Zhihong Fanabc,
Heqin Guoa,
Kegong Fang*a and
Yuhan Sun*d
aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Shanxi, Taiyuan 030001, P. R. China. E-mail: kgfang@sxicc.ac.cn; Tel: +86 351 4040431
bUniversity of Chinese Academy of Sciences, Beijing, 100049, P. R. China
cShanxi Agricultural University, Shanxi, Taigu, 030801, P. R. China
dLow Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201203, P. R. China. E-mail: yhsun@sxicc.ac.cn; yhsun@sari.ac.cn
First published on 3rd March 2015
A series of V2O5/TiO2 composite catalysts (V2O5–TiO2–Al2O3, V2O5–TiO2–SiO2, V2O5–TiO2–Ce2O3 and V2O5–TiO2–ZrO2) were prepared by an improved rapid sol–gel method and the catalytic behavior for dimethoxymethane (DMM) synthesized from methanol selective oxidation was investigated. The physicochemical properties of catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller isotherms (BET), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), NH3 temperature programmed desorption (NH3-TPD), infrared spectroscopy of adsorbed pyridine (Py-IR) and transmission electron microscopy (TEM) techniques. The best catalytic performance was obtained on a V2O5–TiO2–SiO2 catalyst with methanol conversion of 51% and DMM selectivity of 99% at 413 K. Furthermore, the V2O5–TiO2–SiO2 catalyst displayed an excellent catalytic stability within 240 h. Results showed that more Brønsted acidic sites were critical to increasing the DMM yield. The activity of V2O5/TiO2 composite catalysts decreased with increasing Brønsted acidity, but the yield of DMM increased with an increasing amount of Brønsted acidic sites. The excellent performance of the V2O5–TiO2–SiO2 catalyst might come from its optimal acidity and redox properties, higher active surface oxygen species, together with more Brønsted acid sites.
In recent years numerous efforts have been devoted to the selective oxidation of methanol to obtain DMM.8–10 In these reports, two types of active sites in catalysts, including both redox sites and acidic sites, were required for DMM synthesis.11 The redox sites were considered to be involved in the initial formation of formaldehyde (FA) from CH3OH with active lattice oxygen atoms, while acidic sites can catalyze acetalization reactions of FA and CH3OH to DMM. However, methyl formate (MF) could also be produced on redox sites and acidic sites favored another side product of dimethyl ether (DME). Therefore, both the redox property and acidity of catalyst are needed and should match each other in order to obtain DMM with high selectivity.
It was reported that V2O5/TiO2-based (VT-based) catalysts exhibited excellent activity for the selective oxidation of methanol to DMM in mild reaction conditions.12 However, the physiochemical properties and catalytic performance of VT-based catalysts were influenced greatly by several factors, especially the preparation method and support additives.13–16 Traditionally, the VT-based catalysts can be prepared by incipient wetness impregnation,17–20 co-precipitation21 and rapid combustion method.22 In these methods, an additional heat treatment was typically required to obtain the desired phase composition, which usually led to a significant aggregation of catalyst particles. The aggregated catalyst particles might weaken the interaction between vanadium and supports, leading to low reducibility,23,24 which was unfavorable for the first step of DMM synthesis.25,26 Moreover, this aggregation might also decrease the number of acidic sites,25 harmful for the second step for DMM synthesis.27 Generally, the addition of some elements to the support may decrease the aggregation of catalyst particles in some extent. For example, the addition of Al, Si, Ce and Zr to V2O5/TiO2 catalyst may affect both redox and acidic properties and dispersion of the active phase.21,28,29 However, these catalysts were prepared mainly by co-precipitation or incipient wetness impregnation, which may cause the particles aggregation or the metal nonuniform distribution. These may cause the selectivity to DMM usually decreased sharply with the increase of methanol conversion over traditional V2O5/TiO2 catalysts, leading to a low DMM yield.30 Thus, it is necessary to develop new catalyst with improved preparing method for the selective oxidation of methanol to obtain DMM efficiently from the point of scientific significance and economic view.
In this article, we designed an improved rapid sol–gel method, which overcame the demanding and not easy to control of traditional sol–gel method, avoided the deficiency of the above methods and evenly combined the metals on molecular level, to prepare the composite supported V2O5/TiO2 catalysts (V2O5–TiO2–Al2O3, V2O5–TiO2–SiO2, V2O5–TiO2–Ce2O3 and V2O5–TiO2–ZrO2) and get bifunctional catalysts with high DMM selectivity and high methanol conversion simultaneously and excellent life span in methanol selective oxidation. The changed dispersity of vanadium, redox and acid properties of the catalysts prepared by improved sol–gel method with stable three dimensional structures would affect the catalytic property apparently. The physiochemical properties of the catalysts were characterized exhaustively together with the catalytic performance investigation.
For comparison, the V2O5/TiO2 (VTi) catalyst, containing the same vanadium content denoted as VTi, was prepared by the same method.
Measurements of the BET surface area and pore volume of catalysts were performed in a Micromeritics ASAP-2000 instrument by N2 adsorption–desorption. Before analysis, the samples were degassed at 473 K overnight.
The elemental analysis was carried out using ICP optical emission spectroscopy (ICP-OES) with an ACTIVA spectrometer from Horiba JOBIN YVON.
X-ray photoelectron spectroscopy spectra (XPS) were recorded on a XSAM-800 spectrometer using an Al Kα (1486.7 eV) X-ray source.
Infrared spectroscopy of adsorbed pyridine (Py-IR) was applied to determine the kinds of surface acid and the measurements were performed in a Nicolet Magna 550 spectrophotometer. Wafers of 15 mg cm−2 were degassed overnight under vacuum (10−3 Pa) at 673 K for 1 h and then saturated by pyridine. Samples were evacuated to eliminate the physically adsorbed pyridine for 30 min. The amount of Brønsted and Lewis acid sites was calculated from the intensities of the IR bands at ca. 1540 cm−1 and 1450 cm−1, respectively.
Transmission electron microscopy (TEM) images were recorded using a JEOL-2011 microscope operated at 200 kV.
NH3 temperature programmed desorption (NH3-TPD) was performed in a quartz micro-reactor. 200 mg of each sample was heated in Ar at 773 K for 2 h, then NH3 was introduced to the sample after the sample was cooled down to 353 K under Ar flow. After the sample was swept using Ar at 353 K for 1 h to remove the weakly adsorbed NH3, the TPD experiments were carried out with a carrier gas at a flow of 40 mL min−1 Ar and the TPD spectra were recorded using a linear heating rate of 10 K min−1 from 353 K to 900 K by a Shanghai GC-920 equipped with a thermal conductivity detector (TCD). The desorbed NH3 was titrated by 0.01 mol L−1 HCl. The amount of desorbed NH3 was corresponding to the total number of acidic sites. The acidity intensity could be determined according to the desorption temperature of NH3, and the number of acidic sites can be calculated based on the areas of desorption peaks.
Hydrogen temperature-programmed reduction (H2-TPR) was carried out with a mixture of 5% H2/N2 as the reductive gas. A sample of about 200 mg was reduced in a flow of H2/N2 at a heating rate of 10 K min−1 from room temperature to 1000 K. The effluent gas was detected by TCD after the removal of produced water using 5 Å molecular sieves.
The product selectivity was calculated on carbon molar base:
Si = Yini/∑Yini × 100% |
In order to confirm the dispersion of vanadium, the surface and bulk composition of the prepared samples were studied by XPS and ICP techniques (see Table 1). The bulk vanadium contents were similar for all the catalysts. The surface vanadium on VTiSi, VTiZr and VTi catalysts was higher than that of bulk, suggesting the vanadium oxide was mainly dispersed on the surface of the catalysts. However, the surface vanadium on VTiCe and VTiAl catalysts was lower than those in the bulk, indicating that the vanadium mainly existed on the bulk of the catalysts. Calculation shows that V atom numbers per nm2 on VTi catalyst was 5.6, which agrees well with the report that 7–8 V atoms per nm2 was required to form the monolayer-dispersion.34 The values were only 3.6 and 6.4 on VTiSi and VTiZr catalysts, respectively, inferring the high dispersed V species which were half less than the theoretical value. However, the values were 16.5 for VTiCe catalyst, which was almost two times larger than the theoretical value, because of its low surface area. And the values were 11.5 for VTiAl catalyst, which was larger than the theoretical value too. Thus the crystalline V2O5 appeared as confirmed by XRD measurement.
Sample | Texture data | Chemical composition | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
SBET (m2 g−1) | Pore volume (cm3 g−1) | Pore diameter (nm) | V density calculateda (nm−2) | C.A. (wt%) | XPS (wt%) | |||||
V | Ti | Mb | V | Ti | Mb | |||||
a Supposing that all the vanadium atoms locate on the surface.b M = Al, Si, Ce, Zr. | ||||||||||
VTi | 177 | 0.15 | 3.5 | 5.6 | 7.0 | 50.5 | — | 7.4 | 50.4 | — |
VTiAl | 87 | 0.25 | 9.0 | 11.5 | 7.2 | 25.5 | 21.3 | 6.7 | 26.3 | 23.8 |
VTiSi | 277 | 0.55 | 5.3 | 3.6 | 7.4 | 25.5 | 20.7 | 9.0 | 25.1 | 22.1 |
VTiCe | 60 | 0.08 | 3.3 | 16.5 | 7.1 | 24.8 | 35.8 | 6.2 | 20.4 | 28.0 |
VTiZr | 154 | 0.14 | 3.5 | 6.4 | 7.4 | 25.6 | 31.0 | 7.9 | 26.0 | 25.3 |
The isotherms of all samples in Fig. S1† showed the type of IV with the hysteresis loops at relative pressures of 0.4–1.0, which was characteristic for mesoporous materials.35 The pore size distribution for VTi, VTiCe and VTiZr catalysts, centered at about 3.5 nm, while the peak centered at 5.3 and 9.0 nm appeared for VTiSi and VTiAl catalyst respectively (Table 1). The pore volume of VTiAl increased slightly to 0.25 cm3 g−1 compared with that of VTi sample (0.15 cm3 g−1) while the number increased sharply to 0.55 cm3 g−1 for VTiSi sample, which may favor the subsequent reaction. The pore volume of VTiCe, however, decreased to 0.08 cm3 g−1 while that of VTiZr kept almost unchanged.
Table 2 showed the results of binding energies and peak fitting results. The Ti2p3/2 binding energy for all the samples were around 458 eV, which was in reasonable agreement with those for Ti4+ in literature.36 The O1s peak showed two types of oxygen species. The binding energy around 530.8 eV was the characteristic of lattice oxygen species of TiO2 and V2O5, and the binding energy at 532.3 eV could be attributed to active surface oxygen species, including surface oxygen of adsorbed oxygen species, weakly bonded oxygen and hydroxyl-like groups.37–40 Sample VTiSi had much more active oxygen than others. The reference V2p3/2 peak positions for V2O5 and V2O4 were around 517.4 and 516.3 eV, respectively.41,42 The full oxidation state of V5+ was predominant at catalyst surface. It was clearly seen that a greater amount of V4+ species was presented on the VTiSi surface than that of others, indicating that the degree of reduction of V2O5 was enhanced with the Si addition.
Sample | V2p3/2 (eV) | O1s (eV) | Ti2p3/2 (eV) | V4+/(V4+ + V5+) | Osur/(Osur + Olat) | ||
---|---|---|---|---|---|---|---|
V4+ | V5+ | Osura | Olatb | (Area %) | (Area %) | ||
a Osur = surface active oxygen.b Olat = lattice oxygen. | |||||||
VTi | 516.3 | 517.2 | 532.1 | 529.9 | 458.3 | 34.4 | 33.6 |
VTiAl | 516.2 | 517.3 | 532.5 | 529.6 | 458.0 | 29.6 | 22.2 |
VTiSi | 516.5 | 517.4 | 532.3 | 529.8 | 458.4 | 56.0 | 65.8 |
VTiCe | 516.3 | 517.3 | 532.7 | 529.5 | 458.2 | 30.4 | 5.6 |
VTiZr | 516.4 | 517.3 | 532.4 | 529.4 | 458.2 | 32.8 | 23.2 |
The morphology was characterized by the TEM techniques (Fig. 2). It was found that the particles size of VTi was only 5–6 nm. The particle size increased apparently with the addition of Al, Si, Ce and Zr. Such as the particle size of sample VTiAl, VTiSi, VTiCe and VTiZr were about 190, 390, 260 and 200 nm, respectively. Metal additives affected the combination of vanadium and titanium.
Sample | Weak acidity | Middle stronger acidity | Brønsted sites/Lewis sites | ||
---|---|---|---|---|---|
Temp. (K) | Number (μmol g−1) | Temp. (K) | Number (μmol g−1) | ||
VTi | 414 | 462 | 503 | 162 | 0.56 |
VTiAl | 403 | 323 | 499 | 198 | 0.37 |
VTiSi | 387 | 689 | 477 | 256 | 0.74 |
VTiCe | 412 | 147 | 504 | 96 | 0.38 |
VTiZr | 384 | 198 | 480 | 116 | 0.45 |
Chemisorption of pyridine followed by FTIR spectroscopy was useful to probe the presence and nature of surface acid sites on catalysts.45,46 Table 3 showed that VTiSi catalyst exhibited larger amount of Brønsted acidic sites and less Lewis acidic sites compared with those of VTiAl, VTiCe and VTiZr catalysts. Combined to the NH3-TPD results, adding Si element to VTi catalyst brought more acid sites which mainly were Brønsted acidity. However, the addition of Al, Zr and Ce element to VTi catalyst resulted in smaller amount of acid sites since the weak interaction between V and supports.
Table 4 presented the catalytic performances of composite supported V2O5/TiO2 catalysts. The sample VTi exhibited a highest methanol conversion of 43% of and a DMM selectivity of 89% at 413 K. The selectivity for MF was 10% and 1% for DME. The distribution of products on VTi catalyst indicated the surface acidity was not strong enough to effectively catalyze the reaction of FA condensation with methanol to produce DMM, leading to the production of MF and a small amount of DME. The methanol conversion increased sharply with increasing reaction temperature, meanwhile, the selectivity to DMM decreased while the selectivity to MF and DME increased. The sudden drop in DMM selectivity possibly came from the thermodynamic constrains for DMM synthesis.52
Sample | Temp. (K) | Con. of methanol (%) | Selectivity (%) | DMM yield | Consumption of methanol/SBET | ||||
---|---|---|---|---|---|---|---|---|---|
FA | DME | MF | DMM | COx | |||||
VTi | 413 | 43 | 0 | 1 | 10 | 89 | 0 | 38 | 24 |
423 | 50 | 0 | 7 | 26 | 57 | 10 | 29 | 28 | |
433 | 72 | 0 | 19 | 47 | 23 | 11 | 17 | 41 | |
443 | 95 | 0 | 16 | 66 | 1 | 17 | 1 | 54 | |
VTiAl | 413 | 23 | 42 | 1 | 0 | 56 | 1 | 13 | 26 |
423 | 24 | 37 | 3 | 0 | 58 | 2 | 14 | 39 | |
433 | 45 | 34 | 2 | 0 | 62 | 1 | 28 | 52 | |
443 | 48 | 31 | 5 | 0 | 63 | 1 | 30 | 55 | |
VTiSi | 413 | 51 | 0 | 1 | 0 | 99 | 0 | 50 | 18 |
423 | 52 | 0 | 2 | 0 | 98 | 0 | 51 | 19 | |
433 | 77 | 0 | 7 | 37 | 56 | 0 | 43 | 28 | |
444 | 91 | 0 | 6 | 39 | 55 | 0 | 50 | 33 | |
VTiCe | 413 | 15 | 0 | 2 | 0 | 98 | 0 | 15 | 25 |
423 | 20 | 0 | 3 | 0 | 97 | 0 | 19 | 33 | |
433 | 25 | 0 | 7 | 0 | 93 | 0 | 23 | 42 | |
443 | 30 | 0 | 7 | 0 | 93 | 0 | 28 | 50 | |
VTiZr | 413 | 28 | 0 | 1 | 0 | 99 | 0 | 28 | 18 |
423 | 29 | 0 | 1 | 0 | 99 | 0 | 29 | 20 | |
433 | 37 | 2 | 5 | 0 | 93 | 0 | 34 | 24 | |
443 | 40 | 2 | 6 | 0 | 92 | 0 | 37 | 26 |
Compared to VTi catalyst, sample VTiSi showed the highest activity of 51% methanol conversion and DMM selectivity of 99% among all samples at 413 K. Moreover, when the temperature increased to 423 K, the DMM selectivity was still kept at 98% with the methanol conversion of 52%. In addition, the stability was carried for VTiSi catalyst at 413 K (Fig. 5). The result showed that DMM selectivity (99%) and methanol conversion (51%) did not change obviously within 240 h, indicating that the VTiSi catalyst exhibited an excellent stability, which probably related to its bigger particle size. All the catalytic data were better than those data shown in literature.10,11,23,30,53
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Fig. 5 Changes of DMM selectivity and methanol conversion with time on stream on VTiSi catalyst at 413 K. |
However, sample VTiAl showed low activity for DMM synthesis from methanol selective oxidation, 23% of methanol conversion and 56% of DMM selectivity at 413 K. After the ZrO2 doping, the VTiZr sample showed increased DMM selectivity but much lower methanol conversion compared with those of VTiSi sample. In fact, the methanol conversion kept about 28% on VTiZr catalyst when reaction temperature increased from 413 K to 423 K, the DMM selectivity exceeded 99% at 413–423 K and 92% at 433–443 K. The catalytic performance of VTiCe showed a high DMM selectivity (98%) but lowest methanol conversion (15%) was obtained.
The coexistence of different valence states of V on composite VTi catalysts was verified by XPS (Table 2). A greater amount of V4+ species was presented on the VTiSi catalyst surface. Thus, the electron transfer between lattice oxygen and metal cations played a critical role in regenerating the catalyst to the original state by restoring the active lattice oxygen, in accordance with the above discussion.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16727a |
This journal is © The Royal Society of Chemistry 2015 |