Understanding the mechanism of N coordination on framework Ti of Ti-BEA zeolite and its promoting effect on alkene epoxidation reaction

Article abstract of DOI:10.1016/j.mcat.2021.111750

The function of ammonium salts on the epoxidation performance over Ti-BEA zeolite was investigated in detail. Experiments of alkene epoxidation, side reactions of epoxide and decomposition of H₂O₂ with or without ammonium salts were designed, and the UV-Vis spectroscopy was employed to analyze the structure of Ti-hydroperoxo species. It is revealed that the ammonia (or amines) dissociated from the ammonium salt would chelate with the linear Ti-η¹(OOH) species and form a bridged Ti-η²(OOH)-R species, which is more stable, more weaker in epoxide adsorption and acidity as well. Therefore, side reactions and H₂O₂ decomposition would be suppressed, and both alkene conversion and epoxide selectivity would be promoted simultaneously. On the other hand, the excessive NH₃?H₂O (NH₃/Ti>1) or NaOH bond with the Ti-η²(OOH)-R species and generate salt-like Ti-η²(OO)^-M⁺ species, resulting in the deactivation of Ti active center. While for ammonium salts, e.g. NH₄Cl, the limited dissociation degree along with the acidic environment help the Ti active center to maintain in highly active. In short, this work provides a practical Ti active center tuning method for Ti-BEA zeolite, as well as a thorough understanding of its Ti-hydroperoxo species.

Full text of DOI:10.1016/j.mcat.2021.111750

Molecular Catalysis 511 (2021) 111750
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Understanding the mechanism of N coordination on framework Ti of
Ti-BEA zeolite and its promoting effect on alkene epoxidation reaction
Xiaohang Liang, Xinxin Peng*, Dan Liu, Changjiu Xia, Yibin Luo, Xingtian Shu
State Key Laboratory of Catalytic Material and Reaction Engineering, Research Institute of Petroleum Processing, Sinopec, Beijing 100083 PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The function of ammonium salts on the epoxidation performance over Ti-BEA zeolite was investigated in detail.
Experiments of alkene epoxidation, side reactions of epoxide and decomposition of H2O2 with or without
ammonium salts were designed, and the UV-Vis spectroscopy was employed to analyze the structure of Ti-
hydroperoxo species. It is revealed that the ammonia (or amines) dissociated from the ammonium salt would
Ti-BEA
epoxidation
N coordination
Ti-hydroperoxo
UV-Vis spectroscropy
chelate with the linear Ti-
η
1(OOH) species and form a bridged Ti-
η
2(OOH)-R species, which is more stable, more
weaker in epoxide adsorption and acidity as well. Therefore, side reactions and H2O2 decomposition would be
suppressed, and both alkene conversion and epoxide selectivity would be promoted simultaneously. On the other
hand, the excessive NH3•H2O (NH3/Ti>1) or NaOH bond with the Ti-
η
2(OOH)-R species and generate salt-like
Ti-
η
2(OO)-M+ species, resulting in the deactivation of Ti active center. While for ammonium salts, e.g. NH4Cl, the
limited dissociation degree along with the acidic environment help the Ti active center to maintain in highly
active. In short, this work provides a practical Ti active center tuning method for Ti-BEA zeolite, as well as a
thorough understanding of its Ti-hydroperoxo species.
1. Introduction
structure in zeolite crystal, [12–14] nanocrystallization, [15–17] regu-
lating the hydrophilicity and lipophilicity of pore system, [18–20] and
Ti-containing heteroatom zeolite is an important branch of zeolite
family, which possess excellent performance in oxidizing hydrocarbons
selectively to fine chemicals using hydrogen peroxide as oxidant [1,2].
Since the invention of TS-1 (MFI topology) in 1983, the synthesis of
Ti-containing zeolites with other topologies has attracted much aca-
demic interest. Among these materials, Ti-BEA (*BEA topology) zeolite
with three-dimensional 12-member-ring channels gained much atten-
tion due to its superior diffusion ability for larger substrates [3]. And the
development of Ti-BEA zeolite is regarded as a milestone in the design of
large pore oxidation catalysts, [4] as it shows much better catalytic
performance than TS-1 in the epoxidation of bulky alkenes, such as
cyclohexene, cyclooctene, norbornene, linalool and other alkenes, [5–7]
and excellent performance in oxidation of cyclohexane to KA oil. [8] It is
also widely employed in the synthesis of various fine chemicals, such as
oxidation of organic sulphides to sulphoxides and sulphones, [9] isom-
erization of glyceraldehyde to dihydroxyacetone, [10] and isomeriza-
tuning the microenvironment of Ti active sites [21–23] and etc. Among
them, tuning the microstructure of Ti active sites is an easy-to-handle
and effective strategy for the enhancement of catalytic activity and
selectivity.
Generally, the microenvironment of Ti active sites can be modified
by ion exchange, [24] proper solvent, [5] and coordination of N atom
[21,25]. In a commonly accepted epoxidation mechanism, [26] the
five-membered Ti-hydroperoxo intermediates Ti-
η
1(OOH) (Scheme 1
(1)) species is regarded as the active center [27,28]. For the epoxidation
of alkenes over Ti-BEA and Ti-MWW zeolite, acetonitrile (CH3CN) is
preferable solvent, [5,29] and it is interpreted that the CH3CN could
interact with Ti center hence influencing the desorption behavior of
epoxide product [30,31]. Moreover, recent theoretical studies [31–33]
suggest that, for Ti-YNU-1 and Ti-MWW zeolites, the polar molecules (e.
g. CH3CN, NH3) would act as a monodentate ligand (R) to coordinate
with Ti-
η
1(OOH) species and resulting in the formation of another
tion of
α
-pinene oxide to campholenic aldehyde [11], etc.
six-coordinated Ti-
η
2(OOH)-R species (Scheme 1 (2), R=H2O, CH3CN,
In order to acquire Ti-containing zeolites with better catalytic per-
NH3 and etc.). This species has lower product desorption energy and
formance, numerous efforts were paid, such as creating hierarchical
thus beneficial to the removal of epoxide product. What’s more,
* Corresponding author.
E-mail address: pengxx.ripp@sinopec.com (X. Peng).
https://doi.org/10.1016/j.mcat.2021.111750
Received 14 March 2021; Received in revised form 9 June 2021; Accepted 3 July 2021
Available online 21 July 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
Ti-
η
2(OOH)-R species is normally considered as less active in decom-
with reflux condenser at ambient pressure. Typically, 0.2 g of Ti-BEA
(Si/Ti = 34, XRF) zeolite, 480 mmol of acetonitrile, 24 mmol of H2O2
(30 wt%), and 24 mmol of 1 hexene were added in sequence, and then, a
proper amount of additive (if necessary) was introduced in. The mixture
was heated to 40 ◦C under stirring and reacted for 4 h.
position of H2O2 than Ti-
η
1(OOH) species owing to its weaker Lewis
acidity of Ti active sites [22,34,35].
The current research work concerning on tuning the microenviron-
ment of Ti active sites mainly focus on TS-1 and Ti-MWW zeolites, while
the structure and feature of Ti-hydroperoxo species within Ti-BEA
zeolite is rarely investigated. In order to gain a deeper understanding
of the nature of Ti active center and to obtain a better catalytic perfor-
mance of Ti-BEA zeolite, herein, the effect of ammonium salts on the
post-synthesized Ti-BEA zeolite was explored. The effect of amine hy-
drochlorides with different structure were examined for a thorough
analysis. UV-Vis spectroscopy was employed to characterize the struc-
tural evolution of Ti-hydroperoxo species for Ti-BEA zeolite. And the
side reactions of epoxides were tested as well as to understanding the
function of ammonium salts.
Side reactions of epoxide were performed in the same equipment as
epoxidation reaction. Typically, 0.2 g Ti-BEA zeolite, 480 mmol meth-
anol, 24 mmol of H2O2 (30 wt%), and 24 mmol of 1,2-epoxyhexane were
added in sequence. Finally, a certain amount of additives (if necessary)
were fed in. The reaction were performed at 40 ◦C for 4 h.
After the reaction was completed, the product was cooled, centri-
fuged, and quantitatively analyzed by Agilent 6890 GC. The amount of
the unconverted H2O2 was determined by indirect iodimetry.
The conversion, product selectivity, turnover frequency (TOF) and
utilization of H2O2 are calculated as follows:
It is demonstrated that the ammonia (or amine) dissociated from
n0 ꢀ n
1(OOH) species and
conversion =
× 100 %
ammonium salts would coordinate with linear Ti-
η
n0
form a bridged Ti species, e.g. Ti-
η
2(OOH)-NH3 species. The bridged Ti
ni
n0 ꢀ n
species can then effectively suppress the side reactions of the epoxides
and enhance the utility of H2O2, thus the alkenes conversion and ep-
oxides selectivity are concurrently improved. However, the Ti active
center of Ti-BEA would be deactivated in alkaline environment. This
work revealed the function of ammonium compound on Ti-hydroperoxo
species of Ti-BEA, and it shed light on the strategy of constructing su-
perior active center in Ti-containing and other metal-containing
materials.
selectivity =
× 100 %
n0 ꢀ n
nTi × t
TOF =
× 100 hꢀ 1
nepoxide + 2naldehyde + nether + ndiol
utilization of H2O2 =
× 100 %
n
ꢀ n
0,H2O2
H2O2
where n0 and n are the molar contents of the substrate before and after
reaction, ni is the molar contents of a given i product. nTi is the molar
content of framework Ti atom (according to UV-Vis result) in Ti-BEA
zeolite, and t is the reaction time. nepoxide, naldehyde, nether and ndiol are
the molar contents of the 1,2-epoxyhexane, valeraldehyde, hexanediol
methyl ether or 1,2-hexanediol, separately. n0,H O and nH O are the
2. Experimental
2.1. Catalyst preparation
Ti-BEA zeolite was prepared through a liquid-solid impregnation
procedure according to Ref. [36]. Typically, 10 g of parent Al-BEA
zeolite (Si/Al = 11, supplied by Hunan Jianchang Petrochemical Co.,
Ltd) was treated with 400 g of 13 mol/L HNO3 aqueous solution in a 500
mL three-necked flask with reflux condenser at 100 ◦C for 24 h to
remove framework aluminum. The resulting solid was filtered and
thoroughly washed with deionized water, dried at 110 ◦C for 2 h, and
calcined at 550 ◦C for 3 h. Dealuminated *BEA zeolite (DeAl-BEA) with
nest silanol sites was then obtained. In the Ti incorporation process, Ti
precursor liquid was acquired by the reaction of a proper amount of
TiCl4 with anhydrous ethanol, note that TiCl4 will intensely react with
ethanol, it should be taken carefully and added dropwise under stirring.
Then, 2 g of DeAl-BEA was mixed with 4 mL of the Ti precursor liquid
and grinded till the solvent completely volatized. The final Ti-BEA
2
2
2
molar contents of the H2O2 before and after reaction. Note2that two
moles of H2O2 would be consumed as per one mole of valeraldehyde is
formed.
3. Result and discussion
3.1. Effect of NH4Cl on alkene epoxidation reaction
Generally, CH3CN, MeOH and acetone are applied as solvent for al-
kenes epoxidation over titanium silicalite. [3,5] Herein, these three were
selected as solvent of 1-hexene epoxidation over Ti-BEA zeolite with or
without the addition of NH4Cl, and the results are shown in Table 1. The
results confirm that CH3CN is the best solvent with the highest 1 hexene
conversion, which is in accordance with the previous report [3] .
Notably, the epoxide selectivity in MeOH was relatively low (76.0%)
without introducing NH4Cl in, which is mainly caused by solvent
involved side-reaction leading to the formation of ether and diol [37,38]
. While CH3CN and acetone are less likely to react with 1,2-epoxyhex-
ane. When NH4Cl was employed, evidently, the TOF of 1 hexene epox-
idation increased by 31.9% (from 61 to 80), 75.3% (from 24 to 42) and
32.3% (from 46 to 61) with CH3CN, MeOH and acetone as solvent
respectively. And apparently, the epoxide selectivity improved from
76.0 to 96.6% for the reaction using MeOH as solvent, indicating that
side reaction of 1,2-epoxyhexane were greatly inhibited with the addi-
tion of NH4Cl.
◦
◦
sample was collected, dried at 110 C for 2 h and calcined at 550 C
for 3 h. The structure, chemical composition and morphology of various
*BEA zeolites are supplied in Table S1 and Fig. S1.
2.2. Catalyst characterization
UV-Vis spectra were recorded by a Cary 300 Agilent spectropho-
tometer under atmosphere conditions. Prior to the characterization, the
samples were treated with different additives. In a typical treatment, 1 g
of Ti-BEA zeolite was loaded into a 100 mL flask, 40 mL of acetonitrile
(CH3CN) or methanol (MeOH) and 1 g of H2O2 (30 wt%) were added
sequentially. Then, the additives (if necessary) were weighed and fed in
with agitating, and the mixture was reacted at 40 ◦C for 30 min. After the
removal of the solvent by filtration, the solid was collected for charac-
terization. For Fig. 4C and D and Fig. S7B (in SI), the UV-Vis spectra of
Ti-hydroperoxo species were obtained by taking Ti-BEA-0.5 zeolite
sample as background.
Entries 4 to 7 depict the effect of NH4Cl on epoxidation reaction of
various alkene substrates. It is observed that after the addition of NH4Cl,
all the alkenes conversion and epoxides selectivity were enhanced.
Acetone was here selected as solvent for long-chain alkenes (1-dodecene
and 1-hexadecene) due to their relative poor solubility in MeOH and
CH3CN. The result shows that, with the addition of NH4Cl, the TOF of 1-
dodecene and 1-hexadecene epoxidation increased from 19 to 84 and
from 23 to 77, respectively, which was approximately 3.4 and 2.4 times
2.3. Catalytic tests
Epoxidation of alkenes were performed in a 50 mL three-necked flask
2

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
Table 1
Influence of NH4Cl on alkene epoxidation with different solvents and different substrates.
entry
substrate
solvent
conversion/%
TOF/hꢀ 1
selectivity/%c
a
a
a
-
NH4Cl b
84.4
44.4
63.5
88.0
83.7
88.6
80.7
-
NH4Cl b
80
-
NH4Cl b
99.5
96.6
99.5
99.7
99.2
98.5
90.0
1
2
3
4
5
6
7
1-hexene
acetonitrile
methanol
acetone
64.0
25.3
48.0
72.5
71.8
20.3
24.0
61
24
46
69
68
19
23
98.6
76.0
98.6
99.1
97.4
92.3
86.6
1-hexene
42
1-hexene
61
cyclooctene
1-octene
acetone
84
acetone
80
1-dodecene
acetone
84
1-hexadecene
acetone
77
Note: a Without the addition of NH4Cl. bThe molar ratio of NH4Cl to framework Ti atom (according to UV-Vis result) in Ti-BEA zeolite is about 4. c In solvent acetonitrile
and acetone, the amount of two by-products (valeraldehyde, 1,2-hexanediol) are roughly equal, and in solvent MeOH, the amount of three by-products (valeraldehyde,
1,2-hexanediol and hexanediol methyl ether) are also nearly equal.
higher compared with the corresponding contrast tests. These results
demonstrate that NH4Cl is capable of promoting both the alkenes
epoxidation conversion and epoxides selectivity over Ti-BEA zeolite,
especially for long-chain alkenes.
and Table 2).
When weak acidic additive (NH4Cl) was used for 1-hexene epoxi-
dation reaction catalyzed by Ti-BEA zeolite, both of the 1 hexene con-
version and 1,2-epoxyhexane increased from 64.0% to 84.4% and from
98.6% to 99.5%, respectively (Fig. 2). If we adjust the pH value of the
reaction solution by HCl and CH3COOH (Table 2), without addition of
ammonium salt, only slight improvement in catalytic performance could
be observed (Fig. 2), which is also different from prior report [40] .
Nonetheless, this results exclude the effect of H+, Cl- and CH3COO- on
the improvement of 1-hexene epoxidation reaction, and suggest that
NH+4 might be the key issue.
The relationship between 1-hexene conversion and reaction time
with or without NH4Cl in CH3CN is depicted in Fig. 1A. It is calculated
that the initial reaction rate (0–30 min) of the reaction with the addition
of NH4Cl is about 38% higher than the result without NH4Cl, and the 1-
hexene conversion is apparently higher as well. The influence of tem-
perature on 1 hexene epoxidation reaction with or without NH4Cl in
CH3CN is shown in Fig. 1B. Although the 1 hexene conversion was
enhanced obviously with the increasing of the reaction temperature, the
1-hexene conversion and 1,2-epoxyhexane selectivity were further
promoted with the addition of NH4Cl. These results suggest that NH4Cl is
effective in promoting the alkene epoxidation catalyzed by Ti-BEA
zeolite under the reaction conditions of different solvent, substrates
and reaction temperature.
3.3. Effect of different ammonium salts
The 1 hexene epoxidation catalyzed by Ti-BEA zeolite with different
ammonium salts or alkalis were investigated and the results are
exhibited in Fig. 3 and Table S2. The molar ratio of N (nitrogen atom in
ammonium salts) to Ti (framework Ti atom) is denoted as N/Ti. Obvi-
ously, different ammonium salts showed diverse effects. For NH4Cl, the
conversion of 1-hexene increased along with the increasing of NH4Cl
and reached to the maximum value of 84.4% when N/Ti = 4, and then
maintained at about 84% as N/Ti>4. Whereas for NH3•H2O and
CH3COONH4, the maximum conversion (about 83.4%) was achieved at
N/Ti = 1. But as N/Ti>1, 1-hexene conversion was drastically decreased
and even deactivated when N/Ti>4. However, according to previous
study on TS-1, the conversion of 1-hexene epoxidation reaction with the
addition of ammonia was still higher than the blank test even at N/Ti =
9. [25] Our control experiments confirm that the 1-hexene conversion
over TS-1 (44.8% for NH3•H2O and 56.2% for CH3COONH4, respec-
tively) is much higher than the blank test when N/Ti = 4 (Fig. S2).
As CH3COOH has been proved to have little effect on promoting 1
hexene conversion (Fig. 2), perhaps it is NH3•H2O produced by the
hydrolysis of CH3COONH4 that plays a key role in enhancing the
3.2. Influence of additives on 1-hexene epoxidation
In order to investigate the influence of NH4Cl on 1-hexene epoxi-
dation, experiments with the addition of different additives in CH3CN
were designed and the results are displayed in Fig. 2. Previous study
[39] revealed that the Hβ in Ti-
η
1(OOH) species that generated form the
interaction between framework Ti(Ⅳ) species and H2O2 could react with
NaOH, leading to the formation of salt-like Ti-
η
2(OO)-Na+ species and
deactivation of framework Ti within TS-1 zeolite. While with the addi-
tion of proper amount of ammonia, the catalytic performance of alkene
epoxidation over TS-1 and Ti-MWW could be promoted through
hydrogen-bond formation between NH3 and Hβ of Ti-O-O-H [25] .
However, we find that the Ti-BEA zeolite was nearly deactivated under
mild basic condition, no matter ammonium acetate (CH3COONH4) or
alkaline compounds (NH3•H2O and NaOH) was introduced in (Fig. 2
Fig. 1. 1-hexene conversion with the increase of reaction time (A) and temperature (B). (CH3CN is used as solvent. In Fig. B, the bar in grey: without NH4Cl, the bar
in blue: NH4Cl added. Note that the molar ratio of NH4Cl to framework Ti atom (according to UV-Vis result) in Ti-BEA zeolite is about 4.)
3

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
Fig. 2. Influence of different additives on 1-hexene epoxidation. (CH3CN is used as solvent. The molar ratio of additives to framework Ti atom (according to UV-Vis
result) in Ti-BEA zeolite is about 4.)
CH3COONH4. Furthermore, the limited NH3•H2O content insufficient to
Table 2
lead to the deactivation of Ti active center.
pH value of reaction mixture with different additives.
The effect of alkali was further taken into consideration. When NaOH
entry
Additives
HCl
pH
4.1
4.4
6.2
4.8
7.1
7.8
9.2
was added in the reaction system, 1-hexene conversion reached to its
maximum point at Na/Ti = 0.5. And then, it decreased rapidly with the
increasing of Na/Ti ratio. Note that the pH values of the reaction liquid
are 6.4 for Na/Ti = 0.5, 7.9 for Na/Ti = 1 and 8.4 for Na/Ti = 2
(Table S3, entry 4), respectively. It is therefore inferred that the
framework Ti of Ti-BEA is rather sensitive to alkalis, and weak alkalinity
might lead to the deactivation of Ti-BEA zeolite. The speculated deac-
tivation mechanism caused by alkalis is shown in Scheme 3 (2).
1
2
3
4
5
6
7
CH3COOH
NH4Cl
-
CH3COONH4
NH3•H2O
NaOH
In order to verify the coordination effect of N atom on framework Ti
species, several amine hydrochlorides (carbon number≤3) were selected
and applied to 1-hexene epoxidation reaction (Fig. 3B). It is observed
that three linear methylamine hydrochloride (CH3-NH3Cl), ethylamine
hydrochloride (CH3CH2-NH3Cl) and 1-propanamine hydrochloride
(CH3(CH2)2-NH3Cl) have the similar impact on the epoxidation reaction.
When 1≤N/Ti≤2, 1-hexene conversion reached its maximum point
(>80%), which was comparable to NH4Cl, NH3•H2O or CH3COONH4
involved reactions. As N/Ti>2, 1-hexene conversion started to decrease
slowly, but even when N/Ti reached to 8, it was still higher than the
blank test. The effect of three linear amine hydrochlorides on the 1-hex-
ene epoxidation reaction is slightly different from that of NH4Cl, and this
might be explained by the pKa values (Table 3) of corresponding
ammonia/amines. The pKa values of methylamine, ethylamine and 1-
conversion of 1-hexene when N/Ti≤1 (Fig. 3A). For NH3•H2O and
CH3COONH4, the 1-hexene conversion decreased rapidly when N/Ti>1,
however, the pH values are below 7 while N/Ti = 2 (Table S3, entries 2
and 3). This indicates that it is the excessive NH3•H2O (or NH3) would
lead to the deactivation of Ti active center instead of alkalis. Given that
NH3 molecules could coordinate with titanium and forming a six-
coordinated Ti species, [41,42] and further, for NH3•H2O and
CH3COONH4, the reaction condition of N/Ti>1 supplied abundant
NH3•H2O, it is inferred that coordination of excessive NH3•H2O (or
NH3) with framework Ti center hindered the activation of H2O2 on the
framework Ti center and caused its deactivation (Scheme 2). While for
NH4Cl, only a small part of NH4+ would hydrolyze to generate NH3•H2O,
hence, more NH4Cl (N/Ti≥4) is needed to acquire the optimum
enhancement in 1-hexene conversion compared with NH3•H2O and
Fig. 3. Influence of different alkalis and ammonium salts on 1-hexene epoxidation reaction. (CH3CN is used as solvent for all the tests.)
4

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
propanamine are nearly equal to each other, which is higher than that of
ammonia. It means that the alkalinity of the three amines is stronger
than ammonia, and further, the N atom of the corresponding amine
molecules might have relative stronger ability to coordinate with the Ti
active center [31,44] . In addition, the coordination of amine molecules
would promote the hydrolysis of amine hydrochlorides as well. There-
fore, the optimal dosage of three amine hydrochlorides (1≤N/Ti≤2) was
lower than that of NH4Cl (N/Ti≥4). On the other hand, as N/Ti>2,
dissociated amine would accumulate gradually, and the slight decrease
of 1-hexene conversion might due to deactivation of part of Ti active
center by the coordination of excessive amine.
Therefore, a sample of Ti-BEA-0.5 (Si/Ti = 128) with 97.4% of frame-
work Ti species was prepared and chose for further investigation. The
UV-Vis spectra of Ti-BEA-0.5 treated under different conditions are
displayed in Fig. 4.
After the treatment of Ti-BEA-0.5 with CH3CN, the peak at around
206 nm red-shifted to 216 nm. Meanwhile, a broad peak ranging from
300 to 450 nm rose up, which is similar to that of TS-1 (Fig. S5). It was
reported that CD3CN, [5,50] like pyridine, [30] as a weak base probe
molecule can interact with framework Ti(Ⅳ) species, while CH3CN
shows no absorbance peak at around 300–450 nm (Fig. S6), therefore,
the red shift and the new appeared peak might be attributed to the
interaction between CH3CN and Ti active center.
In the case of two branched amine salts, dimethylamine hydrochlo-
ride ((CH3)2NH2Cl) and trimethylamine hydrochloride ((CH3)3NHCl),
the highest 1-hexene conversion was reached when N/Ti=1, and the 1-
hexene conversion was reduced apparently when N/Ti>1, their effect on
1-hexene conversion is a little different form that of three linear amine
hydrochlorides and NH4Cl mentioned above. As the pH values of reac-
tion solutions are all below 7 for all the ammonium salts (Table S3,
entries 1 and 5-9), the 1-hexene conversion might have little correlation
with the pH change of reaction solutions caused by the N/Ti molar ratio.
For the result of (CH3)2NH2Cl, it could also be explained according to the
corresponding pKa value. Dimethylamine has the maximum pKa value
among these amines, which indicates a stronger tendency to coordinate
with the Ti active center among the various amines. Given the consid-
eration of higher steric hindrance of dimethylamine than linear amines
and excessive dimethylamine applied in the reaction, it is more difficult
for framework Ti to activate H2O2, and leading to a positive impact on 1-
hexene conversion when N/Ti>1. As for (CH3)3NHCl with greater steric
hindrance, the maximum of 1-hexene conversion could only achieved
with 71.4% at N/Ti = 1, and it decreases faster than (CH3)2NH2Cl when
N/Ti>1. The steric hindrance effect of amines on 1-hexene epoxidation
could also be seen in Fig. S3.
A shoulder peak at around 340 nm can be observed after treatment
with NH4Cl as well as NH3•H2O, and both NH4Cl and NH3•H2O have no
absorbance peak at around 340 nm (Fig. S6), so this peak might origi-
nated from the interaction of Ti species with NH3•H2O that dissociated
from NH4Cl. It is worth to note that this band cannot be discerned in
NH4Cl (and NH3•H2O) treated TS-1 (Fig. S5), and it might be caused by
the different Lewis acid property between TS-1 and Ti-BEA [7,51].
=
After treatment with H2O2, the peak at around 206 nm red shifted to
222 nm, and a broad band related to Ti-hydroperoxo species emerged at
around 360 nm. This band can be divided into two parts by deconvo-
lution method, [45,47,52] a peak at around 340 nm and another at
around 390 nm (Fig. 4C, in green), which are commonly assigned to the
bridged Ti-
η
2(OOH) and open (or linear) Ti-
η
1(OOH) species,
respectively.
With the sequential addition of H2O2 and NH4Cl, the peak at around
222 nm shifted back to 206 nm, and the broad band at around 360 nm
blue-shifted to 340 nm and became narrower. What’s more, the intensity
of the peak at around 340 nm increased, and the peak at around 390
decreased significantly (Fig. 4C, in blue). Some researchers [22,32]
pointed out that for TS-1, Ti-SBA-15 and Ti-YNU-1 zeolites, the linear
To be brief, the effect of ammonium salt on Ti active center relates to
its dissociation ability, concentration, and also the steric hindrance of
the corresponding ammonia/amines. Excessive ammonia/amines and/
or the alkaline condition would lead to the deactivation of Ti center,
while the amines with greater steric hindrance will bring about the
decline of catalytic activity as well.
Ti-
η
1(OOH) and the bridged Ti-
η
2(OOH) species are isomers and co-exist
with equilibrium in aqueous, and the Ti-
η
1(OOH) is prone to convert
into Ti-
η
2(OOH) species in the existence of polar molecules since the
latter is more stable, and is preferential for the epoxidation reaction
based on the density functional theory study. Accordingly, the change
between the two Ti-hydroperoxo species in Ti-BEA zeolite could be
relate to the interaction of NH3•H2O (generated from NH4Cl) with Ti
3.4. Characterization of Ti-hydroperoxo intermediate
species. More specifically, the Ti-
η
1(OOH) species was converted into
Ti-
η
2(OOH)-NH3 species through the interaction with NH4Cl. And it is
UV-Vis spectroscopy can provide useful information of the coordi-
nation states of Ti on the basis of the ligand to metal charge transfer
(LMCT) from O atom (in Ti4+O2-) to Ti(Ⅳ) atom (in Ti3+O-) [41,42]. The
UV-Vis spectrum of Ti species is sensitive to its coordination environ-
ment, hence, the UV-Vis spectroscopy was widely employed to distin-
guish the different Ti species in Ti-containing zeolites [1,2] and
investigate the interaction between Ti active center and substrates, such
as H2O2, [45–47] NH3, [39,48] NaOH [39,49] and phosphate [22] .
Herein, UV-Vis spectroscopy was adopted to gain a better insight into
the function of ammonium salts in alkene epoxidation reaction. Note
that there are about 19.9% amorphous Ti species (at around 270 nm)
and 14.8% anatase (at around 320 nm) existing in Ti-BEA-2 (Si/Ti = 34)
and 15.1% amorphous Ti species in Ti-BEA-1 (Si/Ti = 62, Fig. S4 and
Table S4), respectively, and these peaks would overlap with the peaks of
NH3 coordinated Ti(Ⅳ) species [48] and Ti-hydroperoxo species [45].
reasonable to attributed the enhancement in substrates conversion and
epoxides selectivity (Table 1) to the increase of Ti-
η
2(OOH)-NH3 species,
which is more efficient in epoxidation than Ti-
η
1(OOH) species. The
plausible transform mechanism of two Ti-hydroperoxo species is dis-
played in Scheme 3(1).
When NH3•H2O was introduced in instead of NH4Cl (Fig. 4B, blue
line), the peak at around 206 nm shifted to shorter wavelength, and
there emerged two peaks at around 250–260 nm and 340 nm. This in-
dicates that the tetrahedral Ti(Ⅳ) species was converted into six-
coordinated Ti species (Scheme 2) and bridged Ti-
η
2(OOH) species.
Also, a transformation from Ti-
η
1(OOH) species to Ti-
η
2(OOH) species
can be observed (Fig. 4D, in blue) as well as NaOH involved reaction
(Figure S7B). It was reported [39,47,53] that the salt-like bridged
Ti-
η
2(OO)-M+ (M+=Na+, NH+4 ) species could be generated from inter-
action of Ti-
bridged Ti-
η
1(OOH) species with basic molecules. That is to say, the
η
2(OO)-M+ species was formed in the presence of excessive
Table 3
NH3•H2O and NaOH (Scheme 3, (2)). It is possible that Ti-
η
2(OO)-M+
pKa value of ammonia and amines [43].
species (Fig. 4D, in blue) shares similar absorption peak at around 340
nm with Ti-
η
2(OOH)-NH3 species (Fig. 4C, in blue), so it is difficult to
entry
Alkali
pKa
1
2
3
4
5
6
NH3•H2O
CH3-NH2
9.25
distinguish the two Ti-hydroperoxo species from each other. As was
discussed above, when NH3/Ti>1 (or Na/Ti>1), the framework Ti
species was almost deactivated, therefore, only the nitrogen (as NH3)
10.62
10.63
10.77
10.57
9.80
CH3CH2-NH2
(CH3)2NH
CH3CH2CH2-NH2
(CH3)3N
chelated Ti-
η
2(OOH)-NH3 species plays a favorable role in alkene
epoxidation reaction, whereas the salt-like Ti-
η
2(OO)-M+ species
5

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
Fig. 4. UV-Vis spectra of Ti-BEA-0.5 interacted with different additives. (The dosage of additives, N/Ti = 4. For interpretation of the references to color in this figure,
the reader is referred to the web version of this article)
Scheme 1. The formation of Ti-hydroperoxo intermediates.
contributes little to the epoxidation. This could be interpreted that the
eletrophilicity of Ti-hydroperoxo species would be further decreased as
Ti-η2(OO)-M+ species is formed in the basic environment, [32,54] this
might be detrimental to the 1-hexene epoxidation and result in the
deactivation.
species to form Ti-
η
2(OOH)-R species as well (Scheme 3, (2)). Moreover,
the nucleophilicity of OH- is stronger than that of NH₃, once the Ti-
η2(OOH)-OH species is formed, the Lewis acidity of Ti active center
along with the eletrophilicity of Ti-η2(OOH)-OH species might be lower
compared with Ti-η2(OOH)-NH₃ species, it might be disadvantageous to
the 1-hexene epoxidation and lead to the deactivation.
On the other hand, as is known, the nucleophilicity of OH- and
CH3COO- are stronger than that of H2O, therefore, the OH- and CH3COO-
Similar trends could also be observed for TS-1 zeolite (Fig. S5), and
all these results together provide reliable spectroscopic proofs for
might act as R like NH3 to interact with Ti-
η
1(OOH) or Ti-
η
2(OOH)-H2O
6

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
Scheme 2. Speculated deactivation mechanism of framework Ti(Ⅳ) caused by coordination of excessive NH3•H2O.
Scheme 3. Proposed rearrangement of Ti-hydroperoxo species under different conditions.
understanding the nature of the Ti-hydroperoxo species.
of valeraldehyde (entry 3 and 6). As both Ti-BEA and H2O2 were
employed, the conversion of the epoxides raised up to 12.1% (entry 6).
On the contrary, after introducing NH4Cl in, the epoxide conversion
decreased to 3.6% (entry 7), which is only a quarter of the result of entry
6, and it suggests that the side reactions of epoxide on the Ti active
center was greatly suppressed with the addition of NH4Cl.
3.5. Side reactions of epoxidation
The above epoxidation evaluation results manifest that the N-
chelated Ti-hydroperoxo species, e.g. Ti-
η
2(OOH)-NH3, would not only
Further, reactions of 1,2-epoxycyclopentane, 1,2-epoxycyclohexane
and 1,2-epoxyoctane with MeOH as solvent were also performed, and
the results are showed in Table 5. As can be seen from the results, the
addition of NH4Cl helped to decrease both epoxide conversion and
aldehyde selectivity. The effect of NH4Cl in inhibiting the side reactions
of epoxides can be also observed in the epoxidation of cyclopentene
(Table S5).
benefit the alkenes conversion but also promote the epoxide selectivity.
For example, the selectivity of 1,2-epoxyhexane raised from 76.0 to
96.6% when it occurred in MeOH (Table 1, entry 2) with the addition of
NH4Cl. In order to investigate how the ammonium salts affect the
epoxidation products selectivity, reactions of epoxide were performed
with MeOH as solvent under different conditions, and the results are
shown in Table 4.
It is argued that Ti-
η
1(OOH) species should be mainly account for the
Generally, there are three side reactions may occur under epoxida-
tion reaction condition, which is oxidation cracking, hydrolysis and
alcoholysis reaction (Scheme 4). As shown in Table 4, either Ti-BEA or
H2O2 was absence in the reaction would lead to a relative low conver-
sion of 1,2-epoxyhexane, and the main product is hexanediol methyl
ether (entry 1 to 5). The involvement of H2O2 resulting in the formation
side reactions of epoxidation, such as alcoholysis, which is owing to its
Brønsted acid properties. [35] Since the proportion of Ti-
η
1(OOH) spe-
cies is relatively low as NH4Cl was added in the Ti-BEA and H2O2 re-
action system (Fig. 4C), the lower epoxide conversion with the addition
of NH4Cl (Table 4, entry 7, and Table 5, entries 2, 4 and 6) should be
associated with the relative low concentration of Ti-
η
1(OOH) species.
This might indicate that the Brønsted acid strength of Ti-
η
1(OOH) spe-
cies is weakened by NH3 coordination. On the other hand, researchers
[31,32] recently point out that the electrophilicity of the
Ti-hydroperoxo species would be decreased by CH3CN (and other li-
gands, e.g., CH3OH and CH3COCH3) coordination. The lower electro-
Table 4
Reactions of 1, 2-epoxyhexane in methanol solvent.
entry
condition
conversion/%
selectivity/%
aldehyde a
ether b
100.0
100.0
76.2
diol c
philicity of the Ti-
η
2(OOH)-CH3CN species would be advantageous to
1
2
3
4
5
6
7
-
0.6
0.9
1.0
5.0
4.1
12.1
3.6
-
-
H2O
-
-
the epoxides desorption and is adverse to the epoxides reabsorption, this
might as well contribute to the prevention of the epoxides ring-opening
H2O2
23.8
0.6
0.7
26.5
17.3
-
Ti-BEA
99.4
-
side reactions. It is therefore inferred that Ti-
η
2(OOH)-NH3 species
Ti-BEA-H2O
Ti-BEA-H2O2
Ti-BEA-H2O2-NH4Cl
99.3
-
might function as the Ti-
η
2(OOH)-CH3CN species in epoxidation and
68.9
4.6
0.2
83.5
more effective, that inhibit the side reactions of epoxides and thus
promote the alkene conversion.
Note: a valeraldehyde. b hexanediol methyl ether. c 1,2-hexanediol.
Reaction conditions: 0.2 g catalyst (if needed), 480 mmol solvent, 24 mmol H2O2
(aqueous, 30 wt%, if needed), 24 mmol 1,2-epoxyhexane, and 0.012 g NH4Cl
(N/Ti = 4), 40 ◦C, 4 h.
7

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
Scheme 4. Major side reactions of epoxidation.
Table 5
Side reactions of other epoxides in methanol.
entry
substrate
N/Ti
conversion/%
selectivity/%
aldehyde a
50.2
ether b
38.2
52.7
66.6
77.5
25.9
19.2
diol c
5.2
others d
1
2
3
4
5
6
1,2-epoxycyclopentane
1,2-epoxycyclohexane
1,2-epoxyoctane
0
4
0
4
0
4
78.6
9.2
6.4
6.9
9.3
10.4
-
31.1
9.3
55.2
12.8
8.5
11.0
13.1
11.5
27.8
36.2
0.6
46.3
2.4
44.6
-
Note: a glutaraldehyde for entries 1 and 2, hexanedial for entries 3 and 4, and heptaldehyde for entries 5 and 6. b 2-methoxycyclopentanol for entries 1 and 2, 2-methox-
ycyclohexanol for entries 3 and 4, and octanediol methyl ether for entries 5 and 6. c 1,2-cyclopentanediol for entries 1 and 2, 1,2-cyclohexanediol for entries 3 and 4,
and 1,2-octanediol for entries 5 and 6. d products of isomerization and hydroxylation.
Reaction conditions: 0.2 g catalyst, 480 mmol solvent, 24 mmol H2O2 (30 wt%), 24 mmol 1,2-epoxycyclopentane (or 24 mmol 1,2-epoxycyclohexane, or 24 mmol 1,2-
epoxyoctane), and 0.012 g NH4Cl (N/Ti = 4), 40 ◦C, 4 h.
3.6. Efficiency of H2O2
and NaOH (entry 7), the efficiencies of H2O2 were barely 8.3% and
2.3%, respectively. The low utilization might due to decomposition of
H2O2 caused by alkaline conditions. Besides, more than 60% (for
NH3•H2O) and 35% (for NaOH) of the H2O2 still retained in the solu-
tions, suggesting that a major part of the framework Ti was inactivated.
In order to investigate the effect of ammonium salts on the efficiency
of H2O2, the efficiency evaluation under various conditions using MeOH
or CH3CN as solvent were carried out, and the results are shown in
Tables 6 and S5.
Decomposition of H2O2 occurred in CH3CN without the addition of
Ti-BEA zeolite (Table 6, entry 1), while Ti-BEA further promoted its
conversion to 19.4% (entry 2). However, the addition of NH4Cl
decreased the H2O2 conversion to 15.7%. As for 1-hexene epoxidation
reaction, the efficiency of H2O2 with CH3CN as solvent was 91.5% (entry
4), whereas the utilization increased to 96.1% as NH4Cl was involved,
indicating that the decomposition of H2O2 was partially depressed.
Some researchers claimed that among the two types of Ti-hydroperoxo
3.7. Reaction mechanism
Based on the above analyses and discussions, the specific effect of the
ammonium salts (take NH4Cl for example) is proposed and exhibited in
Scheme 5. For Ti-BEA zeolite, among the two types of Ti-hydroperoxo
species, the Ti-
η
1(OOH) species is relative more active in alkene epoxi-
dation step and side reactions, it is first generated by the reaction of
Ti–OH and H2O2. After the addition of NH4Cl, the dissociated ammonia
would chelate with framework Ti species and resulting the formation of
species, Ti-
η
1(OOH) species is more active in the catalyzing H2O2
decomposition. [22,55] Accordingly, the suppression of H2O2 decom-
bridged Ti-
η
2(OOH)-NH3 species. This rearranged six-coordinated Ti-
position could be ascribed to the decrease of Ti-
η
1(OOH) species as
hydroperoxo species is relatively more stable and less likely to catalyze
side reactions of epoxide and also the decomposition of H2O2. Therefore,
an improvement in alkene epoxidation conversion and epoxide selec-
tivity as well as H2O2 efficiency could be achieved.
mentioned above. With the presence of excessive NH3•H2O (entry 6)
Table 6
Efficiency of H2O2 under various conditions.
On the other hand, with the existence of excessive NH3•H2O (NH3/
Ti>1), extra NH3•H2O molecule could coordinate with framework Ti
and forming a six-coordinated Ti species, and thus hindered the acti-
entry
condition
additives
conversion of
H2O2/%
10.3
utilization of
a
H2O2/% b
vation of H2O2. In the case of Ti-
η
1(OOH) species with the existence of
1
2
3
4
CH3CN
-
-
excessive NH3•H2O or NaOH, Ti-
η
2(OO)-NH+4 or Ti-
η
2(OO)-Na+ species
CH3CN + Ti-BEA
CH3CN + Ti-BEA
CH3CN + Ti-BEA
+ 1-hexene
-
19.4
-
NH4Cl
-
15.7
-
would be formed. This salt-like Ti species is inert for epoxidation, and
leading to the deactivation of framework Ti of Ti-BEA zeolite.
69.5
91.5
5
6
7
CH3CN + Ti-BEA
+ 1-hexene
NH4Cl
87.1
39.3
64.4
96.1
8.3
4. Conclusion
CH3CN + Ti-BEA
+ 1-hexene
NH3•H2O
NaOH
In this work, the effect of ammonium salts on alkene epoxidation
reaction catalyzed by Ti-BEA was investigated with experimental and
spectroscopic methods. It is revealed that the dissociated ammonia (or
CH3CN + Ti-BEA
2.3
+ 1-hexene
Note: a The dosage of additives, N(Na)/Ti = 4. b Catalytic performance refer to
Fig. 3A.
amines) would promote the transformation of linear Ti-
η
1(OOH) species
to a more stable chelated bridged Ti species, e.g. Ti-
η
2(OOH)-NH3 spe-
Reaction conditions: 0.2 g catalyst (if needed), 480 mmol CH3CN, 24 mmol H2O2
(aqueous, 30 wt%), 24 mmol 1-hexene (if needed), 40 ◦C, 4 h.
cies. The latter Ti-hydroperoxo species can efficiently hamper the
8

X. Liang et al.
Molecular Catalysis 511 (2021) 111750
Scheme 5. Proposed mechanism of the effect of ammonium salt on alkene epoxidation reaction catalyzed by Ti-BEA zeolite.
further reactions of the epoxides and reduce the decomposition of H2O2,
thus leading to an improvement in alkene conversion, epoxide selec-
tivity as well as H2O2 efficiency. What’s more, the proper dissociation of
ammonium salts (especially for NH4Cl and linear amine hydrochlo-
rides), along with the acidic environment help to prevent the framework
Ti from deactivation. Excessive ammonia would coordinate with
framework Ti directly and suppress the activation of H2O2. Alkaline
environment e.g. excessive ammonia/amines or NaOH would facilitate
Supplementary materials
Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.mcat.2021.111750.
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