H. Chiang, A. Bhan / Journal of Catalysis 283 (2011) 98–107
99
how kinetic parameters within the proposed mechanism vary with
OH group environment. The generally accepted pathway of hydro-
isomerization based on the study by Weisz and Swegler [15] in-
cludes dehydrogenation of the linear alkane to form a linear
alkene on the metal surface. This linear alkene is isomerized into
a branched alkene on the acidic site in the zeolite micropore envi-
ronment; subsequently, this branched alkene is hydrogenated over
the metal catalyst to generate a branched alkane. Alkane isomeri-
zation can be catalyzed by only the acidic zeolite, but high temper-
atures are required (573 K on zeolite BEA) [16] for initial
dehydrogenation of the alkane, and in absence of H2 and metal cat-
alysts, high concentrations of alkenes result in excessive oligomer-
ization, cyclization, and aromatization reactions that form
unreactive carbon deposits [8,16–18]. In bifunctional catalyst for-
mulations, however, because the equilibrium of alkane, alkene,
and hydrogen is established by platinum at lower temperatures
(ꢁ473 K), low concentrations of olefins are maintained by adjust-
ing the alkane to H2 ratio in the feed [18].
The detailed mechanism for how linear alkenes are transformed
into branched alkenes over acidic zeolites has also been exten-
sively studied by computational chemistry methods. Hybrid quan-
tum mechanical–molecular mechanical (QM/MM) calculations
show that surface-bound, linear alkoxide intermediates formed
upon chemisorption are more stable than physisorbed linear al-
kenes [19,20] and that the stability of the linear alkoxide interme-
diate increases with increasing carbon number in FER (C3–C5) [19]
and FAU (C2–C8) [20]. Linear alkoxide species have been observed
as stable intermediates in 13C NMR and infrared spectra during
alcohol dehydration and protonation of alkenes on zeolite surfaces
[21–23]. The isomerization of a linear alkoxide into a branch alkox-
ide occurs via an edge-protonated cyclo-propane species as a tran-
sition state which is energetically favored compared to directly
shifting the alkyl group as shown by Demuth et al. [24] and Boro-
nat et al. [25] using density functional theory (DFT).
maintain particle sizes between 180 and 425
l
m (40–80 mesh)
and subsequently treated in dry air (1.67 cm3 sꢀ1 at NTP condi-
tions, ultrapure, Minneapolis Oxygen) to thermally decompose
NHþ4 to H+ and NH3(g) by increasing the temperature from ambient
to 773 K at 0.0167 K sꢀ1 and holding for 4 h. The proton form FER,
MOR, and BEA zeolite samples are abbreviated as H-FER, H-MOR,
and H-BEA, respectively.
c
-Al2O3 (Sasol North America Inc., Lot # C1964, 189 m2 gꢀ1
,
0.44 cm3 gꢀ1 pore volume) was treated in flowing dry air
(1.67 cm3 sꢀ1 at NTP conditions, ultrapure, Minneapolis Oxygen)
to 923 K for 3 h (0.083 K sꢀ1) before adding the metal precursor.
Pt/Al2O3 formulations (1.5 wt.% Pt) were prepared by the incipient
wetness impregnation of c-Al2O3 using chloroplatinic acid solution
(H2PtCl6ꢂ6H2O, 99.95% (metal basis), Alfa Aesar) as precursor. After
impregnation, samples (yellow in color) were treated in dry air
(1.67 cm3 sꢀ1 at NTP conditions, ultrapure, Minneapolis Oxygen)
at 383 K for 9 h and subsequently heated to 823 K (0.083 K sꢀ1
)
for 4 h to thermally decompose the precursors. After decomposing
the precursor, the sample was treated in H2 (3.3 cm3 gꢀ1 sꢀ1, ultra-
pure carrier grade, Airgas) at 723 K (0.083 K sꢀ1) for 2 h and then
cooled to ambient temperatures in dry He flow (1.67 cm3 sꢀ1
,
ultrapure, Minneapolis oxygen). The Pt cluster surface was passiv-
ated by treating Pt/Al2O3 formulations (black color) in mixtures of
dry air (0.1–0.3 cm3 gꢀ1 sꢀ1, ultrapure, Minneapolis Oxygen) and
He (3.3 cm3 gꢀ1 sꢀ1, ultrapure, Minneapolis oxygen) at 298–303 K
for at least 1.5 h.
NH4-MOR (Si/Al = 11.1, 0.5–10 g, CBV 21A, Zeolyst) was mixed
with 1.5 L NaNO3 solution (3.9 ꢃ 10ꢀ3–1.2 ꢃ 10ꢀ2 M, Sigma–Al-
drich) at 353 K for at least 12 h to exchange protons with sodium
cations and then filtered and washed in 5 L deionized water to re-
move unexchanged Na+. The washed sample was dried in ambient
air at 363 K for at least 12 h and then treated in dry air
(1.67 cm3 sꢀ1 at NTP conditions, ultrapure, Minneapolis Oxygen)
at 773 K (0.0167 K sꢀ1) for 5 h.
In this study, three zeolite framework materials (H-BEA, H-FER,
and H-MOR) were chosen to study the effects of zeolite pore con-
nectivity, channel size, and location of OH groups on the rate and
selectivity of n-hexane hydroisomerization over bifunctional cata-
lysts consisting of physical mixtures of zeolites and Pt/Al2O3 (0.9–1
Pt/H+ in molar ratio). The measured rate of isomerization over the
three zeolite materials is a function of n-C6H14/H2 (molar ratio),
consistent with a bifunctional mechanism involving the facile
dehydrogenation of n-hexane on the metal catalyst and a kineti-
cally relevant step involving isomerization of n-hexene on zeolitic
acidic sites. Zeolite BEA has the highest rate among the zeolites
considered because it has lower activation energy than MOR and
higher activation entropy than FER. The rate per proton in the 8-
MR side pockets in MOR is five times higher than the rate in the
12-MR channels because the activation energy in 8-MR pockets
is lower than that in 12-MR channels. The measured entropy of
activation (ꢀ34.7 9.8 J molꢀ1 Kꢀ1) and selectivity to 2-MP and
3-MP (1.55:1) in the 8-MR pockets within MOR are similar to those
in the 12-MR channels of MOR (ꢀ37.4 9.7 J molꢀ1 Kꢀ1 and 1.5:1)
and of BEA (ꢀ33.1 4.3 J molꢀ1 Kꢀ1 and 1.35:1), suggesting that
the n-hexene molecule is only partially confined in the 8-MR pock-
ets of H-MOR.
Chemical titration using dimethyl ether (DME) for the H-FER, H-
MOR, and H-BEA samples used in this study resulted in a DME per
Al ratio of 0.5 0.08 for all three zeolites, showing that the concen-
tration of Brønsted acid sites is nearly identical to the concentra-
tion of Al in these materials. The experimental procedure and
tabulated results of these DME titration studies are described in
Supplemental information.
2.2. Steady-state catalytic reactions of n-hexane-hydrogen mixtures
Steady-state isomerization reactions of n-hexane were carried
out in a tubular packed-bed quartz reactor (10 mm inner diameter)
under atmospheric pressure and differential conditions (<8% for
hydroisomerization). Catalyst samples were supported on a coarse
quartz frit inside the reactor, and the temperature was controlled
using a furnace (National Electric Furnace FA120 type) connected
to a Watlow Temperature Controller (96 series). Catalyst tempera-
tures were measured using a K-type thermocouple touching the
bottom of a well on the external surface of the quartz reactor. Prior
to measurement of n-hexane isomerization rates, catalyst samples
(0.005–0.05 g proton form zeolites physically mixed with Pt/Al2O3
to achieve 0.9–1.0 Pt/H+ molar ratio) were treated in H2 at 673 K
for 4 h (0.0167 K sꢀ1). When catalyst samples were insufficient in
quantity to cover the thermowell, these samples were diluted with
2. Materials and methods
acid-washed quartz particles (0.5–0.8 g, 160–630 lm, European
Commission, washed by 1 M HNO3). Liquid n-hexane (4.6 ꢃ
10ꢀ5 mol sꢀ1) was vaporized at 383 K into a gas flow which con-
tained He (1.8 cm3 sꢀ1 at NTP condition, Minneapolis oxygen), H2
(0.15–1.75 cm3 sꢀ1, ultra-pure carrier grade, Airgas), and a mixture
of Ar/CH4 (0.0137–0.0297 cm3 sꢀ1 at NTP conditions; 75% Ar and
25% CH4, Minneapolis oxygen) as internal standard. The effluent
from the reactor was sent via heated transfer lines to a mass
2.1. Catalyst preparation
FER (Si/Al = 11.5, CP 914c), MOR (Si/Al = 11.1, CBV 21A), and
BEA (Si/Al = 12.0, CP 814 E) zeolite samples from Zeolyst, where
the silicon to aluminum ratio (Si/Al) was determined by elemental
analysis (Galbraith Laboratories), in their NHþ4 form were sieved to