Total hydrogenation of bio-derived furans over supported Ru subnanoclusters prepared via amino acid-assisted deposition

Article abstract of DOI:10.1039/c9gc02943e

Development of a highly efficient and robust catalyst with reduced usage of noble metals is extremely desirable for selective hydrogenations of furan-containing bio-based feedstocks, which represents an attractive and sustainable alternative to petrochemical resources. Herein, we describe a new type of well-dispersed Ru subnanoclusters (ca. 0.50 wt%) supported on commercial P25 TiO₂ material obtained from a facile and effective amino acid-assisted deposition-precipitation strategy. The as-synthesized catalyst exhibits superior catalytic activity and selectivity for direct hydrogenation of industrially important furfural as well as a range of structurally diverse bio-based furanic compounds to their corresponding fully hydrogenated derivatives. An average turnover frequency (ATOF) value as high as 367 h^-1 at 80 °C and 4 MPa H₂ is obtained, which is the highest reported value. This catalyst also shows stable furfural total hydrogenation in 5 reaction cycles conducted at 80 °C (52 mmol-scale, turnover number up to 12?500). In terms of the kinetic and structural characterizations, the key performances of the ultrasmall Ru clusters are proposed to mainly originate from an enhanced number of unsaturated surface Ru atoms and change in local coordination environment. Our work highlights the importance of the subnanometric size of Ru clusters in the advancement of efficient and affordable approaches towards bio-based chemical production.

Full text of DOI:10.1039/c9gc02943e

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Paul T. Anastas et al.
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DOI: 10.1039/C9GC02943E
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ARTICLE
Total hydrogenation of bio-derived furans over supported Ru
subnanoclusters prepared via amino acid-assisted deposition
Yang Qiana†, Ze-Jun Lia†, Xian-Long Dub, Qi Zhanga, Yi Zhaoa, Yong-Mei Liua* and Yong Caoa*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Development of a highly efficient and robust catalyst with reduced usage of noble metals is extremely desirable for
selective hydrogenations of furan-containing bio-based feedstocks, which represents an attractive and sustainable
alternative to petrochemical resources. Herein, we describe a new type of well-dispersed Ru subnanoclusters (ca. 0.50 wt
%) supported on commercial P25 TiO2 material obtained from a facile and effective amino acid-assisted deposition-
precipitation strategy. The as-synthesized catalyst exhibits superior catalytic activity and selectivity for direct
hydrogenation of industrially important furfural as well as a range of structurally diverse bio-based furanic compounds to
their corresponding fully hydrogenated derivatives. An average turnover frequency (ATOF) value as high as 367 h-1 at 80 °C
and 4 MPa H2 is obtained, which is the highest reported value. This catalyst also shows stable furfural total hydrogenation
in 5 reaction cycles conducted at 80 °C (52-mmol-scale, turnover number up to 12,500). In terms of the kinetic and
structural characterizations, the key performances of the ultrasmall Ru clusters are proposed to mainly originate from
enhanced number of unsaturated surface Ru atoms and change in local coordination environment. Our work highlights the
importance of subnanometric size of Ru clusters in the advancement of efficient and affordable approaches towards bio-
based
chemical
production.
Recently, subnanometer-sized metal clusters, having
dimensions between metal atoms and nanoparticles (NPs),
have elicited significant interest in heterogeneous catalysis
due to their high catalytic activity, selectivity and efficient
utilization of critical metals.13 However, owing to the high
mobility and agglomerative character of the small metal
clusters, their synthesis and utilization remains a major
challenge.14 One strategy to achieve improved stability is to
disperse and anchor active metal species on solid supports.15
In this respect, titanium oxide (TiO2), with wide availability,
high thermal stability and promise of strong metal-support
interactions (SMSI) is an attractive support for the preparation
of supported small metal clusters.16 In this rapidly evolving
research area, subnanometer metal catalysts have been
demonstrated to be excellent catalysts for a wide variety of
reactions, including CO oxidation17, carbon dioxide
methanation18, methanol synthesis19, electrochemical20 and
photoelectrochemical reactions21. Nonetheless, upgrading bio-
based feedstocks through subnanometer metal catalysis has
scarcely been investigated.
As a result of an ever-increasing demand for more
affordable and viable technologies suitable for bio-upgrading,
the development of new and cost-effective heterogeneous
catalysts that can capitalize on the use of more abundant
metals is a highly desired goal.22 Due to the limited successes
in finding suitable non-noble metal catalysts sufficiently
efficient for biomass upgrading, special attention has been
directed to the use of less noble metals such as Ru as an
integral part of the catalyst design and process optimization.23
Introduction
In the era of continued depletion of fossil resources, catalytic
hydrogenation has become an increasingly important and
essential part of biorefinery developments that hold promise
for future bio-based economy.1-3 The overall catalytic
hydroprocess requires low cost reductants and 100% atom
efficiency, allowing the potential of a broad application
platform to access multiple desirable chemicals from bio-
derived feedstocks.4,5 One area of particular interest is the
feasible adapting of existing technologies to enable the fossil-
to-bio-based transition in a shorter time frame.6 Despite
tremendous efforts, due to the inherent oxygen-rich nature of
the bio-feedstocks, the successful implementation of such
approach is currently limited by the lack of suitable catalysts
allowing highly efficient and chemospecific hydrogenation
under mild conditions.7,8 Various types of nanoparticulate-
based catalysts, including noble9,10 and non-noble11,12 metals,
have been developed for such a purpose. However, they are
respectively expensive or require harsh conditions, impeding
their widespread implementation.
a. Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Department of Chemistry, Fudan University, Shanghai 200433, China
b. Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai
201204, China
† These two authors contribute equally to this work.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Indeed, a number of recent studies have established Ru-based observations were mostly made with catalysts, Viinew wArthicilcehOnRlinue
materials as potent heterogeneous catalysts for was dispersed as particles no smaller tDhOanI: 1c0a.1.0329/nCm9GCo0r2w94i3thE
transformation of biomass-derived feedstocks to a variety of dispersion no higher than 60-70%.25 A clear
value-added products.24 However, these documented
Scheme 1 Illustration of the preparation of Ru/TiO2-lys-DP and Ru/TiO2-DP
specification of the potential benefits of hydrogenation the potential to provide catalysts containing ultradispersed
catalysis of Ru in the subnanometer regime is not yet available, platinum group metals (PGMs) it still remains largely
probably because of the difficulty to prepare such highly unexplored. Therefore, we began our investigations by
dispersed metal catalysts on conventional supporting oxides.
assessing the viability of Ru subnanocatalysts preparation via
Small Ru clusters with diameters in the size range of 1-2 nm DP method. Preliminary experiments focused on the utilization
dispersed on mesoporous silica or TiO2 have been previously of P25 TiO2 as the underlying support, with the resultant
reported, however the success of such a strategy is highly catalyst denoted as Ru/TiO2-lys-DP. Transmission electron
dependent on using high-cost ruthenium carbonyl cluster microscopy (TEM) images (Fig. 1a, 1b) show that Ru species
complexes as a catalyst precursor.26,27 Herein, we describe the are well dispersed on P25 TiO2 surface. High-angle annular
development of a new methodology for deposition of subnano dark-field scanning transmission electron microscopy (HADDF-
Ru clusters onto the surface of commercial P25 TiO2 for the STEM) (Fig. 1c) shows the subnano-size of Ru clusters (0.9 nm,
first time. Lysine (lys), an essential amino acid, is used as the Fig. 1d), which is favorable for high catalytic performance. In
mediate reagent as illustrated in Scheme 1 (see Experimental contrast, the control catalyst (Ru/TiO2-DP), bereft of lys, shows
section). UV-vis absorption spectra (Fig. S1) show the color the presence of much larger aggregated particles (Fig. S2),
change, provoked by delicate interactions between Ru confirming the important role of lys in mediating the
complexes with lys, occurs upon blending the metallic/lys formation of ultrasmall Ru clusters.
precursor solutions. The subsequent deposition-precipitation
(DP) processing and reduction in H2/Ar atmosphere gives rise
to highly effective and robust catalysts (Ru/TiO2-lys-DP) for the
total hydrogenation of biomass-derived furans and
furan/acetone aldol adducts, which is a key catalytic process
for green biorefinery concepts based on platform chemicals.28
Our facile synthetic approach for attaching subnano catalysts
onto commercially available solid supports not only promotes
the furan hydro-upgrading as a prospective renewable
resource harvesting technology but also provides useful
insights into the design of future high performance catalysts.
Results and discussion
DP has been extensively used as an effective means of
supported Au-based catalyst preparation, displaying superior
metal dispersion.29 However, although DP-based strategy has
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Fig. 1 TEM images (a, b), HAADF-STEM image (c) and size distribution (d) of Ru/TiO -lys-
View Article O2nline
DOI: 10.1039/C9GC02943E
DP
Fig. 2 Structural characterizations of Ru catalysts. (a) C 1s and Ru 3d XPS spectra. (b) Ti 2p and Ru 3p3/2 XPS spectra. (c) O 1s XPS spectra (O1 refers to stoichiometric O in TiO2, O2
to O in reduced TiOx and O3 to organic and hydroxylic O). (d) DRIFT spectra in the carbonyl region of Ru/TiO2-lys-DP and Ru/TiO2-DP. (e) XANES spectra at the Ru K-edge of
Ru/TiO2-lys-DP, Ru/TiO2-DP, RuO2 and Ru foil. (f) Fourier transform (FT) at the Ru K-edge of Ru/TiO2-lys-DP and Ru/TiO2-DP. Note: The quantified fitting results are shown in Table
S2.
To gain greater insight into the nature of the supported Ru observable features from Ru phases being detected. H2-
species, we characterized the catalysts using several methods. temperature programmed reduction (H2-TPR) profiles of
The X-ray diffraction (XRD) scans (data not shown) confirm unreduced Ru-containing samples show a single peak at
that Ru metal is well dispersed in both cases, with no temperatures lower than 120 oC (Fig. S3).30 Although Ru
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species in unreduced Ru/TiO2-lys-DP may appear more easily that signals arising from multicarbonyl species adsorbed on
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reduced compared to Ru/TiO2-DP, analysis of the degree of partially oxidized Ru sites are also ideDnOtifI:ie10d.10in39/RCu9/GTCiO0229-D43PE,
reduction (Table S1) reveals the presence of unreducible Ru despite the fact that both XPS and H2-TPR analyses indicate the
species in Ru/TiO2-lys-DP. As shown in Fig. 2a, Ru 3d X-ray sole presence of metallic Ru species in this material. This could
photoelectron spectroscopy (XPS) further suggests the be a consequence of the high oxygen affinity of Ru, which
coexistence of both metallic and oxidized Ru species (Ru0 and would lead to the re-oxidation of small Ru crystallites on TiO2
Ruδ+) in Ru/TiO2-lys-DP.31 In contrast, only Ru0 is present in surface upon CO exposure during DRIFTS measurements.35
Ru/TiO2-DP. Therefore, since Ru/TiO2-lys-DP has considerably
Ru K-edge of X-ray absorption near-edge structure (XANES)
more Ti3+ defective sites (Fig. 2b, 2c) it confirms strong in Ru/TiO2-DP (Fig. 2e) exhibits a very similar near-edge
interactions between subnano Ru clusters and the underlying structure to Ru foil, which confirms that the same valence
support.32
(Ru0) is in agreement with the above XPS analysis. In
Diffuse reflectance infrared Fourier transform spectroscopy comparison, the Ru K-edge of Ru/TiO2-lys-DP is shifted to
(DRIFTS) study of CO adsorption provides more information higher energy, displaying greater resemblance to RuO2
regarding the chemical state and structure of Ru (Fig. 2d). reference. The Ru local environment was further probed by
DRIFTS spectrum of Ru/TiO2-lys-DP shows a higher population extended X-ray fine structure (EXAFS). The low intensity
of multicarbonyl bands (2130 and 2060 cm-1)33 attributed to oscillations directly following the near-edge region indicates
Run+(CO)x species, indicating the presence of excessive partially the short-range and low coordinate environment of Ru on TiO2
oxidized Ru sites, as well as CO species linearly and bridged in Ru/TiO2-lys-DP.36 In addition, the distances of Ru-O
bonded (2000 and 1950 cm-1, respectively)34 on reduced Ru coordination in Ru/TiO2-DP (1.91 Å) are slightly shorter than
crystallites. Comparison of the relative intensity of the CO-Ru0 Ru/TiO2-lys-DP (1.96 Å) or standard RuO2 (1.98 Å) (Table S2).37
species shows that Ru/TiO2-lys-DP exhibits a higher fraction of The accumulated data,
the linear CO species than Ru/TiO2-DP, confirming a smaller Table 1 Hydrogenation of FFR over various Ru catalysts
domain size of the Ru crystallites in Ru/TiO2-lys-DP. It is noted
Selectivity (%)b
THFA
-
Entrya
Catalyst
t (h)
Temp. (oC)
PH2 (MPa)
Conv. (%)
FA
-
1
2
TiO2
1.5
1.5
1.5
1.5
4
80
80
80
80
80
80
80
80
80
80
80
25
2
4
4
4
4
4
4
4
4
4
4
6
4
4
4
4
<1
Ru/TiO2-lys-DP
Ru/TiO2-lys-DP
Ru/TiO2-DP
>99
>99
>99
>99
>99
>99
98
>99
97
49
>99
89
85
18
6
0
3
3c
4
51
0
5
Ru/TiO2-DP
6
Ru/TiO2-trp-DP
Ru/TiO2-tyr-DP
Ru/TiO2-imp
1.5
1.5
1.5
1.5
1.5
1
11
15
82
94
-
7
8
9
Ru/TiO2-lys-imp
Ru/TiO2-lys-DP
Ru/TiO2-lys-DP
Ru/TiO2-lys-DP
Ru/TiO2-lys-DP
Ru/TiO2-lys-DP
Ru/TiO2-lys-DP
92
10d
11
12
13
14e
15e f
<1
-
>99
>99
>99
>99
>99
>99
>99
>99
97
94
0
8
0
25
12
12
0
80
80
3
6
a Reaction conditions: FFR (5.2 mmol), EtOH (10 mL), catalyst (metal 0.18 mol%). b Determined by GC methods using naphthalene as internal standard. c The 10th reuse
of catalyst recovered from entry 2. d Operated under N2 e FFR (52 mmol), EtOH (45 mL), catalyst (metal 0.04 mol%). t The 4th reuse of catalyst recovered from entry 14.
together with the much weaker amplitude of FT peaks large-scale hemicelluloses.39,40 This transformation has gained
assigned to Ru-Ru shells of Ru/TiO2-lys-DP (Fig. 2f), further recent interest due to THFA being a valuable green solvent, or
support that both Ruδ+ and Ru0 are found in this catalyst.38
a potential precursor in the production of renewable diols.41,42
We evaluated the catalytic ability of the prepared catalysts Amenable catalysts are highly desirable, which promote
to produce tetrahydrofurfuryl alcohol (THFA) via direct concurrent hydrogenation of the aldehyde and furan-ring
hydrogenation of furfural (FFR),
a
key commercially moiety while avoid unwanted decarbonylation, hydrogenolysis
manufactured furan-based building block originating from and ring opening. Current industrial strategies for THFA
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production from FFR rely on sophisticated two-step catalytic give THFA (Table S3). Furthermore, FFR can be quantitatively
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conversion via a furfuryl alcohol (FA) intermediacy.39 A single- converted to THFA over Ru/TiO2-lys-DP within 1.5 h, a
DOI: 10.1039/C9GC02943E
step hydrogenation approach recently became possible with remarkable feat. Moreover, the catalyst can be recovered and
the development of various nanoparticulate based catalysts reused at least ten times with minimal decrease in the yield
43
including noble metals such as Pd/Al2O3 or Ru@C/TiO2,25 (97%; Table 1, entry 3; Fig. S5).
non-noble metals such as Ni/C44 or Ni/NAC45 and alloys such as
With the absence of lys during catalyst preparation, Ru/TiO2-
Pd- Ir46 or Pd-Ni,47 although the majority requires very high DP exhibits a significant decrease in catalytic activity toward
catalyst loadings to attain reasonable THFA yields. hydrogenative conversion of FFR (Table 1, entry 4), although
Table 1 shows the conversion of 5.2 mmol FFR in 10 mL high THFA yield is attainable by extending the reaction time
ethanol using various supported Ru catalysts. Preliminary (Table 1, entry 5). Therefore, lys-mediated precursor
assays conditions consisted of 80 oC, 4 MPa H2, and 0.18 mol% complexation during the DP process is extremely important for
catalyst loading (the amount of the active metal species kinetic enhancement. Interestingly, we found that a somewhat
relative to the substrate). The time-course plot for the similar catalytic enhancement over Ru/TiO2-DP (Table 1, entry
conversion of FFR in the presence of Ru/TiO2-lys-DP (0.18 6, 7) can also be achieved by replacing lys with identical
mol%), as outlined in Fig. S4, showed that the reaction was amounts of tryptophan (trp) and tyrosine (tyr) during the
essentially completed within 1.5 h. It is important to note the above mentioned DP process (Fig. S6). To further assess the
extremely low catalyst loading employed in these assays. The effectiveness of lys in promoting the desired catalysis, we have
reaction did not proceed using Ru-free TiO2 (Table 1, entry 1); prepared Ru/TiO2-lys-DP with various amounts of 0.02 M lys
thus, the presence of ultrasmall Ru is essential for the high solution added in 0.1 mmol RuCl3 precursor solution (Fig. S7)
activity observed in the targeted conversion of FFR to THFA. and discovered that the Ru/TiO2-lys-DP prepared with 0.1
Ru/TiO2-lys-DP proved very effective, displaying an average mmol lys exhibited the best catalytic performance in FFR-to-
turnover frequency (ATOF) of 367 h-1 towards THFA (Table 1, THFA
entry 2), which clearly outperforms all the other catalysts
reported for single-step liquid phase hydrogenation of FFR to
Fig. 3 Kinetic results of Ru catalysts. Reaction profile of Ru/TiO2-lys-DP (a) and Ru/TiO2-DP (b) in FFR hydrogenation. Reaction conditions: FFR (5.2 mmol), EtOH (10 mL), catalyst
(metal 0.18 mol%), H2 (4 MPa), 40 °C. Fitted profiles of TFFR (c) and FA (d) hydrogenation over Ru catalysts. Reaction conditions: TFFR (c) or FA (d) (5.2 mmol), EtOH (10 mL),
catalyst (metal 0.18 mol%), H2 (4 MPa), 40 °C.
conversion (Fig. S8). This suggests that a small amount of lys
It is worth mentioning in this context that a similar lys-
during Ru deposition is sufficient to improve the catalytic mediated promotion was not accomplished in a separate
activity of Ru/TiO2-DP.
experiment focusing on the use of conventional impregnation
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method to prepare Ru/TiO2 (Table 1, entry 8, 9), thus within the present catalytic system is excluded. This is further
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DOI: 10.1039/C9GC02943E
emphasizing the benefits of using DP in the preparation of confirmed by the absence of aldehyde in the post-reaction
ultrafine Ru catalysts. At this point, it is pertinent to point out solutions for those conducted in the presence of both H2 and
that, in line with the above-mentioned DP case, the addition of N2. Since alcohol based solvents promote single-step FFR
small amount of lys in impregnation solution can also promote hydrogenation, we investigated THFA as a solvent, which is
a notable decrease in the average size of supported Ru NPs, as advantageous for the environment and upscaling. Without
verified by TEM analysis (Fig. S9). We postulated that this detailed optimization, 5.2 mmol FFR in 10 mL THFA was
seemingly unexpected finding is due to the adverse effects of hydrogenated with full conversion and >99% selectivity to
trace surface-bound contamination. In fact, additional THFA under standard reaction conditions (4 MPa H2, 80 oC, 4 h;
spectroscopic observations show the presence of residual lys Table S4, entry 10). Thus, the novel process, featuring the sole
molecules adsorbed on the prepared catalyst (Fig. S10). This presence of THFA in the final product, contributes to the
finding, in conjunction with the absence of CO adsorption establishment of a more sustainable and industrially viable
feature in the DRIFT spectrum of the as-prepared Ru/TiO2-lys- process for this particular bio-based transformation.
imp sample (Fig. S11), supports the view that, apart from the
particle size, the creation of clean accessible Ru surface is also DP, the kinetic profile for FFR conversion at 40 C followed by
an essential factor in optimum THFA formation during GC analysis was conducted (Fig. 3a). The reaction process did
To further examine the reaction performed with Ru/TiO2-lys-
o
hydrogenative FFR conversion.
not show the products derived from decarbonylation of the
As a unique titania with biphasic nature, P25 TiO2 is superior carbonyl moiety in furfural or other unwanted by-products
to other kinds of nanocrystalline TiO2 as a support for (such as furan and 2-methylfurfural), indicative of preferential
catalysts. For comparison, Ru clusters deposited on single- activation of C=O and C=C bonds on Ru/TiO2-lys-DP surfaces.
phase anatase- and rutile-TiO2 nanocrystallites were Upon investigating the effect of pressure, we found that the
synthesized in the presence of lys. Surprisingly, despite their reaction time is shortened from 1.5 to 1.0 h when H2 is
similar particle size, distribution and chemical state (Fig. S12, increased from 4 to 6 MPa (Table 1, entry 2, 11). In the case of
S13), both catalysts are inferior to those prepared P25 TiO2 for the effect of the reaction temperature, THFA is quantitatively
o
FFR conversion (Fig. S14), highlighting the strong interaction obtained at 25 C (Table 1, entry 12). Notably, even at a
o
between the active clusters and P25 TiO2. To further subambient temperature of 2 C, quantitative formation of
determine the effect of the choice of the underlying support THFA can be attained within 25 h (Table 1, entry 13). Of yet
on the catalytic performance, we also evaluated Ru deposited further interest is that, in this particular case the specific
on a selection of common supporting materials, including activity based on FFR conversion is 22 h-1, which compares
CeO2, Al2O3 and SiO2 (Fig. S15). In this case all catalysts showed favorably with the state-of-the-art catalyst Pd-Ir/SiO2 (Table
significantly lower activity under the same condition (Fig. S14). S3).46
Therefore, this suggests that the nature of P25 TiO2 as a
The catalytic amount of Ru can be lowered to 0.04 mol%
support is capable of providing a specific interaction with Ru without depleting its inherent catalytic activity for THFA
species, which may be the key factor in facilitating the formation (Table 1, entry 14). Thus, in a gram-scale reaction of
selective production of THFA via single step FFR FFR (52 mmol scale up) for 12 h, 97% yield of THFA was
hydrogenation.
obtained (Fig. S16). After the reaction, Ru/TiO2-lys-DP was
An additional set of experiments was conducted to examine easily separated from the reaction mixture by filtration.
the influence of the solvent on the catalytic performance of Analysis by inductively coupled plasma (ICP) confirmed that no
Ru/TiO2-lys-DP for hydrogenative FFR conversion. Methanol, 2- Ru was present in the filtrate (limit of detection: <24 ppb),
propanol, THFA, THF, DMF, toluene, n-heptane and H2O were thus indicating a strong binding of the subnano Ru clusters to
examined, with ethanol displaying far superior selectivity the TiO2 surface under these reaction conditions. Moreover,
toward THFA formation (Table S4, entries 1-9). This shows that the recovered Ru/TiO2-lys-DP can also be reused at least four
specific nature of the solvent can markedly influence both the times without any major loss in activity, with a total turnover
product distribution and reaction rate. According to the number (TON) of approx. 12,500 (Fig. S17). This value is far
results, nonpolar (toluene and n-heptane) and polar aprotic greater than those in reported catalytic systems.23,25,43-47
solvents (THF and DMF) are detrimental for FFR conversion. Additionally, STEM and XPS confirmed almost no change in the
Despite the relatively high rate of organic solvent-free FFR metal dispersion or chemical state of Ru before and after
hydrogenation observed with H2O (Table S4, entry 9), the reuse (Fig. S18, 19).
selectivity towards THFA decreased by 98% compared to the
To further elucidate the influence of Ru dimensions on the
reaction in ethanol. This decrease in THFA selectivity is total hydrogenation pathway, we evaluated the kinetics of FFR
associated with a 98% increase in the selectivity towards FA, hydrogenation over Ru/TiO2-DP under conditions identical to
which indicates that hydrogenation of C=O bond is more those for Ru/TiO2-lys-DP. Operation under negligible mass
favorable than that of the unsaturated bonds within the furan
ring.
When conducting the reaction under N2 over Ru/TiO2-lys-DP,
no FFR conversion was observed (Table 1, entry 10). Hence,
possibility that the catalytic transfer hydrogenation occurs
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the selectivity before 2 h reaction time favors theViniewteArrmticleedOinalitnee
formation of FA over both catalysts, DsOuIg: 1g0e.1s0ti3n9g/C9tGhCat029C4=3OE
hydrogenation is preferential over C=C hydrogenation within
the furan ring at the initial stage of the reaction. Additionally,
the
two
separate
control
experiments
utilizing
tetrahydrofurfural (TFFR) and FA as the starting substrates,
respectively, showed that TFFR hydrogenation proceeds much
faster than FA over both catalysts (Fig. 3c, 3d).
In contrast to the situation identified for TFFR
hydrogenation, FA hydrogenation proceeds at a higher rate
over Ru/TiO2-lys-DP than Ru/TiO2-DP (Fig. 3d). To examine the
role of Ru particle size, the rate dependence on the H2
pressure and FA concentration was studied. The reaction
orders with respect to FA concentration were calculated as
0.02 and 0.35 for Ru/TiO2-lys-DP and Ru/TiO2-DP, respectively
(Fig. S20, S21), indicating that FA is more strongly adsorbed on
Ru/TiO2-lys-DP surface. The reaction orders with respect to H2
pressure over Ru/TiO2-lys-DP and Ru/TiO2-DP were estimated
as 0.36 and 0.33, respectively (Fig. S22, S23), suggesting that
Fig. 4 Catalytic activities of Ru/TiO2-lys-DP and Ru/TiO2-DP reduced at different Ru dimensions does not induce changes associated with H2
temperatures. Reaction conditions: FFR (5.2 mmol), EtOH (10 mL), catalyst (metal 0.18
mol%), H2 (4 MPa), 80 °C, 1.5 h.
activation. In combination with the infrared (IR) spectroscopic
analysis, we found a greater red shift of furan ring stretching
for furan adsorption on Ru/TiO2-lys-DP relative to Ru/TiO2-DP
transfer resistance was established using the Madon and
(Fig. S24), further supporting the decisive role of subnano Ru
Boudart approach.48 As shown in Fig. 3, the enhanced
sites in facilitating the essential furan ring hydrogenation
step.49
performance of Ru/TiO2-lys-DP relative to Ru/TiO2-DP
originates from the dramatically accelerated hydrogenation of
the furan ring rather than from the reduction of C=O moiety,
presumably owing to the strongly favored interaction of FA
enabled by the surface anchored subnano Ru sites. Notably,
Table 2 Comparison of activities of various furans over Ru/TiO2-lys-DP and Ru/TiO2-DP
Ru/TiO2-lys-DP
Ru/TiO2-DP
Yield (%)
Entry
1a
Substrate
Product
t (h)
2
Yield (%)
ATOF (h-1)
t (h)
10
ATOF (h-1)
52
>99
275
95
88
68
78
64
59
2
3
4
5
6
7
4
6
95
92
90
89
82
78
131
84
71
82
56
36
18
24
36
48
60
72
27
16
12
7
7
6
8
5
12
48
4
a
Reaction conditions: substrate (1 mmol), EtOH (10 mL), catalyst (metal 0.18 mol%), H2 (4 MPa), 80 °C.
HMF (5 mmol)
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DOI: 10.1039/C9GC02943E
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ARTICLE
It is worth mentioning that, as opposed to the kinetic 2,5-dihydroxymethyltetrahydrofuran (DHMTHF), a promising
pattern seen with Ru/TiO2-DP, the reaction rates are renewable source for the production of industrially relevant
practically independent of the FA concentration but distinctly monomer 1,6-hexanediol.51 In addition, the catalytic system
sensitive to the hydrogen pressure for the Ru/TiO2-lys-DP readily adapts to various furan/acetone aldol adducts to afford
sample. These contrastive kinetic parameters imply that the corresponding products in high yields (Table 2, entries 2-
Ru/TiO2-lys-DP breaks through the limitation of FA activation, 7). High yields of the corresponding saturated derivatives are
and hydrogen activation is the rate-controlling step. Therefore, generally obtained with ATOFs at least threefold as high for
it can be concluded that the total hydrogenation of FA over Ru/TiO2-lys-DP than for Ru/TiO2-DP. Hence, Ru/TiO2-lys-DP is
Ru/TiO2-lys-DP is largely limited by the rate of H2 dissociation the most active catalyst for the total hydrogenation of the bio-
due to the high activation barrier to dissociate H2.50 It is based furan derivatives reported to-date.
interesting to note at this point that the degree of rate
acceleration with increasing H2 pressure from 4 to 6 MPa as
indicated by the catalytic data shown in Table 1 seems to infer
Conclusions
In conclusion, we have presented a facile and effective
a first order dependence on H2 pressure. However, it must be
procedure for surface-decorating commercial P25 TiO2
emphasized that the above-discussed kinetic analysis is based
material with ultrasmall Ru clusters, whose size is controlled to
on the dependence of initial reaction rate on the initial
obtain particles smaller than 1 nm. Subnanometric structure of
composition of reaction mixture rather than on the data
RuOx are verified and demonstrated for efficient and robust
obtained at high conversion levels.
single-step total hydrogenation of FFR for the first time. The
To gain greater insight into the influence of the chemical
optimal average TOF of FFR-to-THFA conversion is 367 h-1,
states of Ru clusters and the underlying support on the
which far surpasses those of the current state-of-the-art
performance of Ru/TiO2-lys-DP, a series of control samples
catalysts that usually require considerably rarer and expensive
bearing different levels of Ru and TiO2 reduction were
Ir or Pd as essential components. With its robust total
prepared. A systematic approach was used to modulate the
hydrogenation for a range of structurally diverse bio-based
electronic states of the catalysts with minimal change in Ru
furanic compounds, the as-obtained Ru/TiO2-lys-DP holds
dimensions (Fig. S25, S26). This was achieved by subjecting as-
promise for the development of a practical biorefinery
deposited Ru/TiO2-lys-DP and Ru/TiO2-DP samples to H2/Ar at
technology based on catalytic hydroprocessing.
o
o
o
different temperatures (100 C, 300 C and 500 C). In this
approach, high temperature H2 could reduce Ru species
strongly interacting with the support and promote further
reduction of TiO2 via hydrogen spillover. As corroborated by
XPS analysis (Fig. S27, S28; Table S5, S6), it is possible to
gradually decrease the unreducible Ru species and create
more Ti3+ defective sites by H2 reduction at elevated
temperatures. The performances of the prepared catalysts
(Fig. 4) were studied, confirming that the state of Ru clusters,
rather than TiO2, contributes to the overall reactivity.
Importantly, a moderate amount of Ruδ+ species (ca. 20%) is
essential to achieve a reasonable yield of THFA (Ru/TiO2-DP
with 20% Ruδ+, THFA yield 65%). Therefore, a suitable
Ruδ+/(Ru0+Ruδ+) ratio is one of the key factors in contributing
to the observed high efficiency.
Experimental section
Materials (source and purity)
The reagents used were 5-Hydroxymethylfurfural (99%,
Aladdin), furfuryl alcohol (97%, Aladdin), tetrahydro furfuryl
alcohol (99%, Aladdin), furan (>99.5%, Aladdin), lysine (99%,
Aladdin), tryptophan (99%, Aladdin), tyrosine (98%, Aladdin),
ruthenium chloride hydrate (RuCl3·xH2O, 35%-42%Ru,
Aladdin), CeO2 (99.9%, Degussa), Al2O3(99.9%, Degussa), SiO2
(99.9%, Degussa), P25 TiO2 (99.99%, Degussa), acetone (>99%,
J&K), 2-pentanone (>99%, J&K), 4-methyl-2-pentanone (>99%,
J&K) and tetrahydrofurfural (97%, fluorochem).Furfural (98%,
Aladdin) were distilled under reduced pressure before use.
Lastly, the scope and utility of this new total hydrogenation
methodology was examined, showing that Ru/TiO2-lys-DP
efficiently hydrogenated a range of other furan-based
compounds. As illustrated in Table 2, the reaction proceeds
successfully with a number of sugar-derived furans. For
example, 5-hydroxymethylfural (HMF), one of the most
important C6 platform chemicals derived from cellulosic
biomass, is quantitatively hydrogenated (Table 2, entry 1) to
Catalyst preparation
Preparation of anatase-TiO2: Single-phase anatase-TiO2 was
isolated from commercial P25 TiO2 by chemical treatment.52
Briefly, 6.0 g P25 TiO2 was added to an aqueous mixture of
H2O2 (200 mL, 30 wt%) and NH3 (6 mL, 25 wt%) and stirred at
room temperature for 15 h. To obtain anatase-TiO2 particles,
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ARTICLE
the resultant solid was washed thoroughly with deionized performed with an Escalab 250Xi spectrometer (TVhieewrmArtoiclFe iOsnhliener
DOI: 10.1039/C9GC02943E
Scientific) using 200 W mono-chromated Al Kα radiation
water, dried in vacuum and calcined at 200 °C for 3 h.
Preparation of rutile-TiO2: Single-phase rutile-TiO2 was (1486.6 eV). The catalyst, formed into a small disk (4.0 mm
isolated from commercial P25 TiO2 by chemical treatment.53 diameter), was mounted on the sample stage, reduced in the
Briefly, 1.0 g P25 TiO2 was added to 50 mL HF solution (10 pretreatment chamber at 300 °C for 2 h under 40 mL·min-1
wt%) and stirred at room temperature for 24 h. To obtain flow of 5 vol% H2/N2, and then transferred to the analyzing
rutile-TiO2 particles, the resultant solid was washed thoroughly chamber, where the background pressure was lower than 2
with deionized water, dried in vacuum and calcined at 200 °C x10-9 Torr. The spectra were collected with an analyzer pass
for 3 h.
energy of 23.5 eV and an electron takeoff angle of 45°. The
Preparation of Ru/TiO2-lys-DP: Ru/TiO2-lys-DP was prepared binding energy scale was calibrated by the adventitious C 1s
by a facile modified deposition-precipitation (DP) method. In a peak at 284.6 eV as a reference.
typical synthesis, 0.1 mmol RuCl3 and 0.1 mmol lysine was
Fourier transform infrared spectroscopy (FT-IR) was carried
stirred in 100 mL deionized water for 15 mins before the pH out with a Nicolet iS10 FT-IR spectrometer equipped with a
was adjusted to 8.0 by diluted NaOH solution. After 1.0 g P25 DTGS detector, in the range 400−4000 cm−1, by applying an
TiO2 was dispersed, the pH of the slurry was readjusted to 8.0 optical resolution of 0.4 cm−1. The samples were investigated
by NaOH, followed by a subsequent 18-hour stirring at room by using the KBr pellet technique. Diffuse reflectance infrared
temperature. During deposition, the pH was carefully fourier transform (DRIFT) spectra of chemisorbed CO were
monitored and intermittent addition of NaOH was needed. recorded in an in-situ diffuse reflectance cell (Harick Scientific)
The resulting solid was washed extensively with deionized equipped with a CaF window on a Nicolet 6700 spectrometer
water, dried at 100 °C overnight, and reduced under 5 vol% (Thermo Fisher Scientific), using an Hg-Cd-Te (MCT-A)
H2/Ar (80 mL·min-1) at 300 °C for 2 h to obtain the product detector. The sample was pre-reduced in the cell with flowing
Ru/TiO2-lys-DP (0.49 wt% Ru by elemental analysis). Ru/TiO2- 5 vol% H2/Ar (80 mL·min-1) at 300 °C for 30 mins. The
DP (0.56 wt% Ru) was prepared by the same method except temperature was then lowered to 15 °C for CO chemisorption.
for the absence for lysine.
After collecting a background spectrum of the sample under
Preparation of Ru/TiO2-lys-imp: Ru/TiO2-lys-imp was prepared the Ar flow, the cell was switched to flowing 1 vol% CO/Ar (20
by incipient-wetness impregnation of 1.0 g P25 TiO2 with mL·min-1) for CO adsorption for 45 mins, followed by purging
aqueous mixture of 0.1 mmol RuCl3 and 0.1 mmol lysine. After with Ar. The collected spectra were subtracted from the
a perfect mixing of the slurry, the resulting mixture was sample background spectra in Ar to obtain the spectra for
vigorously stirred at 80 °C for 4 h. Then samples were dried at chemisorbed CO. For furan-FTIR spectra, a vacuum pump was
100 °C overnight and then reduced in 5 vol% H2/Ar (80 mL·min- used to pull the furan vapor through the chamber for in situ
1) at 300 °C for 2 h. Ru/TiO2-imp was prepared by the same FTIR measurements. X-ray absorption fine structure (XAFS)
method except for the absence for lysine. The Ru content of measurements at Ru K3-edge in fluorescence mode were
both catalysts were measured to be 0.92 wt% and 0.90 wt%, performed at the BL14W15 in Shanghai Synchrotron Radiation
respectively.
Facility (SSRF). The electron beam energy was 3.5 GeV and the
stored current was 230 mA (top-up). A 38-pole wiggler with
the maximum magnetic field of 1.2 T inserted in the straight
section of the storage ring was used. Given that the elemental
Characterizations
Elemental analysis of catalysts was conducted with a states and structure of the catalyst might be changed when
PerkinElmer inductively coupled plasma optical emission exposed to air, we carried out these analyses in the absence of
spectrometer (ICP-OES) Optima 8300 DV. Transmission air using a special holder with a gas cell to transfer the sample
electron microscopy (TEM) images were taken by a JEOL JEM from an inert gas environment to the inside of the instrument.
2011 electron microscope operating at 200 kV. Before being XAFS data were collected using a fixed-exit double-crystal Si
transferred into the TEM chamber, the samples dispersed with (311) monochromator. The energy was calibrated using Ru foil.
ethanol were deposited onto a carbon coated copper grid and Data processing was performed by ATHENA.54
then quickly moved into the vacuum evaporator. The size
distribution of the metal nanoparticles was determined by
measuring about 100 random particles in the images. High-
Catalytic activity measurements
angle annular dark-field scanning transmission electron Procedure for the hydrogenation of furanic compounds and
microscopy (HAADF- STEM) images were obtained on a Tecnai reusability test: A mixture of FFR or HMF (5.2 mmol),
G2 F20 S-Twin electron microscope, operating at 200 kV. X-ray supported metal catalysts (metal loading: 0.18 mol%), ethanol
diffraction (XRD) was operated on a Bruker D8 Advance X-ray (10 mL) were charged into a 100 mL Hastelloy-C high pressure
diffractometer using the Ni-filtered Cu Kα radiation source at Parr reactor. The reactant mixtures were then stirred at a rate
40 kV and 40 mA. Temperature programmed reduction (H2- of 800 rpm under given H2 pressure for certain reaction time at
TPR) was conducted using a BEL-CAT (BEL Japan, Inc.) a given temperature. After reaction, the H2 atmosphere was
instrument by heating a 100 mg Ru catalyst at 10 °C ·min-1 removed and 2 mmol naphthalene was added as internal
from room temperature to 750 °C under a 10 vol% H2/Ar flow. standard. The samples were analyzed on a Shimadzu GC-17A
X-ray photoelectron spectroscopy (XPS) measurements were gas chromatograph equipped with a capillary column HP-
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Journal Name
6
W. Li, J. Artz, C. Broicher, K. Junge, H. Hartmann, A.
Besmehn, R. Palkovits and M. BelleDr,OCI:a10ta.1l0.3S9c/Ci.9GTCec0h29n4o3l.E,
2019, 9, 157.
M. J. Gilkey, A. V. Mironenko, D. G. Vlachos and B. J. Xu, ACS
Catal., 2017, 7, 6619-6634.
M. J. Gilkey, A. V. Mironenko, L. Yang, D. G. Vlachos and B. J.
Xu, ChemSusChem, 2016, 9, 3113-3121.
S. S. Poly, S. M. A. H. Siddiki, A. S. Touchy, K. W. Ting, T.
Toyao, Z. Maeno, Y. Kanda and K. Shimizu, ACS Catal., 2018,
8, 11330-11341.
INNOWAX column (30 m × 0.32 mm × 0.25 μm) and FID
detector. For adol adducts, 1 mmol of substrate was subjected
to catalytic test. Each experiment was repeated three times
and average activities were given in the figures and tables. The
carbon balance of each run exceeded 98%, if not especially
stated. For the reusability test, the following procedure was
employed to minimize possible experiment errors arising from
the possible loss of catalyst. 200 mg Ru/TiO2-lys-DP was added
into each of 13 identical reactors for hydrogenation of FFR.
After the reaction run for 1.5 h, the spent catalyst was
collected after filtration and washed thoroughly with ethanol,
dried at 100 °C overnight and reduced under 5 vol% H2/Ar (80
mL·min-1) at 300 °C for 2 h. After that, the same procedure as
the ones described above was performed in 12 reactors, with
each containing same amount of the recovered Ru/TiO2-lys-DP,
and so on.
View Article Online
7
8
9
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Procedure for the gram-scale transformation of FFR and
reusability test: A mixture of FFR (52 mmol), Ru/TiO2-lys-DP
(metal loading: 0.04 mol%), ethanol (45 mL) were charged into
an autoclave (250 mL capacity) and heated to 80 °C in less than
15 minutes. The reactant mixtures were then stirred at a rate
of 800 rpm under 4 MPa H2. After 12 h of reaction, 55 mL of
ethanol was added to dilute the mixture. 5 mL of this diluted
sample was transferred into the vial and 1 mmol naphthalene
was added for further analysis. For the reusability test, the
procedure was identical to that applied on small scale except
that 6 separate reactors operating under otherwise identical
conditions were conducted on the first run.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
Financial support from NSF of China (21773033, 21972024,
91645201), Science & Technology Commission of Shanghai
Municipality (19DZ2270100), and SINOPEC (X514005) is
acknowledged.
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