Synthesis of Metal-Organic Frameworks as Dual-Acid
Catalysts for Glucose Transformation
Thomas W. Chamberlain,a Volkan Degirmenci,b Ryan Oozeerally,b Ralentri Pertiwi,a,b,c Yuni K. Krisnandi,c
Richard I. Waltona,*
a,
Department of Chemistry, University of Warwick, Coventry, CV4 7AL, U.K.
School of Engineering, University of Warwick, Coventry, CV4 7AL, U.K.
c Department of Chemistry, Universitas Indonesia, Depok 16424, Indonesia
*Corresponding author: [email protected]
b
Keywords: MOF, Lewis Acid, Brønsted Acid, Biomass, Hydroxymethylfurfural
1. Introduction
The selective dehydration of glucose to 5hydroxymethylfurfural (HMF) is a key step in the
conversion of biomass-derived organics into reactive
molecules that can then be used as the starting point
for production of numerous useful, and
commercially valuable, functional organic molecules,
such as fuels, drugs, monomers for polymerisation,
flavours and fragrances.1 A major challenge is to
find a solid catalyst to effect the glucose-HMF
transformation at mild temperatures and in water,
Figure 1.
Figure 1. Glucose isomerisation to HMF.
The best known heterogeneous catalyst is based on a
tin-containing zeolite (Sn-beta),2 but this has the
severe drawback of an extremely lengthy synthesis
of the solid catalyst (10s of days), which can only be
shortened by using seed crystals that must be made
separately, and uses high corrosive and toxic
chemicals (such as hydrofluoric acid). For this
catalyst it is suggested that the inclusion of tin
provides Lewis acidic sites, which in combination
with Brønsted acid sites of the zeolite framework is
believed to be responsible for the catalysis
properties.2 Thus new ways to prepare solid catalysts
with ‘dual acidity’, i.e. a combination of Brønsted
and Lewis acidity, are important to consider for
glucose conversion. In this contribution we report on
our work aimed at using metal-organic frameworks
(MOFs) as tunable heterogeneous catalysts with
engineered acid sites. The aim is to make use of the
ease of chemical modification of MOFs, in terms of
both metal centres and organic ligands to create
novel dual acid catalysts. The aim is to produce
hydrothermally stable materials that will allow the
dehydration of glucose in aqueous conditions,
ideally with selectivity towards HMF.
2. Experimental Part
The solvothermal crystallisation of a number of
target MOFs was performed using modified
literature procedures. We focused on materials that
could be crystallised in short reactions (~1 day) and
that were constructed from non-toxic metals and
readily available organic linkers. The frameworks
were also chosen to support coordinatively
unsaturated metal sites and so offer the possibility of
Lewis acid reactivity. We have thus studied the
synthesis of the materials MIL-101,3 MIL-125,4
UiO-66,5 and a recently reported Yb-BDC,6 as
shown in Figure 2.
Figure 2 Structures of the target MOFs for dual-acid
heterogeneous catalysts.
The use of benzene-1,4-dicarboxylate (terephthalate)
as linker was selected since this is known to yield a
number of MOF materials with a variety of metals
and framework topologies and could conceivably be
replaced by a Brønsted acid derivative, 2sulfoterephthalate.
Materials have been characterised using powder Xray
diffraction,
infra-red
spectroscopy,
thermogravimetric analysis and scanning electron
microscopy. Glucose conversion was studied in
water up to 140 oC in the absence and presence of
HCl as a mineral Brønsted acid.
3. Results and discussion
To introduce Lewis acidity we chose materials with
appropriate metal centres (e.g. Yb3+) or studied the
possibility of doping existing frameworks with
appropriate metal ions to prepared isomorphously
substituted materials (e.g. replacing Ti4+ or Zr4+ by
Sn4+, or Fe3+ by Sc3+). Figure 3 shows powder XRD
of two examples of the new materials prepared.
Figure 3. Powder XRD of MIL-101(Fe0.9Sc0.1) and UiO66(Zr0.9Sn0.1) samples.
4. Conclusions
We have prepared some novel dual acid materials by
using the tunable chemistry of metal-organic
frameworks to introduce Lewis acidic sites in
combination with Brønsted acid sites. Preliminary
screening tests show these materials are active for
glucose isomerisation. We will also discuss the
possibility of leaching of the metals by disassembly
of the MOF framework, and how to avoid this issue.
Acknowledgments
We thank the EPSRC Global Challenges Research Fund for
financial support.
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