Selective Activation of C-OH, C-O-C, or Ca? C in Furfuryl Alcohol by Engineered Pt Sites Supported on Layered Double Oxides

Article abstract of DOI:10.1021/acscatal.0c01276

The selective activation of targeted bonds in biomass-derived furfural or furfuryl alcohol with complex chemical linkages (C-C/C-H/C-O, Ca? C/Ca? O, or C-O-H/C-O-C) is of great challenge for biomass upgrading, expecting well-defined catalyst and definite catalytically active sites. This work demonstrates an efficient targeted activation to C-OH, C-O-C, or Ca? C by engineering the structure of catalytic Pt sites, affording 2-methylfuran (2-MF), tetrahydrofurfuryl alcohol (THFA), or 1,2-pentanediol (1,2-PeD) as product in the hydroconversion of furfuryl alcohol. The catalytic Pt sites have been engineered as atomic Pt, coordination unsaturated Pt-Pt in atom-thick dispersion, or coordination unsaturated 3D Pt-Pt by tailoring the Pt dispersion (single atom, 2D cluster, or 3D cluster) on Mg and Al-containing layered double oxide (Mg(Al)O) support. The selective activation of C-OH, C-O-C, or Ca? C has been traced with the FT-IR spectra recorded surface reaction. On atomic Pt, C-O-H is easily activated, with the assistance of Mg(Al)O support, with O-terminal adsorption without affecting furan C-O and Ca? C. However, Ca? C in the furan ring is easier to be activated on coordination-unsaturated Pt-Pt in atom-thick dispersion, resulting in a step-by-step hydrogenation to generate THFA. On coordination-unsaturated 3D Pt-Pt, the hydrogenolysis of furan ring is favored, resulting in the cleavage of furan C-O to produce 1,2-PeD. Also, the Mg(Al)O supports derived from Mg and Al layered double hydroxides (LDHs) here also play a key role in promoting the selectivity to 1,2-PeD by providing basic sites.

Full text of DOI:10.1021/acscatal.0c01276

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Article
Selective Activation of C-OH, C-O-C, or C=C in Furfuryl Alcohol
by Engineered Pt Sites Supported on Layered Double Oxides
Yanru Zhu, Wenfang Zhao, Jian Zhang, Zhe An, Xiaodan Ma, Zhijun Zhang,
Yitao Jiang, Lirong Zheng, Xin Shu, Hongyan Song, Xu Xiang, and Jing He
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c01276 • Publication Date (Web): 29 Jun 2020
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Selective Activation of C-OH, C-O-C, or C=C in
Furfuryl Alcohol by Engineered Pt Sites Supported
on Layered Double Oxides
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Yanru Zhu,† Wenfang Zhao,† Jian Zhang,† Zhe An,† Xiaodan Ma,† Zhijun Zhang,† Yitao Jiang,†
Lirong Zheng,*,§ Xin Shu,† Hongyan Song,† Xu Xiang,† and Jing He*,†
† State Key Laboratory of Chemical Resource Engineering & Beijing Advanced Innovation
Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology,
Beijing 100029, P. R. China
§ Institute of High Energy Physics, the Chinese Academy of Sciences, Beijing 100049, P. R.
China
KEYWORDS: Atomic Pt, 2D Pt cluster, 3D Pt cluster, hydrogenation of furfuryl alcohol,
selective activation
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ABSTRACT
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The selective activation of targeted bonds in biomass-derived furfural or furfuryl alcohol with
complex chemical linkages (C-C/C-H/C-O, C=C/C=O, or C-O-H/C-O-C) is of great challenge
for biomass upgrading, expecting well-defined catalyst and definite catalytically active sites.
This work demonstrates an efficient targeted activation to C-OH, C-O-C, or C=C by engineering
the structure of catalytic Pt sites, affording 2-methylfuran (2-MF), tetrahydrofurfuryl alcohol
(THFA), or 1,2-pentanediol (1,2-PeD) as product in the hydroconversion of furfuryl alcohol. The
catalytic Pt sites have been engineered as atomic Pt, coordination unsaturated Pt-Pt in atom-thick
dispersion, or coordination unsaturated 3D Pt-Pt by tailoring the Pt dispersion (single atom, 2D
cluster, or 3D cluster) on Mg and Al-containing layered double oxide (Mg(Al)O) support. The
selective activation of C-OH, C-O-C, or C=C has been traced with the FT-IR spectra recorded
surface reaction. On atomic Pt, C-O-H is easily activated, with the assistance of Mg(Al)O
support, with O-terminal adsorption without affecting furan C-O and C=C. But C=C in furan
ring is easier to be activated on coordination-unsaturated Pt-Pt in atom-thick dispersion, resulting
in a step-by-step hydrogenation to generate THFA. On coordination-unsaturated 3D Pt-Pt, the
hydrogenolysis of furan ring is favored, resulting in the cleavage of furan C-O to produce 1,2-
PeD. And that, the Mg(Al)O supports derived from Mg and Al layered double hydroxides
(LDHs) here also play a key role in promoting the selectivity to 1,2-PeD by providing basic sites.
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1. Introduction
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The catalytic conversion of biomass as well as biomass-derived platform molecules to fuels
and value-added chemicals has been attracting wide attention for the development of sustainable
chemical industry.1-3 Furfurly alcohol are promising C5 biomass-derived molecule produced by
hydrolysis and dehydration of hemicellulose,4,5 and has already been manufactured from furfural
in a commercial scale.5 Selective hydrogenation and hydrogenolysis are considered to be the
most effective methods for conversion of furfural/furfurly alcohol to valuable chemicals.6-8 1,2-
pentanediol (1,2-PeD)9-12 or 1,5-pentanediol (1,5-PeD)13-15, the most valuable products used as
monomers of resins, has been produced by the selective ring-open. 2-Methylfuran (2-MF), a
potential fuel or fuel additive, has been generated by the selective hydrogenolysis of C-OH.16,17
Tetrahydrofurfuryl alcohol (THFA), used as a green solvent, has been formed by the C=C
hydrogenation.18,19 The complexity of chemical linkages (C-C/C-H/C-O, C=C/C=O, C-O-H/C-
O-C, etc.) in furfural/furfurly alcohol molecules provides great chance to produce various high
value-added chemicals or fuels, but at the same time makes the selective activation of targeted
bonds much more challenging.6 And that, the hydroconversion of furfural/furfurly alcohol
usually involves multiple competitive reactions, leading to a low selectivity to target product.20
Supported noble metals (Pt, Ru, Ir, Pd etc.), transition metals (Cu, Ni, etc.), or complex
components have been developed for the hydroconversion of furfural/furfuryl alcohol.20-22
Hydrotalcite-supported Pt nanoparticles efficiently catalyze the direct hydrogenolysis of furfural
to 1,2-PeD with a selectivity of 73 % and a conversion of 99 %.23 Furfuryl alcohol is completely
transformed to THFA on ultrafine SiO2-MOF supported NiCo alloy, giving 99.1 % selectivity of
THFA.19 In the gas-phase hydrogenation of furfural, Pt-Fe particles supported on SiO2 exhibit a
2-MF selectivity of 63 % with good stability.24 The adsorption mode of furanic compounds and
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the activation of specific chemical bond not only highly depend on the nature of metals and the
catalyst composition but also are sensitive to the microstructures of catalysts and the structures of
catalytically active sites.20-34 TiO2 supported atomic Ir exhibits superior conversion and
selectivity of C3N4 supported atomic Ir in the hydrogenation of furfural to furfuryl alcohol,
attributed to the appropriate strength of interactions between the active metal sites and the
reactant molecules owing to the regulation of the support materials.26 The selectivity to 1,5-
pentanediol from furfuryl alcohol hydrogenolysis increases with increasing TiOy layer thickness
covered on TiO2-supported Co particles.27 Bimetallic Cu-Ni supported on TiO2, with surface Cu-
rich structures, allows for high selectivity to 2-MF or 2,5-dimethylfuran from furfural or 5-
hydroxymethylfurfural.28 Formation of Ir-Rh alloy and modification of ReOx species on Ir-Rh
alloy particles enhance the hydrogenation activity of furfural to THFA.29 The sensitivity of
catalytic activity and/or selectivity to the structure of active sites has been observed in nano-
catalyzed biomass conversion.25,27,30-34 In the hydrogenation of furfural, a selectivity of 97 % to
furfuryl alcohol has been achieved on steps/edges-rich Ni nanoparticles by facilitating the
exclusive activation of HC=O group, while a selectivity of 99 % to THFA is produced on a high
exposure of Ni (111) plane by the activation of both furan ring and C═O group.30 The selective
α-C-O cleavage in 2-furancarboxylic acid hydrogenolysis to synthesize 5-hydroxyvaleric acid
has been achieved on Pt particles with low-index Pt surface (111 plane) at a low temperature of
313 K.31 In the vapor-phase hydrogenation of furfural,32 product selectivity changes as function
of size and shape of Pt nanoparticles. Upon an increase in Pt particle size from 1.5 to 7.1 nm, the
selectivity toward furfuryl alcohol increases from 1 % to 66 %, and the furfuryl alcohol
formation rate increases from 0.001 s-1 to 0.076 s-1.32 Furfuryl alcohol or THFA has been
produced depending on the size of Pd nanoparticles.33 In addition to particle size, chemical states
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of metal sites usually inherently affect the adsorption and activation behavior in catalysis.34
Tuning the chemical state of supported Pt species by varying the exposed lattice-planes of CeO2
support, leads to significantly different selectivity to 1,2-PeD in the hydrogenolysis of furfuryl
alcohol.34
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The effects of metal dispersion, from nanoparticles to single atoms, have been reported for a
variety of industrially promising or important reactions.35-37 The sensitivity of chemical bond
activation to metal dispersion is highly desired for the conversion of biomass and/or biomass-
derived molecules. This work demonstrates the dependence of chemical bond activation in the
conversion of biomass to the structure nature of metal sites, from single atoms to nanoclusters. Pt
sites has been tuned here as dispersed in single atoms, coordination unsaturated 2D clusters, and
coordination unsaturated 3D clusters on Mg and Al-containing layered double oxide (Mg(Al)O),
and the targeted activation to C-OH, C-O-C, or C=C has been investigated. 2-MF, 1,2-PeD, or
THFA has been produced as a result of targeted activation of C-OH, ring C-O-C, or ring C=C in
the hydroconversion of furfuryl alcohol.
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2. Results and Discussion
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The Mg(Al)O supported Pt investigated in this work results from the reduction of Pt2+-
loaded layered double hydroxides (LDHs). The Mg and Al-containing LDHs (MgAl-LDHs) was
first synthesized in-situ on the surface of γ-Al2O3 by a method reported previously by our
group38,39 (denoted MgAl-LDHs@Al2O3), and then [Pt(NH3)4]2+ was loaded as Pt source. In the
X-ray diffraction (XRD) patterns (Figure S1), the reflections characteristic of hydrotalcite
structure and γ-Al2O3 are clearly observed for MgAl-LDHs@Al2O3. From the scanning electron
microscopic (SEM) images (Figure S2), MgAl-LDHs is observed to grow vertically on both
exterior and interior surfaces of Al2O3 spheres with a thickness of ~ 1.15 μm. By varying the
concentration of Pt2+ aqueous solution for incipient-wetness impregnation, Pt2+/MgAl-
LDHs@Al2O3 with input Pt2+ loading of 0.10 wt%, 0.15 wt%, 0.78 wt%, or 1.80 wt% was
prepared. No visible change is observed in the XRD patterns (Figure S1) after Pt loading. By the
in situ reduction (thermal treatment in H2) of Pt2+/MgAl-LDHs@Al2O3 at 400 °C, Mg and Al-
containing layered double oxides (Mg(Al)O) supported Pt (Pt/Mg(Al)O@Al2O3-IR) was
produced. No Pt phase can be observed (Figure S3 a-d) in each reduced sample, indicative of
well-dispersion of Pt on Mg(Al)O@Al2O3. The final Pt loading was determined by inductively
coupled plasma optical emission spectrometer (ICP-OES) technique as being 0.08 wt%, 0.13
wt%, 0.70 wt%, or 1.67 wt% on Mg(Al)O@Al2O3, affording 0.08 %, 0.13 %, 0.70 %, or 1.67 %
Pt/Mg(Al)O@Al2O3-IR (IR represents in situ reduction). The Pt dispersion, calculated on the
basis of hydrogen-oxygen titration (HOT) results, is 92 %, 84 %, 76 %, or 67 % for 0.08 %,
0.13 %, 0.70 %, or 1.67 % Pt/Mg(Al)O@Al2O3-IR (Table S1). For all Pt-containing sample, the
specific surface areas are similar, in the range of 116-133 m2 g-1 (Table S1).
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For 0.08 % Pt/Mg(Al)O@Al2O3-IR, Pt clusters and a few isolated Pt atoms are observed in
the HAADF-STEM image (Figure 1a). Increasing the Pt-loading to 0.13 %, 0.70 %, and 1.67 %,
Pt clusters get more dominant, showing gradually increased density of clusters with average sizes
of 0.9, 1.2, and 1.6 nm (Figure 1b-d). Very few single atoms or > 2.0 nm particles could be
observed for 0.13 %, 0.70 %, or 1.67 % Pt/Mg(Al)O@Al2O3-IR (Figure 1b-d). According to the
brightness intensity profile analysis (Figure S4), two types of Pt clusters in Pt/Mg(Al)O@Al2O3-
IR could be classified: (1) two-dimensional (2D) clusters composed of loose ensembles with
one-atom-thick and (2) three-dimensional (3D) clusters with multi-atom-thick in a size at
maximum distribution of 0.8-2.0 nm. The Pt dispersion, calculated on the basis of hydrogen-
oxygen titration (HOT) results, is 92 % for 0.08 % Pt/Mg(Al)O@Al2O3-IR, and decreases
gradually with increasing Pt loading (Table S1). To obtain atomically dispersed Pt in 100
percentage, 0.08 % Pt/Mg(Al)O@Al2O3-OR (OR represents oxidation in air followed by
reduction in H2) was produced by first calcination of 0.10 % Pt2+/MgAl-LDHs@Al2O3 in air at
400 °C, and then holding in H2 at 300 °C, affording a Pt dispersion of up to 98 % (Table S1). In
the HAADF-STEM image, Pt is observed exclusively as atomic dispersion for 0.08 %
Pt/Mg(Al)O@Al2O3-OR (Figure 1e). Similar procedure has been used to produce supported
atomic Pt in a previous report.40 To estimate the distribution of engineered Pt species, at least
300 of atomic Pt, 2D Pt clusters, and 3D Pt clusters in more than four regions are randomly
selected. The statistic distribution of Pt species for 0.08 %, 0.13 %, 0.70 %, and 1.67 %
Pt/Mg(Al)O@Al2O3-IR and 0.08 % Pt/Mg(Al)O@Al2O3-OR is calculated and illustrated in
Figure 1f. For Pt/Mg(Al)O@Al2O3-IR, Pt sites changes from atom-thick dispersion (2D cluster)
coexisting with 3D clusters gradually to 3D clusters with Pt loading increasing from 0.08 to 1.67
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%. The percentage of 2D/3D Pt clusters has been estimated as being 36/53, 23/69, 13/80, and
3/95 for 0.08 %, 0.13 %, 0.70 %, and 1.67 % Pt/Mg(Al)O@Al2O3-IR.
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Pt coordination was characterized by extended X-ray absorption fine structure (EXAFS)
spectra (Figure 1g). The parameters of Pt-containing samples based on the EXAFS analysis are
shown in Table 1. The detailed fitting results of R-space and K-space are presented in Figure S5
and Figure S6. For 0.08 % Pt/Mg(Al)O@Al2O3-IR, Pt-O contribution (NPt-O = 2.0) is observed
much predominant over Pt-Pt contribution (NPt-Pt = 3.7) from the EXAFS spectrum. The Pt-Pt
contribution is hardly resolved despite the co-existence of 3D Pt clusters with atomic Pt and
atom-thick Pt clusters. The Pt-Pt coordination number for 0.08 % Pt/Mg(Al)O@Al2O3-IR is
much lower than for bulk Pt (coordination number of 12) as well. The results indicate that the 3D
clustered Pt sites here are highly coordination-unsaturated, even close to Pt-Pt coordination in
atom-thick dispersion, which is highly consistent with a Pt dispersion of 92 %. Pt-Pt contribution
becomes more visible (Figure 1g) and Pt-Pt coordination number (Table 1) increases with
increasing Pt loading. Pt-Pt contribution predominates over Pt-O contribution for 1.67 %
Pt/Mg(Al)O@Al2O3-IR, with a Pt-Pt coordination number of 6.2, corresponding to the
coordination-unsaturated atoms at corners/edges41 of Pt particles (NPt-Pt = 4-7). Atomically
dispersed Pt (0.08 % Pt/Mg(Al)O@Al2O3-OR) possesses a Pt-O coordination of 3.7 and no Pt-Pt
coordination, indicative of the strong interaction between atomic Pt and Mg(Al)O (Table 1). By
combining HAADF-STEM and XAFS techniques, atomic Pt, coordination-unsaturated Pt-Pt in
atom-thick dispersion (2D clusters), and coordination-unsaturated 3D Pt-Pt (3D clusters) have
been identified.
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In the X-ray absorption near-edge structure (XANES) spectra (Figure 1h), each
Mg(Al)O@Al2O3 supported Pt displays a white line intensity between PtO2 and Pt foil,
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indicative of electron transfer due to Pt-O contribution and high dispersion of Pt. The white line
intensity of supported Pt (Table S2) decreases in the sequence of single atoms > 2D clusters >
3D clusters (0.08 % Pt/Mg(Al)O@Al2O3-OR > 0.08 % > 0.13 % ≈ 0.70 % > 1.67 %
Pt/Mg(Al)O@Al2O3-IR). The white line results from the electron transition from 2p orbitals to
5d orbitals. A higher intensity of white line indicates larger amount of unoccupied 5d orbitals of
Pt, and thus more positively charged Pt. The atomic Pt in 0.08 % Pt/Mg(Al)O@Al2O3-OR shows
a white line intensity adjacent to PtO2, suggesting that the atomic Pt is partially positively-
charged. 0.08 % Pt/Mg(Al)O@Al2O3-IR shows stronger white line intensity than 0.13, 0.70, and
1.67 % Pt/Mg(Al)O@Al2O3-IR, suggesting more electron transfer from the Pt-Pt in atom-thick
dispersion to support than from coordination-unsaturated 3D Pt-Pt. The Pt-Pt sites in atom-thick
dispersion are electron-deficient in comparison to coordination-unsaturated 3D Pt sites. The 3D
Pt clusters (1.67 % Pt/Mg(Al)O@Al2O3-IR) show a white line intensity adjacent to Pt foil.
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Figure 1. HAADF-STEM images of (a) 0.08 % Pt/Mg(Al)O@Al2O3-IR, (b) 0.13 %
Pt/Mg(Al)O@Al2O3-IR, (c) 0.70 % Pt/Mg(Al)O@Al2O3-IR, (d) 1.67 % Pt/MgAlO@Al2O3-IR,
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and (e) 0.08 % Pt/Mg(Al)O@Al2O3-OR; the inserts for b-d are the size distribution of Pt. (f)
Distribution statistics of Pt dispersion based on brightness intensity results. (g) Fourier-transform
EXAFS spectra and (h) normalized XANES spectra at Pt LIII-edge.
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Table 1. EXAFS fitting parameters at the Pt LIII-edge for various samples
σ2
ΔE0
R
factor
(%)
Sample
Shell
N a
R (Å) b
(Å2·10-3) c
(eV)d
0.08 % Pt/Mg(Al)O@Al2O3-IR
Pt-O
Pt-Pt
Pt-O
Pt-Pt
Pt-O
Pt-Pt
Pt-O
Pt-Pt
Pt-O
2.0
3.7
1.8
4.8
1.6
5.1
1.0
6.2
3.7
2.02
2.67
2.02
2.73
2.02
2.73
1.99
2.70
2.03
2.9
11.7
4.2
1.0
1.9
2.8
1.0
12.0
7.6
0.13 % Pt/Mg(Al)O@Al2O3-IR
0.70 % Pt/Mg(Al)O@Al2O3-IR
1.67 % Pt/Mg(Al)O@Al2O3-IR
9.7
12.1
5.8
8.3
10.1
8.0
8.6
2.5
5.5
12.0
7.7
5.1
0.08 % Pt/Mg(Al)O@Al2O3-OR
1.67 % Pt/Mg(Al)O@Al2O3-NR
13.75
0.8
1.0
Pt-O
0.6
1.94
2.0
-4.0
Pt-Pt
Pt-Pt
Pt-O
Pt-Pt
7.3
12
6
2.74
2.76
2.01
3.09
6.7
3.7
4.1
9.8
7.0
6.1
7.2
2.1
Pt foil
PtO2
0.3
0.7
6
a
b
c
N: coordination numbers; R: bond distance; σ2: Debye–Waller factor value, where the high
d
σ2 suggests a high degree of disorder of Pt; ΔE0: inner potential correction to account for the
difference in inner potential between the sample and the reference compound. R factor: goodness
of fit; better is the curve fitted, smaller is the R-factor value got; generally, an acceptable R-
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factor should be smaller than 5 %. Ѕ0 were respectively set as 0.858/0.9 for Pt-Pt/Pt-O, which
were obtained from the experimental EXAFS fit of Pt foil/PtO2 reference by fixing CN as the
known crystallographic value and was fixed to all the samples. Error bounds (accuracies) of the
above structural parameters were estimated as N, ± 20 %; R, ± 1 %; σ2, ± 20 %; ΔE0, ± 20 %.
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Pt/Mg(Al)O@Al2O3 was then used as catalyst for the hydroconversion of furfuryl alcohol in
o
a continuous-flow system using a fixed-bed reactor at 200 C with a WHSV of 0.12 h-1. The
1
qualitative identifications of each product are based on the results from H and 13C nuclear
magnetic resonance (NMR) spectra and mass spectra (MS) (see details in the Supporting
Information, Appendix). No visible change in the furfuryl alcohol conversion or product
selectivity is observed in a 6-h online reaction for each sample (Figure S7). Thus initial
conversion and selectivity are selected for discussion (Table 2). Atomic Pt (0.08 %
Pt/Mg(Al)O@Al2O3-OR) affords a selectivity of 93 % to 2-methylfuran (2-MF), with a furfuryl
alcohol conversion of 87 %. That means the hydrogenolysis of C-OH is preferred on atomic Pt,
affording 2-MF. On coordination-unsaturated Pt-Pt in atom-thick dispersion coexisting with
highly coordination-unsaturated 3D Pt (0.08 % Pt/Mg(Al)O@Al2O3-IR), both of
tetrahydrofurfuryl alcohol (THFA) and 1,2-pentanediol (1,2-PeD) have been observed as the
main products, with a selectivity of 43 % to 1,2-PeD and 34 % to THFA under 41 % conversion
of furfuryl alcohol. 1,5-pentanediol (1,5-PeD) is detected as major by-product. With
coordination-unsaturated 3D Pt sites gradually predominating (0.13, 0.70 to 1.67 %
Pt/Mg(Al)O@Al2O3-IR), the selectivity to 1,2-PeD increases, and the selectivity to either THFA
or 1,5-PeD decreases. The conversion of furfuryl alcohol also increases with increasing content
of 3D Pt clusters. A selectivity of up to 86 % to 1,2-PeD has been achieved with complete
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conversion of furfuryl alcohol on the Pt sites almost completely present in 3D clusters (1.67 %
Pt/Mg(Al)O@Al2O3-IR). According to previous reports, H-transfer hydrogenation of furfural-to-
furfuryl alcohol, furfural-to-2-methylfuran or ethanol-to-n-butanol is efficient using alcohol as
hydrogen donor, for liquid-solid phase reaction in the autoclave42-44 or gas-solid phase reaction
under N245,46. To better understand the role of Pt in this work, Mg(Al)O@Al2O3 (without Pt) has
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been produced by the thermal treatment of MgAl-LDHs@Al2O3 in H2 at 400 C. The hydro-
conversion of furfuryl alcohol on Mg(Al)O@Al2O3 has been tested, showing barely conversion
of furfuryl alcohol and no product (Table 2). The result indicates the effect of H-transfer
hydrogenation with ethanol as hydrogen carrier could be ignored in our system.
Table 2. Hydrogenation of furfuryl alcohol at 200 oC over each sample
Sel. (%)
Sample
Con. (%)
87 (88)
THFA 2-MF 1,5-PeD 1,2-PeD Others
0.08 % Pt/Mg(Al)O@Al2O3-OR
6 (7) 93 (92)
0 (0)
0 (0)
1 (1)
0.08 % Pt/Mg(Al)O@Al2O3-IR
0.13 % Pt/Mg(Al)O@Al2O3-IR
0.70 % Pt/Mg(Al)O@Al2O3-IR
41
65
98
34
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7
8
2
1
15
13
7
41
64
80
2
6
5
1.67 % Pt/Mg(Al)O@Al2O3-IR
0.08 % Pt/Mg(Al)O@Al2O3-OR-R
1.67 % Pt/Mg(Al)O@Al2O3-NR
0.70 % Pt/Al2O3-IR
> 99
41
6
24
14
15
-
1
4
5
12
8
86
56
73
62
-
2
4
2
4
-
> 99
96
3
16
-
3
Mg(Al)O@Al2O3
< 1
-
Reaction conditions: 0.2 mol/L furfuryl alcohol (ethanol as solvent), 1 g of catalysts, WHSV =
o
0.12 h-1, 200 C, 3 MPa H2. Pentanol and HPO was included in others. The data in parentheses
are the reproduced experimental data.
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To allow selectivity comparison, the conversion is controlled at the same level (~ 20 %) by
changing the contact time or under a mild condition (160 oC) (Table 3). The calculated turnover
frequency (TOF) for atomic Pt (0.08 % Pt/Mg(Al)O@Al2O3-OR) is 45 h-1 (Table 3, entry 1),
with a selectivity of 17 % and 35 % to THFA and 1,2-PeD under 160 oC. Lower selectivity to 2-
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o
o
MF under 160 C than under 200 C indicates that higher temperature is beneficial to the
selective activation of C-OH in furfurly alcohol on atomic Pt. On Pt-Pt site, the TOF value
gradually decreases with increasing content of 3D Pt clusters (Table 3, entry 2-5), from 66 h-1 for
0.08 % Pt/Mg(Al)O@Al2O3-IR with 36 % of 2D cluster and 53 % of 3D cluster to 48 h-1 for 1.67
% Pt/Mg(Al)O@Al2O3-IR with 91 % of 3D cluster, indicative of the higher activity of
coordination-unsaturated Pt-Pt in atom-thick dispersion than coordination-unsaturated 3D Pt-Pt.
On 0.13 % Pt/Mg(Al)O@Al2O3-IR, 1-hydroxypenta-2-one (HPO) was detected in a selectivity of
28 % under 160 C and a WHSV of 0.19 h-1 while in 7 % (Table 3, entry 6) under a longer
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contact time (WHSV of 0.12 h-1). Under a much shorter contact time (WHSV of 0.36 or 1.44 h-
1), HPO was also detected on 0.70 % Pt/Mg(Al)O@Al2O3-IR or 1.67 % Pt/Mg(Al)O@Al2O3-IR
(Table 3, entry 7 or 8). But the selectivity to total HPO and 1,2-PeD hardly changes with contact
time at 160 oC, and is also similar to the selectivity to 1,2-PeD at 200 oC, indicating HPO might
serve as an important intermediate for the formation of 1,2-PeD. By prolonging the contact time
at 160 C, HPO is converted to 1,2-PeD. In a previous report,47 1,2-PeD or 1,5-PeD was
o
produced from THFA with high H2 pressure. Employing THFA as the substrate here (Table S3),
THFA is hardly transformed with no product detected over 1.67 % Pt/Mg(Al)O@Al2O3-IR. The
results indicate that THFA is not a necessary intermediate for the production of pentanediols
from furfuryl alcohol in this work. Employing furfural as the substrate (Table S3), 1,2-PeD is
also produced in a yield of 78 %. The conversion of furfuryl alcohol to 1,2-PeD (a selectivity of
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86 %) or PeDs (a selectivity of 91 %) on coordination-unsaturated 3D Pt sites in this work is
superior to those of the state-of-the-art catalysts.
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Table 3. Hydrogenation of furfuryl alcohol at 160 oC over each Pt sample
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Sel. (%)
Con.
(%)
TOF
(h-1)
Entry Sample
2-
MF
1,5-
PeD
1,2-
PeD
1,2-PeD
+ HPO
THFA
HPO
Others
0.08 % Pt/Mg(Al)O
17
41
35
(35)
1
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7
8
15 (17)
21
5 (5) 1 (1)
36 (36) 1 (1)
45
66
60
51
48
@Al2O3-OR
(17) (43)
0.08 % Pt/Mg(Al)O
@Al2O3-IR
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20
15
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4
1
1
1
1
1
14
9
1
43
41
50
68
64
70
44
69
80
85
71
82
1
1
1
1
2
1
0.13 % Pt/Mg(Al)O
@Al2O3-IR§
18
28
30
17
7
0.70 % Pt/Mg(Al)O
@Al2O3-IR#
21
5
1.67 % Pt/Mg(Al)O
@Al2O3-IR*
19
4
0.13 % Pt/Mg(Al)O
@Al2O3-IR
30
16
11
10
5
0.70 % Pt/Mg(Al)O
@Al2O3-IR
52
12
1.67 % Pt/Mg(Al)O
@Al2O3-IR
1
(1)
85
(85)
96 (95) 6 (5)
6 (6) 1 (1)
86 (86) 2 (2)
Reaction conditions: 0.2 mol/L furfuryl alcohol (ethanol as solvent), 1 g of catalysts, WHSV =
0.12 h-1, WHSV = 0.19 h-1, WHSV = 0.36 h-1, *WHSV= 1.44 h-1, 160 C, 3 MPa of H2.
§
#
o
Pentanol was included in others. The data in parentheses are the reproduced experimental data.
Relating the product selectivity with the Pt dispersion on Mg(Al)O under similar conversion
of furfurly alcohol, it can be found that the selectivity of (HPO + 1,2-PeD) increases with the
increase in the percentage of coordination-unsaturated 3D Pt (3D cluster) (Figure 2a), while the
selectivity of THFA resulting from the hydrogenation of ring C=C increases (Figure 2b) with
increasing percentage of coordination unsaturated Pt-Pt sites in atom-thick dispersion (2D
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cluster). The results for the hydroconversion of furfuryl alcohol on 0.08 % Pt/Mg(Al)O@Al2O3-
OR unambiguously prove that 2-MF are generated by the hydrogenolysis of C-OH on atomic Pt.
Interestingly, the selectivities to total of 1,2-PeD and HPO are similar under varied conversions
over each clustered Pt sample (Pt/Mg(Al)O@Al2O3-IR) in this work, indicative of the
enhancement of activity without sacrificing selectivity. In heterogeneous catalysis, achieving
supported metal with simultaneously enhanced activity and stability is of vital importance and
remains a challenging goal. The achievement in this work is probably attributed to the dilute
solution of furfuryl alcohol in ethanol, which leads to the insensitive of selectivity to reactant
concentration. The role of solvent and reactant concentration on selectivity deserves an in-depth
investigation, which is scheduled in our subsequent research.
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Figure 2. Variation of (a) the selectivity of (HPO + 1,2-PeD) with the percentage of 3D clusters
and (b) THFA selectivity with the percentage of 2D clusters over 0.08 % Pt/Mg(Al)O@Al2O3-IR
(black square), 0.13 % Pt/Mg(Al)O@Al2O3-IR (red circle), 0.70 % Pt/Mg(Al)O@Al2O3-IR (blue
triangle), and 1.67 % Pt/Mg(Al)O@Al2O3-IR (green inverted triangle).
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To further confirm the sensitivity of the selective activation of furan C-O-C, branched C-
OH, or ring C=C to the nature of Pt sites, the as-prepared 0.08 % Pt/Mg(Al)O@Al2O3-OR was
re-treated by calcination in H2 at 400 oC (denoted 0.08 % Pt/Mg(Al)O@Al2O3-OR-R) to convert
Pt dispersion from single atoms to clusters. Pt clusters are clearly observed for 0.08 %
Pt/Mg(Al)O@Al2O3-OR-R in the HAADF-STEM image (Figure 3a), with 32 % of 2D cluster
and 59 % of 3D cluster according to the brightness intensity profile analysis. Based on the
2D/3D percentage, the Pt dispersion for 0.08 % Pt/Mg(Al)O@Al2O3-OR-R is similar to that for
0.08 % Pt/Mg(Al)O@Al2O3-IR. In the hydrogenation of furfuryl alcohol at 200 oC (Table 2), the
major product has been changed from 2-MF (93 % of selectivity) on atomic Pt to 1,2-PeD and
THFA in the selectivity of 56 % and 24 % on 3D and 2D Pt clusters. Just as expected, the
activation of chemical bonds in furfurly alcohol highly depends on the dispersion of Pt sites. On
the other hand, Pt particles with higher Pt-Pt coordination number (Table 1) have been prepared
by first thermal treatment of 1.80 % Pt2+/MgAl-LDHs@Al2O3 in N2 at 400 °C and then reduction
in H2 at 400 °C (denoted 1.67 % Pt/Mg(Al)O@Al2O3-NR, NR represents thermal treatment in N2
followed by reduction in H2). In the XRD pattern (Figure S3f), the reflection of Pt (111) at 40.1 o
is observed for 1.67 % Pt/Mg(Al)O@Al2O3-NR, indicative of the existence of Pt nanoparticles.
Based on TEM/HR-TEM images, the average size of Pt particles is 3.5 nm (Figure 3b), with
exposure of 100 and 111 planes of cub-octahedrons (Figure 3c). Comparing Pt particles (1.67 %
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Pt/Mg(Al)O@Al2O3-NR)
with
coordination-unsaturation
3D
clusters
(1.67
%
Pt/Mg(Al)O@Al2O3-IR) in the hydrogenation of furfuryl alcohol (Table 2), the selectivity to 1,2-
PeD decrease from 86 % to 73 % with Pt dispersion changing from coordination-unsaturation 3D
clusters to nanoparticles, while the selectivity to THFA increases from 6 % to 14 %. The results
indicate that the formation of 1,2-PeD requires coordination-unsaturated 3D Pt-Pt like at
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corners/edges, rather than terrace Pt-Pt. Our observation with Pt particles (1.67 %
Pt/Mg(Al)O@Al2O3-NR) is consistent with previous report30 that the hydrogenation of C=C
bonds in furfural is favored on terrace (such as 111 plane) metal sites. Based on the above results,
the reaction pathways for the hydrogenation of furfuryl alcohol over varied Pt sites on Mg(Al)O
are proposed as Figure 4, which are to be further discussed and proved in the next context.
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Figure 3. HAADF-STEM or TEM images of (a) 0.08 % Pt/Mg(Al)O@Al2O3-OR-R, (b and c)
1.67 % Pt/Mg(Al)O@Al2O3-NR, and (d) 0.72 % Pt/Al2O3-IR.
Figure 4. Proposed reaction paths for the hydrogenation of furfuryl alcohol over engineered Pt
sites.
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To clarify the Pt-dispersion dependent hydrogenation of furfuryl alcohol, the FT-IR spectra
recorded surface reaction experiments are performed (Figure 5 and 6). On atomic Pt (0.08 %
Pt/Mg(Al)O@Al2O3-OR), the absorption bands assigned48,49 to C1-O at 1076 cm−1, C5-O linearly
adsorbed with O terminal at 1102 cm−1, C1-O-C4 at 1149 cm−1, C2-C3 at 1221 cm−1, C1=C2 at
1504 cm−1, and C3=C4 at 1596 cm−1 in furfuryl alcohol are observed (Figure 5; see assignments
of the absorption bands in the Supporting Information, Appendix). With on-stream time under
hydrogenation conditions, the absorption intensity of C3=C4 at 1596 cm−1 gradually increases
relative to that of C1=C2 at 1504 cm−1 with the vanishment of O-terminal adsorption at 1102
cm−1, and new bands assigned to methyl emerge at 2877 and 2961 cm−1 and become more visible
with time. Other change of furan-ring are hardly observed. According to previous observation,48
the IR absorption of C3=C4 in 2-MF is visible while very weak in furfuryl alcohol. So the gradual
increase in the absorption intensity of C3=C4 originates from the conversion of furfuryl alcohol to
2-MF. As discussed above, the atomic Pt sites are electron-deficient. The electron-deficient
atomic Pt sites prefer an O-terminal adsorption, leading to the targeted activation of C5-O (Figure
5). The targeted activation of C5-O with O-terminal adsorption on atomic Ptδ+ sites accounts for
the C5-O cleavage without affecting the furan C-O and ring C=C, affording 2-MF as major
product as illustrated in Figure 4.
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Figure 5. FT-IR spectra recorded hydrogenation progress of furfuryl alcohol at 80 oC at intervals
in H2/Ar flow on 0.08 % Pt/Mg(Al)O@Al2O3-OR; the insert is the proposed adsorption model.
On coordination-unsaturated 3D Pt-Pt sites (3D clusters in 1.67 % Pt/Mg(Al)O@Al2O3-IR),
the hydrogenation of furfuryl alcohol occurs much more rapidly than on atomic Pt, with the
transformation of chemical bonds almost finished in 10 min. The absorption of furan C1-O-C4 at
1149 cm−1, ring C2-C3 and C1=C2 at 1221 and 1504 cm−1 in furfuryl alcohol gradually decrease
in intensity with on-stream time and eventually disappears in 10 min under hydrogenation
conditions (Figure 6A), indicative of the hydrogenation of ring C1=C2 and the hydrogenolysis of
whole furan structure. Meanwhile, new bands at 1064 and 1694 cm−1 are observed to emerge
since 1 min and gradually increase in intensity, and a band at 1128 cm−1 emerges at 10 min. The
bands at 1064 and 1128 cm−1 well agree with the C-O vibrations in 1,2-PeD,48 indicating the
formation of 1,2-PeD. The band at 1694 cm−1, assigned42 to C=O in O-terminal adsorption,
originates from HPO here, as observed in the catalytic results (Table 3) as key intermediate of
1,2-PeD. The adsorbed C=O bond likely results from the transformation from C3=C4-OH,
formed by the cleavage of C1-O, to C3-C4=O. On the basis of the observation above, the
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conversion of furfuryl alcohol to 1,2-PeD can be proposed to start from the hydrogenolysis of
furan structure. Unfortunately, it is hard to clearly distinguish step-by-step hydrogenolysis and
one-step hydrogenolysis for now because no specific intermediates have been detected in the -
FT-IR spectra. But from the incomplete vanishment of C1-O at 1076 cm−1 and C3=C4 at 1596
cm−1, 1,2-dihydrofurfuryl alcohol (1,2-DHFA) can be deduced as an intermediate. It has also
been reported9 that C1=C2 bond of furfuryl alcohol can be hydrogenated to form 1,2-DHFA. In
1,2-DHFA, C1-O linkage, located adjacent to the just-formed C1-C2 linkage, is easier to get
adsorbed on coordination-unsaturated 3D Pt-Pt sites. Furthermore, C1-O is weakened due to the
p-π conjugation of O-C4=C3 linkage, facilitating the cleavage of C1-O to result in ring-opening.
The ring-opened species from the cleavage of C1-O is quickly converted to HPO, and 1,2-PeD is
produced from the hydrogenation of C=O of HPO, as illustrated in Figure 4.
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On 2D cluster coexisting with highly coordination-unsaturated 3D cluster (0.08 %
Pt/Mg(Al)O@Al2O3-IR), the emergence of C=O of HPO at 1694 cm−1 and C-O vibration in 1,2-
PeD at 1128 cm−1 indicate the formation of 1,2-PeD owing to the hydrogenolysis of furan ring
via HPO (Figure 6B), just as observed on coordination-unsaturated 3D Pt-Pt. The absorption of
C3=C4 at 1596 cm−1 becomes intensive with on-stream time in the case of no 2-MF formation
and C1=C2 at 1504 cm−1 becomes weaker till to disappearance, indicating the formation of semi-
hydrogenation product. So the THFA observed in the catalytic results (Table 3) can be proposed
to produce by step-by-step hydrogenation via 1,2-DHFA as the intermediate, as illustrated in
Figure 4. The incompletely vanished C1-O-C4 at 1149 cm−1 and the gradually intensified C1-O at
1074 cm−1 indicate the preservation of furanic ring structure. In the FT-IR spectrum of THFA,
the band around 1074 cm−1 is the only absorption in the range of 1000~1200 cm−1,48 consistent
with our conclusion. THFA has also been observed on supported Pt particles (1.67 %
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Pt/Mg(Al)O@Al2O3-NR). But it can be seen from the FT-IR spectra recorded surface reaction
(Figure S8) that the intensity of C1=C2 at 1504 cm−1 and C3=C4 at 1596 cm−1 simultaneously
decrease in the hydrogenation, indicative of one-step hydrogenation of furfurly alcohol to THFA.
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Figure 6. FT-IR spectra recorded hydrogenation progress of furfuryl alcohol at 80 oC at intervals
in H2/Ar flow on (A) 1.67 % Pt/Mg(Al)O@Al2O3-IR and (B) 0.08 % Pt/Mg(Al)O@Al2O3-IR;
the inserts are the proposed adsorption models.
To further testify the reaction mechanism, FT-IR spectra recorded surface reaction on 3D Pt
clusters (1.67 % Pt/Mg(Al)O@Al2O3-IR) is broken off by stopping H2/Ar flow. After reaction in
H2/Ar flow for 2 min, Ar flow is introduced (Figure 7). The absorption bands of furan C1-O-C4 at
1149 cm−1, C2-C3 and C1=C2 at 1221 and 1504 cm−1 in furfuryl alcohol gradually decrease in
intensity but incompletely vanish, and the absorption bands of C=O at 1694 cm−1 and C-O at
1128 cm−1 in HPO gradually increase in intensity under Ar flow, indicating the initially
converted furfuryl alcohol could further be transformed to HPO. The results prove the observed
bands in the FT-IR spectra represent the active species. However, the absorption band of HPO
(C=O) continuously becomes enhanced under Ar flow, while no visible band for 1,2-PeD (C-O
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at 1064 cm−1) is observed, indicating the hydrogenation of C=O in HPO to produce 1,2-PeD is
restrained with the lack of H2.
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Figure 7. FT-IR spectra recorded hydrogenation progress of furfuryl alcohol at 80 oC on 1.67 %
Pt/Mg(Al)O@Al2O3-IR. After reaction in H2/Ar flow for 2 min (a, b), Ar flow was introduced
for 10 min (c, d) with the FT-IR spectra recorded at intervals.
As is well known, the surface property of the support could affect the catalytic performance.
Therefore, in this work, the role of Mg(Al)O support toward hydrogenation of furfuryl alcohol
has also been investigated. 0.72 % Pt/Al2O3-IR (without LDHs) has been prepared as a control
by reduction of Pt2+-loaded Al2O3. 3D Pt clusters with average size of 1.6 nm, which is similar to
that for 1.67 % Pt/Mg(Al)O@Al2O3-IR, are observed for 0.72 % Pt/Al2O3-IR (Figure 3d). The
percentage of 2D/3D Pt clusters has been estimated as being 6/81 for 0.72 % Pt/Al2O3-IR. This
indicates that on one hand Mg(Al)O promotes Pt dispersion more effectively than Al2O3. In the
XANES spectra (Figure S9 and Table S2), 0.72 % Pt/Al2O3-IR displays the similar white line
intensity as 1.67 % Pt/Mg(Al)O@Al2O3-IR. According to the CO2-TPD profiles (Figure S10),
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1.67 % Pt/Mg(Al)O@Al2O3-IR have stronger basicity and more basic sites than 0.72 %
Pt/Al2O3-IR. In the hydrogenation of furfurly alcohol, 2-MF has been produced in a selectivity of
16 % over 0.72 % Pt/Al2O3-IR (Table 2), which is hardly observed over 1.67 %
Pt/Mg(Al)O@Al2O3-IR. In comparison to 1.67 % Pt/Mg(Al)O@Al2O3-IR, the selectivity to 1,2-
PeD decreases and the selectivity to THFA increases on 0.72 % Pt/Al2O3-IR. The selectivity to
1,2-PeD is positively related with the amount of basic sites in this work, which is consistent with
the observation in an earlier report.10 The basic site on Mg(Al)O is supposed to help abstracting
H+ from C-OH50,51, affording electron-rich alkoxide species (C-O-). The electron-rich alkoxide
species on one hand reinforces C-O- and on the other hand makes furan C-O-C more
approachable to coordination-unsaturated Pt sites through donating electron, thus inhibiting the
cleavage of C-O- and enhancing the breakage of furan C-O-C. In the FT-IR spectra for the
hydrogenation of furfuryl alcohol over 0.72 % Pt/Al2O3-IR (Figure S11), with the decrease in
intensity and/or gradual vanishment of C1=C2 at 1504 cm-1, C2-C3 at 1221 cm-1, and C1-O-C4 at
1149 cm-1, the absorption originating from C=O of HPO at 1701 cm-1 and C-O of 1,2-PeD at
1128, and 1064 cm-1 emerge, indicating the hydrogenation of ring C1=C2 and hydrogenolysis of
furan C1-O-C4, accounting for the production of 1,2-PeD via HPO intermediate as well. In the
meantime, the absorption originating from C3=C4 at 1596 cm-1 gradually increasing in intensity,
and adsorbed C5-O at 1102 cm-1 disappears since 2 min on-stream, consistent with the formation
of 2-MF through the cleavage of C5-O. The FT-IR spectra recorded surface reaction in this work
could partially support the realistic catalytic results. Operando investigations under realistic
condition could provide more powerful evidence, and the experiments as well as the equipments
are being designed in our laboratory.
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To investigate the dispersion stability of Pt, the spent atomic Pt or 3D Pt cluster, which has
undergone a 6-h furfuryl alcohol hydrogenation reaction, has been characterized using HAADF-
STEM technique. No visible change in the dispersion of Pt isolated-atoms in 0.08 %
Pt/Mg(Al)O@Al2O3-OR or 3D clusters in 1.67 % Pt/Mg(Al)O@Al2O3-IR is observed after 6-h
reaction at 200 °C (Figure 8), as well as no aggregated ensemble in low magnification image
(Figure 8). The results also indicate the engineered Pt sites really serve as active sites during the
reaction.
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Figure 8. HAADF-STEM images with varied magnifications of spent (a, b) 0.08 %
Pt/Mg(Al)O@Al2O3-OR and (c, d) 1.67 % Pt/MgAlO@Al2O3-IR after 6-h furfuryl alcohol
hydrogenation reaction; the insert for (d) is the size distribution of Pt.
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3. Conclusions
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In summary, this work demonstrates an efficient selective activation to C-OH, C-O-C, or C=C on
atomic Pt, coordination-unsaturated 2D Pt-Pt, or coordination-unsaturated 3D Pt-Pt in the
hydroconversion of furfuryl alcohol, producing 2-MF, THFA or 1,2-PeD. The Pt sites are
controlled by tailoring the Pt dispersion (single atom, 2D cluster, or 3D cluster) on Mg(Al)O
support. The Pt-site dependent intrinsic reaction mechanisms have been traced with the FT-IR
spectra recorded surface reaction. On atomic Pt, C-O-H is easily activated with O-terminal
adsorption without affecting furan C-O and C=C. On coordination-unsaturated 2D Pt-Pt sites, the
selective activation of C=C step-by-step to generate THFA takes place. On coordination-
unsaturated 3D Pt-Pt sites, the hydrogenolysis of furan ring occurs through the cleavage of
weakened and adsorbed furan C-O, affording 1,2-PeD as major product. The Mg(Al)O support
also play a key role in promoting the selectivity to 1,2-PeD by providing basic sites. 1,5-PeD has
also been detected on Pt clusters, but unfortunately in a low selectivity. The catalytically active
sites for targeted activation of furan C-O to produce 1,5-PeD from furfural/furfuryl alcohol is
worth deep exploring.
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AUTHOR INFORMATION
Corresponding Author
* zhenglr@ihep.ac.cn (L. Zheng)
* hejing@mail.buct.edu.cn or jinghe@263.net.cn (J. He)
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Notes
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The authors declare no competing financial interest.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
Detailed experimental procedures, characterization and catalytic test methods; XRD patterns,
SEM and STEM images, catalytic tests, EXAFS fitting curves, FT-IR spectra recorded surface
1
reactions, and TPD profiles; appendix of H and 13C NMR spectra, mass spectrum, and
assignments of the absorption bands in FT-IR spectra (PDF).
ACKNOWLEDGMENT
Financial supports from National Natural Science Foundation of China (21521005, 91634120),
National Key R&D Program of China (2017YFA0206804), and the Fundamental Research
Funds for the Central Universities (ZY1936 and XK1802-6) are gratefully acknowledged. The
authors also thank the support from Beijing Engineering Center for Hierarchical Catalysts.
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