Effect of alkali-treated HZSM-5 zeolite on the production of aromatic hydrocarbons from microwave assisted catalytic fast pyrolysis (MACFP) of rice husk
Graphical abstract
Introduction
With the modernization of society, the demand for energy has increased intensely. This may lead to an energy crisis in the future if alternative options for fuels are not developed. As a renewable resource, biomass is abundant worldwide and can be converted into liquid fuels called bio-oil via the fast pyrolysis process (Zhang et al., 2018b). However, bio-oil has undesirable properties that prevent it from being used as transportation fuel, including high viscosity, chemical instability, low heating value, and corrosiveness (Wan et al., 2018, Zhang et al., 2018c). In order to convert bio-oil into a fuel that is compatible with the current transportation infrastructure, an upgrading process is required, which involves the conversion of oxygenated molecules into hydrocarbons. The transformation of biomass to high-value aromatic hydrocarbons, in particular, benzene, toluene, ethylbenzene, and xylene (BTEX) is of crucial importance to the development of drop-in fuels, for instance, BTEX is an important gasoline additives contributing to booster its octane number (Qiao et al., 2017).
Fast pyrolysis and upgrade of the pyrolysis vapors can be combined, in a process known as catalytic fast pyrolysis (CFP), which converts biomass to hydrocarbons for fuels and chemicals (Wang et al., 2017, Guo et al., 2017, Abnisa and Wan Daud, 2015). CFP requires heating of the biomass, which can be performed in a variety of ways. Microwave heating has been reported as more rapid and uniform than heat transfer via conduction and convention (Wang et al., 2018b), especially when microwave absorbers are used. Several researchers reported that microwave assisted catalytic fast pyrolysis (MACFP) has advantages compared to traditional CFP (Zhang et al., 2015b, Dong et al., 2018, Zhang et al., 2015a). A comparison of the mechanisms involved in conventional methods and microwave heating for pyrolysis of biomass are shown in Fig. 1. In the conventional method, heat is transferred from the heat source through external layers of the biomass towards the interior. In contrast, the MACFP method involves two mechanisms that take place simultaneously: the first one involves microwave irradiation results in an energy-efficient internal heating by direct application of microwave energy on the molecules of the reaction mixture, causing a rapid rise in temperature due to dipole rotation and ionic conduction (Sweygers et al., 2018). The second mechanism involves absorption of microwave heating by absorbents followed by conduction from the absorbent to the biomass. Several researchers reported the use of microwave to pyrolyze biomass: Nam et al. (2018) reported that microwave vacuum pyrolysis of palm kernel shell generated a biochar yield of up to 36 wt%, which was carbon-dense with high surface area and pore volume; Mahari et al. (2018) performed microwave co-pyrolysis of waste cooking oil and waste polyolefin and obtained a product with high energy content (42–49 MJ/kg) and low moisture content (<1 wt%). Liquid oil shows great potential for fuels. However, lignocellulosic biomass is characterized by a low dielectric loss factor, meaning that these materials are not effective in converting incident microwave energy into heat (Zhang et al., 2018a). To promote more efficient heat transfer, exogenous microwave absorbents can be used. Usually, silicon carbon (SiC) is used as absorbent during MACFP absorbing heat and then transferring it to the biomass. The ability of a material to be heated in the presence of a microwave field is defined by its dielectric loss tangent: tanδ. The larger the value of tanδ, the stronger the adsorption capacity of the material for microwaves. The high capacity of carbon materials to absorb microwave energy and convert it into heat is illustrated in Table S1, where the dielectric loss tangents of several carbon materials are listed (Menendez et al., 2010). Fig. S1 shows a comparison of the heating rates for rice husk (RH) with and without SiC under microwave assisted heating, the heating for rate MACFP in the presence of SiC is 17 times the heating rate without SiC.
In addition, several researchers reported that heat transfer in MACFP is more efficient than conventional CFP in a fixed bed reactor. Farag et al. (2016) reported the differences between the yield and composition of bio-oils obtained from microwave and conventional pyrolysis of kraft lignin. Compared to conventional pyrolysis, microwave pyrolysis showed an increase in yield of 40% for organic chemicals, and a 27% reduction in the yield of water. Wang et al. (2018b) performed a comparison between MACFP and CFP, reporting the advantages in MACFP for agricultural waste: The heating rate for MACFP is higher, the residence time of primary pyrolysis vapors is shortened, the coke content is reduced, and the energy content of the bio-oil is improved. Fernandez and Menéndez (2011) report that the microwave heating method consumes less energy and time. Time savings ranged from 40% to 60%, depending on temperature and raw materials. The energy consumption values were found to be lower than 0.004 kW/h for microwave heating and higher than 0.005 kW/h for the conventional method. In summary, the main advantages of the microwave method in the presence of SiC are: (1) no need to grind, agitate, or fluidize the particle, (2) fast thermal response during start-up and shutdown, (3) easier to control and operate, (4) cost effective and energy efficient (Zhang et al., 2017b).
The most common catalyst used for production of hydrocarbons from biomass is HZSM-5, known for its effectiveness in CFP, with excellent selectivity of aromatic hydrocarbons (Lee et al., 2016, Mullen et al., 2014, Li et al., 2018). However, when the primary pyrolysis vapors originated from biomass contact the HZSM-5 catalyst, large oxygenated compounds are unable to access the microporous structure of the HZSM-5, thus, polymerizing and forming coke on the surface (Ahmadpour and Taghizadeh, 2015, Shao et al., 2013). The deposited coke leads to the blockage of the pore mouth and covers the active sites, leading to loss of catalytic activity (Zhang et al., 2017a). One potential solution for this issue is to treat the HZSM-5 with an alkali solution, which promotes desilication of the zeolite, generating mesopores on its surface. The structure of the mesopores provides a place in which large molecules crack, leading to improved rates of diffusion for reactants and products (Ding et al., 2016). Therefore, the creation of mesoporous structures to the microporous zeolite can potentially reduce coke formation. Xiao, et al. (2015) studied the influence of HZSM-5 modified by varying concentrations of NaOH solutions on the performance of the aromatization of glycerol. In addition, they found that HZSM-5 treated with 0.3 mol/L NaOH solution was the optimum catalyst for the transformation of glycerol/methanol to aromatics. Ding, et al. (2017) prepared alkali-treated HZSM-5 zeolites and focused on the production of aromatic hydrocarbons from CFP of waste cardboard over modified HZSM-5. They found that HZSM-5 treated by 0.3 mol/L NaOH solution was the most effective catalyst, resulting in a 44% increase of carbon yield of aromatics. Both of these studies kept the concentration of NaOH solutions at a low level (<1.0 mol/L). Currently, organic alkali-treatment is used for the synthesis of TS-1 zeolites. Shen, et al. (2018) explored the cooperative structure direction of organosilanes and tetrapropylammonium hydroxide (TPAOH) to generate ZSM-5 zeolite with a controlled porous structure. The modification of the catalysts by the alkali treatment consists in desilication of the molecular sieves by the alkali solution. The organic alkali leads to a moderate treatment, which has the significant advantage of finely regulating the treatment by selectively removing only the strong acid sites on the outer surface of the catalyst, without removing the weak sites inside the pores, in contrast to NaOH. Moreover, the weak alkaline caused less structural disruption and acid site removal at the low concentration relative to treatment with NaOH. (Qiao et al., 2017). In addition, the modified catalyst does not require NH4NO3 ion exchange, simplifying its synthesis. Previous studies reported catalytic fast pyrolysis of biomass over alkali-treated HZSM-5 at low concentrations (<1.0 mol/L) of inorganic base solution, without the assistance of microwave irradiation. In contrast, the present work makes use of higher concentrations of TPAOH (>1.0 mol/L) to generate an effective catalyst for MACFP of biomass, with the goal of maximizing the yields of aromatics and reduce coke yield. Additionally, we performed extensive characterization of the TPAOH-treated catalysts designed in the present study to show the effect of the treatment in the structure and acidity properties of the catalysts.
According to the Food and Agriculture Organization of the United Nations (FAO), in 2017, more than 745 million metric tons of rice and approximately 80 million tons of rice husk were produced all over the world. This large availability, along with its low price, make RH a promising type of lignocellulosic biomass for energy generation (Zhang et al., 2015b).
In the present work, we performed MACFP of RH over alkali-treated HZSM-5 zeolite in order to upgrade pyrolysis vapors and produce hydrocarbons. We evaluated the effect of temperature and the concentration of alkaline solution used to modify the HZSM-5, with the purpose of varying the topography and structure of the resulting catalysts. In addition, we compared the selectivities of aromatics and coke formation obtained with the organic based (TPAOH) with those obtained for alkali-treated HZSM-5 with an inorganic base (NaOH). To the best of our knowledge, this is the first work to perform these comparisons.
Section snippets
Materials
A sample of RH was collected from Suzhou, Jiangsu province, China. Before the experiments, we dried the sample at 105 °C for 24 h, and then ground it mechanically to pass a 60 mesh screen (250 µm). The ultimate analysis results for RH (dry basis) indicate 40.5 wt% carbon, 0.9 wt% nitrogen, 5.7 wt% hydrogen, and 36.5 wt% oxygen. The proximate analysis and higher heating value (HHV) of RH (dry basis) results show 16.4 wt% ash, 16.5 wt% fixed carbon, 67.1 wt% volatile matter and 16.2 MJ/kg HHV. We
XRD results
In Fig. 3a, the XRD pattern shows the characteristic peaks of the parent HZSM-5 and the TPAOH alkali-treated HZSM-5. All the catalysts show characteristic peaks of a typical MFI zeolite framework in the ranges of 7°–9° (100, 101) and 22.5°–25.0° (200, 201, 300). The peaks are smaller for the sampled treated with TPAOH compared to the parent HZSM-5. As the concentration of TPAOH solution increases from 1.0 mol/L to 2.0 mol/L, the crystal structure seems unaffected, as the characteristic peaks
Summary
In the present work, we address one of the most intriguing problems in the development of pyrolysis technology, which is the design of novel catalysts to increase productivity of hydrocarbons, especially focusing on the production of aromatic species from biomass feedstocks. The approach used in the present work involves the modification of a commercial HZSM-5 catalysts with alkali solution, which changes the porosimetry and physicochemical properties of the catalyst. More specifically, alkali
Conclusions
We performed microwave assisted catalytic fast pyrolysis (MACFP) of rice husk (RH) over alkali-treated HZSM-5 zeolite. The organic alkaline (TPAOH) treatment process led to a moderate modification of the catalyst compared with the inorganic alkaline (NaOH) modified HZSM-5. The catalysts were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption-desorption and temperature programmed ammonia desorption (NH3-TPD). The XRD results show that increases
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This paper sponsored by the National Natural Science Fund Program of China (51776042 and U1361115), the Scientific Research Foundation of Graduate School of Southeast University (YBPY1904) and the Program of China Scholarships Council (201806090028).
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