Resin-supported iridium complex for low-temperature vanillin hydrogenation using formic acid in water

Article abstract of DOI:10.1039/d1ra01460a

Biorefinery seeks to utilize biomass waste streams as a source of chemical precursors with which to feed the chemical industry. This goal seeks to replace petroleum as the main feedstock, however this task requires the development of efficient catalysts capable of transforming substances derived from biomass into useful chemical products. In this study, we demonstrate that a highly-active iridium complex can be solid-supported and used as a low-temperature catalyst for both the decomposition of formic acid (FA) to produce hydrogen, and as a hydrogenation catalyst to produce vanillyl alcohol (VA) and 2-methoxy-4-methylphenol (MMP) from vanillin (V); a lignin-derived feedstock. These hydrogenation products are promising precursors for epoxy resins and thus demonstrate an approach for their production without the need for petroleum. In contrast to other catalysts that require temperatures exceeding 100 °C, here we accomplish this at a temperature of <50 °C in water under autogenous pressure. This approach provides an avenue towards biorefinery with lower energy demands, which is central to the decentralization and broad implementation. We found that the high activity of the iridium complex transfers to the solid-support and is capable of accelerating the rate determining step; the decomposition of FA into hydrogen and carbon dioxide. The yield of both VA and MMP can be independently tuned depending on the temperature. The simplicity of this approach expands the utility of molecular metal complexes and provides new catalyst opportunities in biorefinery.

Full text of DOI:10.1039/d1ra01460a

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Resin-supported iridium complex for low-
temperature vanillin hydrogenation using formic
Cite this: RSC Adv., 2021, 11, 15835
acid in water†
Christene A. Smith,‡ Francesco Brandi, ‡ Majd Al-Naji * and Ryan Guterman
*
Biorefinery seeks to utilize biomass waste streams as a source of chemical precursors with which to feed the
chemical industry. This goal seeks to replace petroleum as the main feedstock, however this task requires
the development of efficient catalysts capable of transforming substances derived from biomass into useful
chemical products. In this study, we demonstrate that a highly-active iridium complex can be solid-
supported and used as a low-temperature catalyst for both the decomposition of formic acid (FA) to
produce hydrogen, and as a hydrogenation catalyst to produce vanillyl alcohol (VA) and 2-methoxy-4-
methylphenol (MMP) from vanillin (V); a lignin-derived feedstock. These hydrogenation products are
promising precursors for epoxy resins and thus demonstrate an approach for their production without
ꢀ
the need for petroleum. In contrast to other catalysts that require temperatures exceeding 100 C, here
ꢀ
we accomplish this at a temperature of <50 C in water under autogenous pressure. This approach
provides an avenue towards biorefinery with lower energy demands, which is central to the
decentralization and broad implementation. We found that the high activity of the iridium complex
transfers to the solid-support and is capable of accelerating the rate determining step; the
decomposition of FA into hydrogen and carbon dioxide. The yield of both VA and MMP can be
independently tuned depending on the temperature. The simplicity of this approach expands the utility
of molecular metal complexes and provides new catalyst opportunities in biorefinery.
Received 23rd February 2021
Accepted 14th April 2021
DOI: 10.1039/d1ra01460a
rsc.li/rsc-advances
is possible, but the process is oen costly and time consuming.
Immobilization of molecular catalysts with known high activity
Introduction
Catalysis is useful for a variety of transformations to synthesize on solid supports makes their recyclability accessible in
pharmaceuticals and other value-added chemicals. Tradition- industry and is one strategy to overcome their troublesome
ally, metal complexes are used in catalysis because they are recovery. The potential of catalysis is oen limited by the scale
easily studied and modied to increase reactivity. One salient up necessary for industrial application and this is particularly
1–5
problem with them is their removal, which requires additional the case in biorenery, which is an approach geared towards
steps, solvent, and columns, nullifying the benets of lower upgrading lignocellulosic biomass to valued-added chemicals
3,6
heat and time that they provide. Heterogeneous catalysts do not in the so-called biorenery processing. Within this process,
have the advantage of obtaining mechanistic insight and lignocellulosic biomass undergoes physico-chemicals trans-
7
undergoing optimization to the degree of homogeneous cata- formation toward low-carbon footprint ne products. Among
lysts, but they are easily removed from solution and do not these reactions, catalytic hydrogenation is a key reaction which
8
–10
require the intense work up procedures, unlike homogeneous allows the production of a wide range of compounds.
Typi-
catalysts. They also have the advantage of recyclability, which is cally, biomass heterogeneously-catalyzed hydrogenation
a
crucial on an industrial scale for both environmental and reaction is conducted using relatively high temperature (100–
ꢀ
economic reasons. Immobilization of known molecular cata- 200 C), external pressurized H
as reductive agent and sup-
2
8,9,11–14
lysts onto a solid support harnesses the benets of both ported metals as catalyst, such as Pt, Pd, Ru and Ni.
homogeneous and heterogeneous catalysts with rational design Heterogeneous catalysis is the preferred method because it
and ease of removal. Recyclability with homogeneous catalysts allows for the implementation of more economically efficient
and environmentally benign reactor designs. However, bio-
renery processes are expensive when compared to traditional
reneries. In fact, the complexity of lignocellulosic biomasses
requires a large number of cost-effective processing steps.
Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Am
M u¨ hlenberg 1, 14476 Potsdam, Germany. E-mail: Ryan.guterman@mpikg.mpg.de
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra01460a
Reducing the reaction temperature and pressure using new
2
14–18
catalysts, replacing H
2
with formic acid (FA),
and achieving
These authors contributed equally to the manuscript.
©
2021 The Author(s). Published by the Royal Society of Chemistry
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this using solid-supported catalysts are three viable strategies to and H
2
pressure (2–2.5 MPa). However, the utilization of alter-
reduce the cost of biorenery. Innate difficulties in transporting native and sustainable hydrogen sources e.g., FA, and the
and storing H poses a signicant hurdle to biorenery, where performance of V hydrogenation at room temperature was scarcely
2
1
4,34,35
hydrogenation reactions are invariably needed to convert reported.
oxygen-rich biomaterial into useful substances for the chemical low-pressure methods offer reduced net energy demand, more
For biorenery in particular, low-temperature and
19
industry. Formic acid has emerged as a solution since it can be simplistic setups and nally the need to centralize the biorenery
easily stored as a liquid and can be produced at low temperature process in order to make the heat requirements economically
20
2 2
from CO and H or by biomass processing. Furthermore, feasible. At the moment, biorenery centralization is required in
current efforts to efficiently produce formic acid seeks to bolster order to reduce net energy demands by recycling heat, however
the “formic acid economy” as a pillar for future endeavours in low-temperature approaches allows for decentralization and thus
21
green energy and biorenery. Formic acid decomposition to reduces transportation costs for raw materials. For example,
hydrogen needs the assistance of a strong catalyst in order to a study examining the cost of straw valorization in Baden-
lower the overall temperature of the reaction. Ru and Ir Wuerttemberg, Germany, found that a centralized biorenery
molecular catalysts are among the most commonly used for this approach results in a transportation cost of 10% of the total cost
36
purpose a wide variety of structures producing different dehy- for straw. Furthermore, they explained that smaller, decentral-
drogenation capabilities with dehydrogenation occurring at low ized bioreneries offer better price-to-quantity ratios and are less
22
temperature in many cases. Iridium is known to be active in sensitive to feedstock cost uctuations. This is in contrast to larger
23
this reaction as known by Himeda. It is also known to be an bioreneries that require a constant supply of larger volumes of
efficient hydrogenation catalyst of carbonyls as shown by low-cost material and thus become less protable when feedstock
24
Albrecht and co-workers. Therefore it is an ideal catalyst not value increase in price. For these reasons, efforts towards smaller,
just to convert formic acid to hydrogen, but to also convert decentralized bioreneries via lower heating requirements and
a potential lignin feedstock to various other value-added simpler setups can improve the protability of biorenery.
chemicals at low temperature.
In this context, we report the use of a solid-supported
Vanillin (V) is nowadays the major lignin-derived feedstock iridium complex as a low-temperature catalyst for V upgrad-
already produced in industrial scale, mostly by Borregaard ing using FA. Utilizing a molecular complex provides the benet
2
5,26
company.
Vanillin itself is already used as a avor in the of harnessing developed methodologies for homogenous cata-
27
food and cosmetic industry. Vanillin has the potential to be lysts, but with the convenience of a heterogeneous catalyst for
upgraded to value-added compounds via catalytic hydrogena- separation. The resin supported Ir catalyst (IrPYA) was able to
ꢀ
tion. Indeed, the hydrogenation products of vanillin, i.e. Vanillyl decompose FA into hydrogen at low temperature (50 C), with
alcohol (VA) and 2-methoxy-4-methylphenol (MMP), can be maximum FA conversion of 76 mol%. The supported iridium
used respectively, as avor and bio-fuel, while both are prom- catalyst was also found to be active for V hydrogenation toward
8,28,29
ising precursors of sustainable epoxy resins.
VA and MMP in the presence of FA as a hydrogen source.
Vanillin hydrogenation proceeds toward VA and then pres- Product selectivity was found tunable with temperature,
ꢀ
ents a hydrodeoxygenation step to MMP, viz. in Scheme 1. To yielding a maximum of 54 mol% of VA at 25 C and a maximum
ꢀ
date, several studies have studied V hydrogenation, using sup- of 24 mol% of MMP at 50 C. These results demonstrate how
ported metal catalysts (Pd, Ru, Rh, Pd, Au, Pt and Ni) under metal complexes may be harnessed on solid-supports and their
8
,30–33
pressurized H
2
.
For example, Bindwal et al. reported VA use expanded in low-temperature and low-pressure biorenery.
ꢀ
and MMP yield of 90 mol% and 10 mol% respectively, at 65 C
with Ru/C and 2.1 MPa of H
on a complete conversion of V to MMP at 150 C and 2.5 MPa
using Ni supported on nitrogen doped carbon catalysts in
a continuous ow system. As it can be seen, most reported First 3,5-dimethoxy-N-(pyridin-4-yl)benzamide was reacted with
studies of V hydrogenation reported a high V conversion and VA Merrield resin in acetonitrile at 75 C for 18 hours (Scheme 2).
and MMP yields but at relatively high temperatures (60–150 C) Aer the reaction, the resin was washed several times with hot
33
2
.
Similarly, we recently reported
Results and discussion
IrPYA resin synthesis
ꢀ
8
ꢀ
ꢀ
acetonitrile to remove any precursor and then dried to give PYA
[HCl] resin. Reaction completion was observed by FTIR (Fig. S1 in
the ESI†). Then an altered procedure for proton removal was done
by reacting the PYA[HCl] resin in a solution of KOH in EtOH at
ꢀ
5
0 C overnight to give the PYA resin (Scheme 2). The resin was
washed with hot EtOH and then H
material was then washed with acetone to remove any H
2
O to remove any KOH. The
O and
2
then dried in a vacuum oven. The prepared ligand resin (PYA resin)
was observed by FTIR (Fig. S2 in the ESI†). Then metalation of the
supported ligand was done by reacting the PYA resin with
[
IrCp*Cl ] and NaOAc in dichloromethane at room temperature
2 2
Scheme 1 Hydrogenation of vanillin (V) to vanillyl alcohol (VA) and
subsequently to 4-methyl-2-methoxyphenol (MMP) using formic acid
as a reducing agent over IrPYA resin catalyst.
for 18 hours (Scheme 2). The material was then washed with
dichloromethane, H O, and acetone before drying. The nal
2
15836 | RSC Adv., 2021, 11, 15835–15840
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Scheme 2 Synthesis of the IrPYA resin.
supported catalyst material IrPYA resin was conrmed by FTIR synthesized IrPYA resin catalyst in the hydrogenation of V to VA
Fig. S3 in the ESI†). In all cases FTIR was compared with and the subsequent product MMP was studied using FA as
a molecular species for conrmation of reaction completion a reducing agent in the presence of KOH (Scheme 1). The
Fig. S1–S3 in the ESI†) and in the case of the metalation reaction performance of the catalyst was initially investigated at room
(
(
ꢀ
XPS analysis was also performed on both the molecular and sup- temperature (25 C), with samples collected at different reaction
ported catalyst for additional conrmation (Fig. 1). times (aer 1 h, 2.5 h and 5 h), viz. in Fig. 2. Aer 1 h, the
The XPS spectra of the IrPYA resin material matched with conversion of V was 99 mol% with a VA and MMP yield of
that of the homogeneous IrPYA complex, showing a single 69 mol% and 5.2 mol% respectively. This demonstrates that
oxidation state (Fig. 1). The IrPYA resin has a 4f5/2 peak at a single catalyst can both dehydrogenate FA and hydrogenate V
6
4.8 eV and the 4f7/2 peak at 61.8 eV (red, Fig. 1), and the IrPYA and VA within a reaction vessel. The solution aer the reaction
complex shows a 4f5/2 peak at 64.8 eV and a 4f7/2 peak at 61.7 eV remained colourless, indicating that the strongly coloured
black, Fig. 1). This shows that the synthesized material metal center remained bound to the resin. Increasing the
contains a molecular catalyst and no nanoparticle formation is reaction time from 1 h to 2.5 h did not result in a signicant
(
observed on the surface of the resin.
increase of YVA (from 69 mol% to 70 mol%) and YMMP (from
.2 mol% to 5.8 mol%), cf. Fig. 2. To correlate catalyst perfor-
mance in V hydrogenation to FA decomposition towards H and
CO , a separate experiment was conducted to investigate the FA
5
2
Catalyst evaluation
2
An iridium complex was successfully supported on Merrield degradation rate in absence of V, cf. Fig. S6 at ESI.† In this case,
resin and used as a catalyst for the simultaneous decomposition FA conversion, increases from 6.7 mol% to 15.7 mol% aer
of FA and production of VA and MMP from V. The activity of the 2.5 h, cf. Fig. S6 in ESI.† These results combined with the
insignicant change in V conversion between 1 and 2.5 h
Fig. 2 The conversion of vanillin (X
V
) and the yield of vanillyl alcohol
and 4-methyl-2-methoxyphenol (YVA and YMMP) as a function of the
reaction time; reaction conditions: c
V
¼ (715 mL, 1 wt%, 0.047 mmol),
ꢀ
Fig. 1 Ir 4f region of the XPS spectra of the IrPYA resin (red) and the
IrPYA complex (black).
cFA ¼ (293 mL, 7.78 mmol), cKOH (515 mL, 2 M, 1.03 mmol), T ¼ 25 C and
autogenous pressure (<1.0 MPa).
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indicates that the Ir active sites are occupied rstly by FA. A
further extension of the reaction time to 5 h resulted in an
increase of YMMP from 5.8 mol% to 8.2 mol% combined with
reduction of YVA from 71 mol% to 66 mol% as a result of its
conversion to MMP. This increase of MMP yield is correlated to
the increase in FA conversion from 15.7 mol% to 21 mol%,
which liberated the active Ir sites to V that undergoes to
hydrogenation reaction (Fig. S6†). This observation indicates
2 2
that the decomposition of FA to H and CO is the rate deter-
mining step for the hydrogenation of VA to MMP and is in-line
with our previous studies for hydrogenation of levulinic acid and
14
V as a model mixture, as well as g-valerolactone hydrogenation to
14
pentanoic acid using formic acid as a reducing agent. Moreover,
the 21 mol% conversion of FA at room temperature is a compa- Fig. 4 A comparison in the conversion of formic acid (XFA) at 25 ꢀC and
ꢀ
rable value to similar studies conducted with homogeneous Ir- 50 C after 5 h of reaction time; reaction conditions; cFA ¼ (293 mL, 7.78
3
7
mmol), cKOH (515 mL, 2 M, 1.03 mmol), and autogenous pressure (<1.0
MPa).
catalyst, but here obtained with immobilized IrPYA which can
potentially open new application in the eld of heterogeneous
38–40
catalysis and fuel cells.
As this catalyst uniquely contains the
IrPYA supported on Merrield resin (the catalyst is in solid form)
and the IrPYA not used as homogenous catalyst, as well as the
ꢁ
23 mol% is attributed to deposition of the VA and MMP on the
41
ꢀ
surface of the catalyst due to its low solubility in water. This
MMP is relatively low if compared to a similar study conducted
at same temperature with pressurized H as the hydrogen
source or with FA as the hydrogen source but at higher
reaction conditions is mild (T ¼ 50 C and autogenous pressure),
Y
we anticipate that no difference in the catalytic performance
should be observed. In addition, research on this issue coupled
with advanced characterization techniques are ongoing at our
laboratory to further understand the unique performance and its
potential scalability.
2
14,35
temperature, but is based using milder condition.
the selectivity toward VA or MMP can be tuned according to
temperature.
In order to conrm that our supported molecular catalyst,
IrPYA resin, was indeed decomposing formic acid we subjected
it to the same reaction conditions as in the V to VA and MMP
reactions at both 25 C and 50 C (Fig. 4). At 25 C we observed
a decomposition XFA of 21% and at 50 C a XFA of 76%. Although
the conversion of formic acid at room temperature is approxi-
mately a third of its conversion at 50 C, this still provides an
Moreover,
In order to maximize the Y
increased from 25 C to 35 C and subsequently to 50
Fig. S6–S8 in the ESI†). The experiment conducted at 35
the reaction temperature was
MMP
ꢀ
ꢀ
ꢀ
C
C
ꢀ
(
resulted in only slight improvement of YMMP when compared to
ꢀ
ꢀ
ꢀ
ꢀ
the 25 C experiment (Fig. 3 and S9 in the ESI†). A further
ꢀ
ꢀ
increase in reaction temperature to 50 C, reduced YVA from
54 mol% to 44 mol% and increased YMMP from 8 mol% to
ꢀ
2
4 mol%, cf. Fig. 3 and S8 in the ESI.† This high Y
is
MMP
ꢀ
excess of hydrogen for the conversion of V to VA and MMP in
our reaction, as we used a large excess. Moreover, the FA
decomposition at this lower temperatures, i.e. 25 C and 50 C,
correlated to the high XFA (76 mol%) at 50 C aer 5 h of reac-
tion time (Fig. 3), which leads to a higher availability of active
ꢀ ꢀ
ꢀ
ꢀ
sites with respect to the experiments at 25 C and 35 C. Finally
and noteworthy to be mentioned, the mass balance loss of
22
is typical of Ir complexes used as homogeneous catalysts. This
nding conrm that immobilized IrPYA has the same catalytic

activity as homogenous Ir catalysts. Extending these ndings in
biorenery will require ow reactors to be designed that take
advantage of the dual behaviour of IrPYA supported on Merri-

eld resin. Using this catalyst in our previously reported
continuous ow system for hydrogenation of vanillin will result
ꢀ
ꢀ
in lower reaction temperature (from 150 C to 50 C), as well as
a reduction in system pressure from 6.0 MPa to below 1.0 MPa.
All these will lead to efficient and selective process coupled with
high economic efficiency biorenery process (due to mild
operating conditions). In addition, this type of hydrogenation
with IrPYA supported on Merrield resin can be expanded to
wide range of biomass-derived compounds such as glucose, 5-
hydroxymethylfurfural, levulinic acid and g-valerolactone.
Fig. 3 The conversion of vanillin (X
and 4-methyl-2-methoxyphenol (YVA and YMMP) as a function of the
reaction temperature; reaction conditions: c
mmol), cFA ¼ (293 mL, 7.78 mmol), cKOH (515 mL, 2 M, 1.03 mmol),
reaction ¼ 5 h and autogenous pressure (<1.0 MPa).
V
) and the yield of vanillyl alcohol
Conclusions
V
¼ (715 mL, 1 wt%, 0.047
An iridium complex was successfully supported on Merrield
resin and used as a catalyst for the simultaneous decomposition
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of FA and production of VA and MMP from V. This approach
harnesses the high activity and tunability of molecular metal
complexes, but with the convenience and ease of separation of
heterogeneous catalysts. Catalytic production of MMP typically
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ꢀ
takes place at temperatures exceeding 100 C, however here it
ꢀ
was accomplished at temperatures <50 C. This was achieved by
taking advantage of the high activity of the iridium complex,
which is capable of accelerating the rate determining step; the
decomposition of FA to H
genation of V and VA proceeds using the same iridium complex
2
and CO
2
. Once this occurs, hydro-
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Conflicts of interest
1
2
2
There are no conicts to declare.
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