Xu Xin; Liu Wenge; Qi Jian; Yu Lei; Han Jiaye; Li Zhi

(1. China Coal Information Institute, Beijing 100029;2. State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190;3. University of Chinese Academy of Sciences, Beijing 100049)

Abstract: La-Al2O3 and Ce-Al2O3 binary monolithic column materials with dual functions of carrier and promoter were prepared in one-step by the sol-gel method. Using the as-synthesized binary monolithic column materials as carrier, the 9%Mn/M-Al2O3 (M=La/Ce) catalyst was prepared by the impregnation method. The binary carrier and catalyst were characterized by BET, SEM, XRD, H2-TPR, and O2-TPD techniques, and the methane combustion performance was tested in a micro fixed-bed reactor. Results showed that the Mn-based binary carrier catalyst with dual functions of carrier and promoter had higher catalytic activity than that with single alumina acting as the carrier. The catalyst activity gradually increased with an increasing content of La, while there was no signi ficant effect with the increase in Ce content. When the ratio of La content in La-Al2O3 is 11%, 1% of methane contained in the ventilation air can be completely converted over the 9%Mn/11% La-Al2O3 catalyst at a temperature as low as 550 °C.

Key words: binary carrier; catalytic combustion; VAM; Mn/La-Al2O3; Mn/Ce-Al2O3

1 Introduction

A large amount of ventilation gas is discharged during coal mining, and the main component is methane.Methane has become the second source of greenhouse gas emissions after CO2, about 28 times the CO2amount at the 100-year scale. Therefore, it is of great signi ficance to realize methane catalytic combustion in low temperature environment for controlling greenhouse gas emission[1-3].However, methane is stable and difficult to be activated.When the methane concentration is lower than 3%,the traditional flame cannot oxidize it; thus a high temperature is required for catalytic combustion, so there is a high demand for catalyst with good activity.

The active components of catalysts can be generally divided into noble metals and non-noble metals for methane catalytic combustion. Although noble metals have good low-temperature activity, they are expensive and have poor stability, which can hardly satisfy the largescale use in industry. Therefore, it is of great practical value to replace noble metals with non-noble metals catalyst such as Co, Mn, Cu, etc[4-7]. For example, Hu, et al.[8]studied the effects of different Mn loading and carriers on methane catalytic combustion. The results showed that 15% Mn/Al2O3catalyst exhibited better catalytic activity for methane oxidation, and Mn4+species played a major role. Choudhary, et al.[9]thought that Mn2O3-ZrO2catalyst was the most promising catalyst for methane, and the experimental results showed that when the atomic ratio of Mn/ZrO2is 0.5, the catalyst had the highest activity. In addition to the active components, carriers and promoters in a certain ratio range can strengthen and improve the performance of the catalyst. In recent years,alumina monolithic columns with multi-stage channels have attracted much attention due to their unique pore structure, high specific surface area, and mechanical strength, which are regarded as one of the most promising catalyst carrier. Guo, et al.[10]prepared Al2O3-TiO2binary monolithic column material through the sol-gel process accompanied by phase separation. The results showed that the pore size of the material is between 2—3 μm, and the porosity is over 60%. It has a typical macroporous structure and smooth skeleton and therefore has a great potential application prospect in the fields of catalysis, adsorption and other scienti fic fields. Li, et al.[11]prepared Al2O3-SiO2binary monolithic column material through the sol-gel process accompanied by phase separation. Their material is lamellar and porous with distinct macroporous structure and slender skeleton structure, which can also provide the basis for binary monolithic column materials in the field of catalysis. It is proved that La and Ce additives can improve the performance of carriers and catalysts because of their fine physicochemical properties. Since La2O3is one of the most commonly used alumina stabilizers, it will inhibit the sintering of Al2O3carrier at high temperature to maintain the catalytic activity of the catalyst in the process of methane oxidation[12]; CeO2can provide lattice oxygen to the active component through Ce4+/Ce3+ion pair oxidation-reduction cycle, which improves the electron transfer between active sites, thus enhancing the activity of the catalyst.

In this paper, we designed and synthesized a kind of multistage porous La-Al2O3and Ce-Al2O3monolithic column materials with dual functions of carrier and promoter acting as catalyst carrier, and selected environmentalfriendly, inexpensive non-noble metal serving as active components. Mn/M-Al2O3(M=La/Ce) binary carrier catalyst was prepared by the impregnation method. The effect of catalytic performance on low concentration methane combustion was studied and the influence of addition of La2O3, CeO2was discussed. The research work in this paper provides design principles for the systematic development of efficient non-noble metal ventilation gas combustion catalyst for engineering application.

2 Experimental

2.1 Catalyst preparation

Carrier preparation：Initial content of binary carrier preparation is shown in Table 1. At first, PEO (Mv=1×106)was dissolved in ethanol (95%) at 35 °C followed by the slow addition of deionized water under magnetic stirring (at 300 r/min). After the PEO was completely dissolved, the solution was transferred into an ice bath system and the stirring speed was increased to 400 r/min. 1, 3, 7, and 11 mol% of the promoter metal salt Ce(NO3)·6H2O, respectively, LaCl3·7H2O, and the carrier metal salt AlCl3·6H2O were then added in sequence. After the metal salt was completely dissolved,PO was added, while the stirring speed was continuously increased to 500 r/min and stirring did not terminated until the separation layer interface disappeared. The assynthesized sol was put into a 10-ml centrifuge tube and was subject to aging for 24 h at 40 °C followed by drying for more than 7 days at 40 °C. The dried gel obtained thereby was then calcined at 800 °C for 2 h.Then the catalyst carrier was prepared. Carriers with different initial metal additions were named as 1%, 3%,7%, and 11% La-Al and Ce-Al, respectively.

Active component loading: A certain amount of carrier was impregnated into the manganese nitrate solution to make an active component loading of 9% in a propriety container. Then the container was placed in a boiling water bath for 1 h prior to being stored in a dark place at room temperature for 24 h. After being filtered by suction, the filter cake product was dried at 40 °C for 6 h and calcined at 500 °C for 3 h. The catalyst was obtained,named as Mn/La-Al and Mn/Ce-Al, respectively.

Table 1 Initial content of binary carrier preparation m, g

2.2 Catalyst characterization

A scanning electron microscope (SEM: HITACHI S-4800,Japan) was employed to examine the morphology of the samples. The X-ray diffraction (XRD) patterns were obtained with a Bruker D8-Advance diffractometer using Cu Kα radiation. The speci fic surface area and porosity of the samples was characterized by N2isotherms measured using a JW-BK200 instrument. The H2temperatureprogrammed reduction (H2-TPR) and O2programmedtemperature desorption (O2-TPD) tests were carried out on an AutoChem 1 II 2920 chemisorption analyzer(Micromeritics, US).

2.3 Catalytic performance measurements

The catalytic performance for CH4combustion was tested in a fixed-bed quartz reactor with a diameter of 4 mm,in which 0.1 g of catalyst was packed. The reaction temperature was controlled by an intelligent temperature regulator. The low concentration methane gas containing 1 vol.% of CH4in air was supplied to the catalyst bed through a mass flow controller at a gas hourly space velocity (GHSV) of 6 000 mL/(h·g). The gas compositions were also analyzed by a GC-7890 gas chromatograph equipped with a FID and TCD detectors. The conversion of CH4was calculated by the following equation: X = (Cin- Cout) / Cin× 100%, in which X presents CH4conversion rate, Cinrepresents initial CH4concentration at the inlet,and Coutrepresents the CH4concentration at the outlet.

3 Results and Discussion

3.1 Binary carrier characterizations

The La-Al2O3and Ce-Al2O3binary monolithic materials with dual function of carrier and promoter were prepared in one step by the sol-gel method. The physical and chemical properties of binary monolithic material were illustrated by using 1% and 11% La-Al and Ce-Al binary carrier as examples. N2adsorption and desorption curves of the samples are shown in Figure 1. La-Al and Ce-Al binary carriers had the typical IV and H1 hysteresis loops,indicating the typical mesoporous structure in the binary carriers. Binary carrier pore size distribution curves are shown in the inset picture of Figure 1, denoting that the pore size distribution of binary carrier was uniform.Combined with Table 2, the most probable pore size of the binary carrier was 11.34—19.35 nm, and the pore volume was 0.31—0.49 cm3/g. It can be seen from the speci fic surface area data that when the content of La and Ce was 1%, the speci fic surface area of binary carrier had no obvious change. The specific surface area of binary carrier decreased obviously when the content of La and Ce was 11%, which might be caused by the blockage of partial pore structure.

Figure 1 Adsorption-desorption curves and pore diameter distribution of binary carriers

Table 2 Samples and their pore structure characters

SEM images of Al2O3carrier (a), 1% La-Al binary carrier(b), 11% La-Al binary carrier(c), 1% Ce-Al binary carrier(d), and 11% Ce-Al binary carrier (e) are shown in Figure 2. It can be seen that the binary carriers had macroporous crosslinking structure similar to alumina monolithic column, and the pore size was about 1 µm. The SEM images of the binary carrier with 1% La were consistent with those of Al2O3carrier. When the doping amount of La was 11%, the carrier skeleton became thinner, and a lot of nano-spheres were formed in the framework and pore channels. These nano-spheres were not generated by the separation of lanthanum hydroxide species, but by the secondary phase separation[13], which changed the microstructure of the carrier and led to the decrease of specific surface area. According to the SEM images of Ce-Al binary carrier, Ce doping led to a uniform coating of nanoparticles on the original carrier framework, and 11% Ce doping could lead to the growth of nanoparticles,which changed the carrier microstructure and resulted in a decreased speci fic surface area.

Figure 3 shows the XRD patterns of La-Al and Ce-Al binary carrier. It can be seen from the XRD pattern(Figure 3a) that with the addition of La, the characteristic peak of the original Al2O3gradually weakened, and the characteristic peak of LaAlO3was not obvious, because LaAlO3was highly dispersed. It can be seen from the XRD pattern (Figure 3b) that the characteristic peak of the original Al2O3decreased with the increase of Ce content. When the content of Ce was 1%, it was basically consistent with the original Al2O3, and no new crystal phase appeared. It is possible that the amount of Ce doping was too little to reach the detection limit of the instrument. When the content of Ce reached 11%, the typical diffraction peaks of CeO2appeared at 2θ= 28.5°and 47.5° and the growth trend of CeO2characteristic diffraction peak appeared with the increase of Ce content.

Figure 2 SEM patterns of binary carriers

Figure 3 XRD patterns of binary carriers·—Al2O3; ♦—CeO2

3.2 Physical characterization of Mn based binary carrier catalyst

On the basis of obtaining binary carrier with special three-dimensional cross-linked structure of macroporous multi-stage pores, the Mn based binary carrier catalyst was synthesized by the impregnation method. The SEM images of Mn/Al2O3catalyst (a), Mn/1% La-Al catalyst(b), Mn/11% La-Al catalyst (c), Mn/1% Ce-Al catalyst(d), and Mn/11% Ce-Al catalyst (e) are shown in Figure 4. It can be seen from Figure 4b and Figure 4c that the active component Mn was agglomerated, but its carrier skeleton gradually became thinner with the increase of La content, thus providing sufficient space for pore growth, and improving the mass and heat transfer rate and the area of catalytic reaction sites. It can be seen from Figure 4d and Figure 4e that the active component Mn also agglomerated, resulting in the decrease of catalytic activity area. With the increase of Ce content, the skeleton of the carrier became thicker, thus blocking a part of the pores and reducing the mass and heat transfer.

Figure 4 SEM patterns of Mn-based binary carrier catalysts

Figure 5 XRD patterns of Mn-based binary carrier catalysts

The XRD diagram of Mn based binary carrier catalyst is presented in Figure 5. It can be seen from the XRD diagram (Figure 5a) of Mn/La-Al binary carrier catalyst that the characteristic peak of MnO2gradually weakened with the addition of La. The characteristic peaks of LaAlO3appeared at 2θ = 23.5° and 33.5°, and La2O3did not react with the main components of active components. Combined with Figure 3a, it can be seen that the La-Al binary carrier had the characteristic peak of LaAlO3. It can be seen from the XRD diagram of Mn/Ce-Al (Figure 5b) that the characteristic peak of MnO2decreased with the increase of Ce content. When the content of Ce reached 11%, the typical diffraction peaks of CeO2appeared at 2θ = 28.5° and 47.5°, respectively.With the increase of Ce content, CeO2characteristic diffraction peaks showed an obvious growth trend,which was consistent with the characterization results of Ce-Al binary carrier in Figure 4b. However, CeO2did not react with the main components of the active component.

3.3 H2-TPR properties of Mn based binary carrier catalyst

Combined with the above catalyst characterization, we selected Mn/11%La-Al and Mn/1%Ce-Al catalysts to study their reduction behaviors by H2-TPR testing in the temperature range of 100—700 °C. In Figure 6, it can be clearly observed that H2-TPR profile of Mn/11%La-Al catalyst showed three reduction peaks, which might be ascribed to a three-step reduction via MnO2to MnO.The peak at 344 °C should be ascribed to the reduction step of MnO2to Mn2O3, and the peak at 416 °C should be ascribed to the reduction step of Mn2O3to Mn3O4. The peak at 538 °C might be ascribed to the reduction step of Mn3O4to MnO[14-15]. The H2-TPR profile of Mn/1%Ce-Al showed two reduction peaks, which might be ascribed to a two-step reduction of MnO2to Mn3O4. The peak at 350 °C should be ascribed to the reduction step of MnO2to Mn2O3, and the peak at 412 °C should be ascribed to the reduction step of Mn2O3to Mn3O4. Both Mn/11%La-Al and Mn/1%Ce-Al catalysts exhibited excellent reduction performance.

Figure 6 H2-TPR pro files of Mn/11%La-Al and Mn/1%Ce-Al catalysts

3.4 O2-TPD properties of Mn based binary carrier catalyst

Figure 7 shows the O2-TPD pro files of Mn/11%La-Al and Mn/1%Ce-Al catalysts. In fact, the oxygen desorption property is important for the study on the mobility of surface active oxygen species. Lai, et al.[16]proved that the peak below 400 °C should be classi fied as the desorption of surface adsorbed oxygen and partial lattice oxygen species, whereas the desorption peaks in the temperature range of 400—600 °C might be attributed to the subsurface lattice oxygen species and the one at higher temperatures might be ascribed to the bulk lattice oxygen species. In Figure 7, it can be observed that there were three oxygen desorption peaks at 374 °C,529 °C, and 675 °C for Mn/11%La-Al catalyst, which could be assigned to the desorption of surface adsorbed oxygen, subsurface lattice oxygen, and bulk oxygen.The Mn/1%Ce-Al had two desorption peaks at 517 °C and 608 °C, which would be assigned to the desorption of subsurface lattice oxygen and bulk oxygen. It was found that Mn/11%La-Al and Mn/1%Ce-Al catalysts had excellent oxygen desorption performance.

Figure 7 O2-TPD pro files of Mn/11%La-Al and Mn/1%Ce-Al catalysts

3.5 Catalytic activity of Mn based binary carrier catalyst for ventilation air methane

The SEM and XRD results showed that there were fine microstructures both in La-Al and Ce-Al carriers. In order to explore the relationship between the microstructure of binary carrier and the activity of catalyst, a 9%Mn binary carrier catalyst was prepared by the impregnation method.The catalyst methane conversion rate was tested in a fixed bed quartz reactor, as shown in Figure 8.

The methane conversion rate of Mn/La-Al binary carrier catalyst (a) and Mn/Ce-Al binary carrier catalyst (b) at a GHSV of 6 000 mL/(h·g) is shown in Figure 8. The methane complete conversion temperature over Mn/Al2O3catalyst was 700 °C; while the methane complete conversion temperature over Mn/La-Al and Mn/Ce-Al binary carrier catalyst was about 600 °C. This phenomenon means that the combustion performance of binary carrier Mn/La-Al and Mn/Ce-Al catalysts was better than the single carrier Mn/Al2O3catalyst. As for the Mn/La-Al binary carrier catalyst, the catalytic activity gradually increased with the increase of La content. When the La content reached 11%, CH4started to be subject to conversion at 350 °C and was completed converted at 550 °C with the help of the Mn/11% La-Al binary carrier catalyst. In the Mn/Ce-Al binary carrier catalyst,the addition of 1% Ce had achieved a better catalytic performance, but the catalytic performance did not decrease signi ficantly with the increase of Ce doping amount. As for the Mn/11%La-Al and Mn/1%Ce-Al catalysts, there were no obvious change of methane light-off temperature and the complete conversion temperature was con firmed when the GHSV reached 10 000 mL/(h·g) (Figure 9).

Figure 8 Catalytic methane combustion conversion of Mn/ binary carrier catalysts■—Mn/Al2O3; ●—Mn/1%M-Al; ▲—Mn/3%M-Al; ▼—Mn/7%M-Al; ◆—Mn/11%M-Al

3.6 Stability of Mn based binary carrier catalyst for ventilation air methane

The stability is another important indicator to evaluate the performance of the catalysts. In order to assess the Mn based catalyst with different binary carriers, the stability testing of Mn/1%Ce-Al and Mn/11%La-Al catalysts was carried out at the complete methane conversion temperature. As shown in Figure 10, it can be observed that both Mn/1%Ce-Al and Mn/11%La-Al catalysts exhibited high stability. No obvious deactivation occurred for Mn/1%Ce-Al catalyst at the end of 72-h-testing. As for Mn/11%La-Al catalyst, the CH4conversion ratio was decreased from 97.8% to 93.6% after reaction for 72 hours.

4 Conclusions

Figure 9 Catalytic methane combustion conversion of Mn/ binary carrier catalysts

Figure 10 Stability test of Mn/1%Ce-Al and Mn/11%La-Al catalysts operating at complete methane conversion temperature

(1) The La2O3-Al2O3and CeO2-Al2O3binary porous monolithic column materials with dual functions of carrier and promoter were prepared in one step by the sol-gel method. The Mn based binary carrier catalyst performance is better than that of Mn/Al2O3catalyst.

(2) La and Ce promoters have different effects on the catalytic activity. The amount of Ce has no significant effect on the catalytic activity of ventilation gas combustion. With the increase of La content, the catalyst activity gradually increased.

(3) When the content of La reaches 11%, the 9% Mn/11%La2O3-Al2O3catalyst displayed an initial methane conversion temperature of 350 °C and a complete methane conversion temperature of 550 °C. The main reason was that the addition of 11% La2O3in binary carrier changed the skeleton structure of catalyst carrier,and increased the mass and heat transfer rate and reaction site area. Both Mn/1%Ce-Al and Mn/11%La-Al catalysts exhibited high stability.

Acknowledgements: This work was supported by the National Youth Science Foundation (51704086), the National Natural Science Foundation of China (51772294,51972306). We are also appreciating for the help of Wang Haiwang Group in Northeastern University at Qinhuangdao during the catalytic stability test process.

500 kt/a Aromtization Moving-Bed Unit Comes on Stream at CNOOC Ningbo Daxie Petrochemical Company

On July 10, 2020 the 500 kt/a light naphtha aromatization moving-bed unit jointly developed by the SINOPEC Research Institute of Petroleum Processing (RIPP) and the SINOPEC Guangzhou (Luoyang) Petrochemical Engineering Company has been put on stream at the CNOOC Ningbo Daxie Petrochemical Company,which is the first in China moving-bed unit for naphtha aromatization with independent intellectual property rights.This project, besides adoption of the moving bed technology for aromatization of light naphtha, applies the new-generation continuous aromatization catalyst RF-4. This technology mainly uses the mixed C3-C7hydrocarbons as the feed to manufacture the high addvalue benzene, toluene and mixed xylenes coupled with hydrogen serving as the byproduct.

In order to overcome the disadvantages of the fixedbed light naphtha aromatization technology such as low aromatics yield, high carbon deposition, and short operating cycle, while concurrently meeting the demand of the moving-bed technooogy for enhancement and upgrading of process cycle length, the research team of RIPP on the basis of previous experiences in dealing with light naphtha aromatization process has developed a newgeneration non-noble metals-based spherical catalysts with high zeolite content, featuring high reactivity,high aromatic selectivity and high crushing strength,good circulating mobility and hydrothermal stability,and satisfactory anti-coking and good regeneration performance.

Compared with the fixed-bed technology, the moving-bed aromatization technology can be adapted to a broad range of feedstocks along with some specific features, such as high catalytic activity, high selectivity, high catalytic stability and good anti-attrition performance. It is learned that this technology coupled with its catalysts and the associated facilities is for the first time commercialized inside China, which is expected to increase its economic bene fits and reduce the production cost of the enterprise.