A hydrothermally stable ytterbium metal-organic framework as a bifunctional solid-acid catalyst for glucose conversion

Article abstract of DOI:10.1039/c9cc05364f

Yb₆(BDC)₇(OH)₄(H₂O)₄ contains both bridging hydroxyls and metal-coordinated waters, possessing Br?nsted and Lewis acid sites. The material crystallises from water at 200 °C. Using the solid as a heterogenous catalyst, glucose is converted into 5-hydroxymethylfurfural, via fructose, with a total selectivity of ～70percent after 24 hours at 140 °C in water alone: the material is recyclable with no loss of crystallinity.

Full text of DOI:10.1039/c9cc05364f

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A hydrothermally stable ytterbium metal–organic
framework as a bifunctional solid-acid catalyst for
Cite this:DOI: 10.1039/c9cc05364f
glucose conversion†
Received 12th July 2019,
Accepted 12th August 2019
a
David L. Burnett,
Ryan Oozeerally,b Ralentri Pertiwi,c Thomas W. Chamberlain,ab
b
c
Nikolay Cherkasov,
Guy J. Clarkson,a Yuni K. Krisnandi,
Volkan Degirmenci *b and Richard I. Walton
*
a
DOI: 10.1039/c9cc05364f
rsc.li/chemcomm
Yb6(BDC)7(OH)4(H2O)4 contains both bridging hydroxyls and metal- used.17 Although Sn-beta is hydrothermally stable at low pH and
coordinated waters, possessing Brønsted and Lewis acid sites. The the solid can be recycled, a significant disadvantage in its use, in
material crystallises from water at 200 8C. Using the solid as a hetero- particular on an industrial scale, is that the synthesis of the
genous catalyst, glucose is converted into 5-hydroxymethylfurfural, catalyst is a lengthy process requiring the use of seed crystals or a
via fructose, with a total selectivity of B70% after 24 hours at 140 8C 40 day hydrothermal synthesis reaction using hydrofluoric acid,
in water alone: the material is recyclable with no loss of crystallinity. an extremely toxic and corrosive substance.16 Furthermore, in
continuous flow reactors Sn-beta can be unstable and is rapidly
The conversion of biomass-derived glucose to organic molecules deactivated.18
that may be used as precursors for the production of a variety of
Metal–organic frameworks have been considered as solid
useful products for everyday life is important to provide sustain- acid catalysts.19 The dehydration of glucose to 5-HMF using
able production of chemicals. 5-Hydroxymethylfurfural (5-HMF) MIL-101(Cr) functionalised with sulfonic acid groups (–SO3H)
is one such organic molecule formed from glucose, via isomer- was reported by Herbst and Janiak in 2016.20 The highest
isation to fructose as a common pathway, and is considered a 5-HMF yield of 29% was obtained at 130 1C with a 5 hour
platform molecule for the formation of a variety of useful chemicals reaction time in tetrahydrofuran : water (39 : 1) mixture as solvent.
ranging from plastics to drugs to fragrances and agrochemicals.1 Su et al. studied the same catalyst and found the highest conver-
The two-step conversion of glucose requires both Lewis and sions in mixed organic/aqueous solvent systems and demon-
Brønsted acid catalysis. Various studies have probed the possible strated the effectiveness of the solid in a fixed-bed, continuous
pathways in solution when homogeneous, solution acids are reactor.21 MIL-101(Cr)–SO3H showed high efficiency with more
used as catalysts in aqueous conditions,2–6 and in ionic than 90% yield of 5-HMF when DMSO was used as solvent.22
liquids.7–13 Heterogeneous, solid-acid catalysts would, however, A composite of MIL-101(Cr) and chromium hydroxide showed
be desirable for this reaction if large-scale processes are to be optimum isomerisation of glucose into fructose in ethanol as
developed,14,15 avoiding significant amounts of mineral acids solvent.23 Other MOFs that have been used for glucose conversion
and metal salts, with easy of separation of the catalyst from the include the zirconium-based NU-1000, with phosphate modifica-
solution reagents. The use of zeolite-based catalysts that contain tion to induce Lewis acidity,24 and the zirconium material UiO-66
both Lewis and Brønsted acid sites has been studied and in with sulfonyl-modified ligands and inherent Lewis acidity from
particular tin-substituted zeolite beta (Sn-beta) has been shown defects notably allowing conversion of glucose to fructose, along
as a particularly effective catalyst for the isomerisation of glucose with a significant amount of 5-HMF in water alone,25 or in
into fructose in water,16 or the direct formation of 5-HMF from alcohols as solvents.26 While MIL-101 is constructed from the
glucose when a biphasic water/tetrahydrofuran reactor system is cheap and benign ligand benzene-1,4-dicarboxylate, the metal Cr
is known to be toxic to humans and harmful to the environment;
although these detrimental properties are largely associated with
the +6 oxidation state, Cr3+ is considered an irritant, and its
release into the environment is clearly undesirable if it were to
encounter oxidising conditions.27 Replacement of the chromium
in MIL-101 by other trivalent cations leads to an instability of
the material,28 and we have therefore explored the chemistry of
water-stable metal–organic frameworks containing more benign
metals. Herein, we focus on materials that contain Yb3+ as the
a Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK.
E-mail: r.i.walton@warwick.ac.uk
b School of Engineering, University of Warwick, Coventry, CV4 7AL, UK.
E-mail: v.degirmenci@warwick.ac.uk
c Department of Chemistry, Universitas Indonesia, Depok, Indonesia
† Electronic supplementary information (ESI) available: Synthesis details, crystal-
lographic information and further experimental characterisation. CCDC 1892235 (3),
1892236 (4), 1892237 (2). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9cc05364f
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metal centre, with several reasons for the choice of this metal: isolated Yb centres, bridged by carboxylate functions of the
(i) ytterbium triflate is well-known Lewis acid used for various BDC in a Z,Z-m2-Z1:Z1 mode, to give extended chains, BDC,
organic transformations,29 (ii) YbCl3 in solution has already been Fig. 1c. The chains run in two perpendicular directions, to give
used as a homogeneous Lewis acid catalysts for glucose a pseudo-layered structure, that are crosslinked by the bridging
conversion,30,31 and (iii) metal–organic frameworks based on BDC ligands.
the rare-earth cations are known to be relatively chemically and
The synthesis of phase-pure (2), was subsequently achieved
thermally robust, as well as showing a wide structural variety.32 by addition of extra YbCl3Á6H2O to provide the correct ratio of
At least three different ytterbium MOFs crystallise from Yb : BDC as required by the chemical composition (see ESI†),
N,N-dimethylformamide (DMF)–water solutions of Yb3+ and Fig. 2. We also considered the possibility of inclusion of a
benzene-1,4-dicarboxylic acid (H2BDC), depending on the solvent Brønsted acid functionality by use of the sulfonic-acid modified
composition and temperature.33,34 This was the starting point for 2-monosulfo-benzene-1,4-dicarboxylate (MSBDC) linker, using
considering materials that might be water stable and thus the the sodium salt of the linker as precursor, as has found to be
hydrated material Yb2(BDC)3(DMF)2(H2O)2 (1) was selected, previously successful for modifying MOFs such as UiO-66 and
Fig. 1a, that crystallises from 50: 50 DMF : water solutions at MIL-101. This, however, yielded a novel crystalline structure
100 1C34 to investigate its behaviour in pure water. Hydrothermal only when DMF was used as solvent, [Yb(MSBDC)(DMF)2] (4),
treatment of (1) at 200 1C (ESI†) produced single crystals of and in this material the sulfonic acid functionality is deproto-
Yb6(BDC)7(OH)4(H2O)4 (2), whose structure, determined by sin- nated and coordinated to the Yb3+ centres, along with the
gle crystal X-ray diffraction was found to be analogous to yttrium carboxylate functionalities to yield a rather dense coordination
and ytterbium materials reported by Weng et al.35 The structure polymer (ESI†). The material (4) was also found to be unstable
of (2) is completely different to (1). The latter, Fig. 1a contains under hydrothermal conditions, dissolving completely upon
dimers of 7-coordinate Yb3+ centres bridged the dicarboxylates, being heated in water above 100 1C.
crosslinked by further BDC linkers to give a-Po type net that is
The coordination polymers (2) and (3) both crystallise in
interpenetrated with another such net, while (2) contains three water above 200 1C in 3 day synthesis reactions, so are extre-
distinct 8-coordinate Yb3+ centres bridged by hydroxides and mely hydrothermally stable materials. For (2) the structure has
BDC, two of which have terminal waters, with BDC ligands channels containing occluded water, and thermogravimetric
crosslinking denser regions of the structure, Fig. 1b. The building and thermodiffraction analysis reveals zeolitic behaviour with
unit of the structure is a hexameric cluster, shown on Fig. 2, removal of water on heating to 200 1C (ESI†). Importantly,
where six Yb centres are linked therein directly by shared (2) has the potential for solid-acid properties with both bridging
oxygens. We also discovered that by moderating the synthesis hydroxyls (Brønsted acidity) and coordinatively unsaturated
conditions (see ESI†) the novel salt Yb2(BDC)3 (3) crystallised Yb3+ once bound water is removed to lower the coordination
directly from water at 200 1C. The structure of (3) contains number to seven (Lewis acidity), Fig. 2. In contrast (3) is a dense
structure with fully saturated Yb centres, containing only the
linker and the metal cation with no occluded species or addi-
tional ligands. We therefore chose (2) as a potential water-stable
Fig. 1 The structures of (a) Yb2(BDC)3(DMF)2(H2O)2 (1) viewed along a,
showing one of the 3D connected networks of the interpenetrated
structure, (b) Yb6(BDC)7(OH)4(H2O)4 (2) along b, and (c) Yb2(BDC)3 (3)
along b. The Yb atoms are olive, carbon grey, oxygen red and nitrogen
blue. Hydrogen atoms are omitted for clarity and broken-off bonds
represent the extended connectivity.
Fig. 2 Powder XRD Le Bail fit of a bulk sample of Yb6(BDC)7(OH)4(H2O)4
(2) (see ESI† for crystal structure parameters). The data points are black
triangles, the fitted pattern the red line, the difference curve the grey line
and the blue ticks the positions of allowed Bragg peaks. The inset shows a
view of the hexameric cluster unit in the material, with the same atom key
as for Fig. 1, except for inclusion of hydrogen atoms as white spheres.
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solid-acid catalyst, and examined the conversion of glucose in conversion of glucose is associated with a much lower selectivity
deionised water (pH = 7) at 140 1C, with the results summarised towards the desired products, suggesting decomposition of
in Fig. 3 (see ESI† for full data). For comparison, we studied an glucose by other pathways.
aqueous solution of YbCl3 as a homogeneous catalyst and Yb2O3
We examined the possibility of recycling the catalyst, and
as a heterogeneous comparison (with the same total Yb content after recovering the solid, drying and adding to a fresh reaction,
used for each). While the homogeneous solution of YbCl3 gives virtually the same conversions and selectivities were obtained
the highest conversion of glucose, the selectivity towards fruc- over four runs, Fig. 4a. SEM shows little change in the particle
tose and 5-HMF is not high (48.3%); in fact previous work has morphology before and after use, with a distribution of crystal-
shown that the pH needs to be lowered to obtain more signifi- lites micron-sized and less (Fig. 4b and c), while powder XRD,
cant conversion to the desired products from metal chloride Fig. 4d, shows that the material remains intact after use as a
salts, and then only at 170 1C using a biphasic solvent catalyst, albeit contaminated with some amorphous humins as
mixture.30,31 Solid Yb2O3 shows a low conversion and only a byproducts, consistent with the brown colour of the material
small amount of fructose with no detectable 5-HMF in the product after catalysis.
mixture after 3 hours, as expected in the absence of any Brønsted
The ytterbium MOF (2) acts as a catalyst for glucose conver-
acid sites. In contrast (2) shows a reasonable conversion of glucose sion in water alone, with no additional mineral acid necessary.
(B16%) after 3 hours and a selectivity towards fructose and The fact that significant amounts of 5-HMF are produced under
5-HMF of 50%. Since it may be expected that the kinetics of these conditions, and is indeed the majority product after
the heterogeneous reaction may be retarded compared to the 24 hours, clearly indicates that the solid provides the necessary
homogeneous case, we examined long reactions. As can be seen Brønsted acidity. Most other MOFs that have been reported so
on Fig. 3, extending the reaction time to 24 hours with (2), far have been studied in mixed water/organic solvents or with
increases the conversion to a useful 28%, with a notable 65% the addition of HCl (see Table S4, ESI†), but for the cases where
selectivity towards 5-HMF, and 70% total selectivity towards just water has been used the ytterbium MOF outperforms
fructose and 5-HMF. In all cases only negligible amounts of the zirconium MOF NU-1000, which gave 60% conversion of
mannose are produced, a possible byproduct from the epimer- glucose but yields of only 19% fructose and 2.3% 5-HMF (at
isation of glucose.
140 1C for 5 hours).24 Sulfonyl-modified UiO-66 gave a similar
The presence of acid sites in (2) was proven using ammonia 35.9% conversion to the Yb MOF in water alone and an initially
temperature programmed desorption, which is consistent with high selectivity, but the efficiency dropped on recycling the
the presence of two types of acid sites associated with the solid.25 Comparisons with other solid acids used for glucose shows
material, and pyridine adsorption that indicates both Brønsted that our materials offer significant advantages. For example,
and Lewis acid sites (ESI†). The fact that a significant amount of conventional solid-acid catalysts, such as sulfated zirconium
5-HMF is produced, and that the proportion of this vs. fructose oxide,36 or sulfated niobium oxide,37 require concentrated
increases with extended reaction time, implies the presence of mineral acids, or other noxious reagents, for their synthesis.
Brønsted acidity inherent to the material. Interestingly, lowering Tsapatsis and co-workers have recently shown that a composite
the pH to 2.5 with HCl offers no advantage: as shown on Fig. 3, of MIL-101(Cr) and Cr(OH)3 gave high conversion of glucose but
the total conversion and distribution of products is similar to at this was in a two-step process with ethanol as the solvent for
pH = 7 at short reaction times, while after 24 hours the higher isomerisation to ethyl fructoside, followed by addition of water
to allow hydrolysis to fructose, each performed at 100 1C for
24 hours with almost 60% of the product fructose.23 The use of
multiple batch reaction steps requiring long reaction times is
Fig. 3 Results of catalytic conversion of glucose using [Yb6(BDC)7(OH)4-
(H2O)4] (2) and compared with a homogeneous solution of YbCl3 and with
solid Yb2O3.
Fig. 4 (a) Recyclability of the catalyst (2) with SEM (b) before and (c) after
heating in glucose at 140 1C for 24 hours (the scale bar is 1 mm) and
(d) powder XRD (l = 1.5406 Å) after reuse.
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likely to be less attractive than continuous flow systems on
scale-up. In fact, a lower conversion with higher selectivity may
be useful if applied within continuous flow recycle reactors.
Further work is needed to understand the molecular-scale
mechanisms of the conversions of sugars over MOF catalysts:
this includes the identity of intermediates and byproducts, and
the possibility of disassembly of the surface of the MOF. This
would then allow optimisation of activity and implementation
of the materials in real processes. The use of hydrothermally
stable materials containing the smaller lanthanide cations in
heterogeneous solid-acid catalysis will also be of wider relevance
to other industrial conversions, such as condensations and dehy-
drations, and applications where stability in water is needed,38 for
which metal–organic frameworks are only beginning to be being
explored.39,40
We thank the Royal Society (Challenge Grant CH160099) and
the EPSRC grant EP/P511432/1, Global Challenge Research
Fund (GCRF) Institutional Award for the University of Warwick,
for funding. Support from a Research Grants for Higher Educa-
tion 2016 from DGHE, Indonesia is also gratefully acknowl-
edged (No. 1136/UN2.R12/HKP/05.00/2016). The I11 beamtime
was obtained through the Diamond Light Source Block Alloca-
tion Group award ‘‘Oxford Solid State Chemistry BAG to probe
composition–structure–property relationships in solids’’ (EE13284)
and we thank Dr Mark Senn and Mr Gabriel Clarke for their
assistance. The research data supporting this publication can
be accessed at: https://wrap.warwick.ac.uk/124044.
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There are no conflicts to declare.
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