Kinetics and selectivity of 2-propanol conversion on oxidized anatase TiO2

Article abstract of DOI:10.1006/jcat.1997.1467

The steady-state kinetics of 2-propanol decomposition on oxidized anatase TiO₂ have been determined at temperatures ranging from 448 to 598 K and 2-propanol partial pressures from 8.9 to 102.7 Torr. The effects of the addition of O₂ and water to the carrier gas were also investigated. The steady-state reaction results primarily in the formation of a dehydration product, propylene, and a dehydrogenation product, acetone, with small amounts of carbon oxides also being observed. Depending on the reaction conditions, the selectivity to either propylene or acetone can range between 5 and 95%. The rate of dehydrogenation increases dramatically with the addition of both O₂ and water, while the dehydration rate is unaffected by their presence. Accordingly, the kinetics of 2-propanol decomposition were investigated using both air and an inert carrier. Using air as the carrier gas, the dehydration and dehydrogenation reactions were determined to be approximately one-half order with respect to 2-propanol partial pressure. The activation energies determined for the two processes are substantially different, 68 kJ mol^-1 for dehydrogenation and 130 kJ mol^-1 for dehydration, as evidenced by the strong temperature dependence of the decomposition selectivity. Using an inert carrier, the reaction kinetics depend in a complex fashion on the conversion of 2-propanol. The dependence on conversion was found to arise from the influence of water on the dehydrogenation kinetics. The presence of water, whether produced by 2-propanol dehydration or added independently, was found to increase the rate of 2-propanol dehydrogenation. The results of the present study can be reconciled with previously reported steady-state and temperature-programmed desorption investigations of 2-propanol on TiO₂ by recognizing the influence of both surface hydroxyls and the use of an oxidizing carrier gas on the dehydrogenation and dehydration pathways at steady state.

Full text of DOI:10.1006/jcat.1997.1467

JOURNAL OF CATALYSIS 165, 57–72 (1997)
ARTICLE NO. CA971467
Kinetics and Selectivity of 2-Propanol Conversion
on Oxidized Anatase TiO2
James E. Rekoske and Mark A. Barteau1
Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Received April 30, 1996; revised September 16, 1996; accepted September 19, 1996
as one such probe (2) and, as a result, has been studied
extensively utilizing a wide variety of experimental tech-
niques including infrared spectroscopy (3–5), temperature-
programmed desorption (TPD) (3, 4, 6) and steady-state
kinetic methods (7–16), microcalorimetry (17, 18), and
ultra-high vacuum surface science (19–23). In particular, 2-
propanol decomposition on metal oxide surfaces has been
employed frequently as a probe of surface acid–base prop-
erties (8, 11, 14, 15, 24–32). The dehydration to propylene
is assumed to proceed at acidic sites, while dehydrogena-
tion to acetone is presumed to be catalyzed, in a concerted
fashion, by both acidic and basic sites (27, 33). The acidic
and basic properties of a wide variety of metal oxides, as
determined by the relative rates of dehydration and de-
hydrogenation of 2-propanol, have been correlated with
activity and selectivity changes for a number of acid- and
base-catalyzed reactions (27, 29, 32).
Despite its frequent application as a chemical probe of
surface properties, considerable disagreement exists over
the absolute and relative rates of dehydration and dehy-
drogenation on a number of simple metal oxide materials,
including TiO2. For example, Gervasini and Auroux (8) and
Cunningham et al. (34) both studied the decomposition of
2-propanol on samples of anatase TiO2 at equivalent con-
ditions of temperature, total and partial pressure. Compar-
ison of the reported rates per unit surface area for dehydra-
tion and dehydrogenation show differences of nearly two
orders of magnitude for dehydration and over two orders
of magnitude for dehydrogenation. The ratio of dehydro-
genation to dehydration, reported to be the important pa-
rameter in determining the basic properties of the catalyst
(8), differs by approximately a factor of three between the
two investigations.
The steady-state kinetics of 2-propanol decomposition on oxi-
dized anatase TiO2 have been determined at temperatures ranging
from 448 to 598 K and 2-propanol partial pressures from 8.9 to
102.7 Torr. The effects of the addition of O2 and water to the carrier
gas were also investigated. The steady-state reaction results pri-
marily in the formation of a dehydration product, propylene, and
a dehydrogenation product, acetone, with small amounts of carbon
oxides also being observed. Depending on the reaction conditions,
the selectivity to either propylene or acetone can range between 5
and 95%. The rate of dehydrogenation increases dramatically with
the addition of both O2 and water, while the dehydration rate is un-
affected by their presence. Accordingly, the kinetics of 2-propanol
decomposition were investigated using both air and an inert car-
rier. Using air as the carrier gas, the dehydration and dehydro-
genation reactions were determined to be approximately one-half
order with respect to 2-propanol partial pressure. The activation en-
ergies determined for the two processes are substantially different,
68 kJ molꢀ1 for dehydrogenation and 130 kJ molꢀ1 for dehydration,
asevidenced bythestrongtemperaturedependenceofthedecompo-
sition selectivity. Using an inert carrier, the reaction kinetics depend
in a complex fashion on the conversion of 2-propanol. The depen-
dence on conversion was found to arise from the influence of water
on the dehydrogenation kinetics. The presence of water, whether
produced by 2-propanol dehydration or added independently, was
found to increase the rate of 2-propanol dehydrogenation. The re-
sults of the present study can be reconciled with previously reported
steady-state and temperature-programmed desorption investiga-
tions of 2-propanol on TiO2 by recognizing the influence of both
surface hydroxyls and the use of an oxidizing carrier gas on the de-
c
hydrogenation and dehydration pathways at steady state.
ꢁ 1997
Academic Press, Inc.
INTRODUCTION
The existence of empirical correlations between acidic
properties and activities of catalysts for acid-catalyzed reac-
tions has been well documented (1). It is therefore enticing
to be able to characterize the properties of materials by us-
ing simple probe reaction chemistries. The adsorption and
reaction of alcohols on metal oxide surfaces has been used
Such differences could largely be explained by the use of
different samples, and possibly different purities and con-
taminants, between the two research groups. For example,
Cunningham et al. (34) studied 2-propanol decomposition
on three different TiO2 samples and found rates varying by
1-2 orders of magnitude for both dehydration and dehydro-
genation paths, with no clear dependence on bulk crystal
structure or oxide source. However, both Bond and Flamerz
1 Fax: 302-831-1048. E-mail: barteau@che.ude.edu.
57
0021-9517/97 $25.00
c
Copyright ꢁ 1997 by Academic Press, Inc.
All rights of reproduction in any form reserved.

58
REKOSKE AND BARTEAU
(9) and Cunningham et al. (34) studied 2-propanol decom- liquid held in a constant temperature bath. The bath was
position on the same commercial sample of TiO2 (Degussa, maintained at constant temperature using a MGW/Lauda
P-25). While the dehydration rate was comparable in both RM6 thermostated bath containing either a 50/50 mixture
investigations, the rate of dehydrogenation was insignifi- of methanol and water or water, depending on the temper-
cant in the work of Bond and Flamerz, while Cunningham ature desired. Sufficient gas-liquid contact time is necessary
et al. had a substantial dehydrogenation rate and resulting to achieve reproducible partial pressures of liquid reactant
acetone selectivity (96% ). If the dehydration and dehydro- by this method. Accordingly, only a narrow range of gas
genation rates are taken as indicators of surface properties, flow rates through the saturator were used in this study,
the acid-base properties determined for this material by and the alcohol partial pressures were verified for satura-
two different groups are in complete opposition. Presum- tion against an absolute standard via gas chromatography.
ably, the common TiO2 sample used by the two groups could
The reactor was a 25-cm-long, 6-mm-diameter quartz
not itself be responsible for such considerable differences in tube which was flared to 13 mm in the center over a dis-
catalytic properties, and an alternative explanation must be tance of 2 cm. This middle section of the reactor holds the
sought. In part because of this variability of observed rates, catalyst particles, sandwiched between two plugs of quartz
a detailed interpretation of the kinetics of 2-propanol de- wool which had been washed with warm nitric acid and
composition on most metal oxidesisnot presentlyavailable. baked at 773 K for 24 h. The wall of the reactor is indented
Such fundamental understanding would seem essential in into the center of the catalyst bed for insertion of a ther-
order to confidently utilize the results of this or any probe mocouple probe for measurement of the bed temperature.
reaction for catalyst characterization purposes.
The reactivityofallcatalyst-supportingmaterialswastested
In this contribution, we begin a systematic investigation at all flow rates and partial pressures used in this study;
of the kinetics of 2-propanol decomposition on anatase conversion due to support materials was always less than
TiO2, with particular emphasis on the relative and abso- 0.02% .
lute rates of dehydration and dehydrogenation and their
The analysis section consisted of two on-line sam-
dependence upon typical experimental variables. As both ple valves with additional flow switching valves within a
air and helium are often used as carrier gases in alcohol Hewlett-Packard 5890 Series II Plus gas chromatograph to
reaction studies on metal oxides, we have investigated the allow simultaneous injection and analysis for C1 to C10 hy-
effect of carrier gas composition on the activity and selec- drocarbons and oxygenates, permanent gases and water.
tivity during 2-propanol decomposition. The steady-state Analyses for hydrocarbons and oxygenates were carried
decomposition products of 2-propanol were composed of out on a 100 meter, 0.25 mm o.d. HP-1 capillary column us-
a relatively simple combination of dehydration and dehy- ing a flame ionization detector. Permanent gases and water
drogenation products, with minor contributions at selected were completely separated by sequencing the analysis on
conditions from complete oxidation and bimolecular ether two 1/8 in stainless steel packed columns, an 8 ft HaySep
formation reactions. A strong dependence of the relative Q column and a 10 ft molecular sieve 13X column, and
rates of dehydrogenation and dehydration on the chosen were detected by a thermal conductivity detector. Switch-
reaction conditions was observed. The two factors which able valves allowed analysis of the gas stream either imme-
largely control selectivity are the reaction temperature and diately following or prior to the reactor, providing a precise
the presence or absence of O2 in the reaction mixture, with determination of the reactor feed composition as well.
the rate of dehydrogenation being particularly dependent
A commercially available anatase titanium dioxide
upon the latter. Variations in these two experimental pa- (Aldrich, BET surface: 10.0 m2/g) was used in this work. In
rameters among different research groups are most likely several cases, the kinetics results obtained with this sample
responsible for the apparent discrepancies in previous re- were compared to two other anatase titanium dioxide sam-
ports. Water was also found to have an important effect on ples (Fisher Scientific, 10.1 m2/g; American Instruments,
the reaction selectivity, as described below.
10.3 m2/g). The rates and kinetic behavior were found to
be nearly identical with all three samples. Pretreatment of
the titanium dioxide, performed in the reactor system, con-
sisted of heating the catalyst sample to 673 K in 50 cm3/min
dry air (Matheson) and oxidizing for 2 h. After oxidation,
the sample was cooled to the temperature of interest in
50 cm3/min dry air. When the temperature became stable
at the desired reaction temperature, the flow was switched
to the carrier gas. Carrier gas streams consisting of dry air
(Matheson), high-purity helium (Matheson, 99.999% ) and
0–20% oxygen (Matheson, Extra Dry) mixed with high-
purity helium have been used in this study.
EXPERIMENTAL
Steady-state kinetic investigations were performed in
a typical quartz microreactor flow system at atmospheric
pressure. The apparatus consisted of three sections. In the
first section, mass flow control valves regulate the flow of
gases into a manifold, allowing mixtures of gases to be
admitted to the reactor. Liquid reactants, in this case 2-
propanol (Sigma-Aldrich, HPLC Grade, 99.5% ), are mixed
into the manifold gas stream by saturation of the gas with

ISOPROPANOL CONVERSION ON TiO2
59
The stability of each catalyst was verified using the min-
RESULTS
imum carrier flow rate, maximum 2-propanol partial pres-
sure, and both the highest and lowest temperatures for a
minimum of 48 h, during which time no change in activity
was observed. These tests were repeated for each combina-
tion of carrier gas mixture and catalyst. During collection
of the kinetic data, each catalyst was used for no more than
36 h on-stream time before regeneration in air at 673 K.
Amounts of catalyst were varied between 0.1 and 0.7 g, de-
pending on the conversion desired at a given set of reaction
conditions.
1. Dry Air as 2-Propanol Carrier
The reaction of 2-propanol on oxidized titanium dioxide
in dry air results in products from dehydrogenation, de-
hydration and complete oxidation reaction pathways. The
selectivity of each pathway depends significantly on reac-
tion temperature but only weakly on conversion. Conver-
sion data and selectivity to carbon-containing products for
temperatures from 448 to 598 K and a 2-propanol partial
pressure of 8.9 Torr are presented in Table 1. The product
TABLE 1
Conversion, Selectivity, and Rate of Reactant Consumption and Product Formation for 2-Propanol Decomposition
on Oxidized Anatase TiO2 at a Partial Pressure of 8.9 Torr, Using Air as the Carrier
Selectivity
CO2
Rate of formation (sꢀ1
CO2
)
Rate of
T
(K)
Conv.
(% )
consumption (sꢀ1
(CH3)2CHOH
)
C3H6
(CH3)2CO
C3H6
(CH3)2CO
448
473
498
523
548
573
598
0
100
100
100
100
100
2.73 ꢂ 10ꢀ5
2.73 ꢂ 10ꢀ5
7.75 ꢂ 10ꢀ5
1.92 ꢂ 10ꢀ4
4.79 ꢂ 10ꢀ4
8.28 ꢂ 10ꢀ4
1.79 ꢂ 10ꢀ3
4.37 ꢂ 10ꢀ3
0.34
1.89
4.30
6.17
0
1.3
1.2
1.0
1.0
0.9
98.7
98.8
99.0
99.0
99.1
1.1 ꢂ 10ꢀ6
5.57 ꢂ 10ꢀ6
3.02 ꢂ 10ꢀ5
1.00 ꢂ 10ꢀ4
3.76 ꢂ 10ꢀ4
1.28 ꢂ 10ꢀ3
7.64 ꢂ 10ꢀ5
1.86 ꢂ 10ꢀ4
4.46 ꢂ 10ꢀ4
7.03 ꢂ 10ꢀ4
1.36 ꢂ 10ꢀ3
2.95 ꢂ 10ꢀ3
0.29
1.81
3.71
5.89
0
2.9
2.8
2.7
2.5
2.3
0.0
0.0
0.1
0.1
0.1
97.1
97.2
97.2
97.4
97.6
0.31
1.95
3.91
5.78
0
6.3
6.1
5.8
5.3
4.4
0.5
0.5
0.5
0.5
0.5
93.2
93.4
93.7
94.2
95.1
2.40 ꢂ 10ꢀ6
2.48 ꢂ 10ꢀ5
5.55 ꢂ 10ꢀ5
1.35 ꢂ 10ꢀ4
0.41
2.17
4.05
5.77
0
12.1
12.0
11.9
10.1
9.7
3.0
3.1
3.1
3.2
3.2
84.9
84.9
85.0
86.7
87.1
0.32
1.75
4.24
6.00
0
21.0
20.7
22.3
19.5
18.7
3.1
3.2
3.4
3.0
2.9
75.9
76.1
74.3
77.5
78.4
0.28
1.96
4.17
6.27
0
29.3
29.0
28.7
29.0
26.4
3.1
3.4
4.0
5.0
4.9
67.6
67.6
67.4
66.0
68.7
0.45
2.17
5.09
6.89
Note. Turnover frequency (tof) calculated based upon 2-propanol saturation coverage of 2.22 molecules per nm2 (6).

60
REKOSKE AND BARTEAU
TABLE 2
Typical Mass and Atom Balance for 2-Propanol Decomposition Reaction on Anatase TiO2 Using Air
as the Carrier
Inlet
Outlet
H2O
0.0216
O2
(CH3)2CHOH
1.149
O2
(CH3)2CHOH
1.078
(CH3)2CO
0.0652
C3H6
CO2
10.340
10.315
0.0020
0.0008
C
H
O
0.6894
0.1532
10.646
C
H
O
0.6892
0.1534
10.640
Total
11.4890
Total
11.4826
Note. All values are mg per minute. Conditions: 523 K, 8.9 Torr 2-propanol, balance air, 40 cm3 minꢀ1 total flow.
selectivities are based on carbon content and are defined as products, however, show differing activation energies, as
the number of moles of 2-propanol reacted to each prod- expected by the temperature dependence of the product se-
uct per 100 moles of 2-propanol consumed. Table 2 shows a lectivities, with values ranging from 68 kJ/mol for acetone to
typical mass balance from the analyses at the inlet and out- 148 kJ/mol for carbon dioxide (Fig. 2b). The measured acti-
let of the reactor, which all show 98.5+% closure. Acetone, vation energy for propylene (130 kJ/mol) was significantly
propylene, carbon dioxide, and water with trace amounts higher than that determined for acetone.
(less than 0.05% carbon selectivity) of carbon monoxide
The initial rate of reaction and selectivity were deter-
were the only products observed. Although the reactor ef- mined in a similar fashion from differential conversion data
fluent was carefully analyzed for the presence of hydrogen, collected at partial pressures of 2-propanol varying from 8.9
methane and isopropyl ether, these products were not ob- to 103 Torr at 548 and 598 K. The results, shown in Figs. 3
served when using air as the carrier gas. In all experiments and 4, permit determination of the orders of the overall re-
using air as the reactant carrier, the molar amount of water action and the individual product formation channels with
detected in the exit stream was always equal to the sum of respect to 2-propanol over the pressure range investigated.
the molar amounts of propylene and acetone.
Although the observed order is slightly higher at 548 K
Data were also collected under differential conversion (0.61) than at 598 K (0.52), the overall reaction appears to
(0.1 to 0.8%) conditions at temperatures ranging from 448 be approximately one-half order in 2-propanol at both 548
to 598 K and used to estimate the product initial selectiv- and 598 K. Similarly, the reaction orders of the products
ities, listed as the selectivity at 0% conversion in Table 1.
The product selectivities were observed to be only slightly
dependent upon 2-propanol conversion, with selectivity to
acetone minimally increasing with increasing conversion at
the expense of propylene formation. These differential data
were also used to determine initial reaction rates by plot-
ting the fractional conversion versus the catalyst-to-flow
ratio, with the slope of a line through the origin providing
the initial rate (35), as illustrated in Fig. 1. Initial rates for
2-propanol consumption and product appearance are cal-
culated from these data and presented in Table 1. The rate
is presented as a turnover frequency (tof), in which the to-
tal number of active sites has been set equal to the number
of sites active for the adsorption of 2-propanol determined
from gravimetric and TPD experiments (6).
The initial rate data of Table 1 can also be plotted in
Arrhenius form to provide a measure of the apparent ac-
tivation energy of reaction and to more clearly illustrate
the temperature dependence of the individual products, as
shown in Fig. 2. The temperature dependence of the con-
FIG. 1. Determination of initial rate of 2-propanol decomposition
using dry air as the carrier gas. The fractional conversion of 2-propanol is
plotted versus the ratio of the catalyst mass to the volumetric flow rate at
constant temperature and inlet partial pressure of 2-propanol. The initial
sumption of 2-propanol follows Arrhenius behavior with
an apparent activation energy of 75 kJ/mol (Fig. 2a). The
rate of reaction is given by the slope of a line passing though the data and
the origin.

ISOPROPANOL CONVERSION ON TiO2
61
genation products; however, the selectivity of the compet-
ing paths is now strongly dependent on both reaction tem-
perature and conversion. Conversion and selectivity data
collected between 548 and 598 K and at a 2-propanol par-
tial pressure of 8.9 Torr are reported in Fig. 5. These data
show selectivities for either acetone or propylene can range
from 5 to 95% with inert carrier, depending on the reaction
conditions. As expected, oxidation products such as carbon
monoxide and carbon dioxide are not observed with an in-
ert (helium) carrier gas. No products other than propylene,
acetone, water and hydrogen were detected during experi-
ments which yielded the data of Fig. 5. Under all conditions
using an inert carrier gas, the detected amounts of dehy-
dration products (water and propylene) were equivalent,
as were the amounts of dehydrogenation products (hydro-
gen and acetone). A representative mass balance of reac-
tor inlet and outlet streams is shown in Table 4; closure of
all balances (total mass, C, H and O) was always greater
than 98% .
FIG. 2. Arrhenius plots for determining the apparent activation en-
ergy of the initial rate for (a) the consumption of 2-propanol and (b) the
formation of dehydrogenation (acetone, ), dehydration (propylene, ᭢),
and combustion (carbon dioxide, ᭜) products. Temperature was varied
between 448 and 598 K, and dry air was used as the carrier gas.
acetone and propylene are slightly higher at 548 K (0.63
and 0.48) than at 598 K (0.55 and 0.45) and approximately
one-half order. The rate of formation of carbon dioxide,
however, seems to shift from near zero-order at 548 K (0.15)
to one-half order at 598 K (0.4).
As the small difference in reaction order suggests, there
is a slight increase in selectivity to acetone formation at
higher partial pressures of 2-propanol at both 548 and
598 K as indicated in Table 3. This increased selectivity is
again only slightly dependent on 2-propanol conversion at
higher partial pressures, which is also illustrated in Table 3.
Again, the selectivity to acetone marginally increases
with increasing 2-propanol conversion at the expense of
propylene formation.
FIG. 3. Determination of the reaction order with respect to 2-
propanol for (a) the overall consumption of 2-propanol and (b) the forma-
tion of the dehydrogenation (acetone, ), dehydration (propylene, ᭢), and
combustion (carbon dioxide, ᭜) products at 548 K. The partial pressure
2. Dry Helium as 2-Propanol Carrier
The reaction of 2-propanol on oxidized titania using dry
helium as carrier also produces dehydration and dehydro- was varied between 8.9 and 120 Torr, and dry air was used as the carrier.

62
REKOSKE AND BARTEAU
TABLE 3
Dependence of 2-Propanol Decomposition Selectivity on Partial
Pressure at 548 and 598 K on Oxidized Anatase TiO2 with Air
Carrier
Selectivity
Temp.
(K)
Pressure
(Torr)
Conv.
(% )
C3H6
CO2
(CH3)2CO
548
8.9
31.3
0
1.75
0
1.98
0
2.21
12.1
11.9
11.6
9.1
10.0
6.0
3.0
3.1
2.5
2.5
1.1
1.0
84.9
85.0
85.9
88.4
88.9
93.0
102.7
598
8.9
31.3
0
6.89
0
7.53
0
8.21
29.3
26.4
27.9
26.8
24.6
24.0
3.1
4.9
2.9
4.4
2.7
4.0
67.6
68.7
69.2
68.8
72.8
72.0
102.7
which point (temperature) is chosen for calculation of the
slope. The selectivity to acetone is highest in the region
of low observed activation energy (i.e., low temperature,
75 kJ/mol), while propylene selectivity is greatest in the re-
gion of highest observed activation energy (i.e., high tem-
perature, 115 kJ/mol). These values approach the values
of 70 and 130 kJ/mol found for the dehydrogenation and
dehydration rates, respectively, using air as the carrier.
Increasing the partial pressure of 2-propanol increases
the dehydrogenation selectivity at constant conversion at
the expense of the dehydration products. Figure 7 shows
the dependence of the dehydrogenation and dehydration
selectivities on 2-propanol partial pressure and conversion
at 548 K and a partial pressure range of 8.9 to 120.7 Torr. A
similar trend was observed using air as the carrier: the de-
hydrogenation selectivity increased with increasing partial
FIG. 4. Determination of the reaction order with respect to 2-
propanol for (a) the overall consumption of 2-propanol and (b) the forma-
tion of the dehydrogenation (acetone, ), dehydration (propylene, ᭢), and
combustion (carbon dioxide, ᭜) products at 598 K. The partial pressure
was varied between 8.9 and 120 Torr, and dry air was used as the carrier.
Because of the strong conversion dependence of the de-
hydration and dehydrogenation selectivities, initial rate de-
terminations with dry helium carrier provide little insight.
Instead, Table 5 reports the selectivities and formation rates
of the dehydrogenation and dehydration products deter-
mined at representative conversions from 523 to 598 K at
a 2-propanol partial pressure of 8.9 Torr. Also reported in
Table 5 is the rate of consumption of 2-propanol at the in-
dicated conversion.
An Arrhenius plot for 2-propanol consumption as a func-
tion of temperature constructed at a constant conversion
(5% ) is shown in Fig. 6. With increasing temperature, the
apparent activation energy increases and a similar increase
in the selectivity to propylene at constant conversion (see
Fig. 5) is observed. This is consistent with the dehydration
rate having a stronger temperature dependence than the
dehydrogenation rate, as was observed in the air-carrier ex-
periments. Indeed, the activation energy determined from
FIG. 5. Conversion dependence of the 2-propanol decomposition se-
lectivity as a function of reaction temperature at 8.9 Torr 2-propanol using
Fig. 6 varies between 75 and 115 kJ/mol, depending upon helium as the carrier gas.

ISOPROPANOL CONVERSION ON TiO2
63
At the combination of the highest partial pressure (120.7
Torr) and the highest temperature (598 K) investigated,
isopropyl ether was observed as a product when using he-
lium as carrier with a selectivity ranging from 4.0 to 1.4% ,
decreasing with increasing conversion. These data are indi-
cated at the bottom of Table 5. Isopropyl ether was not ob-
served under any other conditions, nor was isopropyl ether
observed under the identical conditions when air was used
as carrier.
TABLE 4
Typical Mass and Atom Balance for 2-Propanol Decomposition
Reaction on Anatase TiO2 Using Helium as the Carrier
Inlet
Outlet
H2 H2O (CH3)2CO C3H6
(CH3)2CHOH (CH3)2CHOH
1.149
1.003
0.0007 0.0340
0.0307
0.0799
C
H
O
0.6894
0.1532
0.3064
C
H
O
0.6893
0.1528
0.3062
3. Wet Helium as 2-Propanol Carrier
Total
1.1490
Total
1.1483
Early in the investigation it was recognized that the
amount of water vapor in the reactor could play an im-
portant role in directing the 2-propanol decomposition se-
Note. All values are mg per minute. Conditions: 548 K, 8.9 Torr
2-propanol, balance helium, 40 cm3 minꢀ1 total flow.
pressure of 2-propanol (see Table 3). Identical behavior lectivity. Since decomposition in air carrier results in both a
was observed at all temperatures investigated, though the higher dehydrogenation selectivity and a larger amount of
dehydration rate was much less dependent on partial pres- water than reaction in helium carrier, it was hypothesized
sure than on temperature. Accordingly, only small selectiv- that enhanced dehydrogenation selectivities may result
ity changes were observed with increasing partial pressure from increased levels of water. Such an effect of water could
at higher temperatures.
also be responsible for the increase in dehydrogenation
TABLE 5
Conversion, Selectivity, and Rate of Reactant Consumption and Product Formation for 2-Propanol Decomposition
on Oxidized Anatase TiO2 at a Partial Pressure of 8.9 Torr, Using Helium as the Carrier
.
Selectivity
IPEa
Rate of formation (sꢀ1
IPEa
)
Rate of
Temp.
(K)
Conv.
consumption (sꢀ1
(CH3)2CHOH
)
(% )
C3H6
(CH3)2CO
C3H6
(CH3)2CO
523
548
573
598
548b
598b
0.23
0.73
1.63
3.85
38.1
18.2
9.2
61.9
81.8
90.8
94.7
2.47 ꢂ 10ꢀ5
2.33 ꢂ 10ꢀ5
2.01 ꢂ 10ꢀ5
1.93 ꢂ 10ꢀ5
4.01 ꢂ 10ꢀ5
1.05 ꢂ 10ꢀ4
1.99 ꢂ 10ꢀ4
3.45 ꢂ 10ꢀ4
6.48 ꢂ 10ꢀ5
1.28 ꢂ 10ꢀ4
2.19 ꢂ 10ꢀ4
3.64 ꢂ 10ꢀ4
5.3
0.53
0.91
1.56
3.92
78.6
56.2
37.6
20.7
21.4
43.8
62.4
79.3
9.59 ꢂ 10ꢀ5
8.65 ꢂ 10ꢀ5
7.97 ꢂ 10ꢀ5
7.68 ꢂ 10ꢀ5
2.61 ꢂ 10ꢀ5
6.75 ꢂ 10ꢀ5
1.32 ꢂ 10ꢀ4
2.94 ꢂ 10ꢀ4
1.22 ꢂ 10ꢀ4
1.54 ꢂ 10ꢀ4
2.12 ꢂ 10ꢀ4
3.71 ꢂ 10ꢀ4
2.15
2.49
3.71
5.88
79.8
77.3
64.2
50.9
20.2
22.7
35.8
49.1
3.93 ꢂ 10ꢀ4
3.43 ꢂ 10ꢀ4
3.23 ꢂ 10ꢀ4
2.79 ꢂ 10ꢀ4
9.94 ꢂ 10ꢀ5
1.01 ꢂ 10ꢀ4
1.80 ꢂ 10ꢀ4
2.69 ꢂ 10ꢀ4
4.92 ꢂ 10ꢀ4
4.44 ꢂ 10ꢀ4
5.03 ꢂ 10ꢀ4
5.48 ꢂ 10ꢀ4
5.57
7.07
9.17
92.9
90.3
85.5
78.1
7.1
9.7
14.5
21.9
1.20 ꢂ 10ꢀ3
1.15 ꢂ 10ꢀ3
1.07 ꢂ 10ꢀ3
9.37 ꢂ 10ꢀ4
9.16 ꢂ 10ꢀ5
1.23 ꢂ 10ꢀ4
1.81 ꢂ 10ꢀ4
2.63 ꢂ 10ꢀ4
1.29 ꢂ 10ꢀ3
1.27 ꢂ 10ꢀ3
1.25 ꢂ 10ꢀ3
1.20 ꢂ 10ꢀ3
12.67
0.13
0.16
0.26
0.42
73.7
69.1
56.8
47.6
26.3
30.9
43.2
52.4
2.59 ꢂ 10ꢀ4
2.26 ꢂ 10ꢀ4
2.17 ꢂ 10ꢀ4
1.99 ꢂ 10ꢀ4
9.23 ꢂ 10ꢀ5
1.01 ꢂ 10ꢀ4
1.65 ꢂ 10ꢀ4
2.19 ꢂ 10ꢀ4
3.51 ꢂ 10ꢀ4
3.28 ꢂ 10ꢀ4
3.82 ꢂ 10ꢀ4
4.18 ꢂ 10ꢀ4
1.98
2.49
3.08
4.12
84.9
85.7
85.0
84.5
3.8
2.9
2.5
1.4
11.3
11.4
12.5
14.1
4.49 ꢂ 10ꢀ3
4.26 ꢂ 10ꢀ3
4.13 ꢂ 10ꢀ3
3.72 ꢂ 10ꢀ3
9.94 ꢂ 10ꢀ5
7.27 ꢂ 10ꢀ5
6.31 ꢂ 10ꢀ5
3.05 ꢂ 10ꢀ5
5.98 ꢂ 10ꢀ4
5.65 ꢂ 10ꢀ4
6.05 ꢂ 10ꢀ4
6.24 ꢂ 10ꢀ4
5.19 ꢂ 10ꢀ3
4.90 ꢂ 10ꢀ3
4.80 ꢂ 10ꢀ3
4.37 ꢂ 10ꢀ3
Note. Turnover frequency (tof) calculated based upon 2-propanol saturation coverage of 2.22 molecules per nm2 (6).
a IPE: isopropyl ether.
b 2-propanol partial pressure of 120.7 Torr.

64
REKOSKE AND BARTEAU
FIG. 8. Conversion dependence of the 2-propanol decomposition se-
FIG. 6. Arrhenius plot for determining the apparent activation en-
lectivity to propylene as a function of the partial pressure of added water at
548 K using helium as the carrier gas. The partial pressure of 2-propanol
in the feed stream was held constant at 8.9 Torr. Acetone is the only
other carbon-containing product formed; consequently, acetone selectiv-
ity is simply 100 minus the selectivity to propylene.
ergy for 2-propanol decomposition at a constant conversion of 5% . The
activation energy varies between 70 and 131 kJ molꢀ1 in the temperature
range investigated (523 to 598 K) with helium as the carrier gas. Over the
entire range, the average apparent activation energy is 85 kJ molꢀ1
.
selectivity which is observed with increasing conversion.
Therefore, to investigate the role of water in the depen-
dence of 2-propanol decomposition selectivity on conver-
sion, varying amounts of water were added to the dry
helium carrier gas used in the experiments. This was accom-
plished by simply mixing 2-propanol with water in varying
amounts and placing this mixture into the saturator. The
temperature of the saturator was then adjusted to pro-
vide the proper partial pressure of 2-propanol, as mea-
sured by the on-line gas chromatograph using the thermal
conductivity detector. The amount of water in the feed was
then also determined by this same method; the ratio of wa-
ter to 2-propanol could be conveniently altered and care-
fully monitored by this simple procedure.
Addition of relatively small amounts of water produces a
profound effect on the selectivity of the 2-propanol decom-
position pathways at constant conversion. Figure 8 shows
that the dehydration selectivity of 2-propanol decreases at
constant conversion as additional water is fed to the reac-
tor at 548 K. The observed effect is strong enough to com-
pletely invert the product selectivities at 1% conversion or
less upon the addition of 10 Torr of water. The effect ob-
served is caused by some phenomenon other than a shift in
reaction equilibrium, as the maximum conversion in these
experiments (5% ) is still well removed from the theoreti-
cal 2-propanol equilibrium conversion of 98% at the reac-
tion conditions employed. Most interesting is the weakened
conversion dependence of the 2-propanol decomposition
selectivity with increasing water content for the range of
conversions investigated. Analysis of the product rates of
formation at 0.5% 2-propanol conversion, shown in Table 6,
indicates the selectivity shift with increasing water content
is due to an increased rate of acetone formation; propylene
continues to be produced at a nearly constant rate. This
suggests that the presence of water or an adsorbed form of
water may aid in the formation of acetone from 2-propanol
on oxidized anatase TiO2.
4. Oxygen in Dry Helium as 2-Propanol Carrier
The disparate nature of the effect of conversion on selec-
tivity for the decomposition pathways of 2-propanol when
using dry helium or dry air as carrier gas was further inves-
tigated. By mixing dry oxygen and dry helium together in
varying amounts, the oxygen content of the carrier could
be precisely controlled, measured using a gas chromato-
graph with a thermal conductivity detector, and utilized as
FIG. 7. Conversion dependence of the 2-propanol decomposition se-
lectivity to propylene as a function of 2-propanol partial pressure at 548 K
using helium as the carrier gas. At 548 K, acetone is the only other carbon-
containing product formed; consequently, acetone selectivity is simply 100
minus the selectivity to propylene.

ISOPROPANOL CONVERSION ON TiO2
65
TABLE 6
Conversion, Selectivity, and Rate of Reactant Consumption and Product Formation for 2-Propanol Decom-
position at 548 K on Oxidized Anatase TiO2 at a Partial Pressure of 8.9 Torr, Using Helium as the Carrier with
Added Water
Rate of
consumption (sꢀ1
(CH3)2CHOH
Selectivity
(CH3)2CO
Rate of formation (sꢀ1
)
PWater
(Torr)
Conv.
(% )
)
C3H6
C3H6
(CH3)2CO
0
2.0
10.0
0.53
0.61
0.47
78.6
45.7
37.9
21.4
54.3
62.1
9.59 ꢂ 10ꢀ5
9.48 ꢂ 10ꢀ5
9.45 ꢂ 10ꢀ5
2.61 ꢂ 10ꢀ5
1.13 ꢂ 10ꢀ4
1.55 ꢂ 10ꢀ4
1.22 ꢂ 10ꢀ4
2.07 ꢂ 10ꢀ4
2.49 ꢂ 10ꢀ4
Note. Turnover frequency (tof) calculated based upon 2-propanol saturation coverage of 2.22 molecules per nm2 (6).
a carrier gas in the decomposition of 2-propanol over oxi- and the selectivity of the dehydrogenation and dehydra-
dized anatase titanium dioxide. tion paths for 2-propanol were monitored at 548 K. The
The partial pressure of oxygen in the carrier gas was var- results, shown in Fig. 9a, indicate that the selectivity shifts
ied from 0 to 20% , while the partial pressure of 2-propanol dramatically from the dominance of a dehydration path in
(8.9 Torr) and the conversion (1% ) were held constant, an oxygen-free carrier to dehydrogenation dominance in an
oxygen-rich environment at a constant, low (1% ) conver-
sion. Further, this shift is due to a dramatic increase in the
rate of acetone formation with increasing carrier oxygen
content, while the rate of propylene formation is indepen-
dent of the amount of oxygen in the carrier gas as shown
in Fig. 9b. As was the case when air was used as reactant
carrier, the addition of even the smallest amount of oxygen
results in the complete elimination of hydrogen formation;
the molar amount of water observed in the effluent was
equivalent to the sum of the molar amounts of acetone and
propylene.
DISCUSSION
1. General Observations and Comparisons
The characterization of surface acid–base properties by
2-propanol decomposition is typically carried out between
450 and 550 K with approximately 1–3% alcohol in a large
excess of air (8, 27). The results presented above indicate
the need for caution when interpreting dehydration and
dehydrogenation selectivities from 2-propanol decomposi-
tion under these typical conditions in terms of surface acid
and base properties of metal oxides. The dehydrogenation
selectivity can vary substantially with reaction temperature,
from nearly 100% at 473 K and below to 68% at 598 K, as
indicated in Table 1. Less dramatic, but nonetheless impor-
tant, partialpressure and conversion have also been demon-
strated to impact the relative rates of the dehydration and
dehydrogenation pathways (see Tables 1 and 3). Further,
as illustrated by the wide range of rates and selectivities
obtained, the choice of inert or oxidizing carrier gas has
a profound effect not only on the reaction selectivity but
also on the underlying kinetics of the decomposition pro-
FIG. 9. Dependence of 2-propanol decomposition selectivity (a) and
product rates (b) as a function of oxygen incorporated into dry helium
cess. Consequently, the interpretation of selectivity or rate
data from one catalyst to another in terms of acid–base
carrier gas at 548 K. 2-propanol partial pressure is 8.9 Torr, and conversion
is constant at 1(ꢃ0.05)% .

66
REKOSKE AND BARTEAU
properties can be substantially complicated by the condi- mation at 523 K varies from 6.5 ꢂ 10ꢀ5 to 3.6 ꢂ 10ꢀ4 sꢀ1 as
tions chosen to perform the characterization.
the conversion is increased from 0.2 to 3.9% .
The observation of high selectivity to acetone for 2-
Better agreement is observed when comparing the rate
propanol decomposition on TiO2 is consistent with several of propylene formation. Cunningham and co-workers (34)
previous observations. Bond and Flamerz (9) reported an report rates which, when converted to tof, have values
initial selectivity to dehydrogenation of 66% at 475 K us- from 6.3 ꢂ 10ꢀ6 to 5.2 ꢂ 10ꢀ5 sꢀ1 at 475 K in an N2/O2 car-
ing an unwashed sample of anatase TiO2 with helium as rier, again depending on the source of TiO2. Gervasini and
the carrier gas. Cunningham and co-workers (34) observed Auroux (8) report a rate with tof values of 1.8 ꢂ 10ꢀ6 sꢀ1 for
a 96% selectivity to acetone formation at 475 K on a high propylene formation at 475 K in an air carrier. These values
surface area titania in steady state experiments using a com- are quite similar to our tof for propylene of 1.1 ꢂ 10ꢀ6 sꢀ1
bination of N2 and O2 as a carrier. Grzybowska-Swierkosz measured at 473 K in air carrier. Bond and Flamerz (9)
(10) reported a 71% selectivity to acetone in pulse experi- report rates of propylene formation for three different
ments with inert N2 as the carrier at 443 K. Cunningham and TiO2 samples which correspond to tof values ranging from
Hodnett (36) studied the photoassisted decomposition of 7.8 ꢂ 10ꢀ5 to 8.2 ꢂ 10ꢀ4 sꢀ1 using pure N2 as the carrier at
2-propanol on TiO2 at low temperature (348 K) and found 475 K. These rates are only slightly higher than values at
nearly 100% selectivity toward the dehydrogenation route the identical conditions which can be estimated from our
in the presence of oxygen. These experiments, including the results presented in Table 5. Comparison of the results of
results reported here, were all performed with TiO2 sam- Cunningham et al. (34) and Gervasini and Auroux (8) (ox-
ples prepared either by chloride or alkoxide processes (37). idizing carrier gases) with those of Bond and Flamerz (9)
These results contrast with those of Gervasini and Auroux (inert carrier), as well as the results of the present study,
(8), who observed low selectivity to acetone formation on clearly indicate the difference in the carrier gas composi-
TiO2 using air as the carrier at 450 K. The sample of TiO2 tion is likely responsible for the apparent inconsistencies
used by Gervasini and Auroux was reported to contain 1% noted here.
SO4 (17), leading to the relatively high acidity reported for
this material (8, 17).
The activation energy for the dehydrogenation pathway
was determined in the present study to be 70 kJ molꢀ1. This
The kinetics determined in the present study are also con- value is similar to values previously reported when using an
sistent with several previously reported kinetic parameters oxidizing carrier gas, which range from 63 to 109 kJ molꢀ1
for 2-propanol decomposition on TiO2; however, significant (8, 9, 34) depending on the method of determination and
variations in experimental results for the dehydrogenation TiO2 source. Similar to the rate observations, literature val-
pathway are noted. In general, the rate of acetone forma- ues for the activation energy for dehydration of 2-propanol
tion in the present study is larger than reported in previ- on TiO2 are less varied, ranging from 126 to 149 kJ molꢀ1
ous studies. Even here, however, substantial ambiguity is (8, 9, 34). These compare quite favorably to the present re-
present. Cunningham et al. (34), for example, report rates sults, in which the activation energy for dehydration was
for acetone formation which, when converted to a turnover determined to be 130 kJ molꢀ1
frequency (tof) using the adsorption density of 2-propanol The overall decomposition of 2-propanol on TiO2 was
.
on TiO2 (6), range from 1.8 ꢂ 10ꢀ5 to 1.5 ꢂ 10ꢀ3 sꢀ1 for dif- determined to be one-half order with respect to the reac-
ferent samples of TiO2 at 475 K using an N2/O2 carrier gas tant partial pressure in an air carrier. This contrasts to the
mixture. Gervasini and Auroux (8) report a rate, converted usual assumption of zero-order kinetics in alcohol decom-
to a tof as described above, of 1.2 ꢂ 10ꢀ7 sꢀ1 for acetone for- position on metal oxides (8, 24). This assumption, however,
mation at 475 K in an air carrier. These values bracket our appears to be based upon a crude determination of reac-
observed turnover frequencyof7.7 ꢂ 10ꢀ5 sꢀ1 at 473K using tion order for 2-propanol decomposition on a MoO3 P2O5
–
air as the carrier. Bond and Flamerz report a rate of acetone catalyst (32). The reaction order was also determined by
formation corresponding to a tof value of 1.5 ꢂ 10ꢀ3 sꢀ1 on Kochloefl and Kno¨zinger (12) for 2-propanol decomposi-
an unwashed sample of a low surface area (9.6 m2/g) com- tion on several metal oxides, including TiO2, in an inert car-
mercial (Tioxide) TiO2 using an inert carrier gas (N2) at rier gas, by varying the partial pressure within the range of
475 K. This is significantly larger than our estimated value 100 to 700 Torr. These authors found the dependence upon
of 7.3 ꢂ 10ꢀ5 sꢀ1 at 4% conversion, which was obtained by the partial pressure of 2-propanol to be a complex func-
extrapolation of our results with an inert carrier to the same tion, with positive order at low partial pressures and zero or
conditions of Bond and Flamerz. It must be stressed, how- negative order kinetics at high partial pressures. The range
ever, that Bond and Flamerz report only that conversions of pressures used by Kochloefl and Kno¨zinger is consider-
were typically below 15% in their experiments; our results ably higher than in the present study, in which the order
would indicate the acetone rate and selectivity will increase was determined by varying the partial pressure between
dramatically with increasing conversion at these conditions. 8.9 and 120.7 Torr; values ranging from 5 to 100 Torr are
This is illustrated in Table 5, where the tof for acetone for- more commonly used in 2-propanol decomposition studies

ISOPROPANOL CONVERSION ON TiO2
67
on metal oxides (8, 9, 24, 34). It is likely, however, that This value should be similar to the steady-state reaction
if sufficiently high partial pressures are employed, surface selectivity ratio below about 525 K at low conversion. In-
saturation will occur and the reaction would approach zero deed, this correspondence is observed, as the selectivity ra-
order.
tio at steady-state was observed to be 1.7 : 1 at 523 K (see
The kinetics results are also consistent with results from Table 5).
this laboratory (6, 20, 21) and others (3, 4, 38) employ-
Aside from dehydration and dehydrogenation products,
ing TPD methods. When using helium as carrier gas, the total oxidation products and isopropyl ether were also ob-
dehydration of 2-propanol dominates in both the TPD and served in very small quantities. Total oxidation products
steady-state experiments. However, when a mixture of 20% were observed only in the presence of excess O2, consistent
O2 in inert is used as carrier, the amount of acetone formed with the known poor oxidation activity of TiO2 (39–41).
is increased and the quantity of propylene is simultaneously Isopropyl ether formation was only formed in inert carrier
decreased (38), consistent with the observed selectivity shift gas streams; under otherwise identical conditions, reactions
in the steady-state experiments. The desorption peak for with inert carrier gas would produce small amounts of iso-
propylene is observed to be preceded slightly in temper- propyl ether while reactions in an oxidizing carrier would
ature by the desorption of acetone (555 K for acetone to not. Further, the formation of isopropyl ether was observed
570 K for propylene (6)). This is qualitatively consistent only at high partial pressures of 2-propanol, indicating a re-
with the lower observed activation energyfor dehydrogena- action order for ether formation from 2-propanol higher
tion than dehydration found in steady state experiments than those for dehydration and dehydrogenation.
using both helium and air as carriers.
Bimolecular reactions of alcohols to form ethers on TiO2
The magnitude of the activation energy difference for have been shown to require doubly coordinatively unsat-
dehydration and dehydrogenation estimated from TPD urated surface cations (20) in order to allow coupling of
experiments (6–8 kJ molꢀ1) is smaller than suggested by the adsorbed alkoxide species. Such sites have been shown to
steady-state experiments, though estimation in the latter exist on the surface of anatase and rutile TiO2 powders
case is complicated by the strong conversion dependence of (6) giving rise to the formation of ethers from methanol
the dehydrogenation reaction rate. This apparent discrep- and ethanol in temperature-programmed desorption ex-
ancy may be related to method of kinetic measurement and periments on these materials. However, the absence of iso-
the demonstrated complications created by the presence of propyl ether formation at low partial pressures is not un-
water vapor (see Fig. 8 and discussion below), a product of expected because of the larger steric hindrance associated
the dehydration reaction. Further, the TPD measurement with branched alkoxides. The higher surface coverages as-
effectively probes reaction kinetics at very low conversions, sociated with higher partial pressures of 2-propanol require
as low concentration vapor- and surface-phase products are alkoxide species to overcome this steric inhibition toward
present during a typical TPD experiment, as well as at low attachment of two alkoxides at a common cation, and bi-
reaction rates, since each site is responsible for essentially molecular reactions can begin to occur. The presence of
one turnover. Conversely, the steady-state experiments an oxidizing environment results in the formation of larger
are performed at conversions (ca. 5% ), which provide quantities of water. While alcohols can effectively compete
non-negligible amounts of products in both the vapor and with water for adsorption sites on TiO2, the presence of wa-
the adsorbed state, as well as requiring multiple turnovers ter may add to the steric hindrance, shifting the onset of at-
at each reactive site. Therefore, any effect of water, for tachment of the second alkoxide ligand to a single Ti cation
example, would likely be absent in TPD experiments but to even higher partial pressures. This suggestion is consis-
present in steady-state experiments, resulting in apparently tent with the results of Lusvardi et al. (42) who showed
inconsistent kinetic observations. We discuss possibilities that the concentration of ethoxide species formed from
for the role of water in the decomposition of 2-propanol ethanol adsorption on oxidized TiO2 decreases with pread-
on oxidized TiO2 powders in the following section.
sorption of water. Also, Gamble et al. (43) demonstrated
Analysis of the leading-edge of the thermal desorption that water plays a significant role in the high-temperature
spectra of acetone and propylene from 2-propanol decom- (>500 K) reaction chemistry of ethanol adsorbed on an oxi-
position on TiO2 of Lusvardi et al. (6) and Larson et al. dized TiO2(110) single crystal. As a result, conditions which
(38) provides another interesting comparison to the steady- permit the onset of bimolecular reactions in an inert atmo-
state reaction results presented above. The acetone desorp- sphere may be insufficient for such reactions in an oxidizing
tion traces reported by both groups show very asymmetric carrier or when sufficient concentrations of water vapor are
shapes, with desorption rates for acetone being larger than present.
propylene at temperatures on the leading-edge of the ace-
tone desorption feature. Quantitative comparison of the
2. The Oxidative Role of Oxygen and Water
leading-edges of these peaks provides an estimated 2 : 1 ra-
It has been suggested (8) that utilizing air as the car-
tio of the formation rate of acetone to propylene at 525 K. rier gas in 2-propanol decomposition studies on reducible

68
REKOSKE AND BARTEAU
metal oxides is essential to avoid surface reduction, and lectivity obtained when using air as the carrier gas. Further,
thus alteration of selectivities during the course of the char- the rates observed at higher conversion in Table 5 approach
acterization reaction. While the tendency for alcohols to the values obtained when using air as a carrier gas.
undergo deoxygenation reactions on reduced metal oxide
As water is generated in increasing amounts with increas-
surfaces has been demonstrated (20) and would be consis- ing 2-propanol conversion, the in situ formation of water
tent with the higher dehydration selectivity observed us- through the dehydration of 2-propanol may explain the in-
ing an inert carrier, TiO2 surface reduction seems unlikely crease in acetone selectivity and rate with increasing con-
under the relatively mild conditions of alcohol decompo- version of 2-propanol in an inert carrier. As more water is
sition. Indeed, the present results underscore the improb- produced via the dehydration route, more water is present
ability of surface reduction during alcohol decomposition. in the reactor and the selectivity to acetone increases. With
Propylene, the dominant product of 2-propanol reaction the addition of water to the helium carrier gas, a shift to
with reduced TiO2 surfaces, is formed at identical rates re- higher dehydrogenation selectivity is observed, as shown in
gardless of the use of air or helium as the carrier gas (see Fig. 8. More importantly, the selectivity becomes much less
Tables 1 and 5 and Fig. 9). Further, surface reduction by strongly dependent upon the conversion of 2-propanol at
2-propanol would be accompanied by formation of water low conversions, while the selectivity at higher conversions
and perhaps carbon oxides or surface carbon deposition. In still plateaus at the same value regardless of the amount of
helium carrier experiments, the amount of water detected water added. The value of this selectivity plateau at high
always equaled the molar amount of propylene formed, conversion is also identical to that observed when no addi-
and no carbon oxides were detected. Deposition of sur- tional water is fed to the reactor, as indicated in Fig. 8. In
face carbon would likely result in a diminution of the re- effect, the presence of added water expands the range of
action rate over time; however, the catalysts were tested 2-propanol conversion in which the selectivity is indepen-
and found stable at all conditions for on-stream times in dent of reaction extent; the addition of more water shifts the
excess of 40 h. On the basis of these results, the role of onset of this relative independence to lower conversions.
oxygen in the decomposition pathways of 2-propanol on When using air as the carrier, the water concentration is
TiO2 cannot be ascribed solely to the elimination of surface higher than when using an inert carrier when compared at
reduction.
identical conversions, because water is formed from both
The lack of hydrogen production and the equivalence of dehydration and dehydrogenation routes. As a result, the
the molar amount of water produced to the sum of acetone effects of increasing conversion are diminished when using
and propylene suggests the dehydrogenation of 2-propanol air as carrier.
in the presence of O2 proceeds via an oxidative route. This
The addition of 10 Torr water to the inert carrier also
conclusion is also supported by the experiments in which results in an increased rate of dehydrogenation by an or-
O2 was added in varying amounts to dry helium and fed der of magnitude, while leaving the rate of dehydration
to the reactor. As illustrated in Fig. 9, the addition of even unchanged (see Table 6). This is the opposite to the effect
small amounts of O2 to helium results in a dramatic shift in expected if the dehydration equilibrium were shifted by the
the selectivity, from the dominance of the dehydration path addition of water: the apparent dehydration rate should
in the absence of air to 85+% selectivity to the dehydro- decrease, leaving the dehydrogenation rate unaffected. A
genation product with the addition of 10 Torr of O2. Even kinetic effect, rather than a thermodynamic one, is thus in-
more striking is the independence of the rate of propylene dicated by the rate data. The oxidation of propylene to form
formation on the amount of O2 in the carrier gas while the acetone has been reported on mixed metal oxides under re-
formation of acetone increases by an order of magnitude action conditions similar to those employed here (44) and
with the addition of 10 Torr of O2. All of these features would be consistent with many of the experimental obser-
point to the opening of a new, more rapid pathway for the vations. However, at temperatures and pressures consistent
formation of acetone in the presence of O2, with oxidative with those of the 2-propanol decomposition results, propy-
dehydrogenation providing a plausible explanation.
lene and water do not react to any measurable degree over
As Table 5 indicates, high selectivities to acetone can be TiO2.
achieved without the presence of O2 in the carrier gas when
In many respects, the role of water as observed in the
the conversion is appropriately high. However, a compari- present results, both added and generated by reaction, is
son of the results in Tables 1 and 5 indicates that superior similar to the oxidative role of O2 in the decomposition of
acetone yields are obtained with air as carrier, regardless 2-propanol on oxidized TiO2. The presence of O2, of added
of conversion. Interestingly, the changes in selectivity with water or of substantial amounts of water from high conver-
increasing conversion of 2-propanol seem to slow and even- sions of 2-propanol all dramatically increase the rate of ace-
tually plateau at sufficiently high conversions. The position tone formation from 2-propanol on oxidized anatase TiO2
of this selectivity plateau, at least for operation at 523 and while leaving the propylene rate essentially unchanged.
548 K where it is clearly defined, is nearly identical to the se- Further, the observed rates of product formation are quite

ISOPROPANOL CONVERSION ON TiO2
69
similar in the presence of O2, added water, or water gener- 3. Comments on the Mechanism of
ated through dehydration at high conversion, resulting in
very similar product distributions under these conditions.
Therefore, it seems likely that the effect of added O2 does
not result from the direct interaction of O2 with the reac-
tants or products, but from the increased amounts of water
created during the course of reaction with O2 present. Ei-
ther way, these results suggest the presence of water (or a
derivative of water, i.e., hydroxyl groups) and/or the pres-
ence of O2 opens a new or enhances an existing route for
the formation of acetone from 2-propanol.
Several examples of hydroxyl-assisted reaction pathways
on metal oxides can be found in the literature. Chadwick
and Zheng (45) reported a significant influence of adsorbed
water and hydroxyls on methanol decomposition pathways
on ZnO during temperature-programmed desorption ex-
periments. The authors observed that the decomposition of
methanol on dehydroxylated ZnO results in the formation
of CO and H2. When water was coadsorbed, the product
slate included CO2;asthe preadsorbed water wasincreased,
the amount of CO formed decreased linearly, while the
amount of CO2 increased. The authors suggested that this
changing product spectrum represents the opening of a new
reaction channel to CO2 formation as the surface hydroxyl
concentration increases, probably through a direct reaction
between adsorbed methoxy or a related intermediate and
surface OH groups. Shido and Iwasawa (46) have similarly
proposed a reactant-promoted reaction mechanism for the
water–gasshift reaction over ZnO in which adsorbed CO re-
acts with surface hydroxyl groups to produce intermediate
formate species. Both of these studies illustrate the poten-
tially important role that surface hydroxyl groups and/or
water may play in reactions on metal oxides.
The increase caused by the external addition of water,
however, is not as pronounced as the increase caused by
even smaller amounts of water generated in situ from the
dehydration pathway or when the reaction is carried out
in the presence of O2. The partial pressure of water gener-
ated in situ from dehydration is typically less than 0.5 Torr,
as calculated from the reaction mass balances, compared
to 2–10 Torr in the addition experiments. These observa-
tions can be easily reconciled if one assumes the species
active for enhanced dehydrogenation are hydroxyl groups
rather than adsorbed water. The direct synthesis of hy-
droxyl groups from alcohol dehydration (or formed un-
der oxidative conditions) would likely occur more rapidly
than the dissociation of water adsorbed from the gas phase,
especially considering the known inhibition of water ad-
sorption by adsorbed alcohols and alkoxides (42, 43). As
a result, a larger concentration of externally added wa-
ter would be required in the gas phase to achieve the
same surface hydroxyl concentration (and the correspond-
ing dehydrogenation rate increase) that occurs from in situ
generation.
2-Propanol Decomposition
The decomposition of 2-propanol has been extensively
utilized as a probe reaction for determining the acid–base
properties of metal oxides. It has often been asserted that
dehydrogenation occurs as a concerted process on both
acidic and basic sites, while dehydration occurs predomi-
nantly on acidic sites (27, 33). Regardless of the nature of
the active site or sites, the present results indicate the rate
of dehydration is stable and independent of the conversion
and the carrier gas composition. Thus, it appears there is
some fraction of the adsorbed species which readily dehy-
drates. While the rate of the dehydration route is dependent
upon temperature and 2-propanol pressure, the rate is inde-
pendent of the carrier gas composition and water content.
In contrast, the presence or absence of water or hydroxyl
groups dramatically enhances the dehydrogenation selec-
tivity of 2-propanol decomposition on TiO2. We propose
that this enhancement occurs through an ꢀ-H abstraction
step involving an alkoxide and a hydoxyl group bound to
the same Ti cation:
CH3
CH3
CH – D
D
C
3
ꢀ
C
O
CH – –H
H
,OH


3
O
l
OH ꢀꢀꢀ→
l
l
l
l Ti ,
Ti
.
,
,
l
The existence of a hydroxyl-mediated dehydrogenation
step would consolidate many of the present experimental
observations. Such a step would be consistent with the ob-
served dehydrogenation selectivity increase with increas-
ing water concentration in an inert carrier. Additionally,
the conversion dependence of the decomposition selectiv-
ity can be explained: larger quantities of in situ water and
hydroxyls are present at higher conversions, resulting in the
generation of additional species responsible for dehydro-
genation. The kinetics of the dehydrogenation and dehy-
dration reactions determined at steady state are consider-
ably different. The order with respect to 2-propanol, while
near one-half in both cases, is always higher for dehydro-
genation, and the activation energies are nearly a factor of
two different. These observations alone suggest the decom-
position of 2-propoxide toward dehydration and dehydro-
genation products must involve different active sites and/or
different intermediates.
As both alkoxides and hydroxyls are present on the same
cation in the proposed scheme, increasing the partial pres-
sure of 2-propanol may result in increased hydoxyl popula-
tions from dissociation of the alcohol, further enhancing the
rate of dehydrogenation. This trend is observed using both
air and inert carrier gases (see Tables 3 and 5 and Fig. 7).
One might argue that increased 2-propanol partial pressure
could also lead to lower concentrations of surface hydroxyls

70
REKOSKE AND BARTEAU
from site competition. While this may in fact may reduce
Above 525 K in the TPD experiment, water or hydrox-
the total hydroxyl coverage of the surface, only hydroxyls yls are generated, as the dehydration of the alkoxide to
bound with alkoxides to a common Ti cation are proposed propylene begins to occur. These hydroxyls can participate
to be active for dehydrogenation. If alcohols could displace in the ꢀ-H abstraction step if they are bound to a Ti cation
these hydroxyls, two alkoxides would be bound to a com- also containing an alkoxide, resulting in dehydrogenation
mon Ti cation, resulting in bimolecular reactions such as to acetone. This step is also in competition with a surface
ether formation (6, 20, 21, 47). In the present study, iso- hydroxyl disproportionation step to form water. The des-
propyl ether was only formed in inert carrier at the combi- orption rate maxima of all three species, propylene, acetone
nation of the highest temperature and highest partial pres- and water, are observed in TPD within the same tempera-
sure of 2-propanol investigated; further, the selectivity to ture region, 550–580 K, as would be expected for the above
isopropyl ether never exceeded 4% . Based on the reaction described processes.
products observed, the surface population of two alkoxides
In TPD of 2-propanol adsorbed on TiO2 powders, read-
bound to the same Ti cation must be exceedingly small. sorption of 2-propanol desorbing in the 350–550 K range
TPD experiments have shown strong steric barriers to ac- can occur. This readsorption, if it is dissociative, would
commodation of pairs of 2-propoxide ligands on individual almost certainly ensure that the hydroxyl and alkoxide
surface cations (6, 21, 47). We conclude that 2-propanol formed are bound to the same Ti cation. At the upper end
could not effectively compete with hydroxyls for binding to of this temperature range, the ꢀ-H abstraction process can
a Ti cation which already contains one bound alkoxide.
proceed at measurable rates, and dehydrogenation should
One of the key results from previous studies on oxide be observed. In fact, the desorption trace for acetone from
single crystals and powders is that both dehydrogenation anatase and rutile TiO2 powders shows an asymmetric lead-
and dehydration of alcohols occur via initial dissociative ing edge, indicating desorption of acetone is occurring at
adsorption to form alkoxides (6, 19–21). The involvement measurable rates as low as 450 K. This is substantially lower
of hydroxyl groups in the dehydrogenation of 2-propanol than the temperature at which the desorption rate of propy-
via adsorbed alkoxides is also consistent with previous stud- lene becomes measurable, leading to a high predicted se-
ies of alcohol decomposition on single-crystal and powder lectivity to dehydrogenation. Results from the steady-state
samples from this laboratory (6, 21, 47). In TPD experi- experiments confirm this observation: at temperatures be-
ments with 2-propanol adsorbed on oxidized single crystal low 523 K, the selectivity to acetone approaches 100% in
TiO2 surfaces in UHV, little or no dehydrogenation to form both inert and air carrier experiments.
acetone was observed, but substantial dehydration did oc-
The increased rate of dehydrogenation observed with in-
cur (21). On oxidized polycrystalline powders of anatase creasing oxygen partial pressure is also consistent with the
and rutile TiO2, TPD of adsorbed 2-propanol resulted in proposed hydroxyl-mediated ꢀ-H abstraction step. As the
the formation of small amounts of dehydrogenation prod- partial pressure of O2 is increased, less H2 and more H2O
uct, with dehydration being the main decomposition route are observed in the effluent of the reactor. These higher
(6, 47). Both of these experiments can be considered to have concentrations of H2O will act in the manner described
been performed in an inert (i.e., UHV or helium) environ- above for increasing conversion in an inert carrier or the
ment. In both studies, substantial amounts of 2-propanol water addition experiments. This is consistent with the pre-
were desorbed at 350–550 K; this desorption feature has viously discussed strong similarities in the observed effect
been ascribed to a reaction between hydroxyls and alkox- of both water and O2 addition on the 2-propanol decom-
ides, resulting in the re-formation of the alcohol which des- position kinetics: O2 is not directly responsible for higher
orbs (6, 21). This pathway effectively eliminates hydroxyls dehydrogenation rates, yet assists dehydrogenation in some
which may be bonded with alkoxides to the same Ti cation fashion.
during the TPD experiment. The hydroxyl-mediated dehy-
The mechanism by which O2 produces higher effec-
drogenation route does not proceed at measurable rates tive concentrations of water and thus higher dehydrogena-
until higher temperature (ꢄ 475 K). Therefore, little or no tion rates, while having no effect on the dehydration rate
dehydrogenation via this route would take place during the (see Fig. 9), is difficult to establish unequivocally from
low temperature portion of the TPD process. Similar ar- the present results. The rate of 2-propanol consumption
guments can be made to explain the lack of an effect of is also observed to increase with the addition of O2 to
hydroxyl coverage on the high temperature decomposition the carrier gas. A most likely effect of the presence of O2
routes of ethanol found on a TiO2 single crystal (43) and would be to replace any surface oxygen consumed dur-
rutile TiO2 powder (42). A sufficient population of hydrox- ing the steadystate reaction. To explain these effects, we
yls added at room temperature does not survive to high propose that the role of O2 is to replace the most labile
temperature during the TPD process to produce a measur- surface oxygen surface species which are either coordi-
able effect; the hydroxyls either combine with alkoxides to nated or consumed during steady-state reaction. Oxygen
desorb alcohol, or disproportionate to form water.
adsorption provides additional active sites for adsorption

ISOPROPANOL CONVERSION ON TiO2
71
and subsequent dissociation of the alcohol, increasing the tal variables which are often overlooked in heterogeneous
rate of 2-propanol consumption. Alcohol dissociation, in catalysis research.
turn, increases the density of surface hydroxyl groups, thus
increasing the rate of OH-mediated ꢀ-H abstraction, pro-
moting the dehydrogenation route. This role of O2 is con-
ACKNOWLEDGMENT
We gratefully acknowledge the support of the National Science Foun-
dation (Grant CTS-9410965) for this research.
sistent with the observed kinetics: the reaction order with
respect to 2-propanol is not zero, suggesting the rate would
be enhanced by an increased alcohol adsorption rate and/or
higher alkoxide concentration.
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