Two-step catalytic conversion of lignocellulose to alkanes

Article abstract of DOI:10.1039/c9ra03174j

Direct conversion of lignocellulose to alkanes is challenged by the complex and recalcitrant nature of the starting material. Generally, alkanes are obtained from one of the main lignocellulose constituents (cellulose, hemicellulose or lignin) after their separation, and platform chemicals derived therein. Here we describe a two-step methodology, which uses unprocessed lignocellulose directly, targeting a mixture of alkanes. The first step involves the near-complete conversion of lignocellulose to alcohols, using a copper doped porous metal oxide (Cu-PMO) catalyst in supercritical methanol. The second step comprises a novel solvent exchange procedure and the exhaustive hydrodeoxygenation (HDO) of the complex mixture of aliphatic alcohols, obtained upon depolymerization, to C₂-C₁₀ alkanes by either HZSM-5 or Nafion at 180 °C in conjunction with Pd/C in dodecane. This describes an unprecedented two-step process from lignocellulose to hydrocarbons, with an overall carbon yield of 50%.

Full text of DOI:10.1039/c9ra03174j

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Two-step catalytic conversion of lignocellulose to
alkanes†
Cite this: RSC Adv., 2019, 9, 23727
Zhuohua Sun,* Daniel Buwalda and Katalin Barta
*
Direct conversion of lignocellulose to alkanes is challenged by the complex and recalcitrant nature of the
starting material. Generally, alkanes are obtained from one of the main lignocellulose constituents
(cellulose, hemicellulose or lignin) after their separation, and platform chemicals derived therein. Here
we describe a two-step methodology, which uses unprocessed lignocellulose directly, targeting
a mixture of alkanes. The first step involves the near-complete conversion of lignocellulose to alcohols,
using a copper doped porous metal oxide (Cu-PMO) catalyst in supercritical methanol. The second step
comprises a novel solvent exchange procedure and the exhaustive hydrodeoxygenation (HDO) of the
Received 28th April 2019
Accepted 10th July 2019
2
complex mixture of aliphatic alcohols, obtained upon depolymerization, to C –C10 alkanes by either
DOI: 10.1039/c9ra03174j
ꢀ
HZSM-5 or Nafion at 180 C in conjunction with Pd/C in dodecane. This describes an unprecedented
rsc.li/rsc-advances
two-step process from lignocellulose to hydrocarbons, with an overall carbon yield of 50%.
21
would be easily implemented into existing infrastructures.
Indeed, several novel methods have been recently investigated
Introduction
With the decreasing oil reserves and concerns related to global regarding the conversion of lignocellulose to alkanes, most of
warming due to CO
emissions, it is vital to explore more which preferentially focused on the selective catalytic conver-
sustainable sources of energy, including renewables, for sion of either lignin itself or cellulose derived platform chem-
2
1,2
producing liquid transportation fuels. The conversion of raw icals. For example, organosolv lignin was converted to
biomass to liquid transportation fuels would mean the acqui- cycloalkanes in one-pot with high selectivity (>99%) by sup-
2
2,23
24
sition of a fossil fuel replacement that is both CO neutral and ported Ni catalysts
or Ru-based bifunctional catalysts via
2
3,4
renewable as it can be extracted from plant material. Ligno- a cascade HDO reaction sequence. Liquid straight-chain
cellulose, which is already produced in large quantities (about alkanes were obtained directly from cellulose with high yield
3
68 million dry tons annually could be produced on forestlands (up to 82%) in a biphasic catalytic system using tungstosilicic
5
ꢀ
25
in the US alone) would be a starting material for the production acid and Ru/C catalyst at 220 C. Such systems have been also
6
26
of transportation fuels. Indeed, a variety of research directions integrated into existing renery structures. Regarding the
have been established in the past few decades to attain this (hemi)cellulose platform, several elegant routes have been
7
goal. Prominent research directions include thermal methods developed for the transformation of single compounds
ꢀ
8
27,28
29,30
31
such as gasication (typically 600–1200 C) or pyrolysis (typi- HMF,
furfural
or the derived cyclopentanone into long
ꢀ
9
32
cally 450–800 C) which require higher temperatures, or liquid chain alkanes that are suitable to use as jet fuels. These
phase approaches that generally involve the fractionation selective catalytic transformations generally comprise an acid or
1
0
(
pretreatment) of lignocellulose to its main constituents – base catalysed aldol condensation or coupling, followed by deep
cellulose, hemicellulose or lignin and further conversion of HDO. Novel dehydrogenation/aldol condensation/
these fractions. Important developments have been achieved in hydrogenation sequences have also been reported for the
11,12
the production of second generation bioethanol,
butanol as well as alternative energy carriers such as GVL,
bio- conversion of the ABE fermentation mixtures to long chain
13
14–16
33,34
ketones followed by deep HDO.
Very recently we have
17,18
19,20
levulinates
or furanics
obtained from cellulose or hemi- developed a method for the coupling of mixtures of aliphatic
cellulose derived platform chemicals.
alcohols obtained from the ‘Ligno-Flex’ process using a Cu and
Producing liquid alkanes specically has the advantage of Ni doped porous metal oxide catalyst.
35–37
delivering already known products, and such drop-in alkanes
Approaches that use raw lignocellulose directly may
circumvent the relatively energy-, time- and material-intensive
lignocellulose pretreatment/fractionation process, albeit the
difficulty of product separation moves downstream. Recently,
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands. E-mail: k.barta@rug.nl; sunzhuohua@yahoo.com
†
Electronic supplementary information (ESI) available: Characterizations of several elegant examples of direct conversion of lignocellulose
catalysts and the list of chemicals detected in the GC-MS. See DOI:
to alkanes have been reported. Wang and co-workers described
1
0.1039/c9ra03174j
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the direct HDO of raw lignocellulose sources into liquid alkanes 0.12 mol) in deionized water (0.2 L) was added to a solution
38
ꢀ
over a multifunctional Pt/NbOPO catalyst. Ma and co-workers containing Na CO (5.30 g, 0.05 mol) in water (0.3 L) at 60 C
4
2
3
reported the efficient conversion of lignocellulose into gasoline under vigorous stirring. The pH was kept between 9 and 10 by
alkanes (hexanes and pentanes), monophenols and related addition of small portions of a 1 M solution of NaOH. The
ꢀ
cycloalkanes over layered LiTaMoO
phosphoric acid medium.
6
and Ru/C in aqueous mixture was vigorously stirred at 60 C for 72 h. Aer cooling to
room temperature, the light blue solid was ltered and re-
39
Copper doped porous metal oxides (Cu-PMOs) derived from suspended in a 2 M solution of Na
2
CO
3
(0.3 L) and stirred for
ꢀ
synthetic hydrotalcites (HTC) have been established by Ford overnight at 40 C. The solids were ltered and washed with
and co-workers as a highly interesting catalyst class for the deionized water until chloride free. Aer drying the solid for 6 h
ꢀ
conversion of renewable resources in supercritical meth- at 100 C, 15.07 g of the hydrotalcite (HTC) was obtained. 4 g of
40–43
ꢀ
anol.
hydrogen (from reforming a portion of the solvent) is benecial and yielded 2.5 g of Cu20-PMO catalyst.
Supercritical methanol, as well as the in situ generated obtained hydrotalcite was then calcined at 460 C for 24 h in air
for achieving hydrothermal conditions for the full conversion of
The catalyst prepared in this procedure is a porous metal
lignocellulose solids into methanol soluble liquids under oxide (PMO), denoted as Cu20-PMO, which indicates that in
4
0,43
2+
selected reaction conditions.
liquid phase consist of a predominantly C
alcohols from cellulose and C -range, cyclic aliphatic alcohols content were prepared in the same procedure but using
from lignin and their methylated derivatives as methanol different amounts of Cu(NO ) $2.5H O and MgCl $6H O salt
The main products of the a 3 : 1 Mg/Al hydrotalcite precursor 20 mol% of the Mg ions
2
+
2
6
–C range aliphatic were replaced with Cu ions. Other catalysts with different Cu
9
3
2
2
2
2
40,43
solution.
detailed composition analysis of these mixtures, and investigate
their conversion to more uniform mixtures of alkanes. First, we diffractometer using Cu Ka radiation and the spectra were
Inspired by this precedence, we have attempted the and named as Cu5-PMO and Cu10-PMO respectively.
Powder X-ray analysis was performed on a Bruker XRD
ꢀ
ꢀ
attempted to study the effect of different Cu-PMO catalyst recorded in the 2q angle range of 10 –90 . Elemental analysis
compositions on the product distribution and the average chain was performed on a PerkinElmer instrument (Optima 7000DV).
lengths of the obtained alcohols. Second, we aimed to nd ideal
novel catalytic methods capable of deep HDO of such complex
mixtures of predominantly aliphatic alcohols to liquid alkanes
Extraction of lignin from pine lignocellulose
that would possess higher heating value than the corresponding Extraction of lignin from pine lignocellulose was carried out in
44
oxygenates. Such a system has not yet been previously studied, a 500 mL autoclave with an overhead stirrer and temperature
very likely due to the methanol solvent, which may signicantly controller. Typically the reactor was charged with 30 g of pine
36
complicate such downstream processing efforts. Herein we lignocellulose, 250 mL of methanol at room temperature. The
ꢀ
describe the successful, two-step conversion of lignocellulose to reactor was sealed and stirred for 24 h at 170 C. During the
mixtures of alkanes. In addition, we have evaluated the reac- reaction 25 bar of autogenous pressure was developed. Aer
tivity of organosolv lignin extracted from pine lignocellulose as completion of the extraction, the reactor was cooled down to
well as cellulose separately in order to compare the product room temperature. The mixture was collected in a 1000 mL
portfolio to the lignocellulose conversion runs. For the corre- beaker by rinsing the reactor several times with methanol and
sponding HDO reactions, model compounds were used for then ltered. The solids were washed with methanol and the
investigating reactivity and optimization of the catalytic systems combined solution was concentrated to 100 mL by rotary
before moving to the more complex, real lignocellulose derived evaporator and precipitated with ice-cold water and then stirred
mixtures.
overnight. The mixture was then centrifuged and the solids
Lignin) were collected by decanting the solutions which con-
(
tained hemicellulose. The obtained solid was nally dried
under vacuum yielding 0.66 g organosolv lignin.
Material and methods
General considerations
Pine lignocellulose was purchased from Bemap Houtmeel B. V.,
elemental content C: 42.74%; H: 6.19%; O: 46.42%. All reagents
and solvents are used as received without further purication.
Catalytic conversion of pine lignocellulose (Step 1)
Pine sawdust (100 mg) and Cu-PMO catalysts (100 mg) were
added to a 10 mL Swagelok stainless steel microreactor with
2 2 5
ZrO , Nb O , Naon, Pd/C (5%) catalysts was purchased from
3
mL methanol as solvent. The reaction vessel was placed in
Sigma-Aldrich. Zeolite ZSM-5 (Si : Al 30 : 1) was purchased from
ꢀ
ꢀ
a heating block at 320 C for 6 h. The reactor was subsequently
rapidly cooled down in an ice-water bath to terminate the
reaction. The contents of the reaction vessel were transferred to
a centrifuge tube and the solid was separated. The liquid layer
was collected in a round bottom ask and the solid was washed
Alfa Asea, before use it was calcined in the oven at 500 C for 5
hours. USY-600 was purchased from Zeolyst International.
Preparation and characterization of Cu-PMO catalysts
The HTC (hydrotalcite) catalyst precursor was prepared by twice with methanol. All liquid products were combined and
45
a typical co-precipitation method. In a typical procedure, methanol was removed by rotatory evaporation until approxi-
solution containing AlCl $6H (12.07 g, 0.05 mol), mately 5 mL liquid le. The products were then analyzed by GC-
Cu(NO $2.5H O (6.98 g, 0.03 mol) and MgCl $6H O (24.40 g, MS.
a
3
2
O
3
)
2
2
2
2
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ATTENTION: Handle with care!! Upon heating to the indi- gaseous products were collected into a gas bag and analyzed by
cated reaction temperature, autogenous pressure (>200 bar) GC-TCD.
develops in these vessels due to the supercritical methanol and
in situ formed hydrogen.
Analysis of liquid and gas products
Products of the liquid phase, obtained in Step 1 (alcohol
mixtures) and Step 2 (alkanes) are analyzed by GC-MS and GC-
FID. GC-MS was performed using a Shimadzu GC-2010 plus
HDO of model compounds
Cyclohexanol (0.1 mL, 1 mmol) or alcohol mixture 1 (Mix1
containing 2-methylcyclohexanol, cyclohexanol, 4-methyl-2-
system equipped with a GCMS QP2010 GC SE detector and a HP-
ꢀ
5
column. The column was maintained at 30 C for 5 min,
pentanol,
2-methyl-1-butanol,
2-ethylhexanol,
2-cyclo-
ꢀ
ꢂ1
ꢀ
ꢀ
ꢂ1
raised at 10 C min to 90 C, then raised at 25 C min to
50 C and maintained for 10 min. GC-FID was performed on
hexylethanol, 0.17 mmol for each and in total 0.128 mL) or
alcohol mixture 2 (Mix2 containing 1-pentanol, 1-hexanol, 1-
octanol, and 2-cyclohexylethanol, 0.25 mmol for each and in
total 0.136 mL) were added to a reaction vessel with 50 mg solid
acid catalyst and 10 mg Pd/C in 3 mL dodecane. The reaction
vessel was placed in a high pressure stainless steel reactor
ꢀ
2
a Hewlett Packard 6890 (HP-5 capillary column and a ame
ꢀ
ionization detector (FID)). The column was maintained at 30 C
ꢀ
ꢂ1
ꢀ
for 5 min, raised at 10 C min to 90 C, then raised at
2
ꢀ ꢀ
ꢂ1
5 C min to 250 C and maintained for 10 min. A fraction of
short chain alkanes obtained in Step 2 are normally gaseous at
room temperature. These were analyzed by GC-TCD upon
collection in a gas bag. A HP5890 Series II GC equipped with
a CP Porabond Q (50 m ꢃ 0.5 mm, lm thickness 10 mm) and
a CP-Molsieve 5A (25 m ꢃ 0.53 mm, lm thickness 50 mm)
column was used for this purpose. The injector temperature was
(Endeavor©) and carefully purged with N
2
for 3 times. Then the
reactor was lled with H (10 bar) and heated to desired
2
temperature. Next, the reaction mixture was transferred to a GC
vial by ltration with a glass pipette equipped with a short pad
of Celite and cotton pad. The products were analyzed by GC-MS
and GC-FID.
ꢀ
ꢀ
set at 150 C and the detector temperature at 90 C. The oven
ꢀ
temperature was kept at 40 C for 2 min, then increased to 90 at
ꢀ
ꢂ1
2
0 C min , and kept at this temperature for 2 min.
HDO of the product mixtures obtained by depolymerization of
pine lignocellulose (Step 2)
Calculation of the overall yield of obtained alkanes
Solvent exchange. Dodecane (5 mL) was added to the
methanol solution obtained directly aer lignocellulose depo-
lymerization in Step 1. Then methanol was carefully evaporated
by rotary evaporator to leave a dodecane solution behind which
contains most of the products from Step 1. Small amount of
precipitate (ꢁ10 mg) was also observed, which was not further
treated.
The amount of obtained alkanes was calculated based on peak
area in GC-FID with eicosane as internal standard.
Mass yield of obtained alkanes is calculated as follows:
Walkanes
Yieldðwt%Þ ¼ W
lignocellulose
Walkanes means the weight of total obtained alkanes,
Wlignocellulose means the weight of total lignocellulose used in
Step 1.
HDO. The dodecane solution obtained above was then
transferred to reaction vessel containing 50 mg solid acid
catalyst and 10 mg Pd/C. The reaction vessel was placed in
a high pressure stainless steel reactor (Endeavor©) and purged
with N for 3 times. Then the reactor was lled with H (10 bar)
C
alkanes
YieldðC%Þ ¼ C
2
2
lignocellulose
and heated to the desired temperature. The reaction mixture
was next transferred to a GC vial by ltration over a PTFE lter
and analyzed by GC-MS.
means the carbon content of total obtained alkanes,
lignocellulose means the carbon content of total lignocellulose
used in Step 1. The carbon content of the pine lignocellulose
used in this study is 42.74%.
C
Upscaled HDO procedure. In order to more precisely calcu-
late the nal carbon yield and the yield of obtained alkanes,
crude reaction mixtures from 10 separate small scale, 0.1 g pine
lignocellulose depolymerization runs (see above Step 1) were
combined. Then (10 mL) dodecane was added to this methanol
solution and methanol was carefully evaporated by rotary
evaporation resulting in a dodecane solution that contains most
of the products from Step 1. The dodecane solution was then
transferred to reaction vessel containing 0.5 g solid acid catalyst
and 0.1 g Pd/C and subsequently, 10 mg eicosane was added as
internal standard. The reaction vessel was placed in a 100 mL
Results and discussion
With the aim of converting raw lignocellulose to alkanes we
have set to investigate a specic depolymerization/HDO
sequence shown in Fig. 1, which consists of the
Parr reactor that was subsequently pressurized with 10 bar H .
2
ꢀ
Aer reaction was conducted for 4 hours at 180 C, then the
vessel was cooled to room temperature and depressurized. Next,
the reaction mixture was transferred to a GC vial by ltration
over a PTFE lter then analyzed by GC-MS and GC-FID. The Fig. 1 Two-step catalytic conversion of lignocellulose to alkanes.
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depolymerization of raw lignocellulose to alcohols (Step 1)
Upon careful analysis of the product mixtures obtained by
using a Cu-PMO catalyst of varying composition in supercritical using catalysts of varying Cu content, the results of products
methanol and nding an appropriate method for the HDO of distributions as function of catalyst composition was summa-
the produced alcohols to alkanes (Step 2).
rized in Fig. 3 and Table S4,† which clearly shows that the
conversion of raw lignocellulose (from 89% to 97%) and selec-
tivity of alcohols (from 52.5% to 66.1%) increased with the
increase of copper content. This is very likely due to different
Depolymerization of lignocellulose with Cu-PMO catalysts
In order to investigate the inuence of Cu content on reactivity, extent of methanol reforming leading to different hydrogen
three different (HTC) precursors were prepared by co- pressure, and therefore different extent of lignocellulose
precipitation, gradually replacing 5 mol%, 10 mol% and deconstruction. At the same time, products with chain length of
2
+
2+
2
0 mol% of Mg by Cu while keeping a constant 3 : 1 ratio more than 7 decreased slightly with the increase of copper
between divalent and trivalent metal ions. The porous metal content – this is also attributed to hydrogenolysis processes
oxide catalysts were next obtained upon calcination of the HTC being more pronounced with increasing hydrogen pressure in
ꢀ
precursors at 460 C for 24 hours.
the case of Cu20-PMO as expected, while Cu5-PMO should
The composition of HTC (Table S1†) were in good agreement resemble more to a classical hydrotalcite, preferably promoting
33
with the theoretically expected values, indicating good incor- aldol condensation steps that lead to chain elongation.
poration of the metal ions. The formation of the double-layered
HTC structure was conrmed by powder XRD measurements
(Fig. S1a†). Aer calcination, the HTC structure was trans-
formed into an amorphous mixed-oxide composition (Fig. S1b†)
ꢀ
ꢀ
ꢀ
where the broad peaks at 37 , 43 and at 63 indicate the
formation of spinels MgAlO and CuAlO .
2
2
Depolymerization of pine lignocellulose in supercritical
methanol (sc-MeOH) was carried out based on procedures
43
described previously. Aer reaction, the transparent methanol
solution of organic compounds was subjected to a detailed GC-
MS identication, whereby the major products were unambig-
uously identied by the use of authentic standards, and the
remaining signals were assigned based on GC-MS library search
with a similarity index above 70%. A GC-MS trace of the sample
aer depolymerization of pine lignocellulose over Cu20-PMO,
labeled with the chemical structures are shown in Fig. 2.
Detail product analysis by different catalysts were listed in
Tables S2 and S3.† As shown here, most of products are alcohols
or hydroxyl ethers with small amount of aromatics also detec-
Fig. 3 Conversion, product distribution and average chain length of
alcohols in various product mixtures obtained upon catalytic depoly-
merization of different substrates in sc-MeOH over various PMO
catalysts. Reaction conditions: substrate 100 mg, catalyst 100 mg,
ꢀ
ted. The formation of hydroxyl ethers is attributed to the further methanol 3 mL, 320 C, 6 h. (a) Conversion is determined by gravi-
reaction of methanol with formed alcohols.
metric analysis. (b) Lignin was obtained from pine lignocellulose.
Fig. 2 GC-MS trace of a product mixture obtained upon depolymerization of pine lignocellulose using Cu20-PMO in sc-MeOH. (Product labels:
green ¼ confirmed by the use of an authentic standard, blue ¼ similarity index above 90%, red ¼ similarity index between 70% and 90%.)
23730 | RSC Adv., 2019, 9, 23727–23734
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ꢀ
2
Organosolv lignin, separately obtained by lignocellulose C under the same conditions (140 C, 4 h, 10 bar H ). As can be
fractionation, as well as pure cellulose were also reacted under seen in Table 1, the two zeolites (HZSM-5 and USY-600) as well
the same reaction conditions with Cu20-PMO catalyst to eval- as Naon were found to be promising candidates for the
uate the products obtained from these starting materials, conversion of cyclohexanol, giving full substrate conversion at
ꢀ
serving as appropriate control experiments. When pure cellu- 140 C aer 4 hours.
lose was used as starting material, the generated product
Moving toward real lignocellulose-derived mixtures that
mixtures had very similar distributions in the early retention contain both secondary and primary alcohols of varying chain
time range, to those obtained from pine lignocellulose (Fig. 3) length and complexity, we rst prepared model mixtures Mix1
resulting in aliphatic alcohols as main products with 69.3% and Mix2 consisting of compounds relevant for the real ligno-
selectivity. Depolymerization of organosolv lignin gave good cellulose depolymerization mixture. Mix1 consisted of equi-
conversion of 90% and a product mixture mainly containing of molar amount of primary and secondary alcohols whereas Mix2
substituted cyclohexanols and various phenolics (Fig. 3).
consisted of several primary alcohols (see Experimental section
In summary, even though the mixture of products obtained for detailed composition). Clear differences were seen in the
with Cu5-PMO displayed slightly higher average chain length, reactivity of these mixtures (Fig. 5a vs. Fig. 5b) and especially for
the use of Cu20-PMO was most benecial for achieving nearly Mix1, the individual compounds have generally shown different
full lignocellulose conversion (97% vs. 89% for Cu5-PMO). reactivity pattern as well, depending on the catalyst used. While
Therefore further experiments focused on the use of this cata- HZSM-5 was highly efficient leading to almost full conversion of
ꢀ
lyst composition. It has to be pointed out, that more research all components of Mix1 at 140 C, the other two solid acids
ꢀ
should focus on improving substrate/catalyst ratio for industrial tested showed signicantly lower conversion at 140 C, espe-
application. The development of well-dened Cu nanoparticles cially for primary alcohols (Fig. 5a). Increasing the reaction
ꢀ
ꢀ
or Cu containing bimetallic particles on appropriate supports temperature further to 160 C and 180 C lead to full conversion
would be desired.
of all alcohols and in several cases formation of ethers was also
seen. To further investigate the formation of ethers and the
reactivity of primary alcohols, Mix2 containing only primary
alcohols was tested (Fig. 5b). As shown in Fig. 5b, generally poor
HDO of aliphatic alcohol model compounds
Since the product mixtures obtained upon catalytic conversion
of pine lignocellulose in sc-MeOH using Cu20-PMO consisted of
mainly aliphatic alcohols, establishing a suitable HDO proce-
dure seemed feasible. Indeed, several catalytic systems,
including the use of bifunctional catalysts
tion of a solid acid and hydrogenation catalyst
reported for HDO of aliphatic alcohols and phenolics to
alkanes. In our case, the large excess of methanol and the
complexity of product mixtures and possible etherication
reactions are expected to be the major challenge. Therefore,
ꢀ
reactivity was seen at 140 C followed by a marked increase in
ꢀ
primary alcohol conversion at 160 and 180 C. While the
substrate conversion increased dramatically for all substrate
mixtures, product composition varied depending on the catalyst
used in the case of Mix2. While HZSM-5 and Naon gave almost
46,47
or the combina-
4
8,49
have been
ꢀ
the same results at 180 C, and the yield of alcohols accounted
for 86% and 85% respectively, with USY-600 as solid acid,
substantial amount of ether by-product was found.

rst, suitable model compound mixtures were used to discern
HDO of alcohol mixtures from depolymerization of
lignocellulose
optimal catalytic conditions and catalyst type for the desired
transformation and maximize the yield of alkanes. Inspired by
systems that have previously shown high efficiency for HDO of
lignin derived phenolics,
different solid acids (e.g. Naon, HZSM-5, USY-600, ZrO and
Since both HZSM-5 and Naon both performed well with model
ꢀ
mixtures at 180 C, they were both selected for the following
48,49
we have selected to evaluate
tests for the upgrading of lignocellulose derived products to
alkanes. The products from depolymerization of pine lignocel-
lulose, cellulose and pine organosolv lignin with Cu20-PMO
were used as starting materials. Since the presence of
2
Nb
2
O
5
) in combination with Pd/C as preferred hydrogenation
ꢀ
catalyst, in a broader temperature range (140–180 C).
The one pot alcohol to alkane conversion consists of the
reaction sequence shown on Fig. 4, which starts with an acid
catalyzed dehydration to yield the corresponding olen, fol-
lowed by a C]C hydrogenation by Pd/C. For establishing suit-
able reaction conditions, cyclohexanol was identied as
suitable model substrate, and its reactivity was studied in the
presence of different solid acids in combination with 10 mg Pd/
Table 1 Conversion of cyclohexanol using different solid acid cata-
a
lysts with Pd/C
Selectivity%
Entry Solid acid Conversion% Cyclohexane Methylcyclopentane
1
2
3
4
5
HZSM-5
USY-600
99
99
8
0
99
93
100
100
0
6
0
0
0
1
ZrO
Nb
2
2
O
5
Naon
99
a
Fig.
4
Reaction sequence in the HDO of model compound
Reaction condition: 0.1 mL (1 mmol) cyclohexanol, 50 mg solid acid,
ꢀ
cyclohexanol.
10 mg Pd/C, 140 C, 4 h, 10 bar H
2
.
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Fig. 5 Product distribution upon conversion of alcohol model compounds ((a) Mix1 and (b) Mix2) using different solid acid catalysts. Reaction
conditions: 0.128 mL alcohol mixture 1 or 0.138 mL alcohol mix 2, 50 mg solid acid catalyst, 10 mg Pd/C, 4 h, 10 bar H
2
.
methanol would hamper reactivity in further HDO reactions, we using lignocellulose may be also attributed to the formation of
rst have developed a simple solvent exchange method, that formic acid from hemicellulose, as formic acid may act as good
consists of adding dodecane and careful removal of methanol hydrogen donor for biomass conversion, facilitating the HDO

50
through rotary evaporation, whereby the vast majority of process.
compounds remained soluble. The dodecane solution, con- Next, the in-depth quantitative and qualitative analysis of the
taining most of the products, was then used for the further alkane product mixtures was performed using GC-MS, GC-FID
upgrading to alkanes. As can be seen in Fig. 6, the product and GC-TCD methods. In order to calculate the yield of the
mixture from pine lignocellulose was converted to alkanes in obtained alkanes more accurately, the HDO method was
high selectivity (>83%) with both HZSM-5 and Naon as solid upscaled using a crude mixtures from depolymerization of 1 g
ꢀ
acid catalyst at 180 C while ethers and esters remained pine lignocellulose. As shown in Fig. 7, the obtained alkanes
unconverted under these reaction conditions. Slightly more of reect the general structure of lignocellulose: acyclic straight or
the latter two compound categories as well as other by-products branched alkanes mainly originate from cellulose or hemi-
(
ꢁ30%) were formed when using cellulose and pine organosolv cellulose while most cycloalkane components are predomi-
lignin derived product mixture as substrate. The by-products nantly lignin-derived. Furthermore, the chain length of the
are mainly ethers likely formed by etherication of the obtained alkanes range from 2 to 10, while the collected gas
produced alcohols and diols. In the case of organosolv lignin, samples contained mainly ethane and propane. Using Naon as
aromatic monomers or dimers may have also decreased catalyst solid acid catalyst resulted in higher selectivity (30.3% vs.
activity for further HDO reactions. The higher alkane yield when 16.2%) of long chain alkanes (>8) as shown in Fig. 7. This could
be explained with the pore size differences between Naon (>10
51
nm) and HZSM-5 (<1 nm). Furthermore, since HZSM-5 is also
52
a good cracking catalyst, it may have facilitated the conversion
of longer alkanes to shorter chain analogues. As shown in Table
2, the yield of the obtained alkanes using the Pd/C + Naon
reached 25.6 wt%, which is slightly higher than that obtained
with the Pd/C + HZSM-5 catalyst system. The total carbon yield
of the obtained alkanes was 48.7% and 52.9% respectively. A
comparison of the alkane yield with other related research is
summarized in Table S5.†
Possible reasons for loss of mass balance in this two-step
process are: (a) hydrogenolysis of the lignin-methoxy groups
would lead to gaseous products (e.g. methane) or methanol
during Step 1; (b) loss of highly volatile organics during opening
of Swagelok units as well as the solvent transfer procedure; (c)
potential loss of compounds during solvent exchange (d) partial
loss of highly volatile alkanes (<C ) obtained in Step 2 before
analysis. In order to address these points, more elaborate
4
Fig. 6 Distribution of products after HDO of mixtures obtained upon
pine lignocellulose, cellulose or pine organosolv lignin depolymer-
ization. Reaction conditions: 3 mL products mixture in dodecane, equipment that allows for careful collection of each fraction and
ꢀ
5
0 mg solid acid catalyst, 10 mg Pd/C, 180 C, 4 h, 10 bar H
2
.
23732 | RSC Adv., 2019, 9, 23727–23734
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Fig. 7 GC-MS trace of the obtained alkanes after HDO of alcohols obtained by pine lignocellulose depolymerization. Distribution of chain
lengths of the obtained alkane products with HZSM-5 + Pd/C versus Nafion + Pd/C.
Table 2 Quantification of the obtained alkanes after Step 1 + Step 2 from 1 g pine lignocellulose
Weight of obtained alkanes (mg)
Yield
Wt%
Catalysts for HDO
C
2
C
3
C
4
C
5
C
6
C
7
C
8
C
9
C
10
Total
C%
Pd/C + HZSM-5
Pd/C + Naon
22
14
28
31
18
10
31
20
47
44
20
22
37
35
26
42
14
39
192.7
257.5
19.3
25.6
48.7
52.9
upscaling of these reactions will be required for future studies. This work is part of the research program Talent Scheme (Vidi)
Furthermore, testing of the combustion and fuel properties of with project number 723.015.005 (K. B.), which is partly
these mixtures, which benecially contain small amounts of nanced by the Netherlands Organisation for Scientic
methyl-ethers, as well as good amount of cyclic alkanes, should Research (NWO). Z. Sun is grateful for the nancial support by
be determined in order to further asses their potential as bio- the China Scholarship Council.
fuel alternatives.
Conclusions
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