M. K. Gnanamani et al.
authors to believe that 1,6 ring closure was the preferred
mechanism. However, Sivasankar and Padalkar [14] con-
sidered 1,5 ring closure in addition to 1,6 ring closure for
dehydrocyclization of n-hexane and n-heptane. There are
number of factors which determine the product selectivity
obtained from dehydrocyclization of n-alkanes over a
particular catalyst. Most of the research to date has been
devoted to studies of the mechanism of dehydrocylization
of n-alkanes by comparing with 1- or 2-substituted iso-
alkanes in order to define the effect of those alkyl sub-
stituent(s) on ring closure. Hanch [10] discussed the
dehydrocylization reaction that involved polynuclear aro-
matics and several heterocycles that contain nitrogen,
oxygen, or sulfur compounds. However, studies of the
effect of functional groups in the alkyl chain of the C6
hydrocarbon for the dehydrocyclization reaction are rare.
Cornet and Gault [15] used 10 % Pt or Pd/pumice catalyst
to study the dehydrocylization of methyl propyl ketone.
These authors found a maximum yield of about 2 % of
purchased from Sigma Aldrich with 99.9 % purity. Typi-
cally, 0.50 g of calcined Pt/(Na)-Al O catalyst was packed
2
3
between the two layers of glass wool. The temperature of
the catalyst bed was monitored by a thermocouple (Fe–Cr)
and maintained by a temperature controller (Omega CN
3251-R). Prior to testing, the catalyst was first reduced in
H (50 sccm) at 500 °C for 2 h. Products were condensed
2
using a cold trap maintained at 5 °C and the effluent gases
were analyzed using a micro GC with a TCD detector.
Liquid samples were collected at intervals and analyzed by
gas chromatography using a DB-5 or SPB-5 column. A
5973 N MSD coupled to the 6890 GC (GC–MS) from
Agilent was employed in the product identification.
3 Results and Discussion
Table 1 shows the dehydrocyclization of n-hexane over Pt/
(Na) Al O catalyst. The percentage conversion of n-hex-
2
3
2
-methyl furan at 340 °C and concluded that the acetone
ane decreased from 49.5 to 30.4 during 3 h on-stream. The
corresponding data are displayed in Table 1. Benzene is the
primary dehydrocyclized product obtained from n-hexane
and the cracking products like methane and C –C
that is formed by the hydrogenolysis of methyl propyl
ketone could easily poison the active metal sites. In this
study, the effect of the presence of a hydroxyl or keto
substituent at C-2 position in the C6 hydrocarbon was
evaluated for dehydrocylization activity and selectivity and
the results are compared with those of n-hexane. For this,
2
5
hydrocarbons are also observed initially which then
declined slowly with time. The n-hexane also undergoes
dehydrogenation to form 1- and 2-hexenes and isomeriza-
1
.2 % Pt–1.0 % Na/c-Al O catalyst was used. 1.0 wt% Na
tion to i-C . The distribution of the cyclized products
6
2
3
was introduced before adding Pt to minimize the acidity of
obtained with n-hexane are shown in Fig. 1. Benzene was
found to be the major cyclic product whose contribution
varied between 70 and 78 %. The fraction of cyclopentane
and methyl cyclopentanes among the cyclic products
increased from 20 to 30 % with increasing time-on stream
with only a negligible amount of toluene being formed
under these experimental conditions.
the commercial c-Al O support.
2
3
2
Experimental
The sodium containing Pt/alumina catalyst (intended cat-
alyst composition: 1.2 wt% Pt–1.0 wt% Na/Al O ) was
Table 2 shows conversion and the product selectivity
data for 2-hexanol over Pt/(Na) Al O catalyst. The initial
2
3
prepared by the incipient-wetness impregnation (IWI)
using an aqueous solution of tetrammine Pt(II) nitrate,
sodium carbonate as platinum and sodium sources,
respectively. c-Al O (Sasol Catalox-150) was used as
2
3
conversion of 2-hexanol after an hour of time-on stream
was 65 % and then it dropped to 55 % in 3 h time period.
The 2-hexanol was predominantly converted to hexenes (1-
and 2-hexenes) by dehydration reaction whose selectivity
increased from 72.1 to 90.7 % in the 3 h time period.
However, 2-hexanol yielded little cracked products, such
as methane and lower hydrocarbons like C –C . Only a
2
3
support. The dried catalyst was calcined under flowing air
at 500 °C for 6 h. The elemental analysis of a calcined
sample indicates that the sample contains 1.1 wt% Na and
1
1
.1 wt% Pt. Therefore the actual catalyst composition is
2
5
.1 % Pt–1.1 %Na/c-Al O . The catalyst is denoted as Pt/
trace amounts of CO and CO were detected during the
2
2
3
(
Na)-Al O . The reaction was carried out in a conventional
2
initial testing period. A noticeable amount of 2-hexanone
was produced from 2-hexanol by a dehydrogenation reac-
tion over Pt/(Na)Al O catalyst. More importantly, even a
3
flow apparatus as described previously [16]. Runs were
made at a temperature of 482 °C and atmospheric pressure
2
3
(
1 atm). The reactant (n-hexane, 2-hexanol or 2-hexanone)
trace amount of phenol (direct dehydrocyclization product)
was not found under these conditions. Benzene,
cyclopentane and methyl cyclopentanes are the only
cyclized products found for dehydrocylization of 2-hex-
anol. The cyclic product distributions obtained with
2-hexanol are shown in Fig. 2. The fraction of benzene
was continuously fed to the top of a reactor at a constant
molar flow rate of 8.0 mmol/h using a syringe pump
(
Thermo Scientific, Model Orion M361) via a 1/16 in.
needle with a side-port hole. All chemicals (n-hexane,
-hexanol and 2-hexanone) used in this study were
2
1
23