Catalytic oxidation of n-hexane on Mn-exchanged zeolites: Turnover rates, regioselectivity, and spatial constraints

Article abstract of DOI:10.1016/j.jcat.2006.10.019

The effects of channel structure and spatial constraints on n-hexane oxidation rates and regioselectivity were examined on Mn cations within channels of acidic zeolites. Active Mn cations were placed at exchange sites within channels in 8-membered (ZSM-58), 10-membered (ZSM-5 and ZSM-57), and 12-membered ring (MOR) channels by sublimation of MnI₂. Synthesis rates for hexanols (ROH), hexanal/hexanones (R({single bond}H){double bond, long}O), and acids were proportional to hexylhydroperoxide (ROOH) concentrations on all Mn-zeolite catalysts, except Mn-ZSM-58, on which products formed exclusively via noncatalytic autoxidation because of restricted access to Mn cations present within small channels (0.36 nm). Catalytic decomposition of ROOH intermediates occurs on intrachannel Mn cations and is the kinetically relevant step in alkane oxidation. ROOH decomposition rate constants were 2.5, 1.4, and 0.41 mol (mol-Mn h)^-1 (mM-ROOH)^-1 on Mn-ZSM-5, Mn-ZSM-57, and Mn-MOR (403 K; 0.4 MPa O₂), respectively. Regioselectivity was influenced by the constrained environment around Mn cations, which increased terminal selectivities above the values predicted from the relative bond energies of methyl and methylene C{single bond}H bonds in n-hexane. Mn cations within 10-ring channels gave higher terminal selectivities (Mn-ZSM-5: 24%, k_prim / k_sec = 0.42; Mn-ZSM-57: 14%, k_prim / k_sec = 0.22) than those within 8-membered or 12-membered rings (Mn-MOR, Mn-ZSM-57: 8-10%, k_prim / k_sec = 0.12 - 0.14), because of restricted access in ZSM-58 and unconstrained transition states for C{single bond}H bond activation in MOR. Terminal selectivities decreased with increasing alkane conversion, because unselective noncatalytic autoxidation pathways prevail as ROOH concentrations concurrently increase. ROOH intermediates can be scavenged from the extracrystalline liquid phase using H-zeolites with accessible protons, which inhibit unselective noncatalytic reactions and maintain higher terminal selectivities as conversion increases, albeit with a concomitant decrease in the rate of oxidation steps also involving ROOH intermediates.

Full text of DOI:10.1016/j.jcat.2006.10.019

Journal of Catalysis 245 (2007) 316–325
www.elsevier.com/locate/jcat
Catalytic oxidation of n-hexane on Mn-exchanged zeolites:
Turnover rates, regioselectivity, and spatial constraints
Bi-Zeng Zhan a, Björn Modén a, Jihad Dakka b, José G. Santiesteban b, Enrique Iglesia a,∗
a
Department of Chemical Engineering, University of California at Berkeley, 201 Gilman Hall, Berkeley, CA 94720, USA
Corporate Strategic Research, ExxonMobil Research and Engineering Co., Route 22 East, Annandale, NJ 08801, USA
b
Received 2 August 2006; revised 18 October 2006; accepted 21 October 2006
Available online 28 November 2006
Abstract
The effects of channel structure and spatial constraints on n-hexane oxidation rates and regioselectivity were examined on Mn cations within
channels of acidic zeolites. Active Mn cations were placed at exchange sites within channels in 8-membered (ZSM-58), 10-membered (ZSM-5 and
ZSM-57), and 12-membered ring (MOR) channels by sublimation of MnI . Synthesis rates for hexanols (ROH), hexanal/hexanones (R(–H)=O),
2
and acids were proportional to hexylhydroperoxide (ROOH) concentrations on all Mn-zeolite catalysts, except Mn-ZSM-58, on which products
formed exclusively via noncatalytic autoxidation because of restricted access to Mn cations present within small channels (0.36 nm). Catalytic
decomposition of ROOH intermediates occurs on intrachannel Mn cations and is the kinetically relevant step in alkane oxidation. ROOH decompo-
−1
−1
sition rate constants were 2.5, 1.4, and 0.41 mol (mol-Mn h) (mM-ROOH) on Mn-ZSM-5, Mn-ZSM-57, and Mn-MOR (403 K; 0.4 MPa O ),
2
respectively. Regioselectivity was influenced by the constrained environment around Mn cations, which increased terminal selectivities above the
values predicted from the relative bond energies of methyl and methylene C–H bonds in n-hexane. Mn cations within 10-ring channels gave
higher terminal selectivities (Mn-ZSM-5: 24%, k
membered rings (Mn-MOR, Mn-ZSM-57: 8–10%, k
/k = 0.42; Mn-ZSM-57: 14%, k
/k = 0.22) than those within 8-membered or 12-
sec
sec
prim
prim
/k = 0.12–0.14), because of restricted access in ZSM-58 and unconstrained transition
sec
prim
states for C–H bond activation in MOR. Terminal selectivities decreased with increasing alkane conversion, because unselective noncatalytic
autoxidation pathways prevail as ROOH concentrations concurrently increase. ROOH intermediates can be scavenged from the extracrystalline
liquid phase using H-zeolites with accessible protons, which inhibit unselective noncatalytic reactions and maintain higher terminal selectivities
as conversion increases, albeit with a concomitant decrease in the rate of oxidation steps also involving ROOH intermediates.
© 2006 Published by Elsevier Inc.
Keywords: n-Hexane oxidation; Terminal selectivity; Zeolite; Exchange; Sublimation
1. Introduction
tivities (∼20%) on spatially constrained Mn(III) centers in
metalloporphyrins [2]. Oxidation of n-hexane by iodosoben-
zene (PhIO) on 5,10,15,20-tetrakis (2ꢀ,4ꢀ,6ꢀ-triphenylphenyl)-
porphyrinato-manganese(III) acetate [(MnTTPPP(OAc)] gives
19% 1-hexanol among alkanols, with a corresponding pri-
mary selectivity index (kprim/ksec) of 0.31 [3]. These kprim/ksec
values are much greater than on Mn(III)-porphyrins with
less constrained active centers (kprim/ksec = 0.03; 5,10,15,20-
tetraphenyl-porphyrinato-manganese(III) acetate). Molecular
simulations suggest that 2ꢀ,4ꢀ,6ꢀ-triphenylphenyl substituents
on porphyrin macrocycles form a 0.4- to 0.5-nm pocket, which
imposes constraints favoring selective oxygen insertion at ter-
minal methyl groups [2]. Random C–H bond activation would
give kprim/ksec values of unity (43% terminal selectivity for
n-hexane), whereas relative C–H bond strengths and linear free
The design of inorganic catalysts for selective oxidation
of alkanes to alcohols, ketones, and acids using O2 remains
a formidable challenge. Enzymes with nonheme iron active
centers (e.g., ω-hydroxylase) catalyze alkane oxidation with
O2 with high terminal selectivity [1], apparently because
ligands near active centers favor specific docking of reac-
tants at such active sites. Recent studies have tried to repli-
cate these properties for nonbiological systems with modest
success. Linear alkanes react with moderate terminal selec-
*
Corresponding author. Fax: +1 510 642 4778.
E-mail addresses: iglesia@berkeley.edu, eiglesia@aol.com (E. Iglesia).
0021-9517/$ – see front matter © 2006 Published by Elsevier Inc.
doi:10.1016/j.jcat.2006.10.019

B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
317
energy relations predict kprim/ksec values of ∼0.10 (∼7% ter-
2. Experimental
minal selectivity for n-hexane) [4].
Sterically hindered homogeneous 2,4-dichloro-3,5-dinitro-
benzoic carboxylate Rh complexes gave some preferential ter-
minal selectivity (31%) for carbene insertion into n-hexane
(kprim/ksec = 0.60) [5]. Higher terminal selectivities have been
reported for borylation reactions of alkanes. Bis(pinacolato)-
diborane (B2pin2) or pinacolborane (HBpin) react with n-
octane on Rh complexes with bulky ligands (Cp*Rh(η4-
C6Me6)) to give 65% yields of the terminal reaction product
(n-octanyl-1-Bpin) [6].
2.1. Synthesis and modification of catalysts
NH4-mordenite (MOR) was obtained from Zeolyst (CBV
21A; SiO2/Al2O3 = 20). NH4-ZSM-5 (MFI) was obtained
from ALSI-PENTA Zeolithe GmbH (SM-27; SiO2/Al2O3 =
24). ZSM-58 (DDR) was prepared in its Na-form as described
previously [23]; its organic template was removed by heating
to 803 K at 0.167 K s−1 in flowing dry air (Praxair, UHP;
1.67 cm3 s−1) and holding at 803 K for 10 h. Its Na form was
exchanged with NH+4 by stirring in 1.0 M NH4NO3 (Aldrich,
99.99+% in deionized water, 5 cm3 (g-zeolite)−1) for 3 h at
ambient temperature, filtering, and washing the solids with
deionized water. This exchange procedure was carried out three
times. ZSM-57 (MFS) was prepared as the Na form as reported
previously [24]. It was converted to its H form using the same
procedures as for H-ZSM-58. The NH4 form of each zeolite
was converted to its H form by heating to 803 K at 0.167 K s−1
in flowing He (Praxair, UHP; 1.67 cm3 s−1) and holding for 4 h.
Mn2+ cations were exchanged onto H-zeolites using sub-
limation methods [25], because diffusion of charged Mn-
oxo species in aqueous solutions is very slow within small-
and medium-pore zeolites [26,27]. Zeolite samples were held
within a glass ampoule (2 g) and treated at 573 K for 0.5 h in dy-
namic vacuum (∼1.3 × 10−2 Pa) to remove adsorbed species,
cooled to ambient temperature, and finally mixed with anhy-
drous MnI2 (Aldrich, 99.9%); the glass ampoule was sealed in
vacuum, heated to 803 K at 0.167 K s−1, and held at 803 K
for 10 h. The samples were then dispersed in deionized water at
323 K, and the HI thus formed was removed by rinsing with hot
deionized water (323 K). Mn-exchanged zeolites were heated
in flowing dry air (Praxair, UHP; 1.67 cm3 s−1) to 393 K, held
for 2 h, and then heated to 803 K at 0.167 K s−1 and held for
another 2 h before catalytic and characterization tests.
Crystalline aluminosilicates contain cages and channels of
varying size and shape, which can influence selectivity in
catalytic reactions [7,8]. These zeolite materials also have
negatively charged frameworks, which can be compensated
for by redox-active cations [9,10] or porphyrin or phthalo-
cyanine cationic complexes [11–14]. Framework substitutions
can also introduce redox-active centers, as in the case of Co
and Mn redox-active sites within microporous aluminophos-
phates (AlPO) [15,16]. Modest terminal selectivities have
been reported for alkane oxidation with O2 on these inor-
ganic systems [17]. For example, terminal selectivities of
21% (kprim/ksec = 0.54) were measured for n-octane oxida-
tion by O2–H2 mixtures on Fe/Pd-zeolite 5A; these kprim/ksec
values increased (to 0.67) after selective titration of uncon-
strained sites at external surfaces with 2,2ꢀ-bipyridine [17].
The reaction products, however, could be removed only by
dissolving the zeolite framework, because they cannot leave
intracrystalline voids by diffusion through the 8-ring windows
(∼0.4 nm) in zeolite A [17]. Unprecedented terminal selectiv-
ities were reported on MnAPO-18 and CoAPO-18 structures
with small 8-ring windows (0.38 nm) [16]. Terminal selec-
tivities as high as ∼65% (kprim/ksec = 2.5) were claimed for
n-hexane (and n-octane) oxidation using O2, with hexanoic
acid as the predominant product; however, these findings have
remained unconfirmed on catalysts with identical composition
and structure [4,18]. Modest terminal selectivities were also re-
ported for alkane oxidation with H2O2 on zeolites modified by
redox-active cations. n-Octane reactions gave 45% terminal se-
lectivity (kprim/ksec = 1.6) on Fe-ZSM-5 [19]. On vanadium
silicalite (VS-2), n-hexane reactions with H2O2 gave contra-
dictory results; one study reported 32% terminal selectivity
(kprim/ksec = 0.63) [20], whereas another reported much lower
values (11%; kprim/ksec = 0.16) [21].
2.2. Structural and chemical characterization
Micropore volumes were measured from N2 uptakes at its
normal boiling point (Autosorb 6, Quantachrome) after treating
samples at 573 K in dynamic vacuum (∼4 Pa, 4 h). Isotopic ex-
change of D2 with acidic and silanol OH groups was used to
measure the number of OH groups remaining after exchange
with Mn2+ [28]. Samples were treated in 1.67 cm3 s−1 dry
air (Praxair, UHP) within a quartz cell by heating to 803 K at
0.167 K s−1 and holding for 1 h. The samples were then cooled
to ambient temperature and exposed to 5% D2/Ar (Matheson;
0.67 cm3 s−1) while increasing the temperature to 803 K at
0.167 K s−1. Intensities at 2–4 amu (H2, HD, and D2), 16–
20 amu (water isotopomers), and 40 amu (Ar, internal standard)
were measured by mass spectrometry (Orion Compact, MKS
Instruments) at 12-s intervals.
A preference for oxygen insertion at terminal C–H bonds,
despite their greater bond strength, appears to require steric
effects arising from bulky ligands [2,5,6] or small channels
[16,17]. Such preferences may reflect a specific mode of attach-
ment of alkanes at redox-active sites [16], the inhibited termina-
tion of primary radicals [22], or the selective decomposition and
regeneration of specific organoperoxide isomers constrained by
the space surrounding active centers. Here we explore n-hexa-
ne–O2 reactions on Mn cations present as exchanged species
within zeolite channels of varying size and connectivity. In
these systems, Mn redox-active sites decompose ROOH inter-
mediates in kinetically relevant steps that occur within environ-
ments with varying degrees of spatial constraints.
Magic-angle spinning (MAS) 27Al nuclear magnetic reso-
nance (NMR) spectra were measured with a Bruker 500 spec-
trometer (11.7 T field) at 14 kHz after hydrating samples at am-
bient temperature to weaken quadrupole interactions that tend
to broaden 27Al NMR lines [26]. 27Al spectra were acquired

318
B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
with a 1.1-µs (π/15 flip angle) pulse width and a 1-s pulse delay
and referenced to aqueous Al(NO3)3 (0 ppm). Infrared spectra
were measured using a Mattson spectrometer (RS-10000) af-
ter dehydrating samples at 803 K in flowing He (Praxair, UHP;
1.67 cm3 s−1) for 1 h; infrared spectra were acquired at 803 K
in He with a 4-cm−1 resolution and 256 scans.
with mass selective and flame ionization detectors) using a DB-
WAX capillary column (60 m×0.25 mm×0.5 µm film; Agilent
J&W Scientific). The oxidation products detected (3-hexanone,
2-hexanone, hexanal, 3-hexanol, 2-hexanol, 1-hexanol, acetic
acid, propionic acid, butyric acid, valeric acid, and hexanoic
acid) were identified by their MS fragmentation patterns and
confirmed by chromatographic retention times measured for the
pure compounds. Each sample was analyzed twice, before and
after reaction with triphenylphosphine (Ph3P; Fluka, ꢀ98.5%),
which converts hexylhydroperoxide isomers quantitatively to
the corresponding alcohols [29–31]. Alcohols (ROH), ketones
and aldehyde (R(–H)=O), and peroxides (ROOH) were mea-
sured from these two chromatograms by assuming that ther-
mal decomposition of hexylhydroperoxides during chromatog-
raphy leads to equimolar alcohol–ketone (or aldehyde) mixtures
in underivatized samples. C2–C5 acids formed during reac-
tion via oxidative C–C bond cleavage of ROH and R(–H)=O
products; these are reported based on the number of n-hexane
molecules appearing as a given product, taking into account
that two shorter acids are formed when a ROH or R(–H)=O
is cleaved. All rates and selectivities are reported based on
the number of n-hexane molecules converted to each prod-
uct.
2.3. Catalytic rates and selectivities
n-Hexane (ꢀ99.0%, absolute, Fluka; 25 cm3) oxidation
rates and selectivities were measured in a shielded high-
pressure glass reactor (Andrews Glass; 100 cm3). Catalysts
were transferred into the reactor immediately after treatment
in dry air (Praxair, UHP) at 803 K for 2 h. 1,2-Dichlorobenzene
(0.20 cm3, 99.8%, Fisher Scientific) was used as an inter-
nal standard. The reaction unit was flushed three times by
O2 (Airgas, UHP), after which the pressure was increased to
0.3 MPa using O2 at ambient temperature. The reactor temper-
ature was raised to 403 K, which led to a final reactor pressure
of 0.7 MPa (0.4 MPa O2 and 0.3 MPa n-hexane); this O2 pres-
sure was maintained constant by adding O2 periodically as it
was consumed. Noncatalytic oxidation rates were measured un-
der identical conditions but without adding a catalyst. Reactant
and product concentrations were measured by periodically ex-
tracting a sample (∼0.50 cm3) of clear liquid from the reactor.
3. Results and discussion
3.1. Exchange of Mn ions onto Brønsted acid sites
2.4. Gas chromatographic analysis
Table 1 shows compositions and micropore volumes for ze-
olite samples before and after Mn exchange. The measured
Mn/Al ratios were similar to those in the starting MnI2–H-
The concentrations of reactants, products, and internal stan-
dard were measured by gas chromatography (Agilent 6890,
Table 1
Elemental analysis and pore volumes for Mn-zeolite catalysts
a
b
Entry
Zeolite
structure
Channel/window
size (nm)
Si/Al
(atomic ratio)
Mn/Al
Mn/Al
Mn content
Micropore volume
(cm g
f
−1
3
−1
)
(atomic ratio)
(atomic ratio)
(mmol g
)
1
H-MOR
0.65 × 0.70
10
10
10
10
13
13
–
–
–
0.206
0.163
0.164
0.165
0.112
0.105
0.34 × 0.48
2
3
4
5
6
Mn-MOR-3
Mn-MOR-2
Mn-MOR-1
H-ZSM-5
0.65 × 0.70
0.34 × 0.48
0.020 (0.020)
0.040 (0.040)
0.088 (0.083)
–
–
0.029
0.060
0.093
–
0.65 × 0.70
–
0.34 × 0.48
0.65 × 0.70
0.34 × 0.48
0.17
–
0.53 × 0.56
0.51 × 0.55
Mn-ZSM-5
0.53 × 0.56
0.51 × 0.55
0.088 (0.10)
0.14
0.10
7
8
9
H-ZSM-57
Mn-ZSM-57
H-ZSM-58
Mn-ZSM-58
0.51 × 0.54
0.51 × 0.54
0.36 × 0.44
0.36 × 0.44
21
21
66
66
–
–
–
0.149
0.139
0.131
0.131
0.12 (0.13)
–
–
0.13
–
–
10
a
0.47 (0.52)
0.47
0.11
Mn/Al ratios were calculated using elemental analysis results (from Galbraith Laboratory); numbers in the parentheses reflect the Mn/Al molar ratio calculated
from the MnI and H-zeolite in the starting physical mixture.
2
b
27
Framework Al (Al ) measured from solid state Al-NMR.
f

B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
319
Table 2
27
OH groups measured by D –OH and OH/Al by Al NMR
2
f
a
b
Mn/Al
OH/Al
OH
/Mn
OH/Al
consumed
f
(molar ratio)
(molar ratio)
(molar ratio)
H-ZSM-58
Mn-ZSM-58 0.47
0
3.4
2.4
–
2.1
3.4
2.4
a
All OH groups (Brønsted and silanol) measured by D –OH exchange.
2
b
OH
is the difference between Mn-zeolite and its H-form.
consumed
Fig. 1. Infrared spectra of H-ZSM-58 (—) and Mn-ZSM-58 (- - -). Samples
3
−1
were dehydrated in flowing He (1.67 cm s ) at 803 K for 1 h before recording
spectra at 803 K in He.
zeolite physical mixtures, indicating that sublimation led to sto-
ichiometric exchange for 12-ring, 10-ring, and 8-ring channels,
because any unexchanged MnI2 would have been removed dur-
ing subsequent washing with water (323 K) or by sublimation
during subsequent thermal treatment in air at 803 K for 2 h [27].
The Si/Al ratios in zeolites were unchanged by these synthesis
protocols. N2 adsorption uptakes indicated that Mn incorpora-
tion decreased micropore volumes slightly for all Mn/Al ratios
on MOR (entries 1–4, Table 1). These changes were smaller for
zeolites with higher Si/Al ratios (e.g., ZSM-57; entries 7 and 8
in Table 1); the micropore volume was unchanged by exchange
for ZSM-58 (Si/Al = 66) (entries 9 and 10 in Table 1).
27
Fig. 2. Solid state MAS Al NMR of zeolites before and after MnI exchange.
2
Framework tetrahedral Al: Al
; extraframework octahedral Al: Al
.
tetra
oct
were detected in Mn-ZSM-5 (39% of total Al; Fig. 2) and Mn-
MOR (47% of total Al) samples with relatively low Si/Al ratios,
suggesting that some significant dealumination occurred by HI
formed in the sublimation, as also can be inferred from N2 up-
take measurements (Table 1).
The extent of exchange and the possible extraction of some
framework Al were probed by the intensity of the OH stretches
in infrared spectra, by 27Al NMR lines for tetrahedral and octa-
hedral Al-species, and by D2 exchange with OH groups remain-
ing after MnI2 sublimation. Fig. 1 shows that infrared bands for
Brønsted acid centers (OH groups at Al centers) in H-ZSM-
58 became less intense after contact with MnI2(g) (Mn/Al =
0.47), indicating that MnI2(g) can access internal zeolite re-
gions even through 8-ring windows. Silanol bands at 3720 and
∼3500 cm−1 interfered with those for acidic OH groups at
3600 cm−1 (because of the high Si/Al ratio of 66) [32,33], pre-
venting accurate measurements of the number of O–H groups
replaced by O–Mn using infrared spectroscopy. Therefore, iso-
topic exchange of OH groups with D2(g) was used as an al-
ternate method for this sample. Table 2 shows that 2.1 protons
were removed per Mn2+ for Mn-ZSM-58 (column 4, Table 2),
as expected from Mn2+ monomers bridging Al-exchange sites.
OH/Alf ratios were above unity (Alf, framework Al measured
by 27Al-NMR; OH, measured by D2 exchange) in both H-ZSM-
58 and Mn-ZSM-58 (column 5 in Table 2) because of the large
relative number of silanols at these high Si/Al ratios (66), con-
sistent with infrared spectra (Fig. 1) and with similar previous
reports on high-silica zeolites [32,33].
3.2. Noncatalytic oxidation reactions of n-hexane
First, we provide n-hexane oxidation rates and selectivities
without a catalyst as a benchmark for regioselectivity compar-
isons. n-Hexane reacted slowly with O2 to form oxygenates
at 403 K and 0.7 MPa total pressure, even without a cata-
lyst (Fig. 3). Hexylhydroperoxide (ROOH) formation rates in-
creased with time in the early stages of reaction (Fig. 3b). This
induction period for ROH + R(–H)=O formation was longer
than for ROOH formation (∼3 h vs 1 h; Fig. 3a), consistent with
the intermediate role of ROOH in the formation of products.
Hexylhydroperoxides were the only products detected during
the first 0.5 h (Fig. 3b), consistent with autoxidation pathways
involving ROOH as the initial product,
RH + O2 → ROOH → ROH + R(–H)=O → ··· .
(1)
Peroxide (e.g., tert-butylhydroperoxide) initiators eliminate
these induction periods [34,35]. The terminal selectivity among
all ROOH intermediates was 8.2% (kprim/ksec = 0.12) dur-
ing this initial period, as would be expected given the relative
27Al NMR did not detect extra-framework Al (Aloct) in H-
ZSM-58 or Mn-ZSM-58 (Fig. 2), indicating that dealumination
did not occur during exchange protocols (Table 1). Aloct species

320
B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
(a)
(a)
(b)
(b)
Fig. 4. n-Hexane oxidation on Mn-ZSM-57: (a) product concentrations: ROOH
(Q), ROH+R(–H)=O ("), and acids (F); (b) total ROOH formation rates (net
ROOH measured + its products) vs lumped ROOH concentration along with
Fig. 3. Non-catalytic oxidation of n-hexane with O : (a) ROOH (Q),
2
ROH + R(–H)=O ("), and acids (F) including hexanoic acid and smaller
acids formed via C–C bond cleavage; (b) r
(Q), r
("), and
ROOH
ROH+R(–H)=O
3
blank (non-catalytic) reaction for comparison. Reaction conditions: 25 cm
3
3
ROOH selectivity (F). Reaction conditions: 25 cm n-hexane, 0.20 cm di-
3
n-hexane, 0.20 cm dichlorobenzene, 1.00 g Mn-ZSM-57, 403 K, and 0.7 MPa.
chlorobenzene, 403 K, and 0.7 MPa.
(Fig. 4b) show that Mn-ZSM-57 also increased total ROOH
formation rates (the sum of ROOH and its products) but to a
lesser extent than ROOH decomposition rates, leading to lower
prevalent ROOH concentrations than are present during non-
catalytic autoxidation. Turnover rates for the synthesis of ROH,
R(–H)=O, and acids were proportional to ROOH concentra-
tion (Fig. 5), consistent with the intermediate role of ROOH
and with the identity and kinetic relevance of its decomposition
steps, as shown earlier for alkane oxidation on MnAPO cata-
lysts [4,34]. The kinetically relevant ROOH decomposition is
catalyzed by redox-active Mn sites (Step I or Iꢀ in Scheme 1).
Product formation rates were proportional to ROOH con-
centrations on Mn-MOR and Mn-ZSM-5 as well (Fig. 5).
Pseudo-first-order rate constants for ROOH decomposition to
abundance and energies of C–H bonds in n-hexane and lin-
ear free-energy treatments of chemical reaction rates (∼7%,
kprim/ksec = 0.10) [4].
3.3. Catalytic oxidation of n-hexane on Mn-exchanged
zeolites
Fig. 4 shows formation rates of ROOH and products [ROH,
R(–H)=O, and acids] as a function of reaction time (and ROOH
concentration) for n-hexane oxidation on Mn-ZSM-57. Mn-
ZSM-57 gave lower ROOH concentrations and shorter induc-
tion periods for ROH + R(–H)=O formation than noncatalytic
reactions (Fig. 4a vs Fig. 3a), because Mn sites provide catalytic
routes for ROH+R(–H)=O formation from ROOH. These data

B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
321
within 8-ring channels inaccessible to ROOH [36,37]. These n-
hexane oxidation rate constants are similar to those reported
on MnAPO-5 and MnAPO-18 [0.31 and 0.38 mol (g-atom-
Mn h)−1 (mM-ROOH)−1, respectively] at similar reaction con-
ditions [4]. Product formation rates showed a larger than lin-
ear increase with ROOH concentration on 8-ring Mn-ZSM-58
(Fig. 5), a kinetic behavior characteristic of noncatalytic au-
toxidation pathways [35]. These effects of zeolite structure on
n-hexane oxidation rates indicate that ROOH decomposition
rates on Mn active centers are influenced by confining Mn
redox-active sites within channels of varying size. Zeolites with
10-ring structures gave higher ROOH decomposition rates and
lower ROOH concentration than 8-ring and 12-ring zeolites
(Fig. 5).
The effects of spatial constraints on regioselectivity were
probed by measuring terminal oxidation selectivities at low
ROOH concentrations (short reaction times, <0.5 h). These
conditions minimize unselective noncatalytic autoxidation path-
ways, because these processes depend more sensitively on
ROOH concentration than catalytic pathways. Table 3 gives
initial terminal selectivities for Mn-zeolites with different chan-
nel/window sizes and for noncatalytic reactions. All catalysts
except Mn-ZSM-58 demonstrated terminal oxidation selectiv-
ities greater than those for noncatalytic pathways, suggesting
that spatial constraints influence the position of oxygen inser-
tion and increase selectivity toward the activation of stronger
C–H bonds in terminal CH3 groups via reactions with Mn-
bound reactive species (Scheme 1). Terminal selectivities (and
kprim/ksec ratios) reached a maximum for 10-ring channels
(Table 3) but were much lower than those reported on 8-
ring (0.38 × 0.38 nm windows) MeAPO-18 catalysts (up to
65%; kprim/ksec = 2.5) [16]. Terminal selectivities on 8-ring
Fig. 5. Turnover rates (TOR) for combined formation of ROH, R(–H)=O,
and acids on Mn sites vs lumped ROOH concentration: Mn-ZSM-58 ("),
Mw-MOR (e.g., Mn-MOR-1 in Table 1) (2), Mn-ZSM-57 (Q), and Mn-ZSM-5
(F). Reaction conditions: 25 cm n-hexane, 0.20 cm dichlorobenzene, 1.00 g
Mn-zeolites, 403 K, and 0.7 MPa. Amplified profile for Mn-ZSM-5 is inserted
in the left corner.
3
3
form ROH, R(–H)=O, and acids (from data in Fig. 5) were
2.5, 1.4, and 0.41 mol (g-atom-Mn h)−1 (mM-ROOH)−1 on
10-ring Mn-ZSM-5 and Mn-ZSM-57 catalysts and 12-ring Mn-
MOR catalysts, respectively. The lower ROOH decomposition
rates on Mn-MOR relative to those on Mn-ZSM-5 and Mn-
ZSM-57 may reflect the presence of some Mn cations at sites
zeolites (ZSM-58: 0.36 × 0.44 nm; 8.4% or kprim/ksec
=
0.12) resemble those for noncatalytic autoxidation (8.2%, or
Scheme 1. Proposed ROOH decomposition mechanism on zeolite-confined Mn sites. ROOH = hexylhydroperoxides, ROH = hexanol, R(–H)=O = hexanone and
hexanal; Mn–OOR, Mn–OR, and Mn–R are Mn-bound reactive intermediates.

322
B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
Table 3
a
Initial terminal selectivity (within 0.5 h) of n-hexane oxidation on Mn-zeolites compared to non-catalytic pathways (without a catalyst)
ZSM-58
ZSM-5
ZSM-57
MOR
Without catalyst
Open windows (nm)
0.36 × 0.44
0.760/0.359
8.4
0.53 × 0.56
0.630/0.464
24
0.51 × 0.54
0.675/0.531
14
0.65 × 0.70
0.664/0.639
9.5
–
–
8.2
0.12
b
c
D
i
/d
(nm)
max
Initial terminal selectivity (%)
Primary selectivity index (k
/k
sec
)
0.12
0.42
0.22
0.14
prim
a
b
c
3
3
25 cm n-hexane, 0.20 cm dichlorobenzene, 1.00 g Mn-zeolites, 403 K, and 0.7 MPa.
Maximum included sphere diameter D .
i
Maximum free sphere diameters d
.
max
3.4. Reaction pathways and elementary steps in n-hexane
oxidation catalyzed by Mn zeolites
Brønsted acid sites catalyze ROOH decomposition via in-
tramolecular rearrangements that form ROH and R(–H)=O
without radical chains [40,41]. Cyclohexyl hydroperoxide de-
composition rates increased markedly when Na+ cations were
replaced by H+ on Y zeolite [40]. Infrared bands for cyclo-
hexyl hydroperoxide (1367 cm−1) were detected during cyclo-
hexane photooxidation on Na-Y but not on H-Y, because cy-
clohexyl hydroperoxide decomposed rapidly on H+ sites [40].
Thus, acidic OH groups can inhibit the accumulation of perox-
ide intermediates in the liquid phase as the reaction proceeds.
In fact, ROOH was not detected even after 44 h during n-
hexane oxidation on H-MOR at conditions leading to substan-
tial ROOH formation even for noncatalytic reactions (16 mM
or 1% conversion after 6 h). The absence of ROOH during
n-hexane oxidation on H-MOR is in contrast to its detectable
presence when Mn-MOR is used, because ROOH is regener-
ated on Mn active sites via a subcycle within the catalytic se-
quence (steps IV and V, Scheme 1) [4,34]. H-MOR scavenges
ROOH intermediates, as also occurs for 10-ring zeolites (H-
ZSM-57 and H-ZSM-5). Thus, volumetric n-hexane oxidation
rates (∼3 × 10−10 molL−1 s−1; see the slope in Fig. 7) on H-
MOR reflect noncatalytic ROOH formation rates via direct re-
actions of RH with O2 [e.g., Eq. (1)] without ROOH-mediated
autoxidation propagation cycles. In contrast, 8-ring windows in
H-ZSM-58 (0.36×0.44 nm) prevent access to intrachannel H+
sites by RH or ROOH [38,39], a requirement for ROOH decom-
position. Thus, H-ZSM-58 cannot scavenge ROOH intermedi-
ates, and, consequently, ROOH decomposition rate constants
on H-ZSM-58 and Mn-ZSM-58 resemble those for noncatalytic
autoxidation reactions of n-hexane (Fig. 8).
Fig. 6. Terminal selectivities for n-hexane oxidation vs reaction time: non-
catalytic oxidation (Q), Mn-ZSM-57 ("), and Mn-ZSM-5 (F). Reaction con-
ditions: 25 cm n-hexane, 0.20 cm dichlorobenzene, 1.00 g Mn zeolites,
3
3
403 K, and 0.7 MPa.
kprim/ksec = 0.12); these data are consistent with previous re-
ports that the 8-ring channels in ZSM-58 are inaccessible to
small alkanes [38,39]. The 12-ring zeolites gave only slightly
higher terminal selectivities than those for noncatalytic path-
ways (MOR: 0.65 × 0.70 nm; 9.5% or kprim/ksec = 0.14),
because their large channels do not introduce spatial con-
straints that disfavor the activation of the weaker secondary
C–H bonds.
Terminal selectivities on Mn cations within 10-ring zeolites
(ZSM-57: 0.54 nm; 14% or kprim/ksec = 0.22; and ZSM-5:
0.56 nm; 24% or kprim/ksec = 0.42) were significantly higher
than for noncatalytic pathways, but decreased with increasing
reaction time (or ROOH concentration; Fig. 6), because unse-
lective noncatalytic oxidation pathways prevail as ROOH con-
centrations increase. Thus, high terminal selectivity requires
predominant contributions from catalytic pathways on Mn sites
contained within microporous channels (such as 10-ring zeo-
lites), which provide the required spatial constraints on catalytic
steps (Scheme 1). In what follows, we examine the potential
effects of Brønsted acid sites, present in all Mn-exchanged ze-
olites (Tables 1 and 2), on ROOH-mediated oxidation path-
ways. We also explore the influence of ROOH concentra-
tion on terminal oxidation selectivity for these zeolite struc-
tures.
The role of H+ sites within 10-ring and 12-ring zeolite chan-
nels as ROOH scavengers was confirmed by reacting n-hexane-
O2 mixtures on Mn-ZSM-5 for a prespecified period, during
which ROOH concentrations increased and terminal selectiv-
ity concurrently decreased, and then adding H-ZSM-5 (Fig. 9)
to scavenge ROOH molecules already formed. ROOH con-
centrations decreased immediately (from 0.02 to 0.005 mM)
on the addition of H-ZSM-5 (0.1 g) to a reacting mixture
containing Mn-ZSM-5 (1.0 g) after 0.05% n-hexane conver-
sion (Fig. 9b). The initial terminal selectivity was ∼24% (or
k
prim/ksec = 0.42) in these experiments and decreased to ∼16%
(kprim/ksec = 0.25) after 0.05% n-hexane conversion. It re-
mained constant at this value (up to 0.13% conversion) after

B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
323
(a)
3
Fig. 7. n-Hexane oxidation on H-MOR. Reaction conditions: 25 cm n-hexane,
0.20 cm dichlorobenzene, 0.5 g H-MOR, 403 K, and 0.7 MPa.
3
(b)
Fig. 9. Terminal selectivity and measured ROOH concentration during
n-hexane oxidation on Mn-ZSM-5: 1.00 g Mn-ZSM-5 (", !) and 1.00 g
3
Mn-ZSM-5 + 0.10 g H-ZSM-5 (Q, P). Reaction conditions: 25 cm n-hexane,
3
Fig. 8. ROH + R(–H)=O formation rates vs [ROOH] on Mn-ZSM-58 (1.00 g,
F), H-ZSM-58 (1.00 g, Q), and blank (non-catalytic, "). Reaction conditions:
25 cm n-hexane, 0.20 cm dichlorobenzene, 403 K, and 0.7 MPa.
0.20 cm dichlorobenzene, 403 K, and 0.7 MPa; for the control experiment,
0.10 g H-ZSM-5 was added after n-hexane oxidation on 1.00 g Mn-ZSM-5 for
1 h.
3
3
portional to ROOH concentrations, as we also report here for
n-hexane oxidation on Mn active sites exchanged onto 10- and
12-ring zeolites (Fig. 5). These rates are comparable to those
measured for n-hexane oxidation on MnAPO-18 and MnAPO-
5 under similar reaction conditions [4], as discussed above.
These data and the structural resemblance between Mn-zeolites
and MnAPO materials indicate that n-hexane oxidation pro-
ceeds via similar catalytic sequences (Scheme 1). ROOH de-
composes on Mn sites to form either Mn-bound RO species
(Mn–OR; steps I and II in Scheme 1) or Mn-bound ROO inter-
mediates (Mn–OOR; step Iꢀ in Scheme 1). Mn–OR then reacts
with RH to give ROH and Mn-bound alkyl species (Mn–R; step
III), which in turn react with O2 to reform ROO on Mn centers
introducing H-ZSM-5, which scavenged ROOH and minimized
contributions from unselective noncatalytic pathways. These
terminal selectivities were much greater than on Mn-ZSM-5
alone (∼10% or kprim/ksec = 0.15) at this higher conversion
level (0.13%; Fig. 9a). Thus, H-ZSM-5 scavengers maintain
high terminal selectivities by decreasing ROOH concentrations
and concomitant noncatalytic routes, albeit with a concurrent
marked decrease in catalytic alkane oxidation rates, which are
proportional to the prevalent ROOH concentration.
Kinetic and isotopic studies have shown that alkane reac-
tions with O2 on MnAPO-5 proceed via heterogeneous chain-
transfer pathways limited by organoperoxide decomposition on
Mn redox-active sites [34]. Product formation rates were pro-

324
B.-Z. Zhan et al. / Journal of Catalysis 245 (2007) 316–325
(Mn–OOR; step IV). Mn–OOR decomposes to form aldehyde
or ketone [R(–H)=O] to regenerate Mn sites (step VI) or reacts
with RH to form another ROOH molecule (step V). Steps IV
and V provide a subcycle within the catalytic sequence, form-
ing additional ROOH molecules that maintain the ROOH con-
centration and propagate the catalytic cycle. These branching
pathways are unavailable during ROOH decomposition on H-
zeolites because of their inability to regenerate ROOH [40,41],
as discussed above (Fig. 5 vs Fig. 7).
In this oxidation sequence, the position of oxygen insertion
can be influenced by spatial constraints that orient alkanes as
they approach Mn-bound reactive intermediates so as to form
terminal 1-RO, 1-R, and 1-ROOH species (via steps II, III, and
V) preferentially over species containing O atoms at weaker
secondary C–H bonds. These steps reflect spatial constraints on
ROOH decomposition and some preference for the activation of
terminal C–H bonds by Mn-bound organic species involved in
the heterogeneous chain-transfer processes of Scheme 1 rather
than the direct terminal C–H bond activation by unoccupied Mn
redox-active sites.
species, leading to enhanced terminal selectivities. In particular,
Mn-ZSM-5 gives a three-fold increase in initial terminal selec-
tivity compared with unselective noncatalytic pathways (24%
or kprim/ksec = 0.42 vs 8.2% or kprim/ksec = 0.12). Terminal
selectivities decrease with increasing reaction time (or ROOH
concentration) due to the increasing contribution from unselec-
tive autoxidation, which is more sensitive to the ROOH con-
centration than heterogeneous catalytic steps. Sustained high
terminal selectivity was attained by the addition of 10-ring and
12-ring H-zeolites, which scavenge ROOH intermediates; ter-
minal selectivity was maintained with increasing conversion,
albeit at comparably lower conversion rates.
Acknowledgments
The authors acknowledge financial support and permission
to publish these results from ExxonMobil Research and Engi-
neering Co.
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