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Catalytic Conversion of Biomass for Aromatics Over HZSM-5 Modified by Dawson-Type Phosphotungstic Acid

  • Yongsheng FanEmail author
  • Lele Fan
  • Lei Zhu
  • Jiawei Wang
  • Wei Ji
  • Yixi Cai
  • Weidong Zhao
Article
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Abstract

The multi-functional zeolites modified with different phosphotungstic acid (PW) ratios were prepared and characterized. Thermogravimetric tests coupled with kinetics analysis were performed to evaluate catalytic pyrolysis behavior of biomass; The presence of HZSM-5 inhibited the product diffusion, causing a slight delay of pyrolysis; The PW modification made the reaction more complex; reaction order increased from 1.96 to 2.12. The activation energy decreased from 53.32 to 31.15 kJ/mol with the increase of PW. Then, fixed-bed catalytic experiments were further conducted to investigate aromatics production; 10%PW modification gave the appropriate acidic distribution and pore structure, resulting in oxygen being more likely to be removed in the form of COx. Although the organic yield was only 10.73%, HHV reached 38.01 MJ/kg. The organic phase catalyzed by 10%PW/HZSM-5 exhibited higher aromatization degree, and the structures of benzene ring were mainly single-rings. The oxygenates (especially for phenols from 16.88% to undetected) reduced obviously with increasing PW loading, and the 10%PW modification gave the highest content (peak area, %) of desirable mono-aromatic hydrocarbons (52.29%). The GC/MS analysis results were basically consistent with the 1H/13C NMRs. Besides, the 10%PW/HZSM-5 had the highest catalytic stability and the spent catalyst could recover high activity after regeneration. Therefore, catalytic pyrolysis of biomass using Dawson-structured PW-modified HZSM-5 is a promising approach for production of light aromatic hydrocarbons.

Keywords

Rapeseed shell Catalytic pyrolysis HZSM-5 Phosphotungstic acid Aromatics 

Abbreviations

PW

Phosphotungstic acid

TGA

Thermogravimetric analysis

FT/IR

Fourier-transform infrared

GC/MS

Gas chromatography/mass spectroscopy

XRD

X-ray diffraction

BET

Brunner-Emmet-Teller

BJH

Barrett-Joyner-Halenda

NH3-TPD

NH3 temperature programmed desorption

TCD

Thermal conductivity detector

Py-IR

Pyridine infrared

TG

Thermogravimetric

DTG

Differential thermogravimetric

GC

Gas chromatography

NIST

National Institute of Standards and Technology

NMR

Nuclear magnetic resonance

NOE

Nuclear Overhauser effect

HHV

Higher heating value

MAH

Monocyclic aromatic hydrocarbon

PAH

Polycyclic aromatic hydrocarbon

LAH

Light aliphatic hydrocarbon

(H/C)eff

Effective hydrogen to carbon ratio

BTEX

Benzene, toluene, ethylbenzene, xylene

RP

Relative proton

RC

Relative carbon

Symbols

n

Reaction order

E

Activation energy, kJ/mol

A

Frequency factor, s−1

B

Brønsted

L

Lewis

R2

Square of correlation coefficient

Introduction

So far, a large proportion (more than 80%) of energy consumption around the world comes from petroleum-derived fuels, so the environmental issues are serious such as global warming due to the emission of greenhouse gases or acid rain by releasing harmful gases (e.g., NOx and SO2). Biomass, as a promising alternative energy source, is the only renewable carbon-based source, CO2 neutral, and cost effective. Among the many forms of biomass, lignocellulosic biomass is an attractive feedstock attributed to its abundance, low cost, and non-competitiveness with the food chain. Pyrolysis, as a thermo-chemical conversion route, can be easy to handle and utilize biomass source in the absence of oxygen and produce high yield of bio-oil [1]. Nevertheless, the pyrolysis bio-oil is a complex mixture of various oxygenates derived from the decomposition of cellulose, hemicellulose, and lignin and a typical bio-oil composed of hundreds of different compounds [1, 2] whose molecular weight ranges from 18 to 5000 g/mol or more [3], thus limiting the application of bio-oil. Catalytic pyrolysis, which integrates catalysis with lignocellulosic biomass pyrolysis, is the most promising method to improve the bio-oil quality [4]. In this regard, HZSM-5 zeolite is seen to be an efficient catalyst for degradation of biomass; however, oxygenates converted on the acid sites of HZSM-5 by cracking, oligomerization, cyclization, and dehydrogenation are easy to form coke to cover active sites, or even block the pore channels due to excessive acidity and diffusion stop, further resulting in the severe deterioration of the catalytic efficiency or deactivation of coke deposition [5].

Hence, it is necessary to adjust the acidity distribution (including acid intensity and type) and the adsorption properties (including pore volume, pore size, etc.) of original HZSM-5 to adapt to the catalytic pyrolysis of lignocellulosic biomass. So far, many researchers have carried out a lot of chemical modification researches on HZSM-5, involving metallic elements such as Zn, Ni, Co, Sn, Mo, Pd, Pt, Ce, Mg, Al, Fe, Cu, Ag, and Na etc. and non-metallic elements such as P and B. The modification of these elements is conducive to the reasonable deployment of the acidity and adsorption of catalyst, the improvement of the diffusion and escape performance of products, the improvement of the selectivity of the target compounds, and the enhancement of the structural stability of catalyst. For example, P modification can increase the Lewis acid sites, promote the migration of hydrogen atom, and facilitate the generation of aromatic hydrocarbon, and P modification can enhance the stability of catalyst framework [6]. The modification of Zn, Co, Ni, and Sn is beneficial to change part of B acid sites into L acid sites and improve the aromatization performance under the condition of keeping the total acid sites unchanged [6]. The modification of Mo, Pd, Pt, and Ce can also increase the proportion of L acid sites and then improve the aromatization performance [7, 8]. The modification of Na, Mg, Al, and B decreases the amount of total acid and weak acid sites, increases the amount of medium-strength acid sites, changes little in strong acid sites, and enhances the selectivity to olefins [9, 10]. The modification of Fe, Cu, and Ag can improve the cracking and deoxidization ability of the catalyst, improve the selectivity of aromatic hydrocarbon or olefin, and enhance the catalyst stability [11, 12]. In addition, HZSM-5 has also been studied on the composite modification of some elements, most of which are the physical superposition modification of two different elements. The results showed that catalytic performance has been improved to varying degrees, and the law of modification is basically similar. However, there are few studies on the modification of HZSM-5 with multi-element and multi-functional crystal compounds, but there is no report on the application of this kind of modified HZSM-5 in biomass conversion. The PW is a kind of multi-functional material with excellent properties, which not only has the structural characteristics of complexes and metal oxides but also has acidity and catalytic activity. It has the advantages of high catalytic activity and good selectivity and is widely used in various catalytic reaction systems and Dawson-structured PW (H6P2W18O62) showed a better catalytic activity than Keggin-structured PW (H3PW12O40) in many catalytic reactions [13, 14]. In 1953, B. Dawson firstly investigated the structure of K6[α-P2W18O62]14H2O and determined the Dawson-structured model [15]. Saturated Dawson-structured anions are a common type of heteropoly metal oxygen clusters. They can be seen as derivatives of two three-vacancy Keggin-structured anions. Their general formula [X2M18O62]6−(X = PV, SiV, GeV, AsV; M = Mo, W) has D3h symmetry, and 18 coordination atoms are divided into two groups: six atoms are located in the polar position and 12 atoms are in the equator position. Dawson-structured anions can be used as excellent electron acceptors to introduce the organic groups with unpaired electrons into polyanion skeletons to react, which may be conducive to the adsorption and catalysis of organic functional groups. Zeng et al. [16] tried to modify Y-zeolite by PW and determined the catalytic activity of olefins removal from aromatics, and the results showed that the catalytic activity (especially for alkylation) of Dawson-structured PW was higher than that of Kiggen-structured PW.

Moreover, a good understanding of catalytic pyrolysis of lignocellulosic biomass should be comprehensively evaluated in terms of two aspects of thermogravimetric kinetics and reaction pyrolysis. Currently, some publications only reported the catalytic pyrolysis kinetics of lignocellulosic biomass [17, 18]; simultaneously, other studies about catalytic pyrolysis had focused on the operating parameters, biomass compositions, or catalyst types or product selectivity [19, 20], and the online conjoint analysis technologies, such as TGA-FT/IR [21], Py-GC/MS [22], are often adopted. However, these studies have defects in the study of the final composition and yield of bio-oil, which are of great value to analyze and evaluate the practicability and economy of the conversion process. Therefore, the HZSM-5 modified by Dawson-structured PW was prepared and employed as the catalyst. The first focus herein was to investigate the characteristics and kinetics of the catalytic pyrolysis of rapeseed shell. Thermal behavior of biomass pyrolysis using HZSM-5 with different PW loadings (0%, 5%, 10%, and 20%) and different heating rates (10 °C/min, 20 °C/min, and 40 °C/min) was determined by TGA. In addition, the kinetics evaluation was further conducted based on TGA data, and Kissinger [23] and Coats-Redfern [24] methods are specifically applied for n, E, and A determinations because the combination of the two methods could make the calculation be both efficient and accurate [25]. Then, the second objective was to evaluate the yields, properties, and compositions of products during catalytic pyrolysis of rapeseed shell over modified HZSM-5 via a fixed-bed reactor. Specifically, given that the reactor was simple in structure and efficient in operation, in situ upgrading method was applied in this study. To date, lots of studies had been conducted over various aspects of in situ upgrading that covered reaction factors, feedstock composition, catalyst modification, and so on and achieved some beneficial effects [26, 27]. Hu et al. [27] compared ex situ and in situ catalytic pyrolysis of biomass over HZSM-5 in a two-stage fluidized-bed/fixed-bed combination reactor and the results showed that ex situ method gave a similar carbon yield of aromatics + olefins with in situ method. In addition, to maintain consistency with TGA analysis, in situ upgrading method was necessarily used in fix-bed experiments. Overall, the aim was to provide new insight on the implementation possibility and experimental basis of catalytic pyrolysis of rapeseed shell to produce bio-fuels with high added value.

Materials and Methods

Materials

Biomass

The rapeseed shell is easily collected (rape cultivation of China ranks second in the world, nearly 8 million hectares) during the harvest of rapeseed, the shape of natural small strip decides it can be crushed easily, and it has the advantages of clean and low ash content. So far, detailed analysis of rapeseed shell has not been reported yet. Hence, rapeseed shell was chosen as the lignocellulosic biomass feedstock in this study, and the samples were obtained from a farm located in Jiangsu Province. Prior to conversion, the samples were firstly crushed to 100–150 μm range, and then processed with a series of analysis including proximate, ultimate, compositional, and calorific value analysis, and the results together with the related standard methods are listed in Table 1.
Table 1

Results of proximate, ultimate, and compositional analysis and calorific value of biomass together with related methods

Analysis

Item

Standard method

Value

Basis

Proximate

Moisture

ASTM E1756-08. Standard Test Method for Determination of Total Solids in Biomass

5.68wt.%

Received

Volatile matter

ASTM E872-82. Standard Test Method for Volatile Matter in the Analysis of Particulate Wood Fuels

75.77wt.%

Ash

ASTM E1755-01. Standard Test Method for Ash in Biomass

2.45wt.%

Fixed carbon

Calculated by difference

16.10wt.%

Ultimate

C

ASTM E777. Standard Test Method for Carbon and Hydrogen in the Analysis Sample of Refuse-Derived Fuel

46.17wt.%

Dry

H

6.08wt.%

N

ASTM E778-15. Standard Test Method for Nitrogen in Refuse-Derived Fuel Analysis Samples

0.23wt.%

O

Calculated by difference

47.52wt.%

Compositional

Cellulose

NREL/TP-510-42618. Sluiter A, Hames B, Ruiz R, Scarlata C, Sluiter J, Templeton D, Crocker D, Determination of Structural Carbohydrates and Lignin in Biomass

49.81wt.%

Dry

Hemicellulose

21.79wt.%

Lignin

25.10wt.%

Others

3.30wt.%

Calorific value

ASTM E711-87. Standard Test Method for Gross Calorific Value of Refuse-Derived Fuel by the Bomb Calorimeter

18.18MJ/kg

Dry

Catalyst

The modified catalyst was prepared by testing dipping method. The HZSM-5 powder (particle size of 5.0 μm) was calcined at 550 °C for 2 h to remove possible template agent and some moisture, and then immersed in a certain amount of phosphotungstic acid solution. Stirred at 80 °C for 4 h, and transferred to a drying oven and dried at 110 °C for 4h, and then roasted at 550 °C for 2h. Finally, the modified catalyst was obtained and three kinds of catalysts with different loads were prepared, including 5%PW/HZSM-5, 10%PW/HZSM-5, and 20%PW/HZSM-5.

The XRD (Bruker-AXS D8 Advance, Germany) was employed to analyze crystal structure of modified catalysts, with Cu Kα (λ = 0.15406 nm) as the radiation source, tube voltage of 40 kV, current of 30 mA, scanning speed of 5°/min, and scanning range 2θ = 5–80°. The N2 adsorption and desorption tests (conducted at − 196 °C) for specific surface area and pore volume of modified catalysts were measured using a surface area and porosity analyzer (Builder SSA4300, China), and specific surface area was calculated by BET method and pore volume was calculated based on BJH model. The NH3-TPD (Finesorb-3010, China) was employed to characterize the acid intensity on the catalysts; The sample was heated to 300 °C in helium (He) atmosphere, activation for 1 h, then cooled to 100 °C, adsorption of NH3 for 1 h, and then switched to argon (Ar) blowing sweep. Removed the excess NH3 until the detector baseline became stable and then heated at 10 °C/min up to 600 °C, while recorded a TCD signal. The Py-IR tests, by in situ adsorption of pyridine for determination of acid site types, were performed. The concentrations of the B and L acid sites were semi-quantitatively calculated according to empirical formulas (1) and (2):
$$ {C}_{\mathrm{B}}=1.88{A}_{\mathrm{B}}{r}^2/W $$
(1)
$$ {C}_{\mathrm{L}}=1.42{A}_{\mathrm{L}}{r}^2/W $$
(2)
where CB and CL are B and L acid concentrations [mmol/g catalyst], AB and AL represent integrated absorbance of B and L bands [cm−1], r is radius of catalyst disk [cm], and W is weight of catalyst disk [mg].

Thermogravimetric Kinetics

Experiments

The TGA of rapeseed shell without and with catalysts was conducted in a thermo-gravimetric analyzer (Mettler Toledo TGA 1, Swiss). High-purity N2 (99.999%) was used as carrier gas at a flow rate of 50 mL/min. The samples of biomass, catalyst, and biomass mixed with catalyst were placed in Al2O3 crucibles and pyrolyzed by duplicate at the heating rates of 10 °C/min, 20 °C/min, and 40 °C/min. In order to compare the catalytic performance of different catalysts, the same heating rate of 20 °C/min was adopted. Blank runs at the respective heating rates were performed, and the results were used to subtract from the sample runs to remove any instrument artifact. During the experiments, TG and DTG curves were recorded synchronously.

Kinetic Analysis

The Coats-Redfern method combined with Kissinger method had been selected for the kinetic analysis. On the basis of our previous case study of processing TGA data for kinetic calculation [25], in the first place, both the duplicate and increasing trend points included in the TGA data were removed. Then, the mass loss datasets were normalized and the degree of conversion (α) was obtained from the Eq. (3):
$$ \alpha =\frac{m_0-m}{m_0-{m}_z}\times 100\% $$
(3)

where m0 is the initial mass of biomass [mg], m is the actual mass [mg], and mz is the final mass [mg].

Then, the initial and final stages of biomass pyrolysis were removed since the mechanism of both stages is different from the pyrolysis. And the derivative conversion data, which should be used and analyzed in the kinetic calculation, was obtained from the numerical differentiation of the conversion data (dα/dT). It should be noted that the derivative conversion data usually contains data noises, and consequently, it tends to numerically unstable. Hence, to reduce the impact of data noises on the further analyses, a smoothing operation is needed. Before the calculation of E and A, the n should be given according to the Kissinger method. For a case of nth order reaction, n could be calculated by Kissinger index of shape equation (\( n=1.26\sqrt{s} \)) [23]. The shape index s is defined as the absolute value of the ratio of the slope of tangents to the curve at inflection points on the derivative conversion data.

The Coats-Redfern method is based on the general kinetic Eq. (4), and the basic equation can be obtained by rearranging as the Eqs. (5) and (6):
$$ \frac{d\alpha}{d t}=A\exp \left(-\frac{E}{RT}\right)f\left(\alpha \right) $$
(4)
$$ -\ln \left[\frac{-\ln \left(1-\alpha \right)}{T^2}\right]=-\ln \left[\frac{AR}{\beta E}\left(1-\frac{2 RT}{E}\right)\right]+\frac{E}{RT}\kern1em \left(n=1\right) $$
(5)
$$ -\ln \left[\frac{1-{\left(1-\alpha \right)}^{1-n}}{T^2\left(1-n\right)}\right]=-\ln \left[\frac{AR}{\beta E}\left(1-\frac{2 RT}{E}\right)\right]+\frac{E}{RT}\left(n\ne 1\right) $$
(6)

when T is temperature [K], R is constant [8.314 × 10−3 kJ·(K mol)−1], and the heating rate β is a constant. 2RT/E is far less than 1 for biomass pyrolysis, and \( -\mathit{\ln}\left[\frac{AR}{\beta E}\left(1-\frac{2 RT}{E}\right)\right] \) can be regarded as a constant. Hence, interpolating \( -\ln \left[\frac{-\ln \left(1-x\right)}{T^2}\right] \) to \( \frac{1}{T} \) when n = 1, and interpolating \( -\ln \left[\frac{1-{\left(1-x\right)}^{1-n}}{T^2\left(1-n\right)}\right] \) to \( \frac{1}{T} \) when n ≠ 1. The least square method was employed to make linear fitting of the data. After the correct n was determined through the Kissinger method, a straight with high linear correlation can be obtained, whose gradient is E/R and intercept is \( -\mathit{\ln}\left[\frac{AR}{\beta E}\left(1-\frac{2 RT}{E}\right)\right] \). Finally, the values of E and A can be calculated.

Reaction Chemistry

Experiments

The catalytic pyrolysis of rapeseed shell with catalysts was performed in a vacuum pyrolysis system, as shown in Fig. 1. The pyrolysis reactor was a fixed-bed type heated by an electric heating sleeve. A blind tube was installed in the center to help control the reaction temperature. The gas products formed by catalytic pyrolysis were pumped by vacuum pump and filtered into a cold bath. The medium of the cold bath was ethylene glycol, and the cooling temperature was controlled at around − 10 °C. The condensable gas liquefied to form liquid products. The non-condensable gas was gone under the suction of the vacuum pump. Simultaneously, the non-condensable gas was collected by the gas sampling device. The moisture content of liquid product was determined by Karl-Fischer titration (ASTM E203-08), and the ultimate analysis was carried out in an elemental analyzer (FLASH 1112A, Italy).
Fig. 1

The experimental system for catalytic pyrolysis of biomass

Then, the obtained liquid products were weighed, extracted, and separated, and the organic phase was obtained. The dichloromethane (CH2Cl2) reagent was used to for extraction and separation. The organic products were detected and analyzed by FT/IR, GC/MS, and 1H/13C NMR, while the gas products were analyzed by GC. During the experiments, ground reactants were prepared by evenly mixing the biomass and catalysts with a biomass to catalyst ratio of 5 which was the intermediate value based on above literatures [26, 27] and tested in the pre-experiments. Besides, the reaction temperature of 500 °C, heating rate of 20 °C/min and system pressure of 5 kPa were employed, which was proven to be better in previous studies [28, 29]. The organic phases obtained by using 0%PW/HZSM-5, 5%PW/HZSM-5, 10%PW/HZSM-5, and 20%PW/HZSM-5 were respectively labeled as OP-I, OP-II, OP-III, and OP-IV. After the experiments, the spent catalysts were separated to subject to the TGA analysis. In addition, the spent catalyst with the optimal performance was regenerated through 700 °C high-temperature roasting (12 h) and the regenerated catalyst was subjected to the BET analysis to evaluate the activity recovery.

Gaseous Product Analysis

The gas products were analyzed by GC (Agilent 7890A, USA) with carrier gas of high purity argon (Ar), gas flow rate of 45 mL/min, injection volume of 20 mL, column temperature of 55 °C, TCD detector of 60 °C, and bridge current of 60 mA. The main gas products of CO, CO2, CH4, and H2 were determined respectively.

Organic Phase Analysis

The chemical functional groups of organic phases were characterized by FT/IR (Thermo Nicolet iS5, Swiss). A certain amount of potassium bromide (KBr) was taken and pressed to obtain the tablet, and about 0.1 mL of organic phase was added to the tablet. Then, the tablet was measured, and the wavenumber scan range was 4000–400 cm−1 with a resolution of 0.1 cm−1.

The molecular compositions of organic phases were detected by GC/MS (Thermo Trace DSQ II, USA). Conditions: Carrier gas of high purity helium (He), gas flow rate of 1 mL/min, temperature at injection port of 250 °C, sample injection volume of 1 μL. Ion source temperature of 230 °C, MS transfer line temperature of 250 °C, ionization mode of EI, ionization energy of 70 eV, mass scan range of 30–500 m/z, and scan time of 1 s. The column temperature was maintained at 30 °C for 2 min, then increased to 100 °C at 15 °C/min and then to 250 °C at 10 °C/min and kept 250 °C for 3 min. Solvent (CH2Cl2) delay time was 3 min. The identification of the peaks in the chromatogram was based on the comparison with the standard spectra of compounds in the NIST library. The contents of the products were also calculated, which corresponded to the respective relative peak area among all of the detected peaks.

Furthermore, the chemical compositions of organic phases were characterized and confirmed by NMR (Bruker 600M, Germany), a 5-mm tube was used, 80 mg of sample was taken and dissolved in 300 μl deuterium acetone reagent, and testing temperature was 298 °C. The 1H spectrum: Scanning width was 6000 Hz, sampling points were 66,000, scanning times were 16, and delay time was 1.0 s. The 13C spectrum conditions: Scan width 36,000 Hz, sampling points were 66,000, scanning times were 512, delay time was 2.0 s, and using reverse-gated de-duplication method to eliminate NOE effect. A semi-quantitative analysis was made to compare the distribution of aliphatic hydrogen/carbon and aromatic hydrogen/carbon in different bio-oils based on the same amount of solvent (CH2Cl2).

Spent and Regenerated Catalyst Evaluation

The TGA was also used to determine the coke deposited on the spent catalysts. The air was used as the carrier gas, flow rate was 50 mL/min, and heat rate was 10 °C/min from 40 to 800 °C. The TG and DTG curves were recorded synchronously. In addition, the BET analysis was carried out for the regenerated catalyst and the conditions were also the same as above.

Results and Discussion

Modified Catalyst Characterizations

The specific results of catalyst characterizations are listed in the Supplementary Material. The XRD patterns of catalysts showed that the typical diffraction lines of Dawson-structured PW (H6P2W18O62) were detected clearly by referencing JCPDS card: PDF 37-0570 and intensity was enhanced with the increasing loading. The N2 adsorption and desorption tests showed that the adsorption and desorption isotherms of the catalysts coincided at lower relative pressure (p/p0) and showed a sudden increase. H4-type hysteresis loops appeared in the range of 0.45–0.90 p/p0, indicating the coexistence of micropores, mesopores, and macropores, which might be composed of the micropores of the catalyst itself and the stacked mesopores or macropores between the grains. Table 2 shows that the specific surface area of catalysts increased from 66.031 to 245.733 m2/g, and the total pore volume increased from 0.107 to 0.220 cm3/g, in which the micropore volume increased from 0.025 to 0.117 cm3/g, and the micropore volume increased obviously. This is mainly because acid treatment removed amorphous particles from the inter-grain accumulation channels and opened up more micro-channels, resulting in a large increase in specific surface area, pore volume (especially micropore volume), and mean pore radius. The NH3-TPD of catalysts showed that there are two peaks on each profile, indicating the catalysts have two kinds of acidity sites: The low temperature desorption peak near 200 °C represents a weak acidity site, and the high temperature desorption peak near 400 °C represents a strong hydroxyl-related acidity site. With the increase of PW loading, the weak acid intensity decreased obviously, while the strong acid intensity increased obviously, which was related to the transfer of P–O bond to the surface of zeolite by PW modification to form P=O bond [30]. The Py-IR semi-quantitative results are listed in Table 3. The absorption peak near 1450 cm−1 is caused by L acid sites, peak near 1545 cm−1 is caused by B acid sites, and peak near 1490 cm−1 is caused by B + L acid sites [31]. Besides, an absorption peak near 1616 cm−1 was detected and it belongs to Lewis acid sites, which might be closely related to the aromatization of catalyst [32]. Table 3 shows that the B acid sites increased sharply first and then decreased slowly, while the L acid sites decreased with the increase of PW loading, and the B + L acid with moderate acid strength increased gradually, but the increase gradually slowed down. The P–O bond in the catalyst migrates to the surface of the catalyst and forms P=O bond structure after calcination, which is the main reason for the rapid increase of B acid sites [30]. The decrease of the B and L acid sites is mainly due to the interaction between phosphorus and zeolite with the increase of PW content, resulting in the removal of skeleton Al and the removal of acid hydroxyl groups on the surface [33]. Al from the zeolite skeleton was transferred to the surface and existed in the form of amorphous Al, which covered the active sites. Besides, more PW covered the zeolite surface or blocked the catalyst pore, resulting in the decrease of acid sites.
Table 2

Specific surface area and pore volume of catalysts

Sample

Specific surface area/m2 g−1

Pore volume/cm3 g−1

Mean pore radius/μm

Total

Micropore

0%PW/HZSM-5

66.031

0.107

0.025

0.324

5%PW/HZSM-5

71.503

0.112

0.030

0.314

10%PW/HZSM-5

148.936

0.189

0.064

0.254

20%PW/HZSM-5

245.733

0.220

0.117

0.179

Table 3

Py-IR semi-quantitative results of catalysts

Sample

B acid/μmol g−1

L acid/μmol g−1

B + L/μmol g−1

L1450

L1616

0%PW/HZSM-5

20.4

193.0

279.2

62.6

5%PW/HZSM-5

92.8

17.4

168.0

103.3

10%PW/HZSM-5

118.1

10.5

134.7

121.4

20%PW/HZSM-5

113.6

8.3

115.1

122.0

Thermogravimetric Kinetics

Thermogravimetric Analysis

The TG and DTG curves of rapeseed shell pyrolysis without catalyst are shown in Fig. 2a, b. The main pyrolysis transformation stage (TG curve) of biomass shifted to the low-temperature side with the increase of heating rate, and the corresponding DTG curve showed the same shift law and the peak value gradually increased. Generally speaking, heating rate has two positive and negative effects on biomass pyrolysis: With the increase of heating rate, the response time of biomass particles to the temperature required for pyrolysis reaction becomes shorter, which is beneficial to pyrolysis, but the reaction time becomes shorter and the reaction degree decreases. Besides, thermal hysteresis will occur when the temperature difference between inside and outside the particle increases, and the pyrolysis products in the outer layer of the particle will not have time to diffuse, which hinders the internal pyrolysis process [34]. Therefore, the migration direction of TG and DTG curves depends on the combined influence of two aspects. Maschio et al. [35] studied the effect of particle size on biomass pyrolysis characteristics, and they found that the pyrolysis process of biomass particles with particle size less than 1 mm was mainly controlled by intrinsic dynamic rate, and the effect of heat and mass transfer in the particles could be neglected, and the pyrolysis process was controlled by heat and mass transfer and chemical dynamic rate when the particle size was larger and the heating rate was higher. The particle size of biomass used in this study was 0.10–0.15 mm, so there was no heat and mass transfer hysteresis effect, and the pyrolysis process shifted to the low-temperature side. At the same time, it can be seen from Fig. 2a, b that the deviation of conversion curve was not directly proportional to the increase of heating rate, which further indicated that high heating rate did increase the effect of heat and mass transfer on pyrolysis process.
Fig. 2

The TG and DTG curves of rapeseed shell pyrolysis without catalyst (a, b), with catalyst at different heating rates (c, d), and with different catalysts at same heating rate (e, f)

The TG and DTG curves of rapeseed shell pyrolysis with catalyst (0%PW/HZSM-5) are shown in Fig. 2c, d. They show that when 0%PW/HZSM-5 was added to in the catalytic pyrolysis of biomass, the effect of heating rate on catalytic conversion process was similar with the pyrolysis process. The difference is that the addition of zeolite catalyst can reduce the pyrolysis temperature of biomass and the temperature corresponding to the maximum weight-loss rate. HZSM-5 is a typical acidic zeolite catalyst with special pore structure and acid distribution. Hence, there are two aspects of the influence of zeolite catalysts on biomass pyrolysis: The strong acidity of the catalyst can easily decompose the compounds in biomass and advance the pyrolysis conversion process [36], and the larger specific surface area of zeolite has stronger adsorption on pyrolysis products, and the special pore structure has shape-selective catalysis on pyrolysis products, these two factors can slow down the release of products [37]. After adding 0%PW/HZSM-5 zeolite alone, the effect of acidity on the pyrolysis process was more obvious, and the TG and DTG curves shifted to the side of low temperature. Simultaneously, the strong acidity catalysis accelerated the conversion process and caused the maximum weight-loss rate to increase further. When PW-modified zeolites with different loadings were added to the conversion process, the specific TG and DTG curves are shown in Fig. 2e, f. With the increase of PW loading, the acidity distribution of zeolite was adjusted (mainly reflected in the distribution of B acid and L acid). This adjustment could not be simply evaluated as the increase or decrease of acidity but played a role in increasing or decreasing the selectivity of a certain reaction. At this time, the effect of catalyst acidity on biomass pyrolysis characteristics should be much smaller than that of specific surface area and pore structure. PW modification increased the specific surface area of zeolite from 66.031 to 245.733 m2/g and the pore volume from 0.107 to 0.220 cm3/g (in which the micropore volume increased from 0.025 to 0.117 cm3/g). This obvious change enhanced the absorption and catalysis of pyrolysis products and decreased the volatilization and precipitation rate of products. Therefore, the conversion process was slightly delayed compared with 0%PW/HZSM-5 catalysis, and the TG and DTG curves shifted back to the high-temperature side to some extent. At the same time, the maximum weight-loss rate was reduced due to the inhibition of product diffusion.

Kinetics

By means of the Kissinger method, the kinetic n was first determined, and then through the Coats-Redfern method, the E and A were calculated as listed in Table 4. For the pyrolysis trend, it was observed that initially, (a) the n value was 1.96–1.97 and the change was small, which showed that the complexity of cracking reaction did not change significantly with the increase of heating rate, and then (b) the E of rapeseed shell pyrolysis was distributed in the range of 50.40–53.92 kJ/mol with the A value of 8.72E4–2.18E5 s−1, which presented a similar trend of the E value compared with the studies on the pyrolysis of other lignocellulosic biomass [38, 39]. For the catalysis trend with 0%PW/HZSM-5, it was observed that (a) the n slightly increased to 2.02–2.06 attributed to the reason of catalysis reaction introduction, (b) while the E gradually decreased to 35.23–39.42 kJ/mol with a maximal decrease of 34.66%, and the reaction rate (characterized by A) also decreased obviously with the improvement of reaction complexity. This dependency for rapeseed shell pyrolysis over 0%PW/HZSM-5 was similar to that reported in the literature [40]. When the PW/HZSM-5 used as catalyst, (a) the n continued to increase slightly at all loading degrees, indicating that PW increased the complexity of catalytic reaction further; (b) conversely, the E and A continued to decrease, and compared with pyrolysis process, the maximal decrease for E reached 41.58% when 20%PW/HZSM-5 was employed at the heating rate of 20 °C/min. Considering the level of activation energy reduction, the PW/HZSM-5 catalysis had much better performance when compared to the activation energies reported using other catalysts. For example, the activation energy for catalytic pyrolysis of wheat straw with the Ni-Mo-HUSY/γ-Al2O3 catalyst was 18.6% lower than those for non-catalytic pyrolysis [41], and the activation energy for catalytic pyrolysis of pine wood over alkali-treated CaO/HZSM-5 was about 11.23–13.72% lower than those for non-catalytic pyrolysis [42]. Generally, the addition of PW became a competitive route to promote pyrolysis reaction.
Table 4

Results of calculated kinetics parameters for different conversion methods

Methods

Interval/°C

Fitting equation

R2

Number

E(kJ mol−1)

A/s−1

Pyrolysis

10°C/min

309–433

y = 6485.12x + 4.49

0.9910

1.96

53.92

8.72 × 104

20°C/min

272–397

y = 6413.59x + 3.56

0.9985

1.97

53.32

2.18 × 105

40°C/min

243–353

y = 6061.98x + 3.57

0.9895

1.97

50.40

2.04 × 105

Catalysis: 0%PW/HZSM-5

10°C/min

240–360

y = 4237.16x + 6.97

0.9936

2.06

35.23

4.79 × 103

20°C/min

195–330

y = 4650.36x + 5.30

0.9886

2.05

38.66

2.78 × 104

40°C/min

188–308

y = 4741.64x + 4.56

0.9905

2.02

39.42

5.93 × 104

Catalysis: 20 °C/min

5%PW/HZSM-5

212–340

y = 4610.71x + 4.58

0.9856

2.09

38.33

5.68 × 104

10%PW/HZSM-5

225–355

y = 4156.25x + 5.81

0.9964

2.06

34.56

1.50 × 104

20%PW/HZSM-5

260–374

y = 3746.59x + 8.45

0.9899

2.12

31.15

9.61 × 102

Reaction Chemistry

Product Distribution

The main composition of gas and the yield, elemental composition, and calculated HHV of organic phase are listed in Table 5. The main gaseous products including H2, CH4, CO, and CO2 were detected; of course, other light hydrocarbons mainly C2-C6 light hydrocarbons were included in significantly smaller quantities, which had little influence on the analysis of catalytic process. Gas products increased in the presence of all PW/HZSM-5 catalysts as compared to the HZSM-5 catalytic runs, and the total of uncondensed gas increased gradually when the PW loading was added. As expected, under the presence of the catalyst CO and CO2 were the dominant components (70–80%), and CO production was higher than CO2 production with the increasing of PW concentration, and the increase observed for oxycarbide implied a beneficial removal of oxygen from pyrolysis vapors. The removal of oxygen in the form of CO2 is the most preferable route because only one carbon atom is required for the removal of two oxygen atoms, whereas in the case of CO formation, one carbon atom is required for each oxygen atom that is removed. Note that the increase of gas for acidic catalysts, including HZSM-5, FCC, and silica alumina, is mainly attributed to the increase in the production of CO (decarbonylation) while the increase in CO2 production is relatively low [43]. Besides, CH4 was gradually decreased, which was attributed to the reason that Dawson-structured PW had a higher activity of alkylation [16].
Table 5

The main composition of gas and the yield, elemental composition, and calculated HHV of organic phase

Catalysis

Gas composition/%

Organic phase

H2

CH4

CO

CO2

Othera

Yield/%

C/%

H/%

Oa/%

HHV/MJ kg−1

0%PW/HZSM-5

1.80

12.22

29.43

40.10

16.45

18.18

73.56

8.89

17.55

34.34

5%PW/HZSM-5

1.55

8.89

35.91

39.46

14.19

13.10

76.51

9.06

14.43

35.89

10%PW/HZSM-5

2.16

6.50

42.12

39.08

10.14

10.73

80.01

9.48

10.51

38.01

20%PW/HZSM-5

1.92

7.01

40.59

37.87

12.61

11.83

79.42

9.32

11.26

37.54

aBy difference

The yields of organic phases firstly presented an obvious trend of decrease from 18.18 to 10.73%, and then it went up slightly to 11.83% with the increase of PW loading. Generally, oxygen is removed by zeolite catalysts from the biomass pyrolysis vapors in the form of COx and H2O, and deoxidization reactions (decarbonylation, decarboxylation and dehydroxylation) processed simultaneously, so the highest production of CO and CO2 (81.20%) was obtained when the PW loading reached 10%. As for the little uptrend of organic phase yield, the main reason was associated with the catalyst properties: (a) acid treatment removed amorphous particles from the inter-grain accumulation channels and channels were dredged; (b) changes in acid distribution caused by transport of skeleton aluminum and removal of acid hydroxyl groups improved the hydrophilicity of catalyst. Although more channels opened up more micropores, which were beneficial to improve the conversion performance of catalytic products, the increase of hydrophilicity limited the reactions of organic compounds. From the perspective of element composition, the level of oxygen content showed a similar change law with the yield of organic phase, which further confirmed the reason for the yield change. The highest productions of COx resulted in the lowest content of oxygen (10.51%) for 10%PW/HZSM-5 catalysis. Despite the hydrogen content increased gradually, the relative content of hydrogen evaluated by the (H/C)eff (1.45, 1.42, 1.42, and 1.41 respectively) varied little or even decreased slightly with the increase of PW loading. In particular, the level of relative hydrogen content in this study was higher than the results reported by employing other metal-modified HZSM-5 [44, 45], which was also contributed by the catalytic alkylation performance of Dawson-structured PW [16]. Additionally, the HHVs of organic phases were calculated based on the elemental compositions [46] as shown in Eq. (7):
$$ \mathrm{HHV}\left(\mathrm{MJ}/\mathrm{kg}\right)=0.3491\mathrm{C}+1.1738\mathrm{H}+0.1005\mathrm{S}-0.1034\mathrm{O}-0.1510\mathrm{N}-0.0211\mathrm{ash} $$
(7)
where the ash, sulfur, and nitrogen contents of organic phase were ignored in calculation. The results showed that the HHV of OP-III reached the highest value of 38.01 MJ/kg, but there was still a certain gap between organic phase and gasoline/diesel oil due to the limit of relative hydrogen content.

FT/IR Analysis

The FT/IR technique was employed to investigate the chemical structure of the obtained organic phases. The FT/IR spectrums of the organic phases and the identified band positions, functional group absorbance type, and corresponding chemical species are summarized in Supplementary Material. The functional groups of each oil phase were similar, but there are still some differences. The first broad band around 3380 cm−1 corresponded to the O–H stretching vibration mainly caused by phenolic and alcoholic functional groups, and another band indicating O–H bending vibration was all detected in obtained organic phases, which further showed the inevitable existence of alcohols. Differently, when PW was introduced, the intensity of the band decreased slightly, whose change rule was more consistent with the change of oxygen content, indicating that the hydroxyl functional groups also showed the law of decreasing first and then increasing. Then, the bands between 2864 and 2850cm−1 caused by the stretching vibration of the saturated alkyl hydrogen C–H bonds, simultaneously, accompanied by the bending vibration (bands between 1470 and 1350cm−1), indicating that there are many –CH3 functional groups in all organic phases. In addition, an absorption band around 3056 cm−1 was examined in OP-III, which was caused by =C–H stretching, indicating that the alkenyl functional group had a more existence. Thirdly, an obvious band around 1700 cm−1 was caused by C=O stretching vibration, which showed the existence of carbonyl groups from ketones or carboxylic acids, so the stability of organic phase needed further improvement. Fourthly, the vibration absorption bands within the range of 1600–1500cm−1 were caused by the skeleton structure of aromatic C=C bonds; meanwhile, the absorption bands located within the range of 870–625cm−1 was caused by the bending vibration of aromatic =C–H hydrogen bonds, and interestingly, the C=C and =C–H bond types of benzene rings in OP-II, III were relatively few, but the band intensities were significantly enhanced (especially for OP-III), indicating that the benzene ring structures in organic phases may be different, which was mainly reflected in the monocyclic or polycyclic structure. It can be inferred that there are more aromatic compounds with monocyclic structure in OP-III. Last point, the absorption bands between 1300 and 1000cm−1 were mainly caused by the stretching vibration of C–O bond in the alkyl aryl ethers, alcohols, esters, or aliphatic ethers, and generally, the C–O bond stretching vibration of bio-oil after catalytic upgrading is mainly caused by methoxy.

GC/MS Analysis

Then, organic phases were analyzed by GC/MS technique, and more than 75% of hydrocarbons were detected in all organic phases, so hydrocarbons were the main products, and aromatic hydrocarbons were the main components of hydrocarbon products, which became the most important factor affecting the physicochemical properties of organic phase. Therefore, the detected aromatic hydrocarbons were summarized in Supplementary Material. Generally, aromatic hydrocarbons can be classified as MAH and PAH, in order to better analyze the hydrocarbon composition of organic phase. MAH is further divided into MAH-I (contains only one benzene ring without any other ring structure) and MAH-II (contains one benzene ring with one or more hydrogenated benzene ring) in this study, from the perspective of fuel grade and environmental protection; MAH-II and PAH should belong to undesirable products. The change of catalyst made the selection of products inconsistent, but some products belonged to similar compounds, and there are 14 kinds of MAH-I compounds, five kinds of MAH-II compounds and 14 kinds of PAH compounds in all detected aromatics. Vinyl-benzene as a typical high content (6.52%) of MAH-I was only obtained in OP-III, which verified the FT/IR results of =C–H stretching peak around 3056 cm−1, and from this point, the quantitative analysis of organic phase chemical constituents using GC/MS technique compromised with the FT/IR qualitative results. The critical diameter (width), maximum diameter (length), and kinetic diameter of the oxygenates, olefins, and aromatics were reported in literature [47], and it was found that benzene, toluene, naphthalene, indene, ethylbenzene, and p-xylene are sufficiently small to diffuse into the zeolite micro-pores, while the larger aromatic products are most likely formed in the particle gaps.

In order to make the difference of hydrocarbon composition in organic phase clearer, the data of hydrocarbon composition were analyzed statistically, and the results are shown in Fig. 3. As can be seen in Fig. 3, the content of LAH in organic products was low, and only a small amount of LAH was detected in OP-III and IV, further indicating that the catalytic performance of zeolite catalyst was mainly reflected in aromatization. Specifically, the undesired PAH presented a trend of decreasing first and then increasing, while the MAH-II basically showed a trend of gradual increase, and the variation of desired MAH-I was just opposite to that of PAH. The explanation of change laws should be divided into two parts: (a) from OP-I to OP-III, actually, the reduction of PAH contributed to the increase of MAH, which due to the enhancement of absorbance performance (including specific surface area and pore volume) and the increase of B acid and decrease of L acid (especially for L1616), on the one hand, the conversion process was more complete, and on the other hand, the aromatization performance was obviously reduced; (b) from OP-III to OP-IV, despite that the weakening of acidity might be helpful to inhibit the aromatization, the increase of specific surface area and pore volume made the catalytic reaction more sufficient. Therefore, the combined effect of the two factors made the aromatization performance not be inhibited, but obviously strengthened, which led to the increase of the undesired MAH-II and PAH. Another notable phenomenon is that the increase of MAH-II essentially meant the increase of hydro-saturated benzene ring, while the slight decrease of (H/C)eff indicated the reduction of hydrogen retention, so the increasing PW load could promote hydrogenation reaction (or hydrogen transfer reaction) to a certain extent.
Fig. 3

The hydrocarbon compound distribution in the organic phases

In the composition of MAH-I, BTEX is the basic chemical raw materials in great demand and can be blended into gasoline with higher octane number. Hence, it is necessary to investigate the selectivity of PW/HZSM-5 to BTEX individually, the total content of BTEX was counted, and the proportion of BTEX in each sample was calculated, as shown in Fig. 4. As can be seen from Fig. 4 that only 6.15% in the OP-I, and more than half (52.37%) in the OP-III, and the OP-II, IV accounted for 20.16% and 21.32% respectively. As discussed above, the increase of BTEX selectivity mainly attributed to the release of more micro-pores after introducing PW modification; of course, the promotion of aromatization should also be considered. Then, the decrease of BTEX selectivity after 20%PW modification owed to the increase in pore structures, and although the increasing micro-pores might further increase the BTEX production, the increasing meso-pores or macro-pores could result in a great increase in the possibility of product polymerization to generate PAH [48].
Fig. 4

The BTEX distribution in the organic phases

Furthermore, the oxygenate compounds detected in the organic phase were classified and analyzed as shown in Fig. 5. As can be seen from Fig. 5, the oxygenate compounds mainly involved in four categories: ketones, phenols, alcohols and others (mainly ethers), which were much less than those obtained by other catalytic methods, such as Stefanidis et al. [43] examined 10 types of oxygenated organics during the in situ upgrading of biomass vapors using various modified ZSM-5 on a fixed bed reactor, and Asadieraghi et al. [49] identified nine kinds of oxygenate compounds by employing meso-HZSM-5 and Ga/meso-HZSM-5 in a multi-zone fixed-bed reactor. Firstly, the ketones decreased first and then increased with the increase of PW loading and the content of ketones in OP-I was the highest (4.97%) and that in OP-III was the lowest (2.47%). The decreasing amplitude reached 50%, but the change was relatively small, because the content was low so that the incompleteness of the transformation was relatively high. Secondly, phenols showed a significant decrease trend, and even no phenols were detected in OP-III, IV. This was due to that PW modification enhanced catalytic alkylation and dehydroxylation, and on the other hand, because hydrogen transfer or hydrogenation enhanced the conversion of some phenols to alcohols, which also explained the increase of alcohols. Thirdly, the decrease of alcohols and the change of other organics were mainly attributed to incomplete deoxygenation and alkylation reactions so that some methoxy-containing compounds (ethers) were produced. In addition, the oxygen atom distributions in the organic phases were also classified as shown in Fig. 6. As can be seen from Fig. 6, compounds containing two oxygen atoms were substantially reduced until they were undetectable, and some were not completely deoxygenated, increasing the number of compounds containing one oxygen atom. In terms of deoxygenation degree, 10%PW/HZSM-5 had reached the highest catalytic performance, and the increase of PW loading could not further enhance the deoxygenation capacity, and even rebounded slightly, which was basically consistent with the results of above elemental analysis.
Fig. 5

The main oxygenated compound distribution in the organic phases

Fig. 6

The oxygen atom distribution in the organic phases

1H/13C NMR Analysis

The larger compounds in the organic phases cannot be validated and recognized in GC-MS, and this defect can be remedied by NMR technique through the functional groups’ confirmation. The 1H NMR and 13C NMR experiments (C3D6O, 600 MHz) were conducted, and the specific results are shown in Fig. 7. Figure 7a shows the 1H NMR spectra of organic phases, which provided the detailed information about organic components. According to the spectra, it was indicated that the main chemical shift (δ) regions for 1H NMR were 0.5–3.0 ppm and 6.5–8.0 ppm. The former region mainly is related to aliphatic protons and the latter is attributed to aromatic protons [50]. Specifically, the region from 0.5 to 1.5 ppm corresponds to protons on aliphatic carbon atoms at least two bonds away from C=C or heteroatom, the region from 1.6 to 2.5 ppm corresponds to alkyl protons, and the peaks located in the range of 2.5–2.8ppm are caused by protons on a-position of aromatic rings. Besides, the weakened peaks in the 3.0–4.5 ppm and 4.5–6.0 ppm ranges respectively represented methoxyl protons and ether protons. Lastly, the peaks located in the range of 6.0–8.5ppm corresponded to the aromatic protons. To judge specific changes, the RP (%) of the main chemical shift region was calculated by integral normalization based on the proton of CH2Cl2 occurred (peak at 5.63 ppm) in each sample, and the results are also listed in Fig. 7a. The ratios of aliphatic proton to aromatic proton in OP-I, II, III, and IV were 3.0, 3.6, 8.3, and 2.0, respectively. This change confirmed the cracking effect of appropriate amount of PW, and the chemical shift for aliphatic proton of OP-III was mainly located in the 1.6–2.5 ppm, which further indicated that Dawson-type PW had strong alkylation performance. Besides, as shown in Fig. 7b, the main chemical shift regions of 13C NMR were 10–40 ppm and 120–150 ppm, mainly corresponding to aliphatic carbon and aromatic carbon respectively [51]. The peaks in the range of 22–29ppm were attributed to carbon branched methyl carbon. The peaks, in the range of 120–150ppm, were attributed to the aromatic carbon. Also, the RC (%) of the main chemical shift region was calculated according to the carbon of CH2Cl2 occurred (peak at 55 ppm) in each sample, and the calculation results are listed in Fig. 7b. The change rule of the RC was in accordance with the RP change rule. Generally, the change rules of organic groups demonstrated from NMR results were basically consistent with the discussion in GC/MS analysis.
Fig. 7

The NMR spectra of organic phases: a1H NMR spectra and b13C NMR spectra

Spent and Regenerated Catalyst Analysis

The TG and DTG curves of the spent catalysts are shown in Fig. 8. Figure 8 shows that the total weight losses of 0%PW/HZSM-5(s), 5%PW/HZSM-5(s), 10%PW/HZSM-5(s), and 20%PW/HZSM-5(s) were 18.62%, 16.58%, 11.56%, and 14.81%, respectively. The weight loss processes could be roughly divided into three stages. The first stage below about 250 °C was caused by the release of water and light species, the second stage between 25 and 700 °C was attributed to the burning of deposited coke, and the third stage above about 700 °C belonged to the decomposition of heavy carbonaceous species and the slow stabilization process. To identify the characteristics of cokes more accurately, the second stage needed to be analyzed. Based on the DTG curves, the specific temperature ranges of main weight loss in the second stage for the four spent catalysts were 350–700 °C, 265–635 °C, 250–510 °C, and 300–650 °C respectively, which showed that PW modification could reduce the activation energy of the catalytic reaction, while the coking reaction also shifted to the low-temperature direction, and the shift degree varied with the amount of PW. For the corresponding coke contents, 12.13%, 11.27%, 5.25%, and 8.47% were respectively determined, which showed that the 10%PW modification not only obtained the higher conversion proficiency, but it could also enhance the catalyst stability. Generally, the in situ pyrolysis-catalysis method could produce two types of coke on the catalyst including thermal coke and catalytic coke, and the decomposition temperature of catalytic coke was relatively high [52], which could be verified through the acromion on the DTG curve. The thermal coke usually deposited on the catalyst surface and its formation was induced by thermal and weak acid sites (or inert sites), and the catalytic coke was formed in the inner pores during the aromatization and reformation [53]. In this study, 10%PW modification dramatically increased the B acid sites and the pore volume which could obviously decrease the coke contents, while 20%PW modification led to the decrease of B acid sites and further increase of specific surface area (the diffusion performance of products was deteriorated), and both factors finally resulted in the recovery of coke content.
Fig. 8

The TG and DTG curves of the spent catalysts

To evaluate the regeneration performance of the catalysts, the optimal catalyst 10%PW/HZSM-5 was subjected to the BET analysis after roasting regeneration. The specific surface area and pore volume of the fresh catalyst were 148.936 m2/g and 0.220 cm3/g, and those of the regenerated catalyst were 145.241 m2/g and 0.214 cm3/g. The properties recovered 97.52% and 97.27%, which showed that the catalyst had high regeneration activity, and it could be continuously regenerated and reused.

Conclusion

The thermogravimetric behavior, kinetic analysis, and reaction pyrolysis of rapeseed shell without and with Dawson-structured PW-modified HZSM-5 was investigated. The modified PW delayed the weightlessness behavior, thermogravimetric curves slightly shifted back to the high-temperature side, and the maximum weight-loss rate was reduced. Additionally, the reaction order, activation energy, and frequency factor for rapeseed shell conversion without and with PW/HZSM-5 were determined by combining the Kissinger and Coats-Redfern methods. The introduction of zeolite and PW increased the reaction complexity and hence the reaction order was increased slightly (from 1.96–1.97 to 2.06–2.12), while the activation energy decreased from 50.40–53.92 to 31.15–38.33 kJ/mol, and the frequency factor decrease indicated that the rate of reaction was also significantly reduced.

Furthermore, the reaction pyrolysis of rapeseed shell with PW/HZSM-5 was evaluated in a fixed-bed reactor with respect to organic phase yield, deoxygenation ability, and selectivity towards hydrocarbons. The suitable loading for PW was 10% and the modified catalyst formulation with the higher surface area, pore volume, and more reasonable acidity distribution, which yielded the least organic phase (10.73%), although the oxygen content and calorific value reached the optimum levels of 10.51% and 38.01 MJ/kg respectively. With increasing PW, more oxygen was removed in the forms of COx and the structures of benzene ring tended to single rings. The 10%PW/HZSM-5 catalysis gave the highest content of desired MAH-I (52.29%) and the lowest content of undesired PAH (12.32%); simultaneously, the selectivity to BTEX compounds was improved. The cracking and alkylation of the appropriate amount of PW-supported HZSM-5 contributed to the significant reduction of oxygenate compounds, which was further confirmed by 1H/13C NMR analysis. Besides, the 10%PW/HZSM-5 had the lowest coke content and it could recover high activity after regeneration, so the catalyst could be used repeatedly. Overall, this study provided a promising technological path for efficient utilization of rapeseed shell on a laboratory scale and laid the theoretical foundation and experimental basis for large-scale conversion research.

Notes

Acknowledgments

The authors thank the Analysis and Testing Center of Yancheng Institute of Technology for the technical support.

Funding information

This work is currently supported by the National Natural Science Foundation of China (51806186) and the Scientific Research Project for the Introduction Talent of Yancheng Institute of Technology (XJ201708).

Supplementary material

12155_2019_10075_MOESM1_ESM.doc (1.7 mb)
ESM 1 (DOC 1775 kb)

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  1. 1.School of Automotive EngineeringYancheng Institute of TechnologyYanchengPeople’s Republic of China
  2. 2.Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu ProvinceYancheng Institute of TechnologyYanchengPeople’s Republic of China
  3. 3.School of Automotive and Traffic EngineeringJiangsu UniversityZhenjiangPeople’s Republic of China

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