110th Anniversary: Near-Total Epoxidation Selectivity and Hydrogen
Peroxide Utilization with Nb-EISA Catalysts for Propylene
Epoxidation
Swarup K. Maiti,
†
Anand Ramanathan,
†
and Bala Subramaniam*
,†,‡
†
Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66047, United States
‡
Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas 66045, United States
*
S
Supporting Information
ABSTRACT: The Nb-EISA catalyst with relatively low Nb loadings (∼2 wt %) shows exceptional propylene epoxidation
performance with H
2
O
2
as oxidant at 30−40 °C, 5−9 bar propylene pressure with nearly total propylene oxide (PO) selectivity
(>99%), H
2
O
2
utilization (>99%) toward PO formation, high productivity (∼3200 mg/h/g), and mild Nb leaching (3−6%).
The predominantly Lewis acidic nature of the Nb-EISA catalysts favors epoxidation while their relatively low Brønsted acidity
inhibits H
2
O
2
decomposition and Nb leaching. At higher Nb loadings (8−17 wt %), the catalytic performance deteriorates.
However, significant performance improvements were achieved when the Nb-EISA materials are calcined in N
2
(instead of air)
during synthesis, depositing a carbon layer in the pores. The resulting pore hydrophobicity not only inhibits epoxide ring
opening but also increases propylene concentration inside the pores resulting in higher EO productivity and lower H
2
O
2
decomposition. The carbonized Nb-EISA materials also show improved stability to leaching.
1. INTRODUCTION
Propylene oxide (PO) is one of the most important chemical
intermediates for producing many essential fine chemicals,
such as polyurethane plastics, polyglycol esters, unsaturated
resins, and surfactants.
1−3
The global demand for PO is high,
being >10 million metric tons/year in 2012 and growing at an
annual rate of 5%.
4,5
In general, PO is produced via
epoxidation of propylene, an industrially significant substrate.
Current commercial routes to PO production are mainly the
chlorohydrin process, different hydroperoxide based processes,
and the hydrogen peroxide propene oxide (HPPO) process.
While the chlorohydrin process suffers from high environ-
mental impact due to much waste generation, the hydro-
peroxide-based processes depend on stable byproduct (tert -
butyl alcohol) value for economic viability. Among these
processes, the HPPO process has less environmental impact as
it produces only H
2
O as byproduct and uses mild operating
conditions (40−50 °C and 20−30 bar). The HPPO process
provides a maximum PO selectivity of 95% using the titanium
silicate (TS-1) catalyst
6−8
in methanol solvent. While the TS-1
catalyst is active, it is expensive and undergoes deactivation.
9
Further, the extent to which H
2
O
2
decomposes on the acidic
TS-1 catalyst is unknown. Stoichiometric utilization of H
2
O
2
toward PO formation with near-complete PO selectivity is vital
to making the process economically viable. Hence, there
continues to be interest in alternative PO processes using
inexpensive and robust catalysts that maximize PO selectivity
and H
2
O
2
utilization.
Various catalysts have been investigated for propylene
epoxidation with hydrogen peroxide, including heteropolya-
cids,
10,11
methyltrioxorhenium (MTO),
9
tungsten b ased
homogeneous catalysts,
12
and various titanium containing
zeolites, viz., Ti-MWW
13
and TiCl
4
-modified HZSM-5,
14
in
addition to TS-1.
6−8
For example, Xi et al. performed
propylene epoxidation
10
at 65 °C using in situ formed H
2
O
2
Received: June 27, 2019
Revised: August 28, 2019
Accepted: September 2, 2019
Published: September 2, 2019
Article
pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. 2019, 58, 17727− 17735
© 2019 American Chemical Society 17727 DOI: 10.1021/acs.iecr.9b03461
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with 85% PO yield in a 6 h batch run. The catalyst, [π-
C
5
H
5
NC
16
H
33
]
3
[PO
4
(WO
3
)
4
], used in this system is insoluble
but can form a soluble active species in the presence of H
2
O
2
.
When the H
2
O
2
is consumed, the catalyst precipitates enabling
easy recycling. Lee et al. developed a liquid phase propylene
epoxidation process
9
using the MTO catalyst under mild
operating conditions similar to those of the HPPO process
producing PO with 98% yield and complete H
2
O
2
utilization
for PO formation. In contrast, most of the Ti-based
heterogeneous catalysts show lower H
2
O
2
utilization toward
PO formation compared to MTO. For example, the H
2
O
2
utilization toward PO formation is reported to be 94% and
96% respectively over Ti-MCM-42 and Ti-MWW catalysts.
These Ti-based catalysts are often tedious and expensive to
synthesize. Similar to Ti, Nb-
15−30
and W-based catalysts
21,29
are also active for olefin epoxidation using H
2
O
2
as oxidant.
However, in studies of liquid-phase ethylene epoxidation with
H
2
O
2,
we found that metal leaching is a major problem being
more severe for W
29
in general compared to Nb-based
silicates.
21
The epoxidat ion activity of Nb-silicates is in general
attributed to the presence of well dispersed Nb(V) sites in
tetrahedral coor dination. We dem onstrated recently that
improved Nb dispersion can be achieved using the Evaporation
Induced Self-Assembly (EISA) synthesis method with
relatively low Brønsted acidity. The Nb-EISA catalysts show
remarkable activity for cyclohexene epoxidation with high
epoxidation selectivity compared to Nb-catalysts prepared by
either one-pot or impregnation methods.
31
Further, carbon-
ization of the Nb-EISA catalyst
31
via calcination in the
presence of N
2
remarkably stabilizes the epoxide from ring
opening reactions while also improving H
2
O
2
utilization
toward epoxide formation.
Invariably almost all reported epoxidation studies with Nb-
based catalysts involve substrates such as cyclohexene,
cyclooctene, limonene, and vegetable oil that are liquids at
ambient conditions. Our work addresses the important
question of how these epoxidation catalysts fare in the case
of a lower olefin such as propylene that requires higher
pressures to ensure adequate solubility in the liquid phase. As
propylene (P
c
= 46 bar, T
c
= 92.2 ± 0.8 °C) is close to its
critical tempe rature at ambient conditions, it d issolves
significantly in the liquid phase containing methanol (as in
the HPPO process and our experiments) depending on the
pressure. This causes the volume of the liquid phase to change
with pressure, which must be experimentally quantified to
reliably design experiments and to interpret the results. In the
only reported study of Nb-based heterogeneous catalyst for
liquid phase epoxidation of propylene, such important details
are not addressed.
32
Motivated by the foregoing considerations, we report here
systematic investigat ions of Nb-EISA catalysts and their
carbonized versions, C−Nb-EISA, for propylene epoxidation
under similar operating conditions as those employed in the
HPPO process. Remarkably, the Nb-EISA catalysts show
exceptional propylene epoxidation activity, stability, and H
2
O
2
utilization compared to Nb-silicates prepared by other one-pot
and impregnation methods. Optimized Nb-EISA catalysts
perform significantly better for propylene epoxidation than Ti-
based catalysts providing >99% epoxide selectivity and H
2
O
2
utilization.
2. EXPERIMENTAL SECTION
2.1. Materials. Triblock copolymer (Pluronic P123, EO
20
−
PO
70
−EO
20
, with an average molecular weight ∼5,800,
Aldrich), ethanol (Absolute, 200 Proof, Acros organic s),
methanol (Sigma-Aldrich), tetraethyl orthosilicate (TEOS)
(98% Acros organics), conc. hydrochloric acid (37%, Fisher),
and niobium(V) chloride (Alfa Aesar) were used as received.
Acetonitrile (HPLC grade, 99.9%, Fisher) and H
2
O
2
(50 wt %
in water, Fisher) were used as received for catalytic propylene
epoxidation with H
2
O
2
. Ferroin indicator solution, ceric sulfate
(0.1 N), and trace metal grade sulfuric acid (99.9 wt %) were
purchased from Fischer Scientific and used as received. The
PO standard, 1-methoxy-2-propanol, and propylene glycol
were purchased from Sigma-Aldrich, whereas 2-methoxy-1-
propanol was purchased from Chem-Bridge Chemical.
Propylene was purchased from Matheson Tri-Gas Co
(polymer grade).
2.2. Synthesis of Nb-EISA and Catalyst Character-
ization. The Nb-EISA catalysts were prepared as described
previously.
31
Briefly, TEOS was added to the acidified
ethanolic solution containing P123, followed by the required
amounts of niobium(V) chloride predissolved in ethanol. After
stirring the mixture for a couple of hours, it is transferred to a
Pyrex Petri dish, and the solvent is allowed to evaporate for 2−
4 days. The resulting rigid flakes were then calcined in either
flowing air or nitrogen at 550 °C for 5 h at a heating rate of 1.5
°C/min to obtain either Nb-EISA or Nb-EISA, respectively.
The calcined samples were characterized with a comprehensive
suite of complementary analytical techniques including SAXS,
XRD, XRF, N
2
physisorption, diffuse reflectance UV−vis
spectroscopy, transmission electron microscopy (TEM, SEM),
NH
3
-TPD, and FTIR of adsorbed pyridine.
31
The metal contents in the fresh and spent solid catalysts
were determined by inductively coupled plasma optical
emission spectrometry (ICP-OES). Prior to analysis, the
catalyst (60 mg) was digested in the presence of HF (2 g),
H
2
SO
4
(3 g), and deionized water (5 g) in an autoclave at 100
°C for 3 h. The resulting solutions were analyzed by the ICP-
OES technique using appropriate calibration standards. The
ICP-OES analysis offers high sensitivity in calculating metal
concentrations in the catalysts (±1.6 ppm for Nb and ±0.78
ppm for Si).
2.3. Volumetric Expansion Studies. When pressurized
propylene is dissolved in methanol, i t shows enhanced
solubility resulting in propylene-expanded liquids. A 50 mL
high-pressure Jerguson cell rated to ∼400 bar at 100 °C
33
was
used for volumetric expansion studies. Either pure methanol or
a methanol + 50% H
2
O
2
/H
2
O mixture is placed in the view
cell which is then submerged in a constant temperature bath.
To facilitate the mixing of cell contents, the loaded liquid is
agitated by a piston. Once the desired temperature is attained,
propylene is pumped as a liquid from an ISCO pump into the
cell to the desired pressure. Mixing of the gas and liquid phases
by a piston expedites the attainment of equilibrium (as inferred
from constant P and T). At equilibrium, the volume of a
propylene expanded liquid phase is measured visually on a
calibrated external linear scale.
2.4. Catalytic Epoxidation Studies. The catalysts were
tested for propylene epoxidation in a semibatch mode in a 50
mL Parr reactor equipped with a magnetically driven stirrer,
pressure transducer, and thermocouple. Reactor details and the
operating pr ocedure are described else where,
21
and a
Industrial & Engineering Chemistry Research Article
DOI: 10.1021/acs.iecr.9b03461
Ind. Eng. Chem. Res. 2019, 58, 17727−17735
17728
schematic of the reactor unit is given in the Supporting
Information (Figure S1). In a typical reaction, a mixture
containing a 50% aqueous H
2
O
2
(10 mmol) solution, MeOH
(625 mmol), acetonitrile (3 mmol) as internal standard, and a
solid catalyst (fresh or spent) was loaded into the Parr reactor.
A blank experiment using 1,2-dimethoxyethane (DME) as
internal standard confirmed the inertness of acetonitrile. The
solution was heated with mild stirring to attain the desired
temperature (35 °C). Thereafter, propylene was charged from
an external reservoir pressurizing the reactor up to 0.9 MPa.
The impeller speed was kept at 1400 rpm to eliminate gas−
liquid mass transfer limitations. Isothermal semibatch reactions
lasting up to 3 h were performed at constant pressure. The
liquid-phase reaction mixture was analyzed by online GC to
determine the concentrations of the desired product (PO) and
the byproducts [1-methoxy-2-propanol (1M2P), 2-methoxy-1-
propanol (2M1P), and propylene glycol (PG)]. The following
definitions are used in evaluating the performance of the tested
catalysts
P
m
m
S
n
nn n n
U
nn n n
nn
X
nn
n
(batchtime)( )
100%
100%
100%
PO
PO
metal
PO
PO
PO 1M2P 2M1P PG
HO
PO 1M2P 2M1P PG
HO
0
HO
HO
HO
0
HO
HO
0
22
22 22
22
22 22
22
i
k
j
j
j
j
j
y
{
z
z
z
z
z
=
=
+++
×
=
+++
−
×
=
−
×
where P
PO
, S
PO
, U
H
2
O
2
, and X
H
2
O
2
denote PO productivity (mg
PO h
−1
g
−1
metal), PO selectivity, H
2
O
2
utilization toward PO
formation, and H
2
O
2
conversion, respectively; m
PO
and m
metal
represent the mass of PO formed and the mass of metal in the
catalyst, respectively; n
PO
, n
1M2P
, n
2M1P
, and n
PG
denote the
molar amounts of PO, 1M2P, 2M1P, and PG formed,
respectively; and n
H
2
O
2
0
and n
H
2
O
2
denote the initial and the
final molar amounts of H
2
O
2,
respectively.
3. RESULTS AND DISCUSSION
3.1. Catalyst Characterization. Detailed physicochemical
characterization of Nb-EISA samples and their carbonized
versions (C−Nb-EISA) may be found in the Supporting
Information (Figures S2−S9) and elsewhere.
31
Only the
salient features are summarized here. The mesoporous nature
of Nb-EISA and C−Nb-EISA samples was confirmed by N
2
sorption revealing typical type IV isotherm and H
2
hysteresis.
The physicochemical characteristics are summarized in Table
1. For Nb-EISA and C−Nb-EISA samples, the surface area
ranges from 615 to 680 m
2
/g and from 418 to 571 m
2
/g,
respectively, decreasing with increased Nb content (1.6−22.0
wt %). These materials possess an average pore diameter of
about 2.7−3.4 nm. Further, the total acidity (0.10−0.31 mmol
NH
3
/g) was found to be lower compared to other Nb
containing mesoporous silicates. Although an increase in the
relative amounts of Lewis acid sites was inferred from the
FTIR spectra of adsorbed pyridine, relatively low amounts of
Brønsted acid sites were observed (Table 1). Further, the weak
Brønsted acidity is confirmed by complete desorption of
pyridine at mild temperature (250 °C). Diffuse reflectance
UV−vis characterization reveals the presence of bands at 200−
204 nm attributed to ligand-to-metal charge transfer in isolated
NbO
4
tetrahedra and another band at 242−267 nm due to
oligomeric NbO
x
tetrahedral.
31
The lack of a band at 290−320
nm implies the absence of bulk Nb
2
O
5
species which is also
evidenced from powder XRD patterns.
31
3.2. Volumetric Expansion Studies. While conventional
liquid phases are noncompressible, gas-expanded liquid phases
are compressible. The compressibility depends on the extent of
gas d issolution in the liquid phase . Hence, volumetric
expansion data are important to calculate the propylene
concentration in the liquid phase precisely. Such data also
facilitate reliable interpretat ion of the effects o f various
parameters such as temperature, propylene pressure, and
aqueous H
2
O
2
concentration on the extent of dissolution of
propylene in the liquid phase. As shown in Figure 1, the
volume of initial liquid phase containing either pure methanol
or a representative reaction mixture (containing methanol and
Table 1. Physical Properties of Nb-EISA and C−Nb-EISA
Catalysts
no. catalyst
Nb
wt %
a
C
wt %
b
S
BET
c
(m
2
/g)
V
tp
d
(cm
3
/g)
d
P, BJH
e
(nm)
acidity
NH
3
(mmol/g)
1 Nb-
EISA
1.8 680 0.45 3.0 0.10
2 3.7 618 0.46 3.4 0.18
3 6.0 962 1.11 4.6 0.18
4 10.2 681 0.54 3.4 0.24
5 22.0 615 0.47 3.4 0.31
6C−Nb-
EISA
1.6 16.5 571 0.40 2.9 0.06
7 3.3 21.5 473 0.36 2.9 0.12
8 7.8 23.4 426 0.41 3.2 0.18
9 16.6 24.7 418 0.37 3.2 0.22
10 Nb-
EISA
f
1.9 557 0.36 2.7 ND
j
11 Nb-
EISA
g
1.8 549 0.35 2.7 ND
j
12 Nb-
EISA
h
1.4 512 0.32 2.7 ND
j
13 C−Nb-
EISA
i
1.4 NM
i
555 0.41 2.9 ND
j
a
wt % in sample determined by XRF.
b
wt % in sample determined by
TG/DTA.
c
S
BET
= BET specific surface area from adsorption isotherm
at P/P
0
between 0.05 and 0.30.
d
V
tP
= total pore volume at 0.99 P/P
0
.
e
d
P,BJH
= average pore diameter calculated from adsorption branch of
N
2
isotherms using the BJH model.
f
Another batch of Nb-EISA
(1.9%).
g
After 10 h reaction.
h
After 4 recycle runs.
i
NM = not
measured.
j
ND = not determined.
Figure 1. Volumetric expansion plot with propylene pressure at 35
°C.
Industrial & Engineering Chemistry Research Article
DOI: 10.1021/acs.iecr.9b03461
Ind. Eng. Chem. Res. 2019, 58, 17727−17735
17729
50% aqueous H
2
O
2
) increases with increasing propylene
pressure, implying increased propylene dissolution in the
liquid phase. At a fixed temperature, the extent of volumetric
expansion decreases with increasing H
2
O
2
concentration in the
methanol + H
2
O
2
(50% aqueous) mixture. At 35 °C and 9 bar
propylene pressure, the volumetric expansion in the case of
pure methanol is 29%. In the case of methanol + H
2
O
2
(aqueous) mixtures, the volumetric expansion caused by
propylene dissolution at 9 bar and 35 °C is 27%, 23%, and
20% when the H
2
O
2
contents in the mixture are 5, 10, and 15
mmol, respectively. As expected, increased concentrations of
polar H
2
O
2
in the liquid phase decrease the solubility of
nonpolar propylene. Even though the main product, PO, was
not included in the volumetric expansion studies, PO
formation during reaction (3 h) constitutes only ∼0.8 wt %
of the reaction mixture and, hence, is not expected to alter the
propylene dissolution significantly.
3.3. Epoxidation of Propylene. 3.3.1. Epoxidation of
Propylene by Nb-EISA Catalysts. Nb-EISA catalysts with
different Nb loadings were screened for propylene epoxidation,
and the results are summarized in Table 2 (entries 1−5). The
Nb-EISA catalyst with 1.8 wt % Nb loading displayed nearly
total S
PO
(>99%) and U
H
2
O
2
(>99%) with relatively low Nb
leaching (3.5%) during a 3 h run. With an increase in Nb
loading up to 10.2 wt %, the yield of PO increased and then
decreased at the high Nb loading of 22 wt %. U
H
2
O
2
values also
decreased with increased Nb loading. The decreases in P
PO
and
U
H
2
O
2
values at the higher Nb loadings are attributed to the
formation of oligomeric NbOx species that are not as active as
the isola ted NbOx species for epoxidation. Further, the
Brønsted acidic nature of the oligomeric NbOx species also
decomposes H
2
O
2
, adversely affecting its utilization for PO
formation.
We prepared different batches of Nb-EISA materials with
similar Nb loadings (1.9−2.0 wt %) and tested for catalyst
stability during extended runs (Table 2, entries 6−8). Similar
P
PO
was observed for the extended run (10 h) suggesting that
the catalyst is stable under the reaction conditions. Further, the
decreased Nb leaching (5.7% at 10 h vs 3.5% at 3 h) does not
scale with reaction time suggesting that the leaching decreases
with time. Additionally, Nb-EISA catalysts prepared in two
different batches with almost identical Nb loadings show
similar catalytic activities (Table 2, entries 6 and 8) under
similar reaction conditions. This indicates that the catalyst
synthesis technique is reproducible for similar Nb loadings.
We also tested various mesoporous Nb-silicate catalysts
(viz., Nb-KIT-5, Nb-KIT-6, and Nb-TUD-1) that were active
for ethylene epoxidation.
21,29
As shown in Table 3 (entry 1),
Nb-KIT-5 (2.2 wt % Nb) exhibits moderate PO productivity
(P
PO
) (2500 mg/h/g) and PO selectivity (S
PO
) (88%). The
H
2
O
2
utilization efficiency (U
H
2
O
2
) is 70%, and the Nb leaching
during the 3 h run was 10.3%. In contrast, Nb-TUD-1 (0.8 −
3.8 wt %) displayed higher P
PO
(4,075−11,680 mg/h/g) and
PO selectivity (S
PO
> 99%) with moderate to high U
H
2
O
2
(75−
90%) values. However, the leaching of Nb-TUD-1 catalysts
(20−24%) was more severe compared to Nb-KIT-5 (10%). In
the case of Nb-TUD-1 with 3.8 wt % Nb loading, an increase
in temperature (from 308 to 313 K) decreases P
PO
from 4,075
to 3,329 mg/h/g and U
H
2
O
2
from 89.4% to 74.6%, probably
due to the decrease of propylene concentration in the liquid
phase and/or H
2
O
2
decomposition (Table 3, entries 2 and 3).
At even lower Nb loadings (0.8 wt %), P
PO
was significantly
enh anced to 11,680 mg/h/g. This is attributed to the
dominant presence of Lewis acid sites compared to Brønsted
acid sites at low Nb loadings.
21
However, the Nb leaching
(∼25%) was still significant. Bn-Nb-TUD-1,
30
prepared by
covalent capping of benzyl groups on Nb-TUD-1 (3.8 wt %
Nb), displayed a better performance than that of the uncapped
catalyst with dramatically reduced metal leaching from 21% to
3.6%. However, this capped catalyst showed almost 4-fold less
PO productivity (907 vs 4075 mg/h/g) attributed to the
reduction of Lewis acid sites during the passivation step.
21
In
contrast, the Nb-EISA catalysts show nearly total epoxide
selectivity and H
2
O
2
utilization with significantly lower Nb
leaching (Table 2). This enhanced performance is attributed to
the predominance of Lewis acid sites with relatively low
Brønsted acidity in the Nb-EISA catalysts.
Table 2. Performances of Nb-EISA with Different Nb Loadings and Reaction Times for Propylene Epoxidation
b
no. Nb-EISA, Nb wt % time (h) Y
PO
(mmol) (±3%) P
PO
(±3%) S
EO
%(±3%) X
H
2
O
2
%(±3%) U
H
2
O
2
%(±3%) leaching % (±5%)
1 1.8
a
3 0.63 2340 >99 5.3 >99 3.5
2 3.7 3 1.20 2071 >99 12.7 94.5 5.4
3 6.0 3 2.84 3050 >99 28.7 96.3 9.3
4 10.2 3 1.23 777 91.8 22.7 59.0 13.7
5 22.0 3 0.84 247 86.6 31.5 31.1 4.5
6 2.0
a
3 0.94 3171 >99 9.1 >99 3.5
7 2.0
a
10 2.85 2900 >99 27.9 >99 5.7
8 1.9
a
3 0.93 3174 >99 9.3 >99 3.3
a
Different batches with similar Nb loadings.
b
Reaction conditions: MeOH = 624 mmol, H
2
O
2
= 10 mmol, AN = 3 mmol, catalyst loading = 300
mg, T =35°C, propylene P = 9 bar (maintained constant), 1400 rpm.
Table 3. Performances of Different Nb-Silicates for
Propylene Epoxidation
b
no. catalyst
Nb
wt %
P
PO
(±3%)
S
EO
%
(±3%)
X
H
2
O
2
%
(±3%)
U
H
2
O
2
%
(±3%)
leaching
%(±5%)
1 Nb-
KIT-5
2.2 2500 88 15.6 70 10.3
2 Nb-
TUD-1
3.8 4075 >99 27.1 89.4 20.9
3 Nb-
TUD-
1
a
3.8 3329 >99 26.3 74.6 22.2
4 Nb-
TUD-1
0.8 11680 >99 17.0 80.1 24.8
5 Bn-Nb-
TUD-1
2.7 907 >99 4.1 93.6 3.6
a
Reaction temperature is 40 °C.
b
Reaction conditions: MeOH = 624
mmol, H
2
O
2
= 10 mmol, AN = 3 mmol, catalyst loading = 300 mg, T
=35°C, propylene P = 9 bar (maintained constant), t = 3 h, 1400
rpm.
Industrial & Engineering Chemistry Research Article
DOI: 10.1021/acs.iecr.9b03461
Ind. Eng. Chem. Res. 2019, 58, 17727−17735
17730
Parametric studies including temperature, propylene pres-
sure, H
2
O
2
concentration, and catalyst loading were performed
with the Nb-EISA catalyst with 2 wt % Nb, the best-performing
catalyst (Table 2). Table 4 summarizes temperature effects.
When the reaction temperature is increased from 30 to 35 °C,
the PO yield increased approximately 2-fold. Notably, both PO
selectivity and H
2
O
2
utilization remained high (>99%) at 35
°C. However, at higher temperatures (40 and 45 °C), both PO
productivity and H
2
O
2
utilization efficiency decreased. The
lower activity at higher temperatures (40 and 45 °C) is
attributed to the lower propylene dissolution in the gas-
expanded liquid phase as well as increased H
2
O
2
decom-
position, resulting in decreased availability of the two key
reactants.
At 35 °C, PO yield increases with propylene pressure. At 35
°C, the PO yields are 0.30, 0.48, and 0.94 mmol at propylene
pressures of 5, 7, and 9 bar, respectively (Table 4, entries 2, 5,
and 6). Notably, nearly total (>99%) S
PO
and U
H
2
O
2
were
achieved in this pressure range, and Nb leaching remained low
(within 3−4%). The increased PO yield at higher propylene
pressures is attributed to enhanced propylene solubility in the
methanolic liquid phase (Figure 2). Given that propylene at 35
°C is below its critical temperature (92.2 ± 0.8 °C),
pressurization with N
2
beyond the saturation vapor pressure
would cause propylene condensation and dissolution in the
liquid phase. For example, when propylene at 9 bar was
pressurized with 11 bar of N
2
at 35 °C, the PO yield increased
approximately 10% more (Table 4, entries 2 and 7).
The effect of H
2
O
2
concentration was also studied. As
inferred in Table 4 (entries 2, 8, and 9), PO productivity
increased significantly with an increase in H
2
O
2
concentration.
However, Nb leaching also increased. While negligible H
2
O
2
decomposition was observed when 5 or 10 mmol of H
2
O
2
was
used, significant H
2
O
2
decomposition occurred when 15 mmol
of H
2
O
2
was used.
When the catalyst loading is increased by 2-fold (from 150
to 300 mg, entries 10 and 2 in Table 4), the PO yield doubled,
resulting in similar PO productivity. However, when the
catalyst loading was further increased by 4- and 6-fold (from
150 mg to 600 and 900 mg, entries 10−12), the corresponding
increases in PO yield were 2.6× and 3.2×, respectively. These
are also reflected in the lower PO productivity at higher
catalyst loadings (beyond 300 mg). Thus, lower temperature
(∼35 ° C), lower Nb loadings (∼2 wt %), and moderate
propylene pressure (∼9 bar) are preferred to maximize PO
productivity and H
2
O
2
utilization while minimizing Nb
leaching. Figure 2 compares key performance measures of
the Nb-EISA catalyst and its carbonized version at different Nb
loadings. The figure more clearly displays how carbonized Nb-
EISA (C−Nb-EISA) catalysts show better catalytic perform-
ance than neat Nb-EISA, becoming more prominent at higher
Nb loadings.
3.3.2. Epoxidation of Propylene by Carbonized Nb-EISA
Catalysts (C−Nb-EISA). In order to improve H
2
O
2
utilization
and epoxide selectivity at higher N b loadings, the as-
synthesized Nb-EISA materials were carbonized by calcination
in the presence of N
2
(rather than air) at 550 °C. As expected,
the C−Nb-EISA (carbonized version of Nb-EISA) catalysts
showed better P
PO
,S
PO
, U
H
2
O
2
, and resistance to metal leaching
compared to Nb-EISA with similar Nb loadings (Table 5). For
the Nb-EISA catalyst with 10.2 wt % Nb, the values of P
PO
,
S
PO
, U
H
2
O
2
, and Nb leaching were 777 mg/h/g, 92%, 59%, and
Table 4. Effect of Temperature, Pressure, H
2
O
2
Concentration, and Catalyst Loading for Propylene Epoxidation over Nb-EISA
(2.0 wt % Nb)
c
no.
T
(°C)
P
(bar)
H
2
O
2
(mmol)
catalyst amt
(mg)
Y
PO
mmol
(±3%)
P
PO
(±3%)
S
EO
%
(±3%)
X
H
2
O
2
%
(±3%)
U
H
2
O
2
%
(±3%)
leaching %
(±5%)
1 30 9 10 300 0.48 1618 >99 4.67 >99 5.2
2 35 9 10 300 0.94 3171 >99 9.08 >99 3.1
3 40 9 10 300 0.83 2809 >99 8.28 97.1 4.9
4 45 9 10 300 0.58 1964 >99 9.04 65.2 4.0
5 35 7 10 300 0.48 1618 >99 4.67 >99 5.2
6 35 5 10 300 0.30 1032 >99 3.0 >99 4.0
73520
a
10 300 0.97 3232 >98 11.4 86 NM
b
8 35 9 5 300 0.22 733 >99 2.2 >99 1.5
9 35 9 15 300 1.41 4776 >99 11.7 80.1 4.0
10 35 9 10 150 0.47 3188 >99 4.5 >99 5.2
11 35 9 10 600 1.21 2052 >99 11.4 >99 3.6
12 35 9 10 900 1.54 1760 >99 14.7 >99 3.8
a
9 bar propylene and 11 bar N
2
.
b
NM = not measured.
c
Reaction conditions: MeOH = 624 mmol, AN = 3 mmol, propylene P = maintained
constant, t = 3 h, 1400 rpm.
Figure 2. Comparative epoxidation performances of Nb-EISA
catalysts and its carbonized versions (C−Nb-EISA) at different Nb
loadings.
Industrial & Engineering Chemistry Research Article
DOI: 10.1021/acs.iecr.9b03461
Ind. Eng. Chem. Res. 2019, 58, 17727−17735
17731