Next Article in Journal
Effect of Ethanol on the Morphology and Textual Properties of ZSM-5 Zeolite
Previous Article in Journal
Approaches for Selective Oxidation of Methane to Methanol
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 10
Issue 2
10.3390/catal10020197
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

High-Performance Vapor-Phase Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate over SiO2 Supported PMoVNb Oxides

by
Xiaoyu Liu
1,
Qi Wang
2,
Chenghao Liu
1,
Xiaolong Wang
2,
Cuina Peng
1,
Rong Liu
2 and
Cheng Yang
1,*
1
College of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
2
Huaneng Clean Energy Research Institute, Beijing 102209, China
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(2), 197; https://doi.org/10.3390/catal10020197
Submission received: 29 December 2019 / Revised: 20 January 2020 / Accepted: 22 January 2020 / Published: 6 February 2020
Browse Figures
Versions Notes

Abstract

:
This paper describes the application of P-Mo-V-Nb/SiO2 catalysts for the selective oxidation of ethyl lactate (EL) to ethyl pyruvate (EP). The P-Mo-V-Nb/SiO2 catalysts exhibit superior performance for EP selectivity than the corresponding samples of binary V-Nb/SiO2 ternary P-Mo-V/SiO2 and P-Mo-Nb/SiO2 catalysts at same temperatures. The origin of high EP selectivity of the P-Mo-V-Nb/SiO2 catalysts is explored and attributed to the synergistic effect of P, Mo, V, and Nb mixed oxides presented on the surface of the catalyst. The highly dispersive sites separated active species under the action of phosphorus, suppressing over oxidation and improving the selectivity. The existence of MoO3 to provide higher oxidation for catalyst. The redox cycle of V and Nb oxides could be completed through electron transfer between lattice oxygen and metal cations. Moreover, the weak acidity of catalyst surface is favorable to avoid the decarboxylation reaction to target a high selectivity of EP. Therefore, the P-Mo-V-Nb/SiO2 catalyst obtained the maximum yield of 91.8% with a selectivity of 93.8% and a conversion of 99.0% simultaneously at 280 °C.

1. Introduction

In recent years, biomass has attracted increasing interest due to rising fossil fuel prices and global warming [1]. Conversion of biomass-based platform compounds to prepare valuable chemicals is a promising utilization approach. Lactic acid is an important compound which has been commercially produced by the fermentation of biomass. Lactic acid, its esters, and salts are feed for many downstream products such as pyruvate.
Pyruvate and its esters are important chemicals widely used in food, spice, cosmetics, pharmaceutical and other industries [2,3,4]. Currently, pyruvate and its esters are still produced mainly via the energy-intensive pyrolysis of tartaric acid with KHSO4 as a dehydrating agent [5,6]. Unfortunately, this process obtains a yield of only 50–54% but obviously negative environmental effect due to the use of numerous dehydrating agents. Pyruvic chemicals can also be produced through enzymatic conversion or glucose fermentation [7,8,9]. However, these approaches are difficult to industrialize due to critical conditions including reaction temperature and acidity. Lactic acid and its esters can be selectively oxidized to pyruvates and corresponding esters. This is a green and atom-economical reaction route which has received extensive attention from the scientific and industrial circles. This highly atom-efficient catalytic oxidative dehydrogenation process includes liquid and vapor phase strategies [5,10,11,12,13,14].
The liquid-phase process employs various metal oxides as catalysts at a low temperature. Fe2O3, TiO2, V2O5/MgO-Al2O3, ZrO2, CeO2, and ZnO were demonstrated alternative catalysts, in which TiO2 was the most efficient [15]. Zhang et al. reported a one-pot synthesis of mesoporous vanadia-titania, acting as an efficient and recyclable catalyst for the conversion of ethyl lactate (EL) to ethyl pyruvate (EP) [16]. This reaction was carried out with molecular oxygen at 130 °C for 8 h. The selectivity of EP decreased from 91% to 58% when the conversion of EL increased from 20% to 70% with the reaction time on stream. Lu et al. discovered a more efficient catalyst under mild conditions. Tremendously, 100% EL conversion and 97.8% EP yield were obtained after reaction at 50 °C for 9 h [17]. However, the use of H2O2 aqueous solution (molar ratio of H2O2 to ELA 2.80) reduces likelihood of industrialized application because of the expense of H2O2 and the explosive risk of peroxides. Moreover, a low reaction temperature was demanded strictly because higher reaction temperature (70 °C) lead to fast hydrolysis and decarboxylation of EP. In addition, the space time yield is usually not high in liquid-phase process, and the recycle of catalysts is strenuous.
In contrast, selective oxidation is more likely industrialized in vapor phase, which is advantage in continuous operation, process control and space time yield. Several vapor-phase catalysts have been studied and disclosed for the selective oxidation of EL to EP. Sugiyama’s research team [10] reported a series of bimetallic catalysts containing MoO3, SnO2-MoO3, Fe2O3-MoO3, and Te-MoO3, where SnO2-MoO3 obtained the highest yield of 36% at 250 °C. They proposed the collaborative participation of lattice and gas-phase oxygen according the XPS results. Thereafter, Zhao et al. [18] employed Mo0.61V0.31Nb0.08Ox/TiO2 as a catalyst to obtain a yield of 47% at 170 °C. They discovered that oxygen concentration was the only factor affecting the reaction rate rather than the concentration of ethyl lactate on the surface. This apparent first-order reaction is consistent with the Mars–van Krevelen redox mechanism.
However, the application is limited due to the low activity and selectivity. The separation of EL and EP is energy intensive, because the difference of their boiling point is only 1 K. Other byproducts from decarboxylation added the energy consumption of separation. It remains a great challenge to develop a highly effective and selective catalyst.
MoVOx-based composite oxides exhibits excellent catalytic activity and selectivity in the oxidation of light alkanes at relatively low temperatures [19,20,21]. Mo-V-P-Ox/Al2O3 and Mo-V-Te-Nb-O composite oxides catalysts [22,23] are efficient in the ammoxidation of propane to acrylonitrile. MoVNbOx show high catalytic performance for the oxidation of ethane [24], ethylene [25], and ethanol [26]. Despite synergistic effect in multi-component oxides is still indistinct, multiple oxides may be inexpensive and selective catalysts for the selective oxidation. To the best of our knowledge, there is no report about the oxidative dehydrogenation of EL to EP over P-Mo-V-Nb catalysts.
Here we report a P-Mo-V-Nb catalyst supported on SiO2 for the selective oxidation of ethyl lactate to ethyl pyruvate. This quaternary component oxide was compared with binary V-Nb and ternary P-Mo-V and P-Mo-Nb catalysts prepared with an equal volume impregnation method. The resulting catalysts were systematically characterized by nitrogen adsorption, XRF, XRD, H2-TPR, XPS, NH3-TPD, pyridine FT-IR and Raman and evaluated in a fixed bed to understand the effect of the molybdenum and vanadium species of structure and redox properties.

2. Results and Discussion

2.1. Structural Properties

The elemental analysis results of all samples by XRF are shown in Table 1. The molar ratio of P/Si, Mo/Si, V/Si, and Nb/Si remained almost unchanged from that of the experimental design (0.89, 0.74, 0.16, and 0.02). The equal volume impregnation method used is efficient to introduce binary V-Nb, ternary P-Mo-V, P-Mo-Nb, and quaternary P-Mo-V-Nb mixed oxide into silica.
The N2 adsorption-desorption isotherms and the textural properties of catalyst samples are given in Figure 1 and Table 1. The SiO2 and V-Nb/SiO2 samples exhibit type II adsorption isotherm patterns, which display no adsorption saturation at relatively higher pressure with H3-type hysteresis loops, indicating the existence of the large mesoporous channel structure. The P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2 samples have H4 hysteresis loops which adsorption branch is a composite of type I and type II isotherms patterns, suggesting the channel structure with micropores and mesopores [27,28]. Table 1 lists the detailed mesoporous sizes of the sample, which are consistent with the N2 adsorption isotherm analysis. From the pore-size distribution curves (Figure 1B) of these samples, it could be seen that the nanopore size of silica was a bit large with a peak value at 11 nm. The V-Nb/SiO2 had a wide distribution range at 5–40 nm. Meanwhile, the nanopore size of P-Mo-V /SiO2, P-Mo- Nb/SiO2, and P-Mo-V-Nb/SiO2 in Figure 1B were dominantly distributed in the range of 1–4 nm, and the peak values respectively at 1.8 nm, 2.0 nm and 1.5 nm. In addition, it could be obviously observed that the samples showed smaller pore sizes and smaller Brunauer–Emmett–Teller (BET) surface areas with increasing the amount of active species. The BET surface areas of the four catalysts ranged 127 to 9.7 m2/g with the pore volume of 1.2 to 0.1 cm3/g smaller than those (208.76 m2/g and 1.2 cm3/g) of the supported silica (Table 2). This could demonstrate that the active species was well dispersed in the surface of the support as well as in the channels.
Figure 2 gives diffraction patterns obtained with different catalysts. Only two crystallites V2O5 and Nb2O5 were observed in Figure 1B, and the other catalysts showed a large peak at 23.8° like carrier silica but the diffraction intensity was weaker than it. The diffraction disappeared for P-Mo-Nb/SiO2, P-Mo-V/SiO2, and P-Mo-V-Nb/SiO2 catalysts, indicating the high dispersion of active component on the surface of the carrier silica. Considering the three catalysts have a common P species different from V-Nb/SiO2, the above results could attributed to the fact that P species can make the other active species dispersed on the surface of the carrier better.
The Raman spectra of all catalyst samples are shown in Figure 3. No typical Raman peaks of silica was detected, but the right side of the baseline shift to higher may be caused by the fluorescence of the silica. For the V-Nb/SiO2, the band at 1019 cm−1 is assigned to V=O stretches in monovanadates and polyvanadates, and the bands at 998, 690, 516, and 268 cm−1 are assigned to crystallized V2O5 [29,30]. The P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2 show the intense band at 964, 880, and 395 cm−1, corresponding to crystallized MoO3. Besides, the broad band at 1052–1294 cm−1 is assigned to M=O terminal stretches [31,32]. For P-Mo-V/SiO2 and P-Mo-V-Nb/SiO2 in Figure 3, the bands at 1025 cm−1 is implied to mixed Mo-O-V phases. Because of P having a lone pair of electrons, it could tend the electron to the Mo site or V site and increased the negative charge of oxygen on Mo=O or V=O, which made the redox process easier [33]. For P-Mo-Nb/SiO2 and P-Mo-V-Nb/SiO2, the band near 920 cm−1 is characteristic of Mo5O14-type structure [34], this would be consistent with the promotion of such structure upon niobium doping [35].

2.2. Redox Properties

The reducibility of the samples was studied by H2-temperature-programmed reduction (TPR) (Figure 4). The position of the reduction peak represents the ease of catalyst reduction, and the lower the peak temperature, the easier it is to reduce, that is, the stronger redox ability. The peak area was the amount of H2 consumption by the reduction of the catalyst, representing the total amount of species that can be reduced. From the Figure 4a, the carrier silica has a very weak redox peak, and the V-Nb/SiO2 is observed a single peak near 440–748 °C, which is assigned to reduction of V5+ to V3+ in polyvanadates. The P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2 could be observed two reduction peaks obviously. The first H2 consumption peak (Figure 4c,e) at the range of 356–706 °C, which could be assigned to the overlapped reduction peaks of Mo6+ and V5+ in Mo-O-V structures to Mo4+ and V3+, respectively [35,36]. It could be assigned to reduction of Mo6+ to Mo5+ in Figure 4d. Compared to the V-Nb/SiO2, the redox ability of P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2 is improved as Tmax shifting to lower values. The addition of P and Mo improves the mobility of oxygen species, so that the transfer of lattice oxygen can be performed at a lower temperature.The two types of reduction peaks Mo6+ to Mo4+ and V5+ to V4+ could explain this phenomenon [36,37]. Besides, the addition of P promotes the active species dispersed as well as make the redox process easier [37,38]. The peak ranged at 650–900 °C undergo three stages upon reduction: (a) strongly dispersed monomeric octahedral coordinated Mo and polymolybdates Mo6+→Mo5+ (600–750 °C); (b) MoO3 ctystal and tetrahedral coordinated Mo5+→Mo4+(750–900 °C); and (c) Mo4+→Mo (>900 °C) [39].
The chemical state of Mo and V on the surface of the catalyst has a great influence on the catalytic activity of ethyl lactate to ethyl pyruvate. The energy spectra of V 2p3/2 and Mo 3d and the results of the fitting curves are shown in Figure 5A,B and Table 3. For comparing, the V-Nb/SiO2 and P-Mo-V-Nb/SiO2 has more V4+ species than others, which indicates the addition of niobium enhances the degree of reduction of vanadium species. At the same time, it can be observed that the Mo5+ species appear on P-Mo-Nb/SiO2. The results reveal the interaction of V-Nb and Mo-Nb can enhance the degree of reduction of V2O5 and MoO3, induced more oxygen vacancies. However, molybdenum in P-containing catalysts mainly exists as Mo6+ [22]. Partially reduced vanadium-molybdenum species can lead to more oxygen vacancies, which are beneficial to the improvement of catalytic performance due to vacancies facilitate electron transfer to enhance the adsorption of gaseous oxygen [40,41]. In our case, the V4+/(V4+ + V5+) ratio increased to 0.68 on P-Mo-V-Nb/SiO2, because Mo and Nb oxide components could increase the accessibility and reducibility of VOx species [21].
The spectra of O 1s in catalysts can be shown in Figure 5C. With the increase in the number of supported oxides, it can be observed that the mobility of lattice oxygen become better due to the reduced binding energy (535 eV to 532 eV), which significantly improves the catalyst activity for the conversion of ethyl lactate to ethyl pyruvate. Lattice oxygen on the catalyst surface is consumed in the oxidation reaction, then O2 in the gas phase is replenished into the vacant site to complete the cycle of oxygen species. Simultaneously the oxygen species, which are located at the surface defects, are more active and therefore more easily reduced [42,43]. The obtained dependence of conversion rate in ethyl lactate selective oxidation reaction on the concentration of oxygen is in accordance with a MvK redox mechanism, where the irreversible C–H bond activation steps occur on catalyst surface exposing predominantly hydroxyl groups [18]. The high binding energy like V-Nb/SiO2, the poor circulation of oxygen species during the reaction process leads to deep oxidation for lower selectivity of the target product.

2.3. Acidic Properties

The surface acidity intensity distribution of the catalyst samples measured by NH3-TPD is shown in Figure 6. Two weak peaks in range 100–300 °C and 600–800 °C, respectively are observed in the samples, indicative to two types of acidic sites on the surface of samples. However, the first peak may be caused by physical adsorption. When active species are loaded on the support surface, the two types of acidic sites disappear gradually in curve c–e, which implies that the addition of molybdenum and phosphorus could cover acidic sites [44]. This is related to the good dispersion of the active species on the surface of the support. At the same time, weak acidic sites and medium-strong acidic sites appeared on Figure 4c–e, indicating that the catalyst has a wide weak acid distribution on the surface. In short, the acidic sites on the surface have a significant effect on the oxidation of ethyl lactate to ethyl pyruvate. Therefore, the surface acidity of the catalyst is prone to adsorption of ethyl lactate without hydrolysis, and the weak acidic sites and medium-strong acidic sites can promote the desorption of ethyl pyruvate to reduce the rate of hydrolysis reaction, which is beneficial to the selectivity of ethyl pyruvate.
Figure 7 shows the FTIR spectra of pyridine adsorption of the catalysts, which is carried out to explore the nature of acidic sites and their effects on the selectivity of ethyl pyruvate. The band observed at 1450 cm−1 is characteristics of Lewis acidic sites, and 1488 cm−1 band is related to the coexistence of Brönsted and Lewis acidic sites [37]. The SiO2 support only has Lewis acidic sites in Figure 4a. It could be observed that Lewis acidic sites and the coexistence of Brönsted and Lewis acidic sites on V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2. However, there is no good difference in the acidity of each catalyst, and the effect on the reaction is not significant. The P-Mo-V-Nb/SiO2 with the best catalytic performance displayed very weak Lewis and Brönsted acidic sites, and the coexistence of Brönsted and Lewis acidic sites intensity were weaker than others and V-Nb/SiO2 catalyst activity after it. Therefore, we speculate that the surface acidity of the interaction between V and Nb is favorable for the adsorption of ethyl lactate for conversion.

2.4. Catalytic Performance

Figure 8 displays conversion of ethyl lactate and selectivity of ethyl pyruvate product under different reaction temperature. It could be observed that ethyl lactate conversion is positively correlated with reaction temperature. The P-Mo-V-Nb/SiO2 had a highest conversion, which the conversion of the EL to EP increased from 85% to >98% when the temperature increased from 220 °C to 280 °C.
As shown in Figure 8B, the ethyl pyruvate selectivity of P-Mo-V/SiO2 and P-Mo-Nb/SiO2 was highest at 240 °C and decreased gradually with increasing temperature. The P-Mo-V-Nb/SiO2 reached to 93% at 260 °C, while the selectivity of V-Nb/SiO2 began to decrease from 220 °C. We also plotted the yield of major products against ethyl lactate conversion over V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2 at optimized temperature. As shown in Figure 9A, it was observed the worst EP yield at 40% on V-Nb/SiO2 obviously, which could produce many by-products: ethanol, acetic acid, ethyl acetate, acetaldehyde, and carbon dioxide. In addition, the EP yield of P-Mo-V/SiO2 and P-Mo-Nb/SiO2 respectively reached to 49% and 56%, but the P-Mo-V-Nb/SiO2 increased to 90%. On the basis of these results, we propose a reaction network for the oxidation of ethyl lactate, involving oxidative dehydrogenation, hydrolysis and decarboxylation steps (Figure 9B). In Figure 9A, the total content of acetic acid, acetaldehyde and CO2 is about the same as ethanol in the reaction on V-Nb/SiO2, which confirms this reaction network from the side.
Subsequently, the stability of the catalyst samples was investigated while the temperature was fixed at the optimum results (260 °C). The effect of reaction time was studied at different time intervals (Figure 10). The conversion of EL was slightly reduced to 90% over 100 h, while the EP selectivity was lower than 80% from 16 h and continued to decrease to 67%. The result shows that P-Mo-V-Nb/SiO2 catalyst maintains a high activity for the oxidation of EL, however, the improvement of EP selectivity is further studied for industrial applications in the future.

2.5. Relationship between Catalyst Structure and Catalytic Performance

The P-Mo-V-Nb/SiO2 oxides catalyst showed higher ethyl lactate conversion and ethyl pyruvate selectivity in the oxidation of ethyl lactate compared to other catalysts. These could be clarified from the redox properties of the catalysts. H2-TPR result reveled that the total reduced amount of active components of the samples descended in the order of P-Mo-V-Nb/SiO2 > P-Mo-Nb/SiO2 > P-Mo-V/SiO2 > V-Nb/SiO2. It seemed that the catalytic activity was generally relevant to the rate of hydrogen consumption, which depends on the reducibility of the catalyst. On the other hand, according to the above XRD and Raman results, three highly dispersed phases over the catalysts could be observed: MoO3, V2O5, and Nb2O5, in which the cooperation of MoO3 and V2O5 had a good effect on the product selectivity and the Nb2O5 could increase the conversion of ethyl lactate significantly. Oligomeric MoOx species can produce by-product such as acetaldehyde [13].
The reactivity and reducibility (Table 4 and Figure 9) reveal that the EP yield is directly proportional to the H2 consumption of catalysts. The P-Mo-V-Nb/SiO2 H2 consumption is at the maximum among all tested samples (Table 4), which accounts for its highest EP yield among these samples. Moreover, the EP yield is improved with the increase of H2 consumption (Figure 11).
In addition, for V-Nb/SiO2 catalyst, a sharp decrease of the ethyl pyruvate selectivity could be observed, which was attributed to the ethyl pyruvate hydrolysis and decarboxylation to by-produces due to Lewis acidic sites of V2O5 over the catalyst.

3. Experimental

3.1. Preparation of the Catalyst

A series of SiO2 supported catalysts were prepared with an equal volume impregnation method. After sufficient impregnation, they were dried at 110 °C and calcined at 600 °C in a muffle furnace. The V-Nb/SiO2 dipping solution was obtained from 3.2 g of ammonium metavanadate dissolved by the oxalic acid solution which added 1.2 g of niobium ammonium oxalate. The P-Mo-V/SiO2 dipping solution was obtained from 3.2 g of ammonium metavanadate dissolved by the oxalic acid solution which added 20.6 g of ammonium heptamolybdate tetrahydrate and then 14.5 g of phosphoric acid dropwise. The P-Mo-Nb/SiO2 dipping solution came from 20.6 g of ammonium heptamolybdate tetrahydrate dissolved adding 1.2 g of niobium ammonium oxalate and 14.5 g of phosphoric acid dropwise. P-Mo-V-Nb/SiO2 dipping solution was prepared from 3.2 g of ammonium metavanadate dissolved by the oxalic acid solution adding 20.6 g of ammonium heptamolybdate tetrahydrate, 1.2 g of niobium ammonium oxalate, and 14.5 g of phosphoric acid dropwise.

3.2. Catalyst Characterization

The elemental analysis was determined by X-ray fluorescence spectroscopy (XRF) on ZSX Primus Ⅱ X-ray fluorescence analyzer (Rigaku) using rhodium target X-ray tube (40 kV and 10 mA).
Surface areas were measured with an SSA-6000 analyzer (Biaode Electronic Technology) with the multipoint Brunauer–Emmett–Teller (BET) sorption method.
Powder X-ray diffraction (XRD) was recorded on a Bruker diffractometer (Rigaku) using Cu-Kα radiation (40 kV and 40 mA). Each sample was prepared by gently grinding to powder, and placed into a quartz glass holder for measurement.
Raman spectra were obtained on an inVia (Renishaw) dispersive Raman microscope with a laser wavelength of 532 nm.
H2-TPR was performed in a ChemiSorb 2720 system (Micromeritics). A sample in a quartz tube was pretreated at 300 °C for 3 h under helium atmosphere. After it was cooled to room temperature, 10% hydrogen in argon substituted the helium until the signal baseline stable. The sample was then heated to 900 °C at a rate of 10 K·min−1. The exhaust gas was detected by a TCD detector after dehydrated by an isopropanol/liquid nitrogen cold trap.
X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 spectrometer (Thermo Fisher Scientific) using Al-Ka radiation with a pass energy of 30 eV. The binding energies were corrected with respect to C1s peak of contaminated carbon at 284.6 eV.
NH3-TPD was tested using a ChemBET Pulsar chemisorber (Quantachrome) to determine the acid property of catalyst surfaces. 100 mg of sample was swept in helium at 500 °C for one hour. After being cooled to 50 °C, the sample adsorped NH3 adequately. Then helium removed physically absorped NH3 at 50 °C. It was heated to 850 °C at a rate of 10 K·min−1 in a helium stream, and the signal was detected by a TCD detector after cooled to room temperature.
FT-IR spectra with pyridine adsorption were measured on a Nicolet 6700 spectrometer (Thermo Fisher Scientific) to characterize the acid properties of the catalysts. Before measurement, a sample was tableted as a self-supporting thin wafer. The sample was evacuated in the analysis cell at 400 °C for 2 h to remove water and impurities. After cooled to 25 °C, its infrared spectrum was collected as a background reference. Then, pyridine was introduced to the analysis cell. After about 30 min, the catalyst was saturated by pyridine. Subsequently, pyridine was desorped by evacuation at 25 °C, 150 °C and 400 °C respectively. The FT-IR spectrum of the samples were recorded with the same spectrometer.

3.3. Catalytic Test

The catalysts were evaluated in an atmospheric fixed-bed reactor (inner diameter 8 mm) at 220–280 °C. A catalyst (1 g of catalyst on 0.5 g of SiO2 support) was placed into the reactor center. Liquid ethyl lactate enters the preheater by constant flux pump and then gas ethyl lactate together with air (molar ratio of O2 to EL as 2:1) was introduced into the reactor. The liquid hourly space velocity (LHSV) of EL was 7.5 h−1. After the conversion and selectivity were steady, the product exhaust was cooled and collected in an ice water bath. The product was analyzed in a gas chromatograph equipped with an FID detector and a 30QC3/AC20-0.5 capillary column.

4. Conclusions

We developed a quaternary component P-Mo-V-Nb catalyst supported on SiO2 with an equal volume impregnation method for the selective oxidation of ethyl lactate to ethyl pyruvate. The XRD and N2 adsorption data between V-Nb and other three phosphorous catalysts suggested high dispersion of active sites under the action of phosphorus. As a result, the highly dispersive sites separated active species, suppressing over oxidation and improving the selectivity. XPS and Raman spectra indicated the existence of MoO3 to provide higher oxidation for catalyst. The interaction of V and Nb enhanced the degree of reduction of V2O5, which may induce more oxygen vacancies, facilitating the transfer of gaseous oxygen to lattice oxygen.
According to the Mars-van Krevelen mechanism, the mobility of lattice oxygen is crucial for the selective oxidative dehydrogenation. Compared with binary V-Nb and ternary P-Mo-V and P-Mo-Nb catalysts, the P-Mo-V-Nb catalyst exhibited the best oxygen mobility according to the H2-TPR and XPS results. Moreover, the weak acidity of catalyst surface is favorable to avoid the decarboxylation reaction to target a high selectivity of EP. Therefore, the P-Mo-V-Nb/SiO2 catalyst obtained the maximum yield of 91.8% with a selectivity of 93.8% and a conversion of 99.0% simultaneously at 280 °C. Further study should focus the long-period stability of this P-Mo-V-Nb/SiO2 catalyst. The proportion of the four elements can be optimized.

Author Contributions

Conceptualization, X.L.; validation, X.L. and C.L.; formal analysis, X.L.; investigation, X.L. and C.L.; resources, Q.W., X.W., C.P., R.L. and C.Y.; writing—original draft preparation, X.L.; writing—review and editing, C.Y. and Q.W.; supervision, Q.W., X.W., R.L. and C.Y.; project administration, C.Y. and C.P.; funding acquisition, C.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ji Nan Science & Technology Bureau (no. 201602103), Natural Science Fund of Shandong Province (ZR2016YL010) and the Key R&D Program of Shandong Province (GG201809160031).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Dusselier, M.; Van Wouwe, P.; Dewaele, A.; Makshina, E.; Sels, B.F. Lactic acid as a platform chemical in the biobased economy: the role of chemocatalysis. Energy Environ. Sci. 2013, 6, 1415. [Google Scholar] [CrossRef]
  2. Murlykina, M.V.; Sakhno, Y.I.; Desenko, S.M.; Shishkina, S.V.; Shishkin, O.V.; Sysoiev, D.O.; Kornet, M.N.; Schols, D.; Goeman, J.L.; Van der Eycken, J.; et al. Study of the Chemoselectivity of Multicomponent Heterocyclizations Involving 3-Amino-1,2,4-triazole and Pyruvic Acids as Key Reagents, and Biological Activity of the Reaction Products. Eur. J. Org. Chem. 2015, 2015, 4481–4492. [Google Scholar] [CrossRef]
  3. Dennig, A.; Busto, E.; Kroutil, W.; Faber, K. Biocatalytic One-Pot Synthesis of l-Tyrosine Derivatives from Monosubstituted Benzenes, Pyruvate, and Ammonia. ACS Catal. 2015, 5, 1–5. [Google Scholar] [CrossRef]
  4. Muller, A.J.; Duhadaway, J.B.; Daniel, J.; Peter, C.; Richard, M.; Prendergast, G.C. Immunotherapeutic suppression of indoleamine 2,3-dioxygenase and tumor growth with ethyl pyruvate. Cancer Res. 2010, 70, 1845–1853. [Google Scholar] [CrossRef] [Green Version]
  5. Sugiyama, S.; Fukunaga, S.; Ito, K.; Ohigashi, S.; Hayashi, H. Catalysts for vapor-phase dehydration of ethylene glycol and their application to pyruvic acid synthesis. J. Catal. 1991, 129, 12–18. [Google Scholar] [CrossRef]
  6. Shigeru Sugiyama, S.F.K.K.H.H. Catalytic Conversion of Diethyl Tartrate into Pyruvate over Silica-Supported Potassium Disulfate. Bull. Chem. Soc. JPN 2006, 65, 2083–2085. [Google Scholar] [CrossRef]
  7. Miyazaki, M.; Shibue, M.; Ogino, K.; Nakamura, H.; Maeda, H. ChemInform Abstract: Enzymatic Synthesis of Pyruvic Acid from Acetaldehyde and Carbon Dioxide. Chem. Commun. 2001, 33, 1800–1801. [Google Scholar] [CrossRef]
  8. MIYATA; YONEHARA. Improvement of fermentative production of pyruvate from glucose by Torulopsis glabrata IFO 0005. J. Ferment. Bioeng. 1996, 82, 475–479. [Google Scholar] [CrossRef]
  9. Miyata, R.; Yonehara, T. Breeding of high-pyruvate-producing Torulopsis glabrata and amino acid auxotrophic mutants. J. Biosci. Bioeng. 2000, 90, 137–141. [Google Scholar] [CrossRef]
  10. Sugiyama, S.; Shigemoto, N.; Masaoka, N.; Suetoh, S.; Hayashi, H. Vapor-Phase Oxidation of Ethyl Lactate to Pyruvate over Various Oxide Catalysts. Cheminform 1993, 66, 1542–1547. [Google Scholar] [CrossRef]
  11. Ai, M. Catalytic activity of palladium-doped iron phosphate in the oxidative dehydrogenation of lactic acid to pyruvic acid. Appl. Catal. A Gen. 2002, 232, 1–6. [Google Scholar] [CrossRef]
  12. Chen, Z.; Tao, W.; Ding, Y. Oxidative dehydrogenation of lactic acid to pyruvic acid over Pb-Pt bimetallic supported on carbon materials. Appl. Catal. A Gen. 2017, 533, 59–65. [Google Scholar]
  13. Liu, K.; Huang, X.; Pidko, E.A.; Hensen, E.J.M. MoO3–TiO2 synergy in oxidative dehydrogenation of lactic acid to pyruvic acid. Green Chem. 2017, 19, 3014–3022. [Google Scholar] [CrossRef] [Green Version]
  14. Lomate, S.; Bonnotte, T.; Paul, S.; Dumeignil, F.; Katryniok, B. Synthesis of pyruvic acid by vapour phase catalytic oxidative dehydrogenation of lactic acid. J. Mol. Catal. A Chem. 2013, 377, 123–128. [Google Scholar] [CrossRef]
  15. Ramos-Fernandez, E.V.; Geels, N.J.; Shiju, N.R.; Rothenberg, G. Titania-catalysed oxidative dehydrogenation of ethyl lactate: effective yet selective free-radical oxidation. Green Chem. 2014, 16, 3358–3363. [Google Scholar] [CrossRef] [Green Version]
  16. Zhang, W.; Innocenti, G.; Oulego, P.; Gitis, V.; Wu, H.; Ensing, B.; Cavani, F.; Rothenberg, G.; Shiju, N.R. Highly Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate Catalyzed by Mesoporous Vanadia-Titania. ACS Catal. 2018, 8, 2365–2374. [Google Scholar] [CrossRef] [Green Version]
  17. Lu, T.; Zou, J.; Zhan, Y.; Yang, X.; Wen, Y.; Wang, X.; Zhou, L.; Xu, J. Highly Efficient Oxidation of Ethyl Lactate to Ethyl Pyruvate Catalyzed by TS-1 Under Mild Conditions. ACS Catal. 2018, 8, 1287–1296. [Google Scholar] [CrossRef]
  18. Zhao, X.; Zhang, C.; Xu, C.; Li, H.; Huang, H.; Song, L.; Li, X. Kinetics study for the oxidative dehydrogenation of ethyl lactate to ethyl pyruvate over MoVNbOx based catalysts. Chem. Eng. J. 2016, 296, 217–224. [Google Scholar] [CrossRef]
  19. Jo, B.Y.; Kum, S.S.; Moon, S.H. Performance of WOx-added Mo–V–Te–Nb–O catalysts in the partial oxidation of propane to acrylic acid. Appl. Catal. A Gen. 2010, 378, 76–82. [Google Scholar] [CrossRef]
  20. Ramli, I.; Botella, P.; Ivars, F.; Pei Meng, W.; Zawawi, S.M.M.; Ahangar, H.A.; Hernández, S.; Nieto, J.M.L. Reflux method as a novel route for the synthesis of MoVTeNbOx catalysts for selective oxidation of propane to acrylic acid. J. Mole. Catal. A Chem. 2011, 342–343, 50–57. [Google Scholar] [CrossRef]
  21. Botella, P.; Dejoz, A.; Lopeznieto, J.; Concepcion, P.; Vazquez, M. Selective oxidative dehydrogenation of ethane over MoVSbO mixed oxide catalysts. Appl. Catal. A Gen. 2006, 298, 16–23. [Google Scholar] [CrossRef]
  22. Baek, M.; Lee, J.K.; Kang, H.J.; Kwon, B.J.; Lee, J.H.; Song, I.K. Ammoxidation of propane to acrylonitrile over Mo-V-P-Oy/Al2O3 catalysts: Effect of phosphorus content. Catal. Commun. 2017, 92, 27–30. [Google Scholar] [CrossRef]
  23. Shiju, N.R.; Guliants, V.V. Microwave-assisted hydrothermal synthesis of monophasic Mo-V-Te-Nb-O mixed oxide catalyst for the selective ammoxidation of propane. Chemphyschem 2007, 8, 1615–1617. [Google Scholar] [CrossRef] [PubMed]
  24. Li, X.; Iglesia, E. Support and promoter effects in the selective oxidation of ethane to acetic acid catalyzed by Mo-V-Nb oxides. Appl. Catal. A Gen. 2008, 334, 339–347. [Google Scholar] [CrossRef]
  25. Li, X.; Iglesia, E. Synergistic Effects of TiO2 and Palladium-Based Cocatalysts on the Selective Oxidation of Ethene to Acetic Acid on Mo–V–Nb Oxide Domains. Angew. Chem. 2007, 119, 8803–8806. [Google Scholar] [CrossRef]
  26. Li, X.; Iglesia, E. Selective catalytic oxidation of ethanol to acetic acid on dispersed Mo-V-Nb mixed oxides. Chemistry 2007, 13, 9324–9330. [Google Scholar] [CrossRef]
  27. Cassiers, K.; L, T.; Mathieu, M.; Benjelloun, M.; Schrijnemakers, K.; van der Voort, P.; Cool, P.; Vansant, E.F. A Detailed Study of Thermal, Hydrothermal, and Mechanical Stabilities of a Wide Range of Surfactant Assembled Mesoporous Silicas. Chem. Mater. 2002, 14, 2317–2324. [Google Scholar] [CrossRef]
  28. Fang, K.; Liu, L.; Zhang, M.; Zhao, L.; Zhou, J.; Li, W.; Mu, X.; Yang, C. Synthesis of Three-Dimensionally Ordered Macroporous NiCe Catalysts for Oxidative Dehydrogenation of Propane to Propene. Catalysts 2018, 8, 19. [Google Scholar] [CrossRef] [Green Version]
  29. Jeziorowski, H.K. Raman spectra of cobalt molybdenum oxide supported on silica. J. Phys. Chem. 1980, 84, 1825–1829. [Google Scholar] [CrossRef]
  30. Pei, S.; Yue, B.; Qian, L.; Yan, S.; Cheng, J.; Zhou, Y.; Xie, S.; He, H. Preparation and characterization of P–Mo–V mixed oxide-incorporating mesoporous silica catalysts for selective oxidation of methane to formaldehyde. Appl. Catal. A Gen. 2007, 329, 148–155. [Google Scholar] [CrossRef]
  31. Kim, T.; Wachs, I. CH3OH oxidation over well-defined supported V2O5/Al2O3 catalysts: Influence of vanadium oxide loading and surface vanadium–oxygen functionalities. J. Catal. 2008, 255, 197–205. [Google Scholar] [CrossRef]
  32. Zhao, C.; Wachs, I. Selective oxidation of propylene over model supported V2O5 catalysts: Influence of surface vanadia coverage and oxide support. J. Catal. 2008, 257, 181–189. [Google Scholar] [CrossRef]
  33. Grasselli, R.K.; Lugmair, C.G.; Volpe Jr, A.F.; Andersson, A.; Burrington, J.D. Enhancement of acrylic acid yields in propane and propylene oxidation by selective P Doping of MoV(Nb)TeO-based M1 and M2 catalysts. Catal. Today 2010, 157, 33–38. [Google Scholar] [CrossRef]
  34. Jeziorowski, H.; Knoezinger, H. Raman and ultraviolet spectroscopic characterization of molybdena on alumina catalysts. Cheminform 1979, 10, 1166–1173. [Google Scholar] [CrossRef]
  35. Yang, S.; Iglesia, E.; Bell, A.T. Oxidative dehydrogenation of propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: structural characterization and catalytic function. J. Phys. Chem. B 2005, 109, 8987. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  36. Solsona, B.; Dejoz, A.; Garcia, T.; Concepción, P.; Nieto, J.M.L.; Vázquez, M.I.; Navarro, M.T. Molybdenum–vanadium supported on mesoporous alumina catalysts for the oxidative dehydrogenation of ethane. Catal. Today 2006, 117, 228–233. [Google Scholar] [CrossRef]
  37. Wang, Y.; Ma, F.; Chen, S.; Chen, F.; Lu, W.M. Performance of Mo-V-Te-P catalysts supported on the SiC for propane selective oxidation to acrolein. Chin. Chem. Lett. 2011, 22, 1321–1325. [Google Scholar] [CrossRef]
  38. Voge, H.H.; Adams, C.R. Catalytic Oxidation of Olefins. Adv. Catal. 1967, 17, 151–221. [Google Scholar]
  39. Thomas, R.; Oers, E.M.V.; Beer, V.H.J.D.; Medema, J.; Moulijn, J.A. Characterization of γ-alumina-supported Molybdenum oxide and tungsten oxide; reducibility of the oxidic state versus hydrodesulfurization activity of the sulfided state. J. Catal. 1984, 84, 275–287. [Google Scholar] [CrossRef] [Green Version]
  40. Pernicone, N.; Lazzerin, F.; Liberti, G.; Lanzavecchia, G. On the mechanism of CH3OH oxidation to CH2O over MoO3Fe2(MoO4)3 catalyst. J. Catal. 1969, 14, 293–302. [Google Scholar] [CrossRef]
  41. Li, J.; Zhu, H.; Razzaq, R.; Zhu, M.; Li, C.; Li, Z. Effect of zirconium addition on the structure and properties of CuO/CeO2 catalysts for high-temperature water–gas shift in an IGCC system. Int. J. Hydrogen Energ. 2012, 37, 15914–15924. [Google Scholar]
  42. Kang, M.; Park, E.D.; Kim, J.M.; Yie, J.E. Manganese oxide catalysts for NOx reduction with NH3 at low temperatures. Appl. Catal. A Gen. 2007, 327, 261–269. [Google Scholar] [CrossRef]
  43. Lee, K.J.; Kumar, P.A.; Maqbool, M.S.; Rao, K.N.; Song, K.H.; Ha, H.P. Ceria added Sb-V2O5/TiO2 catalysts for low temperature NH3 SCR: Physico-chemical properties and catalytic activity. Appl. Catal. B Environl. 2013, 142–143, 705–717. [Google Scholar] [CrossRef]
  44. Meng, Y.; Wang, T.; Chen, S.; Zhao, Y.; Ma, X.; Gong, J. Selective oxidation of methanol to dimethoxymethane on V2O5–MoO3/γ-Al2O3 catalysts. Appl. Catal. B Environ. 2014, 160–161, 161–172. [Google Scholar] [CrossRef]
Catalysts 10 00197 g001 550
Figure 1. (A) N2 adsorption-desorption isomers of samples: SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2; and (B) Barrett–Joyner–Halenda (BJH) measurements of SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2.
Figure 1. (A) N2 adsorption-desorption isomers of samples: SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2; and (B) Barrett–Joyner–Halenda (BJH) measurements of SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2.
Catalysts 10 00197 g001
Catalysts 10 00197 g002 550
Figure 2. X-ray diffraction (XRD) patterns of a–e: SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Figure 2. X-ray diffraction (XRD) patterns of a–e: SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Catalysts 10 00197 g002
Catalysts 10 00197 g003 550
Figure 3. Raman spectra of (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2.
Figure 3. Raman spectra of (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, and P-Mo-V-Nb/SiO2.
Catalysts 10 00197 g003
Catalysts 10 00197 g004 550
Figure 4. H2-temperature-programmed reduction (TPR) patterns of the samples (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Figure 4. H2-temperature-programmed reduction (TPR) patterns of the samples (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Catalysts 10 00197 g004
Catalysts 10 00197 g005 550
Figure 5. XPS spectra and peak fitting results of (A) Mo 3d (B) V 2p and (C) O 1s signals of the samples (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Figure 5. XPS spectra and peak fitting results of (A) Mo 3d (B) V 2p and (C) O 1s signals of the samples (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Catalysts 10 00197 g005
Catalysts 10 00197 g006 550
Figure 6. NH3-temperature-programmed desorption (TPD) patterns of the samples of (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Figure 6. NH3-temperature-programmed desorption (TPD) patterns of the samples of (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2.
Catalysts 10 00197 g006
Catalysts 10 00197 g007 550
Figure 7. Pyridine-FTIR spectra of (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2. (A): 25 °C; (B):150 °C; (C):400 °C
Figure 7. Pyridine-FTIR spectra of (a–e): SiO2, V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2. (A): 25 °C; (B):150 °C; (C):400 °C
Catalysts 10 00197 g007
Catalysts 10 00197 g008 550
Figure 8. Changing curves of the catalytic performance as a function of temperature over the catalysts. (A): the ethyl lactate conversion; (B): the ethyl pyruvate selectivity.
Figure 8. Changing curves of the catalytic performance as a function of temperature over the catalysts. (A): the ethyl lactate conversion; (B): the ethyl pyruvate selectivity.
Catalysts 10 00197 g008
Catalysts 10 00197 g009 550
Figure 9. (A): An overview of product yield at 260 °C reaction temperature over the catalysts of: V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2a. (B): Proposed reaction pathway for the oxidation of ethyl lactate.
Figure 9. (A): An overview of product yield at 260 °C reaction temperature over the catalysts of: V-Nb/SiO2, P-Mo-V/SiO2, P-Mo-Nb/SiO2, P-Mo-V-Nb/SiO2a. (B): Proposed reaction pathway for the oxidation of ethyl lactate.
Catalysts 10 00197 g009
Catalysts 10 00197 g010 550
Figure 10. Dependence of EL conversion and EP selectivity on time stream for P-Mo-V-Nb/SiO2 catalyst at 260 °C.
Figure 10. Dependence of EL conversion and EP selectivity on time stream for P-Mo-V-Nb/SiO2 catalyst at 260 °C.
Catalysts 10 00197 g010
Catalysts 10 00197 g011 550
Figure 11. Correlation between reducibility and the EP yield of the catalysts.
Figure 11. Correlation between reducibility and the EP yield of the catalysts.
Catalysts 10 00197 g011
Table
Table 1. Molar ratio of P/Si, Mo/Si, V/Si, and Nb/Si by XRF
Table 1. Molar ratio of P/Si, Mo/Si, V/Si, and Nb/Si by XRF
SamplesMolar Ratio
P/SiMo/SiV/SiNb/Si
SiO2----
V-Nb/SiO2--0.170.02
P-Mo-Nb/SiO20.910.750.18-
P-Mo-V/SiO20.890.73-0.02
P-Mo-V-Nb/SiO20.880.740.150.03
Table
Table 2. Textural parameters of the samples
Table 2. Textural parameters of the samples
SamplesSBET (m2/g) aVP (cm3/g) aDP (nm) a
SiO22081.211.6
V-Nb/SiO21271.218.3
P-Mo-Nb/SiO217.20.218.3
P-Mo-V/SiO29.10.117.1
P-Mo-V-Nb/SiO29.70.115.1
a SBET: Brunauer–Emmett–Teller (BET) surface area; Dp: pore diameter; Vp: volume.
Table
Table 3. Relative chemical composition of catalysts calculated from XPS results
Table 3. Relative chemical composition of catalysts calculated from XPS results
SamplesSurface Atomic Ratio (%)Mo5+/(Mo5++Mo6+)V4+/(V4++V5+)
Mo/SiV/Si
SiO2----
V-Nb/SiO2-0.02-0.24
P-Mo-V/SiO20.380.05-0.48
P-Mo-Nb/SiO20.23-0.33-
P-Mo-V-Nb/SiO20.710.09-0.68
Table
Table 4. Hydrogen consumption of the temperature-programmed reduction (TPR) peaks for the various samples
Table 4. Hydrogen consumption of the temperature-programmed reduction (TPR) peaks for the various samples
SamplesH2 Consumption (mmol·g−1) (Temperature before 700 °C)
SiO20.04
V-Nb/SiO21.06
P-Mo-V/SiO21.53
P-Mo-Nb/SiO21.48
P-Mo-V-Nb/SiO21.72

Share and Cite

MDPI and ACS Style

Liu, X.; Wang, Q.; Liu, C.; Wang, X.; Peng, C.; Liu, R.; Yang, C. High-Performance Vapor-Phase Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate over SiO2 Supported PMoVNb Oxides. Catalysts 2020, 10, 197. https://doi.org/10.3390/catal10020197

AMA Style

Liu X, Wang Q, Liu C, Wang X, Peng C, Liu R, Yang C. High-Performance Vapor-Phase Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate over SiO2 Supported PMoVNb Oxides. Catalysts. 2020; 10(2):197. https://doi.org/10.3390/catal10020197

Chicago/Turabian Style

Liu, Xiaoyu, Qi Wang, Chenghao Liu, Xiaolong Wang, Cuina Peng, Rong Liu, and Cheng Yang. 2020. "High-Performance Vapor-Phase Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate over SiO2 Supported PMoVNb Oxides" Catalysts 10, no. 2: 197. https://doi.org/10.3390/catal10020197

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop