Epoxidation of crotyl alcohol using Ti-containing heterogeneous catalysts: Comments on the loss of Ti by leaching

Article abstract of DOI:10.1006/jcat.2000.3139

The leaching of Ti from the redox molecular sieve TS-1 is compared and contrasted with Ti leaching from Ti-Alβ, Ti-MCM-41, and a Ti-xerogel under both batch and continuous flow reaction conditions for the oxidation of crotyl alcohol with hydrogen peroxide. The product distributions for the catalysts are similar, with epoxide being formed initially and, subsequently, the secondary reaction products of the triol and ether diols. Ti leaching is more pronounced under continuous flow conditions and the order of stability is TS-1 > Ti-Alβ > Ti-MCM-41 > Ti-xerogel, for both continuous flow and batch reaction conditions. TS-1 leaching is shown to be caused by the reaction of triol with TS-1 in the presence of hydrogen peroxide. A possible mechanism in which the triol byproduct chelates the Ti and breaks the Ti-O-Si framework bonds, leading in turn to the formation of a Ti species in solution, is proposed. The soluble Ti species is found to be an active homogeneous catalyst giving triol as the major product from crotyl alcohol. Under batch reaction conditions, it is possible that an equilibrium is established between the solution Ti species and Ti on the catalyst surface. This equilibrium is perturbed when continuous flow conditions are used, and this leads to increased Ti leaching.

Full text of DOI:10.1006/jcat.2000.3139

Journal of Catalysis 198, 319–327 (2001)
doi:10.1006/jcat.2000.3139, available online at http://www.idealibrary.com on
Epoxidation of Crotyl Alcohol Using Ti-Containing Heterogeneous
Catalysts: Comments on the Loss of Ti by Leaching
�
�
Lucinda J. Davies, Paul McMorn, Donald Bethell,† Philip C. Bulman Page,‡ Frank King,§
�
,1
Frederick E. Hancock,§ and Graham J. Hutchings
�
Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, United Kingdom; †Leverhulme Centre for Innovative Catalysis,
Department of Chemistry, University of Liverpool, Liverpool L69 3BX, United Kingdom; ‡Department of Chemistry, Loughborough University,
Loughborough, Leicestershire LE11 3TU, United Kingdom; and §Synetix P.O. Box 1, Billingham, Cleveland TS23 1LB, United Kingdom
Received September 12, 2000; revised November 16, 2000; accepted November 28, 2000; published online February 13, 2001
Ti-containing zeolites, e.g., Ti-� (7, 8), and mesoporous Ti-
The leaching of Ti from the redox molecular sieve TS-1 is com- containing MCM-41 (9–12) materials. In addition, titania-
pared and contrasted with Ti leaching from Ti–Al�, Ti–MCM-41, silica co-gels, which have nonregular pore structures (13,
and a Ti–xerogel under both batch and continuous flow reaction
1
4), have been extensively studied for oxidation reactions
conditions for the oxidation of crotyl alcohol with hydrogen per-
oxide. The product distributions for the catalysts are similar, with
epoxide being formed initially and, subsequently, the secondary re-
action products of the triol and ether diols. Ti leaching is more pro-
nounced under continuous flow conditions and the order of stability
is TS-1 > Ti–Al� > Ti–MCM-41 > Ti–xerogel, for both continuous
flow and batch reaction conditions. TS-1 leaching is shown to be
caused by the reaction of triol with TS-1 in the presence of hydro-
by Baiker and co-workers. (15, 16). The use of these Ti-
containing catalysts for oxidation is, therefore, well estab-
lished at this time (17), and the range of active catalysts has
been extended to include other transition metal-containing
microporous and mesoporous materials (18, 19).
Although significant effort has been given to establish-
ing the reactivity of these Ti-containing catalysts, relatively
gen peroxide. A possible mechanism in which the triol byproduct little attention has been given to the stability of these ma-
chelates the Ti and breaks the Ti–O–Si framework bonds, leading terials under typical reaction conditions. Recently, Sheldon
in turn to the formation of a Ti species in solution, is proposed. The et al. (20) have considered the possibility of leaching of
soluble Ti species is found to be an active homogeneous catalyst
giving triol as the major product from crotyl alcohol. Under batch
reaction conditions, it is possible that an equilibrium is established
metal cations from the framework sites of these catalysts.
This leaching is due to solvolysis by polar molecules, ei-
ther reactants or products, reacting with the metal–oxygen
between the solution Ti species and Ti on the catalyst surface. This
framework, resulting in the removal of the metal into solu-
equilibrium is perturbed when continuous flow conditions are used,
tion. Sheldon et al. (20) proposed two types of leaching:
and this leads to increased Ti leaching.
�c 2001 Academic Press
(
a) the metal may leach but is not active as a homoge-
Key Words: epoxidation of allylic alcohols; heterogeneous epox-
idation using TS-1; heterogeneous epoxidation using Ti-�; hetero-
geneous epoxidation using Ti-MCM-41; heterogeneous epoxidation
using Ti-xerogel; Ti-leaching.
neous catalyst and hence there is no interference of the
soluble metal complex with the catalyzed reaction of inter-
est, and (b) the metal leaches and provides an active homo-
geneous oxidation catalyst and, in this case, the observed
catalyst activity is a complex mixture of homogeneous and
heterogeneous processes. Alternatively, the catalyst struc-
ture can be stable under reaction conditions and the re-
INTRODUCTION
Ti-containing materials have been used extensively as ox- action can be considered to be wholly heterogeneously
idation catalysts for both gas phase and liquid phase reac- catalyzed. However, very low levels of leaching may oc-
tants. The interest in the use of titanium silicates as het- cur over a long period of time, particularly for catalysts
erogeneous catalysts was prompted by the discovery that tested under standard batch reaction conditions. Low con-
the microporous titanium silicalite TS-1 (1) was effective centrations of metals in solution can be very active catalysts
for the oxidation of hydrocarbons (2–4) and alcohols (5, 6) with a range of oxygen donors. For example, 1–2 ppm of
using hydrogen peroxide as oxidant. Since then, consider- chromium leached from CrAPO-5 was found to be respon-
able attention has been given to oxidations using wider pore sible for the oxidation activity of this catalyst (21). Recent
studies have addressed this issue, and a large number of
1
redox transition metal microporous and mesoporous cata-
lysts have been shown to leach metals that subsequently
To whom correspondence should be addressed. E-mail: hutch@
cardiff.ac.uk.
319
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

3
20
DAVIES ET AL.
act as homogeneous catalysts. These include VS-1 (22, 23), outlined by Koyano and Tatsumi (35). Tetrabutyl orthoti-
VAPO-5 (24, 25), V- and Cr-MCM-41 (26), and CoAPO-5 tanate (0.27 g, Aldrich) was added dropwise with stir-
�
(
27, 28). Although TS-1 is remarkably stable to leaching of ring to tetraethyl orthosilicate (14.4 g, Merck) at 0 C.
Ti, it has been observed during the ammoxidation of cyclo- Cetyltrimethylammonium hydroxide (62 ml, 25% in wa-
hexanone with ammonia and hydrogen peroxide (29) and ter, prepared by ion exchange of cetyltrimethylammonium
the epoxidation of propene (30). Purely siliceous MCM-41 bromide, BDH), was added dropwise and the mixture was
�
�
materials are very stable at elevated temperatures in air stirred (25 C, 1 h) and then heated (80 C, 4 h) to remove the
�
or oxygen; however, even in the presence of low concen- ethanol. The solution was heated in an autoclave (120 C,
trations of water, studies have shown that the mesoporous 48 h) and the white solid formed was recovered by filtra-
�
structure can collapse due to silicate hydrolysis (31–33). tion, washed with deionized water, dried (100 C, 16 h), and
Hence, Ti-MCM-41 materials may be expected to be less heated under nitrogen (550 C, 8 h).
�
stable in the presence of hydrogen peroxide than TS-1.
In this paper, we present a detailed study of the rela-
tive stability of Ti-containing heterogeneous catalysts for
the epoxidation of crotyl alcohol with hydrogen peroxide,
under both batch and flow reactor conditions.
Synthesis of Ti–xerogel. Ti–xerogel (2 mol% Ti) was
prepared using a procedure based on that reported by Klein
et al. (36). Tetraethyl orthosilicate (10.41 g, Merck) was hy-
�
drolyzed (0 C, 30 min) by reaction with HCl (1.8 g, 0.05 M)
in propan-2-ol (2.5 mol). Tetrabutyl orthotitanate (0.336 g,
�
Aldrich) was added dropwise with vigorous stirring (25 C,
EXPERIMENTAL
1 h). Aqueous ammonia was added until a gel was formed;
this was dried (135 C, 2 h) and calcined (550 C, 24 h).
�
�
Synthesis of Materials
Catalyst Testing
Synthesis of TS-1. TS-1(2.4 mol% Ti) was prepared ac-
cording to the method of Tarramasso et al. (1) Tetraethyl or-
thotitanite (3 g, Aldrich) was added dropwise to tetraethyl
orthosilicate (91 g, Merck) and the mixture was stirred for
Catalysts were studied for the epoxidation of crotyl alco-
hol using the following standard procedures.
Batch reactor. Crotyl alcohol (0.72 g) and aqueous hy-
drogen peroxide (1.35 g, 30 vol% ) were stirred together
in methanol (50 ml) in a round-bottomed flask in a ther-
2
h. To the clear solution, tetrapropylammonium hydroxide
(
100 g, Alfa, 40% in water, K < 1 ppm, Na < 3 ppm) and
�
deionized water (60 g) were added with stirring at 25 C. The
�
�
�
mostatted oil bath at 30 C. The catalyst was added (0.1 g) to
clear gel was stirred at 25 C for 1 h and then heated at 80 C
for 5 h to aid hydrolysis; during this procedure, the volume
was maintained by addition of deionized water. The result-
ing solution was heated in a Teflon-lined, stainless steel au-
initiate the reaction and, at subsequent times, small aliquots
were removed, filtered hot through a layer of celite, cooled,
centrifuged to remove any catalyst particles, and analyzed
using gas chromatography using dimethylsulfone as an in-
ternal standard. Ti leaching was determined using ICP-MS
analysis. Blank experiments using anatase and silicalite as
catalysts showed no product formation under standard re-
action conditions. In addition, anatase was observed to be
insoluble in the reaction mixture and no Ti leaching was
observed.
�
toclave at 175 C for 10 days. The crystalline material was
recovered by centrifugation, washed with deionized water,
�
�
dried at 100 C, and calcined in air at 550 C for 24 h.
Synthesis of Ti–Al�. Ti–Al� (3.2 mol% Ti) was syn-
thesized by a method similar to that of Camblor et al.
(
34). Aqueous HCl (0.1 M, 4.51 g) was added dropwise to
�
tetraethylorthosilicate (12.5 g, Merck) and stirred (25 C,
Continuous flow reactor. Catalysts were pelleted for use
in the continuous flow reactor. Silica (5 g, fumed, BDH)
was added to the Ti catalyst (1 g) and the mixture pelleted,
washed, and sieved to give particles (1–3 mm). Premixed re-
actants (crotyl alcohol (100 g), aqueous hydrogen peroxide
4
0 min). After cooling in ice (15 min) the mixture was
added dropwise with stirring to a cooled solution of tetra-
butyl orthotitanate (0.65 g, Aldrich) in propan-2-ol (9 g)
�
�
and stirred for a further period (0 C, 20 min; 25 C, 25 min).
Tetraethylammonium hydroxide (9.8 g, 35% in water, Alfa,
K < 1 ppm) was added dropwise with stirring until a white
solid formed, which was dried (100% , 4 h). Aluminium iso-
propoxide (0.03 g) and the remainder of the template were
(
30% vol, 173 g), dimethylsulfone (5 g, internal standard),
and methanol (100 ml)) were fed continuously (0.2 ml/h) to
an upflow reactor (15 ml total volume) using a Watson and
Marlow multihead 505S pump. Exit samples were stored at
�
added with stirring at 25 C. The resulting solid was placed in
�
0
C prior to analysis by gas chromatography for product
�
a Teflon-lined, stainless steel autoclave and heated at 135 C
selectivity, pH determination, hydrogen peroxide content,
and the concentration of Ti using ICP-MS. The reactor was
maintained at 50 C using a thermostatically controlled wa-
for 192 h. Ti–Al� was recovered as a white crystalline solid
�
by centrifugation, dried (100 C, 16 h), and calcined in air
�
�
(
550 C, 24 h).
ter bath. A burette was installed at the top of the reactor
Synthesis of Ti–MCM-41. Ti–MCM-41 (1.2 mol% Ti) to monitor oxygen evolution due to hydrogen peroxide de-
was prepared using a procedure based on the method composition. The premixed reactant solution was regularly

Ti-CONTAINING HETEROGENEOUS CATALYSTS
321
monitored to ensure the composition remained constant.
The conditions used were as follows:
TABLE 1
Oxidation of Crotyl Alcohol with Hydrogen Peroxide in Methanol
at 50 C under Batch Reaction Conditions Using TS-1 as Catalyst
�
TS-1,
pellets (4.5g, 100ml), residence time 8.3 h;
pellets (3.7 g, 7.0 ml), residence time 5.8 h;
Ti–Al�,
Conversion %
Yield %
Ti–MCM-41, pellets (4.2 g, 7.0 ml), residence time 5.8 h;
Ti–xerogel, pellets (3.5 g, 6.0 ml), residence time 5.0 h.
Time Crotyl Hydrogen
Ether
Ti leaching
(ppm)
(
h)
alcohol peroxide Epoxide Triol diols pH
Characterization
0
0
0
0
0
0
0
10.7
45.6
72.5
81.6
80.4
83.5
86.5
0
0
0
3.5
5.6
10.0
9.3
9.3
10.3
10.2
4
5
5.5
4
3
1.5
1.5
1
1
1
0.1
0.2
0.3
0.2
tr
0.3
tr
tr
0
2
.8
.0
14.1
20.6
31.9
55.5
83.0
90.9
89.7
93.8
97.1
25.0
24.4
44.0
35.7
73.2
81.0
95.8
97.0
99.4
14.1
20.6
17.7
4.3
0.5
tr
tr
tr
0.4
Powder X-ray diffraction was carried out on a Siemens
D5000 diffractometer using CuK� radiation using quartz as
an internal standard. Infrared spectra were obtained using
a Perkin–Elmer system 2000 FTIR spectrometer fitted with
a Mir TGS detector. Samples were diluted with KBr (1%
sample) and pressed into discs. MAS NMR spectroscopy
8.3
0.4
8.4
17.3
17.1
90.9
2
6
1
2
2
0.5
0.6
was carried out using a 400-MHz Chemagenetics CMX- 331.5
infinity spectrometer with zirconia rotors. DR-UV spectra
were obtained with a Cary UV-visible spectrophotometer
using boron sulfate as a reference.
Hydrogen peroxide decomposition is observed and some
Ti leaching (0.15% of total Ti) after reaction for 170 h. For
the Ti-xerogel catalyst (Table 4), a similar reaction profile to
TS-1 was observed, except that the catalyst was less active
initially. During the initial period, the epoxide is formed se-
lectively and an increase in pH is observed. Subsequently,
triol and ether diols are formed and the pH decreases. For
the Ti–xerogel, significant leaching of Ti occurs, and 1.5%
of the total titaniums is leached during the 234-h reaction
period.
RESULTS
Epoxidation of Crotyl Alcohol Using Batch
Reactions Conditions
The epoxidation of crotyl alcohol with hydrogen perox-
ide in methanol solvent was investigated using TS-1, Ti–
Al�, Ti–MCM-41, and Ti–xerogel as catalysts and the re-
sults are given in Tables 1–4. For TS-1 (Table 1), initially
the epoxide was selectively formed and, at this stage, the
pH increased. Subsequently, the epoxide reacted to form Epoxidation of Crotyl Alcohol Using Continuous Flow
ether diols and triol and the pH decreased:
Reaction Conditions
The oxidation of crotyl alcohol with hydrogen peroxide
in methanol solvent was investigated for the four catalysts
using continuous flow conditions, as stated under the ex-
perimental conditions, and the results are given in Figs. 1–4.
For TS-1 (Figure 1), initially the epoxide was formed at low
conversion but, after 2–3 days time-on-stream, the crotyl
TABLE 2
Oxidation of Crotyl Alcohol with Hydrogen Peroxide in
�
Methanol at 50 C under Batch Reaction Conditions Using Ti–Al�
as Catalyst
No titanium leaching was observed, within experimental
error (<0.5 ppm), throughout the 330-h period of the ex-
periment. For Ti–Al� (Table 2) again the epoxide is formed
selectively in the initial period and an increase in pH is ob-
served. Subsequently, as with TS-1, the epoxide reacts and
the triol becomes the dominant product. For Ti–Al�, some
leaching of Ti is observed but, after 190 h reaction, only
Conversion %
Yield %
Time Crotyl Hydrogen
Ether
Ti leaching
(ppm)
(
h)
alcohol peroxide Epoxide Triol diols pH
0
0
0
7.0
19.1
71.9
97.7
98.3
0
0
0
0
49.3
72.7
70.7
0
0
0
9.3
16.4
16.4
4.5
4.5
6
4
3
0
8.7
9.7
14.2
22.7
26.8
0.3
1.0
5.0
4.1
5.0
4.1
tr
1.2
0.8
0
4
.12% ofthe totalTiisremoved from Ti–Al�. For Ti–MCM-
1 (Table 3), no catalytic activity for the direct formation
2
28.7
89.6
1.1
58.6
90.3
87.9
1
1
of epoxide is observed under these reaction conditions but,
eventually, triol, diol, and traces of epoxide are formed.
2

3
22
DAVIES ET AL.
TABLE 3
Oxidation of Crotyl Alcohol with Hydrogen Peroxide in
�
Methanol at 50 C under Batch Reaction Conditions Using Ti–
MCM-41 as Catalyst
Conversion %
Yield %
Time Crotyl Hydrogen
Ether
Ti leaching
(ppm)
(
h)
alcohol peroxide Epoxide Triol diols pH
0
1
3
5.4
4.0
69.1
0
0
0
0
26.2
83.9
0
0
0
0
0
0
1.0
0
0
0
0
23.2
67.0
0
0
0
0
3.0
15.9
4.5
4.5
4.5
4.5
4
0
2.9
3.7
10.4
17.2
18.0
.7
.4
18.1
29.8
68.7
88.0
97.0
1
6
1
2
alcohol conversion increased to 100% and the epoxide se-
lectivity decreased and the triol and ether diols became
the dominant products. Interestingly, some leaching of Ti
was observed under continuous flow conditions, whereas
none was observed under batch reaction conditions. The
Ti leaching reached a maximum after ca. 4 days time-on-
stream before declining steadily throughout the remainder
of the 45-day experiment. Between 2 and 23 days time-on-
stream, the reactor effluent became yellow in color, which
is indicative of a Ti–peroxy complex in solution.
Ti–Al� (Fig. 2) shows very similar trends in reactivity and
Ti leaching to that of TS-1. However, in the case of Ti–Al�,
some deactivation is observed after ca. 20 days time-on-
stream and the Ti leaching is considerably more significant,
with a broad maximum occurring at ca. 8 days time-on-
stream.
After an initial induction period, Ti–MCM-41 (Fig. 3),
formed triol and ether diols as the main products, together
with some epoxide. As with Ti–Al�, decolorization is ob-
served after ca. 20 days reaction time. For Ti–MCM-41,
FIG. 1. Reaction of crotyl alcohol with hydrogen peroxide in
significant Ti-leaching is observed under continuous flow methanol at 50�C with TS-1 under continuous flow conditions (residence
conditions.
time 8.3 h). (a) Crotyl alcohol conversion and product selectivity: ᭜, crotyl
alcohol conversion; ᭿, epoxide; ᭡, triol; , ether diols, (b) pH, and (c) Ti
leaching.
TABLE 4
The oxidation of crotyl alcohol with hydrogen peroxide
under continuous flow conditions with Ti–xerogel as cata-
lyst also exhibits an initial induction period but, after ca.
Oxidation of Crotyl Alcohol with Hydrogen Peroxide in
Methanol at 50 C under Batch Reaction Conditions Using Ti–
Xerogel as Catalyst
�
2
days, steady conversion is observed with the product being
Conversion %
Yield %
mainly the triol and the ether diols (Fig. 4). After ca. 30 days
operation, catalyst deactivation was observed. Ti leaching
with the Ti–xerogel under these test conditions was most
marked and, interestingly, the conversion of crotyl alcohol
appears to correlate with the concentration of Ti in solution.
Time Crotyl Hydrogen
Ether
Ti leaching
(ppm)
(
h)
alcohol peroxide Epoxide Triol diols pH
0
0
3.7
0
1.8
0
0
0
0
9.2
45.0
87.8
0
0
0
1.7
7.2
12.2
4.5
5
5
2
1
0.1
5.4
7.2
22.7
27.4
165.2
0
1
.9
.9
3.7
3.6
1.9
0
3.6
14.1
22.7
54.6
91.6
Ti-Leaching Experiments
4
2.5
12.8
52.2
100.0
1
2
17.1
34.1
TS-1wasused asa test materialsince, under experimental
conditions this material was found to be the most resistant
0
1

Ti-CONTAINING HETEROGENEOUS CATALYSTS
323
TABLE 5
Ti-Leaching Experiments for TS-1 (Crystallized 2 Days)
Conditions
Ti leaching (% )a
Hydrochloric acid (1 M)
Hydrogen peroxide
Crotyl alcohol
0.002
0.010
0.001
0.005
0.005
0.003
0.013
16.2
2,3 butanediol in methanol
Glycerol and hydrochloric acid
Glycerol, hydrochloric acid, and methanol
Glycerol and tert-butylhydroperoxide
Glycerol, hydrogen peroxide, and methanol
a
Fresh TS-1 (2-day crystallization contains 22,060 ppm Ti).
in solution due to Ti leaching. For the initial period (ca.
h), epoxide selectivity was high but, subsequently, the
3
pH decreased to pH 2.0 after ca. 20 h and ether diols and
triol were observed as products. The titanium complex also
FIG. 2. Reaction of crotyl alcohol with hydrogen peroxide in
�
methanol at 50 C with Ti–Al� under continuous flow conditions (resi-
dence time 5.8 h). (a) Crotyl alcohol conversion and product selectivity:
᭜
, crotyl alcohol conversion; ᭿, epoxide; ᭡, triol; , ether diols, (b) pH,
and (c) Ti leaching.
to Ti leaching. A range of reagents was examined to inves-
tigate the factors that influence Ti leaching. Reagents were
�
stirred with TS-1 at 50 C for 35 h, using batch reaction con-
ditions. The results in Table 5 show that, for leaching to be
observed, a combination of a triol together with hydrogen
peroxide is required. Similar experiments with Ti–Al� in-
dicated that much higher Ti leaching is observed for this
material. For example, heating with glycerol and tert-butyl
�
hydroperoxide in methanol at 50 C leads to leaching of
0
.013% of the Ti from TS-1, but leads to loss of 0.33% of
the Ti from Ti–Al�.
Oxidation Using Titanyl Acetylacetonate
FIG. 3. Reaction of crotyl alcohol with hydrogen peroxide in
methanol at 50 C with Ti–MCM-41 under continuous flow conditions (res-
idence time 5.8 h). (a) Crotyl alcohol conversion and product selectivity:
, crotyl alcohol conversion; ᭿, epoxide; ᭡, triol; , ether diols, (b) pH,
Crotyl alcohol oxidation with hydrogen peroxide was in-
vestigated using a solution of titanyl acetylacetonate com-
plex at 50 C (Table 6). This titanium complex was selected
�
�
᭜
as a model for solution Ti species that could be formed and (c) Ti leaching.

3
24
DAVIES ET AL.
the concentration of Ti dissolved in the reaction medium,
observed in the experiments with Ti-containing catalysts,
could contribute to a homogeneously catalyzed reaction
pathway that could be as significant as the heterogeneously
catalyzed process. Furthermore, the homogeneous reaction
leads exclusively to the formation of triol from the reac-
tion of crotyl alcohol with hydrogen peroxide. Similar ex-
periments using tert-butyl hydroperoxide with the titanyl
acetylacetonate complex gave only trace levels of product
formation and this suggests that this oxidant, and other or-
ganic hydroperoxides, may be preferred for the observation
of heterogeneously catalyzed reactions with Ti-containing
catalysts, other than TS-1.
Catalyst Characterization
Catalysts before and after testing in the continuous flow
experiments were selected for study, since these conditions
gave the highest levels of Ti leaching.
TS-1. Powder X-ray diffraction of TS-1 before and after
catalyst testing showed no change and the diffraction pat-
tern was consistent with literature data (1). Similarly, 29Si
MAS NMR spectroscopy showed no differences for fresh
and used samples. FT-IR spectroscopy revealed a small dif-
�
1
ference in the absorption at 963 cm for the material used,
which is consistent with the loss of Ti from the structure.
Transmission electron microscopy showed that the TS-1
crystallites were highly crystalline, but the surface of the
crystalswasdecorated with small (approximately 1 nm) par-
ticles considered to be anatase. Raman spectroscopy con-
firmed the presence of anatase in the sample (Raman shifts:
�
1
6
42, 517, 387 cm ), but no changes were observed between
fresh and used catalysts.
Diffuse reflectance UV spectroscopy of the fresh cata-
lyst shows a major absorption at 208 nm, corresponding
to tetrahedral Ti in the silicate framework (Fig. 5). There
is also a broad absorption at 290–350 nm corresponding
FIG. 4. Reaction of crotyl alcohol with hydrogen peroxide in
methanol at 50 C with Ti–xerogel under continuous flow conditions (res-
idence time 5.0 h). (a) Cotyl alcohol conversion and product selectivity:
�
᭜
, crotyl alcohol conversion; ᭿, epoxide; ᭡, triol; , ether diols, (b) pH,
and (c) Ti leaching.
TABLE 6
gave decomposition of hydrogen peroxide. From this ex-
periment, it is clear that solution titanium species formed
from leaching could play an active role in catalyst testing
experiments with these catalysts. To examine this further,
filtrates from the reaction mixtures of TS-1 catalyzed exper-
iments were studied. The filtrates from the TS-1 catalyzed
reaction were selected since TS-1 demonstrated the highest
stability under reaction conditions of the materials exam-
ined. Filtrates containing 4 and 25 ppm Ti were reacted with
crotyl alcohol and hydrogen peroxide under standard con-
ditions. After 80 h reaction, 55% crotyl alcohol conversion
was observed with the 4-ppm Ti filtrate (yields: epoxide
Oxidation of Crotyl Alcohol with Hydrogen Peroxide in Meth-
anol under Batch Reaction Conditions Using Titanyl Acetylaceto-
nate as Catalyst at 50 C
�
Conversion %
Yield %
Triol
Time
(h)
Crotyl
Hydrogen
peroxide
Ether
diols
alcohol
Epoxide
pH
0
0
0
0
0
0
5
0
2
15.2
26.7
.9
.6
5.3
4.5
8.9
5.3
4.5
tr
0
0
0
0
3
2.5
2
2
2
25.9
94.9
96.2
97.9
98.1
98.7
57.0
45.7
57.9
53.0
64.1
45.8
34.6
42.2
38.0
47.4
11.2
11.0
11.8
13.5
15.1
tr
37.9
3.9
1.6
1.5
0
% , triol 54% , ether diols 1% ), and the 25-ppm Ti filtrate
1
1
10.7
86.9
2
2
gave 89% crotyl alcohol conversion (yields: epoxide 0% ,
triol 83% , ether diols 6% ). These experiments demonstrate

Ti-CONTAINING HETEROGENEOUS CATALYSTS
325
1
.053 wt.% Ti and the used catalyst contained 0.403 wt.%
Ti, indicating that 61.8% of the total Ti present was leached
during reaction.
DISCUSSION
The reaction profiles for the oxidation of crotyl alcohol
under batch reaction conditions using hydrogen peroxide
for all the samples show that, although the epoxide can be
formed initially, the secondary products of triol and ether
diols dominate possibly due to ring-opening reactions of
the oxirane, as we have previously discussed (38). The sec-
ondary products of triol and ether diols also dominate as
products under continuous flow conditions. The formation
of these secondary products is associated with a decrease
in pH and this is observed for both the batch and contin-
uous flow reaction conditions. Crotyl alcohol was selected
as the substrate as it exhibits low reactivity with respect to
epoxidation, compared with simple alkenes, and hence the
formation of secondary products relative to the formation
of the primary epoxide product can be readily studied.
This study demonstrates that Ti leaching can be relatively
facile for the reaction of Ti-containing catalysts. Further-
more, solutions containing 4 and 25 ppm Ti have been
shown to be effective homogeneous catalysts leading to
the formation of triols and ether diols for the reaction of
crotyl alcohol with hydrogen. This was confirmed by exper-
iments using titanyl acetylacetonate as a model compound.
Ti leaching from TS-1 was investigated for a range of sol-
vent combinations, but leaching was observed only when
TS-1 was stirred in the presence of a triol and hydrogen
peroxide. It is, therefore, apparent that trace levels of tri-
ols could lead to Ti leaching for other oxidation reactions
and that the effect will be autocatalytic, since the soluble Ti
complex formed will catalyze the formation of further tri-
ols. However, this study confirms the stability of TS-1 under
most reaction conditions.
FIG. 5. Diffuse reflectance UV spectra of TS-1 (a) fresh, and (b) after
reaction under continuous flow conditions for 45 days.
to extra framework TiO2 (37). For the used catalyst, the
absorption at the lower wavelength decreases in intensity
relative to the absorption at the higher wavelength (Fig. 5).
This is consistent with loss of framework Ti from the sample.
ICP-MS analysis showed that the fresh catalyst contained
.206 wt.% Ti and the used catalyst contained 2.145 wt.%
Ti, indicating that 2.8% of the total Ti present had been
leached from the catalyst under continuous flow conditions.
2
Ti–Al�. As with the characterization studies for TS-1,
29
powder X-ray diffraction, Si-MAS NMR, and Raman
spectroscopy showed no differences between fresh and
used catalyst samples. FT-IR spectroscopy of the used cata-
lyst showed a decrease in intensity of the absorption at
�
1
9
62 cm , which is consistent with Ti loss from the struc-
tures. ICP-MS analysis confirmed that significant Ti leach-
ing occurred under continuous flow conditions. The fresh
catalyst contained 2.200 wt.% Ti and the used catalyst con-
tained 1.204 wt.% Ti, indicating that 45.2% of the total
titanium had been leached from the catalyst.
For TS-1 and Ti–Al�, it is considered unlikely that the
nonframework Ti, as observed by electron microscopy and
Raman spectroscopy, is the source of the leached Ti. In ad-
dition, for Ti–MCM-41 and Ti–xerogel, TiO2 was observed
not to be present in the fresh samples. First, model ex-
periments using anatase showed that it is insoluble in the
Ti–MCM-41. Powder X-ray diffraction, 29Si-MAS
NMR, and Raman spectroscopy showed no differences be-
tween fresh and used samples. FT-IR spectroscopy showed
reaction mixture under these conditions. Second, the FT-
�
1
that the absorption at 967 cm was absent in the used mate-
rial. ICP-MS analysis confirmed that significant Ti leaching
occurred under continuous flow conditions. The fresh cata-
lyst contained 1.200 wt.% Ti and the used catalyst contained
� 1
IR spectra show the loss of the absorption at 963 cm
,
which is consistent with loss of Ti from framework sites in
the zeolite structure. We consider that the titanium that is
removed from TS-1, and also from Ti–Al�, during reac-
tion with crotyl alcohol and hydrogen peroxide, originates
from defect sites in the framework where the framework
0
.674 wt.% Ti, indicating that 43.8% of the total Ti present
had been leached from the catalyst.
Ti–xerogel. FT-IR spectroscopy of the Ti–xerogel sam- is not perfectly coordinated to four -OSi group. These de-
�
1
ple following use showed that the absorption at 947 cm
fects have been proposed by Lamberti et al. (39) However,
was no longer present. ICP-MS analysis of the catalysts con- an alternative titanium species in TS-1, Ti(OSi)2(OH)2, has
firmed significant Ti leaching was caused by reaction under been proposed by Sinclair and Catlow (40) and, if present,
continuous flow conditions. The fresh catalyst contained this may also be removed under reaction conditions.

3
26
DAVIES ET AL.
A key observation from this study is that Ti leaching un- drolyzed/broken and this then leads to the loss of Ti from
der continuous flow conditions is considerably more pro- the structure. With TS-1, interaction with a triol, a potential
nounced than under batch reaction conditions. For ex- tridentate ligand, is required to cause Ti loss, and a diol or
ample, under batch reaction conditions, leaching of only ether diol does not cause this effect. With the more defec-
0
.6 ppm Ti from TS-1 was observed after 13 days, whereas tive structures of Ti–Al�, Ti–MCM-41, and Ti-xerogels, a
under continuous flow conditions at an equivalent exper- similar Ti leaching mechanism can be expected to operate,
imental time, a steady leaching of approximately 5 ppm except that the effect can be observed in the absence of
Ti was observed. Leaching of Ti from TS-1, and the other tridentate ligand.
Ti-containing catalysts, may be decreased under batch re-
A key result of the comparison of batch and flow reac-
action conditions due to either (i) adsorption of organic tion conditions is that Ti leaching is minimized by using
molecules onto the surface (41) or (ii) an equilibrium being controlled batch reaction conditions. This may be of signif-
established between Ti in solution and Ti adsorbed on the icance with respect to the utilization of these materials as
surface of the catalyst. It is known that reversible hydro- catalysts in the fine chemicals industry.
lysis of Ti–O–Si bonds can take place to form Ti–OH and
Si–OH (42). Both of these processes would be expected to
be perturbed by the use of continuous flow conditions.
With Ti–xerogel, Ti leaching is observed at the beginning
of the reaction, in agreement with previous literature (43).
Ti–xerogels are known to be rapidly deactivated by reaction
with water due to hydrolysis of Ti–O–Si bonds. The leached
Ti can then form Ti–O–Ti bonds by condensation reactions
or form a soluble yellow titanium peroxo species (44).
Catalyst deactivation is observed under continuous flow
conditions with Ti–Al�, Ti-MCM-41, and the Ti–xerogel
catalyst. Previous studies with Ti–xerogels have suggested
that dimerization and polymerization of the epoxide prod-
uct and/or the alkene could lead to deactivation. However,
in the present study, the deactivation observed using con-
tinuous flow conditions is considered to be related to the ex-
tensive loss of Ti from the catalysts. For the materials tested,
the order of stability is TS-1 > Ti–Al� > Ti–MCM-41 > Ti–
xerogel under both batch and continuous flow condition.
For TS-1 under batch conditions, the Ti leaching is negligi-
ACKNOWLEDGMENTS
We thank Synetix, the EPSRC, and the DTI/LINK Programme for
financial support.
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1