Conformal sulfated zirconia monolayer catalysts for the one-pot synthesis of ethyl levulinate from glucose

Article abstract of DOI:10.1039/c4cc04594g

Here we describe a simple route to creating conformal sulphated zirconia monolayers throughout an SBA-15 architecture that confers efficient acid-catalysed one-pot conversion of glucose to ethyl levulinate.

Full text of DOI:10.1039/c4cc04594g

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Conformal sulfated zirconia monolayer catalysts
for the one-pot synthesis of ethyl levulinate from
glucose†
Cite this: Chem. Commun., 2014,
50, 11742
Received 17th June 2014,
Accepted 7th August 2014
Gabriel Morales,a Amin Osatiashtiani,b Blanca Hernandez,a Jose Iglesias,a
´
Juan A. Melero,*a Marta Paniagua,a D. Robert Brown,c Marta Granollers,c
Adam F. Leeb and Karen Wilson*b
DOI: 10.1039/c4cc04594g
www.rsc.org/chemcomm
Here we describe a simple route to creating conformal sulphated
The one-pot production of alkyl levulinates directly from C6
zirconia monolayers throughout an SBA-15 architecture that confers sugars (Scheme 1) would thus provide a highly desirable alter-
efficient acid-catalysed one-pot conversion of glucose to ethyl native approach, especially attractive if abundant cellulose-
levulinate.
derived glucose could be reacted directly with an alcoholic
medium, obviating the need for an LA feedstream. Use of
The US DoE has identified various platform chemicals obtain- an alcoholic medium would also help to minimise humin
able via chemical or biochemical transformation of lignocellu- formation.15 Recent attempts to synthesize alkyl levulinates
losic biomass1,2 as renewable alternatives to existing fossil fuel via homogeneous and heterogeneous acid catalysis16–21 suffer
derived chemicals. Key chemical intermediates that can be serious drawbacks including poor yields, high reaction tem-
synthesised from biomass derived sugars include furanic peratures which induce undesired dehydration of the alcohol
components, such as 5-hydroxymethylfurfural (5-HMF) and solvent/reactant, and poor catalyst recyclability.22 Previous
furfural,3 and levulinic acid (LA) which offers a larger family studies have also focused solely on the use of lower alcohols
of derivatives with significant commercial application (e.g. (methanol or ethanol), and not bulkier alcohols which enhance
solvents, fuel additives and monomers). The latter’s chemical the properties of fuel end-products. Improvements in process
versatility, reflecting the carboxylic acid and ketone functional efficiency for alkyl levulinate production require more active
groups, low cost and ready availability from 5-carbon and and selective solid acid catalysts. In particular, there is a need
6-carbon sugars, suggests that LA could be a building block for bifunctional catalysts capable of directing the Lewis acid
of central importance within future biorefineries.4 For example, catalysed isomerisation of alkyl glucoside intermediates to alkyl
¨
alkyl levulinates have been proposed as excellent candidates for fructosides, and their subsequent Bronsted acid catalysed
‘second generation’ biofuels.5–8 Synthesis of such levulinates dehydration to 5-alkoxymethylfurfural and esterification to
from cellulosic biomass has been previously reported,9–11 nota- form alkyl levulinate and formate (Scheme 1). The correct
¨
bly the acid catalysed esterification of LA with lower alkyl balance of Lewis and Bronsted acid sites is critical to the
alcohols under severe reaction conditions utilising e.g. hetero- success of this complex tandem transformation.
polyacids,12,13 Amberlyst-15, acid zeolites and sulphated oxides;14
We recently reported the preparation of bifunctional SO4/ZrO2
or sulfonic acid silicas,8 however such routes necessitate multi- (SZ)23 catalysts with tunable acid site distributions for 5-HMF
step processes and a source of high-purity LA (non-trivial to obtain production from glucose via a related reaction pathway, involving
due to contamination by polymeric humins).15
Lewis acid catalysed glucose isomerisation to fructose, followed by
a Department of Chemical and Energy Tech, Chemical and Environmental Tech,
Mechanical Tech and Analytical Chemistry, Universidad Rey Juan Carlos,
´
C/Tulipan. s/n. E-28933, Mostoles, Madrid, Spain. E-mail: juan.melero@urjc.es;
Fax: +34-91-488-74-68
b European Bioenergy Research Institute, Aston University, Birmingham B4 7ET, UK.
E-mail: k.wilson@aston.ac.uk; Tel: +44 (0)121 204 5456
c School of Applied Sciences, University of Huddersfield, Huddersfield, HD1 3DH,
UK. E-mail: d.r.brown@hud.ac.uk; Tel: +44 (0)1484 473397
† Electronic supplementary information (ESI) available: Experimental details,
porosimetry, XRD, TEM, XPS, ICP-OES, FTIR-pyridine, NH3-TPD and additional
reaction data. See DOI: 10.1039/c4cc04594g
Scheme 1 Proposed reaction pathway for the acid-catalyzed conversion
of glucose to ethyl levulinate in ethanol.16
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Fig. 2 (a) Si 2p XP attenuation demonstrating layer-by-layer growth of
B0.42 nm thick ZrO2 sheets. Dashed line shows the theoretical fit and
associated film thickness to achieve the observed attenuation; (b) Bronste-
¨
d:Lewis acid ratio from pyridine titration and H+ density determined by TPD.
Fig. 1 (a) N2 adsorption isotherms and pore size distributions; and (b, c)
HRTEM of SBA-15 with grafted SZ layers.
of a (111) oriented monolayer of monoclinic ZrO2 (0.42 nm),
¨
Bronsted acid catalyzed dehydration to 5-HMF. The utility of such confirming the successful growth of conformal ZrO2 mono-
non-porous SZ catalysts is however limited by their inherently low layers over SBA-15.
surface areas (typically o150 m2 gÀ1), and hence methodologies
The composition, morphology and surface acidity of these
are required to stabilise either a highly porous or dispersed SZ grafted ZrO2 layers upon sulfation with 0.075 M H2SO4 was further
phase. Here we report the preparation of high surface area, probed by XRD, XPS, NH3 calorimetry, and pyridine titration. The
thermally robust SZ conformal monolayers with tunable Lewis:- absence of wide angle XRD patterns (Fig. S1b, ESI†) confirms that
¨
Bronsted acid site densities over a mesoporous SBA-15 template, large SZ crystallites were not formed upon sulfation and calcination.
for the tandem conversion of glucose to alkyl levulinates in Furthermore, the Zr : Si atomic ratios from XPS and EDX (Fig. S3,
alcoholic media.
ESI†) remain unchanged by sulfation, suggesting there is minimal
Zirconia monolayers were first grown in a consecutive fashion sintering of the ZrO2 film during impregnation with H2SO4 and
over SBA-15, via sequential grafting and hydrolysis cycles re-calcination. Bulk and surface Zr and S contents for each
employing a zirconium isopropoxide precursor. The uniformity sample (Table S4, ESI†) show the S : Zr ratio falls with ZrO2
of the subsequently sulfated grafted layers was confirmed by thickness, consistent with surface sulfation.
manometric porosimetry (Fig. 1a) which reveals a progressive
Acid site strength and loading was probed by NH3 calori-
decrease in pore volume, mean pore size and BET surface area metry (Fig. S5 and Table S5, ESI†). The 1 and 2 ML samples
with each grafting cycle, consistent with a layer-wise growth. Full exhibit a small proportion of sites at low NH3 coverage with
analysis of the N2 adsorption–desorption isotherms via the t-plot ÀDHads B 180–200 kJ molÀ1, attributable to strong Lewis
method enables micro-, meso- and macropore volumes to be acid sites.24 All grafted SZ/SBA-15 samples also exhibit sites
assessed and hence the influence of the grafting process on the of moderate acidity having a ÀDHads B 100–120 kJ molÀ1
,
¨
textural properties of final materials. t-plot analysis (Tables S1 consistent with Bronsted acid sites, with the 2 ML SZ/SBA-15
and S2, ESI†) reveals a progressive decrease in pore volume on sample possessing the highest acid site loading. It is also
each grafting cycle, which is mainly associated with loss of interesting to note that pyridine titration (Fig. 2b and Fig. S7,
¨
mesoporosity and consistent with the preferential incorporation ESI†) shows that the Bronsted:Lewis acid site distribution
of Zr onto the surface of the mesoporous channels. This is increases with each ZrO2 grafting cycle, suggesting that the
supported by pore wall thickness calculations (Table S3, ESI†). initial ZrO2 layers are more defective or electronically perturbed
HRTEM (Fig. 1b and c and Fig. S1a and S2, ESI†) confirms the by the underlying SiO2 substrate. Fig. 2b also shows the varia-
long range order of the hexagonal SBA-15 phase is retained, and tion in acid site loading (derived from NH3 TPD) with ZrO2 film
that the SBA-15 pore walls also remain uniform upon Zr grafting thickness, which exhibits a maximum of 0.40 mmol gÀ1 for the
and sulfation, with no evidence for large crystallite deposition 2 ML sample. This suggests that a complete surface coverage of
even after three grafting cycles.
acid sites is achieved once the second layer is completed, and
XPS was employed to quantify the growth mode of the the third grafted layer does not further enhance acid properties.
parent ZrO2 film, with attenuation of the Si 2p signal providing The requirement for a ZrO2 bilayer to optimise acid site loading
a direct measure of film thickness. Fig. 2a shows an exponential may reflect different speciation of Zr on the SBA-15 support.
decay in the SBA-15 substrate intensity with consecutive Zr Crystallization of Zr into a tetragonal structure is necessary to
deposition, indicative of a layer-by-layer growth mode. The first achieve superacidity,25 and might be expected to require a
and second grafting cycles attenuate the substrate by 35.6 and bilayer to achieve the correct atomic arrangement. Indeed
51.9%, equating to 0.5 and 0.84 nm thick adlayers of ZrO2 the envelope for the Zr 3d XP spectra (Fig. S4, ESI†) shifts
respectively. This is in excellent agreement with the thickness slightly to lower binding energy with increasing layer thickness,
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Fig. 4 Comparison of different alcohols for the conversion of glucose to
alkyl levulinates over 2 ML grafted SZ/SBA-15 solid acid catalyst. Reaction
conditions: 140 1C; 24 h; 0.25 g of glucose; 2 : 1 glucose : catalyst mass
ratio; 1 : 100 glucose : ROH molar ratio.
Fig. 3 Glucose conversion to ethyl levulinate as a function of reaction
temperature over 2 ML grafted SZ/SBA-15. Reaction conditions: 24 h; 0.25 g
of glucose; 2 : 1 glucose : catalyst mass ratio; 1 : 100 glucose : EtOH molar ratio.
EL yield increased slightly using fructose instead of glucose
(Table S6, ESI†), indicating that ethyl glucoside isomerisation
to ethyl fructoside occurs more slowly than the subsequent
dehydration of ethyl fructoside to 5-ethoxymethylfurfural.
In contrast, HMF conversion to EL was comparatively poor at
140 1C, reflecting its instability with respect to humins, but
conferred EL yields four times greater achieved with glucose at
130 1C (Fig. S9, ESI†). A ZrO2 co-catalyst enhanced this slow
isomerisation step and net EL yield (Table S7, ESI†).
consistent with electronic perturbation of the zirconia mono-
layer at the SiO2 interface.
The catalytic performance of the 2 ML SZ/SBA-15, which
¨
possessed the optimum balance of Bronsted:Lewis acidity and
acid site density, was subsequently evaluated for the direct
production of ethyl levulinate (EL) from glucose (Fig. 3).
Complete glucose conversion was observed under all condi-
tions, however the EL yield was sensitive to reaction temperature,
displaying a volcano dependence with a maximum of 25 mol%
around 140–150 1C. The fall in EL yield at high temperature
coincided with the appearance of polymers from product degra-
dation. Dispersing sulphated zirconia bilayers over a nanoporous
SBA-15 template dramatically enhances the number (and unifor-
mity) of accessible activÀe1sites, resulting in ethyl levulinate yields
up to 141 molEL molZr (Fig. 4). This represents a significant
improvement on the best literature performance, wherein amor-
phous bulk SZ, employed with poor Zr : glucose molar ratios of
B0.7, confer ethyl levulinate yields of only 44 molEL molZrÀ1 at far
higher temperatures 4200 1C which drive undesired intermole-
cular dehydration of ethanol to diethyl ether.16
Therefore, a clear advantage of our catalysts is the capability
of reaching relatively high EL yields under moderate tempera-
tures, while avoiding ethanol losses as diethyl ether. We
attribute the enhanced low temperature activity of grafted
SZ/SBA-15 to the presence of strong Lewis acid sites which
drive glucose isomerization. Catalyst reusability was confirmed
over three consecutive runs (Fig. S8, ESI†), with intermediate
calcination of the used catalyst at 550 1C to remove organic
deposits, which revealed the yield to EL was maintained or even
slightly increased. This evidences the stability of the sulfated ZrO2
monolayers grafted on SBA-15, and overcomes the extended
leaching problems of commercial sulfated zirconias.
The versatility of grafted SZ monolayers towards alkyl levuli-
nate production was further explored using methanol and iso-
propanol (Fig. 4). Methanol conferred a similar yield to ethanol,
whereas isopropanol was less reactive, presumably a result of
the bulkier intermediates and products in the production of
isopropyl-levulinate. Since there are no previous reports on the
telescopic production of isopropyl-levulinate from glucose, this
represents an exciting development in the catalytic production of
more complex alkyl-levulinates which would find wide application
as plasticizing agents, solvents, and speciality chemicals.
Conformal SZ monolayers with tuneable surface acid
strength and site density can be dispersed over a mesoporous
SBA-15 framework through a simple wet chemical grafting/
hydrolysis protocol. A bilayer SZ/SBA-15 material exhibits the
¨
maximum surface acidity and balance of Lewis:Bronsted sites,
and exhibits good performance in the one-pot conversion of
glucose to alkyl levulinates under mild conditions.
Financial support from the Spanish Ministry of Economy
and Competitiveness through the project CTQ2011-28216-
C02–01 is kindly acknowledged. KW thanks the Royal Society
for an industry fellowship and the EPSRC for funding (EP/
K000616/2 and EP/K014676/1). AFL thanks the EPSRC for a
Leadership Fellowship (EP/G007594/4).
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