Received Date: April 01, 2016; Accepted Date: April 28, 2016; Published Date: March 05, 2016
Citation: Wang W, Zhang Q, Gao X, Zhang Z, Gu Y, et al. (2016) VOx Modified H-Beta Zeolite for Dimethyl Ether Direct Oxidation to Polyoxymethylene Dimethyl Ethers. Chem Sci J 7:124. doi:10.4172/2150-3494.1000124
Copyright: © 2016 Wang W, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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The catalytic performances of bifunctional vanadium oxide supported on Hβ zeolite with redox and acid properties were investigated for dimethyl ether (DME) direct oxidation to polyoxymethylene dimethyl ethers (DMMx) (x=1-2). The best catalytic performance was obtained over 15%V2O5/Hβ. The catalysts were characterized by XRD, BET, NH3-TPD, H2-TPR, XPS and FT-IR in order to provide the further insight in the relationship of structure and activity. When the loading content of vanadium oxide was 15%, the presence of oxide species was dominantly the polymeric vanadia. The addition of vanadium oxide to the H-beta zeolite not only provided redox sites, but also changed the acid strength and distribution of H-beta zeolite as confirmed by NH3-TPD measurement, and the positive correlation was found between the DMMx selectivity and the ratio of weak and strong acid sites. The introduction of vanadium oxide with appropriate amount to the H-beta zeolite promoted the balance between the acid sites and redox sites, which was very vital for obtaining high DMMx selectivity.
Dimethyl ether; Direct oxidation; VOx/H-beta; Polyoxymethylene dimethyl ethers
Polyoxymethylene dimethyl ethers (DMMx) have attracted intensive research interest because of their promising properties, such as excellent environmental friendly solvent with low toxicity, low boiling point, good solubility, and the high oxygen content and octane value. DMMx can effectively improve the engine combustion performance of diesel oil to be beneficial to reduce the CH, CO content in the exhaust emissions, therefore, it is considered extremely as the promising green diesel additive in the application [1,2]. Traditionally, the dehydration-condensation of methanol with formaldehyde over the acid catalysts was the main method to synthesize DMMx. Compared with the aldolization reaction, the researchers proposed the one-step oxidation of methanol to DMM, which was considered to be more environmental and economical advantages. For instance, SbRe2O6 , heteropolyacids , Cu-ZSM-5  and Fe-Mo catalyst  were applied. Particularly, the V2O5/TiO2 catalysts have been further studied because of the outstanding catalytic performance in direct oxidation of methanol to DMM [7-9].
In addition, new synthesis routes of DMMx were put forward including the direct oxidation of dimethyl ether (DME, CH3-O-CH3) [10-12]. DME can be synthesized via a one-step process at low cost from syngas generated from coal, biomass and natural gas, and it has been known as a clean fuel and a potential non-petroleum oil route chemical synthesis material [13,14]. It is a very competitive and green route that dimethyl ether as reactant is used to synthesize DMMx.
In our previous work, we reported the synthesis of DMMx via the direct oxidation of DME over Re/Sm/Mn modified heteropolyacids catalysts [15,16]. We also proposed the possible mechanism that the formation of CH3OCH2OCH3 may be from the combination of CH3OCH2 group generated on the redox sites and CH3O dissociated over acid sites or the dissociation and recombination of C-O bonds via the aldol condensation with methanol and formaldehyde under the synergistic effects of the acid and redox sites of the catalysts. Hence, designing a catalyst including the acid sites and the redox sites into one system was in favor of high DMMx selectivity to make the most use of the bifunctional components.
Compared with the heteropolyacids, zeolites have drawn great attention as the acid catalysts, due to their high surface area, specific pore structure, high thermal stability and eco-friendly nature. In particular, the HBEA zeolite dominated by weak acid sites should be in favor of the selective oxidation reaction of DME according to our previous research . At the same time, the redox sites are provided by vanadium oxide that has very important redox properties in catalysts due to the capacity to adopt multiple oxidation states. Vanadium oxide-based catalysts are widely used in catalytic oxidation, such as the selective oxidation of methanol to formaldehyde , oxidation of SO2 to SO3  and oxidative dehydrogenation of propane . So vanadium oxide modified H-beta catalyst is applied to the selective oxidation of DME to DMMx, and the related work is seldom reported, in addition, we observed the formation of DMM2, indicating that the further growth of C-O occurred in our research.
In this paper, we designed and prepared H-beta zeolite supported vanadium oxide catalyst with different loading content of vanadium oxide, and investigated its catalytic performance in the direct oxidation of DME to DMMx. The catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller isotherms(BET), temperatureprogrammed desorption of NH3 (NH3-TPD), temperature programmed reduction of H2 (H2-TPR), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT–IR) to provide the further insight in the influence of structure and surface properties on the activity of VOx/H-beta.
Commercial H-beta zeolite (Nankai University Catalyst Company, China, SiO2/Al2O3=26) was initially calcined at 823 K for 5 h. The VOx/ H-beta zeolite was prepared by the incipient wetness impregnation method. Briefly, a certain amount of ammonium metavanadate (ammonium metavanadate: oxalic acid molar ratio=1:2) were dissolved in oxalic acid solution at the temperature of 333 K in the water bath, then was added by H-beta zeolite. The slurry was kept at room temperature for 12 h, dried overnight at 110 °C, and finally calcined in air at 673 K for 5 h before being crushed to 20-40 mesh.
Catalytic oxidation of DME
The catalytic oxidation of DME was carried out in a continuous flow type fixed-bed reactor. The catalyst (3 ml, 20-40 mesh) was diluted with ground quartz to prevent over-heating of the catalyst due to exothermic reaction. The catalyst was pretreated in flow of O2 (30 ml/ min) for 1 h before reaction. The reactant mixture consisted of DME and O2 with molar ratio of O2: DME=1: 1. The reaction products were analyzed by gas chromatography GC-9A (Shimadzu Co.) equipped with flame ionization detector (30 m × 0.32 mm, PEG-40M), GC- 4000A with thermal conductivity detector (Porapak T column) and GC-4000A (TDX-01 column) with thermal conductivity detectors. The data of the entire work was calculated based on carbon balance, and the carbon balances of most experiments were within 90-95%.
X-ray diffraction (XRD) measurements were carried out on a Rigaku mini Flex II De X-ray Diffractometer scanning from 5° to 70° at a rate of 4°/min using a Cu Kα radiation source. The applied voltage and current were 30 KV and 30 mA, respectively. The BET surface area and pore volume of the zeolite samples were determined by N2 adsorption desorption at -196°C in Micromeritics TriStar II 3020 equipment.
The acid sites of the catalysts were quantified by ammonia temperature-programmed desorption (NH3-TPD). The NH3-TPD profiles were obtained in Tianjin XianQuan company tp-5080 type automatic adsorption instrument with a thermal conductivity detector. In all experiments, 100 mg of sample was pretreated at 873K under N2 flow (30 ml/min) for 30 min, then cooled down to 373K, at this temperature, NH3 was introduced into the flow system till saturation, then the sample was flushed with N2 at 373K for 2 h to remove excess ammonia. The NH3-TPD profiles were recorded from 373 to 873K at a rate of 10 K/min.
The H2-TPR measurements were taken by using tp-5080 type automatic adsorption instrument with a thermal conductivity detector (TCD) in Tianjin Xian Quan Company. In all experiments, 50 mg of sample was pretreated at 573K under N2 flow (30 ml/min) for 1 h, then cooled down to 303K, the TPR measurement was carried out using H2/N2 as reducing gas mixture, the H2-TPR profiles were recorded from 303 to 1123K at a rate of 5 K/min.
The X-ray photoelectron spectra (XPS) were measured on Kratos AXIS Ultra DLD spectrometer equipped with Al anode (Al Kα hv=1486.6 eV) powered at 150 W. The detection area analyzed was 700 × 300 μm. FT-IR measurements were recorded with BRUKER TENSOR 27, in the 4000-600 cm-1 range, with a resolution of 2 cm-1 and using 64 scans. The samples were measured before activation in situ at 673K under the atmosphere of Ar.
The XRD spectra show that the structure of the BEA zeolite was maintained after impregnation of VOx. The marked peaks (20.3°, 34.3° and 51.4°) corresponded to crystallites of V2O5. The characteristic diffraction peaks of crystalline V2O5 were not detected when the content of vanadium oxide was lower than 15%, suggesting that the VOx species were dispersed highly on the zeolite. The peaks of crystalline V2O5 appeared when the loading content exceeded 15%, which can be explained by the agglomeration of the polymeric vanadium oxide with the increase of loading content, but when the content was 15%, the intensity of the diffraction peak at 20.3° was ambiguous, manifesting that the proportion of crystalline V2O5 was very low. The structure of surface vanadium species supported on the HBEA zeolite depended on the loading content. Below 15%, both monomeric and polymeric species were present, while above 15%, the polymeric vanadia species turned into crystalline vanadium oxide. This could be confirmed by FTIR providing more detailed information. Structure properties of VOx/ Hβ catalysts are given in Table 1. The BET surface area continuously decreased from 558 to 397 m2g-1 with the gradual increase in the loading content of vanadium oxide, while the surface area of 25% V2O5/ Hβ sample went up slightly compared with the 20% V2O5/Hβ, maybe due to the assembly of VOx reducing the blocking of the zeolite pore under the conditions of high loading content. The variation of pore volume also could prove this phenomenon. The results were consistent with the analysis of XRD (Figure 1).
|Catalysts||SBETa (m2g-1)||Smicrob (m2g-1)||Dporeb (nm)||Vmicroa (cm3g-1)|
Table 1: Structure properties of VOx/Hβ catalysts.
In Figure 2 displays the FT-IR spectra of the prepared catalysts. The catalysts were first treated by high temperature under the atmosphere of Ar, therefore, the influence of hydration on the supported vanadium oxide could be excluded. Through the comparison of the pure zeolite and the catalysts with different loading content of vanadium oxide, the peak at 1023 cm-1 appeared when the vanadium oxide loading content reached 15%, and the intensity gradually enhanced with the increasing content of vanadium oxide. Combining with the XRD results and the literature , the band at 1023 cm-1 was considered as the V=O characteristic stretching band of crystalline V2O5. For all catalysts except pure zeolite, the band at 894 cm-1 possibly assigning to V-O-V linkage was observed according to the variation tendency, and the intensity of the band increased along with the increase content of surface vanadium oxide, while it did not changed more after the formation of crystalline V2O5.
In Figure 3 presents the binding energies of V2p3/2 and the results of vanadium species analyzed by V2p3/2 peak-fitting from the XPS spectra. Two BE positions were obtained by peak fitting of V2p3/2 line at 517.8 eV and 516.9 eV, which were ascribed to two different components V2O5 and V2O4 species, respectively. Among the sample, V4+ possessed 20%, which may be resulted from the interaction between vanadium and surface atom of zeolite or the combination exposure to air and water vapor, according to the report of Terán- Escobar et al. . The coexistence and shift of V4+ and V5+ oxidation states prompted the catalytic oxidation cycles, moreover, Tokarz- Sobieraj et al.  considered that V4+ could be more effective owning to the more available oxygen vacancies and the higher electron density of vanadium that would improve the redox capability to facilitate the oxidation reaction.
NH3-TPD was used to analyze the strength and distribution of the surface acidity of the catalysts. The temperature region of 400-600K was attributed to the weak acid sites, while 600-800K belonged to the strong acid sites as shown in Figure 4. The results were obtained by the integration from the NH3-TPD spectra. The profiles showed that the weak acid sites dominated in H-beta zeolite, and the amount of the weak acid sites mounted up, then decreased with the increasing content of vanadium oxide impregnated on the HBEA support, whereas, the intensity of weak acid sites was higher than that of parent zeolite. Addition of vanadium oxide to the H-beta zeolite decreased the strong acid sites distinctly due to the overlapping, which in turn favored the increase of weak acid sites because of the weak acidity generated by vanadium oxide. However, when the loading content exceeded 20%, the agglomeration of the polymeric vanadium oxide induced the contrary results that 25% V2O5/Hβ had more strong acid sites than the low loading content catalyst. The possible explanation could be the exposure afresh of the strong acid sites because of the increase proportion of crystalline V2O5. Thus, the ratio of weak acid and strong acid sites presented the volcanic type with the various content of vanadium oxide, and the maximum ratio was 2.77 when the loading content of VOx was 20%, similar to the 15% V2O5/Hβ.
As described in Figure 5 for 3300-3800 cm-1 region of FT-IR profiles, the intensity of these bands at 3600, 3662, 3782 cm-1 weakened obviously and even vanished after the introduction of low loading vanadium oxide content. The band at 3600 cm-1 was corresponding to the bridging hydroxyl groups (Si-(OH)-Al), and the features at 3662 and 3782 cm-1 were assigned to two kinds of Al-OH groups in which Al atoms belonged to EFAL species and tricoordinated Al atoms linked to the framework by two O atoms, respectively . Thus, VOx species mainly anchored to the bridging hydroxyl groups (Si-(OH)-Al) and Al-OH groups which supported the strong acid sites, and the results were in agreement to the literature . These adequately interpreted the variation of the ratio of W/S in line with the results of NH3-TPD (Table 2).
|WA / SA|
Table 2: Results of NH3-TPD profiles integration.
In Figure 6 shows H2-TPR profiles of VOx/Hβ with different V2O5 content. From the comparison of the TPR profiles of the catalysts, it was observed that the reduction temperature of impregnated vanadium oxide shifted to higher value with the surface vanadium oxide coverage increasing. Meanwhile, the area of hydrogen consumption peak to some extent went up. In the TPR profiles, the reduction peaks exhibited one single peak. Different vanadia species such as monomeric, polymeric and crystalline VOx species may coexist due to different loading content of the vanadium oxide on the surface species of the VOx/Hβ system. Thus, the peaks did not necessarily represent different steps of reduction of the same species but should rather be considered to result from a superposition of different reduction steps of several species . In addition, when the vanadium loading content was beyond 20%, the reduction peak was asymmetric with left-hand shoulder that may be ascribed to the reduction of dispersed polymeric vanadium species which were more easily reduced than crystalline V2O5 formed on the surface of zeolite, which was confirmed by XRD and FT-IR.
DME selective oxidation to DMMx
In Table 3 presents the catalytic performance of the pure zeolite and H-beta zeolite supported vanadium oxide catalysts in the DME direct oxidation to DMMx (x=1-2). For pure Hβ zeolite, DME primarily underwent hydrolysis reaction to produce methanol over the acid sites, while the target products were obtained after the introduction of vanadium oxide, suggesting the redox sites provided by vanadium oxide played an importance role in the formation of DMMx from DME oxidation. With the increasing loading content of vanadium oxide, the enhanced selectivity of DMMx was contributed to an increase in the number of active sites. The optimum catalytic performance was achieved over 15%V2O5/Hβ, and the total selectivity of DMM and DMM2 reached 46.0% with DME conversion of 12.4%. As the vanadium loading amount further up to 20%, the selectivity of DMM and DMM2 hardly changed. Routray et al.  pointed out that the decrease in activity above monolayer coverage resulted from the low reactivity of crystalline V2O5 compared to the surface vanadium oxide species. Higher vanadium loading content could promote the conversion of DME but it resulted in more redox sites than acid sites, which accelerated the formation of formaldehyde.
Table 3: Effects of different V2O5 content on the activity of V2O5/ Hβ for DME catalytic oxidation to DMMx.
From the H2-TPR profiles, the presence of reduction peak exhibited the redox properties. With the surface vanadium coverage increasing, the area of hydrogen consumption peak gradually went up, which provided more and more redox sites for methoxy group or methanol to form HCHO. The appropriate value of redox sites facilitated the formation and transform of HCHO. The excess VOx species leads to the imbalance between redox and acid sites, and further inhibited the synthesis of DMM and DMM2. The impregnation of vanadium oxide not only provided the redox sites, but also changed somewhat the acid strength and distribution of H-beta zeolite as confirmed by NH3-TPD measurement. The oxide reduced the number of strong acid sites, and simultaneously increased the amount of weak acid sites, which resulted in the difference of the ratio of weak and strong acid sites. The selectivity of DMMx was compared with the ratio of W/S, and the high ratio of weak and strong acid site was in favor the formation of DMMx (x=1-2) which was coincident with our previous results . When the ratio was similar between 15% and 20%V2O5/Hβ, the selectivity of target product was almost the same. These adequately explained the importance role of weak acid site of H-beta zeolite adjusted by VOx in the DME oxidation reaction. Hence, the formation of DMM and DMM2 was attributed to two types of active sites offered by oxide/ zeolite catalyst exhibiting complementary properties.
For the reaction mechanism, the literature  pointed out that the reaction began with the molecular chemisorption of methanol on the V5+ cations. The adsorbed molecules dissociated to form the methoxy species, the following oxidation of the methoxy species led to the formation of adsorbed formaldehyde, which can transform into the surface dioxymethylene species. The further reaction of dioxymethylene species with either the methoxy species or methanol generated dimethoxymethane. The formation of the species was accompanied by the shift of the vanadium oxidation state. However, in our research, we inferred that HCHO formed over the redox sites of polymeric VOx species desorbed when the reaction temperature was higher than what Kaichev et al. reported , and then transported to the near weak acid site afforded by the H-beta zeolite support, where DMM and DMM2 were formed via the acetalization reaction of methanol with the desorbed formaldehyde. When DME proceeded over 15%V2O5/ Hβ, the major products were formaldehyde and methanol, but they did not further reacted over the active sites of VOx species with the coexistence of formaldehyde and methanol so that dimethoxymethane was not detected under the same reaction conditions. The results above forcefully proved the speculation. The weak acid sites and redox properties supported by VOx species were considered to be responsible for the formation of DMMx.
H-beta zeolite supported vanadium oxide was applied in DME direct oxidation to DMMx (x=1-2), the best catalytic performance were obtained and the total selectivity of DMM and DMM2 reached 46.0% with the DME conversion of 12.4% over 15%V2O5/Hβ.
After the impregnation of 15% vanadium oxide on the H-beta zeolite, the polymeric vanadia was the dominant species from the analysis of XRD and FT-IR spectra. The addition of vanadium oxide offered redox sites, and concurrently influenced the acidic strength and distribution of H-beta zeolite as shown by H2-TPR and NH3-TPD. Comparing the selectivity of DMMx with the ratio of weak and strong acid sites, they presented the positive correlation. Thus, the appropriate balance between the acid sites and the redox sites played an important role in achieving high DMMx selectivity.
This work is supported by the National Natural Science Foundation of China (No. 21373253, 20903114) and Youth Innovation Promotion Association CAS (No. 2014155).