Water-promoted hydrocarbon activation catalyzed by binuclear gallium sites in ZSM-5 zeolite

Article abstract of DOI:10.1002/anie.200702463

Steamy alkane activation: Propane dehydrogenation over Ga/ZSM-5 is substantially improved upon addition of steam to the hydrocarbon feed, which is due to an increase of the steady-state concentration of hydroxylated reaction intermediates. Quantum-chemical computations indicate that H₂ recombination, required to initiate the catalytic cycle, is more favorable over binuclear oxygen-bridged Ga³⁺ complexes than over mononuclear GaO⁺ ions (see picture). (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200702463

Angewandte
Chemie
DOI: 10.1002/anie.200702463
Alkane Activation
Water-Promoted Hydrocarbon Activation Catalyzed by Binuclear
Gallium Sites in ZSM-5 Zeolite**
Emiel J. M. Hensen,* Evgeny A. Pidko, Neelesh Rane, and Rutger A. van Santen
Dedicated to Süd-Chemie on the occasion of its 150th anniversary
Catalytic reactions of hydrocarbons in zeolites can be
profoundly affected by extraframework transition-metal
cationic clusters. MFI zeolites containing dehydrogenating
species such as gallium, zinc, and platinum form the basis for
light alkane aromatization (Cyclar, Aroforming, Z-forming)
processes.[1,2] Combining partial alkane dehydrogenation with
subsequent olefin conversion is another option.[3] An addi-
tional appealing example is the incorporation of methane into
pair formed by a Ga3+ ion and the extraframework oxygen
atom. The following elementary reaction steps [Eqs. (1)–(4)]
may be proposed for alkane (RH) dehydrogenation by the
gallyl ion.
þ
½Ga3þO2ꢀ
Š
þ þ RH ! ½Ga3þðRꢀÞðOHꢀފ
ð1Þ
þ
½Ga3þðRꢀÞðOHꢀފ ! ½Ga3þðHꢀÞðOHꢀފþ þ olefin
ð2Þ
ð3Þ
ð4Þ
gasoline products in the MTG process over Ga/HZSM-5.[4]
A
þ
½Ga3þðHꢀÞðOHꢀފ ! ½Ga3þO2ꢀ
Š
þ þ H2
catalytic issue relates to the nature of the cationic clusters.
Are the alkanes activated by isolated cations, or do the
extraframework oxygen atoms also play a role? Current
literature on Zn- and Ga-promoted catalysis in zeolites
mainly refers to systems in which the state of the cationic
cluster is not well-defined.[1,2] As a starting point for this study,
a zeolitic material has been chosen in which Ga+ ions can be
considered as single-site catalysts. The water-induced
enhancement in the rate of alkane activation by Ga/ZSM-5,
as discussed herein, relates to transformation of these single
Ga+ centers to binuclear cationic Ga complexes involving a
Ga2O22+ core.
þ
½Ga3þðHꢀÞðOHꢀފ Ð Gaþ þ H2O
Earlier studies[6,10] have shown that H2O formation
[Eq. (4)] is energetically favored over H2 desorption
[Eq. (3)] catalyzed by these mononuclear GaO+ species.
Indeed, oxidized Ga/ZSM-5 rapidly looses its high initial
activity in propane conversion because of Ga3+ reduction to
Ga+.[8] According to this reaction scheme, GaO+ can be
stabilized by addition of steam. Herein, we report exper-
imental results showing that addition of steam indeed
substantially increases the rate of alkane dehydrogenation
over Ga/ZSM-5 and results in stable operation. Amechanistic
model is proposed, which is supported by DFT calculations of
the stability of various possible cationic Ga-containing species
formed upon hydroxylation. Also, energy barriers associated
with H2 recombination are reported.
Recent experimental[5–8] and computational[9–13] studies
have contributed to our understanding of the mechanism of
alkane activation by single Ga Lewis acid sites. As candidate
active sites, various mononuclear Ga-containing cations (Ga+,
GaH2 , GaH2+, GaO+) have been considered.[8–13] Quantum-
+
chemical calculations[13] showed that C H bond activation by
ꢀ
reduced Ga/ZSM-5 proceeds over the Lewis acid–base pair
formed by the Ga+ ion and a zeolite-framework basic oxygen
anion (Ga+···ZOꢀ). The experimentally observed increase in
activity of Ga+ species in ZSM-5 upon oxidation at 473 K by
nitrous oxide was ascribed to formation of gallyl (GaO+)
The results of catalytic activity measurements of propane
conversion as a function of the steam partial pressure over
initial Ga+ ions in ZSM-5 are summarized in Table 1. Clearly,
Table 1: Conversion (X) and molar hydrocarbon product distribution
during reaction of propane over Ga/ZSM-5 as a function of the steam
partial pressure and over parent HZSM-5.[a]
ions.[8] Their higher reactivity is derived from more facile C
H bond cleavage catalyzed by the stronger Lewis acid–base
ꢀ
[c]
PH O
[kPa]
X
[%]
CH4
[%]
C2H4
[%]
C3H6
[%]
C4H8
[%]
BTX[b]
[%]
H2
2
[*] Dr. E. J. M. Hensen, E. A. Pidko, N. Rane, Prof. R. A. van Santen
Schuit Institute of Catalysis
Ga/ZSM-5
0.00
0.01
0.05
0.3
0.5
1
7
14
16
19
16
14
11
5
7
8
10
11
11
11
9
15
16
19
19
19
21
80
65
63
58
61
2
4
3
2
2
2
1
3
7
7
7
4
4
2
1.06
1.10
1.16
1.19
1.11
1.08
0.98
Eindhoven University of Technology
P.O. Box 513, Eindhoven (The Netherlands)
Fax: (+31)40-245-5054
E-mail: e.j.m.hensen@tue.nl
Homepage: http://www.catalysis.nl
62
65
[**] The authors thank the European Union for financial support from
the IDECAT framework and Dr. Pieter Magusin for NMR measure-
ments. NWO-NCF and NWO-DUBBLE are acknowledged for
computing and XAS facilities, respectively. E.H. thanks the Tech-
nology Foundation STW for financial support.
4
HZSM-5
14
0.00
5
42
42
–
–
0.24
[a] 50 mg catalyst, atmospheric pressure, T=823 K, WHSV=
11.8 gpropane gcatalystꢀ1 hꢀ1 (WHSV=weight-hourly space velocity); [b] ben-
zene, toluene, xylenes; [c] moles of H2 per mole converted C3H8.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 7273 –7276
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7273

Communications
cofeeding of water leads to a substantial increase of the
propane conversion. The conversion passes a maximum at
bered ring of the wall of the sinusoidal channel of MFI zeolite.
The cluster approach was chosen, because modeling of the
unit cell of MFI zeolite with 96 T atoms is computationally
prohibitive. On the other hand, the cluster with 14 T atoms is
large enough to account for all significant interactions
between the cationic species and the zeolite framework as
well as to describe activation of small molecules such as H2
and H2O. This conclusion is supported by recent quantum-
chemical calculations with this cluster.[12] Water dissociation
at cluster I is exothermic (DE = ꢀ102 kJmolꢀ1, Figure 1) and
pH O of 0.3 kPa. Catalyst performance in the presence of
2
steam (0.3 kPa) was found to be stable over a period of at
least 4 h. The very different product composition between
Ga+/ZSM-5 in the absence of steam and HZSM-5 points to a
change in the mechanism from protolytic hydrocarbon
ꢀ
ꢀ
activation involving C H and C C bond cleavage for
ꢀ
HZSM-5 to dominant C H activation by the Lewis acidic
Ga cations.[8]
Addition of water to the feed strongly increases the
activity. The main product remains propylene, although with
increasing water content slightly increased amounts of
methane and ethylene are formed. The formation of methane
and ethylene is due to protolytic cracking of propane. This
finding indicates some regeneration of Brønsted acid protons,
which are formed upon hydrolysis of gallium species.[6] More
ethylene than methane is formed because of additional
proton-catalyzed oligomerization/cracking reactions of pro-
pylene. The carbon selectivity to dehydrogenated products
remains well over 85%. The increased dehydrogenation
activity of Ga/ZSM-5 compared to HZSM-5 is obvious from
the increased amount of hydrogen formed. No carbon oxides
were detected in the reactor effluent. The activity decrease at
higher pH O is likely due to more complete hydrolysis of the
2
active intrazeolitic Ga sites. 27Al NMR spectroscopy meas-
urements indicate that prolonged steam exposure does not
lead to massive redistribution of tetrahedral aluminium
species (see the Supporting Information).
The promoting effect of water is due to the formation of
reactive partly hydrolyzed gallium species. For mononuclear
gallium sites, the catalytic cycle is initiated by desorption of
H2 from [Ga3+(Hꢀ)(OHꢀ)]+ to form the GaO+ ion. However,
Table 2 shows that the energy barrier for hydrogen recombi-
Figure 1. Relative stability of binuclear cationic Ga species stabilized
by the eight-membered ring of ZSM-5 zeolite. For clarity, the zeolite
clusters are not included in the depicted structures. All of the
structures involve bidentate coordination of each Ga cation to two
zeolite-framework oxygen anions of the 14-T-atom zeolite cluster (see
the Supporting Information).
Table 2: Comparison of reaction energies (DE) and activation barrier
energies (DE°) for H2 and H2O desorption over various Ga clusters.
leads to formation of structure II, which contains
a
[Ga3+(Hꢀ)(OHꢀ)]+ ion in which the hydroxyl group coordi-
nates to the neighboring Ga+ ion. This structure can rearrange
by migration of the hydroxyl H atom to the Ga+ ion to give
structure III, an oxygen-bridged binuclear [{Ga3+(Hꢀ)}2O]2+
ion. The latter rearrangement allows an additional stabiliza-
tion energy of 44 kJmolꢀ1. Reaction of a second water
molecule with structure III results in further stabilization of
the extraframework gallium dimer by 69 kJmolꢀ1 and gives
structure IV (Figure 1). Structure IV contains an almost
square and planar Ga2O2 core with hydride ions and protons
bound to the extraframework Ga cations and O anions,
respectively. Rearrangement of structure IV to the conven-
tional situation with two isolated [Ga3+(Hꢀ)(OHꢀ)]+ ions
(structure V) interacting through a hydrogen bond costs
68 kJmolꢀ1.
Reaction[a]
DE [kJmolꢀ1
]
DE° [kJmolꢀ1
]
II!VI+H2
147
102
127
113
242
128
263
230
211
142
305
259
II!I+H2O
IV!VII+H2
IV!II+H2O
[Ga3+(Hꢀ)(OHꢀ)]+!GaO+ +H2
[b]
[Ga3+(Hꢀ)(OHꢀ)]+!Ga+ +H2O[b]
[a] Roman letters refer to structures in Figure 1; [b] reference [10].
nation is prohibitively high (305 kJmolꢀ1), which is due to the
very low acidity of the hydrogen atom attached to the oxygen
atom. In agreement with earlier computations,[11] water
formation is strongly favored. We propose that multinuclear
Ga sites are formed. To clarify whether such species can be
generated upon water adsorption and whether their forma-
tion facilitates hydrogen desorption, we studied the inter-
action of two distantly placed Ga+ ions with H2O by quantum-
chemical cluster calculations in the DFT formalism. The
Ga+ ions are stabilized by two negatively charged Al-con-
taining oxygen tetrahedra embedded in a cluster of 14
T atoms (T= Si, Al) representing the elongated eight-mem-
Since hydrogen-atom recombination at the hydroxylated
structures II and IV is required to initiate alkane dehydrogen-
ation, the corresponding energetics were evaluated. Reaction
energies and activation barriers are listed in Table 2. Coor-
dination of the hydroxyl group of the [Ga(Hꢀ)(OHꢀ)]+ ion to
the neighboring gallium cation results in a significant decrease
of the reaction energy for H2 recombination (147 kJmolꢀ1)
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compared to that from the isolated [Ga(Hꢀ)(OHꢀ)]+ ion
(242 kJmolꢀ1).
of Al pairs in the ring structures of ZSM-5 zeolite for Si/Al
ratios up to 22.[15]
In contrast, the effect on H2O desorption is much smaller.
Very similar trends are noted for H2 and H2O desorption from
structure IV. The strong decrease in the activation energy for
H2 recombination on the Ga dimer is due to the increased
acidity of the OH group. Compared to the [Ga3+(Hꢀ)(OHꢀ)]+
In summary, addition of water to reduced Ga cations
stabilizes binuclear hydroxyl-bridged Ga-containing cationic
reaction intermediates. The increased acidity of the bridging
hydroxyl bond and the stable tetrahedral configuration
around Ga3+ ions facilitate H2 desorption. In essence, the
result that H2 recombination is facilitated at Ga sites is in
agreement with earlier kinetic studies over reduced Ga/
HZSM-5.[16] The higher activity of dimeric oxygen-bridged
Ga species compared to single-site Ga+ species is in line with
the lower energy for H2 recombination.[13] Dehydrogenation
of an alkane over the Ga2O2 dimer proceeds by initial
heterolytic dissociation by the Lewis acid pair formed by Ga3+
and the basic extraframework oxygen atom (Scheme 1) with
ꢀ
ion, the O H bond is weaker in the dimer, which can be
understood from the bond-order conservation principle.[14]
Moreover, for the dimer complex, H2 recombination does
not result in complete decomposition of the stable tetrahedral
arrangement around the Ga cations. Hydrogen recombina-
tion from the isolated [Ga3+(Hꢀ)(OHꢀ)]+ ion results in less
stable three-fold coordinated Ga.
These theoretical insights imply that H2 desorption should
be facile from hydroxylated Ga clusters in ZSM-5. To verify
this assertion, we carried out a temperature-programmed
experiment over Ga+/ZSM-5 in an atmosphere of 0.3 vol%
H2O in He. Hydrogen evolution is observed around 600 K
(see the Supporting Information). In a similar experiment
with HZSM-5, no hydrogen was formed. The desorption
energy estimated from the H2 desorption maximum gives a
value of 140 ꢁ 12 kJmolꢀ1. This result agrees well with the
computed value for H2 recombination from the fully hydroxy-
lated cluster (IV!VII, DE = 127 kJmolꢀ1, Table 2). Struc-
tural information about the Ga species in Ga/ZSM-5 was
obtained by extended X-ray absorption fine structure
(EXAFS) spectra at the Ga K edge. Table 3 lists the
Scheme 1. Proposed reaction cycle for alkane dehydrogenation over
binuclear Ga-containing cations in the presence of water.
Table 3: EXAFS fit parameters for Ga+/ZSM-5 oxidized by nitrous oxide
at 473 K.[a]
subsequent olefin desorption and hydrogen recombination.
Similar to earlier findings for the single-site Ga+ catalyst,[13]
the last two steps may take place in a concerted manner.
Continuous addition of water is required to maintain a high
steady-state concentration of the hydroxylated reaction
intermediate.
Backscatter
N
R [Š]
Ds2 [”10ꢀ3 Š2]
DE0 [eV]
Ga-O
4.05
4
1.84
1.799–2.250
8.48
8.3
Ga-O (VII)[b]
Ga-Ga
1.09
1
2.98
2.816
14.2
12.5
ꢀ12.3
Ga-Ga (VII)[b]
Ga-Al
1.11
1
2.72
2.772–2.998
9.8
Ga-Al (VII)[b]
Experimental Section
[a] Estimated accuracies: N ꢁ20%, R ꢁ0.04 Š, Ds2 ꢁ20%, DE0 ꢁ
Ga/ZSM-5 was prepared according to reference [8]. In Ga/ZSM-5 the
Brønsted protons have been quantitatively replaced by Ga+ ions.
Propane conversion was carried out in a single-pass atmospheric
quartz reactor by passing a mixture of C3H8 (5 kPa) in He with
WHSV= 11.8 gpropane gcatalystꢀ1 hꢀ1 at 823 K. Water was added via a
well-thermostated saturator. X-ray absorption spectroscopic meas-
urements were carried out at the Ga K edge (Dubble, ESRF).
Structural information was extracted from the k1- and k3-weighted
EXAFS functions by multiple-shell fitting in R space. Quantum-
chemical cluster calculations at the DFT level were carried out to
compute the relative stability of various Ga cationic species
coordinating to a zeolite cluster. Azeolite cluster representative of
the elongated eight-membered ring of the sinusoidal channel of MFI
10%; [b] from DFT calculations.
EXAFS fit parameters for a model catalyst obtained by
oxidation of Ga+/ZSM-5 by nitrous oxide at 473 K that shows
a high initial activity in propane conversion.[8] Four O atoms
ꢀ
are detected at a distance of 1.84 Š. AGa Ga coordination is
identified at an interatomic distance of 2.98 Š with a
ꢀ
coordination number close to one. AGa Al backscatterer
at a distance of 2.72 Š with a coordination number of 1.1 is
included. These structural data attest to the presence of a
Ga2O2 core structure coordinating to the zeolite oxygen
anions with a structure in reasonable agreement with the DFT
calculations. While EXAFS analysis provides averaged struc-
tural data, we note that the computations have been carried
out for one specific ring structure. Adetailed study of the sites
of bivalent cations has shown a preference for the occurrence
zeolite and containing two framework substitutions (Al3+ for Si4+
)
was employed. Further experimental and computational details are
given in the Supporting Information.
Received: June 6, 2007
Revised: August 8, 2007
Published online: August 27, 2007
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Communications
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Keywords: gallium · heterogeneous catalysis ·
hydrocarbon activation · Lewis acids · zeolites
.
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