Aerobic oxidation of Α-pinene catalyzed by homogeneous and MOF-based Mn catalysts

Article abstract of DOI:10.1016/j.apcata.2017.07.047

Manganese catalysts were investigated for the liquid-phase aerobic oxidation of α‐pinene. First, the influence of reaction parameters such as time, solvent, temperature, oxidant flow rate and catalyst concentration on conversion, yield and selectivity were evaluated using Mn(III) acetate as homogeneous catalyst. Mn(III) acetate afforded pinene oxide as the main product along with small amounts of verbenol and verbenone. The optimized reaction conditions were likewise applied to a novel mixed-linker metal-organic framework (MOF) based on MIL-53-NH₂(Al), which was successfully synthesized and modified in a two-step post-synthetic reaction using maleic anhydride and Mn(III) acetate. The performance of the Mn-containing MOF catalyst was directly compared to its homogeneous counterpart Mn(III) acetate, showing very similar activity in a mixture of diethyl carbonate/dimethylformamide (DEC/DMF) as solvent. In both cases, a conversion of 31% and the formation of 17% pinene oxide (55% selectivity) were observed after 6 h. The heterogeneous MOF catalyst was easily removed from the reaction mixture by filtration and reused for at least five catalytic cycles without significant loss of activity. A hot filtration test showed that the catalysis mainly proceeded heterogeneously, although minor contributions of homogeneous species could not be completely excluded.

Full text of DOI:10.1016/j.apcata.2017.07.047

Accepted Manuscript
Title: Aerobic oxidation of ␣-pinene catalyzed by
homogeneous and MOF-based Mn catalysts
Authors: Yasmin S. Raupp, Ceylan Yildiz, Wolfgang Kleist,
Michael A.R. Meier
PII:
S0926-860X(17)30367-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.07.047
APCATA 16352
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Applied Catalysis A: General
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Accepted date:
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26-7-2017
28-7-2017
Please cite this article as: Yasmin S.Raupp, Ceylan Yildiz, Wolfgang
Kleist, Michael A.R.Meier, Aerobic oxidation of ␣-pinene catalyzed
by homogeneous and MOF-based Mn catalysts, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.07.047
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Aerobic oxidation of α-pinene catalyzed by homogeneous and MOF-
based Mn catalysts
1
2
2,3,4*
1,*
Yasmin S. Raupp, Ceylan Yildiz, Wolfgang Kleist,
Michael A. R. Meier
1
) Karlsruhe Institute of Technology (KIT), Institute of Organic Chemistry (IOC), Laboratory of
Applied Chemistry, Straße am Forum 7, 76131 Karlsruhe, Germany; E-mail:
m.a.r.meier@kit.edu
2
3
) Karlsruhe Institute of Technology (KIT), Institute for Chemical Technology and Polymer
Chemistry (ITCP), Engesserstraße 20, 76131 Karlsruhe, Germany
) Karlsruhe Institute of Technology (KIT), Institute of Catalysis Research and Technology
(IKFT), Herrmann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
4
) Present address: Ruhr-University Bochum, Laboratory of Industrial Chemistry,
Universitätsstraße 150, 44801 Bochum, Germany; Email: wolfgang.kleist@rub.de
Graphical abstract
Highlights
homogeneous and heterogenous catalysts for aerobic alpha-pinene oxidation were compeared to one
-
another
-
-
environmentally benign reaction conditions for the aerobic oxidation of alpha-pinene were developed
reaction parameters were optimized, resulting in a robust and applicable direct oxidation procedure
Abstract
Manganese catalysts were investigated for the liquid-phase aerobic oxidation of α-pinene. First, the
influence of reaction parameters such as time, solvent, temperature, oxidant flow rate and catalyst
concentration on conversion, yield and selectivity were evaluated using Mn(III) acetate as
homogeneous catalyst. Mn(III) acetate afforded pinene oxide as the main product along with small
amounts of verbenol and verbenone. The optimized reaction conditions were likewise applied to a
novel mixed-linker metal-organic framework (MOF) based on MIL-53NH
2
(Al), which was
successfully synthesized and modified in a two-step post-synthetic reaction using maleic anhydride

and Mn(III) acetate. The performance of the Mn-containing MOF catalyst was directly compared to its
homogeneous counterpart Mn(III) acetate, showing very similar activity in a mixture of diethyl
carbonate/dimethylformamide (DEC/DMF) as solvent. In both cases, a conversion of 31% and the
formation of 17% pinene oxide (55% selectivity) were observed after 6 h. The heterogeneous MOF
catalyst was easily removed from the reaction mixture by filtration and reused for at least five catalytic
cycles without significant loss of activity. A hot filtration test showed that the catalysis mainly
proceeded heterogeneously, although minor contributions of homogeneous species could not
completely be excluded.
Keywords: terpenes, pinene, pinene oxide, aerobic oxidation, homogeneous catalysis, heterogeneous
catalysis, manganese, metal-organic framework
Introduction
The selective oxyfunctionalization of olefins is an important step in the chemical value chain, as cheap
and readily available substances can be converted into a wide variety of valuable products [1-3]. In
times of depleting fossil reserves and an ever increasing emission of greenhouse gases, the
development of catalytic and sustainable processes for the chemical modification of renewable raw
materials has become a major focus, especially for the synthesis of oxygen-containing compounds [4].
Among the available renewable raw materials, terpenes constitute an abundant and versatile supply of
building blocks for the fine chemical industry [5-7]. Especially monoterpenes like α- and β-pinene,
which are the main components of gum turpentine, a by-product obtained from the pulp and paper
industry, represent inexpensive starting materials for the synthesis of various flavors, fragrances,
agrochemicals and therapeutically active substances. The autoxidation of α-pinene proved to be
radical-propagated and produces fine chemicals such as pinene oxide, verbenol and verbenone [8].
These oxygenated derivatives can be used as precursors for citral, menthol, sandalwood fragrance or
taxol as well as for the vitamins A and E [9-11].
Anastas and Warner recommend solvent-free procedures in their 12 principles of Green Chemistry
[12]. However, since this is often not possible, the use of a solvent becomes inevitable. As reviewed
by Mallat and Baiker, the involvement of organic solvents in redox reactions is rarely considered,
although the ‘sacrificial’ solvent may take over the role of the actual oxidizing agent leading to false
mechanistic interpretations of the results [13]. Moreover, the formation of solvent related by-products
that do not belong to the actual oxidation mechanism effects the atom economy and increases the E
factor. In particular, sulfoxides (e.g. DMSO), nitriles (e.g. acetonitrile) and amides (e.g. DMF) are
known to act as sacrificial solvents. Caps et al. also identified methylcyclohexane as an active part in
the epoxidation mechanism of trans-stilbene [14]. Considering these studies, the choice of an
appropriate reaction medium constitutes a demanding task.

Recently, a lot of effort has been made to replace conventional homogeneous catalytic systems by
their heterogeneous equivalents to benefit from the possibility of catalyst regeneration. This
substitution contributes to reduce problems concerning waste disposal and to avoid or simplify steps,
such as catalyst separation, etc. [5, 15]. For this purpose, various strategies for immobilizing redox-
active elements in a solid matrix have been investigated. For instance, the isomorphous substitution in
the crystallographic positions of an inorganic framework, the impregnation of metal compounds onto a
surface, tethering metal complexes to the surface via a spacer ligand, the encapsulation into a solid
matrix, etc. proved to be suitable approaches [16]. More recently, metal-organic frameworks have
been considered as promising and effective catalysts for various types of oxidations in the liquid phase
(e.g., alcohol oxidation [17-19], epoxidation [20-23], hydrocarbon oxidation [24], etc.).
Advantageously, the well-distributed and well-defined isolated metal centers of MOFs enable them to
potentially combine the advantages of heterogeneous and the efficiency of homogeneous catalysts [25,
2
6]. In this regard, this work demonstrates the development of a novel Mn-containing MOF catalyst
and its implementation for a more sustainable catalytic process.
In the literature, numerous catalytic systems for the oxidation of terpenes have been reported.
However, a direct comparison of the catalytic performance is rarely possible, since these systems
differ in many aspects such as oxidizing agent (e.g. molecular oxygen, hydrogen peroxide (H
2
2
O ), tert-
butyl hydroperoxide (TBHP)), homogeneous or heterogeneous catalysis as well as reaction conditions.
Nardello-Rataj et al. applied eco-friendly solvents (cyclopentyl methyl ether and 2-methyl
tetrahydrofuran) and reusable amphiphilic polyoxometalate nanoparticles for the epoxidation of
various terpenes using H
2
O [27]. In some other studies, catalytic systems for the oxidation of
2
terpenes, which do not require the use of any solvent, were developed. Lajunen and coworkers
demonstrated the aerobic oxidation of olefins under solvent-free conditions using a Co(II) pyridine
complex as catalyst and, in contrast to several previous papers [28], without co-oxidant [29].
Gusevskaya et al. also performed the reaction under solvent-free conditions with molecular oxygen,
but promoted by heterogeneous chromium containing mesoporous molecular sieves MCM-41 [30].
Additionally, no metal leaching of the catalyst was observed, making recovering easy. Kholdeeva and
colleagues explored the potential of Fe- and Cr-containing MOFs (MIL-100 and MIL-101) for the
solvent-free oxidation of alkenes [24, 31]. The applied reaction conditions were rather mild and
afforded mainly the allylic oxidation products. The authors also investigated the heterogeneous nature
of the catalysis and discovered that Fe-MIL-101 leached iron into the solution at temperatures higher
than 50 °C [32].
In the last twenty years, various Mn(III) acetate mediated oxidative free-radical reaction protocols
have been developed for inter- or intramolecular reactions [33, 34]: Usually, Mn(III) acetate is used
for oxidative free-radical cyclizations [3] and for α-keto-acetoxylations [35]. Some studies also
utilized Mn(III) acetate for allylic oxidation reactions. One example is given by Bégué et al., who
applied Mn(III) acetate as a catalyst for the aerobic epoxidation of various olefins including limonene

with pivalaldehyde as sacrificial aldehyde in fluorous solvents [36]. Likewise, Shing et al. applied
Mn(III) acetate for allylic oxidations of simple and complex olefins applying pure oxygen and TBHP
in decane as co-oxidant [37]. Mn has also been employed for the heterogeneously catalyzed
epoxidation of alkenes with air as oxidant. For instance, Raja et al. designed redox molecular sieve
catalysts for the selective oxidation of α-pinene among other alkenes using benzaldehyde as sacrificial
aldehyde [38]. To the best of our knowledge, the aerobic oxidation of α-pinene by a Mn-containing
MOF catalyst has not been reported before.
In this study, Mn(III) acetate was investigated as homogeneous catalyst for the oxidation of α-pinene
and compared to the performance of a novel Mn-containing MOF catalyst. We focused on a more
benign reaction procedure by, i.e., avoiding the use of stoichiometric reagents such as organic
hydroperoxides, which are generally toxic, hazardous and produce serious amounts of waste and
directly using oxygen from air as the sole oxidant. The influence of different reaction conditions on
the conversion and selectivity was studied in detail for the homogeneous model catalyst. Regarding
sustainability, the choice of appropriate reaction conditions for the catalytic reaction was considered
throughout. For this purpose, diethyl carbonate was employed as solvent, since organic carbonates are
considered to be more sustainable alternatives to volatile organic compounds or fluorinated solvents.
Furthermore, we report on a new strategy for the immobilization of Mn complexes via post-synthetic
modification of MIXMIL-53-NH
2
(50) [39, 40] with maleic anhydride and Mn(III) acetate. The
(50)-Mal-Mn as heterogeneous catalyst was
efficiency of the Mn-containing MOF MIXMIL-53-NH
2
explored for the liquid-phase oxidation of α-pinene using the reaction conditions developed for the
homogenous model system, allowing for a direct comparison. Moreover, the heterogeneous nature of
the catalytic system was assessed by a hot filtration test and the recyclability of the catalyst as well as
its stability were investigated.
Results and Discussion
We started our investigations on the manganese-catalyzed aerobic oxidation of α-pinene by
determining optimal reaction conditions for Mn(III) acetate as a homogeneous counterpart to the
metal-organic framework MIXMIL-53-NH (50)-Mal-Mn MOF that was subsequently applied as
2
heterogeneous oxidation catalyst (Figure 1). Conversion, yield and selectivity were optimized by
variation of one reaction parameter at a time (reaction time, solvent composition, catalyst
concentration, temperature, oxidant flow rate). Reaction products were identified using GC-MS by
comparison with authentic samples and quantified by GC techniques (for details see experimental
part). In the first set of experiments, the oxidation of α-pinene was monitored at 130 °C for 24 h with
continuous air flow through the solution (50 mL/min) and a 50:50 mixture of DMF/toluene as solvent
(
Figure 2). The solvent mixture DMF/toluene was chosen on the basis of reports on aerobic

epoxidations of olefins without radical initiators or sacrificial aldehydes using DMF as solvent [41-
6]. In these reports, the use of DMF was essential for the progress of the catalytic reaction and the
4
following mechanism was suggested: a metal-DMF complex is produced, which coordinates
molecular oxygen to form a superoxo species, which then transfers the molecular oxygen to the olefin.
On that account, first, the suitability of DMF was tested. For practical reasons, the addition of a lower
boiling solvent was required to achieve adequate reflux conditions. Since toluene has been widely
used for the aerobic oxidation of alcohols at least 50% toluene were added as solvent [13].
Applying the above-mentioned conditions, the reaction gave pinene oxide as the main product together
with verbenone and verbenol as minor side-products.
First, the impact of the reaction time was studied. An increase in reaction time from 6 to 24 h
improved the α-pinene conversion from 30 to 51%. In contrast, only a slight increase of pinene oxide
yield was observed, resulting in a remarkable decrease of the selectivity from 60 to 36%. This
observation is in accordance with the studies of Jasra et al. [42] and Yu et al. [47], who explained this
observation by the high reactivity of epoxides and thus the conversion of pinene oxide into isomeric
products. Therefore, longer reaction times were not favorable for the formation pinene oxide.
Next, the influence of the solvent was subject of investigation. Therefore, the DMF/toluene ratio was
varied (50:50, 40:60, 30:70, 20:80, 10:90), proving the 10:90 mixture to be the most efficient solvent
system. This observation might be explained by a lower concentration of amines, formed by DMF
decomposition at high temperature, which can block the free metal sites of the catalyst and thus limit
the oxidation reaction. However, a certain amount of DMF seems to have a positive impact on the
conversion and yield of pinene oxide, as the latter dropped if only toluene was used. This observation
was also made in earlier studies when we discovered that the STA-12(Co)-catalyzed epoxidation of
(
E)-stilbene did not occur in toluene, whereas mixtures of toluene and DMF permitted some catalytic
activity [23], most likely due to a solvent co-oxidation mechanism. A strong indication for this theory
was the detection of N-formyl-N-methylformamide (FMF) as a by-product by GC-MS, which was also
found in this study. This is in accordance with the studies of Baiker et al., who reported that olefin
transformation can be accompanied by significant DMF oxidation, leading to FMF formation [13, 48].
In this case, the intermediate N-(hydroperoxymethyl)-N-methylformamide derived from DMF can act
as an oxygen-transfer agent, forming FMF as result. The autoxidation of amides is accelerated by
transition metal ions and thus by the presence of the catalyst [49]. However, also the oxidation of the
olefin mediated by a transition metal peroxo species and the following regeneration of the catalyst
through oxidation of the solvent could explain the strong influence of the solvent and its oxidation
[23].
Ensuing investigations were focused on the effect of five different catalyst concentrations on the
conversion and selectivity. The results of the performed reactions with catalyst amounts varying
between 2.00 and 0.25 mol% are summarized in Table 1. A reaction without catalyst was performed as
blank test (Table 1, entry 6). The blank test gave 5% conversion with only 2% product formation,

conforming that the successful formation of pinene oxide in the other reactions was truly induced by
the catalyst.
Interestingly, the best conversion (62%) and pinene oxide yield (40%) were achieved by the lowest
catalyst amount (0.25 mol%). Conversion, yield and selectivity observed for the reactions containing
1.50, 1.00 and 0.50 mol% catalyst were in a similar range but slightly lower. The reaction with the
highest amount of catalyst (2.00 mol%) resulted in only 46% conversion. These findings are in
accordance with the study of Lajunen et al., who applied Co(II) complexes for the aerobic oxidation of
α-pinene. The authors also reported that the oxidation with the largest catalyst amount was slower and
produced fewer product than the reactions with less catalyst [29].
In further experiments, the dependency of conversion and yield on the reaction temperature was
investigated (Figure 3). α-Pinene conversion and pinene oxide yield increased at higher temperatures.
By increasing the temperature from 100 to 130 °C, conversion and pinene oxide yield almost tripled
from 19 and 13% to 53 and 36%, respectively. As expected, the increase of the reaction temperature
led to an increased reaction rate, but did not result in a significant difference in selectivity, as only a
marginal decrease from 69 to 67% was detected for the pinene oxide selectivity [50].
Next, the applied air flow rates were varied from 25 to 100 mL/min (Figure 4). The analysis showed
that high flow rates were advantageous for the overall conversion. By doubling the oxidant flow rate
from 25 to 50 mL/min, the conversion increased from 36 to 54% and also the yield of pinene oxide
increased to 35%. However, an increase in flow rate from 50 to 100 mL/min did not result in further
enhancement of conversion, yield and selectivity to pinene oxide. The selectivity, which amounted
almost 70% in all three experiments, did not depend on the flow rate. It can thus be concluded that a
high oxygen availability is crucial to this reaction, although an upper limit for the positive impact was
discovered. Another cause of the benefits from high oxidant flow rates might be assigned to the faster
removal of volatile catalyst poisons, such as amines, that could be produced by DMF degradation.
Combining the findings of the performed screening reactions, a reaction time of 6 h, DMF/toluene
-
1
(10:90) as solvent mixture, 130 °C, 0.25 mol% catalyst concentration and an air flow of 50 mL min
gave the best result for the aerobic α-pinene oxidation catalyzed by Mn(III) acetate. With the
optimized reaction conditions, a α-pinene conversion of 62% and a pinene oxide yield of 40% were
achieved, resulting in a turnover number (TON = [nproduct/ncatalyst]) of 160 and a selectivity of 65%.
However, both DMF and toluene are not ideal solvents for this reaction (considering sustainability
aspects), although DMF turned out to be an essential component for the epoxidation mechanism. In
addition to the by-product formed by DMF oxidation, GC-MS analysis of the reaction mixture also
revealed that toluene was not an inert solvent under the applied reaction conditions, leading to
considerable amounts of benzyl alcohol and benzaldehyde as by-products. To find a more sustainable
alternative for toluene, dimethyl carbonate and diethyl carbonate were evaluated as co-solvents under
the same reaction conditions (Table 2). The 10:90 mixture of DMF and dimethyl carbonate resulted in

the lowest conversion (7%) and yield of pinene oxide (1%), which might be caused by the lower
boiling point (90 °C) compared to toluene (111 °C) and diethyl carbonate (126 °C). The 90:10 mixture
of diethyl carbonate and DMF gave an α-pinene conversion of 38% and a pinene oxide yield of 23%.
This mixture should be preferred over toluene/DMF for sustainability reasons (see general discussion
above and safety (S), health (H) and environment (E) scores by CHEM21 selection guide: toluene:
S = 5, H = 6, E = 3, overall ranking = problematic; dimethyl carbonate: S = 4, H = 1, E = 3, overall
ranking = recommended [51]), even if the conversion was slightly lower (compare Table 2, entries 1
and 3). Although these data are for dimethyl carbonate (data for diethyl carbonate is not available),
they suggest a better sustainability for the latter. Moreover, no additional by-products related to
solvent oxidation were produced (i.e. carbonates were inert solvents under these conditions). However,
as already experienced for toluene as the sole solvent, diethyl carbonate without addition of DMF gave
poor conversion and yield. Moreover, if only a stoichiometric amount of DMF (Table 2, entry 4) was
used (in relation to α-pinene), both conversion (20%) and pinene oxide yield (9%) were low compared
to the reaction using DMF (Table 2, entry 3).
For those reasons, it should be noted that, from an environmental point of view, diethyl carbonate as a
co-solvent is the better choice, since no additional side products are formed and the solvent has
sustainability advantages. The reaction employing the above-mentioned conditions with diethyl
carbonate resulted in a conversion of 38% (TON = 23) and 59% selectivity for pinene oxide.
After having investigated appropriate reaction conditions for the aerobic α-pinene oxidation with
Mn(III) acetate as homogeneous catalyst, these reaction conditions were subsequently applied for the
reaction using the novel Mn-containing MOF MIXMIL-53-NH
2
(50)-Mal-Mn as heterogeneous
catalyst to enable direct comparison of the catalyst performances.
For this purpose, a mixed linker metal-organic framework with MIL-53(Al) structure containing equal
amounts of terephthalate and 2-aminoterephthalate linkers was synthesized in a modified two-step
post-synthetic modification (PSM) that has been previously reported for the generation of MOF-based
single-site Pd catalysts [26]. First, the amine groups of the organic linker molecules were covalently
modified using maleic anhydride, resulting in a chelating side group at the linkers that was
subsequently used for the immobilization of manganese complexes. The XRD pattern (Figure 5) of the
resulting catalyst revealed a crystalline structure that is very similar to the simulated diffractogram for
MIL-53(Al) with minor deviations that can be explained by a breathing behavior of the pores after the
introduction of the immobilized guest species in the pore structure [39]. Attenuated total reflection
(ATR) IR spectra (Figure S2) were recorded to exclude residual free acid (C=O band expected around
-
1
-1
1
685 cm ) or solvent molecules (DMF; C=O band expected around 1650 cm ) in the pores of the
-
1
frameworks. The N-H stretching vibrations of the amine functionalities (around 3500 cm and
-
1
-1
3
389 cm ), the OH stretching vibrations of the Al-OH-chains (around 3621 cm ) and the OH

-
1
1
stretching vibrations of the maleate side group (around 3692 cm ) were observed. H-NMR spectra of
the MOF before and after immobilization of the Mn species (Table 3 and Figure S3-S5) proved the
presence of the two linker molecules in the desired ration (1:1) and revealed a modification degree of
6% of the amine groups. Results from physisorption experiments (Table 3) proved that the catalyst
still featured a high specific surface area and accessible micropore volume after the PSM reaction. The
manganese content of the catalyst was determined by atomic absorption spectroscopy (AAS), resulting
in a loading of 0.9 wt% (Table 3).
In a first attempt, the Mn-containing MOF catalyst showed notable activity for the aerobic oxidation of
α-pinene in a solvent mixture of DMF and DEC with a continuous feed of air into the reaction mixture
for 6 h at 130 C. Pinene oxide, verbenone and verbenol were detected as the major products by GC-
MS and comparison with authentic samples (Figure 1). Additionally, the formation of the side-
products verbenone and verbenol was also observed. Subsequently, the catalytic performance of the
MOF based catalyst was compared to the homogeneously dissolved Mn salt (Figure 6). The
heterogeneous catalyst exhibited a good activity and showed similar results as achieved by 0.5 mol/%
Mn(III) acetate under the same conditions. Indeed, after 6 h, the conversion was 31% with the
homogeneous system as well as with the MOF-containing catalyst. The selectivities of Mn(III) acetate
and the MOF-containing catalyst to pinene oxide (55% and 54%, respectively), verbenone (7% and
8
%, respectively) and verbenol (6% and 3%, respectively) were also similar. The mass balances
accounted for 68% and 65%, respectively. The remainder of the converted α-pinene can be assigned to
pinene oxide isomerization products since the GC chromatogram showed several additional, very
small peaks in the same region as the main oxidation products pinene oxide, verbenol and verbenone.
GC-MS analysis was performed to analyze these additional side products. In consistence with the
mass spectra campholenic aldehyde, pinocarvone and carveol were detected.
To benefit from the main advantages of heterogeneous catalysis, the Mn-containing MOF must be
easily recoverable from the reaction medium and allow for a straightforward regeneration. If these
criteria are fulfilled, heterogeneous catalysis facilitates waste prevention and conservation of
resources. Therefore, the stability of the catalyst under the applied reaction conditions has to be
ensured. For the assessment of its stability, the present catalyst’s reusability was tested by using it in
five successive catalytic reactions. After each catalytic reaction, the catalyst was filtered from the
reaction mixture, washed, dried at room temperature and reused in another α-pinene oxidation reaction
under the same conditions. The results of the recycling test show that the catalyst was active for at
least five reaction cycles without significant loss of activity (Figure 7).
The successful reuse of MIXMIL-53-NH (50)-Mal-Mn may be interpreted as a first indication that the
2
catalytic active sites of the catalyst neither became inactive during the reaction nor that significant
amounts of Mn leached out of the catalyst. However, a truly heterogeneous reaction pathway can only

be asserted by a heterogeneity test as proposed by Sheldon and Schuchardt et al. [32]. Therefore, a hot
filtration test was carried out by filtering the catalyst from the hot reaction mixture after 3 h and
continuing the reaction for another 5 h. At the same time, a control experiment was conducted in
which the catalyst remained in the reaction mixture. In the case of exclusive heterogeneous catalysis,
conversion and yield should remain constant after catalyst removal. In our experiments, the conversion
and yield of pinene oxide further increased slightly after removal of the MIXMIL-53-NH (50)-Mal-
2
Mn catalyst (Figure 8). However, in comparison to the control experiment, the reaction rate was
remarkably lower, indicating that the reaction was mainly catalyzed heterogeneously. The observed
activity despite catalyst filtration might arise from solid catalyst particles that were too small to be
removed by the filter, although minor amounts of homogeneous species cannot be completely ruled
out as an additional catalytically active species. In summary, the recycling experiments demonstrated
the reusability of the catalyst and the advantages of a heterogeneous system. In the five consecutive
runs, an overall TON of 166 was obtained.
Conclusion
Mn(III) acetate was investigated for the liquid-phase aerobic α-pinene oxidation and the influence of
reaction time, solvent, temperature, oxidant flow rate and catalyst concentration on conversion, yield,
-
1
and selectivity were evaluated. High temperatures (130 °C) and oxidant flow rates (50 mL min ), low
catalyst concentrations (0.25 mol%) and a DMF/toluene solvent mixture emerged as the most
favorable reaction conditions. Longer reaction times did not enhance the formation of pinene oxide,
since epoxides tend towards isomerization processes at high temperatures. Toluene was exchanged by
diethyl carbonate, which did not undergo any side reaction under the applied conditions and is
regarded as a more sustainable solvent. Furthermore, the novel mixed-linker metal-organic framework
MIXMIL-53-NH (50)-Mal-Mn was successfully synthesized and structurally characterized. NMR
2
spectroscopy confirmed that the detected linker ratio of terephthalate and 2-aminotherephtalate
matched to the applied 1:1 ratio, which proved that both linker molecules were incorporated into the
framework with the same preference. As shown by physisorption experiments the high specific surface
area was maintained after the post-synthetic modification reactions leading to a potentially highly
active catalytic material. We could show that the framework with its immobilized and well-defined
manganese(III) complexes featured notable catalytic activity for the oxidation of α-pinene with
molecular oxygen in DEC/DMF applying the aforementioned reaction conditions. The reaction
resulted in high selectivity for pinene oxide as the main product, along with smaller amounts of
verbenol and verbenone. The novel MOF catalyst gave good results, which were comparable to the
conversion and yield achieved by Mn(III) acetate. The accumulated TON of the MOF catalyst (166)
was considerably higher compared to the homogeneous catalyst system (23). A hot filtration test
indicated that the reaction mainly proceeded through a heterogeneous pathway, but an involvement of

homogeneous species could not be completely excluded. Moreover, the Mn-containing MOF showed
constant activity over at least five catalytic cycles, proving its recyclability and underlining the
stability of the MOF under the applied reaction conditions.
Despite significant constraints due to the necessity of DMF, as a sacrificial reductant, this work
provides evidence that the Mn-containing MOF MIXMIL-53-NH
2
(50)-Mal-Mn can be considered a
promising solid catalyst for aerobic oxidations.

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2
Figure 1. Catalytic oxidation of α-pinene by MIXMIL-53-NH (50)-Mal-Mn resulting in pinene oxide, verbenone and
verbenol as main products. (conversion: 31%, selectivities: 54, 8 and 3%, respectively). Reaction conditions: α-pinene
(
1.0 mmol), MIXMIL-53-NH
2
(50)-Mal-Mn (0.5 mol% Mn), DMF/diethyl carbonate (10:90, 30 mL), compressed air (50 mL
-1
min ), 130 °C, 6 h.
70
60
50
40
30
20
10
0
conversion -pinene
yield pinene oxide
selectivity pinene oxide
0
5
10
15
20
25
reaction time [h]
Figure 2. Conversion, yield and selectivity of aerobic α-pinene oxidation catalyzed by Mn(III) acetate. Reaction conditions:
-1
α-pinene (1.0 mmol), catalyst (1.0 mol% Mn), DMF/toluene (50:50, 30 mL), compressed air (50 mL min ), 130 °C.

60
50
40
30
20
10
0
conversion (130 °C)
yield (130 °C)
conversion (100 °C)
yield (100 °C)
0
1
2
3
4
5
6
reaction time [h]
Figure 3. Dependency of reaction temperature on conversion and yield. Reaction conditions: α-pinene (1.0 mmol), Mn(III)
-1
acetate (1.0 mol%), DMF/toluene (10:90, 30 mL), compressed air (50 mL min ).
conversion -pinene
70
60
50
40
30
20
10
0
yield pinene oxide
yield verbenone
yield verbenol
25
50
100
-
1
air flow [mL min ]
Figure 4. Influence of air flow rate on conversion and yield for the catalytic oxidation of α-pinene. Reaction conditions: α-
pinene (1.0 mmol), catalyst (0.5 mol% Mn), DMF/toluene (10:90, 30 mL), compressed air (flow as indicated), 130 °C, 6 h.

MIXMIL-53(Al)-NH (50)-Mal-Mn
2
MIL-53(Al)_simulated
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
2
 [°]
Figure 5. XRD patterns of simulated MIL-53(Al) and MIXMIL-53(Al)-NH
2
(50)-Mal-Mn.
35
30
25
20
15
10
5
0
conversion (MIXMIL-53-NH (50)-Mal-Mn)
2
yield pinene oxide
(
MIXMIL-53-NH (50)-Mal-Mn)
2
conversion (Mn(OAc)3)
yield pinene oxide
(
Mn(OAc)3)
0
1
2
3
4
5
6
reaction time [h]
2
Figure 6. Conversion and yield of aerobic α-pinene oxidation catalyzed by MIXMIL-53-NH (50)-Mal-Mn and homogeneous
Mn(III) acetate, respectively. Reaction conditions: α-pinene (1.0 mmol), catalyst (0.5 mol% Mn), DMF/DEC (10:90, 30 mL),
-1
compressed air (50 mL min ), 130 °C.

50
45
40
35
30
25
20
15
10
5
0
conversion -pinene
yield pinene oxide
1st
2nd
3rd
4th
5th
run
2
Figure 7. Catalytic performance of MIXMIL-53-NH (50)-Mal-Mn in five cycles of α-pinene epoxidation. Reaction
conditions: α-pinene (1.0 mmol), MIXMIL-53-NH (50)-Mal-Mn (0.5 mol% Mn), DMF/diethyl carbonate (10:90, 30 mL),
2
-1
compressed air (50 mL min ), 130 °C, 6 h.
conversion (heterogeneity test)
40
30
20
10
0
yield pinene oxide (heterogeneity test)
conversion (control experiment)
yield pinene oxide (control experiment)
0
2
4
6
8
reaction time [h]
Figure 8. Comparison of conversion and yield of α-pinene epoxidation with and without catalyst removal after 3 h (as
indicated by vertical line). Reaction conditions: α-pinene (1.0 mmol), MIXMIL-53-NH (50)-Mal-Mn (0.5 mol% Mn),
2
-1
DMF/diethyl carbonate (10:90, 30 mL), compressed air (50 mL min ), 130 °C.

Table 1. Effect of catalyst concentration on conversion and selectivity. Reaction conditions: α-pinene (1.0 mmol), Mn(III)
-1
acetate (as indicated), DMF/toluene (10:90, 30 mL), compressed air (50 mL min ), 130 °C, 6 h.
Entry
Catalyst conc.
mol%]
Conversion [%]
Selectivity (yield) [%]
[
Pinene oxide
67 (31)
67 (35)
66 (35)
66 (34)
65 (40)
52 (2)
Verbenone
Verbenol
5 (2)
1
2
3
4
5
6
2.00
46
53
53
52
62
5
4 (2)
8 (4)
5 (3)
7 (3)
7 (4)
8 (0,4)
1.50
6 (3)
1.00
4 (2)
0.50
5 (3)
0.25
5 (3)
0 (blank test)
6 (0,3)
Table 2. Solvent effect in the aerobic epoxidation of α-pinene. Reaction conditions: α-pinene (1.0 mmol), Mn(III) acetate
-1
(
1.0 mol%), solvent (30 mL), compressed air (50 mL min ), 130 °C, 6 h.
Entry
Solvent
Conversion
%]
Selectivity (yield) [%]
[
Pinene oxide
66 (35)
Verbenone
Verbenol
4 (2)
1
2
Toluene/DMF (90:10)
Dimethyl
53
7
5 (3)
14 (1)
6 (0.4)
5 (0.3)
carbonate/DMF (90:10)
3
4
5
Diethyl carbonate/DMF 38
90:10)
Diethyl carbonate/DMF 20
99.5:0.5)
Diethyl carbonate
59 (23)
43 (9)
14 (2)
5 (2)
4 (1)
(
7 (1)
8 (2)
(
11
4 (0.4)
5 (0.3)
Table 3. Specific surface areas and micropore volumes of modified MIL-53(Al) materials obtained from nitrogen
physisorption measurements. Mn content of MIXMIL-53-NH (50)-Mal-Mn determined by AAS measurements.
2
2
-1
Entry
S
BET (m g ) Micropore volume Mn content [wt%]
3
-1
[
cm g ]
1
MIXMIL-53-NH
2
(50)
840
480
0.23
-
2
3
MIXMIL-53-NH
2
2
(50)-Mal
0.21
0.15
-
MIXMIL-53-NH
(50)-Mal-Mn 380
0.9