Coke formation in the catalytic cracking of bio-oil model compounds

Authors

  • Shanling Li,

    1. Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, People's Republic of China
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  • Suping Zhang,

    Corresponding author
    1. Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, People's Republic of China
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  • Zhanyuan Feng,

    1. Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, People's Republic of China
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  • Yongjie Yan

    1. Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Research Center for Biomass Energy, East China University of Science and Technology, Shanghai, People's Republic of China
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Abstract

Catalytic cracking plays an important role in upgrading crude bio-oil. However, coke formation during the upgrading of bio-oil can deactivate catalysts and is a major problem. In this work, the catalytic cracking of bio-oil models was conducted to study the coke formation using a fixed-bed reactor. The reaction conditions including temperature and weight hourly space velocity (WHSV) have significant influence on the coke yield. The liquid product was analyzed to illustrate the main reaction pathway occurring in the catalytic cracking of the oxygenated compounds in the bio-oil. In order to clarify the molecular structure of the coke precursor, the trichloromethane extraction of the deactivated catalyst was analyzed by 13C NMR. © 2014 American Institute of Chemical Engineers Environ Prog, 34: 240–247, 2015

INTRODUCTION

Biomass is a promising renewable energy resource for its abundance in nature and being considered friendly to the environment. Fast pyrolysis liquid named bio-oil from biomass can be stored for later use or transported as a more energy dense liquid to other locations for use as fuel or chemical feedstock. However, the utilization of bio-oil is limited by problems associated with low heating value, immiscibility with hydrocarbon fuels, chemical instability, high viscosity and corrosiveness [1]. It must be upgraded to obtain high grade fuel.

Current upgrading processes [2], include catalytic hydrogenation, catalytic cracking, esterification, separation technologies, such as molecular distillation [3] and extraction. From a process point of view, catalytic cracking is extremely attractive to researchers because it is performed at atmospheric pressure in the absence of hydrogen to remove oxygen through cracking reactions. Nevertheless, this process is confined to laboratory and pilot scale because of high coke formation and catalyst deactivation [4].

Therefore, large quantities of studies have been conducted to explore the way to minimize the coke yield in the process of catalytic cracking. The previous literatures [5] indicate the coke deposition is influenced by the following operating conditions: catalyst activity, space time, reaction temperature, and H/C ratio in the feed. Coke is principally formed through polymerization and polycondensation reactions, such formation results in the blockage of the pores in the zeolites and the catalyst deactivation [6, 7]. The coke deposited on the catalyst has a significant content of oxygenates and oxo-aromatics and consists of thermal and catalytic coke [8]. Gayubo et al. [9] studied the different role of the bio-oil components in the coke formation, identifying the aldehydes and phenols as the main contributors of coke. The cracking mechanism of bio-oil oxygenated compositions still remains to be researched.

Bio-oil is a complex mixture of water and different oxygenated compounds, such as aldehydes, ketones, alcohols, acids and phenols [10]. As a result, the conversion of the bio-oil to hydrocarbon involves a large number of reactions [11]. It explains why most of the research relevant is limited to the study of pure components as model oxygenates of the bio-oil. In this paper, acetic acid, cyclopentanone and guaiacol were selected as bio-oil model compounds to study the coke formation using a fixed-bed reactor. The effects of temperature and WHSV on the coke yield were investigated. The liquid products obtained in the cracking of bio-oil model compounds were analyzed and the possible cracking pathway was proposed to explain the formation of those products. The compositions of the coke precursors were analyzed to illustrate the coke formation mechanism.

EXPERIMENTAL

Materials

Acetic acid (Analytical Reagent, Shanghai Lingfeng Chemical Reagent Company Limited), cyclopentanone (Chemically Pure, Sinopharm Chemical Reagent Company Limited) and guaiacol (Chemically Pure, Shanghai Runjie Chemical Reagent Company Limited) are chosen as bio-oil model compounds. The model compounds were selected based on the composition of bio-oil derived from the conventional pyrolysis of pine sawdust, selecting compounds that represent some of the most important chemical groups, such as acids, ketones and phenols, respectively.

Catalyst

This experiment uses CHZ-4 which is a kind of molecular sieve catalyst produced by the Sinopec Catalyst Company. The CHZ Series catalyst is one of heavy oil catalytic cracking catalysts. The characteristics of catalyst are showed in Table 1.

Table 1. Characteristics of catalyst.
PropertiesAl2O3NaFeSO4DensityBET surface area
Index≥45% (m)≤0.3% (m)≤0.4% (m)≤2.0%0.85 (g/mL)≥230 (m2/g)

Experimental Instrument

Experimental instruments include fixed bed, feeding pump, heating component, temperature control, condenser, gas collector and liquid collector. Figure 1 is the flow sheet of experimental installation.

Figure 1.

Flow sheet of experimental installation. 1: N2, 2: Valve, 3: Rotor flowmeter, 4,5: Feeding pump, 6: Temperature control, 7: Insulation component, 8: Heating component, 9: Thermocouple, 10: Fixed bed, 11: Condenser, 12: Gas collector, 13: Liquid collector.

Experimental Procedure

The catalytic cracking was carried out in a fixed-bed reactor which was produced by Biomass Energy Research Center of East China University of Science and Technology. First, the packing (porcelain rings) and catalyst were put into the reactor. The heating rate was 15°C/min, and total time was 40 min. Secondly, when the reaction temperature (460–560°C) reached the set temperature, the feed was injected by feeding pump and WHSV (3–7 h−1) of feed was set according to the need. WHSV was defined as the mass flow rate of the feed divided by the mass of the catalyst in the reactor. The gas product out of the reactor was partially condensed by cool water. The condensate was collected in a container. Finally, the liquid and gas products were analyzed.

Product Analysis

The liquid products were identified using a PE CLARUS500 GC-MS with helium as the carrier gas. A HP-5MS 30 m × 0.25 mm i.d. × 0.25 µm film capillary column was used with a 50:1 split ratio. The temperature program used was: 80°C for 3 min, ramping from 80 to 300°C at 16°C/min, and then holding at 300°C for 15 min. The mass-to-charge ratio (m/z) ranged from 33 to 500.

The composition of the coke precursor was determined by AVANCE500 Nuclear Magnetic Resonance Analyzer (NMR).

Definition

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RESULTS AND DISCUSSION

The Effects of Temperature on the Coke Yield

The catalytic cracking of acetic acid, cyclopentanone and guaiacol, selected as typical bio-oil models, was tested with the CHZ-4 catalyst. As shown in Figure 2, the reaction temperature played a key role in the formation of coke. In the lower temperature (<520°C), the coke yield decreased with increasing temperature, indicating that cracking rate of the oxygenated compounds was faster than the polymerization rate [12, 13]. In the higher temperature (>520°C), however, the coke yield presented a positive temperature dependence. This implied that the secondary polymerization of the cracking fragments occurred in the high temperature, leading to the increase of the coke [14]. It was also noticed that the catalytic cracking of guaiacol produced more coke, as compared with acetic acid and cyclopentanone. This indicated that the formation of coke was closely associated with the chemical structure and the hydrogen to carbon effective ratios of the feedstock.

Figure 2.

The effects of temperature on the coke yield (temperature from 460 to 560°C under the WHSV of 5.0 h−1). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

The Effects of WHSV on the Coke Yield

Figure 3 showed the effects of WHSV on the coke yield in the catalytic cracking of acetic acid, cyclopentanone and guaiacol. With the increase of WHSV from 3 to 7 h−1, the coke formation reduced from 16.76 to 7.37% for guaiacol. The coke yield in the catalytic cracking of acetic acid and cyclopentanone followed a similar trend as that of guaiacol. The increase of WHSV generally shortened the reactants residence time in the catalyst bed, leading to the decrease in the conversion of the oxygenated compounds. Meanwhile, a shorter reaction time would slow down the secondary reactions of the cracking products [11, 15].

Figure 3.

The effects of WHSV on the coke yield (WHSV from 3.0 to 7.0 h−1 under the temperature of 520°C). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

The Cracking Pathway of Acetic Acid

Considering that acetic acid is one of the most abundant compounds in bio-oil, the reaction paths for the catalytic cracking of acetic acid were investigated in detail. As shown in Table 2, the main products consisted primarily of acetone, toluene, and o-xylene and other heavier aromatics. Acetone should be originated from the decarbonylation and dehydration reactions of the oxygenated compounds over the catalyst, because the formation of CO2 and H2O were observed. The lighter aromatics such as toluene and xylenes were obtained from complex reactions, mainly including deoxygenation (decarbonylation, decarboxylation, and dehydration), cracking, hydrogen transfer, cyclization, and aromatization reactions. The gas products mainly included CO, CO2, and CH4 along with C2–C4 alkanes and C2–C4 olefins. CO and CO2 were formed via the decarbonylation and decarboxylation reactions of the oxygenated compounds [16]. For the gaseous alkanes and light olefins, the oxygenated organic could be directly cracked to smaller hydrocarbons through the deoxygenation and cracking processes (i.e., CxHyOz → CnH2n (n = 2–4) + CnH2n+2 (n = 1–4) + H2O/CO/CO2) [17]. These gas olefins converted to lighter aromatics (C6–C8 aromatics) through the aromatization of olefins over the catalyst. For the heavier aromatics such as naphthalenes, were formed via the oligomerization reactions of lighter aromatics. Further oligomerization of heavier aromatics resulted in the formation of coke (Figure 4).

Table 2. The liquid compositions of acetic acid cracking (520°C, 5 h−1).
Liquid compositionsContent (%)
image
Acetone77.46
image
Toluene5.86
image
o-xylene5.37
image
4-methylpent-3-en-2-one3.19
image
(E)-hex-3-en-2-one2.49
image
p-xylene1.21
image
1,2,3-trimethylbenzene1.12
image
1,2,4-trimethylbenzene1.06
image
4-hydroxy-4-methylpentan-2-one1.05
image
Naphthalene0.50
image
1,2,3,4-tetrahydronaphthalene0.41
image
1,2,4,5-tetramethylbenzene0.29
Figure 4.

The cracking pathway of acetic acid.

The Cracking Pathway of Cyclopentanone

As shown in Table 3, the main liquid products were 2-cyclopentenone and naphthalene together with other heavier aromatics. 2-cyclopentenone was the most abundant in the liquid product (21.2%), formed via the dehydrogenation reaction of cyclopentanone (Figure 5). Adjaye et al. suggested that cyclopentanone converted into hydrocarbons by decarboxylation, followed by further aromatization into aromatic hydrocarbons [18]. Based on the main organic products distribution, it could be inferred that cleavage of the cyclopentanone may be low and the breaking of carbonyl to form C4 hydrocarbon fragments should be dominated. These linear C4 hydrocarbons formed lighter aromatics such as benzene and phenol through the aromatization reactions [7]. Subsequently, the lighter aromatics converted into heavier aromatics such as naphthalene and its derivatives, and the coke formed through the oligomerization of heavier aromatics.

Table 3. The liquid compositions of cyclopentanone cracking (520°C, 5 h−1).
Liquid compositionsContent (%)
image
2-cyclopentenone24.31
image
Naphthalene12.21
image
[1,1′-bi(cyclopentan)]-2-one7.73
image
(3aS,4S,7R,7aR)−3a,4,7,7a-tetrahydro-1H-4,7-methanoindene6.89
image
p-xylene5.22
image
1,2,3,4-tetrahydronaphthalene5.02
image
3a,4,5,6,7,7a-hexahydro-1H-4,7-methanoinden-6-ol4.78
image
1,3-dimethyl-2-vinyl-benzene3.92
image
1,2,3,4-tetrahydronaphthren-9-ol3.78
image
2-methylnaphthalene3.67
image
[1,1′-bi(cyclopentylidene)]-2-one2.97
image
1-vinyl-3-ethyl-benzene2.75
image
7a-methyl-5,6,7,7a-tetrahydro-1H-inden-2(4H)-one2.24
image
Indane2.13
image
1-methyl-naphthalene2.03
image
p-cresol1.97
image
5,7-dihydrodibenzo[c,e]oxepine1.73
image
3,4,5,6,7,8-hexahydronaphthalen-1(2H)-one1.64
image
[1,1′-bi(cyclopentane)]-1,1′-diene1.33
image
7-methyl-1,2,3,5,8,8a-hexahydronaphthalene1.24
image
1,4-dihydronaphthalene1.03
image
[1,1′-bi(cyclopentan)]-1-ene0.80
image
5,6,7,8-tetrahydronaphthalen-1-ol0.60
Figure 5.

The cracking pathway of cyclopentanone.

The Cracking Pathway of Guaiacol

Catalytic cracking liquid products were measured by GC–MS, the results were shown in Table 4. Under different conditions, the composition of liquid products kept the same. But the content was a little different. The cracking pathway was speculated under the appropriate conditions. The polycyclic aromatic hydrocarbons accounted for a large proportion. It could be speculated that the methoxyl of guaiacol was unstable and easy to crack [19]. And there were many isomers of methyl-phenol in which the p-cresol was relatively stable. Generated phenol was easy to polymerize and generate multiaromatic compounds. Hydrogen transfer reactions played an important role in the conversion of guaiacol [20, 21]. The basic reaction scheme of guaiacol cracking on the CHZ-4 catalyst involved transformation into catechol, phenol, benzene, cyclohexene, and cyclohexane as well as methyl-substituted compounds (Figure 6) [22].The heavier products (coke precursor) might be formed during the reaction especially with the acidic catalytic systems [6].

Table 4. The liquid compositions of guaiacol cracking (under different conditions).
Liquid compositionsContent (%)
460°C5 h−1520°C5 h−1560°C5 h−1520°C 3 h−1520°C 6 h−1
image
4-methyl-2H-benzo[h] chromen-2-one15.4814.288.7011.3012.19
image
p-cresol19.8810.8327.5513.8111.01
image
m-cresol21.6010.798.2312.909.46
image
Benzofuran8.079.202.903.234.31
image
Ethylphenol7.528.997.514.903.93
image
o-hydroxylphenol14.267.8222.5810.8317.05
image
Phenol2.265.4810.804.8812.46
image
dibenzo[b,e]oxepin-11(6H)-one0.804.491.609.430.55
image
1-([1,1′-biphenyl]-4-yl) ethanone1.304.141.362.099.70
image
2-methoxy-9H-fluorene1.103.241.322.211.35
image
9H-xanthene1.082.901.316.025.23
image
1-(4-phenoxyphenyl) ethanone1.052.870.720.830.91
image
4-phenoxybenzaldehyde0.812.870.644.073.14
image
Methylcoumarone1.812.420.412.710.66
image
9,9-dimethyl-9H -xanthene0.802.340.620.760.82
image
dibenzo[b,d]furan0.592.330.430.910.94
image
2,2′-(ethane-1,2-diyl) diphenol0.442.150.471.040.98
image
2,3-dihydro-1H -inden-1-one0.572.050.511.320.89
image
Styrene0.590.800.644.873.64
Figure 6.

The cracking pathway of guaiacol.

The Composition of the Coke Precursor

The deactivated catalyst for analysis was acquired in the condition that reaction temperature was 520°C and reaction time was 120 min. Precursor of coke on deactivated catalyst was dissolved by trichloromethane for 24 h. The trichloromethane extraction was analyzed by 13C NMR instrument (Table 5). Comparison of aliphaticity and aromaticity indicated that the content of aromatic carbon was much more than aliphatic carbon. These observations showed that the composition of the coke precursor was mainly aromatic hydrocarbon [23]. The result was corresponding to the cracking pathway which the coke formed through the polymerization of heavier aromatics.

Table 5. 13C NMR analysis of trichloromethane extraction.
 Carbonyl CAromatic CAliphatic C
Chemical shift (ppm)160100–14020–40
Peak area29.03298.212.18
content (%)8.8190.530.66

CONCLUSIONS

To understand the coke formation mechanism, the catalytic cracking of acetic acid, cyclopentanone and guaiacol, selected as typical bio-oil models, have been investigated in detail. The catalytic cracking of guaiacol produces more coke as compared with acetic acid and cyclopentanone, which can account for the ring structure of guaiacol and direct polymerization on the catalyst surface to form coke. The deoxygenation and aromatization mainly occur in the catalytic cracking of acetic acid, producing derivatives of benzene, naphthalene. Catalytic cracking of cyclopentanone primarily forms aromatic hydrocarbons and other C5 cycloalkyl compounds. Catalytic cracking of guaiacol mainly produces polycyclic aromatic hydrocarbons and phenols. The precursor of coke was mainly aromatic hydrocarbons. The coke formation was mainly due to the polymerization and polycondensation of large amounts of aromatic compounds. The improvement of catalyst and process conditions in the catalytic upgrading can minimize coke formation on the catalyst.

ACKNOWLEDGMENTS

The study supported by the National Natural Science Foundation of China (51176049), the program for New Century Excellent Talents (NCET-11-0642) in University and the Fundamental Research Funds for the Central Universities are greatly appreciated.

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