S. Wang, E. Iglesia / Journal of Catalysis 345 (2017) 183–206
201
coverages (Fig. 8a) is consistent with the absence of detectable
Fig. 9 depicts free energies along the reaction coordinate for the
ketonization elementary steps in Scheme 5 (523 K, 1 bar ethanoic
⁄
AcOH bands in the infrared spectrum of TiO
2
(r) during ethanoic
⁄
acid ketonization reactions (Fig. 5b, Section 3.3).
acid, 1 ML AcO ) relative to two gaseous ethanoic acids and a bare
⁄
The proposed elementary steps for ethanoic acid ketonization
2
TiO (a) surface (enthalpies and entropies; Section S17, SI). AcO
⁄
2
(Scheme 5) were examined on TiO (a) (101) surfaces at 1 ML AcO
species exhibit the lowest free energy among adsorbed species,
⁄
coverages (hacid, Eq. (15)) using DFT methods; such high coverages
seek to rigorously account for the saturated surfaces prevalent dur-
consistent with AcO as the most abundant surface intermediate
⁄
(MASI). DFT-derived adsorption free energies for AcO
ꢀ1
ing catalysis. The first step involves the cleavage of the
bond in AcO to form 1-hydroxy enolates (Step 3 in Scheme 5).
a
-CAH
(DG
AcO
ꢃ
ꢀ50 kJ mol , Eq. (10); Fig. 9) are slightly more negative
⁄
ꢀ1
than measured values (ꢀ33 ± 1 kJ mol , Table 3), a trend that
reflects the overbinding tendencies of the Grimme’s D3BJ disper-
sion corrections in PBE functionals [43,44]; this is also evident
The H-atom is abstracted by the vicinal O2c site; the
tance is 0.113 nm and the -CAH distance is 0.150 nm at the TS
that mediates the formation of the 1-hydroxy enolate (Sche-
me 11a). The similar -HAO2c distances at the TS and in the bound
-hydroxy enolate (0.113 vs. 0.110 nm) demonstrate the late nat-
ure of the enolate formation TS on TiO (a) (101); such a late TS
reflects the strong -CAH bond in carboxylic acids and the
a-HAO2c dis-
a
from the more negative
D
H
AcO
ꢃ
values derived from theory
(ꢀ140 kJ mol , Table 6) compared with those obtained from the
regression of the temperature dependence of measured K values
ꢀ1
a
1
1
ꢀ1
2
(ꢀ114 ± 3 kJ mol , Table 3).
The CAC coupling TS (TS
highest free energy along the reaction coordinate, consistent with
its kinetic relevance in ketonization reactions on TiO (a). The
CC,a value (Eq. (14)), given by the difference between
a
4
in Fig. 9; Step 4, Scheme 5) gives the
endothermic nature of such elementary steps. The transition states
for enolate formation from carbonyl compounds in aldol condensa-
2
à
tion reactions on TiO
coordinate than for acid reactants because the
weaker in carbonyl compounds than in carboxylic acids [24].
2
(a) (101) occur earlier along the reaction
DFT-derived
DG
⁄
a-CAH bonds are
the free energies of the CAC coupling TS and two bound AcO spe-
cies, is 160 kJ mol (523 K, 1 bar ethanoic acid, 1 ML AcO ; Table 6);
its respective enthalpy and entropy components (
(Table 6). Measured
CC,a values are 166 ± 1 kJ mol
(Tables 2 and 3), in good agreement with these
DHCC,a difference between DFT-derived and mea-
is much smaller than for DHAcOꢃ
(27 kJ mol ), because overbinding tendencies influence the bound
ꢀ1
⁄
à
à
1
-Hydroxy enolates can attack the carboxylic C-atom in a vici-
D
H
CC,a
,
G
D
S
CC,a) are
⁄
ꢀ1
ꢀ1 ꢀ1
à
à
nal AcO via its nucleophilic b-C-atom to form a new CAC bond
128 kJ mol and ꢀ63 J mol
K
D
CC,a
ꢀ1
,
DH
CC,a
à
ꢀ1
(
(
Step 4, Scheme 5). The two C-atoms lie farther apart at the TS
0.234 nm, Scheme 11b) than in the product state (0.150 nm), but
and
D
S
,
137 ± 1 kJ mol
, and
ꢀ1
ꢀ1
ꢀ56 ± 1 J mol
DFT estimates. The
sured values (9 kJ mol
K
à
are closer than their combined van der Waals radii (0.340 nm),
indicating that the CAC coupling TS lies at an intermediate point
along the reaction coordinate. The OH group in the acid moiety
of the CAC coupling TS structure is stabilized by H-bonding with
a neighboring lattice O2c atom and with the bound OH species
ꢀ1
)
ꢀ
1
à
reactant and TS for
DHCC,a, but only the product state in the case
of DHAcOꢃ . DFT-derived kinetic isotope effects for the CAC coupling
⁄
*
formed via dissociation of coadsorbed ethanoic acid to form AcO
step (k
4
K
3
, Eq. (13)) and thermodynamic isotope effects for ethanoic
at vicinal Ti5cAO2c pairs (Scheme 11b).
*
⁄
The
a
-hydroxy
c
-carboxy alkoxide formed via CAC coupling
(a)
O(g) (Steps 5–
, Scheme 5). The carboxyl H-atom in the alkoxide first transfers
acid dissociation to AcO (K
1
, Eq. (9)) are both near unity ((k
4
3 H
K ) /
*
undergoes an intramolecular H-shift mediated by the TiO
101) surface to form a b-keto carboxylate and H
2
(k
4
K
3
)
D
= 1.1, (K
1
)
H
/(K
1
)
D
= 0.9; 523 K, Table 6), in agreement with
(
7
2
*
*
3 H 3 D 1 H 1 D
4 4
experiments ((k K ) /(k K ) = 1.1, (K ) /(K ) = 1.0; 523 K, Table 2).
DFT-derived structures for bound intermediates and transition
to a vicinal O2c (Step 5, Scheme 5); this TS occurs very early along
the reaction coordinate, as evidenced by the similar carboxyl OAH
bond lengths in the reactant and the TS (0.104 vs. 0.105 nm, Sche-
me 11c). The H-shift TS is stabilized by interactions with the bound
⁄
states in Scheme 5 are similar at 1/3 ML and 1 ML AcO coverages.
The CAC bond at the CAC coupling TS (Step 4, Scheme 5) is only
slightly shorter at 1/3 ML (0.224 nm, Scheme 11e) than at 1 ML
⁄
(
0.234 nm, Scheme 11b). DFT-derived free energies along the reac-
OH species derived from coadsorbed AcO (Scheme 11c). The H-
⁄
tion coordinate (Section S18, SI) show that AcO remains the MASI
atom that shifts to the O2c site then combines with the
in the alkoxide to form H O (Step 6, Scheme 5). This latter step is
mediated by a late TS with a nearly-formed H O molecule at the
TS (TS structures in Section S16, SI), consistent with the endother-
a-OH group
and the CAC coupling remains the kinetically-relevant step at both
2
à
coverages. The
(
D
G
CC,a value, however, is larger at lower coverages
2
ꢀ1
181 vs. 160 kJ mol , Table 6), a difference that predominantly
reflects the stabilizing effects of H-bonding on
erages (149 vs. 128 kJ mol , Table 6). We conclude that the preva-
lent high acid coverages are requisite for ketonization catalysis
because of the preferential stabilization of the CAC bond formation
à
ꢀ1
DHCC,a at higher cov-
mic nature of this reaction (+94 kJ mol , 523 K, Section S17, SI).
The b-keto carboxylate species then reprotonates to form a
b-keto acid (Step 8, Scheme 5), which decarboxylates to propen-
ꢀ
1
2
-olate and CO
the carboxylate and the enolate moiety at the decarboxylation TS
CAC = 0.243 nm; Scheme 11d) is longer than that in the b-keto
2
(Steps 9–11, Scheme 5). The CAC bond between
2
TS over its relevant precursors on TiO (a) (101) by H-bonding, which
become most evident at near-saturation coverages. Such coverage
effects provide yet another demonstration of how dense monolayers
allow facile turnovers for reactions that would proceed much more
slowly, or not at all, at lower coverages, because crowded surfaces
favor TS structures over those of the relevant precursors [57].
(d
carboxylate reactant (dCAC = 0.152 nm). The elongation of this
CAC bond at the decarboxylation TS is consistent with the sequen-
tial nature of the formation of the H O and CO decomposition
2 2
products (Steps 5–10, Scheme 5) and indicative of the stable
nature of discrete b-keto carboxylate intermediates formed via
A concerted form of the two-step H
2
O elimination reaction
(
(
Steps 5–6, Scheme 5) of -hydroxy -carboxy alkoxide species
a
c
endothermic dehydration steps (Steps 5–6, Scheme 5) on TiO
101).
The propen-2-olate product formed in the decarboxylation of
2
(a)
formed in the CAC coupling step) to b-keto carboxylates becomes
(
⁄
kinetically-accessible only at low acid coverages (1/3 ML AcO ,
Scheme 12). This concerted route is mediated by a six-membered
ring TS (Scheme 11f) and involves a direct shift of the carboxyl
b-keto carboxylates (Step 10, Scheme 5) reprotonates to form
acetone (Step 12, Scheme 5). The TS structure (Section S16, SI)
⁄
H-atom to the leaving
lower free energy at the TS than for the H
a
-OH group. This route exhibits a slightly
O formation TS involved
resembles that for enolate formation from AcO in its ketonization
reactions (Scheme 11a), but with a shorter
-HAO2c bond (0.148 vs. 0.150 nm, 0.114 vs. 0.113 nm, respec-
tively). Acetone desorption (Step 13, Scheme 5) then completes
the catalytic ketonization turnover on TiO (a) surfaces.
a-CAH and a longer
2
in the two-step route (Step 6, Scheme 5), but only at acid coverages
much lower than those prevalent during practical ketonization
a
ꢀ
1
catalysis (65 vs. 69 kJ mol , 1/3 ML, Section S18, SI). The concerted
2