Catalytic Conversion of Model Oxygenates in X Oil from Caprolactam

Article pubs.acs.org/EF

Catalytic Conversion of Model Oxygenates in X Oil from Caprolactam Manufacture Naixin Wang,* Xieqing Wang, Zelong Liu, and Yuxia Zhu Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China S Supporting Information *

ABSTRACT: X oil, a byproduct from caprolactam manufacture, is a complex mixture of organic oxygenates. In order to transform X oil into hydrocarbon fuel, the catalytic cracking reaction pathway and product composition of model oxygenates in X oil were investigated. The model compounds included cyclohexanol, cyclohexanone, bicyclic ketones, oxydicyclohexane, and cyclohexyl butyrate. The product distribution of mixed feedstock could be predicted, as the product compositions would not be influenced when the model compounds had a catalytic cracking reaction together. Oxygen balance during the catalytic conversion process was also studied. The main deoxygenation way of these five model compounds was dehydration, and the next was decarbonylation. The intent was to obtain a reaction pathway which could be used for modeling the catalytic cracking of X oil.

1. INTRODUCTION Cyclohexanone and cyclohexanol are the main feedstocks of caprolactam manufacture. In the synthesis process of cyclohexanone and cyclohexanol as shown in Figure 1, a kind of byproduct, commonly known as X oil, is formed, about 100− 150 kg per 1 t of caprolactam. X oil, which is dark brown and black in color and unpleasant in smell, includes cyclohexanol, cyclohexanone (some cyclohexanol and cyclohexanone remained in X oil due to difficult separation of the target product), bicyclic ketones, tricyclic ketones, oxydicyclohexane, cyclohexanediol, and a certain quantity of unidentified oxygenates.1 Although some compounds, such as bicyclic ketones and tricyclic ketones could be original material of plasticizer, the isolation of individual compounds from the complex is economically infeasible. Because of the technical difficulty in hydrolysis, oxidation, hydrogenation, and modification, X oil was commonly sold by caprolactam manufacturers as an inexpensive fuel oil or collector reagent for coal flotation. Essentially, X oil is a complicated mixture of oxygenates. Great attention has been paid to the catalytic conversion of oxygen-containing organic compounds, such as vegetable oil, fast pyrolysis bio-oil, lignin, and glycerol.2−6 Catalytic conversion of oxygenates represents a promising approach to yield organic distillate products rich in hydrocarbons and useful chemicals. Attempts with the aid of model compounds have been made to better understand the behaviors of different oxygenates during the catalytic cracking. It was postulated by Vonghia et al. that the deoxygenation of triglycerides occurred via two mechanisms during the catalytic transformation. One was γ-hydrogen transfer on one or more of the triglyceride chains directly producing alkenes. The second was β-elimination, which produced carboxylic acids. Then the carboxylic acids formed symmetric ketones, which preferably underwent a further γ-hydrogen transfer to produce alkenes and methyl ketones. Methyl ketones produced acetone, 1-octene, and alcohol, all of which dehydrated to alkenes.7 Gayubo et al. studied the catalytic conversion of model oxygenates in biomass pyrolysis oil, which included alcohols, phenols, aldehydes, ketones, and acids. © 2017 American Chemical Society

Alcohols followed a route to form hydrocarbon constituents of gasoline and light olefins. Phenols and aldehydes had low reactivity to catalytic conversion into hydrocarbons and noticeable deposition of coke. The transformations of ketones and acids mainly occurred through decarboxylation, decarbonylation, and dehydration.8,9 Wang et al. evaluated catalytic deoxygenation of cyclohexanone. The reaction started with hydrogenation of cyclohexanone to cyclohexanol on metal active sites, followed by dehydration of cyclohexanol to the corresponding alkene over acidic sites and hydrogenation of cyclohexene to cyclohexane on metal active sites.10 Horne found when the reaction temperature was between 300 and 400 °C, the removal of oxygen appeared to favor dehydration. However, higher temperature appeared to increase the removal of oxygen as carbon oxides at the expense of H2O formation.11 The literatures illustrate that mixed oxygenate feedstock is a new kind of alternative raw material of catalytic cracking to produce hydrocarbon fuels and chemicals. The objective of this paper is to verify that X oil from caprolactam manufacture also has feasibility as a supplementary material for catalytic cracking. We elucidated products, reaction mechanisms, and deoxygenation routes of five model compounds in X oil, including cyclohexanol, cyclohexanone, bicyclic ketones, oxydicyclohexane, and cyclohexyl butyrate. Furthermore, the catalytic cracking product composition of the mixed feedstock was also discussed. The information provided by this paper about catalytic cracking of oxygenates containing naphthenic rings would be helpful for further research of oxygenate catalytic conversion.

2. EXPERIMENTAL SECTION 2.1. Model Compounds and Catalyst. The model compounds, cyclohexanol (98%), cyclohexanone (99%), bicyclic ketones (85%), oxydicyclohexane (80%), and cyclohexyl butyrate (99%), were Received: November 13, 2016 Revised: February 2, 2017 Published: February 3, 2017 3029

DOI: 10.1021/acs.energyfuels.6b03008 Energy Fuels 2017, 31, 3029−3037

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Energy & Fuels

Figure 1. Formation of X oil. O0 represents the oxygen in the feedstock. O represents the oxygen in the OLP.

purchased from J&K. The mixed feedstock is composed of these model compounds with the ratio of (cyclohexanol/cyclohexanone/ bicyclic ketones/oxydicyclohexane/cyclohexyl butyrate = 45:25:5:20:5), according to the percentage of alcohols, ketones, ethers, and esters in X oil reported before.1 Table 1 summarizes the physical and chemical

yield (wt %) = (P /F ) × 100% deoxygenation rate (%) = (O0 − O)/O0 × 100%

Table 1. Physical and Chemical Characteristics of Catalyst property

value

Al2O3 (wt %) SiO2 (wt %) surface (m2·g−1) mircoporous volume (cm3·g−1) lattice parameters (nm−1)

53.8 38.6 259 0.181 2.462

3. RESULTS AND DISCUSSION 3.1. Catalytic Cracking of Cyclohexanol. Product composition from the catalytic conversion of cyclohexanol at various temperatures is shown in Table 2. The products consisted of gas, coke, H2O, and OLP. The yield of gas increased as the reaction temperature increased. The yield of H2O reached 20% at the temperature of 350 °C and no further increase at higher temperatures. In contrast to gas, the content of coke was higher at low temperature and decreased with the increase of temperature. This phenomenon has been reported before.4,11,13 Dehydration, decarbonylation, and decarboxylation proceed more rapidly than C−C bond cleavage in saturated hydrocarbon.14 During the catalytic cracking reaction, deoxygenation occurred immediately and formed hydrocarbon intermediates. The hydrocarbon intermediates were the precursor of coke and preferred to decompose into light hydrocarbons at higher temperature. Therefore, the yield of coke was higher when the reaction temperature was low. In some of the literature, coke deposits contained complex oxygenates when the raw material of catalytic conversion was phenol, furan, or furfural2,8,15−19 and the variation of coke yield with temperature was different from this work. This meant that the composition and characteristic of coke strongly depended on the type of the catalytic conversion feed. Therefore, a deeper study on the coke of catalytic conversion of oxygenates is needed. Components of the gas phase and OLP are also shown in Table 2. The components of gas products included hydrogen and C1−C4 hydrocarbons, which were similar to gas products from the catalytic cracking of hydrocarbon feedstock. There was no CO in the gas phase, indicating there is no decarbonylation during the catalytic cracking of cyclohexanol. There was a small amount of CO2 (less than 0.5%) in the products. No carboxyl group is in cyclohexanol and the yield of CO2 was not influenced by temperature, so it can be deduced that CO2 may be produced by impurities in the raw material or some insignificant reactions. At the temperature of 350 °C,

characteristics of the catalyst (supplied by Research Institute of Petroleum Processing) used in this study. 2.2. Experimental Setup. Catalytic cracking experiments were carried out on an advanced cracking evaluation (ACE) unit supplied by M/s Kayser Technologies inc., USA. The operational principle was reported before.12 Feedstock was pumped into reactor and then contacted with catalyst particle flow. After the completion of reaction, a period of time of stripping with N2 at the reaction temperature with the aim of eliminating the components that might remain absorbed on the catalyst was provided. During the process of catalytic cracking and stripping, liquid product was collected in glass receivers and maintained at −10 °C. Gaseous product was collected by a water displacement method. To vary the cracking severity and thus conversion, reaction runs were carried out at 350, 400, 450, 500, and 550 °C at atmosphere. A constant amount of catalyst, 9 g, fixed catalyst-to-feed ratio, 5, and fixed stripping time, 800 s, were loaded for each experiment. 2.3. Product Analysis. Products consisted of gaseous and liquid fractions. Analytical procedure involved measuring the yield of each product and identifying various compounds using a combination of gas chromatography (GC; Agilent 6890 equipped with two detectors, TCD and FID) and gas chromatography−mass spectrometry (GC−MS; Agilent 6890/5973 with a column of DB-5MS, 30 m × 0.25 mm × 0.25 μm). The amount of coke deposited on the catalyst was measured by regeneration reaction at 700 °C in the presence of air. Elemental analysis of the organic liquid product (OLP) was carried out on a Vario Micro Cube (Elementar). The yield of product H2O was calculated by closing the mass balance.13 2.4. Data Processing. The yield and deoxygenation rate were calculated by following relations. P represents the amount of product (g). F represents the amount of feedstock (g). 3030


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cyclohexene was the main product in the OLP. The content of cyclohexene decreased with the increasing of temperature, while the contents of methylcyclopentene, cyclohexane, and multiple aromatics (including benzene, toluene, xylene, naphthalenes, and three−five ring aromatics) increased. As for methylcyclopentane, the highest content, about 30 wt %, was at the reaction temperature of 450 °C. The elemental composition and deoxygenation rate are also given in Table 2. The deoxygenation rate reached 100% even at a low temperature of 350 °C. Therefore, the OLP only consisted of hydrocarbon compounds. This indicated that it was easy for cyclohexanol to convert into hydrocarbon gas, hydrocarbon liquid, and H2O. The proposed main catalytic cracking reaction pathway of cyclohexanol based on the composition of products is shown in Figure 2. Dehydration to cyclohexene was the first step. In this step, a cyclohexene cation was formed, followed by the release of H2O. As reported before, a series of cationic dehydrogenation and removal of hydrides by other cations completed the deprotonation of cyclohexene cation.20 Cyclohexene turned into methylcyclopentene through an isomerization reaction, and methylcyclopentene would convert into methylcyclopentane by hydrogen transfer. These cyclic hydrocarbons were prone to transferring into aromatics, especially at high temperature. The second route was thought to proceed through opening of naphthenic rings to produce straight-chain alcohols, followed by dehydration to a mixture of alkenes. These alkene products were responsible for cracking, isomeriaztion, cyclization, and aromatization, which resulted in the formation of short-chain hydrocarbons and aromatic compounds. These reactions involved were similar to the catalytic conversion reactions of other alcohols, such as methylcyclohexanol,20 propanol, and butanol.8 3.2. Catalytic Cracking of Cyclohexanone. The product composition from the catalytic conversion of cyclohexanone as a function of reaction temperature is provided in Table 3. The gas fraction mainly consisted of C1−C4 hydrocarbons, CO2, and CO. The amounts of CO and hydrocarbon gas increased with temperature, while the yield of CO2 remained unchanged. This indicated that decarbonylation and cracking reaction could

Table 2. Product Composition from the Catalytic Conversion of Cyclohexanol at Various Temperatures temperature, °C 350

400

450

500

Yields of Products, wt % of Cyclohexanol Feedstock gas 0.9 1.7 3.1 5.7 coke 3.4 2.4 2.0 1.9 H2O 19.9 19.0 18.8 18.3 OLP 75.8 76.9 76.1 74.1 Gas Product Components, wt % of Cyclohexanol Feedstock hydrogen 0.01 0.01 0.02 0.04 methane 0 0 0.05 0.19 ethane 0 0 0.05 0.10 ethylene 0.05 0.07 0.16 0.36 propane 0.11 0.18 0.22 0.47 propylene 0.19 0.37 0.75 1.45 n-butane 0 0.05 0.13 0.26 isobutane 0.22 0.52 0.95 1.53 1-butene 0 0.05 0.10 0.21 isobutylene 0 0.06 0.17 0.33 cis-2-butene 0 0.06 0.10 0.22 trans-2-butene 0 0.10 0.18 0.32 CO2 0.31 0.20 0.27 0.29 CO 0 0 0 0 OLP Components, wt % of Cyclohexanol Feedstock methylpentene 4.6 3.8 2.8 2.2 methylcyclopentene 8.1 11.8 15.3 15.7 methylcyclopentane 20.6 27.5 30.1 28.2 cyclohexene 32.0 19.5 8.8 3.6 cyclohexane 0.5 1.4 1.5 2.0 benzene 2.0 2.5 3.0 3.2 toluene 0.2 0.5 1.3 2.6 xylene 0.3 0.9 2.1 3.9 naphthalenes 6.1 7.0 8.0 8.5 3−5 ring aromatics 1.4 2.1 3.1 4.0 Elemental Composition, wt % of OLP carbon 86.9 87.1 87.3 87.5 hydrogen 13.1 12.9 12.7 12.5 oxygen 0 0 0 0 deoxygenation rate, % 100 100 100 100

550 9.9 1.9 18.1 70.1 0.22 0.55 0.22 0.73 0.81 2.64 0.38 2.05 0.35 0.57 0.34 0.50 0.43 0 1.9 16.0 20.9 2.2 2.2 3.6 4.4 5.5 8.6 4.9 87.9 12.1 0 100

Figure 2. Proposed main catalytic cracking reaction pathway of cyclohexanol. 3031


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be improved by the rise of temperature and the CO2 may be produced due to the impurities in raw material or insignificant reactions, just like cyclohexanol. The yield of coke decreased from 8.6% at 350 °C to 3.9% at 550 °C. In addition to hydrocarbon compounds, some oxygenates were also included in the OLP, such as cyclohexanone, phenol, and dibenzofuran. Therefore, the deoxygenation rate could not reach 100% even at the high temperature of 550 °C as shown in Table 3. At 350 °C, the conversion rate of cyclohexanone was low. Unconverted cyclohexanone accounted for almost half of the OLP. The conversion and deoxygenation rate were promoted by the rise of temperature. The proposed main catalytic cracking reaction pathway of cyclohexanone is presented in Figure 3. The condensation of two cyclohexanone molecules removed 50% of the oxygen by releasing H2O and forming bicyclic ketones. This step was an aldol condensation reaction, which involved the reaction of intermediate enol and protonated carbonyl compound.20 Deep condensation continued with dehydration and producing tricyclic ketones. Deeper condensation would be constrained by the pore-size constraint of the catalyst. In Figure 3 CO and short chain alkenes were produced by decarbonylation and direct cracking of cyclohexanone. The third reaction mechanism was the transformation to enol structure, followed by dehydrogenation into phenol and dehydration into cyclohexadiene, which converted into aromatics finally. 3.3. Catalytic Cracking of Bicyclic Ketones. The product composition from catalytic conversion of bicyclic ketones at various temperatures is shown in Table 4. It is similar to the product composition in Table 3. It was worth noting that some cyclohexanone remained in the OLP, while no bicyclic ketones remained. At the same temperature, the deoxygenation rate of cyclohexanone was lower than that of bicyclic ketones. This meant that, compared with cyclohexanone, it was easier for bicyclic ketones to crack into hydrocarbons. Figure 4 shows the proposed main catalytic cracking reaction pathway of bicyclic ketones. First of all, two structures of bicyclic ketones, 2-(1-cyclohexen-1-yl)-cyclohexanone and 2-cyclohexylidene-cyclohexanone, were interchangeable. 2-(1-Cyclohexen-1-yl)-cyclohexanone preferred to crack into cyclohexene and cyclohexanone. The cracking mechanisms of cyclohexene and cyclohexanone are discussed in Figures 2 and 3. The other reaction pathway was direct dehydration by producing

Table 3. Product Composition from the Catalytic Conversion of Cyclohexanone at Various Temperatures temperature, °C 350

400

450

500

Yields of Products, wt % of Cyclohexanone Feedstock gas 0.7 1.2 2.2 4.1 coke 8.6 6.8 5.8 4.7 H2O 11.4 14.4 14.1 14.4 OLP 79.3 77.6 77.9 76.8 Gas Product Components, wt % of Cyclohexanone Feedstock hydrogen 0 0 0.08 0.26 methane 0 0 0.09 0.20 ethane 0.12 0.22 0.33 0.52 ethylene 0 0 0.08 0.25 propane 0.11 0.25 0.50 0.96 propylene 0 0 0.00 0.10 n-butane 0 0.12 0.25 0.51 isobutane 0 0.01 0.02 0.03 1-butene 0 0 0.06 0.12 isobutylene 0 0 0.08 0.13 cis-2-butene 0 0 0.06 0.13 trans-2-butene 0 0.05 0.09 0.17 CO2 0.35 0.38 0.39 0.39 CO 0.08 0.13 0.20 0.34 OLP Components, wt % of Cyclohexanone Feedstock methylcyclopentene 3.8 9.3 10.8 11.9 methylcyclopentane 3.2 8.3 12.4 13.1 cyclohexene 11.9 10.2 6.2 3.6 cyclohexane 0.8 1.2 1.6 2.1 benzene 1.4 1.8 3.9 6.2 toluene 0.3 0.9 1.8 2.8 xylene 0.2 0.5 0.9 1.6 cyclohexylbenzene 4.9 2.2 0.7 0.4 1,3-hexadien-1-yl-benzene 2.7 3.3 2.5 1.4 naphthalenes 9.9 13.0 14.1 14.1 3−5 ring aromatics 4.3 4.7 5.0 5.5 cyclohexanone 33.9 18.7 13.3 8.3 phenol 1.1 2.0 3.4 5.1 dibenzofuran 0.9 1.5 1.2 0.6 Elemental Composition, wt % of OLP carbon 81.5 84.3 84.9 86.5 hydrogen 11.3 11.0 10.8 10.5 oxygen 7.2 4.7 4.3 3.0 deoxygenation rate, % 62.3 75.7 78.4 81.2

550 7.2 3.9 14.6 74.3 0.64 0.41 0.84 0.57 1.83 0.21 0.88 0.06 0.21 0.27 0.20 0.29 0.37 0.53 14.2 8.9 2.0 2.6 8.0 4.4 2.4 0.4 0.9 13.4 5.5 5.6 5.6 0.4 87.4 10.0 2.6 82.9

Figure 3. Proposed main catalytic cracking reaction pathway of cyclohexanone. 3032


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Energy & Fuels alkylbenzenes, which would form aromatics through cyclization, aromatization, and cracking. 3.4. Catalytic Cracking of Oxydicyclohexane. The product composition from catalytic conversion of oxydicyclohexane

at various temperatures is shown in Table 5. It was observed that the conversation of oxydicyclohexane could be accomplished at 350 °C, indicating that oxydicyclohexane was easy to convert into hydrocarbons by catalytic cracking just like cyclohexanol. It was interesting that the product distribution and elemental composition in Table 5 were almost the same as those of cyclohexanol as shown in Table 2. Based on these results, the proposed main catalytic cracking reaction pathway of oxydicyclohexane is shown in Figure 5. Cyclohexanol and cyclohexene were produced by initial cracking of oxydicyclohexane. As described above, cyclohexene was the catalytic cracking product of cyclohexanol. The following reaction steps of cyclohexanol and cyclohexene are shown in Figure 2. This result explains why cyclohexanol and oxydicyclohexane had the same catalytic cracking product distribution. 3.5. Catalytic Cracking of Cyclohexyl Butyrate. Table 6 shows the product composition from catalytic conversion of cyclohexyl butyrate as a function of temperature. The product distribution of cyclohexyl butyrate catalytic cracking was slightly different from other model oxygenates. First, an appreciably higher gas product yield was obtained, especially at high temperature. It reached almost 40% at the temperature of 550 °C. Different from other model oxygenates, the yield of CO2 (more than 1%) was improved by temperature. Since cyclohexyl butyrate has an ester functional group, the CO2 was produced by decarboxylation of raw material. At 350 and 400 °C CO2 was formed to a greater extent than CO. At 450 °C the yields of CO and CO2 were basically the same. At 500 and 550 °C the yield of CO was higher than that of CO2. It could be indicated that, as for cyclohexyl butyrate, higher temperature promoted decarbonylation to a greater extent than decarboxylation in the catalytic cracking reaction. The second difference was that the yield of H2O increased gradually with temperature, while as for other model oxygenates, the yields of H2O were almost not influenced by reaction temperature. The deoxygenation rate in Table 6 increased greatly from 15.2% at 350 °C to 96.8% at 550 °C. In contrast to gas and H2O, the production of the OLP was reduced with the rise of temperature. The oxygen in the OLP was mainly provided by butyric acid and heptanone. According to the above results, it was concluded that the reaction temperature had greater influence on the product distribution in the catalytic cracking of cyclohexyl butyrate. It could be found that the OLP components in Table 6, except for butyric acid and heptanone, were similar to the OLPs produced by other model oxygenates. In other words, the catalytic cracking hydrocarbon product compositions of these five model oxygenates were essentially the same.

Table 4. Product Composition from the Catalytic Conversion of Bicyclic Ketones at Various Temperatures temperature, °C 350

400

450

500

Yields of Products, wt % of Bicyclic Ketones Feedstock gas 0.9 1.7 3.3 5.6 coke 11.2 6.2 4.8 4.1 H2O 8.5 9.1 8.8 8.6 OLP 79.4 83.0 83.1 81.7 Gas Product Components, wt % of Bicyclic Ketones Feedstock hydrogen 0.01 0.01 0.02 0.04 methane 0 0.03 0.10 0.28 ethane 0 0.11 0.20 0.32 ethylene 0.14 0.25 0.42 0.75 propane 0 0.10 0.22 0.47 propylene 0.18 0.38 0.68 1.18 n-butane 0.00 0 0.09 0.18 isobutane 0.09 0.24 0.47 0.79 1-butene 0 0 0.09 0.14 isobutylene 0 0 0.06 0.12 cis-2-butene 0 0.05 0.09 0.15 trans-2-butene 0 0.08 0.13 0.21 CO2 0.32 0.22 0.28 0.30 CO 0.16 0.26 0.46 0.68 OLP Components, wt % of Bicyclic Ketones Feedstock methylcyclopentene 2.6 3.7 4.6 5.9 methylcyclopentane 3.7 11.5 15.3 14.0 cyclohexene 27.3 20.6 15.3 10.7 cyclohexane 1.2 1.7 2.1 2.9 benzene 4.1 8.3 12.1 16.0 toluene 0.2 0.2 2.2 3.8 xylene 0.2 0.6 1.2 2.0 cyclohexylbenzene 4.1 2.9 2.0 0.9 1,3-hexadien-1-yl-benzene 10.8 7.8 5.6 3.4 naphthalenes 11.5 12.5 11.0 10.4 3−5 ring aromatics 5.2 6.1 6.0 5.8 cyclohexanone 7.2 5.1 3.7 2.5 phenol 1.0 1.6 1.7 2.9 dibenzofuran 0.4 0.5 0.5 0.6 Elemental Composition, wt % of OLP carbon 86.6 88.0 88.4 88.8 hydrogen 11.4 10.7 10.5 10.2 oxygen 2.0 1.3 1.1 1.0 deoxygenation rate, % 82.3 89.2 90.7 92.6

550 9.5 3.7 8.8 78.0 0.10 0.72 0.55 1.23 0.81 2.09 0.28 1.11 0.23 0.26 0.22 0.33 0.46 1.08 7.0 9.7 5.4 3.7 19.7 5.9 3.0 0.4 2.1 10.8 5.2 1.7 2.9 0.5 89.2 9.9 0.9 93.8

Figure 4. Proposed main catalytic cracking reaction pathway of bicyclic ketones. 3033


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Energy & Fuels Table 5. Product Composition from the Catalytic Conversion of Oxydicyclohexane at Various Temperatures

Table 6. Product Composition from the Catalytic Conversion of Cyclohexyl Butyrate at Various Temperatures

temperature, °C 350

400

450

temperature, °C 500

550

350

Yields of Products, wt % of Oxydicyclohexane Feedstock gas 0.8 1.6 3.5 6.2 10.8 coke 3.5 2.6 2.3 2.0 2.0 H2O 10.2 10.2 10.6 10.3 9.7 OLP 85.5 85.6 83.6 81.5 77.5 Gas Product Components, wt % of Oxydicyclohexane Feedstock hydrogen 0 0.01 0.01 0.03 0.06 methane 0 0 0.07 0.22 0.61 ethane 0 0 0.07 0.13 0.29 ethylene 0.04 0.09 0.19 0.39 0.81 propane 0 0.08 0.38 0.53 1.11 propylene 0.16 0.35 0.76 1.48 2.76 n-butane 0 0.05 0.15 0.31 0.49 isobutane 0.21 0.52 1.03 1.71 2.48 1-butene 0 0.06 0.10 0.21 0.37 isobutylene 0 0.07 0.15 0.32 0.59 cis-2-butene 0 0.06 0.12 0.23 0.37 trans-2-butene 0.05 0.10 0.19 0.34 0.53 CO2 0.28 0.18 0.23 0.29 0.32 CO 0 0 0 0 0 OLP Components, wt % of Oxydicyclohexane Feedstock methylpentene 4.1 3.4 2.6 1.8 1.0 methylcyclopentene 6.5 10.3 12.5 13.9 14.6 methylcyclopentane 23.5 31.0 33.5 30.8 23.6 cyclohexane 38.8 22.4 11.2 4.9 2.5 cyclohexene 1.0 1.5 2.3 3.9 4.1 benzene 2.7 3.6 4.4 4.5 5.5 toluene 0.2 0.5 1.3 2.8 4.6 xylene 0.3 0.9 2.2 3.9 5.7 naphthalenes 6.6 9.5 9.9 10.0 9.8 3−5 ring aromatics 1.7 2.7 3.7 5.1 5.9 Elemental Composition, wt % of OLP carbon 87.0 87.2 87.4 87.6 87.9 hydrogen 13.0 12.8 12.6 12.4 12.1 oxygen 0 0 0 0 0 deoxygenation rate, % 100 100 100 100 100

400

450

500

550

Yields of Products, wt % of Cyclohexyl Butyrate Feedstock gas 2.8 6.3 13.5 25.8 36.5 coke 5.9 5.5 5.8 6.0 5.9 H2O 2.2 5.7 10.2 14.0 15.3 OLP 89.1 82.5 70.5 54.2 42.3 Gas Product Components, wt % of Cyclohexyl Butyrate Feedstock hydrogen 0.02 0.02 0.03 0.04 0.06 methane 0 0.04 0.19 0.60 1.29 ethane 0 0.09 0.27 0.50 0.64 ethylene 0.05 0.13 0.30 0.65 1.27 propane 0.13 0.61 1.90 3.56 4.72 propylene 0.40 0.96 2.57 6.33 10.29 n-butane 0.06 0.10 0.19 0.39 0.57 isobutane 0.07 0.17 0.38 0.92 1.46 1-butene 0.16 0.31 0.57 0.92 1.04 isobutylene 0.00 0.09 0.28 0.60 0.80 cis-2-butene 0.19 0.35 0.57 0.86 0.95 trans-2-butene 0.28 0.53 0.89 1.32 1.42 CO2 1.18 2.09 2.68 3.22 3.10 CO 0.21 0.85 2.69 5.88 8.86 OLP Components, wt % of Cyclohexyl Butyrate Feedstock methylcyclopentene 3.0 6.1 8.2 9.4 8.1 methylcyclopentane 2.5 3.1 3.7 5.0 3.3 cyclohexene 36.5 33.3 19.8 8.2 2.5 cyclohexane 0.4 1.3 3.0 2.9 3.2 benzene 1.7 2.6 3.4 3.6 3.9 toluene 0.2 0.5 1.0 2.1 2.8 xylene 1.4 2.0 3.9 2.8 2.3 naphthalenes 4.2 6.5 8.5 8.9 7.2 3−5 ring aromatics 3.1 4.5 6.1 8.0 8.5 butyric acid 28.7 13.1 10.4 1.6 0.2 heptanone 7.3 9.6 2.5 1.6 0.3 Elemental Composition, wt % of OLP carbon 71.5 74.9 79.6 84.9 86.7 hydrogen 11.4 11.5 11.7 11.9 12.1 oxygen 17.1 13.6 8.7 3.2 1.2 deoxygenation rate, % 15.2 36.2 62.4 89.5 96.8

which produced CO2 and propylene. The other was dehydration, in which an acylium species was formed. The acylium ion suffered nucleophilic attack by butyric ion and thus eliminated CO2 and heptanone, which produced hydrocarbon by further decarbonylation. As mentioned above, the catalytic cracking hydrocarbon products of these five model oxygenates were basically the same. The hydrocarbon products included methylcyclopentene, methylcyclopentane, cyclohexene, cyclohexane, BTX, naphthalenes, and three−five ring aromatics. From the proposed main reaction pathways shown in Figures 2−6, it was found that all of these five oxygenates could convert into cyclohexene during the catalytic cracking reaction. Other hydrocarbons were produced by cyclohexene through the reaction of isomerization, hydrogen transfer, and aromatization. Therefore, the reason why these five model compounds had similar hydrocarbon product distributions was that they had the same intermediate hydrocarbon product, cyclohexene. 3.6. Catalytic Cracking of Mixed Feedstock. The study of model compounds provided partial information on the actual

Figure 5. Proposed main catalytic cracking reaction pathway of oxydicyclohexane.

Based on these results, the proposed main catalytic cracking reaction pathway of cyclohexyl butyrate is shown in Figure 6. It was postulated that the deoxygenation of cyclohexyl butyrate occurs via two mechanisms. The first one was γ-hydrogen transfer on the butyric acid chain to produce cyclohexanol and acetaldehyde. Acetaldehyde produced unsaturated aldehydes by an aldol condensation reaction. Then unsaturated aldehydes continued to produce hydrocarbons by decarbonylation. The other possible mechanism was β-elimination, producing cyclohexene and butyric acid. There were also two mechanisms for the deoxygenation of butyric acid. One was decarboxylation, 3034


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Figure 6. Proposed main catalytic cracking reaction pathway of cyclohexyl butyrate.

temperature but could not reach 100% even at 550 °C. In order to identify whether the product compositions would be affected when these model oxygenates had catalytic cracking reaction together, the correlation between experimental yields and calculated yields of CO, propylene, benzene, and coke at different temperature were plotted in Figure 7. The data of experimental yields was from Table 7. The calculated yields were sums of the products of every model oxygenate ratio in the mixed feedstock and its corresponding product yield from Tables 2−6. It was interesting to note that all the points were close to the line of Y = X. In other words, the calculated yields were basically the same as the experiment yields at different temperatures. Therefore, the product yields of the mixed feedstock could be predicted by the ratio of every model oxygenate in the feedstock and the product yields of individual oxygenate feedstocks from Tables 2−6. 3.7. Oxygen Balance. According to the product distributions in Tables 2−6, the oxygen balances of these model compounds during the catalytic cracking reaction were calculated. Table 8 shows the oxygen balance at different temperatures. Obviously, dehydration was the main deoxygenation reaction for all model compounds. The existence of decarbonylation and decarboxylation depended on the chemical structure of the raw material. According to refs 9 and 18, CO was produced by decarbonylation and CO2 was produced by decarboxylation. Decarbonylation existed in the catalytic cracking reaction of oxygenates with carbonyl functional groups, such as ketones and esters. Likewise, decarboxylation existed in the catalytic cracking reaction of oxygenates with carboxyl functional groups (or ester functional groups) such as carboxylic acids and esters. As for cyclohexanol and oxydicyclohexane, with no carbonyl or ester group, the small amount (less than 0.5%) of product CO2 may be caused by impurities in the raw material or some insignificant reactions. In other words, the decarbonylation or decarboxylation could be neglected in the deoxidation process of cyclohexanol and oxydicyclohexane. Similarly, little CO2 (less than 0.5%) existed in the products from catalytic conversion of cyclohexanone and bicyclic ketones, with no carboxyl or ester functional group. Decarboxylation could also be neglected in their deoxidation process. As for cyclohexyl butyrate, with an ester functional group, decarbonylation and decarboxylation existed simultaneously during the catalytic cracking process. Thus, the yield of CO2 from the catalytic conversion of mixed feedstock strongly depended on the cyclohexyl butyrate content. According to the above, besides dehydration, decarbonylation also existed in the catalytic cracking reaction of

Table 7. Product Composition from the Catalytic Conversion of Mixed Feedstock at Various Temperatures temperature, °C 350

400

450

500

Yields of Products, wt % of mixed Feedstock gas 0.7 1.7 3.1 5.6 coke 7.3 4.7 3.8 3.0 H2O 11.1 11.1 11.3 11.0 OLP 80.9 82.5 81.8 80.4 Gas Product Components, wt % of Mixed Feedstock hydrogen 0.01 0.01 0.02 0.04 methane 0 0.03 0.10 0.28 ethane 0 0.11 0.20 0.32 ethylene 0.14 0.25 0.42 0.75 propane 0 0.10 0.22 0.47 propylene 0.18 0.38 0.68 1.18 n-butane 0 0 0.09 0.18 isobutane 0.09 0.24 0.47 0.79 1-butene 0 0 0.09 0.14 isobutylene 0 0 0.06 0.12 cis-2-butene 0 0.05 0.09 0.15 trans-2-butene 0 0.08 0.13 0.21 CO2 0.20 0.26 0.29 0.44 CO 0.08 0.15 0.29 0.47 OLP Components, wt % of Mixed Feedstock methylcyclopentene 2.3 1.7 1.2 0.9 methylcyclopentane 1.8 2.4 3.4 4.1 cyclohexene 15.0 24.5 29.3 28.1 cyclohexane 38.0 23.2 11.2 3.5 benzene 2.2 3.2 4.3 5.4 toluene 6.7 9.5 13.0 16.1 xylene 0.4 0.9 1.9 3.7 naphthalenes 1.9 1.8 1.9 2.6 3−5 ring aromatics 10.9 13.9 14.2 14.2 cyclohexanone 1.2 0.7 0.4 0.2 phenol 0.5 0.7 0.9 1.6 Elemental Composition, wt % of OLP carbon 87.5 88.0 88.4 88.7 hydrogen 11.5 11.3 11.1 11.0 oxygen 1.0 0.7 0.5 0.3 deoxygenation rate, % 92.2 94.3 96.2 97.4

550 9.1 2.6 11.3 77.0 0.10 0.72 0.55 1.23 0.81 2.09 0.28 1.11 0.23 0.26 0.22 0.33 0.54 0.68 0.9 4.0 21.4 1.1 5.9 17.6 6.4 3.8 13.2 0.1 2.6 89.3 10.4 0.3 98.2

reactivity of X oil. In this section, the reactivity of a mixture of model compounds was investigated. Table 7 shows the product composition from catalytic conversion of mixed feedstock at various temperatures. The result was similar to the results when they were studied separately. Deoxygenation rate increased with 3035


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Figure 7. Experimental and calculated yields of CO, propylene, benzene, and coke at different temperatures.

Table 8. Oxygen Balance from the Catalytic Conversion of Model Oxygenates at Different Temperatures cyclohexanol

cyclohexanone

bicyclic ketones

oxydicyclohexane

cyclohexyl butyrate

temp, °C

dehydration, %

decarbonylation, %

decarboxylation, %

in OLP, %

350 400 450 500 550 350 400 450 500 550 350 400 450 500 550 350 400 450 500 550 350 400 450 500 550

99.9 99.9 99.9 99.9 99.9 76.2 79.2 81.1 85.9 87.8 86.9 87.2 87.4 87.3 85.5 99.8 99.8 99.8 99.8 99.8 28.9 39.6 58.7 70.6 71.9

0 0 0 0 0 0.3 0.4 0.7 1.2 1.9 1.0 1.6 2.9 4.5 6.8 0 0 0 0 0 0.8 2.7 8.4 18.5 23.9

0.1 0.1 0.1 0.1 0.1 0.3 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.4 0.2 0.2 0.2 0.2 0.2 0.6 0.8 1.0 1.2 1.3

0 0 0 0 0 23.2 20.2 18.0 12.7 10.1 11.8 11 9.5 7.9 7.3 0 0 0 0 0 69.7 56.9 31.9 9.7 2.9

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cyclohexanone, bicyclic ketones, and cyclohexyl butyrate, which had carbonyl functional groups. For the model compounds having no carboxyl or ester group, such as cyclohexanol and oxydicyclohexane, decarbonylation and decarboxylation could be neglected. The deoxygenation of cyclohexanol and oxydicyclohexane could complete at 350 °C. That is why no oxygen was left in their OLPs. Cyclohexanone, bicyclic ketones, and cyclohexyl butyrate produced many other oxygenates, such as phenols, furans, acids, and ketones during the catalytic cracking reaction, which provided oxygen in the OLP.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03008. Boiling point ranges of OLPs from catalytic conversion of cyclohexanol, cyclohexanone, bicyclic ketones, oxydicyclohexane, and cyclohexyl butyrate at various temperatures, Figures S1−S5 (PDF)

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4. CONCLUSIONS In this paper, cyclohexanol, cyclohexanone, bicyclic ketones, oxydicyclohexane, and cyclohexyl butyrate were used as model compounds for studying the catalytic cracking of oxygenates. It supplies a theoretical proof for the catalytic conversion of X oil, a byproduct from caprolactam manufacture, into a high value product. The catalytic cracking products of these five model compounds included H2O, gas, coke, and OLP. With the exception of a certain amount of CO and CO2, the gas product composition was basically the same as that of hydrocarbon catalytic cracking. The yield of coke decreased with the increasing of reaction temperature. The catalytic cracking OLPs of cyclohexanol and oxydicyclohexane were hydrocarbon compounds, while the OLPs of cyclohexanone, bicyclic ketones, and cyclohexyl butyrate were the mixture of hydrocarbon compounds and oxygenates, including ketones, phenols, furans, and acids. Although these five oxygenates went through different deoxygenation reactions during the catalytic cracking, they could transform into the same hydrocarbon intermediate, cyclohexene. As a result, their hydrocarbon product compositions were basically the same. Cyclohexene continued to go through isomerization, hydrogen transfer, cracking, and aromatization. Higher reaction temperature was favorable to the conversion of alkenes to aromatics in the OLP. The main deoxygenation way of these five model compounds was dehydration. As for cyclohexanone, bicyclic ketones, and cyclohexyl butyrate, decarbonylation also occurred to produce CO. Decarboxylation of cyclohexanol, cyclohexanone, bicyclic ketones, and oxydicyclohexane was not significant, even at high temperature. In contrast, the decarboxylation of cyclohexyl butyrate was obvious and was affected by temperature. It could be recognized that the way of deoxygenation depended on the chemical structure of oxygenates. By comparing the experimental yield results with the calculated yield results, a phenomenon emerged that the product compositions would not be influenced by mixing the model oxygenates together as the catalytic cracking feedstock. According to this result, the product distribution could be predicted by the composition of the mixed feedstock. This feature has considerable practical significance for X oil catalytic cracking. In order to expand the scope of application of X oil and explore more information about catalytic conversion of oxygenates, much work requires further investigation, for example, the effect of catalyst properties on product composition and the differences between thermal cracking and catalytic cracking. What is more, the feasibility of X oil coprocessing with vacuum gas oil for fuel production in fluid catalytic cracking units should also be studied for the extensive use of this byproduct. The research in these areas will be reported in future work.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Naixin Wang: 0000-0001-5250-0902 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program of China (973 Program, No. 2012CB224801).

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REFERENCES

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