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Hydrocracking of the crude oil from thermal pyrolysis of municipal wastes over bi-functional Mo–Ni catalyst
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Hydrocracking of the crude oil from thermal pyrolysis of municipal wastes over bi-functional Mo–Ni catalyst Junhui Li, Hui Xia, Qian Wu, Zhonghua Hu, Zhixian Hao, Zhirong Zhu ∗ Department of Chemistry, College of Environmental Science and Engineering, Tongji University, 1239, Siping Road, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 30 March 2015 Received in revised form 4 August 2015 Accepted 7 August 2015 Available online xxx Keywords: Mo–Ni/SiO2 –Al2 O3 Bi-functional catalyst Pyrolysis oil Hydrocracking Municipal solid wastes
a b s t r a c t Bi-functional Mo–Ni/SiO2 –Al2 O3 catalyst was prepared by loading bimetal Ni–Mo over large porous and acidic SiO2 –Al2 O3 substrate via impregnation–precipitation–calcination–sulfurization procedure. Highly dispersed NiO clusters were loaded with urea as a precipitator. The catalytic performance for the hydrocracking of the crude oil from thermally pyrolysis of municipal solid wastes was investigated in a fixed bed reactor. It is found that SiO2 –Al2 O3 composites with the mid-acidity and large pores were more suitable as the substrate than Al2 O3 and Al2 O3 –Y(Y zeolite). Mo–Ni/SiO2 –Al2 O3 catalyst with 8 wt% Mo and 14 wt% Ni showed the higher reactivity and higher yield of fuel oil. Moreover, its hydrocracking reactivity could be further improved by partial sulfurization of Ni and Mo. Besides, the bi-functional Mo–Ni/SiO2 –Al2 O3 showed a good stability, and the deactivated catalyst could be easily regenerated by on-line combustion. Under the optimized conditions of 10 MPa, WHSV 0.3 h−1 and 698 K, the conversion of pyrolysis oil was up to 86% with 66% yield of fuel oil, greatly better than a commercial hydrocracking catalyst. The overall hydrocracking refinement converts most of the olefin, acid, ketones and poly-aromatic presented in the pyrolysis oil into alkanes, alcohols, and alkyl-aromatics, with a significant decrease of N and S contents as well as viscosity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The disposal of municipal solid wastes is a great challenge in the environmental technology over the world. The traditional treatment is landfilling or combustion; the former causes land resource-wasting, and the latter brings about many waste gases and huge amount of CO2 . A desirable way is converting the wastes to useful materials in the viewpoint of environment, economic and sustainable development. Municipal solid wastes consist of a lot of hydrocarbon-containing materials, such as scrap food and plastics. They can be converted to crude oil with light CO2 emission through pyrolysis [1–4] instead of landfill or combustion, and this process has come into a plot-scale unit in plant. As potential substitute for petroleum energy, biomass fuels, a major renewable energy source, have attracted much attention [5–10]. Many kinds of bio-fuels, such as bio-diesel and bio-ethanol produced from the conventional biomass were reported in the last decades [11–16], but it consumes a lot of foodstuffs and is not suitable to the large-population countries, such as China and India. A feasible way is utilizing waste materials to produce bio-fuels. So,
∗ Corresponding author. E-mail address: [email protected] (Z. Zhu).
any high hydrocarbon-containing wastes could be considered. It is reported that municipal solid wastes are important sources for producing biomass fuels [17–19]. Crude oil produced from municipal solid wastes through pyrolysis is a complicated mixture with a low fluidity, and cannot be used directly [20–22]. It is necessary to be refined in order to obtain normal fuels by catalytic process [23–25]. Among various refining techniques, catalytic hydrocracking is a preferred method [26–29]. It may significantly improve the quality of refined oil by hydrodesulfurization, hydrodenitrogenation, hydrodeasphaltenization, hydrodemetallization and decomposition of heavy components. The refining catalysts for the petroleum-based heavy oil are well developed and presented in the literature [30–33]. However, different from petroleum, the crude pyrolysis oil from municipal wastes contains a lot of non-hydrocarbon components in rich of nitrogen and oxygen. Therefore, new hydrocracking catalyst is needed for the refining of this type of complicated crude oil. In this work, a Mo–Ni/Al2 O3 catalyst was prepared and the influence of substrate, active components, loading method and post-treatment on the properties of the catalyst was investigated. The hydrocracking reactivity of the catalyst for crude oil from municipal solid wastes and the regeneration of used catalyst were studied.
http://dx.doi.org/10.1016/j.cattod.2015.08.034 0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Li, et al., Hydrocracking of the crude oil from thermal pyrolysis of municipal wastes over bi-functional Mo–Ni catalyst, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.034
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reaction temperature to get final sulfurized catalyst. The sulfur content was measured by chemical analysis [35–37].
2.3. Characterization and analysis
Fig. 1. Schematic diagram of the special made fluidized-bed used for the pyrolysis of the divers waste plastics separated from municipal solid wastes.
2. Experimental 2.1. Preparation of crude oil from municipal solid wastes The high hydrocarbon-containing stuffs, mainly divers waste plastics separated from municipal solid wastes, were pyrolyzed in a special-made fluidized bed unit at 693–773 K in the absence of oxygen. As shown in Fig. 1, the fluidized bed unit consisted of the fluidized tube bed in which a special Archimedean screw conveyer was fixed, and it was placed in tilting. In this way, the powdered wastes were fluidized with N2 flow, and the block wastes were also conveyed out. In the pyrolysis processing, together with the liquid condensed products, some gases and a little solid char were also produced. The liquid products were separated into water and oil phase. The oil phase was analyzed by the GC–MS (Agilent 5973N) and Elemental Analyzer (EuroVector EA3000), and it was chosen as the feedstock (crude oil) for hydrocracking reaction in this study.
2.2. Preparation of refining catalysts Mo–Ni modified catalyst was prepared by a modified impregnation–calcination method. First, support (Al2 O3 or SiO2 –Al2 O3 ) was impregnated with Ni(NO3 )2 aqueous solution at the room temperature. The Ni(NO3 )2 concentration was adjusted according to Ni loading. Support particles were put into the mixed solution of Ni(NO3 )2 and urea (the molar ratio of Ni(NO3 )2 and urea is 1:2). The mixture was stirred for 1 h and filtered. Then the solid was heated up to 363 K in a flask; dried at 393 K and calcined at 773 K to obtain NiO/Al2 O3 and NiO/SiO2 –Al2 O3 . Secondly, MoO3 was deposited on NiO modified catalysts by ordinary method according to the reference [34] to obtain highly dispersed NiO–MoO3 /Al2 O3 and NiO–MoO3 /SiO2 –Al2 O3 . The post-treatment of H2 reduction for Mo–Ni modified catalyst was performed in a H2 gas flow (1500 h−1 ) at 723 K for 6 h. The post-treatment of sulfurization for Mo–Ni modified catalyst was carried out in a mixed gas flow (1000 h−1 ) of H2 S and H2 at programmed temperature from 453 K to 653 K for 6 h. The extent of sulfurization was adjusted by varying H2 S concentration and
The particle morphology of samples was observed by means of scanning electron microscope (SEM, S-4800, Hitachi). The porosity parameters of Mo–Ni modified catalyst was measured by N2 adsorption at 77 K, with an automatic gas adsorption apparatus (Micromeritics ASAP 2000 V 2.5). The specific surface area and pore size distribution was calculated by BET and Barrett–Joyner–Halenda (BJH) method, respectively. The crystal property was studied by X-ray Diffraction (XRD) (D/MAX-2400 diffractometer with Cu K␣), and the crystallite sizes of NiO (dXRD ) was calculated by Scherrer formula [dXRD = k/(ˇ − ˇ0 ) cos , where the particle shape factor k was 0.89, was 43◦ , the wavelength was 0.154 nm and the half peak width ˇ was corrected for the instrumental line broadening ˇ0 ]. The acidity of the catalyst was studied by temperature-programmed desorption of ammonia (NH3 -TPD) (Altamira-100 Characterization System). After the sample was pretreated in helium at 773 K for 2 h, ammonia was adsorbed over the sample at 400 K. TPD of adsorbed NH3 was carried out in the carrier-gas flow of 25 ml/min helium from 393 K to 973 K with a heating rate of 12 K/min. Thermal conduction detection (TCD) signal and temperature corresponding to NH3 desorption were recorded simultaneously. The strength and the amount of the acidic sites were determined according to the temperature and the area of NH3 desorption peaks [38]. Temperature-programmed reduction with hydrogen (H2 -TPR) was performed by an automatic Altamira-100 Characterization System as well. After the sample was pretreated in helium at 773 K for 2 h, H2 -TPR was conducted in the flow of 10% H2 –N2 (25 ml/min) from 473 K to 973 K with a heating rate of 12 K/min. The TGA data were determined using a thermogravimetric analysis (Netzsch STA 449C, Germany). The dynamic viscosity and density for oil were measured by using an automated viscometer (YT-265Z, Yutong, Shanghai) and a density analyzer (SYD-1884, Zuofei, Shanghai) respectively.
2.4. Catalytic refining of pyrolysis oil The refining of pyrolysis oil (crude oil obtained as Section 2.1) was carried out in a stainless steel tubular fixed-bed reactor with 1.5 cm inner diameter. After the prepared catalyst (8.0 g) was packed into the reactor, it was post-treatment of sulfurization or H2 reduction. Then reactants were pumped into the reactor by a metering pump. In order to improve the fluidity of the feedstock, toluene was added as a solvent at 1/3 weight ratio of toluene to crude oil. Hydrocracking reaction was carried out under 693 K and 10 MPa and 0.3 h−1 WHSV of pyrolysis oil, at a hydrogen flow of 1800/1 (V/V). The product was distillated and the compounds with boiling point less than 673 K were collected and defined as fuel oil. According to the pyrolysis oil injected, the obtained fuel oil (except solvent toluene) and residue, the definitional conversion of pyrolysis oil and the yield of fuel oil were calculated as following: Conversion (%) = (Wpyrolysis oil − Wresidue )/Wpyrolysis oil × 100
(1)
Yield (%) = Conversion × Wfuel oil /(Wpyrolysis oil − Wresidue )
(2)
where Wpyrolysis oil , Wfuel oil and Wresidue are the weights of pyrolysis oil, fuel oil and residue, respectively. The composition of the obtained fuel oil was analyzed by the GC–MS (Agilent 5973N) and Elemental Analyzer (EuroVector EA3000).
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Table 1 Porosity and hydrocracking results of the catalysts prepared with different Al2 O3 supports. Catalyst
Support
SBET /m2 /g
D/nm
V/cc/g
Conversion/%
Yield/%
Cata-LP Cata-SP
LP-Al2 O3 SP-Al2 O3
184 206
9.7 4.5
0.460 0.315
70.1 57.2
49.8 40.3
LP-: large porous; SP-: small porous; SBET : BET specific surface area; D: average pore size; V: pore volume.
3. Results and discussion 3.1. Effect of Al2 O3 support on the catalytic activity In the refining course, hydrogenation and cracking are two main reactions taking place on the catalyst simultaneously. Porous Al2 O3 is often used as support for hydrocracking catalysts in the petrochemical processing [30]. In the present work, a large-pore and a small-pore Al2 O3 were chosen as support to prepare the catalysts with 10 wt% Mo and 12 wt% Ni loadings. The porosity of the two resulting catalysts was estimated by N2 adsorption. The porosity parameters and the hydrocracking performance of the two catalysts are listed in Table 1. It is evident that large-pore Al2 O3 -based catalyst (Cata-LP) has the larger average pore size and larger pore volume, but smaller specific surface area than small-pore Al2 O3 -based catalyst (Cata-SP). It can be found that Cata-LP exhibits a higher reactivity of hydrocracking and higher yield for fuel oil than Cata-SP. This result suggests that large-pore benefits to the diffusion of big molecules in pyrolysis oil. Though Cata-SP has a larger surface area, its smaller pores have a negative influence on the diffusion and utility of active sites, especially for large molecules. Similar result has been reported for supported Co–Mo catalysts in hydrotreating reaction [39,40]. Thus we can conclude that large-pore Al2 O3 is more suitable as support for hydrocracking catalyst. The support acidity generally plays a vital role in hydrocracking [41]. To improve the acidity of Al2 O3 , the selected Al2 O3 -LP was modified by adding 10 wt% SiO2 . The resulting SiO2 –Al2 O3 was used as support to prepare the Mo–Ni supported catalyst. SEM images (Fig. 2) showed that the catalysts prepared with SiO2 –Al2 O3 support have higher porosity and smaller particles than that with parent Al2 O3 , which may facilitate the diffusion of reactants with large molecular size. The acidity of the obtained Mo–Ni/SiO2 –Al2 O3 catalyst was estimated by NH3 -TPD, and the results were shown in Fig. 3. For comparison, a commercial hydrocracking catalyst Mo–Ni/Al2 O3 –Y(Y zeolite) with strong acidity and large porous Al2 O3 -dased catalyst were also tested. It is evident that the acidity is in the order of Mo–Ni/Al2 O3 –Y > Mo–Ni/SiO2 –Al2 O3 > Mo–Ni/Al2 O3 . The quantitative acidity and hydrocracking results for the pyrolysis oil are listed in Table 2. The change in the conversion had same order as that of acidity, suggesting that the catalyst with stronger acidity has the higher conversion. However, the yield of fuel oil is in the order of Mo–Ni/SiO2 –Al2 O3 > Mo–Ni/Al2 O3 –Y > Mo–Ni/Al2 O3 . The reason could be that too strong acidity led to over-cracking of the hydrogenated products to produce gaseous products of small molecular weight. As a result, a decrease in the yield of fuel oil was observed for Mo–Ni/Al2 O3 –Y. Therefore, it can be concluded that the acidity of support shows an obvious influence on the performance of hydrocracking catalyst [42,43]. Here the mid-acidity of Mo–Ni/SiO2 –Al2 O3 is suitable for improving both the hydrocracking reactivity and the yield of fuel oil.
impregnated with the aqueous solution of Ni(NO3 )2 was dried at 393 K, and calcinated at 773 K to produce NiO/Al2 O3 (NiO was reduced to Ni in hydrogen before hydrocracking tests). Usually, the metal oxides in the form of small particles were obtained on the support. Especially, metal oxides with high loading easily aggregate in the calcination process. In order to avoid the aggregation, the impregnation–precipitation procedure was modified to achieve highly dispersed metal oxides over the porous substrate. SiO2 –Al2 O3 substrate was put in the mixed solution of Ni(NO3 )2 and urea. After filtration, the impregnated SiO2 –Al2 O3 was dried at 363 K in the flask. During drying, urea was decomposed to produce ammonia providing an alkalescent medium. As a result, Ni(NO3 )2 was gradually transformed to highly dispersed Ni(OH)2 over SiO2 –Al2 O3 . In the conventional impregnation–calcination process, NiO is derived directly from Ni(NO3 )2 . Whereas, in the
3.2. Effect of preparation methods on catalytic reactivity In the conventional impregnation method for preparing the supported metal oxide catalysts [44–47], the Al2 O3 support
Fig. 2. SEM images of the catalysts with different Al2 O3 supports: (1) catalyst with parent Al2 O3 ; (2) catalyst with SiO2 –Al2 O3 .
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Fig. 3. Acidity estimated by NH3 -TPD for three Mo–Ni modified catalysts with different support: (1) SiO2 –Al2 O3 ; (2) Al2 O3 ; (3) Al2 O3 –Y.
Fig. 5. TPR with hydrogen of Mo–Ni/SiO2 –Al2 O3 catalysts prepared (1) ordinary impregnation; (2) impregnation–precipitation.
by ordinary impregnation. The former catalyst showed a higher reduction temperature than that of the latter one, indicating that strong interaction between metal oxides and SiO2 –Al2 O3 support due to the high dispersion of Mo–Ni loaded via the modified impregnation–precipitation procedure. The porosity and hydrocracking results of Mo–Ni/SiO2 –Al2 O3 catalysts prepared by two methods are listed in Table 3. The two catalysts showed similar textural properties including BET specific surface area, pore size, pore volume. The conversion of pyrolysis oil and the yield of fuel oil over IP-Cata were higher than those of OI-Cata, suggesting that the preparation method has a significant influence on the catalytic reactivity. 3.3. Effect of loading components on the catalytic reactivity Fig. 4. XRD patterns of the deposited 10%Mo–12%Ni catalyst over SiO2 –Al2 O3 : (1) prepared by impregnation–precipitation; (2) prepared by ordinary impregnation.
improved process, NiO is derived from Ni(NO3 )2 via the intermediate of Ni(OH)2 . Fig. 4 shows XRD patterns of the two Mo–Ni/SiO2 –Al2 O3 catalysts. The catalyst prepared by ordinary impregnation has evident peaks of NiO crystal. The crystallite sizes of NiO (dXRD ) (calculated by Scherrer formula) is about 14 nm. However, there is not NiO peak for the catalyst prepared by impregnation–precipitation, suggesting highly dispersed amorphous NiO species instead of crystallized NiO over SiO2 –Al2 O3 . Therefore, the impregnation–precipitation was an effective way for preparing the supported metallic catalyst with a good dispersion, especially for metal loading at the high content. Fig. 5 shows the H2 -TPR profiles of the two Mo–Ni/SiO2 –Al2 O3 catalysts prepared by impregnation–precipitation via urea and
Though noble metals Pt, Pd, Rh have high reactivity for the hydrogenation of crude oil, they are hardly used in the industrial scale due to the high cost [48]. Combining two less-cost metals (the individual has less reactivity than noble metals) as active components was proved to be effective for the hydrocracking in the petrochemical processing [49,50], such as bimetallic Ni–Cu catalyst for hydrocracking of pyrolysis oil from biomass [31]. The choice of metals as active component is highly related to the support, feedstock and the objective products. In this work, five metals, Ni, Co, Mo, Cr or W, were used to prepare bimetallic catalysts (using SiO2 –Al2 O3 as support), through impregnation–precipitation procedure. The total metal content was set at about 24 wt% referring to a commercial sample, Mo (16 wt%)–Ni (7 wt%)/Al2 O3 . Their catalytic performance is listed in Table 4. As can be seen, Mo–Ni catalyst had the highest reactivity and yield of fuel oil; Co–Mo had a high reactivity, but low
Table 2 Effect of substrate acidity on activity and yield in hydrocracking of pyrolysis oil. Catalyst
Acidic amount/mmol/g
Acidic strength
Conversion/%
Yield/%
Mo–Ni/Al2 O3 Mo–Ni/SiO2 –Al2 O3 Mo–Ni/Al2 O3 –Y
0.29 0.61 0.82
Weak Mid Strong
70.2 78.6 81.0
49.3 57.9 54.5
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Table 3 Porosity and hydrocracking results of Mo–Ni/SiO2 –Al2 O3 catalysts prepared by different impregnation methods. Catalyst
SBET /m2 /g
D/nm
V/ml/g
Conversion/%
Yield/%
OI-Cata IP-Cata
177 194
9.5 9.8
0.453 0.489
69.8 77.1
49.2 56.0
OI-: ordinary impregnation; IP-: impregnation–precipitation; SBET : BET specific surface area; D: average pore size; V: pore volume.
selectivity due to the formation of more gaseous by-products than other bimetallic catalysts. Furthermore, the influence of different loadings of Ni–Mo on the reactivity was also studied, as listed in Table 5. It is found that the catalyst with 8 wt% Mo and 14 wt% Ni showed the best performance for hydrocracking of the pyrolysis oil. Both samples with lower (Cata-4) or higher (Cata-5) total amount of Mo–Ni showed relative low reactivity. The former one might lack sufficient active materials; while the latter one could undergo metal aggregation or blockage of pores due to excessive loading. 3.4. Effect of sulfurization treatment on catalytic reactivity As is known, the sulfurized metallic catalyst has higher reactivity than the pristine catalyst in hydrogenation of petrochemical oil [42,51–53]. In this work, we also tested the effect of sulfurization treatment on the hydrocracking activity. The Mo–Ni/SiO2 –Al2 O3 catalyst was partially sulfurized in the ratio of 20%, 40% and 60% using H2 S as sulfurizing agent. The details are presented in Section 2. A commercial catalyst, the sulfided Mo–Ni/Al2 O3 (16 wt% Mo and 7 wt% Ni) was also used for comparison. The hydrocracking reactivity of sulfurized catalysts is listed in Table 6. It is clear that the sulfurization can obviously increase the reactivity, the conversion changes from 80.8% to 83.7–86.9% and the yield various from 57.1% to 61.4–65.8%. The catalyst sulfurized in a ratio of 40% (Mo–Ni–S-40) shows the best performance. Besides, it should be noted that both the sulfurized Mo–Ni/SiO2 –Al2 O3 and the precursor showed better catalytic performance than the commercial one. By means of an elemental analyzer, it is found that the pyrolysis oil from municipal solid wastes contains more than 15 wt% N, S and O elements, indicating a lot of non-hydrocarbon components. This composition is greatly different from pyrolysis oil of single waste plastics or petroleum products, shown in Table 7. Compared to petroleum-derived oil, it contains more nitrogen and oxygen compounds, but containing less sulfur compounds.
Table 4 Hydrocracking reactivity of different bimetallic catalysts. Catalyst components
Conversion/%
Yield/%
Mo–Ni W–Ni Cu–Ni Co–Mo W–Mo
77.4 70.2 68.5 75.9 72.0
56.1 52.6 51.7 52.3 51.8
Table 5 Hydrocracking reactivity of different loading Mo–Ni catalysts. Catalyst
Cata-1 Cata-2 Cata-3 Cata-4 Cata-5 Commercial
Composition/wt% Mo
Ni
10 8 6 6 10 7
12 14 16 10 18 16
Conversion/%
Yield/%
78.0 81.2 77.4 72.6 79.1 77.8
56.3 58.6 55.9 50.8 57.0 54.3
We have monitored the components in pyrolysis oil by GC–MS and the results (Table 8) suggest that a lot of carboxylic acids and olefins were present. These compounds are more easily hydrogenated by supported bimetallic catalysts compared with the corresponding mono-metallic form. Moreover, the metal sulfides are even more active for hydrodeoxygenation and hydrodenitrogenation of such compounds as listed in Table 8 [51,54], which well explained the observed superior performance of sulfurized Mo–Ni/SiO2 –Al2 O3 catalysts. Another finding from Table 8 is that alkane dominated (51%) in the fuel oil by the hydrocracking, which was only 17% in the pyrolysis oil. Accordingly, substantial decreases of olefins and acid/ketone from 17% and 21% to 2.4% and 3.3% were noted, respectively. Slight increase of alkyl-aromatics and alcohols were also observed. The significant change in components led to great decrease of dynamic viscosity and density (as shown in Table 9). Moreover, the N, S and O content of the fuel oil reduces greatly, especially for N and S. The obvious increase of alkane content mainly comes from hydrogenation of olefins, hydrodenitrogen and hydrodeoxygen. This result is consistent with previous findings that main reactions in the refinement of pyrolysis oil are hydrogenation of olefins and aromatics, hydrodesulfurization, hydrodeoxygen, hydrodenitrogenization and cracking reactions [42,55]. 3.5. Stability and regeneration of the catalyst The influence of processing conditions on the hydrocracking of pyrolysis oil in a fix-bed reactor was investigated. According to a number of experiment tests, the optional hydrocracking conditions are of 693 K, 0.3 h−1 WHSV, 10 MPa and hydrogen/oil 1800/1 (V/V) for the hydrocracking. The stability of Mo–Ni/SiO2 –Al2 O3 catalyst was continuously monitored with time on-stream as shown in Fig. 6. After the test for 100 h, the conversion and yield decrease slightly from 86% to 84% and from 68% to 63% respectively, suggesting that the catalyst showed rather high stability. It is attributed to the stable structure of Mo–Ni/SiO2 –Al2 O3 , in which Mo–Ni components were highly dispersed on Al2 O3 with a strong interaction caused by the impregnation–precipitation procedure, preventing metal deposition which can cause the decrease of sulfide-phase active sites [53]. Moreover, the large porous Al2 O3 support used not only shows the high coke capacity, but also is favorable to prevent coke formation during the hydrocracking [53]. The deactivation of Mo–Ni/SiO2 –Al2 O3 was accelerated in the hydrocracking process under the conditions of high temperature 723 K, high 0.6 h−1 WHSV and low 900/1 (V/V) ratio of hydrogen to oil. As a result, the conversion (tested again at the normal conditions) reduced by 72.4% after the reaction time of 50 h. The deactivated Mo–Ni/SiO2 –Al2 O3 catalyst was investigated by TGA test in the air flow, as shown in Fig. 7. The weight loss is small Table 6 Hydrocracking reactivity over the Mo–Ni catalysts with the partial sulfurization. Catalyst
Sulfurization ratio/%
Conversion/%
Yield/%
Mo–Ni–S-60 Mo–Ni–S-40 Mo–Ni–S-20 Mo–Ni Commercial
60 40 20 0 –
84.2 86.9 83.7 80.8 77.5
63.0 65.8 61.4 57.1 54.2
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Table 7 The elemental composition of the pyrolysis oil and the fuel oil product. Oil
C/%
H/%
S/%
N/%
O/%
Pyrolysis oil Fuel oil
73.5 81.0
10.7 12.8
0.34 0.05
0.90 0.13
14.6 5.9
Table 8 The compound composition (%) of the pyrolysis oil and the fuel oil obtained. Oil
Olefin
Alkane
Poly-aromatics
Alkyl-aromatics
Acid and ketone
Alcohol and ether
Nitrogen compound
Other
Pyrolysis oil Fuel oil
17 2.4
17 51
5.4 1.8
7.0 10
21 3.3
19 21
3.5 0.40
10 9.6
Table 9 The characteristics of pyrolysis oil and fuel oil. Oil
Initial boiling point (K)
Final boiling point (K)
Density (g/cm3 )
Viscosity (323 K, cst)
Pyrolysis oil Fuel oil
335 320
783 673
0.95 0.83
197 18
Table 10 Catalytic reactivity of the catalyst before and after regeneration.
Conversion or yield (%)
90
80
Conversion Yield 70
60
50 20
40
60
80
100
Time on stream / h Fig. 6. Changes of conversion and yield as a function of reaction time in the hydrocracking with Mo–Ni/SiO2 –Al2 O3 catalyst.
Fig. 7. TGA analysis of the deactivated Mo–Ni catalyst in hydrocracking process (1) the fresh catalyst; (2) the deactivated catalyst.
Catalyst
Conversion/%
Yield/%
Fresh Deactivated After regeneration
87.1 72.4 86.5
65.8 53.0 65.3
at the temperatures lower than 623 K due to desorption of moisture and hydrocarbons with small molecular weight. A significant weight loss appeared at 623 K to 873 K, indicating that the coke was completely oxidized and combusted at temperatures lower than 873 K. This suggests that the regeneration can be performed at temperature lower than 873 K. The regeneration of deactivated Mo–Ni catalyst was performed by a temperature-programmed calcination with a rising rate of 2 K/min to 823 K for 1 h in the nitrogen atmosphere containing 5% oxygen. The catalytic activity of the regenerated catalyst is listed in Table 10. It is found that the reactivity could be recovered by more than 99%, indicating that Mo–Ni/SiO2 –Al2 O3 had an excellent regeneration capability. 4. Conclusions Highly efficient hydrocracking Mo–Ni/SiO2 –Al2 O3 catalyst for the refinement of pyrolysis oil from municipal wastes was prepared by a modified precipitation–impregnation–calcination procedure using urea as precipitation reagent. The presence of urea benefits to loading highly dispersed NiO species. The acidity and porous properties of the supports played an important role in hydrocracking. SiO2 –Al2 O3 composite with the mid-acidity and large pores is suitable for the diffusion and conversion of large molecules in pyrolysis oil. The bifunctional catalyst using this support with the loading of 8 wt% Mo and 14 wt% Ni, and 40% partial sulfurization of Mo and Ni, showed the best performance for refining the pyrolysis oil, greatly better than a commercial hydrocracking catalyst. Under the optimized conditions of 10 MPa, WHSV 0.3 h−1 and 698 K, the yield of refining oil (fuel oil) was up to 66% with 86% conversion of the pyrolysis oil. Besides, the resultant Mo–Ni/SiO2 –Al2 O3 showed a high stability. The conversion and yield decreased slightly from 86% to 84% and 68% to 63% respectively, during the time on-stream of 100 h in a fixed reactor. In addition, the deactivated catalyst could be easily regenerated by combusting coke and the reactivity could be recovered by more than 99% of the fresh catalyst. The bi-functional Mo–Ni/SiO2 –Al2 O3 presented is a promising hydrocracking catalyst, especially for refining the complicated raw oil,
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Hydrocracking of the crude oil from thermal pyrolysis of municipal wastes over bi-functional Mo–Ni catalyst Junhui Li, Hui Xia, Qian Wu, Zhonghua Hu, Zhixian Hao, Zhirong Zhu ∗ Department of Chemistry, College of Environmental Science and Engineering, Tongji University, 1239, Siping Road, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 30 March 2015 Received in revised form 4 August 2015 Accepted 7 August 2015 Available online xxx Keywords: Mo–Ni/SiO2 –Al2 O3 Bi-functional catalyst Pyrolysis oil Hydrocracking Municipal solid wastes
a b s t r a c t Bi-functional Mo–Ni/SiO2 –Al2 O3 catalyst was prepared by loading bimetal Ni–Mo over large porous and acidic SiO2 –Al2 O3 substrate via impregnation–precipitation–calcination–sulfurization procedure. Highly dispersed NiO clusters were loaded with urea as a precipitator. The catalytic performance for the hydrocracking of the crude oil from thermally pyrolysis of municipal solid wastes was investigated in a fixed bed reactor. It is found that SiO2 –Al2 O3 composites with the mid-acidity and large pores were more suitable as the substrate than Al2 O3 and Al2 O3 –Y(Y zeolite). Mo–Ni/SiO2 –Al2 O3 catalyst with 8 wt% Mo and 14 wt% Ni showed the higher reactivity and higher yield of fuel oil. Moreover, its hydrocracking reactivity could be further improved by partial sulfurization of Ni and Mo. Besides, the bi-functional Mo–Ni/SiO2 –Al2 O3 showed a good stability, and the deactivated catalyst could be easily regenerated by on-line combustion. Under the optimized conditions of 10 MPa, WHSV 0.3 h−1 and 698 K, the conversion of pyrolysis oil was up to 86% with 66% yield of fuel oil, greatly better than a commercial hydrocracking catalyst. The overall hydrocracking refinement converts most of the olefin, acid, ketones and poly-aromatic presented in the pyrolysis oil into alkanes, alcohols, and alkyl-aromatics, with a significant decrease of N and S contents as well as viscosity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The disposal of municipal solid wastes is a great challenge in the environmental technology over the world. The traditional treatment is landfilling or combustion; the former causes land resource-wasting, and the latter brings about many waste gases and huge amount of CO2 . A desirable way is converting the wastes to useful materials in the viewpoint of environment, economic and sustainable development. Municipal solid wastes consist of a lot of hydrocarbon-containing materials, such as scrap food and plastics. They can be converted to crude oil with light CO2 emission through pyrolysis [1–4] instead of landfill or combustion, and this process has come into a plot-scale unit in plant. As potential substitute for petroleum energy, biomass fuels, a major renewable energy source, have attracted much attention [5–10]. Many kinds of bio-fuels, such as bio-diesel and bio-ethanol produced from the conventional biomass were reported in the last decades [11–16], but it consumes a lot of foodstuffs and is not suitable to the large-population countries, such as China and India. A feasible way is utilizing waste materials to produce bio-fuels. So,
∗ Corresponding author. E-mail address: [email protected] (Z. Zhu).
any high hydrocarbon-containing wastes could be considered. It is reported that municipal solid wastes are important sources for producing biomass fuels [17–19]. Crude oil produced from municipal solid wastes through pyrolysis is a complicated mixture with a low fluidity, and cannot be used directly [20–22]. It is necessary to be refined in order to obtain normal fuels by catalytic process [23–25]. Among various refining techniques, catalytic hydrocracking is a preferred method [26–29]. It may significantly improve the quality of refined oil by hydrodesulfurization, hydrodenitrogenation, hydrodeasphaltenization, hydrodemetallization and decomposition of heavy components. The refining catalysts for the petroleum-based heavy oil are well developed and presented in the literature [30–33]. However, different from petroleum, the crude pyrolysis oil from municipal wastes contains a lot of non-hydrocarbon components in rich of nitrogen and oxygen. Therefore, new hydrocracking catalyst is needed for the refining of this type of complicated crude oil. In this work, a Mo–Ni/Al2 O3 catalyst was prepared and the influence of substrate, active components, loading method and post-treatment on the properties of the catalyst was investigated. The hydrocracking reactivity of the catalyst for crude oil from municipal solid wastes and the regeneration of used catalyst were studied.
http://dx.doi.org/10.1016/j.cattod.2015.08.034 0920-5861/© 2015 Elsevier B.V. All rights reserved.
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reaction temperature to get final sulfurized catalyst. The sulfur content was measured by chemical analysis [35–37].
2.3. Characterization and analysis
Fig. 1. Schematic diagram of the special made fluidized-bed used for the pyrolysis of the divers waste plastics separated from municipal solid wastes.
2. Experimental 2.1. Preparation of crude oil from municipal solid wastes The high hydrocarbon-containing stuffs, mainly divers waste plastics separated from municipal solid wastes, were pyrolyzed in a special-made fluidized bed unit at 693–773 K in the absence of oxygen. As shown in Fig. 1, the fluidized bed unit consisted of the fluidized tube bed in which a special Archimedean screw conveyer was fixed, and it was placed in tilting. In this way, the powdered wastes were fluidized with N2 flow, and the block wastes were also conveyed out. In the pyrolysis processing, together with the liquid condensed products, some gases and a little solid char were also produced. The liquid products were separated into water and oil phase. The oil phase was analyzed by the GC–MS (Agilent 5973N) and Elemental Analyzer (EuroVector EA3000), and it was chosen as the feedstock (crude oil) for hydrocracking reaction in this study.
2.2. Preparation of refining catalysts Mo–Ni modified catalyst was prepared by a modified impregnation–calcination method. First, support (Al2 O3 or SiO2 –Al2 O3 ) was impregnated with Ni(NO3 )2 aqueous solution at the room temperature. The Ni(NO3 )2 concentration was adjusted according to Ni loading. Support particles were put into the mixed solution of Ni(NO3 )2 and urea (the molar ratio of Ni(NO3 )2 and urea is 1:2). The mixture was stirred for 1 h and filtered. Then the solid was heated up to 363 K in a flask; dried at 393 K and calcined at 773 K to obtain NiO/Al2 O3 and NiO/SiO2 –Al2 O3 . Secondly, MoO3 was deposited on NiO modified catalysts by ordinary method according to the reference [34] to obtain highly dispersed NiO–MoO3 /Al2 O3 and NiO–MoO3 /SiO2 –Al2 O3 . The post-treatment of H2 reduction for Mo–Ni modified catalyst was performed in a H2 gas flow (1500 h−1 ) at 723 K for 6 h. The post-treatment of sulfurization for Mo–Ni modified catalyst was carried out in a mixed gas flow (1000 h−1 ) of H2 S and H2 at programmed temperature from 453 K to 653 K for 6 h. The extent of sulfurization was adjusted by varying H2 S concentration and
The particle morphology of samples was observed by means of scanning electron microscope (SEM, S-4800, Hitachi). The porosity parameters of Mo–Ni modified catalyst was measured by N2 adsorption at 77 K, with an automatic gas adsorption apparatus (Micromeritics ASAP 2000 V 2.5). The specific surface area and pore size distribution was calculated by BET and Barrett–Joyner–Halenda (BJH) method, respectively. The crystal property was studied by X-ray Diffraction (XRD) (D/MAX-2400 diffractometer with Cu K␣), and the crystallite sizes of NiO (dXRD ) was calculated by Scherrer formula [dXRD = k/(ˇ − ˇ0 ) cos , where the particle shape factor k was 0.89, was 43◦ , the wavelength was 0.154 nm and the half peak width ˇ was corrected for the instrumental line broadening ˇ0 ]. The acidity of the catalyst was studied by temperature-programmed desorption of ammonia (NH3 -TPD) (Altamira-100 Characterization System). After the sample was pretreated in helium at 773 K for 2 h, ammonia was adsorbed over the sample at 400 K. TPD of adsorbed NH3 was carried out in the carrier-gas flow of 25 ml/min helium from 393 K to 973 K with a heating rate of 12 K/min. Thermal conduction detection (TCD) signal and temperature corresponding to NH3 desorption were recorded simultaneously. The strength and the amount of the acidic sites were determined according to the temperature and the area of NH3 desorption peaks [38]. Temperature-programmed reduction with hydrogen (H2 -TPR) was performed by an automatic Altamira-100 Characterization System as well. After the sample was pretreated in helium at 773 K for 2 h, H2 -TPR was conducted in the flow of 10% H2 –N2 (25 ml/min) from 473 K to 973 K with a heating rate of 12 K/min. The TGA data were determined using a thermogravimetric analysis (Netzsch STA 449C, Germany). The dynamic viscosity and density for oil were measured by using an automated viscometer (YT-265Z, Yutong, Shanghai) and a density analyzer (SYD-1884, Zuofei, Shanghai) respectively.
2.4. Catalytic refining of pyrolysis oil The refining of pyrolysis oil (crude oil obtained as Section 2.1) was carried out in a stainless steel tubular fixed-bed reactor with 1.5 cm inner diameter. After the prepared catalyst (8.0 g) was packed into the reactor, it was post-treatment of sulfurization or H2 reduction. Then reactants were pumped into the reactor by a metering pump. In order to improve the fluidity of the feedstock, toluene was added as a solvent at 1/3 weight ratio of toluene to crude oil. Hydrocracking reaction was carried out under 693 K and 10 MPa and 0.3 h−1 WHSV of pyrolysis oil, at a hydrogen flow of 1800/1 (V/V). The product was distillated and the compounds with boiling point less than 673 K were collected and defined as fuel oil. According to the pyrolysis oil injected, the obtained fuel oil (except solvent toluene) and residue, the definitional conversion of pyrolysis oil and the yield of fuel oil were calculated as following: Conversion (%) = (Wpyrolysis oil − Wresidue )/Wpyrolysis oil × 100
(1)
Yield (%) = Conversion × Wfuel oil /(Wpyrolysis oil − Wresidue )
(2)
where Wpyrolysis oil , Wfuel oil and Wresidue are the weights of pyrolysis oil, fuel oil and residue, respectively. The composition of the obtained fuel oil was analyzed by the GC–MS (Agilent 5973N) and Elemental Analyzer (EuroVector EA3000).
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Table 1 Porosity and hydrocracking results of the catalysts prepared with different Al2 O3 supports. Catalyst
Support
SBET /m2 /g
D/nm
V/cc/g
Conversion/%
Yield/%
Cata-LP Cata-SP
LP-Al2 O3 SP-Al2 O3
184 206
9.7 4.5
0.460 0.315
70.1 57.2
49.8 40.3
LP-: large porous; SP-: small porous; SBET : BET specific surface area; D: average pore size; V: pore volume.
3. Results and discussion 3.1. Effect of Al2 O3 support on the catalytic activity In the refining course, hydrogenation and cracking are two main reactions taking place on the catalyst simultaneously. Porous Al2 O3 is often used as support for hydrocracking catalysts in the petrochemical processing [30]. In the present work, a large-pore and a small-pore Al2 O3 were chosen as support to prepare the catalysts with 10 wt% Mo and 12 wt% Ni loadings. The porosity of the two resulting catalysts was estimated by N2 adsorption. The porosity parameters and the hydrocracking performance of the two catalysts are listed in Table 1. It is evident that large-pore Al2 O3 -based catalyst (Cata-LP) has the larger average pore size and larger pore volume, but smaller specific surface area than small-pore Al2 O3 -based catalyst (Cata-SP). It can be found that Cata-LP exhibits a higher reactivity of hydrocracking and higher yield for fuel oil than Cata-SP. This result suggests that large-pore benefits to the diffusion of big molecules in pyrolysis oil. Though Cata-SP has a larger surface area, its smaller pores have a negative influence on the diffusion and utility of active sites, especially for large molecules. Similar result has been reported for supported Co–Mo catalysts in hydrotreating reaction [39,40]. Thus we can conclude that large-pore Al2 O3 is more suitable as support for hydrocracking catalyst. The support acidity generally plays a vital role in hydrocracking [41]. To improve the acidity of Al2 O3 , the selected Al2 O3 -LP was modified by adding 10 wt% SiO2 . The resulting SiO2 –Al2 O3 was used as support to prepare the Mo–Ni supported catalyst. SEM images (Fig. 2) showed that the catalysts prepared with SiO2 –Al2 O3 support have higher porosity and smaller particles than that with parent Al2 O3 , which may facilitate the diffusion of reactants with large molecular size. The acidity of the obtained Mo–Ni/SiO2 –Al2 O3 catalyst was estimated by NH3 -TPD, and the results were shown in Fig. 3. For comparison, a commercial hydrocracking catalyst Mo–Ni/Al2 O3 –Y(Y zeolite) with strong acidity and large porous Al2 O3 -dased catalyst were also tested. It is evident that the acidity is in the order of Mo–Ni/Al2 O3 –Y > Mo–Ni/SiO2 –Al2 O3 > Mo–Ni/Al2 O3 . The quantitative acidity and hydrocracking results for the pyrolysis oil are listed in Table 2. The change in the conversion had same order as that of acidity, suggesting that the catalyst with stronger acidity has the higher conversion. However, the yield of fuel oil is in the order of Mo–Ni/SiO2 –Al2 O3 > Mo–Ni/Al2 O3 –Y > Mo–Ni/Al2 O3 . The reason could be that too strong acidity led to over-cracking of the hydrogenated products to produce gaseous products of small molecular weight. As a result, a decrease in the yield of fuel oil was observed for Mo–Ni/Al2 O3 –Y. Therefore, it can be concluded that the acidity of support shows an obvious influence on the performance of hydrocracking catalyst [42,43]. Here the mid-acidity of Mo–Ni/SiO2 –Al2 O3 is suitable for improving both the hydrocracking reactivity and the yield of fuel oil.
impregnated with the aqueous solution of Ni(NO3 )2 was dried at 393 K, and calcinated at 773 K to produce NiO/Al2 O3 (NiO was reduced to Ni in hydrogen before hydrocracking tests). Usually, the metal oxides in the form of small particles were obtained on the support. Especially, metal oxides with high loading easily aggregate in the calcination process. In order to avoid the aggregation, the impregnation–precipitation procedure was modified to achieve highly dispersed metal oxides over the porous substrate. SiO2 –Al2 O3 substrate was put in the mixed solution of Ni(NO3 )2 and urea. After filtration, the impregnated SiO2 –Al2 O3 was dried at 363 K in the flask. During drying, urea was decomposed to produce ammonia providing an alkalescent medium. As a result, Ni(NO3 )2 was gradually transformed to highly dispersed Ni(OH)2 over SiO2 –Al2 O3 . In the conventional impregnation–calcination process, NiO is derived directly from Ni(NO3 )2 . Whereas, in the
3.2. Effect of preparation methods on catalytic reactivity In the conventional impregnation method for preparing the supported metal oxide catalysts [44–47], the Al2 O3 support
Fig. 2. SEM images of the catalysts with different Al2 O3 supports: (1) catalyst with parent Al2 O3 ; (2) catalyst with SiO2 –Al2 O3 .
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Fig. 3. Acidity estimated by NH3 -TPD for three Mo–Ni modified catalysts with different support: (1) SiO2 –Al2 O3 ; (2) Al2 O3 ; (3) Al2 O3 –Y.
Fig. 5. TPR with hydrogen of Mo–Ni/SiO2 –Al2 O3 catalysts prepared (1) ordinary impregnation; (2) impregnation–precipitation.
by ordinary impregnation. The former catalyst showed a higher reduction temperature than that of the latter one, indicating that strong interaction between metal oxides and SiO2 –Al2 O3 support due to the high dispersion of Mo–Ni loaded via the modified impregnation–precipitation procedure. The porosity and hydrocracking results of Mo–Ni/SiO2 –Al2 O3 catalysts prepared by two methods are listed in Table 3. The two catalysts showed similar textural properties including BET specific surface area, pore size, pore volume. The conversion of pyrolysis oil and the yield of fuel oil over IP-Cata were higher than those of OI-Cata, suggesting that the preparation method has a significant influence on the catalytic reactivity. 3.3. Effect of loading components on the catalytic reactivity Fig. 4. XRD patterns of the deposited 10%Mo–12%Ni catalyst over SiO2 –Al2 O3 : (1) prepared by impregnation–precipitation; (2) prepared by ordinary impregnation.
improved process, NiO is derived from Ni(NO3 )2 via the intermediate of Ni(OH)2 . Fig. 4 shows XRD patterns of the two Mo–Ni/SiO2 –Al2 O3 catalysts. The catalyst prepared by ordinary impregnation has evident peaks of NiO crystal. The crystallite sizes of NiO (dXRD ) (calculated by Scherrer formula) is about 14 nm. However, there is not NiO peak for the catalyst prepared by impregnation–precipitation, suggesting highly dispersed amorphous NiO species instead of crystallized NiO over SiO2 –Al2 O3 . Therefore, the impregnation–precipitation was an effective way for preparing the supported metallic catalyst with a good dispersion, especially for metal loading at the high content. Fig. 5 shows the H2 -TPR profiles of the two Mo–Ni/SiO2 –Al2 O3 catalysts prepared by impregnation–precipitation via urea and
Though noble metals Pt, Pd, Rh have high reactivity for the hydrogenation of crude oil, they are hardly used in the industrial scale due to the high cost [48]. Combining two less-cost metals (the individual has less reactivity than noble metals) as active components was proved to be effective for the hydrocracking in the petrochemical processing [49,50], such as bimetallic Ni–Cu catalyst for hydrocracking of pyrolysis oil from biomass [31]. The choice of metals as active component is highly related to the support, feedstock and the objective products. In this work, five metals, Ni, Co, Mo, Cr or W, were used to prepare bimetallic catalysts (using SiO2 –Al2 O3 as support), through impregnation–precipitation procedure. The total metal content was set at about 24 wt% referring to a commercial sample, Mo (16 wt%)–Ni (7 wt%)/Al2 O3 . Their catalytic performance is listed in Table 4. As can be seen, Mo–Ni catalyst had the highest reactivity and yield of fuel oil; Co–Mo had a high reactivity, but low
Table 2 Effect of substrate acidity on activity and yield in hydrocracking of pyrolysis oil. Catalyst
Acidic amount/mmol/g
Acidic strength
Conversion/%
Yield/%
Mo–Ni/Al2 O3 Mo–Ni/SiO2 –Al2 O3 Mo–Ni/Al2 O3 –Y
0.29 0.61 0.82
Weak Mid Strong
70.2 78.6 81.0
49.3 57.9 54.5
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Table 3 Porosity and hydrocracking results of Mo–Ni/SiO2 –Al2 O3 catalysts prepared by different impregnation methods. Catalyst
SBET /m2 /g
D/nm
V/ml/g
Conversion/%
Yield/%
OI-Cata IP-Cata
177 194
9.5 9.8
0.453 0.489
69.8 77.1
49.2 56.0
OI-: ordinary impregnation; IP-: impregnation–precipitation; SBET : BET specific surface area; D: average pore size; V: pore volume.
selectivity due to the formation of more gaseous by-products than other bimetallic catalysts. Furthermore, the influence of different loadings of Ni–Mo on the reactivity was also studied, as listed in Table 5. It is found that the catalyst with 8 wt% Mo and 14 wt% Ni showed the best performance for hydrocracking of the pyrolysis oil. Both samples with lower (Cata-4) or higher (Cata-5) total amount of Mo–Ni showed relative low reactivity. The former one might lack sufficient active materials; while the latter one could undergo metal aggregation or blockage of pores due to excessive loading. 3.4. Effect of sulfurization treatment on catalytic reactivity As is known, the sulfurized metallic catalyst has higher reactivity than the pristine catalyst in hydrogenation of petrochemical oil [42,51–53]. In this work, we also tested the effect of sulfurization treatment on the hydrocracking activity. The Mo–Ni/SiO2 –Al2 O3 catalyst was partially sulfurized in the ratio of 20%, 40% and 60% using H2 S as sulfurizing agent. The details are presented in Section 2. A commercial catalyst, the sulfided Mo–Ni/Al2 O3 (16 wt% Mo and 7 wt% Ni) was also used for comparison. The hydrocracking reactivity of sulfurized catalysts is listed in Table 6. It is clear that the sulfurization can obviously increase the reactivity, the conversion changes from 80.8% to 83.7–86.9% and the yield various from 57.1% to 61.4–65.8%. The catalyst sulfurized in a ratio of 40% (Mo–Ni–S-40) shows the best performance. Besides, it should be noted that both the sulfurized Mo–Ni/SiO2 –Al2 O3 and the precursor showed better catalytic performance than the commercial one. By means of an elemental analyzer, it is found that the pyrolysis oil from municipal solid wastes contains more than 15 wt% N, S and O elements, indicating a lot of non-hydrocarbon components. This composition is greatly different from pyrolysis oil of single waste plastics or petroleum products, shown in Table 7. Compared to petroleum-derived oil, it contains more nitrogen and oxygen compounds, but containing less sulfur compounds.
Table 4 Hydrocracking reactivity of different bimetallic catalysts. Catalyst components
Conversion/%
Yield/%
Mo–Ni W–Ni Cu–Ni Co–Mo W–Mo
77.4 70.2 68.5 75.9 72.0
56.1 52.6 51.7 52.3 51.8
Table 5 Hydrocracking reactivity of different loading Mo–Ni catalysts. Catalyst
Cata-1 Cata-2 Cata-3 Cata-4 Cata-5 Commercial
Composition/wt% Mo
Ni
10 8 6 6 10 7
12 14 16 10 18 16
Conversion/%
Yield/%
78.0 81.2 77.4 72.6 79.1 77.8
56.3 58.6 55.9 50.8 57.0 54.3
We have monitored the components in pyrolysis oil by GC–MS and the results (Table 8) suggest that a lot of carboxylic acids and olefins were present. These compounds are more easily hydrogenated by supported bimetallic catalysts compared with the corresponding mono-metallic form. Moreover, the metal sulfides are even more active for hydrodeoxygenation and hydrodenitrogenation of such compounds as listed in Table 8 [51,54], which well explained the observed superior performance of sulfurized Mo–Ni/SiO2 –Al2 O3 catalysts. Another finding from Table 8 is that alkane dominated (51%) in the fuel oil by the hydrocracking, which was only 17% in the pyrolysis oil. Accordingly, substantial decreases of olefins and acid/ketone from 17% and 21% to 2.4% and 3.3% were noted, respectively. Slight increase of alkyl-aromatics and alcohols were also observed. The significant change in components led to great decrease of dynamic viscosity and density (as shown in Table 9). Moreover, the N, S and O content of the fuel oil reduces greatly, especially for N and S. The obvious increase of alkane content mainly comes from hydrogenation of olefins, hydrodenitrogen and hydrodeoxygen. This result is consistent with previous findings that main reactions in the refinement of pyrolysis oil are hydrogenation of olefins and aromatics, hydrodesulfurization, hydrodeoxygen, hydrodenitrogenization and cracking reactions [42,55]. 3.5. Stability and regeneration of the catalyst The influence of processing conditions on the hydrocracking of pyrolysis oil in a fix-bed reactor was investigated. According to a number of experiment tests, the optional hydrocracking conditions are of 693 K, 0.3 h−1 WHSV, 10 MPa and hydrogen/oil 1800/1 (V/V) for the hydrocracking. The stability of Mo–Ni/SiO2 –Al2 O3 catalyst was continuously monitored with time on-stream as shown in Fig. 6. After the test for 100 h, the conversion and yield decrease slightly from 86% to 84% and from 68% to 63% respectively, suggesting that the catalyst showed rather high stability. It is attributed to the stable structure of Mo–Ni/SiO2 –Al2 O3 , in which Mo–Ni components were highly dispersed on Al2 O3 with a strong interaction caused by the impregnation–precipitation procedure, preventing metal deposition which can cause the decrease of sulfide-phase active sites [53]. Moreover, the large porous Al2 O3 support used not only shows the high coke capacity, but also is favorable to prevent coke formation during the hydrocracking [53]. The deactivation of Mo–Ni/SiO2 –Al2 O3 was accelerated in the hydrocracking process under the conditions of high temperature 723 K, high 0.6 h−1 WHSV and low 900/1 (V/V) ratio of hydrogen to oil. As a result, the conversion (tested again at the normal conditions) reduced by 72.4% after the reaction time of 50 h. The deactivated Mo–Ni/SiO2 –Al2 O3 catalyst was investigated by TGA test in the air flow, as shown in Fig. 7. The weight loss is small Table 6 Hydrocracking reactivity over the Mo–Ni catalysts with the partial sulfurization. Catalyst
Sulfurization ratio/%
Conversion/%
Yield/%
Mo–Ni–S-60 Mo–Ni–S-40 Mo–Ni–S-20 Mo–Ni Commercial
60 40 20 0 –
84.2 86.9 83.7 80.8 77.5
63.0 65.8 61.4 57.1 54.2
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Table 7 The elemental composition of the pyrolysis oil and the fuel oil product. Oil
C/%
H/%
S/%
N/%
O/%
Pyrolysis oil Fuel oil
73.5 81.0
10.7 12.8
0.34 0.05
0.90 0.13
14.6 5.9
Table 8 The compound composition (%) of the pyrolysis oil and the fuel oil obtained. Oil
Olefin
Alkane
Poly-aromatics
Alkyl-aromatics
Acid and ketone
Alcohol and ether
Nitrogen compound
Other
Pyrolysis oil Fuel oil
17 2.4
17 51
5.4 1.8
7.0 10
21 3.3
19 21
3.5 0.40
10 9.6
Table 9 The characteristics of pyrolysis oil and fuel oil. Oil
Initial boiling point (K)
Final boiling point (K)
Density (g/cm3 )
Viscosity (323 K, cst)
Pyrolysis oil Fuel oil
335 320
783 673
0.95 0.83
197 18
Table 10 Catalytic reactivity of the catalyst before and after regeneration.
Conversion or yield (%)
90
80
Conversion Yield 70
60
50 20
40
60
80
100
Time on stream / h Fig. 6. Changes of conversion and yield as a function of reaction time in the hydrocracking with Mo–Ni/SiO2 –Al2 O3 catalyst.
Fig. 7. TGA analysis of the deactivated Mo–Ni catalyst in hydrocracking process (1) the fresh catalyst; (2) the deactivated catalyst.
Catalyst
Conversion/%
Yield/%
Fresh Deactivated After regeneration
87.1 72.4 86.5
65.8 53.0 65.3
at the temperatures lower than 623 K due to desorption of moisture and hydrocarbons with small molecular weight. A significant weight loss appeared at 623 K to 873 K, indicating that the coke was completely oxidized and combusted at temperatures lower than 873 K. This suggests that the regeneration can be performed at temperature lower than 873 K. The regeneration of deactivated Mo–Ni catalyst was performed by a temperature-programmed calcination with a rising rate of 2 K/min to 823 K for 1 h in the nitrogen atmosphere containing 5% oxygen. The catalytic activity of the regenerated catalyst is listed in Table 10. It is found that the reactivity could be recovered by more than 99%, indicating that Mo–Ni/SiO2 –Al2 O3 had an excellent regeneration capability. 4. Conclusions Highly efficient hydrocracking Mo–Ni/SiO2 –Al2 O3 catalyst for the refinement of pyrolysis oil from municipal wastes was prepared by a modified precipitation–impregnation–calcination procedure using urea as precipitation reagent. The presence of urea benefits to loading highly dispersed NiO species. The acidity and porous properties of the supports played an important role in hydrocracking. SiO2 –Al2 O3 composite with the mid-acidity and large pores is suitable for the diffusion and conversion of large molecules in pyrolysis oil. The bifunctional catalyst using this support with the loading of 8 wt% Mo and 14 wt% Ni, and 40% partial sulfurization of Mo and Ni, showed the best performance for refining the pyrolysis oil, greatly better than a commercial hydrocracking catalyst. Under the optimized conditions of 10 MPa, WHSV 0.3 h−1 and 698 K, the yield of refining oil (fuel oil) was up to 66% with 86% conversion of the pyrolysis oil. Besides, the resultant Mo–Ni/SiO2 –Al2 O3 showed a high stability. The conversion and yield decreased slightly from 86% to 84% and 68% to 63% respectively, during the time on-stream of 100 h in a fixed reactor. In addition, the deactivated catalyst could be easily regenerated by combusting coke and the reactivity could be recovered by more than 99% of the fresh catalyst. The bi-functional Mo–Ni/SiO2 –Al2 O3 presented is a promising hydrocracking catalyst, especially for refining the complicated raw oil,
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Please cite this article in press as: J. Li, et al., Hydrocracking of the crude oil from thermal pyrolysis of municipal wastes over bi-functional Mo–Ni catalyst, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.034