Valorization of lignin waste from hydrothermal treatment of biomass: Towards porous carbonaceous composites for continuous hydrogenation

Article abstract of DOI:10.1039/c5ra06635b

Alkali lignin has been accumulated as a by-product mixed with barium salts during the hydrothermal treatment of rye straw with Ba(OH)₂. Direct heat treatment followed by acid washing of such mineralized lignin were performed in order to obtain

Full text of DOI:10.1039/c5ra06635b

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Valorization of lignin waste from hydrothermal
treatment of biomass: towards porous
carbonaceous composites for continuous
hydrogenation†
Cite this: RSC Adv., 2015, 5, 63691
Received 13th April 2015
Accepted 20th July 2015
*
Gianpaolo Chieffi, Nina Fechler and Davide Esposito
DOI: 10.1039/c5ra06635b
www.rsc.org/advances
Alkali lignin has been accumulated as a by-product mixed with barium additional heat-treatment. Final washing with 1 N HCl provided
salts during the hydrothermal treatment of rye straw with Ba(OH)2. porous carbon with very high surface area (ꢀ3000 m2 gꢁ1),
Direct heat treatment followed by acid washing of such mineralized which was successfully tested as an efficient Ni(II) adsorbent
lignin were performed in order to obtain a porous material that was from wastewater.8
further exploited for the synthesis of a carbonaceous supported FeNi
Metal loaded lignin has been also used as precursor for the
nanoparticle composite as active catalysts for continuous preparation of supported nanoparticles (NPs). Mesoporous
hydrogenation.
activated carbons, obtained by chemical activation of KL with
H3PO4, were employed as supports for the synthesis of a carbon-
based Pd material. The composites displayed activity for a broad
range of transformations including the oxidation of toluene and
xylene,9 selected example of hydrogenations and the Suzuki–
Miyaura cross-coupling.10
Introduction
Lignin is one of the most abundant organic polymers on our
planet and the third major component of lignocellulosic
biomass.1 Commercially, lignin is mostly obtained as the side
product of cellulose pulp mills.2 Different pre-treatment
processes that enable the isolation of the lignin fraction from
the biomass have been reported.3 These are usually classied in
accordance to the technology used and, among others, solvent
fractionation (e.g. organosolv process), physical treatment (e.g.
ball milling), biological treatment or chemical pre-treatment
(oxidative, acidic or alkaline) have received great attention.
Basic reagents such as sodium, calcium or ammonium
hydroxide have been used for the effective chemical deligni-
cation of biomass.4 Such treatments favor to a good extent the
cleavage of a- and b-ether linkages, producing a so-called alkali
lignin mainly composed of oligomers in the range of 1–5 kDa.
Lignin has been recently used for the production of activated
carbon (AC) by thermal treatment in the presence of alkali metal
hydroxides.5,6 For instance, Fierro et al. studied the optimiza-
tion of AC manufacture from Kra lignin (KL) using NaOH or
KOH as the activating agents.7 Wang et al. used a two-step
procedure involving the pre-carbonization of the lignin, which
is successively mixed with KOH before undergoing an
Besides the use of rare and expensive metals, composites
based on iron, the fourth most abundant element in the litho-
sphere, have also been prepared. In this regard, Rodriguez et al.
used FeCl3 both as the activating agent for lignin isolated from
alkaline-pulping black liquors as well as precursor for the
introduction of stable Fe nanoparticles in the carbonaceous
matrix. This system was used as catalyst in wet peroxide
oxidation of phenol.11 Interestingly, carbonaceous supported
FeNi NPs have also proved to be an efficient hydrogenation
catalyst for a variety of substrates.12–14
Recently, our group developed an efficient alkaline hydro-
thermal treatment of biomass e.g., rye straw, which affords the
platform chemical lactic acid in high yields.15,16 The method
relies on the use of barium hydroxide excess and generates
alkali lignin as a by-product. The latter can be recovered in the
form of a mixture with barium inorganic salts.
In order to further increase the value chain and inspired by
other groups that reported on the use of carbonaceous residues
from biomass for the preparation of heterogeneous catalysts or
chromatographic supports,17–19 we here utilized the waste
stream of the alkaline hydrothermal digestion of biomass, i.e.
mineralized lignin, as direct “all-in-one” starting material for
the preparation of porous carbonaceous material where the
inherently included barium salts are used as porogen.20–23
The obtained materials were then used as support for iron-
nickel metal nanoparticles which exhibited remarkable
Max-Planck-Institute of Colloids and Interfaces, 14424 Potsdam, Germany. E-mail:
davide.esposito@mpikg.mpg.de
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra06635b
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catalytic activities for the continuous ow hydrogenation of
nitrobenzene and phenylacetylene, commonly reduced using
scarce and expensive noble metal supported catalysts.24
Result and discussion
The rst step of our process was the hydrothermal treatment of
biomass (rye straw) using Ba(OH)2, which despite its toxicity
compared to other alkaline compounds, is known to promote
the formation of lactic acid in high yields,15 and led to the
recovery of a precipitate containing lignin-derived oligomers
(see the ESI†). In this study, we focused on the valorization of
this otherwise waste crude by-product consisting in a lignin/
barium salt mixture (ML: mineralized lignin) as “all-in-one”
precursor for porous carbonaceous materials. Initially, ML,
which precipitates aer the hydrothermal process, could be
easily obtained by ltration (Scheme 1, step 1).
Fig. 1 XRD patterns of the mineralized lignin (a), of the carbonized/
demineralized lignin (b) and of the carbonized/demineralized lignin
impregnated with the salt precursors of Fe and Ni and then heat-
treated at 800 ꢂC for 2 h under N2 (c).
The X-ray diffraction (XRD) pattern of the crude lter cake
revealed the typical diffraction peaks of barium carbonate
(Fig. 1a). Carbonates are classical by-products of alkali hydro-
thermal treatments of biomass.25 The crystallite size of the salt
was calculated by Scherrer equation to be approx. 26 nm in
diameter. The presence of this compound was additionally
conrmed by comparison of the IR spectrum of the ML (Fig. 2a)
with the one of the pure BaCO3 (Fig. 2b). The absorption bands
at 400–1800 cmꢁ1, also visible in ML, were attributed to vibra-
2ꢁ
tions of CO3
and reect the asymmetric C–O stretching
(1427 cmꢁ1), out of plane bending (856 cmꢁ1) and in plane
bending vibration (692 cmꢁ1).26
Besides BaCO3, the IR spectrum of ML also conrmed the
presence of some characteristic infrared bands attributed to
lignin: a broad band at 3340 cmꢁ1 corresponding to the O–H of
alcoholic and phenolic groups stretching vibrations, and the
shoulder at 1644 cmꢁ1 caused by the stretching of carbonyl
groups conjugated with aromatic rings. Aromatic skeleton
vibration was also visible and appeared at 1594 cmꢁ1
.
27
Elemental analysis of the ML (see Table S1†) revealed a
residual carbon content of ꢀ17 wt% and the relative TGA
analysis under inert atmosphere (see Fig. S1†) showed a weight
Fig. 2 IR spectra of the mineralized lignin (a), of BaCO3 (b), of the
carbonized/demineralized lignin (c), of SiO2 (d), and of the carbonized/
demineralized lignin impregnated with the salt precursors of Fe and Ni
and then heat-treated at 800 ꢂC for 2 h under N2 (e).
loss of 38% at 1000 C associated with release of volatiles.28
ꢂ
At this stage, the organic–inorganic solid ML was directly
used as carbonaceous precursor and heat-treated in a nitrogen
oven at 800 ꢂC for 2 h where BaCO3 served as the in situ
Scheme 1 Process chain for the synthesis of porous lignin derived carbonaceous material and the respective supported FeNi NPs composite.
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activation and porogen agent. The carbonized product was further
washed with diluted hydrochloric acid in order to remove the
inorganic content and free the pores of the resulting carbona-
ceous material (CDL: carbonized demineralized lignin), which
was isolated as a black powder in 16 wt% yield aer simple
vacuum drying (Scheme 1, step 2). The removal of crystalline
components was conrmed by XRD (Fig. 1b). However, IR
revealed the presence of an adsorption band centered at
1060 cmꢁ1, which could be ascribed to the asymmetric bending
modes of SiO2, pointing to residual amounts of silica (see Fig. 2c
and d) commonly found in straw ashes.29,30 Scanning electron
microscopy (SEM) revealed a different morphological structure for
crude ML (Fig. 3a and a0) compared to CDL (Fig. 3b and b0).
The crude ML material possessed a ake-like morphology
with extended thick sheets over several microns. In contrast,
CDL showed much ner structures which suggested the
successful removal of the barium salts thus giving access to the
actual carbonaceous material. In order to assess the porosity in
more detail, nitrogen sorption measurements were performed
(Fig. 4a and b).
Here, the Brunauer–Emmett–Teller (BET) calculation for ML
gave a negligible surface area of 11 m2 gꢁ1 whereas the nal
carbonaceous material CDL showed an increased surface area
of 539 m2 gꢁ1 conrming the assumptions from the SEM
images. Furthermore, the isotherm of CDL displayed high
nitrogen uptake already at low relative pressures indicative for a
reasonable amount of micropores. The isotherm of CDL shows
a type IV behavior which is usually ascribed to the presence of
mesopores. Average pore size was calculated from the nitrogen
sorption isotherms using the NLDFT model for slit pores to be
of 5 nm in diameter (see Fig. S2†). Here, it is to be mentioned
that this size is much smaller than the barium carbonate crys-
tallite size obtained from XRD measurements which is attrib-
uted to the deviating properties of the carbon described here
from the model on which calculations are based as well as to
cavitation effects.
Fig. 4 Nitrogen sorption isotherms of the ML (a), CDL (b) and FeNi–
CDL (c).
Eventually, the direct heat treatment of ML followed by acid
washing could be exploited for the synthesis of high surface
area carbonaceous materials with micro- and mesoporosity.
Such features are highly desirable for catalytic reactions where
high mass transport is required.
With this is mind, the CDL was thus considered as a catalyst
precursor and further functionalized with iron nickel nano-
particles (FeNi–CDL). This was achieved by impregnation of CDL
with an alcoholic solution of the respective metal nitrate salts
followed by heat-treatment at 800 ꢂC for 2 h under N2 atmosphere
(Scheme 1, step 3). This yielded a ne black powder and the XRD
conrmed the formation of a fcc crystalline phase of a FeNi alloy
with a NP crystallite size of 17 nm (Fig. 1c).31 Besides the main
phase, an additional less intense peak found at 44.75q revealed
the presence of a concomitant bcc phase of the same alloy.32 The
morphology of the FeNi–CDL resembled quite well the one of the
Fig. 3 Low magnification (a–c) and high magnification (a0–c0) SEM images of the ML (a and a0), CDL (b and b0) and FeNi–CDL (c and c0).
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Also nitrogen sorption analysis still revealed an isotherm
shape similar to the one of the initial CDL, i.e. particles are
deposited on the outer surface, pointing to the successful
functionalization of CDL under preservation of the porosity as
well as morphology. The somewhat lowered surface area of
198 m2 gꢁ1 could be ascribed to the increased mass of the
additional metals (Fig. 4c).
Inductively coupled plasma-optical emission spectroscopy
(ICP-OES) analysis of FeNi–CDL displayed the simultaneous
presence of Fe (14 wt%) and Ni (22 wt%) in the initially
employed ratio of the metal salt precursors as well as residues of
Ba and Si from the ML (see Table S1†). As mentioned earlier, the
focus of the present contribution was more the facile and
sustainable valorization of the alkali hydrothermal process
chain value. Therefore, a standardized washing procedure was
applied without optimizing conditions for a full removal of
inorganic impurities. Moreover, as it will be discussed in the
following, the residual presence of Si and Ba was not detri-
mental for the catalytic performances of the FeNi–CDL, and
possible additional effects caused by these inorganic impurities
will be evaluated in future studies.
Having established the protocol for the preparation of FeNi–
CDL from the alkali lignin, we assessed the catalytic perfor-
mance of such material for hydrogenation reactions. For this
purpose, the FeNi–carbonaceous composite powder was packed
into a cartridge and directly employed as a catalyst within a
bench-top continuous ow reactor (H-Cube Pro™) provided
with an internal H2 source and equipped with a liquid feed.
Table 1 reports the conversion and selectivity for the
hydrogenation of two different model substances. The ow rate
and the molarity of the different substrates were kept constant
Fig.
5 Low (a) and high (b) magnification TEM images of the
carbonized/demineralized lignin impregnated with the salt precursors
of Fe and Ni and then heat-treated at 800 ꢂC for 2 h under N2.
original carbonaceous material, however, well visible bright
metal nanoparticles with an average size of 48 nm in diameter
could be detected (Fig. 3c and c0). The presence of iron and nickel
was conrmed by EDX analysis (see Fig. S3 in the ESI†).
TEM analysis of FeNi–CDL revealed a relatively broad size
distribution and the coexistence of metal particle conglomer-
ates in the micrometer range, together with round shape
nanoparticles embedded in the carbonaceous matrix (see
Fig. 5a and b). The dispersed non-aggregated nanoparticles
visible at high magnication showed an average size distribu-
tion of 48 nm (the size distribution graph is reported in the ESI
in Fig. S4†). The fact that the mean particle size observed from
both SEM and TEM images was larger than the average crys-
tallite size calculated using the Scherrer equation from the XRD
pattern (17 nm) conrmed that these nanoparticles were partly
conglomerates.
Table 1 Conversion, selectivity and optimized temperature and pressure for model continuous hydrogenation reactions using the FeNi–CDL as
catalyst
Entry
1a
Substrate
Product
Conversione (%)
>99
Selectivitye (%)
95
T (ꢂC)
p-H2 (bar)
20
125
2a,b
8
99
95
75
89
125
150
50
20
50
10
10
3a
>99
84
4c
5c,d
>99
50
6c,d
>99
82
25
1
a
b
d
Conditions: 50 mM in ethanol, 0.3 mL minꢁ1
.
CDL was used as catalyst. c 100 mM, 0.5 mL minꢁ1
.
Lindlar catalyst. e Liquid feed compositions
are expressed as uncorrected GC/MS peak area percentages.
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(50 mM and 0.3 mL minꢁ1), while temperature and hydrogen phenylacetylene to styrene under mild conditions without the
pressures were varied to optimize conversion and selectivity. use of any additives, unlike the commercial Lindlar catalyst.
The catalytic hydrogenation of nitroaromatic compounds is Therefore, the approach described in this work demonstrates
a crucial reaction for the production of anilines, which repre- nicely the efficient valorization of a waste stream from an
sent an important intermediate for the synthesis of pharma- independent biomass conversion scheme and opens the way to
ceuticals, polyurethanes, dyes and agricultural products.33,34 As a fully integrated biorenery with increased chain value. Future
a model for such transformation, the reduction of nitrobenzene studies will focus on the preparation of analogs of this novel
was evaluated. The latter compound could be quantitatively catalytic system and the evaluation of their activity for addi-
converted into aniline (entry 1) at 125 ꢂC and 20 bar of H2. The tional catalytic transformations. Moreover, the possible syner-
GC chromatograms and the corresponding mass spectra of the gistic effects of the inorganic residues contained in the waste
major products from Table 1 are reported in Fig. S5 and S6 in biomass on the overall catalytic performance of the nal
the ESI.†
Control experiments performed with the non-functionalized
composites will be further evaluated.
CDL did not show effective hydrogenation of nitrobenzene
under the same experimental conditions, conrming in a
preliminary way that the presence of inorganic residues inher-
Acknowledgements
ently present from the staring material was negligible with The authors gratefully thank Hendrik Wetzel for ICP OES
regard to the catalytic performance and underlined the active measurements (Fraunhofer Institute for Applied Polymer
role of FeNi NPs in the catalytic cycle (entry 2).
Research).
Phenylacetylene was also tested as substrate for the contin-
uous hydrogenation in order to prove the versatility of the FeNi–
CDL for the reduction of additional functional groups (entry 3).
The complete conversion of the starting material at high
selectivity toward ethylbenzene was achieved at 150 ꢂC and
50 bar of H2.
Notes and references
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Interestingly, the selectivity of the reaction could be tuned
towards styrene at milder conditions (entry 4) and increasing
the molarity (100 mM) as well as the ow rate (0.5 mL minꢁ1).
Traditionally, similar processes are performed using commer-
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under the same experimental conditions (entry 5), ethylbenzene
was obtained. The selective hydrogenation to styrene could not
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temperature and with 1 bar of H2 (entry 6), revealing our FeNi–
CDL composite as an interesting candidate for selective
hydrogenation processes.
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