A Cationic Ru(II) Complex Intercalated into Zirconium Phosphate Layers Catalyzes Selective Hydrogenation via Heterolytic Hydrogen Activation

Article abstract of DOI:10.1002/cctc.202100599

Catalytic hydrogenations constitute economic and clean transformations to produce pharmaceutical and a multitude of fine chemicals in chemical industry. Herein, we report a cationic Ru(II) complex intercalated into zirconium phosphate (ZrP) layers that enables the efficient catalytic conversion of furfural and other biomass-derived carbonyl compounds into the corresponding alcohols through selective hydrogenation of C=O group. The ZrP layers acted not only as a support for the Ru-complex, but also as the new ligands to tune the Ru(II) center via forming Ru?O bond. The resulting catalysts exhibit excellent catalytic performance and can be easily recycled for six times without significant loss of activity and selectivity. The Ru(II) complex-intercalated catalysts have been characterized by XRD, SEM, HRTEM, HAADF-STEM, XPS, FT-IR, DR-UV/Vis, EXAFS and XANES. Especially, it is observed that the appropriate interlayer spacing between ZrP layers is favorable to stabilize the Ru(II) complex. Notably, on the basis of the further characterization and density functional theory (DFT) calculation, it is identified that the interaction of cationic Ru(II) complex and P?OH group within ZrP layers leads to the high catalytic performance in selective hydrogenation, and the newly formed Ru?O?P species plays a crucial role in the heterolytic hydrogen activation and selective hydrogenation of biomass-derived compounds containing a carbonyl group.

Full text of DOI:10.1002/cctc.202100599

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A Cationic Ru(II) Complex Intercalated into Zirconium
Phosphate Layers Catalyzes Selective Hydrogenation via
Heterolytic Hydrogen Activation
Manyu Chen+,[a] Jie Xia+,[a] Huan Li,[b] Xiuge Zhao,[a] Qingpo Peng,[a] Jiajia Wang,[a]
Honghui Gong,[a] Sheng Dai,*[c] Pengfei An,*[d] Haifeng Wang,*[a] and Zhenshan Hou*[a]
Catalytic hydrogenations constitute economic and clean trans-
formations to produce pharmaceutical and a multitude of fine
chemicals in chemical industry. Herein, we report a cationic
Ru(II) complex intercalated into zirconium phosphate (ZrP)
layers that enables the efficient catalytic conversion of furfural
and other biomass-derived carbonyl compounds into the
corresponding alcohols through selective hydrogenation of
C=O group. The ZrP layers acted not only as a support for the
Ru-complex, but also as the new ligands to tune the Ru(II)
center via forming RuÀ O bond. The resulting catalysts exhibit
excellent catalytic performance and can be easily recycled for
six times without significant loss of activity and selectivity. The
Ru(II) complex-intercalated catalysts have been characterized by
XRD, SEM, HRTEM, HAADF-STEM, XPS, FT-IR, DR-UV/Vis, EXAFS
and XANES. Especially, it is observed that the appropriate
interlayer spacing between ZrP layers is favorable to stabilize
the Ru(II) complex. Notably, on the basis of the further
characterization and density functional theory (DFT) calculation,
it is identified that the interaction of cationic Ru(II) complex and
PÀ OH group within ZrP layers leads to the high catalytic
performance in selective hydrogenation, and the newly formed
RuÀ OÀ P species plays a crucial role in the heterolytic hydrogen
activation and selective hydrogenation of biomass-derived
compounds containing a carbonyl group.
Introduction
the depletion of fossil fuel resources, which also forces scientists
to seek green and renewable energy sources.[1–4] Development
of biomass energy conversion from renewable resources such
as starch, lignocellulose, sugars (alcohols), vegetable oils, etc.
has attracted more and more attention. Biomass-derived
molecules such as furfural, 5-hydroxymethyl furfural, levulinic
acid, phenols, carboxylic acids/esters and polysaccharide alco-
hols compound etc. can be easily obtained via acid catalysis/
metal catalysis or enzymatic catalysis of biomass raw
materials.[5,6] Therefore, exploring efficient and green catalytic
hydrogenation systems and new methods to convert these
biomass-derived molecules into high value-added oxygenated
fuels and fine chemicals can gradually liberate the industry
from dependence on petroleum.
Fuels and chemicals derived from biomass resources are renew-
able and has promising an effective approach to realize the
sustainable development of human society. In addition, Envi-
ronmental pollution is one of the most serious challenge with
[a] M. Chen,+ J. Xia,+ X. Zhao, Q. Peng, J. Wang, H. Gong, Prof. H. Wang,
Prof. Z. Hou
Key Laboratory for Advanced Materials
Research Institute of Industrial Catalysis
School of Chemistry and Molecular Engineering
East China University of Science and Technology
Shanghai 200237 (P. R. China)
E-mail: hfwang@ecust.edu.cn
houzhenshan@ecust.edu.cn
The catalytic hydrogenation reaction is one of the most
basic and important type of reactions in the fields of
petrochemical industry, fine chemical industry, and drug syn-
thesis, etc.[7–9] Bulk chemicals and fine chemicals can be
obtained through catalytic hydrogenation. In the near future,
water electrolysis using solar energy is expected to enable the
scale-up of “green H2” hydrogen production without CO2
emissions.[10,11] As a clean hydrogen source, H2 can be catalyti-
cally activated by a metal catalyst while platinum group metals
(PGMs) are arguably the best ones.[12–14] Undoubtedly, among
the PGMs, Ru-based catalysts plays a crucial role in both
theoretical research and practical industrial applications.[15,16] On
the one hand, Ru is cheaper and more available than other
PGMs (Pt, Rh, Pd, Ir, Os). On the other hand, compared to the
heterogeneous Ru catalyst, homogeneous Ru complex is a
single-site molecular catalyst, which can expose more coordina-
tion centers and exhibit a higher turnover frequency (TOF).[17–19]
[b] Dr. H. Li
Institute of Crystalline Materials
Shanxi University
Taiyuan 030006, Shanxi (P. R. China)
[c] Dr. S. Dai
Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist
Joint Research Center
Institute of Fine Chemicals
School of Chemistry & Molecular Engineering
East China University of Science and Technology
Shanghai 200237 (P. R. China)
E-mail: shengdai@ecust.edu.cn
[d] Dr. P. An
Institute of High Energy Physics
Chinese Academy of Sciences
Beijing Synchrotron Radiation Facility (BSRF)
Beijing 100049 (P. R. China)
E-mail: anpf@ihep.ac.cn
[+] These authors contributed equally.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202100599
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Furthermore, the steric and electronic structure of various
ligands also endow Ru complexes with adjustable catalytic
performance.[20–22]
catalytically active Ru complexes intercalated into ZrP layers
were rarely reported for the catalytic application. It can be
envisioned that the desirable properties can be achieved from a
deep understanding of the intercalation structure.[50]
Many Ru (II) complexes have been reported to act as
catalysts in homogeneous hydrogenation and transfer hydro-
genation reactions.[23–25] Among them, arene-Ru (ΙΙ) complexes
represented a well-established class of active catalysts for
efficient hydrogenation of carbonyl groups in ketones, alde-
hydes, esters, C=N bonds in imines, or C=C bonds in styrene or
furfuryl alcohol.[26,27] However, these Ru (ΙΙ) complexes are
mostly applied in catalytic transfer hydrogenation systems,
which require a relatively expensive liquid hydrogen source
(formic acid, isopropanol, ethanol etc.).[28,29] In contrast, H2 as
hydrogen source demonstrated some advantages that it is
cheaper and clean, and catalytic H2 hydrogenation was really an
environmentally friendly reaction. Therefore, developing an
easily available Ru (ΙΙ) catalytic systems with excellent hydro-
genation performance by using molecular H2 under mild
condition would be highly promising.
As a continuation of our efforts to design an easily
accessible, highly active and stable ZrP catalysis,[51–54] in this
work we report the design and preparation of a recyclable
catalyst by the intercalation of a Ru(II) complex into ZrP layers
(Ru-ZrP). The structure of catalyst was investigated in detail and
active species was further identified by experimental study and
DFT calculation. It is found that that owning to the newly
formed Ru-OÀ P active species arisen from the specific inter-
action of cationic Ru(II) complex and P-OH group within ZrP
layers, the intercalated Ru-ZrP catalyst afforded high catalytic
performance in the selective hydrogenation of furfural (FAL)
with molecular H2, which was in obvious contrast to that of the
sole Ru-phen complex that barely converted the FAL. The
intercalation catalyst can also be extended for highly selective
hydrogenation of other biomass-derived molecules. Based on
the activity test, the catalyst characterization and DFT calcu-
lation, the reaction mechanism has been also proposed.
Homogenous catalysts are in many respects superior to
heterogeneous catalysts. Despite this, the inherent difficulty in
separating and recycling homogeneous catalysts has hampered
their further utilization.[30] The development of functional
inorganic-organic supramolecular materials and the immobiliza- Results and discussion
tion of suitable guest complexes on solid surfaces have been
extensively investigated.[31,32] Both steric and electrostatic con-
straints can change the structure and reactivity of the guest
complexes, which frequently leads to unexpected physicochem-
ical characteristics in comparison with their properties in
solution.[33–35] Among various inorganic host materials, ZrP, as a
family of transition metal phosphate materials, increasingly
attracts research interest due to its outstanding physical and
chemical properties, including an extremely high ion-exchange
capability and excellent thermal stability. These superior proper-
ties, along with simple preparation and easy functionalization,
making ZrP-based materials promising candidates for a wide
range of applications.[36]
Catalyst characterization
In this work, the [(p-cymene)Ru(phen)Cl]Cl (Ru-phen) (p-cym-
ene=p-MeC6H4Pri, phen=1,10-phenanthroline) complex was
prepared (Figure S1) and then intercalated in ZrP layers through
three steps as follows: the preparation of α-ZrP; the exfoliation
of α-ZrP by a quaternary salt tetrabutylammonium hydroxide
(TBAOH) [Eq. (1)] and intercalation of a cation Ru (II) complex
into ZrP nanosheets [Eq. (2)]. α-ZrP have the high cation-
exchange capability (ca. 6.64 mmol H+/g) owning to the
reactive surface hydroxyl groups.[36] As shown in Table 1, the
contents (C, H, Cl, N, P, Ru and Zr) of ZrP-based catalysts were
analyzed by elemental analysis, ICP-AES analysis and ion
chromatography, respectively. Consequently, the chemical for-
mulas of the exfoliated ZrP (ex-ZrP_0.4) and Ru-phen interca-
lated catalysts were given in Table S1. As a typical example, it
can be seen from Eq. (1) that after a crystalline α-ZrP was
exfoliated by TBAOH, a partial TBA could be substituted by the
Concretely, Zr(HPO4)2·H2O (α-ZrP) is the most widely inves-
tigated because it possess trenchant plates, and the thin plates
of α-ZrP have the same structure and parallel with each
other.[37] In recent years, more and more researchers have
utilized the ion-exchange properties of α-ZrP to optimize its
applications in different fields.[38–41] α-ZrP has
a layered
structure, in which Zr atoms lie almost in plane and are bridged
by the phosphate moiety. Each phosphate group coordinates
three of its O atoms to three Zr atoms, whereas the fourth is
bonded to an exchangeable H atom. The distance between the
layers in α-ZrP is 7.6 Å.[42]
Table 1. BET area and elemental analysis of the ZrP-based catalysts.[a]
Catalysts
SBET
Found [wt./%]
P/Zr
[m2 gÀ 1
]
C
H
N
Ru
Cl[b]
The metal-complexes such as [Ru(bpy)3]2+ [43]
,
[Pt(tpy)Cl]+,[44]
[molar ratio]
and Re(phen)(CO)3Cl[45] have been intercalated into ZrP inter-
layers acting as a host for achieving luminescent materials.
Some metal-complex intercalated into ZrP materials have been
reported for catalysis, e.g. Rh-complex for hydroformylation
reaction,[46] Re-complex for CO2 reduction,[47] and Wilkinson-type
catalyst for hydrogenation reaction.[48] However, although the
α-ZrP
27
1.81
1.79
1.72
1.74
1.71
1.69
–
7.36
6.23
–
–
–
–
–
–
ex-ZrP_0.4 32
Ru-ZrP_0.2 34
Ru-ZrP_0.4 29
Ru-ZrP_0.6 32
Ru-ZrP_1.0 30
2.78 0.48
1.73 0.51 1.72 0.68
11.76 3.04 0.89 1.86 0.66
13.07 3.02 1.17 1.77 0.61
21.35 4.63 1.81 1.71 0.65
[a] The contents of Zr, Ru, and P were determined by ICP-AES while C, H,
and N by elemental analysis. [b] The content of Cl was determined by ion
chromatography.
2+
intercalation of Ru(bpy)3 within the layers of ZrP has been
constructed for influencing photophysical properties,[49] the
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cation Ru complex due to the intercalation. Afterwards, the
TBACl was removed by fully washing, affording the (Ru-ZrP_0.4)
catalyst accordingly [Eq. (2)].
The thermal stability of the ZrP-based samples was
evaluated by TGA. As shown in Figure S2, an obvious weight
several days).[56,57] The possible reasons for the difficulty in direct
intercalation of metal complexes into α-ZrP were relatively
higher van der Waals forces between the layers and the
diffusion-limiting process of intercalating agents to migrate
toward the center of the gallery.[58]
°
loss of approximately 10% between 200 and 800 C for α-ZrP
sample. It can be assigned to the condensation of P-OH under
high temperature and further dehydration (Figure S2a). The
The phase structure of the pristine α-ZrP, ex-ZrP, and the
corresponding Ru-ZrP catalysts were determined by XRD
patterns (Figure 1). As illustrated in Figure 1a, the pattern of α-
°
°
weight loss of ex-ZrP_0.4 (ca. 17%) between 200–800 C was
attributed to the decomposition of the TBA ions (Figure S2b).
The Ru-ZrP_0.4 showed the decomposition temperature started
ZrP displayed three main strong diffraction peaks at 11.7
°
°
(interlayer distance of 7.6 Å), 19.8 and 24.9 , corresponding to
(002), (110) and (112) planes of monoclinic α-ZrP (space group-
P21/n, JCPDS-ISDD No.–70-0561), respectively.[59] It could be
°
at 230 C with a subsequent weight loss of ca. 20% (Figure S2c),
which was assigned to the decomposition of the organic
ligands on Ru-phen moiety and also TBA ions.
°
seen that a new peak at 5.9 emerged in the patterns of ex-ZrP_
0.2 and ex-ZrP_0.4 samples while the peak at 11.7 remained
°
The 1H NMR and 13C NMR spectra corroborated the
successful synthesis of Ru-phen complex (Figure S1).[55] In
addition, ICP-AES analysis proved that ca. 85% of Ru complex
precursor has been incorporated into the as-obtained Ru-ZrP
catalysts (Table 1), revealing that the cationic Ru complex had a
strong affinity toward anionic ZrP nanosheets. The pristine α-
ZrP sample exhibited the BET surface area (27 m2 gÀ 1), which
showed a slight increase up to 32 m2 gÀ 1 after the exfoliation
with TBAOH (Table 1, ex-ZrP_0.4), suggesting that ZrP inter-
layers spacing was likely expanded due to exchanging protons
of ZrP by bulk ions (TBA). However, after the intercalation of Ru-
phen complex, the BET surface area remained unchanged,
implying that although a cationic Ru-phen complex might
replace a part of TBA, the texture of the Ru-ZrP did not undergo
any significant change. Notably, the present intercalation of a
cationic metal complex into ex-ZrP layers cost less time (6 h) as
compared with that reported in previous references (usually
but became weak. The appearance of the new peak (5.9 ,
°
15.0 Å) revealed the partial exfoliation state,[60] which was due
to intercalation of single layer of TBA into α-ZrP nanosheets in
line with a slight increase of BET surface area (Table 1).
Moreover, as the molar ratio of TBAOH to α-ZrP were raised
from 0.2 to 0.4, the exfoliated ZrP catalysts offered a weaker
°
peak at 5.9 (15.0 Å), as compared with that of α-ZrP, which
meant higher extent of exfoliation degree for ex-ZrP_0.4 sample
(Figure 1a).[60] As the molar ratio of TBAOH to α-ZrP increased
further, the resulting Ru-ZrP_0.6 and Ru-ZrP_1.0 catalysts gave
a new strong peak appearing at 5.1 (17.1 Å) and 5.0 (17.6 Å),
respectively. The new peaks around 5.0 could be caused by the
restacking of exfoliated α-ZrP nanoplatelets, with larger inter-
layer spacing than peak at 5.9 (15.0 Å) appearing on ex-ZrP_0.2
and ex-ZrP_0.4 samples, suggesting a single-layer arrangement
of TBA ions plus some water molecules in the restacked
layers.[58] Especially, for the ex-ZrP_1.0 sample, the crystal
°
°
°
°
Figure 1. XRD spectra of (a) α-ZrP and the ex-ZrP, (b) The Ru-phen intercalated catalysts.
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morphology tended to be amorphous and α-ZrP phase barely
remained.
On the other hand, Figure 1b displayed the XRD spectra of
the Ru complex intercalated ZrP catalysts. It was obvious that
band at 530 cmÀ 1 was due to the (P)-OÀ H deformation vibration
and the band around 1045 cmÀ 1 was corresponding to the PÀ O
stretching vibration. A weak absorption at 1249 cmÀ 1 was
attributed to vibration of P=O.[61] In addition, the characteristic
bands to water were observed around 3500 cmÀ 1 and
1622 cmÀ 1 in all samples. For the FT-IR spectra of Ru-phen, the
bands from 650 cmÀ 1 to 900 cmÀ 1 was ascribed to the out-of-
plane bending vibration of CÀ H on the aromatic rings, and the
skeleton vibration absorption bands of aromatic rings were
observed around 1430 cmÀ 1 to 1600 cmÀ 1. It could be seen that
the absorption bands of ligands on Ru-phen were also
appeared on the spectra of Ru-ZrP_0.4, confirming the immobi-
lization of Ru-phen on ZrP layers. Additionally, 1024 cmÀ 1 was
attributed to CÀ N bonds, which was overlapped with PÀ O
vibration of ZrP. Besides, the absorption bond at 2964 cmÀ 1
(CÀ H stretching vibration) was also observed in the samples of
ex-ZrP, revealing that TBA existed in the exfoliated ZrP sample.
All these results above confirmed that the Ru-phen complex
was not subject to an obvious structural change, in line with
that of elemental analysis (Table 1 and Table S1).
°
the weak peak (5.6 , 15.7 Å) of Ru-ZrP_0.2 was similar with the
°
case of Ru-ZrP_0.4 (5.6 , 15.8 Å), indicating the intercalation of
Ru complex into ZrP layers via replacing TBA ions in the
interlayers in the two samples. However, more exfoliation did
not result an obvious change of the interlayer spacing (about
15.5 Å) for Ru-ZrP_0.6. Especially, the interlayer spacing became
smaller as compared with that of ex-ZrP_0.6 (17.1 Å, Figure 1a).
This result suggested that Ru-phen have replaced the partial
TBA ions and the water molecules in the restacked layers due to
the Ru complex of intercalation into ex-ZrP_0.6, but the
structure remained almost untouched. In addition, Ru-ZrP_1.0
showed an almost amorphous structure, which was quite
different from other samples, and the interlayer spacing
increased up to 17.5 Å (Figure 1b). For comparison, Ru was
deposited on α-ZrP by direct impregnation, and the resulting
°
Ru/α-ZrP catalyst only showed a very weak peak at 6.0 (14.7 Å),
implying that the exfoliation was very important prior to the
intercalation of Ru-phen complexed into ZrP layers. As a result,
the α-ZrP exfoliated by TBAOH and further intercalated with the
cationic Ru (II) complex leaded to expansion of the interlayers
for Ru-ZrP_0.2, Ru-ZrP_0.4, and Ru-ZrP_0.6 catalysts, enabling
the cationic Ru(II) complex to be immobilized into the
expanded layers. However, when the excessive exfoliation was
caused by increasing the molar ratio of TABOH/α-ZrP to 1:1,
the crystalline phase of α-ZrP completely disappeared and the
resulting samples became amorphous whether the cationic
Ru(II) complex was intercalated or not (Figure 1a and b).
Solid-state 31P MAS NMR spectra of ZrP-based samples were
performed to evaluate the coordination state of the phosphorus
atoms in ZrP layers. As shown in Figure 3, the spectrum of the
α-ZrP showed a single peak at À 19.1 ppm, which can be
attributed to [(OH)P-(OZr)3].[62] For ex-ZrP_0.4, beside the peak
19.1 ppm, a downfield shifted peak appeared at À 17.7 ppm,
corresponding to the partial deprotonation of the phosphate
groups denoted as [TBA+···À OÀ P-(OZr)3].[63] For the spectrum of
Ru-ZrP_0.4 sample, the peak (at À 14.5 ppm) shifted further
downfield in comparison with the ex-ZrP sample, which could
be ascribed to the interaction of phosphate group with Ru(ΙΙ)
center and likely lead to forming [Ru-OÀ P-(OZr)3] species,
causing a significant de-shielding of the phosphorus. Besides,
the peak around À 17.7 ppm became weaker on Ru-ZrP_0.4
The FT-IR spectra of α-ZrP, ex-ZrP_0.4, Ru-ZrP_0.4 and Ru-
phen catalysts were shown in Figure 2. The absorption band of
α-ZrP was also found in ex-ZrP_0.4 and Ru-ZrP_0.4 samples. The
Figure 2. FT-IR spectra of Ru-phen complex, α-ZrP, ex-ZrP_0.4 and Ru-ZrP_0.4 catalyst.
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crystallinity. It could be observed from Figure 4c that the ex-
ZrP_0.4 material showed as the same layered structure as that
of α-ZrP, indicating that there was no change in surface
morphology but layered ZrP were separated into smaller
nanosheets after exfoliation with TBAOH. Moreover, Ru-ZrP_0.4
also appeared structured layered configuration. HRTEM images
of the three materials were depicted in Figure 4b, 4d, and 4f
respectively, all of which showed thin nanosheets stacked
together and remained uniform in size and shape. To identify
clearly the distribution of Ru element on the ZrP layers, HAADF-
STEM and EDS were employed. As shown in Figure 5, the crystal
morphology of Ru-ZrP_0.4 catalyst was lamellar, whose width
and length were about 10 nm and 300 nm, respectively. The
elemental mappings demonstrated that the Ru, Zr and P
species were distributed uniformly and adjacent to each other.
AFM was also employed to measure nanoplatelet thickness of
α-ZrP, ex-ZrP_0.4, and Ru-ZrP_0.4. As shown in Figure S3, the
thickness of α-ZrP nanoplatelet was detected as ca. 10 nm,
similar to the reported (around 8 nm),[64] while the thickness of
ex-Zr 0.4 (approximate 30 nm) was much larger than that of α-
ZrP (10 nm), which was caused by the intercalation of TBA
cations within ZrP interlayers or nanoplatelet stack. However,
after Ru-phen complex was introduced the thickness of the
resultant Ru-ZrP_0.4 catalyst was slightly decreased to ca.
20 nm via co-assembly of the exfoliated ZrP nanoplatelet and
Ru-phen complex molecules. All these results above were in
line with the characterization by XRD and HRTEM (Figure 1 and
Figure 4).
Next, the chemical state of Ru center was evaluated by XPS.
The XPS spectra, including the survey, Ru 3p region and Zr 3d
region of Ru-ZrP catalysts, were shown in Figure S4-S8. The
peak of Ru 3p3/2 around 462.4 eV were attributed to Ru (ΙΙ)
species, confirming the Ru center remained Ru2+ state for all
the Ru-ZrP catalysts.[65] And the Zr 3d5/2 and Zr 3d3/2 peaks
corresponding to the binding energy of 183.1-183.3 eV and
185.5-185.6 eV, respectively, which can be attributed to the Zr
(IV) species.[66] Ru/Zr molar ratios as obtained from XPS and ICP-
AES were displayed in Table S2, respectively. It can be seen that
Ru/Zr ratios obtained from XPS results exhibited lower values
than that determined by ICP analysis for all Ru catalysts,
suggesting that Ru-phen complex has been intercalated into
interlayers of ZrP to some degree. Especially, the Ru/α-ZrP gave
the maximum molar ratio of Ru/Zr (0.042) as obtained from XPS
analysis among the intercalated catalysts, meaning more Ru-
complex was located on the surface of α-ZrP. Conversely,
RuÀ Zr 0.4 afforded the smallest molar ratio of Ru/Zr (0.028),
which suggests that Ru-phen complex was bonded within
interlayers of ZrP, agreed with XRD results (Figure 1) and AFM
(Figure S3).
Figure 3. 31P MAS NMR of α-ZrP, ex-ZrP_0.4 and Ru-ZrP_0.4 catalysts.
catalyst due to the substitute of a partial TBA ions by the
cationic Ru complex. It was observed that the characteristic 31P
resonance signal of α-ZrP (-19.9 ppm) was still dominant in the
spectra of ex-ZrP and Ru-ZrP_0.4 samples, which implied that
[(OH)P-(OZr)3] species are still retained even after the intercala-
tion of cationic Ru (II) complex (Figure 3).
Figure 4 displayed the SEM and HRTEM images of α-ZrP, ex-
ZrP_0.4, Ru-ZrP_0.4 catalysts. The surface morphology of
materials was firstly revealed by SEM. As displayed in Figure 4a,
α-ZrP showed trenchant plates in which the disks have well-
defined shapes with very smooth surface, appearing as good
Next, the catalysts were characterized by UV/Vis spectra. As
shown in of Figure S9, the UV/Vis spectra of [Ru(p-cymene)Cl2]2
and Ru-phen in CH2Cl2 solution exhibited absorption bands in
the high energy region (200-320 nm) (the inset), which could be
assigned to intra-ligand π-π* transitions. The moderately low-
energy bands ranging from 340–550 nm ascribed to metal-to-
ligand charge transfer (MLCT), abounded in [Ru(p-cymene)
Cl2]2.[67] The diffuse-reflectance (DR)-UV/Vis spectra for ex-ZrP_
Figure 4. SEM and HRTEM images of catalysts. (a, b) α-ZrP, (c, d) ex-ZrP_0.4,
(e, f) Ru-ZrP_0.4.
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Figure 5. HAADF-STEM image of Ru-ZrP_0.4 catalyst and corresponding elemental mappings of Ru, Zr, and P.
0.4 and Ru-ZrP_0.4 were also presented in Figure S9. No clear
absorption was observed in the absence of Ru complex on ex-
ZrP_0.4 sample. Conversely, all the absorption bands of the Ru-
ZrP_0.4 catalyst agreed with those of the free Ru complex.
0.4 were used as catalysts, FAL was barely hydrogenated under
the reaction conditions (Table 2, entries 1 and 2). Additionally,
the water-soluble Ru-phen complex also afforded very poor
conversion of FAL (<5%), which meant that it was incapable to
activate and dissociate H2 under the given conditions (Table 2,
entry 3). The Ruthenium acetylacetone (Ru(acac)3) complex and
the Ru(acac)3/ex-ZrP_0.4 catalysts were also evaluated for FAL
hydrogenation, which showed that the catalysts were inactive
(Table 2, entry 4 and 5). Besides, RuCl3/ex-ZrP_0.4 exhibited
poor catalytic performance as well (Table 2, entry 6). These
results confirmed that these catalysts were not capable of H2
activation or dissociation (Table 2, entry 4–6). Commercial Ru/C
was tested for FAL hydrogenation, and it was found that 86%
of FAL was consumed with 65% selectivity to tetrahydrofurfuryl
alcohol (THFA), and 34% selectivity to furfuryl alcohol (FOL),
respectively (Table 2, entry 7). This result indicated that
although Ru0 sites can activate H2 and provide moderate activity
in FAL hydrogenation, FOL could be further hydrogenated into
THFA, resulting low selectivity toward FOL. If Ru-phen complex
was directly impregnated into α-ZrP, the resulting Ru/α-ZrP
catalyst showed only 12% of conversion but FOL was detected
as the only product (Table 2, entry 8). Notably, the Ru-phen
intercalated ZrP catalysts provided the enhanced activity
(Table 2, entry 9–12). The optimal Ru-ZrP_0.4 catalyst afforded a
full conversion of FAL and 99% selectivity to FOL. However, Ru-
ZrP_0.6 and Ru-ZrP_1.0 catalysts showed 86% and 71%
conversions of FAL respectively, which implied that a disorder
crystal phase would be not beneficial to activity enhancement
(Figure 1). Besides, the catalytic activity of Ru-ZrP_0.2 were
Catalytic performance
The FAL was firstly chosen as a model substrate for catalytic
hydrogenation with the different catalysts in the presence of H2
°
in H2O at 100 C. As shown in Table 2, when α-ZrP and ex-ZrP_
Table 2. Catalytic hydrogenation of FAL over different catalysts.
Entry
Catalyst
Conv.[%][a]
Sel.[%]
1
2
3
4
α-ZrP
ex-ZrP_0.4
Ru-phen
Ru(acac)3
Ru(acac)3/ex-ZrP_0.4
RuCl3/ex-ZrP_0.4
Ru/C
<5
<5
<5
<5
<5
<5
86
12
68
99
86
99
99
99
99
99
99
34
99
99
99
99
99
5
6
7b
8
Ru/α-ZrP
9
Ru-ZrP_0.2
Ru-ZrP_0.4
Ru-ZrP_0.6
Ru-ZrP_1.0
10
11
12
71
[a] Reaction conditions: H2O 6 ml, FAL 1.2 mmol, Ru:FAL=1:150, H2
°
1.5 MPa, 8 h, 100 C. Ru(acac)3 =Ruthenium acetylacetone. Furfuryl alcohol
(FOL) was detected as a sole product. [b] Tetrahydrofurfuryl alcohol
(THFA) was also detected with a selectivity of 65%.
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lower than that of Ru-ZrP_0.4 for FAL hydrogenation (Table 2,
entries 9 and 10), confirming that either the less or more
exfoliation could decrease the catalytic activity, and an
appropriate exfoliation was extremely important for achieving
high activity (Table 2, entries 9–12). It should be noted that
either ex-ZrP_0.4 or a sole Ru-phen complex lacked the active
sites for H2 activation and dissociation (Table 2, entries 2 and 3).
In contrast, the cationic Ru complex intercalated ZrP catalysts
can activate H2 and offer excellent hydrogenation activity,
suggesting strongly that the catalytic active sites arise from a
strong coordination interaction between the Ru complex with
ZrP layers. In particular, the catalytic activity was highly close to
the extent of exfoliation considering that the ZrP-based
catalysts with suitable interlayer spacing and regular crystalline
form (Ru-ZrP_0.4) afforded the best activity and selectivity
(Table 2, entries 10). Conversely, the Ru-ZrP_1.0 catalyst with
larger interlayer spacing or disorder structure showed much
lower catalytic activities than that of Ru-ZrP_0.4 (Table 2,
entry 10 vs 12).
The effect of reaction pressure on reaction was shown in
Figure S10. It can be seen that that the conversion of FAL
increased with H2 obviously and reached up to 99% as the
pressure ranged from 0.5 MPa to 1.5 MPa. Nevertheless, further
enhancing the H2 pressure had no effect on the reaction,
indicating that the present intercalation catalyst can show high
activity even under low H2 pressure while maintaining excellent
selectivity even under higher H2 pressure. Subsequently, the
reaction kinetics were also investigated for hydrogenation of
FAL on Ru-ZrP_0.4 catalyst. To eliminate the diffusion limita-
tions, the effect of stirring speeds and the amount of catalyst
on the reaction rate was investigated, and the results are shown
in Figure S11a and b. As a result, kinetics studies have been
performed by choosing the appropriate stirring speed (1000)
and catalyst amount (40 mg) under the given reaction time,
respectively. The relationship between the reaction rate (R0) and
concentrations of the substrate was displayed in Figure S11c
and d. All reaction rates (R0) for the kinetic studies were
determined at conversion lower than 15% (initial rate method).
The studies showed the reasonable first-order plot for the
concentration of FAL (CFAL =0.1 M-0.6 M). The concentration of
hydrogen was considered to be a constant since hydrogen was
excessive. Therefore, the reaction rate can be calculated by
using Eq. (3) at a constant concentration of hydrogen, in which
R0 and k represent the reaction rate and rate constant,
respectively.
1
R0 ¼ À ½d ðCFALÞ=dt� ¼ k � ½CFAL
�
(3)
Additionally, the dependence of the reaction rate of FAL on
the temperature (Arrhenius plots, 353–383 K) was shown in
Figure 6. The Arrhenius equation enable the determination of
apparent activation energies of FAL hydrogenation (Ea) by
following the Eq. (4). The Ea was obtained according to the
linear correlation between ln k and 1/T (Figure 6),
k ¼ AeðÀ Ea=RTÞ
(4)
which was given as Ea=40.1 kJ/mol. The activation energy
under the present condition was lower than or comparable
with that of previous work on FAL hydrogenation.[68,69]
Substrate scope and recyclability of the catalyst
Overall, the Ru-ZrP_0.4 catalyst offered superior catalytic
performance in selective hydrogenation of C=O bond outside
of the furan ring in the FAL molecule. Subsequently, further
studies have greatly broadened the substrate scope to a series
of other carbonyl compounds on the Ru-ZrP_0.4 catalyst. As
displayed in Table 3, the Ru-ZrP_0.4 catalyst displayed superior
performance for the hydrogenation of these substrates. For
example, with the electron withdrawing -Ph group, the
Figure 6. (a) Conversion/time profile of FAL hydrogenation catalyzed by Ru-ZrP at different temperatures. (b) Arrhenius plots for the hydrogenation reaction
of FAL. The observed rate constants (k) were calculated with the initial rates from Figure 6a at different temperatures. Reaction conditions: H2O 6 ml, FAL
°
1.2 mmol, Ru-ZrP_0.4 40 mg, H2 1.5 MPa, 100 C. FOL was detected as the sole product.
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Table 3. The hydrogenation of different substrates over Ru-ZrP_0.4 catalyst.[a]
Entry
Substrates
T[h]
Conv.[%]
99
Products
Sel.[%]
99
1
8
2
3
8
8
99
99
99
99
4
5
10
10
83
87
99
99
6
7
8
10
10
10
86
88
92
99
97
99
9[b]
10
6
85
92
99
72
10[b]
11[c]
8
85
86
12[d]
13[e]
8
6
99
85
69
25
14
10
87
99
°
[a] Conditions: H2O 6 ml, Substrate 1.2 mmol, Ru-ZrP_0.4 40 mg, H2 1.5 MPa, 100 C. [b] H2 2.0 MPa. [c] The 4-(furan-2-yl) butan-2-ol was also detected by
GC-MS as a minor by-product. [d] H2 5.0 MPa, n-hexanol was also detected as a by-product. [e] Ru/C as a catalyst, H2 2.0 MPa, phenyl propanol and phenyl
propyl aldehyde were also detected as by-products, respectively.
benzaldehyde was also hydrogenated to benzyl alcohol with
high conversion (99%) and selectivity (99%) (Table 3, entry 1).
The conversion of cyclohexanone was still high (99%) and only
produced cyclohexanol (Table 3, entry 2). The acetophenone
molecule bearing -Ph and -CH3 group was hydrogenated to the
desired alcohol in high activity (99%) and selectivity (99%) as
well (Table 3, entry 3). Acetophenone derivatives containing -R
or -OR groups were tested with longer reaction time (10 h), and
the conversions were relatively lower than that of acetophe-
cinnamaldehyde and bio-derived platform molecules, including
5-hydroxymethyl-furfural (HMF), also afforded the correspond-
ing alcohol with good selectivity under a slightly higher
hydrogenation pressure (2.0 MPa) (Table 3, entry 9). Similarly,
the electron donating group À CH2OH made the carbon atom of
C=O in HMF relatively less positive than that of FAL molecule.
For cinnamaldehyde, a nearly full conversion was achieved and
the cinnamyl alcohol was produced with 72% of selectivity
(Table 3, entry 10). Besides, furfurylideneacetone was also
none (Table 3, entry 4–8). The electron donating groups (À R and hydrogenated to the desired unsaturated alcohol with 85%
À OR) reduced the reactivity of C=O in these substrates likely
due to the heterolytic activation of H2,[70] which weakened the
attack of HÀ to the C atom. Besides, the hydrogenation of
conversion and 86% selectivity on Ru-ZrP_0.4 catalyst (Table 3,
entry 11), and also hydrogenation of trans-2-hexenal gave a
nearly full conversion and the selectivity toward trans-2-hexenol
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was up to 69% over Ru-ZrP_0.4 catalyst (Table 3, entry 12),
forming via the selective hydrogenation of C=O group. For
comparison, Ru/C catalyst was employed in cinnamaldehyde
hydrogenation (Table 3, entry 13), which gave slightly lower
conversion and poor selectivity to cinnamyl alcohol (25%),
along with the major by-products (the selectivity to phenyl
propanol and phenylpropyl aldehyde was 51% and 24%,
respectively.). Another bio-derived platform molecule methyl
levulinate was also employed as a substrate and afforded γ-
valerolactone as a sole product with 87% of conversion within
10 h on Ru-ZrP_0.4 catalyst, which included a C=O hydro-
genation step and an intramolecular cyclization reaction
(Table 3, entry 14). The superior catalytic selectivity toward the
hydrogenation of carbonyl group on the Ru-ZrP_0.4 catalyst
had an implication of the heterolytic activation of H2, which was
highly favorable for the selective hydrogenation of polar
carbonyl groups than nonpolar carbon-carbon groups.[71] On
the basis of the results from Table 3, Ru-ZrP_0.4 catalyst had an
excellent activity in hydrogenation of carbonyl groups tethered
in organic molecules, including simple aldehydes and ketones,
and α,β-unsaturated aldehyde, ketone etc., as well as the other
biomass-derived platform molecules.
images, which was similar with that of fresh one (Figure 4f vs
S12b). The spent Ru-ZrP_0.4 catalyst was also characterized by
using the methods of XRD, FT-IR and XPS. As displayed in
Figure S13a, the XRD spectra revealed that the crystal phase of
the spent Ru-ZrP_0.4 catalyst barely changed, compared to the
fresh catalyst (Figure 1b). The absorption bands corresponding
to α-ZrP phase still existed in the FT-IR spectra in spent Ru-ZrP_
0.4 catalyst, and the absorption bands of ligands on Ru-phen
were also appeared on spent Ru-ZrP_0.4 after six consecutive
catalytic recycles, confirming the stability of the Ru-ZrP_0.4
catalyst (Figure S13b). XPS analysis (Figure S13c) confirmed the
peak of Ru 3p3/2 at 462.8 eV of the spent catalyst was attributed
to Ru (ΙΙ) species, confirming that the Ru (II) complex was still
present rather than Ru0 nanoparticles even after six catalytic
recycles in hydrogenation. The characterizations above revealed
that the present Ru (II) intercalated catalyst was very stable
structurally and recyclable in hydrogenation reaction.
Identification of active sites in Ru-phen complex intercalated
ZrP catalysts
The stability and recyclability of the catalyst were indispen-
sable for further application. Thus, the reusability of Ru-ZrP_0.4
catalyst was evaluated. As displayed in Figure 7, the test was
conducted within 6 h for six times with a moderate decrease of
activity in the hydrogenation of FAL to FOL. It is noteworthy
that less than 2 ppm of Ru was detected after each run by ICP-
AES analysis, reflecting negligible leaching of the transition
metal. SEM and HRTEM images of the spent Ru-ZrP_0.4 catalyst
was displayed in Figure S12. SEM images confirmed that the
surface morphology of the spent Ru-ZrP_0.4 catalyst did not
show an obviously change in comparison with the fresh catalyst
(Figure 4e vs S12a). In addition, it could be seen that no Ru
nanoparticles but only ZrP layers were found in the HRTEM
Although HRTEM measurement showed no nanoparticles in the
Ru-ZrP_0.4 catalyst, the possible formation of Ru0 species could
not be excluded during hydrogenation reaction. Hg0 is a well-
known poison for metal nanoparticles due to the formation of
an amalgam, whereas molecular metal complexes remain
unaltered.[72] In the next step, Hg0 poisoning test was carried
out to identify the catalytically active sites on the Ru-ZrP_0.4
catalyst. Figure S14 showed the effect of reaction time on FAL
conversion. It could be observed clearly that the conversion
increased with time. It can be seen that catalytic activity cannot
be suppressed at all upon the addition of Hg0 to a reaction
mixture, which provided a strong evidence for catalytic hydro-
genation facilitating not by Ru0 nanoparticles but Ru complex
Figure 7. Recyclability and Ru leaching of the Ru-ZrP_0.4 catalyst on the hydrogenation of FAL. Reaction conditions: H2O 6 ml, FAL 1.2 mmol, Ru-ZrP_0.4
°
40 mg, H2 1.5 MPa, 100 C, 6 h. FOL was detected as the sole product.
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species. This result was highly consistent with XPS character-
ization of the spent Ru-ZrP_0.4 catalyst, in which the Ru species
always existed in the form of Ru (ΙΙ) state (Figure S6 and
Figure S13c).
the sample Ru/α-ZrP. These results revealed that intercalation
of Ru-phen into pre-exfoliated ZrP layers led to the new Ru
complex structure with RuÀ O coordination bond, while direct
impregnation of Ru-phen into α-ZrP resulted in a minor of new
substituted Ru complex containing RuÀ O coordination bond,
and free Ru-phen still largely existed. Combined with catalytic
performance of these catalysts (Table 2, entry 3, 8 and 10), it
was observed that the sole Ru-phen complex only containing
RuÀ Cl coordination shells barely catalyzed the FAL hydro-
genation reaction under the given conditions (Table 2, entry 3),
while Ru (II) complex intercalation catalysts containing the
RuÀ O species could afforded excellent catalytic activity (Table 2,
entry 10), in which most RuÀ Cl coordination bond was replaced
by RuÀ O coordination bond (Table 2, entry 10 and Figure 8a).
Comparatively, the Ru/α-ZrP catalyst only afforded 12% con-
version of FAL, in which the RuÀ Cl coordination bond still
abounded, and the RuÀ O species existed in a small amount
(Table 2, entry 8 and Figure 8a). Therefore, the above evidence
supports strongly that the RuÀ O species rather than the free
Ru-phen contributed mainly to the hydrogenation catalysis on
the intercalation catalyst.
To gain a deeper understanding of the involvement of the
Ru species in catalysts, transmission-mode X-ray absorption
spectroscopy was also performed at the Ru K-edge (22.12 keV).
The radial distribution function obtained by Fourier trans-
formation of the k3-weighted EXAFS data of catalysts were
shown in Figure 8, together with that of Ru foil for comparison.
It was obvious that samples containing Ru-phen complex were
all different from that of Ru foil, which could be attributed to
the different coordination environment of the Ru species.
Apparently, no metallic RuÀ Ru shell peak at 2.4 Å could be
observed in Ru-phen, Ru-ZrP_0.4 or Ru/α-ZrP samples. This was
further supported by the X-ray absorption near edge structure
(XANES, Figure 8b). The XANES spectra of Ru-phen, Ru-ZrP_0.4
and Ru/α-ZrP catalysts were all different from that of Ru foil as
well. The intensity of the absorption edge, often referred to as
“white line”, can reflect the oxidation state of Ru. In contrast to
the Ru foil, Ru-phen, Ru-ZrP_0.4, and Ru/α-ZrP displayed higher
“white lines“, indicating they had higher chemical state than Ru
foil (Ru0), agreeing with XPS results (Figure S4 and S6).[73] For
Ru-phen, the strong peak at approximately 1.8 Å was attributed
to RuÀ Cl coordination shell, while the major peak at 1.5 Å of Ru-
ZrP_0.4 catalyst was related to RuÀ O shell (the enlarged region
in Figure 8a),[74,75] which revealed that most of chloride ligands
in Ru-phen have been substituted by the layer’s phosphate
groups in the course of intercalation reaction, being consistent
with the results of 31P MAS NMR (Figure 3). In the EXAFS spectra
of Ru/α-ZrP, two major peaks centered around 1.5 Å and 1.8 Å
respectively related to RuÀ O and RuÀ Cl coordination shells
were also observed, indicating that only a small amount of Ru-
phen complex could also intercalate into ZrP layer even without
exfoliation (with RuÀ O and RuÀ Cl shell) (Figure 8a). Additionally,
the peak height of RuÀ Cl was larger than that of RuÀ O,
denoting that the main Ru species was free Ru-phen complex in
DFT calculations
To further understand the correlation between the RuÀ O
species and catalytic activity, the computational analysis based
on first-principles DFT calculations was conducted. Firstly, the
structures of the Ru-phen complex and Ru-ZrP_0.4 interface
were optimized, respectively. The optimized lattice of α-ZrP was
displayed in Figure S15. As shown in Figure 9a, in the Ru-phen
complex [(p-cymene)Ru(phen)Cl]+ClÀ , the phenanthroline li-
gand is coordinated to the ruthenium center in a η2-N,N-
fashion, while the p-cymene presents a η6-conjugation config-
uration. For the formation of Ru-ZrP_0.4 catalyst, the bonding
of Ru-phen complex with ex-ZrP_0.4 essentially involves the
removal of TBACl molecule with the RuÀ O bond formed
Figure 8. (a) k3-weighted Fourier-transformed EXAFS of different catalysts. (b) Ru K-edge XANES spectra of different catalysts.
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Figure 9. Optimized structures of (a) Ru-phen and (b) Ru-ZrP_0.4, and corresponding final states configurations of H2 dissociation. White, grey, royal blue, red,
violet, blue and green balls stand for H, C, N, O, P, Zr and Ru atoms. Pink and yellow balls highlight terminal O on Ru-ZrP_0.4 and dissociated H.
simultaneously at the Ru-phen/ZrP interface, corresponding to
activation energy data showed that H2 activation on Ru-ZrP
catalyst was facile compared with the previous work.[76,77] This
suggested that the interfacial RuÀ O could act as an active site
for H2 activation (RuÀ O+H2!H1À Ru+OÀ H2). According to the
Bader charge analysis, the H atom dissociated onto the interface
O (i.e. the H2 atom) presents a charge state of Hδ+ (δ=0.62 j
ej), leaving the other H1 atom on Ru as HδÀ (δ=0.13 jej). It is
worth noting that this heterolytic activation of H2, yielding both
HδÀ and Hδ+ at RuÀ O interface, could facilitate the hydro-
genation of polar unsaturated carbonyl bond. The conclusion
can also be supported by the results that heterolytic activation
of H2 was highly beneficial to the selective hydrogenation of
polar carbonyl groups than nonpolar carbon-carbon bonds
(Table 3, entries 10 and 12).
Quantitatively, we explored the hydrogenation mechanism
of FAL to FOL on Ru-ZrP_0.4 catalyst. The energy profile and
the related key optimized structures are illustrated in Figure 10.
With the heterolytic cleavage of H2, calculation results revealed
that the hydrogenation process follows a collaborative process
involving both Ru-HδÀ and OÀ Hδ+ simultaneously. Specifically,
the carbonyl of furfural interacted with HδÀ and Hδ+ to form a
weak adsorption state by electrostatic force with an exother-
micity of 27 kJ/mol, where the HδÀ ···C and Hδ+···O distances are
2.220 Å and 1.570 Å, respectively. Subsequently, Ru-HδÀ at-
tacked the C-end of the terminal C=O group of FAL with O-Hδ+
attacking the O-end to complete hydrogenation; in the
transition state (TS-2, Figure 10), the CÀ HδÀ and OÀ Hδ+
distances were shortened to 1.300 Å and 1.098 Å, respectively.
This corresponded to a moderate barrier of 100 kJ/mol to
achieve the formation of FOL, and the reaction enthalpy was
16 kJ/mol. Overall, these computational results demonstrated
(p-cymene)Ru(phen)Cl]+ +TBA+
À OÀ P-(OZr)3![(p-cymene)
Ru(phen)-OÀ P-(OZr)3]+ + TBACl. Specifically, the optimized
binding configuration of Ru-phen on ex-ZrP_0.4 is shown in
Figure 9b, which is calculated to be the most stable structure
(Figure S16). Due to the strong orientation of coordination
RuÀ O bond, the single RuÀ O bond configuration a is energeti-
cally more stable (Eads=À 122 kJ/mol).
Secondly, we compared the H2 activation on Ru-phen and
Ru-ZrP_0.4 catalysts to probe the catalytic activity of Ru-ZrP_
0.4. On Ru-phen complex, it was found that H2 activation could
hardly proceed, largely owing to the coordination saturation of
Ru center. Accordingly, here two possible H2 activation path-
ways were considered; in pathway 1 (Figure 9a, P1), the ligand
Cl was removed to provide sites for H2 chemisorption/
dissociation, corresponding to a large dissociation energy of
148 kJ/mol. Differently, in pathway 2 (Figure 9a, P2) RuÀ Cl bond
was broken at a lower energy cost of 12 kJ/mol with the
removal of the chloride. In pathway 3 (Figure 9a, P3), one RuÀ N
bond was broken to accommodate hydrogen, which was
endothermic by 88 kJ/mol. Obviously, both these three proc-
esses were thermodynamically unfavorable.
Different from the Ru-phen complex, the existence of the
interfacial RuÀ O bond endowed the Ru center in Ru-ZrP_0.4
with improved H2 activation ability. Specifically, the H2
activation occurred at the interfacial RuÀ O bond with an energy
release by À 53 kJ/mol (Figure 9b, P1), while the RuÀ N cleavage
that then trigger H2 dissociation was endothermic by 34 kJ/mol
(Figure 9b, P2). Moreover, the dissociation of H2 at the RuÀ O
sites of Ru-ZrP_0.4 interface gave a moderate barrier of 79 kJ/
mol (see TS-1 in Figure 10). To the best of our knowledge, this
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Figure 10. Energy profiles and optimized structures of furfural hydrogenation catalyzed by Ru-ZrP_0.4, in which TS-1 is the transition state for H2 activation on
the interfacial RuÀ O site and TS-2 is the transition state for furfural hydrogenation. Grey, royal blue, red, violet, blue, green, and white balls stand for C, N, O, P,
Zr, Ru and H atoms, respectively, and the yellow balls highlight the dissociated H.
that the RuÀ O bond at Ru-ZrP_0.4 interface can serve as H2
heterolytic activation site and promote furfural hydrogenation,
which rationalized the experimental observations and provides
insight into the reaction mechanism.
Based on the results above, the RuÀ O active sites for H2
activation and dissociation were disclosed by EXAFS character-
ization and DFT calculation in FAL hydrogenation. Nevertheless,
excessively small or and large interlayer spacing (14.7 Å and
17.5 Å) was likely incapable to confine the Ru-complex between
layers and
a suitable interlayer spacing was crucial for
stabilizing the Ru-complex and the re-coordination of RuÀ O
bond as shown in Figures 10.
it was worth discussing that the extent of exfoliation of ZrP Conclusion
could affect the catalytic activity. As shown in Table 2, Ru-ZrP_
0.4 catalyst afforded the highest catalytic activity among the
catalysts, reflecting that an appropriate exfoliation was very
important for achieving high catalytic performance. ICP analysis
showed that after six consecutive recycles, the 51% and 43% of
Ru loading on the Ru-ZrP_0.2 and Ru-ZrP_1.0 catalysts were
leached into effluents, respectively. In contrast, Ru-ZrP_0.4 and
Ru-ZrP_0.6 catalysts could stabilize the Ru-phen complex more
effectively, and it was found that only ca. 8%–10% of Ru
loading was lost after catalytic recycles. As a result, the
The cationic Ru(ΙΙ) complex [(p-cymene)Ru(phen)Cl]Cl has been
intercalated into the interlayer space of the exfoliated ZrP by
using α-ZrP as the precursor, which provided an efficient
layered catalyst (Ru-ZrP_0.4) with unique active sites for
catalytic hydrogenation. It was demonstrated that the intercala-
tion catalyst showed high catalytic performance in the selective
hydrogenation of FAL and other carbonyl compounds under
the mild conditions. Especially, the catalyst could be easily
recycled for six times along with maintaining good catalytic
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[3] X. Wu, N. Luo, S. Xie, H. Zhang, Q. Zhang, F. Wang, Y. Wang, Chem. Soc.
Rev. 2020, 49, 6198–6223.
activity. In view of the catalyst characterization and DFT
calculation, it is indicated that remarkable performance can be
attributed to the coordination interaction of P-OH groups
within ZrP interlayers and Ru center, leading to RuÀ O bond,
which enabled H2 to be activated and dissociated via heterolytic
cleavage for the selective hydrogenation of carbonyl groups.
Moreover, an appropriate exfoliation would be crucial to
enhance activity and catalyst stability. The suitable interlayer
spacing was needed to immobilize the Ru-complex and favor to
re-coordination of the active RuÀ O species. Undoubtedly, this
work may expand the use of ZrP as supporting platforms for a
more variety of the cationic metal complexes and offer
alternative approaches to facilitate the catalyst separation and
recycling with unique activity and selectivity.
[4] S. A. Akhade, N. Singh, O. Y. Gutierrez, J. Lopez-Ruiz, H. Wang, J. D.
Holladay, Y. Liu, A. Karkamkar, R. S. Weber, A. B. Padmaperuma, M.-S.
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FULL PAPERS
M. Chen, J. Xia, Dr. H. Li, X. Zhao, Q.
Peng, J. Wang, H. Gong, Dr. S. Dai*,
Dr. P. An*, Prof. H. Wang*, Prof. Z.
Hou*
1 – 15
Selective Hydrogenation: The
furfural and other biomass-derived
carbonyl compounds into the corre-
sponding alcohols through selective
hydrogenation of C=O group.
A Cationic Ru(II) Complex Interca-
lated into Zirconium Phosphate
Layers Catalyzes Selective Hydro-
genation via Heterolytic
cationic Ru(II) complex was interca-
lated into ZrP layers, which enables
the efficient catalytic conversion of
Hydrogen Activation