Effects of the supported FeOOH on catalyzing gasification of oily sludge in supercritical water

FeOOH/AC-, FeOOH/Al2O3- and FeOOH/ZnO-supported catalysts were synthesized by immersion, hydrothermal hydrolysis, and hydrothermal methods, and used for supercritical water gasification (SCWG) of oily sludge. Under the same conditions, compared to FeOOH/Al2O3 and FeOOH/ZnO, the catalytic effect of FeOOH/AC was superior. When the concentration is 5% and the loading is 10%, the hydrogen production rate with FeOOH/AC composite is the highest, which is 4.638 mmol/g, and the hydrogen gasification efficiency (HE) is 89.50%. The reaction mechanism of FeOOH composites in the SCWG was also investigated. These results indicate that FeOOH composites are potential catalysts for oily sludge treatment.


Introduction
Oily sludge is a type of solid waste generated during oil exploration and development, which contains a variety of poisonous and dangerous materials, including members of the benzene series, phenols, anthracene, and pyrene, which is extremely harmful to the environment [1].The Supercritical Water Gasification (SCWG) process is regarded as a superior method for addressing large-scale oil sludge contamination.This technology refers to the steam reforming reaction of raw materials or a certain concentration of organic wastes in supercritical water (SCW) (T ≥ 374.1 ℃, P ≥ 22.1 MPa) to generate H 2 , CH 4 , CO, CO 2 , and other hydrogen-rich gases.FeOOH is used for catalytic gasification of oily sludge in SCWG because of its low cost and no toxicity, but it is easy to agglomerate and coke under high temperature and pressure, which reduces its catalytic activity.AC, ZnO and Al 2 O 3 are often used as carriers due to their well-developed pore structure and huge specific surface area.The activity and stability of the catalyst can be effectively improved by loading FeOOH on the carrier, improving the dispersion of FeOOH particles and preventing the agglomeration of catalyst particles through the interaction with the carrier.

Preparation of oily sludge and catalyst
The lab-made oily sludge for the experiment came from Xi'an Sewage Treatment Plant and crude oil from Changqing Oil Field Co., Ltd. in China.Both were pretreated to ensure homogeneity and repeatability.Crude oil was heated at 80 °C for 2 h, then cooled and dried for 48 h.The sludge was dried at 110 °C, crushed, filtered, and dried again.The oily sludge was mixed with water in a 5:3:2 ratio and kept stable for 24 h before use.
All analytical-grade chemicals were used without further purification.The concentrations of FeCl 3 and NaOH solutions were created.In a conical flask, FeCl 3 solution was added, and the pH was raised to 12 by gradually introducing NaOH solution.A reaction kettle was used after the mixture had been stirred for an hour.The mixture was heated to a hydrothermal reaction temperature of 100-130 °C in a kettle that was put inside an oven for 8 hours.FeOOH was produced by filtering, neutralizing washing the precipitate after the reaction, and then drying it for 24 hours at 60 °C.
FeOOH/Al 2 O 3 catalyst was synthesized through a hydrothermal hydrolysis process using FeCl 3 •6H 2 O as the metal precursor.FeCl 3 •6H 2 O and urea were dissolved in deionized water, and Al 2 O 3 was added to the reaction system in varying molar ratios (1:6:54, 1:6:27, 1:6:13.5, 1:6:8.8).The mixture was brought to a pH of 1.6 using HCl and kept at 100 °C for 4 hours in a water bath before being cooled to room temperature and cleaned with deionized water and ethanol.
After preparing a FeCl 3 solution with a specific concentration, 20 nm-sized zinc ions or ZnO particles were added following the molar ratios of Fe to Zn (Fe:Zn) of 1.0:22, 1.0:11, 1.0:5.5, and 1.0:3.7.FeOOH/ZnO was obtained using the same preparation procedure parameters as for FeOOH.
The X-ray diffraction pattern was obtained by Cu-Ka on a Shimadzu XRD6000 X-ray powder diffractometer.Scanning electron microscopy (TESEM) (TESCAN MAIA3) was used to determine the catalysts' morphologies.

Supercritical water gasification process
The reaction was conducted in a self-constructed supercritical water intermittent experimental setup that primarily comprises a reactor, a heating furnace, and a control box, with Hastelloy as the material, the reactor with a volume of 160 mL, the design pressure at 40 MPa, and the design temperature at 650 °C.
After a safety check and cleaning, the reactor was filled with an oily sludge sample, water, and a catalyst.By purging and closing the reactor, the reactor's air was replaced with high-purity argon.The valves were closed, and high-purity argon was introduced to achieve the desired reaction pressure.Once the heating process was complete and the temperature met experimental requirements, the reaction proceeded for the predetermined residence time.A cooling hose was used to bring the reactor down to room temperature.The drainage gas collection method was used to gather the reaction's gas phase products.
In previous studies, our group has obtained some optimal conditions for SCWG of oily sludge.Operating conditions include a temperature of 555 °C, an initial reaction pressure of 1.0 MPa, an oily sludge feed concentration of 10 wt%, and a reaction time of 135 min [2].In this experiment, three types of supported catalysts, FeOOH/Al 2 O 3 , FeOOH/ZnO, and FeOOH/AC, were prepared for investigating their catalytic performance and mechanisms during SCWG of oily sludge.
Gas products are analyzed using gas chromatography (GC-7820A), each sample was examined three times, and the information presented in this report represents the average of the results from those tests.In order to analyze the functional groups present in both liquid and solid phase products, a Fourier transform infrared spectrometer (iCAN9, Tianjin Energy Spectroscopy Technology Co., Ltd.) was employed.

Data interpretation
Gasification efficiency (GE), carbon gasification efficiency (CE), and hydrogen gasification efficiency (HE), were utilized as indicators to assess the gasification characteristics of SCWG for oily sludge.These three indicators were determined using the following equations: GE % = all of the gaseous products combined mass the mass of oil sludge dry basis ×100% CE % = the total carbon in gaseous products the total carbon in oil sludge dry basis ×100% HE % = the total hydrogen in gaseous products the total hydrogen in oil sludge (dry basis) ×100% (3)

Crystal structure and morphological characteristics
The as-synthesized 10% FeOOH/Al 2 O 3 , 10% FeOOH/ZnO and 10% FeOOH/AC were characterized by the XRD (Figure 1).The crystalline phase of FeOOH is consistent with results from a previous study [3].The samples showed more pronounced diffraction lines at 2θ=21.FeOOH/AC all had a favorable impact on the response, significantly increasing H 2 yield and HE.FeOOH/AC exhibited the highest gas yield and HE, compared to FeOOH/Al 2 O 3 and FeOOH/ZnO, particularly when the concentration was 5% and the loading was 10%.Under these conditions, the H 2 yield increased from 2.716 mmol/g to 4.638 mmol/g, representing a 70.77% increase, while the HE increased from 66.39% to 89.50%, representing a 34.81% increase.The results indicated that the supported catalyst had better activity than FeOOH or the support alone due to its higher specific surface area and stability, preventing coke formation and agglomeration under high temperature and pressure.The catalytic effect of FeOOH/AC was superior to that of FeOOH/Al 2 O 3 and FeOOH/ZnO due to the weaker interaction between AC and metal oxides, increasing in pore size and specific surface area.A larger specific surface area increases the contact area, improves component distribution, and enhances catalyst activity [5].Figures 5(a) and 5(c) depict the FTIR spectra of solid products obtained using different catalysts and catalyst loadings.The C≡C-H stretching vibration, which is unique to alkynes, is shown by the absorption bands at 3613.9 cm -1 .Both the absorption bands at 1432.9 cm -1 and 2923.6 cm -1 are indicative of alkanes, which are representative of the C-H bending vibration and the -CH 2 -asymmetric stretching vibration, respectively.The C=C stretching vibration, which is typical of aromatics, is shown by the absorption bands at 1585.2 cm -1 .The C-O stretching vibration, which is found in alcohol and phenol, is shown by the absorption bands at 1033.7 cm -1 .The olefin-specific =C-H out-of-plane bending is shown by the absorption bands at 798.4 cm -1 .The C=O stretching vibration, which is typical of secondary amides, is shown by the absorption bands at 464.8 cm -1 .The addition of FeOOH-supported catalysts reduced the strength of each functional group, making it easier to break and accelerate the cracking of oily sludge compared to the reacting conditions without catalysts.The intensity of the characteristic peaks of aromatic hydrocarbons, olefins, and alkanes decreased significantly, while the intensity of the characteristic peaks of alcohols and phenols decreased less significantly.The catalytic cracking ability of FeOOH/AC for oily sludge was found to be significantly superior to that of FeOOH/Al 2 O 3 and FeOOH/ZnO under the same conditions, as shown in Figures 5(a) and 5(b).Furthermore, at the same concentration, the FeOOH/AC loading quantity of 10% had the strongest catalytic cracking capacity, with the strength of each functional group at its lowest, which was consistent with the gas phase results.After a comprehensive analysis, it can be said that oily sludge's C-C, C-H, and C-O bonds are mostly broken by FeOOH composite material, which encourages the production of light components like H 2 and CH 4 .This results in the complete breakdown and use of oily sludge in the supercritical water state.

Conclusion
FeOOH composite materials were successfully synthesized in this work.The three different types of supported catalysts, as well as their supports and active components, have a positive catalytic effect on the gasification reaction, according to the experimental results, but their combined effects are stronger than their individual effects.The as-synthesized FeOOH/AC exhibited improved catalytic efficiency, activity, and higher reliability than FeOOH, FeOOH/Al 2 O 3 and FeOOH/ZnO.The FeOOH/AC catalytic gasification of oily sludge under the same reaction circumstances produces the most hydrogen and has the highest gasification efficiency, carbon gasification efficiency, and hydrogen gasification efficiency.Additionally, during the SCWG catalytic reaction process, FeOOH effectively encourages the formation of gas-phase products, the cracking of heavy components in the oil sludge, and the breaking of C-C and C-H bonds.
Overall, this study provides valuable insights into the catalytic gasification of oily sludge, and the FeOOH composites show great potential for practical application.These findings are of great significance for the development of more efficient and environmentally friendly catalytic gasification technology for industrial waste treatment.
22°, 33.24°, 34.70°, 36.65°,53.24°, 59.02° and 61.38°.It is consistent with the position of the FeOOH diffraction line in the international diffraction data standard card (JCPDS 29-0713).The FeOOH and the prepared supported catalyst were analyzed by field emission electron microscopy (FESEM).The results shown in Figure 2(a) are the electron microscopic images of FeOOH, Figures 2(b), 2(c) and 2(d) are the electron microscopic images of FeOOH/ZnO, FeOOH/AC and FeOOH/Al 2 O 3 with 10% load respectively.Figure 2(a) shows that the FeOOH particles are needleshaped and uniform in size, which is consistent with the micrographs of crystals previously reported [4].Figures 2(b), 2(c) and 2(d) show that there are uniform needle-shaped structures in the pores of the support material, indicating that FeOOH has been successfully loaded into the pores of the support material to prepare heterogeneous catalysts.

Figure 3 .
Figure 3.Effect of catalyst type on H 2 yield (a), GE (b) (The catalysts concentration is 5%) Figures 4(a) and 4(b) show the catalytic activity of different catalysts at varying loadings.FeOOH/AC exhibited significantly higher activity than FeOOH/Al 2 O 3 and FeOOH/ZnO under the same conditions.With a 10% loading and a 5% concentration, FeOOH/AC had an H 2 yield of 4.638 mmol/g

Figure 4 .
Figure 4.The effect of the catalyst load on H 2 yield (a), HE (b) (The catalysts concentration is 5%)

Figure 5 .
Figure 5. FTIR spectra of products under different catalysts (a: solid phase), (b: liquid phase) FTIR spectra of products under different catalyst loadings (c: solid phase), (d: liquid phase) (The catalysts concentration is 5%) On the other hand, Figures 5(b) and 5(d) show the FTIR spectra of liquid phase products obtained using different catalysts and catalyst loadings.The C-H stretching vibration is represented by the absorption bands at 2954.4 cm -1 , the -CH 2 -asymmetric stretching vibration is represented by the absorption bands at 2923.6 cm -1 , and the -CH 2 -symmetric stretching vibration is represented by the absorption bands at 2852.2 cm -1 , all of which are characteristic of alkanes.Alkanes also exhibit the -CH 2 -shear vibration, which is represented by the absorption bands at 1463.7 cm -1 and the C≡C stretching vibration, respectively, at 2163.7 cm -1 .Ketones exhibit the C=O stretching vibration, which is shown by the absorption bands at 1716.3 cm -1 .The C=C stretching vibration, which is typical of aromatic hydrocarbons, is represented by the absorption bands at 1525.4 cm -1 , whereas the C-O stretching vibration, which is typical of alcohol and phenol is represented by the absorption bands at 1270.9 cm -1 .The addition of FeOOH-supported catalysts reduced the strength of each functional group, making it easier to break and accelerate the cracking of oily sludge compared to the reacting conditions without catalysts.The intensity of the characteristic peaks of aromatic hydrocarbons, olefins, and alkanes decreased significantly, while the intensity of the characteristic peaks of alcohols and phenols decreased less significantly.The catalytic cracking ability of FeOOH/AC for oily sludge was found to be significantly superior to that of FeOOH/Al 2 O 3 and FeOOH/ZnO under the same conditions, as shown in Figures5(a) and 5(b).Furthermore, at the same concentration, the FeOOH/AC loading quantity of 10% had the strongest catalytic cracking capacity, with the strength of each functional group at its lowest, which was consistent with the gas phase results.After a comprehensive analysis, it can be said that oily sludge's C-C, C-H, and C-O bonds are mostly broken by FeOOH composite material, which encourages the production of light components like H 2 and CH 4 .This results in the complete breakdown and use of oily sludge in the supercritical water state.