanol,
acetonitrile, acetone, THF, and DMF).
Figure 1. Molecular structure of the cluster dianion of 2. Thermal ellipsoids are
drawn at 50 % probability level. Sodium cations and stabilizing THF molecules
are omitted for clarity. An overall image is provided as Figure S1 in the
Supporting Information. Selected distances (Å) and angles (°): Rh1-Rh2
2.877(3), Rh1-Mn1 2.640(6), Rh1-Rh2-Rh3 60.26(7), Mn1-Rh1-Mn3
173.08(16).
Scheme 1. Synthesis of Na2[Rh3Mn3(CO)18] 2.
Single crystals suitable for an X-ray diffraction (XRD) analysis
were obtained by diffusion of pentane vapors into a concentrated
solution of 2 in THF at –37 °C. Compound 2 crystallized as a
separate ion-pair; the dianionic Rh3Mn3 cluster core consists of
an array of the six metal atoms, which are distributed as a central
triangle of three rhodium atoms with a bridging manganese atom
on each site (Fig. 1).
Transformation of Supported Precursors into StE Catalysts
Compound 2 was used as a molecular SSP for three different
silica-supported RhMnOx catalysts. In the first case, a solution of
2 in acetonitrile was impregnated via incipient wetness method on
davisil, a commercially available silica with a BET surface area of
480 m2/g.
This hexanuclear metal core has a nearly planar geometry
with three roughly straight angles of Mn-Rh-Mn (av. 174.0(9)°),
leading to a mean deviation of 0.14 Å to the best plane. The latter
cluster core is stabilized by 12 terminal and 6 bridging CO ligands
that additionally link the Mn atoms with the Rh3 subunit. These
bridging CO ligands protrude out of the plane and are coordinated
to a Na ion on each site resulting in a small bending to the middle
of the hexanuclear Rh3Mn3 metal core (Fig. S1 in Supporting
Information). The terminal CO ligands are distributed over the Mn
as well as Rh atoms. Each Mn atom carries three terminal CO
ligands, one of them within and the others out of the plane. The
remaining three terminally bound COs are in-plane and each
attached to a Rh atom. They are slightly bended with an av. Rh-
C-O bond angle of 174.1°. The central Rh3 triangle has an almost
equilateral structure with an average bond angle of 60.00(7)°. The
mean Rh–Rh distance of 2.870(3) Å is significantly longer than
those observed in other rhodium species, such as
Rh3(µ-CO)2(Cp)3 (av. 2.59 Å) or Rh3(µ-CO)3(Cp)3 (av.
2.65 Å).[18,19] The six Rh-Mn bonds have an average bond length
of 2.638(4) Å and are slightly shorter than the other reported
Rh-Mn bond (2.70 Å).[20] The dianionic Rh3Mn3 cluster core of 2 is
highly symmetric with an overall C3 symmetry, which is in
accordance with a singlet resonance observed in the 55Mn-NMR
spectrum at δ = -2702.5 ppm.
FT-IR measurements confirmed the integrity of 2 on the silica-
support (Fig. S2). The present Rh–Mn bonds in 2 could be
advantageous for the preparation of a metallic Rh catalyst with a
close proximity to MnOx sites generated through low-temperature
decomposition. To identify the proper conditions for such a mild
transformation of supported 2,
a temperature-programmed
decomposition (TPDe) investigation was performed after
impregnation (Fig. S3). A two-step decarbonylation could be
observed which begins at approx. 70 °C and reaches its
maximum at 96.4 °C. This might be attributed to the weaker
bonded bridging CO ligands, whereas the decarbonylation of the
terminal CO ligands occurs at the second maximum at 113 °C.
Based on these observations, silica-supported 2 has been
transformed into the NaRhMnOx/SiO2 catalyst 3 with a nominal
loading of 1.25 wt% Na, 1.50 wt% Mn, 2.80 wt% Rh by a low-
temperature decomposition in 10 % dihydrogen with two heating
steps at 75 and 115 °C, respectively. These temperatures are far
below of a classical preparation route using single metal (Rh, Mn)
nitrates (350 °C) which might avoid the sintering of Rh particles at
temperatures above StE reaction conditions (260 °C).
To investigate the role of sodium on the RhMn system, two
different routes to sodium-free RhMnOx/SiO2 catalysts were
pursued: a) the Na ions were removed from as-synthesized 3 by
a Soxhlet extraction with methanol leading to a sodium-free
RhMnOx/SiO2 catalyst 4, and b) the Na cations in 2 were replaced
by the trimethylbenzylammonium cation to give another sample of
sodium-free RhMnOx/SiO2 catalysts 5. The absence of sodium in
both catalysts was confirmed by elemental analysis via inductively
coupled plasma optical emission spectrometry (ICP-OES).
The structural features of the Rh3Mn3 core of 2 are
reminiscent of the reported hexanuclear Na3[Cu3Fe3(CO)12] and
Os6(CO)21 compounds.[21–25]
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Figure 3. Composition analysis of the as-synthesized RhMnOx/SiO2 catalysts 5
after reduction in H2 at 260°C. ADF-STEM with EDX mapping: Small particles
of metallic rhodium (red) are closely contacted to a MnOx species (green). No
separated MnOx or Rh nanoparticles could be observed.
show homogeneously dispersed nanoparticles with mean
particles sizes of 1.30, 2.80 and 1.55 nm in the domains 1, 2 and
3 (Fig. S10), respectively. It also confirms a narrow size
distribution of the synthesized nanoparticles (Fig. S10). The
composition analyses were focused on domain 2, since particles
smaller than 2 nm are difficult to study due to the insufficient EDX
signal. The EDX mapping (Figure 3; for more EDX mappings
(Fig. S11) and an EDX line-scan profile (Fig. S12) indicate
homogeneously dispersed Rh nanoparticles, which are
surrounded by a finely distributed manganese species. Thus, Rh
is in close proximity to manganese and no separated Rh or MnOx
particles are found in all four domains (Figs. 3 and S10).
Since no crystalline Mn particles are observed in ADF-STEM
images, it is assumed that Mn is in an oxidized state. This
assumption was confirmed by XPS measurements without
contact to an oxidative atmosphere (Fig. S8) and is also known
for related RhMnOx catalysts.[16] However, the chemical state of
Mn under reaction conditions is controversially discussed
assuming that zero-valent Mn atoms might be alloyed with Rh.[14]
Furthermore, a theoretical study described RhMn alloys as active
species in the conversion of StE.[27] In this study, it can be shown
that the formation of a supported RhMn nanoalloy is unfavourable
on silica supports.
Not even the use of Na2[Rh3Mn3(CO)18] 2 as molecular SSP
with both metals in low oxidation states and predefined metal-
metal bonds led to the formation of an alloy. The considerations
that a reduction of Mn is induced by a close proximity to Rh might
be attributed to classical preparation methods with precursors
bearing Mn in high oxidation states. In this case, it cannot be
distinguished whether the oxidation state of Mn in as-synthesized
catalysts is caused by the used metal precursor or through the
oxidation by an oxide support. On this account, the decomposition
of silica-supported Na[Mn(CO)5] 1 metallate bearing Mn in the low
oxidation state of -1 could not lead to the preparation of a zero-
valent Mn species. The weakly Brønsted acidic silanol groups are
likely to cause the immediate oxidation of the metallate 1 (Fig. S9).
Figure 2. (HR-)TEM images of the as-prepared silica-supported RhMnOx/SiO2
catalyst
5 after reduction in H2 at 260 °C: a,b) Rh nanoparticles are
homogenously dispersed on silica and surrounded by a MnOx matrix; c,d) the
indicated lattice fringes of the crystalline particles correspond to the fcc{111},
fcc{200} planes of metallic Rh.
Powder XRD measurements were accomplished for phase
identification of the as-synthesized materials 3–5 (Figs. S4 and
S5). No specific reflections could be identified due to the limit of
XRD for the detection of supported, crystalline nanoparticles
(around 3.0 nm).[26] On this account, high-resolution transmission
electron microscopy (HR-TEM) and annular dark-field scanning
transmission microscopy (ADF-STEM) combined with energy-
dispersive X-ray (EDX) mapping were conducted to determine the
structure, morphology and atomic composition of 5.
The overview TEM image showed relatively small
nanoparticles with a mean particle size of 1.55 nm, which are
homogeneously distributed on the silica-support (Fig. 2a). Lattice
fringes analyses were then performed on the HR-TEM images to
determine the atomic structure of the nanoparticles, which are
size-representative according to the particles size distribution in
Figure 2b. The crystalline particles show lattice fringes with d-
spacings of 2.2 (Fig. 2c), 1.9 Å (Fig. 2d). They are consistent with
fcc{111}, fcc{200} planes of metallic Rh in the space group Fm-
3m (Fig. 2c,d). Metallic Mn or crystalline MnOx particles could not
be observed. This leads to the assumption that Mn is in an
amorphous phase. In addition, it is not possible to distinguish
between MnOx and SiO2 due to the low loading and the weak
phase contrast compared to that of the silica-support.
The structural information based on lattice fringes are not
sufficient to differentiate between metallic Rh and RhMn alloy
particles. Therefore, ADF-STEM in combination with EDX was
conducted to investigate the composition of catalytic particles.
The ADF-STEM images recorded from three different domains
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It is therefore suggested that the oxidation of Mn to give MnOx
species is inevitable on silica, even under reaction conditions.
adsorption sites which, in turn, facilitate the formation of ethanol
(Scheme. S1). In fact, previous IR studies suggested that CO
adsorbs in a tilted manner at Rh-MnOx interfaces.[32,33] Moreover,
results from diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) and kinetic investigations confirmed the
enhancement of ethanol selectivity due to the tilted adsorption
mode on RhMnOx which promotes CO dissociation on Rh0
sites.[13,28] Furthermore, density functional theory (DFT)
calculations proposed the lowering of the activation barrier for CO
Catalytic Performance of the As-synthesized RhMnOx
Catalysts
The as-prepared materials 3–5 were tested as catalysts for
the direct conversion of StE in a fixed-bed parallel test setup at
common
reaction
conditions
for
syngas
chemistry
(GHSV = 3500 h-1, p = 54.0 bar, and T = 243–260 °C; see
Experimental Section). The catalysts were directly compared with
a Mn-promoted Rh catalysts 6 prepared by incipient wetness
impregnation of the respective metal nitrates and subsequent
calcination at 350 °C, and Rh/SiO2 8 prepared in a similar fashion
(Tab. S2). For this comparison, different CO conversion levels
were realized by variation of the reaction temperature from 243–
260 °C.
insertion into adsorbed CHx fragments due to
a tilted
adsorption.[14] The Na content in RhMn SSP has an influence on
activity and selectivity but only at very low conversions. At higher
conversions both SSP-originated catalysts 3 and 4 exhibit similar
ethanol selectivities.
Methane is the main product for all tested catalysts and its
selectivity is reduced by factors of 0.83, 0.92, and 0.84 for
catalysts 3–5, respectively. The lower methanation might also be
attributed to the close proximity of Rh and MnOx. With a methane
selectivity of 37.1 %, the Na containing catalyst 3 lowers the
methane formation in a significant amount compared to the
reference catalyst 6 with 44.8 %. Further products are other light
hydrocarbons, methanol and oxygenates, mainly acetaldehyde
and acetic acid.
Compared to the monometallic Rh/SiO2 catalyst
8
(XCO = 4.8 %, SEtOH = 6.5 %; Tab. S2), all Mn-promoted catalysts
show a substantially higher activity and ethanol selectivity. The
enhancement of the catalytic performance due to the close
proximity between Rh and MnOx was also observed for other
reported RhMnOx catalysts.[13,28] The catalytic results show a
lower CO conversion of the SSP-originated catalysts 3–5
compared to the ‘traditionally’ prepared catalyst 6. This finding
might be attributed to a reduced metal loading of 0.6 wt% Rh
which was confirmed by ICP-OES. However, the catalysts 3–5
show an increase in ethanol selectivity compared to the reference
catalyst 6 as well as the mentioned literature-reported catalysts at
iso-conversion denoted with hollow symbols in Figure 4a. In the
case of 5, the selectivity towards ethanol could be substantially
enhanced by a factor of 1.54 and is – to our knowledge – the
highest reported ethanol selectivity compared to literature
examples (Fig. 4b).[2,13,29–31] We reason that the enhancement in
selectivity is due to a higher dispersion of Rh nanoparticles in a
tightly surrounded MnOx matrix in the SSP-originated catalysts 3–
5, because this situation can increase the number of tilted CO
Stability of the Synthesized RhMnOx Catalysts
After several hours time-on-stream (TOS), the catalysts 3
(105 h; Fig. S14) and 5 (220 h; Fig. S15) show a slight decrease
in CO conversion due to deactivation. In the case of 5, a notable
deactivation in the first 20 h TOS can be observed (XCO = 27.7 to
17.5 %). After this initial deactivation phase, the catalyst remains
quite stable after additional 200 h TOS. However, the selectivity
patterns remain similar and a difference in ethanol selectivity was
not observed (Tab. S2, entry 3 and 7). The aforementioned
deactivation might be induced by the formation of some larger
particles due to sintering (Fig. 5a). For those larger particles a
local enrichment of C was observed by ADF-STEM with EDX
Figure 4. a) Ethanol selectivities (mol%) as function of CO conversions. Different CO conversion levels were realized by application of three different reaction
temperatures. The range of iso-conversion which was chosen to compare the product selectivities are highlighted in grey b,c) Product selectivities (mol%) of
measurements points marked with a hollow symbol at iso-conversion (11–16 %). The CO2 selectivity is not shown here, as it is below 1 % for all tested catalysts.
Measuring conditions: 243–260 °C, 54.0 bar, p(H2) = 32.4 bar, p(CO) = 10.8 bar, GHSV 3500 h-1. CH4, HCs, MeOH, C2+ oxy, and EtOH abbreviate methane,
hydrocarbons, methanol, C2+ oxygenates, and ethanol, respectively. S(C2+ oxy) mainly includes acetaldehyde and acetic acid. For more details about the products,
see Tab. S2 in Supporting Information.
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purification system (MBraun SPS-800) or purified using conventional
procedures, freshly distilled under argon atmosphere and degassed prior
to use.
NMR spectroscopy. NMR samples of air- and moisture-sensitive
compounds were prepared in a J. Young NMR tube (sealed with a
polytetrafluoroethylene valve) under nitrogen atmosphere inside a glove
box. The deuterated solvents were dried over a 4 Å molecular sieves and
degassed prior to use. The 13C- and 55Mn-NMR spectra were recorded on
a Bruker AvanceIII (13C: 125 MHz, 55Mn: 124 MHz) spectrometer. The
13C{1H}-NMR spectra were referenced to residual solvent signal as internal
standards (acetonitrile-d3: δC = 1.32 ppm) and the 55Mn-NMR spectra were
referenced to a solution of KMnO4 in D2O. The abbreviations used to
denote the multiplicity of the signals are singlet (s) and multiplet (m).
Figure 5. a,b) ADF-STEM images of the spent catalyst 5: after 220 h TOS,
some big particles are formed due to sintering but most of the particles remain
relatively small. c) EDX mapping and EELS measurements (Figure S13)
suggest the presence of a carbide phase in the bigger particles.
mapping (Fig. 5c), while respective EELS measurements suggest
the presence of a rhodium carbide RhCx phase (Fig. S13).
However, most of the particles remain relatively small (Fig. 5b),
which is the reason for the long-term stability of the system.
FT-IR spectroscopy. IR spectra (4000–400 cm-1) of air- or moisture
sensitive samples were recorded on a Nicolet iS5 spectrometer inside a
glove box. Solid samples were measured with an attenuated total
reflectance sampling technique and are directly placed on an ATR crystal
(diamond). All other samples were placed as solid or as drop of a solution
on an ATR crystal (diamond) of a Bruker ALPHA FT-IR spectrometer. The
spectra were collected as data point tables by using OMNIC (Thermo
Fisher Scientific Inc.) or OPUS (Bruker Corporation) software and were
analyzed with Origin (OriginLab Corporation). Intensities of absorption
bands were assigned on basis of visual inspection and were abbreviated
as very strong (vs), strong (s), medium (m) and weak (w).
Conclusions
In summary, the first carbonyl RhMn cluster 2 with highly labile
CO ligands and predefined Rh–Mn bonds could be realized and
successfully used for the preparation of the silica (davisil)-
supported RhMnOx catalysts 3–5. The utilization of 2 as molecular
SSP facilitates the formation of small Rh nanoparticles embedded
in a finely distributed MnOx matrix. This close proximity between
Rh and MnOx leads to a superior selectivity towards ethanol and
also to a lower methane formation compared to reference as well
as reported RhMnOx catalysts. The RhMn SSP catalysts 3–5
show the best performance of a Mn-promoted Rh catalyst in the
conversion of StE reported as yet. In addition, the usage of the
RhMn cluster 2 as molecular SSP shows that the formation of a
RhMn alloy is unfavorable on silica supports due to the inevitable
oxidation of Mn.
Long-term stability tests showed only a moderate deactivation
after an initial phase and several hours TOS. Based on ADF-
STEM images with EDX mapping and EELS measurements, the
formation of a rhodium carbide RhCx phase is suggested which
might lead to the deactivation of RhMnOx catalysts besides
sintering of Rh nanoparticles.
Mass spectroscopy. Mass spectra were recorded using HR-ESI-MS-
method on an LTQ Orbitrap XL spectrometer.
Elemental analyses. The determination of the mass fractions of carbon,
hydrogen, nitrogen and sulfur were carried out on a Thermo Finnigan
FlashEA 1112 Organic Elemental Analyzer. Air- or moisture sensitive
samples were prepared in silver capsules in a glove box. Mass fractions
of transition or alkali metals were achieved by ICP-OES with a Varian 715
Emission Spectrometer.
Single crystal X-ray diffraction. An analytically pure crystal was directly
placed on a glass capillary with perfuorinated oil and measured in a cold
nitrogen steam. The data of all measurements were collected on an Agilent
Technologies SuperNova diffractometer at 150 K (Cu-Kα-radiation,
λ = 1.5418 Å). The structure was solved by direct methods using
SHELXT,[35] space group determinations were performed with XPREP and
crystal structure refinements with SHELXL.[36] All aforementioned
programs were handled with Olex2 Crystallography Software.[37] The
CCDC number 1868848 (2) contain the supplementary crystallographic
data for this paper. The data can be obtained free of charge by contacting
The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
The molecular SSP approach is therefore a powerful tool for
solid catalysts design resulting in highly dispersed metals
embedded in oxide matrices and might lead to a better
understanding of catalyst structure-function relationships. In this
manner, it might be applicable to other catalysts systems, such as
silica-supported RhFeOx and Rh(Mn,Fe)Ox catalysts by choosing
suitable molecular SSPs, e.g. Na[RhFe2(CO)11] or a [RhMnFe]
carbonyl cluster.[34]
Powder X-ray diffraction. Powder X-ray diffraction (XRD) measurements
were performed in Bragg-Brentano geometry on a D8 Advance II
theta/theta diffractometer (Bruker AXS), using Ni-filtered Cu Kα1,2 radiation
and a position sensitive energy dispersive LynxEye silicon strip detector.
The sample powder was filled into the recess of a cup-shaped sample
holder, the surface of the powder bed being flush with the sample holder
edge (front loading).
Experimental Section
Electron microscopy. The samples were investigated by an aberration-
corrected JEOL JEM-ARM200CF transmission electron microscope. The
microscope is equipped with a high angle silicon drift EDX detector with
the solid angle of up to 0.98 steradians from a detection area of 100 mm2
that allows for EDX measurement.
General Considerations. All manipulations involving air- and moisture
sensitive organometallic compounds were carried out under an
atmosphere of dry, oxygen-free argon by using standard Schlenk
techniques or glove boxes (MBraun LABmaster Pro) under a purified
nitrogen or argon atmosphere. There solvents were taken from a solvent
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Temperature-programmed
decomposition
(TPDe).
TPDe-MS
ꢀꢁ,ꢂꢃꢄꢄꢅꢂꢆꢅꢇ is the corrected mole fraction of compound ꢐ . ꢀꢁ,ꢄꢅꢎꢂꢆꢃꢄ and
ꢀꢁ,ꢋꢌꢍꢎꢏꢏ are the mole fractions of compound ꢐ originally obtained by the
gas chromatograph sampling the respective reactor or the bypass line.
experiments were performed with a MicrotracBEL BelCat II setup, which
was coupled to a mobile Pfeifer Vacuum QMG 220 mass spectrometer. In
a typical experiment, a certain amount of the sample (around 50 mg) was
placed in the sample cell inside a glove box and placed in the BelCat II
setup. Helium was used as carrier gas with a flow rate of 54 mL/min and
hydrogen was used for the decomposition with a flow rate of 6 mL/min.
The temperature was linearly and continuously increased from RT to 873 K
with a temperature ramp of 5 K/min. The reaction products were analyzed
by the computer controlled quadrupole mass spectrometer.
Additionally, the results for all compounds detected by the FID were
corrected using the mole fraction of methane obtained by FID and TCD
according to equation 2.
ꢀꢑꢅꢆℎꢎꢒꢅ,ꢓꢉꢔ
ꢀꢁ = ꢀꢁ,ꢂꢃꢄꢄꢅꢂꢆꢅꢇ
∙
(2)
ꢀꢑꢅꢆℎꢎꢒꢅ,ꢕꢖꢔ
ꢀꢁ is the mole fraction of compound ꢐ that is finally used for further
calculations. ꢀꢑꢅꢆℎꢎꢒꢅ,ꢓꢉꢔ and ꢀꢑꢅꢆℎꢎꢒꢅ,ꢕꢖꢔ are the methane concentrations
detected by TCD and FID after correction with the Ar standard (equation
1).
X-ray photoelectron spectroscopy. XPS was measured on K-Alpha™ +
X-ray Photoelectron Spectrometer (XPS) System (Thermo Scientific), with
Hemispheric 180° dual-focus analyzer with 128-channel detector. X-ray
monochromator is Micro focused Al-Kα radiation. For the measurement,
the as-prepared samples were directly loaded on the sample holder for
measurement. The data was collected with X-ray spot size of 200 μm, 20
scans for survey, and 50 scans for regions.
Carbon monoxide conversion ꢗꢉꢘ was calculated indirectly from the
summation of carbon numbers in all products rather than directly from the
CO concentration as at small conversions the quantification of CO via TCD
was not precise enough (equation 3).
Catalytic Testing for Synthesis Gas Conversion. The catalytic testing
of the syngas to ethanol reaction was performed in a 4-fold parallel testing
unit. 0.5 g (~1 mL) catalyst with a particle size of 100-200 µm were loaded
into each stainless steel reactor with an effective inner diameter of
6.25 mm. The reaction temperature was monitored by temperature
sensors with three thermocouples along the catalyst bed.
∑
ꢀꢙꢚ
ꢙ
ꢗꢉꢘ
=
(3)
ꢀꢉꢘ,0
ꢀꢉꢘ,0 is the mole fraction of CO in the inlet gas and Ci is the carbon number
of the product i. The selectivity S for each product i was determined based
on the number of C atoms by equation 4.
Four mass flow controllers were used to adjust the flow rates of the inlet
gases N2 (99.999 %), CO (99.997 %), H2 (99.999 %) and Ar (99.999 %, all
Air Liquide). The CO feed line was equipped with a carbonyl trap in order
to remove all metal carbonyls that might be formed by high pressure of CO
in contact with stainless steel. The carbonyl trap consisted of a U-shaped
½” stainless steel tube filled with Al2O3 and heated to 170 °C by a heating
sleeve.
ꢀꢁꢚꢁ
ꢛꢁ =
(4)
∑
ꢀꢁ,ꢙꢚꢁ,ꢙ
Synthesis of Na[Mn(CO)5]∙0.5THF (1). Metallate 1 was synthesized by a
modified literature-known preparation.[38] Metallic sodium (3.04 g,
0.133 mol) was cut in small pieces and suspended in 50 mL THF. This
suspension was stirred at r.t. overnight. A yellow solution of manganese
carbonyl Mn2(CO)10 (5.18 g, 13.3 mmol) in dry THF (45 mL) was added to
the suspension. The mixture was stirred at r.t. for 72 h, during which time
a color change from yellow over red to brown was observed. The
suspension was then filtered and washed with additional 25 mL THF. The
combined filtrates were evaporated to dryness in vacuum for 12 h,
providing a light green solid. Yield: 4.60 g, 21.1 mmol, 81.2 % (Based on
Mn2(CO)10). Note: THF molecules are still coordinated to Na, which was
quantified by dynamic flash combustion analysis. This material was used
for subsequent procedures without further purification. Elemental analysis
(%): calcd for C7H4O5.5NaMn: C, 33.10; H, 1.59 Found: C, 32.36; H, 1.86.
13C{1H} NMR (125 MHz, CD3CN, 298 K, ppm): δ = 238.68 (s, Mn-CO).
55Mn NMR (124 MHz, CD3CN, 298 K, ppm): δ = -2702.57. ESI-MS: m/z:
calcd for [M–Na]-: 194.9, found: 194.9. IR (ATR, cm-1): ν = 1936 (w), 1884
(m), 1826 (s).
Compounds in the effluent gas that condense below 180 °C were removed
by a coalescence filter in the downstream oven. All remaining compounds
in the effluent gas were analyzed with an online gas chromatograph
(Agilent 7890B) equipped with two thermal conductivity detectors (TCD)
and one flame ionization detector (FID) using He as the carrier gas. TCDs
detect the inlet gases H2, Ar, N2, CO and in addition methane, CO2 and
H2O. The FID is used to detect a large variety of paraffins, olefins and
oxygenates (alcohols, acetaldehyde, acetic acid) with a combination of a
Porabond Q and an RTX-Wax column. The carbon balance was between
96–102% for all measurements.
The catalysts are reduced in-situ at 54 bar, 265 °C with 5 % H2 in N2 for
1 h with a volume flow of 58.3 mL min-1. Subsequently, the temperature
was decreased to 243 °C and synthesis gas feedstock mixture containing
CO:H2:N2:Ar (20:60:10:10%, v:v) was admitted. The volume flow was kept
constant to achieve a GHSV of 3500 h-1. The temperature was increased
to 260 °C in three steps and subsequently decreased in the same manner.
Each step was held constant for at least 15 h to allow the catalysts to
equilibrate.
Synthesis of Na2[Rh3Mn3(CO)18]∙2THF (2). Rhodium(III) chloride RhCl3
(602.4 mg, 2.88 mmol) was suspended in 100 mL ethanol (EtOH) and
cooled down to -78 °C in an acetone/dry ice bath. A solution of
Na[Mn(CO)5] (1, 1.79 g, 4.12 mmol) in 25 mL EtOH was added dropwise.
The suspension was warmed up slowly to r.t. within 6 h, during which time
a color change from green to dark brown and precipitation of a crystalline
solid was observed. After stirring for 24 h, the mixture was filtered through
a glass frit and the residue was washed two times with additional 10 mL of
EtOH. The combined filtrates were evaporated under vacuum at 38 °C and
dried in vacuum. This crude product was washed four times with 25 mL
n-hexane and dried under vacuum overnight. The title compound was
obtained as dark brown solid. Analytically pure black block-shaped crystals
The obtained concentrations of all compounds were corrected for volume
changes due to the reaction and the following N2 dilution. Therefore, the
mole fraction of Ar was used as inert internal standard according to
equation 1.
ꢀꢊꢄ,ꢋꢌꢍꢎꢏꢏ
ꢀꢁ,ꢂꢃꢄꢄꢅꢂꢆꢅꢇ = ꢀꢁ,ꢈꢉ
∙
(1)
ꢀꢊꢄ,ꢄꢅꢎꢂꢆꢃꢄ
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10.1002/cctc.201801978
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were obtained by diffusion of pentane vapors into a concentrated and
filtered solution in THF at -37 °C. Yield: 520.3 mg, 4.66 mmol, 48.6 %
(based on RhCl3). Elemental analysis (%): calcd for C26H16O20Na2Mn3Rh3:
C, 26.70; H, 1.38 Found: C, 26.70; H, 1.37. 13C{1H} NMR (125 MHz,
CD3CN, 298 K, ppm): δ = 210.69 (s, Mn-CO), 214.50 (s, Mn-CO), 216.67
(m, μ-CO). 55Mn NMR (124 MHz, CD3CN, 298 K, ppm): δ = -2606.78. ESI-
MS: m/z: calcd for [M-Na]-: 1000.3, found: 1000.4. IR (ATR, cm-1): ν = 2077
(w), 2025 (m), 2001 (m), 1917 (m), 1798 (br, m). ATR-FTIR of a drop the
n-hexane washing solution confirmed [Mn(CO)4Cl]2 as by-product. IR of
[Mn(CO)4Cl]2 (ATR, cm-1): 2115 (w), 2045 (m), 2006 (vs), 1934 (vs). Bands
are in accordance to literature, IR (chloroform solution, cm-1): 2104, 2047,
2006, 1977.[39] After serval days, Mn2(CO)10 crystallized from the solution
in form of yellow needles due to the disproportion of [Mn(CO)4Cl]2 to
Mn2(CO)10 and MnCl2. The formation of Mn2(CO)10 was confirmed by
single-crystal XRD. The lability of [Mn(CO)4Cl]2 in n-hexane solution and
the subsequent disproportion was also observed elsewhere.[40]
265 °C in-situ prior the catalyst testing. Metal loadings by ICP-OES (wt%):
2.6 Rh, 1.3 Mn.
Synthesis of [NBnzMe3]2[Rh3Mn3(CO)18] (7). Benzyltrimethylammonium
chloride [NBnzMe3]Cl (155.0 mg, 0.835 mmol) was suspended in 15 mL
acetone and a solution of Na2[Rh3Mn3(CO)18] (2, 133.5 mg, 0.120 mmol)
in 10 mL acetone was added dropwise to the suspension. The suspension
was stirred at room temperature for 24 h and filtered through a glass frit.
The residue was washed with additional 10 mL of acetone. The combined
filtrates were evaporated under vacuum at 45 °C and then dried in vacuum
for 12 h. The crude product was washed three times with 5 mL toluene and
dissolved in 10 mL THF. Evaporation of the solvent and drying in vacuum
overnight led to a tacky compound which was not isolatable. Yield: 71.4
mg, 0.04 mmol, 31.8 % (Based on cluster 2). This material was used for
subsequent procedures without further purification. IR (ATR, cm-1): ν =
2068 (w), 2020 (w), 1992 (m), 1958 (m), 1917 (m), 1793 (m). ESI-MS: m/z:
calcd for [M-NBnzMe3]-: 1071.6, found: 1070.8.
Synthesis of NaRhMnOx/SiO2 SSP (3). A solution of 2 (141.1 mg,
0.12 mmol) in acetonitrile (3.70 mL) was added to 3.26 g SiO2 (sieve
fraction 100–200 µm) under mixing with a spatula. The addition of the
solution was conducted in three steps á 1.23 mL which corresponded to
the specific pore volume of the support (0.90 mL/g) according to the
incipient wetness impregnation method. After impregnation, the silica-
supported precursor was dried under high vacuum for 12 h at room
temperature. The pre-catalyst was activated by thermal treatment in
flowing 10 % H2/Ar (total flow 500 mL/min). A temperature program in
accordance to the TPDe studies (Fig. S2) was chosen (Tab. 1). The final
NaRhMnOx/SiO2 catalyst (3) was kept under inert gas atmosphere prior
use. Metal loadings by ICP-OES (wt%): 2.21 Rh, 1.34 Mn.
Synthesis of Rh/SiO2 Reference Catalyst (8). The Rh/SiO2 reference
catalyst (8) was prepared in a similar manner as reference catalyst 6.[16]
A
aqueous solution of rhodium(III) nitrate Rh(NO3)3•xH2O in water (HPLC
grade) was impregnated on silica (pre-treated at 550 °C in air for 6 h). A
subsequent calcination in synthetic air was conducted providing the
Rh2O3/SiO2 pre-catalyst. The precatalyst was activated in-situ prior the
catalyst testing. Metal loading by ICP-OES (wt%): 2.8 Rh.
Acknowledgements
This work was conducted in the framework of the BasCat
collaboration between BASF SE, TU Berlin, Fritz Haber Institute
of the Max Planck Society, and the Cluster of Excellence UniCat
(EXC 314-2; financed by the Deutsche Forschungsgemeinschaft).
We thank Dr. Somenath Garai for crystal structure refinement,
Dr. Frank Girgsdies for XRD, Shuang Li for XPS measurements,
and Stephen Lohr for technical assistance.
Table 1. Temperature Program of Catalyst Activation.
Step
Temperature (°C)
Holding Time (h)
1
2
3
75
4
4
1
115
260
Keywords: Heterobimetallic Catalysis • Hydrogenation •
Rhodium • Manganese • Oxygenates
Synthesis of RhMnOx/SiO2 SSP washed (4). The NaRhMnOx/SiO2 SSP
catalyst (3) was washed with methanol by a soxhlet extraction. The
washed pre-catalyst was dried in high vacuum yielding the final
RhMnOx/SiO2 SSP (4). Metal loadings by ICP-OES (wt%): 1.82 Rh,
0.87 Mn.
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Entry for the Table of Contents
FULL PAPER
Phil Preikschas, Julia Bauer, Xing
Huang, Shenglai Yao, Raoul Naumann
d’Alnoncourt, Ralph Kraehnert, Annette
Trunschke, Frank Rosowski, and
Matthias Driess*
Page No. – Page No.
Catalysts Birth. The first carbonyl RhMn cluster Na2[Rh3Mn3(CO)18] has been
synthesized, structurally characterized and utilized as well-defined molecular single-
source precursor (SSP) for the low-temperature preparation of selective high-
performance RhMn catalysts for the conversion of syngas to ethanol (StE). Highly
dispersed Rh nanoparticles in a tightly surrounded MnOx matrix lead to a superior
selectivity towards ethanol.
From a Molecular Single-Source
Precursor to a Selective RhMnOx
High-Performance Catalyst for the
Conversion of Syngas to Ethanol
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