Catalytic Cracking of Soybean Oil for Biofuel over γ-Al₂O₃/CaO Composite Catalyst

Abstract

In this paper, we report the catalytic cracking of soybean oil for biofuel over γ-Al₂O₃/CaO composite catalysts. The influence of catalysts, cracking temperature and weight hourly space velocity (WHSV) on the products distribution were investigated. The maximum yield (70.0 wt.%) of biofuel with low acid value (6.7 mg KOH g^-1) and oxygen content (5.6%), as well as high calorific value (44.2 MJ kg^-1) was achieved over 35 wt.% γ-Al₂O₃/CaO at 480 ºC and 3.72 h^-1. The paper focused on the variation of biofuel composition and cracking pathway caused by γ-Al₂O₃/CaO composite catalysts via gas chromatography-mass spectrometry (GC-MS) and thermogravimetric (TG) analysis. Calcium oxide would react with fatty acid to yield calcium carboxylates at 300-350 ºC, which were subsequently decomposed into hydrocarbons (57.9 wt.%) and ketones (22.6 wt.%) at 415-510 ºC. As for 35 wt.% γ-Al₂O₃/CaO, the addition of γ-Al₂O₃ was beneficial to generate alkenes (38.2 wt.%), arenes (10.6 wt.%) and alcohols (12.3 wt.%) with ketones decreasing (16.5 wt.%) via γ-hydrogen transfer reaction and disproportination.

Keywords:
soybean oil; catalytic cracking; γ-Al₂O₃/CaO catalysts; biofuel

Introduction

Severe energy security and environmental pollution have been caused due to extensive use of fossil fuels. Thus, there is a considerable interest in developing renewable and environmentally friendly energy throughout the world.¹ As an important renewable energy source, lipid is a potential feedstock for preparing environmental friendly and carbon neutral biofuel since it possesses a similar structure to the diesel fuel.^1,2 The methods for producing biofuel from lipid mainly include transesterification and cracking.^3-6 It is well known that biodiesel is composed of fatty acid methyl esters (FAME) produced by transesterification of triglycerides and alcohols. The preparation technology is quite mature, but there are still some drawbacks such as poor cold flow properties and relatively low calorific value.^7,8 In this regard, hydrocracking and catalytic cracking have been attempted to convert lipid into biofuel.^8,9 The second-generation biodiesel containing C_15-18 linear paraffins with high quality and yield is achieved by hydrodeoxygenation, decarboxylation and hydroisomerization of triglycerides, whereas the high cost of hydrogen and the rigorous demand for catalyst activity restrict its industrialization.¹⁰ Among all the preparation technologies of biofuel, catalytic cracking is regarded as a promising method for its simple process and similar liquid product to petroleum-based fuels.^11,12 In addition, catalytic cracking can be applied to produce gasoline, kerosene and diesel with suitable catalysts, and presents good compatibility with various feedstock, as well as relatively lower cost.^13,14 Currently, for purpose of reaching the demand of vehicle fuel standards, catalytic cracking still need to reduce the acid value and oxygen content of the biofuel for improving fuel properties including density, calorific value, dynamic viscosity, etc. There is no doubt that the catalysts play a vital role in the catalytic cracking of lipid for biofuel.

Varieties of catalysts that can be divided into basic and acidic solid catalysts in general have been applied to get desirable products in past decades. Na₂CO₃ or K₂CO₃ basic catalysts were reported for catalytic cracking of soybean oil and crude palm oil for biofuel.^5,15,16 The results indicate that traditional basic catalysts could reduce acid value effectively. However, inorganic salts would dissolve in biofuel, leading to large loss of catalysts and corrosion of the equipment.¹⁷ Xu et al.¹⁸ demonstrate that calcium oxide (CaO) possesses considerable decarboxylation capacity with low solubility in biofuel. Moreover, previous work in our group¹⁹ shows that CaO is beneficial to reducing acid value, meanwhile, decreasing the content of aromatic hydrocarbons by rupturing ringed structure. Nevertheless, CaO inhibits the pyrolysis of esters and tends to produce ketones, resulting in the high content of total oxygenated compounds.

Huge effort has been devoted to studying the effects of acidic solid catalysts such as Al₂O₃, HZSM-5 and MCM-41 on catalytic cracking of lipid for biofuel.^5,20-26 The acid sites on catalysts favor deoxygenation of fatty acids and the C-C and C=C bond cleavage of molecules, while leads unavoidably to aromatization due to the high cracking temperature and Brönsted acidity.^21,27-29 The liquid products obtained by the acidic solid catalysts contain high concentration of hydrocarbon compounds, as well as low oxygen content and viscosity. However, the pyrolytic oils with excessive acid value seriously affect the fuel properties. Otherwise, the aromatic hydrocarbons in product reduce the cetane number, and the low H/C ratio results in increasing the production of CO during burning in the engine. Furthermore, among the acidic solid catalysts, the application of Al₂O₃ shows good effect of deoxygenation with low aromatization, due to the acid sites of Al₂O₃ associated with Lewis acidity and very weak Brönsted acidity.²¹ Thus, in an attempt to overcome the defects of CaO catalysts, and avoiding the production of excessive aromatic hydrocarbon by acidic solid catalysts as well, it might be a practicable method to integrate the required acid and base characters by preparing Al₂O₃/CaO composite catalyst.

Thus, catalytic cracking of soybean oil (SO) for biofuel over γ-Al₂O₃/CaO (A/C) composite catalysts has been conducted in this paper. The γ-Al₂O₃/CaO (A/C) composite catalysts were prepared by mechanical mixing with CaO as primary catalysts and γ-Al₂O₃ as additives. The main objective was to obtain hydrocarbon fuels with low acid value and oxygen content. The effects of cracking temperature and weight hourly space velocity (WHSV) were investigated according to the yield of products. Moreover, the influences of different γ-Al₂O₃/CaO composite catalysts on composition and physical-chemical properties of organic liquid product (OLP) were studied.

Experimental

Materials

CaO (AR, CAS 1305-78-8) and γ-Al₂O₃ (AR, CAS 1344-28-1) were purchased from the Aladdin Bio-Chem Technology Co., Ltd. in Shanghai, China and used as the basic and acidic solid catalysts, respectively. Soybean oil was obtained commercially without further purification and its composition and properties were determined by our previous work and listed in Tables 1 and 2, respectively.

Table 1
Fatty acid composition in soybean oil²⁶

Fatty acid	Content / wt.%
Palmitic acid (C16:0)	12.5
Stearic acid (C18:0)	2.9
Oleic acid (C18:1)	34.8
Linoleic acid (C18:2)	49.1
Others	0.6

Table 2
Properties of soybean oil

Property	Densitya / (kg m-3)	Calorific value / (MJ kg-1)	Viscositya / cSt	C / %	H / %	O / %
Soybean oil	922	38.6	58.3	77.9	10.9	11.2

• ^a
  Determined at 20 °C.

Preparation of composite catalysts

The γ-Al₂O₃/CaO composite catalysts were prepared by mechanical mixing. Firstly, powdery CaO was calcined at 800 ºC in a muffle furnace due to CaO might partially hydrate and carbonate by contacting with room air. For activating CaO, the temperature of outgassing should reach 700 ºC as Granados et al.³⁰ reported, and the powdery γ-Al₂O₃ was not pretreated. Secondly, powdery CaO and powdery γ-Al₂O₃ were pelleted, grinded and sieved into 20-40 mesh, respectively. Finally, the γ-Al₂O₃/CaO composite catalysts were prepared by mechanical mixing of granular CaO and granular γ-Al₂O₃. For instance, the composite catalyst containing 3.9 g of CaO and 2.1 g of γ-Al₂O₃ was denoted as 35 wt.% A/C, where A and C represented γ-Al₂O₃ and CaO, respectively, and the amount of catalyst was 6.0 g for each experiment. The other composite catalysts were denoted as 10, 20 and 50 wt.% A/C.

Catalytic cracking of soybean oil

The experiments were conducted under atmospheric pressure in a fixed-bed reactor (Figure 1). The tubular was made of stainless steel (316) with internal diameter of 16 mm and length of 380 mm. Approximately 6.0 g of catalyst was placed in the middle of the reactor for each experiment. Prior to reaction, the reactor was purged of air by nitrogen, then it was heated to the desired temperature by an electric furnace. When the temperature of the catalyst bed reached a steady state, approximately 24.0 g of soybean oil was injected into reactor by a peristaltic pump at different rate of feedstock. The formula for WHSV was as follow (equation 1):

(1) $\mathrm{WHSV}=\frac{r\times {\rho }_{\mathrm{so}}\times 60}{m_{\mathrm{cat}.}}$

Figure 1
Schematic diagram of the experimental setup: (1) gas cylinders; (2) oil tank; (3) peristaltic pump; (4) reducing valve; (5) gas rotor flowmeter; (6) reactor; (7) heating jacket; (8) thermocouple; (9) condenser; (10) liquid product receiving bottle; (11) gas buffer bottle.

where WHSV was weight hourly space velocity (h^-1), r was the rate of feedstock (mL min^-1), ρ_so was the density of soybean oil (g mL^-1) and m_cat. was the amount of catalyst (g).

Once catalytic cracking occurred, the soybean oil (M₁) would become vapor and react with the catalysts, subsequently condensed into a liquid product (M₂) via a condenser. The liquid product consisted of the OLP (M₃) and water (M₆), after static separation, the water was obtained by a syringe, and the organic phase was collected for further analysis. The non-condensable gas was collected by a gas sampling bag. The residue (M₄) in the reactor was mainly comprised by catalyst (M₅) and solid product. Thus, the products from catalytic cracking of soybean oil included OLP, gas, solid and water. The soybean oil, OLP, water product and solid product were weight via a digital balance with the precision of 0.01 g and the mass of gas product was given by mass balance. The formula for the yield of each product was as follow (equations 2-5):

(2) $\mathrm{Yield}\left(\mathrm{water}\right)\%=\frac{M_6}{M_1}\times 100$

(3) $\mathrm{Yield}\left(\mathrm{OLP}\right)\%=\frac{M_2-M_6}{M_1}\times 100$

(4) $\mathrm{Yield}\left(\mathrm{Solid}\right)\%=\frac{M_4-M_5}{M_1}\times 100$

(5) $\mathrm{Yield}\left(\mathrm{gas}\right)\%=\frac{M_1-M_2-M_4}{M_1}\times 100$

where M₁, M₂, M₃, M₄, M₅ and M₆ represented the mass (g) of soybean oil, liquid product, organic liquid product, residue, catalyst and water, respectively.

Thermogravimetric analysis (TG, Netzsch, Germany) was applied to analyze the catalytic cracking of soybean oil with different catalysts. The catalyst (approximate 25 wt.% of soybean oil) was added to soybean oil and homogenized by stirring and ultrasonic shaking. Then, approximately 10 mg of sample was used for analysis at a temperature range of 30-700 ºC with 30 ºC min^-1 heating rate. Furthermore, nitrogen (99.99%) was used as carrier gas at a constant flow rate of 40 mL min^-1.

The reusability of 35 wt.% A/C composite catalyst has been carried out. The used catalyst was directly calcined after reaction in a muffle furnace at 650 ºC for 4 h. Then, the regenerative catalyst was applied to the catalytic cracking of soybean oil at 480 ºC and 3.72 h^-1 again. The amount of catalyst was 6.0 g.

Characterization of liquid product

The composition of OLP was determined by Agilent GC7890A-MS5975C. Separations were performed on a capillary column DB-5MS (30 m × 0.25 mm × 0.25 µm). Helium with the purity of 99.999% was used as carrier gas (1.0 mL min^-1), the injector temperature was 280 ºC with a split ratio of 300:1. The temperature started at 50 ºC (6 min), increased with the gradient of 3 ºC min^-1 to 200 ºC (2 min) and 5 ºC min^-1 to 280 ºC (2 min). The composition of OLP was determined on the basis of NIST library.

In addition, fuel properties were measured: acid value according to GB/T5530-2005, calorific value was determined by a IKA-C2000 basic according to GB/T384, viscosity according to GB/T5530-2005, density according to GB/T19147-2003, the carbon, hydrogen and oxygen contents of OLP were determined on a vario MACRO cube Elemental Analyser.²⁶

Results and Discussion

Effects of cracking temperature and WHSV on yield of the products

The effects of cracking temperature on yield of the products were shown in Figure 2. The results illustrated that cracking temperature played a significant role on product yield. The yield of liquid product increased from 67.4 (450 ºC) to 74.9 wt.% (480 ºC), and the yield of solid product was lower. However, when the temperature exceeded 480 ºC, the liquid product yield decreased dramatically from 74.9 to 58.7 wt.%, on the contrary, the yield of gas product increased noticeably. It can be concluded that high temperature would promote the fracture of carbon chain, resulting in generating a large number of C₁-C₅ hydrocarbons.³¹ In sum, the cracking temperature of 480 ºC was suitable for biofuel production.

Figure 2
The effects of cracking temperature on yield of products with 35 wt.% A/C at 3.72 h^-1.

The relationship between products yield and WHSV at 480 ºC was presented in Figure 3. The maximum yield of liquid product has been achieved at 3.72 h^-1 with lower yield of gas and solid product. Smaller WHSV prolonged contact time between the catalysts and the vapor of soybean oil, leading to producing more gaseous hydrocarbons such as methane, ethane and propylene.²² Otherwise, the olefins and aromatic hydrocarbons generated from the secondary cracking would polymerize to form coke at smaller WHSV.³² However, it is worth mentioning that larger WHSV would decrease the residence time of vapor in the reactor, resulting in the incomplete cracking of soybean oil. Meanwhile, the cracking temperature would be unstable due to the excessive temperature difference. As a result, the WHSV of 3.72 h^-1 was appropriate for catalytic cracking of soybean oil in this work.

Figure 3
The effects of WHSV on yield of products with 35 wt.% A/C at 480 °C.

Effects of catalysts on yield of products and distribution of OLP

The effects of different catalysts on yield of products and distribution of OLP were shown in Table 3. Higher yield of OLP (82.5 wt.%) had been obtained by thermal cracking of soybean oil, but the content of oxygenated compounds mainly composed of fatty acids was excessive. Catalytic cracking facilitated secondary cracking of soybean oil to generate hydrocarbon, thereby the physicochemical properties were remarkably better than that of thermal cracking as shown in Tables 3 and 4.

Table 3
The effects of different catalysts on yield of products and distribution of organic liquid product^a

Comparison group	Yield of product / wt.%					Distribution of OLP / wt.%
	OLP	Water	Solid	Gas		HCb	OCc	C / %	H / %	O / %
Thermal	82.5	2.8	0.4	14.3		46.7	53.3	78.0	11.4	10.6
γ-Al2O3	63.6	6.1	10.2	20.1		75.9	24.1	83.2	12.3	4.5
50 wt.% A/C	64.6	5.3	10.6	19.5		70.9	29.1	82.1	11.9	6.0
35 wt.% A/C	70.0	4.9	10.5	14.6		66.5	33.5	82.5	11.9	5.6
20 wt.% A/C	68.8	3.6	8.3	19.3		62.7	37.3	82.6	12.2	5.2
10 wt.% A/C	68.1	3.1	7.2	21.6		65.1	34.9	82.0	12.0	6.0
CaO	67.1	2.3	9.8	20.8		57.9	42.1	81.4	11.7	6.9

• ^a
  Reaction condition: m_SO = 24.0 g, m_cat. = 6.0 g, 480 °C, 3.72 h^-1;
• ^b
  HC represents hydrocarbon;
• ^c
  OC represents oxygenated compounds; OLP: organic liquid product; A/C: γ-Al₂O₃/CaO.

Table 4
Fuel properties

Property	Acid value / (mg KOH g-1)	Calorific value / (MJ kg-1)	Density (20 ºC) / (kg m-3)	Kinematic viscosity (20 ºC) / (mm2 s-1)
Thermal	104.7	41.2	880.1	12.2
γ-Al2O3	13.5	44.2	809.8	6.3
50 wt.% A/C	8.2	44.3	810.3	7.1
35 wt.% A/C	6.7	44.2	810.6	8.0
20 wt.% A/C	4.7	43.5	828.3	6.1
10 wt.% A/C	7.0	43.9	854.6	8.5
CaO	6.3	43.4	849.9	8.4
Biodiesela	0.8	38.0	860-900	1.9-6.0
Dieselb	–	43.0	820-860	3.0-8.0

• ^a
  According to ASTM6751-03;
• ^b
  according to GB/T19147-2003. A/C: γ-Al₂O₃/CaO.

The content of oxygenated compounds and oxygen decreased from 53.3 to 24.1 wt.% and 10.6 to 4.5 wt.%, respectively, with γ-Al₂O₃ as catalyst, but the yield of OLP decreased sharply to 63.6 wt.% compared with thermal cracking. It is demonstrated that γ-Al₂O₃ had good deoxygenated effect and showed excessive secondary cracking. The deoxygenated effect of CaO was inferior to γ-Al₂O₃, the content of oxygenated compounds and oxygen was 42.1 and 6.9 wt.%, respectively, while the yield of OLP was slightly higher than gγ-Al₂O₃. As for γ-Al₂O₃/CaO composite catalysts, by increasing the amount of γ-Al₂O₃ from 10 to 35 wt.%, the yield of OLP increased from 68.1 to 70.0 wt.%. However, the yield of OLP decreased significantly from 70.0 to 64.6 wt.% by increasing the amount of γ-Al₂O₃ from 35 to 50 wt.%. The phenomenon illustrated that appropriate amount of γ-Al₂O₃ contributed to the improvement of OLP yield. In addition, the elemental analysis also indicated that γ-Al₂O₃/CaO composite catalysts possessed better deoxygenation compared with non-catalyst or CaO. Furthermore, the yield of water increased accompanied by the increase of the amount of γ-Al₂O₃, demonstrating that the presence of acid site on γ-Al₂O₃ was beneficial to dehydration, enhancing the deoxygenated effect.²³

Effects of catalysts on fuel properties

The purpose of this study is to obtain liquid hydrocarbon fuels that can be used in furnace or engines. Therefore, the main fuel properties of OLP gotten from thermal cracking and catalytic cracking of soybean oil were measured and shown in Table 4, and compared with biodiesel and the fuel properties specified in China as well.

It can be found from Table 4 that the acid value of OLP obtained from catalytic cracking of soybean oil was slightly higher than the specification of biodiesel and diesel fuels in China, but it was significantly lower than that of the thermal cracking. Moreover, the 20 wt.% A/C possessed the best decarboxylated effect with the acid value of 4.7 mg KOH g^-1. In sum, the results of calorific value, density and kinematic viscosity presented acceptable values compared to the petroleum-based fuel.

Effects of catalysts on the component of OLP

For a detailed understanding of the OLP, the components and contents of OLP were determined by gas chromatography-mass spectrometry (GC-MS) and quantified by area normalization method. The components of OLP could be classified into eight categories: alkanes, alkenes, arenes, fatty acids, esters, ketones, alcohols and ethers, which were shown in Table 5 and Figure 4. The generation process of the main products during the catalytic cracking of soybean oil over γ-Al₂O₃/CaO composite catalysts has been proposed in Scheme 1.

Table 5
The main components and content of OLP by thermal and catalytic cracking

Compound	Molecular formula	Relative content / wt.%
		Thermal	g -Al2O3	CaO	35 wt.% A/C
Alkane
Octane	C8H18	1.7	2.7	2.0	2.8
Nonane	C9H20	–	1.5	–	2.9
Decane	C10H22	0.6	1.0	0.8	1.0
Undecane	C11H24	0.6	1.1	0.8	1.1
Dodecane	C12H26	3.8	1.1	6.5	1.3
Tridecane	C13H28	0.9	1.4	1.2	1.6
Tetradecane	C14H30	0.9	1.2	1.0	1.4
Pentadecane	C15H32	2.3	2.5	2.3	2.8
Heptadecane	C17H36	1.1	1.1	1.1	2.1
Alkene
2-Octene	C8H16	2.5	1.4	2.4	2.6
1-Nonene	C9H18	2.3	1.4	2.2	3.0
1-Decene	C10H20	1.6	2.0	1.6	2.0
1-Undecene	C11H22	1.9	2.2	1.6	2.1
5-Undecene	C11H22	0.6	3.5	2.0	1.9
3-Dodecene, (Z)-	C12H24	1.5	4.3	0.3	2.1
1-Tetradecene	C14H28	3.6	2.7	4.5	3.6
7-Tetradecene	C14H28	–	2.2	–	–
1-Pentadecene	C15H30	1.6	2.0	1.6	2.3
17-Pentatriacontene	C35H70	–	–	2.6	–
Arene
Toluene	C7H8	1.3	2.4	1.2	1.6
Ethylbenzene	C8H10	0.7	1.2	0.8	1.0
Benzene, butyl-	C10H14	0.4	1.1	0.6	0.8
Benzene, pentyl-	C11H16	1.2	2.0	1.1	1.6
Benzene, 1-methylbutyl-	C11H16	–	1.3	0.8	1.3
Benzene, hexyl-	C12H18	–	1.3	0.9	1.2
Benzene, 1,3-dimethylbutyl-	C12H18	–	1.2	0.9	1.1
Fatty acid
Heptanoic acid	C7H14O2	3.6	–	–	–
Octanoic acid	C8H16O2	2.1	–	–	–
n-Decanoic acid	C10H20O2	2.8	–	–	–
n-Hexadecanoic acid	C16H32O2	8.3	–	–	0.7
cis-Vaccenic acid	C18H34O2	6.8	–	1.8	–
Oleic acid	C18H34O2	6.7	–	–	–
Ester
Butyl acetate	C6H12O2	3.8	–	2.3	1.9
7-Methyl-Z-tetradecen-1-ol acetate	C17H32O2	–	–	1.2	2.1
Z,Z-3,15-Octadecadien-1-ol acetate	C20H36O2	3.1	1.1	0.9	–
Ketone
2-Hexanone	C6H12O	–	0.5	0.6	0.7
2-Heptanone	C7H14O	–	0.5	1.0	1.0
2-Octanone	C8H16O	0.7	1.3	2.3	2.2
2-Nonanone	C9H18O	–	0.8	1.3	1.4
3-Nonanone	C9H18O	–	–	0.4	0.5
2-Fluorenone	C10H20O	–	–	–	1.3
4-Fluorenone	C10H20O	–	–	0.3	–
2-Undecenone	C11H22O	–	–	2.6	–
2-Dodecone	C12H24O	–	–	2.3	–
6-Dodecone	C12H24O	–	–	0.2	–
7-Tridecanone	C13H26O	–	–	0.4	–
7-Hexadecanone	C16H32O	–	–	0.4	–
2-Heptadecanone	C17H34O	2.9	3.3	5.4	6.0
2-Nonadecanone	C19H38O	1.4	1.2	2.3	2.5
Z-5-Methyl-6-heneicosen-11-one	C22H42O	–	1.2	–	–
Alcohol
E-2-Octen-1-ol	C8H16O	–	–	2.5	–
Trans, trans-nona-2,4-dienol	C9H16O	–	1.1	–	0.8
2-Methyl-1-undecanol	C12H26O	–	–	–	2.9
E-2-Hexadecacen-1-ol	C16H32O	1.4	2.2	0.6	–
1-Hexadecanol	C16H34O	–	1.7	0.3	2.1
E-2-Octadecadecen-1-ol	C18H36O	2.0	2.6	2.8	1.7
12-Methyl-E, E-2,13-octadecadien-1-ol	C19H36O	1.8	1.6	–	3.1
Behenic alcohol	C22H46O	–	0.4	0.6	1.5

• A/C: γ-Al₂O₃/CaO.

Figure 4
The effect of catalysts on component of OLP.

Scheme 1
The generation process of the main products during the catalytic cracking of soybean oil over γ-Al₂O₃/CaO composite catalysts.

It is known to all that soybean oil consists of triglycerides, which are three fatty acid chains connected via ester groups to glycerol.³³ As shown in Figure 5, the fatty acid, acrolein and ketones have been produced during the primary cracking of soybean oil by breaking the ester groups at high temperature (Scheme 1, reactions 1a-1d). During the secondary cracking, fatty acids have been converted to hydrocarbons, oxygenated compounds, water, H₂, CO, CO₂, as well as coke through the chemical reactions such as decarboxylation, decarbonylation, aromatization, dehydrogenation, dehydration over γ-Al₂O₃/CaO composite catalysts.

Figure 5
Stability of 35 wt.% A/C composite catalyst: (a) yield and component of OLP; (b) component of oxygenated compounds (reaction conditions: m_SO = 24 g, m_cat.. = 6 g, 480 °C, 3.72 h^-1).

The chemical composition of soybean oil is C_16:0 (12.5 wt.%), C_18:0 (2.9 wt.%), C_18:1 (34.8 wt.%) and C_18:2 (49.2 wt.%) fatty acid derivatives based on fatty acid composition of soybean oil (Table 1). Therefore, n-C₁₅ and n-C₁₇ deoxygenated compounds have been formed by decarboxylation and decarbonylation of palmitic acid derivatives (C_16:0) and stearic acid derivatives (C_18:0). However, C₁₈ fatty acid derivatives are mainly composed of oleic acid derivatives (C_18:1) and linoleic acid derivatives (C_18:2), the unsaturated bond is instable at high cracking temperature and in the existence of catalyst. Thus, through β-scission of C=C bonds, various kinds of active carbenium ions were produced, followed by radical reactions such as disproportion, condensation, decarboxylation and decarbonylation to produce different hydrocarbons and oxygenated compounds.^21,29 However, catalytic cracking of soybean oil was a non-equilibrium reaction, in other words, the radical reactions were unidirectional, resulting in the complexity of the products. As can be seen from Table 5, the alkanes were mainly consisted of straight-chain paraffin. While the composition of alkenes was complex, terminal straight-chain alkenes were the main component. Besides, methyl ketones and primary alcohols were the main composition of ketones and alcohols, respectively.

Furthermore, Figure 4 shows the effects of catalysts on components of OLP, the hydrocarbons and fatty acids were found to be the main components of the OLP produced without catalyst. The content of fatty acid reached 37.7 wt.%, demonstrating that the effect of decarboxylation was unsatisfactory by thermal cracking. The fatty acids dramatically converted into hydrocarbons via catalytic cracking, and it was also produced a certain content of oxygenated compounds such as ketones, alcohols and esters. Calcium oxide tended to convert part of fatty acids into ketones (symmetrical ketones and methyl ketones) as Zhang et al.¹⁹ and Chen et al.³⁴ reported (Scheme 1, reaction 5d). The addition of γ-Al₂O₃ could change the reaction pathway mainly caused by the accessible Lewis acid sites,²⁸ enhancing the generation of olefins, aromatic hydrocarbons and alcohols with the decrease of ketones, thereby, the content of hydrocarbons increased significantly. For instance, the OLP obtained from catalytic cracking of soybean oil over 35 wt.% A/C compared with that of CaO, the olefins, aromatic hydrocarbons and alcohols increased from 33.1 to 38.2 wt.%, from 8.0 to 10.6 wt.% and from 9.7 to 12.3 wt.%, while ketones decreased from 22.6 to 16.5 wt.%. Noteworthy, in comparison to the CaO, the content of C_6-9 methyl ketones showed an increase in the OLP with 35 wt.% A/C as catalyst accompanied with symmetrical ketones decreasing to zero (Table 5). This might be due to the report that the γ-hydrogen transfer reaction of symmetrical ketones occurred via bonding of oxygen to the Lewis acid sites on γ-Al₂O₃ to come into being 1-alkenes and enolic form (Scheme 1, reactions 2e-2f).^21,22 Enolic form was an intermediate product that was easy to be converted into methyl ketones by keto-enol tautomerism (Scheme 1, reaction 2g). In addition, the migration of the carbonyl group of methyl ketones would theoretically generate aldehydes, but no aldehydes have been identified, while alcohols increased. The phenomenon was attributed to aldehydes been converted into primary alcohols immediately by disproportionation during the γ-hydrogen transfer reaction. Moreover, the content of aromatic hydrocarbons increased due to the presence of accessible Lewis acid sites on γ-Al₂O₃, which were beneficial for further dehydrogenation of cyclic hydrocarbons produced during the Diels-Alder reaction (Scheme 1, reaction 4b) and free radical cyclization (Scheme 1, reactions 4k, 4r, 4s).³⁵

Thermogravimetric analysis of the catalytic cracking process

It is extremely important to understand the reaction changes caused by γ-Al₂O₃/CaO composite catalysts. Thus, the TG experiments of catalytic cracking soybean oil over γ-Al₂O₃, CaO and 35 wt.% A/C were conducted, and for comparison thermal cracking of soybean oil was studied as well.

As can be seen from TG-DTG curves in Figure 6, γ-Al₂O₃/CaO composite catalysts could change the reaction pathway mainly caused by CaO, since the curves of 35 wt.% A/C composite catalyst was similar to that of CaO. The primary cracking of soybean oil (Scheme 1, reactions 1a-1d) started around 300 ºC. Compared to thermal cracking of soybean oil, the maximum mass loss rate of SO + A, SO + C and SO + A/C decreased from 49.51 (430 ºC) to 34.06 wt.% min^-1 (430 ºC), 40.45 (475 ºC) and 35.49 wt.% min^-1 (475 ºC), respectively. That is to say, the soybean oil adsorbed on the catalysts and the secondary cracking occurred in the meantime. However, the mass loss step between 300 and 350 ºC was shown in the TG curves of SO + A/C and SO + C, but not of SO and SO + A. It is illustrated that calcium oxide could promote the primary cracking of soybean oil and immediately reacted with fatty acid to yield calcium carboxylates.³⁵ Calcium carboxylates was instable in high temperature because the strong electron-withdrawing groups have been introduced into the carboxyl groups by saponification. Thus, in the case of SO + A/C and SO + C, the mass loss step between 415 and 510 ºC could be attributed to calcium carboxylates decomposed into CaCO₃, ketones and hydrocarbons.^19,34 As a result, the fatty acids were remarkably converted to hydrocarbons and ketones by decarboxylation and decarbonylation (Scheme 1, reactions 5a-5e). Furthermore, the temperature of the maximum mass loss step (T_max) of SO + A/C and SO + C was 475 ºC, higher than T_max of SO and SO + A (430 ºC). It can be concluded that the further decomposition of fatty acid calcium into hydrocarbons and ketones needed more energy. It is worthwhile mentioning that there was a good correspondence between the T_max of 475 ºC of SO + A/C and the suitable cracking temperature of 480 ºC.

Figure 6
TG-DTG curves of soybean oil catalytic cracking over non-catalyst (SO), γ-Al₂O₃ (SO + A), CaO (SO + C) and 35 wt.% A/C (SO + A/C).

Furthermore, the mass loss step between 350 and 415 ºC of SO + A/C was caused by the deoxygenation of γ-Al₂O₃. Noteworthy, the maximum mass loss rate at 475 ºC of 35 wt.% A/C as catalyst compared with pure CaO decreased from 40.45 to 35.49 wt.% min^-1. This is because the decomposition of fatty acid calcium and the adsorption of CO₂ over calcium oxide lead to forming CaCO₃, and the increment of CaO amount promoted the CaO carbonation reaction.³⁶ In addition, there was a slight weight loss at higher temperatures (575-700 ºC) associated with the decomposition of CaCO₃.³⁷ It was reported that the decomposition of CaCO₃ occurred between 700 to 900 ºC in presence of N₂ was similar to this study.³⁸ The decomposition shift to lower temperature in this study might be related to the presence of H₂O produced by catalytic cracking soybean oil, because water vapor could bind to the surface of CaCO₃ and weaken the bond energy between CaO and CO₂, thereby, promoting the decomposition rate and reducing the decomposition temperature.³⁹ Moreover, the above results implied that the regeneration of γ-Al₂O₃/CaO composite catalysts could be achieved by calcining the used catalysts in a steam atmosphere for reducing energy consumption and CO₂ emission.

Table 6
Characteristic parameters of soybean oil catalytic cracking

Tmaxa / °C	The maximum mass loss rate / (wt.% °C-1)
	SO	SO + A	SO + C	SO + A/C
333	–	–	–13.63	–10.21
408	–	–	–5.13	–8.06
430	–49.51	–34.06	–	–
475	–	–	-40.45	–35.49

• ^a
  T_max: temperature of the maximum mass loss step; SO: soybean oil; SO + A: SO + γ-Al₂O₃; SO + C: SO + CaO; SO + A/C: SO + 35 wt.% A/C. A/C: γ-Al₂O₃/CaO.

Stability of γ-Al₂O₃/CaO composite catalyst

The reusability of 35 wt.% A/C composite catalyst at 480 ºC and 3.72 h^-1 has been conducted to investigate the stability of the composite catalysts and the result was shown in Figure 5. As can be seen from Figure 5a, the OLP yield of regenerative 35 wt.% A/C reached 70.4, 69.3 and 68.9 wt.% as similar as that of fresh 35 wt.% A/C during three cycles. However, the content of alkanes, alkenes and arenes decreased from 17.7 to 13.9 wt.%, from 38.2 to 30.4 wt.% and from 10.6 to 7.9 wt.% after three cycles, while the content of oxygenated compounds increased from 33.5 to 47.8 wt.%. The behavior could be attributed to a handful of coke and CaCO₃ remaining in the regenerative composite catalyst. Nevertheless, as shown in Figure 5b, the main components of oxygenated compounds were ketones, alcohols and esters, and the fatty acids content of OLP obtained by regenerative 35 wt.% A/C was remarkably lower than that of OLP from thermal cracking of soybean oil. The above results suggested that the regenerative catalyst could be applied to catalytic cracking of soybean oil with considerable catalytic effect. Anyway, it is essential to further study the regeneration of γ-Al₂O₃/CaO composite catalyst in purpose of reducing the cost of cracking production.

Conclusions

The catalytic cracking of soybean oil for biofuel was studied over the γ-Al₂O₃/CaO composite catalysts. The base sites of CaO exhibited significant fatty acid-binding performance which was beneficial to produce alkenes and ketones (symmetrical ketones and methyl ketones). The acid sites of γ-Al₂O₃ had a positive impact on generation of alkenes, arenes, methyl ketones and primary alcohols via dehydrogenation and deoxygenation. The γ-Al₂O₃/CaO composite catalysts were prepared by mechanically mixing with CaO as primary catalysts and γ-Al₂O₃ as additives. The mixed acid-base sites showed a synergic effect. The OLP yield of composite catalysts was higher than that of single CaO and single γ-Al₂O₃. The maximum yield of 70.0 wt.% have been achieved over 35 wt.% A/C. Compared with CaO, the introduction of acid sites promoted the deoxygenation process, ketones content of OLP obtained by 35 wt.% A/C significantly decreased from 22.6 to 16.5 wt.%, while hydrocarbon content increased from 57.9 to 66.5 wt.%. Compared with γ-Al₂O₃, 35 wt.% A/C reduced the arenes content of OLP with a decrement of 4.7 wt.%. Furthermore, the fuel properties and chemical composition of the biofuel was similar to that of petroleum-based fuel, whereas free of sulfur, nitrogen and heavy metal impurities, presenting great potential for being alternative of fossil fuels. Reusability study indicated that the regenerative 35 wt.% A/C composite catalyst showed considerable reactivity after three cycles with OLP yield (> 68.9 wt.%), hydrocarbon content (> 52.2 wt.%), and fatty acids content (< 3.5 wt.%). Nevertheless, the regeneration method of γ-Al₂O₃/CaO composite catalysts could be optimized in a further work.

Acknowledgments

The authors appreciatively thank the financial support from Natural Science Foundation of Zhejiang Province (No. LQ18B060006) and the National Natural Science Foundation of China (No. 21808207). The research was supported by Biodiesel Laboratory of China Petroleum and Chemical Industry Federation and Zhejiang Province Key Laboratory of Biofuel.

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Publication Dates

• Publication in this collection
  Feb 2019

History

• Received
  19 June 2018
• Accepted
  21 Sept 2018