Conversion of 1-tetralone over HY zeolite: An indicator of the extent of hydrogen transfer

Article abstract of DOI:10.1016/j.apcata.2010.09.019

The conversion of pure 1-tetralone and its mixtures with n-decane, decalin, tetralin, or 1,5-dimethyl tetralin (DMT) has been investigated over HY zeolite. The dominant reactions undergone by 1-tetralone are the dehydrogenation to 1-naphthol and the subsequent isomerization to 2-naphthol. In the presence of hydrocarbons, the hydrogen transfer/dehydration of naphthols is accelerated, and naphthalene is formed in different amounts, depending on the nature of the co-fed hydrocarbon. In this contribution, it is demonstrated how the product distribution from the tetralone conversion can be used as an indicator of the hydrogen transfer ability of a particular hydrocarbon, or mixture of hydrocarbons. The relative order of hydrogen transfer ability of the various hydrogen donating compounds, as inferred from the naphthalene-to-naphthol product ratio, is DMT > tetralin ≈ decalin > n-decane. This trend agrees well with the hydride dissociation energy of individual donors calculated by DFT.

Full text of DOI:10.1016/j.apcata.2010.09.019

Applied Catalysis A: General 389 (2010) 140–146
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Conversion of 1-tetralone over HY zeolite: An indicator of the
extent of hydrogen transfer
a
a
b
Teerawit Prasomsri , Roberto E. Galiasso Tailleur , Walter E. Alvarez ,
c
a,∗
Tawan Sooknoi , Daniel E. Resasco
Center for Biomass Refining and, School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, OK 73019, USA
ConocoPhillips Technology Center, Bartlesville, OK 74004, USA
a
b
c
Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology, Ladkrabang, Bangkok 10520, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2010
Received in revised form
The conversion of pure 1-tetralone and its mixtures with n-decane, decalin, tetralin, or 1,5-dimethyl
tetralin (DMT) has been investigated over HY zeolite. The dominant reactions undergone by 1-tetralone
are the dehydrogenation to 1-naphthol and the subsequent isomerization to 2-naphthol. In the pres-
ence of hydrocarbons, the hydrogen transfer/dehydration of naphthols is accelerated, and naphthalene
is formed in different amounts, depending on the nature of the co-fed hydrocarbon. In this contribu-
tion, it is demonstrated how the product distribution from the tetralone conversion can be used as an
indicator of the hydrogen transfer ability of a particular hydrocarbon, or mixture of hydrocarbons. The
relative order of hydrogen transfer ability of the various hydrogen donating compounds, as inferred from
the naphthalene-to-naphthol product ratio, is DMT > tetralin ≈ decalin > n-decane. This trend agrees well
with the hydride dissociation energy of individual donors calculated by DFT.
1
6 September 2010
Accepted 17 September 2010
Available online 25 September 2010
Keywords:
Oxygenated aromatics
1
-tetralone
Hydrogen transfer
HY zeolites
FCC
© 2010 Elsevier B.V. All rights reserved.
1
. Introduction
The extent of hydrogen transfer in fluid catalytic cracking (FCC)
ent zeolite catalyst parameters, such as zeolite structure, number
of Al per unit cell, and acidity strength, rather than the intrinsic
properties of the different hydrocarbons that may act as H-donors.
A simple method that can predict a-priori the ability of a given
feed component or a mixture of components may have interesting
applications in FCC technology. This is the focus of this contribution.
Being important components of typical FCC catalysts, HY, REY,
and USHY zeolites have been the subject of many studies related
to cracking and hydrogen transfer [10]. In our previous study on 1-
tetralone conversion on HY zeolites [11], we showed that naphthol
is a dominant product, which can further transform to naphthalene
via hydride transfer-dehydration reactions. It was suggested that
naphthalene formed predominantly in the presence of hydrogen
donor molecules. In this study, we have investigated the reac-
tivity of 1-tetralone in the presence of different hydrogen donor
molecules, which typically exist in FCC feed stream such as paraf-
fins, naphthenics, and aromatics. Using tetralone as an indicator
is highly convenient since it only produces a small number of
products, which facilitates the analysis and the quantification of
the effects of the hydrogen donor. We have selected molecules
with similar number of carbon atoms but with varying functional-
ities, which we anticipate that results in varying hydrogen transfer
capacity [12–16]. The relative rates of the possible conversion paths
of 1-tetralone were found to depend on the hydrogen-donation
capacity of the hydrocarbons co-fed to the reactor. DFT calcula-
processes has an important effect on the product distribution and
catalyst stability [1–4]. While hydrogen transfer has well known
benefits, such as improved liquid yield and reduce coke formation,
an excessive rate of hydrogen transfer may have negative effects. A
major concern is the potential loss of olefinicity in the product by
excessive hydrogen transfer, which has the undesired consequence
of reducing octane number in the gasoline fractions [5–7].
It is widely accepted that, in the transition state, the carbenium
ions on the acid sites can accept hydride ions from hydrogen-donor
molecules and that the extent of this transfer can be modified by
the nature of the catalyst, e.g., density or proximity of acid sites,
presence of rare earth, etc. [8,9]. However, hydrogen transfer is
a bimolecular reaction; therefore, the nature of both the hydride
donor and hydride acceptor can impact the hydrogen transfer pro-
cess. Therefore, it is anticipated that varying the nature of the
hydrogen donor, the extent and rate of hydrogen transfer can be
varied. In the majority of the previous studies about the extent of
hydrogen transfer [8,9], the focus has been on the effect of differ-
∗ Corresponding author. Tel.: +1 405 325 4370; fax: +1 405 325 5813.
E-mail address: resasco@ou.edu (D.E. Resasco).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.09.019

T. Prasomsri et al. / Applied Catalysis A: General 389 (2010) 140–146
141
Table 1
tions were used to provide a correlation between the energy of
hydride dissociation and the extent of hydrogen transfer observed
experimentally.
Catalytic reaction of pure compounds over HY zeolites.
Feed
1-Tetralone n-Decane Decalin Tetralin DMT
Conversion
97.7
9.4
19.3
26.3
100.0
2
. Experimental
Product yields (wt%)
C1–C6
Cyclic olefins (C6–C10)
Alkylbenzenes
n-Decane
trans-Decalin
cis-Decalin
Tetralin
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
4.4
2.3
74.0
11.3
8.1
9.4
0.0
0.0
90.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
10.7
6.1
2.6
0.0
65.4
15.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
3.5
3.3
5.9
0.0
0.0
0.0
73.7
0.0
0.0
13.6
0.0
0.0
0.0
0.0
32.1
0.0
44.1
0.0
0.0
0.0
0.0
0.0
23.8
0.0
0.0
0.0
0.0
0.0
2.1. Catalytic activity testing
The model hydrogen donors used in this study were n-decane,
decalin, tetralin, and 1,5-dimethyl tetralin (DMT), while 1-tetralone
was used as a hydrogen acceptor. These compounds are fairly heavy
and their boiling points varied in a wide range. To avoid using a sol-
vent that might interfere with the process of hydrogen transfer and
minimize condensation in the system, the feed was introduced to
the reactor using a vapor saturation vessel made of glass through
which the carrier gas was bubbled. By adjusting the temperature of
the saturator, the vapor pressure of the individual compounds can
be readily controlled. When mixtures were co-fed into the reactor,
each component was controlled independently in a separate satu-
rator. The feed rate was regulated by the carrier gas flow, and an
additional inert gas line was added to keep the total gas flow rate
constant.
1,5-Dimethyltetralin
Alkylnaphthalenes
Naphthalene
1-Tetralone
1-Naphthol
2-Naphthol
Coupling products
Reaction conditions: 450 C, 1 atm He, HY (Si/Al = 15), W/F = 1.6 h, TOS 0.5 h.
calculations for the doublet states of the radicals (i.e., protonated
species) employed an unrestricted formalism. This study was com-
puted at B3PW91 density functional with using the 6–31 + G(d,p)
basis set, which were claimed to provide a good estimation for
hydrogen transfer reactions [19].
Since, as discussed below, naphthol plays a central role in the
hydride transfer mechanism [11], it was important to determine
which protonated intermediate is most likely to form. Using DFT,
the proton affinity (PA) of naphthol in different protonated struc-
tures was compared, according to the expression [20,21]:
ꢀ
ꢀ
The reactions were conducted in a ¼ stainless steel reactor.
Commercial HY zeolite from Zeolyst International (Si/Al = 15 and
4
0) used as the catalyst was placed at the center of the reactor.
Since the zeolites were purchased in the proton form, the only
pretreatment needed was to heat them up in-situ under He flow
at 450 C for 1 h to remove adsorbed water or any other contam-
inants. The top and bottom parts of the packed bed were filled
with 3 mm-diameter glass beads. The operating conditions were as
follows: atmospheric pressure, 450 C, He carrier gas, space-time,
W/F = 1.6 h, with respect to the mass flow rate of 1-tetralone in
both pure and mixed feeds. Prior to each run, the feed was sent
through the by-pass line until the concentration stabilized. At this
point, it was switched to enter the reactor. The carbon mass balance
was checked for every run by the analysis of the by-pass and the
reactor outlet. The reaction products were sampled every 0.5 h on
stream and analyzed on-line using an HP5890 gas chromatograph,
equipped with an HP-5 column and an FID detector. A carbon ele-
mental analyzer was used to measure the amount of coke deposits
on the catalysts.The acid properties of the various zeolites were
characterized by temperature programmed desorption of ammo-
◦
+
+
◦
PA = −{H(C10H O ) − [H(C H O) + H(H )]}
(1)
9
10
8
+
where H(i) is the enthalpy of species i, being C10H O = the proto-
9
nated naphthol in each specific structure; C10H O = naphthol; and
8
+
H = proton. That is, a large positive PA indicates a stable compound.
To compare the experimentally observed trends with the theo-
retical predictions, hydride dissociation energies of various donor
molecules were calculated as follows:
+
Edis = {[H(R ) + H(hydride)] − H(R − H)}
(2)
+
where R − H is the donor; and R is a corresponding carbenium ion.
The relative activation energy of hydride dissociation (E ) can
A
be expressed by a ratio of hydride dissociation energy (Edis) for
each donor molecule over that for the C–H breaking of methane
482.6 kJ/mol);
nia (NH -TPD), conducted in a 0.25 in. o.d. quartz reactor. Before
each experiment, the 50 mg of zeolite sample was pretreated for
3
(
◦
0.5 h in flowing He (30 mL/min) at 600 C to eliminate any adsorbed
◦
Edis(x)
water. Then, the temperature was lowered to 100 C, and the sam-
ple exposed to a 2% NH /He, 30 mL/min stream for 30 min to reach
saturation. After exposure, pure He was passed for 0.5 h over the
sample to remove any weakly adsorbed NH . To conduct the TPD,
EA(x) =
(3)
Edis(CH4)
where x is a particular donor molecule.
3
3
A lower EA value indicates a better hydrogen transfer ability.
In this simplified analysis, proton affinity and relative activation
energy values are taken as intrinsic properties of the individual
molecules and were determined in the gas phase basis, indepen-
dent of their interaction with the solid catalyst.
◦
the temperature was increased at a heating rate of 10 C/min up to
◦
6
50 C. The evolution of desorbed species was continuously mon-
itored by a Cirrus mass spectrometer (MKS) recording the signals
m/z = 17 and 16 (corresponding to NH ). The density of acid sites
was quantified by calibrating the MS signals with the average of ten
3
5
-mL-pulses of 2% NH /He.
3
3. Results
2.2. Computational methods
3.1. Catalytic reaction of pure compounds over HY zeolites
To correlate the different extent of hydride transfer observed
The total conversion and product distribution resulting from
feeding the pure compounds (1-tetralone, n-decane, decalin,
tetralin, and DMT) onto the HY (Si/Al = 15) zeolite are shown
in Table 1. Observed cracking products are linear and branched
alkenes/anes (C –C ), cyclic olefins (C –C10), and alkylbenzenes
experimentally for the various compounds with their intrinsic
molecular properties, density functional theory (DFT) calculations
were carried out using the Gaussian 03 suite of programs [17].
Molecular geometries and zero-point energies (ZPE) for all species
were optimized by the Berny analytical gradient method [18], and
verified to be local minima by computing the Hessian matrix. DFT
1
6
6
(e.g., ethylbenzene, toluene, and xylene). It is seen that DMT and 1-
tetralone are significantly more reactive than the other compounds

1
42
T. Prasomsri et al. / Applied Catalysis A: General 389 (2010) 140–146
Table 2
Table 3
Overall yields from catalytic reactions of mixture of hydrocarbons and 1-tetralone
over HY zeolites.
Conversion and selectivity from catalytic reactions of mixture of hydrocarbons and
1-tetralone over HY zeolites.
Co-feed
n-Decane
Decalin
Tetralin
DMT
Co-feed
n-Decane
98.1
Decalin
Tetralin
DMT
Product yields (wt%)
C1–C6
Cyclic olefins (C6–C10)
A Iky I benzenes
n-Decane
trans-Decalin
cis-Decalin
Tetralin
Conversion of 1-tetralone
Conversion of hydrocarbons
95.0
14.7
96.0
57.3
94.7
98.4
2.3
0.0
0.0
49.4
0.0
0.0
0.0
0.0
0.0
5.1
0.9
31.1
8.9
2.4
1.7
2.0
2.7
0.0
35.1
9.3
1.2
0.0
0.0
27.3
2.4
1.5
1.3
2.2
0.0
0.0
0.0
23.4
0.0
0.0
52.5
1.8
5.4
0.0
5.1
0.0
0.0
0.0
0.0
0.9
44.2
34.5
2.4
5.6
0.7
1.2
4.4
Selectivity of 1-tetralone products (wt%)
Naphthalene
10.7
65.4
18.8
5.1
59.7
27.4
3.9
9.0
1.91
60.3
25.3
8.3
6.0
1.79
82.1
13.4
1.6
2.9
5.47
1
2
-Naphthol
-Naphthol
Coupling products
Naphthalene/Naphthol ratio
0.13
1
,5-Dimethyltetralin
Alkyl naphthalenes
Naphthalene
Selectivity of hydrocarbons products (wt%)
C1–C6
Cyclic olefins (C –C10)
Alkylbenzenes
n-Decane
trans-Decalin
cis-Decalin
Tetralin
100.0
0.0
0.0
–
0.0
0.0
0.0
0.0
0.0
0.0
22.3
26.2
35.6
0.0
–
4.9
4.3
7.0
0.0
0.0
0.0
–
0.0
0.0
83.8
9.8
0.0
9.4
0.0
0.0
0.0
0.0
–
1
1
2
-Tetralone
-Naphthol
-Naphthol
6
12.5
1.8
4.1
11.0
3.6
2.6
Coupling products
–
Reaction conditions:
Si/Al = 15), W/F = 1.6 h, TOS 0.5 h.
%
co-fed concentration = 50 ± 5%, 450 C, 1 atm He, HY
15.8
0.0
0.0
0.0
(
1,5-Dimethyltetralin
Alkylnaphthalenes
Naphthalene
80.8
0.0
in the series. The crackability, i.e., conversion to lighter products,
follows the order DMT > decalin > tetralin > n-decane. As expected,
the presence of tertiary carbons in DMT and decalin increases the
crackability of these molecules, as previously observed [22–24].
Dehydrogenated products (i.e., alkylnaphthalenes) were obtained
from DMT, but in a lower extent. Some naphthalene was also
obtained from dehydrogenation of tetralin, but not from decalin.
In contrast with the hydrocarbons, a major part of 1-tetralone
conversion proceeded through dehydrogenation to form naph-
thols, without cracking. This is consistent with our previous study
Reaction conditions:
(Si/Al = 15), W/F = 1.6 h, TOS 0.5 h.
%
co-fed concentration = 50 ± 5%, 450 C, 1 atm He, HY
The integrated amounts of coke formed over 1 h on stream are
rather low (Table 4). Therefore, it can be expected that the contri-
bution of to hydrogen from coke to the total to hydrogen transfer is
negligible compared to the significant changes observed in product
distribution.
In parallel to the changes in 1-tetralone products as a con-
sequence of the enhanced hydrogen transfer, changes in overall
conversions and product distribution from the individual hydrocar-
bons were clearly observed. For example, in the conversion of DMT
in the presence of tetralone, dehydrogenated products (i.e., alkyl-
naphthalenes) became dominant rather than cracking products,
which dominated in the pure DMT feed reaction. Similar behavior
was observed with tetralin. In this case, the conversion significantly
increased with a higher selectively towards the dehydrogenated
product, naphthalene. The dehydrogenation of DMT and tetralin
can be ascribed to their intrinsic hydride donor ability, which is
promoted in the presence of the hydride acceptor, 1-tetralone.
In contrast to DMT and tetralin, no significant dehydrogenation
products (i.e., tetralin or alkylbenzene) were obtained from decalin
despite the high naphthalene/naphthol ratio observed in the par-
allel 1-tetralone reaction. In fact, this ratio was as high as that
obtained when co-feeding tetralin. This suggests that the hydrogen
transfer occurs from the cracking products, most of which end up as
cycloalkenes and alkylbenzenes. Also, isomerization of (cis-, trans-)
decalin, which involves hydrogen exchange but not net hydrogen
donation may accelerate the hydrogen transfer process for the con-
version of 1-tetralone into naphthalene and water. Therefore, a
[
11], in which dehydrogenation of 1-tetralone to 1-naphthol was
found to be the dominant reaction pathway, and isomerization of
1
atures (i.e., >400 C). Only a small amount of naphthalene can be
produced from 1-tetralone when fed alone.
The extent of overall dehydrogenation follows the order 1-
tetralone > DMT > tetralin. That is, it appears that the presence of
the aromatic ring and substituted groups (e.g., methyl) on the naph-
thenic ring readily improves the hydrogen transfer ability of the
molecule, leading to an increased degree of dehydrogenation [25].
It can be seen that coupling products (e.g., multi-ring oxygenates)
are only produced from 1-tetralone.
-naphthol toward 2-naphthol was observed only at high temper-
◦
3
.2. Catalytic reaction of hydrocarbon with co-fed 1-tetralone
over HY zeolites
In this experiment, 1-tetralone was co-fed with the hydrocar-
bons, i.e., n-decane, decalin, tetralin, and DMT. The overall yields
from catalytic reactions of the mixture feeds are shown in Table 2.
Significantly different conversions and product selectivities, as
compared with those from the pure feeds, were observed for each
corresponding hydrocarbon (Table 3). Although the conversion of
1
-tetralone remained close to 100%, the product selectivity, par-
Table 4
ticularly towards naphthalene, was very sensitive to the nature
of the hydrocarbon co-fed in to the reactor. It appears that,
in the presence of a hydrocarbon that may act as a hydrogen
donor, the naphthols deriving from 1-tetralone behave as hydro-
gen acceptors and subsequently undergo dehydration to produce
naphthalene, as previously proposed [11]. The extent of this trans-
formation depends on the co-fed hydrocarbon. Therefore, the
intrinsic hydride transfer ability of a particular hydrocarbon species
can be represented by the naphthalene-to-naphthol ratio derived
from tetralone (Table 3). A clear trend is observed in this ratio,
DMT > tetralin ≈ decalin > n-decane. It is likely that a relatively rapid
formation of coke occurred at the beginning of the run (i.e., ∼first
Amount of coke on spent catalysts determined by an ele-
mental analyzer.
Feed
Coke (%)
Tetralone
n-Decane
Decalin
Tetralin
1.28
0.98
1.16
1.77
1.59
1.96
2.21
2.07
1.12
DMT
n-Decane + Tetralone
Decalin + Tetralone
Tetralin + Tetralone
DMT + Tetralone
1
0 min), whereas a quasi-plateau in activity was obtained later [26].
Reaction conditions: 450 C, 1 atm He, HY (Si/Al = 15).

T. Prasomsri et al. / Applied Catalysis A: General 389 (2010) 140–146
143
8
6
4
2
0
0
0
0
0
0.25
2.5
2.0
1.5
1.0
0.5
0.0
a
b
0.20
0.15
0.10
0.05
HY(Si/Al=15)
HY(Si/Al=40)
40
0.00
0
20
60
0
20
40
60
%
Co-fed decalin
% Co-fed decalin
Fig. 1. Effect of % co-fed decalin on (a) yield of naphthalene (NP); and (b) 2-naphthol to 1-naphthol ratio, and NP to naphthol ratio . Reaction conditions: 450 C, 1 atm He,
TOS = 0.5 h, HY (Si/Al = 15) (ꢁ), HY (Si/Al = 40) (᭹).
high yield of naphthalene was observed from 1-tetralone without
a prominent change in the product distribution of decalin.
In the case of n-alkanes co-feed, their weaker hydrogen-donor
ability had little effect in the naphthalene/naphthol product ratio
of 1-tetralone (i.e., only a slight increase from about 0.05 for pure
increasing either the concentration of decalin (Fig. 1a) or the
acidity of the catalyst (Fig. 2a). The contribution to naphthalene
from decalin dehydrogenation is negligible under the reaction
conditions investigated. Therefore, the enhanced concentration of
naphthalene product can be solely ascribed to the tetralone con-
version path, enhanced by hydrogen transfer. Since, in this path,
hydrogen transfer enhances the conversion of naphthol to naph-
thalene, a good indicator of the extent of hydrogen transfer is
the naphthalene/naphthol ratio. This ratio clearly increases with
either increasing concentration of the hydrogen donor (Fig. 1b) or
increasing acidity of the catalyst (Fig. 2b). At the same time, since
conversion of naphthol to naphthalene competes with isomeriza-
tion, the 2-naphthol/1-naphthol ratio was found to decrease with
increasing decalin concentration (Fig. 1b). By contrast, when the
acid density increases, both 1-naphthol isomerization and naph-
thalene production (i.e., hydrogen transfer) increase (Fig. 2b), as
expected [27,28].
1
-tetralin alone to 0.13 for the mixed feed with added n-decane).
It can be suggested a small amount of hydrogen transfer from n-
decane can take place, essentially associated with cracking and coke
formation. However, the cracking of both n-decane and decalin,
lacking aromatic structure, is significantly suppressed in the pres-
ence of tetralone, due to a competitive adsorption of the oxygente.
At the same time, the yields of coupling products observed
from all of the mixture feeds were significantly lower than that
with pure 1-tetralone. This can be explained, again, by enhanced
hydrogen transfer from the co-fed hydrocarbon, as well as by the
dilution effect. The latter may be more important with the weaker
hydrogen-donors (i.e., n-decane). However, a part of cracking prod-
ucts from decalin may participate in coupling formation, and result
in a higher yield as compared to others.
3.4. Proton affinity of naphthol from DFT calculation
3
1
.3. Effect of decalin concentration on the product distribution of
-tetralone conversion over HY zeolites with varying acidity (Si/Al
As mentioned above, naphthalene formation occurs via the acid-
catalyzed dehydration of naphthol. Therefore, it is important to
determine what kind of protonated naphthol species is the most
probable structure. Fig. 3 illustrates various possible protonated
structures of naphthol and their corresponding proton affinity,
which can be taken as a measure of their stability, i.e., the larger
positive value the more stable. The various protonated species
tested include the protonated keto-forms of 1-naphthol (1) and 2-
ratio 15 and 40)
In addition to varying the nature of the hydrogen donor, we
investigated the effect on hydrogen transfer of varying the con-
centration of the hydrocarbon as well as the acidity density of
the catalyst. Increases in naphthalene yield were observed by
Fig. 2. Effect of acidity on (a) yield of naphthalene (NP); and (b) 2-naphthol to 1-naphthol ratio, and NP to naphthol ratio. Reaction conditions: co-fed concentration = 50 ± 5%,
50 C, 1 atm He, TOS = 0.5 h.
4

1
44
T. Prasomsri et al. / Applied Catalysis A: General 389 (2010) 140–146
have low PA values. The favorable configuration of the protonated
keto-forms difference could be ascribed to the direct conjugation
of the positive charge on the oxygen atom with the ␲-system of the
aromatic ring. Hence, the high electron density in the ring makes it
favorable to shift towards the protonated site, enhancing stability
of the oxonium ion species [20,21]
1
(900)
2 (910)
3 (780)
3
.5. Hydrogen transfer ability of hydrocarbon species from DFT
calculations
The hydride dissociation energies of various donor molecules
calculated according to the Eqs. (2) and (3), relative to the C–H bond
breaking in methane are shown in Fig. 4 for different positions in
each of the hydrogen donor molecules investigated. We can antici-
pate that the lower is the dissociation energy for a given molecular
position, the more effective the hydrogen transfer should be. For
a particular molecule, the energy required to dissociate the first
hydride is lowest at the tertiary carbon, e.g., those in decalin and
methyl tetralin, while the terminal carbon shows highest energy,
e.g., those in n-decane, as expected [29,30]. The dissociation from
secondary carbons is relatively less probable as compared to that
from tertiary carbons. Finally, the probability is particularly low
4
6
(780)
(750)
5 (760)
7 (760)
(i.e., highest activation energy) for those from the aromatic ring, due
to the higher electron density of carbons in the aromatic structure.
Fig. 3. Possible C10H9O+ ions. Protonation of naphthol is compared for the keto-
forms of 1-naphthol (1) and 2-naphthol (2) to form carbenium ions; the oxiranium
cation (3); and the enol-forms of 1-naphthol (4,5) and 2-naphthol (6–7). The corre-
sponding PAs in kJ/mol are shown in the parentheses.
2
The hydrogen on the aromatic ring is stabilized by sp hybridization
and least likely to be removed. By contrast, the hydrogen disso-
ciation from the benzylic carbon is preferentially favorable since
the positive charge formed can be stabilized by the delocalized ␲-
electrons of the resonance structure. This effect is more pronounced
by combining this effect with the presence of a tertiary carbon, as
observed in methyl tetralin. According to the calculations, methyl
tetralin should be the best hydrogen donor. Indeed, the trend of
hydrogen transfer ability (DMT > tetralin ≈ decalin > n-decane) pre-
dicted according to the experimental naphthalene/naphthol ratios
is consistent with the calculated relative activation energies for
hydride dissociation.
naphthol (2) to form carbenium ions; the oxiranium cation (3); the
protonated enol-forms of 1-naphthol (4, 5) and 2-naphthol (6, 7).
The results of the calculations show that protonated keto-forms
are the most favorable species (PA = 900 and 910 kJ/mol for (1) and
2), respectively). By contrast, the species protonated at C C bond
enol-form of both 1- and 2-naphthol (4, 6)), as well as that proto-
(
(
nated at the oxygen atom (5, 7) are less likely to be formed, i.e., they
Fig. 4. Relative activation energy of hydride dissociation at different locations of the donor molecules. The calculated values relative to methane, and computed by DFT.

T. Prasomsri et al. / Applied Catalysis A: General 389 (2010) 140–146
145
4
. Discussion
feeds or predict what changes in olefinicity and octane number one
could expect when the feed is changed.
The catalytic conversion of 1-tetralone yields 1-naphthol as
a primary product. Over strong acid sites, 1-naphthol can be
isomerized to 2-naphthol, most probably via an oxiranium cation
DMT performed as the best hydrogen donor since it contains
hydrogen on a tertiary benzylic carbon, which shows low relative
activation energy for hydride dissociation (EA ∼ 0.693, Fig. 4). Like-
wise, tetralin exhibits a high rate of hydrogen transfer due to the
presence of benzylic hydrogens (EA ∼ 0.719). Both DMT and tetralin
convert mainly to their corresponding dehydrogenated product
when co-fed with 1-tetralone. Due to the presence of tertiary
carbons in its structure, decalin also possesses low relative acti-
vation energy for hydride dissociation (EA ∼ 0.725), but unlike the
molecules containing one aromatic ring, decalin facilitates hydro-
gen transfer via isomerization (cis- and trans-). The small increase in
the production of alkylbenzene from decalin is also an indication of
(
3) as shown in the scheme below. At the same time, among
all of the possible protonated naphthol species, the most stable
structure is then most probable participant in the hydride transfer
process. Therefore, the two keto-forms of naphthols (1,2) can the
protonated species that can act as hydride acceptors, leading to
dehydrogenation and formation of naphthalene. Consequently,
the rate of isomerization is low since the oxiranium cation (3) is
less stable and therefore less probable in the intermediate state
than the keto-form species.
Depending on their hydrogen transfer ability, hydrocarbons can
donate a hydride to the carbonyl carbon of the protonated keto-
form of naphthol, generating the corresponding alcohol which can
be rapidly dehydrated to produce naphthalene, as summarized in
the scheme above. Since dehydration is relatively fast over acid
catalysts [31], the rate at which naphthalene is produced can be
determined by the rate of hydrogen transfer. The hydrocarbon act-
ing as hydrogen donor is also converted according to their hydrogen
transfer ability, giving rise to products that are also enhanced by
the hydrogen transfer reaction, i.e., dehydrogenation and skele-
ton isomerization. From the applications point of view, the yield
of naphthalene from 1-tetralone can be taken as a measure of the
hydrogen transfer. So, the product distribution of 1-tetralone con-
version can be used as an indicator of the hydrogen transfer ability
of a particular hydrocarbon or mixture of hydrocarbons for a given
catalyst. A simple method like this could give refiners a tool to select
hydrogen transfer. When co-feeding weak hydrogen donors, such
as n-decane, the effects are much less apparent. Moreover, the com-
petitive adsorption of 1-tetralone and its products on the surface
readily suppress the overall conversion of n-decane. This inhibition
is due to strong adsorption of 1-tetralone and naphthol on the acid
sites of the zeolite compared to a rather weak adsorption of the
paraffin, lowering its reaction rate.
5. Conclusion
The acid-catalyzed reaction of 1-tetralone produces 1-naphthol
as primary product, followed by secondary isomerization of 1-
naphthol to 2-naphthol. The presence of hydrocarbons in the feed,
which can act as a hydrogen donor, promotes hydrogen trans-
fer reaction to the surface protonated naphthol, facilitating the
dehydration of the corresponding alcohols to form naphthalene.

1
46
T. Prasomsri et al. / Applied Catalysis A: General 389 (2010) 140–146
Therefore, the yield of naphthalene from 1-tetralone can be taken
as a measure of the hydrogen transfer. So, the product distribu-
tion of 1-tetralone conversion can be used as an indicator of the
hydrogen transfer ability of a particular hydrocarbon or mixture
of hydrocarbons. Both, the experimental naphthalene/naphthol
ratios obtained experimentally and the DFT calculations show that
the hydrogen transfer ability of hydrocarbon species follows the
order DMT > tetralin ≈ decalin > n-decane. It has been shown that
the hydrogen transfer reaction is also sensitive to acid site density
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Acknowledgements
This work has been supported, in part, by the NSF EPSCoR award
EPS 0814361 and by the Oklahoma Center for the Advancement
of Science and Technology (OCAST). Generous allocations of com-
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