Continuous reductive amination of biomass-derived molecules over carbonized filter paper-supported FeNi alloy

Article abstract of DOI:10.1002/cssc.201500804

This paper reports the continuous reductive amination of different molecules, including biomass-related compounds, over carbon-supported FeNi nanoparticles obtained on the basis of inexpensive and abundant metal precursors and cellulose. A biorefinery case study for the preparation of pyrrolidones via acid-catalyzed hydrolysis of glucose followed by reductive amination of the obtained levulinic acid is described. Burn in the fire of filters: Continuous reductive amination of biomass-derivable molecules is achieved using a carbonized filter paper-supported FeNi catalyst in combination with EtOH, MeOH, and 2-methyl THF as green solvents. A biorefinery case study for the preparation of pyrrolidones using crude levulinic acid feeds is also investigated.

Full text of DOI:10.1002/cssc.201500804

DOI: 10.1002/cssc.201500804
Communications
Continuous Reductive Amination of Biomass-Derived
Molecules over Carbonized Filter Paper-Supported FeNi
Alloy
Gianpaolo Chieffi, Max Braun, and Davide Esposito*[a]
This paper reports the continuous reductive amination of dif-
ferent molecules, including biomass-related compounds, over
carbon-supported FeNi nanoparticles obtained on the basis of
inexpensive and abundant metal precursors and cellulose.
A biorefinery case study for the preparation of pyrrolidones via
acid-catalyzed hydrolysis of glucose followed by reductive ami-
nation of the obtained levulinic acid is described.
Interestingly, FeNi alloys have proven to be convenient cata-
lysts for hydrogenation.[15] We recently introduced a carbonized
filter paper (CFP)-supported bimetallic FeNi alloy where the
metal nanoparticles were enwrapped and held together by
graphitic shells. This material exhibited excellent catalytic activ-
ity for the continuous hydrogenation of different classes of
substances, including biomass-derived molecules.[16] FeNi cata-
lysts are expected to follow the conventional mechanistic pic-
ture in which the metal coordinates the unsaturated substrate,
facilitates the dissociation of hydrogen, and finally delivers hy-
dride species to the substrate. In addition, it has been pro-
posed that the iron atoms act as the preferential adsorption
sites for hydrogen on the surface of FeNi alloys.[17] Our CFP-
supported FeNi alloy, which is synthesized using inexpensive
iron and nickel salts and uses the renewable and abundant cel-
lulose as support precursor, constitutes a promising non-pre-
cious alternative to conventional noble catalysts. For this
reason, we focused in this work on expanding the spectrum of
transformations that such catalytic systems could perform. In
particular, we investigated the reductive amination of benzal-
dehyde or biomass-derivable aldehydes (furfural, hydroxyme-
thylfurfural) using different amine sources, including aminopro-
panol, aniline, nitrobenzene, and even amino acids, by employ-
ing a continuous fixed bed reactor (see Scheme 1, path A).
Selective catalytic transformations of biomass-derived com-
pounds are crucial for the sustainable production of value-
added chemicals. In this context, the amino functionalization
of lignocellulose-derived molecules, which intrinsically lack ni-
trogen-containing functionalities, represents an interesting
possibility for the sustainable synthesis of biologically active
amines. Here, the identification of renewable nitrogen sources
remains crucial. Several methodologies have been developed
in the last years to access substituted amines via alkylation of
ammonia, N-alkylation of primary amines with alcohols,[1] or
Buchwald–Hartwig coupling,[2] but the direct reductive amina-
tion of carbonyl compounds still constitutes a very practical so-
lution.[3,4] The toxicity and disposal problems related to classi-
cal reducing agents for reductive amination such as sodium cy-
anoborohydride,[5] sodium triacetoxyborohydride,[6] or pyri-
[7]
dine–BH3 suggest the use of the inexpensive and environ-
mentally benign molecular hydrogen.[4,8] In this case, however,
the sustainability of the employed catalyst becomes essential.
In the recent literature, methodologies for reductive amination
have been disclosed, which make use of expensive and rare
metals such as Ru, Ir, Pd, Pt, or Au as the catalyst.[9–12] The re-
placement of these elements with inexpensive and abundant
alternatives is highly desirable. Beller et al. already described
the reductive amination of aldehydes using nitroarenes as the
nitrogen-containing compound in combination with a carbon-
supported iron–phenanthroline complex as the catalyst.[3] Al-
ternatively, the use of a cobalt-based catalyst has been report-
ed.[13] In addition, Alonso et al. employed nickel nanoparticles
(NPs) as active catalytic species for the reductive amination of
aldehydes through transfer hydrogenation using isopropanol
at mild temperatures.[14]
Scheme 1. Continuous reductive amination using a carbon-supported FeNi
catalyst.
The encouraging results pushed us to widen the scope of
our reductive amination method to include even the more
challenging keto acids. In this regard, the reaction between
the platform chemical levulinic acid (LA) and phenethylamine
(PEA) was investigated to showcase the synthesis of pyrroli-
dones (see Scheme 1, path B). Even the use of a crude LA feed
obtained via acidic hydrolysis of glucose showed very good re-
sults, proving the catalyst to be a promising candidate for
a real integrated biorefinery process.
[a] Dr. G. Chieffi, M. Braun, Dr. D. Esposito
Department of Colloid Chemistry
Max-Planck-Institute of Colloids and Interfaces
14424 Potsdam (Germany)
E-mail: davide.esposito@mpikg.mpg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500804.
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The CFP-supported FeNi powder was prepared through car-
bothermal reduction of a cellulose filter paper soaked with an
aqueous solution of the iron and nickel salt precursors (see the
Supporting Information for the complete procedure). Induc-
tively coupled plasma–optical emission spectroscopy (ICP–OES)
analysis of the hybrid material confirmed the presence of iron
(21 wt%) and nickel (31 wt%) in the initially employed ratio of
the metal salt precursors. Such optimizable loading, although
relatively high compared to traditional heterogeneous cata-
lysts, was chosen as a good compromise to ensure encapsula-
tion without hindering catalytic activity as lower loading
would result in the formation of graphitic shells that excessive-
ly enwrap the metal NPs. The thus obtained powder was
packed into a cartridge and directly employed in combination
with a bench-top continuous flow reactor (H-Cube Pro) with
an internal H2 source and equipped with a liquid feed. Consid-
ering the relatively small size of this non-customizable reactor,
the chosen high metal loading also allowed performing the
catalytic tests at reasonable molarities (millimolar range) in
quite short residence times. An initial assessment of the ability
of the catalyst to perform reductive amination was accom-
plished using aminopropanol (50 mm) and benzaldehyde
(1 equiv.). The solvent of choice was ethanol that beyond
being known as a good solvent for reductive amination reac-
tions is relatively cheap and considered as “green”.[18] Full con-
version of the reactants with high selectivity for 3-(benzylami-
no)propan-1-ol was achieved by optimizing the reaction pa-
rameters up to 10 bar H2, a flow rate of 0.3 mLminÀ1, and
1508C, corresponding to a retention time scale of roughly
3 min and to a weight hourly space velocity (WHSV) of 24 hÀ1
(Table 1, entry 1). The rapidity at which the reactions occurred
under these conditions constitutes an advantage compared to
the corresponding catalyzed processes in batch, which usually
require several hours.
optimized for each single molecule to achieve high conver-
sions. Below the temperature threshold identified for each spe-
cific reaction, the only byproduct observed in the reaction mix-
ture was the corresponding unreacted imine, which suggests
the reduction of the latter was the rate-limiting step. For in-
stance, the reductive amination of aniline and benzaldehyde
required lower energy and was performed at 1008C with the
same flow rate and hydrogen pressure (entry 2). A slightly
lower conversion of the starting materials was observed
whereas the selectivity towards the final secondary amine, N-
benzylaniline, was further increased. The reduced conversion
can be explained by considering the decreased reactivity of ar-
omatic amines towards imine formation.[3,19]
Nitrobenzene was suggested as a substitute for anilines in
reductive amination. This compound, commonly used as
a cheap feedstock[20] for the synthesis of aromatic and primary
amines by hydrogenations, undergoes a cascade of reactions,
which commence with the quantitative reduction of the nitro
group to afford the amine, which subsequently condenses
with benzaldehyde to provide the imine. The latter compound
is hydrogenated in situ, affording the desired product. In our
particular case, N-benzylaniline was obtained in good yields
whereas benzyl alcohol was also obtained as a byproduct
when performing experiments using an excess of benzalde-
hyde (entry 3). As discussed before, amine functionalization of
lignocellulose-derived molecules is highly desired to expand
the applicability of bio-based chemicals. Having identified the
experimental conditions for an effective continuous reductive
amination, we attempted to extend our method to the proc-
essing of targeted biomass-derivable molecules. Furan deriva-
tives can be obtained from renewable biomass feedstock and
possess a sustainable potential to replace petrochemicals for
the production of a wide range of products (e.g., polymers,
surfactants, pharmaceuticals, plant-protection agents).[21]
In this study, furfural and hydroxymethylfurfural (HMF) were
first used for reductive amination with aminopropanol and
then with an abundant and renewable source of amines as the
amino acids. Furfural was select-
In general, in the case of aldehydes, we observed that the
pressure did not play a crucial role for the reaction whereas
the temperature, in turn, was the determining parameter to be
Table 1. Conversion, selectivity, and optimized temperature for reductive amination reactions using a carbon-
ed as one of the top 30 platform
chemicals by the US Department
of Energy.[22] Similarly, HMF was
defined as the “sleeping giant”
due to its potential for the pro-
duction of biomass-derived in-
termediates.[23] This chemical is
usually obtained from renewable
resources such as fructose in
very high yields; recently, its
direct synthesis from cellulose
(44% yield) was demonstrat-
ed.[24,25] Surprisingly, reductive
amination involving furfural and
HMF were poorly investigated in
the literature. Previously, small li-
braries of derivatives from HMF
and different aromatic/aliphatic
amines were produced in batch
supported FeNi alloy.[a]
Entry
1[c]
Amine/nitro
compound
Aldehyde
Product
Conversion
T
[8C]
(selectivity)[b] [%]
>99 (97)
150
2[c]
90 (>99)
>99 (83)
100
125
3[d]
4[c]
>99 (90)
>99 (78)
125
100
5[c]
[a] Conditions: 0.3 mLminÀ1, WHSV=24 hÀ1, 10 bar H2. [b] Liquid feed compositions are calculated using uncor-
rected GC–MS peak area percentages. [c] Aldehyde (50 mm), amine (50 mm) in EtOH. [d] 3 equiv. of aldehyde.
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using a homogenous ruthenium-
based catalyst with hydrogen as
reducing agent or, alternatively,
Table 2. Conversion, selectivity, and optimized temperature for the reductive amination of HMF, furfural, and
amino acids using a carbon-supported FeNi alloy.[a]
Entry
Amine
Aldehyde
Product
Conversion
(yield)[b] [%]
T
employing
stoichiometric
[8C]
NaBH4.[12,18] In the current study,
high conversions and selectivi-
ties towards aminomethylfurans
were afforded when furfural
(Table 1, entry 4) and HMF
(Table 1, entry 5) were used. This
class of compounds that are
commonly obtained by reaction
of an epoxide with a primary or
secondary amine showed high
1[c]
>99 (77)
125
2[d]
3[d]
4[d]
>99 (60)
>99 (50)
90 (<10)
85
125
100
potentiality
agents.[26]
as
antifilarial
At this point we decided to
focus on sustainable amines, and
the preparation of furfurylamine
derivatives via reductive amina-
tion with amino acids was at-
tempted. In a first experiment,
alanine sodium salt was mixed
[a] Conditions: 0.3 mLminÀ1, WHSV=24 hÀ1, 10 bar H2, aldehyde (50 mm), amino acid (50 mm). [b] Yields deter-
mined using NMR spectroscopy. [c] In EtOH. [d] In MeOH.
with stoichiometric amounts of HMF in ethanol and the ob-
tained imine was treated for reductive amination in the flow
reactor, affording the corresponding secondary amine in 77%
yield according to 1H NMR analysis (Table 2, entry 1, see the
Supporting Information for the complete procedure).
achieved by relatively long batch reactions that employ rare
and expensive supported metals such as Pt-Mox/TiO2, Au/ZrO2,
or homogenous cyclometalated iridium complexes.[31–33] In this
study, we mainly focused on the optimization of the reaction
parameters for the reductive amination of LA with phenethyla-
mine to maximize the conversions towards the corresponding
pyrrolidone. Unlike the case of aldehydes discussed above,
using equimolar amounts of amine and LA, the selectivity to-
wards the corresponding pyrrolidone was quite low and the
formation of the imine appeared to be the rate-limiting step
(Table 3, entry 1). The conversion of the starting materials and
the reaction selectivity were enhanced by using an excess of
LA (2 equiv., Table 3, entry 2) and when the H2 pressure was in-
creased from 25 up to 50 bar (entry 3). Interestingly, using
a mixture of EtOH and 2-methylfuran (2-MTHF) as the solvent
(1:1 v/v, entry 4), the selectivity of the reaction could be slightly
improved. Therefore, we evaluated also the use of the pure 2-
MTHF as the solvent, which gave rise to the best results at
a flow rate of 0.3 mLminÀ1 in combination with a H2 pressure
of 85 bar (entry 5). The use of the latter solvent, which can be
derived from bio-based platform chemicals, is particularly con-
venient from a sustainability standpoint. Besides being a very
effective solvent for hydrogenation, 2-MTHF has been proven
to be an excellent candidate to perform biomass fractionation
and extractions due to its very low solubility in water.[34–36] For
these reasons, the robustness of our protocol for the reductive
amination of LA was finally tested for a real biorefinery scheme
in which LA is initially obtained via acidic hydrolysis of glucose
(~40% yield, see the Supporting Information for the complete
procedure) and further extracted in 2-MTHF[37] before applying
the proper conditions for reductive amination in the presence
of PEA without further purifications.
Only a few examples of this type of synthesis are reported in
the literature, and the best result was obtained by Villard et al.
who described a method for the reductive amination of l-ala-
nine and HMF under hydrogen atmosphere using Ni Raney as
catalyst with lower yields.[27] Interestingly, the so obtained fur-
furylamine derivative is an important intermediate for the
preparation of amino acid-based taste enhancers and pyridini-
um-based ionic liquids.[28] Unlike HMF, furfural showed lower
selectivity for similar reactions due to the competitive hydro-
genation of the furan ring. Interestingly, under the same exper-
imental conditions, ring hydrogenation of pure furfural did not
take place, as observed in a control experiment. In particular,
the use of glycine or b-alanine afforded the corresponding
amine with a selectivity of 60% and 50%, respectively (en-
tries 2 and 3). Unfortunately, when leucine was employed in
the reductive amination with furfural, we observed poor selec-
tivity due to competing ring saturation and imine cleavage
leading to free amino acid (entry 4).
At this point, we challenged our methodology to evaluate
the reductive amination of molecules with different reactivity,
namely keto acids. For this purpose, we chose LA, which can
be obtained usually through the enzymatic or acid-catalyzed
conversion of polymeric carbohydrates and represents a versa-
tile building block for the synthesis of industrial and pharma-
ceutical compounds.[29,30] We were particularly interested in the
synthesis of pyrrolidones, which have shown an enormous po-
tential as reaction media for low- and high-temperature reac-
tions, fungicides, herbicides, and nootropic pharmaceuticals
(e.g., piracetam). Usually, the synthesis of these compounds is
In this case, 84% conversion of the starting amine with a se-
lectivity of 90% toward pyrrolidone could be obtained
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Figure 1. a) Conversion and selectivity to 5-methyl-1-phenethylpyrrolidin-2-one versus time on stream evaluated for the continuous-flow reductive amination
of LA and 2-phenylethylamine (conditions: see Table 3, entry 2). b) XRD patterns of the FeNi nanoparticles obtained through impregnation of lab-grade cellu-
lose filter paper with a solution of Fe(NO3)3·9H2O and Ni(NO3)2·6H2O and then heat-treated at 8008C (108CminÀ1) for 2 h under nitrogen flux before (blue
curve) and after 60 (red curve) and 112 h (black curve) of reductive amination reaction.
and selectivity as a function of the time on stream.
Table 3. Conversion, selectivity, optimized parameters, and WHSV for the reductive
The FeNi alloy demonstrated good durability as con-
version remained stable and selectivity was mostly
unchanged over a period of further 52.5 h (see Fig-
ure 1a). Additionally, X-ray analysis confirmed that
the crystalline phase as well as the crystallite size (cal-
culated from the peak [111] using the Scherrer equa-
tion) remained essentially unaffected (~16 nm) up to
approximately 112 h (see Figure 1b). On the basis of
these results, the catalyst is expected to show deacti-
vation only at a considerably longer time on stream.
In conclusion, we described the successful devel-
opment of a continuous flow reductive amination
scheme which relies on the use of a carbon-sup-
ported catalyst system prepared using inexpensive
and abundant metals such as iron and nickel. This
sustainable catalyst showed high potential for the
conversion of bio-based compounds, including
amination of LA and PEA using a carbon-supported FeNi alloy.[a]
Entry Conversion
T
P
Flow rate
WHSV Solvent
[hÀ1
]
H2
(selectivity)[b] [%] [8C] [bar] [mLminÀ1
]
1[c]
2[d]
3[d]
4[d]
5[d]
6[e]
54 (69)
85 (79)
92 (91)
93 (93)
91 (99)
84 (90)
150
150
150
150
150
150
25
25
50
85
85
85
0.5
0.5
0.5
0.3
0.3
0.1
39
39
39
24
25
9
EtOH
EtOH
EtOH
EtOH/2-MTHF 1:1 (v/v)
2-MTHF
2-MTHF
[a] Conditions: LA (50 mm). [b] Conversion and selectivity are calculated using Equa-
tions (1) and (2) defined in the Supporting Information. [c] 50 mm PEA. [d] 25 mm PEA.
[e] 34 mm LA and 17 mm PEA.
(entry 6). The slightly reduced conversion compared to pure
LA could be ascribed to the presence of acid traces and organ-
ic impurities that originated from the hydrolysis of glucose to
LA and that were silent to GC–MS analysis. Nevertheless, these
results are highly valuable for a preparative purpose and show-
case for the first time the synthesis of pyrrolidones on the
basis of crude LA feeds obtained by direct processing of re-
newable feedstock.
HMF, furfural, amino acids, and LA. The latter was used to
showcase the preparation of pyrrolidones that are attractive
value-added products and could find multiple uses in differ-
ent application fields. In this case, even the use of LA crude
feeds obtained via hydrolysis of glucose gave rise to very
good results. Furthermore, the time-on-stream test proved
that the stability of the catalyst remained almost unchanged
over 52.5 h. All in all, the methodology described in this
work showed a very high potential for application in real bio-
refineries. Future studies will focus on the expansion of the
library of obtainable products employing other biomass-de-
rived molecules, the evaluation of different solvents systems
(e.g., water) as well as the design of different integrated bio-
refinery schemes based on the use of second-generation car-
bonized filter paper-supported FeNi nanoparticles.
The stability of an active catalytic material is an important
feature for its prolonged utilization. Therefore, a preliminary
postcatalysis characterization of the FeNi nanocomposite was
performed after roughly 60 h on stream, which corresponds to
more than 20 mol reactants converted per mol of metal in the
catalyst. Interestingly, ICP–OES measurements showed only
limited metal leaching (~1 wt% Ni and 2 wt% Fe). SEM and
TEM investigations confirmed that the catalyst morphology
was preserved, and no sintering of the FeNi nanoparticles was
observed (see Figures S5 and S6).
Experimental Section
Finally, the durability of the catalyst was tested for the re-
ductive amination of LA and PEA by monitoring the conversion
Experimental details are given in the Supporting Information.
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Published online on September 18, 2015
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