Diels-Alder and dehydration reactions of furan derivatives with ethylene catalyzed by liquid Br?nsted acids and Lewis acids

Article abstract of DOI:10.1016/j.molcata.2016.04.023

The tandem Diels-Alder and dehydration reactions of furan derivatives with ethylene represent a promising approach for the production of renewable aromatics. Herein, a series of haloacetic acids and rare-earth metal triflates are employed as liquid Br?nsted and Lewis acid catalysts, respectively, in the Diels-Alder and dehydration reactions of 2,5-dimethylfuran with ethylene to get an insight of this type of acid-catalyzed reaction. With the carefully selected catalysts and solvent, a quasi-homogeneous reaction system can be formed, where accurate kinetic studies could be performed. Haloacetic acids exhibit considerable activity in the Diels-Alder and dehydration reactions with a similar apparent activity energy value of ～8.4 kcal/mol, and CF₂ClCOOH with a proper Br?nsted acidity is optimized for the reaction. Rare-earth metal triflates also exhibit considerable activity in the Diels-Alder and dehydration reactions with a similar apparent activity energy value of ～13.0 kcal/mol, and Sc(OTf)₃ with the highest Lewis acidity is optimized. On the basis of kinetic analysis results, the impacts of acidity on the performance of catalysts are discussed in detail and the reaction network is proposed. Besides, the substrate scope of furan derivatives for the Diels-Alder and dehydration reactions with ethylene is investigated using CF₂ClCOOH and Sc(OTf)₃ as catalysts for possible extensions of the reaction.

Full text of DOI:10.1016/j.molcata.2016.04.023

Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Diels-Alder and dehydration reactions of furan derivatives with
ethylene catalyzed by liquid Brønsted acids and Lewis acids
Song Songa, Guangjun Wua, Weili Daia, Naijia Guana,b, Landong Lia,b,∗
a School of Materials Science and Engineering & National Institute for Advanced Materials, Nankai University, Tianjin 30071, PR China
b Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering,
Tianjin 30071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2016
Received in revised form 8 April 2016
Accepted 20 April 2016
Available online 22 April 2016
The tandem Diels-Alder and dehydration reactions of furan derivatives with ethylene represent a promis-
ing approach for the production of renewable aromatics. Herein, a series of haloacetic acids and rare-earth
metal triflates are employed as liquid Brønsted and Lewis acid catalysts, respectively, in the Diels-Alder
and dehydration reactions of 2,5-dimethylfuran with ethylene to get an insight of this type of acid-
catalyzed reaction. With the carefully selected catalysts and solvent, a quasi-homogeneous reaction
system can be formed, where accurate kinetic studies could be performed. Haloacetic acids exhibit con-
siderable activity in the Diels-Alder and dehydration reactions with a similar apparent activity energy
value of ∼8.4 kcal/mol, and CF2ClCOOH with a proper Brønsted acidity is optimized for the reaction. Rare-
earth metal triflates also exhibit considerable activity in the Diels-Alder and dehydration reactions with
a similar apparent activity energy value of ∼13.0 kcal/mol, and Sc(OTf)3 with the highest Lewis acidity
is optimized. On the basis of kinetic analysis results, the impacts of acidity on the performance of cata-
lysts are discussed in detail and the reaction network is proposed. Besides, the substrate scope of furan
derivatives for the Diels-Alder and dehydration reactions with ethylene is investigated using CF2ClCOOH
and Sc(OTf)3 as catalysts for possible extensions of the reaction.
Keywords:
Furan derivatives
Diels-Alder dehydration
Haloacetic acids
Rare-earth metal triflates
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
recently disclosed [11], and then triggered widespread research
interests [12–22]. It was first proposed by Dauenhauer et al. that
With decreasing fossil resources and increasing demand for
energy and materials, it is quite urgent to find alternative sup-
plies for the production of fuels and chemicals [1–6]. In the past
decades, great attention has been focused on the renewable fuels
and chemicals derived from biomass [7,8]. Glucose and xylose are
the main sugar monomers present in biomass, and a lot of platform
molecules, such as 5-hydroxymethylfurfural, 2,5-dimethylfuran
and levulinic acid, can be produced from these sugar monomers [9].
For example, 2,5-dimethylfuran can be produced via a combined
isomerization, dehydration and hydrodeoxygenation route from
glucose [10]. With these platform molecules, dozens of renewable
fuels and chemicals can be further produced.
Diels-Alder reaction of ethylene and 2,5-dimethylfuran occurred
without catalytic active sites, followed by the dehydration to p-
xylene catalyzed by Brønsted acid sites. In this case, the rate
of p-xylene formation is controlled by the initial non-catalytic
cycloaddition step [11]. In contrast, Wang et al. demonstrated that
acidic oxides, e.g. WOx-ZrO2, exhibited higher activity than zeolite
catalysts and the formation rate of p-xylene showed good depen-
dence on the density of Brønsted acid sites [13]. More recently,
Vlachos et al. reported the kinetic regime change in the tandem
Diels-Alder and dehydration reactions reaction on the basis of
experimental results and QM/MM calculations. It was observed that
the p-xylene production is limited by the homogeneous Diels-Alder
reaction at high solid acid concentrations whereas the overall rate
is limited by the heterogeneously catalyzed dehydration reaction
at low acid concentrations [16].
A very attractive route for the production of renewable p-xylene
via the Diels-Alder and dehydration reactions of biomass-derived
2,5-dimethylfuran with ethylene catalyzed by acidic zeolites was
Heterogeneous Brønsted acids, e.g. zeolites [11,12,14–16] and
mixed oxides [13,17], are reported to be active in the Diels-Alder
and dehydration reactions of 2,5-dimethylfuran with ethylene.
Heterogeneous Lewis acids, e.g. Sn-Beta zeolite [18–20], are
recently disclosed to be also active in the reaction. It appears that
∗
Corresponding author at: School of Materials Science and Engineering & National
Institute for Advanced Materials, Nankai University, Tianjin 30071, PR China.
E-mail address: lild@nankai.edu.cn (L. Li).
http://dx.doi.org/10.1016/j.molcata.2016.04.023
1381-1169/© 2016 Elsevier B.V. All rights reserved.

S. Song et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
135
heterogeneous Brønsted acids and Lewis acids show comparable
activity in the Diels-Alder and dehydration reactions and a direct
comparison between them is lacking. Meanwhile, it should be men-
tioned that both Brønsted acid sites and Lewis acid sites exist in
some catalysts, e.g. H-Beta [21], and this would further compli-
cate the problem. In addition to the reaction of 2,5-dimethylfuran
with ethylene, the reaction scope has been successfully extended
to the Diels-Alder and dehydration reactions of other furan deriva-
tives, e.g. oxidized variants of 5-hydroxymethylfurfural [18,19],
with ethylene or even furan derivatives with other nucleophilic
reagents, e.g. acrolein [23] and acrylic acid [24].
Despite significant achievements made so far, several key issues
on the Diels-Alder and dehydration reactions of furan derivatives
with ethylene are to be investigated. Since the dehydration reaction
is recognized as an acid-catalyzed reaction, it is most important to
know the effects of acidity, e.g. type and strength of acid sites, on
the catalytic performance. To avoid the adsorption and diffusion
problems in gas-liquid-solid reactions, a quasi-homogeneous reac-
tion system is designed by using soluble acid in selected solvent.
Haloacetic acids, that have been reported as efficient catalysts in
biomass conversion [25], and rare-earth metal triflates [RE(OTf)3]
[26,27], are selected as Brønsted and Lewis acid catalysts, respec-
tively. It is revealed that the Diels-Alder and dehydration reactions
of biomass-derived DMF with ethylene can be performed at low
temperature of 433–513 K. Besides, the problem of product selec-
tivity during the Diels-Alder and dehydration reactions is discussed
in detail and the substrate scope of furan derivatives is investigated.
then collected for analysis by gas chromatography (Shimadzu GC-
2010) and gas chromatography–mass spectrometry (Shimadzu
GCMS-QP2010 SE), both with a DB-1 column (30 m, 0.25 mm
i.d., stationary phase thickness 0.25 ␮m). n-Dodecane or durene
was used as an internal standard for quantification. The follow-
ing temperature program was employed: isothermal heating at
313 K for 10 min, heating to 553 K with a rate of 20 K/min, and
isothermal heating at 553 K for 8 min. For the reactions involving
carboxylic acids, the substrates and products were analyzed by high
performance liquid chromatography (Shimadzu LC-20AT) with a
Carbomix H NP column (7.8 × 300 mm, stationary phase thickness
10 ␮m) and 1H nuclear magnetic resonance spectroscopy (Varian
NMR System 400 MHz) with DMSO-d6 as solvent and tetraethylsi-
lane as an internal standard.
The substrate conversion and product yield are directly mea-
sured and the product selectivity is calculated as:
Product
[
]
Product selectivity % =
( )
× 100%
Substrate
− Substrate
[
]
[
]
after
before
Here, the carbon balance is 100% and the calculated value is the
true selectivity toward desired product with the consideration of
all by-products and coke.
3. Results and discussion
3.1. Haloacetic acids catalyzed Diels-Alder and dehydration
reactions
2. Experimental
According to literature report [16] and our experimental
observations (vide infra), the reaction network for the tandem
Diels-Alder and dehydration reactions of typical furan derivatives,
i.e. DMF, with ethylene is shown in Fig. 1, which can be described
as follows:
2.1. Chemicals
The following reagents were used in the experiments without
further purification: furan (Acros, >99%), 2-methylfuran (MF; Acros,
>99%), 2,5-dimethylfuran (DMF; Acros, 99%), methyl 5-methyl-2-
furoate (MMF; Ark, 97%), furan-2,5-dicarboxylic acid (FDCA; Alfa,
98%), dimethyl furan-2,5-dicarboxylate (DMFDC; Ark, 99%), 1,4-
dioxane (DOX; Acros, >99%), ␥-valerolactone (GVL; Acros, 98%),
n-dodecane (DOC; Acros, >99%), ethylene glycol diethyl ether
(EGDE; Acros, >98%), trifluoroacetic acid (CF3COOH; Acros, 99%),
dichloroaceticacid(CHCl2COOH;Acros, >99%), chlorodifluoroacetic
acid (CF2ClCOOH; Acros, >98%), trichloroacetic acid (CCl3COOH;
Acros, 99%), acetic acid (CH3COOH, >99%), Sc(OTf)3 (Acros, 99.9%),
Yb(OTf)3 (Acros, 99.9%), La(OTf)3 (Acros, 99.9%), Ce(OTf)3 (Acros,
99.9%) and Sm(OTf)3 (Acros, 99.9%).
(A) Cycloaddition of DMF with ethylene to form 7-oxabicyclo
[2.2.1] hept-2-ene-type adducts;
(B) Acid-catalyzed dehydration of cycloadducts to p-xylene;
(C) Alkylation of aromatic product p-xylene with ethylene to form
the by-product 2-ethyl-p-xylene and 4-propyltoluene;
(D) Alkylation of p-xylene with p-xylene to produce 2,2ꢀ,5,5ꢀ-
tetramethylbiphenyl;
(E) An equilibrium between the DMF and their hydrolysis product
2,5-hexanedione;
(F) Condensation of 2,5-hexanedione to oligomers;
(G) Condensation of DMF to oligomers.
2.2. Diels-Alder and dehydration reactions
Previous reports on the Diels-Alder and dehydration reactions of
DMF with ethylene are mainly based on the use of alkane solvents.
However, it has been reported that polar aprotic solvents show very
positive impacts on several acid-catalyzed reactions, e.g. xylose
conversion into furfural [25] and cellulose dehydration to levoglu-
cosenone [28]. Therefore, the solvent effects on the Diels-Alder and
dehydration reactions of DMF with ethylene are first investigated
using CF2ClCOOH as a Brønsted acid catalyst. Here, different kinds
of solvents from polar aprotic to non-polar aprotic are selected and
the results are summarized in Table 1.
Water is not a good solvent because it can induce the serious
hydrolysis of DMF and, accordingly, hinder the cycloaddition reac-
tion. For nonaqueous solvents, a positive correlation between the
solvent polarity and the p-xylene formation can be observed when
using CF2ClCOOH as catalyst. According to the Brønsted–Lowry
acid–base theory, the dissociation equilibrium of acid HA is shown
below.
The Diels-Alder and dehydration reactions of furan derivatives
with ethylene were performed in a 15 mL high-pressure auto-
clave reactor equipped with a magnetic stirrer (800 r/min) and
electronic heater. In a typical experiment, certain amount catalyst
(substrate/catalyst = 10/1 in case of haloacetic acids as catalysts,
substrate/catalyst = 100/1 in case of metal triflates as catalysts or
4 mg H-beta) and 5 mL of 0.2 M furan derivatives in dioxane or
other solvent were mixed in the autoclave reactor. The reactor
was purged with pure nitrogen at room temperature for five times.
Then, 3.5 MPa of ethylene was introduced into the reactor and the
reactor was heated to desired temperature within 10 min to initiate
the reaction.
2.3. Product analysis
After the reaction, the autoclave reactor was rapidly cooled
down to room temperature in ice water and the high-pressure
reaction gases were carefully vented. The reaction solution was
HA ↔ H+ + A−

136
S. Song et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
Fig. 1. Reaction network for the Diels-Alder and dehydration reactions of furan derivatives with ethylene.
Table 1
The Diels-Alder and dehydration reactions of DMF with ethylene catalyzed by CDFA in different solventsa.
c
d
Entry
Catalyst
Pethylene (MPa)
Solvent
Polarityb
TOF (h−1
)
Rate (molg−1 h−1
)
1
2
3
4
5
6
7
8
CF2ClCOOH
CF2ClCOOH
CF2ClCOOH
CF2ClCOOH
CF2ClCOOH
CF2ClCOOH
CF2ClCOOH
CF2ClCOOH
H-Betaf
1.0 (1.8)e
2.0 (3.3)
3.0 (4.8)
3.5 (5.2)
3.5 (5.0)
3.5 (5.1)
3.5 (5.0)
3.5 (6.5)
3.5 (5.2)
3.5 (5.0)
3.5 (5.2)
3.5 (5.0)
DOX
DOX
DOX
DOX
GVL
EGDE
DOC
Water
DOX
DOC
DOX
DOC
4.8
4.8
4.8
4.8
4.2
0.26
0.38
0.42
0.45
0.42
0.18
0.15
0
2.0
2.9
3.2
3.4
3.2
1.4
1.2
0
2.0
2.1
2.9
3.0
2.9
∼1.0
10.2
4.8
1.0
4.8
9
2.7 × 103
2.8 × 103
1.9 × 104
2.0 × 104
10
11
12
H-Betaf
Sn-Betag
Sn-Betag
1.0
a
Reaction conditions: 5 mL 0.2 M DMF in different solvents, T = 473 K, t = 4 h.
b
c
d
e
f
From Industrial Solvents Handbook.
Turnover frequencies defined as the yield of p-xylene per mole catalyst per hour.
Production rate of p-xylene, average value within 4 h.
Final reaction pressures shown in the parentheses.
Si/Al = 12.5, Brønsted acid sites = 0.75 mmolg−1
.
g
Si/Sn = 50, Lewis acid sites = 0.15 mmolg−1, prepared via the strategy reported in Ref. [21].
Acid can dissociate into proton (H+) and its conjugate base (A−),
proton concentration in haloacetic acid solution is proposed as fol-
lows: CF3COOH (pKa = 0.23) > CF2ClCOOH (pKa = 0.40) > CCl3COOH
(pKa = 0.66) > CHCl2COOH (pKa = 1.30) > CH3COOH (pKa = 4.76).
CH3COOH is completely inactive in the Diels-Alder and dehy-
dration reactions of DMF with ethylene and no p-xylene could
be detected even after reaction at 473 K for 24 h, probably due
to its low acidity not enough to catalyze the dehydration reac-
tion. In contrast, haloacetic acids show noticeable activity in the
reaction. CF2ClCOOH and CCl3COOH appear to be efficient cata-
lysts, with similar p-xylene production rate of 3.4 mmol h−1 g−1
(TOF = 0.45 h−1). These values are higher than those from H-Beta
(2.0 mmol h−1 g−1) and Sn-Beta (2.9 mmol h−1 g−1) under the same
reaction conditions (Table 1). It is interesting to note that CF3COOH
with highest proton concentration shows rather low activity in
the Diels-Alder and dehydration reactions with p-xylene produc-
tion rate of 1.1 mmol h−1 g−1 (TOF = 0.17 h−1). The effects of proton
concentrations on the Diels-Alder and dehydration reactions of
DMF with ethylene to produce p-xylene are summarized in Fig. 2.
Obviously, a proper proton concentration can be optimized for
the Diels-Alder and dehydration reactions catalyzed by haloacetic
acids. Too high or too low proton concentration will result in poor
and the polarity of the medium shows significant impacts on this
equilibrium. Generally, a higher degree of acid dissociation can be
achieved in a solvent with higher polarity. That is to say, we will get
a higher proton concentration at unit mole of monocarboxylic acid
in the solvent with higher polarity. Under such circumstances, the
production rate of p-xylene correlates well with the proton concen-
tration, i.e. the polarity of solvent, and is consistent with literature
reports on the Diels-Alder and dehydration reactions catalyzed by
Brønsted acids with low concentrations [16]. DOX appears to be the
optimized solvent for the reaction, which is used in the sections that
follow.
In order to further confirm the effects of the proton concen-
tration on the Diels-Alder and dehydration reactions of DMF with
ethylene, a series of ␣-haloacetic acids spanning a wide range of
pKa values, i.e. proton concentrations, are employed as catalysts
for the reaction. Generally, halogen substituents at ␣-C mean a
strong electron-withdrawing inductive effect on carboxyl, which
would make the hydrogen dissociate from the carboxyl easier
and therefore a higher proton concentration could be obtained.
Similarly, a stronger electron withdrawing group at ␣-C means a
higher proton concentration. According to these considerations, the

S. Song et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
137
the tandem reaction should come from their impact on the first
Diels-Alder step. A possible speculation is that protons will pro-
mote the transformation of DMF to 2,5-hexanedione (E, Fig. 1) and,
therefore, hinder the formation of cycloadduct.
The kinetics of the Diels-Alder and dehydration reactions of DMF
with ethylene over selected haloacetic acids are shown in Fig. 3.
For all cases, linear increases in the p-xylene yield with reaction
time are observed at p-xylene yield below 20%. Meanwhile, the p-
xylene selectivities of ca. 65% are kept nearly unchanged during
the reaction over all Brønsted catalysts investigated, indicating that
the most by-products are formed in parallel with the cycloaddition
reaction (Fig. 1). This is in great contrast to previous reports of the
gradual increase in p-xylene selectivity with the progress of reac-
tion [14], which should be explained from the different reaction
conditions employed. In our quasi-homogeneous reaction system,
the regional concentration difference induced by the adsorption
and diffusion could be excluded and accurate kinetic studies could
be performed.
Fig. 4 shows the Arrhenius plots of p-xylene production from
the Diels-Alder and dehydration reactions of DMF with ethylene
over haloacetic acid catalysts. Apparent activation energy values
of 8.43, 8.35 and 8.41 kcal/mol could be calculated for CF2ClCOOH,
CHCl2COOH and CF3COOH, respectively. These three catalysts all
belong to monocarboxylic acid and the halogen substituents only
influence the proton dissociation from carboxyl, i.e. proton con-
centration (vide supra). Besides, they have no extra impacts on
the Diels-Alder and hehydration reactions pathway. That is, the
Diels-Alder and dehydration reactions of DMF with ethylene under-
goes the same reaction pathway using haloacetic acid catalysts,
and, therefore, shares the similar apparent activation energy of
∼8.4 kcal/mol, which appear to be lower than the previous report
using solid Brønsted acids as catalysts (10.8 2.1 kcal/mol) [16].
Fig. 2. Effect of proton concentrations on the Diels-Alder and dehydration reactions
of DMF with ethylene to produce p-xylene. Reaction conditions: 5 mL 0.2 M DMF in
dioxane, 0.1 mmol catalyst, T = 473 K, t = 4 h, P = 3.5 MPa ethylene.
reaction activity, distinctly different from the previous reports on
solid Brønsted acid catalysts [11,13,16].
Taking into account both the effects of solvents and catalysts, we
come to the conclusion that a proper proton concentration should
be good for the tandem Diels-Alder and dehydration reactions.
A narrow range of proton concentration employed for research
might be misleading. The second step of dehydration is a typical
acid-catalyzed reaction and a higher concentration of protons is
undoubtedly desired. In this case, the negative role of protons in
Fig. 3. Kinetics of the Diels-Alder and dehydration reactions of DMF with ethylene over haloacetic acid catalysts. Reaction conditions: 5 mL 0.2 M DMF in dioxane, 0.1 mmol
catalyst, T = 453K–513 K, P = 3.5 MPa ethylene.

138
S. Song et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
Fig. 5. Relationship between p-xylene production rate and the ionic diameter in
rare-earth metal triflate catalysts. Reaction conditions: 5 mL 0.2 M DMF in dioxane,
0.01 mmol catalyst, T = 473 K, t = 4 h, P = 3.5 MPa ethylene.
Fig. 4. Arrhenius plots of p-xylene production from the Diels-Alder and dehydration
reactions of DMF with ethylene over haloacetic acid catalysts. Reaction conditions:
5 mL 0.2 M DMF in dioxane, 0.1 mmol catalyst, T = 453K–513 K, P = 3.5 MPa ethylene.
ethylene. CF2ClCOOH is active in the Diels-Alder and dehydration
reactions of MMF with ethylene with a methyl p-toluate production
rate of 0.9 mmol h−1 g−1 (Entry 4, Table 2), compared to the previ-
ous reports using Sn-Beta as catalyst [18,19]. The ∼90% selectivity
toward target product indicates that replacing of methyl group by
carboxylate ester group efficiently inhibit the side reactions. The
electron-withdrawing and steric hindrance effect of carboxylate
ester substituent could make the furan ring stable and difficult
to hydrolysis (In fact, no hydrolysis products could be detected).
Meanwhile, this would greatly retard the desired cycloaddition, as
reflected by the lower conversion of MMF (19.9%) compared to that
of DMF (72.1%) under similar conditions. FDCA, the fully oxidized
product of HMF, and its ester DMFDC could not reaction with ethy-
lene under our reaction conditions and no desired products could
be detected even after 24 h, in consistence with previous report
with Sn-Beta as a catalyst [18]. The main reason should be the
strong electron-withdrawing inductive effects of two carboxyl or
carboxylic ester groups that completely supress the formation of
cycloadduct.
Table 2
The Diels-Alder and dehydration reactions of furan derivatives DMF with ethylene
catalyzed by CF2ClCOOHa.
b
Entry Substrate
Product
Conversion (%) Yield (%) Rate (molg−1 h−1
)
1
2
3
72.1
65.1
43.0
49.9
23.1
4.6
2.0
1.2
0.3
4
5
19.9
0
18.1
0
0.9
0
6
0
0
0
a
Reaction conditions: 5 mL 0.2 M DMF in DOX solvent, 0.1 mmol CDFA catalyst,
3.5 MPa ethylene, T = 473 K, t = 24 h.
b
Average value within 4 h.
On the basis of the above-mentioned results, CF2ClCOOH
appears to be an efficient catalyst for the Diels-Alder and dehydra-
tion reactions of DMF with ethylene in DOX solvent. We therefore
investigated the Diels-Alder and dehydration reactions of several
furan derivates with ethylene catalyzed by CF2ClCOOH and the
results are summarized in Table 2. We find that the production of p-
3.2. Metal triflates catalyzed Diels-Alder and dehydration
reactions
Rare-earth metal triflates, known as versatile Lewis catalysts
[32], were employed for the Diels-Alder and dehydration reactions
of furan derivatives with ethylene. Here a high substrate/catalyst
ratio of 100 was employed, and metal triflates were soluble in
DOX solvent at elevated temperatures to form so-called quasi-
homogeneous reaction system. Under our reaction conditions, all
rare-earth metal triflates employed are active in the Diels-Alder
and dehydration reactions of DMF with ethylene and TOF values of
2.3–4.1 h−1 could be achieved for p-xylene production. Moreover,
the ionic radii of metals in triflates show significant impacts on the
TOF values. As shown in Fig. 5, a higher TOF value is obtained with
a smaller ionic radius of metal in triflates. The relative Lewis acid-
ity of rare-earth metal triflates has been evaluated based on their
competitive dissociation observed by tandem mass spectrometry
and a stronger Lewis acidity of triflates is obtained with smaller
metal ionic radius [33]. Hence, it can be concluded that stronger
Lewis acidity of rare-earth metal triflates would lead to the higher
activity in the Diels-Alder and dehydration reactions of DMF with
ethylene (Fig. 5). This is the first report of the effects of Lewis acidity
on the tandem Diels-Alder dehydration reaction.
xylene (2.0 mmol h−1 g−1) is faster than toluene (1.2 mmol h−1 g−1
)
and then benzene (0.3 mmol h−1 g−1) under identical reaction con-
ditions. That is to say, the reaction rate depends on the methyl
substituents on the diene. Methyl substituent, as an electron donat-
ing group, on the diene will force the furan ring toward forming
[4 + 2] cycloadduct [13,29,30]. The presence of methyl substituent
on diene can also promote the dehydration of cycloadduct because
the charged intermediates in the dehydration of the bicyclic adduct
could be stabilized by the methyl substituents [13,31]. We also
find that the selectivity of furan toward benzene (10.7%) is much
lower than that of MF to toluene (35.5%) and DMF to p-xylene
(65.3%). The lack of methyl substituent would result in the for-
mation of more reactive dicarbonyl compounds via hydration (like
E, Fig. 1; aldehyde groups are more active than ketone groups),
which are easy to form dimers and oligomers (like F, Fig. 1). These
results are in good agreement with literature reports [13,14], and
are also supported by the darker colour of solution of furan than
MF and DMF after reaction. Furan derivatives with different func-
tional groups, MMF, FDCA and DMFDC, were further investigated
as substrates for the Diels-Alder and dehydration reactions with
The kinetics of the Diels-Alder and dehydration reactions of
DMF with ethylene over rare-earth metal triflates are shown in

S. Song et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
139
Fig. 6. Kinetics of the Diels-Alder and dehydration reactions of DMF with ethylene over rare-earth metal triflate catalysts. Reaction conditions: 5 mL 0.2 M DMF in dioxane,
0.1 mmol catalyst, T = 433K–493 K, P = 3.5 MPa ethylene.
Fig. 6. A linear increase in the p-xylene yield with reaction time
could be observed over all catalysts employed at 433–493 K with p-
xylene yield below 30%. In contrast to the cases of haloacetic acids
catalyzed reaction processes, the p-xylene selectivities gradually
increase with the progress of the reaction, indicating the different
pathways for the formation of by-products as compared with those
catalyzed by haloacetic acids.
The Arrenius plots of p-xylene production from the Diels-Alder
and dehydration reactions of DMF with ethylene over rare-earth
metal triflates are shown in Fig. 7. Very similar apparent activation
energy values of 13.0, 12.9 and 13.1 kcal/mol could be calculated
for Sc(OTf)3, Yb(OTf)3 and La(OTf)3, respectively. These values are
somewhat higher than that reported for the reaction catalyzed by
solid Lewis acid Zr-Beta (10.5 kcal/mol) [19]. Moreover, the similar
apparent activation energy values obtained indicate that the Diels-
Alder and dehydration reactions of DMF with ethylene undergoes
the same process over rare-earth metal triflate catalysts and the
Lewis acidity does not change the reaction pathway at all.
Since Sc(OTf)3 is developed to be an efficient Lewis acid catalyst
for the Diels-Alder and dehydration reactions of DMF with ethy-
lene, the substrate scope of furan derivatives is further investigated.
Fig. 7. Arrhenius plots of p-xylene production from the Diels-Alder and dehydration
reactions of DMF with ethylene over rare-earth metal triflate catalysts. Reaction con-
ditions: 5 mL 0.2 M DMF in dioxane, 0.01 mmol catalyst, T = 433K–493 K, P = 3.5 MPa
As shown in Table 3, the Diels-Alder and dehydration reactions
of furan and MF with ethylene could be achieved with benzene
and toluene production rate of 5.4 and 1.3 mmol h−1 g−1, respec-
tively. The trends in the activity and selectivity with Sc(OTf)3 as a
Lewis catalyst is the same as those with CF2ClCOOH as a Brønsted
acid catalyst (vide supra). The Diels-Alder and dehydration reac-
tions of MMF with ethylene could also be realized using Sc(OTf)3
as a Lewis catalyst, and a methyl p-toluate production rate of
7.7 mmol h−1 g−1 with perfect selectivity of ∼99% could be obtained
ethylene.
(Entry 4, Table 3). Although Sc(OTf)3 shows very promising catalytic
activity in the Diels-Alder and dehydration reactions of furan, MF,
DMF and MMF with ethylene, it cannot catalyzed the reaction of
FDCA or DMFDC due to the strong electron-withdrawing of two
carboxyl or carboxylic ester groups.

140
S. Song et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
Table 3
The Diels-Alder and dehydration reactions of furan derivatives with ethylene cat-
a
alyzed by Sc(OTf)3
.
b
Entry Substrate
1
Product
Conversion (%) Yield (%) Rate (molg−1 h−1
)
76.6
53.7
8.3
2
3
76.1
48.3
35.3
8.3
5.4
1.3
4
5
50.2
0
49.5
0
7.7
0
6
0
0
0
a
Reaction conditions: 5 mL 0.2 M DMF in DOX solvent, 0.01 mmol Sc(OTf)3 cata-
lyst, 3.5 MPa ethylene, T = 473 K, t = 24 h.
b
Average value within 4 h.
Fig. 9. GC–MS analysis of final products from the Diels-Alder and dehydration
reactions of DMF with ethylene catalyzed by CF2ClCOOH and Sc(OTf)3. Reaction con-
ditions: 5 mL 0.2 M DMF in dioxane, 0.1 mmol CF2ClCOOH or 0.01 mmol Sc(OTf)3,
T = 473 K, t = 48 h, P = 3.5 MPa ethylene.
For more information on the overall reaction network in the
Diels-Alder and dehydration reactions of DMF with ethylene and
especial the side reaction, the final reaction products were ana-
lyzed by GC–MS. As shown in Fig. 9, the final products appear
to be very clean and the target product p-xylene (1) of >95%
among all detectable products can be achieved by using both
CF2ClCOOH and Sc(OTf)3 as catalysts. For the reaction catalyzed by
CF2ClCOOH, traces of 2,5-hexanedione (2) from the ring-opening
hydrolysis of DMF, 2-ethyl-p-xylene (3) and 4-propyltoluene (4)
from the alkylation of p-xylene product with ethylene and 2,2ꢀ,5,5ꢀ-
tetramethylbiphenyl (5) from the self-alkylation of p-xylene could
be observed. In zeolite-catalyzed reaction system, the concentra-
tion of 2,5-hexanedione increases first and then decreases with
increasing the conversion of DMF [14–16]. While the concentra-
tion of 2,5-hexanedione is very low and kept constant during the
reaction. Clearly, the hydrolysis of DMF to 2,5-hexanedione can be
efficiently suppressed by liquid monocarboxylic acids as catalysts.
For the reaction catalyzed by Sc(OTf)3, only traces of 2-ethyl-
p-xylene and 2,2ꢀ,5,5ꢀ-tetramethylbiphenyl could be observed as
by-products by GC–MS analysis. However, the p-xylene selectiv-
ity calculated from the internal standard n-dodecane only gives a
value of ∼70%, indicating the formation of a fair amount of bulky
products that could not be analyzed by GC–MS. These bulky prod-
ucts should come from the condensation of DMF or its hydrolysis
product 2,5-hexanedione. On the basis of these results and the for-
mer kinetic data, a simplified reaction network can be established,
as shown in Fig. 1.
Fig. 8. Time-on-stream p-xylene yield from the Diels-Alder and dehydration reac-
tions of DMF with ethylene over CF2ClCOOH and Sc(OTf)3 till the complete
conversion of DMF. Reaction conditions: 5 mL 0.2 M DMF in dioxane, 0.1 mmol
CF2ClCOOH or 0.01 mmol Sc(OTf)3, T = 473 K, P = 3.5 MPa ethylene.
3.3. Brønsted acids versus Lewis acids for Diels-Alder and
dehydration reactions
Solid Brønsted acids, e.g. zeolites and mixed oxides, and solid
Lewis acids, e.g. Sn-Beta and Zr-beta, have been reported to be
efficient heterogeneous catalysts in the tandem Diels-Alder and
dehydration reactions of DMF with ethylene. In this study, we
report liquid Brønsted acids, i.e. haloacetic acids, and Lewis acids,
i.e. rare-earth metal triflates, to be active in the reaction. With
the careful selected DOX solvent, a quasi-homogeneous reaction
system could be formed, and a direct comparison between liq-
uid Brønsted and Lewis acids catalyzed reaction could be made. As
shown in Figs. 3 and 6, comparable p-xylene yield is obtained using
optimized CF2ClCOOH and Sc(OTf)3 catalysts at 473 K for 4 h. Con-
sidering that a ten times higher substrate/catalyst ratio is employed
using Sc(OTf)3 than CF2ClCOOH as catalyst, the TOF of p-xylene
production catalyzed by Sc(OTf)3 (4.13 h−1) is calculated to be ca.
nine times higher than that catalyzed CF2ClCOOH (0.45 h−1). In such
circumstances, Sc(OTf)3 is undoubtedly a more attracting catalyst
than CF2ClCOOH for the tandem Diels-Alder and dehydration reac-
tions of DMF with ethylene although it bears a higher apparent
activation energy (13.0 kcal/mol) than CF2ClCOOH (8.4 kcal/mol).
After the complete conversion of DMF, a final p-xylene yield of
∼70% can be obtained over Sc(OTf)3, in contrast of the ∼65% over
CF2ClCOOH (Fig. 8). Obviously, these values are determined by the
final selectivity toward target product.
It should be noted that more bulky by-products would be formed
when solid acids were used as catalysts and a low carbon balance
value was reported if these bulky by-products were not calculated
as the by-products [13]. Under our reaction conditions, the final
selectivities toward p-xylene in the Diels-Alder and dehydration
reactions of DMF with ethylene were determined to be ∼40 and
∼50% using H-Beta and Sn-Beta as solid acid catalysts, respectively.
These values are distinctly lower than those using liquid acid cata-
lysts in a quasi-homogeneous reaction system.

S. Song et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 134–141
141
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