P. Su, et al.
MolecularCatalysis475(2019)110460
attentions for some years. For instance, a regioselective halogenation of
benzyl nitriles is achieved by using palladium catalyst, where a cyclo-
palladation of substrate is proposed as intermediate [10]. An ortho-
halogenation of aryltetrazines is realized by using palladium too [11].
Simultaneously, ortho-selective halogenations could be found in many
other palladium-catalyzed reactions [12–14]. Obviously, there is a sy-
nergy of palladium with ortho-substituent of substrate in catalytic cycle.
With development of research, more and more metals or materials
are developed into catalyst, including rhodium [15], manganese [16],
iron [17], copper [18], SAPO [19], MOF [20], or carbon nanotube-
supported palladium [21], providing more opportunities and space for
halogenation. But meanwhile, there are still several problems around
this transformation. At first, most CeH halogenations exhibit poor
tolerance of functional group on substrate, still presenting a challenge
to future research, mainly due to high reactivity of known halogen
sources, including molecular halogen, tetrahalomethane, as well as N-
halo compound [9]. Then, use of noble metals such as palladium would
limit the large-scale application. Lastly, some known catalysts are
composed of sophisticate ligands and metal salts [1,10–15], which are
often lack of heterogeneity, and meanwhile raise production cost. Ob-
viously, there is a large room for developing efficient, benign and cost-
effective catalyst for improvement.
The preparation and application of mixed metal oxides have
aroused growing interests in recent years, due to their enhanced or
unexpected properties compared with those of individual oxides [22].
In this field, as n-type semiconductor, TiO2 could adsorb ultraviolet
light, showing values in many fields such as photovoltaic cell and
wastewater treatment [23]. At the same time, the attempts on up-
grading of TiO2 are still ongoing, where doping of foreign metal ion or
functional component into TiO2 framework may create newly-gener-
ated properties. For example, incorporation of Pd and Ag ions shows
much higher activity for photocatalytic degradation of organic pollu-
tant like polybrominated biphenyl than pure TiO2 [24]. Furthermore,
when transition metals like Nb or Cr are introduced into nanostructured
TiO2, the resulting material provides much better stability than pure
TiO2 or commercial carbon black, showing potential for immobilizing
Pt in Proton Exchange Membrane (PEM) fuel cells [25]. Moreover,
metal-doped TiO2 materials have drawn immense attentions due to
their fascinating multifunctional properties, which could be ascribed to
crystal defects that formed during incorporation [26].
Among other endeavors, in order to circumvent shortcomings of
pure TiO2, like poor surface area and moderate thermal stability, along
with very limited machinability, Al2O3 is put forward as a promising
dopant for improvement [27]. Moreover, the surface characteristics of
mixed oxide, such as structure, morphology and acidity could be
modulated by changing composition of preparative solution [28].
Therefore, taking into account inertness of CeH bond, developing Ti-Al
mixed oxides as catalyst may bring about new reactivity in halogena-
tion.
This study intends to provide an efficient, environmental-friendly
and cost-effective halogenation system through merging merits of Ti
and Al. During this process, Ti-Al binary oxides are prepared through
sol-gel, and influences of preparative conditions over product structure
are fully discussed. The synthetic samples are then employed as catalyst
in CeH halogenation. The reaction parameters are well studied, and
catalyst recycling is tested too. Additionally, a catalytic mechanism is
proposed based on experimental results in order to further understand
present transformation. In general, this study would contribute to the
development of heterogeneous CeH activation reactions.
Al(NO3)3·9H2O (99+ %), NaBH4 (99%), NH3∙H2O (25 wt.%), TiO2
(99%) and Al2O3 (99%) are commercially available from Adamas. The
phenol (99%), 4-tert-butylphenol (99%), 2,4-di-tert-butylphenol (99%),
benzaldehyde (99%), propionaldehyde (99%), acetophenone (99%),
cyclohexanone (99%) and trifluoroacetic acid (99%) are all purchased
from Acros. Formic acid (99%), acetic acid (99.5%), copper(II) chloride
dihydrate (CuCl2∙2H2O, 98%), copper(II) bromide (CuBr2, 99%) and
copper(I) iodide (CuI, 99.5%) are commercially available from Fluka.
Other metal salts and organic solvents are provided by local supplier,
and distilled water is prepared in our laboratory.
2.2. Instruments
The ultrasonic bath is performed on KQ-100DE ultrasonic cleaner,
Kunshan Ultrasonic Instruments Co., with power of 100 W, frequency of
40 kHz. Scanning electron microscopy (SEM) is carried out on JEOL
JSM-6700F at 20.0 kV without Au coating. Atomic Force Microscopy
(AFM) is performed on a Veeco Nano Scope IV Multi-Mode AFM system.
The X-ray photoelectron spectroscopy (XPS) is measured on Kratos Axis
Ultra DLD, using monochromatic Al Kα X-ray (1486.6 eV) as irradiation
source, and binding energy scale is calibrated with C 1s peak at 284.8
eV as standard. The peaks are fitted by employing Gaussian-Lorentz (G
/ L) product function with 30% Lorentzian. The low-angle (2θ = 0.5° -
10°) and wide-angle (2θ = 10° - 80°) x-ray diffractions (XRD) of pow-
dered sample are measured on Philips X’Pert Pro diffractometer using
Cu-Kα radiation (λ = 1.5418 Å), having interval of 0.05° s−1. The static
contact angle is detected according to conventional sessile drop method
by a charge-coupled device (CCD) camera (Sony XC-ST70CE).
The acid amount of solid sample is determined by n-butylamine
titration [29]. The real-time monitoring of solvated particle size and ζ
(zeta) potential are performed on Zetasizer Nano ZS90 spectrometer,
Malvern. FT-IR is detected on Bruker Tensor 27 having wave numbers
of 400-4000 cm−1, where sample is dispersed into KBr pellet. UV–vis
spectroscopy is collected on UV 1800, Shimadzu. Thermo-gravimetric
analysis (TGA) is performed on NETZSH TG 209C featuring TASC 414/
4 controller under N2 protection, with a heating rate of 10 °C/min at
30–800 °C. Differential scanning calorimetry (DSC) is carried out on
NETZSH DSC 214 under N2 protection, with a heating rate of 10 °C/min
at 30–300 °C.
GC–MS is tested on GCMS-QP2010 Plus, Shimadzu, with Rxi-5 ms
capillary column having length of 30 m, internal diameter of 0.25 mm.
For GC part, column temperature is set to 60 °C, injection port tem-
perature is 250 °C, sampling mode is split-flow, split-ratio is 26, carrier
gas selects He. For MS part, ion source temperature is set to 200 °C, and
interface temperature is 250 °C.
2.3. Synthesis of Ti-Al binary oxides
The synthetic route is provided in Scheme 1. In practice, PVP (0.8 g,
for C1, C2, C4 and C5) is dissolved into distilled water (30 mL) in a
round-bottomed flask (250 mL) under magnetic stirring at 25 °C. After
continuous stirring for 0.5 h, this mixture is further being placed in an
ultrasonic bath (using ultrasonic cleaner of 100 W and 40 kHz, Sect.
2.2.) for 0.5 h. Then, Ti(OBu)4 (0.3 mL, 0.29 g, 0.88 mmol) and Al
(NO3)3·9H2O (0.33 g, 0.88 mmol) are introduced, and resulting solution
is stirred for 5 min. The base (NaBH4, 0.1 g, 2.6 mmol, for C1 and C4; or
NH3∙H2O, 25 wt.%, 2.6 mmol, 10 mL, for C2, C3, C5 and C6) is in-
troduced at this time, and stirring is continued.
After being stirred at 25 °C for 2 h, this mixture is completely
transferred to an autoclave (100 mL), and being aged at preset tem-
perature (100 °C for C1, C2 and C3; 200 °C for C4, C5 and C6) for 24 h.
The solids are collected by centrifugation, washed with ethanol
(3 × 3 mL) and distilled water (3 × 3 mL) carefully. After being dried at
60 °C for 12 h in air, the products are obtained as powders (C1, white,
0.13 g; C2, white, 0.11 g; C3, white, 0.10 g; C4, faint yellow, 0.11 g; C5,
faint yellow, 0.12 g; C6, faint yellow, 0.11 g).
2. Experimental
2.1. Starting materials
The polyvinylpyrrolidone (PVP, average M.W. = 58,000) and tita-
nium(IV) n-butoxide (Ti(OBu)4, 99+ %) are both bought from Alfa. The
2