3.1. Catalysts characterization
Table 2 shows the N2 adsorption & desorption isotherms of parent and cobalt-modified USY and highlights the textural properties, respectively. Both of catalysts showed typical IV isotherms with hysteresis loops, indicating the existence of mesoporous (Zheng et al., 2019). In the low-pressure range, the adsorption amount increased approximately linearly with pressure because the nitrogen molecules underwent single-layer adsorption on the inner surface of mesopores. When the relative pressure was in the medium- pressure region, the adsorption amount jumped due to the capillary condensation of nitrogen molecules in mesopores. When the pressure continued to increase, multiple layers of adsorption occurred. As can be seen from Table 1, the specific surface areas of parent and cobalt-modified USY were 787.97 and 750.61 m2/g, respectively, and the pore volumes were 0.37 and 0.34 cm3/g, respectively. Compared to the parent USY, the cobalt-modified USY had a reduced specific surface area, pore volume, and pore diameter. This reduction was because Co ions were distributed on the surface and in the pores of the catalyst, covering part of the surface of the carrier pores; thus, the specific surface area was reduced. The Co covering the pores of the carrier blocked a portion of the pores, which narrowed the pores so that the average pore volume and average pore diameter of the molecular sieve were lowered (Gamliel et al., 2016; Lee et al., 2017). Moreover, Co ions covered the outer surface of the catalyst, expanding the external specific surface area of the molecular sieve carrier. The number of weak acid sites in the modified USY increased and shifted to a high temperature zone. The strong acid desorption peak disappeared, indicating that the modified catalyst had mild acidity and that the overall amount of acid was lowered. Cobalt ions were highly dispersed and oligomerized in the pores or on the outer surface of the USY molecular sieve. This support was a critical factor in increasing the catalytic activity of the catalyst.
Table 2
Textural properties of the newly designed catalysts.
Catalyst
|
SBET
(m2/g)
|
Sext
(m2/g)
|
Smicro
(m2/g)
|
Vtotal
(cm3/g)
|
Vext
(cm3/g)
|
Vmicro
(cm3/g)
|
Dave
(nm)
|
USY
|
787.97
|
53.55
|
734.42
|
0.37
|
0.11
|
0.26
|
1.88
|
Co-USY
|
750.61
|
67.83
|
682.78
|
0.34
|
0.08
|
0.26
|
1.83
|
SBET: Total pore area; Sext: External surface area; Smicro: Micropore surface area; Vtotal: Total pore volume; Vext: External pore volume; Vmicro: Micropore volume; Dave: Average pore size |
Fig. 1 and Table 3 show the NH3-TPD curve and acid site properties of the parent and cobalt-modified USY. The presence of certain solid acid sites in the molecular sieve was of great significance for the catalysis of organic matter. USY had two desorption peaks at temperatures of 132°C and 343°C. Compared with USY, the weak acid desorption peak in the modified USY was significantly enhanced, indicating that the number of weak acid sites in the modified USY increased and shifted to a high temperature zone, moving from 132°C to 153°C, which meant that the acidity of weak acids was also enhanced. The strong acid desorption peak disappeared, indicating that the modified catalyst had mild acidity and that the overall amount of acid was lowered. This decrease occurred because Co caused a slight dealumination of the backbone of the catalyst during the impregnation process, promoting a decrease in the acidity of strong acid sites (Zhao et al., 2019). The addition of cobalt reduced the number of acid sites in USY, and the effect on the strong acid sites was greater than that on the weak acid sites. This effect was the result of the interaction of cobalt with Al (OH) Si, and some Al-OH was replaced by Co-OH (Che et al., 2019; Zhang et al., 2007); thus, the number of strongly acidic sites was significantly reduced.
Table 3
The amount of parent and phosphorus-modified USY.
Catalyst
|
Weak acid site (mmol/g)
|
Strong acid site (mmol/g)
|
Total acid site (mmol/g)
|
USY
|
1.118
|
0.533
|
1.651
|
Co-USY
|
1.518
|
-
|
1.518
|
X-ray diffraction (XRD) was used to study the patterns of the parent and cobalt-modified USY. The USY molecular sieve had distinct characteristic diffraction peaks at 2θ = 10.18, 11.94, 15.72, 18.78, 20.46, 23.76, 27.18, and 31.56. The characteristic diffraction peaks of the cobalt-modified USY catalyst did not change significantly in position, which indicated that the loading of Co did not destroy the phase structure of the USY catalyst (Li et al., 2016; Zhang et al., 2018). From the results of catalyst activity and X-ray diffraction, the intensity of the characteristic diffraction peaks was significantly weakened, but the characteristic diffraction peak of cobalt ions did not appear, which indicated that the cobalt ions were highly dispersed and oligomerized in the pores or on the outer surface of the USY molecular sieve (Zhao et al., 2019; Zhang et al., 2018).
3.2. Products yields
Products distribution over without or with the parent and cobalt-modified USY during GBR pyrolysis are shown in Fig.2.The results showed that the yields of the bio-oil, gas products and biochar obtained by pyrolysis without using a catalyst were 36.27%, 28.88%, and 34.85%, respectively. In contrast, catalysts caused a significant change in the distribution of pyrolysis products. The yield of gaseous products in the products obtained by catalytic pyrolysis using the parent and cobalt-modified USY catalysts increased by 8.5% and 11.65%, respectively; the yield of bio-oil decreased by 6.26% and 9.05%, respectively; and catalytic pyrolysis did not have a significant effect on the yield of biochar. These changes were mainly due to the large pore size and specific surface area of the catalyst.These enhanced parameters increase the residence time of the pyrolysis gas in the pyrolysis furnace, increase the mass transfer and heat transfer effects, and affect the heating rate and precipitation rate of volatile materials (Zhou et al., 2019a). At the same time, macromolecules in the pyrolysis gas can enter the pores of the catalyst and undergo cracking to form small molecular substances.Acids can be converted to hydrocarbon compounds by a decarboxylation reaction with the aid of a catalyst. Oxygenates can be converted into small molecules, such as CO2, CO and H2O, which reduce the molecular size of the bio-oil and make the bio-oil more stable (Skoblik et al., 2012). As a result, the amount of gaseous product in the catalytic pyrolysis product increases, and the corresponding amount of bio-oil decreases.
3.3. Gas compositions
Figure 3 shows the composition of the gaseous product of GBR pyrolysis under different pyrolysis conditions. Under the action of catalysts, the yield of CO, CH4 and CO2 in the gas product increased significantly. The yield of H2 was lowered. The yield of C2 − 4 was slightly increased. During the pyrolysis process, large amounts of hydroxyl groups in GBR underwent dehydration, which provided conditions for the formation of carbonyl groups. Then, the unstable carbonyl groups were broken at a higher temperature to form CO by reforming and isomerization reactions. Less CO was formed in the initial stage of pyrolysis, while a large amount of CO was generated by the secondary cracking of primary products(Ren et al., 2018). The carboxyl groups formed by the isomerization of ketene and olefin aldehydes were further decomposed to obtain CO2. Under the action of catalysts, the yields of CO and CO2 increased to different degrees, and the yield of CO2 was higher than that of CO, which also proved that the increased secondary reactions had a more obvious effect on the increase in CO2 production (Liu et al., 2017). Compared with USY, USY modified with cobalt not only contained the acid active sites of the parent but also added the active sites of some cobalt ions. This active site distribution facilitated the intermediate release and disconnection reactions of large molecules, as well as the condensation reactions of ketones, aldehydes, etc., thereby increasing the deoxidation effect in the form of CO and CO2 gases. CH4 was mainly derived from the cleavage and reforming of functional groups such as methyl and methylene, while C2 − 4 was mainly derived from the secondary cleavage of volatiles. Therefore, USY modified with cobalt had a certain promoting effect on these reactions, so the yields of CH4 and C2 − 4 increased.
3.4. The selectivity of catalyst to aromatics
Gaining knowledge toward the quantitative chemical characterization of bio-oils is crucial in the evaluation of its applications as an extended resource of advantageous bio-fuels and bio-chemicals (Hassan et al., 2016; Pirbazari et al., 2019). To determine the chemical compositions of the catalytic and non-catalytic bio-oils, GC-MS analysis was carried out as appropriate. The results of the GC-MS analysis are summarized in Table 4. Due to the complex structure of biomass and the various reactions that may occur during thermochemical processes, the composition of bio-oil obtained from the pyrolysis of GBR is complex. To figure out the selectivity of the products, they have divided into acids, ketones, esters, alcohols, furans, aldehydes, phenols and aromatics, based on their main organic functional groups (Lazaridis et al., 2018).
Table 4
Classification of the identified materials by GC-MC analysis.
Types
|
Library/ID
|
Peak area percentage (%)
|
Non- Cat.
|
USY
|
Co-USY
|
Acids
|
Acetic acid
|
5.01
|
2.15
|
0.98
|
Propanoic acid
|
3.15
|
2.16
|
0.27
|
Octadecanoic acid
|
-
|
-
|
0.37
|
Tetradecanoic acid
|
1.21
|
0.48
|
-
|
7,8,12-Tri-O-acetyl ingol
|
1.89
|
0.81
|
0.51
|
Oleic Acid
|
-
|
0.71
|
-
|
Ketonesz
|
1-Hydroxy-2-propanone
|
1.57
|
0.76
|
-
|
1-Hydroxy-2-butanone
|
0.82
|
-
|
-
|
2-Methyl-2-cyclopenten-1-one
|
0.53
|
0.28
|
0.11
|
1,2-Cyclopentanedione
|
2.43
|
2.17
|
2.05
|
2-Hydroxy-2-cyclopentanon
|
2.68
|
1.27
|
1.04
|
1-Indanone
|
1.24
|
-
|
-
|
Esters
|
Propanoic acid, 2-oxo-,methyl ester
|
0.68
|
-
|
-
|
Pentanoic acid, 1-methylpropyl ester
|
1.22
|
0.87
|
-
|
D-Glucuronicacid. gamma.-lactone
|
5.79
|
1.94
|
2.19
|
Oleic acid, eicosyl ester
|
0.26
|
-
|
-
|
Propanoic acid, 2-oxo-, methyl ester
|
0.48
|
-
|
-
|
Valtrate
|
0.15
|
-
|
-
|
Alcohols
|
2-Propanone, 1-hydroxy-
|
-
|
1.39
|
1.2
|
Methylazoxy methanol acetate
|
-
|
1.48
|
0.55
|
Maltol
|
-
|
1.32
|
|
Furans
|
2,3-Dihydrofuran
|
1.19
|
0.64
|
0.75
|
5-Hydroxymethylfurfural
|
1.04
|
0.52
|
1.26
|
2-Methyl-furan
|
2.41
|
1.07
|
1.86
|
Aldehydes
|
Cyclooctanecarboxaldehyde
|
-
|
2.22
|
4.33
|
Furfural
|
2.52
|
-
|
-
|
Pentanal
|
1.33
|
0.77
|
0.23
|
2-Methyl-valeraldehyde
|
3.53
|
1.87
|
0.19
|
Phenols
|
Phenol
|
4.29
|
3.36
|
3.31
|
Phenol, 2-methyl-
|
2.93
|
-
|
-
|
Phenol, 3-methyl-
|
|
|
3.94
|
Phenol, 4-methyl-
|
5.25
|
4.57
|
3.85
|
Phenol, 2-methoxy-
|
|
2.78
|
|
Phenol, 4-ethyl-
|
0.73
|
|
|
1,2-Benzenediol
|
11.69
|
11.15
|
9.34
|
Hydroquinone
|
-
|
-
|
|
1,4-Benzenediol
|
5.97
|
4.12
|
5.37
|
Phenol, 3,5-dimethoxy-
|
-
|
0.16
|
-
|
Aromatics
|
Benzene
|
-
|
1.81
|
-
|
Toluene
|
1.26
|
6.13
|
7.55
|
p-Xylene
|
2.32
|
4.92
|
6.37
|
Indene
|
-
|
-
|
0.63
|
Naphthalene
|
-
|
5.64
|
3.19
|
1-Methyl-naphthalene
|
-
|
3.04
|
2.25
|
2,7-Dimethyl-naphthalene
|
-
|
2.09
|
-
|
Anthracene
|
-
|
0.75
|
0.96
|
2-Methyl-anthracene
|
-
|
1.46
|
0.94
|
2-Methyl-phenanthrene
|
-
|
1.65
|
-
|
As shown in Fig. 4, the acids in the catalytic pyrolysis products of USY modified with Co were significantly reduced compared with the pyrolysis products of USY. This result indicated that Co facilitated the removal of acid species during pyrolysis. However, under the action of co-modified USY, the content of aldehydes and sugars in the pyrolysis products changed little, while the content of alcohols increased slightly, indicating that the modified catalyst could reduce the viscosity of the bio-oil. The levels of esters and ketones were slowly reduced to 2.19% and 3.2%, respectively. Compared with pyrolysis without a catalyst, the content of phenols in the bio-oil obtained by pyrolysis under catalysis with the parent and cobalt-modified USY catalysts was reduced by 4.72% and 5.05%, respectively, indicating that the stability of the bio-oil had been improved. Under the USY catalysis of the parent, the aromatic hydrocarbon content was 27.49%.eventually the aromatic hydrocarbon content was reaching to 21.89% under the catalysis of Co-USY. Co loading was unfavourable for increasing the content of aromatic hydrocarbons in the bio-oil. The content of aromatic hydrocarbons in the bio-oil obtained by pyrolysis under catalysis by the parent and cobalt-modified USY was 27.49% and 21.89%, respectively, indicating that the use of the modified catalyst hindered the formation of aromatic hydrocarbons.
Compared with the pyrolysis products of USY, the content of acetic acid remained basically unchanged under cobalt-modified USY catalysis, and the content of propanoic acid decreased slightly. The content of 2-methyl-furan and 5-hydroxymethylfurfural increased; however, the content of 2, 3-dihydrofuran was not affected by the modification of the catalyst. The contents of 2-methyl-2-cyclopenten-1-one and 2-hydroxy-2-cyclopentanone slightly decreased, which was consistent with the trend of the total content of ketones. The pentanal and 2-methyl-valeraldehyde quantities were also reduced by similar amounts. It could be speculated that the increased content of aldehydes in the bio-oil may come from the conversion of furfural.
The study found that Co-modified USY had no promoting effect on increasing the total content of aromatic hydrocarbons. Therefore, it was necessary to study the effect of this catalyst on the relative content and selectivity of four typical aromatic hydrocarbons. From Fig. 5(a), it was found that the contents of toluene and xylene increased, whereas the contents of naphthalene and 1-methylnaphthalene slightly decreased, which indicated that Co ions were not conducive to the formation of aromatic hydrocarbons; Co ions hindered the addition or polymerization of benzene rings, thereby reducing the formation of naphthalene and its derivatives (Chen et al., 2019a). Conversely, Co ions could promote the formation of monocyclic or small-molecular-chain hydrocarbons. The calculation of product selectivity is shown in Eq. (1).
(1)
Figure 5(b) shows that the selectivity and content of the four aromatic hydrocarbons are almost identical. The selectivity of toluene and xylene increased, and the selectivity of naphthalene and 1-methylnaphthalene was gradually reduced. The composition of aromatic hydrocarbons was simplified under the action of Co-modified USY. The selectivity ratios of toluene and xylene were 34.35% and 28.96%, respectively, which were much higher than those of the other two aromatic hydrocarbons.