Selective Production of Toluene from Biomass-Derived Isoprene and Acrolein

Article abstract of DOI:10.1002/cssc.201601267

Toluene is a basic chemical that is currently produced from petroleum resources. In this paper, we report a new route for the effective synthesis of toluene from isoprene and acrolein, two reactants readily available from biomass, through a simple two-step reaction. The process includes Diels–Alder cycloaddition of isoprene and acrolein in a Zn-containing ionic liquid at room temperature to produce methylcyclohex-3-enecarbaldehydes (MCHCAs) as intermediates, followed by M (M=Pt, Pd, Rh)/Al₂O₃-catalyzed consecutive dehydrogenation–decarbonylation of the MCHCAs at 573 K to generate toluene with an overall yield up to 90.7 %. Model reactions indicated that a synergistic inductive effect of the C=C double bond and the aldehyde group in MCHCA plays a key role in initiating the consecutive dehydrogenation–decarbonylation, and that methyl benzaldehydes are the key intermediates in the gas-phase transformation of MCHCAs. Microcalorimetric adsorption of CO on different catalysts showed that decarbonylation of the substrate occurs more likely on the strong adsorption sites. To the best of our knowledge, it is the first report of Pt/Al₂O₃-catalyzed consecutive dehydrogenation–decarbonylation of a given compound in one reactor. This work provides a highly efficient and environmental friendly route to toluene by utilizing two compounds that can be prepared from biomass.

Full text of DOI:10.1002/cssc.201601267

DOI: 10.1002/cssc.201601267
Full Papers
Selective Production of Toluene from Biomass-Derived
Isoprene and Acrolein
Tao Dai,[a, b] Changzhi Li,*[a] Bo Zhang,[a] Haiwei Guo,[a, b] Xiaoli Pan,[a] Lin Li,[a] Aiqin Wang,[a]
and Tao Zhang*[a]
Toluene is a basic chemical that is currently produced from pe-
troleum resources. In this paper, we report a new route for the
effective synthesis of toluene from isoprene and acrolein, two
reactants readily available from biomass, through a simple
two-step reaction. The process includes Diels–Alder cycloaddi-
tion of isoprene and acrolein in a Zn-containing ionic liquid at
room temperature to produce methylcyclohex-3-enecarbalde-
hydes (MCHCAs) as intermediates, followed by M (M=Pt, Pd,
Rh)/Al2O3-catalyzed consecutive dehydrogenation–decarbony-
lation of the MCHCAs at 573 K to generate toluene with an
overall yield up to 90.7%. Model reactions indicated that a syn-
ergistic inductive effect of the C=C double bond and the alde-
hyde group in MCHCA plays a key role in initiating the consec-
utive dehydrogenation–decarbonylation, and that methyl
benzaldehydes are the key intermediates in the gas-phase
transformation of MCHCAs. Microcalorimetric adsorption of CO
on different catalysts showed that decarbonylation of the sub-
strate occurs more likely on the strong adsorption sites. To the
best of our knowledge, it is the first report of Pt/Al2O3-cata-
lyzed consecutive dehydrogenation–decarbonylation of
a given compound in one reactor. This work provides a highly
efficient and environmental friendly route to toluene by utiliz-
ing two compounds that can be prepared from biomass.
Introduction
Benzene, toluene, and xylenes (BTX) are very important basic
chemicals in the petrochemical industry.[1] The worldwide
demand for these hydrocarbons is increasing with economic
development. Amongst the basic chemicals, p-xylene is one of
the highest-volume chemical precursors for terephthalic acid
and dimethyl terephthalate. Therefore, alternative approaches
to obtain p-xylene from renewable resources are of great im-
portance.[2] Despite the fact that toluene is a rather low-value
product compared to p-xylene, it is a key precursor to numer-
ous value-added chemicals and drugs; for example, it is a sub-
strate for toluene diisocyanate (used in the production of poly-
urethane foam) and trinitrotoluene.[3] It is also used as an
octane booster in gasoline fuels and as a common solvent for
paints, paint thinners, silicone sealants, and adhesives. Current
production of toluene is derived entirely from fossil sources
through a catalytic reforming process.[4] With the depletion of
fossil reserves, there is urgent demand for the development of
new routes for the production of aromatics from renewable
feedstock.
Biomass is the only renewable organic carbon resource in
nature that has the potential to produce carbon-based prod-
ucts.[5] Alternative approaches to produce BTX from biomass
were developed.[6] Catalytic fast pyrolysis of biomass and plat-
form chemicals over zeolites provides a direct approach to aro-
matic hydrocarbons.[7] However, the wide variety of products[8]
and the deactivation of the zeolite catalyst are issues that
need to be resolved. Lignin-derived phenols can also be con-
verted into BTX through hydrodeoxygenation.[9] Recently, Flou-
das and co-workers proposed a multi-step reaction network
from biomass to BTX, which includes biomass gasification, syn-
thesis gas to methanol, and methanol to aromatics as the key
steps.[10] Another way to produce aromatics from biomass-de-
rived platform compounds was reported by the groups of
Frost,[2b] Dauenhauer,[11] Davis,[12] and other researchers.[13] This
is a typical Diels–Alder (DA) reaction of an aliphatic diene or
substituted furan with an alkene or its derivative followed by
dehydration or dehydrogenation to produce aromatics. This
route provides good selectivity compared to the pyrolysis
strategy. More recently, Bruijnincx and co-workers modified
this method by using a three-step procedure consisting of DA
addition, hydrogenation, and aromatization of methyl furan
with maleic anhydride to obtain a good overall yield of (substi-
tuted) phthalic anhydrides.[14] Despite the above advances, the
selective production of toluene from biomass remains a chal-
lenge. So far, the highest yield of 35.2% was obtained using
gas-phase co-feeding of propylene and furan over ZSM-5.[13a]
Bio-derived isoprene can be produced by fermentation of
glucose and lignocellulose,[15] or through the isoprene-biosyn-
thetic pathway direct from CO2,[16] which shows potential for
[a] T. Dai, Dr. C. Li, Dr. B. Zhang, H. Guo, Dr. X. Pan, Dr. L. Li, Prof. A. Wang,
Prof. T. Zhang
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian 116023 (P.R. China)
E-mail: licz@dicp.ac.cn
taozhang@dicp.ac.cn
[b] T. Dai, H. Guo
University of Chinese Academy of Sciences
Beijing, 100049 (P.R. China)
Supporting Information for this article can be found under:
http://dx.doi.org/10.1002/cssc.201601267.
ChemSusChem 2016, 9, 1 – 8
1
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
Scheme 1. Transformation of acrolein and isoprene to toluene through a simple two-step method.
commercial development.[17] Acrolein[18] is the dehydration
product of glycerol and currently overproduced as a byproduct
in biodiesel production. The aim of this work is to develop an
approach for the selective production of toluene from acrolein
and isoprene through a simple two-step method: DA cycload-
dition of acrolein and isoprene in a Zn-containing ionic liquid
followed by consecutive gas-phase dehydrogenation–decar-
bonylation catalyzed by Pt/Al2O3 (Scheme 1). The DA reaction
usually produces two regioselective cycloadducts, 4- and 3-
methylcyclohex-3-enecarbaldehyde (MCHCA), as para and
meta isomers, which both afford toluene as the final product
after dehydrogenation–decarbonylation. The process reported
here offers an environmentally benign alternative to the cur-
rent non-selective production of toluene from biomass.
Figure 1. DA cycloaddition of acrolein and isoprene in [Bmim]Zn2Cl5 ionic
liquid.
Results and Discussion
Diels–Alder cycloaddition
genation and decarbonylation to form an aromatic ring and
remove the aldehyde group. The overall reaction possibly in-
volves the release of 2 equivalents of H2 and 1 equivalent of
CO. As the para isomer is the main isomer of the obtained
MCHCAs, reaction 1 in Scheme 2 only shows the transforma-
tion of 4-MCHCA, but the reaction of 3-MCHCA is analogous.
Despite many reports in literature on the dehydrogenation of
cyclohexane, cyclohexene, and cyclohexadiene,[19] the combi-
nation of dehydrogenation and decarbonylation in one reactor
was not reported thus far. Noble metals are known to catalyze
a variety of dehydrogenation reactions. Therefore, Pt/Al2O3, Pd/
Al2O3, Rh/Al2O3, and Ni/Al2O3 were tested for the consecutive
gas-phase dehydrogenation–decarbonylation of MCHCAs in
this study.
For the production of toluene from acrolein and isoprene, the
first step is to prepare the six-membered-ring precursor
MCHCA through DA cycloaddition. Interestingly, the ionic
liquid [Bmim]Zn2Cl5 with moderate acid strength shows excel-
lent performance as both catalyst and green solvent for the re-
action of acrolein and isoprene. After the treatment of acrolein
with isoprene in [Bmim]Zn2Cl5 at 298 K for 24 h, MCHCAs were
detected as the main products (para/meta=94:6) with an
overall yield up to 99.6% based on acrolein (for detailed re-
sults, see Figure S1 and Table S1 in the Supporting Informa-
tion). For reactions catalyzed by common Lewis acids, such as
AlCl3, ZnCl2, FeCl3, and TiCl4, we observed immediate polycon-
densation of acrolein to form a white turbid liquid owing to
the instability of acrolein. After completion of the reaction, the
products formed a separate phase on top of the ionic liquid
and were therefore easily isolated by decanting. The remaining
ionic liquid was washed with hexane and recycled in subse-
quent runs. No noticeable decrease in activity was found even
if it was reused five times (Figure 1), implying that the Zn-con-
taining ionic liquid is relatively stable. This setup helps to over-
come disadvantages of homogeneous Lewis acids
Table 1 shows the product distribution for different noble
metal catalysts. Almost no reaction of the MCHCAs occurred in
the blank reaction (Table 1, entry 1). The reaction with Al2O3 in
the absence of a noble metal catalyst afforded a very low con-
version of 16.4% with 4-methyl benzaldehyde (MBA) as the
major product (entry 2). For all experiments with noble metal
catalysts, much higher conversions of MCHCAs (91.1–100%)
concerning side reactions and generation of environ-
mentally hazardous waste streams. The isolated
MCHCAs were further purified by distillation under
reduced pressure to remove residual substrates
(mainly isoprene).
Transformation of MCHCAs to toluene
In light of their structural differences, the transforma-
tion of MCHCAs to toluene requires both dehydro- Scheme 2. Transformation of 4-MCHCA to toluene and p-xylene.
&
ChemSusChem 2016, 9, 1 – 8
www.chemsuschem.org
2
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
Table 1. Transformation results of MCHCAs to toluene over different cata-
lysts.[a]
Entry Catalyst Conv. [%] Molar yield [%]
toluene p-xylene MBA others[b] total[c]
1
2
3
4
5
6
7[d]
8[e]
9[f]
none
Al2O3
4.3
16.4
99.9
99.5
91.1
17.5
2.1
2.6
91.1
80.8
72.0
0.8
88.7
88.2
37.2
1.1
0.7
3.4
17.4
11.8
0.1
0.9
1.1
0.2
0
12.3
0
0
0
15.7
0
2.3
25.6
1.1
0.8
5.5
1.8
7.2
0.9
10.2
8.3
4.6
3.2
3.3
Pt/Al2O3
Pd/Al2O3
Rh/Al2O3
Ni/Al2O3
Pt/Al2O3 100
Pt/Al2O3 100
94.5
98.1
83.8
16.6
89.6
91.6
63.0
Pt/Al2O3
67.6
[a] Reaction conditions unless otherwise specified: catalyst (1 g), carrier
gas (N2, 20 mLminꢀ1), MCHCAs (liquid, 1 mLhꢀ1), 573 K, LHSV 1 mLgꢀ1 hꢀ1
.
[b] Methyl cyclohexene, methyl cyclohexadiene, and 1,4-dimethyl cyclo-
hexene (Figure S2). [c] Overall mole yield of toluene, p-xylene, and MBA.
Figure 2. Transformation of MCHCAs to toluene with the co-products of CO
and H2.
[d] N2 flow 100 mLminꢀ1, LHSV 1 mLgꢀ1 hꢀ1. [e] N2 flow 200 mLminꢀ1
,
LHSV 2 mLgꢀ1 hꢀ1. [f] N2 flow 400 mLminꢀ1, LHSV 4 mLgꢀ1 hꢀ1
.
MCHCAs were the dominant reactions over Pt/Al2O3. To identify
the sequence of the two reactions and probe the reaction
pathway, conversion of MCHCAs at different residence times
was explored. As shown in entries 3 and 7 in Table 1, a N2 flow
rate below 100 mLminꢀ1 [liquid hourly space velocity (LHSV)
1 mLgꢀ1 hꢀ1] generated toluene as the major product without
any MBA (dehydrogenation product). If the reaction was per-
formed at a shorter residence time of 200 mLminꢀ1 (LHSV
2 mLgꢀ1 hꢀ1) under otherwise identical conditions, MBA was
detected in a yield of 2.3% (Table 1, entry 8). Further shorten-
ing the residence time to 400 mLminꢀ1 (LHSV 4 mLgꢀ1 hꢀ1) in-
creased the MBA yield to 25.6% although the conversion was
decreased to 67.6% (Table 1, entry 9). Only trace amounts of
methyl cyclohexene (decarbonylation product) were formed in
all the reactions compared above. These results suggest that
the MCHCAs first underwent dehydrogenation to form MBA as
an intermediate, which was subsequently transformed to tolu-
ene through decarbonylation. Results from model reactions
using MBA as the substrate over Pt/Al2O3 also supported the
proposed reaction pathway. Under the same conditions as
those in Table 1 (entry 3), complete conversion of MBA was ob-
tained to afford a toluene yield higher than 90%, regardless of
whether the carrier gas was N2 or H2 (Scheme 3, reaction 1).
For Pd/Al2O3- and Rh/Al2O3-catalyzed reactions, Figure 2
shows that the amount of gaseous products (total yield 16.3
and 15.1 mmol, respectively) was lower than that generated
over Pt/Al2O3 (Figure S3). This could be attributed to the lower
were obtained, affording toluene, p-xylene, and MBA as the
major products with the overall yields of 83.8–98.1% (en-
tries 3–5). The conversion efficiency of MCHCAs over various
catalysts decreased in the order of Pt/Al2O3 >Pd/Al2O3 >Rh/
Al2O3. In terms of product selectivity, Pt-, Pd-, and Rh-based
catalysts afforded toluene as the main product with yields of
91.1, 80.8, and 72.0%, respectively, and the overall yields were
90.7, 80.5, and 71.7% based on acrolein (entries 3–5). Ni/Al2O3
is known as a widely used alternative for noble-metal catalysts
for hydrogen-related reactions. However, in the transformation
of MCHCAs, Ni/Al2O3 performed comparably to pure Al2O3 for
both the dehydrogenation and decarbonylation (entry 8 vs. 2).
In addition to the liquid product mixture, gaseous products
were released during the above-mentioned reactions. Figure 2
shows the distribution of a gaseous product resulting from
conversion of MCHCAs at 573 K over different catalysts. For the
reaction over Pt/Al2O3, the major gas component was syngas
(7.18 mmol CO, 14.33 mmol H2) with a total amount of
22.1 mmol. The amount of CO accounted for decarbonylation
of 89.0% of the substrate (8.06 mmol) and was very similar to
the yield of toluene (91.1%, Table 1, entry 3). Moreover, the
molar ratio of H2/CO was approximately 2.0, implying that
during the transformation of MCHCAs to toluene 2 equivalents
of H2 and 1 equivalent of CO were released. The above results
demonstrate that dehydrogenation and decarbonylation of
Scheme 3. Comparative experiments over Pt/Al2O3 using different substrates.
ChemSusChem 2016, 9, 1 – 8
www.chemsuschem.org
3
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
selectivity towards toluene and increased p-xylene yield for Pd
and Rh (Table 1, entries 4–5) because in the transformation of
4-MCHCA to p-xylene dehydrogenation and in situ hydrodeox-
ygenation only generate water as byproduct without any gas
formation (reaction 2 in Scheme 2). Nevertheless, CO and H2
were the main gas components because toluene was still the
major liquid product in the two reactions. Other gas compo-
nents were small amounts of CH4 and CO2 that were generated
from side reactions (methanation or water-gas-shift reac-
tion).[20] As for the Ni/Al2O3-catalyzed reaction, the lowest
MCHCA conversion of the tested catalysts afforded the corre-
sponding lowest amount of gaseous products.
Based on the analysis of liquid and gaseous product distri-
butions and the comparative experiments, a possible reaction
pathway for the conversion of MCHCAs to toluene is proposed.
Taking 4-MCHCA as an example, this compound first under-
goes two consecutive dehydrogenations to generate MBA as
well as 2 equivalents of H2 gas. The MBA adsorbs on the cata-
lyst with the carbonyl group parallel to the surface of the
metal species[21] and subsequently undergoes two competitive
reactions to form different products. The first route is the
direct decarbonylation: MBA preferentially converts into a acyl
species, in which the carbon atom of the carbonyl remains at-
tached to the surface [h1(C)-acyl].[22] This acyl species may be
a precursor for the decarbonylation reaction. The stronger the
interaction between the carbon atom of the carbonyl and the
Pt species, the easier the decarbonylation reaction occurrs (re-
flected in the CO microcalorimetric adsorption experiments
discussed below). In the second competitive reaction, the
h2(C,O)-aldehyde can convert to p-xylene through intramolecu-
lar hydrodeoxygenation with comparatively lower activity than
for the first reaction pathway, using hydrogen species that
were generated in situ from 4-MCHCA dehydrogenation.
Medlin and co-workers[23] demonstrated that the terrace sites
of Pd/Al2O3 preferentially catalyzed aldehyde decarbonylation,
whereas aldehyde hydrogenation and hydrodeoxygenation oc-
curred on edges and defect sites. Based on this finding, we
cautiously assumed that decarbonylation of MCHCAs occurred
on the terrace sites of the catalysts whereas edges and defects
sites were active for hydrodeoxygenation of the aldehyde
group to produce p-xylene. Additionally, size and shape effects
of the catalyst may affect the selectivity of the products.[24]
To investigate the difference in selectivity of the various cat-
alysts, microcalorimetric adsorption experiments were per-
formed. Figures 3 and 4 show plots of differential heat versus
adsorbate coverage with CO and histograms of the differential
heat distributions for adsorption of CO on different catalysts.
As can be seen in Figure 3, the initial differential heats on the
catalysts were in the range of 90–147 kJmolꢀ1 for CO adsorp-
tion. The value of the initial differential heat for CO adsorption
on Pt/g-Al2O3 was 147 kJmolꢀ1, which was similar to a previous-
ly reported value for a Pt/SiO2 catalyst.[25] The data decreased
Figure 3. Differential heats of CO adsorption on various catalysts at 313 K.
Figure 4. Distribution of differential heats for CO adsorption on different cat-
alysts at 313 K.
differential heat of adsorption decreased with the increase of
CO coverage as reflected in Figure 3.
Figure 4 exhibits histograms of the differential heat distribu-
tions for adsorption of CO on different catalysts. These histo-
grams were generated by the differential heat data with
a least-quadruplicates-fitted polynomial and subsequent use of
this polynomial to determine the amount of CO adsorbed
within a given range of differential heat. Figure 4c shows that
Rh/Al2O3 contains the largest number of sites for CO adsorp-
tion (73 mmolgꢀ1); interaction of CO with Rh produced the
major differential heats at 100–130 kJmolꢀ1. The amount of
sites for CO adsorption on Pt/Al2O3 (36 mmolgꢀ1) and Pd/Al2O3
(27 mmolgꢀ1) is lower than for Rh/g-Al2O3, but the two catalysts
contain more active sites that interact strongly with CO mole-
cules (majority of differential heats for Pt at 120–150 kJmolꢀ1
and for Pd at 110–140 kJmolꢀ1). The difference in strong ad-
sorption sites correlates well with the activity of the different
catalysts for the decarbonylation reaction (Table 1, entries 3–5),
suggesting that the strong adsorption sites of the metal cata-
lysts are responsible for the decarbonylation of the substrate.
Therefore, the strength of CO adsorption might correlate with
in
the
order
of
Pt/Al2O3
(147 kJmolꢀ1)>Pd/Al2O3
(136 kJmolꢀ1)>Rh/Al2O3 (128 kJmolꢀ1)>Ni/Al2O3 (90 kJmolꢀ1),
which was consistent with the trend of the catalyst per-
formance for toluene production. As a result of adsorption on
weaker sites and/or interaction between adsorbed species, the
&
ChemSusChem 2016, 9, 1 – 8
www.chemsuschem.org
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
the ease of decarbonylation, related to the strength of interac-
tion with the gas product (CO) of this reaction in the manner
of a Polanyi–Evans relation, as reported by Mavrikakis and co-
workers.[26] Ni/Al2O3, however, lacks adsorption sites, and the
differential heat of adsorption for CO is the lowest of the inves-
tigated catalysts (70–100 kJmolꢀ1). Accordingly, the weak inter-
action between nickel species and reactant molecules is insuffi-
cient to catalyze the dehydrogenation and decarbonylation of
MCHCAs.
Notably, both the C=C bond and the aldehyde group in
MCHCAs are crucial to the formation of toluene. As shown in
Scheme 3 (reaction 2) 4-methyl cyclohexane aldehyde, an ana-
logue of 4-MCHCA without the C=C bond, underwent decar-
bonylation to afford methylcyclohexane with over 95% yield
under identical conditions as for the reaction detailed in
Table 1 (entry 3) suggesting that dehydrogenation cannot
occur if the substrate contains no C=C bond in the six-mem-
bered ring. In addition, no reaction occurred on methyl cyclo-
hexene under identical conditions, implying that the aldehyde
group also plays a key role in the dehydrogenation of MCHCAs
(Scheme 3, reaction 3). In terms of the structure of MCHCAs,
the presence of the C=C bond makes the allylic CꢀH bond in
C2-position easy to activate. Moreover, the aldehyde group in
C1-position acts as a directing group, assisting the activation
of the CꢀH bond in C1-position.[27] This interesting synergistic
inductive effect favors the dehydrogenation of MCHCAs to
MBA, whereas either of the individual groups is insufficient to
promote the dehydrogenation.
Figure 5. MCHCAs conversion and liquid yield of different products (toluene,
p-xylene, MBA, and others) over 1% Pt/g-Al2O3 catalyst as function of reac-
tion time. Reaction conditions: 1% Pt/Al2O3, 573 K, LHSV 1 mLgꢀ1 hꢀ1, N2 as
carrier gas (20 mLminꢀ1).
the fresh catalyst (approximately 1.4 nm), which might be
caused by particle migration. From the conversion results and
characterization obtained so far, it can be concluded that at
573 K with a LHSV of 1 mLgꢀ1 hꢀ1 Pt/Al2O3 showed good activi-
ty and stability in the transformation of MCHCAs to toluene. To
reflect the real activity/stability of the catalyst, we also per-
formed the reaction of MCHCA under conditions that did not
afford full conversion (Figure S5). The results showed that the
conversion decreased gradually with increasing reaction time.
However, the selectivity towards toluene did not decrease
even after 72 h. By simple calcination and reduction of the cat-
alyst, its activity was easily recovered.
Stability of Pt/Al2O3
In the recycling experiments, CO and H2 were always the
main gaseous products with a molar ratio of H2/CO of approxi-
mately 2.0. This ratio matches the gas composition of syngas
required for the synthesis of methanol.[28] Hence, highly selec-
tive production of toluene with co-production of syngas from
isoprene and acrolein was realized in our catalytic system.
Catalyst reusability is one of the key factors affecting the appli-
cation potential of the catalytic process. Therefore, we studied
the stability of Pt/Al2O3 in the conversion of MCHCAs. As
shown in Figure 5, MCHCAs could be completely converted at
573 K with a LHSV of 1 mLgꢀ1 hꢀ1 even if the catalyst was
tested for 24 h. The yield of toluene remained nearly constant
at approximately 90%, indicating that the activity of Pt/Al2O3
for the consecutive dehydrogenation–decarbonylation trans-
formation remained stable.
Conclusions
A method for the production of toluene from renewable sour-
ces was reported through Diels–Alder cycloaddition of bio-
mass-derived isoprene and acrolein followed by Pt/Al2O3-cata-
lyzed gas-phase dehydrogenation–decarbonylation with overall
yields up to 90.7%. Model reactions and monitoring of the
product formation indicated that 4-methyl benzaldehyde was
the key intermediate in the gas-phase transformation of 3-
methylcyclohex-3-enecarbaldehydes (MCHCAs) to toluene.
Both the C=C and the aldehyde group in MCHCAs played
a key role in initiating the consecutive dehydrogenation–decar-
bonylation of MCHCAs. Ionic liquid [Bmim]Zn2Cl5, acting as sol-
vent and catalyst for the cycloaddition of isoprene and acrole-
in, could be easily recycled and Pt/Al2O3 showed high stability
and selectivity in the consecutive dehydrogenation–decarbon-
ylation. Through microcalorimetric adsorption experiments and
comparative model reactions, the difference in selectivity for
the various systems was explained and a possible mechanism
for the gas-phase dehydrogenation–decarbonylation of
The high selectivity and good stability of the catalyst was
also reflected in the distribution of gaseous products, as the
released H2 was always in the range of 13.3–15.2 mmol and
the molar ratio of H2/CO remained approximately 2.0 (Fig-
ure S4). Characterization of the catalyst further confirmed the
good stability. XRD characterization showed no significant dif-
ference between the fresh catalyst and the one after 24 h reac-
tion time. Both catalysts exhibited no diffraction peak for Pt,
indicating that the Pt particles were highly dispersed and that
the reaction did not induce particle sintering (Figure S6). X-ray
photoelectron spectroscopy (XPS) characterization (Figure S7)
also supported this point because no clear difference was
found in the peak number and position of Pt/Al2O3 before and
after reaction. High-angle annular dark-field scanning transmis-
sion electron microscope (HAADF-STEM) characterization (Fig-
ure S8) indicated that the mean particle diameter of the used
catalyst (approximately 1.8 nm) was slightly larger than that of
ChemSusChem 2016, 9, 1 – 8
www.chemsuschem.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
MCHCAs was proposed. The simple experimental procedure
and facile product isolation as well as the easily reusable cata-
lysts may contribute to the development of green chemistry.
Acknowledgements
Support from the National Natural Science Foundation of China
(21473187, 21506214) and the Strategic Priority Research Pro-
gram of the Chinese Academy of Sciences (XDB17020100) is
gratefully acknowledged.
Experimental Section
Keywords: biomass · decarbonylation · dehydrogenation ·
Diels–Alder reaction · toluene
Reagents and catalysts: Anhydrous ZnCl2 was prepared by using
SOCl2 to dehydrate ZnCl2·2H2O, and after the remaining SOCl2 was
removed by distillation under vacuum, the obtained anhydrous
ZnCl2 was kept in a vacuum desiccator with KOH overnight prior
to use. Acrolein (purchased from Alfa, 96.0%) was stabilized with
hydroquinone. Isoprene (purchased from TCI, 99.0%) was stabilized
with 4-tert-butylcatechol. The ionic liquid [Bmim]Zn2Cl5 was pre-
pared according to a previous study[29] by mixing 1-butyl-3-methyl-
imidazolium chloride and anhydrous ZnCl2 (2 equiv.) followed by
heating at 373 K for 12 h under N2 with vigorous stirring. Catalysts
were prepared by means of incipient wetness impregnation as de-
scribed in Supporting Information.
[1] a) P. C. Bruijnincx, B. M. Weckhuysen, Angew. Chem. Int. Ed. 2013, 52,
11980–11987; Angew. Chem. 2013, 125, 12198–12206; b) A. M. Niziolek,
O. Onel, C. A. Floudas, AIChE J. 2016, 62, 1531–1556.
[2] a) T. W. Lyons, D. Guironnet, M. Findlater, M. Brookhart, J. Am. Chem.
Soc. 2012, 134, 15708–15711; b) K. K. Miller, P. Zhang, Y. Nishizawa-Bren-
nen, J. W. Frost, ACS Sustainable Chem. Eng. 2014, 2, 2053–2056.
[3] P. V. Cline, J. J. Delfino, P. S. C. Rao, Environ. Sci. Technol. 1991, 25, 914–
920.
[4] Y. Fong, A. Abdullah, A. Ahmad, S. Bhatia, Chem. Eng. J. 2008, 139, 172–
193.
[5] a) D. M. Alonso, S. G. Wettstein, J. A. Dumesic, Chem. Soc. Rev. 2012, 41,
8075–8098; b) C. Li, X. Zhao, A. Wang, G. W. Huber, T. Zhang, Chem.
Rev. 2015, 115, 11559–11624.
General procedure for the Diels–Alder reaction: In a typical reac-
tion, isoprene (30 mmol) was added slowly to a solution of Zn-con-
taining ionic liquid (2.5 mmol) and acrolein (25 mmol) at 298 K
under vigorous stirring for 24 h. After completion of the reaction,
two phases formed and the upper clear liquid was pipetted off for
gas chromatographic analysis [Agilent Technologies 6890N, flame
ionization detector (FID), 30 mꢁ0.32 mmꢁ0.25 mm HP-5 column,
N2 as carrier gas] with external standard (Figure S1). The isolated
products were further purified by distillation (398 K, 2 KPa) to
afford the two regioselective cycloadducts para- and meta-MCHCA.
The ionic liquid phase (bottom) was washed with hexane (3ꢁ
3 mL), purified under vacuum, and directly reused in subsequent
runs.
[6] A. Maneffa, P. Priecel, J. A. Lopez-Sanchez, ChemSusChem 2016, 9,
2736–2748.
[7] a) A. J. Foster, J. Jae, Y. T. Cheng, G. W. Huber, R. F. Lobo, Appl. Catal. A
2012, 423–424, 154–161; b) C. A. Mullen, A. A. Boateng, ACS Sustain-
able Chem. Eng. 2015, 3, 1623–1631.
[8] H. Zhang, Y.-T. Cheng, T. P. Vispute, R. Xiao, G. W. Huber, Energy Environ.
Sci. 2011, 4, 2297.
[9] a) A. M. Robinson, G. A. Ferguson, J. R. Gallagher, S. Cheah, G. T. Beck-
ham, J. A. Schaidle, J. E. Hensley, J. W. Medlin, ACS Catal. 2016, 6, 4356–
4368; b) Q. Tan, G. Wang, L. Nie, A. Dinse, C. Buda, J. Shabaker, D. E. Re-
sasco, ACS Catal. 2015, 5, 6271–6283; c) C.-J. Chen, W.-S. Lee, A. Bhan,
Appl. Catal. A 2016, 510, 42–48.
[10] A. M. Niziolek, O. Onel, Y. A. Guzman, C. A. Floudas, Energy Fuels 2016,
30, 4970–4998.
Catalytic conversion of MCHCAs: The experiments were per-
formed in a fixed-bed reactor (quartz tube, inner diameter 10 mm,
total length 350 mm). The Pt/Al2O3 catalyst was transferred into
the reaction tube and the gas flow rate was set to 20 mLminꢀ1
using a mass flowmeter. MCHCAs were supplied to the catalyst
layer at 1 mLhꢀ1 using a constant-flow piston pump together with
N2 flow. All runs were performed at atmospheric pressure. A con-
denser with an icepack was used to trap the liquid products. At
the same time, the gas products were collected in an airbag. The
condensers were washed with chloroform and the liquid products
in the chloroform were analyzed qualitatively using GC–MS and
quantified using GC–FID. The gas products were detected using
GC with a thermal conductivity detector.
[11] a) C. L. Williams, C.-C. Chang, P. Do, N. Nikbin, S. Caratzoulas, D. G. Vla-
chos, R. F. Lobo, W. Fan, P. J. Dauenhauer, ACS Catal. 2012, 2, 935–939;
b) C. C. Chang, S. K. Green, C. L. Williams, P. J. Dauenhauer, W. Fan, Green
Chem. 2014, 16, 585–588; c) C. C. Chang, H. Je Cho, J. Yu, R. J. Gorte, J.
Gulbinski, P. Dauenhauer, W. Fan, Green Chem. 2016, 18, 1368–1376.
[12] a) J. J. Pacheco, M. E. Davis, Proc. Natl. Acad. Sci. USA 2014, 111, 8363–
8367; b) J. J. Pacheco, J. A. Labinger, A. L. Sessions, M. E. Davis, ACS
Catal. 2015, 5, 5904–5913.
[13] a) Y. T. Cheng, G. W. Huber, Green Chem. 2012, 14, 3114; b) A. Zheng, Z.
Zhao, S. Chang, Z. Huang, K. Zhao, H. Wu, X. Wang, F. He, H. Li, Green
Chem. 2014, 16, 2580; c) E. Mahmoud, J. Yu, R. J. Gorte, R. F. Lobo, ACS
Catal. 2015, 5, 6946–6955; d) R. Lu, F. Lu, J. Chen, W. Yu, Q. Huang, J.
Zhang, J. Xu, Angew. Chem. Int. Ed. 2016, 55, 249–253; Angew. Chem.
2016, 128, 257–261; e) M. Shiramizu, F. D. Toste, Chem. Eur. J. 2011, 17,
12452–12457; f) I. F. Teixeira, B. T. W. Lo, P. Kostetskyy, M. Stamatakis, L.
Ye, C. C. Tang, G. Mpourmpakis, S. C. E. Tsang, Angew. Chem. Int. Ed.
2016, 55, 13349–13349; Angew. Chem. 2016, 128, 13255–13260.
[14] S. Thiyagarajan, H. C. Genuino, J. C. van der Waal, E. de Jong, B. M.
Weckhuysen, J. van Haveren, P. C. Bruijnincx, D. S. van Es, Angew. Chem.
Int. Ed. 2016, 55, 1368–1371; Angew. Chem. 2016, 128, 1390–1393.
[15] a) G. M. Whited, F. J. Feher, D. A. Benko, M. A. Cervin, G. K. Chotani, J. C.
McAuliffe, R. J. LaDuca, E. A. Ben-Shoshan, K. J. Sanford, Ind. Biotechnol.
2010, 6, 152–163; b) C. Yang, X. Gao, Y. Jiang, B. Sun, F. Gao, S. Yang,
Metab. Eng. 2016, 37, 79–91.
Methods of data processing: The yields (Y) of toluene, p-xylene, 4-
or 3-methyl benzaldehyde, and other identified products, along
with the conversion (X) of MCHCAs and the LHSV were defined as
shown in Equation (1), (2), and (3):
DmolðMCHCA before and after reactionÞ
ð1Þ
ð2Þ
ð3Þ
X½%ꢁ ¼
Y½%ꢁ ¼
LHSV ¼
ꢂ 100%
molðMCHCA after reactionÞ
[16] X. Gao, F. Gao, D. Liu, H. Zhang, X. Q. Nie, C. Yang, Energy Environ. Sci.
2016, 9, 1400–1411.
[17] C. T. Matos, L. Gouveia, A. R. C. Morais, A. Reis, R. Bogel-Lukasik, Green
Chem. 2013, 15, 2854–2864.
[18] a) B. Katryniok, S. Paul, F. Dumeignil, ACS Catal. 2013, 3, 1819–1834;
b) S. H. Chai, H.-P. Wang, Y. Liang, B.-Q. Xu, Green Chem. 2007, 9, 1130;
c) R. Liu, S. T. Lyu, T. F. Wang, J. Ind. Eng. Chem. 2016, 37, 354–360; d) L.
molðproductsÞ
ꢂ 100%
molðMCHCA before reactionÞ
flow rate of MCHCA ½mL hꢀ1
weight of catalyst ½gꢁ
ꢁ
&
ChemSusChem 2016, 9, 1 – 8
www.chemsuschem.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
Liu, X. P. Ye, J. J. Bozell, ChemSusChem 2012, 5, 1162–1180; e) B. Katry-
niok, S. Paul, M. Capron, F. Dumeignil, ChemSusChem 2009, 2, 719–730.
[19] a) S. T. Qi, W. T. Yu, W. W. Lonergan, B. L. Yang, J. G. Chen, Chin. J. Catal.
2010, 31, 955-960; b) R. Ahuja, B. Punji, M. Findlater, C. Supplee, W.
Schinski, M. Brookhart, A. S. Goldman, Nat. Chem. 2011, 3, 167–171.
[20] a) G. Garbarino, D. Bellotti, P. Riani, L. Magistri, G. Busca, Int. J. Hydrogen
Energy 2015, 40, 9171–9182; b) P. Panagiotopoulou, D. I. Kondarides,
X. E. Verykios, Appl. Catal. A 2008, 344, 45–54; c) J. Vecchietti, A. Boni-
vardi, W. Xu, D. Stacchiola, J. J. Delgado, M. Calatayud, S. E. Collins, ACS
Catal. 2014, 4, 2088–2096.
[25] R. D. Cortright, J. A. Dumesic, J. Catal. 1994, 148, 771–778.
[26] P. Ferrin, D. Simonetti, S. Kandoi, E. Kunkes, J. A. Dumesic, J. K. Nørskov,
M. Mavrikakis, J. Am. Chem. Soc. 2009, 131, 5809–5815.
[27] X. Y. Liu, X. Y. Li, H. Liu, Q. Guo, J. B. Lan, R. L. Wang, J. S. You, Org. Lett.
2015, 17, 2936–2939.
[28] M. Behrens, F. Studt, I. Kasatkin, S. Kuhl, M. Havecker, F. Abild-Pedersen,
S. Zander, F. Girgsdies, P. Kurr, B. L. Kniep, M. Tovar, R. W. Fischer, J. K.
Nørskov, R. Schlçgl, Science 2012, 336, 893–897.
[29] D. H. Yin, C. Z. Li, B. M. Li, L. Tao, D. L. Yin, Adv. Synth. Catal. 2005, 347,
137–142.
[21] S. Sitthisa, W. An, D. E. Resasco, J. Catal. 2011, 284, 90–101.
[22] R. D. Srivastava, A. K. Guha, J. Catal. 1985, 91, 254–262.
[23] S. H. Pang, C. A. Schoenbaum, D. K. Schwartz, J. W. Medlin, Nat.
Commun. 2013, 4, 2448.
Manuscript received: September 10, 2016
Revised: October 17, 2016
[24] V. V. Pushkarev, N. Musselwhite, K. An, S. Alayoglu, G. A. Somorjai, Nano
Lett. 2012, 12, 5196–5201.
Final Article published: && &&, 0000
ChemSusChem 2016, 9, 1 – 8
www.chemsuschem.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

FULL PAPERS
T. Dai, C. Li,* B. Zhang, H. Guo, X. Pan,
L. Li, A. Wang, T. Zhang*
Green toluene: A route for the effective
synthesis of toluene from isoprene and
acrolein, two reactants readily available
from biomass, is reported. A two-step
reaction affords toluene in an overall
yield up to 90.7% by combining Diels–
Alder cycloaddition of the two sub-
strates in a Zn-containing ionic liquid
with dehydrogenation–decarbonylation
of the resulting methylcyclohex-3-ene-
carbaldehyde intermediates.
&& – &&
Selective Production of Toluene from
Biomass-Derived Isoprene and
Acrolein
&
ChemSusChem 2016, 9, 1 – 8
www.chemsuschem.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!