Catalytic consequences of composition in polyoxometalate clusters with keggin structure

Article abstract of DOI:10.1002/anie.200701292

(Graph Presented) Rate constants for acid-catalyzed butanol dehydration on Keggin-type polyoxometalate clusters H_8-nXⁿ⁺W ₁₂O₄₀ (X=P, Si, Al, Co) increase as the oxidation state of the central atom increases and the number of cluster protons concurrently decreases (see diagram). These trends reflect the lower deprotonation enthalpies of clusters with high valent central atoms and their stability as the anionic conjugate base in ionic transition states involved in dehydration catalysis.

Full text of DOI:10.1002/anie.200701292

Communications
DOI: 10.1002/anie.200701292
Cluster Catalysis
Catalytic Consequences of Composition in Polyoxometalate Clusters
with Keggin Structure**
Josef Macht, Michael J. Janik, Matthew Neurock, and Enrique Iglesia*
Reliable correlations among structure, composition, and
function in heterogeneous catalysis require well-defined
atomic connectivity within active structures and the assess-
ment of the specific elementary steps and reaction inter-
mediates responsible for the relevant catalytic function. The
non-uniform nature of typical active structures creates
significant challenges because probes of structure and func-
tion average such heterogeneity in complex ways. Polyoxo-
metalate (POM) clusters with stable Keggin structures and
well-defined atomic connectivity provide the compositional
diversity required for a rigorous assessment of the conse-
quences of composition on catalytic reactivity.
butanol with another butanol molecule are unreactive
spectators. Measured turnover rates depend on the rate
ꢀ
constant for C O cleavage and on the equilibrium constant
for dimer formation; their values were obtained from the
measured effects of 2-butanol pressure on dehydration rates.
Both constants increased with increasing valence of the
central atom, as the deprotonation enthalpy—a measure of
the relative stability of the conjugate base—decreased.
Supported POMclusters catalyze 2-butanol dehydration
at low temperatures (333–373 K) without detectable deacti-
vation or structural changes. Reaction rates decreased sharply
with increasing 2-butanol pressure (Figure 1) on all POM
We describe herein the effects of the central atom X (P, Si,
n+
Al, and Co) in Keggin-type POMclusters (H
X W12O40;
8ꢀn
H8ꢀnXW) on acid strength based on calculated deprotonation
enthalpies, which reflect intrinsic acid strength, and reactivity,
based on a rigorous analysis of elementary rate constants,
using 2-butanol dehydration as a probe reaction. Previous
studies have not reported intrinsic acid properties for these
materials and treated reactivity merely in terms of measured
rates without the mechanistic interpretations required for
meaningful composition–function relations.[1]
The ubiquitous aggregation and incomplete and environ-
ment-dependent accessibility[2,3] of POMclusters was mini-
mized by dispersing them onto SiO2 supports. The number of
accessible protons, required for rigorous measurements of
turnover rates, was determined by titration with pyridine
ꢀ
during catalysis. We conclude that C O bond breaking in
chemisorbed butanol monomers is the kinetically relevant
step, while butanol dimers that form by solvation of adsorbed
Figure 1. 2-Butanol dehydration rate r (in 10ꢀ3 molecules 2-butanol
POMꢀ1 sꢀ1) as a function of 2-butanol pressure for 0.04H3PW/Si ( ),
*
~
!
&
0.04H4SiW/Si ( ), 0.04H5AlW/Si ( ) ,and 0.04H6CoW/Si ) (343 K,
0.04POMnmꢀ2, conversion <10%).
[*] J. Macht, Prof. E. Iglesia
Department of Chemical Engineering
University of California at Berkeley
Berkeley, CA 94720 (USA)
catalysts, as reported also for ethanol dehydration on bulk
crystalline H3PW12.[4] This behavior reflects solvation of
reactive C2H5OH2 intermediates to form less-reactive
+
Fax: (+1)510-642-4778
E-mail: iglesia@berkeley.edu
(C2H5OH)2H+ dimers. 13C NMR and infrared spectra, and
ethanol uptakes confirmed these conclusions.[4,5] The mea-
sured kinetic response for 2-butanol dehydration also reflects
the formation of unreactive co-adsorbed 2-butanol dimers.
Theoretical estimates of the enthalpy of formation of butanol
Prof. M. J. Janik
Department of Chemical Engineering
Pennsylvania State University
University Park, PA 16802 (USA)
Prof. M. Neurock
Department of Chemical Engineering, University of Virginia
Charlottesville, VA 22904 (USA)
dimers by interactions of butanol with
a monomer
(ꢀ88.1 kJmolꢀ1 for H3PW) confirmed the stable and unreac-
tive nature of such dimers.
[**] Support by the Chemical Sciences, Geo Sciences, Bio Sciences
Division, Office of Basic Energy Sciences, Office of Science US
Department of Energy under grant number DE-FG02-03ER15479 is
gratefully acknowledged. We also thank Dr. Cindy Yin for the
synthesis of bulk H5AlW and H6CoW samples.
The identity of the central atom influenced 2-butanol
dehydration rates on SiO2-supported POMclusters
(0.04 H8ꢀnXW/Si; 0.04 POMnmꢀ2 surface density; Figure 1).
At 2-butanol pressures below 0.1 kPa, dehydration rates
decreased in the sequence: H3PW> H4SiW> H5AlW>
H6CoW, whereas these trends were essentially reversed at
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
higher pressures (H5AlW> H4SiW> H6CoW> H3PW). Thus,
composition–function relations and the underlying effects of
acid strength cannot be discerned by mere inspection of these
rates, without their rigorous interpretation in terms of rate
and equilibrium constants for elementary steps.
actions with the anionic Keggin structure (Figure 2). The
ꢀ
calculated activation energy for C O bond breaking is
132 kJmolꢀ1 (H3PW), whereas the 2-butene desorption acti-
vation energy is significantly lower (88 kJmolꢀ1). The level of
A plausible sequence of elementary steps includes 2-
butanol adsorption on Brønsted acid sites, its irreversible
decomposition through E1 or E2 elimination pathways, the
reversible desorption of butene isomers, and the formation of
unreactive protonated dimers (Scheme 1). At low conver-
Figure 2. DFT-calculated transition state for 2-butanol dehydration on
H3P W to asec-butyl alkoxide and a weakly bound water molecule.
substitution at the carbon atom bearing the OH group,
increased by using 1-butanol, 2-butanol, and tert-butanol as
reactants, markedly increased the dehydration rates (k1-buta-
nol:k2-butanol:ktert-butanol = 1:2000:1 ” 105 (343 K)), consistent with
Scheme 1. Proposed sequence of elementary steps for 2-butanol dehy-
dration on POM clusters with Keggin structure.
ꢀ
the kinetic relevance of C O bond cleavage and with the ionic
character of the E1 elimination transition state.
sions, water coadsorption does not influence measured rates
because of the large excess of 2-butanol and the lower
adsorption energies calculated for H2O (ꢀ67 kJmolꢀ1) rela-
tive to 2-butanol monomers (ꢀ77 kJmolꢀ1).
The elementary steps in Scheme 1 lead, with the assump-
tions of a quasi-equilibrated step 4 and adsorbed 2-butanol
(step 1) and 2-butanol dimers (step 4) as most abundant
surface species, to Equation (1) for rates measured at low
conversions (see Supporting Information for derivation).
Elimination can occur through E1 or E2 pathways.[6,7] E2
ꢀ
ꢀ
routes involve concerted cleavage of C O and C H bonds in
butanol monomers using acid–base pairs and form one butene
molecule, one OH group, and adsorbed water which is
k2½HþŠ
ð1Þ
r ¼
1 þ K4 ½C4H9OHŠ
ꢀ
subsequently desorbed. E1 pathways cleave C O bonds to
form water molecules and adsorbed butoxides; the latter
undergo H-abstraction and desorb as butene isomers. On
0.04H3PW/Si, the cis/trans ratios in 2-butenes formed by 2-
butanol dehydration and through 1-butene (double-bond
isomerization; extrapolated to zero conversion) are similar
(0.95 vs. 0.97) and much larger than the thermodynamic value
(0.40). These similar stereoselectivities reflect a common sec-
butyl alkoxide intermediate as the source of butenes in these
two reactions, and indicate the prevalence of E1 routes that
involve them. 1-Butene isomerization rates were much larger
(0.8 (POMs)ꢀ1) than 2-butanol dehydration rates (0.02–
0.09 (POMs)ꢀ1) at 343 K on 0.04H3PW/Si, indicating that
H-elimination from butoxide intermediates, required also in
double-bond isomerization turnovers, occurs much faster
than butene formation by 2-butanol dehydration. Thus, the
The k2 and K4 terms represent the 2-butanol decomposi-
tion rate constant and the equilibrium constant for dimer
formation, respectively (Scheme 1), and [H+] is the number of
accessible proton sites. Equation (1) accurately describes the
pressure dependence of measured rates, as shown by the
linear dependence of inverse rates on 2-butanol pressure
(Figure 39. At low 2-butanol pressures (< 0.1 kPa), the value
of k2[H+] determines dehydration rates; at higher 2-butanol
pressures, rates depend on (k2[H+])/K4. As a result, the
ranking of catalysts and the role of central atom and of acid
strength are substantially different at high and low reactant
pressures.
A regression analysis of the pressure dependence of
dehydration rates (per POM) gave accurate estimates for
k2[H+] and K4. [H+] values were determined by titration with
pyridine during the catalytic reaction, because accessibility
constraints within secondary POMstructures can depend
sensitively on the size and polarity of reactants and prod-
ucts.[2,3] The number of accessible protons sites [H+] was
ꢀ
step that forms these butoxide intermediates (C O cleavage
in E1 pathways) must be the kinetically relevant step in
ꢀ
dehydration catalysis. DFT calculations also indicate that C
O cleavage is kinetically relevant and that reactions occur
through carbenium ion transition states stabilized by inter-
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Figure 4. 2-Butanol decomposition rate constant k2 (in 10ꢀ3 molecules
2-butanol (H+)ꢀ1 sꢀ1) (closed symbols) and dimer formation equilibri-
um constant K4 (open symbols) as a function of deprotonation
enthalpy, defined as DHrxn of HA!Aꢀ +H+ (HA is the acid, and Aꢀ is
Figure 3. Inverse 2-butanol dehydration rate rI (in 103 POM s (mole-
cules 2-butanol)ꢀ1) as a function of 2-butanol pressure for 0.04H3PW/
~
!
&
*
Si ( ), 0.04H4SiW/Si ( ), 0.04H5AlW/Si ( ), and 0.04H6CoW/Si (
)
*
the conjugate base) and calculated by DFT. (0.04H3PW/Si ( ),
(343 K, 0.04POMnmꢀ2, conversion <10%).
~
!
&
0.04H4SiW/Si ( ), 0.04H5AlW/Si ( ), 0.04H6CoW/Si ( ), and H-
^
BEA ( ). c denotes the deprotonation energy range reported for
different zeolite catalysts.[10]
similar to that expected from stoichiometry in all samples (e.g.
0.04H3PW12/Si, 2.9[H+] measured; Table 1), consistent with
intact clusters and with weak interactions between protons
and silanols on silica surfaces.
Table 2: DFT-calculated deprotonation enthalpies DPE (DHrxn of HA!
Aꢀ +H+), activation energies for the alkoxy formation step (Ea2,calcd), and
2-butanol dimer formation enthalpies (DH4,calcd) (see Scheme 1) for 2-
n+
Table 1: Proton site density [H+], alkoxy formation rate constant k2, and
butanol dehydration on H
X W/Si.
8ꢀn
DPE[b]
Ea2,calcd
DH4,calcd
[b]
[b]
the 2-butanol dimer formation equilibrium constant K4 (see Scheme 1)
n+
for 2-butanol dehydration on 0.04H
X
W/Si.[a]
8ꢀn
H3PW
H4SiW
H5AlW
1087
1105
1121
132.4
140.0
145.8
83.7
76.7
70.3
[H+]
k2
[10ꢀ3 (sH+)ꢀ1 [c]
K4
(per POM)[b]
]
[kPaꢀ1 [c]
]
[a] HA is the acid, and Aꢀ is the conjugate base; X=P5+, Si4+, and Al3+
[b] In kJmolꢀ1
.
0.04H3PW/Si
0.04H4SiW/Si
0.04H5AlW/Si
0.04H6CoW/Si
2.9
3.8
5
60.3
28.7
16.0
6.0
21
6
2
.
6
2
[a] X=P5+, Si4+, Al3+, and Co2+; 0.04 POMnmꢀ2 surface density; 343 K,
101 kPa He, 0.05–0.6 kPa 2-butanol. [b] Determined by titration with
pyridine during 2-butanol dehydration reaction (343 K, 0.5 kPa 2-butanol,
0.9 Pa pyridine). [c] Determined by fitting Eq. (1) to the experimental
data.
cluster concurrently increased. Deprotonation enthalpies
rigorously rank solid Brønsted acids in terms of their acid
strength. The values estimated by DFT suggest that Keggin-
type POMclusters (1087–1143 kJmol ꢀ1) are stronger acids
than H2SO4 (1293 kJmolꢀ1) and CF3SO3H (1248 kJmolꢀ1)
and comparable to the CB11H12H carborane acid
(1084 kJmolꢀ1).[9]
The values of k2 and K4 were similarly affected by POM
deprotonation enthalpy because butyl carbenium ion tran-
sition states and unreactive protonated 2-butanol dimers are
significantly ionic in character and benefit from the effective
delocalization of the concomitant negative charge by POM
clusters.[8] In contrast, charge is highly localized on inorganic
insulators, such as the silicate framework in zeolites, and
deprotonation enthalpies are significantly higher for zeolites
(ranging from 1171 (zeolite Y) to 1200 kJmolꢀ1 (ZSM-5)[10]
than for POMclusters. On H-BEA, the higher deprotonation
enthalpy led also to lower k2 and K4 values; these data fell on
the same correlation with deprotonation enthalpy as the
various H8ꢀnXW clusters (Figure 4). We note that the range of
deprotonation enthalpies among various zeolite structures (Y,
CHA, MOR, MFI) (1171–1200 kJmolꢀ1; 29 kJmolꢀ1 range)[10]
is smaller than for the various POMstructures reported
herein (1087–1143 kJmolꢀ1; 56 kJmolꢀ1 range). POMclusters
Both k2 and K4 decreased in parallel with decreasing
oxidation state of X in H8ꢀnXW12 clusters (P > Si > Al > Co,
Table 1) and as the number of charge-balancing protons
ꢀ
increased. Activation barriers calculated for C O cleavage in
2-butanol and dimer formation energies from DFT follow
trends with central atom similar to those measured exper-
imentally. The observed trends in k2 and K4 values parallel the
effects of central atom on the enthalpy for removing the first
proton from H8ꢀnXW12 clusters (Figure 4, and Table 2). The
deprotonation enthalpy is defined as that for AH!Aꢀ + H+
(AH is the neutral cluster; Aꢀ is the deprotonated conjugate
base). Deprotonation enthalpies were calculated using den-
sity functional theory (DFT);[8] they reflect the relative
stability of the conjugate base and the intrinsic acid strength
of the neutral cluster. These enthalpies increased
(1087 kJmolꢀ1 (H3PW), 1143 kJmolꢀ1 (H6CoW)), and the
conjugate base became less stable, as the oxidation state of
the central atom X decreased and the number of protons per
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reactants (Sigma-Aldrich, 99.5% (2-butanol), 99.8% (1-butanol),
therefore provide a greater range of acid strengths useful in
the practice of Brønsted acid catalysis.
The central atom strongly influences the reactivity of
protons in Keggin-type POMclusters through the combined
99.5% (tert-butanol, anhydrous)) were introduced as a liquid using a
syringe pump (Cole Parmer, 74900 series) and vaporized at 393 K by
injection into flowing He (Praxair, UHP). 1-Butene (Scott Specialty
Gases, 99%) flow rates, liquid 2-butanol introduction rates and He
flow rates were adjusted to give desired reactant pressures and to
keep conversions low (< 10%) and relatively constant among various
catalyst samples. Reactant and product concentrations were mea-
sured by gas chromatography using flame ionization detection
(Agilent 6890N GC, 50 m HP-1 column). Only butene products of
dehydration reactions were detected (1-butene, cis-2-butene, and
trans-2-butene). Brønsted acid sites were titrated by introducing
liquid mixtures of 2-butanol reactants (Sigma-Aldrich, 99.5%,
anhydrous) with pyridine (Aldrich, 99.9%) into flowing He to give
0.5 kPa 2-butanol and 0.9 Pa pyridine. The amount of titrant adsorbed
on the catalyst was measured from measurements of its concentration
in the effluent stream using the chromatographic protocols described
above for 2-butanol dehydration.
ꢀ
effects on the rate constant of C O bond breaking k2 and the
equilibrium constant for the formation of unreactive 2-
butanol dimers K4. Both increased in parallel as the oxidation
state of the central atom X in H8ꢀnXn+W increased (Co < Al <
Si < P) and the deprotonation enthalpy concurrently
decreased, because of the ionic character of the transition
ꢀ
state in C O cleavage and of the 2-butanol dimer. Reaction
rates reflect k2 and K4 values in a manner that leads to
compensating effects and to rates that benefit from stronger
acids at low butanol pressures but from weaker acids at higher
pressures. 2-Butanol dehydration rates (at 0.5 kPa 2-butanol
pressure) increased by a factor of 2.6 as deprotonation
enthalpies decreased by 34 kJmolꢀ1 (H5AlW!H3PW;
Figure 1).
Calculations were carried out using a periodic plane wave density
functional theory code VASP.[15,16] The generalized gradient approx-
imation of the Perdew-Wang form (PW91) was used to correct
exchange energies.[17] A cut off energy of 396.0 eV defined the plane
wave basis set expansion and ultrasoft pseudopotentials[18] were used
to model the electron–ion interactions. The Keggin structure was
Earlier, van Santen and Kramer proposed, based on
electronic structure calculations, a relation between the
stability of cationic species present as transition states and
the deprotonation energies of Brønsted acids.[11] Our study
provides experimental verification for this proposal for the
dehydration of 2-butanol on POMclusters in terms of a
rigorous analysis of turnover rates in terms of rate and
equilibrium constants for elementary steps. The relationship
between the stability of transition states and of intermediates
and the intrinsic acid strength is essential to design materials
with specific reactivity and selectivity in acid catalysis. Indeed,
activation barriers for steps involving ionic transition states
benefit from lower deprotonation enthalpies, but these steps
may not limit overall catalytic rates. Deprotonation enthal-
pies also influence the stability of ionic intermediates of
varying reactivity, leading to compensating effects that cause
rates that increase or decrease with increasing deprotonation
enthalpy depending on the relative concentrations of reactive
and unreactive intermediates.
3
placed in the center of a 20 ” 20 ” 20 Š supercell to allow for a
sufficient vacuum region between neighboring Keggin structures. A
single G-point was found to be sufficient to sample the first Brillouin
zone.[8] All reported structures were optimized to force values below
0.05 eV per atom. The climbing nudged elastic band method was used
to locate transition states.[19]
Received: March 23, 2007
Revised: July 25, 2007
Published online: September 7, 2007
Keywords: acid catalysis · alcohols · cluster compounds ·
.
dehydration · polyoxometalates
[1] Hammet indicator methods (e.g. T. Okuhara, C. Hu, M.
Hashimoto, M. Misono, Bull. Chem. Soc. Jpn. 1994, 67, 1186)
and liquid phase acid dissociation values (e.g. I. V. Kozhevnikov,
Chem. Rev. 1998, 98, 171; T. Okuhara, N. Mizuno, M. Misono,
Adv. Catal. 1996, 41, 133; M. N. Timofeeva, Appl. Catal. A 2003,
256, 19, and references therein) are solvent dependent and thus
no measures of intrinsic acid strength. None of the work
summarized in the aforementioned review papers discusses the
effect of central atom on the catalytic function in terms of the
rates of specific elementary steps.
Experimental Section
H3PW12O40 (Aldrich), H4SiW12O40 (Aldrich, 99.9%), H5AlW12O40
(prepared as in Ref. [12]), and H6CoW12O40 clusters (prepared as in
Ref. [13,14]) were deposited onto SiO2 (Cab-O-Sil, 304 m2 gꢀ1, pore
volume 1.5 cmꢀ3 gꢀ1; washed three times in 1m HNO3 and dried in Air
(Praxair, extra-dry, 573 K, 5 h, 20 cm3 gꢀ1)) by incipient wetness
impregnation with 1.5 cm3 of ethanol (Aldrich, anhydrous 99.5%)—
H3PW H4SiW H5AlW, or H6CoW solutions per gram of dry SiO2.
[2] N. Mizuno, M. Misono, Chem. Rev. 1998, 98, 199.
[3] M. Misono, N. Mizuno, K. Katamura, A. Kasai, K. Sakata, T.
Okuhara, Y. Yoneda, Bull. Chem. Soc. Jpn. 1982, 55, 400.
[4] K. Y. Lee, T. Arai, S. Nakata, S. Asoka, T. Okuhara, M. Misono,
J. Am. Chem. Soc. 1992, 114, 2836.
,
,
Impregnated samples were treated in flowing dry air (Praxair, extra-
dry) at 323 K for 24 h. H-BEA (Zeolyst) with Si/Al 12.5:1 was used.
Catalytic 2-butanol dehydration rates and selectivities were
measured at 343 K in a quartz flow cell (1.0 cm inner diameter)
containing samples (1–100 mg of catalysts (125–180 mm) diluted with
acid-washed quartz ( ꢁ 50 mg, 125–180 mm)) held on a porous quartz
disc. Temperatures were measured using K-type thermocouples and
set using a Watlow controller (Series 982) and a resistively-heated
furnace. Samples were treated in flowing He (80 cm3 minꢀ1, Praxair,
UHP (He), extra-dry (air)) at 343 K for 1 h before catalytic measure-
ments. Thermal treatments in He or air (80 cm3 minꢀ1, Praxair, UHP)
at 373–575 K did not influence measured rates. Transfer lines were
held at 393 K to prevent adsorption or condensation of reactants,
products, and titrants before chromatographic analysis. Butanol
[5] The additional stability gained by the formation of [R1-O-H-O-
R2]+ cationic hydrogen bonds is well known, see Molecular
Structure and Energetics, Vol. 4 (Eds.: J. F. Liebman, A. Green-
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+
therein. The well-known H5O2 ion is another example.
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[7] S. Delsarte, P. Grange, Appl. Catal. A 2004, 259, 269.
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Chem. B 1998, 102, 10817; M. J. Janik, A. C. Kimberly, B. B.
Bardin, R. J. Davis, M. Neurock, Appl. Catal. A 2003, 256, 51.
[9] I. A. Koppel, P. Burk, I. Koppel, I. Leito, T. Sonoda, M. Mishima,
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[10] M. Brꢀndle, J. Sauer, J. Am. Chem. Soc. 1998, 116, 5428.
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Communications
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