1. Introduction
The pyrolysis process is considered a promising way to produce economical renewable fuels and chemicals starting from different sources of biomass [
1,
2,
3]. To date, the major application of pyrolysis that have mainly been studied is bio-oil production, which is considered a second-generation energy carrier [
4]. However, the liquid fraction including oil phase consists of a mixture of oxygenated compounds and hydrocarbons [
5,
6], and the oil phase usually has a high acidity and viscosity, low stability, corrosive nature, and low vapor pressure. All these are the major limitations that make it impossible to use directly as a bio-fuel, as shown in studies [
7,
8,
9,
10]. There are two major routes to improve the quality of bio-oil: upgrading the biomass raw materials through pretreatment before pyrolysis [
11,
12,
13] and a catalytic method of deoxygenating the bio-oil created by the pyrolysis process [
14,
15,
16,
17,
18]. As shown in references [
15,
19], the biomass washing with a dilute acid could remove over 90% of ash content; therefore, acid washing should have a significantly positive influence on biomass quality. The catalytic processing of pyrolysis vapors is the second way used to improve the quality of the bio-oil [
20,
21,
22,
23,
24].
The catalytic pyrolysis process can be performed in two different ways, depending on how the catalyst is placed in the reactor: in situ, when the catalyst is mixed with the biomass, and ex situ, when the catalyst is located outside the area containing the biomass, experiments of this type being used in a number of studies as well [
25,
26,
27,
28,
29,
30,
31,
32].
Different nanocatalysts such as zeolites have shown good performances in the deoxygenation reaction of oxygenate compounds from the bio-oil [
33,
34]. However, other studies [
35,
36,
37] have mentioned that the in situ pyrolysis of biomass causes the formation of a larger amount of coke on the surface of the catalyst, which leads to the deactivation of the catalyst, a phenomenon due to the fact that the catalyst is mixed with biomass and forms a single zone. In addition, because the catalyst is mixed with biomass, its regeneration is more difficult, compared to the ex situ process where the catalyst can be easily removed from the reaction zone of the reactor for regeneration and reuse.
The results from previous studies [
34,
35,
37,
38,
39,
40] have suggested that the deoxygenation reactions of oxygenated compounds from the bio-oil composition can be efficiently performed in the in situ and ex situ processes.
On the other hand, zeolites are also known as being very effective molecular sieve materials. Some examples of zeolites showing sharp sieving properties (acting as molecular sieves) are shown in references [
41,
42,
43]. In addition to being an effective molecular sieve material, Na-13X zeolite also has the ability to retain water and CO
2 from air and inert gases. Due to its large surface area and defined pore diameter, it can also be used to support catalysts in the biomass conversion processes. Currently, few studies of this have been reported.
Therefore, aiming at the chemical treatment of the biomass by acid washing, Zn/13X zeolite and Cr/13X zeolite nanocatalysts were prepared. Their deoxygenation potential of the oxygenated compounds present in the bio-oil resulting from the pyrolysis processes (in situ, ex situ, or combined) of PTCCB was demonstrated. These two nanocatalysts are different from what is known, for example, comparing the characteristics of ZSM-5 zeolite (one of the most studied and used zeolites in biomass conversion processes) with 13X zeolite characteristics: it was found that 13X zeolite has a bigger specific surface area than ZSM-5, 682 m
2/g compared to 420 m
2/g, respectively [
44]. In addition, the average pore size of the 13X zeolite is bigger than that corresponding to the ZSM-5 zeolite (1 nm compared to 0.54–0.56 nm, respectively) [
45]. Considering these characteristics and that biomass components have bigger molecules, 13X zeolite could be a suitable catalytic support to obtain effective nanocatalysts in the pyrolysis process, favoring bio-oil production and its quality improvement. Thus, the use of acid-treated biomass (using 0.1 M H
2SO
4) and these nanocatalysts in a combined process (in situ and ex situ variants) [
46] make it possible to produce a bio-oil product with an improved quality.
The rationale for selecting Cr and Zn as doping metals was based on the fact that the main challenge for converting biomass to liquid fuels is in the design of new catalysts to promote the reactions of deoxygenation, cracking, and aromatization with high efficiency and to resist catalytic activity degradation.
Among the four most active metals, Fe, Co, Ru, and Ni [
47,
48,
49], Fe- or Co-doped catalysts have been found to form clusters quite easily by sintering, destabilizing the surface and reducing the catalytic efficiency. Various studies [
50,
51,
52] have shown that zinc, if added as a promoter to iron-based catalysts, prevents iron cluster formations from sintering and stabilizes the surface area of iron oxide, which led to the idea that this metal could have a good behavior and activity in the process of the pyrolysis of biomass. Additionally, Zn/ZSM-5 has been reported to be an excellent aromatization catalyst with, in some cases, a performance superior to that of Ga/ZSM-5 [
53,
54], and bimetallic Zn/ZSM-5 catalysts have been reported to show increased aromatics selectivity and increased stability compared to the parent Zn/ZSM-5 [
55,
56].
Despite a large number of studies concerning noble metals, their use is not ideal, owing to some restrictions. First, the price of these metals may limit their use. Secondly, it has been established that they are not resistant to severe thermal treatment: the sintering of metal particles is associated with a loss of catalytic activity. Thus, new active phases less sensitive to high temperature in the presence of water are desirable. In this sense, transition metals appear to be promising candidates. Chromium metal is notable for its high corrosion resistance and hardness, as evidenced by many studies [
57,
58,
59]. Therefore, in the present work, chromium was chosen as a doping metal in the catalyst design.
2. Materials and Methods
2.1. Corn Cobs Biomass
In this study, corn cob biomass (CCB), harvested from the local area, were crushed and sieved by 60 mesh sieve and then were air-dried in the oven at 110 °C for 3 h (
Figure 1a–c). The CCB used was a fraction of ≤60 mesh (0.25 mm) to minimize the heating and mass transfer impact and to reduce the temperature gradients into the CCB sample. A sulfuric acid (0.1 M H
2SO
4) solution was used to wash the biomass. Then, 100 g of CCB was immersed in an Erlenmeyer flask containing 500 mL of H
2SO
4 (0.1 M) and maintained at room temperature (~22 °C) for 6 h.
The washed CCB was filtered through a filter funnel, the separated solid was washed with distilled water until the filtrate was neutral to pH and was dried at 110 °C for 4 h, and the resulting material was labeled as pretreated corn cob biomass (PTCCB).
2.2. Corn Cob Biomass Analysis
CCB proximate analysis was made according to ASTM standards D 2016 74, D3174 89, and D1102 84 to determine the fixed carbon, moisture, volatile and ash content. The final analysis was performed according to ASTM D 5373 to determine the content of carbon, hydrogen, nitrogen, sulfur, and oxygen using a Carlo Elba 1106 instrument. The composition was established according to ASTM D 3176, and using ASTM D240-02 standard, the gross calorific value (GCV) was determined. The extractives and lignocellulosic were determined using TAPPI test method, as is described in reference [
60].
The metals content was determined by atomic absorption analysis using a spectrophotometer type Analytic Yena Nova 300. The Organic structure of CCB and PTCCB was obtained through Fourier infrared spectroscopy using a FTIR spectrometer Nicolet iS50. CCB and PTCCB were subjected to thermogravimetric analysis using a Setaram Setsys Evolution instrument coupled with a PC under N2 of 99.99% purity, at a flow rate of 50 mL/min, and with a heating rate of 20 °C/min up to a final temperature of 700 °C.
In all tests, the sample mass was of 50 mg, and each analysis/test was repeated three times. The reported values represent the average of these three tests, the relative error was less than 1.5%, and the data standard deviations are provided.
2.3. Preparation and Characterization of Nanocatalysts
A commercial zeolite type 13X, in pellets form (Ꝋ = 3 mm), with a Si/Al ratio = 3.2, was purchased from FLUKA of Sigma-Aldrich Holding AG (
Figure 2). The zeolite pellets were milled, sieved to particles with a size ≤ 0.5 mm, and calcined at 500 °C for 3 h. According to previous studies [
61,
62,
63], a metal loading value in the range of 8 to 10 wt.% in the catalyst is necessary to obtain a sufficient number of strong catalytic active sites for deoxygenation reactions. A higher metal content reduces the number of these active sites and inhibits the catalytic activity. In the work, the nanocatalysts containing a metal load around 9 wt.% were prepared by the wet impregnation method.
To prepare 50 g of Zn/13X zeolite or Cr/13X zeolite, 20.47 g Zn(NO3)2·6H2O or 20.60 g Cr(NO3)3·9H2O was dissolved in 75 mL of distilled water, and then 45.5 g of 13X zeolite was added in the solution and mixed for 8 h under constant magnetic stirring. To evaporate the water, the mixture was heated at 85 °C for 2 h and dried in an oven at 110 °C for 8 h. The resulting material was further calcined at 500 °C for 6 h to produce the final nanocatalyst. The atomic absorption analysis results showed that 8.85 wt.% of Zn and 9.72 wt.% of Cr were present in Zn/13X and Cr/13X zeolite nanocatalysts, respectively.
The prepared Zn/13X zeolite and Cr/13X zeolite nanocatalysts were characterized by XDR, using D/max-2200/PC, Rigaku, Japan, and copper KR radiation (40 kV, 20 mA) as the X-ray source. The size of metallic crystallite was determined based on Scherrer Formula (1):
where d
crystallite size is determined in nm, B is full width at half maximum of the most intense peak from spectrum (FWHM), λ is considered 1.5405 Å, and 2θ angle is in the range 10 to 90. Using Quantachrome Inst., Nova 2200e analyzer, the N
2 adsorption/desorption isotherms at (−196.15 °C) were determined, and the Brunauer–Emmett–Teller method was employed to determine the specific surface area of catalysts.
The total pore volume (Vtp) was established from the N2 quantity adsorbed for a relative pressure (P/PO) of 0.99. The nanocatalyst morphology was analyzed by scanning electron microscopy (SEM), using a microscope JSM-7500 F (JOEL-Japan) operated at 10 kV, and gold coating was used. The nanocatalyst acidity was analyzed by the ammonia temperature programmed desorption method (NH3-TPD). For the TPD-NH3 measurements, a Micrometrics Autochem 2920 instrument equipped with a thermal conductivity detector (TCD) was used.
2.4. Pyrolysis Experiments
All pyrolysis experiments were carried out under an N
2 atmosphere in a fixed bed reactor (length of 450 mm, inner diameter of 12 mm) made of stainless steel. A scheme of the experimental system is shown in
Figure 3. To ensure an isothermal regime, the reactor was heated by an electrical oven. The experiments were performed at 500 °C, with 50 °C/min heating rate and a N
2 gas flow rate of 60 mL/min, and for each experiment, 20 g of PTCCB with particle size ≤0.25 mm were used. For pyrolysis tests, the nanocatalyst was mixed with PTCCB in different ratios, as is shown in
Table 1.
During in situ tests, the area corresponding to the ex situ process was kept empty. After the series of in situ and ex situ tests were performed with each type of catalyst, the obtained results were compared, and the catalysts with the best performance in in situ and ex situ tests were chosen. These catalysts were further evaluated in the combined pyrolysis process.
In the ex situ pyrolysis experiments, the catalyst bed was placed after the biomass bed. In the combined tests, Zn/13X zeolite was employed in the in situ pyrolysis process, and Cr/13X zeolite was employed in the ex situ pyrolysis process.
The particle size of catalysts used in all experiments was ≤0.5 mm. Due to the way biomass and catalyst are placed in the reactor, in the ex situ experiment, the thermal pyrolysis of the biomass took place first, and then the released vapors (primary product) passed over the catalyst bed. The resulting vapors were then passed through a metallic sieve and ceramic filter to retain any solid particles and through traps to collect the liquid phase.
The bio-oil contained in the liquid fraction was separated by dissolving in dichloromethane (DCM) and then filtered over glass wool and calcium chloride. The bio-oil mass was determined by weighing, the solid phase consisting of bio-char, and nanocatalyst together with the coke deposited on and into nanocatalyst mass were also weighed to determine the pyrolysis yield for this product. Each pyrolysis test was carried out at least three times; an average of these values was considered as the final result, and the measurement error was less than 0.5%. The bio-oil composition was identified using GCMS-QP2010SE Gas Chromatograph/Mass Spectrometer, and for the identification of the chromatographic peaks the NIST mass spectrum library was used.
The detected compounds were classified into 9 main groups: aliphatic hydrocarbons, aromatic hydrocarbons, acids, aldehydes, ketones, phenols, furans, nitrogen compounds, and other compounds (compounds containing halogen, sulfur and silicon).
2.5. Coke Deposition on the Nanocatalysts
To analyze the coke formation and its deposition on catalysts after the pyrolysis process, a thermogravimetric device type TGA/DSC Stare, from Mettler Toledo, Ltd. was used. Oxidation-based analysis at the programmed temperature (TPO) was performed for spent catalyst, and the obtained results were compared with the fresh catalyst.
A quantity of 20 mg catalyst was heated from 20 °C (~room temperature) to a temperature of 900 °C with a heating rate of 10 °C/min, in an atmosphere of air at a flow rate of 80 mL/min, and nitrogen gas at a flow rate of 15 mL/min, respectively. The coke amount was established as a percentage calculated by the difference between the mass of the initial sample and the mass of the residual sample.
4. Conclusions
This study showed that washing with sulfuric acid improved the quality of corn cob biomass. The content of cellulose and hemicellulose from biomass composition influences the process of pyrolysis and the quality of resulted products, and an increased content obtained means that an important part of extractives by acid washing was removed. This change had a favorable influence on the pyrolysis process and bio-oil quantity and quality. Zeolite 13X, having a larger specific surface area and an average pore diameter larger than that corresponding to the ZSM-5 zeolite, proved to be a suitable catalytic support, itself having a catalytic activity.
By loading metals (Cr/Zn) on 13X zeolite, efficient catalysts for the pyrolysis process of biomass to produce bio-oil were obtained. The Cr/13X zeolite catalyst used in in situ pyrolysis favored the production of aliphatic hydrocarbons, while the Zn/13X zeolite catalyst used in ex situ pyrolysis favored the production of aromatic hydrocarbons. The present study also showed that Zn/13X zeolite and Cr/13X zeolite catalysts achieved a conversion of the compounds containing oxygen over 97% in a combined process (in situ and ex situ pyrolysis variants), which is higher than either in situ or ex situ variant. Overall, this research study shows that the combined pyrolysis process using Zn/13X zeolite and Cr/13X zeolite catalysts and acid-pretreated biomass would be very effective in achieving a good conversion rate of the compounds containing oxygen to aliphatic and aromatic hydrocarbons and, therefore, to produce bio-oil of improved quality.