Supporting Information
Controllable fabrication and catalytic activity of highly
b-oriented HZSM-5 coatings
Meiling Ji, Guozhu Liu,* Li Wang, and Xiangwen Zhang
Key Laboratory for Green Chemical Technology of Ministry of Education, School of
Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
*
Email: gliu@tju.edu.cn
1
Experimental
1.1 Catalysts preparation.
TiO2 sol (20 wt% in ethanol) was supplied by Hang Zhou Wan Jing New Material
Co. Ltd. Tetraethylorthosilicate (TEOS), aluminium nitrate (Al(NO3)3•9H2O),
Tetra-n-butyl titanate (TBOT), methanol and toluene were purchased from KRS Fine
Chemical Co. Ltd, Tetrapropylammonium (TPAOH) from Alfa Aesar, and n-dodecane
from Shuangshu Fine Chemical Plant. Chemicals were used as received. 304
Non-porous stainless steel tubes (SST) (2 mm in inside diameter and 30 cm in length)
were used as supports. The SST was pretreated by acetone solvent to remove
adsorbed impurities.
Sample HZ-M: HZ-M is the sample of b-oriented HZSM-5 monolayer on inner
wall of stainless steel tube, which is prepared by hydrothermal method. A TiO2 layer
was deposited on the inner surface of SST with TiO2 sols by wash-coating method to
provide a smooth surface for the formation of HZ-M. Resulting TiO2/SST was filled
with synthesis solution (1TEOS: 0.01Al (NO3)3:0.32TPAOH: 165H2O), sealed and
vertically placed in an oven at 165 ºC for 24 h. After that, the residual sol was dropped
and b-oriented HZSM-5 monolayer obtained was thus dried at room temperature
overnight, followed by calcination at 450 °C for 2 h with a heating and cooling rate of
0.5 °C·min-1 to remove the template.
Sample HZ-B-D: Direct secondary growth on the as-prepared HZ-M gave a new
top layer of a-oriented crystals, this sample was denoted as HZ-B-D where B refers to
bilayer and D refers to direct hydrothermal growth.
Sample HZ-B-C: HZ-B-C refers to the sample of HZSM-5 bilayer which prepared
with a coating of TiO2 interlayer. For the preparation of these samples, the as-prepared
HZ-M was coated again with a thin TiO2 layer by wash-coating method, and then
followed
by
a
secondary
crystallization
procedure
using
the
same
0.32TPAOH:0.01Al(NO3)3:1TEOS:165H2O solution. To optimize the orientation of
the coating, several TiO2 sols diluted with different amount of ethanol were used. The
TiO2 sol/ethanol mass ratios (T/E) were varied from 1:2 to 1:16. For the sample
HZ-B-C1, T/E= 1/2 was used, and for HZ-B-C2 and HZ-B-C3, T/E = 1/8 and 1/16
was used respectively.
Sample HZ-B-M: HZ-B-M refers to the samples of HZSM-5 bilayer which
prepared with a modification method. For these samples, the HZ-M was modified
with a surface sol-gel process using titanium alkoxide solution to tether the Ti-OH on
the surface. A schematic diagram of the modification process is shown in Scheme 1.
The modification solution, with molar ratio of 1.0 TBOT: 47 toluene: 124 methanol,
was prepared by adding TBOT to an anhydrous mixture of methanol and toluene.
Subsequently, the HZ-M was then filled with the prepared metal alkoxide solution of
titanium, and sealed for 10 min. After dropping the residual solution out, the coating
was then washed with anhydrous ethanol, hydrolyzed with deionized water, and dried
with N2 flow. The above sequential process can be repeated for several cycles to
adjust the coverage of the Ti-OH groups on the surface of HZ-M. After the
modification, repeated synthesis was then carried out following the same
crystallization procedure described above. The samples of HZ-B-M1, HZ-B-M2 and
HZ-B-M3 were prepared finally by carrying out 5 cycles, 3 cycles and 1cycles of
modification.
1.2 Characterization
The surface coverage and morphology of zeolite coatings were observed through a
scanning electron microscopy (SEM, FEI Nano-SEM 430 field emission gun scanning
electron microscope). The microscope was also equipped with an energy-dispersive
X-ray spectroscopy (EDS) detector for elemental analysis, which was used to estimate
the Si/Al ratio of coatings. The synthesized coatings were also examined by X-ray
diffraction (XRD, D/MAX-2500) for phase identification and crystal orientation,
using the Cu Kα radiation, over a 2θ range of 5–50° with at a scanning speed of
5ºmin-1. X-ray photoelectron spectroscopy (XPS) was used to characterize the
chemical changes of zeolite coatings after modification, using a PHI5000 Versa Probe
electron spectrometer (Mg-Ka, 15 kV, 18 mA). For all samples, the loading of the
coatings include only the mass loading of zeolite (not the mass loading of TiO2 layer).
1.3 Catalytic cracking of n-dodecane.
In the last few years, there has been a growing interest in the catalytic cracking of
hydrocarbon fuels at high temperature and elevated pressure due to the potential for
enhancing engine performance over the entire spectrum of flight regimes1,2. For future
hypersonic aircraft, hydrocarbon fuels can serve as not only source of propulsion
power through combustion, but also coolant through the cracking reaction to remove
the waste heat from aircrafts3,4. Therefore, catalytic cracking of supercritical
n-dodecane (550 ºC, 4 MPa) was selected as a model reaction to evaluate catalytic
performances of the zeolite coatings5,6. Supercritical condition was selected to
enhance the heat transfer, whereas the diffusion of reactants within the zeolite pores
became a rate-limited step in that the diffusivity of hydrocarbons was significantly
reduced (i.e., liquid-like behavior)7.
The catalytic cracking of n-dodecane was carried out in a flowing reactor using the
prepared tubular zeolite coating as catalysts. The experimental apparatus used in this
work was described in our previous work1. n-Dodecane was fed to SST with a HPLC
pump at a flow rate of 10 mL·min-1. The reaction pressure was kept at 4 MPa by a
backpressure valve. The tube reactor was heated by direct current power. The reaction
temperature was maintained at 550 ºC and measured by K-type thermocouples.
The liquid products were analyzed by HP7890 gas chromatography using a flame
ionization detector (FID) and a PONA column (50 m×0.20 mm). Cracking conversion
was defined as the ratio of reacted n-dodecane to the entering amount of n-dodecane,
i.e.
, where w0 and w represent the mass of the fed n-dodecane and
the mass of the n-dodecane after cracking.
2
The calculation of Fb for the as-prepared coatings.
The fraction of b-oriented crystals (Fb) was defined by the ratio of the (h00) peak
and (0k0) peak and being normalized according to the peak intensities in a randomly
oriented sample (Fig. S1). The calculated formula was given as follows,
,
where I' is the intensity of peaks according to the XRD pattern of the test samples,
and I is that from a randomly oriented sample (Fig. S1). Fb describes the fraction of
crystals oriented with b-perpendicular to the substrate surface. The (1000) and (0100)
peaks are thus chosen as the suitable peaks in this present work since they are well
resolved from each other.
Fig. S1 XRD pattern of a randomly oriented HZSM-5 coating.
3
SEM and XRD characterization of HZ-B-C1, HZ-B-C3, HZ-B-M2 and
HZ-B-M3.
3.1 HZ-B-C1 and HZ-B-C3.
For HZ-B-C1, a thick TiO2 interlayer was introduced. Fig. S2a shows the smooth
TiO2 surfaces on the HZ-M, together with 580 nm in thicknesses as measured by SEM.
After a secondary hydrothermal growth, the newly formed HZSM-5 layer is still
b-oriented (Fig. S2a'). Only peaks corresponding to the (0k0) reflections were
detected in the XRD pattern (Fig. S2c), indicating the high b orientation. And the
calculated Fb is as high as 89.7%.
For HZ-B-C3, a thin TiO2 interlayer was introduced. Fig. S2b shows the surface of
HZ-M covered with a very thin TiO2 interlayer that can’t fully cover the under-layer
of HZ-M. Fig. S2b' indicates the much a-oriented crystals synthesized on the thin
TiO2 interlayer. The visible (h00) reflections on XRD pattern also reveal the large
amount of a-oriented crystals, which resulting a low Fb of 48.4%.
Fig. S2 (a) and (a') shows the SEM images of TiO2 and the corresponding HZSM-5
bi-layers for HZ-B-C1; (b) and (b') for HZ-B-C3; (e) shows their XRD patterns.
3.2 HZ-B-M2 and HZ-B-M3.
When the coverage of Ti-OH groups reduced to 67.85%, the zeolite bilayer appears
some a-oriented crystals, as shown in Fig. S3a. Further reducing the coverage of
Ti-OH groups to 43.31%, much more a-oriented crystals appeared on the HZ-B-M3
(Fig. S3b). Both (0k0) and (h00) XRD reflection can be observed on the HZ-B-M3 as
shown in Fig. S3c, revealing the newly formed twin a-oriented crystals.
Fig. S3 SEM images of (a) HZ-B-M2 and (b) HZ-B-M3; (c) shows their XRD
patterns.
4
Impacts of Ti-OH groups on catalytic performance.
In comparison, we carried out the experiment of HZ-M sample modified only with
titanium alkoxide treatment (TBOT/HZ-M) to check the possible effect of the
titanium alkoxide modification in catalytic activity, and found that TBOT/HZ-M gives
a conversion of 19.2% ± 0.3%, which is almost completed same as the original HZ-M
sample (conversion = 19.8% ± 1.0%), suggesting that the titanium alkoxide
modification has very limited effect in the catalytic activity of coating.
5
XPS results of HZ-B-M1, HZ-B-M2, HZ-B-M3.
Fig. S4 High resolution XPS spectra of O 1s region for HZ-B-M1, HZ-B-M2,
HZ-B-M3.
Table S1 Curve fitting result of high resolution XPS spectra for O 1s region.
Sample No.
O1s (Si-O)
O1s (Ti-OH)
O1s (Ti-O)
Eb (ev)
532.2
531.0
529.4
ri (%)
12.31
71.85
15.83
Eb (ev)
532.3
531.5
529.7
ri (%)
13.33
67.85
18.82
Eb (ev)
532.7
531.3
529.6
ri (%)
45.01
43.31
11.61
ZH-B-M1
ZH-B-M2
ZH-B-M3
a
ri represents the ratio Ai/ΣAi, where Ai is the area of each peak.
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