Reductive dealkylation of anisole and phenetole: Towards practical lignin conversion

Article abstract of DOI:10.1002/ejoc.201101015

We present and develop alternative catalysts for biomass conversion and specifically lignin conversion into aromatics. Unlike the conventional CoMo and NiMo formulations, our catalysts can convert low-sulfur feedstocks. A set of five magnesia-alumina mixed oxides were screened in the hydrodealkylation of alkyl phenyl ethers as lignin model compounds. The typical selectivity to phenol is 30-75 %. Interestingly, we saw that the more basic the catalyst, the higher the selectivity for phenol. The results concur with the formation of phenoxide (PhO^-) and RH₃⁺ fragments on the catalyst surface. These can then react with H⁺ and H^- species formed by the hydrogen dissociation on the MgO surface, giving phenol and hydrocarbons. We conclude that magnesia-alumina mixed oxides are attractive candidates for catalyzing lignin breakdown. These catalysts are highly stable, inexpensive, and readily available .

Full text of DOI:10.1002/ejoc.201101015

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101015
Reductive Dealkylation of Anisole and Phenetole: Towards Practical Lignin
Conversion
[a]
[a]
[a]
Zea Strassberger, Stefania Tanase, and Gadi Rothenberg*
Keywords: Heterogeneous catalysis / Sustainable chemistry / Reduction / Biomass / Bulk chemicals
We present and develop alternative catalysts for biomass
conversion and specifically lignin conversion into aromatics.
Unlike the conventional CoMo and NiMo formulations, our
catalysts can convert low-sulfur feedstocks. A set of five mag-
nesia–alumina mixed oxides were screened in the hydro-
selectivity for phenol. The results concur with the formation
–
+
of phenoxide (PhO ) and RH
3
fragments on the catalyst sur-
+
–
face. These can then react with H and H species formed by
the hydrogen dissociation on the MgO surface, giving phenol
and hydrocarbons. We conclude that magnesia–alumina
dealkylation of alkyl phenyl ethers as lignin model com- mixed oxides are attractive candidates for catalyzing lignin
pounds. The typical selectivity to phenol is 30–75%. Interest-
ingly, we saw that the more basic the catalyst, the higher the
breakdown. These catalysts are highly stable, inexpensive,
and readily available.
Introduction
With this in mind, we searched for an alternative hydro-
deoxygenation (HDO) catalyst that needs no sulfiding and
is capable of converting low-sulfur feedstocks. Here we re-
port a new type of mixed magnesia–alumina catalyst for the
reductive dealkylation of anisole and phenetole, two lignin
model compounds, and discuss the pros and cons of their
application.
Lignin, the glue that holds trees together, is the most
abundant natural resource of aromatic compounds.
[
1–3]
In
that respect, it is a far more advanced resource than crude
oil. This is because lignin already contains aromatic func-
tional groups. Crude oil must first undergo cracking and
then reformation, both of which are energy intensive and
[
4–9]
costly.
Thus, catalytic conversion of lignin into high-
value aromatics is not only politically attractive but also an
Results and Discussions
economically viable option.[6,10,11]
As lignin model compounds, we chose two simple alkyl
phenyl ethers, namely, anisole (methoxybenzene) and phen-
etole (ethoxybenzene). These are also two important and
actual breakdown products of the lignin structure. As cata-
lysts, we used five different mixed alumina–magnesia oxides
The problem is that lignin is typically over-function-
alized. Its polymeric structure must first be broken down to
dimeric and monomeric components.[12,13] These must then
be transformed into the desired functional aromatics. Inter-
estingly, changing the hydrocarbon feedstock from petro-
leum and coal into biomass also requires new types of cata-
lysts. CoMo and NiMo are typically used for catalyzing
(catalysts A–E, see Table 1). In a typical reaction [Equa-
tion (1)], a solution of the alkyl phenyl ether was treated
crude oil hydrodesulfurization (HDS),[
14,15]
with 40 bar H at 350 °C for 3 h in the presence of the cata-
but these refin-
2
lyst (10 wt.-%). The main reactions observed were hydrode-
alkylation and alkyl rearrangement [Equations (2), (3), and
ery catalysts rely on feedstocks with high sulfur content.
The lower sulfur content of biomass causes catalyst deacti-
vation via reduction of sulfided Co or Ni followed by cok-
(4)]. The latter is very interesting, as it leads to the forma-
ing.[ This can be prevented by adding sulfur donor com-
16]
tion of new C–C bonds. Until now, only acidic hetero-
geneous catalysts have been reported, including cation-ex-
[
17,18]
pounds to the feed,
which are then converted into H S,
2
[
19]
[20]
changed montmorillonites,
Nafion,
and zeolites. To
but it is a “degenerate” solution. Thus, new catalysts are
[
21]
needed for these new feedstocks.[
10]
investigate the effect of basic sites, we used MgO
and
Table 1. Catalysts tested in hydrodealkylation.
[
a] Van’t Hoff Institute for Molecular Sciences, University of
Entry
Catalyst
% MgO
2 3
% Al O
Amsterdam,
Science Park 904, 1098 XH Amsterdam, The Netherlands
Fax: +31-20-525-5604
1
2
3
4
5
A
B
C
D
E
0
100
40
34
25
20
60
66
75
80
E-mail: g.rothenberg@uva.nl
Re-use of this article is permitted in accordance with the Terms
and Conditions set out at http://onlinelibrary.wiley.com/journal/
10.1002/(ISSN)1099-0690/homepage/2046_onlineopen.html
5246
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 5246–5249

Reductive Dealkylation of Anisole and Phenetole
MgO–Al O mixed oxides at various Mg/Al ratios. In the disproportionation reaction between two ortho-cresol mole-
2
3
3
+
[22]
latter case, the presence of Al ions is expected to change cules [Equation (4)].
the acidic character of MgO.
The high selectivity towards phenol can be interpreted
by considering the adsorption of anisole, a weak Lewis
base, onto the acidic Al O sites. This makes the anisole
2
3
prone to nucleophilic attack. The most reactive nucleophilic
site is the oxygen bound to magnesium, which can attack
the methyl group of the anisole molecule. This gives phen-
–
+
(
1) oxide (PhO ) and CH3 fragments on the surface
+
(
Scheme 1). These fragments can then react with H and
–
H species formed by hydrogen dissociation on the MgO
surface, giving phenol and methane [Equation (2)]. Partici-
pation of H and H is documented in based-catalyzed hy-
drogenation. Our studies show, however, that anisole can
also be converted into phenol with good selectivity in the
+
–
[
23]
(
(
(
2)
absence of hydrogen. Therefore, we do not rule out the pos-
+
sibility of a nucleophilic interaction between CH cations
3
and an O2– anion from the MgO surface. This would give
[23]
a formate surface species, which may decompose into CO
[
24]
3) and H at high temperatures.
2
4)
Both anisole and phenetole gave high conversions with
00% alumina. For anisole, the main products of the de-
1
2 3
Scheme 1. Proposed interaction of phenol with the Al O –MgO
methylation were phenol, ortho-cresol, and 2,6-xylenol support.
Table 2). ortho-Cresol is obtained by the isomerization of
anisole [Equation (2)]. We did not observe any meta-cresol.
(
Recent studies have shown that Lewis acid sites play a
+
[25]
This is expected if we consider that the meta position is key role in the formation of CH3 fragments. Indeed, we
thermodynamically the most favored for the substitution in also see that Al O3 is necessary for activating anisole.
2
phenol rings, whereas the ortho position is kinetically pre- Table 2 shows that lowering the Al O content decreases an-
2
3
ferred due to its reactivity towards electrophilic substitu- isole conversion. MgO alone does not catalyze the conver-
tion. The formation of 2,6-xylenol can be explained by the sion of anisole. We also studied the rearrangement of phen-
Table 2. Product distribution for anisole and phenetole conversion using catalysts A–E.[a]
[
a] Standard reaction conditions: anisole (1 mL, 9.2 mmol) in cis/trans-decahydronaphthalene (20 mL), H
tion conditions: substrate [1 mL anisole (9.2 mmol) or phenetole (7.9 mmol)] in cis/trans-decahydronaphthalene (20 mL), H
00 °C, 3 h. [c] Yield determined by GC analysis by using n-octane as an external standard.
2
(40 bar), 350 °C, 3 h. [b] Reac-
2
(15 bar),
3
Eur. J. Org. Chem. 2011, 5246–5249
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5247

Z. Strassberger, S. Tanase, G. Rothenberg
SHORT COMMUNICATION
etole. As seen in Table 2, phenetole is less reactive than ani- Table 3. Temperature pretreatment effects on conversion and
yield.[
a]
sole. Nevertheless, a similar trend is observed for both sub-
strate conversion and product distribution.
% Yield[b]
o/p-Cresol 2,6-Xylenol
Entry Activation
temp.
Anisole
conversion
Table 2 also shows a definite synergistic behavior increas-
ing the selectivity in the conversion of anisole. The more
basic the catalyst, the higher the selectivity for phenol. The
price is a sharp lowering of the conversion of anisole. Such
a decrease can be explained by considering that anisole mo-
lecules bind to the Lewis acid sites. When the number of
these sites (Al O sites) decreases, they are rapidly saturated
Phenol
1
2
3
4
r.t.
23.6
26.9
29.8
29.5
15
6.8
7.7
8.1
8.1
1.9
2.1
2
200
400
600
17.1
19.6
19.1
2.3
[
a] Standard reaction conditions: anisole (1 mL, 9.2 mmol) in
cis/trans-decahydronaphthalene (20 mL), H (40 bar), 350 °C, 3 h.
b] Yield determined by GC analysis by using n-octane as an exter-
2
2
3
[
and subsequent anisole molecules can interact only via hy-
nal standard.
[
25]
drogen bonding. Therefore, less anisole molecules will be
activated, lowering the conversion.
Studies on the effect of temperature and hydrogen pres-
sure were carried out by using catalyst A (Table 2, Entry 2).
The conversion of anisole decreased at lower temperature
and lower hydrogen pressure. However, the selectivity
towards phenol remained unchanged, although less methyl-
ated and dimethylated products were observed.
Hydrogen affects the distribution of products when using
both decaline and hexadecane as solvents (Figure 1). The
yields of ortho-cresol and xylenol are higher compared with
those obtained under an atmosphere of argon. However,
the selectivity towards phenol is lower in hexadecane com-
pared with decaline. This suggests the involvement of deca-
line as a hydrogen donor. The use of decaline as a solvent
gives a similar conversion of anisole for both argon and
hydrogen atmospheres. Higher yields of ortho-cresol and xy-
lenol are achieved.
Conclusions
Magnesia–alumina mixed oxides are attractive candi-
dates for catalyzing lignin breakdown reactions. These cata-
lysts are highly stable, inexpensive, and readily available.
Pure alumina is not the preferred catalyst because it shows
low selectivity for phenol, but 60:40 magnesia–alumina
shows high selectivity for phenol at reasonable conversion.
These mixed oxides can be used alternatives for petrochemi-
cal feedstock catalysts in the conversion of biomass and
bio-oils.
Experimental Section
Materials and Instrumentation: Gas chromatography (GC) analysis
was performed by using an Interscience GC-8000 gas chromato-
graph equipped with a flame ionization detector (FID), 14% cya-
nopropylphenyl and 86% dimethyl polysiloxane capillary column
(
Rtx-1701, 30 m; 25 mm ID; 1 μm df). Samples for GC analysis
were diluted in pentane (1 mL). Reactants and products were quan-
tified by using octane as an external standard. GC conditions: iso-
–
1
therm at 50 °C (2 min); ramp at 2 °Cmin to 70 °C; ramp at
–
1
–1
7
2
0 °Cmin to 140 °C; ramp at 10 °Cmin to 260 °C; isotherm at
60 °C (2 min). All reactions were performed under 40 bar of hy-
drogen using a 40-mL stainless steel autoclave. Unless otherwise
noted, all chemicals used were purchased from commercial sources
and used as received. All products were identified by comparing
their GC retention times to those of authentic samples. The Al
2 3
O –
MgO mixed oxides were provided by Eurosupport.[
26]
Procedure for Alkyl Transfer of Anisole and Ethyl Benzene Ether:
Screening of the different supports (0.1 g) was performed in a 40-
mL stainless steel autoclave. A solution of anisole (1 mL 9.2 mmol)
in cis/trans-decahydronaphthalene (20 mL) was charged into the re-
Figure 1. Effect of the solvent on the conversion of anisole.
actor. The pressure was increased to 40 bar with H
the reactor was heated at the desired reaction temperature (300–
50 °C). All the supports were tested at 350 °C for 3 h. The effect
of the reaction temperature and pressure (300 °C and 15 bar H
2
, after which
3
In another set of experiments, we studied the role of the
2
)
pretreatment temperature on 2:1 MgO–Al O (catalyst C).
2
3
was studied only on catalyst A. After the reaction, the reactor was
cooled down to room temperature by using an ice bath. Liquid
samples were analyzed by GC.
We expected that the molecules covering the surface would
desorb successively according to their interaction strength
with the surface sites. The evolution of water and carbon
dioxide continues up to 800 K for MgO.[ Consequently,
stronger basic sites should form at higher temperatures.
Table 3 shows indeed that anisole conversion increases
Procedure for Catalyst Activation: Each catalyst sample was heated
at 200 °C under an atmosphere of N flow for 2 h prior catalytic
2
tests. This precaution was taken to avoid any differences on the
catalyst surface related to water or other species deposition that
23]
slightly with the pretreatment temperature, but the product could interfere. Different temperatures of activation were also
distribution remains unchanged. tested, from r.t. to 600 °C always under N flow. Table 3 shows only
2
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Eur. J. Org. Chem. 2011, 5246–5249

Reductive Dealkylation of Anisole and Phenetole
a slight increase in the conversion of anisole and the yield of phe-
nol. Because the difference were minor and because the procedure
time consuming, we used 200 °C as a standard temperature of acti-
vation.
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Received: July 12, 2011
Published Online: August 11, 2011
Eur. J. Org. Chem. 2011, 5246–5249
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5249