A DFT study of the acid-catalyzed conversion of 2,5-dimethylfuran and ethylene to p-xylene

Article abstract of DOI:10.1016/j.jcat.2012.09.017

We present a detailed mechanism for the conversion of DMF (2,5-dimethylfuran) and ethylene to p-xylene, obtained by gas-phase DFT electronic structure calculations. The conversion consists of Diels-Alder cycloaddition and subsequent dehydration of the cycloadduct, an oxa-norbornene derivative. We present the energetics of both the uncatalyzed and acid-catalyzed (Bronsted and Lewis) reactions. Even though the DMF-ethylene cycloaddition is thermally feasible, we show that Lewis acids can further lower the activation requirements by decreasing the HOMO-LUMO gap of the addends. The catalytic effect may be significant or negligible depending on whether the Diels-Alder reaction proceeds in the normal or the inverse electron-demand direction. We also show that Bronsted acids are extremely effective at catalyzing the dehydration of the oxa-norbornene derivative, which, according to our calculations, cannot proceed uncatalyzed. On the other hand, we conclude that Bronsted acids do not catalyze the cycloaddition. Although strong Lewis acids like Li⁺ can catalyze the dehydration, our calculations indicate that relatively elevated temperatures would be required as they are not as effective as Bronsted acids. We argue that the specific synthetic route to p-xylene is kinetically limited by the Diels-Alder reaction when Bronsted acids are used and by the dehydration when a Lewis acid is used, with the latter being slower than the former. Finally, we adduce experimental data that corroborate the theoretical predictions: we observe no activity in the absence of a catalyst and a higher turnover frequency to p-xylene in the Bronsted acidic zeolite HY than in the Lewis acidic zeolite NaY.

Full text of DOI:10.1016/j.jcat.2012.09.017

Journal of Catalysis 297 (2013) 35–43
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
A DFT study of the acid-catalyzed conversion of 2,5-dimethylfuran and ethylene
to p-xylene
a
a
a,
⇑
a
b
Nima Nikbin , Phuong T. Do , Stavros Caratzoulas , Raul F. Lobo , Paul J. Dauenhauer ,
a,
⇑
Dionisios G. Vlachos
a
Department of Chemical Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, USA
Department of Chemical Engineering and Catalysis Center for Energy Innovation, University of Massachusetts, 686 North Pleasant Street, Amherst, MA 01003, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2012
Revised 29 August 2012
Accepted 18 September 2012
Available online 2 November 2012
We present a detailed mechanism for the conversion of DMF (2,5-dimethylfuran) and ethylene to p-
xylene, obtained by gas-phase DFT electronic structure calculations. The conversion consists of Diels–
Alder cycloaddition and subsequent dehydration of the cycloadduct, an oxa-norbornene derivative. We
present the energetics of both the uncatalyzed and acid-catalyzed (Brønsted and Lewis) reactions. Even
though the DMF–ethylene cycloaddition is thermally feasible, we show that Lewis acids can further lower
the activation requirements by decreasing the HOMO–LUMO gap of the addends. The catalytic effect may
be significant or negligible depending on whether the Diels–Alder reaction proceeds in the normal or the
inverse electron-demand direction. We also show that Brønsted acids are extremely effective at catalyz-
ing the dehydration of the oxa-norbornene derivative, which, according to our calculations, cannot pro-
ceed uncatalyzed. On the other hand, we conclude that Brønsted acids do not catalyze the cycloaddition.
Keywords:
Diels–Alder reaction
Dehydration
DFT
2
,5-Dimethylfuran
+
p-Xylene
Although strong Lewis acids like Li can catalyze the dehydration, our calculations indicate that relatively
elevated temperatures would be required as they are not as effective as Brønsted acids. We argue that the
specific synthetic route to p-xylene is kinetically limited by the Diels–Alder reaction when Brønsted acids
are used and by the dehydration when a Lewis acid is used, with the latter being slower than the former.
Finally, we adduce experimental data that corroborate the theoretical predictions: we observe no activity
in the absence of a catalyst and a higher turnover frequency to p-xylene in the Brønsted acidic zeolite HY
than in the Lewis acidic zeolite NaY.
Ó 2012 Elsevier Inc. All rights reserved.
1
. Introduction
In 1982, Brion realized that furan can be converted into six-
cycloaddition in the liquid phase and often in the presence of Lewis
acid catalysts. Aromatization was exclusively achieved through
base catalyzed b-elimination.
membered carbon rings in a simple two-step mechanism [1]. The
key step in order to achieve such a structurally complex conversion
is Diels–Alder cycloaddition between the furan and an alkene.
The cycloadduct – an oxa-norbornene – may then undergo b-
elimination of the heteroatom bridge and form cyclohexenols or
cyclohexenediols, depending on the substituents on the alkene.
In an attempt to produce difluorocyclohexadienol, using ethyl
It was not until recently that an alternative route was discov-
ered, based on similar principles, which would lead to aromatics.
It was proposed that substituted benzenes can be synthesized by
dehydration of oxa-norbornene derivatives which again may be
synthesized from furan derivatives and alkenes by the Diels–Alder
cycloaddition [4,5]. In the context of unraveling sustainable routes
for the synthesis of base chemicals, this discovery can potentially
have a tremendous impact as it enables the production of aromat-
ics exclusively from renewable resources, if we consider that
cellulosic biomass can be depolymerized into glucose which in
turn may be dehydrated to hydroxymethylfurfural (HMF) – with
or without prior isomerization to fructose – [6,7] while hydrodeox-
ygenation of HMF can yield alkylated furans [8].
3
2
,3-difluoroacrylate as the alkene, Leroy et al. identified ethyl
-fluoro-3-hydroxybenzoate – an aromatic compound – and again
explained the mechanism through Diels–Alder cycloaddition
followed by b-elimination [2]. Numerous studies followed in which
substituted furans and alkenes were converted to a variety of oxa-
norbornenes and 6-membered aromatic or non-aromatic rings [3].
All these reactions commonly proceeded through Diels–Alder
Generally, the Diels–Alder reaction can easily take place under
mild thermal conditions. The small activation energy is owed to
the reaction being symmetry allowed, as one can readily see by
applying the conservation of orbital symmetry theory of Woodward
and Hoffmann [9]. It is a reaction whose mechanism has been
⇑
Corresponding authors.
E-mail addresses: cstavros@udel.edu (S. Caratzoulas), vlachos@udel.edu (D.G.
Vlachos).
0
021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.09.017

3
6
N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
studied extensively both experimentally and theoretically (ab initio
or by DFT) and has sparked vigorous debates on whether it occurs
in a concerted or in a stepwise (through a diradical intermediate)
fashion. Notable are the femtosecond experiments of Zewail and
co-workers and the calculations of Houk and co-workers on the
retro-Diels–Alder of norbornene, in which the stepwise pathway
has been found to lie higher in energy [10–14]. Computational
studies of various Diels–Alder systems in the presence of Lewis
acids have also been carried out in the gas phase or with dielectric
solvent models [15–24]. A convenient framework to understand
the observed Diels–Alder activity patterns at the electronic level
is provided by the frontier molecular orbital (FMO) theory of Fukui
CH3
O
CH3
CH3
O
+
C2H4
-H O
2
Diels-Alder
Reaction
Dehydration
CH3
CH3
CH3
2
,5-Dimethylfuran
Oxa-norbornene cycloadduct
p-Xylene
Fig. 1. Proposed reaction for the conversion of DMF to p-xylene (adapted from
Williams et al. [5]).
[
25]. Despite its qualitative character, it can provide very valuable
insights that pertain to reactivity and selectivity [26,27]. Extracting
from Fukui’s work, the relevant passages about Diels–Alder reac-
tions, two simple postulates can be formulated: (1) the smaller
the energy gap between the highest occupied molecular orbital
Furthermore, we investigate whether extra-framework alkali
cations can achieve Lewis acid catalysis and whether the latter
can be an alternative to Brønsted acid catalysis. Lewis acids are
known to have a catalytic effect on dehydration, for example, alkali
cation-exchanged X and Y zeolites promote the dehydration of
short alcohols [43]. We use experiments to test our theoretical
predictions about different catalytic systems and their turnover
frequencies to p-xylene. To the best of our knowledge, no compu-
tational studies of the dehydration of oxa-norbornene or its deriv-
atives have been published to date, either in the uncatalyzed or in
the catalyzed case.
(
HOMO) of the electron-rich reactant and the lowest unoccupied
molecular orbital (LUMO) of the electron-deficient reactant, the
lower the activation barrier; and (2) the symmetries of the inter-
acting molecular orbitals must be compatible, that is, adequate
for their mixing. The first statement implicates that if a Diels–Alder
reaction is to be accelerated, then the HOMO–LUMO gap needs to
be minimized. By supplying electron-withdrawing substituents to
the more electron-deficient reactant and by adding electron-
donating substituents to the electron-rich reactant, the electron-
content disparity of the addends can be further increased, as for
example in the Diels–Alder reaction between furan and acrylic
monomers [28]. As a consequence, the polarity of the transition
state – which has been shown to be a measure of the HOMO–
LUMO gap as well as of the reaction rate – increases [27,29,30].
Lewis acids have been the catalysts of choice and the most effi-
cient way to enhance the Diels–Alder reaction rates [31–35] –
more so than solvent effects [36]. By lowering the frontier orbital
energies of the coordinated addend, they may accelerate the reac-
tion by several orders of magnitude. The associated effect of Lewis
acids on the system’s geometry may range from leaving it largely
unaffected to forcing the reaction to proceed through a stepwise
mechanism, depending on the system of reactants and catalyst as
described elsewhere [32,33]. Among the numerous Lewis acids
that were tested, alkali and alkaline perchlorates showed particu-
larly promising results. Alkali and alkaline perchlorates [37] were
employed as catalysts, recording dramatic rate improvements over
the uncatalyzed reaction. The observed rates for the different cat-
2
. Methods
2
2
.1. Experimental setup
.1.1. Catalyst preparation
Two commercial zeolitic catalysts from Zeolyst International –
NaY (CBV100, Si/Al = 2.55) and HY (CBV600, Si/Al = 2.6) – have
been used in our experiments. The NaY sample has been washed
3
at 298 K with 0.05 M NaHCO solution for 30 min in order to pre-
clude trace Brønsted acidity. Subsequently, the two samples were
calcined at 823 K for 4 h in 100 cc/min helium flow to remove
any organic residual. The total acid sites and BET surface areas
were quantified using techniques described elsewhere [5].
2.1.2. Measurement of catalytic properties
The reaction of DMF (99% purity, Sigma–Aldrich) with ethylene
(Scotts Specialty Gas, Inc) was carried out in a 45 mL-closed Parr
reactor (series 4740). The vessel was heated by a Chemglass oil-
bath unit. The mixing of both reactants was done by use of a mag-
netic stirrer. 7.2 g (0.075 mol) of DMF and 4.69 MPa (680 psig) of
ethylene (0.07 mol) were charged to the reactor and reacted at
493 K for 6 h. n-Decane was found to be inactive under the exper-
imental conditions and thus was used as an internal standard. After
the end of reaction, product mixtures were collected, filtered with
ions were found to follow the order Mg2 > Ba P Li > Na and
+
2+
+
+
to be in direct correlation with the respective charge/radius ratios
[
38].
It is well established that the hydrophobic effect increases the
rate of the Diels–Alder reaction [39]. Thus, it is not surprising that
confinement of the addends, like inside zeolite pores, can have the
same effect on the rate [40]. In the case of zeolites, it has also been
reported that extra-framework cations can be beneficial, presum-
ably because they function as Lewis acid catalysts. Murthy and
Pillai found that when furan was mixed with acrolein in Na-
containing zeolite Y, the Diels–Alder yield increased [41]. Imachi
and Onaka have observed higher Diels–Alder activity in solvent-
free NaY than in organic or aqueous media [42].
0.2 lm-syringe filter, and diluted in ethyl acetate (HPLC-grade,
Sigma–Aldrich) for the subsequent gas chromatography (GC) anal-
ysis (7890A Agilent). Product identification was confirmed with an
Agilent 6850 series GC, a 5973 MS detector and injection of the
standards. Details for the determination of the acid site density
in the zeolitic catalysts can be found elsewhere [5].
In a recent study, Williams et al. introduced a procedure to syn-
thesize p-xylene from DMF and ethylene in zeolite HY [5]. The
authors reported selectivity of 51% to p-xylene at 573 K, which
they were able to increase to 76% with the use of an aliphatic sol-
vent. They proposed that the reaction proceeds by Diels–Alder
cycloaddition and subsequent dehydration of the resulting cyc-
loadduct. (Fig. 1) In this paper, we study the specific reaction by
carrying out electronic structure calculations and investigate how
Brønsted acid catalysis alters the mechanism and energetics.
2.2. Computational model
All the calculations were performed at the restricted-DFT
M062X/6-311+(d,p) theory level [44,45] with the Gaussian soft-
ware package (Gaussian 09, Revision A.2) [46]. Rubidium and
Cesium atoms were modeled with the effective core potential
LANL2DZ. All the structures were fully relaxed, and vibrational
frequencies were calculated. Transition states were confirmed by
the presence of a single imaginary frequency and by IRC (intrinsic

N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
37
reaction coordinate) calculations. All reported energies are zero-
point-corrected electronic energies and in units of kcal/mol.
In the present study, the calculations were carried out in the gas
phase. Both for the Brønsted and for the Lewis acid catalysts, we
have used simple models that do not consider the zeolitic environ-
ment. Thus, in the case of Brønsted catalysis, we have calculated
reaction pathways when each of the two addends is protonated.
In the case of Lewis acid catalysis, we have considered only bare al-
kali cations, either furan- or ethylene-coordinated. Therefore, the
calculated activation energies do not account for solvation or
confinement effects. In light of recent work by Wannakao et al.,
we expect this to be a fairly good approximation in the case of
the alkali-metal-containing zeolite Y [47].
LUMOethylene À HOMODMF gap of 0.290 Hartree is smaller than the
LUMODMF À HOMOethylene gap of 0.350 Hartree; thereby, the uncat-
alyzed reaction proceeds in the normal electron-demand direction.
The ethylene molecule approaches the furan ring from above, so as
to achieve maximal orbital overlap. The interacting complex pos-
sesses C
through the oxygen atom of the furan and bisects the C
of DMF and the C –C bond of ethylene (nomenclature shown in
s
symmetry with respect to the mirror plane that passes
2
3
–C bond
5
6
Fig. 2). This concerted mechanism, with an energy barrier of
24.7 kcal/mol, conforms to the postulates of the Woodward–Hoff-
mann theory [9]. The transition state structure is shown in Fig. 3a.
3.2.1.2. Lewis-acid-catalyzed Diels–Alder reaction. We have consid-
ered all three possible arrangements between the addends and
the catalyst: DMF bound to the alkali cation; ethylene bound to
the catalyst; or both reactants bound to the cation. Our calcula-
tions have shown that the reaction pathway associated with the
third configuration lies significantly higher in energy than the
other two and thus we will not discuss it herein.
We have performed calculations for all the alkali cations from Li
to Cs, regardless of whether such materials can be manufactured or
whether they can expose the extra-framework cations as potential
active sites [48]. In all the cases, we calculated similar geometries.
Specifically, when the cation binds the ethylene molecule, the
resulting complex has C2v symmetry with the cation lying in the
plane of the double bond (see Fig. 3b). When the cation binds the
DMF molecule, the complex has C symmetry with the cation lying
s
in the symmetry plane and slightly closer to the C2 and C3 carbons
than to C1 and C4, by about 0.05 Å (see Fig. 3c).
Irrespective of the coordination of the cation, our calculations
have identified only Diels–Alder transition states of C symmetry,
s
that is, the two new sigma bonds form synchronously. The calcu-
lated zero-point-corrected energies are reported in Table 2 along
with the corresponding HOMO–LUMO and HOMO–(LUMO + 1)
gaps. In Fig. 4a and b, we depict the correlation between the calcu-
lated activation barrier and the FMO energy gaps; in Fig. 4c, we
show the correlation between the computed barrier and the effec-
tive ionic radius. The reported orbital energy gaps show that the
ethylene–cation binding activates the normal electron-demand
3
. Results and discussion
3.1. Experimental results
Table 1 reports the conversion of DMF and the corresponding
turnover frequencies into p-xylene for the uncatalyzed reaction
and for reaction in the zeolites HY and NaY. All conversions were
maintained as low as 10%. Turnover frequencies were calculated
as moles of p-xylene over moles of zeolite acidity. Because the
3
NaY samples were treated with dilute solution of NaHCO to de-
plete any existing trace Brønsted acidity, we assume that the total
acidity of this sample is entirely due to the Lewis acidity of the ex-
tra-framework Na cations. Product analysis revealed significant
side reactivity, including hydrolysis of DMF, alkylation of p-xylene
and condensation reactions; details have been given elsewhere [5].
The present study is mainly concerned with the effect of different
catalysts on the productive pathways to p-xylene.
While the conversion of DMF in HY is only slightly higher than
in NaY, the turnover frequency (TOF) is significantly higher, by
ꢀ
60%. Much less DMF reacted when no catalyst was present, and
only traces of p-xylene were produced.
3
3
.2. Computational results
.2.1. Diels–Alder reaction
A Diels–Alder reaction in which the electron richness of the
mechanism and, clearly, the catalytic ability diminishes as we
diene (the nucleophile) and the electron deficiency of the dieno-
phile (the electrophile) trigger the mixing of the LUMOdienophile
and the HOMOdiene is defined as to be following the normal elec-
tron-demand direction. Conversely, the LUMOdiene À HOMOdienophile
gap becomes indicative of the activation barrier height in Diels–
Alder reactions that follow the so-called inverse electron-demand
mechanism in which the nucleophile is the dienophile. This situa-
tion may arise when the diene has electron-withdrawing substitu-
ents or is coordinated with an acid. The catalyzed Diels–Alder
reaction of DMF with ethylene could therefore follow either the
normal or the inverse electron-demand direction, depending on
which of the two addends binds to the acid catalyst.
move down the group of the alkali metals, with Li+ performing
the best as the barrier drops from 24.7 kcal/mol for the uncata-
lyzed reaction to 16.9 kcal/mol. The inverse mechanism is in play
when the catalyst binds to the DMF. The trends are the same, with
+
+
the exception of Na which is marginally better than Li , by ca.
kcal/mol. We also observe that the inverse demand mechanism
1
+
is always slower, and in the case of Cs , there is no catalysis at all.
It is instructive to take a closer look at the reaction from the
point of view of the molecular orbitals involved. Irrespective of
the binding geometry, binding to the cation does not impose any
symmetry restrictions and thereby the catalyzed Diels–Alder reac-
tion is symmetry allowed. In Fig. 5, we show the LUMO and
LUMO + 1 of the isolated ethylene–cation complex (panels (b)
and (c)), the HOMO of the isolated DMF (panel d), and the HOMO
of the Diels–Alder transition state complex (panel e). Unlike the
uncatalyzed reaction, the LUMO of the ethylene–cation complex
3.2.1.1. Uncatalyzed Diels–Alder reaction. Owing to the conjugated p
electrons, the DMF molecule is the more nucleophilic reactant in a
reaction with ethylene. This is confirmed by the fact that the
Table 1
Catalyst characterization, experimentally determined conversion of DMF and turnover frequencies (TOF) for the production of p-xylene per hour per active site in zeolites with
Lewis (NaY) and Brønsted (HY) acidity. Reactions were carried out at 493 K and an ethylene partial pressure of 4.69 MPa (680 psig) for 6 h.
2
Catalyst
Masscatalyst (g)
Si/Al ratio (–)
BET area (m /g)
Total acid sites (mmol/g)
ConversionDMF (–) (%)
TOFp-xylene (1/site h)
No catalyst
NaY (CBV 100)
HY (CBV 600)
–
0.10
0.02
–
2.55
2.55
–
900
730
–
1.99
2.46
2.0
10.1
10.7
–
7.37
11.61

3
8
N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
H C
the region of the cation due to binding to the alkali cation and
CH3
3
H
some contribution from the cation’s p-orbital. We note that the
reason we have to consider the HOMO–(LUMO + 1) interaction
rather than the HOMO–LUMO one is merely a consequence of
the fact that we have only considered the bare cation in our model
and thereby the LUMO is basically the empty s-orbital of the cat-
ion. That should not be the case if one models the Lewis acid as
part of a zeolite cluster; such calculations are currently underway
in our lab and we shall report them in a forthcoming article. It is
also worth pointing out that the LUMO of the transition state is
the LUMO of the ethylene–cation complex, albeit slightly distorted
(not shown).
According to the FMO energy gaps shown in Table 2, the DMF–
cation binding geometry closes the HOMO–LUMO gap to a lesser
extent, and as a result, the cycloaddition reaction proceeds slower.
That was anticipated, because in this configuration, the cation
withdraws electron density from the DMF molecule and thus the
addend becomes less electron rich, and that makes cycloaddition
to an already electron-deficient addend (the ethylene) less facile.
In Fig. 6, we show the LUMO and LUMO + 1 of the isolated
H
H
H
H
C4
C
3
C
4
C
C
3
C
5
C
5
H
H
CH2
O
O
C
2
CH2
C6
C
1
2
C
6
C1
CH3
H
H C
3
Fig. 2. Nomenclature for DMF and ethylene atoms.
is symmetric with respect to the mirror plane and thereby does not
possess the right symmetry in order to interact with the HOMO of
DMF, which is antisymmetric. However, the LUMO + 1 of the ethylene-
cation complex does have the proper symmetry and thereby the
HOMO of the transition state is the bonding combination between
the LUMO + 1 of the ethylene-cation complex and the HOMO of
DMF, as can clearly been seen in panel (e). The LUMO + 1 of the cat-
of the alkene double
bond, as FMO theory would dictate, but with augmented lobes in
Ã
ion–ethylene complex is in essence the
p
(a) Thermal Diels-Alder reaction
(b) Lewis acid coordinated to ethylene
(c) Lewis acid coordinated to DMF
2
.57 Å
97.4˚
9
7.3˚
2.57 Å
.07 Å
2.16 Å
2.16 Å
2.73 Å
2
.13 Å
2.13 Å
2
2
.07 Å
97.0˚
2
.73 Å
+
Fig. 3. Selected geometrical properties of the transition states for the uncatalyzed and Lewis-acid-catalyzed (here shown for Na ) Diels–Alder reaction.
Table 2
Zero-point-corrected electronic energy barriers for the Diels–Alder reaction for different alkali cations bound either to ethylene or to DMF. The relevant orbital energy gaps are
printed in boldface. Reff denotes the effective ionic radius.
Cation
R
eff (pm)
Cation coordinated with ethylene
FMO gaps
Cation coordinated with DMF
FMO gaps
Barrier (kcal/mol)
A (Hartree)
B (Hartree)
C (Hartree)
Barrier (kcal/mol)
A (Hartree)
B (Hartree)
C (Hartree)
None
24.65
16.90
18.65
20.55
21.64
22.11
0.290
0.050
0.053
0.080
0.084
0.097
0.350
0.577
0.549
0.519
0.505
0.494
+
Li
76
0.058
0.089
0.112
0.125
0.135
23.23
22.58
24.02
24.01
24.86
0.494
0.466
0.453
0.439
0.432
0.153
0.143
0.168
0.167
0.178
0.185
0.204
0.216
0.219
0.224
+
Na
102
138
152
167
+
K
+
Rb
+
Cs
A: LUMOeth+cation À HOMODMF
B: LUMO + 1eth+cation À HOMODMF
C: LUMODMF À HOMOeth+cation
A: LUMOeth À HOMODMF+cation
B: LUMODMF+cation À HOMOeth
C: LUMO+1DMF+cation À HOMOeth
(
a) 30
(b)30
25
(c)30
Rb+ Cs+
+
Li+
K
25
20
15
10
5
25
20
15
10
5
Na
+
20
15
no catalyst
10
5
no catalyst
Cation coordinated with ethylene
Cation coordinated with DMF
Cation coordinated with ethylene
Cation coordinated with DMF
Cation coordinated with ethylene
Cation coordinated with DMF
0
0
0
0
10
20
30
0
5
10
15
20
0
50
100
150
200
(
LUMOelectrophile-HOMOnucleophile)-1
(
LUMO+1electrophile-HOMOnucleophile)-1
Effective Ionic Radius [pm]
Fig. 4. Correlation between the zero-point-corrected Diels–Alder barriers and (a) the inverse of the HOMO–LUMO gap; (b) the inverse of the relevant HOMO–(LUMO + 1) gap;
c) the effective ionic radius.
(

N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
39
+
(b) LUMO of isolated
ethylene-cation complex
(c) LUMO+1 of isolated
ethylene-cation complex
(d) HOMO of isolated
DMF
(
a) Ethylene-Na & DMF
(
e) HOMO of Diels-Alder TS
Fig. 5. (a) Ethylene–Na+ complex (top) and DMF (bottom); (b) LUMO of isolated ethylene–cation complex; (c) LUMO + 1 of isolated ethylene–cation complex; (d) HOMO of
isolated DMF; and (e) HOMO of Diels–Alder transition state (TS). All orbitals are illustrated for a surface isovalue of 0.01.
DMF–cation complex (panels b and c), the HOMO of isolated
ethylene (panel d), and the HOMO of the Diels–Alder transition
state complex (panel e). The HOMO of the Diels–Alder transition
state is the bonding combination between the HOMO of the
ethylene and the LUMO + 1 of the DMF–cation complex. The LUMO
of the DMF–cation complex does have the same symmetry as the
HOMO of ethylene (symmetric with respect to the mirror plane),
but it is mostly localized around the cation and thus does not lead
to bonding between DMF and ethylene. In the LUMO + 1, note also
the non-bonding p-orbital on O that is out of phase with the
on a realistic laboratory time scale. When the proton is bound to
the DMF, it may attach to more than one site, namely, the oxygen
atom, the C
1
carbon, or the C
state, H -on-C , the C carbon does not participate in conjugated
bonding and it may thus not play a part in the cycloaddition. Sim-
2
carbon (Fig. 8). In the most populated
+
1
1
p-
+
2 1
ilarly, with the proton on C (DE (relative to H -on-C ) = +5.8 kcal/
mol), the Diels–Alder reaction of interest may not occur either.
However, we have located a reaction path of a fast, competing
reaction shown at the bottom of Fig. 8, which may contribute to
the side products. When the proton is bound to the oxygen, it low-
ers the Diels–Alder reaction barrier height to 19.9 kcal/mol. How-
p-symmetry orbital on the ring and also out of phase with the
HOMO of the approaching ethylene molecule, a situation non-
conducive to maximal overlap between the two molecular orbitals.
ever, this configuration is a lot less stable than the other two (DE
(relative to H -on-C ) = +28.7 kcal/mol). When the system is in
1
+
any one of the two stable states, a proton shift to the oxygen is
rather unlikely, as it requires an activation of 54.5 kcal/mol.
Thus, our calculations predict that Brønsted sites cannot catalyze
the DMF–ethylene cycloaddition, no matter which of the two reactants
binds to the site. This is in agreement with recent kinetic studies by
Williams et al. [5] who synthesized p-xylene using the Brønsted
acidic zeolite HY.
3
.2.1.3. Cycloadduct rearrangement on Lewis acid site. After the
Lewis-acid-catalyzed Diels–Alder reaction has taken place, the
cation may be either located behind the two carbon atoms of the
ethylene addend (after normal electron-demand reaction, Fig. 7)
or coordinated with the oxygen (inverse electron-demand reaction,
Fig. 7), the latter being energetically much more favorable. These
two stationary points are separated by a small barrier which, for
all the alkali cations considered in this work, is never higher than
3.2.2. Dehydration
2
kcal/mol.
The cycloadduct (1,4-Dimethyl-7-Oxabicyclo[2.2.1]hept-2-ene,
Thus, we conclude that regardless of which reactant initially binds
an oxa-norbornene derivative) needs to be dehydrated to yield
p-xylene. The mechanism and associated energetics are depicted
in Fig. 9. We remind the reader that the atom nomenclature can
be found in Fig. 2.
to the cation, the cycloadduct will be coordinated with the cation
through the oxygen atom before desorption from the catalyst or dehy-
dration occurs.
Barring solvation effects, not accounted for in the present study,
our gas-phase calculations show that the uncatalyzed dehydration
3.2.1.4. Diels–Alder reaction with protonated reactants. The Diels–
Alder reaction between the protonated ethylene and DMF has a
very high barrier of 71.8 kcal/mol and thus is unlikely to occur
is a very slow reaction, as one must first break the C
providing 59 kcal/mol of activation energy (see Fig. 9a). Both
1
–O bond by
(
a) DMF-Na+ & Ethylene
(b) LUMO of isolated
DMF-cation complex
(c) LUMO+1 of isolated
DMF-cation complex
(d) HOMO of isolated
DMF
(e) HOMO of Diels-Alder
Fig. 6. (a) DMF–Na+ complex (top) and ethylene (bottom); (b) LUMO of isolated DMF–cation complex; (c) LUMO + 1 of isolated DMF–cation complex; (d) HOMO of isolated
ethylene; and (e) HOMO of Diels–Alder transition state (TS). All orbitals are illustrated for a surface isovalue of 0.01.

4
0
N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
CH3
H
CH3
O
H
H
H
NormalED
DAreaction
18.6 // 19.5
H
H
C
C
H C
2
+
+
O
LA
LA
H C
2
H
H
CH3
CH3
+
LA
CH3
CH
3
H
H
Inverse ED
DAreaction
H
H
C
C
CH2
H
+
LA
O
O
C
C
CH2
22.6 // 38.9
H
H
CH3
H
3
CH
Fig. 7. Schematic representation of Lewis-acid-catalyzed Diels–Alder reaction and cycloadduct rearrangement. The indicated Forward/Backward barriers are for the Na+ case
and are in units of kcal/mol.
H3C
CH3
among lone-pair NBOs. At the same time, the migrating H is still
formally bonded to the C carbon, but the corresponding anti-
bonding orbital has substantial occupancy of 0.142 owing to the
geometry of the system, which is conducive to a favorable overlap
H
H
H
H
H
H
DA reaction
w/ ethylene
H
H
6
+
+
OH
OH
19.9 // 55.9
CH3
H3C
between the C
the O that leads to the O–H bond, once the system is past the tran-
4
sition state. In the last step, an H shift, from C to the OH on C ,
6
–H anti-bonding orbital and a lone-pair p-orbital on
5
completes the reaction to p-xylene. Once the system is over, the
corresponding barrier of 47.6 kcal/mol height, that is, once the H
shift is complete, the incipient water breaks off without additional
energy requirements. On the basis of the transition state NPA
charges (+0.472 on the transferring H, À0.86 on O and À0.57 on
CH3
O
CH3
H
H
H
Reaction
w/ ethylene
Protonation
+
O
5
C ), we characterize the H migration as a proton transfer – but with
H
H
some reservations. Our interpretation of the occupancy of the
NBOs is not entirely consistent with a proton transfer, namely, a
neutral H transfer may not be precluded, because during the requi-
site transition-state re-hybridization, we find a lone-pair NBO on
CH3
H3C
5
the C carbon with occupancy 1.33, which could suggest formation
of a diradical in a singlet spin state. Unrestricted calculations have
not been of much help in this respect as they have converged to the
restricted solution. We believe that a clearer answer to this ambiv-
alent case may be offered by more expensive, multiconfiguration
interaction (e.g., CASSCF) calculations.
CH3
H
CH3
O
H
H
Reaction
H
+
O
w/ ethylene
H
8
.1 // 42.3
H
CH3
H3C
The dehydration of the protonated oxa-norbornene derivative is
facile and happens in two steps rather than three (see Fig. 9b).
Fig. 8. Reactions of ethylene and protonated DMF; Forward/Backward barrier in
After protonation of the O atom, modeling Brønsted catalysis, it
kcal/mol.
+
takes only 7.9 kcal/mol of energy to break the C
1
–OH bond. The
bond breaks heterolytically and leads to the formation of a carbo-
cation. The NPA charge on C is +0.47 (in contrast, C has a natural
charge of +0.22). In the second step, a proton transfer from C to
1
5
restricted and unrestricted calculations on the singlet state con-
6
verge to the same solution and show that the C
1
–O bond breaks
the OH, requiring 17.0 kcal/mol, completes the reaction. According
to NBO analysis, in the transition state, the shifting hydrogen is not
bonded to any atoms and its NPA charge is +0.48. As in the uncat-
alyzed case, upon completion of the proton transfer, the incipient
heterolytically; not an unexpected result, given the disparity in
electronegativity between the two atoms. According to NBO (Nat-
ural Bond Orbital) analysis of the first transition state, the oxygen
atom possesses three lone pairs of electrons as well as a formal
water breaks off the ring; in the gas phase, the C
is 3.4 Å. For a complete catalytic cycle, the proton left on C
4
–water separation
, after
bond to the C
sition state, the oxygen is formally bonded to the C
in a geometry that has the O located over the ring and proximal to
a hydrogen atom on the C carbon, while the double bond shifts
from C –C to C –C . The second step is slow, too, requiring
6.8 kcal/mol of activation energy. This step entails proton transfer
4
carbon. In the resulting intermediate, past this tran-
5
3
carbon as well,
the release of water, would have to return to an oxygen atom of
the zeolite framework to re-generate the Brønsted acid site.
The use of a Lewis acid, unlike a Brønsted acid, does not alter the
dehydration mechanism, which, like in the uncatalyzed case, in-
volves three transition states (see Fig. 9c). The reaction starts by
6
2
3
1
2
4
from C
6
to the O atom, coupled with the breaking of the C
4
–O bond.
cleavage of the C
transfers: one from C
a second from C to the OH on C
1
–O bond and continues with two sequential proton
to O, coupled with the C –O dissociation; and
to form water. The corresponding
We have ascertained that the H shift is indeed a proton transfer by
carrying out an NBO analysis of the transition state, according to
which the NPA (Natural Population Analysis) charges on the trans-
ferring H, the O, and the C
respectively. In the transition state, the O atom is formally bonded
only to the C carbon while the rest of its electrons are distributed
6
4
5
4
zero-point-corrected activation energies are summarized in Table 3
for all the alkali cations considered in this study. The catalytic
activity diminishes with the effective ionic radius, but even the
6
carbon are +0.4, À0.746, and À0.526,
+
4
Cs – which proved a very poor catalyst for the Diels–Alder

N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
41
(
a)
H
CH
3
CH
3
H
H
3
H C
OH
CH
3
H
H
st
H
2nd Proton
Transfer
C-O bond
cleavage
1 Proton
H
H
H
H
H
H
H
H
H
H
H
Transfer
O
O
59.0 // 54.9
46.8 // 55.9
47.6 // 71.8
H
H
H
H
CH
3
CH
3
CH
3
CH
3
H
H
+
H
3
C
O
H
H
H
(
b)
H
H
H C
CH3
H C
3
H
3
H
H
H
H
H
st
+
C-O bond
cleavage
1 Proton
H
C
H
H
H
H
CH
3
+
HO
C
Transfer
OH
+
(transition state structure)
H
7.9 // 12.0
17.0 // 30.4
H
H
H
H
CH3
H
H C
H C
3
3
(
c)
H
+
+
LA
CH
3
LA
H
CH
3
st
2nd Proton
Transfer
CH
3
C-O bond
cleavage
H
1 Proton
OH
H
3
C
H
H
H
H
H
H
+
H
Transfer
LA
O
H
H
O
H
35.8 // 33.6
30.1 // 36.3
30.3 // 55.8
H
H
H
H
H
H
CH
3
H
H
CH
3
CH
3
CH
3
Fig. 9. Mechanism and energetics of the dehydration of 1,4-Dimethyl-7-Oxabicyclo[2.2.1]hept-2-ene. (a) Uncatalyzed; (b) with Brønsted acid; and (c) with Lewis acids
indicated energies are for the Na+ case); Forward/Backward barriers are in units of kcal/mol.
(
reaction – manages to increase the dehydration rate by lowering the
–O bond cleavage barrier by ca. 15 kcal/mol. In neither of the ele-
3.3. Discussion
C
1
mentary steps does the Lewis acid participate in formal bonds,
according to NBO analysis. It does, however, affect the electron den-
sity of the oxygen atom and of the neighboring bonds. By drawing
The energy profiles of the Diels–Alder reaction and of the subse-
quent dehydration for the uncatalyzed and catalyzed cases are
summarized in Fig. 11. The uncatalyzed reaction exhibits barriers
as high as 60 kcal/mol, and therefore, no p-xylene should be pro-
duced at typical experimental temperatures (200–300 °C), which
is in agreement with our experimental findings (Table 1).
1 4
electron density away from the regions around the C –O and C –O
bonds, the cation weakens these bonds and thus facilitates the req-
uisite C–O dissociation. We have mapped the electrostatic potential
on an isosurface of electron density equal to 0.1, and in Fig. 10a, we
In earlier experiments with both low and high siliceous HY
(Si/Al 2.6 and 40, respectively), it was observed that the catalytic
activity evidently does not correlate with the Brønsted site density
[5]. This is in line with our findings, as we observe that, in the case
of Brønsted acid catalysis, the elementary step that determines the
overall rate of the process is the uncatalyzed Diels–Alder reaction,
with activation energy of 24.7 kcal/mol. In the case of Lewis acid
catalysis, like in the NaY experiments presented in this article,
the rate is limited by the first dehydration step, with activation
+
+
show the mappings for the two extreme cases, viz. Li and Cs . The
electrostatic repulsion between the C and O nuclei is the largest
1
+
in the case of Li and evidently decreases with the effective ionic
radius of the cation, that is, the bond weakens with increasing Lewis
acidity. The same type of analysis shows that in the ensuing step, viz.
coupled proton transfer to O and C
repulsion between the C and O nuclei is larger in the case of Li+
see Fig. 10b). Similarly, Fig. 10c shows the larger, positive electro-
3
–O cleavage, the electrostatic
3
(
+
+
static potential around the C
4
–O bond in the case of Li .
energy of 35.8 kcal/mol for Na . This explains why the observed
In summary, both Lewis acid and Brønsted acid can have a sig-
nificant catalytic effect on the dehydration reaction. In fact, a
strong Lewis acid may reduce the activation energy requirements
turnover frequencies are higher in the zeolite HY than in NaY.
According to our calculations, Li would be a better Lewis acidic
catalyst, even though it would still not achieve the performance
of Brønsted acids.
+
+
by >50% (e.g., Li ). This number can climb up to 70% in the case
of Brønsted catalysis. The energetically most difficult step changes
from the first proton transfer in the case of Brønsted acid to the
C –O cleavage in the case of Lewis acid.
1
In view of an earlier work [5], which inter alia highlighted the
importance of confinement, and the computational data presented
here, we envisage an optimized, bifunctional catalyst that would
Table 3
Zero-point-corrected electronic energy barriers for the three elementary dehydration reaction steps in the presence of different alkali cations. Reff denotes the effective ionic
radius.
Cation
None
R
eff (pm)
76
1st barrier (kcal/mol)
2nd barrier (kcal/mol)
3rd barrier (kcal/mol)
59.0
28.7
35.8
39.2
43.2
44.6
46.8
22.7
30.1
32.7
35.6
36.5
47.6
20.9
30.3
35.0
39.1
41.0
+
Li
+
Na
102
138
152
167
+
K
+
Rb
+
Cs

4
2
N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
(
a) before C- -O bond cleavage
(b) before 1 st proton shift
(c) before 2nd proton shift
1
Li
O
Li
Li
C6
O
C1
O
C3
C2
C1
C4
C5
C4
C2
C3
C6
Cs
Cs
O
Cs
O
C3
C1
O
C2
C1
C4
C5
C4
C2
C3
C6
0
.3
0.7
Fig. 10. Mapping of the electrostatic potential on the isosurface of electron density equal to 0.1 before each of the three elementary dehydration steps, in the case of Li+ (top)
and of Cs+ (bottom).
6
4
2
0
0
0
Lewis-acid-catalyzed
inverse electron-demand Diels-Alder reaction
Lewis-acid-catalyzed
normal electron-demand Diels-Alder reaction
0
20
40
60
Lewis-acid-catalyzed dehydration
Brønsted-acid-catalyzed dehydration
no catalyst
-
-
-
Diels-Alder
reaction
Dehydration
Ads.
Fig. 11. Zero-point-corrected electronic energy profiles for the conversion of DMF and ethylene to p-xylene relative to the reactants’ energy at infinite separation. In the case
+
of Lewis acid catalysis, we only show the Na data. The binding energies for the DMF–ethylene complex, the Lewis acid–ethylene–DMF complex, and the Lewis acid–DMF–
ethylene complex are, respectively, 2.21, 8.08, and 3.55 kcal/mol. This state is found by performing a ground-state optimization following an IRC calculation in the direction of
the reactants.
combine Lewis acidity, for fast Diels–Alder cycloaddition, with
Brønsted acidity, for fast dehydration of the cycloadduct. Given
that the most efficient Lewis acid catalysis can be achieved only
by the normal electron-demand mechanism, it is imperative that
the ethylene partial pressure be maintained at high levels. The
effect that this may have on the acceleration of side reactions is
yet to be investigated.
constant stabilization of ꢀ10 kcal/mol. Yet, the Lewis-acid-cata-
lyzed dehydration barriers are significantly lower, which implies
that the catalytic effect is mainly due to the stabilization of the
transition state.
4. Conclusion
It is important to point out that upon comparison of the
uncatalyzed dehydration energy profile with that for Lewis-
acid-catalyzed dehydration we see that the cycloadduct, the
p-xylene, and all the intermediates in-between experience a
Using gas-phase DFT electronic structure calculations, we have
presented a detailed mechanism for the conversion of DMF and
ethylene to p-xylene, consisting of Diels–Alder cycloaddition and
subsequent dehydration of the oxa-norbornene derivative

N. Nikbin et al. / Journal of Catalysis 297 (2013) 35–43
43
cycloadduct, and investigated the Lewis and Brønsted acid cataly-
sis of these reactions.
tions and at the Center for Functional Nanomaterials, Brookhaven
National Laboratory, which is supported by the U.S. Department
of Energy, Office of Basic Energy Sciences, under Contract No.
DE-AC02-98CH10886.
Even though the DMF–ethylene cycloaddition is thermally fea-
sible, owing to being symmetry allowed, we have nevertheless
found that Lewis acids can further lower the activation require-
ments by decreasing the HOMO–LUMO gap of the reacting ad-
dends. The catalytic effect may be significant or negligible
depending on whether the Diels–Alder reaction proceeds in the
normal or the inverse electron-demand direction; the former hap-
pens when ethylene binds to the Lewis acid, as the electron poor
addend becomes more so. Out of the alkali cations studied in this
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microkinetic picture.
a more complete
Acknowledgments
The work of N.N. and D.G.V. was supported partially by the
Department of Energy under Grant Number DE-FG02-05ER25702.
The work of P.T.D., S.C., P.J.D., and R.F.L. was based on work finan-
cially supported as part of the Catalysis Center for Energy Innova-
tion, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under award number DE-SC0001004. Calculations were
performed with use of the TeraGrid resources provided by the
University of Illinois’ National Center for Supercomputing Applica-
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