Article pubs.acs.org/IECR
Catalytic Upgrading of Corn Stalk Fast Pyrolysis Vapors with Fresh and Hydrothermally Treated HZSM‑5 Catalysts Using Py-GC/MS Bo Zhang, Zhaoping Zhong,* Kuan Ding, Yuanyuan Cao, and Zhichao Liu Key Laboratory of Energy Thermal Conversion and Control of the Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: Fresh HZSM-5 catalyst was hydrothermally treated with 20% steam partial pressure at 450, 500, and 550 °C. Catalytic upgrading of corn stalk fast pyrolysis vapors with fresh and hydrothermally treated HZSM-5 catalysts was studied by means of analytical Py-GC/MS. Hydrothermal treatment caused a reduction in the surface area, micropore surface area, pore volume, micropore volume, pore size, total acid sites (both weak acid sites and strong acid sites), and coke yield of the HZSM-5 catalyst. The product distribution and the quality of the pyrolysis vapors were also affected. HZSM-5 catalyst with higher hydrothermal treatment temperature resulted in a higher relative content of hydrocarbons and lower relative contents of acids, esters, sugars, and furans. Esters, sugars, and furans were completely converted when HZSM-5 catalyst with 550 °C hydrothermal treatment was used. The relative contents of alcohols, carbonyls, and phenols first increased and then decreased sharply with a rise of hydrothermal treatment temperature. The increase of hydrothermal treatment temperature was also conducive to the decrease of oxygen content in the organic pyrolysis vapors.
1. INTRODUCTION High crude oil prices, national energy security issues, and the global climate change crisis have stimulated considerable interest in the development of renewable energy production from lignocellulosic biomass. Fast pyrolysis of biomass is widely used technology to produce alternatives for fossil fuels, called bio-oil.1−4 However, certain obvious drawbacks of bio-oil restrict its wide application, such as its low heating value, poor thermal stability, high viscosity, acidity, and corrosiveness.5−7 Therefore, it is necessary to upgrade bio-oil so that the fuel usual standard specifications can be reached. Catalytic upgrading of the fast pyrolysis vapors in the presence of acidic catalysts (HZSM-5, Al-MCM-41, Al2O3, etc.) is one of the most effective methods,8−10 which has attracted great attention. Among all of the selected acidic catalysts, HZSM-5 catalyst has shown attractive performance in the catalytic fast pyrolysis of biomass.11−13 The addition of HZSM-5 catalyst accomplishes the removal of oxygenated organic compounds in bio-oil through catalytic cracking reactions and so on, thus improving the quality of bio-oil.14,15 However, as for the HZSM-5 catalyst, hydration and dehydration reactions may occur in the presence of steam and high temperature (hydrothermal condition), leading to the hydrolysis of framework aluminum.16,17 Consequently, acidic characteristics and catalytic activities of HZSM-5, which are closely associated with framework aluminum,18,19 will be changed. The catalytic fast pyrolysis process of biomass will inevitably result in the formation of severe hydrothermal conditions, because H2O is one of the main products and high temperature is needed. Therefore, HZSM-5 catalyst will be continuously influenced by hydrothermal conditions. For the sake of long-term reliability in engineering, it is necessary to carry out research to evaluate the influence of hydrothermal conditions on the HZSM-5 catalyst. © 2014 American Chemical Society
Hydrothermal treatment is commonly used in the laboratory to simulate the influence of hydrothermal conditions on catalysts. The influence of hydrothermal treatment on the catalyst has been studied widely in the petrochemical industry,20−24 but few studies have been done in the field of catalytic fast pyrolysis of biomass. Mante et al.25 treated FCC catalyst and ZSM-5 additive with 100% steam at temperatures of 732 and 788 °C for 4 h, and the hydrothermally treated catalysts were used for the catalytic pyrolysis of poplar. Their results showed that hydrothermal treatment led to a specific surface area loss of FCC catalyst and ZSM-5 additive to some degree. Meanwhile, the product distribution was also affected. For the FCC catalyst, hydrothermal treatment increased the yields of gas and organic liquid and reduced the yields of coke and water; for the ZSM-5 additive, hydrothermal treatment increased the yield of organic liquid and reduced the yield of the gas, whereas the yields of char and coke were affected slightly. Iliopoulou et al.26 studied the influence of hydrothermal treatment on the physical and chemical properties of Al-MCM-41 catalyst and catalytic re-forming of bio-oil. They found that hydrothermal treatment contributed to the decrease in the specific surface area and the acid sites of Al-MCM-41 catalyst, but had little effect on the bio-oil catalytic re-forming. In this work, HZSM-5 catalyst was treated under a hydrothermal condition, and quantitative pyrolysis−gas chromatography/ mass spectrometry (Py-GC/MS) experiments were conducted to evaluate the influence of hydrothermal treatment on the catalytic upgrading of corn stalk fast pyrolysis vapors. The chemical compositions of the upgraded vapors were analyzed, and their relative contents were compared and discussed. Received: Revised: Accepted: Published: 9979
December 29, 2013 May 26, 2014 May 27, 2014 May 27, 2014 dx.doi.org/10.1021/ie404426x | Ind. Eng. Chem. Res. 2014, 53, 9979−9984
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of 275 °C and interface temperature of 300 °C were held. A capillary column named HP-5MS (30 m × 0.25 mm × 0.25 μm) was used for the GC separation. The split ratio was set at 1:80. The MS was operated with the ionization energy of 70 eV and the scan rate of 35−550 amu/s. The experiments were repeated at least three times. 2.4. Data Processing. During the experiments, the selectivity of aromatics and the oxygen content in organic pyrolysis vapors were calculated and analyzed. The selectivity of a certain kind of aromatics was defined and calculated as
2. EXPERIMENTAL SECTION 2.1. Biomass. Corn stalk used in the experiments was from the city of Zhumadian in Henan province, China. Prior to the beginning of the experiments, the corn stalk was smashed and sieved through a 40 mesh sieve. Then the feedstock was dried at 105 °C for 24 h. The elemental composition of the corn stalk (air-dry, ash-free basis) was 46.73 wt % carbon, 6.11 wt % hydrogen, 0.59 wt % nitrogen, and 46.57 wt % oxygen (by difference). The proximate analysis of the corn stalk (air-dry basis) was 9.27 wt % moisture, 71.73 wt % volatile, 6.06 wt % ash, and 12.94 wt % fixed carbon (by difference). 2.2. Catalyst. The HZSM-5 zeolite catalyst (silicon-toaluminum ratio is 50) used in this study was bought from the Catalyst Plant of Nankai University. The catalyst was hydrothermally treated in a horizontal tubular reactor with 20% steam partial pressure (H2O/N2) at 450, 500, and 550 °C for 4 h, respectively. Fresh HZSM-5 catalyst, which was not hydrothermally treated, was labeled HZSM-5(NHT), and HZSM-5 catalysts from 450, 500, and 550 °C hydrothermal treatments were labeled HZSM-5(450), HZSM-5(500), and HZSM-5(550), respectively. The hydrothermal conditions applied in this experiment (20% steam partial pressure; 450, 500, and 550 °C) were selected for the simulation of steaming atmosphere, which occurred during the catalytic upgrading of biomass fast pyrolysis vapors at the temperatures of 450−550 °C. 2.3. Methods. As a very practical technique, Py-GC/MS can provide critical information about the properties of pyrolysis vapors. Fast pyrolysis of corn stalk and catalytic upgrading of primary pyrolysis vapors were carried out in a CDS Pyroprobe 5200 pyrolyzer. Prior to the experiments, the open-ended quartz tube was successively filled with some quartz wool, 0.50 mg of HZSM-5 catalyst samples, some quartz wool, 0.50 mg of powdered corn stalk samples, some quartz wool, 0.50 mg of HZSM-5 catalyst samples, and some quartz wool. The HZSM-5 catalyst samples were placed at both sides of the corn stalk sample. Thus, the noncatalytic fast pyrolysis took place first and then the product vapors were upgraded when they passed through the HZSM-5 catalyst samples. The schematic cross section of the quartz tube is shown in Figure 1. During the
Saromatic =
caromatic ∑ caromatic
(1)
where Saromatic is the selectivity of a certain kind of aromatic, such as benzene or toluene, caromatic is the relative content of a certain kind of aromatic, and ∑caromatic is the relative content of all aromatics. The oxygen content in organic pyrolysis vapors was calculated on the basis of the relative content and oxygen content of each chemical composition. The oxygen content was defined and calculated as Cox =
∑ (ci × Ci ,ox)
(2)
where COX is the oxygen content in organic pyrolysis vapors, ci is the relative content of a certain kind of chemical compositions, and Ci,ox is the oxygen content of a certain kind of chemical compositions.
3. RESULTS AND DISCUSSION 3.1. Influence of Hydrothermal Treatment on HZSM-5 Catalyst. Porosity and acidity characteristics of HZSM-5 catalysts were analyzed by N2 porosimetry and ammonia TPD. The BET equation was used in the calculation of the surface area, and the t-plot method was used for the analysis of the microporosity; the results are listed in Table 1. As can be seen, hydrothermal treatment resulted in a loss of surface area, micropore surface area, pore volume, micropore volume, and pore size of the HZSM-5 catalyst, and all of these parameters decreased with a rise in the hydrothermal treatment temperature. The results showed reductions of 4.07, 6.84, and 8.83% in the surface area of the HZSM-5 catalyst after hydrothermal treatment at 450, 500, and 550 °C respectively. The HZSM-5 catalyst framework would be dealuminated through high-temperature hydrolysis reaction by hydrothermal conditions,27 leading to the generation of hydroxy holes (reaction 3). Meanwhile, a small amount of HZSM-5 catalyst would inevitably suffer the collapse of crystal structures in the process of hydrothermal treatment, producing some amorphous SiO2. The amorphous SiO2 would fill those hydroxy holes through a dehydration reaction (reaction 4). Taken together, part of the framework aluminum (Al) was replaced with silicon (Si).
Figure 1. Schematic cross section of the quartz tube.
experiments, the feedstock was pyrolyzed at the temperature of 550 °C, which was held for 20 s. Besides, the heating rate was set at 20 °C/ms. Meanwhile, high-purity helium (99.999%) was used as carrier gas at a constant flow of 1.0 mL/min. The pyrolysis vapors flowed from the quartz tube into a gas chromatograph/mass spectrometer (7890A/5975C, Agilent) via the helium sweeper gas stream. The GC/MS injector temperature 9980
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Table 1. Porosity and Acidity Characteristics of HZSM-5 Catalysts porosity characteristics
acidity characteristics
catalyst
surface area (m2/g)
micropore surface area (m2/g)
pore volume (cm3/g)
micropore volume (cm3/g)
pore size (nm)
HZSM-5(NHT) HZSM-5(450) HZSM-5(500) HZSM-5(550)
308.08 295.53 287.02 280.89
179.46 169.82 163.73 158.45
0.2114 0.1701 0.1421 0.1166
0.1669 0.1283 0.1076 0.0890
2.7447 2.3023 1.9803 1.6604
The Si−O bond length (0.166 nm) was shorter than the Al−O bond length (0.175 nm). As a result, the crystal cell shrank, pore size narrowed, and pore volume decreased. Therefore, surface areas, micropore surface areas, pore volume, micropore volume, and pore size of HZSM-5 catalyst decreased. Moreover, a higher hydrothermal treatment temperature was more conducive to the dealumination reactions in the HZSM-5 catalyst framwork, so the surface areas, micropore surface areas, pore volume, micropore volume, and pore size decreased with the increase in hydrothermal treatment temperature. In addition, as the acidity of the HZSM-5 catalyst was caused by framework aluminum,24,26 hydrothermal treatment, therefore, could result in a loss of catalyst acid sites through the dealumination reaction. As shown in Table 1, hydrothermal treatment led to a decrease in the total acid sites of HZSM-5 catalyst (both weak acid sites and strong acid sites). Besides, both weak acid sites and strong acid sites decreased with the increase of hydrothermal treatment temperature. However, it was not clear whether the combined effect of hydrothermal treatment was positive or negative, and it had been studied by Py-GC/MS experiment in this paper. 3.2. Product Yields. It was known that products could not be collected during the Py-GC/MS experiments, so it seemed that the yields of total products could not be analyzed quantitatively. However, the changes of product yields could be revealed by comparison of the total chromatographic peak areas,28,29 as the masses of corn stalk and HZSM-5 catalyst and the operating parameters of the Py-GC/MS equipment were kept exactly the same during each experiment. The results are shown in Figure 2. Compared to HZSM-5(NHT) catalyst, as illustrated in Figure 2, hydrothermally treated HZSM-5 catalysts led to a reduction in product yields. Moreover, the product yields decreased dramatically with the increase in hydrothermal treatment temperature. Hydrothermal treatment promoted the reactivity of the HZSM-5 catalyst. Therefore, HZSM-5(450), HZSM-5(500), and HZSM-5(550) catalysts were benificial for the upgrading of primary product vapors. The three-dimensional pore system of HZSM-5 zeolite contained 10-membered rings with Z-shaped channels of 0.51 × 0.54 nm and straight channels of 0.54 × 0.56 nm. The primary product vapors were adsorbed on the surface of the HZSM-5 catalyst, and series reactions occur (such as deoxygenation reaction) inside the internal structure, forming the light vapors. These reactions would result
total acid sites (mmol NH3/g)
weak acid sites (300 °C, mmol NH3/g)
1.435 0.711 0.573 0.418
0.578 0.411 0.371 0.292
0.857 0.300 0.202 0.126
Figure 2. Total peak areas.
in a decrease of product vapors yield and an increase of light gas and water yields. The spent HZSM-5 catalyst samples were collected and dried at 120 °C for 1 h in a drying oven. Then the dried samples were put into a muffle furnace for combustion (650 °C, 2 h) so that the coke yields were determined. The results of coke yields are shown in Table 2. From Table 2, we can see that a lower coke Table 2. Coke Yields catalyst
HZSM5(NHT)
coke yield (%)
4.2
HZSM-5(450) HZSM-5(500) HZSM-5(550) 3.4
2.8
2.6
yield was obtained because of the hydrothermal treatment of the HZSM-5 catalyst, and the coke yield continued to decline with the increase in hydrothermal treatment temperature. 3.3. Chemical Compound Analysis. The chemical compounds discriminated and analyzed by GC/MS could be classified into several groups as carbon dioxide, hydrocarbons, acids, alcohols, esters, carbonyls, phenols, sugars, and furans. Figure 3 shows the relative contents of different groups of chemicals in upgraded pyrolysis vapors from the fast pyrolysis of corn stalks using different HZSM-5 catalysts. HZSM-5 catalyst had shown excellent catalytic upgrading effect, and the acidity characteristics of HZSM-5 catalyst had a critical influence on the catalytic effect. Fresh HZSM-5 catalyst without hydrothermal treatment (HZSM-5(NTH)) had potent activity; as a result, coke was generated easily, and pore mouth blockage could occur on the external surface of the HZSM-5 catalyst, preventing the subsequent shape-selective reactions. As for HZSM-5(450), HZSM-5(500), and HZSM-5(550) catalysts, the hydrothermal treatment process led to dealumination reactions in the HZSM-5 framwork, reducing the catalyst acidity 9981
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relative contents of carbon dioxide and hydrocarbons compared to the HZSM-5(NHT) catalyst. Moreover, the relative contents of carbon dioxide and hydrocarbons increased with the increase of hydrothermal treatment temperatures, consistent with the above disscussion. When the hydrothermal treatment temperature was 550 °C, the relative content of hydrocarbons could be as high as 50.5%. It is worth noting that the HZSM-5 pore diameter was similar to the dynamics diameters of benzene, toluene, and xylene, so the HZSM-5 catalyst had a strong shape-selective effect on this kind of product. Detailed selectivity is presented in Table 3. It can be seen that the hydrothermally treated HZSM-5 catalysts have an obvious promoting effect on the selectivity of the five kinds of aromatic hydrocarbons. Note that polycyclic aromatic hydrocarbons (PAHs) were carcinogenic toxins, and special attention should be paid to the increase in the selectivity of PAHs. Due to the optimization of surface acidity and activity of the HZSM-5 catalyst through the hydrothermal treatment process, the catalytic effect was improved. The hydrothermally treated HZSM-5 catalysts dramatically reduced the relative contents of acids, esters, sugars, and furans. Their relative contents reduced with the increase in hydrothermal treatment temperatures. Esters, sugars, and furans were completely converted when the HZSM-5(550) catalyst was used. The relative content of phenols is shown in Figure 4. The relative content of phenols tended to increase at first and then decrease dramatically with the rise of hydrothermal treatment
Figure 3. Relative contents of different groups of chemicals using different HZSM-5 catalysts.
and improving the pore mouth blockage. Besides, the higher hydrothermal treatment temperature was more conducive to the dealumination reactions and the optimization of surface acid site density and the activity of catalyst, which promoted the quality of the pyrolysis products. As can be seen in Figure 3, the hydrothermally treated HZSM-5 catalysts resulted in a remarkable increase in the
Table 3. Detailed Selectivity of Aromatics over Different HZSM-5 Catalysts catalysts compound
HZSM-5(NHT)
HZSM-5(450)
HZSM-5(500)
HZSM-5(550)
benzene toluene xylene indene PAHs
3.584 7.207 9.380 0.195 3.735
3.785 7.778 8.537 0.322 5.043
3.922 8.161 13.281 0.550 9.289
4.731 12.092 13.746 1.351 8.520
Figure 4. Relative contents of phenols using different HZSM-5 catalysts. 9982
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Table 4. Oxygen Content in Organic Pyrolysis Vapors catalysts oxygen content (%)
HZSM-5(NHT)
HZSM-5(450)
HZSM-5(500)
HZSM-5(550)
19.62
18.78
12.85
8.48
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temperature. Phenols were mainly from the pyrolysis of lignin. The HZSM-5(450) catalyst increased the relative content of phenols in the pyrolysis gas, which was due to the further catalytic pyrolysis of lignin derivatives with the HZSM-5(450) catalyst. Compared to the HZSM-5(450) catalyst, the HZSM5(500) and HZSM-5(550) catalysts resulted in a remarkable decrease in the relative content of phenols, which was due to the better deoxidization capacity of the HZSM-5(500) and HZSM-5(550) catalysts. The HZSM-5(500) and HZSM-5(550) catalysts generated fewer methoxy-containing compounds (e.g., 2-methoxyphenol, 2-methoxy-4-methylphenol, 4-ethyl-2methoxyphenol, 2-methoxy-4-vinylphenol, 4-methoxybenzene1,2-diol) and more monofunctional phenols (e.g., phenol and 4-ethylphenol). It was reported that methoxy-containing compounds acted as the polymerization precursors of bio-oils.30 These results illustrated that the catalytic upgrading of corn stalk fast pyrolysis vapors using HZSM-5(500) and HZSM-5(550) catalysts could produce a more stable bio-oil. The relative contents of alcohols and carbonyls showed a similar tendency. The oxygen content in organic pyrolysis vapors could be calculated on the basis of the relative contents of various chemical compositions (see Table 4). As shown in Table 4, hydrothermally treated HZSM-5 catalysts led to a decrease in the oxygen contents in organic pyrolysis vapors, compared to the HZSM-5(NHT) catalyst. The increase of hydrothermal treatment temperature resulted in a remarkable decrease in the oxygen contents in organic pyrolysis vapors. The HZSM-5(550) catalyst contributed to a >10% decrease in the oxygen content, decreasing from 19.62 to 8.48%.
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4. CONCLUSION The effect of hydrothermal conditions on catalytic upgrading of corn stalk fast pyrolysis vapors was studied. Reductions in the surface area, micropore surface areas, pore volume, micropore volume, pore size, total acid sites (both weak acid sites and strong acid sites), and coke yield of the HZSM-5 catalyst were achieved after hydrothermal treatment at 450, 500, and 550 °C, respectively. Besides, the HZSM-5 catalyst with higher hydrothermal treatment temperature produced higher relative contents of carbon dioxides and hydrocarbons and lower relative contents of acids, esters, sugars, and furans. The relative contents of alcohols, carbonyls, and phenols first increased and then decreased sharply with a rise of hydrothermal treatment temperature. The increase of hydrothermal treatment temperature was also conducive to the decrease of oxygen content in organic pyrolysis vapors.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*(Z.Z.) E-mail:
[email protected] or
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by the National Basic Research Program of China (973 Program) (Grant 2013CB228106). 9983
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