Selective Hydrogenolysis of α-C-O Bond in Biomass-Derived 2-Furancarboxylic Acid to 5-Hydroxyvaleric Acid on Supported Pt Catalysts at Near-Ambient Temperature

Article abstract of DOI:10.1021/acscatal.9b04074

Hydrogenolysis of the α-C-O bond in abundantly available biomass-based furfural and its derivatives provides a viable route for sustainable synthesis of valuable C5 compounds, particularly with two terminal oxygen-containing functional groups. However, efficient cleavage of this bond under mild conditions still remains a crucial challenge, primarily because of the competing cleavage of the α-C-O bond and hydrogenation of furan ring. Here, we report that supported Pt catalysts were extremely active for the selective α-C-O cleavage in 2-furancarboxylic acid (FCA) hydrogenolysis to synthesize 5-hydroxyvaleric acid (5-HVA), affording a high yield (～78%) on Pt/SiO₂ with a Pt particle size of 4.2 nm at an unprecedentedly low temperature of 313 K. In this reaction, the turnover rate and 5-HVA selectivity sensitively depend on the size of the Pt nanoparticles and the underlying support, as a consequence of their effects on the exposed Pt surfaces. Combined reaction kinetic, infrared spectroscopic, and theoretical assessments reveal that while the exposed high-index Pt surfaces (containing higher fraction of step sites) facilitate the kinetically relevant addition of the first H atom to the unsaturated C atom in furan ring and thus the hydrogenolysis activity, the low-index surfaces (containing higher fraction of terrace sites), together with the electron-withdrawing effect of the carboxylic substituent in FCA, favorably stabilize the dangling C2 atom in the transition states of α-C-O cleavage and lower their activation barriers, leading to the observed high 5-HVA selectivity. Such pivotal roles of the intrinsic properties of metal surfaces and substituents in tuning the reaction pathways will provide a viable strategy for highly selective upgrading of furan derivatives and other biomass-based oxygenates.

Full text of DOI:10.1021/acscatal.9b04074

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Article
Selective hydrogenolysis of #-C-O bond in biomass-
derived 2-furancarboxylic acid to 5-hydroxyvaleric acid
on supported Pt catalysts at near-ambient temperature
Qianhui Sun, Shuai Wang, and Haichao Liu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b04074 • Publication Date (Web): 30 Oct 2019
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Selective hydrogenolysis of α-C-O bond in
biomass-derived 2-furancarboxylic acid to
5-hydroxyvaleric acid on supported Pt catalysts at
near-ambient temperature
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Qianhui Suna,b,†, Shuai Wangc,†, and Haichao Liua,*
a Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, China
b Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
c State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation
Center of Chemistry for Energy Materials, and National Engineering Laboratory for Green
Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China
ABSTRACT: Hydrogenolysis of α-C-O bond in abundantly available biomass-based furfural
and its derivatives provides a viable route for sustainable synthesis of valuable C5
compounds particularly with two terminal oxygen-containing functional groups. However,
efficient cleavage of this bond under mild conditions still remains a crucial challenge,
primarily due to the competing cleavage of δ-C-O bond and hydrogenation of furan ring.
Here, we report that supported Pt catalysts were extremely active for the selective α-C-O
cleavage in 2-furancarboxylic acid (FCA) hydrogenolysis to synthesize 5-hydroxyvaleric acid
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(5-HVA), affording a high yield (~78%) on Pt/SiO2 with a Pt particle size of 4.2 nm at an
unprecedentedly low temperature of 313K. In this reaction, the turnover rate and 5-HVA
selectivity sensitively depend on the size of the Pt nanoparticles and underlying support, as a
consequence of their effects on the exposed Pt surfaces. Combined reaction kinetic, infrared
spectroscopic and theoretical assessments reveal that while the exposed high-index Pt
surfaces (containing higher fraction of step sites) facilitate the kinetically-relevant addition of
the first H atom to the unsaturated C atom in furan ring and thus the hydrogenolysis activity,
the low-index surfaces (containing higher fraction of terrace sites), together with the
electron-withdrawing effect of the carboxylic substituent in FCA, favorably stabilize the
dangling C2 atom in the transition states of α-C-O cleavage and lower their activation
barriers, leading to the observed high 5-HVA selectivity. Such pivotal roles of the intrinsic
properties of metal surfaces and substituents in tuning the reaction pathways will provide a
viable strategy for highly selective upgrading of furan derivatives and other biomass-based
oxygenates.
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KEYWORDS: selective hydrogenolysis, 2-furancarboxylic acid, 5-hydroxyvaleric acid, Pt
catalyst, exposed surface effect, substituent effect
1. INTRODUCTION
Aliphatic compounds with two terminal oxygen-containing functional groups, such as
adipic acid and 1,4-butanediol, are important commodity chemicals with versatile
applications particularly as monomers in the production of polyesters and polyurethanes,
etc.1,2 While they are commercially manufactured from non-renewable fossil resources,
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increasing attention has been paid recently to exploring alternative routes for sustainable
synthesis of these important chemicals from biomass-derived feedstocks.3-5 As such, largely
available furfural and its derivatives represent promising precursors because of their
structural advantage. For example, via specific cleavage of its α-C-O bond (i.e. the C-O bond
in the furan ring at the α position of the CHO group), furfural can be converted to
1,5-pentanediol (1,5-PeD), and by further oxidation even to 1,5-pentanedioic acid.6
However, selective activation of the α-C-O bond in the furan ring still remains
challenging.7,8 From the recent studies on hydrogenolysis of furfural9-11 and its derivatives,
furfuryl alcohol (FA)12-17 and tetrahydrofurfuryl alcohol (THFA)18-20, it is clear that the
formation of 1,5-PeD in these reactions is significantly limited by the competing reaction
channels, mainly including parallel hydrogenolysis of δ-C-O bond to 1,2-pentanediol
(1,2-PeD) and hydrogenation of the furan-ring on metal catalysts. For example, furfural
hydrogenolysis on Pt/CeO2 formed 57.8% 1,2-PeD and 28.3% THFA with only 4.0%
1,5-PeD at 100% conversion (443 K, 1.0 MPa H2).11 In FA hydrogenolysis, a 1,5-PeD
selectivity of ~44% was obtained on Cu-based catalysts in ethanol with 1,2-PeD as a major
by-product (~18% at 100% conversion).15 On the other hand, high 1,5-PeD yields (~80%)
were recently reported in THFA hydrogenolysis (393 K, 8.0 MPa H2),18,21 in which acidic
ReOx-modified Rh or Ir catalysts and high H2 pressures were however applied to activate
THFA due to its high chemical stability. These studies clearly show that more effective
catalysts are highly desirable for selectively breaking the α-C-O bond in furfural and its
derivatives under mild conditions.
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Here, we report hydrogenolysis of 2-furancarboxylic acid (FCA) to synthesize
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5-hydroxyvaleric acid (5-HVA) on supported Pt catalysts. FCA can be readily formed from
furfural via oxidation or Cannizaro disproportionation.22-24 5-HVA is an important
compound, and it can be efficiently converted to 1,5-PeD (via hydrogenation),
1,5-pentanedioic acid (via oxidation), and δ-valerolactone (via dehydration), etc., all being
useful monomers in synthesis of polymers.25-27 However, catalytic conversions of FCA into
valuable products, especially 5-HVA, have received limited attention and achieved limited
success to date.28,29 In this work, we find that the cleavage of the α-C-O bond can be
facilitated by the strong electron-withdrawing effect of -C(=O)OH substituent at the α-C
position in FCA, and achieve high 5-HVA yields on supported Pt catalysts at near-ambient
temperatures and mild H2 pressures. Reaction kinetic, spectroscopic, and theoretical methods
are combined to further assess the effects of Pt facets on FCA hydrogenolysis activity and
selectivity, revealing that the terrace sites predominantly present on low-index Pt surfaces
provide extra Pt sites to stabilize the dangling C atom in the transition state of α-C-O
cleavage, and account for the high 5-HVA selectivity. This work demonstrates an effective
strategy of tuning reaction selectivity via control of exposed metal catalyst surfaces and
coordination of functional groups in the oxygenate reactants, generally useful for developing
novel routes to catalytic upgrading of biomass feedstocks.
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2. EXPERIMENTAL SECTION
2.1. Catalyst Preparation. Pt, Ru, Pd and Ir nanoparticles supported on oxides, including
TiO2 (P25, 99%, Evonik), ZrO2 (monoclinic, 99%, Alfa Aesar) and SiO2 (99%, Alfa Aesar),
were prepared by a deposition-precipitation method, using H2PtCl6·6H2O (A.R.),
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RuCl3·3H2O (A.R.), PdCl2 (A.R.) and H2IrCl6·6H2O (A.R.) as the metal precursors,
respectively (all purchased from Sinopharm Chemicals). Taking the Pt/TiO2 catalyst as an
example for the catalyst preparation procedure, a certain amount of TiO2 was added into a
diluted aqueous solution of H2PtCl6·6H2O, into which, under vigorous stirring at a speed of
700 rpm at 333 K, a diluted ammonia solution was added dropwise until the pH value of the
solution was over 11. After agitation for 8 h at 333K, the supported catalyst precursors in the
solution were centrifugally separated, and then fully washed using deionized H2O and
ethanol. Afterwards, the resulting powders were treated in a vacuum oven at 333 K overnight,
heated in flowing air (40 mL min-1) at 623 K (at a ramping rate of 5 K min-1) for 3 h, and
subsequently treated in flowing 20% H2/N2 (>99.99%, Beijing Huayuan, 40 mL min-1) at 373
K (at a ramping rate of 5 K min-1) for 1 h.
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Pt particle sizes of the Pt/SiO2 catalysts were modulated by changing Pt loadings and
thermal treatment conditions of the dried precursors (Table 1), and the catalysts were denoted
as Pt/SiO2-Xnm, where Xnm represents the average Pt particle size.
Table 1. Specific preparation protocols for Pt/SiO2 catalysts with different Pt particle sizes
Pt loadinga
(%)
Pt particle sizec
(nm)
Catalyst
Thermal treatmentb
Pt/SiO2-2.6nm
Pt/SiO2-4.2nm
2.0
1. in 20% H2/N2 at 523 K for 6 h.
1. in air at 623 K for 3 h;
2.6±0.4
2.0
3.9
5.7
4.2±0.7
5.7±1.3
7.5±1.5
2. in 20% H2/N2 at 373 K for 1 h.
1. in air at 673 K for 4 h;
Pt/SiO2-5.7nm
Pt/SiO2-7.5nm
2. in 20% H2/N2 at 373 K for 1 h.
1. in air at 673 K for 4 h;
2. in 20% H2/N2 at 373 K for 1 h.
b
aMeasured by inductively coupled plasma atomic emission spectrometry (ICP-AES). The flow rate of 20%
c
H2/N2 and air is 40 mL min-1, and the heating rate of each treatment is 5 K min-1. Measured by transmission
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electron microscope (TEM).
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2.2. Catalyst Characterization. The compositions of catalysts were measured by
inductively coupled plasma atomic emission spectrometry (ICP-AES; Teledyne Leeman
Laboratories). X-ray diffraction (XRD) patterns for supported noble metal catalysts were
collected on a Rigaku D/MAX-2400 diffractometer within the 2θ range of 10°-80°, using Cu
Kα1 radiation (λ = 1.5406 Å), operated at 40 kV and 100 mA. The noble metal particle sizes
of the catalysts were measured by transmission electron microscope (TEM). TEM images
were obtained at an acceleration voltage of 300 kV on Philips Tecnai F30 FEGTEM.
Dispersions of Pt particles were calculated by a simplified formula assuming spherical
particles and a Pt atom density of 1.25 × 1019 atoms m-2,30 which is given by
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Pt dispersion = 1.13/Pt particle size
(1)
CO in-situ diffuse reflectance infrared Fourier transform spectra (CO-DRIFTS) were
collected by averaging 100 scans in the 650-4000 cm−1 range with a 2 cm−1 resolution on a
Bruker Tensor-27 Fourier infrared spectrometer equipped with a Hg-Cd-Te (MCT) detector.
Fresh samples (~0.05 g) were treated in an in-situ diffuse reflectance cell (Harrick Scientific
Products Inc.) in flowing He (20 mL min-1) at 573 K for 0.5 h to eliminate absorbed H2O and
other impurities on the surface by the unavoidable exposure to ambient air, and cooled to 298
K for background spectra collection. The treated samples were exposed in flowing 5%
CO/He (20 mL min-1) at room temperature for 0.5 h, and then purged by a He flow (20 mL
min-1) for 0.5 h to remove physically absorbed CO before CO-DRIFTS measurements at 298
K.
2.3. Hydrogenolysis reactions of 2-furancarboxylic acid (FCA). Catalytic reactions
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were carried out in a 40 mL Teflon-lined stainless steel autoclave with vigorous stirring at a
speed of 700 rpm. A certain amount of FCA (A.R., J&K) and fresh catalyst were added into
the autoclave and diluted with 10 mL deionized water. The autoclave was pressurized with
H2 (>99.99%, Beijing Huayuan) after fully purging out the air, and heated to the reaction
temperatures (313-373 K) monitored by a thermocouple inserted in the autoclave. The
concentrations of FCA and hydrogenolysis products were quantified by high-performance
liquid chromatography (HPLC; Shimadzu LC-20A) using a RID detector and an Alltech
OA-1000 organic acid column (0.001 mol L-1 H2SO4 mobile phase, 0.6 mL min-1 flow rate,
and 323 K oven temperature). Turnover rates (molFCA (molsurface-Pt ∙ h)−1) were reported as
molar FCA conversion rates per mole of exposed Pt atoms (estimated from the TEM images
as described in Section 2.2), and product selectivities were reported on a carbon basis. For
recycling tests, the used catalysts were fully washed with deionized H2O and acetone, treated
in flowing air (40 mL min-1) at 573 K (5 K min-1) for 3 h after drying in a vacuum oven at
ambient temperature, and then directly used in the next cycle.
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2.4. Computational Methods. Theoretical treatments of the elementary steps involved in
FCA hydrogenolysis on Pt(111) and Pt(211) surfaces were carried out using periodic
plane-wave density functional theory (DFT) methods31,32 with the Perdew-Burke-Ernzerhof
(PBE) exchange-correlation functional33 as implemented in the Vienna ab initio simulation
package (VASP). The 5d96s1, 2s22p4, 2s22p2, and 1s1 electrons were treated explicitly for Pt,
O, C and H atoms; core electrons were treated using projector augmented-wave (PAW)
pseudo-potentials with an energy cutoff of 400 eV.34 The Monkhorst-Pack sampling method35
was used to generate the k-mesh for integration of the first Brillouin zone (i.e. 3×3×1 for
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Pt(111) and Pt(211) surfaces). The energy between successive self-consistent iterations was
converged to 1×10-4 eV, while the structures were optimized until forces were below 0.04 eV
Å-1.
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Nudged elastic band (NEB)36 and dimer37 methods were used to optimize transition state
(TS) structure and energy for each elementary step. The starting point for each TS structure
was obtained by NEB method with 8 images along the reaction coordinate. The electronic
structures were converged self-consistently to energies of 1×10-3 eV using a single Γ-centered
k-point, and the corresponding maximum force on each atom converged to 0.1 eV Å-1. The
starting TS structure was then optimized by the dimer method, using a 3×3×1
Monkhorst-Pack k-point mesh and more stringent convergence criteria for electronic energies
and forces (1×10-4 eV and 0.04 eV Å-1, respectively).
The experimental lattice constants (3.92 Å) of crystalline Pt38 were used to build Pt(111)
and Pt(211) surfaces. The Pt(111) and Pt(211) surfaces were modeled by 4 atomic-layer
slabs, which were repeated in 3×3 and 1×4 surface supercells, respectively. During all
calculations, the bottom two layers of Pt(111) and three layers of Pt(211) were fixed to the
bulk positions, while the rest layers were allowed to fully relax. To avoid interactions
between slabs, all slabs were separated by a vacuum gap of 20 Å.
The adsorption energy (Eads) of an adsorbate to the surface slabs was calculated as
follows:
Eads = Eslab+i - Eslab - Ei
(2)
where Eslab+i, Eslab, and Ei are the DFT-derived electronic energies of the adsorption complex,
clean surface slab and gas-phase adsorbate i, respectively. The reaction activation barriers
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(Ea) were calculated as follows:
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Ea = ETS – EIS
(3)
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where ETS and EIS were the DFT-derived electronic energies of the transition state and the
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initial state of a given elementary reaction, respectively.
3. RESULTS AND DISCUSSION
3.1. Activity and Selectivity of Aqueous-Phase 2-Furancarboxylic Acid
Hydrogenolysis on Supported Metal Catalysts. TiO2-supported Pt, Ru, Pd and Ir catalysts
with similar metal particle sizes of ~2 nm (TEM micrographs shown in Fig. 1a-d) were
prepared by a deposition precipitation method, and used to examine the intrinsic selectivity of
FCA hydrogenolysis on different noble metal catalysts. As shown in Table 2, on Pt/TiO2,
FCA hydrogenolysis formed 5-HVA and THFCA with selectivities of 59.3% and 38.0% at
~26% conversion, respectively, at 313 K and 3 MPa H2. Their selectivities kept almost
constant within the whole range of FCA conversions (15-100%, Fig. S1a, SI), indicating that
both 5-HVA and THFCA are the primary products. We thus surmise that 5-HVA was formed
from direct hydrogenolysis of the α-C-O bond in FCA, instead of secondary C-O
hydrogenolysis of THFCA after the furan ring in FCA was completely saturated (Scheme 1).
The THFCA hydrogenolysis pathway for 5-HVA formation appears to be kinetically
unfavorable because of the low ring-opening reactivity of THFCA, most likely due to the
weak adsorption of THFCA on the Pt surface through its tetrahydrofuran ring. It is worth
noting that no ring-opening products derived from hydrogenolysis of the δ-C-O bond in FCA
(e.g. 2-hydoxyvaleric acid) were detected, reflecting the preferential cleavage of the α-C-O
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bond in FCA conferred by the strong electron-withdrawing effect of the carboxylic group
attached to the α-C atom. Such hydrogenolysis selectivities for the furan-ring C-O bonds on
Pt catalysts were completely different when the carboxylic substituent was replaced by an
electron-denoting hydroxy methyl group (i.e. furfural alcohol as reactant),13 as discussed in
the introduction section, and also observed in our control experiments that showed similar
selectivities for the cleavage of α-C-O and δ-C-O bonds on the Pt catalysts (313 K, 3 MPa
H2; Fig. S2, SI).
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(a)
(c)
(e)
(b)
(d)
(f)
2.0±0.3 nm
1.4±0.2 nm
2.0±0.3 nm
1.5±0.4 nm
1.7±0.3 nm
2.6±0.4 nm
Figure 1. Transmission electron microscopy (TEM) micrographs and histograms of metal
particle size distribution for (a) 2% Pt/TiO2, (b) 2% Ru/TiO2, (c) 2% Pd/TiO2, (d) 2% Ir/TiO2,
(e) 2% Pt/ZrO2, (f) 2% Pt/SiO2-2.6nm (prepared by the protocol described in Section 2.1)
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O
+H2
HO
OH
5
6
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HO
OH
+H2
pening
1,5-pentanediol
(1,5-PeD)
5-hydroxyvaleric acid
(5-HVA)
O
O
ring-o
9
OH
+H
2
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ring-sa
2-furancarboxylic acid
(FCA)
O
O
OH
O
+H2
t
u
ration
OH
tetrahydrofuran-2-carboxylic acid
(THFCA)
Tetrahydrofurfuryl alcohol
(THFA)
Scheme 1. Ring-opening and ring-saturation pathways for FCA hydrogenolysis on Pt
catalysts.
Table 2. Selectivities of FCA hydrogenolysis on Pt, Ru, Pd, and Ir-based catalysts.
Selectivity (on a carbon basis, %)
Particle size
(nm)
Conversion
(%)
Catalyst
5-HVAc
59.3
1,5-PeDd
THFCAe
THFAf
n.d.
2%Pt/TiO2
2.0±0.3
25.7a
96.6b
91.9b
100b
n.d.
38.0
57.3
10.4
n.d.
n.d.
3.6
0.4
39.2
87.9
99.9
97.5
n.d.
1.3
2%Ru/TiO2
2%Pd/TiO2
2%Ir/TiO2
1.5±0.4
1.4±0.2
1.7±0.3
n.d.
n.d.
n.d.
n.d.
95.3b
Reaction conditions: a 0.5 g FCA in 10 mL water, 0.1 g catalyst, 313 K, 3 MPa H2, 3 h; b 0.5
c
g FCA in 10 mL water, 0.2 g catalyst, 373 K, 4 MPa H2, 10 h; 5-HVA = 5-hydroxyvaleric
d
e
f
acid; 1,5-PeD = 1,5-pentanediol; THFCA = tetrahydrofuran-2-carboxylic acid; THFA =
tetrahydrofurfuryl alcohol.
Even at 373 K and nearly complete FCA conversion (96.6%), the selectivities to 5-HVA
and THFCA on Pt/TiO2 remained essentially unchanged (at 57.3% and 39.2%, respectively;
Table 2), although a small amount of 1,5-PeD (3.6% selectivity) was formed via secondary
hydrogenation of 5-HVA (Scheme 1). In contrast, the main product of FCA hydrogenolysis
on Ru/TiO2 was THFCA (87.9% selectivity), and the 5-HVA selectivity decreased to 10.4%
(Table 2). On Pd/TiO2 and Ir/TiO2, THFCA was almost exclusively formed from FCA
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reaction (selectivity > 97%). The different selectivity on these catalysts with similar metal
particle sizes reflects that the relative abilities for catalyzing C-O bond cleavage and C=C
bond hydrogenation are sensitive to the nature of metal sites. Such selectivity dependence
appears to be relevant to the oxygen affinity of the metal sites that determines the
accessibility of the furan-ring O atom in the hydrogenolysis process as suggested from the
following theoretical calculations shown in Section 3.4. Only on Pt surface, which has a
lower oxygen affinity than the other VIII group metals,39 the C-O bond cleavage is preferred
over the C=C bond hydrogenation, exhibiting the uniquely high selectivity to 5-HVA in FCA
hydrogenolysis (Table 2).
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Figure 2 compares reaction rates and selectivities of FCA hydrogenolysis on TiO2, ZrO2
and SiO2-supported Pt particles of ~2 nm (TEM micrographs shown in Fig. 1a, e and f)
within the kinetically controlled regime (at ~20% conversion). The three oxide supports are
resistant to acid corrosion, making them suitable for FCA hydrogenolysis. Similar to Pt/TiO2,
FCA hydrogenolysis on Pt/ZrO2 and Pt/SiO2 showed combined selectivities of 5-HVA and
THFCA above 96% at 313 K and 3 MPa H2, indicating that the hydrogenolysis pathways of
FCA on Pt surfaces are not significantly affected by the oxide supports. The FCA reaction
rates normalized by the exposed surface Pt atoms (i.e. turnover rates) decreased in an order of
Pt/TiO2 (322.0 molFCA (molsurface-Pt∙h)−1), Pt/ZrO2 (168.2 molFCA (molsurface-Pt∙h)−1) and Pt/SiO2
(108.0 molFCA (molsurface-Pt∙h)−1), whereas the corresponding selectivities to 5-HVA on these
catalysts were 59.3%, 65.7% and 73.3% (Fig. 2), which indicates that selective formation of
5-HVA requires moderate hydrogenolysis activity. These support effects on catalytic activity
and selectivity are ascribable to the change of exposed Pt surfaces determined by the
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interaction between Pt particles and oxide support, as evidenced from spectroscopic
characterization and theoretical calculations shown below in Sections 3.2 and 3.4,
respectively.
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Figure 2. Turnover rates and selectivities of FCA hydrogenolysis on Pt particles supported
by TiO2, ZrO2, and SiO2 (0.5 g FCA in 10 mL water, 313 K, 3 MPa H2, 3 h, ~20 % FCA
conversion obtained by varying catalyst amount).
The reaction rates and selectivities of FCA hydrogenolysis are also tunable by changing
the size of Pt particles. As depicted in Figure 3, the turnover rates of FCA conversion on
Pt/SiO2 decreased gradually from 108.0 to 99.6 molFCA (molsurface-Pt∙h)−1 at 313 K and 3 MPa
H2 when the average size of Pt particles increased from 2.6 to 7.5 nm (controlled by varying
the Pt loading and treatment condition as described in Section 2.1; TEM micrographs shown
in Fig. S3, SI). In concomitance with this activity change, the selectivity to 5-HVA increased
from 73.3% to 80.0% with increasing the Pt particle size in the examined range, whereas the
selectivity to THFCA declined from 23.7% to 17.5%. Such size effects are consistent with
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the higher 5-HVA selectivities observed on less active Pt sites (Fig. 2), and thus imply that
5-HVA is favorably formed on lower-index Pt surfaces that generally prevail for larger Pt
particles.
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Figure 3. Effects of Pt particle size on turnover rates and selectivities in FCA hydrogenolysis
on Pt/SiO2 (0.5 g FCA in 10 mL water, 313 K, 3 MPa H2, 3 h, ~20 % FCA conversion
obtained by varying catalyst amount).
The above results show that SiO2-supported Pt particles of moderate sizes (e.g. ~4 nm)
are more appropriate for catalyzing FCA hydrogenolysis to produce 5-HVA than other Pt
particle sizes or oxide supports, because of the balance between the catalytic activity of FCA
hydrogenolysis and the selectivity to 5-HVA. The Pt/SiO2 catalyst with an average Pt particle
size of 4.2 nm remained a high 5-HVA selectivity of 78.1% at ~100% FCA conversion (313
K, 3 MPa H2; Fig. S1b), reflecting its intrinsically high selectivity to 5-HVA. Moreover, both
turnover rate and 5-HVA selectivity of this Pt/SiO2 sample kept almost unchanged after four
consecutive cycles of FCA hydrogenolysis within the kinetically controlled regime (Fig. 4).
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No leaching of Pt was detectable in the reaction solution after recycling (by ICP-AES), and
the average size of the Pt particles in the spent Pt/SiO2 catalyst increased slightly to 5.4 nm
(TEM micrographs shown in Fig. S4, SI; XRD patterns shown in Fig. S5, SI), consistent with
the high stability of Pt/SiO2 during FCA hydrogenolysis.
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Figure 4. Turnover rates and selectivities to 5-HVA for four consecutive cycles of FCA
hydrogenolysis on Pt/SiO2-4.2nm (0.5 g FCA in 10 mL water, 0.2 g Pt/SiO2-4.2nm, 313 K, 3
MPa H2, 3 h).
3.2. Infrared Spectroscopic Assessment of Exposed Pt Sites via CO Adsorption. CO
adsorption was used here to discern the Pt sites exposed on the oxide supports, because the
vibrational frequency of CO is sensitive to the chemical state of bound metal sites.40 The
stretching bands of adsorbed CO appeared in the range of 2000-2150 cm-1 for the Pt/TiO2,
Pt/ZrO2 and Pt/SiO2 samples at ambient temperature (Fig. 5a), which are assigned to linearly
atop adsorption modes at Pt surface sites.40 On Pt/TiO2, adsorbed CO molecules showed
vibrational bands at 2037, 2063, 2086 and 2115 cm-1, indicative of different extents of
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back-donation of electrons from the Pt surface sites to the CO 2π* orbitals. The lower the CO
band frequency is, the higher d-band electron density the exposed Pt sites have. Specifically,
the three bands between 2037-2086 cm-1 are ascribed to CO adsorption on the metallic Pt
sites with different coordination numbers (e.g. those in the terraces, steps, and edges of the
exposed Pt surfaces),41 while the band at 2115 cm-1 is ascribed to the CO adsorption on the
cationic Ptδ+ sites that are strongly bound to the lattice oxygen of Pt-TiO2 interfaces.42,43
Similar CO stretching bands were also observed for Pt/ZrO2 with slight blue shifts to 2041,
2065, 2090 and 2124 cm-1, respectively.
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(a)
(b)
Figure 5. CO-DRIFTS spectra of (a) Pt catalysts on different supports and (b) Pt/SiO2 with
different Pt particle sizes.
In contrast, the CO adsorption on Pt/SiO2 showed the main vibrational band at 2083 cm-1
with a little shoulder at 2071 cm-1 (Fig. 5a). The absence of CO bands above 2100 cm-1
reflects weak interaction between Pt and SiO2 in the Pt/SiO2 sample,44,45 whereas the low
integrated intensity of CO bands below 2070 cm-1 suggests that Pt/SiO2 lacks high-index Pt
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surfaces that possess abundant step and corner sites of high d-band electron densities.41,
46-48Consistently, the main CO vibrational bands shifted from 2083 to 2091 cm-1 as the
average Pt particle size of Pt/SiO2 increased from 2.6 to 7.5 nm (Fig. 5b), and such increase
of the Pt particle size leads to higher portion of terrace sites that are predominant on
low-index surfaces.46,47,50
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These observed differences in the infrared spectra of CO adsorption for these supported
Pt catalysts imply that the low-index Pt surfaces contribute to the high selectivity of 5-HVA.
Next, kinetic and theoretical methods are combined to provide molecular-level insights into
this structural requirement of Pt catalysts for selective FCA hydrogenolysis.
3.3. Kinetic Assessment of FCA Hydrogenolysis on Pt catalysts. The effects of
reactant concentrations on FCA hydrogenolysis rate and selectivity were examined at 313 K
on the Pt/SiO2 (with Pt particle size of 4.2 nm, donated as Pt/SiO2-4.2nm) and Pt/TiO2 (with
Pt particle size of 2.0 nm, donated as Pt/TiO2-2.0nm) catalysts, representing the
aforementioned low-index and high-index Pt surfaces, respectively, in order to assess their
intrinsic difference in catalyzing FCA hydrogenolysis. As shown in Figure 6a, at similar FCA
conversions (~20%), the turnover rates on Pt/SiO2-4.2nm increased with increasing the
concentration of FCA, and then reached a plateau around 100 molFCA (molsurface-Pt∙h)−1 at
higher FCA concentrations of above 0.20 mol L-1, reflecting high coverages of FCA or its
derived species on the Pt surfaces during catalysis. In contrast, the turnover rates on
Pt/TiO2-2.0nm increased to a maximum value of 335.7 molFCA (molsurface-Pt∙h)−1 with
increasing the FCA concentration to 0.08 mol L-1, which then declined gradually as the FCA
concentration further increased. We surmise that the adsorption of FCA on the Pt/TiO2-2.0nm
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catalyst is so strong that high FCA concentration can hinder the adsorption and dissociation
of H2 on the same Pt surfaces required for the proceeding of FCA hydrogenolysis, which
accounts for the observed decrease of FCA hydrogenolysis rates at FCA concentrations above
0.08 mol L-1. The selectivities to 5-HVA and THFCA were essentially insensitive to the FCA
concentration in the examined range (0.039-0.634 mol L-1) for both Pt/SiO2 and Pt/TiO2
catalysts (Figs. S6a and S7a, SI), indicating that 5-HVA and THFCA are formed apparently
from identical FCA-derived intermediates adsorbed on Pt surfaces with an identical reaction
order.
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(a)
(b)
Figure 6. Effects of (a) FCA concentration and (b) H2 pressure on turnover rates in FCA
hydrogenolysis on Pt/SiO2-4.2nm and Pt/TiO2-2.0nm (313 K, 3 h, ~20 % FCA conversion
obtained by varying catalyst amount; (a) 3.0 MPa H2; (b) 0.2 g FCA in 10 mL water, i.e.,
0.198 mol L-1). Dashed curves represent regressed fits to the functional form of Equation 4.
For the kinetic effects of H2 pressure on the Pt/SiO2 and Pt/TiO2 catalysts (Fig. 6b),
increasing H2 pressure led to an increase of the turnover rates, which then became less
sensitive to the H2 pressures higher than ~1 MPa. This trend indicates that H atoms are
readily accumulated on the Pt surfaces under the reaction conditions. Similar to the effect of
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FCA concentrations, the selectivities to 5-HVA and THFCA kept nearly constant within a
large range of H2 pressures (1-5 MPa, Figs. S6b and S7b, SI), further supporting that the two
products are formed from the identical FCA-derived surface intermediates.
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By referring to the previous reports on hydrogenolysis reactions of furan derivatives,14,51
we propose that FCA hydrogenolysis on Pt surfaces proceeds involving the following
elementary steps (Scheme 2). First, FCA adsorbs molecularly on multiple Pt sites that is
closely related to the nature of the exposed Pt surface, while a H2 molecule dissociates on two
Pt sites to form H atoms. The H atoms then attack the C=C bonds of the adsorbed FCA,
yielding partially hydrogenated intermediates (FHn(α*); n is the number of added H atoms
and α is the number of Pt surface atoms to which each intermediate binds). These FHn(α*)
intermediates can be further hydrogenated to form THFCA via full saturation of the furan
ring or be hydrogenolyzed to form 5-HVA via selectively breaking the C-O bond of the furan
ring.
Scheme 2. Proposed sequence of reaction steps for FCA hydrogenolysis on metal surface.a
FCA adsorption
H2 dissociation
FCA + m* ⇌ FCA(m*)
H2 + 2* ⇌ 2H(*)
Partial hydrogenation
FCA(m*) + nH(*) →FHn(α*) + (m+n-α)*
FHn(α*) + H(*) → FHn+1(β*) + (α+1-β)*
FHn+1(β*) + (3-n)H(*) → THFCA + (β+4-n)*
FHn(α*) + H(*) → FHn+1-ro(β’*) + (α+1-β’)*
FHn+1-ro(β’*) + (5-n)H* → 5-HVA + (β’+5-n)*
Ring-saturation
Ring-opening
a
m represents the average bound Pt atoms per FCA for its adsorption on a given Pt catalyst.
FHn represents the partially hydrogenated intermediates, where n is the number of added H
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atoms. α or β is the number of Pt surface atoms that each intermediate binds to; FHn-ro
5
6
represents the ring-opening intermediate via breaking the C-O bond of the furan ring.
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On the assumptions that the addition of the first H atom to the furan ring is the kinetically
relevant step14,51 and the bound FCA and H atom are the prevailing surface species, from the
proposed elementary steps, we derived an expression of the FCA hydrogenolysis rate
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equation as
1
[ ]2
푘푎푝푝[FCA] H2
Rate =
(4)
1 + m
1
1
1
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2[ ]2
(1 + 퐾FCAm[FCA]m + 퐾H2 H2
)
Here, kapp is the apparent kinetic constant for the H-addition of FCA catalyzed by Pt, KFCA
and KH2 are the respective equilibrium constants for FCA adsorption and H2 dissociation on
the Pt surfaces, and m represents the average bound Pt atoms per FCA for its adsorption on a
given Pt catalyst. The measured FCA hydrogenolysis rates obtained on the Pt/SiO2 and
Pt/TiO2 catalysts fitted well with Equation 4 (Fig.6 and Fig. S8, SI), supporting the kinetic
relevance of the addition of the first H atom to the bound FCA species. Furthermore, these
data suggest that alternate assumption of the subsequent addition of H atoms to the adsorbed
FCA species did not involve in the kinetically relevant steps of FCA hydrogenolysis, because
of poor regression fittings for such cases.
The regressed parameters of Equation 4 for the Pt/SiO2 and Pt/TiO2 catalysts were
compared in Table 3. The kapp value of Pt/TiO2 was about two times higher than that for
Pt/SiO2 (1.5×106 vs. 5.3×105 L mol-1 MPa-0.5 h-1), reflecting that the Pt sites on the high-index
surfaces are intrinsically more active than those on the low-index surfaces. Compared with
Pt/SiO2, such high-index Pt surfaces on Pt/TiO2 also led to a slight decrease of the average
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bound Pt atoms per FCA (m: 2.6 vs. 2.9) and to stronger adsorption of FCA (KFCA: 4.4 ×102
vs. 3.6 ×102 L mol-1), consistent with the presence of more step sites on the high-index
surfaces.52 On the other hand, the H2 dissociation constant (KH2) for Pt/TiO2 was smaller than
that for Pt/SiO2 (KH2: 0.41 vs. 0.78 MPa-1), which implies that the co-adsorption of strongly
bound FCA species weakens the stability of H atoms on the high-index Pt surfaces. In spite
of such differences in these adsorption constants between Pt/SiO2 and Pt/TiO2, their total
coverages of bound FCA and H species were always above 80% under the examined reaction
conditions, unveiling that FCA hydrogenolysis occurred on densely covered Pt surfaces.
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Table 3. Regressed kinetic parameters for FCA hydrogenolysis on Pt/SiO2-4.2nm and
Pt/TiO2-2.0nm catalysts at 313 K.
푘푎푝푝
(L mol-1 MPa-0.5 h-1)
5.3×105
퐾FCA
퐾퐻2
(MPa-1)
0.78
m
Catalyst
(L mol-1)
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4.4×102
Pt/SiO2-4.2nm
Pt/TiO2-2.0nm
2.9
2.6
1.5×106
0.41
Based on the above kinetic analysis, the addition of the first H atom to the furan ring
determines how fast FCA converts on the Pt surfaces, whereas the kinetic constants of
partially hydrogenated FCA intermediates for the branching furan-ring saturation and
opening steps determine the selectivities to form THFCA and 5-HVA products. It is worth
nothing that the ratios of these kinetic constants can be regarded as a descriptor for the
intrinsic selectivities of furan-ring saturation and opening on a given Pt catalyst, which are
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directly measurable from experiments and insensitive to the concentrations of FCA and H2.
Specifically, the ratios for Pt/SiO2 and Pt/TiO2 are 4.3 and 1.5, respectively (Figs. S6a and
S7a, SI), reflecting the higher 5-HVA selectivity on low-index Pt surfaces. However, these
ratios are unable to provide molecular details on the relevant surface intermediates, because
these H-addition events occur after the kinetic limiting step. Next, theoretical treatments were
applied to assess the elementary steps of FCA hydrogenolysis on Pt surfaces and the effects
of Pt facets on the hydrogenolysis activity and selectivity.
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3.4. Theoretical Assessment of FCA Hydrogenolysis on Pt Surfaces. Theoretical
simulations of FCA hydrogenolysis on Pt surfaces were carried out using Pt(111) and Pt(211)
planes in order to examine the effects of Pt facets on hydrogenolysis rates and selectivities.
Uniform Pt sites with a coordination number (CN) of 9 are exposed on the flat low-index
Pt(111) planes, whereas the high-index Pt(211) planes are corrugated surfaces consisting of
step (CN: 7), terrace (CN: 9), and corner (CN: 10) sites.53 Structural optimization of FCA
adsorption on the two Pt planes showed that parallel adsorption models via coordination of
furan ring to the Pt surface are most stable for both planes. Specifically, FCA prefers to bind
to three terrace sites of the Pt(111) surface, but to two step sites of the Pt(211) surface (Fig.
7). Calculated adsorption energies for FCA on Pt(111) and Pt(211) planes were -0.52 and
-1.37 eV (Fig. 8), respectively, reflecting stronger stability of adsorbed FCA on the
coordinately more unsaturated Pt sites. The corresponding adsorption energies for a H atom
on the vicinal Pt sites of the adsorbed FCA molecules were -0.22 eV for Pt(111) and -0.21 eV
for Pt(211) (Fig. 8; gaseous H2 as the energy reference), both of which are much lower than
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for FCA. The weaker stability of the adsorbed H atom on Pt(211) than on Pt(111) may be
attributed to steric hindrance of the neighboring FCA species that binds more strongly on
Pt(211). These stability differences for adsorbed FCA and H-atom on the Pt(111) and Pt(211)
planes agree well with those observed from the kinetic assessments on Pt/SiO2 and Pt/TiO2
(Section 3.3), indicating that the Pt(111) and Pt(211) planes are suitable models for studying
the effects of Pt facets on FCA hydrogenolysis.
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(a)
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Figure 7. Calculated structures of absorbed FCA on (a) Pt(111) surface and (b) Pt(211)
surface. (Atom legend: Pt, blue; C, gray; O, red; H, white; some specific atoms are labeled.)
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(b)
Figure 8. Calculated energy profiles for possible ring-saturation (black line) and
ring-opening (red line) pathways in FCA hydrogenolysis on (a) Pt(111) surface and (b)
Pt(211) surface.
FCA hydrogenolysis on the Pt(111) surface is initiated by the attack of a H atom to the
adsorbed FCA species. This H atom can either be added to one of the four C atoms in the
furan ring (C2-C5, labeled in Fig. 7) for ring saturation or the furan-ring O atom for C-O
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bond cleavage. DFT-derived changes of reaction energies and activation barriers show that
the first H-addition prefers the C4 position both kinetically and thermodynamically (see
details in Table S1, SI). On the other hand, the transition state (denoted as TS1 in Fig. 8a) for
the H-addition at the C4 position has significantly higher energy (0.05 eV, referred to a bare
Pt surface and gaseous FCA and H2 reactants) than those involved in the following FCA
hydrogenation steps (TS2, TS3, and TS4 in Fig. 8a) as discussed below, indicative of kinetic
relevance of this step on the Pt(111) surface.
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Subsequent reaction of the surface species formed from the H-addition of FCA at the C4
position (denoted as FH1*) with a second H atom has several possible products. DFT
calculations show that the attacks of the second H atom at the other three C positions in the
furan ring have similar activation barriers (Ea: 0.94-0.99 eV, with respect to FH1* and H*;
Table S2, SI), in which the C5 position (through TS2 in Fig, 8a; DFT-structure shown in Fig.
9a) is slightly preferred over the other two C positions, consistent with the relatively higher
stability of the H-addition product at the C5 position (i.e. 4,5-dihydrofuran-2-carboxylic acid,
denoted here as FH2*). In contrast, the attacks at the O position of the furan ring lead to
cleavages of the O1-C2 or O1-O5 bonds with activation barriers of 0.76 eV (TS-ro1 in Fig,
8a; DFT-structure shown in Fig. 9a) and 0.83 eV (details in Table S2, SI), respectively. The
difference between these activation barriers for the O-C bond cleavage is attributed to the
electron-withdrawing effect of the carboxylic group attached to the C2 position, reflecting
that FCA is an inherently selective precursor for the formation of 5-HVA via hydrogenolysis.
Moreover, these ring-opening activation barriers are lower than those for the ring saturation
channels of FH1*, which indicates that C-O bond cleavage is kinetically favorable compared
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to the ring-saturation on the Pt(111) surface, consistent with the experimental observations
(Section 3.1).
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DFT-derived activation barriers for the H-addition of FH2* show that the C2 position
(through TS3 in Fig, 8a; DFT-structure shown in Fig. 9a) is preferred over the C3 position in
accepting the third H atom for ring-saturation (Ea: 0.75 vs. 1.02 eV; Table S3, SI), while the
competitive H addition to the O1 position of FH2* mostly leads to cleavage of the O1-C2
bond with an activation barrier of 0.76 eV (TS-ro2 in Fig, 8a; DFT-structure shown in Fig.
9a). The similar activation barriers for the ring-saturation and ring-opening pathways of FH2*
imply that FH2* would contribute equally to the selectivities of the THFCA and 5-HVA
products. On the other hand, the product of H addition to the C2 position of FH2* (denoted as
FH3*) can further react with the fourth H atom at the C3 position to form THFCA (Ea: 0.99
eV). However, this intermediate has to overcome a much higher barrier for the H attack at the
O1 position, because of the large distance of the O1 atom in FH3* from the Pt(111) surface
(0.39 nm, Fig. S11, SI). Taken together, FH2* and FH3* are excluded as the key
intermediates that determine the selectivities of THFCA and 5-HVA on Pt(111) surface,
which in turn suggests the preference of O1-C2 bond cleavage in FH1* hydrogenation
accounts for the high selectivity of 5-HVA on the low-index Pt(111) surface.
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DFT-derived ring-saturation and ring-opening pathways for FCA hydrogenation on
Pt(211) surface (Fig. 8b) are similar to those found for Pt(111). Specifically, the first H atom
prefers to attack the C3 and C4 positions with similar activation energies of 0.64 eV (Table
S4, SI). As for the Pt(111) surface, the first H-addition step appears to be the kinetically
relevant step for FCA hydrogenation on Pt(211), as evidenced from the relatively higher
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energy of the transition state (TS1’) of this step compared to those involved in the subsequent
H-addition steps (TS2’, TS3’ and TS4’ in Fig. 8b). However, the transition state (TS1’ in Fig.
8b) of the H-addition on Pt(211) is more stable than that (TS1 in Fig.8a) on Pt(111) by 0.99
eV (referred to the corresponding bare Pt surface and gaseous FCA and H2 reactants),
reflecting the stronger bonding of such transition states to the step sites exposed on the
Pt(211) surface. This stability difference of the kinetically-relevant H-addition transition
states on Pt surfaces, mainly ascribed to the difference of the electronic density between the
step and terrace sites, is consistent with the higher FCA hydrogenolysis activity measured on
the higher-index Pt surfaces (Section 3.3).
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DFT calculations suggest that the preferential sequence of the H addition positions for
ring saturation of FCA on the stepped Pt(211) surface follows C4→C3→C5→C2 (detailed
results shown in Table S4-S6, SI), whereas the corresponding sequence on the flat Pt(111)
surface is C4→C5→C2→C3 as described above. Here, FHn’* is denoted as the
corresponding product formed after the addition of the nth H atom on the Pt(211) surface.
Analogous to the branching ring-saturation and ring-opening pathways on Pt(111), FH1’* and
FH2’* are the reaction intermediates relevant to the selectivities of THFCA and 5-HVA. As
shown in Figure 8b, the activation barriers for ring-saturation and ring-opening of FH1’* are
0.51 and 1.34 eV (TS2’ vs. TS-ro1’ in Fig. 8b; DFT-structure shown in Fig. 9b), respectively,
implying the preference of FH1’* to form THFCA. In contrast, the two activation barriers for
FH2’* is inverted (TS3’: 0.93 eV vs. TS-ro2’: 0.76 eV, Fig. 8b; DFT-structure shown in Fig.
9b), which results in its ring-opening and the favored formation of 5-HVA over THFCA on
Pt(211).
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As demonstrated above, the FH1* and FH2* intermediates contribute to the formation of
5-HVA from FCA hydrogenolysis on Pt(111), whereas FH2’* is nearly the sole intermediate
that leads to the formation of 5-HVA on Pt(211). Therefore, the Pt(111) surface is more
selective to 5-HVA than the stepped Pt(211) surface, although the activation barrier
difference between the ring-saturation and ring-opening pathways for FH1* on Pt(111) and
FH2’* on Pt(211) is similar (0.18 vs. 0.17 eV, Fig. 8). These DFT assessments are in good
agreement with the experimental results, and also suggest that the high activation barrier for
the C-O bond cleavage in FH1’* hydrogenation accounts for the lower selectivity of 5-HVA
on Pt(211) compared to Pt(111). DFT treatments reveal that such hydrogenative ring-opening
reactions involve late transition states, in which the bond between the attacking H atom and
the O1 atom of the furan ring is nearly fully formed and the O1-C2 bond of the furan ring is
partially cleaved (TS-ro1, TS-ro2, TS-ro1’ and TS-ro2’ in Fig. 9). Accordingly, the
stabilization of the dangling C2 atom by the surface Pt atoms at the transition state is critical
to the activation barrier. On Pt(211) surfaces, the dangling C2 atom has to share a step Pt site
with the unsaturated C3 atom of FH1’*, because the neighboring Pt atoms of this step site are
too far to form effective bonding with the C2 atom (distance > 0.32 nm, TS-ro1’ in Fig. 9b).
In contrast, the dangling C2 atom and the unsaturated C3 atom of FH1* at the ring-opening
transition state (TS-ro1 in Fig. 9a) can be stabilized by separate terrace Pt atoms on Pt(111)
surfaces, benefiting from the flat nature of the low-index surface. We surmise that such
coordination difference between the transition state and the Pt surface results in the different
C-O cleavage activation barriers on the Pt(111) and Pt(211) surfaces, which finally leads to
the observed higher 5-HVA selectivity on Pt(111). The above theoretical calculations thus
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indicate the catalytic activity and selectivity of FCA hydrogenolysis on Pt surfaces are
determined by the electronic and geometric factors of the Pt surface sites, respectively,
providing guidance to the rational design of novel catalysts for selective hydrogenolysis of
FCA and its derivatives.
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(a)
(b)
Figure 9. Calculated structures of transition states (a) TS2, TS3, TS_ro1, and TS_ro2 on
Pt(111) surface and (b) TS2’, TS3’, TS-ro1’, and TS-ro2’ on Pt(211) surface (Atom legend:
Pt, blue; C, gray; O, red; H, white).
4. CONCLUSIONS
The α-C-O bond of the furan ring is selectively cleaved in FCA hydrogenolysis to form
5-HVA in a high yield (~78%) on supported Pt catalysts at near-ambient temperature (i.e. 313
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