Catalytic wet oxidation treatment of aspartame wastewater

The wastewater produced in the aspartame production process has high concentration and complex components, and is a typical high-concentration refractory organic wastewater. At present, the methods for aspartame production wastewater treatment mainly include:

One is to directly enter the biochemical system after a large amount of dilution. The disadvantage is that the processing volume cannot be effectively controlled, thereby increasing the processing cost. In addition, this method cannot recover the active ingredients;

The second is through membrane treatment, but the COD concentration of this type of wastewater is very high, which is extremely easy to cause membrane pollution, thus greatly reducing the service life of the membrane, resulting in high treatment costs.

The third is the traditional advanced oxidation method such as Fenton oxidation method, but there are also shortcomings such as low degradation efficiency and unstable operation.

Catalytic wet oxidation method is to use molecular oxygen (air or pure oxygen) to deeply oxidize high-concentration and refractory organic matter in wastewater under the combined action of high temperature and high pressure and catalyst, so that the organic matter is oxidized and decomposed into CO2, H2O and N2 and other harmless substances or small. Molecular organic matter, an advanced oxidation method to achieve the purpose of purifying water quality.

The method has the advantages of wide application range, no secondary pollution and high treatment efficiency. Catalyst is the key to this technology. The quality of the catalyst is directly related to the degradation efficiency, operation process, equipment process and economic cost of the entire reaction system. Therefore, efficient and stable heterogeneous catalysts have become the current research hotspot.

At present, heterogeneous catalysts mainly include precious metal series, transition metal series and rare earth metal series. Precious metal catalysts are too expensive to be further applied. Copper-containing composite metal oxide catalysts have been widely used at present, but there is a problem of dissolution of active components during use, and the activity and stability of the catalyst are limited.

Therefore, enhancing the stability of wet oxidation catalysts is an urgent problem to be solved.

According to reports, rare earth oxides represented by Ce have been widely used in heterogeneous catalysts. CeO2 can improve the surface dispersion of metals, and its excellent oxygen storage capacity can play a role in stabilizing the crystal structure, thereby improving the Catalyst activity and stability.

Therefore, this study attempts to prepare CuCeOx catalyst for catalytic oxidation treatment of aspartame production wastewater.

In this work, CuCeOx/TiO2-ZrO2 supported catalysts were prepared using TiO2-ZrO2 composite metal oxides as supports, and were characterized by XRD, BET and XPS.

Taking aspartame production wastewater as the treatment object, the effects of active component loading, calcination temperature and Ce addition on the treatment effect were investigated, and the relationship between catalyst structure and catalytic performance was analyzed and discussed.

Experimental part

1.1 Main instruments and reagents

1.2 Preparation of catalysts

1.2.1 Preparation of TiO2-ZrO2 support

According to the method in the literature, first add TiO2 powder to the beaker and add deionized water until dissolved to make the liquid suspended, and then slowly add the prepared ZrOCl2 8H2O solution dropwise to make the amount of ZrOCl2 8H2O and TiO2 The ratio is 1:1, and ammonia water is added dropwise above the suspension while stirring, and the pH value is adjusted to about 7.

The gel was allowed to stand overnight, then rinsed with deionized water and filtered several times, and the rinse water was titrated with silver nitrate. When the white precipitate no longer appeared, the titration end point was reached.

The gel was placed in an oven to dry for 10 hours, and the dried gel was ground until powdery. The ground powder was calcined in a muffle furnace at a temperature of 650 °C for 5 h, so that the final TiO2-ZrO2 composite oxide could be obtained.

1.2.2 Preparation of supported CuCeOx/TiO2-ZrO2 catalyst

According to a certain proportion, Cu(NO3)2·3H2O and Ce(NO3)3·6H2O were weighed and prepared into a solution of a certain concentration, and then the pretreated carrier was immersed in the pre-prepared solution for 24h, and then The catalyst was dried in an oven at 100° C. for 10 hours, and finally calcined in a muffle furnace for 5 hours to obtain the prepared catalyst.

1.3 Experimental method

Add a certain amount of waste water into the reaction kettle, and then add the weighed catalyst; fill the reaction kettle with oxygen until it reaches the specified pressure; open the reaction kettle control device, set the required reaction temperature, and adjust the stirring to a certain speed, Start to heat up; when the temperature rises to the set temperature, start timing;

After the reaction, sampling and analysis were carried out, COD was measured and the reaction temperature and reaction pressure were recorded.

1.4 Analysis method

COD was determined by potassium dichromate method (HJ828-2017); pH was measured by glass electrode method; metal ion concentration in wastewater after reaction was measured by TAS-990 flame atomic absorption spectrophotometer.

1.5 Catalyst Characterization Analysis

1.5.1 BET Characterization

The specific surface area of the catalyst was measured on a Belsorp II specific surface area meter from BEL Japan.

Experimental method: Grind the catalyst into powder, weigh 0.2 g, carry out the dehydration process at a pressure of 0.5 Pa and a temperature of 200 °C, and then use the liquid nitrogen adsorption capacity method. The relative partial pressure of the desorption branch and the adsorption branch of the adsorption isotherm is Measure within the range of 0.001 to 0.99.

1.5.2 XRD characterization

  将样品放置在Cu-Kα辐射(λ=1.7890Å)的条件下,设置管电流40mA,管电压30kV,扫描范围为10~80°,扫描速率为5°/min。

1.5.3 XPS Characterization

X-ray photoelectron spectroscopy (XPS) was performed on a ThermoESCALAB250Xi instrument. The test used a double-anode Al/Mg target. The pressure of the analysis chamber was 6.5×10-5Pa. All XPS test narrow-sweep data were calibrated at C1s284.8eV.

Results and discussion

In a 1L autoclave, take oxygen as the oxidant, load 500mL of waste water, add 2.5g of the prepared catalyst, and carry out the CWAO experiment at a stirring speed of 200r/min. In addition, the reaction temperature is 200°C, and the oxygen partial pressure is 2MPa. The time is 2h.

2.1 Catalyst characterization results

2.1.1 XRD characterization results

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Figure 1 shows the XRD patterns of different catalysts calcined at five temperatures of 450°C, 550°C, 600°C, 700°C and 800°C with a total loading of 6%.

After comparative analysis, it can be seen that the CuCeOx/TiO2-ZrO2 catalyst has obvious characteristic peaks of ZrTiO4 crystal phase with 2θ=24.7° and 30.6°. In addition, after the catalyst was calcined at high temperature, three strongest diffraction peaks of CuO appeared on the obtained XRD pattern, at 2θ=35.6°, 38.8° and 48.7°, respectively, indicating that the Cu(NO3)2 precursor was calcined at high temperature. It has been completely decomposed and distributed on the surface of the support in the form of CuO.

With the increase of calcination temperature, the diffraction peaks of CuO appear stronger and stronger, indicating that the catalyst crystallization tends to be complete, and the defects of the crystal lattice are conducive to the improvement of catalyst activity. It shows that the sintering phenomenon occurs on the surface of the catalyst, which will lead to the reduction of catalytic activity.

In addition, three strongest diffraction peaks of CeO2 were found on the XRD pattern of the catalyst, which were located at 2θ=28.6°, 33.1° and 56.2°, respectively. Narrowing indicates that the grains grow and the crystallization becomes more and more complete, and at the same time, the specific surface area of the catalyst is reduced, so that the catalyst activity is also reduced.

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Figure 2 shows the XRD patterns of the CuCeOx/TiO2-ZrO2 catalysts with Ce additions of 0.5%, 1%, 1.5%, 2%, 2.5% and 3%, respectively.

It can be seen that when the Ce content increases, the diffraction peak of CeO2 is narrowed by the broad side, indicating that crystal grains are being formed;

At the same time, the shape of the diffraction peak of CuO becomes wider, and the intensity of the diffraction peak gradually becomes weaker, which indicates that the addition of a small dose of Ce improves the dispersion of the active component CuO of the catalyst on the surface of the catalyst, which reduces the grain size of the catalyst and reduces the The catalyst activity was observed, which was consistent with the BET analysis results.

2.1.2 XPS Characterization Results

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Figure 3 is the XPS characterization result of Cu2p of the catalyst.

As shown in the figure, Cu2p has two main peaks near 934eV and 953.8eV, and a strong shake-up peak appears between 940 and 945eV. According to previous studies, it has a higher binding energy value (higher than 933.1 eV) and a strong shake-up peak are distinct features of divalent copper oxide species (mainly crystalline copper oxide).

This shows that the copper species of the catalyst mainly exists in the divalent form, which is consistent with the XRD analysis results. In addition, with the addition of Ce, the Cu2p3/2 peak gradually shifted to the direction of lower binding energy, which indicated that the addition of Ce changed the surface structure of the catalyst, possibly promoting the formation of solid solution.

In addition, the intensity of the shake-up peak also gradually weakened. When the Ce addition amount was 4%, the shake-up peak disappeared.

Generally, the low binding energy of Cu2p3/2 (932.2~933.1 eV) and the absence of shake-up peaks are the characteristics of the presence of Cu2O in copper-cerium catalysts.

This shows that the copper oxide in the copper-decorated catalyst exists in a highly dispersed form, and has a strong interaction with cerium oxide, which makes the valence state of copper element change, thus producing Cu+ species.

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The XPS spectra of O1s on the catalyst surface prepared with different Ce additions are shown in Fig. 4. It can be seen from the figure that O1s on the catalyst surface has similar peaks and is asymmetric, which indicates that there are different oxygen states on the catalyst surface. Combined with the XPS analysis results, it can be divided into three types: lattice oxygen, hydroxyl oxygen and adsorbed oxygen.

The XPS analysis results of O1s on the catalyst surface with different Ce additions are shown in Table 1.

It can be seen from the table that with the increase of Ce content, the content of adsorbed oxygen on the catalyst surface increases, but when the amount of Ce added is greater than 1.5%, the content of adsorbed oxygen decreases again.

In addition, it can be seen that the higher the chemisorbed oxygen on the catalyst surface, the higher the activity of the catalyst, which plays an important role in the CWAO of organic compounds.

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2.2 Influence of active ingredient loading

The loading amount has a great influence on the performance of the catalyst in the preparation of the catalyst.

If the load is too large, it is easy to block the micropores on the surface of the carrier, which greatly reduces the catalytic activity. In addition, during the reaction process, the active components are more likely to be lost, which not only affects the catalytic efficiency, but also causes secondary pollution.

On the contrary, if the loading amount is too small, the active centers will also decrease, and the catalyst activity will also decrease. In this paper, catalysts with active component contents of 2%, 4%, 6%, 8%, 10% and 12% were prepared respectively. )=1):1].

Table 2 shows the effect of different loadings on catalyst activity. It can be seen from the results in the table that when the loading amount is 6%, the catalytic effect of the catalyst is the best, and the corresponding specific surface area is also the largest. When the catalyst loading exceeded 6%, the COD removal rate gradually decreased with the increase of the catalyst loading.

In addition, when the loading amount is greater than 10%, the specific surface area at this time is smaller than that of the blank carrier, so too large a loading amount will easily lead to the blockage of the micropores of the carrier. In addition, excessive loading will also cause the dissolution of Cu ions to gradually increase, because the determination of the appropriate catalyst loading plays a key role in the activity and stability of the catalyst.

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2.3 Influence of calcination temperature

The different calcination temperature will affect the pore structure, grain size, surface composition and chemical morphology of the catalyst, which will directly change the activity, stability and mechanical strength of the catalyst. In this paper, catalysts calcined at 450°C, 550°C, 600°C, 700°C and 800°C were prepared, respectively.

Table 3 shows the effect of different calcination temperatures on the catalyst activity.

It can be seen from the table that the COD removal rate is the highest when the calcination temperature is 600 °C. When the temperature is higher than 600 °C, the COD removal rate gradually decreases with the increase of the calcination temperature, indicating that the excessive calcination temperature is easy to cause the sintering phenomenon of the catalyst, which will reduce the catalytic activity. The amount of dissolution is reduced, and the stability of the catalyst is improved.

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2.4 Effect of Ce addition

The outermost electronic structure of rare earth elements has a 5d empty orbital, which can better improve the electron transfer orbit, so it has higher catalytic activity.

In addition, studies have shown that CeO2 not only has good catalytic activity, but also has very strong acid resistance. It combines with metal oxides to form a stable solid solution at high temperatures, and the stability of the catalyst can be greatly improved.

In this paper, catalysts with different amounts of Ce were prepared (the total loading of active components was 6%, and the calcination temperature was 600 °C) for CWAO experiments. The experimental results are shown in Table 4.

It can be seen from the table that the catalyst with Ce addition of 1.5% has the best catalytic activity, and then with the increase of Ce addition, the catalytic activity gradually decreases, indicating that adding a certain amount of Ce addition plays a role in promoting the catalytic activity. However, excessive Ce addition will reduce the catalytic activity of the catalyst, which may be caused by the reduction of the active component CuO on the surface of the catalyst due to excessive CeO2.

In addition, with the increase of Ce content, the dissolution amount of Cu ions will be reduced, indicating that the addition of Ce can effectively improve the catalytic activity and the stability of the catalyst.

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2.5 Reusability of catalysts

In order to explore the reuse performance of the catalyst, the reacted catalyst was centrifuged, washed with deionized water and dried, and then reused for many times under the same experimental conditions.

Parts: The total loading of active components is 6%, the calcination temperature is 600°C, and the Ce addition amount is 1.5%. The experimental results are shown in Figure 5.

It can be seen from the figure that the COD removal rate did not decrease significantly after the catalyst was used for many times compared with the initial use, indicating that the catalyst has good stability, can be reused, and has certain industrial application value.

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Conclusion

The prepared CuCeOx/TiO2-ZrO2 catalyst has the highest catalytic activity when the total loading of active components is 6%, the calcination temperature is 600℃, and the addition amount of Ce is 1.5%.

Under the same experimental conditions of reaction temperature of 200 °C, oxygen partial pressure of 2 MPa, and reaction time of 2 h, the catalyst was used repeatedly for many times, and the COD removal rate remained above 85%, showing good activity and stability. It has certain industrial application value.

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