Photocatalytic Denitrogenation over Modified Waste FCC Catalyst

Zheng Liuping; Lin Mei; Huang Yingying; Yan Guiyang,2,3; Zheng Binquan; Li Ling

(1. Chemistry and Materials Science, Fujian Normal University, Fuzhou, 350007; 2. Ningde Teachers College, Ningde, 352100; 3. Fujian Provincial Key Laboratory of Photocatalysis–State Key Laboratory Breeding Base, Fuzhou, 350002)

Photocatalytic Denitrogenation over Modified Waste FCC Catalyst

Zheng Liuping1; Lin Mei1; Huang Yingying1; Yan Guiyang1,2,3; Zheng Binquan1; Li Ling1

(1. Chemistry and Materials Science, Fujian Normal University, Fuzhou, 350007; 2. Ningde Teachers College, Ningde, 352100; 3. Fujian Provincial Key Laboratory of Photocatalysis–State Key Laboratory Breeding Base, Fuzhou, 350002)

The strontium modified waste FCC catalyst was prepared by magnetic stirring method and characterized by X-ray diffractometry (XRD), UV-Vis diffuse reflectance spectrometry (DRS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Meanwhile, its photocatalytic denitrogenation performance was evaluated in terms of its ability to degrade the N-containing simulation oil under visible light. A mixture of strontium nitrate solution (with a concentration of 0.5 mol/L) and waste FCC catalyst was calcined at 400 ℃ for 5 h prior to taking part in the photocatalytic denitrogenation reaction. The test results showed that the photocatalytic degradation rate of pyridine contained in simulation oil in the presence of the strontium modified FCC catalyst could reach 92.0% under visible light irradiation for 2.5 h.

waste FCC catalyst; photocatalysis; denitrogenation

1 Introduction

There exist different kinds of non-hydrocarbon compounds in petroleum products, such as nitrogen-containing compounds, oxygen-containing compounds and sulfur-containing compounds. These compounds account for 10%—20% of the total compounds in oil[1], among which the nitrogen compounds have negative impact on the refining process of crude oil as well as the color and stability of petroleum products[2]. Meanwhile, the toxic materials produced by the use of N-containing oil products are harmful to people and environment. Many researches have been conducted worldwide on denitrogenation of petrochemical products, but only a few possess the practical application conditions. Plenty of studies indicate that photocatalysis exhibits excellent effects on degrading organics into non-poisonous materials, and thereby photocatalysis has a good development potential and bright prospect[3-4].

So far, the total capacity of catalytic cracking units (CCU) in China has already surpassed 100 Mt/a and each year the discharge of waste FCC catalyst could reach more than 105metric tons[2,5]. As a solid waste, a part of the waste FCC catalyst is consumed naturally, the rest of them end up in landfills, leading to wastage of recyclable resources. On the other hand, the high content of heavy metals may endanger the public health and the surrounding environment. So the reclamation of hazardous solid wastes can increase the economic benefits of oil refinery as well as reduce the cost for disposal of waste FCC catalyst, which would benefit the society significantly.

Herein, we have utilized the strontium modified waste FCC catalyst to degrade the pyridine contained in the simulation oil as well as studied its photocatalytic denitrogenation performance under visible light.

2 Experimental

2.1 Apparatus and chemical reagents

The waste FCC catalyst was collected from a batch of spent equilibrium FCC catalyst provided by a petroleum refinery; and the chemical reagents included pyridine; normal octane; strontium nitrate; and deionized water.

2.2 Preparation of catalysts

The strontium species were loaded via impregnation of the waste FCC catalyst with an aqueous solution of strontium nitrate. The impregnation loading was carried out according to the following procedure. The ground waste FCC catalyst powder (0.5 g) and a small amount (10 mL) of strontium nitrate solution (with a concentration of 0.5 mol/L) were put into a beaker. The suspension was stirred magnetically for 2 h, after filtration the dried waste FCC catalyst powder adsorbing strontium nitrate on the surface was calcined in a muf fle furnace at 400℃ for 5 h to decomposeions. Finally, the catalyst was cooled down to the room temperature, ground and sieved.

2.3 Physico-chemical properties of the catalyst

X-ray diffraction (XRD) patterns of catalyst samples were recorded on a Philips X’Pert MPD diffractometer using Cu Kα radiation at a tube voltage of 40 kV and a tube current of 40 mA. Theλof copper Kα radiation was 0.154 nm, the scanning angle (2θ) was 10°—80° with a scanning speed of 2(°)/min. To study the absorption spectra of the photocatalyst samples, UV-vis diffuse reflectance spectra were analyzed on a UV-vis spectrophotometer (CARY-500, VARIAN) with an integrating sphere attachment at room temperature using BaSO4as the re flectance standard and the spectra were recorded in the range of 200 nm～800 nm. X-ray photoelectron spectroscopy (XPS) was performed to identify the composition and the chemical state of surface elements. Spectra were recorded in a VG MiltiLab 2000 spectrometer using a monochromatic Mg Kα X-ray source.

2.4 Photocatalytic denitrogenation reaction

Evaluation of the catalyst activity was carried out in a self-made photocatalytic reaction device[6]. During the degradation studies, 0.05 g of catalyst was dispersed in 50 mL of pyridine solution at an initial mass concentration of 100 mg/g, the suspension was continuously stirred in dark for 30 min prior to visible light irradiation. At regular time intervals (every 0.5 h), 3 mL of suspension was sampled and centrifuged at a proper speed to separate the photocatalyst particles from the solution. The supernatant was detected by recording the variations in absorbance at a characteristic absorption band (251 nm) in addition to scanning over the wavelength ranging from 200 nm to 800 nm using a TU-1810 spectrophotometer. The degradation rate of pyridine was used to evaluate the photocatalytic activity of the strontium modified catalyst.

3 Results and Discussion

3.1 Compositional analysis of the waste FCC equilibrium catalyst

The waste FCC catalyst used in the experiments was collected from an oil refinery and the chemical composition of the said FCC catalyst is showed in Table 1.

Table 1 Chemical composition of waste FCC catalystw, %

3.2 Comparison of physico-chemical properties of waste FCC catalyst before and after modification

3.2.1 XRD

The XRD patterns of different catalyst samples are similar to each other as shown in Figure 1. The bread-shaped amorphous area between spectral peak and the base line indicates that the main components γ-Al2O3and SiO2

were crystalline as well as amorphous in structure. This may be ascribed to the fact that the lattice defect, vacancy and dislocation on the particle surface could result in superficial nonuniformity, thus leading to polarized deformation, rearrangement, lattice distortion as well as descending structural ordering[5], which could develop both in scope and depth, and then the molecular structure of the samples intended to maintain an amorphous state, bringing about bread-like shape instead of spectral peak in the XRD patterns. This part of the molecule belongs to an unstable and upper state, and its high activity, particle diffusion as well as powerful solid-phase reaction and sintering driving force at high temperature could promote the formation and sintering of the product[7]. Owing tothe evenly dispersed doping component on the surface of base material, these two spectrograms differed a little and none of the characteristic peaks of strontium appeared. The possible reason to explain the strong diffraction peak around 11o of the modified catalyst sample is that a kind of molecular sieve was formed after the secondary restructuring of Al2O3and SiO2.

3.2.2 DRS

Diffuse reflection spectra of different catalyst samples are presented in Figure 2. It can be seen from Figure 2 that the absorption edge of strontium modified waste FCC catalyst showed a certain degree of red-shift compared to the unmodified sample. In addition, the absorption intensity was stronger, indicating that the absorption of the modified catalyst was more significant in the visible light region, which was the base of visible light induced catalytic activity. Furthermore, the steep absorption band edge demonstrates that the size distribution of the catalyst sample was narrow and well-distributed[8].

Figure 1 XRD pattern of different catalyst samples

Figure 2 UV-vis absorption spectra of different catalyst samples

3.2.3 XPS

Figure 3 shows the XPS spectra of the strontium modified waste FCC catalyst. It can be seen from the full spectrum diagram at the left of Figure 3 that strontium has been doped into the waste FCC catalyst successfully, the detected elements are in conformity with the results of chemical composition analysis. Because the amount of doped strontium is small, the peak shape is not obvious in the configuration diagram of strontium 3d at the right, however, the binding energy at 134 eV can be attributed to the energy level diagram of Sr2+[9].

Figure 3 XPS spectra of strontium modified catalyst and Sr (3d)

3.2.4 SEM

Figure 4 shows the SEM photomicrographs of the waste FCC catalyst before and after modification. We can learn from the electron photomicrographs that the surface topography of the two catalyst samples did not differ much, but a part of the catalyst reunited after modification, which was in compliance with the results of XRD and DRS analyses. In line with the crystal bridge theory[10], the evaporation of liquid made these particles close to each other during desiccation of the catalyst particles, and at the same time, the hydroxide radical could reunite with the crystal bridge into pieces. It has been reported[11]that the structure and interface energy of particles can also give rise to the agglomeration of particles.

Figure 4 SEM photomicrographs of the waste FCC catalyst before and after modification

3.3 Photocatalytic denitrogenation performance

3.3.1 Comparison of photocatalytic activity of the waste FCC catalyst before and after modif ication

Figure 5 shows the comparison on photocatalytic activity of the waste FCC catalyst before and after modification. It can be seen from Figure 5 that under similar conditions, the catalytic activity of strontium modified waste FCC catalyst increased distinctly compared to the unmodified catalyst. The denitrogenation rate over the modified FCC catalyst attained 92.0%, indicating that the strontium modification of the FCC catalyst is one of the efficient routes to improve the photocatalytic denitrogenation performance of the waste FCC catalyst[12].

Figure 5 Comparison of photocatalytic activity between different catalysts

3.3.2 Influence of different amounts of doped strontium metal on the activity of catalyst

The influence of different amount of doped strontium metal on the photocatalytic denitrogenation performance of waste FCC catalyst is depicted in Figure 6. The degradation of pyridine over the modified waste FCC catalyst increased at first and then decreased with further increase of the concentration of strontium nitrate in the FCC catalyst. The photocatalytic denitrogenation performance of the strontium modified FCC catalyst could reach 92.0% when the concentration of the aqueous solution of strontium nitrate was 0.50 mol/L, demonstrating that the optimal amount of doped strontium was restricted by the structure of the base material[1].

Figure 6 Influence of different amount of doped strontium on denitrification activity

3.3.3 Influence of different calcination temperature on the activity of catalyst

Figure 7 demonstrates the influence of different calcination temperature on the activity of catalyst. It can be seen from Figure 7 that the denitrogenation ability of the strontium modified FCC catalyst was at first enhanced with an increasing temperature, and then declined when the temperature was higher than 400℃. Since the photocatalytic activity was related with the disordered motion ofparticles in the base materials at different calcination temperature, the highest degradation rate appeared at 400℃, which complied well with the result of XRD analysis.

Figure 7 Influence of calcination temperature on denitrification activity of catalyst

3.3.4 Influence of different calcination time on the catalyst activity

The influence of calcination time on the catalyst activity is presented in Figure 8. The denitrogenation ability of the modified waste FCC catalyst at first increased with an increasing calcination time, and then declined when the calcination time exceeded 5 h. This manifests that neither incomplete sintering nor excessive sintering could favor the reorganization of particles in the base materials as well as the control over specific surfac.

Figure 8 Influence of different calcination time on denitrification activity

4 Conclusions

A mixture of strontium nitrate solution (with a concentration of 0.5 mol/L) and waste FCC catalyst was calcined at 400 ℃ for 5 h prior to taking part in the photocatalytic denitrogenation reaction. The test results showed that the photocatalytic degradation rate of pyridine contained in the simulation oil in the presence of the strontium modified FCC catalyst could reach 92.0% under visible light irradiation for 2.5 h. This research results have provided a theoretical basis for photocatalytic denitrogenation.

Acknowledgements:This work was financially supported by the Science and Technology Project of Fujian Province (No. CE0015), the Ningde Normal University Project on Serving the Western Coast to the TW Strait (No. 2010H103), and the National-level College Students’ Innovative Entrepreneurial Training Plan Project of Fujian Normal University (Nos. 201210394005 and 201310394015).

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Recieved date: 2013-03-09; Accepted date: 2013-04-30.

Prof. Yan Guiyang, E-mail: ygyfjnu@163.com.