N-Hexane activation over zeolites: Aromatic alkylation to 1-phenylhexane

Article abstract of DOI:10.1039/b922789j

Terminal 1-phenylhexane was observed during alkylation of benzene with n-hexane over 10-membered ring zeolites at moderate temperature, whereas it was not observed when 1-hexene was the reactant. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b922789j

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n-Hexane activation over zeolites: aromatic alkylation to 1-phenylhexane
Nadiya Danilina, Elisabeth L. Payrer and Jeroen A. van Bokhoven*
Received (in Cambridge, UK) 30th October 2009, Accepted 8th December 2009
First published as an Advance Article on the web 23rd December 2009
DOI: 10.1039/b922789j
Terminal 1-phenylhexane was observed during alkylation of
benzene with n-hexane over 10-membered ring zeolites at
moderate temperature, whereas it was not observed when
1-hexene was the reactant.
benzene with propane over bifunctional catalysts, and its
formation was explained by the isomerization6,9 of cumene
and by transalkylation by a bifunctional mechanism.17–19
We alkylated benzene with n-hexane over ZSM-5 and
observed a significant fraction of the terminal alkylated
product, 1-phenylhexane. The alkylation reaction was
performed at 205 1C under autogeneous pressure in a batch
reactor at a benzene-to-hexane ratio of one. After 24 h, the
reaction mixture was quenched in cold water and analyzed by
GC/MS. Various zeolites of different pore sizes and including
a (de)hydrogenation function were compared.w Table 1 shows
the sample characterization and the results of the alkylation
reaction. Over H-ZSM-5, 20% of the n-hexane reacted. The
selectivity to alkylation products was 34%, about a third of
which was 1-phenylhexane. The rest of the products were
cracking and multialkylated products. 3-Phenylhexane was
not observed; its formation was probably suppressed by the
spatial restraints in the pores of ZSM-5. For alkylation over
zeolites with larger pores, such as mordenite and faujasite,
considerable amounts of 3-phenylhexane were observed. No
phenylhexanes were detected during the reaction without a
catalyst or during reaction of pure benzene or pure n-hexane,
indicating that phenylhexanes originate from both reactants.
To understand how 1-phenylhexane formed, the reaction
was carried out over the less acidic Na-ZSM-5 and metal-
modified Pt/H-ZSM-5. The Si/Al ratio, the BET surface area,
and the microporosity of those samples were very similar
(Table 1). The conversion of n-hexane was about 30%
for both samples. This increase in conversion compared
to H-ZSM-5 can be explained by increased cracking and
isomerization over Na-ZSM-5 and Pt/H-ZSM-5, respectively.
No alkylation products were observed over Na-ZSM-5,
suggesting that acid sites catalyze the alkylation. The presence
of Pt induced the isomerization of n-hexane, suppressed multi-
alkylation, and increased the selectivity to 2-phenylhexane.
The increase in the formation of 2-phenylhexane probably
originated from the bifunctional activation of n-hexane.
Reacting n-hexane and benzene over metal-free zeolites of
different pore size showed that 10-membered ring zeolites
(MFI, TUN) showed higher selectivity to 1-phenylhexane than
12-membered rings (MOR, FAU) (Table 1). Obviously,
shape selectivity plays an important role in the formation of
1-phenylhexane.
Alkylation of aromatics and cyclic molecules is an important
chemical reaction to produce fine chemicals, dyestuffs,
detergents, and scents.1 In many industrial processes alkylation
reactions are still performed in the presence of the toxic and
corrosive liquids, hydrofluoric and sulfuric acids. To make
alkylations more effective, environmentally friendly, and
cheaper, research has been devoted to replacing liquid acids
by zeolites. Today, heterogeneously catalyzed alkylation of
aromatics with olefins is a well-developed process.2,3 Because
of the fast oligomerization of olefins and subsequent catalyst
coking, it is necessary to work at low olefin-to-aromatic ratios.
When alkylation takes place with olefins, terminal alkylation
products do not form, because the primary carbenium ion
intermediate is much less stable than the secondary and
tertiary ion intermediates.
Replacing alkanes by olefins in aromatic alkylation leads to
reduced coke formation, enables the process to proceed at a
stoichiometric aromatic/alkylation agent ratio, simplifies
reactor design, decreases the cost of the feed, and may enhance
catalytic stability. Because alkanes are much more difficult to
activate than olefins, super acids and bifunctional catalysts are
required. The synthesis of alkylaromatics from alkanes and
aromatics in superacidic media4 and over zeolites at moderate
temperature has been reported.5–15 In homogeneously catalyzed
reactions, C2–C5 alkanes were used as alkylating agents;
conversions of up to 10% were reached after six hours at
ambient temperature.4 Tertiary, secondary, and primary C–H
and C–C bonds underwent protolysis.16 The formation of the
terminal product was explained by the reaction of protonated
benzene with alkane by an alkylation-cleavage mechanism.16
The heterogeneously catalyzed aromatic alkylation with
alkanes has not been studied in depth. Until now,
ethane5,6,13–15 and propane6–10,12 have generally been used.
Over some bifunctional Pt-7,8 and Ga-modified9,10 ZSM-5
zeolites, there was considerable conversion of propane,
however, the yield was usually quite low. The rationale
for using a bifunctional catalyst is to dehydrogenate the alkane
on the metal and to perform alkylation by means of a
carbenium ion intermediate adsorbed on the acid site of a
zeolite. n-Propylbenzene was observed during the alkylation of
There are a few possible explanations for the formation of
terminal alkylation products, notably by (a) the formation and
reaction of a primary carbenium ion on the alkane, (b) the
isomerization of secondary alkylaromatics, (c) ring opening of
cyclohexyl benzene, (d) transalkylation, (e) a radical mechanism,
and (f) the formation and reaction of a benzenium ion
(Wheland intermediate).20 We performed preliminary studies
ETH Zurich, Institute for Chemical and Bioengineering,
Wolfgang-Pauli-Str. 10, HCI, Zurich, Switzerland.
E-mail: j.a.vanbokhoven@chem.ethz.ch; Fax: +41 44 6331361;
Tel: +41 44 6325542
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Table 1 Characterization and alkylation of benzene with n-hexane over the zeolite samples
Selectivity (%)
BET surface
Conversion
1PHb 2PHc 3PHd CHBe Isomerizationf Cracking Higherg
Sample
Si/Al area/m2 gꢁ1 Vmicro/cm3 gꢁ1 (%)a
H-ZSM-5
Na-ZSM-5
Pt/H-ZSM-5 25
H-TNU-9
H-MOR
H-Y
24
25
400
400
370
334
508
493
0.12
0.12
0.13
0.13
0.18
0.18
20
28
30
22
16
21
13
0
13
21
3
21
0
37
2
47
19
0
0
1
0
10
10
1
0
1
0
9
10
0
0
22
0
0
0
12
100
15
76
23
53
0
11
0
8
37
8
30
2.6
0
24
a
b
c
d
e
f
g
Conversion of n-hexane. 1-Phenylhexane. 2-Phenylhexane. 3-Phenylhexane. Cyclohexylbenzene. To methylpentanes. Multialkylated
products.
to determine the possible mechanism. The formation of a
Notes and references
carbenium ion from the alkane requires its protonation and
w H-ZSM-5 (PZ-2/50 H) and Na-ZSM-5 (PZ-2/50 Na) were obtained
from Zeochem, Switzerland. Pt/H-ZSM-5 was synthesized by wet
dehydrogenation. Alkylation with olefins is known to proceed
by this intermediate. Thus, if the alkylation of benzene with
n-hexane takes place by this mechanism, then we expect
1-phenylhexane to form during alkylation with an olefin.
During alkylation of benzene with 1-, 2-, or 3-hexene, we
never observed the formation of 1-phenylhexane. Furthermore,
monomolecular activation of n-hexane to hexenes at 205 1C
was not observed; thus, the presence of benzene aids the
activation of n-hexane. The hypothesis that 1-phenylhexane
forms by isomerization of 2- or 3-phenylhexane was tested by
exposing H-ZSM-5 to 1-phenylhexane or to a mixture of
2- and 3-phenylhexane at 205 1C. Isomerization did not occur.
To test the ring opening route, cyclohexyl benzene was
reacted.z No 1-phenylhexane was detected in the product
mixture. Transalkylation, by which n-propylbenzene forms
from cumene and benzene,17–19 is unlikely. We reacted a
mixture of 2- and 3-phenylhexane with benzene and found
that 1-phenylhexane did not form. To investigate the radical
nature of the reaction, reactions were performed in the
presence of potassium persulfate, iodine, and hydroxyquinone
as radical scavengers or propagators. The addition of potassium
persulfate and iodine did not affect the reaction significantly,
whereas the addition of hydroxyquinone decreased the
conversion and selectivity towards 1- and 2-phenylhexane. A
decrease in radicals and poisoning of the acid sites by basic
hydroxyquinone may explain this. The aromatic carbenium
activation of alkane requires acid sites, which we showed to be
crucial for the formation of 1-phenylhexane. The reaction
of benzene alone gave biphenyl, showing that benzene is
activated on the zeolite. Furthermore, hydrogen–deuterium
exchange on the aromatic ring over acidic zeolites is rapid at
200 1C.21–23 This reaction occurs by means of a benzenium
intermediate, which we assume is present in the reaction
mixture. The alkylation of benzene with propane by a
benzenium ion has been discussed elsewhere.1,7,8
impregnation of H-ZSM-5 with [Pt(NH3)4](NO3)2 solution,
subsequent drying in air at 300 1C and reduction in H2 at 250 1C. Pt
loading was 2.3 wt%. The average Pt particle size was 1.6 nm. H-MOR
(HSZ660HOA2) was provided by Tosoh Corp., Japan. H-Y was
obtained by calcination of CBV600, obtained from Chevron.
H-TNU-9 was kindly provided by Suk Bong Hong from the Hanbat
National University in Taejon, South Korea.
z Cyclohexylbenzene was detected in the reaction mixture over some
catalysts (Table 1). It was shown experimentally that it can form from
benzene only.
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This journal is The Royal Society of Chemistry 2010
1510 | Chem. Commun., 2010, 46, 1509–1510