J. Ding, P. Wu / Applied Catalysis A: General 488 (2014) 86–95
87
restricted by 12-membered ring (MR) windows together with inner
connected 8-MR channels. It severs as an efficient catalyst in the
ammoximation of cyclohexanone [11], methyl ethyl ketone [12],
and acetaldehyde [13], but not yet applied to DMK ammoximation.
In this work, post-synthesized Ti-MOR was used as the cata-
lyst for the ammoximation of DMK. The experimental parameters
governing the activity have been systematically studied, and the
optimized conditions were further applied to a continuous slurry
reactor. By comparing the catalytic behaviors among Ti-MOR,
Ti-WWW and TS-1, we found Ti-MOR possessed great advan-
tages over other titanosilicates in terms of selective production of
DMKO. The deactivated Ti-MOR was investigated to give a better
understanding of the deactivation mechanism of Ti-MOR in DMK
ammoximation.
of catalyst (0.05 g), 20 mmol DMK, 27 wt% H O2 (20–36 mmol),
2
NH3 (20–36 mmol) and water or 85% t-BuOH as solvent (0–10 g)
were heated and stirred at 333 K in a 50 mL flask for 1.5 h. After
cooling and removing the solid catalyst, the reaction mixture was
subjected to GC analysis on a gas chromatograph (Shimadzu 2014,
FID detector) equipped with a 50 m FFAP capillary column. The
by-products were extracted with CH Cl2 and further identified by
2
gas chromatography–mass spectrometry (Agilent 6890 series GC
system, 5937 network mass selective detector).
The continuous ammoximation of DMK was carried out in a
160 mL glass slurry reactor equipped with a glass sand filter and
a magnetic stirrer. For a typical run, 2.0 g of catalyst powder and
120 mL of t-BuOH aqueous solution (85 wt%) were added in the
reactor and heated under stirring at 333 K. The mixture of DMK and
8
5 wt% t-BuOH aqueous solution (weight ratio of 1:3) and 27 wt%
2
. Experimental
H2O2 were then fed into the reactor separately with a micro-pump.
−
1
The feeding rate of DMK was always kept constant at 0.21 mol h
.
2.1. Titanosilicate materials
Meanwhile, ammonia gas (99.9%) was charged into the reaction
system with a mass flowmeter. The molar ratios of H O /ketone
2
2
Following previously reported methods [11–13], Ti-MOR cat-
alysts were prepared by gas–solid isomorphous substitution
between highly dealuminated mordenite (Del-MOR, Si/Al molar
and NH3/ketone were kept at 1.05 and 1.7, respectively. With the
reaction proceeding, the reaction mixture overflowed from the out-
let filter and the catalyst powder remained in the reactor. The
ammonia unconverted and not soluble in the reaction mixture was
exhausted through a condenser vent. The organic products were
analyzed with GC to calculate the conversion of DMK and the selec-
tivity of DMKO. The content of unconverted H2O2 was determined
by iodometric titration.
ratio of 160) and TiCl vapor. Del-MOR (2 g) placed in a quartz tube
4
reactor (ø 3 cm) was pretreated at 673 K in a dry N2 stream for 2 h.
TiCl4 vapor was then brought into the reactor by N2 flow, treating
dehydrated Del-MOR at 673 K for 2 h. After the treatment, the sam-
ple was purged with pure N2 at the same temperature for 1 h to
remove any residual TiCl4 from the zeolite powder. After cooling
to room temperature in N , the treated sample was washed with
deionized water and dried in air at 353 K overnight, giving rise to
MOR-type titanosilicate Ti-MOR (Si/Ti = 60).
2
3
. Results and discussion
3.1. A summary of catalyst characterization
For control experiment, TS-1 (Si/Ti = 30) [14] and Ti-MWW
(
Si/Ti = 32) [15] were hydrothermally synthesized following lit-
The crystalline structures of three titanosilicates (Ti-MOR, TS-
and Ti-MWW) were investigated with XRD technique. The XRD
erature methods. With the purpose of enhancing the catalytic
performance, TS-1 was further re-crystallized with tetrapropylam-
monium hydroxide (TPAOH) following the literature method [16].
All catalysts were finally calcined at 823 K in air to remove the
organic species.
1
patterns indicate that all the samples possessed a high crystallinity
without any impurity phase (see supporting information Fig. S1).
The UV–visible and IR spectra of these samples showed characteris-
−
1
tic adsorption bands at 220 nm and 960 cm , respectively, which
are assigned to the tetrahedral Ti species isolated in the zeolite
2
.2. Characterization methods
framework [17]. Base on the N adsorption isotherms, Ti-MOR, TS-1
2
2
−1
,
and Ti-MWW had a specific surface area of 565, 534 and 525 m g
Powder X-ray diffraction (XRD) was employed to check the
respectively. These physicochemical properties verified that these
titanosilicates were qualified as liquid-phase oxidation catalysts
structure and crystallinity of the zeolites. The XRD patterns were
collected on a Rigaku Ultima IV diffractometer using Cu K␣ radia-
tion at 35 kV and 25 mA in the 2Â angle range of 5–35 . Scanning
with aqueous H O2 as an oxidant.
◦
2
electron microscopy (SEM) was performed on a Hitachi S-4800
microscope to determine the morphology. The Ti, Al and Si contents
were determined by inductively coupled plasma emission spec-
trometry (ICP) on a Thermo IRIS Intrepid II XSP atomic emission
spectrometer after dissolving the samples in HF solution. The spe-
cific surface area was measured by N2 adsorption at 77 K on a
BELSORP-MAX instrument equipped with a precise sensor for low-
pressure measurement. The samples were activated at 573 K under
vacuum for at least 10 h. The specific surface area was determined
using Langmuir method. The UV–visible diffuse reflectance spectra
were recorded on a Shimadzu UV-2550 spectrophotometer using
3.2. Reaction pathways in DMK ammoximation
Scheme 1 summarizes the reaction pathways in the ammoxi-
mation of DMK. With a relatively active nature, DMK undergoes
easily non-catalytic condensation with NH3 and H2O2. NH3
reacts with DMK in ambient temperature and form 2,2,6,6-
tetramethylpiperidin-4-one (TMPDO). When the temperature is
higher, a mixture of acetone amines such as diacetone amine,
triacetone diamine, triacetone amine and other products can
also be formed [18,19]. Bearing a carboxyl group, TMPDO could
also go through ammoximation reaction like other ketones to
form 2,2,6,6-tetramethylpiperidin-4-one oxime (4-hydroxyimino-
TMPD). H2O2 may react with organic solvents to produce dangerous
peroxides. In the ammoximation, a high-concentration mixture
of DMK and H2O2 is an especially hazardous mixture which
may form various explosive peroxides. The primary explosive
peroxide synthesized by DMK and H O is 3,3,6,6,9,9-hexamethyl-
BaSO as a reference. The IR spectra were collected on Nicolet Nexus
4
6
70 FT-IR spectrometer in absorbance mode at a spectral resolution
−
1
of 2 cm using KBr technique (25 wt% wafer). Transmission elec-
tron microscope (TEM) (FEI-Tecnai G2F30) was used to investigate
the crystalline structure of the deactivated Ti-MOR catalyst.
2.3. Catalytic reactions
2
2
1
,2,4,5,7,8-hexaoxonane (TATP), which could further decompose
The liquid-phase ammoximation of DMK was first carried
out in a batch-type reactor. For typical reactions, the mixture
into O2 and DMK, causing a rapid increase in reaction system pres-
sure [20,21].