Selective hydrogenation of biomass-based 5-hydroxymethylfurfural over catalyst of palladium immobilized on amine-functionalized metal-organic frameworks

Article abstract of DOI:10.1021/cs5012926

A catalyst of palladium [Pd/MIL-101(Al)-NH₂] supported on amine-functionalized Metal-Organic Frameworks (MOFs) allows selective hydrogenation of biomass-based 5-hydroxymethylfurfural (HMF) to 2,5-dihydroxymethyl-tetrahydrofuran (DHMTHF) with 2,5-dihydroxymethylfuran (DHMF) as an observed "intermediate". The Pd/MIL-101(Al)-NH₂ was prepared by using a direct anionic exchange approach and subsequent gentle reduction. The presence of free amine moieties in the frameworks of MIL-101(Al)-NH₂ is suggested to play a key role on the formation of uniform and well-dispersed palladium nanoparticles on the support. The adsorption experiments reveal that the amine-functionalized MOF supports show preferential adsorption to hydrogenation intermediate DHMF than in the case of reactant HMF owing to an enhanced hydrophilic nature of DHMF as well as improved hydrogen bonding interactions between DHMF and the MOF support, which promotes a further hydrogenation of DHMF to DHMTHF upon the in situ formation of DHMF over Pd/MIL-101(Al)-NH₂. Moreover, our results also indicate that the observed high selectivity toward DHMTHF form HMF is closely related to the cooperation between the metallic site and the free amine moiety on the MOF support. Under the optimal conditions, a maximum DHMTHF yield of 96% with a full conversion of HMF is obtained by using Pd/MIL-101(Al)-NH₂ (Pd 3.0 wt %) catalyst at a low reaction temperature of 30 °C in aqueous medium. The research thus highlights new perspectives for aluminum-based and amine-functionalized MOF material for biomass transformation.

Full text of DOI:10.1021/cs5012926

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Article
Selective Hydrogenation of Biomass Based 5-
Hydroxymethylfurfural over Catalyst of Palladium Immobilized
on Amine-Functionalized Metal–Organic Frameworks
Jinzhu Chen, Ruliang Liu, Yuanyuan Guo, Limin Chen, and Hui Gao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs5012926 • Publication Date (Web): 16 Dec 2014
Downloaded from http://pubs.acs.org on December 20, 2014
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Selective Hydrogenation of Biomass Based 5-Hydroxymethylfurfural
over Catalyst of Palladium Immobilized on Amine-Functionalized
Metal–Organic Frameworks
Jinzhu Chen,*,† Ruliang Liu,†,§ Yuanyuan Guo,† Limin Chen‡ and Hui Gao†
†CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese
Academy of Sciences, Guangzhou 510640, P.R. China
‡College of Environment and Energy, South China University of Technology, Guangzhou 510006,
P.R. China
§University of Chinese Academy of Sciences, Beijing 100049, P.R. China
*Corresponding author. Tel./Fax: (+86)ꢀ20ꢀ3722ꢀ3380. Eꢀmail address: chenjz@ms.giec.ac.cn (J.
Chen)
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ABSTRACT:
A catalyst of palladium [Pd/MILꢀ101(Al)ꢀNH2] supported on amineꢀfunctionalized Metal–
Organic Frameworks (MOFs) allows selective hydrogenation of biomassꢀbased
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5ꢀhydroxymethylfurfural (HMF) to 2,5ꢀdihydroxymethylꢀtetrahydrofuran (DHMTHF) with
2,5ꢀdihydroxymethylfuran (DHMF) as an observed “intermediate”. The Pd/MILꢀ101(Al)ꢀNH2 was
prepared by using a direct anionic exchange approach and subsequent gentle reduction. The presence
of free amine moieties in the frameworks of MILꢀ101(Al)ꢀNH2 is suggested to play a key role on the
formation of uniform and wellꢀdispersed palladium nanoparticles on the support. The adsorption
experiments reveal that the amineꢀfunctionalized MOF supports show preferential adsorption to
hydrogenation intermediate DHMF than in the case of reactant HMF owing to an enhanced
hydrophilic nature of DHMF as well as improved hydrogen bonding interactions between DHMF
and the MOF support, which promotes a further hydrogenation of DHMF to DHMTHF upon the in
situ formation of DHMF over Pd/MILꢀ101(Al)ꢀNH2. Moreover, our results also indicate that the
observed high selectivity towards DHMTHF form HMF is closely related to the cooperation between
metallic site and free amine moiety on the MOF support. Under the optimal conditions, a maximum
DHMTHF yield of 96% with a full conversion of HMF is obtained by using Pd/MILꢀ101(Al)ꢀNH2
(Pd 3.0 wt%) catalyst at a low reaction temperature of 30 °C in aqueous medium. The research thus
highlights new perspectives for aluminumꢀbased and amineꢀfunctionalized MOF material for
biomass transformation.
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KEYWORDS:
Biomass, Hydrogenation, 5ꢀHydroxymethylfurfural, Palladium, Metal–Organic Frameworks
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INTRODUCTION:
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Recently, the productions of fuels and chemicals from renewable biomass as an alternative to
petroleumꢀbased productions has attracted much interest, owning to diminishing fossil resources and
growing concerns of environment degradation.1ꢀ6 Currently, 5ꢀhydroxymethylfurfural (HMF),
obtained from the dehydration of sugars, is regard as one of the most important biomassꢀderived
platform compounds.7ꢀ8 Recently, the hydrogenation of HMF has been investigated extensively with
the conventional catalyst.9ꢀ16 Depending on the catalyst system, the hydrogenated products can be
2,5ꢀdihydroxymethylfuran (DHMF), 5ꢀhydroxymethylꢀ2,3,4,5ꢀtetrahydroꢀ2ꢀfuraldehyde (HMTF),
2,5ꢀdihydroxymethylꢀtetrahydrofuran (DHMTHF), and ringꢀopened products such as
1,2,6ꢀhexanetriol (1,2,6ꢀHT), 1,2,5ꢀhexanetriol (1,2,5ꢀHT), and 1,2,5,6ꢀhexanetetrol (1,2,5,6ꢀHT).
Among these hydrogenation products, DHMTHF is a typical useful chemical with wild applications
as a solvent and monomer.17,18 For example, a mixed solvent system of DHMTHF and water was
recently developed as a sustainable solvent system for a tandem catalytic approach to continuous
production of DHMTHF from fructose.17 In addition, DHMTHF is an important precursor to the
production of 1,6ꢀhexanediol which is a valuable chemical in the manufacture of polymer
plastics.11,19ꢀ24
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In the literature search for the selective hydrogenation of HMF to DHMTHF, Schiavo et al.
obtained DHMTHF with high selectivities (80–100%) by using Ni, Cu, Pt, Pd, and Ru based
heterogeneous catalysts.10 Tomishige and coꢀworkers reported a total hydrogenation of HMF to
DHMTHF with the selectivity of 96% by using Ni–Pd bimetallic catalysts.11 It is suggested that the
HMFꢀtoꢀDHMTHF transformation proceeds either by the hydrogenation of aldehyde first (forming
DHMF), or by the hydrogenation of ring first (yielding HMTF). Dumesic et al. studied the selective
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hydrogenation of HMF to DHMTHF over supported Ru, Pd, and Pt catalysts in monophasic and
biphasic reactor systems.12 The highest yields (88–91%) to DHMTHF are achieved by using Ru
supported on the materials with high isoelectric points; moreover, the selectivity to DHMTHF is
affected by the acidity of the aqueous solution. Xu and coꢀworkers investigated a direct conversion
of fructose into DHMTHF by using a combination of acid and hydrophobic Ru/SiO2 in a
water/cyclohexane biphasic system.13 Notably, harsh conditions such as high hydrogen pressure,
high reaction temperature, high catalyst loading, and acidic reaction medium were usually required
for the selective conversion of HMF to DHMTHF.9 Therefore, the development of greener process
with the attainment of high DHMTHF selectivity under environmentally benign conditions is still a
great challenge. Obviously, the development of efficient catalyst plays a key role in this important
biomassꢀrelated HMF transformation.
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Recently, the biomass valorization with Metal–Organic Frameworks (MOFs) as catalysts was
investigated owning to their high surface area, tunable pore sizes, controllable structures, and easy
functionalization through postsynthetic modification (PSM).25ꢀ32 For instance, the phosphotungstic
acid (PTA)ꢀencapsulated MILꢀ101(Cr) [PTA/MILꢀ101(Cr)] was reported as a solid acid catalyst for
fructose dehydration to HMF.33 The sulfonic acid functionalized MILꢀ101(Cr) [MILꢀ101(Cr)ꢀSO3H]
was investigated as a solid acid for cellulose hydrolysis.34 In addition, we recently systematically
studied fructoseꢀtoꢀHMF transformation over sulfonic acid functionalized MOFs including
MILꢀ101(Cr)ꢀSO3H, UIOꢀ66(Zr)ꢀSO3H, and MILꢀ53(Al)ꢀSO3H.35 Later, a series of amine
functionalized MOF materials were demonstrated by us as solid bases for a liquid phase
transesterification of triglycerides and methanol.36 Very recently, the acidꢀmetal bifunctional catalyst
of RuꢀPTA/MILꢀ100(Cr) was synthesized by us to achieve a oneꢀpot conversion of cellobiose and
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cellulose into sorbitol.37
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The above developed MOFꢀbased catalysts are, however, mainly based on PSM of known
MOFs. The development of a more general and facile method to obtain a catalyst by supporting
transition metal nanoparticles on MOFs remains a big challenging, due to the easy agglomeration
and leach of these nanoparticles without protecting groups on the frameworks of MOF support.38,39
Therefore, the encapsulation of highly dispersed noble metal nanoparticles such as palladium and
platinum on the amineꢀfunctionalized MOFs were recently developed for catalysis purpose. For
example, Chang and Férey reported Heck reaction by using Pd/APSꢀMILꢀ101(Cr) and
Pd/EDꢀMILꢀ101(Cr) (APS and ED indicate 3ꢀaminopropyltrialkoxysilane and ethylenediamine,
respectively).40,41 Suzuki–Miyaura crossꢀcoupling reaction was recently investigated by the catalysts
of Pd/MILꢀ53(Al)ꢀNH2,42 Pd/UIOꢀ66ꢀNH2,43 and Pd/MILꢀ101(Cr)ꢀNH2.44 In addition, the
Pd/MILꢀ101(Cr)ꢀNH2 was further developed for the dehalogenation of aryl chlorides.45 Yamashita
and coꢀworkers demonstrated the dehydrogenation of formic acid for hydrogen production at
ambient temperature over Pd/NH2ꢀMILꢀ125.46 The catalyst of Pt supported on PTA encapsulated in
NH2ꢀMILꢀ101(Al) [Pt/PTAꢀNH2ꢀMILꢀ101(Al)] was recently developed for the reactions of the
preferential oxidation of CO in the presence of H2, and the hydrogenation of toluene.47 The catalyst
of Pt/UIOꢀ66ꢀNH2 was reported for chemoselective hydrogenation of cinnamaldehyde.48 The above
researches thus demonstrate that amineꢀfunctionalized MOFs offer excellent possibilities for
nanoparticle engineering in catalyst fabrication. Moreover, recent studies further suggested that the
catalyst with nitrogenꢀcontaining support could efficiently modify its acidity/basicity,
solubility/dispersability, surface area, and especially the selectivity towards the target products in
hydrogenation reactions.49ꢀ52 Inspired by these works, an amineꢀfunctionalized MOF material
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MILꢀ101(Al)ꢀNH2 is targeted herein as a support to construct a bicomponent catalyst for selective
hydrogenation of HMF to DHMTHF. In addition to the presence of amine functional groups on the
frameworks of MILꢀ101(Al)ꢀNH2, other properties such as crystalline structure, high porosity, and
high chemical/thermal stability (up to 377 °C) make MILꢀ101(Al)ꢀNH2 a particularly attractive for
the fabrication of hydrogenation catalyst.53
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In this research, we report the immobilization of uniform and highly dispersed palladium
nanoparticles over the MILꢀ101(Al)ꢀNH2 through the reaction pathway of anionic exchange followed
by gentle reduction (Figure 1a). In addition, the resulting composite Pd/MILꢀ101(Al)ꢀNH2 shows
excellent catalytic performance towards the selective hydrogenation of HMF to DHMTHF (Figure
1b). Under the optimal conditions, a maximum DHMTHF selectivity of 96% with a full conversion
of HMF is obtained by using Pd/MILꢀ101(Al)ꢀNH2 catalyst at a low reaction temperature of 30 °C.
The presence of free amine moieties in the frameworks of MILꢀ101(Al)ꢀNH2 is suggested to play a
key role on the formation of uniform and wellꢀdispersed palladium nanoparticles on the support
(Figure 1a). Moreover, our results also indicate that the observed high selectivity towards DHMTHF
form HMF is closely related to the cooperation between metallic site and free amine moiety on the
MOF support (Figure 1b).
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Figure 1. (a) Synthetic procedure to Pd/MILꢀ101(Al)ꢀNH2. (b) Molecular interactions in the
hydrogenation of HMF over Pd/MILꢀ101(Al)ꢀNH2. Gray and blue balls represent carbon and
nitrogen atoms, respectively. The aluminum octahedra is yellow in the MILꢀ101(Al)ꢀNH2. Hydrogen
atoms in the MILꢀ101(Al)ꢀNH2 are omitted for clarity. The palladium nanoparticles are golden in the
Pd/MILꢀ101(Al)ꢀNH2.
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EXPERIMENTAL SECTION:
Materials
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Unless otherwise stated, all chemicals in this work were commercial available and used without
further purification. Terephthalic acid, 2ꢀaminoterephthalic acid, 5ꢀhydroxymethylfurfural (HMF),
sodium hydroxide, hydrochloric acid (36~38 wt% aqueous solution), aluminum chloride hexahydrate
(AlCl3·6H2O), aluminum nitrate nonahydrate [Al(NO3)3·9H2O], and chromium nitrate nonahydrate
[Cr(NO3)3·9H2O] were purchased from Aladdin Industrial Inc. (Shanghai, P. R. China).
N,Nꢀdimethylformamide (DMF), methanol, sodium borohydride (NaBH4) and acetone were
purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, P. R. China). Palladium chloride
(PdCl2) was provided by Kunming Boren Precious Metals Co. Ltd. (Kunming, P. R. China).
Hydrogen gas (>99.999%) was obtained from Huate Co. Ltd (Foshan, P.R. China). Deꢀionized pure
water from MilliporeꢀMilli Q Plus System was used as solvent.
Characterization
The BrunauerꢀEmmettꢀTeller (BET) surface area measurements were performed with N2
adsorptionꢀdesorption isotherms at 77 K (SIꢀMPꢀ10/PoreMaster 33, Quantachrome). After degassed
under vacuum at 393 K for 24 hours and then measured over the range of 10−6 < P/P0 < 0.1. The
specific surface areas were evaluated using the BET method in the P/P0 range 0.05 to 0.3. The
powder Xꢀray diffraction (XRD) patterns of the MOF samples were obtained by a Bruker Advance
D8 diffractometer at 40 KV and 40 mA, using Niꢀfiltered CuꢀKα radiation with a scan speed of 0.3
sec/step, a step size of 0.02° in 2
θ, and a 2
θ
range of 5–50. Approximately 15 mg sample was
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dehydrated under high vacuum at 100
°C overnight before XRD analysis. The Xꢀray photoelectron
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spectroscopy (XPS) spectra analysis was performed with a Kratos Axis Ultra (DLD) photoelectron
spectrometer operated at 15 KV and 10 mA at a pressure of about 5×10−9 torr using AlKa as the
exciting source (1486.6 eV). C 1s photoelectron peak (BE = 284.2 eV) was used for the binding
energy calibration. The palladium contents in the catalysts Pd/MOFs were quantitatively determined
by inductively coupled plasmaꢀatomic emission spectroscopy (ICPꢀAES) analysis on the
PerkinElmer Optima 8000 instrument. The samples were digested in the sodium hydroxide and
then aqua regia was added to the mixture to digest the noble metal. The morphological analysis of
the catalyst was carried out using transmission electron microscopy (TEM, JEMꢀ2010HR). Samples
for TEM studies were prepared by placing a drop of the suspension of Pd/MOFs sample in ethanol
onto a carbonꢀcoated copper grid, followed by evaporating the solvent. High performance liquid
chromatographyꢀmass spectrometer (HPLCꢀMS) analysis were performed by using an Agilent
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HPLCꢀMS instrument equipped with a Column Comp. G1316C (column temperature: 40 °C) and
MS QꢀTOF G6540B (ion source: dual ESI). The 1H NMR and 13C{1H} NMR spectra of DHMF and
DHMTHF were recorded on Bruker AV III 400 at 25 °C with DMSOꢀd6 as solvent.
Catalyst preparation
MILꢀ101(Al)ꢀNH2,53 MILꢀ53(Al)ꢀNH2,54,55 MILꢀ53(Al),56 and MILꢀ101(Cr)40,41,57 were
prepared according to the literature methods.
Pd/MIL-101(Al)-NH2: The activated MILꢀ101(Al)ꢀNH2 (500 mg) in H2O (40 mL) was treated
with hydrochloric acid to tune the pH around 4. A solution of H2PdCl4 [containing ca. 3.0 wt% of Pd
relative to MILꢀ101(Al)ꢀNH2] was added to the above acidulated slurry under vigorous agitation for
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10 min. The mixture was then stirred for another 8 hours. The solid was filtered and washed with
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deionized water. The resulting gray sample [MILꢀ101(Al)ꢀNH3 ]2[PdCl4]2− were then reduced by
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sodium borohydride (NaBH4, 40 mg) at 0
The Pd/MILꢀ101(Al)ꢀNH2 (Pd 3.0 wt% based on ICPꢀAES) was further dried under vacuum
overnight at 120 C before use.
°C for 2 hours to yield Pd/MILꢀ101(Al)ꢀNH2 (Figure 1a).
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°
Pd/MIL-53(Al)-NH2: Pd/MILꢀ53(Al)ꢀNH2 (Pd 3.0 wt% based on ICPꢀAES) was prepared
following the same synthetic procedure as for Pd/MILꢀ101(Al)ꢀNH2 used except that
MILꢀ101(Al)ꢀNH2 was replaced by MILꢀ53(Al)ꢀNH2.
Pd/MIL-53(Al) and Pd/MIL-101(Cr): An aqueous solution of H2PdCl4 (containing ca. 3.0 wt%
Pd) was added to the activated MOFs [500 mg, MILꢀ53(Al) or MILꢀ101(Cr)] which was suspended
in water (40 mL) under vigorous agitation for 10 min. The mixture was then stirred for another 24
hours. The solid was centrifuged and washed with deꢀionized water and ethanol, respectively. The
resulting MOF samples containing palladium salts were then reduced by sodium borohydride
(NaBH4, 40 mg) at 0 °C for 2 hours to obtain Pd/MILꢀ53(Al) (Pd 1.7 wt% based on ICPꢀAES) or
Pd/MILꢀ101(Cr) (Pd 1.4 wt% based on ICPꢀAES).
Catalytic Performance
The selective hydrogenation of HMF was carried out in a 60 mL stainless steel reactor equipped
with a magnetic stirrer. In a typical run, HMF (126 mg, 1.0 mmol), Pd/MILꢀ101(Al)ꢀNH2 (20 mg)
were added to water (8.0 mL) in the stainless steel reactor. After purging the reactor three times with
hydrogen, the outlet valve was then closed to maintain 1.0 MPa of hydrogen pressure (ambient
temperature). The reaction was conducted at 30 °C for 12 hours with a stirring speed of 600 rpm.
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After the reaction was halted, the reactor was cooled to room temperature. The mixture was filtered,
decanted into a volumetric flask using water as diluent, and then analyzed by high performance
liquid chromatography (HPLC) and high performance liquid chromatographyꢀmass spectrometer
(HPLCꢀMS). The analysis of products was conducted on HPLC (Shimadu LCꢀ20AT) with a Shodex
Sugar SHꢀ1011 column (300×8 mm) using HPLC grade H2SO4 (0.005 M) water solution as the
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eluent at a column temperature of 50 °
C and a flow rate of 0.5 mL min−1. The conversion of HMF
was determined by a UV detector (320 nm); whereas, the concentrations of products such as DHMF,
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and DHMTHF were monitored by a refractive index (RI) indicator. DHMF: H NMR (400 MHz,
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DMSOꢀd6) δ 6.18 (s, 1H), 5.17 (t,
J
= 5.6 Hz, 1H), 4.35 (d,
J
= 5.4 Hz, 2H). C{1H} NMR (101
1
MHz, DMSOꢀd6) δ 154.7, 107.4, 55.8. Mass spectrum (EI) m/z 128.1 (M+). DHMTHF: H NMR
(400 MHz, DMSOꢀd6) δ 4.56 (d,
J = 5.1 Hz, 1H), 3.90–3.73 (m, 1H), 3.34 (t, J = 4.6 Hz, 2H), 1.85–
1.52 (m, 2H). 13C{1H} NMR (101 MHz, DMSOꢀd6) δ 79.8, 64.0, 27.3. Mass spectrum (EI) m/z
133.1 (MH+).
Adsorption experiments
All of the investigated MOFs were dried at 80 °C under high vacuum for 24 hours before
adsorption experiments. Adsorption of HMF over various MOFs was investigated by addition of
activated MOFs (100 mg) to the solutions of HMF (0.6 mmol) in water (25 mL). The mixture was
stirred at 25 °C for 4.0 hours. The amount of adsorbed HMF over MOFs was determined
chromatographically (HPLC) by the corresponding concentration change of HMF in water. The
adsorption of DHMF over various MOFs was performed with the same procedure to that of HMF
except that HMF was replaced by DHMF.
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RESULTS AND DISCUSSION:
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Preparation and Characterization of Pd/MOF Catalysts
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In this research, both amineꢀfunctionalized MOFs [MILꢀ101(Al)ꢀNH2 and MILꢀ53(Al)ꢀNH2]
and MOF materials without amine functional groups on the frameworks [MILꢀ53(Al) and
MILꢀ101(Cr)] were introduced to Pd catalyst as the supports. The influence of the MOF support on
the catalytic performance of Pd catalysts for the HMF hydrogenation and especially the selectivity
towards DHMTHF was investigated. For the comparison of MILꢀ101(Al)ꢀNH2 (Figure 2a) and
MILꢀ53(Al)ꢀNH2 (Figure 2c), both of them are aluminum based MOF materials and contain amine
functional groups on the frameworks with, however, different structures. According to the literature,
the MILꢀ101(Al)ꢀNH2 possesses two different types of quasispherical mesoporous cages formed by
12 pentagonal and 16 faces, respectively.53 Whereas, the MILꢀ53(Al)ꢀNH2 is built up from infinite
chains of cornerꢀsharing AlO4(OH)2 octahedra interconnected by dicarboxylate groups.55 In the cases
of MILꢀ53(Al)ꢀNH2 (Figure 2c) and MILꢀ53(Al) (Figure 2d), both of them share a similar structure;
however, MILꢀ53(Al)ꢀNH2 contains amine functional groups on the frameworks, owning to the
different connecting organic linker of 2ꢀaminoterephthalic acid for MILꢀ53(Al)ꢀNH2 and terephthalic
acid for MILꢀ53(Al), respectively.55,56 As the MILꢀ101 series, both MILꢀ101(Al)ꢀNH2 (Figure 2a)
and MILꢀ101(Cr) (Figure 2b) share a same topology of the framework. However, the connecting
organic linker of 2ꢀaminoterephthalic acid in the MILꢀ101(Al)ꢀNH2 is replaced by terephthalic acid
in the MILꢀ101(Cr), and the metal site of aluminum in MILꢀ101(Al)ꢀNH2 is replaced by chromium
in MILꢀ101(Cr).40,41,53,57 Therefore, the structure of MILꢀ101(Al)ꢀNH2 is similar to that of
MILꢀ101(Cr) but possesses the amine groups on the frameworks.
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d
c
Figure 2. The structures of (a) MILꢀ101(Al)ꢀNH2, (b) MILꢀ101(Cr), (c) MILꢀ53(Al)ꢀNH2, and (d)
MILꢀ53(Al). Gray and blue balls represent carbon and nitrogen atoms, respectively. The aluminum
octahedra is yellow in the MILꢀ101(Al)ꢀNH2, MILꢀ53(Al)ꢀNH2, and MILꢀ53(Al). The chromium
octahedra is green in the MILꢀ101(Cr). Hydrogen atoms are omitted for clarity.
As shown in Figure 1a, the synthetic procedure of Pd/MILꢀ101(Al)ꢀNH2 consists of the
neutralization of the amine groups on the MILꢀ101(Al)ꢀNH2 with an aqueous HCl solution, anionic
exchange reactions between the chloride anions of [MILꢀ101(Al)ꢀNH3 ]Cl− and anionic [PdCl4]2− of
+
+
H2PdCl4, and finally the gentle reduction of palladium metal of [MILꢀ101(Al)ꢀNH3 ]2[PdCl4]2− with
NaBH4 at low reaction temperature (Figure 1a). Pd/MILꢀ53(Al)ꢀNH2 was prepared following the
same synthetic procedure as for Pd/MILꢀ101(Al)ꢀNH2 used except that MILꢀ101(Al)ꢀNH2 was
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replaced by MILꢀ53(Al)ꢀNH2. In the cases of Pd/MILꢀ53(Al) and Pd/MILꢀ101(Cr), a conventional
incipient wetness impregnation was involved for their preparation with the corresponding supports.
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Table 1 Textural properties of various MOF support and Pd/MOF catalysts
surface area a pore volume b
Samples
[m2 g−1]
1025.8
[cm3 g−1]
1.36
0.96
0.69
0.55
0.51
1.81
1.09
1.75
0.38
1.86
0.71
MILꢀ101(Al)ꢀNH2
Pd/MILꢀ101(Al)ꢀNH2 (Pd 0.9 wt%) 871.9
Pd/MILꢀ101(Al)ꢀNH2 (Pd 3.0 wt%) 615.7
Pd/MILꢀ101(Al)ꢀNH2 (Pd 2.8 wt%) c 542.8
Pd/MILꢀ101(Al)ꢀNH2 (Pd 4.4 wt%) 529.2
MILꢀ53(Al)
1494.2
1046.5
217.6
87.4
Pd/MILꢀ53(Al) (Pd 1.7 wt%)
MILꢀ53(Al)ꢀNH2
Pd/MILꢀ53(Al)ꢀNH2 (Pd 3.0 wt%)
MILꢀ101(Cr)
1517.0
864.8
Pd/MILꢀ101(Cr) (Pd 1.4 wt%)
a
Langmuir surface area was evaluated using the BrunauerꢀEmmettꢀTeller (BET) method in the p/p0
b
range 0.05 to 0.3. Pore size distribution curves were calculated using the adsorption branch of the
isotherms and the BarrettꢀJoynerꢀHalenda (BJH) method, pore sizes were obtained from the peak
c
positions of the distribution curves. Recovered Pd/MILꢀ101(Al)ꢀNH2 with the reaction conditions
described in Figure 11b.
The specific surface areas of the Pd/MOF catalyst samples were measured by N2
adsorption/desorption at 77 K and the results are presented in Table 1. Compared with the bare
MILꢀ101(Al)ꢀNH2, the resulting pore modification is visible on the N2 adsorption/desorption
isotherms after the Pd loading (Figure 3). As expected, the Pd/MILꢀ101(Al)ꢀNH2 exhibits a
significant decrease of porous surface area with the increase of palladium loading levels. After the
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loading of palladium nanoparticles, the BrunauerꢀEmmettꢀTeller (BET) surface area of
MILꢀ101(Al)ꢀNH2 (1025.8 m2 g−1) significantly decreases to 871.9 m2 g−1 for Pd content of 0.9 wt%,
and to 529.2 m2 g−1 for Pd content of 4.4 wt% (Table 1). The appreciable decreases in nitrogen
adsorption amount and surface area indicate that the cavities of MILꢀ101(Al)ꢀNH2 are presumably
occupied by highly dispersed Pd nanoparticles or Pd nanoparticles deposited on the pore surface of
MILꢀ101(Al)ꢀNH2.58 The similar results were also observed on other Pd/MOF catalysts, the BET
surface area of Pd/MOF significantly decreased when compared with its corresponding bare MOF
support (Table 1 and Figure S1 Supporting Information).
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MIL-101(Al)-NH
2
Pd/MIL-101(Al)-NH (Pd 0.9 wt%)
2
1200
800
400
0
Pd/MIL-101(Al)-NH (Pd 3.0 wt%)
2
recovered Pd/MIL-101(Al)-NH (Pd 2.8 wt%)
2
Pd/MIL-101(Al)-NH (Pd 4.4 wt%)
2
0.0
0.2
0.4
0.6
0.8
1.0
P/P
0
Figure 3. N2 adsorptionꢀdesorption isotherms of MILꢀ101(Al)ꢀNH2, Pd/MILꢀ101(Al)ꢀNH2 with
different Pd loading levels (Pd 0.9–4.4 wt%), and recovered Pd/MILꢀ101(Al)ꢀNH2 (Pd 2.8 wt%)
with the reaction conditions described in Figure 11b
To identify the active species for the hydrogenation, MILꢀ101(Al)ꢀNH2 and
Pd/MILꢀ101(Al)ꢀNH2 were further compared by powder Xꢀray powder diffraction (XRD). The
powder XRD pattern of MILꢀ101(Al)ꢀNH2 matches well with the published results (Figure 4a).53
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After the immobilization of Pd nanoparticles, the structure of MILꢀ101(Al)ꢀNH2 is mostly preserved
regardless of the increased Pd content in the material Pd/MILꢀ101(Al)ꢀNH2 (Figure 4a). However,
some variations of the Bragg intensities were observed, indicating the successful loading of Pd
nanoparticles in the MILꢀ101(Al)ꢀNH2. In addition, no diffraction peaks corresponding to Pd
crystallite can be observed over the Pd/MILꢀ101(Al)ꢀNH2, which implies that the highly dispersed
Pd species exist in the very small nanoparticles or in amorphous structure. Moreover, Figure 4b
shows the XRD patterns of other Pd/MOF catalysts and the corresponding MOF supports, and
similar results were also observed.
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b
a
(e)
(f)
(e)
(d)
(c)
(b)
(d)
(c)
(b)
(a)
(a)
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50
10
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2θ
degree
2θ
degree
Figure 4. a) XRD patterns of (a) MILꢀ101(Al)ꢀNH2, (b) Pd/MILꢀ101(Al)ꢀNH2 (Pd 0.9 wt%), (c)
Pd/MILꢀ101(Al)ꢀNH2 (Pd 3.0 wt%), (d) Pd/MILꢀ101(Al)ꢀNH2 (Pd 4.4 wt%), (e) recovered
Pd/MILꢀ101(Al)ꢀNH2 (Pd 2.8 wt%) with the reaction conditions described in Figure 11b. b) XRD
patterns of (a) MILꢀ53(Al)ꢀNH2, (b) Pd/MILꢀ53(Al)ꢀNH2 (Pd 3.0 wt%), (c) MILꢀ53(Al), (d)
Pd/MILꢀ53(Al) (Pd 1.7 wt%), (e) MILꢀ101(Cr), (f) Pd/MILꢀ101(Cr) (Pd 1.4 wt%)
As indicated in the transmission electron microscopy (TEM) images, the Pd nanoparticles were
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highly dispersed on MILꢀ101(Al)ꢀNH2 with a uniform size (Figure 5a–f). Notably, the TEM image of
Pd/MILꢀ101(Al)ꢀNH2 (Pd 0.9 wt%) shows that the Pd nanoparticles are embedded inside the crystals
and are homogeneously distributed throughout the crystals of MILꢀ101(Al)ꢀNH2 (Figure 5a), further
indicating the encapsulation of Pd nanoparticles within the interior of MILꢀ101(Al)ꢀNH2. Moreover,
the average nanoparticle size of Pd in Pd/MILꢀ101(Al)ꢀNH2 only slightly increased with the Pd
loading levels from 2.8 nm for Pd content of 0.9 wt% to 3.1 nm for Pd content of 4.4 wt% (Figure
5a–f). In the case of Pd/MILꢀ53(Al)ꢀNH2, again, well dispersed and uniform Pd nanoparticles were
observed throughout MILꢀ53(Al)ꢀNH2 support with an average nanoparticle size of 2.4 nm (Figure 5i,
j). In sharp contrast to the MOF support without amine group on the frameworks, the Pd
nanoparticles were readily accumulated over the MILꢀ53(Al) with a mean nanoparticle size of 4.0
nm for 1.7 wt% loading level of Pd in the Pd/MILꢀ53(Al) (Figure 5k, l), further indicating an
efficient stabilization effect of the MILꢀ101(Al)ꢀNH2 and MILꢀ53(Al)ꢀNH2 supports on the
palladium nanoparticles. For Pd/MILꢀ101(Cr) (Pd 1.4 wt%), the size of the Pd nanoparticles was
around 3.7 nm (Figure 5m, n). Moreover, these larger Pd nanoparticles were found accumulated
mainly on the surfaces of the MILꢀ101(Cr) crystals, as shown by the TEM (Figure 5m).
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b30
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f
d
Pd/MIL-101(Al)-NH2
(Pd 0.9 wt%)
d = 2.84 nm
Pd/MIL-101(Al)-NH2
Pd/MIL-101(Al)-NH2
(Pd 4.4 wt%)
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(Pd 3.0 wt%)
d = 2.91 nm
d = 3.12 nm
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0
0
0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Partical size / nm
Partical size / nm
Partical size / nm
g
i
k
25
20
15
10
5
recovered
Pd/MIL-101(Al)-NH2
j 40 Pd/MIL-53(Al)-NH2
(Pd 3.0 wt%)
l
h
25
20
15
10
5
Pd/MIL-53(Al)
(Pd 1.7 wt%)
d = 3.96 nm
(Pd 3.0 wt%)
d = 3.26 nm
d = 2.41 nm
30
20
10
0
0
0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
1.0
1.5
2.0
3.5
4.0
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Partical2.s5ize /3n.0m
Partical size / nm
Partical size / nm
m
n
Pd/MIL-101(Cr)
(Pd 1.4 wt%)
d = 3.73 nm
30
20
10
0
2
3
4
5
6
Partical size / nm
Figure 5. TEM images and particleꢀsize distribution of (a, b) Pd/MILꢀ101(Al)ꢀNH2 (Pd 0.9 wt%), (c,
d) Pd/MILꢀ101(Al)ꢀNH2 (Pd 3.0 wt%), (e, f) Pd/MILꢀ101(Al)ꢀNH2 (Pd 4.4 wt%), (g, h) recovered
Pd/MILꢀ101(Al)ꢀNH2 (Pd 2.8 wt%), (i, j) Pd/MILꢀ53(Al)ꢀNH2 (Pd 3.0 wt%), (k, l) Pd/MILꢀ53(Al)
(Pd 1.7 wt%), (m, n) Pd/MILꢀ101(Cr) (Pd 1.4 wt%).
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The Xꢀray photoelectron spectroscopy (XPS) survey spectra of the Pd/MILꢀ101(Al)ꢀNH2
sample is shown in Figure 6a. The photoelectron peaks of main elements on the surface of
Pd/MILꢀ101(Al)ꢀNH2 appear at the binding energies of 74.4 eV (Al 2P), 281.8 eV (C 1s), 335.6 eV
(Pd 3d), 399.2 eV (N 1s), and 530.4 eV (O 1s), respectively, further confirming the presence of
amine group on the framework of MILꢀ101(Al)ꢀNH2 and palladium nanoparticles over
MILꢀ101(Al)ꢀNH2. The Pd 3d XPS spectrum of Pd/MILꢀ101(Al)ꢀNH2 shows two main peaks
appeared at 340.9 eV and 335.6 eV correspond to Pd 3d3/2 and Pd 3d5/2, respectively (Figure 6b),
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+
indicating that the Pd(II) cations in the [MILꢀ101(Al)ꢀNH3 ]2[PdCl4]2− were successfully reduced to
Pd(0) in the Pd/MILꢀ101(Al)ꢀNH2 after the reduction by sodium borohydride.42
Pd
a
b
Pd/MIL-101(Al)-NH
5/2
2
Pd/MIL-101(Al)-NH
2
Pd 3d
C 1s
Pd
O 1s
3/2
0
N 1s
Pd 3d
Pd
Al 2p
800
700
600
500
400
300
200
100
0
346
344
342
340
338
336
334
332
Binding Energy / eV
Binding Energy / eV
Figure 6. XPS spectra of Pd/MILꢀ101(Al)ꢀNH2 (Pd 3.0 wt%) (a) survey spectra, and (b) Pd 3d.
HMF Hydrogenation over Pd/MOF Catalysts
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DHMTHF,
DHMF,
HT,
unidentified products
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o
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Pd 3.0 wt%
Pd/MIL-101(Al)-NH
40
Pd 4.4 wt%
30
100
100
60
80 100 C
40
Pd 0.9 wt%
60
80
30
40
60
30
2
Figure 7. Influences of the reaction temperature and Pd loading levels in the catalyst
Pd/MILꢀ101(Al)ꢀNH2 on the HMF hydrogenation. Reaction conditions: HMF (126 mg, 1.0 mmol),
Pd/MILꢀ101(Al)ꢀNH2 (20 mg, Pd 0.9–4.4 wt%), water (8 mL), H2 (1.0 MPa), reaction temperature
and time (30 °C for 12 hours and 40–100 °C for 4 hours).
The prepared Pd/MOF catalysts were investigated for selective hydrogenation of HMF.
Hydrogenation of HMF leads to the formation of DHMF, a further hydrogenation of DHMF results
in the transformation of DHMF to DHMTHF (Figure 1b). In addition to DHMF and DHMTHF,
other polyols such as hexanetriol (HT) were observed by HPLC as byꢀproducts during the process of
HMF hydrogenation.12,59 In this research, we performed the hydrogenation of HMF under
pressurized hydrogen conditions in water using Pd/MOF as the catalyst. An initial experiment was
carried out to investigate the effect of the reaction temperature and Pd loading levels in the
Pd/MILꢀ101(Al)ꢀNH2 catalyst on the HMF hydrogenation. The HMF hydrogenation was performed
over Pd/MILꢀ101(Al)ꢀNH2 with the Pd content ranging from 0.9 wt% to 4.4 wt% at the reaction
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temperature from 30
°C to 100
°C (Figure 7). Generally, a full conversion of HMF was observed
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Pd/MILꢀ101(Al)ꢀNH2 towards HMF. The selectivity of DHMTHF increased with reaction
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temperature ranging from 40
selectivity of DHMF decreased with the reaction temperature from 40
conditions, suggesting that increasing the reaction temperature promotes a further hydrogenation of
DHMF to DHMTHF (Figure 7). In the case of the hydrogenation performed at 100 C, a full
°
C to 80
°
C at all Pd loading levels of the catalysts. Whereas, the
°
C to 80 C under the above
°
°
conversion of DHMF and a significantly increased selectivity to HT were observed over the
Pd/MILꢀ101(Al)ꢀNH2 with various Pd loading levels, suggesting that raising the reaction temperature
promotes the DHMF hydrogenation as well as the HT formation.
The influence of the Pd loading levels on the HMF hydrogenation indicated that the
transformation rates both for DHMFꢀtoꢀDHMTHF and DHMFꢀtoꢀHT increased with the Pd content
in the catalyst under the specified reaction temperature (Figure 7). Moreover, the selectivity of HT
increased from 6% to 12% at 100 °C with the increasing Pd loading level from 0.9 wt% to 4.4 wt%,
indicating that the increasing Pd content in the catalyst leads to an increased selectivity to HT.
Obviously, low reaction temperature, prolonged reaction time and moderate loading level of Pd in
the catalyst favor a high selectivity to DHMTHF. Under the optimal conditions, a maximum
DHMTHF selectivity of 96% with full conversions of both HMF and DHMF were obtained by using
the Pd/MILꢀ101(Al)ꢀNH2 (Pd 3.0 wt%) at an optimal reaction temperature of 30
time of 12 hours (Figure 7).
°C and reaction
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0.5
1.0
1.5
2.0
2.5
3.0
H
pressure / MPa
2
Figure 8. The effect of H2 pressure on the HMF hydrogenation over Pd/MILꢀ101(Al)ꢀNH2. Reaction
condition: HMF (126 mg, 1.0 mmol), Pd/MILꢀ101(Al)ꢀNH2 (20 mg, Pd 3.0 wt%), water (8 mL),
reaction temperature (100 °C), reaction time (2 hours).
The influence of H2 pressure on the reaction revealed that the increase in H2 pressure from 0.5
MPa to 1.0 MPa significantly increased the hydrogenation rates both for HMFꢀtoꢀDHMF and the
subsequent DHMFꢀtoꢀDHMTHF transformations (Figure 8). HMF achieved a full conversion at 1.0
MPa; meanwhile, DHMTHF reached a maximum selectivity of 86% under the investigated
conditions. DHMF was observed to be formed with a maximum selectivity of 67% at 0.5 MPa and
later consumed with the increased H2 pressure. The selectivity of HT significantly increased from 3%
at a H2 pressure of 0.5 MPa to 26% at the H2 pressure of 3.0 MPa, suggesting that increasing the H2
pressure promotes hydrolytic hydrogenation of DHMF as well as hydrogenolysis of DHMTHF to
give HT.
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HT Selec.
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Time / h
Figure 9. Selectivity to DHMF, DHMTHF and HT as a function of reaction time for the
hydrogenation of HMF over Pd/MILꢀ101(Al)ꢀNH2. Reaction condition: HMF (126 mg, 1.0 mmol),
Pd/MILꢀ101(Al)ꢀNH2 (20 mg, Pd 3.0 wt%), water (8 mL), reaction temperature (30
MPa).
°C), H2 (1.0
Figure 9 shows a typical reaction profile for the hydrogenation of HMF over
Pd/MILꢀ101(Al)ꢀNH2 at low reaction temperature of 30 C in aqueous media. HMF was quickly
°
converted into DHMF and DHMTHF with the selectivities of 67% and 32%, respectively, at 1.0 hour.
After this time the selectivity of DHMF quickly declined with the increased selectivity of DHMTHF,
owing to a further hydrogenation of DHMF. HMF was completely converted into DHMF and
DHMTHF at the reaction time of 5.0 hours with, however, the primary product of DHMTHF rather
than DHMF. The selectivity of DHMTHF reaches a maximum of 96% at the reaction time of 12.0
hours with a full conversion of DHMF. HT was observed as byproduct with the selectivities ranging
from trace to 4%. The above reaction profile further suggested that the hydrogenation of HMF was
stepwise with DHMF as the observed “intermediate” (Figure 1b).
Effect of MOF support
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Pd/MIL-101(Al)-NH
Pd/MIL-53(Al)-NH
Pd/MIL-53(Al)
(Pd 1.7 wt%)
Pd/MIL-101(Cr)
(Pd 1.4 wt%)
2
2
2
(Pd 0.9 wt%)
(Pd 3.0 wt%)
(Pd 3.0 wt%)
Figure 10. The effect of MOF support on the selective hydrogenation of HMF. Reaction condition:
HMF (126 mg, 1.0 mmol), catalyst [20 mg, Pd/MILꢀ101(Al)ꢀNH2, Pd/MILꢀ53(Al)ꢀNH2,
Pd/MILꢀ53(Al) or Pd/MILꢀ101(Cr)], water (8 mL), reaction temperature and time (30
hours, and 60–100 C for 4 hours).
°C for 12
°
Figure 10 shows the influence of the MOF support on the catalytic performance of Pd catalyst
for the HMF hydrogenation. A full conversion of HMF was generally observed under all investigated
condition. Pd/MILꢀ101(Al)ꢀNH2 catalysts with the Pd contents of 0.9 wt% and 3.0 wt% afforded
DHMTHF selectivities of 93% and 96%, respectively at 30 °C after 12 hours. In addition to
Pd/MILꢀ101(Al)ꢀNH2, Pd/MILꢀ53(Al)ꢀNH2 (Pd 3.0 wt%) also showed good catalytic performance
towards HMF hydrogenation, yielding DHMTHF and DHMF selectivities of 88% and 8%,
respectively, with a full conversion of HMF. In sharp contrast, the DHMTHF selectivities of 61%
and 76% were observed over Pd/MILꢀ53(Al) (Pd 1.7 wt%) and Pd/MILꢀ101(Cr) (Pd 1.4 wt%),
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respectively, under the investigated conditions. Therefore, Pd/MILꢀ101(Al)ꢀNH2 is considerably
more active and selective to afford DHMTHF than Pd/MILꢀ53(Al)ꢀNH2, Pd/MILꢀ53(Al) and
Pd/MILꢀ101(Cr) under all specified conditions. In addition, Pd/MILꢀ101(Al)ꢀNH2 shows higher
catalytic performance than Pd/MILꢀ53(Al)ꢀNH2; while, Pd/MILꢀ101(Cr) is more efficient to afford
DHMTHF when compared with Pd/MILꢀ53(Al). These results can presumably be attributed to an
inherent drawback of MILꢀ53 series on the strong adsorption and diffusion limitations in their 1ꢀD
pore structure (Figure 2c,d) when compared with a quasispherical mesoporous cage system of
MILꢀ101 series (Figure 2a,b).60 In addition, MILꢀ53 series generally possess relatively smaller
surface area in comparison to the corresponding MILꢀ101 series (Table 1). Notably, as described in
Figure 10, the influence of MOF support indicates that the Pd catalysts immobilized over
amineꢀfunctionalized MOFs [Pd/MILꢀ101(Al)ꢀNH2 and Pd/MILꢀ53(Al)ꢀNH2] generally show higher
selectivity to DHMTHF when compared with their analogues without amine groups on the MOF
frameworks [Pd/MILꢀ101(Cr) and Pd/MILꢀ53(Al)].
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The above results thus clearly indicate that the observed selectivity towards the formation of
DHMTHF presumably relies on the presence of free amine moieties in the frameworks of
MILꢀ101(Al)ꢀNH2 and MILꢀ53(Al)ꢀNH2. Notably, recent researches also indicate that the catalysts
with nitrogenꢀcontaining support can efficiently modify their catalytic performance towards the
selectivity of target product in the hydrogenation reactions.50ꢀ52 In our case, Figure 1b illustrates the
special function of the support MILꢀ101(Al)ꢀNH2 and demonstrates the step sequence for the
selective hydrogenation of HMF. In the initial stage, HMF can interact with the MILꢀ101(Al)ꢀNH2
surface through hydrogen bonds to form O–H···N interactions between the hydroxyl group of the
HMF and the amine group of the MILꢀ101(Al)ꢀNH2. H2 is presumably activated by the palladium
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nanoparticles supported on MILꢀ101(Al)ꢀNH2. The aldehyde group of HMF is then hydrogenated to
the hydroxyl group and HMF is accordingly converted into DHMF (Figure 1b). There is, however, a
stronger interaction between MILꢀ101(Al)ꢀNH2 and DHMF than that of MILꢀ101(Al)ꢀNH2 and HMF,
owning to more hydroxyl groups in DHMF molecule and a much more hydrophilic nature of DHMF
than in the case of HMF. Therefore, the in situ obtained DHMF can hardly be replaced by a fresh and
more weakly binding HMF molecule, thus leading to a further hydrogenation to DHMTHF (Figure
1b).
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Table 2 Adsorptions of HMF and DHMF over various MOFs
HMF
[mmol g−1]
0.25
HMF
DHMF
[mmol g−1]
0.79
DHMF
[mmol m−2]
7.70×10−4
6.34×10−3
2.21×10−4
2.44×10−4
Samples
[mmol m−2]
2.44×10−4
1.01×10−3
3.75×10−4
1.05×10−4
MILꢀ101(Al)ꢀNH2
MILꢀ53(Al)ꢀNH2
MILꢀ53(Al)
0.22
1.38
0.56
0.33
0.16
0.37
MILꢀ101(Cr)
The above assumption was further supported by adsorption experiments of HMF and DHMF
over various MOFs as described in Table 2. Adsorptions of HMF or DHMF over various MOFs were
investigated by addition of activated MOF to a solution of HMF or DHMF in water. The mixture was
stirred and the amounts of adsorbed HMF or DHMF over the MOFs were determined
chromatographically by the corresponding concentration change of HMF or DHMF in water. In the
cases of amineꢀfunctionalized MOFs, both MILꢀ101(Al)ꢀNH2 and MILꢀ53(Al)ꢀNH2 show significant
adsorption of DHMF than HMF (Table 2), suggesting an enhanced interaction between DHMF and
amineꢀfunctionalized MOFs through the hydrogen bonding. Moreover, the adsorption of DHMF over
amineꢀfunctionalized MOFs [MILꢀ101(Al)ꢀNH2 and MILꢀ53(Al)ꢀNH2] is high than in the case of
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unfunctionalized MOFs [MILꢀ101(Cr) and MILꢀ53(Al)], indicating an improved hydrophilic nature
both for DHMF and amineꢀfunctionalized MOFs when compared with HMF and unfunctionalized
MOFs, respectively. For instance, the adsorbed DHMF over MILꢀ53(Al)ꢀNH2 and MILꢀ53(Al) are
1.38 and 0.33 mmol g−1 (Table 2), respectively, although the MILꢀ53(Al)ꢀNH2 shows dramatically
low BET area of 217.6 m2 g−1 when compared with the BET area of 1494.2 m2 g−1 for MILꢀ53(Al)
(Table 1). The above adsorption experiments thus indicate that the adsorptions of HMF or DHMF
over various MOFs are enhanced by the hydrophilic nature of substrate molecule as well as the
hydrogen bonding interactions between the substrate molecule and MOF support.
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Therefore, from the point view of both metal sites and substrate molecules, the presence of
amine group in the MILꢀ101(Al)ꢀNH2 brings multifunctional advantages to the catalytic reaction
system in this research. For the metal sites, the presence of amine groups on the frameworks lead to
the formation of uniform and highly dispersed palladium nanoparticles over the MILꢀ101(Al)ꢀNH2
through the reaction pathway of anionic exchange and gentle reduction (Figure 1a). In the case of
substrate molecules, owing to the enhanced hydrogen bonding interactions between
MILꢀ101(Al)ꢀNH2 and the “intermediate” DHMF than in the case of reactant HMF,
MILꢀ101(Al)ꢀNH2 promotes the further hydrogenation of DHMF to DHMTHF upon the in situ
formation of DHMF under the reaction conditions, and thus improves the selectivity to the desired
DHMTHF production (Figure 1b). Finally, recent research revealed that the formation of HT during
the HMF hydrogenation is presumably obtained from DHMF by acid catalyzed reactions, followed
by hydrogenation of ringꢀopening products.12 In our case, the presence of amine group and the weak
basicity of the support MILꢀ101(Al)ꢀNH2 can thus suppress the DHMFꢀtoꢀHT transformation and,
therefore, improve the selectivity to DHMTHF.
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HT
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Figure 11. The reusability of Pd/MILꢀ101(Al)ꢀNH2. Reaction condition: HMF (126 mg, 1.0 mmol),
Pd/MILꢀ101(Al)ꢀNH2 (20 mg, Pd 3.0 wt%), water (8 mL), H2 (1.0 MPa), reaction temperature (30
°
C), reaction time [1 hour for (a) and 12 hours for (b)].
In order to probe the reusability of Pd/MILꢀ101(Al)ꢀNH2, a fiveꢀcycle experiment was
performed for HMF hydrogenation at 1 hour (Figure 11a) and 12 hours (Figure 11b), respectively, to
investigate the catalyst activity decay. Each time, the Pd/MILꢀ101(Al)ꢀNH2 was recovered by filter,
washed thoroughly with methanolꢀwater and then recycled for the next reaction. In the case of the
recyclability tests performed at 1 hour, almost identical results were observed as shown in Figure 11a,
indicating no efficiency loss for the catalyst. Under the above hydrogenation conditions, HMF
conversion was low than 82% with the major hydrogenation product of DHMF (Figure 11a). Notably,
a full conversion of HMF can always be achieved during the fiveꢀcycle experiments after 12 hour
hydrogenation of HMF (Figure 11b). However, the measured selectivity for the predominant product
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DHMTHF reduced from 96% to 80% (Figure 11b). The selectivity of DHMF increased from 2% to
17%. Moreover, the selectivity of HT was kept around 3% during the recycling experiments.
Inductively coupled plasmaꢀatomic emission spectroscopy (ICPꢀAES) analysis shows that only 0.03%
of the total amount of Pd had leached from Pd/MILꢀ101(Al)ꢀNH2 after the fiveꢀcycle experiment,
indicating an efficient stabilization effect of the MILꢀ101(Al)ꢀNH2 support on the palladium
nanoparticles (Figure 11b). However, the XRD analysis revealed that the XRD patterns of the
recovered Pd/MILꢀ101(Al)ꢀNH2 show some slight variations than the fresh one (Figure 4a),
indicating partial loss of crystallinity in the support MILꢀ101(Al)ꢀNH2 for the recovered catalyst
after a hydrogenation time of 60 hours (Figure 11b). Moreover, the BET analysis indicates that the
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specific surface area of Pd/MILꢀ101(Al)ꢀNH2 decreases from 615.7 m2g− for the fresh one to 542.8
1
m2g− for the recovered one (Figure 3 and Table 1), suggesting the adsorption of insoluble polymer
generated from the reactive DHMF intermediate on the surface of the catalyst.12 The TEM analysis
of fresh Pd/MILꢀ101(Al)ꢀNH2 (Pd 3.0 wt%) reveals that highly dispersed Pd nanoparticles were well
deposited on the surface of MILꢀ101(Al)ꢀNH2 with an average diameter size of 2.9 nm (Figures 5c,
d). In contrast, the recovered Pd/MILꢀ101(Al)ꢀNH2 catalyst after fiveꢀtime recycling (Figure 11b)
had the average particle sizes of 3.3 nm (Figures 5g, h).
CONCLUSIONS:
In summary, we have developed an efficient heterogeneous catalyst of palladium immobilized
on amineꢀfunctionalized Metal–Organic Frameworks [Pd/MILꢀ101(Al)ꢀNH2] for selective
hydrogenation
of
biomassꢀbased
5ꢀhydroxymethylfurfural
(HMF)
to
2,5ꢀdihydroxymethylꢀtetrahydrofuran (DHMTHF) in aqueous phase. The Pd/MILꢀ101(Al)ꢀNH2 was
prepared by using a direct anionic exchange approach and subsequent gentle reduction. The presence
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