Zirconium-doped porous magadiite heterostructures upon 2D intragallery in situ hydrolysis-condensation-polymerization strategy for liquid-phase benzoylation

Article abstract of DOI:10.1039/c5ra10911f

Novel zirconium-doped porous magadiite heterostructures (PMH-xZr, x = Zr/Si molar ratio) are fabricated by two-dimensional intragallery cosurfactant-directing in situ hydrolysis-condensation-polymerization method of TEOS and Zr-n-propoxide from synthetic Na-magadiite and characterized systematically by XRD, SEM/(HR)TEM, ²⁹Si MAS NMR, BET, UV-vis DRS, NH₃-TPD, pyridine FT-IR, and XPS techniques. The results indicate that the obtained PMH-xZr materials possess high surface area and high thermal stability upon effective assembly of interlayer Zr-doped meso-structural silica and the layers of magadiite. The PMH-xZr samples with x < 0.2 show successful incorporation of Zr into the lattice of interlayer mesostructural silica framework leading to considerably generated Bronsted sites Zr-O(H)-Si and obviously increased Lewis sites Zr-O-Si along with well-kept layered supermicro-mesostructure, while PMH-0.2Zr shows delaminated layers. PMH-0.1Zr exhibits the highest liquid-phase benzoylation activity of anisole with benzoyl chloride (Conv. 99.5%) and yield for 4-methoxybenzophenone (4-MBP) (94.1%) due to the strongest synergy between the high concentration of surface Bronsted sites and supermicro-mesostructure. PMH-0.1Zr can be reused by no further chemical treatment for at least five runs with a slightly reduced 4-MBP yield. These PMH-xZr materials can serve as a promising solid acid catalyst and/or acidic support with high surface area and thermal stability in broad range of catalysis applications.

Full text of DOI:10.1039/c5ra10911f

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Zirconium-doped porous magadiite
heterostructures upon 2D intragallery in situ
hydrolysis–condensation–polymerization strategy
for liquid-phase benzoylation†
Cite this: RSC Adv., 2015, 5, 67853
Yuewen Ma, Huan Sun, Qi Sun and Hui Zhang*
Novel zirconium-doped porous magadiite heterostructures (PMH-xZr, x ¼ Zr/Si molar ratio) are fabricated
by two-dimensional intragallery cosurfactant-directing in situ hydrolysis–condensation–polymerization
method of TEOS and Zr-n-propoxide from synthetic Na-magadiite and characterized systematically by
29
3
XRD, SEM/(HR)TEM, Si MAS NMR, BET, UV-vis DRS, NH -TPD, pyridine FT-IR, and XPS techniques. The
results indicate that the obtained PMH-xZr materials possess high surface area and high thermal stability
upon effective assembly of interlayer Zr-doped meso-structural silica and the layers of magadiite. The
PMH-xZr samples with x < 0.2 show successful incorporation of Zr into the lattice of interlayer
mesostructural silica framework leading to considerably generated Brønsted sites Zr–O(H)–Si and
obviously increased Lewis sites Zr–O–Si along with well-kept layered supermicro-mesostructure, while
PMH-0.2Zr shows delaminated layers. PMH-0.1Zr exhibits the highest liquid-phase benzoylation activity
of anisole with benzoyl chloride (Conv. 99.5%) and yield for 4-methoxybenzophenone (4-MBP) (94.1%)
due to the strongest synergy between the high concentration of surface Brønsted sites and supermicro-
mesostructure. PMH-0.1Zr can be reused by no further chemical treatment for at least five runs with a
slightly reduced 4-MBP yield. These PMH-xZr materials can serve as a promising solid acid catalyst and/
or acidic support with high surface area and thermal stability in broad range of catalysis applications.
Received 9th June 2015
Accepted 30th July 2015
DOI: 10.1039/c5ra10911f
www.rsc.org/advances
acidity modication of PCHs derived from pure siliceous clay
1
. Introduction
magadiite was rarely studied probably due to its higher layer
1
1
Porous clay heterostructures (PCHs), derived from cationic charge density thus difficulties resulting from swelling.
layered clays upon 2D clay intragallery co-surfactant directing in
Magadiite, a kind of hydrous sodium silicate found in lake
situ hydrolysis–condensation–polymerization of TEOS, has beds at Lake Magadi, can be easily prepared by hydrothermal
14
15,16
aroused gradually increasing interest in adsorption and sepa- route in lab.
Upon synthetic silicic magadiite, a good
2,3
4
5
6,7
ration, electrochemistry, as supports and catalysts due to candidate for PCHs can be expected with excellent resistance to
1
its considerable high surface area, high thermal stability, acid and hydrothermal stability compared to common clays.
1,8,9
1
combined supermicro- and mesoporous structure.
It is noted Pinnavaia et al. have ever mentioned magadiite in their rst
that the surface acidity of the materials plays a critical role in report on PCHs from various clay hosts but without detailed
7
,17
the catalysis process especially acid-catalytic reactions. study. It is noted that Kwon et al.
reported a mesoporous
+
However, there's none or a small concentration of acid sites silica-pillared H -magadiite via amine-catalyzed hydrolysis of
located in the interlayer silica framework apart from those TEOS without pre-swelling step, but the obtained silicic mate-
10
aroused from the parent clay. Several studies have been pub- rials yet show very low or no acidity. Our recent study rst
lished aiming at further enhancement of acidity of PCHs, such reported the porous magadiite heterostructures (PMH) and its
1
1
as acid treatment of clays prior to PCH synthesis, incorpo- post-graing aluminated derivatives (xAl-PMH) from NaAlO
2
6
12
18
rating heteroatoms (e.g. Al, Ti ) into the silica pillars, and the showing greatly enhanced Friedel–Cras alkylation activity.
use of various clay hosts (e.g. uorohectorite, montmoril-
1
Transition metal oxide ZrO with high corrosion resistance,
2
6,12
10,13
13
lonite,
synthetic saponite
and vermiculite ). While the low thermal conductivity and high melting point has attracted
19
special attention in catalysis, but suffered from low surface
area and non-shape selective nature. The common way to
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, Beijing 100029, China. E-mail: huizhang67@gst21.com; Tel: +86 10 6442 overcome these limitations lies in supporting ZrO
2
onto high
20,21
5
†
1
872
surface oxides.
So far, there are a few reports on Zr–Si
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra10911f
22
mixture pillared clays/PCHs. Kosuge et al. studied zirconia–
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silica-based pillared H-ilerite by intercalating octylamine and magnetic stirring for 1 h forming an intermediate suspension
Zr-butoxide and found the formation of high surface products followed by adding premixed solution of tetraethyl-
with uniform micropores and irregular gallery heights upon Zr orthosilicate (TEOS, 99.5%) and zirconium n-propoxide
a
23
contents. Han et al. reported microporous Si–Zr-based pillared (Zr-nPro, 70 wt% in 1-propanol) with designed Zr/Si ratio to
ꢀ
clays upon the reaction of Na-montmorillonite with colloidal react at 25 C for 8 h. In above systems, the QM/OA/TEOS/Zr-
2 2
SiO –ZrO particles showing limited pore size used for selective nPro molar ratios were designed as 1/20/150/0, 1/20/148.2/1.85,
2
adsorption of toluene and mesitylene. Pinto et al. reported 1/20/146.3/3.66, 1/20/142.9/7.14, 1/20/136.4/13.64 and 1/20/125/
montmorillonite-based PCH modied with Zr by copolymeri- 25, respectively. The resultants were centrifuged, washed by
ꢀ
zation of TEOS with Zr-ethoxide for separation of aromatic absolute ethanol and dried at 60 C for 24 h giving the precur-
molecules but without detailed structure and acidity studies. sors named as QOTM-xZr (x ¼ Zr/Si, refers to Zr/Si molar ratio as
24
While Cecilia et al. prepared natural montmorillonite-based 0, 0.0125, 0.025, 0.05, 0.1 and 0.2). The TG plots (Fig. S1†) of the
PCHs with varied Zr contents by long time sol–gel process typical precursors QOTM and QOTM-0.1Zr show that the calci-
ꢀ
(
10 days) of TEOS and Zr-propoxide in 1-propanol and studied nation at the temperatures $ 550 C could remove the organic
their texture, structure and acidity by SEM, BET, XRD, FTIR, TG- templates. Thus in step (III), the precursors were calcined at
ꢀ
DTA, NH
3
-TPD and 1-butene isomerization. To the best of our 600 C for 6 h resulting in Zr-doped porous magadiite hetero-
knowledge, there is no report on Zr-doped porous magadiite structure catalysts PMH-xZr.
heterostructures, far from its acidic catalytic applications such
as Friedel–Cras acylation, which is highly desired to be
explored.
2.2 Characterization
In the present work, we report novel Zr-doped porous mag- X-ray diffraction (XRD) patterns were recorded on a Rigaku
adiite heterostructures (PMH-xZr) by cosurfactant-directing D/Max 2500 VB2+/PC diffractometer using Cu Ka radiation
ꢀ
two-dimensional intragallery in situ hydrolysis–condensation– (40 kV, 50 mA) in 2q range of 1–10 (small angle) and Cu Ka
ꢀ
polymerization strategy of TEOS and Zr-n-propoxide from a radiation (40 kV, 200 mA) in 2q range of 15–60 (wide angle).
synthetic Na-magadiite, which were studied by XRD, SEM/ The FT-IR spectra were recorded on a Bruker Vector22 FT-IR
2
9
ꢂ1
ꢂ1
3
HRTEM, Si MAS NMR, BET, UV-vis DRS, XPS, NH -TPD and spectrometer in the range of 400–4000 cm with 4 cm reso-
pyridine FT-IR techniques. The effect of Zr-doping on the lution and 60 scans by the standard KBr disk technique
texture, acidity, and benzoylation performance of anisole with (sample/KBr ¼ 1/100) with almost identical mass of samples.
benzoyl chloride of PMH-xZr materials is thoroughly investi- Thermogravimetric analyses were taken on a Mettler Toledo
ꢀ
gated. Moreover, the recycle efficiency of the PMH-xZr catalysts TGA/DSC 1/1100 SF thermo analyzer at a heating rate of 10 C
ꢂ1
ꢀ
ꢂ1 29
is also studied.
min in 25–1000 C in N ow (25 mL min ). Si MAS NMR
2
spectra were obtained at 79.5 MHz using a 7 mm zirconia rotor
and a spinning frequency of 5 kHz. A pulse duration of 4.00 ms
2
. Experimental
2
9
and a pulse delay of 60 s were used. The Si chemical shi is
reported with respect to Kaolin standard (d ¼ ꢂ91 ppm).
2.1 Preparation of catalysts
The synthesis of the catalysts follows the strategy shown in Textural analysis was done by N adsorption–desorption at 77 K
2
Scheme 1. Initial Na-magadiite was prepared via hydrothermal using a Quadrasorb SI automated gas adsorption system. Prior
route using silicate gel and NaOH according to the previous to the analysis, the samples were degassed under vacuum at
18,25
ꢀ
In detail, the starting mixture composed of SiO2 300 C overnight. The specic surface area was estimated using
work.
(
Ludox suspension, HS-40, Sigma-Aldrich)/NaOH/H O with the BET method, micropore volume by using the t-method of
2
molar ratio of 9/2/75 was stirred for 3 h and then transferred DeBoer, and the pore size distribution upon both the BJH and
into a steel autoclave with Teon lining for hydrothermal density functional theory (DFT) models. X-ray photoelectron
ꢀ
treatment at 150 C for 48 h. The product was washed with spectra (XPS) were recorded on a VG ESCALAB250 X-ray
ꢀ
deionized water till pH of 7 and air-dried at 60 C for 24 h giving photoelectron spectrometer at a base pressure in the analysis
ꢂ9
the Na-magadiite with an empirical formula Na1.9Si14
O
29
-
chamber of 2 ꢃ 10 Pa using Al Ka source (1486.6 eV) and the
7,18
$
9.2H
2
O (Table S1†), close to previously reported,
based on
TG (Fig. S1†) and EDX data. Then, in step (I), 5.0 g of Na-
magadiite and 5.0 g of cetyltrimethylammonium bromide
(
CTMABr) were suspended in 100 mL of deionized water. The
ꢀ
suspension was heated to 80 C and kept for 4 h and the
resultant was washed with deionized water till pH 7 and air-
ꢀ
+
dried at 60 C for 24 h giving CTMA -magadiite (QM in short).
The empirical formula of QM were estimated as (C19
29$2.45H O (Table S1†) upon the empirical formula of
H
43N)2.1-
Si14
O
2
+
magadiite, assuming that the amount of CTMA ions exchanged
for Na corresponds to the cation exchange capacity of clay. In
step (II, II ), proper amount of QM was added into proper
+
0
Scheme 1 Schematic of the synthesis strategy of the PMH-xZr
volume of n-octylamine (OA, 99.5%, ꢁ0.5% H
2
O) under catalysts.
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binding energies corrected using C 1s line of aliphatic carbon centrifugation (5000 rpm), washed thoroughly with absolute
ꢀ
contamination at 285.0 eV. SEM/EDX results were obtained on ethanol, dried at 120 C overnight, and subjected to next run.
an Oxford Instruments INCAx-act EDX detector attached to a Commercial AlCl was used for comparison. The product was
3
1
Zeiss Supra 55 eld emission scanning electron microscopy identied by H NMR (Brucker Avance 600 MHz). As shown in
using a 20 kV electron beam and 60 s acquisition time. Trans- Fig. S2,† the main product was conrmed as 4-MBP (small
mission electron microscope (TEM) and HRTEM images were amount of 2-MBP may be evaporated in evaporation step due to
obtained with Hitachi-800 and JEM-2010, operated at an accel- its similar boiling point to anisole).
erating voltage of 200 kV. Temperature-programmed desorption
3
of NH was performed on a Thermo Electron Corporation TPD/
R/O 1100 series instrument. Prior to NH sorption, 0.1 g sample 3. Results and discussion
3
ꢀ
ꢂ1
was out-gassed at 300 C in pure N ow (20 mL min ) for 1 h.
Then, the temperature was naturally cooled to 100 C, and NH
2
3.1 Crystal structure and textural properties
ꢀ
3
Fig. 1 shows the XRD patterns of the PMH-xZr catalysts in small
and wide 2q range. The XRD of the intermediates aer each step
and pristine magadiite are shown in Fig. S3A.† The XRD of the
was adsorbed by exposing sample to a stream of pure NH
3
for
ꢀ
0.5 h. It was then ushed at 100 C with N
2
for another 0.5 h to
remove physisorbed NH
3
. The desorption of NH
3
was carried
ꢀ
ꢂ1
magadiite shows a strong peak at 2q 5.72 (d001 ¼ 1.54 nm) and
out in pure N
2
ow (20 mL min ) by increasing the tempera-
ꢀ
ꢀ
ꢀ
ꢂ1
ve-nger-shaped peaks in 20–30 indicating the formation of
ture to 600 C at a heating rate of 10 C min . The pyridine
adsorption FT-IR studies were carried out on a Nicolet 380 FT-IR
spectrometer. The IR spectra (in absorbance mode) were
recorded using self-supporting pellets of the sample powder.
The pellet (diameter 13 mm) of 70 mg was placed in an IR cell
designed to carry out spectroscopic measurement at varied
1,15,16
well-dened magadiite (JCPDS 42-1350).
The QM shows an
+
enhanced d001 of 2.96 nm on intercalation of CTMA . Most
other peaks of the magadiite layers seem to suffer from coa-
lescence into broad bands, a phenomenon reminiscent of the
turbostraticulate stacking occurred for many clay minerals.
Then, a large excess of co-surfactant OA was added to stable the
swelling gallery favoring the 2D intragallery hydrolysis–
condensation–polymerization of subsequently added TEOS/Zr-
nPro upon a hydrated silica structures formed around a
temperatures and equipped with CaF
were rst heated to 400 C at a heating rate of 10 C min in a
2
windows. The samples
ꢀ
ꢀ
ꢂ1
2
pure N ow and kept for 2 h and then cooled to room
temperature. The pyridine vapor was introduced under N ow
2
+
1
monolayer of micellar CTMA and OA assemblies. Then the
QOTM precursor shows a similar d001 of 2.94 nm to QM, while
the Zr-doped samples QOTM-xZr show gradually increased
interlayer space from 3.09 to 3.36 nm with Zr-doping, implying
the incorporation of Zr into the intragallery silica framework
for 1 h; subsequently weakly adsorbed pyridine was ushed for
0
2
.5 h under N ow. The FT-IR spectra were recorded at 100,
2
00, 300, 400 C and a resolution of 8 cm is attained aer
ꢀ
ꢂ1
averaging over 64 scans for all the FT-IR spectra reported here.
The UV-vis diffuse reectance spectra (DRS) were obtained on a
Hitachi U-3900 VIS spectrophotometer equipped with the Var-
ian diffuse reectance accessory in 200–800 nm. Barium sulfate
was used as a reference material.
8,15
given the thickness 1.12 nm of magadiite layer.
Aer calcination, the obtained pure PMH shows further
expanded d001 as 3.38 nm, together with broadened peak shape,
which is probably due to the stabilization role of the co-
templates though with somewhat disturbance of layer
ordering owing to the removal of the gallery cosurfactants
during calcinations for the completion of dehydroxylation and
2.3 Catalytic studies
The liquid phase benzoylation reactions of anisole with benzoyl
chloride (BzCl) was performed in a 50 mL three-necked glass
1,12
cross-linking of intragallery assembled silica structure. This
expanding effect is well-retained in the nal PMH-xZr samples
as revealed by a clear broad (001) peak in low 2q angle though

ask, equipped with a magnetic stirrer, a reux condenser, a
thermometer and a CaCl guard tube. Reaction conditions:
2
anisole (12 mL, 108 mmol), BzCl (324 mL, 2.65 mmol), the
catalyst (0.2 g), n-tetradecane (as a GC internal standard, 0.1 g,
ꢀ
9
3
9.5% purity), reaction temperature, 110–140 C, reaction time,
–7 h. The liquid phase (0.2 mL) was analyzed before and aer
reaction by an Agilent 7890A GC, equipped with an Agilent J & W
HP-5 capillary column (5% phenyl polysiloxane, 30 m ꢃ 0.32
ꢀ
mm ꢃ 0.25 mm) and a ame ionization detector (FID: 340 C,
ꢀ
ꢀ
ꢀ
injector: 320 C, temperature program: 120 (3 min) to 220 C
(
ꢂ1
1 min) by 20 C min ). Conversion is given as the conversion
into 4-methoxybenzophenone (4-MBP) and 2-methox-
ybenzophenone (2-MBP); benzoic acid (BA) and phenyl
benzoate (PB) were the only by-products. The selectivity to the
desired product is given as the amount of 4-MBP divided by the
amount of total products. The control experiment conrmed
that the reaction does not take place in the absence of catalyst.
Fig. 1 XRD patterns of PMH (a) and PMH-xZr (b–f: x ¼ 0.0125, 0.025,
The used catalyst was separated from reaction system by 0.05, 0.1, 0.2) at small (A) and wide (B) angle range.
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with reduced intensity probably due to the partial loss of crys-
tallinity upon the thermal decomposition of interlayer organic
moieties. With increasing Zr-doping (x ¼ 0.0125–0.1), the (001)
lines remain clear and the d value increases and reaches the
0
01
maximum 3.82 nm for PMH-0.05Zr, and then slightly reduced
to 3.69 nm for PMH-0.1Zr, implying the uniformly ordered
incorporation of zirconium into the interlayer mesostructure of
PMH. While PMH-0.2Zr shows almost completely disappeared
(001) line, implying the delaminated structure due to its
maximal Zr-doping, similar to previously reported PCHs with
12,24
titanium and zirconium.
These results indicate that the
incorporation of Zr into the lattice of interlayer meso-structural
silica strongly affects the order of the stacking layers along the c-
axis, causing a dramatic decrease of the coherent diffraction
domain along this direction with increased Zr-doping. Note that
silica-based PMH with proper Zr-doping can hold well-retained
layered structure assembled with intragallery mesostructure
such as PMH-0.1Zr and PMH-0.05Zr.
Fig. 2 FT-IR spectra of PMH (a) and PMH-xZr samples (b–f: x ¼
0
.0125, 0.025, 0.05, 0.1, 0.2) (inset refers to the amplified IR spectra
Fig. 2 depicts the FT-IR spectra of PMH and PMH-xZr
samples, while those of the intermediates and magadiite shown
in Fig. S3B.† It can be seen from Fig. S3B† that magadiite shows
ꢂ1
between 4000–3200 cm ).
ꢂ1
a broad band at ꢁ3440 cm assigned to n(OH) and a small
ꢂ1
25,26
sharp one at 3663 cm to free OH group,
implying the probably upon the formation of Si–O–Zr linkages. Moreover, the
27
ꢂ1
presence of terminal silanol groups. The intense band at 1083 intensity of the band at 1083 cm and its fair-dened shoulder
cm can be assigned to nas(Si–O–Si), whereas two overlapped (1237 cm ) due to the Si–O–Si framework is greatly reduced,
peaks at 820 and 781 cm
motions, two ones at 620 and 573 cm to the bending modes of Zr-doping is increased especially for the samples with high Zr
single and double Si–O–Si rings of SiO unit,
ꢂ1
ꢂ1
ꢂ1
to n (Si–O–Si) involving silicon and gradually becomes narrow with slight downshi as
s
ꢂ1
2
6,27
and a sharp one content (x > 0.05). Typically for PMH-0.1Zr and PMH-0.2Zr, the
4
ꢂ1
26
ꢂ1
at 462 cm to n
s
(Si–O–Si). For QM, two sharp peaks at 2852 n (Si–O–Si) occurs at ꢁ1077 cm . Two other bands, at 811 and
as
ꢂ1
ꢂ1
and 2921 cm are due to n
s
(C–H) and nas(C–H) of –CH
2
– groups, 454 cm
for PMH, characteristic of Si–O bonds in a ring
ꢂ1
26,27,29
respectively, and a sharp one at 1469 cm to CH
3
antisym- structure,
exhibit similar band downshis and intensity
2
metric deformation modes and CH scissoring modes, implying decrease for PMH-xZr with Zr-doping, also reecting a gradual
+
the intercalation of CTMA . For QOTM and QOTM-xZr samples, increase of Zr in the Si–O network in forming Zr–O–Si hetero-
strong IR bands for organic groups are clearly seen due to the linkages. While PMH-0.2Zr shows the minimum absorption
introduction of cosurfactant and inorganic precursors. While intensity and the maximum band downshi in the ring struc-
ꢂ1
ꢂ1
the strong n (Si–O–Si) at 1083 cm
of magadiite clearly tural Si–O bands (799 and 449 cm ) corresponding to the
as
ꢂ1
reduces and slightly downshis to ꢁ1079 cm , and two bands delaminated structure of the sample. These results are probably
ꢂ1
at 820 and 781 cm to n
s
(Si–O–Si) show similar decrease in due to the superimposition of stretching and rocking vibrations
20,31
intensity and merge in one peak with gradually reduced wave- of Si–O–Si and Si–O–Zr bonds.
numbers from 808 cm of QOTM to 803 cm of QOTM-0.2Zr
ꢂ1
ꢂ1
29
Fig. 3 shows the Si MAS-NMR spectra of pure PMH and
with Zr-doping. These phenomena may be due to the formation PMH-xZr samples. All the spectra present a broad signal between
of Si–O–Zr linkages, similar to those occurred in aluminated ꢂ80 and ꢂ140 ppm, which can be deconvoluted to three main
28
29
LAPONITE®-derived PCHs or Zr-doped silica, upon the components with chemical shis at ca. ꢂ92, ꢂ102, and ꢂ111
2
3
4
intercalation of silica or Zr-doped silica molecular sieve-like ppm resulted from Q , Q and Q silicon nuclei, respectively,
i
clusters between the magadiite layers.
where the Q corresponds to silicon nuclei with i siloxane link-
2
3
As for PMH and PMH-xZr (Fig. 2), a weak broad band in 3420– ages, i.e., Q to disilanol Si(OSi) (OR) , where R is H or Zr, Q to
450 cm can be assigned to n(OH) and a quite small sharp one silanol Si(OSi) (OR), and Q to Si(OSi) in the framework. The
pure PMH exhibits a prominent Q Si(OSi)
While a strong band at 1086 cm accompanied by a shoulder at ppm, a moderate Q Si(OSi)
2
2
ꢂ1
4
3
3
4
ꢂ1
27,30
4
at 3745 cm (inset in Fig. 2) to the terminal Si–OH groups.
4
signal near ꢂ111.1
ꢂ1
3
3
(OH) one near ꢂ102.0 ppm and a
ꢂ1
2
17,18,32
29
1
206 cm of PMH can be ascribed to n (Si–O–Si) and Si–O–Si weak Q Si(OSi) (OH) near ꢂ92.3 ppm.
The Si MAS NMR
as
2
2
stretching modes of ve-membered rings in calcined spectra of the PMH-xZr samples are quite similar to that of pure
25–27
samples,
respectively, implying considerable structure PMH. One may see that the samples modied with low Zr-
stability of magadiite sheets in PMH materials. Interestingly, the doping (x < 0.2) show a larger population of the Q and Q
sharp Si–OH signals at ꢁ3745 cm of PMH are variously coordination, thus a bigger value of (Q + Q )/Q , that strongly
reduced for the PMH-xZr samples with increasing Zr-doping, indicates more Si ions being replaced by Zr ions in interlayer
2
3
ꢂ1
3
2
4
3
2
4
implying the incorporation of Zr into the interlayer space of silica framework. While the value of (Q + Q )/Q of PMH-0.2Zr
PMH which may cause varied amount of Si/Zr–OH groups reduces to 0.33 (Table S2†), similar to the value of PMH, being
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33,35
the possible nitrogen lls in micropores.
Therefore, another
PSD calculation model, density functional theory (DFT) model
usually applied to characterize mesoporous materials contain-
36
ing micropores such as mesoporous carbon, is used to illus-
trate the PSD. The DFT distributions (Fig. S4c†) show obvious
existence of the multi-modal pore structure, involving some
supermicropores with optimal size of ꢁ0.84, 0.92, and 1.15 nm,
and small mesopores at ꢁ1.89–1.97 nm near to the gallery
height of samples upon XRD. The quite similar average pore
size implies the uniform Zr-incorporation into the interlayer
meso-silica framework except PMH-0.2Zr with a little larger one
possibly due to its delaminated layers.
The detailed textural data are listed in Table 1. The specic
surface areas (SBET) of PMH-xZr samples are greatly increased
2
ꢂ1
with Zr-doping. The SBET reaches to the maximum 603.3 m g
ꢂ1
with the relatively larger pore volume of 0.40 mL g for PMH-
0.05Zr, implying more terminal oxygen atoms from the
bridging ones, leading to structural stabilization of SiO tetra-
hedral in an interlayer 3D silica network. Then the SBET is
4
29
29
Fig. 3
Si MAS NMR spectra for PMH (a), PMH-0.0125Zr (b), PMH-
0
.05Zr (c), PMH-0.1Zr (d) and PMH-0.2Zr (e).
contrarily reduced with further increased Zr-doping probably
4
+
due to the increased incorporation of larger Zr ion (radius
4
+
0
.084 nm) than Si (0.026 nm) into the gallery SiO
leading to obviously enhanced interlayer packing density of
SiO –ZrO molecule sieve-like clusters. The great SBET drop for
2
framework
attributed to the higher Zr-doping leading to small amount of
extra-framework Zr oxides (also veried by later UV-vis DRS).
2
9
2
2
Therefore, it can be deduced upon the Si MAS-NMR and FT-IR
data that the zirconium atoms are incorporated into the silica
framework via Si–O–Zr linkages in the PMH-xZr samples. This is
the rst report on the fabrication of Zr-doped PMH via in situ 2D
intragallery cosurfactant-directing TEOS/Zr-nPro hydrolysis–
condensation–polymerization method.
PMH-0.2Zr, consistent with its lower amount of super-
micropores than others, suggests the presence of some amor-
phous ZrO -like species (veried by later UV-vis DRS) causes
2
delamination of the layered PMH material as XRD showed.
Clearly, PMH-0.05Zr and PMH-0.1Zr simultaneously possesses
relatively larger SBET and Zr-doping and well-kept layered
supermicro-mesoporous heterostructures.
The N adsorption–desorption isotherms (Fig. S4a†) of the
2
PMH-xZr samples clearly display a type IV isotherm with a H4
hysteresis loop, implying a certain mesoporosity related to the
33
3.2 Morphology
slit-shaped pores (IUPAC). Interestingly, a gradual increase in
sorption observed in the relative pressure (p/p ) of 0.05–0.3
suggests the presence of supermicropores and small meso-
N
2
0
Fig. 3 shows the SEM/EDX images of PMH-xZr samples. The
morphologies of magadiite and intermediates are shown in
Fig. S5 and S6.† Clearly, magadiite exhibits rosette morphology
of ꢁ15 mm (inset in Fig. S5†) constructed by contact packing of
1,5,33
pores.
The BJH pore size distributions (PSD) indicate that
the six distributions all mainly center at ꢁ1.89 nm (Fig. S4b†).
34
32
Considering the tensile strength effect, that is, the current
pore data ꢁ1.89 nm might be the combined effect of the
micropores and mesopores, BJH model is not completely suit-
able for the samples. It is noted that all the desorption inec-
tion points (Fig. S4a†) are very smooth and desorption
well-dened plates, similar to previously reported. The QM
shows loose discrete rectangle plates with inhomogeneous sizes
originated from broken rosette morphology of magadiite due to
+
the intercalation of CTMA . The HRTEM images further reveal
the (001) lattice fringes as 1.54 and 2.72 nm for magadiite and
QM, respectively, in line with the XRD data. Then, the QOTM
0
hysteresis loops at p/p < 0.25 are not closed, strongly indicating
Table 1 Textural data and related structure parameters of the catalysts
2
ꢂ1
a
2
ꢂ1
3
ꢂ1
a
3
ꢂ1
b
c
Samples
S
BET/m g
S
mico /m g
V
T
/cm g
V
micro /cm g
RBJH/nm
RDFT/nm
GH /nm
Zr/Si
PMH-0.2Zr
PMH-0.1Zr
PMH-0.05Zr
PMH-0.025Zr
PMH-0.0125Zr
PMH
242.8
401.6
603.3
557.7
533.6
442.8
179.1
340.8
458.8
487.5
457.8
353.3
0.241
0.288
0.400
0.388
0.395
0.354
0.093
0.174
0.227
0.246
0.231
0.177
1.92
1.89
1.89
1.89
1.89
1.91
1.97
0.92, 1.15, 1.89
0.84, 0.92, 1.15, 1.89
0.84, 0.92, 1.15, 1.89
0.84, 0.92, 1.16, 1.89
0.84, 0.92, 1.15, 1.90
—
0.130
0.056
0.038
0.021
0.013
—
2.571
2.705
2.492
2.460
2.260
a
b
Microporous area and volume obtained by t-plot method. Gallery height (GH) obtained by subtracting the thickness 1.12 nm of magadiite layer
c
from d001 upon XRD analysis. Based on EDX analysis.
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shows considerably swelling plate-like morphology, though the 0.05Zr and PMH-0.1Zr, respectively, similar to ꢁ1.18 nm of
14,32
plates are quite irregular and some show an edge-curved shape magadiite layer (Fig. S5†) and literature value 1.12 nm,
due to the hydrolysis–condensation–polymerization of TEOS in strongly implying the transformation of the precursors into
D gallery of the magadiite. The morphological similarity of PMH-0.05Zr and PMH-0.1Zr with no change of original layer
2
QOTM to QM implies that the intercalation of cosurfactants and structure of the starting magadiite. The (001) lattice fringes of
TEOS occurs in a topotactic fashion. The SEM of Zr-doped 3.25 and 3.12 nm for PMH-0.05Zr and PMH-0.1Zr, respectively,
precursors QOTM-xZr exhibit similar platy morphology to are in good agreement with the XRD data.
QOTM except QOTM-0.2Zr showing very thin broken layers due
to its delaminated layers upon its largest Zr-doping content.
3.3 Surface acidity
On calcinations, the PMH (Fig. 4a) shows ake like layers
with a smooth surface and regular packing probably due to the
decomposition of interlayer organic groups and proper layer
rearrangement during the calcinations. The EDX of PMH (inset
in Fig. 4a) indicates the existence of Si and O with Si/O ratio of
NH -TPD was used to study the acid strength distribution of the
3
samples and, furthermore, to obtain the quantitative amount of
acid sites in the specied temperature range. The pyridine FT-IR
was performed to determine the contribution of Brønsted (B)
and Lewis (L) acid sites.
4/8.1, higher than that of pristine magadiite (4/8.2), implying
It is generally accepted that the acid strength depends on the
the intercalation of ordered meso-silica into the interlayer space
of magadiite. However, the SEM images of PMH-xZr samples
ꢀ
NH
4
3
desorption temperature: weak (100–200 C), medium (200–
ꢀ
ꢀ
37
00 C) and strong (>400 C). The NH -TPD prole of PMH
3
(Fig. 4(b)–(f)) clearly describe an irregularly shaped aggregate of
(
Fig. 6) shows almost undetectable desorption peak, implying
rugged and irregular platy particles. Differently, PMH-0.2Zr
shows obviously isolated irregularly shaped small platy parti-
cles, in line with its possible delaminated layer structure as the
XRD shown. These morphological changes of the PMH-xZr
samples are more likely due to the incorporation of Zr as silica–
zirconia molecular sieve-like clusters into the interlayer space of
the magadiite. A typical EDX of PMH-0.1Zr (inset in Fig. 4e)
clearly indicates the incorporation of Zr in PMH.
Fig. 5 presents the TEM and HRTEM images of PMH-0.05Zr
and PMH-0.1Zr. The HRTEM images indicate that the gallery
space appears in light in contrast to the dark layers and the
mean thickness of the dark layer is 1.25 and 1.23 nm for PMH-
ꢂ1
its very small amount (0.071 mmol g ), similar to previously
1,12
reported PCHs from montmorillonite and saponite et al. The
removal of surfactant from the as-prepared PMH by calcination
leads to the formation of protons, which are necessary to
balance the clay layer charge, resulting in pure PMH intrinsi-
cally acid. However, all PMH-xZr samples (Fig. 6) exhibit clearly
enhanced acidity. The low Zr-doped samples PMH-xZr (x ¼
ꢀ
0
.0125 and 0.025) show only one maximum at ꢁ160 C with
clearly enhanced TCD signal, implying the formation of weaker
acid sites. With increased Zr-doping, PMH-0.05Zr, PMH-0.1Zr
and PMH-0.2Zr present appropriate two-maxima type of TPD-
ꢀ
ꢀ
prole, i.e. two maxima at ꢁ190 C and 360 C, implying the
formation of weak and medium-strong acid sites. These curves
3
are broad and the complete NH -desorption is not reached even
Fig. 4 SEM/EDX results of PMH (a) and PMH-xZr (b–f: x ¼ 0.0125, Fig. 5 (HR)TEM images of PMH-0.05Zr (a and b) and PMH-0.1Zr (c and
.025, 0.05, 0.1, 0.2). d) and their corresponding FFT diffractograms (insets).
0
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ordered packing of interlayer SiO
2
–ZrO
2
clusters in largely
extended gallery space as FT-IR, XRD and HRTEM showed, thus
the maximum concentration of acid sites. It should be noted
2
that chemisorbed NH (assuming 0.15 nm occupied per NH
3
3
12
molecule ) covers the fraction of the sample surface lower than
2% except delaminated PMH-0.2Zr assuming the formation of
2
monolayer (Table 2).
Pyridine FT-IR was used to further probe the nature of the
acidity of the samples (Fig. 7). The amount of both B and L sites
ꢂ1
+
was derived from the intensity of the band at 1540 cm (PyH
ꢂ1
pyridinium ions) and 1445 cm (pyridine bonded onto Lewis
sites, PyL) with extinction coefficient 0.73 and 1.11 cm
ꢂ1 39,42
mol
0
,
respectively. The PMH-xZr samples with x in 0.025–
Fig. 6 NH
3
-TPD profiles of PMH (a) and PMH-xZr samples (b–f: x ¼
.2 contains both Brønsted and Lewis sites, however the former
0
.0125, 0.025, 0.05, 0.1, 0.2).
are much weaker than the latter. Particularly, PMH-0.1Zr shows
the maximum amount and strength of acid sites of the PMH-xZr
3
catalysts, in accordance with its NH -TPD result.
ꢀ
at 500 C, suggesting a heterogeneous distribution of acid
strengths, though the weak-to-medium acid sites are majority
Thermodesorption of pyridine was followed by FT-IR to
estimate the acid strength of both Brønsted and Lewis sites. The
position of the pyridine IR bands is independent on the
strength of the site, and depends only on the type of bonding,
therefore the relative intensities of the bands of pyridine were
taken to estimate the relative acid strength of acid sites in the
examined samples. The FT-IR of PMH (Fig. S7†), aer pyridine
considering the ammonia desorption mainly occurred between
00 and 450 C. It is noted that the acidity is increased with the
ꢀ
1
amount of Zr incorporated to the framework, being the PMH-
.1Zr the most acidic among the PMH-xZr samples (Table 2),
and the lower acidity of PMH-0.2Zr with the most Zr-doping may
be due to the delaminated layer structure. Clearly, all PMH-xZr
samples exhibit a total acidity much higher than PMH, pure
mesoporous silica, Zr-doped mesoporous silica, a hexagonal
ordered mesoporous silica graed with 20 wt% ZrO (ref. 40)
and a ZrO -pillared montmorillonite. The presence of acid
sites is mainly due to the incorporation of Zr into the interlayer
mesoporous silica network where supercial Zr species with
low coordination exist. Generally, the high concentration of
acid sites exposed on the catalyst can be related to the high
surface area. However, in the present system, the highest acid
0
ꢀ
adsorption and outgassing at 100 C shows two weak absorp-
ꢂ1
tions at 1444 and 1592 cm , assigned to pyridine adsorbed on
38
39
3
9
weak Lewis sites or to weakly hydrogen-bonded pyridine,
2
ꢀ
which are nearly vanished aer outgassing at 200 C, indicating
the trace or no acid sites on pure PMH. However, for PMH-xZr
samples with x in 0.025–0.2 (Fig. 7), the IR spectra present not
41
2
4
+
ꢂ1
only greatly enhanced absorptions at 1445 and 1592 cm , but
ꢂ1
also a gradually strengthened band at ꢁ1538 cm
due to
pyridine bonded to B sites (pyridinium ions), and a band at
ꢂ1
1
489 cm to a combined contribution from both B and L
ꢂ1
amount is found for PMH-0.1Zr (0.968 mmol g ), though it
39,43
sites,
implying the formation of both B and L acid sites aer
2
ꢂ1
only shows relatively larger SBET (401 m g ). This phenom-
enon can be explained as that PMH-0.1Zr simultaneously holds
higher Zr-doping and relatively higher SBET closely related to its
Zr-doping. These bands can be clearly seen aer evacuation at
ꢀ
2
00 C. With further increased temperatures, the intensities of
3
Table 2 Acid concentration and acid distribution of the samples upon NH -TPD and pyridine-IR
a
b
Weak-medium/ Strong/
Total acidity/ Acidity density / Coveragesurf. by
B sites/
Desorp./ C mmol g
L sites/
mmol g
ꢂ1
ꢂ1
ꢂ1
2
ꢀ
ꢂ1
ꢂ1
Samples
PMH
PMH-0.0125Zr 0.480
PMH-0.025Zr
mmol g
mmol g
mmol g
NH
3
/nm
NH
3
/%
T
B/L ratio
0.071
—
—
—
0.071
0.480
0.552
0.097
0.542
0.596
1.4
8.1
9.0
200
200
200
—
—
0.054
—
0.065
—
—
—
—
0.19
—
0.18
—
0.223
0.284
0.081
0.358
0.155
0.127
0.373
0.205
0.161
0.351
0.180
0.552
300
PMH-0.05Zr
0.506
0.238
0.246
0.23
0.744
0.968
0.795
0.743
1.452
11.1
21.8
29.6
200
3
4
00
00
—
—
PMH-0.1Zr
0.722
0.565
200
0.071
0.056
—
0.070
0.040
0.19
0.27
—
0.20
0.22
3
4
00
00
PMH-0.2Zr
1.972
200
00
3
a
2
ꢂ1
2
ꢂ1
b
Acidity density (NH
3
molecules per nm ) ¼ [acidity (mol g ) ꢃ N
A
/SBET (nm g )]. Percentage of the surface covered by a monolayer of
2
12
3
chemisorbed ammonia. It was assumed that the surface occupied by one NH molecule is 0.15 nm .
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The former can be attributed to the ligand-to-metal charge
2
ꢂ
4+
transfer (LMCT) from an O (2p) to an isolated Zr (4d) ion in a
tetrahedral conguration,
46,48
while the later assigned to small
Zr O clusters in the extra-silica framework (probably seven-
x
y
2
coordinate Zr species like those of monoclinic ZrO ) specially
48
for PMH-0.2Zr. While PMH-xZr with low Zr-doping (x < 0.05)
only show a broad and quite weak band covering 200–350 nm
4
+
compatible with that of nearly isolated eight- or seven-fold Zr
ions in oxides,
48,49
and approximately detected band near
200 nm especially for PMH-0.05Zr due to the LMCT transitions.
These results clearly indicate that Zr species in PMH-xZr are
mainly incorporated into the interlayer silica–zirconia frame-
work and the higher Zr-doping leading to small amount of extra-
2
9
framework zirconium oxides, in line with the above Si MAS
NMR results.
As a powerful technique to distinguish the nature of Zr ions
presented in micro- and mesoporous Zr substituted molecular
19,50
sieves and similar materials,
XPS analysis was employed to
further explore the nature of the acid sites of the catalysts. Fig. 9
shows the Zr 3d XPS spectra of PMH-xZr samples, while the
corresponding Si 2p and O 1s XPS shown in Fig. S8.† With Zr-
Fig. 7 Concentration of surface acid sites determined for PMH-xZr
(
x ¼ 0.025, 0.05, 0.1, 0.2). Insets, IR spectra of the samples pre- doping, Si 2p and O 1s binding energy (BE) maintain constant
adsorbed with pyridine.
for low Zr-doped samples (x < 0.05) and slightly shi to lower BE
for high Zr-doped samples compared to pure PMH, and Zr 3d
exhibits similar changing trend, implying the slight change of
the characteristics of both B and L sites reduce, quite similar to electron density around Si, O and Zr cores with Zr-doping. In
29
the Zr-modied silica reported by Teo and Zeng.
detail, obviously asymmetric O 1s peak can be deconvoluted into
It is well-known that pure silica framework of MCM-41 and two components, implying varied environments for oxygen, i.e.
4
4,45
ZrO
2
are Lewis solids without B acid site,
and the fact that Si–O–Si (ꢁ533.04 eV) and Si–O–Zr (ꢁ530.76 eV) (Table S3†). The
the pure PMH nearly does not hold B and L sites; therefore, the intensity/area ratios of these two peaks closely match to the
B and L sites of the PMH-xZr samples are caused by the pres- surface Si/Zr ratio. In such Zr-substituted analogues of siliceous
4
+
ence of Zr atoms replaced the Si ones. Given that the Zr ions PMH materials, the question arises as to the true insertion of Zr
4
+
has much larger radius (0.084 nm) than Si (0.026 nm); when into the silica framework, as opposed to its presence as a
4
+
4+
the smaller Si ions are replaced by the larger Zr ions in the segregated oxide phase. Here, the BE of O 1s for PMH-0.1Zr at
framework, the bond length of Zr–O–Si clearly differs from that ꢁ530.76 eV (Table S3†) is at least 0.56 eV higher than that of pure
50
of Si–O–Si, and this phenomenon must leads to structural ZrO (530.2 eV), strongly evidencing the presence of framework
2
microstrain within the lattice cell. Changes in the electron
density around Si, due to charge unbalance, or differences in
electronegativity or local structure deformation resulting from
4
+
the introduction of Zr ion into the vicinity of the hydroxyls
carrying Si, may weaken the SiO–H bond; this is one of the
possible origins resulting in the B acid sites on the Zr-modied
PMH materials. Table 2 gives the contributions of B and L sites
of the PMH-xZr samples. The considerably created B sites and
greatly increased L sites of the PMH-xZr are highly desirable for
extended solid acid catalytic applications.
3.4 Nature of the acidity
UV-vis DRS is commonly used to detect the framework and
extra-framework Zr species in hybrid solids, particularly
dispersed oxides and metal ions in constrained environment
46
47
such as silica, zeolites and alumosilicate materials to obtain
information on coordination environment, oxidation state of
the embedded transition metal ions. As shown in Fig. 8, PMH-
0
.1Zr and PMH-0.2Zr exhibit a strong absorption at 207 nm and
Fig. 8 UV-vis DRS spectra of the PMH-xZr catalysts (a–e: x ¼ 0.0125,
a broad band at 275 nm with increased strength in the latter. 0.025, 0.05, 0.1, 0.2).
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Zr via Si–O–Zr linkage. Moreover, the Zr 3d5/2 peak shis to high Zr enrichment is observed for the sample with Zr/Si ratio 0.05
51
53
BE (183.47 eV) compared to ZrO2 (Zr 3d5/2 182.6 eV), also (9.3 wt% as ZrO2).
implying the formation of Si–O–Zr bond. Meantime, the higher Based on above analysis, it can be deduced that the proper Zr-
Zr 3d5/2 BE of PMH-0.1Zr indicates a higher ionic character of doping into interlayer silica framework renders the PMH-xZr
3
9,52
the bonding Zr–O
which benets for creating a stronger materials unique layered supermicro-mesoporous structure and
greatly enhanced B acid sites related to Si–O(H)–Zr linkage and L
acidity in the catalyst as pyridine-IR revealed.
Comparing with bulk composition Zr/Si ratio of the samples acid sites (Si–O–Zr), strongly implying the efficient acidic
from EDX, the surface Zr/Si ratios (Table S3,† XPS peak area tunability of transition metal-doped PMH materials on in situ
ratios normalized by the atomic sensitivity factors) upon XPS 2D-magadiite interlayer co-surfactant directing hydrolysis–
data suggests that Si-rich on the surface is observed for the low condensation–polymerization method for acid catalysis process.
Zr-doping samples as PMH-0.0125Zr and PMH-0.025Zr, while
Zr-rich on the surface for the highest Zr-doping sample PMH-
3.5 Catalytic activity
0.2Zr, and PMH-0.05Zr and PMH-0.1Zr shows similar bulk
and surface Zr/Si ratio, implying their best interlayer Zr-doping The Friedel–Cras acylation reaction is of great importance in
silica molecular sieve-like heterogeneous structure. Specially, ne chemical industry owing to create functionalized aromatic
the surface Zr/Si ratio of PMH-0.1Zr is 0.077, close to the theo- ketones such as 4-MBP (para-methoxybenzophenone) from the
retical bulk value 0.056, revealing no signicant elemental benzoylation of anisole with BzCl (Scheme 2) mainly used as a
enrichment on the surface and the homogeneous doping of Zr valuable perfumery intermediate and a precursor for antioxi-
into the silica matrix. This result is quite similar to an XPS dants oen used in cosmetics, unsaturated polyesters, PVC, and
54–56
observation for the ZrO –SiO system prepared using Zr-n- alkylated resins.
The catalytic activity of the catalysts was
propoxide and TEOS in n-propanol, but different from the tested in liquid phase benzoylation of anisole. Primarily, the
oxide gel system upon ZrOCl and TEOS in ethanol, in which a dependency of the conversion degree of the acylation on the
great enrichment of Si ions on the surface is found for the reaction time and temperature and reactants ratio were per-
2
2
29
2
sample with Zr/Si ratio 0.1 (17.0 wt% as ZrO
2
), whereas a surface formed over PMH-0.025Zr. It can be seen from Fig. S9† that the
benzoylation activity shows pronounced dependent on reaction
temperature and mole ratio of reactants. With constant anisole/
BzCl ratio of 40 : 1, the BzCl conversion is increased from 75.6%
ꢀ
ꢀ
at 110 C to 93.5% at 140 C aer 3 h. The 4-MBP yield reaches
ꢀ
ꢁ
93.5% at 140 C aer 5 h, and slightly increases aer 7 h of
reaction. The BzCl conversion increases substantially with
reaction temperature, suggesting that the reaction was intrinsi-
cally kinetically controlled. If one takes into account the higher
conversion and shorter reaction time, the reaction temperature
ꢀ
of 140 C and the reaction time of 3 h should be the better
option. Then, with a varied anisole/BzCl ratio (7 : 1), a signicant
decrease in both BzCl conversion and reaction rate is observed at
ꢀ
1
40 C, and the conversion of BzCl only reaches 42.4% aer 3 h,
much lower than those with anisole/BzCl ratio of 40 : 1. This can
be explained as that the lower anisole/BzCl ratio, i.e. the higher
concentration of BzCl make the active sites of the catalyst
become saturated with BzCl, leading to quantitatively low
formation of active electrophilic benzoylinium to react with
anisole existed in bulk phase, thus showing a decrease in
conversion of BzCl and rate of reaction. Therefore, the reaction
ꢀ
temperature of 140 C, reaction time of 3 h and anisole/BzCl
ratio of 40 : 1 is selected as the optimal reaction conditions.
A series of novel PMH-xZr catalysts was evaluated at optimal
reaction conditions and the detailed activity data are summa-
rized in Table 3 (entries 1–6). Beside the ketones, BA and PB
were also formed as side-products. The pure PMH shows quite
low BzCl conversion of 58.4% probably due to its quite low
intrinsic B acidity. Then for the Zr-doped sample PMH-
0.0125Zr, greatly enhanced BzCl conversion of 80.4% can be
attributed to its greatly increased total acidity especially B sites
resulting from the formation of Zr–OH and Zr–O–Si–OH link-
ages though pyridine IR fail to detect the B acid site due to steric
hindrance for larger pyridine molecules. Then for PMH-0.025Zr,
Fig. 9 Zr 3d XPS of the PMH-xZr catalysts (a–e: x ¼ 0.0125, 0.025,
.05, 0.1, 0.2).
0
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out that reaction exclusively proceeds at Brønsted acid sites,
57
and Sakthivel et al. reported that sulfation of ZrO predomi-
2
nantly generates B acidity responsible for catalytic activity in the
benzoylation reaction over SO /Zr Sn O . Given that pure
4
1ꢂx
x 2
2
ZrO has no activity in the present reaction system, the excellent
benzoylation activity of anisole with BzCl over the novel PMH-
xZr catalysts can be attributed predominantly to the Brønsted
acid sites generated upon Zr-doping to PMH, while the Lewis
acids show less inuence on conversion of the benzoylation
agents into the desired aromatic ketones under the applied
conditions.
The selectivity to 4-MBP of 92.2–94.6% over all the catalysts
with varied Zr-doping seems to be owing to the similar PSD and
optimal pore size. However, PMH-0.1Zr and PMH-0.05Zr with
high yield of 4-MBP show much higher/comparable ratio of para-
Scheme
benzoyl chloride (BzCl).
2 Friedel–Crafts benzoylation reaction of anisole with
to ortho-MBP (P/O) as 26 and 24 than in AlCl
3
system (16) and
further increased BzCl conversion of 93.5% can be attributed to
its further increased acidity especially detectable B sites. While
further increasing Zr-doping, PMH-0.05Zr and PMH-0.1Zr show
BzCl conversion of 98.5% and 99.5%, respectively. The
comparable BzCl conversion of the former to the latter can be
ascribed to the higher specic surface area thus more available
B acid site to reactants compensated to its lower acidity than the
latter. As for the contrarily reduced BzCl conversion of PMH-
2ꢂ
52
previously reported superacid SO
4
2
/ZrO (24), reecting the
less diffusion resistance of the 4-MBP suffered in these catalysts.
This is because the open gallery framework of PMH-xZr (x < 0.2)
samples with considerable amounts of layered supermicropores
near 0.84, 0.92 and 1.15 nm (Fig. S4†) affords to smaller diffu-
sion resistance for narrow-shaped 4-MBP molecules (0.51 ꢃ 1.21
nm estimated from ChemOffice Ultra 2004 in Fig. S10†) than for
58
near square-shaped 2-MBP (0.72 ꢃ 0.98 nm). Deutsch et al.
0.2Zr (89.1%), it can be attributed to its reduced available acid
have previously reported obvious pore size dependent perfor-
sites upon delaminated layer structures as XRD indicated. It is
also noted hat the PMH-0.05Zr and PMH-0.1Zr even present
BzCl conversion of 98.5% and 99.5%, quite near to conventional
mance of the sulfated-ZrO with larger mesopores (ꢁ6.8 nm) for
2
the acylation of anisole with benzoic anhydride compared to
commercial microporous H-BEA (0.76 nm) and H-mordenite
3
AlCl (99.9%), implying excellent distribution of acid sites on
(0.70 nm) upon steric hindrance effect.
these two catalysts, in line with their NH -TPD data. It is
3
In order to further compare the activity of the present PMH-
xZr catalysts with previous report, PMH-0.05Zr was also tested
in the benzoylation of anisole with BzCl, under the reaction
generally believed that benzoylation activity of anisole with BzCl
is dependent on both typical Lewis acids (e.g. AlCl ) and
3
Brønsted acids (e.g. H
heterogeneous system, Quaschning et al. reported benzoyla-
tion activity of anisole with BzCl over SO
2
SO
4
) in homogeneous phase. While in
ꢀ
conditions of anisole/BzCl ¼ 7 : 1, Wcat. 0.4 g, and 110 C,
52
55
similar to previous report, and shows extremely high activity of
2
ꢂ
4
/ZrO
2
and pointed
ꢂ1 ꢂ1
14.8 mmol g
h
with much faster kinetics probably due the
a
Table 3 Friedel–Crafts benzoylation reaction results of anisole with benzoyl chloride on the catalysts
Sel./%
b
ꢂ1 ꢂ1
No. Catalysts
Conv./% 4-MBP 2-MBP BA PB
P/O
Yield4-MBP/% Activity /mmol g
h
Refs
1
2
3
4
5
6
7
8
9
1
1
PMH
58.4
80.4
93.5
98.5
99.5
89.1
37.2
99.9
86
92.3
92.2
93.7
94.3
94.6
93.1
92.6
75.2
96
5.6
5.6
4.6
4.0
3.7
5.9
4.7
4.7
4
1.8
1.9
1.5
1.6
1.5
0.8
2.6
0.3 16.5 53.9
0.3 16.5 74.1
0.2 20.4 87.6
0.1 23.6 92.9
0.2 25.6 94.1
0.2 15.8 83.0
0.1 19.7 34.4
2.6
3.6
4.1
4.4
This work
This work
This work
This work
This work
This work
This work
This work
52
PMH-0.0125Zr
PMH-0.025Zr
PMH-0.05Zr
PMH-0.1Zr
PMH-0.2Zr
—
3.9
14.8
—
1.6
1.7
c
PMH-0.05Zr
AlCl
3
4.1 26.0 16
75.1
85.6
30
2
ꢂ
ꢂ
d
SO
SO
4
/ZrO
2
—
—
—
24
—
—
2
e
0
1
4
/Zr0.95Sn0.05
O
2
38
37/100
78.9
100
—
—
57
55
f
20% w/w Cs2.5
H
0.5PW12
O
40/K-10
37/100
7.0/—
a
ꢂ3
c
Reaction conditions: anisole 108 mmol (11.7 g), BzCl 2.65 mmol (0.373 g), catalysts loading (Wcat.) 0.2 g (0.016 g cm ), reaction temperature 140
ꢀ
b
ꢂ1 ꢂ1
C, reaction time 3 h. Activity (mmol g
h
) ¼ [BzCl (mmol) ꢃ conversion]/[mass of catalyst (g) ꢃ reaction time (h)]. Anisole 108 mmol (11.7 g),
ꢀ
d
BzCl 15.88 mmol (2.24 g), Wcat. 0.4 g, reaction temperature 110 C, reaction time 1 h. From ref. 52 reaction time 7 h, others same as this work.
e
ꢀ
f
From ref. 57 reaction temperature 60 C, others same as this work. From ref. 55, anisole 270 mmol (29.3 g), BzCl 39.7 mmol (5.59 g), Wcat.
ꢂ3
ꢀ
ꢀ
ꢀ
1
.05 g (0.03 g cm ), 37% Conv. obtained at 70 C and 2 h, 100% Conv. at 110 C and 4 h. Activity was estimated upon 37% BzCl Conv. at 70 C
and 2 h.
67862 | RSC Adv., 2015, 5, 67853–67865
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unique PMH structure (Table 3, entry 7). This value is much amount of benzoate species on the catalyst. Therefore, the
ꢀ
higher than those of previously reported superacid catalysts PMH-0.1Zr-reused5 was recalcined at 600 C for 1 h, and the
2
ꢂ
2ꢂ
57
SO4 /ZrO (ref. 52) and SO /Zr Sn0.05O , and similar to obtained PMH-0.1Zr-recalcined becomes white again and
2
4
0.95
2
ꢂ1 ꢂ1 59
WO /Zr-SBA-15 (11.9 mmol g
h ), though lower than het- displays almost restored initial activity. The XRD of PMH-0.1Zr-
3
eropoly acid-loaded montmorillonite 20% Cs2.5H0.5PW12O40/K- recalcined shows clear (001) line with similar d001 to initial
ꢂ1 ꢂ1 55
1
0 (28.4 mmol g
h ). These results strongly suggest the PMH-0.1Zr and the corresponding IR describes completely
ꢂ
ꢂ
s
remarkably enhanced benzoylation activity of the present PMH- disappeared absorptions due to nas(COO ) and n (COO ), indi-
xZr catalyst, which can be mainly ascribed to considerably cating the complete removal of the adsorbed carbonaceous
created Brønsted acidity along with the considerably high species. These results strongly demonstrate the high thermal
surface area.
stability and supermicro-mesoporous structure stability of the
Combined the characterization analyses, we can deduce that PMH-xZr catalysts upon the protection of the layers of the
the Brønsted acid sites affect the benzoylation activity of anisole staring magadiite thus easy recovery and high cycling efficiency,
using PMH-xZr as the heterogeneous catalysts, along with the fundamentally originated from the uniformly incorporated Zr
well-kept layered supermicro-mesoporosity. In view of these species in interlayer of PMH strongly reducing Zr leaching
results and discussion, the schematic illustration of the PMH- during the repeated tests.
xZr catalyst and the nature of the acid sites of the catalyst
affording to the benzoylation mechanism of anisole with BzCl
are tentatively proposed and presented in Fig. 10.
4. Conclusions
It is very interesting that novel group of the porous magadiite
heterostructures (PMH) intercalated with SiO –ZrO molecular
2
2
3.6 Recyclability
sieve-like clusters were gropingly fabricated via a 2D gallery in
situ hydrolysis–condensation–polymerization strategy of TEOS
and Zr-n-propoxide from synthetic Na-magadiite. The obtained
PMH-xZr catalysts are characterized with respect to their
textural parameters, chemical nature of Zr-incorporated into
the PMH structure as well as surface acidity. Both the pure
silicic PMH and PMH-xZr samples intercalated with silica and
silica–zirconia molecular sieve-like clusters, respectively, is
One advantage of heterogeneous catalysts over homogeneous
catalysts is their potential reusability. The optimal catalyst
PMH-0.1Zr was recovered and used for ve consecutive ben-
zoylation of anisole with BzCl (Fig. S11†). Aer each run, the
used catalyst was separated from reaction system by centrifu-
gation, washed thoroughly with absolute ethanol, dried at
ꢀ
1
20 C overnight, and subjected to next run. Some loss in
activity occurred during the reaction, and the conversion was
lowered to 93.2% aer ve cycles; however, the selectivity was
still as high as 93.2%. The loss of activity might be due to the
deposition of carbonaceous species (namely, benzoate species)
at the active site of the catalyst aer the reaction. Indeed, the
originally white catalyst became a little pink aer the reaction,
indicative of some adsorbate on it. The XRD and IR data of the
used catalyst PMH-0.1Zr-reused5 well support above analyses.
As shown in Fig. S12,† the XRD of PMH-0.1Zr-reused5 shows a
broad (001) line with slightly reduced intensity and the corre-
sponding IR spectrum exhibits very weak but distinguishable
ꢀ
found to be thermally stable up to 600 C. Photoemission peak
shapes and binding energy values suggest the formation of the
Zr–O(H)–Si bonds in the PMH-xZr catalysts. The in situ incor-
poration of Zr into the PMH signicantly increases surface
acidity of PMH by generation of both considerably more
Brønsted acid sites as well as obviously increased Lewis acid
sites upon Zr-doping. The liquid-phase Friedel–Cra acylation
study of the PMH-xZr catalysts for benzoylation of anisole with
benzoyl chloride shows that PMH-0.1Zr exhibits the highest 4-
MBP yield owing to its most Brønsted acid sites and optimal
layered supermicro-mesoporous structure. The simply recov-
ered sample exhibits some loss in activity owing to the reduced
acid sites of the catalyst due to the adsorption of carbonaceous
species, while the recalcined catalyst almost restored the initial
activity even aer ve cycles, implying a highly stable and effi-
cient acid catalytic performance of PMH-xZr heterostructures by
an in situ 2D gallery-template methodology. The present work
may open a new avenue for preparing sustainable, recyclable
and eco-friendly heterogeneous catalysts with high acidity and
large surface area associated with transition metal doped
layered supermicro-mesostructures for wide catalysis
applications.
ꢂ1
ꢂ
absorptions at 1605 and 1385 cm ascribing to nas(COO ) and
ꢂ
n
s
(COO ), respectively, implying the deposition of quite small
Acknowledgements
This work was supported by the 973 Program (No.
2
011CBA00508), National Natural Science Foundation of China
Fig. 10 Schematic structure of the PMH-xZr catalysts and the nature
of Brønsted and Lewis acid sites on PMH-xZr.
(No. 21276015), the Fundamental Research Funds for the
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