Abstract
A Ni/Zr-MOF catalyst supported on Zr-metal organic framework (Zr-MOF) was prepared by a homogeneous precipitation method and was used in the co-gasification of wet sludge and straw. The Ni/Zr-MOF catalyst was characterized via thermogravimetric, X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, and Brunauer-Emmett-Teller analyses. The experimental results illustrated that the Zr-MOF crystals were an octahedral structure with a specific surface area of 806 m2/g, and had mesoporous structure. Nickel was uniformly dispersed on the surface of the catalyst, and most of the Ni/Zr-MOF crystals maintained an octahedral morphology. Compared with non-catalyst biomass gasification, the H2 yield increased from 0.39 mol/kg to 11.87 mol/kg using the Ni/Zr-MOF catalyst at 500 °C. After 10 instances of reuse, the H2 yield was still as high as 10.11 mol/kg. The Ni/Zr-MOF catalyst exhibited high catalytic activity and stability for biomass gasification at low-temperature.
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Hydrogen-rich Syngas Production via Catalytic Gasification of Biomass Using Ni/Zr-MOF Catalyst
Shuang Shang, Zhenhua Qin, Kui Lan, Yan Wang, Juanjuan Zhang, Tao Xiong, Weitao He, and Jianfen Li *
A Ni/Zr-MOF catalyst supported on Zr-metal organic framework (Zr-MOF) was prepared by a homogeneous precipitation method and was used in the co-gasification of wet sludge and straw. The Ni/Zr-MOF catalyst was characterized via thermogravimetric, X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, and Brunauer-Emmett-Teller analyses. The experimental results illustrated that the Zr-MOF crystals were an octahedral structure with a specific surface area of 806 m2/g, and had mesoporous structure. Nickel was uniformly dispersed on the surface of the catalyst, and most of the Ni/Zr-MOF crystals maintained an octahedral morphology. Compared with non-catalyst biomass gasification, the H2 yield increased from 0.39 mol/kg to 11.87 mol/kg using the Ni/Zr-MOF catalyst at 500 °C. After 10 instances of reuse, the H2 yield was still as high as 10.11 mol/kg. The Ni/Zr-MOF catalyst exhibited high catalytic activity and stability for biomass gasification at low-temperature.
Keywords: Gasification; Biomass; Zr-MOF; Catalyst; Syngas
Contact information: School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, China; *Corresponding author: lijfen@163.com
INTRODUCTION
Energy and environment issues are current topics of concern in society. With the rapid development of the economy, fossil fuels, as non-renewable sources of energy, have been unable to meet the growing demand for energy. Long-term development and use of fossil energy will face the problem of fuel depletion, and oil reserves are estimated to be depleted by 2050 (Saxena et al. 2009). Biomass is a renewable material that can be converted into solid, liquid, and gaseous fuels through chemical processes (Guan et al. 2016). Biomass pyrolysis or gasification is one of the most effective ways to convert biomass into fuel (Ismail and El-Salam 2017). Pyrolysis can make full use of biomass raw materials. The products are gas, tar, and char, and the process does not form toxic and harmful organic compounds (Xu et al. 2017). The problem with pyrolysis is its low content of flammable gases in the product gas, such as H2 and CO. Additionally, a large amount of tar in the gas phase will condense at low temperatures, blocking the downstream pipeline of the process. In addition, tar can cause a lot of energy loss and some environmental problems related to tar (Han and Kim 2008). Catalytic cracking has high cracking tar efficiency and gas reforming ability. Converting tar into useful syngas is a technically feasible and economical gas purification technology (Zhou et al. 2017).
Ni-based catalysts have attracted much attention for tar cracking and gas purification due to their high catalytic activity and relatively low cost. Some studies also show that Ni-based catalysts can reverse ammonia reaction; thus, it is possible to reduce NOx emission during biomass gasification and tar reforming (Shen and Yoshikawa 2013). The traditional Ni-based catalyst supports are mainly metal oxide (γ-Al2O3, CeO2, and MgO) and natural materials (palygorskite, dolomite, and olivine). The accommodation of Ni in the support could improve Ni dispersion, suppress the aggregation of metal particles, and control the particle size efficiently (Zhang et al. 2018).
In recent years, char has also been used as the support for Ni-based catalysts due to its pore structure and large specific surface area. Hu et al. (2018) prepared Ni/char catalysts using pine sawdust char as the catalyst support. The results showed that the char-supported nickel catalysts had high catalytic activity in biomass gasification and tar removal. Shen et al. (2014) prepared a Ni-Fe/RHC catalyst using rice husk char (RHC) as the support. It was found that rice husk char can be prepared as a highly porous carbonaceous material, like activated char. The support with a larger specific surface area can promote the dispersion of metal elements on the surface of the catalyst; at the same time, a larger pore size is beneficial to the diffusion of reactants and products in the channel of the catalyst. Xie et al. (2018) used carbon nanofiber (CNF) as the catalyst support to prepare an Fe-Ni/CNF catalyst and applied it to the catalytic conversion of tar from biomass gasification. The experimental results indicated that the Brunauer-Emmett-Teller (BET) surface area and average pore diameter of Fe-Ni/CNF was 283 m2/g and 3.73 nm, respectively. The syngas yield increased from 0.3 to 0.7 L/g using an Fe-Ni/CNF catalyst. For the same metal loading, a support with a larger specific surface area can provide more active sites, and the catalytic activity of the resulting catalyst is higher. Therefore, to improve the catalytic activity of catalysts, researchers have prepared many synthetic materials with high specific areas, such as SBA-15, ZSM-5, MCM-41, etc. Zhang et al. (2019) prepared Ni/Al2O3 and Ni/SBA-15 catalysts with 20% nickel loading using Al2O3 and SBA-15 as a support to compare the effect of support on catalytic activity. The results showed that the BET surface area of Ni/SBA-15 (333.1 m2/g) was higher than that of Ni/Al2O3 (139.9 m2/g), and the Ni/SBA-15 catalyst showed superior activity to that of the Ni/Al2O3 catalyst. Wu et al. (2011) prepared Ni/MCM-41 catalysts using MCM-41 as support for sawdust gasification to produce hydrogen. The results showed that Ni/MCM-41 catalysts had higher specific surface areas (> 737 m2/g), and the NiO particles had good dispersion on the surface of the support.
The Zr-metal organic framework (Zr-MOF) material has an octahedral structure, and its specific surface area could reach 1367 m2/g (Ren et al. 2014). The uniform structure and larger specific surface area of Zr-MOF material enable the metal to be evenly dispersed on the surface of the support, and the rich pore structure can make the tar fully contact with the catalyst. In addition, presently, Zr-MOF material is mainly used for hydrogen storage, but it has not been used in the study of biomass catalytic gasification for hydrogen production. Therefore, in this work, an Ni/Zr-MOF catalyst was prepared using synthesized Zr-MOF materials as support and applied to the co-gasification of wet sludge and wheat straw.
The Ni/Zr-MOF catalyst was characterized by thermogravimetric (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and BET analyses to explore the mechanism of the catalyst. A series of experiments were performed to explore the effects of the catalyst on the composition and yield of gases, Ni loading, cycle stability of catalyst and catalyst regeneration. Meanwhile, a non-catalyst was also studied as the baseline for comparison.
EXPERIMENTAL
Materials
The wet sludge used in this study was collected from a domestic sewage treatment plant in Wuhan, Hubei Province, China. The sludge was dried at 105 °C for 24 h in an oven to ensure that their mass was constant. The wet sludge with a moisture content of 80% was obtained by uniformly mixing the dry sludge with a certain amount of water. The wheat straw selected from Wuhan, China was pulverized to the size of 0.3 mm to 4.0 mm with a small crusher to eliminate the influence of the particle size on the experimental results. The biomass sample obtained by mixing wheat straw and wet sludge with a moisture content of 80% was used as the raw material for hydrogen-rich syngas production. The proximate and ultimate analyses of dry sludge and wheat straw are listed in Table 1 and presented on an air-dried basis.
Table 1. Proximate and Ultimate Analysis of Dry Sludge and Wheat Straw
M (Moisture); A (Ash); V (Volatile); CF (Fixed carbon); ad Air-dried; c Calculated by difference (O% = 100%－Ash%－C%－H%－N%－S%)
Methods
Catalysts preparation
The highly ordered mesoporous Zr-MOF was synthesized by the solvothermal method reported previously (Ren et al. 2014). Approximately 2.34 g of ZrCl4 and 1.66 g of terephthalic acid were ultrasonically dissolved in 100 mL of N,N-dimethylformamide (DMF) solvent. Approximately 38 mL of formic acid was added as the modulator. The solution was transferred to a Teflon-lined pressure vessel before being heated up to 120 °C and maintained at that temperature for 24 h under static conditions. After cooling, the white product was collected by centrifugation, washed twice with 200 mL DMF and 200 mL ethanol, and then dried for 12 h in an oven at 60 °C.
The Ni/Zr-MOF catalyst was prepared by the homogeneous precipitation method using Ni(NO3)2·6H2O as the nickel precursor. The nickel loading was 20 wt%. A certain proportion of Ni(NO3)2·6H2O and Zr-MOF material was added in a 500-mL flask. Then, 4 g of potassium hydroxide as precipitant and 400 mL of distilled water were added. The blend was stirred at 115 °C for 3 h and precipitated for 1 h. After filtering and drying overnight at 105 °C, the Ni/Zr-MOF catalyst was devolatilized in a flowing nitrogen atmosphere from ambient temperature to 500 °C at 10 °C/min, and then held at 500 °C for 2 h before being stored and used further.
Apparatus and procedure for biomass gasification
The experiments were performed using a fixed bed reactor facility, as shown in Fig. 1. The biomass sample (1 g of wet sludge with a moisture content of 80% and 1 g of wheat straw) and quartz wool (Daiermeng Science and Technology, Wuxi, China) containing 1 g of Ni/Zr-MOF catalyst was placed in a porcelain boat. Before the experiment, high-purity nitrogen gas was introduced into the quartz tube for 30 min to exhaust the air in the quartz tube. The experiments were performed at 500 °C. The holding time was set to 10 min. Once the desired temperature was reached, the heating furnace was pushed to the position where the sample was located, and it was instantly gasified at the specified temperature. The non-condensable gases were collected with a gas collecting bag and the volume of produced gas was measured by a gas flow meter. The composition and content of the gas were measured by a gas analyzer.
Fig. 1. Schematic diagram of the experimental system
Methods
Analysis
The ultimate analysis of the samples was conducted with a CHNS/O analyzer (FLASH2000; Thermo Fisher Scientific, Waltham, MA, USA). This analysis simultaneously gave the mass percentages of carbon, hydrogen, nitrogen, and sulfur in the samples, and the mass percent of oxygen was determined by the difference. The proximate analysis method of coal, according to GB/T 212 (2008), was used to obtain a proximate analysis of the sludge and wheat straw (i.e., moisture, volatile matter, fixed carbon, and ash content of the material). The material structure of the Ni/Zr-MOF catalyst was analyzed using an X-ray diffractometer (XRD-7000; Shimadzu Corp., Kyoto, Japan). The thermal stability of the support and catalyst were evaluated using a thermogravimetric analyzer (SDT Q600; TA Instruments, New Castle, DE, USA). Scanning electron microscopy (S9000; Tescan, Ltd., Brno, Czech) equipped with EDS was used to observe the surface morphology of the support and catalyst. The surface area and pore properties of the support and catalyst were determined by a surface area analyzer (ASAP 2020 HD88; Micromeritics Instrument Corp., Norcross, GA, USA). The gas composition analysis was conducted with an infrared gas analyzer (Gasboard-3100P; Hubei Cubic-Ruiyi Instrument Co., Wuhan, China). The lower heating value (LHV) of the product gas is defined by Lv et al. (2004) as follows,
(1)
where H2, CO, CH4, and CnHm are the gas concentrations (vol%) of the product gas.
RESULTS AND DISCUSSION
Characterization of Catalysts
TG analysis
Figure 2 shows the thermogravimetric analysis of the Zr-MOF support and Ni/Zr-MOF catalyst under the N2 atmosphere. As shown by the dotted line in Fig. 2, with the increase of temperature, the TG profile of Ni/Zr-MOF catalyst was an almost horizontal straight line, which showed that the quality of the catalyst was almost unchanged. Consequently, the Ni/Zr-MOF catalyst had a high thermal stability. As shown by the solid line in Fig. 2, the Zr-MOF support presented mainly two stages in weight loss. The TG profile of Zr-MOF support obtained in this experiment was consistent with the results reported by Ren et al. (2014). A continuous mass loss of Zr-MOF support before 350 °C likely corresponded to the removal of all organic material, including the evaporation of guest molecules from the pores such as the solvent DMF. Between temperatures of 350 °C and 500 °C the mass loss was almost linear, which indicated that the Zr-MOF support had high thermal stability under 500 °C. The Zr-MOF support maintained their structure up to 500 °C and after that a sudden weight loss was observed, which was attributed to decomposition of the Zr-MOF to ZrO2.
Fig. 2. TG analysis of Zr-MOF support and Ni/Zr-MOF catalyst under the N2 atmosphere
XRD analysis
Figure 3 shows the XRD patterns of three catalysts and Ni/Zr-MOF catalysts with different Ni loading. As shown in Fig. 3(a), in the Ni/Zr-MOF catalyst, the peaks at 2θ = 37.3°, 43.3°, 62.8°, 75.4°, and 79.5° were observed and identified as NiO (PDF#89-5881). According to the Scherrer formula, the average size of the NiO grains in the Ni/Zr-MOF catalyst was approximately 10 nm, which indicated that NiO had good dispersion on the Zr-MOF support. After catalytic cracking, the active components of Ni/Zr-MOF catalyst had changed. The characteristic diffraction peaks of Ni (111), Ni (200), and Ni (220) at 2θ = 44.4°, 51.6°, and 76.1° (PDF#01-1258), respectively, were observed in the used Ni/Zr-MOF catalyst. The average size of the Ni grains was approximately 12 nm, which indicated that the metal nickel might agglomerate after 10 instances of reuse. The conversion of NiO to Ni was attributed to carbothermal reduction and hydrogenation reduction. Hu et al. (2018) showed that NiO was transformed into Ni, which contributed to the enhancement of tar conversion. In addition, the strong peaks at 2θ = 30.1°, 34.9°, 50.2°, and 59.7° were observed and identified as ZrO2 (PDF#89-9069), which indicated that the Zr-metal organic framework was decomposed into ZrO2, and the metal organic framework might collapse. The characteristic diffraction peaks of NiO at 2θ = 37.2°, 43.9°, 62.9°, 75.4°, and 79.5° (PDF#89-7131), respectively, were observed in the regenerated catalyst. What’s more, the characteristic peak intensity of NiO in the regenerated catalyst was higher than that of the Ni/Zr-MOF catalyst. As shown in Fig. 3(b), the NiO characteristic peaks became stronger and narrower with the increase of Ni loading.
Fig. 3. XRD patterns of (a) three catalysts, and (b) Ni/Zr-MOF catalysts with different Ni loading
SEM-EDS analysis
Figure 4 shows the SEM images of the Zr-MOF support, Ni/Zr-MOF catalyst, and used Ni/Zr-MOF catalyst. Figure 4(a) shows that all of the Zr-MOF crystals took an octahedral shape with particle sizes of approximately 100 nm. The analysis of Ni/Zr-MOF catalyst via SEM-EDS indicated that the octahedral morphologies of the Zr-MOF crystals were maintained, and nickel was uniformly dispersed on the surface of the Zr-MOF support, which provided more active sites for the Ni/Zr-MOF catalyst. Moreover, the pore structure of the Ni/Zr-MOF catalyst was still clear, which is conducive to the diffusion of tar in the catalyst. This is noticeable in the SEM image and was confirmed by the EDS analysis (Fig. 4(b)). According to EDS analysis, the metal nickel had been successfully loaded on the Zr-MOF support. However, it can be observed from Fig. 4(c) that most of the octahedron structure of the Ni/Zr-MOF catalyst collapsed after reuse. Additionally, only the Zr-MOF crystal with the larger particle size retained the octahedron structure, and the agglomeration of nickel resulted in the increase of particle size.
BET analysis
The N2 adsorption and desorption isotherms of the Zr-MOF support, Ni/Zr-MOF catalyst, and used Ni/Zr-MOF catalyst, are presented in Fig. 5. The pore sizes of the three samples were mainly distributed between 0 nm to 50 nm. Additionally, all the isotherms were of the type 4 pattern according to the IUPAC classification, which corresponded to the mesoporous structure of the three samples. The mesoporous structure of the Ni/Zr-MOF catalyst had adsorption capacity for tar, which could promote the contact between tar and active components to produce more hydrogen-rich syngas from cracking tar.
Fig. 4. SEM image of (a) Zr-MOF support, SEM-EDS images of (b) Ni/Zr-MOF catalyst, and (c) used Ni/Zr-MOF catalyst
The textural parameters of Zr-MOF support, Ni/Zr-MOF catalyst, and used Ni/Zr-MOF catalyst, including BET surface area, total pore volume, and average pore diameter, are listed in Table 3. It was found that the BET surface area of Zr-MOF support was 806 m2/g, the pore volume was 0.47 cm3/g, and the pore diameter was 20.14 nm. A large number of studies have shown that supports with larger BET surface areas can promote the dispersion of metal elements on the surface of a catalyst, and a larger pore diameter is beneficial to the diffusion of reactants and products in the channel of catalyst, so as to improve the catalytic activity of the catalyst (Shen et al. 2011). Furthermore, it was found that the BET surface area and pore volume of the Ni/Zr-MOF catalyst decreased after reuse. Carbon deposition of the Ni/Zr-MOF catalyst blocked some micropores, which resulted in a decrease in the BET surface area. Compared with the fresh Ni/Zr-MOF catalyst, the used Ni/Zr-MOF catalyst had a larger pore diameter due to the destruction of the Zr-MOF support structure after reuse.
Fig. 5. N2 adsorption-desorption isotherm plots and pore distribution of Zr-MOF support (a), Ni/Zr-MOF catalyst (b), and used Ni/Zr-MOF catalyst (c)
Table 2. Textural Parameters of Zr-MOF Support, Ni/Zr-MOF Catalyst, and Used Ni/Zr-MOF Catalyst
Application and Analysis of Ni/Zr-MOF Catalyst in Biomass Gasification
The important reactions that took place during catalytic gasification of wet sludge and wheat straw were summarized as follows (Hu et al. 2015; Yang et al. 2018):
Figure 6 shows the processes of biomass catalytic gasification. First, the drying process of the wet sample took place at a high temperature, which formed in-situ steam around the sample (Eq. 2). Meanwhile, the dry sample was rapidly pyrolyzed to gas, tar, and char (Eq. 3), followed by secondary cracking of the tar to form gas and small molecule organic matter (Eq. 4), steam gasification reaction, methane reforming reaction, water gas shift reaction, char steam gasification reaction, and the Boudouard reaction (Eqs. 5 through 9).
Fig. 6. Mechanism of catalytic gasification of biomass using Ni/Zr-MOF catalyst
The primary gas was produced from the thermal scission of chemical bonds in the individual constituents of biomass, when catalytic reforming reactions cleaved the C―C and C―H bonds of the carbohydrate backbone to produce CO and H2 (Kunkes et al. 2008). [41]Due to the presence of in-situ steam, the char steam gasification reaction not only could refresh the active surface area of the catalyst by reducing the carbon deposit of the catalyst, but it also increased the CO and H2 content in the gas.
Influence of catalyst on gas composition
The Ni/Zr-MOF catalyst was used for tar cracking and improving gas quality in the catalytic in-situ steam gasification of wet sludge and straw, compared with the case without catalyst. The influence of th