Efficient conversion of cotton stalks over a Fe modified HZSM-5 catalyst under microwave irradiation

Abstract

Microwave-assisted catalytic liquefaction is a promising technology, which converts biomass to liquid valuable fuels via a fast and efficient way. In this study, HZSM-5 based catalysts were usefully obtained through the liquefaction of cotton stalk. Results showed that compared with the conventional thermal liquefaction process, microwave irradiation could promote the cleavage of C–C bonds, and the yield of aromatic compounds increased from 8.4% to 14.0%; doped amounts of Fe species to HZSM-5 distinctly improved the catalytic performance of the catalyst due to enhanced total and weak acid sites. It was found the increased total acid sites apparently facilitated the liquefaction yield of cotton stalk by 19.7%, while the weak acid sites promote the formation of aromatics and long-chain compounds.

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Introduction

With the rapid depletion of traditional fossil fuels and increasing severe problems on the environment, finding a renewable and recyclable resource is extremely urgent.^1,2 Biomass has become one of the most promising substitutes because it is the only renewable source that can be made into liquid carbon-based fuels.^3,4 Bioconversiontechnology was a major technology in producing first generation biofuel from biomass, and this kind of biofuel was mainly obtained from plant seeds, which is diverged from stressfully food problem. The biomass such as crop residues, branches and leaves were difficult to transform to bio-oil by biodegradation. In addition, many factors can critically influence the bioconversion process of biomass, such as the microbe type, acidity and alkaline, temperature, nitrogen, phosphorus, the presence of phenolic compounds and other inhibitors.⁵ Therefore, the large scale industrialization of bio-oil from biomass by bioconversion was stagnant for the reasons mentioned above.⁶ Recently, catalytic liquefaction technology has proved to be an effective method for the transformation of biomass to bio-oil.^7,8

Normally, biomass liquefaction processes were carried out on the use of conduction/convection as the thermal resource, which often suffered from time-consuming and low liquefaction rates.⁹ Compared with the conventional (conduction/convection) thermal resource, microwave irradiation is based on interfacial heat transfer, and uses the ability of direct heating of the target object due to the applied electromagnetic field. Recently, microwave irradiation has been increasingly used to reduce the reaction temperature, shorten the reaction time and improve the yield of target product during liquefaction technology.^10,11

During the microwave assisted liquefaction process, strong corrosive acids such as concentrated sulfuric acid, p-toluenesulfonic, and phosphoric acid were often used as the catalyst in most of the previous studies, which can cause extreme damage to reaction equipment and steel tubes in the actual production.^12,13 Since solid catalysts especially HZSM-5 are preferred over homogeneous acid catalysts, numerous studies have been conducted for the production of bio-oil or platform chemicals using various solid acid catalysts.^14–17 Cotton stalk, as one of the most widespread and abundant crop residues in China, should be paid more attention in order to fulfil the large-scale industrial production of bio-energy,¹⁸ the total amount of cotton stalk biomass energy resources in China are huge with millions of tons produced every year,¹⁹ the efficient utilization of them will reduce the millions of dollars spent on petroleum consumption every year and alleviate greatly the pressure on the environment, however, the low energy density and extensive distribution make the utilization of cotton stalk biomass energy not easy, the liquid bio-fuels are easily preserved and transported, therefore, a study of catalytic liquefaction of cotton stalk to bio-oil is quite necessary. Thus, the main purpose of our work is to investigate the possibility of obtaining liquid fuels from cotton stalk by microwave assisted liquefaction technology with solid acid catalysts, and to the best of our knowledge, research about microwave assisted liquefaction technology of biomass with solid acid catalysts has seldom been reported in the literature.

Results and discussion

Effects of catalysts on bio-oil yield and composition

As mentioned previously, HZSM-5 has been reported to have a tunable acidity, three-dimensional porosity, and the capability of stabilizing active metal clusters.¹⁶ Therefore, HZSM-5 was selected first as the solid acid catalyst to compare the influence of the thermal resource on the liquefaction of cotton stalk.

The results showed that the liquefaction yield of cotton stalk markedly increased from 15.5% of the conventional reaction into 30.4% of the microwave irradiation. To further analyse the influence of the thermal resource, the obtained liquefied products were analysed by GC-MS. The GC-MS data showed that more than 100 kinds of organic species were observed from each sample, among which chemicals with a relative peak area of more than 1% are listed in Tables S1 and S2 (ESI†). The major compounds in the bio-oils can be classified as aliphatic hydrocarbons, aromatics and some nitrogen containing compounds, as well as some other oxygen containing compounds including alcohols, carboxylic acids, which is consistent with previous studies.^19,20 As can be found from Tables S1 and S2,† the thermal resource has a clear influence on the bio-oil components. Compared with the conventional thermal liquefaction process, microwave irradiation increases the yield of aromatic compounds from 8.4% to 14.0% under the same conditions.

As is well known, microwave irradiation provides homogeneity of temperature in the reactor compared with conventional heating. Besides this thermal effect, specific non-thermal effects of microwave irradiation in chemical reactions are basically ascribed to the increase of the pre-exponential factor and the decrease of the activation energy in the Arrhenius equation. Consequently, the cleavage of the C–C bond in the liquefaction reaction which is difficult by conventional heating can be easily performed under microwave irradiation, and hence more aromatic compounds were formed, which is in accordance with previous studies.¹¹

Since, microwave irradiation promotes the cleavage of C–C bonds, and increases the liquefaction yield of cotton stalk, the liquefaction yield of cotton stalk over HZSM-5 was only 30.4%. Nevertheless, such a result is still under our expectations. As is well known, the yield of the liquefaction bio-oils mainly depends on the acid sites of the catalyst. Based on the above analysis, to further improve the catalytic performance of HZSM-5, the acidic sites of the catalyst should be modified. The incorporation of metals into a HZSM-5 support might bring significant changes in their acid, which would lead to a strong influence on the catalytic performance. In view of this, as one of the most used metal cations in catalysts, Fe was employed to modify HZSM-5 to obtain an excellent catalyst for microwave-assisted liquefaction of cotton stalk, and the results of the modified catalyst are listed in Fig. 1. As described in Fig. 1, with the addition of Fe to the catalyst, the liquefaction yield significantly increased, and the maximum liquefaction yield 36.4% was obtained when the concentration of Fe(NO₃)₃ solution is 0.02 mol L⁻¹. These results demonstrated that the catalytic properties of the catalyst in the liquefaction process of cotton stalk were evidently influenced by the introduced Fe. To better understand the effects of doped Fe, the chemical components of the bio-oils which were produced in the presence of 0.02-Fe/HZSM-5 were analyzed by GC-MS and the obtained results are listed in Table S3 (ESI†).

Fig. 1 Influence of Fe loading on the liquefaction yield.

Table S3† indicated that the introduced Fe remarkably changed the component distribution in the liquid oils. When 0.02-Fe-HZSM-5 was used as the catalyst, the total content of C₁₉H₃₆O₂ and C₂₄H₃₈O₄ accounted for almost half of the sample, while components of the bio-oil in Table S2† mainly contained C₃H₆O₂, C₁₉H₃₆O₂, and C₂₄H₃₈O₄, demonstrating that the Fe-HZSM-5 catalyst promoted the formation of long chain compounds. It is clear that the formation of those long chain compounds could apparently decrease the oxygen content and help to improve the quality of the bio-oil.²¹ Another significant influence of the doped Fe to HZSM-5 is the formation of aromatics, as can be found in Table S3,† the aromatics in the product increased to 33.0% from the original 14.0%, among which bis(2-ethylhexyl)phthalate increased greatly. And this may be due to the catalyst promoted degradation of lignin of cotton stalk components.²² In summary, for the Fe modified HZSM-5 catalyst, the content of long-chain compounds and aromatic species were remarkably increased, contributing to a distinct decrease in oxygen content. Since, Fe-HZSM-5 exhibited an excellent performance for liquefaction of cotton stalk, the obtained catalysts were investigated by physical methods.

Catalyst characterization

Textural properties. The textural properties of the parent and modified HZSM-5 catalysts were investigated by nitrogen sorption. The values of the surface area and microporous volume are given in Table 1. The obtained data showed that the BET surface area and the microporous volume of HZSM-5 were apparently decreased with the introduction of Fe, which may be attributed to the small oxide aggregates on the external surface and a partial blockage incorporation of metal ions into the microporous space of HZSM-5. However, this results is consistent with previous studies, indicating that the Fe was successfully incorporated in to HZSM-5.¹²

Table 1 The textural properties of the parent and Fe modified HZSM-5 catalysts

Sample	Surface area (m^2 g^−1)	Micro pore volume (cm^3 g^−1)
HZSM-5	368	0.158
0.01-Fe-HZSM-5	321	0.116
0.02-Fe-HZSM-5	306	0.108
0.03-Fe-HZSM-5	285	0.098
0.04-Fe-HZSM-5	227	0.057

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Structure and morphology. The XRD patterns of the parent HZSM-5 and the Fe modified HZSM-5 catalysts are shown in Fig. 2. All samples exhibited the typical diffraction pattern corresponding to the MFI structure, confirming that the main crystalline structure of HZSM-5 was preserved during the modification process.

Fig. 2 XRD patterns for (a) HZSM-5, (b) 0.01-Fe-HZSM-5, (c) 0.02-Fe-HZSM-5, (d) 0.03-Fe-HZSM-5, (e) 0.04-Fe-HZSM-5.

It was reported that the doped Fe species would be oxidized during the calcination process, and result in the formation of iron oxide.¹⁴ To confirm the type of iron oxide, X-ray photoelectron spectroscopy (XPS) of 0.02-Fe-HZSM-5 was carried out and the obtained spectra are presented in Fig. S1 and S2 (ESI†). As shown in Fig. S1,† the peak at about 711.5 and 724.0 eV corresponding to the spin–orbit split doublet of Fe 2p_3/2 and Fe 2p_1/2, indicated the presence of iron element on the catalyst surface mainly in the form of Fe³⁺ species. The O 1s XPS of the sample showed a single peak at 532.2 eV corresponding to the oxide oxygen (O²⁻) in Fe₂O₃. Therefore, these results indicated that the Fe species were predominantly in the form of Fe₂O₃, which is in accordance with previous studies.²³

The TEM images (ESI, Fig. S3†) of the catalyst demonstrated that the Fe₂O₃ dispersed well on the surface of HZSM-5, and with increasing concentration of Fe(NO₃)₃ solution, the average size of the Fe₂O₃ sites is slightly increased. However, the average size of Fe₂O₃ of all the catalysts are no more than 20 nm. Furthermore, compared with the parent HZSM-5, no new peak appeared in the XRD patterns of Fe modified catalysts, suggesting that the Fe₂O₃ were highly dispersed as very small particles on the zeolite, which is in accordance with the TEM results.

FT-IR. In order to investigate the structural properties of HZSM-5 and Fe-HZSM-5 catalysts, FT-IR analyses were carried out in the range of 4000–400 cm⁻¹. From Fig. 3, it can be observed that there was no significant band position shift, which indicated that the introduction of Fe did not significantly affect the structure of HZSM-5. This suggested that almost no loss of crystallinity occurred in the zeolite samples after metal modification, which was also in agreement with the XRD results. It was reported that the IR spectra of Fe₂O₃ can be detected at 445, 460 and 635 cm⁻¹.²⁴ However, no obvious IR spectra of iron oxides were observed for all the zeolite catalysts as depicted in Fig. 3. This may was due to the amount of Fe₂O₃ being too small to be detected by FT-IR. The typical band of zeolite at around 3450 cm⁻¹ indicated the existence of acidic sites on the materials.²³ The strong IR band within this region signified that the catalyst possesses a substantial amount of acid sites, which is essential for the catalytic activity to take place. As observed from the FTIR spectra, the intensity of this band was reduced for all Fe-HZSM-5 catalysts as the Fe cation interacted with the OH group of HZSM-5.

Fig. 3 FTIR spectra of (a) HZSM-5, (b) 0.01-Fe-HZSM-5, (c) 0.02-Fe-HZSM-5, (d) 0.03-Fe-HZSM-5, (e) 0.04-Fe-HZSM-5.

TGA. Thermal gravimetric analysis of HZSM-5 and Fe modified HZSM-5 zeolite catalysts as shown in Fig. 4 indicates the mass variations in the catalyst samples from 30 to 800 °C at a heating rate of 10 °C min⁻¹ along with 20 mL min⁻¹ of nitrogen purging. Two mass variation stages can be found over the TG curves, which is consistent with the typical TG curves of the zeolite samples.^25,26 Clearly the first stage in the temperature range of 30–120 °C can be attributed to the release of water molecules from the large cavities of the zeolite, while the second stage of mass loss taking place in the temperature range of 200–800 °C can be assigned to the dehydroxylation of –OH groups on the zeolite surface. No significant mass losses are observed for all the Fe modified catalysts over 550 °C. From this analysis, the calcination of the Fe-HZSM-5 zeolite catalysts is sufficient at 550 °C to validate the interaction of Fe with HZSM-5 zeolite through the ion exchange process, and also signified the excellent thermal stability of these catalysts. Fig. 4 also shows that the weight of adsorbed water on these catalysts slightly decreased as the concentration of Fe(NO₃)₃ solution increased. This may be due to a loss of pore volume of the modification catalyst, however, this result is consistent with textural properties as shown in Table 1.

Fig. 4 TGA profiles for (a) HZSM-5, (b) 0.01-Fe-HZSM-5, (c) 0.02-Fe-HZSM-5, (d) 0.03-Fe-HZSM-5, (e) 0.04-Fe-HZSM-5.

NH₃-TPD. The acidity of the parent HZSM-5 and Fe-HZSM-5 was determined by NH₃-TPD, and the results are shown in Fig. 5. All of these catalysts exhibited two typical desorption peaks, which could be assigned to the weak (120–300 °C) and strong acid sites (above 400 °C), respectively.^27–29 The total amount of ammonia desorption is listed in Table 2, which represents the total sum of the weaker and stronger acid sites. With the addition of Fe, the intensity of the low desorption temperature peak was increased, suggesting a distinct increase in the weak acid sites, which may be attributed to the creation of Lewis acid sites of Fe(OH)²⁺.³⁰ While the acidity of strong acid sites over HZSM-5 were sharply suppressed by Fe species, this may due to the Brønsted acid protons of HZSM-5 being substituted by Fe³⁺ during the ion exchange process.³¹ Table 2 also displayed that, with the increased concentration of Fe(NO₃)₃ solution, the total amount of ammonia desorption increased to 1.226 and then slightly decreased. The total acidity of 0.03-Fe-HZSM-5 and 0.04-Fe-HZSM-5 were lower than that of 0.02-Fe-HZSM-5 catalyst. This is probably due to the presence of Fe₂O₃ covering the acid sites on HZSM-5 catalyst surface as can be found from the nitrogen sorption results in Table 1. The same trend has also been reported previously, where metal oxides probably partially covered the acidic sites, thus reducing the catalyst acidity.^23,25

Fig. 5 NH₃-TPD profiles for (a) HZSM-5, (b) 0.01-Fe-HZSM-5, (c) 0.02-Fe-HZSM-5, (d) 0.03-Fe-HZSM-5, (e) 0.04-Fe-HZSM-5.

Table 2 The total amount of ammonia desorption of different catalysts

Sample	Weak ammonia acidity (mmol g^−1)	Strong ammonia acidity (mmol g^−1)	Total ammonia acidity (mmol g^−1)
HZSM-5	0.730	0.456	1.186
0.01-Fe-HZSM-5	0.743	0.444	1.187
0.02-Fe-HZSM-5	0.783	0.443	1.226
0.03-Fe-HZSM-5	0.803	0.413	1.216
0.04-Fe-HZSM-5	0.769	0.316	1.085

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During the liquefaction process, the acidity of the catalyst was responsible for alkylation, isomerization, cyclization, and aromatization reactions. In addition, the aromatics were thought to be derived from the secondary catalytic cracking of some lignin-like compounds by breaking the C–O and C–C bonds, and this cracking can be facilitated by the acid sites of the catalyst. As can be found from the NH₃-TPD results, when metal was introduced into the zeolite, the acid sites of the catalyst were greatly increased. So the amount of aromatics and long-chain compounds increased with the increase of acid sites of the catalysts. Clearly, these results are consistent with the GC-MS analysis results as can be found in Tables S2 and S3.†

Experimental

Materials and catalyst preparation

The cotton stalk was collected from Shenxian, Shandong province, China. The sample was sun dried for several days and milled to 60 mesh, then it was stored in sealed containers for experiment. The parent HZSM-5 powder with a nominal Si/Al ratio of 25 was purchased from Nankai University catalyst Co., Ltd. The iron nitrate was purchased from Tianjin Guangfu Fine-chemical institute. All the commercially available solvents and reagents were used without further purification.

X-Fe-HZSM-5 (X means the concentration of the nitrate solution used for the ion exchange) were prepared by an ion exchange method immersing the HZSM-5 in an aqueous solution of Fe(NO₃)₃ at different concentrations. For example, 0.02-Fe-HZSM-5 was prepared as follows: 300 mL 0.02 mol L⁻¹ Fe(NO₃)₃ solution was prepared, HZSM-5 was added into the solution to form a suspension, then the suspension was stirred for 24 h. After that it was filtered and washed with 200 mL deionized water 4 times, and then the residue was dried in air at 110 °C for 6 h, calcined at 550 °C for 5 h.

Catalytic test

The conventional liquefaction reaction was conducted in a 100 mL three-necked flask as follows: 4.0 g cotton stalk powder, 60.0 mL ethylene glycol (EG) and 2.0 g HZSM-5 were mixed thoroughly before being put into the flask, then the flask was placed in an oil bath (150 °C) with refluxing and stirring for 30 min.

Microwave-assisted liquefaction reaction was carried out in a MAS-II microwave oven system which was purchased from Sineo Microwave Chemical Technology Co. Ltd. The reaction mixture which contained 4.0 g cotton stalk powder, 60.0 mL EG and 2.0 g catalyst was mixed completely via magnetic stirring during liquefaction. The reaction oven was quickly heated to 150 °C under 500 W power. After 30 min, the reaction vessels were allowed to cool at room temperature and the reaction mixture was collected and separated by filtration at about 90 °C, the reaction residue was dried at 110 °C overnight and then was weighed at room temperature. The bio-oil yield was calculated according to our previous studies.⁹ In addition, no gas products were observed during the reaction due to the low reaction temperature.

The compositions of the bio-oils were analyzed with a HP 6890/5793 gas chromatograph and mass spectrometer (GC-MS). Chromatographic conditions: the GC was equipped with a 30 m × 0.25 mm × 0.25 μm fused 5% benzyl polysiloxane quartz capillary column. High-purity helium was used as carrier gas with a constant flow of 1.0 mL min⁻¹, and the split ratio was 0, the injector temperature was 280 °C, the main GC oven temperature program was set at 70 °C and held for 2 minutes, then heated to 280 °C at 10 °C min⁻¹ and maintained for 15 minutes. Mass spectrometer conditions: the ionization mode of the GC-MS was electron impact ionization (EI) with 70 eV of electron energy, the temperature of the ion source and quadrupole rod, respectively, are 250 °C and 130 °C. The quality of the scanning mode was full scan with a standard tuning mode, the scanned area was set from 10 amu to 550 amu. The standard mass spectrometry search library was NIST05.

The bio-oil was analyzed by GC-MS after filtration without any further treatment, and the amount of each testing sample was 0.2 μL. The particulars of its chemical components would be given by contrasting the mass spectrogram with the NIST05 search library and verifying with a mass spectrometer atlas. The relative mass fraction of each composition in the bio-oil was detected by GC area normalization method. Each sample was detected 3 times by the same procedure in order to validate the reproducibility of the GC-MS data.

Catalyst characterization

The crystal structures of the catalysts were tested by using an X-ray diffraction system (XRD, Bruker D8 ADVANCE, Germany) with a maximum power of 18 kW, voltage 40 kV, current 40 mA, Cu/K-α, and scanning speed of 5° min⁻¹ of 2θ from 5° to 80°. X-ray photoelectron spectroscopy (XPS) was carried out with a PHI1600 spectrometer. High-resolution transmission electron microscopy (HR-TEM) images were observed using a JEOL electron microscope (JEM-2010). NH₃-temperature programmed desorption (NH₃-TPD) was carried out on a TP-5080 instrument with a thermal conductivity detector (TCD). The BET surface areas of the catalysts were determined through a N₂ adsorption–desorption study performed at −196 °C using a Quantachrome Autosorb IQ-C instrument. Before the measurement, the samples were evacuated at 200 °C for 2 h. Fourier transform infrared (FT-IR) spectra of the samples pressed in KBr pellets were collected in a Thermo Nicolet 6700 FTIR spectrometer in the region of 4000–400 cm⁻¹. The thermal stability of the catalysts were studied by Pyris 1 TGA at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C.

Conclusions

In this paper, the liquefaction of cotton stalk to bio-oil was investigated over HZSM-5 based catalysts. Microwave irradiation could promote the cleavage of C–C bonds compared with the conventional thermal liquefaction process, and increase the yield of aromatic compounds under the same conditions. Doped amounts of Fe to HZSM-5 catalyst facilitated the liquefaction of cotton stalk by 19.7% during the microwave-assisted liquefaction process and GC-MS analysis indicated that the content of aromatics and long-chain compounds increased greatly. Therefore, the Fe species modified HZSM-5 shows good potential and a beneficial nature for the preparation of bio-oil from cotton stalk with high efficiency.

Acknowledgements

Supported by National Natural Science Foundation of China (Grant No. 21406103), and Natural Science Foundation of Shan Dong Province (Grant No. ZR2015BM014).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26918k

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