Cooperative catalysis at the metal-MOF interface: Hydrodeoxygenation of vanillin over Pd nanoparticles covered with a UiO-66(Hf) MOF

Article abstract of DOI:10.1039/c9dt01371g

Cooperative catalysis has been demonstrated over metal-MOF hybrids for the conversion of vanillin (biomass based platform molecules) into value-added 2-methoxy-4-methylphenol. Over a Pd@UiO-66(Hf) core-shell catalyst, cooperativity between Br?nsted acidic μ₃-OH groups and Pd active sites present at the interface has rendered a catalytic performance of >99% vanillin conversion and >99% 2-methoxy-4-methylphenol selectivity at 90 °C under 3 bar H₂ in water. An enhanced cooperative effect has been observed over a core-shell catalyst compared to a support catalyst.

Full text of DOI:10.1039/c9dt01371g

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Cooperative Catalysis at Metal-MOF Interface:
Hydrodeoxygenation of Vanillin over Pd Nanoparticles Covered
with UiO-66(Hf) MOF
Received 00th January 20xx,
Accepted 00th January 20xx
Vasudeva Rao Bakuru,a,b Deljo Davis a, and Suresh Babu Kalidindi a*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cooperative catalysis has been demonstrated over metal-MOF
hybrids for the conversion of vanillin (biomass based platform
molecules) into value-added 4-hydroxy-3-methoxybenzaldehyde.
Over Pd@UiO-66(Hf) core-hsell catalyst, cooperativity between
Brønsted acidic μ3-OH groups and Pd active sites present at the
interface has rendered a catalytic performance of >99% vanillin
conversion and >99% 2-methoxy-4-methylphenol selectivity at 90
˚C under 3 bar H2 in water. Enhanced coperative effect has been
observed over core-shell catalyst compared to support catalyst.
Porous crystalline metal-organic frameworks (MOFs) with
chemically well-defined pore surface recently attracted great deal
of attention as a new class of heterogeneous catalysts because of
the presence of much desired and clearly demarcated active
sites.1,2,3,4 The active sites can be present in MOF either on the
secondary building units (SBUs) or on the organic linker.5,6,7
Combing MOFs with metal nanoparticles (MNPs) is an interesting
prospect as this allows rational designing of novel multifunctional
catalysts through synergization of active sites present on both
entities.8,9 To date good number of metal-MOF hybrid catalysts
have been reported for
a
wide number of chemical
transformations.10,11,12 However, in most of these reports MOFs
behave as stabilizers for MNPs and the studies that deal with
cooperative catalysis between the active sites of MOF and MNPs
are very few.13,14,15
Scheme 1. a) Schematic representation of Pd@UiO-66(Hf) core-shell and Pd/UiO-
66(Hf) materials. Pd and μ3-OH active sites present at the metal-MOF interface is
also shown; b) Vanillin hydrodeoxygenation reaction along with the steps
involved is shown.
interface in metal-MOF hybrids is to grow the MOF directly onto the
Cooperative catalysis involves simultaneous activation of
reactants by different catalytic functional sites which accelerates
the rate of the reaction (compared to either functional sites
operating independently) with precise control over the products.15
This can be realized by apt selection of functional sites and their
precise installation in close proximity. In this regard, the metal-MOF
interface is potentially poised for cooperative catalysis because of
the presence of active sites of the both entities in close
proximity.16,17,18 The best way to maximize the metal-MOF
preformed metal nanoparticles (bottle around ship approach).11,16,17
10,19,16
This kind of metal-MOF core-shell architectures
have been
recently studied for catalysis due to the obvious advantages such
as sieving of reactant molecules,20,12 immobilization of reactants
around active centers15 and tunable environment around active
centers.21 Though positive effect of MOF coating over metal
nanoparticles for catalysis is demonstrated to good extent,
cooperative catalysis at metal-MOF interface has not been
elaborately investigated. 11,16,17
Vanillin (4-hydroxy-3-methoxybenzaldehyde) is
a famous
a.
Materials Science Division, Poornaprajna Institute of Scientific Research,
Devanahalli, Bangalore Rural- 562164, India. Email:
sureshk@poornaprajna.org/ksureshu@gmail.com
flavouring agent used in cosmetics, food, and pharmaceuticals as
well. Even though this can be extracted from plants like vanilla
planifolia the extraction process is inefficient and the current
demand is more or less is met from chemical synthesis 22 or by acid
hydrolysis of lignin derived from biomass.23 Hydrodeoxygenation of
vanillin is highly sought-after for their conversion into value-added
b.
Manipal Academy of Higher Education, Manipal-576104, India.
Supporting information for this article is given via a link at the end of the
document.† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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measured to be 755 ±50 m2/g(core-shell) and 772±50 Vmie2w/gA(rtsiculepOpnolrinte)
from N2 isotherms at 77K(Figure S3 and SD4O).I:In10d.u10c3ti9v/eCl9yDcTo0u13p7le1Gd
plasma optical emission spectrometry (ICP-OES) analysis revealed
presence of 5.2±0.3 wt% Pd and 4.7±0.4 wt% Pd in Pd@UiO-66(Hf)
and Pd/UiO-66(Hf) materials, respectively. Scannin electron
microsopy (SEM) analysis suggest that both core-shell and support
catalysts have neraly spherical morphology(Figure S5). The
transmission electron microscopy (TEM) bright field images suggest
that Pd nanoparticles in Pd@UiO-66(Hf) are covered with MOF
matrix(Figure 1a, Figure S6). Whereas, most of the Pd NPs are
clearly present on the edges of the crystal for Pd/UiO-66(Hf) (Figure
1b). The detailed transmission electron microscopy characterization
confirming the respective nanostructures of the hybrids is reported
earlier.32 Presence of Brønsted acidic sites in Pd@UiO-66(Hf)
Figure 1. TEM bright field image of a) Pd@UiO-66(Hf) core-shell and b) Pd/UiO-
66(Hf) support material.
materials is evident from the acid-base titrations(Table S1, Figure
34,35
S7).
Similar to UiO-66(Hf), first derivative curve of Pd@UiO-
66(Hf) showed three peaks at pKa = 3.4, 5.7 and 7.3 which are
assigned to μ3–OH, –OH2 (coordinated at missing linker defect) and
–OH protons. 34,35 Therefore, Pd-UiO-66(Hf) hybrids contain both Pd
active sites and acidic sites.
Vanillin is derived from lignin (cross-linked phenolic bio-
polymers) pyrolysis and is considered as a model compound for
36,37
biofuel upgrading.
Initially, Pd NPs activate the hydrogen and
reduce aldehyde group to give vanillin alcohol. In next step,
deoxygenation step may be accelerated in the presence of acidic
sites to give the desired 2-methoxy-4-methylphenol(MMP)(Scheme
1).38,18 However, HDO of vanillin is generally accompanied by
vanillin alcohol (especially at temperatures around 90˚C) as a result
of partial hydrogenation, while a high selectivity to MMP has been
rarely reported.40
Figure 2. Comparison between the catalytic performances: a) Conversion and
product selectivity for vanillin hydrodeoxygenation over Pd@UiO-66(Hf) core-
shell, Pd/UiO-66(Hf) support, Pd+UiO-66(Hf) physical mixture. Reaction
conditions: 0.5 mmol of vanillin, 5 mL of water and 1.2 mol% of Pd catalyst with
respect to vanillin at 90 ˚C and 3 bar H2 for 2h.
We carried out HDO of vanillin in water at 90 ˚C over hybrid
catalysts and physical mixture under 3 bar H2 gas. The comparison
between the catalytic performances of three catalysts is shown in
figure 2. Pd@UiO-66(Hf), Pd/UiO-66(Hf) and Pd NPs+UiO-66(Hf)
showed a vanillin conversion and MMP selectivity of 99.8%, 99.8%;
91.3%, 60.3%; and 66 %, 33% respectively. Pd@UiO-66(Hf) is one of
the rare examples over which >99% vanillin conversion and >99%
MMP selectivity has been achieved at temperatures around 90
˚C(Tabel S3). Other catalysts reported with similar catalytic
chemicals.24,25 Closely placed metal and Brønsted acidic sites
are found to accelerate the hydrodeoxygenation reaction.26,18,27,28
In this context, we selected chemically robust UiO-66(Hf) MOF
29
{Hf6O4(OH)4[(O2C-C6H4-CO2]
}
6
MOF for the present study. This
MOF contains μ3-OH groups on secondary building unit (SBU)
which are Brønsted acidic in nature owing to higher oxophilicty of
Hf(IV).30 We recently demonstrated the Brønsted acidic character of
μ3-OH groups through the facile synthesis of solketal from glycerol
and acetone over UiO-66(Hf) with very high turnover frequency
(13886 h-1).31 Therefore, Pd-UiO-66(Hf) hybrids could be potential
metal-acid bifunctional catalysts for the HDO of vanillin (Sheme 1).
Among various metal-MOF catalysts, Pd-MOF hybrids are one of the
performance
include Pd/PRGO/Ce-MOF,41 Pd@NH2-UiO-66,26
Pd/SO3H-MIL101(Cr),18 Pd/N-doped porous carbon derived from
ZIF-8.42 The calculated turnover frequency values were 742.4 h-1,
446.9 h-1, and 245.2 h-1 for Pd@UiO-66(Hf), Pd/UiO-66(Hf) and
PdNPs+UiO-66(Hf), respectively (Figure S8). Both hybrid materials
clearly outperformed physical mixture catalyst in terms of activity
and selectivity towards MMP. This emphasizes the role played by
UiO-66(Hf) in enhancing the catalytic properties of Pd NPs and
confirms that the observed catalytic properties of hybrid catalysts
are not the simple liner combination of their individual
counterparts.
13,
well explored systems especially for hydrogenation reactions.
20,12,15,21
We very recently synthesized Pd-UiO-66(Hf) hybrids
(support and core-sehll, Scheme 1) and shown their effectiveness
for dehydrogenation of ammonia borane in non-aqueous
medium.32 In the present work, we used these Pd@UiO-66(Hf) core-
shell and Pd/UiO-66(Hf) support metal-MOF interfaces for vanillin
HDO.
Palladium nanoparticles33 were supported on UiO-66(Hf)
crystals external surface by stirring MOF powders with colloidal Pd
NPs in dimethylformamide(DMF) to give Pd/UiO-66(Hf) supported
material. On the other hand, Pd@UiO-66(Hf) material was obtained
by carrying out UiO- 66(Hf) MOF) sysnthesis(using HfCl4·6H2O and
benzene dicarboxylic acid in the solvent(DMF) containing colloidal
Pd NPs (For details S.I section2). The powder X-ray diffraction (XRD)
data of the samples confirmed the presence of UiO-66(Hf) in both
hybrid materials(Figure S1 and S2) and the surface areas were
Further, clear differences were observed between the catalytic
performances of Pd/UiO-66(Hf) and Pd@UiO-66(Hf). Time resolved
studies were carried out over these two catalysts to get insight into
the reaction pathway. A vanillin conversion of >99% is reached at
30 min over Pd@UiO-66(Hf), whereas over Pd/UiO-66(Hf) a vanillin
conversion of only 30.1% has been achieved in the same reaction
time(Figure 3a and 3b). Over both catalysts, vanillin alcohol is the
dominate product in the initial stages of the reaction. During the
course of the reaction, vanillin alcohol got converted into MMP.
This suggests that vanillin HDO is majorly a stepwise process and
formation of MMP proceeds through vanillin alcohol
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intermediate.43 In Pd@UiO-66(Hf), at the interface all the Pd active
sites are surrounded by MOF pores(nanoreactors). Therefore,
confinement effects will come into force and can accelerate the
rate of the reaction.
Conclusions
View Article Online
DOI: 10.1039/C9DT01371G
Figure 4. a) ) Recyclability of Pd@UiO-66(Hf) catalyst, b) Hot filtration test for
Pd@UiO-66 (Hf) catalyst. Hot filtration test was carried out by separating the
catalyst from reaction mixture after 10 min of reaction time. The reaction was
continued further in the absence of catalyst, which showed no further vanillin
conversion
In summary, cooperativity between the active sites (Pd and μ3-
OH) present at the metal-MOF interface has been demonstrated for
vanillin hydrodeoxygenation. Pd@UiO-66(Hf) core-shell catalyst
outperformed Pd/UiO-66(Hf) support catalyst and the physical
mixture, and exhibited >99% vanillin conversion and >99% MMP
selectivity at 90 ˚C in water. Catalyst poisoning experiments
revealed the positive role played by Brønsted acidic μ3-OH groups in
the conversion of vanillin alcohol into MMP. The Pd@UiO-66(Hf)
hybrid material retained its structure even after 5 cycles. The
results discussed here highlights the versatility of metal-MOF
interface for co-operative catalysis and stimulates further research
in this direction.
Figure 3. Time resolved studies: vanillin hydrodeoxygenation over a) Pd@UiO-
66(Hf) and b) Pd/UiO-66(Hf). c) conversion of vanillin alcohol to 2-methoxy-4-
methylphenol over Pd@UiO-66(Hf), Pd/UiO-66(Hf) and NH3 treated Pd@UiO-
66(Hf). Reaction conditions: 0.5 mmol of substrate, 5 mL of H2O and 1.2 mol% of Pd
catalyst w. r. t. substrate, at 90 ˚C and 3 bar H2.
Further, these nanoreactors contain Brønsted acidic μ3-OH
groups(present on SBUs) which could accelerate deoxygenation of
vanillin alcohol into MMP. A high MMP selectivity of 99.8%
observed for Pd@UiO-66(Hf) can be an outcome of
the
cooperativity between Pd and μ3-OH sites. To establish the role
played by μ3-OH sites, we selectively poisoned the μ3-OH sites using
aqueous NH3 (Figure S9).43 Under the same reaction conditions,
MMP selectivity has dropped to 23.3% from 99.8% while the
conversion was around 95% over Pd@UiO-66(Hf)-NH3
catalyst(Figure S10). Further, we carried out deoxygenation of
vanillin alcohol over Pd/UiO-66(Hf), Pd@UiO-66(Hf) and Pd@UiO-
66(Hf)-NH3 catalysts. The comparison between the performances is
shown figure 3c. Clearly, deoxygenation step is more facile over
Pd@UiO-66(Hf) and μ3-OH sites played vital in the acceleration of
the reaction. All these outcomes points out at the cooperative
catalysis that has taken place at metal-MOF interface. The possible
mechanism involves H+ attack on the –OH group which leads to the
cleaving of the alkyl–OH bond through the elimination of
water(Scheme S1).28 Subsequent interaction of the intermediate
with atomic hydrogen present on Pd gives MMP product.
Conflicts of interest
“There are no conflicts to declare”.
Acknowledgement
SBK thanks Science and Engineering Research Board Govt.of India,
for funding this work under early carrier research
award(ECR/2015/000350). SBK also thanks Department of science
and technology (DST), for INSPIRE faculty award (IFA-14/CH-166).
BVR is thankful to Manipal University, India, for accepting this
research as a part of a Ph.D. program and also to PPISR providing
facility for research work.
Recyclability and leaching tests were carried over Pd@UiO-
66(Hf) to confirm the stability of material and heterogeneity of the
reaction. As evident from figure 4a, both activity and selectivity of
the catalyst were more or less retained after 5 cycles. X-ray
diffraction and N2 sorption analysis of the used catalyst confirm that
the catalyst is quite stable (Figure S11, S12 and 13). Also XPS
analysis suggest that oxidation state of palladium remains as zero
even after catalysis (Figure S14). Further, hot filtration test (Figure
4b) and ICP-OES analysis (supernatants did not show any
detectable amounts of Pd and Hf) reinforced the the stability of the
catalyst heterogeneous nature of the reaction.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
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