Research Article | Open Access
Highly Dispersed in Porous Silica for CO Hydrogenation
A facile method has been developed to prepare highly dispersed in porous silica. By utilizing C atoms in methyl modified silica supports, was obtained via insitu carburization. The obtained samples exhibited high activity for CO hydrogenation.
The development of new heterogeneous catalysts is a highly attractive topic in current chemical research [1, 2]. Molybdenum carbides have been found to show high catalytic activities as noble metals and may be an inexpensive alternative to noble metal catalysts [3, 4]. In recent years, molybdenum carbides highly dispersed in mesoporous silica molecular sieves have been synthesized  however, to insert molybdenum species during the synthesis of mesoporous silica supports, the routes are somewhat complicated and difficult. Molybdenum source, MoO3 or polymolybdates, needs to be treated into low-nuclearity molybdenum peroxo complexes via the peroxo route at the initial synthesis of mesoporous silica supports . Furthermore, to carburize molybdenum species into its carbides, one of the most widely used methods is the temperature-programmed reduction (TPR) of the transition metal oxide by gaseous hydrocarbons, such as CH4 , C2H6 , C3H8 , and C4H10 , as pioneered by Levy and Boudart . Few studies have been carried out on the utilization of other carbon sources.
Herein, we report a facile synthesis of highly dispersed in porous silica through insitu carburization of molybdenum species on methyl modified silica (Mo-M-silica) in Ar gas. Mo-M-silica was synthesized via sol-gel route using Si(OC2H5)4 (TEOS) and polymethylhydrosiloxane (PMHS) as silica sources without using any surfactants. (NH4)6Mo7O244H2O (AHM) as molybdenum source was introduced without peroxo treatment. CO hydrogenation reaction was used as a probe reaction: the obtained /SiO2 exhibited high catalytic activity and selectivity to alcohols in CO hydrogenation.
2.1. Synthesis of Samples
The typical synthesis method is as follows. (NH4)6Mo7O244H2O was dispersed into 80 mL ethanol to obtain a white suspension. Then, 1.5 mL PMHS was added under vigorous stirring. Si–H bonds in PMHS interacted with transitionmetal (Mo), and the suspension turned from white to gray. Ethylene diamine (EDA) was added as catalyst to allow residual Si–H bonds to react with part of the C2H5OH and release H2. After being stirred for 12 h at room temperature, 5 mL TEOS and determined deionized water were added under vigorous stirring. The resulting gray solid was dried at 100°C in a vacuum oven to remove the EtOH and EDA. The carburization of Mo-M-silica was carried out under atmospheric pressure in Ar gas at 700°C. Then, the sample was quenched to room temperature and gradually passivated with 1% (v/v) O2/N2 before exposure to air. Similarly, another sample was carburized in 10% (v/v) CH4/H2 gas for comparison.
2.2. Characterization Methods
X-ray patterns were measured using Ni filtered Cu K radiation () and a Rigaku D Max III VC diffractometer equipped with a rotating anode operated at 40 kV and 30 mA. Changes of Mo-M-SiO2 during carburization were monitored by thermogravimetric analysis (TG, Rheometric Scientific STA 1500 equipped with a simultaneous thermal analyzer). A small quantity (approximately 30 mg) of Mo-M-SiO2 was loaded into a small platinum crucible and placed into the balance in the instrument. N2 adsorption isotherms were obtained at 77.3 K on a Tristar 3000 sorptometer, using static adsorption procedures. Samples were degassed at 423 K and under a vacuum of 10~6 Torr for at least 12 h prior to the measurement. BET specific surface areas were calculated using the adsorption data in a relative pressure (P/P°) range from 0.05 to 0.20. The XPS was recorded on a VG MultiLab 2000 spectrometer with Mg K (1253.6 eV) radiation as the excitation source. The experimental setup consisted of a preparation chamber connected to a UHV system. The sample holder with the tablets was placed onto a vertical tube that could be pushed up into a quartz reactor. The bores in the sample holder stub ensured that the gases flew through the sample and exited from the setup through a valve in the lower end of the vertical tube. From time to time, the carburization process was interrupted at given temperatures, the reactor was evacuated, and the sample was transferred into the UHV system via an insertion mechanism. After spectrum acquisition, the samples were drawn back to the chamber, which was kept at the given temperature in the meantime. The chamber was flooded with the gas mixture, the flow was set, and the sample was pushed into the reactor cell. Afterwards, heating to the following temperature began and the treatment cycle was repeated until reaching the final temperature. TEM images were taken on a JEOL 100CX microscope operating at an accelerating voltage of 200 kV. Chemical analyses were carried out at the Service Central d’ Analyse (CNRS-Lyon) by inductive coupling plasma atomic emission spectroscopy (ICP-AES) after alkaline fusion with Li2B4O7.
2.3. CO Hydrogenation
CO hydrogenation was carried out using a stainless fixed-bed reactor with 4 mL of catalyst. The effluents were analyzed by 1790-GC and among these H2, CO, CH4, and CO2 were analyzed by thermal conductivity detector (TCD) equipped with a TDX-101 column; C1–C4 alkanes were detected by flame ionization detector (FID) with a chromosorb 101 column; C1–C4 alkenes were detected by FID with a packed column C-18; the water and methanol in liquids were also detected by TCD with a GDX-401 column; the alcohols and hydrocarbons were analyzed by FID with a porapak-Q column. hydrocarbons were detected by FID with a capillary column. The mass balance was based on carbon, and the error of the balance of oxygen and hydrogen was within 5%. The selectivity and products distribution (alcohols and hydrocarbons) were calculated on a CO2 free basis.
3. Results and Discussion
3.1. Structural Properties
Figure 1 shows the powder XRD patterns of the three samples. The XRD pattern of Mo-M-silica shows none-Mo-related XRD diffraction peaks, but a very wide diffraction peak of amorphous SiO2 at 24-25°. It can be concluded that molybdenum species are not crystallized and highly dispersed in SiO2. The two carburized samples, respectively, in Ar gas and in CH4/H2 gas, show similar XRD patterns, which are in agreement with the reported by Bouchy et al. . The intensity of (111) peak is higher than that of the (200) one. So, it can be concluded that the carburization of molybdenum species can be realized by only utilizing C atoms in silica supports (sample B).
Table 1 shows the surface area and pore volume of the three samples. It can be seen that, BET surface area increases after carburization, respectively, from 77 to 339 m2 g−1 in Ar gas and to 311 m2 g−1 in CH4/H2 gas. The large increase of micropore volume after carburization perhaps resulted from the exhaustion of methyl in silica during carburization. Table 1 also shows the content of Mo and C in the three samples. It can beseen that the content of C in uncarburized Mo-SiO2 reaches 7.23 wt%, and the /SiO2 carburized in Ar gas decreased to 0.62 wt%, while the /SiO2 carburized in CH4/H2 gas only decreased to 0.78 wt%, which is little higher than that of /SiO2 carburized in Ar gas. This may be the cause of amorphous carbon formed by the CH4 pyrolytic reaction , which is accumulated on molybdenum carbides.
Figure 2 shows the representative HRTEM images of /SiO2 obtained in Ar gas, taken at different magnification. Clearly, Figure 2(a) shows a 3D wormhole-like structure with a disorganized porous network. At higher magnification, it can be seen from Figure 2(b) that nanoparticles with diameter of ~3 nm are highly dispersed in silica supports. The d-value of 0.21 nm matches the value determined from the XRD analysis, which is in agreement with other reports .
3.2. XPS Study of the Carburization Process of Mo-M-SiO2
XPS were used to investigate the change of molybdenum species during carburization. The corresponding results are shown in Figure 3. The 3d5/2 and 3d3/2 binding energies are 232.4 and 235.5 eV for Mo6+, 230.9 and 234.0 eV for Mo5+, 229.1 and 232.2 eV for Mo4+, 228.4 and 231.6 eV for Mo2+, and 227.6 and 230.8 eV for Mo0. The values for Mo6+-Mo4+ ions are almost the same as those used by Ward et al.  for Mo/SiO2. The values for Mo2+ and Mo0 are taken from the work of Yamada et al. . C 1s binding energy is located at 284.6 eV and used as an energy reference (Figure 3(d)). It can be seen that before carburization Mo6+ species exist in a simple chemical characteristic in SiO2 from the high resolution of Mo 3d doublet in XP spectrum for the unheated, according to the explanation by Liu et al. . Heating this sample in Ar gas causes practically no alteration in the spectral shape with respect to the starting state up to 400~600 K, indicating no appreciable reduction of Mo6+. Dramatic changes take place after heating above this temperature range, when a three-peak structure appears on the spectrum with peak maxima at 228.6, 232.6 and 236.2 eV at 623 K. By further heating to 823 K, the three peaks all shift to lower energy, indicating a further reduction of molybdenum species. The deconvolution of Mo 3d for the resultant at 623 and 823 K is, respectively, presented in Figures 3(b) and 3(c). It clearly shows that the carburization of molybdenum species proceeds through several states of Mo. At 623 K a series of reduced Mo ions comes forth expert Mo0, which is reduced with temperature further getting high to 823 K. At 973 K, the Mo three peaks are converted into two peaks with maxima at 227.8 and 231.2 eV. The two-peak position has not changed but the lower-energy peak becomes stronger at higher temperature, indicating the further reduction of Mo and the formation of carbides. At 973 K, the C 1s peak shifts to 283.1 eV, corresponding to the energy of the carbon 1s orbital measured earlier in containing samples . Finally, it can be concluded that molybdenum species are almost carburized at heating temperature above 973 K. Meanwhile, a minor C 1s peak at 284.9 eV indicates the existence of a little coke.
Changes of Si 2p and O 1s peaks during the carburization are similar. Both peaks shift to higher energy upon heating to 823 K and have no more shifts but become stronger at higher heating temperature. The shift of Si 2p peaks below 823 K reflects the gradual pyrogenation of Si-C bonds. And the change of O 1s peaks below 823 K is mainly caused by the gradual reduction of Mo species. Above 823 K, Si 2p and O 1s peaks are fixed, respectively, at 103.3 and 532.6 eV and reflected to complete pyrogenation of Si-C bonds leaving unhybrid SiO2 .
3.3. Changes in the Mo-M-SiO2 during Carburization
Figure 4 shows TG-DTA-DTG studies during the course of the carburization of Mo-M-SiO2 to /SiO2. The weight-loss stages of Mo-M-SiO2 in this carburization reaction are caused by two aspects: changes of molybdenum species and pyrogenation of methyl groups in SiO2. The main five weight-loss stages are observed. The first weight-loss stage occurs below 388 K, and the weight loss of 1.88% is the physical absorbed water leaving. From 388 to 504 K, a weight loss of 3.01% is observed, corresponding to chemical absorbed water and en leaving. The weight loss from 504 to 694 K is about 4.61%, which can be attributed to the formation of MoO2. At 921 K, a further weight loss of 4.32% is found, suggesting the formation of an immediate phase, molybdenum oxycarbide [, ], between MoO2 and the final carbides. The final weight-loss stage occurs above 921 K, in which molybdenum oxycarbide is converted to the molybdenum carbide .
3.4. Catalytic Properties
The results of CO hydrogenation over the three samples, one uncarburized and two carburized, are listed in Table 2. It can be seen that the uncarburized sample shows the lowest activity and alcohol selectivity among the three samples. It is notable that both the catalytic activity and selectivity to alcohols remarkably increase in /SiO2 than those of uncarburized Mo-SiO2 sample, however, the Mo-SiO2 carburized in Ar gas showed highest activity and alcohol selectivity. Thus, it can be concluded that the higher the surface area and pore volume, the higher activity and alcohol selectivity. the amorphous carbon formed by the CH4 pyrolytic reaction may reduce the activity and alcohol selectivity.
|Free of CO2.|
3.5. Formation Mechanism
We propose a formation mechanism as shown in Figure 5 to explain the /SiO2 formation. Si–H bonds are easy to be activated through interaction with transition-metal (tM), which makes activated H atoms transferred from Si to tM. In EtOH, Si–OC2H5 bonds are formed . In our experiment, AHM is hard to dissolve in EtOH, just forming a white suspension, but still a little dissolved AHM exits. The dissolved Mo complexes interact with Si–H bonds in PMHS, which make PMHS alkyloxidized and Mo–H bonds form. Mo–H bonds react with EtOH to form Mo–OC2H5 bonds and release H2, and further transform to Mo–O–Si bonds through hydrocondensation with alkyloxidized PMHS, which make Mo species connected with C long chains of PMHS (Mo–PMHS). With dissolved AHM reacting with PMHS, more AHM dissolve into EtOH and further react with PMHS. EDA as catalyst is added to activate residual Si–H bonds to form Si–OC2H5 bonds and release H2. Finally, porous Mo–M–silica is obtained from hydrocondensation of alkyl oxidized Mo-PMHS and TEOS. By utilizing C atoms of methyl in SiO2 supports as carbon sources, /SiO2 can be obtained just by heating in Ar gas.
In summary, we facilely fabricated highly dispersed with an even diameter of 3 nm in micro/mesoporous silica by insitu carburization. The porous methyl modified silica supports were obtained using mixed TEOS and PMHS without additional introduction of any surfactants. AHM as molybdenum source was introduced into silica supports, and obtained high dispersion without peroxo treatment. Furthermore, the sample exhibited obvious high catalytic activity and alcohol selectivity in CO hydrogenation reaction.
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Copyright © 2013 Minglin Xiang and Juan Zou. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.